RU2599636C2 - Method of purified terephthalic acid producing - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method of producing purified terephthalic acid, in which it is produced without using a dryer, wet crystals of crude terephthalic acid are purified in the form of a suspension with the help of an aqueous liquid without drying damp crystals to a crystalline powder. Here take place stages of washing of wet crystals of crude terephthalic acid with water and regeneration of acetic acid, contained in waste flush fluid is effectively made in the energy saving mode. More specifically, after the crystals are separated from the suspension of crystals of crude terephthalic acid and washed, crystals containing no acetic acid are collected essentially, waste flush fluid is collected separately brought out during the water washing. Then, acetic acid is extracted from used flushing fluid using the extracting solvent, without dehydration distillation usage, and water is separated. After that acetic acid extract is supplied into the column for acetic acid dehydration distillation to extract acetic acid.

EFFECT: it is found that there is a significant correlation between content of acetic acid in the spent flushing fluid and the extracting solvent (i/e relative flow of solvent), and a really executable method of water washing is developed to reduce power consumption and losses of acetic acid.

9 cl, 5 dwg, 4 tbl, 3 ex

Description

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to improvements in the method for producing purified terephthalic acid. More specifically, the method includes the steps of obtaining crude terephthalic acid using para-xylene as a starting material, and then crude terephthalic acid is hydrotreated in a combined process.

State of the art

When crude terephthalic acid is obtained prior to the purification step, para-xylene is subjected to liquid phase oxidation as a starting material, and crystals of the resulting crude terephthalic acid are obtained in high yield in an acetic acid solvent. Then, the acetic acid solvent separated from the crude terephthalic acid crystals is also regenerated. It is important to involve acetic acid in recycling in order to minimize the loss of acetic acid during regeneration. Thus, an economical, industrial-scale process was required, in particular a low-cost manufacturing process, based on minimizing the loss of acetic acid.

In a typical process for the preparation of purified terephthalic acid, para-xylene dissolved in an acetic acid solvent is first subjected to liquid phase oxidation and then crystals of crude terephthalic acid (CTA) are obtained. The obtained crystals of crude terephthalic acid are subjected to solid phase liquid separation, washing and drying to obtain a powder of substantially acetic acid-free crystals. After that, the crude terephthalic acid crystals thus collected are sent to the purification step, whereby purified terephthalic acid (PTA) is obtained.

In this method, crystals that have undergone solid-liquid separation from a suspension of crude terephthalic acid crystals are collected as a filter cake (or wet crystals) containing a reaction solvent (i.e., an acetic acid solvent containing, for example, a catalyst and reaction by-products). Consequently, the crystals in the filter cake can be washed with an acetic acid solvent (e.g., regenerated acetic acid from a dehydration distillation column and fresh industrial acetic acid to replenish) to obtain an acetic acid-containing crystalline filter cake, which essentially does not contain catalyst and reaction by-products. This filter cake is wet terephthalic acid-containing wet crystals of acetic acid (for example, with an acetic acid content of about 8 to 13% by weight). These wet crystals are dried to evaporate and remove the acetic acid contained therein, and then a substantially crude acetic acid-free terephthalic acid is obtained in the form of a crystalline powder. These steps are hereinafter referred to as the "step for the preparation of crude terephthalic acid."

In the crude terephthalic acid production step, the acetic acid contained in the collected crystalline filter cake (or in wet crystals) is evaporated and separated. For this purpose, it is necessary to have drying equipment in order to obtain dried crystalline powder. More specifically, examples of the drying equipment needed to evaporate and separate the acetic acid contained in the wet crystals of crude terephthalic acid include a fluidized bed dryer column and a rotary dryer type with heating tubes, with a heating unit that circulates the heated inert gas.

In addition, special equipment is required that is used to process the powder and is needed to transfer, weigh, store or transport large quantities of dried crystalline powder. Specific examples of equipment include belt or bucket conveyor systems, and a conveyor device based on low density or high density gas that uses an inert gas to transport crystalline powder. In addition, examples of additional equipment needed include a powder storage tank, or a container for weighing and storing a powder product, and a powder hopper.

In addition, during the crude terephthalic acid production step, the separated mother liquor that is collected in the solid phase liquid separation step, the withdrawn acetic acid spent washing liquid collected in the acetic acid washing step of the separated filter cake, and the acetic acid solvent that was collected in the washing step gas (for example, circulating gas during the drying process and the exhaust gas from the oxidation reaction) can be reused as part of the solution ator for oxidation reactions used for the oxidation of para-xylene. However, in this regard, the acetic acid solvent, with the exception of the reusable one, is sent to the regeneration stage of the acetic acid solvent, and a high purity regenerated acetic acid free of impurities is obtained.

The step of regenerating the acetic acid solvent includes concentrating and separating the oxidation catalyst contained in the reaction solvent, an aromatic by-product such as benzoic acid, and a high molecular weight reaction by-product such as a resinous polymer by-product. Meanwhile, low molecular weight by-products such as water and methyl acetate are separated by distillation in a dehydration distillation column, and then the acetic acid solvent is regenerated. In a dehydration distillation column, water and methyl acetate, for example, are distilled off from the top of the column, and acetic acid is withdrawn from the bottom of the column as bottoms. In this way, the acetic acid solvent is recovered in the form of regenerated acetic acid with high purity and high regeneration yield (for example, about 92% or higher). Thus acetic acid regenerated in this way is recirculated to the crude terephthalic acid production step.

To reduce the amount of acetic acid that is distilled off with water distillate from the top of the column, the dehydration distillation column should be a distillation column with an increased number of plates (about 80 plates), and reflux liquid with a high reflux number (4 to 7) is needed. This operation requires a large amount of energy. In addition, aqueous components (water as a reaction product) that were separated and distilled from the top of the column include low boiling compounds (e.g. methyl acetate). As a result, the distilled aqueous components are fed back into the solvent recovery column, and subjected to evaporation and distillation to separate and collect a fraction with a low boiling point, such as methyl acetate. Then, the stream exiting from the bottom of the solvent recovery column is discharged as an aqueous reaction product formed in the crude terephthalic acid production step, and is treated as waste water.

However, the water distilled from the top of the dehydration distillation column contains a certain amount of acetic acid solvent (i.e., about 1 wt.% Or lower). Meanwhile, the acetic acid solvent, not separated and not removed as the outlet stream of the solvent recovery column, must thereby be sent to waste. This leads to the loss of acetic acid solvent. Accordingly, the necessary equipment for obtaining crude terephthalic acid includes: a step in which the content of acetic acid in the aqueous distillate from the dehydration distillation column is reduced; and a step for regenerating an acetic acid solvent to obtain high-purity acetic acid as a bottoms product, both of which act simultaneously in the preparation of crude terephthalic acid. An important objective of the cost-effective industrial production of crude terephthalic acid is to minimize the loss of acetic acid in the dehydration distillation column during the regeneration stage of the acetic acid solvent, and to reduce the amount of energy required for the dehydration purification of acetic acid (i.e., to obtain highly pure regenerated acetic acid).

Given the above circumstances, such as the increased demand for purified terephthalic acid recently, the capacity of equipment for the production of crude terephthalic acid has increased. As a consequence, the powder processing equipment used after the drying process of the above wet crystals of crude terephthalic acid has become larger units or multiple units. Thus, there was an increasing need for technological improvement, which is called the process without using a dryer. In a process without using a dryer, crude terephthalic acid crystals are subjected to solid phase liquid separation and washing to obtain wet crystals; then the wet crystals are mixed with an aqueous solvent to purify as is; and then the mixture is used as a suspension to obtain purified terephthalic acid.

In the method without using a dryer, the drying process of wet crystals (or crystalline filter cake) and the crystalline powder transfer process are excluded. The method without using a dryer does not need drying equipment and transfer process, which can save on equipment costs, reduce technological problems during powder handling, so operating costs are also reduced. The following patent documents describe technology based methods without using a dryer.

Patent Document JP-H05-065246A discloses a method for preparing a suspension of crude terephthalic acid crystals, the method comprising the steps of: filtering a suspension of crude terephthalic acid crystals formed in an acetic acid solvent using a filter (or belt filter) using a moving tape as a filter material; using an aqueous medium to sufficiently wash the filter cake (or first wet cake) on the filter belt; and suspending the collected crystalline filter cake (or second wet cake) in fresh water (e.g., pure water, demineralized water) to obtain a suspension of crude terephthalic acid crystals.

In this technology, the crystalline filter cake (or first wet cake) after filtration using a filter is subjected to countercurrent washing, in which the acetic acid fraction, the catalytic component, and the residual component stored in the crystalline filter cake are washed repeatedly using spent washing liquid in the direction opposite to moving the movable tape. Finally, the crystals after washing in countercurrent mode are washed with fresh water (i.e., pure water) in the final washing zone (i.e., on the exit side of the crystals). This allows you to collect crude terephthalic acid in the form of a crystalline filter cake, which is essentially wetted with water. In addition, another aqueous medium (i.e., an aqueous hydrotreating solvent) is added to the collected crystalline filter cake to obtain a slurry. Then the suspension is heated to form an aqueous solution. It is further described that after this, the resulting aqueous solution can be sent to the step of purifying the crude terephthalic acid.

In addition, JP-H06-327915A discloses that a rotating filter (e.g., a rotating drum filter) with a rotating filter medium is used to filter a suspension of crude terephthalic acid crystals in an acetic acid solvent, while the filter medium is rotated to obtain a crystalline filter sediment (or wet crystals) on the filter medium. The obtained crystalline filter cake was washed with fresh water in the final washing zone. After washing, the effluent is used to repeatedly wash the crystalline filter cake in a direction that is opposite to the direction of rotation of the filter medium. This method is called countercurrent flushing. Using this method, wet crystals of crude terephthalic acid can be collected from the final wash zone. Then the collected crystals are treated using an aqueous medium to obtain a suspension. It is described that this suspension can be directed directly to the step of obtaining purified terephthalic acid.

In this technology, the output stream of the final spent washing liquid is used to repeatedly perform washing in countercurrent mode. Thus, this technology makes it possible to reduce the amount of fresh water used. Repeated rinsing provides an increase to high levels of acetic acid concentration in the spent wash liquid. Accordingly, it is desirable that this spent washing liquid can be directly used for the para-xylene oxidation reaction without dehydration distillation.

In addition, JP-2620820B discloses that: a filter cell type filter (e.g., a BHS-FEST pressure filter manufactured by BHS-WERK Inc.) having a special filtration zone is used to subject a suspension of crude terephthalic acid crystals in an acetic acid solvent to a highly efficient filtering and rinsing with water (pressure displacement flushing); and the suspension is also subjected to repeated washing in countercurrent mode using spent washing liquid. Describes that this technology provides the ability to wash a small amount of water and allows to obtain wet crystals of crude terephthalic acid with a residual amount of acetic acid ^-1 5000 million (i.e. 0.5 wt.%) Or less, based on the crude terephthalic acid. It should be noted that the crude terephthalic acid crystals can be directly sent to the purification step in a subsequent step.

In this method, the amount of newly added wash water is also small (i.e., the wash ratio = 0.15 to 0.35 (mass ratio): “wash water / crude terephthalic acid crystals”). The spent washing liquid, withdrawn from the final washing stage, contains acetic acid with a high concentration level. As a result, the spent wash liquid is sent to the terephthalic acid production process and reused as is. Alternatively, the spent washing liquid is sent to a dehydration distillation column to recover acetic acid as regenerated acetic acid, and the regenerated acetic acid is recirculated in the manufacturing process. In addition, crude terephthalic acid crystals containing acetic acid may be mixed with an aqueous solvent for purification and may be submitted as a suspension of crystals to the purification step. In this case, the assumption that this method leads to the loss of acetic acid solvent is justified. An acetic acid solvent can also act as a catalyst poison for catalytic hydrotreatment, so that an acetic acid solvent must be removed to a level (e.g., ^-1 5000 million (0.5 wt.%) Or less), which corresponds substantially to the absence of acetic acid in the crystals of crude terephthalic acids.

Here, the aforementioned patent documents JP-H05-065246A, JP-H06-327915A and JP-2620820B disclose technologies for filtration-washing methods using an aqueous solvent (i.e., in these methods, a layer of filtered crystals is placed on top of the filter medium; and the washing water is forced out washing time). However, in these methods, it is difficult to completely remove acetic acid from crude terephthalic acid crystals. Then the technology disclosed in patent document JP-3859178B appeared. Patent Document JP-3859178B is a combination of a suspension washing method and a filtration washing method. In this method, crystals of crude terephthalic acid in an acetic acid solvent are subjected to solid phase liquid separation. The aqueous wash solvent is then used for stirring and mixing in order to resuspend the crystals (i.e. to form a re-suspension). Then the resuspended solution is subjected to filtration and washing.

In this technology, a suspension of crude terephthalic acid crystals in an acetic acid solvent is subjected to filtration separation and filtration washing using a first rotary drum filter. The crystalline filter cake collected after washing is then re-suspended and washed (or washed with a second suspension) using an aqueous separated medium and spent washing liquid from a crystalline filter cake of a second rotary drum filter and additional fresh water. Then, a second rotary drum filter is used to repeat filtration separation and filtration washing, so that sufficient washing with water is carried out. The crystalline filter cake collected in this process is wet, including water with a low content of acetic acid. In addition, in this technology, the amount of water newly added for suspension washing is relatively large (i.e., the washing ratio = 1.0 (mass ratio): “washing water / crude terephthalic acid crystals”). Accordingly, the technology can reduce the content of acetic acid in the recovered crystalline filter cake (for example, the ratio of acetic acid to crude terephthalic acid crystals = 0.1 wt.%). At this time, the spent washing liquid is sent to a dehydration distillation column in the stage of regeneration of the acetic acid solvent and subjected to dehydration. It is then described that acetic acid is regenerated as a bottoms product flowing through the bottom of the column.

The above patent documents describe methods without using a dryer, which are effective methods for producing purified terephthalic acid. However, the newly added water for washing the crystalline filter cake is discharged in the form of spent washing liquid, according to which the spent washing liquid contains acetic acid. In view of this, while the spent washing liquid containing acetic acid with a high concentration can be directly used for the oxidation reaction, the content of acetic acid in the newly added washing water had to be somehow eliminated. More specifically, the newly added water, together with the aqueous reaction product formed during the oxidation reaction, should be sent to the recovery stage of the acetic acid solvent and subjected to dehydration and separation using a dehydration distillation column.

Thus, the water added again to wash the crystalline filter cake results in an increase in the amount of water that is dehydrated and separated using a dehydration distillation column in any of the above methods. Therefore, increased energy consumption for dehydration distillation. This increase in energy consumption for dehydration in a dehydration distillation column is one of the reasons why the method without using a dryer did not find industrial application in the method for producing crude terephthalic acid.

Meanwhile, patent documents JP-S62-041219B and WO 96/06065 disclose techniques for improving the process during the regeneration step of an acetic acid solvent. During the distillation of acetic acid using a dehydration distillation column, an azeotropic agent is added that forms an azeotropic composition with water to perform dehydration by azeotropic distillation. Within the framework of these technologies, it is proposed to reduce the energy consumption for dehydration and to reduce the loss of acetic acid obtained from the top of the column. In the regeneration step of the acetic acid solvent disclosed in these patent documents, the input stream of the water-containing solution of acetic acid that flows from the reaction mixture of the oxidation process to be directed to a dehydration distillation column is a water-containing solution of acetic acid with a water content of 30 wt.% or less (i.e., the content of acetic acid is 70 mass% or more) (see patent document JP-S62-041219B). In addition, the above feed stream is a water-containing solution of acetic acid with a water content of from 20 to 40 wt.% (That is, with an acetic acid content of from 60 to 80 wt.%) (See patent document WO 96/06065). Accordingly, it should be noted that in patent documents JP-S62-041219B and WO 96/06065, the azeotropic distillation method is ultimately used for a water-containing acetic acid solution with a high content of acetic acid.

In addition, patent document WO 2007/038258 proposed an energy-saving dehydration technology in which the azeotropic agent used for the dehydration distillation process using azeotropic distillation is used as an extraction solvent, which is used to extract acetic acid from an aqueous solution of acetic acid. Thus, after separating water from it, the water-containing fraction of acetic acid is fed to a dehydration distillation column.

However, the above patent documents JP-S62-041219B and WO 96/06065 describe that an acetic acid solution originating from a reaction solvent during the oxidation process in the crude terephthalic acid production step has a lower water content (i.e., a relatively high content of acetic acid in the solution) . Here, in the extraction separation method disclosed in patent document WO 2007/038258, the extracting solvent (or azeotropic agent) is mixed with a solution of acetic acid having a lower water content (i.e., with a highly concentrated solution of acetic acid), whereby the mixture will become a uniform solution . Thus, the mixture cannot be separated into two immiscible phases, so it will be difficult to extract acetic acid using an azeotropic agent (i.e., an extraction solvent). Therefore, the energy-saving dehydration technology presented in patent document WO 2007/038258, performed using a direct extraction process to separate water from a high concentration of acetic acid solution obtained from an oxidation reaction, and then performed in combination with a dehydration distillation process, is hardly applicable as such.

SUMMARY OF THE INVENTION

In view of the above circumstances, the present inventors have found that when the crystalline filter cake separated in the crude terephthalic acid production step is washed with an aqueous solvent and fresh water, the resulting aqueous solution containing acetic acid has an acetic acid content of from 5 to 30 mass%; and thereby, an extraction process of water using an organic solvent without dehydration distillation is applicable. In addition, the authors of the present invention further found that this method is combined with a method without using a dryer for the above crude terephthalic acid crystals; and this combination can improve the method of obtaining crude terephthalic acid. As a result of this, the inventors of the present invention have developed a technological system in which such a washing process in which a crystalline filter cake subjected to solid-liquid separation in the crude terephthalic acid production step is washed with water, combined with an extraction process in which acetic acid is extracted from the spent aqueous washing liquid, and water is separated. This process system has been found to be suitable as a new process without using a dryer.

Here, the authors of the present invention focused on the fact that the acetic acid filter cake (wet crystals containing acetic acid) formed in the crude terephthalic acid production step has an acetic acid content of about 8 to 13 wt.% (I.e., an acetic acid content of about 8 , 7 to 14.9 wt.% In dry crystals). Accordingly, in the process in which acetic acid containing the wet crystals can be washed with an aqueous solvent using the newly added water and can be recovered substantially free of acetic acid wet crystals containing water (the content of acetic acid 5000 million ^-1 (0, 5 wt.%) Or less), the content of acetic acid present in the spent wash liquid is usually from about 5 to 30 wt.%, Although the content of acetic acid may vary depending on the amount of newly added Lena water. It should be noted that the term “newly added water” refers to newly added water, other than an aqueous solvent that is repeatedly used for washing with water by a method of repeated filtration washing in countercurrent mode using the above filter, or a washing method in which suspension washing is combined and filtration washing. Examples of newly added water include pure water, such as demineralized water.

Meanwhile, an extraction solvent may be used to extract acetic acid from the above spent wash liquid (containing an aqueous solution of acetic acid), and to separate the water. When the characteristics of the extracting solvent are taken into account (for example, phase equilibrium in a system of three components), this procedure is a method effective for saving energy for dehydration, as is the process of separating water by forming two immiscible phases. Here, the authors of the present invention considered the design of a technological system that combines the extraction of acetic acid to extract acetic acid by separating water from the above spent wash liquid and a process (method without using a dryer) without the above drying and transferring steps.

Firstly, the present inventors have considered the situation that the slurry of crude terephthalic acid crystals are subjected to solid-liquid separation and then washing with water to separate the water-containing wet crystalline filter cake is substantially free of acetic acid (acetic acid content being 5000 million ^-1 ( 0.5 wt.%) Or less). The authors of the present invention additionally considered the option of extracting, separately, the entire volume of the spent wash liquid containing acetic acid without mixing with the separated mother liquor and acetic acid solution (acetic acid effluent) to wash acetic acid from crude terephthalic acid crystals, or without mixing these acetic acid solutions ( effluent) with the above spent flushing fluid.

In addition, the authors of the present invention considered a method in which acetic acid is extracted from the collected waste wash liquid (with an acetic acid content of about 5 to 30 wt.% In an aqueous solution containing acetic acid) according to an extraction process using an organic solvent; and considered a method of obtaining an aqueous solution with a markedly reduced content of acetic acid and separating water as a raffinate. Moreover, the authors of the present invention set the goal of using an extraction process without distillation separation, to achieve such a content of acetic acid in the raffinate as the content of acetic acid (0.7 wt.% Or less), equivalent to the content in an aqueous solution from dehydration distillation obtained in a dehydration distillation process using a two-component acetic acid-water system (for the method of two-component distillation separation, please see the above Patent Documents JP-S62-041219B and WO 2007/038258).

The acetic acid extract, which was obtained by extracting acetic acid from an aqueous solution containing acetic acid using an organic solvent, is fed into the regeneration stage of the acetic acid solvent, which is part of the crude terephthalic acid production step. After the extraction solvent has been separated in a dehydration distillation column, acetic acid is removed from the bottom of the dehydration distillation column. Meanwhile, the raffinate of the aqueous solution is fed to the solvent regeneration column during the regeneration step of the acetic acid solvent. After the contained (dissolved) extracting solvent is separated by evaporation (evaporation) or distillation, the residue is treated as waste water. As a result, all the acetic acid contained in the raffinate is lost. Accordingly, in order to optimize the method of regeneration of acetic acid present in the spent washing liquid, it is necessary to reduce the content of acetic acid (0.7 wt.% Or less) found in the raffinate. Here, an object of the present invention is to provide an energy-saving method for reducing losses of acetic acid in a process, comprising: a step in which separation / washing of the collected crystals of substantially crude acetic acid-free terephthalic acid is carried out; and a step for regenerating the acetic acid solvent using an extraction / separation process in which acetic acid is recovered from the spent washing liquid withdrawn during washing of the crude terephthalic acid crystals and water is separated to facilitate the industrial use of the process without using a dryer.

The extraction solvent (i.e., an organic solvent) used in the extraction of acetic acid to extract acetic acid from the waste acetic acid-containing wash liquid (i.e., an acetic acid aqueous solution) should allow separation of the mixture (i.e., the acetic acid-water-extracting system) solvent ") with an aqueous solution containing acetic acid in at least two phases (that is, two immiscible phases into the organic phase and the aqueous phase). To achieve this, the concentration of acetic acid must be set to a concentration of the point of complete mixing of partially miscible liquids or less, at which the mixture of the three components is homogeneous. Here, the term “point of complete mixing of partially miscible liquids” refers to the point where the boundary between the aqueous phase and the organic phase disappears, as described further in FIG. 3. More specifically, when an acetic acid extraction process is carried out using a suitable organic solvent, the content of acetic acid in the mixture of the three components appears to be no more than about 50 wt.%, Or less, or about 40 wt.% Or less. Accordingly, when a water-containing solution of acetic acid with such a high acetic acid content is used, the addition of the extracting solvent may not form two-phase layers, or a large amount of extracting solvent should be added to it.

Usually, after solid-phase liquid separation of a suspension of crude terephthalic acid crystals in wet crystals containing acetic acid, from 8 to 13 wt.% (From about 8.7 to 14.9 wt.% Calculated on dry crystals) remain acetic acid and containing acetic acid acid, wet crystals after the formation of the above crystals are additionally washed with an acetic acid solvent. As a result, the amount of acetic acid contained in the spent washing liquid discharged during repeated filtration washing in countercurrent mode is assumed to be from about 16 to 29 wt.%, While the amount of added water is half the mass of crystalline terephthalic acid powder. In addition, it is assumed at this time is obtained substantially free of acetic acid Crude terephthalic acid (acetic acid content being 5000 million ^-1 (i.e., 0.5 wt.%) Or less).

In addition, by adding water in an amount of 0.75, 1.0, 1.5, or 2 crystal masses, a filtration washing method or a washing method combining suspension washing and filtering washing can be performed. In this case, the content of acetic acid is assumed to be from about 11 to 19 wt.%, From about 8 to 14 wt.%, From about 5 to 10 wt.%, Or from about 4 to 7 wt.%, Respectively. It should be noted that when fresh water is added in an amount of 0.4 to the weight of the crystals of crude terephthalic acid and multiple filtration washing is carried out in countercurrent mode, the content of acetic acid in the output spent washing liquid is calculated to vary from about 20 to 36 wt.% .

Thus, the wet crystals are substantially free of acetic acid, it is assumed that water-containing (i.e., the content of acetic acid ^-1 5000 million (i.e. 0.5 wt.%) Or less) may be prepared by a method in which fresh water is added to an amount ranging from 0.5 to 2 weight percent of wet crystals of crude terephthalic acid containing acetic acid, and performing multiple filtration washing in countercurrent mode, and / or multiple washing in countercurrent mode in a manner combining suspension washing washing and filtration. At this time, the aqueous spent washing liquid having an acetic acid content of from about 4 to 29 wt.% (About 5 to 30 wt.%) Is sent to the extraction of acetic acid. In view of the foregoing, a method for extracting acetic acid from a spent wash liquid (or an aqueous solution of acetic acid) containing acetic acid with a low concentration using the above suitable organic solvent to separate water is an energy-saving method that provides reduced loss of acetic acid. Therefore, this method becomes a method of separating water, suitable for aqueous spent wash liquid.

More specifically, one aspect of the present invention is a method for producing purified terephthalic acid.

The method includes the step of producing crude terephthalic acid, wherein crystals of crude terephthalic acid are formed by oxidation of para-xylene in an acetic acid solvent, and crystals of crude terephthalic acid are isolated by washing after separation using solid phase liquid separation;

an acetic acid solvent regeneration step for recovering the acetic acid solvent contained in the acetic acid solution withdrawn during solid phase liquid separation in the crude terephthalic acid production step;

a crude terephthalic acid purification step, in which the hydrotreating the aqueous solution is carried out after mixing and dissolving the crude terephthalic acid crystals obtained in the crude terephthalic acid preparation step with the aqueous solvent, and hydrotreating is performed, and then crystals of the purified terephthalic acid in the solution are obtained, and the crystals are separated crystals of purified terephthalic acid by solid phase-liquid separation from the solution (or suspension).

More specifically, at the appropriate stages, the following operations (1) to (3) are carried out.

(1) Step for the Preparation of Crude Terephthalic Acid

Crude terephthalic acid crystals obtained by solid phase liquid separation using a solid phase liquid separator are transferred to the first washing zone and washed at least once with an acetic acid solvent, and the washed crystals obtained are transferred to the second washing zone. In the second washing zone, the crystals are washed at least once with fresh water and / or an aqueous solvent (i.e., spent washing liquid) obtained from the zone in the direction of transfer of the crystals, and then the washed crystals are transferred to the third washing zone. In the third washing zone (i.e., the final washing zone), substantially moist acetic acid-free crude terephthalic acid crystals are recovered after they are washed with fresh water.

The mother liquor separated in the solid phase liquid separator is collected in a separate reservoir for the mother liquor, and the wastewater obtained by washing with an acetic acid solvent in the first washing zone is collected in the washer fluid reservoir, and the mother liquor and wastewater are used as acetic acid solvent for the para-xylene oxidation reaction.

The spent washing liquid withdrawn during washing with fresh water and / or an aqueous solvent in the second washing zone is collected and reused at least once as washing water in the second washing zone, and the extracted aqueous spent washing liquid is collected in a waste tank flushing fluid. The spent washing liquid obtained by washing in the third washing zone (i.e., in the final washing zone) is collected and then used as an aqueous solvent for washing in the second washing zone, which is located in the opposite direction to the crystal transfer direction.

(2) Stage of regeneration of acetic acid solvent

The aqueous spent washing liquid that has been removed from the crude terephthalic acid separation / washing process and collected in the spent washing liquid tank is fed to an acetic acid extraction column using an extracting solvent that forms two immiscible phases with water. Then, acetic acid in the aqueous spent wash liquid is extracted as an extract in an acetic acid extraction column, and the acetic acid extract is recovered as an organic phase, and the raffinate is obtained as an aqueous phase.

The acetic acid extract is sent to the top of the dehydration distillation column. Acetic acid is then recovered from the bottom of the dehydration distillation column, and a condensate containing an extraction solvent is obtained from the top of the dehydration distillation column. The collected condensate is fed to two-phase separation using a water-oil mixture separation tank to separate and regenerate the extraction solvent, and the extraction solvent is reused as the extraction solvent in the acetic acid extraction column, while a portion of the extraction solvent is recirculated as reflux liquids in a dehydration distillation column.

The raffinate withdrawn from the acetic acid extraction column is fed to the solvent recovery column, and the dissolved extraction solvent is recovered.

(3) Purification step of crude terephthalic acid

Wet crystals of crude terephthalic acid removed from the third washing zone are sent directly to the slurry preparation tank. Crude terephthalic acid crystals are mixed with an aqueous hydrotreating solvent to obtain a suspension of crude terephthalic acid crystals. Then the suspension is fed to the purification step of crude terephthalic acid.

According to the present invention, the following advantageous effects are expected.

There is no need for drying equipment, which is traditionally provided at the stage of obtaining crude terephthalic acid. Drying equipment includes a rotary dryer such as a drum with heating tubes, and a fluidized bed dryer equipped with a unit for circulating heated inert gas to vaporize and separate acetic acid from the wet filtration cake of crude terephthalic acid. In addition, there is also no need to use special equipment that transports the powdered product. Examples of equipment include: a belt or bucket mechanical powder conveyor that transfers crystals of dry powder of crude terephthalic acid; pneumatic conveying device that conveys powdered crystals; weight tank; storage tank; powder tank; and a powder hopper.

This makes it possible to significantly reduce equipment costs and maintenance costs. In addition, since there is no need for such equipment, the frequency of malfunctions due to complications in the operation of such equipment can thereby be reduced. This greatly contributes to reliable operation during the production of purified terephthalic acid.

Until now, it was inevitable that the process of separating water to regenerate acetic acid from the spent wash liquid be achieved in order to achieve the predominant effect obtained by eliminating the above drying and transfer stages. However, according to the present invention, it is possible to create a system in which the energy-consuming dehydration distillation process can be avoided and the water separation process using extraction combined with a washing process included in a process in which crude terephthalic acid crystals are separated and washed. This can create a system in which only the column for the extraction of acetic acid is additionally introduced into the existing stage of regeneration of the acetic acid solvent, while the need to improve the newly added equipment is minimized.

In addition, this technological system is designed as an all-in-one integrated system, including the process of separating and washing crystals of crude terephthalic acid and the process of separating water and regenerating acetic acid from the spent wash liquid. This provides an opportunity to save energy spent when water is separated and acetic acid is regenerated from the spent wash liquid. It also simplifies the practical implementation of the economical method without using a dryer, which reduces the loss of acetic acid to a level equivalent to that achieved in the process of dehydration distillation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a method for producing purified terephthalic acid according to one embodiment of the present invention.

FIG. 2 illustrates an extraction / separation process, in particular during the regeneration step of an acetic acid solvent.

FIG. 3 is a graph of liquid-liquid equilibrium measurement results in an acetic acid-isobutyl acetate system, according to A. Chafer et al. The chart is drawn with the transformation of data into triangular coordinates, having the shape of a right-angled triangle (i.e. represented in mass%).

FIG. 4 is a graph of the results represented by acetic acid concentration curves at the outlet of each theoretical plate in countercurrent multistage extraction, modeled to supply an aqueous solution containing 10 wt.% Acetic acid using isobutyl acetate as an extracting solvent, and the amount of extracting solvent as a variable parameter.

FIG. 5 is a graph of the results represented by acetic acid concentration curves at the outlet of each theoretical plate in countercurrent multistage extraction, modeled to supply an aqueous solution containing 20 wt.% Acetic acid using isobutyl acetate as an extracting solvent, and the amount of extracting solvent as a variable parameter.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

First, the contents of one embodiment of the present invention will be schematically presented, at the same time as comparing with standard technologies. Then, the content of the embodiment will be described more specifically.

Previously disclosed methods without using a dryer were known as a method of washing (i.e., filtering washing or a combination of suspension washing and filtering washing) of a crystalline filter cake after a suspension of crude terephthalic acid crystals was subjected to solid phase liquid separation, as described above, to recover crystals (for example, the above patent documents: JP-H05-065246A, JP-H06-327915A, JP-2620820B and JP-3859178B). In these methods, fresh water is used for washing in the final crystal washing zone, and the aqueous spent washing liquid (i.e., an aqueous solvent) from it is used for washing as washing water, which is repeatedly used in the zone in the opposite direction to the crystal transfer direction. This allows the spent washing liquid with a higher acetic acid content, and thus makes it possible to obtain substantially free of acetic acid wet crystals (e.g., crystalline filter cake with acetic acid content being 5000 million ^-1 (i.e., 0.5 wt.% ) or less).

The above feature is the same as in the method in which wet crystals containing water are collected after a process (i.e., solid phase liquid separation and washing with water) of separating and washing crystals of terephthalic acid in a suspension of crude terephthalic acid crystals during the process of obtaining purified terephthalic acid according to to this embodiment. However, the production method according to one embodiment of the present invention has the following differences in the nature of the operation from traditional methods without using a dryer.

Firstly, (1) wet crystals obtained immediately after a suspension of crude terephthalic acid crystals was subjected to solid-liquid separation (i.e., a separated mother liquor from an acetic acid solvent is collected) is washed at least once with acetic acid (then is an acetic acid solvent containing regenerated acetic acid and fresh acetic acid) in the first washing zone. This removes the separated mother liquor (i.e., an acetic acid solvent) containing the oxidation catalyst and oxidation by-products. The wet crystals are then washed with an aqueous solvent (i.e., an aqueous spent washing liquid and / or fresh water) in a second washing zone. After that, the wet crystals are washed with fresh water (i.e. pure water, such as demineralized water) in the third washing zone.

Further, (2) only all of the aqueous spent washing liquid obtained during washing with water is collected. It should be noted that the aforementioned aqueous spent washing liquid is collected separately from the above-separated separated mother liquor and the washing wastewater using an acetic acid solvent.

Accordingly, a single combined filter (e.g., a rotary drum filter, a belt filter, a BHS-FEST filter) having a solid-liquid separation zone and a washing zone can be used for the filtration separation and washing process. During the washing process, the washing zone of the filter, which ensures the performance of the two above-mentioned operations (1) and (2), is appropriately divided, and the outlet channel (outlet) for its washing wastewater (aqueous spent washing liquid) is improved. Then two of the above operations can be carried out.

More specifically, the following enhancements should be made to the equipment. The washing zones located behind the filter zone are divided into at least three zones. In the initial washing zone, washing is performed using an acetic acid solvent at least once in the first washing zone. In subsequent washing zones (i.e., second and third washing zones), washing may be carried out using an aqueous solvent and / or fresh water. The equipment and auxiliary equipment should be improved as follows: the outlet channel for the washing wastewater from the first washing zone is separated from the outlet channel for the aqueous spent washing liquid from the subsequent water washing zones, which allows the mother liquor and the spent washing liquid to be collected separately. In addition, the equipment should be improved so that the aqueous spent wash liquid from the final wash zone (third wash zone) using fresh water can circulate as wash water to the second wash zone. In addition, if necessary, in the second washing zone, the equipment can be improved so as to enable the use of aqueous spent washing liquid, which is repeatedly used in the opposite direction to the crystal transfer direction.

The above method of washing and filtering washing can be combined in the process of solid-liquid separation and washing. In this process, a single filter is used having a solid phase liquid separation zone and a wash zone. Then the filter, the suspension washing tank and another filter can be placed in this order for the two above operations (1) and (2). More specifically, a suspension of crude terephthalic acid crystals is fed to a first filter. After solid phase-liquid separation, the wet crystals are washed at least once with an acetic acid solvent in the washing zone (first washing zone).

Then, the washed wet crystals containing acetic acid are fed into the suspension washing tank. To the crystals, a part of the liquid spent during the washing separation from the suspension washing removed from the second filter, the aqueous spent washing liquid obtained by washing with fresh water, and, if necessary, fresh water are added. The mixture is then mixed and mixed to perform suspension washing in the second washing zone. After the agitated suspension was applied to the second filter and subjected to solid phase liquid separation, the obtained wet crystals containing water were washed additionally with fresh water in the third washing zone. Finally, substantially wet acetic acid free crystals are recovered. In addition, the spent liquid from the washing separation is collected in the form of an aqueous spent washing liquid, and thereby, the spent liquid is separated from the separated mother liquor and the washing wastewater from the first filtration device.

According to the above method, the spent washing liquid obtained by washing with fresh water in the final crystal removal zone (i.e., in the third washing zone) is reused at least once as either washing water (or an aqueous solvent) in the washing zone in the opposite direction relative to the direction of transfer of the crystals. Accordingly, in the second washing zone, a multiple washing process is carried out with water. Due to the above, the amount of newly added water (that is, 0.5-2 times the weight of the crystals of crude terephthalic acid) corresponds to the amount of spent wash liquid, which determines the content of acetic acid in the aqueous spent wash liquid and / or the content of acetic acid in the recovered water containing wet crystals.

Then, the collected separately spent washing liquid is transferred to the extraction / separation process for the extraction of acetic acid. After separating the water, the extract is sent to a dehydration distillation column in the stage of regeneration of the acetic acid solvent of the collected extracted acetic acid. Finally, acetic acid is recovered with an acetic acid solvent for the oxidation reaction. A portion of the separated mother liquor and wash wastewater that contains an acetic acid solvent removed during the solid phase separation and washing of the crude terephthalic acid crystal slurry is directly reused as part of the conventional solvent in the oxidation reaction. The remaining amount is sent to the regeneration stage of the acetic acid solvent to recover acetic acid.

Below, in relation to the washing method, which combines suspension washing and filtration washing, as described above, FIG. 1 is used to specifically illustrate how purified terephthalic acid is prepared according to one embodiment of the present invention.

FIG. 1 schematically illustrates a method for producing purified terephthalic acid according to one embodiment of the present invention. The step of producing crude terephthalic acid, including a separation / washing process, is used to produce crude terephthalic acid. The crude terephthalic acid thus obtained is purified in the crude terephthalic acid purification step to obtain purified terephthalic acid. It should be noted that the acetic acid of the spent washing liquid withdrawn from the crude terephthalic acid production step is extracted and collected in the regeneration step of the acetic acid solvent including the extraction / separation process.

First, para-xylene (p-xylene) as a starting material and a heavy metal catalyst are dissolved in an acetic acid solvent. This acetic acid solvent contains acetic acid recovered in the regeneration step of the acetic acid solvent. Then para-xylene is subjected to oxidation using air in an oxidation reactor 1 to form crude terephthalic acid. At this time, the excess air and the escaped acetic acid are liquidated using a cooling condenser 2 and are subjected to gas-liquid separation using a gas-liquid separator 3. While the separated gas is discharged as reaction exhaust gas, the separated liquid is returned to the oxidizing reactor 1 and is also fed to a subsequent dehydration distillation column 44.

The crude terephthalic acid formed in the oxidation reactor 1 is subjected to further oxidation in the additional oxidation reactor 4. This process allows the unreacted para-xylene to be completely converted to the crude terephthalic acid. In the same manner as in the oxidation reactor 1, with the additional oxidation reactor 4, a cooling condenser 5 and a gas-liquid separator 6 are used to separate the gas from the liquid. While the separated gas is withdrawn as a reaction off-gas, the separated liquid is returned to the additional oxidation reactor 4.

The crude terephthalic acid is then fed to the crystallizer 7. In the crystallizer 7, the solution containing the crude terephthalic acid is evaporated and cooled to precipitate the crude terephthalic acid crystals. In the same manner as in the additional oxidation reactor 4, a cooling condenser 8 and a gas-liquid separator 9 are used to separate the gas in the mold 7 into gas and liquid. While the separated gas is withdrawn in the form of exhaust gas, the separated liquid is returned to the crystallizer 7.

A suspension of crude terephthalic acid crystals formed in the crystallizer 7 is fed to a solid phase liquid separator (i.e., a single rotary drum filter 10 having a filtration zone and a wash zone). In the solid phase liquid separator 10, the crude terephthalic acid crystals in solution are subjected to filtration separation and filtration washing with acetic acid (i.e., in the first washing zone). In this process, wet crystals (or crystalline filter cake) containing acetic acid can be obtained. In addition, the separated mother liquor obtained by filtration separation is collected in a reservoir 11 for the separated mother liquor. In addition, the wash wastewater discharged during the washing with acetic acid is collected in the wastewater wash tank 12.

It should be noted that part of the separated mother liquor stored in the reservoir 11 for the separated mother liquor is reused as an acetic acid solvent for the para-xylene oxidation reaction. The remaining amount is fed to the concentration column 43 in the stage of regeneration of the acetic acid solvent for the recovery of acetic acid. It should also be noted that the wash wastewater stored in the wastewater wash tank 12 is reused as part of the acetic acid solvent for the para-xylene oxidation reaction.

Acetic acid-containing wet crystals of crude terephthalic acid, which were separated and washed in a solid phase liquid separator 10 (i.e., a rotary drum filter), were sent to a tank 1A for re-suspension. At this time, an aqueous spent washing liquid separated in the second solid phase liquid separator 2A described below, a spent washing liquid withdrawn after washing with fresh water from the second solid phase liquid separator 2A, and fresh water are supplied. The resulting mixture is mixed and mixed in the tank 1A for re-suspension, and subjected to suspension washing in the second washing zone.

Then, the suspension obtained by resuspension in the resuspension tank 1A is fed to a second solid phase liquid separator 2A (i.e., a rotary drum filter). The crystals of crude terephthalic acid are then subjected to filtration washing. After that, the crystals are washed with fresh water in the third washing zone. Finally, substantially wet acetic acid-free water-containing crystals are recovered. At this time, the wet crystals thus collected containing water are fed into a reservoir 23 for preparing a suspension for the crude terephthalic acid purification step. The crystals are then mixed and mixed with an aqueous hydrotreating solvent to obtain a suspension of crude terephthalic acid.

The suspension slurry spent liquid discharged during filtration separation using the second solid-phase liquid separator 2A is discharged into the spent washing liquid tank 3A. A portion of the spent washing liquid is repeatedly used as an aqueous solvent for suspension washing. The unused suspension slurry wastewater is taken from the spent wastewater tank 3A and fed to the extraction / separation process. All spent washing liquid obtained by washing with fresh water in a second solid phase liquid separator 2A is reused as an aqueous solvent for suspension washing.

As described above, the aqueous spent washing liquid can be reused as an aqueous washing solvent. In this washing method, the amount of water newly added in the third washing zone (i.e., the washing zone for the second solid phase liquid separator 2A) and in the second washing zone (i.e., the suspension washing zone) is in balance with the amount of spent washing liquid withdrawn from reservoir 3A for waste flushing fluid. Due to this, the content of acetic acid in the spent wash liquid from the spent waste tank 3A becomes a process parameter for regulating and controlling the amount of newly added water. Accordingly, the amount of newly added wash water affects the content of acetic acid in the wet crystals of crude terephthalic acid thus collected containing water, so that its amount can be controlled.

It should be noted that, as described in Example 1 below, wet crystals containing acetic acid obtained after filtration separation and washing with acetic acid suspension of crude terephthalic acid crystals were used for repeatedly repeated suspension washing and filtration washing. Here, during filtration washing in the third washing zone, the amount of fresh water added was 0.5 of the mass of terephthalic acid crystals. Thus, to perform the suspension washing, an amount of an aqueous solvent was used, which was 1.9 times the mass of terephthalic acid crystals.

Then, the spent liquid of suspension washing was removed in an amount of about 1.0 of the mass of terephthalic acid crystals with an acetic acid content of about 11 wt.%. At the same time, wet crystals of crude terephthalic acid (essentially with acetic acid content of 0.41 wt.%) Were obtained essentially containing no acetic acid and containing water.

Here, in one example of a washing method, a special filtration washing device (i.e., counterflow pressure displacement washing), described in JP-2620820B, was used. Here, for repeatedly washing, an aqueous solution is used containing newly added water, the amount of which is 0.3 times the mass of crystals of crude terephthalic acid. Thereafter recovered substantially free of acetic acid wet crystals (e.g., acetic acid content of about 4000 million ^-1 (i.e., 04 wt.%)). Therefore, in JP-2620820B, it is assumed that the spent wash liquid has an acetic acid content of from about 27 to 48% by weight.

On the contrary, according to the present invention, it is not necessary to achieve a high content of acetic acid in order to carry out the extraction process of acetic acid from the spent washing liquid. Thus, the method according to the present invention is performed without using such a special filter disclosed in patent document JP-2620820B.

In addition, in one example of the washing method disclosed in JP-3859178B, two standard filters (i.e., a rotary drum filter) were used for suspension washing. The amount of water used thereby for suspension washing is about 1.5 times the mass of crystals of crude terephthalic acid. As a result of this, substantially moist water-containing acetic acid-free crystals are collected (for example, with an acetic acid content of about 0.1 wt.%). Although the washing method presented in JP-3859178B is different from the method according to the present invention, it is believed that the spent washing liquid has an acetic acid content of about 8% by weight.

During the washing with water of crystals of crude terephthalic acid according to the present invention, the amount of newly added water is adjusted to 0.5-2 the weight of the crystals of crude terephthalic acid. As a result of this, the content of acetic acid in the spent washing liquid to be extracted is assumed to be from about 5 to 30 wt.%. However, in order to collect a substantially crude acetic acid-free crude terephthalic acid (e.g., with an acetic acid content of 0.5 mass% or less), it is assumed that the spent wash liquid to be extracted has an acetic acid content of 8 mass%.

In addition, according to the present invention, it is not necessary to use a special device for filtering washing, which involves reducing the amount of newly added water in order to collect the spent washing liquid with the above-described acetic acid content (i.e., from about 27 to 48 wt.%, When the amount of newly added water is 0.3 mass% of crude terephthalic acid).

Accordingly, it is preferable to control the content of acetic acid in the spent washing liquid, which was removed from the separation and washing of the crude terephthalic acid crystals, by a value of from about 8 to 25 wt.%. Here, to repeat the washing without using a special filtration / washing process, a combination of conventional filtration methods is used, and the amount of newly added water is controlled. As a result of this, it was confirmed that crystals of crude terephthalic acid essentially free of acetic acid were successfully obtained.

As shown in FIG. 1, as a solid-phase liquid separator 10 for a suspension of crude terephthalic acid crystals, a single rotary drum filter having both filtration and washing zones therein is used. The crystals are then subjected to filtration separation and washing with acetic acid (i.e., regenerated acetic acid, fresh acetic acid) in the first washing zone. In this way, wet crystals containing acetic acid can be obtained in which the separated mother liquor remaining in the separated crystals is displaced by the acetic acid solvent used for washing. Until the crystals are washed with acetic acid, a separated mother liquor remains in the crystals. Consequently, the spent washing liquid withdrawn during the subsequent washing with water (or an aqueous solvent) contains an oxidation catalyst and / or high boiling point substances, such as reaction by-products included in the separated mother liquor.

In this case, during the extraction / separation process in the regeneration step of the acetic acid solvent, contaminants such as an oxidation catalyst cannot be extracted and the impurities remain in the raffinate. Then the raffinate is treated like wastewater. In addition, since the oxidation catalyst may be lost, the oxidation catalyst must be regenerated. Therefore, the crystals after separation using a solid phase liquid separator 10 must be washed at least once using an acetic acid solvent. In addition, the separated mother liquor and wastewater from the washing with acetic acid must be separately removed so as not to mix with the aqueous spent washing liquid obtained during the subsequent washing with an aqueous solvent.

Next will be described in detail the purification step of crude terephthalic acid. At this stage, the crude terephthalic acid obtained in the crude terephthalic acid production step is purified.

The wet crystals containing water are fed from a second solid phase liquid separator 2A to a reservoir 23 for preparing a slurry. As a result, 23 wet crystals containing water are mixed and mixed with the aqueous solvent used for hydrotreating, thereby forming a slurry. This aqueous solvent contains a separated mother liquor from solid phase liquid separators 29 and 32, water after washing the crystals and / or newly supplied water. Then, the suspension is sufficiently dissolved in the dissolution apparatus 25, and then fed to the hydrotreatment reactor 26. The hydrotreating reactor 26 is filled with a hydrotreating catalyst made of a noble metal deposited on activated carbon. The aqueous terephthalic acid-containing aqueous solution flows downward with high pressure hydrogen gas through the catalyst bed from the top of the hydrotreating reactor 26 for hydrotreating.

An aqueous solution containing purified terephthalic acid is supplied to numerous crystallizers 27 and 28, which are further arranged in series, the pressure and temperature gradually decreasing as they move downstream. It should be noted that in FIG. 1, not all crystallizers are shown, for example, with 5 or 6 crystallizers. In the first mold 27, depressurization and cooling occurs when the generated vapor is released together with excess hydrogen gas. Then part of the terephthalic acid crystals precipitates to form a crystal-containing suspension. Then, in the second, third and other molds, successive pressure reduction and cooling are performed. In the last crystallizer 28, crystals of almost all of the terephthalic acid are precipitated, and ultimately a suspension of crystals of purified terephthalic acid is obtained.

This crystal-containing suspension is passed through a solid-liquid separator 29 (eg, Super Decanter), a re-suspension tank 31 and a solid-liquid separator 32 (eg, Super Decanter), and fed to the dryer 34. In this process, the separated mother liquor (or the upper layer of a transparent liquid) formed in the solid-phase liquid separator 32 is stored in the reservoir 30 for the separated mother liquor. In addition, the separated mother liquor (or upper transparent liquid layer) formed in the solid-phase liquid separator 32 is stored in the reservoir 33 for spent washing liquid.

Wet crystals sent to dryer 34 are subjected to a process for evaporating and removing water using dryer 34 (i.e., water is discharged as condensate). The wet crystals are thereby dried to obtain purified terephthalic acid in the form of a crystalline powdery product. The resulting purified terephthalic acid as a product is transferred to a powder supply device 35, and then mixed with an inert gas for shipment. After this, the product is passed through powder conveying equipment (not shown) and sent to a powder storage tank 36. In the tank 36 for storing powder to remove inert gas using a gas-liquid separator 37.

Next will be described in detail the stage of regeneration of the acetic acid solvent.

After the separated mother liquor from the reaction solution of the oxidation process is collected in the separated mother liquor tank 11, the solution is fed to the concentration column 43 for evaporation. Concentration column 43 includes an evaporator 41 and a heat exchanger 42. This configuration makes the acetic acid solvent volatile and enables the removal of non-evaporated contaminants as a residue. Then, the evaporated acetic acid solvent is fed into the dehydration distillation column 44. It should be noted that, as described above, the acetic acid solvent from the oxidation reactor 1 is also supplied to the dehydration distillation column 44.

The dehydration distillation column 44 is a typical distillation system. Accordingly, the acetic acid solvent is removed from the bottom of the column, and the aqueous components are separated through the top of the column. FIG. 1 illustrates how azeotropic distillation is performed with the addition of an azeotropic agent that forms an azeotropic composition with water. First, a vaporous distillate forming an azeotropic composition with an azeotropic agent is withdrawn from the top of the column and introduced into a cooling condenser 46 and into a reservoir 47 for separating the water-oil mixture. Then, the oil fraction (i.e., the organic phase) is separated from the aqueous fraction (i.e., the aqueous phase), and each of them is collected. The oil fraction thus collected is used as reflux liquid to perform irrigation in a dehydration distillation column 44. The aqueous fraction is fed to the solvent regeneration column 48. The dissolved azeotropic agent and oxidation by-products such as methyl acetate are then collected and the aqueous components are removed from the bottom of the column.

At this time, the stream (i.e., water components after dehydration) exiting the bottom of the solvent recovery column 48 contains a small amount of acetic acid. This acetic acid does not evaporate using a heat exchanger 49, and it is treated like wastewater, which leads to the loss of acetic acid. As a result, the water distillate from the top of the dehydration distillation column must be adjusted to the content of acetic acid of 0.7 wt.%.

Next, with reference to FIG. 2, an acetic acid solvent regeneration step provided for in an extraction / separation process will be described in detail.

In the column 4A for the extraction of acetic acid during the extraction / separation process, a countercurrent multistage extraction device (or countercurrent multistage extraction system) is used, such as an extraction column with perforated plates. The aqueous spent washing liquid is withdrawn from the separation / washing process included in the crude terephthalic acid production step, and is continuously fed to the column 4A for extraction of acetic acid from its upper part. In addition, an extraction solvent containing an organic solvent is fed to column 4A to extract acetic acid from its bottom. At this time, an organic solvent, divided into two phases composed of an organic phase containing extracted acetic acid and a separate aqueous phase, is selected as the solvent extracting acetic acid in the liquid-liquid equilibrium of the system "acetic acid - water - extracting solvent".

In the column 4A for the extraction of acetic acid, multistage contact occurs repeatedly in countercurrent mode and the phases are separated using the supplied spent washing liquid and an extraction solvent, as a result of which acetic acid is extracted. Then, the organic phase of the acetic acid extract flows from the top of the column and is collected. The acetic acid extract (or organic phase) thus collected is refluxed as reflux liquid until it reaches the top of the dehydration distillation column 44, while acetic acid, together with a large amount of acetic acid solvent, is removed from the oxidation reaction solvent from the bottom of the dehydration distillation columns 44. That is, the extracted acetic acid is recovered as an acetic acid solvent (that is, regenerated acetic acid acid) to be reused in the oxidation reaction. At the same time, a raffinate (or aqueous phase) with an acetic acid content of 0.7% by mass or less is separated and removed from the bottom of the column 4A for the extraction of acetic acid.

The acetic acid extract, which flowed and was collected from column 4A for the extraction of acetic acid, is supplied as reflux liquid to the upper part (i.e., to the azeotropic distillation zone) of the dehydration distillation column 44. Then, the extraction solvent and acetic acid are subjected to distillation separation. Acetic acid is withdrawn as a bottom product from the bottom of the dehydration distillation column 44. Here, an azeotropic agent (or organic solvent) is used to stimulate dehydration of acetic acid in the dehydration distillation column 44, which provides azeotropic distillation with water. Accordingly, an azeotropic distillation process is used to regenerate acetic acid. In view of the above, an azeotropic agent is used as the extracting solvent, which ensures the formation of an azeotropic composition with water (i.e., with the aqueous product of the oxidation reaction). Then, the extraction solvent is distilled off in the form of an azeotropic composition with water from the top of the dehydration distillation column 44.

Thus, the vaporous distillate, which was distilled off in the form of an azeotropic composition containing water and an extracting solvent (or an azeotropic agent) from the top of the column, is cooled and condensed using a cooling condenser 46, and then introduced into the tank 47 to separate the water-oil mixture . The composition is then separated into two phases from the aqueous and oil fractions (i.e., the organic phase containing the extracting solvent). After the extraction solvent (or azeotropic agent) is collected as an oil fraction (or organic phase) forming the upper phase in the reservoir 47 for separating the water-oil mixture, the extraction solvent is returned and recirculated as an extraction solvent to column 4A for countercurrent multistage extraction. In addition, the extraction solvent thus recovered is also used as reflux liquid for the dehydration distillation column 44.

Examples of the acetic acid extraction column 4A used in the extraction / separation process include an extraction column with perforated plates, an extraction column with multiple pulsating plates, a multi-plate extraction column with a mixer, a multi-plate extraction column with cap plates, and a packed extraction column. Among them, a multi-plate extraction column with a mixer and an extraction column with numerous pulsating plates having an increased productivity of the plates during extraction are preferred. In addition, for reasons of intensification of the production of purified terephthalic acid and enlargement of column 4A for the extraction of acetic acid, a packed extraction column, a multi-plate extraction column with cap plates and an extraction column with perforated plates, all of which are an autonomous system suitable for enlarging the column, are also preferred. .

In the extraction / separation process, the azeotropic agent that is used for dehydration by azeotropic distillation in the dehydration distillation column 44 and the extraction solvent that is used in the extraction / separation process are preferably the same substance. It should be noted that, even if the extracting solvent and the azeotropic agent are not the same, each of them can be collected separately using the difference in the boiling point during distillation (or the boiling point of the azeotropic mixture) in the upper part of the dehydration distillation column 44, so that each of They could be reused.

In addition, if the dehydration distillation column 44 is a two-component distillation system, an organic solvent that promotes the formation of an azeotropic composition with water can be used as an extraction solvent. In this case, modifications to the upper distillation portion of the dehydration distillation column 44 allow for simple conversion of the column into an azeotropic distillation system, thereby increasing the dewatering rate. Examples of an azeotropic agent that can be used in the dehydration distillation of acetic acid include organic solvents such as acetates (e.g. butyl acetate, isobutyl acetate, propyl acetate, isopropyl acetate) and aromatic hydrocarbons (e.g. benzene, toluene, para-xylene, meta-xylene )

Here, the authors of the present invention investigated the following process: the above azeotropic agent was used as an extraction solvent; acetic acid was extracted from the spent wash liquid with an acetic acid content of from about 5 to 30 wt.%; water was separated; and acetic acid was recovered from the acetic acid extract (or organic phase). As a result, the authors of the present invention concluded that examples of a method for producing a raffinate from this wash process water (or aqueous phase) having an acetic acid content of 0.7 mass% or less (i.e., with an acetic acid content equivalent to that in the dehydration process distillation of a two-component system “acetic acid - water”), include a multi-stage extraction process for applying a large amount of extraction solvent to perform multi-stage extraction using mations fresh extracting solvent. In addition, the authors of the present invention have come to understand an effective extraction process for performing countercurrent multistage extraction using an appropriate volume of extraction solvent.

Among the above methods, the last process of countercurrent multistage extraction involves the use of mixing and extraction tanks. Further, with respect to these provided mixing and extraction tanks, the spent washing liquid flows through from one end of the tank, and the extraction solvent flows through from the other end of the tank, so that these flows are continuously supplied in the opposite direction to counter flows. In each mixing and extraction tank, the process of mixing - extraction - phase separation is repeated at each of the many stages to perform extraction processing. Then, the acetic acid extract (or organic phase) flows from the upper part of the phase separation zone of the extraction tank at the end of the waste wash fluid supply. In addition, a raffinate with an acetic acid content of 0.7% by mass or less is obtained from the lower part of the phase separation zone at the other end of the extraction tank (i.e., at the place of supply of the extraction solvent to the extraction tank).

This way in the column 4A for the extraction of acetic acid using the multi-plate extraction column system illustrated in FIG. 2, a multi-stage countercurrent contact is repeated using the spent wash liquid supplied from the top of the column and an extraction solvent (or organic phase) supplied from the bottom of the column. Accordingly, the acetic acid extract (or organic phase), in which the aqueous component is separated (i.e., its content decreases), flows and is discharged from the top of the column. In contrast, a raffinate (for example, with an acetic acid content of about 0.7 mass% or less), which is an aqueous phase having a reduced acetic acid content, flows from the bottom of the column. Acetic acid extract, withdrawn from the top of the extraction column, is fed to the dehydration distillation column 44. Then, acetic acid, together with a large amount of acetic acid solvent, is removed from the reaction solvent of the oxidation process from the bottom of the dehydration distillation column 44 (see FIG. 1; FIG. 2; in FIG. 2; not shown).

In addition, an extraction solvent separated from acetic acid is distilled off from the top or top of the enrichment portion of the dehydration distillation column 44 to form an azeotropic composition with water. The azeotropic composition condenses in a cooling condenser 46 and is separated into an organic phase and an aqueous phase in a reservoir 47 for separating a water-oil mixture. The azeotropic agent (or extraction solvent) is recovered as an organic phase and sent through the bottom of column 4A to extract acetic acid and is recycled.

The raffinate, which was removed from the aqueous phase in the bottom of the acetic acid extraction column 4A, is fed to the solvent recovery column 48 in an acetic acid solvent recovery step. Then, in the solvent recovery column 48, a small amount of the dissolved extracting solvent (about 1% by weight) is subjected to distillation separation. Then, the separated water obtained is withdrawn from the bottom of the column 48 to recover the solvent. As described above, the spent wash liquid is supplied to the acetic acid extraction column 4A provided in the acetic acid solvent regeneration step illustrated in FIG. 2. This allows you to separate it without the use of a dehydration distillation column, thereby saving more heat energy than in the traditional method.

And furthermore, the acetic acid extract from the acetic acid extraction column 4A is supplied to the dehydration distillation column 44. But the water should not be separated in the dehydration distillation column 44. Here, the extraction solvent (or organic phase) is separated from the acetic acid. Thus, the extracted acetic acid, together with a large amount of the acetic acid solvent used in the oxidation reaction, is removed from the bottom of the dehydration distillation column 44. Accordingly, the extraction solvent and water (or the aqueous reaction product) form an azeotropic composition, and are enriched and distilled from the top dehydration distillation column 44. The azeotropic composition is cooled, condensed and introduced into the tank 47 to separate the water-oil mixture. After that, the extraction solvent is reused as described above. This way, this method allows you to individually collect acetic acid and an extracting solvent from an acetic acid extract (for example, with a water content of 1 wt.% Or less), while at the same time saving more energy than in the traditional method.

Here FIG. Figure 3 shows the liquid-liquid equilibrium curve in the system of acetic acid - isobutyl acetate - water (from A. Chafer et al., Fluid Phase Equilibria, Volume 271, pp. 76-81 (2008)). Using this curve, extraction was simulated, including the steps to obtain a raffinate with an acetic acid content of 0.4 wt% or less by subjecting an aqueous solution of acetic acid to multi-stage extraction of acetic acid. It should be noted that isobutyl acetate is a representative extraction solvent among the acetates used for the extraction of acetic acid. Although the details are described below, based on an extraction simulation, Table 1 summarizes the amount of extraction solvent for the extraction of acetic acid, the number of theoretical plates and the loss of acetic acid when the concentration of acetic acid changes.

Table 1 shows the results of the fact that to obtain a raffinate with a content of acetic acid of 0.4 wt.%, The following quantities were required to perform countercurrent multistage extraction. The amount of extracting solvent used was 2 or more by weight of an aqueous solution of acetic acid with an acetic acid content of 30% by weight. The amount of extraction solvent used was 2.5 or more by weight of an aqueous solution of acetic acid with an acetic acid content of 20% by weight. The amount of extraction solvent used was 3 or more by weight of an aqueous solution of acetic acid with an acetic acid content of 10% by weight. The amount of extracting solvent used was 3 or more by weight of an aqueous solution of acetic acid with an acetic acid content of 5% by weight.

In addition, when acetic acid is extracted from an aqueous solution of acetic acid with an acetic acid content of 5 to 30 wt.%, Fewer theoretical plates simplify extraction. Based on the results from Table 1 below, model example 1 has the least number of theoretical plates. That is, when the amount of extracting solvent is 5.5 times by weight (i.e., relative solvent consumption), acetic acid can be particularly easily extracted using a countercurrent multistage extraction system. In addition, from the same point of view, when the amount of extracting solvent is 4.2 times the mass (see model examples 2, 5, 10 and 14), the number of theoretical plates is about 5 in each case. Thus, it appears that acetic acid can be easily extracted.

Here, isobutyl acetate can be used as an extraction solvent, and a countercurrent multistage extraction system can be used to extract acetic acid from the spent wash liquid. In this case, it should be taken into account that the spent washing liquid has an acetic acid content of 5 to 30% by weight. The simulation results showed that the extraction is effective using an extraction solvent in an amount of at least 2-3 times the mass, and not more than 5.5 times the mass.

Meanwhile, the amount of raffinate (for example, with an acetic acid content of 0.4 wt.%) Obtained from the spent wash liquid with each acetic acid content (for example, from 5 to 30 wt.%) Is used to calculate the loss of acetic acid. When the spent wash liquid has an acetic acid content of 5% by weight, the loss is about 7.5% (see model examples 1 and 2 in Table 1 below). When the spent wash liquid has an acetic acid content of 10% by weight, the loss is about 3.5% (see model examples 4-6 in Table 1 below). When the spent wash liquid has an acetic acid content of 20% by weight, the loss is about 1.5% (see model examples 8-10 in Table 1 below). In view of the above, when the content of acetic acid in the spent wash liquid doubles from 5 to 10 wt.%, The loss of acetic acid becomes half as small. When the spent wash liquid has an acetic acid content of 30 wt.%, The loss can be reduced to about 1% (see model examples 11-14 in Table 1 below).

Accordingly, when these data are taken into account, the amount of acetic acid loss from the spent wash liquid can be reduced to 5% or less. That is, the degree of extraction of acetic acid can be increased to 95% or more. To achieve this, the spent washing liquid (i.e., the spent washing liquid collected in the spent washing liquid tank 3A shown in FIG. 1) removed during washing with water of crude terephthalic acid crystals must have an acetic acid content of 7.5% by weight or more. In addition, in order to provide an increase in the degree of extraction of acetic acid to about 98.5%, it was found that the spent washing liquid preferably has an acetic acid content of 20 wt.%.

Thus, the results of modeling acetic acid extraction shown in Table 1 demonstrate that during the extraction / separation process, the amount of extracting solvent is at least 3 or more of the weight of the spent wash liquid; when the spent washing liquid has an acetic acid content of 20% by mass or more, the amount of extracting solvent is 2.5 or more than the mass of the spent washing liquid; and it seems preferable to carry out the extraction of acetic acid using the amount of extracting solvent, a maximum of 5.5 times the mass of the spent wash liquid. In addition, in order to achieve an extraction level of acetic acid of 95% or more (i.e., a loss of acetic acid of 5%), the spent washing liquid with an acetic acid content of 7.5 to 30 wt.% Can be sent to the extraction / separation process separation / washing. Moreover, in order to achieve a degree of extraction of acetic acid of 98.5%, the volume of the wash water during the separation / washing process can be adjusted so that the spent wash liquid has an acetic acid content of 20% by weight.

In such a case, during the separation / washing process, wet crystals of substantially crude acetic acid-free terephthalic acid containing water can be obtained. Although a special filtration rinse method (i.e., a countercurrent pressure-flow rinse method (see JP-2620820B)) was used to produce spent wash liquid containing a high concentration of acetic acid to reduce the amount of wash water, a wash method ( that is, a filtration washing method and a suspension washing method) using a standard filter (e.g., a rotary drum filter, a belt filter). With this design, the content of acetic acid in the spent wash liquid can be controlled and controlled at a level of from 5 to 30 wt.%, And preferably from 7.5 to 30 wt.%.

In this way, crystals of substantially crude acetic acid-free terephthalic acid can be obtained, while at the same time reducing the loss of acetic acid. In addition, the amount of newly added wash water can be set to a value of 0.75 by weight of the crude terephthalic acid mass. This way you can implement a simpler washing method, which can give crystals essentially crude acetic acid-free terephthalic acid, and thus the spent washing liquid can have an acetic acid content of 20 wt.% Or less. Due to this, if in this preferred washing method the content of acetic acid in the spent washing liquid can be further adjusted and set at a level of from 7.5 to 20 wt.%, The method can be performed without using a dryer with a reduced loss of acetic acid.

A method without using a dryer according to one embodiment of the present invention includes a process for separating and washing terephthalic acid crystals using a conventional common solid-liquid separator, for reasons of simplification of industrial applications. Here, examples of this separation / washing process include: a multi-stage process comprising suspension washing using a rotary drum filter; a process using a belt filter in the washing zone; a multi-stage process, including suspension washing using a centrifuge; and a multi-stage process, including suspension washing using a centrifuge and a filtration device. For reasons of using a common separator (or common filtering device) and the existing separation / washing step, it is preferable that the spent washing liquid supplied to the extraction / separation process has an acetic acid content of 5 to 30% by weight.

More specifically, the relationship between the content of acetic acid in the spent wash liquid and the amount of wash water demonstrates that the amount of wash water added again during the separation / washing step is preferably 0.5 or more, but not 0, of the crude terephthalic acid crystal mass. 4 or less than the mass of crude terephthalic acid, which amount is the amount of wash water when a special filter is used. In addition, the amount of wash water used is preferably adjusted to 1.5 times the mass or less, but not to 2 times the mass or more, to sufficiently wash the crude terephthalic acid crystals. These settings make it possible to ensure the content of acetic acid in the spent washing liquid supplied to the extraction / separation stage at the level of the above concentration.

It should be noted that situations with the use of isobutyl acetate as an extracting solvent are described above. However, when butyl acetate is used as the extracting solvent, similar results should also be obtained. More specifically, butyl acetate was used to construct a liquid-liquid equilibrium curve in a three-component system (see the authors E. Ince and S.I. Kirbaslar, Brazilian J. Chem. Eng., Volume 19, pp. 243-254 (2002)). In addition, toluene was used from aromatic hydrocarbons to construct a liquid-liquid equilibrium curve in a three-component system (see the authors S. Javad et al., J. Chem. Thermdyn., Volume 57, pp. 76-81 (2013)). The shapes of these curves are similar to the shape of the liquid-liquid equilibrium curve from FIG. 3, for which isobutyl acetate was used. Examples of an extraction solvent include: the above-described isobutyl acetate and butyl acetate; and organic solvents selected from acetates (e.g., propyl acetate, isopropyl acetate) and aromatic hydrocarbons (e.g., benzene, toluene, para-xylene, meta-xylene). When any of the above extraction solvents is used, the above description is illustrated by the use of isobutyl acetate as one example.

In addition, such an extracting solvent forms an azeotropic composition with water, so that dehydration by azeotropic distillation can be performed in a dehydration distillation column. Due to this, such an extraction solvent like isobutyl acetate can be used as an extraction solvent for the extraction of acetic acid, and can also be used as an azeotropic agent for dehydration distillation. Therefore, such an extracting solvent can be reused.

As described previously, during the process of separating and washing crystals of crude terephthalic acid formed in the crude terephthalic acid production step, the spent washing liquid and acetic acid waste liquid, such as a separated mother liquor, are stored separately. The spent washing liquid is sent to column 4A for extraction of acetic acid without subjecting it to processing in a dehydration distillation column. An extracting solvent is then used to recover a high degree of extraction of acetic acid. Meanwhile, raffinate is removed, which is an aqueous phase with a significantly reduced content of acetic acid (for example, with an acetic acid content of 0.7% by weight or less). This process does not require energy to separate water by dehydration, and is thus an economical way to regenerate acetic acid. In view of this, the acetic acid extraction column 4A, including a countercurrent multi-plate extraction column, can be further connected to existing equipment for carrying out the acetic acid solvent regeneration step. In addition, auxiliary equipment for recovering and recycling acetic acid and an extracting solvent in a dehydration distillation column 44 and a solvent recovery column 48 can be improved. This enables highly efficient recovery of acetic acid.

EXAMPLES

Modeling Acetic Acid Extraction

Regarding the extraction / separation process for extracting acetic acid from the spent wash liquid, the goal was to investigate the feasibility of extracting acetic acid according to, for example, the content of acetic acid in the raffinate, and the extraction process using an extraction solvent. The set numerical values were then assigned to the conditions of the spent washing liquid, such as the concentration of acetic acid in the spent washing liquid, and the extraction conditions, such as the amount of extraction solvent (for example, relative solvent consumption (or mass ratio): amount of extraction solvent based on the quantity of spent flushing fluid). Then, the extraction of acetic acid was simulated under each of the technological conditions.

In this simulation, the subject of the study was waste aceous acid containing acetic acid removed from the separation / washing step. Isobutyl acetate was used as the extraction solvent, and a countercurrent, multi-plate extraction column was modeled as 4A column for extraction of acetic acid. After that, graphical modeling was performed to calculate the concentration of acetic acid in the raffinate output at the outlet of each extraction stage and the number of theoretical plates in which the final raffinate had an acetic acid concentration of 0.4 mass% or less. The liquid-liquid equilibrium curve in the system "isobutyl acetate - acetic acid - water" shown in FIG. 3, was used to model the extraction of acetic acid using isobutyl acetate as an extracting solvent. Graphical analysis was used to sequentially calculate the concentration of acetic acid in the raffinate and the concentration of acetic acid in the extract at each extraction stage of countercurrent extraction of acetic acid. The simulation was repeated to calculate the number of theoretical extraction plates until the concentration of acetic acid in the raffinate reached 0.4 wt.%. In addition, the amount of raffinate with a concentration of acetic acid of 0.4 wt.% Was used to calculate the amount of loss of acetic acid. The amount of acetic acid loss was calculated (see Table 1) relative to each spent wash liquid (i.e., an aqueous solution containing acetic acid).

The amount of extracting solvent (i.e., expressed as the relative solvent flow rate) was changed in each aqueous solution having an acetic acid concentration of 5, 10, 20, and 30 wt.%. Table 1 shows the results of modeling the number of theoretical plates required to obtain a raffinate with a concentration of acetic acid of 0.4 wt.%, And the loss of acetic acid (in wt.%), Calculated using raffinate, in which acetic acid was sent to waste extraction process.

Table 1 Model example The concentration of acetic acid (wt.%) The amount of extracting solvent (relative solvent consumption) The number of theoretical plates (stages) The loss of acetic acid (wt.%) one 5 3.0 14.7 7.4 2 5 4.2 5,0 7.7 3 10 2.5 Impossible - four 10 3.0 11,4 3.8 5 10 4.2 5.8 3.4 6 10 5.5 3.0 3,5 7 twenty 2.0 Impossible - 8 twenty 2.5 17.3 1,5 9 twenty 3.0 9.6 1,2 10 twenty 4.2 6.6 1.9 eleven thirty 2.0 19,4 0.8 12 thirty 2.5 10.0 0.9 13 thirty 3.0 7.3 0.8 fourteen thirty 4.2 4.9 0.8

FIG. 4 and 5 illustrate the results of modeling the concentration of acetic acid in the raffinate from each extraction stage, when the concentration of acetic acid in the aqueous solution is 10 wt.% And 20 wt.% For each amount of extracting solvent (expressed by the relative flow rate of the solvent) as a process parameter.

FIG. 4 shows that when the aqueous solution had an acetic acid concentration of 10 wt.%, The concentration of acetic acid in the raffinate reached 0.4 wt.%, As follows: in the case of a relative solvent consumption of 3.0, the number of theoretical plates was 12 (exact number = 11.4); and in the case of a relative solvent consumption of 4.2, the number of theoretical plates was 6 (exact number = 5.8). However, in the case of a relative solvent consumption of 2.5, the decreasing concentration of acetic acid was equalized with the concentration of acetic acid of 4 wt.% On the sixth theoretical plate. Therefore, it was not possible to obtain a raffinate with an acetic acid content of about 3.5 wt.% Or less. Thus, it was almost impossible to obtain a raffinate with a concentration of acetic acid of 0.4 wt.% Under conditions of a relative solvent consumption of 2.5 (see model example 3). As a result, the number of theoretical plates in model example 3 in Table 1 was designated as “impossible”.

In addition, FIG. 5 shows that when the aqueous solution had an acetic acid concentration of 20 wt.%, The concentration of acetic acid in the raffinate reached 0.4 wt.%, As follows: in the case of a relative solvent consumption of 2.5, the number of theoretical plates was 18 (exact number = 17.3) (not shown in FIG.); and in the case of a relative solvent consumption of 3.0, the number of theoretical plates was 10 (exact number = 9.6). In addition, in the case of a relative solvent consumption of 4.2, the number of theoretical plates was 7 (exact number = 6.6). Thus, it was possible to obtain a raffinate with a concentration of acetic acid of 0.4 wt.% Or less with such plates. However, in the case of a relative solvent consumption of 2.0 (see model example 7), it was not possible to obtain a raffinate with a concentration of acetic acid of 0.4 wt.%. As a result, the number of theoretical plates in model example 7 in Table 1 was designated as “impossible”.

In addition, Table 1 shows that when the aqueous solution had an acetic acid concentration of 30 wt.% And the relative solvent consumption was 2.0, it was possible to obtain a raffinate with an acetic acid concentration of 0.4 wt.% Or less using an extraction column with a large number of theoretical plates 20 (exact number = 19.4).

Thus, when isobutyl acetate is used as the extracting solvent for the spent wash liquid, the raffinate may have a concentration of acetic acid of 0.4% by weight or less. To achieve this, when the concentration of acetic acid in the spent wash liquid is increased, the extraction can be performed at a lower relative flow rate of the solvent. Then, for the spent wash liquid with an appropriate concentration of acetic acid (from 5 to 30 wt.%), The amount of extracting solvent is required at a relative solvent flow rate of at least 2- (for spent wash liquid with an acetic acid concentration of 30 mass%) to 3 -fold value of mass or more (for spent washing liquid with a concentration of acetic acid of 5 wt.%). When the flow rate of the extracting solvent relative to the spent wash liquid is a maximum of 5.5 times the mass, countercurrent multistage extraction can be used to easily extract acetic acid.

The above simulation results were obtained by calculating the number of theoretical plates in liquid-liquid equilibrium. The efficiency of mass transfer on contact plates may vary, for example, depending on the shape of the multi-plate extraction column and the flow rate of the liquid flow in the organic phase and in the aqueous phase associated with the extraction system. Consequently, the system-specific extraction performance of acetic acid (or plate efficiency) must be taken into account. Accordingly, the actual number of plates of the extraction column is determined based on the number of theoretical plates. Then a multi-plate extraction column can be designed. However, under extraction conditions in which the above liquid-liquid equilibrium curve was used to perform the simulation and extraction was assumed impossible (model examples 3 and 7), it is not possible to extract acetic acid to an acetic acid content of 0.4 mass% or less.

Table 1 shows the loss of acetic acid when simulating the ratio of acetic acid (i.e. 0.4 wt% of acetic acid in the raffinate) that remained in the extraction / separation process to the amount of an aqueous solution of acetic acid as a raw material for extraction. When the content of acetic acid in the spent wash liquid doubled from a concentration of acetic acid of 5 wt.% In an aqueous solution to a content of 10 wt.%, The loss of acetic acid became half lower. Thus, the loss of acetic acid during the extraction / separation process should be considered. The loss of acetic acid content in the spent wash liquid can be adjusted to 5% or less (that is, with an extraction efficiency of acetic acid of 95% or higher). To achieve this, it is considered preferable that the spent wash liquid have an acetic acid content of at least 7.5 wt.% Or more (i.e., an acetic acid concentration intermediate between 5 and 10 wt.%). An upper limit is assumed to be preferred at a level of 30 mass% or less. In order to ensure an extraction efficiency of acetic acid of 98.5%, the spent washing liquid preferably has a concentration of acetic acid of 20% by weight.

Further, assuming for the separation / washing process the steps of “filtration separation / washing → suspension washing → filtration separation / washing”, one example of washing will be described, including suspension washing of crystals of crude terephthalic acid, as follows.

Example 1

About 2.5 kg of a suspension of crude terephthalic acid crystals obtained in the para-xylene oxidation reaction in the crude terephthalic acid production step was taken as a sample from a crystallization tank (i.e., crystallization tank 7 shown in FIG. 1). Then, 300 g of the crystal-containing suspension were placed separately in each of six flasks with a capacity of 500 ml, with stirring. Then, one (300 g) suspension thus collected containing crystals was heated to a temperature of about 60 ° C and subjected to suction filtration using a Buchner funnel with a diameter of 70 mm (using filter paper 5A; then, a filter of the same type was used for filtering). After that, the resulting suspension was washed with regenerated acetic acid in an amount of 50 g (with a water content of 5.2 wt.%) At a temperature of 60 ° C in order to collect wet crystals containing acetic acid (or filter cake). Individually measured by weighing the weight and dry weight of this wet filter cake, and determined 112 g and 100 g (i.e. moisture content: 10.7 wt.%), Respectively. It should be noted that all amounts of the separated mother liquor withdrawn during suction filtration and the washing wastewater obtained during the washing with acetic acid were collected and sent to waste.

These wet crystals containing acetic acid were transferred to a 500 ml flask and 188 g of water was added to prepare the suspension. The resulting suspension was stirred at a temperature of about 60 ° C for about 10 minutes, and subjected to suspension washing. Then, this aqueous suspension of crude terephthalic acid crystals was filtered with suction using a Buchner funnel with a diameter of 70 mm. Then, the entire amount of the separated filtrate (first aqueous filtrate) was collected. Then, 50 g of water was added to the wet crystals (or filter cake) on a Buchner funnel at a temperature of about 60 ° C. to perform filtering washing. After that, the entire amount of wastewater (i.e., the first spent wash liquid) was collected and wet crystals containing water (i.e., first wet crystals) were obtained.

A second individual suspension containing crystals (300 g) was heated to a temperature of about 60 ° C and similarly subjected to suction filtration using a Buchner funnel with a diameter of 70 mm. Then, the obtained wet crystals were washed with 50 g of acetic acid at a temperature of 60 ° C. to obtain wet crystals of acetic acid. Acetic acid containing wet crystals was transferred to a 500 ml flask, and 50 g of fresh water, the entire amount (about 50 g) of the first spent wash liquid using water, and 88 g of the first aqueous filtrate were added to the resulting suspension. The resulting mixture was stirred at a temperature of about 60 ° C for about 10 minutes, and subjected to suspension washing. It should be noted that a residue of about 100 g of the first aqueous filtrate was withdrawn in the form of an aqueous spent wash liquid containing acetic acid.

Then, this turbulent suspension of crude terephthalic acid crystals was filtered with suction using a Buchner funnel with a diameter of 70 mm. Then, the entire amount of the second aqueous filtrate was collected. Then, 50 g of fresh water was added to the wet crystals on a Buchner funnel at a temperature of about 60 ° C to perform filtration washing. After that, the entire amount of the second spent washing liquid was collected, and the second wet crystals containing water were obtained.

Each of the individual third (300 g), fourth (300 g), fifth (300 g) and sixth (300 g) crystal-containing suspensions was similarly filtered with suction and washed with acetic acid to obtain wet crystals containing acetic acid. Then, 50 g of fresh water, the entire amount (about 50 g) of the second spent washing liquid, and 88 g of the second aqueous filtrate were added to the third wet crystals in a similar manner to perform suspension washing. Then, suction filtration was carried out in a similar manner, and 50 g of fresh water was added to perform filtration washing. After that, the entire amount of the third aqueous filtrate and the entire amount of the third spent washing liquid, each separately, were collected and the third wet crystals containing water were obtained.

In addition, in the same manner as described above, the fourth wet crystals containing acetic acid were used to collect the fourth aqueous filtrate and the fourth spent wash liquid, and then to obtain the fourth wet crystals containing water.

In addition, in the same manner as described above, fifth wet crystals containing acetic acid were used to collect a fifth aqueous filtrate and a fifth spent wash liquid, and then fifth wet crystals containing water were obtained.

Six wet crystals containing acetic acid were transferred to a 500 ml flask, and 50 g of fresh water, the entire amount (about 50 g) of the fifth spent wash liquid, and 88 g of the fifth aqueous filtrate were added to prepare the suspension. The resulting suspension was stirred at a temperature of about 60 ° C for about 10 minutes, and subjected to suspension washing. This turbulent suspension of crude terephthalic acid crystals was then filtered with suction using a Buchner funnel with a diameter of 70 mm. Then the entire amount of the sixth aqueous filtrate was collected. Then, 50 g of fresh water (at a temperature of about 60 ° C) was added to the wet crystals on a Buchner funnel to perform filtration washing. After that, the entire amount of the sixth spent wash liquid was collected, and at the same time, sixth wet crystals containing water were then obtained.

The above operation is a simulation in which a washing process (i.e., multiple washing with a spent washing liquid) is carried out according to FIG. one.

The content of acetic acid in each of the fifth and sixth aqueous filtrate was determined, and the content of acetic acid in each of the fifth and sixth wet crystals containing water. Table 2 shows the results.

table 2 The content of acetic acid in the aqueous filtrate (wt.%) The content of acetic acid in wet crystals containing water (wt.%) Acetic acid content (converted) in crude terephthalic acid crystals (wt.%) Fifth time 11.1 0.37 0.42 Sixth time 11,2 0.37 0.41

Table 2 shows that when the spent wash liquid was reused, the content of acetic acid in each of the fifth and sixth wet crystals containing water and the content of acetic acid in each of the fifth and sixth aqueous filtrate seemed to be almost constant. Wet crystals of crude terephthalic acid containing acetic acid were subjected to repeated suspension washing with an aqueous solvent in an amount of about 1.9 times (i.e., the total amount of water for suspension washing was 1.88 times the weight, including fresh water in an amount of about 0 5 times the mass value, the previous waste wash liquid in an amount of about 0.5 times the weight and the previous aqueous filtrate about 0.88 times the weight), and, cr IU of fresh water used in an amount 0.5 times the weight of crystals to be subjected to a filtration washing crystals. The results demonstrated the successful isolation of crystals of crude terephthalic acid with an acetic acid content of about 0.41 wt.%.

In addition, 100 g (dry weight) of crystals were washed using water in an amount of 1 times the mass of crystals (i.e. using fresh water in an amount (50 g) of 0.5 times the mass of crystals for suspension washing, and fresh water in an amount (50 g) of 0.5 times the mass of crystals for filtration washing). As a result of this, it was confirmed that then the suspended spent washing liquid was withdrawn in an amount (about 100 g) of 1 times the mass of crystals (100 g of dry weight) and with an acetic acid content of 11.2 wt.%.

Example 2

An extraction column was used with perforated plates with an inner diameter of 60 mm, a height of about 2600 mm, and 25 plates (from an acrylic short column (90 mm) sandwiched between perforated plates and with a drain pipe made of stainless steel), and an aqueous a solution with an acetic acid content of 15 wt.% was supplied at a flow rate of 170 g / min from the top of the column. Meanwhile, butyl acetate as an extraction solvent was continuously fed (i.e. for countercurrent extraction) with a flow rate of 526 g / min (i.e., the ratio of extraction solvent to aqueous solution with acetic acid was 3.1 times the mass) from the bottom of the column. Then, the acetic acid extract flowed from the top of the column, and the raffinate was removed from the bottom of the column. In this way, a continuous extraction of acetic acid using butyl acetate was tested. It should be noted that the extraction column, which was used for the test, had an average temperature of about 35 ° C.

Table 3 shows the results of the excretion rate and the content of acetic acid, and the excretion rate of the extract of acetic acid (or the organic phase) removed from the top of the column, and the content of acetic acid in the raffinate (or aqueous phase) discharged from the bottom of the column.

Table 3 Excretion rate (g / min) The content of acetic acid (wt.%) Acetic acid extract (organic phase) 568 4.4 Raffinate (aqueous phase) 127 0.4

Table 3 shows that the acetic acid extract (acetic acid extract) had an acetic acid content of 4.4% by weight. This result demonstrates that the extraction efficiency of acetic acid was about 98%. In addition, the raffinate had an acetic acid content of 0.4% by weight. This result showed that almost all acetic acid was extracted from an aqueous solution of acetic acid.

Example 3

Used the same as in Example 2, the extraction column with perforated plates. An aqueous solution with an acetic acid content of 10 wt.% Was continuously fed with a flow rate of 100 g / min from the top of the column. Meanwhile, isobutyl acetate as an extracting solvent was continuously fed with a flow rate of 300 g / min (i.e., the ratio of the extracting solvent to the aqueous solution with acetic acid was 3 times the mass) from the bottom of the column. Then, in the same manner as in Example 2, a continuous extraction of acetic acid was tested. It should be noted that the extraction column, which was used for the test, had an average temperature of about 35 ° C. Table 4 shows the results.

Table 4 Excretion rate (g / min) The content of acetic acid (wt.%) Acetic acid extract (organic phase) 317 3,1 Raffinate (aqueous phase) 83 0.3

Table 4 shows that the acetic acid extract had an acetic acid content of 3.1% by weight. This result demonstrates that the extraction efficiency of acetic acid was about 97%. In addition, the raffinate had an acetic acid content of 0.3% by weight. This result showed that almost all acetic acid was extracted from an aqueous solution of acetic acid. It should be noted that the plate efficiency in this example was found to be about 46% when the simulated number of theoretical plates was used.

Legend List

1. Oxidation reactor.

2. Cooling condenser.

3. Gas-liquid separator.

4. Additional oxidation reactor.

5. Cooling condenser.

6. Gas-liquid separator.

7. The mold.

8. Cooling condenser.

9. Gas-liquid separator.

10. Solid-phase liquid separator (including the first washing zone).

11. The reservoir for the separated mother liquor.

12. The tank for washing wastewater.

23. The tank for the preparation of the suspension.

24. The heat exchanger.

25. Installation for dissolution.

26. Hydrotreating reactor.

27. The first mold.

28. Nth crystallizer.

29. The first solid-phase liquid separator.

30. The reservoir for the separated mother liquor.

31. The tank for re-suspension.

32. The second solid-phase liquid separator.

33. The tank for waste flushing fluid.

34. The dryer.

35. A device for supplying powder.

36. A reservoir for storing powder (purified terephthalic acid).

41. Evaporator.

42. Heat exchanger.

43. Concentration column.

44. Dehydration distillation column.

45. Heat exchanger.

46. Cooling condenser.

47. The tank for the separation of water-oil mixture.

48. Column for the regeneration of the solvent.

49. Heat exchanger.

50. Cooling condenser.

1A. Resuspension tank (including second wash zone).

2A. The second solid-phase liquid separator (including the third washing zone).

3A. Waste fluid reservoir.

4A. Acetic acid extraction column.

Claims (9)

1. The method of obtaining purified terephthalic acid, including:
a step for producing crude terephthalic acid comprising oxidation of para-xylene in an acetic acid solvent, subsequent additional oxidation, crystallization to obtain a suspension of crystals, separation of the obtained crystals of crude terephthalic acid by washing after separation using a solid phase liquid separator;
an acetic acid solvent regeneration step in which the acetic acid solvent contained in the acetic acid solution withdrawn during solid phase liquid separation in the crude terephthalic acid production step is regenerated;
a crude terephthalic acid purification step, in which a hydrotreating the aqueous solution is carried out after mixing and dissolving the crude terephthalic acid crystals obtained in the crude terephthalic acid preparation step with an aqueous solvent, and purified terephthalic acid crystals are formed in the solution, and then purified terephthalic acid crystals are isolated from solution solid-liquid separation, in which operations (1) to (3) are carried out in the corresponding stages of the method:
(1) in the step of preparing crude terephthalic acid, crystals of crude terephthalic acid obtained by solid phase liquid separation using a solid phase liquid separator are transferred to the first washing zone and washed at least once with an acetic acid solvent, and the washed crystals obtained are transferred to the second washing; in the second washing zone, the crystals are washed, at least once, with fresh water and / or an aqueous solvent, that is, spent washing liquid obtained from the zone in the direction of transfer of the crystals, and then the obtained washed crystals are transferred to the third washing zone; in the third washing zone, that is, in the final washing zone, the crystals are washed with fresh water and substantially wet terephthalic acid crystals free of acetic acid are isolated;
the separated mother liquor recovered using a solid phase liquid separator is collected in a separate mother liquor tank, and the wastewater obtained by washing with an acetic acid solvent in the first washing zone is collected in a wastewater washer, and the mother liquor and wastewater are used as an acetic acid solvent for the oxidation reaction of para-xylene;
the spent washing liquid withdrawn during washing with fresh water and / or an aqueous solvent in the second washing zone is collected and reused at least once as washing water in the second washing zone, and the extracted spent washing liquid is collected in the spent washing tank liquids
the spent washing liquid obtained by washing in the third washing zone, that is, in the final washing zone, is collected and then used as an aqueous solvent for washing in the second washing zone, which is located in the opposite direction to the crystal transfer direction;
(2) in the regeneration step of the acetic acid solvent, the aqueous spent washing liquid in the above spent washing liquid tank, which was withdrawn and collected from the crude terephthalic acid separation / washing process, is fed to the acetic acid extraction column, an extracting solvent forming two immiscible phases with water, used for the extraction of acetic acid from the aqueous spent washing liquid in the column for the extraction of acetic acid, and the acetic acid extract ekayut as an organic phase and raffinate obtained in the form of an aqueous phase;
the acetic acid extract is sent to the top of the dehydration distillation column, acetic acid is recovered from the bottom of the dehydration distillation column, and a condensate containing an extracting solvent is obtained from the top of the dehydration distillation column; the condensate obtained is fed to two-phase separation using a reservoir for separating the water-oil mixture to separate and regenerate the extraction solvent, and the extraction solvent is reused as the extraction solvent in the acetic acid extraction column, while part of the extraction solvent is recirculated as reflux liquids in a dehydration distillation column;
the raffinate withdrawn from the acetic acid extraction column is fed to the solvent recovery column, and the dissolved extraction solvent is recovered; and
(3) in the crude terephthalic acid purification step, wet crude terephthalic acid crystals removed from the third washing zone are sent directly to the slurry preparation tank, the crude terephthalic acid crystals are mixed with an aqueous hydrotreating solvent to obtain a suspension of crude terephthalic acid crystals, and a suspension of crude terephthalic acid crystals is sent to the hydrotreating step in the presence of a noble metal catalyst supported on activated carbon, then stvlyayut crystallization in several crystallizers, separation using a solid-liquid separator and drying. 2. The method of obtaining purified terephthalic acid according to claim 1, wherein in the step of obtaining crude terephthalic acid,
multiple washing in the opposite direction to the transfer direction of the crude terephthalic acid crystals was carried out using the spent washing liquid from the third and / or second washing zones so that the aqueous spent washing liquid collected in the waste washing liquid tank has an acetic acid content of 7 or more 5 to 30 mass. % 3. The method of obtaining purified terephthalic acid according to claim 2, in which the aqueous spent washing liquid collected in the tank for the spent washing liquid has an acetic acid content of from 7.5 to 20 mass. % 4. The method of obtaining purified terephthalic acid according to claim 1, wherein the washing process in the second washing zone comprises at least one suspension washing. 5. The method for producing the purified terephthalic acid according to claim 1, wherein the extracting solvent, which forms two immiscible phases with water, used in the regeneration step of the acetic acid solvent, is at least one organic solvent selected from the group consisting of butyl acetate, isobutyl acetate, propyl acetate, isopropyl acetate, benzene, toluene, para-xylene and meta-xylene. 6. The method for producing the purified terephthalic acid according to claim 1, wherein the amount of extracting solvent used in the acetic acid extraction column is 3-5, 5 times the weight of the aqueous spent washing liquid collected in the spent washing liquid tank. 7. The method for producing the purified terephthalic acid according to claim 1, wherein the extracting solvent used in the acetic acid extraction column and the azeotropic agent used for dehydration by azeotropic distillation in a dehydration distillation column are the same. 8. The method of obtaining purified terephthalic acid according to claim 1, in which
extraction of acetic acid in an acetic acid extraction column is carried out using a countercurrent multistage extraction system using an extracting solvent and an aqueous spent washing liquid collected in the spent washing liquid tank; and
acetic acid is extracted until the raffinate has a residual content of acetic acid of 0.7 mass. % or less. 9. The method for producing purified terephthalic acid according to claim 8, wherein the countercurrent multistage extraction system operates in a countercurrent multicare extraction column.

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