KR101194067B1 - Crystallization and solid-liquid separation process in preparation of terephthalic acid - Google Patents

Abstract

The present invention provides a crystallization and solid-liquid separation process for separating the terephthalic acid into a solid phase from a slurry containing terephthalic acid and a reaction intermediate produced by oxidation of paraxylene, wherein the reaction intermediate includes paratoluic acid and 4-carboxybenzaldehyde. And crystallizing the slurry and filtering in a high temperature high pressure filter to separate the terephthalic acid into a solid phase and to separate the mother liquor including the reaction intermediate into a liquid phase, and the operating temperature of the high temperature high pressure filter is 130 to 220 ° C. It provides a crystallization and solid-liquid separation process, characterized in that the operating pressure is 10 ~ 20kgf / cm ² g. The paratoluic acid in the slurry may be contained 10wt% or more, preferably 20wt% or more, the crystallization may be carried out in a crystal bath.

Description

Crystallization and solid-liquid separation process in preparation of terephthalic acid

The present invention relates to a method for separating terephthalic acid having high purity by an effective method when the result of the oxidation reaction of paraxylene includes a large amount of reaction intermediates such as paratoluic acid in addition to terephthalic acid.

Terephthalic acid is a useful compound used as a raw material for a wide range of products and is used as the main raw material for polyethylene terephthalate (PET), polyester fibers, and polyester films for packaging and containers. Terephthalic acid is manufactured in more than 50 million tons annually worldwide and can be produced from 100,000 to 800,000 tons annually in a single plant.

In general, terephthalic acid can be produced by the liquid phase exothermic oxidation of paraxylene in an acetic acid solvent using a gas containing air or other oxygen molecules as the oxidant as a source and using at least one heavy metal compound and at least one reaction promoter compound. have. Based on this method, the so-called Amoco MC process, which is the most widely used commercially to date, is described in detail in US Pat. No. 2,833,816 to Saffer et al.

When the method described in U.S. Patent No. 2,833,816 is reacted in the oxidation reactor to react paraxylene and oxygen molecules, solid terephthalic acid is produced and the yield is very high, 95% or more. In addition to terephthalic acid through oxidation, reaction intermediates such as p-tolualdehyde, p-toluic acid, 4-hydroxymethylbenzoic acid, and 4-carboxybenzaldehyde 4-carboxybenzaldehyde) is produced in small amounts. Thus produced impurities are not high in amount, and as shown in FIG. 1, after controlling the particle size of terephthalic acid through the crystallization process (1, 2, 3), the terephthalic acid and the liquid solvent and the dissolved solid are passed through the vacuum filter (4). Impurities are separated. Terephthalic acid separated into the solid phase contains some liquid solvent, so that it is completely dried and further dried to recover the solvent (5).

On the other hand, in recent years, in addition to social movements on environmental regulations and restrictions on the use of toxic substances, terephthalic acid production methods have been continuously attempted to develop environmentally friendly methods that do not use acetic acid and bromine. Typical of these methods is the oxidation reaction using a water solvent and a bromine free catalyst system.

For example, when Hanotier et al. Produce terephthalic acid using water and a reaction intermediate as a solvent, as disclosed in US Pat. No. 4,334,086, the material produced after the reaction is a large amount of 4-carboxybenzaldehyde, paratoluic acid, etc. in addition to terephthalic acid. It will include the reaction intermediate of. In this case, US Pat. No. 4,334,086 suggested sedimentation columns 6 and 9 as shown in FIG. 2 as a method for removing such high concentration of impurities.

The basic principle of sedimentation column is to put slurry containing impurities at the top of the column and send it down to the bottom of the column, while at the bottom of the column, water or other washing water is put up and sent to the top of the column. It is to meet the washing action of impurities. In other words, as a method of solid-liquid separation, by operating the hydraulic pressure from the bottom to the top lower than the gravity of the solid, the solid particles are gradually lowered to the bottom while being washed.

The advantage of this method is that it is possible to maximize the cleaning effect for solid particles containing high concentrations of impurities by increasing the residence time of the solid particles in the wash water and flowing the wash water in the opposite direction to the solid particles.

The above-mentioned patent preferentially removes most impurities from the primary sedimentation column 6, and then dissolves or melts the impurities by sufficiently raising the temperature using the heater 7 for the slurry flowing out to the bottom of the column. Recrystallization process is carried out in (8) to precipitate relatively high purity terephthalic acid. Then, the second settling column 9 is passed again to obtain very high purity terephthalic acid solid particles of 700 ppmw of 4-carboxybenzaldehyde from the bottom of the column.

However, as a problem of the sedimentation column, first, it is practically difficult to design. The sedimentation column is not commercialized and operated, and it is easy to determine optimal values such as residence time, diameter and height of the column, shape and size of the inlet and outlet, and space velocity to maximize the cleaning effect in terms of design. not. This is because these values vary greatly depending on the temperature and pressure during operation and the flow rate of the slurry and washing water introduced.

Second, it is not easy to operate as a large facility. In particular, it is very important and difficult to operate the hydraulic pressure of the washing water introduced from the bottom to the top lower than the gravity of the solid particles from the top to the bottom. Gravity of the solid particles is substantially determined by the particle size, density, and shape of the solid particles, and the gravity resistance of the solid particles also varies depending on the density and viscosity of the washing water. In addition, since the physical properties of the wash water are again determined by the temperature and pressure in the column, there are many factors that need to be adjusted in order to perform proper operation, and there are some variables that are not practically adjustable.

On the other hand, in general, impurities produced when oxidizing paraxylene and air in a liquid phase to prepare terephthalic acid are reaction intermediates before conversion to terephthalic acid, that is, 4-carboxybenzaldehyde, paratoluic acid, and the like. These impurities, when included in the terephthalic acid product, not only cause the polymerization performance of the polyester polymer to be degraded, but also have a negative effect on the color and transparency, and therefore, the content of the terephthalic acid should be minimized. However, methods other than US Pat. No. 2,833,816, ie, methods of completely converting these reaction intermediates to terephthalic acid through oxidation without using a catalyst combination of acetic acid solvent and cobalt / manganese / bromine have not been chemically known.

Technical Challenge

When the oxidation reaction is performed using a water solvent and a bromine free catalyst system, the amount of reaction intermediates such as paratoluic acid and 4-carboxybenzaldehyde is relatively high during the conversion of paraxylene to terephthalic acid. The present inventors have found that separation and purification are difficult with conventional methods such as vacuum filters or hydrogen reduction tablets when the amount of reaction intermediates such as paratoluic acid or 4-carboxybenzaldehyde is high.

Accordingly, the present invention can effectively separate the terephthalic acid from the reaction intermediate in order to increase the yield and ensure quality when the result of the oxidation reaction contains a large amount of reaction intermediates such as paratoluic acid in the production of terephthalic acid by the oxidation reaction of paraxylene. It is an object of the present invention to provide a method for crystallization and solid-liquid separation.

Technical solution

The present invention provides a crystallization and solid-liquid separation process for separating terephthalic acid into a solid phase from a slurry containing terephthalic acid and a reaction intermediate produced by oxidation of paraxylene, wherein the reaction intermediate is paratoluic acid and / or And containing 4-carboxybenzaldehyde, crystallizing the slurry and filtering in a high temperature high pressure filter to separate the terephthalic acid into a solid phase and to separate the mother liquor containing the reaction intermediate into a liquid phase. It provides a crystallization and solid-liquid separation process characterized by a temperature of 130-220 ° C. and an operating pressure of 10-20 kgf / cm ² g.

The terephthalic acid slurry referred to in the present invention is preferably a slurry produced by an oxidation reaction using a catalyst system containing no bromine under a water solvent, and an oxidation reaction of paraxylene without using an acetic acid solvent. In addition, it is preferable to apply to a slurry containing 10wt% or more, preferably 20wt% or more of the paratoluic acid in the slurry produced by the oxidation reaction of the paraxylene. The 4-CBA content in the slurry produced by the oxidation reaction of the paraxylene is not important because it is not generated much in the actual reaction, and is about 1 to 5 wt%.

Hereinafter, the process of the present invention will be described in detail with reference to FIGS. 3 and 4 so that those skilled in the art can easily carry out the process of the present invention.

The development of a technology capable of completely separating the reaction intermediate generated in the oxidation reaction of paraxylene from the terephthalic acid and returning it to the reactor is very important for increasing the yield of terephthalic acid and securing the quality of the final product.

Since paratoluic acid is solid at room temperature and has a melting point of 180 °, in order to recover the paratoluic acid in a liquid form when the paratoluic acid is present in more than 10wt%, the separation temperature must be close to the melting point of the paratoluic acid (180 °). However, since the vacuum filter operates at room temperature, it is impossible to recover the liquid in the presence of a large amount of paratoluic acid.

Therefore, the present invention uses a high temperature and high pressure filter because the reaction intermediate such as paratoluic acid or 4-carboxybenzaldehyde must be sufficiently raised in the presence of a large amount to separate the liquid into a liquid phase.

Impurities that are the most important reaction intermediates produced during the conversion of paraxylene to terephthalic acid by oxidation are 4-carboxybenzaldehyde and paratoluic acid. Melting points of 4-carboxybenzaldehyde and paratoluic acid are very high at 256 ° C and 182 ° C, respectively. Especially, 4-carboxybenzaldehyde is very difficult to separate from terephthalic acid due to its very similar properties to terephthalic acid. However, since terephthalic acid does not melt even when heated, it is sublimed at 402 ° C. Therefore, if the temperature is raised to 180 ° C or higher, terephthalic acid remains in a solid phase and paratoluic acid is converted into a liquid. 4-carboxybenzaldehyde is a solid, but partially dissolved in paratoluic acid to be converted into a liquid phase, it is possible to separate relatively high purity terephthalic acid through a high temperature solid-liquid separation method.

In the case of the filter, when the solid-liquid separation is performed at a high temperature, a vacuum system that can only be performed at room temperature cannot be used, and therefore, the filtering must be performed by applying a pressure. Because of the thermodynamic characteristics, a certain pressure is required to maintain the equipment temperature at a high temperature, and basically, the temperature must be lowered to create a vacuum state, so the vacuum filter cannot be used and a high temperature can be maintained by applying pressure.

Therefore, in the present invention as a facility for such a high temperature solid-liquid separation, it is possible to effectively separate the solid terephthalic acid by operating the high-temperature high-pressure filter 12 mounted on the rear end of the crystal bath 11.

In general, the high temperature high pressure filter is equipped with a large number of small cell type filters in a circular drum mounted in a high pressure vessel, and slowly rotates to fill the slurry, filter, wash, dry, discharge the cake, and recover the mother liquor. Will be performed by zone (see Figure 5), and the detailed operating principle will vary by manufacturer.

The high temperature and high pressure filter has no significant difference in operation principle from the vacuum filter except that the filtering is performed by applying pressure at a high temperature. On the other hand, the high pressure filter in the present invention may be another feature that performs a drying process for the filter cake using steam.

The temperature range for performing the filtering is preferably from 130 to 220 ° C, preferably from 150 to 200 ° C. When the temperature range is lower than this, the reaction intermediate does not completely turn into a liquid phase, and solid-liquid separation is not performed correctly. When the temperature is higher than this, it is preferable to operate the given range because energy is unnecessarily consumed. In order to maintain the operating temperature, the operating pressure should be adjusted to a level of 10 to 20 kgf / cm ² g, preferably 12 to 18 kgf / cm ² g. If the pressure range is lower than this, the temperature is lowered and the separation performance is lowered. If the pressure range is higher than this, the thickness of the equipment is unnecessarily thick and the filter may be clogged due to excessive pressure. In addition, it should be borne in mind that for most pressure filter installations it is difficult to raise the difference between the atmospheric pressure and the pressure inside the filter above a certain limit.

One of the most important points in successfully performing the filtering is the particle size of the solid. Therefore, before operating the high temperature high pressure filter, a process of enlarging the particles of the solid is required while the mixed slurry of the solid and the liquid is held in the crystal bath for a specific time. Therefore, in the present invention, the crystal bath 11 may be connected to the front end of the high temperature and high pressure filter to adjust the particle size. It is preferable that the particle size of a solid particle is 30-120 micrometers. However, the number of crystal baths can be used within the range of 0-4, Preferably 1 group is suitable. This is because the operating temperature and pressure range of the high temperature high pressure filter are very high, similar to the operating temperature and pressure range of the oxidation reactor. Because it does not.

The operating temperature range of a crystal bath is 150-250 degreeC, Preferably 170-210 degreeC is suitable. If it is lower than this range, the reaction intermediate is precipitated to reduce the separation effect, and when it is high, the effect of increasing the particle size through crystallization is insufficient.

On the other hand, terephthalic acid is still low in water solubility at high temperature of more than 180 ℃ while paratoluic acid is greatly increased, so that the hot water and steam can be used in the high temperature high pressure filter device to maximize the separation and washing effect. Can be. The water used here can be used at no additional cost by introducing some of the water produced by the oxidation reaction into the crystal bath and filter.

The amount of water introduced into the crystal bath and the filter should be such that it does not excessively increase the water concentration when it is included in the mother liquor after filtering and refluxed into the oxidation reactor. Crystal water tank is 30 ~ 80wt% internal water concentration, the amount of water to be added to the filter as a washing liquid is suitable for the level of 0.2 ~ 2 tons per tonne of solid. The temperature of the cleaning solution is also an important factor in maximizing the separation performance, and using a cleaning solution in the range of 170 ~ 200 ℃ is most suitable when considering the solubility of impurities.

The slurry and wash liquor are preferably in contact with each other in countercurrent.

That is, in the filtering and washing operation, as shown in FIG. 4, the slurry and the washing liquid are filtered and washed with a small amount of washing water while continuously contacting the filters in the counter currents from 2 to 5 stages (13, 14, 15, and 16). The effect can be maximized. In other words, the final filter is in contact with the freshest washing liquid and the initial filter is in contact with the most washed washing liquid, so as to meet with the cleaner cleaner from the cake with more impurities to the cleaning with less cake.

The slurry passed through the high temperature and high pressure filter is separated into the cake and the mother liquor. The moisture content of the cake can be reduced to 10 wt% or less, and the content of paratoluic acid and 4-carboxybenzaldehyde is also reduced to 10 wt% or less of the total weight of the cake. When the high-temperature, high-pressure filter treatment of the slurry of paratoluic acid 20-40 wt% and 4-carboxybenzaldehyde 2-5wt% is performed, the reaction is performed at a level of about 2wt% of paratoluic acid and about 1wt% of 4-carboxybenzaldehyde. The concentration of intermediates can be significantly reduced. In particular, by using a high temperature and high pressure filter, compared with 4-carboxybenzaldehyde which is co-precipitated during the crystallization process of terephthalic acid, the solubility in water is high and paratoluic acid mainly formed outside the terephthalic acid particles has a high removal rate. Notable

In some cases, when the drying function is added using steam, the moisture content of the cake may be lowered to 5 wt% or less. The separated cake may be again subjected to a multi-step high temperature high pressure filter or purified by chemical methods to produce high purity terephthalic acid. Separate mother liquor containing water and reaction intermediates can further produce terephthalic acid by refluxing with an oxidation reactor. At this time, some of the mother liquor may be purged out of the system to achieve a quality level.

If the result of the oxidation reaction contains a large amount of the reaction intermediate, the cake may contain several wt% of the reaction intermediate even after performing the high temperature and high pressure filter. In this case, as described in US Pat. No. 2,833,816, acetic acid solvent and cobalt may be used. The total amount of the reaction intermediate can be lowered to 5,000 ppm or less by further oxidizing the reaction intermediate at 160-220 ° C. using a catalyst system comprising manganese and bromine. In addition, after performing the high temperature and high pressure filter, various purification techniques may be applied to further lower the total amount of the reaction intermediate to 200 ppm or less.

Favorable effect

According to the process of the present invention, when the reaction product generated from the oxidation reaction contains a large amount of the reaction intermediate, it is possible to ensure product quality by effectively separating the solid terephthalic acid from them, the liquid reaction intermediate is refluxed to the oxidation reactor By doing so, the yield of terephthalic acid can be increased. In addition, the very simple configuration of the crystallization and solid-liquid separation process can reduce the capital investment and more effectively manage the process. In addition, the solid-liquid separation process can be easily and reliably applied and operated on a slurry containing a large amount of reaction intermediate.

1 is a process diagram for separating terephthalic acid from the product of the oxidation of paraxylene according to US Pat. No. 2,833,816.

2 is a process diagram for separating terephthalic acid from the product of oxidation of paraxylene according to US Pat. No. 4,334,086.

3 is a process diagram for separating terephthalic acid from the product of the oxidation of paraxylene according to one embodiment of the present invention.

Figure 4 is a process for filtering, washing, cake discharge and mother liquor recovery of the product slurry of the oxidation reaction of para xylene in a high temperature and high pressure filter according to an embodiment of the present invention.

5 is a structural diagram of a high pressure filter that can be used in one embodiment of the present invention.

Description of the Related Art

1: First Decision Group 2: Second Decision Group

3: Article 3

4: Rotary Vacuum Filter

5: dryer

6: first settling column 7: heater

8: Decision Tree 9: Second Settlement Column

11: crystal

12: High-Temperature Pressure Filter

13: first filter part 14: second filter part

15: third filter unit 16: fourth filter unit

17: 4th stock solution 18: 3rd stock solution

19: second mother liquor storage 20: first mother liquor storage

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in detail with reference to the following Examples. However, the following examples are merely to illustrate the present invention and the present invention is not limited by the following examples.

[Example]

In the following examples, the method proposed in the present invention is applied to a terephthalic acid slurry containing a high concentration impurity having a total concentration of paratoluic acid and 4-carboxybenzaldehyde of 50 wt% or more, and a terephthalic acid slurry containing a low concentration impurity of 50 wt% or less, respectively. The effect was compared.

Example 1 Separation of Intermediate of High Concentration

After passing through a crystal bath having an internal temperature of 170 ° C. and a high temperature high pressure filter having an internal temperature of 185 ° C. and an internal pressure of 15 kgf / cm ² g, the terephthalic acid, paratoluic acid and 4-carboxybenzaldehyde were produced in the reactor. Before and after concentration (wt%) change was measured, and the results are shown in Table 1 below. However, cake washing and drying were not performed.

Table 1

Example 2 Isolation of Intermediate at Low Concentration

Terephthalic acid, paratoluic acid and 4-carboxybenzaldehyde were passed through a crystal bath having an internal temperature of 165 ° C. and a high temperature high pressure filter having an internal temperature of 180 ° C. and an internal pressure of 14 kgf / cm ² g. Before and after the concentration (wt%) change was measured, and the results are shown in Table 2 below. However, cake washing and drying were not performed.

Table 2

There is no case where a vacuum filter is applied to a terephthalic acid slurry containing a large amount of reaction intermediates and separation is not possible. However, as can be supported by the above examples, 4-carboxybenzaldehyde is 2.3 to 2.5 wt% at 4.3 wt% when filtering is carried out at high temperature and high pressure regardless of the amount of the intermediate. Paratoluic acid is reduced to 185 degrees, 78.3wt% to 5.6wt% at 15KG, and 180 degrees, at 37KG to 37.4wt% to 6.6wt%, filtering is performed at higher temperature and pressure conditions. Is far superior in terms of its effectiveness.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, And it goes without saying that the invention belongs to the scope of the invention.

Claims (12)

A crystallization and solid-liquid separation process for separating the terephthalic acid into a solid phase from a slurry containing terephthalic acid and a reaction intermediate produced by oxidation of paraxylene, The reaction intermediate comprises paratoluic acid and 4-carboxybenzaldehyde, Crystallizing the slurry and then filtering in a high temperature and high pressure filter to separate the terephthalic acid into a solid phase and to separate the mother liquor including the reaction intermediate into a liquid phase. The operating temperature of the high temperature high pressure filter is 130 ~ 220 ℃, the operating pressure is 10 ~ 20kgf / cm ² g, The paraxylene oxidation is carried out using a catalyst system containing no bromine under a water solvent, crystallization and solid-liquid separation process characterized in that no acetic acid solvent. The crystallization and solid-liquid separation process according to claim 1, wherein the crystallization proceeds in a crystal bath. delete The process of claim 1 or 2, wherein the paratoluic acid in the slurry contains 10 wt% or more. The crystallization and solid-liquid separation process according to claim 1 or 2, wherein the slurry is filtered and washed using a washing liquid and steam containing water in a high temperature high pressure filter. 6. The crystallization and solid-liquid separation process according to claim 5, wherein the slurry and the washing liquid are brought into contact with each other in opposite directions. The crystallization and solid-liquid separation process according to claim 2, wherein the operation temperature of the crystal bath is 150 to 250 ° C. The crystallization and solid-liquid separation process according to claim 2, wherein the internal water concentration of the crystal bath is 30 to 80 wt%. The crystallization and solid-liquid separation process according to claim 2, wherein the number of crystal baths is 1-4. The crystallization and solid-liquid separation process according to claim 1 or 2, wherein the amount of the washing liquid used in the high temperature and high pressure filter is 0.2 to 2 tons per ton of solid, and the washing liquid has a temperature of 170 to 200 ° C. The process of claim 1 or 2, wherein the mother liquor comprising the reaction intermediate separated into a liquid phase after the high temperature and high pressure filter is refluxed to an oxidation reactor. The cake containing terephthalic acid separated after the high temperature and high pressure filter is further oxidized at 160 to 220 DEG C using a catalyst system comprising acetic acid solvent and cobalt, manganese and bromine. Crystallization and solid-liquid separation process characterized by reducing the total concentration to 5,000 ppm or less.

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