CN113354536A

CN113354536A - Process for producing terephthalic acid esters

Info

Publication number: CN113354536A
Application number: CN202010147297.XA
Authority: CN
Inventors: 王群朋
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-03-05
Filing date: 2020-03-05
Publication date: 2021-09-07
Anticipated expiration: 2040-03-05
Also published as: CN113354536B; TW202134210A

Abstract

The present invention provides a method for preparing terephthalic acid esters, and specifically provides a method for purifying crude dibasic ester prepared by reacting terephthalic acid with alcohol in the presence of an esterification catalyst, wherein the method comprises the following steps: providing the crude ester; treating the crude ester with an aqueous solution of a base at a temperature of 14 ℃ to 165 ℃, preferably at a temperature of 145 ℃ to 160 ℃, more preferably at a temperature of 150 ℃ to 155 ℃ to obtain a base-treated crude ester, wherein the amount of the aqueous solution of the base used is 0.5 wt% to 2 wt%, preferably 0.6 wt% to 1.5 wt%, more preferably 0.8 wt% to 1.2 wt%, most preferably 1 wt%, relative to the weight of the crude ester; dehydrating the base-treated crude ester to obtain a dehydrated crude ester; and filtering the dehydrated crude ester to obtain a filtered crude ester.

Description

Process for producing terephthalic acid esters

Technical Field

The present invention relates to a process for producing terephthalic acid esters, in particular dioctyl terephthalate (abbreviation: DOTP).

Background

Existing terephthalate production processes generally involve first preparing a crude ester, followed by a purification treatment of the crude ester, including dealcoholization, neutralization, water washing, stripping, and filtration.

In the case of dioctyl terephthalate, terephthalic acid and octanol (i.e., 2-ethylhexanol, hereinafter referred to as octanol) are reacted in an esterification reactor at 180-230 ℃ in the presence of a titanate catalyst (typically 0.05-0.15% of the product, 0.117-0.353% of PTA based on PTA) to produce dioctyl terephthalate. When the terephthalic acid solid was completely dissolved and the acid value (mg KOH/g consumed per gram of material at the end of titration of the crude ester with potassium hydroxide solution) reached 0.2mgKOH/g or less, esterification was complete to give the crude ester. This step produced 9 wt% (based on finished product) of by-product water; the COD (chemical oxygen demand) of this by-product water was about 9000-10000mg/L because of the presence of dissolved organics.

After the crude ester is obtained, it is dealcoholated in an esterification reactor or in a dealcoholation column under reduced pressure, in which the major part of the octanol is evaporated off and condensed by a condenser for recycling. Typically, the crude ester after dealcoholization contains about 0.5 to 5 wt% octanol.

The dealcoholized crude ester is then neutralized. This neutralization step neutralizes the monoester acid in the crude ester thereby reducing the acid number of the crude ester (e.g., to ≦ 0.02mgKOH/g) and hydrolyzes the catalyst to form hydrated titanium dioxide. Cooling the dealcoholized crude ester to 80-95 ℃, adding alkali solution with the amount of about 20-30% (wt) of the crude ester under stirring for neutralization, and then allowing the oil phase and the water phase to separate, wherein the monoester salt and titanium dioxide hydrate mostly enter the water phase. The aqueous phase was separated as aqueous alkaline waste. The monoester salt has both oleophilic and hydrophilic properties, which carry a portion of the dioctyl terephthalate into the aqueous phase; the titanium dioxide hydrate is flocculent and also carries part of dioctyl terephthalate into the water phase, so that the COD value of the waste alkaline water generated in the step is about 40000-50000 mg/L.

The neutralized crude ester is then washed with water. To the neutralized crude ester is added water in an amount of about 20 to 30 wt% of the crude ester and stirred. The alkali, mono-ester acid salt and titanium dioxide hydrate enter the aqueous phase. The oil and water phases were allowed to separate and the water phase was separated as wastewater. The COD value of the waste water produced in this step was about 15000-20000 mg/L.

The neutralized and washed crude ester can be further stripped of octanol and water, for example, by stripping. The alcohol and water are condensed in a condenser and then subjected to alcohol-water separation, the separated octanol can be recycled, and the separated water is used as waste water. The COD of the wastewater produced in this step was about 2000-3000 mg/L. After the stripping step, the stripped crude ester may be dried to further remove water. After stripping and drying, the crude ester may optionally be further filtered to yield the final terephthalate product.

As can be seen from the above process, a large amount of wastewater is generated during the production of terephthalic acid esters. The neutralization and water washing steps produce a wastewater yield of 40-50% by weight, with the combined wastewater COD being as high as about 30000 mg/L. Such wastewater is alkaline, has a high COD, and contains titanium dioxide hydrate, resulting in complicated wastewater treatment and high costs.

CN101184721A discloses a process for the purification of a crude ester, which process comprises treating the crude ester with an alkaline aqueous solution of an alkali metal salt at a temperature of 100-140 ℃, the amount of water present during the treatment being from 0.7 to 1.4% by weight based on the weight of the crude ester. The process disclosed in this document, although overcoming the problem of large amounts of waste water by using low levels of water in the neutralisation process, also leads to additional problems. The process of this reference uses less than stoichiometric amounts of alkali metal relative to the acidity of the crude ester, which tends to result in insufficient neutralization, making the acid number of the final product too high. The process of this document requires the use of low levels of titanium catalyst, up to 0.07 wt% based on the amount of acid or anhydride reagent used in the esterification reaction. It is believed that the process of this reference limits the amount of base and catalyst used because the sodium bicarbonate crystals and titanium dioxide crystals formed by neutralization tend to form small crystals, which tend to cause filtration plugging problems. Using low levels of base and catalyst can reduce the filtration burden, enabling the process of this reference to be carried out.

In the process of CN101184721A, under the condition, the titanate catalyst forms floccules after deactivation, which is difficult to filter, and a filter aid is required to perform the filtration, resulting in increased consumption and increased cost.

Since the process of CN101184721A uses low levels of catalyst, it is not suitable for the production of terephthalate. For the reaction of terephthalic acid and octanol, the terephthalic acid is insoluble in octanol, the reaction is a solid-liquid reaction, the reaction speed is slow, and more catalysts are required to be added to promote the reaction. Typically, the amount of titanate is from 0.117 wt% to 0.353 wt% of the amount of terephthalic acid. This high level of catalyst resulted in high levels of titanium dioxide crystals present during purification, which exceeded the filtration capacity of the CN101184721A process.

This results in the process being practically unusable in the production of terephthalic acid dibasic esters (mainly dioctyl terephthalate) with high catalyst usage (0.117-0.353 wt%, based on the amount of terephthalic acid), because the process forms a dense filter layer in the filtration step, which makes the production impossible.

Accordingly, there remains a need to provide a process for the preparation of terephthalates that overcomes one or more of the disadvantages of the prior art processes.

Disclosure of Invention

The present application provides an improved process for the production of terephthalates which reduces the amount of wastewater, is not plagued by filtration problems (excellent filtration capacity), and results in terephthalate end products with advantageous properties including acid number, color, purity, moisture, flash point, volume resistivity, and the like.

The application provides a preparation method of terephthalic acid dibasic ester, in particular a purification method of crude ester obtained after esterification. According to one embodiment, the present application provides a process for purifying a crude dibasic ester prepared by reacting terephthalic acid with an alcohol in the presence of an esterification catalyst, the process comprising the steps of:

providing the crude ester;

treating the crude ester with an aqueous solution of a base at a temperature of 145 ℃ to 165 ℃, preferably at a temperature of 145 ℃ to 160 ℃, more preferably at a temperature of 150 ℃ to 155 ℃ to obtain a base-treated crude ester, wherein the amount of the aqueous solution of the base used is 0.5 wt% to 2 wt%, preferably 0.6 wt% to 1.5 wt%, more preferably 0.8 wt% to 1.2 wt%, most preferably 1 wt%, relative to the weight of the crude ester;

dehydrating the base-treated crude ester to obtain a dehydrated crude ester; and

filtering the dehydrated crude ester to obtain a filtered crude ester.

According to one embodiment of the present application, the above crude filtered ester may be further treated, including subjecting the crude filtered ester to a stripping treatment to obtain a stripped crude ester, and drying the stripped crude ester to provide an ester product. Optionally, the ester product may also be filtered to ensure that the ester product does not contain any solid particles.

According to one embodiment of the present application, the treatment of the crude ester with an aqueous base in the process of the present application is carried out at an elevated pressure sufficient to prevent evaporation of water. The pressure should be greater than or equal to the saturation vapor pressure of water at the treatment temperature to maintain the aqueous base solution in a liquid state. Preferably, the pressure is from 0.3MPa to 3MPa, more preferably the pressure is from 0.4 to 1.0 MPa.

According to one embodiment of the present application, the crude ester may be subjected to a dealcoholation treatment in the process of the present application before being treated with an aqueous solution of a base. This dealcoholation treatment is an operation known in the art. The alcohol content of the crude ester is preferably reduced to not more than 5% by weight of the crude ester, preferably not more than 3% by weight of the crude ester, more preferably not more than 1% by weight of the crude ester.

As for the base used in the method of the present application, the method of the present application is not particularly required, and a base generally used in the art may be used. According to one embodiment, the base is selected from the group consisting of hydroxides, carbonates and bicarbonates. Preferably, the base is a hydroxide, carbonate or bicarbonate of an alkali metal and calcium. More preferably, the base is selected from alkali metal carbonates and bicarbonates, such as sodium carbonate, sodium bicarbonate, potassium carbonate and potassium bicarbonate. Most preferably the base is selected from sodium or potassium carbonate. According to one embodiment, the amount of base used in the treatment is between 100% and 200%, preferably between 105% and 180%, more preferably between 110% and 150%, most preferably between 110% and 120% of the theoretical amount calculated on the acid number of the crude ester before treatment.

According to one embodiment, the water content of the dehydrated crude ester does not exceed 0.2 wt.%, preferably does not exceed 0.1 wt.%, more preferably does not exceed 0.08 wt.%.

A filter aid may be used to facilitate filtration of the dehydrated crude ester. According to one embodiment, a filter aid is added to the crude ester before, during or after treating the crude ester with the aqueous solution of the base. Preferably, the filter aid is selected from the group consisting of attapulgite, diatomaceous earth, calcium carbonate, activated carbon, kaolin, and activated clay. The filter aid is added in an amount of 0.01 to 0.15%, more preferably 0.02 to 0.05% by weight of the crude ester.

As the esterification catalyst, various catalysts known in the art can be used. According to one embodiment, the esterification catalyst is a titanium catalyst. Preferably, the titanium catalyst is a titanate catalyst, more preferably the titanium catalyst is selected from the group consisting of tetramethyl titanate, tetraethyl titanate, tetra-n-propyl titanate, tetra-isopropyl titanate, tetra-n-butyl titanate, tetra-isobutyl titanate, tetra-pentyl titanate, tetra-hexyl titanate, tetra-heptyl titanate, tetra-octyl titanate, tetra-nonyl titanate, tetra-decyl titanate, tetra- (2-propylheptyl) titanate, tetra (dodecyl) titanate, tetra (hexadecyl) titanate, tetra (octadecyl) titanate and tetraphenyl titanate. The titanate catalyst is used in an amount of 0.117 to 0.353%, preferably 0.235 to 0.353% by weight, relative to the weight of the terephthalic acid.

As the alcohol used for preparing the ester, various alcohols generally used in the art may be used. Preferably, the alcohol is an alcohol having 4 to 15 carbon atoms, preferably octanol, i.e. 2-ethylhexanol.

The terephthalate product obtained by the process of the present invention has advantageous properties and can be used in a variety of applications, including as a plasticizer for polymeric materials such as PVC, as an additive, and the like.

Detailed Description

The present application provides improved processes for the preparation of terephthalic acid esters, and more particularly, processes for the purification of crude esters obtained after the esterification reaction is complete. The method of the present application comprises the steps of: providing a crude ester; treating the crude ester with an aqueous solution of a base at a temperature of 145 ℃ to 165 ℃ to obtain a base-treated crude ester, wherein the amount of the aqueous base solution used is 0.5 wt% to 2 wt% relative to the weight of the crude ester; dehydrating the base-treated crude ester to obtain a dehydrated crude ester; and filtering the dehydrated crude ester to obtain a filtered crude ester.

By crude ester in the process of the present application, it is meant an esterification reaction product containing impurities obtained after the esterification reaction, comprising a terephthalic acid dibasic ester product as a main component, and unreacted raw materials such as terephthalic acid mono-ester and alcohol, by-product water, and an esterification catalyst.

The crude ester may be prepared by the in situ reaction of terephthalic acid and octanol (2-ethylhexanol), or the crude ester may be from other sources, such as from other plants, and the like. The preparation of the crude ester is known in the art and the crude ester can be prepared using various methods known in the art. For example, terephthalic acid and excess 2-ethylhexanol can be used as starting materials and reacted in an esterification reactor at elevated temperatures, for example 180 ℃ and 230 ℃ in the presence of a catalyst, such as a titanate.

According to one embodiment, the esterification reaction may be carried out under the protection of an inert gas. For example, nitrogen can be used as the protective gas. It is believed that the use of inert gas can improve the color properties of the terephthalate product and can even allow the omission of the decolorization procedure using activated carbon.

As the alcohol used for preparing the terephthalate ester, various alcohols generally used in the art can be used. According to one embodiment, the alcohol used in the process of the invention is a linear or branched primary or secondary alcohol having from 1 to 18 carbon atoms, preferably a primary or secondary alkanol. According to one embodiment, the alcohol is an alcohol having from 4 to 15 carbon atoms. By way of example, alcohols that may be mentioned include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, hexanol, heptanol, 2-ethylhexanol, nonanol, decanol and the like, and mixtures thereof. According to one embodiment the alcohol is 2-ethylhexanol.

While various known catalysts can be used to prepare the terephthalate, according to one embodiment, the crude ester is preferably prepared using a titanium catalyst. Preferably, the titanium catalyst is a titanate catalyst, more preferably the titanium catalyst is selected from the group consisting of tetramethyl titanate, tetraethyl titanate, tetra-n-propyl titanate, tetra-isopropyl titanate, tetra-n-butyl titanate, tetra-isobutyl titanate, tetra-pentyl titanate, tetra-hexyl titanate, tetra-heptyl titanate, tetra-octyl titanate, tetra-nonyl titanate, tetra-decyl titanate, tetra- (2-propylheptyl) titanate, tetra (dodecyl) titanate, tetra (hexadecyl) titanate, tetra (octadecyl) titanate and tetraphenyl titanate.

For the esterification reaction, the skilled person can select the catalyst and the amount of catalyst selected. According to one embodiment, a titanate catalyst is chosen and is used in an amount of 0.117 to 0.353%, preferably 0.235 to 0.353% by weight, relative to the weight of terephthalic acid.

When the esterification reaction is carried out in the esterification reactor to a desired extent, it can be considered that the esterification reaction has been completed. According to one embodiment, when the terephthalic acid solids are completely dissolved in the esterification reactor and the acid value of the feed reaches 0.2mgKOH/g or less, the esterification is considered complete and further processing can be performed.

Optionally, the crude ester may be subjected to a dealcoholation treatment to remove residual alcohol from the crude ester prior to treatment of the crude ester with an aqueous base solution. The dealcoholization treatment may be carried out in an esterification reactor or in a dealcoholization column. Typically, the crude ester is distilled under reduced pressure, for example at 1 to 5KPa absolute, to remove the alcohol. By the dealcoholation treatment, most of the alcohol in the crude ester is evaporated. The evaporated alcohol is condensed and the condensed alcohol can be recycled. According to one embodiment, the alcohol content of the crude ester is reduced to not more than 5% by weight of the crude ester, preferably not more than 3% by weight of the crude ester, more preferably not more than 1% by weight of the crude ester.

The treatment of the crude ester (after dealcoholation or without dealcoholation) with aqueous base is carried out at higher temperatures, for example from 145 ℃ to 165 ℃. This treatment neutralizes the monoester acid in the crude ester, reducing the acid value of the crude ester. According to one embodiment, the acid value of the crude ester is reduced to not more than 0.02 mgKOH/g. In addition, the treatment may hydrolyze the titanate catalyst to produce hydrated titanium dioxide.

The choice of temperature is important for the process of the present application. According to one embodiment, the crude ester is treated with an aqueous base at a temperature of 145 ℃ to 165 ℃, preferably at a temperature of 145 ℃ to 160 ℃, more preferably at a temperature of 150 ℃ to 155 ℃.

Generally, it is advantageous to use higher temperatures, since this may reduce the heat energy loss and may accelerate the hydrolysis of the titanate. However, the applicants have surprisingly found that when temperatures in excess of 165 ℃ are employed, the acid number of the treated crude ester increases abnormally. It is believed that this unusual increase in acid number is due to the fact that when the temperature exceeds 165 ℃, the titanate activity increases causing the terephthalic acid dibasic ester to react in the reverse direction in the presence of water. This reverse reaction results in hydrolysis of the terephthalic acid dibasic ester to the monoester acid and alcohol, causing an increase in the acid value of the crude ester. By using the temperatures required in this application, the reverse reaction can be reduced or eliminated, maintaining rapid hydrolysis of the titanate and at the same time reducing heat energy losses.

According to one embodiment of the present application, the treatment of the crude ester with an aqueous base in the process of the present application is carried out at an elevated pressure sufficient to prevent evaporation of water. The pressure should be greater than or equal to the saturation vapor pressure of water at the treatment temperature to maintain the aqueous base solution in a liquid state. Preferably, the pressure is from 0.3MPa to 3MPa, preferably the pressure is from 0.4 MPa to 1.8MPa, more preferably the pressure is from 0.4 MPa to 1.0 MPa.

When treating the crude ester with an aqueous base, the amount of the aqueous base is important because water is the reactant for the hydrolysis of the titanate to produce titanium dioxide hydrate. The amount of aqueous solution used must not be too small, since too little water will not hydrolyze the titanate completely, leading to clogging of the filter screen during subsequent filtration operations. However, water is also an impurity in the crude ester and should be removed in a subsequent operation. Therefore, too much aqueous solution cannot be added. According to one embodiment, the aqueous solution of the base is used in an amount of 0.5% to 2% by weight, preferably 0.6% to 1.5% by weight, more preferably 0.8 to 1.2% by weight, most preferably about 1% by weight, relative to the weight of the crude ester.

Generally, the aqueous alkali solutions of the present application have a relatively low alkali concentration. According to one embodiment, the concentration of the aqueous base solution herein is between 0.2 wt% and 5 wt%, preferably between 0.5 wt% and 3 wt%, more preferably between 1 wt% and 2 wt%.

As the base used in the aqueous solution of the base, a base generally used in the art may be used. According to one embodiment, the base is selected from the group consisting of hydroxide, carbonate, bicarbonate or mixtures thereof. Preferably, the base is a hydroxide, carbonate or bicarbonate of an alkali metal and calcium. More preferably, the base is selected from alkali metal carbonates and bicarbonates, such as sodium carbonate, sodium bicarbonate, potassium carbonate and potassium bicarbonate. Most preferably, the base is selected from sodium or potassium carbonate. The amount of base used by those skilled in the art can be used in accordance with the amount of base used when treating the crude ester with an aqueous solution of the base. According to one embodiment, the amount of base used in the treatment is between 100% and 200%, preferably between 105% and 180%, more preferably between 110% and 150%, most preferably between 110% and 120% of the theoretical amount calculated on the acid number of the crude ester before treatment.

Those skilled in the art will recognize that the aqueous solution of base used may be adjusted depending on the type and concentration of catalyst used and the base used, including the amount of the aqueous solution used and the concentration of the base in the aqueous solution. These adjustments can be readily determined and implemented by those skilled in the art. Preferably, adjustments are made within the concentration and amount ranges disclosed herein, although deviations from the ranges disclosed herein may also be made as necessary.

The crude ester may be treated with an aqueous solution of a base, with stirring. The skilled person can select a suitable stirring means as long as it is possible to facilitate the treatment process.

After treating the crude ester with an aqueous solution of a base, the treated crude ester is dehydrated to obtain a dehydrated crude ester. The purpose of dehydrating the crude ester after treatment with an aqueous alkali solution is to reduce the water content of the crude ester. According to one embodiment, the water content of the dehydrated crude ester is not more than 0.2 wt.%, preferably not more than 0.1 wt.%, preferably not more than 0.08 wt.%, more preferably not more than 0.05 wt.%. Dehydration may be carried out by means generally known in the art, including but not limited to distillation, flash distillation, vacuum distillation, and the like.

According to one embodiment, the dehydration operation is carried out immediately after the treatment of the crude ester with the aqueous solution of the base. Thus, the temperature of the crude ester is slightly lower when the dehydration is carried out than when the crude ester is treated with an aqueous solution of a base (since the addition of water and the cooling by natural heat dissipation, is not exclusively carried out). The temperature at which the dehydration operation is carried out is therefore a temperature of from 120 ℃ to 155 ℃, preferably a temperature of from 125 ℃ to 155 ℃, more preferably from 130 ℃ to 150 ℃. According to one embodiment, after treating the crude ester with an aqueous solution of a base, the crude ester is fed to a distillation column (dehydration column) at a temperature of 130 ℃ to 150 ℃; the distillation column is operated at reduced pressure, for example 10 to 30KPa absolute, to distill off water, thereby reducing the water content of the crude ester to the desired level.

During this dehydration process, the mono-ester acid salt, such as the sodium salt of the mono-ester acid, crystallizes to form crystals and the hydrated titanium dioxide loses water to form titanium dioxide crystals due to the reduction of water. It is believed that the conditions employed in the process of the present application, including the amount of aqueous base, the concentration of the aqueous base, the temperature of the aqueous base at which the crude ester is treated, the dehydration operation, and the like, result in crystals formed during dehydration having a larger crystal size, which facilitates subsequent filtration operations. The process of the present application may even allow good filtration operations to be achieved without the use of filter aids. In addition, it is believed that the crystallization of titanium dioxide to form crystals can adsorb polar materials such as colored materials in the crude ester as well as adsorb mono-ester acid salts. Since these crystals are removed by subsequent filtration, this additionally facilitates the removal of polar species, which is advantageous for reducing the color of the product and for improving the volume resistance of the product. In the industrial production, under the condition of nitrogen protection, no active carbon and filter aid are added, and the color number of the product can reach 10-15 (platinum-cobalt color number).

The COD of the water removed by this dehydration step was about 2000-3000mg/L and was salt-free. According to one embodiment, the water removed by the dewatering step is used to formulate a solution of the base used above, thereby further reducing wastewater discharge, which is particularly advantageous.

After dehydrating the crude ester to obtain a dehydrated crude ester, the dehydrated crude ester is filtered. The crystals in the dehydrated crude ester were filtered off by filtration.

Filtration may be performed using filtration means commonly known in the art for plasticizers. For example, filtration can be carried out using a plate-type closed filter. The filter residue obtained by filtering contains titanium dioxide, monoester salt and terephthalic acid dibasic ester. According to one embodiment, the filter residue contains 30-35 wt% of dibasic terephthalate, about 30 wt% of titanium dioxide and 35-40 wt% of monoester salt.

It is believed that in the process of the present application, the crystals formed in the crude ester have better properties (size and morphology) so that filtration can be smoothly performed. In the process of the present application, a dense and low porosity filter cake is not formed during the filtration process. In the prior art processes, a dense and low porosity filter cake is easily formed, resulting in failure to continue filtration.

According to one embodiment, the temperature of the dehydrated crude ester is 110-130 ℃ (the temperature is reduced due to heat absorption of water evaporation and natural heat dissipation, and no special temperature reduction is needed), and the crude ester is filtered by a plate-type closed filter to obtain clear crude ester.

While the process of the present application achieves advantageous filtration, according to one embodiment, a filter aid may also be used to further facilitate filtration of the dehydrated crude ester. In general, filter aids have the function of preventing crystals from forming a dense and low-porosity filter layer on the filter cloth and thus rapidly clogging the filter. According to one embodiment, a filter aid may be added to the crude ester before, during or after treating the crude ester with the aqueous solution of the base. When a filter aid is added after the treatment of the crude ester with the aqueous solution of the base, it is preferably added immediately after the treatment with the aqueous solution of the base. The present application uses filter aids commonly used in the art. Preferably, the filter aid is selected from the group consisting of attapulgite, diatomaceous earth, calcium carbonate, activated carbon, kaolin, and activated clay. The filter aid is used in the amounts normally used in the art. According to one embodiment, the filter aid is added in an amount of 0.01 to 0.15%, more preferably 0.02 to 0.05% by weight of the crude ester.

In a further embodiment, an adsorbent such as activated carbon may also be added to the crude ester before, during or after treatment of the crude ester with an aqueous base. Preferably, the adsorbent is added together with the filter aid. Typically the adsorbent provides a product having a lighter colour and/or a lower metal content. The present application uses an adsorbent generally used in the art. As examples of the adsorbent, alumina, activated clay, activated carbon, magnesia, activated alumina and silica can be mentioned. Preferably, activated carbon is used as the adsorbent.

In one embodiment, the filter aid and/or adsorbent may be added together at the same time as or immediately after treatment with the aqueous base.

According to one embodiment of the present application, the crude ester after filtration above may also be subjected to further processing in order to further improve the quality of the ester product. The further treatment may comprise subjecting the filtered crude ester to a stripping treatment to obtain a stripped crude ester, and drying the stripped crude ester to provide an ester product. Optionally, the ester product may also be subjected to microfiltration to ensure that the ester product does not contain any solid particles.

For the stripping treatment, this is a common treatment in the field of ester plasticizer preparation. According to one embodiment, the filtered crude ester is stripped to further remove the alcohol and water therefrom. The stripping may use water vapor or an inert gas, such as nitrogen. According to one embodiment, steam is used for stripping.

According to one embodiment, the filtered crude ester is heated to 145-165 ℃ and then stripped using steam, for example in a stripper, to further remove light components therein, such as octanol, water, etc. The stripper operates at reduced pressure, for example, 5 to 15KPa absolute pressure. Direct steam is introduced into the stripping tower, and the direct steam can reduce the gas phase partial pressure of the octanol and is beneficial to removing the octanol. Generally, the direct steam is added in an amount of 5 to 10 wt% based on the amount of crude ester. In the lower part of the stripper, an evaporator, such as a falling film evaporator, was provided to heat the crude ester to maintain the crude ester at a temperature of 145-165 ℃. The evaporated alcohol and water are condensed in a condenser and then enter an alcohol-water separation tank. The separated octanol can be used as raw material for recycling, and the water is sent to a wastewater treatment system. The COD of the wastewater separated in this step was 2000-3000 mg/L.

In the case of steam stripping, the stripped crude ester contains traces of water. Thus, the stripped material may be subjected to a drying treatment to remove water. Drying treatments commonly used in the field of ester plasticizers may be employed. For example, the stripped crude ester can be treated using a drying column at 1 to 3KPa absolute to further remove moisture.

Generally, after drying the stripped crude ester, the resulting ester product can be used as the final product. Optionally, however, after said drying, the ester product may also be subjected to further filtration. According to one embodiment, the further filtration is a microfiltration to ensure that the final ester product does not contain solid particles.

The process of the present application may be carried out in a batch, semi-continuous or continuous manner. According to one embodiment, at least a portion of the process of the present application is carried out in a continuous manner. For example, the esterification reaction may be carried out in a batch manner to produce and provide the crude ester, but the steps of treating the crude ester with an aqueous solution of a base, dehydrating the crude ester, and filtering the dehydrated crude ester are carried out in a continuous manner. Optionally, the subsequent stripping, drying and secondary microfiltration steps are also carried out in a continuous manner. Alternatively, the entire process of the method of the present application is carried out in a continuous manner.

According to one embodiment, the method of the present application may be implemented in the following manner: after the esterification reaction of terephthalic acid is finished, the crude ester is directly neutralized by an aqueous solution of alkali in a neutralization reactor at the temperature of 140-160 ℃ under the pressure of 0.4-1.5MPa after being optionally dealcoholized; adding an aqueous alkali solution with the amount of about 1 percent of the crude ester during the neutralization; the base is Na₂CO₃Or NaHCO₃Preferably Na₂CO₃；Na₂CO₃The concentration is 150 percent of the theoretical value calculated according to the acid value of the crude ester; then removing water in the crude ester (the removed water COD is 2000-3000mg/l, contains no salt, can be used for preparing alkali solution and can be recycled); then filtering to remove the sodium monoester acid salt in the crude ester and the titanium dioxide formed by catalyst deactivation; then steam stripping and drying are carried out by using water vapor; most preferablyAnd then carrying out precision filtration to obtain a final product.

The inventors of the present application have found that with the process of the present application, ester products can be provided having particularly advantageous properties, including acid number, color, purity, moisture, flash point, volume resistivity, Ti content, and the like. On the one hand, the quality of the ester product prepared by the method can reach or even be higher than that of the product prepared by the continuous neutralization and water washing process in the prior art. On the other hand, the quality of the ester product prepared by the method can reach or even exceed the standard of HG/T2423-2008 premium grade products.

Meanwhile, the method can obviously reduce the discharge of waste water. By adopting the method, except the byproduct water of the esterification reaction and the steam stripping condensed water, no wastewater is discharged in the process. Compared with the prior art which adopts the continuous neutralization and water washing process, the COD discharge of the method is reduced by about 90 wt%, and the sewage quantity is reduced by about 57 wt%. The reduction in wastewater and COD emissions was calculated as follows:

the neutralization and water washing steps of the prior art produce 40-50 wt% of wastewater and the COD of the mixed wastewater is about 30000 mg/L. In the prior art method, the neutralization and water washing treatment uses byproduct water of esterification reaction and steam stripping condensed water as water sources for neutralization and water washing. In the method of the application, the COD of the byproduct water of the esterification reaction is about 9000-10000mg/L, and the amount is about 9 percent of the amount of the crude ester; the COD of the stripped condensed water was about 2000-3000mg/L, the amount was about 8% of the amount of crude ester; the total amount after mixing was 17% of the amount of crude ester, and the COD was about 6000-7000 mg/L. Thus, the amount of wastewater corresponds to about 43% (0.17 ÷ 0.40 ═ 42.5%,) of the prior art process, and the COD corresponds to about 9.2% [ (0.17 × 6500)/(0.4 × 30000) ] of the prior art process.

In addition, it is worth noting that the wastewater discharged by the method of the application is salt-free and free of solid suspended matters, so that the wastewater can be directly subjected to biochemical treatment. Moreover, the sludge amount that produces among the waste water treatment process of this application reduces by a wide margin.

Finally, it is worth noting that there is no problem of filtration when the dehydrated crude ester is filtered after the crude ester has been dehydrated. This demonstrates the excellent filterability of the dehydrated crude ester produced by the process of the present application. This is a significant improvement over prior art methods.

The process of the present application is further illustrated below using examples.

Examples

Example 1

Synthesis of dioctyl terephthalate

Terephthalic Acid (PTA) and octanol (2-ethylhexanol) are used as raw materials, the molar ratio of PTA to octanol is 1:3, titanate (tetraisopropyl titanate or tetra-n-butyl titanate) with the weight percent of 0.117-0.353, preferably 0.235-0.353 (based on the weight of PTA) is used as a catalyst, and esterification reaction is carried out in an esterification kettle at 230 ℃ at 180 ℃. When the PTA solid is completely dissolved and the acid value of the material reaches less than or equal to 0.2(mgKOH/g material), the esterification is finished. After the water generated in the reaction is condensed by a condenser, the water is separated from alcohol in an alcohol-water separation tank, the alcohol flows back into an esterification kettle for continuous reaction, and the separated water is discharged as waste water. The amount of water in this portion was about 9% of the amount of crude ester, and the COD was about 9000-10000 mg/l. This water is condensed water, free of salts and solid impurities, and has a pH of about neutral.

Dealcoholization of crude esters

The crude ester after completion of the esterification contained, in molar ratio, dioctyl phthalate: 2-ethylhexanol ═ 1: 1, 2-ethylhexanol. Dealcoholizing the crude ester in a dealcoholization tower under the absolute pressure of 1-5 KPa; wherein most of 2-ethylhexanol was evaporated and then condensed by a condenser, and used as a raw material. The dealcoholated crude ester contained less than 1 wt% 2-ethylhexanol. The temperature of the crude ester at this point was about 180 ℃ and 210 ℃.

Neutralization

Cooling the crude ester after dealcoholization to 140-160 ℃, and adding Na into a neutralization kettle₂CO₃(or NaHCO)₃) Neutralizing the solution. The amount of aqueous alkali solution is about 1% of the amount of crude ester and the amount of alkali is about 100-150% of the theoretical value calculated from the acid value of the crude ester (the concentration of aqueous alkali solution is 1-2%), usually about 110-120% of the theoretical value. The pressure of the neutralization kettle is 0.4-1.0MPa (more than or equal to the saturated vapor pressure of water at the temperature); under the condition, the alkali liquor is kept in liquid state, and the monoester acid in the crude ester reacts with sodium carbonate to generate the monoesterSodium ester acid salt and titanate are hydrolyzed into titanium dioxide hydrate.

Dewatering

The neutralized crude ester enters a dehydration tower at the temperature of 130-150 ℃. The dehydration column was operated at 10-30KPa absolute pressure where the water evaporated and the water content in the crude ester was reduced to below 0.08 wt%. The evaporated water is condensed in a condenser. The dehydration process is also the process of the recrystallization of the sodium salt of the monoester acid and the recrystallization of the hydrated titanium dioxide to form the titanium dioxide. And the process of titanium dioxide crystallization can adsorb polar substances in the crude ester, thereby removing partial substances forming color and sodium monoester acid salt, which is beneficial to reducing the chroma and volume resistance of the product.

Filtration

The temperature of the dehydrated crude ester is 110-130 ℃ (the temperature is reduced by water evaporation and heat absorption and natural heat dissipation, and special temperature reduction is not needed), and a plate-type closed filter is used for filtering to obtain clear crude ester which is used as a stripping raw material. The filter residue contains 30-35% of DOTP, about 30% of titanium dioxide and 35-40% of monoester acid sodium salt.

Steam lift drying

Heating the filtered crude ester to 145-165 ℃, and removing light components such as octanol and water in a stripping tower. The stripping tower is filled with direct steam under 5-15KPa absolute pressure, and the direct steam can reduce the gas phase partial pressure of octanol and is beneficial to removing the octanol; the amount of direct steam added is 5-10 wt.% of the amount of crude ester. In the lower part of the stripping column, there was a falling film evaporator which heated the crude ester to maintain the temperature of the crude ester at 145-165 ℃. The evaporated alcohol and water are condensed in a condenser and then enter an alcohol-water separation tank, and the separated octanol is recycled as a raw material. The water enters a wastewater treatment system. The COD of the wastewater separated in this step was about 2000-3000 mg/L. The crude ester contained a trace amount of water due to the steam introduced into the stripper. And drying the stripped crude ester in a drying tower, and further removing the water in the drying tower under the absolute pressure of 1-3 KPa.

Precision filtration

The stripped and dried crude ester is filtered through a fine filter to obtain the final product. The precision filter is used as a safety device for preventing incomplete filtration in the early stage, and can ensure that the product does not contain solid particles.

In an industrialized DOTP (dioctyl terephthalate) device with 10 ten thousand tons per year, no filter aid and activated carbon are added in the production process, and the test result of the final product in HG/T2423-:

Claims

1. a process for the purification of crude esters of dibasic esters prepared by reacting terephthalic acid with an alcohol in the presence of an esterification catalyst, said process comprising the steps of:

providing the crude ester;

filtering the dehydrated crude ester to obtain a filtered crude ester.

2. The method of claim 1, further comprising:

the filtered crude ester is subjected to a stripping treatment to obtain a stripped crude ester,

drying the stripped crude ester to provide an ester product, and

optionally, the ester product is filtered.

3. The process according to claim 1 or 2, wherein the treatment with an aqueous solution of a base is carried out at an elevated pressure sufficient to prevent evaporation of water, preferably the pressure is from 0.3MPa to 3MPa, more preferably the pressure is from 0.4 to 1.0 MPa.

4. The process according to claim 1 or 2, wherein the crude ester is subjected to a dealcoholation treatment prior to treatment with the aqueous solution of a base, preferably to reduce the alcohol content of the crude ester to not more than 5% by weight of the crude ester, preferably not more than 3% by weight of the crude ester, more preferably not more than 1% by weight of the crude ester.

5. The process according to claim 1 or 2, wherein the base is selected from the group consisting of hydroxides, carbonates and bicarbonates, preferably the base is selected from the group consisting of hydroxides, carbonates and bicarbonates of alkali metals and calcium, more preferably from the group consisting of carbonates and bicarbonates of alkali metals, most preferably from sodium carbonate or potassium carbonate.

6. The process according to claim 1 or 2, wherein the aqueous solution of base comprises an amount of base of 100-200%, preferably 105-180%, more preferably 110-150%, most preferably 110-120% of the theoretical amount calculated on the acid number of the crude ester before treatment.

7. The process according to claim 1 or 2, wherein the water content of the dehydrated crude ester is not more than 0.2 wt.%, preferably not more than 0.1 wt.%, more preferably not more than 0.08 wt.%.

8. The process of claim 1 or 2, wherein the esterification catalyst is a titanium catalyst, preferably the titanium catalyst is a titanate catalyst, more preferably the titanium catalyst is selected from the group consisting of tetramethyl titanate, tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetra-n-butyl titanate, tetraisobutyl titanate, tetrapentyl titanate, tetrahexyl titanate, tetraheptyl titanate, tetraoctyl titanate, tetranonyl titanate, tetradecyl titanate, tetra- (2-propylheptyl) titanate, tetradodecyl titanate, tetra-hexadecyl titanate, tetra-octadecyl titanate and tetraphenyl titanate, more preferably the titanate catalyst is used in an amount of 0.117-0.353% relative to the weight of the terephthalic acid.

9. The process according to claim 1, wherein the alcohol is an alcohol having 4 to 15 carbon atoms, preferably octanol, i.e. 2-ethylhexanol, the finished product being dioctyl terephthalate (DOTP for english abbreviation).