JP2002326001A - Azeotropic distillation method - Google Patents

Abstract

(57) [PROBLEMS] To provide a method for performing azeotropic distillation with improved or reduced energy consumption. SOLUTION: A substance having a lower boiling point (hereinafter referred to as "low boiling point substance")
) And a higher boiling substance (hereinafter referred to as "high boiling substance") using an entrainer capable of forming an azeotropic mixture with at least one of the above substances. (1) supplying an object to be distilled to an azeotropic distillation column from at least two supply ports having different heights, and (2) supplying a lower one of the two supply ports. Feeding a low-boiling substance concentration low-boiling substance from the inlet, and supplying a high-boiling substance high-distillation substance from the upper supply port; and (3) at least 3 as a distillate of the azeotropic distillation column. The fractions having different boiling points are withdrawn so that the lowest boiling fraction can be obtained from the top portion, the highest boiling fraction can be obtained from the bottom portion, and the middle distillate can be obtained from the middle stage. Azeotropic distillation method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an azeotropic distillation method. The invention finds particular use in recovering aliphatic carboxylic acids by separating water from an aqueous feed stream containing aliphatic carboxylic acids, such as acetic acid, by azeotropic distillation. More specifically, the present invention relates to a method suitable for recovering an aliphatic carboxylic acid used as a solvent in a method for producing an aromatic carboxylic acid by a liquid phase oxidation reaction in a reaction medium containing the aliphatic carboxylic acid.

[0002]

2. Description of the Related Art In an azeotropic distillation method, a substance (azeotropic agent or entrainer) which forms an azeotropic mixture with any of the component substances is added to a mixture which is difficult to separate by distillation. Is improved. As an industrial application example of the azeotropic distillation method, there is a method of obtaining highly pure acetic acid from a mixture of acetic acid and water by adding n-propyl acetate or n-butyl acetate which forms an azeotropic mixture with water.

[0003] One of the fields where the azeotropic distillation method can be applied is the production of aromatic carboxylic acids. That is, azeotropic distillation can be applied in the process of recovering the reaction medium from the production process of the aromatic carboxylic acid. The production of an aromatic carboxylic acid such as terephthalic acid is generally performed in a reaction medium containing an aliphatic carboxylic acid such as acetic acid. However, water is generated in the step, so that water accumulates in the reaction system. Need to be prevented. For this purpose, a mixed vapor of aliphatic carboxylic acid and water is taken out of the reactor, and a feed stream containing a condensate of this vapor is distilled to separate water from the aliphatic carboxylic acid,
An operation such as recycling at least a part of the dehydrated aliphatic carboxylic acid to the reaction raw material liquid preparation tank is performed.

[0004] Of the aliphatic carboxylic acids, acetic acid, which is widely used as the above reaction medium, is usually
Rectification is used to separate water from acetic acid, but azeotropic distillation is more advantageous depending on equipment and variable costs. The main aspects of the technical development of azeotropic distillation are roughly divided into separation, controllability, reduction of the reflux ratio, and post-treatment of the recovered top liquid. In general, the higher the reflux ratio, the better the operation stability, and the lower the reflux ratio, the lower the operation stability. Further, when the reflux ratio falls below a certain limit value, the separability of the azeotropic distillation itself rapidly deteriorates. This limit is commonly referred to as the minimum reflux ratio, the value of which depends on the feed composition, the type of entrainer, the location of the feed, the number of feed lines, the method of returning the reflux, the method of returning the entrainer, and the like.

[0005] It is easy to satisfy controllability and separability by operating at a very high reflux ratio, but such operation consumes a large amount of energy and is economically disadvantageous. The operation is performed with the reflux ratio as close to the minimum reflux ratio as possible to reduce the energy consumption as much as possible. For example, even when acetic acid is obtained from a mixture of water and acetic acid by azeotropic distillation, the purity of the liquid removed from the bottom of the azeotropic distillation column (hereinafter referred to as “bottom liquid”) and the purity of the condensate at the top are actually measured. To achieve the required level, a certain level of separation is required. On the other hand, economic requirements call for a reduction in the reflux ratio. When the reflux ratio is reduced, the separability tends to decrease, and when the reflux ratio is reduced while maintaining the separability, the controllability tends to be poor. Actually, when the difference between the water reflux ratio and the minimum water reflux ratio becomes 1.0 or less, the stability starts to deteriorate. Further, when the error is 0.9 or less, the degree of the deterioration increases. Becomes very bad. If the state is further continued, the control becomes completely ineffective, resulting in undesired results such as mixing of acetic acid into the top liquid or mixing of the entrainer into the bottom liquid.

On the other hand, when the aromatic hydrocarbons retained in the azeotropic distillation column are extracted from the middle stage of the column, the entrainer contained therein is lost, so that the aromatic hydrocarbon extracted from the middle stage of the column is lost. The smaller the trainer (entrainer concentration), the better. This entrainer concentration depends on the water reflux ratio, and decreases as the water reflux ratio increases. In other words, if the amount of energy used is increased by increasing the water reflux, the loss of the entrainer is reduced, and the phenomenon in the opposite direction is also correct. Thus, there is a trade-off between energy usage and entrainer loss. In the operation of the azeotropic distillation column, when the reflux ratio is lowered, this increase in the entrainer concentration is the earliest in the deteriorating separability, so that the water reflux ratio can be further reduced within a range where the loss is not so large. I have to give up.

An azeotropic distillation method for separating acetic acid and water is described in JP-A-10-504556, WO 98/4.
No. 5239, Korean Patent Publication No. 94-14292, and Japanese Patent Publication No. 62-41219. Japanese Patent Publication No. Hei 10-504556 proposes a method of reducing the water reflux ratio by selecting the type of entrainer used for azeotropic distillation. Here, an azeotropic agent having a boiling point from the boiling point of isobutyl acetate to the boiling point of n-propyl acetate is used. This is also one method aiming at the effect of reducing the water reflux ratio. However, there is only one supply port for the distillation target, and this does not give any knowledge about the supply from a plurality.

[0008] WO98 / 45239 and Korean Patent Publication No. 94-14292 each disclose a method for recovering methyl acetate and a method for treating a condensed oil phase of the overhead vapor, which are characterized by the method of treating the overhead vapor. A method for recovering methyl acetate having the following has been proposed. Among them, examples are shown in which a plurality of kinds of feeds are supplied to an azeotropic distillation column. Further, Japanese Patent Publication No. 62-41219 proposes a method for recovering organic components from an aqueous phase liquid condensed at the top of the column, and discloses an example in which a plurality of types of feeds are supplied to an azeotropic distillation column. I have. In the above example, the upper side of the two types of supplies has a high water concentration. But,
There is no description of the effect of these multiple supplies.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and aims to provide a method for effectively performing azeotropic distillation while achieving reduced energy consumption. is there.

[0010]

The inventors of the present invention have conducted intensive studies on the phenomena associated with the azeotropic distillation operation of the above-mentioned system. As a result, the feed having a high water concentration obtained by dividing the distillation target was obtained. Has a new finding that has the same effect as reflux water,
The idea was reached to shift the trade-off between the entrainer concentration and the water reflux ratio by combining the middle-stage extraction of aromatic hydrocarbons and the split supply. On the basis thereof, the present inventors prepared at least two or more types of distillation objects while extracting aromatic hydrocarbons from the middle stage in the above-mentioned azeotropic distillation operation of the system, and prepared the distillation objects having a higher water concentration. By supplying to the higher position of the azeotropic distillation column than the one with low water concentration, the entrainer in the middle-stage extract is the same as the total supply of each component compared to the method of supplying all at once. It has been found that it is possible to improve the separability based on the concentration as an index, or to operate at a low reflux ratio with the same entrainer concentration. The present invention has been achieved based on these findings.

[0011] That is, the gist of the present invention is to distill an object containing two substances, a substance having a lower boiling point (hereinafter, referred to as a "low boiling substance") and a substance having a higher boiling point (hereinafter, referred to as a "high boiling point substance"). In a method of performing azeotropic distillation using an entrainer capable of forming an azeotrope with at least one of the substances, (1) feeding an object to be distilled to an azeotropic distillation column with at least two different heights From the supply port,
(2) Of the two supply ports, supply the distillation target having a low concentration of the low-boiling substance from the lower supply port, and supply the distillation target having a high concentration of the low-boiling substance from the upper supply port. And (3) distillates of at least three kinds having different boiling points as distillates of the azeotropic distillation column, the lowest boiling fraction from the top part, the highest boiling fraction from the bottom part, and intermediate Azeotropic distillation method characterized in that the fraction is extracted so as to be obtained from the middle stage.

[0012]

Embodiments of the present invention will be described below in detail. In the present specification, "distillation target"
The term "entrainer" means a mixed solution containing a plurality of substances having different boiling points, a mixed gas or a two-phase mixed gas liquid, and the "entrainer" means a third component added for performing azeotropic distillation. Further, the “azeotropic region” means a region where the concentration of the entrainer is at least 0.1% by weight in the entire composition existing as a liquid phase in the region, and the “azeotropic distillation column” , Means a distillation column for distilling the above-mentioned distillation target and entrainer. The “range of the azeotropic region” means a space part which is an azeotropic region in the azeotropic distillation column.

First, the distillation target and the entrainer will be described. The distillation target in the present invention is a mixture containing a plurality of substances having different boiling points, and furthermore, an entrainer to be added and at least one of the above substances form an azeotropic mixture. There is no particular limitation as long as the mixture has a large difference in boiling point from the other substance. Further, it may contain a substance which does not substantially affect the azeotropic distillation.

The entrainer in the present invention is not particularly limited as long as the effect is exhibited. It does not need to be a single component, and may be a mixture of two or more components that form a heterogeneous azeotrope with one substance in the distillation target, or a part of a decomposition product of the component. May be contained. The azeotropic distillation column in the present invention may be either a packed column or a tray column. Although the supply position of the distillation target is not particularly limited, it is usually the middle stage of the azeotropic distillation column, and the optimum position may be determined in consideration of the composition in the column in order to optimize the separation efficiency. The operation of the azeotropic distillation column can be carried out under any conditions of normal pressure, increased pressure, or reduced pressure, and the system may be a batch system or a continuous system. More preferably, it is carried out continuously under normal pressure.

In the present invention, (1) the distillation target is supplied to the azeotropic distillation column from at least two supply ports having different heights, and (2) the lower one of the two supply ports is provided. Supply a low-boiling substance concentration low distillation target from the supply port of
A distillation target having a high low-boiling substance concentration is supplied from an upper supply port. Further, (3) at least three types of distillates having different boiling points as distillates of the azeotropic distillation column, the lowest-boiling fraction from the column top, the highest-boiling fraction from the column bottom, and intermediate , So that the fraction can be obtained from the middle stage.

By the azeotropic distillation, a bottom liquid containing the other substance (substance (B)) having a reduced concentration of one substance (substance (A)) in the distillation target is obtained from the column bottom, From the top of the column, vapor of an azeotropic mixture mainly consisting of the substance (A) and the entrainer is obtained. The vapor obtained from the top is usually condensed and separated into substance (A) and an entrainer. The separation means is not particularly limited as long as the separation of the substance (A) and the entrainer is achieved, but a heterogeneous azeotrope in which the substance (A) and the entrainer are not uniformly mixed, in a preferred embodiment, Those providing an aqueous layer and an oil layer are preferred.

[0017] Of the two phases separated, the phase mainly comprising the entrainer is recycled to the azeotropic distillation column. The method of returning the entrainer includes a method of returning the entire amount to the top of the tower, and a method of partially returning the entrainer to the middle of the tower. On the other hand, a part of the phase mainly containing the substance (A) is discarded, and the rest is returned to the azeotropic distillation column as a reflux liquid. In addition, after the above-mentioned one-material phase is reused in the process, a part thereof may be discarded. The method of returning the reflux liquid includes, for example, a method of returning to the top of the column and a method of returning to the middle stage of the column. The entrainer may provide a fresh supply as compensation for the loss.

The object of distillation to which the method of the present invention is applied is a mixture of a substance having a lower boiling point ("low-boiling substance") and a substance having a higher boiling point ("high-boiling substance"). The high-boiling substance is, for example, a saturated or unsaturated aliphatic carboxylic acid having 2 to 6 carbon atoms, preferably a saturated aliphatic carboxylic acid having 2 to 4 carbon atoms. Examples of the acid include acetic acid, propionic acid, and butyric acid. Water is an example of the low-boiling substance.

The entrainer used for the above-mentioned combination of distillation objects is selected in consideration of the kind of the coexisting aliphatic carboxylic acid, and the azeotropic mixture of the mixture of the aliphatic carboxylic acid and water is selected. Known compounds used for distillation can be used. For example, butyl formate, acetic acid n
Esters such as -propyl, isobutyl acetate, n-butyl acetate, amyl acetate, n-butyl propionate, isobutyl propionate, dichloromethyl ether, ethyl isoamyl ether, allyl isoamyl ether, di-
ethers such as n-butyl ether; halogenated hydrocarbons such as ethylene dichloride and chlorobenzene; ketones such as acetone chloride, dipropyl ketone, methyl butyl ketone and allyl acetone; aromatic hydrocarbons such as toluene, xylene and ethyl benzene. Compounds capable of forming an azeotrope with water, such as hydrogen, are commonly used. Of these entrainers, esters are preferably used. For example, n-propyl acetate or n-acetate
The use of butyl is preferred. The entrainer may contain other substances such as p-xylene and methyl acetate derived from the azeotropic distillation raw material, and decomposition products of the entrainer.

The azeotropic distillation of the above-mentioned combination of distillation objects will be specifically described. The composition of the aliphatic carboxylic acid and water in the distillation target is arbitrary, but usually the water content is 4%.
To 99% by weight, preferably 10 to 70% by weight
The process according to the invention is applied to mixtures consisting of aliphatic carboxylic acids and water in the range As a specific distillation target to which the method of the present invention is applied, a step of producing an aromatic carboxylic acid by performing liquid phase oxidation and purification using an alkyl-substituted aromatic hydrocarbon as a raw material in a reaction medium containing an aliphatic carboxylic acid is used. And those recovered in the above. For example, a mixture obtained by condensing and recovering mixed vapor from a reactor, a case where vapor acetic acid in waste gas is absorbed with water, a case where at least a part of the collected reaction mother liquor is evaporated and collected, and a reaction with an aromatic carboxylic acid An arbitrary solution and amount are selected from a reaction mother liquor separated and collected from the mother liquor or a washing solution used and collected in the step. These contain water and an aliphatic carboxylic acid at least in part and have an aliphatic carboxylic acid content of 1
Use the one by weight or more. In a more preferred embodiment, it contains acetic acid and water recovered in the step of producing terephthalic acid by performing liquid phase oxidation and purification using p-xylene as a raw material in a reaction medium containing acetic acid, and the acetic acid content is 1% by weight. % Of the mixture is used.

When the concentration of water in a mixture containing an aliphatic carboxylic acid and water is reduced by azeotropic distillation,
Usually, distillation is carried out so as to obtain a bottom liquid containing an aliphatic carboxylic acid having a reduced amount of water from the bottom of the column, and to obtain an azeotrope vapor mainly composed of water and an entrainer from the top of the column. At this time, for the purpose of reusing the aliphatic carboxylic acid, the concentration of the entrainer in the bottoms is preferably 100 ppm or less. Is preferably 1,000 ppm or less. A part of the bottom liquid is recycled as a raw material preparation liquid to the liquid phase oxidation reaction system of the alkyl-substituted aromatic hydrocarbon. The vapor obtained from the top of the column is usually condensed to obtain a condensed liquid separated into two phases, an oil phase and an aqueous phase. After this is separated into liquid and liquid by a decanter or the like, the oil phase liquid is recycled to an azeotropic distillation column as an entrainer. The amount of the entrainer recycled to the azeotropic distillation column is given a theoretical value by the amount of water to be discharged from the distillation column and the composition of the azeotropic mixture. In practice, the optimum amount of entrainer is determined based on the aliphatic carboxylic acid concentration in the overhead liquid and the entrainer concentration in the bottoms. The entrainer may make a new supply as compensation for the loss. The optimum amount depends on the operating conditions and may vary. A part of the aqueous phase liquid is discarded, and if necessary, a part is returned to the azeotropic distillation column as a reflux liquid. Further, the aqueous phase liquid may be used in a purification step relating to the production of an aromatic carboxylic acid before being discarded and refluxed. The ring flow rate of water is usually set to about 0.1 to 3 depending on the ratio (reflux water amount / discharge water amount). When an oil phase component including an entrainer is mixed into the water phase side, the oil phase component is sent to a wastewater treatment device through a process of removing the oil phase component by blowing steam or gas or treating with activated carbon. At this time, the oil phase component in the aqueous phase liquid may be reduced by stripping as described in JP-B-62-41219.

In addition, as described in JP-B-61-31091, the effect of changing the operating conditions can be obtained by dividing the circulating flow of the entrainer and returning one to the top of the column and the other to the middle of the column. A method that facilitates the reflection and speeds up the response may be used. Further, as described in JP-A-10-504556, a method of returning the refluxed liquid to the middle stage of the column and controlling the impurity concentration at the bottom of the column by manipulating the amount thereof can also be used. Further, as described in WO 98/45239, partial condensation of the vapor at the top of the azeotropic distillation column was performed,
The acetate impurities may be removed by distillation of the remaining uncondensed vapor in a continuous column.

Next, a method for producing an aromatic carboxylic acid to which the azeotropic distillation method of the present invention is applied will be described. First, the method for producing the aromatic carboxylic acid itself will be described.The aromatic carboxylic acid as the target compound is an aromatic monocarboxylic acid, an aromatic dicarboxylic acid, an aromatic tricarboxylic acid, or the like, and these are monoalkylbenzene, dialkylbenzene, It is produced by liquid-phase oxidation of an alkyl-substituted aromatic hydrocarbon such as trialkylbenzene. In particular, the method of the present invention
It is preferably applied when the aromatic carboxylic acid is terephthalic acid. In this case, p-xylene is mentioned as the alkyl-substituted aromatic hydrocarbon as a raw material.

As the aliphatic carboxylic acid which is a solvent for the liquid phase oxidation reaction, for example, acetic acid is preferable, and the amount of the solvent used is usually 2 to the starting alkyl-substituted aromatic hydrocarbon.
6 times by weight. The water concentration in the oxidation reaction system is
Usually, it is 4 to 25% by weight, preferably 7 to 20% by weight. In an oxidation reaction for producing an aromatic carboxylic acid by subjecting an alkyl-substituted aromatic hydrocarbon to liquid-phase oxidation, a transition metal compound such as manganese, cobalt, iron, chromium, and nickel is generally used as a catalyst. Further, a bromine compound may be used as a co-catalyst. When a bromine compound catalyst is not used, acetaldehyde, methyl ethyl ketone, or the like is used as a promoter for the cobalt catalyst. Molecular oxygen, usually air, is used as the oxidizing agent. It is also possible to use air in which oxygen concentration is increased by mixing oxygen gas, and conversely, air in which oxygen concentration is decreased by mixing inert gas such as nitrogen gas.

The reaction temperature of the liquid phase oxidation is usually from 120.degree.
A range of 20 ° C. is employed, and the pressure may be any range as long as the solvent acetic acid can maintain a liquid phase. In the oxidation method not using a bromine compound catalyst, the reaction temperature is generally 160 ° C. or lower. The heat of the oxidation reaction is mainly removed by flash evaporation of the aqueous acetic acid solvent. That is, the exhaust gas from the oxidation reactor mainly contains evaporated acetic acid and water, and also contains a small amount of low-boiling products among the by-products of the oxidation reaction and unreacted alkyl-substituted aromatic hydrocarbons. This vapor is cooled by a condenser and condensed into a liquid,
It is refluxed again in the oxidation reactor as the oxidation reaction solvent,
A part thereof is sent to a dehydration tower for the purpose of removing water generated by the oxidation reaction, and is subjected to the azeotropic distillation of the present invention.

The liquid-phase oxidation of alkyl-substituted aromatic hydrocarbons is usually carried out in one or more reactors. If necessary, the reaction solution after the oxidation reaction is sent to one or two or more successively depressurized crystallizers, and cooled to a temperature corresponding to each pressure by a flash cooling action of the solvent to produce a reaction solution. Most of the aromatic carboxylic acid crystallized as crystals to form a slurry solution. The slurry solution is separated into an aromatic carboxylic acid cake and an oxidation reaction mother liquor by a crystal separation means, for example, a rotary vacuum filter method, a centrifugal separation method or another suitable separation method. The aromatic carboxylic acid cake is washed with acetic acid or water as needed, and the attached solvent is removed with a dryer. The aromatic carboxylic acid cake is further re-slurried, if necessary, in a reaction mother liquor mainly composed of water, purified through a hydrogenation step, crystallized, separated, washed and dried, and then dried. An acid is obtained.

FIG. 1 is a flow chart showing an example of a distillation process for applying the method of the present invention. Numeral 11 denotes an azeotropic distillation column, and numeral 14 denotes a reboiler. Distillates having different compositions are supplied from the supply ports 41 and 42 having different heights, and the entrainer is supplied from the line 15 to perform azeotropic distillation. Here, the distillation target is a mixture containing at least two substances, a low-boiling substance (specifically, water) and a high-boiling substance (specifically, acetic acid). A mixture having a low concentration of the low-boiling substance is supplied, and a mixture having a high concentration of the low-boiling substance is supplied from an upper supply port.

The azeotropic vapor containing the low-boiling substance and the entrainer is supplied from the top of the azeotropic distillation column 11 to the cooler 12.
, Where it is condensed and separated into two phases in a liquid-liquid separation tank 13. An appropriate separation means is selected depending on the nature of the azeotrope. If the azeotropic mixture does not separate into two phases in a liquid phase, a distillation column or the like is installed as a step of separating the entrainer. In the process shown in FIG. 1, in the liquid-liquid separation tank 13, the phase mainly composed of a low-boiling substance is sent to the stripping column 21 through the line 17, where the organic component is recovered and used or recycled through the line 18. Discarded. The line 15 and the line 16 are connected to the top of the azeotropic distillation column 11 or the middle of the column, if necessary, and the number thereof may be one or more. In the line returning to the azeotropic distillation column 11, a common line is used for returning the liquid mainly composed of the entrainer and the distillation target mainly composed of the low-boiling substance to the azeotropic distillation column 11. It may be used or by individual lines.
From the bottom of the azeotropic distillation column 11, a liquid mainly containing a high-boiling substance having a reduced low-boiling substance content is extracted through a line 19. Further, a mixture mainly composed of water and aromatic hydrocarbons is extracted from the middle line 22 of the tower.

In the method of the present invention, as described above, the reduced water reflux ratio or the improved water reflux ratio can be improved by separately supplying the distillation target having a low boiling point substance concentration, specifically, a water concentration, to the azeotropic distillation column. Operation with separation can be realized. A mixture having a higher water concentration has an action closer to that of a water reflux liquid. For example, when two distillation objects having different water concentrations are supplied and when the two are mixed and supplied, the total amount of each component is Even if they are equal, improved operation can be realized when the distillation target is supplied in two parts. Also,
This effect is not limited to two divisions, and the same effect can be obtained even if the division number is three or more. In dividing the distillation target, the difference in water concentration between the feeds is usually 5% or more, preferably 10% or more, and more preferably 20% or more.

The route from which the divided feed is derived is not particularly limited, for example, when it comes from the same aromatic carboxylic acid production process. For example, those obtained by condensing and recovering the vapor of the reactor, those obtained by collecting acetic acid contained in the waste gas from the reaction step, the crystallization step or the separation step, the collected reaction mother liquor mainly composed of aliphatic carboxylic acid and water, or the same. Steam, those recovered in the separation step, those recovered in the purification step, and others. These can be supplied as they are, in their entirety, in selected forms, or partially mixed to reduce the total number of supplies, or further divided to increase the total number of supplies, etc. . Further, each supply may be a whole or a part thereof. These distillation objects mainly consist of an aliphatic carboxylic acid and water, and contain at least 1% or more of each. Also, the feeds listed here need not be completely liquid and may contain steam.

The supply position of the distillation target is not limited except that it is supplied from the upper stage of the column in order of increasing water concentration.
Although there is a difference in the effect depending on the positional relationship of the divided supplies, the feature of the present invention resides in the divided supply itself, and the presence or absence of the difference causes a large difference. The supply position may be optimized using the entrainer concentration in the mixture mainly consisting of water and aromatic hydrocarbons extracted from the middle stage of the column as an index.

The extraction from the middle stage is performed to extract the aromatic hydrocarbons accumulated in the column. By extracting from the middle stage, a mixture mainly composed of water and an aromatic hydrocarbon is obtained, which contains an entrainer and its decomposition product. Since the contained entrainer is lost, the entrainer is extracted from the middle stage so as to obtain a mixture having a reduced entrainer concentration. For example, when the extraction position is lowered from the top of the tower to the lower side, the concentration of the aromatic hydrocarbon increases, and at the same time, the concentration of the entrainer decreases. This tendency continues while the condensate withdrawn from the middle stage forms two liquid phases, but when the condensate is further lowered to one liquid phase, the concentration of the aromatic hydrocarbons drops sharply, and the aromatic hydrocarbons are substantially recovered. Can not achieve its purpose. Therefore,
The withdrawal position is, for example, below the midpoint where two liquid phases can be taken, and preferably near the lowest point where two liquid phases can be taken. Also, since this lowest point is linked to the lower limit position of the azeotropic region,
It is also possible to set conditions by moving the lower limit position of the azeotropic region while fixing the middle stage extraction position. For example, the conditions at which the middle-stage extraction position is lower than the middle point of the azeotropic region, preferably the aromatic hydrocarbon and entrainer concentration in the middle-stage extraction product are set so that the lower limit of the azeotropic region comes near the middle-stage extraction position. To optimize. In addition, the length of the azeotropic region in the height direction of the column and the water reflux ratio also affect the composition of the middle-stage extract, so that the conditions are determined so as to achieve the target. The longer the azeotropic region and the higher the water reflux ratio, the more advantageous the composition of the middle extract. However, these have inherent disadvantages such as raising the tower height and increasing energy use, so they are not set excessively, and the composition of the middle-stage extract is set to the minimum allowable range that achieves the target. Is done. The concentration of the aromatic hydrocarbon is preferably 50% in the middle-stage withdrawn oil phase.
Or more, more preferably 80% or more, and still more preferably 9% or more.
0% or more. The entrainer concentration is preferably 50% or less, more preferably 1% or less, in the middle-stage withdrawn oil phase.
It is at most 5%, more preferably at most 5%. Further, the middle-stage withdrawn mixture is distinguished from the overhead recovered by the concentration of the aromatic hydrocarbon in the oil phase. The middle-stage discharged mixture has a higher concentration of aromatic hydrocarbons in the oil phase than that recovered from the top.

According to the method of the present invention, it is possible to carry out the operation with a reduced water reflux ratio or the middle stage extraction with a more advantageous composition. Normally, the water reflux ratio is set to the minimum from the viewpoint of energy loss.However, when aromatic hydrocarbons are recovered by middle-stage extraction, the water reflux ratio is set so that the entrainer or aromatic hydrocarbons in the middle-stage extraction can be tolerated. The ratio is reduced, which is the minimum water reflux ratio. When the feed liquid division of the present invention is performed, a decrease in the entrainer concentration or an increase in the aromatic hydrocarbon concentration in the middle extract is observed, and aromatic hydrocarbons can be recovered from the middle extract at an advantageous concentration. Thereby, it is possible to reduce the use of energy in equipment for concentrating aromatic hydrocarbons and to omit the equipment itself as shown in WO 97/29068. Further, after the liquid supply is divided, the composition of the middle-stage withdrawn material may be deteriorated again within an allowable range, and the water reflux amount may be reduced instead. At that time, the entrainer concentration in the middle-stage extract does not change, but the amount of heat used can be reduced by lowering the water reflux ratio. In addition, it is also possible to find a condition for matching by reducing both the water reflux ratio and the entrainer concentration.

The present invention has a greater effect in azeotropic distillation in which conditions are set to minimize energy consumption. For example, usually, in order to further reduce the reflux ratio of the system to be carried out, a loss must be allowed in the recovery of the aromatic hydrocarbon, but in the method of the present invention, the energy consumption is reduced without allowing the disadvantage. It is possible to reduce or reduce the disadvantage.

In a further preferred embodiment of the above-mentioned application of the present invention, the aliphatic carboxylic acid concentration in the fraction withdrawn from the top of the azeotropic distillation column is 1% or less, preferably 1,000 ppm or less. The entrainer concentration in the liquid (bottom liquid) withdrawn from the bottom of the boiling distillation column is 1
A method for operating an azeotropic distillation column that gives a reduced minimum water reflux ratio of not more than 00 ppm and an entrainer concentration in the middle extract (fraction) of not more than 20%, preferably not more than 5%.

[0036]

Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples unless it exceeds the gist. Example 1 The method of the present invention was carried out by a continuous distillation method according to the process shown in FIG. 2 using a water mixture containing acetic acid as a distillation target and n-butyl acetate as an entrainer. The aromatic hydrocarbon extracted and recovered from the middle stage is p-
Xylene was used. As the azeotropic distillation column 11 for performing azeotropic distillation, a packed column having 17 theoretical plates was used.

As a method for starting continuous azeotropic distillation, 82.9% by weight of acetic acid, 16.0% by weight of water, 0.03% by weight of n-butyl acetate, 0% by weight of p-xylene A mixed solution consisting of 0.2% by weight and 0.9% by weight of methyl acetate was filled, and heated by a reboiler 14 to perform a total reflux circulation. Thereafter, an azeotropic distillation system of three components of acetic acid-water-n-butyl acetate was formed by gradually supplying n-butyl acetate as an entrainer. The amount of the entrainer was adjusted, and after the azeotropic region reached a desired range, the supply of raw materials was started from the two raw material supply lines 41 and 42 in the azeotropic distillation column 11.

In the steady state of operation, the distillation target supplied was 82.9% by weight of acetic acid, 16.0% by weight of water, 0.03% by weight of n-butyl acetate, 0.2% by weight of p-xylene, and acetic acid. Feed 1 consisting of 0.9% by weight of methyl and 32.2% by weight of acetic acid, 67.7% by weight of water, 0.1% of methyl acetate.
Feed 2 consisting of 2% by weight. The flow rates were 435 parts by weight for feed 1, 15 parts by weight for feed 2 per unit time,
The feed port is at a position where feed 1 is at 13% of the tower height with reference to the bottom (line 42), and where feed 2 is at 55% of the tower height (line 4).
Installed in 1). Line 19 from the bottom of the azeotropic distillation column 11
As a result, concentrated acetic acid was withdrawn as a bottom liquid at 400 parts by weight per unit time. A vapor containing water having an azeotropic composition and an entrainer was obtained from the top of the azeotropic distillation column 11, which was cooled by the cooler 12 and collected in the decanter 13. The aqueous phase liquid of the two liquid phases separated by the decanter 13
Through an azeotropic distillation column 11 and one returned to an azeotropic distillation column 11 by a line 16. Amount discharged (W
w) and the amount returned as reflux water (Wr), Wr / W
The amount of reflux water was determined so that the water reflux ratio defined by w became a desired value. In this embodiment, the amount (Ww) of the discharged aqueous phase liquid is 53 parts by weight per unit time,
r) was 42 parts by weight and the water reflux ratio was 0.8. The entrainer sets the amount corresponding to the amount extracted from the line 18 after the liquid-liquid separation in the decanter 13
5 to the azeotropic distillation column 11. The position of the middle stage withdrawn from the line 22 was 43% of the tower height based on the bottom of the tower, and the lower limit of the azeotropic region was within 3% above and below 39%. The feed rate of the entrainer was adjusted to control this lower limit position. The middle stage was extracted at 2 parts by weight per unit time. The results are shown in Table 1.

In a state where the water reflux ratio was as low as 0.8, the entrainer in the liquid oil phase withdrawn at the middle stage was 0.6%, which was a good state. Incidentally, 97% of p-xylene in the oil phase
In the aqueous phase, p-xylene was scarcely present, and was 1% or less. Comparative Example 1 Example 1 was repeated except that feeds 1 and 2 were mixed and fed to a position (line 42) at 13% of the tower height with respect to the bottom of the tower. The results are shown in Table 1.

The concentration of the entrainer in the middle stage was significantly deteriorated. This result indicates that when the operation was attempted with the same water reflux ratio, that is, with the same energy, Example 1 was used.
Means that it must be allowed up to 70 times the entrainer loss allowed. Comparative Example 2 In Example 1, feeds 1 and 2 were mixed and supplied to a position (line 42) at 13% of the tower height with respect to the bottom of the column.
The procedure was performed in the same manner as in Example 1 except that the water reflux ratio was increased to 1.3. In Comparative Example 1, the entrainer concentration at the middle stage was changed, but Comparative Example 2 was performed to verify the water reflux ratio necessary to prevent the entrainer concentration from changing. The results are shown in Table 1.

As a result, it was confirmed that in order to prevent a change in the entrainer concentration, the water reflux ratio had to be drastically increased to 1.6 times the energy consumption. Thus, it was confirmed that either energy or entrainer losses would have to be tolerated without a split supply. Thus, split feed can be said to be an effective approach to providing an improved process.

[0042]

[Table 1]

[0043]

According to the present invention, an azeotropic distillation operation with improved or reduced energy of separation can be achieved. The method of the present invention has a great effect on variable costs or environmental protection.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating an example of a distillation process for practicing the present invention.

FIG. 2 is a flowchart showing a distillation process used in Examples.

[Explanation of symbols]

 11 azeotropic distillation tower 13 liquid-liquid separation tank (decanter) 21 stripping tower

Continued on front page F-term (reference) 4D076 AA16 AA22 BB08 CB02 EA02Y EA03Y EA03Z EA04Y EA04Z EA05Y EA05Z EA20Y EA20Z FA02 FA12 GA01 JA03 4H006 AA02 AC47 AD12 BA16 BA19 BA20 BD33 BD34 BD53 BJ50 BS30 4

Claims (9)

[Claims] 1. A substance having a lower boiling point (hereinafter referred to as "low boiling point substance")
) And a higher boiling substance (hereinafter referred to as "high boiling substance") using an entrainer capable of forming an azeotropic mixture with at least one of the above substances. (1) supplying the distillation target to the azeotropic distillation column from at least two supply ports having different heights, and (2) supplying a lower one of the two supply ports. (I) supplying a low-boiling substance concentration low-boiling substance concentration from a port, and supplying a high-boiling substance concentration high-distillation substance from an upper supply port; and (3) at least 3 as distillate of an azeotropic distillation column. The fractions having different boiling points are withdrawn so that the lowest boiling fraction can be obtained from the top portion, the highest boiling fraction can be obtained from the bottom portion, and the middle distillate can be obtained from the middle stage. Azeotropic distillation method. 2. The azeotropic distillation method according to claim 1, wherein the low-boiling substance is water, the high-boiling substance is an aliphatic carboxylic acid, and the distillation target contains an aromatic hydrocarbon. 3. The azeotropic distillation method according to claim 1, wherein the aromatic hydrocarbon concentration in the fraction withdrawn from the middle stage is 50% or more. 4. The aliphatic carboxylic acid concentration in the fraction withdrawn from the top is 1% or less, the entrainer concentration in the liquid withdrawn from the bottom is 100 ppm or less, and the entrainer concentration in the middle fraction is 20% or less. %.
4. The azeotropic distillation method according to any one of items 1 to 3. 5. The azeotropic distillation method according to claim 1, wherein the distillation target is generated from a process for producing an aromatic carboxylic acid. 6. The azeotropic distillation method according to claim 1, wherein the reflux of the entrainer is supplied to an azeotropic distillation column in addition to the distillation target. 7. The azeotropic distillation method according to claim 2, wherein the reflux of water is supplied to the azeotropic distillation column in addition to the distillation target. 8. A method for producing an aromatic carboxylic acid comprising oxidizing an alkyl-substituted aromatic hydrocarbon in a reaction medium containing the aliphatic carboxylic acid, wherein the aliphatic carboxylic acid containing water resulting from the oxidation reaction is distilled. A method for producing an aromatic carboxylic acid, comprising performing the azeotropic distillation according to any one of claims 1 to 7 as an object. 9. The method according to claim 8, wherein the aliphatic carboxylic acid is acetic acid, the alkyl-substituted aromatic hydrocarbon is p-xylene, and the aromatic carboxylic acid is terephthalic acid.