WO2002068083A1 - Azeotropic distillation method - Google Patents

Abstract

A method of performing the azeotropic distillation of a distilled object including two substances comprising a low boiling point substance and a high boiling point substance by increased dissolubility or reduced energy consumption and by using an entrainer capable of forming an azeotrope with at least one of the two substances, characterized by comprising the steps of supplying the distilled object to an azeotropic distillation tower from two or more feed ports of different heights, supplying the distilled object having the low boiling point substance with low density from the lower feed ports, supplying the distilled object having the low boiling point substance with high density from the upper feed ports, and extracting the distilled object so that a fraction with the lowest boiling point can be provided from the top of the azeotropic distillation tower, a fraction with the highest boiling point can be provided from the bottom thereof, and a fraction with intermediate boiling point can be provided from the intermediate part thereof.

Description

 Description Azeotropic distillation method Technical field>

 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.

<Background technology>

 In an azeotropic distillation method, a substance (azeotropic agent or entrainer) that forms an azeotropic mixture with any of the component substances is added to a mixture that is difficult to separate by distillation, so that the separation of distillation is improved. Be improved. Examples of industrial applications of the azeotropic distillation method include 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 that forms an azeotropic mixture with water. .

 One of the fields where the azeotropic distillation method can be applied is in 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 carried out in a reaction medium containing an aliphatic carboxylic acid such as acetic acid.However, since water is generated in this step, water is contained in the reaction system. It is necessary to prevent accumulation. For this reason, a mixed vapor of the 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 and to obtain a dehydrated aliphatic carboxylic acid. An operation such as recirculation of at least a part of the mixture to the reaction raw material liquid preparation tank is performed.

Of the aliphatic carboxylic acids, acetic acid, which is widely used as the above-mentioned reaction medium, is focused. Usually, rectification is used to separate water from acetic acid. The azeotropic distillation is more advantageous depending on the moving cost.

 The major aspects of azeotropic distillation technology development can be broadly classified into separation, controllability, reduction of reflux ratio, and post-treatment of the liquid at the top of the recovered tower. In general, the higher the reflux ratio, the better the operating stability, and the lower the reflux ratio, the worse the operating stability. Furthermore, when the reflux ratio is below a certain limit, the separability of the azeotropic distillation itself deteriorates rapidly. This limit is commonly referred to as the minimum reflux ratio, and the value depends on the feed composition, type of entrainer, location, number of supply lines, how to return the reflux, how to return the entrainer, etc. .

 It is easy to satisfy controllability and separability by operating at a very high reflux ratio.However, such an operation consumes a large amount of energy and is economically disadvantageous. The recirculation ratio is set as close to the minimum recirculation ratio as possible to reduce the energy consumption as much as possible.

 For example, 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 tower (hereinafter referred to as “bottom liquid”) and the condensate at the top of the column are actually measured. In order to achieve the required level, a certain level of separation is required. On the other hand, economic demands require a reduction in the reflux ratio. Decreasing the reflux ratio tends to decrease the separability, while decreasing the reflux ratio while maintaining the separability tends to result in poor controllability. Actually, when the difference between the water reflux ratio and the minimum water reflux ratio becomes less than 1.0, the stability starts to deteriorate, and when the error is 0.9 or less, the degree of deterioration increases, and when the error is 0.8 or less, the stability decreases. Becomes very bad. If this condition is continued, the control will not be effective at all, resulting in undesired results such as the incorporation of acetic acid into the top liquid or the entrainer into the bottoms.

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 the entrainer contained in the aromatic hydrocarbons extracted from the middle stage of the column is lost. (Entrainer concentration) should be as low as possible. 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 end trainer will decrease, and the opposite phenomenon will be correct. Therefore, energy consumption and There is a trade-off with the best loss. In the operation of the azeotropic distillation column, when the reflux ratio is lowered, the entrainer concentration increases at the earliest due to the deterioration of the separability, so that the water reflux ratio must be further increased within a condition where the loss is not so large. We have to give up the reduction.

 Regarding the azeotropic distillation method for separating acetic acid and water, see Japanese Patent Application Laid-Open No. H10-5044556, W09844523, Korean Patent Publication 94-142429. No. 2 and Japanese Patent Publication No. Sho 62-412129 are also disclosed.

 Japanese Patent Application Laid-Open No. 10-5504556 proposes a method of reducing the water reflux ratio by selecting one type of entrainer used for azeotropic distillation. Here, an azeotropic agent having a boiling point from the boiling point of isoptyl acetate to the boiling point of n-propyl acetate is used. This is another method aimed at reducing the water reflux ratio. However, there is only one supply port for the distillation target, and this does not give any knowledge on the supply from multiple sources.

 WO984 / 452339 and Korean Patent Publication No. 941-142292 each disclose a method for recovering methyl acetate, which is characterized by a method for treating overhead vapor, and a method for recovering overhead vapor. A method for recovering methyl acetate characterized by a method for treating a condensed oil phase has been proposed. In these, examples are shown in which all types of feed are supplied to an azeotropic distillation column. In addition, Japanese Patent Publication No. Sho 62-412129 proposes the recovery of organic components from the aqueous phase liquid condensed at the top, and provided multiple types of feed to the azeotropic distillation column. An example of paying is shown. In the above example, the water concentration is higher at the upper side of the two types of feed. However, there is no mention of the effect of these multiple supplies.

 The present invention has been made in view of the above circumstances, and has as its object to provide a method for effectively performing azeotropic distillation while achieving reduced energy consumption. Invention disclosure>

The present inventors have conducted intensive studies on the phenomena associated with the azeotropic distillation operation of the above system, and as a result, the feed with high water concentration obtained by dividing the distillation target has the same effect as reflux water. With the new knowledge that it has fruit, and reached the idea of shifting the trade-off relationship between the entrainer concentration and the water reflux ratio by combining middle-stage extraction of aromatic hydrocarbons with split supply. did. On the basis of this, the present inventors prepared at least two or more types of objects to be distilled while extracting aromatic hydrocarbons from the middle stage in the azeotropic distillation operation of the above-mentioned system, and prepared the object to be distilled having a higher water concentration. By supplying to a higher position in the azeotropic distillation column than the one with low water concentration, compared to the method of supplying all at once, the amount of each component supplied is the same, but the entrainer in the middle extract It has been found that it is possible to improve the separation performance using the toner concentration as an index, or to operate at a low reflux ratio if the same entrainer concentration is used. The present invention has been accomplished based on these findings.

 That is, the gist of the present invention is to provide a distillation target 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”). Azeotropic distillation using an entrainer capable of forming an azeotrope with at least one of the substances,

 (1) supply of the distillation target to the azeotropic distillation column from at least two supply ports having different heights;

 (2) Of the above two supply ports, supply the distillation target with low low-boiling substance concentration from the lower supply port, and supply the distillation target with high low-boiling substance concentration from the upper supply port. , And

 (3) As distillates of the azeotropic distillation column, at least three types of distillates having different boiling points, the lowest-boiling fraction from the top, the highest-boiling fraction from the bottom, and the middle distillate So that the minutes can be obtained from the middle,

 An azeotropic distillation method characterized by the above. Brief description of drawings>

 FIG. 1 is a flowchart showing an example of a distillation process for carrying out the present invention. FIG. 2 is a flowchart showing the distillation process used in the examples.

In the figure, reference numeral 11 denotes an azeotropic distillation column, 13 denotes a liquid-liquid separation tank (decanter), and 21 denotes It is a stripping tower. BEST MODE FOR CARRYING OUT THE INVENTION>

 Hereinafter, embodiments of the present invention will be described in detail.

In this specification, the term "distillation target" refers to a mixed solution, a mixed gas or a two-phase mixed gas liquid containing a plurality of substances having different boiling points, and "entrainer" performs azeotropic distillation. Means the third component added. Further, the "azeotropic region", the concentration of E emissions trainers in the entire composition is present as a liquid phase in the region is meant at least 0.1 area by weight _^0/0, "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, the entrainer to be added and at least one of the above substances form an azeotropic mixture, and as a result, the azeotropic temperature There is no particular limitation as long as the mixture has a large difference in boiling point with the other substance. Further, it may contain a substance that does not substantially affect the azeotropic distillation.

 The end trainer 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 supply of the distillation target to the azeotropic distillation column is performed at least in different heights. (2) Of the above two supply ports, supply the distillation target having a low concentration of the low boiling substance from the lower supply port, and supply the low boiling substance from the upper supply port. Provide a highly concentrated distillation target. 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 Withdraw so that an intermediate fraction is obtained from the middle stage. By the azeotropic distillation, a bottom liquid containing the other substance (substance (B)) in which the concentration of one substance (substance (A)) in the distillation target is reduced is obtained from the bottom of the column, and from the top of the column Gives an azeotropic vapor consisting mainly of substance (A) and an entrainer. The vapor from the top is usually condensed and separated into substance (A) and 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 is preferable. In the embodiment, those providing an aqueous layer and an oil layer are preferable.

 Of the two phases separated, the phase mainly containing the entrainer is recycled to the azeotropic distillation column. There are two methods for returning the trainer: returning the whole amount to the top of the tower, or returning the entrainer to the middle part of the tower. On the other hand, a part of the phase mainly composed of the substance (A) is discarded, and the rest is returned to the azeotropic distillation column as a reflux liquid. In addition, the phase mainly composed of the substance (A) may be partly discarded after being reused in the process. 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 supply new as compensation for losses.

 The distillation target 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 also an example of a low-boiling substance.

The entrainer used for the combination of the above-mentioned distillation objects is selected in consideration of the type of the aliphatic carboxylic acid that coexists, but the azeotropic distillation of a mixture of the aliphatic carboxylic acid and water is performed. The well-known compound used for can be used. Give an example Then, esters such as butyl formate, n-propyl acetate, isobutyl acetate, n-butyl acetate, amyl acetate, n-butyl propionate, and isobutyl propionate, dichloromethyl ether, ethyl isoamyl ether, and aryl Ethers such as isoamyl ether and di-n-butyl ether; halogenated hydrocarbons such as ethylene dichloride and chlorbenzene; ketones such as acetone, dipropyl ketone, methyl butyl ketone, and arylacetone; toluene, xylene Compounds that can form an azeotrope with water, such as aromatic hydrocarbons such as ethylbenzene, are commonly used. Of these end trainers, the use of esters is preferred. For example, the use of n-propyl acetate or n-butyl acetate is preferred. The entrainer may contain a decomposition product of another substance such as p-xylene or methyl acetate derived from the azeotropic distillation raw material.

 The azeotropic distillation of the distillation object in the above combination will be specifically described.

 The composition of the aliphatic carboxylic acid and water in the distillation target is arbitrary, but usually, a fat having a water content in the range of 4 to 99% by weight, preferably 10 to 70% by weight. The method of the present invention is applied to a mixture consisting of an aliphatic carboxylic acid and water.

As a specific distillation target to which the method of the present invention is applied, an aromatic carboxylic acid is produced by subjecting an alkyl-substituted aromatic hydrocarbon as a raw material to liquid phase oxidation and purification in a reaction medium containing an aliphatic carboxylic acid. Examples include those recovered in the process. For example, aromatic carboxylic acids obtained by condensing and recovering mixed vapor from the reactor, absorbing acetic acid in waste gas with water, and condensing and recovering by evaporating at least part of the recovered reaction mother liquor An arbitrary solution and amount are selected from the reaction mother liquor obtained by solid-liquid separation of the reaction mother liquor and the recovered mother liquor or the washing liquid used and recovered in the step. These contain water and an aliphatic carboxylic acid at least partially, and have an aliphatic carboxylic acid content of 1 weight. Use the one with / ₀ or more. In a more preferred embodiment, acetic acid and water recovered in the step of producing terephthalic acid by liquid phase oxidation and purification using ρ-xylene as a raw material in a reaction medium containing acetic acid, and the acetic acid content is 1% Use a mixture of more than 1% by weight.

The azeotropic distillation reduces the concentration of water in the mixture containing aliphatic carboxylic acids and water. In order to reduce the amount of water, it is usually necessary to obtain a bottoms containing aliphatic carboxylic acid with a reduced amount of water from the bottom of the column, and to obtain a vapor of an azeotropic mixture mainly composed of water and entrainer from the top of the column. Distillation is carried out. At this time, for the purpose of reusing the aliphatic carboxylic acid, etc., it is preferable that the concentration of the entrainer in the bottom liquid is 100 ppm or less. Therefore, the concentration of the aliphatic carboxylic acid in the overhead liquid is preferably not more than 1,000 ppm. Part of the bottom liquid is recycled as a raw material preparation liquid to the liquid-phase oxidation reaction of alkyl-substituted aromatic hydrocarbons. The vapor obtained from the top of the column usually condenses to obtain a condensate separated into two phases, an oil phase and an aqueous phase. After this is separated into liquid and liquid by a decanter, the oil phase liquid is recycled to the azeotropic distillation column as an end trainer. The amount of 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 from the concentration of aliphatic carboxylic acid in the top liquid and the concentration of entrainer in the bottoms. Entrainers may supply new supplies to compensate for losses. The optimal 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 related 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 (recirculated water amount / discharged water amount). If oil phase components including entrainers are mixed into the water phase side, wastewater treatment equipment is passed through processes such as removing steam by injecting steam or gas, or treating with activated carbon. Sent to At this time, the oil phase component in the aqueous phase liquid may be reduced by stripping as described in Japanese Patent Publication No. 62-41219.

In addition, as described in Japanese Patent Publication No. 63-13101, the operating conditions were changed by dividing the circulating flow of the entrainer and returning one to the top of the tower and the other to the middle of the tower. A method in which the effect is easily reflected and the response is made faster may be used. Further, as described in Japanese Patent Application Laid-Open No. H10-0504556, 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 is also used. be able to. Further, as described in WO98 / 445239, partial condensation of vapor at the top of an azeotropic distillation column is The impurities in the acetate ester may be removed by performing the 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, which is the target compound, is an aromatic monocarboxylic acid, an aromatic dicarboxylic acid, an aromatic tricarboxylic acid, or the like. It is produced by subjecting an alkyl-substituted aromatic hydrocarbon such as trialkylbenzene to liquid phase oxidation. In particular, the method of the present invention is preferably applied when the aromatic carboxylic acid is terephthalic acid. In this case, the alkyl-substituted aromatic hydrocarbon as a raw material includes P-xylene.

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 is usually 2 to 6 times by weight based on the starting alkyl-substituted aromatic hydrocarbon. The water concentration in the oxidation reaction system is usually 4 to 25% by weight, preferably 7 to 20% by weight. / ₀ .

 In an oxidation reaction for producing an aromatic carboxylic acid by subjecting an alkyl-substituted aromatic hydrocarbon to liquid-phase oxidation, usually, a transition metal compound such as manganese, cobalt, iron, chromium, and nickel is used as a catalyst. A bromine compound may be used as a co-catalyst. When a bromine compound catalyst is not used, acetoaldehyde-methylethyl ketone or the like is used as a promoter for the cobalt catalyst. Oxygen is molecular oxygen, usually air. 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 reduced by mixing inert gas such as nitrogen gas.

The reaction temperature for the liquid phase oxidation is usually in the range of 120 ° C. to 220 ° C., and the pressure may be at least as high as the solvent acetic acid can maintain the liquid phase. In an oxidation method that does not use a bromine compound catalyst, the reaction temperature is generally 160 ° C. or less. 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 other Low boiling point products and unreacted alkyl-substituted aromatic hydrocarbons. This vapor is cooled by the condenser and condensed into a liquid, which is returned to the oxidation reactor as a solvent for the oxidation reaction, but a part of it is dehydrated to remove the water produced by the oxidation reaction. It is sent to a column and subjected to the azeotropic distillation of the present invention. 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 reduced pressure crystallizers, where it is cooled to the temperature corresponding to each pressure by the flash cooling action of the solvent to produce 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 means of crystal separation, for example, a rotary-vacuum filter method, a centrifugal method or other suitable separation method. The cake of aromatic carboxylic acid is washed with acetic acid or water as necessary, 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 flowchart showing an example of a distillation process for applying the method of the present invention. 11 is an azeotropic distillation column, and 14 is its 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 low-boiling substances is supplied from, and a mixture having a high concentration of low-boiling substances is supplied from an upper supply port.

The azeotropic vapor containing the low-boiling substances and the entrainer is sent to the cooler 12 from the top of the azeotropic distillation column 11 where it is condensed and separated into two phases in the liquid-liquid separation tank 13. It is. An appropriate separation means is selected depending on the nature of the azeotrope. If the azeotrope does not separate into two phases in the liquid phase, a distillation column or the like is installed as a step for separating the entrainer. In the process shown in Fig. 1, low-boiling The phase mainly composed of the point substance is sent to the stripping tower 21 through the line 17, where the organic component is recovered and used or discarded through the line 18. The line 15 and the line 16 are connected to the top of the azeotropic distillation column 11 or the middle stage of the azeotropic distillation column 11 if necessary, and the number thereof may be one or more. In addition, in the line returning to the azeotropic distillation column 11, when 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, Lines or individual lines. From the bottom of the azeotropic distillation column 11, a liquid containing a high-boiling substance having a reduced low-boiling substance content as a main component is extracted through a line 19. Also, 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, the reduced water reflux ratio or the improved water reflux ratio is achieved by separately supplying the distillation target having a low boiling point substance concentration, specifically, a water concentration, to the azeotropic distillation column as described above. Operation with separation can be realized. A mixture with a higher water concentration has an action closer to that of a water reflux liquid.For example, when two distillation objects with different water concentrations are supplied and when the two are mixed and supplied, the total amount of each component is Even if 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 with three or more divisions. 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, the reactor vapor is condensed and recovered, the acetic acid contained in the waste gas from the reaction process, the crystallization process or the separation process is recovered, the recovered mother liquor mainly consisting of aliphatic carboxylic acid and water or Vapors, those recovered in the separation process, those recovered in the purification process, and others. These can be supplied as they are, in their entirety, in selected forms, or partially mixed to reduce the total supply, or divided further to increase the total supply. it can. Also, each supply is May be a whole amount or a part thereof. These distillation objects mainly consist of aliphatic carboxylic acid and water, each containing at least 1% or more in concentration. Also, the feeds listed here need not be completely liquid but may contain vapor.

 The supply position of the distillation target is not limited, except that it is supplied from the upper side of the column in the 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 provides a large difference. The supply position may be optimized using the concentration of the entrainer in the mixture mainly consisting of water and aromatic hydrocarbons extracted from the middle stage of the tower as an index.

Extraction from the middle stage is performed to extract aromatic hydrocarbons accumulated in the tower. By extracting from the middle stage, a mixture mainly consisting of water and aromatic hydrocarbons is obtained, which contains the entrainer and its decomposition products. Since the contained entrainer is lost, it is extracted from the middle stage so as to obtain a mixture having a lower entrainer concentration. For example, when the extraction position is lowered from the top of the tower to the lower side, the concentration of aromatic hydrocarbons 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 the first liquid phase, the concentration of the aromatic hydrocarbon drops sharply, and the aromatic hydrocarbon concentration substantially decreases. The purpose of collection cannot be achieved. Therefore, the withdrawal position is, for example, below the midpoint where two liquid phases can be obtained, and preferably near the lowest point where two liquid phases can be obtained. In addition, 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, a condition that the middle-stage extraction position is lower than the middle point of the azeotropic region, preferably by setting the lower limit of the azeotropic region near the middle-stage extraction position, to reduce the aromatic hydrocarbon and entrainer concentration in the middle-stage extraction 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 extract, so the conditions are determined to achieve the targets. The longer the azeotropic region and the higher the water reflux ratio, the more advantageous the composition of the middle extract. However, these have the disadvantages of increasing tower height and increasing energy use. Therefore, it is not set excessively, and the composition of the middle extract is set to the minimum allowable range required to achieve the target. The aromatic hydrocarbon concentration in the middle-stage withdrawn fraction (oil phase) is preferably 50% or more, more preferably 80% or more, and even more preferably 90% or more. The concentration of the entrainer is preferably 50% or less, more preferably 15% or less, and still more preferably 5% or less, in the middle-stage withdrawn oil phase. The middle withdrawn mixture is distinguished from the overhead recovered by the concentration of aromatic hydrocarbons in the oil phase. The middle-stage discharged mixture has a higher concentration of aromatic hydrocarbons in the oil phase than that recovered at 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 content of the entrainer or aromatic hydrocarbons in the middle-stage extraction should be reduced to an acceptable concentration. The reflux ratio can be reduced, which is the minimum water reflux ratio. When the feed liquid division of the present invention is performed, a decrease in the concentration of the entrainer 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. As a result, it is possible to reduce the use of energy in equipment for concentrating aromatic hydrocarbons and to omit the equipment itself, which is described in WO97 / 29068. In addition, after the liquid supply is divided, the composition of the middle extract can be deteriorated again within an acceptable range, and the amount of water reflux can 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 possible to find a condition for reconciliation by reducing both the water reflux ratio and the entrainer concentration.

 The present invention has a greater effect in azeotropic distillation where conditions are set to minimize energy usage. For example, in order to further reduce the reflux ratio of the system to be carried out, losses must be allowed in the recovery of aromatic hydrocarbons. It is possible to reduce the amount of use or reduce the disadvantage itself.

As a further preferred embodiment of the application example of the present invention, from the top of the azeotropic distillation column The concentration of the aliphatic carboxylic acid in the fraction to be withdrawn is l ° / o or less, preferably 1.0 OO ppm or less, and the ene in the liquid (bottom liquid) withdrawn from the bottom of the azeotropic distillation column. An azeotropic distillation column having a trainer concentration of less than 100 ppm and a middle extract (fraction) of less than 20%, preferably less than 5%, an azeotropic distillation column giving a reduced minimum water reflux ratio. Driving method is mentioned. Examples>

 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

Using a water mixture containing acetic acid as a distillation target and n-butyl acetate as an end trainer, the method of the present invention was carried out by a continuous distillation method according to the process shown in FIG. The aromatic hydrocarbon extracted and recovered from the middle stage was p-xylene. As an 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, acetic acid 82.9% by weight, water 16.0% by weight, n-butyl acetate 0.03% by weight, p- A mixed solution consisting of 0.2% by weight of xylene and 0.9% by weight of methyl acetate was spread, and heated by a reboiler 14 to perform a total reflux circulation. Thereafter, by gradually feeding n-butyl acetate, which is an entrainer, an azeotropic distillation system of three components of n-butyl acetate / monoacetate / acetic acid was formed. The amount of the entrainer was adjusted, and after the azeotropic region was in the desired range, the feed was started from the feed feed lines 41 and 42 provided in the azeotropic distillation column 11. In the steady state of operation, the supplied distillation target is acetic acid 82.9% by weight, water 16.0% by weight, n-butyl acetate 0.03% by weight, p-xylene 0.2% by weight, vinegar 0.9 weight of methyl acid. Feed 1 consisting of / ₀ and feed acid 2 consisting of 32.2% by weight of sulfuric acid, 67.7% by weight of water and 0.2% by weight of methyl acetate. The flow rate is 1 unit of power S 4 35 parts by weight per unit time, 2 units of 15 parts by weight of supply 2 units, and the supply port is at a position where feed 1 is 13% of the tower height with respect to the bottom (line 4 2) Feed 2 was installed at 55% of the tower height (line 41). As effluent from the bottom of the azeotropic distillation column 11 through line 19 Concentrated acetic acid was extracted at 400 parts by weight per unit time. From the top of the azeotropic distillation column 11, water containing an azeotropic composition and steam containing an entrainer was obtained, which was cooled by the cooler 12 and collected in the decanter 13. Of the two liquid phases separated in the decanter 13, the aqueous phase liquid is divided into two, one discharged through the line 17 and one returned to the azeotropic distillation column 11 by the line 16. From the amount discharged (Ww) and the amount returned as reflux water (Wr), the amount of reflux water was determined so that the water reflux ratio defined by Wr / Ww became a desired value. In this example, the operation was carried out under the conditions that the amount (Ww) of the discharged aqueous phase liquid was 53 parts by weight per unit time, the reflux water (Wr) was 42 parts by weight, and the water reflux ratio was 0.8. did. The entrainer supplied an amount corresponding to the amount extracted from the line 18 after the liquid-liquid separation in the decanter 13 to the azeotropic distillation column 11 from the line 15. The position of the middle stage withdrawn from line 22 was set at 43% of the tower height based on the bottom of the tower, and the lower limit of the azeotropic region was set at 3% above and below 39%. In order to control this lower limit position, the feed rate of the entrainer was adjusted. The middle stage was extracted at 2 parts by weight per unit time. Table 1 shows the results.

 The water reflux ratio was as low as 0.8, and the entrainer in the liquid oil phase withdrawn at the middle stage was 0.6%, which was a good state. In addition, p-xylene in the oil phase was 97%, and almost no p-xylene was present in the aqueous phase, which was less than 1%. Comparative Example 1

 Example 1 was carried out in the same manner as in Example 1 except that feeds 1 and 2 were mixed and fed to a position (line 42) of 13% of the tower height based on the bottom of the tower. The results are shown in Table 1.

Significant deterioration was found in the concentration of the entrainer from the middle stage. This result means that if the operation is to be performed with the same water recirculation ratio, that is, the same energy, up to 70 times the entrainer loss allowed in Example 1 must be allowed. Comparative Example 2 In Example 1, except that feeds 1 and 2 were mixed and fed to a position (line 42) of 13% of the tower height with respect to the bottom of the tower, and the water reflux ratio was increased to 1.3. Was carried out in the same manner as in Example 1. In Comparative Example 1, the concentration of the entrainer extracted from the middle stage changed, but in Comparative Example 2, the experiment was performed to verify the water reflux ratio necessary to prevent the entrainer concentration from changing. Table 1 shows the results.

 As a result, it was confirmed that in order to prevent a change in the concentration of the entrainer, the water recirculation ratio had to be drastically increased to 1.6 times the energy consumption. Thus, it was confirmed that without split supply, either energy or entrainer loss must be allowed. Therefore, split supply can be said to be an effective way to provide improved processes.

Table 1

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

 This application is based on a Japanese patent application filed on Feb. 27, 2001 (Japanese Patent Application No. 2001-051554), the contents of which are incorporated herein by reference. Industrial applicability>

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

Claims

The scope of the claims 1. Distillation target containing two substances, a substance with a lower boiling point (hereinafter referred to as “low-boiling substance”) and a substance with higher boiling point (hereinafter referred to as “high-boiling substance”), is combined with at least one of the above substances. In an azeotropic distillation method using an entrainer capable of forming an azeotrope,  (1) supply of the distillation target to the azeotropic distillation column from at least two supply ports having different heights;  (2) Of the above two supply ports, supply the distillation target with low low-boiling substance concentration from the lower supply port, and supply the distillation target with high low-boiling substance concentration from the upper supply port. , And  (3) As distillates of the azeotropic distillation column, at least three types of distillates having different boiling points, the lowest-boiling fraction from the top, the highest-boiling fraction from the bottom, and the middle distillate So that the minutes can be obtained from the middle,  An azeotropic distillation method characterized in that: 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 or 2, wherein the concentration of the aromatic hydrocarbon in the fraction extracted from the middle stage is 50% or more. 4. The concentration of aliphatic carboxylic acid in the fraction withdrawn from the top is 1% or less, the concentration of entrainer in the liquid withdrawn from the bottom is lOO ppm or less, and the concentration of entrainer in the middle fraction is 20%. The azeotropic distillation method according to any one of claims 1 to 3, which is as follows. 5. The distillation target is generated from the aromatic carboxylic acid production process. The azeotropic distillation method according to any one of claims 1 to 4. 6. The azeotropic distillation method according to any one of claims 1 to 5, 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 any one of claims 2 to 6, wherein a reflux of water is supplied to the azeotropic distillation tower in addition to the distillation target. 8. A method for producing an aromatic carboxylic acid, which comprises oxidizing an alkyl-substituted aromatic hydrocarbon in a reaction medium containing an aliphatic carboxylic acid, wherein an aliphatic carboxylic acid containing water resulting from the oxidation reaction is used as a distillation target. A method for producing an aromatic carboxylic acid, comprising performing the azeotropic distillation according to any one of claims 1 to 7. 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.