Title of Invention

A METHOD OF AZEOTROPIC DISTILLATION

Abstract 1. A method of azeotropic distillation of distillation objectives containing two substances of a substance having a lower boiling point (hereinafter referred to as "low-boiling substance" ) and a substance having a higher boiling point (hereinafter referred to as "high-boiling substance") using an entrainer capable of forming an azeotropic mixture together with at least one of the substances, said distillation objectives containing the low-boiling substance in an amount of 4 to 99% by weight and the high boiling substance in an amount of 1% by weight or more, wherein the method comprises: (1) feeding the distillation objectives into an azeotropic distillation column from at least two feed ports having a different height;
Full Text FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS (AMENDMENT) RULES 2006
COMPLETE SPECIFICATION
[See Section 10; rule 13] "A METHOD OF AZEOTROPIC DISTILLATION"
MITSUBISHI CHEMICAL CORPORATION, of 5-2, Marunouchi 2-chome, Chiyoda-ku, Tokyo 100-0005, Japan,

8-9-2007
GRANTED
The following specification particularly describes the nature of the invention and the manner in which it is to be performed:-



ORIGINAL 2 8 FEB 2007.

DESCRIPTION
AZEOTROPIC DISTILLATION METHOD

The present invention relates to an azeotropic distillation method. Especially, the invention is utilized in the case of recovering an aliphatic carboxylic acid by azeotropic distillation by separating water from an aqueous feed flow containing an aliphatic carboxylic acid such as acetic acid. In detail, the invention relates to a method suitable for recovery of an aliphatic carboxylic acid to be used as a solvent in a process of producing an aromatic carboxylic acid by liquid phase oxidation reaction in an aliphatic carboxylic acid-containing reaction medium.

In the azeotropic distillation method, by adding a substance (azeotropic agent or entrainer) capable of forming an azeotropic mixture together with any one of component substances to a mixture that is hardly subjected to distillation separation, separation property of distillation is enhanced. Industrial application examples of the azeotropic distillation method include a method in which n-propyl acetate or n-butyl acetate capable of


forming an azeotropic mixture together with water is added to obtain acetic acid having a high purity from a mixture of acetic acid and water.
One field to which the azeotropic distillation method is considered applicable is the production of aromatic carboxylic acids. That is, the azeotropic distillation can be applied in the recovery step of a reaction medium from the production process of aromatic carboxylic acids.
The production of aromatic carboxylic acids such as terephthalic acid is in general carried out in a reaction medium containing aliphatic carboxylic acids such as acetic acid, but since water is formed during the step, it is necessary to prevent accumulation of water in the reaction system. For achieving this, there is employed an operation in which a vapor comprising a mixture of aliphatic carboxylic acids and water is taken out from a reactor, a feed flow containing a condensate of this vapor is distilled to separate water from the aliphatic carboxylic acids, and at least a part of the dehydrated aliphatic carboxylic acids is recirculated into a reaction raw material preparation tank.
Of the aliphatic carboxylic acids, acetic acid that is widely used as the above-described reaction medium will be noticed. Usually, rectification is used in the separation of water from acetic acid. However, azeotropic distillation is advantageous depending on the cost of
.

equipment and the csost of fluctuation.
Major viewpoints of technical development of the azeotropic distillation are roughly classified into separation property, control property, reduction in reflux ratio, and post treatment of recovered overhead liquid. In general, when the reflux ratio is large, the operation stability is good, whereas when the reflux ratio is small, the operation stability becomes worse gradually. Further, when the reflux ratio is a certain critical value or less, the separation property of the azeotropic distillation per se is rapidly deteriorated. This critical value is in general called a minimum reflux ratio, and that value varies depending on the composition of feed material, the kind of entrainer, the feed position, the number of feed lines, the method Qf returning a reflux liquid, the method of returning an entrainer, and the like.
When the operation is carried out at a very high reflux ratio, it is easy to satisfy the control property and separation property, but such an operation consumes a large amount of energy so that it is economically disadvantageous. Accordingly, the operation is actually carried out in such a manner that the reflux ratio is made closed to the minimum reflux ratio as far as possible, thereby reducing the energy consumption as far as possible.
For example, even in the case where acetic acid is obtained by azeotropic distillation from a mixture of

water and acetic acid, for making the purity of each of a liquid taken out from the column bottom of an azeotropic distillation column (this liquid being hereinafter referred to as "bottom liquid") and an overhead condensate reach an actually required level, separation property of a certain extent or more is required. On the other hand, reduction in the reflux ratio is required from the economical demand. When the reflux ratio is reduced, the separation property is liable to be lowered, and when the reflux ratio is reduced while keeping the separation property, the control property is liable to become poor. Actually, when a difference between a water reflux ratio and a minimum water reflux ratio is 1.0 or less, the stability begins to be deteriorated, and when the error is 0.9 or less, the degree of deterioration increases, and when it is 0.8 or less, the stability becomes very worse. When this state is further continued, the control is no more effective, leading to undesired results such as incorporation of acetic acid into the overhead liquid and incorporation of the entrainer into the bottom liquid.
On the other hand, during extraction of aromatic hydrocarbons remaining within an azeotropic distillation column from a middle stage of the column, the entrainer contained therein is a loss. Accordingly, it is desired that the entrainer (entrainer concentration) contained in the aromatic hydrocarbons to be extracted from a middle

stage of the column is low as far as possible. This entrainer concentration relies upon the water reflux ratio and decreases as the water reflux ratio increases. That is, when the water reflux is increased to increase the amount of energy to be used, the loss of the entrainer becomes small, and a phenomenon of the reverse direction is also correct. Therefore, the amount of energy to be used and the loss of the entrainer are in a trade-off relation. In operating the azeotropic distillation column, when the reflux ratio is decreased, an increase of the entrainer concentration reveals most rapidly among the deterioration of separation property. Accordingly, one cannot stop giving up a further reduction of the water reflux ratio within the range under which the loss is not so large.
The azeotropic distillation method for separation of acetic acid from water is also disclosed in JP-T-10-504556, WO 98/45239, KR-A-94-14292, JP-B-62-41219, etc.
A method of decreasing the water reflux ratio by selecting the kind of an entrainer to be used in azeotropic distillation is proposed in JP-T-10-504556. Here, an azeotropic agent having a boiling point of from the boiling point of isobutyl acetate to the boiling point of n-propyl acetate is used. This is also one measure of aiming at an effect of reducing the water reflux ratio. However, a feed port of the distillation objective is one,

but any knowledge for feeding from a plurality of ports is not given.
In WO 98/45239 and KR-A-94-14292, are proposed a method of recovery of methyl acetate, the characteristic of which resides in a treatment method of overhead vapor, and a method of recovery of methyl acetate, the characteristic of which resides a treatment method of a condensed oil phase of overhead vapor. In any of them, there are given examples of feeding plural kinds of feed materials into an azeotropic distillation column. In addition, JP-B-62-41219 proposes the recovery of organic components from an aqueous phase liquid obtained by condensation in a column head and gives examples of feeding plural kinds of feed materials into an azeotropic distillation column. Incidentally, the foregoing examples are concerned with two kinds of feed materials in which the feed material in the upper stage side has a higher water concentration. However, there are no descriptions at all regarding the effects to be brought by a plurality of feedings.
Under these circumstances, the invention has been made and is aimed at providing a method of effectively carrying out azeotropic distillation while attaining a reduced amount of energy to be used.


The present inventors made extensive and intensive investigations about phenomena attendant upon the azeotropic distillation operation in the above-describe systems. As a result, they have obtained a new finding that a feed material having a higher water concentration as obtained by dividing distillation objectives has the same effect as in reflux water and reached an idea of shifting the trade-off relation between entrainer concentration and water reflux ratio by combining extraction of aromatic hydrocarbons from the middle stage with divided feeding. Then, the present inventors have found based on this finding that in the azeotropic distillation operation in the above-describe systems, by preparing at least two kinds of distillation objectives and feeding a distillation objective having a higher water concentration into an azeotropic distillation column in a higher position than a distillation objective having a lower water concentration while extracting aromatic hydrocarbons from the middle stage, it is possible to enhance the separation property while taking an entrainer concentration in the extract from the middle stage as an index, or it is possible to perform the operation at a low reflux ratio if the entrainer concentration is identical, nevertheless the total feed amount of the respective components is the same as compared with the method of collective feeding. The invention has been accomplished

based on these findings.
Specifically, the gist of the invention resides in (1) feeding the distillation objectives into ar
azeotropic distillation column from at least two feec
ports having a different height;
(2) feeding a distillation objective having a lov concentration of the low-boiling substance from the lowei feed port of the two feed ports and feeding a distillation objective having a high concentration of the low-boiling substance from the upper feed port thereof; and
(3) extracting at least three kinds of fractions having a different boiling point as distillates from the azeotropic distillation column such that a fraction having the lowest boiling point is obtained from a column head, a fraction having the highest boiling point from a column bottom, and an intermediate fraction from the middle stage respectively.


Fig. 1 is a flow chart showing one example of the distillation process for carrying out the invention.
Fig. 2 is a flow chart showing the distillation process used in the Examples.
In the drawings, reference numerals 11, 13 and 21 denote an azeotropic distillation column, a liquid-liquid separation tank (decanter) and a stripping column, respectively.

The mode for carrying out the invention will be described below in detail.
In this description, the term "distillation objective" means a mixed solution, mixed gas or mixed gas-liquid two-phase material containing plural substances having a different boiling point; and the term "entrainer" means a third component to be added for performing azeotropic distillation. Further, the term "azeotropic region" means a region where the concentration of the entrainer is at least 0.1 % by weight in the whole of the composition present as a liquid phase within that region; and the term "azeotropic distillation column" means a distillation column for distilling the foregoing distillation objective and entrainer. The term "range of azeotropic region" means a spatial site that is the


azeotropic region within the azeotropic distillation column.
First of all, the distillation objective and entrainer will be described.
In the invention, the distillation objective is a mixture containing plural substances having a different boiling point and is not particularly limited so far as it is a mixture in which the entrainer to be further added and at least one of the foregoing substances form an azeotropic mixture, and as a result, a difference in boiling point from the other substance becomes large according to the azeotropic temperature. Further, substances that do not substantially affect the azeotropic distillation may be additionally contained.
In the invention, the entrainer is not particularly limited so far as it reveals its effects. Further, it is not always required to be a single component but may be a mixture of two kinds or more of components forming a heterogeneous azeotropic mixture together with one substance in the distillation objectives, or may contain a part of decomposition products of the foregoing component.
In the invention, the azeotropic distillation column may be any of a packed column or a plate column. The feed position of the distillation objectives is not particularly limited but is usually a middle stage of the azeotropic distillation column. For optimizing the


separation efficiency, an optimum position may be determined taking into account the composition within the column. The operation of the azeotropic distillation column can be carried out under any condition of an atmospheric pressure, an elevated pressure or a reduced pressure, and its mode may be a batch mode or a continuous mode. More preferably, the operation is carried out in a continuous mode at atmospheric pressure.
In the invention, (1) feeding of the distillation objectives into the azeotropic distillation column is carried out from at least two feed ports having a different height; and (2) a distillation objective having a low concentration of the low-boiling substance is fed from the lower feed port of the two feed ports, and a distillation objective having a high concentration of the low-boiling substance from the upper feed port thereof. Further, (3) at least three kinds of fractions having a different boiling point as distillates from the azeotropic distillation column are extracted such that a fraction having the lowest boiling point is obtained from a column head, a fraction having the highest boiling point from a column bottom, and an intermediate fraction from the middle stage, respectively.
By the azeotropic distillation, a bottom liquid containing a reduced concentration of one substance (substance (A)) and the other substance (substance (B)) in

the distillation objectives is obtained from the column bottom, and a vapor of the azeotropic mixture composed mainly of the substance (A) and the entrainer is obtained from the column head. The vapor obtained from the column head is usually condensed and separated into the substance (A) and the entrainer. The separation measure is not particularly limited so far as the separation of the substance (A) and the entrainer is attained but is preferably one giving a heterogeneous azeotropic mixture where the substance (A) and the entrainer do not uniformly intermingle with each other, and an aqueous phase and an oil phase in a preferred embodiment.
Of the two phases as separated, the phase composed mainly of the entrainer is recycled into the azeotropic distillation column. The method of returning the entrainer includes a method of returning the whole into the column head and a method of returning a part as divided into the middle stage of the column. On the other hand, a part of the phase composed mainly of the substance (A) is disposed, and the reminder is returned as a reflux liquid into the azeotropic distillation column. Further, the phase composed mainly of the substance (A) may be reused within the process, a part of which may be then disposed. Examples of the method of returning the reflux liquid include a method of returning it into the column head and a method of returning it into the middle stage of

the column. The en trainer may be newly fed as compensation for a loss.
The distillation objective to which the method of the 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"). Specific combinations to which the invention is preferably applied will be described. Examples of high-boiling substances include saturated or unsaturated aliphatic carboxylic acids having from 2 to 6 carbon atoms, and preferably saturated aliphatic carboxylic acids having from 2 to 4 carbon atoms such as acetic acid, propionic acid, and butyric acid. Examples of low-boiling substances include water.
The entrainer to be used for the combination of the foregoing distillation objectives is selected taking into account the kind of the aliphatic carboxylic acid to be co-present, and known compounds that are used in azeotropic distillation of a mixture comprising an aliphatic carboxylic acid and water can be used. Examples include usual compounds capable of forming an azeotropic mixture together with water, including esters such as butyl formate, n-propyl acetate, isobutyl acetate, n-butyl acetate, amyl acetate, n-butyl propionate, and isobutyl propionate; ethers such as dichloromethyl ether, ethyl isoamyl ether, allyl isoamyl ether, and di-n-butyl ether;

halogenated hydrocarbons such as ethylene dichloride and chlorobenzene; ketones such as acetone chloride, dipropyl ketone, methyl butyl ketone, and allyl acetone; and aromatic hydrocarbons such as toluene, xylene, and ethylbenzene. Of these entrainers are preferably used esters. For example, use of n-propyl acetate or n-butyl acetate is preferable. The entrainer may contain other substances derived from the azeotropic distillation raw material, such as p-xylene and methyl acetate, and decomposition products of the entrainer.
The azeotropic distillation of the foregoing combined distillation objective will be specifically described.
The composition of an aliphatic carboxylic acid and water in the distillation objective is arbitrary, but the method of the invention is usually applied to mixtures comprising an aliphatic carboxylic acid and water and having a content of water in the range of from 4 to 99 % by weight, and preferably from 10 to 70 % by weight.
As specific distillation objectives to which the method of the invention is applied, are enumerated ones recovered in the step of producing aromatic carboxylic acids by liquid phase oxidation and purification in an aliphatic carboxylic acid-containing reaction medium using an alkyl-substituted aromatic hydrocarbon as the raw material. For example, ones obtained by condensation and recovery of mixed vapors from the reactor, ones obtained

by absorbing an acetic acid vapor in the waste gas with water, ones obtained by evaporating at least a part of the recovered reaction mother liquor, followed by condensation and recovery, and reaction mother liquors obtained by solid-liquid separation of an aromatic carboxylic acid and a reaction mother liquor, followed by recovery or washings used in the step and then recovered, are arbitrarily selected along with the amounts. These contain at least partly water and an aliphatic carboxylic acid, and ones having the content of the aliphatic carboxylic acid of 1 % by weight or more are used. In a more preferred embodiment, a mixture containing acetic acid recovered in the step of producing terephthalic acid by liquid phase oxidation and purification in an acetic acid-containing reaction medium using p-xylene as the raw material and water and having the content of acetic acid of 1 % by weight or more is used.
In the case of reducing the concentration of water in a mixture containing an aliphatic carboxylic acid and water by azeotropic distillation, the distillation is usually carried out in a manner such that a bottom liquid containing an aliphatic carboxylic acid and having a reduced amount of water is obtained from the column bottom, whereas a vapor of an azeotropic mixture composed mainly of water and the en trainer is obtained from the column head. During this, for the purposes of reuse of the

aliphatic carboxylic acid and the like, the concentration of the entrainer in the bottom liquid is preferably not higher than 100 ppm, and from the economic demand, etc., the concentration of the aliphatic carboxylic acid in the overhead liquid is preferably not higher than 1,000 ppm. A part of the bottom liquid is recycled as a raw material preparation liquid into the liquid phase oxidation reaction system of alkyl-substituted aromatic hydrocarbon. The vapor obtained from the column head is usually condensed to obtain a condensate in the two-phase form of an oil phase and an aqueous phase. The resulting condensate is subjected to liquid-liquid separation by a decanter, etc., and the oil phase liquid is then recycled as the entrainer into the azeotropic distillation column. For the amount of the entrainer to be recycled into the azeotropic distillation column, a theoretical value is given from the amount of water to be discharged from the distillation column and the composition of azeotropic mixture. Actually, the optimum amount of the entrainer is judged from the concentration of the aliphatic carboxylic acid in the overhead liquid and the concentration of the entrainer in the bottom liquid. The entrainer may be newly fed as compensation for a loss. The optimum amount relies upon the operation conditions and may be accompanied with fluctuation. A part of the aqueous phase liquid is disposed, and if desired, a part thereof is

returned as a reflux liquid into the azeotropic distillation column. Further, the aqueous phase liquid may be used in the purification step regarding the production of aromatic carboxylic acid prior to the disposal and reflux. The reflux amount of water is usually set up at from about 0.1 to 3 in terms of its ratio (reflux water amount/discharge water amount). In the case where the oil phase components containing the entrainer are intermingled into the aqueous phase side, the aqueous phase liquid is delivered into a waste water treatment device through a step such as removal of the oil phase components by blowing water vapor or a gas and treatment with active carbon. Moreover, during this, the oil phase components in the aqueous phase liquid may be reduced by stripping, as described in JP-B-62-41219.
Besides, there may be employed a method in which the circulating flow of the entrainer is divided, one of the divided flows is returned into the column head, and the other is returned into the middle stage of the column, thereby making it easy to reflect the effects with changed operation conditions and hastening the response, as described in JP-B-61-31091. Further, there may be employed a method in which the reflux liquid is returned into the middle stage of the column, and the concentration of impurities in the column bottom is controlled by an operation at that amount, as described in JP-T-10-504556.

Moreover, impurities of acetic acid esters may be removed by a method in which the column head vapor of the azeotropic distillation column is partially condensed, and the remaining uncondensed vapor is distilled within continuous columns, as described in WO 98/45239.
Next, the production process of aromatic carboxylic acids to which the azeotropic distillation method of the invention is applied will be described.
First of all, the production process per se of aromatic carboxylic acids will be described. Examples of aromatic carboxylic acids as the desired compound include aromatic monocarboxulic acids, aromatic dicarboxylic acids, and aromatic tricarboxylic acids, and they are produced by liquid phase oxidation of alkyl-substituted aromatic hydrocarbons such as monoalkylbenzenes, dialkylbenzenes, and trialkylbenzenes. Especially, it is preferred to apply the method of the invention in the case where the aromatic carboxylic acid is terephthalic acid, and in this case, p-xylene is enumerated as the raw material alkyl-substituted aromatic hydrocarbon.
As the aliphatic carboxylic acid as the solvent of the liquid phase oxidation reaction, for example, acetic acid is preferable, and the amount of the solvent to be used is usually from 2 t 6 times the raw material alkyl-substituted aromatic hydrocarbon. Further, the water content within the oxidation reaction system is usually


from 4 to 25 % by weight, and preferably from 7 to 20 % by weight.
In the oxidation reaction for the production of aromatic carboxylic acids by liquid phase oxidation of alkyl-substituted aromatic hydrocarbons, transition metal compounds such as manganese, cobalt, iron, chromium, and nickel are usually used as a catalyst. Further, bromine compounds may possibly be used as a co-catalyst. In the case where no bromine compound catalyst is used, acetaldehyde, methyl ethyl ketone, etc. are used as an accelerator for the cobalt catalyst. As an oxidizing agent, is used molecular oxygen, and usually air. Air in which oxygen gas is mixed to increase the oxygen concentration, or air in which an inert gas such as nitrogen gas is mixed to conversely decrease the oxygen concentration, can be used.
As the reaction temperature of the liquid phase oxidation, the range of from 120 °C to 220 °C is usually employed, and as the pressure, the range wherein the acetic acid as the solvent can kept the liquid phase or more may be employed. In the oxidation method in which no bromine compound catalyst is used, the reaction temperature is in general not higher than 160 °C. An oxidation reaction heat is mainly removed by flash evaporation of a water-containing acetic acid solvent. That is, the waste gas from the oxidation reactor mainly


contains evaporated acetic acid and water and additionally contains slight amounts of low-boiling products of oxidation reaction by-products and an unreacted alkyl-substitute aromatic hydrocarbon. This vapor is cooled and condensed into a liquid by a condenser and then again refluxed as the oxidation reaction solvent into the oxidation reactor. For the purpose of removing water formed by the oxidation reaction, the water-containing acetic acid is delivered into a dehydration column, and a part thereof is provided for azeotropic distillation of the invention.
The liquid phase oxidation of the alkyl-substituted aromatic hydrocarbon is usually carried out in one or more reactors. The reaction mixture after completion of the oxidation reaction is optionally delivered into one crystallizer or two or more continuous crystallizers in which the pressure is successively reduced, and cooled due to a flash cooling action with the solvent to temperatures corresponding to the respective pressures, and the major part of the formed aromatic carboxylic acid is crystallized as a crystal to form a slurry solution. The slurry solution is separated into a cake of aromatic carboxylic acid and an oxidation reaction mother liquor by a crystal separation measure such as the rotary vacuum filter method and the centrifugation method or other proper separation method. If desired, the cake of

aromatic carboxylic acid is rinsed with acetic acid or water, and the deposited solvent is removed by a dryer. If further desired, the cake of aromatic carboxylic acid is again slurried in the reaction mother liquor composed mainly of water and purified through a hydrolysis step, followed by crystallization, separation, rinsing and drying to obtain an aromatic carboxylic acid.
Fig. 1 is a flow chart showing one example of the distillation process for applying the method of the invention. 11 denotes an azeotropic distillation column, and 14 denotes its reboiler. Distillation objectives having a different composition are respectively fed from feed ports 41 and 42 having a different height, and an entrainer is fed from a line 15, thereby performing azeotropic distillation. Here, the distillation objective is a mixture containing at least two substances of a low-boiling substance (specifically water) and a high-boiling substance (specifically acetic acid), and in the method of the invention, a mixture having a low concentration of the low-boiling substance is fed from the lower feed port 42, and a mixture having a high concentration of the low-boiling substance is fed from the upper feed port.
The azeotropic mixture vapor containing the low-boiling substance and the entrainer is delivered into a condenser 12 from a column head of the azeotropic distillation column 11, condensed therein, and then

separated into two phases in a liquid-liquid separation tank 13. The separation measure is properly selected according to the nature of the azeotropic mixture. In the case where the azeotropic mixture is not separated into two phases in the liquid phase, a distillation column, etc are established as a step of separating the entrainer. In the process as shown in Fig. 1, in the liquid-liquid separation tank 13, the phase composed mainly of the low-boiling substance is delivered into a stripping column 21 through a line 17, and organic components are recovered therein and then utilized or disposed through a line 18. If desired, a line 15 and a line 16 are connected to the column head or middle stage of the azeotropic distillation column 11, and the number of lines thereof may be respectively one or plural. In a return line into the azeotropic distillation column 11, return of the liquid composed mainly of the entrainer and the distillation objective composed mainly of the low-boiling substance into the azeotropic distillation column 11 may be carried out using a common line or individual lines. A liquid composed mainly of the high-boiling substance in which the content of the low-boiling substance is reduced is extracted from the column bottom of the azeotropic distillation column 11 through a line 19. Further, a mixture composed mainly of water and the aromatic hydrocarbon is extracted from a line 22 of the middle

stage of the column.
In the method of the invention, as described above, by separately feeding the distillation objectives having a different concentration of the low-boiling substance, and specifically water into the azeotropic distillation column, it is possible to realize the operation with a reduced water reflux ratio or enhanced separation. As the water concentration in the mixture increases, the mixture has an action more closed to the water reflux liquid. For example, in comparison between the case where two distillation objectives having a different water concentration are fed and the case where these two are mixed and fed, even when the total amount of the respective components is equal, an enhanced operation can be realized in the case where the distillation objective is divided two and then fed. Further, this effect is not limited to the case where the division number is two, but the same effect is obtained even when the division number is three or more. Incidentally, during dividing the foregoing distillation objective, a difference in water concentration between the feed materials is usually set up at 5 % or more, preferably 10 % or more, and more preferably 20 % or more.
What passage is derived the divided feed material is not particularly limited in the case where, for example, it is derived from the production process of the same


aromatic carboxylic acid. Examples include ones obtained by condensation and recovery of a vapor in the reactor, ones obtained by recovery of acetic acid contained in a waste gas from the reaction step, crystallization step or separation step, recovered reaction mother liquors composed mainly of an aliphatic carboxylic acid and water or vapors thereof, ones recovered in the separation step, ones recovered from the purification step, and others. These may be all fed as they are, or they may be provided as feed material by selecting in a plural number, partly mixing to reduce the total feed number, or further dividing to increase the total feed number. Further, the respective feed materials may be the whole or a part thereof. These distillation objectives are composed mainly of an aliphatic carboxylic acid and water and contain them in concentrations of 1 % by weight or more, respectively. Moreover, the feed materials enumerated herein are not necessarily completely liquid but may contain vapors.
The feed position of the distillation objective is not particularly limited, except for feeding one having a higher water concentration from the upper side of the column in that order. Although the effect is different depending on the position relation of the divided feed material, the characteristic of the invention resides in the divided feeding itself, and the presence or absence

thereof brings about a large difference. Incidentally, the feed position may be optimized while taking as an index an entrainer concentration in the mixture composed mainly of water and an aromatic hydrocarbon, which is to be extracted from the middle stage of the column.
The extraction from the middle stage is carried out for the purpose of extracting the aromatic hydrocarbon accumulated within the column. Though the mixture composed mainly of water and an aromatic hydrocarbon is obtained by extraction from the middle stage, the mixture also contains the entrainer and its decomposition products. Since the entrainer as contained is a loss, the extraction from the middle stage is carried out so as to obtain a mixture having a reduced entrainer concentration. For example, when the extraction position is made to get down from the column head toward the lower side, the concentration of the aromatic hydrocarbon increases, and at the same time, the entrainer concentration is lowered. This tendency continues during the time when the condensate extracted from the middle stage forms a two-part liquid phases. When the position is further made to get down so that the condensate forms a one-part liquid phase, the concentration of the aromatic hydrocarbon is rapidly lowered, whereby the object of recovering the aromatic hydrocarbon cannot be substantially attained. Accordingly, for example, the extraction position is lower

than an intermediate point where a two-part liquid phase can be taken, and preferably in the vicinity of the lowest point where a two-part liquid phase can be taken. Since this lowest point is connected with the lower limit position of the azeotropic region, it is possible to set up the condition by moving the lower limit position of the azeotropic region while fixing the extraction position from the middle stage. For example, the concentrations of the aromatic hydrocarbon and entrainer in the extract from the middle stage are optimized by setting up the condition such that the extraction position from the middle stage is lower than an intermediate point of the azeotropic region, and preferably, the lower limit of the azeotropic region is positioned in the vicinity of the extraction position from the middle stage. Further, since the length of the azeotropic region occupying in the height direction of the column and the water reflux ratio also affect the composition of the extract from the middle stage, the respective conditions are determined so as to attain the targets. The longer the azeotropic region and the higher the water reflux ratio, the more advantageous the composition of the extract from the middle stage is. However, since these immanently include demerits such as an increase of column height and an increase of the energy to be used, they are set up within the tolerant range of minimum requirements for attaining the target of the


composition of the extract from the middle stage, without being excessively set up. Incidentally, the concentration of the aromatic hydrocarbon in the fraction (oil phase) extracted from the middle stage is preferably 50 % or more, more preferably 80 % or more, and further preferably 90 % or more. Further, the concentration of the entrainer in the oil phase extracted from the middle stage is preferably not more than 50 %, more preferably not more than 15 %, and further preferably not more than 5 %. Moreover, the mixture extracted from the middle stage is distinguished from the recovered material in the column head according to the concentration of the aromatic hydrocarbon in the oil phase. The mixture extracted from the middle stage has a higher concentration of the aromatic hydrocarbon in the oil phase than the recovered material in the column head.
According to the method of the invention, it is possible to carry out the operation with a reduced water reflux ratio or the extraction from the middle stage with a more advantageous composition. Usually, the water reflux ratio is set up at minimum from the viewpoint of energy loss. In the case of recovering the aromatic hydrocarbon by extraction from the middle stage, the water reflux ratio can be reduced to the concentration that the entrainer or aromatic hydrocarbon in the extract from the middle stage can tolerate, and this becomes a minimum


rater reflux ratio. According to the division of feed
iquid of the invention, a reduction of the entrainer
loncentration or an increase of the aromatic hydrocarbon
:oncentration in the extract from the middle stage is
ound so that it is possible to recover the aromatic
ydrocarbon from the extract from the middle stage in an
advantageous concentration. Thus, it is possible to
attain a reduction o£ the energy to be used in the
equipment for concentration of the aromatic hydrocarbon as
shown in WO 97/29068, or an omission of the equipment per
se. Further, after division of the feed liquid, it is
possible to again deteriorate the composition of the
extract from the middle stage within the tolerant range
and reduce the water reflux amount in tern. During this,
although the entrainer concentration in the extract from
the middle stage does not change, it is possible to reduce
the amount of heat to be used by reducing the water reflux
ratio. Moreover, it is also possible to find out a
compromised condition by reducing both the water reflux
ratio and the entrainer concentration.
The invention brings about larger effects in the azeotropic distillation, the condition of which is set up such that the amount of energy to be used becomes minimum. For example, in general, for further reducing the reflux ratio of a system to be practiced, a loss must be tolerated in the recovery of the aromatic hydrocarbon.

However, in the method of the invention, it is possible to reduce the amount of energy to be used without tolerating this demerit or to reduce the demerit per se.
As a further preferred embodiment of the above-described application example of the invention, is enumerated an operation method of the azeotropic distillation column giving a reduced minimum water reflux ratio, in which the concentration of the aliphatic sarboxylic acid in the fraction extracted from the column head of the azeotropic distillation column is not more than 1 %, and preferably not more than 1,000 ppm; the concentration of the entrainer in the liquid extracted from the column bottom of the azeotropic distillation column (bottom liquid) is not more than 100 ppm; and the concentration of the entrainer in the extract (fraction) from the middle stage is not more than 20 %, and preferably not more than 5 %.

The invention will be more specifically described selow with reference to the Examples. However, it should se construed that the invention is never limited to these Examples unless exceeding the gist of the invention.
Sxample 1
Using an acetic acid-containing water mixture as a

distillation objective and n-butyl acetate as an entrainer, the method of the invention was carried out by the continuous distillation method according to the process as shown in Fig. 2. p-Xylene was defined as an aromatic hydrocarbon to be extracted and recovered from the middle stage. A packed column having the number of theoretical plates of 17 was used as an azeotropic distillation column 11 for carrying out the azeotropic distillation.
As a method of initiating the continuous azeotropic distillation, a mixture consisting of 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 0.9 % by weight of methyl acetate was filled in the column bottom of the azeotropic distillation column 11 and heated by a reboiler 14 to undergo total reflux circulation. Thereafter, n-butyl acetate as the entrainer was gradually fed to form an azeotropic distillation system of three components of acetic acid-water-n-butyl acetate. After the azeotropic region had reached the desired range by adjusting the amount of the entrainer, it was started to feed the raw material from two raw material feed lines 41 and 42 provided in the azeotropic distillation column 11.
In the stationary state of the operation, the distillation objective to be fed comprises a feed material 1 consisting of 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 0.9 % by weight of methyl acetate and a feed material 2 consisting of 32.2 % by weight of acetic acid, 67.7 % by weight of water, and 0.2 % by weight of methyl acetate. The flow rate per unit time was set up at 435 parts by weight for the feed material 1 and 15 parts by weight for the feed material 2, respectively, and the feed port was established at a position of 13 % of the column height (line 42) for the feed material 1 and a position of 55 % of the column height (line 41) for the feed material 2, respectively on the column bottom standard. Concentrated acetic acid was extracted as a bottom liquid in an amount of 400 parts per unit time through a line 19 from the column bottom of the azeotropic distillation column 11. A vapor with an azeotropic composition containing water and the entrainer was obtained from the column head of the azeotropic distillation column 11, cooled in a condenser 12, and then recovered into a decanter 13. In a two-part liquid phase separated in the decanter 13, an aqueous phase liquid is separated into two parts of one to be discharged through a line 17 and one to be returned into the azeotropic distillation column 11 through a line 16. A reflux water amount was determined from an amount to be discharged (Ww) and an amount to be returned as the reflux water (Wr) such that a water reflux ratio defined in terms of Wr/Ww fell within the desired value. In this example, the operation

was carried out under conditions such that the amount of the aqueous phase liquid to be discharged (Ww) and the amount of the reflux water (Wr) are respectively 53 parts by weight and 42 parts by weight per unit time, with the water reflux ratio being 0.8. The entrainer was fed into the azeotropic distillation column 11 through a line 15 in an amount corresponding to the amount of the entrainer subjected to liquid-liquid separation in the decanter 13 and then extracted through a line 18. Further, the extraction position from the middle stage through a line 22 was set up at 43 % of the column height on the column bottom standard, and the lower limit of the azeotropic region was set up within + 3 % of 39 %. For controlling the lower limit position, the feed rate of the entrainer was adjusted. The extraction amount from the middle stage was set up at 2 parts by weight per unit time. The results are shown in Table 1.
In the state where the water reflux ratio was low as 0.8, there was obtained a good state that the content of the entrainer in the oil phase extract from the middle stage was 0.6 %. Incidentally, the oil phase contained 97 % of p-xylene, and the aqueous phase did not substantially contain p-xylene, the amount of which was not more than 1 %.
Comparative Example 1

The same procedures as in Example 1 were followed,
except that in Example 1, the feed materials 1 and 2 were
mixed and fed at a position of 13 % of the column height
(line 42) on the column bottom standard. The results are
shown in Table 1.
It was found that the concentration of the en trainer extracted from the middle stage was largely deteriorated. This result means that in the case where it is intended to carry out the operation at the same water reflux ratio, i.e., the same energy, up to 70 times of the loss of the entrainer tolerated in Example 1 must be tolerated.
Comparative Example 2
The same procedures as in Example 1 were followed, except that in Example 1, the feed materials 1 and 2 were mixed and fed at a position of 13 % of the column height (line 42) on the column bottom standard and that the water reflux ratio was increased to 1.3. Though in Comparative Example 1, the concentration of the entrainer extracted from the middle stage changed, Comparative Example 2 was practiced for the purpose of verifying the water reflux ratio necessary for not generating a change of the entrainer concentration. The results are shown in Table 1.
As a result, it was confirmed that in order to generate no change in the entrainer concentration, the amount of energy to be used should be largely increased

such that the water reflux ratio be 1.6 times.
Thus, it was confirmed that either the energy or entrainer loss must be tolerated in the case of no divided feeding. Accordingly, it can be said that the divided feeding is an effective measure for providing an improved process.
Table 1

Division of
liquid
feeding Water reflux ratio Concentration
of entrainer
from the middle
stage
Example 1 Yes 0.8 0.6 %
Comparative Example 1 No 0.8 43 %
Comparative Example 2 No 1.3 0.7 %
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
This application is based on a Japanese patent application filed February 27, 2001 (Japanese Patent Application No. 2001-051554), the contents of which are incorporated herein by reference.


WE CLAIM:
1. A method of azeotropic distillation of distillation objectives containing two substances of a substance having a lower boiling point (hereinafter referred to as "low-boiling substance" ) and a substance having a higher boiling point (hereinafter referred to as "high-boiling substance") using an entrainer capable of forming an azeotropic mixture together with at least one of the substances, said distillation objectives containing the low-boiling substance in an amount of 4 to 99% by weight and the high boiling substance in an amount of 1% by weight or more, wherein the method comprises:
(1) feeding the distillation objectives into an azeotropic distillation column from at least two feed ports having a different height;
(2) feeding a distillation objective having a low concentration of the low-boiling substance from the lower feed port of the two feed ports and feeding a distillation objective having a high concentration of the low-boiling substance from the upper feed port thereof; and
(3) extracting at least three kinds of fractions having a different boiling point as distillates from the azeotropic distillation column such that a fraction having the lowest boiling point is obtained from a column head, a fraction having the highest boiling point from a column bottom, and an intermediate fraction from the middle stage, respectively, and
characterized in that the fraction extracted from the column head has the high-boiling substance concentration of not more that 1%, the liquid extracted from the column bottom has an entrainer concentration of not more than 100 ppm, and the intermediate fraction has an entrainer concentration of not more than 20%, wherein an oil phase extracted from the middle stage has an aromatic hydrocarbon concentration of 50% or more.
2. The method of azeotropic distillation as claimed in claim 1, wherein the low-boiling substance is water, the high-boiling substance is an aliphatic carboxylic acid, and the distillation objective contains an aromatic hydrocarbon.

3. The method of azeotropic distillation as claimed in any of the preceding claims, the distillation objective is one generated from the production process of aromatic carboxylic acid.
4. The method of azeotropic distillation as claimed in any of the preceding claims, wherein in addition to the distillation objective, the reflux of the entrainer is fed to the azeotropic distillation column.
5. The method of azeotropic distillation as claimed in any of the preceding claims, wherein in addition to the distillation objective, the reflux of water is fed to the azeotropic distillation column.
6. A process of producing an aromatic carboxylic acid including oxidizing an alkyl-substituted aromatic hydrocarbon in an aliphatic carboxylic acid-containing reaction medium, wherein it comprises carrying out the azeotropic distillation as claimed in any of the preceding claims using an aliphatic carboxyl acid containing water generated by oxidation reaction as a distillation objective.
7. The process of producing an aromatic carboxylic acid as claimed in claim 6, wherein the aliphatic carboxylic acid is acetic acid, the alkyl-substituted aromatic hydrocarbon is p-xylene, and the aromatic carboxylic acid is terephthalic acid.
Dated this August 08, 2003.
OF REMFRY AND SAGAR ATTORNEY FOR THE APPLICANTS

Documents:

759-mumnp-2003-cancelled page(02-09-2007).pdf

759-mumnp-2003-claim(granted)-(08-09-2007).doc

759-mumnp-2003-claim(granted)-(08-09-2007).pdf

759-mumnp-2003-correspondence(14-09-2007).pdf

759-mumnp-2003-correspondence(ipo)-(22-08-2007).pdf

759-mumnp-2003-drawing(08-09-2007).pdf

759-mumnp-2003-form 1(28-02-2007).pdf

759-mumnp-2003-form 18(02-12-2005).pdf

759-mumnp-2003-form 2(granted)-(08-09-2007).doc

759-mumnp-2003-form 2(granted)-(08-09-2007).pdf

759-mumnp-2003-form 3(08-08-2003).pdf

759-mumnp-2003-form 3(27-10-2003).pdf

759-mumnp-2003-form 3(28-02-2007).pdf

759-mumnp-2003-form 5(08-08-2003).pdf

759-mumnp-2003-form-pct-ipea-409(07-08-2003).pdf

759-mumnp-2003-petition under rule 137(28-02-2007).pdf

759-mumnp-2003-power of authoirty(08-08-2003).pdf

759-mumnp-2003-power of authoirty(28-02-2007).pdf

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Patent Number 214035
Indian Patent Application Number 759/MUMNP/2003
PG Journal Number 13/2008
Publication Date 28-Mar-2008
Grant Date 24-Jan-2008
Date of Filing 08-Aug-2003
Name of Patentee MITSUBISHI CHEMICAL CORPORATION
Applicant Address 5-2, MARUNOUCHI 2- CHOME, CHIYODA-KU, TOKYO 100-0005,
Inventors:
# Inventor's Name Inventor's Address
1 MOTOKI NUMATA 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI, FUKUOKA 806-0004,
2 TAKAYUKI ISOGAI 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI, FUKUOKA 806-0004,
PCT International Classification Number B01D 3/36
PCT International Application Number PCT/JP01/11382
PCT International Filing date 2001-12-25
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 2001-051554 2001-02-27 Japan