Title of Invention	AN AZEOTROPIC DISTILLATION METHOD
Abstract	An azeotropic distillation method including at least the following steps (1) to (5): (1) a step of azeotropically distilling an objective distillation solution containing water, an saturated aliphatic carboxylic acid having from 2 to 4 carbon atoms and an aliphatic carboxylic acid ester having from 2 to 4 carbon atoms using an entrainer selected from the group consisting of butyl formate, n-propyl acetate, isobutyl acetate, n-butyl acetate, amyl acetate, n-butyl propionate and isobutyl propionate, to obtain an aliphatic carboxylic acid with a reduced water content and a water concentrated overhead distillate; (2) a step of condensing the overhead distillate to obtain a gas and a condensate Separating into two phases of an aqueous phase and an oil phase; (3) a step of dispensing the aqueous phase and the oil phase of the condensate; (4) a step of distilling a part or the whole of the aqueous phase obtained by the dispensation to obtain an overhead distillate containing the aliphatic carboxylic acid ester and with a reduced water content; and (5) a step of distilling the overhead distillate obtained in the step (4) together with a part or the whole of the oil phase dispensed in the step (3) and recovering a part or the whole of the said aliphatic carboxylic acid ester obtained by the distillation outside the system.

Title of Invention

AN AZEOTROPIC DISTILLATION METHOD

Abstract

An azeotropic distillation method including at least the following steps (1) to (5): (1) a step of azeotropically distilling an objective distillation solution containing water, an saturated aliphatic carboxylic acid having from 2 to 4 carbon atoms and an aliphatic carboxylic acid ester having from 2 to 4 carbon atoms using an entrainer selected from the group consisting of butyl formate, n-propyl acetate, isobutyl acetate, n-butyl acetate, amyl acetate, n-butyl propionate and isobutyl propionate, to obtain an aliphatic carboxylic acid with a reduced water content and a water concentrated overhead distillate; (2) a step of condensing the overhead distillate to obtain a gas and a condensate Separating into two phases of an aqueous phase and an oil phase; (3) a step of dispensing the aqueous phase and the oil phase of the condensate; (4) a step of distilling a part or the whole of the aqueous phase obtained by the dispensation to obtain an overhead distillate containing the aliphatic carboxylic acid ester and with a reduced water content; and (5) a step of distilling the overhead distillate obtained in the step (4) together with a part or the whole of the oil phase dispensed in the step (3) and recovering a part or the whole of the said aliphatic carboxylic acid ester obtained by the distillation outside the system.

Full Text	FORM 2 THE PATENTS ACT 1970 [39 OF 1970] COMPLETE SPECIFICATION [See Section 10 ; rule 13] AN 'AZEOTROPIC DISTILLATION METHOD" MITSUBISHI CHEMICAL CORPORATION, of 5-2, Marunouchi 2-chome, Chiyoda-ku, Tokyo 100-0005, Japan, The following specification particularly describes the nature of the invention and the manner in which it is to be performed:- GRANTED 9-3-2005 ORIGINAL 758/MUMNP/2003 DESCRIPTION The present invention relates to an azeotropic distillation method. Especially, the invention gives rise to an effect 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. 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 solvent. 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 a mixed solution of acetic acid and water, followed by azeotropic distillation to obtain acetic acid having a high purity from the mixed solution 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 solvent 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 solvent containing aliphatic carboxylic acids such as acetic acid. However, since water is formed during the production step, it is necessary to prevent accumulation of water in the reaction system. For achieving this, there is employed an operation in which a mixed vapor 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 solvent will be noticed. Usually, rectification is used in the separation of water from a mixture of acetic acid and water. However, azeotropic distillation is advantageous depending on the cost of equipment and the cost 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 a condensate of an overhead distillate of an azeotropic distillation column. 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 liquid, the kind of entrainer, the feed position (height) of a feed liquid into an azeotropic distillation column, the number of liquid feed lines, the method of 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 The azeotropic distillation method for separation of acetic acid from water is also disclosed in JP-B-62-41219, KR-A-94-14292, JP-A-2000-72714, WO 98/45239, etc. As a method in which after subjecting an overhead vapor condensate of an azeotropic distillation column to liquid-liquid separation, oil phase components to be entrained in the aqueous phase side are reduced, JP-B-62-41219 pays attention especially to isobutyl acetate and aliphatic carboxylic acid esters as impurities as oil phase components and strips an aqueous phase liquid to recover them. However, this method is aimed at a countermeasure of a loss due to the entrainment of the oil phase components into the aqueous phase but is not a technology for overcoming all of the problems derived from the aliphatic carboxylic acid esters as impurities. The above-described aliphatic carboxylic acid esters are Entrained with the feed liquid, enter a distillation column, and are accumulated in the oil phase. The method described in JP-B-62-41219 is a technology for preventing oss caused by the entrainment of. a part of 'the accumulated aliphatic carboxylic ,acid..esters as impurities from the aqueous phase but does not mention a problem such that the above-described aliphatic carboxylic acid esters are accumulated in the oil phase, whereby the concentration thereof increases. The increase of the concentration of the aliphaticcarboxylic acid esters deteriorates the separation property between acetic acid and water in azeotropic distillation to cause a loss of the effective component (acetic acid). Since then, respective corporations have proposed technologies for preventing the accumulation of impurities (aliphatic carboxylic acid esters) in the oil phase. For example, KR-A-94-14292 proposes a method in which after subjecting a condensate of an overhead vapor of an azeotropic distillation column to liquid-liquid separation, the aliphatic carboxylic acid esters as impurities to be entrained in the oil phase side are distilled and then recovered. However, this method involves a problem such that since the separation property of the carboxylic acid esters from the entrainer in the oil phase is poor, a large amount of energy is required for the separation of the aliphatic carboxylic acid esters. Further, JP-A-2000-72714 proposes a method in which an overhead vapor of an azeotropic distillation column is partially condensed, and aliphatic carboxylic acid esters as impurities in a residual gas are absorbed in acetic acid and recovered. However, according to this method, since the entrainer component is entrained by the partial condensation, it is impossible to take out only the aliphatic carboxylic acid esters, leading to a loss of the effective component (entrainer component) and contamination of the product. WO 98/45239 proposes a method in which an overhead vapor of an azeotropic distillation column is partially condensed, and the residual gas is distilled to recover aliphatic carboxylic acid esters as impurities. While this technology brings about an improvement on the point of the amount of heat to be used, it is impossible to isolate the aliphatic carboxylic acid esters, and it involves the same problem as the technologies described in other patents on the point of providing an exclusive passage for recovery. Further, in the subject patent publication, since the water concentration in the azeotropic composition is low, there is involved a problem such that for removing water by azeotropic distillation, a large amount of the entrainer must be used. Though the amount of the entrainer to be used in the azeotropic distillation is determined according to the azeotropic composition, it is in general one time or more, for example, from 2 to 5 times in terms of a weight ratio against water. Accordingly, in addition to the matter that the amount of the entrainer per se is large, since the entrainer is in general used upon circulation, materials that have a lower boiling point than acetic acid and are sparingly soluble in water (for example, methyl acetate and p-xylene in the production of terephthalic acid) are accumulated in the entrainer side. Thus, when the amount of the contained impurities increases, the amount of the oil phase largely increases. For this reason, a tolerant width wherein the aliphatic carboxylic acid esters as one kind of impurities can be contained is narrow. In the light of the above, with respect to the recovery of aliphatic carboxylic acid esters as impurities, it is preferred to correspond to all of problems such that entering and coming out of the distillation system be balanced; the amount of energy to be used for separation and recovery be decreased; recovery with a high purity be performed; and a loss of fluctuation be avoided by simple equipment. For achieving this, equipment exclusive for recovery of aliphatic carboxylic acid esters as impurities has been proposed. Under these circumstances, the invention has been made based on a new way of thinking attained from the point to which an attention is paid regarding a ratio of the concentration of aliphatic carboxylic acid esters as impurities to the concentration of an entrainer in an aqueous phase of a condensate obtained by condensing an overhead distillate of an azeotropic distillation column. Though it is known that the recovery of organic components from the aqueous phase is a substantially essential operation, the invention has found a technology for positively applying it and depressing an increase of the aliphatic carboxylic acid esters as impurities. And, the invention is constructed from this technology and is to provide an azeotropic distillation method capable of depressing the amount of energy to be used to a low level and giving rise to an excellent separation performance by introducing a simple process. The invention relates to recovery of aliphatic carboxylic acid esters as impurities accumulated in an entrainer present within an azeotropic distillation column in the operation of azeotropic distillation. The present inventors paid attention to the matter that a ratio of the concentration of the aliphatic carboxylic acid esters to the concentration of the entrainer in an aqueous phase of a condensate of an overhead distillate, of the azeotropic distillation column is high as compared with that in an oil phase and found that by recovering the aliphatic carboxylic acid esters from the aqueous phase, the aliphatic carboxylic acid esters can be recovered simply and with a low energy as compared with a method of recovery from the passage to which attention has hitherto been paid. The invention has been attained based on these findings. Specifically, the gist of the invention resides in an azeotropic distillation method including at least the following steps (1) to (5): Step (1): A step of azeotropically distilling an objective distillation solution containing water, an aliphatic carboxylic acid and an aliphatic carboxylic acid ester using an entrainer, to obtain an aliphatic carboxylic acid with a reduced water content and a water-concentrated overhead distillate; Step (2): A step of condensing the overhead distillate to obtain a gas and a condensate separating into two phases of an aqueous phase and an oil phase; Step (3) : A step of dispensing the aqueous phase and the oil phase of the condensate; Step (4) : A step of distilling a part or the whole of the aqueous phase obtained by the dispensation to obtain an overhead distillate containing the aliphatic carboxylic acid ester and with a reduced water content; and Step (5) : A step of distilling the overhead distillate obtained in the step (4) together with a part or the whole of the oil phase dispensed in the step (3) and recovering a part or the whole of the aliphatic carboxylic acid ester obtained by the distillation outside the system. Fig. 1 is a flow chart showing one example of the distillation process for carrying out the invention. In the drawing, reference numerals 11, 13, 21 and 24 denote an azeotropic distillation column, a liquid-liquid separation tank (decanter), a stripping column and an entrainer recovery column, respectively. The mode for carrying out the invention will be described below in detail. In this description, the term "objective distillation solution" means a mixed solution containing a desired substance to be purified and a substance the concentration of which is to be reduced (hereinafter referred to as "substance to be reduced") . 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. The term "azeotropic distillation column" means a distillation column for distilling the foregoing objective distillation sortition and entrainer. [Description of the step (1)] In the invention, first of all, the step (1) includes a step of azeotropically distilling an objective distillation solution containing water, an aliphatic carboxylic acid and an aliphatic carboxylic acid ester using an entrainer, to obtain the desired substance, aliphatic carboxylic acid with a reduced water content and a water-concentrated overhead distillate. For reducing the concentration of water in the mixture containing an aliphatic carboxylic acid and water by azeotropic distillation, distillation is in general carried out so as to obtain a bottom liquid containing an aliphatic carboxylic acid with a reduced amount of water from the column bottom and to obtain a vapor of an azeotropic mixture mainly containing water and the entrainer from the column head. During this, for the purposes of reuse of the aliphatic carboxylic acid, etc., 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 condensate of the overhead distillate of the azeotropic distillation column 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 an alkyl-substituted aromatic hydrocarbon. Next, the objective distillation solution and the entrainer will be described. In the invention, the objective distillation solution is not particularly limited so far as it is a mixed solution containing the desired substance and the substance to be reduced, and the entrainer to be added and the substance to be reduced form an azeotropic mixture, and the azeotropic temperature thereof is lower than the boiling point of the desired substance. Further, the objective distillation solution may further contain a substance that does not substantially affect the subject azeotropic distillation. 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 the substance to be reduced, 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 objective distillation solution 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. The concentration of the aliphatic carboxylic acid ester in the azeotropic distillation is arbitrary, but it is preferred that the aliphatic carboxylic acid ester is contained in an amount of from 3 to 50 % by weight in the oil phase containing the entrainer on the point of separating water from the aliphatic carboxylic acid such as acetic acid stably and in a high concentration. In the invention, the desired substance to be purified is an aliphatic carboxylic acid, and is usually a saturated or unsaturated aliphatic carboxylic acid having from 2 to 6 carbon atoms. Preferred examples include saturated aliphatic carboxylic acids having from 2 to 4 carbon atoms such as acetic acid, propionic acid, and butyric acid. The substance to be reduced, the concentration of which is to be reduced, is water. Further, in the invention, the kind of aliphatic carboxylic acid esters as impurities contained in the objective distillation solution is not particularly limited, but it usually includes esters of the desired substance, aliphatic carboxylic acid, and specifically, for example, methyl acetate. The entrainer 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 mixed solutions containing 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_Jbutyl 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 chlprobenzene; 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. Incidentally, in the invention, in the case where the objective distillation solution is a solvent used in the production of aromatic carboxylic acids such as terephthalic acid, the objective distillation solution may contain other substances derived from the raw materials of the aromatic carboxylic acids, such as p-xylene and methyl acetate, and decomposition products of the entrainer. The composition of the aliphatic carboxylic acid and water in the objective distillation solution is arbitrary, but the invention is usually applied to mixed solutions containing 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 objective distillation solutions 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 of an alkyl-substituted aromatic hydrocarbon in an aliphatic carboxylic acid-containing solvent. For example, ones obtained by condensation and recovery of mixed vapors of an alkyl-substituted aromatic hydrocarbon from the liquid phase oxidation reactor, ones obtained by absorbing an acetic acid vapor in the waste gas with water from the foregoing reactor, ones obtained by evaporating 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 subject step and then recovered, are arbitrarily selected and used along with the amounts. These solutions may be mixed and individually treated. [Description of the steps (2) and (3)] In the invention, subsequent to the step (1) , the step ' (2) includes a step of condensing the overhead distillate to obtain a gas and a condensate separating into two phases of an aqueous phase and an oil phase; and the step (3) includes a step of individually dispensing each of the phases of the condensate. In the invention, according to the azeotropic distillation of the step (1), a bottom liquid containing the desired substance with a reduced concentration of the substance to be reduced is obtained from the column bottom, and a vapor of the azeotropic mixture (overhead distillate) composed mainly of the substance to be reduced and the entrainer is obtained from the column head. As the step (2) , the vapor obtained from the column head is condensed to obtain a condensate and a gas. The condensate is usually obtained as a two-phase liquid of an aqueous phase composed mainly of the substance to be reduced and an oil phase composed mainly of the entrainer. Subsequently, as the step (3) , this two-phase condensate is dispensed into the aqueous phase and the oil phase. As dispensation measures is numerated liquid-liquid separation by a decanter or the like, but there are no particular limitations so far as the object can be attained. For making it possible to dispense the two phases, it is preferred to use a substance to be reduced and an entrainer capable of giving a heterogeneous azeotropic mixture such that the aqueous phase and the oil phase constituting the condensate are not mixed homogeneously with each other. In the aqueous phase and the oil phase of the condensate obtained in the step (2) , it is preferred that the amount of the aliphatic carboxylic acid ester recovered from the oil phase is not more than 20 times the amount of the aliphatic carboxylic acid ester recovered from the aqueous phase. This is, for example, obtained by " (flow rate per unit time of the oil phase) x (concentration of the aliphatic carboxylic acid ester in the oil phase)" and " (flow rate per unit time of the aqueous phase) x (concentration of the aliphatic carboxylic acid ester in the aqueous phase)". When the foregoing value exceeds 20 times, the amount of the aliphatic carboxylic acid ester that can be recovered from the aqueous phase is low so that the effect of the invention may possibly be low. Especially, the value is preferably not more than 18 times, and particularly preferably not more than 15 times. Of the dispensed two phases, at least a part of the phase (oil phase) composed mainly of the entrainer is preferably 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. 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 azeotropic distillation column and the composition of the azeotropic mixture. Actually, the optimum amount of the entrainer may be determined from the concentration of the aliphatic carboxylic acid in the condensate of the overhead distillate of the azeotropic distillation column and the concentration of the entrainer in the bottom liquid. Incidentally, the entrainer may be newly fed as compensation for a loss of the entrainer come out the system. On tne other hand, at least a part of the phase (aqueous phase) composed mainly of the substance to be reduced is provided in the step (4) described later. The residual aqueous phase may be returned as a reflux liquid into the azeotropic distillation column. Incidentally, the whole of the aqueous phase may be provided in the step (4) , a part of which is then returned as a reflux liquid into the azeotropic distillation column. Further, water as the main component in the phase (aqueous phase) composed mainly of the substance to be reduced may be reused within the process, a part of which is then disposed. Examples of the method of returning water as the reflux liquid include a method of returning it into the column head of the azeotropic distillation column and a method of returning it into the middle stage of the column. 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). [Description of the step (4)] In the step (4), a part or the whole of the dispensed aqueous phase is distilled to obtain an overhead distillate containing the aliphatic carboxylic acid ester with a reduced water content. Specific examples include a method in which the aqueous phase (composed mainly of water as the substance to be reduced) dispensed from the condensate of the overhead distillate of the azeotropic distillation column is delivered into a distillation column, and the aliphatic carboxylic acid with a reduced water content is obtained from a column head of the distillation column. A material obtained from a column bottom of the distillation column by the distillation is mainly water. This water may be disposed outside the system, or may be partially returned as a reflux liquid into the azeotropic distillation column. [Description of the step (5)] In the step (5) , the overhead distillate obtained in the step (4) is distilled together with a part or the whole of the oil phase dispensed in the step (3) , and a part or the whole of the aliphatic carboxylic acid ester obtained by the distillation is recovered outside the system. In the step (5) , the distillation column is aimed at recovery of the entrainer. In the step (5) , the overhead distillate of the step (4) and a part or the whole of the oil phase dispensed in the step (3) from the condensate of the overhead distillate of the azeotropic distillation column are fed into the distillation column and distilled therein. By this distillation, the entrainer is separated from the aliphatic carboxylic acid ester as an impurity. Incidentally, in this step, it is preferred to control the feeding of components to be distilled into the step (5) such that 20 % or more of the aliphatic carboxylic acid ester is derived from the overhead material obtained in the step (4) on the point of energy efficiency. The entrainer is recovered from the column bottom, and a part or the whole thereof may be recovered outside the system, or a part or the whole thereof may be returned into the azeotropic distillation column. Further, a part or the whole of the aliphatic carboxylic acid ester as an impurity may be recovered outside the system. Further, in the step (5) , as oil phase components other than a part or the whole of the dispensed oil phase, the condensate of the overhead distillate of the azeotropic distillation column (namely, a mixture of the entrainer and the aliphatic carboxylic acid ester) and the gas obtained in the step (2) may be fed into the distillation column along with the oil phase. Incidentally, with respect to the oil phase dispensed in the step (3) and the entrainer obtained in the step (5), there may be employed a method in which the circulating flow of the entrainer into the azeotropic distillation column is divided, one of the divided flows is returned into the column head of the azeotropic distillation column, 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, with respect to the water obtained in the step (2) or step (4), there may be employed a method in which water is returned as the reflux liquid into the azeotropic distillation column 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, for recovering the alkyl-substituted aromatic hydrocarbons accumulated in the entrainer, they may be directly separated from the entrainer, or extracted from the middle stage of the azeotropic distillation column as described in JP-T-10-504556. Additionally, the extract can be purified as described in WO 97/29068. Next, the production process of aromatic carboxylic acids of the invention, 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. The aromatic carboxylic acid as the desired compound means an arbitrary aromatic carboxylic acid. Examples include aromatic monocarboxylic acids, aromatic dicarboxylic acids, and aromatic tricarboxylic acids. For example, when these have a benzene ring as the aromatic ring, they are produced by oxidation, and preferably liquid phase oxidation of corresponding alkyl-substituted aromatic hydrocarbons such as monoalkylbenzenes, dialkylbenzenes, and trialkylbenzenes. Especially, it is preferred to apply the method of the invention in the case of the aromatic dicarboxylic acid, especially 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. In carrying out the oxidation reaction in the liquid phase, an aliphatic carboxylic acid is usually used as a solvent. 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, a catalyst is usually used. Examples of the catalyst include transition metal compounds such as manganese, cobalt, iron, chromium, and nickel. Further, bromine compounds may 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 or air, 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. The reaction temperature of the liquid phase oxidation may be properly selected but is usually from 120 °C to 220 °C. For example, in the oxidation method in which no bromine compound catalyst is used, the preferable reaction temperature is in general not higher than 160 °C. Similarly, the pressure may be properly selected and may be within the range where the solvent (such as acetic acid) can be present in the gas state. An oxidation reaction heat is mainly removed by flash evaporation of a water-containing acetic acid solvent. That is, the fraction (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, a part of which is usually then again refluxed as the oxidation reaction solvent into the oxidation reactor. Further, for the purpose of removing water formed by the oxidation reaction, other part thereof is delivered into a dehydration column and provided for azeotropic distillation of the invention, and dehydrated acetic acid is again delivered as the oxidation reaction solvent into the oxidation reactor. The liquid phase oxidation of the alkyl-substituted aromatic hydrocarbon is usually carried out in one reactor, or plural reactors, if desired. 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. The slurry is separated into a cake of an aromatic carboxylic acid crystal and an oxidation reaction mother liquor by an arbitrary crystal separation measure such as the rotary vacuum filter method and the centrifugation method or other proper separation method. If desired, the cake of the aromatic carboxylic acid crystal is rinsed with acetic acid or water, and the deposited solvent is removed by a dryer. If further desired, the resulting aromatic carboxylic acid crystal is again slurried in the reaction mother liquor composed mainly of water and recrystallized through a hydrolysis step to reduce impurities in the crystal, thereby achieving purification. Then, operations such as solid-liquid separation, rinsing, and drying are carried out to obtain an aromatic carboxylic acid. Next, the azeotropic distillation method of the invention and the production process of aromatic carboxylic acids using it will be described with reference to Fig. 1. Fig. 1 is a flow chart showing one example of the distillation process for carrying out the invention. 11 denotes an azeotropic distillation column. An objective distillation solution containing water, an aliphatic carboxylic acid (such as acetic acid) and an aliphatic carboxylic acid ester as an impurity are fed into the azeotropic distillation column through lines 40, 41, 42, etc. (in the case of feeding objective distillation solutions through a plurality of lines, the composition of each of the objective distillation solutions is usually different from each other), and an entrainer is fed thereinto through a line 15, thereby carrying out azeotropic distillation. For adding a heat necessary for the distillation, heating is carried out using a heat exchanger 14. As heating sources are used heated oils and heated vapors. Here, an example of using a vapor having a temperature higher than the boiling point at atmospheric pressure of acetic acid, specifically 0.35 MPa vapor is shown. The azeotropic mixture vapor (overhead distillate of the azeotropic distillation column) containing the substance to be reduced, the concentration of which is to be reduced by azeotropic distillation, and the entrainer is delivered into a condenser 12 from a column head of the azeotropic distillation column 11 and condensed therein. The resulting condensate is separated into two phases in a liquid-liquid separation tank 13, followed by dispensation. The separation measure is properly selected according to the nature of the azeotropic mixture. In Fig. l in the liquid-liquid separation tank 13, a part of the phase (aqueous phase) composed mainly of the substance to be reduced is delivered into a stripping column 21 through a line 17. Further, a part of the aqueous phase may be made a water reflux into the azeotropic distillation column 11 through a line 16. The entrainer is circulated through the line 15. If desired, the line 15 and the line 16 are connected to the column head or middle stage of the azeotropic distillation column 11. The line number of each of the lines 15 and 16 into the azeotropic distillation column 11 may be one or plural Further, the entrainer and the substance to be reduced may be returned together into the azeotropic distillation column 11 using a common line or individual lines. A liquid composed mainly of the desired substance (for example, an aliphatic carboxylic acid) with a reduced water content is taken out from the column bottom of the azeotropic distillation column 11 through a line 19. Further, when the desired substance is the aliphatic carboxylic acid used in the production process of the aromatic carboxylic acid of the invention, a liquid composed mainly of water and an alkyl-substituted aromatic hydrocarbon as the raw material of aromatic carboxylic acid is extracted from a line 22 of the middle stage of the column. In the stripping column 21, the organic component (mainly an aliphatic carboxylic acid ester as an impurity in the production process of aromatic carboxylic acid of the invention) is recovered, and the substance to be reduced, which is separated by azeotropic distillation, such as water, is circulated into the production process of aromatic carboxylic acid through a line 18 and effectively utilized there or disposed. The organic component recovered in the stripping column 21 is delivered into an entrainer recovery column 24 through a line 23 and separated therein into the entrainer and the aliphatic carboxylic acid ester as an impurity. The entrainer is recovered from the column bottom and returned into the azeotropic distillation column 11 through lines 25 and 15. The aliphatic carboxylic acid ester is recovered through a line 26. The aliphatic carboxylic acid ester may be stored, or a part or the whole thereof may be circulated in the oxidation reaction step for producing aromatic carboxylic acids according to the desire. Further, at least a part of the mixture of the entrainer and the aliphatic carboxylic acid ester branched in a line 27 is directly introduced into the entrainer recovery column 24. Moreover, it is also possible to introduce a gas generated in the condenser 12 into the entrainer recovery column 24, though such is not illustrated. The method of recovering the aliphatic carboxylic acid ester as an impurity according to the invention will be specifically described with reference to Fig. 1. The amount of the aliphatic carboxylic acid ester to be recovered outside the system, i.e., the amount as obtained in the step (5), is expressed as an amount of the subject substance to be discharged from the line 26. The discharge flow may contain the entrainer, water and impurities such as aliphatic hydrocarbons and is a liquid or gas or a mixture thereof. The amount of the aliphatic carboxylic acid ester to be introduced into the recovery step via the aqueous phase liquid of the condensate phase of the overhead distillate of the azeotropic distillation column 11 is expressed as an amount of the subject substance flowing through the line 17. It is preferred from the viewpoint of energy efficiency to introduce the aliphatic carboxylic acid ester into the recovery step through the aqueous phase at a recover rate of 20 % or more, preferably 30 % or more, and more preferably 40 % or more against the total amount of the aliphatic carboxylic acid ester to be recovered outside the system. Incidentally, in the entrainer recovery step, the aliphatic carboxylic acid ester can be entrained even in an extremely low amount in a bottom liquid 25 of the entrainer recovery column 24. According to the invention, the above-described recovery rate is calculated in terms of a rate of the amount of the aliphatic carboxylic acid ester via the aqueous phase in the amount of the aliphatic carboxylic acid ester to be introduced in the recovery step via the aqueous phase and the oil phase. A flow of the aliphatic carboxylic acid ester not via the aqueous phase is introduced into the entrainer recovery column 24 through the line 27. This flow may be any of a gas or a liquid or a mixture thereof and can be used in combination of a gas generated in the condenser 12 as described in WO 98/45239. Incidentally, a concentration ratio of the aliphatic carboxylic acid ester as an impurity to the entrainer in the oil phase to be extracted from the liquid-liquid separation tank 13 is usually from 1:100 to 1:2, and it is disadvantageous to recover the aliphatic carboxylic acid ester only from the oil phase. On the other hand, turning now to the concentration in the aqueous phase, this ratio is usually from 1:1 to 15:1, and such is advantageous for recovery of the aliphatic carboxylic acid ester. Accordingly, when the recovery rate of the aliphatic carboxylic acid ester from the aqueous phase decreases, a load in the recovery step increases, and when the recovery rate is too low, an increase of the amount of energy to be used becomes remarkable. Accordingly, more preferably, the recovery rate is 20 % or more. It is possible to adjust the recovery rate by adjusting the flow rate in the passage shown by the line 27 or the flow rate of the passage shown by a line 28. Incidentally, in the case of adjustment by the passage shown by the line 28, it is preferred to adjust the recovery rate such that the total amount of the water reflux does not change. Further, it is possible to recirculate water discharged from the stripping column 21 into the liquid-liquid separation tank 13 to increase the recovery rate. Incidentally, in the invention, for example, during the liquid-liquid separation in the liquid-liquid separation tank 13, with respect to distribution of the aliphatic carboxylic acid ester as an impurity, the higher the rate expressed by (total amount of the subject substance in the aqueous phase)/(total amount of the subject substance in the oil phase), the larger the effect of the method of the invention is. Such can be, for example, obtained by [(flow rate per unit time of the aqueous phase) x (concentration of the aliphatic carboxylic acid ester in the aqueous phase)]/[(flow rate per unit time of the oil phase) x (concentration of the aliphatic carboxylic acid ester in the oil phase)]. While the concentration of the aliphatic carboxylic acid ester in the oil phase is usually from 1 to 50 % by weight, this concentration relies upon the amount of the aliphatic carboxylic acid ester to be recovered from the passage shown by the line 25 and a distribution rate of the subject substance into each of the aqueous phase and the oil phase. In the case where the kind of the entrainer is changed under the condition that the amount of the subject substance recovered from the aqueous phase is identical, when the distribution rate of the entrainer into the oil is high, the absolute amount of the aliphatic carboxylic acid ester in the oil phase increases. This means that the amount of the subject substance entrained in the oil phase becomes high, i.e., the total amount of the oil phase containing the entrainer increases, leading to generation of demerits from the standpoints of facilities and energy. Especially, with respect to the energy, since the whole amount of the entrainer is recovered from the column head, the amount of heat to be used increases. However, in the process carrying out the water reflux as in the line 16, by reducing the amount of the water reflux of the line 16 while keeping the amount of the subject substance flowing out from the line 17, it is possible to reduce the increase of the heat amount corresponding to the increase of the entrainer. WO 98/45239 describes an example in which the amount of the aliphatic carboxylic acid ester as an impurity, which is entrained in the oil phase, is high, and no water reflux is carried out. Accordingly, in the invention, it is preferred that the amount of the aliphatic carboxylic acid ester as an impurity to be entrained in the oil phase is not more than 20 times, preferably not more than 17 times, and more preferably not more than 15 times the amount of the aliphatic carboxylic acid ester to be entrained in the aqueous phase or that the water reflux is carried out. Incidentally, the entrainment amount as referred to herein is determined by the product of the concentration of the subject substance and the flow rate of the liquid flowing though the passage. Incidentally, in Pig. 1, while the recovery step is denoted by two distillation columns, it is possible to modify the step such that the column 21 is connected to the column 24, thereby omitting a reboiler of the column 24. The invention brings about a larger effect in azeotropic distillation requiring separation and recovery of the aliphatic carboxylic acid ester as an impurity. As a further preferred embodiment of the above-described application example of the invention, is enumerated an operation method of the azeotropic distillation column such that the concentration of the aliphatic carboxylic acid in the overhead liquid of the azeotropic distillation column is not more than 1,000 ppm, the concentration of the entrainer in the bottom liquid of the azeotropic distillation column is not more than 100 ppm, and the concentration of the aliphatic carboxylic acid ester as an impurity in the entrainer is not more than 25 %. The specific embodiments of the invention will be described below with reference to the Examples more in detail. However, it should be construed that the invention is never limited to these Examples unless exceeding the gist of the invention. Example 1 Using a water mixture containing acetic acid and methyl acetate as an objective distillation solution and n-butyl acetate as an entrainer, a continuous distillation treatment was carried out according to the distillation process as shown in Fig. 1. The objective distillation solutions were ones recovered from the respective steps of the process of producing terephthalic acid by liquid phase oxidation of p-xylene, which are each a mixture composed of 116.9 parts by weight of acetic acid, 25.5 parts by weight of water and 1.3 parts by weight of methyl acetate in terms of the total feed amount per unit time, and were respectively fed as three feeds having a different composition from one another into the 7th plate, 16th plate and 21st plate of an azeotropic distillation column having the number of theoretical plates of 21. Concentrated acetic acid was extracted in an amount of 128.2 parts by weight per unit time as a bottom liquid from the column bottom of the azeotropic distillation column. The concentration of butyl acetate in the bottom liquid was made not more than 250 ppm. p-Xylene was extracted and recovered from the middle stage through the line 22, and methyl acetate was distilled and recovered from the entrainer recovery column 24 having the number of theoretical plates of 13. A vapor containing water and the entrainer with an azeotropic composition was obtained from the column head of the azeotropic distillation column 11 and cooled to recover in the liquid-liquid separation tank 13. Of the two phases separated in the liquid-liquid separation tank 13, the aqueous phase liquid was introduced in the whole amount into the stripping column 21 having the number of theoretical plates of 8 through the line 17, and after removing the organic component, was divided into the line 18 and the line 28. Waste water was flown into the line 18, and reflux water was fed into the azeotropic distillation column 11 though the line 28. A reflux water amount was determined from an amount of waste water (Ww) and an amount to be returned as the reflux water 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 17.3 parts by weight and 11 parts by weight per unit time, with the water reflux ratio being 0.64. The organic component recovered in the stripping column 21 is transferred into the entrainer recovery column 24 through the line 23. The entrainer was subjected to liquid-liquid separation in the liquid-liquid separation tank 13 and then circulated and fed into the azeotropic distillation column 11 through the line 15. A loss of the entrainer was supplemented through a line 30. For recovering methyl acetate, was provided the line 27 to introduce the entrainer into the 8th plate of the entrainer recovery column 24 depending upon the desire. By adjusting the amount of the entrainer flowing through the line 27, a ratio of the amount of methyl acetate to be recovered outside the system to the amount of methyl acetate to be introduced into the recovery step via the aqueous liquid phase of the overhead condensate was set up. Specifically, the amount of methyl acetate to be recovered outside the system and the amount of methyl acetate to be introduced into the recovery step via the aqueous phase liquid of the overhead condensate were calculated as the amount of methyl acetate flowing through the line 26 and the amount of methyl acetate flowing through the line 17, respectively. The amount of methyl acetate to be introduced into the recover step via the aqueous phase liquid of the overhead condensate was 24 % against the amount of methyl acetate to be recovered outside the system; the concentration of n-butyl acetate in methyl acetate recovered through the line 26 was 0.90 %; and the amount of heat to be used of the entrainer recovery column 24 was 1.3 Gcal/hr. Example 2 A distillation treatment was carried out in the same manner as in Example 1, except that in Example 1, the amount of methyl acetate to be introduced into the recover step via the aqueous phase liquid of the overhead condensate was reduced to 15 % against the amount of methyl acetate to be recovered outside the system. The amount of heat to be used of the entrainer recovery column 24 was 2.0 Gcal/hr. Thus, it was confirmed that when the rate of recovery of methyl acetate directly from the oil phase increases, the amount of heat to be used increases. 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. 2001-051553), the contents of which are incorporated herein by reference. According to the invention, an azeotropic distillation operation can be carried out at a reduced energy without using complicated distillation processes. Accordingly, the method of the invention brings about large effects on the cost of fluctuation or environmental protection. In addition, the method of the invention is high in stability because of its simplicity and brings about an effect of reducing outflow of the active components. We claim: 1. An azeotropic distillation method including at least the following steps (1) to (5): (1) a step of azeotropically distilling an objective distillation solution containing water, an saturated aliphatic carboxylic acid having from 2 to 4 carbon atoms and an aliphatic carboxylic acid ester having from 2 to 4 carbon atoms using an entrainer selected from the group consisting of butyl formate, n-propyl acetate, isobutyl acetate, n-butyl acetate, amyl acetate, n-butyl propionate and isobutyl propionate, to obtain an aliphatic carboxylic acid with a reduced water content and a water concentrated overhead distillate; (2) a step of condensing the overhead distillate to obtain a gas and a condensate Separating into two phases of an aqueous phase and an oil phase; (3) a step of dispensing the aqueous phase and the oil phase of the condensate; (4) a step of distilling a part or the whole of the aqueous phase obtained by the dispensation to obtain an overhead distillate containing the aliphatic carboxylic acid ester and with a reduced water content; and (5) a step of distilling the overhead distillate obtained in the step (4) together with a part or the whole of the oil phase dispensed in the step (3) and recovering a part or the whole of the said aliphatic carboxylic acid ester obtained by the distillation outside the system. 2. The azeotropic distillation method as claimed in claim 1, wherein the concentration of the aliphatic carboxylic acid ester in the oil phase in the azeotropic distillation is from 3 to 50 % by weight. 3. The azeotropic distillation method as claimed in claim 1 or 2, wherein at least a part of the oil phase dispensed in the step (3) is returned into the azeotropic distillation column. 4. The azeotropic distillation method as claimed in any of the preceding claims, wherein 20 % or more of the said aliphatic carboxylic acid ester obtained in the step (5) is derived from the overhead distillate obtained in the step (4). 5. The azeotropic distillation method as claimed in any of the preceding claims, wherein not more than 80 % of the aliphatic carboxylic acid ester obtained in the step (5) is derived from a part or the whole of the dispensed oil phase. 6. The azeotropic distillation method as claimed in any of the preceding claims, wherein a part of the aliphatic carboxylic acid ester obtained in the step (5) is returned into the azeotropic distillation column. 7. The azeotropic distillation method as claimed in any of the preceding claims, wherein at least a part of the aqueous phase dispensed in the step (3) is returned as a reflux liquid into the azeotropic distillation column. 8. The azeotropic distillation method as claimed in any of the preceding claims, wherein in the step (5), the fit overhead distillate obtained in the step (4) is distilled together with a part or the whole of the oil phase dispensed in the step (3) and the gas obtained in the step (2) . 9. The azeotropic distillation method as claimed in any of the preceding claims, wherein the amount of the aliphatic carboxylic acid ester recovered from the oil phase of the condensate obtained in the step (2) is not more than 20 times the aliphatic carboxylic acid ester recovered from the aqueous phase of the condensate. 10. A process of producing an aromatic carboxylic acid by oxidizing an alkyl-substituted aromatic hydrocarbon in an aliphatic carboxylic acid solvent, characterized by carrying out the azeotropic distillation as claimed in any of the preceding claims using a fraction mainly containing water obtained by *t)xidation reaction and an aliphatic carboxylic acid as an objective distillation solution. 11. The process of producing an aromatic carboxylic acid as claimed in claim 10, wherein at least a part of the aliphatic carboxylic acid ester recovered in the step (5) is recycled into anyone of the steps of oxidizing an alkyl-substituted aromatic hydrocarbon to produce an aromatic carboxylic acid. Dated this 8th Day of August, 2003 (RANJNA MEHTA-DUTT) OF REMFRY & SAGAR ATTORNEYS FOR THE APPLICANTS

Full Text

FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
COMPLETE SPECIFICATION
[See Section 10 ; rule 13]
AN 'AZEOTROPIC DISTILLATION METHOD"
MITSUBISHI CHEMICAL CORPORATION, of 5-2, Marunouchi 2-chome, Chiyoda-ku, Tokyo 100-0005, Japan,
The following specification particularly describes the nature of the invention and the manner in which it is to be performed:-

GRANTED

9-3-2005

ORIGINAL
758/MUMNP/2003

DESCRIPTION

The present invention relates to an azeotropic distillation method. Especially, the invention gives rise to an effect 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. 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 solvent.

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 a mixed solution of acetic acid and water, followed by azeotropic distillation to obtain acetic acid having a high purity from the mixed solution 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 solvent 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 solvent containing aliphatic carboxylic acids such as acetic acid. However, since water is formed during the production step, it is necessary to prevent accumulation of water in the reaction system. For achieving this, there is employed an operation in which a mixed vapor 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 solvent will be noticed. Usually, rectification is used in the separation

of water from a mixture of acetic acid and water. However, azeotropic distillation is advantageous depending on the cost of equipment and the cost 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 a condensate of an overhead distillate of an azeotropic distillation column. 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 liquid, the kind of entrainer, the feed position (height) of a feed liquid into an azeotropic distillation column, the number of liquid feed lines, the method of 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 The azeotropic distillation method for separation of acetic acid from water is also disclosed in JP-B-62-41219, KR-A-94-14292, JP-A-2000-72714, WO 98/45239, etc.
As a method in which after subjecting an overhead

vapor condensate of an azeotropic distillation column to liquid-liquid separation, oil phase components to be entrained in the aqueous phase side are reduced, JP-B-62-41219 pays attention especially to isobutyl acetate and aliphatic carboxylic acid esters as impurities as oil phase components and strips an aqueous phase liquid to recover them. However, this method is aimed at a countermeasure of a loss due to the entrainment of the oil phase components into the aqueous phase but is not a technology for overcoming all of the problems derived from the aliphatic carboxylic acid esters as impurities. The above-described aliphatic carboxylic acid esters are Entrained with the feed liquid, enter a distillation column, and are accumulated in the oil phase. The method described in JP-B-62-41219 is a technology for preventing oss caused by the entrainment of. a part of 'the accumulated aliphatic carboxylic ,acid..esters as impurities from the aqueous phase but does not mention a problem such that the above-described aliphatic carboxylic acid esters are accumulated in the oil phase, whereby the

concentration thereof increases. The increase of the concentration of the aliphaticcarboxylic acid esters deteriorates the separation property between acetic acid and water in azeotropic distillation to cause a loss of the effective component (acetic acid).
Since then, respective corporations have proposed technologies for preventing the accumulation of impurities (aliphatic carboxylic acid esters) in the oil phase.
For example, KR-A-94-14292 proposes a method in which after subjecting a condensate of an overhead vapor of an azeotropic distillation column to liquid-liquid separation, the aliphatic carboxylic acid esters as impurities to be entrained in the oil phase side are distilled and then recovered. However, this method involves a problem such that since the separation property of the carboxylic acid esters from the entrainer in the oil phase is poor, a large amount of energy is required for the separation of the aliphatic carboxylic acid esters.
Further, JP-A-2000-72714 proposes a method in which an overhead vapor of an azeotropic distillation column is partially condensed, and aliphatic carboxylic acid esters as impurities in a residual gas are absorbed in acetic acid and recovered. However, according to this method, since the entrainer component is entrained by the partial condensation, it is impossible to take out only the aliphatic carboxylic acid esters, leading to a loss of the

effective component (entrainer component) and contamination of the product.
WO 98/45239 proposes a method in which an overhead vapor of an azeotropic distillation column is partially condensed, and the residual gas is distilled to recover aliphatic carboxylic acid esters as impurities. While this technology brings about an improvement on the point of the amount of heat to be used, it is impossible to isolate the aliphatic carboxylic acid esters, and it involves the same problem as the technologies described in other patents on the point of providing an exclusive passage for recovery. Further, in the subject patent publication, since the water concentration in the azeotropic composition is low, there is involved a problem such that for removing water by azeotropic distillation, a large amount of the entrainer must be used. Though the amount of the entrainer to be used in the azeotropic distillation is determined according to the azeotropic composition, it is in general one time or more, for example, from 2 to 5 times in terms of a weight ratio against water. Accordingly, in addition to the matter that the amount of the entrainer per se is large, since the entrainer is in general used upon circulation, materials that have a lower boiling point than acetic acid and are sparingly soluble in water (for example, methyl acetate and p-xylene in the production of terephthalic

acid) are accumulated in the entrainer side. Thus, when the amount of the contained impurities increases, the amount of the oil phase largely increases. For this reason, a tolerant width wherein the aliphatic carboxylic acid esters as one kind of impurities can be contained is narrow.
In the light of the above, with respect to the recovery of aliphatic carboxylic acid esters as impurities, it is preferred to correspond to all of problems such that entering and coming out of the distillation system be balanced; the amount of energy to be used for separation and recovery be decreased; recovery with a high purity be performed; and a loss of fluctuation be avoided by simple equipment. For achieving this, equipment exclusive for recovery of aliphatic carboxylic acid esters as impurities has been proposed.
Under these circumstances, the invention has been made based on a new way of thinking attained from the point to which an attention is paid regarding a ratio of the concentration of aliphatic carboxylic acid esters as impurities to the concentration of an entrainer in an aqueous phase of a condensate obtained by condensing an overhead distillate of an azeotropic distillation column. Though it is known that the recovery of organic components from the aqueous phase is a substantially essential operation, the invention has found a technology for

positively applying it and depressing an increase of the aliphatic carboxylic acid esters as impurities. And, the invention is constructed from this technology and is to provide an azeotropic distillation method capable of depressing the amount of energy to be used to a low level and giving rise to an excellent separation performance by introducing a simple process.

The invention relates to recovery of aliphatic carboxylic acid esters as impurities accumulated in an entrainer present within an azeotropic distillation column in the operation of azeotropic distillation. The present inventors paid attention to the matter that a ratio of the concentration of the aliphatic carboxylic acid esters to the concentration of the entrainer in an aqueous phase of a condensate of an overhead distillate, of the azeotropic distillation column is high as compared with that in an oil phase and found that by recovering the aliphatic carboxylic acid esters from the aqueous phase, the aliphatic carboxylic acid esters can be recovered simply and with a low energy as compared with a method of recovery from the passage to which attention has hitherto been paid. The invention has been attained based on these findings.
Specifically, the gist of the invention resides in an

azeotropic distillation method including at least the
following steps (1) to (5):
Step (1): A step of azeotropically distilling an objective
distillation solution containing water, an aliphatic
carboxylic acid and an aliphatic carboxylic acid ester
using an entrainer, to obtain an aliphatic carboxylic acid
with a reduced water content and a water-concentrated
overhead distillate;
Step (2): A step of condensing the overhead distillate to
obtain a gas and a condensate separating into two phases
of an aqueous phase and an oil phase;
Step (3) : A step of dispensing the aqueous phase and the
oil phase of the condensate;
Step (4) : A step of distilling a part or the whole of the
aqueous phase obtained by the dispensation to obtain an
overhead distillate containing the aliphatic carboxylic
acid ester and with a reduced water content; and
Step (5) : A step of distilling the overhead distillate
obtained in the step (4) together with a part or the whole
of the oil phase dispensed in the step (3) and recovering
a part or the whole of the aliphatic carboxylic acid ester
obtained by the distillation outside the system.

Fig. 1 is a flow chart showing one example of the distillation process for carrying out the invention.

In the drawing, reference numerals 11, 13, 21 and 24 denote an azeotropic distillation column, a liquid-liquid separation tank (decanter), a stripping column and an entrainer recovery column, respectively.

The mode for carrying out the invention will be described below in detail.
In this description, the term "objective distillation solution" means a mixed solution containing a desired substance to be purified and a substance the concentration of which is to be reduced (hereinafter referred to as "substance to be reduced") . 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. The term "azeotropic distillation column" means a distillation column for distilling the foregoing objective distillation sortition and entrainer.
[Description of the step (1)]
In the invention, first of all, the step (1) includes a step of azeotropically distilling an objective distillation solution containing water, an aliphatic

carboxylic acid and an aliphatic carboxylic acid ester using an entrainer, to obtain the desired substance, aliphatic carboxylic acid with a reduced water content and a water-concentrated overhead distillate.
For reducing the concentration of water in the mixture containing an aliphatic carboxylic acid and water by azeotropic distillation, distillation is in general carried out so as to obtain a bottom liquid containing an aliphatic carboxylic acid with a reduced amount of water from the column bottom and to obtain a vapor of an azeotropic mixture mainly containing water and the entrainer from the column head. During this, for the purposes of reuse of the aliphatic carboxylic acid, etc., 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 condensate of the overhead distillate of the azeotropic distillation column 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 an alkyl-substituted aromatic hydrocarbon.
Next, the objective distillation solution and the entrainer will be described.
In the invention, the objective distillation solution is not particularly limited so far as it is a mixed

solution containing the desired substance and the substance to be reduced, and the entrainer to be added and the substance to be reduced form an azeotropic mixture, and the azeotropic temperature thereof is lower than the boiling point of the desired substance. Further, the objective distillation solution may further contain a substance that does not substantially affect the subject azeotropic distillation.
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 the substance to be reduced, 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 objective distillation solution 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.
The concentration of the aliphatic carboxylic acid ester in the azeotropic distillation is arbitrary, but it is preferred that the aliphatic carboxylic acid ester is contained in an amount of from 3 to 50 % by weight in the oil phase containing the entrainer on the point of separating water from the aliphatic carboxylic acid such as acetic acid stably and in a high concentration.
In the invention, the desired substance to be purified is an aliphatic carboxylic acid, and is usually a saturated or unsaturated aliphatic carboxylic acid having from 2 to 6 carbon atoms. Preferred examples include saturated aliphatic carboxylic acids having from 2 to 4 carbon atoms such as acetic acid, propionic acid, and butyric acid. The substance to be reduced, the concentration of which is to be reduced, is water.
Further, in the invention, the kind of aliphatic carboxylic acid esters as impurities contained in the objective distillation solution is not particularly limited, but it usually includes esters of the desired substance, aliphatic carboxylic acid, and specifically, for example, methyl acetate.
The entrainer 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 mixed solutions containing 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_Jbutyl 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 chlprobenzene; 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.
Incidentally, in the invention, in the case where the objective distillation solution is a solvent used in the production of aromatic carboxylic acids such as terephthalic acid, the objective distillation solution may contain other substances derived from the raw materials of the aromatic carboxylic acids, such as p-xylene and methyl acetate, and decomposition products of the entrainer.
The composition of the aliphatic carboxylic acid and water in the objective distillation solution is arbitrary, but the invention is usually applied to mixed solutions containing 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 objective distillation solutions 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 of an alkyl-substituted aromatic hydrocarbon in an aliphatic carboxylic acid-containing solvent. For example, ones obtained by condensation and recovery of mixed vapors of an alkyl-substituted aromatic hydrocarbon from the liquid phase oxidation reactor, ones obtained by absorbing an acetic acid vapor in the waste gas with water from the foregoing reactor, ones obtained by evaporating 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 subject step and then recovered, are arbitrarily selected and used along with the amounts. These solutions may be mixed and individually treated.
[Description of the steps (2) and (3)]
In the invention, subsequent to the step (1) , the step ' (2) includes a step of condensing the overhead distillate to obtain a gas and a condensate separating

into two phases of an aqueous phase and an oil phase; and the step (3) includes a step of individually dispensing each of the phases of the condensate.
In the invention, according to the azeotropic distillation of the step (1), a bottom liquid containing the desired substance with a reduced concentration of the substance to be reduced is obtained from the column bottom, and a vapor of the azeotropic mixture (overhead distillate) composed mainly of the substance to be reduced and the entrainer is obtained from the column head. As the step (2) , the vapor obtained from the column head is condensed to obtain a condensate and a gas. The condensate is usually obtained as a two-phase liquid of an aqueous phase composed mainly of the substance to be reduced and an oil phase composed mainly of the entrainer. Subsequently, as the step (3) , this two-phase condensate is dispensed into the aqueous phase and the oil phase. As dispensation measures is numerated liquid-liquid separation by a decanter or the like, but there are no particular limitations so far as the object can be attained. For making it possible to dispense the two phases, it is preferred to use a substance to be reduced and an entrainer capable of giving a heterogeneous azeotropic mixture such that the aqueous phase and the oil phase constituting the condensate are not mixed homogeneously with each other.

In the aqueous phase and the oil phase of the condensate obtained in the step (2) , it is preferred that the amount of the aliphatic carboxylic acid ester recovered from the oil phase is not more than 20 times the amount of the aliphatic carboxylic acid ester recovered from the aqueous phase. This is, for example, obtained by " (flow rate per unit time of the oil phase) x (concentration of the aliphatic carboxylic acid ester in the oil phase)" and " (flow rate per unit time of the aqueous phase) x (concentration of the aliphatic carboxylic acid ester in the aqueous phase)". When the foregoing value exceeds 20 times, the amount of the aliphatic carboxylic acid ester that can be recovered from the aqueous phase is low so that the effect of the invention may possibly be low. Especially, the value is preferably not more than 18 times, and particularly preferably not more than 15 times.
Of the dispensed two phases, at least a part of the phase (oil phase) composed mainly of the entrainer is preferably 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. 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 azeotropic distillation column and the composition of the azeotropic mixture. Actually, the optimum amount of the entrainer may be determined from the concentration of the aliphatic carboxylic acid in the condensate of the overhead distillate of the azeotropic distillation column and the concentration of the entrainer in the bottom liquid. Incidentally, the entrainer may be newly fed as compensation for a loss of the entrainer come out the system.
On tne other hand, at least a part of the phase (aqueous phase) composed mainly of the substance to be reduced is provided in the step (4) described later. The residual aqueous phase may be returned as a reflux liquid into the azeotropic distillation column. Incidentally, the whole of the aqueous phase may be provided in the step (4) , a part of which is then returned as a reflux liquid into the azeotropic distillation column. Further, water as the main component in the phase (aqueous phase) composed mainly of the substance to be reduced may be reused within the process, a part of which is then disposed. Examples of the method of returning water as the reflux liquid include a method of returning it into the column head of the azeotropic distillation column and a method of returning it into the middle stage of the column. 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).
[Description of the step (4)]
In the step (4), a part or the whole of the dispensed aqueous phase is distilled to obtain an overhead distillate containing the aliphatic carboxylic acid ester with a reduced water content.
Specific examples include a method in which the aqueous phase (composed mainly of water as the substance to be reduced) dispensed from the condensate of the overhead distillate of the azeotropic distillation column is delivered into a distillation column, and the aliphatic carboxylic acid with a reduced water content is obtained from a column head of the distillation column. A material obtained from a column bottom of the distillation column by the distillation is mainly water. This water may be disposed outside the system, or may be partially returned as a reflux liquid into the azeotropic distillation column.
[Description of the step (5)]
In the step (5) , the overhead distillate obtained in the step (4) is distilled together with a part or the whole of the oil phase dispensed in the step (3) , and a part or the whole of the aliphatic carboxylic acid ester obtained by the distillation is recovered outside the system.

In the step (5) , the distillation column is aimed at recovery of the entrainer. In the step (5) , the overhead distillate of the step (4) and a part or the whole of the oil phase dispensed in the step (3) from the condensate of the overhead distillate of the azeotropic distillation column are fed into the distillation column and distilled therein. By this distillation, the entrainer is separated from the aliphatic carboxylic acid ester as an impurity. Incidentally, in this step, it is preferred to control the feeding of components to be distilled into the step (5) such that 20 % or more of the aliphatic carboxylic acid ester is derived from the overhead material obtained in the step (4) on the point of energy efficiency.
The entrainer is recovered from the column bottom, and a part or the whole thereof may be recovered outside the system, or a part or the whole thereof may be returned into the azeotropic distillation column. Further, a part or the whole of the aliphatic carboxylic acid ester as an impurity may be recovered outside the system.
Further, in the step (5) , as oil phase components other than a part or the whole of the dispensed oil phase, the condensate of the overhead distillate of the azeotropic distillation column (namely, a mixture of the entrainer and the aliphatic carboxylic acid ester) and the gas obtained in the step (2) may be fed into the distillation column along with the oil phase.

Incidentally, with respect to the oil phase dispensed in the step (3) and the entrainer obtained in the step (5), there may be employed a method in which the circulating flow of the entrainer into the azeotropic distillation column is divided, one of the divided flows is returned into the column head of the azeotropic distillation column, 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, with respect to the water obtained in the step (2) or step (4), there may be employed a method in which water is returned as the reflux liquid into the azeotropic distillation column 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, for recovering the alkyl-substituted aromatic hydrocarbons accumulated in the entrainer, they may be directly separated from the entrainer, or extracted from the middle stage of the azeotropic distillation column as described in JP-T-10-504556. Additionally, the extract can be purified as described in WO 97/29068.
Next, the production process of aromatic carboxylic acids of the invention, 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.
The aromatic carboxylic acid as the desired compound means an arbitrary aromatic carboxylic acid. Examples include aromatic monocarboxylic acids, aromatic dicarboxylic acids, and aromatic tricarboxylic acids. For example, when these have a benzene ring as the aromatic ring, they are produced by oxidation, and preferably liquid phase oxidation of corresponding alkyl-substituted aromatic hydrocarbons such as monoalkylbenzenes, dialkylbenzenes, and trialkylbenzenes. Especially, it is preferred to apply the method of the invention in the case of the aromatic dicarboxylic acid, especially 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.
In carrying out the oxidation reaction in the liquid phase, an aliphatic carboxylic acid is usually used as a solvent. 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, a catalyst is usually used. Examples of the catalyst include transition metal compounds such as manganese, cobalt, iron, chromium, and nickel. Further, bromine compounds may 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 or air, 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.
The reaction temperature of the liquid phase oxidation may be properly selected but is usually from 120 °C to 220 °C. For example, in the oxidation method in which no bromine compound catalyst is used, the preferable reaction temperature is in general not higher than 160 °C. Similarly, the pressure may be properly selected and may be within the range where the solvent (such as acetic acid) can be present in the gas state.
An oxidation reaction heat is mainly removed by flash evaporation of a water-containing acetic acid solvent. That is, the fraction (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, a part of which is usually then again refluxed as the oxidation reaction solvent into the oxidation reactor. Further, for the purpose of removing water formed by the oxidation reaction, other part thereof is delivered into a dehydration column and provided for azeotropic distillation of the invention, and dehydrated acetic acid is again delivered as the oxidation reaction solvent into the oxidation reactor.
The liquid phase oxidation of the alkyl-substituted aromatic hydrocarbon is usually carried out in one reactor, or plural reactors, if desired. 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. The slurry is separated into a cake of an aromatic carboxylic acid crystal and an oxidation reaction mother liquor by an arbitrary crystal separation measure such as the rotary vacuum filter method and the

centrifugation method or other proper separation method. If desired, the cake of the aromatic carboxylic acid crystal is rinsed with acetic acid or water, and the deposited solvent is removed by a dryer. If further desired, the resulting aromatic carboxylic acid crystal is again slurried in the reaction mother liquor composed mainly of water and recrystallized through a hydrolysis step to reduce impurities in the crystal, thereby achieving purification. Then, operations such as solid-liquid separation, rinsing, and drying are carried out to obtain an aromatic carboxylic acid.
Next, the azeotropic distillation method of the invention and the production process of aromatic carboxylic acids using it will be described with reference to Fig. 1. Fig. 1 is a flow chart showing one example of the distillation process for carrying out the invention. 11 denotes an azeotropic distillation column. An objective distillation solution containing water, an aliphatic carboxylic acid (such as acetic acid) and an aliphatic carboxylic acid ester as an impurity are fed into the azeotropic distillation column through lines 40, 41, 42, etc. (in the case of feeding objective distillation solutions through a plurality of lines, the composition of each of the objective distillation solutions is usually different from each other), and an

entrainer is fed thereinto through a line 15, thereby carrying out azeotropic distillation.
For adding a heat necessary for the distillation, heating is carried out using a heat exchanger 14. As heating sources are used heated oils and heated vapors. Here, an example of using a vapor having a temperature higher than the boiling point at atmospheric pressure of acetic acid, specifically 0.35 MPa vapor is shown.
The azeotropic mixture vapor (overhead distillate of the azeotropic distillation column) containing the substance to be reduced, the concentration of which is to be reduced by azeotropic distillation, and the entrainer is delivered into a condenser 12 from a column head of the azeotropic distillation column 11 and condensed therein. The resulting condensate is separated into two phases in a liquid-liquid separation tank 13, followed by dispensation. The separation measure is properly selected according to the nature of the azeotropic mixture.
In Fig. l in the liquid-liquid separation tank 13, a

part of the phase (aqueous phase) composed mainly of the substance to be reduced is delivered into a stripping column 21 through a line 17. Further, a part of the aqueous phase may be made a water reflux into the azeotropic distillation column 11 through a line 16. The entrainer is circulated through the line 15. If desired, the line 15 and the line 16 are connected to the column

head or middle stage of the azeotropic distillation column 11. The line number of each of the lines 15 and 16 into the azeotropic distillation column 11 may be one or plural Further, the entrainer and the substance to be reduced may be returned together into the azeotropic distillation column 11 using a common line or individual lines.
A liquid composed mainly of the desired substance (for example, an aliphatic carboxylic acid) with a reduced water content is taken out from the column bottom of the azeotropic distillation column 11 through a line 19. Further, when the desired substance is the aliphatic carboxylic acid used in the production process of the aromatic carboxylic acid of the invention, a liquid composed mainly of water and an alkyl-substituted aromatic hydrocarbon as the raw material of aromatic carboxylic acid is extracted from a line 22 of the middle stage of the column.
In the stripping column 21, the organic component (mainly an aliphatic carboxylic acid ester as an impurity in the production process of aromatic carboxylic acid of the invention) is recovered, and the substance to be reduced, which is separated by azeotropic distillation, such as water, is circulated into the production process of aromatic carboxylic acid through a line 18 and effectively utilized there or disposed. The organic component recovered in the stripping column 21 is

delivered into an entrainer recovery column 24 through a line 23 and separated therein into the entrainer and the aliphatic carboxylic acid ester as an impurity. The entrainer is recovered from the column bottom and returned into the azeotropic distillation column 11 through lines 25 and 15. The aliphatic carboxylic acid ester is recovered through a line 26. The aliphatic carboxylic acid ester may be stored, or a part or the whole thereof may be circulated in the oxidation reaction step for producing aromatic carboxylic acids according to the desire. Further, at least a part of the mixture of the entrainer and the aliphatic carboxylic acid ester branched in a line 27 is directly introduced into the entrainer recovery column 24. Moreover, it is also possible to introduce a gas generated in the condenser 12 into the entrainer recovery column 24, though such is not illustrated.
The method of recovering the aliphatic carboxylic acid ester as an impurity according to the invention will be specifically described with reference to Fig. 1.
The amount of the aliphatic carboxylic acid ester to be recovered outside the system, i.e., the amount as obtained in the step (5), is expressed as an amount of the subject substance to be discharged from the line 26. The discharge flow may contain the entrainer, water and impurities such as aliphatic hydrocarbons and is a liquid

or gas or a mixture thereof. The amount of the aliphatic carboxylic acid ester to be introduced into the recovery step via the aqueous phase liquid of the condensate phase of the overhead distillate of the azeotropic distillation column 11 is expressed as an amount of the subject substance flowing through the line 17. It is preferred from the viewpoint of energy efficiency to introduce the aliphatic carboxylic acid ester into the recovery step through the aqueous phase at a recover rate of 20 % or more, preferably 30 % or more, and more preferably 40 % or more against the total amount of the aliphatic carboxylic acid ester to be recovered outside the system.
Incidentally, in the entrainer recovery step, the aliphatic carboxylic acid ester can be entrained even in an extremely low amount in a bottom liquid 25 of the entrainer recovery column 24. According to the invention, the above-described recovery rate is calculated in terms of a rate of the amount of the aliphatic carboxylic acid ester via the aqueous phase in the amount of the aliphatic carboxylic acid ester to be introduced in the recovery step via the aqueous phase and the oil phase.
A flow of the aliphatic carboxylic acid ester not via the aqueous phase is introduced into the entrainer recovery column 24 through the line 27. This flow may be any of a gas or a liquid or a mixture thereof and can be used in combination of a gas generated in the condenser 12

as described in WO 98/45239.
Incidentally, a concentration ratio of the aliphatic carboxylic acid ester as an impurity to the entrainer in the oil phase to be extracted from the liquid-liquid separation tank 13 is usually from 1:100 to 1:2, and it is disadvantageous to recover the aliphatic carboxylic acid ester only from the oil phase. On the other hand, turning now to the concentration in the aqueous phase, this ratio is usually from 1:1 to 15:1, and such is advantageous for recovery of the aliphatic carboxylic acid ester. Accordingly, when the recovery rate of the aliphatic carboxylic acid ester from the aqueous phase decreases, a load in the recovery step increases, and when the recovery rate is too low, an increase of the amount of energy to be used becomes remarkable. Accordingly, more preferably, the recovery rate is 20 % or more. It is possible to adjust the recovery rate by adjusting the flow rate in the passage shown by the line 27 or the flow rate of the passage shown by a line 28. Incidentally, in the case of adjustment by the passage shown by the line 28, it is preferred to adjust the recovery rate such that the total amount of the water reflux does not change. Further, it is possible to recirculate water discharged from the stripping column 21 into the liquid-liquid separation tank 13 to increase the recovery rate.
Incidentally, in the invention, for example, during

the liquid-liquid separation in the liquid-liquid separation tank 13, with respect to distribution of the aliphatic carboxylic acid ester as an impurity, the higher the rate expressed by (total amount of the subject substance in the aqueous phase)/(total amount of the subject substance in the oil phase), the larger the effect of the method of the invention is. Such can be, for example, obtained by [(flow rate per unit time of the aqueous phase) x (concentration of the aliphatic carboxylic acid ester in the aqueous phase)]/[(flow rate per unit time of the oil phase) x (concentration of the aliphatic carboxylic acid ester in the oil phase)].
While the concentration of the aliphatic carboxylic acid ester in the oil phase is usually from 1 to 50 % by weight, this concentration relies upon the amount of the aliphatic carboxylic acid ester to be recovered from the passage shown by the line 25 and a distribution rate of the subject substance into each of the aqueous phase and the oil phase.
In the case where the kind of the entrainer is changed under the condition that the amount of the subject substance recovered from the aqueous phase is identical, when the distribution rate of the entrainer into the oil is high, the absolute amount of the aliphatic carboxylic acid ester in the oil phase increases. This means that the amount of the subject substance entrained in the oil

phase becomes high, i.e., the total amount of the oil phase containing the entrainer increases, leading to generation of demerits from the standpoints of facilities and energy. Especially, with respect to the energy, since the whole amount of the entrainer is recovered from the column head, the amount of heat to be used increases. However, in the process carrying out the water reflux as in the line 16, by reducing the amount of the water reflux of the line 16 while keeping the amount of the subject substance flowing out from the line 17, it is possible to reduce the increase of the heat amount corresponding to the increase of the entrainer. WO 98/45239 describes an example in which the amount of the aliphatic carboxylic acid ester as an impurity, which is entrained in the oil phase, is high, and no water reflux is carried out.
Accordingly, in the invention, it is preferred that the amount of the aliphatic carboxylic acid ester as an impurity to be entrained in the oil phase is not more than 20 times, preferably not more than 17 times, and more preferably not more than 15 times the amount of the aliphatic carboxylic acid ester to be entrained in the aqueous phase or that the water reflux is carried out. Incidentally, the entrainment amount as referred to herein is determined by the product of the concentration of the subject substance and the flow rate of the liquid flowing though the passage.

Incidentally, in Pig. 1, while the recovery step is denoted by two distillation columns, it is possible to modify the step such that the column 21 is connected to the column 24, thereby omitting a reboiler of the column 24.
The invention brings about a larger effect in azeotropic distillation requiring separation and recovery of the aliphatic carboxylic acid ester as an impurity. As a further preferred embodiment of the above-described application example of the invention, is enumerated an operation method of the azeotropic distillation column such that the concentration of the aliphatic carboxylic acid in the overhead liquid of the azeotropic distillation column is not more than 1,000 ppm, the concentration of the entrainer in the bottom liquid of the azeotropic distillation column is not more than 100 ppm, and the concentration of the aliphatic carboxylic acid ester as an impurity in the entrainer is not more than 25 %.

The specific embodiments of the invention will be described below with reference to the Examples more in detail. However, it should be construed that the invention is never limited to these Examples unless exceeding the gist of the invention.

Example 1
Using a water mixture containing acetic acid and methyl acetate as an objective distillation solution and n-butyl acetate as an entrainer, a continuous distillation treatment was carried out according to the distillation process as shown in Fig. 1.
The objective distillation solutions were ones recovered from the respective steps of the process of producing terephthalic acid by liquid phase oxidation of p-xylene, which are each a mixture composed of 116.9 parts by weight of acetic acid, 25.5 parts by weight of water and 1.3 parts by weight of methyl acetate in terms of the total feed amount per unit time, and were respectively fed as three feeds having a different composition from one another into the 7th plate, 16th plate and 21st plate of an azeotropic distillation column having the number of theoretical plates of 21. Concentrated acetic acid was extracted in an amount of 128.2 parts by weight per unit time as a bottom liquid from the column bottom of the azeotropic distillation column. The concentration of butyl acetate in the bottom liquid was made not more than 250 ppm.
p-Xylene was extracted and recovered from the middle stage through the line 22, and methyl acetate was distilled and recovered from the entrainer recovery column 24 having the number of theoretical plates of 13. A vapor

containing water and the entrainer with an azeotropic composition was obtained from the column head of the azeotropic distillation column 11 and cooled to recover in the liquid-liquid separation tank 13. Of the two phases separated in the liquid-liquid separation tank 13, the aqueous phase liquid was introduced in the whole amount into the stripping column 21 having the number of theoretical plates of 8 through the line 17, and after removing the organic component, was divided into the line 18 and the line 28. Waste water was flown into the line 18, and reflux water was fed into the azeotropic distillation column 11 though the line 28. A reflux water amount was determined from an amount of waste water (Ww) and an amount to be returned as the reflux water 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 17.3 parts by weight and 11 parts by weight per unit time, with the water reflux ratio being 0.64. The organic component recovered in the stripping column 21 is transferred into the entrainer recovery column 24 through the line 23. The entrainer was subjected to liquid-liquid separation in the liquid-liquid separation tank 13 and then circulated and fed into the azeotropic distillation column 11 through the

line 15. A loss of the entrainer was supplemented through a line 30. For recovering methyl acetate, was provided the line 27 to introduce the entrainer into the 8th plate of the entrainer recovery column 24 depending upon the desire. By adjusting the amount of the entrainer flowing through the line 27, a ratio of the amount of methyl acetate to be recovered outside the system to the amount of methyl acetate to be introduced into the recovery step via the aqueous liquid phase of the overhead condensate was set up. Specifically, the amount of methyl acetate to be recovered outside the system and the amount of methyl acetate to be introduced into the recovery step via the aqueous phase liquid of the overhead condensate were calculated as the amount of methyl acetate flowing through the line 26 and the amount of methyl acetate flowing through the line 17, respectively.
The amount of methyl acetate to be introduced into the recover step via the aqueous phase liquid of the overhead condensate was 24 % against the amount of methyl acetate to be recovered outside the system; the concentration of n-butyl acetate in methyl acetate recovered through the line 26 was 0.90 %; and the amount of heat to be used of the entrainer recovery column 24 was 1.3 Gcal/hr.

Example 2
A distillation treatment was carried out in the same manner as in Example 1, except that in Example 1, the amount of methyl acetate to be introduced into the recover step via the aqueous phase liquid of the overhead condensate was reduced to 15 % against the amount of methyl acetate to be recovered outside the system. The amount of heat to be used of the entrainer recovery column 24 was 2.0 Gcal/hr.
Thus, it was confirmed that when the rate of recovery of methyl acetate directly from the oil phase increases, the amount of heat to be used increases.
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. 2001-051553), the contents of which are incorporated herein by reference.

According to the invention, an azeotropic distillation operation can be carried out at a reduced

energy without using complicated distillation processes. Accordingly, the method of the invention brings about large effects on the cost of fluctuation or environmental protection. In addition, the method of the invention is high in stability because of its simplicity and brings about an effect of reducing outflow of the active components.

We claim:
1. An azeotropic distillation method including at least the following steps (1) to (5):
(1) a step of azeotropically distilling an objective distillation solution containing water, an saturated aliphatic carboxylic acid having from 2 to 4 carbon atoms and an aliphatic carboxylic acid ester having from 2 to 4 carbon atoms using an entrainer selected from the group consisting of butyl formate, n-propyl acetate, isobutyl acetate, n-butyl acetate, amyl acetate, n-butyl propionate and isobutyl propionate, to obtain an aliphatic carboxylic acid with a reduced water content and a water concentrated overhead distillate;
(2) a step of condensing the overhead distillate to obtain a gas and a condensate Separating into two phases of an aqueous phase and an oil phase;
(3) a step of dispensing the aqueous phase and the oil phase of the condensate;
(4) a step of distilling a part or the whole of the aqueous phase obtained by the dispensation to obtain an overhead distillate containing the aliphatic carboxylic acid ester and with a reduced water content; and
(5) a step of distilling the overhead distillate obtained in the step (4) together with a part or the whole of the oil phase dispensed in the step (3) and recovering a part or the whole of the said aliphatic

carboxylic acid ester obtained by the distillation outside the system.
2. The azeotropic distillation method as claimed in claim 1, wherein the concentration of the aliphatic carboxylic acid ester in the oil phase in the azeotropic distillation is from 3 to 50 % by weight.
3. The azeotropic distillation method as claimed in claim 1 or 2, wherein at least a part of the oil phase dispensed in the step (3) is returned into the azeotropic distillation column.
4. The azeotropic distillation method as claimed in any of the preceding claims, wherein 20 % or more of the said aliphatic carboxylic acid ester obtained in the step (5) is derived from the overhead distillate obtained in the step (4).
5. The azeotropic distillation method as claimed in any of the preceding claims, wherein not more than 80 % of the aliphatic carboxylic acid ester obtained in the step (5) is derived from a part or the whole of the dispensed oil phase.
6. The azeotropic distillation method as claimed in any of the preceding claims, wherein a part of the aliphatic carboxylic acid ester obtained in the step (5) is returned into the azeotropic distillation column.
7. The azeotropic distillation method as claimed in any of the preceding claims, wherein at least a part of the aqueous phase dispensed in the step (3) is returned as a reflux liquid into the azeotropic distillation column.
8. The azeotropic distillation method as claimed in any of the preceding

claims, wherein in the step (5), the fit overhead distillate obtained in the step (4) is distilled together with a part or the whole of the oil phase dispensed in the step (3) and the gas obtained in the step (2) .
9. The azeotropic distillation method as claimed in any of the preceding claims, wherein the amount of the aliphatic carboxylic acid ester recovered from the oil phase of the condensate obtained in the step (2) is not more than 20 times the aliphatic carboxylic acid ester recovered from the aqueous phase of the condensate.
10. A process of producing an aromatic carboxylic acid by oxidizing an alkyl-substituted aromatic hydrocarbon in an aliphatic carboxylic acid solvent, characterized by carrying out the azeotropic distillation as claimed in any of the preceding claims using a fraction mainly containing water obtained by *t)xidation reaction and an aliphatic carboxylic acid as an objective distillation solution.
11. The process of producing an aromatic carboxylic acid as claimed in claim 10, wherein at least a part of the aliphatic carboxylic acid ester recovered in the step (5) is recycled into anyone of the steps of oxidizing an alkyl-substituted aromatic hydrocarbon to produce an aromatic carboxylic acid.
Dated this 8th Day of August, 2003
(RANJNA MEHTA-DUTT)
OF REMFRY & SAGAR
ATTORNEYS FOR THE APPLICANTS

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Patent Number

209495

Indian Patent Application Number

758/MUMNP/2003

PG Journal Number

38/2007

Publication Date

21-Sep-2007

Grant Date

31-Aug-2007

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	C/O MITUBISHI CHIMICAL CORPORATION 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI, FUKUOKA 806-0004,
2	TAKAYUKI ISOGAI	C/O MITUBISHI CHIMICAL CORPORATION 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI, FUKUOKA 806-0004,

PCT International Classification Number

C07C51/46

PCT International Application Number

PCT/JP02/01372

PCT International Filing date

2002-02-18

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2001-051553	2001-02-27	Japan