Title of Invention	PROCESS FOR PRODUCING AROMATIC CARBOXYLIC ACID
Abstract	A process for producing an aromatic carboxylic acid under high pressure and high temperature by subjecting an alkylaromatic compound to a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst in a reaction solvent comprising an aliphatic carboxylic acid in an oxidizing reactor while recovering the aliphatic carboxylic acid by ditillation of the oxidation exhaust gas, in which the recovery of the aliphatic carboxylic acid can be attained efficiently under separation thereof from by-products, such as water and alcohols, with permission of easier waste water treatment, wherein the process comprises proceeding the reaction while removing the water and the alcohol formed by the oxidation reaction by means of membrane-separation.

Title of Invention

PROCESS FOR PRODUCING AROMATIC CARBOXYLIC ACID

Abstract

A process for producing an aromatic carboxylic acid under high pressure and high temperature by subjecting an alkylaromatic compound to a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst in a reaction solvent comprising an aliphatic carboxylic acid in an oxidizing reactor while recovering the aliphatic carboxylic acid by ditillation of the oxidation exhaust gas, in which the recovery of the aliphatic carboxylic acid can be attained efficiently under separation thereof from by-products, such as water and alcohols, with permission of easier waste water treatment, wherein the process comprises proceeding the reaction while removing the water and the alcohol formed by the oxidation reaction by means of membrane-separation.

Full Text	SPECIFICATION FIELD OF THE INVENTION The present invention relates to a process for producing an aromatic carboxylic acid by a liquid phase oxidation of an alkylaryl compound having one or more substituent alkyl or partially oxidized alkyl groups by a molecular oxygen-containing gas. BACKGROUND OF THE INVENTION Aromatic carboxylic acids are important as fundamental chemical product and, in particular, aromatic dicarboxylic acids are useful for the starting material of fibers, resins and the like. For example, terephthalic acid has found growing demand as the starting material for polyester fiber in recent years. For producing aromatic carboxylic acids, a process has hitherto been employed in general in which an alkyl-substituted aromatic compound is subjected to a liquid phase oxidation by bringing it into contact with a molecular oxygen-containing gas in a reaction solvent comprising a lower aliphatic carboxylic acid, such as acetic acid, in the presence of an oxidation catalyst composed of a heavy metal compound and a bromine compound in an oxidizing reactor. In this production 1A process, a mixture composed of an alkyl-substituted aromatic compound, such as paraxylene, as the starting material, acetic acid as the reaction solvent and a catalyst is supplied to the oxidizing reactor while introducing thereinto a molecular oxigen-containing gas, such as air, to cause oxidation, whereby an aromatic carboxylic acid, such as terephthalic acid, is produced. There has been proposed a process in which condensate formed in a condensation step by condensing the exhaust gas from the oxidizing reactor is guided into a distillation column to subject to distillation and the resulting fractions containing the reaction solvent are returned to the oxidation reactor. There has also been proposed a process, wherein a distillation column is arranged so as to communicate with the reactor upper part to subject the oxidation exhaust gas to rectification under utilization of the enthalpy of. the oxidation exhaust gas to recover the reaction solvent by returning it to the oxidation reactor. In Indian Patent no. 133997 and Indian Patent no 180885. In this process, the overhead efflux gas from the distillation column top is cooled by cooling water in a condenser to condense steam contained in the efflux gas and the resulting condensate is refluxed to the distillation column. In such a technique comprising distillation of the oxidation exhaust gas or its condensate, the height of the distillation column should be large enough for 2 recovering the reaction solvent, such as acetic acid, by separating it completely from water and alcohol in the distillation column, as a lower column height may bring about transference of a part of the reaction solvent to the condensate side. Since water is formed by the oxidation reaction, a part of the condensate should be withdrawn from the reaction system, wherein a difficulty may be encountered in the waste water treatment due to the entrained reaction solvent and so on. A process has further been proposed, in which esters of aliphatic carboxylic acids, such as methyl acetate etc., formed on the oxidation reaction are collected by causing them to be absorbed in scrubbing water and are hydrolyzed using an ion-exchange resin as the hydrolysis catalyst to recover corresponding aliphatic carboxylic acid to reuse as the reaction solvent. However, this may endure a difficulty of separation of the alcohol with the aliphatic carboxylic acid, DISCLOSURE OF THE INVENTION An object of the present invention is to provide a process for producing an aromatic carboxylic acid, in which an efficient recovery of the aliphatic carboxylic acid can be attained under efficient separation thereof from by-products, such as water and alcohol, with simultaneous attainment of easy waste water treatment. 3 Another object of the present invention is to provide a process for producing an aromatic carboxylic acid, in which useful ingredients can be recovered under efficient hydrolysis of the ester of aliphatic carboxylic acid formed in the reaction system. Accordingly, process for producing an aromatic carboxylic acid, such as herein described, under high pressure and high temperature by subjecting an alkylaromatic compound, such as herein described, to a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst, such as herein described, in a reaction solvent, such as herein described, comprising an aliphatic carboxylic acid in an oxidizing reactor, comprising proceeding the reaction while removing the water and the alcohol formed by the oxidation reaction by means of membrane-separation. (2) The process as defined in the above (1), wherein the process comprises the steps of oxidizing an alkylaromatic compound by a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst in a reaction solvent comprising an aliphatic carboxylic acid in an oxidizing reactor to form an aromatic carboxylic acid under high pressure and high temperature, performing distillation of the oxidation exhaust gas or of the condensate thereof in a distillation column by guiding the oxidation exhaust gas from the oxidizing reactor into the distillation column or passing it to an oxidation exhaust gas 4 condenser and guiding the resulting condensate to the distillation column, while returning the distillation fractions- containing the reaction solvent to the oxidizing ractor, condensing the distillation overhead gas by-cooling it in a condenser to form a condensate and membrane-separating the condensate to cause water and alcohol to permeate through a membrane to the permeate side and to cause the aliphatic carboxylic acid to be retained on the concentrate side to recover it. (3) The process as defined in the above (2), wherein the process comprises further an absorption step comprising causing the efflux gas from the condensation step to contact with an absorbent liquor to absorb aliphatic carboxylic acid esters therein and supplying the so-absorbed liquor to the membrane-separation step. (4) The process as defined in the above (2), wherein the process comprises further an absorption step comprising causing the efflux gas from the oxidation exhaust gas condenser to contact with an absorbent liquor to absorb aliphatic carboxylic acid esters therein and supplying the so-absorbed liquor to the membrane-separation step. (5) The process as defined in any one of the above (2) to (4), wherein the process comprises further a hydrolysis step comprising causing the condensate and/or the absorbed liquor and/or the membrane-separated permeate and/or the concentrate to contact with a hydrolysis catalyst to decompose the aliphatic 5 carboxylic acid esters while supplying the resulting hydrolysate to the membrane-separation step. (6) The process as defined in any one of the above (1) to (5), wherein the membrane-separation is performed using a reverse-osmosis membrane. (7) The process as defined in any one of the above (4) to (6), wherein the hydrolysis catalyst is an ion-echange resin. (8) The process as defined in any one of the above (1) to (7), wherein the whole or a part of the condensate is subjected to the membrane-separation and the concentrate is returned to the distillation step. (9) The process as defined in any one of the above (1) to (8), wherein the membrane-separation is performed on a plurality of separation stages. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow diagram of an embodiment of the process for producing terephthalic acid. Fig. 2 is a flow diagram of another embodiment of the prcess for producing terephthalic acid. Fig. 3 is a flow diagram of a further embodiment of the process for producing terephthalic acid. Fig. 4 is a flow diagram of a still further embodiment of the process for producing terephthalic acid. Fig. 5 is a flow diagram of a still further embodiment of the process for producing terephthalic acid. 6 THE BEST MODE FOR EMBODYING THE INVENTION According to the present invention, the oxidation reaction is carried out while removing the reaction water and the alcohol formed upon the reaction by membrane-separation. Here, it is favorable to incorporate a distillation step in combination with a membrane-separation , in order to separate efficiently the aliphatic carboxylic " acid to be used as the reaction solvent, such as acetic acid, from unnecessary ingredients, such as the reaction water and the alcohols, formed upon the reaction. If the separation is realized only by distillation, it is necessary to design the distillation column to have larger column height, as mentioned previously. On the other hand, if the separation is realized only by membrane-separation, the separation efficiency is inferior and, therefore, a multistage membrane-separator is required. However, by employing a distillation column having lower column height to effect a predominant part of the separation and performing a membrane separation for the resulting distillate of lower concentration, an efficient separation can be attained by means of a small scale arrangement. As the raw material to be oxidized for producing an aromatic carboxylic acid by the process according to the present invention, aromatic compounds having one or more substituent alkyl or partially oxidized alkyl groups (hereinafter, sometimes referred to merely as the oxidizable raw material) may be used. Such an 7 aromartic compound may either be a monocyclic or polycyclic one. As the substituent alkyl group, there may be exemplified alkyl groups having 1 - 4 carbon atoms, such as methyl, ethyl, n-propyl and isopropyl. As the partially oxidized alkyl group, there may be exemplified aldehydo, acyl, carboxyl and hydroxyalkyl. Concrete examples of the alkyl-substituted aromatic hydrocarbon include di- and polyalkylbenzenes having 2-4 alkyl groups of 1 - 4 carbon atoms, such as m-diisopropylbenzene, p-diisopropylbenzene, m-cymene, p-cymene, m-xylene, p-xylene, trimethylbenzenes and tetramethylbenzenes; di- and polyalkylnaphthalenes having 2-4 alkyl groups of 1 - 4 carbon atoms, such as dimethylnaphthalenes, diethylnaphthalenes and diisopropylnaphthalenes; and polyalkylbiphenyls having 2-4 alkyl groups of 1 - 4 carbon atoms, such as dimethylbiphenyls. The aromatic compound having one or more partially oxidized substituent alkyl groups is a compound in which one or more substituent alkyl groups are partially oxidized into aldehydo, acyl, carboxyl or hydroxyalkyl as mentioned above. Concrete examples thereof include 3-methylbenzaldehyde, 4-methylbenz-aldehyde, m-toluic acid, p-toluic acid, 3-formylbenzoic acid, 4-formylbenzoic acid and 2-methyl-6-formyl-naphthalene. They may be employed either individually or in a combination of two or more of them. In the process according to the present invention, a heavy metal compound and a bromine compound are used as the catalyst. For such compounds, 8 the followings may be exemplified: Thus, for the heavy metal in the heavy metal compounds, for example, cobalt, manganese, nickel, chromium, zirconium, copper, lead, hafnium and cerium, may be enumerated. They may be used either alone or in a combination of two or more of them, wherein preferance is given to a combination of cobalt and manganese. For such heavy metal compounds, there may be exemplified acetates, nitrates, acetylacetonates, naphthenates, stearates and bromides, wherein particular preference is given to acetates. For the bromine compounds, there may be exemplified molecular bromine; inorganic bromine compounds, such as hydrogen bromide, sodium bromide, potassium bromide, cobalt bromide and manganese bromide; and organic bromine compounds, such as methyl bromide, methylene bromide, bromoform, benzylbromide, bromo-methyltoluene, dibromoethane, tribromoethane and tetrabromoethane. They may be used either alone or in a combination of two or more of them. The catalyst constituted of a combination of the heavy metal compound and the bromine compound according to the present invention may favorably have a proportion of the bromine atom to the heavy metal atom in the range from 0.05 to 10 moles, preferably from 0.1 to 2 moles, of bromine per 1 mole of the heavy metal. Such a catalyst may favorably be used usually in an amount, as the concentration of heavy metal, in the range from 10 to 10,000 ppm by weight, preferably from 100 to 5,000 ppm by weight. 9 In the process according to the present invention, an aromatic carboxylic acid as a product of manufacture is obtained by subjecting the aromatic compound used as the oxidizable raw material to a liquid phase oxidation in the oxidation step with a molecular oxygen-containing gas in a reaction solvent comprising a lower aliphatic carboxylic acid in the presence of the above-mentioned catalyst in an oxidizing reactor. As the molecular oxygen-containing gas, for example, oxygen gas and air may be recited, wherein air is used favorably for practical convenience. The molecular oxygen-containing gas is supplied to the reaction in excess of the requisite amount for oxidizing the aromatic compound to be used as the oxidizable raw material into the aromatic carboxylic acid. When air is used as the molecular oxygen-containing gas, it may favorably be supplied to the reaction system at a rate in the range from 2 to 20 Nm3 , preferably from 2.5 to 15 Nm3 , per one kg of the aromatic compound to be used as the oxidizable raw material. As concrete examples of the lower aliphatic carboxylic acid to be used as the reaction solvent, there may be recited acetic acid, propionic acid and butyric acid. The lower aliphatic carboxylic acid may be used as the reaction solvent either individually or in a mixture with water. Concrete examples of the reaction solvent may comprise acetic acid, propionic acid, butyric acid and mixtures of them and, further, 1 0 mixtures of them with water. Among them, preference is given to a mixture of acetic acid with water, in particular/ a mixture of 1 - 20 parts, preferably 5 -15 parts, by weight of water with 100 parts by weight of acetic acid. The temperature for the oxidation reaction may, in general, favorably be in the range from 100 to 250 °C, preferably from 150 to 220 °C. For the pressure for the oxidation reaction, any pressure above a value at which the reaction system is maintained in a liquid phase may be permissible. By performing the oxidation reaction in the manner as above, an aromatic carboxylic acid corresponding to the aromatic compound used as the oxidizable raw material can be obtained. As concrete examples of the aromatic carboxylic acid, there may be recited aromatic dicarboxylic acids, such as terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid and 4.4'-biphenyl dicarboxylic acid; aromatic tricarboxylic acids, such as trimellitic acid and trimesic acid; and aromatic polycarboxylic acids, such as pyromellitic acid and so on. The process for producing aromatic carboxylic acid according to the present invention may favorably be applied to the production of aromatic dicarboxylic acids or to the production of aromatic carboxylic acids insoluble or difficultly soluble in the reaction solvent, in particular, to the production of terephthalic acid. The thereby formed aromatic carboxylic acid, 1 1 such as terephthalic acid, will deposit out as crystals to form a slurry which is guided out from the oxidizing reactor to solid/liquid separation to recover the crystals to obtain a crude product, such as crude terephthalic acid. In the crystals of the so-obtained crude product, entrained impurities including intermediates of the oxidation reaction are contained, so that the crude product is then subjected to purification step comprising dissolving the crude product, treating by oxidation, treating by reduction and crystallizing the objective product, i.e. terephthalic acid, in order to obtain a slurry containing the crystals of the product. By collecting the crystals from the slurry, purified objective product, such as pure terephthalic acid, can be obtained. In the distillation step, it is favorable to carry out the distillation by guiding the oxidation exhaust gas into a distillation column (high-pressure distillation column) connected to the top of the oxidizing reactor by making use of the heat of the exothermic oxidation reaction, while it is permissible to carry out distillation of a condensate formed in an oxidation exhaust gas condenser, in which the oxidation exhaust gas is subjected to condensation, by supplying this condensate to a distillation column (normal pressure distillation column). In both cases, the fractions containing the reaction solvent are collected in . the column bottom and are returned to the oxidizing reactor, while the column overhead gas containing water 1 2 vapor and non-condensing gases is exhausted out at the column top. The distillation column may be one which is independent of the oxidation reactor, as disclosed in Japanese Patent Publication Sho 54-14098 B, or one which is connected directly to the top of the oxidizing reactor, as disclosed in Japanese Patent Kokai Hei 6-279353 A. While the distillation column may be a plate column, preference is given to a packed column which may favorably has a means for collecting fine solid particles, such as crystals of the aromatic carboxylic acid, namely, for example, one or more solid matter collecting trays, arranged at lower part of the packed column. The distillation column may favorably be constructed in such a manner that a plurality of subsidiary columns are connected successively to effect distillation of the exhaust gas from the preceding sub-column in the subsequent sub-column successively, while refluxing the distillate of the subsequent sub-column to the preceding sub-column, wherein it is preferable that distillates may be withdrawn out of the system at intermediate portions between the sub-columns upon emergency cessation of the operation, in order to prevent reduction of the concentration and temperature of the reaction liquor in the oxidizing reactor. By using such a distillation column to perform distillation of the oxidation exhaust gas or of the condensate from an oxidation exhaust gss condenser, in which the oxidation exhaust gas is subjected to condensation, the fractions containing the reaction 1 3 solvent entrained in the oxidation exhaust gas are returned to the oxidizing reactor. These fractions contain portions of the unreacted alkyl aromatic compound, aromatic carboxylic acid formed, catalyst and so on in a concentrated state, in addition to the reaction solvent, and are collected in the column bottom, from which they are returned to the oxidizing reactor. Among these components, solid matters, . such as the crystals of the aromatic carboxylic acid and the catalyst, and components having higher boiling points are collected at lower portion of the distillation column and the reaction solvent, such as an aliphatic carboxylic acid having low boiling point, is withdrawn at a relatively high portion. While these fractions may be refluxed as such to the oxidizing reactor, it is possible to withdraw a fraction having higher concentration of acetic acid from a lower portion of the distillation column by arranging a liquid fraction withdrawal means at a lower portion of the distillation column, in order to utilize the so-withdrawn fraction as an absorbent liquor for absorbing esters of the aromatic carboxylic acid and, further, as a washing liquor for washing the crystals separated on the solid/liquid separation of the slurry withdrawn from the oxidizing reactor. When such a fraction is withdrawn out of the system upon an emergency cessation of the reaction, it is possible to prevent such a circumstance that a large amount of the refluxing liquor is introduced into the oxidizing reactor to dilute the reaction liquor. 1 4 In the condensation step, the distillation overhead gas discharged from the distillation column top is cooled by cool ing water in a condenser to condense steam contained in the discharged gas into a condensate. The condenser may also be constituted of a unit of single construction or a unit subdivided into a plurality of sub-units. The temperature on the condensation may be such that steam can be condensed, wherein esters of the aliphatic carboxylic acid may or may not be condensed. The resulting condensate may be guided either wholly to the membrane-separation step or partly to the membrane-separation step while returning a part thereof to the distillation column. In the membrane-separation step, a part or the whole of the condensate is subjected to a membrane-separation to cause water and the by-products, such as alcohols, to permeate the membrane to the permeate side while retaining the aliphatic carboxylic acid on the concentrate side to concentrate therein, in order to attain separation of them. For the separation membrane for realizing such separation, a reverse-osmosis membrane may be employed. As the reverse-osmosis membrane, there may be recited those based on polyamide, aromatic polyamide, polyacrylonitrile, polypropylene, polyvinylidene fluoride, polyfluoroethylene, polyvinyl alcohol, polyester, polyimide, polysulfone, polyether sulfone, cellulose acetate, cellulose esters, triacetyl cellulose, polyether urate, polypiperic azamides, polyfurane and polyethyleneimine, wherein a particular preference is given to reverse-osmosis membranes based 1 5 on cross-linked aromatic polyamide. The separation mambrane may be in any voluntary configuration including plane, tubular and spiral forms as well as hollow fibrous form. The separation membrane functions to attain the separation based on the difference in the molecular size, ionic activity or so on, for which appropriate one permitting permeation of water and the by-products, such as the alcohols, therethrough to the permeate side and retaining the aliphatic carboxylic acid on the concentrate side to concentrate therein may be employed. The membrane permits to permeate the esters of the aliphatic carboxylic acid therethrough by a proportion of about 20 % and, thus, to retain them on the concentrate side by a proportion of about 80 %. The membrane-separation step may be realized on one single stage but, preferably, on plural stages. By subjecting the condensate to membrane-separation, unnecessary ingredients by-produced in the oxidation reaction, such as water and alcohols, such as methanol, can be separated by permeating through the membrane to the permeate side. The aliphatic carboxylic acid, such as acetic acid, to be used as the reaction solvent is retained on the concentrate side and is concentrated therein. If the separation of acetic acid from water is carried out only by distillation, the distillation column to be used should have a large height, since the boiling point of water is close to that of acet ic acid. By the membrane-separation, however, they can be separated easily due to the difference in the molecular size and in the 1 6 ionic activity. If the separation is incomplete, the membrane separation may be performed on plural stages. The resulting permeate can be treated by an adequate separation means to separate water and the by-produced products, such as alcohols, and the separated water may be discharged out of the system or be utilized, for example, as washing liquor for washing the aromatic carboxylic acid produced. The concentrate is returned to the oxidation step, wherein it is favorable to return a part or 'the whole of the condensate to the top of the distillation column to support the distillation. By restricting the condensation temperature in the condensation step, it is possible to cause the esters of the aliphatic carboxylic acid to be condensed or to be transferred to the side of the efflux gas. In case the esters of the aliphatic acid are to be transferred to the efflux gas, they are absorbed in an absorption step in an absorbent liquor by contacting the gas with the absorbent liquor. As the absorbent liquor, water may be used, while acetic acid may also be used. The absorbed liquor may be guided as such to the membrane-separation step, while it is permissible to guide it to the membrane-separation step after it has been subjected to hydrolysis in a hydrolysis step. In the hydrolysis step, the condensate and/or the absorbed liquor and/or the permeate and/or the concentrate resulting from the membrane-separation of them are caused to contact with a hydrolysis catalyst to effect hydrolysis of the esters of the aliphatic 1 7 carboxylic acid. In the case of using a normal pressure distillation column, it is preferable to subject the whole of the permeate resulting from the membrane-separation to the hydrolysis. As the hydrolysis catalyst, an ion-exchange resin in H-form, preferably a strongly acidic cation-exchange resin, in particular, that of macroreticular type may be used favorably, though a catalyst consists of acid or of alkali may also be used. The esters of the aliphatic carboxylic acid will be decomposed into corresponding alcohols and aliphatic carboxylic acid, so that they can be separated on subjecting the hydrolysate to the membrane-separation, as described above. On treating the permeate or the concentrate from a membrane-separation stage by hydrolysis, the resulting hydrolysate will be subjected to membrane-separation on the subsequent membrane-separation stage or, for the concentrate, the membrane-separation may also be attained after being returned to the oxidation step and then distilled to form a condensate which is guided to the membrane-separation. According to the present invention, the oxidation reaction can be realized, while the water and the alcohols formed upon the reaction are removed by membrane-separation, whereby the objective carboxylic acid can be recoverd under an efficient separation from components unnecessary for the reaction, such as water and by-products, such as alcohols etc., with simultaneous attainment of easy waste water treatment. 1 8 By combining distillation with a membrane-separation, aromatic carboxylic acid can be obtained while being separated efficiently from components unnecessary for the reaction, such as water and by-products, such as alcohols etc., even though a distillation column having small column height is employed, with attainment of easy waste water treatment. By incorporating an absorption step, esters of aliphatic carboxylic acid can be recovered from the efflux gas from the condensation step. By subjecting the esters of the aliphatic carboxylic acid to hydrolysis, the efficiency of utilization can be increased by separating the aliphatic carboxylic acid and the alcohols by membrane-separation. Here, the separation of the aliphatic carboxylic acid from the alcohols is made easy by combining the hydrolysis step with the membrane-separation step. Moreover, esters of the aliphatic carboxylic acid present in the oxidation exhaust gas can be recovered and brought into effective utilization by combining the absorption step with the hydrolysis step and the membrane-separation step, with simultaneous attainment of increase in the efficiency of separation of the aliphatic carboxylic acid from the alcohols. EMBODIMENT OF THE INVENTION BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Below, the present invention will be described by way of embodiments of the process for producing terephthalic acid with reference to the drawings 1 9 appeended. Figs. 1 to 3 show each an embodiment of the invention using a high-pressure distillation column in a flow diagram. Figs. 4 and 5 show each an embodiment of the invention using a normal pressure distillation column in a flow diagram. In Figs. 1 to 3, 1 denotes an oxidizing reactor which is connected at the top directly to a distillation column 2 as a high-pressure distillation column. 3 is a condenser, 4 and 5 are each a cooler, 6 is an absorption column and 7 is a pump. 8 and 8a denote each a membrane-separation unit having a separation membrane 9 or 9a, respectively. 10 and 10a indicate each a hydrolysis vessel having a catalyst layer 11 or lla consisting of an ion-exchange resin, respectively. In the process for producing terephthalic acid, of which embodiments are given by the flow diagrams of Figs. 1 to 3, there are supplied to the oxidizing reactor 1 paraxylene, as an alkyl aromatic compound to be used as the raw material, acetic acid, as the reaction solvent, and a heavy metal compound and a bromine compound, as the catalyst, via a line LI, while supplying thereto air, as an molecular oxygen-containing gas, via a line L2, to effect a liquid phase oxidation at a high temperature under a high pressure to form terephthalic acid. The resulting terephthalic acid deposits out as crystals to form a slurry which is withdrawn via a line L3. In Fig. 1, the oxidation exhaust gas is guided in a state of high-temperature and high-pressure to the 2 0 distillation column 2, in which distillation is realized during the gas passes through a packing layer 12. Most part of the solvent will be distilled out together with the raw material, paraxylene, and the catalyst contained in the oxidation exhaust gas and the fractions containing these ingredients are refluxed to the oxidizing reactor 1. The efflux gas containing a part of acetic acid and the lower boiling by-products, such as methanol etc., is guided into the condenser 3, where it is cooled and acetic acid, water, methanol and other by-products are condensed to form a condensate. A part of methyl acetate, which is an ester of the aliphatic carboxylic acid, is also condensed here. The condensate is refluxed partly to the distillation column 2 as such and collected partly on a collector 13 and withdrawn via a line L4 and is cooled by the cooler 4 before being transferred to a concentrate chamber 14 of the membrane-separation unit 8 under pressure elevation by the pump 7. By the membrane-separation, non-ionic low molecular weight ingredients, such as water and methanol, are caused to permeate through the membrane 9 into a permeate chamber 15. The permeate is discharged out via a line L5 and the concentrate is returned to the top of the distillation column 2 via a line L6. The efflux gas from the top of the condenser 3 is guided into the cooler 5 via a line L7 where it is cooled before being introduced into the absorption column 6, in which it is brought into contact, on passing through the packing layer 16, with an absorbent 2 1 liquor supplied thereto via a line L8, whereby methyl acetate contained in the efflux gas is absorbed in the absorbent liquor. The efflux gas deprived of methyl acetate is discharged out via a line L9. The absorbent liquor having absorped methyl acetate flows through a line L10 and joins with the condensate flowing in a line L4, whereupon the resulting mixture is supplied to membrane-separation unit 8. Methyl acetate may either be retained on the concentrate side or permeate through the separation membrane 9 in accordance with the separation performance of the membrane used, wherein the hydrolysis thereof can be attained by a suitable technique for both the cases. In the embodiment of Fig. 2, the hydrolysis vessel 10 containing the catalyst layer 11 is disposed above the cooler 5. In this embodiment, methyl acetate present in the cooled efflux gas from the cooler 5 is hydrolyzed into methanol and acetic acid upon passing through the hydrolysis vessel 10. The portions of acetic acid and methyl acetate remaining in the gas passed through the hydrolysis vessel 10 are recovered in the absorption column 6 on contacting the gas with water supplied from the line L8. When the hydrolysate is guided via the line 10 into the membrane-separation unit 8 together with the condensate from the line L4, acetic acid is retained on the concentrate side and methanol is caused to permeate to the permeate side. Thus, even in the case of using acetic acid as the absorbent liquor in the absorpotion column 6, recovery of acetic acid is attained while being sent to the 2 2 distillation column 2. Other constructions and operations of the arrangement correspond to those in the embodiment of Fig. 1. In the embodiment of Fig. 3, the whole of the efflux gas from the distillation column 2 is guided into the condenser 3 via the line L7 and is subjected to condensation therein. The resulting condensate is transferred as a mixture with the absorbenet liquor which has absorbed methyl acetate in the absorption column 6 to the membrane-separation unit 8 via the line L10, where it is separated by membrane-separation into each component, i.e. acetic acid, water and methanol. A part of methyl acetate is retained on the concentrate side and the other part permeates to the permeate side. The permeate is guided into the hydrolysis vessel 10a, where methyl acetate is hydrolyzed into acetic acid and methanol by the catalyst layer lla. The hydrolysate is subjected to a membrane-separation in the membrane-separation unit 8a, wherein acetic acid is retained on the concentrate side and methanol and water are caused to permeate to the permeate side. The concentrates in the membrane-separation unit 8 and 8a are put together and guided via the line L6 to the hydrolysis vessel 10, where hydrolysis of methyl acetate is performed, and are then returned to the top of the distillation column 2. During circulation of acetic acid and methanol returned to the oxidizing reactor in accompaniment with the treatment cycle of the oxidation exhaust gas, they are separated from each other in the membrane-separation unit 8. 2 3 In the embodiments of Figs. 4 and 5, the oxidizing reactor 1 is connected at its top with a normal pressure distillation column as the distillation column 2 under interposition of an oxidation exhaust gas condenser 3a and a pressure reduction valve 18. The distillation column 2 is connected with a condenser 3. To the oxidation exhaust gas condenser 3a are connected a cooler 5 and an absorption column 6. For producing terephthalic acid by the apparatuses of Figs. 4 and 5, the reactor 1 is supplied via the line L1 with paraxylene, as the raw material of alkylaromatic compound, with acetic acid, as the reaction solvent, and with a heavy metal compound and a bromine compound, as the catalyst, while supplying thereto via the line L2 with air, as the molecular oxygen-containing gas, to effect a liquid phase oxidation under high temperature and high pressure to form terephthalic acid. The thereby formed terephthalic acid deposits out as crystals to form a slurry which is withdrawn via the line L3. In the embodiment of Fig. 4, the oxidation exhaust gas is guided in the state of high temperature and high pressure into the oxidation exhaust gas condenser 3a via a line L11 and is cooled there. A part of the condensate is returned as such to the oxidizing reactor 1 and the other part is guided into the distillation column via the line L12 through the pressure reduction valve 18 and is subjected to distillation by being heated by a reboiler 17. The efflux gas from the oxidation exhaust gas condenser 3a 2 4 is guided via a line L13 to a cooler 5, where it is cooled and is then transferred to an absorption column 6, in which it is brought into contact with an absorbent liquor supplied thereto via a line L8 while it passes through a packing layer 16 to thereby remove methyl acetate by absorption. The resulting efflux gas deprived of methyl acetate is discharged out via a line L9. The absorbent liquor having absorbed methyl acetate is guided through a line L10 to a line L12 and is put together with the condensate, whereupon the resulting mixture is fed to the distillation column 2. During distillation in the distillation column 2, a predominant part of the reaction solvent, i.e. acetic acid, contained in the oxidation exhaust gas is returned to the oxidizing reactor 1 under rectification. The distillation overhead efflux containing the reaminder of acetic acid and the low-boiling by-products, such as methanol etc., is guided via a line L14 to a condenser 3, where it is cooled to condense the remaining acetic acid, steam, methanol and other by-products to form a condensate. Here, a part of methyl acetate, which is an ester of the aliphatic carboxylic acid, is also condensed. The condensate is transferred as such to a concentrate chamber 14 of a membrane-separation unit 8 under pressure elevation by a pump 7, whereby nonionic low molecular weight substances, such as water and methanol, are caused to permeate through the separation membrane 9 to the permeate chamber 15 by the membrane-separation. The permeate is discharged out via a line 2 5 L5 and the concentrate is returned to the top of the distillation column 2 via a line L6. In the embodiment of Fig. 5, a hydrolysis vessel 10 having a catalyst layer 11 is installed in the line L5 with ommission of cooler 5. In this arrangement, the permeate from the membrane-separation unit 8 is subjected to hydrolysis of methyl acetate contained therein on passing through the hydrolysis vessel 10 and the resulting hydrolysate containing methanol and acetic acid is returned to the distillation column 2 at its upper portion. EXAMPLES Below, the present invention will be described by way of Examples. In the Examples, % and ppm are on the weight basis so long as not specifically mentioned. Reference Example 1 In the oxidizing reactor of Fig. 1, paraxylene was oxidized at 190 °C under 1.2 MPa to produce terephthalic acid. The oxidation exhaust gas was subjected to diatillation in the distillation column 2, with condensation in the condenser 3 at 160 °C. The overhead efflux gas from the distillation column 2 was brought into contact with an absorbent liquor consisting of 10 kg/hr of water at 35 °C in the absorption vessel 6 to absorb methyl acetate therein, whereupon 50 kg/hr of the resulting absorbed liquor . were brought together with 55 kg/hr of the condensate 2 6 guided from the condenser and the resulting 105 kg/hr of the mixed liquid were discharged out of the system. The mixed liquid contained 1.5 % of acetic acid, 2,650 ppm of methyl acetate and 1040 ppm of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 1020 ppm by volume of methyl acetate and 106 ppm by volume of methanol. Example 1 The procedures of Reference Example 1 were changed such that 205 kg/hr of condensate from the condenser 3 were brought together with 50 kg/hr of the absorbed liquor from the absorption column 6 and the resulting 255 kg/hr of the mixed liquid were introduced into the membrane-separation unit 8 under pressure elevation up to 4.9 MPa to subject to a membrane-separation, whereupon 150 kg/hr of the concentrate were returned to the distillation column 2 and 105 kg/hr of the permeate were discharged out of the system. As the separation membrane, a reverse-osmosis membrane SU-820 (trademark, a product of Toray Industries, Inc.) constituted of a cross-linked polyamide composite membrane was used. The permeate contained 0.3 % of acetic acid, 303 ppm of methyl acetate and 1010 ppm of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 1530 ppm by volume of methyl acetate and 305 ppm by volume of methanol. Example 2 The apparatus of Fig. 2 was operated in such as manner that the absorbent water was supplied to the 2 7 absorption column 6 at a rate of 50 kg/hr and 90 kg/hr of hydrolysate liquor from the hydrolysis vessel 10 resulting after the hydrolysis of the absorbed water were brought together with 205 kg/hr of the condensate from the condenser 3, whereupon the resulting 295 kg/hr of the mixed liquid were subjected to a membrane-separation in the membrane-separation unit 8, wherein 150 kg/hr of the concentrate were returned to the distillation column 2 and 145 kg/hr of the permeate were discharged out of the system. The hydrolysis catalyst consisted of a strongly acidic cation exchange resin of macroreticular type (Amberlist 15, trademark, of Japan Organo Co., Ltd.). Other procedures were the same as in Example 1. The permeate contained 0.5 % of acetic acid, 550 ppm of methyl acetate and 1700 ppm of methanol. The efflux gas from the absorption column contained 0.5 ppm by volume of acetic acid, 103 ppm by volume of methyl acetate and 1100 ppm by volume of methanol. Example 3 The apparatus of Fig. 3 was operated in such as manner that the absorbent water was supplied to the absorption column 6 at a rate of 10 kg/hr and 900 kg/hr of the absorbed condensate were passed to the membrane-separation units 8 and 8a and to the hydrolysis vessels 10 and 10a under pressure elevation up to 7.8 MPa by the pump 7. The concentrates of the membrane-separation units 8 and 8a were brought together at flow rates of 650 kg/hr and 145 kg/hr, respectively, and the resulting mixture was returned to the distillation column after 2 8 passing through the hydrolysis vessel 10, while 105 kg/hr of the permeate from the membrane-separation unit 8a were discharged out of the system. Other procedures were the same as in Example 2. The permeate contained 0.1 % of acetic acid, 50 ppm of methyl acetate and 2010 ppm of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 212 ppm by volume of methyl acetate and 1050 ppm by volume of methanol. Reference Example 2 In the oxidizing reactor of Fig. 4, paraxylene was oxidized at 190 ° C under 1.2 MPa to produce terephthalic acid. The oxidation exhaust gas was cooled in the oxidation exhaust gas condenser 3a at 120 °C to condense it. The efflux gas from the oxidation exhaust gas condenser 3a was cooled in the cooler 5 to 40 ° C and was brought into contact with an absorbent liquor consisting of 10 kg/hr of water at 35 °C in the absorption column 6 to absorb methyl acetate therein, whereupon 210 kg/hr of the resulting absorbed liquor were brought together with 465 kg/hr of the condensate guided from the condenser 3a and the resulting 685 kg/hr of the mixed liquid were subjected to distillation in the distillation column 2 at a reflux ratio of 4 and 105 kg/hr of the distilate from the column top were discharged out of the system. The discharged liquid contained 1.5 % of acetic acid, 1.6 % of methyl acetate and 0.1 % of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 500 ppm by volume of methyl acetate and 30 ppm by 2 9 volume of roethanol. Example 4 The procedures of Reference Example 2 were changed such that the condensate from the condenser 3 was introduced into the membrane-separation unit 8 under pressure elevation up to 4.9 MPa to subject to a membrane-separation, whereupon 525 kg/hr of the concentrate were returned to the distillation column 2 and 105 kg/hr of the permeate were discharged out of the system. As the separation membrane, a reverse-osmosis membrane SU-820 (trade name, a product of Toray Industries, Inc.) constituted of a cross-linked polyamide composite membrane was used. The permeate contained 0.3 % of acetic acid, 0.3 % of methyl acetate and 0.08 % of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 700 ppm by volume of methyl acetate and 50 ppm by volume of methanol. 3 0 WE CLAIM: 1. A process for producing an aromatic carboxylic acid, such as herein described, under high pressure and high temperature by subjecting an alkylaromatic compound, such as herein described, to a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst, such as herein described, in a reaction solvent, such as herein described, comprising an aliphatic carboxylic acid in an oxidizing reactor, comprising proceeding the reaction while removing the water and the alcohol formed by the oxidation reaction by means of membrane-separation. 2. The process as claimed in claim 1, wherein the process comprises the steps of oxidizing an alkylaromatic compound by a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst in a reaction solvent comprising an aliphatic carboxylic acid in an oxidizing reactor to form an aromatic carboxylic acid under high pressure and high temperature, performing distillation of the oxidation exhaust gas or of the condensate thereof in a distillation column by guiding the oxidation exhaust gas from the oxidizing reactor into the distillation column or passing it to an oxidation exhaust gas condenser and guiding the resulting condensate to the distillation column, while returning the distillation fractions containing the reaction solvent to the oxidizing reactor, condensing the distillation overhead gas by 3 1 cooling it in a condenser to form a condensate and membrane-separating the condensate to cause water and alcohol to permeate through a membrane to the permeate side and to cause the aliphatic carboxylic acid to be retained on the concentrate side to recover it. 3. The process as claimed in claim 2, wherein the process comprises further an absorption step comprising causing the efflux gas from the condensation step to contact with an absorbent liquor to absorb aliphatic carboxylic acid esters therein and supplying the so-absorbed liquor to the membrane-separation step. 4. The process as claimed in claim 2, wherein the process comprises further an absorption step comprising causing the efflux gas from the oxidation exhaust gas condenser to contact with an absorbent liquor to absorb aliphatic carboxylic acid esters therein and supplying the so-absorbed liquor to the membrane-separation step. 5. The process as claimed in any one of claims 2 to 4, wherein the process comprises further a hydrolysis step comprising causing the condensate and/or the absobed liquor and/or the membrane-separated permeate and/or the concentrate to contact with a hydrolysis catalyst to decompose the aliphatic carboxylic acid esters while supplying the resulting hydrolysate to the membrane-separation step. 6. The process as claimed in any one of claims 1 to 5, wherein the membrane-separation is performed using a reverse-osmosis membrane. 7. The process as claimed in any one of claims 4 3 2 to 6, wherein the hydrolysis catalyst is an ion-exchange resin. 8. The process as claimed in any one of claims 1 to 7, wherein the whole or a part of the condensate is subjected to the membrane-separation and the concentrate is returned to the distillation step. 9. The process as claimed in any one of claims 1 to 8, wherein the membrane-separation is performed on a plurality of separation stages. 3 3 10. A process for producing an aromatic carboxylic acid, substantially as herein described, particularly with reference to the accompanying drawings. - 34 - A process for producing an aromatic carboxylic acid under high pressure and high temperature by subjecting an alkylaromatic compound to a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst in a reaction solvent comprising an aliphatic carboxylic acid in an oxidizing reactor while recovering the aliphatic carboxylic acid by ditillation of the oxidation exhaust gas, in which the recovery of the aliphatic carboxylic acid can be attained efficiently under separation thereof from by-products, such as water and alcohols, with permission of easier waste water treatment, wherein the process comprises proceeding the reaction while removing the water and the alcohol formed by the oxidation reaction by means of membrane-separation.

Full Text

SPECIFICATION
FIELD OF THE INVENTION
The present invention relates to a process for producing an aromatic carboxylic acid by a liquid phase oxidation of an alkylaryl compound having one or more substituent alkyl or partially oxidized alkyl groups by a molecular oxygen-containing gas.
BACKGROUND OF THE INVENTION
Aromatic carboxylic acids are important as fundamental chemical product and, in particular, aromatic dicarboxylic acids are useful for the starting material of fibers, resins and the like. For example, terephthalic acid has found growing demand as the starting material for polyester fiber in recent years. For producing aromatic carboxylic acids, a process has hitherto been employed in general in which an alkyl-substituted aromatic compound is subjected to a liquid phase oxidation by bringing it into contact with a molecular oxygen-containing gas in a reaction solvent comprising a lower aliphatic carboxylic acid, such as acetic acid, in the presence of an oxidation catalyst composed of a heavy metal compound and a bromine compound in an oxidizing reactor. In this production
1A

process, a mixture composed of an alkyl-substituted aromatic compound, such as paraxylene, as the starting material, acetic acid as the reaction solvent and a catalyst is supplied to the oxidizing reactor while introducing thereinto a molecular oxigen-containing gas, such as air, to cause oxidation, whereby an aromatic carboxylic acid, such as terephthalic acid, is produced.
There has been proposed a process in which condensate formed in a condensation step by condensing the exhaust gas from the oxidizing reactor is guided into a distillation column to subject to distillation and the resulting fractions containing the reaction solvent are returned to the oxidation reactor. There has also been proposed a process, wherein a distillation column is arranged so as to communicate with the reactor upper part to subject the oxidation exhaust gas to rectification under utilization of the enthalpy of. the oxidation exhaust gas to recover the
reaction solvent by returning it to the oxidation
reactor. In Indian Patent no. 133997 and Indian Patent no 180885.
In this process,
the overhead efflux gas from the distillation column top is cooled by cooling water in a condenser to condense steam contained in the efflux gas and the resulting condensate is refluxed to the distillation column.
In such a technique comprising distillation of the oxidation exhaust gas or its condensate, the height of the distillation column should be large enough for
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recovering the reaction solvent, such as acetic acid, by separating it completely from water and alcohol in the distillation column, as a lower column height may bring about transference of a part of the reaction solvent to the condensate side. Since water is formed by the oxidation reaction, a part of the condensate should be withdrawn from the reaction system, wherein a difficulty may be encountered in the waste water treatment due to the entrained reaction solvent and so on.
A process has further been proposed, in which esters of aliphatic carboxylic acids, such as methyl acetate etc., formed on the oxidation reaction are collected by causing them to be absorbed in scrubbing water and are hydrolyzed using an ion-exchange resin as the hydrolysis catalyst to recover corresponding aliphatic carboxylic acid to reuse as the reaction solvent. However, this may endure a difficulty of separation of the alcohol with the aliphatic carboxylic acid,
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a process for producing an aromatic carboxylic acid, in which an efficient recovery of the aliphatic carboxylic acid can be attained under efficient separation thereof from by-products, such as water and alcohol, with simultaneous attainment of easy waste water treatment.
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Another object of the present invention is to provide a process for producing an aromatic carboxylic acid, in which useful ingredients can be recovered under efficient hydrolysis of the ester of aliphatic carboxylic acid formed in the reaction system.
Accordingly, process for producing an aromatic carboxylic acid, such as herein described, under high pressure and high temperature by subjecting an alkylaromatic compound, such as herein described, to a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst, such as herein described, in a reaction solvent, such as herein described, comprising an aliphatic carboxylic acid in an oxidizing reactor, comprising
proceeding the reaction while removing the water and the alcohol formed by the oxidation reaction by means of membrane-separation.
(2) The process as defined in the above (1), wherein the process comprises the steps of
oxidizing an alkylaromatic compound by a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst in a reaction solvent comprising an aliphatic carboxylic acid in an oxidizing reactor to form an aromatic carboxylic acid under high pressure and high temperature,
performing distillation of the oxidation exhaust gas or of the condensate thereof in a distillation column by guiding the oxidation exhaust gas from the oxidizing reactor into the distillation column or passing it to an oxidation exhaust gas
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condenser and guiding the resulting condensate to the distillation column, while returning the distillation fractions- containing the reaction solvent to the oxidizing ractor,
condensing the distillation overhead gas by-cooling it in a condenser to form a condensate and
membrane-separating the condensate to cause water and alcohol to permeate through a membrane to the permeate side and to cause the aliphatic carboxylic acid to be retained on the concentrate side to recover it.
(3) The process as defined in the above (2),
wherein the process comprises further an absorption step
comprising causing the efflux gas from the condensation
step to contact with an absorbent liquor to absorb
aliphatic carboxylic acid esters therein and supplying
the so-absorbed liquor to the membrane-separation step.
(4) The process as defined in the above (2),
wherein the process comprises further an absorption
step comprising causing the efflux gas from the
oxidation exhaust gas condenser to contact with an
absorbent liquor to absorb aliphatic carboxylic acid
esters therein and supplying the so-absorbed liquor to
the membrane-separation step.
(5) The process as defined in any one of the above
(2) to (4), wherein the process comprises further a
hydrolysis step comprising causing the condensate
and/or the absorbed liquor and/or the membrane-separated
permeate and/or the concentrate to contact with a
hydrolysis catalyst to decompose the aliphatic
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carboxylic acid esters while supplying the resulting hydrolysate to the membrane-separation step.
(6) The process as defined in any one of the above
(1) to (5), wherein the membrane-separation is
performed using a reverse-osmosis membrane.
(7) The process as defined in any one of the above
(4) to (6), wherein the hydrolysis catalyst is an
ion-echange resin.
(8) The process as defined in any one of the above
(1) to (7), wherein the whole or a part of the
condensate is subjected to the membrane-separation and
the concentrate is returned to the distillation step.
(9) The process as defined in any one of the above
(1) to (8), wherein the membrane-separation is
performed on a plurality of separation stages.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a flow diagram of an embodiment of the process for producing terephthalic acid.
Fig. 2 is a flow diagram of another embodiment of the prcess for producing terephthalic acid.
Fig. 3 is a flow diagram of a further embodiment of the process for producing terephthalic acid.
Fig. 4 is a flow diagram of a still further embodiment of the process for producing terephthalic acid.
Fig. 5 is a flow diagram of a still further embodiment of the process for producing terephthalic acid.
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THE BEST MODE FOR EMBODYING THE INVENTION
According to the present invention, the oxidation reaction is carried out while removing the reaction water and the alcohol formed upon the reaction by membrane-separation. Here, it is favorable to incorporate a distillation step in combination with a membrane-separation , in order to separate efficiently the aliphatic carboxylic " acid to be used as the reaction solvent, such as acetic acid, from unnecessary ingredients, such as the reaction water and the alcohols, formed upon the reaction. If the separation is realized only by distillation, it is necessary to design the distillation column to have larger column height, as mentioned previously. On the other hand, if the separation is realized only by membrane-separation, the separation efficiency is inferior and, therefore, a multistage membrane-separator is required. However, by employing a distillation column having lower column height to effect a predominant part of the separation and performing a membrane separation for the resulting distillate of lower concentration, an efficient separation can be attained by means of a small scale arrangement.
As the raw material to be oxidized for producing an aromatic carboxylic acid by the process according to the present invention, aromatic compounds having one or more substituent alkyl or partially oxidized alkyl groups (hereinafter, sometimes referred to merely as the oxidizable raw material) may be used. Such an
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aromartic compound may either be a monocyclic or polycyclic one. As the substituent alkyl group, there may be exemplified alkyl groups having 1 - 4 carbon atoms, such as methyl, ethyl, n-propyl and isopropyl. As the partially oxidized alkyl group, there may be exemplified aldehydo, acyl, carboxyl and hydroxyalkyl.
Concrete examples of the alkyl-substituted aromatic hydrocarbon include di- and polyalkylbenzenes having 2-4 alkyl groups of 1 - 4 carbon atoms, such as m-diisopropylbenzene, p-diisopropylbenzene, m-cymene, p-cymene, m-xylene, p-xylene, trimethylbenzenes and tetramethylbenzenes; di- and polyalkylnaphthalenes having 2-4 alkyl groups of 1 - 4 carbon atoms, such as dimethylnaphthalenes, diethylnaphthalenes and diisopropylnaphthalenes; and polyalkylbiphenyls having 2-4 alkyl groups of 1 - 4 carbon atoms, such as dimethylbiphenyls.
The aromatic compound having one or more partially oxidized substituent alkyl groups is a compound in which one or more substituent alkyl groups are partially oxidized into aldehydo, acyl, carboxyl or hydroxyalkyl as mentioned above. Concrete examples thereof include 3-methylbenzaldehyde, 4-methylbenz-aldehyde, m-toluic acid, p-toluic acid, 3-formylbenzoic acid, 4-formylbenzoic acid and 2-methyl-6-formyl-naphthalene. They may be employed either individually or in a combination of two or more of them.
In the process according to the present invention, a heavy metal compound and a bromine compound are used as the catalyst. For such compounds,
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the followings may be exemplified: Thus, for the heavy metal in the heavy metal compounds, for example, cobalt, manganese, nickel, chromium, zirconium, copper, lead, hafnium and cerium, may be enumerated. They may be used either alone or in a combination of two or more of them, wherein preferance is given to a combination of cobalt and manganese.
For such heavy metal compounds, there may be exemplified acetates, nitrates, acetylacetonates, naphthenates, stearates and bromides, wherein particular preference is given to acetates.
For the bromine compounds, there may be exemplified molecular bromine; inorganic bromine compounds, such as hydrogen bromide, sodium bromide, potassium bromide, cobalt bromide and manganese bromide; and organic bromine compounds, such as methyl bromide, methylene bromide, bromoform, benzylbromide, bromo-methyltoluene, dibromoethane, tribromoethane and tetrabromoethane. They may be used either alone or in a combination of two or more of them.
The catalyst constituted of a combination of the heavy metal compound and the bromine compound according to the present invention may favorably have a proportion of the bromine atom to the heavy metal atom in the range from 0.05 to 10 moles, preferably from 0.1 to 2 moles, of bromine per 1 mole of the heavy metal. Such a catalyst may favorably be used usually in an amount, as the concentration of heavy metal, in the range from 10 to 10,000 ppm by weight, preferably from 100 to 5,000 ppm by weight.
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In the process according to the present invention, an aromatic carboxylic acid as a product of manufacture is obtained by subjecting the aromatic compound used as the oxidizable raw material to a liquid phase oxidation in the oxidation step with a molecular oxygen-containing gas in a reaction solvent comprising a lower aliphatic carboxylic acid in the presence of the above-mentioned catalyst in an oxidizing reactor.
As the molecular oxygen-containing gas, for example, oxygen gas and air may be recited, wherein air is used favorably for practical convenience. The molecular oxygen-containing gas is supplied to the reaction in excess of the requisite amount for oxidizing the aromatic compound to be used as the oxidizable raw material into the aromatic carboxylic acid. When air is used as the molecular oxygen-containing gas, it may favorably be supplied to the reaction system at a rate in the range from 2 to 20 Nm3 , preferably from 2.5 to 15 Nm3 , per one kg of the aromatic compound to be used as the oxidizable raw material.
As concrete examples of the lower aliphatic carboxylic acid to be used as the reaction solvent, there may be recited acetic acid, propionic acid and butyric acid. The lower aliphatic carboxylic acid may be used as the reaction solvent either individually or in a mixture with water. Concrete examples of the reaction solvent may comprise acetic acid, propionic acid, butyric acid and mixtures of them and, further,
1 0

mixtures of them with water. Among them, preference is given to a mixture of acetic acid with water, in particular/ a mixture of 1 - 20 parts, preferably 5 -15 parts, by weight of water with 100 parts by weight of acetic acid.
The temperature for the oxidation reaction may, in general, favorably be in the range from 100 to 250 °C, preferably from 150 to 220 °C. For the pressure for the oxidation reaction, any pressure above a value at which the reaction system is maintained in a liquid phase may be permissible.
By performing the oxidation reaction in the manner as above, an aromatic carboxylic acid corresponding to the aromatic compound used as the oxidizable raw material can be obtained. As concrete examples of the aromatic carboxylic acid, there may be recited aromatic dicarboxylic acids, such as terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid and 4.4'-biphenyl dicarboxylic acid; aromatic tricarboxylic acids, such as trimellitic acid and trimesic acid; and aromatic polycarboxylic acids, such as pyromellitic acid and so on.
The process for producing aromatic carboxylic acid according to the present invention may favorably be applied to the production of aromatic dicarboxylic acids or to the production of aromatic carboxylic acids insoluble or difficultly soluble in the reaction solvent, in particular, to the production of terephthalic acid.
The thereby formed aromatic carboxylic acid,
1 1

such as terephthalic acid, will deposit out as crystals to form a slurry which is guided out from the oxidizing reactor to solid/liquid separation to recover the crystals to obtain a crude product, such as crude terephthalic acid.
In the crystals of the so-obtained crude product, entrained impurities including intermediates of the oxidation reaction are contained, so that the crude product is then subjected to purification step comprising dissolving the crude product, treating by oxidation, treating by reduction and crystallizing the objective product, i.e. terephthalic acid, in order to obtain a slurry containing the crystals of the product. By collecting the crystals from the slurry, purified objective product, such as pure terephthalic acid, can be obtained.
In the distillation step, it is favorable to carry out the distillation by guiding the oxidation exhaust gas into a distillation column (high-pressure distillation column) connected to the top of the oxidizing reactor by making use of the heat of the exothermic oxidation reaction, while it is permissible to carry out distillation of a condensate formed in an oxidation exhaust gas condenser, in which the oxidation exhaust gas is subjected to condensation, by supplying this condensate to a distillation column (normal pressure distillation column). In both cases, the fractions containing the reaction solvent are collected in . the column bottom and are returned to the oxidizing reactor, while the column overhead gas containing water
1 2

vapor and non-condensing gases is exhausted out at the column top. The distillation column may be one which is independent of the oxidation reactor, as disclosed in Japanese Patent Publication Sho 54-14098 B, or one which is connected directly to the top of the oxidizing reactor, as disclosed in Japanese Patent Kokai Hei 6-279353 A. While the distillation column may be a plate column, preference is given to a packed column which may favorably has a means for collecting fine solid particles, such as crystals of the aromatic carboxylic acid, namely, for example, one or more solid matter collecting trays, arranged at lower part of the packed column.
The distillation column may favorably be constructed in such a manner that a plurality of subsidiary columns are connected successively to effect distillation of the exhaust gas from the preceding sub-column in the subsequent sub-column successively, while refluxing the distillate of the subsequent sub-column to the preceding sub-column, wherein it is preferable that distillates may be withdrawn out of the system at intermediate portions between the sub-columns upon emergency cessation of the operation, in order to prevent reduction of the concentration and temperature of the reaction liquor in the oxidizing reactor.
By using such a distillation column to perform distillation of the oxidation exhaust gas or of the condensate from an oxidation exhaust gss condenser, in which the oxidation exhaust gas is subjected to condensation, the fractions containing the reaction
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solvent entrained in the oxidation exhaust gas are returned to the oxidizing reactor. These fractions contain portions of the unreacted alkyl aromatic compound, aromatic carboxylic acid formed, catalyst and so on in a concentrated state, in addition to the reaction solvent, and are collected in the column bottom, from which they are returned to the oxidizing reactor. Among these components, solid matters, . such as the crystals of the aromatic carboxylic acid and the catalyst, and components having higher boiling points are collected at lower portion of the distillation column and the reaction solvent, such as an aliphatic carboxylic acid having low boiling point, is withdrawn at a relatively high portion.
While these fractions may be refluxed as such to the oxidizing reactor, it is possible to withdraw a fraction having higher concentration of acetic acid from a lower portion of the distillation column by arranging a liquid fraction withdrawal means at a lower portion of the distillation column, in order to utilize the so-withdrawn fraction as an absorbent liquor for absorbing esters of the aromatic carboxylic acid and, further, as a washing liquor for washing the crystals separated on the solid/liquid separation of the slurry withdrawn from the oxidizing reactor. When such a fraction is withdrawn out of the system upon an emergency cessation of the reaction, it is possible to prevent such a circumstance that a large amount of the refluxing liquor is introduced into the oxidizing reactor to dilute the reaction liquor.
1 4

In the condensation step, the distillation overhead gas discharged from the distillation column top is cooled by cool ing water in a condenser to condense steam contained in the discharged gas into a condensate. The condenser may also be constituted of a unit of single construction or a unit subdivided into a plurality of sub-units. The temperature on the condensation may be such that steam can be condensed, wherein esters of the aliphatic carboxylic acid may or may not be condensed. The resulting condensate may be guided either wholly to the membrane-separation step or partly to the membrane-separation step while returning a part thereof to the distillation column.
In the membrane-separation step, a part or the whole of the condensate is subjected to a membrane-separation to cause water and the by-products, such as alcohols, to permeate the membrane to the permeate side while retaining the aliphatic carboxylic acid on the concentrate side to concentrate therein, in order to attain separation of them. For the separation membrane for realizing such separation, a reverse-osmosis membrane may be employed. As the reverse-osmosis membrane, there may be recited those based on polyamide, aromatic polyamide, polyacrylonitrile, polypropylene, polyvinylidene fluoride, polyfluoroethylene, polyvinyl alcohol, polyester, polyimide, polysulfone, polyether sulfone, cellulose acetate, cellulose esters, triacetyl cellulose, polyether urate, polypiperic azamides, polyfurane and polyethyleneimine, wherein a particular preference is given to reverse-osmosis membranes based
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on cross-linked aromatic polyamide. The separation mambrane may be in any voluntary configuration including plane, tubular and spiral forms as well as hollow fibrous form. The separation membrane functions to attain the separation based on the difference in the molecular size, ionic activity or so on, for which appropriate one permitting permeation of water and the by-products, such as the alcohols, therethrough to the permeate side and retaining the aliphatic carboxylic acid on the concentrate side to concentrate therein may be employed. The membrane permits to permeate the esters of the aliphatic carboxylic acid therethrough by a proportion of about 20 % and, thus, to retain them on the concentrate side by a proportion of about 80 %. The membrane-separation step may be realized on one single stage but, preferably, on plural stages.
By subjecting the condensate to membrane-separation, unnecessary ingredients by-produced in the oxidation reaction, such as water and alcohols, such as methanol, can be separated by permeating through the membrane to the permeate side. The aliphatic carboxylic acid, such as acetic acid, to be used as the reaction solvent is retained on the concentrate side and is concentrated therein. If the separation of acetic acid from water is carried out only by distillation, the distillation column to be used should have a large height, since the boiling point of water is close to that of acet ic acid. By the membrane-separation, however, they can be separated easily due to the difference in the molecular size and in the
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ionic activity. If the separation is incomplete, the membrane separation may be performed on plural stages.
The resulting permeate can be treated by an adequate separation means to separate water and the by-produced products, such as alcohols, and the separated water may be discharged out of the system or be utilized, for example, as washing liquor for washing the aromatic carboxylic acid produced. The concentrate is returned to the oxidation step, wherein it is favorable to return a part or 'the whole of the condensate to the top of the distillation column to support the distillation.
By restricting the condensation temperature in the condensation step, it is possible to cause the esters of the aliphatic carboxylic acid to be condensed or to be transferred to the side of the efflux gas. In case the esters of the aliphatic acid are to be transferred to the efflux gas, they are absorbed in an absorption step in an absorbent liquor by contacting the gas with the absorbent liquor. As the absorbent liquor, water may be used, while acetic acid may also be used. The absorbed liquor may be guided as such to the membrane-separation step, while it is permissible to guide it to the membrane-separation step after it has been subjected to hydrolysis in a hydrolysis step.
In the hydrolysis step, the condensate and/or the absorbed liquor and/or the permeate and/or the concentrate resulting from the membrane-separation of them are caused to contact with a hydrolysis catalyst to effect hydrolysis of the esters of the aliphatic
1 7

carboxylic acid. In the case of using a normal pressure distillation column, it is preferable to subject the whole of the permeate resulting from the membrane-separation to the hydrolysis. As the hydrolysis catalyst, an ion-exchange resin in H-form, preferably a strongly acidic cation-exchange resin, in particular, that of macroreticular type may be used favorably, though a catalyst consists of acid or of alkali may also be used.
The esters of the aliphatic carboxylic acid will be decomposed into corresponding alcohols and aliphatic carboxylic acid, so that they can be separated on subjecting the hydrolysate to the membrane-separation, as described above. On treating the permeate or the concentrate from a membrane-separation stage by hydrolysis, the resulting hydrolysate will be subjected to membrane-separation on the subsequent membrane-separation stage or, for the concentrate, the membrane-separation may also be attained after being returned to the oxidation step and then distilled to form a condensate which is guided to the membrane-separation.
According to the present invention, the oxidation reaction can be realized, while the water and the alcohols formed upon the reaction are removed by membrane-separation, whereby the objective carboxylic acid can be recoverd under an efficient separation from components unnecessary for the reaction, such as water and by-products, such as alcohols etc., with simultaneous attainment of easy waste water treatment.
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By combining distillation with a membrane-separation, aromatic carboxylic acid can be obtained while being separated efficiently from components unnecessary for the reaction, such as water and by-products, such as alcohols etc., even though a distillation column having small column height is employed, with attainment of easy waste water treatment.
By incorporating an absorption step, esters of aliphatic carboxylic acid can be recovered from the efflux gas from the condensation step. By subjecting the esters of the aliphatic carboxylic acid to hydrolysis, the efficiency of utilization can be increased by separating the aliphatic carboxylic acid and the alcohols by membrane-separation. Here, the separation of the aliphatic carboxylic acid from the alcohols is made easy by combining the hydrolysis step with the membrane-separation step. Moreover, esters of the aliphatic carboxylic acid present in the oxidation exhaust gas can be recovered and brought into effective utilization by combining the absorption step with the hydrolysis step and the membrane-separation step, with simultaneous attainment of increase in the efficiency of separation of the aliphatic carboxylic acid from the alcohols.
EMBODIMENT OF THE INVENTION
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Below, the present invention will be described by way of embodiments of the process for producing terephthalic acid with reference to the drawings
1 9

appeended.
Figs. 1 to 3 show each an embodiment of the invention using a high-pressure distillation column in a flow diagram. Figs. 4 and 5 show each an embodiment of the invention using a normal pressure distillation column in a flow diagram. In Figs. 1 to 3, 1 denotes an oxidizing reactor which is connected at the top directly to a distillation column 2 as a high-pressure distillation column. 3 is a condenser, 4 and 5 are each a cooler, 6 is an absorption column and 7 is a pump. 8 and 8a denote each a membrane-separation unit having a separation membrane 9 or 9a, respectively. 10 and 10a indicate each a hydrolysis vessel having a catalyst layer 11 or lla consisting of an ion-exchange resin, respectively.
In the process for producing terephthalic acid, of which embodiments are given by the flow diagrams of Figs. 1 to 3, there are supplied to the oxidizing reactor 1 paraxylene, as an alkyl aromatic compound to be used as the raw material, acetic acid, as the reaction solvent, and a heavy metal compound and a bromine compound, as the catalyst, via a line LI, while supplying thereto air, as an molecular oxygen-containing gas, via a line L2, to effect a liquid phase oxidation at a high temperature under a high pressure to form terephthalic acid. The resulting terephthalic acid deposits out as crystals to form a slurry which is withdrawn via a line L3.
In Fig. 1, the oxidation exhaust gas is guided in a state of high-temperature and high-pressure to the
2 0

distillation column 2, in which distillation is realized during the gas passes through a packing layer 12. Most part of the solvent will be distilled out together with the raw material, paraxylene, and the catalyst contained in the oxidation exhaust gas and the fractions containing these ingredients are refluxed to the oxidizing reactor 1. The efflux gas containing a part of acetic acid and the lower boiling by-products, such as methanol etc., is guided into the condenser 3, where it is cooled and acetic acid, water, methanol and other by-products are condensed to form a condensate. A part of methyl acetate, which is an ester of the aliphatic carboxylic acid, is also condensed here.
The condensate is refluxed partly to the distillation column 2 as such and collected partly on a collector 13 and withdrawn via a line L4 and is cooled by the cooler 4 before being transferred to a concentrate chamber 14 of the membrane-separation unit 8 under pressure elevation by the pump 7. By the membrane-separation, non-ionic low molecular weight ingredients, such as water and methanol, are caused to permeate through the membrane 9 into a permeate chamber 15. The permeate is discharged out via a line L5 and the concentrate is returned to the top of the distillation column 2 via a line L6.
The efflux gas from the top of the condenser 3 is guided into the cooler 5 via a line L7 where it is cooled before being introduced into the absorption column 6, in which it is brought into contact, on passing through the packing layer 16, with an absorbent
2 1

liquor supplied thereto via a line L8, whereby methyl acetate contained in the efflux gas is absorbed in the absorbent liquor. The efflux gas deprived of methyl acetate is discharged out via a line L9. The absorbent liquor having absorped methyl acetate flows through a line L10 and joins with the condensate flowing in a line L4, whereupon the resulting mixture is supplied to membrane-separation unit 8. Methyl acetate may either be retained on the concentrate side or permeate through the separation membrane 9 in accordance with the separation performance of the membrane used, wherein the hydrolysis thereof can be attained by a suitable technique for both the cases.
In the embodiment of Fig. 2, the hydrolysis vessel 10 containing the catalyst layer 11 is disposed above the cooler 5. In this embodiment, methyl acetate present in the cooled efflux gas from the cooler 5 is hydrolyzed into methanol and acetic acid upon passing through the hydrolysis vessel 10. The portions of acetic acid and methyl acetate remaining in the gas passed through the hydrolysis vessel 10 are recovered in the absorption column 6 on contacting the gas with water supplied from the line L8. When the hydrolysate is guided via the line 10 into the membrane-separation unit 8 together with the condensate from the line L4, acetic acid is retained on the concentrate side and methanol is caused to permeate to the permeate side. Thus, even in the case of using acetic acid as the absorbent liquor in the absorpotion column 6, recovery of acetic acid is attained while being sent to the
2 2

distillation column 2. Other constructions and operations of the arrangement correspond to those in the embodiment of Fig. 1.
In the embodiment of Fig. 3, the whole of the efflux gas from the distillation column 2 is guided into the condenser 3 via the line L7 and is subjected to condensation therein. The resulting condensate is transferred as a mixture with the absorbenet liquor which has absorbed methyl acetate in the absorption column 6 to the membrane-separation unit 8 via the line L10, where it is separated by membrane-separation into each component, i.e. acetic acid, water and methanol. A part of methyl acetate is retained on the concentrate side and the other part permeates to the permeate side.
The permeate is guided into the hydrolysis vessel 10a, where methyl acetate is hydrolyzed into acetic acid and methanol by the catalyst layer lla. The hydrolysate is subjected to a membrane-separation in the membrane-separation unit 8a, wherein acetic acid is retained on the concentrate side and methanol and water are caused to permeate to the permeate side. The concentrates in the membrane-separation unit 8 and 8a are put together and guided via the line L6 to the hydrolysis vessel 10, where hydrolysis of methyl acetate is performed, and are then returned to the top of the distillation column 2. During circulation of acetic acid and methanol returned to the oxidizing reactor in accompaniment with the treatment cycle of the oxidation exhaust gas, they are separated from each other in the membrane-separation unit 8.
2 3

In the embodiments of Figs. 4 and 5, the oxidizing reactor 1 is connected at its top with a normal pressure distillation column as the distillation column 2 under interposition of an oxidation exhaust gas condenser 3a and a pressure reduction valve 18. The distillation column 2 is connected with a condenser 3. To the oxidation exhaust gas condenser 3a are connected a cooler 5 and an absorption column 6.
For producing terephthalic acid by the apparatuses of Figs. 4 and 5, the reactor 1 is supplied via the line L1 with paraxylene, as the raw material of alkylaromatic compound, with acetic acid, as the reaction solvent, and with a heavy metal compound and a bromine compound, as the catalyst, while supplying thereto via the line L2 with air, as the molecular oxygen-containing gas, to effect a liquid phase oxidation under high temperature and high pressure to form terephthalic acid. The thereby formed terephthalic acid deposits out as crystals to form a slurry which is withdrawn via the line L3.
In the embodiment of Fig. 4, the oxidation exhaust gas is guided in the state of high temperature and high pressure into the oxidation exhaust gas condenser 3a via a line L11 and is cooled there. A part of the condensate is returned as such to the oxidizing reactor 1 and the other part is guided into the distillation column via the line L12 through the pressure reduction valve 18 and is subjected to distillation by being heated by a reboiler 17. The efflux gas from the oxidation exhaust gas condenser 3a
2 4

is guided via a line L13 to a cooler 5, where it is cooled and is then transferred to an absorption column 6, in which it is brought into contact with an absorbent liquor supplied thereto via a line L8 while it passes through a packing layer 16 to thereby remove methyl acetate by absorption. The resulting efflux gas deprived of methyl acetate is discharged out via a line L9. The absorbent liquor having absorbed methyl acetate is guided through a line L10 to a line L12 and is put together with the condensate, whereupon the resulting mixture is fed to the distillation column 2.
During distillation in the distillation column 2, a predominant part of the reaction solvent, i.e. acetic acid, contained in the oxidation exhaust gas is returned to the oxidizing reactor 1 under rectification. The distillation overhead efflux containing the reaminder of acetic acid and the low-boiling by-products, such as methanol etc., is guided via a line L14 to a condenser 3, where it is cooled to condense the remaining acetic acid, steam, methanol and other by-products to form a condensate. Here, a part of methyl acetate, which is an ester of the aliphatic carboxylic acid, is also condensed.
The condensate is transferred as such to a concentrate chamber 14 of a membrane-separation unit 8 under pressure elevation by a pump 7, whereby nonionic low molecular weight substances, such as water and methanol, are caused to permeate through the separation membrane 9 to the permeate chamber 15 by the membrane-separation. The permeate is discharged out via a line
2 5

L5 and the concentrate is returned to the top of the distillation column 2 via a line L6.
In the embodiment of Fig. 5, a hydrolysis vessel 10 having a catalyst layer 11 is installed in the line L5 with ommission of cooler 5. In this arrangement, the permeate from the membrane-separation unit 8 is subjected to hydrolysis of methyl acetate contained therein on passing through the hydrolysis vessel 10 and the resulting hydrolysate containing methanol and acetic acid is returned to the distillation column 2 at its upper portion.
EXAMPLES
Below, the present invention will be described by way of Examples. In the Examples, % and ppm are on the weight basis so long as not specifically mentioned.
Reference Example 1
In the oxidizing reactor of Fig. 1, paraxylene was oxidized at 190 °C under 1.2 MPa to produce terephthalic acid. The oxidation exhaust gas was subjected to diatillation in the distillation column 2, with condensation in the condenser 3 at 160 °C. The overhead efflux gas from the distillation column 2 was brought into contact with an absorbent liquor consisting of 10 kg/hr of water at 35 °C in the absorption vessel 6 to absorb methyl acetate therein, whereupon 50 kg/hr of the resulting absorbed liquor . were brought together with 55 kg/hr of the condensate
2 6

guided from the condenser and the resulting 105 kg/hr of the mixed liquid were discharged out of the system. The mixed liquid contained 1.5 % of acetic acid, 2,650 ppm of methyl acetate and 1040 ppm of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 1020 ppm by volume of methyl acetate and 106 ppm by volume of methanol. Example 1
The procedures of Reference Example 1 were changed such that 205 kg/hr of condensate from the condenser 3 were brought together with 50 kg/hr of the absorbed liquor from the absorption column 6 and the resulting 255 kg/hr of the mixed liquid were introduced into the membrane-separation unit 8 under pressure elevation up to 4.9 MPa to subject to a membrane-separation, whereupon 150 kg/hr of the concentrate were returned to the distillation column 2 and 105 kg/hr of the permeate were discharged out of the system. As the separation membrane, a reverse-osmosis membrane SU-820 (trademark, a product of Toray Industries, Inc.) constituted of a cross-linked polyamide composite membrane was used. The permeate contained 0.3 % of acetic acid, 303 ppm of methyl acetate and 1010 ppm of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 1530 ppm by volume of methyl acetate and 305 ppm by volume of methanol. Example 2
The apparatus of Fig. 2 was operated in such as manner that the absorbent water was supplied to the
2 7

absorption column 6 at a rate of 50 kg/hr and 90 kg/hr of hydrolysate liquor from the hydrolysis vessel 10 resulting after the hydrolysis of the absorbed water were brought together with 205 kg/hr of the condensate from the condenser 3, whereupon the resulting 295 kg/hr of the mixed liquid were subjected to a membrane-separation in the membrane-separation unit 8, wherein 150 kg/hr of the concentrate were returned to the distillation column 2 and 145 kg/hr of the permeate were discharged out of the system. The hydrolysis catalyst consisted of a strongly acidic cation exchange resin of macroreticular type (Amberlist 15, trademark, of Japan Organo Co., Ltd.). Other procedures were the same as in Example 1. The permeate contained 0.5 % of acetic acid, 550 ppm of methyl acetate and 1700 ppm of methanol. The efflux gas from the absorption column contained 0.5 ppm by volume of acetic acid, 103 ppm by volume of methyl acetate and 1100 ppm by volume of methanol. Example 3
The apparatus of Fig. 3 was operated in such as manner that the absorbent water was supplied to the absorption column 6 at a rate of 10 kg/hr and 900 kg/hr of the absorbed condensate were passed to the membrane-separation units 8 and 8a and to the hydrolysis vessels 10 and 10a under pressure elevation up to 7.8 MPa by the pump 7. The concentrates of the membrane-separation units 8 and 8a were brought together at flow rates of 650 kg/hr and 145 kg/hr, respectively, and the resulting mixture was returned to the distillation column after
2 8

passing through the hydrolysis vessel 10, while 105 kg/hr of the permeate from the membrane-separation unit 8a were discharged out of the system. Other procedures were the same as in Example 2. The permeate contained 0.1 % of acetic acid, 50 ppm of methyl acetate and 2010 ppm of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 212 ppm by volume of methyl acetate and 1050 ppm by volume of methanol. Reference Example 2
In the oxidizing reactor of Fig. 4, paraxylene was oxidized at 190 ° C under 1.2 MPa to produce terephthalic acid. The oxidation exhaust gas was cooled in the oxidation exhaust gas condenser 3a at 120 °C to condense it. The efflux gas from the oxidation exhaust gas condenser 3a was cooled in the cooler 5 to 40 ° C and was brought into contact with an absorbent liquor consisting of 10 kg/hr of water at 35 °C in the absorption column 6 to absorb methyl acetate therein, whereupon 210 kg/hr of the resulting absorbed liquor were brought together with 465 kg/hr of the condensate guided from the condenser 3a and the resulting 685 kg/hr of the mixed liquid were subjected to distillation in the distillation column 2 at a reflux ratio of 4 and 105 kg/hr of the distilate from the column top were discharged out of the system. The discharged liquid contained 1.5 % of acetic acid, 1.6 % of methyl acetate and 0.1 % of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 500 ppm by volume of methyl acetate and 30 ppm by
2 9

volume of roethanol. Example 4
The procedures of Reference Example 2 were changed such that the condensate from the condenser 3 was introduced into the membrane-separation unit 8 under pressure elevation up to 4.9 MPa to subject to a membrane-separation, whereupon 525 kg/hr of the concentrate were returned to the distillation column 2 and 105 kg/hr of the permeate were discharged out of the system. As the separation membrane, a reverse-osmosis membrane SU-820 (trade name, a product of Toray Industries, Inc.) constituted of a cross-linked polyamide composite membrane was used. The permeate contained 0.3 % of acetic acid, 0.3 % of methyl acetate and 0.08 % of methanol. The efflux gas from the absorption column contained 0.1 ppm by volume of acetic acid, 700 ppm by volume of methyl acetate and 50 ppm by volume of methanol.
3 0

WE CLAIM:
1. A process for producing an aromatic carboxylic acid,
such as herein described, under high pressure and high
temperature by subjecting an alkylaromatic compound, such
as herein described, to a liquid phase oxidation by a
molecular oxygen-containing gas in the presence of an
oxidation catalyst, such as herein described, in a
reaction solvent, such as herein described, comprising an
aliphatic carboxylic acid in an oxidizing reactor,
comprising
proceeding the reaction while removing the water and the alcohol formed by the oxidation reaction by means of membrane-separation.
2. The process as claimed in claim 1, wherein the
process comprises the steps of
oxidizing an alkylaromatic compound by a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst in a reaction solvent comprising an aliphatic carboxylic acid in an oxidizing reactor to form an aromatic carboxylic acid under high pressure and high temperature,
performing distillation of the oxidation exhaust gas or of the condensate thereof in a distillation column by guiding the oxidation exhaust gas from the oxidizing reactor into the distillation column or passing it to an oxidation exhaust gas condenser and guiding the resulting condensate to the distillation column, while returning the distillation fractions containing the reaction solvent to the oxidizing reactor,
condensing the distillation overhead gas by
3 1

cooling it in a condenser to form a condensate and
membrane-separating the condensate to cause water and alcohol to permeate through a membrane to the permeate side and to cause the aliphatic carboxylic acid to be retained on the concentrate side to recover it.
3. The process as claimed in claim 2, wherein the
process comprises further an absorption step comprising
causing the efflux gas from the condensation step to
contact with an absorbent liquor to absorb aliphatic
carboxylic acid esters therein and supplying the
so-absorbed liquor to the membrane-separation step.
4. The process as claimed in claim 2, wherein the
process comprises further an absorption step comprising
causing the efflux gas from the oxidation exhaust gas
condenser to contact with an absorbent liquor to absorb
aliphatic carboxylic acid esters therein and supplying
the so-absorbed liquor to the membrane-separation step.
5. The process as claimed in any one of claims 2
to 4, wherein the process comprises further a hydrolysis
step comprising causing the condensate and/or the
absobed liquor and/or the membrane-separated permeate
and/or the concentrate to contact with a hydrolysis
catalyst to decompose the aliphatic carboxylic acid
esters while supplying the resulting hydrolysate to the
membrane-separation step.
6. The process as claimed in any one of claims 1
to 5, wherein the membrane-separation is performed using
a reverse-osmosis membrane.
7. The process as claimed in any one of claims 4
3 2

to 6, wherein the hydrolysis catalyst is an ion-exchange resin.
8. The process as claimed in any one of claims 1
to 7, wherein the whole or a part of the condensate is
subjected to the membrane-separation and the concentrate
is returned to the distillation step.
9. The process as claimed in any one of claims 1
to 8, wherein the membrane-separation is performed on a
plurality of separation stages.
3 3

10. A process for producing an aromatic carboxylic acid, substantially as herein described, particularly with reference to the accompanying drawings.

- 34 -
A process for producing an aromatic carboxylic acid under high pressure and high temperature by subjecting an alkylaromatic compound to a liquid phase oxidation by a molecular oxygen-containing gas in the presence of an oxidation catalyst in a reaction solvent comprising an aliphatic carboxylic acid in an oxidizing reactor while recovering the aliphatic carboxylic acid by ditillation of the oxidation exhaust gas, in which the recovery of the aliphatic carboxylic acid can be attained efficiently under separation thereof from by-products, such as water and alcohols, with permission of easier waste water treatment, wherein the process comprises proceeding the reaction while removing the water and the alcohol formed by the oxidation reaction by means of membrane-separation.

Documents:

00141-cal-2001-abstract.pdf

00141-cal-2001-claims.pdf

00141-cal-2001-correspondence.pdf

00141-cal-2001-description(complete).pdf

00141-cal-2001-drawings.pdf

00141-cal-2001-form-1.pdf

00141-cal-2001-form-18.pdf

00141-cal-2001-form-2.pdf

00141-cal-2001-form-3.pdf

00141-cal-2001-form-5.pdf

00141-cal-2001-g.p.a.pdf

00141-cal-2001-letters patent.pdf

00141-cal-2001-others document.pdf

00141-cal-2001-priority document others.pdf

00141-cal-2001-reply f.e.r.pdf

141-CAL-2001-FORM-27.pdf

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Patent Number

206900

Indian Patent Application Number

141/CAL/2001

PG Journal Number

20/2007

Publication Date

18-May-2007

Grant Date

16-May-2007

Date of Filing

08-Mar-2001

Name of Patentee

MITSUI CHEMICALS,INC

Applicant Address

2-5,KASUMIGASEKI 3-CHOME,CHIYODA-KU,TOKYO 100-6070,

Inventors:

#	Inventor's Name	Inventor's Address
1	NAKAO FUJIMASA	C/O MITSUI CHEMICALS ENGINEERING CO. LTD 1-2,WAKI 6-CHOME,WAKICHO,KUNGA-GUN,YAMAGUCHI,740-0061,
2	UMEDA MICHIO	C/O MITSUI CHEMICALS INC. 1-2,WAKI 6-CHOME,WAKICHO,KUNGA-GUN,YAMAGUCHI,740-0061,
3	SUZUKI HIROSHI	C/OMITSUI CHEMICALS,INC 2-5,KASUMIGASEKI 3-CHOME,CHIYODA-KU,TOKYO 100-6070,
4	YAMANE HIROSHI	C/O MITSUI CHEMICALS INC. 1-2,WAKI 6-CHOME,WAKICHO,KUNGA-GUN,YAMAGUCHI,740-0061,

PCT International Classification Number

C07C 51/265

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	77833/2000	2000-03-15	Japan