Title of Invention

"AN AZEOTROPIC DISTILLATION PROCESS"

Abstract

To provide a process for azeotropically distilling a mixture to be distilled comprising water, acetic acid and methyl acetate by using an azeotropic agent which is azeotropic with water. An azeotropic distillation process for separating water, acetic acid and methyl acetate into the respective components by distillation, which comprises the following steps (1) to (5): (1) a step of distilling a mixture to be distilled comprising water, acetic acid and methyl acetate in the presence of an azeotropic agent under a column top pressure higher thnri atmospheric pressure to separate the mixture into a bottom recovery fractioii containing acetic acid and a top distillate vapor containing water, methyl acetate and the azeotropic agent, (2) a step of condensing the top distillate vapor obtained in the step (1), (3) a step of evaporating methyl acetate from the condensed liquid obtained in the step (2) by releasing the pressure, to recover methyl acetate, (4) a step of subjecting the residual liquid obtained in the step (3) to a liquid-liquid separation into a water phase and an oil phase, and (5) a step of supplying the oil phase obtained in the step (4) to the step (1) as the azeotropic agent.

Full Text

TECHNICAL FIELD The present invention relates to a process for azeotropically distilling a mixture to be distilled comprising water, acetic acid and methyl acetate by using an azeotropic agent which is azeotropic with water. BACKGROUND ART Water, acetic acid, methyl acetate, etc. formed by a disproportionation reaction of acetic acid, are contained in a mother liquor, or its vapor, obtained by solid-liquid separation of e.g. a reaction vapor or its condensed liquid, or a reaction product slurry, formed in a process for producing terephthalic acid by oxidation of paraxylene in acetic acid as a solvent. It is common to employ a method for dehydration distillation to recover the acetic acid solvent from such a mixture. In such distillation, water and acetic acid have poor separability as the relative volatility is close to 1, and an azeotropic distillation method employing an azeotropic agent which is azeotropic with water, is accordingly used. In such azeotropic distillation, if methyl acetate contained in the above mixture is accumulated in the azeotropic agent to be recycled, there will be a problem such that the separation performance deteriorates, and a method for solving such a problem is disclosed in Patent Document 1. Further, a method for recovering energy by generating low pressure steam by means of the heat of condensation of a distillate vapor formed by distillation, is disclosed in e.g. Patent Documents 2 and 3. Especially, an azeotropic distillation method tends to be disadvantageous for the generation of low pressure steam if the temperature of the distillate vapor becomes low, as compared with a usual distillation method employing no azeotropic agent. Accordingly, a method of increasing the temperature of the distillate vapor by pressurizing the column top for azeotropic distillation, is disclosed in Patent Document 3. In the Patent Document 3, at least a part of the azeotropic agent to be recycled is subjected to distillation treatment in order to remove methyl acetate accumulated in the azeotropic agent to be recycled. Patent Document 1: JP—A-2002-326001 Patent Document 1: JP-A-5-213816 Patent Document 3: US Patent Application 2003-0150706 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED THE BY THE INVENTION However, in the azeotropic distillation, it was not efficient to remove methyl acetate by distillation treatment of the azeotropic agent to be recycled, as it required an energy. Further, from the viewpoint of the installation, a distillation column for the treatment and its incidental facilities were required. Under the circumstances, the present invention is to propose a new process for recovering methyl acetate, which is capable of simplifying the installation and reducing energy consumption at the time of azeotropic distillation under an elevated pressure of a mixture to be distilled comprising water, acetic acid and methyl acetate. MEANS TO SOLVE THE PROBLEMS According to the present invention, the above problems have been solved by adopting an azeotropic distillation process for azeotropically distilling water, acetic acid and methyl acetate into the respective components via the following steps (1) to (4). Namely, the gist of the present invention resides in the following 1 to 8. 1. An azeotropic distillation process for separating water, acetic acid and methyl acetate into the respective components by distillation, which comprises the following steps (1) to (5): (1) a step of distilling a mixture to be distilled comprising water, acetic acid and methyl acetate in the presence of an azeotropic agent under a column top pressure higher than atmospheric pressure to separate the mixture into a bottom recovery fraction containing acetic acid and a top distillate vapor containing water, methyl acetate and the azeotropic agent, (2) a step of condensing the top distillate vapor obtained in the step (1), (3) a step of evaporating methyl acetate from the condensed liquid obtained in the step (2) by releasing the pressure, to recover methyl acetate, (4) a step of subjecting the residual liquid obtained in the step (3) to a liquid-liquid separation into a water phase and an oil phase, and (5) a step of supplying the oil phase obtained in the step (4) to the step (1) as the azeotropic agent. 2. The azeotropic distillation process according to the above 1, wherein the temperature of the top distillate vapor is from 95 to 130°C. 3. The azeotropic distillation process according to the above 1 or 2, wherein the evaporated material obtained in the step (3) is distilled to recover methyl acetate contained in the above evaporated material. 4. The azeotropic distillation process according to the above 1 or 2, wherein the evaporated material obtained in the step (3) is distilled together with the water phase obtained in the step (4) to recover methyl acetate contained in the evaporated material and the water phase. 5. The azeotropic distillation process according to any one of the above 1 to 4, wherein the mixture to be distilled comprising water, acetic acid and methyl acetate is a mixture of water, acetic acid and methyl acetate obtained in a process for production of terephthalic acid by oxidation of paraxylene in acetic acid as a solvent, and the recovered methyl acetate is retuned to the oxidation step. 6. The azeotropic distillation process according to any one of the above 1 to 5, wherein the azeotropic agent to be used in the azeotropic distillation is an acetate having 5 or 6 carbon atoms. 7. The azeotropic distillation process according to any one of the above 1 to 5, wherein the azeotropic agent to be used in the azeotropic distillation is at least one member selected from the group consisting of n-propyl acetate, i-propyl acetate, n-butyl acetate and i-butyl acetate. 8. The azeotropic distillation process according to any one of the above 1 to 7, wherein the heat of condensation generated in the step (2) is used for heat exchange to produce steam. EFFECTS OF THE INVENTION According to the present invention, a new process for recovering methyl acetate can be provided which is capable of simplifying the installation and reducing the energy consumption in the azeotropic distillation under an elevated pressure of a mixture to be distilled containing acetic acid and methyl acetate. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic view showing a flowchart to be used for the azeotropic distillation process of the present invention. Fig. 2 is a schematic view showing a flowchart to be used for the conventional azeotropic distillation process (Comparative Example). Meanings of symbols 1: azeotropic distillation column 2: condenser 3: condensed liquid tank : pressure-releasing tank 5: second distillation column 6: cooler 7: oil-water separator A: mixture to be distilled B: bottom recovery fraction C: column top distillate vapor D: condensed liquid Dl: water D2: steam E: evaporated material F: liquid component G: oil phase H: water phase I: gas line BEST MODE FOR CARRYING OUT THE INVENTION The azeotropic distillation process of the present invention is a process wherein a mixture to be distilled comprising water, acetic acid and methyl acetate, is azeotropically distilled by using an azeotropic agent which is azeotropic with water. The above mixture to be distilled comprising water, acetic acid and methyl acetate may, for example, be a mother liquor or its vapor, obtained by solid-liquid separation of a reaction vapor or its condensed liquid, or a reaction product slurry, formed in the process for producing terephthalic acid by oxidation of paraxylene to terephthalic acid in acetic acid as a solvent. In this case, such a mixture contains water formed by the oxidation reaction, acetic acid as the solvent and methyl acetate formed by a disproportionation reaction of acetic acid. With the above mixture to be distilled, the respective components are azeotropically distilled via the following step (1). (1) a step of distilling the mixture to be distilled comprising water, acetic acid and methyl acetate in the presence of an azeotropic agent under a column top pressure higher than atmospheric pressure to separate the mixture into a bottom recovery fraction containing acetic acid and a top distillate vapor containing water, methyl acetate and the azeotropic agent. The above azeotropic agent is a compound capable of forming an azeotropic mixture together with water. By using such an azeotropic agent, azeotropic distillation of water and acetic acid can efficiently be carried out. As an azeotropic agent to be used, a heterogeneous azeotropic agent may be mentioned. Specifically, an acetate having 5 or 6 carbon atoms such as n-propyl acetate, i-propyl acetate, n-butyl acetate or i-butyl acetate may be mentioned. Among them, n-butyl acetate is particularly preferred. One or more types of such azeotropic agents may be employed, but it is more preferred to employ only one type. The minimum azeotropic point with water under atmospheric pressure (1 atm) is 85°C with n-propyl acetate, 77°C with i-propyl acetate, 91°C with n-butyl acetate or 88°C with i-butyl acetate. Accordingly, in a case where the minimum azeotropic point is to be increased by pressurizing, n-butyl acetate is capable of reaching a prescribed temperature under a low pressure level as compared with other azeotropic agents. The higher the pressure level, the smaller the vapor capacity load in the distillation column, such being advantageous from the viewpoint of a compact column. However, if the pressure is too high, at the acetic acid recovery portion at the column bottom, the temperature becomes high as the pressure level is high irrespective of the type of the azeotropic agent, and a special care for prevention of corrosion such as use of a high grade material, will be required, such being undesirable. The "under a pressure higher than atmospheric pressure" in the above step (1) means that the column top pressure of the distillation column used in the above step (1) exceeds atmospheric pressure, and specifically, it is meant for a pressure under which the column top distillate vapor temperature will be from 95°C to 130°C, preferably a pressure under which the column top distillate vapor temperature will be from 95°C to 110°C. When the pressure for distillation operation is increased in such a manner, the vapor load in the column will be reduced, whereby downsizing of the distillation column will be possible. Such an effect is substantial especially when the production ability of a chemical plant is being increased. Further, the boiling point of the azeotropic composition will be substantially higher than the boiling point under atmospheric pressure, whereby the pressure grade of a low pressure steam which generates the heat of condensation of the distillate vapor can also be increased. For example, it is possible to recover steam (90°C) under 0.07 MPa (absolute pressure) thereby to efficiently recover an energy by a steam turbine. If the above pressure is too low, the above effect (to increase the pressure grade of the low pressure steam) tends to be hardly sufficiently obtainable. On the other hand, if the above pressure is too high, the column bottom of the distillation column tends to be a severe temperature region from the viewpoint of the corrosive environment by acetic acid, and further, high pressure instruments will be required, whereby the costs for the instruments tend to be high. By the distillation in the above step (1) , an azeotropic composition comprising water and the azeotropic agent, and methyl acetate are distilled, and the above acetic acid is recovered from the column bottom. Accordingly, as the bottom, a mixture containing acetic acid as the main component and having a water content smaller than the mixture to be supplied to the distillation column, for example, having a water content of at most 10 wt% while the water content in the mixture to be supplied is from 20 to 30 wt%, is recovered. And, in a case where the mixture to be distilled is a mother liquor or its vapor, obtainable by solid-liquid separation of e.g. a reaction vapor or its condensed liquid, or a reaction product slurry, in the process for producing terephthalic acid by oxidation of paraxylene to terephthalic acid in acetic acid as a solvent, the recovered acetic acid may be returned to the process for producing terephthalic acid and reused as a solvent or cleaning liquid. Further, the column top distillate vapor distilled from the above distillation column comprises water, methyl acetate and the azeotropic agent as the main components (usually comprises at least 50%, preferably at least 70%, of water, methyl acetate and the azeotropic agent) and contains acetic acid in a small amount of at most 1 wt%. Here, methyl acetate is a product formed by a disproportionation reaction of acetic acid. In the present invention, wthe column top portion" is meant for a portion corresponding to at least 1/3 of the entire column from the top of the distillation column. Usually, the distillate vapor is withdrawn from the column top, but it may be withdrawn from a portion corresponding to at least 1/3 of the entire column from the column top, preferably from a portion corresponding to at least 1/10 of the entire column from the column top. As a treating method for the above top distillate vapor, the following steps (2) to (5) may be employed. (2) a step of condensing the top distillate vapor obtained in the step (1), (3) a step of evaporating methyl acetate from the condensed liquid obtained in the step (2) by releasing the pressure, to recover methyl acetate, (4) a step of subjecting the residual liquid obtained in the step (3) to a liquid-liquid separation into a water phase and an oil phase, and (5) a step of supplying the oil phase obtained in the step (4) to the step (1) as the azeotropic agent. The above step (2) is a step of condensing the top distillate vapor which is at a temperature of from 95 to 130°C. In the step (3), methyl acetate is evaporated by releasing the pressure of the condensed liquid. Accordingly, in the step (2), it is preferred to maintain the state of a pressure higher than atmospheric pressure (substantially the same pressure as the operation pressure in the distillation column). The above step (2) is a step of condensing the above top distillate vapor, and this condensation is carried out usually by a condenser. As such a condenser, a kettle type heat exchanger or a thin film evaporator may, for example, be used, whereby the heat of condensation formed during the condensation of the above top distillate vapor may be utilized to generate low pressure steam, for example, steam of 0.07 atm (absolute pressure} (90°C). In such a manner, the heat of condensation can effectively be utilized, such being preferred from the viewpoint of the energy efficiency. Here, the low pressure steam is meant for low pressure steam obtained when water functioning as a cooling medium in the above condenser, is heated by heat exchange in such a condenser, As mentioned above, the temperature at the top of the distillation column is usually from 95 to 130°C. Accordingly, by utilizing the heat of condensation, it is possible to generate low pressure steam of from 0.05 to 0.20 MPa as a pressure grade having a temperature within a range of from 80 to 120°C. If the temperature of steam generated in the above condenser is within the above range, it may, for example, be supplied to a steam turbine to effectively recover the energy, such being preferred as the utilization range increases. The higher the pressure grade of steam, the larger the energy to be recovered per unit mass, such being preferred. In the above step (3), the above condensed liquid in a pressure state higher than atmospheric pressure, is subjected to flush evaporation by reducing the pressure, whereby it is subjected to gas-liquid separation. Specifically, the condensed liquid obtained in the above step (2) is supplied to a pressure-releasing tank set under a prescribed pressure for gas-liquid separation. Such a prescribed pressure is preferably atmospheric pressure, but it may be set to be a pressure less than atmospheric pressure by e.g. an ejector. In such a case, methyl acetate being a low boiling component will be evaporated and recovered. Here, in order to evaporate methyl acetate by pressure-releasing distillation, the temperature for condensation in the above condensation step (2) becomes important. In a case where the pressure set for the pressure release is atmospheric pressure, the condensation temperature is set to be at the lowest "a temperature higher than 56°C being the boiling point of methyl acetate under atmospheric pressure". As a result of this flush evaporation, the concentration of methyl acetate in the residual liquid in the above step (3) will be lowered, and in the oil-water separation step (4), the content of methyl acetate in the oil phase will also be lowered, whereby accumulation of methyl acetate in the distillation system can be suppressed. By suppressing the accumulation of methyl acetate in the oil phase (the azeotropic agent), the acetic acid dehydration efficiency will be improved, and further, the load to the distillation column can be reduced. From such viewpoints, the concentration of methyl acetate in the oil phase is maintained (suppressed) to be at most 20 wt%, preferably at most 15 wt%. In such a case, the evaporated material obtained in the above step (3) contains not only methyl acetate but also an azeotropic composition comprising water and the azeotropic agent. Accordingly, it is preferred that this evaporated material is distilled to distill and recover the above methyl acetate. For example, in order to distill and separate methyl acetate (b.p.=56°C) and the azeotrope of water and butyl acetate (b.p.=91°C) under atmospheric pressure, a common distillation method employing a plate column or a packed column, may be used. The condensed liquid having the pressure decreased by the pressure-release in the above step (3) is composed mainly of the azeotropic agent and water and therefore is subjected to oil-water separation in the above step (4). Prior to the oil-water separation, the condensed liquid obtained in the step (3) may be cooled, as the case requires, whereby not only the oil-water separation efficiency may be increased, but also methyl acetate in the condensed liquid may be separated to the water phase in the oil-water separation as the temperature becomes low, and accumulation of methyl acetate in the azeotropic agent to be recycled can further be suppressed. The oil phase contains the above azeotropic agent as the main component, and the separated oil phase is recycled as the zeotropic agent to the azeotropic distillation column in the above step (1) (the above step (5)). On the other hand, the water phase is mainly water but contains small amounts of methyl acetate and the azeotropic agent, and accordingly, it is preferred to distill and recover the methyl acetate and the azeotropic agent by distillation treatment. At that time, it is further preferred that the evaporated material evaporated in the above step (3) is introduced to the distillation step to distill a part or whole of the water phase obtained in the above step (4), whereby separation and recovery of methyl acetate and the azeotropic agent in the evaporated material obtained in the above step (3) can be carried out at the same time as the separation and recovery of methyl acetate and the azeotropic agent in the water phase obtained by oil-water separation in the above step (4). In a case where as the above mixture to be distilled, a mother liquor or its vapor obtainable by solid-liquid separation of a reaction vapor or its condensed liquid, or a reaction product slurry, formed in a process for producing terephthalic acid by oxidation of paraxylene to form terephthalic acid in acetic acid as a solvent, is used, the recovered methyl acetate may effectively be utilized by converting it to another compound such as methanol by e.g. hydrolysis, but is preferably returned directly to the above oxidation reaction step. If it is returned to the above oxidation reaction step, it is possible to suppress formation of methyl acetate by a disproportionation reaction of acetic acid used as a solvent in this oxidation reaction step, whereby the loss of acetic acid can be reduced. Now, one embodiment of the azeotropic distillation process of the present invention will be specifically described with reference to the flowchart shown in Fig. 1, wherein the above mixture A to be distilled, is formed in a process for producing terephthalic acid by oxidation of paraxylene to form terephthalic acid in acetic acid as a solvent. Here, by the above oxidation, water will be formed, and by the disproportionation reaction of acetic acid, methyl acetate will be formed. The azeotropic agent is n-butyl acetate. And, the mixture A to be distilled comprises water, acetic acid and methyl acetate, as the main components. The mixture A to be distilled and the azeotropic agent (n-butyl acetate) are introduced into a distillation column 1 to carry out separation of water and acetic acid by azeotropic distillation. And, as the bottom recovery fraction B, acetic acid is recovered from the bottom of the distillation column 1. On the other hand, the top distillate vapor D distilled by distillation, is permitted to be distilled from the column top. The above top distillate vapor C comprises water, n-butyl acetate and methyl acetate, as the main components, and further contains a low concentration of acetic acid and an inert gas such as nitrogen. At that time, a non-condensed gas containing an inert gas such as nitrogen as the main component, is discharged from the column top. The pressure is controlled by a valve of a gas line I so that the temperature at the column top will be within a range of from 95 to 130°C. Then, the above top distillate vapor C is sent to a condenser 2 and condensed. At that time, water D! receives the heat of condensation from the above top distillate vapor C, and steam D2 will be generated. The condensed liquid D condensed by the above condenser 2 and a non-condensed gas will be stored once in a tank 3, and after gas-liquid separation, will be sent to a pressure-releasing tank 4. This pressure- releasing tank 4 is a tank wherein the pressure is set to be lower than the pressure of the condensed liquid D, and as the pressure decreases, a part of the condensed liquid D will be evaporated. The pressure in the pressure-releasing tank 4 at that time is acceptable so long as it is lower than the vapor pressure of methyl acetate at the temperature of the condensed liquid D, but it is preferably atmospheric pressure or lower than atmospheric pressure. The above evaporated material E is composed mainly of methyl acetate but contains an azeotropic composition of water and n-butyl acetate. Such an evaporated material E will be sent to the second distillation column 5 and subjected to azeotropic distillation. The liquid component F remaining in the above pressure-releasing tank 5 is, as the case requires, further cooled by a cooler 6 and then sent to an oil-water separator 7, whereby it will be separated into an oil phase G and a water phase H. The oil phase G is composed mainly of n-butyl acetate, and the water phase H is composed mainly of water. And, methyl acetate remaining in the liquid component F will be contained in the oil phase G, but a part thereof will also be contained in the water phase H. The above oil phase G will be recycled as an azeotropic agent to the distillation column 1, and the above water phase H will be sent to the above second distillation column 5 and subjected to azeotropic distillation. In the second distillation column 5, the above evaporated material E and the water phase H are distilled, whereby methyl acetate will be recovered from the column top, n-butyl acetate will be recovered from a middle portion of the column and water will be recovered from the column bottom. The recovered n-butyl acetate can be reused as an azeotropic agent. Further, the recovered methyl acetate is preferably returned to the above oxidation reaction step, whereby it is possible to suppress conversion of acetic acid by a disproportionation reaction to methyl acetate in this oxidation reaction thereby to reduce the loss of acetic acid as the solvent. EXAMPLES Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by the following Examples. EXAMPLE 1 An acetic acid solution containing p-xylene and catalysts (an acetic acid solution of cobalt acetate and manganese acetate, and hydrogen bromide), a separated mother liquor recycled from a later stage solid-liquid separation step, and air, are continuously supplied to an agitation tank, and an oxidation reaction was carried out by adjusting the liquid level so that the residence time would be 1 hour at an operation temperature of 190°C under an operation pressure of 1.23 MPa {absolute pressure). Further, the distillate vapor was cooled by multistage condensers finally to 40°C, and the operation was carried out by adjusting the oxygen concentration in the exhaust gas to be 2.5 vol%. Further, condensed liquids obtained from the respective condensers were put together and refluxed to the oxidation reactor, and a part thereof was withdrawn so that the water concentration in the mother liquor of the slurry withdrawn from the reactor would be 10 wt%. The slurry concentration of the slurry withdrawn from the oxidation reactor was 35 wt%, and the cobalt/manganese/bromine concentrations in the reaction mother liquor were 300/300/1000 wtppm. The slurry withdrawn from the oxidation reactor was continuously supplied to an agitation tank together with air, and a low temperature additional oxidation reaction was carried out by adjusting the liquid level so that the residence time would be 15 minutes at an operation temperature of 181°C under an operation pressure of 1.15 MPa (absolute pressure). Further, the distillate vapor was cooled by multistage condensers finally to 40°C, and the operation was carried out by adjusting the oxygen concentration in the exhaust gas to be 6 vol%. Further, the condensed liquids obtained from the respective condensers were put together and refluxed to the low temperature additional oxidation reactor. The slurry withdrawn from the low temperature additional oxidation reactor was subjected to crystallization up to 90°C, and then, the slurry obtained by this crystallization was supplied to a rotary vacuum filter and subjected to solid-liquid separation and washing. The operation pressure here was atmospheric pressure. The separated crude terephthalic acid cake was dried by a steam rotary dryer to obtain crude terephthalic acid crystals. Using a mixture of water containing acetic acid as the mixture to be distilled and n-butyl acetate as the azeotropic agent, a continuous distillation process was carried out in accordance with the process shown in Fig. 1. As an azeotropic distillation column 1 to carry out azeotropic distillation, a distillation column having 70 plates was used. The supplied mixtures to be distilled, were 38.2 parts by weight per unit time of mixture 1 comprising 88 wt% of acetic acid and 12 wt% of water, 70.9 parts by weight per unit time of mixture 2 comprising 19 wt% of acetic acid, 3 wt% of methyl acetate and 78 wt% of water, and 5.7 parts by weight per unit time of mixture 3 comprising 64.8 wt% of acetic acid, 0.1 wt% of methyl acetate and 35.1 wt% of water. The mixture 1 was supplied to the 70th plate (the lowest plate) from the top, the mixture 2 was supplied to the 60th plate from the top, and the mixture 3 was supplied to the 40th plate from the top. The operation pressure at the top of the azeotropic distillation column 1 was controlled to be 0.16 MPa by a valve installed for a non-condensed gas obtained by gas-liquid separation of condensation treated material of the top distillate vapor. The temperature of the top distillate vapor at that time was 98°C. In the azeotropic distillation column 1, 101.0 parts by weight per unit time of recovered acetic acid B containing 8 wt% of water was withdrawn as the bottom from the column bottom. This recovered acetic acid was sent to an acetic acid tank and reused as acetic acid to be used in various terephthalic acid production processes, such as an oxidation reaction solution, a cleaning liquid, an absorbing liquid, etc. From the top of the azeotropic distillation column 1, a distillate vapor comprising an azeotropic composition of water and n-butyl acetate, methyl acetate and a very small amount of acetic acid, was obtained and the vapor was treated for condensation by a condenser 2 in a state where the operation pressure at the top of the azeotropic distillation column 1 was maintained. Here, water was used as the cooling medium, and steam was obtained by means of the heat of condensation to carry out recovery of energy. This steam was saturated steam at a temperature of 90°C under a pressure of 0.07 MPa, and the generated amount was 30.0 parts by weight per unit time. The condensation-treated mixture was sent to a tank 3 and treated for gas-liquid separation. The non-condensed material was further subjected to cooling treatment (not shown) to obtain a gas containing an inert gas as the main component. A control valve was installed on the gas line I to discharge such an inert gas out of the system, and the pressure at the top of the distillation column was thereby controlled to be 0.16 MPa (absolute pressure). Accordingly, the above condensation treatment was carried out substantially under the same pressure as at the column top. The condensed liquid obtained in the tank 3 was subjected to flush evaporation treatment in the pressure-releasing tank 4. The operation temperature of the pressure releasing tank at that time was 85°C. By the flush evaporation, an evaporated material containing 37 wt% of methyl acetate was obtained in an amount of 4.0 parts by weight per unit time. This evaporated material was supplied to the second distillation column 5 and subjected to distillation treatment. On the other hand, the liquid obtained in the pressure-releasing tank 4 was cooled to 50°C by a cooler prior to liquid-liquid separation into the oil phase and the water phase by the oil-water separator 7. This cooling not only increases the oil-water separation efficiency, but it is thereby possible that as the temperature lowers, methyl acetate in the condensed liquid will be separated to the water phase side in the oil-water separation, whereby accumulation of methyl acetate in the azeotropic agent to be recycled can further be suppressed. In the oil-water separator 7, oil-water separation was carried out, and the oil phase containing n-butyl acetate as the main component, was recycled to the top of the azeotropic distillation column 1 (65 parts by weight per unit time). Further, the water phase was supplied to the second distillation column and subjected to distillation treatment together with the evaporated material obtained in the pressure-releasing tank 4, thereby to obtain a distillate liquid containing 90 wt% of methyl acetate (2.1 parts by weight per unit time) from the column top, a side cut liquid containing 24 wt% of n-butyl acetate (4.2 parts by weight per unit time) from a middle portion, and water containing a very small amount of acetic acid as the bottom (20.4 parts by weight per unit time) from the column bottom. The obtained distillate liquid (methyl acetate) was1 sent to "the acetic acid solution containing catalysts" in the oxidation reaction step in the process for producing terephthalic acid crystals, and the side cut liquid was sent to the oil-water separator 7. Further, an amount corresponding to 41 wt% of the bottom was refluxed to the top of the azeotropic distillation column 1 as reflux water (reflux ratio of water: 0.7) . And, distillation-treated water corresponding to 59 wt% of the bottom was obtained. In this manner, in the azeotropic distillation method pressurizing the column top, by subjecting the condensed liquid having a pressure to flush evaporation, it is possible to introduce a part of methyl acetate contained in the condensed liquid into the second distillation column 5 without passing through the oil-water separator, whereby it is possible not only to reduce the energy consumption at the cooler 4 or at the reboiler for the second distillation column 5 but also to reduce the amount of methyl acetate to be accompanying the oil phase side in the oil-water separator. As a result, without requiring a distillation column to remove methyl acetate from the oil phase liquid, or its peripheral equipments, or consumption of an energy required for distillation, it was possible to carry out the operation constantly over 7 months in a state where the concentration of methyl acetate (13 wt%) in the recycled azeotropic agent was maintained within the range not to deteriorate the separation performance of the azeotropic distillation. COMPARATIVE EXAMPLE 1 The condensed liquid obtained in the tank 3 was sent directly to the cooler 6 without being sent to the pressure-releasing tank 4; oil-water separation was carried out in the oil-water separator 7; an amount corresponding to 30% of the oil phase G containing n-butyl acetate as the main component was supplied to a third distillation column 8; methyl acetate was distilled and recovered from the oil phase J so that methyl acetate in the recycled azeotropic agent to be supplied to the azeotropic distillation column 1 became 13 wt%; and the bottom cake was mixed with the oil phase G and recycled to the azeotropic distillation column 1. Other than these, the operation was carried out in the same manner as in (Table Removed) As a result, in Examples 1 and Comparative Example 1, the relative values of the consumption of energy required for the reboiler to recover methyl acetate, were as shown in Table 1. Thus, by adopting the technology of the present invention, there will be no necessity of the third distillation column and its peripheral equipments, and further the consumption of energy required to recover methyl acetate within the distillation system can be substantially reduced. As is evident from the result of the above Example, it was possible to operate azeotropic distillation of acetic acid and water constantly without a distillation installation to remove methyl acetate from the azeotropic agent to be recycled and without consumption of energy (calorie) required for such distillation. The entire disclosure of Japanese Patent Application No. 2004-029483 (February 5, 2004) including specification, claims, drawings and summary is incorporated herein by reference in its entirety. WE CLAIM: 1. An azeotropic distillation process for separating water, acetic acid and methyl acetate into the respective components by distillation, which comprises the following steps (l)to(5): (1) a step of distilling a mixture to be distilled comprising water, acetic acid and methyl acetate in the presence of an azeotropic agent under a column top pressure higher than atmospheric pressure to separate the mixture into a bottom recovery fraction containing acetic acid and a top distillate vapor containing water, methyl acetate and the azeotropic agent, (2) a step of condensing the top distillate vapor obtained in the step (1), (3) a step of evaporating methyl acetate from the condensed liquid obtained in the step (2) by releasing the pressure, to recover methyl acetate, (4) a step of subjecting the residual liquid obtained in the step (3) to a liquid-liquid separation into a water phase and an oil phase, and (5) a step of supplying the oil phase obtained in the step (4) to the step (1) as the azeotropic agent. 2. The azeotropic distillation process as claimed in Claim 1, wherein the temperature of the top distillate vapor is from 95 to 130°C. 3. The azeotropic distillation process as claimed in Claim 1 or 2, wherein the evaporated material obtained in the step (3) is distilled to recover methyl acetate contained in the above evaporated material. 4. The azeotropic distillation process as claimed in Claim 1 or 2, wherein the evaporated material obtained in the step (3) is distilled together with the water phase obtained in the step (4) to recover methyl acetate contained in the evaporated material and the water phase. 5. The azeotropic distillation process as claimed in any one of Claims 1 to 4, wherein the mixture to be distilled comprising water, acetic acid and methyl acetate is a mixture of water, acetic acid and methyl acetate obtained in a process for production of terephthalic acid by oxidation of paraxylene in acetic acid as a solvent, and the recovered methyl acetate is retuned to the oxidation step. 6. The azeotropic distillation process as claimed in any one of Claims 1 to 5, wherein the azeotropic agent to be used in the azeotropic distillation is an acetate having 5 or 6 carbon atoms. 7. The azeotropic distillation process as claimed in any one of Claims 1 to 5, wherein the azeotropic agent to be used in the azeotropic distillation is at least one member selected from the group consisting of n-propyl acetate, i-propyl acetate, n-butyl acetate and i-butyl acetate. 8. The azeotropic distillation process as claimed in any one of Claims 1 to 7, wherein the heat of condensation generated in the step (2) used for heat exchange to produce steam.

Documents:

4472-delnp-2006-Abstract-(15-12-2011).pdf

4472-delnp-2006-abstract.pdf

4472-delnp-2006-Claims-(15-12-2011).pdf

4472-delnp-2006-claims.pdf

4472-DELNP-2006-Correspondence Others-(04-05-2011).pdf

4472-delnp-2006-Correspondence Others-(15-12-2011)..pdf

4472-delnp-2006-Correspondence Others-(15-12-2011).pdf

4472-delnp-2006-correspondence-others-1.pdf

4472-delnp-2006-correspondence-others.pdf

4472-delnp-2006-description(complete).pdf

4472-delnp-2006-Drawings-(15-12-2011).pdf

4472-delnp-2006-drawings.pdf

4472-delnp-2006-Form-1-(15-12-2011).pdf

4472-delnp-2006-form-1.pdf

4472-delnp-2006-form-18.pdf

4472-delnp-2006-Form-2-(15-12-2011).pdf

4472-delnp-2006-form-2.pdf

4472-delnp-2006-form-26.pdf

4472-DELNP-2006-Form-3-(04-05-2011).pdf

4472-delnp-2006-form-3.pdf

4472-delnp-2006-form-5.pdf

4472-delnp-2006-pct-210.pdf

4472-delnp-2006-pct-301.pdf

4472-delnp-2006-pct-304.pdf

4472-delnp-2006-Petition-137-(15-12-2011).pdf