Title of Invention | "METHOD OF AZEOTROPIC DISTILLATION" |
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Abstract | The invention provides a method of stabler azeotropic distillation which is controlled based on a new index. Namely, the invention provides a method of azeotropic distillation in which a solution to be distilled is distilled in an azeotropic distillation column using an entrainer to recover, as a distillate, azeotrope components including the entrainer and to recover, as bottoms, a liquid reduced in the content of the azeotrope components, wherein a target value of entrainer residence amount is fixed beforehand so as to result in an azeotropic region extending over a given range in the azeotropic distillation column, the actual amount of the entrainer residing in the azeotropic distillation column is determined, and the conditions for operating the distillation column are controlled so that the actual residence amount becomes the target value. |
Full Text | Description METHOD OF AZEOTROPIC DISTILLATION The present invention relates to a method of azeotropic distillation. More particularly, the invention relates to a method for conducting azeotropic distillation while controlling the conditions for operating the distillation column using entrainer residence amount as an index. Examples of applications of the invention include a method of distillation in a process for producing an aromatic carboxylic acid in a reaction medium containing an aliphatic carboxylic acid, wherein a vapor comprising the water yielded during the production and the aliphatic carboxylic acid is partly taken out of the reactor and the water contained in the condensate of the vapor is separated by azeotropic distillation, while controlling the conditions for operating the distillation column using entrainer residence amount as an index, for the purpose of recovering the aliphatic carboxylic acid from the condensate. In methods of azeotropic distillation for a mixture comprising substances difficult to separate, a substance which forms an azeotrope with any of these is added to the mixture to thereby improve the separating ability of distillation. Examples of industrial applications of azeotropic distillation methods include (1) a method in which high-purity acetic acid is obtained from a liquid mixture of acetic acid and water by adding n-propyl acetate or n-butyl acetate, which form an azeotrope with water, (2) a method in which high-purity benzene is obtained from a liquid mixture of benzene and cyclohexane by adding acetone, which forms an azeotrope with cyclohexane, (3) a method in which high-purity toluene is obtained from a liquid mixture of toluene and a paraffin by adding methanol, which forms an azeotrope with paraffins, (4) a method in which high-purity butanol is obtained by the azeotropic distillation of a liquid mixture of water and butanol, and the like. An industrial field where a method of azeotropic distillation described above is thought to be applicable is a process for producing an aromatic carboxylic acid. More particularly, a method of azeotropic distillation is applicable to a step for recovering the reaction medium from the process for producing an aromatic carboxylic acid. An aromatic carboxylic acid, e.g., terephthalic acid, is generally produced in a reaction medium containing an aliphatic carboxylic acid, e.g., acetic acid. However, since water generates during the process, it is necessary to prevent water accumulation in the reaction system. This is accomplished by taking a vapor of a mixture of the aliphatic carboxylic acid and water out of the reactor, distilling a feed flow comprising the condensate of the vapor to separate the water from the aliphatic carboxylic acid, and recycling at least part of the resultant dehydrated aliphatic carboxylic acid to a feedstock liquid preparation vessel. In the case of acetic acid, which among aliphatic carboxylic acids is extensively used as the reaction medium, rectification is usually used for separating water from acetic acid. However, azeotropic distillation can be advantageous depending on equipment cost and variable expenses. The main standpoints for technical developments concerning azeotropic distillation are roughly divided into separating ability, controllability, reduction in reflux ratio, and post-treatment of a recovered distillate. In general, higher reflux ratios result in better stability in operation, and as the reflux ratio is reduced, the operation stability gradually becomes poor. Furthermore, when the reflux ratio decreases beyond a certain critical value, the separating ability of the azeotropic distillation itself is worsened abruptly. This critical value is generally called minimum reflux ratio, which varies depending on the composition of the feed solution, kind of the entrainer, feed positions, number of feed lines, method for returning a reflux, method for returning the entrainer, etc. When an operation is performed using an extremely high reflux ratio, it is easy to satisfy controllability and separating ability. However, since such an operation is economically disadvantageous, operations are virtually being performed using a reflux ratio which is as close as possible to the minimum reflux ratio. For example, also in the case of obtaining acetic acid from a liquid mixture of water and acetic acid by azeotropic distillation, a separating ability not below a given level is necessary for enabling the liquid discharged from the bottom of the azeotropic distillation column (hereinafter this liquid will be referred to as "bottoms") to have a purity on an actually required level. On the other hand, a reduction in reflux ratio is desired from an economical standpoint. A reduction in reflux ratio tends to result in a reduced separating ability, and reducing a eflux ratio while maintaining a separating ability tends to result in poor controllability. When the difference between the actual water reflux ratio and the minimum water reflux ratio decreases to 1.0 or smaller, stability begins to become poor. When that difference decreases to 0.5, stability is further worsened. When that difference reaches 0.3, stability is significantly worsened. Worsened stability of operation leads to undesirable results such as inclusion of acetic acid into the distillate or inclusion of the entrainer into the bottoms due to fluctuations in operation. Methods of azeotropic distillation for separating acetic acid from water are disclosed in Japanese Patent Publication No. 41219/1987, WO 98-45239, Published Unexamined International Patent Application No. 504556/1998, Japanese Patent Publication No. 31091/1986, etc. Japanese Patent Publication No. 41219/1987 proposes a method in which a condensate of vapor from an azeotropic distillation column is subjected to liquid/liquid separation and the oil-phase ingredients which have come into the aqueous phase are diminished by stripping the aqueous-phase liquid. However, since the aqueous-phase liquid is usually discarded, the inclusion of oil-phase ingredients is undesirable from the standpoints of variable expenses and environment. WO 98-45239 proposes an improved method for removing impurities from an entrainer, which comprises partly condensing vapors from the top of an azeotropic distillation column and distilling the entrainer contained in the vapors remaining uncondensed and the aqueous phase in a continuous column. These techniques each are improvements concerning a step for the post-treatment of vapors recovered from the top of an azeotropic distillation column, and the operation for controlling the azeotropic distillation is the same as that heretofore in use. No technique for improving controllability is disclosed therein. Published Unexamined International Patent Application No. 504556/1998 proposes a method in which the condensate of vapors from the top of an azeotropic distillation column is supplied as a feed flow to an azeotropic region to thereby omit water refluxing. Since a reduction in water reflux amount can reduce the amount of heat to be used in the azeotropic distillation column, advantages in equipment cost and variable expenses are expected. With respect to control, it has been simultaneously proposed that a water reflux is returned to a middle part of the column and the amount thereof is regulated to thereby control the water concentration of bottoms. However, those proposals do not give an improved method from the standpoint of controlling the amount of an entrainer contained in the liquid discharged from the bottom of an azeotropic distillation column or the amount of acetic acid contained in the distillate. There has hence been a desire for a method of control by which the amount of an entrainer in the bottoms and the amount of an impurity in the distillate can be regulated to a value in a desired range or be reduced while satisfying the economic requirement. Japanese Patent Publication No. 31091/1986 proposes a method of control with enhanced response which comprises dividing a circulating entrainer flow and returning one to the column top and the other to a middle part of the column to thereby enable the effect of a change in operating conditions to be readily produced. Specifically, the technique disclosed is to measure the temperature in the column or the concentration of the entrainer, acetic acid, or water and, according to a change in any of these factors, change the amount of the entrainer flow supplied to a middle part of the column to thereby improve stability. However, this effect is attained only when the entrainer flow is divided. This technique necessitates a special passage in an azeotropic distillation process and hence has poor suitability for general use. Furthermore, in the method disclosed in that reference, how to regulate the en trainer flow rate is judged based on, as an index, the temperature in the column or a change in entrainer concentration itself. However, investigations made by the present inventors have revealed the following. When an operation is performed using a reduced reflux ratio because of an economic requirement, much time is required for the effect of entrainer flow rate regulation to be reflected in those indexes. Because of this, there is a tendency to excessively change the flow rate in flow rate regulation. In addition, since the temperature distribution in the column tends to irregularly fluctuates depending on the history of entrainer flow rate change, it is extremely difficult to conduct a stable operation. Attaining improvements in separating ability and controllability, a reduction in flux ratio, etc. in azeotropic distillation while reconciling these is extremely difficult as in the above-described examples of methods of azeotropic distillation for the separation of water from acetic acid, and is one of the important technical subjects. In particular, there is a desire for an index as a substitute for a temperature distribution or change in the concentration of an entrainer or another substance, for the purpose of using a reduced reflux ratio and thereby conduct a stable operation. The invention has been achieved in view of these circumstances. An aim thereof is to provide a method of stabler azeotropic distillation which is controlled based on a new index. The present inventors have found that when azeotropic distillation is controlled in such a manner that the actual amount of an entrainer residing in the azeotropic distillation column is calculated and the conditions for operating the azeotropic distillation column are controlled so as to keep the actual entrainer residence amount at a target value, then a far stabler operation than in the conventional methods in which the temperature or concentration itself in the azeotropic distillation column is used as an index for regulating the entrainer flow rate can be attained without installing a special passage in the process. The invention has been achieved based on these findings. Namely, the essential points of the invention reside in the following (1) to (10). (1) A method of azeotropic distillation in which a solution to be distilled is distilled in an azeotropic distillation column using an entrainer to recover, as a distillate, azeotrope components including the entrainer and to recover, as bottoms, a liquid reduced in the content of the azeotrope components, characterized in that a target value of entrainer residence amount is fixed beforehand so as to result in an azeotropic region extending over a given range in the azeotropic distillation column, the actual amount of the entrainer residing in the azeotropic distillation column is determined, and the conditions for operating the distillation column are controlled so that the actual residence amount becomes the target value. (2) The method described in (1) characterized in that the actual entrainer residence amount is determined from the mass balance between the liquids which are entering or leaving the azeotropic distillation column. (3) The method described in (1) characterized in that the actual entrainer residence amount is determined by subtracting the actual amount of the entrainer residing in the distillation system other than the azeotropic distillation column from the amount of the entrainer which has been introduced into the azeotropic distillation column. (4) The method described in any one of (1) to (3) characterized in that the target value of entrainer residence amount is fixed based on the range of the azeotropic region determined from a composition change or temperature change in the azeotropic distillation column. (5) The method described in any one of (1) to (4) characterized in that the target value of entrainer residence amount is fixed based on measurements of the impurity amounts in the distillate and in the bottoms. (6) The method described in any one of (1) to (5) characterized in that the entrainer residence amount is controlled by the amount of the entrainer which is returned to the distillation column from an entrainer circulation part attached to the azeotropic distillation column. (7) The method described in any one of (1) to (6) characterized in that the entrainer residence amount is controlled by regulating the amount of a reflux which is returned to the azeotropic distillation column. (8) The method described in any one of (1) to (7) characterized in that the entrainer residence amount is controlled by regulating the amount of heat applied to the azeotropic distillation column. (9) The method described in any one of (1) to (8) characterized in that the solution to be distilled is a mixture comprising an aliphatic carboxylic acid and water, and that azeotrope components comprising water and the entrainer are recovered as a distillate and the concentrated aliphatic carboxylic acid is recovered as bottoms. (10) The method described in (9) characterized in that in fixing the target value of entrainer residence amount, the entrainer residence amount at which the concentration of the aliphatic carboxylic acid in the distillate from the azeotropic distillation column is at least 1,000 weight ppm of the weight of the distillate is taken as the lower limit and the entrainer residence amount at which the concentration of the entrainer in the bottoms from the azeotropic distillation column is at least 100 weight ppm of the weight of the bottoms is taken as the upper limit. Fig. 1 is a diagrammatic view of a process example for practicing the invention. Fig. 2 is a diagrammatic view of a process example for practicing the invention. Fig. 3 is a graph showing temperature changes with time in respective parts in the azeotropic distillation column described in an Example. Fig. 4 is a graph showing temperature changes with time in respective parts in the azeotropic distillation column described in an Example. In the Figs., numeral 1 denotes an azeotropic distillation column, 2 a liquid/liquid separation tank, 3 a distillation column, 4 an entrainer recovery tank, 5-11 a piping line, 12 a condenser, 13 a reboiler, 14-16 a piping line, 51 an azeotropic distillation column, 52 a decanter, 53 an entrainer tank, 54 a heat exchanger, 55-62 a piping line, 63 a reboiler, and 64-66 a piping line. The invention will be explained below in detail. In the invention, the term "solution to be distilled" means a mixed solution comprising the substance to be purified and an impurity whose concentration is desired to be reduced. The term "entrainer" means a third ingredient which is added in order to conduct azeotropic distillation. The term "azeotropic region" means the region in which the composition present therein as a liquid phase as a whole has an entrainer concentration of at least 0.1% by weight. The term "range of azeotropic region" means that spatial area in the azeotropic distillation column over which an azeotropic region extends. Furthermore, in the invention, the term "azeotropic distillation column" means a distillation column in which the solution to be distilled and the entrainer are distilled. The term "entrainer circulation part attached to the azeotropic distillation column" means that part of the apparatus which is intended to recover the entrainer and recycle it to the azeotropic distillation column. Moreover, in the invention, the term "entrainer residence amount" means the amount of the entrainer residing in the distillation column in which azeotropic distillation is conducted. In particular, the term "actual amount of the entrainer residing" means the value obtained by actually calculating, from various data, the amount of the entrainer residing in the distillation column at the time during operation of the azeotropic distillation column. The term "target value of entrainer residence amount" means the target value or target range of the amount of the entrainer to be allowed to reside in the azeotropic distillation column during operation of the distillation column. First, the solution to be distilled and the entrainer will be explained. The solution to be distilled in the invention is not particularly limited as long as it is a mixed solution which comprises the substance to be purified and an impurity whose concentration is desired to be reduced and in which the impurity, together with the entrainer added, form an azeotrope whose azeotropic point is lower than the boiling point of the substance to be purified. The solution may further contain substances which exert substantially no influence on the azeotropic distillation. The entrainer in the invention is not particularly limited as long as it produces its effect. From the standpoint of profitability, it is preferred to recover the entrainer. When this entrainer recovery is taken in account, a preferred combination of the solution to be distilled and an entrainer is one which gives a heterogeneous azeotrope in which the impurity contained in the solution to be distilled and the entrainer are not evenly mixed together in a liquid state. Furthermore, as in the case of concentrating butanol by the azeotropic distillation of a liquid mixture of water and butanol, the substance itself to be purified can be caused to function as an entrainer. However, in view of the precision of the index for stably controlling azeotropic distillation, the entrainer desirably is a compound different from the substance to be purified. With respect to the solution to be distilled and entrainer, preferred combinations suitable for application of the invention thereto, for example, include (i) methods of azeotropic distillation in which the substance to be purified is an aliphatic carboxylic acid such as acetic acid, propionic acid, butyric acid, or the like, the impurity whose concentration is desired to be reduced is water, and the entrainer is an ester of an aliphatic carboxylic acid, such as n-propyl acetate, n-butyl acetate, or the like and (ii) methods of azeotropic distillation in which the substance to be purified is an aromatic hydrocarbon such as benzene, toluene, or the like, the impurity whose concentration is desired to be reduced is a paraffin such as n-hexane or the like, and the entrainer is a lower alcohol such as methanol, ethanol, isopropyl, or the like or an oxygen-containing polar organic compound such as acetone, methyl ethyl ketone, nitromethane, or the like. Especially preferred of these are, for example, (iii) a method of azeotropic distillation in which the substance to be purified is acetic acid, the impurity whose concentration is desired to be reduced is water, and the entrainer is n-propyl acetate or n-butyl acetate, (iv) a method of azeotropic distillation in which the substance to be purified is benzene, the impurity whose concentration is desired to be reduced is cyclohexane, and the entrainer is acetone, and (v) a method of azeotropic distillation in which the substance to be purified is toluene, the impurity whose concentration is desired to be reduced is a paraffin, and the entrainer is methanol. Next, the azeotropic distillation column and the entrainer circulation part attached to the azeotropic distillation column will be explained. The azeotropic distillation column in the invention may be either a packed column or a plate column. Although the position in which the solution to be distilled is supplied is not particularly limited, it is usually a middle part of the azeotropic distillation column. An optimal position may be determined in order to optimize a plate efficiency, while taking account of compositions in the column. The azeotropic distillation column can be operated under any of ordinary-pressure, elevated-pressure, and reduced-pressure conditions, and the operation may be batchwise or continuous. More preferably, the operation is conducted continuously at ordinary pressure. As a result of the azeotropic distillation, bottoms comprising the substance to be purified which has a reduced impurity concentration are obtained through the bottom of the column, and a vapor of an azeotrope consisting mainly of the impurity and entrainer is obtained through the top of the column. The vapor obtained through the column top is usually condensed and separated into the impurity and the entrainer. Techniques for this separation are not particularly limited as long as the purpose is accomplished However, in the case of a combination in which the impurity and the entrainer are not evenly mixed together, examples include a technique in which the condensate is subjected, e.g., to liquid/liquid separation with a decanter. Of the two phases separated, the phase mainly comprising the entrainer is recycled to the azeotropic distillation column. Methods for returning the entrainer include a method in which the entrainer is wholly returned to the column top and a method in which part of the entrainer is separated and returned to a middle part of the column. On the other hand, the phase mainly comprising the impurity is partly discarded, and the remainder is returned as a reflux to the azeotropic distillation column. Methods for returning the reflux include, for example, a method in which the reflux is returned to the column top and a method in which it is returned to a middle part of the column. The entrainer may be freshly supplied in compensation for the loss. In the case where the azeotropic distillation results in accumulation of a fourth ingredient, which is an ingredient other than the impurity whose concentration is desired to be reduced or than the substance to be purified, in the phase mainly comprising the entrainer, then this phase is sent to a separate distillation column and returned to the azeotropic distillation column after the accumulation is removed. It is also possible to remove the impurity by a method in which the vapor from the top of the azeotropic distillation column is partly condensed and the vapor remaining uncondensed is distilled in a continuous column, as described in WO 98-45239. Furthermore, a method for enhancing response may be used which comprises dividing a circulating entrainer flow and returning one to the column top and the other to a middle part of the column to thereby enable the effect of a change in operating conditions to be readily produced, as described in Japanese Patent Publication No. 31091/1986. Moreover, a method may be used which comprises returning a reflux to a middle part of the column and controlling the impurity concentration in the column bottom by regulating the reflux amount, as described in Published Unexamined International Patent Application No. 504556/1998. In Fig. 1 is shown an embodiment of azeotropic distillation processes to which the method of the invention is applicable. An azeotrope vapor comprising an impurity, the concentration of which is desired to be reduced by azeotropic distillation, and an entrainer is sent from the top of an azeotropic distillation column 1 to a condenser 12, where the vapor is condensed. The resultant condensate is separated into two phases in a liquid/liquid separation tank 2. A suitable separation technique is selected according to the nature of the azeotrope. In the case where the azeotrope in a liquid state does not separate into two phases, a distillation column or the like is installed as a step for separating the entrainer. In the process shown in Fig. 1, the phase comprising the entrainer as the main component, of the liquids separated in the liquid/liquid separation tank 2, is supplied through a line 9 to a distillation column 3 in order to remove the impurity accumulated therein. A liquid reduced in accumulated-impurity concentration is recovered through the bottom of the distillation column 3. This liquid passes through a line 10, resides in an entrainer recovery tank 4, and is then returned to the azeotropic distillation column 1 through a line 14. Although it is preferred to install the distillation column 3 and the tank 4, these are not essential. On the other hand, the phase comprising as the main component the impurity whose concentration is desired to be reduced by azeotropic distillation is partly returned to the azeotropic distillation column 1 through a line 6 in order to attain a desired reflux ratio. The remaining part passes through a line 15 and a treating apparatus installed according to need, and is then discarded or reused outside the process. The line 6 and the line 14 are connected to the top or a middle part of the azeotropic distillation column 1, according to need, and the number of pipes may be either one or more. When the liquid comprising the entrainer as the main component and the liquid comprising as the main component the impurity whose concentration is desired to be reduced by azeotropic distillation are returned to the azeotropic distillation column 1, use may be made of the same line or separate lines. A solution to be distilled is supplied to the azeotropic distillation column 1 through a line 7. The line 7 is disposed in a desired position, and the number of pipes may be either one or more. A liquid comprising as the main component the substance to be purified is obtained through the bottom of the azeotropic distillation column 1. In the invention, the azeotropic region is regulated so as to extend over a given range in the azeotropic distillation column to thereby realize a stable operation. The actual procedure comprises determining the actual amount of the entrainer residing in the azeotropic distillation column, fixing a target value of entrainer residence amount beforehand so as to result in an azeotropic region extending over a given range in the azeotropic distillation column, and regulating the actual amount of the entrainer residing in the azeotropic distillation column to the target value. With respect to the amount of the entrainer residing in the azeotropic distillation column, there is no entrainer residing therein. A strict coincidence has an effect that a calculated entrainer residence amount and a separated state always correspond to each other strictly in 1:1. However, when the relationship between the entrainer residence amount and separation is suitably corrected based on the results of actual separation or a temperature distribution in the column which can be substituted for the results of actual separation, then the relationship between the entrainer residence amount and separation can be properly maintained. Consequently, when a change in the actual amount of the entrainer residing in the azeotropic distillation column can be detected, then the purpose of the invention which is to keep separation in an optimal state can be accomplished even when the calculated value of entrainer residence amount and the absolute value of actual residence amount do not strictly coincide with each other. Methods for determining the actual amount of an entrainer residing in an azeotropic distillation column in the invention will be explained. For this purpose can be used, for example, the following three techniques. In the first technique, the difference between the amount of the entrainer leaving the azeotropic distillation column 1 and the amount of the entrainer entering the azeotropic distillation column 1 is measured to calculate the actual entrainer residence amount. For example, in Fig 1, the actual residence amount is calculated from the difference between the amount of the entrainer leaving through the line 5 and line 8 and the amount of the entrainer entering the azeotropic distillation column 1 through the line 6, line 7, and line 14. Each time the number of lines connected to the azeotropic distillation column increases according to the apparatus, the lines additionally installed are classified into introduction lines and discharge lines and added to the lines used for entrainer amount estimation. However, with respect to the lines which contain the entrainer in a slight amount, e.g., the line 6, line 7, and line 8, calculation may be omitted. Furthermore, the amount of the entrainer flowing through the line 9 and the amount of the entrainer residing in the separation tank 2 can be substituted for the amount of the entrainer flowing through the line 11, which is for the condensate of the vapor which has passed through the line 5 The amount of the entrainer flowing through each line is determined from the entrainer composition determined for the line according to need and from the flow rate measured. The composition is determined, for example, by a method in which a sample is taken out and analyzed by gas chromatography or the like or a method in which a detection terminal is installed in the line to obtain a spectrum. The second method for calculating the actual amount of an entrainer residing in an azeotropic distillation column comprises determining the actual amount of the entrainer residing in the entrainer circulation part and calculating the actual entrainer amount in the column from that value. Subtracting the amount of the entrainer in the circulation part from the total amount of the entrainer in the system gives the amount of the entrainer residing in the azeotropic distillation column 1. For example, in Fig. 1, an entrainer leaves the top of the azeotropic distillation column 1 and returns to the azeotropic distillation column 1 through the line 5, separation tank 2, line 9, distillation column 3, line 10, tank 4, and line 14. The actual amount of the entrainer residing in the azeotropic distillation column 1 can be calculated from the actual amount of the entrainer residing in the line 5, separation tank 2, line 9, distillation column 3, line 10, tank 4, and line 14. For determining the amount of the entrainer residing in the tanks and column may be used either a method in which the liquid level is read to determine the amount or a method in which the amount is calculated from the difference between entering and leaving amounts. For example, in Fig. 1, the amount of the entrainer residing in the tank 4 may be determined by either a method in which the liquid level in the tank 4 is read to determine the amount or a method in which the amount is calculated from the difference between the amount of the entrainer entering the tank 4 through the line 10 and the amount of the entrainer leaving the tank 4 through the line 14. With respect to the actual amount of the entrainer residing in a line, the volume determined from line dimensions, e.g., the inner diameter and length, may be substituted for that amount. In the case where a significant amount of the impurity has come into the entrainer, the actual entrainer residence amount can be determined from the entrainer composition determined according to need and from the residence amount of the liquid mixture measured. The composition is determined, for example, by a method in which a sample is taken out and directly analyzed by gas chromatography or the like or a method in which a detection terminal is installed in the tanks and column to obtain a spectrum. In the third method for calculating the actual amount of an entrainer residing in an azeotropic distillation column, this amount is calculated from the residence amounts in the azeotropic distillation column 1 of the substance to be purified and of the impurity whose concentration is desired to be reduced by azeotropic distillation. The amounts of the substance to be purified and impurity which reside in the azeotropic distillation column 1 are calculated from the amounts of the substance to be purified and impurity which enter the azeotropic distillation column 1 and from the amounts of the substance to be purified and impurity which leave the azeotropic distillation column 1. The amount of the entrainer residing in the azeotropic distillation column 1 can be calculated by calculating the total liquid volume in the azeotropic distillation column 1 and subtracting the amounts of the substance to be purified and impurity residing therein from the total liquid volume. For example, in Fig. 1, the amounts of the substance to be purified and the impurity which reside in the azeotropic distillation column 1 are calculated from the amounts of the substance to be purified and the impurity which enter through the line 6, line 7, and line 14 and from the amounts of the substance to be purified and the impurity which leave through the line 5 and line 8. Each time the number of lines connected to the azeotropic distillation column increases according to the apparatus, the lines additionally installed are classified into introduction lines and discharge lines and the amounts of the substance to be purified and the impurity which reside therein are added to the values used for calculating the amounts of these ingredients residing in the azeotropic distillation column 1. However, with respect to the lines in which the substance to be purified is contained in an extremely small amount, e.g., the line 5, line 6, and line 14, calculation may be omitted. Furthermore, the amounts of the substance to be purified and the impurity which flow through the line 16 and the amounts of the substance to be purified and the impurity which reside in the separation tank 2 can be substituted for the amounts of the substance to be purified and the impurity which flow through the line 5. Next, methods for fixing beforehand a target value of entrainer residence amount in an azeotropic distillation column in the invention will be explained. Actually, conditions are fixed so that the amount of an entrainer residing in the azeotropic distillation column 1 is an optimal value or in an optimal range in order to attain desired separation. Specifically, a target value of entrainer residence amount in the azeotropic distillation column is fixed so that the concentration of an undesirable substance which has come into the distillate or bottoms from the azeotropic distillation column becomes optimal. For this purpose can be used, for example, the following two techniques. In the first technique, the concentration of an undesirable substance which has come into the distillate or bottoms from the azeotropic distillation column is actually measured and a target value of entrainer residence amount in the azeotropic distillation column is fixed based on that. Examples of the undesirable substance which has come into the distillate recovered include the substance to be purified, and examples of the undesirable substance which has come into the bottoms include the entrainer. The composition of the distillate or bottoms is determined, for example, by a method in which a sample is taken out and directly analyzed by gas chromatography or the like or a method in which a detection terminal is installed in the tanks and column to obtain a spectrum. In the second method for fixing a target value of entrainer residence amount in the azeotropic distillation column, the temperature distribution in the azeotropic distillation column is measured to fix a temperature distribution range in which the concentration of an undesirable substance which has come into the distillate or bottoms from the azeotropic distillation column can be kept optimal. The target value of entrainer residence amount in the azeotropic distillation column is determined from an allowable temperature distribution range. This temperature distribution measurement is made with a temperature detection terminal installed in the column. One temperature detection terminal is disposed or two or more temperature detection terminals are disposed in the column in different positions in the height direction. The temperature measurement is made with respect to the whole column, preferably parts including the column top and column bottom and a part around a lower part of the azeotropic region. For example, a part around a lower part of the azeotropic region shows an abrupt temperature change as shown in Japanese Patent Publication No. 31091/1986. Consequently, the azeotropic region can be grasped based on the temperature distribution. Next, in the invention, methods for regulating the entrainer residence amount in the azeotropic distillation column to the target value will be explained. For this purpose can be used, for example, the following three techniques. In the first method for regulating the entrainer residence amount so as to be the target value or in the target range, the amount of the entrainer which is returned to the azeotropic distillation column from an entrainer circulation part attached to the distillation column is regulated. For example, in Fig. 1, the amount of the entrainer to be returned to the column through the line 14 is regulated so that the entrainer residence amount becomes the target value or comes to be in the target range. The target value or target range of entrainer residence amount may be suitably corrected according to the separated state or temperature distribution in the column. For the purpose of regulating the impurity concentration of the bottom, the amount of heat to be applied with the reboiler 13 or the amount of the flux to be returned through the line 6 may be regulated. The second method for regulating the entrainer residence amount in the azeotropic distillation column to the target value therefor is to regulate the amount of a reflux. For example, in Fig. 1, the amount of the reflux to be returned to the column through the line 6 is regulated so that the actual entrainer residence amount calculated becomes the target value. The target value of entrainer residence amount may be suitably corrected according to the separated state or temperature distribution in the column. For the purpose of regulating the impurity concentration of the bottoms, the amount of heat to be applied with the reboiler 13 may be regulated. The third method for regulating the entrainer residence amount in the azeotropic distillation column to the target value therefor is to regulate the amount of heat to be applied to the azeotropic distillation column. For example, in Fig. 1, the amount of heat which is applied with the reboiler 13 is regulated so that the actual entrainer residence amount calculated becomes the target value or comes to be in the target range. The target value of entrainer residence amount may be suitably corrected according to the separated state or temperature distribution in the column. For the purpose of regulating the impurity concentration of the bottoms, the amount of the reflux to be returned through the line 6 may be regulated. In control loops for azeotropic distillation, the amount of bottoms to be discharged and the amount of reboiler steam are usually determined respectively from the amount of the bottoms residing in the column and the temperature of the column bottom. The invention is applicable also to the control loop proposed in Published Unexamined International Patent Application No. 504556/1998. The invention brings about a higher effect in a system having poor controllability. For example, as the reflux ratio in a system which is to be practiced becomes close to the minimum reflux ratio, controllability worsens. The method of azeotropic distillation of the invention can be applied to the azeotropic distillation of any substance. However, the invention can be preferably used, for example, for reducing the water concentration in a mixture comprising an aliphatic carboxylic acid and water. This aliphatic carboxylic acid means a saturated or unsaturated aliphatic carboxylic acid having 2 to 6 carbon atoms. The invention is especially preferably used as a method for reducing the water concentration in a mixture comprising a saturated aliphatic carboxylic acid having 2 to 4 carbon atoms and water. The solution to which the invention is to be applied is preferably a liquid obtained by recovering and condensing a vapor mixture discharged from the reactor in a step in which an aromatic carboxylic acid is produced from an aromatic hydrocarbon as a starting material by liquid-phase oxidation in a reaction medium comprising an the mother liquor separated from the solid aromatic carboxylic acid produced or a liquid which has been optionally used in the step and comprises an aliphatic carboxylic acid and water may be supplied. In the mixed solution which comprises an aliphatic carboxylic acid and water and is supplied to an azeotropic distillation column, the proportion of each component may be any value. However, the method of the invention is usually applied to a mixed solution which comprises an aliphatic carboxylic acid and water and which has a water content in the range of from 5 to 95% by weight, preferably from 10 to 70% by weight, based on the weight of the mixed solution. In reducing the water concentration in the mixture comprising an aliphatic carboxylic acid and water, azeotropic distillation is conducted to obtain, from the column bottom, bottoms comprising the aliphatic carboxylic acid and reduced in water amount and further obtain, from the column top, a vapor of an azeotrope mainly comprising water and an entrainer, as described above. In this operation, from the standpoints of, e.g., reusing the aliphatic carboxylic acid, the concentration of the entrainer in the bottoms is preferably not higher than 100 weight ppm of the weight of the bottoms. Furthermore, from the standpoints of profitability, etc., the concentration of the aliphatic carboxylic acid in the distillate is preferably not higher than 1,000 weight ppm of the weight of the distillate. Although there is no particular lower limit on the two concentrations, the concentration of the entrainer in the bottoms is preferably not lower than 0.05 weight ppm of the weight of the bottoms and the concentration of the aliphatic carboxylic acid in the distillate is preferably not lower than 0.5 weight ppm of the weight of the distillate. The bottoms are partly recycled as a feedstock preparation liquid to the liquid-phase oxidation reaction of the aromatic hydrocarbon. The vapor obtained through the column top is usually condensed and subjected to liquid/liquid separation with a decanter or the like. Thereafter, the oil-phase liquid is recycled as an entrainer to the azeotropic distillation column, and the aqueous-phase liquid is partly discarded and partly returned as a reflux to the azeotropic distillation column. The entrainer may be freshly supplied in compensation for the loss. In the case where one or more oil-phase components including the entrainer come into the aqueous phase, this aqueous phase is subjected to a step such as the removal of the oil-phase components by blowing steam or a gas or treatment with active carbon, and is then sent to a wastewater-treating apparatus. In this case, the oil-phase components in the aqueous-phase liquid may be diminished by stripping as described in Japanese Patent Publication No. 41219/1987. A more preferred embodiment of the above-described application example of the invention is provided. This is a method in which in fixing a target value of entrainer residence amount, the entrainer residence amount at which the concentration of the aliphatic carboxylic acid in the distillate from the azeotropic distillation column is at least 1,000 weight ppm of the weight of the distillate is taken as the lower limit and the entrainer residence amount at which the concentration of the entrainer in the bottoms from the azeotropic distillation column is at least 100 weight ppm of the weight of the bottoms is taken as the upper limit, by which conditions for operating the azeotropic distillation column are controlled. The entrainer to be used in applying the invention for reducing the water concentration of a mixture comprising an aliphatic carboxylic acid and water is not particularly limited as long as it produces its effect. Virtually, the kind of the coexistent aliphatic carboxylic acid is taken into account in selecting an entrainer. Specifically, known compounds for use in the azeotropic distillation of mixed solutions mainly comprising acetic acid and water are preferred. Compounds capable of forming an azeotrope with water are usually used. Examples thereof include esters such as butyl formate, n-propyl acetate, isobutyl acetate, n-butyl acetate, amyl acetate, n-butyl propionate, isobutyl propionate, and the like, ethers such as dichloromethyl ether, ethyl isoamyl ether, allyl isoamyl ether, di-n-butyl ether, and the like, halogenated hydrocarbons such as ethylene dichloride, chlorobenzene, and the like, ketones such as acetone chloride, dipropyl ketone, methyl butyl ketone, and the like, and aromatic hydrocarbons such as toluene, xylene, ethylbenzene, and the like. Preferred of these entrainers are esters. For example, it is preferred to use n-propyl acetate or n-butyl acetate. The entrainer may contain an ingredient derived from the feedstock for azeotropic distillation, such as p-xylene, methyl acetate, or the like. The method of the invention was practiced with the apparatus shown in Fig. 2 by the continuous distillation method using an aqueous solution containing acetic acid as a solution to be distilled and using butyl acetate as an entrainer. As the azeotropic distillation column for conducting azeotropic distillation therein, a plate column was used in which the total number of plates was 60. Continuous azeotropic distillation was initiated in the following manner. A liquid mixture consisting of 75.0% by weight acetic acid, 24.9% by weight water, and 0.1% by weight butyl acetate was fed to an azeotropic distillation column 51 and a reboiler 63 of the azeotropic distillation column was used to conduct heating. Thus, total refluxing was performed. Thereafter, butyl acetate as an entrainer was gradually fed from an entrainer tank 53 to thereby form a three-component azeotropic distillation consisting of acetic acid/water/butyl acetate. After the azeotropic region was caused to extend over a desired range by regulating the entrainer amount, a feedstock was begun to be supplied to the azeotropic distillation column through a feedstock feed line 57. The feedstock consisted of 75.0% by weight acetic acid, 24.9% by weight water, and 0.1% by weight butyl acetate, and the flow rate was 10.5 T/hr. A feed opening was disposed between the 44th plate and 45th plate of the azeotropic distillation column. Concentrated acetic acid was withdrawn as bottoms from the bottom of the azeotropic distillation column through a line 58 at a rate of 8.2 T/hr. Through the top of the azeotropic distillation column was obtained a vapor having an azeotropic composition comprising water and the entrainer. This vapor was cooled and recovered with a decanter 52. The decanter 52 had an interface meter installed therein, and the amount of an aqueous-phase liquid (Ww) discharged through a line 66 and line 60 was regulated so that the interface level was always kept constant. An aqueous-phase liquid which branched off from the line 66 was returned as a reflux water (Wr) through a line 56 (disposed at the 1st plate) to the azeotropic distillation column 51. The amount of the reflux water was regulated so that the water reflux ratio defined by Wr/Ww became a given value. In this Example, azeotropic distillation was conducted under such conditions that the amount of the aqueous-phase liquid discharged (Ww) was 2.3 T/hr, the amount of the reflux water (Wr) was 1.15 T/hr, and the water reflux ratio was 0.5. The entrainer was separated by liquid/liquid separation in a decanter 52, passed after overflow through a line 59, an entrainer tank 53, and a line 64 (disposed at the 1st plate) , and was fed to the azeotropic distillation column 51. The line 64 was branched and part of the entrainer was passed through a line 65 and fed to between the 15th plate and 16th plate of the azeotropic distillation column 51. The amount of the entrainer fed through the line 65 was always kept constant at 0.5 T/hr and the amount of the entrainer fed through the line 64 was suitably regulated according to the separated state. The actual amount of the entrainer residing in the azeotropic distillation column 51 was calculated through a measurement of the amount of the entrainer residing in the circulation system. Specifically, the entrainer was returned to the azeotropic distillation column 51 through the line 61, decanter 52, line 59, tank 53, line 64, and line 65. The amounts of the entrainer residing in the line 61, line 59, line 64, and line 65, among those, were always constant, and the values corresponding to the volumes of the lines were used therefor. In the decanter 52, the interface was always on a constant level and the discharge was by overflow. For this decanter also, an always constant value was hence used as the residence amount. With respect to the tank 53, the liquid level was measured to calculate the actual residence amount. The amount of the entrainer actually residing in the azeotropic distillation column 51 was calculated by subtracting the actual amount of the entrainer residing in those lines and tanks from the total amount of the entrainer which had been fed before initiation of the operation and resided in the azeotropic distillation system. In this Example, the amount of the entrainer residing in the tank 53 was substantially regulated in the manner described above to thereby control the entrainer amount in the azeotropic distillation column 51. The target value of the entrainer residence amount in the tank 53 and the target value of the entrainer residence amount in the azeotropic distillation column were fixed at 2.10 T and 1.15 T, respectively, and continuous azeotropic distillation was continuously conducted for 8 hours. During this operation, slight temperature fluctuations were observed at the 23rd plate. However, the temperatures as measured at the 20th plate and 26th plate were stable, and no temperature fluctuations were observed in the parts above these (e.g., the 43rd plate, 52nd plate, and 60th plate) and in the parts below these (e.g., the 1st plate and 11th plate) . Fig. 3 shows changes with time of a temperature distribution in the azeotropic distillation column. The extremely stable temperature distribution in the distillation column revealed that the range of the azeotropic region had not fluctuated considerably. Thus, a satisfactory separated state was maintained. COMPARATIVE EXAMPLE 1 Azeotropic distillation was conducted in the same manner as in Example 1, except that the actual amount of the entrainer residing in the azeotropic distillation column 51 was not calculated and the entrainer flow rate was regulated directly based on changes in temperature distribution. During the operation, considerable temperature changes were observed throughout the range of from the 14th plate to the 32nd plate. (Considerable temperature fluctuations were not observed in the parts above these (e.g., the 43rd plate, 52nd plate, and 60th plate) and in the parts below these (e.g., the 1st plate and 11th plate) .) Fig. 4 shows changes with time of a temperature distribution in the azeotropic distillation column. Thus, the temperature distribution was unstable, and it was unavoidable to operate the azeotropic distillation column with considerable fluctuations. Factors causative of the fluctuations which could not be inhibited include irregular delays in changing in temperature distribution with changes in entrainer flow rate. It is thought that since a temperature distribution changes depending on the stable state in the column and the history of entrainer change, changes in temperature distribution become irregular. Consequently, in the invention, the amount of an entrainer residing in the column is used as an index in place of a temperature distribution to thereby produce an effect that a control loop which responds quickly is established and distillation is conducted stably. It was found that in controlling an azeotropic distillation column, regulating the amount of an entrainer residing in the column is more effective in realizing a stable operation than regulating the entrainer flow rate directly based on the separated state or temperature distribution. As a result, it has become possible to stably conduct azeotropic distillation, and the inclusion of acetic acid into the distillate or inclusion of the entrainer into the bottoms due to operational fluctuations can be minimized. The method of the invention prevents effective ingredients from being lost and thereby brings about a considerable effect in variable expenses or environmental protection. In conventional methods, when an azeotropic distillation column is operated at a reduced reflux ratio in order to meet the desire for a reduction in variable expenses by reducing the amount of heat to be used, then the results are slow responses, unstable control, and sacrifices with respect to the increase in the inclusion of acetic acid into the distillate or inclusion of the entrainer into the bottoms. In contrast, according to the invention, a stable operation can be attained even with reduced refluxing. Furthermore, the invention does not necessitate a special or complicated passage in azeotropic distillation processes, has suitability for various uses, and is applicable to most azeotropic distillation processes. WE CLAIM : 1. A method of azeotropic distillation in which a solution to be distilled is distilled in an azeotropic distillation column using an entrainer to recover, as a distillate, azeotrope components having the entrainer and to recover, as bottoms, a liquid reduced in the content of the azeotrope components, characterized in that a target value of entrainer residence amount is fixed beforehand so as to result in an azeotropic region extending over a given range in the azeotropic distillation column, the actual amount of the entrainer residing in the azeotropic distillation column is determined, and the conditions for operating the distillation column are controlled so that the actual residence amount becomes the target value. 2. The method as claimed in claim 1, wherein the actual entrainer residence amount is determined from the mass balance between the liquids which are entering or leaving the azeotropic distillation column. 3. The method as claimed in claim 1, wherein the actual entrainer residence amount is determined by subtracting the actual amount of the entrainer residing in the distillation system other than the azeotropic distillation column from the amount of the entrainer which has been introduced into the azeotropic distillation column. 4. The method as claimed in any one of claims 1 to 3, wherein the target value of entrainer residence amount is fixed based on the range of the azeotropic region determined from a composition change or temperature change in the azeotropic distillation column. 5. The method as claimed in any one of claims 1 to 4, wherein the target value of entrainer residence amount is fixed based on measurements of the impurity amounts in the distillate and in the bottoms. 6. The method as claimed in any one of claims 1 to 5, wherein the entrainer residence amount is controlled by the amount of the entrainer which is returned to the distillation column from an entrainer circulation part attached to the azeotropic distillation column. 7. The method as claimed in any one of claims 1 to 6, wherein the entrainer residence amount is controlled by regulating the amount of a reflux which is returned to the azeotropic distillation column. 8. The method as claimed in any one of claims 1 to 7, wherein the entrainer residence amount is controlled by regulating the amount of heat applied to the azeotropic distillation column. 9. The method as claimed in any one of claims 1 to 8, wherein the solution to be distilled is a mixture comprising an aliphatic carboxylic acid and water, and that azeotrope components comprising water and the entrainer are recovered as a distillate and the concentrated aliphatic carboxylic acid is recovered as bottoms. 10. The method as claimed in claim 9, wherein in fixing the target value of entrainer residence amount, the entrainer residence amount at which the concentration of the aliphatic carboxylic acid in the distillate from the azeotropic distillation column is at least 1,000 weight ppm of the weight of the distillate is taken as the lower limit and the entrainer residence amount at which the concentration of the entrainer in the bottoms from the azeotropic distillation column is at least 100 weight ppm of the weight of the bottoms is taken as the upper limit. |
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in-pct-2002-01173-del-abstract.pdf
in-pct-2002-01173-del-claims.pdf
in-pct-2002-01173-del-complete specification granted.pdf
in-pct-2002-01173-del-correspondence-others.pdf
in-pct-2002-01173-del-correspondence-po.pdf
in-pct-2002-01173-del-description (complete).pdf
IN-PCT-2002-01173-DEL-Drawings.pdf
in-pct-2002-01173-del-form-1.pdf
in-pct-2002-01173-del-form-13.pdf
in-pct-2002-01173-del-form-19.pdf
in-pct-2002-01173-del-form-2.pdf
in-pct-2002-01173-del-form-3.pdf
in-pct-2002-01173-del-form-5.pdf
in-pct-2002-01173-del-pct-338.pdf
in-pct-2002-01173-del-pct-409.pdf
in-pct-2002-01173-del-petition-137.pdf
in-pct-2002-01173-del-petition-138.pdf
Patent Number | 233321 | |||||||||
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Indian Patent Application Number | IN/PCT/2002/01173/DEL | |||||||||
PG Journal Number | 13/2009 | |||||||||
Publication Date | 27-Mar-2009 | |||||||||
Grant Date | 28-Mar-2009 | |||||||||
Date of Filing | 02-Dec-2002 | |||||||||
Name of Patentee | MITSUBISHI CHEMICAL CORPORATION | |||||||||
Applicant Address | 33-8, SHIBA 5-CHOME, MINATO-KU, TOKYO 108-0014, JAPAN. | |||||||||
Inventors:
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PCT International Classification Number | B01D 3/36 | |||||||||
PCT International Application Number | PCT/JP01/05280 | |||||||||
PCT International Filing date | 2001-06-20 | |||||||||
PCT Conventions:
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