JPH0459006B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for separating volatile mixtures, specifically, a method for separating volatile mixtures consisting of two or more types of volatile components using a combination of a distillation column and a gas separation membrane. It relates to a method for separating the components into their constituent components by a process of

[Conventional technology]

Conventionally, a distillation method has generally been adopted as a method for separating volatile mixtures.

In addition, in recent years, a pervaporation method has been proposed as an energy-saving method for dehydrating organic aqueous solutions, and a dehydration method that combines this method with the above-mentioned distillation method has also been proposed (Japanese Patent Laid-Open No. 54-33279 Publication No. 167702, Japanese Patent Publication No. 167702, Japanese Patent Publication No. 16770-
(See Publication No. 48427). This pervaporation method uses a separation membrane, supplies an organic aqueous solution as a liquid to one side of the membrane, and maintains the other side at reduced pressure or supplies an inert gas to remove water vapor. This is a method of selectively transmitting light.

[Problem that the invention seeks to solve]

In the conventional distillation method, in order to increase the concentration of the distillate taken out from the distillation column, it is necessary to install many stages of the distillation column, which increases the size of the equipment and requires a large amount of energy. It has problems such as being necessary.

In addition, the above-mentioned pervaporation method and a dehydration method that combines this method and a distillation method can reduce energy costs compared to the distillation method, but
Since the separation membrane comes into direct contact with the organic aqueous solution, the separation membrane swells, resulting in problems such as a decrease in permselectivity and a loss of long-term durability.

Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a method for separating volatile mixtures into their constituent components, which can process volatile mixtures in large quantities on an industrial scale, and can easily control quality and at low cost. The object of the present invention is to provide a method for separating volatile mixtures.

[Means for solving problems]

As a result of various studies, the present inventors extracted a mixed vapor from the stages of a distillation column, separated the extracted mixed vapor into low-boiling point substances and high-boiling point substances using a gas separation membrane, and transferred the low-boiling point substances to the above-mentioned distillation column. It has been found that the above object can be achieved by returning the high-boiling substances to the concentration stage of the distillation column and returning the high-boiling substances to the recovery stage of the distillation column.

The present invention was made based on the above findings, and
A method for separating a volatile mixture consisting of at least two types of volatile components using a distillation column and a gas separation membrane, the method comprising: supplying a raw material volatile mixture to the distillation column; A part or the entire amount of a mixed vapor consisting of at least two types of volatile components is extracted from the mixture, and the extracted mixed vapor is supplied to one side of a gas separation membrane having selective permeability to the constituent components, and By maintaining the other side of the gas separation membrane at reduced pressure, the membrane-permeable fraction and the non-membrane permeable fraction are separated, and the membrane-permeable fraction and/or the non-membrane permeable fraction are separated into their compositions. The present invention provides a method for separating a volatile mixture, characterized in that the volatile mixture is returned to the concentration stage or recovery stage of the distillation column, depending on the situation.

The volatile mixture to which the separation method of the present invention can be applied is not particularly limited as long as it can be separated in a distillation column; for example, methanol, ethanol, n
-propanol, isopropanol, butanol, pentanol, acetone, acetonitrile,
Aqueous solutions of organic substances such as acrylonitrile, methyl ethyl ketone, tetrahydrofuran, dioxane, ethyl formate, ethyl acetate, butyl acetate, benzene, toluene, xylene, acetic acid, and carbonic acid, mixtures of acetone and n-hexane, mixtures of ethanol and acetone, and styrene. and ethylbenzene, and a mixture of benzene and aniline.

Hereinafter, the method for separating volatile mixtures of the present invention will be described in detail with reference to the flow sheet of FIG. 1, which outlines a preferred embodiment thereof.

To carry out the method for separating a volatile mixture of the present invention, first, the volatile mixture is supplied from line A to the intermediate stage (intermediate section) 2 of the distillation column 1.

The volatile mixture supplied to the intermediate stage 2 of the distillation column 1 is indirectly heated by a heat source such as steam supplied to the bottom of the distillation column 1, and a part of it is vaporized and rises inside the distillation column 1 as a mixed vapor. , the remainder flows down the distillation column 1 as a mixed liquid.

Next, part or all of the mixed vapor generated by vaporizing the volatile mixture from the intermediate stage 2 of the distillation column 1 is transferred from line B to the superheater 9, and the superheater is heated to prevent the mixed vapor from condensing. After raising the temperature at step 9, the gas is supplied to one side (primary side) 4a of the gas separation membrane 4, and at the same time, by maintaining the other side (secondary side) 4b of the gas separation membrane 4 at reduced pressure. , the high boiling point substances in the mixed vapor are selectively permeated through the secondary side 4b of the gas separation membrane 4, and the mixed vapor is converted into a membrane-permeable fraction rich in high boiling point substances (membrane-permeable volatile components). and a membrane non-permeable fraction rich in low boiling point substances (membrane non-permeable volatile components).

At this time, in addition to the reheating operation using the superheater 9,
By increasing the pressure and temperature of the mixed vapor supplied to the gas separation membrane 4 within a range in which the mixed vapor does not condense, the amount of membrane-permeable volatile components that permeate through the gas separation membrane 4 is increased, and the gas separation membrane 4 The degree of separation can be increased. Considering these points, the pressure of the mixed vapor supplied to the gas separation membrane 4 is
Preferably, the temperature is 760 to 5000 mmHg and the temperature is 70 to 150°C. In the conventional pervaporation method mentioned above, it is necessary to supply the organic matter aqueous solution to be separated to the separation membrane in liquid form, so it is generally not necessary to use high-temperature and
It is not possible to use high pressure to increase the amount of permeation through the separation membrane. An advantage of the present invention is that by operating the gas separation membrane 4 at high temperature and high pressure, the amount of membrane-permeable volatile components permeable can be increased.

Further, the higher the degree of pressure reduction on the secondary side 4b of the gas separation membrane 4, the greater the amount of membrane-permeable volatile components that permeate through the separation membrane, and it is preferable to set the degree of pressure reduction to such a degree that at least the membrane-permeable fraction does not condense. . In order to ensure the necessary degree of reduced pressure, the system pressure on the secondary side 4b of the gas separation membrane 4 is usually 200 mmHg or less, preferably 100 mmHg or less.
Keep it below mmHg.

Further, to maintain the reduced pressure on the secondary side 4b of the gas separation membrane 4, the membrane-permeated fraction that has passed through the gas separation membrane 4 is transferred from the line D to the cooler 5, and is indirectly cooled with a refrigerant in the cooler 5. This is done by a method of condensation.
This method allows the vacuum pump 7 to be removed only once at the start of operation.
If the pressure is reduced on the secondary side 4b of the gas separation membrane 4 by driving the There is no need to drive the vacuum pump 7 at the same time, which is effective because the power cost is reduced.

Next, the membrane non-permeable fraction rich in low-boiling substances is transferred from line C to the concentration stage (concentration section) of distillation column 1.
2', and the membrane-permeated fraction rich in high-boiling substances is transferred from line D to a cooler 5, condensed in the cooler 5, passed through a vacuum tank 6, and then sent from line E to a distillation column 1. It is returned to the collection stage (recovery section) 2''.

The membrane non-permeable fraction returned to the concentration stage 2' of the distillation column 1 is distilled in the distillation column 1, ascends within the distillation column 1, and is distilled out from the top of the distillation column 1. Distillation column 1
The vapor distilled from the top of the column is transferred from line F to condenser 3, where it is condensed. The condensate is
A portion is returned to the top of the distillation column 1 from line G, and the remainder is taken out from line H as a distillate containing a high concentration of low-boiling substances.

In addition, the membrane-permeable fraction returned to the recovery stage 2'' of the distillation column 1 is distilled in the distillation column 1 and
The liquid flows down through the distillation column 1 and is distilled out from the bottom of the distillation column 1. A portion of the liquid distilled from the bottom of the distillation column 1 is transferred from the line J to the reboiler 8, heated and evaporated in the reboiler 8, and then returned to the bottom of the distillation column 1. It is removed from line I as a concentrated distillate.

The return positions of the membrane-permeable fraction and non-membrane-permeable fraction are as follows:
determined depending on their composition. That is, those rich in low-boiling substances are returned to the concentration stage (concentration section), and those rich in high-boiling substances are returned to the recovery stage (recovery section). Therefore, contrary to the embodiment shown in the flow sheet of FIG. The fraction is rich in low boiling point substances and the membrane non-permeable fraction is rich in high boiling point substances, so the membrane permeable fraction is returned to the concentration stage 2', and the membrane non-permeable fraction is sent to the recovery stage. Return to 2″.

Furthermore, the membrane-permeable fraction and the membrane-unpermeable fraction do not necessarily need to be returned to the distillation column; for example,
When the volatile mixture is an aqueous solution of organic matter and the membrane-permeable fraction is water containing almost no organic matter, the membrane-permeable fraction may be discharged from the system without being returned to the distillation column.

Further, the position for extracting the mixed vapor from the distillation column may be any intermediate stage or concentration stage of the distillation column, or may be the uppermost stage of the concentration section.

Furthermore, the gas separation membrane used in the present invention may be any gas separation membrane that has selective permeability to some of the volatile components constituting the volatile mixture, such as an inorganic membrane such as a porous ceramic membrane. film,
Examples include organic films made of polyamide, cellulose, cellulose acetate, polyimide, etc., and are appropriately selected depending on the type of volatile mixture. For example, when the volatile mixture is an aqueous solution of organic substances such as alcohols, ketones, ethers, and esters, it has excellent water vapor selective permeation performance, is easy to modularize, and can have a large membrane area per unit volume.
Among organic membranes, aromatic polyimide gas separation membranes are preferred because of their excellent heat resistance and solvent resistance.

As the gas separation membrane, an aggregate of hollow fibers having a large effective membrane area is preferable, but a flat membrane may also be used.

The outer diameter of the hollow fiber used as a gas separation membrane is usually 50 to 2000μ, preferably 200 to 1000μ. If the outer diameter of the hollow fiber is too small, pressure loss will increase, and if it is too large, the effective membrane area will decrease.
In addition, as for the above hollow fiber, (thickness/outer diameter) = 0.1
It is preferable to use one that satisfies the condition of ~0.3.
Note that the above thickness = (outer diameter - inner diameter)/2. If the thickness of the hollow fiber is small, the pressure resistance will be insufficient, and if the thickness is large, the gas selective permeability will be poor.

The aromatic polyimide gas separation membrane that can be particularly advantageously used as the gas separation membrane in the present invention contains an aromatic tetracarboxylic acid skeleton and an aromatic diamine skeleton, and can be produced by a known method.

As the aromatic tetracarboxylic acid skeleton,
3,3',4,4'-benzophenonetetracarboxylic acid, 2,3,3',4'-benzophenonetetracarboxylic acid, pyromellitic acid, 3,3',4,4'-biphenyltetra carboxylic acid, and 2,3,3',4'-
Biphenyltetracarboxylic acid, and acid dianhydrides and esters of these aromatic tetracarboxylic acids.
Examples include carboxylic acid skeletons derived from salts and the like. Among these, acid dianhydride of 3,3',4,4'-biphenyltetracarboxylic acid and 2,3,
Uses an aromatic polyimide gas separation membrane whose main acid skeleton is an acid skeleton derived from biphenyltetracarboxylic dianhydride, such as 3',4'-biphenyltetracarboxylic acid dianhydride. The present invention is particularly useful in such cases.

Further, as the aromatic diamine skeleton, p-
phenylenediamine, m-phenylenediamine,
2,4-diaminotoluene, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, o-tolidine, 1,4-bis(4-aminophenoxy)benzene, o-tolidine sulfone, bis( Examples include aminophenoxy-phenyl)methane and bis(aminophenoxy-phenyl)sulfone.

As a method for producing a gas separation membrane made of aromatic polyimide, for example, an aromatic diamine component consisting of the above aromatic diamine (which may contain other aromatic diamines) and the above biphenyltetracarboxylic acid component may be abbreviated. Polymerize and imidize equimolar phenolic compounds in an organic solvent at a temperature of about 140°C or higher to produce an aromatic polyimide, and turn the aromatic polyimide solution (concentration: about 3 to 30% by weight) into a dope solution. A thin film (flat film or hollow fiber) of the dope solution is formed by coating or casting on a substrate at a temperature of about 30 to 150°C or extruding it into a hollow fiber film, and then the thin film is converted into a coagulating liquid. immersion to form a coagulated film,
An example of a membrane forming method is to wash and remove the solvent, coagulating liquid, etc. from the coagulated membrane, and finally perform heat treatment to form an asymmetric gas separation membrane made of aromatic polyimide.

〔Example〕

EXAMPLES Hereinafter, the present invention will be explained in more detail by giving examples of the present invention.

Example 1 This example is an example in which the method of the present invention was applied to the separation and concentration of an aqueous ethanol solution, and was carried out according to the embodiment shown in the flow sheet of FIG.

An aqueous ethanol solution with an ethanol concentration of 30% by weight was supplied from line A to the middle stage (9th tray from the top) of a distillation column 1 equipped with 14 trays at a rate of 80 kg/hour. The mixed vapor was extracted from the same stage as the raw material supply stage of the distillation column 1. The mixed vapor extracted from distillation column 1 has an ethanol concentration of 63% by weight and a temperature of
It was 85℃. After this mixed steam is heated to 90°C in the superheater 9, it is heated to the primary side 4a of the gas separation membrane 4 every hour.
Supplied in 80Kg. As the gas separation membrane 4, an aromatic polyimide hollow fiber membrane (an assembly of hollow fibers) with an outer diameter of 500 μm and an effective membrane area of 70 m ² was used. The pressure on the secondary side 4b of the gas separation membrane 4 was reduced to 100 mmHg.

A mixed vapor with an ethanol concentration of 90% by weight is obtained as a membrane non-permeable fraction on the primary side 4a of the gas separation membrane 4,
The mixed vapor was returned to the stage one above the raw material supply stage of the distillation column 1 (the eighth tray from the top).

Further, a mixed vapor with an ethanol concentration of 4% by weight is obtained as a membrane permeation fraction on the secondary side 4b of the gas separation membrane 4, and after being cooled and condensed in the cooler 5, it is returned to the recovery stage of the distillation column 1. did.

As a result, concentrated ethanol with an ethanol concentration of 92% by weight was obtained from line H as a distillate at a rate of 26 kg/hour. In addition, the amount of water discharged from Line I is 54% per hour.
kg, and the wastewater contained 0.2% by weight of ethanol.

Comparative Example 1 The same distillation column as that used in Example 1 (number of plates
An aqueous ethanol solution with an ethanol concentration of 30% by weight was distilled using a 14-stage plate. The above ethanol aqueous solution was supplied to the ninth tray from the top at a rate of 80 kg/hour in the same manner as in Example 1.

As a result, the ethanol concentration was 89.6 as a distillate.
% ethanol aqueous solution was obtained at 26.5 Kg per hour. In addition, the amount of water discharged is 53.5Kg per hour, and during this drainage,
Contained 0.5% ethanol by weight.

In addition, when distillation was carried out using a distillation column (20 plates) with an additional 6 trays added to the concentration plate side, the ethanol concentration and amount of ethanol were almost the same as that of the distillate obtained in Example 1. An aqueous solution was obtained.

〔Effect of the invention〕

According to the method for separating volatile mixtures of the present invention, volatile mixtures can be processed in large quantities on an industrial scale, and the volatile mixtures can be separated into constituent components with easy quality control and at low cost.

That is, according to the method for separating volatile mixtures of the present invention, the following effects are achieved.

(1) Due to the separation and concentration effect of the volatile mixture by the gas separation membrane, the number of stages in the distillation column can be reduced compared to the case where a distillation column is used alone, and the concentration of the distillate can be increased.

(2) Since the gas separation membrane separates high-boiling point substances and low-boiling point substances, the amount of low-boiling point concentrated vapor returned to the concentrating stage of the distillation column can be significantly reduced, and the diameter of the concentrating stage of the distillation column can be reduced. can do.

(3) Compared to the case where a distillation column is used alone, the amount of reflux can be made smaller, so the amount of distillate can be increased.

(4) Compared to the case where a distillation column is used alone, thermal energy consumption is lower and energy saving is possible.

(5) Since the gas separation membrane is separated in a gaseous state, the durability of the gas separation membrane is good, and by increasing the temperature and pressure of the mixed vapor supplied to the gas separation membrane, the amount of membrane-permeable volatile components that permeate through the gas separation membrane can be reduced. increase,
Separation performance can be improved (driving force can be easily increased).

[Brief explanation of drawings]

FIG. 1 is a flow sheet outlining a preferred embodiment of the method for separating volatile mixtures of the present invention. 1... Distillation column, 2, 2', 2''... Stage, 3... Condenser, 4... Gas separation membrane, 5... Cooler, 6...
Decompression tank, 7...vacuum pump, 8...reboiler, 9...superheater.

Claims (1)

[Claims] 1. A method for separating a volatile mixture consisting of at least two types of volatile components using a distillation column and a gas separation membrane, the method comprising: supplying a raw material volatile mixture to the distillation column; From the intermediate stage to the concentration stage of the column,
Part or all of a mixed vapor consisting of at least two types of volatile components is extracted, the extracted mixed vapor is supplied to one side of a gas separation membrane having selective permeability to the constituent components, and By maintaining the other side of the gas separation membrane at reduced pressure, it is separated into a membrane permeable fraction and a membrane non-permeable fraction, and the membrane permeable fraction and/or the membrane non-permeable fraction are separated according to their composition. A method for separating a volatile mixture, characterized in that the volatile mixture is returned to the concentration stage or recovery stage of the distillation column. 2. The method for separating a volatile mixture according to claim 1, wherein one component of the volatile mixture is water. 3. The method for separating a volatile mixture according to claim 2, wherein the gas separation membrane is an aromatic polyimide gas separation membrane. 4. The method for separating a volatile mixture according to claim 1, wherein the reduced pressure on the other side of the gas separation membrane is maintained by indirectly cooling and condensing the membrane-permeable fraction with a refrigerant.