JPS592522B2 - Water separation method using membranes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the membrane separation of water from aqueous mixtures of organic and/or inorganic substances having a higher boiling point than water.

In another aspect, the invention relates to a method of separating water by evaporating a liquid whose volumetric heat of vaporization is less than one-fourth that of water instead of evaporating water.

In yet another aspect, the present invention relates to a method of concentrating an aqueous mixture by removing at least some water from the aqueous mixture.

The separation of water from aqueous mixtures such as dispersions, emulsions, etc. of organic and/or inorganic substances having a boiling point higher than that of water is often carried out by distillation.

However, since water has an abnormally high heat of vaporization, its distillation requires a large amount of energy.

For example, in the process of producing ethylene glycol from ethylene oxide, 20 moles or more of water is used per mole of ethylene oxide, and a huge amount of water is separated from the resulting aqueous ethylene glycol solution by distillation.

It is hoped that energy-saving processes will be developed to separate water from aqueous mixtures of substances with boiling points higher than water without distilling the water.

An object of the present invention is to provide an energy-saving method for separating water from an aqueous mixture.

According to the present invention, water is efficiently separated from an aqueous mixture by using a polymeric membrane that is selectively permeable to water with a water-miscible sweeping liquid.

In this case, by using a sweep liquid with a boiling point lower than water, it is possible to convert the system (water + substance with a boiling point higher than water) into the system (water + substance with a boiling point lower than water). water can be easily separated from the latter by distilling the low boilers or sweep liquid.

The present invention consists of the following steps. (a) contacting an aqueous mixture of substances having a higher boiling point than water with a first surface of a polymeric membrane selectively permeable to water; (b) allowing a portion of * to permeate through the membrane; and (c) The second surface of the membrane has a boiling point lower than that of water, and has a volumetric heat of vaporization that is one-quarter or less than that of water;
and in contact with a water-miscible liquid (sweep liquid), (
d) extracting the water that has passed through the membrane into a sweep liquid from a second surface of the membrane; (e) withdrawing a water-enriched sweep liquid from the second surface side of the membrane; and (f) extracting a water-rich sweep liquid from the second surface of the membrane. (g) separating the water and the sweep liquid by distilling the sweep liquid; and (g) recycling the separated sweep liquid to the second surface side of the membrane.

If the heat of vaporization per unit volume of water and sweep liquid is Qw and Qsa, respectively, then in order to operate the method of the present invention in an energy-saving manner, the final concentration of water permeating into the sweep liquid on the second surface side of the membrane must be (volume fraction) Q s/
It is necessary to make it more than (Qs + Qw).

If the concentration of water in the sweep liquid is below this value, a large amount of heat is required for the evaporation of the sweep liquid itself, and the
It is advantageous to evaporate water directly from the aqueous mixture on the surface side.

In other words, the method of the invention is economically advantageous if the concentration of water in the aqueous mixture fed to the first surface side of the membrane is relatively high.

In order to effectively operate the method of the present invention, it is extremely important to select a polymer membrane that is selectively permeable to water.

As such a membrane, for example, polymers such as polyvinyl alcohol, ethylene-vinyl alcohol copolymer, cellulose, polyhydroxyethyl methacrylate, nylon 6, nylon 66, or copolymers thereof are used.

All of these polymer membranes have excellent water permeability, but in order to further increase their selectivity, they require appropriate heat treatment and
In some cases, it may be necessary to perform treatments such as crosslinking or grafting.

These polymer membranes can be used as flat membranes or as hollow systems.

When used as a flat membrane, it may be used as it is, or it may be supported on another porous support.

In the method of the invention, the content of the aqueous mixture fed to the first surface side of the membrane is a substance having a higher boiling point than water.

Such solutes include, for example, ethylene glycol,
1.2-7'Ropile7 ReIJcol, ■, 3-propylene glycol, ■, 4-butynediol, 1,4-butanediol, ■.

3-butanediol, 3-methyl-1,3,-butanediol, glycerin, 3-methylpentane-1,3,5
- various alcohols such as toljol and phenol; various carboxylic acids such as acetic acid, propionic acid, citric acid, tartaric acid, malic acid, fumaric acid, itaconic acid, and maleic acid;
In addition, there are various sugars, various amino acids, etc.

Furthermore, the solute in the aqueous mixture on the first surface side may be a colloidal substance such as an emulsion of various polymers, colloidal silica, or colloidal alumina.

Examples of inorganic compounds include phosphoric acid, polyphosphoric acid, sulfuric acid, and salts thereof.

As mentioned above, the higher the concentration of water in the aqueous mixture on the first surface side, that is, the lower the concentration of solute, the more economically advantageous the method of the present invention becomes.

Kinetically, the water permeation rate per unit membrane area is proportional to the difference μm - μ2 between the chemical potential of water on the first surface side (μm) and the chemical potential of water on the second surface side (μ2). , μm, that is, the higher the concentration of water on the first surface side, the higher the water permeation rate becomes.

In addition, from an equilibrium perspective, the final concentration of water on the second surface side at equilibrium (μm-μ2) can increase as the concentration of water in the stock solution supplied to the first surface side increases.

The higher the final concentration of water on the second surface side, the smaller the amount of circulation of the sweep liquid used, which means that the amount of evaporation of the sweep liquid per unit amount of water removed becomes smaller.

Therefore, from both kinetic and equilibrium standpoints, it is desirable that the concentration of solute in the stock solution supplied to the first surface be as low as possible.

The desired solute concentration in the stock solution varies depending on the type of sweep liquid used, but is generally 70 volume percent or less, preferably 50 volume percent or less, and more preferably 30 volume percent or less.

For example, ethylene oxide pope ethylene glycol f
In the fa process, the concentration of water in the liquid coming out of the ethylene oxide hydrolysis tower is about 85% by volume, and the remaining 15% by volume is products such as ethylene glycol.

The method of the present invention is most suitable for purposes such as removing water from such a system and concentrating the solute such as ethylene glycol to a concentration of 60 to 70 percent by volume.

As the sweep liquid to be supplied to the second surface side of the membrane, a liquid is used that has a boiling point lower than that of water, has a volumetric heat of vaporization that is one-fourth or less of that of water, and is miscible with water.

The boiling point of the sweep liquid must be lower than the boiling point of water, but if it is too close to the boiling point of water, separation by distillation will be difficult.

Furthermore, if the boiling point of the sweep liquid is too low, a large amount of cooling energy is required to condense the gas in the sweep liquid during distillation, which is undesirable.

Therefore, the boiling point of the sweep liquid is preferably 100 DEG C. or less under atmospheric pressure, preferably 90 DEG to 10 DEG C., and more preferably 75 DEG to 25 DEG C.

In the present invention, the volumetric heat of vaporization is defined as the heat of vaporization of a unit volume of a liquid at the boiling point of the liquid under atmospheric pressure.

The volumetric heat of vaporization of water is 539 K cal /l.

It is necessary that the volumetric heat of vaporization of the sweep liquid is less than a quarter of that, ie less than 135 K cal/l.

If the volumetric heat of vaporization of the sweep liquid is more than a quarter of that of water, it will take a lot of energy to evaporate the sweep liquid when distilling the water-rich sweep liquid taken from the second surface side of the membrane. However, the purpose of energy conservation cannot be achieved.

Sweep liquid volumetric heat of evaporation 6i 135 K cal/
As long as it is less than l, the smaller the value, the more desirable.

The sweep liquid must be sufficiently miscible with water.

This (!:) does not necessarily mean that the solubility of water in the sweep liquid is infinite, but
It is desirable that 100 parts by volume or more of water can be dissolved per 00 parts by volume.

Furthermore, it is desirable that the sweep liquid not be azeotropic with water.

In addition, chemical stability and the ability to withstand long-term circulation are also desirable conditions for a sweep liquid.

Sweep liquids that satisfy these conditions include, for example, t-butylamine (45,2; 72) and acetone (5
6,3;94), diethylamine (55,9;64),
Tetrahydrofuran (66; 84), dioquinrane (7
8;87) etc.

Here, the first term in parentheses is the boiling point at atmospheric pressure (℃),
The second term is the volumetric heat of vaporization (Kcal/
#) is.

The sweep liquid does not necessarily need to be a single compound, but may be a mixture of multiple compounds that meet the above conditions.

As an application example of the method of the present invention, FIG. 1 shows a process diagram in which a membrane extractor is incorporated into the ethylene oxide method ethylene glycol manufacturing process.

The liquid coming out of the ethylene oxide hydrolysis tower 1 consists of products such as ethylene glycol and a large amount of water.

This liquid is fed to the first surface side 2 of the membrane extractor.

Acetone is introduced as a sweep liquid into the second surface side 3 of the membrane extractor, and water is membrane extracted into the acetone by the membrane 4 from the first surface side.

In this case, trace amounts of ethylene glycol are also extracted into acetone through the polymer membrane.

The water-rich sweep liquid taken out from the second surface side is sent to the distillation column 5, where acetone is obtained from the top of the column and water and a trace amount of ethylene glycol are obtained from the bottom of the column.

Acetone from the top of the tower is returned to the membrane extractor 3, and water + a trace amount of ethylene glycol from the bottom of the tower is returned to the water-splitting group.

The liquid coming out from the first surface side of the membrane extractor consists of ethylene glycol + a small amount of water + a trace amount of acetone, which is sent to the next distillation step where it is separated into acetone, water and ethylene glycol, and the acetone is passed through the membrane extractor. It is recycled to the second surface side of the extractor.

By this method, for example, a 15% ethylene glycol aqueous solution can be easily dehydrated to a 60% ethylene glycol aqueous solution.

The method of the invention is further illustrated by the following examples, which are illustrative only and are not intended to limit the invention.

Example 1 50 cc of a 20% by weight ethylene glycol aqueous solution was placed on the first surface side of a dialysis cell sandwiching a polyvinyl alcohol membrane with a thickness of 20 microns and an effective area of 3.14 i, and 50 cc of acetone was placed on the second surface side. Both solutions were allowed to stand at room temperature with thorough stirring, and after 1 hour, the water melt in the acetone on the second surface side was 349/13, and the concentration of ethylene glycol was 0.19/11.

Further, the concentration of acetone on the first surface side was 0.149711.

Example 2 When the membrane extraction experiment of Example 1 was continued for 24 hours, the second
The concentration of water on the surface side is 286. ? /#, the concentration of ethylene glycol is 3. lf: l/11.

Further, the concentration of water on the first surface side was 6509/11, and the concentration of acetone was 4.6ji/11.

By distilling the liquid on the second surface side, 49 cc of acetone and 20 cc of (water + trace amount of ethylene glycol)
was gotten.

That is, in this case, about 20 cc of water could be separated by distilling 49 cc of acetone.

Considering that the volumetric heats of vaporization of water and acetone are 539 and 94 K calA, respectively, this means that water could be separated using 43% of the energy required to distill water in this case.

Example 3 Thickness 20 microns, effective area 3.14 CI? When 50 cc of an acetic acid aqueous solution of 50% by weight was placed on the first surface side of a dialysis cell sandwiching a L polyvinyl alcohol membrane, and 50 cc of acetone was placed on the second surface side, and the mixture was left at room temperature with stirring, the reaction time was 1 hour. The concentration of water in the second surface side acetone after<t i s g7tI.

Acetic acid concentration is 0.62g/l! , and the acetone concentration on the first surface side was 0.159/l.

Example 4 When the experiment of Example 2 was continued for 48 hours, the concentration of water on the second surface side was 210 g/11, and the concentration of acetic acid was 6.8F!
/iI, and the concentration of water on the first surface side is 479f! /11
．． Acetone concentration was 20 g/11.

By distilling the liquid on the second surface side, 49 cc of acetone and 13 cc (water + trace amount of acetic acid) were obtained.

In this case, water could be separated using 66% of the energy required to distill water.

Example 5 50 cc of an aqueous dispersion of 10% by weight silica colloid was placed on the first surface side of a dialysis cell sandwiching a cellophane membrane with a thickness of 20 microns and an effective area of 3.14 cIIf, and 50 cc of an aqueous dispersion of silica colloid was placed on the second surface side. When cc of tetrahydrofuran was added and left at room temperature for 24 hours with stirring, the concentration of water on the second surface side was 286 g/11.

The concentration of water on the first surface side is 8809/13°, and the concentration of tetrahydrofuran is 4. It was Ofl/11.

By distilling the liquid on the second surface side, 49 cc of tetrahydrofuran and 20.5 cc of tetrahydrofuran were obtained.

The volumetric heat of vaporization of tetrahydrofuran is 85 Kcal/
Considering that A', in this case water can be separated using only 38% of the energy required to distill water.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a process for implementing the present invention. 1...Hydrolysis tower, 2...First surface side of the membrane extractor, 3... Second surface side of the membrane extractor, 4...
... Membrane, 5... Distillation column.

Claims (1)

[Claims] 1. An aqueous mixture of substances having a boiling point higher than water is brought into contact with the first surface side of a polymer membrane that selectively permeates water, and an aqueous mixture of substances having a boiling point lower than water and having a volumetric heat of vaporization is brought into contact with the second surface side of the membrane. contacting a sweep liquid that is less than one-fourth that of water and miscible with water; extracting a water-enriched sweep liquid from the second surface side; and distilling the water-enriched sweep liquid. 1. A method for separating water from an aqueous mixture of organic and/or inorganic substances, characterized in that the water and the sweep liquid are separated by this process, and the separated sweep liquid is recycled to the second surface side of the membrane.