JP2685928B2 - Recovery method of dilute ethanol - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for recovering dilute ethanol, and more specifically,
It relates to a method for economically recovering dilute ethanol.

[Problems to be solved by conventional technology and invention]

Generally, in order to recover dilute ethanol of 5% by weight (hereinafter abbreviated as%) or less by distillation operation, the reflux ratio rapidly increases in inverse proportion to the alcohol concentration, and the recovery cost exceeds the value of recovered ethanol. Economy is not established. Therefore, although there are many studies, documents and patents related to the method for recovering dilute ethanol, there is still no industrially satisfactory method.

Among related researches, the membrane separation process has been a promising method from long ago, but the permeation pressure of the ethanol / water mixture is high, and the permeation flux of the membrane itself decreases due to consolidation and hydrolysis. It has not been put to practical use because of a decrease in separation rate and a difficulty in durability.

However, some of the recently developed new reverse osmosis membranes cover the drawbacks so far, suggesting the possibility of a separation process incorporating membrane separation.

Generally, when dilute ethanol is concentrated, the membrane separation is used only for the first-stage concentration because of the high osmotic pressure of the ethanol / water mixture. Osmotic pressure of ethanol / water is 10%
It is about 100 atm in an aqueous solution, and it is considered uneconomical to increase the concentration any more with conventional 1-pass reverse osmosis. or,
If the ethanol concentration on one side increases, the rejection also decreases rapidly, so in this sense, there is an optimum value in terms of economic efficiency. It is necessary to have a larger pressure difference across the membrane than the osmotic pressure difference of the immersion liquid separated by the membrane. The maximum pressure that the membrane can withstand is governed by the membrane material. Some commercially available osmosis membranes can withstand up to 100 atm, but at higher pressures, industrial use is impossible.

[Means for solving the problem]

As a result of intensive research to solve the above problems, the present inventors have found that evaporation-reverse osmosis-distillation-rectification (or Pervaporizatio
The inventors have found that dilute ethanol can be economically recovered by a method that is composed of the unit operations of n) and is a combination of all known techniques, and has completed the present invention.

That is, the present invention provides an evaporative concentration step of evaporating and concentrating a dilute ethanol aqueous solution of 1% by weight or less by utilizing a salt effect, and a membrane module including a reverse osmosis membrane having a removal rate of 20 to 70% for ethanol. The permeate from the first-stage membrane module is arranged in parallel on the permeate side and the concentrate side in multiple stages, and the permeate from the first-stage membrane module is sequentially supplied to the next-stage membrane module on the permeate side for concentration. The liquid is returned to the membrane module of the previous stage, and the concentrated liquid from the membrane module of the first stage is sequentially supplied to the membrane module of the next stage on the concentrated liquid side, and the permeated liquid is returned to the membrane module of the previous stage, so that the final stage A reverse osmosis step of obtaining a permeate and a concentrated solution from the membrane module and concentrating the ethanol aqueous solution to a concentration equal to or higher than the economical level of distillation concentration, and a distillation step of concentrating the ethanol aqueous solution to about 95% by weight by a distillation operation. Perform esterification by reactive distillation with acetic acid or acetic anhydride in concentrated ethanol, finally there is provided a method of recovering dilute ethanol, characterized in that comprising the esterification step of recovering as ethyl acetate.

In addition, the present invention provides an evaporative concentration step of evaporating and concentrating a dilute ethanol aqueous solution of 1% by weight or less by utilizing a salt effect, and a membrane module provided with a reverse osmosis membrane having a removal rate of ethanol of 20 to 70%. The permeate from the first-stage membrane module is arranged in parallel on the permeate side and the concentrate side in multiple stages, and the permeate from the first-stage membrane module is sequentially supplied to the next-stage membrane module on the permeate side for concentration. The liquid is returned to the membrane module of the previous stage, and the concentrated liquid from the membrane module of the first stage is sequentially supplied to the membrane module of the next stage on the concentrated liquid side, and the permeated liquid is returned to the membrane module of the previous stage, so that the final stage A reverse osmosis step of obtaining a permeate and a concentrated solution from the membrane module and concentrating the ethanol aqueous solution to a concentration equal to or higher than the economical level of distillation concentration, and a distillation step of concentrating the ethanol aqueous solution to about 95% by weight by a distillation operation. There is provided also a method of recovering dilute ethanol, comprising the dehydration step of recovering the dehydrated ethanol with azeotropic distillation.

By optimizing the above-mentioned known combinations, the present inventors can economically take out ethanol, which has been conventionally disposed of by sludge treatment or the like as a valuable resource, because it is not economical. It was

Generally, the ethanol aqueous solution that is the object of recovery is 1%
The above is common sense, and there are almost no cases of concentration below the reverse osmosis method described in the background of related technology.
When the aqueous ethanol solution is concentrated with a membrane, water becomes the permeate, and therefore when the extremely thin liquid is concentrated, the amount on the process side is extremely reduced. For this reason, the device becomes a Christmas tree type, but when it is applied to a liquid that is too thin, unless the number of divisions is increased, the number of Reynolds on the process side extremely decreases, and the permeation flux only decreases. Without
Unnecessary pressure is applied in the low osmotic pressure region, which causes disadvantages such as consolidation of the membrane and reduction of the permeation flux. Therefore,
The concentration rate in one pass naturally has a limit, and it seems that it is considerably expensive in terms of equipment to directly concentrate an extremely thin solution of 1000 ppm to 2000 ppm by the reverse osmosis method.

From the above viewpoint, introducing an extremely thin aqueous ethanol solution into the reverse osmosis process as it is is not considered to be a good idea comprehensively, but the energy consumption increases slightly, but the ethanol-water vapor-liquid equilibrium is Using the property of rapidly rising at a low concentration, we considered introducing a one-step evaporative concentration before reverse osmosis.

Since there are almost no measurement examples of gas-liquid equilibrium data in the dilute concentration range of ethanol to water, and general literature values cannot be used, actual measurement was performed, and the results shown in FIG. 2 were shown. That is, it was found that the vapor-liquid equilibrium in the low-concentration region exhibits a gas phase composition about 2-3 times higher than the vapor-liquid equilibrium measured in the high-concentration region (1% or more). In addition, in the case of this method, the higher the gas phase concentration is, the more the vapor effect is considered, and the vapor-liquid equilibrium in the dilute region in the presence of salt (MgSO ₄ ) was measured considering the use of the salt effect. It showed a good result. That is, it was found that the gas phase composition was about 20 to 70% higher than that of the system without salt. As a result, even in a dilute ethanol concentration range of about 2000 ppm, 500 p
A distillate of about 10,000 ppm can be obtained for a liquid phase concentration of about pm. As a result, the recovery rate of ethanol is 50%.
From the above, it was found that the amount of the treatment liquid was about 1/10 and the concentration was about 5 times, and that the operation by the reverse osmosis method could be greatly advantageously performed. In addition, the dilute ethanol solution that comes out from the actual industrial process usually contains other trace components, salts, etc., and when introduced into the reverse osmosis process as it is,
Since the osmotic pressure of impurities and salts increases and the operating pressure increases, the concentration limit of ethanol also decreases, which is not preferable. Therefore, evaporation energy is applied, and in this step, an uneconomical evaporation operation is introduced, and the distillate obtained by evaporation is brought into the reverse osmosis step to avoid the above-mentioned problems, and the equipment cost in the reverse osmosis process. It has been found that the cost can be saved, the running cost can be reduced, and the overall merit can be brought out. In the evaporative concentration step utilizing such a salt effect, the concentration of the salt is not particularly limited, but the effect becomes higher as it approaches saturation.

If the distillate concentrated by the evaporation operation is 5000 ppm or more, it is technically possible to concentrate up to about 10% by the reverse osmosis method using a membrane that has already been developed. It will be advantageous. Therefore, as the process, a process of concentrating up to about 10% by a combination of evaporation operation and reverse osmosis is adopted.

In the reverse osmosis step of the present invention, a membrane module equipped with a reverse osmosis membrane having a removal rate for ethanol of 20 to 70% is arranged in multiple stages from the first stage membrane module in parallel with the permeate side and the concentrate side. Then, the permeate from the first-stage membrane module is sequentially supplied to the next-stage membrane module on the permeate side, the concentrated solution is returned to the preceding membrane module, and the concentrated solution from the first-stage membrane module is removed. A multi-stage reverse osmosis process is adopted in which the permeated liquid and the concentrated liquid are sequentially supplied to the next-stage membrane module on the side of the concentrated liquid and the permeated liquid is returned to the preceding membrane module to obtain the permeated liquid and the concentrated liquid, respectively. A preferred embodiment of such a multi-stage reverse osmosis process will be described with reference to FIG.

The multi-stage reverse osmosis process shown in FIG.
The permeate side is connected in two stages in series by 15 and the membrane module 16 in the next stage, and the concentrate side is also connected in six stages by the membrane modules 17 to 21 including the membrane module 15 in the first stage. And the permeated liquid side and the concentrated liquid side are connected in a total of 7 stages. The number of stages of the permeate-side and concentrate-side membrane modules in the multi-stage reverse osmosis process of the present invention is not particularly limited and can be arbitrarily selected.

In the multi-stage reverse osmosis process shown in FIG. 3, first, the dilute ethanol aqueous solution obtained from the evaporative concentration process is supplied to the first-stage membrane module 15 by the pump 22, and the permeate thereof is pumped to the permeate side. The concentrate is supplied to the membrane module 16 of the second stage, while the concentrate is supplied to the membrane module 17 of the second stage on the concentrate side by the pump 24. The permeate from the second-stage membrane module 16 on the permeate side is finally recovered, and the concentrated liquid is returned to the preceding membrane module 15.

On the other hand, the concentrated liquid from the first-stage membrane module 15 is supplied to the concentrated liquid-side second-stage membrane module 17 by the pump 24, and the permeated liquid from the second-stage membrane module 17 is That is, it is returned to the membrane module 15 of the first stage. The concentrated liquid from the second-stage membrane module 17 on the concentrated liquid side is pump 25
Is supplied to the next stage, that is, the third stage membrane module 18,
The permeate from the third-stage membrane module 18 is
It is returned to the second stage membrane module 17. In this way,
The concentrated liquid is passed from the first-stage membrane module 15 through the concentrated liquid-side membrane modules 17 to 21 in multiple stages in series, and is obtained from the final-stage membrane module 21 at a required high concentration.

As described above, in the multistage reverse osmosis step of the present invention,
The membrane module is connected in multiple stages from the first-stage membrane module to the permeate side and the concentrate side, and on the permeate side, the permeate of each stage is supplied to the next-stage membrane module, and Is returned to the previous membrane module.On the other hand, on the concentrate side, the concentrated solution of each stage is supplied to the next membrane module, and the permeate is returned to the previous membrane module, and the permeate is returned from the last membrane module. A liquid and a concentrate are obtained.

Since the reverse osmosis membrane used in such a reverse osmosis membrane process has a relatively low removal rate for ethanol of 20 to 70%, the ethanol aqueous solution and the permeated liquid supplied to the membrane module at any stage are The difference in concentration between them is not so large, and therefore the difference in osmotic pressure between the raw material ethanol aqueous solution and the permeate is not so large. As a result, such a multi-stage reverse osmosis process does not require so large operating pressure in any stage,
Concentration treatment of aqueous solution by reverse osmosis treatment can be performed,
As described above, in each stage, a concentrated ethanol aqueous solution having a high concentration can be finally obtained while sequentially performing the concentration process in multiple stages at a relatively low operating pressure.

Examples of the reverse osmosis membrane used in the present invention include a cellulose acetate membrane, a crosslinked composite membrane, and the like.
40486, JP-A-54-107882, JP-A-56-40403, JP-A-55-147106, and polyamides, polyimides, polyethers, and polyvinyl alcohols found in JP-A-58-64102 and the like. System-based membranes can be used. These membranes can be used as membrane modules of various shapes such as a flat membrane, a tube, a hollow fiber, and a spiral.

The ethanol solution concentrated to about 10% in the reverse osmosis process can be easily concentrated to about 95% by a normal distillation operation. It is generally considered that an ethanol solution concentrated to 95% is dehydrated by azeotropic distillation by adding a third component such as benzene to recover ethanol at a concentration of 99% or more. Since the boiling point of ethanol is 78 ° C. as compared with the azeotropic temperature of 70 ° C., and the difference in boiling points between the limiting components in this separation is only 8 ° C., a considerable number of separation stages is actually required.

The present inventor has provided a considerable number of separation stages as described above,
Although ethanol can be recovered, it is assumed that it may not always be necessary to recover it as ethanol depending on the situation.When recovering valuables, ethanol is esterified in the system and converted to ethyl acetate or the like. Focusing on the fact that the ease of separation changes significantly by changing it, we found that ethanol can be recovered at low cost by combining the method of making the final concentration step an esterification column, and in addition to the above The present invention completes the present invention by configuring a process.

The basic structure of the dilute ethanol recovery based on the method of the present invention is shown in FIG.

 Hereinafter, this step will be described with reference to FIG.

Dilute aqueous ethanol solution 1 of 1% or less is charged into an evaporator 2, and an appropriate amount according to the concentration of the charge is evaporated and distilled off.
It becomes stream 3. At this time, as the heating source necessary for evaporation, in addition to the usual use of low-pressure steam or the like, it is also possible to effectively use low-temperature low-grade energy from the viewpoint of energy saving. In that case, the evaporator 2 Is a slight depressurization operation. Regarding the evaporation rate, for example, when treating an ethanol solution of 2000 ppm, by adding magnesium sulfate or the like to distill about 15%,
It is possible to obtain a distillate of 1000 ppm.

Due to the salt effect, the concentrated aqueous ethanol solution is introduced into the multi-stage reverse osmosis step 4 on the basis of the gas-liquid equilibrium relationship which is greatly improved over the normal ethanol-water gas-liquid equilibrium.

In the multi-stage reverse osmosis process 4, keep the ethanol recovery rate high within the range where the rejection can be kept as high as possible.
Concentrate to a concentration of over%. Since the operating conditions at this time vary greatly depending on the membrane used, selection of the membrane is important.

Ethanol solution 5 concentrated to 5% or more in reverse osmosis step 4
Is concentrated to about 95% in the ethanol concentration tower 6. Since the ethanol-water vapor-liquid equilibrium has a tangent azeotrope, it cannot be concentrated to 95% or more. Since the required reflux ratio of the concentrating column 6 depends on the ethanol concentration of the stream 5, the higher the ethanol concentration of the stream 5, the better.

The ethanol concentrated in the ethanol concentration tower 6 is 60 to 95
Although the concentration will be about%, it is also possible to dehydrate it by azeotropic distillation with a third component such as benzene and ethyl acetate to recover ethanol, but as shown in FIG. It is also possible to recover as ethyl. In this case, sulfuric acid and acetic acid may be charged from the stream 8 into the preliminary reaction tank 9 and preliminarily reacted with the stream 7 and then charged into the esterification tower 11, or may be directly charged into the esterification tower 11 as it is.

The partially esterified liquid 10 is charged into the esterification tower 11 by the preliminary reaction tank 9, is esterified, and is distilled off azeotropically with ethyl acetate / water from the top of the tower. The distillate is a decanter 12
And the phase is separated and recovered as the upper layer liquid 13.

〔Example〕

Hereinafter, the present invention will be described in more detail by examples,
The present invention is not limited to these examples.

Example 1 In the manufacturing process of the present invention, the material balance around the evaporator 2, the multi-stage reverse osmosis step 4, the ethanol concentration tower 6, the preliminary reaction tank 9, the esterification tower 11 and the decanter 12 as shown in FIG. 4 was measured. did.

That is, 0.12% ethanol solution in magnesium sulfate
0.80% and acetic acid 0.03% were added, and after the mixture was evaporated and concentrated by the evaporator 2, the multi-stage reverse osmosis step 4, and the ethanol concentrated liquid concentrated by the ethanol concentration tower 6 were added to the preliminary reaction tank 9,
The concentrations and flow rates of ethanol, ethyl acetate, acetic acid, water, magnesium sulfate, and sulfuric acid at the points (a) to (q) shown in FIG. 4 when esterified by the esterification tower 11 and recovered as ethyl acetate were measured. .

The membrane used in the multi-stage reverse osmosis step is a polyamide-based spiral membrane (GRA-98; manufactured by Filmtec).

 Table 1 shows the results.

Example 2 The mass balance around each membrane module of the multi-stage reverse osmosis step 4 used in Example 1 was measured.

That is, the concentrations and flow rates of ethanol, ethyl acetate, acetic acid, water, and magnesium sulfate at each point (a) to (t) shown in FIG. 5 were measured.

 Table 2 shows the results.

[Brief description of the drawings]

FIG. 1 is a process diagram of dilute ethanol recovery based on the method of the present invention, and FIG. 2 is ethanol-water system and ethanol-water-MgSO 4.
₄ is a graph showing vapor-liquid equilibrium in a dilute concentration region of ₄ systems, FIG. 3 is a schematic diagram showing an example of a multi-stage reverse osmosis process used in the method of the present invention, and FIGS. 4 and 5 are graphs of Examples 1 and 2, respectively. It is a figure which shows the measurement point of material balance. 1: Dilute ethanol, 2: Evaporator 4: Multi-stage reverse osmosis process, 6: Ethanol concentration tower 9: Preliminary reaction tank, 11: Esterification tower 12: Decanter 15-21: Membrane module 22-28: Pump

Claims (2)

(57) [Claims] 1. A membrane module comprising an evaporative concentration step of evaporating and concentrating a dilute ethanol aqueous solution of 1% by weight or less by utilizing a salt effect, and a membrane module equipped with a reverse osmosis membrane having a removal rate of 20 to 70% for ethanol. The permeated liquid from the first-stage membrane module is sequentially supplied to the next-stage membrane module on the permeated liquid side by sequentially arranging the permeated liquid from the first-stage membrane module in parallel with the permeated liquid side and the concentrated liquid side in parallel. Back to the previous membrane module,
The concentrate from the first-stage membrane module is sequentially supplied to the next-stage membrane module on the concentrate side, and the permeate is returned to the previous-stage membrane module, so that the permeate and the concentrate are obtained from the final-stage membrane module, respectively. A reverse osmosis step of concentrating the aqueous ethanol solution to a concentration equal to or higher than the economical level of distillation and concentration.
It consists of a distillation step of concentrating the ethanol aqueous solution to about 95% by weight by a distillation operation, and an esterification step of adding acetic acid or acetic anhydride to concentrated ethanol to effect esterification and finally recovering as ethyl acetate. A method for recovering dilute ethanol characterized by. 2. A first membrane module comprising an evaporative concentration step of evaporating and concentrating a dilute ethanol aqueous solution of 1% by weight or less by utilizing a salt effect, and a reverse osmosis membrane having a removal rate for ethanol of 20 to 70%. The permeated liquid from the first-stage membrane module is sequentially supplied to the next-stage membrane module on the permeated liquid side by sequentially arranging the permeated liquid from the first-stage membrane module in parallel with the permeated liquid side and the concentrated liquid side in parallel. Back to the previous membrane module,
The concentrate from the first-stage membrane module is sequentially supplied to the next-stage membrane module on the concentrate side, and the permeate is returned to the previous-stage membrane module, so that the permeate and the concentrate are obtained from the final-stage membrane module, respectively. A reverse osmosis step of concentrating the aqueous ethanol solution to a concentration equal to or higher than the economical level of distillation and concentration.
A method for recovering dilute ethanol, comprising a distillation step of concentrating an aqueous ethanol solution to about 95% by weight by a distillation operation, and a dehydration step of dehydrating by azeotropic distillation to recover ethanol.