JPH025441B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for dewatering a mixture of organic liquid and water by distillation and permeation. More specifically, it relates to a method in which the water content is first reduced by distillation, and the remaining water is removed by permeation through a pervaporation membrane. BACKGROUND OF THE INVENTION Many organic liquids can be mixed with water, often without restriction. Complete dehydration by distillation requires a large amount of energy. Another question then arises as to whether this organic liquid forms an azeotrope with water. Examples of azeotropes include ethanol/water, isopropanol/water, ethyl acetate/water, and pyridine/water.
There's water. In this case, an entrainer is generally used.
is used. A traditional example is the use of benzene as an azeotropic agent for the dehydration of ethanol. [Problem to be Solved by the Invention] However, some of the entrainer is inevitably lost along with the exhaust air and waste water, and even if cost is not considered, the loss of this entrainer has a negative impact on the environment. Therefore, other methods of dehydrating organic liquid and water mixtures are being explored. In principle, the use of membrane permeation for dehydration has long been known. However, all the methods actually used so far are not available. An object of the present invention is to provide a method that can dehydrate an aqueous system by membrane separation in a pervaporation process. [Means for Solving the Problems] The method of the present invention for dehydrating a mixture of organic liquid and water involves first reducing the water content of this mixture by distillation, and then passing it through a pervaporation membrane to remove the remaining water. This is a method of dehydration. Its distinctive features are:
The amount of material retained during membrane permeation depends on the heat exchange in the same system as the distillation, and the amount of heat required for water to permeate through the membrane is circulated through the system during heat exchange. It is the amount that is conducted to the retained object. The invention further relates to an apparatus for dewatering a mixture of organic liquid and water, comprising distillation means and membrane permeation means of a pervaporation process. The apparatus is characterized in that it has a heat exchanger circuit for exchanging heat between the material retained by the membrane permeation means and the distillate from the distillation means. Preferably, the apparatus of the invention comprises a rectification column having a preheater for preheating the starting material mash, a heat exchanger for exchanging heat with the retentate from the membrane module of the pervaporation, and a condenser for condensing the top stream of the membrane module; a pressure pump and a vacuum pump for creating the differential pressure between the retentate side and the permeate side necessary to operate this membrane module and another membrane module; It is characterized by being equipped with pressure maintaining means and a condenser for condensing the permeate from the membrane module. The process of the invention is particularly suitable for the dehydration of organic liquids which form azeotropes with water and are therefore extremely difficult to separate by distillation. From the point of view of energy costs, we will specifically discuss the dehydration of ethanol. The method of the invention is significantly cheaper than conventional methods. In addition, since no entrainer is used, there is no negative impact on the environment. The membrane actually used in the method of the invention is a so-called membrane module of pervaporation. The membrane module is in the form of a flat plate or a bundle of tubes, and if a small effective pressure difference is applied between the retentate and the permeate,
Preferably, a short flow path is obtained so that only a small pressure drop occurs to allow the permeate to flow. The membrane itself can be, for example, a modified cellulose acetate as described in Chemie-Technik 8 (1979) 611-617 for isopropanol/water separation. In a preferred embodiment, the retentate circulation capacity is
The temperature at which the retentate leaves the membrane or membrane module is selected to be well above the cooling water temperature. 15 or more
A typical cooling water temperature of 30°C means that the retentate discharge temperature must be well above this, for example 50°C. In another preferred embodiment, the membrane permeation is carried out in two stages, the first stage using a membrane with low resolution and the second stage using a membrane with high resolution.
As a membrane with a relatively low degree of separation, cellulose triacetate is preferred because it has a high permeation rate and a short saponification time so that most of the water is separated as quickly as possible. As a membrane with a high degree of separation, one made from cellulose triacetate, which has a long saponification time, is preferable because the amount of water to be separated is small. This compensates for the fact that the high resolution of the membrane obtained by the long saponification time slows down the permeation rate. Details of the dependence of permeation rate on saponification time can be found in Chemie-Technik, supra. If the membrane permeation is carried out in two stages, to avoid the additional cost of a second heat exchange circuit,
In the retentate heat exchange circuit connected to the distillation stage,
Preferably, only the retentate from the first membrane permeation stage is drawn. Depending on the degree of anhydrity desired in the final product,
Preferably, the amount of water in the mixture introduced into the second membrane permeation stage is selected such that its heat capacity is sufficient for the permeation of the residual water. To this end, it is further preferred to ensure that the temperature at which the retentate leaves the second membrane permeation stage is well above the cooling water temperature. In yet another embodiment, the method of the invention is used for the dehydration of ethanol starting from agricultural conventional mash. In this method, the pine nuts are dehydrated in the distillation stage to reduce the ethanol content to approximately 80% by weight, the ethanol content is reduced to approximately 95% by weight in the membrane permeation stage of the first pervaporation, and the ethanol content is reduced to approximately 95% by weight in the membrane permeation stage of the second pervaporation. It becomes almost anhydrous in the permeation stage. [Example] Next, the present invention will be described with reference to the accompanying drawings. In the figure, a preferred two-stage membrane separation apparatus for carrying out the method of the invention is shown. In the figure, the starting material is ordinary ethanol mash with an ethanol content of 8.8% by weight. This matshu is introduced via line 1 and passes through a product condenser 2 and a preheater 3 into a separation section 5 of a rectification column 4. The rectification column also includes a concentration section 6 and a heat exchanger 7. High-boiling components, primarily fusel oil, can be discharged as side stream 8. The ethanol mash is heated to the boiling point in the preheater 3. This heat source is the distillation residue in the sump 9 of the rectification column 4. This residual liquid is introduced into the preheater 3 by a pump 10 and discharged through a pipe 12. The rectification column 4 is supplied with energy by a steam-heated circulation evaporator 11. The product stream with an ethanol content of 80% by weight is sent to rectification column 4.
is extracted as a top stream through line 13. Condensation takes place in the product condenser 2 and, depending on the capacity of this condenser, the remaining condensation takes place in another condenser 14. The more volatile components are transferred to the final condenser 15
It is condensed in Uncondensed gas and vapor are discharged via line 16. The condensate remaining in condenser 15 joins through line 17 with the product stream exiting from condensers 2 and 14 via lines 18 and 19. All flow is transferred to line 2 by pumps 21 and 23.
Pervaporation membrane module 2 through 0
4. The reflux required for column 4 is at point 22
It is collected in The membrane used in this membrane module 24 has a high permeation rate and can rapidly separate water. Relatively low demands are made on the degree of separation of the membrane. On the “retention” side of the membrane module 24, the pump 2
A pressure of approximately 3 bar generated at points 1 and 23 acts.
On the permeate side, a pressure of approximately 70 mbar is maintained via line 25 by means of a vacuum pump (not shown). Essentially this permeate contains 10% ethanol by weight.
It consists of water (steam) containing up to This permeate is introduced into a condenser 27 via line 26, where it is condensed and then returned to the original process via line 28, pump 29 and line 28a. At point 30, the product stream is turned around and directed via line 31 to a membrane module 32 of a separate pervaporation section. In this module 32,
Higher resolution membranes are used to obtain the desired purity of product. Anhydrous ethanol (ethanol content 99.8% by weight) is discharged via line 33 and pressure maintenance valve 34.
The permeate remaining in membrane module 32 is essentially water (steam) and contains up to 10% by weight ethanol. This permeate passes through line 35 to condenser 3
6, where it is condensed and then returned to the original process through line 37. A circulating flow occurs from the membrane module 24 via the line 38, the heat exchanger 7 and the pump 23. At point 39 the top stream from the rectification column is introduced and at point 30 a substream of product is removed. In the mass balance, the total amount of product fraction removed at point 30 and permeate discharged via line 26 is equal to the amount of overhead stream from the rectification column directed to point 39. Membrane module 24 yields 95% ethanol on the "retentate" side. A partial stream is withdrawn from this at point 30 and led to a second membrane module 32, where the ethanol content is present at 99.8% by weight. As is clear from the following table, under the conditions actually carried out, the flow rate in the circulation path of membrane module 24-pipe 38-heat exchanger 7-pump 23-membrane module 24 was 8122 Kg/hr. The top stream from the rectifier (1421
Kg/hr). Membrane module 2
Since the amount of water separated at point 4 is 251 Kg/hr, the amount of product (95% ethanol) leaving this circuit at point 30 is only 1170 Kg/hr. The operation described above has two important advantages. First, the concentration on the feed side of membrane module 24 varies from 80% by weight of ethanol in the overhead stream from the rectification column.
Increased to 92.5% (after mixing into circulating flow). Second, by providing the circulation path, the heat of condensation that would otherwise be lost to the cooling water can be utilized for the operation of the rectification column 4. Furthermore, it is possible to operate at a reflux ratio as low as only about 2, compared to the reflux ratio of the rectifier of 3.5 to 4 in conventional processes. Effects of the Invention As a result of these advantages, the method of the invention can be carried out with considerable energy savings compared to conventional processes. In terms of energy balance, the energy requirements of the process of the invention are only 1.6 Kg per alcohol in terms of steam consumption compared to the conventional process which requires a steam consumption of about 5 Kg per alcohol. In other words, the energy requirements of the method of the invention are only one-third of the energy requirements of conventional processes.

[Table] *: Numbers indicate the symbols on the attached drawings.

[Brief explanation of drawings]

The figure is a configuration diagram of an apparatus using two-stage pervaporation membrane separation according to an embodiment of the present invention. 2: product condenser, 3: preheater, 4: rectification column,
7: Heat exchanger, 10, 21, 23, 29: Pump, 11: Circulation evaporator, 14, 15, 27, 3
6: Condenser, 24, 32: Membrane module (membrane separator).

Claims (1)

[Scope of Claims] 1. A method for dewatering a mixture of an organic liquid and water, the mixture comprising first distilling the mixture to reduce the water content and then substantially removing the remaining water by membrane permeation in a pervaporation process. At least a portion of the substance retained by the membrane separator of the pervaporation process is drawn out and passed through the interior of a rectification column to be placed in an indirect heat exchange relationship,
The withdrawn material is returned to the separator to recirculate at least a portion thereof, the amount of the recycle stream being such that substantially all of the heat required for membrane permeation is transferred to the heat exchanger within the rectification column. A method for dewatering a mixture of organic liquid and water, characterized in that an amount of water is conducted into this recirculating stream during the dehydration process. 2. The method of claim 1, wherein the amount of recycle stream is large enough to cause the temperature of the retentate leaving the membrane to be significantly higher than the temperature of the cooling water. 3. Claim 1, characterized in that membrane permeation is carried out in two stages, a membrane with a low degree of separation being used in the first stage, and a membrane with a high degree of separation being used in the second stage. Method described. 4. Process according to claim 3, characterized in that the recycle stream of retentate is withdrawn only from the first membrane permeation stage. 5. The retentate stream drawn from the first membrane separator is passed through a heat exchanger in the rectification column and then split, a part of which is recycled to the first membrane separator and the remaining part is recycled to the first membrane separator. The method according to claim 4, characterized in that the method is passed through two membrane separators. 6. The water content of the mixture fed to the second membrane permeation stage is selected such that the heat capacity of the mixture is sufficient to pass through the remaining water. The method described in section. 7 Aqueous ethanol mash is dehydrated to an ethanol content of approximately 80% by weight in the distillation stage, ethanol content is reduced to approximately 95% by weight in the first membrane permeation stage, and almost anhydrous in the second membrane permeation stage. Claim 3, which is used for dehydrating the mash
The method described in section. 8 A rectification column and a purifier connected in series to first distill a mixture of organic liquid and water and then membrane separate said mixture to remove residual water in the distillate from the rectification column. a membrane separator for a vaporization process, further comprising a recirculation path for withdrawing at least a portion of the retentate of the separator, and a recirculation path for indirectly exchanging heat with a portion of the withdrawn retentate. and a recirculation conduit for returning at least a portion of the heated withdrawn retentate coming from the heat exchanger to the membrane separator. A dehydrating device for the mixture as described above. 9 comprising two membrane separators, the recirculation path connecting the first membrane separator to the heat exchanger so as to supply the entire retentate of the first membrane separator to the heat exchanger; The recirculation conduit is configured to recirculate a portion of the heated retentate from the first membrane separator to the first membrane separator and supply the remainder to the second membrane separator. 9. The apparatus according to claim 8, wherein one separator and said second separator are connected.