METHOD FOR SEPARATING A LIQUID COMPONENT FROM A SOLUTION CONTAINING TWO OR MORE LIQUID COMPONENTS

Abstract: An improved method for separating a particular liquid component from a solution containing two or more liquid components having different boiling points. The method employs a vessel divided into a solution chamber and a vacuum chamber by a permeable membrane which is selectively and preferentially permeable to the vapors of the components having a lower boiling point than those of the other components. The solution is placed in the solution chamber out of direct contact with the permeable membrane at which condition a vacuum is applied to the vacuum chamber to evaporate the solution in the chamber and draw the resulting vapors through the permeable membrane into the vacuum chamber, thereby collecting the particular component from the solution in a vapor-to-vapor permeation through the permeable membrane. The improvement is to maintain the solution in the solution chamber at a higher temperature than an environment, and particularly the permeable membrane for increased separation efficiency.

Description: TECHNICAL FIELD The present invention relates to a method for separating a liquid component from a solution containing two or more liquid components through the use of a permeable membrane, and more particularly to an improved method of separating the component in vapor-to-vapor permeation environment. BACKGROUND ART As a method of separating a particular liquid component from a solution or mixture such as an organic liquid containing two or more liquid components there has been developed a pervaporation (permeation evaporation) method. The principle of the permeation method will be briefly discussed with reference to FIG. 5 which employs a separation vessel 20 divided into an upper solution chamber 22 and a lower vacuum chamber 23 by means of a permeable membrane 24 . By reducing a pressure in the vacuum chamber 23 while keeping the solution S introduced into the solution chamber 22 in contact with the membrane 24 , a particular component in the liquid solution S will permeate and diffuse through the permeable membrane 24 selectively in preference to the other components so that the component to be separated selectively permeates through the membrane 24 and evaporate from the surface of the permeable membrane 24 into the vacuum chamber 23 . However, in this permeation method in which the liquid solution S is kept in direct contact with the permeable membrane 24 to effect the permeation in a liquid-to-vapor phase environment, there arises a serious problem that the permeable membrane 34 which is generally made from a macromolecular material is likely to be swelled due to the direct contact with the solution S . When the permeable membrane is swelled, its permeability will be considerably degraded to thereby lower the separation efficiency. To overcome the above problem, the inventor has proposed in the prior, not pre-published European application No. 0 273 267 A2 an evapomeation (evaporation permeation) method which is directed to effect the permeation in vapor-to-vapor phase environment. That is, as schematically shown in FIG. 6, the evapomeation method utilizes a like vessel 30 divided into a lower solution chamber 32 and an upper vacuum chamber 33 by a permeable membrane 34 and effects the permeation separation while keeping the solution S from which a particular component is to be separated out of contact with the permeable membrane 34 . Thus, the vapors of the particular component will selectively permeate through the membrane 34 in a vapor phase into the vacuum chamber 33 when drawn by vacuum through the membrane 34 . With this result, the evapomeation method can be free from the swelling which is seen in the conventional pervaporation method and therefore prevents a lowering in separation efficiency. Nevertheless, the evapomeation method may be found insufficient in some cases due to limited separation efficiency inherent to the permeable membrane employed. A process for the dehydration of a water containing medium is known from DE-C-0 851 048, wherein any contact between the water and the membrane is to be avoided and difference between the partial pressures of the water vapor on the inner and on the outer side of the container is caused, the latter being the lower. However, the membrane is always maintained on a higher temperature as the dew point of the water vapor on the inner side of the membrane. In Journal of Membrane Science, Vol. 16, Nrs. 1-3, pages 269-284, the ethanol-water separation by pervaporation is described in its basic principles without any hint to the solution of the above mentioned problems. A further specific pervaporation process with specific membranes for specific purposes is described in FR-A-1529382. DISCLOSURE OF THE INVENTION The present invention eliminates the above problems and provides an improved permeation separation method for further enhancing separation efficiency. The method of the present invention utilizes a vessel which is divided by a permeable membrane into a solution chamber and a vacuum chamber. The permeable membrane is a membrane selectively and preferentially permeable to the vapors of the component which is intended to be separated and which has a lower boiling point than those of the other components. The solution is placed in the solution chamber out of direct contact with the permeable membrane. Then, a vacuum is applied to the vacuum chamber to evaporate the solution in the solution chamber and draw the resulting vapors through the permeable membrane into the vacuum chamber, whereby the components to be separated selectively separates through the membrane in a vapor-to-vapor phase environment. The improvement resides in heating the solution in the solution chamber to a temperature higher than the environment while said zone adjacent to the solution chamber side of said membrane or said membrane itself is cooled to a temperature lower than that of said solution in the solution chamber. According to the improved method of providing a temperature difference between the solution and the permeable membrane or the zone adjacent thereto, a remarkable increase in separation efficiency will result. Although an exact mechanism or reason for the increased separation efficiency is not yet known, it is assumed that when reaching a cooled zone adjacent to the permeable membrane the vapors of the components not intended or having a higher boiling point are likely to aggregate more than those of the intended component or having a lower boiling point to thereby form a greater aggregation of molecules which is difficult to enter the membrane or even return back to the liquid phase. Thus, the vapors of the intended component having the lower boiling point are allowed to preferentially enter or permeate through the membrane while those of the other components are rejected so that the vapors of the intended component can be collected at a greater amount in the vacuum chamber than those of the components not intended. Accordingly, it is a primary object of the present invention to provide an improved permeation separation method in which a particular liquid component can be separated from solution containing two or more liquid components at an increased separation efficiency. The above and other objects and advantages of the present invention will become more apparent from the following description of the invention with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing an apparatus employed for the present invention; FIGS. 2 to 4 are respectively graphs illustrating separation efficiency and concentration of ethanol separated with differing temperature conditions from a 10 % aqueous solution of ethanol with respect to 9 (nine) samples in accordance with Example 1 of the present invention; FIG. 5 is a schematic view illustrating a prior pervaporation method; and FIG. 6 is a schematic view illustrating a basic concept of an evapomeation method. MODES FOR CARRYING OUT THE INVENTION Referring to FIG. 1 which illustrates the principle of the "evapomeation (evaporation permeation)" separation method of the present invention, a separation vessel 10 is divided by means of a permeable membrane 14 into a lower solution chamber 12 and an upper vacuum chamber 13 . A solution S , for example, an organic solution containing two or more volatile or evaporizable components having different boiling points is introduced into the solution chamber 12 and is kept out of direct contact with the permeable membrane 14 . The vacuum chamber 13 is provided with a port 15 which is connected to a source of vacuum such as a vacuum pump for evacuating the vacuum chamber and in turn for evaporating the solution S in the solution chamber 12 and drawing the resulting vapors through the permeable membrane 14 . The vapors permeating through the membrane 14 into the vacuum chamber 13 is collected or recovered by a suitable manner. Since the permeable membrane 14 allows a particular component to selectively permeate in preference to the others, it is readily possible to complete the separation either by collecting the component having passed into the vacuum chamber 13 through the permeable membrane 14 or by collecting the remaining solution in the solution chamber 12 from which the particular component has been removed. The permeable membrane 14 utilized in the present invention may be a membrane which is generally called a non-porous membrane including those utilized in the conventional pervaporation method. For example, the permeable membrane may be made from alginic acid, chitosan, cross-linked chitosan, chitosan acetate, quaternized chitosan, polystyrene, polyvinylidenechloride, polydimethylsiloxane, polyvinylidenefluoride, cellulose nitrate, cellulose acetate, cross-linked polyvinylalcohol, polyamic acid, poly-1-trimethylsilyl-1-propyne, graft copolymer of stylenedimethylsiloxane. In the present invention, the permeable membrane is selected to be permeable preferentially to a liquid component having a lowest boiling point among the others in a solution. For instance, when an aqueous solution of alcohol, e.g. ethanol (b.p. 78.3 °C) or methanol (b.p. 64.1°C) is intended to be separated, a permeable membrane of polydimethylsiloxane, poly-1-trimethylsilyl-1-propyne, or graft copolymer of stylene-dimethylsiloxane is selected to be permeable to alcohol in preference to water. For an aqueous solution of dimethylsulfoxide (b.p. 189°C), dimethylformamide (b.p. 153°C), dimethylacetamide (b.p. 165.5°C), or acetic acid (b.p. 117.8°C), a permeable membrane of alginic acid, chitosan, cross-linked chitosan, cross-linked polyvinylalcohol, or polyamic acid is selected to be permeable to water in preference to the other component. The permeable membrane 14 is normally provided as a thin fragile membrane so that it is difficult to withstand a pressure difference developed thereacross at the time of evacuating the vacuum chamber 13 . For reinforcement and prevention of the break of the permeable membrane, it is preferred to support the membrane by means of a porous member such as non-woven fabric of polypropylene or polyester, porous film of Teflon or polysulfone, porous glass plate, or porous ceramic plate. In operation, as a vacuum is applied to the vacuum chamber 13 , the solution chamber 13 is correspondingly evacuated to vaporize the liquid components in the solution S . The resulting vapors are drawn to the vacuum chamber 12 through the permeable membrane 14 . When reaching the membrane 14 , the vapors of the component having a lower boiling point than the other are caused to permeate and diffuse preferentially through the membrane 14 and are eventually drawn into the vacuum chamber 13 . For example, when ethanol is intended to be selected from its aqueous solution by a suitable permeable membrane having selective permeability to ethanol in preference to the water. The vapors of ethanol having a lower boiling point are caused to permeate and diffuse through the membrane in preference to the water, to thereby increase ethanol concentration within the vacuum chamber 13 . Thus, ethanol of increased concentration can be recovered from the vacuum chamber 13 . During the above separation process, the solution S is maintained at a raised temperature for enhancing evaporation of the component to be separated and therefore obtaining the intended component of increases concentration. To this end, a heating bath 16 is provided to surround the solution chamber 12 , as shown in FIG. 1. The heating of the solution may be effected alternatively by providing a heating jacket around the solution chamber 12 , providing an immersion heater in the solution chamber 12 , or the like heating device. The heating of the solution, in the present invention, means to heat the solution S to a temperature above that of an environment or room temperature and preferably that of the permeable membrane 14 . Thus, the heating is preferably carried out in such a manner as to raise the temperature of the solution S while maintaining the permeable membrane 14 at a lowered temperature. For positively providing a temperature difference between the solution S and the permeable membrane 14 , the apparatus of FIG. 1 includes a cooling jacket 18 surrounding the permeable membrane 14 for cooling the membrane 14 by a cooling medium continuously fed through the jacket 18 . By cooling the permeable membrane 14 while heating the solution S , a more enhanced separation could result. The exact reason or mechanism for such enhanced separation is not known, but it is assumed that the vapors of the components having a higher boiling point are most likely to aggregate when reaching adjacent the cooled membrane and return to liquid or at least form a large sized aggregation which is difficult to permeate through the membrane, thus enabling substantially only the vapors of the component having a lower boiling point to permeate through the membrane. It should be noted at this time that, although the apparatus of FIG. 1 is intended to cool the permeable membrane 14 and therefore the adjacent zones on the opposite sides thereof, the cooling could be limited only to the zone adjacent the solution side of the permeable membrane 14 for achieving the above enhanced separation, and the cooling is not necessarily made to the opposite zone in the vacuum chamber or the permeable membrane itself. Therefore, any other cooling scheme or means could be utilized to effect such limited cooling while retaining the equally enhanced separation. The separation method of the present invention could be adapted for separation of a wide variety of solutions containing two or more liquid components, and is found particularly useful for separation of solutions which are difficult to be separated by conventional methods. Such useful adaptation includes, for example, separation of an alcohol, aromatic compounds or ether from an azeotropic mixture of water-alcohol, water-aromatic compound, or water-ether; separation of an organic solvent from a solution containing two or more organic solvents of differing but closing boiling points; separation of a structural isomer from a mixture containing two or more such isomers such as o-, m-, and p-xylenes; separation of an optical isomer from a mixture containing two or more such isomers such as dextro- and levo-rotary ones; separation of a mixture containing thermally decomposable or deformable substances such as medicines, living organisms, fruit juices, polymerizable monomers; recovery of a reaction product from a reaction system in equilibrium for accelerating the reaction; separation of volatile organic substances such as ammonia, amine, hydrogen sulfide, carbon dioxide, sulfur dioxide, or the mixture thereof from a waste liquid; separation of alcohols form a biomass; separation of an expensive organic solvent, e.g. dimethylfromamide, dimethylacetoamide, or dimethylsulfoxide from a spinning solution of synthetic fibers; separation of a solvent from a waste paint resulting in the preparation of a paint or in a paint processing line; separation of alcohols, organic solvents, or salts from an alcohol or solvent containing mixture resulting in a chemical plant; separation of trichlene from a dry-cleaning fluid; concentration of an emulsion solution; concentration and separation of ions of rare earth elements from a solution containing radio-active elements; concentration of virus or bacteriophage; manufacture of sterile water or non-pyrogenic water for use in blood dialysis or other pharmaceutical or therapeutic purposes; and manufacture of ultrapure water for use in an electronic industry. The present invention will be discussed with reference to the following examples, which are provided by way of illustration and not by way of limitation. All percent and parts are by weight. Example 1 A 10 % aqueous solution of ethanol was separated by the use of a permeable membrane of polydimethylsiloxane which shows selective permeability to ethanol in preference to water. The polydimethylsiloxane membrane was prepared from the mixture of 7 parts of silicon (sold by SHINETSU KAGAKU, Japan as KE10), 0.25 parts of hardening agent (sold by the same as catalyst RA), and 49 parts of benzene. The mixture was stirred followed by placed onto a stainless-steel made membrane former and dried thereon at 25°C for 6 hours to obtain a 100 µ thick permeable membrane. Thus obtained membrane was fixed in the apparatus of FIG. 1 while the solution is placed in the solution chamber 12 out of direct contact with the membrane. The separating operations were carried out while keeping the vacuum chamber 13 at a reduced pressure of 1.5 x 10⁻² Torr. and varying the temperatures of the heating bath 16 and the cooling jacket 18 , as listed in Table 1, to provide nine (9) samples of ethanol-rich solutions recovered through the membrane 14 with differing temperature conditions. The solution collected for 30 minutes after the start of separation was discarded and each sample was collected from the vacuum chamber 13 for 2 hours after an elapse of initial 30 minutes from the start of separation. Then, separation factor α and ethanol concentration for each sample (Nos. 1 to 9) were obtained as listed in Table 1. The separation factor α is defined by the following equation: wherein X H2O and X ETOH are fractions of water and ethanol respectively in aqueous ethanol solution placed in the solution chamber 12 , while Y H2O and Y ETOH are fractions of water and ethanol respectively for each sample solution collected in the vacuum chamber 13 . As apparent from the above relation, when α is greater than 1, it means that ethanol has passed through the membrane 14 in a greater amount than water and therefore that ethanol is allowed to pass preferentially through the membrane to a larger extent as the value α becomes greater. samples solution temp.(°C) membrane temp.(°C) concentration (wt%) separation factor [α] No. 1 25 25 38.20 5.56 No. 2 40 40 38.51 5.64 No. 3 40 25 61.47 14.36 No. 4 40 0 73.15 24.52 No. 5 40 -10 84.94 50.76 No. 6 55 55 43.19 6.84 No. 7 55 25 68.52 19.59 No. 8 70 25 71.80 22.91 No. 9 70 70 44.18 7.12 In Table 1, samples Nos. 1, 2, 6, and 9 were obtained by maintaining the solution and the permeable membrane at the same temperatures, while samples Nos. 3-5, 7, and 8 were obtained by maintaining the permeable membrane at a lower temperature than that of the solution. Each sample was analyzed to provide separation factor α and ethanol concentration, the results of which are shown in graphs of FIGS. 2, 3, and 4, in which a line marked by a triangle shows the concentration of each sample, a line marked by a rectangular shows the separation factor α of each sample, upper and lower figures in a brace indicate the solution temperature and membrane temperature, respectively. FIG. 2 shows the graph for a first group of samples Nos. 1, 2, 6, and 9 in which the solution and the membrane were at the same temperatures. Sample No. 1 was obtained by leaving the solution and membrane at a room temperature of 25°C, and the other samples Nos. 2, 6, and 9 were obtained by raising the solution and membrane temperatures, respectively to 40, 55, 70°C. As seen from the figure that raising the solution temperature will results in a superior separation effect as confirmed both in ethanol concentration and separation factor α. FIG. 3 shows the like graph for a second group of samples Nos. 1, 3, 7, and 8, which were obtained by fixing the membrane temperature at 25°C, while varying the solution temperature respectively to temperatures of 25, 40, 55, and 70°C. As seen from the figure, it is confirmed that samples Nos. 3, 7 and 8 obtained by raising the solution temperatures, respectively to 40, 55, 70°C exhibit superior separation results both in ethanol concentration and separation factor α, as compared to sample No. 1 in which no temperature difference was seen between the solution and membrane. Also noted from the figure is that the superior separation results are correspondingly enhanced as the solution temperature is raised. FIG. 4 shows the like graph for a third group of samples Nos. 2, 3, 4, and 5, which were obtained by fixing the solution temperature at 40°C while varying the membrane temperature, respectively to 40, 25, 0, and -10°C. It is also confirmed from the figure that the separation effect is more enhanced as the temperature difference between the solution and the membrane is increased while fixing the solution temperature. Example 2 A 10 % aqueous solution of methanol was separated in the same apparatus and by the same permeable membrane of polydimethylsiloxane as employed in Example 1. The like separating operations were carried out with a vacuum level of 2.0 x 10⁻² Torr. in the vacuum chamber 13 , so as to obtain samples No. 10 and 11 with varying membrane temperatures but with a fixed solution temperature, as listed in Table 2. The samples were analyzed to provide the respective ethanol concentrations which are also listed in Table 2. From the above results, it is further confirmed that the separation effect also for ethanol is greatly enhanced by providing the temperature difference between the solution and the membrane. samples solution temp.(°C) membrane temp.(°C) concentration (wt%) No. 10 40 40 35.50 No. 11 40 5 72.50 LIST OF REFERENCE NUMERALS 10 vessel 12 solution chamber 13 vacuum chamber 14 permeable membrane 15 port 16 heating bath 18 cooling jacket 20 vessel 22 solution chamber 23 vacuum chamber 24 permeable membrane 30 vessel 32 solution chamber 33 vacuum chamber 34 permeable membrane

Claims: A method for separating a liquid component from a solution containing two or more liquid components having different boiling points, comprising: a) providing a vessel (10) divided by a permeable membrane (14) into a solution chamber (12) and a vacuum chamber (13), said membrane being selectively and preferentially permeable to the vapors of the component having a lower boiling point than those of the other components; b) placing the solution (S) into the solution chamber out of direct contact with the permeable membrane; and c) applying a vacuum to the vacuum chamber to evaporate the solution in the solution chamber and draw the resulting vapors through the permeable membrane into the vacuum chamber, whereby the components to be separated selectively permeates through the membrane in a vapor-to-vapor phase environment, characterized in that the solution in the solution chamber is heated to a temperature higher than the environment while said zone adjacent to the solution chamber side of said membrane or said membrane itself is cooled to a temperature lower than that of said solution in the solution chamber. A method according to claim 1, characterized in that the solution is an aqueous solution of alcohol. A method according to claim 2, wherein the permeable membrane is a polydimethylsiloxane membrane selectively and preferentially permeable to ethanol.

Inventors: URAGAMI, TADASHI
Applicants: LIGNYTE CO., LTD.
Application year: 1989
Main classification: B01D61
IPC codes: B01D61, B01D53, C07C31, C07C29, C07B63