Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
  • B01D61/36—Pervaporation; Membrane distillation; Liquid permeation
  • B01D61/365—Osmotic distillation or osmotic evaporation

Definitions

• This invention relates to an osmotic distillation process for concentrating a liquid. More specifically, it relates to an osmotic distillation process in which a nonporous membrane of a high free volume polymer composition is utilized to transfer a volatile component from a liquid to be concentrated into a strip solution.
• Osmotic distillation is a type of membrane separation process in which a component of a liquid on one side of a porous or microporous membrane is transported through the membrane to another liquid on the opposite side of the membrane. It differs from other more widely recognized types of membrane separation processes, such as reverse osmosis, ultrafiltration, and pervaporation in that the membrane is not wettable by either of the two liquids, and that the volatile component passes through the membrane in a vapor state. Accordingly, the driving force for transfer of the volatile component is the difference between the vapor pressure of the volatile component over the "sending" liquid and the lower vapor pressure of the component over the "receiving" liquid.
• a significant feature of osmotic distillation is that the transfer of the volatile component does not require a substantial system pressure or temperature gradient across the membrane. Therefore, this process advantageously can be carried out at ambient temperatures and pressures.
• Such low temperature and low pressure process conditions render osmotic distillation ideal for increasing the concentration in an initially dilute liquid phase of a temperature and/or pressure sensitive component.
• These are materials which have limited stability to elevated temperatures and shear stresses.
• Such a component is one which would likely be adversely affected or destroyed if concentrated at elevated temperatures or pressures required by other processes.
• osmotic distillation recently has gained much favorable attention in particular for the potential to concentrate liquid foodstuffs, cosmetics (e.g., fragrances), pharmaceutical products and thermally labile biological substances.
• An excellent survey of osmotic distillation technology is presented in Hogan, Paul A., A New Option: Osmotic Distillation, Chemical Engineering Progress, July, 1998, pp. 49-61, which is incorporated herein by reference in its entirety.
• the concentration of beverages such as fruit and juices and alcoholic beverages is a primary application for osmotic distillation. Perhaps the most notable reasons for concentrating beverages are that the concentrates do not contain large amounts of solvent and they are stable much longer than in the dilute state. The concentrates thus can be shipped less expensively for long distances and remain fresh far longer than if diluted.
• oils or other components that reduce the surface tension of the juice e.g., surfactants.
• These oils and other surface tension reducing components are collectively referred to herein as “oils” or “oily components”.
• orange juice contains a substantial amount of dissolved limonene oils.
• the presence of dissolved oils in a primarily aqueous juice solution can be problematic for osmotic distillation because the oily components tend to wet the membrane surface, fill the pores and reduce or altogether block desired transmission of the volatile component, thereby preventing further concentration of the starting material. Oils may also eventually penetrate the membrane and allow the fluids on either side of the membrane to mix, which is undesirable.
• Fruit juices as well as other liquids can be pulpy. That is, they contain solids suspended in the liquid. As the juice concentrates, the solid concentration increases. Solids can also block substantial portions of the membrane surface so as to occlude the pores and hinder osmotic distillation to the extent that the rate of transmission of the volatile component is greatly reduced.
• a process for concentrating a liquid feed mixture comprising the steps of providing a feed mixture of components present in initial concentration, the mixture comprising a volatile component in which mixture said volatile component is soluble and over which mixture said volatile component has a first vapor pressure, and a strip solution over which the volatile component has a second vapor pressure different from the first vapor pressure; providing a two sided membrane structure comprising a nonporous membrane on one side and coextensive with a microporous substrate on the second side, the nonporous membrane being of a composition which is permeable to the volatile component and not wettable by either the feed mixture or the strip solution; contacting the feed mixture with the nonporous membrane while contacting the strip solution with the microporous substrate; maintaining the second vapor pressure below the first vapor pressure, thereby
• an osmotic distillation apparatus for concentrating a liquid feed mixture of components present in initial concentration, the mixture comprising a volatile component which is soluble in the feed mixture and over which said volatile component has a first vapor pressure
• the apparatus comprising, a two sided membrane structure comprising a nonporous membrane of a composition permeable to the volatile component, and a microporous substrate adjacent and coextensive with the nonporous membrane, means for contacting the nonporous membrane with the feed mixture, means for contacting the microporous substrate with a strip solution comprising the volatile component which is present at a second vapor pressure different from the first vapor, pressure; and means for maintaining the second vapor pressure below the first vapor pressure, thereby causing the volatile component to permeate the membrane from the feed mixture to the strip solution, in which apparatus the composition of the nonporous membrane is not wettable by either the feed mixture or the strip solution.
• Fig. 1 is a plot of specific volume versus temperature of a polymer exhibiting excess free volume.
• Fig. 2 is a schematic diagram of an apparatus utilized to practice osmotic distillation according to an embodiment of this invention.
• Fig. 3 is a plot of conductivity in millisiemens vs. weight of feed tank contents in grams during osmotic distillations to concentrate limonene-containing sucrose solution.
• Fig. 4 is a plot of transmembrane flux during osmotic distillations to concentrate limonene-containing sucrose solution.
• Fig. 5 is a plot of transmembrane water vapor flux and conductivity of the feed stream during osmotic distillation of coffee according to an embodiment of this invention.
• the novel osmotic distillation process basically calls for transferring a volatile component from a first liquid through a nonporous membrane to a second liquid, thereby concentrating the residual components which remain in the first liquid and diluting the second liquid by addition of the transferred volatile component. Often the principal product of this process is the residual-enriched first liquid rather than the diluted second liquid.
• the first liquid which is the liquid from which the volatile component is removed, i.e., the concentrating liquid, as the "feed” and further, to refer to the second, or liquid diluted by the volatile component, as the "strip solution".
• the latter term may be deemed to have been derived from its function of stripping the volatile component from the feed.
• the feed is a liquid state mixture comprising at least one volatile component and at least one other component which becomes concentrated by removing some of the volatile component.
• Solids can be present in the feed mixture, which thus will be a slurry or suspension.
• the proportion of solids to liquid is not critical provided that the feed mixture remains a freely flowing fluid.
• the solids are usually uniformly, or at least well dispersed in the feed mixture.
• Fruit juice with pulp, i.e, pieces of fruit, is a typical example of a solids-containing feed.
• the feed mixture can have one or more residual components which are concentrated by operation of the osmotic distillation process.
• Reference herein to the concentrating component in singular is intended to encompass the plural unless stated otherwise.
• the concentrating component can be a liquid or a solid.
• the concentrating component should be miscible with the volatile component.
• the vapor pressure of the concentrating component should be low compared to that of the volatile component to prevent substantial transfer of the former to the strip solution. If the concentrating component is a solid, it can be completely or partially soluble in the volatile component.
• the driving force for transfer of the volatile component through the membrane is the gradient of the volatile component vapor pressure between the feed mixture and the strip solution. Therefore the volatile component should provide a high vapor pressure over the concentrating solution so that a large vapor pressure gradient can be obtained.
• the novel osmotic distillation process is well suited to concentrating initially dilute solutions of low- or non-volatile solutes of moderate to high molecular weight, such as sugars, polysaccharides, carboxylic acid salts, proteins, and the like.
• the solvent in many industrially important cases is frequently water or an organic solvent, typically a polar organic solvent such as an alcohol, e.g., ethyl alcohol. The actual solvent used will depend on the feed mixture to be treated.
• the feed mixture is selected from such compositions as liquid foodstuffs, including soups and fruit, alcoholic, or caffeinated beverages, cosmetics (e.g., fragrances), pharmaceutical products, neutriceuticals, latices and thermally labile biological substances, such as animal body fluids including, blood, urine, cerebrospinal fluid, and the like.
• the strip solution should be a solution of preferably a high osmotic activity, nonvolatile solute dissolved in the volatile component. That is, the nonvolatile solute should have a low equivalent weight and high solubility in the volatile component.
• the strip solution should be maintained at a relatively high concentration of the nonvolatile solute so as to depress the vapor pressure of the volatile component at the strip solution side of the membrane. This promotes the rapid transfer of the volatile component to the strip solution.
• the vapor pressure of the volatile component in the strip solution will increase as transfer through the membrane progresses.
• the volatile component concentration in the strip solution should be kept low. This can be accomplished by restocking the strip solution with virgin, low volatile component concentration strip solution. For economically attractive operation, the strip solution can be reconcentrated by removing excess volatile component and reused.
• the nonvolatile solute preferably should be stable at high temperatures which may be used to accelerate reconcentration
• the strip solution can also be restocked from brine wells or bodies of water with high salt concentration
• the nonvolatile solvent of the strip solution is also desirable for the nonvolatile solvent of the strip solution to be nontoxic, noncorrosive and inexpensive
• the nonvolatile strip solution solute is an alkali hydroxide, a water soluble salt selected from the group consisting of alkali halide salt, and alkaline earth metal halide salt or mixtures thereof
• Representative water soluble salts include sodium chloride, calcium chloride, magnesium chloride, monopotassium phosphate, dipotassium orthophosphate, magnesium sulfate, lithium chloride, lithium bromide, lithium iodide, potassium iodide, sodium iodide and mixtures thereof
• the osmotic distillation process utilizes a membrane structure comprising a nonporous gas permeable membrane in contrast to a porous membrane that has heretofore been used in conventional osmotic distillation
• a membrane structure comprising a nonporous gas permeable membrane in contrast to a porous membrane that has heretofore been used in conventional osmotic distillation
• This feature renders the novel process exceptionally useful to concentrate liquid mixtures which have oily components and/or suspended solids
• Membrane pores can be wetted by oils in the feed mixture, especially as the mixture concentrates
• a porous membrane becomes "wetted out", i.e., saturated with the oily components the oils can penetrate the membrane to occlude and/or coat the pores
• Such occlusion diminishes transfer of the volatile component and such coating can also allow the strip solution and/or the feed mixture to "breakthrough" the membrane and thus contaminate the other
• solids in the concentrating solution can occlude the pores
• nonporous membranes were not thought suitable for use in osmotic distillation.
• transfer of the volatile component charactistically occurs by permeation of a gas through a membrane.
• Conventional materials for nonporous membranes generally do not have high gas permeabilities. Therefore, it has been traditionally observed in the art that transmission of a volatile component through a nonporous membrane would be too slow to make osmotic distillation practicable. It has now been found that certain materials can be formed into nonporous membranes which provide acceptable transmission rates for osmotic distillation and thus permit the benefit of reduced wetting derived from utilizing nonporous membranes in osmotic distillation to be achieved.
• Free volume is a characteristic of a polymer which can be understood by inspection of a plot of specific volume V versus temperature T as seen in the Fig 1.
• the term "free volume" refers to the volume of the polymer not actually occupied by the molecules.
• the volume of polymer occupied by the molecules is represented by region 8 below line 7 that defines the volume-temperature relationship of molecules which make up a polymeric article.
• Line 4 defines the volume-temperature relationship of the article formed of the polymer. Therefore, the region between line 4 and line 7 represents the space between molecules within the physical dimensions of the polymeric article. Further, the region above line 4 represents the volume of space outside the bounds of the article. This uppermost region would include the volume of the pores throughout a porous article of polymer.
• the plot of V vs. T defines a linear relationship 4.
• a discontinuity 1 in the V-T plot is seen at the glass transition temperature, Tg. That is, below the Tg, where the polymer is glassy, specific volume increases linearly with increasing temperature along line 4.
• the rate of increase is higher. That is, the slope of the specific volume vs. temperature line 2 above the Tg, is steeper than line 4 below the Tg.
• region 5 is designated the "excess free volume”.
• glassy polymers at temperatures below the Tg can have a large excess free volume which provides an overall free volume greater than the expected free volume between lines 6 and 7 that is especially high.
• High free volume polymers are preferred for use as nonporous membranes for osmotic distillation.
• high free volume is meant that the free volume of the polymer at the temperature of use is at least about 15% and preferably at least about 28%.
• the temperature of use will be the temperature at which osmotic distillation is performed. It is desirable for the high free volume polymer to have a glass transition temperature above normal room temperature, preferably above about 30°C, and more preferably above about 115°C.
• High free volume polymers which are particularly preferred for carrying out osmotic distillation are polytrimethylsilylpropyne, silicone rubber, and certain amorphous copolymers of perfluoro-2,2- dimethyl- 1,3-dioxole, the latter being especially favored.
• the membrane can be formed from an amorphous copolymer of a certain perfluorinated dioxole monomer, namely perfluoro-2,2-dimethyl- 1,3-dioxole ("PDD").
• the copolymer is copolymerized PDD and at least one monomer selected from the group consisting of tetrafluoroethylene (“TFE”), perfluoromethyl vinyl ether, vinylidene fluoride and chlorotrifluoroethylene.
• TFE tetrafluoroethylene
• the copolymer is a dipolymer of PDD and a complementary amount of TFE, especially such a polymer containing 50-95 mole % of PDD. Examples of dipolymers are described in further detail in U.S. Patents Nos. 4,754,009 of E. N. Squire, which issued on June 28, 1988; and 4,530,569 of E. N. Squire, which issued on July 23, 1985.
• Perfluorinated dioxole monomers are disclosed in U.S. Patent No. 4,565,855 of B.C. Anderson, D.C. England and P R. Resnick, which issued January 21, 1986. The disclosures of all of these U.S. patents are hereby incorporated herein by reference.
• the amorphous copolymer can be characterized by its glass transition temperature which will depend on the composition of the specific copolymer of the membrane, especially the amount of TFE or other comonomer that may be present. Examples of Tg are shown in FIG. 1 of the aforementioned U.S. Patent No. 4,754,009 of E.N.
• perfluoro-2,2-dimethyl- 1,3-dioxole copolymers according to this invention can be tailored to provide sufficiently high Tg that a membrane of such composition can withstand exposure to steam temperatures.
• membranes of this invention can be made steam sterilizable and thereby suitable for various uses requiring sterile materials, especially those involving biological materials.
• the gas permeable membrane is nonporous. Absence of porosity can be measured by various methods known in the art, including for example, microscopic inspection of the membrane surface.
• PDD copolymers are particularly advantageous in this regard because they are intrinsically selectively gas permeable. Specifically, nonporous membranes of PDD copolymers exhibit an oxygen/nitrogen gas selectivity of greater than about 1.4. Hence it is possible to measure the difference in flux rates of two gases, for example oxygen and nitrogen, through a PDD copolymer membrane to verify that it is selectively gas permeable, and therefore, intact and nonporous over the membrane surface.
• the nonporous membrane can be an unsupported monolithic gas permeable membrane structure.
• a multilayer composite of a nonporous gas permeable layer supported on a porous or microporous substrate layer is utilized in which the nonporous membrane is adjacent and coextensive with the supporting porous substrate.
• the porous support provides structural integrity for the nonporous membrane. Any porous substrate material which is nonwettable by the strip solution and feed and that offers such support is suitable provided that it also is not degraded by the feed mixture, the volatile component or the strip solution and does not impede the transmission of the volatile component through the nonporous membrane. Additionally, materials that are by themselves wettable by the strip solution or feed can be treated to be made nonwettable.
• porous substrate materials include polyolefins, such as polyethylene and polypropylene, polytetrafluoroethylene, polysulfone, and polyvinylidene fluoride.
• the shape of the membrane can be in a diverse variety of forms and typically can be in sheet form, such as a perforated sheet; porous woven or nonwoven fabric; microporous polymer film.
• the sheet can be deployed as a flat sheet, or the sheet can be pleated or rolled into a spiral to increase the surface to volume ratio of the separation unit.
• the membrane can also be in tube or tube ribbon form.
• Membrane tubes and tube ribbons are disclosed in U.S. Patent No. 5,565.166 which is incorporated herein by reference.
• the nonporous gas permeable membrane for use in osmotic distillation is applied as a thin layer on a support of a microporous hollow fiber.
• Such composite hollow fibers beneficially provide a very large surface area per unit of membrane structure volume and thus are able to produce extremely high gas flux in small occupied space.
• This surface to volume ratio benefit can be exploited further by assembling a plurality of composite hollow fibers in a so-called fiber module.
• Such module typically includes a bundle of many hollow fibers in substantially parallel alignment. The ends of the fibers are potted in a fixation medium, for example an epoxy resin. The bundle is then sliced through the potted ends and the bundles can be mounted within a casing to form a shell and tube modular unit.
• the nonporous membrane can be formed on the hollow fibers before bundling and assembling the module, it is preferred to form the membrane on the hollow fibers after installing them within a module.
• An osmotic distillation apparatus was set up as shown in Fig 2.
• Calcium chloride strip solution in water was circulated in a brine loop 20 from brine feed tank 22 through a membrane module 30 via a Cole-Parmer Model 7144-05 variable speed, brine feed pump 24.
• the brine feed tank was equipped with a magnetic stirrer 23 to maintain a homogeneous solution concentration in the tank.
• the brine was pumped through a heat exchanger in a VWR model 1140 constant temperature bath 25 maintained at 25°C and an Avecor AffinityTM blood oxygenator (Medtronic, Inc., Minneapolis, Minnesota) 26 to remove excess gases in the strip solution.
• the latter step was accomplished by passing the strip solution through the side of the oxygenator which normally receives blood while drawing about 25 inches Hg vacuum gauge on the side of the oxygenator which normally receives gas with a Welch Duo-SealTM model 1400 vacuum pump. Temperature of the circulating strip solution was measured at thermocouple 27 prior to entry into membrane module 30. Pressure of the circulating strip solution at pressure gauge 28 was controlled by manually throttling valve 29 at return of the solution to the feed tank.
• the feed was circulated in a feed loop 41 from supply tank 40 through the membrane module 30 via another Cole-Parmer Model 7144-05 pump 42 and a second heat exchanger in the same VWR model 1140 constant temperature bath 43. Temperature 44 and pressure 45 were monitored prior to entry into the module. In all examples, brine and feed temperatures entering the module were kept within a narrow range of each other that did not exceed about 2°C and was typically about 0.5°C. Conductance of the feed was continuously measured by a Yellow Springs Instrument Model 32 conductance meter 46. Hence, a breakthrough of electrically conductive strip solution could be detected by an increase in conductivity of the feed. Mass of the concentrating solution in the supply tank was determine from mass balance calculations based on weight measurements taken using an Acculab® Model V-4800 electronic balance (0.1 g sensitivity) 47 placed under the supply tank. Flow of the feed stream entering the feed tank provided agitation.
• the membrane module 30 also was a Avecor AffinityTM blood oxygenator. This unit contained within a cylindrical case 33 multiple, microporous, polypropylene hollow fibers with a total of about 25 square feet of membrane transfer area which are collectively represented by schematic element 32.
• Pore size was believed to be about 0.04 ⁇ m and the hollow fibers had a bore diameter of about 230 ⁇ m and an outer diameter of about 300 ⁇ m
• the apparatus was configured to direct the strip solution through the fiber bores and feed over the outer surface of the fibers to so as to pass these fluids through separate zones 34 and 36 on opposite sides of the membrane while in the module
• solid calcium chloride was added intermittently to the brine feed tank as needed to maintain solid salt present in the tank and thus to keep the concentration of strip solution at or near saturation Pressure of the strip solution was controlled to at least 0 5 pounds/sq in , "psi", (3 5 KPa) greater than the feed stream This was done to assure that any breach of the membrane would cause bulk strip solution to contaminate the feed which then could be detected by conductivity as mentioned earlier
• Example 2 The osmotic distillation procedure of Example 1 was repeated except that the feed stream consisted of 20 ml r-limonene added to 4 liters of 10 wt./wt.% sucrose solution in water (Ex. 4). For comparison, this procedure was also repeated using the membrane module of Comparative Example 1 (Comp. Ex. 2).
• Fig. 3 shows a plot of conductivity, Q, in millisiemens vs. weight of feed tank contents,
• Example 4 demonstrates that the existence of a nonporous membrane on the microporous polypropylene fibers prevented the r-limonene oil from "wetting out” the membrane so that effective osmotic distillation could be carried out to concentrate the feed.
• Limonene is a naturally occurring oil in many fruit juices, particularly citrus juices.
• Example 5 The apparatus and procedures of Example 2 were repeated with the following differences. Four thousand grams of freshly brewed coffee was circulated through the feed stream circulation loop and supply tank to fill the system. Initial inventory in the supply tank was 3586 grams, implying that the balance of 414 grams was resident in the circulation loop volume. Osmotic distillation was performed until the brewed coffee concentrated to a residue mass of 295 grams in the supply tank.
• Fig. 5 is a plot of water vapor flux (data “E") through the membrane calculated from weight loss in the supply tank and conductivity (data “F”) of the feed stream as a function of time during osmotic distillation.
• Water vapor flux rose rapidly to a steady state rate at about 90 minutes then decreased very gradually until about 10 hours of distillation when the transmission rate began to drop precipitously.
• the level of inventory in the feed tank dropped very close to the feed loop intake. This caused air to be entrained into the feed and is believed to have caused the flux drop seen in the last two recorded points. Because coffee contains electrolytes, conductivity would be expected to rise as the feed became more concentrated. This is seen in Fig.

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• Engineering & Computer Science (AREA)
• Water Supply & Treatment (AREA)
• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Separation Using Semi-Permeable Membranes (AREA)
• Tea And Coffee (AREA)
• Detergent Compositions (AREA)
• Non-Alcoholic Beverages (AREA)

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