ASYMMETRIC SUPPORTED MEMBRANE FOR DIRECT OSMOTIC CONCENTRATION

Abstract: There is disclosed an asymmetric, hydrophilic, membrane having thin surface layer and a porous support layer having an embedded mesh embedded therein, which membrane is suitable for use in direct osmotic concentration (DOC) but not in reverse osmosis (RO).

Description: ASYMMETRIC SUPPORTED MEMBRANE FOR DIRECT OSMOTIC CONCENTRATION TECHNICAL FIELD OF THE INVENTION The present invention provides an asymmetric, hydrophilic, membrane having thin surface layer and a porous support layer having an embedded mesh embedded therein, which membrane is suitable for use in direct osmotic concentration (DOC) but not in reverse osmosis (RO). BACKGROUND OF THE INVENTION Concentration of food products is a difficult task in that it is important to preserve the quality of the food product while removing as much water or solvent as possible to reduce transportation costs and increase stability. Therefore, it is important to prevent oxidation and avoid heating. In both reverse osmosis (RO) and direct osmotic concentration (DOC), membranes selectively allow small molecules to cross while blocking the transfer of larger molecules. Such selectivity is due to the properties of the polymer in the membrane. For polymers such as cellulose acetate and polyamide, the long intertwined molecules provide a network of small channels through the membrane that are a few angstroms (A) wide. These channels are large enough for small molecules, such as water or ethanol, to enter. However, larger molecules, such as sugars or colors, are blocked. In RO, water is forced through the membrane by high pressure. In DOC, water moves from one side of the membrane to the other due to diffusion. For both processes, the economics are highly dependent on the rate of water transfer per unit of membrane area (flux). Even though RO and DOC often use identical polymers, fabrication and design of economical membranes for the two processes differ. Differences in designs of the membranes arise due to different physical processes causing water transfer. In both processes, flux is controlled by pressure (RO) and osmotic concentration gradients (DOC). However, in RO pressure gradients are of primary importance. In DOC, flux is almost entirely due to osmosis. Flux in DOC can be quantified as: Fw = wherein Fw is water flux, kw is a constant for the flux through a particular membrane, P is pressure, π 1 is osmotic pressure, and the I and II superscripts refer to the solutions on each side of the membrane. Osmotic pressure is a measure of the tendency of water to diffuse through the membrane from a region of high water concentration to a region of lower water concentration. In general, a good approximation is: cRT, where c is molar concentration of non-water species in a solution, R is a gas constant, and T is absolute temperature. In RO, the value of kw is determined by molecular-scale hydrodynamic resistance to water flow through a polymer matrix. To minimize this resistance, RO systems use extremely thin membranes (< 0.01 mm) supported by an external backing (the backing provides the structural strength needed to withstand the high applied pressure). The pores in the external backing are much larger than the channels in the membrane, therefore the external backing contributes little to the flow resistance and does not significantly retard flux. In contrast, DOC kw values are primarily controlled by diffusion rates. Therefore, in DOC, the porous membrane external backing causes significant resistance and is the major impediment to flux. This is why a membrane designed for RO procedures provides a poor DOC membrane due to slow flux rates. RO flux is primarily dependent on the properties of the rejection layer (the thin layer of polymer on top of the porous backing), producers of RO membranes have found it advantageous to cast the membrane on top of a dense hydrophobic fabric. Such a secondary support gives the membrane the mechanical strength needed to withstand the high applied pressures. The advantages of this design in RO is the secondary support underlies all portions of the membrane, thus giving the maximum resistance to compaction and tearing caused by the high applied pressure, characteristic of RO. Such a membrane performs poorly, however, in DOC. A major reason for this is the secondary backing. The secondary backing makes the membrane thicker, which increases the resistance to diffusion and reduces flux in the absence of the high pressures characteristic of RO. In addition, the hydrophobic nature of the backing inhibits wetting, resulting in vapor-locked pores and further reduction in flux. An unbacked membrane, however, lacks the structural strength to operate for extended periods of time, even in the lower pressure environment of DOC. Lateral forces caused by pressure or fluid shear on an unbacked membrane tend to be concentrated in the thin rejection layer. Because of the fragility of this layer, the membrane is extremely prone to stretching and ripping. The requirements of DOC require: 1) Most of the membrane in an economically viable cell design needs to have unobstructed fluid contact on both sides. 2) To prevent flapping, one fluid needs to be at a higher pressure than the other. 3) To prevent fouling, the fluid needs to have a cross-flow velocity in excess of 0.1 m/s. Requirement #1 results in a cell design in which the membrane is suspended between supports. In the suspended region, the shear and pressure forces must be withstood by the tensile strength of the membrane. The relation for pressure-induced tension in this region shows that the distance between supports and the operating pressure are of primary importance in controlling membrane failure. In addition, failure can occur due to shear forces. Shear is caused by fluid viscosity and it tends to pull the membrane from the inlet of the cell toward the outlet. The important parameters contributing to shear-induced tension are: (1) fluid viscosity; (2) fluid velocity; and (3) cell dimensions. The cell dimensions are the important parameter rather than the distance between the membrane supports because the membrane can slide freely over the smooth supports. The only thing stopping the membrane's migration toward the cell outlet is the mechanical strength of the membrane. Therefore, there is a need in the art for DOC membranes useful for, for example, food processing activities, that combine high flux rates, low fouling and utility for solutions that are highly viscous and have high amounts of suspended solids. The present invention provides an asymmetric membrane designed primarily for DOC applications. SUMMARY OF THE INVENTION The present invention provides an asymmetric supported direct osmotic concentration (DOC) membrane, comprising a thin surface layer of polymeric material, and a porous support layer of polymeric material, wherein the porous support layer further comprises a flexible mesh material of woven or non-woven fibers having an open structure having a plurality of open holes having a distance between fiber centers of between about 0.5 mm to about 10 mm and having at least a 50% void area. Preferably, the present invention substitutes an open weave cloth backing for a tight weave sail-cloth backing that is common in RO membranes, to change an RO membrane into a lower pressure DOC membrane. The net result is a much faster flux membrane having lower tensile strength better suited for a DOC environment. Preferably, the polymeric material is cellulosic. Preferably, the thickness of the thin surface layer is from about 5 μm to about 20 μm. Preferably, the polymeric porous support layer is cast to have a thickness above the flexible mesh of about 35 to about 300 μm. The flexible mesh material generally has a thickness of from about 0.15 to about 1 mm. The present invention further provides a method for casting a DOC membrane, comprising: (a) providing a flexible mesh backing composed of woven or non- woven fibers, having an open structure having a plurality of open holes having a distance between fiber centers of between about 0.5 mm to about 10 mm and having at least a 50% void area cast onto a surface a spinning drum partially immersed in water; (b) casting a thin film of a liquid polymeric material onto the flexible mesh backing onto a surface of a spinning drum partially submerged in a tank of water, wherein the casting occurs above a water line, to form a DOC membrane on the surface of the spinning drum, and wherein the drum rotates at a speed to cast of from about 15 to about 150 linear meters per hour; and (c) drying and removing the finished DOC membrane from the surface of the spinning drum. Preferably, the polymeric material is a cellulosic material. Preferably, the cellulosic polymeric material is selected from the group consisting of cellulose acetate, cellulose diacetate, cellulose triacetate, regenerated cellulose, cellulose butyrate, cellulose proprionate, and combinations thereof. Most preferably, the flexible mesh backing is first saturated with a solvent in which the liquid polymeric material is soluble. Preferably, the solvent is selected from the group consisting of ethanol, methanol, acetone, isopropyl alcohol, other alcohols having no more than 4 carbon atoms, and combinations thereof. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a diagram of the membrane casting process by an immersion precipitation procedure, showing the product membrane (G) completed after being cast on a rotating casting drum (D) and annealed under water on a series of rollers (F). The membrane begins with a fabric backing on a fabric support roll (A) that is wetted in solvent (B) and eventually applied to the rotating casting drum. Membrane resin (C) is applied to the fabric support in a uniform layer. Figure 2 illustrates a schematic cross section of the inventive DOC membrane. The asymmetric membrane has a thin surface layer and a porous support layer having fabric support embedded therein. PETAHJED DESCRIPTION OF THE INVENTION The present invention provides an asymmetric supported direct osmotic concentration (DOC) membrane, comprising a thin surface layer of polymeric material, and a porous support layer of polymeric material, wherein the porous support layer further comprises a flexible mesh material of woven or non-woven fibers having an open structure having a plurality of open holes having a distance between fiber centers of between about 0.5 mm to about 10 mm and having at least a 50% void area. Preferably, the present invention substitutes an open weave cloth backing for a tight weave sail-cloth backing that is common in RO membranes, to change an RO membrane into a lower pressure DOC membrane. The net result is a much faster flux membrane having lower tensile strength better suited for a DOC environment. Preferably, the polymeric material is cellulosic. Preferably, the thickness of the thin surface layer is from about 5 μm to about 20 μm. Preferably, the polymeric porous support layer is cast to have a thickness above the flexible mesh of about 35 to about 300 μm. The flexible mesh material generally has a thickness of from about 0.15 to about 1 mm. The present invention provides a DOC membrane having a flexible mesh backing, and composed of cellulose polymeric material. The membrane is imbedded in a strong, flexible mesh. As a result, there are two radically different membrane regions. The part of the membrane between the mesh fibers has flux identical to that in an unbacked membrane, while the part contacting the fibers has flux similar to sailcloth backed membrane. Tests with a 7.0 mil, inventive DOC membrane cast on a polyethylene mesh (with 50% void area) show fluxes between those of the sailcloth backed RO membrane and unbacked membrane. For water versus 70 brix corn syrup at 20 °C, the flux has been measured to be 8 LMH (i.e., liters of water removed through one square meter of membrane each hour). The present invention further provides a method for casting a DOC membrane, comprising. (a) providing a flexible mesh backing composed of woven or non- woven fibers, having an open structure having a plurality of open holes having a distance between fiber centers of between about 0.5 mm to about 10 mm and having at least a 50% void area cast onto a surface a spinning drum partially immersed in water; (b) casting a thin film of a liquid polymeric material onto the flexible mesh backing onto a surface of a spinning drum partially submerged in a tank of water, wherein the casting occurs above a water line, to form a DOC membrane on the surface of the spinning drum, and wherein the drum rotates at a speed to cast of from about 15 to about 150 linear meters per hour; and (c) drying and removing the finished DOC membrane from the surface of the spinning drum. Preferably, the polymeric material is a cellulosic material. Preferably, the cellulosic polymeric material is selected from the group consisting of cellulose acetate, cellulose diacetate, cellulose triacetate, regenerated cellulose, cellulose butyrate, cellulose proprionate, and combinations thereof. Most preferably, the flexible mesh backing is first saturated with a solvent in which the liquid polymeric material is soluble. Preferably the solvent is selected from the group consisting of ethanol, methanol, acetone, isopropyl alcohol, other alcohols having no more than 4 carbon atoms, and combinations thereof. The mesh material can be any fiber-based material. Examples of mesh materials include nylon, polyester, polyethylene, polypropylene, cotton, silk and combinations and blends thereof. The mesh must be woven of intertwined such that it provides support for the membrane. Preferably, the mesh has a tensile strength of from about 10 to about 20 N/mm. The present invention provides an improved modification to RO membranes by substituting an open-weave fibrous backing material in place of a closed weave material common to RO membranes. This results in an inventive DOC membrane with much higher flux rates needed for DOC applications but unable to withstand the higher pressures of an RO process. Therefore, the inventive DOC membranes are not useful for RO applications or any pressures above 690 kPa, and preferably not above 170 kPa. DOC membranes useful for the present invention are made by an immersion precipitation process shown in Figure 1. In this process, the membrane is formed by spreading a thin layer of a membrane casting resin (35 to 300 microns) over a surface. Preferably the surface is the flexible mesh material. The resin consists of the desired membrane polymer or mixture of polymers dissolved in a solvent or mixture of solvents. For example, one resin, cellulose triacetate, is shown in Example 1 below. A thin resin layer is immersed in a water bath. Contact with the water causes precipitation of the resin in solvent to form a very thin layer (about 7 microns) of solid polymer at the water-resin interface in a very short time (milliseconds). The formation of this thin layer impedes water penetration to the remainder of the resin, such that precipitation of the remaining polymer occurs over the next 60 seconds. This precipitation rate is slow enough such that the membrane polymer tends to agglomerate before precipitation, forming a bubbly, porous structure underneath an unbroken surface layer. Preferably, the drum rotates at a speed to cast of from about 15 to about 150 linear meters per hour. Membrane selectively is due to the properties of the thin surface layer (Figure 2). Use of a cellulosic polymer for DOC provides a highly hydrophobic membrane that is able to absorb water into its "plastic-like" matrix. In a DOC process, for example, a salt brine is allowed to diffuse through the bubbly porous region from the back of the membrane. If the osmotic strength of the brine is high enough, the brine can dehydrate the polymer in the surface layer. The surface layer of the membrane then rehydrates itself by drawing water from the product in contact with the outer skin surface layer, thus dewatering the product. Larger molecules, such as flavor agents or sugars, are unable to pass through the plastic-like matrix of the thin surface layer. In the membrane, the porous, bubbly layer provides mechanical strength to support the extremely thin surface layer. Further tensile strength and mechanical support is provided by having a fabric support embedded within the resin of the porous layer, but within the porous layer accomplished by having the resin immersed in water. The inventive DOC membranes are cast in a continuous process by casting the resin on a rotating drum partially immersed in water. This process is illustrated in Figure 1. Example 1 This example illustrates the preparation of a DOC-type cellulosic membrane. Acetone (23.7 kg) and p-dioxane (44.6 kg) were placed in a 30 gallon industrial mixer. The blade was started and set to rotate slowly. Cellulose triacetate (type 435-755, 13.0 kg) resin was added slowly through a port on top of the mixer. The ingredients were allowed to mix for 2.5 hr that allowed the polymer to dissolve completely. A solvent solution was formed with methanol (7.0 kg) solution and by added maleic acid (3.4 kg) and lactic acid (3.4 kg), and slowly added to the mixer. Mixing was continued for another 30 min. The solution was filtered through a 5 micron polypropylene canister-type filter and into a holding vat where the filtered solution was allowed to stand overnight and deaerate. The resin solution was again filtered through a 5 micron filter and then cast on a support cloth, consisting of a calendared, 150 μm polyester net using a casting knife with a 10 mil opening and a casting speed of 3 feet per min (0.9 m per min). The solution cloth composite entered an evaporation chamber where dry air was swept across the surface of the solution for 20 sec. The membrane entered a coagulation bath of water at 18 °C for 7 min. The membrane then progressed through a hot water bath where it was annealed at 40.5 °C for 12 min. A DOC membrane, according to the present invention was formed. Example 2 This example illustrates properties of the DOC membrane formed in example . Tests have been conducted with cellulose triacetate (CTA) membranes to determine the effect of the secondary backing on DOC flux rates. Asymmetric, 3.5 mil membranes with and without a dense sailcloth backing were tested in a DOC cell. In both experiments, water was introduced to one side of the membrane and 70 brix high fructose corn syrup was introduced to the other. At 21 °C, the flux with the unbacked membrane was 14 liters/hour-m 2 (LMH) while the flux with the backed membrane was 4 LMH.

Claims: We claim: 1. An asymmetric supported direct osmotic concentration (DOC) membrane, comprising a thin surface layer of polymeric material, and a porous support layer of polymeric material, wherein the porous support layer further comprises a flexible mesh material of woven or non- woven fibers having an open structure having a plurality of open holes having a distance between fiber centers of between about 0.5 mm to about 10 mm and having at least a 50% void area. 2. The DOC membrane of claim 1 wherein the membrane material is polymeric. 3. The DOC membrane of claim 2 wherein the polymeric material is a cellulosic material. 4. The DOC membrane of claim 3 wherein the cellulosic polymeric material is selected from the group consisting of cellulose acetate, cellulose diacetate, cellulose triacetate, regenerated cellulose, cellulose butyrate, cellulose proprionate, and combinations thereof. 5. The DOC membrane of claim 1 wherein the thickness of the thin surface layer is from about 5 μm to about 20 μm. 6. The DOC membrane of claim 1 wherein the polymeric porous support layer is cast to have a thickness above the flexible mesh of about 35 to about 300 μm. 7. The DOC membrane of claim 1 wherein the flexible mesh material generally has a thickness of from about 0.15 to about 1 mm. 8. A method for casting a DOC membrane, comprising: (a) providing a flexible mesh backing composed of woven or non- woven fibers, having an open structure having a plurality of open holes having a distance between fiber centers of between about 0.5 mm to about 10 mm and having at least a 50% void area cast onto a surface a spinning drum partially immersed in water; (b) casting a thin film of a liquid polymeric material onto the flexible mesh backing onto a surface of a spinning drum partially submerged in a tank of water, wherein the casting occurs above a water line, to form a DOC membrane on the surface of the spinning drum, and wherein the drum rotates at a speed to cast of from about 15 to about 150 linear meters per hour; and (c) drying and removing the finished DOC membrane from the surface of the spinning drum. c , The method of claim 8 wherein the polymeric material is a cellulosic material. 10. The method of claim 9 wherein the cellulosic polymeric material is selected from the group consisting of cellulose acetate, cellulose diacetate, cellulose triacetate, regenerated cellulose, cellulose butyrate, cellulose proprionate, and combinations thereof. 11. The method of claim 8 wherein the flexible mesh backing is first saturated with an organic solvent in which the liquid polymeric material is soluble. 12. The method of claim 11 wherein the solvent is selected from the group consisting of ethanol, methanol, acetone, isopropyl alcohol, other alcohol's having no more than 4 carbon atoms, and combinations thereof.

Inventors: BEAUDRY EDWARD G, HERRON JOHN R, BEAUDRY, EDWARD, G., HERRON, JOHN, R.
Applicants: OSMOTEK INC, OSMOTEK, INC.
Application year: 1997
Main classification: B01D63
IPC codes: B01D63, B01D69, B01D71, B32B5