WO1997033681A1 - Membrane asymetrique sur support destinee a la concentration osmotique directe - Google Patents

Membrane asymetrique sur support destinee a la concentration osmotique directe Download PDF

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Publication number
WO1997033681A1
WO1997033681A1 PCT/US1997/003574 US9703574W WO9733681A1 WO 1997033681 A1 WO1997033681 A1 WO 1997033681A1 US 9703574 W US9703574 W US 9703574W WO 9733681 A1 WO9733681 A1 WO 9733681A1
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WO
WIPO (PCT)
Prior art keywords
membrane
doc
cellulose
polymeric material
flexible mesh
Prior art date
Application number
PCT/US1997/003574
Other languages
English (en)
Inventor
Edward G. Beaudry
John R. Herron
Original Assignee
Osmotek, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Osmotek, Inc. filed Critical Osmotek, Inc.
Priority to EP97916730A priority Critical patent/EP0888171A4/fr
Priority to JP9532701A priority patent/JP2000506439A/ja
Priority to AU25275/97A priority patent/AU724712B2/en
Publication of WO1997033681A1 publication Critical patent/WO1997033681A1/fr

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/10—Supported membranes; Membrane supports
    • B01D69/107—Organic support material
    • B01D69/1071—Woven, non-woven or net mesh
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/10—Supported membranes; Membrane supports

Definitions

  • 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).
  • DOC direct osmotic concentration
  • RO reverse osmosis
  • 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.
  • RO reverse osmosis
  • DOC direct osmotic concentration
  • 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.
  • 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.
  • RO water is forced through the membrane by high pressure.
  • DOC water moves from one side of the membrane to the other due to diffusion.
  • the economics are highly dependent on the rate of water transfer per unit of membrane area (flux).
  • 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.
  • flux is controlled by pressure (RO) and osmotic concentration gradients (DOC).
  • RO pressure gradients are of primary importance.
  • DOC flux is almost entirely due to osmosis.
  • 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.
  • 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.
  • cRT molar concentration of non-water species in a solution
  • R is a gas constant
  • T absolute temperature.
  • 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.
  • 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.
  • 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.
  • the hydrophobic nature of the backing inhibits wetting, resulting in vapor-locked pores and further reduction in flux.
  • An unbacked membrane 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the polymeric material is cellulosic.
  • the thickness of the thin surface layer is from about 5 ⁇ m to about 20 ⁇ m.
  • the polymeric porous support layer is cast to have a thickness above the flexible mesh of about 35 to about 300 ⁇ m.
  • 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;
  • the polymeric material is a cellulosic material.
  • 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.
  • the flexible mesh backing is first saturated with a solvent in which the liquid polymeric material is soluble.
  • 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.
  • 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.
  • FIG. 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.
  • 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.
  • DOC direct osmotic concentration
  • 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.
  • the polymeric material is cellulosic.
  • the thickness of the thin surface layer is from about 5 ⁇ m to about 20 ⁇ m.
  • the polymeric porous support layer is cast to have a thickness above the flexible mesh of about 35 to about 300 ⁇ m.
  • 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.
  • 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.
  • the polymeric material is a cellulosic material.
  • 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.
  • the flexible mesh backing is first saturated with a solvent in which the liquid polymeric material is soluble.
  • 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.
  • 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.
  • 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.
  • the membrane is formed by spreading a thin layer of a membrane casting resin (35 to 300 microns) over a surface.
  • 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.
  • 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).
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 ⁇ m 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.
  • 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.
  • CTA cellulose triacetate
  • Asymmetric, 3.5 mil membranes with and without a dense sailcloth backing were tested in a DOC cell.
  • water was introduced to one side of the membrane and 70 brix high fructose corn syrup was introduced to the other.
  • the flux with the unbacked membrane was 14 liters/hour-m 2 (LMH) while the flux with the backed membrane was 4 LMH.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Separation Using Semi-Permeable Membranes (AREA)
  • Materials For Medical Uses (AREA)

Abstract

Membrane hydrophile asymétrique, comprenant une couche superficielle mince et une couche de support poreuse dans laquelle est incorporée un élément à mailles, ladite membrane se prêtant à l'utilisation pour la concentration osmotique directe (COD) mais non pour l'osmose inverse (OI).
PCT/US1997/003574 1996-03-11 1997-03-11 Membrane asymetrique sur support destinee a la concentration osmotique directe WO1997033681A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
EP97916730A EP0888171A4 (fr) 1996-03-11 1997-03-11 Membrane asymetrique sur support destinee a la concentration osmotique directe
JP9532701A JP2000506439A (ja) 1996-03-11 1997-03-11 直接浸透濃縮化のための非対称の支持された膜
AU25275/97A AU724712B2 (en) 1996-03-11 1997-03-11 Asymmetric supported membrane for direct osmotic concentration

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US61387896A 1996-03-11 1996-03-11
US08/613,878 1996-03-11

Publications (1)

Publication Number Publication Date
WO1997033681A1 true WO1997033681A1 (fr) 1997-09-18

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Family Applications (1)

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PCT/US1997/003574 WO1997033681A1 (fr) 1996-03-11 1997-03-11 Membrane asymetrique sur support destinee a la concentration osmotique directe

Country Status (5)

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EP (1) EP0888171A4 (fr)
JP (1) JP2000506439A (fr)
AU (1) AU724712B2 (fr)
CA (1) CA2248256A1 (fr)
WO (1) WO1997033681A1 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9186627B2 (en) 2009-08-24 2015-11-17 Oasys Water, Inc. Thin film composite heat exchangers
JP2016147260A (ja) * 2009-08-24 2016-08-18 オアシス ウォーター，インコーポレーテッド 正浸透膜

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6299777B1 (en) * 1999-08-17 2001-10-09 Cms Technology Holdings, Inc. Osmotic distillation process
KR101892108B1 (ko) * 2010-09-30 2018-08-27 포리페라 인코포레이티드 정삼투용 박막 복합 멤브레인 및 이의 제조 방법

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3876738A (en) * 1973-07-18 1975-04-08 Amf Inc Process for producing microporous films and products
US4707265A (en) * 1981-12-18 1987-11-17 Cuno Incorporated Reinforced microporous membrane
US5275725A (en) * 1990-11-30 1994-01-04 Daicel Chemical Industries, Ltd. Flat separation membrane leaf and rotary separation apparatus containing flat membranes

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2860833B2 (ja) * 1990-11-28 1999-02-24 日東電工株式会社 分離膜
JPH06346A (ja) * 1992-06-17 1994-01-11 Nitto Denko Corp 複合半透膜及びスパイラル型膜分離装置
US5522991A (en) * 1994-07-20 1996-06-04 Millipore Investment Holdings Limited Cellulosic ultrafiltration membrane

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3876738A (en) * 1973-07-18 1975-04-08 Amf Inc Process for producing microporous films and products
US4707265A (en) * 1981-12-18 1987-11-17 Cuno Incorporated Reinforced microporous membrane
US5275725A (en) * 1990-11-30 1994-01-04 Daicel Chemical Industries, Ltd. Flat separation membrane leaf and rotary separation apparatus containing flat membranes

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9186627B2 (en) 2009-08-24 2015-11-17 Oasys Water, Inc. Thin film composite heat exchangers
JP2016147260A (ja) * 2009-08-24 2016-08-18 オアシス ウォーター，インコーポレーテッド 正浸透膜
US9463422B2 (en) 2009-08-24 2016-10-11 Oasys Water, Inc. Forward osmosis membranes

Also Published As

Publication number Publication date
EP0888171A1 (fr) 1999-01-07
CA2248256A1 (fr) 1997-09-18
JP2000506439A (ja) 2000-05-30
AU724712B2 (en) 2000-09-28
AU2527597A (en) 1997-10-01
EP0888171A4 (fr) 2000-01-05

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