CA2248256A1 - Asymmetric supported membrane for direct osmotic concentration - Google Patents
Asymmetric supported membrane for direct osmotic concentration Download PDFInfo
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- CA2248256A1 CA2248256A1 CA002248256A CA2248256A CA2248256A1 CA 2248256 A1 CA2248256 A1 CA 2248256A1 CA 002248256 A CA002248256 A CA 002248256A CA 2248256 A CA2248256 A CA 2248256A CA 2248256 A1 CA2248256 A1 CA 2248256A1
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- 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
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- 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
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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
wo 97/33681 PCT/USg7/03574 ASYMMETRIC SUPPORTED MEMBRANE FOR DIRECT OSMOTIC
CONCENTRATION
5 TECEINICAL ~IELD OF TEE INVENTION
The present invention provides an as~ .e~lic, hydlopllilic, me,nb.~e having thinsurface layer and a porous support layer having an emheclded mesh embedded therein, which ,.,e...b.~le is suitable for use in direct osmotic col c~ .lion (DOC) but not in reverse osmosis (RO).
BACKGROUND OF T~IE INVENTION
Concenllalion 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 l.~r.spo~l~lion costs and increase stability. Th~,~efore, it is important to prevent oxidation and 15 avoid h~q~ing In both reverse osmosis (RO) and direct osmotic cQnC~ontration (DOC), ~--e...b.~ es selectively allow small molecules to cross while bloc~in~ the l-~1sÇer of larger - ~ mo1ecl)les Such selectivity is due to the plope-lies ofthe polymer in the .--t;--.b.~le. For polymers such as c~ lose acetate and polyamide, the long intertwined molecules provide a r.~,lwo.k of small rhqnn~l~ through the ...~,mb.~1e that a re a few ang~-o---s (A) wide. These channels are large enough for small molecul~r., such as water or ethqnol, to enter. However, larger mo'eP-lec such as sugars or colors, are blocked. In RO, water is forced through the .nemb.~le by high pressure. In DOC, water moves from one side of the meml~-~ e to the other due to diffusion. For both proce~ses, the ecol om;cs are highly dependent on the rate of water t~ srer per unit of l,.e.nbl ~e area (fiux).
Even though RO and DOC often use identi~l polymers, fabrication and design of economi~,q-l l--e.-lblal1es for the two processes differ. Di~l el1ces in designs of the 111~ 1 ~les arise due to di~le.l~ physical processes causing water ~-~1srer. In both processes, flux is controlled by pressure (RO) and osmotic collcenl~alion gradients (DOC). However, in RO
pres~ul e gradients are of primary importance. In DOC, flux is almost entirely due to osmosis.
Flux in DOC can be quantified as: Fw = kW[(pI-7~I)-(pII-~II)]~ wh~l eh1 Fw is water flux, kw is a cQll~lA~I for the flux through a particular membrane, P is pressure, ~c~ is osmotic pressure, and the I and II superscripts refer to the solutions on each side of the Illelllb- ~le.
Osmotic pressure is a measure of the tendency of water to diffuse through the ~llt;lllbl~ule from a region of high water concentration to a region of lower water concentration.
3s In general, a good appr<: Ai.llation is: cRT, where c is molar concentration of non-water species in a solution, R is a gas constant, and T is absolute telll~)e~al~lle. In RO, the value of kw is determined by molecular-scale hydrodynamic resi~t~nce to water flow through a polymer matrix. To n-inimi7e this re~i~t~nce, RO systems use eA~-~lnely thin membranes (< 0.01 mm) supported by an external backing (the ba~l~ing provides the structural ~l,englh needed to with.~t-qnd the high applied pressure). The pores in the external bDcl~ing are much larger than the c~qnnçlc in the Inem~.~ne, lhelerore the external ba~l~ine contributes little to the flow rçS;~ ce and does not ci~ifi~As~ntly retard flux. In contrast, DOC kw values are primarily controlled by diffusion rates. Therefore, in DOC, the porous ll.el..l).alle external b~-c~ing causes significant rçsi~tqnce and is the major impediment to flux. This is why a n~c.llblane desi~ned for RO procedures provides a poor DOC membrane due to slow flux rates.
RO flux is primarily dependent on the plop~llies ofthe rejection layer (the thin layer of polymer on top ofthe porous b~.e~in~), producers of RO .llc,llbl~ es have found it advantageous to cast the men.b.~e on top of a dense h~drophobic fabric. Such a secondary support gives the ",en.l"~le the ~erllqnir.ql sLI~ h needed to w;~ the high applied pressures. The advantages of this design in RO is the second&~ support underlies all portions ofthe me."b,~e, thus giving the nla~ un, ree C~ e to co~ ction and tearing caused by the high applied pressure, characteristic of RO. Such a ~c~bl~ e pcl~lllls poorly, however, in DOC. A major reason for this is the secQ~ y bPclring The scconda.~ bncl~in~ makes the I"e."b,~ne thicker, which hlcleascs the recist~ n~e to ~iffil-~ion and reduces flux in the -bsence ofthe high l~ressLIl~,s characteristic of RO. In ~d~ition the hydlophobic nature ofthe baç~in~
inhibits ~._lli"g, res~lti~ in vapor-locked pores and further reduction in flux.An UDkACL'edlll~ ale, however, lacks the structural ,Il~.lglh to operate for PYt~nded periods oftime, even in the lower pres~lle envi~o"~n~ of DOC. Lateral forces caused by pressure or fluid shear on an ~ a--l ~ membrane tend to be col-ce~ dled in the thin rejectioll layer. Because of the fragility of this layer, the ",wnb,ane is c.~lle.llely prone to ~ ,LCLII-g and ripping. The requirements of DOC require: I) Most ofthe ,llelllblane in an economically viable cell design needs to have unobstructed fluid contact on both sides. 2) To prevent napl)ing, one fluid needs to be at a higher pres;~uc; than the other. 3) To prevent fouling, the fluid needs to have a cross-flow velocity in excess of 0.1 m/s. Require,nenl #1 results in a cell design in which the ,llembl~ne is suspentled bel~,en s. ppolls. In the s~ ~pended region, the shear and pless~t; forces must be withstood by the tensile ;,llenglh of the m~.lll).~e. The relation for pr~,ss. ,e-indllced tension in this region shows that the ~ nce ~ . ~n supports and the ope,a~ g pressure are of plill~ importance in controlling Illelll~lane failure.
In addition, failure can occur due to shear forces. Shear is caused by fluid viscosity and it tends to pull the m.,lllbl~e from the inlet of the cell toward the outlet. The important pal,ul~ s contributing to shear-inrll~ced tension are: (1) fluid viscosity; (2) fluid velocity; and (3) cell dimensions. The cell d;.,.Pn~;ons are the illlpoll~ull palanltler rather than the rl-~t~nce bel~c~,n the nlelllblal e supports because the Ill~;lllblane can slide freely over the smooth supports. The only thing stopping the mell,b.ane's migration toward the cell outlet is the mechAIlical strength of the In.,ll~l~l ane. Thc. ~;rO~ e, there is a need in the art for DOC
,s useful for, for PY~mrle, food processing activities, that cOIl~ lle high flux rates, low fouling and utility for solutions that are highly viscous and have high amounts of W O 97/33681 PCTrUS97/03574 suspended solids. The present invention provides an asymmetric Illemblane designPd plillla.;ly for DOC applications.
SUM~IARY OF THE INVENTION
s The present invention provides an a~ ,t,ic supported direct osmotic cQrlcpntration (DOC) ..I~ e, comprising a thin surface layer of polymeric material, and a porous support layer of polymeric material, whe-eill the porous support layer further col.l~lises a flexible mesh material of woven or non-woven fibers having an open structure having a plurality of open holes having a ~ t~qnce between fiber centers of between about 0.5 mm to about 10 mrn and 0 having at least a 50~/0 void area. Preferably, the present invention s~;lules an open weave doth backing for a tight weave sail-cloth b ~cL iny that is CQ~ 01) in RO l"cmb- ~les, to change an RO l,-e-,-bl ~e into a lower pressure DOC lll~,-lll,l aile~ The net result is a much faster flux m~ e having lower tensile ;,ll -,n~ better suited for a DOC en~iro~ Preferably, the polymeric material is ce~ osic. Preferably, the thickness of the thin surface layer is from about 5 ~m to about 20 ~lm. Preferably, the polymeric porous support layer is cast to have a L .~ess above the fiexible mesh of about 35 to about 300 llm. 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 ",~,n,l~,~ne, C~
(a) providing a flexible mesh bacL ing col"~osed of woven or non-woven fibers, having an open structure having a plurality of open holes having a distqnce bel~,el fiber centers of b~ ~n about 0.5 mm to about 10 mm and having at least a 50% void area cast onto a - surface a ~~ ng drum partially i".. l,e,sed in water;
(b) casting a thin film of a liquid polymeric material onto the flexible mesh ~a~l ing 25 onto a surface of a sp;l-nil~ drum partially subl-,e,ged in a tank of water, v~L~,I ~1 the casting occurs above a water line, to form a DOC n~ "b~ e on the surface ofthe spi~ ¢ drum, and Wh~leill the drum rotates at a speed to cast offrom about 15 to about 150 linear meters per hour; and (c) drying and removing the finished DOC me~ ~ from the surface of the spinning 30 drum.
Preferably, the polymeric material is a cellulosic material. PleÇe~ably, the cPll--losic polymeric material is sPlected from the group cQns;s~ 8 of celh)lQse acetate, cP~ lose diq-cetqte, cplhllose triacetvte, lege~.elaled cellulose, ce~l~llose butyrate, cellulose proprionate, and co,..l.il-~l;on~ thereof. Most pleré-~bly, the flexible mesh bacl~ing is first saturated with a 35 solvent in which the liquid polymeric material is soluble. Plerélably, the solvent is sele~ted from the group col-~ g of ethq-nol methqllol, lce:onpJ isoprop~l alcohol, other alcohols having no more than 4 carbon atoms, and combin-q-tionc thereof.
W O 97/33681 PCT~US97/03574 BRIEF DESCRIPTION OF TEIE DRAWINGS
Figure 1 illustrates a diagram of the Illel~b~ ane casting process by an ;~ ;on preci~ ;on procedure, showing the product ",e",l"~e (G) co~ lPted after being cast on a rolalh~g casting drum (I)) and Anne~led under water on a series of rollers (F). The mGIlll~ G
5 begins with a fabric b ~-t ine on a fabric support roll (A) that is wetted in solvent (B) and eventually applied to the rolali"~, casting drum. Mc~ e resin (C) is applied to the fabric support in a unir~"" layer.
Figure 2 illustrates a sçhe~ l ;c cross section of the inventive DOC ,,,~,.,II,l ~ne. The a~". "el,ic me."l"~e has a thin surface layer and a porous support layer having fabric 10 support f-.mhedded therein.
DETAILED DESCRIPTION OF T~IE INVENTION
The present invention provides an ~,~.nQIel.ic ~uppolled direct osmotic collce ~l~alion (DOC) me.llblane7 co",p,isi"g a thin surface layer of polymeric material, and a porous support layer of polymeric material, ~hcrcin the porous support layer further CO"I~IiSCS a flexible mesh material of woven or non-woven fibers having an open structure having a plurality of open holes having a ~ nce be~ ,n fiber centers of bcl~.~n about 0.5 mm to about 10 mm and having at least a 50% void area. Preferably, the present invention ~ es an open weave cloth ba-L-in~ for a tight weave sail-cloth ba~L ;.~ that is co.. ol- in RO "lem~lanes, to change 20 an RO l"~ e into a lower p~ JrG DOC IllGlllbll~e. The net result is a much faster flux ,llel~llJla~e having lower tensile ~I,englh better suited for a DOC environment. Prefe,~ly, the polymeric material is c~ los;c PIGrGlably, the th;cL-ness ofthe thin surface layer is from - about 5 ~lm to about 20 ~lm. Pl erGI ~bly, the polymeric porous support layer is cast to have a Ih-~L ~ C above the flexible mesh of about 35 to about 300 llm. The flexible mesh material generally has a thicL ness of from about 0.15 to about 1 mm.
The present invention provides a DOC me.~bl~u~e having a flexible mesh b~ct ;.~9 and composed of cellulose polymeric material. The ~ ."blane is imhedded in a strong, flexible mesh. As a result, there are two radically di~G,enl me,l,bl~ e regions. The part ofthe l,lelllblane bGI~en the mesh fibers has flux ident~ to that in an lmbact-ed n,~ml,.~ le, while the part cont~cting the fibers has flux similar to sailcloth backed ml ,.lbrane. Tests with a 7.0 mil, inventive DOC mem~,~ne cast on a polyethylene mesh (with 50% void area) show fluxes between those of the SPilrloth backed RO ",e",bl ~u~e and l-nbac~ed J~cmbl ~e. 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 Illellll~ e each hour).
3s The present invention further provides a method for casting a DOC men~bl~le, comprismg:
(a) providing a flexible mesh bac~in~ composed of woven or non-woven fibers, having an open structure having a plurality of open holes having a ~ t~nce between fiber centers of W O 97/33681 PCTrUS97/03574 between about 0. 5 mm to about l 0 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 sul,n.e. ged in a tank of water, wherein the casting occurs above a water line, to form a DOC ~.-e,nb.i.ne on the surface of the spinning drum, and wl,e,~i" the drum rotates at a speed to cast of from about l S to about l 50 linear meters per hour; and (c) drying and removing the fini~hed DOC ,.,cl.lb.~,le from the surface of the S~ g drum.
0 Plerelhbly, the polymeric material is a cellulosic material. Preferably, the ce~ 1Qs;r polymeric material is selected from the group COI.~ of cP1ll~lose acetate, cellutcse et~qte, cellulose triq-cetqte, legGllGI~led cçllulos~p~ ce~ lose butyrate, cellulose pr~,p,iollale, and CO~I1J; ~;on~ thereof. Most preferably, the flexible mesh ~ ine is first saturated with a solvent in which the liquid polymeric material is soluble. Preferably the solvent is se1ectP~d fromthegroupconci~tingofethq-~ol 1~ 1.anol ac~olle,isop,o~"~lq-lcQhQl,otherqlcoh~
having no more than 4 carbon atoms, and co",bin~tions thereo~
The mesh material can be any fiber-based material. Examples of mesh materials include nylon, polyester, polyethylene, pol~"opylene, cotton, silk and cG~ ;ons and blends thereof. The mesh must be woven of intertwined such that it provides support for the ~ b~ule. Fle~bly, the mesh has a tensile sllel~lh of from about l0 to about 20 N/mm.
The present invention provides an improved mollific~qtion to RO n.en~ es by sul)~ g an open-weave fibrous b~c~ing material in place of a closed weave material - common to RO ,ne"lbranes. This results in an inventive DOC me"ll,.~le with much higher flux rates needed for DOC appli~tions but unable to w;l ~ ntl the higher pressures of an RO
25 process. Th~r~rore, the inventive DOC lncn~b,~les are not useful for RO applic~;ol ~ or any pressures above 690 kPa, and preferably not above 170 I*a.
DOC IllGllll,l ~ es useful for the present invention are made by an illllllGI sion pree;~ ;on process shown in Figure 1. In this process, the ~G~ ~ e is formed by spreading a thin layer of a ",e",~ e casting resin (35 to 300 microns) over a surface. P~erelably the 30 surface is the flexible mesh material. The resin consists of the desired ~..em~l~ e polymer or mixture of polymers dissolved in a solvent or mixture of solvents For example one resin, ce11111Qse triac-et~qte is shown in Example l below. A thin resin layer is immersed in a water bath. Contact with the water causes preç;p:~l;on ofthe 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 35 (m~ ecQn~s) The forrnation ofthis thin layer impedes water pen~ l,alion to the rPmqindPr of the resin such that prec;~ ;on of the ,~ g polymer occurs over the next 60 seconds This prec ;~ ;on rate is slow enough such that the ~,.e..,l,l ~e polymer tends to agglomerate before p,Gc~ l;Qn~ îo",.n~g a bubbly, porous structure unde.,.~ll an unbroken surface layer .... ..
W O 97133681 PCTrUSg7!03574 Preferably, the drum rotates at a speed to cast of from about 15 to about 150 linear meters per hour.
Men,l,. ane selectively is due to the pl ope. lies of the thin surface layer (Figure 2). Use of a cellulosic polymer for DOC provides a highly hydrophobic me.l.bl ~1e that is able to s 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 lllel..b- ~e. If the osmotic ~ nglh of the brine is high enough, the brine can dehydrate the polymer in th surface layer. The surface layer ofthe ~ne.l~ e then l~,h~dlales itselfby dlav~r;ng water from the product in contact with the outer skin surface layer, thus dewatering the product. Larger 0 molePlles such as flavor agents or sugars, are unable to pass through the plastic-like matrix of the thin surface layer.
In the lllc.lll,l~ e, the porous, bubbly layer provides ~f'C'~3~ lleny~lh to support the e,~ ..ely thin surface layer. Further tensile sl-~,nglll and ,..r~ ic~l support is provided by having a fabric support emhedded within the resin of the porous layer, but within the porous 5 layer acco.lli)lished by having the resin ;.I--lle ~ed in water. The inventive DOC ll.e ,.l.l~es are cast in a continllous process by casting the resin on a rolalillg drum partially ill-...~..sed in water This process is illustrated in Figure 1.
E~amole 1 This CA r'e illustrates the plepd alion of a DOC-type cplh~losic l"~ ~ e. l'cetonP, (23.7 kg) and p-~ioy~ne (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 ofthe mixer. The ingredients were allowed to mix for 2.5 hr that allowed the polymer to dissolve co~ r' lPly. A solvent so!uti~n was formed with ,...,~
2s (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 contim~ed for another 30 min.
The solution was filtered through a 5 micron pol~,-o~"/lene cd~ster-type filter and into a holding vat where the filtered ssl-~tion was allowed to stand overnight and deaerate. The resin so~ 1tion was again filtered through a 5 micron filter and then cast on a support doth, 30 CQ~ il;~ of a c~lPn(lqred, 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 sol-ltion cloth composite entered an evaporation çl~ el where dry air was swept across the surface of the soh~tion for 20 sec The ~--eml,-~1e entered a coa~ tiQn bath of water at 18 ~C for 7 min. The ".emb-~e then progressed through a hot water bath where it was ~nn~led at 40.5 ~C for 12 min. A DOC
3s ..le...b.~le, acco-ding to the present invention was formed E~ample 2 This example illustrates propt;. lies of the DOC n~l..b.~1e formed in PYqmrle 1. Tests have been con-luGted with cellulose tri~cet~te (CTA) ...el--b-~nes to determine the effect of the secondary bac~in~ on DOC flux rates. Asymmetric, 3.5 mil me.l,~l~1es with and without a dense sailcloth backing were tested in a DOC cell. In both cA~el illlents, water was introduced to one side of the me.l~ e and 70 brix high fructose corn syrup was introduced to the other.
At 21 ~C, the flux with the unbacked Illt;lllb~-e was 14 liters/hour-m2 (Ll~) while the flux 5 with the backed me.llb~ e was 4 LMH.
CONCENTRATION
5 TECEINICAL ~IELD OF TEE INVENTION
The present invention provides an as~ .e~lic, hydlopllilic, me,nb.~e having thinsurface layer and a porous support layer having an emheclded mesh embedded therein, which ,.,e...b.~le is suitable for use in direct osmotic col c~ .lion (DOC) but not in reverse osmosis (RO).
BACKGROUND OF T~IE INVENTION
Concenllalion 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 l.~r.spo~l~lion costs and increase stability. Th~,~efore, it is important to prevent oxidation and 15 avoid h~q~ing In both reverse osmosis (RO) and direct osmotic cQnC~ontration (DOC), ~--e...b.~ es selectively allow small molecules to cross while bloc~in~ the l-~1sÇer of larger - ~ mo1ecl)les Such selectivity is due to the plope-lies ofthe polymer in the .--t;--.b.~le. For polymers such as c~ lose acetate and polyamide, the long intertwined molecules provide a r.~,lwo.k of small rhqnn~l~ through the ...~,mb.~1e that a re a few ang~-o---s (A) wide. These channels are large enough for small molecul~r., such as water or ethqnol, to enter. However, larger mo'eP-lec such as sugars or colors, are blocked. In RO, water is forced through the .nemb.~le by high pressure. In DOC, water moves from one side of the meml~-~ e to the other due to diffusion. For both proce~ses, the ecol om;cs are highly dependent on the rate of water t~ srer per unit of l,.e.nbl ~e area (fiux).
Even though RO and DOC often use identi~l polymers, fabrication and design of economi~,q-l l--e.-lblal1es for the two processes differ. Di~l el1ces in designs of the 111~ 1 ~les arise due to di~le.l~ physical processes causing water ~-~1srer. In both processes, flux is controlled by pressure (RO) and osmotic collcenl~alion gradients (DOC). However, in RO
pres~ul e gradients are of primary importance. In DOC, flux is almost entirely due to osmosis.
Flux in DOC can be quantified as: Fw = kW[(pI-7~I)-(pII-~II)]~ wh~l eh1 Fw is water flux, kw is a cQll~lA~I for the flux through a particular membrane, P is pressure, ~c~ is osmotic pressure, and the I and II superscripts refer to the solutions on each side of the Illelllb- ~le.
Osmotic pressure is a measure of the tendency of water to diffuse through the ~llt;lllbl~ule from a region of high water concentration to a region of lower water concentration.
3s In general, a good appr<: Ai.llation is: cRT, where c is molar concentration of non-water species in a solution, R is a gas constant, and T is absolute telll~)e~al~lle. In RO, the value of kw is determined by molecular-scale hydrodynamic resi~t~nce to water flow through a polymer matrix. To n-inimi7e this re~i~t~nce, RO systems use eA~-~lnely thin membranes (< 0.01 mm) supported by an external backing (the ba~l~ing provides the structural ~l,englh needed to with.~t-qnd the high applied pressure). The pores in the external bDcl~ing are much larger than the c~qnnçlc in the Inem~.~ne, lhelerore the external ba~l~ine contributes little to the flow rçS;~ ce and does not ci~ifi~As~ntly retard flux. In contrast, DOC kw values are primarily controlled by diffusion rates. Therefore, in DOC, the porous ll.el..l).alle external b~-c~ing causes significant rçsi~tqnce and is the major impediment to flux. This is why a n~c.llblane desi~ned for RO procedures provides a poor DOC membrane due to slow flux rates.
RO flux is primarily dependent on the plop~llies ofthe rejection layer (the thin layer of polymer on top ofthe porous b~.e~in~), producers of RO .llc,llbl~ es have found it advantageous to cast the men.b.~e on top of a dense h~drophobic fabric. Such a secondary support gives the ",en.l"~le the ~erllqnir.ql sLI~ h needed to w;~ the high applied pressures. The advantages of this design in RO is the second&~ support underlies all portions ofthe me."b,~e, thus giving the nla~ un, ree C~ e to co~ ction and tearing caused by the high applied pressure, characteristic of RO. Such a ~c~bl~ e pcl~lllls poorly, however, in DOC. A major reason for this is the secQ~ y bPclring The scconda.~ bncl~in~ makes the I"e."b,~ne thicker, which hlcleascs the recist~ n~e to ~iffil-~ion and reduces flux in the -bsence ofthe high l~ressLIl~,s characteristic of RO. In ~d~ition the hydlophobic nature ofthe baç~in~
inhibits ~._lli"g, res~lti~ in vapor-locked pores and further reduction in flux.An UDkACL'edlll~ ale, however, lacks the structural ,Il~.lglh to operate for PYt~nded periods oftime, even in the lower pres~lle envi~o"~n~ of DOC. Lateral forces caused by pressure or fluid shear on an ~ a--l ~ membrane tend to be col-ce~ dled in the thin rejectioll layer. Because of the fragility of this layer, the ",wnb,ane is c.~lle.llely prone to ~ ,LCLII-g and ripping. The requirements of DOC require: I) Most ofthe ,llelllblane in an economically viable cell design needs to have unobstructed fluid contact on both sides. 2) To prevent napl)ing, one fluid needs to be at a higher pres;~uc; than the other. 3) To prevent fouling, the fluid needs to have a cross-flow velocity in excess of 0.1 m/s. Require,nenl #1 results in a cell design in which the ,llembl~ne is suspentled bel~,en s. ppolls. In the s~ ~pended region, the shear and pless~t; forces must be withstood by the tensile ;,llenglh of the m~.lll).~e. The relation for pr~,ss. ,e-indllced tension in this region shows that the ~ nce ~ . ~n supports and the ope,a~ g pressure are of plill~ importance in controlling Illelll~lane failure.
In addition, failure can occur due to shear forces. Shear is caused by fluid viscosity and it tends to pull the m.,lllbl~e from the inlet of the cell toward the outlet. The important pal,ul~ s contributing to shear-inrll~ced tension are: (1) fluid viscosity; (2) fluid velocity; and (3) cell dimensions. The cell d;.,.Pn~;ons are the illlpoll~ull palanltler rather than the rl-~t~nce bel~c~,n the nlelllblal e supports because the Ill~;lllblane can slide freely over the smooth supports. The only thing stopping the mell,b.ane's migration toward the cell outlet is the mechAIlical strength of the In.,ll~l~l ane. Thc. ~;rO~ e, there is a need in the art for DOC
,s useful for, for PY~mrle, food processing activities, that cOIl~ lle high flux rates, low fouling and utility for solutions that are highly viscous and have high amounts of W O 97/33681 PCTrUS97/03574 suspended solids. The present invention provides an asymmetric Illemblane designPd plillla.;ly for DOC applications.
SUM~IARY OF THE INVENTION
s The present invention provides an a~ ,t,ic supported direct osmotic cQrlcpntration (DOC) ..I~ e, comprising a thin surface layer of polymeric material, and a porous support layer of polymeric material, whe-eill the porous support layer further col.l~lises a flexible mesh material of woven or non-woven fibers having an open structure having a plurality of open holes having a ~ t~qnce between fiber centers of between about 0.5 mm to about 10 mrn and 0 having at least a 50~/0 void area. Preferably, the present invention s~;lules an open weave doth backing for a tight weave sail-cloth b ~cL iny that is CQ~ 01) in RO l"cmb- ~les, to change an RO l,-e-,-bl ~e into a lower pressure DOC lll~,-lll,l aile~ The net result is a much faster flux m~ e having lower tensile ;,ll -,n~ better suited for a DOC en~iro~ Preferably, the polymeric material is ce~ osic. Preferably, the thickness of the thin surface layer is from about 5 ~m to about 20 ~lm. Preferably, the polymeric porous support layer is cast to have a L .~ess above the fiexible mesh of about 35 to about 300 llm. 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 ",~,n,l~,~ne, C~
(a) providing a flexible mesh bacL ing col"~osed of woven or non-woven fibers, having an open structure having a plurality of open holes having a distqnce bel~,el fiber centers of b~ ~n about 0.5 mm to about 10 mm and having at least a 50% void area cast onto a - surface a ~~ ng drum partially i".. l,e,sed in water;
(b) casting a thin film of a liquid polymeric material onto the flexible mesh ~a~l ing 25 onto a surface of a sp;l-nil~ drum partially subl-,e,ged in a tank of water, v~L~,I ~1 the casting occurs above a water line, to form a DOC n~ "b~ e on the surface ofthe spi~ ¢ drum, and Wh~leill the drum rotates at a speed to cast offrom about 15 to about 150 linear meters per hour; and (c) drying and removing the finished DOC me~ ~ from the surface of the spinning 30 drum.
Preferably, the polymeric material is a cellulosic material. PleÇe~ably, the cPll--losic polymeric material is sPlected from the group cQns;s~ 8 of celh)lQse acetate, cP~ lose diq-cetqte, cplhllose triacetvte, lege~.elaled cellulose, ce~l~llose butyrate, cellulose proprionate, and co,..l.il-~l;on~ thereof. Most pleré-~bly, the flexible mesh bacl~ing is first saturated with a 35 solvent in which the liquid polymeric material is soluble. Plerélably, the solvent is sele~ted from the group col-~ g of ethq-nol methqllol, lce:onpJ isoprop~l alcohol, other alcohols having no more than 4 carbon atoms, and combin-q-tionc thereof.
W O 97/33681 PCT~US97/03574 BRIEF DESCRIPTION OF TEIE DRAWINGS
Figure 1 illustrates a diagram of the Illel~b~ ane casting process by an ;~ ;on preci~ ;on procedure, showing the product ",e",l"~e (G) co~ lPted after being cast on a rolalh~g casting drum (I)) and Anne~led under water on a series of rollers (F). The mGIlll~ G
5 begins with a fabric b ~-t ine on a fabric support roll (A) that is wetted in solvent (B) and eventually applied to the rolali"~, casting drum. Mc~ e resin (C) is applied to the fabric support in a unir~"" layer.
Figure 2 illustrates a sçhe~ l ;c cross section of the inventive DOC ,,,~,.,II,l ~ne. The a~". "el,ic me."l"~e has a thin surface layer and a porous support layer having fabric 10 support f-.mhedded therein.
DETAILED DESCRIPTION OF T~IE INVENTION
The present invention provides an ~,~.nQIel.ic ~uppolled direct osmotic collce ~l~alion (DOC) me.llblane7 co",p,isi"g a thin surface layer of polymeric material, and a porous support layer of polymeric material, ~hcrcin the porous support layer further CO"I~IiSCS a flexible mesh material of woven or non-woven fibers having an open structure having a plurality of open holes having a ~ nce be~ ,n fiber centers of bcl~.~n about 0.5 mm to about 10 mm and having at least a 50% void area. Preferably, the present invention ~ es an open weave cloth ba-L-in~ for a tight weave sail-cloth ba~L ;.~ that is co.. ol- in RO "lem~lanes, to change 20 an RO l"~ e into a lower p~ JrG DOC IllGlllbll~e. The net result is a much faster flux ,llel~llJla~e having lower tensile ~I,englh better suited for a DOC environment. Prefe,~ly, the polymeric material is c~ los;c PIGrGlably, the th;cL-ness ofthe thin surface layer is from - about 5 ~lm to about 20 ~lm. Pl erGI ~bly, the polymeric porous support layer is cast to have a Ih-~L ~ C above the flexible mesh of about 35 to about 300 llm. The flexible mesh material generally has a thicL ness of from about 0.15 to about 1 mm.
The present invention provides a DOC me.~bl~u~e having a flexible mesh b~ct ;.~9 and composed of cellulose polymeric material. The ~ ."blane is imhedded in a strong, flexible mesh. As a result, there are two radically di~G,enl me,l,bl~ e regions. The part ofthe l,lelllblane bGI~en the mesh fibers has flux ident~ to that in an lmbact-ed n,~ml,.~ le, while the part cont~cting the fibers has flux similar to sailcloth backed ml ,.lbrane. Tests with a 7.0 mil, inventive DOC mem~,~ne cast on a polyethylene mesh (with 50% void area) show fluxes between those of the SPilrloth backed RO ",e",bl ~u~e and l-nbac~ed J~cmbl ~e. 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 Illellll~ e each hour).
3s The present invention further provides a method for casting a DOC men~bl~le, comprismg:
(a) providing a flexible mesh bac~in~ composed of woven or non-woven fibers, having an open structure having a plurality of open holes having a ~ t~nce between fiber centers of W O 97/33681 PCTrUS97/03574 between about 0. 5 mm to about l 0 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 sul,n.e. ged in a tank of water, wherein the casting occurs above a water line, to form a DOC ~.-e,nb.i.ne on the surface of the spinning drum, and wl,e,~i" the drum rotates at a speed to cast of from about l S to about l 50 linear meters per hour; and (c) drying and removing the fini~hed DOC ,.,cl.lb.~,le from the surface of the S~ g drum.
0 Plerelhbly, the polymeric material is a cellulosic material. Preferably, the ce~ 1Qs;r polymeric material is selected from the group COI.~ of cP1ll~lose acetate, cellutcse et~qte, cellulose triq-cetqte, legGllGI~led cçllulos~p~ ce~ lose butyrate, cellulose pr~,p,iollale, and CO~I1J; ~;on~ thereof. Most preferably, the flexible mesh ~ ine is first saturated with a solvent in which the liquid polymeric material is soluble. Preferably the solvent is se1ectP~d fromthegroupconci~tingofethq-~ol 1~ 1.anol ac~olle,isop,o~"~lq-lcQhQl,otherqlcoh~
having no more than 4 carbon atoms, and co",bin~tions thereo~
The mesh material can be any fiber-based material. Examples of mesh materials include nylon, polyester, polyethylene, pol~"opylene, cotton, silk and cG~ ;ons and blends thereof. The mesh must be woven of intertwined such that it provides support for the ~ b~ule. Fle~bly, the mesh has a tensile sllel~lh of from about l0 to about 20 N/mm.
The present invention provides an improved mollific~qtion to RO n.en~ es by sul)~ g an open-weave fibrous b~c~ing material in place of a closed weave material - common to RO ,ne"lbranes. This results in an inventive DOC me"ll,.~le with much higher flux rates needed for DOC appli~tions but unable to w;l ~ ntl the higher pressures of an RO
25 process. Th~r~rore, the inventive DOC lncn~b,~les are not useful for RO applic~;ol ~ or any pressures above 690 kPa, and preferably not above 170 I*a.
DOC IllGllll,l ~ es useful for the present invention are made by an illllllGI sion pree;~ ;on process shown in Figure 1. In this process, the ~G~ ~ e is formed by spreading a thin layer of a ",e",~ e casting resin (35 to 300 microns) over a surface. P~erelably the 30 surface is the flexible mesh material. The resin consists of the desired ~..em~l~ e polymer or mixture of polymers dissolved in a solvent or mixture of solvents For example one resin, ce11111Qse triac-et~qte is shown in Example l below. A thin resin layer is immersed in a water bath. Contact with the water causes preç;p:~l;on ofthe 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 35 (m~ ecQn~s) The forrnation ofthis thin layer impedes water pen~ l,alion to the rPmqindPr of the resin such that prec;~ ;on of the ,~ g polymer occurs over the next 60 seconds This prec ;~ ;on rate is slow enough such that the ~,.e..,l,l ~e polymer tends to agglomerate before p,Gc~ l;Qn~ îo",.n~g a bubbly, porous structure unde.,.~ll an unbroken surface layer .... ..
W O 97133681 PCTrUSg7!03574 Preferably, the drum rotates at a speed to cast of from about 15 to about 150 linear meters per hour.
Men,l,. ane selectively is due to the pl ope. lies of the thin surface layer (Figure 2). Use of a cellulosic polymer for DOC provides a highly hydrophobic me.l.bl ~1e that is able to s 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 lllel..b- ~e. If the osmotic ~ nglh of the brine is high enough, the brine can dehydrate the polymer in th surface layer. The surface layer ofthe ~ne.l~ e then l~,h~dlales itselfby dlav~r;ng water from the product in contact with the outer skin surface layer, thus dewatering the product. Larger 0 molePlles such as flavor agents or sugars, are unable to pass through the plastic-like matrix of the thin surface layer.
In the lllc.lll,l~ e, the porous, bubbly layer provides ~f'C'~3~ lleny~lh to support the e,~ ..ely thin surface layer. Further tensile sl-~,nglll and ,..r~ ic~l support is provided by having a fabric support emhedded within the resin of the porous layer, but within the porous 5 layer acco.lli)lished by having the resin ;.I--lle ~ed in water. The inventive DOC ll.e ,.l.l~es are cast in a continllous process by casting the resin on a rolalillg drum partially ill-...~..sed in water This process is illustrated in Figure 1.
E~amole 1 This CA r'e illustrates the plepd alion of a DOC-type cplh~losic l"~ ~ e. l'cetonP, (23.7 kg) and p-~ioy~ne (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 ofthe mixer. The ingredients were allowed to mix for 2.5 hr that allowed the polymer to dissolve co~ r' lPly. A solvent so!uti~n was formed with ,...,~
2s (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 contim~ed for another 30 min.
The solution was filtered through a 5 micron pol~,-o~"/lene cd~ster-type filter and into a holding vat where the filtered ssl-~tion was allowed to stand overnight and deaerate. The resin so~ 1tion was again filtered through a 5 micron filter and then cast on a support doth, 30 CQ~ il;~ of a c~lPn(lqred, 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 sol-ltion cloth composite entered an evaporation çl~ el where dry air was swept across the surface of the soh~tion for 20 sec The ~--eml,-~1e entered a coa~ tiQn bath of water at 18 ~C for 7 min. The ".emb-~e then progressed through a hot water bath where it was ~nn~led at 40.5 ~C for 12 min. A DOC
3s ..le...b.~le, acco-ding to the present invention was formed E~ample 2 This example illustrates propt;. lies of the DOC n~l..b.~1e formed in PYqmrle 1. Tests have been con-luGted with cellulose tri~cet~te (CTA) ...el--b-~nes to determine the effect of the secondary bac~in~ on DOC flux rates. Asymmetric, 3.5 mil me.l,~l~1es with and without a dense sailcloth backing were tested in a DOC cell. In both cA~el illlents, water was introduced to one side of the me.l~ e and 70 brix high fructose corn syrup was introduced to the other.
At 21 ~C, the flux with the unbacked Illt;lllb~-e was 14 liters/hour-m2 (Ll~) while the flux 5 with the backed me.llb~ e 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 sudmerged 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.
9, 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.
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 sudmerged 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.
9, 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.
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 |
|---|---|
| CA2248256A1 true CA2248256A1 (en) | 1997-09-18 |
Family
ID=24459033
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002248256A Abandoned CA2248256A1 (en) | 1996-03-11 | 1997-03-11 | Asymmetric supported membrane for direct osmotic concentration |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0888171A4 (en) |
| JP (1) | JP2000506439A (en) |
| AU (1) | AU724712B2 (en) |
| CA (1) | CA2248256A1 (en) |
| WO (1) | WO1997033681A1 (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6299777B1 (en) * | 1999-08-17 | 2001-10-09 | Cms Technology Holdings, Inc. | Osmotic distillation process |
| MX2012002393A (en) | 2009-08-24 | 2012-04-11 | Oasys Water Inc | Forward osmosis membranes. |
| EP2621615B1 (en) * | 2010-09-30 | 2020-07-15 | Porifera Inc. | Thin film composite membranes for forward osmosis, and their preparation methods |
| KR20130101533A (en) | 2010-10-04 | 2013-09-13 | 오아시스 워터, 인크. | Thin film composite heat exchangers |
Family Cites Families (6)
| 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 |
| JP2860833B2 (en) * | 1990-11-28 | 1999-02-24 | 日東電工株式会社 | Separation membrane |
| US5275725A (en) * | 1990-11-30 | 1994-01-04 | Daicel Chemical Industries, Ltd. | Flat separation membrane leaf and rotary separation apparatus containing flat membranes |
| JPH06346A (en) * | 1992-06-17 | 1994-01-11 | Nitto Denko Corp | Composite semipermeable membrane and spiral-type separation membrane device |
| US5522991A (en) * | 1994-07-20 | 1996-06-04 | Millipore Investment Holdings Limited | Cellulosic ultrafiltration membrane |
-
1997
- 1997-03-11 EP EP97916730A patent/EP0888171A4/en not_active Withdrawn
- 1997-03-11 JP JP9532701A patent/JP2000506439A/en active Pending
- 1997-03-11 CA CA002248256A patent/CA2248256A1/en not_active Abandoned
- 1997-03-11 AU AU25275/97A patent/AU724712B2/en not_active Ceased
- 1997-03-11 WO PCT/US1997/003574 patent/WO1997033681A1/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| EP0888171A1 (en) | 1999-01-07 |
| EP0888171A4 (en) | 2000-01-05 |
| JP2000506439A (en) | 2000-05-30 |
| WO1997033681A1 (en) | 1997-09-18 |
| AU724712B2 (en) | 2000-09-28 |
| AU2527597A (en) | 1997-10-01 |
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