Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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.
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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
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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
.... ..
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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
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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.
