Osmotic Distillation Using Liqui-Cel

Osmotic distillation — a separa-tion process in which a liquidmixture containing a volatilecomponent is contacted with a

microporous, non-liquid-wettable mem-brane whose opposite surface is exposedto a second liquid phase capable of ab-sorbing that component — is nearingcommercialization for the concentrationof beverages and other liquid foodstuffs,and is under evaluation for the concentra-tion of aqueous solutions of thermally la-bile pharmaceutical products and biologi-cals. Its primary advantage lies in its abil-ity to concentrate solutes to very highlevels at low temperature and pressure,with minimal thermal or mechanicaldamage to or loss of those solutes. Theprocess also can enable the selective re-moval of a single volatile solute from anaqueous solution (for instance, ethanolfrom wine and other ferments) usingwater as the extracting solvent. Low-al-cohol-content beverages have been pro-duced in this manner with minimal lossesof volatile flavor and fragrance compo-nents. Osmotic distillation (OD) promisesto become an attractive complement oralternative to other athermal or low-temperature separations techniques suchas ultrafiltration (UF), reverse osmosis(RO), pervaporation, and vacuum freezedrying.

PROCESS FUNDAMENTALS OD, which is also called “isothermal

membrane distillation,” is a membranetransport process in which a liquid phase(most commonly an aqueous solution)

containing one or more volatile compo-nents is allowed to contact one surface ofa microporous membrane whose poresare not wetted by the liquid, while the op-posing surface is in contact with a secondnonwetting liquid phase (also usually anaqueous solution) in which the volatilecomponents are soluble or miscible. Themembrane thereby functions as a vaporgap between the two liquid phases, acrosswhich any volatile component is free tomigrate by either convection or diffusion.The driving potential for such transport isthe difference in vapor pressure of eachcomponent over each of the contactingliquid phases. The mechanism is illustrat-ed schematically in Figures 1 and 2. If thesole or primary volatile component in so-lution is the solvent, then evaporation ofsolvent from the solution of higher vaporpressure into that of lower vapor pressurewill result in concentration of the formerand dilution of the latter. Thus, the rate oftransport of solvent from one liquid phaseto the other will increase as the solventvapor pressure over the receiving phase isreduced. If the solvent vapor pressureover the liquid being concentrated dropsto a value equal to that over the receivingphase, no further transport will occur.

In most applications of practical inter-est, the solutions to be concentrated con-tain relatively low concentrations of non-volatile solutes of moderate to highmolecular weight (sugars, polysaccha-rides, carboxylic acid salts, proteins, andso on) which have limited stability to ele-vated temperatures and shear stresses.Because of the low osmotic activity of

CHEMICAL ENGINEERING PROGRESS • JULY 1998 ©Copyright 1998 American Institute of Chemical Engineers. All rights reserved. Copying and downloading permitted with restrictions.

A New Option: Osmotic Distillation

This novel athermalmembrane-

separation processretains flavors and fragrances better

than thermal techniques. It

promises to play animportant role in theprocessing of foods,

pharmaceuticals,and other products.

Paul A. Hogan,Wingara Wine GroupR. Philip Canning,

Zenon Environmental, Inc.Paul A. Peterson,

Hoechst Celanese Corp.Robert A. Johnson,

Queensland University of TechnologyAlan S. Michaels,

Alan Sherman Michaels, Sc. D., Inc.

ON THE HORIZON

http://www.aiche.org/docs/publication/permiss.htm

such solutes, the vapor pressure ofwater over such solutions is verynearly that of pure water, and de-creases quite slowly with increasingsolute concentration. Hence, if the re-

ceiving or “strip” solution on the op-posite membrane face contains a highconcentration of nonvolatile solute ofhigh osmotic activity (meaning a so-lute of low equivalent weight and

high water solubility), its water vaporpressure will be low and will increaseslowly on dilution. This makes it anattractive candidate for favoring rapidtransfer of water vapor through themembrane.

The basic transport process is il-lustrated schematically on a macroscale in Figure 3. OD is uniqueamong membrane-separation process-es in that it involves the transport ofvolatile components between two in-herently miscible liquid streams,driven by differences in componentactivity between those streams. Itsclosest analogs are probably dialysisand membrane solvent extraction, al-though the former involves transportof solutes (whether volatile or non-volatile) between two miscible liquidphases, and the latter transport of so-lutes between two immiscible liquids.

Inasmuch as the strip solution, fol-lowing its dilution by water trans-ferred from the feed stream, must bereconcentrated by evaporation so thatit can be recycled and reused in theOD operation, it is important that thestrip solute itself be thermally stableto quite high temperatures — and alsopreferably nontoxic, noncorrosive,and of low cost. Water-soluble saltsare the most attractive prospects forthis purpose; those that have beenmost frequently employed are the al-kali and alkaline earth metal halides(particularly sodium and calciumchloride). Sodium chloride, however,has relatively low water solubility anda rather low temperature coefficient ofsolubility, while calcium chloride issensitive to precipitation in the pres-ence of carbon dioxide; both are quitecorrosive to ferrous alloys at elevatedtemperature. Salts that display largeincreases in solubility with tempera-ture are desirable, because they can beevaporatively concentrated to veryhigh levels without danger of crystal-lization in the evaporator or duringstorage prior to recycle.

We have found that, for osmoticconcentration of foodstuffs and phar-maceutical products, the most attrac-tive strip solutes are the potassium

CHEMICAL ENGINEERING PROGRESS • JULY 1998

ON THE HORIZON

■ Figure 2.Mechanism of osmotic distillationthrough a microporous hydrophobic membrane.

ConcentratedFeed Out

DiluteFeed In

ConcentratedBrine In

Diluted Brine Out

Membrane

Increasing Water Vapor Pressure

Decreasing Water Vapor Pressure

■ Figure 3. The basic transport process in osmotic distillation.


ConcentratedSalt Solution In

Diluted SaltSolution Out

DiluteFeed In

MembraneContactor

■ Figure 1. In osmotic distillation, a semipermeable membrane acts as a vapor gapthat allows migration of volatiles in a single direction.


DiluteFeed In

Feed Channel

ConcentratedBrine InDiluted

Brine OutBrine Channel

Water Transport Pathways

Semipermeable Membrane

salts of ortho- and pyrophosphoricacid. These have quite low equivalentweights, very high water solubilities,and very steep positive temperaturecoefficients of solubility. They alsohave the advantage of being normallypresent in biological fluids and, thus,safe for food or drug use when pre-sent in low concentrations.

The vapor pressure/concentrationrelationships for a representative feedto be concentrated (for example, anaqueous sucrose solution) and severalcandidate brines as strip solutions areshown in Figure 4. The equivalentweights of the salts increase in theorder NaCl > CaCl2 > K2HPO4, as dotheir water solubilities. Because the“osmotic activity” of a salt is deter-mined by the ratio of its water solu-bility to its equivalent weight, this inpart accounts for the attractiveness ofconcentrated dipotassium orthophos-phate brine for this application.

Process thermodynamicsThe water transport process across

the membrane takes place in threeconsecutive steps: (1) evaporation ofwater at the liquid meniscus at a poreentry; (2) diffusional or convectivetransport of water molecules as vaporthrough the membrane pore; and (3)condensation of water vapor on thebrine-side liquid meniscus at the poreexit. The evaporative process requiresthe supply of the latent heat of vapor-ization at the upstream meniscus; thisonly can be provided as sensible heatvia conduction or convection fromthe bulk upstream liquid, or via con-duction across the solid phase com-prising the membrane. Conversely, atthe downstream face of the mem-brane, condensation of water vaporinto the strip requires removal of theheat of condensation by the samemechanisms. Supplying or removingthis energy by conduction/convectionfrom the bulk liquid phases would, ofcourse, cool the feed and heat thestrip, thereby reducing the drivingforce for water transport.

Fortunately, however, the thermalconductance of the membrane is suf-

ficiently high that virtually all the en-ergy of vaporization can be suppliedby conduction across the membraneat a quite low temperature gradient.As a consequence, under normal op-erating conditions, the temperaturedifference between the liquids on op-posite sides of the membrane (“tem-perature polarization”) is quite small— seldom greater than 2°C. Hence,the process is essentially isothermalwith respect to both liquid streams.For this reason, membranes preparedfrom solids of high thermal conduc-tivity and of minimum practicalthickness are desirable. It is interest-ing that the situation is exactly theopposite for the process of “mem-brane distillation,” where a warm so-lution to be concentrated is separatedvia a microporous membrane from acold liquid into which condensationis to occur. In that case, a membraneof very low thermal conductance isnecessary to prevent heat flow byconduction from the warm to the coldliquid stream.

Many liquid feeds whose concen-tration is desired (such as fruit andvegetable juices, and vegetable ex-tracts such as tea or coffee) also con-tain small concentrations of essentialvolatile, lipophilic organic solutes(flavor and fragrance components),the loss of which would make theproduct unpalatable and unmar-ketable. While such products can beconcentrated by evaporation, losses

of these essential volatiles with thewater vapor are severe. Condensationof the vapor mixture, followed byrectification to recover these volatilesfor reblending with the concentrate,can offset this somewhat — but wors-ens thermal deterioration of thesecomponents and results in a signifi-cant incremental processing cost.

With OD, several factors make itpossible to achieve concentration byselective removal of water withoutsignificant depletion of these impor-tant flavor/fragrance components.First, if the concentration is carriedout at low temperature, the vaporpressure of these components (rela-tive to that of water) is substantiallydepressed, reducing the driving forcefor transmembrane transport of thesesolutes. Second — and perhaps moreimportant — the solubilities of theselipophilic solutes are substantiallylower in concentrated saline solutionsthan in pure water; as a consequence,the vapor pressures of these soluteswhen present in any given concentra-tion in such a solution are much high-er than they are over water at thesame concentration. Thus, the vapor-pressure driving force for vapor-phase transfer of these solutes fromthe feed to the strip is far lower thanthat encountered in simple evapora-tion. Additionally, because the molec-ular weights of these solutes are farhigher than those of water, their dif-fusive permeabilities through the


■ Figure 4.Generalized

vapor-pressure relationships for

sugar and salt solutions at 25°C.

25

20

15

10

50 10 20 30 40 50 60 70

Wt. % Solute

Vapo

r Pre

ssur

e of

Wat

er, m

m H

g

CaCl2

K2HPO4NaCl

Sucrose

membrane are much lower. The endresult of these factors is that volatileflavor and fragrance losses from suchfeeds during OD often are too low tobe significant. Indeed, concentrate,when rediluted with distilled water toits original volume, is organoleptical-ly very similar to that of the originalfeed; this, of course, makes OD par-ticularly attractive for food and bev-erage processing.

Membrane transport kineticsFor successful operation of OD, it

is essential that (1) liquid is preventedfrom penetration into and passagethrough the membrane pores, and (2)unimpeded vapor-phase transport ofvolatile components from feed tostrip solution can occur. The first re-quirement can be satisfied if themembrane surface is sufficiently hy-drophobic such that neither the feednor strip solutions can wick by capil-lary forces into the pores (requiringan angle of contact between the liquidand solid phases of greater than 90°),and the surface tensions of the liquidphases are sufficiently high that thecapillary penetration pressure of liq-uid into a pore is well in excess of themaximum pressure difference acrossthe membrane that might be encoun-tered in operation. Liquid penetrationinto the pores will lead to unaccept-able gross mixing of feed and stripsolutions.

The critical penetration pressure is defined by the classical KelvinEquation:

∆p = 2γ cos θ/r (1)where ∆p is the pore-entry pressure, γthe liquid surface tension, θ the con-tact angle, and r the pore radius.Thus, the higher the surface tensionof the liquid, the larger the contactangle (in excess of 90°), and thesmaller the pore radius, the greaterthe intrusion pressure. For most con-centrated aqueous solutions of miner-al salts, the surface tension far ex-ceeds that of pure water; so, intrusionof such solutions into microporoushydrophobic membranes is unlikelyat any reasonable pressures. Many

aqueous feeds of vegetable or animalorigin, however, contain amphiphilicsolutes that may depress the surfacetension and reduce the wetting angleon hydrophobic surfaces. For suchsolutions, membranes fabricated frommaterials with very high contact angles with respect to water, and ofsufficiently small pore size to meetthe penetration-pressure limitationimposed by Eq. 1, will be required.Transmembrane pressure differenceslikely to be encountered in OD mem-brane modules are in the range of 140kPa (20 psig); for a liquid with a sur-face tension of 50 dynes/cm and acontact angle of 110°, the maximumtolerable pore radius to prevent liquidpenetration is about 250 nanometers— corresponding to a pore diameterof about 0.5 micron. Commerciallyavailable microporous membranesfall well within this range of pore di-mensions. Membranes of smallerpore size than this, of course, willwithstand higher hydrostatic pres-sures without liquid intrusion.

The most suitable materials forOD membranes, thus, are apolarpolymers with low surface free ener-gies. These include the polyolefins,particularly polyethylene and poly-propylene, and the perfluorocarbons,especially polytetrafluorethylene(PTFE) and polyvinylidene difluoride(PVDF). Microporous membranesfabricated from these materials areavailable with pore sizes and pore-size distributions in acceptable rangesfor this application. For reasons to beelaborated on later, small-dia., hol-low-fiber membranes with thin wallsappear to be the best candidates, be-cause they offer high area/volume ra-tios and do not require supports andspacers in modular geometries. Mostof the studies described in this articlehave been conducted with microp-orous, polypropylene hollow-fibermodules made of Celgard (a fibermanufactured by Hoechst Celanese).These fibers are approximately 0.3mm in external dia. with a wall thick-ness of about 0.030 mm; they have amean pore diameter of about 30

nanometers (0.03 micron), and aporosity of about 40%. The intrusionpressure of water into the pores iswell in excess of 100 psig. Burststrength of the fibers exceeds 200psig; they can tolerate external pres-sures greater than 100 psig withoutcollapse.

Gas and vapor transport throughmicroporous membranes of this struc-ture takes place principally by molec-ular diffusion. For pores of dia. in thenanometer range, at ambient tempera-ture and gas-phase pressures of theorder of the vapor pressure of waterat that temperature (approximately 20mm Hg or 2.7 kPa), the mean freepath of gas molecules is significantlygreater than the pore diameter. Underthese circumstances, molecular trans-port occurs by Knudsen diffusion.The rate equation for such transport isapproximated by:

dn/Adθ ∝ ∆ p/(ΜRΤ)1 × εr/τt (2)where the lefthand side of the equa-tion is the molar flux of vapor(mol/cm2s) with n the number ofmoles, A the area, and ∆θ the time in-terval; ∆p is the vapor pressure differ-ence across the membrane, M themolecular weight of the vapor, R thegas constant, T the absolute tempera-ture, ε the membrane porosity, τ thepore tortuosity, and t the membranethickness. For a vapor pressure differ-ence of about 10 mm Hg (1.3 kPa)and a membrane of porosity, poresize, and thickness indicated above,the water vapor flux through themembrane is estimated to be about 5L/m2h. Measured experimental waterfluxes through this membrane underthese conditions (that is, pure wateron one side and a concentrated brinewith a vapor pressure about half thatof pure water on the other) are ap-proximately of this magnitude.

These calculations and measure-ments assume that the only gaseousspecies present in the pore space iswater vapor. In practice, however,both the feed liquid and brine stripare likely to be saturated with air atambient temperature. Under theseconditions, there also will be air at


ON THE HORIZON

about atmospheric pressure in themembrane pore space. In the pres-ence of inert gas, the Knudsen diffu-sion condition no longer applies, be-cause gas/gas collisions dominateover gas/wall collisions; water vapor,thus, will transit the pores by simpleFickian diffusion. The water vaporflux in this case will be significantlylower than that predicted by theKnudsen relation at the same vapor-pressure gradient, although it be-comes independent of pore radius.Experimental measurements of waterflux during OD in the presence andabsence of air in the pores clearlyconfirm this difference: with air pre-sent at atmospheric pressure, thewater fluxes are lower by a factor oftwo to three than in its absence.

Obviously, water vapor transportwould be favored by removal of dis-solved air from the liquid phases; theadditional cost of air removal, how-ever, combined with the losses of es-sential volatiles from many candidatefeed streams consequent to air strip-ping, make this an unattractive op-tion. Thus, the majority of OD opera-tions are being carried out with non-condensable gas present (usually air,although not infrequently nitrogen orcarbon dioxide), with average ODwater fluxes in the range of 0.2–3.0L/m2h.

As Eq. 2 indicates, vapor transportkinetics can be improved by selectingmembranes of higher porosity andsmaller thickness (and also largerpore size when transport is in theKnudsen diffusion regime). Unfortu-nately, such changes reduce mem-brane strength and durability. Also,according to Eq. 1, increasing poresize reduces the pressure required tocause liquid intrusion into the mem-brane pores and promotes membrane“wet-out.” Recent development oftechniques for surface modification ofmicroporous polymeric membranesthat render their surfaces highly hy-drophobic (and, thus, more resistantto liquid penetration) may yield im-proved membrane structures for usein this process.

The rather low permeation fluxesattainable with OD relative to thoseachievable via pressure-driven mem-brane processes such as microfiltra-tion (MF), UF, and RO (where per-meation fluxes in the range of 10–200L/m2h often are achievable) wouldappear to make the process economi-cally uncompetitive with them.When, however, the product to beconcentrated is a solution containingmacrosolutes or hydrocolloids sensi-tive to shear degradation or a solutionof microsolutes of moderate molecu-lar weight (such as simple sugars),there are serious limitations to the de-gree of concentration achievable viathese methods without significantproduct deterioration or costly pro-cess equipment. It is for preciselythese applications where OD has im-portant advantages.

By way of illustration, to concen-trate by RO an aqueous glucose solu-tion to 70 wt. % sugar from 20%would require a transmembrane pres-sure difference well in excess of theapproximate osmotic pressure ofsugar in that solution (about 150atm). In contrast, the vapor pressureof water over a 70% glucose solutionis only about 80% that of pure waterat the same temperature; so, the ODwater flux (using a concentrated brineas the stripping solution) would beonly slightly depressed over that con-centration range.

OD offers another potential advan-tage over pressure-driven membraneconcentration for producing highlyconcentrated products, because it is aliquid/liquid transport process duringwhich the strip solution is undergoingdilution while the product stream isbeing concentrated. Hence, if themembrane contactor is operated incounterflow mode, where the productstream encounters increasingly con-centrated strip solution as it flowsthrough the contactor, it is possible tomaintain the vapor-pressure drivingforce for water transport across themembrane (and, thus, the transmem-brane flux) large and nearly constantthroughout the contactor. This is in

contrast to the usually rapid andmarked decline in transmembrane fluxexperienced during UF and RO as thefeed stream becomes increasinglyconcentrated. Thus, this mode of op-eration can compensate to a degreefor the inherently low fluxes of OD.

BOUNDARY-LAYER ISSUES As is the case with all membrane

separations processes, boundary-layerproblems are encountered in OD andmust be mitigated to achieve maxi-mum process performance. The na-ture and magnitude of these prob-lems, however, are rather different forOD than for the more common pres-sure-driven membrane processes —and require rather different engineer-ing strategies.

Polarization moduliDuring concentration of aqueous

solutions by OD, removal of waterfrom the feed and its transfer into thestrip creates a polarization boundarylayer at the upstream membrane sur-face of increasing solute concentra-tion and also one on the strip side ofdecreasing concentration of salt. Thisreduces the transmembrane water fluxby depressing the vapor pressure ofwater over the feed solution contact-ing the membrane (relative to itsvalue in the bulk liquid) and, con-versely, increasing the water vaporpressure over the strip solution con-tacting the downstream membranesurface. The “polarization moduli”(that is, the ratios of the solute con-centration at the membrane surface tothat in the adjacent bulk liquid) areexponentially dependent upon theratio of the transmembrane volumeflux of liquid solvent across the mem-brane to the mass-transfer coefficientof the solute in the feed and stripchannels. Because the inherent trans-membrane fluxes in OD are low, andeconomics demand the use of mem-brane contactors with the highest pos-sible membrane area per unit volume(such as hollow-fiber contactors),concentration polarization (becauseof both low transmembrane flux and


high mass-transfer coefficients) ismuch less important as an additionalresistance to interphase transport thanit is in MF, UF, and RO.

Another important — and para-doxical — feature of OD that alsotends to mitigate boundary-layer resistances to solvent transport is the thermodynamic consequence ofchanging solute concentration on sol-vent vapor pressure. To a first approx-imation, the vapor pressure of waterover an aqueous solution is deter-mined by Raoult’s Law — that is, thevapor pressure is directly proportion-al to the mole fraction of water in themixture. Because water is a majorcomponent of these solutions and isthe lowest-molecular-weight compo-nent present, the effect of changes insolute concentration over moderateranges (as likely to be encountered inthe boundary layers) on water vaporpressure is quite small. Hence, polar-ization on either side of the mem-brane has relatively little effect on thevapor-pressure difference across themembrane and, thus, on the OD flux.Yet another factor reducing boundaryresistances in OD, particularly infood and beverage applications, isthat the majority of the nonvolatilesolutes are compounds with relativelylow molecular weight and rather highaqueous solution diffusivities, there-by contributing further to high mass-transfer coefficients.

Viscous polarizationWhen highly concentrated prod-

ucts are desired from OD (for juicesand beverages, for example, removalof 90% of the water is necessary), anunusual boundary-layer problem de-velops on the feed side of the mem-brane. This problem, which seldom isencountered in other membrane con-centration processes, can have a verydetrimental effect on process perfor-mance. Many of these feed streamscontain hydrophilic solutes such assugars, polysaccharides, and proteins,that, when concentrated to high lev-els, yield solutions of anomalouslyhigh viscosity (and, frequently, non-

Newtonian rheologic behavior). Forexample, sucrose solutions undergolarge increases in viscosity at roomtemperature as the sugar concentra-tion rises above about 50 wt. %,reaching values at a concentration of70 wt. % about two orders of magni-tude greater than that of water. De-spite this enormous viscosity rise,however, the vapor pressure of waterover the solution decreases veryslowly; indeed, the OD flux of waterout of a 70% sucrose solution of vis-cosity of 100 cP is not much less thanthat from a 20% sucrose solution witha viscosity of about 2 cP.

As such a solution passes througha membrane-bounded channel under-going OD, solution near the mem-brane surface becomes increasinglyconcentrated and, upon reaching acritical concentration level, its viscos-ity begins to rise very rapidly withfurther water removal. Viscous trans-port of this layer along the channelbecomes progressively slower, be-cause it is bounded by flowing liquidof lower solute concentration and vis-cosity. Ultimately, stagnation of fluidin the boundary layer occurs, and theless-viscous, more-dilute solution“fingers” through the center of thechannel with increasing velocity. Theend result is channeling of dilute,low-viscosity feed through the cen-ters of channels bounded by mem-brane coated with essentially station-ary films of concentrate. Reduction insolution residence time in the contac-tor and blockage of access of that so-lution to the membrane surface leadto very large decreases in transmem-brane water transport below expectedperformance. (This “viscous finger-ing” process is quite analogous to thatobserved in multiphase flow throughporous petroleum reservoirs when thedisplacing fluid (water) is less vis-cous than that (oil) initially occupy-ing the pore space.)

This problem, incidentally, is ac-centuated if an attempt is made tomaximize water removal from a dilutefeed stream during a single passthrough a contactor. This operating

strategy maximizes the viscositychanges during transit and, thus, thedegree of viscous polarization and per-formance deterioration attributable toit. Experience has shown that the ef-fect can be minimized if a contactor isoperated either in continuous feed-siderecycle mode where the rise in soluteconcentration per pass can be held tomodest levels, or in batch-recyclemode where the increase in concentra-tion per pass is also small but the aver-age solute concentration is allowed torise to the desired final value. Fortu-nately, these strategies usually becomenecessary only when the solute con-centration increases to levels (typicallyaround 50 wt. % for most sugars)where the viscosity begins to rise veryrapidly with concentration.

By far the most effective meansfor minimizing the “viscous polariza-tion” is by proper fluid managementof the feed-side channel of the mem-brane contactor. Fluid mechanicalmodeling of flows in membrane-bounded channels makes it clear thatlaminar flow in a channel of uniformcross-section (of even small dimen-sions) is a poor option to minimizeviscous fingering, because there areno normal stresses to promote mixingof the viscous boundary layer withthe lower viscosity fluid moving overit. Indeed, even turbulent flow at highvelocities in such channels is relative-ly ineffective until wall shear stressesreach unreasonably high values. Ex-perimental measurements of OD per-formance with such solutions in hol-low-fiber reactors with feed flowthrough the lumen (tube) sides of thefibers (a strategy initially and mistak-enly considered to be the best alterna-tive) clearly support this conclusion.

If, however, the feed solution isforced to flow normal to the long axisof a hollow membrane fiber, in achannel designed to maintain an as-sembly of such fibers in uniform, rel-atively closely spaced array, withmeans for assuring uniform flow offluid normal to all fibers, then, evenin laminar flow, each fluid element issubjected to stagnation and transport


ON THE HORIZON

around each fiber with eddying on thedownstream side before it encountersthe next fiber in the array. The resultis both rather efficient displacementof the boundary layer around eachfiber by fresh fluid and efficient mi-cromixing of fluid elements through-out the transit process.

Hoechst Celanese Corp. (Charlotte,NC) within the past few years has de-veloped a hollow-fiber-membranecontactor, called Liqui-Cel, that pro-vides a shell-side fluid channel ar-rangement of precisely this configura-tion (see Figure 5). Modules contain-ing nominal membrane areas of 1.4,19.2, and 135 m2 (Figure 6) are com-mercially available. The membranefibers first are woven into a fabricconsisting of a warp of parallel hol-low fibers held in fixed position by awoof of small-dia., widely spacedmonofilaments, as shown in Figure 7.The spaces between the hollow fibersare about equal to the external fiberdia. This fabric is spirally woundaround a perforated tube or mandrelto yield a cylinder whose central tubeserves as a conduit for liquid entranceand exit, and an annulus filled withclosely and uniformly spaced hollowfibers parallel to the cylinder axis. Thehollow fibers typically occupy about50% of the volume of the annulus.

Next, this cylinder is inserted intoa tubular shell and the ends pottedand sliced to provide access to the lu-mens of all fibers for fluid entry andexit. The central tube and shell arebaffled so that fluid entering at oneend is forced to flow radially outwardthrough the shell annulus, along thelong axis of the cylinder, and then ra-dially back into the tube prior to dis-charge. As a consequence, nearly allthe fluid delivered to the shell is con-strained to travel normal to fiber axes.

For OD applications, the feed tobe concentrated enters the shell sideof the module, and the brine strip thelumen side with its flow countercur-rent to the feed. Because the brinestrip exhibits quite low viscosity overthe entire concentration range em-ployed, there is virtually no signifi-

cant polarization on the brine side.Despite the small internal dia. of thefibers (0.24 mm), the brine pressuredrop required to provide adequatebrine flow for the concentration pro-cess is well within the burst limit ofthe fibers and below the pressurelevel for liquid intrusion into themembrane pores.

On the shell side, the hydraulic re-sistance to feed flow — even forhigh-viscosity concentrates — is lowenough to allow adequate flow rateswith pressure drops in the 2–5 psi(20–50 kPa) range. Despite the lowmass velocities in the shell space(and, thus, low Reynolds numbers),boundary-layer turnover and mi-cromixing are sufficient to achievetransmembrane OD fluxes withinone-half to two-thirds of those pre-dicted from base-line measurementsusing pure water as the feed — evenfor juice concentrates containing ashigh as 70 wt. % sugar. The flux loss-es attributable to viscous polarization,thus, are sufficiently low to render theprocess both technically and econom-ically feasible for production of suchhigh-solids concentrates.

OD membrane contactors generallyare operated with a slightly higher pres-sure on the shell (feed) side of the mod-ule than on the lumen (strip) side, sothat the pressure difference across themembrane at all points in the contactoris greater on the shell side. This is to in-sure that, in the event of a fiber failureor liquid intrusion into the membranepores, feed liquid will leak into thestrip, and not vice versa. This avoids

contamination of the product with thestrip solute. In most instances in prac-tice, such leakage is too small to have adetectable impact on product yield. Pe-riodic sampling of diluted strip andanalysis for a specific feed componentcan be used to detect leakage; beforeleakage becomes serious, it can be cor-rected by module replacement.

PROCESS DESIGN The objective of an OD process

for concentration of an aqueous feedis to remove water from the feed viatransfer of a large fraction of the


■ Figure 5. Construction detail of the membrane contactor. (Drawing courtesy ofHoechst Celanese.)

■ Figure 6. Membrane contactors areavailable in 1.4-, 19.2-, and 135-m2

modules. (Photo courtesy of HoechstCelanese.)

■ Figure 7. Photo enlargement of asection of a hollow-fiber array. (Courtesy of Hoechst Celanese.)

water into a saline strip solution,yielding a product of the desired so-lute concentration and a diluted strip.The essential design parameters are:(1) the required plant capacity indaily volume of feed to be concen-trated, (2) the solute concentrations inthe feed and final concentrate, (3) thewater vapor pressure/concentrationrelationship for the feed stream, (4)the water vapor pressure/salt concen-tration relationship for the strip solu-tion, and (5) the intrinsic water vaporpermeability of the OD membrane,expressed as liters of water transmit-ted per sq. m. of membrane area perunit of water vapor-pressure differ-ence across the membrane. These re-lationships allow determination of themaximum and minimum concentra-tions of salt in the strip, and the aver-age rate of removal of water from thefeed. Then, the mode of feed andbrine supply to the membrane contac-tors (for instance, single-pass orfeed/bleed recycle of the feed or stripstreams) can be selected; this, in turn,enables computation of the mem-brane area and strip volume require-ments, feed- and strip-pump capaci-ties, and the evaporative load for stripreconcentration.

A schematic for a simple juice-con-centration unit is shown in Figure 8.The process employs partial batch recy-cle on the feed side (to minimize largefeed viscosity changes through themembrane contactors) and continuouscountercurrent recycle plus evaporativereconcentration of the brine strip. Thefeed side of the system usually operatesin batch mode for food and pharmaceu-tical products, because such productstypically are processed in batches, andperiodic cleaning, sanitizing, and steril-ization of equipment are essential.

The contactors can be arranged ina series, parallel, or series-parallelarray, as process requirements dictate.Because feed-side viscous polariza-tion results in OD flux decline withincreasing feed-side solution viscosi-ty, economics usually favor a taperedseries cascade with increasing num-bers of contactors in parallel towardthe feed-discharge end. This results inan elevated average OD flux and,thus, greater water-removal capacityper unit membrane area.

Feed- and strip-solution pretreatment

The flow channel dimensions inthe Liqui-Cel contactors are quite

small (on the order of 200–300 mi-crons). Liquids containing suspendedparticles (or that may precipitate suchparticles during concentration) mayplug those channels — particularlyon the feed (shell) side of the modulewhere the fiber fabric behaves inmany respects as a deep-bed filter.Hence, prefiltration of both feed andstrip prior to delivery to the OD arrayis a necessity. Cartridge or sheet-membrane filters capable of removingparticles larger than 5 microns havebeen found to be adequate, althoughsecondary backup with a 1-micronfilter is a recommended precaution.Obviously, only feed streams withvery low concentrations of such par-ticulates (such as clear juices andvegetable extracts, and drug or bio-logical solutions devoid of suspendedcells or cell debris) are amenable toprocessing in this manner. Feeds con-taining high concentrations of sus-pended solids can be concentrated byOD, but require different pretreat-ments, as will be discussed later.

Certain clear solutions contain dis-solved solutes of limited water solu-bility that, in the course of OD con-centration, will precipitate or crystal-lize during transit through the contac-tor and deposit on or in the mem-brane. An important example is grapejuice, many species of which containpotassium bitartrate. Such precipi-tates or crystals may form obstructivelayers on the membrane surface or, insome cases, actually nucleate andgrow into the membrane pores wherethey can promote wet-out and liquidleakage through the membrane. Theproblem can be eliminated by re-stricting the degree of concentrationof the feed to a level just short of thesaturation solubility of that solute atthe processing temperature. Anotherapproach that appears to be effectiveis to concentrate the feed to the solu-bility limit via OD, collect and chillthe concentrate to allow precipitationto occur in the storage vessel, andthen decant or filter, rewarm, and re-turn the solution to the OD unit forfurther concentration. Sometimes, the


ON THE HORIZON

■ Figure 8. Typical OD system for juice concentration.

JuiceConcentrate

Evaporator

Single Strength Juice

Diluted Brine

Osmotic DistillationMembrane Array

Reconcentrated Brine Condenser

CondensateTo Waste

rate of precipitation during concentra-tion is sufficiently slow that it doesnot occur during transit through thecontactor — in such instances, pro-viding a holding tank for the (super-saturated) concentrate to allow pre-cipitation to take place external to thecontactor, and then prefiltering or cy-clone separating the precipitate priorto recycling the supernatant for fur-ther concentration may be effective.

Feed pretreatment of particulate suspensions

Many feed streams that are candi-dates for OD processing containrather high concentrations of particu-late solids that neither the OD contac-tors nor conventional prefilters canhandle properly. These include pulpyfruit and vegetable juices (citrus,pineapple, tomato, and the like), andfermentation broths containing mi-crobial cells or cell debris. In somecases, the suspended solids are im-portant components of the final con-centrate; in others, they are contami-nants. The most convenient means forpreprocessing such feeds is tangentialor vortex-flow MF or UF, to yield aparticle-free permeate for delivery tothe OD contactor array, and a reten-tate of high-suspended-solids content.This process does not alter the com-position of the suspending medium.OD concentration of the permeatethen can be carried out as with anyclear solution.

If the suspended solids must be in-cluded in the concentrate, then the re-tentate labelled as “concentrated sus-pension” in Figure 9 would be re-blended with the product concentrate.This, of course, will cause dilution ofthe final concentrate by the (uncon-centrated) serum present in the reten-tate, the extent dependent upon therelative volumes of the two streams.This two step process necessarilylimits the achievable degree of con-centration (as well as increases theprocessing cost) and, thus, may be in-appropriate for certain pulpy juices.

If, on the other hand, the suspend-ed solids are contaminants, the reten-

tate generated on pretreatment can bediscarded as waste, as indicated inFigure 9. It, of course, will containproduct in solution; the fraction ofproduct lost will approximately equalthe ratio of the retentate volume tothat of the original feed. Usually, thisfraction is too small to be worth re-covering. If its recovery is desirable,this easily can be accomplished by di-afiltration of the retentate in a secondcross-flow-filtration stage, with thepermeate so produced delivered tothe OD contactor array for addition tothe main permeate stream prior toconcentration.

Hybrid processesAs mentioned earlier, OD is inher-

ently more costly for concentration ofaqueous mixtures than such conven-tional processes as evaporation, UF,or RO. Its advantage stems from itsability to selectively remove waterrelative to other low-molecular-weight solutes (volatile or non-volatile) and, thus, to yield concen-trates of superior quality. It also al-lows preparation of more concentrat-ed products than are achievable by

RO. On the other hand, RO, where itcan be carried out at relatively lowpressure, is among the least costlymeans for water removal for feeds oflow solute concentration. Althoughusing RO will cause losses in volatileand nonvolatile flavors, fragrances,and other essential microsolutes, suchlosses can be minimized by limitingthe fraction of water removed. Testresults indicate that, with most liquidfoodstuffs, acceptable concentratesnot exceeding 30 wt. % solids can beproduced economically in this man-ner. Therefore, a hybrid process, in-volving preconcentration of the feedby RO followed by further concen-trating the RO retentate by OD,should yield a high-solids productconcentrate of quality comparable tothat achieved by OD alone, but at sig-nificant reduction in processing cost.

This hybrid process is also illus-trated in Figure 9. Optimization cal-culations indicated that RO precon-centration of an 18° Brix (18 wt. %sugar) fruit-juice feed to about half itsoriginal volume (yielding ≈30° Brixretentate at a permeation flux of about10 L/m2h) would cut the amount of


■ Figure 9. Hybrid concentration system for food and pharmaceutical products containing suspended solids utilizes UF, RO, and OD.

Condenser

BrineEvaporator

ProductConcentrate

FeedSuspension

(100 L)

(20 L)

ReverseOsmosis Stack

OsmoticDistillationMembrane

Array

Reconcentrated Brine

ROConcentrate

(40 L)

ClearFiltrate(95 L)

Dilute Brine

Concentrated Suspension (5 L)

RO PermeateTo Waste (55 L)

Crossflow/MicrofilterUltrafilter

To Waste or Recovery

water to be removed by OD to delivera 68° Brix concentrate by about 56%,with undetectable change in concen-trate quality. This reduces the evapo-rator capacity requirements by one-half, and membrane area for OD byover 30% — leading to a decrease incapital and operating costs that farmore than compensate for the incre-mental cost of the OD pretreatmentstage. Obviously, for feeds of lowersugar or solute content, removal oflarger fractions of the water by RO ispractical, with even further improve-ment in process economics. Pilot-scale operations with this hybrid sys-tem have confirmed these predictions.

Thus, it appears that a hybridRO/OD system is probably the mosteconomic choice for feeds whose pri-mary solutes are retained by ROmembranes. A first-stage UF treat-ment for removal of suspended solidsand colloids is also generally desir-able to minimize possible fouling ofeither the RO or OD units. For feedscontaining only macromolecular orintermediate-molecular-weight so-lutes (such as solutions of biologicalsor pharmaceuticals), preconcentrationby UF (in this case, discarding thepermeate) without intermediate ROconcentration may suffice.

APPLICATIONS AND LIMITATIONS

At present, the most thoroughlyevaluated and successfully demon-strated application of OD has beenthe concentration of fruit and veg-etable juices, conducted in pilot-plant facilities located in Milduraand Melbourne, Australia. The Mel-bourne facility is a hybrid plant(consisting of UF and RO pretreat-ment stages, an OD section contain-ing two 19.2-m2 Liqui-Cel mem-brane modules, and a single-stagebrine evaporator) designed and fab-ricated by Zenon Environmental(Burlington, Ont.). This unit canconcentrate fresh fruit juices at anaverage throughput of 50 L/h toyield a 65–70° Brix (65–70 wt. %)concentrate, and has evaporative ca-

pacity for brine reconcentration ofabout 100 L/h. Each membranestage is mounted independently onits own wheeled chassis with quick-connect sanitary fittings and flexi-ble-hose connections betweenstages, to facilitate changes in sys-tem layout for differing feedstocks.The system occupies about 400 ft2

of floorspace and requires headroomof about 12 ft.

This pilot facility has been usedprincipally to develop operationalparameters and economic data forthe design of full-scale plants and,secondly, for the production of sam-ple concentrates from feedstockssupplied by potential customers fortheir testing and evaluation.

The primary focus to date hasbeen on the concentration of varietalgrape juices used for the productionof high-quality vintage varietalwines (Chardonnay, Cabernet Sauvig-non, Merlot, Sauvignon Blanc, andthe like). These are clear juices re-quiring relatively modest pretreat-ment to remove suspended particu-lates or hydrocolloids. Concentratesof these juices are of great interest topremium winemakers, because theycan be stored for long periods with-out deterioration and can be used asblending stocks to adjust the sugarcontent of freshly harvested grapesto minimize variations in alcoholcontent of the resulting wine fromvintage to vintage. Another impor-tant advantage of concentrates is thatthey can be shipped over long dis-tances at relatively low cost andused to produce high-priced varietalwines in locations and at timeswhere local grape supplies are un-available, inadequate, or too costly.

An important economic motiva-tion for the use of varietal-grape-juice concentrates for winemaking isthe extraordinarily high cost of suchgrapes in many areas of the worldand the wide disparity in cost fromreg ion to reg ion . F resh ju ices frequently sell in the range of$0.50–$1.50/L. As one liter of freshjuice yields about 200 mL of 70°

Brix concentrate, the base value ofconcentrate is between $2.50 and$7.50/L. The total processing cost ofOD concentration (at commercial-scale production levels) is estimatedto be well below $1.00/L of concen-trate. There are, therefore, opportu-nities for high profits to be derivedfrom producing quality varietal-grape-juice concentrates by OD fromfruit available at low cost and ship-ping and selling concentrate to wine-makers serving high-price markets.

Other fruit and vegetable juicesthat have been concentrated by ODin this facility include high-valueclear tropical fruit juices, as well asapple and carrot juices. Interest alsohas developed in the use of the pro-cess for preconcentration of coffeeand tea prior to relatively costlyfreeze drying.

Concentration of citrus and otherpulpy juices has sparked some inter-est, but it is uncertain whetherdepulping and recombination afterconcentration will be technically andeconomically feasible, and whetherthe resulting concentrate would besufficiently superior in quality tojustify its higher cost. Moreover, cit-rus juices contain peel oils and otherhighly lipophilic flavor componentsthat reduce their surface tension andpromote wetting of hydrophobic sur-faces. Such fluids, in most cases,will promote wet-out and liquid pen-etration into polypropylene ODmembranes. Development work cur-rently is underway to attempt to pro-duce hollow-fiber microporousmembranes from more hydrophobicpolymers such as PTFE or PVDF orto make laminate membranes thatwill prevent liquid intrusion withoutimpeding vapor transport. Success inthis area should broaden the utilityof OD for feedstocks of this nature.

Pharmaceuticals and biologicals

Many manufacturing operations inthe drug and biologicals industriesproduce relatively dilute solutions ofmacromolecular products (such as


ON THE HORIZON

vaccines, peptide hormones, recombi-nant proteins, enzymes, and nucleicacids) or of intermediate-molecular-weight products generated by fermen-tation (like antibiotics, fungicides,and oligopeptides). Most of theseproducts exhibit rather poor thermalstability or are susceptible to denatu-ration and inactivation with changesin their solution environment. Isola-tion and recovery of these products inthe dry state often is essential tomaintain their activity and shelf sta-bility for extended periods. Today,preconcentration of these products byRO, nanofiltration, and UF is in wideuse, although the degree of concen-tration achievable without product ac-tivity loss via these techniques fre-quently is limited. Second-stage con-centration of these solutions by ODmay allow the production of bioac-tive product concentrates from whichsolids can be more easily recovered(by crystallization or extraction).

By far the most successful meansof producing a stable solid productfrom these solutions is lyophilization(vacuum freeze drying). Lyophiliza-tion, however, is a very costly and te-dious process, because it usually re-quires the removal of water by icesublimation at very low temperatures— where the sublimation rate is ex-ceedingly slow and the energy re-quirements for maintaining the neces-sary low pressures and temperaturesare substantial. In addition, the capi-tal and maintenance costs of industri-al-scale units are quite high.

There is growing interest in thepossibility of employing OD as a pre-concentration step for such productsprior to lyophilization, to remove asignificant fraction of the water fromsolution without product deteriorationand, thus, to reduce the water-re-moval load during freeze drying. Ef-forts currently are underway to evalu-ate, in small-scale tests, OD concen-tration of a vaccine or biological so-lution, the bioactivity of the resultingconcentrate, and the final activity andstability of the dry product made bylyophilizing this concentrate.

Dealcoholization and removalof volatile solutes

Another potentially important ap-plication of OD is the selective re-moval of a volatile solute from anaqueous solution. Here, the solute ofinterest is evaporated from the feed atthe membrane surface, transported byvapor diffusion through the mem-brane pores, and condensed into astrip liquid on the opposite face of the membrane. Most commonly, thestripping liquid is pure water or anaqueous solution containing a lesserconcentration of the solute beingtransferred. This process more pre-cisely can be described as “evapora-tive pertraction.” It functionally re-sembles dialysis, but is selective onlyfor volatile solutes.

The primary application nowunder active evaluation is the re-moval of ethanol from fermentedbeverages such as wine or beer. Thereis considerable worldwide interesttoday, for health reasons, in a methodfor selective removal of alcohol fromsuch beverages without adversely af-fecting their taste, odor, or mouthfeel.For wines, which typically contain11–15 vol. % ethanol, evidence sug-gests that reducing the alcohol con-tent to about 6% will yield a beveragethat closely resembles in organolepticproperties the original ferment, solong as the flavor and fragrancevolatiles (principally organic acids,

ketones, aldehydes, alcohols, and es-ters) are retained. Decreasing ethanolto below 6% in a wine may drastical-ly alter its taste and aroma.

OD of a high-alcohol-content wineat a temperature of 10–20°C usingplain water as the stripping liquid canrapidly reduce its alcohol content tolevels down to 6% with minimal lossof its flavor and fragrance compo-nents. (Of course, the process permitsremoval of nearly all the ethanol, ifthat is desired.) The mechanism ofthe process, as illustrated in Figure10, takes advantage of three factors:(1) ethanol is the most volatile com-ponent in the wine and the mostrapidly diffusing species across themembrane, (2) the vapor pressure ofthe flavor/fragrance components islow and, thus, so is their OD flux, and(3) the solubilities of the flavor/fra-grance components in alcohol/watersolutions are substantially higher (andtheir vapor pressures correspondinglylower) than they are in plain water.As a consequence of this last proper-ty, the driving force and transport rateof these components from wine tostrip is reduced even further. In addi-tion, because the vapor pressures ofwater over the wine and over the stripsolution are nearly identical, there isvirtually no traffic of water from thestrip into the wine. The variations invapor pressures of ethanol and waterover ethanol/water solutions are illus-


■ Figure 10.Mechanism of

selective alcoholremoval by evaporative pertraction.

DealcoholizedWine Out

Wine In

Water InDilute AlcoholSolution Out

Membrane

Increasing Alcohol Partial Pressure

Decreasing Alcohol Partial Pressure

WaterAlcohol

trated in Figure 11, from which therelative OD fluxes of alcohol andwater during dealcoholization can beeasily visualized.

Another advantage of this processis that it generates a strip solutionfrom which food-grade (190 proof)ethanol can be recovered by rectifica-tion. This is a potential blendingstock for production of fortifiedwines, liqueurs, and whiskeys.

Such a system for controlled deal-coholization is suitable for wineriesof any size, and is very simple andnearly foolproof to operate. The es-sential features of the system areshown in Figure 12. Wine from achilled storage tank is continuouslyrecycled through the shell side of theOD contactor array, while strip solu-tion (water) from a second storagetank is continuously recycled throughthe lumen side. The volume of waterprovided to the storage tank is adjust-ed so that, when the concentrations ofalcohol in the strip and wine reachequality, the wine will have the de-sired alcohol content. Thus, a 10,000-L tank of 15%-ethanol wine treatedwith 15,000 L of water will yield10,000 L of 6%-ethanol wine and15,000 L of 6% ethanol. This systemcan run unattended, except for occa-sional analysis of the strip forethanol, until the transfer stops.

Winemakers also encounter situa-tions where a dry-season vintage pro-vides high-sugar-content grapeswhich, on fermentation, yield inordi-nately high-alcohol-content wines.Such wines are considered “hot” tothe palate and, so, have reduced mar-ket value. Dilution with water cancorrect this, but is considered adulter-ation in many jurisdictions. Thus,there is a need for a means for “trim-ming” the wine — that is, removingalcohol selectively to a level of ≈12%without altering its flavor and fra-grance characteristics. Trimming byOD dealcoholization, using a smallrelative volume of strip water, accom-plishes this task rapidly and readily.

The cost of OD dealcoholization(per liter of ferment processed) is

considerably below that of OD con-centration, because the vapor trafficper liter of feed is much lower, pump-ing and instrumentation requirementsare minimal, and the ancillary costsof feed pretreatment and brine recon-centration are avoided. Moreover,rates of transmembrane permeation of ethanol are far higher than those of water; so, OD membrane-area requirements are much reduced. Re-covery of marketable food-gradeazeotropic ethanol is another costbenefit.

Opportunities also exist for selec-

tive removal of organic volatiles fromaqueous solutions in the drug indus-try. For example, miscible solvent ex-traction of intracellular products fromwhole fermentation broths is widelyemployed today. After separating outthe biomass from this mixture, itoften is necessary to remove and re-cover part of the organic solvent fromthe product-rich supernatant. This fre-quently needs to be done at low tem-perature to protect the product fromthermal degradation. Solvent removalby OD using water as the strip solu-tion is a convenient means for accom-


ON THE HORIZON

■ Figure 11.Vapor pressure

relationships forevaporative

pertraction ofethanol.

10

10

20

30

00 2 4 6 8 10 12 14

% Ethanol

Vapo

r Pre

ssur

e of

Eth

anol

, mm

Hg

Vapo

r Pre

ssur

e of

Wat

er, m

m H

g

Driving Force forWater Transport

20˚C

Driving Force forEthanol Transport

5

20

15

30

25

■ Figure 12. Dealcoholization of ferments by evaporative pertraction.

24 ProofWine

4-6 Proof Alcohol Solution

10-12 ProofWine

PertractionMembrane Array

Water

Condenser

190 ProofEthanol

RectifyingColumn

plishing this. The solvent-containingstrip then can be rectified to recoverthe solvent in high purity, leavingclean water for recycle.

Similar needs exist in the isolationand purification of antibiotics, hor-mones, and biologicals by hydropho-bic or inverse-phase chromatography,which frequently employs water/or-ganic solvent mixtures as eluting sol-vents. Eluant cuts containing prod-ucts of interest can be processed byOD to remove and recover the organ-ic cosolvent without product damage,providing an aqueous solution of theproduct for final recovery.

Commercial statusOver the past year, process devel-

opment studies with the pilot plantnow operating in Melbourne, andproduction in that facility of testquantities of juice concentrates anddealcoholized ferments for customerevaluation have confirmed the relia-bility and economic feasibility of theprocess for such applications and thesuperior quality of the resulting prod-ucts. Design and costing of a com-mercial-scale hybrid OD plant re-cently has been finished and con-struction of such a unit is scheduledfor completion later this year. Nego-tiations currently are underway to es-tablish a collaborative advanced-commercial-development and prod-uct-marketing program with a majorinternational producer of wines andfruit juices that has U.S. productionfacilities for varietal-grape and otherfruit-juice concentrates and low-alco-hol wines.

As noted above, the process ap-pears to have considerable promise

for the concentration of heat-sensi-tive pharmaceutical and biologicalproducts such as vaccines and antibi-otics. Selective removal and recov-ery of volatile organic solutes fromaqueous solution by OD also mayhave promising applications in thepharmaceutical industry for recoveryand isolation of thermally labilebioactive substances. The pilot ODfacilities now available are expectedto be used for evaluations of suchapplications in cooperation with sev-eral pharmaceutical manufacturerswho have expressed interest in thistechnology.

It should be emphasized that ODis a relatively more costly process for

water removal from solution thanmore-conventional processes such asdistillation, UF, and RO. Therefore, itis unlikely to compete with these con-ventional concentration processes ifthe concentrates they produce are ad-equate. Its unique ability, however, toproduce highly concentrated solu-tions of heat- or stress-sensitive prod-ucts with minimal deterioration ap-pears to make the incremental costjustifiable for many high-value prod-ucts. Hybrid processes, involving theuse of pressure-driven membrane-preconcentration processes such asUF and RO, promise to reduce theoverall costs of OD concentration andbroaden its utility. CEP


P. A. HOGAN was a senior process developmentengineer, with primary responsibility for thedevelopment of osmotic distillation processes,for the Wingara Wine Group, Melbourne,Australia, at the time this article was written. He since has entered the graduate degreeprogram in business administration at the Univ. of Melbourne (61-3-9531 5690; E-mail:[email protected]). He received a BE (hons) in chemical engineering as well as a PhD in industrial chemistry from the Univ. of New South Wales.

R. P. CANNING is Engineering Manager of ZenonEnvironmental, Inc., Burlington, Ont.(905/639–6320; Fax: 905/639–1812; E-mail:[email protected]), a company thatspecializes in the manufacture andimplementation of membrane technologies.During 1996 and 1997, he was seconded toPalassa Pty. in Melbourne, Australia, where he was responsible for the engineeringdevelopment and commercialization of osmotic distillation technology for fruit juiceconcentration and wine dealcoholization. Hereceived a B. Eng. Sc. in chemical engineeringfrom the Univ. of Western Ontario. He is aregistered Professional Engineer in Ontario, anda member of Professional Engineers of Ontario.

P. A. PETERSON is Marketing Manager for Liqui-Celmembrane contactors for the SeparationProducts Div. of Hoechst Celanese Corp.,Charlotte, NC (704/587–8584; Fax: 704/587–8610:E-mail: [email protected]). In his 17years with the firm, he also has served as aprocess/project engineer and a technical serviceengineer. He holds a BS in chemical engineeringfrom Worcester Polytechnic Inst., and is workingtowards an MBA. A Professional Engineer inTexas, he is a member of AIChE.

R. A. JOHNSON is a Lecturer in physical,inorganic, and analytical chemistry at theCentre for Instrumental and DevelopmentalChemistry at the Queensland University of Technology, Brisbane, Australia(07/38642016; Fax: 07/38641804; E-mail:[email protected]). He has considerableexperience in industrial research, much of it involving the development of membraneprocesses, particularly for the sugar, dairy,fruit juice, coffee, and fermentationindustries. He received a doctorate inchemistry from the Univ. of Queensland and completed a post-doctoral researchfellowship in coal research at the Univ. of Newcastle, Australia.

A. S. MICHAELS is President of Alan ShermanMichaels, Sc. D., Inc., Chestnut Hill, MA(617/323–9188; Fax: 617/323–3778; E-mail:[email protected]), a firm that providesconsulting services to the food,pharmaceutical, and chemical industries. He has held senior positions in both industry and academia: President of Amicon Corp., President of Pharmetrics, and Sr. Vice President of ALZA Corp., as well as Professor of ChemicalEngineering at Massachusetts Inst. ofTechnology, Professor of ChemicalEngineering and Medicine at Stanford Univ., and Distinguished University Professor at North Carolina State Univ. He received a doctorate in chemicalengineering from Massachusetts Inst. of Technology. He is a Fellow of AIChE and of the American Inst. of Medical and Biological Engineering, a member of the National Academy of Engineering, and a Director of the North AmericanMembrane Soc.

Further ReadingKunz, W., A. Behabiles, and R. Ben-Aim,

“Osmotic Evaporation through Macrop-orous Hydrophobic Membranes: a Sur-vey of Current Research and Applica-tions,” J. Membr. Sc., 121, pp. 25–36(1996).

Documents

Osmotic Distillation Using Liqui-Cel