CA1290257C - Osmotic concentration by membrane - Google Patents

Abstract

A B S T R A C T

A hydrophobic semi-permeable membrane process (e.g., using hydrophobic microporous hollow fibers) for the concentration of a dilute solution of low osmotic pressure by osmotic distillation (also known as membrane distillation or membrane evaporation), which is a separation process using as its driving force the difference in the osmotic pressure between two fluids (e.g., fruit juice and seawater) separated by a hydrophobic semi-permeable membrane to achieve a concentration of the fluid having the lower osmotic pressure (e.g., fruit juice). When coupled with reverse osmosis means there is provided a method for recovery of solvent (e.g., potable water) from the process, e.g., desalination of seawater. A process is also provided to recover part of the energy available between the two fluids i.e., between two different potential energies, due to the difference in osmotic pressure between the two fluids.

Description

The presont inventlon relat-~ to the conc-ntr~tlon of B l~quid by utlllslng the diffor~nc- of o~motic pres~ure betw~n two liquld4, and to app~ratu~
~uitablo therefore ~ho applic~tion for rovor40 o-~o-i- to the concentratlon of olutlon~, for xample dilute food products, ~nd to the produ~tion of potable wat~r from ~e~w~ter h~6 ln m~ny c~ r--ult-d in ~ignificant co~t b~noflt~ wh-n compared wlth tho conv-nt~on~l v~poratlon and o di~tillation pro~e~e# re6pectlvely Nov~rth~ , ther- i8 ~cope for a furth-r ~ub~tantla~ co~t roduction by ollmlnatlng the fouling probl-m a-~oc~ated wlth hydrophllic r-v-r-e o~mo41s membr~ne~ Th- ~-tt~bllllty o~ the~e membrane~ provlde~ for clo~- cont~t wlth ~n adherence of foul~nts ~uch ~ organlc colo~rant~, varlou~ lnor~nlc ~alt~, anC ~u~pendet ~nd colloldal m-tt~r ~oul~ng h~s in some oaseæ b~n allovlat~d by pr-clplt~tion and filtr~tion pr-tr-~tmont proco~40s ~o~evor, thl~ ~dC~ ~ub~t~nt~lly to th- co-t o~ the overall proee4~, thus r-duc~ng th-attract$vene~s of the reverse o~mo~l~ technlque.
Hydrophobic mombr~no- of variou~ porc dlmen~ion~
h~ve ~een developed ~or varlou~ ~pplicatlon~, namely mombrano dl-tillatlon And ~rtificlAl oxygen~tlon.
Hydrophoblc m mbr~n-- ~r- g-n-r~lly mDd- from ~n hydrophobic polymor, o g polytetra~luoroethylene *tTEF~ON) or polyp~opylon~, ~uch a~ m~mbrano~ producod undor the n~me *CE~GARD
Hydrophoblc mombrano~ can al40 be ~ade from hydrophilic polymor~ whlch h~vo boon ~peclally troated to havo r-atioul~tlon of flurocarbon radlc~ t tho urf~co of ~h~ polym-r Hydrophoblc m-mbran-- ~re widoly u--d in tho ~cdlcal lndustry for ~rtlflclal oxyg-n~tlon of blood Such artificial oxygan~tlon unlt4 ~re dlocJrded ~ft-r u~, an~
aro ro~dlly ~vallablo from ~04pltAl~ after u4e * denotes trade mark 1;~90~57 It is an object of this invention to provide a method of, and apparatus for, the concentration of a liquid by utilising the difference of osmotic pressure between two liquids.
It is another object of the present invention to recycle such membrane oxygenation units for use in a membrane separation process using as the driving force the difference of the osmotic pressure between two liquids to achieve a concentration of the liquid having the lower osmotic pressure, or to recover part of the energy available between the two different potential energies of the two different liquids due to the difference in osmotic pressure between the two liquids.
It is yet a further ob~ect of this invention to provide a method of, and apparatus for, the concentration of a dilute solution by a membrane evaporation or osmotic distillation process by the transfer of solvent from the dilute solution in a vapour state through a hydrophobic (non-wettable) membrane into a concentrated solution of higher osmotic pressure.
It is yet another object of this invention to provide a process for the fractionation of, or recovery of a solvent from, a solution by a combination of osmotic distillation and reverse osmosis processes.
These and other ob~ects of the invention will be apparent from the following disclosure of the invention.
According to one aspect of the present invention there is provided a method of osmotic distillation for the concentration of a first aqueous solution of relatively low osmotic pressure comprising: circulating said solution on one side of a hydrophobic porous barrier; simultaneously circulating a second aqueous solution of a relatively high osmotic pressure on the opposite side of the porous barrier;
sol~ent from said first aqueous solution beinq transferred across the porous barrier in the vapour state under the influence of an osmotic pressure qradient to the second aqueous solution resulting in concentration of the first solution.

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According to another aspect of ~he invention there i9 prov.ided a method for recovery of a solvent from a primary aqueous solution of relatively low osmotic concentration, comprising:-(i~ subjecting the primary solution to osmotic distillation with a porous barrier which separates the primary solution from a second aqueous solution of higher osmotic concentration, characterizedi in that said porous barrier comprises a hydrophobic porous matrix and wherein an osmotic pressure gradient exists between said primary and second aqueous solutions sufficient that solvent from the primary solution on one side of the porous barrier is transferred under the influence of said gradient through the porous barrier in a vapour state to the second solution on the opposite side of the porous barrier; and (ii) further subjecting the diluted second solution from the previous step to reverse osmosis by reverse osmosis means whereby the solvent is separated and the second solution is recovered and concentrated suitable for recycling.
According to a further aspect of the invention there is provided a method for desalination of seawater, comprising:-(i) subjectingthe seawater to osmotic distillation against a salt solution of higher osmotic pressure than the seawater, wherein the seawater and the salt solution are separated by a hydrophobic porous barrier, sufficient that water from the seawater is transferred through the porous barrier in a vapour state under the influence of an osmotic pressure gradient and into the salt solution on the opposite side o~ the porous barrier thereby diluting the salt solution;
and (ii~ sub-Jecting the diluted salt solution to reverse osmosis treatment whereby water is separated from the salt solution, and the salt solution is concentrated suitable for recycling.

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~ ~9~2~7 According to yet another aspect of the presen~
invention there is provided an osmotic distillation apparatus for concentrating a first aqueous solution of relatively low osmotic pressure comprising:
a hydrophobic porous barrier and means for circulating said first solution on one side of said hydrophobic porous barrier;
means for simultaneously circulating a second aqueous solution of a relatively high osmotic pressure on the side of said porous barrier opposite the side on which the first solution circulates including means for causing solvent from said first solution to be transferred across said porous barriex in a vapour state under the influence of an osmotic pressure gradient to the second solution resulting in concentration of the first solution.
Osmotic distillation is a process by which dilute solutions such as seawater, fruit juices, milk and coffee can be concentrated by transfer of water in the vapour state through a hydrophobic ~non-wettable), non-fouling membrane into concentrated brine ~e.g. magnesium sulphate, MgSO~
solution) which is free of fouling material. The latter solution may then be treated by reverse osmosis for reconcentration and recycling to the membrane distillation unit and, in the case of seawater treatment, for ~he recovery of potable water. In this way the expected life of reverse osmosis membranes can be extended significantly.
The invention will be further described with reference to the drawings, in which:-Figure lschematically illustrates the concentration of ~ruit ~uice against seawater or concentrated brine by osmotic distillation.
Figure 2 schematically illustrates apparatus according to the invention wherein an osmotic distillation i -- 5 ~
l.Z~ 57 tube i~ ~ouple~ to a rever~e oomo~l~ unit for u~e in ~ .g .
~alinatlon - i .e ., t~e p~odu~tlon of potabl~ wa~r o~ high purity from the br~cki~h w~t~r or ~awater~
~iguro 3 1~ ~n enl~rg~d ~iew of th~ m~rnbrane w~ll of ~he o~motic di~tl~ tlon a~ tus of ~i~ure 2, ~chematlc~lly lllu~trating th~t on~ o~ th~ m~in d2; ivlng for~es or the trans~er o~ ~Ato~ fro~n the oe~w~ter a~ro~
the m0mbr~ne w~ nd into the M~SO~ ~olut ion i~ a v~pour pr0~ure gr~d lent ~
~l~ur6 4 is a ~raph I llu~t~atlrlg the ~everoe 09mo5is~ i~lux v~r~us MD~04 conc~ntratLon at ~n opera~in~
pr~ur~ o~ 4130 kPa ( ~0. 8 ~tn~ of a~ r~tu3 of the typ~
illu~tr~t~d ln Fl~ure 2.
Flgure 5 i8 a ~raph illu~tratin~ th~ o~motic di~tillation flux v~us MgSO~ conc~ntration or a diJtilled w~torfM~80~ sy3tem ~at 30 kPa wat~r b~k pres~ur~).
Pigur~ G i~ ~ ~r~ph illu~tr~ting the o~moti~
~listillation ~lux v~r~l~8 the t~mpora~ur~ dif~erqnce b~tw~n the tamp~rature of the inlet di~tllled water ~nd the temp~r~ur~ o~ ho inlet Mg~04 aolution tat 30 kPa ~4at~r baok ~ ur0) .
Figure 7 i~ a ~r~ph illu~tratln~ ~h~ oomot~
di~till4~ion flux ve~u~ ~9~O~ ~oncentration for a soawat~r/M~04 ~y~m (at 30 kPa a~aw~e~ b~k pr~ssure).
Fl~ur~ 8 i~ ~ ~ra~ lllu~tratin5~ tho o~moti~
di~till~t~on flux ver~u~ the te~lpers~ur~ ~lfference be~ween ~e temp~ratur~ of the lnlot ~eawater and the t~mp~r~tur~ of the lnle~ 04 ~oll~tlon ~t 30 kP~ ~eaw~te~ 4~ck prea~ure)~
Fls~u~ 9 14 a g~aph illu~tr~ting ~h~ o~moti~
dlotlll~tlon ~lux ver4u~ 8eAwate~ ~low ruto ~.144 Mg~04, 40 kP~ SeAwat~r back pr~a~ur~, 0.71 l~min MyS04).
Figur~ 10 i~ a grA~ 111u~tr~1ng the o~mot~-c di~till~tion ~lux ver~u~ M~S04 ~luk$on ~lo~ rat~ (14~
MgSO,~, 40 kPa ll-a~at~r b~k praa~ure, 1.36 l/mln 8eawater l~low r~to).

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Fi~ure 11 ~ ~ d ~r~ph illu~tr~atln~ the rolAtion hl;p b0~w~en the 03mo'cio dl~t~llation ~lux And ~S~SO"
~on~en~ration under optlMum condieion~.
In th~ win~, Figure 1 ~homatlc~lly illu~tral:e~ the conoentratlon o~ orat~e ~uice in a hollow f ibr3 o~motlc di8~illation unit whorRln :Ere~h or~nge ~uic~
at a oon~ntra~lon of 12 ~r~ x i~ pa~ll3e~ th~ough ~ho ~ntre lumen~ ef ~ha bundle o~ hollow f ibres in the~ 04motio di~tillat~on unlt ~nd 6~wa~r i~ pa~3d ~oun~ ur~en~ in th~ ~acket~d ~p~ce o~ t~ o~motic di~ tion unit ~urroundin~ the bur~dle of hollow f ibr~ . Th~ o~mc~
di~tlll~tlon unlt oo2n~ri~ a ~ylindric~l polyc~rbon~te oute~ 4holl ~ t~r 100 mm, l~ngth~ 300 mm) wlth ~onlcal ~ntry (top) and ~xit ~ottom) chamb~rs Por the orang~ julce beln~ ~c)no~ntra~t. Th~ or~try and ~xlt ~hamber are ~ink~d by ca. 62,000 ~yd~o~hobl~ polypropylene hollow fibre membrane~ t in~lde di~nletor ~oO uln; w~ll hiQkn~ 2~ um, ~ffe~ e l~n~th 140 mm, sver ~o pore diam~t~r 700 A, ~oro~i~y 50 ~er ~n~ total ~r~a 5.4 m2~ w~h ~re ~ Qd toS~h~r ~t bo~h 63nds in a polyurethana r~3~in (potting compound) ~o . hAt ~i~ey a~ ~on~lned in ~ j~cket ~i~h en~ry an~ ex~ por~. Filto~ed ~eawa~er cont~inlng no su~pen~e~
or collold~l ma~erl~ p~lmp~d throu~h this ~a~kat approxlmat~ly count~rcurront to the ~n~rnal ~lowO

2 5 Th6 pure or~nge ~ui~ pumpod through the 091110tiC
d~tlll~tion ~ube ~nd~r low ~r~ure be~ome~ ~oncentra~ed il8 it pa~e~ thrwgll t~e llollow flb~ a to ~ traE~er of ~ter Acrc~ the ~lbre w~ll lnto the s~w~tor. 'rhe drivin5~
for~ for thl8 tran8far i~ the ~ombinod effect of ~ vapour pre~ura ~or o~moti~) ~rAdi~nt 21n~ a teml?~ratur~ ~radient, brought elbout by an ~l~va~ion of ~h~ our pr~ r~ o~ ~he o~an~e ~ulce by g~ntl~ he~tln~ b~or~ lt 2nt~r3 th~ ~ub~, and ~ depro~ed ~aawator vapour pr~sure by virtu~ of it~
rolatively ht~h o0motic prs~ur~ The hy~lrophobicity o~ th~
men~bran~ pr~vents tran~r o~ llqul~ wat~r and th~ i~oulin~

~ ~90~S~7 pro~lem~ ociated wit~ hydrophobil$c m~mbr~n~
Figure 2 ~howa tha ~o~pling o~ ~n 04I110tiC
di~tillatioFl tub~ and rever~e o~lno~i~ unit ~ulta41~ ~or de~alinAtlon of sflaw~tcr or b~a~ki~h water.
A ~uitabl~ o~mo~1~ di~t~lla~ion tubo com~ri~es a cylin~rlc~l ~olycarbona'cc out~r ~holl ~di~meter~ lO0 mm~
langth 300 mm~ wlth cor~ical entry ~op) and exit ~bottoln) ch~mber~ ~or the ~olu~ion b~ng concentr~t~d. Tho entry and ~xi~ ch~mb~ link0d by ¢al. 62,Q00 hydrophobit~
~olyp~opylene hollow i~ibro mombrano~ (ln~lde dl2mt0r ~OU um, w~ll thickne4~ 25 um, ef~ectivo len~th 140 m~ verag~ por~
di~m~ter '~00 A, poro~lty 50 por ~nt, total zlr~3a 5~ m2) which ar~ oealad tog~the~ ~t botb ~nd~ in a polyureth~ne re~ln ~ot~ing ~om~ound ) 40 that t~y ar~ con~in~d in a ~ack~t with entL~y and ex1~ po~t~. A ~on~entrat~d brlne 301utlon ~ .9. Iaa~nooium ~UlphPt~ 9 M~S04~) ~ontainir,~ no ~u~pen~ed or Gollold~l mst~rial i~ pump~d ~h~ou~h thi~
ja~k~t ~pprox~mat~ly oount~r~ur~ont ~o t~o inte~n~l ~low.
Norm~l (dllute) 40aw~tor pum~od through ~ho o~mo~lc dl~till~ion tu~ und~r low pr~s~ur~ b~coma concontr~t~d ~18 It ~4e~ ~hrou~ ho hollow fibr~ duo to a 'crar~ r o~ wa'cor acro33 the ~ibro wall~ into tho MgS~d~
~olu~on. Tha drivincl ~ors~ ~or th~ tr~ns~r i3 a v~pour pr~ure gr~ nt, brou~ht abou'c by an olevation og th~
v~our ~re~urs of tho dilut~e ~olution by h2atln~ b~a~oro l'c enter~ th~ tubo, and a d~pr~ d M~S04 ~olutlon ~ou~
~res~u~e by vlr~uo of lt4 hi~h o~mo~ic 1pr~3ur~. ~h~
hydro~hobiaity ol! th~ m~mbran4 provl-ntl~ tr~n~or of li~uid water And tho i!oulin~ probl~m~ ocl~tGd wl~h hydrophili~

3 mom~r~n~ .
Tho dilut~d MggO4 ~olution i~ pump~ to th~
r~v~r~ o~mo~i3 ~R,0, ) un~t wh~r~ ~otabl~ water lo ~xt~acl:~d u~ing Bend R~e~rch hollow fib~ R~0. membrane0. The te4t ~pp~r~u4 ha~ provi~ion for ~cwo RØ men~bran~ modules/ ~ach o~ which cont~ln~ nomln~lly o~. 1300 hs:~llow fibro0 .
. ~ ~............... . . -. ~ .. ; ( . , --' : , ..

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~2s7 ~ e~feativs l~n~h 7~0 mm, to~al ~a per module 0~9 m2~.
M~So~ w~ osen for the ~rine aolutlon becau~e of it~ hlgh rsj~ction by the ~ev~r~e o~m~ mem~ran~.
Figure 3 ~chama~ioally illustrata~ that one of the m~ln dr~vin~ ~or~8 for tha tran~fe~ o wate~ ~ro~ ~he qe~wat~r k~lativ~lY low o~motic pree~ure~ ~c~o~e ~he membrane wall 2nd into th~ M~SO~ ~olution ~rel~ively hi~h o~mot~ ~re~ure) i~ ~ v~pour pr~ur~ g~adi~nt. ~omb~n~d wl~h th~ ~ffact of th~ t~mp~ratur~ ~r~dient ~s~ween th~ t~o lo solu~lon~ th~re 1~ provide~ the me~n~ ~or ~ub4t~n~1~11y in~r~31n~ the flux of wat0r by the proce~ of m~mbran~
di4t111~ion, ~h~ osmotic distlllatlon u~lt pro~uGe~ e ~lux~8 ~t hl~h M~B04 ~oncsn~ra~ions (wh~re the M~04 s~r~m i~ hyd~ated) and at high te~p~r~tur~ dif~eren~ of ~he ~04 ~tream ~n~ ~e~w~te~ ~t~m.
Th~ ~evor3~ osmo~1~ unit p~rorm~ b~t Bt low ~S04 oono~ntr~tion ~o ln ~ou~lifl~ the t~o unit~ a compromi3e had to b~ mad~ to en~bl~ hi~h flux and low M~04 ~ono~ntr~tionq in ~he ~4 p~r~ate~
o~mi~A~ion ~tudios on the con~lton~ of op~ration w~r~ c~r~i~d o~t on both the o~motio ~istillation snd r~ve~ O~mQ~i~ sy4tem~ in ord~r to ~ff~ct an efflci~nt eoupllng o ~he two unit~. ~xp~rim3nts w~re de~l~n~d to ~eer~n t~ ~et o~ oper~tln~ cond~tions whi~h would allow ~he m~xlmum yleld o~ potable wa~r from ~awa~e~.
In the ~xperiment~l ~pparatus compri~ing a Syrinx Ro~ea~ch 4-tube ~motl~ d~t~ tion unit ~oupled wlth a Bond Re~rch ~-tub~ rev~ra~ o~o~ nlt, the reverso o~mo~l~ flux wa~ found to incre~e line~rly wi~h ~eed ~r~13ur~ up to ~h~ maximum safs3 opflratin~ pr~ r~ ~eqt~d o~
4130 kP~ ~40.B 4~m ~r 600 p.~.i.)l All testin~ waa c~rrled 4Ut At th i~3 pretl~ur~ .
Fi~uro 4 illu~r~t~ the reve~se 04mo~ R.O.) ~lux ver~us ~g8O4 ooncen~r~ion ~n~ dhow~ th~t 41gn1ic~nt '~'"''' " ,-.

~L2~57 f l~xe~ cannot b~ obt~in~d a~ the pre3su~ uo~d wi~h ~olut~on~ of M~04 ~ona~n~r~tion~ ~eater than ~ou~ 14 per cent, Reje~tlon ~or retont~on) of the M~SO~ ~y the R.O.
membrane d~cro~e~ a~ it~ con~ontra~ion lncr~a~
Th~4e re~ult~ ~nd~c~t~ for ~hla ornbodiment~ th~t ~or maximum 1ux ~nd M~SO,a r~ection~ th~ R.O,. un~t ~hould be op~rat~d at 4130 kPR u~lnç1 ~he m~nimum p~4~1ble aon~ntration o~' MD~04. ~lowovor, the ~ctual conc~n~r~ion u~od in the coupled sy~tem w~ 11 be l~ ly de~endent ~n the o ~onçentra~i~n raqu~red to ~iv~ an aoeeptabl~ osmotl~
dl~ tlon ~lux~
In order to deto~m1 n~ ~ome ~a~ic opera~ in~
a~a~acterlstl~s ~ the membr~ di~tllla~ion pro~o~ And to more ~ully under~tAnd the bohaviou~ o~ ~eRw~ter in thi3 proce~, the initial ~erl~ o te~t~ involv~d pa~;n~
dl~tilled waSer rather than ~0~waSer th~ou~b t~e hollow f ibro m~mbr~n~a, . . .

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MgS04 solutioil - di~tilled water system.

Referring to ~IG. 5, lt is seen that membrane distillation fl~x increase~ bxponsntially wlth lncreaslng 5 MgS04 concentr~tion. r:rhis 1~ pre~umably due to lncr~ed lowering of the wat~er vapour pres~ure orl the Mg~04 solu~ion slde of the membr}tne with ~ resulting increase in wa~er vapour pressure gradi~rlt acr~ he membrane.
Periodic analysi~ of the outle~ water xtream show~ that 10 there was no le~k~ge of MgS04 acroYs the membrane, thu~
conf irmin~ ~h~ hydrophobicity oE the latter .
FIG . 6 illustrate~ flux versu~ difference b~tween inlet water ten~perature and inl~t~ Mg~0,~ solution temperA~ure~ The observed increase ln ~l ux with in~re~sln~ temperature 15 gradient is a ~onsequ~nce of the enhanced water vapour pressur~ on the w~eK sid~ of the n~em~r~ne rela'cive to tha~c on the M~S04 solu~ion si~e. ;~

(b) MgS~4 solution - se~w~ter system FIG. 7 illu~tr~tes flux ver~us M~So4 con~en~rla~ion. As ln the case o~ distllled wal:er there iq an incroase in flux with increasing ~gS~4 cvncentra~ion. However, th~ seawater fluxe~ are c~nqi~erably le8$ than those or di~till~d w~ter at the same Mg50" concen~rat~ons due to the hi~h ~smoti~
p~e~sure o~ the seawater, ~ndeed~ no ma~s tr~n$~er from seawa~er wa~ ob~erved for M~S0~ concentration~ ~elow ~ to 10 p~rcent.
The latter obse~ation i~ con~i~tent with the known a~mo~ic pre~.~ure~ ~or M~04 ~nd NaCl ~olution~. As sho~n in ~X~. 7 the ~eawater te~ed hehav~ in a simil~r w~y to a 2.6 per cen~ NaCl ~olu~ion. A 2.6 p~r ~ent ~0.44 mol~r) Na~l solution has an osmotic pres~ur~ laa- lg atm) e~uivalent to that o~ a 0.78 molar tg~ per c~nt) MgS0~ ~olutivn ~ 25~.
3S Therefore, ln the ab~ence o~ a temper~ture ~radient a v~po~r pre~sure gradlent acros~ th~ membrane w~ll only exist if the MgSl4 concentr~tion ~xceed~ 9~4 per ~ent.

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~ ~0~57 Flgure ~ illu~tret~ ~lux ver~u~ di~eren~e betwe~n inl~t ~e~w~ter temper~ur~ and l~le~ ~g~O4 ~olutlon t0mper~ture, ~c $~ ar ~rom Fisur~ ~ th~c there 18 ~n obviou~ trend towa~d~ hi~her 1uxes wl~h incre~aln~
~emporature g~adient. For ~ klg5Q,~ ~onc~nt~atlon of 12 pe~
~nt~ ~ ~lux of caO 0.5 ll~re~ p~r ~uara m~Le per d~y 1 ob~e~ved in the absen¢~ of a ~lux ~ t~mp4aratur~ slr~dlen~
~Figure 7). Howe~rer, Fi~ure 8 ahow~ that thi~ ~lux c~ be increa~e~ at l~t- 3-i~old by ~mploying ~ te~nperature gradi~nt of 30 to 35C~ C.
Fi~ur~ ~ illu~r~e~ ~lux ver~u~ ~eaw~ter flow ~at~ . For a M~SO4 ~olution ~low rate of 0. 71 li~r~ per minut~, flux incr0~ with ln~reasln~ ~awatar ~low ~at~7 T~ pr0~umably duo to ~nore r~id repl~o~m~nt of the ~130AW~ fou~dary l~yer in ~hl~l tl~e ~alt çon~entr~tlon - in~r~a~s a~ ~ ~pour kr~nsfo~ o~r~ ~ A build up o~ ~lt ln ~he boundary l~r would ¢au~ a lo~all~ed lnc~e~o ln osms~ic pree~ure and henca a d~cre~e in ~r~pour p~e~u~e ~ra~lent .
Fi~ure 10 ll~ue~r~tos flux v~r3u~ PlgB04 ~olution f low ra~e . For ~ w~ low rat~ of 1. 36 litre~ per minut~, ~lux d~cre~e~ 411gh~1y wlth lnoreasin~ MgS04 solution flow rat~. As in th~ ca~s o ln~re~ing ~eawater 10w rate above, thia pre4um~bly ~u4e8 ~ore rapid r~pla~ n~ of th~ boundary l~y3r, th~ time on th~ Ms~04 ~olutlon sid~ of th~ m~mbran0 . A bu~ ~d u~ o~ ~S0~ in the ~o~ln~ary la~r ~nd hon~ ~n incroo~e ln o~motlc ~re~ure 1 ben~cl4~1 in th~ ino~ the v2~0ur pr~s~ur~ gradient i~ inor~o~ad ~
q~h~ r0~ults o~ 'eho optiml~t~on ~tu~13~ gge~t th~ t~e con~ltion~ ~or o~e~tlon of the o~mo~l~
di~tilla~ion unit ~houl~ b~ ~el~t~d on th~ ba~i3 of ~h~
followln~ ~onsid~r~tlon3l-t 1 ) ~he ~flwo.te~ ck ~r~ur~ ~hould be at 1488t 30 k,PI~.

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5~7 2 ) Th~ temporatu~o ~r~d i~nt ~croo~ t~le menlbr~ne ~hould b~ a~ larg~ ~ p~tl~Ablo . Th~
manu~a~tur~r~ o~ the m~mbr~no h~v~ ~&clfi~d an uppar t~m~r~ur~ limi~ oi~ ~a., 75 C a~d h~n~e a ~awat~r lnlot t~7m~r~ure of ca. 70~ C ~nd ~mblent ~Sg80~ ~olutiorl t~m~ra~ur~ sr~
r~aonunen~e~ ~
3 ) The ~eaw~t~r ~low rata should b~ a~ la~o and the M~04 ~olutlon f low rs~ a~ 3n~all ~ ~ra~tl~flbl~, o In thls rega~d lt ~hould be ~ememb~r0d` thaS th~
~xtent to which the ~gB04 ~olution ~tream i8 ho~ted in th~ tube ~n~r~a~ a~ it~ f low r~t~
d~cr~e~. Exc~iva h~Atlng of t~ t~m redu~ th~ av~rag~ t~m~raturo gra~iont acro~
th~ mbrane and n~o~ltat~s mor~ ~fectlv~
eoolinç~ oro ro~hln~ l:h~ r~ver~e o~mo~l~ unlt.
H~ving r~rd i~or th~e ~a~ors~ w~r flow rat~ of c~ rs~ ~or minut3 and M~0~3 aolu'c~on i~ow ~to o ~. O. 5 litr~ er mlnut~
2 o aro r~oomm~nde~ i 4) ~he M~S04 ~ono~n~,:r~l:ion o ~reatsr than ca ~ 3 to 10 po~ cent i~ r~au~ . Ho~ev~r, ~h~ rno4~
suitabla con~ntr~ion mll~t b~ d~to~min~ by ~xl~r1mont employing th~ optimum cond~tlon~ of w~te~ b~ e4~ur~, t~mp~r~'cure gr~ti~nt and flow ral;~ dig~84~d ~bov0. ~lS0, tha c;~Lqclty o ~h~ r~v~rs~ 0~2;10~i~ unit ~o ~mov~ water ~rom tho ~SQ4 ~olu'eion At th~ 4Amo L~at~ a~ it 1~
r~o~ from th~ wat~ by tho oo~notlç
dl~till~tion unlt n~uut b0 takan into aaooUnt whon dot~ ninu this ~onaon~r~ion, ~h~ ral~tion~3hip b61two~n ~ux ~nd kigS04 401ut1on ~on~en~A~ion unde~ opt1mum con~ition~ 19 8hOWn in ~i~ur~ l l, Tn thi~ c~a~ th~ 1ux 80~ iB
~iv~n ~n lll:r~3 p-r day f~r a 4-tub~ un1t ,. .

~ .

- ~la con~truct~ by syrinx ~ ~ar~h for coupling wlt~
~h~ ~ond R~e~rch revel~ o~lno~i~ unit.
Flsauro~ 4 and 10 ~how th~t ~ 14 pPr ~n~ Mg80,~
~olutlon ~ n osmotlc dl~tillation ~lux o~ 50 lltre~ p~r d~y and ~ha~ the dllutea M~SC)4 ~ol~il;ion ~an b~
re~on~0ntrated by th3 r~v~r~ o~mo~l~ ur)lt p~ovided tha~
both o~ th~ membrano module~ sup~lied by B~nd ~as~rch ~r~
ir~ o~eration. ~t a concon~rat~on o~ 14 p~ c13n~ Mg~0,9,, the combin~d ~lux o~ the two rover~q 03~n~31~ m~ambran~

.
~; . . .
. : ; . .

.

~ 2 ~7 ; module~ Is ca. 2.5 l.Ltre~ par ~q~arc metre pe~ hour ~55 res per d~y). Fi~ure ~ qhows that consi~erahly hiyher ~nembrane di.~ tion fluxe~ ~n be achieved at higher M~S0 ¢oncontra~i~n~ ~e.g. ea. 90 l.itre$ per day at 18 pe~ ¢ent MgS0~)~ How~ver, the particular ~embrane modules used In this particul~r embodiment ~Ire in~pa~la of with~tanding the hi~h pressure required to overcome the osmo~ie pres~ure of su~h ~olu~lons.
In summary t~ result~ ~how th~ he pres~ure ciiferen~e acro~s ~.he mernhrane i9 pe~h~ the main contributin~ variable determinin3 ~he Elux. The p~e~sure ~i~ference ls due to osmotic pre6~ure dlfferenc~ an~ te~pera ture di~ferenc~s a~ross ~he membrane.
The experimental r~sult.s ~how that M~ S04 concentration and temp~r~tures of the M~S04 ~tream and ~awater stre~m ~re very impor~ant v~rl~les in gover~ing t.he flux. This is because the Mçl~0~ ¢oncentration i~ related dire¢tly to the osmotic~ pre~ure of ~he st~eam at a ~iven te~rature.
Another important v~riable is the seawater flowrata.
The o~otiç pres~ure of ~ liquid is linked to the vapour pressure at equilibrium by t.~e re1a~10n~hip, Vl e RT ln P (1 P
where:-7r = os~otic pr~4~ure v1 = p~rtial molar volum~ o~ ~he so1ution R - Universa]. ~ C~nstant T = Ah~olute T~mp~rature ~ and P ~ the VapOUL pr~ ures ~f th~ ~wo cli~eren~
state~ of the 1ic~lld ~l~e. with and wlthout solute ) Thl~ rel~tior~hip 1~ ~pplicab1e prvvided ~hat tha vapour behave3 a~ nn ideal ~a~ and the eo1uti~n i~ lncornpre~ib1e.
I~ the ~olu~lon ~nd~r ~nsidar~ti~n beh~ves idaa11y ~hen ~aoult's Law i~ o~ey~ ~nd~PO ~ 1 ~ x2 where x2 is the mola fraction of the ~olute~

~29~5~7 nl ~ ''2 wh~re n = number of mol~cules in a ~iven volu~e nl ~ number of mole~ of ~clvent~
n2 = n~mber oP moles o~ solute.
~or thi.s reason, any 501~te low~r~ the vapour pr~sure of a solver.t, and ther~ is at ~he ga~ interfac~ hetween two li~uids havin~ diferent o~rnotic pressures a tran~er of sol~ent from the lower o~mo~:ic pres~ure liquid to the h1g~er o~motic pre3sure li~uld.
T~is process ean be illu~i~r~ed usin~ an a~ti~iclal oxyger)a~r oE the ~ollowLng typt3 developed ~y Terumo A 15 C~Pr~tlon of Japan -Type: TERUMO C~PIOX XX 1~ - ~ollow ; Fi~re oxyge~tor with Int~rated Heat ~xchanger Code ~o~: CX*MP16 2~ E~ectiue surEa~e Are~: 1.6m2 Maxi~um Blood Flow R~te: 2 l~min.
Maximum Operating Pres~ur~: 1,000 mm Hg.
The hollow ~ibre m~3m~rançs are m~de from polypropylenel and have pore dlmension~ of 700 An~$~ro~. Thls equipment is normally used for artificlal oxygenation of blood, with blood bein~ circ~lated through th~ hollow fibres and oxy~en gas being circ~ ted in the j~cket surroundin~ the hollow ribres, i.e. cn the outside ~f the hollow f~rqs~ In thl~ ~ase ther~
is transer of oxy~en ~as rom the out~l~e of the hollow ~0 ~ibr(3s to the bloo~ circulattn~ ln~id~ the hollow ~i~re~.
~ or thi~ typ~ oE rnedic~l applicAtion, ~u~h hollow f1~re axygenator unit.~ are norn~11.y di~po.~od of a~r a ~lngle Use.
However, ~ecordiny to the ~chin~ o~ one a3pect of the pre~ent invent.ion lt i~ po~sihle to re~ycle ~u~h hollow ~lbre oxygenator uni~ for di~erent applic~tion~
Takin~ such a u~ed oxygen~to~ unlt, and ¢leanin~ tha uni~ with ~uita~la alean1ng a~ent ~ e.g,*PYRONE~, dryin~ the *a/e~Jc~ 25 ~ v/e, rnc~k .. .. . .
. , ~ , : , .

.~ .
;' unit a~ inere~slng the hydro~ho~i~1ty v~ the ~ibre by chemical treatment, we obtain a unit whleh can be used for osrnotic concentr~tion.
The ehemical tre~tment of the hollow fibre~ to in~rea~e the hydrophobici~y is desira~le t~ maintain the long~term workir~g proper~.ies of the un~t ln osmut~c ~oncentration proee~ses.
An example of the chemical treatment of ~he hollow fibres is given hS follows:-1. Pre-irradiation of lhe hollow ~ibre unit ~illed wi~h pure nitrogen, u~in~ a Cobalt 60 source at a ~adlatlan lavel o~ 2 meg~rad.
2. ~illir-g t.he unlt w;.~h a 1uorocarbon ga~ of the Freon ~eries.
3~ Rinsin~ with di~tilled w~ter.
If in su~h a unit pure w~er i~ circulated ins1de th~
hollow fibre.s, ~nd seawater i9 circulated in the jacketed r~pace surroun~ing the hvllow fibre~, the pure water would evapora~e lnsid~ the pores and i~ conden~ed on the Yeawa~er side, with a flux of approximate}y 12 1/m2/d~y c~ room temperature.
In the sa~e manner, i~ the pure water stream ~ B replace~
by any li(juid having an o.cjmot.lo pre~Yure lower than the osmotic pre~sure of the ~eawaterr then th~ said liquid will be carlcentrated~ as a result of ~:he contin~l~us evaporation o~
it~ water eontent, ~nd thb condens~ation thereo~ on th~
~eawate~ si~e.
By reference to equ~tion (1~, above, the flux o ~olvent thro~gh the system ls proportional to the ~ e~en~e P-P~
which is in re:lation to the di~ren~e in o~motic pre$sure7r.
The ~lux is ~lso ln1u~nc~t3 by ~Ottl the thermic ~ra~1~n~ and tt~e ~re~sure ~3radien~ ~etwe~n tdle two ~olut.ions. For example, usiny the ~am~ me~hr~ne, if we cir~ulRtd a solution of 20~ grams of N~Cl in water OU~8;~ th~ ~lbre~, an~ ~
coloured solu~ipn o~ 1~ methylene blu~ ln di~tille~ w~ter in~d~ the ~ibr~s, ~nd u~ing ~n lnn~ ~re~sure ~or t~1~
solution o~ lOOkPa and without s~plyin~ ~ny pre~sure to t~e ~2~ 7 Yodium ~hlo~ide solutlon, ar~d usin~ a telnperature diff~ren~e bet~een the two sol~tions o~ 2~~ ~e.9. 40C ~or water +
methylene blue and 20C or the ~odium chloride ~olution), the flux ob~aine~ i.s approxim~tely 85 l/m~/day.
The present invention wlll be further described with re~erence to ~he following non~limiting diselosur~ o~ sorne o~
the oth~r possible applicaticns of the invention.
Concentr~lon o ~ of R ~ ow Osmotic Press~re Using Sea~at.er _ _ .
Examples of li~uids ~hich can be concent~ed by this met~od ln~ de ~oo~tu~9 ~uch as milk~ whey, coffee, tea, ~ruit and ve~etable ~ul~e3, arld sugar cane juice. Other ex~mples inclu~e appllcati.on~ in the phar~aceuti~al an~ ~lne chemical~ ind~lstrles where heat labile prod~cts can be eoneentrated ~y o~motic dis~illatlon. The liquid to be concentrated ls circula~ed on one side o~ the rne~brane and ~ea~ater is ci~culat~d on the other side, An in~rease 3f ~lux is ~hieved by an increase of ~tatic press~re of the l lq~lid to be concentra.ed, ~nd pre~erably an lncrease of the 20 temperatllre thereof ~ompa~ed to the temperature o~ ~eawPte~, ~or example by the u~e o~ ~e~v~ered wa~t~ heat ~rom other sour~es in the factory ~ In thi~ re~ard, most artif ical oxygranator unit~ are ~lready equipped wit:h soms form of ~eat exohan~er which ~arl b~ ~;ed for ~his pu~po~e.
25 o~motic Con~ntra~
__ Concen~ratiotl o~ liquids of relatlvely low o~n~tl~
presYure, -~ch a~ milk, whey, ~ruit Mr)d vegetahle juic~.~ and oane ~u1~e, as de~cribed above, can be obtained hy a prooe~
u,~in~ ~he diffr~ren~ in o~motic pressure h~ween ~uch 30 solution.~ an~ a hi~ly concerll:r~ked solution of a ~alt e.g.
NaCl. In ~hl.~ case, ~h~ conren~a~i~n of the Na ~olution i~
mairltainefl by one o~ the ~ollowi-lg method~:
I i ) ~y u~r.~ ~f a solar ponr~ wher~ ~he SOld~ e~aporation of th~ water mRintaln~ th~ hi~h r~oncrantrfltion of the ~alt nece~ry tO maintaln the high 0!3lnotl~ gr~dLent which exi~tY b~ween the ~olution ~v be concentrated ~nd th~
oon~ntratl3d ~lt ~olutlon.

1~
0~ ~7 In this r~ar~, the ~rocess provide~ a way to recover ~nd to utili2e solar en~rgy for eoncentr~tion of liquidsO For ~xa~plo, w~ilst it i~ not po~sible to use sol~r energy directly to concentln~te mllk it ca~ be use~
in this proce~s to m~lntain the corl~en~ratlon of a salt solution whiGh ln turn is used in the osmotic concentr~.ioll of milk.
In this re~ard the techl~olo~y lq oE great import~nee for countries like I~rael and Jordan wh~re ~here is a hLgh pr~duction o fr~it juice and a ~eadily available supply of concer~trated ~lt water, e.g. the De~d SeA. In thi~
re~3ard the inv~ntion provides a w~y to utili~e the high po~entLal eners1~ the concentr~ited ~al~ water for industri~il application, in ~n ~re~ where th~ altern~tive eources of energy ~re relatively very expen~fv~, p~rticula~ly wh~re h~ting ~pplica~ions are requlred, su~h as for the concentration of fr~it juice~.
(ii) T}le u~e of a re~erqe 03~0$i~ unl.t to maintain the concentration of s~lt. of the hi~h o~motic ~re~ re solution 4y elLmination of water using ~ ~re~sure-driven process. This applicAtion is o~ p~rtlcular import~nce since reverse OSmOSi~ cannot be used dlrectly because of the problerns o~ ~caling ~ncg: oulln~ o rever~e osmosi~
membran~s i~ used ~or th~ ~lre~t eoncentration o~ a large numher o~ uidx7 e.g. milk~ In thl~
ap~ &~l~n, the mllk ~r fruit jui~e i~ cbn~entrat~d ~y osmotic concentr~t;~)n throu~h the membrane without ~caling ( the meml~rane ~eing hydrophoblc does not coma in~o contact with t;hb liquid), without he~t diqsip~tlon ~which 1~ im~ort:ant, ~or example fo~ pre ~ncentratlon a s~lut:ion befor~ *~n, e.g~ ~or the produc~ion of in~t~qnl: chE~ee), ln a circuit which i~
totally Ins~lat~d rom th~ ~ut~ide, bçins1 ~eparated by a ~ag phase. On the other hAn~, the rever~e o~mo 1~ ~Ini~
~5 ha~ only to maintAin the ~alt concentration of ~ pure ~olution, e.~ Na~l in w~er which do~ not ~en0rate ~ny ~calin~, and whe~ein the ~lembran~ lifo ~or the reverse ,~
-; . , ,:, ~.~ 9~ 57 osmosis n~embrane under ~he~e conditions could ~e of the order of up to about 10 ye~r~ Due to the fact tha~
rev~r~e o~mosls melnbrane~ do not yive 100~ re~ectlon, the choi~e of ~ hiyll reJeetin~ membrane, e.g. 99.5~
important, the loss of ~alt belng ~ompeT~ted b~ the small addition required.
Production and Recover~ o~ Energy The invention ca~ also ~e ~g~d ~o produce and to recover ener~y, part o~ the energy potentlal whioh ex1sts between two streams, one o~ hi3h osrnotic ~re~sure e.g~ seaw~ter, brlnes etc., an~ a s~ream of low o~motLc pre~ure o.g. fresh water from river.s, including ~r~ckl~h wa~r, and al~o wa~er from ~eweraye et~. In all o~ thf~se applic~klons, the transfer of water from the low vsmokic press~re ~ide to the high osmotic pressure 5ide is u~ed to pressurize th~ hlgh oSmo~i~ pressure llguid, ~nd the energy is recovered by any clas~ical means using two liqulds of ~i~ferent pressure e.g. high pressure turblnes, pie~oelectric eell.s, using ~n oscillatiny pres~ure.
Use a~ a Com~lem~nt of Reverse Osmo3i~
A ~urther aspect of the inventi~n is ~he ~se ~ the pro¢es~ a$ a complement of revers~ osmosis proces~es to produce pure water froin seawater or bracki~h w~ter.
Moqt rever~e o~mosi~ membranes hava rejectlo~
coe~ficient~ o the order oP ~S~ Por NR~1 (wh~ ~embranes are designe~ ~o h~ve high specific flux) b~t h~ve hi~h reje~t$on char~cteristlcs, ~or example ~3~7%, for other ~alts ~e.g~
Magnesium sulphate).
A ~urther u~e of the invention i8 to u~e a highly concentrated solution o~ a Fialt, which i ea~ily re~ected by the reversc osmosi~ membralle, a~ th~ high o~moti~ pre~u~e ~lution, and to use s~nw~t~r ~or br~cki~h w~ter) ~g the low o~motie pr~ure ~oluti.on (relative to the ~her solution) and to ~xtr~ct water thererom.
In thls ~a~,e, wlthout pre~treatment of the ~eawa~er 3s stream, pure water i~ produced by rever~e o~mo~is of the hi~h pre~ure solution. In thi~. re~ard th~ lnv~nti~n p~ovide~ nn efficient de~lination proce~ ~o~ the recovery Of pot~ble .

~ ~9~)~57 water from sea~ter or ~rackiqh water. The r~covered w~ter can be su~plemented ~here reqlllred by the ~ddi~ion o~ 3alt8, minerals ~nd/or other addit~ves.
Further, by a selective choice of salt~ for the concentrated salt solution i~ is pos~ible ~o use more ~pen mernbrar1es to concentra~e and recycle the dlluted hlgh 05~0tiC
pres$ure solution an~Jor to recover the water there~rom. For example, selectin~ a sa~ which ln ~olution h~ a lArge anion ~ and a small cation te.~. acotinie acid, which has a lar~e anion an~ a ree acid f~nction) allows the cholce of ~ mor~
open membrane, but h~vin~ a pore size su~ lent to reject ~he large anior~ this membrAne is positlvely chArged by sulphochlorination and an~ination there is provided a porou~
~embrane which eejects ~ ion~ by the charge effect and rej~c~s anionq by si~e.
8imilarly where the solu~ion contain~ a ba~e ~ompri~in~
a large cation and a s~all ani~n (eOg. OH ~ th~ po~ous membrane i~ chosen ~uch that the large c~tion i~ re~ec~ed by ~ ~læe and wherein the ~ oan be re~ec~ed by ne~a~ively char~ing the membrane.
Ch~racteristics of the Membr~ne 1. To maintain the hydrophobicity o~ ~e me~brane, pore3 no ~rea~er than about S ~rnicron~ are required. High speci~ic porosit'y~ t~h~ flux~m2 bein~ ~roportlon~l to ~he surfac~ are~ of ~he pores.
2. The me~bran~ is as thin ~s pos3ible, pre~erably below 50 mi~rons thickness. ~ec~u~e o~ the dif~erence in vapour pre~sure established A~ross the membrane the flux iB
directly related tc~ the length o~ the pore where the pres~ure differe~ce is est~bli~h~
3~ The ~mbran~ shvuld ~e totaJ.I.y hydropho~ic to m~intain a ~as lock b~tw~e~ the two li~uid~, otherwise ~lffusion of s~lts will occur and this 1~ b-le. To avoi~ air lockci, and to en~lre ~hat the por~ ~re ~llled only by ~he ~olv~n~ ln ga~eou~ ~orm/ the ~nlt i5 initially illed with a ya~ which ls highl~ s~luhlo in the ~olvent, ~.9~ C02.

, ,:

, ~ . .

To malrlt~in a hi~h degree o~ hydrophobiclty, a surface tre~tment o~ the membrane is usu~lly reyuired to en~ure lon~evlLy, even wher, the mem~rane comprises an hydrophobic polymer. 4. The membrane i~ required to have sufficient che~ic~l~therlnic re~ist~nce to the liq~lds belng treated and under the condition~ of treatment - i.e. the mem~ran~ h~s to be used below the gl~sæ transition polnt of the polymer of whlch t~e mem~ane i~ comprised. 0. S. The cleaning a~ents must be carefully selected in order to avold wa~in~ o~ ~.he polymer. For example, ethyl alcohol wlll wet polypropylHne ancl will destroy the hydropho~ y~
~. The pore ~ize of the membrane ha~ to be s~fficiently small compared to khe hubble point - the pore size has to be suffi~iently ~mall to ~ive a ~ubble ~oint pressure ~bove the pressure u~ed in the prOcefis.
Althou~h the lnventiorl has been de~rihed above with referen~e to examples and to pre~orred embodiments, 1~ wlll be appreci~ted that the invention may be embodied in okher ~or~s or carried out in other ways without d0partin~ from the spirit or essen~ial ch~r~cteris~ics thereof. The ~bove descrLption 1~ there~ore ~o be conæidered a~ in ~11 respe~t~, illustrative and n~t restrlctive, and ~11 changes w~ich come within tha mean~ng an~ ran~e of equiv~lency are lnt~nded to be emb~ced therein.

Claims (11)

1. A method of osmotic distillation for the concentration of a first aqueous solution of relatively low osmotic pressure comprising: circulating said solution on one side of a hydrophobic porous barrier; simultaneously circulating a second aqueous solution of a relatively high osmotic pressure on the opposite side of the porous barrier;
solvent from said first solution being transferred across the porous barrier in the vapour state under the influence of an osmotic pressure gradient to the second solution resulting in concentration of the first solution.

2. A method according to Claim 1, wherein the porous barrier comprises a hollow fibre tube unit, wherein said first solution is circulated through inner channels or lumens of a bundle of hollow fibres and wherein the second solution is circulated around the bundle of hollow fibres, whereby solvent from said first solution is transferred across a microporous matrix structure in the walls of the hollow fibres under the influence of an osmatic pressure gradient.

3. A method according to Claim 2, wherein the second solution circulating around the bundle of fibres is seawater or a brine solution.

4. A method according to Claim 2 or Claim 3, wherein the second solution is taken from a solar pond.

5. A method according to Claim 1, wherein the two solutions have different potential energies as the result of the difference in osmatic pressure between the two solutions, and wherein energy is recovered therefrom by the use of high pressure turbines or piezoelectric cells or by any other suitable means.

6. A method according to Claim 1, wherein the diluted second solution is subsequently subjected to reverse osmosis treatment to separate the solvent and to concentrate the second solution suitable for recycling.

7. A method for recovery of a solvent from a primary aqueous solution of relatively low osmotic concentration, comprising:-(i) subjecting the primary solution to osmotic distillation with a porous barrier which separates the primary solution from a second aqueous solution of higher osmotic concentration, characterized in that said porous barrier comprises a hydrophobic porous matrix and wherein an osmotic pressure gradient exists between said primary and second solutions sufficient that solvent from the primary solution on one side of the porous barrier is transferred under the influence of said gradient through the porous barrier in a vapour state to the second solution on the opposite side of the porous barrier; and (ii) further subjecting the diluted second solution from the previous step to reverse osmosis by reverse osmosis means whereby the solvent is separated and the second solution is recovered and concentrated suitable for recycling.

8. A method for desalination of seawater, comprising:-(i) subjecting the seawater to osmotic distillation against a salt solution of higher osmotic pressure than the seawater, wherein the seawater and the salt solution are separated by a hydrophobic porous barrier, sufficient that water from the seawater is transferred through the porous barrier in a vapour state under the influence of an osmotic pressure gradient and into the salt solution on the opposite side of the porous barrier thereby diluting the salt solution; and (ii) subjecting the diluted salt solution to reverse osmosis treatment whereby water is separated from the salt solution and the salt solution is concentrated suitable for recycling.

9. An osmotic distillation apparatus for concentrating a first aqueous solution of relatively low osmotic pressure comprising:
a hydrophobic porous barrier and means for circulating said first solution on one side of said hydrophobic porous barrier;
means for simultaneously circulating a second aqueous solution of a relatively high osmotic pressure on the side of said porous barrier opposite the side on which the first solution circulates including means for causing solvent from said first solution to be transferred across said porous barrier in a vapour state under the influence of an osmotic pressure gradient to the second solution resulting in concentration of the first solution.

10. Apparatus according to Claim 9, wherein said hydrophobic porous barrier comprises one or more hollow fibre membrane distillation units, each comprising a tubular housing;
a bundle of osmotic distillation microporous hollow fibres arranged within the housing in the axial direction thereof; a countercurrent fluid chamber formed between the outer surfaces of the hollow fibre bundle and the inner surface of the housing; first inlet and outlet ports for passing the second solution into and out of the said fluid chamber; partition walls supporting the hollow fibre bundle, separating the open ends of the hollow fibre fluid chamber and defining the length of the fluid chamber; and second inlet and outlet ports communicating with the interior space or lumen of each of the hollow fibres, each hollow fibre having an outer wall comprising a microporous matrix whereby solvent from the primary solution on one side of the matrix may be transferred through the matrix in the vapour state to the second solution on the opposite side of the matrix under the influence of an osmotic pressure gradient.

11. Apparatus according to Claim 9, further comprising a diffusion barrier for reverse osmosis whereby the second solution diluted by solvent from the first solution may be separated to reconstitute and recycle the second solution and recover the solvent transferred from the primary solution.