Lecture 17. Membrane Separations [Ch. 14]

Transcription

1 Lecture 17. embrane Separatons [Ch. 14] embrane Separaton embrane aterals embrane odules Transport n embranes -Bulk flow - Lqud dffuson n pores - Gas dffuson - onporous membranes

2 embrane Separaton Separaton by means of a sempermeable barrer (membrane) through whch one or more speces move faster than another or other speces Characterstcs - The two products are usually mscble - The separatng agent s a sempermeable barrer - A sharp separaton s often dffcult to acheve

3 Hstory of embrane Separaton Large-scale applcatons have only appeared n the past 60 years s: separaton of 235 UF 6 from 238 UF 6 (porous fluorocarbons) s: reverse osmoss for seawater desalnzaton (cellulose acetate), commercal ultrafltraton membranes : hollow-fber membrane for gas separaton (polysulfone) s: commercalzaton of alcohol dehydraton by pervaporaton Replacement of more-common separatons wth membrane - Potental: save large amounts of energy -Requrements producton of hgh-mass-transfer-flux, defect-free, long-lfe membranes on a large scale fabrcaton of the membrane nto compact, economcal modules of hgh surface area

4 Characterstcs of embrane Separaton Dstllaton vs. gas permeaton : energy of separaton for dstllaton s usually heat, but for gas permeaton s the shaft work of gas compresson Emergng (new) unt operaton : mportant progress s stll beng made for effcent membrane materals and packagng embrane separator vs. other separaton equpment - more compact, less captal ntensve, and more easly operated, controlled, and mantaned - usually modular n constructon: many parallel unts requred for large-scale applcatons Desrable characterstcs of membrane (1) good permeablty, (2) hgh selectvty, (3) chemcal and mechancal compatblty, (4) stablty, freedom from foulng, and useful lfe, (5) amenablty, (6) ablty to wthstand large pressure dfferences

5 embrane aterals Typcal membrane materals - atural polymers: wool, rubber, and cellulose - Synthetc polymers - Inorganc materals: mcroporous ceramcs, metals, and carbons Almost all ndustral membrane materals are made from polymers : lmted to temperatures below 200 and chemcally nert mxture Types of polymer membrane - Dense amorphous membrane pores, f any, less than a few Angstroms n dameter dffusng speces must dssolve nto the polymer and then dffuse through the polymer - croporous membrane (mcrofltraton, ultrafltraton, nanofltraton) contans nterconnected pores of μm n dameter for small molecules, permeablty for mcroporous membranes s hgh but selectvty s low

6 Asymmetrc Polymer embrane Asymmetrc membrane - thn dense skn (permselectve layer) about μm n thck formed over a much thcker mcroporous layer (support) Caulked membrane Thn-flm composte (slcone rubber)

7 embrane odules (1) Common membrane shapes Flat asymmetrc or thn-flm composte Tubular Provde a large membrane surface area per unt volume Hollow fber onolthc

8 embrane odules (2) Common membrane modules Plate and flame Spral-wound Four-leaf spral-wound Tubular Hollow-fber onolthc

9 embrane odules (3) Typcal characterstcs of membrane modules Plate and frame Spral-wound Tubular Hollow-fber Packng densty, m 2 /m ,000 Resstance to foulng Good oderate Very good Poor Ease of cleanng Good Far Excellent Poor Relatve cost Hgh Low Hgh Low an applcaton D, RO, PV, UF, F D, RO, GP, UF, F RO, UF D: dalyss, RO: reverse osmoss, GP: gas permeaton, PV: pervaporaton, UF: ultrafltraton, F: mcrofltraton D, RO, GP, UF

10 Transport n embranes olar transmembrane flux P ( drvng force) l P : permeablty, P : Types of membrane: macroporous, mcroporous, dense echansms of transport n membranes Bulk flow through pores o separaton Dffuson through pores P ( drvng force) permeance Restrcted dffuson through pores Sze excluson, sevng Soluton dffuson through dense membranes

11 Bulk Flow Hagen-Poseulle law (for lamnar flow) 2 D v ( P0 PL ) 32L (D: pore dameter L: length of the pore) Pressure dfference P 0 Porosty (vod fracton) nd 2 / 4 (n: pores per unt cross secton) P L Superfcal flud bulk-flow flux (mass velocty) 2 4 D n D v ( P0 PL ) ( P0 PL ) 32l 128l Tortuosty factor, τ If pore length s longer than the membrane thckness, l l (l : membrane thckness)

12 Lqud Dffuson n Pores When dentcal total pressures but dfferent component concentratons exst no bulk flow, but dfferent dffuson rates separaton odfed form of Fck s law De ( c c ) 0 L l Effectve dffusvty Concentraton drvng force D e D d m Restrctve factor Kr 1,( dm / dp) 1 d p effect of pore dameter, d p, n causng nterferng collsons of the dffusng solutes wth the pore wall K r 4 c,o c,l

13 Gas Dffuson If total pressure and temperature on ether sde are equal Dc e ( p p ) 0 Partal-pressure drvng force L Pl De ( p p ) 0 L RTl c, total concentraton of the gas mxture (=P/RT by the deal-gas law) D e 1 ( 1/ D) ( 1/ DK ) Ordnary dffuson Knudsen dffuson Collsons occur prmarly between gas molecules and the pore wall

14 onporous embranes echansm - Absorpton of gas or lqud components nto the membrane - Dffuson through the sold membrane - Desorpton at the downstream face Dffusvtes of water (cm 2 /s at 1 atm, 25 ) - Water vapor n ar : Water n ethanol lqud : Water n cellulose acetate sold : Soluton-dffuson model : The concentratons n the membrane are related to the concentratons or partal pressures n the flud adjacent to the membrane faces thermodynamc equlbrum for the solute between the flud and membrane materal at the nterfaces

15 Soluton-Dffuson for Lqud xtures Porous membrane onporous membrane Concentraton profle s contnuous K If the mass-transfer resstances n the boundary layers are neglgble K D s the permeablty, P, for the soluton-dffuson model o c o L c o K D ( c c ) 0 L l L KD ( c c ) 0 L l KD ( c c ) F P l c c L

16 Soluton-Dffuson for Gas xtures Porous membrane onporous membrane Contnuous partalpressure profle H If the external mass-transfer resstances are neglgble P o HD c p o o c H L p HD ( p p ) 0 L l HD ( p p ) F P l P ( p p ) F P l L L