N e w s a n d V i e w s

Improved m e m b r a n e for h a e m o d i a l y s i s Hydrophll ic funct ional group polyvinyl alcohol (PVA), hydra t ion , permeabfl lW a n d blood compat ibi l i W all increase wi th increas ing hydrolys is t ime for vinyl ace ta te p l a s m a depos i ted membranes .

A haemodia lys i s m e m b r a n e p repa red by p l a s m a depos i t ing v inylaceta te onto annea led nylon 4 m e m b r a n e followed by hydrolys is t rea tment , is desc r ibed in J. Biomed. Mat. Res., 27, 8, 1993, pp 983-989. This is sa id to improve the blood compat ib i l i ty of nylon 4 a n d overcomes the hydrolys is problem of PVA.

In addi t ion to convent ional p l a s m a deposi t ion, the p l a s m a ini t ia ted in te rpene t ra t ing network method was also invest igated.

Further Informatlon from: J.Y. Lal and C.Y. Shlh, Department of Chemical Engineering, Chung Yuan Christian University, Chung Li, Talwan 32023.

Membrane separates vapours from gas s tream A method of s epa ra t ing condensab le vapours from a gas s t r eam by m e a n s of a m e m b r a n e has been pa t en t ed by Bend Research Inc, b a s e d in Bend, Oregon, USA. The vapours m a y be water or volatile organic mater ia ls . This me thod differs from earl ier sys t ems in tha t it does not require compressors .

The technique (European pa ten t 0 532 368) is ba sed on a hollow fibre m e m b r a n e which has a feed side and a pe rmea te side. The flow to be t rea ted contac t s the feed side of the m e m b r a n e which has a cooled ext rac t ion fluid on the pe rmea te side. The total p r e s su re in the sys tem is such tha t the p r e s s u r e on the pe rmea te s ide is less t han tha t on the feed. The fluid to be ext rac ted is condensed from the gas s t r eam on the pe rmea te s ide of the membrane .

This technique is sa id to be s imple a n d energy-eff ic ient It u s e s a cool ext rac t ion fluid both to ext rac t hea t a n d en t ra in fluid on the pe rmea te side of the m e m b r a n e while the feed side of the m e m b r a n e is in contac t with the gas s t ream. The pe rmea te side of the m e m b r a n e serves as a hea t s ink removing sensible and la tent hea t removed from the gas s t ream.

When the gas in contac t with the m e m b r a n e is c o d e d pas t i ts dew point the vapour condenses from it and the vapour a n d la tent hea t from the condensa t ion are then en t ra ined in the m e m b r a n e wall.

The ext rac t ion fluid is thro t t led at the inlet. This ma in t a in s a p r e s s u r e lower t han tha t of the warm feed

gas s t r eam which provides the driving force for the condensa t e to p e r m e a t e the m e m b r a n e wall.

In a tes t a wa rm mois t gas f e e d c o m p r i s l n g air with a water vapour a t a dry bu lb t empe ra tu r e of 23°C a n d a dew point of 18°C was fed a t a n ambien t p r e s su re (I a tmosphere) a t a ra te of 0 .006 N m a / m 2 to a 1 m 2 hollow fibre module. The m e m b r a n e compr ised 7050 regenera ted cel lulose hollow fibres with a n outs ide d iamete r of 230 m m (Model CF 15-1 I, Baxter Heal thcare Corp).

The p r e s s u r e d rop of the wa rm air feed th rough the module was 0.03 a tmospheres .

An ext rac t ion fluid compr is ing cool water a t 8°C a n d 0.96 a t m o s p h e r e s was d rawn through the lumens of the hollow fibre m e m b r a n e s a t 250 m l / m i n u t e . The combined cooling water a n d condensed vapour s t ream exit ing the module h a d a d ry bu lb t empera tu re of 120C a n d a dew point t empe ra tu r e of 8°C.

Further Informatlon from: Bend Research Inc, 64550 Research Road, Bend, OR 97701, USA.

Organic mat ter foul ing in UF m e m b r a n e s Recent r e sea rch has been conduc ted on the effect of organic ma t t e r adsorp t ion on ul t raf l l t ra t ion m e m b r a n e blocking and fouling (Journal of Membrane Science, 8 4 , pp 61-77, 1993). This follows the increased use of UF m e m b r a n e s in dr ink ing water t rea tment , and fouling p rob lems c a u s e d by organic compounds tha t p lay a par t in tha t t rea tment .

Tests were carr ied out on a hollow fibre hyflrophilic cellulose derivative m e m b r a n e and a hollow fibre hydrophobic acrylic polymer membrane . Exper iments were per formed us ing synthet ic , par t ic le free solu t ions of a low molecular weight dex t r an a n d t ann ic acid. The effect on fouling of p r e t r e a t m e n t of the synthet ic so lu t ions by preozonat lon a n d by p readso rp t ion on powder ac t iva ted ca rbon (PAC) was a lso evaluated.

The effect of preozonat ion was eva lua ted with respect to the n a t u r e of the organic c o m p o u n d s and their in terac t ions with the m e m b r a n e mater ia l . Preadsorp t ion on PAC was expected to ac t competi t ively with the m e m b r a n e in t e r m s of adsorpt ion . The products , which otherwise would adso rb on the m e m b r a n e and foul it, would preferably adso rb on the PAC a n d therefore m e m b r a n e fouling would be prevented.

The s tudy showed tha t the h ighes t level of adsorp t ion occurred in the ul t ra lHtrat ion of t h e aromat ic compounds th rough the hydrophoblc membrane . Preozonat ion was very effective in prevent ing adsorp t ion of tannic acid by -p roduc t s on the hydrophi l lc membrane , b u t only par t ia l ly r educed the fouling caused by adsorp t ion of the hydrophobic membrane . Similarly, p r eadso rp t ion on PAC was efffective when the m e m b r a n e u sed was hydrophtllc.

Further Informatlon from: G. Crozes, Montgomery Watson, 161 E. Mallard Drive, Boise, ID 83706, USA.

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