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Diffusional Phenomena in Membrane Separation Processes*

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Diffusional Phenomena in Membrane Separation Processes* Journal of Membrane Sczence, 73 (1992) 103-118 103 Elsevler Science Publishers B V , Amsterdam Review Diffusional phenomena in membrane separation processes* G B van den Berg” and C A Smoldersb “Unzlever Research, Process Engzneerzng Sectzon, Colworth House, Sharnbrook, Bedford, MK44 1LQ (UK) bDept of Chemzcal Engzneerzng, Unzverszty of Twente, P 0 Box 217, 7500 AE Enschede (Netherlands) (Received June 25, 1991, accepted m revised form April 6,1992) Abstract Nowadays membrane filtration processes are used mdustrlally as an alternative to conventional sep- aration methods Membrane separation methods can be divided into classes according to their separation characterlstlcs (1) separation by sieving action, (11) separation due to a difference m affinity and dlf- fuslvlty, (111) separation due to a difference m charge of molecules, (m) carrier-fachtated transport, and (v) the process of (time-) controlled release by diffusion In all these cases dlffuslon processes play an important role m the transport mechanism of the solutes Various mechanisms have been distinguished to describe the transport m membranes transport through bulk material (dense membranes), Knudsen diffusion m narrow pores, viscous flow m wide pores or surface diffusion along pore walls In practice, the transport can be a result of more than only one of these mechanisms For all of these mechamsms models have been derived The characterlstlcs of a membrane, e g its crystalhmty or its charge, can also have major consequences for the rate of diffusion m the membrane, and hence for the flux obtained Apart from the diffusion transport processes zn membranes mentloned above, other important diffusion processes are related to membrane processes, viz diffusion zn the boundary layer near the membrane (concentration polanzatlon phenomena) and diffusion durzng membrane formatzon The degree of con- centration polarlzatlon IS related to the magnitude of the mass transfer coefficient which, m turn, 1s influenced by the diffusion coefficient The effect of concentration polarlzatlon can be rather different for the various membrane processes The phase mverslon membrane formation mechanism 1s determined to a large extent by the kinetic aspects during membrane formation, which are diffusion of solvent and of non-solvent and the kinetics of the phase separation itself Keywords diffusion, theory, review Introduction used mdustrlally as an alternative to conven- tional separation methods such as dlstlllatlon, Membrane filtration processes nowadays are centrlfugatlon and extraction. Since the first asymmetric reverse osmosis membranes be- Correspondence to G B van den Berg, Umlever Research, came avallable m the early slxtles membrane Process Engineering Sectlon, Colworth House, Sharn- technology has developed enormously This is brook, Bedford MK44 lLQ, UK expressed m the vast amount of research that *Paper presented at the Int Symp “Progress m Membrane Science and Technology”, Enschede, Netherlands, June 25- has gone mto developmg the right membrane 28,199l type and module for different kinds of separa- 0376-7388/92/$05 00 0 1992 Elsevler Science Publishers B V All rights reserved 104 G B van den Berg and CA Smolders/J Membrane Scr 73 (1992) 103-118 tlon processes, developing new processes and rier membranes Carrier-facilitated transport the best possible circumstances for separation can also be determined by diffusion only, which These efforts have resulted m the present day depends for example on the kmd of membrane commerclahzation of ultrafiltration (UF), ml- applied Furthermore, the technique of con- crofiltratlon (MF), reverse osmosis or hyper- trolled release of drugs from (biodegradable) filtration (RO), gas separation (GS), perva- reservoirs is a process that is mainly deter- poratlon (PV), (kidney-) dialysis and mined by diffusion electrodlalysls (ED ) Applications of already commercialized A membrane separation process can be gen- techniques include erally represented as m Fig 1 - Food industry: whey processmg (RO and UF ) , The various membrane separation methods concentration of milk for cheese production can be divided mto three classes according to (UF), clarification and/or sterilization of their separation characteristics various fluids such as wine, vmegar and apple (1) UF and MF use the size of the solutes to Juice (MF ) and whey desalting (ED ) separate particles by sieving action, with a - Water treatment production of high reslstiv- pressure difference as the driving force, the ity ( > 18 M&cm) water for the electronics membranes used m UF can have pores from 1 industry (MF and RO) and production of to 50 nm, while for MF the pore range is from clean boiler feedwater, potable water and 005tolOpm clean waste water (RO and ED ) (u) RO, gas separation and pervaporatlon, - Others oil-water separation (UF and MF), which are associated with (partly) dense mem- recovery of pamt and latices from waste water brane structures (pores < 1 nm), make use of effluents (UF), hemodlalysls, membrane a difference m affimty between several feed electrodlalysis and recovery of gases (GS) components and the membrane, and of a dlf- The membranes used m the various mem- ference m dlffuslvlty through the membrane, brane processes can be very different, both the the drlvmg force is a pressure difference m case material and the configuration (modules) offer of RO and gas separation, and m case of dl- several posslblhtles A membrane can be made alysls and pervaporatlon the driving force is a out of a polymeric or inorganic material Well concentration gradient known polymer materials are polysulfone, cel- (111) Electrodlalysls uses anion- and cation se- lulose-acetate, polycarbonate, polypropylene lective membranes to separate charged mole- and polyacrylomtrlle A large variety of poly- cules from uncharged ones The ions are trans- meric membranes are produced to optimize ported by a diffusional mechanism, as a result their permeability and separation charactens- of an applied potential difference tics. Inorganic membranes (usually MF-type) Some other membrane processes are under can be made from e g a-Al,O, or silica ( S102) development at the moment, for instance, fa- eventually supphed with a less porous ~-A1~0~ cilitated transport by hqmd and fixed site car- top layer Membranes can be subdivided m symmetric and asymmetric membranes Symmetric mem- dnvmg force branes can be dense, or can have straight or feed AP AC etc permeate sponge-like pores Polymeric asymmetric cross-flow membranes, usually made by the phase-mver- velocity flux slon method, consist of a thm dense skin layer t membrane (0 l-l ,um thickness, contaunng pores or not) on top of a much more porous sublayer The Fig 1 Schematx representation of a membrane process thinness of the skm layer results m a low resls- G B van den Berg and CA Smolders/J Membrane SCL 73 (1992) 103-118 105 tance for transport through the membrane gulshed m relation to membrane structure Another example of an asymmetric membrane -transport through bulk material (dense 1s a composite membrane, which 1s usually made membranes), of a very permeable UF membrane mto which -Knudsen diffusion, m narrow pores, a thm dense layer, often of another polymer -viscous flow, m wide pores, type, 1s applied and which 1s chosen for its high -surface dlffuslon, along pore walls selectivity In practice, the overall transport can be the re- Apart from the diffusion processes men- sult of more than only one of these mecha- tioned above, that determine the actual mem- nisms, this 1s for example the case m an asym- brane separation, other important diffusion metric membrane, as shown m Fig 2. processes are related to membrane processes, It 1s not possible to define fixed pore-dlmen- viz diffusion m the boundary layer near the slons coupled to the various transport mecha- membrane (concentration polarlzatlon phe- nisms that occur Apart from the case m which nomena) and diffusion during membrane for- there are no pores at all (dense membranes m mation: the inflow and outflow of solvents and which sorption and diffusion of the permeants non-solvents during the phase mverslon pro- occurs ) , transport will be a combmatlon of the cess determine the structure of the membrane various mechanisms mentloned. In general, the to be formed transport can be considered to be mainly of the The various diffusion processes that can oc- Knudsen diffusion type when the pore radius r cur during processes m which membranes are 1s smaller than lo-’ m (=lO nm) at ambient involved are summarized m Table 1. pressures, and it will be mainly viscous (Pol- The aim of this paper 1s to show the lmpor- seullle) flow when r 1s larger than 10m5 m ( = 10 tance of diffusion related to membrane sepa- ,um) These values also depend on the applied ration processes A description of the different pressure and temperature In between these diffusion processes will be given m the para- pore sizes the flow 1s a combmatlon of Knudsen graphs to come and Polseullle flow [ 1 ] 1. Diffusion in membranes 1 1 Transport In dense membranes When describing transport m membranes In the literature usually one of the followmg various possible mechanisms can be dlstm- three theories/models are used to describe the TABLE I Survey of the various areas where diffusion occurs 1 In membranes (a) m dense membranes (gas separation, pervaporatlon) (b) m porous membranes -gases -1lqulds -faclhtated transport (liquid membranes ) -solutes (controlled release) -electrodlalysls 2 In the boundary layer near the membrane 3 During membrane formation
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