Mass transfer ultrafiltration

Concentration Polarization The equations governing cross-flow mass transfer are developed in the section describing ultrafiltration. 

Fluidized beds have also been used to promote mass-transfer in both ultrafiltration and reverse osmosis. Smolders et al (22) ran 18mm i.d. UF tubes and 12mm i.d. RO tubes with and without fluidized beds (Ballotinl glass spheres). ... 

For a detailed description of the separation processes that may take place at the sensing microzone, the foundation of which is closely related to non-chromatographic continuous separation techniques based on mass transfer across a gas-liquid (gas diffusion), liquid-liquid (dialysis, ultrafiltration) or liquid-solid interface (sorption), interested readers are referred to specialized monographs e.g. [3]). 

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

This chapter will focus on three types of membrane extracorporeal devices, hemodialyzers, plasma filters for fractionating blood components, and artificial liver systems. These applications share the same physical principles of mass transfer by diffusion and convection across a microfiltration or ultrafiltration membrane (Figure 18.1). A considerable amount of research and development has been undertaken by membrane and modules manufacturers for producing more biocompatible and permeable membranes, while improving modules performance by optimizing their internal fluid mechanics and their geometry. 

Lysaght, M.J., Ford, C.A., Colton, C.K. et al. (1977) Mass transfer in clinical blood ultrafiltration devices, in Technical Aspects of Renal Dialysis (ed. T.M. Frost), Pitman Medical, London, UK. 

Noordman et al. [90] assessed the performance enhancement of ultrafiltration of 10 g/L BSA solution by fluidized particles of different materials and size using a polysulfone mbular membrane with inside diameter of 14.4 mm and 1.75 m length and an MWCO of 10 kDa. Table 8.4 summarizes the fluxes and the mass transfer coefficients obtained under different experimental... 

Phattaranawik, J., Jiraratananon, R., and Eane, A.G. Effects of net-type spacers on heat and mass transfer in direct contact membrane distillation and comparison with ultrafiltration studies, J. Membr. Sci., 217, 193, 2003. 

Aimar, P., Taddei, C., Lafaille, J.P., and Sanchez, V., Mass transfer limitations during ultrafiltration of cheese whey with inorganic membranes, J. Membr. Sci., 38, 203, 1988. 

Ghosh, R. Cui, Z.F. Mass transfer in gas-sparged ultrafiltration upward slug flow in tubular membranes. J. Membr. Sd. 1999, 162, 91-102. 

Manno, P. Moulin, P. Rouch, J.C. Clifton, M. Aptel, P. Mass transfer improvement in helically wound hollow-fibre ultrafiltration modules bentonite and yeast suspensions. Sep. Purif Tech. 

Membrane operation is a specific, but not exotic, operation. In fact it is a hybrid of classical heat and mass transfer processes (Figure 4.1). Direct contact mass transfer operations tend to reach equilibrium due to a difference of chemical potential between two phases that are put into contact. In the same way, temperature equilibrium is aimed at during heat transfer operations, for which driving force is a temperature gradient. In contrast, for membrane operations, by using the specific properties of separation of the thin layer material that constitutes the membrane, under the particular driving force that is applied, it is possible to deviate from the equilibrium that prevails at fluid-to-fluid interphase with classical direct contact mass exchange systems and to reorientate the mass transfer properties. In particular, this is the case with classical operations such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), gas separation (GS), pervaporation (PV), dialysis (DI) or electrodialysis (ED), for which a few characteristics are recalled in Table 4.1. 

In practice, however there could be differences between the observed and estimated flux. The mass transfer coefficient is strongly dependent on diffusion coefficient and boundary layer thickness. Under turbulent flow conditions particle shear effects induce hydrodynamic diffusion of particles. Thus, for microfiltration, shear-induced difflisivity values correlate better with the observed filtration rates compared to Brownian difflisivity calculations.Further, concentration polarization effeets are more reliably predicted for MF than UF due to the fact diat macrosolutes diffusivities in gels are much lower than the Brownian difflisivity of micron-sized particles. As a result, the predicted flux for ultrafiltration is much lower than observed, whereas observed flux for microfilters may be eloser to the predicted value. 

Dialysis involves the mass transference between two miscible liquid phases (the donor and acceptor solutions) separated by a liquid membrane through which some chemical species are likely to pass. Miscibility between the donor and acceptor solutions is inherent to dialysis and distinguishes it from e.g., liquid—liquid extraction, osmosis and ultrafiltration [271], These latter two membrane-based separation approaches tend to occur concomitantly with dialysis and involve the solvent rather than the solute crossing the membrane. In osmosis, the driving force towards separation is the concentration difference involved whereas in ultrafiltration, also called reverse osmosis, the driving force is an applied pressure that forces the solution across the membrane. 

Ultrafiltration (UF) is used to filter any large molecules (e.g., proteins) present in a solution by using an appropriate membrane. Although the driving potential in UF is the hydraulic pressure difference, the mass transfer rates will often affect the rate of UF due to a phenomenon known as concentration polarization (this will be discussed later in the chapter). 

The modelling of enzymatic membrane reactors follows, in general, the same approach as described previously. In enzymatic membrane reactors the catalyst is a macromolecule (enzyme). It can be found either in a free form in the reactor or supported on the membrane surface, or inside the membrane porous structure by grafting it or in the form of a gel obtained by ultrafiltration. As in the case of the whole-cell membrane bioreactors discussed above, the proper calculation of the mass transfer characteristics is of great importance for the modelling of this type of reactor. One of the earliest models of enzymatic membrane bioreactors is by Salmon and Robertson [5.108]. These authors modelled an enzymatic membrane bioreactor, which was made of four coaxial compartments the enzyme is confined within one of the compartments, and one of the substrates is fed in a gaseous form. 

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