Functionalized membranes diffusion dialysis

In many processes, including those in nature, transport proceeds via diffusion rather than convection. Substances diffuse spontaneously from a high to a low chemical potential. Processes which make use of a concentration difference as the driving force are gas separation, vapour permeation, pervaporation, dialysis, diffusion dialysis, carrier mediated processes and membrane contactors (In pervaporation, gas separation and vapour permeation it is preferred to express the driving force as a partial pressure difference or an activity difference rather than concentration difference). On the basis of differences in structure and functionality it is possible to distinguish between processes that use a synthetic solid (polymeric or sometimes ceramic or zeolitic) membrane (gas separation, dialysis and pervaporation) and those that use a liquid (with or without a carder) as the membrane. 

In hemodialysis, blood from the patient flows on one side of a membrane and a specially prepared dialysis solution is fed to the other side. Waste material in the blood such as urea, excess acids, and electrolytes diffuse into the dialysate the blood is then returned to the patient, as shown in Fig. 48. A patient typically undergoes dialysis three times per week in sessions lasting several hours each. Modern dialysis systems combine sophisticated monitoring and control functions to ensure safe operation. Regenerated cellulose was the first material used in hemodialysis membranes because of its biocompatibility and low cost it remains the most popular choice. Subsequently, high-permeability dialysis membranes derived from cellulose esters, modified polysulfone, or polyacrylonitrile copolymers have also gained wide acceptance because of the shorter sessions they make possible. 

Dialysis is a diffusion-based separation process that uses a semipermeable membrane to separate species by vittue of their different mobilities in the membrane. A feed solution, containing the solutes to he separated, flows ou one side of the membrane while a solvent stream, die dialysate, flows on die other side (Fig. 21. -1). Solute transport across the membrane occurs by diffusion driven by the difference in solme chemical potential between the two membrane-solution interfaces. In practical dialysis devices, no obligatory transmembrane hydraulic pressure may add an additional component of convective transport. Convective transport also may occur if one stream, usually the feed, is highly concentrated, thus giving rise to a transmembrane osmotic gradient down which solvent will flow. In such circumstances, the description of solute transport becomes more complex since it must incorporate some function of die trans-membrane fluid velocity. 

The relative transfer of two solutes across a dialysis membrane is a function of bod) their diffusivitics in die membrane and their driving forces. Separations will be efficient only for species that differ significantly in diffusion coefficient. Since diffusion coefficients are a relatively weak function of molecular size. 

FIGURE 21.1-4 Ratio of solute difftisivity in the membmae (3DW) to solute diffusivity in solution (X>) as a function of solute molecular weight for three cullulosic dialysis membranes. 

On the basis of either model, ii is clear ihat dialytic transport will dectesse with increasing solute molecular size, not only beeause of the smeller solution diffusivity of a large molecule bur also because of incransmg values of q. This effect is seen in Fig. 21.1-4. where the ratio of diffusivity in the membrane (3Dm) to difliisivity in solution (3D) is plotted as a function of solute Stokes radii for several dialysis membranes. 

In membranes with pore sizes of >2nm, the separation is based on size exclusion, and thus, these membranes are suited for separation of components with sufficient size difference, for example, in dialysis, waste water treatment, and functional clothing. " For separation of components with comparable sizes or the separation of ions from water, solution-diffusion membranes are used. ... 

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