Dialysis mass transfer coefficient

As was shown in the BAHLM model for transport kinetics, the values of the overaU mass-transfer coefficients govern the location (i ax) and the maximum quantity (Ofemax) of the metal species in the LM phase (see Eqs (27) and (28)). At Ormax Qf, the BAHLM is working mainly as a Donnan dialysis system, in which the loaded carrier solution is a treated feed. In this... 

One real application of membrane dialysis is the recovery of sodium hydroxide from a textile mill. The percent recovery was reported to be between 87.3% and 94.6%. However, dialysis is limited to smaller flowrates since the mass transfer coefficient (K) is relatively small. See Refs. [11,13] for additional information. 

Also in dialysis the transport resistance is not determined only by the membranebut frequendy bound layer iesisthnees have to be taken into account. This is drawn schematically in jf re VI - 43b. The overall mass transfer coefficient k(> is obtained by the sum of the threje resistances according to... 

Profiles in which this latter profile can be found are electrodialysis, per/aporation, gas separation, dialysis, diffusion dialysis, facilitated transport or carrier mediated transport and membrane contactors. The extent of the boundary layer resistance varies from process to process and even for a specific process it is quite a lot dependent on application. Table Vn.2 summarises the causes and consequences of concentration polarisation in various membrane processes. The effect of concentration polarisation is very severe in microfiltration and ultrafiltration both because the fluxes (J) are high and the mass transfer coefficients k (= EV8) are low as a result of the low diffusion coefficients of macromolecuiar solutes and of small particles, colloids and emulsions. Thus, the diffusion coefficients of macromolecules are of the order of lO ° to 10 m /s or less. The effect is less severe in reverse osmosis both because the flux is lower and the mass transfer coefficient is higher. The diffusion coefficients of low molecular weight solutes are roughly of the order of 10 m /s. In gas separation and pervaporation the effect of concentration polarisation is low or can be neglected. The flux is low and the mass transfer coefficient high in gas separation (the diffusion coefficients of gas molecules are of the... 

Concentration polarisation is not generally severe in dialysis and diffusion dialysis because of the low fluxes involved (lower than in reverse osmosis) and also because the mass transfer coefficient of the low molecular solutes encountered is of the same order of magnitude as in reverse osmosis. In carrier mediated processes and in membrane contactors the effect of concentration polarization may become moderate mainly due to the flux through the membrane. Finally, the effect of concentration polarisation may become ver severe in electrodialysis. In the following sections concentration polarization will be described more in detail. In some module configurations such as plate-and-frame and spiral wound spacer materials are used in the feed compartment (see chapter VIII). These spacers effect the mass transfer coefficient and can be considered as turbulence promoters. 

Dialysis and diffusion dialysis are membrane processes that consumes generally not much energy. The energy consumption Ep is determined by the pumps to circulate the feed and permeate (dialysate) stream along the membrane (eq. VIII - 87). If concentration polarisation becomes severe higher cross-flow velocities may be required to increase the solute mass transfer coefficient and consequently the energy consumption will increase. 

In this model, there are mass transfer eflFects between the CSF and extracellular compartments (given by the mass transfer coefficient K2) and between the extracellular compartment and the intracellular compartment (Ki). G is the natural rate of generation of the contaminant urea, L is the flow rate of CSF fluid that flows directly into the extracellular fluid, and F is the rate of urea removal due to dialysis with an artificial kidney. 

In dialysis and similar processes through permeable membranes, it has become convenient to replace the permeabilities, which were defined in Chapfer 3, by an effective mass transfer coefficient, termed which equals the ratio of membrane diffusivity over membrane thickness. Thus,... 

To implement buffer exchange, coutercurrent dialysis has been explored to eliminate 0.05 M sodium pbospbate from a 1 mg/ml solution of bovine serum albumin (BSA) and 25 mM Tris buffer at a pH of 7 (via NaOH). Tbe dialyzing buffer composition is 25 mM Tris and tbe pH is 7 (via HCl). The dialysate flow rate is 100 ml/min. Two membrane modules are in series, each having a surface area of 0.2 m. The value of (Ci/o/Cj/i) for sodium phosphate at a feed flow rate of 10 cm /min was found to be 246. Determine the value of the overall mass-transfer coefficient Kg. (Ans. 1.5 x 10 cm/min.)... 

Salt is to be removed from a solution containing lOOg/Uter of salt and 200g/liter of rafBnose by dialysis in a hollow fiber dialyzer operating countercurrentiy. The overall mass-transfer coefficient for the salt was determined in the dialyzer with 200 cm /min feed flow rate and 500 cm /min pure water dialysate flow rate to be 0.0415 cm/min. If 90% of the salt is to be removed for the same feed and dialysate flow rates, what is the hollow fiber membrane area required Assume that the overall transport coefficient remains unchanged due to any changed conditions. (Ans. 14 860 cm. )... 

The terminology for characterizing dialysis membranes is somewhat unique to the dialysis field. Instead of being characterized in terms of hydrauhc penneabUity, diffusive membrane permeabihties, and solute rejection coefficients, dialyzers arc generally characterized in terms of an ultrafiltration coefficient (Kuf), solute clearances, and the product of the mass transfer coefficient times the surface area (KoA). 

Another term used to characterize the transport properties of dialysis membranes is the so-called mass transfer area coefficient (MTAC), which is the product of the mass transfer coefficient (Ko) times the membrane surface area (A), or KoA. Usually, the terms MTAC and KoA reported are those for urea. While Ko should equal the maximum clearance obtained at high blood and dialysate flow rates, reports in the dialysis Hterature (Leypoldt et al., 1997) discuss the variation of KoA with dialysate flow rate. Such reports reflect the manufacturers or others inappropriate extrapolation of KoA from data obtained at typically clinically relevant flow rates, which arc not high enough to minimize boundary layer resistance. While the measurement of in vivo rather than in vitro characteristics of dialyzers is meant to provide more accurate or realistic information, it can be misleading in this context. 

T. Ktari, C. Larchet and B. Auclair, Mass transfer characterization in Donnan dialysis, J. Membr. Sci., 1993, 84, 53-60 H. Miyoshi, Diffusion coefficient of ions through ion exchange membrane in Donnan dialysis using ions of different valence, J. Membr. Sci., 1998, 141, 101-110. 

Drugs are cleared from the blood during peritoneal dialysis in an analogous fashion to the clearance of creatinine or other plasma constituents. Transport between the blood and the peritoneal cavity is symmetric in the sense that transport rates are equivalent in each direction, and therefore if the PA (mass transfer-area coefficient) for a substance of equivalent molecular weight is available, the transfer rate can be estimated from... 

Keshaviah P, Emerson PF, Vonesh EF, Brandes JC. 1994. Relationship between body size, fill volume, and mass transfer area coefficient in peritoneal dialysis. / Am Soc Nephrol 4(10) 1820-1826. 

---