Average rejection coefficient

For a polydisperse polymer solute, both W2 and R2 are functions of the solute molecular weight, M2. If f(M2) is the solute differential weight distributions and W(M2) th corresponding hindrance, the average rejection coefficient, R2 is defined through ... 

A relation is needed between these measurable quantities and R, the intrinsic rejection coefficient defined by Equation 4. First, is defined as the local axial solute flux averaged over the fiber cross-section with axial diffusion negligible. 

Here An = RT cosm, Lp = permeability for solute (cm s 1), AP = hydrostatic pressure difference, An = osmotic pressure, a = reflection coefficient, Acs = solute concentration difference, and cs = average solute concentration in upstream solution. Membranes with a reflection coefficient cj —> 0 are permeable to all components whereas a membrane with <7—> 1 rejects all solutes. 

Rejection or retention coefficient This describes the ability of the membrane to retain the desired species from the feed on the membrane surface. Since the rejection is often dependent on membrane characteristics and operating parameters, these must be clearly stated so that a fair comparison can be made between different types of membranes for a given application. It is defined as / = 1- Cp/Cn, where, Cp is the concentration of the species in permeate and Q is its concentration in the retentate. If a significant passage of the species occurs, then an average concentration is used. 

The mass transfer coefficient was determined as a function of membrane t)pe and stirrer speed. Results obtained for various membranes are within 5%, and thus no impact of the membrane was found. The average mass transfer coefficient at 400 rpm was (1.81 0.10) 10 ms h Rejection of Dextran was >99.5% for all membranes used. 

This permeation relationship can be used in the separation calculations for liquid mixtures by assuming that P, = Dj/zRT in the gas-phase format and all units are consistent, with the permeability incorporating the averaged or mean compressibility factor for the reject and permeate. Using the mean compressibility factor partially offsets the effect of pressure difference on the flux relationship for component /. In other words, the permeability coefficient itself can be perceived as dependent on the initial pressure P and final pressure Py and changes with the particular operating conditions. 

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