Ultrafiltration processing variables

The optimum time cycle and the relative diafiltration volume in the ultrafiltration-diafiltration process can be expressed as a function of three variables, P, Q, and R. P and Q are simple functions of the initial volume, membrane area, and flux (P = mA/Vo, Q = bA/Vo), and R is the solute recovery. From these, the time cycle and relative diafiltration volume (Vd/Vo) can be solved at various values of m, b, Vo, A, and R (m and b are respectively the slope and intercept of the flux, J = m In Vo/V + b). At a fixed recovery, the optimum time cycle and the relative diafiltration volume become functions of only two variables P and Q. Thus, the optimum operating condition can be simply plotted as function of P and Q. These plots, providing convenient and sufficient information, can be used as a guide in the design and operation of the ultrafiltration process. 

A major objective of fundamental studies on hollow-fiber hemofliters is to correlate ultrafiltration rates and solute clearances with the operating variables of the hemofilter such as pressure, blood flow rate, and solute concentration in the blood. The mathematical model for the process should be kept relatively simple to facilitate day-to-day computations and allow conceptual insights. The model developed for Cuprophan hollow fibers in this study has two parts (1) intrinsic transport properties of the fibers and (2) a fluid dynamic and thermodynamic description of the test fluid (blood) within the fibers. 

The effect of osmotic pressure in macromolecular ultraflltra-tlon has not been analyzed in detail although many similarities between this process and reverse osmosis may be drawn. An excellent review of reverse osmosis research has been given by Gill et al. (1971). It is generally found, however, that the simple linear osmotic pressure-concentration relationship used in reverse osmosis studies cannot be applied to ultrafiltration where the concentration dependency of macromolecular solutions is more complex. It is also reasonable to assume that variable viscosity effects may be more pronounced In macromolecular ultra-filtration as opposed to reverse osmosis. Similarly, because of the relatively low diffuslvlty of macromolecules conqiared to typical reverse osmosis solutes (by a factor of 100), concentration polarization effects are more severe in ultrafiltration. 

Several physicochemical methods and biological have been used to remove organic compounds in industrial effluents. Application of membrane filtration systems and adsorption processes in water treatment and effluent was used by a group of researchers. They developed a system for removal of phenol from an aqueous solution through a combined process of ultrafiltration and adsorption using kaohnite and montmorillonite. The adsorption experiments were performed in batch with 0.2 g of day and 100 mL of water contaminated in the range of variable concentration of the organic compound from 20 to 1000 mg L l, stirred for 12 h at 25°C. The results showed that the phenol removal efficiency was 80% and a maximum adsorption cap>adty equal to 40 mg (Lin et al., 2005). 

Let us take polysulfone as an example. This is a polymer which is frequently used as a membrane material, both for microfiltration/ultrafiltration as well as a sublayer in composite membranes. These applications require an open porous structure, but in addition also asymmetric membranes with a dense nonporous top layer can also be obtained which are useful for pervaporation or gas separation applications. Some examples are given in table ni.S which clearly demonstrate the influence of various parameters on the membrane structure when the same system, DMAc/polysulfone(PSf), is employed in each case. How is it possible to obtain such different structures with one and the same system To understand this it is necessary to consider how each of the variables affects the phase inversion process. The ultimate structure arises through two mechanisms i) diffusion... 

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