Ultrafiltrate fluxes, must

Sugar rejection was always zero. Polyphenol rejection increased from 9% to 57% with decreasing membrane cut-off. Difference in polyphenol rejection have been observed when the must is ultr filtered in the first UF step with the BMR 100515 and after with the BMR 021006, as reported in table III. Fig.3 shows the typical behaviour of the ultrafiltrate flux observed in these experiments. A constant flux was generally obtained after two hours. Table IV shows the final must ultrafiltrate flux values. All the experiments were carried out at the same axial velocity and at the same applied pressure. Table V shows results obtained with tubular membranes (Abcor-USA). 

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

Direct flow filtration has certain Umitations. The flux (filtration flow rate per unit membrane area) decreases over time as the process continues because the filtering media is loaded with more contaminant particles, as illustrated in Figure 14.1. Moreover, when the concentration of the contaminant in the feed stream is high, the filtering media must be replaced very frequently, which can be economically impractical. Also when the contaminant matter to be separated is small in size, requiring ultrafiltration or reverse osmosis membranes with much smaller pores, then direct filtration is less feasible as the flux declines very rapidly over time, again requiring frequent filter replacement. 

The filtration rate must be as high as possible so that the required membrane area is minimum. Flux rate (/) through the ultrafiltration membrane is related to the applied pressure AP by Darcy s law ... 

In the counter-current mode the magnitude of this difference is set by the construction of the dialyzer and the dialysate pressure control and is generally on the order of 50 mm Hg or greater. This minimum pressure will induce an absolute minimum ultrafiltration rate of 350 ml/hr for a typical high flux membrane. Thus, when the patient has lost sufficient water or perhaps when he does not need to lose any water during dialysis, the patient must continuously be given sterile saline to make up for the minimum ultrafiltration loses. 

Boundary Layer Theory. The Reynolds number for flow-through hollow fibers during our experiments was at most about 0.02 cm (diameter) x 4 cm/sec (velocity) x 1.0 g/cm (density)/ 0.007 poise (viscosity) 11 therefore, a boundary layer theory is needed for laminar flow in tubes. Because of its simplicity, the most attractive available theory is an approximate result of thln-film theory. This theory is restricted to a description of boundary layers that are thin in comparison to the tube radius. Furthermore, the ultrafiltrate velocity, J, must not vary along the tube length (uniform-wall-flux theory). At the centerline or axis of the fiber, the impermeable solute concentration C = C... 

For ultrafiltration as well as microfiltration the permeate flux is controlled by a gel layer-not by the membrane itself. This must be concluded from Figure 6.33 because the permeate flux for steady state conditions is almost exactly the same for the three tested membranes (for comparison, the data of Figure 6.33 have to be corrected for the same temperature and the same flow conditions). 

Xylans can be coextracted from wheat straw and bran in a twin-screw extruder. The best results for both the production yield and the extract properties are obtained with low alkali content, as the majority of the xylans conies from bran. The desired concentration of the extract solution can be achieved by ultrafiltration. The membrane configuration and molecular weight cut off (MWCO) must be adapted to each solution to limit fouling and concentration polarisation. A permeate flux of 20 dmVh.nr was obtained at a final concentration ratio of 2. Ultrafiltration allows for a partial demineralization of the solutions but does not change the properties of the final powder. 

The fluxes obtained with ultrafiltration membranes are not very high (about 20 L/h.nr) because of the viscosity of the solutions and because of fouling. Ultrafiltration can be introduced for concentration and partial purification, but the final concentration does not allow direct atomization. New trials must be done to reach a CR of 4 using other membranes with a higher MWCO perhaps up to 100 kD, and under optimized hydrodynamic conditions. 

Obviously batch ultrafiltration or one-stage discontinuous diafiltration is a more efficient technique for puriiying the protein of a low molecular weight impurity. However, one must ensme that the required solvent flux level is achievable in batch UF when the volume is reduced. 

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