Ultrafiltration retention limit

Walch et al.58) in Hoechst fabricated a membrane for hemofiltration from 2,2,4-trimethylhexamethylenediamine-2,4,4-trimethylhexamethylenediamine-terephthalic acid copolymer 13. The membrane has an ultrafiltration capacity for cattle blood of 1640 1/m2 day bar, a retention value of 44 % for dextran 7000, and molecular weight limit of 58000 + 6000. 

Equations (22-86) and (22-89) are the turbulent- and laminar-flow flux equations for the pressure-independent portion of the ultrafiltration operating curve. They assume complete retention of solute. Appropriate values of diffusivity and kinematic viscosity are rarely known, so an a priori solution of the equations isn t usually possible. Interpolation, extrapolation, even prediction of an operating curve may be done from limited data. For turbulent flow over an unfouled membrane of a solution containing no particulates, the exponent on Q is usually 0.8. Fouhng reduces the exponent and particulates can increase the exponent to a value as high as 2. These equations also apply to some cases of reverse osmosis and microfiltration. In the former, the constancy of C aji may not be assumed, and in the latter, D is usually enhanced very significantly by the action of materials not in true solution. 

P. Aimar, C. Taddei, J.P. LafaiUe and V. Sanchez, Mass transfer limitations during ultrafiltration of cheese whey with inorganic membranes. /. Membr. Sci., 38 (1988) 203 A.D. Marshall, P.A. Munro and G. Tragardh, The effect of protein fouling in microfiltration and ultrafiltration on permeate flux, protein retention and selectivity A literature review. Desalination, 91 (1993) 65. 

The limiting oil-concentration for the first stage, microfiltration, is determined by the tolerable concentration of oil in the recycled product. Since the oil retention capacity of the ultrafiltration stage is independent of feed concentration, the final concentration of the retentate is solely determined by the phase inversion "oil/water -> water/oil". At this concentration, the overall water recovery of the process is above 97.5%. However, the retentate of the process still contains too much water if incineration or refining is considered. Therefore, an evaporation step must be included in the process. Almost certainly evaporation will not be economical for the small capacities indicated in Figure 6.34 and a central evaporation station for the tentates from several production lines should be considered. 

Regarding the economical viability of the plant, the retention and stability of acylase are essential features for the process. An ultrafiltration unit retains acylase as the mobile catalyst in the reactor. Alternatively, acylase can be immobilized in a fixed or fluidized bed. A mobile catalyst system is preferred compared to the immobilized form, as the mobile catalyst system avoids mass-transfer limitations. Additionally, regeneration of the catalyst and scale-up of the reactor are much easier as compared to the process with the immobilized acylase. With respect to the deactivation of the catalyst, the thermal as well as the operational stability of acylase has been evaluated extensively [128, 129]. At a pH of 7, acylase appears to be sufficiently stable for L-amino acid manufacture. 

All the trials were done in a concentration mode, as the aim was to reduce the volume of the solution in order either to limit the amount of alcohol used for the precipitation or to replace precipitation by spray-diying. Concentration mode means that the retentate is recycled in the feed tank but the permeate is eliminated, thus leading to a volume decrease. Permeate flow rate was periodically evaluated by weight measurements and concentration ratios (CR) calculated from these measures. After concentration, extracts were precipitated with alcohol to evaluate the influence of ultrafiltration on the final powder composition. 

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