Ultrafiltration, molecular weight limit

With decreasing packing size in SEC columns, the probability of physical entrapment of macromolecules increases. To estimate the molecular weight limit above which ultrafiltration will occur, we must first calculate an average radius of the interstices formed in a packed bed. This is done by assuming that the packed column consists of a bundle of capillaries in which the capillary radius can be estimated from the bed hydraulic radius ... 

Ultrafiltration molecular weight >1000 >100-L volumes ambient temperature, contamination limited throughput 15-17... 

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. 

The "slurry reactor, which was introduced by Bossow and Wandrey for enzymatic formation of C-C bonds (25), is basically a stirred ultrafiltration cell (10 mL) containing a membrane with a defined molecular weight limit of 100 or 300 kD, which serves as barrier. The components of the galactosylation mixture are continuously pumped through the reactor and pass through this barrier, whereas the immobilized catalyst is retained in the reactor. The immobilized enzyme is held in suspension by continuous stirring. 

A limitation to the more widespread use of membrane separation processes is membrane fouling, as would be expected in the industrial application of such finely porous materials. Fouling results in a continuous decline in membrane penneation rate, an increased rejection of low molecular weight solutes and eventually blocking of flow channels. On start-up of a process, a reduction in membrane permeation rate to 30-10% of the pure water permeation rate after a few minutes of operation is common for ultrafiltration. Such a rapid decrease may be even more extreme for microfiltration. This is often followed by a more gradual... 

TWO kinds of cellulose acetate ultrafiltration tubular membranes (T2/A and T4/A) produced by Paterson Candy International, Limited, England, were employed in this study. Six kinds of solutes, polyethylene glycol (PEG//4000), vitamin B12, raffinose, sucrose, glucose, and glycerin, were used. Molecular weights, diffusivities and molecular radii of these solutes are shown in Table 1. The experimental apparatus is shown schematically in Fig. 7. 

Ultrafiltration (UF), a pressure-driven separation process on the basis of molecular size using a membrane. Suspended solids and solutes of high molecular mass are retained. UF is used for the concentration of protein solutions and for the separation of molecules with significant mass differences. The nature of the membrane determines between compounds which permeate and those which are retained. UF membranes are classified according to the molecular mass limit ( molecular weight... 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

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