Membrane transmembrane processes

Dense nonporous isotropic membranes are rarely used in membrane separation processes because the transmembrane flux through these relatively thick membranes is too low for practical separation processes. However, they are widely used in laboratory work to characterize membrane properties. In the laboratory, isotropic (dense) membranes are prepared by solution casting or thermal meltpressing. The same techniques can be used on a larger scale to produce, for example, packaging material. 

Figure 13.15 Flow scheme and performance data for a membrane distillation process for the production of water from salt solutions [31]. Feed salt solution is heated to 100 °C and passed counter-current to cool distillate that enters at 42 °C. The distillate product is almost salt-free as shown by its low conductivity. The distillate flux is almost constant up to salt concentrations as high as 20 % NaCI. Reprinted from J. Membr. Sci. 39, K. Schneider, W. Holz, R. Wollbeck and S. Ripperger, Membranes and Modules for Transmembrane Distillation, p. 25. Copyright 1988, with permission from Elsevier...

Ultrafiltration (UF) and diafiltration (DF) are very popular methods for conditioning the clarified culture broth for chromatography. As general rule, dilute animal cell culture supernatants are concentrated up to a factor of 20 and even more prior to fractionation. Ultrafiltration is a process in which a solution containing macromolecules is passed across a functional membrane, while macromolecules are retained by the membrane. The process is driven by the pressure drop across the membrane also called transmembrane pressure (PT) expressed as... 

Acetyl CoA is the donor for the 7- and 9-O-acetylation of sialic acids in the Golgi membrane. Neither free acetate nor acetyl CoA crosses the Golgi membrane, and the reaction appears to be a transmembrane process, with intermediate acetylation of a membrane component that then accumulates... 

Ultrafiltration of whey is a major membrane-based process in the dairy industry however, the commercial availability of this application has been limited by membrane fouling, which has a concomitant influence on the permeation rate. Ultrasound cleaning of these fouled membranes has revealed that the effect of US energy is more significant in the absence of a surfactant, but is less markedly influenced by temperature and transmembrane pressure. The results suggest that US acts primarily by Increasing turbulence within the cleaning solution [91]. 

Vaccum membrane distillation, such as any membrane distillation process, is a thermally driven process in which a feed solution is bought into contact with one side of a microporous membrane and a vacuum is created on the opposite side to create a driving force for transmembrane flux (Figure 19.8). The microporous membrane only acts as a support for a vapor-liquid interface, and does not affect the selectivity associated with the vapor-liquid equilibrium [76]. 

If recently synthesized phospholipid molecules remained only on the cytoplasmic surface of the ER, a monolayer would form. Unassisted bilayer transfer of phospholipid, however, is extremely slow. (For example, half-lives of 8 days have been measured across artificial membrane.) A process known as phospholipid translocation is now believed to be responsible for maintaining the bilayer in membranes (Figure 12F). Transmembrane movement of phospholipid molecules (or flip-flop), which may occur in as little as 15 seconds, appears to be mediated by phospholipid translocator proteins. One protein (sometimes referred to as flippase) that transfers choline-containing phospholipids across the ER membrane has been identified. Because the hydrophilic polar head group of a phospholipid molecule is probably responsible for the low rate of spontaneous translocation, an interaction between flippase and polar head groups is believed to be involved in phosphatidylcholine transfer. Translocation results in a higher concentration of phosphatidylcholine on the lumenal side of the ER membrane than that... 

Dynamic filtration processes counteract the formation of a covering layer over the active surface of the membrane This is defined as gelpolarization of fouling In modules with tubular and capillary membranes, transmembrane pressure can be calculated as follows ... 

While aU the above mentioned membrane separation processes utiHze the transmembrane pressure drop as the driving force, there are other membrane separation processes based on different driving forces. 

The opposite situation (T/L is small) corresponds to slow reactions and small distances between Ae membrane and the chamber walls. In this case, the correction factor can be very large. The rate of the transmembrane processes, then, is not a function of any diffusion coefficient but is determined only by the rates of the chemical processes. This situation could be important for different systems of biological membranes and subcellular structures. 

Cystic fibrosis, a disease of the Caucasian population, is associated with defective CL regulation and is essentially a disorder of epithehal cells (113,114). The defect arises at several levels in the CL ion transporter, ie, the cystic fibrosis transmembrane regulation (CFTR), and is associated with defective CL transport and defective processing, whereby the protein is not correctiy incorporated into the cell membrane. The most common mutation, affecting approximately 60% of patients, is termed F 608 and designates the loss of phenylalanine at this position. This mutation appears to be at least 50,000 years old, which suggests that its survival may have had evolutionary significance (115). 

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