Membranes general

Process Description Gas-separation membranes separate gases from other gases. Some gas filters, which remove hquids or sohds from gases, are microfiltration membranes. Gas membranes generally work because individual gases differ in their solubility and diffusivity through nonporous polymers. A few membranes operate by sieving, Knudsen flow, or chemical complexation. 

Two common types of membrane materials used are cellulose acetate and aromatic polyamide membranes. Cellulose acetate membrane performance is particularly susceptible to annealing temperature, with lower flux and higher rejection rates at higher temperatures. Such membranes are prone to hydrolysis at extreme pH, are subject to compaction at operating pressures, and are sensitive to free chlorine above 1.0 ppm. These membranes generally have a useful life of 2 to 3 years. Aromatic polyamide membranes are prone to compaction. These fibers are more resistant to hydrolysis than are cellulose acetate membranes. 

Health Hazards Information - Recommended Personal Protective Equipment Data not available Symptoms Following Exposure Irritating to skin and mucous membranes General Treatment for Exposure Data not available Toxicity by Inhalation (ThresholdLimit Value) Not pertinent Short-Term Exposure Limits Not pertinent Toxicity by Ingestion Grade 2 oral rat LDjq 3500 mg/kg Late Toxicity Not pertinent Vapor (Gas) Irritant Characteristics Not pertinent Liquid or Solid Irritant Characteristics Data not available Odor Threshold Not pertinent. 

Health Hazards Information - Recommended Personal Protective Equipment Self-contained breathing apparatus complete protective clothing safety glasses face shield Symptoms Following Exposure Inhalation causes severe irritation of upper respiratory system. Contact with liquid or vapor causes severe bums of eyes and can cause ulcers and blindness. Contact with skin causes severe burns. Ingestion causes severe burns of mucous membranes General Treatment for Exposure Get immediate... 

The most important application of semi-permeable membranes is in separations based on reverse osmosis. These membranes generally have pores smaller than 1 nm. The pressure across the semi-permeable membranes for reverse osmosis is generally much larger than those for ultrafiltration, for example. This is because reverse osmosis is usually used for small molecules which have a much higher osmotic pressure, because of the higher number density, than the colloids separated in ultrafiltration. As a result reverse osmosis membranes have to be much more robust than ultrafiltration membranes. Since the focus of our discussion in this chapter will be on reverse osmosis based separations, we will describe these membranes in greater detail. 

Molecular dispersion < 1.0 nm Particles invisible by electron microscopy pass through semipermeable membranes generally rapid diffusion Oxygen molecules, potassium and chloride ions dissolved in water... 

Colloidal dispersion 1.0 nm-1.0 pm Particles not resolved by ordinary microscope but visible by electron microscopy pass through filter paper but not semipermeable membranes generally slow diffusion Colloidal silver sols, surfactant micelles in an aqueous phase, aqueous latices and pseudolatices... 

Simple diffusion is another mechanism by which substances cross membranes without the active participation of components in the membranes. Generally, lipid-soluble substances employ this method to enter cells. Both simple diffusion and filtration are dominant factors in most drug absorption, distribution, and elimination. 

Koresh, J. E. and A. Sofler. 1987. The carbon molecular sieve membranes. General properties and the permeability of CHJHj mixtures. Separation Scietice and Technology 22(2 3) 972-82. 

The inorganic poly(phosphazene) backbone has received attention as a PEM candidate. This is an attractive system for study due to its ease of synthesis and subsequent modification by many functional groups. However, these membranes generally show low glass transition temperatures and somewhat poor mechanical properties, and they require cross-linking to enhance their performance in hydrated environments. 

However, the variability of these membranes was extremely high in regards to salt rejection, and the membranes generally exhibited low flux. 

Because the plasma membrane generally lacks transporters for phosphorylated sugars, the phosphorylated glycolytic intermediates cannot leave the cell. After the initial phosphorylation, no further energy is necessary to retain phosphorylated intermediates in the cell, despite the large difference in their intracellular and extracellular concentrations. 

Cytoplasmic membranes Generally do not contain sterols Sterols present... 

The main limitation of this wet phase inversion technique for membrane imprinting is that the use of the resulting imprinted membranes is limited to an aqueous medium, since the membrane generally swells in organic solvents, resulting in a loss of recognition sites. 

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

Figure 8.30 The relative size and condensability (boiling point) of the principal components of natural gas. Glassy membranes generally separate by differences in size rubbery membranes separate by differences in condensability...

Note that the activity data in Table 11.2 combines pharmacodynamic and pharmacokinetic data. Lipophilicity trends are observed in assays that require the molecule to cross a membrane, generally whole cell and animal tests, not most biochemical assays. The... 

Inflammation of the respiratory organs and internal membranes generally. One ounce of herb in 1 1/2 pints of water simmered down to 1 pint. Dose, wineglassful every two or three hours. Used externally as a poultice for inflamed surfaces, boils, burns and skin eruptions. 

---