Selectivity of membranes

Selectivity of Membranes Membrane potentials result from a chemical interaction between the analyte and active sites on the membrane s surface. Because the signal depends on a chemical process, most membranes are not selective toward... 

Permeabilities measured for pure gases can serve as a rough guide for selection of membrane materials. For design, data must be obtained on gas mixtures, where selectivities are often found to be much lower than those calculated from pure-component measurements. This effect is often due to plasticisation of the membrane by sorption of the most soluble component of the gas. This allows easier penetration by the less-permeable components. The problem of concentration polarisation, which is often encountered in small-scale flow tests, may also be responsible. Concentration polarisation results when the retention time of the gas in contact with the membrane is long. This allows substantial depletion of the most permeable component on the feed side of the membrane. The membrane-surface concentration of that component, and therefore its flux through the membrane, decreases. 

Many efforts to improve the efficiency and the selectivity of membrane processes are based on molecular recognition properties. 

Confirm the selection of membrane and gasket material for a full-scale facility. 

For practical purposes it is appropriate to characterize the selectivities of membranes of similar insoluble salts (primarily silver halides) in terms of selectivity coefficients. These are, of course, only of empirical character and are found using (3.1.7). An approximation for the values of these constants is... 

The selectivity of membranes between two cations of the same charge (in the absence of other interfering species) can be conveniently characterized by the observed zero-current membrane potential E through a measurement of the EMF of a cell of the type shown in Fig. 1. The potential difference between solutions 1 and 2, the so-called membrane potential E, is ideally described by 18-21... 

The technology to fabricate ultrathin high-performance membranes into high-surface-area membrane modules has steadily improved during the modem membrane era. As a result the inflation-adjusted cost of membrane separation processes has decreased dramatically over the years. The first anisotropic membranes made by Loeb-Sourirajan processes had an effective thickness of 0.2-0.4 xm. Currently, various techniques are used to produce commercial membranes with a thickness of 0.1 i m or less. The permeability and selectivity of membrane materials have also increased two to three fold during the same period. As a result, today s membranes have 5 to 10 times the flux and better selectivity than membranes available 30 years ago. These trends are continuing. Membranes with an effective thickness of less than 0.05 xm have been made in the laboratory using advanced composite membrane preparation techniques or surface treatment methods. 

Natural gas is usually produced from the well and transported to the gas processing plant at high pressure, in the range 500-1500 psi. To minimize recompression costs, the membrane process must remove impurities from the gas into the permeate stream, leaving the methane, ethane, and other hydrocarbons in the high-pressure residue gas. This requirement determines the type of membranes that can be used for this separation. Figure 8.30 is a graphical representation of the factors of molecular size and condensability that affect selection of membranes for natural gas separations. 

Concentration polarization plays a dominant role in the selection of membrane materials, operating conditions, and system design in the pervaporation of VOCs from water. Selection of the appropriate membrane thickness and permeate pressure is discussed in detail elsewhere [50], In general, concentration polarization effects are not a major problem for VOCs with separation factors less than 100-200. With solutions containing such VOCs, very high feed velocities through... 

Membranes are very finely porous structures and like all such porous structures used in an industrial context are susceptible to fouling caused by adhesion of components of the materials being processed. This fouling can be minimised or avoided if suitable polymers are used in membrane manufacture. However, the selection of membrane polymers suited to particular separations has until now been a matter of experience (and failure) rather than science. However, an AFM used with the colloid probe technique [23] can provide a rapid means of assessing the adhesion of solutes to membrane materials and is hence a powerful tool for the membrane technologist. 

Is the membrane hydrophobic or hydrophilic Has the proper selection of membrane tied to the property of the slurry ... 

The module operation efficiency is determined on the one hand, by selectivity of membrane elements and sorption characteristics of ion-exchangers and on the other hand, by physico-chemical and radionuclide composition, concentration of suspended particles, salts and radionuclide activity levels. The integral decontamination-purification coefficients Kpurj t, depending on LRW radionuclide, physical and chemical composition, vary within 10 - 10 . Depending on composition of initial LRW and in compliance with on-line control data, MMSF can operate either imder the full-cycle mode involving all basic modules or imder reduced-cycle mode using only some of modules. 

Limiting glucose diffusion without affecting oxygen diffusion by careful selection of membranes (88, 129, 180). 

The correct choice of membrane should be determined by the specific objective, such as the removal of particulates or dissolved solids, the reduction of hardness for the production of ultra pure water or the removal of specific gases/chemicals. The end use may also dictate the selection of membranes in industries such as potable water, effluent treatment, desalination, or water supply for electronic or pharmaceutical manufacturing. 

Membrane fouling is primarily a result of membrane-solute interaction that is generally irreversible and requires chemical cleaning to restore the original separation properties of the membrane. Fouling effects can be accenmated or minimized by proper selection of membrane material properties such as hydrophobicity/hydrophUicity or surface charge, as well as membrane pore size. A discussion on the membranes that are typically used in beer processing was included in Section... 

It is essential to understand the basic characteristics of the membrane, such as membrane materials and their formation, so that proper selection of membrane separation processes can be made. 

Chemical surface modification methods of gas-separation membranes include treatment with fluorine, chlorine, bromine, or ozone. These treatments result in an increase in membrane selectivity with a decrease in flux. Cross-linking of polymers is often applied to improve the chemical stability and selectivity of membranes for reverse osmosis, pervaporation, and gas-separation applications (41). Mosqueda-Jimenez and co-workers studied the addition of surface modifying macromolecules, and the use of the additive... 

Selectivity is one of the essential characteristics of the membrane electrodes. Accordingly, it is important that by-products, degradation products, metabolites, and compressing components (excipients) do not interfere, and thus the ISMEs can be successfully used for raw material assays as well as for raw material determinations in pharmaceutical formulations. To improve the selectivity, it is necessary to choose another counter ion for membrane construction, because the selectivity of membrane electrodes can be explained through the stability constants of the ion pair complexes between the counter ion and the ions from the solution. To be selective over one ion from the solution it is necessary for the stability constant of the ion pair complex to be less than the stability constant of the ion pair complex obtained between the main ion and counter ion. 

The extrinsic pathway is activated by tissue injury and is not of major concern in the clinical use of membrane devices. The intrinsic pathway, however, is initiated by a multitude of factors. Including interactions between serum proteins and exogenous materials. Hydrodynamic forces acting on platelets may also lead to the release of platelet factors that trigger the intrinsic pathway. Thus, the selection of membrane materials to minimize thrombogenesls cannot be fully separated from the design of devices to contain them because of this potential for shear forces to activate the clotting cascade. 

Native soy protein isolate may be produced by ultrafiltration of an aqueous extract of defatted soy bean meal, Q), (5 ). The process layout is shown in Fig. 1. A careful selection of membrane parameters such as flow velocity, pressure drop, temperature, and of the type of membrane and modules is important in order to obtain a bean protein isolate by a direct ultrafiltration of the clarified extract ( 5). The protein isolate has a protein-dry matter ratio higher than 90% (N x 6.25), when using this process. 

In most cases the membrane material serves as a classification for reviews and surveys [6-9]. The selectivity of membranes is based on the pore size and various properties of the membrane material such as hydrophilicity or hydrophobicity, thus giving the ability, depending on the particle size, to recover soluble catalysts either by reverse osmosis or by ultrafiltration. In the case of ultrafiltration, the molecular weight of the catalyst has to be increased by suitable methods such as deri-... 

Figure 3.99 shows Wang s data on the hydrolysis of starch by a-amylase using the same MWCO membrane. Here the difference in carbohydrate concentrations between retentate and permeate is much larger because the products of a-amylase hydrolysis are predominantly maltose and larger dextrins. Eventually the dextrins are hydrolyzed to glucose, but at a slower rate. Thus, the carbohydrate concentration in the reactor increases with time. These results suggest that the selection of membrane MWCO can control the products of the reaction. 

This general technique has been widely used for many years it is unfortunately limited by the selection of membrane materials available. The rate of gas flow in all these devices is controlled by changing the temperature of an external heater. This is satisfactory for constant flow systems, but too slow for accurate dosing of small quantities. 

---