Permeability membrane performance

The foregoing equations assume that membrane performance is time independent. In some cases, a noticeable reduction in permeability occurs over time primarily due to membrane fouling. In such cases, design and operational provisions are used to maintain a steady performance of the system (Zhu et a ., 1997). 

When the membrane performs only a separation function and has no catalytic activity, two membrane properties arc of importance, the permeability and the selectivity which is given by the separation factor. In combination with a given reaction, two process parameters are of importance, the ratio of the permeation rate to the reaction rate for the faster permeating component (c.g. a reaction product such as hydrogen in a dehydrogenation reaction) and the separation factors (permselectivities) of all the other components (in particular those of the reactants) relative to the faster permeating gas. These permselectivities can be expressed as the ratios of the permeation rates of... 

The membrane performance for separations is characterized by the flux of a feed component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a feed mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity or separation factor. Selectivity (0 can be defined as the ratio of the permeabilities of the feed components across the membrane (i.e., a/b = Ta/Tb, where A and B are the two components). The permeability and selectivity of a membrane are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent... 

Cell microarrays have also been fabricated. Ziauddin and Sabatini (2001) demonstrated the ability to transfect cells cultured onto plasmid DNA arrayed in gelatin on a standard DNA microarray slide. Xu (2002) printed down cells in the form of high density microarrays on permeable membranes and demonstrated phenotypic assay performance with the immobilized cells. The commercialization of viable cell arrays will permit an even closer look at cell-mediated events during the drug discovery process. 

DMFC low methanol permeability membrane, high performance catalyst. 

The requirement of hydrophilicity in barrier materials has been widely accepted, but the mechanism by which it affects membrane performance, especially for the permselectivity, is not fully understood. Cellulose acetate and some kinds of polyamides and their analogues featured in the present review have both hydraulic permeability and permselectivity, while most highly hydrophilic materials have high permeability for water and show unselective permeation for ions and organic solutes. 

A major advance in the performance of amperometric oxygen sensors has been achieved by placing both the cathode and the anode behind the oxygen-permeable membrane (Fig. 7.4). This sensor is known as the Clark oxygen electrode. 

Table II lists the feed gas compositions used for the study to determine the effects of feed composition on membrane performance. The different compositions are representative of diving gas mixtures used at various depths. Both the individual component permeabilities and the mixture permeabilities were evaluated at several different pressure drops up to 1200 psig feed pressure.

Other system variables that will have an effect on the separation process are temperature and relative humidity of the gas. Increasing the temperature raises most permeabilities by about 10 to 15% per 10°C and has little effect on separation factors. The effect of relative humidity is variable depending upon the membrane used. High relative humidities, greater than 95%, are generally detrimental due to membrane plasticization. Contamination with liquid water has been found to dramatically reduce membrane performance for cellulose acetate ... 

Equation (9.1) is the preferred method of describing membrane performance because it separates the two contributions to the membrane flux the membrane contribution, P /C and the driving force contribution, (pio — p,r). Normalizing membrane performance to a membrane permeability allows results obtained under different operating conditions to be compared with the effect of the operating condition removed. To calculate the membrane permeabilities using Equation (9.1), it is necessary to know the partial vapor pressure of the components on both sides of the membrane. The partial pressures on the permeate side of the membrane, p,e and pje, are easily obtained from the total permeate pressure and the permeate composition. However, the partial vapor pressures of components i and j in the feed liquid are less accessible. In the past, such data for common, simple mixtures would have to be found in published tables or calculated from an appropriate equation of state. Now, commercial computer process simulation programs calculate partial pressures automatically for even complex mixtures with reasonable reliability. This makes determination of the feed liquid partial pressures a trivial exercise. 

Equation (9.11) identifies the three factors that determine the performance of a pervaporation system. The first factor, pevAp, is the vapor-liquid equilibrium, determined mainly by the feed liquid composition and temperature the second is the membrane selectivity, G-men, an intrinsic permeability property of the membrane material and the third includes the feed and permeate vapor pressures, reflecting the effect of operating parameters on membrane performance. This equation is, in fact, the pervaporation equivalent of Equation (8.19) that describes gas separation in Chapter 8. 

This chapter will focus on three types of membrane extracorporeal devices, hemodialyzers, plasma filters for fractionating blood components, and artificial liver systems. These applications share the same physical principles of mass transfer by diffusion and convection across a microfiltration or ultrafiltration membrane (Figure 18.1). A considerable amount of research and development has been undertaken by membrane and modules manufacturers for producing more biocompatible and permeable membranes, while improving modules performance by optimizing their internal fluid mechanics and their geometry. 

Another favorable aspect of stirred batch reactors is the fact that they are compatible with most forms of a biocatalyst. The biocatalyst may be soluble, immobilized, or a whole-cell preparation in the latter case a bioconversion might be performed in the same vessel used to culture the organism. Recovery of the biocatalyst is sometimes possible, typically when the enzyme is immobilized or confined within a semi-permeable membrane. The latter configuration is often referred to as a membrane reactor. An example is the hollow fiber reactor where enzymes or whole cells are partitioned within permeable fibers that allow the passage of substrates and products but retain the catalyst. A hollow-fiber reactor can be operated in conjunction with the stirred tank and operated in batch or... 

Mohamed, N. A., and Al-Dossary, A. O. H. (2003), Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and mefa-phenylene units. Part III Preparation and properties of semi-permeable membranes for water desalination by reverse osmosis separation performance, Eur. Polym. /., 39,1653-1667. 

In liquid filtration using micro-, ultra-, and nanofiltration porous membranes, the driving force for transport is a pressure gradient. Solvent permeability and separation selectivity are the two main factors characterizing membrane performance. Convective flux is predominant with macroporous and mesoporous membrane strucmres, the selectivity being controlled by a... 

Spinning conditions affect membrane physical dimensions and morphology, which in turn influence membrane performance such as permeability. A satisfactory spinning process would produce fibers having the requisite surface pore size and mechanical strength. Parameters involved in a spinning process are ... 

Figure 6 accentuates the effect of the width of the psd. The three parameteriza-tions with s = 0.15,0.3, 0.6, labeled as (1), (2), (3) in Table 1 and Fig. 5 are used in Fig. 6. All other parameters of the calculation are specified in Table 2. The width of the psd decreases upon increasing s. The first moments of the psds are, respectively, r = 14.9, 3.0, 1.2 nm. In Fig. 6, yp and U are scaled to Jm ccr, defined in Eq. (19). Table 1 indicates that the permeability of the saturated membrane, K(ws), varies strongly with s. However, the effect of the psd width on membrane performance is accumulated in the scaling parameter Jm, as shown in Fig. 6. Here, the critical current densities are found at ypc/7m 0.66, 0.65, 0.67, practically independent of the psd with this scaling to Jm.

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