Membrane permeability and selectivity

Figure 8.3 Oxygen/nitrogen selectivity as a function of oxygen permeability. The upper-bound line represents the point above which no better membranes are known [12]. This line shows the trade-off relationship between membrane permeability and selectivity.

Some of the variables that are important for the subsequent discussion are recalled here. The membrane properties are related to the mass transport of the different chemical species through the membrane itself or its separating layer (for an asymmetric or multilayer membrane). Permeability and selectivity were defined for the mass transport by permeation both depend on the membrane nature and morphology that impose the specific transport mechanism driving the permeation of which it is characteristic. Table 13.2 reports the permeability coefficient, selectivity and permeating driving force of some permeation mechanisms. 

This brief overview of mass transfer and separation mechanisms involved in ceramic membrane processes will be useful not only for a better understanding of actual operating conditions of ceramic membranes, but also for anticipating future applications. For example, a same microporous membrane can serve theoretically as liquid or gas separation membrane. However, transport mechanisms and operating conditions being totally different, a good membrane permeability and selectivity in the former case cannot be systematically transposed to the second case. 

The flux of each component is proportional to the concentration gradient and the diffiisivity in the dense layer. However, the concentration gradient is often nonlinear because the membrane swells appreciably as it absorbs liquid, and the diffusion coefficient in the fully swollen polymer may be 10 to 100 times the value in the dense unswollen polymer. Furthermore, when the polymer is swollen mainly by absorption of one component, the diffusivity of other components is increased also. This interaction makes it difficult to develop correlations for membrane permeability and selectivity. 

Synthetic separation membranes are either nonporous or porous. For nonpor-ous membranes, permeability and selectivity are based on a solution-diffusion mechanism examples for technical membrane separations are gas separation, reverse osmosis, or pervaporation. For porous membranes, either diffusive or convective How can yield a selectivity based on size, for larger pore sizes typically according to a sieving mechanism examples for technical membrane separations are dialysis, ultrafiltration, or microfiltration. It is important to note that additional interactions between permeand and membrane, e.g., based on ion exchange or affinity, can change the membrane s selectivity completely membrane adsorbers with a pore structure of a microfiltration membrane are an example. 

Representative data on membrane permeability and selectivity using selected membranes for various components in gases, liquids, and solutions (or suspensions) are presented in Appendix 1. The spreadsheet layout used illustrates and summarizes the diversity of membrane information. This information, mostly of a random nature, is adapted from the appropriate tabulations in the more readily available literature, and the corresponding references are cited. 

As mentioned, membranes ean be classified according to their material components and structure as being porous or dense. What can be understood as dense membranes depends on die scale to which they are studied. The performance of dense membranes (permeability and selectivity) is determined by the intrinsic properties of the material by solution-diffusion flirough the molecular interstices in the membrane material. When solution-diffusion is not the main transport mechanism, the membranes are considered porous. 

In 2009 Scovazzo published a significant paper where literature data obtained using the proposed model to predict gas solubility and gas permeability was summarized, along with adding new data, on the SILMs membranes permeabilities and selectivities for the gas pairs CO2/N2, CO2/CH4, O2/N2, ethylene/ethane, propylene/propane, 1-butene/butane, and 1,3-butadiene/butane, with the object as to serve as guide for future researches in this area. 

FIG. 22-76 Constant -cost lines as a function of permeability and selectivity for CO2/CH4, Cellulose-acetate membrane mscf is one thousand standard cubic feet, CouHesy VP R. Grace.)... 

The factors to consider in the selection of cross-flow filtration include the cross-flow velocity, the driving pressure, the separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, and the hydrodynamic conditions within the flow module. Again, since particle-particle and particle-membrane interactions are key, broth conditioning (ionic strength, pH, etc.) may be necessary to optimize performance. 

In situ perfusion studies assess absorption as lumenal clearance or membrane permeability and provide for isolation of solute transport at the level of the intestinal tissue. Controlled input of drug concentration, perfusion pH, osmolality, composition, and flow rate combined with intestinal region selection allow for separation of aqueous resistance and water transport effects on solute tissue permeation. This system provides for solute sampling from GI lumenal and plasma (mesenteric and systemic) compartments. A sensitive assay can separate metabolic from transport contributions. 

Despite concentrated efforts to innovate polymer type and tailor polymer structure to improve separation properties, current polymeric membrane materials commonly suffer from the inherent drawback of tradeoff effect between permeability and selectivity, which means that membranes more permeable are generally less selective and vice versa. 

Immobilization or labeling technique. The polymeric supports should not significantly affect indicator performance, such as response time or selectivity, and if affected the changes should be reproducible. For cellular applications, indicator should be membrane permeable and/or retained by the cells. 

Table 6.2 shows some gas permeabilities and permselectivities for several gases through a membrane, activated at 9S0°C. From these data it is clear, that by simple thermochemical treatment, permeability and selectivity can be influenced. The product of permeability and selectivity is among the highest ever reported. 

Large area membranes that demonstrate high permeability and selectivity Reproducibility of synthesis ... 

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... 

Mixed matrix membranes have been prepared from ABS and activated carbons. The membranes are intended for gas separation. A random agglomeration of the carbon particles was observed. A close interfacial contact between the polymeric and filler phases was observed. This morphology between inorganic and organic phases is believed to arise from the partial compatibility of the styrene/butadi-ene chains of the ABS copolymer and the activated carbon structure. A good permeability and selectivity for mixtures of carbon dioxide and methane has been reported (91,92). 

Considerable research and development effort is being placed on a chlorine-resistant membrane that will maintain permeability and selectivity over considerable time periods (years). 

This so-called "active" layer has characteristics similar to those of cellulose acetate films but with a thickness of the order of 0.1 micrometer (jjm) or less, whereas the total membrane thickness may range from approximately 75 to 125 ym (see Figure 1). The major portion of the membrane is an open-pore sponge-like support structure through which the gases flow without restriction. The permeability and selectivity characteristics of these asymmetric membranes are functions of casting solution composition, film casting conditions and post-treatment, and are relatively independent of total membrane thickness. 

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. 

---