Permeability and Permselectivity

Permselectivity is crucial to the utility of any types of membranes. If the permselectivity toward a particular reaction species is high, the separation is quite clean and the need for further separation processing downstream of the membrane reactor is reduced. When a permeate of very high purity is required in some cases, dense membranes are preferred. While a high permselectivity is generally desirable, there may be situations where a high permeate flux in combination with a moderate permselectivity is a better alternative to a high permselectivity with a low permeability, particularly when recycle streams are used. 

Material design and selection. The permeate flux of a membrane has an enormous impact on the process economics. This can be particularly essential when dense membranes are used due to their extremely high permselectivity. When choosing a dense 

Effects of alloying metal on the hydrogen permeability of a palladium-based membrane 

Alloying metal content (%) Relative hydrogen permeability  

Note a Remaining content being Pd b Relative permeability = hydrogen permeability of Pd alloy membrane/hydrogen permeability of pure Pd [Iloh, 1990] 

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Pore size plays a key role in determining permeability and permselectivity (or retention property) of a membrane. The structural stability of porous inorganic membranes under high pressures makes them amenable to conventional pore size analysis such as mercury porosimetry and nitrogen adsor-ption/desorption. In contrast, organic polymeric membranes often suffer from high-pressure pore compaction or collapse of the porous support structure which is typically spongy . 

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. 

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. 

FIGURE 22.6 Permeability and permselectivity of vaginal and buccal epithelia in the rabbit, (a) Flux of 6-carboxyfluoroscein, a hydrophilic molecule, by in vitro perfusion studies steady-state flux ( xg/cm2/h x 106), (b) resistance (fl cm2 x 10 2), (c) thickness (p,m x 10-2), and (d) ratio of potassium transport number to chloride transport number, which is calculated from electrical measurements, used as indicative of the epithelium selectivity for positively charged molecules. (Modified from Sayani, A.P. and Chien, Y.W., Crit. Rev. Ther. Drug Carrier Syst. 13, 85, 1996.)... 

K. Tanaka, H. Kita, M. Okano and K. Okamoto, Permeability and Permselectivity of Gases in Fluorinated and Non-fluorinated Polyimides, Polymer 33, 585 (1992). 

Li Y, Wang X, Ding M, Xu J (1996) Effects of molecular structure on the permeability and permselectivity of aromatic polyimides. J Appl Polym Sci 61(5) 741... 

Michaels et al (5) further studied the barrier effects of the skin in terms of the composition of the pathways, as well as the microstructure, permeability and permselectivity of the particular pathways. In particular, the barrier to permeation in the stratum corneum was attributed not only to the interstitial lipids, but also to their structure as ordered multilayers for nonpolar alkanols. 

Permeability and permselectivity of oxygen, nitrogen and sulfur hexafluoride through modified carbon molecular sieve membranes... 

The openness (e.g., volume fraction) and the nature of the pores affect the permeability and permselectivity of porous inorganic membranes. Porosity data can be derived from mercury porosimetry information. Membranes with higher porosities possess more open porous structure, thus generally leading to higher permeation rates for the same pore size. Porous inorganic membranes, particularly ceramic membranes, have a porosity... 

Possible transport mechanisms in a fluid system through the membrane pores are multiple. They vary to a great extent with the membrane pore size and, to a less extent, with chemical interaction between the transported species and the membrane material. Under the driving force of a pressure gra nt, permeants (whether in the form of solvents, solutes or gases) can transport across a membrane by one or more of the mechanisms to be discussed below. The degree by which they affect permeability and permselectivity depends on the operating conditions, membrane characteristics and membrane-permeating species interactions in the application environment. 

Permeability and permselectivity of a membrane depend on its pore size distribution. But equally important, they are applications specific and determined by the interactions between the process stream and the pore or membrane surface. However, for general characterization purposes, some model permeants (solvents and molecules) are often used to obtain a generic" permeability and permselectivity for that membrane. Water is... 

Thermal and hydrothermal exposures can change the ix>re size and its distribution, porosity and tortuosity of a porous membrane which in turn influence the separation properties of the membrane such as permeability and permselectivity. Several ceramic membranes have been investigated for their responses to thermal and hydrothermal environments. 

Effect of alkali exposure on membrane permeability and permselectivity... 

Perhaps the most important operating consideration related to membrane processes is how to enhance, maintain and restore membrane flux (or permeability) and permselectivity. They can be approached by the following techniques. 

In most of the industrially important gas separation applications, the feed streams to be processed occur at high temperatures. It is very desirable not to ramp down the stream temperature and then ramp up again after the treatment It is exactly this reason that inorganic membranes are attractive due to their inherent thermal stabilities. Operation at high temperauires, however, not only confounds the above issues but also can affect the phases and microstructures of the membrane materials. All these factors have implications on the permeabilities and permselectivities. 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

The combination of high temperature and chemical exposure poses a very challenging material problem that is quite common in high-temperature membrane reactor applications. The consequence of structural degradation as a result of such a combination not only affects the permeability and permselectivity but also leads to physical integrity or mechanical properties. These issues apply to both metal and ceramic membranes. 

When combining the separator and the reactor functions into one compact physical unit, factors related to catalysis need to be considered in addition to those related to selective separation discussed in previous chapters. The selection of catalyst material, dispersion and heat treatment and the strategic placement of catalyst in the membrane reactor all can have profound impacts on the reactor performance. The choice of membrane material and its microstructure may also affect the catalytic aspects of the membrane reactor. Furthermore, when imparting catalytic activity to inorganic membranes, it is important to understand any effects the underlying treatments may have on the permeability and permselectivity of the membranes. 

Besides the critical issue of containment and sealing, the choice of the materials for the membrane and other membrane reactor components affects the permeability and permselectivity, operable temperature, pressure and chemical environments and reaction performance. Important material parameters include the particular chemical phase, thickness, thermal properties and surface contamination of the membrane, membrane/support microstructure, and sealing of the end surfaces of the membrane elements and of the joining areas between elements and module components. The conventional permeability versus permselectivity dilema associated with membranes needs to be addressed before inorganic membrane reactors are used in bulk processing. 

Like all other chemical processes, the separation processes by inorganic membranes have two major cost issues capital investments and operating costs. Capital costs are affected by the membrane area required (which in turn arc determined by the permeability and permselectivity), compression or recompression energy requirements as dictated by the operating pressure, piping and vessels, instrumentation and control, and any pretreaunent requirements (depending on the nature of the feed material and the membrane). The operating cosLs arc determined by the required membrane replacement... 

In addition to the considerations on permeability and permselectivity, another important parameter is the ratio of filtering surface to membrane volume, because... 

A list of permeabilities and permselectivities of inorganic membranes is given in Table 2. 

Table 2 Permeabilities and Permselectivities of Some Inorganic Membranes...

Several investigators have faced the problem of modeling of membrane reactors either to achieve a proper interpretation of their experimental data or to assess the role of the various operating parameters (temperature, membrane permeability and permselectivity, feed flow rates, and concentrations, etc.) on the performance of membrane reactors. In some other cases [61,138] modeling studies helped to point the way toward future experimental work concerning, e.g., the need for thinner or more permeable or more stable membranes to outperform conventional technologies for given applications. 

The relationship between A b.p. (nonsolvent boiling point -solvent boiling point) of the two solvents acetone (b.p. 56°C) and dloxolane (b.p. 75 C) and the level of a single nonsolvent isobutanol (b.p. IIO C) required to produce equivalent skinned membranes (as deduced from their equivalency in permeability and permselectivity) is shown in Table II. 

In the case of a composite membrane consisting of a skinless porous substrate and a dense film, permeability and permselectivity may be determined solely by the resistance of the denser film. Different membrane polymers may therefore be employed for the thin barrier layer and the thick support structure. This permits a combination of properties which are not available in a single material. Such membranes were initially developed for desalination by reverse osmosis where they are known as thin- or ultrathin-film composites or nonlntegrally-skinned membranes. A second type of composite membrane is utilized for gas separations. It is a composite consisting of an integrally-skinned or asymmetric membrane coated by a second, more permeable skin which is used to fill skin defects. The inventors of the latter have entitled their device a resfstanee model membrane, but the present author prefers the term coated integrally-skinned composites. 

Many scientific papers (J ) on membranes are primarily concerned with phenomenological correlations between the preparation of the membrane and its performance, including fluid permeability and permselectivity to solutes. The performance of the membrane is principally governed by its pore characteristics in a complicated manner (2 ). These pore characteristics in turn are influenced by the molecular characteristics of the polymer and the preparative method ( , ). 

Factors on dehydrogenation of ethylbenzene to styrene in ZSM-5 type zeolite membrane reactors were studied. About 18% conversion of ethylbenzene increase in the Fe-ZSM-5 membrane reactor can be obtained over the fixed-bed reactor. This result is better than that obtained in the other membrane reactors. The bigger is the permeability and permselectivity of ZSM-5 membranes, the higher is the conversion of ethylbenzene. The order of membrane stability for ethylbenzene dehydrogenation is silicalite-1 > Fe-ZSM-5 > Fe/ZSM-5 > ZSM-5. 

Park MK, Deng S, Advincula R (2004) Permeability and permselectivity control in photo-cross-linkable polyelectrolyte ultrathin films containing pH-switchable and benzophenone functional groups. Polym Mater Sci Eng 90 133-134... 

Pis are attractive membrane materials for gas separation because of their good gas separation and physical properties. Many attempts have been made to modify the chemical structure of Pis in order to construct both highly permeable and permselective membrane materials. Blends of Pis have been demonstrated to exhibit improved performance in gas separation applications. ... 

Komatsuka, T. and Nagai, K. (2009) Temperature dependence on gas permeability and permselectivity of poly(lactic acid) blend membranes. Polymer Journal, 41, 455-458. 

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