Ideal membrane selectivity

Table 8.1 are measured with pure gases the selectivity obtained from the ratio of pure gas permeabilities gives the ideal membrane selectivity, an intrinsic property of the membrane material. However, practical gas separation processes are performed with gas mixtures. If the gases in a mixture do not interact strongly with the membrane material, the pure gas intrinsic selectivity and the mixed... 

Under ideal conditions with a negligible downstream pressure of both components, the separation factor can be equated to the ideal membrane selectivity factored into its mobility and solubility controlled contributions, viz.,... 

Consider an undesirable scenario where the atmosphere has 400ppmv (parts per million by volume) of CO2 and the rest may be assumed to be N2 from the perspective of membrane separation. How bigb should the ideal membrane selectivity be to recover a 90% CO2 stream in the permeate for the purpose of sequestering the CO2 (Ans. 22500.)... 

More economically competitive if ideal permselectivity is >15 (highly dependent on membrane selection) indication of feasibiUty obtained with information on critical temperature and van der Waals volume. 

State of the Art A desirable gas membrane has high separating power (ot) and high permeability to the fast gas, in addition to critical requirements discussed below. The search for an ideal membrane produced copious data on many polymers, neatly summarized by Robeson [J. Membrane ScL, 62, 165 (1991)]. Plotting log permeability versus log selectivity (ot), an upper bound is found (see Fig. 22-73) which all the many hundreds of data points fit. The data were taken between 20-50°C, generally at 25 or 35°C. 

The conclusion above is valid for ideally selective membranes. Real membranes in most cases have limited selectivity. A quantitative criterion of membrane selectivity for an ion to be measured, relative to another ion M +, is the selectivity coefficient The lower this coefficient, the higher the sefectivity wifi be for ions relative to ions An electrolyte system with an imperfectly selective membrane can be described by the scheme (5.16). We assume, for the sake of simplicity, that ions and have the same charge. Then the membrane potential is determined by Eq. (5.17), and the equation for the full cell s OCV becomes... 

Below we present a well-known calculation of membrane potential based on the classical Teorell-Meyer-Sievers (TMS) membrane model [2], [3]. The essence of this model is in treating the ion-selective membrane as a homogeneous layer of electrolyte solution with constant fixed charge density and with local ionic equilibrium at the membrane/solution interfaces. In spite of the obvious idealization involved in the first assumption the TMS model often yields useful results and represents in fact the main tool for practical membrane calculations. We shall return to TMS once again in 4.4 when discussing the electric current effects upon membrane selectivity. In the case of our present interest, the simplest TMS model of membrane potential for a 1,2 valent electrolyte reads... 

Ion-selective electrodes, discussed in the remainder of this chapter, respond selectively to one ion. These electrodes are fundamentally different from metal electrodes in that ion-selective electrodes do not involve redox processes. The key feature of an ideal ion-selective electrode is a thin membrane capable of binding only the intended ion. 

It appeared that only for the membranes with very high selectivity the correction for water transport is sufficient to explain the difference between the ideal membrane potential according to (46) and the value... 

For a defect-free ideal membrane, the selectivity is independent of thickness, and either permeability ratios or permeance ratios can be used for comparison of selectivi-ties of different materials. Nonideal module flow patterns, defective separating layers, impurities in feeds, and other factors can lower the actual selectivity of a membrane compared to tabulated values based on ideal conditions (Koros and Pinnau, 1994). 

At low permeate pressures typical of ideal pervaporation, the same form for membrane selectivity applies as for gas or vapor separation of components A and B, which will be discussed later. This simple selectivity equals the inverse ratio of the resistances of the membrane to permeation of components A and B [S2B / 2A from Eq. (1)]. This ratio can be shown to simply equal to the ratio of permeabilities of the two components in the material comprising the selective layer of the membrane. 

The membrane selectivity toward an osmotic agent and water, described by the osmotic reflection coefficient a. An ideal semipermeable membrane has the a value of 1, which means that it allows the passage of only water molecules. In contrast, a leaky semipermeable membrane with a value approaching zero does not exhibit such selectivity and permits the transport of not only water, but also an osmotic agent. 

Only if —Z,pd = Lp, then (AP)j =0 = All. This is the condition for an ideal semiperrneable membrane, which blocks the transport of solute no matter what the value of AP and All is. When this is not the case, the membrane allows some solute to pass, Zpd/Zp < 1. The ratio —LpdJLp is called the reflection coefficient a. The value cr = 1 indicates that all solute is reflected (ideal membrane) the solute cannot cross the membrane. When a < 1, on the other hand, some of the solute is reflected and the rest crosses the membrane when cr = 0, the membrane is completely permeable and is not selective. If we introduce cr into Eq. (10.22a), we have... 

Selectivity and productivity depend on sorption and diffusion. Sorption is dictated by thermodynamic properties, namely, the solubility parameter of the solute(s)/membrane material system. On the other hand, the size, shape, molecular weight of the solute, and the availability of inter/intra molecular free space of the polymer largely govern the second property, the diffusion coefficient. For an ideal membrane, both the sorption and diffusion processes should favor the chosen solute. If one step becomes unfavorable for a given solute the overall selectivity will be poor [28]. 

Many studies demonstrate that Pd-based membranes with ideally infinite selectivity for H2 can be used to increase the equilibrium conversion of steam reforming of methane by H2 removal [10,11]. The driving force in this process is the partial pressure difference. 

Permeation characteristics of Knudsen diffusion membranes, consisting of a support and two consecutive layers, have been used to calculate the performance of the ceramic membrane reactor, see also Section 14.2.1 [17,31]. The pore size of the separation layer of these membranes is 4 nm in diameter [31,38]. Ideal membranes which remove all the hydrogen formed do not exist (possible Pd-based membranes will come close to the required characteristics), but are used as a basis for calculating the maximum possible increase in conversion and selectivity. 

Theoretical infinite selectivity can be achieved for hydrogen separation by Pd or Pd-alloy membranes [40]. The reaction is carried out at high temperature, about above 1000 K, because of the equihbrium of the endothermic reactions involved in the process. Many studies demonstrate that Pd-based membranes with ideally infinite selectivity for H2 can be used to increase the equihbrium conversion of methane steam to H2 removal [36, 40]. The driving force in this process is the partial pressure difference. The high cost, hmited hfetime, and low permeabihty are relevant hmits of Pd and Pd-ahoy membrane. To overcome these drawbacks, the studies have been carried out for the preparation of supported metaUic membranes in which a thin metallic layer is supported on a thicker sublayer [73]. 

The ratio of permeabilities for a binary mixture is the membrane selectivity OL (also called the ideal separation factor) ... 

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