Gas permeance

We have been studying the novel process for CO2 separation named membrane/absorption hybrid method. The advantages of this process are that high gas permeance and selectivity were obtained. The concept of this process is shown in Fig. 1. Both feed gas and absorbent solution are supplied to the inside of hollow fibers. While Ae liquid flows upward inside the hollow fibers, absorbent solution absorbs CO2 selectively and it becomes a rich solution. Most of rich solution permeates the membrane to the permeate side maintained at reduced pressure, where it liberated CO2 to become a lean solution. Compared to a conventional gas absorption... 

Figure 10.6 Temperature dependency of gas permeances for unmodified MFI-type zeolite membrane (closed symbols on solid line gas permeances for single permeation, open...

Gas permeance through the membranes was measured in the pressure-controlled dead-end mode [18], The disc-shaped membranes were placed in the commercially permeance cells, K250 and K500 as mentioned before. Maximum operation temperatures were 300 and 600°C respectively. The membrane was fitted in the cell with the microporous top-layer at the gas feed side. The pressure difference over the membrane was adjusted by an electronic pressure controller. The gas flow through the membrane was measured by electronic mass flow meters. A schematic representation of the permeance set-up is given in Figure 5. 

Effect of hydrothermal exposure of silica-modified alumina membrane on gas permeance... 

A membrane reactor preceded or followed bv a conventional reactor. Consider a typical commercial porous membrane currently available that exhibits a moderate to low gas separation factor and a high gas permeance even for the gas intended to be the retentate. Much of this less permeable gas in the feed stream is lost to the permeate (low pressure) side in the entrance section of the reactor due to its high partial pressure difference across the membrane layer. This leads to the undesirable effect of a low reactant conversion in that section. An effective way of reducing this reactant loss is to have a membrane enclosed section preceded by an impermeable reaction zone. To achieve a maximum total conversion, the impermeable length relative to the membrane length needs to be optimized. 

FIGURE 10.13 (See color insert following page 588.) Experimental system for the calculation of single-gas permeances on zeolite mbular membranes. 

FIGURE 10.14 Qualitative diagram showing the dependence of single-gas permeance with temperature. 

In the ideal molecular sieving regime the interaction of the permeating species with the zeolite is minimal, and the separation takes place simply because the size and shape of some of the molecules prevent them from entering the membrane pores or move across them with substantial velocity [2]. In that regime adsorption does not play an important role, and, therefore, separation selectivity (ratio of permeances between species in the mixture) should coincide with ideal selectivity (ratio of single-gas permeances). 

Mol/m s (note that flux is represented not permeance). Ratio of single-gas permeances. 

Korikov AP, Sirkar KK, Otto H. Membrane gas permeance in gas-liquid membrane contactor systems for solutions containing a highly reactive absorbent. J Membr Sci 2005 246 27-37. 

If the starting sol becomes below supersaturation before the membrane is fully developed, growth must be repeated with a fresh sol. Two or three hydrothermal syntheses are often required to gel good quality membranes. In many cases, large quantity of non connected zeolite crystals can be observed on the surface of infiltrated composite membranes, after several synthesis steps. The methods for evaluating the membrane quality will be detailed in the Characterization section. In the case of MFI membranes, a very simple method can be used at this step the membrane is washed, dried at about 150-200 °C and tested by single gas permeation (lie or N2). As far as the TP A ions are blocking the zeolite channels, no gas permeance should be detected if there are no defects. 

The single gas permeance ratios N2/SF6 or FF/SFf, are often considered as representative of the zeolite membrane quality. However these values can be quite low even for high quality membranes [131]. Indeed, these ratios are affected by the defect distribution but can also depend on other factors such as the applied feed pressure, the film thickness and the support type. For example the N2/SF6 permeance ratio increases with pressure due to non-linear adsorption of SF6 and also increases with the film thickness due to the mass transfer resistance of the support. In the same way, the ideal or mixture selectivity /-butane/n-butane is often considered as a good indication of the membrane quality, but varies largely (from 10 up to 200) even for good quality membranes. It is then difficult to use only one of these criteria to compare zeolite membranes. 

Depending on the considered gas, membrane thickness, crystal orientation and experimental conditions, the gas permeance through zeolite membranes typically lies in the range 10 5-10"9 mol.m 2.s 1.Pa 1. 

Figure 17.5 Single gas permeance trend as a function of the temperature.

The viscous flow occurs when gas molecules collide exclusively with each-another. In this case no separation can be obtained. Equation 17.2 describes the gas permeance in presence of the viscous flow ... 

Figure 17.7 Gas permeance and CO2/N2 separation factor as a function of the temperature for equimolar CO2-N2 mixture in dry and moist conditions. Reprinted from X. Gu, J. Dong and T. M. Nenoff, Synthesis of defect free FAU-type zeolite membranes and separation for dry and moist CO2/ N2 mixtures, Industrial and Engineering Chemistry Research, 44, 937-944, 2005, with permission from ACS.

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