Permeation studies, membrane material

Since A. Pick observed that gas molecules tend to pass through thin nitrocellulose films at different permeation rates for each gas, membrane-based gas separation has been studied extensively to look for better membrane materials and to apply this property to gas separation on an industrial scale. Numerous scientists have studied membrane materials with respect to transport... 

As the pore size is reduced to 1 nm or less, gas permeation may exhibit a thermally activated diffusion phenomena. For example, in studies at Oak Ridge National Laboratory, for a certain proprietary membrane material and configuration, permeation of helium appeared to increase much faster than other gases resulting in an increase in Helium to C02 selectivity from 5 at 25°C to about 48.3 at 250°C (Bischoff and Judkins, 2006). Hydrothermal stability of this membrane in the presence of steam, however, was not reported. 

The process shown in Figure 9.21 was first developed by Separex, using cellulose acetate membranes. The separation factor for methanol from MTBE is high (>1000) because the membrane material, cellulose acetate, is relatively glassy and hydrophilic. Thus, both the mobility selectivity term and the sorption term in Equation (9.5) significantly favor permeation of the smaller molecule, methanol, because methanol is more polar than MTBE or isobutene, the other feed components. These membranes are reported to work well for feed methanol concentrations up to 6%. Above this concentration, the membrane is plasticized, and selectivity is lost. More recently, Sulzer (GFT) has also studied this separation using their plasma-polymerized membrane [56],... 

My introduction to membranes was as a graduate student in 1963. At that time membrane permeation was a sub-study of materials science. What is now called membrane technology did not exist, nor did any large industrial applications of membranes. Since then, sales of membranes and membrane equipment have increased more than 100-fold and several tens of millions of square meters of membrane are produced each year—a membrane industry has been created. 

Yet another unique class of inorganic membrane materials called pillared clay and carbon composite membranes has been studied for gas separation [Zhu et al., 1994]. The permeation rates of benzene, chlorobenzene and 1,3-dichlorobenzene vapors through these membranes can be different by orders of magnitude as indicated earlier. This may open the door for these types of membranes for separating organic mixtures. 

The solubility parameter has found previous use in membrane science. Casting solution components and composition have been selected using the Hansen solubility parameters (68-71). The total Hansen solubility parameter, which is equivalent to the Hildebrand parameter (.72), has been used to explain permeation and separation in reverse osmosis (23). Hansen s partial parameters have also been used to explain permeation and separation in pervaporatlon (61). The findings of these studies (61,73) plus those reported elsewhere in this volume (74) do lend credence to the use of 6, 6, and 6, for membrane material selection. 

Both theoretical and experimental studies have been performed on palladium-based membrane reactors for the water-gas shift reaction. Ma and Lund simulated the performance achievable in a high temperature water-gas shift membrane reactor using both ideal membranes and catalysts [18]. By comparing the results obtained with those related to the existing palladium membrane reactors, they concluded that better membrane materials are not needed, and that higher performances mainly depend on the development of a water-gas shift catalyst not inhibited by CO2. Marigliano et al. pointed out how the equilibrium shift conversion in membrane reactors is an increasing function of the sweep factor (defined as the ratio between the flow rate of the sweep at the permeate side and the flow rate of CO at the reaction side) [19]. The ratio is an index of the extractive capacity of the system. 

A material for which the chemical stability in the presence of CO2 and SO2 has been extensively studied is BSCF, as it is one of the membrane materials with the highest flux. The presence of CO2 causes a reversible decline in oxygen flux. Even at concentrations of 500 ppm CO2 (ambient air), a slight decrease in the oxygen permeation rate is observed [34]. This decrease is caused by the formation of carbonates of alkaline earth metals on the exposed surface of the membrane. The carbonate formation of perovskite materials AB03 g in the presence of CO2 can be expressed as follows [72] ... 

Slow relaxation processes appear to be the dominant factors causing the long times required to reach steady state permeation rates. Transient permeation experiments would yield incorrect diffusion coefficients for membrane materials exhibiting this behavior. Relaxation processes, highly concentration dependent diffusion coefficients and solubility coefficients, therefore, require a more detailed approach to studying transport. This paper describes a preferred method of analyzing diffusion process. 

Stability, but at the expense of a lower O2 permeation [153]. Moreover, the complete substitution of Ga " by Cr " and Ti " in the B sites of LSF materials can promote to an important extent the structural stability of the membranes [102]. Finally, a formulation including Ce in A sites and A1 in B sites, that is, Lao.ssCeo.iGao.sFeo.esAlo.osOs-a (LCGFA), has revealed to be very stable in the presence of CO2 (20% (v/v) diluted in He for 100 h) [154]. However, to our knowledge, no permeation study on this material has been reported to date. 

Permeation studies have been performed using membranes of different materials (regenerated cellulose, RC polyethersulfone, PES), with a distinct cut-off, at different transmembrane pressures (TMP) [1, 2, 25]. P-Lactoglobulin and horseradish peroxidase were used as model proteins for studying the impact of... 

---