24-10-2012, 10:43 AM
CO2 Selective Ceramic Membrane for Water-Gas Shift Reaction with Concomitant Recovery of CO2
ABSTRACT
CO2 diffusivity through hydrotalcite materials at 200 to 250 C was determined based upon the weight pick-up vs time.
D/r(sup 2) (diffusivity/radius(sup 2)) for CO2 ranges from 3 x 10(exp -4) to 1 x 10(exp -3) depending upon the temperature.
This range of diffusivity is consistent with the diffusivities through nanoporous materials, such as pillard clays and carbon
molecular sieve, reported in the literature. Further the activation energy calculated based upon the diffusivity as a function of
temperature is(approx)12 kcal/mole CO2, indicating activated diffusion for CO2 transport through the intracrystalline region
of hydrotalcite. More importantly nitrogen diffusivity determined based upon the same methodology is negligible. This implies
that the hydrotalcite materials have a strong affinity to CO2, but not nitrogen although the kinetic diameters for both molecules
are similar. This result supports our proposed concept on the use of the hydrotalcite membrane for selective permeation of
CO2. In the next quarter, we will conduct more calculation to determine the CO2 permeability of an ideal hydrotalcite
membrane. This theoretical analysis will provide a quantitative basis for the design of a hydrotalcite membrane. Further, the
theoretical diffusivity thus obtained can be used as a tool to (1) gauge the degree of defects of experimental membranes
prepared, and (2) direct the future membrane synthesis and improvement.