Membrane processes separate gas mixtures continuously, selectively and energy-efficiently. Thus, this separation process represents an interesting alternative in comparison to conventional, energy-intensive gas cleaning processes using scrubbers. However, particularly for high temperature applications the material and process related requirements on the membrane system are increasing which limits the material selection enormously. Up to now there is no temperature stable separation process available which separates CO2 of synthesis gas continuously and selectively. The usage of hydrotalcite membranes represents a promising approach for CO2 separation of prepurified synthesis gases.
This work presents the systematic development of inorganic multilayer hydrotalcite membranes which can be used for CO2 selective separation from gas flows under high temperature and high pressure conditions (T > 350 °C, p ≤ 80 bar). The processes CO2 sorption and CO2 desorption on the membrane surface respectively, as well as diffusion properties of the membrane were investigated separately.
First, CO2 sorption equilibrium data for pure and potassium carbonate doped hydrotalcite were recorded with a sorption/pressure reactor at temperatures and pressures between 200–500 °C and 20–80 bar respectively. Maximum CO2 capacities of 1.2 mol/kg for pure hydrotalcite and 2.0 mol/kg for K-doped hydrotalcite using dry gas or 1.95 mol/kg for pure hydrotalcite and 5.7 mol/kg for K-doped hydrotalcite using wet gas were detected respectively. Desorption properties were determined with cyclic CO2 sorption experiments. After several sorption cycles a constant working capacity of two thirds of the initial CO2 sorption capacity was found for both pure and K-doped hydrotalcite using dry or wet feed gas.
Synthesis of hydrotalcite membranes occurred on Al2O3 substrates using a urea hydrolysis process. As a result, growth of hydrotalcite crystals was achieved directly on the substrate surface and a homogenous hydrotalcite membrane was synthesized. Defects of the membrane could be reduced by an additional hydrotalcite layer. CO2 permeances of 3.03·10-7 mol/(m2·s·Pa) at 200 °C and 1.06·10-6 (mol/(m2·s·Pa) at 500 °C were determined with a high temperature membrane module. In addition to Knudsen diffusion, solution diffusion of CO2 was identified as a further transport mechanism through the hydrotalcite membrane. With respect to N2, H2 und CO the ideal selectivities were therefore slightly above the Knudsen selectivities. Compared to N2, a selectivity of 1.31 was detected at temperatures of 350 °C and thus a partial selective CO2 separation was achieved with the synthesized hydrotalcite membranes.