(134a) Pure Carbon-Dioxide Gas Absorption In a Counter-Current Foam-Bed Reactor

A foam-bed reactor is a gas-liquid contactor which offers low pressure drop and high gas hold-up. Thus it can be used for the treatment of large volumes of gaseous contaminants. Present work attempts to study the removal of carbon-dioxide gas, which could be obtained from power plant exhausts, by absorbing it in aqueous barium-sulfide using a counter-current foam-bed reactor.

The difficulty in scaling up and commercializing foam-bed reactors in spite of the advantages like very high interfacial area is the limitation of foam height. When the carbon-dioxide gas is sparged through the storage section i.e. a mixture of aqueous barium sulfide and a surfactant, the generated foam travels upwards into the foam section. The gas-liquid chemical reaction takes place in the foam section. The liquid hold-up in foam being small and chemical reaction being fast, after a particular height, the liquid is consumed by the chemical reaction, while unreacted gas is still left in the foam. This unreacted gas eventually leaves the reactor at the top where foam is broken. Thus, very tall foam columns cannot be used in practice because of the occurrence of this “depletion condition”. This scale-up difficulty could be overcome by adding the solution of liquid-phase reactant from the top into foam, in addition to that introduced into the storage section.

Experiments were carried out for the carbonation of barium sulfide specifically to verify the anticipated advantages offered by such reactors and their effectiveness in removing carbon-dioxide gas. The parameters explored were the foam height, barium sulfide flow rate into the foam section, and number of inlets used for charging the barium sulfide solution into the foam section.

With the increase in foam height, the conversion of barium sulfide increased. An optimum value of barium sulfide flow rate into the foam section was obtained above and below which the conversion decreased. When two inlets were used at different locations to charge the same amount of the aqueous barium sulfide as in case of a single inlet, the conversion of barium sulfide increased marginally.

The experimental results were also compared with the pure-gas model of Gaikwad and Bhaskarwar (2008). The pure-gas absorption model predicted the experimental data to well within an accuracy of 20%. Comparison of conversions in semi-batch and counter-current foam-bed reactors showed that the latter perform better than the semi-batch reactors. Thus, with the counter-current mode of operation of the foam-bed reactors, there is a potential for large-scale treatment of carbon-dioxide gas.