Capture of CO2 from high humidity flue gas by vacuum swing adsorption with zeolite 13X

Capture of CO2 from flue gas streams using adsorption processes must deal with the prospect of high humidity streams containing bulk CO2 as well as other impurities such as SOx , NOx , etc. Most studies to date have ignored this aspect of CO2 capture. In this study, we have experimentally examined the capture of CO2 from a 12% synthetic flue gas stream at a relative humidity of 95% at 30 degrees C. A 13X adsorbent was used and the migration of the water and its subsequent impact on capture performance was evaluated. Binary breakthrough of CO2/water vapor was performed and indicated a significant effect of water on CO2 adsorption capacity, as expected. Cyclic experiments indicate that the water zone migrates a quarter of the way into the column and stabilizes its position so that CO2 capture is still possible although decreased. The formation of a water zone creates a "cold spot" which has implications for the system performance. The recovery of CO2 dropped from 78.5% to 60% when moving from dry to wet flue gas while the productivity dropped by 22%. Although the concentration of water leaving the bed under vacuum was 27%(vol), the low vacuum pressure prevented condensation of water in this stream. However, the vacuum pump acted as a condenser and separator to remove bulk water. An important consequence of the presence of a water zone was to elevate the vacuum level thereby reducing CO2 working capacity. Thus although there is a detrimental effect of water on CO2 capture, long term recovery of CO2 is still possible in a single VSA process. Pre-drying of the flue gas steam is not required. However, careful consideration of the impact of water and accommodation thereof must be made particularly when the feed stream temperature increases resulting in higher feed water concentration.