A Combined Process of Vacuum and Temperature Swing Adsorption for Post-Combustion CO2 Capture

Abstract:

It is widely acknowledged that CO₂ emissions from major industries, such as the power, steel and cement plants; make the major contribution to global warming and their mitigation is urgently needed. Several technologies are available to capture CO₂ from flue gases, among which absorption and adsorption are the leading candidates.

Some recent studies have highlighted the potential of adsorption technology for post-combustion CO₂ capture. The pressure/vacuum swing adsorption (PSA/VPSA), temperature swing adsorption (TSA) and electrical swing adsorption (ESA) have attracted much research effort with the development of the novel CO₂ adsorbents materials.

In this paper, a combined process of vacuum and temperature swing adsorption (VPTSA) is developed for post-combustion CO₂ capture using modified 13X zeolite. In the combined process, it needs neither to adopt a deeper vacuum degree for regeneration when compared with VPSA nor to use a higher regeneration temperature when compared with TSA. The power consumption of vacuum pump and the vacuum equipments cost can be reduced significant.  

Experimental research for the combined VPTSA process with a simple six-step cycle (adsorption, heating, vacuum, purge at vacuum, pressurization, and cooling) are presented for post-combustion CO₂ capture, using modified 13X zeolite with a mixture 85%N₂-15%CO₂ modeling flue gas at atmosphere pressure and room temperature. In the set-up experimental apparatus, the stainless steel adsorber is packed with 268g of adsorbents. The temperatures of heating and cooling indirectly adsorbents are controlled by the heat-conducting oil in thermostat bath. The diaphragm vacuum pump is used to adjust the vacuum pressure in the adsorber at the vacuum step and the purge step with nitrogen. The gas mixture composition is determined from the outlet of the column by gas chromatography (TCD detector) and CO₂ infrared analyzer. The cyclic steady state of the combined process can be reached after running 10 times of the cyclic adsorption/desorption operation.

Commercial adsorbent 13X zeolite has a working capacity of 3.1 mol/kg and CO₂/N₂ selectivity of 35 for the CO₂ capture from the flue gas at room temperature. Both the CO₂ adsorption capacity and the selectivity of CO₂/N₂ can be improved by modified 13X zeolite with Li ion exchange.

To capture CO₂ from flue gas at room temperature, it needs the vacuum pressure less than 1kPa reaching 90% regeneration of adsorbents when adopting VPSA process alone; and it also needs to heat the adsorbents above 180-200^oC obtaining 90% regeneration when using TSA process alone. However, the regeneration conditions in the combined VPTSA process become gentler when compared with those in both VPSA and TSA processes. Keeping the same regeneration efficiency of adsorbents (that is 90%), with the aid of the heating (100-130^oC), a relatively higher vacuum pressure (3-5kPa) in the combined VPTSA process can be adopted for the efficient regeneration of adsorbents. Therefore, both the power consumption of vacuum pumps and vacuum equipments cost can be reduced significantly. Moreover, the industrial waste heat (100-150^oC) can be utilized as the heating medium to further reduce the energy consumption when applied into industries.

Keywords: adsorption, VPSA, TSA, Combined VPTSA process, CO₂ capture, CO₂ mitigation , 13X zeolite, mathematical model, modeling