Gongkui Xiao

CO2 capture at elevated temperatures by cyclic adsorption processes

Capture of CO2 from high temperature process streams, such as those arising in the Integrated Gasification Combined Cycle (IGCC) process, is receiving increasing attention. In this paper, we report the performance of zeolite 13X for CO2 capture from IGCC streams at elevated temperatures. Isotherms of CO2 adsorption on zeolite 13X at 90 °C, 120 °C, and 200 °C were measured and the fitted parameters were used for cyclic adsorption process design. Four different cycles were designed and tested with gas mixtures representing syngas compositions from a coal gasification pilot plant. These four cycles were then applied to CO2 separation from real coal gasification syngas. The experiments carried out with real syngas showed that zeolite 13X can be used for CO2 capture with acceptable performance even with impurities present in the syngas.

Adsorption equilibria and kinetics of CH4 and N2 on commercial zeolites and carbons

Adsorption equilibria and kinetics are two sets of properties crucial to the design and simulation of adsorption based gas separation processes. The adsorption equilibria and kinetics of N2 and CH4 on commercial activated carbon Norit RB3, zeolite 13X, zeolite 4A and molecular sieving carbon MSC-3K 172 were studied experimentally at temperatures of (273 and 303) K in the pressure range of (5–120) kPa. These measurements were in part motivated by the lack of consistent adsorption kinetic data available in the literature for these systems, which forces the use of empirical estimates with large uncertainties in process designs. The adsorption measurements were carried out on a commercial volumetric apparatus. To obtain reliable kinetic data, the apparatus was operated in its rate of adsorption mode with calibration experiments conducted using helium to correct for the impact of gas expansion on the observed uptake dynamics. Analysis of the corrected rate of adsorption data for N2 and CH4 using the non-isothermal Fickian diffusion (FD) model was also found to be essential; the FD model was able to describe the dynamic uptake observed to better that 1% in all cases, while the more commonly applied isothermal linear driving force model was found to have a relative root mean square deviation of around 10%. The measured sorption kinetics had no dependence on gas pressure but their temperature dependence was consistent with an Arrhenius-type relation. The effective sorption rates extracted using the FD model were able to resolve inconsistencies in the literature for similar measurements.

Multi-objective optimisation of hybrid CO2 capture processes using exergy analysis

Carbon dioxide (CO2) purification is an essential step in the carbon capture and storage (CCS) process. The leading technology consists of a solvent absorption carbon capture process followed by a multi-stage CO2 gas compression into supercritical state for sequestration. This study considers a hybrid system of vacuum swing adsorption (VSA), membranes and cryogenic separation. Replacing the multi-stage gas compression with the cryogenic separation has two main advantages: firstly, it further purifies the CO2 stream, which is valuable for both VSA and membrane processes since both processes struggle to achieve high purity product. Secondly, it produces liquid CO2 that can be pumped to the supercritical state, which is required for transport and sequestration. Due to the higher degree of freedom available in hybrid processes, a new methodology using multi-objective optimisation combined with exergy analysis was used to optimise the process. This allowed different decision variables to be considered to find the range of optimum operating conditions for each of the processes. It was determined that the refrigerant flow rate, multi-stage compression and cryogenic minimum temperature had the biggest impact on the recovery rate. Furthermore, it was observed that the total specific shaft work had a linear relationship with the specific exergy loss rate.