| It is widely acknowledge that CO2 is one of the most important greenhouse gases and its mitigation is urgently needed. Capture of CO2 from major industries such as power plants and steel mills has been considered as the most effective solution. Moreover, the capture cost may be reduced by the industrial use of CO2. Among the various capture approaches, absorption, membranes, cryogenic, and adsorption, CO2 capture by vacuum pressure swing adsorption (VPSA) process is a promising option for separating CO2 from flue gas, since it has a number of advantages, such as possible low energy requirement, low capital investment cost and easy to achieve automated operation. The major problem of VPSA process is the cost is relative high for CO2 capture, which restricts its industrial applications. The crucial point to reduce the CO2 capture cost is the use of new adsorbents and the optimization of adsorption process. Thus, this paper was focused on the design and optimization of VPSA process with the use of novel CO2 adsorbent material (pitch-based activated carbon beads), which was aimed to reduce the capture cost.Firstly, adsorption isotherms of CO2 and N2 on AC beads were gravimetrically measured at 303,333,363,393 and 423 K in the range of 0-100 kPa and isotherms at 303 and 333 K were also measured up to 4000 kPa in a magnetic suspension microbalance. A good CO2 capacity (at 100 kPa and 303K,qCO2=2.2 mmol/g) and good selectivity of CO2/N2 can be observed on AC beads. Virial equation can fit the isotherms quite well both at low and high pressure ranges, while Multi-site Langmuir model only showed good fitting at low pressures. The fitting parameters were reported and can be employed in the prediction of multi-component adsorption equilibrium, which provided basic data to optimize a VPSA unitSecondly, adsorption kinetics of CO2/N2 on AC beads was measured by diluted breakthrough curves at 303,333,363,393 and 423 K, respectively. An isothermal mathematical model was established for components diffusion in bi-disperse adsorbent. It was showed that the model can fit the breakthrough curves very well, and micropore diffusions were obtained by the numerical fitting. At the above mentioned temperatures, the diffusion of N2 on AC beads was much faster than CO2, and micropore resistances controlled the diffusion mechanism for both CO2 and N2. With the parameters of the mathematical model for diffusion, diffusivity at various temperature can be calculated which provided another important basic data for the design of VPSA process.Thirdly, a mathematical model taking into account mass balance, energy balance, and Ergun relation for pressure drop, was derived to describe the VPSA process. Single-column experiments were performed to verify the model and to evaluate the effect of different operating parameters on the VPSA performance (purity, recovery, productivity and specific energy consumption). Moreover, effects of operating parameters and process configurations on the multi-bed VPSA performance were investigated theoretically. It was showed that the mathematical model described the pressure histories, temperature change and CO2 flowrate very well. Using the AC beads, CO2 purity of (40-60)% with recovery of (40-96)% was obtained using a four-step Skarstrom cycle VPSA.Finally, a multi-bed two-stage VPSA process was designed for CO2 capture. At the lst-stage, CO2 was concentrated to about (40-60)% when the flue gas feeding at almost atmospheric pressure and employing four-step Skarstrom cycle. Then the product gas of the first stage was compressed to (2-5) bar and feed to the second stage VPSA process, where CO2 was further concentrated to above 95%. The 2nd-stage VPSA unit operated with a cycle with feed pressurization, adsorption, pressure equalization, blowdown and pressure equalization. With the designed two-stage VPSA process, a CO2 purity of 95.29% was obtained with 74.36% recovery. The total specific power consumption of the two-stage VPSA process is 723.56 kJ/kg-CO2, while the unit productivity is 0.85 mol-CO2/kg/h. |
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