| An increasing concern about controlling carbon dioxide emissions to curb the global warming has been proposed by scientists and it is critical for humanbeing to develop feasible technologies to mitigate the issues of greenhouse gases. CO2capture by adsorption based technology has great potential in CO2capture area due to their system simplicity and low operational&capital costs and much progress has been made in this technology in the recent years.Among various adsorbents, activated carbon (AC) and13X zeolite are the most popular and traditional commercial candidates that can be applied in CO2-VSA system.13X is the better adsorbent for the bulk CO2capture from dry flue gas, due to its favorable isotherm shape and equilibrium selectivity. However, since water adsorption is much stronger than CO2adsorption on13X zeolite, the presence of water vapor can largely displace CO2and reduce the adsorption capacity. Most of the post-combustion carbon capture projects deal with highly humid flue gases in the real industrial plant containing5-10%saturated water vapor. Conventional approaches using a pre-treatment/drying apparatus to remove moisture from post-combustion flue gas would considerably increase the overall capture cost, which is already the dominant portion of the overall carbon capture and storage (CCS) cost. Thus in order to solve the water issues simultaneously, two methods are applied in this study. One of them is multiple-layering VSA using at least two different adsorbents to capture H2O and CO2separately. Another method is to use the hydrophobic material, activated carbon for CO2capture from wet flue gas. Both simulation analysis and experimental work were envisaged to achieve a systematic result.CO2, N2and H2O adsorption isotherms onto different adsorbents were shown the advantage and disadvantage of those materials in VSA. Results indicated that due to the relative large CO2adsorption amount and better adsorption selectivity over CO2/N2,13X zeolite and LiX zeolite were good candidates for CO2capture. The H2O isotherms onto BASF Sorbeads WS, F200alumina and ACF illustrated they were very good desiccant for water removel. And the CO2, N2and H2O isotherms on activated carbon showed that AC is not only good at CO2capture but it will be the latest affected by water vapor. Following, both single and binary H2O/CO2breakthrough curves showed faster adsorption kinetics of CO2on all the materials than H2O on different adsorbents. Temperature swing profiles in the adsorption bed indicated that heat released by H2O adsorption was much larger than CO2adsorption.In the multiple-layering CO2-VSA process, the simulator MINSA was firstly explored to simulate two double-layered work choosing Sorbead and F200as protective layer materials individually and13X as main adsorption layer. Results showed that the both of the two pre-layer adsorbents can stop water vapor from further moving into13X zone, therefore protecting the main layer adsorbent13X zeolite. LEBT equations and CDS&CMMS equations were successfully fitted to the H2O isotherms on F200and Sorbead, respectively. The simulated9-step VSA cycles yielded a good CO2purity of91%from original feed CO2concentration of12%, with a good recovery of80%. Then the VSA process work was applied to a single bed3-stpe rig to study the feasibility of three different layering conditions. Results showed that comparing with13X+Sorbead double-layer experiment, the LiX triple-layer experiment can improve the CO2recovery and productivity, however decrease the CO2purity. On the other hand, the ACF triple-layer experiment could shallow the height of pre-layer material placement. But because of the pressure drop caused by ACF cloth, energy consumption was increased and CO2purity was slightly decreased. The simple single bed VSA process can get the result of68%CO2purity and81%CO2recovery. Lastly, the13X+Sorbead double layered VSA process was applied into our pilot-scale3-bed apparatus. The pressure and temperature profiles were tested and H2O distribution inside the adsorption bed was analysized. The6-step and9-step VSA cycles were used to improve CO2purity. Results showed that the evacuation pressure, feed flow rate and ratio of13X to Sorbead were three most important operation parameters in determining CO2capture and water removal. The3-bed VSA process can retain water adsorption in the pre-layer, providing a CO2recovery of85.4%, and a purity of72.3%achievable with6-step cycles at a vacuum pressure of3.5kPa. For a9-step cycle employing product purge, the best CO2recovery and purity were80.5%and96.1%, respectively.The activated carbon based VSA process is to study the feasibility and advantage of CO2 capture from wet flue gas streams along with simultaneously moisture removal. Through experiment and analysis, water vapor hardly influenced the CO2capture performance. The process can be operated under a relatively high vacuum pressure. Due to the relative low adsorption selectivity over CO2/N2, the preliminary results showed that our pilot-scale three bed VSA process using activated carbon as adsorbent could yield a reasonable CO2purity of57%CO2with the recovery of68%. Then base on the pros and cons that AC showed, a two-stage VSA process was investigated employing AC as the first stage material and13X as the second stage material. The simple cycles could largely save the industrial operation cost. The tow-stage technique could not only solve the water issues exiting in real flue gas, but achieve a excellent CO2purity of over99%, with a great CO2recovery of over90%, which will satisfy the requirement of carbon capture and storage. The two stage VSA process has a good potential in industrial application. |
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