Abstract
Capture of CO2 by aqueous alkanolamine solution was carried out in a rotating packed bed. The conventional packed bed absorber and stripper were replaced by a rotating packed bed in this study for reducing the volumes of conventional absorber and stripper and the regeneration energy of chemical absorbent in the CO2 capture process. There are six chapters in this dissertation, including introduction, capture of CO2 by alkanolamine solution in a rotating packed bed, the effect of operating variables on the amount of dissolved oxygen in the alkanolamine solution in the rotating packed bed operation, the simulation of CO2 capture process in a rotating packed bed, thermal regeneration of alkanolamine solutions in a rotating packed bed, and summary. In chapter 1, chemical absorption and higee technique were described, and the reason why choosing these techniques for CO2 capture was illustrated. In chapter 2, capture of CO2 from flue gases with 1000 ppm, 10%, or 30% CO2, respectively, by alkanolamine solution in a rotating packed bed system was studied. The results showed that increasing the liquid flow rate and decreasing the gas flow rate were favorable for enhancing the CO2 capture efficiency and reducing the height of transfer unit because the molar ratio of liquid to gas was increased. Because the reaction rate constant was increased with the increase of temperature, and the absorbent did not achieve the maximum CO2 loading in the CO2 absorption, the increase of temperature was also favorable for enhancing the CO2 capture efficiency and reducing the height of transfer unit. Besides, a proper rotational speed existed because of the trade off between gas-liquid contact area and contact time. The results also showed the absorbent with higher CO2 reaction rate performed a better CO2 capture efficiency because of the short residence time of gas and liquid in the rotating packed bed. Compared with the conventional packed bed, the height of transfer unit of rotating packed bed is much smaller than the former, explained why the volume of rotating packed bed could be much smaller than that of conventional packed bed. In chapter 3, the effect of operating variables on the amount of dissolved oxygen in the alkanolamine solution was examined by experimental design method for inferring the effect of rotating packed bed on oxygen degradation of absorbent. The results showed that the gas flow rate was the most dominating factor affecting dissolved oxygen, followed by the content of oxygen scavenger Na2SO3 in the solution, temperature, and rotational speed. Besides, the interaction between factors was not obvious. In a circulation operation, dissolved oxygen was increased in the beginning of experiment through physical absorption. However, dissolved oxygen in the solution with the presence of Na2SO3 was smaller than that in the solution with the absence of Na2SO3 in the starting period, due to the reaction between dissolved oxygen and Na2SO3. After a certain period of time, Na2SO3 was mostly consumed by O2, and Na2SO3 in the solution was eventually exhausted. Because the degradation rate of alkanolamine is proportional to dissolved oxygen, the addition of the scavenger Na2SO3 definitely beneficial for CO2 removal. With this concern, addition of more Na2SO3 is needed if the absorbent solution is to be re-circulated for subsequent use. In chapter 4, two models were built up for simulating the CO2 capture process in a rotating packed bed for the scale up of rotating packed bed in the future. The ranges of operating variables used in the simulation included CO2 concentrations: 1000 ppm to 30%, CO2 capture efficiencies: 40 to 99%, temperatures: 25 to 60 oC, gas flow rates: 6 to 70 L/min, liquid flow rates: 50 to 300 mL/min, respectively. The results showed that both the stirred tanks connected in series model and differential equation model can simulate the outlet CO2 concentration in the rotating packed bed operation accurately, showing that these two models are reliable, and a wide of applicable ranges of operating variables of these models. In chapter 5, CO2-loaded 30wt% monoethanolamine aqueous solution and CO2-loaded 20wt% diethylenetriamine +10wt% piperazine blended aqueous solution were thermally regenerated in a rotating packed bed. The effects of rotational speed, liquid flow rate, reboiler temperature, and pressure on regeneration efficiency and regeneration energy were investigated. The results showed that the reboiler temperature and pressure were the dominant operating factors affecting regeneration efficiency and regeneration energy, and the volume of the rotating packed bed apparatus could be reduced to no more than one-tenth that of a conventional packed bed. The CO2-loaded 20wt% diethylenetriamine +10wt% piperazine blended aqueous solution demonstrated a more effective CO2 capturing ability, higher regeneration efficiency and the lower regeneration energy consumed as compared with the performance of a 30wt% MEA aqueous solution, suggesting that the proper choice of absorbent for CO2 capture was of paramount importance. In chapter 6, a summary based on previous chapters was provided.