Mass Transfer Coefficients for CO 2 Absorption into Aqueous Ammonia Using Structured Packing Wenbin Li, Xingjian Zhao, Botan Liu,* and Zhongli Tang State Key Laboratory for Chemical Engineering and School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ABSTRACT: In this study, the gas phase volumetric overall mass transfer coefficients (K G a e ) for CO 2 absorption into aqueous ammonia solutions were measured to explore the mass transfer performance of a column packed with structured packing. The K G a e values were evaluated over ranges of main operating variables, that are, 1400-2300 m 3 ·m -2 ·h -1 gas flow rate, 20-39 m 3 · m -2 ·h -1 liquid flow rate, up to 8 kPa partial pressure of CO 2 , and 0.27-0.72 kmol·m -3 ammonia concentration. The results show that higher liquid loading and concentration of aqueous ammonia are beneficial to enhance the K G a e values. On the contrary, K G a e value decreases as the CO 2 partial pressure increases. The results also show that gas loading has little influence on K G a e . To allow the mass transfer data to be readily utilized, an empirical K G a e correlation for this system was developed. Finally, Simulations were made for the absorption of CO 2 into aqueous ammonia solutions by using the currently developed computational mass transfer model along with the proposed correlation for K G a e . The simulation results on the gas phase CO 2 concentration are found to be in satisfactory agreement with the experimental measurements. 1. INTRODUCTION The continuously rising demand for energy and increasing emissions of CO 2 have become severe challenges for the sustainable development of the world. The main sources of carbon dioxide emissions include power generation, industrial processes, transportation, and residential and commercial buildings. 1 Power generation, mainly from the use of coal and natural gas, accounts for about a third of CO 2 emissions from fossil fuel use. 2 Therefore, the main application of CO 2 capture is currently expected to be in power generation. Several technologies of CO 2 sequestration for power generation have been developed, including postcombustion capture, oxy combustion, and precombustion capture. It should be noted that only the postcombustion decarbonization based on the mature state of the art chemical solvent absorption process has realized the industrial pilot. 3 The absorption process is one of the most common industrial technologies today. Chemical solvent absorption methods are considered as a reliable method for reducing CO 2 emissions from fossil fuel power plants. 4 The most commonly used absorption solvents are alkanolamines, which were discovered in the late 1920s by Bottoms. 5 Among the alkanolamines, the most used solvent is monoethanolamine (MEA) scrubbing. However, the cost to capture CO 2 from the flue gas of power plants is very high when using MEA scrubbing. 4 It is estimated that the energy penalty from using this solvent for CO 2 capture from coal-fired power plants is about 15% to 35%. 6,7 In addition, it has several major problems, 8-10 including low CO 2 loading capacity, slow absorption rate, high equipment corrosion rate, amine degradation by SO 2 , NO 2 , HCL, and O 2 in the flue gas, etc. In order to improve the above detects, Wolsky et al. 11 suggested the need for a new solvent to be discovered. As early as in 1997, Bai and Yeh 9 found another route of reducing CO 2 emissions from power plants by using ammonia. The performance of the ammonia and MEA solvents for scrubbing CO 2 emissions were compared experimentally by Yeh and Bai. 12 It was shown that both the CO 2 removal efficiency and absorption capacity of ammonia solvent are better than those of MEA solvent under the operating conditions in their study. As stated by Yeh et al., 13 the use of ammonia seems to have avoided the shortcomings of the MEA solvent. Generally speaking, the advantage of aqueous ammonia as solvent includes high loading capacity, less corrosion, no absorbent degradation, and low energy consumption. It is found that thermal energy consumption for CO 2 regeneration using the aqua ammonia process could be at least 75% less than if the MEA process is used for CO 2 absorption and regeneration. Additionally, the three major acid gases (SO 2 , NO x and CO 2 ) plus HCl and HF, which may exist in the flue gas, will be captured in the aqua ammonia process simultaneously. A single process to capture all acidic gases is attractive, promising to reduce the total cost and complexity of emission control system. In recent years, many researchers were interested in the fundamental work for the CO 2 absorption in aqueous ammonia. The UNIQUAC-Non Random Factor (NRF) model was developed by Pazuki et al 14,15 for the NH 3 -CO 2 -H 2 O system to correlate the experimental data reported by Edwards et al. 16,17 and Bernardis et al. 18 Derks and Versteeg 19 studied the kinetics and mechanism of the reaction between CO 2 and ammonia in a well-stirred cell reactor. Puxty et al. 20 presented their latest kinetics results for a reaction in a wetted-wall column reactor. There was a large amount of work 21-24 studying the kinetics and mechanism in depth by contributing more experimental data, and thus robust kinetics constants were fitted and available for the determination of the reaction between CO 2 and ammonia. Received: September 18, 2013 Revised: December 23, 2013 Accepted: March 20, 2014 Published: March 20, 2014 Article pubs.acs.org/IECR © 2014 American Chemical Society 6185 dx.doi.org/10.1021/ie403097h | Ind. Eng. Chem. Res. 2014, 53, 6185-6196