Article pubs.acs.org/IECR
Investigation of Mass-Transfer Performance for CO2 Absorption into Diethylenetriamine (DETA) in a Randomly Packed Column Kaiyun Fu,† Teerawat Sema,‡ Zhiwu Liang,*,†,‡ Helei Liu,† Yanqing Na,† Huancong Shi,†,‡ Raphael Idem,†,‡ and Paitoon Tontiwachwuthikul†,‡ †
Joint International Center for CO2 Capture and Storage (iCCS), Department of Chemical Engineering, Hunan University, Changsha, 410082, People’s Republic of China ‡ International Test Centre for CO2 Capture (ITC), Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada ABSTRACT: The mass-transfer performance of CO2 absorption into aqueous diethylenetriamine (DETA) solutions was investigated in an absorption column randomly packed with Dixon rings at 303−303 K and atmospheric pressure, and compared with that of monoethanolamine (MEA), which is widely considered as a benchmark solvent for CO2 absorption. The masstransfer performance was presented in terms of volumetric overall mass-transfer coefficient (KGav). In particular, the effects of operating parameters, such as inlet CO2 loading, solvent concentration, liquid flow rate, inert gas flow rate, and liquid temperature, were investigated and compared for both MEA and DETA. Over 40 runs of absorption experiments were carried out in this study. The results showed that KGav of DETA was found to be higher than that of MEA. Also, inlet CO2 loading, solvent concentration, liquid flow rate, and liquid inlet temperature had significant effect on KGav for both systems. However, the inert gas flow rate had an insignificant effect on KGav. Lastly, predictive correlations for KGav for DETA−CO2 and MEA−CO2 systems in randomly Dixon ring packed columns were successfully developed. The predicted results were found to be in relatively good agreement with the experimental results, with average absolute deviations (AADs) of 16% and 14%, respectively.
1. INTRODUCTION The accumulation of carbon dioxide (CO2) in the atmosphere leads to an increase in the Earth’s surface temperature, which is recently gaining worldwide public attention. Therefore, the removal of CO2 from industrial process gas streams is crucial in order to mitigate global warming and climate change problems. Currently, absorption of CO2 into aqueous alkanolamine is one of the most attractive approaches for CO2 removal. This is considered to be the most technically and economically feasible method. Several alkanolamines have been used to capture CO2. Monoethanolamine (MEA) and diethanolamine (DEA), which are considered as primary and secondary amines, respectively, have been widely used, because of their high reactivity with CO2. However, their shortcomings of low absorption capacity and high energy requirements for solvent regeneration offset the benefit of good kinetics.1−3 On the other hand, methyldiethanolamine (MDEA), which is a tertiary amine, has a higher CO2 absorption capacity and lower energy requirement for solvent regeneration, but much lower reactivity with CO2. 2-Amino-2-methyl-1-propanol (AMP), which is a primary sterically hindered amine, is also reactive with CO2. In addition, MDEA and AMP offer higher degradation resistance advantages over MEA and DEA.4,5 However, precipitation is the main drawback of AMP. Thus, developing an efficient absorbent has been considered to be very significant in today’s world of serious atmospheric pollution and energy scarcity.6,7 An absorbent that has great potential for CO2 capture should have a fast absorption rate, high absorption capacity, low regeneration energy requirement, negligible vapor pressure, high thermal and chemical stability, and low corrosiveness.8−12 © 2012 American Chemical Society
Recently, diethylenetriamine (DETA), which has two primary amine groups and one secondary amine group, has been considered as a potentially alternative solvent for capturing CO2, because of its fast reaction kinetics and high absorption capacity.8,13−17 In addition, the performance evaluation of any solvent is not only based on solubility and reaction kinetics, but also on the mass-transfer performance, because it is a critical parameter for designing the absorption column. The higher the mass-transfer coefficient, the shorter the absorption column. Therefore, for an effective solvent, a high mass-transfer coefficient is expected. This is because the shorter the absorption column, the lower the capital cost for building the column. In addition, with the shorter absorption column, the operating cost for capturing CO2 can be reduced, because the energy requirement for pumping can be decreased. Thus, the cost of capturing CO2 can be reduced. In the process of post-combustion CO2 capture in packed column systems, in addition to the effective solvent, the highefficiency column internals also play a crucial role. In this study, a Dixon-ring random packing (provided by China Haohua, Tianjin, Univtech Co., Ltd., China) was used; a photograph of Dixon rings is shown in Figure 1. A Dixon ring offers higher surface area and void fraction than conventional random packings, such as Rasching ring and Berl saddle.18,19 The high surface area reflects high gas−liquid phase contact area and the high void fraction helps to reduce operational cost. Received: Revised: Accepted: Published: 12058
March 29, 2012 August 16, 2012 August 20, 2012 August 20, 2012 dx.doi.org/10.1021/ie300830h | Ind. Eng. Chem. Res. 2012, 51, 12058−12064
Industrial & Engineering Chemistry Research
Article
and found to be 2.219 × 10−8, which is very close to zero and can be negligible. Therefore, several mass-transfer studies20,21,23 assumed that the yA* value was zero and could be neglected from the calculation scheme. Also, the same assumption was applied in the present study. The concentration gradient (dY A,G /dZ) can be obtained by measuring the CO 2 concentration along the column and subsequently plotting as the CO2 concentration profile. In such a way, the value of KGav can be calculated at any interesting YA. In this work, the comparison criteria for KGav values were obtained at a CO2 molar ratio of YA = 0.12.
3. EXPERIMENTAL SECTION 3.1. Chemicals. Reagent-grade MEA and DETA with purities of ≥99% were purchased from Tianjin Kermel Chemical Reagent Co., Ltd., China. These amine solutions were prepared to the desired concentration with deionized water. Commercial-grade CO2 cylinder (with a purity of ≥99%) was supplied by Changsha Jingxiang Gas Co., Ltd., China. 3.2. Experimental Apparatus and Procedure. The absorption experiments were conducted in a randomly packed column that was made of acrylic plastic and enwrapped by thermal insulation material. A Dixon ring (made of 304 stainless steel) was filled into the column. In order to minimize the unfavorable effect of wall flow, a liquid redistributor was placed in the middle of packed bed. Besides the packed column, auxiliary equipment including an air compressor, a CO2 cylinder, a gas mass flow meter with ±1% accuracy (Model CS200, Beijing Sevenstar Electronics Co., Ltd., China), a constant liquid-flow pump with ±0.3% accuracy (Model BT1300, Shanghai Qi Te Analytical Instruments Co., Ltd., China), a thermostatic water bath with ±1 K accuracy (Model HHS-11-4, Shanghai Lei Yun Test Equipment Manufacturing Co., Ltd., China), a gas mixer, and a storage tank were required. The experimental setup for the absorption column is similar to our previous work.21 Prior to the experiment, all the variables were prepared to the desired value. Since the density of liquid phase varies with both the solvent concentration and CO2 loading in solution, the liquid flow rate was calibrated by measuring a known volume of liquid phase, which was driven by constant liquid-flow pump over time in a graduated cylinder. CO2 from cylinder and air from air compressor were flowed through mass flow meters at desired rates, then mixed to a desired CO2 concentration before being introduced into the bottom of the column. The liquid solvent was fed from the top of the column. Both the gas and liquid phases were preheated and controlled by the temperature-controlled bath. The gas phase was sampled along the column. Liquid-phase samples were taken at the inlet and outlet of the column. To confirm the validity of each experimental run, a mass balance calculation (the amount of CO2 removed from gas phase versus that of CO2 absorbed into the liquid phase) was conducted. The mass balance error, which is an error in the experiment, obtained in this study was found to be
