Energy Procedia 4 (2011) Energy Procedia 00 (2008) GHGT-10. Jinzhao Liu, Shujuan Wang*, Guojie Qi, Bo Zhao, Changhe Chen

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1 Energy Procedia 4 (011) Energy Procedia 00 (008) Energy Procedia GHG-10 Kinetics and mass transfer of carbon dioxide absorption into aqueous ammonia Jinzhao Liu, Shujuan Wang*, Guojie Qi, Bo Zhao, Changhe Chen Key laboratory for thermal science and power engineering of Ministry of Education singhua University, Beijing , China Elsevier use only: Received date here; revised date here; accepted date here Abstract Aqueous ammonia can be used to capture CO from flue gas of coal-fired power plant with acceptable CO removal efficiency, high loading capacity of CO, low corrosion, low cost, and less degradation, and it could remove other acid gas pollutants at the same time. However, the CO absorption rate and mass transfer in aqueous ammonia still need to study more deeply. he mass transfer characteristics of CO in aqueous ammonia were investigated in this paper. A small wetted wall column (WWC) was used to study the inetics of CO absorbed in aqueous ammonia. he correlation of gas-side mass transfer coefficient g was fitted as Sh! 3.79(Re Sc d / h) based on the experimental data of CO absorption in 30%wt MEA solution for this WWC reactor. he reaction rates of CO in 1 wt%,.5 wt%, 5 wt% to 7.5 wt% aqueous ammonia were measured at the temperatures of 10, 0,30 to 40, and the second-order reaction rate constant was calculated. he termolecular mechanism model was used and discussed according to the experimental results. he inetics constants of and NH in termolecular mechanism were also fitted in this paper. 3 H O c Elsevier Published Ltd. byall Elsevier rights reserved Ltd. Open access under CC BY-NC-ND license. Keywords: Carbon dioxide, Aqueous ammonia, Absorption, Mass transfer, Kinetics, Separations 1. Introduction Alanolamines, such as monoethanolamine (MEA), are common absorbents in CO capture process. he drawbacs of normal alanolamines include easily degradation, high corrosion rate, and a high energy requirement for regeneration. Different from organic amines, aqueous ammonia has recently been considered to have potential as an effective and economic solvent for CO capture since its high loading capacity of CO, low corrosion, low cost, and less degradation, and it could remove other acid gas pollutants at the same time, and has also an acceptable CO removal efficiency [1-9]. Detailed inetic data are required for the optimal design and operation of an absorber. Studies have reported inetic reaction rate constants for different temperatures. A rapid method of measuring the absorption rate was developed and applied to CO absorption in partially carbonated ammonia solution at 0 40 by Andrew [10], and the reaction was considered to be first order for both CO and ammonia. he reaction rate was determined by Pinsent et al [11]. in a bubble reactor for the same temperature range as Andrew [10]. Gibson et al [1]. presented the relationship between mass transfer and the solution flow rate/concentration in a paced column. Hsu [13] studied the rate of r eaction between CO and ammonia in a semi-batch reactor from 5 to 65. he rate of reaction in a spray tower was given by Diao et al []. for * Corresponding author. el.: ; fax: address: wangshuj@tsinghua.edu.cn.s doi: /j.egypro

2 Author name / Energy Procedia 00 (010) J. Liu et al. / Energy Procedia 4 (011) temperatures from 8 to 43.he important parameters for the gas-liquid reaction such as the gas-liquid contact time and area cannot be accurately measured in these studies. Wetted-wall column studies on the inetics of CO absorption in unloaded and partially carbonated ammonia solution were conducted by Qin et al [14]. and Zheng et al [15]. at temperatures from 0 and 50, and the reaction was considered to be first order for both CO and ammonia. he zwitterion mechanism was applied to CO absorption in ammonia solution in a well-stirred cell reactor from 5 to 5 by Ders and Versteeg [16]. In addition, Puxty et al [17]. presented their latest inetics results for a reaction in a wetted-wall column reactor at temperatures from 5 to 0. hough these studies use the reactors with fixed gas-liquid contact area such as wetted-wall column to measure the inetic parameters of CO absorption into ammonia, the data do not agree well, as shown later in this paper, and the experimental ranges of the concentration and temperature of ammonia solution were not wide enough. In this wor, more data will be contributed to the study of the inetics of CO absorption in aqueous ammonia. he gas-side mass transfer coefficient g was fitted based on the experimental data of CO absorption in 30 wt% MEA solution. he termolecular mechanisms were applied to characterize the inetics and mechanisms of the reaction. he inetic experiments, using a wetted-wall column with a contact area of about cm, cover the concentration o f aqueous ammonia from 1wt% to 7.5wt% and temperature from 10 to 40, which expanded the range of temperature and concentration of aqueous ammonia, and also laid an experimental basis for a more comprehensive and systematic description of the absorption process of CO into aqueous ammonia.. Chemical reaction.1 Reaction description he total reaction of C O absorption into aqueous ammonia can be described as the eq. (1): CO # # H O NH4HCO3 (1) he actual process of the reaction is more complicated, which can be described as step-by-step reactions. First of all, the eq. () occurs, that is, CO and NH 3 react to generate NH COONH 4, and then NH COONH 4 hydrolyzes in solution instantaneously. # CO # NH # H O NH COO # NH 3 4 () Secondly, NH + 4 and NH COO - have an irreversible reaction (3) in solution: # NHCOO # NH 4 # HO % NH4HCO3 # H O (3). Reaction mechanism he zwitterion mechanism was proposed to describe the reaction between CO amines firstly by Caplow [17] and further discussed by Dancwerts [18]. Based on the mechanism, amine and CO form zwitterions firstly, and then zwitterion is further deprotonated by a base (B). Eq. (4) and eq. (5) show the reaction between ammonia and CO when the zwitterion mechanism is applied to this non-amine system. CO # NH # NH COO (4) NH COO B NH COO BH (5) # B # 3 # # B he termolecular mechanism was proposed by Croos and Donnellan [19], which concluded that the reaction between amine and CO is single-step and termolecular. he initial product is not zwitterion, but a loosely bound encounter complex. Most of the complexes brea up to give reagent molecules again. he reagent molecules do not react with a second molecule of amine or a water molecule to give ionic products. Bond formation and charge separation occur only in the second step. his mechanism was reviewed by da Silva and Svendsen [0] in their ab initio study of carbamate formation from CO and alanolamines, and the termolecular mechanism was further developed. A single-step third-order reaction mechanism is most liely for the formation of carbamate from CO and alanolamines in solution. he observed broen-order and higher-order inetics can be explained by this mechanism. he mechanism applied to the ammonia system is written as B # CO # # B NH COO # BH (6) B If ammonia and water are the dominating bases, the reaction rate for the termolecular mechanism can be given as follows: rco NH! ( [ 3 NH NH 3 3] # H [ O HO])[ CO][ NH 3] (7) he termolecular mechanism was used to describe the reaction of aqueous ammonia and CO in this wor, and the correlation of, and with temperature were fitted by the experimental data in this paper. H O

3 Author name / Energy Procedia 00 (010) J. Liu et al. / Energy Procedia 4 (011) Correction of the reaction rate for hydroxyl ions Aqueous ammonia is a wea alali solution. CO reacts with both of ammonia and hydroxyl when carbon dioxide dissolves into aqueous ammonia solution. he reaction rate is determined by both reactions as shown in eq. (9). Pinsent et al [11]. suggested that the effect of reaction (8) should be taen into account. OH OH 3 CO # OH &&&% HCO (8) r! r # [ OH ][ CO ] (9) obs CO NH 3 OH r OH CO NH 3 obs! # [ ] (10) OH [ CO ] could be calculated by the following eq. (11). lg( ) / OH! (11) [OH - ] could be calculated by eq.(1), and the correlation of KNH 3 is shown as eq. (13) [1-4]. # [ OH ][ NH 4 ] K NH! (1) 3 [ NH ][ H O ] 3 ln K! A # B / # C ln # D (13) And the value of the parameters A, B, C and D in eq.(13) could be , , and , according to Aspen Plus. So the apparent inetic rate constant for the formation of carbamate in the NH 3 -C O -H O system can be written as:! [ OH ] [ NH ] app obs! (14) OH 3! [ NH ] # [ H O] (15) 3 H O 3. Experimental apparatus and methods 3.1 he wetted-wall column A wetted wall column with a contact area of about cm has been built [5]. he gas-liquid contactor was constructed from a stainless steel tube, measuring 11.0 cm in height and 1. cm in diameter. he gas-liquid contact region is enclosed by a 31.0 cm thic-walled glass tube, separated from a water-bath.he chemical solution is pumped through the inside of the tube, overflows on the top, and is evenly distributed across the outer surface of the tube. After collection at the base of the column, the liquid is pumped to a liquid reservoir. Gas enters near the base of the column, counter-currently contacting with the liquid as it flows up into the gas outlet. Fig.1. Overall experimental flowsheet of WWC. he water/oil-bath, with circulation of the water/oil inside, is used to control the temperature of the inlet gas, liquid and the reactor. wo reservoirs are used in this system, one of which is used for the amine solution storage, and the other is used to holdup the waste solution out of the reactor. A micro-pump pushes the solution from the reservoir through a coil submerged in the water/oil-bath, flowing through a rotameter for flowrate determination. he liquid flow rate was -3 cm 3 /s. After heated, the solution flows into the wetted-wall column, contact with the gas stream, and then returns to the reservoir. 3. Aqueous Ammonia and CO Reactions he mass flux with chemical absorption can be described as follows based on two-film model, where the total resistance to mass transfer was divided as the sum of resistances from gas side and liquid side as eq.(16) #! K ' g G G (16)

4 4 Author name / Energy Procedia 00 (010) J. Liu et al. / Energy Procedia 4 (011) he overall gas transfer coefficient K G can be calculated by the Flux and P and P CO, in,. At the same temperature and concentration of ammonia, the value of K G is fixed and can be calculated by the Flux- CO partial pressure curve. Flux KG! (17) * PCO, b PCO Where, P CO, is the operational partial pressure of CO in the wetted wall column, which was the log mean average, as eq. (18): PCO, in PCO, out PCO, b! (18) Ln( PCO, in / PCO, out ) he gas liquid mass transfer process is enhanced by chemical absorption when the acid gas is treated with amines/ammonia. he mass flux with chemical absorption is as eq.(19): 1 * Flux! ( C A CAb, ) (19) 1 R # E A L Hg Where EA! 1# Ha and D obs A Ha!. L he reaction is considered to be a pseudo-first-order irreversible reaction. If the Hatta number Ha>>3, the expression for the enhancement factor (E A ) based on the two-film model, penetration model and surface renewal model can be simplified as eq. (0) [6-7]. obs DA EA! Ha! (0) 0 L Physiochemical properties of the aqueous ammonia solution required in determination of mass transfer, such as density, viscosity, solubility and diffusivity, were measured or taen from the literature before the experiment. he viscosities of ammonia solutions of different concentrations and water were calculated using eq. (1) and () [8] ! (0.67 # 0.78 x )exp( ) (1) R H exp( ) O! () R he methods used to estimate the diffusion coefficient of C O in aqueous ammonia are described below. he diffusion coefficient of CO is estimated from the solution s viscosity using a modified Stoes-Einstein equation [9]. HO HO HO 0.8 D! D ( ) (3) CO CO NH 3 HO H O D CO! exp( ) (4) In this paper, the concentration of ammonia is very low, so we can use Henry's law constant of CO in water to estimate the Henry's law constant H in aqueous ammonia. Pacheco [30] re-calibrate the Henry's Law constant using CO the following correlation HCO ( atm cm / mol)! exp( ) (5) ( K) If we can calculate the gas-side mass transfer coefficient g, therefore, the value of G can be got by eq. (16), and we can get the value of the observed reaction rate constant obs by eq. (6). 0 ' E obs D l A CO G!! (6) HCO H CO he correlation of gas film mass transfer coefficient g for this WWC is very important and need to be fitted by the experiment. 3.3 he correlation of gas film mass transfer coefficient g he characterization of gas-side mass transfer coefficient g was given by eq. (7). Sh!)(Re Sc d h / h ) (7) Where Sh! Rgdh / D, CO Re! / gvd g h +, Sc!, / D, d g h is the hydraulic diameter of the annulus and h is the length of the column. he parameters of ) and ( were fitted based on the experimental data of CO absorption into MEA solution. he reaction rate between CO and MEA solution is supposed to be high enough that the resistance from film side can be dominated [30].

5 Author name / Energy Procedia 00 (010) J. Liu et al. / Energy Procedia 4 (011) his wor determined the gas phase mass transfer coefficient of the apparatus by absorption of carbon dioxide into 30 wt% MEA solution. he experimental temperature was from 0 to 60, and the gas flow rate range chosen as 3-40 cm 3 /s. Fig.. Correlation of gas-side mass transfer coefficient g fitted by CO absorption into 30 wt% MEA solution from 0 to 60. he g is fitted by fig. and the correlation is given by the eq. (8) for this WWC Sh! 3.79(Re Sc d / h) (8) 4. Results and discussions 4.1 Experimental data of CO absorption rate in aqueous ammonia According to the results of this wor, the reaction is considered to be a pseudo-first-order irreversible reaction. With Hatta number Ha>>3, the expression for the enhancement factor (E A ) based on the two-film model, penetration model and surface renewal model can be simplified as eq. (17), and therefore, can be calculated using the eq. (9) D ' obs CO G! H CO he apparent inetic rate constant app and the second-order reaction rate constant discussed in section can be calculated by eq. (14). 4. Determination of the inetics constants and comparison he second-order reaction rate constant of CO with aqueous ammonia was calculated at different temperatures and ammonia concentrations, shown in fig.3. And 5 wt% aqueous ammonia (m 3 /mol s) was found to be as 8! exp( 3861/ ) (30) obs (9) Fig.3. -emperture curves at ammonia concentration of 1wt%,.5wt% and 5wt%, temperature range from 10 to 40. he second-order reaction rate constant of 5wt% aqueous ammonia in this wor was also compared with the prior researchers results at the same temperature. Fig.4 shows that in this paper was in good agreement with Hsu, Pinsent and Qin, and significantly lower than Versteeg and Ptux [11,13,14,16,31]. Moreover, this wor expanded the range o f

6 6 Author name / Energy Procedia 00 (010) J. Liu et al. / Energy Procedia 4 (011) temperature and concentration of aqueous ammonia, which has laid an experimental basis for a more comprehensive and systematic description of the absorption process of CO into aqueous ammonia. Fig.4. Experiment results comparison of at different temperature. he termolecular mechanism, discussed deeply in section, was used to describe the reaction of aqueous ammonia and CO in this wor, and the correlation of, NH and 3 H O with temperature were fitted by the experimental data in this paper.! [ NH ]# [ H O] (15) he inetics constants 3 H O and H O temperature according to the and the concentrations of aqueous ammonia. in termolecular mechanism were fitted and expressed as a function o f Fig.5 -[NH 3] lines used for and H O fitted at 10, 0, 30 and 40. Fig.6 he correlation of and H O with temperature fitted curves. and H O he correlation of were important constants to describe the reaction process of CO absorption into aqueous ammonia. and H O exp( 3847/ ) H O with temperature were given as eq. (30) and eq. (31) according to fig.6.! exp( 3793/ ) (30)! (31)

7 Author name / Energy Procedia 00 (010) J. Liu et al. / Energy Procedia 4 (011) hese correlations provide an effective method for the calculations of and gas-liquid reaction liquid phase mass transfer coefficient l at different ammonia concentration and temperature. he inetics constants and were also compared with some amines solution. he reaction rate of aqueous H O ammonia was much lower than MEA and some other amines according to the results shown in table 3. able 3 Comparison of inetics constants between aqueous ammonia and amines at 5 Absorbent 10 3 absorbent (m mol s ) HO (m mol s ) Source NH his wor NH Qin, 010 [3] MEA Aboudheir, 003 [33] AEEA Ma mun, 007 [34] EDA a Li, 007 [35] DEA Hartono, 009 [36] PZ Cullinane, 007 [37] 5. Conclusions he reaction rates of CO in 1 to 7.5 wt% aqueous ammonia were determined at the temperatures of 10 to 40. he second-order reaction rate constant of CO with aqueous ammonia was calculated and also compared with the prior researchers results. he second-order reaction rate constant of aqueous ammonia was much lower than MEA and some other active amines. he inetics constants and in termolecular mechanism were fitted and expressed as a function o f H O temperature. he correlations of NH and 3 H O with temperature provide an effective method for the calculations o f second-order reaction rate constant and liquid phase mass transfer coefficient l. Moreover, this wor expanded the range of temperature and concentration of aqueous ammonia and laid an experimental basis for future research. Notation B base species D diffusivity, m s 1 E enhancement factor H Henry constant, Pa m 3 mol 1 Ha Hatta number -1 K G overall mass transfer coefficient, mol pa -1 cm - s -1 G mass transfer coefficient based on gas phase, mol pa -1 cm - s forward reaction constant, m 3 mol 1 s 1 app apparent reaction rate constant, s 1-1 g gas-side mass transfer coefficient, mol pa -1 cm - s HO termolecular reaction rate constant contributed by water, m 6 mol s 1 l liquid-side mass transfer coefficient, m s 1 termolecular reaction rate constant contributed by ammonia, m 6 mol s 1 obs observed reaction rate constant, s 1 OH - reaction rate constant for CO hydration, s 1 N absorption flux, mol m s 1 r reaction rate, mol m 3 s 1 R universal gas constant, J mol 1 K 1 Re Reynolds number Sc Schmidt number Sh Sherwood number emperature, K v velocity, m s 1 η viscosity, Pa s ρ density, g m 3

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