ABSTRACT. KEYWORDS CO 2 and H 2 S absorption, diethanolamine, modeling absorption, gas treating INTRODUCTION

Transcription

1 Simltaneos Absorption of Carbon Dioxide and Hydrogen Slfide into Aqeos Soltions of Diethanolamine On-line nmber 6 Bishn P. Mandal #, Syamalend S. Bandyopadhyay Separation Science Laboratory, Cryogenic Engineering Centre, Indian Institte of Technology haragpr, haragpr 70, India. * Corresponding athor s ssbandyo@hijli.iitkgp.ernet.in (S. S. Bandyopadhyay). # Present address: Department of Chemical Engineering, Indian Institte of Technology Gwahati, North Gwahati 780, India. address: bpmandal@iitg.ernet.in (B.P. Mandal). ABSTRACT In this work, the simltaneos absorption of CO and H S into aqeos soltions of DEA is stdied theoretically and experimentally. The effect of contact time, amine concentration and temperatre on the rate of absorption and the selectivity were stdied by absorption experiments in a wetted wall colmn. The diffsion-reaction processes for CO and H S mass transfer into aqeos DEA are modeled according to Higbie's penetration theory with the assmption that all reactions are reversible. The psedo-first-order rate constant of the reaction between CO and DEA was obtained by adjsting the vale of rate constant in the mathematical model and correlated. Model predicted reslts have been fond to be in good agreement with the experimental reslts of rates of absorption of CO and H S into (DEA+H O). EYWORDS CO and H S absorption, diethanolamine, modeling absorption, gas treating INTRODUCTION Removal of acid gas imprities, sch as CO and H S, from gas streams is very important in natral gas processing, hydrogen prification, and treating refinery off gases and synthesis gas for ammonia and methanol making. In indstrial gas processing, there is an increasing interest in gas absorption processes for the selective removal of H S from sor gas streams. The secondary amine diethanolamine (DEA) is one of the most widely sed alkanolamines for gas treating processes. Aqeos DEA has been sed for many years for the treatment of refinery gases which normally contain appreciable amont of COS and CS, besides H S and CO (ohl and Nielsen, 7). DEA is the main constitent of the SNPA-DEA process, which has been widely sed for the treatment of high pressre natral gasses with high concentration of acid gas components (Rojey et al., 7; ohl and Nielsen, 7). Althogh relatively concentrated aqeos DEA is particlarly sefl for complete removal of both CO and H S, DEA, a secondary amine, may also exhibit good H S- selectivity with appropriate control of operating parameters in the absorber. However, inspite of high commercial significance of aqeos DEA for simltaneos absorption of CO and H S from sor gas streams there is limited pblished literatre on simltaneos absorption of these acid gasses in aqeos DEA, and more so on development of mathematical to describe CO and H S mass transfer in this solvent. In this work, the simltaneos absorption of CO and H S into aqeos DEA is stdied theoretically and experimentally. The effect of contact time, temperatre and amine concentration on the rates of absorption and the selectivity were stdied by absorption experiments in a wetted wall colmn. The

2 diffsion-reaction processes for CO and H S mass transfer into aqeos DEA soltions are modeled according to Higbie's penetration theory with the assmption that all reactions are reversible. SELECTIVITY For the simltaneos absorption process for H S and CO, the selectivity factor S p is expressed as: ( Molar concentration of HS Molar concentration of CO) liqid phase S p = Molar concentration of H S Molar concentration of CO ( ) gas phase REACTION SCHEME AND MATHEMATICAL MODEL The following reactions may take place when CO and H S are absorbed into an aqeos soltion of DEA ( RRNH ) (where, R = CHCHOH )., k + + +, () CO RRNH RRNCOO H -, k - CO + OH HCO, () HCO H + CO, () RRNH + H RRNH, () RRNH + HS HO H S + RRNH, (5) H +OH, (6) where, i ( i =,.,6) is the eqilibrim constant for the reaction (i), and k j (j =,) is the second order forward rate coefficient for reaction (j). Reactions () - () have finite reaction rates and are reversible and reaction () - (6) are considered instantaneos and reversible and at eqilibrim, since they involve only proton transfer. The proposed mechanism for the reaction between CO and DEA (RRNH) involves the formation of zwiterion (RRNH + COO - ) followed by the deprotonation of zwitterion by a base to prodce carbamate (RRNCOO - ) and protonated base (Caplow, 68). The rate coefficient k is viewed as the global rate coefficient for the formation of carbamate. Hence, this representation does not rle ot the possible formation of a zwitterion reaction intermediate (Barth et al., 86). Blk liqid eqilibrim model For convenience, the chemical species in the liqid phase are renamed as follows: = [ CO ], = [ RRNH ], = RRNH, = HCO, 5 = OH, = CO, 7= H, 8= RRNCOO, = HS, 0= [ HS ]. The initial liqid blk concentrations of all chemical species can be estimated from initial concentrations of RRNH, the initial CO and H S loadings of the soltion (α and α 0, respectively), and the assmption that all reactions are at eqilibrim. We have the following eqations for the initial liqid blk concentrations,..., : 0 0 0

3 Overall RRNH balance: = [ RRNH ] initial (7) Overall carbon (from CO ) balance: =α{ [ RRNH ] initial} (8) Overall slfr (from H S) balance: =α0{ [ RRNH ] initial} () Electronetrality balance: =0 (0) All reactions at eqilibrim: =( )/( ) () = /( 5) () =(6 7)/ () =( 7)/ () =( 8)/( 0) (5) =.0/(5 7) (6) There are ten nknowns 0 0,...,0and ten nonlinear algebraic eqations, which can be solved for the liqid blk concentrations. To solve this, we have sed IMSL library Fortran Developer Stdio version.0. Diffsion-reaction model All reactions are treated as reversible reactions. The first two reactions have finite reaction rates, which are given by the following reaction rate expressions (R i is the reaction rate expression for reaction (i)): R = k + ( k / ) (7) 7 8 R = k 5 + ( k/ ) (8) The following eqations describe the diffsion/reaction processes: CO balance: / t= D( / x) + R+ R () Total carbon (from CO ) balance: ( / t) + ( / t) + ( 6/ t) = D ( / x ) + D ( / x ) + D 6( / x ) (0) Total slfr (from H S) balance: (/ t) + ( 0/ t) = D( / x) + D 0( 0 / x) () Total RRNH balance: ( / t) + (/ t) = D( / x) + D( / x) () Eletronetrality balance: (/ t) + (/ 7 t) (/ t) (/ 5 t) (/ 6 t) (/ 8 t) (/ t) = D( / x) + () D( / x) D( / x) D( / x) D( / x) D( / x) D( / x)

4 Carbamate balance: (/ 8 t)=d( 8 / 8 x) R () Instantaneos reactions assmed to be at eqilibrim: =(6 7)/ (5) =( 7)/ (6) 5 =( 8)/( 0) (7) 6 =.0/(5 7) (8) We have ten partial differential/algebraic eqations, which can be solved for the concentrations of the eleven chemical species (,..., 0 ). Initial condition and bondary conditions at x = At t = 0 (for all x 0) and at x = (for all t 0), the concentrations of all chemical species are eqal to their liqid blk concentrations: 0 i = i, i =,...,0 () Bondary conditions at gas-liqid interface (x = 0) At x = 0 (gas-liqid interface), the flxes of the nonvolatile chemical species are eqal to zero, which leads to the following eqations: (/ i x) = 0 at x= 0, t> 0 (0) for all i except i = (CO ) and i = 0 (H S). For the volatile components (CO and H S), the mass transfer rate in the gas near the interface is eqal to the mass transfer rate in the liqid near the interface: D i( i / x) = kg,i(pi Hi i(0, t)) at x = 0, t > 0 () for i = (CO ) and i = 0 (H S). H i is the physical eqilibrim constant (Henry's law constant) of gas i which is defined as the interfacial partial pressre of gas i in the gas phase, P * i, divided by the interfacial concentration of gas i in the liqid, * i. For the case of pre gas i in the gas phase, P * i, is the same as the blk partial pressre of the gas, P i, and there is no mass transfer resistance in the gas phase (k g,i ). Hence, the bondary condition for gas i at the gas-liqid interface redces to: * i(0, t) = i = (P i / H i) at x = 0, t > 0 () Absorption rate and enhancement factor The differential eqations are integrated from t = 0 to t = θ, the contact time. According to Higbie's penetration model, the liqid-phase mass transfer coefficient for physical absorption of gas i is defined as: k L,i = (D i /π θ ) () The time-averaged rate of absorption of gas i per nit interfacial area is then compted from the following eqation: θ i R A = (D i i /θ) (0, t) dt x 0 () And the enhancement factor for absorption of gas i is determined according to the following eqation: E = R /( k * o ( )) (5) i Ai L,i i i

5 Method of soltion The method-of-lines is sed to transform each partial differential eqation into a set of ordinary differential eqations in t. Detailed method of soltion was discssed elsewhere (Mandal et al., 00a). Eqally spaced nodes are sed to discretize the spatial variable x. The typical nmber of nodes sed in this work is 00 and corresponding nodal spacings are of the order 0-7 to 0-8 m. The reslting system of ordinary differential eqations copled with the algebraic eqations is solved by sing the sbrotine DDASSL (Petzold, 8) in doble precision FORTRAN on a Pentim III processor. PHYSICOCHEMICAL PROPERTIES AND MODEL PARAMETERS The physical solbility of CO in the aqeos DEA soltions has been estimated sing the N O analogy method. The solbility and the diffsivity of N O in water and in the aqeos DEA soltions as well as the solbility of CO in water has been measred and described elsewhere (Mandal et al., 00a). The solbility and the diffsivity of H S in the aqeos DEA soltions needed for the analysis, have been obtained from the reliable correlations described by Rinker (7). Diffsion coefficient for all other species, D i, was obtained as follows (Glasscock, 0): D i = 0. D CO (6) The density and viscosity of aqeos DEA soltions has been calclated from the correlations described by Mandal et al. (00b). All physicochemical properties are presented in Table. Table. Densities and viscosities of the aqeos soltions of DEA sed and the diffsion coefficients and Henry s law constants of CO and H S in these soltions. [DEA] [kmol/m ] T [] ρ [kg/m ] η 0 [kg/ms] D CO 0 [m /s] D H S 0 [m /s] H CO [kpa m /kmol] H H S [kpa m /kmol] The vales of the eqilibrim constants - 6 were calclated as follows: The following correlation for 6 was reported by Oloffson and Hepler (75) for the temperatre range 7 57 : log 0(/ 6) = 808- (6.6/T) -.5 log 0(T) + 8 T (7) T T T. Data for ( / 6) were reported by Read (75) for the temperatre range 7 5 and were correlated according to the following eqation: 5

6 log 0( / 6) = T log 0(T) - (75./T). (8) Danckwerts and Sharma (66) correlated the data for over the temperatre range 7 with the following eqation: log 0( ) = T - (0./T). () Vales of and were calclated from the following correlations, which were reported by Barth et al. (86) for the temperatre range 8 log ( ) = (56.7/T) (0) 0 log 0(/ ) = (80.5/T) T () Barbero et al. (8) reported the following correlation for ( / ) 56 over the temperatre range of 7 57 : log ( 6/ ) 0 5 = + (0.8/T) log 0 ( T). () The reaction rate constant k was calclated from the following correlation presented Danckwerts and Sharma (66) over the temperatre range 7 : log 0( k ) =.65 - (85/T) () The other rate coefficient (k ) was obtained by adjstment sing nmerical model and experimental reslts. EXPERIMENTAL Materials DEA was spplied by E. Merck and was 8% pre. To make amine soltions freshly distilled water degassed by prolonged boiling was sed. CO and N gasses with better than % prities were obtained from BOC India. Calibration standard H S was spplied by Hydrogas India as 0% H S and 0% N mixtre. Apparats and procedre The wetted wall colmn sed for measrement of rates of simltaneos absorption of CO and H S into aqeos DEA was similar to the one sed by Mandal et al. (00b). Flow rates of CO, N and H S were controlled by mass flow controllers. A circlator temperatre controller (JULABO FP 55) along with Ptsensors (JULABO Pt-00) were sed to control the liqid film and the gas-film temperatre within ±0. of the desired level. H S and CO dilted with N to make the desired gas phase composition (H S: CO : N = :0:8) was passed throgh the mixing tbe and the coil immersed in the controlled temperatre bath and finally throgh the satrators to satrate the gas-phase with water vapor. The composition of the feed gas was determined from the respective flow rates. The gas, satrated with water vapor at the temperatre of the absorption, was fed to the top of absorption space of the wetted wall colmn. The amine soltion was then fed from the overhead storage to the contactor at the desired flow rate sing a precalibrated rotameter. Once the system reached steady state with respect to the gas and liqid flow rates and the gas phase and liqid film temperatre, samples were collected from the liqid otlet. Three liqid samples each measring abot m were collected at abot 60 s interval for each rn. The samples were immediately analyzed for the H S and CO contents. 6

7 Method of analysis H S contents in the liqid samples were determined by titration with standard AgNO soltion. An atotitrator (Mettler DL-) with a silver electrode (Mettler DM-) was sed for this titration. For the determination of CO content of the liqid, a known volme of the liqid sample was acidlated with 6 (N) HCl and the volme of evolved gas was measred with a gas brette. The evolved gas was the total H S and CO content of the liqid sample. The H S content of the liqid as fond ot earlier by titration was sbtracted from the total gas volme to get the CO content of the liqid sample. RESULTS AND DISCUSSION Liqid flow rate for all rns were fixed at 0-6 m /s. The liqid film Reynolds s nmber (N Re ) at this flow rate has been fond to be well within the laminar region (Dankwerts, 70). Gas flow rate was m /s. Experiments were performed at, 0.5 and. The total pressre in the absorption chamber was abot 00 kpa. The amine concentrations were,.5 and.0 kmol/m. For a particlar amine concentration and temperatre the contact time was varied by changing the absorption length from to m. It has been observed earlier (Mandal et al., 00b) that, while the rate of absorption of CO increased initially with the increase in gas flow rate, it leveled off beyond a gas flow rate of m /s. However, for absorption of H S, inflence of gas flow rate was significant even above a gas flow rate of m /s, the maximm gas flow rate that cold be maintained in the wetted wall colmn experimental facility sed for this work. The gas phase mass-transfer coefficient k g for H S at a gas flow rate of m /s for all test temperatres (presented in Table ), have been determined by measrements of absorption of H S into 0% aqeos hydroxide soltions from a % H S % N gas mixtre, as described by Mandal et al. (00). Reprodcibility in absorption rate measrements was within % as observed from replicate rns. Experimental and model predicted reslts for simltaneos absorption of CO and H S are smmarized in Table. Table shows, the effect of contact time on the specific rates of absorption of CO and H S in aqeos DEA and the selectivity in the amine concentration range of kmol/m at, 0.5 and. While there is little change in the rate of absorption of CO with contact time, that of H S decreases with increasing contact time reslting in a decrease in the selectivity factor. This sggests that the absorption of CO for simltaneos absorption of CO and H S into aqeos DEA soltions nder conditions of negligible interaction in the absorbed phase, conforms closely to the fast psedo-first-order reaction regime. The decrease of H S absorption rate at higher contact times (i.e., lower k L vales) indicates that althogh the gas film resistance has predominant effect on absorption with the instantaneosly fast H S-amine reaction, the liqid side mass transfer coefficient also inflences the rate of absorption significantly. This is expected, since the k L vale in the wetted wall colmn was in the range of m/s, which is sally obtained in a packed colmn. However, in trayed towers, where the k L vales are mch higher, the H S absorption rate is almost entirely gas film controlled. It is observed from Table that, within the amine concentration range of kmol/m, the rate of absorption of CO increased rapidly with the increase in DEA concentration, while that of H S increased to a mch lesser extent. Ths, there is a conseqent decrease in the selectivity factor with the increase in DEA concentration, de to increased kinetic selectivity towards CO. It has also been observed from Table that with the rise in temperatre from to, the rate of absorption of H S decreases slowly becase the reversibility of the H S-amine reaction is more prononced 7

8 at higher temperatres. The rate of absorption of CO on the other hand increases becase the forward rate of reaction between CO and the amine increases with temperatre. This reslts in a decrease in the selectivity factor. Table. Reslts of simltaneos absorption of CO and H S into aqeos soltions of DEA. = (kmol/m s kpa) at ; k = (kmol/m s kpa) at 0.5 ; k S = (kmol/m s kpa) at. S g, H g, H k g, H S [DEA] [kmol /m ] T [] Θ [s] p CO [kpa] p H S [kpa] k L,CO 0 5 [m/s] R CO 0 6 [kmol/m s] Experimental R H S 0 6 [kmol/m s] S p R CO 0 6 [kmol/m s] Model predicted R H S 0 6 [kmol/m s] S p The measred and predicted rates of absorption are compared in the parity plot shown in Figre. There is very good agreement between the model predictions and experimental reslts with an average deviation (AAD%) of abot.5% and.% for CO and H S rates, respectively. The psedo-first-order rate constant k (rate constant of the reaction between CO and DEA) was obtained by adjsting the vale of rate constant in or mathematical model ntil the predicted rate of absorption agreed with the experimentally measred rate of absorption to within %. The estimated k vale obtained by adjstment in the model for CO H S (DEA + H O) in this work has been correlated as follows. ln k =.0- (7./(T/)) () The estimated k vales from the above correlation are in good agreement with the vales reported by Danckwerts and Sharma, (66). 8

9 Typical calclated concentration profiles for absorption of CO and H S into.5 kmol/m DEA are shown in Figre. It is interesting to note that the CO concentration profile in Figre is concave downwards from the interface and becomes slightly convex above a x/δ vale of 0., instead of being consistently concave downwards with respect to the dimensionless distance. This may be de to the fact that, becase of the relatively faster CO -DEA reaction and the instantaneosly fast H S-amine reaction, there is a conseqential slight depletion of hydroxide in the film (as shown in Figre ) and a resltant redction in the contribtion of hydroxide towards the enhancement factor. Predicted Rate of Absorption x 0 6 (kmol/m s) R CO RH S Experimental Rate of Absorption x 0 6 (kmol/m s) Figre. Comparison of model predicted and experimental rates of absorption for CO and H S into aqeos soltions of DEA Concentration (kmol/m ) E- E- E-5 E-6 E-7 E-8 E- E-0 E- CO - HS - DEA DEACOO - DEAH + HCO - E Dimensionless distance from interface, x/δ H + H S OH - CO E- E- E-5 E-6 E-7 E-8 E- E-0 E- E- Figre. Calclated concentration profiles in the film for absorption of CO and H S into aqeos DEA. T=, [DEA]=.5 kmol/m, t=0.7 s. CONCLUSIONS In this work simltaneos absorption of H S and CO from a nitrogen stream into diethanolamine (DEA) soltions has been stdied sing a wetted wall model contactor at constant feed gas composition and 00 kpa total pressre. The gas flow rate, temperatre range, and amine concentration range were m /s, - and kmol/m, respectively. While the absorption of CO has been fond to conform to the fast psedo-first-order reaction regime, the H S absorption has been fond to be inflenced by both the gas side and liqid side may transfer coefficients. In the amine concentration range of - kmol/m, the rate of absorption of CO into aqeos DEA is observed to increase rapidly with the increase in amine concentration, while that of H S increases to a mch lesser extent. Ths, there is a conseqent decrease in the selectivity factor with the increase in amine concentration. With the rise in temperatre from to, the rate of absorption of H S decreases slowly while the rate of absorption of CO increases, reslting in a decrease in the selectivity factor. Model predicted rates of absorption of CO and H S are in very good agreement with the experimental reslts. From this stdy it is sggested, for better H S selectivity of aqeos DEA solvent, it will be desirable to employ hydrodynamic conditions that will give a large vale of k L while keeping a reasonably good gas velocity to maintain a high k g vale for H S.

10 REFERENCES Barbero, J. A.,. G. McCrdy and P. R. Tremaine ; Apparent Molar Heat Capacities and Volmes of Aqeos Hydrogen Slfide and Sodim Hydrogen Slfide near 5 o C: the Temperatre Dependence of H S Ionization, Can. J. Chemistry, 60, , (8) Barth, D., C. Tondre and J.-J. Delpech ; Stopped Flow Investigations of the Reaction inetics of Carbon Dioxide with some Primary and Secondary Alkanolamines Aqeos Soltions, Int. J. Chem. inetics, 8, 5-57, (86) Caplow, M. ; inetics of Carbamate Formation and Breakdown, J. Am. Chem. Soc., 0, (68) Danckwerts, P. V. and M. M. Sharma ; The Absorption of Carbon Dioxide into Aqeos Soltions of Alkalis and Amines (with some notes on Hydrogen Slphide and Carbonyl Slphide), Chem. Engineer, 0, CE-CE80, (66) Danckwerts, P. V., Gas-Liqid Reactions, McGraw-Hill, New York, (70) Glasscock, D. A., Doctoral Thesis, The University of Texas, Astin (0) ohl, A. L. and R. B. Nielsen, Gas Prification, Glf Pblishing Co., Hoston, (7) Mandal, B. P., A.. Biswas and S. S. Bandyopadhyay ; Absorption of Carbon Dioxide into Aqeos Blends of -Amino--methyl--propanol and Diethanolamine, Chem. Eng. Sci., 58, 7 (00a) Mandal, B. P., A.. Biswas and S. S. Bandyopadhyay ; Selective Absorption of H S from Gas Streams Containing H S and CO in Aqeos Soltions of N-Methyldiethanolamine and -Amino--methyl-- propanol. Sep. Prif. Technol., 5, 0, (00b) Mandal, B. P., M. nd and S. S. Bandyopadhyay ; Density and Viscosity of Aqeos Soltions of (N- Methyldiethanolamine + Monoethanolamine), (N-Methyldiethanolamine + Diethanolamine), (-Amino- -methyl--propanol + Monoethanolamine), (N-Methyldiethanolamine + Diethanolamine), J. Chem. Eng. Data., 8, (00b) Mandal, B. P., M. nd, N. U. Padhiyar and S. S. Bandyopadhyay ; Physical Solbility and Diffsivity of N O and CO into Aqeos Soltions of (-Amino--methyl--propanol + Diethanolamine) and (N- Methyldiethanolamine + Diethanolamine), J. Chem. Eng. Data.,, 6 70 (00a) Oloffson, G. and L. G. Hepler ; Thermodynamics of Ionization of Water over Wide Ranges of Temperatre and Pressre, J. Sol. Chem.,, 7-, (75) Petzold, L. R. ; A Description of DASSL: A Differential/Algebraic System Solver in Scientific Compting, pp , North Holland, Amsterdam, (8) Read, A. J. ; The First Ionization Constant of Carbonic Acid from 5 to 50 o C and to 000 Bar, J. Sol. Chem.,, 5-70, (75) Rinker, E. B., Doctoral Thesis, University of California, Santa Barbara, Jne (7) Rojey, A., C. Jaffret, S. C. Gandolphe, B. Drand, S. Jllian and M. Valais, Natral Gas Prodction Processing Transport, Editions Technip, Paris, (7) 0