Simultaneous Absorption of CO2 and H2S Into Aqueous Blends of N

Aug 19, 2006 - Indian Institute of Technology Kharagpur, Kharagpur -. 721302, India. Removal of CO2 from gaseous streams by absorption with chemical ...
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Environ. Sci. Technol. 2006, 40, 6076-6084

Simultaneous Absorption of CO2 and H2S Into Aqueous Blends of N-Methyldiethanolamine and Diethanolamine B I S H N U P A D A M A N D A L * ,† A N D SHYAMALENDU S. BANDYOPADHYAY‡ Department of Chemical Engineering, Indian Institute of Technology Guwahati, North Guwahati - 781039, India, and Separation Science Laboratory, Cryogenic Engineering Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

Removal of CO2 from gaseous streams by absorption with chemical reaction in the liquid phase is usually employed in industry as a method to retain atmospheric CO2 to combat the greenhouse effect. A broad spectrum of alkanolamines and, more recently, their mixtures are being employed for the removal of acid gases such as CO2, H2S, and COS from natural and industrial gas streams. In this research, simultaneous absorption of CO2 and H2S into aqueous blends of N-methyldiethanolamine and diethanolamine is studied theoretically and experimentally. The effect of contact time, temperature, and amine concentration on the rate of absorption and the selectivity were studied by absorption experiments in a wetted wall column at atmospheric pressure and constant feed gas ratio. The diffusion-reaction processes for CO2 and H2S mass transfer in blended amines are modeled according to Higbie’s penetration theory with the assumption that all reactions are reversible. A rigorous parametric sensitivity test is done to quantify the effects of possible errors in the pertinent model parameters on the prediction accuracy of the absorption rates and enhancement factors. Model results based on the kineticsequilibrium-mass transfer coupled model developed in this work are found to be in good agreement with the experimental results of rates of absorption of CO2 and H2S into (MDEA + DEA + H2O).

Introduction Acid gases, primarily CO2 and H2S, are the major impurities in natural and refinery gases, synthesis gas for ammonia production, synthetic natural gas, and hydrogen manufacture. CO2 and H2S concentrations in the sour gas streams may vary widely, from several parts per million to 50% by volume of the gas streams. The cleanup target, the allowable extent of impurity in the treated gas to meet product specifications, varies markedly from process to process and with the nature of the impurities. In natural gas processing, removing CO2 and H2S to a level less than 1% for CO2 and 4 ppm for H2S is essential to avoid corrosion of pipeline and equipment as well as to meet fuel gas * Corresponding author phone: +91-361-2582256; fax: +91-3612690762; e-mail: [email protected]. † Indian Institute of Technology Guwahati. ‡ Indian Institute of Technology Kharagpur. 6076

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specifications. Some of the CO2 is often removed from natural gas because at high concentrations it reduces the heating value of the gas. Moreover, it is costly to pump this extra volume of CO2 when it has no heating value. In refinery operations, H2S must be removed nearly completely from the gas streams due to its toxicity and corrosiveness and to avoid catalyst poisoning. Bio-generated methane from the decomposition of organic material often contains large amounts of CO2, which must be removed at least partially to improve its heating value. Synthesis gases for ammonia plants and polymerization units must be free of CO2 to prevent catalyst poisoning. In natural gas liquefaction plants, gas sweetening processes are designed to meet more stringent standards with respect to the limiting concentrations of about 50 and 4 ppm for CO2 and H2S, respectively, in the treated gas to avoid freezing in the cryogenic equipment. Removal of H2S to a very low level from fuel gases is mandatory for environmental reasons. Thus, the cleanup target plays an important role in the selection of gas treating processes. Normally, aqueous solutions of amines are used to enhance the absorption rate of the process with respect to the physical absorption. Because of their high reactivity and low cost, ease of reclamation, and low absorption of hydrocarbons, monoethanolamine (MEA), diethanolamine (DEA), di-isopropanolamine (DIPA), and N-methyldiethanolamine (MDEA) are among the most widely used amines for removal of CO2 and H2S. Blends of primary and tertiary amines have been shown to outperform conventional singleamine solutions. The blended amines such as mixtures of MDEA and DEA combine the higher loading capacities of MDEA and faster reaction rates of DEA. Thus, aqueous (MDEA+DEA) is considered an attractive blended amine solvent for simultaneous absorption of CO2 and H2S from sour gas streams. Many commercial gas-treating processes are still designed by experience and heuristics resulting in overdesign and excessive energy consumption. Most of the commercial simulators available today are based on models involving simplistic approximations and assumptions. Although performance of these simulators is reasonably satisfactory for gas treating processes with single amine solvents, these certainly cannot effectively represent the coupled influence of mass transfer, chemical kinetics, and thermodynamics in acid gas absorption in complex chemical solvents such as aqueous blends of alkanolamines. Hence, for the rational design and simulation of gas treating units involving blended amine solvents, it is essential to develop mass transferreaction rate based models to describe the CO2 and H2S mass transfer in these solvents. Rinker et al. (1) developed a comprehensive model for the absorption of CO2 into aqueous blends of DEA and MDEA. Their model equation was based on penetration theory and the authors incorporated an extensive set of important reversible reactions and took into account the coupling among chemical equilibrium, mass transfer, and chemical kinetics. Zhang et al. (2) measured the absorption rate of CO2 into aqueous solution of MDEA blended with DEA. They presented a second-order reaction rate constant for the reaction of CO2 with DEA. Yu et al. (3) presented an approximate model for the rate of simultaneous absorption of H2S and CO2 into aqueous solutions of a tertiary amine and compared model predictions with the experimental results of simultaneous absorption. Mandal et al. (4) studied experimentally and theoretically the simultaneous absorption of H2S and CO2 into aqueous solutions of AMP (2-amino10.1021/es0606475 CCC: $33.50

 2006 American Chemical Society Published on Web 08/19/2006

2-methyl-1-propanol) and MDEA. The experimental and model results have been found to be in good agreement. Rascol et al. (5) developed a calculation model to numerically interpret the simultaneous mass transfer of CO2 and H2S into blended alkanolamines. They have used the film theory and approximate film theory model to describe the mass transfer phenomena within the liquid phase, while gas-phase resistance is neglected. However, in spite of the immense commercial significance of the aqueous blended amine solvent (MDEA+DEA) for simultaneous removal of CO2 and H2S from sour natural gas streams, studies on simultaneous absorption of H2S and CO2 into aqueous blends of MDEA and DEA have not been published in the open literature so far. In this work, the simultaneous absorption of CO2 and H2S into aqueous blends of MDEA and DEA is studied theoretically and experimentally. The effect of contact time, temperature, and amine concentration on the rate of absorption and the selectivity were studied by absorption experiments in a wetted wall column. The diffusionreaction processes for CO2 and H2S mass transfer in the blended amine solvents are modeled according to Higbie’s penetration theory with the assumption that all the reactions are reversible. A rigorous parametric sensitivity test is often essential for the critical evaluation of the developed mathematical model and identification of the key systems’ parameters and quantifying their effects on the mass transfer. This is also needed to quantify the effects of possible errors in the pertinent model parameters on the prediction accuracy of the absorption rates and enhancement factors. Parametric sensitivity analyses have been presented using the mathematical model developed in this work.

Model Development The model comprises an equilibrium model for determining the initial bulk concentrations of all species in the liquid phase, and a diffusion-reaction model for predicting concentration profiles of all liquid-phase species and enhancement factors and rates of absorption of CO2 and H2S. Reaction Scheme. The following reactions may take place when CO2 and H2S are absorbed into an aqueous mixed amine solution of DEA (R′′R′′NH) and MDEA (R′R′′R′′N) (where R′ ) CH3 and R′′ ) CH2CH2OH). K1,k21

CO2 + R′R′′R′′N + H2O 798 R′R′′R′′NH+ + HCO3(1) K2,k22

CO2 + HO- 798 HCO3-

(2)

K9

H+ + OH- 798 H2O

(9)

where Ki (i ) 1,....,9) is the equilibrium constant for the reaction i, and k2j (j ) 1,...,3) is the second-order forward rate coefficient for reaction j. Reactions 1-3 have finite reaction rates and are reversible, and reactions 4-9 are considered instantaneous, reversible, and at equilibrium. In the above reaction scheme not all the reaction equilibrium constants are independent. Only seven equilibrium constants (K2, K3, K4, K5, K7, K8, and K9) are independent. The remaining two can be obtained by appropriate combinations of the independent equilibrium constants. Reaction Mechanism. The zwitterion mechanism has become one of the most widely accepted mechanisms for secondary amine reactions with CO2 (6-10). This mechanism involves the formation of a zwitterion intermediate (R′′RNH+COO-), which is subsequently deprotonated by a base to produce carbamate (R′′R′′NCOO-) and protonated base. Reaction 3 represents the overall reaction between CO2 and a secondary amine for producing carbamate. The rate coefficient k23 is viewed as the global rate coefficient for the formation of carbamate. Hence, this representation does not rule out the possible formation of a zwitterion reaction intermediate. Donaldson and Nguyen (11) proposed that the reaction mechanism for CO2 with tertiary amines is a base-catalyzed hydration reaction as shown in reaction 1. The mechanism implies that tertiary amines (e.g., MDEA) do not react directly with CO2. Versteeg and van Swaaij (7), who studied CO2 absorption into nonaqueous solutions of MDEA in ethanol, found that only physical absorption occurs in nonaqueous tertiary amine systems which support the validity of the reaction 1. The reaction between H2S and aqueous amines involves a proton transfer, and can be regarded as reversible and instantaneously fast with respect to mass transfer. Everywhere in the liquid phase, including the interfacial liquid film, H2Samine equilibrium exists always. Bulk Liquid Equilibrium Model. The initial liquid bulk concentrations of all chemical species can be estimated from the initial concentrations of R′R′′R′′N and R′′R′′NH, the initial CO2 and H2S loadings of the solution (RCO2 and RH2S, respectively), and the assumption that all reactions are at equilibrium. For the twelve liquid bulk concentrations, the equations are as follows: overall MDEA balance

[RR′′R′′N]0 + [RR′′R′′NH+]0 ) [MDEA]initial

K3,k23

-

+

R′′R′′NH + CO2 798 R′R′′R′′NCOO + H K4

HCO3- 798 H+ + CO32K5

H2S + R′R′′R′′N 798 R′R′′R′′NH+ + HSK6

H2S + R′′R′′NH 798 R′′R′′NH2+ + HSK7

R′R′′R′′N + H+ 798 R′R′′R′′NH+ K8

R′′R′′NH + H+ 798 R′′R′′NH2+

(10)

(3) overall DEA balance

(4)

(5)

[R′′R′′NH]0 + [R′′R′′NH2+]0 + [R′′R′′NCOO-]0 ) [DEA]initial (11) overall CO2 balance

(6)

[CO2]0 + [HCO3-]0 + [CO32-]0 + [R′′R′′NCOO-]0 ) RCO2{[MDEA]initial + [DEA]initial} (12)

(7)

overall H2S balance

(8)

[HS-]0 + [H2S]0 ) RH2S{[MDEA]initial + [DEA]initial} (13) VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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electroneutrality balance

[R′R′′R′′NH+]0 + [H+]0 + [R′′R′′NH2+]0 - [HCO3-]0 -

[OH-]0 - 2[CO32-]0 - [R′′R′′NCOO-]0 - [HS-]0 ) 0 (14)

all reactions in equilibrium

[HCO3-]0

K2 )

[CO2]0[OH-]0

+ 0

K3 )

[CO2]0[R′′R′′NH]0

K4 )

K5 )

[CO32-]0[H+]0 - 0

[HCO3 ]

K8 )

[R′R′′R′′N]0[H2S]0 [R′R′′R′′NH+]0 [R′R′′R′′N]0[H+]0 [R′′R′′NH2+]0 0

+ 0

[R′′R′′NH] [H ]

K9 )

total MDEA balance

(16)

∂[R′R′′R′′N] ∂[R′R′′R′′NH+] + ) ∂t ∂t 2 ∂ [R′R′′R′′N] ∂2[R′R′′R′′NH+] DR′R′′R′′N + D (27) + R′R′′R′′NH ∂x2 ∂x2

(17)

1.0 [OH-]0[H+]0

(18)

(19)

(20)

(21)

k21 R1 ) - k21[CO2][R′R′′R′′N] + [R′R′′R′′NH+][HCO32-] K1 (22) k22 [HCO3-] R2 ) - k22[CO2][OH ] + K2 -

(23)

The following equations describe the diffusion reaction process: CO2 balance 2

(25)

total carbon (from CO2) balance 6078

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+ ∂[R′R′′R′′NH+] ∂[H+] ∂[R′′R′′NH2 ] ∂[HCO3 ] + + ∂t ∂t ∂t ∂t 2∂[CO3 ] ∂[R′′R′′NCOO ] ∂[HS ] ∂[OH ] -2 ) ∂t ∂t ∂t ∂t 2 + 2 + ∂ [R′R′′R′′NH ] ∂ [H ] DR′R′′R′′NH+ + DH + + 2 ∂x ∂x2 ∂2[R′′R′′NH2+] ∂2[HCO3-] DR′′R′′NH2+ - DHCO32 ∂x ∂x2 ∂2[CO32-] ∂2[OH-] DOH2D CO32∂x2 ∂x2 ∂2[R′′R′′NCOO-] ∂2[HS-] DR′′R′′NCOOD . (29) HS ∂x2 ∂x2

carbamate balance

∂[R′′R′′NCOO-] ∂2[R′′R′′NCOO-] - R3 ) DR′′R′′NCOO∂t ∂x2 (30) total sulfur (from H2S) balance

k23 + [H ][R′′R′′NCOO-] K3 (24)

∂[CO2] ∂ [CO2] + R1 + R2 + R3 ) DCO2 ∂t ∂x2

∂[R′′R′′NH] ∂[R′′R′′NH2 ] ∂[R′′R′′NCOO-] + + ) ∂t ∂t ∂t 2 2 ∂ [R′′R′′NH2+] ∂ [R′′R′′NH] DR′′R′′NH + D + + R′′R′′NH 2 ∂x2 ∂x2 ∂2[R′′R′′NCOO-] DR′′R′′NCOO(28) ∂x2 electroneutrality balance

We have twelve unknowns and twelve nonlinear algebraic equations (eqs 10-21), which can be solved for the liquid bulk concentrations. To solve this we have used IMSL Library, FORTRAN Developer Studio version 4.0. Diffusion-Reaction Model. Higbie’s penetration model is used to set up the diffusion-reaction partial differential equations which describe the simultaneous absorption of CO2 and H2S into aqueous solutions of a tertiary amine R′R′′R′′N and a secondary amine R′′R′′NH. All reactions were treated as reversible reactions. The rates for reactions 1-3 are given by the following rate expressions:

R3 ) - k23[CO2][R′′R′′NH] +

total DEA balance +

[R′R′′R′′NH+]0[HS-]0

K7 )

(15) - 0

[H ] [R′′R′′NCOO ]

∂[CO2] ∂[HCO3-] ∂[CO32-] ∂[R′′R′′NCOO-] + + + ) ∂t ∂t ∂t ∂t ∂2[CO2] ∂2[HCO3-] ∂2[CO32-] DCO2 + D + D + 2HCO CO 3 3 ∂x2 ∂x2 ∂x2 2 ∂ [R′′R′′NCOO ] DR′′R′′NCOO(26) ∂x2

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 19, 2006

∂2[H2S] ∂[HS-] ∂[H2S] ∂2[HS-] + ) DHS+ D (31) H S 2 ∂t ∂t ∂x2 ∂x2 instantaneous reactions assumed to be at equilibrium

K4 )

K5 )

[CO32-][H+] [HCO3-]

[R′R′′R′′NH+][HS-] [R′R′′R′′N][H2S]

K7 )

[R′R′′R′′NH+] [R′R′′R′′N][H+]

(32)

(33) (34)

K8 )

[R′′R′′NH2+] +

[R′′R′′NH][H ]

K9 )

1.0 [OH-][H+]

(35) (36)

Thus, there are twelve partial differential-algebraic equations for the blended amine solvent, (MDEA+DEA), which can be solved for the concentrations of twelve chemical species. For convenience, the chemical species in the liquid phase are renamed as follows:

u1 ) [CO2],

u2 ) [R′R′′R′′N],

u5 ) [OH-],

u6 ) [CO32-], u8 ) [R′′R′′NH],

u3 ) [R′R′′R′′NH+], u4 ) [HCO3-], +

u7 ) [H ],

u9 ) [R′′R′′NH2+], u11 ) [HS-],

Materials and Methods

u10 ) [R′′R′′NCOO-], u12 ) [H2S]

Initial Condition and Boundary Conditions at x ) ∞. At t ) 0 (for all x g 0) and at x ) ∞ (for all t g 0), the concentrations of all chemical species are equal to their liquid bulk concentrations

ui ) ui0, i ) 1,....,12

(37)

Boundary Conditions at Gas-Liquid Interface (x ) 0). At x ) 0 (gas-liquid interface), the fluxes of the nonvolatile chemical species are equal to zero, which leads to the following equations:

∂ui ) 0 at x ) 0, t > 0 ∂x

(38)

for all i except i ) 1 (CO2) and i ) 12 (H2S). For the volatile components (CO2 and H2S), the mass transfer rate in the gas near the interface is equal to the mass transfer rate in the liquid near the interface

-Di

∂ui ) kg,i (Pi - Hiui(0,t)) at x ) 0, t > 0 ∂x

(39)

where kg,i is the gas-phase resistance, Pi is the bulk partial pressure, and Hi is the physical equilibrium constant (Henry’s law constant) of gas i. Absorption Rate and Enhancement Factor. The differential equations are integrated from t ) 0 to t ) θ, the contact time. The contact time and the liquid-phase mass transfer coefficient for physical absorption of CO2 are calculated from the equations as presented by Mandal et al. (10). The time-averaged rate of absorption of gas i per unit interfacial area is then computed from the following equation:

RAi ) -

Di θ

∫

θ

0

∂ui (0,t) dt ∂x

(40)

and the enhancement factor for absorption of gas i is determined according to the following equation:

Ei )

R Ai kL,i(u*i - u0i )

ordinary differential equations in t. Detailed method of solution was discussed elsewhere (10). Equally spaced nodes are used to discretize the spatial variable x. The resulting system of ordinary differential equations coupled with the algebraic equations is solved by using the subroutine DDASSL (12, 13) in double precision FORTRAN on a Pentium IV processor. It uses the implicit backward-differentiation formulas (BDF) of orders one through five in a variable order/ variable step integration mode to solve nonlinear systems of differential/algebraic equations. The typical number of nodes used in this work is 200 and corresponding nodal spacings are of the order 10-7 to 10-8 m. The computational time is about 45 min to obtain converged solution for this system.

(41)

where u*i and u0i are the interfacial and bulk concentrations of gas i in the liquid, respectively. Method of Solution. The method-of-lines is used to transform each partial differential equation into a set of

Reagent grade DEA and MDEA of 98% purity were obtained from E. Merck. For CO2 and N2 commercial gas cylinders were used. Purities of both the gases were better than 99%. Calibration standard H2S was supplied by Hydrogas India as 90% H2S and 10% N2 mixture. Distilled water degassed by boiling was used for making amine solutions. The total amine concentrations of the solutions were determined by titration with standard HCl using methyl orange indicator. Absorption Experiments. The wetted wall column experimental setup used for simultaneous absorption of CO2 and H2S into aqueous blends of MDEA and DEA is shown in Figure S1 (see Supporting Information). The outer diameter of the column was 2.81 × 10-2 m. Absorption measurements were done with 3 kmol/m3 total MDEA and DEA mixture in the temperature range 293-313 K. The gas-phase composition was maintained at 1% H2S, 10% CO2, and 89% N2 (H2S/ CO2/N2 ) 1:10:89). The temperature of absorption was controlled within (0.2 K of the desired level with a circulator temperature controller (JULABO FP 55, FRG). Before each run the absorption surface of the column was thoroughly cleaned with neutral EXTRAN (N) cleaning solution and distilled water. Circulation of the thermostated water from the circulator temperature controller was established through the jacket of the glass shroud, tube-intube heat exchanger of the wetted wall column and the bath accommodating the gas inlet coil and the saturators. H2S and CO2 diluted with N2 to make the desired gas-phase composition (H2S/CO2/N2 ) 1:10:89) was passed through the mixing tube and the coil immersed in the controlled temperature bath and finally through the saturators to saturate the gas phase with water vapor. The composition of the feed gas was determined from the respective flow rates. The gas, saturated with water vapor at the temperature of the absorption, was fed to the top of absorption space of the wetted wall column. The amine solution was then fed from the overhead storage to the contactor at the desired flow rate using a precalibrated rotameter. Once the system reached steady state with respect to the gas and liquid flow rates and the gas phase and liquid film temperature, samples were collected from the liquid outlet. Three samples, each measuring about 50 × 10-6 m3 in volume, were taken in stoppered sample bottles at an interval of about 60 s for each run. The samples were then immediately analyzed for the H2S and CO2 contents. H2S contents in the liquid samples were determined by titration with standard AgNO3 solution. An autotitrator (Mettler DL-21) with a silver electrode (Mettler DM-141) was used for this titration. For the determination of CO2 content of the liquid, a known volume of the liquid sample was acidulated with 6 N HCl and the volume of evolved gas was measured with a gas burette. The evolved gas was the total H2S and CO2 content of the liquid sample. After temperature and pressure correction the H2S content of the liquid as found out earlier by titration was subtracted from the total gas volume to get the CO2 content of the liquid sample. VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Physicochemical Properties of Aqueous Blends of MDEA and DEA [MDEA] (kmol/m3)

[DEA] (kmol/m3)

T (K)

G (kg/m3)

µ × 103 (kg/ms)

DCO2 × 109 (m2/s)

DH2S × 109 (m2/s)

HCO2 (kPa m3/kmol)

HH2S (kPa m3/kmol)

2.8 2.8 2.8 2.6 2.6 2.6 2.2 2.2 2.2 1.5 1.5 1.5 0.8 0.8 0.8 0.4 0.4 0.4 0.2 0.2 0.2

0.2 0.2 0.2 0.4 0.4 0.4 0.8 0.8 0.8 1.5 1.5 1.5 2.2 2.2 2.2 2.6 2.6 2.6 2.8 2.8 2.8

293 301.5 313 293 301.5 313 293 301.5 313 293 301.5 313 293 301.5 313 293 301.5 313 293 301.5 313

1032.8 1028.5 1022.4 1032.9 1028.8 1022.7 1033.2 1029.2 1023.4 1033.9 1030.1 1024.6 1034.7 1031.9 1026.0 1035.4 1032.0 1026.9 1035.7 1032.4 1027.4

4.81 3.55 2.89 4.71 3.49 2.45 4.53 3.38 2.38 4.24 3.19 2.27 3.98 3.02 2.17 3.84 2.93 2.12 3.77 2.88 2.09

0.67 0.87 1.22 0.68 0.88 1.23 0.69 0.90 1.25 0.72 0.93 1.29 0.75 0.96 1.32 0.76 0.98 1.34 0.77 0.99 1.35

0.49 0.68 1.00 0.49 0.69 1.02 0.51 0.71 1.02 0.54 0.74 1.09 0.57 0.78 1.13 0.59 0.80 1.15 0.60 0.81 1.16

3479 3925 4479 3480 3954 4611 3493 3989 4759 3508 4034 4918 3518 4081 5075 3527 4118 5221 3536 4147 5369

1108 1206 1508 1082 1176 1472 1029 1118 1399 942 1023 1278 860 933 1165 815 883 1088 792 859 1072

Physicochemical Properties and Model Parameters Knowledge of the physicochemical and transport properties of the alkanolamines and CO2 and H2S, e.g., density and viscosity of the aqueous amine solutions and diffusivity and physical solubility of CO2 and H2S in the aqueous amine solutions were necessary for analyzing the results of absorption studies using the numerical model developed in this work. These properties for the alkanolamines and CO2 and H2S are determined or evaluated as discussed elsewhere (1416) and presented in Table 1. The equilibrium constants were needed to interpret the results of CO2 and H2S absorption measurement into alkanolamines. Values of the independent equilibrium constants have been found out using reliable correlations and the dependent ones have been estimated by appropriate combination of the independent equilibrium constants. These are calculated as presented by Mandal et al. (10, 17) and listed in Table S1 (Supporting Information). The rate coefficient for CO2 and OH- reaction is estimated using reliable correlation presented by Danckwerts and Sharma (18). The other rate coefficients of CO2-amine reactions were obtained by adjustment using numerical model and experimental results.

Results and Discussion Liquid flow rate for all runs were fixed at 2 × 10-6 m3/s. The liquid film Reynolds number (NRe) has been found to be below 20 throughout this work. Hence, the liquid flow was well within the laminar region since turbulence appears when NRe is greater than 400 (19). The gas flow rate was 180 × 10-6 m3/s. Experiments were performed at 293, 301.5, and 313 K. The total pressure in the absorption chamber was about 100 kPa. The total amine concentration was 3.0 kmol/m3. The contact time was varied in the range 0.30-0.94 s. For a particular amine concentration and temperature the contact time was varied by changing the absorption length from 0.05 to 0.1 m. Under the conditions of this work, the liquid-side mass-transfer coefficient kL has been in the range (3.0-8.6) × 10-5 m/s. It has been observed in our earlier work (4, 20) that the rate of absorption of CO2 into aqueous solutions of both AMP and DEA for simultaneous absorption of CO2 and H2S, as shown in Figures S2-S3 (Supporting Information), increases initially with the increase in gas flow rate, but it levels off above a gas flow rate of 140 × 10-6 m3/s, thus 6080

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indicating that the gas-phase resistance to CO2 mass transfer in these amine solvents under the conditions of the present wetted wall contractor experiments is negligible above a gas flow rate of 140 × 10-6 m3/s. However, for absorption of H2S, influence of gas flow rate was significant even above a gas flow rate of 180 × 10-6 m3/s, the maximum gas flow rate that could be maintained in the wetted wall column experimental facility used for this work. Hence, the gas-phase mass-transfer coefficient kg for H2S at a gas flow rate of 180 × 10-6 m3/s for all test temperatures has been determined by measurements of absorption of H2S into 10% aqueous hydroxide solutions from a 1% H2S/99% N2 gas mixture (4), and all absorption measurement runs were performed at a gas flow rate of 180 × 10-6 m3/s. The determined kg,H2S values are presented elsewhere (17) and also in Table SII (Supporting Information). Analytically solved profiles of various species in the liquid film for acid gas-solvent systems studied in this work have been presented. Detailed parametric sensitivity analyses using the mathematical model developed in this work are performed for CO2 and H2S into blended amine system studied. Kinetics of Aqueous DEA and CO2 Reaction. The experimental results on the physicochemical parameters as well as rates of absorption of CO2 and H2S into aqueous DEA at various temperatures and for various contact times and amine concentrations are presented in Tables SIII and SIV (Supporting Information). The pseudo-first-order rate constant k23 (rate constant of the reaction between CO2 and DEA) is estimated from the correlation presented by Mandal et al. (17) over the temperature range 293-313 K as follows:

ln k23 ) 21.403 -

4271.1 (T/K)

(42)

Kinetics of Aqueous MDEA and CO2 Reaction. The experimental results of rates of absorption of CO2 and H2S into aqueous MDEA have been published earlier (4). The pseudo-first-order rate constant k21 (eq 1) obtained by adjustment in the model for CO2 - H2S - (MDEA + H2O) in this work has been correlated over the temperature range 293-313 K as follows:

ln k21 ) 16.863 -

4343.9 (T/K)

(43)

FIGURE 1. Effect of various parameters on the specific rate of absorption and selectivity factor in aqueous blends of MDEA and DEA at 313 K: (A) effect of contact time, (B) effect of amine concentration, and (C) effect of temperature. Aqueous Blends of MDEA and DEA. The measured rates of absorption of CO2 and H2S into aqueous blends of MDEA and DEA are shown in Figure 1A-C and listed in Table 2A-D along with the absorption rates predicted using the numerical diffusion reaction model. The measured and predicted rates of absorption are compared in the parity plot shown in Figure S4 (Supporting Information). There is very good agreement between the model prediction and experimental results with average deviation (AAD%) being about 4.1% and 2.7% for CO2 and H2S, respectively. The rate coefficients for MDEA and DEA obtained by the single-amine system have been used to predict the rate of absorption of CO2 and H2S into the blends of MDEA and DEA. Calculated concentration profiles for absorption of CO2 and H2S into blends of MDEA and DEA are shown in Figure 2. The contact time in the wetted wall contactor was varied by changing the absorption length of the wetted wall column from 5 × 10-2 to 10 × 10-2 m. Figure 1A shows the effect of contact time on the specific rates of absorption of CO2 and H2S and the selectivity for the blends of MDEA and DEA. While the specific rate of absorption of CO2 does not change with contact time, that of H2S decreases with increasing contact time resulting in a decrease in the selectivity factor. This suggests that the absorption of CO2 for simultaneous absorption of CO2 and H2S into aqueous blends of MDEA and DEA under the conditions of negligible interaction in the absorbed phase remains in the fast pseudofirst-order regime. This is further verified by computing the values of

xM )

(

xm 2+ 1D k

A mn[A*]

kL

m-1

)

[B0]n

and

E)

( ) RA kAA*

for CO2 - (MDEA+DEA+H2O). The Hatta number (xM) and enhancement factor (E) values lie in the range of 17-43 and 14-31, respectively, under the conditions of the present work, thus validating the fast pseudo-first-order regime for this absorption. The decrease of H2S absorption rate at higher contact times (i.e., lower kL values) indicates that although the instantaneously fast H2S-amine reaction is predominantly gas-phase controlled, the rate of absorption is also influenced by kL. The effect of concentration of DEA in the blends of MDEA and DEA on the specific rates of absorption of CO2 and H2S and the selectivity factor is shown in Figure 1B. For the aqueous blends of MDEA and DEA, the rate of absorption of CO2 is seen to increase rapidly with the increase in the DEA concentration in the blends, while that of H2S increases to a much lesser extent. Thus, there is an expected decrease in the selectivity factor with the increase in concentration of DEA. Similar trends of CO2 absorption rates were observed by Rinker et al. (1) and Zhang et al. (2). The effect of temperature on the specific rate of absorption and selectivity factor for aqueous blend of MDEA and DEA is shown in Figure 1C. It is observed that with the rise in VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Results of Simultaneous Absorption of CO2 and H2S Into Aqueous MDEA and DEA Blends (VG ) 180 × 10-6 m3/s) experimental [MDEA] (kmol/m3)

[DEA] (kmol/m3)

T (K)

θ (s)

kL,CO2 × 105 (m/s)

RCO2 × 106 (kmol/m2s)

RH2S × 106 (kmol/m2s)

2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8

293 293 293 301.5 301.5 301.5 313 313 313 293 293 293 301.5 301.5 301.5 313 313 313 293 293 293 301.5 301.5 301.5 313 313 313 293 293 293 301.5 301.5 301.5 313 313 313 293 293 293 301.5 301.5 301.5 313 313 313 293 293 293 301.5 301.5 301.5 313 313 313 293 293 293 301.5 301.5 301.5 313 313 313

0.468 0.656 0.937 0.424 0.594 0.848 0.377 0.528 0.755 0.465 0.651 0.931 0.421 0.590 0.843 0.375 0.526 0.751 0.459 0.643 0.919 0.417 0.584 0.834 0.372 0.520 0.743 0.449 0.629 0.898 0.409 0.572 0.818 0.367 0.512 0.731 0.439 0.615 0.879 0.401 0.562 0.802 0.360 0.504 0.720 0.434 0.608 0.868 0.397 0.556 0.794 0.357 0.500 0.714 0.432 0.604 0.863 0.395 0.553 0.790 0.356 0.498 0.711

4.268 3.607 3.018 5.110 4.319 3.613 6.416 5.423 4.537 4.314 3.646 3.050 5.158 4.356 3.645 6.459 5.459 4.567 4.374 3.696 3.093 5.243 4.431 3.707 6.544 5.530 4.627 4.518 3.819 3.195 5.382 4.549 3.806 6.703 5.665 4.739 4.662 3.940 3.296 5.520 4.665 3.903 6.833 5.775 4.831 4.721 3.990 3.338 5.606 4.737 3.963 6.912 5.842 4.888 4.766 4.028 3.370 5.648 4.773 3.994 6.952 5.875 4.915

1.75 1.70 1.69 1.83 1.82 1.79 1.95 1.92 1.94 2.15 2.16 2.13 2.25 2.21 2.19 2.37 2.33 2.31 2.59 2.54 2.52 2.68 2.69 2.66 2.82 2.80 2.75 2.86 2.87 2.84 2.99 2.97 2.94 3.17 3.14 3.15 3.13 3.09 3.11 3.31 3.29 3.25 3.59 3.56 3.54 3.37 3.35 3.36 3.54 3.52 3.51 3.81 3.79 3.75 3.62 3.60 3.55 3.85 3.82 3.80 3.97 3.94 3.90

2.94 2.69 2.50 2.90 2.70 2.49 2.82 2.51 2.23 3.02 2.76 2.58 2.99 2.78 2.56 2.88 2.55 2.27 3.11 2.84 2.66 3.08 2.85 2.63 2.95 2.60 2.31 3.23 3.00 2.78 3.19 2.96 2.73 3.03 2.67 2.37 3.35 3.14 2.93 3.20 3.06 2.84 3.12 2.74 2.43 3.47 3.26 3.10 3.38 3.15 2.92 3.18 2.78 2.47 3.59 3.38 3.22 3.45 3.23 2.98 3.23 2.83 2.51

temperature from 293 to 313 K, the rate of absorption of H2S decreases slowly while the rate of absorption of CO2 increases, resulting in a decrease in the selectivity factor. Parametric Sensitivity Analysis. Parametric study allows for process optimization by parameter identification. Parametric sensitivity analysis has been performed here using 6082

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model fitting

Sp

RCO2 × 106 (kmol/m2s)

RH2S × 106 (kmol/m2s)

Sp

16.8 15.8 14.8 15.9 14.8 13.9 14.5 13.1 11.5 14.1 12.8 12.1 13.3 12.6 11.7 12.2 10.9 9.83 12.0 11.2 10.6 11.5 10.6 9.89 10.5 9.29 8.40 11.3 10.5 9.79 10.7 9.97 9.29 9.56 8.50 7.52 10.7 10.2 9.42 9.67 9.30 8.74 8.69 7.70 6.86 10.3 9.73 9.23 9.55 8.95 8.32 8.35 7.34 6.59 9.92 9.39 9.07 8.96 8.46 7.84 8.14 7.18 6.44

1.71 1.67 1.62 1.80 1.78 1.75 1.94 1.90 1.87 2.08 2.06 2.01 2.17 2.14 2.12 2.25 2.21 2.17 2.48 2.44 2.37 2.60 2.54 2.50 2.74 2.70 2.62 2.84 2.81 2.77 2.95 2.90 2.86 3.08 3.05 3.01 3.17 3.12 3.09 3.30 3.26 3.21 3.42 3.37 3.33 3.45 3.40 3.36 3.58 3.54 3.51 3.69 3.65 3.60 3.72 3.68 3.65 3.86 3.81 3.76 4.00 3.96 3.91

2.91 2.82 2.65 2.84 2.75 2.58 2.73 2.61 2.48 2.98 2.85 2.71 2.90 2.73 2.60 2.71 2.59 2.42 2.99 2.86 2.70 2.88 2.78 2.60 2.70 2.61 2.45 3.12 3.03 2.91 3.02 2.93 2.81 2.79 2.70 2.59 3.19 3.12 3.03 3.07 3.01 2.90 2.88 2.80 2.70 3.31 3.24 3.15 3.19 3.11 3.01 2.98 2.91 2.82 3.42 3.34 3.25 3.28 3.20 3.11 3.06 2.97 2.86

17.0 16.9 16.4 15.8 15.5 14.7 14.1 13.7 13.3 14.3 13.8 13.5 13.4 12.8 12.3 12.0 11.7 11.2 12.1 11.7 11.4 11.1 11.0 10.4 9.85 9.67 9.35 11.0 10.8 10.5 10.2 10.1 9.83 9.06 8.85 8.61 10.1 10.0 9.81 9.30 9.23 9.03 8.42 8.31 8.11 9.59 9.53 9.38 8.91 8.79 8.58 8.08 7.97 7.83 9.19 9.08 8.90 8.50 8.40 8.27 7.65 7.50 7.32

the numerical model developed for absorption of CO2 and H2S into blended alkanolamine to examine the effects of important parameters on the rates of absorption and also quantify the effects of possible errors in the pertinent model parameters on the prediction accuracy of the absorption rates. The parameters considered for the analyses are CO2 interfacial

FIGURE 2. Calculated concentration profiles in the film for absorption of CO2 and H2S into an aqueous blend of MDEA and DEA at the end of contactor. T ) 313 K, [MDEA] ) 2.6 kmol/m3, [DEA] ) 0.4 kmol/m3, pCO2 ) 9.4 kPa, pH2S ) 0.94 kPa, t ) 0.703 s, kL,CO2 ) 5.053 × 10-5 m/s. concentration (CO2 partial pressure), Henry’s law constant for CO2 and H2S, and diffusivity of CO2 and H2S. The effect of the CO2 interfacial concentration (CO2 partial pressure) on the rates of absorption of CO2 and H2S is shown in Figure S5 (Supporting Information). It is clear that the

rate of absorption of CO2 into the aqueous blended amine increases almost linearly. The H2S absorption rate, as expected, decreases marginally due to increased kinetic selectivity for CO2. Figure 3A shows the effect of errors in Henry’s law constant for CO2 on the predicted rate of absorption. Here, errors of -50% to +50% have been introduced into the value of Henry’s law constant of CO2 and the corresponding errors in the rate of absorption of CO2 have been determined to range from +88% to -31%, respectively, while the H2S absorption rate remains almost unaffected. Hence, it is very important to have accurate values for the Henry’s law constant to get accurate predictions for the CO2 absorption rate and enhancement factor. Similar errors have been introduced into the value of Henry’s law constant of H2S as shown in Figure 3B and the corresponding errors in the rate of absorption of H2S have been determined to range from +3% to -4%, respectively, while CO2 absorption rate remains unaffected by the error in HH2S. Thus RH2S is found to have less sensitivity to the value of HH2S. Figure 3C shows the effect of errors in the value of the diffusion coefficient of CO2 on the predicted rate of absorption. Here, errors of -50% to +50% have been introduced into the value of the diffusion coefficient of CO2, and the corresponding errors in the rate of absorption of CO2 have been determined to be in the range of -29% to +20%, respectively. The H2S absorption rate, however, is not affected. Thus, it is also important to have accurate values for the

FIGURE 3. Effect of errors of various parameters on the predicted rate of absorption of CO2 and H2S into an aqueous MDEA and DEA solution: (A) effect of errors in Henry’s law constant of CO2; (B) effect of errors in Henry’s law constant of H2S; (C) effect of errors in diffusion coefficient of CO2; and (D) effect of errors in diffusion coefficient of H2S. T ) 313 K, [MDEA] ) 2.6 kmol/m3, [DEA] ) 0.4 kmol/m3, pCO2 ) 9.4 kPa, pH2S ) 0.94 kPa, t ) 0.375 s, kL,CO2 ) 6.459 × 10-5 m/s, k21 ) 20 m3/kmol s, kCO2-DEA ) 2400 m3/kmol s, HH2S ) 1472 kPa m3/kmol, HCO2 ) 4611 kPa m3/kmol, DH2S )1.02 × 10-9 m2/s, DCO2 )1.23 × 10-9 m2/s with RCO2 ) 2.37 × 10-6 kmol/m2 s and RH2S ) 2.88 × 10-6 kmol/m2 s. VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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diffusion coefficient of CO2 to obtain accurate predictions for the CO2 absorption rate and enhancement factor in the blended amines. Similar errors have been introduced into the value of the diffusion coefficient of H2S as shown in Figure 3D, and the corresponding errors in the rate of absorption of H2S have been determined to range from -5% to 4%, respectively, while there is no significant change in the rate of absorption of CO2. Thus, RH2S, as expected, is found to be somewhat sensitive to errors in DH2S.

Nomenclature A*

concentration of dissolved gas A at gas-liquid interface, in equilibrium with gas at interface, kmol/m3

B0

concentration of species B in bulk of liquid, kmol/m3

[DEA]

concentration of diethanolamine in the aqueous solution, kmol/m3

[DEA]initial

initial liquid bulk concentration of diethanolamine, kmol/m3

DA

diffusivity of dissolved gas A in the liquid phase, m2/s

Di

diffusion coefficient of species i in the aqueous alkanolamine solution, m2/s

Ei

enhancement factor for absorption of species i

kmn

rate-constant for reaction, mth order in A and nth order in B, (m3/kmol)m+n-1s-1

[MDEA]

concentration of N-methyldiethanolamine in the aqueous solution, kmol/m3

[MDEA]initial initial liquid bulk concentration of N-methyldiethanolamine, kmol/m3 t

independent time variable, s

x

independent spatial variable, m

Greek Letters RCO2

initial CO2 loading of the aqueous amine solution, (kmol CO2/kmol total amine)

RH2S

initial H2S loading of the aqueous amine solution, (kmol H2S/kmol total amine)

Acknowledgments This work was supported by the Centre for High Technology (CHT), Ministry of Petroleum and Natural Gas, New Delhi, India.

Supporting Information Available Figure S1 shows the wetted wall column experimental setup used for simultaneous absorption of CO2 and H2S into aqueous blends of MDEA and DEA. Figure S2 shows the effect of gas flow rate on the specific rates of absorption of CO2 and H2S into aqueous AMP solutions for simultaneous absorption of these gases. Figure S3 shows the effect of gas flow rate on the specific rates of absorption of CO2 and H2S into aqueous DEA solutions for simultaneous absorption of these gases. Figure S4 shows comparison of model predicted rates of absorption to the experimental rates of absorption for CO2 and H2S into aqueous blends of MDEA and DEA. Figure S5 shows the effect of interfacial concentration of CO2 on the predicted rate of absorption of CO2 and H2S into an aqueous MDEA and DEA. Table SI lists the equilibrium constant correlations used for model calculations. Table SII lists the gas-phase mass transfer coefficients of H2S at different 6084

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temperatures. Table SIII lists the physicochemical properties of aqueous DEA solutions. Table SIV lists the experimental results along with the model fitting results of simultaneous absorption of CO2 and H2S into aqueous solutions of DEA. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Rinker, E. B.; Ashour, S. S.; Sandall, O. C. Absorption of carbon dioxide into blends of diethanolamine and methyldiethanolamine. Ind. Eng. Chem. Res. 2000, 39, 4346-4356. (2) Zhang, X.; Zhang, C.-F.; Liu, Y. Kinetics of absorption of CO2 into aqueous solutions of MDEA blended with DEA. Ind. Eng. Chem. Res. 2002, 41, 1135-1141. (3) Yu, W. C.; Astarita, G.; Savage, D. W. Kinetics of carbon dioxide absorption in solutions of methyldiethanolamine. Chem. Eng. Sci. 1985, 40, 1585-1590. (4) Mandal, B. P.; Biswas, A. K.; Bandyopadhyay, S. S. Selective absorption of H2S from gas streams containing H2S and CO2 in aqueous solutions of N-methyldiethanolamine and 2-amino2-methyl-1-propanol. Sep. Purif. Technol. 2004, 35, 191-202. (5) Rascol, E; Meyer, M; Huor, M. H.; Prevost, M. Modelisation and simulation of the absorption of CO2 and H2S into mixed alkanolamine solutions. Hungarian J. Ind. Chem. 1997, 25, 11-16. (6) Blauwhoff, P. M. M.; Versteeg, G. F.; van Swaaij, W. P. M. A study on the reaction between CO2 and alkanolamines in aqueous solutions. Chem. Eng. Sci. 1984, 39, 207-225. (7) Versteeg, G. F.; van Swaaij, W. P. M. On the kinetics between CO2 and alkanolamines both in aqueous and nonaqueous solutions-II. Tertiary amines. Chem. Eng. Sci. 1988, 43, 587-591. (8) Versteeg, G. F.; Oyevaar, M. H. The reaction between CO2 and diethanolamine at 298 K. Chem. Eng. Sci. 1989, 44, 1264-1268. (9) Little, R. J.; Versteeg, G. F.; van Swaaij, W. P. M. Kinetics of CO2 with primary and secondary amines in aqueous solutions-II. Influence on temperature on zwitterion formation and deprotonation rates. Chem. Eng. Sci. 1992, 47, 2037-2045. (10) Mandal, B. P.; Biswas, A. K.; Bandyopadhyay, S. S. Absorption of carbon dioxide into aqueous blends of 2-amino-2-methyl1-propanol and diethanolamine. Chem. Eng. Sci. 2003, 58, 41374144. (11) Donaldson, T. L.; Nguyen, Y. N. Carbon dioxide reaction kinetics and transport in aqueous amine membranes. Ind. Eng. Chem. Fundam. 1980, 19, 260-266. (12) Brenan, K. E.; Campbell, S. L.; Petzold, L. R. Numerical Solution of Initial-Value Problems in Differential-Algebraic Equations; North-Holland: New York, 1989. (13) Petzold, L. R. A Description of DASSL: A Differential/Algebraic System Solver in Scientific Computing; North Holland, Amsterdam, 1983; pp 65-68. (14) Mandal, B. P.; Kundu, M.; Bandyopadhyay, S. S. Density and viscosity of aqueous solutions of (N-Methyldiethanolamine + Monoethanolamine), (N-Methyldiethanolamine + Diethanolamine), (2-Amino-2-methyl-1-propanol + Monoethanolamine) and (2-Amino-2-methyl-1-propanol + Diethanolamine). J. Chem. Eng. Data 2003, 48, 703-707. (15) Mandal, B. P.; Kundu, M.; Padhiyar, N. U.; Bandyopadhyay, S. S. Physical solubility and diffusivity of N2O and CO2 into aqueous solutions of (2-amino-2-methyl-1-propanol + diethanolamine) and (N-methyldiethanolamine + diethanolamine). J. Chem. Eng. Data 2004, 49, 264-270. (16) Rinker, E. B. Acid gas treating with blended alkanolamines. Ph.D. dissertation, University of California, Santa Barbara, CA, 1997. (17) Mandal, B. P.; Bandyopadhyay, S. S. Simultaneous absorption of carbon dioxide and hydrogen sulfide into aqueous blends of 2-amino-2-methyl-1-propanol and diethanolamine. Chem. Eng. Sci. 2005, 60, 6438-6451. (18) Danckwerts, P. V.; Sharma, M. M. The absorption of carbon dioxide into aqueous amine solutions of alkalis and amines (with some notes on hydrogen sulphide and carbonyl sulphide). Chem. Eng. 1966, 10, CE244-CE280. (19) Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970. (20) Mandal, B. P.; Absorption of carbon dioxide and hydrogen sulfide into blended alkanolamines. Ph.D. dissertation, Indian Institute of Technology, Kharagpur, India, 2004.

Received for review March 19, 2006. Revised manuscript received June 14, 2006. Accepted July 21, 2006. ES0606475