874 Ind. Eng. Chem. Res. 1995,34, 874-880

Desorption of COz from MDEA and Activated MDEA Solutions

Guo-Wen Xu,* Cheng-Fang Zhang, Shu-Jun &in, and Bin-Chen Zhu Research Institute of Inorganic Chemical Technology, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People$ Republic of China

A packed column was used for investigating the desorption rate of C02 from aqueous methyldiethanolamine (MDEA) and activated MDEA solutions. Experiments were conducted within the temperature range 30-70 "C, the concentration of MDEA was 4.28 km01/m3, and the concentration of piperazine (PZ) was 0.10 km0Vm3 for aqueous activated MDEA solutions. Experimental data confirmed that the kinetics model of absorption COz into aqueous MDEA and activated MDEA solutions can be applicable to the situations in which desorption occurs, and the desorption rate of model predictions agree well with that of experimental determination.

Introduction The BASF activated methyldiethanolamine (MDEA)

technology for recovery of C02 from gas mixtures was developed in the 1970s, and it was well-known as a low- energy-consumption process (Meissner and Wagner, 1983). Research work about absorption of C02 in aqueous MDEA solutions has been engaged in measur- ing physicochemical properties (Haimour, 19871, deter- mining the equilibrium data (Jou, 1982; Chakma and Meisen, 1987; Xu et al., 19931, and studying the absorp- tion kinetics of C02 in aqueous MDEA solutions (Barth et al., 1981, 1984; Blauwhoff et al., 1984; Yu and Astarita, 1985; Versteeg and Van Swaaij, 1988; Wang et al., 1991; Tomcej and Otto, 1989; Littel et al., 1990).

The kinetics of absorption of COS in activated MDEA solutions which contain piperazine (PZ) as an activator was investigated using a disc column (Xu et al., 1992). Experiments were conducted within the temperature range 30-70 "C, MDEA concentration 1.75-4.21 kmoll m3, and PZ concentration 0.041-0.21 kmol/". The kinetics data agree well with a proposed mechanism which can be regarded as a rapid pseudo-first-order reversible reaction between CO2 and PZ in parallel with the reaction between C02 and MDEA.

Shah and Sharma (1976) reviewed the general theory of gas desorption. Astarita and Savage (1980a) dis- cussed the much more prevalent case of chemical desorption using a film model. Astarita and Savage (1980b) then gave a more detailed analysis of the instantaneous reaction case.

Bosch et al. (1990) studied the desorption of Cog and H2S from loaded MDEA and monoethanolamine (MEA) solutions at various temperatures in a stirred cell reactor. Experiments and analysis show that the theory of absorption with a reversible chemical reaction can also be applied to desorption. Chakravarty et al. (1985), Critchfield and Rochelle (19871, Glasscock and Rochelle (19901, and Glasscock et al. (1991) studied C02 absorp- tion or desorption in mixtures of MDEA with monoeth- anolamine (MEA) or diethanolamine (DEA).

Glasscock et al. (1991) presented a compilation of data and model interpretation of C02 absorptioddesorption with mixtures of MDEA, MEA, and DEA. The data presented included both absorption and desorption conditions and temperatures ranging from 288 to 313 K. The results indicated that the combined mass transfer/equilibrium model can effectively represent C02 mass transfer rates for the mixtures MEA/MDEA and DEA/MDEA under a wide range of conditions.

However, the published fundemental research of desorption of COS from aqueous activated MDEA solu-

OS88-5885/95/2634-0874$09.QQIQ

tions has not been reported so far. In this paper, the desorption of C02 from aqueous MDEA and activated MDEA solutions has been investigated in a experimen- tal packed column, in order to test the applicability of the kinetics model for absorption in which desorption occurs.

Theoretical Analysis

Donaldson and Nguyen (1980) proposed the following reaction mechanism for the reaction of CO2 with MDEA

R3N + CO, + H 2 0 = HC03- + R3NH+ (1)

This reaction mechanism is essentially a base cataly- sis of the C02 hydration reaction and it can be divided into two steps:

First, MDEA combines with CO2 in a liquid film to form an unstable weakly boned COz-nitrogen atom complex as follows:

(2) Then the hydrolytic reaction of R3NCOO takes place

R3NCO0 + H,O = R3NH+ + HC03- (3) Because of the concentration of MDEA in bulk phase

is high, the concentration of MDEA in liquid film is nearly equal to that of bulk phase if the partial pressure of C02 is not very high and the conversion of MDEA is not large. The reaction with respect to MDEA can be regarded as pseudo-first-order and the reaction rate can be expressed as

(4) The activated MDEA solution is composed of aqueous

MDEA solutions and a little activator, PZ PZ can react directly with C02 as follows:

(5) The hydrolytic reaction of R(NHC00)2 takes place

R(NHCOO), + 2H20 = R'(NH2+), + 2HC0,- (6)

Compared with that of MDEA, the concentration of PZ is very low. If the reaction of PZ with C02 is only in parallel with that of MDEA with Cog, a little PZ must be depleted in the liquid film. Experiments found that a little PZ would obviously promote both the absorption rate and desorption rate. Therefore, PZ acts as a

0 1995 American Chemical Society

R3N + CO, = R3NCO0

in the liquid phase in equilibrium as follows:

rnl= k2Cm (Pco, - PCO,*)

R(NH), + 2C02 = R(NHCOO),

in equilibrium in the liquid phase as follows:

Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 875

homogeneous activation mechanism as the results of an earlier study show (Xu et al., 1992). The COS can transfer rapidly from R(NHC00)z to R3NCOO; that is, a simultaneously reversible reaction exists everywhere in the liquid phase and liquid film.

R(NHCOO), + 2R3N = R(NH), + 2R3NCO0 (7) Combining equilibrium eqs 3,6, and 7, a relationship

can be obtained:

where

= (K~K,,VK, (9) It has been proved by the experiments of and absorp-

tion study (Xu et al., 1992) that the value of w is near 1. Equation 8 shows the relationship of conversions dispensed between MDEA and PZ. The total content of C02 absorbed in liquid is

(10)

According to eq 8; the concentration of free PZ is dependent on the conversion of MDEA and the initial concentration of PZ. If the conversion of MDEA is defined at every point in the liquid film, then the conversion of PZ is fixed, and its concentration is not being depleted. Therefore the reaction of free PZ with C02 can be regarded as pseudo-first-order in parallel with that of free MDEA with C02. If conversion of MDEA and PZ is constant in the liquid film, then the reaction rate of activated MDEA can be expressed as

The above reaction mechanism has been used in an absorption study of C02 in activated MDEA solutions in a disc column (Xu et al., 1992). The kinetics data agree well with the foregoing proposed mechanism which can be regarded as a rapid pseudo-first-order reversible reaction between C02 and PZ in parallel with the reaction between C02 and MDEA.

The present study is for testing the applicability of the reaction model to desorption of C02 from MDEA and activated MDEA solutions in an experimental packed column.

Astarita (1983) analyzed the process of chemical desorption and found that chemical absorption theory can be applied to chemical desorption in a fast reaction regime, provided the chemical reaction which takes place during absorption has a forward rate which is linear in the transferring component's concentration.

The differential equations for desorption circum- stances in a liquid film combined with a rapid pseudo- first-order reaction is

for aqueous MDEA solutions

(12)

For aqueous activated MDEA solutions

The two differential equations can be solved by the condition of pseudo-first-order reaction with constant

m

Figure 1. Schematic diagram of the experimental setup: 1, molecular sieve adsorber; 2, soap film meter; 3, mixture tube; 4, saturation flask 5, packed column; 6, cooler; 7, flowmeter; 8, high- level storage; 9, pump; 10, tank.

CA* and the rate of desorption can be obtained:

For aqueous MDEA solutions,

(14)

For aqueous activated MDEA solutions,

Experimental Apparatus Description The main experimental device was a glass-packed

column with a water jacket and an inside diameter of 35 mm and which was packed with 4 mm ceramic Rasching rings. The packed height was varied between 25 and 100 cm. The sketch of the experimental setup is given in Figure 1.

Gas (N2 or C02) from cylinders with a purity of 99% passed through a high-pressure regulator followed by a low-pressure regulator in order to keep the flow rate stable. Gas passed through a molecular sieve adsorber and was then metered by a soap film meter. After passing through a gas-mixture tube, the mixture gas was heated to the desirable temperature and simulta- neously saturated. Gas was introduced into the bottom of the experimental column and contacted with down- ward liquid in the column. The outgas was cooled to room temperature, and then its flow rate was metered by a soap film meter.

The solution was stored in the feed vessel and pumped to a high level storage for steady control of liquid flow rate. The solution was metered by a rotameter and heated to the desirable temperature and then was introduced into the top of the column. The solution was well distributed by a multibarred distributor. The outlet solution from the bottom of the column was cooled and stored in another vessel.

The content of carbon dioxide in the solutions was determined by addition of an excess of sulfuric acid into the solutions and then measurement of the evolved volume of C02 by a graduated buret. The temperature and pressure corrections were incorporated. The con- tent of C02 in mixture gas was determined by an absorption device.

This apparatus can be used for the purpose of absorp- tion and desorption measurements. The gas phase may be N2, C02, or mixtures of N2 and C02 in the different experimental runs.

876 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995

Table 1. Experimental Results for Effective Interfacial Area

1.603 4.318 5.785 6.328 1.075 8.0 128.2 1.020 13.08 2.256 4.626 6.472 7.344 1.288 10.5 140.3 1.068 14.98 2.798 4.936 6.863 7.623 1.422 12.0 147.4 1.087 16.02 4.152 5.452 7.641 8.486 1.780 15.5 165.0 1.104 18.22 5.601 5.941 7.780 8.924 1.911 18.0 170.9 1.150 19.65

Table 2. Experimental Results for Measuring KJ# r, NA x 104, (Ci* - CL)T x IO', (Ci* - CL)B x IO2, ACm x lo2, km x lo3,

kg/(m2-s) kmol/(m3-s) kmol/m3 kmol/m3 km0Um3 11s 1.596 1.789 2.786 2.558 4.133 3.502 5.577 3.775 6.957 4.475

a T, top of column; B, bottom of column.

2.830 2.830 2.830 2.830 2.830

Correction for Characteristics of the Experimental Packed Column Effective Interfacial Area

The effective interfacial area a in the experimental packed column was determined by the method of chemi- cal absorption of pure C02 in a pseudo-first-order irreversible reaction. The absorbent was 0.5 km01/m3 Na2C03-0.5 kmol/m3 NaHC03 carbonate buffer solu- tions containing NaClO as catalyst. The concentration of catalyst ranged from 0.005 to 0.02 kmol/m3. This method has been used by Richard (19641, Danckwerts and Gillham (19661, and Joosten and Danckwerts (1973).

For the pseudo-first-order irreversible reaction, the absorption rate is

(16)

The value of kl was obtained in a wetted-wall column

k, = 0.90 + 1582Ccl0- (17)

Supposing we varied Cclo-, keeping all else constant, and then measured the values of Nco2 with Cclo-. Then, plottingNco,2 against k1, we could get a straight line of slope ( a C ~ o ~ * ) ~ D c o ~ and intercept (aC*co&~)~. From the slope and intercept the value of effective interfacial area a and liquid-side mass transfer coefficient K L can be obtained with known Cco2* and Dco2.

The experiments for measuring the effective interfa- cial area were carried out a t the condition of atmo- spheric pressure at 25 "C. The liquid phase was carbonate buffer solutions and the range of liquid flow rate was from 1.6 to 5.6 kg/m2.s. The gas phase was pure C02, and the absorption rate was determined by the loss of flow rate of pure C02 through the packed column. The packed height was 70 cm. The experi- mental results are shown in Table 1.

It was found that the effective area was increased with liquid rate as shown in Figure 2. They are well correlated by the equation

u = ii5.51-O.~~ (18)

at 25 "C, and it can be expressed as

Liquid-Side Mass Transfer Coefficient

The most simple and effective method of measuring the liquid-side mass transfer coefficient was absorbing

1.59 1.50 1.55 1.68 1.59

2.15 8.32 2.10 12.21 2.13 16.44 2.21 17.08 2.15 20.81

180 t I

I 120 I I 1 I

0 2 4 6 8

r,kg/mZ. s Figure 2. Effective area vs liquid rate.

pure CO2 by distilled water. The absorption rate can be expressed as

(19)

where ACm is the log mean driving force. The experiments were carried out at the condition of

atmospheric pressure at 30 "C. The liquid flow rate was varied from 1.6 to 7.0 kg/(m2*s). The packed height of column was 25 cm. The experimental results are shown in Table 2.

The mass transfer coefficient km can be expressed as a function of liquid flow rate r, and is well correlated by the equation

(20)

Figure 3 shows the comparison of ka of Table 1 and Table 2. It can be seen that the value of k ~ u resulting from chemical absorption experiments is higher than that of pure physical absorption experiments. This is the reason that the wetted area is more effective in chemical absorption with pseudo-first-order reaction and less effective in pure physical absorption.

kLu = 6.46 x io-3r0.6043

Gas-Side Mass Transfer Coefficient

The convenient way to determinate the value of k, is absorption of lean carbon dioxide in aqueous solutions of sodium hydroxide. Wales (1966) has determined both a and k, by the above method. In this paper, we used this method to measure the value of K, in the case of known effective interfacial area.

Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 877 Table 3. Experimental Results for Measuring k,

ci, x 102, Cout x 102, H a x lo7, NA x 104, k, x lo7, G, l/h t , "C km0Um3 km01/m3 PA, kF'a kmo10~V("%;kPa) kmoU(m3.s) kmoU(mWsPa) 25 16.0 4.68 2.22 50 17.5 4.68 1.61 75 19.0 4.68 1.34 100 19.3 5.78 2.25 125 23.5 4.68 1.14

5.44 6.56 6.98 6.48 6.55

20

16

m 'r

"-& 12

%J

t-

d Y

8

0 2 4 6 a .. P,kg/mc.s

Figure 3. Comparison of km between physical (0) and chemical (0) experiments.

0 50 100 150

G,l/h Figure 4. Value of k, vs G.

Dilute COS was absorbed into aqueous solutions of sodium hydroxide. Provided the reaction is pseudo-first- order in COz, the absorption rate is given:

(21) Pco, 1 --- - I + NCO, %P a~JG

It is convenient to obtain the gas-side mass transfer coefficient under various gas flow rates, according to the above equation.

Experiments were carried out at room temperature. The liquid flow rate of aqueous solutions of sodium hydroxide was fixed at 2.8 kg/(m2*s). The concentration of NaOH was varied from 0.02 to 0.08 kmol/m3. The mixture of Nz and C02 was used, and the content of CO2 before entering the bottom of column was about 10 mol % in the course of experiments. The absorption rate was determined by the concentration difference of NaOH between the input and the output. The absorp- tion driving force is the log mean partial pressure of COS between that of the top and the bottom of column. The experimental results are shown in Table 3.

11.93 1.38 7.93 12.45 1.72 9.45 12.68 1.87 10.61 12.69 1.97 11.36 13.36 2.43 13.00

Table 4. Experimental Data of Desorption of COz from MDEA Solutions

~ ~ ~~ ~~ ~ ~~

XUI, xout, no. T, K kmol/m3 kmoUm3 Yin Yout 1 313 1.674 1.643 0 0.051 2 313 1.391 1.345 0 0.043 3 313 1.130 1.113 0 0.029 4 313 0.707 0.669 0 0.013 5 313 0.549 0.517 0 0.005 6 328 1.507 1.412 0 0.130 7 328 1.253 1.241 0 0.095 8 328 1.147 1.088 0 0.081 9 328 1.027 1.021 0 0.075 10 328 0.737 0.668 0 0.053 11 343 0.762 0.592 0 0.145 12 343 0.662 0.546 0 0.115 13 343 0.585 0.547 0 0.082 14 343 0.495 0.417 0 0.061 15 343 0.446 0.384 0 0.050

Table 5. Experimental Data of Desorption COz from Activated MDEA Solutions

~ ~

XUl, xout, no. T, K kmol/ms km0Um3 YIn Yout 16 313 1.904 1.855 0 0.072 17 313 1.849 1.831 0 0.069 18 313 1.699 1.658 0 0.056 19 313 1.208 1.207 0 0.041 20 313 0.918 0.908 0 0.028 21 328 1.602 1.375 0 0.153 22 328 1.477 1.335 0 0.149 23 328 1.337 1.200 0 0.128 24 328 0.839 0.791 0 0.081 25 328 0.619 0.588 0 0.049 26 343 0.963 0.755 0 0.252 27 343 0.783 0.647 0 0.208 28 343 0.713 0.571 0 0.166 29 343 0.458 0.365 0 0.086 30 343 0.354 0.301 0 0.058

The K , change with the G is shown in Figure 4. From the results of experiment, we suggested that the gas- side coefficient K , in the experimental packed column may be given by an expression of the type

(22)

Results and Desorption Experiments in the Packed Column

Desorption experiments were performed unders the condition of aqueous MDEA solutions and aqueous activated MDEA solutions being in counterflow contact with inert N2 in the packed column. Experiments were carried out at temperatures from 40 to 70 "C, and the conversion of MDEA was from 0.07 to 0.45. The concentration of MDEA was 4.28 km01/m3, and the concentration of PZ was 0.10 km0Vm3 for activated MDEA solutions. The height of the packing was 40 cm. During the experiments, the liquid flow rate was fixed at 4.15 kg/(mz*s) and gas flow rate was fixed at 75 Llh (room temperature).

878 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995

Table 6. Desorption Rate of COS from MDEA Solutions

C a m H J ~ x 107, KG x 107, N~~ x 104, N~~~ x 104, kmoV(m&kPa) kmol/(m%*kPa) APm, kPa kmoY(mL) kmol/(m3*s) no. Yam kmol/m3

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15

Table 7.

0.3874 2.622 0.2350 0.2296 0.3196 2.912 0.2685 0.2615 0.2620 3.159 0.2996 0.2908 0.1608 3.592 0.3604 0.3479 0.1220 3.758 0.3861 0.3717 0.3609 2.735 0.3230 0.3124 0.2914 3.033 0.3703 0.3564 0.2610 3.163 0.3924 0.3769 0.2393 3.256 0.4089 0.3921 0.1690 3.556 0.4657 0.4439 0.1573 3.603 0.5890 0.5532 0.1406 3.676 0.6090 0.5708 0.1063 3.824 0.6511 0.6076 0.0968 3.865 0.6632 0.6181 0.0817 3.929 0.6825 0.6349

Desorption Rate of COz from Activated MDEA Solutions

25.73 18.85 13.08 3.88 1.94

56.54 36.08 30.70 26.77 14.28 35.68 28.65 15.86 13.60 9.78

1.069 0.893 0.594 0.262 0.100 2.835 1.992 1.672 1.539 1.062 3.077 2.358 1.621 1.179 0.955

0.975 0.813 0.628 0.223 0.119 2.914 2.122 1.909 1.732 1.046 3.257 2.698 1.590 1.387 1.024

Cam, C,, HJD(L,C, + k C ) x lo7, k~ x lo7, N~~ 104, N~~~ x 104, no. Yam yD kmol/m3 kmol/m3 kmol/(m4Wa) kmol/(m%AGa) AP,, kPa kmol/(m3.s) kmol/(m3.s) 18 0.4215 0.3467 2.476 0.065 0.2680 17 0.4143 0.3434 2.507 0.067 0.2726 18 0.3795 0.2722 2.656 0.073 0.2950 19 0.2763 0.1272 3.098 0.087 0.3652 20 0.2103 0.0662 3.380 0.093 0.4126 21 0.3375 0.2060 2.836 0.079 0.4530 22 0.3198 0.1810 2.911 0.082 0.4705 23 0.2894 0.1423 3.041 0.086 0.5054 24 0.1880 0.0509 3.476 0.095 0.6044 25 0.1398 0.0257 3.682 0.097 0.6562 26 0.1974 0.0570 3.435 0.094 0.7928 27 0.1648 0.0375 3.574 0.096 0.8427 28 0.1480 0.0293 3.648 0.097 0.8687 29 0.0951 0.0110 3.872 0.099 0.9532 30 0.0761 0.0067 3.954 0.099 0.9851

Experimental Results The experimental data for aqueous MDEA and acti-

vated MDEA solutions are shown in Table 4 and Table 5 , respectively. The volume of packing is 3.848 x m3.

During the desorption operation, the differences of component concentration and temperature in the liquid between the top and bottom of column were not large. Therefore, we may simply calculate the desorption rate using the arithmetic mean values of conversion and temperature.

In desorption experiments, the inert gas N2 was used. Thus the gas-side resistance existed and the value of gas-side mass transfer coefficient according to eq 22 was in the range of (9-10) x

According to the desorption model, the total mass transfer coefficient KG is

for aqueousMDEA solutions

kmol(m3-kpa*s).

(23)

for aqueous activated MDEA solutions

(24) KG kg HJD(k,C,, + k,CJ

Because the content of added PZ is very low, the values of H and D in aqueous MDEA solutions were directly substituted for those in aqueous activated MDEA solutions. These physical properties of MDEA solutions were previously reported by Haimour et al. (1987).

- _ - L 1 + 1

0.2610 30.56 1.543 1.316 0.2653 29.67 1.474 1.299 0.2865 26.00 1.184 1.299 0.3523 14.76 0.839 0.858 0.3962 7.82 0.564 0.511 0.4324 51.80 3.414 3.696 0.4483 43.72 3.335 3.234 0.4754 35.97 2.773 2.821 0.5683 17.07 1.677 1.601 0.6139 9.19 0.980 0.930 0.7293 54.14 8.107 8.515 0.7713 37.26 4.765 4.742 0.7930 30.62 3.599 4.007 0.8628 12.30 1.696 1.751 0.8888 7.94 1.122 1.164

The values of kp and k, were reported by Wang et al. (1991) and Xu et al. (19921, respectively.

k, = 5.86 x lo6 exp(-3984/T)

k,, = 2.98 x 10l1 exp(-6424/T) (26)

The values of yam, y,, C,, and Cam were the log mean value between that of liquid in and that of liquid out.

The model value of desorption rate can be given:

(27) where AP, is log mean driving force; a is defined by eq 18.

For the desorption of COS in aqueous MDEA solu- tions, the rate values of experimental and model are listed in Table 6. From Figure 5 , the experimental desorption rate value agreed well with the model value, with mean error no greater than 10%. This indicates that the kinetics of absorption of COn in aqueous MDEA solutions is also applicable to desorption situations.

According to the desorption kinetics of activated MDEA solutions, the reaction between C02 and PZ is parallel with the reaction between C02 and MDEA, and there is a relationship of conversions dispensed between MDEA and PZ. Absorption experiments in a disc column (Xu et al., 1992) assumed that w is hardly changed with temperature and can be regarded as a constant (w = 1.0). In the present work, we calculated the conversions and free concentrations of MDEA and PZ at a certain content of C02 in solutions according t o eq 8 and eq 10 by a trial and error method. The calculated desorption rate is listed in Table 7.

Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 879

k z , k , = second-order rate constant, m3/(kmol*s) N = rate of absorption or desorption, kmol/(m3d p = pressure, W a Re = Reynolds number = Ge/dp Sc = Schmidt number = pleD Xco2, Xi,, Xout = content of COz absorbed in liquid, kmov

Yin, Yout = mole fraction of COZ yam, y , = conversion, mol of COz/mol of MDEA T = temperature, K Greek Symbols

r = liquid flow rate, kg/(m2*s) e = density of liquid, kg/m3 p = viscosity, kg4m.s)

Subscripts

am = methyldiethanolamine cal = calculation exp = experiment G, g = gas L = liquid p = piperazine

Superscripts

* = equilibrium at the interface

Amine Abbreviations

DEA = diethanolamine MDEA = methyldiethanolamine MEA = monoethanolamine PZ = piperazine

m3

0 1 2 3 4

Neal * 0 4 , k m a (m'. 8 )

Figure 5. Comparison of desorption rate between experiment and calculation for MDEA solutions.

0 2 4 6 8

Figure 6. Comparison of desorption rate between experiment and calculation for activated MDEA solutions.

From Figure 6 it has been found that the experimen- tal results are in good agreement with the proposed reaction mechanism, and the value of the experimental rate is close to the value of the desorption model with mean error no greater than 10%. This indicates that the kinetics model of absorption is suitable to that of desorption under experimental conditions. On the other hand, desorption experiments proved that the kinetics model of absorption CO2 into aqueous activated MDEA solutions was correct.

Acknowledgment

Science Foundation of People's Republic of China. This work was supported by the National Natural

Nomenclature a = effective interfacial area, l/m C = concentration in liquid phase, km01/m3 D = diffusivity in liquid phase, mzls G = gas flow rate, m3/s H = solubility coefficient, km0Y(m3;kPa) K,, Kb, K, = equilibrium constant of reaction 7, 3, and 6,

KG = absorption rate coefficient, kmol/(m"s.kPa) k, = mass transfer coefficient of gas side, kmol/(m2s.kPa) k~ = mass transfer coefficient of liquid film, d s kl = first-order rate constant, l/s

respectively

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(61, 1166-1171.

42 (4), 466-474.

Received for review March 15, 1994 Revised manuscript received October 12, 1994

Accepted October 31, 1994@

IE9401591

@ Abstract published in Advance ACS Abstracts, February 1, 1995.