Ind. Eng. Chem. Res. 1992, 31, 921-927

GENERAL RESEARCH

92 1

Kinetics Study on Absorption of Carbon Dioxide into Solutions of Activated Met hyldiet hanolamine

Guo-Wen Xu, Cheng-Fang Zhang,* Shu-Jun &in, and Yi-Wei Wang Research Institute of Inorganic Chemical Technology, East China University of Chemical Technology, Shanghai 200237, People's Republic of China

A disk column was used to investigate the kinetics of carbon dioxide absorption in activated me- thyldiethanolamine (MDEA) solutions which contain piperazine as an activator. Experiments were conducted within the temperature range 30-70 "C, MDEA concentration 1.75-4.21 kmol/m3, and piperazine concentration 0.0414.21 kmol/m3. The kinetic data agree with a proposed mechanism which can be regarded as a rapid pseudo-first-order reversible reaction between COz and piperazine in parallel with the reaction between C02 and MDEA. The absorption rate can be expressed as NCO = HCO,[DCO (k&, + k,Cp)11/2(p~o, - pco?), where k2 = 5.86 X lo6 exp(-3984/T) and k, = 2.98 x 1611 exp(-6424/~).

Introduction The BASF activated methyldiethanolamine (MDEA)

technology for recovery of carbon dioxide from gas mixture was developed in the 1970s, and it was well-known as a low-energy process (Meissner and Wagner, 1983). The energy consumption of the BASF activated MDEA tech- nology is only one-third that of conventional Benfield technology, and the activated MDEA solution is more stable, is not subject to degradation, is noncorrosive to carbon steel, and has a low minimizing solubility of hy- drocarbon.

In general, the overall reaction between COB and MDEA can be written as

COz + HzO + R3N F= R3NH+ + HC03- (1)

The first investigation of the kinetics of C02 reaction with MDEA was reported in the literature by Barth et al. (1981). They studied the kinetics of C02 reaction with MDEA by using a stopped-flow technology, and the con- tribution of MDEA to the absorption was assumed only through the neutralization of the species formed by carbon dioxide hydration.

Blauwhoff et al. (1983) studied the reaction between COP and various alkanolamines including MDEA at 25 "C. Their results showed a significant base catalytic effect of MDEA on hydration of COz.

In light of the findings by Blauwhoff et al. (1983), Barth et al. (1984) reexamined their previous results for the re- action of COP and MDEA (Barth et al., 1981). They found that their new results can be very satisfactorily interpreted by the mechanism supported by Blauwhoff et al. (1983).

Yu and Astarita (1985) have studied the kinetics of the reaction of carbon dioxide in aqueous MDEA solution in a range of MDEA concentrations from 0.25 to 2.5 kmol/m3 and temperatures from 40 to 60 "C. In this study, the authors concluded that MDEA acts as a homogeneous catalyst for COz hydrolysis and then proposed a possible zwitterion mechanism to account for the catalytic effect.

Versteeg and Van Swaaij (1988) studied the kinetics of the reaction between COz and aqueous MDEA solutions

0888-5885/ 921 263 1-092 1 $03.00/0

in a range of temperature from 20 to 60 "C. The results showed that the kinetics is in good agreement with the proposed reaction mechanism which is a pseudo-fmt-order irreversible reaction.

Wang (1988) studied the kinetics of C02 reaction with MDEA in a disk column at atmospheric pressure. The absorption rates of COP into aqueous MDEA solution were measured in a range of MDEA concentrations from 1.75 to 4.28 kmol/m3 and temperatures from 30 to 70 "C. All experimental data of absorption rates in those studies can be described by a relational expression for a rapid pseu- do-first-order reversible reaction.

(2)

(3) The activation energy E2 is 33.13 kJ/mol.

Tomcej and Otto (1989) investigated the rate of ab- sorption of COz into aqueous solutions of MDEA by using a single sphere absorber. Kinetic rate constants were calculated from the absorption rate data by a pseudo- first-order reaction mechanism.

An activator must be added into the absorbent in order to accelerate the absorption or desorption rate. Piperazine was used as an activator in the BASF activated MDEA technology (Appl et al., 1982). It was reported that pi- perazine would be more effective than the conventional absorption accelerators. In this paper a study on kinetics of COz absorption into solutions of MDEA activated by piperazine is presented.

Theoretical Analysis Absorption Reactions. MDEA has a base catalytic

effect on the C02 hydrolytic reaction. According to the zwitterion mechanism reported by Yu and Astarita (1985), MDEA may react with C02 in liquid film to form an unstable weakly bonded C02-nitrogen atom complex as follows:

(4)

Nco, = ~COz~~COz~z~am11'2@C02 - PCOZ*)

kz = 5.86 X lo6 exp(-3984/T) where

R3N + COZ - R3NCOO

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922 Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992

Then the hydrolytic reaction of R3NCO0 takes place in the liquid phase as follows:

(5) Reaction 5 is a homogeneous hydrolytic reaction in equilibrium, and reaction 4 is the dominant reaction of absorption of C02 into activated MDEA solution and generally can be regarded as rapid pseudo first order with respect to R3N if the partial pressure of C02 is not very high and the free concentration of MDEA is not very low.

Simultaneously, the activator piperazine may react with C02 in liquid film to form an intermediate as follows:

R’(NH)2 + 2C02 - R’(NHC00)2 (6) Reaction 6 is a rapid reaction which is parallel with re- action 2. The hydrolytic reaction of R’(NHC00)2 also takes place in equilibrium in the liquid phase as follows:

R’(NHC00)2 + 2Hz0 R’(NH2+) + 2HC03- (7) Equilibrium Relationship in the Liquid Phase.

Piperazine acta as an effective activator in MDEA solution. Its main contribution to absorption of C02 is the fact that C02 can be transferred through the intermediate R’-

R’(NHC00)2 + 2R3N + R’(NH)2 + 2R3NCO0 (8) R’(NHCOO)2 can be renewed to R’(NH)2 by reaction 8

R3NCO0 + H20 + R3NH+ + HC03-

to MDEA

in equilibrium. [R3NC00I2[R’(NH)2]

Ka = [R’(NHC00),][R3NI2

For reaction 5, the equilibrium relationship of R3NCO0 hydrolysis is

For reaction 7, the equilibrium relationship of R’- hydrolysis is

Combining eqs 9-11, we may obtain a relationship as follows:

Consider an aqueous solution initially containing C,” of MDEA and Cpo of piperazine; the conversions of MDEA to R3NH+ and piperazine to R’(NH2+I2 are yam and yp, respectively. We assume the intermediate concentrations of R,NCOO and R’(NHC00)2 are very low and can be neglected; then [R3NH+] = y,C,”, C, = [R3N] = (1 -

- y,)Cpo. Equation 12 may be rearranged to give y,)C,O, [R’(NH2+)21 = ypCpo, and Cp = [R’(NH),] = (1

2 1 - Y p 1 - Y , -=w(i..) YP

where W = K&b2/K, (14)

Equation 13 shows the relationship of conversions dis- pensed between MDEA and piperazine. Therefore, the total content of C02 absorbed in liquid is

x = y,c,o + 2ypcpo (15) When eqs 13 and 15 are combined, the conversions of

MDEA and piperazine can be obtained if X and W are fixed.

Absorption Rate. A kinetics study (Wang, 1988) in- dicated that the absorption of C02 into MDEA solution is a rapid pseudo-first-order reaction with respect to free MDEA. In activated MDEA solution, the concentration of free piperazine will be depleted if C02 does not transfer to MDEA. However, the rate of transfer reaction 8 is so fast that the concentration of free piperazine only depends on the equilibrium of reactions defined by eq 12 and is not subject to depletion. Therefore, the model of reaction can be regarded as two parallel rapid pseudo-first-order re- versible reactions and the overall reaction rate of carbon dioxide is

r = (k2C, + kpCp)(Cco, - CCO2*) (16)

where C, and C, are the free concentration of piperazine and MDEA, respectively; they can be obtained from eqs 13 and 15.

When the diffusion equation in liquid film is combined with a rapid pseudo-first-order parallel reversible reaction, the chemical absorption rate can be expressed as follows:

The absorption rate coefficient k which regards the partial pressure as the driving force can be expressed as follows:

or

Physical Properties The physicochemical properties needed to interpret the

absorption data are the solubility and diffusivity of C02 in aqueous activated MDEA solutions. Because the con- tent of added piperazine is very low, we use the values of Hco, and Dc in MDEA solution directly to substitute for activated M8EA solution. These physical properties of MDEA solution were previously reported by Haimour et al. (1987).

During the course of absorption, electrolyte ions, Le., HC03-, R3NH+, and R’(NH2+)2, would be formed and af- fect the solubility of C02. It can be estimated by the method of Van Krevelen and Hoftijizer (Danckwerts, 1970). In order to estimate the coefficient for R3NH+ and R’(NH2+)2, the value for NH4+ is used for R3NH+ and R’(NH2+),. The diffusivity data of C02 in aqueous MDEA was correlated by Haimour et al. (1987). The solution viscosity was measured by using a capillary viscosity meter. Partial pressure of water vapor and C02 in activated MDEA solutions was measured by Wang (1988).

Experimental Section A disk column, developed by Stephens and Morris

(1951), was used for the kinetic measurements. The sketch of the experimental setup is presented in Figure 1. The principal dimensions of the disk column are shown in Table I.

Carbon dioxide from cylinders with a purity of 99.0% was passed through a molecular sieve adsorber and then a soap film meter to measure the flow rate. After passing through a heating tube, the gas was introduced to the bottom of the disk column. The outgas flow rate was also measured by a soap film meter.

The activated MDEA solution was heated to the desired temperature and then pumped to the top of the disk

Ind. Eng. Chem. Res., Vol. 31, No. 3,1992 923

9 "':" 0 70 OC

9

e 8

h m

W

l n 4

L 3 s 2

1 3 ' Figure 1. Schematic diagram of the apparatus used for absorption experiments: 1, pump; 2, stirred flask; 3, magnetic stirrer; 4, liquid sample valve; 5, heating tape; 6, disk column; 7, molecular sieve adsorber; 8, COP cylinder; 9, soap film meter.

0 1-75 k m l / m 3 - A 3.04 kmol/m3 4.21 kml/m3 !%

Table I. The Principal Dimensions of the Disk Column number of disks disk diameter disk thickness tube diameter free space (dry) absorption surface mean perimeter for liquid flow equivalent diameter for gas flow

40 14.4 mm 4.3 mm 30 mm 85 % 0.02057 m2 35.97 mm 19.3 mm

column. After direct contact with the gas, the solution was introduced into a stirred flask. The solution was recycled to the column, so that the content of C02 absorption in solution increased with run time. We measured the ab- sorption rate at intervals of minutes and took liquid sample to analyze simultaneously. The absorption rate data at any content of C02 in solution can be obtained within a shorter run period and consume only a small amount of activated MDEA solution. The flow rate of solution was fixed at 5 L/h during all experiments.

The absorption rate of C02 was calculated from inlet and outlet flow rates of C02 measured by soap film meters. The concentration of C02 absorption in solution was an- alyzed by chemical method.

The MDEA concentrations in solution used in the ki- netic studies were 1.75, 3.04, and 4.21 kmol/m3, and the piperazine concentrations in solution were 0.041,0.10, and 0.21 kmol/m3. The experiments took place at 30,40,55, and 70 "C.

We measured the liquid-film mass-transfer coefficient of this disk column using a pure carbon dioxide-water system at 20 OC. Experimental results were well correlated as follows:

Results and Discussion Absorption Rate Coefficient k. The absorption rate

of C02 into activated MDEA solution was monitored under different conditions. Figure 2 shows the changes in ab- sorption rate coefficient k at various temperatures. The absorption rate coefficient increases with temperature, since both reaction rate constant and diffusivity increase with temperature. The absorption rate coefficient de- creases with increasing conversion for C,, C, and Hco, decreases with conversion.

Comparing the absorption rate coefficient of activated MDEA aqueous solution and that of MDEA aqueous so- lution without any piperazine given by Wang (1988), we find that a little piperazine will obviously improve the absorption rate. That means the higher the absorption

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924 Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992

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Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 925

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926 Ind. Eng. Chem. Res., Vol. 31, No. 3,1992

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Figure 4. Absorption rate coefficient vs concentration of piperazine in 4.21 kmol/m3 MDEA solution at 55 "C.

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140 - 120 - 100 - 80 -

0 0.10

0 0.21

7 I

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0.02 0.06 0.10 0.14 0.18 0.22

cP, kmol/m3

Figure 5. Apparent rate coefficient vs concentration of piperazine in 4.21 kmol/m3 MDEA solution at 55 "C.

Apparent Rate Constant k,,,,,. According to eq 19, the apparent rate constant k,, can be obtained from the values of k, Hcoz, Dcoz, k2, andPC,. The conversions of MDEA and piperazine are determined by eqs 13 and 15. In order to simplify the model, we assumed that W is hardly changed with temperature and can be regarded as a constant. Then we try to calculate the value of k, from W within the range 0.8-1.2. It is found that the experi- mental results are in good agreement with the proposed reaction mechanism if W = 1.0. The conversions and free concentrations of MDEA and piperazine corresponding to a certain content of COz in solution can be calculated according to eqs 13 and 15 by a trial and error method.

In this paper, we used the value of k 2 given by eq 3 (Wang, 1988). The calculation results are shown in Table 11.

The apparent first-order rate constants are calculated and listed in Figure 5. k o m Figure 5, a linear dependence of the apparent rate constant k,, with concentration of piperazine is observed. Therefore, it can be concluded that the reaction between piperazine and C02 is fmt order with respect to piperazine and is in good agreement with the proposed reaction mechanism.

Second-Order Rate Constant k,. The values of the second-order rate constant k, are shown in Figures 6 and 7. From Figures 6 and 7, k, is nearly constant at the same temperature under various concentrations of MDEA and piperazine. It can be concluded that the reaction between piperazine and COz is a rapid pseudo-fmt-order reversible reaction, and it is parallel with the reaction between MDEA and C02. An Arrhenius plot of the values of the

xya 6oo 500 - 0.04 0.08 0.12 0.16 0.20 0 .24 0

c , k m o l / J P Figure 6. k , vs concentration of piperazine at 55 "C.

P x 100

0.6 1.2 1.8 2.4 3.0 '.6 4.2

c,, kmol/m3

Figure 7. k, vs concentration of MDEA at 40 "C.

3000

2000

1000

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200

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2 r 4

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for k,.

// 0 2 4 6 8 10

k* I O 5 , kmol/( m2, s.Mpa)

Figure 9. Comparison between value of experiment and model.

second-order rate constant k, is shown in Figure 8. An activation energy E of 53.41 kJ/mol is indicated by the data. The fact of 4 > Ez shows that the higher the ab- sorption temperature, the higher the contribution of pi- perazine on absorption rate as shown in Figure 2.

The rate constant k, can be represented by

Experiment value

k, = 2.98 X 10" exp(-6424/T) (21)

According to eq 18 or 19, we use the values of k2 and k,

Ind. Eng. Chem. Res. 1992,31,927-934 927

to calculate the absorption rate coefficient and then com- pare it with the experimental values. The results are presented in Figure 9 with mean error no greater than 10%.

Acknowledmnent

p = corresponding to piperazine Registry No. MDEA, 105-59-9; COz, 124-38-9; piperazine,

110-85-0.

Literature Cited

This work was supported by the Science Foundation of National Education Committee of China.

Nomenclature C = concentration in liquid phase, kmol/m3 Co = initial component concentration, kmol/m3 C* = concentration in equilibrium condition, kmol/m3 D = diffueivity in liquid phase, mz/s E = activation energy, kJ/mol H = solubility coefficient, kmol/(m3.MPa) K,, Kb, K, = equilibrium constants k = absorption rate coefficient, kmol/ (m2.s.MPa) kL = mass-transfer coefficient in liquid film, m/s k2, kp = reaction rate constant of reactions 2 and 7, respec-

kzapp = apparent firsborder rate constant, kZapp = kzC,, k = apparent first-order rate constant, kp,app = kpCp, fi:Pabsorption rate, kmol/(m2.s) p = partial pressure, MPa p* = partial pressure in equilibrium condition, MPa r = reaction rate, kmol/(m3-s) T = absolute temperature, K X = content of COz absorbed in liquid phase, kmol/m3 y = conversion

Greek Symbols r = liquid flow rate per unit width of surface, kg/(ms) p = density of liquid, kg/m3 ec. = viscosity of liquid, Pa/s

Subscripts COz = corresponding to COz am = corresponding to MDEA

tively, m3/(kmol*s)

Appl, M.; Wagner, U.; Henrici, H. J.; Kuessner, K.; Voldamer, K.; Fuerst, E. Removal of C02 And/or H a And/or COS From Gases Containing These Constituents. U S . Patent 4,336,233, 1982.

Barth, D.; Tondre, C.; Lappai, G.; Delpuech, J. J. Kinetic Study of Carbon Dioxide with Tertiary Amines in Aqueous Solutions. J. Phys. Chem. 1981,85, 3660-3667.

Barth, D.; Tondre, C.; Delpuech, J. J. Kinetics and Mechanisms of the Reaction of Carbon Dioxide with Alkanolamines: A Discus- sion Concerning the Cases of MDEA and DEA. Chem. Eng. Sci. i984,39, i753-i757.

I

Blauwhoff, P. M. M.; Versteeg, G. F.; Van Swaaij, W. P. M. A Study on the Reaction Between CO, and Alkanolamines in Aaueous Solutions. Chem. Eng. Sci. 1983,38, 1411-1429.

Danckwerta, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970; p 19.

Haimour, N.; Bidarian, A.; Sandall, 0. C. Kinetics of the Reaction Between Carbon Dioxide and Methyldiethanolamine. Chem. Eng. Sci. 1987,42, 1393-1398.

Meissner, R. E.; Wagner, U. Low-energy Process Recovers C02. Oil Gas J. 1983, Feb, 55-58.

Stephens, E. J.; Morris, G. A. Determination of Liquid-Film Ab- sorption Coefficients. Chem. Eng. Prog. 1951, May, 232-242.

Tomcej, R. A.; Otto, F. D. Absorption of C02 and N20 Into Aqueous Solutions of Methyldiethanolamine. AIChE J. 1989,35,861-864.

Versteeg, G. F.; Van Swaaij, W. P. M. On The Kinetics between C02 and alkanolamines both in aqueous and non-aqueous solutions- 11. Tertiary Amines. Chem. Eng. Sci. 1988, 43, 587-591.

Wang, Y. W. Kinetics of C02 absorption in Activated Methyldi- ethanolamine. M.S. Thesis, East China University of Chemical Technology, 1988.

Yu, W. C.; Astarita, G. Kinetics of Carbon Dioxide Absorption in Solutions of Methyldiethanolamine. Chem. Eng. Sci. 1985, 40,

Received for review March 13, 1991 Revised manuscript received September 17,1991

Accepted November 8, 1991

1585-1590.

Diffusion of Phenylacetic Acid and Vanillin in Supercritical Carbon Dioxide

Tony Wells,* Neil R. Foster, and Rodney P. Chaplin School of Chemical Engineering and Industrial Chemistry, University of New South Wales, P.O. Box 1, Kensington 2033, N S W Australia

Binary diffusion coefficients, Ol2, of phenylacetic acid and vanillin have been determined in su- percritical carbon dioxide using the Taylor-Aris tracer response technique. DI2 values are reported for temperatures ranging from 308 to 318 K and densities between 600 and 850 kg/m3. The binary diffusion coefficients, D12, have a magnitude of m2/s. The influence of pressure, temperature, and carbon dioxide density on the D12 values is examined. The applicability of Stokes-Einstein based correlations and the free volume diffusion model is also evaluated.

Introduction flavor and fragrance industry as it is inexpensive, nontoxic, and nonflammable. Additionally C02 is an effective sol- The advantages of utilizing solvents in a supercritical vent at ne^ ambient conditions (30-60 oc), and conse- state to perform extractions and achieve separations are quently the for thermal degradation, character-

The potential for the use of SCFs in the extraction of The use of supercritical COz, the most commonly employed supercritical fluid (SCF), has considerable potential in the flavor and fragrance has in n u m e r o ~ in-

vestigations of their solubility in COz (e.g., Wells et al., 1990; Vitzthum and Hubert, 1978; Schutz et al., 1984). However, there has been little or no information published on the diffusion and mass-transfer characteristics of such

documented williams lg81; et '*, 1983)' istic of alternative distillation is minimized.

*Address correspondence to this author at Faculty of Chemical Engineering, Delft University of Technology, P.O. Box 5045,2600 GA Delft, The Netherlands.

0888-5885/92/2631-0927$03.00/0 0 1992 American Chemical Society