디에탄올아민수용액에서의 CO2의 반응속도론

박 문기, 서 동수*, 김 성수**, Orville C. Sandall***

경산대학교 환경학부, 부산대학교 화학공학과*, 지산대학 환경공학과**,

Dept. of Chemical Engineering, University of California, Santa Barbara, U.S.A.***

Kinetics of Carbon Dioxide Reaction with Aqueous Diethanolamine

Moon-Ki Park, Dong-Soo Suh*, Sung-Soo Kim**, Orville C. Sandall***

Dept. of Environ. Sci. & Eng., Kyungsan University, Kyungpook, Korea

Dept. of Chem. Eng., Pusan National University, Korea*

Dept. of Environmental Engineering, Jisan College, Pusan, Korea**

Dept. of Chemical Engineering, University of California, Santa Barbara, U.S.A.***

Introduction

The reaction between CO2 and aqueous DEA has been the subject of many experimental studies.  However, there are widely varying opinions in the literature regarding the mechanism of this reaction. The majority of the studies listed in their review reported either first- or second-order kinetics with respect to the DEA concentration, while a couple of studies reported fractional orders between 1 and 2 with respect to the DEA concentration.  All but one of these studies (Savage and Kim, 1985) report fractional orders between 1 and 2 with respect to the DEA concentration.

Theoretical

The carbamate to bicarbonate reversion reaction is very slow and the concentration of carbamate is small over the short contact times of our experiments, the contribution of reaction to the rate of absorption is negligible.  Secondly, since unloaded aqueous DEA solutions were used in the experiments and since the gas-liquid contact times were very short, the concentrations of bicarbonate and carbonate in the aqueous solutions are very small and the contributions of these species to the deprotonation of the zwitterion are also negligible. Consequently, the following rate expression is obtained for the consumption of CO2 according to the zwitterion mechanism with DEA being the only base that deprotonates the zwitterion:

[eqn: r_CO_2 = -k_4 over {1+ 1 over {(k_5 /k_-4 ) [DEA]}} left( [CO_2 ][DEA]- 1 over {K_4 K_5} {[DEAH^+ ][R_2 NCOO^- ]} over [DEA] right )] (1)

Note that (K4K5) is the equilibrium constant of the overall reaction between CO2 and DEA:

[eqn: CO_2 + 2R^(1) R^(2) NH buildrel <-> {scale 80 K_4 K_5} R^(1) R^(2) NH_2 ^+ + R^(1) R^(2) NCOO^-]              (2)

and K4K5 = k4k5 / k-4k-5, R(1)=-CH3 and R(2)=-CH2CH2OH.

In the analysis of the experimental data, the apparent second-order rate coefficient which is defined by the following equation:

[eqn: k_app = k_4 over {1 + 1 over {(k_5 /k_-4 ) [DEA]}}]                                         (3)

was estimated for each data point at a given temperature by adjusting its value in the numerical model until the predicted rate of absorption was within 1% of the measured rate of absorption.  Equation (3) was rearranged into the following form:

[eqn: 1 over k_app = 1 over k_4 + {1 over {(k_4 k_5 /k_-4 ) [DEA]}}]                                     (4)

Experimental

The laminar liquid jet absorber used in this work is similar to that used by Al-Ghawas et al. (1989) for measuring diffusion coefficients of CO2 in alkanolamine solutions. The operation of the laminar jet absorber is identical to the operation of the wetted sphere absorber described above.  The contact times which we obtained in the laminar liquid jet apparatus ranged from 0.5x10-3 to 1x10-2 seconds.

In this work, bone dry grade CO2 which was obtained from Linde Specialty Gases was used.  The DEA was obtained from Fisher Scientific with a minimum purity of 99% and was used without any further purification.  The aqueous DEA solutions were prepared by weight with deionized water and were degassed at elevated temperatures under vacuum.

As a check of the apparatus, the diffusion coefficient of CO2 in water was determined by measuring CO2 absorption rates into water for various contact times and at temperatures of 293 and 313 K.

Results and Discussion

Table 1. Zwitterion rate constant estimates
Reference	T (℃)	lnk4 (m3/kmol.s)	ln(k4k5/k-4) (m6/kmol2.s)
this work	20 25 30 40 50 60 70	8.169 8.316 8.541 8.615 8.697 8.777 9.193	7.000 7.034 7.321 7.388 7.931 8.058 8.520
Versteeg and Oyevaar(1989)	25	8.061	6.579
Littel et al. (1992)	30 45	8.380 8.897	7.170 7.215

The parameters k4 and k4k5/k-4 were determined by fitting 1/kapp vs. 1/[DEA] by means of linear least-squares regression at each temperature.  Although the experiments can be considered to be pseudo-first-order, the cup-mixed concentration of unreacted DEA did change by as much as 5%.  As a result, the average of the inlet and outlet cup-mixed concentrations of unreacted DEA was used in the fitting of kapp.

The estimates of k4 and k4k5/k-4 from this work are listed in Table 1 and are plotted in Figures 2 and 3 respectively.  Also included in Table 1 and Figures 2 and 3 are the estimates of k4 and k4k5/k-4 which were reported by Versteeg and Oyevaar (1989) and Littel et al. (1992) for comparison.  The estimates of k4 reported by Versteeg and Oyevaar (1989) and Littel et al. (1992) show a much stronger temperature dependence than the estimates from the present study, but there is fair agreement in the magnitude of the numbers.  Similarly, the values of k4k5/k-4 reported by these authors are in fair agreement with the present data, but their values show a different temperature dependence and are much more scattered.

The rate coefficient estimates from this work were fitted as functions of temperature by the following Arrhenius equations:

[eqn: lnk_4 = 6.093 - 1544 over T]                                               (5)

[eqn: ln{k_4 k_5} over k_-4 = 14.031 - 2741 over T]                                         (6)

which fit the kinetic data from this work to within 16% and 19%, respectively.  Littel et al. (1992) reports a deviation of about 40% for their Arrhenius fit of k4k5/k-4.  The large scatter observed in this work and the work of Littel et al. (1992) is probably due to the ill-conditioned nature of the zwitterion rate expression.  The estimated values of k4 and k4k5/k-4 are very sensitive to experimental errors in the apparent rate coefficient, especially at low DEA concentrations.

Conclusion

There has been much discussion in the literature regarding the discrepencies in aqueous DEA kinetics with CO2 obtained from different experimental techniques.  Littel et al. (1992) point out that nearly all experimental methods have given surprisingly similar results for aqueous primary and tertiary amines, but for secondary amines there are significant differences between results obtained from different experimental techniques. Our experimental technique is different from that used by Littel et al. (1992), yet our results show good agreement with theirs.  In the interpretation of our experimental absorption data, we have included the effects of parallel and consecutive reversible reactions by using a rigorous numerical mass transfer model.

References

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3. Littel, R.J., van Swaaij, W.P.M., and Versteeg, G.F., 1990, Kinetics of Carbon

Dioxide with Tertiary Amines in Aqueous Solution.  A. I. Ch. E. J., 36,

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