I n d . Eng. Chem. Res. 1988,27, 2237-2241
equations. Furthermore, a good relationship was obtained between the frequency factor of the chain propagation reaction and the molecular structure of the organic compound. It can therefore be concluded that these empirical findings would make it possible to evaluate the activation energy and the frequency factor for a steady-state autoxidation reaction.
Nomenclature A , = frequency factor of reaction rate for the chain propagation, L/(mol-s) D[R-HI = carbon-hydrogen bond dissociation energy in the organic compound, kJ/mol D[ROO-HI = oxygen-hydrogen bond dissociation energy in hydroperoxide, kJ/mol E = activation energy, kJ/mol E , = activation energy for the chain propagation reaction, kJ/mol k , = rate constant of chain propagation, L/(mol.s) k , = rate constant of chain termination, L/(mol-s) Pi = rate of chain initiation, mol/(L.s) AH = enthalpy change, kJ/mol AH, = enthalpy change for the chain propagation reaction, kJ/mol Literature Cited Bateman, T. “Olefin Oxidation”. Q.R. Chem. SOC.1954, 8, 147. Benson, S. W.; Shaw, R. “Thermochemistryof Oxidation Reaction”. Adv. Chem. Ser. 1968, 75, 288. Bolland, J. L. “Kinetics of Olefin Oxidation”. Q.Rev. Chem. SOC. 1949, 3, 1.
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Bolland, J. L. “Kinetic Studies in the Chemistry of Rubber and Related Materials”. Trans. Faraday Soc. 1950, 46, 358. Golden, D. M.; Benson, S. W. “Free-Radicaland Molecule Thermochemistry From Studies of Gas-Phase Iodine Atom Reactions”. Chem. Rev. 1969,69, 125. Howard, J. A. “Absolute Rate Constants for Reactions of Oxy1 Radicals”. Adv. Free Radical Chem. 1972,4, 85-109. Howard, J. A.; Ingold, K. U. “Absolute Rate Constants for Hydrocarbon Autoxidation”. Can. J. Chem. 1965,43,2729; 1967,45,785; 1968,46, 2655. Howard, J. A.; Ingold, K. U.; Symonds, M. Can. J. Chem. 1968,46, 1017. Kamiya, Y. Yuki Sanka hanno; Gihodo: Tokyo, 1973; (a) pp 35-37, (b) P 45, (c) P 50, (d) P 59, (e) P 200, (0 P 224, (9) P 272, (h) PP 296-297, (i) p 342, (j) p 354, (k) p 362, (1) p 366. Knox, J. H. “Rate Constants in the Gas-Phase Oxidation of Alkanes and Alkyl Radicals”. Adv. Chem. Ser. 1968, 76, 1. Korcek, S.;Chenier, J. H. B.; Howard, J. A,; Ingold, K. U. ”Absolute Rate Constants for Hydrocarbon Autoxidation”. Can. J. Chem. 1972,50, 2285. Liang, H.; Tanaka, T. “Simulation of Spontaneous Heating for Evaluating Ignition Temperature and Induction Time”. Kagaku Kogaku Ronbunshu 1987,13,63. Reich, Leo; Stivala, Salvatore, S. “Jido Sanka”; Matsuzaki, H., Ohsawa, Z., Transl.; Maruzen: Tokyo, 1972; p 90. Sajus, L. “Kinetic Data on the Radical Oxidation of Petrochemical Compounds”. Adv. Chem. Ser. 1968, 75, 59. Thomas, J. R.; Ingold, K. U. “Determination of Rate Constants for the Self-Reactions of Peroxy Radicals by Electron Spin Resonance Spectroscopy”. Adu. Chem. Ser. 1968, 75, 258. The Research Group of Organic Peroxide Yuki Kasankabutsu; Kagaku-Kogyosha: Tokyo, 1972; pp 63-67.
Receiued for review February 17, 1988 Accepted September 1, 1988
Kinetics of Carbon Dioxide Reaction with Sterically Hindered 2-Amino-2-methyl-1 -propanol Aqueous Solutions Siu-Ming Yih* and Keh-Perng Shen Department of Chemical Engineering, Chung Yuan Christian University, Chung Li, Taiwan 32023
The kinetics of the reaction between carbon dioxide and an aqueous solution of a sterically hindered primary amino alcohol, 2-amino-2-methyl-1-propanol (AMP), was investigated at 40 “ C using a laboratory wetted wall column. T h e reaction was found t o be first order with respect to both COz and the amine. T h e second-order forward rate constant had a value of 1270 m3/(kmol.s) within the amine concentration range 0.258-3.0 kmol/m3. In addition, the product of the diffusivity and solubility for COz in AMP solution, DA1I2/HA, was found by absorbing NzO into the same AMP solution within the above concentration range. The bulk removal of COz is an important gas-treating process in synthesis gas purification and hydrogen and ammonia manufacture and is commonly accomplished by absorption into aqueous amino alcohol solutions (Astarita et al., 1983). Sterically hindered amines have recently been proposed as commercially attractive new solvents for acid gas treating over conventional amines such as MEA, DGA, DEA, DIPA, TEA, MDEA and as rate promoters for the hot carbonate process (Sartori and Savage, 1978, 1983; Savage et al., 1984; Say et al., 1984; Goldstein et al., 1986). The advantages of these sterically hindered amines are mainly due to their particular structure which results in increased thermodynamic and cyclic capacity for COz absorption. A sterically hindered amine is either a primary amine in which the amino group is attached to a tertiary carbon atom, or a secondary amine in which the amino group is attached to a secondary or a tertiary carbon atom
(Sartori and Savage, 1983). An example of a hindered primary amino alcohol is 2-amino-2-methyl-1-propanol (AMP), which is the hindered form of MEA. Sartori and Savage (1983) have presented VLE data showing that AMP has much better loading capacity for COz than MEA, especially at higher pressures. This is because of the very low carbamate stability constant of AMP as compared to MEA, which means that AMP has a low tendency to form carbamates. The formation of stable carbamates by MEA limits the loading capacity of MEA to about 0.5 mol of COz/mol of MEA from stoichiometry considerations: COZ + 2RNHz s RNHCOO- + RNHS’ (1) Since the formation of carbamates is inhibited by the bulkiness of the group attached to a tertiary or a secondary carbon atom of a hindered amine, the only reaction of importance between COz and the hindered amine would
0888-5885/88/2627-2237$01.50/0 0 1988 American Chemical Society
2238 Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988
be the formation of the bicarbonate ion, the stoichiometry of which allows loading of COPup to 1mol/mol of hindered amine: CO,
+ RNH, + H 2 0 zHC03- + RNH3+
(2)
The above evidence was shown by Chakraborty et al. (1986), who examined the NMR spectrum of a solution of AMP containing chemically combined COPand found the absence of a carbamate peak but a bicarbonate peak was observed. They also presented VLE data for the COPAMP solutions and some preliminary rate data based on the mechanism that the slow step for reaction 2 is the formation of some intermediate by reaction of COP with AMP so that the forward rate can be assumed to be first order with respect to both COPand AMP. Their rate data also indicated that absorption proceeded in the fast reaction regime with a value of the second-order rate constant ( k ) obtained as 100 m3/(kmol.s) at 42 "C. Although Sartori and Savage (1983) have noted that steric hindrance generally has an adverse effect on the CO,-amine reaction rate constants as evidenced from the data of Sharma (1965), the above value of kPobtained by Chakraborty et al. (1986) seems too low as compared to conventional amines. Therefore, this research was undertaken to investigate the kinetic order with respect to both CO, and AMP and to obtain the second-order forward rate constant at 40 "C by using a laboratory wetted wall column. Also, the product of the diffusivity and solubility for COz in AMP solution, DA'/'/HA, was measured by absorbing NPO into the same AMP solution within the amine concentration range 0.258-3.0 kmol/m3. These data have not yet been reported in the literature. Only absorption rates of COz and HPS in AMP have recently been presented by Zioudas and Dadach (1986).
penetration theory gives the specific absorption rate as NA = ~ ( D A / T ~ , ) ' / ~ C A *
(6)
where the contact time (t,) can be derived from wetted wall column hydrodynamics as (7) Specific absorption rates are measured at several different liquid flow rates, L. The slope of the straight line fitting the data of In NA versus In L is used to calculate CA*DA112. If CA* is known, DA can be calculated and vice versa. For a fast chemical reaction between the dissolved gas and a reactant, the specific absorption rate is PAi
N A = k,(pA - PAi) = E ~ L C A = * EkL-
(8)
HA
E is the enhancement factor due to chemical reaction and is a function of the Hatta number (Ha) and the instantaneous reaction enhancement factor (Ei)defined respectively as (9) and
In a region of fast pseudo mnth order reaction in which 3 < H a
