Ind. Eng. Chem. Res. 1990,29, 1721-1725 Physical Sciences Data; Thermodynamic Data for Pure Compounds, Part A Hydrocarbons and Ketones. Elsevier: New York, 1986; Vol. 25. Prausnitz, J. M.; Lichtenthaler, R. N.; De Azevedo, E. C. Molecular Thermodynamics of Fluid Phase Equilibria; Prentice-Hall: Englewood Cliffs, NJ, 1986. Reed, T. M.; Gubbins, K. E. Applied Statistical Mechanics; McGraw-Hill: New York, 1973. Renon, H.; Prausnitz, J. M. Local Compositions in Thermodynamic Excess Functions for Liquid Mixtures. AZChE J. 1968, 14, 135-144.
Rowlinson, J. S.; Swinton, F. L. Liquid and Liquid Mixtures; Butterworth Scientific: London, 1982. Stell, G.; Zhou, Y. Chemical Association in Simple Models of Molecular and Ionic Fluids. J. Chem. Phys. 1989, 91, 3618-3623. Twu, C. H.; Lee, L. L.; Starling, K. E. Improved Analytical Representation of Argon Thermodynamic Behavior. Fluid Phase Equilib. 1980, 4, 35-44. Wertheim, M. S. Fluids with Highly Directional Attractive Forces. I. Statistical Thermodynamics. J. Stat. Phys. 1984a, 35,19-34.
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Wertheim, M. S.Fluids with Highly Directional Attractive Forces. 11. Thermodynamic Perturbation Theory and Integral Equations. J . Stat. Phys. 1984b,35, 35-47. Wertheim, M. S. Fluids with Highly Directional Attractive Forces. 111. Multiple Attraction Sites. J. Stat. Phys. 1986a,42,459-476. Wertheim, M. S. Fluids with Highly Directional Attractive Forces. IV. Equilibrium Polymerization. J. Stat. Phys. 1986b, 42, 477-492.
Wertheim, M. S. Fluids of Dimerizing Hard Spheres, and Fluid Mixtures of Hard Spheres and Dispheres. J.Chem. Phys. 1986c, 85, 2929-2936. Wertheim, M. S. Thermodynamic Perturbation Theory of Polymerization. J. Chem. Phys. 1987,87, 7323-7331. Wilson, G. M. Vapor-Liquid Equilibrium. XI. A New Expression for the Excess Free Energy of Mixing. J.Am. Chem. Soc. 1964, 86, 127-130.
Received f o r reuiew September 6, 1989 Revised manuscript receiued January 23, 1990 Accepted March 23, 1990
Organic Chemistry of Calcium. 3. Steam Stripping of Metal Phenoxides Liberates Phenol and Regenerates the Metal Hydroxide Charles G. Scouten* and H. Warren Dougherty Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801
Treatment of hydroxycalcium phenoxide (HOCaOC6H5),sodium phenoxide, and potassium phenoxide with steam at 350 "C and atmospheric pressure liberated phenol and regenerated the corresponding metal hydroxide. High phenoxide conversions in the range 85-100% were obtained, even though the reaction conditions were not optimized. Essentially quantitative recoveries of the hydroxide bases were obtained. The steam stripping reaction appears to be generally applicable to alkali and alkaline-earth phenoxides. Metal phenoxides are produced when phenol-containing process streams are contacted with basic hydroxides for phenol removal. Alternative phenol recovery methods, such as thermolysis and acid neutralization, are energy intensive and can have detrimental side effects. Thus, steam stripping offers a new general method for recovering phenols from metal phenoxides.
A previous report described the formation of hydroxycalcium phenoxides (Schlosberg and Scouten, 1988). Together with the subsequent thermal decomposition of these salts to phenols and calcium oxide, this chemistry provided the basis of a novel method for separating and recovering phenols from process streams (eq 1-4 (Schlosberg and Scouten, 1981)). formation Ca(OH)2+ ArOH
25 *C
HOCaOAr
drying HOCaOAr
+ entrained liquids
phenol recovery
550 o c
+ entrained liquids
(1)
350 O C
HOCaOAr (dry) (2)
HOCaOAr CaO + ArOH regeneration of hydroxide CaO + H 2 0 Ca(OH)2
(3)
(4)
In the sequence in eqs 1-4, heating to approximately 350 "C frees the calcium salt entrained liquids. Further
* Present address: Amoco Oil Company, P.0. BOX3011, Naperville, IL 60566. 0888-5885/90/2629-1721$02.50/0
heating in a second stage is then required to liberate phenol for recovery. In order to reduce the energy requirements of the new phenol recovery scheme, we sought ways to liberate phenol at lower temperatures, thereby eliminating the need for second-stage heating. The reactions of calcium phenoxides with carbon dioxide and other acids are known to liberate phenol (Fischer and Erhardt, 1919). However, regenerating Ca(OH), from the resulting inorganic salts requires heating to temperatures that approach or exceed 650 "C. Thus, phenol liberation via such acid treatments would not afford the desired reduction in energy requirements. The most direct route from the calcium salt to phenol and hydroxide would be via reaction with steam. Therefore, this report describes our studies on the reactions of steam with hydroxycalcium phenoxide, and with alkali metal phenoxides, to liberate phenol (eq 5 (Scouten, 1986,1987)). For convenience, we term these "steam stripping" reactions. HOCaOAr HCP
steam
ArOH
+ Ca(OH),
(5)
Results and Discussion The fundamental chemistry of steam stripping reactions was studied using hydroxyca~ciumphenoxide (HCP, HOCaOC6H5),sod'lum phenoxide, and potassium phenoxide. The preparation of HCP was described previously (Schlosberg and Scouten, 1988). The sodium and potas0 1990 American Chemical Society
1722 Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 Table I. Elemental ComDosition and Phenoxy GrouD Assay of Metal Phenoxides metal phenoxide sodium potassium found calcd found calcd carbon, wt % 62.66 62.08 56.61 54.51 4.14 3.81 hydrogen, wt % 4.35 4.34 oxygen, wt 7'0 (difference) 13.69 13.78 11.55 12.10 27.7 metal, wt 70 19.3 19.80 29.57 phenoxy assay 8.44 8.61 7.20 7.58 mmol/g 70 of theoretical 98 95
HCP (calcium) found calcd 47.62 47.98 3.91 4.03 21.19 21.31 27.28 26.69 6.32 95
.
7Rsacl~on
TEMPERATURE RECORDER 0 TURNACE
6.66 Zona Thermocouple
DIGITAL TEMPERATURE INDICATOR METER FLOW
Prebmwr -Thrmwple
Liquid Inlet +
I TC
c _
HELIUM SWEEP GAS
FURNACE
VEN
REACTOR TUBE
I I
I
-_--_--_-_-_--______
1-
J
ACCUMULATOR SYSTEM
Figure 1. Schematic drawing of the miniflow pyrolysis unit.
sium salts were prepared from phenol and the corresponding hydroxides, by the procedure of Kornblum and Lurie (1959). Satisfactory elemental analyses were obtained for each compound (Table I). By determining the anisole produced upon alkylation with iodomethane, purities of 98%,95%, and 95% were found for the sodium, potassium, and calcium salts, respectively (Table I). Steam Stripping Reactions in the Miniflow Unit. Steam stripping reactions were carried out in the miniflow pyrolysis unit, a small fixed bed unit equipped for controlled feeds of both liquid and gaseous reagents during the pyrolysis of a solid sample (Figure 1). A typical experiment is outlined to illustrate the general procedure: In a nitrogen-filled glovebox, the solid phenol salt was packed into the center section of the reactor tube, the details of which are shown in Figure 2. The tube was installed in the furnace, while maintaining an inert atmosphere. Under a flow of helium, the reactor was heated to the indicated temperature. Water was then fed from the syringe into the top of the reactor where it was converted to steam and swept through the sample. The unreacted steam and liberated phenol were collected in a cold trap at the reactor exit. The extent of phenol liberation (conversion) was monitored by determining the number of phenol moieties in the salt residue afhr steam stripping. The residue was recovered from the reactor tube and was treated with iodomethane in NJV-dimethylformamide (DMF) solvent for 30 min at 60 "C to quantitatively convert phenol moieties in the residue into anisole. The anisole was determined by GC, using an internal standard technique. In selected cases, the steam and liberated phenol were trapped. The phenol was converted to anisole, which was analyzed to establish the material balance.
Figure 2. Reactor tube used in the miniflow pyrolysis unit.
Thermal Stability of HCP in the Miniflow Pyrolysis Unit. Previous work showed that HCP was stable to approximately 450 "C under inert atmosphere (Schlosberg and Scouten, 1981). This work was generally carried out by using temperature-programmed thermogravimetric analysis (TGA). Exposure of HCP to temperatures in the range of 350 "C was relatively short in these experiments; hence, it was desirable to verity its thermal stability under the conditions used in the present work. Accordingly, a sample of HCP was placed in the reactor tube and heated at 350 "C under a flow of helium for 80 min (approximately twice the heating time used in steam stripping). The residue was recovered and methylated as described above. Analysis showed that essentially all of the phenol moieties (99%) remained in the residue. This verified the thermal stability of HCP in the absence of steam. Effect of Temperature on Steam Stripping. The effect of temperature on steam stripping was studied over the range 150-350 "C at a constant molar ratio of water:HCP of 12:l. In this study, water was fed to the reactor over 10 min using helium (100 mL/min) as the sweep gas. A t a sample temperature of 350 "C, no detectable anisole (x0.5%) was obtained upon methylation of the steam stripped residue. Thus, under these condi-
Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 1723
i
loo
a?
100 mL/min to 43% at 25 mL/min. Thus, at 350 "C, the steam and stripping efficiency is relatively insensitive to the sweep gas flow rate and, hence, to the activity of water in the sweep gas stream-through the total amount of water is important, as discussed above. Steam Stripping of Alkali Metal Phenoxides. Extraction of phenol-containing process streams with aqueous solutions of alkali metal hydroxides (caustic wash) is a common technique for removal of phenols. Evaporation of the caustic extracts affords the corresponding alkali metal phenoxides, which are thermally stable to 500 "C (Schlosberg and Scouten, 1981). Phenols can be recovered from the alkaline extract by neutralizing with an acid, such as carbon dioxide. However, as discussed above in the case of the calcium salt, regenerating the alkali metal hydroxide from the neutral salt is energy intensive and expensive. Accordingly, we have studied the steam stripping reactions of the alkali metal phenoxides, sodium phenoxide, and potassium phenoxide. Thermal stability of sodium and potassium phenoxides in the absence of steam was verified by heating at 350 "C for 80 min as described above for the calcium salt. For the sodium and potassium salts, anisole yields of 97% and 99%, respectively, were obtained, indicating essentially no loss of phenol moieties due to heating at 350 "C in the absence of steam. Steam stripping of sodium phenoxide at 350 "C, using a 12:l molar ratio of water:phenoxide and a helium flow rate of 100 mL/min, was carried out in the miniflow unit, as described above. Analysis of the anisole from residue methylation indicated 85% salt conversion. The liberated phenol was trapped and methylated to verify that molecular phenol was liberated. Summing the anisole obtained on methylations of the residue and trapped phenol gave a phenol balance of 100% in this experiment. Potassium phenoxide was also steam stripped under these conditions. Methylation of the residue and trapped phenol gave 14% and 87% of the theoretical amount of anisole, respectively. Thus, an 87% salt conversion and an essentially quantitative phenol balance (101% ) were obtained from the potassium salt. Titration established that the basic residue from steam stripping was the hydroxide-and not the carbonate, which could arise from inadvertent neutralization and which would not be effective if recycled for phenol removal due to its lower base strength. A residue from steam stripping of potassium phenoxide was dissolved in 50 mL of C02-free water and titrated with 0.5 N HCl using a pH meter to monitor the titration. The resulting smooth titration curve, shown in Figure 5, is characteristic of the neutralization of a strong base like potassium hydroxide with a strong acid. The presence of carbonate, as a contaminant in the hydroxide, would have been indicated by an inflection at a pH of about 10 in the titration curve, due to the buffering effect of bicarbonate ion. The end point indicates essentially quantitative recovery of base (97%) in this experiment. A similarly smooth titration curve was obtained upon neutralization of a residue from steam stripping of sodium phenoxide, indicating the absence of carbonate. Base recovery was only 92% in this case, probably due to handling losses. The above reactions were carried out under conditions found to give quantitative liberation of phenol from the calcium salt. No attempt was made to optimize reaction conditions for the alkali metal phenoxides.
8o
t
I 150
350
250 TEMPERATURE,
O C
Figure 3. Efficiency of phenol liberation in the steam stripping of HCP, being strongly dependent upon the temperature. 100,
OY' 0
7
'
2
'
' 4
'
'
6
'
'
'
8
'
10
'
1
'
12
'
MOLAR RATIO OF WATER:HCP
Figure 4. Conversion of HCP in steam stripping depending linearly on the amount of water added as steam.
tions, steam stripping of phenol from HCP was complete. Methylation of the residue from a comparable experiment carried out at 250 "C afforded 46% of the theoretical amount of anisole; hence, a 54% conversion of HCP was obtained. At a still lower temperature of 150 "C, essentially all the phenol moieties remained in the salt residue, indicating no conversion of HCP. These results, summarized in Figure 3, show that steam stripping is strongly dependent on temperature. Effect of Water:HCPRatio on Steam Stripping. An approximately linear relationship was found between conversion and water:HCP molar ratio when steam stripping was carried out at 350 "C with successively smaller amounts of water to give water:HCP ratios of 9:l and 4:l (Figure 4). The reason for this strong dependence is not understood. In all cases, water was fed in large molar excess, and the contact time was maintained constant (10 min) by adjusting the water feed rate. Moreover, the good reproducibility obtained in duplicate experiments make it unlikely that random channeling led to uneven gas flow and poor contacting of HCP by the steam. This unexpected observation led us to examine the effect of water activity on steam stripping (vide infra). Effect of Sweep Gas Flow Rate and Water Activity. Using a lower sweep gas flow rate, while maintaining a constant water feed rate, would increase the water activity in the sweep gas. This could enhance steam utilization efficiency. Accordingly, the steam stripping of HCP at 350 "C was studied using a sweep gas flow of 50 mL/min and a 9:l molar ratio of water:HCP. Conversion increased slightly, from 72% conversion at a sweep gas flow of 100 mL/min to 78% at 50 mL/min. A similarly small increase in conversion was obtained at a 4:l molar ratio of water:HCP. In this case, the increase was from 36% at
Conclusions Treatment of sodium phenoxide, potassium phenoxide, and hydroxycalcium phenoxide with steam at 350 "C at
1724 Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990
l4
=
I
'r
I
i
I
2k I 0
5
'0 VOLUME OF 0.5
15
20
25
30
N HCI TITRANT, mL
Figure 5. Titration of the residue from steam stripping of potassium phenoxide with a water:HCP molar ratio of 12 at 350 "C, indicating essentially quantitative (97%) recovery of potassium hydroxide. The smooth curve indicated that no more than a trace of potassium carbonate was present.
atmospheric pressure liberates phenol and regenerates the corresponding metal hydroxide. High phenoxide conversions in the range 85100% were obtained, using conditions that were optimized for the calcium salt. Essentially quantitative recoveries of base were obtained in each case. The steam stripping reaction appears to be generally applicable for phenol recovery from metal phenoxides.
Experimental Section The metal phenoxides were always handled under an inert atmosphere of either helium or nitrogen to exclude both carbon dioxide and water. All transfers were carried out under nitrogen in a glovebox. A small, positive pressure of nitrogen was maintained during methylation reactions using an oil bubbler as a pressure relief. Gas chromatographic analyses were carried out by using a Hewlett-Packard 5752B gas chromatograph equipped with a thermal conductivity detector and a 10-ft X 1/8-in. column packed with 10% SP-2100 (poly(dimethylsi1oxane)) on 80-100-mesh Chromosorb W/HP. Product and intemal standard peak areas were integrated electronically. A Chemtrix 6OA pH meter was used to monitor the titrations. Materials. Solvents and reagents were generally of analytical reagent quality and were obtained commercially. Spectrophotometric quality hexane and DMF were obtained from Burdick & Jackson (Muskegon, MI). HCP was prepared from phenol and calcium hydroxide in benzene (Schlosberg and Scouten, 1988). Sodium and potassium phenoxides were prepared by reacting the corresponding hydroxides with phenol in aqueous methanol, evaporating the solvent, and drying to constant weight under vacuum at room temperature (Komblum and Lurie, 1959). The following procedure illustrates the assay of phenoxy moieties in these salts: A 50-mL flask with an injection port was equipped with a magnetic stirring bar and a gas inlet tube with a stopcock. The injection port was capped with a serum stopple, and the flask was placed in a nitrogen-flushed glovebox. The flask was charged with 1.30 g of HCP (8.66 mmol), sealed, and removed from the glovebox. The flask was attached to a low-pressure nitrogen source, using a mineral oil bubbler as a pressure relief. To the flask were added, via syringe, 3.0 mL of iodomethane (6.8 g, 48 mmol), 1.0mL of p-xylene (internal standard for G C ) ,and 15 mL of DMF solvent. The flask was heated for 1h in an oil bath maintained at 60 "C. A l-mL aliquot of the resulting slurry was removed via
syringe and injected into a vial containing 25 mL of 0.1 N HCl and 2 mL of hexene. The solvent and inorganic salts partitioned into the aqueous phase, while the anisole (methyl phenyl ether) product and p-xylene standard were extracted essentially quantitatively into the hexane. The hexane layer was separated and analyzed by GC for anisole and p-xylene. The anisole yield was obtained by comparing the aniso1e:p-xylene peak area ratio to that for a synthetic product mixture. The anisole found in this case was 95% of the theroretical amount. Similar assays were made of the sodium and potassium phenoxides, which gave 98% and 95% of the theoretical amounts of anisole, respectively. The conversions obtained in the steam stripping reactions were based upon these phenoxy group assay values. These data and the elemental analyses of the salts are summarized in Table I. Thermal Stability of HCP in the Miniflow Pyrolysis Unit. Verification of the thermal stability of HCP in the absence of steam is described to illustrate the use of the miniflow pyrolysis unit: The partially disassembled reactor was placed in the glovebox, as the following parts: preheater, reactor tube, spacers, screens, and quartz wool. A spacer (1/4-in.stainless steel tubing), selected to place the sample in the center of the heated zone, was slipped over the thermocouple well on the preheater. This was followed by a screen wrapped with a small amount of quartz wool. The reactor tube was fitted to the preheater, as shown in Figure 2, and the reactor was inverted. Next, 1.30 g of HCP (8.66 mmol) was charged to the reactor. A second screen wrapped in quartz wool and a second spacer were inserted to support the charge. With careful handling so as not to disturb the packed bed, the reactor was capped, removed from the glovebox, and installed in the furnace. A flow of helium sweep gas (100 mL/min) was established, and the accumulator was attached. The preheater was heated to 250 "C. The reactor tube was heated to 350 "C, maintained a t 350 "C for 80 min, and then cooled while maintaining the helium flow. The empty accumulator was disconnected from the reactor. The reactor was then sealed, disconnected from the helium source, and returned to the glovebox. In the glovebox, the salt residue was transferred to a reaction flask for assay of phenoxy groups as described above. The anisole yield was 99% of the theoretical value, showing that no phenoxy groups were lost by heating at 350 "C in the absence of steam. Steam Stripping of HCP in the Miniflow Pyrolysis Unit. The reactor tube was charged with 1.30 g of HCP (8.66 mmol) and mounted in the furnace, as described above. As in the thermal stability study, the preheater and reactor tube temperatures were 250 and 350 "C, respectively. A 30-mL syringe of water was attached to the liquid inlet and fitted to the variable rate syringe pump. The syringe pump was then used to feed 1.87 mL of water (104 mmol, water:HCP molar ratio of 12:l) to the reactor over a period of 10 min. Because the shallow bed was the only restriction to flow, the pressure in the reactor was essentially 1atm throughout these experiments. After adding the water, heating was continued for an additional 30 min to ensure complete removal of volatiles from the nonvolatile residue. As described above, the reactor was cooled, sealed, and returned to the glovebox for assay of the residue for the remaining phenoxy groups. No anisole was detected in the product of residue methylation; hence, complete phenol liberation was obtained in this steam stripping experiment. Thermal Stability of Sodium Phenoxide and Potassium Phenoxide. The thermal stability of sodium
Ind. Eng. Chem. Res. 1990,29, 1725-1728
phenoxide and potassium phenoxide in the absence of steam was verified in the miniflow pyrolysis unit, using procedures analogous to that used to verify the thermal stability of HCP. From the sodium and potassium phenoxide residues, anisole yields were 97% and 99% of the theoretical values, respectively. Comparison of these values to those of the starting phenoxides shows that essentially no phenol was liberated by heating to 350 OC in the absence of steam. Steam Stripping of Sodium Phenoxide and Potassium Phenoxide. Steam stripping of the alkali metal phenoxides was carried out as described above. From the steam-stripped sodium phenoxide residue, the anisole yield was 15% of the theoretical. An 85% yield of anisole was obtained upon methylation of trapped phenol and steam. This confirmed that 85% of the phenol moieties initially present in the salt were removed by steam stripping. Phenol balance was quantitative in this experiment. Comparable steam stripping of potassium phenoxide afforded an 86% phenoxide conversion and a 101% phenol balance. In another experiment, potassium phenoxide (1.35 g, 10.2 mmol) was steam stripped as described above, and the residue was dissolved in 50 mL of C02-freewater. The resulting solution was titrated with 0.50 N HC1, using a pH meter to monitor the titration. The smooth curve obtained from this titration is shown in Figure 5. Similarly,
1725
no carbonate and a 92% recovery of base were obtained from steam stripping of sodium phenoxide. The lower recovery in the latter case may be due to mechanical losses.
Acknowledgment The miniflow pyrolysis unit was constructed by R. H. Schlosberg and A. Kurs, who made it available for these studies. The advice and assistance of these workers are gratefully acknowledged. Also, very helpful discussions with Prof. H. C. Brown (Purdue University) and the late Prof. R. Pettit (University of Texas, Austin) are acknowledged with thanks. Registry No. Phenol sodium salt, 139-02-6;phenol potassium, 100-67-4;phenol, 10895-2;potassium hydroxide, 1310-58-3;phenol calcium salt, 5793-84-0.
Literature Cited Fisher, R.; Erhardt, U. Gesammelte Abh. Kennt. Kohle 1919, 4 , 237-263. Kornblum, N.; Lurie, A. P. J. Am. Chem. SOC.1959,81, 2710. Schlosberg, R. H.; Scouten, C. G. US. Patent 4 256568, 1981. Schlosberg, R. H.; Scouten, C. G. Energy Fuels 1988, 2, 582-585. Scouten, C. G. US.Patent 4595489, 1986. Scouten, C. G . US.Patent 4551637, 1987.
Received for review August 10, 1989 Accepted December 5, 1989
Reaction Mechanism and Kinetics of Aqueous Solutions of 2-Amino-2-methyl-1-propanol and Carbon Dioxide Erdogan Alper Chemical Engineering Department, Kuwait University, P.O. Box 5969, 13060 Safat, Kuwait
The mechanism and kinetics of the reaction between aqueous solutions of COz and a sterically hindered primary alkanolamine, 2-amino-2-methyl-1-propanol (AMP), were investigated a t 278-298 K by using a stopped flow technique. Experiments were also carried out with monoethanolamine (MEA) solution, which is the sterically unhindered form of AMP. The corresponding second-order rate constants a t 298 K were found to be 520 and 5545 m3/ (km01.s) for AMP and MEA with activation energies of 41.7 and 46.7 kJ/mol, respectively. On the basis of this rate constant, the reaction was considered to be formation of carbamate which subsequently hydrolyzed.
Introduction 2-Amino-2-methyl-1-propanol(AMP) is a commercially available primary amine which is the sterically hindered form of monoethanolamine (MEA). Sterically hindered amines cannot form stable carbamates, leading to much higher carbonation ratios (moles of C02/mole of amine). However, the steric hindrance lowers the reaction rate of C02,which may be undesirable. Although thermodynamic properties and industrial applications of sterically hindered amines have been discussed in detail in previous literature (Sartori and Savage, 1983; Sartori et al., 1987; Chakraborty et al., 1986), studies on the reaction kinetics are limited. In the case of AMP, a number of heterogeneous gas absorption studies were carried out previously. Sharma (1965) reported a second-order rate constant of 1048 m3/(kmol.s) at 298 K. Sartori et al. (1987) and Chakraborty et al. (1986) deduced a rate constant of about 100 m3/(kmol.s) at 313 K. The recent study by Yih and Shen (1988) reports a rate constant of 1270 m3/(kmol-s)at 313 K. Absorption rates of C02and H2S(but not direct kinetic data) were also reported for AMP by Zioudas and Dadach (1986). These gas absorption studies were all analyzed by OSSS-5SS5/90/2629-1725$02.50/0
using the methodology of “gas absorption with pseudo first order reaction”, and the agreement between them cannot, in general, be considered as satisfactory. Recently, Bosch et al. (1989) suggested that carbon dioxide absorption into solutions of sterically hindered amines cannot be simplified as above and the process should be considered as a case of mass transfer with parallel reversible reactions. Their rigorous numerical solution interprets the gas absorption results of Chakraborty et al. (1986) and yields a completely different conclusion. It seems, therefore, that there is a need for obtaining data from direct techniques which do not involve mass transfer. Further, there appears to be a confusion about the exact reaction mechanism since some of the authors presume no carbamate formation (Yih and Shen, 1988) while others (Sharma, 1965) consider the reported data as the forward reaction rate constant between carbon dioxide and AMP. On the other hand, Chakraborty et al. (1986) presumes the reaction to be the hydration of C 0 2 which is catalyzed by AMP as in the case of tertiary amines. The aim of this paper is to report the results obtained by a direct technique (that is, stopped flow experiments) 0 1990 American Chemical Society
