Ind. Eng. Chem. Res. 1989,28, 470-475
470
Dietz, P. W.; Melcher, J. R. AIChE Symp. Ser. 1978a, 74(175), 166. Dietz, P. W.; Melcher, J. R. Ind. Eng. Chem. Fundam. 1978b,17,28. Ergun, S. Chem. Eng. Bog. 1952, 48, 89. Grace, J. R.; Harrison, D. Chem. Eng. Sci. 1967, 22, 1337. Harper, W. R. Contact and Frictional Electrification; Clarendon Press: Oxford, 1967. Hendricks, C. D. Charging Macroscopic Particles. In Electrostatics and its Applications; Moore, A. D., Ed.; Wiley: New York, 1973. Henry, P. S. H. Br. J. Appl. Phys., Suppl. 1953,2, S31. Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. Inculet, I. I. Colloid Interface Sci. 1970, 32, 395. Inculet, I. I. Static Electrification of Dielectrics and at Material's Interfaces. In Electrostatics and its Applications; Moore, A. D., Ed.; Wiley: New York, 1973. Johnson, T. W.; Melcher, J. R. Ind. Eng. Chem. Fundam. 1975,14, 146. Katz, H.; Sears, J. T. Can. J. Chem. Eng. 1969, 47, 50. Kisel'nikov, V. N.; Vyalkov, V. V.; Filatov, V. M. Int. Chem. Eng. 1967, 7, 428. Kobayashi, H.; Arai, F.; Izawa, N.; Miya, T. Kagaku Kogaku 1966, 30, 656. Kunii, D.; Levenspiel, 0. Fluidization Engineering; Wiley: New York, 1969.
Kurosaki, S. J. Phys. Chem. 1954,58, 320. Loeb, L. B. Electrical Coronas-Their Basic Physical Mechanisms; University of California Press: Berkeley, 1965. Leva, M. Fluidization; McGraw-Hill: New York, 1959. Milne-Thomson, L. M. Theoretical Hydrodynamics, 4th ed.; The McMillan Company: New York, 1960. Nicklin, D. J. Chem. Eng. Sci. 1962, 17, 693. Nicklin, D. J.; Wilkes, J. 0.;Davidson, J. F. Trans. Znst. Chem. Eng. 1962, 40,61. Richardson, J. F. Incipient Fluidization and Particulate Systems. In Fluidization; Davidson, J. F., Harrison, D., Eds.; Academic: New York, 1971. Shih, Y. T.; Gidaspow, D.; Wasan, D. T. AIChE J . 1987,33, 1322. Soo, S. L. Powder Technol. 1974, 10, 211. Thorp, J. M. Trans. Faraday SOC.1959,55, 442. Wittmann, C. V.; Ademoyega, 0. Ind. Eng. Chem. Res. 1987, 26, 1586. Yates, J. G. Fundamentals of Fluidized-Bed Chemical Processes; Butterworths: London, 1983. Zahedi, K.; Melcher, J. R. Air Pollut. Control Assoc. J . 1976, 26,345.
Received for reuiew December 21, 1987 Accepted December 15, 1988
GENERAL RESEARCH Reactions of Carbonyl Sulfide and Methyl Mercaptan with Ethanolamines Mahmud A. Rahman,t Robert N. Maddox,*J and G. J. Mains* Department of Chemical Engineering and Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078-0537
Carbonyl sulfide and methyl mercaptan were interacted with anhydrous monoethanolamine (MEA), diglycolamine (DGA), diethanolamine (DEA), di-2-propanolamine (DIPA), methyldiethanolamine (MDEA), and dimethylethanolamine (DMEA). The reaction products were analyzed by 'H and I3C NMR. T h e protonated amine:thiocarbamate salt was detected for MEA, DEA, and DGA. The DIPAthiocarbamate product could not be isolated, and no evidence for reaction products from MDEA or DMEA interaction with carbonyl sulfide could be obtained. The contact of MEA, DEA, DGA, DIPA, MDEA, and DMEA with methyl mercaptan did not lead to a reaction. However, pronounced shifts and lump formations of the hydroxyl and amine protons in the 'H NMR spectra indicate that Lewis acid-base adducts are synthesized between the amine and mercaptan molecules. The pressure measurements also corroborate this NMR evidence. The removal of acid gases such as carbonyl sulfide (COS) and methyl mercaptan (CH,SH) is an essential process in the natural gas and petroleum gas processing industry. Monoethanolamine has been a traditional solvent for removal of carbonyl sulfide (Kohl and Riesenfeld, 1979; Maddox, 1977; Danckwerts and Sharma, 1966). However, loss of MEA in the regenerator is a well-known fact: the loss due to an irreversible reaction with COS. Recently, di-2-propanolamine has gained stature as a major COS solvent in the ADIP and Sulfinol processes (Kohl and Riesenfeld, 1979; Maddox, 1977). The amine solvents which are of industrial importance and considered in this study are monoethanolamine (MEA), diethanolamine
* Author t o whom correspondence should be addressed. 'Department of Chemical Engineering. Department of Chemistry.
*
0888-5885/89 /2628-0470$01.50/0
(DEA), /3,/3'-(hydroxyamino)ethyl ether (diglycolamine, DGA), di-Zpropanolamine (DIPA), methyldiethanolamine (MDEA), and dimethylethanolamine (DMEA). The objective of this study is to determine the reaction products of the reactions between COS and the amines and to investigate the possibility of a reaction between methyl mercaptan and amines.
Reactions of Carbonyl Sulfide with Amines The removal of carbonyl sulfide from mixtures of gases by reactive liquid solvents is an important industrial operation. The relevant reactions with a primary amine are RNHz + COS
-
RNHCOS-
RNHz + H+
-
+ H+
RNH3+
(1) (2)
where R is generally an alkanolamine radical. This reac0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 471 tion, analogous to the reaction with C02, also occurs with a secondary amine to yield a thiocarbamate. The similarity in reactions of C 0 2 and COS with an amine stems from the fact C02 and COS have similar electronic structures and the rates of reaction of both of the gases with an amine are comparable (Maddox et al., 1987; Sharma and Danckwerts, 1964; Sharma, 1965). Although the rates of reaction of COS and COP are similar in MEA, the reaction products in the case of COS are not regenerable. The reaction of MEA with COS progresses as follows (Danckwerts and Sharma, 1966; Pearce et al., 1961):
+
HOCH2CH2NH2
O=C=S
+
HOCH2CH2HNC-SH
II 0
HOC H2CH2NHC-SH
II
H20
HOCH2CH2NHC-SH
-
heat
II 0
HOCH2CH2NH2
+
C02
+
(3)
H2S (4)
H 2 C v C H 2
0
0W
,.N
+
H2S ( 5 )
H
c,
II
0 HOCH2CH2NHC-SH
I1 0
+
H~NCHZCH~OH _e
H O C H P C H ~HN C ' =O / H 0CH2CH2NH
-
HOCH$H2N H
,c\ =o
DEA is not a very effective solvent for COS (Kohl and Reisenfeld, 1979; Pearce et al., 1961). The performance of DIPA is generally comparable with DEA (Danckwerts and Sharma, 1966). However, Shell International has patented the ADIP process and the Sulfinol process, both of which employ DIPA as the reactive solvent (Kohl and Riesenfeld, 1979; Danckwerts, 1970; Skoog and West, 1980). In the SulFinol process, DIPA is used in conjunction with a physical organic solvent (sulfolane, tetrahydrothiophene dioxide) and has proven effective for removing COS and CH,SH (Kohl and Riesenfeld, 1979). The ADIP process solvent is 30-40% aqueous DIPA requires a fairly long residence time. It is efficient for sweetening refinery gases and liquids containing H2S and C02, as well as COS. No wastage of amine occurs through side reactions, as in the case of MEA. Furthermore, low regeneration steam requirements and the noncorrosive nature of DIPA solutions make it an attractive option for COS scrubbing. The second-order rate constants for the amine in the case of reactions between COS and MEA, DEA, and DIPA are 16, 11, and 6 L mol-l s-l a t 25 OC, respectively (Danckwerts and Sharma, 1966; Sharma and Danckwerts, 1964; Sharma, 1965). The rate of reaction of COS with amines is reported to be slightly slower than with COz (Sharma and Danckwerts, 1964; Sharma, 1965).
+
+
heat
HOCH2CH2NH
HzS ( 6 )
H20 (7)
H 2 c F N : " .
/
FH2
/
c
II
0
,CH2
Absorption of Methyl Mercaptan in Amines The removal of methyl mercaptan by amines is limited to its physical solubility (Kohn and Riesenfeld, 1979). The difficulty of ionization of the mercaptan hydrogen in weak bases such as amine inhibits ionic interaction. However, some kinetic data are available for mercaptan reactions with aqueous sodium hydroxide solutions (Landry, 1966). The reaction was reported to be very fast, i.e., a forward rate constant of the order of lo5 L mol-' s-l was observed. Experimental Section Rahman (1984) has complete details of the experimental apparatus and techniques. The reaction vessel was a Claisen distillation flask. The experimental volume of the reaction system was 552.5 mL. Amine was added to the system through the capillary nozzel outlet of the highprecision buret. The uncertainty in the amine volume was 0.025 mL. The temperature of the gas was measured at a point just above the gas-liquid interface. A mercury thermometer, range -4 to 220 "C, was used to measure the gas temperature as the reaction progressed. When the reaction appeared to stop, as determined by a constant gas pressure, an aliquot of the reaction mixture was pipetted from the flask and diluted approximately by a factor of 5 for NMR analysis using either DzO or DCCl,. The piping consisted of 'Ia-in. stainless steel, except for the line to the vacuum pump which was lI4-in.internal diameter rubber tubing with lI4-in. thick walls. MEA and MDEA were obtained from Alfa Products 96% and 97% pure, respectively. Aldrich Chemical Company provided DMEA of 99% stated purity. Reagent grade DEA, assayed to be 99.8% pure, was obtained from Baker Chemical Company. ICN Pharmaceuticals supplied technical grade DIPA and DGA, both about 95% pure. Carbonyl sulfide (97.5% pure) and methyl mercaptan (99.5% pure) were obtained from Matheson Corporation.
H2CmCH2 Oii
+
0'
I
C
HOCH~CHPNH
NH
I1
0
+
H20
(8)
" 7/ 7 Nc :
7"' 1
7'
OH
Reaction 3 shows that MEA reacts reversibly with COA to form the thiocarbamate. Addition of water and heat may regenerate the amine (reaction 4) but may also lead to the synthesis of 2-oxazolidone, as indicated by reaction 5. The thiocarbamate may react with MEA to form diethanolurea, as in reaction 6. The diethanolurea cyclization reaction occurs on heating (reaction 7) to yield N(2-hydroxyethy1)imidazolidone.This product can also be synthesized by the reaction of oxazolidone with MEA, as indicated by reaction 8. In industrial operations, the loss of MEA may range from 10% to 100% of that reacting with COS depending on contactor conditions and strength of amine solution (Kohl and Riesenfeld, 1979; Danckwerts and Sharma, 1966; Weber and McClure, 1981; McClure and Morrow, 1979; Pearce et al., 1961).
Reaction between Carbonyl Sulfide and Monoethanolamine The reaction between carbonyl sulfide and MEA OCcurred at a rapid rate and yielded a green product. Figure 1 shows the 'H NMR spectrum of 20% MEA (80% sol-
472 Ind. Eng. Chem. Res., Vol. 28, No. 4,1989
10
9
8
7
6
4
5
3
I
'
2
0
PPM
I zoo
ifio
160
140
izo
Figure 1. 'H NMR spectrum of pure MEA in DCCl:, (20% sample).
io0
ao
60
40
zo
o
PPM
Figure 3. 13C NMR spectrum of pure MEA in D 2 0 (20% sample).
-
9
8
7
6
5
4
3
2
1
0
PPM
Figure 2. 'H NMR spectrum of MEA-COS reaction products in D 2 0 (20% sample).
vent) in DCC1,. For convenience, the MEA skeletal structure is labeled as N--C--C--OH a P 1
In Figure 1,peak 1is an impurity proton associated with DCC1, and peaks 2 and 4 are the p and cy protons, respectively. The protons of the N and OH group are represented by broad absorption peak 3. The 'H NMR scan of MEA and COS reaction products (solvent D20) is shown in Figure 2. A downfield shift of 0.2 ppm was observed for cy and 6 protons (labels 6 and 7 in Figure 2) relative to pure MEA (Figure l ) , and in the case of N protons, the triplet symmetry was not observed. This indicates the presence of a new species on the N atom. The reaction anticipated is 2HO.CH2CH2.NH2 COS HO*CH2*CHyNHCOS-NH3+CHyCH2*OH (9)
+
-
The pure MEA 13C NMR spectrum in D20 solvent is shown in Figure 3. Labels 8 and 9 are the absorption responses of the 6 and a carbons. Figure 4 shows the 13C NMR spectrum of the reaction products in DzO. The P and cy carbons are represented by labels 11 and 12, respectively. Reaction 9 does not go to completion. Therefore, both the reactant R.NH2 and product R-NH3+ (R = CH2-CHz.0H) are present in the reaction product mixture. This is borne out in Figure 4 where label a indicates R.NH2 and R-NH3+is shown as label b. Peak 10 is the 13Cnucleus in the group COS. The cy and p carbons of the R.NHCOS-/ion are represented by peak 13. A similar pattern was observed by Batt (1979) and Batt et al. (1980) in reactions between MEA and COz. The overall reaction between MEA and COS may be written as R.NH2 + COS mR.NH2 + nCOS + (1 - n)R*NH,+ + (1 - n)RNHCOSH- (10) The experimental data show that approximately 55 % of the COS remained unreacted, whereas determination
-
200
i8o
160
140
120
loo
fio
fio
40
20
n
PPM
Figure 4. 13C NMR spectrum of MEA-COS reaction products in D,O (20% sample).
of the fraction of component a is 0.58 and b is 0.68 in the peaks labeled 11 and 12, respectively. Based on this observation, a fair estimate of n is 0.6, and consequently, m was calculated to be 0.2 (reaction 10). The reaction products are not completely regenerable because of degradation of MEA thiocarbamate, when heated, into 2-oxazolidone and diethanolurea (reactions 5 and 6). Detection of the C=O group of oxazolidone and diethanolurea is generally within the 150-200 ppm range of the 13C NMR spectrum. Figure 4 does not show any absorption response within this range, which is not surprising because no additional heat was applied to the system except for the heat release due to the exothermic forward reaction in reaction 10. If any oxazolidone or diethanolurea was produced because of this heat, the amount was too small for detection by NMR. The solid product from the reaction between MEA and COS was separated from the liquid mass by using a centrifuge. Visible, ultraviolet, and infrared spectroscopies were employed to determine the molecular structure of the solid and liquid product mixture. NMR solvents like deuteriochloroform, deuterium oxide, and deuterioacetone did not dissolve the solid reaction product phase significantly; hence they could not be used as internal NMR solvents. The solid product was somewhat miscible in carbon tetrachloride (CC14),and the use of an external D20 locking arrangement rendered the sample amenable to NMR spectroscopic analysis. However, the solubility of the solid and liquid product phases was so low that 13C NMR scans did not show any absorption peak. For further information, infrared spectroscopy analysis was carried out on the solid and liquid reaction product phases dissolved in CC1,. The scans are presented in Figures 5, 6, and 7, respectively. The pure MEA infrared scan (Figure 5) shows the assignment of various groups present in the MEA molecule. Primary amines generally show a broad band of peaks at wavenumbers less than
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 473 1
0
0
~
"
"
"
'
"
1
'
951
I 3500
40LO
3000
2500
1500
2000
, u
,
500
1000
I 10
I
9
E
7
5
6
WAVE NUMBERS
2
3
4
1
0
PPM
Figure 5. Infrared spectrum of pure MEA dissolved in CC14.
Figure 8. 'H NMR spectrum of MEA-CH3SH absorption system in DCCI, (20% sample). 2
*
75
70 40O ;
t
NH2 I
I
3500
1
~
I
3000
2500
'
'
'
2000
'
1500
'
"
1000
YI
I
500
WAVE NUMBERS
Figure 6. Infrared spectrum of solid product dissolved in CCI, (MEA-COS system).
200
180
160
120
140
100
EO
60
40
20
0
PPM
Figure 9. 13C NMR spectrum of pure DEA in D20 (15% sample).
I 1
proton impurity. The SH and CH, protons are indicated by labels 15 and 17, respectively. The NMR scans were obtained with 20% of the reaction sample dissolved in D20, which may be an adequate concentration for the 13C to compute an absorption response of the mercaptan carbon. The weak basicity of MEA does not allow the ionization of methyl mercaptan into H+ and CH3+ ions. Comparison of Figure 1with Figure 8 shows thAt in Figure 8 the nitrogen and hydroxyl protons are distinctly upfield relative to their position in the pure MEA scan. This implies that the nitrogen and oxygen are serving as Lewis acid-base interaction sites. The interaction probably is of the type
ao 75 4000
3500
3000
2500
2000
1500
1000
500
WAVE NUMBERS
Figure 7. Infrared spectrum of liquid residue dissolved in CCl, (MEA-COS system).
1000, which is observed in Figure 5 . Comparison of the pure MEA spectrum with the solid product scan (Figure 6) reveals a new peak of S H centered a t 2300 wavenumbers. The thiocarbamate product of MEA has the structure HO CH2
CH2
NHC=O
I
SH
2
The S H peak was also noticed in the liquid residue spectrum (Figure 7). This indicates that some thiocarbamate was dissolved in the liquid residue. All three infrared spectra indicate strong absorption peaks for wavenumbers less than 800, which signifies appreciable concentrations of the amine group.
Solubility of Methyl Mercaptan in Monoethanolamine The 'H NMR spectrum of the mercaptan-amine system (DCC1, solvent) is illustrated in Figure 8. The absorption peaks centered at labels 16 and 19 are the protons of the @ and a amine carbons, respectively. The OH and N protons are indicated by label 18, and label 14 shows DCCI,
H2N
CH2
CH2
OH I
I
HS *CH3
HS CH3
3
Reaction between Carbonyl Sulfide and Diet hanolamine Pure DEA samples were scanned for protons and carton-13 (DzOsolvent). The 13CNMR spectrum is presented in Figure 9. The skeletal structure of DEA is CH2 H-N'
',
CH2 OH
'sH25H2
OH
4
In Figure 9, labels 20 and 21 indicate the @ and a carbon nuclei absorption responses, respectively. Since the twin a and @ carbon nuclei are in identical magnetic environments, their absorption peaks are observed a t the same position in the 13CNMR spectrum. The reaction products were miscible in D,O and analyzed for proton and carbon-13. The proton spectrum was identical with the pure DEA spectrum except for a downfield shift of 0.5 ppm. Further information was obtained from the 13C NMR spectrum (Figure 10). The peaks labeled 23 and 24 are
474 Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989
I
200
180
160
140
120
100
80
60
40
20
0
J
* O h k - + - 1 4 0
120
,do
60
80
40
20
i
PPM
PPM
Figure 10. 19C NMR spectrum of DEA-COS reaction products in D 2 0 (15% sample).
Figure 11. 13C NMR spectrum of DGA-COS reaction products in D 2 0 (20% sample).
the /3 and a carbon nuclei, respectively. The /3 carbon nuclei in the DEA molecule exhibit an upfield shift of about 3 ppm in Figure 10. The product conversion of COS was only about 10% of the original COS concentration. Label 22 is the COSH product formed according to the following reaction:
similar to that of pure DGA except for an upfield shift of 0.2 ppm. A hydrogen bonding type of interaction probably occurs in this case in a manner similar to structure 2.
CH2*CH2*OH
i
+ cos
-
HO* CHp- CH2 HO ‘CH2
CH2’
-
CH2
/ ‘NCOS-NH2+
C ‘ H2
(CHyCHOHCH2)2NCOS-NH2+(CH&HOH*CHJ~
CH2. OH CH2
0H
(11)
Solubility of Methyl Mercaptan in Diethanolamine After 30 min of reaction, the mercaptan solubility was less than 10%. The ‘H and 13C NMR of the product mixture did not yield any information with regard to hydrogen bonding or a chemical reaction. The relatively large viscosity of liquid DEA causes significant resistance to liquid-phase solute transport, thus leading to poor mercaptan absorption by DEA. Reaction between Carbonyl Sulfide and Diglycolamine The proton scan of the COS-DGA reaction products was not sharply resolved even when diluted to 10% sample strength in D20. The 13C NMR spectrum, however, revealed some new information for this reaction system (Figure 11). The anticipated reaction is of the form 2HOCH2CH2OCH2CH2NHz + COS 0
7
P
a
Reaction between Carbonyl Sulfide and Di-2-propanolamine The reaction rate closely followed that of COP with DIPA; hence, a similar reaction scheme was postulated 2(CH,*CHOHCHz)2*NH+ COS @ P Y
-
HOCH2CH20CH2CH2NHCOS-NH3+CH2CH20CH,CH2 OH (12)
In Figure 11the peaks labeled 25 and 26 are the y and
/3 carbon nuclei, respectively. Peak 29 is that of the
u
carbon, and the CY carbon is represented by peak 30. The COS peak, as in the case of MEA in Figure 4, is represented by label 27. The y and 0carbon nuclei of the DGA thiocarbamate (reaction 12) are represented by peak 28 and the CY carbamate carbon by peak 31. The product conversion is determined by the ratio of the peaks at labels 31 and 30 which is 0.22, whereas the amount of COS reacted according to the experimental run was 28%.
Methyl Mercaptan Absorption in DGA The methyl mercaptan-DGA absorption product system was analyzed for protons and carbon-13. The 13C NMR spectrum did not yield any new information, and the proton scan of the mercaptan-DGA system is basically
(13) The ‘H spectrum of the COS-DIPA reaction products was indistinguishable from that of pure DIPA. Since the N and OH proton absorptions appear as a lump in the pure DIPA spectrum, a significant amount of product formation is required to have a visible effect on the product spectrum. The thiocarbamate caron nuclei could not be isolated with certainty from the I3CNMR spectrum. The low-resolution power of the 100-MHz NMR may cause overlapping of the thiocarbamate peaks with the pure DIPA peaks. An important aspect of DIPA reactions with COS and COz is the steric hindrance caused by the 2-propanol groups. This hampers the transport of the COS molecule to the vicinity of the nitrogen atom. For this reason, DIPA is not as effective as other primary and smaller chained secondary amines for removal of COS. Processes employing DIPA as a solvent require longer residence time relative to other commonly used amines.
Solubility of Methyl Mercaptan in Di-2-pro panolamine
Di-2-propanolamine is midly basic and consequently does not facilitate the dissociation of the methyl mercaptan molecules. However, a strong hydrogen bond is speculated between the amine of the DIPA molecule and the S proton of the CH,SH molecule. In the carbon-13 spectrum, no mercaptan peak could be detected and no shift was noticed of the a , 0,and y DIPA carbon nuclei. The proton scan (Figure 12) shows the adsorption response of the NH and OH protons (peak 33) as a lump. The peaks centered at labels 32, 34, and 36 are the a , 0,and y DIPA protons, respectively. Relative to pure DIPA, there are 1 ppm upfield shifts of the nitrogen proton and hydroxyl protons. This may be attributed to the fact that the nitrogen and hydroxyl protons serve as hydrogen-bonding sites. The bonding may be of the form (CH3 * C H O H * C H 2 ) 2 * N H CH3SH
HSCH3 5
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 475
I
10
9
8
7
6
5 PPM
4
3
2
t
0
Figure 12. 'H NMR spectrum of DIPA-CH3SH absorption system in DCC13 (10% sample).
Reactions Involving tert -Amines No evidence for reaction products from the interaction of either carbonyl sulfide or methyl mercaptan with anhydrous methyldiethanolamine or dimethylethanolamine could be determined by comparison of the 'H and 13C NMR spectra for pure and reacted samples. Pressure drops of about 20% of the initial pressure of the gas charged were observed when these amines were introduced into the reaction flask containing either COS or CH,SH. However, a striking dissimilarity was observed between the reaction of DMEA with COz and COS. The COS gas absorption was 12.6% of the initial COS gas initially charged, whereas only 5.7% of the initially charged C02 was absorbed. This significant difference was unanticipated because of the molecular similarities of COz and COS. MDEA reactions with C02 and COS showed fairly uniform absorption results. Since thiocarbamate formation is not possible for tert-amines and water was not present, the failure to observe any difference between the NMR spectra of these tert-amines before and after COS absorption simply indicates that the Lewis acid-base complex was too low in concentration for detection after venting to the atmosphere. There were small shifts in the location of the 'H and '3C adsorptions for these tert-amines, but in view of the fact that small shifts can be caused by uncertainties in concentration measurements, they cannot positively be attributed to complex formation. These observations suggest that water may play a very important role in stabilizing the Lewis acid-base complex for both CH3SH and COS. 'H NMR analysis of MDEA and DMEA treated with CH3SH indicated either a distinct shift of the hydroxyl proton (MDEA) or formation of a new lump (DMEA). Both of these occurrences are due to Lewis acid-base interaction between the amines and mercaptan molecule. Conclusions Carbonyl sulfide yielded the respective thiocarbamates when reacted with MEA, DEA, and DGA. The thiocarbamate salt of DIPA could not be positively ascertained presumably due to the limitations of the 100-MHz NMR system. In the case of the tert-amines, no evidence of
chemical reaction was obtained from the NMR spectrum for either DMEA or MDEA with COS. The pressure drop trend suggests that Lewis acid-base complexes are formed with these amines, but the complex decomposes when pressure is reduced in the stirred flask reactor to extract samples. The exposure of methyl mercaptan to MEA, DEA, DGA, DIPA, DMEA, and MDEA did not lead to a reaction. However, shifts or lump formation in the proton NMR spectra indicate the probability of Lewis acid-base complex formation with all the amines.
Acknowledgment The authors acknowledge the assistance of Stanley Siege1 with the NMR analysis and Ann Ratcliffe with the infrared analyses. Both instruments were provided by the Department of Chemistry at Oklahoma State University. Fluid Properties Research, Inc. provided financial support for M. A. Rahman. Registry No. MEA, 141-43-5; DGA, 929-06-6; DEA, 111-42-2; DIPA, 110-97-4; MDEA, 108-01-0;DMEA, 105-59-9; COS, 46358-1; CHSSH, 74-93-1.
Literature Cited Batt, W. T. Monoethanolamine Reactions with Selected Acid Gases. M.S. Thesis, Oklahoma State University, Stillwater, 1979. Batt, W. T.; Maddox, R. N.; Mains, G. J.; Rahman, M.; Vaz, R. N. Chemical and Engineering Fundamentals of Ethanolamine Sweetening. Proceedings Gas Conditioning Conference, University of Oklahoma, Norman, March 1980. Danckwerts, P. V. Gas Liquid Reactions; McGraw-Hill: New York, 1970. Danckwerts, P. V.; Sharma, M. M. The Absorption of Carbon Dioxide into Solutions of Alkalis and Amines. Chem. Eng. 1966, Oct,
CE144. Kohl, A.; Riesenfeld, F. Gas Purification, 3rd ed. Gulf Publishing: Houston, TX, 1979. Landry, J. E. The Effect of a Second Order Chemical Reaction on the Absorption of Methyl Mercaptan in a Laminar Liquid Jet. Ph.D. Thesis, Louisiana State University, Baton Rouge, 1966. Maddox, R. N. Gas and Liquid Sweetening, 2nd ed.; Campbell Petroleum Series: Norman, OK, 1977. Maddox, R. N.; Mains, G. J.; Rahman, M. A. Ind. Eng. Chem. Res. 1987,26, 17-31. McClure. G. P.: Morrow. D. C. Amine Process Removes COS from Propane Economically. Oil Gas J. July 2, 1979, 106-8. Pearce, R. L.; Arnold, J. L.; Hall, C. K. Studies Show Carbonyl Sulfide Problem. Hydrocarbon Process. Pet. Refin. 1961,40(8), 121.
Rahman, M. A. Study of Reactions of Carbon Dioxide and Sulfur Containing Compounds with Ethanolamines. Ph.D. Thesis, Oklahoma State University, Stillwater, 1984. Sharma, M. M. Kinetics of Reactions of Carbonyl Sulfide and Carbon Dioxide with Amines and Catalysis by Bronsted Bases of the Hydrolysis of COS. Trans. Faraday Soc. 1965, 61, 681. Sharma, M. M.; Danckwerts, P. V. Absorption of Carbonyl Sulfide in Amines and Alkalis. Chem. Eng. Sci. 1964, 1, 991-92. Skoog, D. A.; West, D. M. Principles of Instrument Analysis, 2nd ed.; Saunders Golden Series; W. B. Saunders: Philadelphia, 1980. Weber, S.; McClure, G. New Amine Process for FCC, Oil Gas J . 1981, 160-163, June 8. Received for review February 9, 1988 Accepted November 14, 1988
