480
Ind. Eng. Chem. Res. 1994,33, 480-487
KINETICS, CATALYSIS, AND REACTION ENGINEERING Mechanism and Kinetics of COS-Induced Diethanolamine Degradationf Olukayode F. Dawodu and Axel Meisen' Department of Chemical Engineering, The University of British Columbia, Vancouver, British Columbia, Canada V6T 124
The degradation of aqueous diethanolamine (DEA) solutions by carbonyl sulfide was examined by using a 600-mL well-stirred reactor operating under the following conditions: DEA concentration 20-40 w t %, temperature 120-180 "C,COS partial pressure 0.3-1.17 MPa. The reaction products were identified by GC/MS, and reaction mechanisms are developed which conform with experimental observations. The reaction rate constants are determined, and a mathematical model for estimating DEA degradation by COS is presented.
Introduction Natural, synthesis and refinery gases frequently contain impurities such as carbon dioxide (COz),hydrogen sulfide (HzS),carbonyl sulfide (COS), and carbon disulfide (CSZ). Alkanolamine-based processes are widely used for the removal of the acidic constituents, with diethanolamine (DEA) being the generally preferred choice when COS is present. An important problem in amine sweetening is "degradation", Le., the irreversible transformation of amines into undesirable products. Degradation not only causes a loss of valuable amine, but it may also contribute to operational problems such as foaming (Smith et al., 1972;Pauley and Hashema, 1989), corrosion (Polderman and Steel, 1956; Moore, 1960; Hall and Barron, 1981; Chakma and Meisen, 1986), and fouling (Chakma and Meisen, 1987a). Although the degradation of DEA by COZ has been studied quite extensively (Smith and Younger, 1972; Polderman and Steel, 1956; Chakma and Meisen, 1987a; Choy, 1978; Kim and Sartori, 1984; Kennard and Meisen, 1985; Hakka et al., 1968; Hsu and Kim, 1985; Kennard, 1983; Chakma, 1987), relatively little is known about DEA degradation due to COS. Orbach and Selleck and Pearce et al. (1961)were unable to detect appreciable amounts of degradation compounds in COS-DEA systems, and they concluded that, unlike monoethanolamine (MEA), DEA is not degraded by COS. The aforementioned results are somewhat surprising since it is wellknown that COS is hydrolyzed in aqueous solutions to HzS and COZ(Sharma, 1965; Thompson et al., 1935) and that COZdegrades DEA. The increased occurrence of COS in natural and other hydrocarbon gases makes it necessary to reevaluate the previous claims regarding DEA's degradation resistance. We recently reported findings that indicate that COS degrades DEA (Dawoduand Meisen, 1991). In the present paper, the reaction mechanisms are proposed and a kinetic model is presented for estimating the concentrations of DEA and the degradation products. Experimental Apparatus and Procedure The degradation experiments were conducted with a 600-mL stainless steel reactor which was described pre+ This is an extended version of a paper presented at the 40th Canadian Chemical Engineering Conference, Halifax, NS, July
15-20, 1990. 0888-588519412633-0480$04.50/0
viously by Dawodu (1991). In a typical run, 250 mL of an aqueous DEA solution with the desired concentration was placed into the reactor. The reactor was then closed, the stirrer started, and the air purged by passing nitrogen through the reactor for about 15 min. The reactor temperature was then brought to the desired value. COS was forced from a pressurized cylinder into the reactor. The cylinder exit pressure was set with apressure regulator, and the COS cylinder remained connected to the reactor throughout the run. Small samples of the liquid and gas phases could be withdrawn from the reactor without interrupting the runs. The samples were subsequently analyzed by gas chromatography (Dawodu and Meisen, 1991;Dawodu, 1991),but an improved method of analysis involving a polyethylene glycol capillary column was recently reported (Dawodu and Meisen , 1993). COS was supplied by Matheson Inc. (Edmonton, AB) and had the following composition: COS, 97.7%; COZ, 1.4%;CS2,0.19%; HzS, 0.01%; 0 ~ ~ 0 . 1 CO % ; and/or Nz, 0.6%. The nitrogen, air, and hydrogen (>99% purity) used for the GC analysis were acquired from Medigas Ltd. (Vancouver,BC). DEA (>99% purity) and the compounds used for the calibration of the gas chromatograph were purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI) with the exceptions of HEOD, HEI, THEED, and BHEI which were unavailable and had to be synthesized (Drechsel, 1957; Moller, 1979; Kennard, 1983; Dawodu, 1991). The chemical names and structures of compounds are given in Table 1.
Results and Discussion Using previously reported GCIMS techniques (Dawodu and Meisen, 1991; Dawodu, 19911, 15 compounds were typically found in degraded solutions (see Figure 1 and Table 2). The products may be conveniently grouped into low and high boiling degradation compounds which elute before and after DEA, respectively. Apart from the low boiling degradation compounds shown in Table 2, methanol, acetaldehyde, acetic acid, methylpyridine, diethyl disulfide, ethylmethylpyridine, and 1,Zdithianewere also found in the partially degraded laboratory solutions. Analysis of the gas phase in the reactor revealed the presence of high molecular weight ketones (suspected to be pentanone and hexanone) as well as the low boiling degradation compounds detected in the liquid phase. In addition to the aforementioned water-soluble degradation 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 481 Table 1. Structures of Some of the Compounds in the
COS-DEA System compound
chemical structure
monoethanolamine carboxymonoethanolamine
diethanolamine
carboxydiethanolamine
-
N f l -bis (hydroxyethyl) ethylenediamine (BHEED)
carboxy-BHEED
NN-bis(hydroxyethy1)imidazolidone (BHEI)
NJV-bis(hydroxyethy1)piperazine (BHEP) N-(hydroxyethy1)imidazolidone(HEI)
N-(hydroxyethy1)oxazolidone(HEOD)
NfljV'-tris(hydroxyethy1)ethylenediamine (THEED) Carboxy-THEED
Reaction Mechanism. The number and variety of degradation products implies that the degradation reactions are complex. In addition to the normal degradation runs, further experiments were therefore conducted to understand the roles and transformations of the various species in the partially degraded solutions. The details of this work aregiven by Dawodu (1991)but the main findings may be summarized as follows: 1. Experiments conducted to determine the effects of operating variables on the degradation reactions showed that MEA, BHEED, and THEED are reaction intermediates whereas BHEP, HEOD, HEI, and BHEI are end products (Dawodu and Meisen, 1992). Since the products of the COS-induced degradation of DEA are similar to thosefound in DEA solutions in contact with gas mixtures of C02 and H2S, it suggests that COS was first hydrolyzed to C02 and H2S and that these compounds are largely responsible for the degradation. 2. Equilibrium COS solubility and hydrolysis experiments conducted to quantify the extent and rate of hydrolysisshowed that equilibrium hydrolysis was attained prior to the commencement of significant degradation and that the CO2 and H2S loadings in the DEA solution were typically about 2 orders of magnitude higher than the COS loadings. 3. Other experiments conducted with DEA solutions spiked with certain degradation compounds prior to degradation provided information on the relationship between certain degradation products. Compounds such as acetaldehyde, acetone, butanone, EAE, HEA, EDEA, and ETAHEAME were formed in low concentrations at temperatures typical of industrial DEA regenerators. For practical purposes, they are minor products and, consequently, the mechanisms for their formation are not discussed but have been reported elsewhere (Dawodu, 1991). On the basis of the experimental results, the mechanism for DEA degradation and the formation of the major degradation compounds can be postulated. (a) Formation of MEA. The formation of MEA and the low boiling degradation compounds appears to be initiated by the absorption and hydrolysis of COS:
+ COS = (HOC,H,),NCOS-H+ (1) (HOC,H,),NCOS-H+ + H,O = (HOC,H,),NH + H,S + (HOC,H,),NH
compounds, the COS-DEA system also gave rise to an insoluble solid product (Dawodu and Meisen, 1991; Dawodu, 1991). Preliminary experiments showed that the degradation reactions could be speeded up by operating in the range of 120-180 "C within which DEA is still known to be thermally stable for the experimental durations used in this study (Kennard, 1983; Dawodu, 1991). As shown by Figure 1, similar degradation products were obtained at all the temperatures used, which suggests that the basic reaction mechanism is unaffected by temperature. Gas chromatographic analysis of the lean aqueous DEA solution from a gas plant treating sour natural gas containing 12.9% C02 and 1.9% H2S (Figure 2) shows degradation compounds such as MEA, HEP, BHEED, BHEP, HEOD, HEI, THEED, and BHEI; HEOD is usually a significant compound in solutions of gas plants utilizing diethanolamine. The very low concentration of HEOD in this particular sample is probably due to the fact that the solution had been dosed with sodium and potassium hydroxide which caused the reversal of HEOD to DEA (Chakma, 1987). The similarity of products found in the laboratory and industrial samples suggests that the same degradation mechanism applies in both cases.
CO, (2) Kinetic experiments reported by Littel et. al. (1992) have confirmed that eq 1 involves the COS analogue of the zwitterion mechanism proposed by Caplow (1968) for the reaction between C02 and secondary amines. The dissolved carbon dioxide gives rise mainly to H+ and HCOs-, whereas hydrogen sulfide yields primarily H+ and HS-: CO,
+ H,O = H+ + HCOL H,S = H+ + HS-
(3)
(4)
The DEA molecules are readily protonated: (HOC,H,),NH
+ H+ = (HOC2H4),NH2+
(5)
The DEA molecules also react with C02 to form carboxyDEA (HOC,H,),NH
+ CO, = (HOC,H,),NCOO-H+
(6)
482 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
14
14
I
t =
t
30h
I
= 50h (b)
(a)
IS
t
* 215h (e)
Figure 1. Chromatograms of partially degraded aqueous DEA solutions of initial DEA concentration of 4 M [(a) T = 180 OC, PCOS= 0.34 MPa; (b) T = 150 “C, Pcos = 0.34 MPa; (c) T = 120 “C, PCOS0.68 MPal. Table 2. Compounds Detected in the COS-DEA System retention peak time,a min compound 1 1.4-1.5 acetone 2 2.2-2.3 butanone 3 3.1-3.3 monoethanomine (MEA) 4 5.2-5.3 ethylaminoethanol (EAE) 5 9.2-10.0 diethanolamine (DEA) 6 10.9-11.1 ethyldiethanolamine (EDEA) 7 11.5-11.6 (hydroxyethy1)acetamide (HEA) 8 12.Ck12.1 (hydroxyethy1)piperazine (HEP) 9 13.4-13.5 ethanethioic acid S-(hydroxyethy1)aminomethyl ester (ETAHEAME) 10 15.2-15.5 bis(hydroxyethy1)ethylenediamine(BHEED) 11 16.2-16.6 NJV’-bis(hydroxyethy1)piperazine(BHEP) 12 16.7-16.9 N-(hydroxyethy1)oxazolidone(HEOD) 13 18.2-18.4 N-(hydroxyethy1)imidazolidone (HEI) 14 19.7-19.8 N,NJV’-tris(hydroxyethy1)ethylenediamine (THEED) 15 21.0-21.3 NJV’-bis(hydroxyethy1)imidazolidone (BHEI) a Based on the GC conditions specified by Dawodu and Meisen (1991). The retention times may vary slightly depending on the age of the column and the concentrations of the compounds.
which establishes an equilibrium with the bicarbonate ions: (HOC,H,),NH
+ HCO;
= (HOC,H,),NCOO-
+ H,O
(7)
Equations 1-5 and 7 can be combined to give the overall reaction for the COS-DEA system: 2(HOC2H4),NH+ COS + H 2 0 = (HOC,H,),NH,+HS- + (HOC,H,),NCOO-H+ (8) The protonated DEA molecule can lose one hydroxyethyl group to form MEA: (HOC,H,);
co2
= HOC2H,NH2
+ “+C2H,0H”
(9)
To confirm that the transformation of higher order alkanolamines to lower order ones as shown by eq 9 is not limited to secondary alkanolamines, a tertiary amine, ethyldiethanolamine (EDEA), was degraded in the usual manner. GC/MS analysis of the partially degraded solution showed the presence of acetone, butanone, acetic acid, ethylamine, and ethylaminoethanol (EAE) in the partially degraded solution. This result indicates thi’t the transformation
Figure 2. Chromatogram of lean aqueous DEA solution from a gas plant treating natural gas containing 12.9% COz and 1.9% H2S.
ethyldiethanolamine
-
ethylaminoethanol
-
ethylamine
occurred and thus confirms that the reaction represented by eq 9 can be generalized to other classes of alkanolamines. It may be noted that the COz involved in the reaction need not be present in the form of carbamates since tertiary amines such as EDEA do not form carbamates. The hydroxyethyl group in eq 9 is written in quotation marks because it does not actually exist in the solutions. It is suggested that it is transformed immediately according to the following scheme: (i) The bisulfide ion formed in eq 4 reacts with C02 to yield the thiobicarbonate ion as proposed by Al-Ghawas et al. (1989): HS- + CO, = HC02S-
(10)
(ii) The thiobicarbonate ion then reacts with the hydroxyethyl group to form an enol of acetaldehyde:
HC0,S-
+ "+C,H,OH"
-
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 483 CH,=CHOH
+ H,S + CO, (11)
This reaction provides a means for the fast transformation of the hydroxyethyl group released in the reaction represented by eq 9 and is probably the driving force for the formation of MEA. The presence of H2S and COZin eq 11indicates that both compounds are not used up, but only catalyze the transformation of DEA to MEA. Equation 11 also explains the lack of formation of MEA in C02-DEA or H2S-DEA systems, since such systems cannot form the thiobicarbonate ion. Chakma (1987) detected ethylene oxide and ethylene glycol in MDEA solutions degraded by COZat elevated temperatures and attributed those compounds to the hydroxyethyl group released from protonated MDEA. The lack of formation of ethylene glycol in the present study, despite the release of the hydroxyethyl group, suggests that, given the composition of the solution in the present study, the reaction described by eq 11is a more favorable route for the transformation of the hydroxyethyl group. (b) Formation of N-(Hydroxyethy1)oxazolidone (HEOD). Kim and Sartori (1984) and Kennard and Meisen (1985) have shown that carboxy-DEA dehydrates to HEOD and that an equilibrium exists between both compounds: DEACOO-H+ = HEOD + H,O
(12)
(c) Formation of N,N,M-Tris(Hydroxyethy1)ethylenediamine (THEED). The production of THEED from DEA or DEA and carboxy- DEA has already been reported by Kennard and Meisen (1985): 2DEA
-
THEED + H,O
or DEA
+ DEACOO-H+
-
THEED
(13)
+ H,O
(14)
Equation 13 represents the thermal route for THEED formation. I t may be discounted at the temperatures used in this study as well as typical conditions in DEA plants. Kim and Sartori (1984) reported a different mechanism which indicates that THEED is formed from the reaction of HEOD and DEA. In deciding which of the reported mechanisms is more applicable to degrading DEA solutions, the limiting cases of Con-rich and COz-deficient solutions are considered: In basic solutions, such as partially degraded DEA solutions deficient in COZ,the HEOD ring is quite unstable, reverting to DEA via carboxy-DEA. THEED could therefore be formed by the reaction of DEA with the unstable HEOD as suggested by Kim and Sartori or through the reaction of DEA and carboxy-DEA produced from the reversal of HEOD in accordance with the mechanism put forth by Kennard and Meisen. As such, both mechanisms may hold under COz limiting conditions. In less basic solutions, as is the case with COz-rich DEA solutions, the stability of the HEOD ring is higher, making it less reactive. Since HEOD maintains an equilibrium with carboxy-DEA, the reaction of DEA with its carboxyDEA may be faster than the reaction of HEOD with DEA. Therefore, the mechanism suggested by Kennard and Meisen appears to be the more favorable one for THEED formation in such solutions. (d) Formation of Bis(hydroxyethy1)ethylenediamine (BHEED).BHEED was only formed in solutions containing MEA, DEA and CO,. Its formation is thus the
result of the reaction between MEA and carboxy-DEA or carboxy-MEA and DEA: MEA + DEACOO-H+
DEA + MEACOO-H+
-
BHEED
+ H,O + CO,
BHEED + H,O
+ CO,
(15) (16)
These reactions result in the N,N and N,N' isomers of BHEED. The thermal formation of BHEED (as a result of the reaction of DEA with MEA) does not proceed readily at the temperatures used in this study. (e) Formation of N,N-Bis(hydroxyethy1)piperazine (BHEP) and N-(Hydroxyethy1)piperazine (HEP). THEED, in the presence of COz, dehydrates to BHEP (Kim and Sartori, 1984; Kennard and Meisen, 1985): THEED + CO,
-
BHEP + CO,
+ H,O
(17)
(f) Formation of Bis(hydroxyethy1)imidazolidone (BHEI). BHEI results from the dehydration of carboxyBHEED: BHEEDCOO-Ht
-
BHEI
+ H,O
(18)
or the reaction of MEA with carboxy-DEAand the reaction of carboxy-MEA with DEA:
+ DEACOO-H+ MEACOO-H+ + DEA MEA
-
-
BHEI + H 2 0
(19)
BHEI + H,O
(20)
Equations 19 and 20 are two step reactions consisting of the coupling of MEA and carboxy-DEA to form an intermediate, and the cyclization of the intermediate product. Carboxy-BHEED appears to be the most likely intermediate product, and since BHEED was a stable intermediate, the formation of BHEI is better represented by eq 18. When an aqueous solution of BHEED was saturated with C02 and maintained at 165"C, the BHEED was converted virtually completely to BHEI within 24 h. Only trace amounts of HEP were detected in the solution. Furthermore, in all the degradation experiments, the formation of BHEED preceded that of BHEI (Dawodu, 1991). (f) Formation of N-(Hydroxyethy1)imidazolidone (HEI). The formation of HE1 proceeds via the reaction between MEA and carboxy-MEA: MEA + MEACOO-H+
-
HE1 + 2H,O
(21)
The same reaction could also produce (hydroxyethy1)ethylenediamine (HEED), but this compound was not detected probably due to its transformation to HE1 through a reaction similar to that represented by eq 18. It is also possible that HEED is embedded in the DEA peak since it elutes just before DEA (Kennard, 1983) and since the DEA peak is very large. Therefore, eq 21 may involve HEED as an intermediate. The above equations represent the overall reactions and may, in detail, involve additional ionic steps. Kinetics. In order to develop a kinetic model based on the reactions, the following simplifying assumptions are invoked. 1. The solubility and hydrolysis reactions governed by eqs 1-6 are much faster than the degradation reactions, and equilibrium acid gas loadings of the liquid phase are
484 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
achieved prior to any significant degradation. As a result, the degradation reactions become the rate-controlling steps. 2. With the exception of eqs 1-6, all reactions are considered to be irreversible because the experimental data suggest that, for the duration of the experiments, equilibrium was still in favor of product formation represented by the forward reactions. 3. Ionic species such as DEAH+ and DEACOO-H+ may be written as resulting from
(25) (26)
(37)
(27)
where i = 1,2, ...,8 denotes the compounds DEA, MEA, BHEED, BHEP, HEOD, HEI, THEED, and BHEI, respectively,in the kinetic expressions. The times at which samples were taken are represented by t = 1 , 2 , ...,N . Yci, Yei, and Ymmidenote the calculated, experimental, and maximum experimental concentrations for compound i, respectively. The term Ymq was introduced as a weighting factor to account for the considerable differences between the concentrations of DEA and the degradation products. A Runge-Kutta differential equation solver (Moore, 1983) was employed to solve the differential equations. The rate constants obtained from the above procedure were functions of temperature and pressure and could be represented by an Arrhenius-type equation. Except for k3 and k5, the dependency on the initial DEA concentration did not follow any specific pattern and was therefore attributed to scatter in the experimental data and/or approximations in the NLPQL and Runge-Kutta routines. The variation with concentration was eliminated by taking the average of the constants (except k3 and k5) for all runs conducted at the same temperatures and COS partial pressures. Table 3 lists the frequency factors and acti-
+ HS- = DEA + H,S DEACOO-H+ = DEA + CO,
4. The reaction between MEA and DEACOO- is equivalent to the reaction between MEACOO- and DEA. 5. The amine solutions are sufficiently dilute so that the concentration of water can be neglected in the kinetic expressions. 6. When C02 or H2S appears on both sides of an equation, they are considered to act as a catalyst and need not be included in the kinetic expressions. 7. Reactions leading to the formation of solids are neglected because elemental analysis showed that the amount of amine responsible for solids formation is small (Dawodu and Meisen, 1991; Dawodu, 1991). The above assumptions lead to the following simplified set of overall reactions which are not necessarily balanced:
ki
MEA + CH,CHO
DEA + MEA DEA + CO,
kz
ks
2MEA + CO,
ks
2DEA
(23)
HEOD + H,O
(24)
HE1 + 2H,O
THEED + H,O
ke
THEED
(22)
BHEED + H,O
k4
BHEP + H,O
k7
BHEED + CO, BHEI + H,O The corresponding rate equations are
(35)
d[BHEIl/dt = k,[BHEEDl [CO,] (36) where [il denotes the concentration of compound i in units of mol/L. Equations 29-36 represent the simplified kinetic model for the DEA degradation by COS. Although the C02 and H2S loadings were obtained from the COS solubility and hydrolysis system described by eqs 1-6 (Dawodu, 19911, these values were assumed to be either constant or not limiting throughout the duration of the runs. Therefore, the acid gas loadings can be lumped into the rate constants where applicable. It is recognized that, as degradation proceeds, the concentration of DEA falls thereby causing a reduction in the C02 and H2S solution loadings. However, since MEA and other degradation compounds such as BHEED, THEED, and BHEP (Chakma and Meisen, 1987b) are also able to absorb acid gases, their presence in the degraded solution will compensate for the reduction in acid gas loadings associated with the reduction in DEA concentration. The acid gas loadings should therefore remain approximately constant. It should be noted that eqs 29-36 are kinetic expressions and do not strictly follow the reaction stoichiometries. For example THEED formation has a molecularity of 2 with respect to DEA according to eqs 14and 26, but the first-order kinetic representation in eq 35 has been found in the past (Kennard, 1983;Chakma, 1987)to represent experimental data better. This was also the case in the present study. In order to solve eqs 29-36, the rate constants kl to k7 must be known. They were determined by first fitting a fourth-order polynomial expression to the concentrationtime measurements for DEA, MEA, BHEED, BHEP, HEOD, HEI, THEED, and BHEI. In this way, the concentrations for each compound could be found at uniform time intervals. For each experimental run, a nonlinear optimization routine, NLPQL (Vassen, 1983), was used to search for the set of rate constants kl to k7 which gave the best agreement with the experimental measurements. The objective function, which was minimized in the search, was defined as
DEAH'
DEA
d[THEED]/dt = k,[DEA] - k,[THEED]
(28)
d[MEAl/dt = kl[DEA] - k4[MEA12[COJ k,[DEAl [MEA] (30) d[BHEEDl/dt = k,[MEAl [DEAI - k7[BHEED][CO,] (31) d[BHEPl/dt = k,[THEED]
(32)
d[HEODl/dt = k3[DEAl [CO,]
(33)
d[HEIl/dt = k,[MEA12[C0,1
(34)
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 485 Table 3. Frequency Factors (A) and Activation Energies (E)in the Reaction Rate Constants Expression kj = A exp(-E/RT)
ki (h-1 or L (mol h)-l) ki kz k3 k3 k3 k4 k5 k5 k5 ks ki
A (h-1 or L (mol h)-l) 4.353 x 104 1.525 X lo7 4.029 X lo3 2.683 X lo3 1.774 X lo2 8.554 X lo8 9.900 x 106 1.931 X lo7 1.343 X lo6 5.398 X lo4 1.412 X lo2
E (J mol-') 56 174 80 282 55 063 54 924 47 272 94 931 77 450 82 670 75 604 59 315 31 077
[DEAlo (mol/L)
4 3 2 4 3 2
tion energi-s of the rate constants for all the runs performed at an initial COS partial pressure of 345 kPa and at temperatures below 165 "C. Values of the activation energies are typical of liquid-phase reactions, but some of the frequency factors are unusually low. The rate constants kl, k2, k ~and , kg for runs conducted at different initial COS partial pressures (e.g., PI and P2), were found to be related by the expression V
0.0
124
I
I
I
I
80.0
130.0
160.0
200.0
TIME (h) Figure 3. Experimental and predicted concentrations of DEA in COS-DEA systems as a function of temperature.
rd The pressure variation of expression
I
40.0
I
,
pl I
I
can be represented by the
1
- PRED.
(39)
Rate constants ks and 12, were fairly independent of pressure. Since DEA and MEA have the highest absorption capacities compared to the other compounds, it is not surprising that the rate constants which control the reactions involving these two compounds are the ones affected by changes in pressure. By using Table 3 with eqs 38 and 39, it was possible to obtain the rate constants governing the degradation reactions under the following conditions: DEA concentration, 20-40 wt % (approximately 2-4 MI; temperature, 120-165 "C; COS partial pressure, 345-1172 kPa. Results of sensitivity analyses (Dawodu, 1991) indicate that the rate constants obtained from the optimization method are accurate within f 2 0 % . The rate constants were then substituted in the kinetic expressions (eqs 2936), and the expressions were solved with a Runge-Kutta routine (Moore, 1983). Figures 3-5 show the predicted and experimental concentrations of DEA as functions of temperature, initial DEA concentration, and initial COS partial pressure. The average absolute percentage deviations (AAPD) for DEA and the major degradation products, within the region of validity of the kinetic model, are shown in Table 4. For DEA, the model predictions were good, with AAPD below 10% and generally about 5%. In the case of the degradation products, the model predictions were satisfactory, with values of AAPD falling generally between 15 and 35 % . The slightly higher deviations for the degradation compounds are due to the fact that, at low concentrations, a difference of 0.01 M between experimental and predicted concentrations may result in a deviation of 25% or more. From a practical standpoint, it is only necessary to obtain estimates of the concentrations of the degradation compounds to avert operational problems. However, an accurate prediction of the concentrations of DEA is of greater importance in order to provide remedial measures that would ensure the achievement of the specifications
0.0
I
I
12.0
24.0
I
1
1
40.0
60.0
TIME (h?" Figure 4. Experimental and predicted concentrations of DEA in COS-DEA systems as a function of initial DEA concentration.
for the final product purity. Consequently, the much better model predictions for DEA are very beneficial. In summary, the model gave satisfactory predictions for the operating conditions indicated above. It should be possible to extend the range of applicability to COS partial pressures lower than 345 kPa without any significant errors.
Conclusions The mechanisms governing the degradation of DEA by COS have been elucidated. I t is shown that the degrading system consists of the fast initial solubility and hydrolysis reactions followed by the comparatively slower degradation and side reactions. Knowledge of the reaction mechanism aids the development of procedures to minimize degradation or purify partially degraded solutions. For example, the extent of degradation in the COS-DEA system can be reduced by hindering the formation of MEA or removing it by suitable means (e.g., distillation) as it accumulates
486 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 Table 4. Average Absolute Percentage Deviations (AAPD) between Experimental and Predicted Concentrations of DEA and the Major Degradation Compounds in D E A C O S Systems average absolute percentage deviation (AAPD) run 1 2
3 4 5 6 7 8 9 10
11 12 a
[DEAJo(M) 4 4 4 4 3 3 3 3 3 2 2 2
T('C) 165 160 150 127 165 150 127 150 150 165 150 127
Pcos(kPa) 345 345 345 345 345 345 345 759 1171 345 345 345
DEA 6.99 1.53 1.66 8.24 7.40 5.62 2.23 4.18 8.82 3.67 1.32 3.74
MEA 22.73 11.87 17.72 15.86 4.58 7.59 2.40 9.22 6.33 25.65 8.87 11.57
BHEED 37.22 50.82 35.50 42.38 16.05 21.25 25.23 17.71 8.89 26.63 28.28 25.16
BHEP 29.36 24.23 23.77 35.89 29.33 57.09 33.02 NAB NAa 33.05 33.65 25.00
HEOD 7.48 8.14 22.85 1.76 14.40 27.94 27.83 2.77 17.89 17.22 18.66 37.75
HE1 54.87 72.61 18.66 60.61 5.25 29.17 19.77 27.62 16.12 51.30 7.33 33.33
THEED 7.36 22.54 36.61 26.99 40.40 15.81 24.62 23.87 23.60 30.26 NAn 29.22
BHEI 43.40 57.05 18.32 53.72 29.17 6.14 21.30 21.63 35.78 29.75 28.39 28.35
Reading unavailable.
PCOS= partial pressure of carbonyl sulfide, kPa or MPa R = universal gas constant, J (mol K)-1 t = time, h GC/MS = gas chromatography/mass spectroscopy Yei= experimental concentration of compound i, mol/L Yci= calculated concentration of compound i, mol/L YmUi= maximum experimental concentration of compound i, mol/L
Id XI
759 k k
= ll7l kPa
Literature Cited
ld
0.0
1P.O
P4.0
48.0
80.0
TIME (hf8" Figure 5. Experimental and predicted concentrations of DEA in COS-DEA systems as a function of initial COS partial pressure.
in solution. It is also shown that a tertiary amine (EDEA) can be degraded in aqueous solutions containing mixtures of COz and H2S. This result has implications regarding the use of methyldiethanolamine (MDEA) for gas sweetening. A simplified kinetic model based on the degradation reactions was developed. The rate constants, which were determined by an optimization routine, were found to follow the Arrhenius expression; the frequency factors and activation energies for the reactions are reported. Some of the rate constants were discovered to depend on the initial amine concentration. The kinetic model satisfactorily predicted the concentrations of DEA and the degradation compounds.
Acknowledgment The financial support provided by the Natural Sciences and Engineering Research Council of Canada and the Canadian Commonwealth Scholarship and Fellowship Plan is gratefully acknowledged. Nomenclature [i] = concentration of compound i, mol/L [DEAI0 = initial concentration of diethanolamine, mol/L ki = rate constant i, h-1 or L (mol h)-1 T = temperature, "C or K
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Abstract published in Advance ACS Abstracts, February 1,
1994.
