Ind. Eng. Chem. Res. 2007, 46, 3729-3733
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Aqueous Amines as Reactive Solvents for Mercaptan Removal Stephen A. Bedell* and Michelle Miller The Dow Chemical Company, 2301 N. Brazosport BouleVard, Freeport, Texas 77566
The solubilities of methyl mercaptan in a variety of aqueous amine solutions have been measured. Mercaptan chemical solubility can be predicted by consideration of acid-base properties of the mercaptan and amine. Chemical equilibrium constants have been used to separate the chemical and physical solubilities from experimental results for 50 wt % methyldiethanolamine. At moderate acid loadings, the chemical solubility is diminished and the physical solubility dominates. This methodology has been extended to aqueous solutions of monoethanolamine, diethanolamine, and diglycolamine. Introduction Solvents have been used for decades for removal of acid gases from a variety of gas streams.1,2 The simplest type of solvent is referred to as a physical solvent, usually a pure organic compound that dissolves the acid gases through nonreactive interactions. The more commonly used solvent systems are aqueous amine solutions which are called chemical solvents. In addition to a certain degree of physical solubility, chemical solvents (aqueous amines in particular) can also react to form thermally regenerable salts with the acid gases. Though amine-based solvent systems have been designed for H2S and CO2, few data exist on the solubilities of mercaptans which are often found in H2S-containing gas streams. Many confusing descriptions of amine-mercaptan interactions have been presented in the literature. For instance, Maddox and coworkers3 state that “the exposure of methyl mercaptan to MEA, DEA, DGA, DIPA, DMEA, and MDEA4 did not lead to a reaction”. Jou et al.5 write “the thiols are much weaker acids than H2S, and they do not react with alkanolamines”. The purpose of this work is to show that mercaptans are scrubbed by aqueous amines due to a combination of physical and chemical solubilities (reactions). Furthermore, the chemical solubility can be predicted by understanding the mercaptan and amine acid-base properties. Experimental Section Fifty grams of each amine solution was loaded with sulfuric acid such that acid-loading ratios of 0, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.4, and 0.6 were obtained. The acid-loading ratio was defined as
acid-loading ratio ) mol of H+/mol of amine in solution (1) Calculations of the loadings for each solution assumed H2SO4 donates both protons. Once the acid-loaded solutions were prepared, 10.0 g of solution was added to a headspace vial, which was sealed with an aluminum crimp cap containing a PTFE septum and star spring. The headspace vial was then purged with nitrogen for 10 min and injected with 0.5 mL of methyl mercaptan using a gastight syringe. After injection with methyl mercaptan, the vials were loaded onto the magazine of the headspace analyzer and sampled by gas chromatography. * To whom correspondence should be addressed. Tel.: 979-2383240. Fax: 979-238-5183. E-mail:
[email protected].
Static headspace gas chromatography measurements were performed with a Perkin-Elmer Turbomatrix-40 headspace analyzer attached to an Agilent 6890N gas chromatograph. A capillary GS-Q column (i.d. 0.53 mm, 30 m) from J&W Scientific provided separation of methyl mercaptan from nitrogen and detection was performed by a standard thermal conductivity detector. The transfer line of the headspace analyzer was connected directly to the capillary column using a universal fused silica Press-Tight (Restek) connector. Since the transfer line and GC column were directly connected, carrier gas flow through the column was supplied through the headspace analyzer. Sample equilibration time prior to injection onto the column was also controlled by the headspace analyzer. Each sample was equilibrated for 30 min (thermostating time) in the oven of the headspace analyzer, which was set at 40 °C. At the end of the thermostating time the vial was pressurized and the headspace sample injected onto the column. Results and Discussion Henry’s Law. Henry’s law states that
pΦ ) xLKH
(2)
where xL is the mole fraction of solute in solution, p is the partial pressure of solute in the gas phase, and KH is Henry’s constant. The experiments reported in this paper were performed at total pressures less than 0.3 MPa and it was assumed that the fugacity coefficient, Φ, was unity. The Henry constant represents the equilibrium constant for degassing. Therefore, lower values of KH translate to higher solubilities of a gas in a solvent. The tendency for a gas to dissolve in a liquid can better be represented by solubility,
Sol ) p × Ksol
(3)
with Sol in units of mol/L with a solubility constant of
Ksol ) mol of solvent/L ÷ KH
(4)
and Ksol in units of mol L-1 MPa-1. This report presents Ksol values as measures of relative solubility at equal partial pressures of mercaptan. In the reaction of acid gases with amines, Henry constants are often used to describe the overall solubility due to both physical (hydrogen bonding, van der Waals forces, etc.) and chemical solubilities. It should be realized that while values of KH based only on physical solubility are likely to be linear with
10.1021/ie0611554 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007
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Table 1. Ksol (mol L-1 MPa-1) for Sulfur Compounds in Water at 25 °C Richon and Mather and Yaws Przyjazny et al.7 compound et al.6 Sekyiamah8 co-worker9 co-workers10 MeSH EtSH iPrSH nPrSH nBuSH tBuSH
2.9 3.5 2.2 2.4 1.1 1.9
4.0 2.8
3.3 2.1
2.5 2.1
1.2 1.1
2.9 2.3 (20 °C) 1.9 (20 °C)
Table 2. Ksol (mol L-1 MPa-1) for Methyl Mercaptan in Various Solvents at 50 °C solvent H2O n-hexane toluene a
MeSH 1.9,a 5.5 17
2.110
EtSH
nPrSH
nBuSH
CH4
1.1 7.7 40
0.67 13 55
14 55
0.006912 0.3113 0.2014
All values from ref 11 unless otherwise indicated.
pressure, those based on reactivity may change with increased partial pressure as reactants in solution are consumed. The purpose of presenting the solubilities in this report is to understand some general principles guiding mercaptan solubility. If these data are going to be used for predictive calculations, the reader is encouraged to consult the original references and select constants most appropriate to a specific application. Mercaptan Solubility in Water. Several values have been reported in the literature for solubilities of methyl mercaptan in water. Original units have been converted to mol L-1 MPa-1. Solubilities of higher mercaptans have not been as well studied. In general, they show a slight decrease in solubility as the mercaptan alkyl groups increase in size. Though Yaws and Sekyiamah (these are not primary sources) both reported a very high KH (low Ksol) value for nBuSH, they may have used the same original source for the value. (See Table 1.) Mercaptan Physical Solubility in Organic (Nonamine) Solvents. Physical solvents (such as Selexol and sulfolane) are used for bulk removal of acid gases. Limited data are available for commercial physical solvents. To better understand the effect of organic solvents on solubility, Table 2 shows solubility constants for several mercaptans in water, n-hexane, and toluene. It is clear that considerably more RSH dissolves in the organic solvents compared to water. It is also apparent that the solubility increases as the size of the mercaptan increases. A major disadvantage of using physical solvents is the cosolubility of hydrocarbons. This is demonstrated by the substantially higher solubility of methane in the organic solvents than in water. Table 2 also shows that the mercaptans are more soluble in the organic solvents than is methane. Solubility of Mercaptans in Aqueous Solutions of Amines. To better understand the reaction between amines and mercaptans, the interactions of amines with the well-studied acid gases H2S and CO2 should be considered. Shown in Figure 1 are the solubility constants of H2S and CO2 as a function of acid gas loading. The sharp inflection in solubilities with loading resembles a titration curve for MDEA. As MDEA is neutralized by the acid gas, the solubility decreases in a way that is predictable from the acid-base properties of the amine and the acid gas. Mercaptans (RSH) are much weaker acids than H2S or CO2, but can similarly react via salt formation with amines to form mercaptide salts. This reaction adds a level of complexity to solubility beyond that encountered in the discussion thus far. This combination of physical solubility and chemical reaction is illustrated in Figure 2.
Figure 1. Ksol (mol L-1 MPa-1) for H2S and CO2 in MDEA at 40 °C. (From ref 15, literature values for KH have been converted using estimates of solution density.)
Figure 2. Schematic of RSH solubility.
Henry constants (or solubilities) in amines are often reported as the overall constants which combine the physical and chemical reaction solubilities. Thus, reports in the literature showing Henry constants or mercaptan solubilities in aqueous amine systems usually report values which are dependent on the acid gas loadings. Mercaptan solubility in an amine solution can be partitioned into physical and chemical solubilities.
SolTotal ) Solchem. + Solphys ) p(Ksol-phys + Ksol-chem)
(5)
The chemical solubility in amines equals the amount of mercaptide ([RS-]) in solution due to salt formation and can be represented by the following relationship:
Solchem. ) pRSH × Ka(RSH) × Ksol-phys/[H+]
(6)
where Ka(RSH) is the acid dissociation constant (Ka(RSH) ) [RS-] [H+]/[RSH]) for the mercaptan and Ksol-phys values are in mol L-1 MPa-1. This relationship has the following consequences: 1. Increased mercaptan removal is achieved by use of more basic amines because they increase the solution pH. Increasing the basicity of the amine will also decrease the reversibility of the amine/RSH reaction and increase the energy needed for regeneration. More basic amines will also remove more CO2, which can be an advantage or disadvantage depending on the situation. Stronger bases such as caustic will result in higher RSH solubilities, but will not be reversible (thermally regenerable in a gas-treating plant). 2. Lower (C1, C2) mercaptans with higher acidities (larger values of Ka(RSH)) will exhibit larger chemical solubilities than higher mercaptans in amine solutions. 3. Because the RSH reaction is pH-dependent, any substantial acid gas (CO2 or H2S) loadings will greatly limit the RSH
Ind. Eng. Chem. Res., Vol. 46, No. 11, 2007 3731 Table 3. Comparative Gas Solubilities in Water gas
solubility at 40 °C (mol L-1 MPa-1)
ref
N2O CO2 CH3SH
0.18 0.24 2.2
17 17 this work
Table 4. Ksol-phys for Various Solvents at 40 °C; Also Shown Are Values of the Acid Dissociation Constant, Ka, from Ref 18 solvent
wt %| water
Ksol-phys (mol MPa-1 L-1)
water 4.2 M MDEA 4.2 M DEA 4.2 M MEA 4.2 M DGA 35% DEA 25% MDEA tetraglyme sulfolane
100 50 56 74 56 65 75 0 0
2.2 3.5 3.3 4.6 4.0 3.1 2.6 32 22
log Ka at 40 °C -8.20 -8.53 -9.08 -9.00 -8.53 -8.20
reaction. Small loadings of the amine cause a steep decrease in solution pH, which results in a lower degree of RSH ionization. In addition to the acid-base reaction, the effect of amines on physical solubility alone should be considered. The N2O analogy16 is probably not appropriate for estimating methyl mercaptan physical solubility. The arguments for comparing water solubilities of reactive gases with the nonreactive N2O assume similar interactions between the solvent and the two gases. As seen in Table 3, this analogy is valid for prediction of CO2 solubilities since the water solubilities of N2O and CO2 are close. CH3SH however exhibits an order of magnitude higher solubility in water than does N2O. To separate the effects of physical and chemical solubilities, an equilibrium model was constructed. Amine dissociation constants and heats of reaction were obtained from the NIST database.18 The equilibrium constants used for the amines are presented in Table 4. Protonation equilibrium constants and heats of reaction for mercaptans were obtained from NIST and from Crampton.19 The value of log Ka at 40 °C used in this study was -10.08. The model takes these values, along with appropriate amine and total mercaptan concentrations as well as total acid loadings and calculates solution speciation. For instance, in the MDEA example shown in Figure 3,
[MDEA]total × acid loading ) [MDEAH+] + [RSH] + [H+] (7) A total mercaptan loading of 0.0005 mol of RSH/mol of MDEA was chosen (close to our experimental values and small relative to total acid gas loadings) and the mass balance was solved by iteration of solution pH. Speciation of the amine and mercaptan was performed at the calculated pH. Figure 3 shows comparative values for total solubility constants from literature and values determined in this study. The model line was constructed by adding the calculated Ksol-chem value to the value of Ksol-phys chosen to best fit the experimental results. The uncorrected model did not show the diminished solubilities at high loadings. This is most likely due to salting out effects on the physical solubility of CH3SH. A salt correction (for sulfate salts) was added to the model calculation, using the modified method of Onda et al.20 The calculated physical solubilities are divided by 10(kI) where k is the sum of constants for the ionic components and dissolved RSH and I is the ionic strength calculated from the solution speciation. The value of k for the 4.2 M MDEA example shown in Figure 3 is 0.070.
Figure 3. Comparison of model and experimental data for CH3SH solubility in 50% MDEA at 40 °C. Data from this work (9) and Jou et al.10 (2), model shown in solid line, corrected for salting out effect.
The new data presented here (using strong acid neutralization) are in fair agreement with the values reported by Jou et al.10 Figure 3 shows those values using total loadings of CO2 + H2S + CH3SH. The comparison of strong acid loading (mol of H+/ mol of amine) with H2S or CO2 loadings is valid for intermediate loadings. At low amine loadings CO2 can contribute two protons, and at high H2S loadings, significant amounts of H2S will remain diprotonated. A correction for degree of protonation may be added to strong acid neutralization headspace data for low and high loadings of H2S or CO2. For the literature values shown in Figure 3, all CO2 and H2S values shown were at intermediate loading in which >90% of the acid gas molecules were in the monoprotonated acid form. The value of Ksol-phys used for the best line fit in Figure 3 was 3.5 mol L-1 MPa-1. The shape of the curve in Figure 3 is similar to that shown in Figure 1 for H2S and CO2. This sharp inflection is indicative of typical acid-base reaction behavior for neutralization of amines. The total solubilities are substantially lower for CH3SH because the mercaptan is a much weaker acid than H2S or CO2 and its Ksol-chem component is much smaller. Both the model and our data show a sharp inflection at the lower loadings that is also seen for the Jou et al.10 data, though not to the same degree. At the low pressures of CH3SH used in the present study, the loadings of mercaptan are much closer to zero than those used in the higher pressure study of Jou et al.10 For that reason, the acid loadings shown in Figure 3 are comprised of the sum of CO2 + H2S + CH3SH. Thus, for the data points in which the H2S and CO2 loadings are zero, there is enough mercaptan loading in the Jou et al.10 study to neutralize some amine and reduce the solubility of CH3SH. The inflection demonstrates how low lean loadings and high circulation rates can be used to more effectively remove mercaptans from gas streams with aqueous amines. At higher loadings, chemical solubility is reduced and physical solubility dominates the removal. This is illustrated in Figure 4 which used the model calculations for 50% MDEA. Further refinements to the model can be made by consideration of amine activities. More rigorous VLE measurements (utilizing loadings with CO2 and H2S as well as liquid-phase analysis) may also be used. The purpose of this exercise is to estimate the effect of various amine structures on physical solubilities. The same technique of experimental fitting was done to arrive at values of Ksol-phys for several other aqueous amine mixtures which are shown in Table 4. Initially, solvent concentrations
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Ind. Eng. Chem. Res., Vol. 46, No. 11, 2007
Improved Amine-Based Solvents. As seen in Figure 3, aqueous amine solvents will lose much of their mercaptan solvency as soon as they react with other acid gases. Hybrid solvents (mixtures of amines with physical solvents) allow for the same lean amine reactions plus sustained greater physical solubility at the higher loadings. Other additives have also been reported to substantially increase mercaptan removal.21,22 Conclusions
Figure 4. Degree of physical and chemical solubility of methyl mercaptan in 50% MDEA at 40 °C with various acid loadings. Acid loading defined in eq 1.
The solubility of mercaptans in amines can be treated as the sum of both a physical solubility and a chemical solubility. The chemical solubility can be predicted from the acid-base properties of both the amine and the mercaptan. At low loadings, the chemical solubility dominates the total solubility. As the acid loadings in the solution increase, chemical solubility is substantially reduced and physical solubility dominates. Literature Cited
Figure 5. Chemical solubility at “zero” loading vs amine properties. Ka ) [amine][H+]/[amineH+], values from ref 18, Ksol-phys values from Table 4. Ksol-chem ) Ksol-experimental - Ksol-phys. Zero loading refers to no sulfuric acid added, mercaptan loadings in all experiments
