Hydrogen Sulfide Hydrate Dissociation in the Presence of Liquid Water

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Hydrogen Sulfide Hydrate Dissociation in the Presence of Liquid Water Kayode I. Adeniyi, Connor E. Deering, G. P. Nagabhushana, and Robert A. Marriott*

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Department of Chemistry, University of Calgary, 2500 Research Road, NW, Calgary, Alberta T2N 1N4, Canada ABSTRACT: Of the major components found in lean sour natural gas (CH4, H2S, CO2, H2O), hydrogen sulfide (H2S) readily forms the one of the highest temperature hydrates in the presence of water (structure I). Thus, when heating wellheads, flowlines, processing and injection facilities, the H2S hydrate formation conditions dictate the design for flow assurance purposes. Due to the high toxicity of H2S, there is a paucity of hydrate formation/dissociation condition data in the literature, especially in the liquid H2S region. In fact, there are only four previous experimental decomposition measurements reported in the literature for liquid H2S. In this work, we report the dissociation of pure H2S hydrate in the presence of water for p = 0.360 to 16.343 MPa and T = 285.23 to 304.04 K. These measurements were carried out in a 25 cm3 stirred autoclave cell, where both the Lw−H−H2S(g) and Lw− H−H2S(l) phase boundaries were autonomously measured by using the phase boundary dissociation method. These new measurements augment our previously reported data, which were below the vapor pressure of H2S. The results obtained were used to fit a semiempirical Clausius−Clapeyron equation for the rapid calculation of H2S hydrate formation conditions. Also, the results were thermodynamically modeled using the reference quality reduced Helmholtz energy equations of state and the van der Waals and Platteeuw model for fluid and hydrate phases, respectively. Finally, the results obtained from the model were compared to available literature where the uncertainty in temperature was found to be within an average ±0.2 K.

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absence of a sII hydrate former, that is, for dry sour fluids with very little hydrocarbon beyond CH4. For an example, the Khuff formation in Abu Dhabi contains dry sour gas with up to 50% H2S.9 Due to the high toxicity and safety concerns associated with working with H2S, there is a paucity of data for its hydrate formation conditions reported in the literature, especially in the liquid H2S region. Prior to this work, four data points were experimentally measured along the liquid H2S-hydrate-liquid water (Lw−H−H2S(l)) triple loci (Scheffer measured one point and Selleck et al. measured three).10,11 These data were subsequently smoothed by Selleck et al.11,12 There is need to expand the database for these conditions and revisit these measurements, because the experimental data are crucial for the calibration of models used for the prevention or mitigation of hydrate formation in sour gas production or acid gas injection. In a previous study, we have reported dissociation data for H2S along the Lw−H−H2S(g) phase boundary for T < 301.53 K and p < 1.96 MPa.8 This work extends the previous measurements up to p = 2.26 MPa and T = 302.68 K, that is, up to the vapor pressure of H2S. Also, we report new dissociation data for H2S hydrate along the Lw−H−H2S(l)

INTRODUCTION Natural gas is one of the most widely explored and produced forms of low-carbon energy, but the composition varies from well to well depending on the location and burial history of biomass.1−3 Primarily, commercially produced gas is composed of methane (CH4), but can contain other light hydrocarbons such as ethane, propane, butane and nonhydrocarbon such as carbon dioxide (CO2), hydrogen sulfide (H2S), nitrogen (N2), water (H2O), and mercaptan.2,4 Natural gas containing an appreciable quantity of H2S is referred to as sour.2 Among the world’s proven conventional natural gas reserves, 40% are estimated to be sour.5 With such a significant amount of sour conventional natural gas reserves and the constant depletion of sweet gas reserves, attention is shifting to the production of sour gas to meet the energy demand. Newer unconventional gas, such as shale gas reserves, are also known to contain problematic amounts of H2S and mercaptan.3,6 The production of natural gas with high H2S and/or CO2 content can present issues not encountered when producing sweet natural gases, such as corrosion, acid gas (CO2 and H2S) removal, processing/disposal and higher temperature clathrate gas hydrate formation.2,4,7 H2S hydrates can form at ambient temperatures (T < 304 K at 20 MPa), as opposed to methane hydrates (T < 291 K at 20 MPa), thus the heat tracing and dehydration within production facilities becomes more important. Like CO2, H2S forms a sI hydrate, but among known hydrate formers, it is an unusually stable former and will kinetically form easily.7,8 An sI hydrate can form in the © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

August 20, 2018 October 19, 2018 October 24, 2018 October 24, 2018 DOI: 10.1021/acs.iecr.8b04017 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research phase boundary for p = 2.40 to 16.34 MPa and T = 302.54 to 304.04 K. The data were modeled using reference quality reduced Helmholtz energy equations of state and the van der Waals and Platteeuw hydrate model and the results are subsequently compared to literature.13,14

pressure stability at the 95% confidence for these hydrate measurements was ±0.059 MPa. The setup was interfaced with Laboratory Virtual Instrument Engineering Workbench (LabVIEW) which records the pressure and internal temperature of the system continuously and averages every 30 s. Experimental Procedure. Prior to any experiment, the system was leak checked using pressurized N2 and then evacuated for 24 h using a vacuum of 2.5 × 10−7 MPa. After evacuation, the system was purged with pure H2S three times before charging the autoclave with the fluid to the desired pressure by opening valves V3 and V1 shown in Figure 1. The stirrer (250 rpm) was then turned on and the pressure was allowed to stabilize for ca. 6 h. For studying the Lw−H-H2S(g) phase boundary measurement, ca.10 mL of polished and degassed H2O was injected into the evacuated autoclave by suction. This amount of H2O corresponds to an approximate mole ratio of 76:1 H2O/H2S (after the final H2S(g) charge) so that H2O(l) is always present, even when 100% occupied hydrate is formed. Because the crystal structure of a Type I hydrate has 46 molecules per 8 cages, any mole ratio greater than 46:8 H2O/H2S ensures that the hydrate will never consume all the dense-phase water. For measurements along the Lw−H−H2S(l) loci, an aliquot of properly degassed water (still in excess) was delivered to the autoclave through a 260D Teledyne ISCO high pressure syringe pump after first charging the autoclave with liquid H2S by opening the valves V6, V5 and V1, respectively. After charging, the H2S + H2O system was mixed for 8 h until the pressure was stable to within ±0.005 MPa. Once the system had reached equilibrium, it was subcooled and held at 285 K, i.e., above the freezing point of water, for 18 h to form hydrates. Figure 2 show an example of the cooling, hydrate formation and heating stages for the H2S + H2O mixture along the Lw−H−-H2S(l) phase boundary. Note that the system remained subcooled to 285 K in the liquid region even though the hydrate formed completely at ca. 298 K. Upon heating, there are two changes in the slope of the p-T plot, due to the hydrate melt: from ca. 302.7 to 304.04 K, the

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EXPERIMENTAL SECTION Materials. All water used was polished to a resistivity of 18 MΩ·cm using an EMD Millipore model Milli-Q Type 1 water purification system and degassed under vacuum for at least 12 h. H2S with a listed purity of 99.6 mol % was supplied by Praxair Inc. and used without further purification. Table 1 Table 1. Measured Gas Impurities (Mole Fraction) in H2S Used for This Study N2

CO2

CH4

COS

0.000508

0.000577

0.0000283

0.00304

shows the experimentally determined compositions of H2S fluid using the Bruker 450-gas chromatograph equipped with a thermal conductivity detector and a flame ionization detector. Measurement Apparatus. The small stirred autoclave used in this work has been discussed in previous studies.8,15 Figure 1 shows a schematic diagram of the experimental setup. The 25 mL Hastelloy C276 autoclave vessel and the lower end of the transducer were submersed inside a PolyScience PP07R40 refrigerated circulating bath. The stirring assembly was controlled by an in-house assembled voltage regulation controller. The platinum resistance thermometer was calibrated according to the International Temperature Scale of 1990 (ITS-90),16 where the overall estimated standard uncertainty in the temperature measurement was found to be ±0.004 K; however, the stability of our temperature measurements for this system is normally ±0.05 K. The Keller Druckmesstechnik PA-33X pressure transducer was found to have an uncertainty of ±0.001 MPa upon comparison to a dead-weight calibrated standard transducer; however, the

Figure 1. Schematic diagram of the setup used for the measurement of pure CO2 and H2S hydrates dissociation in the presence of liquid water. V1, V2, DLC, and PT represent the inlet feed valve, outlet valve, data logging computer, and pressure transducer, respectively. V3, V4, V5, V6, V7, and V8 represent control valves. B

DOI: 10.1021/acs.iecr.8b04017 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Table 2. Experimental Dissociation Data for H2S Hydrate Lw−H-H2S(l)

Lw−H-H2S(g)

T/Ka

p/MPab

T/Ka

p/MPab

302.72 302.92 303.03 303.13 303.23 303.33 303.44 303.54 303.64 303.74 303.84 303.94 304.04

2.401 4.443 5.532 6.596 7.695 8.782 9.924 11.066 12.171 13.288 14.405 15.497 16.343

285.23 291.23 295.23 297.23 299.23 300.23 301.23 301.44 301.64 301.84 302.04 302.24 302.34 302.44 c 302.54 c 302.62 c 302.68

0.360 0.654 0.981 1.217 1.526 1.715 1.933 1.982 2.031 2.081 2.134 2.187 2.211 2.234 2.252 2.263 2.260

Figure 2. Pressure versus temperature curves showing H2S + H2O cooling, hydrate formation and decomposition stages along the Lw− H−H2S(l) phase boundary. a

large p-T slope is controlled by hydrate melting and above 304.04 K the slope is controlled by liquid−liquid equilibria (flash). Thus, according to the Gibbs phase rule, all points along the large slope are hydrate melt points (three phase loci). This method is referred to as the phase boundary method. To our knowledge, this is the first time that the phase boundary dissociation method has been used in a liquid former region. Previous attempts with liquid propane were less successful due to hydrate buoyancy and location issues.15 Note that the rapid hydrate formation in Figure 2, results in a brief temperature increase due to the exothermic enthalpy of hydrate formation. The latter is another indication that H2S hydrate formation is rapid, whereas many other hydrates often show a near isothermal pressure drop during formation. After complete hydrate formation (indicated by a rapid decrease in pressure), temperature was increased incrementally by 0.2 K to melt the formed hydrate while logging the temperature and pressure. Each temperature between increments was held for 6 h. After the experiments, H2S is evacuated through the valves, V2 and V7 and scrubbed with the 20% KOH solution traps. Any remaining trapped hydrate and H2S in the transfer line was further flushed with N2 before evacuating with the vacuum. Safety Measures. All of the researchers involved in this work are certified to use, and are fit-tested for, Self Contained Breathing Apparati. The experiment was carried out in a laboratory that is fully equipped with the state-of-the-art gas detection system.

Uncertainty for T measurements from triplicate experiments for each of the reported data point was estimated to be within an average value of ±0.05 K. bUncertainty for p measurements from triplicate experiments for each of the reported data point was estimated to be within an average value of ±0.059 MPa. cThese three points are within our uncertainty for the H2S vapor pressure; therefore, Lw−H− H2S(g)-H2S(l) or Q2.

Semi-Empirical Model for Hydrate Dissociation Pressure Calculation. It is reasonable for application to use simple and rapidly deployed methods for calculating the dissociation boundary. Therefore, like our previous studies with C3H8(g&l),15 we have included two simple semiempirical closed-form equations. A Clausius−Clapeyron equation was fit to the experimental dissociation results for H2S hydrate in the gas phase and a linear equation in temperature was used for the liquid H2S results. The semiempirical correlations for hydrate dissociation are shown in Table 3, where p is in MPa and T is Table 3. Semi-Empirical Correlations for H2S Hydrate Dissociation Pressure phase boundary

semiempirical correlation

Lw−H−H2S (g) Lw−H−H2S(l)

Pdiss/MPa = Exp[(31.65 ± 0.42) − (9335 ± 125/K) /T] Pdiss/ MPa = (−3252 ± 19) + (10.751 ± 0.063/K) T

in Kelvin. All fits were completed using linest least-squares regression and reproducibility has been quoted at the 95% confidence interval. These equations can be used for swift calculation of the of Lw−H-H2S(g) and Lw−H-H2S(l) phase boundaries. Thermodynamic Modeling. For any rigorous thermodynamic model, the fugacities of all coexisting phases must meet the following equality at equilibrium:

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RESULTS AND DISCUSSION Previous studies conducted using this experimental setup have demonstrated the accurate determination of equilibrium conditions of CH4(g), C3H8(g&l) and H2S(g) gas hydrates.8,15 In this study, we have measured the dissociation conditions of H2S hydrates in the presence of liquid and gaseous H2S, where the results are reported in Table 2. This study reports new measurements up to p = 2.26 MPa and T = 302.68 K on for phase gaseous H2S to add to our previously reported measurements along the Lw−H−H2S(g) triple loci. In addition, new experimental dissociation conditions for H2S hydrates along the Lw−H−H2S(l) phase boundary are reported.

fib = f iaq = fiH

(1)

where f is the fugacity of component i, the superscripts b, aq, and H represent the nonaqueous phase, aqueous phase and hydrate phase, respectively. Fluid Phases. The H2S and aqueous phase fugacities were calculated using the Lemmon and Wagner EOS17 and the Wagner and Pruβ EOS,18 respectively. We note that one can C


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bυ represents the fitting parameters which were fit to the volumetric measurements data of Hnedkovsky et al. and Barbero et al.21,23 The optimized parameters found by Bernard et al.19 are used in eqs 2 to 6 and are shown in Table 4. Note that Bernard et

use mixing coefficients with these equations; however, the current available mixing coefficients give poorly calculated H2S solubilities when compared to the literature. An alternative method was reported by Bernard et al.19 which used infinite dilution fugacity coefficients for the solute in either phase. This is a reference quality model which can be used to calibrate commercial simulation. The comparison and tuning to water content (H2O in H2S) and H2S solubility in the H2O phase was provided in the previous publication; however, the model is explained briefly here.19 According to eq 1, H2S fugacity in the nonaqueous fluid phase is equal to its fugacity in the aqueous phase at equilibrium:19 φH SyH S p = γH Sx H2SKHe 2

parameter

2

ΔGT°r

H2o )/ RT

Tr

(2)

Tr 2

61.56 T

(4)

R1nKH = −

Tr

ÅÄÅ ÅÅ T ÅÅ ÅÅ ÅÅÇ Tr Tr Å

Tr jij dΔCp zyz zz jj 2 jjk dT zz{ ο

+

+

ÄÅ ÅÅi y2 ÅÅjj T zz ÅÅjj zz − 6 T + 3 ÅÅj T z Tr ÅÅk r { ÅÇ

ÄÅ É ÑÉ ÅÅ T ij T yzÑÑÑÑ 1 ÑÑÑÑ ο Å r Å z j − ÑÑ + ΔCp, TrÅÅÅ − 1 + 1njjj zzzÑÑÑÑ ÅÅÇ T T ÑÑÑÖ ÅÇÅ Tr k Tr {ÑÑÖ É Ñ 2 ο 2 ij T yzÑÑÑ T ji d ΔCp zyz T z − r − 21njjjj zzzzÑÑÑÑ + r jjjj zz 2 z j T k Tr {ÑÑÑÖ 12 k dT {T r ÉÑ ÑÑ y i Ñ T jT z + 2 r + 61njjjj zzzzÑÑÑÑ Ñ T k Tr {ÑÑÑÖ ÅÄÅ 1

Å ΔHTοrÅÅÅÅ Å

ρoH2O

1 ρHο O 2

(5)

θjm =

2

−3

−71.7 ± 17.6 J mol−1K−1

C jmfj 1 + ∑j C jmfj

(9)

where f j is the fugacity of pure hydrate former j in cavity m which was calculated by using the equilibrium model of Bernard et al.19 The Langmuir constant (Cjm) can be calculated by using the correlation provided by Parrish and Prausnitz:29 A jm Bjm exp C jm(T ) = (10) T T This correlation relates the Kihara potential fitting parameters (Ajm and Bjm) and can be further optimized by

ο

+ ρHο O κTο,H2ORT[aυ + bυ(e 90ρH2O − 1)]

Tr

ij yz j z Δμwβ− H = RT ∑ vm̅ lnjjj1 − ∑ θjmzzz jj zz m j (8) k { where v̅m represents the number of cavities of type m per water molecule and θjm denotes the fractional occupancy of the guest molecules j within the hydrate cavities m. θjm is calculated by using the Langmuir adsorption equation which is expressed as

where ΔGoTr is the standard Gibbs free energy, ΔCop is the change in heat capacity and Tr represents the reference temperature of 298.15 K. All the coefficients in eq 5 were optimized by Bernard et al. to fit the available experimental solubility data except ΔCop,Tr. Barbero et al. have experimentally measured ΔCop,Tr = −144.3 J mol−1K−1 and this value was used without further optimization.21 The density and compressibility correlation of O’Connell et 3 −1 22 al. was used for calculating υ∞ H2S (m mol ): υH∞2S =

188.5 ± 20.8 J mol−1K−1

ij −Δμ β− H yz zz w f wH = f wβ expjjjj zz j RT z (7) k { β where f w is the reference fugacity of the empty hydrate cavity, Δμβ−H is the difference in chemical potential for water between w a filled hydrate cage and the hypothetical empty hydrate cage. van der Waals and Platteeuw derived Δμβ−H from statistical w thermodynamics and gave the expression as13

The temperature dependence of KH is provided by20 ΔGTοr

dT

al.19 also describe a virial equation for the fugacity of H2O in the H2S phase. The later is used to iteratively calculate all equilibrium concentrations and reduces the fugacity of H2S due to the water content in the H2S phase. The average error found by Bernard et al.19 for xH2S < 0.3 was calculated to be ∂xH2S/xH2S = 3.7%, and for xH2S > 0.3, ∂xH2S/xH2S = 5.7%.19 Hydrate Phase. The fugacity of water in the hydrate phase (f H w ) is calculated using the van der Waals and Platteeuw model13 as outlined by Englezos and Mohammadi et al.:24,25

where βaa = −0.2106 +

dΔC °p

( )

2 Tr2 i jj d ΔC°p yzz j z 2 j dT 2 z k {Tr

(3)

2

16539 ± 157 J mol−1

ΔH°Tr

where φH2S is the fugacity coefficient of H2S in the H2S rich phase, yH2S is the mole fraction of H2S in the H2S rich phase, p is the total pressure, γH2S is the activity coefficient for H2S in the aqueous phase, xH2S is the mole fraction of aqueous H2S, KH denotes the Henry’s law constant for total H2S dissolution in water, υ∞ H2S is the partial molar volume of aqueous H2S at infinite dilution, poH2O is the vapor pressure for pure water and R is the ideal gas constant. φH2S is calculated using the pure fluid (Lemmon and Span equation) EOS, because as yH2S → 0 at infinite dilution, H2S behaves according to Raoultian ideality.17 The γH2S is calculated using Pitzer theory for a H2S solute while assuming minor dissociation from the expression19 γHx S = e 2βaamH2S(1 + 0.0180152·mH2S)

Bernard et al.19 49.6 cm3 mol−2 1.37 cm3 mol−2 147.9208 ± 0.0402 J mol−1

aυ bυ

υH∞ S(p − p ο

2

2

Table 4. Optimized Parameters for the H2S Fluid Phase Calculation Used for This Work

(6)

κoT,H2O

where is the density of pure water (mol m ), is the isothermal compressibility of pure water (Pa−1), while aυ and D


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Ajm(large) = 0.1649 K MPa−1 and Bjm(large) = 3621.46 K, with a of SSE = 0.00022965 Pa2. The optimized parameters are in good agreement with the values reported by Parrish and Prausnitz to within 1% but were less than those reported by Munck et al. (Ajm(small) = 0.0002467K MPa−1; Bjm(small) = 4568 K, Ajm(large) = 0.1613 K MPa−1, Bjm(large) = 3737 K).29,30 These optimized values for Ajm and Bjm can be used to iteratively solve for the hydrate formation temperature at any specified pressure on the hydrate stability boundary. Comparison of Experimental Measurements to This Study’s Model. A pressure versus temperature plot of all experimentally measured conditions for this study, literature data and calculated values along the Lw−H-H2S(g) and Lw− H-H2S(l) phase boundary are shown in Figure 3.8−12,31,32 Our

using the H2S hydrate experimental dissociation data. This optimization procedure will be subsequently discussed. The water fugacity of the hypothetical empty hydrate cavity can be calculated from the expression: β−L y ji −Δμw zz zz f wβ = f wL expjjjj j RT zz k {

(11)

f Lw

where was calculated from the Wagner and Pruß reduced Helmholtz energy EOS.18 Δμβ−L is the difference in chemical w potential for water in a hypothetical empty hydrate cage and a pure water phase. The equation derived by Holder et al. from classical thermodynamics can be used for calculating Δμβ−L w using hexagonal ice (ice Ih) water as a reference point from the relationship,26 Δμw β−L RT

=

Δμwο RTο

−

∫T

T

ο

Δh w RT

2

dT +

∫p

p

ο

Δvw dp − 1na w RT

(12)

Δμwo

where is the experimentally determined reference chemical potential difference between water in the empty hydrate lattice and pure liquid water phase, at reference conditions of T0 = 273.15 K and po = 0.1 MPa. Δvm represents the reference molar volume change for the formation of empty hydrate cage from the pure liquid water phase and aw is the activity of water in H2S rich phase. aw can be calculated through the Gibbs−Duhem relationship from the activity of H2S and xH2S which can be expressed as19 x H2 2S(1 − 55.5087βaa ) − x H2S

a H 2O =

(1 − x H2S)2

(13)

The molar enthalpy change for the formation of the empty hydrate lattice from the liquid water phase (Δhw) is expressed as26 Δhw =

Δhwο

+

ο Δc pw

+ b(T − Tο)

Figure 3. Summary of available literature data for H2S hydrates dissociation conditions in the presence of liquid water (Lw−H− H2S(g) and Lw−H−H2S(l) loci). ●, this study measurements; ____, thermodynamic model reported in this study; ----, H2S vapor pressure calculated with the Lemmon and Wagner EOS;17 ◇, Ward et al.;8 × , Mohammadi and Richon;31 ○, Carroll and Mather;12 Δ, Bond and Russell;32 □, Selleck et al.;11 ■, Scheffer.10

(14)

Δcopw

where is the reference standard difference in heat capacity between ice and liquid water and b represents the temperature dependence of the heat capacity difference. Table 5 shows the constant parameters and coefficients used in the hydrate model for this work.

optimized coefficients were found to be accurate to within an average of ±0.2 K when compared to the measured conditions in both the vapor and liquid regions. As previously mentioned, Ward et al.,8 have measured the H2S hydrate dissociation along the Lw−H−H2S(g) phase boundary and have compared their data to available literature and other available software. In most cases a good agreement (to within ±0.3 K) was observed by Ward et al.,8 except for the data reported by Bond and Russel and Carroll and Maher that shows a deviation greater than 0.7 K.12,32 The difference between the experimental measurements and the model reported in this study along the Lw−H−H2S(l) phase boundary shows an average difference of ±0.05 K. The model shows a much better calculation in the liquid H2S region than that of the gas phase. Scheffer in 1913 reported a single measurement along the Lw−H−H2S(l) locus (p = 3.2 MPa, T = 302.95 K) and the temperature calculated for this pressure using the model reported in this study was found to be only 0.1 K less.10 The three data points of Selleck et al.11in the liquid region were also compared to the model reported in this study. Note that we have only used the raw data points, versus Selleck et al.’s reported smoothed data.12 The

Table 5. Thermodynamic Reference Properties for Structure I Hydrate Used for This Work reference property

structure i

literature source

Δμ° Δh° ΔCp° Δv b

1297 J mol−1 1389 J mol−1 −37.32 J mol−1K−1 4.601 cm3 mol−1 0.179

Dharmawardhana et al.27 Dharmawardhana et al.27 Holder et al.28 Parrish and Prausnitz29 Holder et al.28

Optimization of Kihara Fitting Potential Parameters. The Kihara potential parameters (Ajm and Bjm in eq 10) for the H2S hydrate were optimized using the experimental dissociation data by minimizing the sum squared difference for the fugacities of hydrate and fluid phases. Because H2S occupies the small and large cavities in the sI hydrate, Parrish and Prausnitz reported the value for H2S as (Ajm(small) = 0.029946 KMPa−1; Bjm(small) = 3736 K; Ajm(large) = 0.1652 KMPa−1; Bjm(large) = 3610.9 K).29 Our optimized parameters were found to be Ajm(small) = 0.0299 K MPa−1, Bjm(small) = 3739.5 K, E



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temperatures calculated from this study’s model were within 0.2 K of Selleck et al.’s points.11 The two highest temperature points within Table 2 show an increasing temperature (0.06 K beyond our uncertainty of 0.05 K), whereas the pressure has not increased beyond our uncertainty. Thus, these points must be at the upper quadruple point (there is a pressure halt). The upper quadruple point for this study and other literature are reported in Table 6. All of the quadruple point pressures reported in the literature fall within ±0.004 MPa, and the quadruple point temperature is within ±0.14 K.11,12

author

T/K

p/MPa

302.65 302.55 302.65 ± 0.05

2.2387 2.24 2.261 ± 0.059

Q2 was calculated as an average of the final two points reported in Table 2 for the Lw−H−H2S(g) data.

a

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CONCLUSIONS We have reported new experimental hydrate dissociation measurements for liquid and gaseous H2S in the presence of liquid water using the phase boundary dissociation method. To our knowledge, this is the first time this method has been reported for liquid−liquid-solid hydrate measurements. Data for the Lw−H−H2S(g) and Lw−H−H2S(l) phase boundaries were used to optimize two semiempirical equations for rapid calculation of the loci. Also, the data were used to calibrate Kihara potential parametrized in the more rigorous van der Waals and Platteeuw model. The results obtained from this model were compared to available literature along both loci, where the average deviation was found to be within ±0.2 K. The upper quadruple point was also extrapolated from these models and shown to be in good agreement with previous studies. These equations and measurements can be used to aid in the design of facilities handing wet H2S, where heating and/or dehydration are required to avoid plugging. Specifically, H2S formation is important within acid gas injection and sour gas production. Our next steps will be to investigate the water content above the H2S hydrate or the so-called sub dew point hydrate region where no aqueous phase exists. The later will be used to help understand dehydration targets beyond the liquid water dew point.

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REFERENCES

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Table 6. Upper Quadruple Points from This Study and Literature for H2S Selleck et al.11 Carroll and Mather12 This studya

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robert A. Marriott: 0000-0002-1837-8605 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for funding through the NSERC ASRL Industrial Research Chair in Applied Sulfur Chemistry and the sponsoring companies of Alberta Sulphur Research Ltd. F


Article

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Hydrogen Sulfide Hydrate Dissociation in the Presence of Liquid Water

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