Phase Behavior and Raman Spectroscopic Analysis for CH4 and CH4

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Phase Behavior and Raman Spectroscopic Analysis for CH4 and CH4/ C3H8 Hydrates Formed from NaCl Brine and Monoethylene Glycol Mixtures Gye-Hoon Kwak,† Kun-Hong Lee,† Sang Yeon Hong,‡,⊥ Seong Deok Seo,‡,§ Ju Dong Lee,‡ Bo Ram Lee,*,† and Amadeu K. Sum*,∥ †

Department of Chemical Engineering, Pohang University of Science & Technology, Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 37673, Republic of Korea ‡ Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, Busan 46744, Republic of Korea § Department of Polymer Science and Engineering, Pusan National University, Busan 46241, Republic of Korea ∥ Hydrates Energy Innovation Laboratory, Department of Chemical & Biological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: We present pure CH4 and CH4/C3H8 mixed hydrate phase equilibria formed from a mixture of NaCl (10 wt %) and monoethylene glycol (MEG, 10 and 30 wt %) solutions. As expected for thermodynamic inhibitors, the mixture of salt and glycol causes the hydrate phase equilibrium boundary to shift to lower temperatures and higher pressures, and on increasing the MEG concentration, the hydrate stable region shifted more. The measured experimental data are also compared with a thermodynamic model recently developed, named the Hu−Lee−Sum correlation, showing that the data match well with the predictions. The experimental data were used to calculate the enthalpy of hydrate dissociation. The enthalpies of CH4 hydrates in the mixture of 10 wt % NaCl brine and 10 or 30 wt % MEG were found to be ∼58.7 and 54.63 kJ/ mol, respectively, corresponding to structure I hydrates, whereas for the CH4/C3H8 (91.98:8.02 mol %) mixed gas system, the enthalpies of dissociation were found to be ∼101.10 kJ/mol (10 wt % NaCl + 10 wt % MEG) and 95.34 kJ/mol (10 wt % NaCl + 30 wt % MEG), confirming the mixed hydrates formed structure II. We also performed Raman analysis for CH4 hydrates and CH4/C3H8 mixed hydrates in the NaCl and MEG system and investigated their spectroscopic behavior and hydrate structure.

1. INTRODUCTION

can reach half of the total operating costs, so it is very important to estimate the proper amount of inhibitors to ensure hydrate-free hydrocarbon transportation.2−4 It is equally important to consider the mixture of thermodynamic inhibitors, including high salinity brines, because the produced water is generally a brine with salt concentration as high as ∼30 wt %. Salts are also thermodynamic inhibitors, depressing the hydrate phase equilibria to lower temperature and higher pressure, similar to methanol and MEG.2,5−7 To understand the behavior of MEG and electrolytes on gas hydrates, our group recently published a number of phase equilibrium data and a thermodynamic model (Hu−Lee−Sum (HLS) correlation),8,9 which are applicable up to 200 MPa for single or any mixed salts (up to saturation concentrations).

Gas hydrates are crystalline compounds formed when hydrocarbon gas molecules (guest, i.e., CH4, C2H6, C3H8, etc.) are incorporated into hydrogen-bonded water cages (host) under relatively high pressure and low temperature conditions. The structure of hydrates is determined by the size of gas (guest) molecules, and typically consists of structure I (sI), II (sII), and H (sH).1 Hydrates have many applications involving energy and natural gas because of their unique physical properties. One of the major research areas for gas hydrates is related to flow assurance in the production of oil and gas. Hydrates can unexpectedly form under the condition of low temperature and high pressure in the flowline, causing potential flowline blockage/plug and other operational problems in hydrocarbon transportation.1 To prevent the formation of hydrates, continuous injection of thermodynamic inhibitors such as methanol or monoethylene glycol (MEG) is a preferred option in the oil industry, but the cost of thermodynamic inhibitors © XXXX American Chemical Society

Received: February 25, 2018 Accepted: April 23, 2018

A

DOI: 10.1021/acs.jced.8b00155 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

pressure stainless steel cell (inner volume 270 mL), a supply vessel, a gas booster, and a real-time Raman spectrometer. The operating pressure limit for the system is 20 MPa, and temperature is controlled by a refrigerated/heating circulator between 253.2 and 293.2 K. Two RTDs (resistance temperature detector, ±0.1 K) monitor the temperature of each gas and liquid phases, and the system pressure is monitored by a pressure transducer (WIKA, S-20, 0.125% uncertainty). An anchor-type impeller mixes the cell content (NaCl brine and MEG mixture) with a rotational speed of 400 rpm, which was determined to be proper for the experiments. The mixing was only for the liquid phase; however, we can also expect that the vapor phase had a homogeneous composition as there was sufficient agitation at the gas−liquid interface. A Raman spectrometer (Acton, SP-2556) with a Vector probe (fiber optic) and CCD (charge coupled device) detector was used in this study. The custom-designed probe tip, a 9.5 mm diameter cylindrical stainless steel probe tip with an 8 mm ball lens (NBK7, Edmund optics), was inserted through a fitting in the wall of the reactor. The laser wavelength was 532 nm and spectra resolution was 0.61 cm−1. Further details of the experimental apparatus, including a real-time Raman system, are available elsewhere.10,11 Prior to each hydrate phase equilibrium measurement, the pressure cell was filled with 100 g of aqueous sample (a mixture of 10 wt % NaCl brine and 10 or 30 wt % MEG). The mixture solution was prepared by initially taking a 10 wt % of NaCl solution and then adding MEG to make it 10 or 30 wt % in MEG, gravimetrically.3 The cell was flushed at least three times with the experimental gas to remove any residual air. We used the isochoric method to measure the hydrate phase equilibria, which is based on the variation of the pressure with temperature for a fixed volume system.1 As shown in Figure 2, temperature and pressure data are monitored based on the

In this study, we report phase equilibrium data for CH4 and CH4/C3H8 mixed hydrates in 10 wt % NaCl brine and 10 or 30 wt % MEG mixture and compare the measured data with HLS correlation, which is based on the fundamental principle of freezing point depression, which for hydrate is equivalent to the suppression temperature from the uninhibited systems. From the experimental measurements, the heat (enthalpy) of hydrate dissociation was calculated, and the hydrate structure confirmed in each system. Raman spectroscopic analysis was performed to investigate the gas molecules in hydrate cages with mixtures of NaCl and MEG and verify the hydrate structure, sI for CH4 hydrates and sII for CH4/C3H8 mixed hydrates, which was the same identified from the hydrate dissociation enthalpy.

2. EXPERIMENTAL SECTION Table 1 shows the purity and suppliers of the materials used in this work. In this study, mixtures of 10 wt % NaCl and 10 or 30 Table 1. Purity and Suppliers of Chemicals Used chemical methane (CH4) methane (CH4)/ propane (C3H8) sodium chloride (NaCl) monoethylene glycol (MEG)

suppliers Dae-Duck gas (Korea) Dae-Duck gas (Korea) Kanto Chemical (Japan) Sigma-Aldrich

purity or composition 99.995% (molar) C1/C3 (molar) 91.98(99.995%)/8.02(99.6%) 99.95% (molar) 99.8% (molar)

wt % MEG were considered. Measurements were performed with two gases, CH4 and CH4/C3H8 mixture. Figure 1 represents the schematic of the experimental system for the measurements of hydrate phase equilibrium and Raman spectroscopic analysis. This apparatus consists of a high-

Figure 1. Experimental setup for hydrate phase equilibrium measurement and in situ Raman analysis. B



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following procedure: (a) fast cooling (linear line of pressure− temperature data with no hydrate), (b) hydrate formation, (c) fast heating at a rate of 5 K/h, and (d) slow stepwise heating (0.2 K/1.5 h, verified as proper rate for equilibrium experiments) so the hydrates dissociate and the system returns to its initial state, thermal expansion line (we confirmed the system pressure stabilizes at each temperature during stepwise heating). The inflection point, intersection of the hydrate dissociation line and the heating/cooling line, corresponds to the three-phase (liquid−hydrate−gas) hydrate equilibrium temperature and pressure for a given NaCl and MEG mixture system. The Raman spectroscopic analysis was carried out to investigate the gas in the hydrate cages and hydrate structure during CH4 and CH4/C3H8 hydrate phase equilibrium measurement (step b, hydrate formation) in the mixture of NaCl and MEG. Figure 2. Experimental procedure for the measurements of hydrate phase equilibrium via the isochoric method.

3. RESULTS AND DISCUSSION 3.1. Thermodynamics of Gas Hydrates Formed from NaCl Brine and MEG Mixture. Figure 3 and Table 2 show

Figure 3. Thermodynamics of gas hydrates formed from NaCl brine and MEG mixture. (a) CH4 hydrate phase equilibria. ●, pure CH4 in 10 wt % NaCl + 10 wt % MEG; ▲, pure CH4 in 10 wt % NaCl + 30 wt % MEG; solid lines, predictions by HLS correlation, (b) CH4/C3H8 hydrate phase equilibrium. ○, CH4/C3H8 in 10 wt % NaCl + 10 wt % MEG; △, CH4/C3H8 in 10 wt % NaCl + 30 wt % MEG; solid lines, predictions by HLS correlation; dashed line, prediction by CSMGem, (c) relationship of ΔT/T0T vs T. ●, Pure CH4 in 10 wt % NaCl + 10 wt % MEG; ▲, Pure CH4 in 10 wt % NaCl + 30 wt % MEG; ○, CH4/C3H8 in 10 wt % NaCl + 10 wt % MEG; △, CH4/C3H8 in 10 wt % NaCl + 30 wt % MEG; dashed lines, corresponding to the average value, and (d) average absolute deviation for the HLS correlation with experimental data (this study and literature3,4) for gas hydrate phase equilibria in the mixture of salt and MEG. Red-colored bars, this study; black-colored bars, ref 4; white-colored bars, ref 3. C



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correlation. All parameters used in Figure 3c are in Tables S1−S4. Figure 3d shows the average absolute deviation (AAD) in temperature between the experimental data (this study and literature data) and predicted results from the HLS correlation.

Table 2. Measured Hydrate Phase Equilibrium Data for NaCl Brines with MEG CH4 hydrate phase equilibria T/Ka

solution 10 wt % NaCl + 10 wt % MEG

10 wt % NaCl + 30 wt % MEG

p/MPab

271.1 4.33 272.5 4.94 275.6 7.09 277.6 8.85 279.4 11.14 261.6 3.64 264.7 5.05 267.8 7.17 270.2 9.60 272.1 12.35 CH4/C3H8 hydrate phase equilibria

solution

T/Ka

p/MPab


283.7 286.5 288.2 289.1 289.8 275.8 279.2 280.6 281.4 282.0

4.33 6.83 9.35 11.82 14.31 3.92 6.72 8.98 11.26 13.51


a Uncertainty of temperature: ±0.1 K. 0.125%.

b

N

AAD =

N

(2)

where Tpred and Texp are the predicted and measured hydrate dissociation temperature, respectively, for solutions with the mixture of salt and MEG, and N is the number of equilibrium measurements. As shown in Figure 3d, the HLS correlation provides reliable and accurate results for hydrate phase equilibria over a wide range of pressure (∼200 MPa), temperature (258 ∼ 294 K), and total moles of the salt and MEG with AADs smaller than 0.8 K for this study and 2 K for others. The details for AAD calculation are in Tables S5 and S6. Based on the phase equilibrium pressure and temperature data measured, the enthalpy of hydrate dissociation was calculated with the Clausius−Clapeyron equation.1,12 Figure 4

Uncertainty of pressure:

the CH4 and CH4/C3H8 (feed composition, 91.98%:8.02%) hydrate phase equilibrium data in 10 wt % NaCl and 10 or 30 wt % MEG mixture system. As expected for thermodynamic inhibitors, the mixture of salt and glycol causes the hydrate phase equilibrium boundary to shift to lower temperatures and higher pressures, and on increasing the MEG concentration, the hydrate stable region shifted even more.5,7 Our recently published thermodynamic model, Hu−Lee− Sum correlation,8,9 was also used for each system to compare experimental data and predictions. Detailed information on the model is available elsewhere.8,9 Briefly, this model is a predictive method to estimate hydrate suppression temperature in single or mixed inhibitor systems based on −1 ⎡ ⎛ ΔT ⎞ ⎤ T = T0⎢1 + ⎜ ⎟T0 ⎥ ⎢⎣ ⎝ T0T ⎠ ⎥⎦

∑i = 1 |Tpred − Texp|

Figure 4. Clausius−Clapeyron plots based on CH4 and CH4/C3H8 hydrate equilibrium data from NaCl brine and MEG mixtures. ●, pure CH4 in 10 wt % NaCl + 10 wt % MEG; ▲, pure CH4 in 10 wt % NaCl + 30 wt % MEG; ○, CH4/C3H8 in 10 wt % NaCl + 10 wt % MEG; △, CH4/C3H8 in 10 wt % NaCl + 30 wt % MEG; solid lines, regressions of each data set.

represents the slope from the logarithm of the hydrate dissociation pressure plotted against the reciprocal temperature, which gives the negative enthalpy of hydrate dissociation divided by the product of compressibility and gas constant,

(1)

where ΔT is the hydrate suppression temperature and T0 and T are hydrate dissociation temperature with fresh water and with inhibitor system, respectively. On the basis of the fundamental principle of freezing point depression, which for hydrate is equivalent to the suppression temperature from the uninhibited system, the hydrate suppression temperature with any saline system can be accurately and directly predicted with the calculation of water activity, which is given by the HLS correlation (the quantity ΔT/T0T is directly related to the water activity8,9). As shown in Figure 3c, the form of ΔT/T0T vs T, and each data set falls in a straight horizontal line, revealing the quantity ΔT/T0T is nearly independent of temperature T. The value of ΔT/T0T, or equivalently the water activity, is independent of temperature, which is one of the key findings and assumptions in the Hu−Lee−Sum

d lnP d

1 T

()

=−

ΔHd zR

(3)

where R is the universal constant, z is the compressibility (calculated based on the Peng−Robinson EOS), and P and T are the absolute pressure and temperature of hydrate equilibrium, respectively. The enthalpies of dissociation of CH4 hydrates in the mixture of 10 wt % NaCl brine and 10 or 30 wt % MEG were found to be ∼58.7 and 54.63 kJ/mol, respectively, corresponding to hydrates of structure I (sI), whereas for CH4/C3H8 (91.98%:8.02%) mixed gas, the enthalpies of dissociation were found to be 101.10 kJ/mol (10 wt % NaCl + 10 wt % MEG) and 95.34 kJ/mol (10 wt % NaCl + 30 wt % MEG), confirming the mixed hydrates formed D



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peaks from dissolved MEG could be observed at 2887 and 2945 cm−1, and the intensities decrease significantly after hydrate formation.13,14 Figure 5b shows the Raman spectra of C−H stretching region during hydrate formation, which provides evidence of the hydrate structure. The Raman spectra of CH4 hydrates with pure water show two peaks at 2905 and 2915 cm−1, corresponding to CH4 gas molecules occupying the small and large cages, respectively, in sI hydrates.14 In 10 wt % NaCl and 30 wt % MEG mixture system, as discussed above, initially two large peaks were observed because of dissolved MEG, and after hydrate formation, two small peaks of CH4 hydrates appeared at the same peak positions of the pure water system, representing the same structure, sI. In the CH4/C3H8 mixed gas system, with the spectra of CH4 at 2904 and 2914 cm−1, we additionally observed two more peaks at approximately 2871 and 2880 cm−1, corresponding to sII hydrates.14,15

structure II (sII). Details of the parameters calculated in the equation are in Table S7. 3.2. Raman Spectroscopic Analysis of Gas Hydrates in NaCl Brine and MEG Mixture. The Raman spectroscopic measurement was carried out to verify the hydrate structure estimated by the heat of dissociation determined from the Clausius−Clapeyron equation and also investigate the inclusion of gas molecules in the hydrate cages from NaCl brine and MEG mixtures. First, we investigated the background Raman spectra at atmospheric condition to obtain the baseline spectra, which are for pure water, 10 wt % NaCl solution, 30 wt % MEG solution, and 10 wt % NaCl brine with 30 wt % MEG. Figure 5a

4. CONCLUSIONS This study presented pure CH4 and CH4/C3H8 mixed hydrate phase equilibrium data formed from a mixture of NaCl (10 wt %) and monoethylene glycol (10 and 30 wt %) solutions. The mixture of thermodynamic inhibitors caused the hydrate phase equilibrium boundary to shift to lower temperatures and higher pressures. The HLS correlation was used to compare and verify the experimental data, showing good agreement. From the Clausius−Clapeyron equation, the enthalpy of hydrate dissociation was calculated, and the structure of the samples was confirmed to be sI and sII in CH4 and CH4/C3H8 hydrates, respectively. Raman spectroscopic analysis was also performed to investigate the guest molecules in the hydrate cages and hydrate structure, and it revealed the same hydrate structure as obtained from the heat of dissociation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00155. Detailed parameters used in the HLS correlation, comparison of literature data and the HLS correlation, and in situ data of Raman microscopic analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 5. Raman spectroscopic analysis of gas hydrates formed in NaCl brine and MEG mixture. (a) Baseline Raman spectra in pure or mixed samples and (b) Raman spectra of C−H stretching mode for CH4 or CH4/C3H8 hydrates with pure water or the mixture of 10 wt % NaCl and 30 wt % MEG.

ORCID

Ju Dong Lee: 0000-0002-9567-8396 Bo Ram Lee: 0000-0002-9510-5846 Amadeu K. Sum: 0000-0003-1903-4537 Present Address

shows the typical C−H and O−H stretching regions. NaCl affects both the O−H symmetric and O−H asymmetric stretch, so compared to the pure water system, 10 wt % NaCl brine enhances the structural changes of the O−H stretching band, diminishing the O−H symmetric and increasing the O−H asymmetric stretching vibrations, leading to a spectrum shift toward slightly higher wavenumbers. This can be considered a direct result of the dissolution of NaCl in water, which causes a decrease in the number of hydrogen bonds in the intermolecular structure. In the C−H stretching region, two

⊥

S.Y.H.: R&BD Center, PROSAVE Co., ltd., Gimhae, Gyeongnam 50875, Republic of Korea

Funding

This work was supported by the BK21Plus Program for advanced education of creative chemical engineers of the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP). This research was also partially supported by a project named “Development of WaterTreatment and Recovery of Vital Resources from LNG Cold E



Article

Energy and Gas Hydrate Process (EO180032)” funded by the Korea Institute of Industrial Technology. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (2) Hu, Y.; Makogon, T. Y.; Karanjkar, P.; Lee, K. H.; Lee, B. R.; Sum, A. K. Gas Hydrates Phase Equilibria and Formation from High Concentration NaCl Brines up to 200 MPa. J. Chem. Eng. Data 2017, 62, 1910−1918. (3) Burgass, R.; Chapoy, A.; Li, X. Gas hydrate equilibria in the presence of monoethylene glycol, sodium chloride and sodium bromide at pressures up to 150 MPa. J. Chem. Thermodyn. 2018, 118, 193−197. (4) Kim, H.; Park, J.; Seo, Y.; Ko, M. Hydrate risk management with aqueous ethylene glycol and electrolyte solutions in thermodynamically under-inhibition condition. Chem. Eng. Sci. 2017, 158, 172−180. (5) Hu, Y.; Lee, B. R.; Sum, A. K. Phase equilibrium data of methane hydrates in mixed brine solutions. J. Nat. Gas Sci. Eng. 2017, 46, 750− 755. (6) Hu, Y.; Lee, K.-H.; Lee, B. R.; Sum, A. K. Gas hydrate formation from high concentration KCl brines at ultra-high pressures. J. Ind. Eng. Chem. 2017, 50, 142−146. (7) Hu, Y.; Makogon, T. Y.; Karanjkar, P.; Lee, K.-H.; Lee, B. R.; Sum, A. K. Gas hydrates phase equilibria for structure I and II hydrates with chloride salts at high salt concentrations and up to 200 MPa. J. Chem. Thermodyn. 2018, 117, 27−32. (8) Hu, Y.; Lee, B. R.; Sum, A. K. AIChE J. 2017, 63, 5111−5124. (9) Hu, Y.; Lee, B. R.; Sum, A. K. Universal correlation for gas hydrates suppression temperature of inhibited systems: II. Mixed salts and structure type. AIChE J. 2018, DOI: 10.1002/aic.16116. (10) Kwak, G.-H.; Lee, K.-H.; Lee, B. R.; Sum, A. K. Quantification of the risk for hydrate formation during cool down in a dispersed oilwater system. Korean J. Chem. Eng. 2017, 34 (7), 2043. (11) Lee, J. M.; Cho, S. J.; Lee, J. D.; Linga, P.; Kang, K. C.; Lee, J. Insights into the Kinetics of Methane Hydrate Formation in a Stirred Tank Reactor by In Situ Raman Spectroscopy. Energy Technol. 2015, 3, 925−934. (12) Babu, P.; Yang, T.; Veluswamy, H. P.; Kumar, R.; Linga, P. Hydrate phase equilibrium of ternary gas mixtures containing carbon dioxide, hydrogen and propane. J. Chem. Thermodyn. 2013, 61, 58−63. (13) Park, J.; Shin, K.; Lee, J.-W.; Lee, H.; Seo, Y. S. In situ Raman and 13C NMR spectroscopic analysis of gas hydrates formed in confiend water: application to natural gas capture. Can. J. Chem. 2015, 93, 1−8. (14) Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of Clathrate Hydrates via Raman Spectroscopy. J. Phys. Chem. B 1997, 101, 7371−7377. (15) Uchida, T.; Moriwaki, M.; Takeya, S.; Ikeda, I.Y.; Ohmura, R.; Nagao, J.; Minagawa, H.; Ebinuma, T.; Narita, H.; Gohara, K.; Mae, S. Two-step formation of methane−propane mixed gas hydrates in a batch-type reactor. AIChE J. 2004, 50, 518−523.

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Phase Behavior and Raman Spectroscopic Analysis for CH4 and CH4

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