J. Chem. Thermodynamics xxx (2017) xxx–xxx Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct The impact of amino acids on methane hydrate phase boundary and formation kinetics Cornelius B. Bavoh a,b, Omar Nashed a,b, Muhammad Saad Khan a,b, Behzad Partoon a,b, Bhajan Lal a,b,⇑, Azmi M. Sharif a,b a b Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia CO2 Research Centre (CO2RES), Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 Perak, Malaysia a r t i c l e i n f o Article history: Received 25 April 2017 Received in revised form 31 August 2017 Accepted 1 September 2017 Available online xxxx Keywords: Gas hydrate Amino acids Equilibrium phase boundary Kinetics Methane a b s t r a c t The thermodynamic and kinetic effects of two amino acids (valine and arginine) on methane hydrate formation was evaluated by measuring the dissociation temperature of methane hydrate in the range of 4.5– 10 MPa using the T-cycle method. The kinetics of methane hydrate formation was evaluated at 7.1 MPa and 274.15 K. The experiments were conducted at concentrations of 0.01 and 0.05 mass fraction. Both amino acids showed a slight inhibition effect on the phase boundary of methane hydrate. The predicted methane hydrate phase boundary data in the presence of amino acids was strongly correlated with the experimental data with R = 0.9996 and an AAE less than 0.15 K. However, these amino acids also showed hydrate formation rate enhancement compared to pure water. In addition, the total methane uptake at the end of the experiments was increased in the presence of these amino acids. Ó 2017 Elsevier Ltd. 1. Introduction Gas hydrate are formed by the physical trapping of appropriate gas molecules (such as methane, carbon dioxide etc.) in hydrogen bonded water cages at lower temperatures and higher pressures conditions. The gas molecules are held in the water molecules by Van der Waal forces [1,2]. In oil and gas flow assurance, the formation of gas hydrate is unwanted, because it can lead to plugged pipelines which consequently provoke high removal expense, stoppage in production and in severe scenarios, the possible loss of human life [3]. Generally, gas hydrates inhibitors and promoters are the two main kinds of gas hydrate chemical additives [4–6] that are usually applied to affects gas hydrate formation either thermodynamically (by shifting the hydrate phase boundary conditions) and/or kinetically (by delaying/enhancing the hydrate nucleation and growth process) based on the required area of application. Gas hydrate inhibitors are employed to mitigate gas hydrate formation in pipelines. There are two main classes of gas hydrate inhibitors: thermodynamic hydrate inhibitors (THIs) and low dosage hydrate inhibitors (LDHIs). THIs are usually alcohol-based (methanol and glycols) that inhibit hydrate formation by increasing the hydrate free regions through the shifting of the hydrate phase boundary conditions to higher pressures and/or lower tem- ⇑ Corresponding author at: Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia. E-mail address: bhajan.lal@utp.edu.my (B. Lal). peratures. However, THIs are volatile in nature and are applied in huge quantities (up to over 0.4 mass fraction) [2,7,8], which makes them expensive and environmentally unfriendly. LDHIs are mostly polymers (PVP and PVCap) and are used in lower concentrations (<0.02 mass fraction). They typically delay hydrate nucleation process and/or total gas consumed in hydrate formation [1,9]. However, there is ongoing research for new and effective gas hydrate chemical inhibitors to replace existing ones. Such research outcomes would aid in an efficient application of gas hydrate based technologies. Recently, natural amino acids have been introduced as gas hydrate inhibitors due to their zwitterionic behavior in water and hydrogen bonding affinity for water molecules. Sa et al. [10,11] reported that, in the concentration range of 0.001– 0.09 mol fraction, amino acids such as glycine, alanine, proline, valine, and serine could thermodynamically inhibit methane and carbon dioxide hydrate formation. Proline and valine were found to show the highest inhibition impact for methane and carbon dioxide hydrate, respectively. On the other hand, on the bases of mass fraction, glycine has been reported to thermodynamically inhibit methane hydrates better than alanine, proline, serine, and arginine at 0.1 mass fraction [12]. Further study by Sa & coworkers [13,14], showed that amino acids kinetically reduce carbon dioxide consumption compared to pure water via local water perturbation. Since then, several studies [15–17] have also shown that amino acids kinetically inhibits ethane and THF hydrates. Most recently, Liu et al. [18] studied the effect of natural amino http://dx.doi.org/10.1016/j.jct.2017.09.001 0021-9614/Ó 2017 Elsevier Ltd. Please cite this article in press as: C.B. Bavoh et al., J. Chem. Thermodyn. (2017), http://dx.doi.org/10.1016/j.jct.2017.09.001 2 C.B. Bavoh et al. / J. Chem. Thermodynamics xxx (2017) xxx–xxx Nomenclature AAE T Tf(i) Tf P n DHd a average absolute error temperature, K freezing point temperatures of water, 273.15 K freezing point temperatures of aqueous amino acid solution, K pressure, MPa hydration number, 6.0 dissociation enthalpy, kJ/mol activity Table 1 Chemical structures and details of studied amino acids. Amino acid Chemical structure Purity Supplier Arginine P 99% Sisco Research Laboratories Pvt. Ltd Valine P 98% Sigma-Aldrich m number of data points R universal gas constant, 8.314 J/(mol.K) DHFUS(i) heat of fusion of ice, 6008 J/mol Subscripts aa amino acid w water Cal. calculated Exp. experimental Due to the requirement for new effective gas hydrate additives, more understanding of the effect of amino acids on gas hydrate formation is need. There are some amino acids whose thermodynamic and kinetic effect on gas hydrate formation have not been widely studied. In addition, there is limited study on the prediction of gas hydrate equilibrium phase boundary conditions in the presence of amino acids. Thus, it is motivating to study the effect of such amino acids on methane hydrate formation. Herein, the thermodynamic effect of arginine and valine on the methane hydrate equilibrium phase boundary is predicted by adapting an existing model. 2. Experimental acids as CH4 hydrate promoters at concentrations less than 0.01 mass fraction. They found that, leucine exhibited the highest promotion effect. Their findings motivated Veluswamy et al. [19] to further study the morphological changes mediated by leucine during methane hydrate formation nucleation, growth and dissociation. They suggested that, no hydrate promotion was observed below 0.003 mass fractions. In addition, Bhattacharjee et al. [20] reported that, histidine (a polar amino acid) promotes methane hydrate growth in the same range as SDS at 0.01 mass fraction. However, histidine has also been reported as a kinetic inhibitor for carbon dioxide hydrates [14,21], indicating that, the kinetic inhibition and promotion effects of amino acids are dependent on the type of guest molecules present. 2.1. Materials The chemical structures and details of the amino acids used in this study are shown in Table 1. All chemicals were used without further purification. Methane gas with purity of 99.995% was supplied by Gas Walker Sdn Bhd, Malaysia. All samples were prepared using deionized water. 2.2. Apparatus and procedures A hydrate sapphire cell rector as illustrated in Fig. 1 was used for both hydrate equilibrium point and kinetic measurements. In order to measure the hydrate equilibrium points, the isochoric Fig. 1. Schematic diagram of the experimental setup. Please cite this article in press as: C.B. Bavoh et al., J. Chem. Thermodyn. (2017), http://dx.doi.org/10.1016/j.jct.2017.09.001 3 C.B. Bavoh et al. / J. Chem. Thermodynamics xxx (2017) xxx–xxx 3. Model theory To predict the methane hydrate equilibrium phase points in the presence of arginine and valine, the model by Dickens and QuinbyHunt [28] for electrolytes was adopted since most aqueous amino acids show zwitterionic behavior. This zwitterionic behavior makes them exhibit electrostatic behavior similar to normal electrolytes (salts) [29]. The model is a modification of the Pieroen [30] model and has been previously adapted for modelling the effect of electrolytes and ionic liquid solutions on the methane hydrate phase boundary by Javanmardi et al. [31,32] and Partoon et al. [4], and was developed from classical thermodynamic theory. It assumes that, the amount of methane in the water phase is negligible, and vice versa. Also, the effect of arginine and valine on methane hydrate formation solely decreases water activity (aw) and at small temperature ranges, the hydrate enthalpy of dissociation (DHd) is constant. The derivation details are provided by Dickens and Quinby-Hunt [28] and Pieroen [30]. Based on this model, the effect of amino acid on the methane hydrate formation temperature can be represented as; In aw ¼
DHd 1 nR T w 1 T aa  ð1Þ where, In aw ¼
DHFUSðiÞ 1 R T f ðiÞ 1 Tf  point temperatures of water (273.15 K) and water + aqueous amino acid solution. Tf is calculated as suggested by Dickens and QuinbyHunt [28] using cryoscopic constant for water as 1.853 K
kg/mol. Hence, combining Eqs. (1) and (2), relates the temperature offset of methane hydrate phase condition and the temperature of the ice-water equilibrium condition in any amino acid solution at constant pressure as follows; 
nDHFUSðiÞ 1 1 ¼ T aa T f ðiÞ DHd 1 Tw
1 Tf  ð3Þ Therefore, Eq. (3) can be used to calculate Taa, the hydrate formation temperature in the presence of amino acids. The model’s average absolute error (AAE) is calculated using Eq. (4). AAE ¼ m 1X jT Exp: m i¼1 T Cal: ji ð4Þ 4. Results and discussion 4.1. Thermodynamic effect The measured methane hydrate equilibrium boundary points in the presence of the amino acids are tabulated in Table 2 and graphically presented in Fig. 2. The presence of arginine and valine shifts the methane hydrate equilibrium phase boundary to higher Table 2 The experimental and predicted methane dissociation temperature (Ta) and pressure (Pa) of amino acids at 0.05 mass fraction. Amino acid TExp/K TCal/K P/MPa Arginine 278.80 281.30 283.30 285.90 278.61 281.19 283.22 285.89 4.55 6.15 7.50 9.84 Valine 278.60 281.40 283.00 285.80 278.52 281.31 282.80 285.69 4.60 6.20 7.58 9.65 a Standard uncertainties (U) are U(T) = ±0.15 K, U(P) = ±0.01 MPa, U(mass fraction) = ±0.0003 11 10 9 Pressure / Mpa T-cycle method [4] was employed. 18 ml of 0.05 mass fraction desired aqueous amino acid solution was pumped into the cell via a manual hand pump. A gas booster was then used to supply the cell with methane at the desired pressure. When the system stabilized, the magnetic torque stirrer was switched on, and the system temperature reduced to 274.15 K (at 4 K/hr) to allow hydrate formation. The hydrate formation is detected both visually and via observation by a rapid pressure drop in the system. When hydrates were formed, the system temperature was first heated fast (at 4 K/hr) to about 5 K close to the desired dissociation temperature and then heated stepwise for 3 h at intervals of 0.5 K/step [22–24]. The details of the experimental apparatus and procedures employed are previously reported elsewhere [12,25,26]. To perform the kinetic measurements, the cell was cleaned, vacuumed and loaded with 18 ml of the desired aqueous amino acids. The cell was then pressurized with methane to 7.1 MPa, and the stirrer is turned on. The system temperature was reduced to the experimental temperature (4 K/hr) for hydrate to form simultaneously with the data acquisition system recoding the changes in pressure and temperature of the system (every 10 s). The experimental temperature for all kinetic experiments were fixed at 274.15 K. The experiments were considered completed by observing a constant pressure and temperature in the cell for 2–3 h. The induction time was determined as described in literature [26]. To calculate the moles of methane consumed, the real gas equation was used with the compressibility factor, z, calculated from Peng–Robinson equation of state. The initial apparent rate constant was determined by finding the gradient of the plot of calculated methane mole consumed verses time [27]. 8 7 6 ð2Þ 5 where aw represents water activity, DHd denotes the dissociation enthalpy of methane (58.88 kJ/mol [22]), n is methane hydrate hydration number (6.0 [33]), R is the universal gas constant, and Tw and Taa are the hydrate formation temperatures in pure water and water + amino acid solution. The pure water methane hydrate dissociation temperature can be calculated by any hydrate prediction model as described in literature [9] or CSMGem. DHFUS(i) is the heat of fusion of ice (6008 J/mol), Tf(i) and Tf are the freezing 4 278 279 280 281 282 283 284 285 286 287 Temperature / K Fig. 2. Methane hydrate equilibrium phase boundary in the presences of amino acids: (d) Pure water this wok, (s) Pure water Sabil et al. [2], ( ) Arginine, ( ) Valine. Please cite this article in press as: C.B. Bavoh et al., J. Chem. Thermodyn. (2017), http://dx.doi.org/10.1016/j.jct.2017.09.001 4 C.B. Bavoh et al. / J. Chem. Thermodynamics xxx (2017) xxx–xxx 11 60 10 50 Induction time / min Pressure / Mpa 9 8 7 6 40 30 20 5 10 4 278 280 282 284 286 288 0 Temperature / K Arginine Valine SDS Fig. 3. Experimental and predicted methane hydrate equilibrium phase boundary in the presences of amino acids: (d) Pure water this wok, (s) Pure water Sabil et al. [2], ( ) Arginine, ( ) Valine, ( ) Arginine predicted, ( ) Valine predicted. Fig. 4. The induction time of methane hydrates in the presence of arginine, valine and SDS; the solid line represents pure water sample. pressures and lower temperatures regions. An average methane hydrate equilibrium temperature shift of about 0.5 K is observed, suggesting a thermodynamic methane hydrate inhibition effect. There was a minor inhibition impact (Fig. 2) likely because the amino acids are tested at low concentration (0.05 mass fraction) compared to concentrations of known THIs (>0.15 mass fraction) used in academic laboratories and industrial applications. The inhibition mediated by both arginine and valine was found to be in the same range, with valine slightly higher than arginine. It is possible that the thermodynamic inhibition impact of arginine and valine is due to their ability to form hydrogen bonding with water molecules [11], and therefore influences the activity of water to compete in hydrate formation. This findings further extends and supports the study of Sa et al. [11] and Bavoh et al. [12] on amino acids as THIs for methane hydrate formation. Furthermore, the slight inhibition impact of valine over arginine could be due to their side chain alkyl length and properties [12]. This resulted in a slight improvement of the miscibility of valine with water. The proposed model was then used to predict the impact of arginine and valine on the methane hydrate and for comparison with the equilibrium phase condition point. The predicted and experimental methane hydrate equilibrium points are presented in Fig. 3. As shown in Fig. 3, the model predictions are in good agreement with the experimental data with an AAE of 0.09 K and 0.12 K for arginine and valine, respectively. In addition, a very strong correlation (R = 0.9996 at 95% confidence level) was observed between the experimental and predicted hydrate dissociation temperatures. This further validates the accuracy of the model and thus, supports the used of the model in predicting the hydrate phase behavior in amino acids. the induction time of arginine and water were found to be in the same range as shown in Fig. 4. The effect of arginine and valine on the initial rate of hydrate is estimated and illustrated in Fig. 5. Interestingly, both arginine and valine delayed the rate of hydrate formation compared to pure water. Valine showed the highest methane hydrate formation rate reduction, as it could delay the hydrate formation rate about 3 times more than arginine and 6 times more than pure water. Due to the stochastic nature of hydrate induction time measurements, the use of induction time alone can sometimes be misleading. Therefore, the total methane moles consumed was used to further investigate the effects of arginine and valine on methane hydrate formation. The total methane consumed is taken as the point where the system pressure became stable after hydrate formation. However, in most cases, the stability of the system pressure in hydrate formation is either due to kinetic or thermodynamic constrains. The hydrate formation stability or constant pressures of all experiments were found to be above the hydrate equilibrium pressure of the experimental temperature (274.15 K) as shown in Fig. 6. This suggests that, the hydrate formation stability or completion is not due to thermodynamic constraints but rather, on kinetics. The influence of 0.01 mass fraction of arginine and valine on the total moles of methane consumed is presented in Fig. 7. Contrary to the thermodynamic inhibition effect (shown in Fig. 2), it is observed in Fig. 7 that, the Pure water Arginine 4.2. Kinetic effect The induction time, initial apparent rate of hydrate formation and the total methane moles consumed are studied as kinetic inhibition indicators. Since kinetics studies of gas hydrate is probabilistic, the experiments were repeated two times to ensure repeatability of the experimental results. Induction time was used to evaluate the effect of amino acids on the methane hydrate nucleation process until a detectable hydrate is formed. The induction time in this study is determined as described in literature [26]. Valine showed a longer induction time than pure water. However, Valine SDS 0 0.02 0.04 0.06 0.08 Initial apparent rate of formation / 0.1 0.12 min-1 Fig. 5. The initial apparent rate of methane hydrate formation in the presence of arginine, valine and SDS. Please cite this article in press as: C.B. Bavoh et al., J. Chem. Thermodyn. (2017), http://dx.doi.org/10.1016/j.jct.2017.09.001 C.B. Bavoh et al. / J. Chem. Thermodynamics xxx (2017) xxx–xxx 5 8 7 Pressure / Mpa 6 5 4 3 Fig. 8. Pictures of produced methane hydrate from pure water, arginine, valine, and SDS during hydrate growth. 2 272 274 276 278 280 282 284 Temperature / K Fig. 6. The hydrate formation stability pressures of all experiments in this work against methane hydrate equilibrium condition: (d) Pure water run 1, (s) Pure water run 2, ( ) Arginine run 2, ( ) Arginine run 2, ( ) Valine run 2, ( ) Valine run 2, ( ) SDS run 1, ( ) SDS run 2. 0.025 Methane consumed / mole 0.02 0.015 0.01 0.005 0 0 100 200 300 400 500 600 Time / min Fig. 7. Mole of methane consumed during hydrate growth: (d) Pure water ( Arginine, ( ) Valine, ( ) SDS. ) presences of arginine and valine significantly increased the total moles of methane consumed compared to pure water. However, valine showed the highest impact on methane hydrate promotion. Valine enhanced methane hydrate formation about 10 times more than water. The surprising contrasting behavior of arginine and valine on the thermodynamics and kinetics of methane hydrate formation is similar to methanol (a commercially used thermodynamic hydrate inhibitor). Methanol is well known as a good thermodynamic hydrate inhibitor, however several studies [34] have shown that, methanol sometimes kinetically promotes hydrate formation depending on the applied concentration [35]. The methane hydrate kinetic promotion behavior of arginine and valine is similar to the mechanism of surfactants [18,36]. In addition, both arginine and valine have longer alkyl side chains, which according to Sa et al. [13], a hydrate kinetic promotion impact is sometimes exhibited as amino acids side chain alkyl length increases. Also, the longer alkyl side chains of amino acids make them good candidates to develop bio-surfactants. Thus, just like surfactants (e.g. SDS) effect on hydrate formation, arginine and valine may restrict the aggregation of hydrate at the liquid – vapor interface during hydrate formation. This facilitates the movement of more methane into the liquid phase, therefore, increasing the moles of methane in hydrate formation [27]. This mechanism is supported by the visual observation made during hydrate formation until completion (see Fig. 8). In all experiments containing arginine, valine, and SDS it was visual observed that, the stirrer in the cell continue to rotate till completion. Whereas in experiments without additives, the stirrer stops rotating upon hydrate formation at the liquid/water interface, suggesting hydrate aggregation suppression behavior as the possible cause of the methane hydrate promotion effect in arginine and valine. In addition, the observed promotion variation of arginine and valine is attributed to their different side chain properties. The higher methane hydrate promotion of valine than arginine is because, the methyl R-groups in valine can become incorporated as guests in hydrate cages [37]. This could provide some stability for hydrate formation, hence result in higher hydrate formation than arginine. The surprising high methane hydrate kinetic promotion effect of arginine and valine observed in this study is further compared with SDS (a commercial hydrate promoter) as shown in Figs. 4, 5, and 6. On the bases on induction time, arginine and valine performed poorly as methane hydrate formation nucleation promoters in comparison with SDS (see Fig. 4). On the other hand, the impact of SDS on the initial rate of methane hydrate formation is in the same range as arginine (see Fig. 5). However, both arginine and valine were found to promote methane hydrate uptake more than SDS as shown in Fig. 7. The average methane consumed in the hydrate in the presence of valine is 1.3 times that of SDS. This results agrees with Bhattacharjee et al. [20] suggesting that, the impact of some amino acids (Histidine) on the total moles of methane consumed are in the range with SDS. Furthermore, the methane promotion effect of valine agrees with the findings of Perfeldt et al. [38] who suggested that valine does not kinetically promote methane hydrate formation. 5. Conclusions The effect of arginine and valine on the thermodynamics and kinetics of methane hydrate formation is studied in an isochoric Please cite this article in press as: C.B. Bavoh et al., J. Chem. Thermodyn. (2017), http://dx.doi.org/10.1016/j.jct.2017.09.001 6 C.B. Bavoh et al. / J. Chem. Thermodynamics xxx (2017) xxx–xxx high pressure sapphire cell reactor. It was found that both arginine and valine thermodynamically inhibits methane hydrate formation by shifting the phase boundary to higher pressures and lower temperatures region. An average dissociation temperature shift of 0.5 K is observed in the presence of both arginine and valine. Furthermore, the predicted and experimental methane hydrate equilibrium phase temperature were in good agreement with an AAE of less than 0.15 K. In contrary to the thermodynamic inhibition effect, both arginine and valine kinetically enhanced the total mole of methane consumed compared with pure water and SDS (i.e. kinetic methane hydrate promotion effect). Valine showed the highest average methane hydrate promotion impact of about 10 and 1.3 times higher than pure water and SDS. The findings in this study should be useful for gas hydrate based technological application research such as flow assurance and gas transportation and storage. Acknowledgement This work is support by the Universiti Teknologi PETRONAS through FRGS Research grants (Grant No. FRGS-0153AB-K77) from the Ministry of Higher Education, Malaysia. References [1] C.A. Koh, E.D. Sloan, A.K. Sum, D.T. Wu, Fundamentals and applications of gas hydrates, Annu. Rev. Chem. Biomol. Eng. 2 (2011) 237–257. [2] K.M. Sabil, O. Nashed, B. Lal, L. Ismail, A. Japper-jaafar, Experimental investigation on the dissociation conditions of methane hydrate in the presence of imidazolium-based ionic liquids, Thermodyn. J Chem. 84 (2015) 7–13. [3] E.G. Hammerschmidt, Formation of gas hydrates in natural gas transmission lines, Ind. Eng. Chem. 26 (1934) 851–855. [4] B. Partoon, N.M.S. Wong, K.M. Sabil, K. Nasrifar, M.R. Ahmad, A study on thermodynamics effect of [EMIM]-Cl and [OH-C2MIM]-Cl on methane hydrate equilibrium line, Fluid Phase Equilib. 337 (2013) 26–31. [5] O. Nashed, K.M. Sabil, B. Lal, L. Ismail, A.J. Jaafar, Study of 1-(2-Hydroxyethyle) 3-methylimidazolium Halide as Thermodynamic Inhibitors, Appl. Mech. Mater. 625 (2014) 337–340. [6] Xu. Chun-Gang, Xiao-Sen Li, Research progress of hydrate-based CO2 separation and capture from gas mixtures, RSC Adv. 4 (2014) 18301–18316. [7] E. Broni-Bediako, R. Amorin, C.B. Bavoh, Gas hydrate formation phase boundary behaviour of synthetic natural gas system of the Keta Basin of Ghana, Open Pet. Eng. J. 10 (2017) 64–72. [8] C.B. Bavoh, B. Lal, O. Nashed, M.S. Khan, K.K. Lau, M.A. Bustam, COSMO-RS: an ionic liquid prescreening tool for gas hydrate mitigation, Chinese J. Chem. Eng. 11 (2016) 1619–1624. [9] E.D. Sloan, C.A. Koh, Clathrate Hydrates of Natural Gases, third ed., CRC Press, Baca Raton, 2007. [10] J.H. Sa, B.R. Lee, D.H. Park, K. Han, H.D. Chun, K.H. Lee, Amino acids as natural inhibitors for hydrate formation in CO2 sequestration, Environ. Sci. Technol. 45 (2011) 5885–5891. [11] J.-H. Sa, G.-H. Kwak, K. Han, D. Ahn, S.J. Cho, J.D. Lee, K.-H. Lee, Inhibition of methane and natural gas hydrate formation by altering the structure of water with amino acids, Sci. Rep. 6 (2016) 31582. [12] C.B. Bavoh, B. Partoon, B. Lal, L.K. Keong, Methane hydrate-liquid-vapourequilibrium phase condition measurements in the presence of natural amino acids, J. Nat. Gas Sci. Eng. 37 (2016) 425–434. [13] J.-H. Sa, G.-H. Kwak, B.R. Lee, D.-H. Park, K. Han, K.-H. Lee, Hydrophobic amino acids as a new class of kinetic inhibitors for gas hydrate formation, Sci. Rep. 3 (2013) 2428. [14] J.-H. Sa, G.-H. Kwak, K. Han, D. Ahn, K.-H. Lee, Gas hydrate inhibition by perturbation of liquid water structure, Sci. Rep. 5 (2015) 11526. [15] P. Naeiji, A. Arjomandi, F. Varaminian, Amino acids as kinetic inhibitors for tetrahydrofuran hydrate formation: experimental study and kinetic modeling, J. Nat. Gas Sci. Eng. 21 (2014) 64–70. [16] P. Naeiji, M. Mottahedin, F. Varaminian, Separation of methane–ethane gas mixtures via gas hydrate formation, Sep. Purif. Technol. 123 (2014) 139–144. [17] S.A. Rad, K.R. Khodaverdiloo, M. Karamoddin, F. Varaminian, K. Peyvandi, Kinetic study of amino acids inhibition potential of Glycine and L-leucine on the ethane hydrate formation, J. Nat. Gas Sci. Eng. 26 (2015) 819–826. [18] Y. Liu, B. Chen, Y. Chen, S. Zhang, W. Guo, Y. Cai, B. Tan, W. Wang, Methane storage in a hydrated form as promoted by leucines for possible application to natural gas transportation and storage, Energy Technol. 3 (2015) 815–819. [19] H.P. Veluswamy, Q.W. Hong, P. Linga, Morphology study of methane hydrate formation and dissociation in the presence of amino acid, Cryst. Growth Des. 16 (2016) 5932–5945. [20] G. Bhattacharjee, N. Choudhary, A. Kumar, S. Chakrabarty, R. Kumar, Effect of the amino acid l-histidine on methane hydrate growth kinetics, J. Nat. Gas Sci. Eng. 35 (2016) 1453–1462. [21] H. Roosta, A. Dashti, S.H. Mazloumi, F. Varaminian, Inhibition properties of new amino acids for prevention of hydrate formation in carbon dioxide-water system: experimental and modeling investigations, J. Mol. Liq. 215 (2016) 656–663. [22] M.S. Khan, B. Partoon, C.B. Bavoh, B. Lal, N.B. Mellon, Influence of tetramethylammonium hydroxide on methane and carbon dioxide gas hydrate phase equilibrium conditions, Fluid Phase Equilib. 440 (2017) 1–8. [23] M.S. Khan, C.B. Bavoh, B. Partoon, B. Lal, M.A. Bustam, A.M. Shariff, Thermodynamic effect of ammonium based ionic liquids on CO2 hydrates phase boundary, J. Mol. Liq. 238 (2017) 533–539. [24] C.B. Bavoh, B. Partoon, B. Lal, G. Gonfa, S. Foo Khor, A.M. Sharif, Inhibition effect of amino acids on carbon dioxide hydrate, Chem. Eng. Sci. 171 (2017) 331–339. [25] C.B. Bavoh, B. Partoon, L.K. Keong, B. Lal, C.D. Wilfred, Eeffect of 1-ethyl-3methylimidazolium chloride and polyvinylpyrrolidone on kineticss of carbon dioxide hydrates, Int. J. Appl. Chem. 12 (2016) 6–11. [26] C.B. Bavoh, B. Lal, L.K. Keong, M. Binti Jasamai, M. Binti Idress, Synergic kinetic inhibition effect of EMIM-Cl + PVP on CO2 hydrate formation, Procedia Eng. 148 (2016) 1232–1238. [27] B. Partoon, S.N.A. Malik, M.H. Azemi, K.M. Sabil, Experimental investigations on the potential of SDS as low-dosage promoter for carbon dioxide hydrate formation, ASIA-PACIFIC J. Chem. Eng. 8 (2013) 258–261. [28] G.R. Dickens, M.S. Quinby-Hunt, Methane hydrate stability in pore water: a simple theoretical approach for geophysical applications, J. Geophys. Res. 102 (1997) 773–783. [29] J.G. Kirkwood, Theory of solutions of molecules containing widely separated charges with special application to zwitterions, J. Chem. Phys. 2 (1934) 351– 361. [30] A.P. Pieroen, Gas hydrates- approximate relations between heat formation, composition and equilibrium temperature lowering by inhibitors, Rec. Trav. Chim. 74 (1955) 955–1002. [31] J. Javanmardi, M. Moshfeghian, R.N. Maddox, An accurate model for prediction of gas hydrate formation conditions in mixtures of aqueous electrolyte solutions and alcohol, Can. J. Chem. Eng. 79 (2001) 367–373. [32] J. Javanmardi, M. Moshfeghian, R.N. Maddox, Simple method for predicting gas-hydrate-forming, Energy. 28 (1998) 219–222. [33] G.K. Anderson, Enthalpy of dissociation and hydration number of methane hydrate from the Clapeyron equation, J. Chem. Thermodyn. 36 (2004) 1119– 1127. [34] M.H. Yousif, Effect of underinhibition with methanol and ethylene glycol on the hydrate-control process, SPE Prod. Facil. 13 (1998) 184–189. [35] H.K. Abay, T.M. Svartaas, W. Ke, Effect of gas composition on sII hydrate growth kinetics, Energy & Fuels. 25 (2011) 1335–1341. [36] H.P. Veluswamy, P.Y. Lee, K. Premasinghe, P. Linga, Effect of biofriendly amino acids on the kinetics of methane hydrate formation and dissociation, Ind. Eng. Chem. Res. 56 (2017) 6145–6154. [37] T. Sun, P.L. Davies, V.K. Walker, Article structural basis for the inhibition of gas hydrates by a-helical antifreeze proteins, Biophysj. 109 (2015) 1698–1705. [38] C.M. Perfeldt, P.C. Chua, N. Daraboina, D. Friis, E. Kristiansen, H. Ramløv, J.M. Woodley, M.A. Kelland, N. Von Solms, Inhibition of gas hydrate nucleation and growth: efficacy of an antifreeze protein from the Longhorn Beetle Rhagium mordax, Energy Fuels 28 (2014) 3666–3672. JCT 17-342 Please cite this article in press as: C.B. Bavoh et al., J. Chem. Thermodyn. (2017), http://dx.doi.org/10.1016/j.jct.2017.09.001