Kinetic investigation of methane hydrate in the presence of Imidazolium Based Ionic Liquid solutions

Transcription

1 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University Kinetic investigation of methane hydrate in the presence of Imidazolium Based Ionic Liquid solutions M. Zare 1, A. Haghtalab * 1, A. N. Ahmadi 2, K. Nazari 2, Ali Mehdizadeh 2 1- Department of Chemical Engineering, Tarbiat Modares University, P.O. Box: , Tehran, Iran 2- Center of chemistry and petrochemical, Research Institute of Petroleum Industry, Tehran, Iran haghtala@modares.ac.ir Abstract Methane hydrate formation experiments in the presence of the various imidazolilium based ionic liquid solutions with 0.5wt% concentration including 1-buthyl-3-methylimidazolium methyl sulfate ([BMIM][MeSO 4]), 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM][HSO 4]), 1- ethyl-3-methylimidazolium ethyl sulfate ([EMIM][EtSO 4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4]) and 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate ([OH-EMIM][BF 4]) were conducted in a high pressure reactor at the same temperature. The induction time, gas consumption and temperature were measured.the results of experiments were indicated that [EMIM][EtSO 4] and [BMIM][MeSO 4] had the kinetic inhibiton effects meanwhile the other ionic liquids could be able to apply as the kinetic promoters. Keywords: ionic liquid, hydrate, methane, imidazolium, kinetic, promoter, induction time. Research Highlights Measuring induction time, gas consumption and temperature. [EMIM][EtSO 4]and [BMIM][MeSO 4] solutions with 0.5w% had the kinetic inhibiton effects. [EMIM][HSO 4], [BMIM][BF 4] and [OH-EMIM][BF 4] solutions with 0.5w% could be as the kinetic promoters for methane hydrate formation. 1

2 Kinetic investigation of methane hydrate in the 1. Introduction Gas hydrates are ice-like crystalline compounds in which gas molecules with suitable size are trapped inside cages, that forming from water molecules through hydrogen bonding [1]. Four essential factors are necessary for gas hydrate formation containing gas molecules (guests), water molecules (hosts), and proper conditions including low temperature/high pressures [2,3].The research on gas hydrates have been performed in two opposite directions [4]. The researchers have focused over the last decade, not only on the inhibition of gas hydrates formation, which is related to petroleum and gas industries, but also they have studied to find some means for promoting hydrate formation [5]. The thermodynamic and low dosage kinetic inhibitors are applied to prevent hydrate blockage in pipelines. Although, the thermodynamic inhibitors including alcohols, glycols and electrolytes effectively cause to inhibit the formation of gas hydrates, but large concentrations are required (10-50% of the water phase) so that, using of these inhibitors has become more costly [1,6,7,8]. Low dosage kinetic inhibitors (LDHIs) including anti-agglomerants (AAs) and kinetic hydrate inhibitors (KHIs) as a new group of inhibitors have been proposed to prevent hydrate formation. They not only do not change the thermodynamic conditions of gas hydrate formation but also they are able to prevent and delay hydrate nucleation and slow the rate of hydrate growth [1, 9]. On the other hand, gas hydrates have been considered recently as a new means for storage and transport of natural gas [4]. By using enough amounts of kinetic or thermodynamic promoters, the mass transfer between gas and water is extended, thus the hydrate formation rate can be efficiently increased [10]. Several researches have been reported the hydrate formation rate can be enhanced by using promoters such as sodium dodecyl sulfate [4, 10]. The hydrate formation rate of natural gas hydrates in the presence of the surfactants (anionic, cationic, and nonionic surfactant) with various concentrations were investigated. The hydrate formation rate enhanced with the use of anionic surfactant for all concentrations [5]. Ionic liquids are salts with low melting points which have specific characteristics including low volatility (low vapor pressure) and good thermal stability. Also, they can be designed for a special application by tuning of cation, anion and functional groups [11-13]. The limited researches have been accomplished with respect to the experimental data on ionic liquids as the inhibitors/promoters. Chen et. al investigated the effect of 1-butyl-3- imidazolium tetrafluoroborate ([BMIM] [BF4]) on the formation rates of carbon dioxide hydrate. They presented the rate of gas consumption was increased with the lowering of experimental temperature when the concentration of [BMIM][BF4] was constant [14]. The performance of some imidazolium-based ionic liquids with various cations and anions was investigated by Xiao and their co-workers [15, 16]. They found that ionic liquids, as a new group of gas hydrate inhibitors, were able to act as dual function (kinetic-thermodynamic) inhibitors for methane hydrate. Villano and Kelland showed that 1-butyl-3- methylimidazolium tetrafluoroborate ([BMIM][BF4]) and 1-ethtyl-3-methylimidazolium tetrafluoroborate (EMIM][BF4]) were the weak kinetic inhibitors when they applied to prevent the synthetic natural gas hydrate in the range of 0.5-1wt%. Also, they noted these ionic liquids were good synergists in the presence of the commercial kinetic inhibitors such as Luvicap 55W and RE5131 HIO [17]. The equilibrium conditions of methane hydrate formation in the presence of ionic liquids, based on dialkylimidazolium and tetraalkylammonium cations, was investigated by Li et. al [18]. Also, Mohammadi et al. studied methane and carbon dioxide hydrates in the presence of 2

3 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University tributhylmethylphosphonium methylsulfate experimentally. The result of the experimental data indicated that tributhylmethylphosphonium methylsulfate prevented to form CO2 and CH4 hydrates [19]. The effects of 1-ethyl-3-methyl imidazolium cation of ionic liquids investigated on CO2 hydrate by Makino and his colleagues. They measured induction times of hydrate formation for CO2 + ionic liquid + water systems and showed the induction times of these systems were shorter than the water+co2 system. Indeed, the studied ionic liquids with 0.10 mol% could be as the kinetic hydrate promoters [20]. In this work, the induction time and the rate of gas consumption of methane hydrate in the presence of imidazolium based aqueous ionic liquid solutions are measured by using high pressure reactor and the resuls of tests are compared. 2. Experimental section 2.1. Materials Table 1 shows the specification and the purities of the materials used in this study. Methane gas with the purity of 99.99% was purchased from Roham Gas Company. The ionic liquids are water soluble and purchased from Aldrich except [OH-EMIM][BF4] which was made in this laboratory (Research Institute of Petroleum Industry). The characteristic of this ionic liquid is presented in the reference [21]. Name 1-ethyl-3-methylimidazolium ethyl sulfate 1-ethyl-3-methylimidazolium hydrogen sulfate 1-buthyl 3-methylimidazolium methyl sulfate 1-butyl-3-methylimidazolium tetrafluoroborate 1-(2-Hydroxyethyl)-3-methylimidazolium Tetrafluoroborate Table1. The list of materials used in this work. Symbol [EMIM][EtSO 4] [EMIM][HSO 4] [BMIM][MeSO 4] [BMIM][BF 4] [OH-EMIM][BF 4] Mw (gr/mole) Purity >=95% >=94% >=95% >=97% > Apparatus A stainless steel stirrer reactor is designed with a volume of 750 cm 3 and used to produce gas hydrates. It is surrounded by a cooling jacket, which is connected to a thermostatic bath (HAKKLE F3) with the accuracy of K. A stirrer with a magnetic motor was installed in the reactor to agitate the sample. Pressure in the reactor was measured by an Indumart pressure transducer with an accuracy of less than 0.3%. Resistance thermometer (Pt100) with a precision of 0.1 K was used to measure the temperature inside the reactor in liquid phase. During the test, experimental data including reactor temperature and reactor pressure versus time were recorded on the computer by data acquisition system. 3

4 Kinetic investigation of methane hydrate in the 2.3. Experimental procedure Prior to the test, the reactor was rinsed with the ionized water and then dried. The ionic liquid aqueous solution (300 gr) with desired concentration injected to the reactor. Afterwards, methane gas was charged into the reactor (about 2MPa) and then the gas was purged, because the trace of the remained air in the reactor was eliminated. Again methane gas was injected into the reactor to achieve the desired pressure. At the beginning of the every test, after loading the reactor, the stirring rate was fixed at 200 rpm. At the beginning of the experiment with the initial pressure P0 (the initial pressure of the experiment at K), the temperature of the reactor was kept at K more than 3 hours to achieve thermodynamic equilibrium (where the reactor pressure was not change respect to time). It should be noted that, for the meaningful comparison, all test were started at the same temperature and pressure. Finally, the system was cooled down to the hydrate formation temperature ( K) at the rate of 0.7 K/min. The temperature and pressure of the reactor were recorded after starting the cooling process. The number of the moles of the gas which are consumed at any time (t) is the difference between the number of moles of the gas at t=0 (starting cooling) and the number of moles of the gas at t.for calculating the amount of the gas consumption, the equation (1) was used. n gas P V P V zr T zr T 0 t (1) P, V, R and T are reactor pressure, the volume of the gas phase, the gas universal constant and reactor temperature respectively. z is compressibility factor which computed by Peng Robinson equation of state [22]. It is supposed that the volume of the gas phase (450 cm 3 ) is not changed during the test and constant. The uncertainties in the mole numbers were calculated by using the uncertainties in the measurement of the pressure and temperature. Thus, the uncertainty of the mole numbers is believed to be about 1.6%. 3. Results and Discussion During each experiment, the amount of gas consumed versus time and induction time are determined by using recorded data containing pressure and temperature versus time. The induction time defines the time taken for crystal nuclei. That is, induction time also is the time in which the volume of hydrate or equivalently the consumption of the gas hydrate former is detectable [1]. In each test, after starting the cooling, the initial pressure drop is due to gas solubility and the reactor temperature reduction. The kinetic parameters including the rate of nucleation and induction time are usually related to nucleation [23]. Since the hydrate formation process is a stochastic event, the experiments were repeated at least three times. Moreover, for methane hydrate without any additives, the experiments repeated as least seven times. 4

5 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University 3.1. Methene hydrate in the presence of ionic liquid solutions The measured induction times for methane hydrate in the presence/absence of the ionic liquid solutions with 0.5wt% concentration are shown in Table 2. All experiments were performed at K and two initial pressures (P0 at K) containing 12.1 and 13.7 MPa. Table2. The measured induction times for methane hydrate in the presence of ionic liquid solutions with 0.5 wt% concentration. No System P (MPa) Tr(K) Range of induction Mean induction (at K) 0.2 K time(min) time(min) 1 water [BMIM][BF 4] [OH-EMIM][BF 4] [EMIM][HSO 4] [EMIM][EtSO 4] [BMIM][MeSO 4] water [BMIM][BF 4] [OH-EMIM][BF 4] [EMIM][HSO 4] [EMIM][EtSO 4] [BMIM][MeSO 4] As seen from Fig. 1, comparing the induction time of different ionic liquids at two initial pressures shows that, when the pressure decreases at given temperature, the induction time increases as expected. Fig.1. Comparing measured induction times of methane hydrate in the presence of ionic liquid solutions with 0.5wt% concentration in two initial pressures 13.7 and 12.1 (MPa). 5

6 Kinetic investigation of methane hydrate in the As seen in the Table 2 in the absence of an ionic liquid, the induction time is 112 minutes at P0=13.7MPa. It should be noted that addition of a kinetic inhibitor delayed the onset of hydrate nucleation. The systems including [EMIM][EtSO4] and [BMIM][MeSO4] Solutions with 0.5wt% concentration that extended the induction time. Consequently, they are able to act as the kinetic inhibitors. The induction times of the mentioned ionic liquids, are 230 and 311 minutes respectively therefore they are strong kinetic inhibitors. Moreover, Fig. 2 shows the gas consumptions of [BMIM] [MeSO4] and [EMIM] [EtSO4] are clearly smaller than pure water. It should be noted that a good hydrate inhibitor has the lower gas consumption compared to pure water. As seen in Fig. 2, [BMIM][MeSO4] prolonged the onset of the nucleation much more than [EMIM][EtSO4]. However, the gas consumption rate is smaller. As a result, these ionic liquids are as kinetic inhibitors so that [BMIM][MeSO4] is more effective inhibitor. Fig.2.Hydrate formation rate of methane in the presence of various ionic liquid solutions with 0.5 wt% as kinetic inhibitors, Conditions: Pinitial at K=13.7 MPa, Treactor = K. Although, the knowledge of ionic liquid behavior in an aqueous solution is limited. But researchers have studied the interaction between ionic liquid and water by using molecular modeling. Ficke and Brennecke demonstrated between two anions including [MeSO4] - and [EtSO4] -, the oxygen atom attached to the methyl chain for [MeSO4] - is more negative than the ethyl group for [EtSO4] -. Moreover, the researches in literature showed the hydrophobicity increases through increasing the alkyl chain length on an ionic liquid [24]. Also, the butyl group in [BMIM] [MeSO4] has a stronger interaction than ethyl group with the hydrate surface [25]. Therefore, [BMIM] [MeSO4] has the stronger ionic liquid/water interactions than [EMIM] [EtSO4]. The kinetic inhibition effect of [BMIM] [MeSO4] is more than [EMIM] [EtSO4] due to the aforesaid reasons. 6

7 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University As seen from Table 2, under the same hydrate formation temperature and initial pressure conditions, the induction time is less than pure water when ionic liquid is present as a promoter. The systems containing [BMIM] [BF4], [OH-EMIM] [BF4] and [EMIM] [HSO4] solutions with 0.5wt% promote induction times in comparison with pure water. The induction time of [OH-EMIM] [BF4] is slightly less than [BMIM] [BF4] at the same hydrate formation temperature and P0. Moreover, between these promoters, the induction time of [EMIM] [HSO4] is the least and equals 11 minutes. Fig. 3 shows the gas consumption of [BMIM][BF4] and [OH-EMIM][BF4] for methane hydrate formation are more than pure water. In addition, the hydrate formation rate of [OH- EMIM][BF4] is slightly more compared to [BMIM][BF4]. Fig. 3.Hydrate formation rate of methane in the presence of various ionic liquid solutions with 0.5 wt% as kinetic promoters, Conditions: Pinitial at K=13.7 MPa, Treactor = K. In literature just a few papers have been presented about ionic liquids as promoters. Chen et.al showed that the hydrate formation rate of carbon dioxide in various concentrations of [BMIM][BF4] were more than pure water and the storage capacity of hydrate increased when ionic liquid concentration increased [14]. Also, Makino and his colleagues measured induction times of hydrate formation for CO2 in the presence of ionic liquid solutions. Indeed, the studied ionic liquids including [EMIM] [Br], [EMIM] [TfO], [EMIM][BF4] and [EMIM][NO3] with 0.10mol% could be as hydrate kinetic promoters[20]. Researchers analyzed the properties imidazolium based ionic liquids and found ionic liquids could have surface active properties like that surfactants. Surfactants (surface active agents) molecules in water tend to aggregate and form micelles. The formation of micellar aggregations help in solubilizing the hydrocarbon gases in water and enhances the process of hydrate formation [26]. Ionic liquids maybe are able to reduce the surface tension. Therefore, the hydrate former gas can be solved in water conveniently due to increasing the contact of the two phases (water/gas) and as a result the nucleation process is increased [14]. Fig. 3 7

8 Kinetic investigation of methane hydrate in the indicates the hydrate formation of [OH-EMIM][BF4] is slightly more than [BMIM][BF4] and has a strong promotion effect compared to [BMIM][BF4]. As mentioned before, [EMIM] [HSO4] promotes the hydrate formation process and possesses the least induction time. Moreover, it is appears a strong promoter also could help to form hydrates at higher temperatures (lower driving force). To probe this further some experiments were conducted for methane hydrate formation in the presence/absence of [EMIM][HSO4] aqueous solutions with 0.5wt% at K and at two initial pressures. Comparing the induction times in Table 3 indicates that for pure water, no hydrates formed during 48 hours meanwhile induction times of the system including [EMIM][HSO4] are 244 and 582 minutes at initial pressures 13.7 and 12.1 MPa, respectively. The results show that [EMIM][HSO4] can be considered as the effective promoter. Table3. The measured induction times for methane hydrate in the presence of [EMIM][HSO4] solution with 0.5 wt% concentration, temperature of reactor is K. No System P (MPa) Tr (K) Range of induction Mean induction (at K) 0.2 K time(min) time(min) 1 water No hydrate No hydrate 2 [EMIM][HSO 4] water No hydrate No hydrate 4 [EMIM][HSO 4] Subcooling is computed as the difference between the system temperature and the hydrate equilibrium temperature at the system pressure [1]. Comparing the gas consumption rates of methane hydrate in the presence of [EMIM][HSO4] aqueous solutions with 0.5wt% concentration at different reactor temperatures and initial pressures are presented in Fig. 4. At initial pressure 12.1MPa, the gas consumption rate at K (subcooling= C ) is much more than that of K (subcooling = C ). This behavior also can be observed at P0=13.7MPa. As a result, decreasing the subcooling causes to enhance the gas consumption rate. This finding for the methane hydrate in the presence of tetra-n-butyl ammonium bromide (TBAB) was presented by Gholinezhad et.al [27]. They indicated that at higher subcooling less gas consumed by TBAB [46]. In addition, the induction times of [EMIM][HSO4], as shown in Tables 2 and 3, at (subcooling = C ) and K (subcooling= C ) and at P0=13.7MPa are 11 and 244 minutes respectively. Since subcooling is usually considered as the driving force for the kinetic of hydrate nucleation, therefore the induction time is decreased when the subcooling is increased. 8

9 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University Fig.5.Methane hydrate formation rate in the presence of [EMIM][HSO4]solutions with 0.5 wt% as kinetic promoter in different initial pressures and temperatures. 4. Conclusions Methane hydrate formation experiments in the presence of the various imidazolilium based ionic liquid solutions were investigated. The results showed among the alkylsulfate based ionic liquids, two ionic liquids including [EMIM][EtSO4] and [BMIM][MeSO4] were able to act as the kinetic inhibitors so that [BMIM][MeSO4] was the best. Moreover, comparing the induction times and gas consumption rates of other ionic liquids indicated these had promoter effects so that [EMIM][HSO4] solution with 0.5wt% concentration was better than the others. Moreover, having compare the results of gas consumption rates for methane+[emim][hso4]+water system at two reactor temperature showed at higher subcooling less gas consumed. References [1] Sloan,E. D., Koh C. A., Clathrate Hydrates of Natural Gases, 3rded.; CRC Press, Taylor & Francis Group Boca Raton, FL, [2] Sloan,E. D., Introductory overview: Hydrate knowledge development,. American Mineralogist, vol. 89, , [3] Englezos, P., Clathrate Hydrate, Ind. Eng. Chem. Res. Vol. 32, , [4] Sun, Z., Ma, R., Fan, S., Guo, K., Wang, R., Investigation on Gas Storage in Methane Hydrate, Journal of Natural Gas Chemistry, vol.13, , [5] Karaaslan, U., Parlaktuna, M., Surfactants as Hydrate Promoters, Energy & Fuels,vol.. 14, , [6] Lee, J. D., Englezos, P., Enhancement of the performance of gas hydrate kinetic inhibitors with polyethylene oxide, Chemical Engineering Science, vol. 60, , [7] Lee, J. D., Englezos, P., Unusual kinetic inhibitor effects on gas hydrate formation, Chemical Engineering Science. Vol.61, , [8] CuiPing, T., XingXue, D., JianWei, D, DongLiang, L., XiaoYa, Z., XiangYang, Y., DeQing, L., Kinetic studies of gas hydrate formation with low-dosage hydrate inhibitors, vol.53, no 12, ,

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