Article pubs.acs.org/jced
Experimental Study of Methane Hydrate Equilibria in [EMIM]-NO3 Aqueous Solutions Zhen Long,† Xuebing Zhou,†,‡ Deqing Liang,*,† and Dongliang Li† †
Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: The thermodynamic effect of imidazolium-based aqueous ionic liquid solutions containing 1-ethyl-3-methyl-imidazolium nitrate ([EMIM]-NO3) on methane hydrate phase equilibrium formation conditions at five mass fractions of (0.055, 0.1, 0.2, 0.3, and 0.4) has been determined. The data are obtained in the pressure range of (3.08 to 16.12) MPa and temperature range of (274.0 and 289.7) K by using an isochoric pressure search method. The studied ionic liquid is found to behave well as a thermodynamic inhibitor on the methane hydrate. Results show that the hydrate phase boundary of methane hydrate is shifted to lower temperature at constant pressure about 1.0 K to 7.3 K in the presence of [EMIM]-NO3 aqueous solutions. Moreover, an enhanced increase in inhibition effect is demonstrated with increasing concentrations.
seas.10,11 Hence, it is necessary to make efforts to discover new inhibitors that are more effective than existing inhibitors. Ionic liquids (ILs) are a type of salts, consisting of organic cations and organic/inorganic anions, which usually exist in the liquid state at room temperature and can be prepared from commonly available materials.12,13 Most ILs exhibit some common features including low volatility (i.e., negligible vapor pressure), high thermal stability, relatively low viscosity, nonflammability, high ionic conductivity, and so on. Because of the tunability of cation and anion, ILs can be designed for generating the desired effect and therefore have gained huge interests as green solvents used in various applications.14−17 Recent studies have discovered that ILs have potential as both thermodynamic and kinetic hydrate inhibitors for methane hydrate.17,18 Xiao et al.18 reported the dual function inhibition effectiveness of five imidazolium-based ILs. Later, the effect of six dialkylimidazolium halide ILs at 0.1 mass fraction on the equilibrium methane hydrate dissociation conditions was further experimentally studied by them.19 They identified that the inhibition effect of ILs was dependent on the type of anions and the length of alkyl chain substituents of the cations. Among all the ILs studied as thermodynamic inhibitors, [EMIM]-Cl was the most effective. Li et al.20 found that the hydroxyl group of cations in ILs could enhance the thermodynamic inhibition effect. The same phenomenon was seen by Partoon et al.21 when they compared the performance of [EMIM]-Cl and [OH-EMIM]-Cl and salts at mass fractions of 0.001, 0.005, and 0.01, and the effectiveness of ILs was concentration depend-
1. INTRODUCTION Gas hydrates are ice-like crystalline compounds composed of water molecules, connected through hydrogen bonding, forming a three-dimensional lattice that is occupied by another small guest species. Greatly varying in the cavity size and shape, gas hydrates are divided into three main structures: structure I (sI), structure II (sII) and structure H (sH).1 Methane is wellknown to form sI hydrate. Since hydrate formation was revealed as the main cause of gas pipeline blockages, how to prevent hydrate formation has been a troublesome problem for the oil and gas industry.2,3 Various methods, such as heating, depressurization, water removal, and inhibition, have been developed to destroy the stable conditions that benefit the formation of gas hydrate.3,4 Under certain circumstances, the most common and feasible method is to inject thermodynamic or kinetic inhibitors. The thermodynamic inhibitors, such as methanol, monoethylene glycol (MEG), and sodium chloride (NaCl), avoid gas hydrate formation through shifting the hydrate−aqueous liquid−vapor equilibrium (HLVE) curve toward higher pressures or lower temperatures.5−8 But they are usually used at high concentrations (> 0.4 mass fraction), which thus leads to a huge operation cost (about $220 million annually) and environmental issue.9 Instead of preventing the formation of gas hydrate, kinetic inhibitors hinder the process by delaying hydrate nucleation time and/or slowing down growth rate of crystals. Despite economic and environmental advantages due to a low dosage in the aqueous phase (< 0.01 mass fraction), kinetic inhibitors that are generally water-soluble polymeric compounds (i.e., polyvinylpyrrolidone, abbreviated as PVP and polyvinylcaprolactam, abbreviated as PVCap) are found to be poorly biodegradable and ineffective at higher subcoolings (> 10 °C), meaning they are not qualified for use in deeper © XXXX American Chemical Society
Special Issue: Proceedings of the 19th Symposium on Thermophysical Properties Received: May 21, 2015 Accepted: July 15, 2015
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DOI: 10.1021/acs.jced.5b00435 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 1. Ionic liquid studied in this work
ent.21,22 In addition, a more significant thermodynamic inhibition performance of ILs at higher pressure was observed by Richard et al.,23 and a synergetic effect for the mixtures of [EMIM]-Cl with conventional inhibitors or other IL was likely to occur at higher pressure. To understand the thermodynamic effect of nine ILs on hydrate equilibrium conditions, Sabil et al.24 associated the inhibition efficiency of ILs with electrical conductivity and pH of these aqueous IL solutions. ILs are yet to be comparable to such traditional thermodynamic inhibitors as methanol and monoethylene glycol (MEG) and therefore have not been applied in the oil and gas industry.18,19,22 However, their unique tunable structure provides a great possibility to produce excellent dual inhibition functionality and to compete with other types of hydrate inhibitors. Therefore, more investigations are required to seek the optimum combination of the anion with cation of the ILs. In this work, the equilibrium hydrate formation conditions for methane in the presence of 1-ethyl-3-methyl-imidazolium nitrate ([EMIM]-NO3) aqueous solution are measured at 0.055, 0.1, 0.2, 0.3, and 0.4 mass fractions. The isobaric pressure search method is employed to obtain the experimental data at different pressures ranging from (3.08 to 16.12) MPa.
Figure 1. Schematic of experimental apparatus. V1−V7, valves; T, resistance thermometer; P, pressure transducer; BC, buffer cell; GC, gas cylinder; M, magnetic stirrer.
a platinum resistance (Pt100) thermometer with an uncertainty of ± 0.1 K. The system pressure and temperature are real-time recorded by an Agilent data acquisition system every 10 s and displayed in a computer. 2.3. Procedure. The hydrate dissociation conditions are measured by an isochoric pressure-search method.11,17,25−28 Before each experiment run, the reactor is rinsed by deionized water and fully dried. About 70 mL ionic liquid aqueous solution with the desired concentration is introduced into the crystallizer. Subsequently, the crystallizer is evacuated by a vacuum pump and flushed by the methane gas three times to completely exhaust the air. The methane gas is then charged into the reactor until the reactor is pressurized to the desired pressure, far away from the hydrate stability zone. After the initial temperature and pressure become stable, the stirrer is started, and the temperature is initially rapidly lowered at a high cooling rate of 15 K so as to form hydrate. During the cooling step, the pressure gradually decreases with temperature in an almost linear trend as shown in Figure 2 until the onset of hydrate formation. The hydrate formation is detected by a sharp pressure drop due to the gas consumption. The temperature is then heated, initially in large temperature steps (i.e., 1 K to 5 K). Once the pressure begins to increase sharply, meaning that the hydrate begins to partially dissociate, the temperature is slowly increased with steps of 0.1 K. When the point is reached at which the hydrate completely dissociates, a small increase in the pressure with the increased temperature is observed again. At each temperature step, sufficient time (i.e., 6 h to 10 h) is given to achieve a steady equilibrium. In this way, the pressure−temperature diagram is obtained. The point at which the slope of P−T curve sharply changes is considered as the hydrate dissociation point (see Figure 2).
2. EXPERIMENTAL SECTION 2.1. Materials. The ionic liquid [EMIM]-NO3 studied in this work is purchased from Lanzhou Institute of Chemical Physics with a purity of 0.99 mass fraction. The chemical structure is shown in Table 1. The concentrations of the ionic liquid used are 0.055, 0.1, 0.2, 0.3, and 0.4 mass fractions. Methane gas with a purity of 0.9999 mole fraction is supplied by South China Special Gases Ltd., CO. Deionized water with a resistivity of 18.25 mΩ·cm−1 is made by an ultrapure water system and used for preparing all of the sample solutions. Samples are weighed on an electronic analytical balance with an uncertainty of ± 0.1 mg. 2.2. Apparatus. The experiments are carried out on a high pressure hydrate reaction apparatus whose schematic diagram is shown in Figure 1. It consists of a cylindrical hydrate crystallizer made of 316 stainless steel. The maximum effective volume of the crystallizer is 175 mL. Maximum working pressure is 20 MPa and the temperature ranges from 233.15 K to 383.15 K. A magnetic stirrer is equipped in the crystallizer to ensure the full agitation and to facilitate reaching equilibrium. The stirring speed is set to 600 rpm. The crystallizer is immersed in a programmable temperature-controlled air bath with an uncertainty of ± 0.1 K and insulated to minimize heat transfer from the surroundings. The pressure in the crystallizer can be controlled by injection or withdrawal of water using a highpressure pump and is measured by a CYB-20S relative pressure transducer ranging from (0 to 20) MPa with an uncertainty of ± 0.02 MPa. The temperature in the crystallizer is measured by B
DOI: 10.1021/acs.jced.5b00435 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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using the isobaric equilibrium step-heating pressure-search method. Furthermore, Figure 4 illustrates the deviation
Figure 2. A typical experimental P−T diagram for dissociation point determination for methane hydrate in the presence of [EMIM]-NO3 aqueous solution.
Figure 4. Deviations of experimental equilibrium pressures from those calculated values for methane hydrate in pure water: ■, this work; ○, Adisasmito et al.;29 △, Mohammadi et al.;30 dashed line, uncertainty of the present measurement.
3. RESULTS AND DISCUSSION 3.1. Validation of Apparatus and Procedure. For the purpose of checking the reliability of the experimental apparatus and procedure, the methane hydrate dissociation data in pure water are measured and presented in Table 2 at a
100(Pexp − Pcalc)/Pcalc of the experimental equilibrium pressures Pexp from the calculated values Pcalc, along with the measurement uncertainty curves. Since the experimental pressures are available at different temperatures, to perform the fast but accurate calculation, an empirical three-order polynomial correlation, by fitting the experimental data obtained from the present study, is used to predict the pressure. The uncertainty is 100 multiplied by the ratio of the uncertainty of the pressure (± 0.02 MPa) to Pcalc. In Figure 4, it is obvious that the deviation of the data sourced from the present study is within the uncertainty of the measurement. The absolute deviation is within 8 % between the experimental data obtained in the present study and literature with those predictions. Figures 3 and 4 indicate our apparatus and method are able to well reproduce the existing experimental data. 3.2. Hydrate Dissociation Conditions in the Presence of Ionic Liquid. The dissociation conditions of methane hydrate in the presence of [EMIM]-NO3 at concentrations of 0.055, 0.1, 0.2, 0.3, and 0.4 mass fractions are measured at pressure P ranging from 3 to 16 MPa. The results are reported in Table 3 and demonstrated in Figure 5. Compared with those data in pure water, the addition of [EMIM]-NO3 causes to shift the equilibrium dissociation temperature to the lower value at the same pressure P, which means this IL presents an inhibition effect on the methane hydrate formation as a thermodynamic inhibitor. As the concentration increases from 0.055 to 0.40 mass fractions, the inhibition effect of [EMIM]-NO 3 progressively increases, which can be seen from the increasing temperature shift. The same behavior has been found in the case of [EMIM]-Cl.21,23 A reasonable explanation is given that it is possibly related with a higher activity coefficient of the ILs at lower concentration.21 To well evaluate the thermodynamic inhibition effect of this IL, a standard statistical method19,23 is used to obtain the average hydrate temperature shifts at different concentrations, as shown in Figure 6 along with third-order trend curves. Because it is generally accepted that ILs with a longer alkyl chain substituent present worse thermodynamic inhibition effects than those with a shorter chain substituent, and
Table 2. Experimental Dissociation Data for Methane Hydrate in Pure Watera T/K
P/MPa
278.8 282.9 285.1 287.3 289.4 289.7 a
4.53 6.80 8.65 11.25 14.43 14.99
Uncertainties u are u(T) = ± 0.1 K, u(P) = ± 0.02 MPa.
pressure range of (4.53 to 14.99) MPa and between (278.8 and 289.7) K. The results are also plotted in Figure 3 together with those reported by Adisasmito et al.29 and Mohammadi et al.30
Figure 3. Measured dissociation conditions for methane hydrate in pure water: ■, this work; ○, Adisasmito et al.;29 △, Mohammadi et al.30 C
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Table 3. Experimental Dissociation Data for Methane Hydrate in the Presence of [EMIM]-NO3 at Various Concentrationsa w/mass fraction
T/K
P/MPa
0.055
274.0 278.3 281.1 283.8 287.1 289.7 274.8 278.3 280.8 283.2 286.8 273.7 277.0 279.4 281.5 285.3 271.8 275.1 277.4 279.6 283.4 269.2 272.4 275.0 277.3 281.0
3.08 4.67 6.25 8.21 12.12 16.12 3.50 4.91 6.43 8.40 12.87 3.53 4.93 6.50 8.18 12.86 3.53 4.91 6.42 8.20 12.88 3.41 4.79 6.35 8.11 12.76
0.1
0.2
0.3
0.4
a
Figure 6. Average temperature shifts at different mass fractions. ■, [EMIM]-NO3, this work; ●, [EMIM]-Cl, Xiao et al.19 and Richard et al.;23 ▲, MEG, Haghighi et al.;31 ▼, NaCl, de Roo et al.;32 □, [EMIM]-Br, Xiao et al.;19 ○, [BMIM]-Cl, Xiao et al.;19 △, [BMIM]Br, Xiao et al.;19 ▽, [BMIM]-I, Xiao et al.19
[EMIM]-Cl and obviously superior to other ILs, as seen in Figure 6. With the increased concentration, the difference of the inhibition effect between them can become much greater. It may be due to the bigger size of anion [NO3]− than Cl−, resulting in a weaker interaction between IL with water. Because of the lack of data for other ILs at more concentrations, it cannot be known whether the role of cation type is more important than that of anion type in affecting the thermodynamic inhibition effectiveness of ILs. On the other hand, the average dissociation temperature shifts by these ILs are far lower than those by MEG and NaCl at the same concentration. The performance of [EMIM]-Cl as an inhibitor is supposed to surpass that of MEG at concentrations of > 0.6 mass fraction of inhibitor in aqueous solution according to the trend curve, while the difference of the inhibition effect between [EMIM]-NO3 with the traditional thermodynamic inhibitors becomes larger as the concentration increases. These data provide evidence that ILs interact with hydrate crystals. However, much is still unknown on how these ILs operate. This is one of reasons that limit the enhancement of ILs and their wide application. More investigations on the mechanism are required in our further study.
Uncertainties u are u(T) = ± 0.1 K, u(P) = ± 0.02 MPa.
4. CONCLUSIONS
Figure 5. Dissociation conditions for methane hydrate in the presence of [EMIM]-NO3 at various mass fractions: ○, 0.055; ▲, 0.1; ▼, 0.2; ⧫, 0.3; □, 0.4; ■, pure water.
In this work, the equilibrium conditions of methane hydrate are measured at 0.055, 0.1, 0.2, 0.3, and 0.4 mass fractions of [EMIM]-NO3 in aqueous solution in the temperature range of (274.0 to 289.7) K and pressure range of (3.08 to 16.12) MPa. It is found that the [EMIM]-NO3 can function as a thermodynamic inhibitor to shift the dissociation conditions of methane hydrate toward higher temperature and lower pressure regions. The thermodynamic inhibition effects of [EMIM]-NO3 become more significant at higher concentrations. Among the ionic liquids with the mass fraction of 0.1, the inhibition performance of [EMIM]-NO3 is approximately comparable with that of [EMIM]-Cl.
dialkylimidazolium halide ILs perform a stronger inhibition effectiveness,19−23 herein data for EMIM-halides and BMIMhalides at 0.1 mass fraction available from Xiao et al.19 and EMIM-Cl at concentrations of 0.05, 0.2, 0.3, and 0.4 mass fractions published by Richard et al.23 are selected for comparison. The results for systems containing MEG and NaCl, which are taken from Haghighi et al.31 and de Roo et al.,32 respectively, are also included in Figure 6. Among the present ILs with the mass fraction of 0.1, the inhibition effectiveness of [EMIM]-NO3 is very slightly close to D
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 20 8705 7669. Fax: +86 20 8705 7669. E-mail:
[email protected]. Funding
This work is supported by the National Natural Science Foundation of China (No. 51376182), and the Chinese Academy of Sciences Key Development Program (No. KGZD-EW-301). Notes
The authors declare no competing financial interest.
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