Effect of EMIM-BF4 Ionic Liquid on Dissociation Temperature of

Nov 28, 2018 - Most of the studies done on kinetic hydrate inhibitors (KHIs) are related to the effect of KHIs on hydrate formation, and there are a l...
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Effect of EMIM-BF4 Ionic Liquid on Dissociation Temperature of Methane Hydrate in the Presence of PVCap: Experimental and Modeling Studies Meysam Mardani,† Arezoo Azimi,† Jafar Javanmardi,*,† and Amir H. Mohammadi*,‡ †

Department of Chemical, Petroleum and Gas Engineering, Shiraz University of Technology, Shiraz, Iran Discipline of Chemical Engineering, School of Engineering, University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban 4041, South Africa

Energy Fuels Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/24/18. For personal use only.

‡

ABSTRACT: Most of the studies done on kinetic hydrate inhibitors (KHIs) are related to the effect of KHIs on hydrate formation, and there are a limited number of research works on hydrate dissociation conditions in the presence of KHIs. In the subject of hydrate remediation, knowing the effects of KHIs on hydrate dissociation conditions is necessary. In this work, first, the effect of the presence of poly vinyl caprolactam (PVCap) as a KHI on methane hydrate dissociation conditions has been studied and the results show that dissociation temperature of methane hydrate in the presence of PVCap is higher than the noninhibited system and increases hydrate stability conditions. Then, at the second stage, the effects of EMIM-BF4 as a synergist for PVCap on methane hydrate dissociation conditions have been investigated. Different concentrations of EMIM-BF4 and PVCap have been studied, and a thermodynamic model has been developed to predict hydrate dissociation temperature. The experiments reveal that adding EMIM-BF4 to an aqueous solution of PVCap does not have a significant effect on methane hydrate dissociation conditions. The model has been developed based on the fact that the presence of PVCap changes the large to small cavity ratios, L/S, from its theoretical value, i.e. 3/1. It has been assumed that the number of small cavities to water molecules for methane hydrates in the presence of PVCap depends on PVCap concentration. A correlation is proposed to indicate this dependence. Different activity coefficient models have been tested to calculate water activity in the presence of PVCap and IL. The agreement between the experimental data and model results is found to be satisfactory. ers14−17 have studied gas hydrates formation and dissociation in the presence of KHIs morphologically and kinetically. The used KHIs in their studies are PVP, PVCap, H1W85281, GHI 101, Luvicap-EG, poly ethylene oxide (PEO), and some other biological KHIs like type I and III antifreeze protein (AFP). They all concluded that when KHIs are present in the system, nucleation and/or growth will be delayed and hydrates formed will take longer than the hydrates formed in the presence of pure water to dissociate. Lee and Englezos17 used the term “hard” for formed hydrates in the presence of KHIs versus the term “soft” for formed hydrates in noninhibited systems. Similarly, Sharifi et al.15 called this phenomenon recalcitrance of gas hydrate crystals formed in the presence of KHIs. Studies2,18−20 show that KHIs do not shift the hydrate−aqueous liquid−vapor/gas (HLV) equilibrium curve and they will not be able to prevent the hydrate formation completely. They hinder hydrate nucleation and growth by increasing induction time. Tohidi et al.21 proposed a complete inhibition region (CIR) where gas hydrate formation can be indefinitely prevented or in other words for a very long time when subcooling is within the CIR. Recently, Gulbrandsen and Svartås22,23 conducted experiments on methane hydrate systems in the presence of different concentrations of PVCap to investigate thermodynamic inhibition effects of KHIs. They observed that PVCap concentration affects final dissociation temperature. They

1. INTRODUCTION Gas hydrates, or clathrate hydrates, are ice-like solid crystalline compounds which are formed from water trapping of gases and some low molecular weight volatile liquids under specific temperature and pressure conditions.1,2 There are many advantages and disadvantages for gas hydrate formation. In the oil and gas industry, they may cause flow assurance problems and blockage of pipelines, valves, wellheads, and equipment which lead to loss of production and pressure drop.1,3−5 There are several physical and chemical methods to control pipeline blockage. Nowadays, the most promising strategy used in gas industry is injecting hydrate formation inhibitors to the system. These inhibitors are categorized in two main groups called thermodynamic and low dosage hydrate inhibitors (LDHIs). Thermodynamic hydrate inhibitors (THIs) help preventing hydrate formation by shifting the hydrate equilibrium curve to the severe conditions of low temperatures and high pressures.6−9 These conventional inhibitors need to be injected to the systems at high concentrations. Since they are not much environmentfriendly,7 some other inhibitors called low dosage hydrate inhibitors (LDHIs) have been developed which are effective even at tens of percents lower than the required concentrations for THIs.6,7 LDHIs consist of kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs). KHIs are normally polymeric chemicals like poly vinyl caprolactam (PVCap) and polyvinylpyrrolidone (PVP).10,11 Inhibitors’ performance is connected to the degree of subcooling.12 The KHIs are known to have a desirable subcooling temperature of about 10 °C.7,13 Some research© XXXX American Chemical Society

Received: June 25, 2018 Revised: November 24, 2018 Published: November 28, 2018 A

DOI: 10.1021/acs.energyfuels.8b02176 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

growth rate. This behavior indicates that combinations of some ILs and PVCap lead to synergistic inhibition effects. Another nonpolymeric synergist is butyl ether glycol (BGE) which is not only a new applicable synergist but also a good solvent due to its surfactant-like properties.34 Yang and Tohidi35 suggested that addition of BGE increases PVCap inhibition performance by adsorbing on hydrate growth sites. Based on these new theories, Gulbarndsen and Svartaas34 tested two different mixtures of PVCap and BGE. Dissociation temperatures of methane hydrate formed in a 1:1 solution of PVCap-2k and BGE were lower than the dissociation temperature of methane hydrates formed in the presence of the pure PVCap. On the other hand, when mixing PVCap-6k and BGE, a slight increase in dissociation temperature was observed compared to the system containing PVCap alone. In this work, the effects of PVCap on methane hydrate dissociation temperatures as well as in combination with EMIMBF4 have been investigated. Different concentrations of additives have been studied. Finally, a thermodynamic model based on the van der Waals−Platteeuw (vdW-P) solid solution theory is introduced to calculate hydrate dissociation temperatures for these systems. For this purpose, the Flory−Huggins theory is used to calculate water activity in the presence of PVCap. Due to low concentration of PVCap, however, the water activity coefficient is about unity. Moreover, the effect of IL on water activity is estimated using NRTL and UNIQUAC activity coefficient models. Also, the final equation obtained from vdWP solid solution theory is modified in the presence of PVCap. The details are given in following section.

compared the measured hydrate dissociation temperatures with the equilibrium temperatures for corresponding noninhibited system and observed that the hydrate dissociation temperatures are highly increased even at low heating rates of about 0.0125 °C/h. This observation indicates that PVCap likely has thermodynamic effects in addition to kinetic impacts on such systems. Based on these studies, it can be argued that there could be both kinetic and thermodynamic inhibition effects on methane hydrate formation because of the presence of PVCap in water. There are many different hypotheses in the mechanism of KHI inhibition. One of the ways that KHIs hinder hydrate nucleation and growth is to adsorb onto the surface of hydrate crystals or prevent cavities from being occupied with gas molecules.3 Adsorption occurs when hydrogen bonds are formed between water molecules and KHI pendant groups. In 2000, Freer and Sloan13 correlated the inhibitor performance to van der Waals (vdW) forces. They proposed that by increasing the lactam ring in PVCap, the vdW forces will be increased and the best interactions between the large cavities and lactam rings are when the size of lactam rings is bigger. During Raman spectroscopy tests, Subramanian and Sloan24 and Hong et al.25 found out that the large to small cavity ratio in structure one (SI) methane hydrates does not follow the theoretical trend, i.e., 3/1 in the presence of PVCap unless that amount of PVCap is adsorbed to the hydrate surfaces and its concentration in solution is decreased. Hong et al.25 reported that in the initial stage of hydrate formation in the presence of PVCap, the rate of large cavity encapsulation is affected so the formed amount of large cages is not sufficient. Subramanian and Sloan24 referred to this effect as a rate-limiting step for hydrate formation. Ionic liquids (ILs) are poorly coordinated organic salts which are known as environmental friendly chemicals thanks to their low vapor pressure. They have low melting points of about 100 °C, which means that they are liquid below that temperature.26 The most common ILs consist of a big heterocyclic nitrogen containing cation and both organic anions (e.g., nitrate or methanesulfonate) and inorganic anions (such as halide, dicyanamide, BF 4 − , PF 6 − ). 27,28 Lately, some research works19,29−32 have shown that some kinds of ILs have dual functionality in gas hydrate inhibition. ILs can act as THIs and KHIs simultaneously owing to their electrostatic charges and the possibility of forming hydrogen bonds through their cations and/or anions with water.30,32 On the other hand, Perrin et al.33 first suggested that KHIs need some synergists to enhance their inhibition performance, e.g., 2-butoxyethanol. Some researchers while conducting different experiments found out that ILs can be strong synergists for KHIs. Kang et al.31 investigated synergistic behavior of PVCap and IL combinations. They experimented with three types of inhibitors: pure PVCap, pure IL, and different mixtures of PVCap and ILs. They used these three ILs ([EMIM][BF4], [BMP][BF4], and [HEMP][BF4]) which were appropriate inhibitors for CH4 hydrate. When mixing PVCap with these ILs, the kinetic inhibition performance and induction time were greatly improved compared to solutions of pure ILs or pure PVCap due to the synergistic effects. Lee et al.11 studied induction times of CH4 hydrate formation in the presence of the ILs alone and mixtures of PVCap + ILs. They found out that Br− and BF−4 based ILs show the best thermodynamic and kinetic inhibition effects, respectively. Further investigation of mixtures of some ILs and PVCap resulted in increased induction time or decreased CH4 hydrate

2. EXPERIMENTAL SECTION 2.1. Materials. The specifications of the materials utilized in this study are presented in Table 1. Methane gas with purity of 99.95% was

Table 1. Materials Used in This Work

used. The PVCap used in the experiments was obtained from LuvicapEG (supplied by BASF) which contains 40 wt % PVCap and 60 wt % ethylene glycol as a solvent. The ethylene glycol was evaporated at a temperature about 100 °C during 2 weeks by an electric oven and the pure PVCap was obtained. The molecular weight of PVCap is 7000 g/ mol (PVCap-7k). The ionic liquid used in this work is EMIM-BF4 with purity of more than 98% purchased from the io-li-tec. All the materials were weighed using a digital A&D balance, type HR-200. 2.2. Apparatus and Experimental Procedure. The experimental setup is consisted of an equilibrium cell equipped with a stirrer with a total volume of 75 cm3. The used equilibrium cell is made up of stainless steel 316 which can endure pressures up to 15 MPa. Figure 1 shows a schematic view of the equilibrium cell. The equilibrium cell is inside a cooling/heating bath of alcohol (here is ethanol) in order to control the cell temperature. The temperature of the alcohol bath is adjusted using B

DOI: 10.1021/acs.energyfuels.8b02176 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

of experiments. The isochoric pressure search method was applied to obtain the hydrate dissociation temperature.36−40 Note that, two different heating rates were applied before obtaining the final hydrate dissociation point. At the first step, the system temperature increased relatively quickly with a heating rate of 1 °C/h. At the second step, about 5 °C under the expected equilibrium decomposition temperature, the heating rate decreased to 0.1 °C/h in order to obtain better thermal equilibrium. After conducting several experiments with different amounts of PVCap and/or EMIM-BF4, it was observed that the dissociation temperatures of methane hydrate in the presence of PVCap or simultaneous presence of both additives are higher than the noninhibited system at the corresponding pressure. The temperature shift due to PVCap presence is called ΔTdiss. The parameter is described as the difference between the equilibrium temperature of the noninhibited methane hydrate system in the presence of pure water Tcal.pure and experimental dissociation temperature of methane hydrate in the presence of inhibitors as Texp.in.: Figure 1. Schematic view of the equilibrium cell.

ΔTdiss = Texp.in − Tcal.pure

(1)

where Tcal.pure is described and calculated in section 3 using eq 9.

an intelligent temperature control system with timing ability (Julabo TP-50). The temperature control is achieved via the circulation of alcohol through a circulator. The temperature is recorded using a thermometer (Pt-100) with a deviation of ±0.1 K. To measure the pressure of the system, a pressure transformer (P-2) with precision of ±0.01 MPa is used. The measured temperature and pressure are transferred to the computer with an analog to digital converter (ADC) for recording and storing data. A general plan of the experimental devices is shown in Figure 2. A 25 cm3 portion of aqueous solution which contained a specific weight percent of EMIM-BF4, PVCap, or a mixture of both was injected into the equilibrium cell. After preparing each desired solution, the equilibrium cell was washed and dried with deionized water. All air inside the cell was evacuated using a vacuum pump. Then the pressure of the equilibrium cell reached the desired value by opening the inlet valve of the methane gas cylinder. Although the stirrer has the capability of rotating with the maximum speed of 1000 rpm, it was used for three seconds in every minute during all runs

3. THERMODYNAMIC MODEL The liquid-hydrate-vapor/gas (LHV) equilibrium conditions of a gas hydrate system can be computed by employing the vdW-P solid solution theory41 which was originally developed by Parrish and Prausnitz.42 Based on this theory, water fugacity in both hydrate and liquid phases is equal: f wH = f wl

(2)

The water fugacity in the hydrate phase is related to the chemical potential difference of water in the real filled and hypothetical empty hydrate lattice (β-phase). Thus, the previous equation can be written as follows:

Figure 2. Schematic sketch of the experimental setup. R, regulator; V, valve; P, pressure transducer; T, thermometer; TCS, temperature controller system. C

DOI: 10.1021/acs.energyfuels.8b02176 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Δμwβ − H = Δμwβ − l

enthalpy differences of water between empty hydrate lattice and liquid phase, respectively. P, aw, and T are pressure, water ° activity, and reference temperature (273.15 K at 1 atm). Δhlw is defined by

(3)

According to vdW-P solid solution theory, the left side of eq 3 will be no.of cavity

Δμwβ− H = RT

∑ m=1

ij j νm lnjjj1 + jj k

yz

∑ Cmjf j zzzzz nc

j=1

z {

Δhwl = Δhw° + (4)

∫0

zzr 2 dr zz {

N

(8)

where σ is the collision diameter, a is hard core radius, ε stands for depth of energy well, N is a constant parameter (N = 4, 5, 10, 11). Z, r, and R′ are, respectively, the coordinate number, radius, and mean cavity radius. Thermodynamic model parameters for methane are given in Table 2.2,45 Table 2. Thermodynamic Model Parameters for Methanea,2,45 ε/k (K)

Tc (K)

Pc (MPa)

ω

methane

0.3834

3.1439

155.59

190.56

4.599

0.011

Table 4. UNIQUAC Interaction and Molecular Structure Parameters for Water (i) and EMIM-BF4 (j)a,50

a Notation: a, hard core radius; σ, collision diameter; ε, depth of energy well; k, Boltzmann’s constant; Tc, critical temperature; Pc, critical pressure; ω, acentric factor.

component

r

q

q′

aij (K)

Water EMIM-BF4

0.9200 6.5136

1.400 5.158

1 −

−439.46 5999.30

a

Notation: r, UNIQUAC volume parameter; q and q′, UNIQUAC surface area parameter; aij, UNIQUAC interaction parameter for water and EMIM-BF4.

The right side of eq 3 is defined by thermodynamic properties. The final form46 of eq 3 is

nc ij yz jj ∑ νm lnjjj1 + ∑ Cmjf j zzzzz j z m=1 j=1 k { l T P Δν l ° Δμw Δhw w = − + d T dP − ln a w 2 T RT 0 RT RT (9) ° ° l l where Δμw° , Δhw, Δνw are the chemical potential difference of water at reference temperature between the liquid/aqueous phase and hypothetical empty hydrate lattice, molar volume, and

For mixtures of PVCap + EMIM-BF4, the effect of IL on water activity is just considered, and based on the better results of

2

∫

value 1263.6 −4858.9 4.6

dissociation temperatures of studying systems can be calculated easily using eq 9. In this work, we have used EMIM-BF4 and PVCap as hydrate inhibitors. νm is calculated as described in the following section. Water activity in the presence of PVCap is calculated using the Flory−Huggins theory.47 The details of measurement and thermodynamic modeling of water activity in the presence of PVCap is given by Foruotan and Zarrabi.48 It was understood that at concentrations of PVCap used in this work, i.e., 0.5 wt %, the water activity can be assumed approximately unity. To calculate water activity in the presence of EMIM-BF4, two activity coefficient models are used: NRTL and UNIQUAC.49 These two model parameters are reported in Tables 4 and 5.50,51 The results of applying these two activity coefficient models are reported in the following sections.

and

σ (Å)

property Δμ°w (J/mol) Δhw° (J/mol) Δνlw (cm3/mol)

a Notation: Δμw°, chemical potential difference of water at reference temperature between the liquid/aqueous phase and hypothetical empty hydrate lattice; Δhw°, molar enthalpy difference of water between empty hydrate lattice and ice; Δνlw molar volume difference of water in the liquid phase and the empty hydrate lattice.

(7)

a (Å)

(11)

Table 3. Thermodynamic Properties for Structure I Methane Hydratea,2,42

where k is the Boltzmann’s constant and ω(r) is the cell potential which is calculated by Kihara spherical-core model.44 Therefore, we can have ÄÅ ÉÑ ÅÅÅji σ zy12 ji σ zy6ÑÑÑ z − jj zz ÑÑÑ Γ(r ) = 4εÅÅÅjj ÅÅk r − 2a z{ k r − 2a { ÑÑÑÖ (6) ÅÇ ÄÅ 12 É Ñ ÅÅ σ i σ6 i a 11yz a 5yzÑÑÑ ω(r ) = 2zεÅÅÅÅ 11 jjjδ10 + δ zz − 5 jjjδ 4 + δ zzÑÑÑ ÅÅÇ R′ r k R′ { R′ r k R′ {ÑÑÖ

gas

(10)

ΔCpw is the heat capacity difference of water between the empty hydrate lattice and liquid water. Δμ°w, Δh°w, and Δνlw for structure I methane hydrates are reported in Table 3.2,42 Hydrate

(5)

ÄÅ É −N −N Ñ 1 ÅÅÅÅji r a zy r a zy ÑÑÑÑ i j zz − jj1 + zz Ñ δ = ÅÅjj1 − − − N ÅÅÇk R′ R′ { R′ R′ { ÑÑÑÖ k

ΔCpw dT

ΔCpw = −38.12 + 0.141(T − 273.15)

∞ i − ω(r ) y

jj jj k KT

T

°

where νm is the number of type m hydrate cavity per water molecules in a single hydrate unit, Cmj is the Langmuir constant of guest molecules (j) in cavity type m, and f j is fugacity of hydrate former in the gaseous phase which in this work is calculated by the Peng−Robinson equation of state (PR EoS).43 R and T stand for the universal gas constant and the absolute temperature. The Langmuir constant is expressed as 4π Cmj(T ) = kT

∫T

Table 5. NRTL Parameter, Δg (J/mol), for Binary System of Water (i), and EMIM-BF4 (j) used in the NRTL Equation (α = 0.3)51

∫

D

component

EMIM-BF4 (j)

water (i)

water (i) EMIM-BF4 (j)

5410.1 −

− −4061.1 DOI: 10.1021/acs.energyfuels.8b02176 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels NRTL, only this model has been used in calculating aw for the studying systems. 3.1. Model Parameters. The theoretical values of number of small and large cavities to number of water molecules for SI methane clathrate hydrate are as follows: υsmall =

2 46

(12)

υ large =

6 46

(13)

Raman spectroscopy studies on methane hydrate systems containing PVCap, however, reveal some changes in cage occupation. Hong et al.25 conducted an in situ Raman experiment on an agitated methane hydrate forming system. Results indicate that systems containing PVCap show a different behavior and large to small cavity ratio (L/S) is also different from its theoretical value, i.e. 3/1. As hydrate formation progressed, since the PVCap adsorbs on the hydrate crystals via their lactam rings and PVCap concentration decreases in solution,10 the inhibition effect will be reduced and the cavity ratios will get back to the usual trend. In this work, we have assumed that the number of small cavities to water molecules for methane hydrates in the presence of PVCap is related to PVCap concentration:

Figure 3. Methane hydrate dissociation conditions in the presence of 10 wt % of EMIM-BF4. AAD of NRTL and UNIQUAC activity coefficient models are 0.1 and 0.2 K, respectively.

Table 6. Displacement (Shift) Temperature of Methane Hydrate for Four Different Systemsa no.

system

1

CH4 + 10 wt % of EMIM-BF4

2

CH4 + 0.49 wt % of PVCap-7k

3

CH4 + 0.48 wt % PVCap-7k + 0.56 wt % EMIM-BF4

4

0.51 wt % PVCap-7k + 0.99 wt % EMIM-BF4

B 1 + AC PVCap

υsmall =

23

(14)

where C is PVCap concentration in ppm and A and B are the optimized coefficients based on the available experimental data. Seo et al.52 studied the effects of PVCap molecular weight and molecular weight distribution on methane hydrate formation and found out that decreasing molecular weight will increase hydrate inhibition performance of PVCap. Based on this fact, the coefficients of eq 14 are determined using the available literature data for PVCap-6k and experimental data of this work for PVCap-7k and minimizing of the following objective function: N

OF =

∑ |Texp − Tcal| i=1

(15)

where Tcal is calculated using eqs 9 and 14. Texp is experimental methane hydrate dissociation temperature in the presence of PVCap. These coefficients are given in the following section. Also, the number of large cavity to the water molecules in a unit cell is assumed to be constant and equal to its theoretical value, i.e. 6/46. Trying to optimize this parameter, also confirms this assumption. The average absolute deviation (AAD) which is described in the following equation confirms the agreement of the model results with experimental data: AAD =

1 N

N

∑ |Texp − Tcal| i=1

(16)

Note that N is the number of existing experimental points.

4. RESULTS AND DISCUSSION The experimental methane hydrate dissociation points in the presence of 10 wt % of EMIM-BF4 and their deviation from pure water dissociation points are shown in Figure 3, and their values are reported in Table 6, system no. 1. The results of the measurements indicate that the hydrate equilibrium curve is

Pexp (MPa)

Texp.in (K)

ΔTdiss (K)

ΔTdiss,ave (K)

2.79 3.47 3.76 4.65 5.32 6.49 3.02 3.46 3.82 4.38 4.84 5.51 5.93 6.89 8.06 3.10 3.56 4.00 4.53 4.97 5.55 6.03 6.78 7.08 7.29 7.46 7.61 3.14 3.50 3.93 4.37 4.99 5.59 6.13 6.40 7.45

273.3 275.5 276.1 278.3 279.6 281.4 279.3 280.6 281.6 282.8 283.5 284.4 285.2 287.0 288.2 279.7 280.9 282.2 282.8 283.5 284.6 285.1 286.2 286.7 286.9 287.1 287.5 279.8 280.8 281.8 282.6 283.6 284.2 285.1 285.4 286.8

−0.5 −0.5 −0.7 −0.7 −0.7 −0.8 4.7 4.8 4.6 4.5 4.1 3.7 3.8 4.2 4.0 4.8 4.6 4.7 4.1 3.9 3.9 3.6 3.6 3.7 3.6 3.6 3.8 4.8 4.6 4.5 4.3 3.9 3.4 3.4 3.3 3.4

−0.7

4.3

4.0

4.0

The expanded uncertainty, Uc, is Uc(T) = ±0.1 K, Uc(w) = ±0.01, Uc(P) = ±0.01 MPa (0.95 level of confidence).

a

E

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Energy & Fuels shifted to the left slightly. This leads to a negative temperature shift which proves that EMIM-BF4 can be considered as a thermodynamic inhibitor. A good agreement between methane hydrate dissociation temperature in the presence of EMIM-BF4 predicted by both NRTL and UNIQUAC activity coefficient models and the experimental data is observed which is shown in Figure 3. The experimental and predicted methane hydrate dissociation points in the presence of 0.49 wt % of PVCap-7k obtained in this work are given in Table 6, system no. 2 and Figure 4. The results

Figure 5. Methane hydrate dissociation conditions in the presence of 0.48 wt % PVCap-7k + 0.56 wt % EMIM-BF4.

Figure 4. Methane hydrate dissociation conditions in the presence of 0.49 wt % PVCap-7k.

show that hydrate dissociation temperatures are on average 4.3 K higher than that of the noninhibited system. This can prove that methane hydrate formed in the presence of PVCap as a KHI likely becomes more stable, which results in an increase in the decomposition temperature and/or required time to dissociate versus noninhibited system. In order to investigate the effect of simultaneous presence of PVCap and EMIM-BF4 on methane hydrate dissociation temperatures, two different mixtures of these additives were tested: 0.48 wt % PVCap + 0.56 wt % EMIM-BF4 and 0.51 wt % PVCap + 0.99 wt % EMIM-BF4. The obtained results are reported in Table 6, systems no. 3 and 4. In general, Table 6 shows shift and experimental temperature of methane hydrate systems in the presence of different mixtures of PVCap and/or EMIM-BF4. The dissociation temperatures of methane hydrate in the presence of 0.48 wt % PVCap + 0.56 wt % EMIM-BF4, roughly one to one ratio, and 0.51 wt % PVCap + 0.99 wt % EMIM-BF4, roughly one to two ratio, are on average 4.0 K higher than that of the noninhibited system for both systems. Therefore, the addition of EMIM-BF 4 at the applied concentrations slightly reduces the dissociation temperatures of methane hydrate formed in the presence of PVCap alone, about 0.3 K. Figures 5 and 6 show experimental and predicted dissociation temperatures of the methane hydrate system in the presence of different mixtures of PVCap + EMIM-BF4. Looking at Figures 5 and 6, no improved inhibition performance is investigated. Although adding EMIM-BF4 to the methane hydrate system containing PVCap have synergetic kinetic inhibition performance based on previous studies,11,31,33 there is no thermodynamic inhibition effect on methane hydrate in the presence of PVCap. In other words, adding EMIM-BF4 may just prolong induction time and retard hydrate nucleation and/or growth by forming hydrogen bonds and electrostatic interactions with water molecules but does not make methane

Figure 6. Methane hydrate dissociation conditions in the presence of 0.51 wt % PVCap-7k + 0.99 wt % EMIM-BF4.

hydrate containing PVCap difficult to dissociate. Apparently, EMIM-BF4 does not affect methane hydrate structure. As discussed earlier, results from Gulbrandsen and Svartaas for PVCap-6k23 and the measured data of this work for PVCap7k were used to optimize the coefficients of eq 14. Five points of measured data for PVCap-7k, i.e., system no. 2 in Table 6, and four points of literature data for PVCap-6k23 were used for optimization. The obtained coefficients for PVCap-7k and PVCap-6k are reported in Table 7. Then, the model is verified Table 7. Optimized Values of Parameters of Equation 14 PVCap 0.49 wt % PVCap-7k 0.48 wt % PVCap-7k + 0.56 wt % EMIM-BF4 0.51 wt % PVCap-7k + 0.99 wt % EMIM-BF4 PVCap-6k

A

B

AAD (K)

0.1737

0.1297

0.3 0.2 0.3

0.1483

0.1335

0.2

ref this work this work this work 23

using the remained literature and measured data for PVCap-7k, PVCap-6k, and mixtures of PVCap-7k and EMIM-BF4. Acceptable agreements between experimental and predicted methane hydrate dissociation conditions for each studying system are observed and their error is reported in Tables 7 and 8. Figures 4−6 show the experimental and predicted dissociation temperatures of methane hydrate for 0.49 wt % (4900 ppm) F

DOI: 10.1021/acs.energyfuels.8b02176 Energy Fuels XXXX, XXX, XXX−XXX

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Table 8. Experimental and Predicted Methane Hydrate Dissociation Points in the Presence of PVCap-6k23 concentration (wt %)

Pexp (MPa)

Texp (K)

Tcal (K)

0.004 0.008 0.010 0.075 0.150 0.300 0.600 AAD (K)

9.33 9.40 9.48 9.49 9.55 9.53 9.50

287.3 288.2 288.4 289.1 289.3 289.6 289.5

287.7 288.0 288.1 288.9 289.3 289.6 289.9 0.2

Article

AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected] (J.J.). *Email: [email protected] (A.H.M.). ORCID

Jafar Javanmardi: 0000-0002-4146-1490 Amir H. Mohammadi: 0000-0002-2947-1135 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors wish to thank Professor Bahman Tohidi (HeriotWatt University) for providing us with PVCap and Shiraz University of Technology for supporting of this work.

of PVCap-7k and different mixtures of PVCap-7k + EMIM-BF4, respectively. Table 8 shows experimental23 and predicted methane hydrate dissociation conditions in the presence of PVCap-6k. The results show the accuracy of the model. According to the results, it can be concluded that the change in PVCap molecular weight causes effective changes in the coefficients. In this way, large to small cavity ratio in SI methane hydrates in the presence of PVCap will change from 3 to values between 2 and 2.5 depending on the PVCap concentration and PVCap molecular weight.

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5. CONCLUSION In this work, the effects of PVCap on methane hydrate dissociation temperatures as well as in combination with EMIM-BF4 have been investigated. Different concentrations of additives have been studied. Finally, a thermodynamic model based on the van der Waals−Platteeuw solid solution theory is introduced to calculate hydrate dissociation temperatures for these systems. Different concentrations of PVCap or EMIM-BF4 and mixtures of PVCap and EMIM-BF4, i.e. 0.49 wt % PVCap, 10 wt % EMIM-BF4, 0.48 wt % PVCap + 0.56 wt % EMIM-BF4, and 0.51 wt % PVCap + 0.99 wt % EMIM-BF4 were investigated. Results reveal that for all mixtures containing PVCap, the methane hydrates will dissociate at temperatures higher than the pure water. The dissociation temperatures of methane hydrate in the presence of 0.49 wt % PVCap are on average 4.3 K higher than that of noninhibited system. However, when using EMIMBF4 alone the methane hydrate equilibrium curve is shifted to the left and methane hydrates will dissociate easier. The dissociation temperatures of methane hydrate in the presence of 0.48 wt % PVCap + 0.56 wt % EMIM-BF4 and 0.51 wt % PVCap + 0.99 wt % EMIM-BF4 are on average 4.0 K higher than that of the noninhibited system for both systems. Therefore, the addition of EMIM-BF4 at the applied concentrations slightly reduces the dissociation temperatures of methane hydrate formed in the presence of PVCap alone (about 0.3 K). The results show that despite of eccentric synergetic effect of adding EMIM-BF4 to methane hydrate system containing PVCap on kinetic inhibition performance and induction time of the system, there is no evidence of significant thermodynamic effects on methane hydrate dissociation conditions. Generally, addition of PVCap or mixtures of PVCap and EMIM-BF4 makes methane hydrate difficult to dissociate, and it may cause some serious problems in gas hydrate remediation. On the other hand, thermodynamic modeling proves the performance of additives and confirms that adding PVCap to the methane hydrate system will change the number of small cavities to water molecules from the theoretical value according to PVCap concentration and its molecular weight. G

DOI: 10.1021/acs.energyfuels.8b02176 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.8b02176 Energy Fuels XXXX, XXX, XXX−XXX