**1. Introduction**

Gas hydrates are icelike crystalline solid compounds that could form in the presence of water and gas under favorable thermodynamic temperature-pressure condition [1]. At low-temperature and high-pressure conditions, water molecules (host) will surround the gas molecules (guest) and encapsulate the gas in a hydrogenbonded solid lattice [2]. Depending on the gases trapped, different structures of gas hydrates can be formed. The structure I hydrate trapped methane (CH4), ethane (C2H6), and carbon dioxide (CO2) gases. Structure II usually forms for propane (C3H8) gas, while a mixture of CH4 and butane (C4H10) and other hydrocarbons can be captured by structure H hydrates [3].

In recent decades, hydrates have received plenty of attention, because of its potential to capture and store gas [4–21]. Also, it is discovered that gas hydrates located in subsea as well as permafrost region are a potential source of energy too [15]. However, the formation of natural gas hydrates in oil and gas pipeline is never applauded [4, 15, 18, 22]. This is because hydrate formation in pipelines has resulted in blockage and affected flow assurance of natural gas [23]. In spite of the economic losses caused by the blockage, ecological disasters could occur in severe cases too [24]. To prevent hydrate formation, several methods including isobaric thermal heating, water removal, depressurization, and chemical inhibitor injection [25] have been implemented. The three former methods, however, are not feasible and costly. As a result, chemical inhibitors have been researched and developed a lot in recent years to control the growth of hydrates.

There are generally three types of inhibitors, which are thermodynamic hydrate inhibitor (THI), kinetic hydrate inhibitor (KHI), and anti-agglomerates (AA). THI prevents the formation of the hydrate by shifting the thermodynamic equilibrium curve of gas hydrate to a lower temperature and higher pressure [25]. KHI, on the other hand, does not inhibit hydration formation, but it slows down their nucleation and growth of hydrate. It works on the principle of lengthening the formation time of hydrate to be longer than the residence time of the gas in pipelines [26]. Finally, AA, also a low-dosage inhibitor, allows the formation of hydrate but, through perturbation of water molecules, prevents the hydrate molecules from accumulating and growing larger [27].

Some common THI inhibitors include methanol and sodium chloride. To be effective, THI normally needs to be injected in a high concentration of around 10–50 wt% [28], which leads to high operational cost. Furthermore, sodium chloride corrodes oil and gas pipelines [29]. While KHI inhibitors were able to work effectively at a lower dosage (<1 wt%), Kelland reported that as exploration operation goes into the deeper sea, KHI still has to work together with THI to effectively inhibit hydrate formation [27]. These limitations signify that existing chemical inhibitors are still not performing well, and there is a strong need to develop more effective inhibitor [29, 30].

This leads the oil and gas industry toward ILs which was initially introduced as inhibitors by Chen et al. [31] in 2008, as the team discussed the effect of 1-butyl-3-methylimidazolium tetrafluoroborate in inhibiting CO2 hydrate formation. A year later, Xiao and Adidharma [29] suggested the dual function of ILs inhibitors. The results showed that IL is not only able to shift the hydrate thermodynamic equilibrium curve, but it also retards the formation of the hydrate. Since then, numerous experimental works have been carried out to study the effect of ILs in inhibiting gas hydrates formation, mainly using imidazolium- and pyridinium-based ILs [2, 3, 25, 32]. The targeted ILs of this context are ammonium-based ILs (AILs), which are cheaper and easier to synthesis, but not being studied intensively. Therefore, due to cost economics and more environmentally friendly, AILs are chosen to be studied in this work.

To date, all the testing work of ILs effectiveness is done using an experimental method, which is by measuring the average depression temperature for thermodynamic hydrate inhibitors and by measuring induction time for kinetic hydrate inhibitors. There are generally no other methods available to validate the experimental work or to pre-screen ILs in a shorter time. Due to this reason, it is very desirable if a theoretical method to predict ILs effectiveness as hydrate inhibitors could be established just by analyzing their fundamental properties. And to obtain these fundamental properties, COSMO-RS, a thermodynamic properties predictive tool, is the best option available in the market.

For this purpose, COSMO-RS, which can estimate the fundamental properties of ILs system, has been selected. COSMO-RS is a novel method to predict

**145**

triethylammonium-based ILs.

detailed and specific manner.

**2. Methodology**

ties which provide the way for this current work.

*Pre-Screening of Ionic Liquids as Gas Hydrate Inhibitor via Application…*

the thermodynamic properties of ILs based on quantum chemistry model [33]. COSMO-RS first calculates the charge density of individual molecules based on the structure of each molecule [34]. The charge density will then be distributed onto the entire molecule surface. This distribution will then be described by a onedimensional probability density [35], or more famously known as sigma profile, P(*σ*). Lastly, from the charge density, chemical potential, μ, will be calculated, and it will act as the basis for all other calculations to predict thermodynamic properties such as Henry's law constant and activity coefficient [36]. The calculated properties will then try to be correlated to IL inhibition ability to develop a prediction model

Throughout the years, COSMO-RS model has been successfully applied in numerous works to predict the thermodynamic properties of systems containing ILs, such as liquid-liquid equilibrium [37, 38] and activity coefficient [34, 39]. Therefore, this has prompted a lot of screening efforts of ILs through COSMO-RS for different purposes such as determining extraction solvent and improving separating process [37, 40–42]. Grabda et al. [43], for example, has used COSMO-RS to carry out a screening process for ILs that is used as an extraction solvent for neodymium chloride and dysprosium chloride. Kurnia and Mutalib [44], on the other hand, had screened imidazolium-based ILs for the separation process of benzene from n-hexane through COSMO-RS. Other than screening work, comparison and validation work have been conducted too. Calvar et al. [37], for instance, have compared COSMO-RS prediction of LLE values of ILs with their experimental data and found out that the result is satisfactory. In 2007, Palomar et al. [45] reinforced the applicability of COSMO-RS in predicting density and molar volume of imidazolium-based IL when their predicted values laid close to the experimental data.

To support the application of COSMO-RS in this work, it is found out that many other applications involving ammonium-based and bionic ILs have already been conducted through COSMO-RS [43]. In 2010, Sumon and Henni [46] performed a COSMO-RS study on the properties of ILs for CO2 capture. In this study, 12 ammonium-based cations such as tetramethylammonium (TMA), tetraethylammonium, and tetrabutylammonium (TBS) cations are used to derive ammonium-based ILs to be studied. In 2014, Grabda et al. [43] studied the effectiveness of 4400 ILs for NdCl3 and DyCl3 extraction. Among the many cations used are tetra-n-butylammonium, tetraethylammonium, tetramethylammonium, etc. Dodecyl-dimethyl-3-sulfopropylammonium cation, which is a type of ammonium-based cation, was concluded as the best performing cation in decreasing the chemical potential of NdCl3 and DyCl3, thus increasing their solubility and easing the extraction process. In the same year, Pilli et al. [47] screened out the best ILs to extract phthalic acid from aqueous solution using COSMO-RS. Although ammonium-based cation ILs in this simulation do not give the highest selectivity, they, however, have the highest activity coefficient. Next, through COSMO-RS, Machanová et al. [48] also obtained well-predicted values of excess molar volumes and excess enthalpy for N-alkyl-

As it observed from literature, screening of ILs for gas hydrate inhibition through COSMO-RS is a relatively new and fresh concept, yet, based on the successfulness of previous works [6, 18, 21, 22, 37] in predicting thermodynamic proper-

The research methodology comprises several activities described in even

*DOI: http://dx.doi.org/10.5772/intechopen.86847*

that could predict the inhibition ability of ILs.

#### *Pre-Screening of Ionic Liquids as Gas Hydrate Inhibitor via Application… DOI: http://dx.doi.org/10.5772/intechopen.86847*

*Solvents, Ionic Liquids and Solvent Effects*

recent years to control the growth of hydrates.

ing and growing larger [27].

effective inhibitor [29, 30].

In recent decades, hydrates have received plenty of attention, because of its potential to capture and store gas [4–21]. Also, it is discovered that gas hydrates located in subsea as well as permafrost region are a potential source of energy too [15]. However, the formation of natural gas hydrates in oil and gas pipeline is never applauded [4, 15, 18, 22]. This is because hydrate formation in pipelines has resulted in blockage and affected flow assurance of natural gas [23]. In spite of the economic losses caused by the blockage, ecological disasters could occur in severe cases too [24]. To prevent hydrate formation, several methods including isobaric thermal heating, water removal, depressurization, and chemical inhibitor injection [25] have been implemented. The three former methods, however, are not feasible and costly. As a result, chemical inhibitors have been researched and developed a lot in

There are generally three types of inhibitors, which are thermodynamic hydrate inhibitor (THI), kinetic hydrate inhibitor (KHI), and anti-agglomerates (AA). THI prevents the formation of the hydrate by shifting the thermodynamic equilibrium curve of gas hydrate to a lower temperature and higher pressure [25]. KHI, on the other hand, does not inhibit hydration formation, but it slows down their nucleation and growth of hydrate. It works on the principle of lengthening the formation time of hydrate to be longer than the residence time of the gas in pipelines [26]. Finally, AA, also a low-dosage inhibitor, allows the formation of hydrate but, through perturbation of water molecules, prevents the hydrate molecules from accumulat-

Some common THI inhibitors include methanol and sodium chloride. To be effective, THI normally needs to be injected in a high concentration of around 10–50 wt% [28], which leads to high operational cost. Furthermore, sodium chloride corrodes oil and gas pipelines [29]. While KHI inhibitors were able to work effectively at a lower dosage (<1 wt%), Kelland reported that as exploration operation goes into the deeper sea, KHI still has to work together with THI to effectively inhibit hydrate formation [27]. These limitations signify that existing chemical inhibitors are still not performing well, and there is a strong need to develop more

This leads the oil and gas industry toward ILs which was initially introduced as inhibitors by Chen et al. [31] in 2008, as the team discussed the effect of 1-butyl-3-methylimidazolium tetrafluoroborate in inhibiting CO2 hydrate formation. A year later, Xiao and Adidharma [29] suggested the dual function of ILs inhibitors. The results showed that IL is not only able to shift the hydrate thermodynamic equilibrium curve, but it also retards the formation of the hydrate. Since then, numerous experimental works have been carried out to study the effect of ILs in inhibiting gas hydrates formation, mainly using imidazolium- and pyridinium-based ILs [2, 3, 25, 32]. The targeted ILs of this context are ammonium-based ILs (AILs), which are cheaper and easier to synthesis, but not being studied intensively. Therefore, due to cost economics

and more environmentally friendly, AILs are chosen to be studied in this work.

tool, is the best option available in the market.

To date, all the testing work of ILs effectiveness is done using an experimental method, which is by measuring the average depression temperature for thermodynamic hydrate inhibitors and by measuring induction time for kinetic hydrate inhibitors. There are generally no other methods available to validate the experimental work or to pre-screen ILs in a shorter time. Due to this reason, it is very desirable if a theoretical method to predict ILs effectiveness as hydrate inhibitors could be established just by analyzing their fundamental properties. And to obtain these fundamental properties, COSMO-RS, a thermodynamic properties predictive

For this purpose, COSMO-RS, which can estimate the fundamental properties of ILs system, has been selected. COSMO-RS is a novel method to predict

**144**

the thermodynamic properties of ILs based on quantum chemistry model [33]. COSMO-RS first calculates the charge density of individual molecules based on the structure of each molecule [34]. The charge density will then be distributed onto the entire molecule surface. This distribution will then be described by a onedimensional probability density [35], or more famously known as sigma profile, P(*σ*). Lastly, from the charge density, chemical potential, μ, will be calculated, and it will act as the basis for all other calculations to predict thermodynamic properties such as Henry's law constant and activity coefficient [36]. The calculated properties will then try to be correlated to IL inhibition ability to develop a prediction model that could predict the inhibition ability of ILs.

Throughout the years, COSMO-RS model has been successfully applied in numerous works to predict the thermodynamic properties of systems containing ILs, such as liquid-liquid equilibrium [37, 38] and activity coefficient [34, 39]. Therefore, this has prompted a lot of screening efforts of ILs through COSMO-RS for different purposes such as determining extraction solvent and improving separating process [37, 40–42]. Grabda et al. [43], for example, has used COSMO-RS to carry out a screening process for ILs that is used as an extraction solvent for neodymium chloride and dysprosium chloride. Kurnia and Mutalib [44], on the other hand, had screened imidazolium-based ILs for the separation process of benzene from n-hexane through COSMO-RS. Other than screening work, comparison and validation work have been conducted too. Calvar et al. [37], for instance, have compared COSMO-RS prediction of LLE values of ILs with their experimental data and found out that the result is satisfactory. In 2007, Palomar et al. [45] reinforced the applicability of COSMO-RS in predicting density and molar volume of imidazolium-based IL when their predicted values laid close to the experimental data.

To support the application of COSMO-RS in this work, it is found out that many other applications involving ammonium-based and bionic ILs have already been conducted through COSMO-RS [43]. In 2010, Sumon and Henni [46] performed a COSMO-RS study on the properties of ILs for CO2 capture. In this study, 12 ammonium-based cations such as tetramethylammonium (TMA), tetraethylammonium, and tetrabutylammonium (TBS) cations are used to derive ammonium-based ILs to be studied. In 2014, Grabda et al. [43] studied the effectiveness of 4400 ILs for NdCl3 and DyCl3 extraction. Among the many cations used are tetra-n-butylammonium, tetraethylammonium, tetramethylammonium, etc. Dodecyl-dimethyl-3-sulfopropylammonium cation, which is a type of ammonium-based cation, was concluded as the best performing cation in decreasing the chemical potential of NdCl3 and DyCl3, thus increasing their solubility and easing the extraction process. In the same year, Pilli et al. [47] screened out the best ILs to extract phthalic acid from aqueous solution using COSMO-RS. Although ammonium-based cation ILs in this simulation do not give the highest selectivity, they, however, have the highest activity coefficient. Next, through COSMO-RS, Machanová et al. [48] also obtained well-predicted values of excess molar volumes and excess enthalpy for N-alkyltriethylammonium-based ILs.

As it observed from literature, screening of ILs for gas hydrate inhibition through COSMO-RS is a relatively new and fresh concept, yet, based on the successfulness of previous works [6, 18, 21, 22, 37] in predicting thermodynamic properties which provide the way for this current work.
