Assessing the Alkyl Chain E ﬀ ect of Ammonium Hydroxides Ionic Liquids on the Kinetics of Pure Methane and Carbon Dioxide Hydrates

: In this study, four ammonium hydroxide ionic liquids (AHILs) with varying alkyl chains were evaluated for their kinetic hydrate inhibition (KHI) impact on pure carbon dioxide (CO 2 ) and methane (CH 4 ) gas hydrate systems. The constant cooling technique was used to determine the induction time, the initial rate of hydrate formation, and the amount of gas uptake for CH 4 -AHILs and CO 2 -AHILs systems at 8.0 and 3.50 MPa, respectively, at 1 wt.% aqueous AHILs solutions. In addition, the e ﬀ ect of hydrate formation sub-cooling temperature on the performance of the AHILs was conducted at experimental temperatures 274.0 and 277.0 K. The tested AHILs kinetically inhibited both CH 4 and CO 2 hydrates at the studied sub-cooling temperatures by delaying the hydrate induction time and reducing the initial rate of hydrate formation and gas uptake. The hydrate inhibition performance of AHILs increases with increasing alkyl chain length, due to the better surface adsorption on the hydrate crystal surface with alkyl chain length enhancement. TPrAOH e ﬃ ciently inhibited the induction time of both CH 4 and CO 2 hydrate with an average inhibition percentage of 50% and 84%, respectively. Tetramethylammonium Hydroxide (TMAOH) and Tetrabutylammonium Hydroxide (TBAOH) best reduced CH 4 and CO 2 total uptake on average, with TMAOH and Tetraethylammonium Hydroxide (TEAOH) suitably reducing the average initial rate of CH 4 and CO 2 hydrate formation, respectively. The ﬁndings in this study could provide a roadmap for the potential use of AHILs as KHI inhibitors, especially in o ﬀ shore environs. on pure CH and CO 2 gas hydrate The KHI two di ﬀ erent and 274.0 to study the sub-cooling e ﬀ ect at 1 wt.%, which could represent a moderate to high hydrate prone conditions. selection AHILs their alkyl chain e ﬀ ect. To KHI of the AHILs in all the experiments are performed of 8.0 and 3.50 MPa for CH 4 and CO 2 hydrates, respectively. experiments with the commercial inhibitor (PVP) at high sub-cooling and the obtained results compared with AHILs and other ILs systems


Introduction
Clathrate hydrates generally form by the combination of hydrogen-bonded water (host) and gas molecules (<10 Å) at certain favourable thermodynamic conditions via van der Waals force of attractions [1]. Three types of hydrate structures exist depending on the size and shape of the guest: structure I (sI), structure II (sII), and structure H (sH). sI consist of 5 12 (2) and 5 12 6 2 (6) cages made gaseous systems. The findings in this work would provide more insight into the implementation of ionic liquids as gas hydrate inhibitors in gas production and processing operations.

Materials
The tested chemicals (AHILs) were supplied by Merck milli-pore (Germany) and used without further purification, as outlined in Table 1. Deionized water was used to prepare the concentrations of AHILs solutions for all the studied samples. The CH 4 and CO 2 were purchased from Gas Walker Sdn. Bhd. (Malaysia).

Experimental Setup and Methods
The high-pressure volumetric equilibrium cell having a maximum capacity of 650 mL was used in this study. The apparatus can efficiently operate within the temperature and pressure ranges from 253.0 to 323.0 K and up to 20.0 MPa, respectively. Details about the equipment setup can be found in our earlier reported articles [9,35,40,47]. The setup has a PID controlled thermostat bath, which is used to control the temperature in the cell during experimentation. In addition, an upper and lower temperature sensor is installed in the cell to measure the cell temperature. A pitch impeller stirrer is used to provide enough mixing in the liquid phase at 400 rpm. The pressure in the cell is controlled and monitored with a pressure transducer of accuracy ± 0.01 MPa. The detailed schematic diagram of the apparatus is shown in Figure 1.

Materials
The tested chemicals (AHILs) were supplied by Merck milli-pore (Germany) and used without further purification, as outlined in Table 1. Deionized water was used to prepare the concentrations of AHILs solutions for all the studied samples. The CH4 and CO2 were purchased from Gas Walker Sdn. Bhd. (Malaysia).

Experimental Setup and Methods
The high-pressure volumetric equilibrium cell having a maximum capacity of 650 mL was used in this study. The apparatus can efficiently operate within the temperature and pressure ranges from 253.0 to 323.0 K and up to 20.0 MPa, respectively. Details about the equipment setup can be found in our earlier reported articles [9,35,40,47]. The setup has a PID controlled thermostat bath, which is used to control the temperature in the cell during experimentation. In addition, an upper and lower temperature sensor is installed in the cell to measure the cell temperature. A pitch impeller stirrer is used to provide enough mixing in the liquid phase at 400 rpm. The pressure in the cell is controlled and monitored with a pressure transducer of accuracy ± 0.01 MPa. The detailed schematic diagram of the apparatus is shown in Figure 1.   Table 2 presents the experimental kinetic details of gaseous systems together with the temperature and pressure conditions in the absence and presence of aqueous AHILs solutions. The KHI experimental testing pressures and temperature conditions were selected to represent the typical hydrate plugs challenges encountered in gas transmission lines. Additionally, to assess the effect of sub-cooling (driving force) on CH 4 and CO 2 hydrate formations, the KHI experiments were performed at two (2) different experimental temperatures of 274.0 and 277.0 K. In addition, the performance of the AHILs was compared with PVP at 1 wt.% at for both CO 2 and CH 4 hydrate systems.

Kinetic Measurement Procedure
An isochoric constant cooling technique was employed in all the kinetic experiments for KHI evaluation. Prior to all experimental runs, the cell is cleaned to remove contaminants, after which 100 mL of the AILs solution is poured into the hydrate cell. The hydrate cell is placed in the water bath and simultaneously placed on a vacuum. The target gas was then compressed to the necessary experimental running pressure inside the hydrate cell. Afterward, the system was left to stabilize by allowing it to cool for almost an hour to the appropriate initial test temperature and pressure limits. The stirrer is switched on at 400 rpm and the data logging program is begun continuously upon initiation of the experiment. The experimental testing pressure was 8.00 MPa for CH 4 and 3.50 MPa for CO 2 systems. All the systems were tested at 274.15 and 277.15 K and repeated thrice with their mean then presented. At the stage or time when there is constant pressure in the hydrate cell (for about five hours), the testing is terminated and considered complete, as illustrated in Figure 2. The observation of a sharp decrease in the system's pressure signifies the formation of hydrates.  Table 2 presents the experimental kinetic details of gaseous systems together with the temperature and pressure conditions in the absence and presence of aqueous AHILs solutions. The KHI experimental testing pressures and temperature conditions were selected to represent the typical hydrate plugs challenges encountered in gas transmission lines. Additionally, to assess the effect of sub-cooling (driving force) on CH4 and CO2 hydrate formations, the KHI experiments were performed at two (2) different experimental temperatures of 274.0 and 277.0 K. In addition, the performance of the AHILs was compared with PVP at 1 wt.% at for both CO2 and CH4 hydrate systems.

Kinetic Measurement Procedure
An isochoric constant cooling technique was employed in all the kinetic experiments for KHI evaluation. Prior to all experimental runs, the cell is cleaned to remove contaminants, after which 100 mL of the AILs solution is poured into the hydrate cell. The hydrate cell is placed in the water bath and simultaneously placed on a vacuum. The target gas was then compressed to the necessary experimental running pressure inside the hydrate cell. Afterward, the system was left to stabilize by allowing it to cool for almost an hour to the appropriate initial test temperature and pressure limits. The stirrer is switched on at 400 rpm and the data logging program is begun continuously upon initiation of the experiment. The experimental testing pressure was 8.00 MPa for CH4 and 3.50 MPa for CO2 systems. All the systems were tested at 274.15 and 277.15 K and repeated thrice with their mean then presented. At the stage or time when there is constant pressure in the hydrate cell (for about five hours), the testing is terminated and considered complete, as illustrated in Figure 2. The observation of a sharp decrease in the system's pressure signifies the formation of hydrates.   The time required for an additive to perform poorly, or for hydrate crystal to appear is called the hydrate nucleation time, t i . The nucleation time in this study was calculated using the temperature/pressure versus time profile shown below ( Figure 2) as where t s is the time at initial hydrate testing conditions, and t h is the time when hydrates began to form.

Total Gas (CH 4 and CO 2 ) Uptake
The total CH 4 and CO 2 uptake or moles consumed into hydrates were estimated by employing the real gas equation (see Equation (2)).
where, T, P, V, z, and R are system temperature, pressure, gas-phase volume, compressibility factor, and the gas constant, respectively.

Initial Hydrate Formation Rate
The initial hydrate formation rate was determined using the description expressed in Equation (3) adopted from Nashed et al. [48].
where n t is the moles of methane uptake converted to hydrate at time t of initial rapid hydrate growth with respect to the sharp pressure drop in the system, and n s is the moles of methane uptake at induction time, and dt is the time difference between n t and n s .

Relative Inhibition Efficiency (RIE)
The relative inhibition efficiency (RIE) was determined to comparatively evaluate the inhibition efficiency of the kinetic inhibition parameters (induction time, methane uptake, and rate of hydrate formation) [14,17,42,49]. The values for RIE are calculated relative to pure water and AILs solution samples as fractional inhibition. Positive RIE values represent hydrate inhibition, whereas negative values present hydrate promotion. The RIE values for all the kinetic hydrate inhibition parameters were calculated using Equation (4).

Effect of AHILs on the Induction Time of CH 4 and CO 2 Hydrates
The kinetic formation measurements of CH 4 and CO 2 hydrates in the presence of AHILs were assessed at moderate experimental pressures (8.0 and 3.5 MPa) to evaluate their inhibition strength in simulated seabed sub-cooling conditions. Both the CH 4 and CO 2 hydrate systems were investigated at 277.0 and 274.0 K in the absence and presence of the AHILs to study the influence of sub-cooling on hydrate formation. Figure 3 illustrates the effect of 1 wt.% AHILs on the average induction time of CH 4 hydrates at different experimental temperatures (277.0 and 274.0 K). The induction time of water (without AHILs) was 29.8 min and 37.9 min at 274.0 and 277.0 K, respectively. The variation in the induction times was controlled by the different sub-cooling effect. Thus, hydrate formed faster in the system with a higher sub-cooling than the system with a lower sub-cooling degree.
Energies 2020, 13, x FOR PEER REVIEW 6 of 18 times was controlled by the different sub-cooling effect. Thus, hydrate formed faster in the system with a higher sub-cooling than the system with a lower sub-cooling degree. The measured average induction time of AHILs increases with the increasing alkyl chain length of their cations up to TPrA + (TMAOH < TEAOH < TPrAOH), then decreases in the presence of TBA + cation. Therefore, apart from TBAOH, the induction time of AHILs at 1 wt.% and 277.0 K was found in the following decreasing order: TPrAOH > TEAOH > TMAOH > TBAOH > water. It was expected that TBAOH should exhibit the best AHILs inhibitor to delay the CH4 hydrate nucleation time because it has the longest alkyl chain, since longer chain ionic liquids are known to effectively inhibit hydrate formation [50]. However, TBAOH trail as the best inhibitor compared with the other AHILs because of its long carbon chain causing the formation of micelles within the studied concentration range. According to Tariq et al. [41], ammonium-based ILs with longer chains (>C3) have the tendency to form micelles at low concentrations (≤2 wt.%) in aqueous solution. These micelles act in a mild surfactant nature to slightly prevent aggregation at the gas/liquid interface for CH4 hydrate The measured average induction time of AHILs increases with the increasing alkyl chain length of their cations up to TPrA + (TMAOH < TEAOH < TPrAOH), then decreases in the presence of TBA + cation. Therefore, apart from TBAOH, the induction time of AHILs at 1 wt.% and 277.0 K was found in the following decreasing order: TPrAOH > TEAOH > TMAOH > TBAOH > water. It was expected that TBAOH should exhibit the best AHILs inhibitor to delay the CH 4 hydrate nucleation time because it has the longest alkyl chain, since longer chain ionic liquids are known to effectively inhibit hydrate formation [50]. However, TBAOH trail as the best inhibitor compared with the other AHILs because of its long carbon chain causing the formation of micelles within the studied concentration range. According to Tariq et al. [41], ammonium-based ILs with longer chains (>C3) have the tendency to form micelles at low concentrations (≤2 wt.%) in aqueous solution. These micelles act in a mild surfactant nature to slightly prevent aggregation at the gas/liquid interface for CH 4 hydrate systems, which reduces the inhibition strength of the chemical (TBAOH). Though the inhibition impact of TBAOH in Figure 3 slightly increases with concentration, the presence of micelles formation reduces its performance compared with the other tested AHILs at all concentrations. The micelles formations in the TBAOH aqueous systems were considered as mild for all the tested concentrations (≤2 wt.%) in this study, as suggested by Tariq et al. [41]. However, it can be observed in Figure 3 that the inhibition strength of TBAOH retards with concentration as compared with other AHILs. For example, at 1 wt.% the CH 4 hydrate inhibition impact is about 73%. When the concentration is doubled (2 wt.%), the induction time inhibition impact further increased by only 43%. This decrease in inhibition strength with concentration could probably be due to the mild system micelle formation and/or the hydrate nuclei crystallization orientation with the aqueous TBAOH solution [51]. Further studies on the effect and quantification of micelles formation of AILs with varying concentrations on CH 4 hydrates are recommended.
In addition, the subcooling effect on TBAOH is found to be unusual with regards to its induction time and rate of hydrate formation. The induction time and rate of hydrate formation (Figures 3-6) of CH 4 hydrate are high at 277 and lower at 274 K. In both systems, the TBAOH micelle formation behavior affects the surface activity of the system via the reduction of the hydrophilic effect of the system, as discussed by Zielinski et al. [52]. This guarantees the possibility of hydrate forming during the cooling stage for both systems based on the complex nucleation stochasticity of hydrate formation. It must be stated that, for most of the system in this work, the onset of hydrate formation occurs during the constant cooling stage before reaching the experimental temperatures, which in effect could initiate the hydrate formation arbitrary once the meta-stability in either systems is overcome in the hydrate stability region as cooling continues. Hence, the varying inhibition effect at different experimental temperatures is mainly attributed to the nucleation probabilistic effect, which is described by Sowa et al. [51], has been solely system dependant, i.e., gas type, concentration and temperature dependant. However, it is also possible that the system cooling rate might lead to such an unusual nucleation phenomenon [53]. Subsequently, the gas dissolution in the lower subcooling system might be undersaturated and delay its hydrate formation time as compared to the higher subcooling system.
The impact of the AHILs on the induction time of CH 4 hydrate at higher sub-cooling temperature 274.0 K (higher sub-cooling = 10.8 K) confirms their hydrate inhibition potentials in Figure 3a. Interestingly, the induction time delaying ability of the AHILs for CH 4 hydrate at 274 K was similar to PVP (commercial KHI). The measured induction time of all the AHILs were less at a high sub-cooling temperature (274.0 K) compared to the lower sub-cooling condition (277.0 K), except for TBAOH. The presence of a strong driving force at high sub-cooling explains the short hydrate nucleation time at 274.0 K. At the 274.0 K condition, the induction times of AHILs are established in a following decreasing orders: TPrAOH > TEAOH > PVP > TBAOH > TMAOH > water. Results also suggested that the KHI performance is immensely dependant on the structure of the AHILs. It had been reported earlier in the literature that the longer alkyl chain cations enhance the kinetic hydrate inhibition effect more than the shorter ones (for instance, [BMIM] + exhibited better kinetic inhibition than [EMIM] + ) [23]. Nashed et al. [54] reported a similar trend when they studied the KHI behaviour of imidazolium-based ILs by varying their alkyl chains. They reported that an increasing IL alkyl chain improves gas hydrate formation nucleation time. However, in the case of ammonium salts, a chain length increment above TPrA + results in the formation of micelles at low concentrations, thus, causing the induction time of TBAOH to be less as expected.
Moreover, the kinetic inhibition of the anion is also evident in the comparison between TMACl [42] and TMAOH. At a higher sub-cooling condition (274.0 K), TMAOH (41.55 min) delays hydrate formation nucleation time more efficiently than TMACl (31.25 min). This enhanced inhibition of TMAOH over TMACl is probably due to the ability of the OH − functional group to form hydrogen bond cleavages owing to its high hydrogen bonding affinity with water molecules [23]. The hydrogen-bonded network slightly stemmed from the sub-cooling temperature resulted in a lowered driving force, thus shelving hydrate formation.
The relative inhibition efficiency (RIE) values in Figure 3b quantitatively confirm the kinetic inhibitory efficacy of the tested AHILs. Koh and coworkers [55] proposed a method known as the relative inhibition efficiency (RIE), which could be used to effectively compare the kinetic hydrate inhibition impact of different kinds of systems independent of their experimental conditions. RIE values greater than zero (0) denote the superior inhibitory performance of the KHIs. The RIE data of the AHILs at different experimental conditions are presented in Figure 3b. The RIE results add more clarity to the KHI performance of the AHILs. The effect of sub-cooling was also evident in the RIE attained data. For instance, the RIE of the AHILs is lower at a higher sub-cooling condition (274.0 K), than a lower sub-cooling condition (277.0 K). This is due to the existence of high hydrate formation driving forces induced by the higher sub-cooling temperature at 274.0 K. The KHI performance (RIE) of TPrAOH (0.89) appears to be superior to the PVP (0.78) (commercial KHI). Furthermore, RIE data of considered AHILs were compared with previous studies (with a different class of ILs), as reported by Nashed et al. [54] and Khan et al. [42] in Table 3. All the considered AHILs possess better RIE values than the earlier imidazolium-based ILs studied by Nashed et al. [54].  Figure 3c illustrates the influence of the AHILs concentration on the induction time of CH 4 hydrate. From the induction time data, it can be deduced that almost all the studied AILs systems (except 0.5 wt.% TBAOH) increase the methane hydrate formation induction time with an increasing concentration. This suggests that the hydrate delay (nucleation) seems to be concentration dependent. The improved inhibition performance is found to be attained with increased AHIL concentrations, perhaps attributed due to the enhanced activity of water and AHILs solutions (with increases AHILs concentrations). Therefore, considering Figure 3, TPrAOH was the best KHI inhibitor at all studied concentrations among the AHILs. The reason for improved KHI inhibition of TPrAOH is perhaps due to the optimum alkyl chain length, which can provide sufficient adsorption on gas-liquid and hydrate-gas interfaces together with proficient hydrogen bonding ability to hinder the hydrate formations [57]. The kinetic hydrate inhibition impact is concentration and guest (gas) molecule type dependent [42,51]. Hence, the hydrate inhibition effect on CO 2 was different for CH 4 . The formation pressure of CO 2 hydrates is significantly less compared to CH 4 hydrates due to the bigger diameter size of CO 2 (CO 2 = 5.12 Å or CH 4 = 4.36 Å) [58] together with the higher solubility of CO 2 in water [42,59], making CO 2 prone to hydrate formation.
The average induction time of AHILs on CO 2 hydrates for 1 wt.% concentration at different experimental temperature conditions at 274.0 and 277.0 K is presented in Figure 4. CO 2 generally form hydrates quite fast compared to the CH 4 hydrates at the same experimental temperature conditions, owing to proneness to hydrate formation at smaller driving forces [17,42]. The average induction time of CO 2 hydrates without AHILs is found to be 12.35 and 14.35 min at 277.0 and 274.0 K temperatures, respectively. The extent of AHILs' kinetic inhibition on CO 2 hydrates is different from CH 4 hydrates, as observed in Figures 3 and 4. This means that inconsistent inhibition trends for hydrate nucleation time exist between CH 4 and CO 2 hydrates in the presence of AHILs, thus, suggesting the effect of guest (gas) molecule type on the kinetics of hydrate formation [51]. These findings are consistent with Sowa et al. [51] who indicated that hydrate nucleation or kinetics are greatly influenced by the guest or gas molecule type. Therefore, based on the type of gas and chemical, the hydrate inhibition mechanism could either be via surface adsorption [55] or crystal perturbation activities by a hydrogen bonding effect [60]. For instance, TBAOH in the CH 4 system performs relatively less compared with other AHILs (see Figure 3) because of its micelles formations behavior which reduces its surface activity causing the enhancement of CH 4 dissolution into the liquids phase, thus reducing its performance. However, in the presence of polar gases, such as CO 2 , which could possibly interact with aqueous TBAOH solution, leads to the perturbation of the water structures and delay hydrate nucleation (see Figure 4). According to Zielinski et al. [52], the formation of micelles in ammonium salts favors their hydrophilic effect, which enables them to perturb the local water structure via hydrogen bonding; this then causes the nucleation process in the CO 2 system to increase more than in the CH 4 system [60]. On the contrary, in the CH 4 system a strengthened hydrophilic surface of the TBAOH solutions affects their hydrophobic shield which weakens the gas/liquid interface and causes a favorable CH 4 dissolution in the liquid phase and reduces the hydrate inhibition impact of TBAOH.
Among the tested AHILs aqueous solutions, TBAOH was the most effective kinetic inhibitor in both experimental conditions at 1 wt.%. All the AHILs could prolong the induction time. In the presence of the CO 2 hydrates at 277.0 K, the induction times of AHILs are in the increasing order of water < TMAOH < TEAOH < TPrAOH < TBAOH at 1 wt.%.
At 274.0 K, the induction times of CO 2 -AHILs are reduced when compared to the 277.0 K experimental condition. The induction time delays for CO 2 hydrates at 274 K are in an increasing order, as follows: water < TMAOH < TEAOH < PVP < TPrAOH < TBAOH. The AHILs with longer alkyl chains (TBAOH, TPrAOH) prolong CO 2 hydrates inhibition similar to the CH 4 system; however, there is no critical inhibition chain length for the AHILs in CO 2 hydrate system at 1 wt.%. A comparison between studied anions (OH − and Cl − ) for CO 2 hydrates, TMACl [42] (42.6 min) provide better induction time inhibition than TMAOH (19.0 min) potentially, due to the better surface adsorption of Cl − on the surface of the water, as mentioned by the earlier study [56]. Table 3 shows the RIE results studied for 1 wt.% CO 2 -ILs systems. At lower temperature conditions (274.0 K), the KHI impact (RIE) of longer alkyl chain AHILs (TBAOH (3.8) and TPrAOH (3.5)) was found to be superior to the PVP (1.9) (commercial KHI). Moreover, RIE data of studied AHILs are comparable with prior studies like Chun et al. [12], Bavoh et al. [56], and Khan et al. [42] as shown in Table 3. In fact, most of the considered AHILs (TEAOH, TPrAOH, and TBAOH) possess better RIE values compared to the earlier studied ILs systems. In the case of hydroxyl anions (OH − ), the influence of the alkyl chain can also be observed from the induction time data at 277.0 K. With an increase in the AHILs alkyl chains; the induction time is enhanced. This is due to the better adsorption of the cation at the surface of the gas-liquid and hydrate-liquid interface, reducing the dissolution of gas into the liquid phase [61].
The RIE results of AHILs at lower sub-cooling (277.0 K) temperature show higher RIE values at lower sub-cooling than higher sub-cooling temperature (274.0 K) owing to the less significant driving force (see Figure 4b). The RIE values at lower sub-cooling were related to the alkyl chain of the AHILs [14,42,62,63]. In Table 3, the induction time inhibition performance of AHILs is more evident and effective in the CO 2 hydrate system compared with CH 4 . In addition, the hydrate induction time inhibition time is delayed more as AHILs concentrations increase in both CH 4 and CO 2 hydrate systems.

Effect of AHILs on the Initial Formation Rate of CH4 and CO2 Hydrates
According to Sloan and co-workers [58], hydrate crystal growth periods (initial formation rates) are technically more rational than induction time information due to the intricacy of the hydrate nucleation process. The rate of hydrate formation of CH4 and CO2 hydrate in the presence of 1 wt.% AHILs at different experimental temperatures (274.0 and 277.0 K) are considered and reported in Figure 5a.

Effect of AHILs on the Initial Formation Rate of CH 4 and CO 2 Hydrates
According to Sloan and co-workers [58], hydrate crystal growth periods (initial formation rates) are technically more rational than induction time information due to the intricacy of the hydrate nucleation process. The rate of hydrate formation of CH 4  The estimated initial formation rate of pure water for CH4 hydrates is 0.015 and 0.0276 mol.min −1 at 274.0 and 277.0 K conditions, respectively. However, in the CO2 hydrate system, the initial formation rate was 0.0042 and 0.0079 mol.min −1 for 274.0 and 277.0 K, respectively. For lower subcooling temperature conditions (277.0 K and 1 wt.%), all the studied AHILs significantly reduce the initial formation rate of CH4 hydrates compared with water. The initial formation results further confirmed the superior kinetic performance of TPrAOH, which exhibits the lowest CH4 hydrate formation rate among all considered AILs at 277.0 K and 1 wt.%. The initial formation rates of the studied AHILs (at 277.0 K) are in the following increasing order: TPrAOH < TEAOH <TBAOH < TMAOH < water for CH4 hydrates (see Figure 5a). However, in CO2 hydrates, TBAOH trailed to inhibit the initial formation rates, thus, in CO2-AHILs hydrates (at 277.0 K) the following increasing order of hydrate inhibition was observed: TMAOH < TPrAOH < TEAOH < water < TBAOH ( Figure 6). Furthermore, the initial formation rate of AHILs is also reported at higher sub-cooling (≈ 10.8 K) at 274.0 K experimental temperature and compared with water and commercial kinetic inhibitor, PVP. The initial formation rate of pure water is around 0.015 mol.min −1 at 274.0 K. The presence of all the AHILs solutions at 1 wt.% could reduce the methane hydrate formation rate (see Figure 5a). TEAOH and TBAOH reduced the methane rate of hydrate formation to about 0.0022 mol.min −1 at 1 wt.% and 274.0 K. However, the initial formation rates of AHILs were significantly reduced at the 277.0 K temperature compared to the 274.0 K temperature condition for both CH4 and CO2 systems due to the presence of higher driving force (sub-cooling) which causes rapid hydrate crystal growth at 274.0 K. The initial rate of CH4 hydrate formation in AHILs solutions at 1 wt.% and 274.0 K are found to be in the following increasing order: TBAOH < TEAOH < TPrAOH < TMAOH < PVP < water (Figure 5a). The increment in the AHILs alkyl chain decreased the formation rates. The reason behind this is that, when the chain length of AHILs increases, the Van der Waals forces in the system are also intensified, hence strong interactions occur between the aqueous gas-liquid interfaces [17,64], which provides a reduced rate of hydrate formation on methane hydrates. All the AHILs inhibited the methane hydrate growth rate better than PVP at 1 wt.% and 274.0 K. In Figure 5b, the presence of all the AHILs reduced the initial rate of hydrate formation with increasing concentration for CH4 hydrates, perhaps due to the enhanced activity of AHILs with an increasing concentration.
However, for the CO2 hydrate system (Figure 6b), the initial rate of CO2 hydrate formation increases with concentration for TMAOH and TEAOH. TPrAOH and TBAOH showed their optimum inhibition performance at 1 wt.%. The initial CO2 hydrates formation rates in the tested AHILs The estimated initial formation rate of pure water for CH 4 hydrates is 0.015 and 0.0276 mol·min −1 at 274.0 and 277.0 K conditions, respectively. However, in the CO 2 hydrate system, the initial formation rate was 0.0042 and 0.0079 mol·min −1 for 274.0 and 277.0 K, respectively. For lower sub-cooling temperature conditions (277.0 K and 1 wt.%), all the studied AHILs significantly reduce the initial formation rate of CH 4 hydrates compared with water. The initial formation results further confirmed the superior kinetic performance of TPrAOH, which exhibits the lowest CH 4 hydrate formation rate among all considered AILs at 277.0 K and 1 wt.%. The initial formation rates of the studied AHILs (at 277.0 K) are in the following increasing order: TPrAOH < TEAOH <TBAOH < TMAOH < water for CH 4 hydrates (see Figure 5a). However, in CO 2 hydrates, TBAOH trailed to inhibit the initial formation rates, thus, in CO 2 -AHILs hydrates (at 277.0 K) the following increasing order of hydrate inhibition was observed: TMAOH < TPrAOH < TEAOH < water < TBAOH ( Figure 6). Furthermore, the initial formation rate of AHILs is also reported at higher sub-cooling (≈ 10.8 K) at 274.0 K experimental temperature and compared with water and commercial kinetic inhibitor, PVP. The initial formation rate of pure water is around 0.015 mol·min −1 at 274.0 K. The presence of all the AHILs solutions at 1 wt.% could reduce the methane hydrate formation rate (see Figure 5a). TEAOH and TBAOH reduced the methane rate of hydrate formation to about 0.0022 mol·min −1 at 1 wt.% and 274.0 K. However, the initial formation rates of AHILs were significantly reduced at the 277.0 K temperature compared to the 274.0 K temperature condition for both CH 4 and CO 2 systems due to the presence of higher driving force (sub-cooling) which causes rapid hydrate crystal growth at 274.0 K. The initial rate of CH 4 hydrate formation in AHILs solutions at 1 wt.% and 274.0 K are found to be in the following increasing order: TBAOH < TEAOH < TPrAOH < TMAOH < PVP < water (Figure 5a). The increment in the AHILs alkyl chain decreased the formation rates. The reason behind this is that, when the chain length of AHILs increases, the Van der Waals forces in the system are also intensified, hence strong interactions occur between the aqueous gas-liquid interfaces [17,64], which provides a reduced rate of hydrate formation on methane hydrates. All the AHILs inhibited the methane hydrate growth rate better than PVP at 1 wt.% and 274.0 K. In Figure 5b, the presence of all the AHILs reduced the initial rate of hydrate formation with increasing concentration for CH 4 hydrates, perhaps due to the enhanced activity of AHILs with an increasing concentration.
However, for the CO 2 hydrate system (Figure 6b), the initial rate of CO 2 hydrate formation increases with concentration for TMAOH and TEAOH. TPrAOH and TBAOH showed their optimum inhibition performance at 1 wt.%. The initial CO 2 hydrates formation rates in the tested AHILs solutions at higher sub-cooling conditions (10.8 K and 1 wt.%) are found to be following a decreasing order: water > PVP > TEAOH > TPrAOH > TBAOH > TMAOH as shown in Figure 6a. All the considered AHILs could reduce the initial formation rates of CO 2 hydrate better than PVP at 1 wt.%. Again, the anomaly behaviour of TBAOH as observed in Figure 6a is because of its micelles formation surfactant nature on the rate of CO 2 hydrate formation. Thus, causing the initial rate of hydrate growth to occur within a shorter time period (with high rate of formation) at 277.0 K than at 274.0 K in the presences of 1 wt.% TBAOH.
Energies 2020, 13, x FOR PEER REVIEW 12 of 18 solutions at higher sub-cooling conditions (10.8 K and 1 wt.%) are found to be following a decreasing order: water > PVP > TEAOH > TPrAOH > TBAOH > TMAOH as shown in Figure 6a. All the considered AHILs could reduce the initial formation rates of CO2 hydrate better than PVP at 1 wt.%. Again, the anomaly behaviour of TBAOH as observed in Figure 6a is because of its micelles formation surfactant nature on the rate of CO2 hydrate formation. Thus, causing the initial rate of hydrate growth to occur within a shorter time period (with high rate of formation) at 277.0 K than at 274.0 K in the presences of 1 wt.% TBAOH.

Effect of AHILs on Mole Consumption of CH4 Hydrates
The total gas consumed or uptake defines the maximum amount of gas (moles) trapped into the hydrate lattice structure. Figure 7a shows the effect of AHILs on CH4 uptake during the hydrate formation in this work. The obtained results at 277.0 K experimental temperature indicated that, apart from TBAOH, all AHILs reduce the overall methane hydrate mole consumption into hydrates. Since all the tested AHILs have the same anion (OH -), the effect of their cation chain length causes their effects on the hydrate formation uptake. TEAOH exhibited the least moles of methane hydrate consumption due to the presence of TEA + cations (at 277.0 K and 1 wt.%). However, TBAOH significantly enhanced/promoted the overall methane hydrate moles consumption, due to its surfactant nature resulting from its micelles formation tendency. Therefore, this provides a positive guest dissolution into the liquid phase to form more hydrates [41,51]. Interestingly, hydrate formation was not observed in the presence of PVP after 48 h of experimentation at 277.0 K and 1 wt.%. This indicates that AHILs are poor inhibitors compared with PVP at lower sub-cooling degrees with 7.8 K. Additionally, the mole consumptions of methane hydrate with AHILs are reported at a lower temperature condition (274.0 K; sub-cooling = 10.8 K) and also compared with pure water and PVP (commercial KHI) samples. At higher sub-cooling, the mole consumption of CH4 hydrates is not significantly affected by AHILs. With the increasing alkyl chain of AHILs, the CH4 hydrate mole consumption is found to be increased at 1 wt.%. However, results revealed that all the studied AHILs have lower mole consumptions compared with PVP, which exhibited the highest CH4 moles uptake. Similar behaviour was also reported by Nguyen et al. [57] for PVP, and they indicated that when PVP forms hydrate, it increases the total methane consumption, hence, behaving as a hydrate total uptake promoter. In Figure 7b, the total methane uptake in hydrate formation is reduced with increasing

Effect of AHILs on Mole Consumption of CH 4 Hydrates
The total gas consumed or uptake defines the maximum amount of gas (moles) trapped into the hydrate lattice structure. Figure 7a shows the effect of AHILs on CH 4 uptake during the hydrate formation in this work. The obtained results at 277.0 K experimental temperature indicated that, apart from TBAOH, all AHILs reduce the overall methane hydrate mole consumption into hydrates. Since all the tested AHILs have the same anion (OH − ), the effect of their cation chain length causes their effects on the hydrate formation uptake. TEAOH exhibited the least moles of methane hydrate consumption due to the presence of TEA + cations (at 277.0 K and 1 wt.%). However, TBAOH significantly enhanced/promoted the overall methane hydrate moles consumption, due to its surfactant nature resulting from its micelles formation tendency. Therefore, this provides a positive guest dissolution into the liquid phase to form more hydrates [41,51]. Interestingly, hydrate formation was not observed in the presence of PVP after 48 h of experimentation at 277.0 K and 1 wt.%. This indicates that AHILs are poor inhibitors compared with PVP at lower sub-cooling degrees with 7.8 K. Additionally, the mole consumptions of methane hydrate with AHILs are reported at a lower temperature condition (274.0 K; sub-cooling = 10.8 K) and also compared with pure water and PVP (commercial KHI) samples. At higher sub-cooling, the mole consumption of CH 4 hydrates is not significantly affected by AHILs. With the increasing alkyl chain of AHILs, the CH 4 hydrate mole consumption is found to be increased at 1 wt.%. However, results revealed that all the studied AHILs have lower mole consumptions compared with PVP, which exhibited the highest CH 4 moles uptake. Similar behaviour was also reported by Nguyen et al. [57] for PVP, and they indicated that when PVP forms hydrate, it increases the total methane consumption, hence, behaving as a hydrate total uptake promoter. In Figure 7b, the total methane uptake in hydrate formation is reduced with increasing AHILs concentrations. TMAOH had the best CH 4 mole uptake at 1 and 2 wt.%; however, at 0.5 wt.%, all the AHILs showed methane hydrate promotional effect. For the CO 2 hydrate system, the presence of AHILs shows an inconsistent relationship between the CO 2 moles consumed with concentration, indicating the dependence of hydrate inhibition of AHILs on concentration and guest molecule type as shown in Figures 7b and 8b.  The mole consumption of carbon dioxide hydrates is comparatively lesser than methane hydrates due to the moderate experimental pressure (3.50 MPa) used for the CO2 hydrates systems. The influence of 1 wt.% AHILs (aqueous solutions) on the mole consumption of CO2 hydrates are presented in Figure 8a. The results revealed that AHILs do not significantly inhibit CO2 mole consumption at both experimental temperatures. TMAOH and TBAOH inhibited the CO2 hydrate formation total uptake in both experimental conditions (at 274.0 K and 1 wt.%). However, TMAOH showed the best CO2 moles uptake inhibition. All the remaining AHILs showed a CO2 hydrate total uptake promotional effect, as shown in Figure 8a. The increase in mole consumptions of ammoniumbased ionic liquids in CO2 hydrates agree with the findings of Cha and coworkers [16], who reported that the presence of AILs prolongs the induction time of CO2 hydrates to an extent which allows significant hydrate nucleation sites in the system. This causes more CO2 to be consumed into hydrates once they begin to form (when the AILs inhibitor fails). A similar hydrate uptake promotional effect is exhibited by some commercial hydrate inhibitors like PVP.
The CO2 moles' consumption data of AHILs at different concentrations illustrate that, at a lower strength, the mole consumption of CO2 is reduced for all the studied concentrations ( Figure 8b). Overall, TMAOH exhibited the least CO2 uptake amongst the studied systems for all concentrations. The finding of this study suggested that the studied AHILs possess KHI abilities. Similarly, our earlier studies [9,14,15,65,66] have indicated that these AILs also possess good THI potential, which makes them potential dual-functional gas hydrate inhibitors. However, the THI and KHI properties are highly dependent on their effective alkyl chain length selection [14,42]. Therefore, to get better dualfunctional performance, the appropriate chain length of ILs is essential. This study has shown that the use of AILs can be used as efficient gas hydrate inhibitors in flow assurance applications. The mole consumption of carbon dioxide hydrates is comparatively lesser than methane hydrates due to the moderate experimental pressure (3.50 MPa) used for the CO 2 hydrates systems. The influence of 1 wt.% AHILs (aqueous solutions) on the mole consumption of CO 2 hydrates are presented in Figure 8a. The results revealed that AHILs do not significantly inhibit CO 2 mole consumption at both experimental temperatures. TMAOH and TBAOH inhibited the CO 2 hydrate formation total uptake in both experimental conditions (at 274.0 K and 1 wt.%). However, TMAOH showed the best CO 2 moles uptake inhibition. All the remaining AHILs showed a CO 2 hydrate total uptake promotional effect, as shown in Figure 8a. The increase in mole consumptions of ammonium-based ionic liquids in CO 2 hydrates agree with the findings of Cha and coworkers [16], who reported that the presence of AILs prolongs the induction time of CO 2 hydrates to an extent which allows significant hydrate nucleation sites in the system. This causes more CO 2 to be consumed into hydrates once they begin to form (when the AILs inhibitor fails). A similar hydrate uptake promotional effect is exhibited by some commercial hydrate inhibitors like PVP.
The CO 2 moles' consumption data of AHILs at different concentrations illustrate that, at a lower strength, the mole consumption of CO 2 is reduced for all the studied concentrations ( Figure 8b). Overall, TMAOH exhibited the least CO 2 uptake amongst the studied systems for all concentrations. The finding of this study suggested that the studied AHILs possess KHI abilities. Similarly, our earlier studies [9,14,15,65,66] have indicated that these AILs also possess good THI potential, which makes them potential dual-functional gas hydrate inhibitors. However, the THI and KHI properties are highly dependent on their effective alkyl chain length selection [14,42]. Therefore, to get better dual-functional performance, the appropriate chain length of ILs is essential. This study has shown that the use of AILs can be used as efficient gas hydrate inhibitors in flow assurance applications.

Conclusions
In this work, the formation kinetics of CH4 and CO2 gas hydrates in the presence of four AHILs are reported via a constant cooling approach at 1 wt.%. It was found that the presence of longer alkyl chain AHILs performed better in delaying CH4 and CO2 hydrate nucleation time and reducing their initial hydrate formation rate and gas uptake. TBAOH and TPrAOH were the best performing AHILs, and their inhibition impact was comparable to PVP (a commercial gas hydrate inhibitor). As the AHILs alkyl chain length increases, their surface adsorption affinity is enhanced, hence, providing more adsorption affinity on the hydrate crystal lattices. This causes the AHILs to inhibit the hydrate crystal nucleation and growth. Moreover, the formation kinetic results suggested that the KHI impact is concentration dependent. As the AHILs concentration increases, their inhibition impact is increased, especially in CH4 hydrates systems. In addition, the hydrate inhibition effect of the studied AHILs strongly depends on the concentration, the guest molecule present, and the sub-cooling degree. However, the studied AHILs could inhibit the induction time and hydrate formation rate more than the total gas uptake.  Acknowledgments: This study was conducted at the Chemical Engineering Department, Universiti Teknologi PETRONAS.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In this work, the formation kinetics of CH 4 and CO 2 gas hydrates in the presence of four AHILs are reported via a constant cooling approach at 1 wt.%. It was found that the presence of longer alkyl chain AHILs performed better in delaying CH 4 and CO 2 hydrate nucleation time and reducing their initial hydrate formation rate and gas uptake. TBAOH and TPrAOH were the best performing AHILs, and their inhibition impact was comparable to PVP (a commercial gas hydrate inhibitor). As the AHILs alkyl chain length increases, their surface adsorption affinity is enhanced, hence, providing more adsorption affinity on the hydrate crystal lattices. This causes the AHILs to inhibit the hydrate crystal nucleation and growth. Moreover, the formation kinetic results suggested that the KHI impact is concentration dependent. As the AHILs concentration increases, their inhibition impact is increased, especially in CH 4 hydrates systems. In addition, the hydrate inhibition effect of the studied AHILs strongly depends on the concentration, the guest molecule present, and the sub-cooling degree. However, the studied AHILs could inhibit the induction time and hydrate formation rate more than the total gas uptake.  Acknowledgments: This study was conducted at the Chemical Engineering Department, Universiti Teknologi PETRONAS.

Conflicts of Interest:
The authors declare no conflict of interest.