Materials used for kinetic study of gas hydrates.
This present work highlights the impact of ammonium-based ionic liquid tetramethylammonium hydroxide (TMAOH) on the formation kinetics of carbon dioxide (CO2), methane (CH4), and their binary mixed gas (50–50 mole%) hydrates. The TMAOH (IL) is applied in varying concentrations (0.5, 1, and 2 wt%) at different experimental temperatures, i.e., 1 and 4°C. The kinetic experiments are conducted in a high-pressure reactor equipped with two-bladed impeller, to provide sufficient agitation. The experimental pressures of CO2, CH4, and mixed 50% CO2 + 50% CH4 were 3.50, 8.0, and 6.50 MPa, respectively. Induction time, the initial apparent rate of formation and the total gas consumed are the kinetic parameters used to evaluate the performance of TMAOH as KHI. The results are further compared with commercial KHI (PVP), at higher subcooling condition of 1°C and 1 wt% of all the studied gaseous systems. Furthermore, the KHI performance of TMAOH is also evaluated via the relative inhibition performance (RIP) compared with other ILs for CO2 and CH4 hydrates. Results revealed that TMAOH delays the induction time for all the considered systems. The presence of TMAOH also reduced the total gas consumed and the initial rate of hydrate formation in most of the studied systems.
- CH4 hydrate
- CO2 hydrate
- ionic liquids
- mix gas hydrate
Gas hydrates are solid crystals formed when gas (C1, C2, C3, C4 and CO2) molecules are trapped inside the hydrogen-bonded water cages under thermodynamically favourable conditions (low-temperature and high-pressure conditions) [1, 2]. Typically, three types of gas hydrate structures are known, sI, sII and sH hydrates, depending on the type and size of the encaged gas molecules. For example, pure CH4 and CO2 mostly form sI hydrate, while C2H6 forms sII hydrate.
Gas hydrate formation in the pipelines is considered as one of the most perennial flow assurance problems, which consumes about 70% of flow assurance resources. The accumulation of gas hydrates plugs oil and gas pipelines, disturbing hydrocarbon flow, and causes several safety issues [3, 4, 5]. The removal of hydrate plugs from transmission pipeline amount to about 1 million/day shutdown . The hydrate plugging risk increases when producing and transporting high carbon dioxide (CO2) content natural gas, as CO2 readily forms hydrates than methane (CH4) at the same pressure [6, 7]. High CO2 content natural gas reservoirs are frequently encountered in various areas around the world, such as Central European Pannonian Basin, Colombian Putumayo Basin, Gulf of Thailand, South China Sea, Ibleo platform, Taranaki Basin, Sicily, North Sea South Viking Graben and New Zealand . Malaysia is among the leading natural gas exporters in the world . The J5 and K5 gas fields located in offshore Eastern Malaysia produces about 70–87 mol% CO2 content natural gas . This significant amount of CO2 together with the harsh offshore conditions poses various complications for the exploration and transportation of these enormous reserves. One of the core concerns from CO2-enriched gas systems are their susceptibility to forming gas hydrates in the natural gas production lines [6, 7, 10]. Therefore, in-depth understanding of the mix gas hydrate systems will primarily provide an avenue for safe flow assurance operations when transporting such natural gas systems. Also, this will also provide some fundamental knowledge to design CO2 separation system for the natural gas system together with the storage opportunity for CO2 captured from richer CO2 content natural gas [4, 11, 12].
Four methods can be used to combat gas hydrate formation in flow assurance; this includes removal of water, pressure reduction, thermal heating and chemical injection . In most of the cases, chemical injection is the utmost economical preventive method among the others [8, 13]. These gas hydrate preventive chemicals are known as hydrate inhibitors and are extensively used in oil and gas transmission pipelines. There are three types of gas hydrate inhibitors; thermodynamic hydrate inhibitors (THIs), which mainly shift the hydrate equilibrium curves towards lower-temperature and higher-pressure regions. THIs are mostly required in large concentrations (10–50 wt% of water cuts). Commonly used THIs are methanol and glycols. Although these chemicals are still used in practical field applications till date, they face many drawbacks such as their high operational cost (transportation, storage, injection and pumping quantities and regeneration units) [14, 15].
The drawbacks of above THIs motivated researchers to develop a new kind of hydrate of inhibitors known as low-dosage hydrate inhibitors (LDHIs). This class of inhibitors are typically applied in very less concentration (<2 wt%). LDHIs consist of kinetic hydrate inhibitors (KHIs) and anti-agglomerates (AA). Kinetic hydrate inhibitors (KHIs) are primarily engrossed in delaying the hydrate nucleation time and formation growth rate. It remains quite problematic to evaluate the kinetics of hydrate formation [16, 17, 18], exclusively in the presence of KHIs, as it is a very dynamic and complex process . Commercially employed KHIs are water-soluble polymers such as polyvinylpyrrolidone (PVP) and polyvinyl caprolactam (PVCAP). Karaaslan and Parlaktuna  reported that PVP and PEO work as kinetic hydrate inhibitors but have carcinogenic materials which are capable of causing severe health and safety impact on human health. However, the impact of PEO is relatively less compared to PVP . To address these environmental defies of current inhibitors, the quest for greener inhibitors is actively ongoing. Several chemicals such as ILs (mostly imidazolium-based ILs) have been tested for gas hydrate mitigation. ILs could efficiently work as dual-functional hydrate inhibitors (THIs alongside KHIs).
Xiao and Adidharma  initiated the research on ILs as gas hydrate inhibitors and found a dual functional effect through imidazolium-based ILs. Since ILs are salts in a molten state, their ability to exhibit electrostatic interaction and form hydrogen bonds with water molecules enhances their hydrate crystalline surface adsorption ability by retarding hydrate nucleation process . Kim and Kang  used a high-pressure cell to evaluate pyrrolidinium- and morpholinium-based ionic liquids such as N-hydroxyethyl-N-methyl pyrrolidinium chloride ([HEMP][Cl]), N-hydroxyethyl-N-methyl pyrrolidinium tetrafluoroborate ([HEMP][BF4]), N-butyl-N-methyl pyrrolidinium bromide ([BMP][Br]), N-(2-hydroxyethyl)-N-methyl morpholinium bromide ([HEMM][Br]) and N-(2-hydroxyethyl)-N-methyl morpholinium tetrafluoroborate ([HEMM][BF4]) that can delay methane hydrate nucleation time than PVP and PVCAP at 0.1, 1 and 10 wt%. Furthermore, Nazari and Ahmadi  studied the effects of [BMIM][BF4] and [BMIM][MS] on the CH4 hydrate formation and suggested that [BMIM][BF4] acts as better KHI than [BMIM][MS]. However, poor THI behaviour was observed from [BMIM][BF4] in comparison with [BMIM][MS]. Norland and Kelland  and Lee et al.  elucidate that the type of anion/cation has a significant impact on IL effectiveness. They further suggested that ILs synergistically enhance the kinetic inhibition impact of PVCAP . However, quaternary ammonium salts have also reported as potential KHIs, which open the door for the second-generation IL-based KHIs.
An earlier study suggested that quaternary ammonium salts (QAs) can reduce the nucleation time and hydrate growth rate of tetrahydrofuran (THF) hydrates . The inhibition influence of trimethylpentane, trimethyl hexane and trimethyl octane (QAs) is attributed to the presence of alkyl (CH3) groups in a polar moiety form, which shows high tendency to be adsorbed on the hydrate crystalline surface . Most QAs were found to exhibit kinetic inhibition and anti-agglomeration tendency. However, prior study from Storr et al.  recommended that these chemicals should be tested in mixed gas or natural gas hydrate system to evaluate their suitability for field applications. The best kinetic inhibition performance for the studied QAs on THF hydrates were achieved by tetrapentylammonium bromide (TPAB) followed by tetrabutylammonium bromide (TBAB) [28, 29, 30].
On the other hand, a limited number of studies are found on the effect of ammonium-based ionic liquids (AILs) as gas hydrate inhibitors [4, 12, 31, 32, 33]. Few studies have reported that AILs could efficiently induce thermodynamic hydrate inhibition [3, 4, 14, 34]; however, kinetic evaluations of AILs are rarely available in open literature . Tariq et al.  previously reported that the presence of OH− anion in ILs has a higher inhibition effect as it facilitates more water hydrogen bond cleavage, thus delaying hydrate formation nucleation time. As the search for useful IL hydrate inhibitors is still ongoing, in our recent works [3, 4], tetramethylammonium hydroxide (TMAOH) was reported as an effective THI for both CH4 and CO2 gases with average suppression temperature (∆Ŧ) of 1.53 and 2.23°C, respectively. However, its ability to delay hydrate formation kinetics has not been investigated in the open literature.
Therefore, herein, the kinetic influence of TMAOH on CO2, CH4 and binary mixed gas hydrate (50–50 CO2 + CH4) formation at different concentrations (0.5, 1 and 2 wt%) is comprehensively evaluated and reported for the first time. Induction time, moles consumption and initial apparent rate of hydrate formation are the kinetic parameters that are used to assess the performance of TMAOH at different subcooling experimental temperatures of 1 and 4°C. The obtained results for all gaseous systems are further compared with PVP (commercial KHI) at 1 wt% and 1°C (since the best and commercially applicable concentration of PVP is 1 wt%, and 1°C gives the highest subcooling conditions in the study.) conditions. Also, the performance evaluation of TMAOH via RIP with other ILs is reported. Also, an attempt is made to describe the TMAOH kinetic inhibition mechanism in the presence of the studied hydrate systems.
Table 1 shows the materials used in this study. Only deionized water is used for all the sample preparations during the experiments. The sample concentration measurements are performed using a gravimetrical analytical balance.
2.2. Details of experimental equipment
A high-pressure stainless steel cell [3, 12, 32] with an internal volume of 650 mL, alongside a maximum working pressure of 30 MPa, is used in this study. The cell is immersed in a water bath to regulate the temperature in the cell at desired conditions during experimentations. Two thermocouples are used to measure the temperature inside the cell at an accuracy of ±0.01°C. To achieve appropriate agitation in the liquid phase, two-bladed pitch impeller stirrer is positioned in the equipment with a constant speed of 400 rpm (optimum speed) in all experimentations. The pressure inside the cell is noticed via a pressure transducer with an accuracy of ±0.001 MPa.
2.3. Kinetic measurement procedure
Before each experimental run, the cell is meticulously cleaned with distilled water, and then a 100 mL liquid phase sample (with or without TMAOH) is loaded into the cell. The cell is then introduced into the water bath, and the system is put under vacuum for about an hour to ensure there are no traces of the air in the cell. The desired gas is then pressurized into the cell to the desired experimental operating pressures; after that, the system is allowed to stabilize to the desired initial experimental pressure and temperature conditions by leaving it for about an hour. The mechanical stirrer is turned on at 400 rpm, and data logging system is started simultaneously with the commencement of the experiment (i.e. by reducing the system temperature to the experimentally desired subcooling temperature of 1 or 4°C). The experimental pressures are set at 3.50, 8.00 and 6.50 MPa for CO2, CH4 and binary mixed gas 50–50 CH4 + CO2 hydrate systems, respectively. The experimental temperatures are kept constant at 1 and 4°C for all studied systems and concentrations. All the experiments are repeated for three times, and averages are reported. The kinetic inhibition influence of TMAOH is assessed within the range of 0.5–2 wt%, similar to the typical industrial application of KHIs.
2.4. Kinetic measurement parameter
In all experiments, the hydrate formation is observed through a sudden pressure drop in the pressure-time plot recorded by the data recording system. The experiments are considered complete after observing a constant pressure in the cell at each experimental temperature for about 3–5 hours. A typical pressure and temperature verse time profile during hydrate formation is illustrated in Figure 1. In Figure 1, the general hydrate formation process has three major stages: nucleation, crystal/hydrate growth and complete hydrate formation due to mass transfer limitation. The objective of effective KHIs is to increase the hydrate nucleation time (induction time) and at the same time reduce the hydrate crystal growth rate and the total amount of gas consumed into hydrates . Hence, the kinetic inhibition influence of TMAOH is evaluated based on these three parameters.
2.4.1. Induction time measurement
The induction time is the most significant indicator to assess initiation of gas hydrate crystallization and growth. It is the time elapse for the occurrence of visible hydrate crystals of the critically stable-sized hydrate nucleus. Longer induction time than the fluid retention time would result in hydrate-free transportation for hydrocarbons in pipelines . However, induction time is a probabilistic phenomenon which depends upon the heterogeneous nucleation parameters . The nucleation rate could influence by the numerous factors, such as the existence of particles and impurities in the sample, the roughness of the cell wall and the presence of driving force. Thus, diverse experimental approaches would apparently provide different outcomes. For comparison of measurements of the induction time of hydrate inhibitors, the same apparatus is highly recommended/appropriate, and experimental method should is employed . The induction time is measured in this study from the pressure–time data plotted in Figure 1 as described in the literature [37, 38] as
where ts is the time taken for the system pressure to decrease the experimental pressure and th and ti is the actual induction time for hydrate formation (Figure 1). Usually, the induction time is recognized by the point at which there is a drastic drop in the reactor pressure accompanied by a corresponding sudden spike in the reactor temperature, thus indicating the onset of hydrate formation (see Figure 1 at point th).
2.4.2. Initial apparent rate of hydrate formation
The initial apparent rate of hydrate formation determines the rate of hydrate crystallization after the hydrate nucleation . The initial apparent rate is measured with regard to the rate at which the initial moles of gas is consumed into hydrate formation, thereby defining fast hydrate crystal growth that takes place . In these experiments, the initial rate of hydrate formations is accounted for initial 10 min of hydrate formations. The initial apparent rate of hydrate formation can be given by
where k is the initial apparent rate of any studied gas, n0 is the mole of gas at initial stage 0 and ns is the initial moles of consumed gas. The Peng-Robinson equation is employed for calculation of compressibility factor z, and the real gas equation is also used to calculate the mole of consumed gas.
2.4.3. Moles of gas consumed
Moles of gas consumed during hydrate formation determine the dissolved gas in hydrate phase which could form hydrate plug in the subsea condition. The total amount of consumed gas for complete hydrate formation is calculated by applying the real gas law [38, 41]:
where V, R, P and T denote the system gas phase volume, universal gas constant, pressure and temperature, respectively. ∆ngas denotes the moles of gas consumed, z is the compressibility factor of the gas determined from the Peng-Robinson equation of state and the subscripts ‘0’ and ‘f’ denote the number of n of moles of gas at the time, zero and time of complete hydrate formation.
In the mixed gas hydrate system, the final gas composition in the gas phase is different from the original mix gas composition due to the guest cage occupancy ratio of the mix gas composition. A gas chromatograph (PerkinElmer) is used to accurately calculate the final composition of the mixed gas in the gas phase after complete hydrate formation. The gas chromatograph results are employed to calculate the moles of mixed gas in the gas phase after hydrate completion (denoted as ‘f’ in Eq. (3)). Also, the gas chromatograph values indicated that gas composition is more in the hydrate phase in the presence and absence of TMAOH.
3. Results and discussion
The kinetic inhibition effect of TMAOH on the hydrate induction time, formation growth rate and total gas consumption on the hydrate systems is assessed in this work. This is to efficiently evaluate the kinetic inhibition of TMAOH on the complete hydrate formation and growth stages.
3.1. Effect of TMAOH on the kinetics of CO2 hydrates
The experimental pressure for CO2 hydrates has been fixed at 3.50 MPa with operating temperatures of 1 and 4°C (subcooling of 8.32 and 5.32°C). These conditions are selected to provide sufficient driving force for hydrate formation as typically encountered in offshore operations of oil and gas production. The effect of various concentrations (0.5, 1 and 2 wt %) of TMAOH on the induction time of CO2 hydrate at different experimental temperatures (1 and 4°C) is presented in Figure 2(a) and (b), respectively. Results reveal that TMAOH could delay the growth of CO2 hydrates at almost all studied systems by hindering its nucleation process. In the absence of TMAOH, the induction time of water is observed as 14.33 min at 1°C (see Figure 2(a)), while 4°C condition showed even lesser induction time of 12.35 min for pure water as evident in Figure 2(b). The presence of 0.5 wt% TMAOH shows a slight improvement in hydrate nucleation time (induction time) with the base sample (pure water) for both 1 and 4°C, which are 23.2 and 21.45%, respectively. Upon increasing the concentration to 1 wt%, the induction time of pure water is improved up to 32.5 and 78.1% at 1 and 4°C, respectively, indicating an inhibition ability of TMAOH at the studied subcooling conditions. However, the inhibition impact at 4°C is relatively higher compared to 1°C, primarily due to the significant driving force existed at 1°C (8.32°C). Further increase in TMAOH concentration to 2 wt% resulted in a negligible delay of CO2 hydrate induction time compared to the 1 wt% sample. Thus, the optimum TMAOH concentration to delay CO2 hydrate nucleation is at 1 wt% as shown in Figure 2. For comparison purposes, the induction time data is also equated with PVP at 1°C condition in Figure 2(a). The comparison results suggest that PVP still possess better hydrate holding efficacy than studied aqueous TMAOH solution perhaps due to the enhanced polymer linkage on the liquid-gas surface compared to TMAOH.
Since hydrate formation kinetic is a complex phenomenon, it is hard to compare with the previous studies due to the different experimental conditions and apparatus designs. However,  found the efficient method to compare different types of the system via relative inhibition power (RIP):
The RIP values determine the kinetic inhibitory efficacy of the KHIs, as the RIP>0 corresponds to the better inhibitory performance of KHI. The RIP value of 1 wt% TMAOH concentration at a higher subcooling experimental temperature (1°C) is presented in Figure 3 for CO2 hydrate system. The RIP results are further compared with PVP and some reported ILs, namely, [EMIM][Cl] and [EMIM][BF4] from Bavoh et al.  and Chun et al. , respectively. The RIP results further enlighten the kinetic inhibition performance of TMAOH. In fact, TMAOH possess enhanced RIP value (0.32) compared to [EMIM][BF4] (0.27) earlier studied imidazolium-based IL. However, [EMIM][Cl] provides much better inhibition performance due to the presence of Cl− anion in its structure which leads towards better induction time . However, PVP performs better than all studied ILs in Figure 3.
Figure 4 presents the initial rate constant of CO2 hydrate formation in the presence of various TMAOH concentrations at 1 and 4°C, respectively. It is found that the rate of hydrate formation is inhibited in the presence of TMAOH compared with the pure water sample at all studied concentrations (see Figure 4). The CO2 hydrate formation rate is reduced more at 4°C experimental temperature than 1°C, especially at 0.5 wt% system. This is attributed to the higher driving force existed at 1°C [15, 21]. At both 1 and 4°C, the samples with 0.5 wt% TMAOH concentration shows the maximum inhibition impact on CO2 hydrate formation rates up to 0.0013 and 0.00065 min−1, respectively. Unlike the induction time, whose inhibition impact is enhanced with increasing TMAOH concentration, the inhibition impact on CO2 formation rate is condensed with increasing TMAOH concentrations from 0.5 to 1 and 2 wt% as shown in Figure 4. It is suggested that the hydrate kinetic inhibition impact of TMAOH is concentration and subcooling temperature dependent. The obtained formation rate data of PVP (see Figure 4(a)) suggests that 1 wt% TMAOH performed considerably superior to PVP at 1°C conditions perhaps due to superior steric hindrances ensued in very least formation rates for all the studied CO2 systems.
The total moles of CO2 consumption during hydrate formation in TMAOH solution is depicted in Figure 5. All concentrations of TMAOH solution reduce the CO2 uptakes (moles) into hydrate phase. The maximum inhibition effect on mole consumption of TMAOH for CO2hydrate is observed at 1 wt% for both experimental conditions. The CO2 uptake values are found to be 0.132 and 0.124 moles for 1 and 4°C, respectively. The complete moles consumed at 4°C at all concentration are significantly higher than 1°C, again affirming the effect of the active driving force at 1°C. This considerable driving force attributed further hydrate formation environs cause the formation of more CO2 hydrates. Additionally, the CO2 uptake result of 1 wt% TMAOH is further compared with PVP in Figure 5(a). Referring to the comparison results of CO2 consumed, it is detected that TMAOH can reduce the mole consumption of CO2 more than PVP which signifies its usefulness as a potential KHI for CO2 hydrate formation.
3.2. Effect of TMAOH on the kinetics of CH4 hydrates
CH4 hydrate formation is a bit different from CO2 hydrate formation and required higher pressure (8.0 MPa) compared to CO2 hydrate (3.5 MPa). Figure 6 presents the influence of TMAOH on the induction time of CH4 hydrate formation at various concentrations (0.5, 1 and 2 wt%) and different experimental temperatures (1 and 4°C).
In contrary to CO2, higher concentrations of TMAOH do not reflect the linear induction time with mass concentration perhaps due to probabilistic nature of hydrate formation. Similarly, as overseen in CO2 hydrate systems, the maximum TMAOH inhibition impact is observed at 1 wt% at 1 and 4°C. In the presence of 1 wt% TMAOH, the CH4 hydrate induction time is enhanced nearly 41.4 and 81.1% for 1 and 4°C, respectively, compared to the pure water sample. However, the presence of 0.5 wt% TMAOH ensued little induction time value at 4°C and however slightly enhanced CH4 hydrate nucleation observed at 1°C. Increasing the TMAOH concentration above 1 wt% reduced the induction time inhibition impact on CH4 hydrates (see Figure 6). Furthermore, the induction data of 1 wt% TMAOH at 1°C condition is further compared with PVP data in Figure 6(a). Similar to CO2 hydrate, PVP is able to enhance the induction time of CH4 more than TMAOH.
Additionally, obtained induction time results of 1 wt% TMAOH concentration of CH4 hydrates at 1°C are also compared with recently published imidazolium-based IL data of Nashed et al.  in the form of RIP and perceived in Figure 7. Moreover, RIP data of studied AILs could easily be compatible with the previous study  as shown in Figure 7. TMAOH possess better RIP values compared to most earlier studied imidazolium-based ILs reported by Nashed et al.  (see Figure 7).
The initial rates of CH4 hydrate formation with and without TMAOH mass concentrations (0.5, 1 and 2 wt%) at 1 and 4°C are illustrated in Figures 8(a) and (b), respectively. At 4°C condition, the formation rates of TMAOH found for concentration are driven; as the quantity of TMAOH increases, the rate of CH4 hydrate formation decreases. The similar behaviour was observed in CO2 hydrate systems as well (Figure 4(a)). However, unlike CO2 hydrates (see Figure 4), at 1°C the formation rates of CH4 hydrates are found in non-linear trend (see Figure 8(a)) which demonstrates that the inhibition impact of TMAOH is not only concentration dependent but also significantly dependent on the type of hydrate formerly present. The presence of low driving force (subcooling = 7°C) at 4°C enhanced the inhibition of the rate of hydrate formation than at 1°C, which holds the higher driving force (subcooling = 10°C) resulting in catastrophic crystal growth as discussed in the preceding study .
Furthermore, the formation rate of 1 wt% TMAOH is also compared with PVP data in Figure 8(a). The formation rate data revealed that TMAOH is able to reduce the formation rate moderately than PVP as also evident in the above results of CO2 hydrates in an earlier section (see Figure 4(a)).
Figure 9 presents the CH4 mole consumed into hydrate with and without TMAOH. The presence of TMAOH poorly inhibited the CH4 consumption into hydrate formation at both studied experimental temperatures. Especially at 1°C, all tested TMAOH concentrations significantly enhanced CH4 hydrate formation, thus showing hydrate promotional impact. However, the moles of CH4 consumed into the hydrate at 4°C are similar to the water sample; this may be due to the low driving force existing at that temperature (4°C). Therefore, the presence of TMAOH in CH4 hydrate can inhibit the hydrate induction time and formation rate but will cause hydrate plug upon formation with time due to its CH4 mole consumption promotion effect. Previous studies have shown that some conventional KHIs such as PVP have similar behaviour . They form large hydrate plugs upon hydrate formation (i.e. after hydrate nucleation).
3.3. Effect of TMAOH on the kinetics of binary mixed gas hydrates (50–50 mole% CO2 + CH4)
The kinetic formation of mixed gas 50–50 CO2 + CH4 hydrates in the presence of various quantities of aqueous TMAOH solutions is discussed in this section. The hydrate induction time values for the mix gas hydrate systems are perceived in Figure 10. Results revealed that as TMAOH concentration increases the mixed gas hydrate induction time is further delayed, thus showing a significant inhibition impact for the mixed gas hydrates.
At the lowest concentration (0.5 wt%), TMAOH delayed the base sample induction time by about 1.2 times at both 1 and 4°C, while at 2 wt% about 1.5 and 2.3 times at 1 and 4°C, respectively. Additionally, the KHI impact (induction time) of 1 wt% TMAOH aqueous solution of the mixed gas is also compared with PVP data in Figure 10(a). The comparisons of data are interestingly piercing that TMAOH can delay hydrate nucleation time further than commercial KHI inhibitor PVP. The potential of the improved delay time observed is perhaps due to the larger subcooling (11°C) conditions of studied mixed gas system compared to pure water systems (CO2 = 7.32°C; CH4 = 10°C). This observation also highlighted the limitation of PVP (commercial inhibitor) efficacy over harsh deep-water condition which loses its strength at higher subcooling conditions [38, 47].
Figure 11 refers the initial formation rate of 50–50 CO2 + CH4 mixed gas hydrates. Results revealed that initial rate of hydrate formation for mixed gas hydrates reduced with increasing TMAOH mass concentrations for both 1 and 4°C. The presence of 1 wt% TMAOH shows the optimal hydrate rate diminution. The hydrate formation rate is found to be reduced about five times at 1 wt% TMAOH for all subcooling compared with the mixed gas hydrate (pure water sample). Furthermore, the result of 1 wt% TMAOH is further compared with PVP in Figure 11(a) at a similar concentration. Like the pure gases hydrates, TMAOH can further reduce the initial formation rate compared to the PVP-based system.
In Figure 12, it is observed that the presence of TMAOH significantly reduces the amount of mixed gas consumption in both experimental (subcooling) conditions, unlike the case of pure CH4 hydrates. The inhibition of mixed gas consumed into hydrate is practically observed at all concentrations. However, the much significant inhibition impact is obtained at 2 wt% aqueous TMAOH solution. At 2 wt%, the amount of mixed gas consumed into hydrate is reduced about 1.7 times compared to the pure water system. This notable inhibition together with the THI impact  shows the potentials of applying TMAOH as a novel dual functional inhibitor for practical field operation, especially in high CO2 content reservoir productions of fluid transportation and procession.
A gas chromatography (GC) is used to explore the mixed gas composition after hydrates are entirely formed in all the binary mixed gas experiments. The data are used to calculate the amount of gas remaining in the gas phase after hydrate formation. However, the GC readings are also useful to understand the percentage of each gas entrapped in the hydrate phase in the presence and absence of TMAOH. The inhibition selectivity of TMAOH for the binary mixed gas composition may be suggested. The measured GC readings are tabularized in Table 2. The data revealed that CO2 compositions in the binary mixed gas are diminished after complete hydrate formation compared to original compositions (50–50% CO2 + CH4) for all experiments. Confirming that CO2 hydrates are more formed in mixed gas hydrates compared to CH4 hydrate, which is due to experimental pressure, provides more driving force to CO2 than CH4 hydrates as CO2 is more prone to hydrate formation compared to CH4 gas.
|Sample||Concentration (wt%)||Experimental temperature (°C)||CH4||CO2|
3.4. Kinetic mechanism of TMAOH
The kinetic inhibition influence of TMAOH observed for all studied gas systems in this work presents TMAOH as a potential KHI. TMAOH is speculated to demonstrate a kinetic inhibitory mechanisms effect via (1) coulombic forces of interactions and (2) hydrogen bonding affinity, which probably results in its kinetic hydrate inhibition influence. Firstly, TMAOH is an ionic liquid which can form hydrogen bonding with water molecules and thus disturb the hydrogen-bonded structure of water molecules owing to the presence of its anion and cation ions [4, 12, 15, 48]. TMAOH ionizes in aqueous solution as TMA+ cation and OH− anions which interacts with the dipoles of the aqueous molecules with strong coulombic forces in additions to its hydrogen bonding ability . This combined effect easily overcomes the hydrogen bond structure or the van der Waals forces which causes clustering of water molecules in hydrate formation nucleation and growth [3, 4, 12]. As the water structures are distorted, it promotes the hindrance/delay in hydrate nuclei clustering and gas consumption into hydrate. Additionally, TMAOH possesses four methyl cations [CH3+] which is the least hydrophobic alkyl radical among other alkyl groups, which makes tetramethylammonium [TMA+] cation relatively more hydrophilic [3, 50, 51, 52]. Similarly, hydroxyl [OH−] anion is well known for its strong hydrogen bonding affinity for water molecules [3, 4, 32]. Therefore, TMAOH could adsorb on the gas-water interface, and retarded hydrate nucleation and crystal growth (through hydrogen bonding by the amide group and anion) and sterically block the hydrate formation [15, 53, 54, 55, 56].
It is already proven from previous studies [1, 13, 35, 57, 58] that hydrate formation depends on the presence of a guest molecule. Therefore, the inhibition impact of TMAOH also differs according to the type of gas systems. For instance, CO2 hydrates showed lesser induction time in comparison with CH4 hydrates and 50–50 CO2-CH4 mix gas hydrates in this study. The overall better KHI performance of TMAOH is found with 50–50 CO2-CH4 mix gas hydrates for both experimental temperature conditions (1 and 4°C) for induction time, hydrate formation rate alongside moles of gas consumed. Additionally, although the kinetic inhibition is concentration dependent, as aqueous TMAOH concentration increases, the KHI performance enhances for most of the studied systems. However, for optimum performance evaluation, one% (1 wt%) TMAOH seems to be more efficient as not much differently observed (above results) between 1 and 2 wt% concentrations. It is also evident for all studied system that at higher experimental temperature condition 4°C TMAOH performs better in comparison to lower-temperature condition (1°C). It can be concluded that TMAOH successfully worked as a KHI inhibitor for all the studied system; earlier , TMAOH has already been proven as an efficient THI as well for CH4 and CO2 gas hydrates. Therefore, TMAOH possibly will work as dual-functional hydrate inhibitor and therefore could efficiently be contemplated for flow assurance strategies.
In the present work, ammonium-based ionic liquid, TMAOH, is tested as a potential kinetic inhibitor for pure CO2, CH4 and binary mixed (50–50) CO2 + CH4 hydrate systems at different mass concentrations (0.5, 1.0 and 2.0 wt%). The effect of subcooling was also investigated at two different experimental temperatures of 1 and 4°C at moderate pressures for all the studied TMAOH systems. Experimental results revealed that TMAOH kinetically inhibits hydrate formation by delaying hydrate nucleation through the enhancement of hydrate induction time and TMAOH also lessens the initial apparent rate of hydrate formation and accordingly decreased the mole consumptions for almost all studied systems. Influence of subcooling was also observed as the subcooling increased the hydrate formation due to the presence of excessive driving force. The effect of TMAOH concentration and the type of guest molecules under study are found to affect the TMAOH hydrate inhibition impact significantly. Furthermore, the experimental results of TMAOH were further compared with PVP at 1 wt% concentration at 1°C conditions. The induction time of PVP seems to be higher than TMAOH for both pure gases; however, for binary mixed gas, the induction time of TMAOH is found to be higher than PVP due to enhanced subcooling conditions. The initial formation rate and mole consumption are found to be enhanced with TMAOH compared to the commercial counterpart. The TMAOH inhibitory mechanisms are driven by their coulombic forces of interactions and hydrogen bonding affinity for water molecules in hydrate formation environment. Therefore, the findings of this study highlighted the kinetic impact of TMAOH, which should be beneficial for gas hydrate-based technological applications such as storage and gas transportation, flow assurance and energy productions.
PETRONAS Research Sdn Bhd (PRSB) financially supports this work under the Grant No. 053C1–024.
|LDHI||low-dosage hydrate inhibitor|
|KHI||kinetic hydrate inhibitors|
|RIP||relative inhibition performance|