Cuiping Tang,Deqing Liang*

Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences,Guangzhou 510640,China

Key Laboratory of Gas Hydrate,Chinese Academy of Sciences,Guangzhou 510640,China

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development,Guangzhou 510640,China

Guangzhou Center for Gas Hydrate Research,Chinese Academy of Sciences,Guangzhou 510640,China

Key words:Gas hydrate Inhibition Green inhibitors Kinetic inhibitors Thermodynamic inhibitors

ABSTRACT Natural gas hydrates easily form in pipelines,causing potential safety issues during oil and gas production and transportation.Injecting gas hydrate inhibitors is one of the most effective methods for preventing gas hydrate formation or aggregation.However,some thermodynamic hydrate inhibitors are toxic and harmful to the environment,whereas degradation of kinetic inhibitors is difficult.Therefore,environmentally friendly and easily biodegradable novel green inhibitors have been proposed and investigated.This paper provides a short but systematic review of the inhibitory performance of amino acids,antifreeze proteins,and ionic liquids.For different hydrate formation systems,the influences of the inhibitor type,structure,and concentration on the inhibitory effects are summarized.The mechanism of green inhibitors as kinetic inhibitors is also discussed.The progress described here will facilitate further developments of such green inhibitors for gas hydrate formation.

1.Introduction

Since Hammerschmidt first reported the formation of hydrates in gas pipelines in 1934,the problem of hydrate blockage of flow channels has been of major importance to the energy industry[1].Gas hydrates are crystalline solids in which water molecules act as host molecules to form hydrogen-bonded cage structures that are stabilized by various guest molecules[2].If the cages are filled with natural gas molecules,such as methane(CH4),ethane(C2H6),or propane(C3H8),these clathrates are generally called natural gas hydrates.Natural gas hydrates often form under specific pressures and temperatures(normally,low temperatures and moderate pressures).For example,CH4hydrates can form at 4°C and approximately 3.8 MPa[2].

There are three main gas hydrate crystallographic structures,namely,structure I(sI),structure II(sII),and structure H.sI hydrates often form with CH4or C2H6molecules,whereas sII hydrates mainly include C1-C4 hydrocarbons and structure H hydrates mainly include C1-C5 hydrocarbons as major components[3].However,the relationship between hydrate structure and guest size is not strict；for example,transitions of gas hydrates from sI to sII have been observed in a CH4+C2H6system [4].

The formation of gas hydrate blockages in pipelines may lead to large pressure drops or even cause critical safety issues.Therefore,the prevention of hydrate blockages is of importance in the oil and gas industry.Dehydration,insulation,and depressurization can be used to prevent gas hydrate formation[5,6].However,dehydration is impossible between the well and the dehydration units.Furthermore,in the deep sea,insulation is too difficult and expensive,and depressurization results in decreased transportation capability.Therefore,the most common method for avoiding gas hydrate blockages is the addition of chemicals that can inhibit gas hydrate formation or aggregation.There are two classes of hydrate inhibitors:thermodynamic hydrate inhibitors(THIs)and low-dosage hydrate inhibitors(LDHIs).

THIs shift hydrate-aqueous liquid-vapor equilibrium(HLVE)curves to lower temperatures and higher pressures,forcing the oil or gas system to stay in the hydrate-free region [2,7].Therefore,gas hydrates will not form if a sufficient amount of THI is added.The effectiveness of this approach is well known,but large concentrations,sometimes up to 50 wt%,are needed[5],necessitating additional storage capacity and recycling units.The most frequently used THIs are alcohols and salts,such as methanol,ethylene glycol,and NaCl.Some THIs can cause pipe corrosion as well as being toxic and harmful to humans and the environments.Owing to economic and environmental concerns,LDHIs have been developed to replace THIs.

LDHIs,which consist of antiagglomerants(AAs)and kinetic hydrate inhibitors(KHIs),do not change the phase equilibrium conditions of gas hydrate formation,but delay gas hydrate nucleation or retard growth,thus avoiding gas hydrate blockages for a period longer than the free water residence time in a pipeline.LDHIs are often used at concentrations of 0.1 wt%-1.0 wt%[8].

Owing to their amine and carboxylic acid groups,amino acids can also be used as THIs.Glycine,L-alanine,and L-valine changed the equilibrium conditions of CO2hydrate formation,causing a shift to lower temperatures and higher pressures,and the extent of inhibition increased as the amino acid concentration increased within the solubility limit.However,the inhibition abilities of glycine and L-alanine were slightly lower than those of methanol,and L-valine was as efficient as methanol at the same concentration.The extent of inhibition increased with increases in the size and hydrophobicity of the alkyl chain[35].Park et al.obtained the same result for glycine as a THI[36].The thermodynamic inhibitory effect of five amino acids(glycine,alanine,proline,serine,and arginine)on CO2hydrates was also investigated by Bavoh et al.[37].All of these amino acids exhibited inhibitory effects,with glycine being the best inhibitor.

The development of KHIs has been rapid.However,as the natural degradation of conventional KHIs such as PVP and PVCap is difficult,there is a need to develop environmentally friendly KHIs with enhanced biodegradability to alleviate potential environmental risks.Thus,green inhibitors have been developed as a new class of KHIs.Recently,amino acids,antifreeze proteins(AFPs),and ionic liquids (ILs)have been introduced as novel green inhibitors.In this review,the available published data on novel green hydrate inhibitors are summarized with the aim of facilitating future research in this area.

2.Amino Acids

Amino acids are biodegradable compounds composed of amine and carboxylic acid functional groups with a specific side chain.There are 20 different essential amino acids in nature.According to the chemical nature of their side chains,amino acids can be classified as hydrophobic,hydrophilic or charged[26].

Time passed so quickly for both of them that the youth (for now he was quite a young man, and no more a lad) forgot altogether how long he had been there

As amino acids are capable of forming hydrogen bonds with water molecules,amphiphilic,and environmentally friendly,they have the potential to be used as gas hydrate inhibitors.Table 1 gives the structures of some natural amino acids that have been investigated as gas hydrate inhibitors.The inhibition performance of amino acids as KHIs is often evaluated using CH4or natural gas,CO2,and tetrahydrofuran(THF)hydrates.

2.1.Amino acids as KHIs

The effects of KHIs can be evaluated using many methods.The most commonly used parameter is the induction time,which is defined as the time required for the hydrate volume or equivalently consumption of the hydrate former gas to be detectable[2].However,in some cases,the gas formation or consumption rate and the subcooling temperature have also been used because of the easy influenced nature of the induction time by experimental conditions.

2.1.1.Inhibition of CO2and THF hydrate formation

Hereupon he knew for a certainty that it was his dear wife and his dear child, and he kissed them, and was glad, and said, A heavy stone has fallen from off mine heart

Only a few studies have examined the use of amino acids to inhibit CO2and THF hydrate formation.Sa et al.[27]and Roosta et al.[30]reported different amino acids as KHIs for CO2hydrate formation.Sa et al.[27]identified glycine and L-alanine as more efficient than alanine,valine,and isoleucine.Roosta et al.[30]reported that at the same concentration,the inhibition ability decreased in the order of L-histidine ＞glycine ＞L-proline ≈L-serine ≈L-threonine ＞L-glutamine,and that the growth rate of hydrates decreased as the concentration increased from 0.5 wt% to 1.5 wt%.At a concentration of 1.5 wt%,the inhibitory effect of L-histidine was even better than that of PVP.

Table 1 The structures of amino acids

As inhibitors of THF hydrate formation,glycine performed better than L-leucine.However,when 7.5 vol%acetone was added,the inhibition performance of L-leucine was obviously enhanced.Acetone helped the amino acids to reduce the water activity,thus increasing their inhibition capability[31].

The young man did as he was bid, and when the lion was made the old woman hid the youth in it, and brought it to the king, who was so delighted with it that he wanted to buy it

2.1.2.Inhibition of natural gas hydrate formation

Here she resumed her own gigantic shape, tore up the trees by their roots, threw one of them over her back and clasped the other to her breast, carried them down to the shore and waded12 out with them to the ship

Table 2 shows the performance of various amino acids as KHIs for natural gas hydrate formation.For C2H6hydrate formation,hydrophilic L-histidine and L-glutamine had different effects on the nucleation stage.In contrast,hydrophobic glycine and L-serine showed inhibition ability in three hydrate systems for both the nucleation and growth stages[26].Rad et al.[33]also reported that glycine was a better KHI than L-leucine at various concentrations.Sa et al.[32]reported that glycine and L-serine delayed CH4hydrate nucleation and growth,whereas Lalanine and L-proline did not,similar to the behavior observed for natural gas hydrates.Using molecular dynamics (MD)simulations,Maddah et al.[29]investigated the inhibitory effect of four amino acids (glycine,alanine,proline,and serine)on CH4hydrate formation,and the simulation results were in agreement with available experimental data.These amino acids acted as inhibitors of CH4hydrates,with serine and glycine being more effective hydrate inhibitors than the other two.Overall,these studies indicate that glycine is the best inhibitor among investigated amino acids.

(1)Some amino acids have kinetic inhibition abilities,but their inhibitory effects are not very good.The strongest performance for an amino acid is similar to that of PVP,which is one of the weakest KHIs among conventionally used LDHIs.Therefore,enhancing the inhibitory effects is of importance for amino acid investigations.

But he felt very bitterly parting from the home where he had been born, and where he had at least passed a short but happy childhood, and sitting down on a hill he gazed once more fondly on his native place

Amino acids can be used as KHIs because of their structures,each of which contains a carboxylic acid,an amine,and a unique side chain group.The majority of amino acids have been found to behave as zwitterions.They can interact with water molecules through electrostatic interactions,and their carboxylic acid and amine groups can interact with water molecules through hydrogen bonds[27].

Sa et al.[27,28]proposed that the KHI abilities of some hydrophobic amino acids were attributable to perturbation of the structure of the surrounding water.They showed experimentally that the inhibitory ability of amino acids increased with decreasing hydrophobicity.This trend was similar to that for the extent of perturbation observed in Raman spectra.Therefore,they suggested that amino acids with hydrophilic side chains were likely to be effective in delaying hydrate nucleation and growth owing to disruptions of the water structure.

Using MD simulations,Maddah et al.[29]suggested that interactions and/or electrostatic interactions between the amino acid and the crystal surface inhibited hydrate growth,whereas hydrogen bonds between the amino acid and water disrupted the hydrate cages and prevented further growth of the hydrates.They suggested that the disruption of water networks by amino acids was the predominant mechanism,as also proposed by Sa et al.[28,34].X-ray diffraction data[34]showed that the hydrate crystal lattice expanded with the addition of amino acids,which may indicate that the amino acids were incorporated into part of the hydrate crystal lattice via the formation of hydrogen bonds rather than directly occupying the hydrate cages.Moreover,the solubility of amino acids in water was another effective factor in hydrate growth inhibition[35].

2.2.Amino acids as THIs

AAs are typically surfactants that prevent the formed hydrate particles from aggregating,thus maintaining the flow of multiphase petroleum fluids[9,10].In contrast,most KHIs are water-soluble polymers.Early KHIs in common use include polyvinyl lactams and their derivatives,such as polyvinylpyrrolidone(PVP),polyvinylcaprolactam(PVCap),and Gaffix VC-713[11-15].There are various hypotheses about the mechanism of KHIs,with adsorption-inhibition and perturbation-inhibition mechanisms being two representative examples.The adsorption-inhibition mechanism assumes that KHI molecules adsorb directly onto the hydrate surface and inhibit further crystal growth[12,16-21].Carver et al.[16]reported that PVP molecules could adsorb onto the surface of hydrates through the formation of hydrogen bonds between the pyrrolidone oxygen and the water surface,with the location of the adsorption sites depending on the availability of pendant hydrogens on the hydrate surface.The PVP chains were generally found to lie flat on the surface,thus disturbing hydrate growth.Subsequently,a new inhibition mechanism proposed that perturbation of the liquid water structure by KHIs is a key process in hydrate inhibition[22-25].It is thought the KHI molecules can perturb the water phase and prevent water molecules from completing the hydrate cage.Villano et al.[25]observed perturbation of the liquid water structure by amino acids using polarized Raman spectroscopy.They found that the inhibition performance was related to the extent of perturbation by amino acids.

As THIs,ILs shift the gas hydrate equilibrium conditions to lower temperatures and higher pressures.The formation conditions for CH4,CO2,and natural gas hydrates in the presence of numerous ILs have been investigated using various methods.The available experimental data on the inhibition performance of ILs are summarized in Tables 5-7.The inhibitory effects of more than 80 ILs have been evaluated,mostly for CH4hydrate formation.

Interestingly,because of their hydrophobic and hydrophilic properties,some amino acids acted as promoters.For example,L-histidine and L-glutamine promoted C2H6and CH4/C3H8hydrate growth[26].Furthermore,L-histidine at concentrations of 0.1 wt% and 1 wt% wasfound to enhance CH4hydrate growth significantly,just like sodium dodecyl sulfate[40].Tryptophan had a similar effect on CH4hydrate formation[41].

Table 2 effect of amino acid on natural gas hydrate formation

3.AFPs

AFPs are classes of proteins that can prevent ice crystal growth in organisms at subfreezing temperatures [42].Despite structural differences,they exhibit a common ability to adsorb onto microscopic ice crystals and delay their growth.AFPs are produced by certain fish,insects,plants,and microorganisms that survive at temperatures below 0°C.Because of their characteristics of ice growth inhibition,environmental friendliness,and biodegradation,AFPs have gained interest as potential gas hydrate formation inhibitors.Several AFPs have been shown to act as KHIs for the crystallization of clathrate hydrates.

3.1.Inhibition of CO2hydrate formation

CO2hydrate slurry is a coolant,but its continuous production is difficult to control because of the high rate of CO2hydrate formation.To lower the formation rate,Zhou and Ferreira[43]introduced type III AFPs as inhibitors in a coil heat exchanger.Under a low solution velocity(0.56 m·s-1)with 10 ppm type III AFP,the dissolution rate of CO2gas in the solution decreased significantly and the mass transportation rate of CO2from the bulk liquid to the liquid-crystal layer was also limited.

3.2.Inhibition of THF hydrate formation

(3)ILs have been developed rapidly and widely studied.They are effective both as KHIs and THIs.Furthermore,as synergists,they can increase inhibitory effects both thermodynamically and kinetically.Unlike the limited types and quantities of amino acids and AFPs,there are millions of potential ILs,making ILs a promising prospect for developing effective KHIs and THIs.

3.3.Inhibition of natural gas hydrate formation

Although there are few studies on AFP inhibition of CO2hydrates.Some differences have been observed between THF hydrates and natural gas hydrates.Therefore,the inhibitory effects of AFPs on CH4,C3H8,and natural gas hydrate formation are discussed.

3.3.1.Inhibition of CH4hydrate formation

The available data on CH4hydrate inhibition with AFPs are summarized in Table 3.

Table 3 Inhibition of AFPs on methane hydrate

Using a nuclear magnetic resonance microimaging method,Zeng et al.[48]evaluated the performance of wfAFP using the average conversion ratio.This ratio was only 23% after 64 h,whereas it was 50%after 42 h without wfAFP,indicating that wfAFP was effective for the inhibition of CH4hydrate formation.

Her brothers and sisters were all astonished at her appearance, as they had never expected to see her again, and there was no end to the questions they asked her

Servio and co-workers[49,53]also examined the influence of wfAFP on the gas consumption rate in a high-pressure stainless steel crystallizer.At the same concentration,wfAFP inhibited hydrate growth to the same extent as poly(VP/VC)at 277.15 K and 6500 kPa.Under a high driving force,dendrite hydrates formed without additives,but there were only minor or no dendrites with wfAFP or poly(VP/VC).

Perfeldt et al.[50]studied hydrate formation in NaCl solution with RmAFP in a rocking cell.During the nucleation stage,the inhibitory effect of RmAFP was weaker than that of PVP.However,during the hydrate growth stage,the results were quite different,with RmAFP promoting hydrate growth in the fresh solution.

Using a stainless steel hydrate equilibrium cell,Jensen et al.[51]found that hydrate formation was inhibited effectively by type III HPLC12 and tmAFP.At sufficiently high concentrations of AFPs,a nonlinear growth profile was initially observed,which then changed into a linear growth profile after a certain time,indicating that the AFPs initially available for adsorption on the hydrate surface were no longer present.

Udegbunam et al.[52]examined the inhibition performance of tmAFP,LpAFP,and Maxi.Hydrate film growth velocity data and hydrate morphologies revealed that TmAFP,LpAFP,and Maxi all showed inhibitory effects.

3.3.2.Inhibition of mixed gas hydrate formation

To mimic field conditions,a gas mixture(93%CH4,5%C2H6,and 2%C3H8)was used,sometimes with added saline solution(typically 3.5%NaCl)and liquid hydrocarbon(heptane),as a simple model for sea pipeline conditions.All of the AFPs listed in Table 4 showed inhibitory effects at the gas hydrate growth stage.

Table 4 Inhibition of AFPs on synthetic natural gas hydrate

Daraboina et al.[54]assessed the inhibitory effect of two fish AFPs(AFP-I and AFP-III)and a commercial KHI(H1W85281)using magnetic resonance microscopy and a stirred autoclave.Both the AFPs inhibited hydrate formation,but the inhibitory effect was weaker than that of H1W85281.AFP-I was considered a more effective nucleation and growth inhibitor than AFP-III.

Ohno et al.[55]evaluated LpAFP,AFP-III,and TmAFP at a concentration of 0.1 mmol·L-1.The solutions were dispersed on silica gel beads,which increased the area of the water-gas interfaces and thus facilitated gas hydrate nucleation and growth.AFP-III was superior to TmAFP in terms of nucleation inhibition,but both these AFPs showed similar growth inhibition activity.AFP-III,AFP-III/GFP,and TmAFP were more effective than LpAFP/GFP as sII hydrate growth inhibitors[55].

Sharifi et al.[56]reported the performance of wfAFP and AFP-III in saline solutions in a high-pressure crystallizer.Saline did not appear to influence the structures of the AFPs.The onset of hydrate nucleation was delayed 1.3 and 1.6 times in the presence of wfAFP and AFP-III,respectively,relative to control experiments.The presence of AFPs divided the hydrate growth process into two stages until catastrophic growth occurred.When a liquid hydrocarbon(n-heptane)was added to the above system[57],the induction time for the hydrate nucleation stage was reduced in the presence of WfAFP compared with that of the saline system without inhibitors,whereas n-heptane did not affect the induction time in the presence of AFP-III.After nucleation,both these AFPs inhibited natural gas hydrate growth.Thus,AFP-III still performed better than wfAFP in the presence of n-heptane[57].

Similar experiments were also conducted by Jensen et al.[58]in a high-pressure stirred cell to examine the effect of type III HPLC12 on the nucleation of sI CH4hydrates and sII synthetic natural gas hydrates(mole fraction:O20.24%,N23.68%,CH487.81%,C2H66.60%,C3H81.22%,n-C4H100.17%,i-C4H100.22%,n-C5H120.02%,i-C5H120.03%,and n-C6H140.01%).The addition of salt and heptane to the system increased the hydrate formation rate.HPLC12,which has been identified in ocean pout,was effective at delaying the nucleation of gas hydrates,showing slightly better inhibition performance than PVCap.

3.4.Inhibition mechanism of AFPs

AFPs have a unique structure consisting of a hydrophobic flat binding site that structurally matches the water molecules in ice.The adsorption-inhibition mechanism is predominantly believed to account for the suppression of ice growth by AFPs.First,the proteins adsorb onto the ice crystal surface.Further ice growth is then only possible in the regions between adsorbed proteins,where the ice is forced to grow with a curved interface.This interfacial curvature leads to a reduction in the local freezing temperature owing to the Kelvin effect,while leaving the melting temperature unchanged[59].However,the structure of hexagonal ice is different from the crystalline ice-like cages of water in hydrates.

wfAFP or CfAFP modified the THF clathrate hydrate crystal morphology from octahedral to plate-like,but proteins without antifreeze activity had not effect on the THF crystal morphology.But no antifreeze activity proteins had no impact on THF crystal morphology[45].Further,in the presence of LpAFP the morphology was hopper-like or skeletal[47].Thus,AFPs with inhibition abilities changed the THF crystal growth morphology,revealing that the AFPs adsorbed on the surface of the hydrates.Therefore,Zeng et al.[44]suggested that AFPs inhibited hydrate growth by the same mechanism as for ice growth.

Based on the similarity between the hydrate-like region of Maxi and the sII{100}planes,Sun et al.[60]proposed a model of Maxi binding to sII hydrates,which was supported by experiments[44].Type I AFP and Maxi have backbone amide groups and hydrophobic side chains,similar to the common functional motifs of most KHIs.Thus,they were directly adsorbed onto sII gas hydrates and the hydrophobic side chains of the AFPs occupied cavities on the hydrate surface in the same way as gas guest molecules[60].

Using MD simulations,Bagherzadeh et al.[61]proposed a similar scenario,in which wfAFP was bound to the hydrate surface by cooperative anchoring of the pendant methyl side chain to the half-cages of the hydrate framework.The side chain functional groups could also add an additional entropic barrier for growth of the hydrate lattice.

4.ILs

ILs are liquids composed entirely of ions that are fluid near or below 100°C.At least a million binary ILs and 1018ternary ILs are possible[62],which means that ILs can be selected and designed according to demand.ILs are not intrinsically“green”—some are extremely toxic—but they can be designed to be environmentally benign,with large potential benefits for sustainable chemistry[63],and they have no detectable vapor pressure.Owing to these advantages and their unique structures,ILs have been recently investigated as dual-function gas hydrate inhibitors.

Mickey Mantle7 drove the ball right over the center field wall, he said. Just a straight line climb in right out of the stadium. He looked out the window as if trying to pick the ball out of the cloud formations. I tried to imagine Mickey Mantle wearing an apron.

4.1.ILs as THIs

For CH4and natural gas hydrates,the presence of amino acids also shifted the formation conditions to lower temperatures and higher pressures.Although L-proline was a poor KHI,it was the best THI among proline,L-serine,L-alanine,and glycine [32]for CH4hydrate and mixed gas (93% CH4,5% C2H6,and 2% C3H8)hydrate formation.Bavoh et al.[38]examined the equilibrium of CH4hydrates in the presence of four amino acids(valine,threonine,asparagine,and phenylalanine)at concentrations of 1 wt% and 5 wt%.All of the amino acids inhibited CH4hydrate formation thermodynamically,with the best performance observed for valine.A study of the effects of another five amino acids(glycine,alanine,proline,serine,and arginine)by Bavoh et al.[39]on CH4hydrates revealed similar behavior,with glycine showing the highest inhibitory effect.

Table 5 list of ionic liquids used on methane hydrate and their concentration

4.1.1.Inhibition of CH4hydrate formation

Then the two Dreadful Sisters who were left wakened, and rose in the air like great birds; and though they could not see him because of his Cap of Darkness, they flew after him up the wind, following by the scent31 through the clouds, like hounds hunting in a wood

CH4hydrates have often been used to evaluate ILs.The inhibition of CH4hydrate formation has been investigated using 72 different ILs,as shown in Table 5.

4.1.1.1.Effect of cations.Based on 1-alkyl-3-methylimidazolium chloride,Chu et al.[66]investigated the effect of cation chain length on theinhibition performance of ILs.They found that the shorter the alkyl chain length was,the stronger the inhibitory effect was [66,67].However,Gupta et al.[74]reported that the alkyl chain length of alkylimidazolium-based ILs with the same anion did not play a significant role in hydrate inhibition.

Table 6 list of ionic liquids used on CO2hydrate and their concentration

Table 7 list of ionic liquids used on natural gas hydrate and their concentration

In Table 5,ILs with the same anion are listed together.The greatest amount of data is available for ILs with Cl-,BF4-,and Br-as anions with different cations.As an example,the data for ILs based on BF4-at a concentration of 10 wt%are illustrated in Fig.1.The inhibition ability of the ILs was divided into three groups,excluding methanol.The first group included ethylene glycol and[EMPip][BF4]；the second included[OH-EMIM][BF4],[BMIM][BF4],and[EMMor][BF4]；and[HEMM][BF4],which exhibited poor performance as a THI,belonged to the third group.The inhibitory abilities of all of these ILs were less than that of methanol.

Fig.1.Effect of cations of ILs with[BF4]-ion at the concentration of 10 wt%.

Interestingly,the structures of[HEMM]+and[EMMor]+are similar,expect that[HEMM]+has an extra hydroxyl group,which is helpful for forming hydrogen bonds with water.Li et al.[72]reported that hydroxyl-functionalization of the cation enhanced the inhibition effectiveness,but contrary to this trend,the inhibition ability of[HEMM]+was weaker than that of[EMMor]+.The performance of[EMPip][BF4]was similar to that of ethylene glycol at the same concentration,indicating that[EMPip][BF4]had a strong inhibitory effect.[EMPip]+lacks an oxygen atom in the cyclic group,but otherwise has the same structure as [EMMor]+.Thus,a comparison of the inhibitory effects of these cations suggests that the hydroxyl group of[EMMor]+may have been restricted by an intramolecular hydrogen bond with oxygen in the cyclic group,but this issue requires further investigation.However,as shown in Fig.1,[OH-EMIM][BF4]and [BMIM][BF4]exhibited similar behavior,suggesting that the hydroxyl group here did not have a significant effect.

Only say to your goat, Little goat, bleat3, Little table, appear, and a beautifully spread table will stand before you, with the most delicious food on it, so that you can eat as much as you want

4.1.1.2.Effect of anions.Fig.2 shows the influence of anions for ILs with the same cation([EMIM]+)on CH4hydrate formation,and the equilibrium data are compared with data for water,methanol,and ethylene glycol.All of these ILs acted as THIs,but the inhibitory effects were weaker than those of ethylene glycol and methanol.Under the examined experimental conditions,the thermodynamic inhibition effectiveness of these ILs decreased in the following order:[EMIM][Ac]≈[EMIM][Cl]＞[EMIM][NO3]＞[EMIM][SCN]≈[EMIM][ClO4]≈[EMIM][I]＞[EMIM][HSO4]＞[EMIM][EtSO4].However,the inhibition ability of [EMIM][HSO4]showed a distinct dependence on pressure.When pressure was less than 7 MPa,the performance of[EMIM][HSO4]was better than that of[EMIM][I].

Fig.2.Effect of anions of ILs with[EMIM]+cation at the concentration of 10 wt%.

Thermodynamic hydrate inhibition was caused by strong hydrogen bonding between the inhibitor and water molecules [77].Xiao et al.[67]confirmed experimentally that the thermodynamic inhibition effectiveness decreased in the same order as the hydrogen bonding ability of the anions,namely Cl-＞Br-＞I-＞.Thus,the performance of ILs with the same cations is relative to the ability to form hydrogen bonds or interact with water.Furthermore,similar to conventional THIs,as the concentration of ILs increased,the inhibitory effect increased[69,79,87].

4.1.2.Inhibition of CO2hydrate formation

I d pretend to be rugged3, so fast and so coolWhen actually lost like a blinded old fool.I m getting too old for this tiresome4 gameOf acting5 real hard with no sense of shame.

The influence of ILs on CO2hydrate formation is summarized in Table 6.Although not as much data is available as for CH4hydrates,the effects of cations and anions on CO2hydrate inhibition have been discussed by Shin et al.[93].They analyzed the inhibitory effects of three cations([HEMP]+,[BMIM]+,and[BMMIM]+)with Cl-andIt was suggested that the possibility of hydrogen bonding with water could contribute to thermodynamic inhibitory effects.However,when a methyl group was introduced to replace a hydrogen in the imidazolium ring,thus eliminating hydrogen bonding with water,there was little difference between the inhibition abilities of [BMMIM]+and [BMIM]+.They also compared the effects of various anionswith the same cation.The inhibitory effects increased as the anion size decreased or anion charge density increased,and this inhibitory tendency was related to the number of ions.

ILs are dual-function gas hydrate inhibitors,as they also can be used as KHIs.However,there is less experimental data on ILs as KHIs than as THIs.Most available data are for CH4hydrate formation inhibition,with only two studies on natural gas hydrates and two studies on CO2hydrates.Table 8 summarizes the results for ILs as KHIs of CH4gas hydrates.

Although natural gas is similar in composition to real gas,most IL investigations have focused on CH4hydrates.Table 7 lists the available data for natural gas hydrate formation in the presence of ILs.

Atilhan and co-workers[78,96]reported that the ILs[Ch][Cl],[Ch][Ntf2],and[Ch][OAc]showed a typical thermodynamic inhibitor behavior for the formation of QNG-S1 gas mixture hydrates at two concentrations(1 wt%and 5 wt%).Furthermore,[Ch][Cl]also inhibited QNG-S1+N2hydrate formation at these concentrations[96].Khan et al.[97]investigated the thermodynamic inhibition effect of four ILs([TMA][OH],[TEA][OH],[TPrA][OH],and[TBA][OH])in a CO2-rich mixed gas hydrate system.[TMA][OH]at concentrations of 1 wt%,5 wt%,and 10 wt%,as well as [TEA][OH],[TPrA][OH],and [TBA][OH]at a concentration of 10 wt%,all showed inhibitory effects.

4.2.ILs as KHIs

4.1.3.Inhibition of natural gas hydrate formation

But when the same thing happened the third time, the dragon lost his temper, and went down into the stable and took a whip and gave the horse a good beating

All of ILs shown in Table 8 exhibited inhibitory effects,which were evaluated using the induction time.In addition to the ILs in Table 8,Tariq et al.[89]also investigated the kinetic inhibition ability of TMAA,[Ch][But],[Ch][iB],[Ch][Hex],and[Ch][Oct].At 1 wt%,all of these ILs showed negligible delays or little effect,but at 5 wt%,they performed better,[Ch][Oct]exhibited the best inhibitory effect.Altamash et al.[88]reported inhibitory effects for[EA][Of]and[DMA][Of],which are alkylammonium formate-based protic ILs.It is meaningless to compare induction times obtained from different sources,as this parameter is closely related to specific experimental conditions.

For natural gas hydrate inhibition,Villano and Kelland[102]investigated the inhibitory performance of[EMIM][BF4]and[BMIM][BF4]on sII synthesized natural gas hydrates (mole fraction:CH480.67%,C2H610.20%,C3H84.90%,iso-C4H101.53%,n-C4H100.76%,N20.10%,and CO21.84%).They found that these ILs were poor KHIs but effective synergists under the examined experimental conditions.Lee et al.[103]reported six-based ILs as KHIs for synthetic natural gas hydrates (mole fraction:CH490.07%,C2H64.18%,C3H82.11%,iso-C4H100.82%,n-C4H100.65%,CO21.94%,and N20.23%).All of these ILs exhibited inhibitory effects,but they performed worse than PVCap.

Table 8 list of ionic liquids used on methane hydrate as KHI and their concentration

Shen et al.[104]and Lee et al.[105]investigated the performance of[Py14][Br]and[EMIM[[BF4],respectively,on CO2hydrate formation.Both these ILs showed inhibition abilities within a certain concentration range.

When the jar was filled, Dad would sit at the kitchen table and roll the coins before taking them to the bank. Taking the coins to the bank was always a big production. Stacked neatly5 in a small cardboard box, the coins were placed between Dad and me on the seat of his old truck. Each and every time, as we drove to the bank, Dad would look at me hopefully. Those coins are going to keep you out of the textile mill, son. You re going to do better than me. This old mill town s not going to hold you back. Also, each and every time, as he slid the box of rolled coins across the counter at the bank toward the cashier, he would grin proudly. These are for my son s college fund. He ll never work at the mill all his life like me.

There are a few studies on the inhibition mechanism of ILs.ILs have strong electrostatic charges and their cations and anions can be chosen to allow hydrogen bonding with water.As for conventional KHIs,the adsorption-inhibition mechanism has been hypothesized by many researchers[78,93,106].Based on the adsorption-inhibition mechanism,if water molecules are prevented from getting close to the crystal by bonding of the inhibitor to the crystal surface,Nasrollah et al.[106]suggested that the rate of ionic movement towards the gas hydrate crystal surface would be a key factor for controlling the inhibition ability of ILs.Using MD simulations,they found that cations moved faster than anions towards the crystal surface,so the cation type should have a greater effect on kinetic inhibition than the anion type.However,as this result seems to contradict the work of Shin et al.[93],which showed that the inhibitory effect was influenced mainly by the anion of the IL,this issue requires further investigation.

It should be noted that ILs have also been used as synergists to enhance the inhibitory effect,both thermodynamically and kinetically[69,77,100,102,103,107-109].Some ILs also exhibited a promotion effect for gas hydrate formation[76,94,101,110].Interestingly,when various ILs and PVCap were mixed,their inhibitory effect exceeded that of the ILs or PVCap alone.According to the inhibition mechanism,conventional polymer KHIs like PVCap inhibit hydrate formation by adsorption on hydrate particles or perturbation of the water phase to prevent water molecules from completing the hydrate cage.However,the structures of ILs are quite different from that of PVCap,and they retard hydrate nucleation and growth by interacting with water molecules after dispersion in aqueous solution.For CH4hydrate formation,Kang et al.[100]suggested that ILs approached water molecules or the interface between water and guest molecules,retarding hydrate structure formation,whereas polymers like PVCap delayed the transport of CH4from the bulk phase towards the clusters,which resulted in a good synergistic effect.For a CH4-abundant sII-forming gas mixture,Ohno et al.[4]reported that metastable CH4hydrates could form first and then be converted into sII hydrates.Thus,Lee et al.[103]proposed that PVCap prevents sII hydrate nucleation and/or growth by adsorption onto the hydrate surface,whereas ILs inhibit the nucleation of metastable sI hydrates.

5.Other Green Inhibitors

With the development of LDHIs and green inhibitors,more and more novel inhibitors proposed.Except amino acids,AFPs and ILs,some other new inhibitors were still investigated,such as starch,chitosan,pectin and glycoprotein.

Peter Englezos et al.[111]investigated the kinetic inhibiting effect of a number of cationic starches in hydrate formation experiments with methane and methane/ethane and methane/propane gas mixtures and pointed out tapioca starch delayed of onset of crystallization by an order of magnitude.Mohammad Reza Talaghat[112]reported modified starch could inhibit gas hydrate formation(including C1,C3,i-C4 and CO2)as a kinetic inhibitor and polyethylene oxide and polypropylene oxide enhanced its inhibition ability.Chitosans was found to be a good KHI and delay the onset of gas hydrate nucleation with methane and methane/ethane gas mixtures[5].Pectin was reported to inhibit methane hydrates formation and retard hydrate crystal growth and the induction time of methane hydrate nucleation was much longer with pectin than PVCap at the same concentration[113].A glycoprotein of porcine pancreatic lipase inhibited the formation of THF hydrate and its growth[114].

6.Conclusions and Suggestions

Novel gas hydrate inhibitors are environmentally friendly and easily degradable.Amino acids,AFPs,and ILs at low concentrations can be effective KHIs,whereas ILs at high concentrations can act as THIs.Thus,such inhibitors have the potential for wide applicability.According to currently available data,the following conclusions and suggestions can be made.

2.1.3.Inhibition mechanism

Between articles, he took breaks by watching the nearby traffic and pedestrians6. The coffee shop was next to a huge parking lot. The lot was for customers of a grocery store, movie rental7 store, pharmacy8, bank, and restaurant. Kevin considered his outdoor seat the perfect place for one of his favorite activities—people watching.

(2)As potential inhibitors,AFPs are sometimes as effective as commercial KHIs under specific conditions.However,as it is difficult to obtain AFPs,which limits further research.

THF hydrates can exist at atmospheric pressure,which is convenient for investigating hydrates without using elevated pressure conditions,especially for observing the morphology change of hydrates.Zeng et al.[44-46]investigated THF hydrate formation with two AFPs(wfAFP and CfAFP).Both these AFPs inhibited THF hydrate formation and were found to be even more effective than PVP.Both AFPs and PVP modified the hydrate morphology from octahedral to plate-like,and the addition of AFPs or PVP appeared to restrict the threedimensional growth to two-dimensions [46].LpAFP and type III AFPs also modified THF hydrate crystal morphologies,changing them from octahedral to hopper-like or skeletal,by adhering to the hydrate surface and inhibiting growth in a similar fashion to PVP,which is a KHI[47].

(4)There is a lack of effective software for predicting gas hydrate formation.Currently,hydrate formation under complicated conditions cannot be predicted well with or without inhibitors.Now the software are about thermodynamic condition or thermodynamic hydrate inhibitor,and seldom is about kinetic inhibitors.Therefore,it is necessary to develop more adaptable software.

Nomenclature

AA antiagglomerant

AFP antifreeze proteins

[BMIM]-[Ac]1-butyl-3-methylimidazolium acetate

[BMIM][BF4]1-butyl-3-methylimidazolium tetrafluoroborate

[BMIM][Br]1-butyl-3-methylimidazolium bromide

[BMIM][CF3SO3]1-butyl-3-methylimidazolium trifluoromethane sulfonate

[BMIM][Cl]1-butyl-3-methylimidazolium chloride

[BMIM][ClO4]1-butyl-3-methylimidazolium perchlorate

[BMIM][DCA]1-butyl-3-methylimidazolium Dicyanamide

[BMIM][I]1-butyl-3-methylimidazolium iodide

[BMIM][N(CN)2]1-butyl-3-methylimidazolium dicyanamide

[BMIM][PF6]1-butyl-3-methylimidazolium hexafluorophosphate

[BMMIM][BF4]1-butyl-2,3-dimethylimidazolium tetrafluoroborate

[BMIM][HSO4]1-butyl-3-methyl imidazolium sulphate

[BMIM][HSO4]1-butyl-3-methylimidazolium hydrogen sulfate

[BMIM][MeSO4]1-buthyl-3-methylimidazolium methyl sulfate

[BMP][BF4]N-butyl-N-methylpyrrolidinium tetrafluoroborate

[BMP][Br]1-butyl-1-methylpyrrolidinium bromide

[BMP][Cl]1-butyl-1-methylpyrrolidine chloride

[Bu3NH][HSO4]tri-butyl-ammonium sulfate

CfAFP recombinant AFP from the spruce budworm insect,choristoneura fumiferama

[Ch][But]choline butyrate

[Ch][Cl]choline chloride

[Ch][Hex]choline hexanoate

[Ch][iB]choline iso-butyrate

[Ch][Ntf2]choline bistriflamide

[Ch][OAc]choline acetate

[Ch][Oct]choline octanoate

[DMA][Of]dimethylammonium formate

[DMEA][Of]dimethylethylammonium formate

[DMIM][Cl]1-decyl-3-methylimidazolium chloride

[EA][Of]ethylammonium formate

[EMIM][Ac]1-ethyl-3-methylimidazolium acetate

[EMIM[[BF4]1-ethyl-3-methylimidazolium tetrafluoroborate

[EMIM][Br]1-ethyl-3-methylimidazolium bromide

[EMIM][CF3SO3]1-ethyl-3-methylimidazolium trifluoromethanesulfonate

[EMIM][Cl]1-ethyl-3-methyl-imidazolium chloride

[EMIM][ClO4]1-ethyl-3-methylimidazium perchlorate

[EMIM][EtSO4]1-ethyl-3-methylimidazolium ethyl sulfate

[EMIM][I]1-ethyl-3-methyl-imidazolium iodide

[EMIM][HSO4]1-ethyl-3-methylimidazolium hydrogen sulfate

[EMIM][N(CN)2]1-ethyl-3-methylimidazolium dicyanamide

[EMIM][NO3]]1-ethyl-3-methylimidazolium nitrate

[EMIM][SCN]1-ethyl-3-methylimidazolium thiocyanate

[EMMor][BF4]N-ethyl-N-methylmorpholinium tetrafluoroborate

[EMMor][Br]N-ethyl-N-methylmorpholinium bromide

[EMP][BF4]1-ethyl-1-methylpyrrolidine tetrafluoroborate

[EMP][Br]1-ethyl-1-methylpyrrolidine bromide

[EMP][Cl]1-ethyl-1-methylpyrrolidine chloride

[EMPip][BF4]N-ethyl-N-methylpiperidinium tetrafluoroborate

[EMPip][Br]N-ethyl-N-methylpiperidinium bromide

[Et2NH2][HSO4]di-ethyl-ammonium sulfate

GFP green fluorescent protein

[HEMM][Cl]1-hydroxyethyl-1-methylmorpholinium chloride

[HEMP][Cl]1-hydroxyethyl-1-methylpyrrolidinium chloride

[HEMM][BF4]N-(2-hydroxyethyl)-N-methylmorpholinium tetrafluoroborate

[HEMP][BF4]1-hydroxyethyl-1-methylpyrrolidinium tetrafluoroborate

HLVE hydrate-aqueous liquid-vapor equilibrium

[HMIM][Br]1-hexyl-3-methyl imidazolium bromide

[HMIM][Cl]1-hexyl-3-methylimidazolium chloride

[HMIM][I]1-hexyl-3-methylimidazolium iodide

[H-MIM][HSO4]1-methylimidazolium hydrogen sulfate

IL ionic liquid

KHI kinetic hydrate inhibitor

LDHI low dosage hydrate inhibitor

LpAFP cloned AFP from the perennial grass

Maxi. a fish protein

MD molecular dynamic

[MMIM][I]1,3-dimethyl-imidazolium iodide

[MOIM][Cl]1-methyl-3-octylimidazolium chloride

[N1,1,1,1][Cl]tetramethyl-ammonium chloride

[N1,1,1,eOH][Cl)hydroxyethyl-trimethyl-ammonium chloride

[N2,2,2,2][Cl]tetraethyl-ammonium chloride

name symbol

[OH-EMIM][BF4]1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate

[OH-EMIM][Br]1-(2-hydroxyethyl)3-methylimidazolium bromide

[OHEMIM][ClO4]1-hydroxyethyl-3-methylimidazolium perchlorate

[OH-EMIM][Cl]1-hydroxylethyl-3-methyl-imidazolium chloride

[OH-EMMIM][Cl]1-hydroxyethyl-2,3-dimethylimidazolium chloride

[OMIM][Cl]1-octyl-3-methyl imidazolium chloride

[PMIM][I]1-propyl-3-methylimidazolium iodide

[Pr3NH][HSO4]tri-propyl-ammonium sulfate

PVCap polyvinylcaprolactam

PVP polyvinylpyrrolidone

P(VP/VC)N-vinylpryrrolidone-co-N-vinylcaprolactam

RmAFP recombinant antifreeze protein from the bark beetle R.mordax

[TBA][OH]tetrabutylammonium hydroxide

[TBMA][Of]tributylmethylammonium formate

[TEA][Cl]tetraethyl ammonium chloride

[TEA][I]tetraethylammonium iodide

[TEA][OH]tetraethylammonium hydroxide

THI thermodynamic hydrate inhibitor

THF tetrahydrofuran

[TMA][OH]tetramethylammonium hydroxide

[TMA][Cl]tetramethylammonium chloride

[TPrA][OH]tetrapropylammonium hydroxide

Type III AFP AFP from ocean pout fish

Type III HPLC12 AFP originally identified in ocean pout

tmAFP AFP from a worm,Tenebrio molitor

WfAFP Type I AFP from winter flounder

[3C4C1P][MeSO4]tributylmethylphosphonium methylsulfate

Acknowledgment

This work was supported by National Key Research and Development Plan of China(2017YFC0307306),the National Natural Science Foundation of China(51876211).