Sheshan Bhimrao Meshram,Omkar S.Kushwaha,Palle Ravinder Reddy,Gaurav Bhattacharjee,Rajnish Kumar*

Department of Chemical Engineering,Indian Institute of Technology,Chennai,Tamil Nadu 600036,India

Keywords:Flow assurance Gas hydrates kinetics Hydrate promoters Hydrate inhibitors Inhibition mechanism Gas uptake measurement Induction time

ABSTRACT Gas hydrate reserves are potential source of clean energy having low molecular weight hydrocarbons trapped in water cages.In this work,we report how organic compounds of different chain lengths and hydrophilicities when used in small concentration may modify hydrate growth and either act as hydrate inhibitors or promoters.Hydrate promoters foster the hydrate growth kinetics and are used in novel applications such as methane storage as solidified natural gas,desalination of sea water and gas separation.On the other hand,gas hydrate inhibitors are used in oil and gas pipelines to alter the rate at which gas hydrate nucleates and grows.Inhibitors such as methanol and ethanol which form strong hydrogen bond with water have been traditionally used as hydrate inhibitors.However,due to relatively high volatility a significant portion of these inhibitors ends up in gas stream and brings further complexity to the safe transportation of natural gas.In this study,organic additives such as oxalic acid,succinic acid and L-aspartic acid(all three)having--COOH group(s)with aspartic acid having an additional--NH2group,are investigated for gas hydrate promotion/inhibition behavior.These compounds are polar in nature and thus have significant solubility in liquid water；the presence of weak acidic and water loving(carboxylic/amine groups)moieties makes these organic acids an excellent candidate for further study.This study would pave ways to identify a novel(read better)promoter/inhibitor for gas hydrate formation.Suitable thermodynamic conditions were generated in a stirred tank reactor coupled with cooling system；comparison of gas hydrate formation kinetics with and without additives were carried out to identify the effect of these acids on the formation and growth of hydrates.The possible mechanisms by which these additives inhibit or promote the hydrate growth are also discussed.

1.Introduction

Though natural gas hydrates under the sea bed have immense energy potential,gas hydrate prevention in oil and gas pipelines is one of the major industrial challenges.The plugging problem due to gas hydrate formation was identified in 1930s,the solid compounds initially referred as“pipeline snow”was identified as hydrates formed from methane,ethane,propane and isobutane along with water at appropriate temperature and pressure conditions[1].Gas hydrate is typically defined as non-stoichiometric crystalline solids having trapped gas molecule(s)in the cages formed by the water molecules.The trapped gases are termed as guest molecules and the water molecules forming the cage-like structures as host molecules[2].The mechanical strength of gas hydrates was identified to be 20 times more than that of ice which makes plugging problem severe[3].The increasing energy demand has led to drilling deeper wells resulting in higher water cuts and increased transmission cost.Thus,problems associated with hydrate formation have gained more attention from both researchers and industries.The problems faced due to gas hydrate formation can be broadly classified into three categories (a)in drilling operations,(b)in transportation ships,and(c)in the transmission lines.In a case when natural gas is encountered during deep water drilling in shallow sediments,the gas can interact with drilling fluids leading to plugging of pipe[4].A plug formed in pipeline results into two zones namely high-pressure zone between well or gas-rich zone and the plug,and low-pressure zone is developed above the plug.This leads to dangerous situation wherein the plug behaves as projectile when the pressure difference between two zones becomes very high [5].The other evident problems are caused due to plugging of Choke and Kill line,plugging at or below the blowout preventer(BOP),around the drillstring in the riser,between the drillstring and BOPs or in the ram cavity of the BOPs respectively[6].Some of the problems encountered during drilling operations due to hydrate formation are represented in Fig.1.Further,trade and industry losses can incur by the hydrate formation and have potential to become menace to ships going through ocean or hydrate formation can take place in compressed natural gas stored in the ships[7].Temperature and pressure in transmission lines can cross the equilibrium temperature and pressure leading to hydrate formation,thus plugging the pipelines[8].Plugging in the transmission line often leads to intermittent stoppages in production,hampering the economics of the industries [9,10].It is estimated that prevention of gas hydrates in the transmission line can cause an expense of 500000000 USD to an industry annually and for offshore activities can cause an expense of 1000000 USD per mile for insulation in subsea pipeline conditions[11].Hence,gas hydrate inhibition is important；and the study of the effect of different inhibitors on the formation kinetics of gas hydrates can be instrumental for providing flow assurance solution to the industries.There are three major chemical methods:(a)thermodynamic inhibition,(b)kinetic inhibition,and(c)anti-agglomeration of hydrate[12-14].

Fig.1.Schematic representation of hydrate plugging problems encountered during drilling operations.

The thermodynamic inhibitors(THIs)affect the chemical potential thereby moving equilibrium dissociation curve to less favorable region for hydrate formation.Depending on the water cut and prevailing thermodynamic conditions THIs are used at higher concentration ranging from 10 wt% to 60 wt%.Commonly used thermodynamic inhibitors are methanol,ethanol,mono-,di-,tri-ethylene glycols etc.Apart from high cost associated with the large amount of THI required for hydrate inhibition,problems like recovery cost from wastewater,large storage and injection units,loss of inhibitor(in gas streams)during operations and pollution of hydrocarbon fraction have led to exploration of new class of inhibitors[15,16].

The Kinetic Inhibitors (KHIs)are the additives that reduce the kinetics of hydrate formation mainly by reducing the rate of growth of hydrate,at times it may also delay hydrate nucleation.Thus,the presence of KHI in the pipeline helps transportation of fluids without plugging due to hydrate formation and growth[17].KHIs are employed at lower concentration ranging from 0.1 wt%-1 wt%of active components.Poly(vinylpyrrolidone)(PVP)and poly(vinylcaprolactam)(PVCap)are some common examples of KHI[18].KHIs are water soluble or water dispersible polymers[16].

Anti-agglomerators(AAs)are the new class of inhibitors which are added at lower concentrations which act as dispersants.AAs are not supposed to inhibit hydrate nucleation；rather they prevent hydrate growth and agglomeration of the hydrate crystals thus preventing the plugging of pipeline.The presence of AAs results in transportable slurry of hydrates in pipelines[18].Quaternary Ammonium Bromide(QAB)is one of the new generations of AAs[19].Due to the usage of comparatively lesser amount of inhibitors compared to THIs,KHIs and AAs are also called Low Dosage Hydrate Inhibitors(LDHIs)[18].

Gas hydrate inhibition is mainly concerned with the petroleum industries；however gas hydrate promotion also has diverse potential novel applications in desalination,gas storages,gas separations,gas transport,and gas separations[20-24].In desalination process,the hydrates are formed from saline water which on dissociation results into pure water,and the gas regenerated is recycled back into the system[25,26].The current gas storage costs are very high due to the requirement of maintaining very low temperatures below the freezing point of water,but the gas hydrate technology has huge potential in order to cut the requirement of very low temperatures and huge high pressure containers [27-29].Due to the difference in the gas hydrate forming conditions between gases of different molecular sizes,the gas hydrate technology has immense potential for the separation of gases from their mixtures such as separation of H2from CO2/H2mixture,recovery of CH4from shale gas and coal mine gas,etc.[30-35].Therefore,investigations pertaining to gas hydrate promotion become pivotal.

The present work delves into natural gas hydrate formation kinetics in the presence of ethanedioic acid (oxalic acid),butane dioic acid(succinic acid)and 2-aminobutanedioic acid(L-aspartic acid)having two carboxyl functional group each,which can take part in the hydrogen bonding.Aspartic acid has additional amino group which can also take part in the hydrogen bonding using lone pair of electron present on nitrogen atom.The experiments are carried out at two concentrations(0.1 mol·L-1and 0.5 mol·L-1)to analyze the effect of variation of concentration for extensive study.Here,molar concentration is given priority over weight percent in order to ensure that equal number of molecules of additive are present in the solution at given concentration.The obtained results are compared with pure water gas hydrate system for their kinetic performance.All the experiments were performed at 273.15 K and 3.0 MPa enabling the comparison of kinetics.

2.Experimental

2.1.Materials and apparatus

High purity chemicals(purity 99%)were obtained from SRL Pvt.Ltd.,India.All the chemicals were used without any additional treatment.FTIR Spectra analysis was done for chemicals to check the presence of the different functional groups and the purity.Double distilled water was used in all the experiments.Synthetic Natural Gas (SNG)was obtained from Indo Gas Agency,India having components such as methane,ethane,propane,etc.Detailed composition of natural gas is mentioned in Table 1.

The apparatus is comprised of a jacketed 140 cm3SS-316 reactor having two transparent polycarbonate windows placed to opposing sides for observing the hydrate growth.The temperature in the reactor is controlled by external refrigeration unit(Siskin Profichill)which continuously circulates solution of ethylene glycol and water in the jacket of the reactor.The reactor is placed on the magnetic stirrer(Remi 1ML)which induces rotation in the magnetic pellet(3 cm in length)which is placed inside the reactor.The temperature and pressure in the reactor are monitored using a pressure transducer(Wika,range 0-16 MPa)and thermocouple(RTD-Pt)with an accuracy of±0.1 K.Data acquisition system(PII)is used to display and store the temperature and pressure readings.The location of valves,vent,and data acquisition system is shown in the Fig.2.

2.2.Procedure for hydrate formation experiments

The following procedure was used to form hydrates,assess induction time and kinetics.

I.The reactor was thoroughly cleaned with mild soap solution followed by acetone and distilled water.

II.Aqueous solution was made by dissolving the required weight of additive in distilled water.The aqueous mixture was stirred for sufficient time to get clear solution.For example,to prepare 0.1 mol·L-1of Oxalic Acid,0.45 g of Oxalic Acid was dissolved in 50 ml of double distilled water.Composition of aqueous solutions is shown in the Table 2.

III.50 ml of aqueous solution along with magnetic pellet was loaded in the reactor after drying it completely.

IV.Reactor was sealed tightly and was allowed to cool down and attain desired temperature of 273.15 K by continuously circulating coolant(ethylene glycol+water)through the jacket around the reactor.

V.After attaining the required temperature,the reactor was subjected to flushing by quick pressurization and depressurization cycles of SNG.

Table 1 Composition of the Synthetic Natural Gas(SNG)

Fig.2.Schematic diagram of the setup used for natural gas hydrate formation experiments.Where,SNG is Synthetic Natural Gas Cylinder；S is Magnetic Stirrer；W is Polycarbonate Window；R is Reactor；P is Pressure Transducer；T is Thermocouple,CU is Chiller Unit circulating water and ethylene glycol mixture and DAQ is Data Acquisition System.

VI.Reactor was pressurized to pre-determined experimental pressure of 3 MPa with SNG which is greater than equilibrium pressure 0.685 MPa at 273.15 K,thus providing driving force(ΔP)of 2.315 MPa for hydrate formation.

VII.Once the change in temperature due to pressurization was eliminated due to temperature-controlled water bath,the magnetic stirrer was switched to“ON”position,imparting agitation to solution inside the reactor.The rpm of magnetic stirrer was set at 350 r·min-1which was found to be optimal from the trial experiments with water.

VIII.Constant temperature and pressure were taken as indication for end of hydrate formation.

2.3.Calculation of amount of gas consumed in hydrate formation experiments

Induction time is the time taken for first solid hydrate crystal to nucleate in a gas-liquid system.After hydrate nucleation,quick hydrate formation takes place showing a sharp drop in reactor pressure.We have taken induction time as the time at which sudden pressure drop is observed along with the corresponding increase in the temperature due to the start of hydrate formation.The amount of gas present in the liquid phase at any time“t”is calculated by finding the difference between initial number of moles present in the gaseous phase and the number of moles present in the gaseous phase at time“t”.The formula stated below can be used to find the same[36].

where,

Vr=Volume of gaseous phase in the reactor,

P=Pressure in the reactor,

T=Temperature in the reactor,R=Universal Gas Constant and.

z=compressibility factor which is calculated using Pitzer's correlation[37].

The rate of hydrate formation and water to hydrate conversion was calculated by forward difference formula as reported by Gaurav Bhattacharjee et al.[36]:

Table 2 Structural and experimental details of additives

where,hydration number is taken as 5.67 for structure II hydrate.

According the P.Englezos et al.the rate equation for gas hydrate is given by

where,n is the number of moles of gas consumed in the hydrate phase,t is elapsed time,a is the interfacial area for the hydrate formation,K*is the overall kinetic constant,fgis the fugacity of gas phase,feqis the fugacity at three-phase equilibrium,klis liquid side mass transfer resistance and kfis crystal growth constant[38].

On following the method given by Jebraeel Gholinezhad et al.which is true for systems having vigorous stirring such that rate in only governed by kf,we get following equation[39].

These equations are used to calculate akf(intrinsic rate constant)which is used to compare the intrinsic rate of hydrate formation for different scenarios.

3.Results and Discussion

The experimental observations of gas hydrate formation under stirred conditions of synthetic natural gas are shown in Fig.S2(see the supporting information)and their gas uptake curves are shown in Fig.3.Usually,gas hydrate profile can be distinguished into three stages namely,gas dissolution (it enriches the liquid with gas which is followed with a sharp decline in pressure immediately),hydrate nucleation (a highly dynamic phenomenon,exothermic in nature which is followed by a substantial shift in temperature profile)and hydrate growth(capture of guest molecules in host cages which is reflected by a significant drop in pressure).Fig.3 demonstrates gas uptake behavior of low molecular weight additives containing two carboxylic groups and their comparison with the water system.Three low concentration(0.1 mol·L-1)systems viz.；0.1 mol·L-1oxalic acid,0.1 mol·L-1succinic acid and 0.1 mol·L-1aspartic acid along with pure water show comparative hydrate growth and inhibition activity(figure represents an average of three repeated experiments with an error bar)in Fig.3.Similarly,a comparative hydrate growth is revealed for the same systems at higher concentration of 0.5 mol·L-1viz.；0.5 mol·L-1oxalic acid,0.5 mol·L-1succinic acid and 0.5 mol·L-1aspartic acid along with pure water system.Time zero in the graphs corresponds to the commencement of decrease in the pressure,after sudden hydrate nucleation(Fig.S2,see the supporting information).The temperature and initial pressure for these experiments were fixed at 273.15 K and 3.0 MPa respectively.It is evident that oxalic acid behaves as NGH inhibitor,whereas succinic acid and aspartic acid(zwitterionic in nature)behave as NGH promoter Fig.3 and Table 3.

Fig.3.Comparison of gas uptake obtained for Synthetic Natural Gas (SNG)in the presence of different additives used at two concentrations(0.1 mol·L-1&0.5 mol·L-1)for constant volume system.

Table 3 Induction time,water to hydrate conversion and gas uptake of different system

It is observed from the gas uptake curves that oxalic acid(0.5 mol·L-1)is an effective gas hydrate inhibitor as compared to other systems under study.Succinic acid behaves as a gas hydrate promotor but with a slight activity at both the concentrations when compared to oxalic acid.On the other hand,aspartic acid behaves as strong promotor for hydrate growth.Furthermore,the hydrate growth by oxalic acid and succinic acid shows no significant dependence on concentration range of 0.1 mol·L-1to 0.5 mol·L-1.In the case of aspartic acid,initial rate of hydrate growth was again independent of concentration,however at higher concentration(0.5 mol·L-1)hydrate growth slows down compared to the one obtained at lower concentration(0.1 mol·L-1).After 15 min of nucleation,the efficacy of the hydrate promotor aspartic acid was found to be nearly 3 times higher as compared with the other systems,whereas oxalic acid and succinic acid showed nearly same(slower)hydrate growth than pure water system.The order of gas uptake for gas hydrate formation on the basis of investigation follows:oxalic acid ＜pure water ＜succinic acid ＜aspartic acid.The difference in the behavior of the additives can be explained on the basis of their chemical structures and functional group activity.

The induction time was also determined for natural gas hydrate formation in the presence of oxalic acid,succinic acid and aspartic acid.Fig.4 and Table 3 show the average induction time values at different concentrations of additives.It is observed that,oxalic acid leads to increase in the average induction time in comparison with pure water.The average induction time of hydrate formation in the presence of oxalic acid(0.1-0.5 mol·L-1)is almost 1.5-1.7 times greater as compared with hydrate formation with pure water.In the case of succinic acid,reduction in the average induction time of hydrate formation(0.1-0.5 mol·L-1)is around 1.7-2.0 times greater when compared with hydrate formation with pure water.On the other hand,nucleation of hydrate stimulated by aspartic acid showed a decrease in the average induction time by the factor of 3.On the basis of the average induction times of acids,the decreasing order of induction time:oxalic acid＞pure water ＞succinic acid ＞aspartic acid.

Fig.4.Comparison of the average induction time with standard deviation for the different systems studied.

In order to understand the gas hydrate kinetics,we have investigated the hydrate growth with respect to time by fitting data obtained by the gas uptake calculations for both the initial(0-0.2 h)and during(2.0-2.5 h)time interval and is represented in Fig.5a,b and Table 4.Fig.5a shows the initial growth kinetic curves having high rates as compared to the second one depicting comparatively slower rate of hydrate growth.From linear curve fitting slopes for initial period,oxalic acid is found to have the slowest growth kinetics at 0.1 mol·L-1with slope value of 0.0146 and can be used as hydrate inhibitor whereas the aspartic acid was found to have the highest hydrate kinetics at 0.1 mol·L-1with slope of 0.0643 and is suggested for applications where hydrate formation at higher rates is desirable.The results obtained from the linear curve fitting slopes are in accordance with the intrinsic growth constant(akf)calculated for the same time interval.The intrinsic rate constant has the lowest value of 1.43 × 10-9mol2·s-1·J-1for oxalic acid at 0.1 mol·L-1and highest value of 6.07×10-9mol2·s-1·J-1for aspartic acid at 0.1 mol·L-1as shown in Table 4.

Fig.5.a.Comparison of initial gas hydrate formation kinetics (0-0.2 h)along with linear fit for the different systems studied.b:Comparison of gas hydrate formation kinetics(2-2.5 h)along with linear fit for the different systems studied.

Table 4 Intrinsic growth constants and fitting parameters for straight line equation(y=a+b×x)for Fig.5a and b

After considerably fast kinetics of gas hydrate formation initially,the kinetics of gas hydrate formation decreased significantly as shown in Fig.5b.The rates of hydrate formation are observed to be extremely slow with the least slope value of 0.0016 in the case of oxalic acid having 0.1 mol·L-1concentration.It can be concluded from the linear fitting curves that gas hydrate kinetics is fastest initially till 0.20 h after nucleation and subsequently slowed down with the progress of the reaction.The significant drop in the growth kinetics is observed after 0.5 h,which led to almost stagnant growth from 2.0 h and can be seen as almost flat lines in the Fig.5b.The intrinsic rate constant has lowest value of 7.00×10-11mol2·s-1·J-1for aspartic acid at 0.5 mol·L-1and highest value of 2.317× 10-10mol2·s-1·J-1for oxalic acid at 0.5 mol·L-1.The reversal in the behavior of additives can be attributed to higher gas consumption at initial time period for aspartic acid which led to decrease in pressure(driving force)for hydrate formation in the latter periods.In oxalic acid,since gas consumption was low during initial periods,the hydrate growth sustained for latter periods too.

The effect of additives on the gas hydrate kinetics can also be studied on the basis of gas to hydrate conversion calculations.These studies have many implications on the storage capacity,commercial viability,morphology and rheological behavior of the gas hydrates.While experiments with higher gas uptake rate result in large solid gas hydrate formation with almost no water,thus may end as hydrates blockage.The water to hydrate conversions are shown in Fig.6,where it can be observed that oxalic acid has the lowest conversion values whereas aspartic acid shows the highest water to hydrate conversion which aligns with kinetics as shown in Fig.3.In the case of aspartic acid(0.5 mol·L-1),a decrease in the water to hydrate conversion is observed which was mainly due to the very fast initial hydrate growth resulting in complete coverage of interface by solid hydrate formation.

Fig.6.Comparison water to hydrate conversion for different systems at 2.5 h.

One of the widely accepted mechanisms for inhibitors is that they perturb regular water patterns or the structures.At macro level some of these inhibitors function by slowing down the heat as well as mass transfer rates under gas hydrate formation conditions.Additionally,inhibitors may also reduce the gas dissolution in the bulk water,thus affecting the nucleation/growth/formation of gas hydrates due to poor availability of the gaseous host molecules [40-43].Based upon this hypothesis,we selected two dicarboxylic acids namely oxalic acid and succinic acid which are in dimer form in the solid-state condition but can form network with the water molecules when dissolved in aqueous solvents.The third molecule,aspartic acid has similar structure to that of succinic acid but has an additional amine group which is geminal to one of the carboxylic acids of the succinic acid.

As shown in Fig.7,the FT-IR spectrum of the three acids representing the broad signature peak of the carboxylic acid group (--COOH)typically ranges from 2500 to 3600 cm-1with strong absorption due to presence of two carboxylic groups in each.All the systems show strong C=O stretching frequency near 1700 cm-1.In addition,succinic and aspartic acid show stretching frequency of C--H groups in the range of 2850-2950 cm-1.Furthermore,in aspartic acid,there is an additional amine(--NH2)group with N--H bending vibrations near 1580 cm-1.Aspartic acid is one of the zwitterionic amino acids having wide applications in biological systems.The broad signature peak of the carboxylic acid groups in the range of 2500-3600 cm-1and carbonyl groups--C=O；1680-1780 in the spectrum of the aqueous solutions of the acids can be corroborated with their ability to form strong hydrogen bonds with the water molecules.The observations are in accordance with the Panuszko et al.[44]and Max et al.[45].The maximum transmittance in the acidic region of the FTIR spectrum of the aspartic acid is indicative of long-range ordering of the water molecules surrounding the polar groups,thus maximum kinetics of hydrate growth is observed among three additives.

The water molecules in bulk are coordinated by hydrogen bonds,thus form a long range networked dynamic pattern having liquid phase.Under hydrate forming conditions in pure water systems,these water molecules interact randomly with the gas molecules.The frequent gas water interaction would result in enhanced gas hydrate formation kinetics,whereas the reduced interactions result in the inhibition of the same.We have observed different kinetic behaviors of the additives whose mechanistic details are provided individually.

Fig.7.FT-IR spectrum of oxalic acid,succinic acid,aspartic acid and their aqueous solutions(0.1 mol·L-1)in transmission mode.The carboxylic acid and carbonyl groups can be distinguished.The broad signature peak of the carboxylic acid group (--COOH；2500-3600 cm-1)and carbonyl group (--C=O；1680-1780 cm-1)are visible in the FT-IR spectrum of the aqueous solutions of the acids.

In the case of oxalic acid systems,its inhibitory action can be associated with the fact that oxalic acid contains two acidic groups which are highly polar(can form di-anion upon complete ionization)and have the ability to engage large number of the water molecules of the bulk water via formation of strong hydrogen bonds.Fig.8 shows the molecular modeling and mechanism of the gas hydrate inhibition and formation by the interaction of additives and water.Furthermore,due to the increased polarity by the oxalic acid in the water system,this can further affect(decrease)the dissolution of natural gas in the water,which can be observed from the limited data available in Table 3.Therefore,due to the high-water coordination ability,oxalic acid acts as an inhibitor by slowing down the rate of hydrate growth than the pure water system.

In the case of succinic acid,there are two--COOH functional groups similar to that of oxalic acid but,it behaves as a hydrate promotor.This reversal in the activity of the succinic acid can be explained by the fact that it also has two additional--CH2units which are hydrophobic in nature and thus enhances the gas dissolution in water,thus boost the availability of the gas in the vicinity of succinic acid molecules.Further,due to the presence of the two additional--CH2units,there are greater chances of formation of seven membered intramolecular hydrogen bonded rings shown in Fig.8(B),which in turn may reduce interference with the bulk water.Thus,better gas dissolution and lesser interaction with the bulk water than oxalic acid result in higher gas hydrate kinetics for succinic acid system.

Aspartic acid has two hydrogen attached hydrophobic carbon atoms which are further attached to the terminal--COOH groups and one--NH2group.Aspartic acid is an amino acid and has zwitterionic property due to the presence of amine(basic)and carboxylic(acidic)group together at vicinal carbon atoms.Due to the zwitterionic action,H+transfer takes place from vicinal carboxylic group to the nitrogen of the amine group.This makes one end of the aspartic acid a dipole and the other end still having one carboxylic group which can interact with the bulk water but cannot engage too many water molecules than oxalic acid due to reduced number of acidic group and existence of a hydrophilic strong dipole at the other end.As can be seen in the Fig.8,large number of water molecules are attracted towards the dynamic dipole of the zwitterionic end which enhances the rate of hydrate formation on interaction with the gas molecules.Therefore,the hydrate promotion activity of the aspartic acid can be correlated with the high gas dissolution,dynamic dipole end which facilitates the assembly of large number of water molecules.

4.Conclusions

Natural gas hydrate formation experiments were carried out for two dicarboxylic acids and an amino acid at two different concentrations.Oxalic acid was found to exhibit moderate gas hydrate inhibition property whereas succinic acid and aspartic acid exhibited gas hydrate promotion activity.Kinetic study was found to be in alignment with induction time observations,intrinsic growth constants and water to hydrate conversion values.Oxalic acid behaves as a mild inhibitor for gas hydrate nucleation and growth at both the concentrations (0.1 mol·L-1&0.5 mol·L-1)at 273.15 K and 3.0 MPa.Whereas,based on gas uptake measurement,aspartic acid could be classified as a promoter of hydrate nucleation and growth.Overall,the gas uptake for gas hydrate formation on the basis of investigation follows the order；oxalic acid ＜pure water ＜succinic acid ＜aspartic acid.The probable mechanisms for the gas hydrate inhibition and promotion activity are also discussed.The results obtained in the present study give noteworthy insight about effect of molecular structure of additives on hydrate growth kinetics.

Acknowledgement

Dr.Rajnish Kumar acknowledges the Department of Science and Technology,Science and Engineering Research Board,India for the project grant EMR/2017/000810.

Dr.Kushwaha acknowledges the Department of Science and Technology-Science and Engineering Research Board,India for the award of National Postdoctoral Fellowship(Principal Investigator)and project grant DST-SERB-PDF-2017/003075,and the Director IITMadras for providing infrastructure to carry out the research work.

Fig.8.Proposed mechanism of working of the different additives during hydrate formation.

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2019.02.030.