Journal of Molecular Liquids 294 (2019) 111608

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Journal of Molecular Liquids

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The effects of SDS, SLES and THF on the growth rate, kinetic behaviors andenergy consumption during ethylene hydrate formation process

Ali Al-Sowadi a,b, Hadi Roosta a,b, Ali Dashti a,b,⁎, S. Arash Pakzad a,b, Reza Ghasemian a,b, Mehdi Rajaei a,b

a Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iranb Research Laboratory of Polymer Testing (RPT Lab.), Research Institute of Oil & Gas, Ferdowsi University of Mashhad, Mashhad, Iran

⁎ Corresponding author at: Chemical Engineering DepaFerdowsi University of Mashhad, Mashhad, Iran.

E-mail address: dashti@um.ac.ir (A. Dashti).

https://doi.org/10.1016/j.molliq.2019.1116080167-7322/© 2019 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 March 2019Received in revised form 20 July 2019Accepted 21 August 2019Available online 22 August 2019

In this study, the kinetics of hydrate formation in ethylene-water and ethylene-THF-water systemswith orwith-out SDS and SLESwas investigated and analyzed. The effects of these surfactants were tested in the concentrationrange of 250–1000 ppm. The experimental results indicated that SLES can be introduced as a new promoter ofethylene hydrate formation. The experiments with the surfactants also led to an interesting result in relationto unconventional behavior of the hydrate formation kinetics. The kinetic behavior of hydrate formation wasalso examined and analyzed in ethylene-THF-water system. The results demonstrated that THF plays a crucialrole in the enclathration rate of the ethylenemolecules in the large cages of the hydrate structure. In this regard,the concentrations of 1 and 2mol% THF, had the best performance to promote ethylene hydrate formation kinet-ics. Also, the energy consumptionmeasurements showed that the use of SDS, SLES and THF (at the concentrationsof 1 and 2mol%) can increase the potential of ethylene hydrate in some practical application fields, due to energysaving and the increase of hydrate formation rate.

© 2019 Elsevier B.V. All rights reserved.

Keywords:Gas hydrateKineticsEthyleneWater moleculesSurfactantEnergy saving

1. Introduction

Gas hydrates are crystalline solid compounds composed of waterand gas molecules [1]. Water molecules can form a network ofhydrogen-bonded cages for the enclathration of suitably sized guestmolecules and stabilization of hydrate in structures I (sI), II (sII) and H(sH). Gas hydrates can form in the transportation pipeline and blockthe flow path [2]. However, the hydrate formation is not always consid-ered as a risk factor. Inmany applications such as gas separation [3], hy-drogen storage [4], desalination of seawater [5], natural gastransportation [6], and energy storage [7] gas hydrates can be useful.But one of the most important issues that can limit the use of hydratein the mentioned applications is the slow rate of hydrate formation.Therefore, increasing the growth rate of hydratewith differentmethodscan develop the applications of the gas hydrate technology [1]. Stirring[8], spraying liquid into the gas phase [9], bubbling of gas through liquid[10], hydrate formation in the porous media [11], using nanoparticles[12], applying hydrophilic surfaces [13], addition of CO2 [14], andusing additives are recommendations to increase the hydrate formationrate. In this regard, the application of kinetic promoters (as a maingroup of additives) is superior to other methods [15–17]. Kinetic pro-moters mainly contain surfactants, although some amino acids and

rtment, Faculty of Engineering,

sodium halides can also increase the rate of hydrate formation at lowconcentrations [18–20]. However, the use of surfactants is the mostcommon method for increasing the growth rate of hydrate. In this re-spect, Lin et al. [21] conducted one of the first studies to investigatethe effect of sodium dodecyl sulfate (SDS) on kinetics of formationand decomposition of methane hydrate. The results of their experi-ments indicated that this anionic surfactant can increase the rate of hy-drate formation. Gayet et al. [22] investigated the effect of SDS onmethane hydrate formation in a high-pressure cell without stirrer.They found that the hydrate formation rate was increased comparedto pure water. The promoting effect on the kinetics of methane hydrateformation was also reported in the presences of linear alkyl benzenesulfonate (LABS), cetyltrimethyl ammonium bromide (CTAB),nonoxylated nonylphenol surfactant (ENP) [23], sodium tetradecyl sul-fate (STS), sodium hexadecyl sulfate (SHS) [24], dodecyltrimethyl am-monium bromide (DTAB), triton X-100 [16], tergitol [25], sodiumdodecyl sulfonate (SDSN), and sodium dodecyl benzene sulfonate(SDBS) [26].

The effect of surfactants on hydrate formation was also studied inthe systems including CO2 [27–30]. Unlikemethane hydrate, the surfac-tants had no significant effect in this system. However, a promoting ef-fect of SDS on the CO2 hydrate formation kinetics was reported byPivezhani et al. [8], which applied SDS at high concentrations (1000 to8000 ppm). The effects of surfactants on the kinetics of gas hydrate for-mation in other systems, including ethane [31], chlorodifluoromethane(R22; CHClF2) [32], difluoromethane (R32; CH2F2) [33], sulfur

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hexafluoride (SF6) [34], cyclopentane (CP) [35], methane-propane gasmixtures [36], methane-ethane-propane gas mixture [37], natural gas[38],methane nitrogen oxygenmixture [39], andmethane carbon diox-ide gas mixture [40] were also reported and it was demonstrated thatthe effects of surfactants in the gas mixtures are dependent on thetype of gas and its percentage composition in the system [41].

Hydrate formation in the systems including ethylene can also be ofinterest. It should be noted that in ethylene-water system, the ethylenemolecules form structure I hydrate [42]. However, cage occupancy ofethylene in large and small cages of structure I, is not exactly deter-mined. In this regard, based on the present data by Sloan [1], the guestmolecules such as ethane and ethylene cannot occupy the small cagesof structure I. On the other hand, Sugahara et al. [43] based on Ramanspectra of ethylene hydrate at 950 bar and 303.9 K observed doublepeaks and led to that ethylene molecules can occupy both the smalland the large cages of structure I, although atmoderate pressure, the ex-istence of double peaks was not investigated. Also, Liu et al. [44] re-ported that at above-critical temperatures the ethylene cannot remainin the small cages of hydrate. Ethylene can also be stable in structure IIhydrate with the help THF, such that at moderate pressures it can onlyoccupy large cages of structure II [45]. However, ethylene is an impor-tant industrial gas, especially in petrochemical industry and commonstate of the art of ethylene hydrate are: (i) its application in separation(especially its separation in low boiling systems which requires exces-sive cooling and high cost by the usual distillation methods), (ii) stor-age, and (iii) research interest due to hydrate formation in a widerange of temperature,which even covers the critical temperature of eth-ylene [42,44]. On the other hand, a detailed knowledge of the kinetics ofthe ethylene hydrate formation and increasing the rate of hydrategrowth is of importance in this field. But, there are a few researcheson the kinetics of ethylene hydrate formation, and especially on effectsof surfactants. In this regard, only the effect of SDS was investigated onethylene hydrate formation by Rezaei et al. [46], which their results in-dicated that SDS at the concentrations of 300 and 500 ppm significantlyincreases the ethylene hydrate formation rate. However, in this regard,the effects of other surfactants can be tested for more effective use ofethylene hydrate in the mentioned applications. Also, there is a gap ofresearch about the kinetic behaviors of ethylene hydrate formation, es-pecially in the presences of surfactants and tetrahydrofuran (THF).Therefore, the knowledge of these kinetic behaviors can be helpful forfurther development of the application of ethylene hydrate especiallyin the separation of ethylene from gas mixtures. On the other hand,the purpose of using these additives is the increase of hydrate formationrate to save energy and process time. However, no study is available onthemeasurement of energy consumption during the formation of ethyl-ene hydrate.

In thiswork, the effects of SDS and sodium lauryl ether sulfate (SLES)as anionic surfactants on the kinetics of hydrate formation in ethylene-water and ethylene-THF-water systems were tested. Also, the kineticbehaviors of ethylene hydrate formation in the presences of these sur-factants were investigated and described. In addition, the effects of thepresence of THF at various concentrations on the enclathration rate ofethylene in hydrate structure were examined. Finally, the potential ofapplied additives for more functional use of ethylene hydrate was eval-uated based on the measurements of energy consumption during hy-drate formation in different systems.

2. Experimental

2.1. Materials

The kinetic experiments of gas hydrate formationwere performed inthe presence of ethylene gas with a purity of 99.9%. Also, two anionicsurfactants, including SDS (99%) and SLES (98%) were used in the ki-netic experiments of hydrate formation. THF (99.8%) was also preparedand applied for sII hydrate formation.

2.2. Experimental apparatus

The equipments for hydrate formation mainly contain a high-pressure reactor (made of AISI 304 L stainless steel) equipped with astirrer, circulating cooling system (Lauda Alpha RA 8 with a workingtemperature range of 248.15 to 358.15 K, Germany), vacuum pump,gas flowing system, ethylene gas cylinder, temperature and pressuresensors, data acquisition system, and a computer. The jacketed reactorwas designed with the total volume of 650 mL and could operate inthe pressure range of 0 to 60 bar. Also, the speed of the stirrer could beadjustable from 0 to 1500 rpm by a belt connected to an electric motorand with the help of a speed controller. The reactor temperature couldbe controlled with the circulating cooling bath. The reactor temperaturecould bemeasured by PT100 sensor with a precision of ±0.1 K. Also, thereactor pressure could be detected by a pressure transmitter with a pre-cision of ±0.1 bar. During hydrate formation experiments, the tempera-ture and pressure were monitored with the help of a data acquisitionsystem and saved in a computer. Also, it should be demonstrated thatthe surface tension of the solutions was measured using a Kruss K100tensiometer (Kruss GmbH, Germany). In this setup, the Wilhelmyplate method was used for measuring surface tension. In this regard,the experiments were performed in the temperature of hydrate forma-tion (275 K) and atmospheric pressure. In addition, a watt meter withaccuracy of ±0.01 W was used to measure energy consumption.

2.3. Experimental procedure for hydrate formation

For performing the kinetic experiments of gas hydrate formation,first, the reactor was washed once with acetone (due to washing of lu-brication oils) and three times with de-ionized water. Then, the systemwas evacuated with a vacuum pump for 5 min. After ensuring that theentire system is evacuated, the valves were closed and then the pumpwas switched off. A water injection valve located at the top of the reac-tor was also used to inject 400 mL of water (solution) into the reactor.Then the cooling bath was switched on and it was set at the desiredtemperature to cool the coolant in the jacket of the reactor. When theaqueous solution inside the reactor was reached to the desired temper-ature (275 K), the reactor was pressurized to 21 bar. Then the stirrerwas switched on and the stirring rate was adjusted at 300 rpm. Whenthe stirrer starts to work, the pressure and temperature variationswere recorded and stored in terms of the time. However, the hydrategrowth was associated with an abrupt drop of pressure. Finally, therate of the hydrate growth was detectable and measurable based onthe rate of gas consumption during ethylene hydrate formation. Also,it should be demonstrated that the moles of gas consumed during hy-drate formation were calculated according to following equation:

nci ¼ n0−ni ¼ ð PVZRTÞ0

−ð PVZRTÞi

ð1Þ

In this equation, nci, no, ni, P, T, V, Z, and R aremoles of gas consumedup to time ti, initial moles of gas, moles of gas at time ti, pressure, tem-perature, volume of gas in the cell, compressibility factor, and universalgas constant respectively. Subscripts o and i exhibit the initial conditionsand the conditions at any time, respectively. Also, the Peng-Robinsonequation of state was applied to calculate the compressibility factor[47]. Also, for calculation of dimensionless consumed moles (nci/ncf),the calculated value of nci (obtained from Eq. (1)) was divided to thefinal consumed mole. The final consumed mol was obtained based onfollowing equation.

ncf ¼ n0−nf ¼ ð PVZRTÞ0

−ð PVZRTÞ f

ð2Þ

Subscripts f exhibits the values of pressure or temperature at the endof hydrate formation (at the equilibrium time).

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3. Results and discussion

3.1. The equilibrium pressure of ethylene hydrate formation in the presenceof SDS, SLES and THF

For the understanding of the kinetic behaviors of ethylene hydrateformation in the presence of surfactants and THF, it is better to investi-gate the hydrate equilibrium curves. All experimentswere performed atconstant temperature of 275 K and therefore the equilibrium pressuresof ethylene hydrate were measured at different concentrations of theapplied additives. First, the equilibrium pressures of ethylene hydrateformation in the presence of SDS were investigated. The results showedthat at tested concentrations, SDS has not significant effect on equilib-rium pressure of ethylene hydrate. In fact, the kinetic promoters, suchas SDS are applied at low concentrations, and these concentrations can-not influence on the equilibrium pressure of hydrate. In this regard,some literatures [48,49] also demonstrated that SDS has not evident in-fluence on the equilibrium conditions of hydrate formation. Similarly,SLES even at the highest concentration (1000 ppm) cannot change equi-librium pressure of ethylene hydrate formation.

The effect of THF on the equilibrium pressure of ethylene hydratewas also investigated. Fig. 1 exhibits the equilibrium pressures of ethyl-ene hydrate at different concentrations of THF. As is shown,with the in-crease of THF concentration, the equilibrium pressure of ethylene isenhanced. In fact, although THF is a thermodynamic promoter in somesystems, such as methane-water system [50], but in the ethylene hy-drate leads to the increase of equilibrium pressure. The probable reasonmay be in relation to competition of THF with ethylene to occupy largecages of sII hydrate. In this regard, Zhang et al. [45] demonstrated thatwhen THF and ethylene are present in a system, sII hydrate is formed.They also noted that THF occupies the large cages of the sII hydrate,and ethylene cannot occupy the small cages. Therefore, the ethyleneand THF molecules only can be trapped in large cages of sII hydrate.They also reported that, although THF is a thermodynamic promoterof methane hydrate, but inhibits ethane or ethylene to form hydrate.Based on these descriptions, in this current study, when the hydrate isformed in ethylene-THF-water system, a competition is created be-tween THF and ethylene molecules to occupy large cages of sII hydrate.Therefore, with the presence of THF, less molecules of ethylene can betrapped by the cages. Therefore, THF plays the role of inhibitor (as wasdemonstrated by Zhang et al. [45]), and increases the equilibrium

Fig. 1. The equilibrium pressures of ethylene hydrat

pressure of ethylene hydrate formation, such that with the increase ofTHF concentration, the equilibrium pressure of hydrate is increased.

3.2. Kinetic behavior of formed hydrate in ethylene-water system (struc-tures I of hydrate) with or without surfactants

Kinetic experiments of hydrate formation were performed at a con-stant temperature of 275 K and initial pressure of 21 bar. These condi-tions were favorable because of hydrate formation without tardiness(without induction time) which is suitable in ethylene hydrate applica-tions due to energy and time saving. In this regard, the sI hydrate struc-ture was formed with pure water or aqueous solutions of surfactants(including SDS and SLES as anionic surfactants). First, the results of eth-ylene hydrate formation in the presence of aqueous solutions of SDSwere investigated and compared to hydrate formation with purewater. Fig. 2 shows the effect of SDS at concentrations of 250, 500, 750and 1000 ppm in comparison with pure water. The results evidencethat SDS at a concentration of 250 ppm increases the growth rate of eth-ylene hydrate, although in early time of hydrate formation its effectswas less than pure water. Similarly, SDS at 500 ppm was effective ingeneral. However, with increasing the concentration to 750 and1000 ppm the performance of SDS was improved even at the starttime. The experimental results also confirm that the best performanceof SDS for increasing ethylene hydrate formation rate was occurred at750 ppm.

The experiments on the kinetics of ethylene hydrate formationwerealso performedwith SLES as an anionic surfactant. SLES is one of the sur-factants whose effects on ethylene hydrate have not been studied (evenon other gas systems). However, the effect of SLES was evaluated at theconcentrations of 250, 500, 750 and 1000 ppm, which Fig. 3 exhibits itseffects on ethylene hydrate. According to thisfigure, although the rate ofethylene hydrate formation is low at the start time, but it has led to anincrease in the growth rate of ethylene hydrate. On the other hand, itis observed that with the increase of surfactant concentration, the pro-motion effect of SLES on ethylene hydrate kinetics is increased, so thatat the concentration of 1000 ppm, it had the greatest effect on the rateof hydrate formation.

Fig. 4 compares the average growth rate of ethylene hydrate at dif-ferent concentrations of SDS and SLES during the first half of hydrateformation. As it is seen, SDS has exhibited a better performance com-pared to SLES for kinetic enhancement of ethylene hydrate formation.

e formation at different concentrations of THF.

Fig. 2. The effect of SDS at different concentrations on the rate of ethylene hydrate formation.

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Also, SDS at the concentration of 750 ppm and SLES at the concentrationof 1000 ppm had the best performance for increasing the rate of ethyl-ene hydrate formation among the tested concentrations. According tothe results, at the concentration of 750 ppm, the average growth ratein the presence of SDS was approximately 4 times higher than hydrateformation with pure water. Also, in the concentration of 1000 ppm ofSLES the average growth rate of ethylene hydrate became 2 timesmore than formation of ethylene hydrate with pure water. However,one of the possible reasons in the enhancement of growth rate may bethe reduction of surface tension. In fact, the reduction of surface tensionincreases the rate of diffusion of ethylene from the liquid-gas interfaceto the bulk liquid and subsequently increases the growth rate of hydratecrystals. Therefore, in this regard, the surface tension of the aqueous so-lutions of SDS and SLES was measured. First, the surface tension of SDSsolutions at the concentrations of 250, 500, 750 and 1000 ppm wasmeasured. The obtained results show that in the presence of SDS, thesurface tension has decreased (Fig. 5(a)). This can be led to the increase

Fig. 3. The effect of SLES at different concentration

of the growth rate during ethylene hydrate formation. In fact, the sur-face tension was continuously decreased from 250 to 1000 ppm andthe growth rate of ethylene hydrate was also continuously increasedfrom 250 to 750 ppm. However, the rate of ethylene hydrate formationwas reduced by increasing the concentration of SDS from 750 to1000 ppm, while the surface tension of SDS was reduced in this range.Accordingly, it was expected to increase the rate of ethylene hydrateformation from 750 to 1000 ppm. On the basis of this case, it seemsthat although the reduction of surface tension is an effective factor in in-creasing the rate of formation of ethylene hydrate, but other factors,such as adsorption of surfactants on hydrate surfacemay bemore effec-tive, especially at higher concentrations.

Similarly, the surface tension of SLES solutionswasmeasured at var-ious concentrations. The results are shown in Fig. 5(b) and indicate thatthis anionic surfactant has also reduced the surface tension. Addition-ally, it is observed that the surface tension is almost constant at the con-centrations higher than 250 ppm, while according to previous results,

s on the rate of ethylene hydrate formation.

Fig. 4. Average growth rate of ethylene hydrate at different concentrations of SDS and SLES during the first half of hydrate formation.

Fig. 5. a) Surface tension of aqueous solutions of SDS; b) Surface tension of aqueous solutions of SLES.

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the rate of hydrate formation has continuously increased from 250 to1000 ppm. In fact, although the surface tension was constant at therange of 250 to 1000 ppm, but the hydrate formation rate is increased.This can be again attributed to the fact that the surface tension is notthe only effective factor in increasing the rate of hydrate formation. Inthis respect, the adsorption of surfactants on the surface of hydrate crys-tals is one of the main factors, which will be demonstrated.

By analyzing the results from another perspective, a noticeable dif-ference in kinetic behavior of formed ethylene hydrate is seen betweenthe pure water and the applied surfactants. For better understanding,first the kinetic behavior of ethylene hydrate formation in the presenceof pure water is analyzed and described. In this regard, Fig. 6(a) showsthe growth rate of ethylene hydrate in terms of dimensionless con-sumed moles (ratio of consumed moles at any moment to total con-sumed moles; nci/ncf). In fact, nci/ncf is as an index of hydrateformation progress and changes from 0 to 1. It should be demonstratedthat the growth rate (mmol/min) was calculated based on slope of thecurve of gas consumption versus time. As is shown in Fig. 6(a), therate of ethylene hydrate formation was reduced uniformly from begin-ning to end, and the highest rate of hydrate formation was occurred at

Fig. 6. a) The kinetic behavior of ethylene hydrate formation with pure water; b) The kineticexperiments).

beginning of hydrate formation due to the highest driving force of hy-drate at the start of the process. It should be demonstrated that duringthe hydrate formation at non-isobaric conditions, the pressure is re-duced due to enclathration of gas molecules in hydrate structure andsubsequently the driving force is decreased and the rate of hydrategrowth is reduced uniformly from beginning to end. However, whenSDS and SLES were present in the system, a different behavior was ob-served in the kinetics of ethylene hydrate formation. For example,Fig. 6(b) shows the rate of ethylene hydrate formation in the presence250 ppm of SDS. As is shown, the maximum rate is not occurred at be-ginning of hydrate formation and also the growth rate is enhanced toa specific time of hydrate formation (a specific value of nci/ncf). In fact,although driving force of ethylene hydrate formation is decreased butthe growth rate is increased to a specific time. This unusual behaviorin the kinetics of ethylene hydrate formation is also repeated at otherconcentrations of SDS and also in the presence of SLES. According tothe kinetic behavior, the growth rate of ethylene hydrate is increasedto a specific value of nci/ncf ((nci/ncf)max) and then is decreased. On theother hand, the specific value of (nci/ncf)max which indicates to themax-imum rate of ethylene hydrate formation is different at various

behavior of ethylene hydrate formation in the presence 250 ppm of SDS (two identical

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concentrations of SDS and SLES. In this regard, Fig. 7(a) shows the valuesof (nci/ncf)max at different concentrations of SDS and SLES. Accordingthese results, it is found thatwith increasing of SDS and SLES concentra-tion, the value of (nci/ncf)max is decreased. For more investigation, thevalues of (ti/tf)max that demonstrate to the dimensionless time of max-imum growth rate, were calculated. In this regard, first the values of ti/tfwere obtained by dividing the timevalues by thefinal time (equilibriumtime). Then, a specific value of ti/tf which corresponds to maximumgrowth rate, was considered as (ti/tf)max. According to Fig. 7(b), thevalues of (ti/tf)max are decreasedwith the increase of surfactant concen-tration. In the other words, with the increase of surfactants concentra-tion, the maximum growth rate of ethylene hydrate is occurred in lessvalue of formed hydrate and shorter time of beginning of hydrate for-mation. This indicates that with the increase of concentration, the sur-factant is faster adsorbed on the hydrate surface. In fact, as previouslywas explained the adsorption of surfactants on hydrate surface plays acrucial role in their performance, so that some studies [51] have notedthat its effects is evenmore than the role of the surface tension. Accord-ing to this, the unconventional behavior of kinetics of ethylene hydrateformation in the presences of SDS and SLES may be explainable. Itshould be demonstrated that SDS and SLES can be adsorbed on hydratesurface through hydrogen bonding with water molecules of hydratecrystals. In this regard, the hydrophilic head of surfactant is oriented to-ward crystals and the hydrophobic tail is oriented toward the aqueous

Fig. 7. a) The values of (nci/ncf)max at the different concentrations of SDS and SLE

phase, so that the parallel hydrophobic tails create a hydrophobic regionwhich can stabilize more hydrophobic molecules (such as methane oreven ethylene). These regions can act as nucleation sites and can pro-mote the hydrate growth. However, based on the results of this work,it considers that the adsorption of SDS and SLES is function of timeand their concentration. As is shown in the presented schematic ofFig. 8, the ethylene hydrate formation in the presences of SDS andSLES can be investigated in three separate zones based on its kinetic be-havior. In zone І, with the passage of time and the increase in theamount of formed hydrate (nci/ncf), the growth rate of ethylene hydrateis slightly decreased. In fact, in this zone, the surfactants have not hadenough time for suitable adsorption and creation of nucleation sites.On the other hand, with the hydrate formation, the pressure is reducedand subsequently the driving force is decreased and therefore thegrowth rate of ethylene hydrate is decreased. The decrease of drivingforce is also continued in zone ІІ, but in this zone, with the passage oftime, more surfactant molecules are adsorbed on hydrate surface andalso more crystals are created and the chance of the adsorption of sur-factant molecules on hydrate surface (to create nucleation sites) is in-creased. Therefore, despite the decrease of driving force, the growthrate of ethylene hydrate is increased to a specific time and a specificamount of formed hydrate. However, after the specific time (zone ІІІ),the effect of the decrease of driving force is compatible with the surfac-tant effects and the hydrate growth rate is decreased. So, the

S; b) the values of (ti/tf)max at the different concentrations of SDS and SLES.

Fig. 8. The schematic of the effects of driving force and surfactants on the growth rate of ethylene hydrate in different zones.

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unconventional behavior of ethylene hydrate formation in the pres-ences of SDS and SLES is in relation to the simultaneous effects of a fa-vorable factor (adsorption of more surfactant on crystals of ethylenehydrate) and an undesirable factor (the decrease of driving force of eth-ylene hydrate formation). Based on this description, it can be said that atthe higher concentrations of SDS and SLES themaximum growth rate ofethylene hydrate is occurred faster, which the results of Fig. 7 were inaccordance with this matter. However, it should be demonstrated thatthemechanism of adsorption of anionic surfactants such as SDS was in-ferred based on work of Lo et al. [52]. Their study indicates that duringhydrate formation and with the passage of time, more surfactantswere adsorbed on hydrate surface.

The effects of SDS and SLES on kinetic behavior of ethylene hydrateformation were also tested at different initial pressures of ethylene.The ethylene hydrate was also formed at initial pressures of 18 and25 bar (driving forces of 11.3 and 18.3 bar) in the presence of SDS andSLES. The results indicate that the kinetic behavior of ethylene hydrateformation at the initial pressures of 18 and 25 bar was similar to kineticbehavior of the initial pressure of 21 bar, such that the growth rate ofethylene hydrate in initial time of hydrate formation was low andwith the passage of time was increased and reached to a maximumvalue. However, the value of nci/ncf (as an index of hydrate formationprogress) was dependent on initial pressures. Fig. 9 shows that in thepresence of SDS and SLES the values of nci/ncf are decreasedwith the re-duction of initial pressure of ethylene hydrate formation. In fact, atlower initial pressures, the required time to reach maximum growthrate is shorter. This again confirms that at lower initial pressures(lower driving force), the adsorption of surfactants on hydrate surfaceis probably enhanced and subsequently the maximum growth rate isreached faster.

3.3. Kinetic behavior of formed hydrate in ethylene-THF-water system(structure II of hydrate)

In thiswork, the effect of THF on the kinetics of ethylene hydrate for-mation was also investigated. The kinetic experiments were performedat THF concentrations of 1, 2, 3, 4, and 5.56mol%, which their results areexhibited in Fig. 10. It is observed that the rate of gas consumption dur-ing ethylene hydrate formation in the presence of THF is decreased incomparison with hydrate formation with pure water (without THF).In fact, although the initial conditions in all experiments were 21 barand 275 K, but the equilibrium pressures of hydrate formation and sub-sequently the driving forces were different. As was previously illus-trated in Fig. 1, the equilibrium pressure of hydrate formation isincreased in the presence of THF. Therefore, the driving force of hydrateformation is decreased. In fact, the driving force of ethylene hydrate for-mationwith purewater is about 14.3 bar,while at THF concentrations of1, 2, 3, and 4 mol% the driving force is decreased to 13.7, 10.6, 7.8, and6.4 bar, respectively. Based on these results, it can be said that THF de-creases the driving force and subsequently decreases the hydrate for-mation rate. Fig. 10 also confirms that at high concentrations of THF (3and 4 mol%), the rate of hydrate formation is significantly decreaseddue to the decrease of driving force. Also, the experimental results indi-cated that at the concentration of 5.56 mol%, the mole consumption ofethylene was zero and in other word, the ethylene hydrate was notformed. In fact, although the THF hydrate might be formed but the eth-ylenemolecules could not occupy the cages of hydrate structure. There-fore, for this system, it is more correct to say “the enclathration rate ofethylenemolecules in hydrate structure” instead of “the hydrate forma-tion rate”, because the growth rate of hydrate is also dependent on oc-cupying hydrate cages by THF molecules. However, it can be said that

Fig. 10. The effect of THF at different concentrations on kinetics of hydrate formation.

Fig. 9. The values of (nci/ncf)max at the different initial pressures of ethylene hydrate formation in the presence of SDS and SLES.

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with the increase of the THF concentration, the enclathration rate of eth-ylenemolecules in hydrate structure is decreased. Also, according to thepresented data in Table 1, it is seen that the average rate of ethyleneenclathration in hydrate structure at THF concentrations of 1 and2 mol%, is higher than the concentrations of 3 and 4 mol%. Also at

Table 1The effect of THF concentration on the rate of the ethylene enclathration.

Components E.Ra

at nci/ncf = 0.25(mmol/min)

E.Rat nci/ncf = 0.50(mmol/min)

THF (1 mol%) 1.55 1.25THF (2 mol%) 2.17 1.48THF (3 mol%) 0.32 0.25THF (4 mol%) 0.25 0.21THF (5.56 mol%) No Et.Hc No Et.H

a It demonstrates to enclathration rate (E.R) of ethylene molecules in the hydrate structure.b It demonstrates to average enclathration rate (A.E.R) of ethylene molecules in the hydratec It demonstrates that ethylene hydrate cannot formed (ethylene cannot be enclathrated in

these concentrations, the rate of ethylene enclathration is significantlyhigher (at the moments when the values of nci/ncf are 0.25, 0.5 and0.75, respectively). On the other hand, the values of equilibrium timeof hydrate formation also confirm thatwith 2mol% THF, thehydrate for-mation process was ended during 150min. However, with the increase

E.Rat nci/ncf = 0.75(mmol/min)

A.E.Rb

(mmol/min)tk(min)

0.56 0.63 2530.76 0.80 1500.24 0.19 N4000.07 0.15 N400

No Et.H No Et.H No Et.H

structure.the hydrate structure).

10 A. Al-Sowadi et al. / Journal of Molecular Liquids 294 (2019) 111608

of THF concentration to 3 and 4 mol%, the equilibrium time of hydrateformation is N400 min. These data confirm again that with the increaseof THF concentration to 2 mol%, the enclathration rate of ethylene in sIIhydrate is increased and subsequently with the increase of the concen-tration to 3 and 4 mol% it is decreased and with further increase (to5.56 mol%), the ethylene hydrate cannot be formed. For understandingof the kinetic behavior of ethylene hydrate in the presence of THF, cageoccupancy by THF and ethylene can be investigated. In fact, in the pres-ence of THF, formed hydrate is type II, and THF occupies large cages ofthis structure. On the other hand, due to the molecular size of ethylene,it can also be trapped in large cages of structure II hydrate [45]. There-fore, it considers that there is a competition for occupying of largecages of hydrate by THF and ethylene molecules and subsequently thekinetic behavior of formed hydrate is in relation to this competition. Inthis regard, the amount of enclathrated ethylene in hydrate structureand the ratio of large cages occupied by ethylene to THF (ethylene inlarge cages/THF in large cages) were calculated. As was demonstrated,the sII hydrate is formed in ethylene-THF-water system which THFand ethylene molecules can only occupy the large cages. Every unitcell of sII hydrate is included 8 large cages [1]. Therefore, the percentageof occupation of large cages by THF and ethylene (Et) in n unit cells (uc)of hydrate structure can be calculated by following equations.

%large cages occupied by THF

¼ 1n

Xni¼1

Number of THF molecouls in uci8

!� 100 ð3Þ

%large cages occupied by Et

¼ 1n

Xni¼1

Number of Et molecouls in uci8

!� 100 ð4Þ

However, based on Eqs. (3) and (4) the ratio of large cages occupiedby ethylene to THF can be written according to Eqs. (5) and (6).

%large cages occupied by Et%large cages occupied by THF

¼ total number of Et molecoules in sII hydratetotal number of THF molecoules in sII hydrate

ð5Þ

%large cages occupied by THF%large cages occupied by Et

¼ moles of Et in sII hydratemoles of THF in sII hydrate

ð6Þ

In Eq. (6), themoles of THF in sII hydrate are measured based on ob-tained solution of melted hydrate in the end of hydrate formation pro-cess. Also, the gas consumption during hydrate formation (after initialsolubility) reflects the moles of ethylene in sII hydrate. Therefore, theratio of large cages occupied by ethylene to THF can be calculated(Table 2). The presented results in Table 2 show that with the decreaseof THF concentration in aqueous solution, the amount of enclathratedethylene in hydrate is increased from 0 to 0.16 mol. Also the chance ofoccupying of large cages by ethylene is increased with the decrease ofTHF concentration, such that the ratio of large cages occupied by ethyl-ene to THF is reached to 4.8. On the other hand, at higher concentrationof THF this chance become less, such that at the concentration of

Table 2The occupation of large cages by ethylene and THF in ethylene-THF hydrate formation.

THF concentration inaqueous solution(mol%)

Enclathrated ethylene inhydrate structure (mol)

The ratio of large cagesoccupied by ethylene to THF(ethylene in large cages/THFin large cages)

1 0.16 4.82 0.12 1.93 0.08 0.94 0.06 0.45.56 0 0

5.56 mol% THF in aqueous solution none of the ethylene moleculescan occupy large cages while THF molecules can fill all the large cages.Therefore, ethylene cannot be able to attend in hydrate phase at thisconcentration and in other word the ethylene hydrate is not formed. Itshould be demonstrated that the concentration of 5.56 mol% resultedto a stoichiometric (1THF:17H2O) THF hydrate, which all the largecages are occupied by the THF molecules [53]. Based on these descrip-tions andwith help of the presented schematic in Fig. 11, the kinetic be-havior of formed hydrate in the ethylene-THF-water system can bemore understandable. According to this schematic, when the THF con-centration in aqueous solution is increased from 1 to 2mol%, the poten-tial of sII hydrate formation is increased and subsequently the hydrateformation rate and the enclathration rate of ethylene in the largecages is enhanced. In fact, although with the increase of THF concentra-tion, fewer cages are available for occupying by ethylene but the highpotential of sII hydrate formation leads to the increase of theenclathration rate of ethylene in the large cages. However, with moreincrease of THF concentration to 3 mol%, the available large cages tofill by ethylene are decreased (less of half of the large cages) and there-fore even with the increase in the potential of hydrate formation, theenclathration rate of ethylene is decreased. The conditions for the pres-ence of ethylene molecules in the large cages become even more diffi-cult at the concentration of 4 mol% THF, and therefore theenclathration rate of ethylene is significantly decreased. Finally, at theconcentration of 5.56mol% THF, all cages of sII hydrate structure are oc-cupied with THF and ethylene molecules cannot be present in hydratestructure. In this regard, as was previously reported, the equilibriumpressure of hydrate formation in ethylene-THF-water system was in-creased with the increase of THF concentration in every experiment.In fact, THF inhibits ethylene molecules to occupy large cages [45]. Theperformed experiments at different concentration of THF also con-firmed that with the increase of THF concentration, less molecules ofethylene are enclathrated in hydrate structure. In this regard, at THFconcentration of 5.56 mol%, all large cages are occupied with THF(Fig. 11). Also, in the literature [53], it was confirmed that at this con-centration, all large cages are occupied by THF. Also, it should be demon-strated that, during hydrate formation in ethylene-THF-water system,only one temperature peak was observed. It demonstrates that onlyone type of structure (sII hydrate) can be formed from beginning tothe end of process. Also, the decrease of ethylene pressure and the effectof THF on equilibrium pressure show that ethylene and THF moleculesare present in the hydrate structure, although at higher concentrationsof THF, less ethylene molecules could be present in hydrate structure.

Based on the descriptions and the experimental results, when theTHF concentration in aqueous solution is 2 mol%, the enclathrationrate of ethylene in hydrate structure is maximized. Therefore, the effectof SDS and SLES on the kinetics of hydrate formation in ethylene-THF-water system (structures II hydrate), was also investigated at the THFconcentration of 2 mol%. In this regard, Table 3 presents the rate of eth-ylene enclathration at the moments when the values of nci/ncf are 0.25,0.5 and 0.75, respectively. Also, the average growth rate of ethylene-THFhydrate is showed. According to these data, all surfactants have in-creased the rate of ethylene-THF hydrate formation. The experimentalresults indicate that the effect of SLES in this system is more than SDS,so that at the concentration of 750 ppm, it has enhanced theenclathration rate of ethylene to 3.4 and 5.5 times more (at the mo-ments when the values of nci/ncf are 0.25 and 0.5, respectively) thanwhen the THF is only present in the system. The probable reason forthe better effect of SLES compared to SDS in ethylene-THF-water systemmay be related to formation of micelles in aqueous solutions of SLES. Infact, the formation ofmicelles is an effective factor to enhance of hydrateformation rate due to more solubility of gas molecules and creation ofnucleation sites [54,55]. Therefore, the possibility of micelle formationwas investigated based on the CMC values. In this regard, Marcolongoand Mirenda [56] reported that in the temperature range of 284 to308 K, the CMC of SDS is almost constant (varied of 8.0 to 8.4 mM).

Fig. 11. The schematic of the kinetic behavior of formed hydrate in ethylene-THF-water system based on the concentration of THF in aqueous solution (the black cages are the large cages).

11A. Al-Sowadi et al. / Journal of Molecular Liquids 294 (2019) 111608

Table 3The effects of SDS and SLES on the rate of ethylene enclathration during hydrate formation in the ethylene-THF-water system.

Components E.Ra

at nci/ncf = 0.25(mmol/min)

E.Rat nci/ncf = 0.50(mmol/min)

E.Rat nci/ncf = 0.75(mmol/min)

A.E.Rb

(mmol/min)tk(min)

THFc 2.17 1.48 0.76 0.80 150THF + SDS (500) 4.69 2.74 0.74 1.01 143THF + SDS (750) 6.03 3.92 1.01 1.20 137THF + SLES (500) 6.73 7.02 1.62 1.26 132THF + SLES (750) 7.40 8.16 1.67 1.41 109

a It demonstrates to enclathration rate (E.R) of ethylene molecules in the hydrate structure.b It demonstrates to average enclathration rate (A.E.R) of ethylene molecules in the hydrate structure.c The THF concentration was 2 mol%.

12 A. Al-Sowadi et al. / Journal of Molecular Liquids 294 (2019) 111608

Also, Profio et al. [54] determined that the CMC of SDS under normalpressure and temperature is 8.0 mM (about 2300 ppm). On the otherhand, Tang et al. [57] demonstrated that the CMC of SLES is 0.75 mM(about 300 ppm). Therefore, based on these data, at the applied concen-tration of SDS in this work (below 1000 ppm), the micelle cannot beformed,while themicelles canbe formedduring hydrate formation pro-cess with SLES at the concentrations of 500, 750, and 1000 ppm. There-fore, the more significant effect of SLES in comparison with SDS in thissystem may be in relation to micelles formation in SLES solution (atthe concentrations of 500, 750, and 1000 ppm),which leads tomore sol-ubility of gas molecules. In fact, as was previously demonstrated, inethylene-water-THF system, the competition between THF and ethyl-enemolecules is effective on the hydrate growth rate. Therefore, themi-celle formation in SLES solution leads to the increase of solubility ofethylene and subsequently increases the chance of ethylene moleculesto reach hydrate surface in competition with THF molecules. Therefore,it considers that in ethylene-water-THF system, themicelle formation iseffective on the increase of growth rate of hydrate.

3.4. Analysis of energy consumption during ethylene hydrate formation indifferent systems

Due to economic consideration and the ability to use gas hydratetechnology in aforementioned applications, it is necessary to decreasethe required energy for hydrate formation. One of the purposes ofusing SDS, SLES and THF was the decrease of energy consumption,which can justify the application of ethylene hydrate. But unfortunatelythere is no available study for investigation of energy saving during eth-ylene hydrate formation with the help of additives. Therefore, in thiswork, a focus was also placed on the measurement and analysis of en-ergy consumption and energy saving during ethylene hydrate forma-tion in the presences of SDS, SLES and THF. In this regard, the energyneeded for reducing the cell temperature to 275 K (the required energyfor cooling the cell before hydrate formation), the energy needed formaintaining the temperature of the cell at low temperature of 275 K(during hydrate formation), and the energy needed for stirring of aque-ous solution and formed hydrate in the high pressure cell (before hy-drate formation and during hydrae formation) were measured by awatt meter. The sum of three measured values was considered asamount of energy needed for the formation of ethylene hydrate. How-ever, it was assumed that the required energy for vacuuming, washing,injecting the solution, injecting the gas into the cell, and discharging ofmelted hydrate is almost insignificant, although sumof these cases is al-most the same in all experiments, and is not effective in comparison ofthe results. First, the amount of energy needed for the formation of eth-ylene hydrate in the presences of different concentration of SDS wasmeasured. Fig. 12(a) shows the amount of required energy for hydrateformation with pure water and aqueous solutions made up of SDS.The results indicate that the addition of SDS significantly reduces theamount of energy needed to form ethylene hydrate compared to pure

water, so that even at the concentration of 250 ppm, the amount of en-ergy needed to form hydrate was reached from 7.08 kWh/mol (to formhydratewith purewater) to 2.98 kWh/mol. Thismeans 58% energy sav-ing. However, by increasing the concentration of SDS to 500 ppm, theenergy saving was increased to 63% (Fig. 12(b)). Also, at the concentra-tion of 750 ppm of SDS, the energy savingwas 82%, whichwas very sig-nificant. However, by increasing the concentration of SDS from 750 to1000 ppm, the energy saving became less (71%). Therefore, the concen-tration of 750 ppm can be introduced as optimum concentration of SDSto decrease of the required energy of the ethylene hydrate formation.Similarly, the amount of required energy was decreased at various con-centrations of SLES, so that energy saving was 36%, 37%, 40% and 63% atthe concentrations of 250, 500, 750 and 1000 ppm of SLES, respectively.In fact, when SLES is used, the efficiency increases with increasing con-centrations. However, it is seen that at high concentration of surfactants,the required energy is reduced. In fact, the results of experiments showthat the surfactants can increase the hydrate growth rate. Therefore, thetime of hydrate formation process becomes shorter and subsequentlyless energy is consumed for stirring and maintaining the temperatureof cell at low temperature of 275 K during hydrate formation.

The energy consumption during hydrate formation process in theethylene-THF-water system was also measured. According to the re-sults, in the presences of 1 and 2 mo% THF, the energy consumptionwas decreased in comparison with hydrate formation with purewater, so that the energy saving was 31% and 47%, respectively. How-ever, with the increase of THF concentration to 3 and 4mol%, the energyconsumption during hydrate formation was increased to 15.86 and19.75 kWh/mol, respectively. These values are more than the requiredenergy for ethylene hydrate formation with pure water, and demon-strate again that these concentrations are not suitable for ethylene hy-drate formation. The measurements of energy consumption duringhydrate formation in ethylene-THF-water system showed that the useof SDS and SLES can also reduce energy consumption, so that in thepresences of 500 and 750 ppm of SLES the energy saving was 64% and69%, respectively. Also, SDS has been able to save energy to 59%. There-fore, these surfactants are also useful for hydrate formation in theethylene-THF-water system.

4. Conclusions

The experimental results showed that SDS and SLES can promote thehydrate formation in ethylene-water system, so that the growth rate ofhydrate could be increased to 4 and 2 times higher than hydrate forma-tionwith purewater, respectively. The performance of SLES as newpro-moter of ethylene hydrate formation was even better than SDS inethylene-THF-water system. The results also emphasized that the ki-netic behavior of ethylene hydrate formation in the presences of SDSand SLES is different in comparison with when the hydrate is formedwith pure water. The maximum growth rate of ethylene hydrate withpure water was seen at the onset of hydrate formation, while in the

Fig. 12. a) Comparison of the amount of energy needed to for ethylene hydrate formation with pure water and aqueous solutions of SDS; b) The percentage of energy saving in theformation of ethylene hydrate at the different concentrations of SDS.

13A. Al-Sowadi et al. / Journal of Molecular Liquids 294 (2019) 111608

presences of SDS and SLES the growth rate was maximized at a specifictime after hydrate formation. The analysis of results also demonstratedthat THF plays a crucial role in the enclathration of ethylene in hydratestructure. On the other hand, the energy measurements showed thatSDS and SLESwere able to decrease the required energy for hydrate for-mation in ethylene-water system to 82% and 63%, respectively. The ef-fect of SLES for decreasing of required energy of hydrate formation inethylene-THF-water was even m.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.molliq.2019.111608.

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