DOI: 10.1002/ente.201300064

Effects of Different Surfactants on the Kinetics of Ethane-Hydrate Formation: Experimental and Modeling StudiesReza Karimi,[a] Farshad Varaminian,*[a] Amir A. Izadpanah,[b] andAmir H. Mohammadi *[c, d]

Introduction

Gas hydrates, or clathrate hydrates, are ice-like compoundsthat form if certain molecules (such as methane, ethane, pro-pane, carbon dioxide, etc.) come into contact with waterunder appropriate thermodynamic conditions (high pressureand low temperature). Water molecules can form a latticestructure containing cavities, due to the hydrogen bondingbetween them. This structure is thermodynamically unstable.The inclusion of molecules with favorable shapes and sizesinto cavities can stabilize this structure. In fact, due to thevan der Waals forces between the encaged molecules and thewater lattice, the hydrate structure is thermodynamically sta-bilized. Based on the size, shape, and number of the cavitiesin the unit cell, three different crystalline structures [structur-es I (sI), II (sII), and H (sH)] can be typically formed.[1]

Natural-gas hydrates have been recently studied for uti-lization in industrial systems as a method to store and trans-port natural gas (e.g., 150–180 volumes of gas for eachvolume of hydrate).[2,3] The idea of storing natural gas in theform of a hydrate is advantageous not only because of thehigh storage capacity but also for the preservation of the hy-drate itself.[4] The slow hydrate-formation rate has hinderedthe industrial applications of hydrate storage. Natural gas isslightly soluble in water, so the most straightforward methodfor increasing the hydrate-formation rate is the utilization ofmixing processes (such as spraying water in a continuous gasphase, bubbling gas in water, and stirring) and these process-es can consume significant power.[1]

Many investigations have been conducted to increase thehydrate formation rate by adding surfactants to liquid waterat low stirring rates, particularly for the case of methane hy-drate. Zhong and Rogers[5] studied the effect of sodium do-decyl sulfate (SDS) on the ethane-hydrate-formation rate ina quiescent system. They found that SDS above its criticalmicelle concentration (CMC) increases the rate of ethane-hydrate formation by a factor of greater than 700. Han

et al.[6] investigated the effect of SDS on natural-gas-hydrateformation containing 90 wt % methane. They reported thata concentration of 300 ppm marks the maximum in hydrategas content. Karaaslan et al.[7] investigated the effect oflinear alkyl benzene sulphonic acid in concentrations of 0,0.01 and 0.05 wt % on a mixture of methane and gas (includ-ing 88.17 % propane). They found that the increase in the sIhydrate formation rate is higher than that of sII. Sun et al.[8]

investigated the hydrate-formation rate and storage capacityof hydrates of synthetic natural gas (methane 92.05 mol %,ethane 4.96 mol %, propane 2.99 mol %) in the presence ofanionic and nonionic surfactants . They found that the anion-ic surfactants are more effective than nonionic surfactants.Okutani et al.[9] surveyed the effect of the anionic surfactantsSDS, sodium tetradecyl sulfate (STS), and sodium hexadecylsulfate (SHS) on the methane-hydrate-formation rate. Theyreported that all of these surfactants increased the hydrate-formation rate. These materials differ in their alkyl chainlength. Each of these surfactants increased the solubility of

An experimental study of the ethane-hydrate-formation ki-netics has been performed. First, the effects of stirring veloci-ty and initial pressure on the hydrate-formation rate were in-vestigated. Second, the effects of additives on the ethane-hy-drate-formation rate were also studied. Three surfactants,namely sodium dodecylbenzenesulfonate (SDBS), dodecyltrimethyl ammonium bromide (DTAB), and TritonX-100

(TX-100) were tested. The results show that DTAB not onlydoes not significantly promote the hydrate-formation rate atdifferent concentrations, but actually inhibits the ethane-hy-drate-formation rate, whereas SDBS does increase the hy-drate-formation rate. Furthermore, TX-100 increases theethane-hydrate-formation rate, but to a lesser degree thanSDBS.

[a] R. Karimi, F. VaraminianDepartment of Chemical Engineering, Oil and Gas,Semnan UniversitySemnan (Iran)E-mail: fvaraminian@semnan.ac.ir

[b] A. A. IzadpanahGas and Petrochemical Engineering FacultyPersian Gulf UniversityBushehr (Iran)

[c] Dr. A. H. MohammadiInstitut de Recherche en G�nie Chimique et P�trolier (IRGCP)Paris Cedex (France)E-mail: a.h.m@irgcp.fr

[d] Dr. A. H. MohammadiThermodynamics Research Unit, School of Chemical EngineeringUniversity of KwaZulu-Natal, Howard College CampusKing George V Avenue, Durban 4041 (South Africa)

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methane in water between 1.82–3.75 times; SDS had thestrongest effect on the solubility of methane in water due tohaving the shortest alkyl chain. Daimaru et al.[10] conductedan experimental study on the effect of anionic surfactants(C4, C12, and C18) on the hydrate-formation rates of meth-ane and xenon. They found that the surfactant with theshortest chain length (C4, butanesulfonic acid sodium salt)showed the highest degree of promotion.

Zhang et al.[11] investigated the effect of SDS on the natu-ral-gas-hydrate formation rate. They reported that in thepresence of these surfactants, the hydrate-formation rate andstorage capacity increase. Lin et al.[12] investigated the effectof SDS on the methane-hydrate-formation rate. They report-ed that at 650 ppm concentration of SDS, the methane-hy-drate-formation rate is increased significantly. Di Profioet al.[13] investigated the effect of anionic and cationic surfac-tants on the methane-hydrate-formation rate. They reportedthat all of these materials have positive effects on the rate ofmethane-hydrate formation. Watanabe et al.[14] studied theeffect of SDS on the hydrate formation rate of difluorome-thane (HFC-32). They reported that this surfactant decreasedthe surface tension of the gas–liquid interface and increasedhydrate formation rate. Ganji et al.[15] studied the effect ofSDS, linear alkyl benzene sulfonate (LABS), cationic surfac-tant cetyl trimethyl ammonium bromide (CTAB), and non-ionic surfactant ethoxylated nonylphenol (ENP) in variousconcentrations (300, 500 and 1000 ppm) on hydrate forma-tion rate of methane. They reported that all of these materi-als generally increased the rates of hydrate formation, withthe exceptions of LABS at 300 ppm, and CTAB and ENP atconcentrations of 300 and 500 ppm, for which decreases inthe hydration-formation rate were observed. Li et al.[16] in-vestigated the capture of CO2 from a binary mixture (CO2 +

N2) by forming a hydrate in a 5 wt% solution of tetrabuty-lammonium bromide (TBAB); they found that TBAB in-creased the hydrate-formation rate and enriched the level ofCO2 in the hydrate phase. Lee et al.[17] studied the effect ofSDS, LABS, and Tween 20 on the hydrate-formation rate ofsulfur hexafluoride (SF6). They reported that surfactants in-creased the rates of hydrate formation. Yoslim et al.[18] inves-tigated the effects of surfactants SDS, STS, and SHS on thehydrate-formation rate of a gas mixture of methane and pro-pane. They reported that all of these surfactants increasedthe gas-hydrate-formation rate.

Various driving forces such as differences in temperature,fugacity, and concentration are utilized to initiate hydrateformation. Englezos et al.[19] used the difference between thesolvated-gas fugacity and equilibrium hydrate fugacity at thetemperature of the experiment as a driving force in the ki-netic model of hydrate formation. Skovborg and Rasmus-sen[20] used the difference between the mole fraction of gasat the gas–liquid contact surface and bulk liquid phase as thedriving force. Herri et al.[21] stated that the driving force forgas solubility in water is the difference between the gas con-centration at the gas–liquid interface and the bulk liquid.Varaminian and Danesh[22] expressed the driving force for

hydrate crystal growth as the difference between the equilib-rium temperature and the experimental temperature.

However, investigations of the effects of surfactants on thekinetic behavior of ethane hydrate are relatively scarce inthe literature. Therefore, the main purpose of this study is toinvestigate the effect of various concentrations of sodium do-decylbenzenesulfonate (SDBS) as an anionic (negativecharge) surfactant, dodecyl trimethyl ammonium bromide(DTAB) as cationic (positive charge) surfactant, and tritonX-100 (TX-100) as a nonionic surfactant on the hydrate-for-mation rate of ethane hydrate. In addition, the influences ofthe stirring rate and the initial pressure (driving force) onthe kinetics of ethane-hydrate formation have been investi-gated. A kinetic model based on mass transfer is used tomodel these data and the kinetic coefficient is evaluated.

Results and Discussion

Effect of stirring rate

Stirring is the most straightforward method to enhance themixing of the gas and liquid water phases and increase thehydrate-formation rate. One question should be addressed:What are the effective and optimum conditions for increas-ing the ethane hydrate formation rate at 275.2 K? To answerthis question, ethane hydrate was initially formed at 275.2 K(stirring velocities were 450, 600, and 800 rpm) without sur-factant. Figure 1 represents gas consumption versus time at

different stirring velocities. The results showed that an in-crease in stirring velocity from 450 to 600 rpm and from 600to 800 rpm increased the hydrate formation rate. The ethane-hydrate-formation rate at the increased stirring velocity of800 rpm was almost unchanged. Therefore, for stirring veloci-ties greater than 800 rpm, the liquid mass-transfer resistance

Figure 1. Gas consumption curve for ethane hydrate at different stirring rates.

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Kinetics of Ethane-Hydrate Formation

is almost eliminated 1kL� 1

kf

� �. Table 1 shows the kinetic con-

stants at various stirring velocities. Thus, 800 rpm is the opti-mum stirring velocity at 275.2 K without surfactant.

Effects of initial pressure

Different kinetic experiments were conducted to investigatehow the initial pressure affects the ethane-hydrate-formationkinetics at a stirring velocity of 800 rpm. Figure 2 shows theeffects of various initial pressures on the ethane-hydrate-for-

mation rate at 275.2 K. One can see that increasing the initialpressure from 1.56 to 2.3 MPa resulted in a significant pro-motion of the hydrate formation rate. The kinetic constantsof hydrate formation at various initial pressures are present-ed in Table 2. The kinetic constants increased as the initialpressure was increased. This can be partly interpreted by thefollowing brief discussion. With an increase in initial pres-

sure, the interfacial tension of the aqueous phase with thegas phase is reduced and the interactions with interfacialwater are weakened, so that gas molecules (ethane) could betaken into the aqueous phase more easily than at lower ini-tial pressures.

Effects of surfactants

To investigate the effects of various surfactants on theethane-hydrate-formation rate, experiments were performedwith SDBS as an anionic surfactant, DTAB as a cationic sur-factant, and TX-100 as nonionic surfactant, as mentionedearlier. The results above show that the stirring velocity af-fected the rate of hydrate formation and additional experi-ments with different surfactant concentrations were per-formed at stirring rates of 450 and 800 rpm.

The results of various concentrations of these surfactantson the rate of consumption of ethane (for the formation ofethane hydrate) at 275.2 K and 450 rpm are shown in Fig-ures 3–5. Figure 3 shows the effect of SDBS on the ethane-

hydrate-formation rate, and Figure 4 shows the effect of TX-100 on ethane-hydrate-formation rate. These results indicatethat, at concentrations used in this work (300, 500, and1000 ppm), SDBS effectively increased the ethane-hydrate-formation rate. Figure 5 shows the effect of the DTAB con-centration on ethane-hydrate-formation rate. As indicated inFigure 5, the concentration of DTAB effectively slows therate of formation of ethane hydrate.

The overall kinetic constants at 450 rpm were calculatedand are shown in Table 3. As shown in this table for SDBS,the overall kinetic constant for the ethane-hydrate-formationrate increases by a factor of more than 90 relative to purewater. Also, the results of modeling at a stirring rate of450 rpm and concentration of 500 ppm of surfactant areshown in Figure 6. The results of DTAB show that this sur-

Table 1. The overall kinetic constant at different stirring rates.

Stirring velocity [rpm] aK* [mol2 J�1 s�1]

450 1.3312 � 10�8

600 1.2004 � 10�7

800 [a]1.3973 � 10�6

[a] this value is akf, because at this stirring rate we assume the liquidphase mass resistance is vanished in this case.

Figure 2. Gas consumption curve for ethane hydrate at different initial pres-sures.

Table 2. The kinetic constant of hydrate formation at different initial pres-sures and 800 rpm stirring rate.

Initial pressure [MPa] akf [mol2 J�1 s�1]

1.56 1.8521 � 10�6

1.87 1.9413 � 10�6

2.3 2.1954 � 10�6

Figure 3. Gas consumption curve for ethane hydrate at different concentra-tions of SDBS and 450 rpm stirring rate.

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F. Varaminian and A. H. Mohammadi et al.

factant has the opposite influence on the ethane-hydrate-for-mation rate; by increasing the concentration of this surfac-tant the rate of ethane hydrate formation decreased at a stir-ring rate of 450 rpm.

Similar experiments were conducted at two surfactant con-centrations (300 and 500 ppm) and a stirring rate of 800 rpm.The experiments at a stirring rate of 450 rpm showed that atsurfactant concentrations of 300 and 500 ppm the promotion-al effects were much stronger than at 1000 rpm. Then, similar

experiments were conducted at two surfactant concentrations(300 and 500 ppm) and a stirring rate of 800 rpm. These re-sults are shown in Figure 7–9. As can be seen, the ethane-hy-drate-formation rate at 800 rpm is almost independent of thesurfactant concentration.

The kinetic constants of hydrate formation at 800 rpmwere calculated and the results are reported in Table 4. It isassumed that at 800 rpm the mass transfer resistance in theliquid phase was attenuated and only the kinetic constant af-fected the hydrate formation rate. According to Table 4, at

Figure 4. Gas consumption curve for ethane hydrate at different concentra-tions of TX-100 and 450 rpm stirring rate.

Figure 5. Gas consumption curve for ethane hydrate at different concentra-tions of DTAB and 450 rpm stirring rate.

Table 3. The overall kinetic constant of hydrate formation at different sur-factant concentrations and 450 rpm.

Surfactant concentration [ppm] aK* [mol2 J�1 s�1]SDBS TX-100 DTAB

300 1.1347 �10�6 9.6597 � 10�8 7.0596 �10�8

500 9.7218 �10�7 2.5301 � 10�7 3.1374 �10�8

1000 1.5063 �10�6 4.7860 � 10�7 1.2214 �10�8

Figure 6. Comparison between the model and experimental data at 450 rpmstirring rate at 500 ppm concentrations of SDBS and TX-100.

Figure 7. Gas consumption curve for ethane hydrate at different concentra-tions of SDBS and 800 rpm stirring rate.

Table 4. The kinetic constant of hydrate formation at different surfactantconcentrations and 800 rpm stirring rate.

Surfactant concentration [ppm] akf [mol2 J�1 s�1]SDBS TX-100 DTAB

300 8.5227 �10�6 6.2518 � 10�6 5.4817 �10�6

500 8.2891 �10�6 6.0669 � 10�6 5.9712 �10�6

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Kinetics of Ethane-Hydrate Formation

higher concentrations of SDBS and TX-100 the rate ofethane-hydrate formation slightly decreased wheras forDTAB the ethane-hydrate-formation rate slightly increased.

Comparing these results with the kinetic constants of for-mation at a stirring rate of 800 rpm using pure water showsthat the rate of ethane-hydrate formation was increasedslightly in the presence of surfactants and concentration andtype of surfactant did not influence the rate of ethane hy-drate formation.

Conclusions

In this work, the effects of stirring rate, initial pressure, andconcentration of various surfactants were investigated on theethane-hydrate-formation rate. The results show that the ef-fects of stirring rate and initial pressure were similar to otherstudies: by increasing the stirring rate and initial pressure,the ethane-hydrate-formation rate was increased.

Three surfactants were used in this study, SDBS as anionicsurfactant, DTAB as cationic surfactant, and triton X-100(TX-100) as nonionic surfactant. The results show that SDBSeffectively increases the ethane-hydrate-formation rate atvarious concentrations. DTAB has the opposite effect on theethane-hydrate-formation rate, which decreased with increas-ing DTAB concentration. TX-100 also increased the ethane-hydrate-formation rate at concentrations of 300, 500, and1000 ppm but it is not as effective as SDBS. The results showthat at high stirring rates (800 rpm in this study), the effectof the concentration and type of surfactant do not have greateffects on the rate of ethane-hydrate formation.

Experimental Section

Materials

Analytical-grade ethane supplied by Technical Gas Services(99.99%) and double-distilled water were used. The surfactantsutilized in the experiments were: Sodium dodecylbenzenesulfo-nate (SDBS) as an anionic surfactant with the chemical formulaC18H29NaO3S (Sigma–Aldrich), dodecyl trimethyl ammoniumbromide (DTAB) as a cationic surfactant with the chemical for-mula C15H34BrN (Merck), and TritonX-100 (TX-100) as a nonion-ic surfactant with the chemical formula C14H21(C2H4O)nOH(Merck). A list of chemicals used in the experiments along withtheir their purities and suppliers is given in Table 5.

Apparatus

Figure 10 shows a schematic of the experimental setup used inexperiments. The setup consists of a high-pressure stainless-steelreactor with a capacity of 600 cm3 that is equipped with a four-blade mixer. It is designed for operating pressures up to 10 MPaand temperatures ranging from 253.15 to 373.15 K. A PT100thermometer (Pro-Temp Controls) with an accuracy of betterthan 0.1 K, and a pressure transmitter (BD-Sensors-Str. 1) withan uncertainty of �0.01 MPa were used to measure temperatureand pressure of the reactor, respectively. A data acquisitionsystem was used to record the pressure and temperature of thereactor at preset intervals on the computer. A mixture of waterand ethylene glycol was used as coolant and cooling was ach-ieved by circulating the coolant through the inner and outerlayers of the reactor. A temperature bath was used to control thetemperature of the cooling fluid.

Experimental procedure

The surfactants were measured out using a balance with an accu-racy of 0.1 mg for the preparation of aqueous solutions at the re-

Figure 8. Gas consumption curve for ethane hydrate at different concentra-tions of TX-100 and 800 rpm stirring rate.

Figure 9. Gas consumption curve for ethane hydrate at different concentra-tions of DTAB and 800 rpm stirring rate.

Table 5. List of chemicals used in the experiments.

Chemical Purity [%] Supplier

Ethane 99.95 Arkan gasSDBS �98 Sigma–AldrichTX-100 �98 MerckDTAB �97 MerckWater Distilled Own

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F. Varaminian and A. H. Mohammadi et al.

quired concentrations. First, the reactor was washed and rinsedwith deionized water. A vacuum pump was used for evacuatingthe hydrate-formation cell. Then, pure water (300 cm3) was intro-duced into the cell for the case of evaluating the effect of the ini-tial pressure on the hydrate formation rate. Also, to estimate theinfluence of the stirring rate and surfactant concentration on thehydrate-formation rate, pure water (150 cm3) or an aqueous solu-tion of surfactant was introduced into the cell. As all experi-ments were performed at 275.2 K, the reactor was then pressur-ized to approximately 0.15 MPa below the equilibrium pressureof ethane-hydrate formation at this temperature. The constant-temperature bath was turned on and the reactor was allowed toreach a steady-state temperature. After thermal equilibrium wasreached, the reactor was pressurized to the desired initial pres-sure with ethane from the cylinder. The mixer was started at thedetermined stirring velocity, and the temperature and pressuredata were recorded during the each experiment. Each experi-ment was continued until the hydrate sample reached a constantpressure.During each experiment, the pressure of the reactor was moni-tored and saved on computer by using a data acquisition system.At any time, the gas pressure in the reactor was converted tomoles of gas by the following equation:

n ¼ PVZRT

ð1Þ

in which P is the pressure of the reactor, V is the volume of thegas in the reactor (the difference between the volume of the re-actor and the volume of the liquid in the reactor), T is the abso-lute temperature of the reactor, and R is the gas constant. Thecompressibility factor Z was calculated by using an equation ofstate (Peng–Robinson). The difference between the initial molesof gas and moles of gas at any time in the reactor is the amountof gas consumed during hydrate formation. Then the rate of gasconsumption during hydrate formation is the ratio of the amountof gas consumed to the time interval.

Kinetic model of hydrate forma-tion

In this study, the kinetic modelproposed by Daimaru et al.[10] hasbeen used to describe the kineticsof ethane-hydrate formation. Ac-cording to this model, the chemi-cal potential difference is consid-ered as the driving force for hy-drate formation. The mass trans-fer is involved in the overall gasuptake rate, and the kinetic equa-tion is given by:

� dndt

� �¼ a K* mg � meq

� �ð2Þ

in which a and K* represent theinterfacial area and overall kinet-ic constant, respectively, m is thechemical potential, and the sub-scripts “g” and “eq” designatea guest molecules in the gas andhydrate phases, respectively. The

overall kinetic constant (K*) can be written in terms of the mass-transfer coefficient in the liquid phase (kL) and the hydrate crys-tal growth constant (kf) as follows:

1K*¼ 1

kLþ 1

kfð3Þ

The mass transfer coefficient of the liquid phase increases withincreasing stirring velocity whereas the crystal growth constant isindependent of the stirring velocity. By using vigorous stirringduring the hydrate-formation process, the mass transfer contribu-

tion, 1kL

, can be eliminated 1kL� 1

kf

� �. Under such conditions, the

kinetics of hydrate formation can be given by:

rf ¼ �dndt

� �¼ a kf mg � meq

� �ð4Þ

Thus, the kinetic equation for hydrate formation can be givenby:

rf ¼ �dndt

� �¼ a kf R T ln

fg

feq

� �ð5Þ

in which R is the gas constant, T is temperature, and f is the fu-gacity. The subscripts “g” and “eq” are attributed to guest mole-cules in the gas and in the hydrate phases, respectively. The rateconstant (akf), can be determined from the curve of experimentalpressure versus time by fitting on the basis of Equation (4). It isimpossible to measure interfacial area for hydrate formation inthe present system, so the rate constant is expressed in terms ofakf. For a vigorous stirring velocity:[16]

rf ¼Dn

D t

� �¼ ug

RT

fg � feq

� �D t

ð6Þ

Figure 10. Diagram of the experimental setup used in this study.

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Kinetics of Ethane-Hydrate Formation

Keywords: clathrates · ethane · gas hydrates · kinetics ·surfactants

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Received: June 10, 2013Revised: June 6, 2013

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F. Varaminian and A. H. Mohammadi et al.