anti-agglomeration in cyclopentane hydrates from bio- … pub/3linegadifiroozabadi.pdf ·...

4937r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels 2010, 24, 4937–4943 : DOI:10.1021/ef100622pPublished on Web 08/30/2010

Anti-agglomeration in Cyclopentane Hydrates from Bio- and Co-surfactants

Xiaokai Li,† Latifa Negadi,‡ and Abbas Firoozabadi*,†

†Department of Chemical Engineering, Mason Laboratory, Yale University, New Haven, Connecticut 06520, and‡Faculty of Sciences, Department of Chemistry, University Abou Bekr Belkaid of Tlemcen, Post Office Box 119,

Tlemcen 13000, Algeria

Received May 18, 2010. Revised Manuscript Received July 28, 2010

Hydrate formation in subsea pipelines is a serious problem in gas and oil production for offshore fields.Current methods are mainly based on thermodynamic inhibitors to change bulk phase properties.Thermodynamic inhibitors, such as methanol, are very effective, but large quantities, sometimes as highas a 1:1 volume of alcohol/water, are required. Kinetic inhibitors generally in a 0.005-0.02 volume ratio ofsurfactant/water can either inhibit hydrate formation or reduce the rate of growth. In the sea bed, thesubcooling for hydrates is around 20-25 �C because of the sea bed temperature of about 4 �C. The kineticinhibitors are not effective at such a high subcooling. An effective method is the use of anti-agglomerants,which allow for hydrate formation in the form of small particles and prevent agglomeration of suchparticles. Rhamnolipid biosurfactant and methanol are used recently to demonstrate anti-agglomeration intetrahydrofuran (THF) hydrates. In this work, we present data for cyclopentane hydrates to demonstratethat a mixture of rhamnolipid and methanol is the ideal combination for effective anti-agglomeration. Theformation of cyclopentane hydrates is believed to be closely analogous to methane hydrate formationbecause of the low solubility of cyclopentanes in water and various aspects of crytallization.

Introduction

Natural gas has a high hydrogen/carbon ratio compared topetroleum fuels and coal. It is also a clean-burning fuel, whichresults in low production of CO2. As a result, natural gas is adesirable fuel. A large portion of natural gas is produced fromthe deep sea, where the temperature is low. The low tempera-turemay result in the formation of gas hydrates frommethaneand some other species in the gas and co-produced water. Thehydrates may plug gas pipelines. To prevent hydrate forma-tion, large quantities of alcohols, such as methanol, are addedto water to change bulk phase properties. The use of largequantities of alcohols is costly and damages the environment.An alternative is the use of surfactants to alter surface proper-ties. Surface property changes lead to hydrate kinetic inhibi-tion, which delays nucleation or growth of hydrates. Thelimitation of this process is applicability in low subcooling,1

where subcooling is defined as the difference between thehydrate equilibrium temperature and the operating tempera-ture at a given pressure. In some deep sea environments, thesubcooling can be as high as 25 �C.An attractive alternative isthe use of surfactants that do not prevent hydrate formationbut prevent agglomeration of hydrate particles. Anti-agglom-erants (AAs) are effective at high subcooling in flow condi-tionsor at shut-in conditions, i.e., whenpipeline flow is pausedfor a period of time.2-4 Despite the promise of the process,

AAs have not been studied in the published literature asextensively as kinetic inhibitors.

In two recent papers,5,6 we have reviewed the literature onhydrate anti-agglomeration and have shown that a biosurfac-tant can be very effective at low concentrations.We have alsoshown that methanol at low concentrations serves as a co-surfactant in anti-agglomeration. We have used tetrahydro-furan (THF) hydrateswhen studying anti-agglomerationwiththe biosurfactant and MeOH co-surfactant. THF formsstructure II hydrates and is much more soluble in water thanany species in natural gas. As a result, there may be a concernthat the crystallization inTHFhydratesmaybe different frommethane hydrates. THFhydrates may form in the bulk phase,whereasmethane and propane hydratesmay formon the inte-rface between water and oil phases. The formation of THFhydrates unlike methane hydrates occurs at atmosphericpressure. This gives a vast advantage in conducting experi-ments to improve the understanding of the process. In thiswork, we study the cyclopentane (CP) hydrate former, whichhas a low solubility inwater. CP is known to form the structureIIhydrate at a temperaturenear 280Kat atmospheric pressure.CP hydrates in some respect are close to hydrates from naturalgas species.We also useCPas the oil phase to formwater-in-oilemulsion. Themain goal of this research is to examine whetheranti-agglomeration in CP hydrates can be achieved by a lowconcentration of a surfactant and a co-surfactant.

Experimental Section

Apparatus.The experimental setup used in this work is similarto the one used in our previous work on THF hydrate anti-agglomeration.5-7 The setup is a multiple screening-tube rocking

*To whom correspondence should be addressed. E-mail: [email protected].(1) Kelland, M. A. Energy Fuels 2006, 20, 825.(2) Kelland,M. A.; Svartaas, T. M.; Dybvik, L. A. Proceedings of the

Society of Petroleum Engineers (SPE) 69th Annual Technical Conferenceand Exhibition; New Orleans, LA, 1994.(3) Urdahl, O.; Lund, A.; Mork, P.; Nilsen, T. N. Chem. Eng. Sci.

1995, 50, 863.(4) Huo, Z.; Freer, E.; Lamar, M.; Sannigrahi, B.; Knauss, D. M.;

Sloan, E. D. Chem. Eng. Sci. 2001, 56, 4979.

(5) York, J. D.; Firoozabadi, A. J. Phys. Chem. B 2008, 112, 845–851.(6) York, J. D.; Firoozabadi, A. J. Phys. Chem. B 2008, 112, 10455–

10465.

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Energy Fuels 2010, 24, 4937–4943 : DOI:10.1021/ef100622p Li et al.

apparatus, which consists of a motor-driven agitator with a rackholding up to 20 separate borosilicate glass scintillation vials. Thevial dimensions are 17 mm (diameter) and 60 mm (height)submerged in a temperature bath. Each vial holds roughly 7 mLof a test mixture and a ∼8 mm diameter stainless-steel 316 ball.The ball aids agitation. A Teflon-lined plastic screw cap is usedalong with Teflon tape, around threads, to seal vials. The rackrotates the vials 150� to either side of the vertical direction,completing a cycle every 5 s. The temperature bath used is aHuberCC2-515 vpc filled silicon oil (10 cSt at 24 �C) fromClearcoProducts Co., Inc., Bensalem, PA.

Thermocouples with an accuracy of (0.2 �C from 70 to-20 �C are attached to the outside of the vials when crystal-lization and melting data are desired. The thermocouples areattached to the outside wall with the use of such vials. As wereport in refs 5 and 6, placement of thermocouples inside thevials may serve as a nucleation active site. The temperaturesinside the vial and vial wall are also the same. Therefore, outsideplacement is a method of choice for the temperature measure-ment. An Agilent 34970A data acquisition unit, recordingtemperature every 20 s, and an ice bath as a fixed junctionreference temperature are used with all thermocouples. A sketchof the apparatus is shown in Figure 1.

Chemicals. In all testmixtures, deionizedwater obtained froma Barnstead Nanopure Infinity system with a quality of roughly5.5 � 10-2 μS/cm and 99%þ purity CP (from Acros) are used.CP serves both as the oil phase and the hydrate former. Thesolubility of CP in water is very low, about 160 ppmbyweight at25 �C, and is nearly constant in the temperature range of 0-25 �C.8 The solubility of methane in water is around 1000 ppmby mole at 100 bar and room temperature.

Rhamnolipid biosurfactant (product JBR 425) (Rh) is ob-tained from Jeneil Biosurfactant Co., Madison, WI. It is amixture of two forms at 25 wt% in water. Themixture is dilutedwith water when preparing samples with different Rh concen-trations. Rh is used as supplied and is the same as discussed inref 5. The co-surfactant, 99.8%anhydrousMeOHwith less than5 ppm water, is obtained from Acros.

Procedure. The experimental procedures are the same as inrefs 5-7. A composition of mostly x/1/0.02/y parts by weight ofCP/water/THF/surfactant is used in our tests, where x is avarying amount of CP and y is a varying surfactant concentra-tion in different tests. In the tests with the co-surfactantMeOH,the mixture composition is x/1/0.22/y/z, where z is the weightratio of MeOH/water. Each sample is prepared in duplicate(except those data points without an error bar).

Temperature data are acquired separately from visual ob-servations, because half of the sample vial surface area is covered

when thermocouples are attached. However, the temperaturecontrol of the bath and agitation are the same for both types ofexperiments.

Kinetic/ThermodynamicDataAcquisition.Mixtures are broughtto 11 �C. This temperature is higher than 7 �C, the reportedequilibrium temperature of CP hydrates at 1 atm.9,10We allow theliquid mixture to reach equilibrium, and then a 5 �C/h coolingramp is employed to a desired temperature for a specific sample,where hydrates form without ice. The mixture is then heated backto 15 �Cat 15 �C/h.As themixture is cooled below the equilibriumtemperature, it crystallizes at the crystallization temperature (Tc)and an exothermic heat release begins. Once crystallization hasoccurred, the sample temperature rejoins that of the bath fluid.Dissociationof themixtureduringheating showsas anendotherm,the beginning of which is labeled as the dissociation temperature,Td. This is the same as the equilibrium hydrate temperature.

Agglomeration States. Experiments for gathering visual ob-servations are conducted similarly to crystallization/dissocia-tion testing. Agitated mixtures are allowed to equilibrate at11 �C, and then 5 �C/h cooling is applied to bring themixtures toa temperature where CP hydrates are formed without theformation of ice. The procedure deviates from the kinetic/thermodynamic test, whereby the minimum temperature is heldconstant (some samples are heated to 1.5 �C for ease of obser-vation). Observations are made at 2 and 24 h into this period,mainly with the naked eye but also with the borescope. Theseobservations show whether a dispersion of hydrate crystals isfacilitated by the surfactant or surfactant/co-surfactant oragglomeration occurs.

Emulsion Stability. Mixtures of x/1/0.02/y and x/1/0.02/y/z(weight ratio to water) of CP/water/THF/surfactant and CP/water/THF/surfactant/co-surfactant are prepared and homo-genized by shaking by hand for 1 min. The time that it takes for60 vol%of the initial aqueous phase to separate ismeasured andused as an indicator of emulsion stability.

Results and Discussion

Formation of CP Hydrates. Attempts are made to obtainCP hydrates in mixtures of CP and water. It is found that nocrystallites (CP hydrates) are formed because there is noexothermic peak when cooling the sample to a temperatureabove 0 �C. When cooling the sample to-2 �C or below, iceis formed, indicated by the endothermic peak starting at 0 �C.Whitman et al.10 have reported that, for mixtures of CP andwater as either water-in-oil or oil-in-water emulsion, hydrateand ice always form simultaneously, with ice forming pre-ferentially. Some authors have shown that methane andother gases, which can be incorporated into the smallercavities of the CP hydrate at modest pressures, can serve asa helper molecule in the formation of CP hydrates.11,12 Notethat the equilibrium hydrate formation temperature for CPby Sloan and Koh13 is about 2 �C higher than the valuereported by Lo et al.9 andWhitmanet et al.10 We will discusslater that our results are in agreement with the data from refs9 and 10.

Effect of THF in CP Hydrate Formation. Helper mole-cules, such as methane, are used only under high pressurebecause of the very low solubility of these gases in water at

Figure 1. Multiple screening-tube rocking apparatus.

(7) York, J. D.; Firoozabadi, A. Energy Fuels 2009, 23, 2937–2946.(8) Yaws, C. L. Chemical Properties Handbook; McGraw-Hill: New

York, 1999.

(9) Lo, C.; Zhang, J. S.; Somasundaran, P.; Lu, S.; Couzis, A.; Lee,J. W. Langmuir 2008, 24, 12723.

(10) Whitman, C. A.; Mysyk, R.; Whitea,M. A. J. Chem. Phys. 2008,129, 174502.

(11) Sun, Z. G.; Fan, S. S.; Guo, K. H.; Shi, L.; Guo, Y. K.; Wang,R. Z. J. Chem. Eng. Data 2002, 47, 313.

(12) Tohidi, B.; Danesh, A.; Todd, A. C.; Burgass, R. W. Fluid PhaseEquilib. 1997, 138, 241–250.

(13) Sloan, E.D.;Koh, C.A.ClathrateHydrates ofNatural Gases, 3rded.; CRC Press, Taylor and Francis Group: Boca Raton, FL, 2008.

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atmospheric pressure. In this work, THF is employed as ahelper molecule because of its high solubility in water atatmospheric pressure. As can be seen in Figure 2, CPhydrates are formed by cooling samples of CP/water/THFwith a composition of 0.4/1/x (weight ratio to water), withx=0.01, 0.03, and 0.05, to a temperature of 0.5 �C (where icedoes not form). The results clearly indicate that the presenceof THF as a helper molecule gives rise to hydrate formation.The average dissociation temperature is 7.0 �C for the threeTHF concentrations of 0.01 (6.9 �C), 0.02 (7.0 �C), and 0.03(7.0 �C), in agreement with data from refs 9 and 10. Theresults reveal that THF does not measurably affect the dis-sociation temperature of CP hydrates. At higher concentra-tions than used, the THF concentration may affect thedissociation temperature of CP. The equilibrium hydrateformation temperature of THF is 3.5 �C.5

Effect of Rh in CP Hydrate Formation. Rh is added to theabove samples to study the AA effect in the concentrationrange of 0.001-0.05 (weight ratio to water). The addition ofRh depresses the CP hydrate formation temperature. Nohydrates are formed above 0 �C,with theRhpresence even inlow concentrations (as low as 0.001). The samples are cooledbelow 0 �C to formhydrates. Figure 3 shows the freeze-thawcycle data for a sample of CP/H2O/THF/Rhwith a composi-tion of 0.4/1/0.03/0.01. Because of 3% THF and 1% Rh inthe mixture, no ice forms to a temperature of -4 �C in themixture. For a conclusive study of anti-agglomeration ofhydrates, the formation of ice should be avoided. In thisexperiment, the weight ratio of CP/water is 0.4/1 and themolar ratio is 1/10, which is higher than the hydrate stoichio-metric molar ratio of 1/17.14 Figure 3 shows that the hydrateformation with surfactant Rh is accompanied by a highgrowth rate, as compared to the data in Figure 2 without Rh.

The addition of Rh depresses both the crystallization anddissociation temperatures. Figure 4 shows that the crystal-lization and dissociation temperatures both decrease as theconcentration of Rh increases in the mixture. All of the testspresented in Figure 4 are conducted with the lowest tem-perature held at -4 �C. The decrease in the dissociationtemperature can be explained from bulk-phase thermody-namics. The lower crystallization temperature relates to thelower driving force for hydrate formation from the additionofRh.Although crystallization is not repeatable, the averageof a few runs will give a measure of the effect. In a similar

way, the induction time from a few measurements gives anidea of the delay in the process. The systematic results fromduplicate and various runs show a depression of the cryslti-zation by Rh and its kinetic effect.

Effect of the CP Concentration in Hydrate Formation. Theamount of CP in the mixture affects the ratio of hydrates tothe sample volume. The total volume of the mixture is fixedat 7 mL. For a CP/H2O ratio of 0.4/1 (weight ratio), morehydrates form followed by a ratio of 2:1 and then a ratio of4:1. However, the dissociation temperature should not beappreciably affected. The data in Figure 5 clearly show thatthe CP amount does not affect the dissociation temperaturebecause of very low solubility of CP in water.

Figure 2. Freeze-thaw cycle data for mixtures of CP/H2O/THF of composition 0.4/1/x. The results are for 0.01, 0.03, and 0.05 weight ratio ofTHF/H2O.

Figure 3.Freeze-thaw cycle data for themixture of CP/H2O/THF/Rh of composition 0.4/1/0.03/0.01 (weight ratio to water).

Figure 4. Effect of Rh on the crystallization temperature (Tc) anddissociation temperature (Td) for mixtures of CP/H2O/THF/Rh ofcomposition 0.4/1/0.03/x (weight ratio to water).

(14) Zhang, Y. F.; Debenedetti, P. G.; Prud’homme, R. K.; Pethica,B. A. J. Phys. Chem. B 2004, 108, 16717–16722.

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Effect ofMeOH inCPHydrate Formation.The addition ofMeOH depresses the dissociation temperature of CP hy-drates. The data in Figure 6 reveal a decrease in the dissocia-tion temperature with an increasing concentration of MeOH.There is a synergistic effect of Rh andMeOH in the reductionof Td. The increase of Rh from 0.001 to 0.005 weight ratio towater depresses Td by 0.5 �C; the addition of MeOH by 0.01weight ratio to water lowersTd by 1.9 �C. The combined effectof the concentration increase ofRh from0.001 to0.005 and theadditionofMeOHby0.01 lowersTd by 3.5 �C,which is greaterthan the sum of the contributions from the increase in Rh andMeOH when added individually by about 1 �C. Note that thedissociation temperatures of CP/H2O/TH/Rh with composi-tions of 0.4/1/0.03/0.01 and 1.5/1/0.02/0.01 in Figures 4 and 6,respectively, are about the same (5.7 �C). This is an indicationthat neither the CP amount nor THF has an appreciable effecton the dissociation temperature of CP hydrates within therange of concentrations used in this work.

Agglomeration State. The central theme of this work is thestudy of anti-agglomeration in CP hydrate particles. Webegin the study of the agglomeration state by testing mix-tures of CP/H2O/THF/Rh of composition 4/1/0.02/x and 4/1/0.03/x (weight ratio to water). As Figure 7 shows, disper-sible hydrates are formed with a low concentration of Rh.The samples with 0.02 THF are cooled to -2 �C and thenkept at 1.5 �C for AA state observation, while the sampleswith 0.03 THF are cooled to -3 �C and then kept at 1.5 �Cfor AA state observation. There is no ice formation in thesetests. Stable dispersion is observed for samples with Rh

concentrations of 0.003-0.03 during the observation periodof 24 h.However, for lower and higher concentrations ofRh,there is change in the performance. At -2 �C, the mixtureswith Rh concentrations of 0.001 and 0.002 exhibit agglom-eration in the vial before 24 h.When the concentration of Rhis 0.05, there seems to be a plug when the samples are kept at-3 �C for 24 h. The hydrate plugwould immediately disperseinto stable dispersion when heavy shaking is applied. The“fake plug” may be due to the high viscosity of the solutionfor the Rh concentration of 0.05. By shaking, i.e., by in-creasing the shear force, the high viscosity is overcome andthere is stable dispersion.When the temperature at which themixture is kept for 24 h is compared to the dissociationtemperature shown in Figure 8, it can be seen that the anti-agglomeration effect in some of these samples is tested at asubcooling of about 8-10 �C. According to our work onTHF hydrates,5 subcooling does not appreciably affect AA.AA that performs well at a subcooling of 8 �C may be alsoeffective at a subcooling of 25 �C.

In our recent work on hydrate anti-agglomeration in THFhydrates and the work in the literature, it is widely knownthat the anti-agglomeration process becomes ineffective at ahighwater cut. In this work, we have also examined the effectof the water cut on anti-agglomeration.

For samples with 2 part CP, stable dispersion forms at-3 �C (for sample with 0.01 part of Rh) and -2 �C (forsamples with 0.003-0.005 part of Rh) when the concentra-tion of Rh is 0.003-0.01 without MeOH. When the Rhconcentration is high, in the range of 0.03-0.05, the AA

Figure 5. Effect of the CP amount on the dissociation temperature(Td) for mixtures CP/H2O/THF/Rh of composition x/1/0.03/0.01(weight ratio to water).

Figure 6. Effect of MeOH on the dissociation temperature (Td) formixtures of CP/H2O/THF/Rh/MeOH of composition 1.5/1/0.02/x/y (weight ratio to water).

Figure 7. Agglomeration states for mixtures of CP/H2O/THF/Rhof composition 4/1/x/y (weight ratio to water), where the THFamount x and Rh amount y are control variables: (þ) stabledispersion, (O) dispersible hydrate, hydrates that looks like a plugbut can be dispersed by heavy shaking, and (�) plugging tendency.The plugging tendency means that either a total (i.e., the steel ball isunable to move) or partial (i.e., the steel ball is unable to movethrough the entire length of the vial) plug occurs.

Figure 8. Effect of Rh on the dissociation temperature (Td) formixtures of CP/H2O/THF/Rh of composition 4/1/x/y (weight ratioto water).

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effectiveness decreases probably because of the high viscosityeffect. Plugs, either full or partial, where the steel ball isblocked from moving across the entire length of the vial,appear when the Rh concentration is lower than 0.003.

In ref 6, it is discovered that the addition of small quan-tities of MeOH as a co-surfactant may prevent agglomera-tion of THF hydrates. In this work, we also find that small

quantities of MeOH can be effective when added to sampleswith 2 part CP. MeOH has dual effects at low concentra-tions: co-solvent and co-surfactant, as shown by a recentwork by Moreira and Firoozabadi.15 Figure 9 demonstratesthatMeOHat aweight ratio of 0.005 to water is effective as aco-surfactant in anti-agglomeration where hydrate slurriesare formed at -2 �C. The slurries are also formed with 0.01MeOH.Note that there is agglomeration of hydrates at a lowMeOH concentration of 0.002. The dissociation data inFigure 10 and the temperature of -2 �C where the hydrates

Figure 9. Agglomeration states for mixtures of CP/H2O/THF/Rh/MeOH of composition 2/1/x/y/z (weight ratio to water), where theTHF amount x, Rh amount y, and MeOH amount z are controlvariables: (þ) stable dispersion, (O) dispersible hydrate, hydratesthat looks like a plug but can be dispersed by heavy shaking, and (�)plugging tendency. The plugging tendency means that either a total(i.e., the steel ball is unable to move) or partial (i.e., the steel ball isunable to move through the entire length of the vial) plug occurs.

Figure 10. (a) Effect of THF on the dissociation temperature (Td)for mixtures of CP/H2O/THF/Rh of composition 2/1/x/y (weightratio to water). (b) Effect of Rh and MeOH on the dissociationtemperature (Td) for mixtures of CP/H2O/THF/Rh/MeOH ofcomposition 2/1/0.02/x/y (weight ratio to water).

Figure 11. Stable dispersion observed for mixtures of CP/H2O/THF/Rh of composition 2/1/0.02/x (weight ratio to water), wherethe Rh amount x is 0.003-0.01 for the data shown in Figures 9 and10. The sample shown in the image contains 0.005 wt Rh and 0 wtMeOH. The vial is tilted roughly 60� from the horizontal with thebottom side up.

Figure 12.Agglomeration states for mixtures of CP/H2O/THF/Rh/MeOH of composition 1.5/1/0.02/x/y (weight ratio to water), wherethe Rh amount x and MeOH amount y are control variables: (þ)stable dispersion; (4) hydrates attached to the bottom or wall of thevial, and (�) plugging tendency. The plugging tendency meanseither a total (i.e., the steel ball is unable to move) or partial (i.e.,the steel ball is unable to move through the entire length of the vial)plug occurs.

(15) Moreira, L. A.; Firoozabadi, A. Langmuir 2009, 25, 12101–12113.

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are kept for a period of 24 h show anti-agglomeration for asubcooling of about 6-9 �C.

In Figure 11, we show a typical picture in which hydratesare in dispersed form; the vial wall and bottom are transpar-ent, with no crystallites adhered to the walls. The air bubblecan travel freely in the vial, indicating that there is noagglomeration.

The effectiveness of the MeOH co-surfactant is furtherconfirmed when MeOH is added to samples with 1.5 part ofCP, i.e., even higher water cut. Figure 12 shows the results.Plugs are formed to a Rh concentration of 0.01 when there isno MeOH. When only the concentration of MeOH is 0.005,hydrate slurries are formed at Rh concentrations of 0.005and 0.01. When the concentration of Rh is 0.003, a MeOHconcentration of 0.01 allows the formation of stable disper-sion. In mixtures of 2 and 1.5 part CP, a MeOH concentra-tion of 0.002 MeOH is not effective in anti-agglomeration.The results in Figure 12 correspond to a subcooling of about6-10 �C.We have also carried out experiments for mixturesin which the concentrations of CP are 1 and 0.4 (CP weight/water weight; water cut more than 50%). Hydrate agglom-eration appears in all samples even at relatively high con-centrations of Rh and MeOH. Figure 13 shows an examplewhere there is hydrate agglomeration for a mixture contain-ing 1.5 part CP and 1 part water. We are currently looking

into surfactants and co-surfactants that will allow anti-agglomeration at a high water cut.

Emulsion Stability. In the past, emulsion stability inhydrate anti-agglomeration has been suggested to be veryimportant.16 In refs 5-7, we show that emulsion stabilitymay not be critical. In this work, we use the methodology inrefs 5-7 tomeasure emulsion stability of themixtures.We reportthe average time that it takes to form 60% of the water volume toseparate in the mixture. Table 1 gives emulsion stability resultswith two duplicate tests. The weight ratio of CP/water in themixtures is 1, 1.5, and 4 parts, with Rh at 0.001, 0.002, 0.003,0.005, and 0.01. For samples with 1.5 CP ratio to water, themethanol concentration is 0, 0.002, and 0.005. As can be seenfrom the table, the Rh concentration increases emulsion stabilityin all mixtures. The CP concentration also increases emulsionstability. The addition of methanol generally increases emulsionstability, but the effect is not significant.

Conclusions

In this work, we have demonstrated that anti-agglomera-tion in CP hydrates is similar to THF hydrates. A smallamount of MeOH in the mixture has a significant effect on

Figure 13. Significant hydrates being adhered to the side walls and bottom observed in mixtures of CP/H2O/THF/Rh/MeOH of composition1.5/1/0.02/0.003/0.005 (weight ratio to water) for the data shown in Figures 6 and 12. The vial is tilted roughly 60� from the horizontal with thebottom side up.

Table 1. Emulsion Stability (min)a

Rh (weight ratio to water)

CP (weight ratio to water)

1 1.5 4

MeOH (weight ratio to water)

0.000 0.002 0.005

0.001 1.2( 0.3 2.3( 0.30.002 1.4( 0.2 7.6( 0.90.003 1.3( 0.2 5.9( 0.5 5.3( 0.3 5.3( 0.8 10.7( 1.00.005 1.9( 0.3 6.7( 1.2 7.2 ( 0.6 8.5( 0.8 11.3( 0.50.01 10.0 ( 2.0 11.7( 1.5 16.3( 2.2

aA fresh sample is hand-agitated for 1 min, and the time for separation of 60 vol % of the initial aqueous phase is measured and used as an indicatorof emulsion stability.

(16) Zanota, M. L.; Dicharry, C.; Graciaa, A. Energy Fuels 2005, 19,584–590.

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the anti-agglomeration between hydrate particles.MeOH canlower the concentration ofRh for anti-agglomeration in someof the mixtures. In other mixtures, MeOH results in anti-agglomeration when the concentration increase of biosurfac-tant does not lead to anti-agglomeration. Similar to theprevious work,5-7 a small amount of MeOH is sufficient toprevent agglomeration because of the co-surfactant nature ofthe alcohol.Wehave also shown that the addition of THFas ahelper molecule in the formation of CP hydrates does notaffect the equilibriumhydrate temperature within the range of1-5 wt% of water. In working with THF, a large subcooling(as high as 25 �C) could be imposed without ice formation.With CPs, a subcooling of about 10 �C is studied because

higher subcooling will lead to the formation of ice. However,the fact that, within a period of 24 h, a hydrate slurry can bemaintained may imply effectiveness at high subcooling. Ourdata also confirms that the equilibrium hydrate formationtemperature for CP is around 7 �C. This value is about 2 �Cless than the value suggested in ref 13.

Acknowledgment. We are grateful to Jeneil Biosurfactant Co.for providing the rhamnolipid sample. This work was supportedby themember companies of theReservoir EngineeringResearchInstitute (RERI) in Palo Alto, CA. One of the authors (L.N.)gratefully acknowledges a grant from the U.S. GovernmentFulbright Program.

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