Energy Fuels 2010, 24, 4937–4943 Published on Web 08/30/2010
: DOI:10.1021/ef100622p
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 high as a 1:1 volume of alcohol/water, are required. Kinetic inhibitors generally in a 0.005-0.02 volume ratio of surfactant/water can either inhibit hydrate formation or reduce the rate of growth. In the sea bed, the subcooling for hydrates is around 20-25 °C because of the sea bed temperature of about 4 °C. The kinetic inhibitors 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 such particles. Rhamnolipid biosurfactant and methanol are used recently to demonstrate anti-agglomeration in tetrahydrofuran (THF) hydrates. In this work, we present data for cyclopentane hydrates to demonstrate that a mixture of rhamnolipid and methanol is the ideal combination for effective anti-agglomeration. The formation of cyclopentane hydrates is believed to be closely analogous to methane hydrate formation because of the low solubility of cyclopentanes in water and various aspects of crytallization.
AAs have not been studied in the published literature as extensively as kinetic inhibitors. In two recent papers,5,6 we have reviewed the literature on hydrate anti-agglomeration and have shown that a biosurfactant can be very effective at low concentrations. We have also shown that methanol at low concentrations serves as a cosurfactant in anti-agglomeration. We have used tetrahydrofuran (THF) hydrates when studying anti-agglomeration with the biosurfactant and MeOH co-surfactant. THF forms structure II hydrates and is much more soluble in water than any species in natural gas. As a result, there may be a concern that the crystallization in THF hydrates may be different from methane hydrates. THF hydrates may form in the bulk phase, whereas methane and propane hydrates may form on the interface between water and oil phases. The formation of THF hydrates unlike methane hydrates occurs at atmospheric pressure. This gives a vast advantage in conducting experiments to improve the understanding of the process. In this work, we study the cyclopentane (CP) hydrate former, which has a low solubility in water. CP is known to form the structure II hydrate at a temperature near 280 K at atmospheric pressure. CP hydrates in some respect are close to hydrates from natural gas species. We also use CP as the oil phase to form water-in-oil emulsion. The main goal of this research is to examine whether anti-agglomeration in CP hydrates can be achieved by a low concentration of a surfactant and a co-surfactant.
Introduction Natural gas has a high hydrogen/carbon ratio compared to petroleum fuels and coal. It is also a clean-burning fuel, which results in low production of CO2. As a result, natural gas is a desirable fuel. A large portion of natural gas is produced from the deep sea, where the temperature is low. The low temperature may result in the formation of gas hydrates from methane and some other species in the gas and co-produced water. The hydrates may plug gas pipelines. To prevent hydrate formation, large quantities of alcohols, such as methanol, are added to water to change bulk phase properties. The use of large quantities of alcohols is costly and damages the environment. An alternative is the use of surfactants to alter surface properties. Surface property changes lead to hydrate kinetic inhibition, which delays nucleation or growth of hydrates. The limitation of this process is applicability in low subcooling,1 where subcooling is defined as the difference between the hydrate equilibrium temperature and the operating temperature at a given pressure. In some deep sea environments, the subcooling can be as high as 25 °C. An attractive alternative is the use of surfactants that do not prevent hydrate formation but prevent agglomeration of hydrate particles. Anti-agglomerants (AAs) are effective at high subcooling in flow conditions or at shut-in conditions, i.e., when pipeline flow is paused for a period of time.2-4 Despite the promise of the process,
Experimental Section
*To whom correspondence should be addressed. E-mail: abbas.
[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 Conference and 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. r 2010 American Chemical Society
Apparatus. The experimental setup used in this work is similar to the one used in our previous work on THF hydrate antiagglomeration.5-7 The setup is a multiple screening-tube rocking (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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: DOI:10.1021/ef100622p
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when thermocouples are attached. However, the temperature control of the bath and agitation are the same for both types of experiments. Kinetic/Thermodynamic Data Acquisition. Mixtures are brought to 11 °C. This temperature is higher than 7 °C, the reported equilibrium temperature of CP hydrates at 1 atm.9,10 We allow the liquid mixture to reach equilibrium, and then a 5 °C/h cooling ramp is employed to a desired temperature for a specific sample, where hydrates form without ice. The mixture is then heated back to 15 °C at 15 °C/h. As the mixture is cooled below the equilibrium temperature, it crystallizes at the crystallization temperature (Tc) and an exothermic heat release begins. Once crystallization has occurred, the sample temperature rejoins that of the bath fluid. Dissociation of the mixture during heating shows as an endotherm, 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 observations are conducted similarly to crystallization/dissociation testing. Agitated mixtures are allowed to equilibrate at 11 °C, and then 5 °C/h cooling is applied to bring the mixtures to a temperature where CP hydrates are formed without the formation of ice. The procedure deviates from the kinetic/ thermodynamic test, whereby the minimum temperature is held constant (some samples are heated to 1.5 °C for ease of observation). Observations are made at 2 and 24 h into this period, mainly with the naked eye but also with the borescope. These observations show whether a dispersion of hydrate crystals is facilitated by the surfactant or surfactant/co-surfactant or agglomeration 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 homogenized by shaking by hand for 1 min. The time that it takes for 60 vol % of the initial aqueous phase to separate is measured and used as an indicator of emulsion stability.
Figure 1. Multiple screening-tube rocking apparatus.
apparatus, which consists of a motor-driven agitator with a rack holding up to 20 separate borosilicate glass scintillation vials. The vial dimensions are 17 mm (diameter) and 60 mm (height) submerged in a temperature bath. Each vial holds roughly 7 mL of a test mixture and a ∼8 mm diameter stainless-steel 316 ball. The ball aids agitation. A Teflon-lined plastic screw cap is used along with Teflon tape, around threads, to seal vials. The rack rotates the vials 150° to either side of the vertical direction, completing a cycle every 5 s. The temperature bath used is a Huber CC2-515 vpc filled silicon oil (10 cSt at 24 °C) from Clearco Products 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 crystallization and melting data are desired. The thermocouples are attached to the outside wall with the use of such vials. As we report in refs 5 and 6, placement of thermocouples inside the vials may serve as a nucleation active site. The temperatures inside the vial and vial wall are also the same. Therefore, outside placement is a method of choice for the temperature measurement. An Agilent 34970A data acquisition unit, recording temperature every 20 s, and an ice bath as a fixed junction reference temperature are used with all thermocouples. A sketch of the apparatus is shown in Figure 1. Chemicals. In all test mixtures, deionized water obtained from a Barnstead Nanopure Infinity system with a quality of roughly 5.5 10-2 μS/cm and 99%þ purity CP (from Acros) are used. CP serves both as the oil phase and the hydrate former. The solubility of CP in water is very low, about 160 ppm by weight at 25 °C, and is nearly constant in the temperature range of 025 °C.8 The solubility of methane in water is around 1000 ppm by mole at 100 bar and room temperature. Rhamnolipid biosurfactant (product JBR 425) (Rh) is obtained from Jeneil Biosurfactant Co., Madison, WI. It is a mixture of two forms at 25 wt % in water. The mixture is diluted with water when preparing samples with different Rh concentrations. Rh is used as supplied and is the same as discussed in ref 5. The co-surfactant, 99.8% anhydrous MeOH with less than 5 ppm water, is obtained from Acros. Procedure. The experimental procedures are the same as in refs 5-7. A composition of mostly x/1/0.02/y parts by weight of CP/water/THF/surfactant is used in our tests, where x is a varying amount of CP and y is a varying surfactant concentration in different tests. In the tests with the co-surfactant MeOH, the mixture composition is x/1/0.22/y/z, where z is the weight ratio of MeOH/water. Each sample is prepared in duplicate (except those data points without an error bar). Temperature data are acquired separately from visual observations, because half of the sample vial surface area is covered
Results and Discussion Formation of CP Hydrates. Attempts are made to obtain CP hydrates in mixtures of CP and water. It is found that no crystallites (CP hydrates) are formed because there is no exothermic peak when cooling the sample to a temperature above 0 °C. When cooling the sample to -2 °C or below, ice is formed, indicated by the endothermic peak starting at 0 °C. Whitman et al.10 have reported that, for mixtures of CP and water as either water-in-oil or oil-in-water emulsion, hydrate and ice always form simultaneously, with ice forming preferentially. Some authors have shown that methane and other gases, which can be incorporated into the smaller cavities of the CP hydrate at modest pressures, can serve as a helper molecule in the formation of CP hydrates.11,12 Note that the equilibrium hydrate formation temperature for CP by Sloan and Koh13 is about 2 °C higher than the value reported by Lo et al.9 and Whitmanet et al.10 We will discuss later that our results are in agreement with the data from refs 9 and 10. Effect of THF in CP Hydrate Formation. Helper molecules, such as methane, are used only under high pressure because of the very low solubility of these gases in water at (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 Phase Equilib. 1997, 138, 241–250. (13) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press, Taylor and Francis Group: Boca Raton, FL, 2008.
(7) York, J. D.; Firoozabadi, A. Energy Fuels 2009, 23, 2937–2946. (8) Yaws, C. L. Chemical Properties Handbook; McGraw-Hill: New York, 1999.
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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 of THF/H2O.
atmospheric pressure. In this work, THF is employed as a helper molecule because of its high solubility in water at atmospheric pressure. As can be seen in Figure 2, CP hydrates are formed by cooling samples of CP/water/THF with a composition of 0.4/1/x (weight ratio to water), with x = 0.01, 0.03, and 0.05, to a temperature of 0.5 °C (where ice does not form). The results clearly indicate that the presence of THF as a helper molecule gives rise to hydrate formation. The average dissociation temperature is 7.0 °C for the three THF 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. The results reveal that THF does not measurably affect the dissociation temperature of CP hydrates. At higher concentrations than used, the THF concentration may affect the dissociation temperature of CP. The equilibrium hydrate formation temperature of THF is 3.5 °C.5 Effect of Rh in CP Hydrate Formation. Rh is added to the above samples to study the AA effect in the concentration range of 0.001-0.05 (weight ratio to water). The addition of Rh depresses the CP hydrate formation temperature. No hydrates are formed above 0 °C, with the Rh presence even in low concentrations (as low as 0.001). The samples are cooled below 0 °C to form hydrates. Figure 3 shows the freeze-thaw cycle data for a sample of CP/H2O/THF/Rh with a composition of 0.4/1/0.03/0.01. Because of 3% THF and 1% Rh in the mixture, no ice forms to a temperature of -4 °C in the mixture. For a conclusive study of anti-agglomeration of hydrates, the formation of ice should be avoided. In this experiment, the weight ratio of CP/water is 0.4/1 and the molar ratio is 1/10, which is higher than the hydrate stoichiometric molar ratio of 1/17.14 Figure 3 shows that the hydrate formation with surfactant Rh is accompanied by a high growth rate, as compared to the data in Figure 2 without Rh. The addition of Rh depresses both the crystallization and dissociation temperatures. Figure 4 shows that the crystallization and dissociation temperatures both decrease as the concentration of Rh increases in the mixture. All of the tests presented in Figure 4 are conducted with the lowest temperature held at -4 °C. The decrease in the dissociation temperature can be explained from bulk-phase thermodynamics. The lower crystallization temperature relates to the lower driving force for hydrate formation from the addition of Rh. Although crystallization is not repeatable, the average of a few runs will give a measure of the effect. In a similar
Figure 3. Freeze-thaw cycle data for the mixture 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) and dissociation temperature (Td) for mixtures of CP/H2O/THF/Rh of composition 0.4/1/0.03/x (weight ratio to water).
way, the induction time from a few measurements gives an idea of the delay in the process. The systematic results from duplicate and various runs show a depression of the crysltization by Rh and its kinetic effect. Effect of the CP Concentration in Hydrate Formation. The amount of CP in the mixture affects the ratio of hydrates to the sample volume. The total volume of the mixture is fixed at 7 mL. For a CP/H2O ratio of 0.4/1 (weight ratio), more hydrates form followed by a ratio of 2:1 and then a ratio of 4:1. However, the dissociation temperature should not be appreciably affected. The data in Figure 5 clearly show that the CP amount does not affect the dissociation temperature because of very low solubility of CP in 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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Figure 7. Agglomeration states for mixtures of CP/H2O/THF/Rh of composition 4/1/x/y (weight ratio to water), where the THF amount x and Rh amount y are control variables: (þ) stable dispersion, (O) dispersible hydrate, hydrates that 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 is unable to move through the entire length of the vial) plug occurs.
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) for mixtures of CP/H2O/THF/Rh/MeOH of composition 1.5/1/0.02/ x/y (weight ratio to water).
Figure 8. Effect of Rh on the dissociation temperature (Td) for mixtures of CP/H2O/THF/Rh of composition 4/1/x/y (weight ratio to water).
Effect of MeOH in CP Hydrate Formation. The addition of MeOH depresses the dissociation temperature of CP hydrates. The data in Figure 6 reveal a decrease in the dissociation temperature with an increasing concentration of MeOH. There is a synergistic effect of Rh and MeOH in the reduction of Td. The increase of Rh from 0.001 to 0.005 weight ratio to water depresses Td by 0.5 °C; the addition of MeOH by 0.01 weight ratio to water lowers Td by 1.9 °C. The combined effect of the concentration increase of Rh from 0.001 to 0.005 and the addition of MeOH by 0.01 lowers Td by 3.5 °C, which is greater than the sum of the contributions from the increase in Rh and MeOH when added individually by about 1 °C. Note that the dissociation temperatures of CP/H2O/TH/Rh with compositions 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 indication that neither the CP amount nor THF has an appreciable effect on the dissociation temperature of CP hydrates within the range of concentrations used in this work. Agglomeration State. The central theme of this work is the study of anti-agglomeration in CP hydrate particles. We begin the study of the agglomeration state by testing mixtures 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, dispersible hydrates are formed with a low concentration of Rh. The samples with 0.02 THF are cooled to -2 °C and then kept at 1.5 °C for AA state observation, while the samples with 0.03 THF are cooled to -3 °C and then kept at 1.5 °C for AA state observation. There is no ice formation in these tests. Stable dispersion is observed for samples with Rh
concentrations of 0.003-0.03 during the observation period of 24 h. However, for lower and higher concentrations of Rh, there is change in the performance. At -2 °C, the mixtures with Rh concentrations of 0.001 and 0.002 exhibit agglomeration in the vial before 24 h. When the concentration of Rh is 0.05, there seems to be a plug when the samples are kept at -3 °C for 24 h. The hydrate plug would immediately disperse into stable dispersion when heavy shaking is applied. The “fake plug” may be due to the high viscosity of the solution for the Rh concentration of 0.05. By shaking, i.e., by increasing the shear force, the high viscosity is overcome and there is stable dispersion. When the temperature at which the mixture is kept for 24 h is compared to the dissociation temperature shown in Figure 8, it can be seen that the antiagglomeration effect in some of these samples is tested at a subcooling of about 8-10 °C. According to our work on THF hydrates,5 subcooling does not appreciably affect AA. AA that performs well at a subcooling of 8 °C may be also effective at a subcooling of 25 °C. In our recent work on hydrate anti-agglomeration in THF hydrates and the work in the literature, it is widely known that the anti-agglomeration process becomes ineffective at a high water cut. In this work, we have also examined the effect of 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 (for samples with 0.003-0.005 part of Rh) when the concentration of Rh is 0.003-0.01 without MeOH. When the Rh concentration is high, in the range of 0.03-0.05, the AA 4940
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Figure 9. Agglomeration states for mixtures of CP/H2O/THF/Rh/ MeOH of composition 2/1/x/y/z (weight ratio to water), where the THF amount x, Rh amount y, and MeOH amount z are control variables: (þ) stable dispersion, (O) dispersible hydrate, hydrates that 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 is unable to move through the entire length of the vial) plug occurs.
Figure 11. Stable dispersion observed for mixtures of CP/H2O/ THF/Rh of composition 2/1/0.02/x (weight ratio to water), where the Rh amount x is 0.003-0.01 for the data shown in Figures 9 and 10. The sample shown in the image contains 0.005 wt Rh and 0 wt MeOH. The vial is tilted roughly 60° from the horizontal with the bottom 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), where the Rh amount x and MeOH amount y are control variables: (þ) stable dispersion; (4) hydrates attached to the bottom or wall of the vial, and () plugging tendency. The plugging tendency means either 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.
quantities of MeOH can be effective when added to samples with 2 part CP. MeOH has dual effects at low concentrations: co-solvent and co-surfactant, as shown by a recent work by Moreira and Firoozabadi.15 Figure 9 demonstrates that MeOH at a weight ratio of 0.005 to water is effective as a co-surfactant in anti-agglomeration where hydrate slurries are formed at -2 °C. The slurries are also formed with 0.01 MeOH. Note that there is agglomeration of hydrates at a low MeOH concentration of 0.002. The dissociation data in Figure 10 and the temperature of -2 °C where the hydrates
Figure 10. (a) Effect of THF on the dissociation temperature (Td) for mixtures of CP/H2O/THF/Rh of composition 2/1/x/y (weight ratio to water). (b) Effect of Rh and MeOH on the dissociation temperature (Td) for mixtures of CP/H2O/THF/Rh/MeOH of composition 2/1/0.02/x/y (weight ratio to water).
effectiveness decreases probably because of the high viscosity effect. Plugs, either full or partial, where the steel ball is blocked 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 quantities of MeOH as a co-surfactant may prevent agglomeration of THF hydrates. In this work, we also find that small
(15) Moreira, L. A.; Firoozabadi, A. Langmuir 2009, 25, 12101– 12113.
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Figure 13. Significant hydrates being adhered to the side walls and bottom observed in mixtures of CP/H2O/THF/Rh/MeOH of composition 1.5/1/0.02/0.003/0.005 (weight ratio to water) for the data shown in Figures 6 and 12. The vial is tilted roughly 60° from the horizontal with the bottom side up. Table 1. Emulsion Stability (min)a CP (weight ratio to water) Rh (weight ratio to water)
1
4
1.5 MeOH (weight ratio to water) 0.000
0.001 0.002 0.003 0.005 0.01
1.2 ( 0.3 1.4 ( 0.2 1.3 ( 0.2 1.9 ( 0.3
5.9 ( 0.5 6.7 ( 1.2 10.0 ( 2.0
0.002
5.3 ( 0.3 7.2 ( 0.6 11.7 ( 1.5
0.005
5.3 ( 0.8 8.5 ( 0.8 16.3 ( 2.2
2.3 ( 0.3 7.6 ( 0.9 10.7 ( 1.0 11.3 ( 0.5
a A 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 indicator of emulsion stability.
are kept for a period of 24 h show anti-agglomeration for a subcooling of about 6-9 °C. In Figure 11, we show a typical picture in which hydrates are in dispersed form; the vial wall and bottom are transparent, with no crystallites adhered to the walls. The air bubble can travel freely in the vial, indicating that there is no agglomeration. The effectiveness of the MeOH co-surfactant is further confirmed when MeOH is added to samples with 1.5 part of CP, i.e., even higher water cut. Figure 12 shows the results. Plugs are formed to a Rh concentration of 0.01 when there is no MeOH. When only the concentration of MeOH is 0.005, hydrate slurries are formed at Rh concentrations of 0.005 and 0.01. When the concentration of Rh is 0.003, a MeOH concentration of 0.01 allows the formation of stable dispersion. In mixtures of 2 and 1.5 part CP, a MeOH concentration of 0.002 MeOH is not effective in anti-agglomeration. The results in Figure 12 correspond to a subcooling of about 6-10 °C. We have also carried out experiments for mixtures in which the concentrations of CP are 1 and 0.4 (CP weight/ water weight; water cut more than 50%). Hydrate agglomeration appears in all samples even at relatively high concentrations of Rh and MeOH. Figure 13 shows an example where there is hydrate agglomeration for a mixture containing 1.5 part CP and 1 part water. We are currently looking
into surfactants and co-surfactants that will allow antiagglomeration at a high water cut. Emulsion Stability. In the past, emulsion stability in hydrate anti-agglomeration has been suggested to be very important.16 In refs 5-7, we show that emulsion stability may not be critical. In this work, we use the methodology in refs 5-7 to measure emulsion stability of the mixtures. We report the average time that it takes to form 60% of the water volume to separate in the mixture. Table 1 gives emulsion stability results with two duplicate tests. The weight ratio of CP/water in the mixtures 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, the methanol concentration is 0, 0.002, and 0.005. As can be seen from the table, the Rh concentration increases emulsion stability in all mixtures. The CP concentration also increases emulsion stability. The addition of methanol generally increases emulsion stability, but the effect is not significant. Conclusions In this work, we have demonstrated that anti-agglomeration in CP hydrates is similar to THF hydrates. A small amount of MeOH in the mixture has a significant effect on (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 can lower the concentration of Rh for anti-agglomeration in some of the mixtures. In other mixtures, MeOH results in antiagglomeration when the concentration increase of biosurfactant does not lead to anti-agglomeration. Similar to the previous work,5-7 a small amount of MeOH is sufficient to prevent agglomeration because of the co-surfactant nature of the alcohol. We have also shown that the addition of THF as a helper molecule in the formation of CP hydrates does not affect the equilibrium hydrate temperature within the range of 1-5 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 be maintained may imply effectiveness at high subcooling. Our data also confirms that the equilibrium hydrate formation temperature for CP is around 7 °C. This value is about 2 °C less than the value suggested in ref 13. Acknowledgment. We are grateful to Jeneil Biosurfactant Co. for providing the rhamnolipid sample. This work was supported by the member companies of the Reservoir Engineering Research Institute (RERI) in Palo Alto, CA. One of the authors (L.N.) gratefully acknowledges a grant from the U.S. Government Fulbright Program.
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