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dissociation rates and stability
Journal of Natural Gas Science and Engineering 26 (2015) 1e5
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Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
Effect of maize starch on methane hydrate formation/dissociation rates and stability Saheb Maghsoodloo Babakhani, Abdolmohammad Alamdari* Dept. of Chemical Eng., School of Chemical and Petroleum Eng., Shiraz University, Shiraz, 7193616511, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2015 Received in revised form 21 May 2015 Accepted 23 May 2015 Available online xxx
The effects of maize starch on the methane hydrate formation/dissociation rates and stability were investigated in the present work. Maize starch at 5 concentrations of 200, 400, 600, 800 and 1000 ppm was tested. The results showed that maize starch at low concentrations had no significant effect on hydrate formation; however, at the concentrations higher than 400 ppm, maize starch increased the hydrate formation rates. The most effective concentration of maize starch was 800 ppm. At this concentration, methane can be stored in hydrate 2.5 times more than in the case of pure water when no maize was used. Hydrate stability was studied at ambient pressure and two temperatures below the ice point (272.2 K and 269.2 K). The results showed that for all samples the maximum dissociation rate occurred at the beginning of the process and then the rate decreased. At all concentrations, mole percentage dissociated at 272.2 K was lower than at 269.2 K and the hydrate formed at 800 ppm maize starch had the maximum stability. © 2015 Elsevier B.V. All rights reserved.
Keywords: Maize starch Methane hydrate Formation Dissociation Stability
1. Introduction Safe and cheap transport of natural gas from fields to consumption places is nowadays under the consideration due to increasing energy demand. Transport of natural gas as LNG or through pipes is unsafe or expensive. To reduce the risk in gas transport, hydrate formation technology may be employed. In this technology large volumes of hydrocarbons can be stored in cavities of hydrate matrices. For example 150e180 m3 of methane gas at standard conditions can be stored in one m3 of gas hydrate (Khokhar et al., 1998; Makogon, 1997). Gas storage in hydrate needs no conditions of low temperature compared to the conditions for LNG. Additionally, hydrate formation installations may be performed on the platforms at offshore. It has been reported that transportation of natural gas through hydrate is about 18e24% cheaper than through LNG (Kim et al., 2010; Gudmundsson et al., 1994). Natural gas transportation through hydrate consists of three main steps: formation, transportation, and dissociation. Hydrate formation rate and storage capacity are the critical stages which determine the economics of the process (Hao et al., 2008). Therefore, an increase in formation rate and storage capacity is essential in hydrate research. Researchers have used additives and promoters to enhance the formation rate and * Corresponding author. E-mail address: [email protected] (A. Alamdari). http://dx.doi.org/10.1016/j.jngse.2015.05.026 1875-5100/© 2015 Elsevier B.V. All rights reserved.
capacity (Kwona et al., 2011; Lee et al., 2010; Ganji et al., 2007). Lee et al. (2007) investigated the effect of some cationic starches on the hydrate formation rate of methane mixture with the other natural gas components. They found that the cationic starches did not increase the formation rate. Taheri et al. (2012) studied the effects of hydroxyethyl cellulose on methane hydrate formation and stability at different temperatures. They reported the optimum concentration of 500 ppm for hydrate formation. Fakharian et al. (2012) reported a significant effect of the potato starch on methane hydrate formation and the maximum capacity of 163 volume per volume at 300 ppm concentration of the potato starch. In the present study the effects of maize starch on the methane hydrate formation and dissociation rates and on the hydrate stability were investigated. The bottleneck for hydrate formation technique is to storage as much gas as possible in a unit volume of hydrate. Therefore, finding a good promoter for hydrate formation may help the gas industry to overcome this bottleneck. To the knowledge of authors no research on maize starch as a hydrate promoter has been reported in the literature. 2. Experimental 2.1. Materials High pure methane (99.9%) and deionized water were used for tests. Maize starch (100% pure) from Acros Company (Belgium) was
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used as an additive. The formula of starch was (C6H10O5)n and its molecular structure is (www.acros.com):
Using the method of Williams et al. (1970) for analysis, the amylose-amylopectin content of maize starch was determined as 29 and 71 percents. 2.2. Apparatus The experiments of hydrate formation were carried out in a 250 ml cell. The maximum pressure of 34 MPa might be employed in the cell. The cell was equipped with a thermocouple of 0.1 C accuracy, a pressure sensor of 0.1% accuracy, and an adjustable speed stirrer. The resistant torque exerted by the fluid was transferred to a computer by which the state of mixing in the fluid could be detected and a stirrer jam might be recognized. The schematic diagram of the system used for hydrate formation and dissociation is shown in Fig. 1. 2.3. Procedure 2.3.1. Hydrate formation In order to check the cell for a leakage, it was filled with nitrogen gas to a pressure of 10 MPa. The pressure was maintained for 1 h while no pressure decline was observed. The cell was evacuated by a vacuum pump and a volume of 60 ml water containing a precise concentration of maize starch was injected to the cell. The pressure was raised to 8 MPa by methane injection and the temperature was adjusted to 275.2 K. Then, agitation was started with a stirrer speed of 300 rpm. During the course of experiments for about 24 hours, the temperature, the pressure, and the stirrer speed in the cell were recorded every 3 min. The gas consumption was calculated based on the changes in pressure and temperature as n ¼ PV/zRT where P,
Fig. 1. Schematic diagram of the system used for the hydrate formation and dissociation.
T, and V are the pressure, temperature, and volume of gas, respectively. R is the gas constant and z is the compressibility factor which is calculated through PengeRobinson equation of state (Danesh, 1998). The V was considered as the cell volume minus the water volume. Due to a very low methane solubility in water, the volume increase of liquid water due to methane absorption is quite negligible; however, the volume change of gas phase due to the volume increase of hydrate as it forms was calculated based on the hydrate density and volume difference between water and hydrate. It is pertinent to note that the temperature chosen for hydrate formation was about 3 BC above the ice point; therefore, there was no chance for the solid phase to contain partially or entirely the ice. 2.3.2. Hydrate dissociation The dissociation step started after the constancy of the cell pressure. First, the temperature was maintained at 269.2 K and then the cell was depressurized to the ambient pressure in two steps; a slow reduction of pressure to the equilibrium pressure of methane at 269.2 K, and a rapid reduction of pressure from the equilibrium to the ambient pressure. The variations in pressure build up were recorded for 6 h by which the quantities of gas released were calculated. This procedure was performed to avoid throttling process and also to prevent unwanted-early hydrate dissociation. Then, after the pressure build up, the cell temperature was raised to 272.2 K and the cell was depressurized again to the ambient pressure. Once again the variations in pressure buildup were recorded for 6 h. The subzero temperatures of dissociation were selected in order to eliminate the liquid water vapor pressure. 3. Results and discussion 3.1. Effects of maize starch on hydrate formation Effects of the different concentrations of maize starch on methane hydrate formation/dissociation were studied. Quantity of methane stored in the hydrate was calculated through the variations of pressure and temperature during the course of the process. 3.1.1. Variations of pressure Fig. 2a shows the variations of pressure during the course of hydrate formation process for 600 ppm concentration of maize starch. Fig. 2b shows the pressure variations for all the other maize starch concentrations. As Fig. 2a illustrates, the variations of pressure can be divided into four specific sections. The first section (AB) represents the injection of gas into the cell where the pressure was maintained above 80 atm almost constant. The second section (BC) demonstrates the cooling stage of the gas from the ambient temperature to 275.2 K. Due to reduction in temperature, the pressure in this stage declined from about 80 atm to about 73 atm. When the gas temperature reached 275.2 K, the stirrer was started. The pressure remained constant for a while before a sharp decline. At this pressure, the hydrate was expected to be formed but due to induction time the system came to a halt. At stage CD, the pressure declined sharply indicative of the hydrate formation progress. However, soon after this stage the pressure remained constant indicative of the end of the hydrate formation stage. Deposition of the gas in the hydrate occurs at this stage when the gas is entrapped into the hydrate cages. Inspection of pressure drops in Fig. 2b shows that maximum gas deposition occurred at a maize starch concentration of 800 ppm. However, the starch concentrations less than and more than 800 ppm exhibited less pressure drops. At very low concentrations of maize starch (200 and 400 ppm) the pressure drop is slightly lower than that for pure water, however, the effect is very small that may be covered by experimental error; therefore, the effect was
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temperature rise will fade due to bath performance in maintaining the temperature constant. The trace of temperature rise is because for a short time the heat of nucleation dominates the performance of bath in maintaining the solution temperature at set point (Zhang et al., 2007). Both Figs. 2b and 3b show that in the case of 800 ppm starch concentration, the hydrate formation time illustrates a delay compared to the other cases. In the case 800 ppm starch concentration, the pressure drop at first 2 h of the process is due to gas cooling; while thereafter and before hydrate formation, the pressure drop is probably due to the dissolution of methane in the solution (Fig. 2b). It is obvious that after 5 h, the pressure drop is due to the hydrate formation and growth of hydrate crystals. Regarding the hydrate promotion, it seems that there is a little chance for methane absorption by maize starch at these low concentrations of the present experimental conditions but a more chance of decreasing the surface tension of water at 800 ppm concentration of the starch. A lower chance for methane absorption was postulated since at the starch concentrations higher than 800 ppm the methane incorporation was lower compared to the case of 800 ppm. If absorption was the mechanism, the higher starch concentration would result in the higher methane incorporation.
Fig. 2. a. The variations of pressure during the course of the formation process in the presence of 600 ppm maize starch at 275.2 K. b. The variations of pressure during the hydrate formation process in the presence of different concentrations of maize starch at 275.2 K.
considered to be negligible. There is a possibility that the mechanism of hydrate promotion by maize starch be through ease of methane diffusion from the gaseliquid interface to the bulk of the water phase. Zhang et al. (2013) reported that surfactants reduce the surface tension of water and thereby enhance the gas diffusion into the water. The ease of diffusion provides more methane in water for incorporation into the hydrate cavities. Since methane hydrate is non-stoichiometric, higher concentrations of methane close to the structuring cavities decrease the formation of vacant cavities, and hydration number as well. However, some researchers have reported that the phosphate groups in starch molecules they used enhanced the formation of water hydrogen bonding and cage structuring (Fakharian et al., 2012).
3.1.2. Variations of temperature Fig. 3a and b show the variations of temperature during the course of formation process for 600 ppm and the other concentrations of maize starch, respectively. The temperature declines from 305.2 K to 275.2 K at initial stages of the profile. A temperature rise then after indicates the start of hydrate formation process due to the generation of crystallization heat. Soon after, the trace of this
Fig. 3. a. The variations of temperature during the course of the formation process in the presence of 600 ppm maize starch. b. The variations of temperature during the course of the hydrate formation process in the presence of different concentrations of maize starch at the initial pressure of 8 MPa.
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3.1.3. Gas consumption Rate of hydrate formation was represented as moles of the gas consumed during the course of experiments. Fig. 4 illustrates methane consumed at different concentrations of maize starch during the hydrate formation process. Fig. 4 shows almost no change in the gas consumption for the maize starch concentrations of 200 and 400 ppm compared to the case of hydrate formation from pure water in the absence of starch. However, as the maize starch concentration increases, the gas consumption increases. The gas consumption at the maize starch concentration of 800 ppm shows its maximum value. However, at the higher maize starch concentrations the gas consumption corresponding to the methane stored in hydrate diminishes. The trends of gas consumption illustrated by the shape of the curves for starch concentrations of 200, 400, 600, and 1000 ppm are almost similar to each other. At early stages of the process before start of hydrate formation, the extant of methane dissolved is very low shown by horizontal lines before about hour 3. At the hydrate formation stage a jump is observed and after that again horizontal lines seen are indicative of a stage of equilibrium (Fig. 4). The jump in the gas consumption is due to the gas trap in the cavities of water molecules being formed through the hydrogen bonding. As the system approaches equilibrium, there would be no enough driving force for hydrate formation and the gas consumption approaches to zero illustrated by horizontal lines after hour 5 in Figs. 2 and 4. However, the trend of methane consumption at the 800 ppm maize starch concentration is slightly different from the trend of the other cases (Fig. 4). At the early stage the methane dissolution is more. After the dissolution stage for about 3 h no gas consumption is observed. This halt in the process is considered as the induction time, which hydrate nuclei need for growth to a minimum size corresponding to a minimum gas consumed detectable by the pressure transducer. However, after the induction the gas consumption increases sharply due to generation of a vast number of nuclei which catastrophically grow in a short time and consume a considerable amount of the gas. Thereafter, the hydrate particles continue their growth but at a much lower speed. This stage of growth for the 800 ppm maize starch concentration also appears different compared to the other cases, apparently due to the continuation of gas diffusion in water and completion of the growth process. As the time passes, the cages approach to their highest storage capacity and the gas diffusion from the gaseliquid interface to the bulk of water declines and finally comes to a stop. Fig. 5 illustrates the maize starch concentration versus the methane
Fig. 4. The calculated amount of methane consumption under the influence of different maize starch concentrations at the initial pressure 8 MPa and temperature 275.2 K.
Fig. 5. Moles of methane stored in the hydrate at different concentrations of maize starch and at the initial pressure 8 MPa and temperature 275.2 K.
consumed in the experiments. 3.2. Hydrate stability under the influence of maize starch at subzero temperatures In order to give an insight into the hydrate stability, the dissociation trials were performed. In the first trial of the dissociation experiment, at atmospheric pressure and at minus 4 C, the dissociated gas was collected and its magnitude was calculated. Later on the collected gas was evacuated and the hydrate temperature was increased to minus 1 C and another trial of dissociation was performed. The pressure build-up was recorded and the calculation was implemented. The subzero temperature was chosen because the hydrate transport is expected to be practically performed under subzero conditions to prevent hydrate dissociation. The transport conditions are the pressures higher than and the temperatures lower than those of equilibrium conditions. Since the high pressure practicing is expensive, the optimum conditions for hydrate transport are low temperature conditions. The remaining or occluded water in the reactor which at subzero conditions converts to ice has no effect on experimental results of hydrate dissociation. Therefore, in the present study dissociation was investigated at ambient pressure and minus 4 and minus 1 C temperatures.
Fig. 6. Methane dissociated per stored methane in the hydrate under the influence of different concentrations of maize starch at the ambient pressure and temperature minus 4 C.
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starch. The results obtained at minus 4 C for hydrate stability are almost similar to those obtained at minus 1 C; however, a rise in dissociation at minus 4 C is seen when data from the two figures are compared as illustrated in Fig. 8. As Fig. 8 shows, the dissociation of hydrate at subzero temperatures is in the range 2e5%, and the minimum instability is observed at the 800 ppm maize starch concentration. 4. Conclusions
Fig. 7. Methane dissociated per methane stored in the hydrate under the influence of different concentrations of starch at the ambient pressure and temperature minus 1 C.
The effect of different maize starch concentrations on methane hydrate formation and dissociation was investigated. Results showed that maize starch at low concentrations has not significant effect on hydrate formation rate, but at the high concentrations of 600, 800 and 1000 ppm, maize starch increases the formation rate. At the 800 ppm of maize starch concentration, an increase in the hydrate formation rate up to 2.5 times compared to no addition of maize starch at the same conditions was observed. Hydrate stability was examined through measurement of hydrate dissociation at atmospheric pressure and subzero temperatures of minus 4 C and minus 1 C. Results showed that the stability at minus 4 C was less than that at minus 1 C; however, self-preservation phenomenon prevents instability. Some dissociation about 4e5% occurred before complete self-preservation. Hydrate formed in the starch presence of 800 ppm showed the most stability. References
Fig. 8. Maximum dissociated methane as mole percent for different maize starch concentrations at temperatures minus 4 C and minus 1 C.
Fig. 6 shows the dissociated methane during the course of experiment at minus 4 C. The dissociated gas has been represented as the moles dissociated per the moles stored in the hydrate. The more gas dissociated the less stable the hydrate is. As shown in Fig. 6, the rate of dissociation is highest at the early stages of dissociation. However, after 2 h the rate is at minimum and finally approaches to zero due to the phenomenon of selfpreservation. As the hydrate dissociates, the heat of phase change is absorbed from the environment including the water produced by dissociation. Therefore, a layer of ice covers the hydrate crystals which reduces the rate of further dissociation through the mechanisms of hindering the mass and heat transfer (Fakharian et al., 2012; Stern et al., 2001; Handa, 1986). Fig. 6 also shows that the hydrate is the most stable when the hydrate formation occurred under the influence of the 800 ppm maize starch concentration. It is noticeable that at 800 ppm concentration the stored gas was also the most. At 400 ppm, the highest mole percent of the gas dissociated has been illustrated; thus, the lowest stability corresponds to this concentration of maize starch. Fig. 7 shows the dissociated methane during the course of experiment at minus 1 C for different concentrations of maize
Danesh, A., 1998. PVT and Phase Behavior of Petroleum Reservoir Fluids, first ed. Elsevier Publishing. Fakharian, H., Ganji, H., Naderifar, A., Kameli, M., 2012. Potato starch as methane hydrate promoter. J. Fuel 94, 356e360. Ganji, H., Manteghian, M., Sadaghianizadeh, K., Omidkhah, M., Rahimi Mofrad, H., 2007. Effect of different surfactants on methane hydrate formation rate, stability and storage capacity. J. Fuel 86, 434e441. Gudmundsson, J.S., Parlaktuna, M., Khokhar, A.A., 1994. Storing natural gas as frozen hydrates. J. SPE 9, 69e73. Handa, Y., 1986. Calorimetric determination of the compositions, enthalpies of dissociation and heat capacities in the range of 80e270 K for clathrate hydrates of Xenon and Krypton. J. Chem. Therm. 18, 891e902. Hao, W., Wang, J., Fan, S., Hao, W., 2008. Evaluation and analysis method for natural gas hydrate storage and transportation processes. J. Energy Convers. Manag. 49, 2546e2553. Khokhar, A.A., Gudmundsson, J.S., Sloan, D.E., 1998. Gas storage in structure H hydrates. J. Fluid Phase Equilibria 150, 383e392. Kim, N.J., Lee, J.H., Cho, Y.S., Chun, W., 2010. Formation enhancements of methane hydrate for natural gas transport and storage. J. Energy 35, 2717e2722. Kwona, Y.A., Park, J.M., Jeong, K.E., Kim, C.U., Kim, T.W., Chae, H.J., Jeong, S.Y., Yim, J.H., Park, Y.K., Lee, J.D., 2011. Synthesis of anionic multi-chain type surfactant and its effect on methane gas hydrate formation. J. Ind. Eng. Chem. 17, 120e124. Lee, J., Shin, C., Lee, Y., 2010. Experimental investigation to improve the storage potentials of gas hydrate under the un-stirring condition. J. Energy Fuel 24, 1129e1134. Lee, J.D., Wu, H., Englezos, P., 2007. Cationic starches as gas hydrate kinetic inhibitors. J. Chem. Eng. Sci. 62, 6548e6555. Makogon, Y.F., 1997. Hydrates of Hydrocarbons, first ed. Pennwell Publishing. Company, Oklahoma. Stern, L.A., Circone, S.K., Stephen, H., 2001. Anomalous preservation of pure methane hydrate at 1 atm. J. Phys. Chem. 105, 1756e1762. Taheri, M., Zarringhalam, A., Nazari, K., Gholipour, N., 2012. Methane hydrate stability in the presence of water-soluble hydroxyalkyl cellulose. J. Nat. Gas Chem. 21, 119e125. Williams, P.C., Kuzina, F.D., Hlynka, I., 1970. A rapid calorimetric procedure for estimating the amylose content of starches and flours. Cereal Chem. 4, 411e420. Zhang, J., Lee, S., Lee, J., 2007. Kinetics of methane hydrate formation from SDS solution. J. Ind. Eng. Chem. Res. 46, 6353e6359. Zhang, L., Zhou, S., Wang, S., Wang, L., Li, J., 2013. Surfactant surface tension effects on promoting hydrate formation: an experimental study using fluorocarbon surfactant (Intechem-01) þ SDS composite surfactant. J. Environ. Prot. 4, 42e48.
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
Effect of maize starch on methane hydrate formation/dissociation rates and stability Saheb Maghsoodloo Babakhani, Abdolmohammad Alamdari* Dept. of Chemical Eng., School of Chemical and Petroleum Eng., Shiraz University, Shiraz, 7193616511, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2015 Received in revised form 21 May 2015 Accepted 23 May 2015 Available online xxx
The effects of maize starch on the methane hydrate formation/dissociation rates and stability were investigated in the present work. Maize starch at 5 concentrations of 200, 400, 600, 800 and 1000 ppm was tested. The results showed that maize starch at low concentrations had no significant effect on hydrate formation; however, at the concentrations higher than 400 ppm, maize starch increased the hydrate formation rates. The most effective concentration of maize starch was 800 ppm. At this concentration, methane can be stored in hydrate 2.5 times more than in the case of pure water when no maize was used. Hydrate stability was studied at ambient pressure and two temperatures below the ice point (272.2 K and 269.2 K). The results showed that for all samples the maximum dissociation rate occurred at the beginning of the process and then the rate decreased. At all concentrations, mole percentage dissociated at 272.2 K was lower than at 269.2 K and the hydrate formed at 800 ppm maize starch had the maximum stability. © 2015 Elsevier B.V. All rights reserved.
Keywords: Maize starch Methane hydrate Formation Dissociation Stability
1. Introduction Safe and cheap transport of natural gas from fields to consumption places is nowadays under the consideration due to increasing energy demand. Transport of natural gas as LNG or through pipes is unsafe or expensive. To reduce the risk in gas transport, hydrate formation technology may be employed. In this technology large volumes of hydrocarbons can be stored in cavities of hydrate matrices. For example 150e180 m3 of methane gas at standard conditions can be stored in one m3 of gas hydrate (Khokhar et al., 1998; Makogon, 1997). Gas storage in hydrate needs no conditions of low temperature compared to the conditions for LNG. Additionally, hydrate formation installations may be performed on the platforms at offshore. It has been reported that transportation of natural gas through hydrate is about 18e24% cheaper than through LNG (Kim et al., 2010; Gudmundsson et al., 1994). Natural gas transportation through hydrate consists of three main steps: formation, transportation, and dissociation. Hydrate formation rate and storage capacity are the critical stages which determine the economics of the process (Hao et al., 2008). Therefore, an increase in formation rate and storage capacity is essential in hydrate research. Researchers have used additives and promoters to enhance the formation rate and * Corresponding author. E-mail address: [email protected] (A. Alamdari). http://dx.doi.org/10.1016/j.jngse.2015.05.026 1875-5100/© 2015 Elsevier B.V. All rights reserved.
capacity (Kwona et al., 2011; Lee et al., 2010; Ganji et al., 2007). Lee et al. (2007) investigated the effect of some cationic starches on the hydrate formation rate of methane mixture with the other natural gas components. They found that the cationic starches did not increase the formation rate. Taheri et al. (2012) studied the effects of hydroxyethyl cellulose on methane hydrate formation and stability at different temperatures. They reported the optimum concentration of 500 ppm for hydrate formation. Fakharian et al. (2012) reported a significant effect of the potato starch on methane hydrate formation and the maximum capacity of 163 volume per volume at 300 ppm concentration of the potato starch. In the present study the effects of maize starch on the methane hydrate formation and dissociation rates and on the hydrate stability were investigated. The bottleneck for hydrate formation technique is to storage as much gas as possible in a unit volume of hydrate. Therefore, finding a good promoter for hydrate formation may help the gas industry to overcome this bottleneck. To the knowledge of authors no research on maize starch as a hydrate promoter has been reported in the literature. 2. Experimental 2.1. Materials High pure methane (99.9%) and deionized water were used for tests. Maize starch (100% pure) from Acros Company (Belgium) was
2
S. Maghsoodloo Babakhani, A. Alamdari / Journal of Natural Gas Science and Engineering 26 (2015) 1e5
used as an additive. The formula of starch was (C6H10O5)n and its molecular structure is (www.acros.com):
Using the method of Williams et al. (1970) for analysis, the amylose-amylopectin content of maize starch was determined as 29 and 71 percents. 2.2. Apparatus The experiments of hydrate formation were carried out in a 250 ml cell. The maximum pressure of 34 MPa might be employed in the cell. The cell was equipped with a thermocouple of 0.1 C accuracy, a pressure sensor of 0.1% accuracy, and an adjustable speed stirrer. The resistant torque exerted by the fluid was transferred to a computer by which the state of mixing in the fluid could be detected and a stirrer jam might be recognized. The schematic diagram of the system used for hydrate formation and dissociation is shown in Fig. 1. 2.3. Procedure 2.3.1. Hydrate formation In order to check the cell for a leakage, it was filled with nitrogen gas to a pressure of 10 MPa. The pressure was maintained for 1 h while no pressure decline was observed. The cell was evacuated by a vacuum pump and a volume of 60 ml water containing a precise concentration of maize starch was injected to the cell. The pressure was raised to 8 MPa by methane injection and the temperature was adjusted to 275.2 K. Then, agitation was started with a stirrer speed of 300 rpm. During the course of experiments for about 24 hours, the temperature, the pressure, and the stirrer speed in the cell were recorded every 3 min. The gas consumption was calculated based on the changes in pressure and temperature as n ¼ PV/zRT where P,
Fig. 1. Schematic diagram of the system used for the hydrate formation and dissociation.
T, and V are the pressure, temperature, and volume of gas, respectively. R is the gas constant and z is the compressibility factor which is calculated through PengeRobinson equation of state (Danesh, 1998). The V was considered as the cell volume minus the water volume. Due to a very low methane solubility in water, the volume increase of liquid water due to methane absorption is quite negligible; however, the volume change of gas phase due to the volume increase of hydrate as it forms was calculated based on the hydrate density and volume difference between water and hydrate. It is pertinent to note that the temperature chosen for hydrate formation was about 3 BC above the ice point; therefore, there was no chance for the solid phase to contain partially or entirely the ice. 2.3.2. Hydrate dissociation The dissociation step started after the constancy of the cell pressure. First, the temperature was maintained at 269.2 K and then the cell was depressurized to the ambient pressure in two steps; a slow reduction of pressure to the equilibrium pressure of methane at 269.2 K, and a rapid reduction of pressure from the equilibrium to the ambient pressure. The variations in pressure build up were recorded for 6 h by which the quantities of gas released were calculated. This procedure was performed to avoid throttling process and also to prevent unwanted-early hydrate dissociation. Then, after the pressure build up, the cell temperature was raised to 272.2 K and the cell was depressurized again to the ambient pressure. Once again the variations in pressure buildup were recorded for 6 h. The subzero temperatures of dissociation were selected in order to eliminate the liquid water vapor pressure. 3. Results and discussion 3.1. Effects of maize starch on hydrate formation Effects of the different concentrations of maize starch on methane hydrate formation/dissociation were studied. Quantity of methane stored in the hydrate was calculated through the variations of pressure and temperature during the course of the process. 3.1.1. Variations of pressure Fig. 2a shows the variations of pressure during the course of hydrate formation process for 600 ppm concentration of maize starch. Fig. 2b shows the pressure variations for all the other maize starch concentrations. As Fig. 2a illustrates, the variations of pressure can be divided into four specific sections. The first section (AB) represents the injection of gas into the cell where the pressure was maintained above 80 atm almost constant. The second section (BC) demonstrates the cooling stage of the gas from the ambient temperature to 275.2 K. Due to reduction in temperature, the pressure in this stage declined from about 80 atm to about 73 atm. When the gas temperature reached 275.2 K, the stirrer was started. The pressure remained constant for a while before a sharp decline. At this pressure, the hydrate was expected to be formed but due to induction time the system came to a halt. At stage CD, the pressure declined sharply indicative of the hydrate formation progress. However, soon after this stage the pressure remained constant indicative of the end of the hydrate formation stage. Deposition of the gas in the hydrate occurs at this stage when the gas is entrapped into the hydrate cages. Inspection of pressure drops in Fig. 2b shows that maximum gas deposition occurred at a maize starch concentration of 800 ppm. However, the starch concentrations less than and more than 800 ppm exhibited less pressure drops. At very low concentrations of maize starch (200 and 400 ppm) the pressure drop is slightly lower than that for pure water, however, the effect is very small that may be covered by experimental error; therefore, the effect was
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temperature rise will fade due to bath performance in maintaining the temperature constant. The trace of temperature rise is because for a short time the heat of nucleation dominates the performance of bath in maintaining the solution temperature at set point (Zhang et al., 2007). Both Figs. 2b and 3b show that in the case of 800 ppm starch concentration, the hydrate formation time illustrates a delay compared to the other cases. In the case 800 ppm starch concentration, the pressure drop at first 2 h of the process is due to gas cooling; while thereafter and before hydrate formation, the pressure drop is probably due to the dissolution of methane in the solution (Fig. 2b). It is obvious that after 5 h, the pressure drop is due to the hydrate formation and growth of hydrate crystals. Regarding the hydrate promotion, it seems that there is a little chance for methane absorption by maize starch at these low concentrations of the present experimental conditions but a more chance of decreasing the surface tension of water at 800 ppm concentration of the starch. A lower chance for methane absorption was postulated since at the starch concentrations higher than 800 ppm the methane incorporation was lower compared to the case of 800 ppm. If absorption was the mechanism, the higher starch concentration would result in the higher methane incorporation.
Fig. 2. a. The variations of pressure during the course of the formation process in the presence of 600 ppm maize starch at 275.2 K. b. The variations of pressure during the hydrate formation process in the presence of different concentrations of maize starch at 275.2 K.
considered to be negligible. There is a possibility that the mechanism of hydrate promotion by maize starch be through ease of methane diffusion from the gaseliquid interface to the bulk of the water phase. Zhang et al. (2013) reported that surfactants reduce the surface tension of water and thereby enhance the gas diffusion into the water. The ease of diffusion provides more methane in water for incorporation into the hydrate cavities. Since methane hydrate is non-stoichiometric, higher concentrations of methane close to the structuring cavities decrease the formation of vacant cavities, and hydration number as well. However, some researchers have reported that the phosphate groups in starch molecules they used enhanced the formation of water hydrogen bonding and cage structuring (Fakharian et al., 2012).
3.1.2. Variations of temperature Fig. 3a and b show the variations of temperature during the course of formation process for 600 ppm and the other concentrations of maize starch, respectively. The temperature declines from 305.2 K to 275.2 K at initial stages of the profile. A temperature rise then after indicates the start of hydrate formation process due to the generation of crystallization heat. Soon after, the trace of this
Fig. 3. a. The variations of temperature during the course of the formation process in the presence of 600 ppm maize starch. b. The variations of temperature during the course of the hydrate formation process in the presence of different concentrations of maize starch at the initial pressure of 8 MPa.
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3.1.3. Gas consumption Rate of hydrate formation was represented as moles of the gas consumed during the course of experiments. Fig. 4 illustrates methane consumed at different concentrations of maize starch during the hydrate formation process. Fig. 4 shows almost no change in the gas consumption for the maize starch concentrations of 200 and 400 ppm compared to the case of hydrate formation from pure water in the absence of starch. However, as the maize starch concentration increases, the gas consumption increases. The gas consumption at the maize starch concentration of 800 ppm shows its maximum value. However, at the higher maize starch concentrations the gas consumption corresponding to the methane stored in hydrate diminishes. The trends of gas consumption illustrated by the shape of the curves for starch concentrations of 200, 400, 600, and 1000 ppm are almost similar to each other. At early stages of the process before start of hydrate formation, the extant of methane dissolved is very low shown by horizontal lines before about hour 3. At the hydrate formation stage a jump is observed and after that again horizontal lines seen are indicative of a stage of equilibrium (Fig. 4). The jump in the gas consumption is due to the gas trap in the cavities of water molecules being formed through the hydrogen bonding. As the system approaches equilibrium, there would be no enough driving force for hydrate formation and the gas consumption approaches to zero illustrated by horizontal lines after hour 5 in Figs. 2 and 4. However, the trend of methane consumption at the 800 ppm maize starch concentration is slightly different from the trend of the other cases (Fig. 4). At the early stage the methane dissolution is more. After the dissolution stage for about 3 h no gas consumption is observed. This halt in the process is considered as the induction time, which hydrate nuclei need for growth to a minimum size corresponding to a minimum gas consumed detectable by the pressure transducer. However, after the induction the gas consumption increases sharply due to generation of a vast number of nuclei which catastrophically grow in a short time and consume a considerable amount of the gas. Thereafter, the hydrate particles continue their growth but at a much lower speed. This stage of growth for the 800 ppm maize starch concentration also appears different compared to the other cases, apparently due to the continuation of gas diffusion in water and completion of the growth process. As the time passes, the cages approach to their highest storage capacity and the gas diffusion from the gaseliquid interface to the bulk of water declines and finally comes to a stop. Fig. 5 illustrates the maize starch concentration versus the methane
Fig. 4. The calculated amount of methane consumption under the influence of different maize starch concentrations at the initial pressure 8 MPa and temperature 275.2 K.
Fig. 5. Moles of methane stored in the hydrate at different concentrations of maize starch and at the initial pressure 8 MPa and temperature 275.2 K.
consumed in the experiments. 3.2. Hydrate stability under the influence of maize starch at subzero temperatures In order to give an insight into the hydrate stability, the dissociation trials were performed. In the first trial of the dissociation experiment, at atmospheric pressure and at minus 4 C, the dissociated gas was collected and its magnitude was calculated. Later on the collected gas was evacuated and the hydrate temperature was increased to minus 1 C and another trial of dissociation was performed. The pressure build-up was recorded and the calculation was implemented. The subzero temperature was chosen because the hydrate transport is expected to be practically performed under subzero conditions to prevent hydrate dissociation. The transport conditions are the pressures higher than and the temperatures lower than those of equilibrium conditions. Since the high pressure practicing is expensive, the optimum conditions for hydrate transport are low temperature conditions. The remaining or occluded water in the reactor which at subzero conditions converts to ice has no effect on experimental results of hydrate dissociation. Therefore, in the present study dissociation was investigated at ambient pressure and minus 4 and minus 1 C temperatures.
Fig. 6. Methane dissociated per stored methane in the hydrate under the influence of different concentrations of maize starch at the ambient pressure and temperature minus 4 C.
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starch. The results obtained at minus 4 C for hydrate stability are almost similar to those obtained at minus 1 C; however, a rise in dissociation at minus 4 C is seen when data from the two figures are compared as illustrated in Fig. 8. As Fig. 8 shows, the dissociation of hydrate at subzero temperatures is in the range 2e5%, and the minimum instability is observed at the 800 ppm maize starch concentration. 4. Conclusions
Fig. 7. Methane dissociated per methane stored in the hydrate under the influence of different concentrations of starch at the ambient pressure and temperature minus 1 C.
The effect of different maize starch concentrations on methane hydrate formation and dissociation was investigated. Results showed that maize starch at low concentrations has not significant effect on hydrate formation rate, but at the high concentrations of 600, 800 and 1000 ppm, maize starch increases the formation rate. At the 800 ppm of maize starch concentration, an increase in the hydrate formation rate up to 2.5 times compared to no addition of maize starch at the same conditions was observed. Hydrate stability was examined through measurement of hydrate dissociation at atmospheric pressure and subzero temperatures of minus 4 C and minus 1 C. Results showed that the stability at minus 4 C was less than that at minus 1 C; however, self-preservation phenomenon prevents instability. Some dissociation about 4e5% occurred before complete self-preservation. Hydrate formed in the starch presence of 800 ppm showed the most stability. References
Fig. 8. Maximum dissociated methane as mole percent for different maize starch concentrations at temperatures minus 4 C and minus 1 C.
Fig. 6 shows the dissociated methane during the course of experiment at minus 4 C. The dissociated gas has been represented as the moles dissociated per the moles stored in the hydrate. The more gas dissociated the less stable the hydrate is. As shown in Fig. 6, the rate of dissociation is highest at the early stages of dissociation. However, after 2 h the rate is at minimum and finally approaches to zero due to the phenomenon of selfpreservation. As the hydrate dissociates, the heat of phase change is absorbed from the environment including the water produced by dissociation. Therefore, a layer of ice covers the hydrate crystals which reduces the rate of further dissociation through the mechanisms of hindering the mass and heat transfer (Fakharian et al., 2012; Stern et al., 2001; Handa, 1986). Fig. 6 also shows that the hydrate is the most stable when the hydrate formation occurred under the influence of the 800 ppm maize starch concentration. It is noticeable that at 800 ppm concentration the stored gas was also the most. At 400 ppm, the highest mole percent of the gas dissociated has been illustrated; thus, the lowest stability corresponds to this concentration of maize starch. Fig. 7 shows the dissociated methane during the course of experiment at minus 1 C for different concentrations of maize
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