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Energy & Fuels 2006, 20, 324-326
Natural Gas Hydrate Dissociation by Presence of Ethylene Glycol Shuanshi Fan, Yuzhen Zhang, Genlin Tian,* Deqing Liang, and Dongliang Li Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Nengyuan Road, Guangzhou, 510640, China ReceiVed July 19, 2005. ReVised Manuscript ReceiVed October 11, 2005
In the current article, natural gas hydrate dissociation experiments have been conducted on a self-designed apparatus, which consists of a high-pressure reaction vessel, visualization system, chiller with circulation system, gas supplier, and data acquisition system. Gas hydrate was synthesized in the reaction vessel, and then the dissociation experiment was performed by addition of ethylene glycol (EG) as an inhibitor. The results show that dissociation rate depends on the concentration and flow rate of EG because it reduced the dissociation heat, and less energy is required for dissociation at higher ethylene concentration. Thus, EG concentration and flow rate can be optimized in practical utilization.
Introduction Gas hydrate is considered an important clean energy resource. More and more countries pay special attention to its research and development.1 Now in China, energy consumption mainly depends on coal, with a high percentage of 67%.2 China is the biggest coal-consuming country, with a great amount of discharge of CO2. However, using oil and gas to replace coal and improve energy consumption is limited by resource reserves. Thus, looking for a new energy source is very important in China. It is well-known that natural gas hydrate (NGH) is a potential energy resource with great amounts around the world.3 A primary survey and a series of investigations show the existence of NGH reservoirs, which could be the clean energy of the future.4,5 To improve energy consumption and solve energy demand problems, it is necessary and important to investigate the possible ways to develop NGH. Many potential methods, such as heat exchange, depressurization, and injection of thermoinhibitors, have been employed to exploit NGH with respect to different conditions.6 For instance, in ocean reservoir development, injection of thermoinhibitors is most feasible because of its easy operation. Although ethylene glycol (EG) is believed to be an inhibitor to prevent hydrate formation, we still lack detailed information of
experimental data, and in particular, the mechanism of dissociation of NGH by ethylene glycol is not well understood yet. Another reason to study the influence of EG is because it can also be used as an inhibitor to prevent hydrate formation during natural gas transportation and production.7 EG is widely used in oil and gas industries to prevent hydrate formation in the production tubing and transportation pipes.8,9 Its concentration and amount need to be improved with thermodynamic study. Here we report the study of EG as an inhibitor for the dissociation of NGH by self-made setup. Using different concentrations and flow rates of injected EG, we attempted to obtain the NGH dissociation behaviors under various conditions. Materials and Equipment Distilled water is used in all the experiments. Gas mixture with a composition of C3H8 4.96%, C2H6 4.03%, and CH4 90.01% is obtained from Foshan Huawen Gas Factory, China. Ethylene glycol (CR) is from Guangzhou Chemical Reagent Factory, China. Experiment apparatus is designed and set up in our laboratory. It can be used for both hydrate formation and dissociation test. The schematic diagram is shown in Figure 1. The apparatus mainly consists of a gas supplier, a high-pressure transparent reaction vessel, a video camera, a temperature control system, a vacuum pump, and a data acquisition system.
Experimental Procedures * Corresponding author. E-mail:
[email protected]. (1) Fan, S. S.; Chen, Y.; Liang, D. Overview on the development of NGH. Mod. Chem. Eng. 2003, 23, 1-5. (2) Guo, Y. T. Analysis and prediction of middle and long term coal supply in China. Chin. Coal 2004, 10. (3) Lee, S. Y.; Holder, G. D. Methane hydrates potential as a future energy source. Fuel Process. Technol. 2001, 71, 181-186. (4) Shipley, T. H.; Houston, M. H.; Buffler, R. T.; Shaub, F. J.; McMillen, K. J.; Ladd, J. W.; Worzel, J. L. Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. Am. Assoc. Pet. Geol. Bull. 1979, 63, 2204-2213. (5) Iseux, J. C. Gas Hydrates: Occurrence, Production and Economics. SPE Production Operations Symposium, Oklahoma City, OK, April 7-9, 1991; Paper 21682. (6) (a) Islam, M. R. A new recovery technique for gas production from Alaskan gas hydrate. SPE 22924. (b) Staykova, D. K.; Kuhs, W. F.; Salamatin, A. N.; Hansen, T. Formation of Porous Gas Hydrates from Ice Powders: Diffraction Experiments and Multistage Model. J. Phys. Chem. B 2003, 107, 10299-10311. (c) Sun, Z. G.; Ma, R. S.; Wang, R. Z.; Guo, K. H.; Fan, S. S. Experimental Studying of Additives Effects on Gas Storage in Hydrates. Energy Fuels 2003, 17, 1180-1185. (d) Circone, S.; Stern, L. A.; Kirby S. H. The Role of Water in Gas Hydrate Dissociation. J. Phys. Chem. B 2004, 108, 5747-5755. (e) Moon, C. M.; Taylor, P. C.; Rodger, P. M. Molecular Dynamics Study of Gas Hydrate Formation. J. Am. Chem. Soc. 2003, 125, 4706-4707.
After the whole system is tested, experiments can be conducted as following with several main steps: Vacuum the reaction vessel for more than 30 min to completely remove the inner air, then set up chiller’s temperature at 0 °C to reduce the temperature of the vessel, supply the gas with a small pressure gradient to 3.5-5 MPa, observe the reaction directly and through pressure change, and inject ethylene glycol to observe the dissociation at different rates.
Results and Discussion To conduct dissociation experiments, NGH was synthesized first by mixing gas and water at 0 °C in the presence of a very small amount of SDS as an inducer. The reaction time is as long as 7 days. The NGH sample is a white snowlike crystal, which was observed from the transparent reactor directly. (7) Sloan, E. D. Natural Gas Hydrates. J. Pet. Technol. 1991, 11, 14141417. (8) Yousif, M. H. Effect of Underinhibition with Methanol and Ethylene Glycol on the Hydrate-Control Process. Soc. Pet. Eng., Prod. Facil. 1998, 13, 184-189. (9) Tang, C. P.; Fan, S. S. Advancement on the research of new types of NGH inhibitors. Chem. Eng. Pet. Natural Gas 2004, 33, 157-159.
10.1021/ef0502204 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/08/2005
NGH Dissociation by Presence of Ethylene Glycol
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Figure 1. NGH experiment system.
Figure 2. Dissociation curves of NGH by injection of ethylene solutions.
Figure 3. Dissociation rate to ethylene glycol concentration.
For the dissociation test, water or EG solution is injected into the vessel. Figure 2 shows the pressure changes during dissociation when different EG solutions are injected. Before injection of EG solution, the free gas was removed to reduce the pressure for safety reason, which causes the pressure drop at the very beginning. It can be seen that the pressure gradient is greater at the beginning, and then the slope decreases. This means that the dissociation rate at the beginning is faster because the amount of NGH is more and there is no free water from NGH to dilute the solution. The dissociation rates are nearly constant at the very beginning and then reduce to nearly zero when the reaction is nearly finished. Although dissociation is a dynamic process, it can also been seen from Figure 2 that, with increase in ethylene glycol concentration, dissociation rates also increase. This is because EG can reduce the dissociation heat. For example, when EG concentrations are 10, 20, and 30%, dissociation heats are 73.35, 65.16, and 60.65 KJ‚mol-1, repectively.10 The reduction of dissociation potential energy makes NGH be “solved” quickly. It is known that other substances such as EG can also change phase transition conditions. These make the dissociation rate increase with EG concentration. To know the influence of EG on dissociation rate, average dissociation rates have been drawn in Figure 3. It can be seen
clearly that with increasing EG concentration, the dissociation rate increases greatly. To show the NGH system pressure behaviors upon the inhibitor flow rate, we select the EG solution with a concentration of 20%. Figure 4 shows the system pressure performance when EG is injected at different flow rates. The system pressure was decreased to 0.8 MPa before the valve was closed and EG solution was injected. Among the all of the tested experiments, the dissociation rate is fast at the beginning stage. This is because the lowest system pressure and only a little H2O from dissociation exist. Thus, the injected EG solution is not diluted. But at this stage the driving force for dissociation is the biggest in the whole dissociation process. When dissociation continued, more and more gas formed and resulted inincreasing pressure. Furthermore, the EG concentration decreases at dissociation interface because of the dilution by dissociated H2O. Therefore, it can be seen that there is a plateau in the curve for every test. It should be noted that increasing the EG concentration offers a steeper curve, meaning that the shorter dissociation times are needed in this case. But at the ending stage when all NGH is dissociated, the pressures are nearly all the same in the reaction vessel. Calculated from the data, the dissociation rates are dem(10) Fan, S. S. Technology of NGH storage and transportation; Chemical Industrial Press: Beijing, 2005.
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Figure 4. Relationship of dissociation rate with injection rate.
onstrated to be nearly linear to the flow rate. They are 0.056, 0.075, and 0.085 mol/min at the flow rate of 80, 100, and 120 mL/min, respectively. The above result also shows that at such a flow rate there is a fine contact for the solution and hydrate. Because dissociation rate depends on the flow rate for practical utilization, the dissociation rate should be optimized in practical utilization. From these experiments, it can be seen that ethylene glycol can help hydrate dissociation a lot. Not only important for hydrate development, dissociation is also important to prevent natural gas from forming hydrate during production and transportation. With injection of EG, dissociation heat and energy can also be reduced. Figure 5a indicates the dependence of heat flow during hydrate dissociation on heating and injection of EG at 293 K. It can be seen that heat flow from heater grows at first, corresponding to the NGH with lowest temperature. It is the big temperature gap between NGH and environmental water. However, the absorbed heat was consumed by the dissociation. Since the pressure increases and temperature goes down due to the fast dissociation of NGH, the dissociation rate becomes slow and heat flow reduces. One can easily see that there is a sudden change at 30 min, which indicates that NGH stops the dissociation at this point. The total heat input by circulating water and EG varies with time (Figure 5b). The heat flow increases very fast when dissociation starts and reaches an almost constant value after 3 min. At this stage, the heat flow is mainly contributed to the dissociation. After 8 min, the heat flow starts to decrease suddenly, meaning that the NGH stops dissociation. At the end there is still 500 W heat left in the system. From calculation, it is known that total energy used by heating is:
Qh ) heat flow × time ) 1500 × 2400 ) 3600 kJ Total energy used by injection of EG is:
Qi ) heat flow × time ) 2250 × 480 ) 1080 kJ The energy cost by heating is more than 3 times as much as that by injection of EG. It has been mentioned previously that EG can reduce the dissociation heat, and therefore less energy and heat are needed. By using ethylene glycol as an inhibitor or production media, we can reduce both time and energy.
Figure 5. (a) Heat flow by heating. (b) Heat flow by injection of ethylene glycol.
In the experiment, the amount of NGH for heating was 4.3 mol and for injection of EG it was 5.1 mol, separately. The theoretic dissociation heat efficiency results are:
Qth ) 4.3 × 74.12 ) 318.7 kJ Eh ) Qth/Qh ) 318.7/3600 ) 10.3% Qti ) 5.1 × 74.12 ) 378.0 kJ Ei ) Qti/Qi ) 378.0/1080 ) 35% It can be seen that with injection of ethylene glycol, heat efficiency is nearly 3.5 as high as that by heating, meaning that heat energy can be used more efficiently. Conclusions EG can accelerate the dissociation of natural gas hydrate, mainly because it can reduce the dissociation heat. Dissociation rate can be promoted by increasing both EG concentration and flow rate. Therefore, it can be optimized in practical utilization. Injection of EG not only can increase dissociation rate, but also can save energy by increase in heat efficiency. This study is important not only for NGH development, but also for natural gas transportation and production. EF0502204
