Natural Gas Hydrate, an Alternative for Transportation of Natural Gas

Transcription

1 Page 1 of 6 Þ Natural Gas Hydrate, an Alternative for Transportation of Natural Gas J. Javanmardi 1, Kh. Nasrifar 2, S. H. Najibi 3, M. Moshfeghian 4 1 Chemical Engineering Department, Shiraz University, Shiraz, Iran 2 Institute of Petroleum Engineering, Tehran University, Tehran, Iran 3 University of Petroleum Industry, Ahwaz, Iran 4 Chemical Engineering Department, University of Qatar, Doha, Qatar ABSTRACT: A process for conversion of natural gas to Natural Gas Hydrate, NGH, has been proposed. Based on the energy balances for the process reactor, the heat exchangers heat duties, the compressor power, and other operational conditions of the proposed process have been determined. The effects of other operational conditions such as seawater temperature as a cooling media, and the temperature of the feed gas have been studied. The results of this work can be used for feasibility study of natural gas transportation in the hydrate form. KEYWORDS: Natural Gas, Gas Hydrate, Natural Gas Transportation, Energy INTRODUCTION: Gas hydrate is a lattice-like compound. At the appropriate conditions of temperature and pressure, the host water molecules can form cage structures that guest compounds are entrapped in. Small hydrocarbon molecules or some non-hydrocarbon compounds in gas or liquid phases are examples of these guest compounds. In appearance, this compound is like snow or loose ice. This compound, thermodynamically, is solid solutions. The cage structure of this compound is formed because of water molecules hydrogen bonding. Large guest molecules may decompose this structure where this framework can not entrap the small molecules. The main industrial interest in hydrates lies in preventing their formation and subsequent plugging of gas transmission lines. On the other hand, some new industrial applications of gas hydrates are as follows: It is now well known that huge deposits of natural gas in the form of hydrates are present in the marine sediments throughout the world and also in oil deposits and permafrost regions. These are the new source of natural gas and future energy. Removal of carbon dioxide from atmosphere and converting to the hydrate form is another application. However, decomposition of gas hydrates might also enhance the greenhouse effect. Water desalination, [1], [2] and gas separation and storage, [3], are other applications of the hydrate phenomenon. Transportation of natural gas in the form of frozen hydrate is considered by some researchers [4]. The results of this study can be used in economic evaluation of the transportation of natural gas in hydrate form. PROPOSED PROCESS: The proposed process for production of NGH has been given in Figure 1. In this process, natural gas which is consisted of about 95 mole% CH 4, after passing through the dryer is fed to the reactor. The dryer has two roles: it acts as a direct heat exchanger and a pre-cooler for the natural gas stream. The second function of the dryer is reducing the water content of the waternatural gas hydrate slurry which is coming from the separator. The gas stream leaving the dryer is therefore at its water-dew point. The fresh water, which is assumed to be pure at this stage, is also pumped to the reactor. In the reactor the heat of hydrate formation is removed. For this purpose an

2 Page 2 of 6 external refrigeration cycle is used. The reactor operational conditions will be given later. After that, the slurry of the natural gas hydrate and free water are fed to the separator. The free water, after separation, is recycled to the reactor. The free water content of the water-natural gas hydrate slurry fed to the dryer is assumed to be 12 wt%. It has been investigated that to transport the natural gas hydrate at atmospheric pressure, the temperature of the hydrate slurry should be lowered to about -15 C, [5]. In other words, the hydrate needs not be refrigerated down to equilibrium temperature at atmospheric pressure. At this temperature and using the insulated vessels, because of hydrate decomposition a layer of ice is formed around the vessel. The heat of hydrate formation and the cooling duties of the heat exchanger in Figure 1 have been removed using a refrigeration cycle as indicated in this figure. Propane is used as refrigerant in this cycle. The approach temperature equal to 6 C is considered in the heat exchanger and condenser. Assuming temperature of the hydrate phase leaving the exchanger equal to -15 C and using the above approach temperature, the temperature of the refrigeration cycle evaporator and consequently its pressure are determined. Using seawater as a heat sink, temperature and pressure of the condenser have been determined. The turbine efficiency equal to 0.8 is used to simulate the compressor in this cycle. For every mole of natural gas fed to the process the following parameters are introduced: R, moles of water to the moles of natural gas fed to the reactor F, number of natural gas molecules (or moles) per water molecule (or mole) in hydrate phase F is reciprocal of the hydrate number, n, and always is less than unity. Using the free water content of the water-natural gas hydrate slurry fed to the dryer, the parameter R, can be obtained in the following manner: R 1 F =0.12(MwNG 18 +R ) (1)

3 Page 3 of 6 The models developed by Parrish and Prausnitz [8] or Holder et al. [9] could be used to predict the hydrate formation conditions in the presence of pure water. To evaluate the heat exchanger duty in the proposed process, the hydrate heat capacity reported by Rueff et al. [10] has been used. The total duty of the refrigeration cycle, as shown in Figure 1, is consisted of the reactor and heat exchanger duties. A computer program has been prepared for the simulation of the proposed process. RESULTS: For a typical natural gas with the following composition several different operational conditions of the proposed process have been studied. These conditions have been given in Table 1. The results, including the duties of the exchangers, compressor power, and other operational conditions have been given in Tables 2 and 3. The capacity of the plant for all of these cases is 25 MMSCF/D. As shown in Tables 2 and 3, the seawater temperature as a cooling media and the temperature of the stored hydrate have significant effect on the cooling load of the refrigeration cycle and the compressor power. Many investigations have been done on the self-preservation of hydrate at atmospheric pressure

4 Page 4 of 6 and temperatures higher than its equilibrium value, [11]. Recent studies indicate that this temperature may be increased to above ice point depending on the technique of the hydrate formation. So, the selfpreservation of hydrate at higher temperature may decrease the operating and the fixed cost of natural gas transportation considerably; and so, convert this method as an alternative for natural gas transportation. At present, the LNG method is the best especially for long distances; however, it requires a large reserve of natural gas and considerable capital investment. The effect of hydrate storage temperature on the compressor and condenser duties has been given in Figure 2. Figure 3 shows the effect of seawater and the feed water temperature on these terms. Other operational conditions in this figure are the same as the case a in Figure 1. The hydrate structure and so reactor temperature have been affected by the natural gas composition. For

5 Page 5 of 6 example at higher methane concentration as used in the previous cases, the hydrate structure is changed into S-I and the equilibrium temperature at the same pressure is increased about 6 C. To maximize the hydrate formation rate, 2 C as driving force has been considered in the reactor, i.e., its temperature is 2 C lower than equilibrium value. Some inorganic compounds such as acetone have promoting effect on hydrate formation conditions, [12]. In the presence of these compounds, the hydrate formation occurs at more feasible conditions and therefore lower operational costs are needed. CONCLUSIONS: The storage temperature of hydrate has considerable effect on the energy consumption and the cost of natural gas transportation in hydrate form. Moreover, to compare the NGH method with available methods in natural gas transportation such as LNG, the seawater and the feed water temperature have critical roles. ACKNOWLEDGEMENT: The authors appreciate the financial support of Research and Development Branch of National Iranian Gas Company Grant No SYMBOLS: R, moles of water to the moles of natural gas fed to the reactor F, number of natural gas molecules per water molecule in hydrate phase Mw, molecular weight i, ratio of the no. of type i cavities to no. of water molecules mi, fraction of the type m cavities which are occupied by hydrate former i H, heat of formation of hydrate molecule, J/gr mole C P, heat capacity, J/gr mole.k H, enthalpy, J/gr mole P, pressure, kpa T, temperature, K NGH, natural gas hydrate REFERENCE: 1. Knox, W. G., M. Hess, G. E. Jones and H. B. Smith, (1961), "The Hydrate Process", Chem. Eng. Prog., 57 (2), pp Kubota, H., K. Shimizu, Y. Tanaka and T. J. Makita, (1984), "Thermodynamic Properties of R13 (CClF 3 ), R23 (CHF 3 ), R152a (C 2 H 4 F 2 ) and Propane Hydrates for Desalination of Seawater" Chem. Eng. Japan, vol. 17, no. 4, pp Miller, B. and E. K. Strong, (1946), Amer. Gas Assoc. Monthly, 28 (2), p Gudmundsson, J. S., and A. Borrehaug, (1996), Frozen Hydrate for Transportation of Natural Gas, 2 nd International Conference on Natural Gad Hydrate, June, 2-6, Toulouse, France, pp Gudmundsson, J. S., V. Andersson, and O. I. Levik, (1997), Gas Storage and Transport Using Hydrates, Offshore Mediterranean Conference, Ravenna, March,

6 Page 6 of 6 6. Pieroen, A. P. (1955), Gas Hydrates-Approximate Relations Between Heat of Formation, Composition and Equilibrium Temperature Lowering by Inhibitors, Recueil Trav. Chim. 74, pp Javanmardi, J., M. Moshfeghian and R. N. Maddox, (1998), "Simple Method for Predicting Gas-Hydrate-Forming Conditions in Aqueous Mixed-Electrolyte Solutions", The Journal of Energy and Fuels, 12 (2), pp Parrish, W. R. and J. M. Prausnitz, (1972), "Dissociation Pressures of Gas Hydrates Formed by Gas Mixtures", Ind. Eng. Chem. Proc. Dev., 11 (1), pp Holder, G. D., G. Gorbin, and K. D. Papadopoulos, (1980), "Thermodynamic and Molecular Properties of Gas Hydrates from Mixtures Containing Methane, Argon and Krypton", Ind. Eng. Chem. Fund., 19 (3), pp Rueff, R.M., E.D. Sloan, and V.F. Yesavage, (1988) Heat Capacity and Heat of Dissociation of Methane Hydrates, AIChE J. 34, pp Shirota, H., E. D. Sloan, Jr., P. Bollavaram, D. J. Turner, I. Aya, and S. Namie, Measurement of Methane Hydrate Dissociation for Application to Natural Gas Storage and Transportation, Ng, H. J. and D. B. Robinson, (1994), "New Developments in the Measurement and Prediction of Hydrate Formation for Processing Needs", Annals of the New York Academy of Science, vol. 715, p. 450.