170
Ind. Eng. Chem. Process Des. Dev. 1983, 22, 170-171
Formation of Clathrate Hydrates in Hydrogen-Rich Gases A hydrogen-rich gas obtained from the high-pressure separator system of the SRC-I I process development unit operated by Gulf Science and Technology Co. was contacted with liquid water at pressures up to 6720 kPa (975 psia) and at temperatures below 282.3 K. Experimental results indicate that clathrate hydrates readily form at these conditions and that the potential exists for the formation of clathrate hydrates in many coal conversion processes where high-pressure gas must be cooled to temperatures below approximately 295-300 K. The currently used predictive model significantly underpredicts the experimental dissociation pressures and indicates that further studies of hydrate formation from hydrogen-rich gases are needed. This sensitivity to hydrate formation will be strongly dependent upon composition.
Introduction Gas hydrates are crystalline compounds composed of water and light gases such as methane, ethane, and nitrogen. They belong to a class of compounds known as clathrates in which one species forms a “host” lattice structure with large interstices which entrap the second, “guest”species. In clathrate hydrates, water forms the host lattice and the light gas occupies some of the interstices or cages of this lattice. Physical dispersion forces acting between the guest and host molecules stabilize the clathrate, and although the gas is said to be dissolved in the lattice, the water lattice does not exist in the absence of entrapped gas. The formation of gas hydrates is favored by high pressures and low temperatures, but they can exist at subatmospheric pressures and at temperatures above 300 K depending upon the composition of the gas from which the hydrate is formed. In general, hydrates will form from liquid water and gas phases, although a water-gas mixture in any combination of states can form hydrates (Sloan et al., 1976). The process of hydrate formation and dissociation has been suggested for industrial application in the desalinization of seawater in relatively small quantities (Barduhn, 1967),the underground storage of gas (Parent, 1948),and for gas separation (Barrer and Ruzicka, 1962). In addition, the blocking of natural gas pipelines due to hydrate formation has been a problem in the natural gas industry (Hammerschmidt, 1934). Recently, the possibility of hydrate formation in natural gas and petroleum reservoirs has stimulated research in this area (Holder et al., 1982). The need for a knowledge of the dissociation pressures and temperatures of gas hydrates arises because in any operation in which hydrate formation is or is not desired, the minimum pressure for which hydrates are stable in the presence of a water rich liquid phase will define the thermodynamic limits of the operation. In the present study, the potential for formation of gas hydrates from hydrogen-rich streams produced in coal gasification/liquefaction processes is considered. An important step in many proposed coal conversion processes involves the addition of hydrogen, in gaseous form and at high pressures, to the conversion unit or reactor. Because the effluent from the reactor can contain significant quantities of unreacted hydrogen, the hydrogen must be separated from the converted and unconverted coal fractions and subsequently recycled. In the SRC-I1 process, the raw recycle gas stream (at 13550 kPa) has significant quantities of gases other than hydrogen including hydrocarbons (methane through pentanes), carbon dioxide, and hydrogen sulfide; hydrogen comprises roughly 78 mol % of the raw recycle stream. However, prior to being added to the conversion reactor, the SRC-I1 design calls for upgrading the purity of the 0196-430518311122-0170$0 1.5010
recycle stream to 91 mol % hydrogen. In the proposed SRC-I1 demonstration plant, it was planned to use cryogenic purification to gain the needed purity. In preparation for the cryogenic unit, many of the impurities (including heavy hydrocarbons) must be removed beforehand. The purification steps consist of a water wash followed by washing with liquid butane at temperatures of 278-305 K. This latter step is designed to remove most of the higher boiling point contaminants and it is in this step that the high-pressure gas, being saturated with water, has the potential for forming gas hydrates. In the present work the potential that the raw recycle gas has for forming gas hydrates is examined experimentally.
Experimental Section A standard high-pressure equilibrium apparatus for measuring three-phase clathrate hydrate equilibria was used. This apparatus has been described in detail elsewhere (Holder et al., 1980) and can measure pressure with an error of less than 1% and temperature to h0.3 K. Gases were analyzed by gas chromatography, and analyses of the hydrocarbons are considered accurate to 2% of the amount present. The reported mole fractions of C02 and H,S may have a somewhat greater error. A pressurized l-L cylinder of gas representing the inlet to the butane scrubber at approximately 14 MPa (2000 psia) was obtained from Gulf Science and Technology Company’s SRC-I1 process development unit. The actual experiments were conducted with the apparatus available at the University of Pittsburgh. The experimental results are shown in Table I and Figure 1. Each experimental point represents the minimum pressure (at a fixed temperature) or maximum temperature (at a fixed pressure) for which hydrates can exist. The composition of the gas used is shown in the inset of Figure 1. Trace amounts of nitrogen and carbon monoxide were also present. This represents the composition of the vapor phase (V) which was in equilibrium with a water-rich liquid phase (L) (which is essentially pure water) and a solid hydrate phase (H). The curves in Figure 1 can be safely extrapolated to a t least 20000 kPa. Discussion In order to evaluate the experimental data, the model of van der Waals and Platteeuw (1959), as modified by Parrish and Prausnitz (1972), and Holder and Hand (1982) was used to calculate the three-phase, VLH, equilibrium pressure. This model uses a Langmuir coefficient to describe vapor-hydrate equilibria. We have assumed that structure I1 hydrate has formed in these calculations. The Kihara potential function parameters used for determining the Langmuir constants for these calculations are shown in Table 11. The model assumes that hydrogen and the pentane-plus fraction do not contribute to the stability of 0 1982 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983 171
2000
'0
273
275
277
279
201
203
205
TEMPERATURE (K)
Figure 1. Experimental and predicted dissociation pressures of a hydrogen-rich gas. Solid dots are the experimental points which are connected by a solid line. The dashed line is the predicted dissociation pressure curve. Table I. Experimental Three-phase, Vapor + Water-Rich Liquid + Hydrate Equilibrium Temperatures and Pressures for the Hydrogen Rich Gas Given in Figure 1 T,K P, Wa
282.2 281.6 281.0 280.6 280.2 276.7
6720 6210 6760 6600 6300 3606
Table 11. Kihara Parameters Used for Calculating Hydrate Dissociation Pressures a,pmx u,pmx component 10-2 lo-' elk, K methane" 0.3330 3.1883 150.73 ethane" 0.6680 3.1650 174.77 ethylene 0.4700 3.2910 172.07 propane" 0.6460 3.3200 209.02 isobutane 0.8000 3.1244 220.62 0.7600 3.4862 n-butane 169.41 0.3600 carbon dioxide 2.9861 169.09 hydrogen sulfide 0.3600 3.1668 206.86
c,t
hydrogend a Holder and Hand (1981). Parrish and F'rausnitz (1972). Ng and Robinson (1977). Does not
stabilize the hydrate phase.
the hydrate phase. If the model were to assume that hydrogen could stabilize the hydrate phase, the calculated dissociation pressures would be lower than shown here. Because the calculated dissociation pressures are already lower than the experimental dissociation pressures, the assignment of a nonzero Langmuir constant to hydrogen would increase the discrepancy between the experimental and calculated results. Barrer and Ruzicka (1962) also found that hydratea of hydrogen and chloroform contained a very small (although nonzero) quantity of hydrogen. The degree of stabilization or destabilization of the hydrate phase by hydrogen is certainly an area needing further study, especially since the disagreement between the experimental and predicted values is greater than would normally be expected. However, when temperature is treated as the dependent variable, the uncertainty for process design applications is on the order of only 3 K. For
design purposes, if one desires to avoid hydrate formation, the direction of the error in calculated dissociation pressures (which are too low) means that an estimate of the lowest acceptable design temperature will be too high. The implications of the experimental results are clear: any water-saturated stream containing even small quantities of hydrate forming gases, can, when subject to high pressures and low temperatures (-300 K), form hydrates. Virtually any gas from a coal gasification or liquefaction plant which contains some low molecular weight hydrocarbons, hydrogen sulfide, carbon monoxide, or carbon dioxide, and water could potentially form hydrates, and this must be considered in the design of any process since solid hydrates can easily plug process lines. In the present study, the methane, the propanes and butanes, and the hydrogen sulfide are the gases making the greatest contribution to the stability of the hydrate phase. Because gas mixtures act in a synergistic fashion in stabilizing the hydrate phase, it is not possible to specify which of these species are most important in this stabilization, although the presence of propanes and/or butanes is very significant. For any gas stream which is not saturated with water, hydrates can still form, although higher pressures would be required (Sloan, et al., 1976; Ng and Robinson, 1977). Finally, one must recognize that there are kinetic limitations to the formation of hydrate-for example, appropriate nucleation sites must be available-and in practice a gas stream must be considerably subcooled before hydrate formation is initiated.
Nomenclature a = Kihara core diameter for gas-water interactions, pm = Kihara energy parameter for gas-water interactions, J u = Kihara molecular diameter for gas-water interactions, pm
Literature Cited Barduhn, A. J. Chem. Eng. Pmg. 1967, 63,98.
Barrer, R. M.; Ruzioka, D. J. Trans. Farady Soc. 1962, 58. 2239.
HammerschmM, E. G. Ind. Eng. Chem. 1984, 26, 851. Holder, G. D.; Angert, P. F.; John, V. T.; Yen, S.J . Pet. Techno. 1982, 34, 1127. Holder, G. D.; CorMn, 0.;Papedopoulos, K. Ind. €ng. Chem. Fundem. 1980, 10, 282. Holder, G. D.; Hand, J. H. A I C M J . 1982, 28, 440. Ng, H. J.; Roblnson, D. 0. AIChE J . 1977, 23,477. Parent, J. D. Inst. Gas. Techno/. BUN. 1948, 1 , 1. Parrish, W. R.; Prausnltz. J. M. Ind. €ng. Chem. procesS Des. Dev. 1972, 1 1 , 26. Sloan. E. D.; Khoury, F.; Kobayashi, R. Ind. €178.Chem. Fundam. 1976, 15, 318. van der Waals, J. H.; Platteeuw, J. C. A&. Chem. fhys. 1969, 2 , 1.
Gulf Science and Technology Company Gerald D.Holder Pittsburgh, Pennsylvania 15230 J e r r y L. Stephenson* Solvent Refined Coal International, Inc. Englewood, Colorado 80155 Chemical and Petroleum Engineering Department University of Pittsburgh Pittsburgh, Pennsylvania 15261
Jeffery J. Joyce
Vijay T.John Vidyadhar A. Kamath Shirish Malekar
Received for review January 4, 1982 Revised manuscript received June 30,1982 Accepted July 21, 1982
