Gerald D. Holder

Methane hydrates potential as a future energy source

Gas hydrates are crystalline solids that form from mixtures of water and light natural gases such as methane, carbon dioxide, ethane, propane and butane. They are of considerable interest for their potential as an energy resource and for their potential role in global climate change. From an energy resource point of view, the enormous amounts of methane hydrate under the ocean and beneath arctic permafrost represent an estimated 53% of all fossil fuel (coal, oil, natural gas) reserves on earth, about 10,000 gigatons. The difficulty with recovering this source of energy is that the fuel is in solid form and is not amenable to conventional gas and oil recovery techniques.

Extraction of coal using supercritical water

Abstract An experimental apparatus was developed to inject coal into an autoclave containing preheated supercritical water. The supercritical water appears to act as both solvent and reactant in the conversion of coal to gases and liquids. Experiments were carried out with German brown coal, lignite and bituminous coal and with glucose at both subcritical and supercritical water densities. A significantly larger quantity of char was obtained when operating at subcritical densities and when the coal was mixed with water before heating to supercritical conditions. Smaller amounts of char were obtained as density increased and as reaction time increased.

Solubility of progesterone, testosterone, and cholesterol in supercritical fluids☆

One potentially used application of supercritical fluid extraction (SCFE) is isolating and separating pharmaceuticals such as steroids. Experimental solubility data were obtained for testosterone, progesterone, and cholesterol in supercritical CO2 with and without N2O cosolvent over a pressure rage of 80 to 250 atm at 308.15–333.15 K. The addition of 10% (volume) N2O into CO2 increased the solubilities of these compounds by as much as one order of magnitude with larger effects occurring at lower pressures. Ternary systems (progesterone + testosterone + CO2, progesterone + testosterone + CO2, and testosterone + cholesterol + CO2) were also investigated. The experimental data were correlated by using a modified Peng-Robinson equation-of-state with density-dependent binary mixing rules, treating steroid vapor pressures in the form of the Clapeyron equation as adjustable correlating parameters.

Characterization of Fischer-Tropsch liquids for vapor-liquid equilibria calculations

Abstract K -value formulations are developed for use in performing vapor-liquid equilibria (VLE) calculations on Fischer-Tropsch (F-T) systems. Carbon-number-based pseudocomponents are defined with the aid of the Anderson-Schulz-Flory (ASF) molecular weight distribution. Proper selection of the number and types of pseudocomponents enables the trade-off which exists between accuracy and computing time to be adequately addressed. The properties of a pseudocomponent are based on a hypothetical ‘model’ component in each carbon-number cut. Solubilities of gases and light hydrocarbons in the liquid phase are calculated from correlations for the individual solutes in pure n -paraffin solvents, using average carbon numbers of the liquid phase. Comparisons are made with data for several different F-T reactor compositions and with predictions from literature models. The VLE model developed accurately predicts product phase splits, and is superior to other formulations.

High pressure phase behavior in systems containing CO2 and heavier compounds with similar vapor pressures

Abstract The separation of the para and ortho isomers of xylene as well as the azeotropic mixture of butyl ether/o-xylene using carbon dioxide at high pressure was investigated. The phase behavior of carbon dioxide and each of these compounds along with the ternary systems; CO2/p-xylene/o-xylene and CO2/butyl ether/o-xylene were experimentally determined. The relative volatilities of p-xylene to o-xylene in the CO2/p-xylene/o-xylene system compared favorably with those obtained in distillation. The results also indicated a substantial shift in the butyl ether/o-xylene azeotrope to higher butyl ether concentrations in the presence of carbon dioxide thus indicating a potential for the separation of these mixtures using carbon dioxide at low temperatures. Thermodynamic models using the Peng—Robinson equation of state were developed and better predictions of the bubble point pressures were obtained when the interaction parameter, δij, was allowed to vary with phase density. This approach results in an analytically solvable quartic equation in volume and gives different δijs for the vapor and liquid phases. In this model, the temperature dependence of the binary interaction parameter is contained within its density dependence and, δijs obtained from fitting VLE data at a single temperature could be used for accurate prediction of equilibrium data at other temperatures. The results of such predictions were better than predictions obtained by fitting the actual data using the conventional VDW-1 mixing rules.

Critical and three phase behavior in the carbon dioxide/tridecane system

The PT behavior of the carbon dioxide/normal alkane series exhibits a distinct transition in the CO2/nC13H28 system. This particular diagram is characterized by two liquid—liquid—vapor (l-l-g) loci, a lower liquid-upper liquid (l-l) critical branch extending from high pressures to the upper critical end point (UCEP) and two liquid—vapor (l-g) critical branches which cross near the lower critical end point (LCEP). An experimental PTx diagram in the vicinity of the CO2 critical point, LCEP and K point reveals the emergene at the LCEP of a l-l region which increases in size with temperture while the upper liquid—vapor (L2-g) region diminishes, eventually disappearing at the K point. The l-l-g surface illustrates the compositional changes of each phase with temperture. Detailed Px diagrams at three temperatures between the LCEP and K point are presented and each exhibits two critical points, a l-l-g locus and curves of constant phase volume ratio which show discrete changes in both value and slope at the l-l-g locus. Graphical and numerical methods of determining the phase densities and compositions from three phase volumetric behavior are presented.

A Thermodynamic Evaluation of Thermal Recovery of Gas From Hydrates in the Earth (includes associated papers 11863 and 11924 )

An examination is made of the potential for recovering gas from naturally occurring hydrates. Factors to be considered in such a study are (1) location of the hydrate fields, (2) purity of hydrates in the reservoir, (3) types of media in which hydrates form, (4) thermodynamic conditions of temperature, pressure, and composition, and (5) thermal properties of the reservoir. Based on these considerations, calculations were made to determine the energy needed to dissociate hydrates and the amount of gas recovered per g mol of hydrate dissociated. 14 refs.

Modeling clathrate hydrate formation during carbon dioxide injection into the ocean

Because of carbon dioxides potential role in global warming, there is considerable interest in methods of long-term sequestering of anthropogenic emissions of CO[sub 2] outside of the atmosphere. The work reported here predicts the effect of hydrate formation on the fate of CO[sub 2] droplets discharged into the ocean under hydrate-forming conditions. New information on hydrate growth rates recently determined by one of the authors is incorporated into the model. It is seen that hydrate film formation on CO[sub 2] droplets into the deep ocean will increase estimates of required injection depths and decrease the maximum allowable droplet size suitable for effective sequestration to occur. 13 refs., 2 figs.

Hydrate dissociation pressure minima in multicomponent systems

Abstract Conditions for which gas hydrates have a minimum dissociation pressure are determined analytically. The development allows direct determination of the gas phase compositions which will result in such azeotropic conditions. Results show that the azeotrope is not a manifestation of non-idealities in the hydrate phase but rather a direct result of the hydrate structure. Minima will occur for some ternary mixtures (2 gases + water), but, surprisingly, not for more complex systems. Analysis shows that molecular size can be used as the basis for choosing ternary mixtures which will display dissociation pressure minima.

Adamantane and diamantane; phase diagrams, solubilities, and rates of dissolution

Abstract Properties of adamantane and diamantane relevant to their occurrence in and removal from gas pipelines have been determined. The phase diagrams for these two diamondoids have been determined between ambient temperature and the estimated critical point using vapor pressure measurements, S-S and S-L phase transition observations, and solubilities in dense carbon dioxide. Diamantane exhibited three different solid phases. Although many common organic liquids were strong solvents for adamantane and diamantane, cyclohexane was particularly effective due to the similarity between its structure and the fused cyclohexane ring structure of the diamondoids. A generalized correlation for the mass transfer coefficient governing the dissolution of extremely thick diamantane deposits from the inter wall of pipelines developed in this study can be used to provide accurate estimates of the amount of solvent required to remove the deposit. Reasonable estimates can also be obtained with the analytical solution derived in this study if an invariant, average mass transfer coefficient is used.