P.R. Bishnoi

Kinetics of formation of methane and ethane gas hydrates

Abstract An intrinsic kinetic model with only one adjustable parameter is proposed for the formation of methane and ethane gas hydrates. Experimental formation data were obtained in a semi-batch stirred tank reactor. The experiments were conducted at four temperatures from 274 to 282 K and at pressures ranging from 0.636 to 8.903 MPa. The kinetic model is based on the crystallization theory, while the two-film theory model is adopted for the interfacial mass transfer. Experiments were performed at various stirring rates to define the kinetic regime. The study reveals that the formation rate is proportional to the difference in the fugacity of the dissolved gas and the three-phase equilibrium fugacity at the experimental temperature. This difference defines the driving force which incorporates the pressure effects. The gas consumption rate is also proportional to the second moment of the particle size distribution. The rate constants indicate a very weak temperature dependence.

Kinetics of methane hydrate decomposition

Abstract The kinetics of methane hydrate decomposition was studied using a semibatch stirred-tank reactor. The decomposition was accomplished by reducing the pressure on a hydrate slurry in water to a value below the three-phase equilibrium pressure at the reactor temperature. The data were obtained at temperatures from 274 to 283 K and pressures from 0.17 to 6.97 MPa. The stirring rates were high enough to eliminate mass-transfer effects. Analysis of the data indicated that the decomposition rate was proportional to the particle surface area and to the difference in the fugacity of methane at the equilibrium pressure and the decomposition pressure. The proportionality constant showed an Arrhenius temperature dependence. An estimate of the hydrate particle diameters in the experiments permitted the development of an intrinsic model for the kinetics of hydrate decomposition.

A kinetic study of methane hydrate formation

Abstract The kinetics of methane hydrate formation, after commencement of nucleation, were studied using a semibatch stirred tank reactor. The temperatures studied in the experiments were from 274 to 284 K over a pressure range of 3–10 MPa. The results of the experiments revealed that the formation kinetics were dependent on the interfacial area, pressure, temperature and degree of supercooling. The history of water sample affected the induction delay times for nuclei formation, but it had no observable effects on the overall kinetics of hydrate formation after the nucleation had commenced. A consistent semi-empirical model was formulated to correlate the experimental kinetic data.

Development of a new four-parameter cubic equation of state

Abstract A new four-parameter cubic equation of state is presented which gives improved predictions of phase behaviour for both polar and non-polar fluids. The effect of the number of parameters contained in the equation is discussed in detail. New temperature functions are presented for the “a” and “b” parameters which are thermodynamically consistent and yet simple in form. Discussion of the optimization and generalization procedures used for parameter evaluation is given. Overall comparisons of PVT predictions with ten other recently developed cubic equations of state are made.

Kinetics of gas hydrate formation from mixtures of methane and ethane

Abstract Experimental data on the kinetics of formation of gas hydrates from three mixtures of gaseous methane and ethane are reported. the experiments were conducted in a semi-batch stirred tank reactor at temperatures from 273 to 284 K and of pressures from 0.68 to 5.60 MPa. An intrinsic kinetic model for the growth of the gas hydrate is proposed. It is extension of the model for pure component hydrate formation. The model is based on the crystallization theory coupled with the two-film theory for the gas absorption into the liquid phase. the model does not contain any adjustable parameters. The kinetic rate constants which appear in the model are those obtained previously from pure component formation data. The results indicate that the formation rate is proportional to a lienar combination of the differences in the fugacities of the dissolved gases and their three-phase equilibrium fugacities at the experimental temperature. The effect of the mixture composition is taken into account indirectly through the computation of the three-phase equilibrium conditions and of the fugacities. the total gas consumption rate is proportional to the second moment of the particle size distribution.

Induction phenomena in gas hydrate nucleation

Abstract The induction phenomena in hydrate nucleation have been investigated experimentally and the induction period data are reported for methane, ethane and carbon dioxide hydrates. Based on crystallization considerations, the metastable region for hydrate nucleation is discussed and the driving force for hydrate nucleation is defined. The induction period data for the various hydrates from a total of 93 hydrate formation runs were modelled as a function of the nucleation driving force.

Kinetics of ethane hydrate formation

Abstract The kinetics of ethane hydrate formation after commencement of nucleation were studied at temperatures from 274 to 282 K over a pressure range of 0.6–2.6 MPa. Gaseous ethane was reacted with liquid water in a semi-batch stirred tank reactor. The results of the experiments revealed that the reaction rates were governed by the same kinetic parameters as those found for the methane hydrate formation, i.e. interfacial area, pressure, temperature and degree of supercooling. A semi-empirical model was used to correlate the experimental data.

Extension of the Trebble-Bishnoi equation of state to fluid mixtures

Abstract Mixing rules are established for the Trebble-Bishnoi equation of state which are quadratic in form. Vapour—liquid data for thirty-three binary systems were used to evaluate the extension of the equation to fluid mixtures. As many as four interaction parameters were used to fit the binary data, however good correlation was often obtained with only one interaction parameter. Several isotherms were included for each binary system and many chemical types were included in the list of components evaluated. Extensive volumetric data for the binary system of carbon-dioxide and propane were compared to specific volumes calculated from the Trebble-Bishnoi equation in order to evaluate the accuracy of mixture density predictions. A ternary system of acetone—methanol—water was predicted very accurately using interaction parameters regressed from binary vapour—liquid equilibrium data alone. All of the results compared very favourably to results from both the Peng-Robinson and the Soave-Redlich-Kwong equations of state.

Experimental investigation of hydrate formation behaviour of a natural gas bubble in a simulated deep sea environment

Abstract A vertically flowing, closed circuit, high pressure water tunnel was designed and constructed for holding individual gas bubbles stationary against an opposing flow for detailed observations. Hydrate formation behavior of natural gas bubbles was studied at constant pressure as well as under conditions of controlled decompression designed to simulate buoyant rise of the bubble. A bubble of simulated natural gas suspended in 3°C salt water formed hydrates when the pressure was 4826 kPa or higher. The simulated decompression accompanying buoyant rise had very little effect on hydrate formation behavior of a bubble starting from a pressure of 5516 kPa or above. At lower starting pressures, a slight increase in the reaction rate was detected in the initial stages of a run. The conversion of the simulated natural gas to hydrates was complete in runs starting from a pressure of 4826 kPa or above.

Accuracy and consistency comparisons of ten cubic equations of state for polar and non-polar compounds

Abstract A diverse component library containing extensive PVT data for 75 pure components has been used to critically evaluate ten recently published cubic equations of state. Several of the equations tested showed specific regions of very poor volume predictions and some failures to predict physically meaningful values for volume were even encountered. Temperature dependence in the “b” parameter was included by two of the researchers: Fuller (1976) and Hayen (1981). Investigation revealed that this dependence led directly to the prediction of negative heat capacity in the single phase region. Restraints upon the allowable forms of temperature dependence in the “b” parameter are therefore implied and are discussed in this work.