P. Raj Bishnoi

Formation and decomposition of gas hydrates

The kinetics of hydrate formation and decomposition are explained as understood to date. The formation and decomposition phenomena are complex. Hydrate formation is viewed as a crystallization process that includes the nucleation and growth processes. Hydrate nucleation is an intrinsically stochastic process that involves the formation and growth of gas-water clusters to critical sized, stable hydrate nuclei. Hydrate growth process involves the growth of stable hydrate nuclei as solid hydrates. Hydrate decomposition is a sequence of lattice destruction and gas desorption processes. The process of heat transfer during hydrate decomposition is analogous to nucleate boiling phenomena. The focus of this work is to present the various perspectives on the kinetic processes at a conceptual level. Key issues for research in this area are identified and some possible directions for future work are suggested.

Dissolution of methane in water at low temperatures and intermediate pressures

The solubility of methane in pure water was measured at low temperatures and moderate pressures below the gas hydrate forming conditions. Within the experimental accuracy, Henrys law is obeyed according to the Krichevsky-Kasarnovsky relation. These and other existing data in the literature have been reviewed for derivation of partial molar volumes of aqueous methane at infinite dilution and other thermodynamic properties. Conclusions regarding structuring of water, based on molar volume measurements, are shown to be tentative with the currently available data.

Measuring and modelling the rate of decomposition of gas hydrates formed from mixtures of methane and ethane

The rate of decomposition of gas hydrates formed from mixtures of methane and ethane was measured in a semi-batch stirred-tank reactor. The experimental apparatus was the same as that used by Clarke and Bishnoi (Chem. Eng. Sci. 55 (2000) 4869; Can. J. Chem. Eng. (2000) accepted for publication) to measure the kinetics of decomposition of hydrates formed from pure ethane and methane. Experiments were conducted with methane/ethane mixtures ranging from 25% methane to 75% methane and at temperatures and pressures between 274 and 278K and 6.39 to 14.88bar, respectively. The model of Clarke and Bishnoi was extended to hydrates that are formed from gas mixtures. For hydrates that were known to form structure I, no new parameters were needed and the model was found to accurately predict the data. For the structure II hydrates, it was assumed that the rate constant of ethane was independent of structure because ethane only occupies the large cavities. The intrinsic rate constant and activation energy for methane in structure II were determined to be 8.06×103mol/m2Pas and 77.33kJ/mol, respectively.

A method for the simultaneous phase equilibria and stability calculations for multiphase reacting and non-reacting systems

Abstract A development of the stability criterion for multiphase reacting/non-reacting systems is presented. This new development has led to a formulation of a set of coupled non-linear algebraic equations that describe both the stability and the isothermal-isobaric flash calculations of reacting and non-reacting systems. The formulation has been used to develop an algorithm for the simultaneous computation of stability and multiphase equilibria in reacting /non-reacting systems. The Newton-Raphson procedure is used to solve the stability and the summation equations for the phase fractions and the stability variables. The stability equation has been transformed to alleviate the problems associated with the ill-conditioning and the singularity of the Jacobian near the phase boundaries. The appearance or disappearance of a phase during the computations is handled easily. Simultaneous computation of the stability variables and the phase fractions is particularly suited near phase boundaries and for multiphase reactive systems. The effectiveness of the proposed algorithm is illustrated by examining a mixture of methane, carbon dioxide and hydrogen sulfide, and reacting mixtures typically encountered in methanol synthesis in the presence or absence of heavy oil.

Equilibrium conditions for hydrate formation from binary mixtures of methane and carbon dioxide in the presence of electrolytes, methanol and ethylene glycol

Abstract Ethylene glycol, methanol and electrolytes inhibit hydrate formation. Computation of the inhibition effects of these additives is necessary for the design of industrial operations where it is desired to avoid hydrate formation. Development of thermodynamic methods to calculate the hydrate equilibria conditions requires accurate experimental data. In the present work experimental three phase (aqueous liquid solution, vapor and incipient solid hydrate) equilibrium data for two mixtures of methane and CO 2 in the presence of methanol, ethylene glycol and electrolytes were obtained. The experiments were carried out in the temperature range of 264–282 K and pressure range of 1.5–10.4 MPa. A ‘full view’ fixed volume sapphire cell used earlier to gather data on CO 2 by the authors was converted into a variable volume cell with the help of a movable piston to perform the experiments.

Multiphase equilibrium flash calculations for systems containing gas hydrates

The methodology for multiphase equilibrium flash calculations of Gupta (1988) is adapted for systems containing gas hydrates. The solid hydrate phase is treated as a solid solution. The equilibrium distribution ratios for the components present in the hydrate are defined appropriately. The methodology is used to examine the phases present in a methane-propane-water system and in a condensate with water and methanol.

Estimation of binary interaction parameters for equations of state subject to liquid phase stability requirements

Abstract A methodology is presented for the estimation of equation of state binary interaction parameters subject to no liquid phase separation. The inequality constraint describing the stability of the liquid phase is incorporated in the minimization procedure to determine those interaction parameters which not only ensure prediction of the correct phase behavior but they also correspond to the constrained minimum of the chosen optimality criterion. The methodology has been successfully applied to the n-Hexane-Ethanol system.

Kinetics of Ethane Hydrate Growth on Latex Spheres Measured by a Light Scattering Technique

Abstract: A high pressure, temperature‐controlled sapphire equilibrium cell is used to observe the nucleation and growth behavior of gas hydrates in bulk water or on spheres suspended in water. Gas hydrate crystals are grown on positively and negatively charged latex spheres. The experiments have been conducted on an ethane‐water system that is known to form structure I hydrate. Experiments were carried out at temperatures between 278.0 K and 278.6 K and pressures ranging from 1,300 kPa to 1,500 kPa. A light scattering apparatus was used to monitor the hydrate formation process. In particular, the photomultiplier voltage was recorded over time in order to observe the effects of latex spheres on the nucleation and growth of gas hydrates. More experiments need to be performed to better ascertain the hydrate nucleation and growth behavior in the presence of latex spheres.

Simultaneous multiphase isothermal/isenthalpic flash and stability calculations for reacting/non-reacting systems☆

Abstract A formulation for the computation of stability and multiphase equilibrium in reacting/non-reacting systems permits the simultaneous calculation of the phase fractions and the stability variables. Based on this formulation an algorithm is given for the coupled stability and isothermal-isobaric flash computations in reacting/non-reacting multiphase systems. This algorithm is extended to handle the isenthalpic flash computations in non-reacting systems. The appearance or disappearance of a phase during the computations is easily handled by incorporating a switching policy in the algorithm. Simultaneous computation of the stability variables and the phase fractions is particularly suited near phase boundaries and for multiphase reactive systems. The effectiveness of the proposed algorithms is illustrated by solving several typical multiphase problems.

Simultaneous regression of binary VLE and VLLE data

Abstract A computational methodology for the simultaneous regression of binary vapor—liquid and vapor—liquid—liquid equilibrium data is presented. It is used for the estimation of the interaction parameters in equations of state. An implicit least-squares estimation procedure has been developed which yields the best set of parameter estimates for the equation of state. Subsequently, if the thermodynamic model is capable of representing the data without any systematic deviation, the statistically optimal parameters can be obtained by a proposed implicit maximum likelihood estimation procedure. The key advantage of both the estimation procedures is the absence of any iterative phase equilibrium computations. As a result, the methodology is easily implemented and is computationally very efficient. The application of the method is illustrated with a typical example (hydrogen sulfide—water).