Nicolas Kalogerakis

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 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.

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.

Hydrate plugging problems in undersea natural gas pipelines under shutdown conditions

Abstract Undersea oil and gas transportation pipelines in cold regions often have thermodynamically suitable conditions for the formation of gas hydrates. There is a widespread concern in the petroleum industry that the hydrate formation could result in a partial or complete plug. A shutdown offshore pipeline condition is emulated in the laboratory to obtain methane hydrate plug formation data. The experiments were conducted at a temperature of 274 K and at pressures of 4, 5 and 7 MPa. Furthermore, a mathematical model which couples intrinsic hydrate formation kinetics with heat and mass transfer phenomena is presented to describe the overall hydrate plug formation process. The effective diffusivities of methane gas through hydrate are estimated by matching the model predictions with the experimental data. The results indicate that the plugging problem is not severe for an unplanned pipeline shutdown for periods less than 48 h. However, caution in the restarting procedure is recommended.

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.

Dynamic modelling of mass transfer phenomena with chemical reaction in immobilized-enzyme bioreactors

Abstract This paper presents a dynamic model for a fixed or liquid fluid bed immobilized-enzyme bioreactor, together with a novel method for the solution of the coupled partial differential equations in the real-time domain. Both the tanks-in-series and the dispersion models are used to describe the non-ideal axial mixing in the reactor. The solution, in its final form, comes in both cases as a system of simultaneous ordinary differential equations that can be readily solved using commercially available software packages. Based on this solution, a complete parametric analysis was performed. The analysis revealed the importance of intraparticle and external mass transfer resistances, intraparticle chemical reaction and axial dispersion on the transient behaviour of the reactor. Most important, the analysis revealed ways for parameter estimation and system identification via simple dynamic experiments. The design and optimization implications are demonstrated by using the derived solution to simulate the performance of an immobilized-urease fluidized-bed bioreactor with a recycle loop. Such a configuration is characterized by a time-varying feed concentration and can be used, as part of an extracorporeal artificial kidney device, for the treatment of uremic patients.

Hydrodynamics of liquid fluidized beds including the distributor region

Abstract An important application of liquid—solid fluidized beds has been developed recently in biotechnology, namely, immobilized biocatalyst bioreactors. For this application, experiments have been carried out to investigate the effect of distributor-induced flow nonuniformities on the hydrodynamics of liquid fluidized beds. The influence of important variables associated with the design of distributors has been studied including the effect of the density of the solid particles on the distributor region flow behavior. In the presence of the low-density particles(ϱ S = 1.61 g/cm 3 ), the influence of the distributor region was quite significant whereas, for high-density particles(ϱ s = 2.46 g/cm 3 , it was negligible. A new model has been proposed which accounts for the stirring effects of the high-velocity orifice jets in the distributor region.

Estimation of multiple binary interaction parameters in equations of state using VLE data. application to the Trebble-Bishnoi equation of state☆

Abstract A systematic approach for the estimation of binary interaction parameters for equations of state is presented. A least-squares procedure which is computationally very efficient is advocated for the calculation of the binary interaction parameters. Subsequently, if the calculated phase behavior represents the experimental data without a gross bias, the statistically best parameters can be obtained by maximum likelihood (ML) estimation. For these cases, the use is advocated of an implicit ML estimation procedure which is computationally significantly more efficient than the “error in variables” method. The proposed approach is particularly suitable for equations of state which have more than one interaction parameter. In such cases, the best parameter set is chosen from among several combinations of interaction parameters present in the equation of state.

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.