Krishnaswamy Rajagopal

Consistent evaluation of effective diffusion and reaction in pore networks

Abstract A model is developed based on the equivalent pore network conceptualize to account for diffusion and reaction processes in catalytic pore structures. The chromatographic reactor is adopted as a reference system so that the transit time moments of the response curve can be used to assess the effectiveness of the intraparticle diffusional transport and overall catalyst activity. As a consequence of the detailed description of the phenomenology at the microscopic pore scale, the results can then be expressed in terms of a Thiele modulus (o) which is a purely physico-chemical parameter, not dependent on the structural aspect of the pore space. At particular ranges of o values, a strong influence of the pore structure is detected onthe effective diffusion and effective reaction coefficients. It is then appropriate to say that the modelling strategy devised here not only sets a framework for the selection of a suitable catalyst pore structure. It also indicates that since the structure is known and consistently modelled, there is scope for design in terms of the reactor operational conditions to increase the catalyst performance.

Simulating oil flow in porous media under asphaltene deposition

Abstract A simulator for one-phase flow in porous media near a wellbore is coupled with a thermodynamic model and a network model in order to predict the change in petroleum flow under asphaltene deposition. The thermodynamic model is capable of predicting the quantity of precipitated asphaltene. The network model is used to predict formation damage due to in situ asphaltene deposition. The model is qualitatively evaluated using data from literature. Results are in concordance with expected physical behavior.

Liquid film flow and area generation in structured packed columns

Several correlations are available in the literature for the prediction of wetted area or the effective interfacial area in packed columns. A careful examination shows considerable discrepancies in the calculated areas and conflicting predictions concerning the influence of viscosity on the interfacial areas. In this work, the effect of physical properties of liquids and of surface treatment on wetted area of structured packings was experimentally studied. Several wetting tests were performed on metallic and ceramic plates with flat, smooth or textured surfaces, using a circulation system, specially designed for this purpose. The liquid film width and thickness were measured for solutions with different surface tension and viscosities in a wide range (1 to 200 cP). The experimental results show that the liquid film width, and hence the wetted area, decreased with liquid viscosity, contrary to earlier correlations in the literature. Also the influence of contact angle is not so strong as stated in the literature for random packings. In this study, a new statistical correlation for the estimation of the wetted area and for the liquid film thickness is proposed, reflecting the measured variations with viscosity and advancing contact angles.

Towards a polydisperse molecular thermodynamic model for asphaltene precipitation in live-oil

A polydisperse molecular thermodynamic model is developed to predict the asphaltene precipitation in crude oil under reservoir conditions. Asphaltenes are characterized as a continuous family that follows a distribution function taken from the fractal aggregation theory. Discretization of the continuous family is made by using collocation points chosen as the roots of an orthogonal polynomial. It is assumed that the asphaltenes form a pseudo-liquid phase, free of other components. The parameters of the model were adjusted to fit experimental data available from the literature. A satisfactory representation of these data was obtained, showing a better agreement than the earlier models. A sensitivity analysis is performed in order to evaluate the influence of the model parameters on predicted values.

An evaluation of density corrections for estimating diffusivities in liquids and liquid mixtures

Abstract In this work we analyse the effect of density correction on the estimation of diffusion coefficients in liquids and liquid mixtures using the Carnahan-Starling (1969, J. Chem. Phys. 51 , 635–636) pair correlation function and the correlation of Speedy (1987, Molec. Phys. 62 , 509–515) and Harris (1992, Molec. Phys. 77 , 1153–1167) which have been proposed as models of self-diffusion coefficient of hard-sphere fluids. The hard-sphere diameters of nine liquids were estimated by fitting the experimental self-diffusion coefficients with the smooth-hard-sphere theory and the estimated diameters were used for predicting diffusivities in 13 binary and eight ternary systems. This theory with the density- and temperature-dependent hard-sphere diameter obtained from the Weeks-Chandler-Andersen (Weeks et al. , 1971, J. Chem. Phys. 54 , 5237–5247) perturbation theory of liquids is shown to be an excellent approach for predicting diffusivities in liquids and liquid mixtures. The calculations involved nonideal mixtures as well as systems with high molecular asymmetry. The predicted diffusivities are in good agreement with the experimental data for the binary and also for the ternary systems. The present results are much better than the estimates of Guo and Kee (1991, Chem. Engng Sci. 46 , 2133–2140) for the mutual diffusion coefficient at infinite dilution for any of the density correction methods. It seems, then, that the coupling parameter of the rough hard-sphere theory has no essential role for predicting diffusion coefficients, even for large molecules, if a suitable hard-sphere diameter is used. The methodology proposed here only makes use of pure component information and density of mixtures. The simple algebraic relations proposed are without any binary adjustable parameters and can be readily used for estimating diffusivities in multicomponent liquid mixtures.

MODELING AND SIMULATION OF HYDROGEN SULFIDE REMOVAL FROM PETROLEUM PRODUCTION LINES BY CHEMICAL SCAVENGERS

The in-line scavenging of hydrogen sulfide is the preferred method for minimizing the corrosion and operational risks in offshore oil production. We model hydrogen sulfide removal from multiphase produced fluids prior to phase separation and processing by injection of triazine solution into their gas phase. Using a kinetic model and multiphase simulator, the flow regimes, amounts, and composition of three phases are determined along the horizontal and vertical flow path from subsea well to separator tank. The flow regimes were found to be slug flow or intermittent flow. The highly reactive triazine is destroyed on contact with water phase flowing near the wall. We have simulated the hydrogen sulfide concentration profiles for different amounts of gas injection. The results are compatible with the available field data from an offshore oil well and are useful in determining the injection rates of expensive chemical scavengers and optimal gas injection rates.

Characterization of Functional Groups of Asphaltenes in Vacuum Residues Using Molecular Modelling and FTIR Techniques

Abstract Asphaltenes are components of crude oil, and their average chemical structures are characterized with difficulty. This study shows that simple Fourier transform infrared (FTIR) analysis spectroscopy could be adapted to the determination of aromatic hydrogens in asphaltenes and resins and elucidation of their average molecules. The work demonstrates the existence of a linear correlation in the infrared (IR) intensities of the symmetric and asymmetric aromatic hydrogens in methyl substituted arenes, in the 2,900 to 3,100 cm−1 region and of the out-of-plane deformation in the 700 to 900 cm−1 region.

Steady State Size Distribution of Asphaltenes by Flocculation From Toluene–n-Heptane Mixtures

Abstract The aggregation mechanisms of asphaltenes have been the subject of several recent studies. In this work, we have studied the effect of inhibitors on size distribution of asphaltene particles, in solutions containing toluene and mixtures of n-heptane and toluene. The asphaltenes used have been extracted from Marlim crude oil, using a modified procedure IP-143/82. Several solutions containing one percent volume of asphaltenes have been prepared in pure toluene and in toluene and n-heptane mixtures with and without inhibitors and have been homogenized during a period of 24 h. The particles sizes were determined by filtering the solutions through a set of standard filters ranging from 0.45 to 0.02 µm pore size. In addition, from the saturated filtered solution of asphaltene in toluene, three other solutions were prepared having 10, 20, and 30% volume of n-heptane and were homogenized for 24 h. Again the size distribution of precipitated particles was obtained by filtering. The concentration of asphaltenes remaining in the solution was measured directly by evaporation or by spectroscopy. The results have shown that large part of asphaltenes remain as colloidal particles in the size range tested in toluene solution without inhibitor. On the solutions in which inhibitors were used, one of the inhibitors effectively prevented flocculation, concentrating the asphaltene particles on the smaller size range even on mixtures containing 20 and 30% volume of n-heptane, which is a strong flocculating agent.

Scaling laws in network models: porous medium property prediction during morphological evolution

A 3-D network model was used to represent a porous medium and the macroscopic properties of the network (like permeability) were simulated by Monte Carlo method. It was shown that these macroscopic properties can be related to network parameters (throat-size distribution parameters, network size and connectivity) through power law correlations, as can be inferred from percolation theory. In this way, macroscopic properties evaluation during the morphological evolution of a 3-D network requires less computational effort, which facilitates the incorporation of this model into oil flow simulators.

Shape selectivity in porous catalysts

Abstract The shape selectivity of porous catalysts is examined with reference to the detailed structural arrangement of pore sizes. As a conceptualization of the pellet structure, an array of cylindrical capillaries comprising pores of two sizes with a distribution function specified in terms of the smaller pore is used. A generic case study based on a complex reaction scheme is considered. It involves a set of parallel and reversible first-order reactions which take place in the larger pores, with one of the products being diffusionally limited in the smaller capillaries because of sterically hindered transport. Simulation of the detailed interactions between the kinetics and the diffusion clearly indicates the way in which the structure can have a dramatic influence on the selectivity of the unhindered product. Specifically, it reveals how the pore size distribution can influence the effective tortuosity of the diffusion path, and the way in which bottleneck cavities can be used to improve selectivity.