The measurement of transport coefficients in gas-solid heterogeneous reactions

Non-Idealities in Lab-Scale Kinetic Testing: A Theoretical Study of a Modular Temkin Reactor

The Temkin reactor can be applied for industrial relevant catalyst testing with unmodified catalyst particles. It was assumed in the literature that this reactor behaves as a cascade of continuously stirred tank reactors (CSTR). However, this assumption was based only on outlet gas composition or inert residence time distribution measurements. The present work theoretically investigates the catalytic CO2 methanation as a test case on different catalyst geometries, a sphere, and a ring, inside a single Temkin reaction chamber under isothermal conditions. Axial gas-phase species profiles from detailed computational fluid dynamics (CFD) are compared with a CSTR and 1D plug-flow reactor (PFR) model using a sophisticated microkinetic model. In addition, a 1D chemical reactor network (CRN) model was developed, and model parameters were adjusted based on the CFD simulations. Whereas the ideal reactor models overpredict the axial product concentrations, the CRN model results agree well with the CFD simulations, especially under low to medium flow rates. This study shows that complex flow patterns greatly influence species fields inside the Temkin reactor. Although residence time measurements suggest CSTR-like behavior, the reactive flow cannot be described by either a CSTR or PFR model but with the developed CRN model.

Gas phase dispersion in compost as a function of different water contents and air flow rates

Gas phase dispersion in a natural porous medium (yard waste compost) was investigated as a function of gas flow velocity and compost volumetric water content using oxygen and nitrogen as tracer gases. The compost was chosen because it has a very wide water content range and because it represents a wide range of porous media, including soils and biofilter media. Column breakthrough curves for oxygen and nitrogen were measured at relatively low pore gas velocities, corresponding to those observed in for instance soil vapor extraction systems or biofilters for air cleaning at biogas plants or composting facilities. Total gas mechanical dispersion-molecular diffusion coefficients were fitted from the breakthrough curves using a one-dimensional numerical solution to the advection-dispersion equation and used to determine gas dispersivities at different volumetric gas contents. The results showed that gas mechanical dispersion dominated over molecular diffusion with mechanical dispersion for all water contents and pore gas velocities investigated. Importance of mechanical dispersion increased with increasing pore gas velocity and compost water content. The results further showed that gas dispersivity was relatively constant at high values of compost gas-filled porosity but increased with decreasing gas-filled porosity at lower values of gas-filled porosity. Results finally showed that measurement uncertainty in gas dispersivity is generally highest at low values of pore gas velocity.

Estimation of diffusion parameters in functionalized silicas with modulated porosity

The diffusion parameters of binary gas mixture He (tracer gas)-N2 (carrier gas) in hybrid organic-inorganic SiO2-X porous solids which have suffered gradual functionalization with functional groups X of increasing length (X = psi, [triple bond]Si-H, [triple bond]Si-CH2OH, [triple bond]Si-(CH2)3OH, [triple bond]Si-(CH2)11CH3) are reported. The effective diffusivities Deff, the Henry law constants K as well as the tortuosity factors tau for the examined solids were estimated by a typical pulse gas chromatographic method. Analysis of the experimental results was carried out by the well-known method of linearization of moments. The moments s analysis provides a powerful means for extracting diffusion parameters from the experimental response curves The proposed methodology is simple compared to other similar studies and provides rapidly reliable data. The results of this work indicate that the effective diffusivity Deff in porous networks drops markedly as the initial porosity of the parent SiO2 sample is blocked by the functionalization of the pore surfaces with functional groups of increasing size, [triple bond]Si-H, [triple bond]Si-CH2OH, [triple bond]Si-(CH2)3OH and [triple bond]Si-(CH2)11CH3. The low values of the Henry law constants K found indicate that the adsorption of He on the porous surfaces for all the solids is weak. Also, the tortuosity factor r is proportionally correlated to the pore blocking effects and the percolation phenomena of gases flowing into the porous network.

Diffusional Effects in the Recovery of Methane From Coalbeds

Abstract

This paper presents evidence that the unipore model is inadequate for describing diffusional fluxes from coal over the entire timescale of desorption. Field desorption of methane from coal pieces shows pronounced curvature, which is attributable to the bidisperse pore structure generally found for coat. A model is presented that accounts for the bidisperse pore structure of coal and predicts diffusion rates for both field and laboratory predicts diffusion rates for both field and laboratory desorptions. Transient diffusivities are determined for Pittsburgh bituminous and Madrid (NM) anthracite coals Pittsburgh bituminous and Madrid (NM) anthracite coals by using a pulse tracer technique coupled with Fourier analysis of the elution curves. This technique allows the rapid determination of the diffusion parameters with minimal experimental effort. In addition, steady-state diffusivities are determined to identify mechanisms of diffusion. Diffusion was found to be a combination of bulk, Knudsen, and surface diffusions, depending on the coal pore structure and gas pressure.

Introduction

Over the last decade, increased consideration has been given to coalbed methane as a resource, a result of rising overhead gas paces and declining U.S. conventional natural gas reserves. Recent estimates for the quantity of recoverable coalbed methane in the continental U.S. range from 30 × 1012 to 200 × 1012 cu ft [0.85 to 5.7 × 1012 m3]. Although coalbed methane is produced with conventional drilling and completion produced with conventional drilling and completion technology, the actual gas flow mechanism is quite different from conventional formations. The transport of methane through coal generally is assumed to be a two-stage process:diffusion of methane through the pore structure of the coal to naturally occurring cracks followed bythe simultaneous flow of water and gas through the crack structure.

The relative magnitude of these two steps has been studied by several investigators. However, these investigators, including Kissell and Kuuskraa et al., ignored the coupling of the diffusional and laminar flow processes. Also, both investigators used a "unipore" diffusion model, which we found inadequate for describing methane diffusion in coal. The major conclusions of both works were that pore diffusion could be ignored and that the production of methane from coal is controlled solely by the laminar flow of water/gas through the coal crack structure. Ancell et al. have acknowledged correctly that diffusion effects may not be neglected a priori. Diffusion effects were included by using the unipore diffusion model coupled to Darcy law equations for gas and water flow through the cracks. Although the mechanism of pore diffusion is included, no mention is made concerning the effect of diffusion on the gas/water production rates. production rates. The diffusion of an adsorbing gas through a porous solid may occur as a result of any combination of three mechanisms. These mechanisms are bulk diffusion (molecule/molecule interactions dominate), Knudsen diffusion (molecule/pore wall interactions dominate), and surface diffusion (transport of the adsorbed liquid-like film). To extrapolate findings safely from laboratory diffusion experiments to the high pressures associated with coalbed methane, it is important to have an adequate understanding of these mechanisms. These mechanisms have been identified for several coals and are presented in a following section. In addition, a model alternative to the unipore model has been found that describes desorption from coal over the complete time frame of interest. A straightforward experimental procedure is presented for determining the necessary procedure is presented for determining the necessary diffusion parameters for this model.

Methane Diffusion in Coal

Past investigations of methane diffusion through coal Past investigations of methane diffusion through coal have used two basic types of experiments. One technique is to flow methane or helium through a solid coal disk. With the measured pressure drop and flow rate, a permeability constant (and hence diffusion coefficient) permeability constant (and hence diffusion coefficient) may be obtained. In the second, the rate of desorption from small coal particles after undergoing a step-change in external concentration is used in conjunction with an appropriate model to determine a diffusion coefficient. Thimons and Kissell used the flow disk method for the study of methane and helium diffusion in three different eastern U.S. coals. The study at ambient temperatures and pressures ranging from 0.7 to 2.7 atm [70 to 270 kPa] resulted in the following conclusions.1. Surface transport of adsorbed methane is negligible compared to gas-phase transpose.2. Steady-state diffusion coefficients are on the order of 10–4 to 10–5 cm2/s.3. Transient diffusion coefficients ranged from 0.5 to 10 times the steady-state values. Sevenster conducted a similar study for a variety of gases on a British coal. In contrast to findings of Thimons and Kissell, a large surface transport contribution for methane was observed.

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