Anthony G. Dixon

Comparison of CFD simulations to experiment for convective heat transfer in a gas-solid fixed bed

Abstract Computational fluid dynamics (CFD) studies can improve understanding of fixed bed fluid flow and heat transfer. Our long-term objective is to use the CFD results in the development of tractable reactor models that are based on a good understanding of the fluid flow phenomena. The CFD simulations of fluid flow and heat transfer require verification to increase confidence in their use for model development. Results of a quantitative comparison between CFD results and heat transfer experimental data are given here. Simulations are presented for a model geometry of 44 solid spheres in a tube with tube-to-particle diameter ratio equal to 2. Velocity vector profiles and temperature contours have been obtained with emphasis on the wall–particle region. Comparisons are made with measured temperature profiles in a typical experimental setup with the same geometry. Several correction factors were required to compensate for non-ideal experimental measurements, and for phenomena that were not included in the CFD model. After correction, excellent agreement could be obtained between simulation and experiment.

Adsorption, permeation, and diffusion of gases in microporous membranes. II. Permeation of gases in microporous glass membranes

The gas permeability properties of He, H2, CO2 O2, N2 and CH4 in microporous silica membranes were studied as a function of temperature and pressure. A mathetical model of compressible flow in a hollow fiber tube with permeable walls was developed and solved to describe gas transfer in the membranes. The permeation rate of He in the microporous membrane was comparable to that in industrially produced polymeric membranes. Selectivity factors in the membrane were found to be a function of differences in the gas kinetic diameters. The ideal selectivity factor for He/CH4 was more than 10,000 at 30°C. Selectivity factors decreased with increasing temperature.

Computational fluid dynamics simulations of fluid flow and heat transfer at the wall-particle contact points in a fixed-bed reactor

Abstract An accurate description of the fluid flow and heat transfer within a fixed-bed reactor is desirable. The prevailing models of fluid flow invoke either a constant velocity (plug-flow) profile, or make use of a single axial velocity component with radial variation across the tube diameter. However, difficulties in predicting reactor performance and the wide disagreement between effective heat transfer coefficients suggest that these are oversimplified pictures of the real-flow situation. Computational fluid dynamics is a means that could improve our understanding of fixed-bed fluid flow and heat transfer, by solving the 3D Navier–Stokes equations. Simulations are presented for an improved geometry, compared to previous studies, of 10 solid spheres in a tube with a tube-to-particle ratio of 2.43, that includes both particle to particle and also wall to particle contacts. Simulations are also reported with heat generation from the spheres. The simulation results show strong flow components towards the wall and away from the wall, thereby transporting heat. The flow around the contact points themselves shows stagnant regions, due to the high shear of the solid surfaces. A high velocity gradient in the radial direction is observed between two layers of spheres, which clearly shows how the heat transfer is increased within the bed. Regions of back-flow are also observed, in qualitative agreement with literature experimental studies.

Oxygen-permeable dense membrane reactor for the oxidative coupling of methane

A perovskite material (BaCe0.8Gd0.2O3) in powder form, with both electronic and ionic conductivity, was synthesized by the ethylene glycol method. A dense membrane tube was fabricated using a plastic extrusion technique. The oxygen permeances of the dense membrane tube were measured as functions of temperature and oxygen partial pressure on the feed side. In the temperature range of 688–955°C, the oxygen flux showed an approximately exponential dependence on temperature. The oxygen flux increased proportionally to the natural logarithm of the ratio of oxygen partial pressures across the membrane. Experimental results for the oxidative coupling of methane (OCM) to C2 hydrocarbons, in the absence of additional catalyst, showed that this material has fairly good catalytic activity for the OCM reaction. The maximum yield to C2 hydrocarbons that was obtained was 16%, which compares favorably to prior dense membrane studies.

The determination of zeolite crystal diffusivity by gas chromatography—II. Experimental

Abstract Experimental results are reported for the adsorption of propane and n -butane in a column packed with Linde 5A zeolite crystals, and methane, propan n - and iso -butane in a column packed with silicalite crystals. The adsorption equilibrium constant K and crystal diffusivity D c , we obtained at different temperatures in the range 100–300°C, for each sorbate and packing and the heat of adsorption and diffusional activation energ were determined. The results show that in order to separate the effects of K and the pressure drop, the latter must be independently estimated by a suitable correla

Packed Tubular Reactor Modeling and Catalyst Design using Computational Fluid Dynamics

Abstract Computational fluid dynamics (CFD) is rapidly becoming a standard tool for the analysis of chemically reacting flows. For single-phase reactors, such as stirred tanks and “empty” tubes, it is already well-established. For multiphase reactors such as fixed beds, bubble columns, trickle beds and fluidized beds, its use is relatively new, and methods are still under development. The aim of this chapter is to present the application of CFD to the simulation of three-dimensional interstitial flow in packed tubes, with and without catalytic reaction. Although the use of CFD to simulate such geometrically complex flows is too expensive and impractical currently for routine design and control of fixed-bed reactors, the real contribution of CFD in this area is to provide a more fundamental understanding of the transport and reaction phenomena in such reactors. CFD can supply the detailed three-dimensional velocity, species and temperature fields that are needed to improve engineering approaches. In particular, this chapter considers the development of CFD methods for packed tube simulation by finite element or finite volume solution of the governing partial differential equations. It discusses specific implementation problems of special relevance to packed tubes, presents the validation by experiment of CFD results, and reviews recent advances in the field in transport and reaction. Extended discussion is given of two topics: heat transfer in packed tubes and the design of catalyst particles for steam reforming.

Computational fluid dynamics studies of fixed bed heat transfer

Abstract Fluid flow and heat transfer in a fixed bed of tube to particle ratio 2.86 were studied by solving the 3D Navier–Stokes and energy equations using a commercial finite element code, ANSYS/FLOTRAN. The geometry model consisted of an arrangement of eight spheres. Boundary conditions were given at the wall and the inlet, similar to an experimental setup, to determine the velocity and temperature at various locations. The main objective of this study was to use computational fluid dynamics (CFD) to obtain values for the dimensionless wall heat transfer coefficient, Nuw and the radial effective thermal conductivity ratio, kr/kf, using air as the fluid. Nuw and kr/kf were evaluated from the calculated temperatures at different locations in the bed by fitting these with the analytical solution of the usual two-dimensional pseudo-homogeneous model, using a non-linear least squares analysis. Results were obtained for Reynolds numbers in the range 9–1450. The Nuw and kr/kf values showed reasonable qualitative agreement with both experimental values and values predicted by a model-matching theory based on experimental measurements. Studies were also carried out to verify the depth, pressure and wall temperature dependence of the effective heat parameters. The effect of the temperature profile measurement position above the bed on the estimated parameters, was investigated.

WALL AND PARTICLE-SHAPE EFFECTS ON HEAT TRANSFER IN PACKED BEDS

The effective radial thermal conductivity and apparent heat transfer coefficient for a packed bed were experimentally determined for beds of spheres, full cylinders and hollow cylinders, for flow rates giving particle Reynolds numbers in the range 100-1000, and for tube to particle diameter ratios of 5-12. Over these ranges the radial Peclet number Per showed significant dependence on solid conductivity, gas flow rate and particle shape, while the wall Biot number Bi showed significant dependence on tube to particle diameter ratio, gas flow rate and particle shape. These dependencies were predicted well by equations incorporating the effects of these variables into individual gas and solid phase parameters, which were then combined to give the effective or lumped parameters

Fluid-phase radial transport in packed beds of low tube-to-particle diameter ratio

Abstract Mass transfer experiments were carried out in packed beds of spheres of low tube-to-particle diameter ratio (3 ⩽ d t p ⩽ 12) to determine, by analogy, the fluid-phase contribution to the heat transfer effective radial Peclet number and the apparent wall Biot number. The asymptotic Peclet number was shown to be between 8 and 11 for the entire d t d p range, contradicting standard correlations. A correlation for the wall Sherwood number is given that extends those of previous studies: Sh wf = (1.0−1.5( d p d t ) 1.5 )(Sc) 1 3 (Re) 0.59 . These results are in good agreement with data obtained directly from heat transfer experiments.

The effects of the silica source on the crystallization of zeolite NaX

Abstract The effects of varying the starting silica source on the synthesis of molecular sieve zeolite NaX were investigated, while all other reaction parameters were kept constant. The silica sources were all powders of varying types and purity. Characterizations of the silica sources, the silicate solutions, and the synthesis products were completed by powder X-ray diffraction, electron microscopy, particle-size analysis, n.m.r, spectroscopy, laser light scattering, and elemental analysis. The use of different silica sources significantly influenced the outcome of the synthesis experiments. There were large differ- ences in the results from the various batch synthesis mixtures, even though the batch compositions were all the same (neglecting impurities). The experiments, all conducted at 115°C in sealed 6 ml autoclaves, yielded products of different particle sizes, different amounts of impurity zeolite phases, and different conversion rates to zeolite NaX. The n.m.r. spectra of the dissolved silica sources were all effectively the same; however, the levels of trace impurities were very different. The extent to which nuclei formed in these experiments was shown to correlate to the impurity level of any one of several elements in the silica sources, but not to the turbidity of the filtered solutions as noted by a simple light-scattering measurement.