Hugo S. Caram

Tensile strength of wet granula materials

The dimensionless tensile strength σd/α of wet granular materials with saturation levels in the plateau region of the pendular state was correlated with the void fraction of the agglomerate. The correlation fitted well with experimental measurements carried out on glass beads of diameter 93 μm and with literature data, and was an improvement over the traditional Rumpf model. The tensile strength at other saturations could be easily derived from the proposed correlation. The effects of bed height in the tester cell, liquid saturation levels in the agglomerate, chemical additives and non-uniformity of the packing of the powder mass were also investigated.

The design of reverse flow reactors for catalytic combustion systems

Abstract Periodic reversal of the gas feed to a catalytic plug flow reactor can be used for the autothermal combustion of very dilute volatile organic compounds. This paper examines the system through a series of progressively simplified models. A full dynamic model, a high switching frequency model, and a complete combustion model are presented and discussed. A design procedure for the reverse flow reactor is given. It uses the various simplified models to generate a base design, which is then extended to the full transient model. The design procedure is able to predict the maximum temperature and inert end section sizing.

Bubble formation and gas leakage in fluidized beds

Abstract The Harrison and Leung model for spherical bubble formation in fluidized beds has been extended to account for the dynamics of the emulsion phase. The simplified Davidson-Harrison model is used to obtain the gas and particle velocities in the emulsion phase and includes the effect of the bubble pressure, the change in hydrostatic pressure, grid orifice discharge rate, and gas leakage into the emulsion phase. Computations for the constant flow rate case agree well with measured gas leakage for beds of large particles. For large grid orifices, a simplified model explains the low frequencies observed experimentally for small flow rates.

The simulated moving bed chemical reactor

Abstract A novel method of maintaining a transient cyclic steady state in a fixed bed tubular reactor is examined. This process, the simulated moving bed chemical reactor, utilizes the differences in the thermal and mass velocities of the system to maintain autothermal behavior within the reactor. This system can process low calorific fuels and low temperature feed streams, is insensitive to minor disturbances, and distributes the load evenly over all the catalyst in the bed. This work presents, for the first time, theoretical results concerning the operation of these systems. Both irreversible and reversible reactions are considered, and comparisons are drawn between the new process and the reverse flow reactor.

The application of reverse flow reactors to endothermic reactions

Abstract The concept of the reverse flow reactor traditionally used for mildly exothermic reactions has been extended to endothermic reactions and applied to the dehydrogenation of ethylbenzene. Periodic alternation of a reactant and a regenerating medium countercurrently over a fixed bed of catalyst in endothermic processes can be shown to result in higher conversions than traditional non-periodic systems. With this configuration several new process parameters are introduced such as total cycle time and the ratio of countercurrent flows. By judicious choice of operating conditions the hottest point of the reactor can be placed at the exit of the reactant stream allowing for much more favourable equilibriums and outlet conversions than if it were the coldest spot in the reactor as is common in steady-state processes. Two types of modeling are utilized. The first employs a technique which assumes that the switching is so rapid that the temperature distribution in the catalyst is in time independent steady-state. This high switching frequency model is compared against a more complicated time dependent dynamic model which traditionally is employed in reverse flow systems. Results for the first model are shown to be the limiting case of the dynamic model with fast switching. The models show significant conversion improvement with respect to existing steady state processes.

The spherical reverse flow reactor

A packed bed reactor made up of two hemispheres separated by an orifice plate can be operated as an adiabatic reverse flow reactor with nearly spherical symmetry. The reactor requires little or no insulation with external temperatures limited to the adiabatic temperature rise, but much higher internal temperatures. The operational characteristics of this type of reactor have been studied using a dynamic model and the steady state model obtained for fast switching of the flow direction. Due to the large cooling region, higher conversions than the conventional reverse flow reactor are predicted for reversible exothermic reactions.

Effect of moisture on the yield locus of granular materials: theory of shift

Abstract The yield locus of a wet material was shifted to the left of that of a dry specimen by a constant value equal to the compressive isostatic stress due to pendular bridges. For materials with straight yield loci, the shift was computed from the uniaxial tensile strength, either measured in a tensile strength tester or calculated from the correlation, and the angle of internal friction of the material. The predicted shift in the yield loci due to different moisture contents compare well with the measured shift in the yield of glass beds, crushed limestone, super D catalyst and Leslie coal.

Monitoring Solid Oxide CO2 Capture Sorbents in Action

The separation, capture, and storage of CO2 , the major greenhouse gas, from industrial gas streams has received considerable attention in recent years because of concerns about environmental effects of increasing CO2 concentration in the atmosphere. An emerging area of research utilizes reversible CO2 sorbents to increase conversion and rate of forward reactions for equilibrium-controlled reactions (sorption-enhanced reactions). Little fundamental information, however, is known about the nature of the sorbent surface sites, sorbent surface-CO2 complexes, and the CO2 adsorption/desorption mechanisms. The present study directly spectroscopically monitors Na2 O/Al2 O3 sorbent-CO2 surface complexes during adsorption/desorption with simultaneous analysis of desorbed CO2 gas, allowing establishment of molecular level structure-sorption relationships between individual surface carbonate complexes and the CO2 working capacity of sorbents at different temperatures.

Single bubble eruptions in gas fluidized beds

Abstract The motion of the solids thrown above the bed surface by single bubble eruptions was studied using a high speed video system. Data on the motion of the bubble and emulsion phases are presented as functions of bubble size, type of bed material, fluidizing velocity and bed depth. When plotted in dimensionless form, the trajectories of the tops of the bulge layer, bubble and wake can be condensed into three universal curves which represent the trajectories for these points for the entire range of bed materials, bubble sizes, fluidization velocities and bed depths tested. There appeared to be no significant effect of excess air, bubble size or bed depth on the dimensionless heights reached by the wake and bulge materials. In all cases the bulge material was ejected to a greater height above the bed than the wake.

IGNITION PHENOMENA AND CONTROLLED FIRING OF REACTION-BONDED ALUMINUM OXIDE

Abstract The reaction-bonded aluminum oxide (RBAO) process utilizes the oxidation of attrition-milled Al/Al2O3/ZrO2 powder compacts, that are heat treated in air, to make alumina-based ceramics. A simultaneous mass and energy balance has been used to model the propagation of the ignition front that has been observed during reaction-bonding. The model is used to determine conditions under which ignition can be avoided.