Walter P. Walawender

The measurement of local velocity of solid particles

Abstract Methods for measuring the local solid-particle velocity inside equipment for very high solid loadings are reviewed, and the underlying principles of these methods are discussed in terms of their signal flow diagrams. The fiber optic method as well as a low cost correlator developed by the present authors for use in signal processing are discussed in detail. The choice of any specific measuring method for maximum efficiency is contingent upon its characteristics and the given flow systems. However, the use of the correlation method in conjunction with a fiber optic probe appears to be a promising and simple approach because of the availability of low cost signal processing devices.

Statistical study of the grid zone behavior in a shallow gas—solid fluidized bed using a mini-capacitance probe

Abstract The behavior of the grid zone in a shallow-air-sand fluidized bed was studied by means of a mini-capacitance probe coupled with the on-line correlation technique. Experiments were carried out in a bed with a diameter of 203 mm. The bed had a height of 50 mm and was operated at different air flow rates. A perforated “plexiglas” plate with a hole diameter of 1.59 mm and a thickness of 9.53 mm was used as the distributor. The technique has been found to be effective in determining locally or microscopically the jet penetration depth and the dead zone height. The phenomenon of jetting in a shallow gas—solid fluidized bed is described semi-quantitatively.

Grain-based activated carbons for natural gas storage.

Natural gas has emerged as a potential alternative to gasoline due to the increase in global energy demand and environmental concerns. An investigation was undertaken to explore the technical feasibility of implementing the adsorbed natural gas (ANG) storage in the fuel tanks of motor vehicles with activated carbons from biomass, e.g., sorghum and wheat. The grain-based activated carbons were prepared by chemical activation; the experimental parameters were varied to identify the optimum conditions. The porosity of the resultant activated carbons was evaluated through nitrogen adsorption; and the storage capacity, through methane adsorption. A comparative study was also carried out with commercial activated carbons from charcoal. The highest storage factor attained was 89 for compacted grain-based activated carbons from grain sorghum with a bulk density of 0.65 g/cm(3), and the highest storage factor attained is 106 for compacted commercial activated carbons (Calgon) with a bulk density of 0.70 g/cm(3). The storage factor was found to increase approximately linearly with increasing bulk density and to be independent of the extent of compaction. This implies that the grain-based activated carbons are the ideal candidates for the ANG storage.

The onset of slugging in gas—solid fluidized beds with large particles

Abstract The onset of slugging in gas—solid fluidized beds with relatively large particles was studied by means of pressure transducers coupled with the on-line cross-correlation technique. The rise velocity of bubbles was employed as the criterion for the onset of slugging. Different sizes and types of particles and different column diameters were used to obtain data on the bubble rise velocity. A semiempirical model has been derived to predict the superficial gas velocity at the onset of slugging. The model appears to predict the present and available experimental results reasonably well.

Wood Chip Gasification in a Commercial Downdraft Gasifier

Fixed bed and moving bed gasifiers for the production of low Btu gas from wood and charcoal were widely used in Sweden and other countries during the World War II era. The Swedish experience was compiled by the Swedish Academy of Engineering, and this work was recently translated by Reed and Jantzen.1 After the war, the need for gasifiers dwindled although the Swedes continued their development efforts.

Morphological development of rice-hull-derived charcoal in a fluidized-bed reactor

Abstract Rice hull, a by-product of the rice milling industry, can be a potential fuel for energy self-sufficiency in the rice mills of the developing countries because of its high energy content. Nevertheless, little has been done to gasify this material because of various difficulties involved; such difficulties include high ash content and irregular geometry. Fluidized-bed gasification has been proposed as a useful method for biomass energy resource conversion. The gasification process is a heterogeneous reaction with a solid reactant which changes geometrically as conversion progresses. The use of a fluidized-bed reactor for rice-hull conversion has been proposed as well; however, it appears that the kinetics of the gasification process have not been studied extensively. In the present work, the development of the morphology of the rice-hull charcoal or char has been characterized by observing the changes in the surface areas, surface fractal dimensions and pore sizes at various temperatures of the devolatilization process in a fluidized-bed reactor. The results, which show the highly porous network of the microstructure, are relevant and applicable to the simulation of the reaction kinetics of rice-hull conversion.

Mass and energy balance analyses of a downdraft gasifier

Abstract An empirical stoichiometric equation has been developed for wood chip gasification in a commercial-scale moving bed downdraft gasifier. The equation is based on an analysis of experimental overall and elemental material balance data obtained with the gasifier. A thermodynamic analysis of the gasifier has also been performed, based on the resultant empirical stoichiometric equation. The first-law and second-law thermodynamic efficiencies of the gasifier have been evaluated for four different operating modes at three different output temperatures. The resultant empirical stoichiometry is in agreement with the means of the experimental data within one standard deviation. The highest first-law and second-law thermodynamic efficiencies have been obtained when all products are considered useable; they are 90% and 62%, respectively. The lowest first-law and second-law efficiencies have been obtained when cool dry gas is considered as the only useable product; they are 72% and 53%, respectively. The heat loss from the system to the surroundings has been estimated to be 10% of the energy input; this corresponds to a loss of 7% of the available energy input. The available energy dissipation in the system, due to the various irreversibilities of the gasification process, has been evaluated to be 31% of the available energy input. This dissipation is not recoverable and reflects the nature of the process.

Moving-bed solids flow between two fluidized beds

Abstract A simplified form of the macroscopic momentum balance for two-phase flow is applied to moving-bed solids flow in an inclined pipe connecting two fluidized beds. The resulting equation shows that the gravity force of the material in the pipe is counter-balanced by the pressure drop across the pipe and the wall friction. In order to account for the wall friction, a friction factor analogous to the Fanning friction factor in fluid flow is defined, and a friction factor correlation is established based on the data of Trees [11]. The friction factor is found to be a function of the solids flow rate and the pipe diameter. The flow curves of moving-bed solids were also constructed based on the data of Trees. The resulting straight lines show the power-law type flow behavior of the solids. And from the power law indices, it has been shown that the data fit well with the generalized friction factor correlation by Metzner and Reed [16]. It is also observed that data of different pipe diameters yield separate flow curves, which is unusual for time-independent fluids. However, the possibility that moving-bed solids may be time dependent can be ruled out because data of different pipe lengths do fall on a single line.

A mechanistic kinetic model of the rate of mixing segregating solid particles

Abstract Axial mixing and demixing of three diffrent free-flowing segregating particle systems of Kenics motionless mixers were investigated. The first particle systems contained particles differing in size only, the second in density only, and the third in both size and density. Experiments were conducted in an effort to visualize the kinetics of the mixing—demixing process and to examine the influence of size and/or density differences on the rate of the process. Two previous models proposed by Rose and by Weydanz were tested with the experimental data. Neither model was satisfactory. A new model was proposed in this work and was found to give a good description of the mixing—demixing process in the motionless mixer. Experimental results of the three different cases were compared with the case of identical size and density. The comparison showed that the differences in both particle density and size gave rise to substantially faster mixing and demixing rates than the difference in either particle size or particle density alone.

Blood flow in tapered tubes.

Abstract Steady laminar blood flow in uniformly tapering tubes under non-Newtonian flow conditions, is examined. The pressure-flow relationship is studied both theoretically and experimentally. Blood rheology is assumed to be described by the Casson Model. A theoretical pressure flow relation for a Casson model fluid is derived by a differential-cylinder approach as well as by direct integration of the equation of motion. Both methods result in the same expression. Experimental data for bovine blood flow in 1° and 2° tubes agree well with the theoretical predictions over a flow rate range of 0.01–0.15 cc/sec.