Discrete particle simulation of particulate systems: A review of major applications and findings

Thermal dispersion in a porous medium

Analysis of dispersion effects and non-thermal equilibrium, non-Darcian, variable porosity incompressible flow through porous media

Devolatilization of a lignite and a bituminous coal was studied at high temperatures under rapid heating conditions. Devolatilization rates were measured in a flow furnace designed to yield heating rates of 10 4 –2×10 5 K/s, temperatures of 1000–2100 K time resolution down to a few milliseconds product quenching rates of 10 6 K/s and good particle collection efficiencies. The volatile yields in this study were determined both by difference in the weights of coal fed and char collected and by use of ash as a tracer. Volatile yields of both coals increase significantly with temperature, from about 30% (d.a.f.) at 1260 K and 20ms to about 63% at 2100 K and 25ms. The maximum volatile yields were significantly in excess of the 46% value given by the ASTM proximate volatile test. The yields are even higher when an estimated correction is introduced for condensed material produced during devolatilization that is collected with the char residue. Correlation of the pyrolysis kinetics with a single Arrhenius type rate expression, assuming first order behavior with respect to volatile matter remaining in the char, yielded an apparent frequency factor of about 6.6×10 4 s −1 and an apparent activation energy of about 25 kcal/mole. These results are consistent with an extrapolation to higher temperatures of data previously reported in the literature for similar coals. An empirical model based on two competing overall reactions gave a better fit of the data over all of the conditions of the present study than was obtained using the single-reaction model.

Coal devolatilization at high temperatures

Nanoparticle-enhanced phase change materials (nepcm) with great potential for improved thermal energy storage

Eddy mass diffusivities, effective thermal conductivities, and wall heat transfer coefficients were measured in an 8-in. tube packed with 1/2- and 3/4-in. glass spheres. Superficial mass velocities ranged from 110 to 1,640 Ib./(hr.) (sq. ft.), corresponding to modified Reynolds numbers of 100 to 2,000. Air was the main stream fluid in all cases.

The modified Peclet group (DpV/E*td) was found to be constant at a value of about 12 in the region of fully developed turbulence. At lower Reynolds numbers this group varied with the flow rate. Effective thermal conductivities were correlated by an equation. Modified Peclet numbers for heat transfer were about 25% less than those for mass transfer. The wall heat transfer coefficient varied with the superficial mass velocity as hw = 0.090 (Go0.75).

An explanation is suggested for the similarity in velocity dependence between these values and those for turbulent flow in an empty tube, based on channeling at the wall.

Heat and mass transfer in packed beds

According to the concept of two-phase fluidization, a part of the gas in a fluidized reactor passes through the uniform dispersed solid-gas phase in the form of bubbles, channels, and slugs. Material transport by mixing or diffusion takes place at the phase boundaries. A mass transfer coefficient between the two phases may be used to evaluate the effectiveness of contact between the gas and solid. The reaction rate for the catalytic decomposition of nitrous oxide was determined in a fluidized bed of impregnated alumina particles and compared with the corresponding rate in a fixed bed. Simultaneous rate equations were established based on the assumption that the continuous phase is either completely unmixed or uniformly mixed, and the discontinuous phase passes without mixing. The effects of the velocity of the gas, the particle size, and the bed depth on the transfer coefficient were investigated. Applications to heat transfer in fluidized beds and equipment design are discussed.

Gas-solid contact in fluidized beds

Low Temperature Carbonization Rates in a Fluidized Bed

The absorption and subsequent liquid-phase reaction of oxygen was studied with two types of dispersion apparatus, the Venturi atomizer and the fritted-glass disperser. The systems studied in both devices included the absorption of atmospheric oxygen by catalyzed sodium sulfite solutions and the simultaneous absorption of atmospheric oxygen with nitrogen dioxide and with sulfur dioxide by water.

Very large values of the liquid-film mass transfer coefficient for oxygen absorption were measured in the atomization zone of the Venturi atomizer. Over-all recovery efficiencies were less than 2.3% for nitrogen dioxide but reached as much as 22% for sulfur dioxide. Oxidation efficiencies for sodium sulfite solutions ranged up to 80%, depending on the operating conditions.

The fritted-glass disperser gave recovery efficiencies of nitrogen dioxide as high as 90% from air containing 10% of the gas. The recovery efficiency decreased at low concentrations of nitrogen dioxide for both the Venturi atomizer and the fritted-glass disperser.

Gas absorption and oxidation in dispersed media

The oxidation of ammonia in the presence of manganous oxide-bismuth oxide catalyst supported on small alumina spheres was studied in both fixed and fluidized beds in the temperature range from 205° to 250°C. The column used was 4½ in. in diameter and 43 in. high. The experiments were made so that the transport effects in the fluidized bed might be separated from the chemical kinetic effects.

In accordance with the theory of two-phase fluidization proposed in a previous paper, comparison is made between the reaction rate associated with the discontinuous phase and that associated with the continuous phase as estimated from the results in the fixed bed. The over-all reaction-rate constants in the fluidized bed can be related to those in the fixed bed by an exponential term in the superficial gas velocity, Vn; n is a constant which depends on the reaction system and the size and type of the reactor.

H. F. Johnstone

Papers

Heat and mass transfer in packed beds

Gas-solid contact in fluidized beds

Low Temperature Carbonization Rates in a Fluidized Bed

Gas absorption and oxidation in dispersed media

Low-temperature oxidation of ammonia in fixed and fluidized beds