Craig A. McKnight

Use of dimensional similitude for scale-up of hydrodynamics in three-phase fluidized beds

A new method of scaling three-phase fluidized beds is presented based upon achieving geometric and dynamic similitude with the aid of the Buckingham Pi theorem. This results in five dimensionless groups that must be matched to ensure hydrodynamic similarity between separate units. Experiments have been carried out to validate this technique by comparing gas hold-up and bed expansion in two different columns operating with significantly different fluid and solids properties, but where efforts have been made to match the five dimensionless groups. These comparisons show mostly favourable agreement between the two units.

Dimensional hydrodynamic similitude in three-phase fluidized beds

This study tests the scaling approach for three-phase fluidized bed hydrodynamics proposed by Safoniuk, Grace, Hackman, & McKnight (Chem. Eng. Sci. 54 (1999) 4961) based on geometric and dynamic similitude with a limited number (5) of dimensionless groups. Experiments were carried out in two systems in which all five dimensionless groups were matched: an aqueous glycerol solution with glass beads (system 1) and silicone oil with porous alumina particles (system 2), with air as the gas in both cases. Although bed expansions were similar for the two systems, trends differed. Gas holdups were always slightly higher for system 1. The dimensionless transition velocities from dispersed to coalesced flow were similar. The minimum liquid fluidization velocity Reynolds number was slightly higher for system 1 without gas, but somewhat lower with gas present. Differences between the systems are statistically significant, but generally less than 12%, so the dimensional similitude approach gives a reasonable basis for estimating global hydrodynamic parameters under the present operating conditions. The differences between the two systems are attributed to the complex coalescence behavior of liquid mixtures, suggesting that additional dimensionless groups are needed to fully characterize the local dynamic bed behavior.

Mechanism of coke formation from hydrocracked Athabasca residuum

Abstract Coke forming tendencies of hydrocracked Athabasca residuum were studied. The per cent toluene insolubles formed was taken as an indicator of the coke forming tendency. A coking unit and a tubing reactor were used for experiments under atmospheric pressure and at high pressures, respectively. The types of toluene insolubles formed were analysed using optical microscopy. The paper discusses the results, and correlates the amount and type of toluene insolubles formed with the reaction conditions. At the lowest and highest severity conditions studied, the toluene insolubles formed were predominantly isotropic and anisotropic, respectively. Reaction temperature and time were found to be dominant factors in producing toluene insolubles; of all the variables studied, the reaction temperature was the most important. The optical texture of toluene insolubles produced in the laboratory was compared to that from Syncrudes commercial plant. Anisotropic particles are proposed to be formed by homogeneous nucleation and from the conversion of isotropic to anisotropic particles.