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Dimensionless hydrodynamic simulation of high pressure multiphase reactors subject to foaming Macchi, Arturo

Abstract

Safoniuk (1999) proposed that three-phase fluidized bed hydrodynamics can be scaled based on geometric similarity and matching of five dimensionless groups: the M-group, M = g(p[sub L]-P[sub g])μ[sub L]⁴/(p[sub L]²σ³); a modified Eotvos number, Eo* = g(p[sub L] – p[sub g])d[sub p]²/σ; the liquid Reynolds number, Re[sub L] = P[sub L]d[sub p]U[sub L]/μ[sub L]; a density ratio, P[sub P]/P[sub L] ; and a superficial velocity ratio, Ug/UL- This approach implicitly assumes that the major physical properties of the liquid (density, viscosity and surface tension) are sufficient to characterize the bubble coalescence behaviour and that the influence of the gas density is negligible. Since many commercial reactors operate at high pressure with multicomponent liquids that may be subject to foaming, an experimental program was designed to test whether multiphase systems that match Safoniuk's criteria but differ in interfacial properties and gas density produce the same fluid dynamic parameters. The liquid density, viscosity and surface tension were found to be insufficient to characterize bubble coalescence in multicomponent solutions. Multicomponent and contaminated liquids present interfacial effects that reduce the bubble coalescence rate and hinder the bubble rise velocity resulting in greater gas holdups than in pure monocomponent liquids under similar conditions. The extent of interfacial effects depends on the bubble size and is most important for Eo < 40. Additional liquid physical properties such as dynamic surface tension and dilatational surface elasticity were also found insufficient since surface-active components were well-dispersed and in equilibrium with the gas-liquid interface. Gas density was found to be an important parameter in both gas-liquid and gas-liquid-solid systems. The dispersed bubble flow regime is sustained to higher gas velocities and gas holdups for denser gases. This phenomenon can be attributed to enhanced bubble break-up, rather than to the formation of smaller bubbles with increasing gas density. As it stands, the dimensional similitude approach will fail when the effects of surface-active contaminants are important since the physical properties and forces that effectively characterize the bubble coalescence mechanism in multicomponent/contaminated liquids are still unknown. The effect of pressure via gas density can be taken into account by the dimensionless group p[sub g]/ p[sub L]. As a secondary objective, a study on the role of particles in establishing radial uniformity of fluids that are initially maldistributed was undertaken in a 127 mm inner diameter column with 3.3-mm polymer particles and 3.7-mm glass beads (densities 1280 and 2510 kg/m³, respectively), with water and air as the liquid and gas. The effects of initial gas-liquid spatial maldistribution on overall phase holdups were not very significant for the glass beads since radial non-uniformities seemed to be eliminated relatively quickly. For the lighter polymer beads, maldistribution at the distributor only caused a significant drop in overall bed voidage and gas holdup at higher gas velocities. Finally, the measurement of cross-sectional phase holdups using the attenuation and velocity change of ultrasound was attempted in a 292 mm inner diameter column with air, water and uniform glass beads of 1.3 mm diameter. The approach worked relatively well for gas-liquid and liquid-solid systems. However, signal attenuation greatly limits its use in three-phase fluidized beds, as it is difficult to operate at a frequency that ensures transmission through both dispersed phases. Slurry bubble columns with lower dispersed phase holdups and smaller particles present less attenuative media and are better suited to this technique.

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