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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.
Item Metadata
Title |
Dimensionless hydrodynamic simulation of high pressure multiphase reactors subject to foaming
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
2002
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Description |
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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Extent |
14338826 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-09-23
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0058969
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2002-05
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.