Three phase fluidization

1. THREE PHASE FLUIDIZATION

2. Introduction
• Fluidization has been a pivotal topic of research among the
researchers for the past three decades. Since the Winkler
process for gasification of coal was on track, the focus on
fluidization has been consistently gaining significant
importance in industrial phase as well as in academic
research.
• The concept of fluidization in Winkler process was further
developed into various successful applications such as FCC,
Metallurgical process, Sohio process, synthesis of
polymers, combustion and in the recent years for the
treatment of waste water. Analogous to such industrial
developments, considerable academic research has also
been carried out in the field of fluidization.

3. Three Phase Fluidization
• The three-phase fluidized bed is a type of system that can be used
to carry out variety of multiphase chemical reactions. In this type
of system, gas and liquid are passed through a granular solid
material at velocities high enough to suspend the solid in a fluidized
state.
• Three-phase fluidized beds enjoy widespread use in multiphase
chemical reactions, a few of which include methanol production,
conversion of glucose to ethanol, various hydrogenation and
oxidation reactions, hydro treating and catalytic cracking in
petroleum refinery, conversion of heavy petroleum and synthetic
crude and coal liquefaction.
• Three-phase fluidized beds are used in a wide range of industrial
applications including processing of hydrocarbons, in particular in
the upgrading of heavy oil and high molecular weight feedstock’s,
aerobic wastewater treatment, and direct coal liquefaction.

4. Parameters
• Hydrodynamic properties of three-phase
fluidized beds are important for analyzing its
ultimate performance. These include mainly bed
expansion behavior, phase holdups and pressure
drop. All the three phase holdups (∈g, ∈s, ∈l)
should add to give unity. Bed expansion is
important in determining the size of the system
while the phase holdups are essential in mixing
and studying the overall performance of the
system.

5. Minimum Fluidization Velocity
• For a gas-liquid-solid system, the minimum liquid
fluidization velocity is the superficial liquid velocity at
which the bed becomes fluidized for a given superficial gas
velocity. Above the minimum liquid fluidization velocity,
there is good contact between the gas, liquid, and solid
phases. Concentration and temperature gradients are
minimized, which is important if the solid phase is a
catalyst, a reactant, or an adsorbent, and if the reaction is
endothermic or exothermic as temperature gradients may
degrade reaction selectivity, product quality, and catalyst
activity.
• Most studies on the minimum liquid fluidization velocity in
gas-liquid solid systems have been conducted with solids
whose density is at least twice the liquid density.

6. • It is essential in considering such three-phase systems to be
able to predict the minimum gas and liquid flows needed
to initiate and sustain full fluidization conditions.
• Co-current upward gas-liquid fluidization of coarse solids
(dp> 1 mm) is actuated primarily by the motion of the liquid
at low gas velocities (Ug <0.2 m/s for air and water as the
gas and liquid). For such systems, it is seen that the
minimum liquid fluidization velocity, Ulmf, for a given
modest gas superficial velocity is predicted quite well by a
gas-perturbed liquid model, in which it is assumed that the
solid particles are fully supported by the liquid, the role of
the gas being simply to occupy space, thereby increasing
the effective velocity of the liquid.
• Bubble-induced flow can be neglected under these
conditions. Although the liquids involved in all cases were
Newtonian, a recent study has shown that the same model
can be successfully adapted to non-Newtonian liquids.

7. The gas-perturbed liquid model
• This model, applied to three-phase fluidization involving
Newtonian liquids, equates the liquid-buoyed weight of
solids per unit bed volume to the frictional pressure
gradient given by the Ergun packed bed equation applied
to the liquid-solids part of the incipiently fluidized bed.
where αmf is the gas hold-up divided by the total fluid
(gas+liquid) hold-up at minimum fluidization. By means of
the two approximations by Wen and Yu relating the
minimum fluidization voidage, εmf, to the sphericity, ϕ, i.e.,

8. • In the absence of gas flow, i.e., for αmf =0 , Eq. (la) becomes
equivalent to the Wen and Yu equation for minimum liquidsolid fluidization. For three-phase (gas-liquid- solid)
fluidization, however, an estimate of αmf is required in order to
solve Eq. (1) or (la). A good estimate at minimum fluidization
is provided by the empirical equation of Yang et al. for co
current upward flow of gas and non-foaming liquid through
fixed beds of solids, which can be written as
(2)

9. • Eq. ( 1 ) or Eq. ( 1 a), together with Eq. (2),
must be solved simultaneously or iteratively
for αmf and Ulmf.

10. Gas Holdup
• The gas holdup is one of the most important characteristics
for analyzing the performance of a three-phase fluidized
bed. For chemical processes where mass transfer is the
rate limiting step, it is important to estimate the gas holdup
since this relates directly to the mass transfer.
• Although gas holdup in three phase fluidized beds have
received significant attention as summarized in various
reviews, in most of the previous work air, water, and small
glass beads have been used as the gas, liquid, and solids,
respectively. This combination limits the generality and
usefulness of the results. The gas holdup in such systems is
often considerably lower than for pilot-plant or industrialscale units.

11. • Industrial-scale units operate with high gas holdup and
contain small bubbles: these conditions result from
effects of both high reactor pressures and low surface
tension liquids.
• With the use of high viscous and low surface tension
liquids, the gas hold up is enhanced.
• Higher liquid viscosity exerts higher drag on the gas
bubble; the same is done by lower surface tension of
liquid due to formation of surface tension gradient on
the bubble surface. A higher drag results in lower
bubble rise velocities and hence higher holdup. With
the lower surface tension of the liquid finer bubbles
are formed.
• The gas holdup characteristic depends upon the bubble
size and its dispersion in the bed.

12. • The gas holdup was determined from the pressure drop
measurements using Eq. (1).
• Previous studies on three-phase fluidized beds have
identified ten variables (UL, Ug, μL, σL, ρL, Δρg, ρp, dp, hs, Dc)
which are expected to influence the gas holdup
significantly. The gas density has been incorporated in the
buoyancy (Δρg). Using these ten significant variables which
involve three fundamental dimensions (mass, length and
time), seven independent dimensionless groups can be
formed according to Buckingham Pi theorem. Keeping in
mind the advantage of using groups that are familiar in
multiphase flow, two sets of seven independent
dimensionless groups has been developed by
rearrangement as;

13. The combinations Weber number (We), Reynolds number (Re), and Froude number
(Fr) or Morton number (Mo) and Eötvös number (Eo) are used to characterize the
multiphase flow of bubbles or drops moving in a surrounding fluid. Morton number
(Mo) is the combination of Weber number (We), Reynolds number (Re), and Froude
number (Fr).

14. • The R-square value of the developed equation
is 0.972.
• The R-square value of the developed equation
is 0.997.
• It is precise enough for the prediction of gas
holdup and has been used to optimize the
operating conditions for finding highest
possible gas holdup in the experimental
domain.

15. Other correlations
• Dakshinamurthy(1972)
• Begovich and Watson(1978)
a=2.12, b= 0.41
a=2.65, b= 0.6

16. • Liquid hold up by Kato (1984)

17. Pressure Drop

18. References
• Fluid Bed Technology in materials processing by
C.K.Gupta.
• Minimum liquid fluidization velocity in two and three
phase beds by H. Miura and Y. Kawase.
• Minimum liquid fluidization velocity in gas-liquid-solid
fluidized beds of low-density particles by C. L. Briens,
A. Margaritis and J. Hay.
• Determination of optimum gas holdup conditions in a
three-phase fluidized bed by genetic algorithm by H.
M. Jena, G. K. Roy and S. S. Mahapatra.
• Minimum fluidization velocities for gas-liquid-solid
three-phase systems J.P. Zhang, N. Epstein , J.R. Grace.