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118 3. Heterogeneous Processes and Reactor Analysis

Criteria for suspension of solid particles in the slurry phase
For maximum utilization of the solid phase in a b it is essential that all par- ubble column,
ticles be suspended in the reactor (Ramachandran and Chaudhari, 1984). This means that
the gas velocity should be sufficiently high to enable suspension of all solids in the liquid.
In slurry bubble column reactors, two suspension states e xist:

• complete suspension, in which all particles are suspended in the liquid, and
• homogeneous suspension, in which the solids concentration is uniform throughout the
reactor.

The gas velocities that are required for homogeneous suspension are far greater than
those required for complete suspension. The gas velocity required to achiee complete v
suspension can be obtained from the correlation of Ro y et al . (Ramachandran and
Chaudhari, 1984):

m 6.8
Du 
h  0.23   hu  0.18
s,ma x L,m sG G LG Gte r 3 (3.158)
L 10000
G  u  sG L
    u sG  
where

0.232 0.1788 log( )

2
) 0.1026 log(
L,m L L (3.159)
with:
m s, max the maximum solid loading at a gien gas v g/cm , v elocity 3

L the surface tension of the liquid, dyne/cm
D the reactor diameter
u ter the terminal velocity of the particles
the wetability f which can be considered equal to unity for most , actor
of the catalyst particles.

CGS units should be used with this equation.
Gas holdup
Before analyzing the subject of gas holdup, it has to pointed out that while u b ub- is the b
ble rising velocity and Z / u b is the bubble residence time, the superficial gas velocity pres-
v ent in the following equation is equialent to the superficial gas velocity in f ed beds: ix

Q G
u (3.160)
sG
A
where A is the cross-sectional area of the vessel and Q G the gas v The w rate. olumetric flo
superficial velocity in a slurry, as in the case of a f is not equal to the real v eloc-
ix
ed bed,
ity of the fluid. In fixed beds, the real v elocity is the interstitial v elocity u / while in slur- ,
s
ries the real velocity of the gas phase is the bubble rising v. In case of a fed bed, elocity ix
elocity the bed porosity decreases the fluid v, while in a slurry the decrease of gas v elocity
elocity icial v, is due to its interaction with the liquid phase, e.g. for the same superfthe real