Gas Holdup in Slurry Bubble Columns: Effect of
Column Diameter and Slurry Concentrations
Rajamani Krishna, Jeroen W. A. de Swart, Jurg Ellenberger, Gilbert B. Martina, and Cristina Maretto
Dept. of Chemical Engineering, University of Amsterdam, 1018 WV Amsterdam, The Netherlands
To study the influence of particle concentration on the hydrodynamics of bubble-col-
umn sluny reactors operating in the heterogeneous flow regime, experiments were car-
ried out in 0.10, 0.19, and 0.38-m-dia. columns using paraffinic oil as the liquid phase
and sluny concentrations of up to 36 vol. %. To intelpret experimental results a general-
ization of the “two-phase”mode1 for gas-solid fluid beds was used to describe bubble
hydrodynamics. The two phases identified are: a dilute phase consisting of fast-rising
large bubbles that traverse the column virtually in plug flow and a dense phase that is
identified with the liquid phase along with solid particles and entrained small bubbles.
The dense phase suffers backmixing considerably. Dynamic gas disengagement was ex-
perimented in the heterogeneous flow regime to determine the gas voidage in dilute and
dense phases. Experimental data show that increasing the solid concentration decreases
the total gas holdup significantly, but the influence on the dilute-phase gas holdup is
small. The dense-phase gas voidage significantly decreases gas holdup due to enhanced
coalescence of small bubbles resulting porn introduction of particles. The dense-phase
gas voidage i s practically independent of the column diameter. The dilute-phase gas
holdup, on the other hand, decreases with increasing column diameter, and this depen-
dence could be described adequately with a slight modification of the correlation of
fishna and Ellenberger developed for gas - liquid systems.
In processes for converting natural gas to liquid fuels, bub-
ble-column reactors are finding increasing application. Bub-
ble-column slurry reactors are attractive reactor configura-
tions for Fischer-Tropsch and methanol syntheses (Eisenberg
et al., 1994; Fox, 1990; Jager and Espinoza, 1995; De Swart
and Krishna, 1995). There are considerable reactor design and
scale-up problems associated with these synthesis technolo-
gies; these problems arise because of several special features
of these processes. First, large gas throughputs are involved,
necessitating the use of large-diameter reactors (typically 5-8
m), often in parallel. Second, the processes operate under
high-pressure conditions (typically 10-80 bar). Third, in or-
der to achieve economically high space-time yields, high slurry
concentrations (typically 30-40 vol. %) need to be employed
(Fox, 1990). Fourth, to obtain high conversion levels, large
reactor heights, typically 20-30 m tall, are required. Finally,
most of these processes are exothermic in nature, requiring
Correspondence concerning this article should be addressed to R. Krishna.
C. Maretto is presently with Eniricerche, Milan, Italy.
heat removal by means of cooling tubes inserted in the reac-
tor. Successful commercialization of the bubble-column reac-
tor technology is crucially dependent on the proper under-
standing of the scaling up principles for these conditions,
which fall outside the purview of most published theory and
correlations, as can be ascertained by a careful examination
of the published literature on bubble columns; see, for exam-
ple, the recent comprehensive literature survey of Saxena
(1995). The accurate estimation of the gas holdup under the
conditions outlined in the foregoing is an essential factor in
commercial reactor design for Fischer-Tropsch and methanol
The present experimental study distinguishes itself from
earlier studies on the hydrodynamics of bubble-column slurry
reactors (see, e.g., Bukur et al., 1987, 1992; Deckwer et al.,
1980; Deckwer, 1992; Deckwer and Schumpe, 1993; Fukuma
et al., 1987; Kara et al., 1992; Kelkar et al., 1984; Koide et al.,
1984; ODowd et al., 1987; Saxena et al., 1992a,b; Saxena and
Rao, 1993; Schumpe and Grund, 1986; Schumpe et al., 1987;
Vol. 43, No. 2
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Manuscript receiwd Feb. 15, 1996, and reuision receiwd July 16, 1996.
February 1997 Vol. 43, No. 2