31-08-2012, 02:52 PM
Operating and hydrodynamic characteristics of a reversed flow jet loop bioreactor (RFJLB) with ejector
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ABSTRACT
The hydrodynamic characteristics of RFJLB was studied with superficial liquid velocity (Ul), nozzle diameter
(Dn) and nozzle height (Hn) in the range of 0.0293–0.094 m/s, 17.4–22.0 mm and 50–400 mm,
respectively. For Dn = 17.4 mm, Hn = 50 and 200 mm, with ejector mode and regular operating procedure
i.e. simultaneous entry of gas with increasing liquid velocity, had limitation of not establishing the circulation
loop. To overcome this limitation a modified operating procedure i.e. entry of gas after established
liquid circulation loop is proposed. Also the comparison of gas holdups with ejector and injector mode
proves the effectiveness of ejector mode and can eliminate the supply of compressed gas. Thus proper
choice of Dn, Hn and also the operating procedure becomes necessary.
Introduction
The reversed flow jet loop bioreactor (RFJLB) is a cylindrical vessel
with a concentric draft tube. The gas and liquid are introduced
through a two fluid nozzle from the top of the column and flows co
currently downward through draft tube. The bubbles are forced to
move in a direction opposite to their buoyancy in the draft tube,
while through annulus in upward direction. Near the top opening
of draft tube, a part of gas liquid mixture is sucked into the draft
tube, developing a recirculation loop. RFJL bioreactors are characterized
by simple construction, good mixing, high mass transfer
coefficient, fine gas dispersion and relatively low power requirements
and hence have become of interest in recent years.
The two fluid nozzle consists of two concentric tubes. Entry for
gas is arranged through the center tube and for liquid through the
annulus of two fluid nozzle. This special construction makes it possible
to operate the nozzle either as an injector (the gas is introduced
from compressor) or as an ejector (the gas is inhaled by
liquid jet) (Bhutada and Pangarkar, 1987, 1988; Genenger and
Lohrengel, 1992; Wachsmann et al., 1985). Ejector mode is possible
due to the high momentum of liquid jet.
Experimental setup and operational procedure
The experimental setup used in this investigation is shown in
Fig. 1. It consists of a transparent cylindrical perplex column with
height of 1.8 m and inner diameter of 0.19 m. A draft tube with a
height of 1.6 m and inner diameter of 0.072 m is inserted into
the column. The bottom of reactor is provided with a bowl. The
clearance between the base of bowl and the lower end of draft tube
is 0.06 m. For the entry of gas and liquid, a two fluid nozzle is fixed
axially in the center at the top of column. Gas enters through air
tube of 10 mm I.D. and 12.7 mm O.D., fixed axially in the center
of a two fluid nozzle.
Gas holdup with modified operating procedure
With 17.4 mm diameter nozzle, limited circulation mode is obtained
at Hn = 50 and 200 mm in regular operating procedure. In
modified operating procedure, sucked air in to the reactor is initially
prevented by closing the stop valves (A and B) just above
the inlet of aeration tube. Liquid flow is started, allowed to develop
the liquid circulation loop into the reactor and then air stop valve
(A) is opened to suck the natural air inside the reactor. Once the liquid
is in circulation mode the actual liquid velocity in draft tube
will be more than the superficial liquid velocity in draft tube. The
increased magnitude of liquid flow through draft tube carry along
with it the sucked gas bubbles out of draft tube in annulus and then
forms complete circulation of gas liquid mixture. With 17.4 mm
diameter nozzle, the circulation loop is established for Hn = 50
and 200 mm by adopting this modified operating procedure.
Conclusions
The RFJLR has been operated with ejector mode, to reduce the
operating cost for compressor. In the modified operating procedure
of RFJLB, first liquid was fed at desired flowrate by isolating gas
suction, allowed the liquid circulation loop to develop, and then
permitted the suction of gas into the reactor. This modified operating
procedure is necessary for lower diameter nozzles to obtain
complete circulation loop. The comparison of gas holdups for ejector
and injector modes provides the indirect method to estimate
the gas velocity sucked in during ejector mode.