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Application of airlift gas-liquid-solid reactors in biotechnology
0
Applications
of
Airlift
Gas-Liquid-Solid
M.H. Department
of
SIEGEL
and
Reactors
C.W.
Chemical Engineering, Ontario, Canada
in
ooo9-25cw92 $5.oo+o.M) 1992 Pcrgamon Press Ltd
Biotechnology
ROBINSON
N2L
University 3Gl
of
Waterloo
ABSTRACT Airlift reactors are pneumatically agitated reactors with circulation The possibility in a defined cyclic pattern through a loop of conduits. during fermentation of providing the required aeration and agitation flourishing of with has brought a processes, low energy input, Due to the mildness and airlift reactors. industrial interest in uniformity of turbulence in airlift reactors compared to conventional such as stirred tank reactors and bubble columns, particular reactors, interest is being focused on airlift reactors for use with various cell This paper presents an overview of the current state of cultures. knowledge in airlift reactor design and operation with a focus on Emphasis is placed on a discussion of external-loop airlift reactors. macroscale liquid circulation velocity because it is this parameter which distinguishes airlift reactors from other pneumatically agitated In addition, specific areas where such as bubble columns. reactors, design, reactor gaps presently exist in the knowledge of airlift and scale-up are identified and recommendations are made for operation, future research.
KEYWORDS Airlift, bioreactor, liquid velocity, gas
pneumatically holdup, mass
agitated transfer
reactor,
loop
reactor,
INTRODUCTION Since the airlift reactor was first patented for use as a bioreactor by significant advances have been reported Lefrancois (1955) in 1955, many and many articles have been published describing airlift reactor (ALR) operation, and performance. much of the published data design, However, consistent due to the wide variations in reactor geometries, are not experimental techniques, While the andphysio-chemical systems studied. volume of the data is substantial, much work remains to be done to facilitate a better understanding of ALRs and exploit their potential. With few exceptions, basic transport phenomena studies of ALR behaviour well as synthetic have been conducted on water-air systems, as "representative" fermentation media (usually of Newtonian nature). While many papers have been published discussing various applications of ALRs, very little data has been published on transport phenomena actual have during fermentations. these studies The bulk of concentrated on the growth kinetics during the fermentation process in the ALR. Furthermore, due to the costs and complexities involved in large-scale studies of actual fermentations, these studies usually have been conducted on small bench-scale ALR fermenters which do not provide 3215
32 16
M.
the basic successful
hydrodynamic scale-up of
H.
SIEGEL
and ALRs.
and C. W. ROIIINS~N
mass
transfer
K3
information
critical
for
Therefore, it is not surprising that the biochemical industries hesitate to select ALRs for commercial application. The stirred tank reactor remains the work-horse of the biochemical industry due to its (STR) relatively "off-the-shelf" convenience and its generally well defined characteristics. improved performance and scale-up Reports of fermentation performance on the bench-scale have not convinced the biochemical industry to make widespread use of ALRs due to the remaining question of whether or* not these reactors will exhibit similar performances after scale-up. This paper reviews the current status of knowledge on ALRs, focusing on concepts which can be used for design and scale-up of these reactors. Particular attention will be paid to external-loop ALRs due to their distinct nature as loop bioreactors. Recommendations will also be presented as to the many areas of knowledge which remain to be addressed before ALRs will achieve widespread commercial acceptance. What
are
Airlift
Reactors?
Airlift reactors are pneumatic reactors. They are different from the other commonly used pneumatic reactor, the bubble column, in that ALRs are comprised of four distinct zones, each with its own distinct flow pattern, which divide the reactor into separate upward and downwardtwophase flow regions. The zones, or channels, enable macroscale liquid circulation around the loop. The first zone, in which the gas is sparged, is denoted the "riser", as the gas-liquid dispersion travels upward in co-current, two-phase flow. This section has the higher fractional gas holdup (~~~1 and is where most of the gas-liquid mass transfer takes place. The liquid leaving the top of the riser enters a gas disengagement zone, separator", where, depending the "gas-liquid on its specific design, some or most of the dispersed gas is removed. The gas-free liquid (or a dispersion of lesser holdup, &,,) then flows into the "downcomer" and travels to the bottom of the device, through the "base", where it re-enters the riser. Thus, the liquid phase circulates continuously around the loop. While retaining some of the characteristics of conventional bubble columns, the macroscale liquid circulation exhibited by loop bioreactors is a unique feature. This circulation is an effect caused by the difference in the fractional gas holdup (AeG) that exists between the riser (EGR) and the downcomer (E,~). In turn, this creates a hydrostatic pressure difference between the bottom of the riser and the bottom of the downcomer which acts as the driving force for the fluid circulation. Internal-Loop
Airlifts
External-Loop
;
?G
Concentric
Fig.
1.
TG Tubes
Types
3
TG
Split Vessel
of
airlift
Airli!is
reactors
Applications of airlift gas-liquid--solid
K3
reactors in biotechnology
3217
ALRs
are based on their physical commonly divided into two types structure (Fig. 1). Internal-loop ALRs (IL-ALR) are baffled vessels where baffles are placed within a bubble column to provide the distinct flow channels of the loop. Concentric tube ALRs and split vessel ALRs are examples of IL-ALRs. External-loop ALRs (EL-ALR) are constructed which separate conduits attached at the top and bottom by of are horizontal conduits to form the circulation loop. One fundamental difference between the EL-ALRs and IL-ALRs is the design of the gasIn IL-ALRs, the gas-liquid separator is usually liquid separator. simply an unbaffled extension above the riser and downcomer, allowing for little gas disengagement. In the EL-ALR the gas-liquid separator has a clear region of horizontal flow, either a closed horizontal connection pipe or open reservoir between the riser and downcomer conduits, which allows for either partial or total gas disengagement. The subsequent differences in the operating behaviour of EL-ALRs and ILALRs implies the important role played by the gas-liquid separator in overall reactor performance. However, except for a few mixing studies 1983; Weiland, 1984; Verlaan et al. 1986a; Merchuk (Fields and Slater, and Yunger, 1990) and hydrodynamic and mass transfer studies (Siegel et Siegel and Merchuk, 1991) the influence of the gasal. 1986; 1988, liquid separator has been largely ignored. will profoundly influence the overall The extent of gas disengagement behaviour of ALRs as can be clearly seen by comparing gas holdup, liquid and mass transfer in EL-ALRs and IL-ALRs. Since circulation velocity, the driving force for liquid circulation is the difference in the mean riser and downcomer hydrostatic pressure between the density or EL-ALRs with near total gas disengagement in the horizontal sections, will have much greater liquid circulation gas-liquid separator The consequences of this velocities. due to higher riser liquid velocity It follows that 1983; Merchuk, 1986). ALRs than IL-ALRs due to lower riser results in less gas-liquid interfacial
is reduced gas holdup in the riser (Belle, 1981; Onken and Weiland, mass transfer is also less in ELand downcomer gas holdups which area for mass transfer.
The modelling and scale-up of ALRs is complicated by the interaction of liquid velocity, and principle parameters of holdup and the gas After the initial consequently mass transfer and mixing intensity. rate is usually the are set the influent gas flow geometric parameters At any specific gas flow rate (and sole adjustable operation variable. for a given physio-chemical composition of the fermentation broth) there will be a given AsG between the riser and downcomer which will dictate In order to change the the liquid circulation velocity around the loop. in the ALR the influent gas flow rate must be mass transfer or mixing velocity. changed resulting in a new A&, and liquid circulation Alternatively, in EL--ALRs, a low-pressure-drop throttling valve may be installed in one of the horizontal conduits linking the riser and the downcomer the liquid velbcity independently of the gas to regulate All the geometric factors sparging rate (Popovi& and Robinson, 1987). which influence the pressure drop around the loop (e.g. reactor height, cross-sectional areas ratio of downcomer/riser aspect ratio, (A,/&) r subsequently base diameter, and gas-liquid separator configuration) spite of many design influence ALR operation. Therefore, in the correlations in the literature for liquid circulation velocity, gas holdup, mixing, and mass transfer, are as yet no general there
correlations which are applicable literature in the are type substantially larger data base scale ALRs correlations
during actual be available.
Advantaqes
of
Airlift
The current correlations to all ALRs. until a specific. Not and system of transport phenomena data on large
fermentations
is
established
will
generalized
Reactors
Pneumatically agitated reactors, several advantages over STRs.
such Their
as
ALRs
simple
and
bubble
construction
columns, is
a
offer primary
3218
M.
H. SIEGEL
and C.
W. ROBINSON
K3
advantage. Since there are no moving mechanical parts needed for agitation, there is a reduced danger of contamination through seals or the need for complicated shaft bearings, seals or magnetically-driven agitators. The vertical orientation of these reactors, as well as the lack of internals, facilitate easier cleaning and sterilization. The injected gas serves the dual function of aeration and agitation. This promotes efficiency in the overall energy balance, eliminating the need for a separate expenditure of energy for agitation. When compared with bubble columns, ALRs have the additional advantages of loop reactors, such as increased heat and mass transfer capacity and a reduction of energy consumption for mixing. Heat removal and control is improved by the slender reactor configuration, providing a favourable wall-area-to-volume ratio, as well as high liquid velocity and turbulence at the heat exchange contact surface (Blenke, 1979). In biological processes, the primary advantage of ALRs over bubble columns and STRs is related to the shear stress imposed by the turbulent field on the cells or pellets suspended in the medium. One of the most important aspects of the flow in ALRs is the homogeneous field of shear, which is relatively constant throughout the reactor. In ALRs, fluid motion is induced by differences in the mean densities between the riser and downcomer sections of the reactor. Therefore, there is an overall directionality of liquid flow, even if random movements may be superimposed on it. In contrast, in bubble columns and STRs the energy source inducing fluid motion is focal. The shear forces in bubble columns will be greatest adjacent to the gas sparger and dissipate with distance from the sparger. In STRs, a region of very high shear exists near the agitator, which decreases with increasing distance from the agitator. The lack of uniformity in the field of shear in bubble columns and STRs exposes the organisms to varying shear stresses and environments as they pass through the reactor which can adversely affect the organism. Numerous studies have been conducted investigating the shear effects on microorganisms and cells in an effort to quantify the level of shear various organisms will tolerate (as reviewed by Merchuk, 1991). ALRs have many potential applications in chemical as well as biological processes as either twoor In ALRs, three-phase reactors. the fluidization of solids is not a direct consequence of the bubbling of but rather due to the liquid circulation within the reactor. gas, Therefore, these reactors offer the possibility of very simple and highly effective solids fluidization. This indicates a high potential for application in three-phase processes where gas, liquid, and solids must be brought into contact. solid phase may be one of the The reactants or a catalyst. the ALR is an attractive option for Thus, slurry reactors. DESIGN
AND
OPERATING
CHARACTERISTICS
As discussed below, the rate of liquid circulation, expressed as the linear liquid velocity in the riser (vLR = uJE~~) can be shown from an = (E,, - E,,)~~'. For stable energy balance to be proportional to A.E,'*~ unidirectional liquid circulation, E== always must be greater than &co, and obviously the maximum liquid circulation rate is obtained when E,~ = 0, i.e., when in the upper gas all the dispersed gas is removed disengagement application to aerobic section. However, for fermentations, a detrimental effect on the fermentation kinetics may occur if oxygen the cells are zero dissolved exposed to low or concentration. This may occur in a gas-free downcomer (or even one where E,, is relatively small) if the liquid residence time therein is depleted by sufficiently such oxygen is long that the dissolved microbial uptake (respiration) before the liquid reaches the bottom of the downcomer. Particularly in tall loop bioreactors, i.e., one of industrial scale, it may be necessary to either provide for gas carry-
K3
Applications of airlift gas--liquid-solid
reactors in biotechnology
over from the riser into the downcomer, or to separately the downcomer (at a point above the bottom) such that maximum potential liquid circulation rate may not practice.
3219
sparge gas Thus, E,, > 0. be achievable
into the in
While in the first instance the liquid circulation rate is an effect (i.e., a dependent variable), it in turn becomes the cause of the gas holdup, mass transfer and liquid-phase mixing characteristics exhibited by loop bioreactors compared to bubble columns in which macroscale for optimum design and scaleliquid circulation is absent. Therefore, it is essential to be able to predict the liquid up of loop bioreactors, (usually in terms of the superficial liquid velocity circulation rate in riser, liquid physicochemical properties the phase the ULR ) I non-Newtonian Newtonian vs. (viscosity, interfacial tension, density; rheological and the bioreactor geometry (dispersion characteristics) height, H,; downcomer-to-riser cross-sectional area ratio, A,/A,; the type of gas disengagement section, i.e., the geometry of the riserdowncomer interconnections which influence the frictional resistance to the circulating liquid flow). Thus, it is not surprising that there is no correlation of uLR (or J&, bioreactor which is external-loop etc.) applicable to all %.a, fluid all fermentation systems (i.e., geometries for types of several correlations which can serve as useful However, properties). guidelines for design and scale-up are summarized and briefly discussed in the following sections_ Circulatinq
Liquid
Velocitv
Numerous investigators have developed Newtonian, Low Viscosity Systems. models of which demonstrate the theoretical the fluid hydrodynamics circulation pressure drop, and relationship between the gas hold-up, liquid velocity in the different types of airlift reactors (de Nevers, Freedman and Davidson, 1969; Chakravarty et al., 1974; Kubota et 1968; al., 1978; Merchuk and Stein, 1981; Jones, Hsu and Dudukovic, 1980; et al., 1984; Koide et al., 1984; Verlaan et al., 1986: Lee 1985; Fan 1990; 1987; Calvo, 1989; Joshi et al., et al., Chisti et al., 1988; These models were derived using either an energy Young et al., 1991). balance on the gas input due to the isothermal expansion of the sparged and/or a steady state mechanical macroscopic momentum balances, gas, energy balance, usually in conjunction with the Zuber and Findlay (1965) drift-flux model. total A key parameter in these models is the frictional resistance to fluid flow in the circulating liquid loop, Newtonian including the flow reversals at the top and bottom. For systems, friction coefficients (K,) have been estimated by applying standard relationships for single-phase flow in pipes and bends (Verlaan et al., 1986), two-phase flow (Joshi et al., 1990), or by treating K, as estimated for a particular airlift design by an adjustable parameter While all these models have fit data fitting (Chisti et al., 1988). they have some of the published experimental data to varying degrees, the limitation of empiricism for friction factors, slip velocities, and/or gas which present overall recirculation rates. Also, models circulation well defined loss sometimes on very coefficients, not systems, do not consider the effect of gas-liquid separator design with its consequent effect on Furthermore, experimental gas holdup. verification of these models generally has been restricted to benchor pilot-scale airlifts, and to the air-water systems. Belle liquid
et al. (1984) velocity in
derived the riser V LR
However,
they
did
not
estimate
the
=
following
(3%
KfT,
%,
relat
ionship
for
the
(1)
1 Km)1'3
rather
they
1.inear
correlated
their
air-
M. H.
3220
water
data
empirically
SIEGEL
according V LR
=
and C. W. ROBINSON
K3
to
(2)
a,(A, / A,jb' uG~R/~
where for both internaland external-loop contactors b, - 0.75. For the external-loop case, a, = 1.55 and for the internal-loop a, = 0.66 (m/sj2". Comparison of the values of a, for the two different types of designs shows that the circulating liquid velocity in the external-loop was more than double that in the internal-loop contactor contactor (concentric cylinder draft tube). This reflects the fact that bubble disengagement from the upflowing gas-liquid dispersion at the top of the riser in the former apparatus is considerably more extensive, such that (external loop) < E,, (internal loop). It also should be noted that E i: the internal loop case, a considerable fraction of the total gas holdup in the downcomer was comprised of motionless or near-motionless bubbles, i.e., having a buoyancy force equal to the opposing drag force exerted by the downflowing liquid. these stagnant bubbles In practice, would contribute little or nothing to the gas-liquid oxygen transfer in the overall contactor, process due to eventual depletion or neardepletion of their oxygen content. The model of Verlaan et al. (1986) was developed for the case of &GR 5 On this basis, 0.10 and for E,, = 0. they were able to estimate KLT using as previously had been only single-phase (liquid) flow relationships, Wallis Application of their required an shown by (1969). model interactive calculation procedure from which for specified contactor geometry and gas sparging rate the hydrodynamic parameters uLR, ecR and The two-phase drift-flux model of Zuber and E,, could be predicted. Flndlay (1965) was used in the gas-holdup estimation to account for the effective slip velocity between the bubbles and the liquid which arises from the non-uniformity of the holdup and velocity profiles in the Using two different sizes of external-loop airlifts, radial direction. they obtained good agreement between predicted and experimental values For the smaller contactor (V = 0.165 m', H = 3.23 m, K,, = 1.8), of ULD. for 0.005 1. uGR 5 0.17 m/s; in the they observed 0.14 5 u,, _< 2.1 m/s larger vessel (V = 0.6 ma, H = 10.5 m, K,, = 4.75), 0.1 < uLR 1.8 m/s for u,,(V = 0.6;;;3)/u,,(V = 0.165 0.005 F u,, < 0.075. m/s. At UC;, = 0.05 m/s, whereas from Equation (1) the corresponding ratio of = 1.067; m3) et al. Although the two airlifts used by Verlaan (H,,'K,,)= = 1.073. (gas disengagements) (1986) had different designs for the top sections the agreement with the interconnecting the risers and the downcomers, prediction from Equation (1) suggests that on scale-up the effect of be accounted for by (HD/KfT)1'3. increased height on uIR may reasonably Bello (1981) Chisti et al. (1988) extended the modelling of and low-viscosity liquids, i.e., they considered only the case of Newtonian, could ignore the negligibly small contribution of wall friction losses Only the losses in the top and bottom sections arising from the to K:,. change on fluid flow direction were considered; furthermore, in the case the top and bottom flow reversal losses of external-loop contactors, were considered to be equal. For these contactors, they then obtained ULF7
= {2gH,
(A&,)
/Kfs[
(1-E,,)2)
+
(AD/A,)'
(l/
(1 -E,,J2)
I}'.'
(3)
(3) requires a priori knowledge of both E,, and Application of Equation Chisti (which are system specific), as well as an estimate of K,,. %D et al. (1988) obtained experimental values of uLR for the air-water contactor (V system in an internal-loop (draft tube; annulus sparged) = 1.46 A pulse (V = 0.234 m3). m3) and a split-cylinder airlift was used to measure vLD injection of concentrated HISO, in the downcomer the by means of two pH electrodes separated by a known distance: Apparently, their measurement method for E,, and E,, was not described. own results were compared to the predictions using Equation (3) by be an adjustable parameter evaluated by data taking K,, to to be
K3
Applications
of airlift
gas-liquid--solid
reactors in biotechnology
3221
regression. They also compared the predictions of Equation (3) to the results of several previous investigators of internaland external-loop airlifts et al., 1986). Observed values of (including those of Verlaan the uLR generally agreed with the predictions from Equation (3) to within 30% for both internaland externalloop airlifts. Joshi et al. recently provided an extensive review of the (1990) hydrodynamics of external-loop reactors. In addition, they developed a model for the steady-state circulating liquid superficial velocity (u,,) by equating in differential form the driving force for liquid circulation resisting (i.e. with the forces (dAP,/dz). EGR %D) Homogeneous gas-liquid flow was assumed in downcomer and the heterogeneous flow in the riser. The model (solved by applying a 6-step was used to analyze the effect of various design iterative procedure) parameters on the hydrodynamic and mass transfer performance for a range air-water of essentially for reactor volumes (10 1. V 3 1000 m3, dispersions. They showed that for the case of sparging gas only in the riser there is only one (stable) steady-state value of uLR. On the other two possible values of hand, when gas is also sparged in the downcomer, They also uLR can exist, only one of which (the higher value) is stable. axial location for the downcomer showed that there was a critical sparger, which depended on the airlift geometry and on the relative If gas were to rates of gas sparging in the downcomer and the riser. no stable liquid circulation could be be sparged above this location, established. As a result of their simulation studies for Newtonian, low viscosity is a possibility systems, Joshi et al. (1990) concluded that 'I... there height to diameter ratio of selecting optimum values of reactor volume, "However, the optimum values are different and area ratio" (i.e. AD/A,). liquid circulation rate, for different design objectives such as overall effective interfacial area, mass transfer rate and extent of mixing. consumption per unit These values also depend upon the level of power dissipated by the isothermal expansion of the ;;:;;:I; ,',ise. the power While an extensive which is directly proportionalto uGR). simulation $tudy of the type performed by Joshi et al. (1990) has not been conducted for external-loop airlift contactors for the case of viscous, non-Newtonian results obtained for the latter, as systems, summarized below, indicate that there is no single optimum condition for non-Newtonian external-loop airlift design and operation with viscous, mycelial microorganisms or fermentation systems, eg. cultures of extracellular polysaccharides gum) fermentations. (e.g. xanthan Young et al. (1991) developed a differential, two-fluid hydrodynamic model for two-phase flow in airlift risers using point equations of continuity and motion. Macroscopic energy balance equations were used to describe flow in the downcomcer and gas-liquid separator. By using a differential separated-flow type model to describe flow in the riser along the length of the they incorporated changes in flow phenomena riser. The model has the advantage that the only empirical parameters They compared are those related to friction effects in the reactor. their own experimental results and those of Merchuk and Stein (1981) to predicted model values and found an agreement of approximately 10%. It should be noted that both the reactors of Merchuk and Stein (1981) and Young et al. (1991) used straightening vanes, in the downcomer entrance of the former to achieve more and the riser of the latter, entrance uniform liquid velocity profiles. Using the air-water system to investigate the performances of three 1.09-1.23 and 1.03relatively large airlift contactors (v = 0.16-0.23, 1.88 m3), Siecrel and Merchuk (1991) hiahliahted the sicnificant effect of the gas disengagement section. Conslderlngthat a gas bubble leaving the riser will completely disengage from the liquid phase in a time period t, (related to the terminal rise velocity of the bubble) and time of the gas-liquid dispersion in the defining the mean residence separator as t,, for a rectangular separator Siegel and Merchuk showed CES
47:13/14-E
3222
M.
H.
SIEGEL
and C.
W.
K3
ROBINSON
that
h/ % = (UU)/Vb~)(AD&Ws) They defined circulating studied was
the term A,/L,W, as the "disengagement liquid velocity in the downcomer of the correlated in the form
(4)
ratio", smallest
DR. The contactor
UIJJ = 0.45(P/V$"*33/DRo.*5
In Equation theoretical
(5), P/V,) prediction
=
uGI, such that of Bello (1981)
uLD - uGR0-33,in and Bello et
(5)
agreement al.
with
the
(1984).
By means of using a movable vertical baffle in the gas-liquid separator, Siegel and Merchuk (1991) were able to vary DR such that in a given split-vessel ALR they could achieve a considerable variation in E,,. At low values of t,, high values of E,, (up to 0.12, depending on uGR) were obtained, producing a uLR similar to that of concentric tube IL-ALRs. At higher values of tz, nearly complete gas disengagement was achieved producing a maximum U,, (similar to that of EL-ALRs). f&G, 5 0.02), A high level of disengagement, or essentially complete even gas disengagement can be achieved in practice with moderately-sized disengagement sections for low viscosity liquids, as in the case of the studies by Siegel and Merchuk However, it is unlikely that (1991). nearly complete removal of all the bubbles exiting at the top of the riser can be achieved in the case of viscous fermentation broths. In these latter systems, the bubble size distribution is binodal. The large bubble fraction likely would disengage fairly readily in the gasliquid separator. However, small fraction, which the bubble can comprise ca. 15 percent of the total gas holdups, likely would not disengage during any practical residence time Studies on the (t2) effect of gas separator conditions for viscous, non-Newtonian systems should be conducted. Viscous, non-Newtonian Systems. Popovic andRobinson (1987, 1988, 1989) made extensive investigations of the hydrodynamic and mass transfer behaviours of external-loop airlift contactors, using pseudoplastic Their solutions of various types of carboxymethyl cellulose (CMC) . airlift contactors also could be operated in the bubble column mode (A,/A,=O) by closing the butterfly valves installed in the top and bottom horizontal piping sections joining the riser and the downcomer. The empirical correlations developed by them are listed in Table 1, and were obtained for churn-turbulent to slug flow dispersion conditions in the riser. liquids As shown by Equation (l.l), Table 1, for non-Newtonian the dependency of uLR on uGa (- u~~"-~*) is nearly identical to that non-viscous, Newtonian predicted and verified by Bello (1981) for liquids, namely uLR - uGR0-33. As expected, uLR decreases with increasing liquid-phase viscosity (- q,fL-".3g). The effective viscosity exerts a moderate On the other hand, ?leff exerts a strong influence on uLR. influence on the mass transfer capabilities of an EL-ALR (Equation 1.3, Table 1). Depending on the actual value of qeff, k,a for viscous, nonNewtonian broths in a specified EL-ALR can be up to lo-fold lower than in the case of non-viscous, Newtonian fermentation broths. K,a also decreases with increasing AD/AR, i.e. increasing uLR in the riser where co-current upward flow of gas and liquid exists (see Equation 1.1, Table Thus, k,a always is greater in the corresponding bubble column 1). (A,/A,=O) than in an EL-ALR.
K3
Applications of airlift gas-liquid-solid
Table
reactors in biotechnology
Hydrodynamic and Mass Parameter Correlations of Robinson for Non-Newtonian Airlifts External-Loop Columns.
1.
3223
Transfer Popovic and Systems in or Bubble
Ref. Year
No.
Circulating
1.1
liquid ULR =
1.2
Riser
gas
Volumetric
k,a, = 1.4
0.005
=
O-02
I
0, rleff
Perforated
0.111, 5
uGR0-65 [l
transfer
uGRO.=
interfacial a =
A,/A,
0.50
plate
u,:-~~
(AD/A,) o-g7 fl,Ct-o-39
1989
0.456
mass
Specific
0.23
1988
holdup
EGR = 1.3
velocity
233
0.25,
[l +
+
(A,/A,)]-1-06 T&~-'-~'~
(A,/A,) J -'*" Hetf DLom5 p,l.03 CS-'." 1987
area
u,,'-'~ [l
+
(A,/A,)]-1.52 f7,ft-0-327
0.444
Pa.s; y = 5000 u=(a) sparger; 0.03 5 u G(R)-< 0.26
APPLICATIONS
1989
coefficient
OF
AIRLIFT
m/s
REACTORS
the application of production-scale ALRs in the At the present time, biochemical industries is limited due to the lingering questions related ALRs are less flexible Furthermore, to the scale-up of these reactors. geometric requirements than STRs. Once the t0 changing process parameters of the ALR have been selected for a given process at the time the gas flow rate is the principle, if not sole, adjustable of design, the ALR is less adaptable to parameter during operation. Therefore, liquid velocities, need different other processes which might gas mixing intensities andmasstransfer characteristics than distributions, conventional STRs where the aeration and agitation can be independently The influence of A,/A, (the ratio of downcomer/riser crosscontrolled. sectional areas) has received much attention as a design parameter that Other geometric parameters significantly affects reactor performance. which have been shown to strongly influence ALR performance are gas(Siegel and Merchuk, 1991) and bottom liquid separator configuration While these parameters have been shown clearance (Ladwa et al. 1988). and pilot-scale reactors using air-water to be adjustable in faboratorythey remain largely untested during actual systems and synthetic media, process operations and on production-scale reactors. The applications of ALRs have been reviewed previously by Onken and Margaritis and Wallace (1984), Smart (1984) and Siegel Weiland (1983), and pilot-scale applications of et al. (1988). Recently, many benchALRs have been studied for a variety of microorganism and cell cultures. Many of these studies have been conducted on small bench-scale ALRs focusing on growth kinetics rather than transport phenomena during the fermentation. little work has been conducted to optimize Furthermore, ALR design and operation during actual fermentations. Consequently, little basic hydrodynamic andmass transfer information is available for optimum reactor design and scale-up. What follows is a brief synopsis of some of the more recent works discussing the application of ALRs.
M. H. SIEGEL and C. W. ROBINSON
3224 Frdhlich
EC3
al. (1991, 1991a) examined the cultivation of Saccharomyces m3 in a 4 m3 concentric tube pilot plant ALR and a 0.08 They laboratory ALR using both batch and continuous modes of operation. examined the global axial mixing (Frijhlich et al. 19911, and local gas bubble diameter and bubble velocity (Frohlich et al. 1991a) holdup, during the yeast cultivation and in model media. This work represents the first report of the measurement of local properties of the dispersed gas phase in a pilot-scale reactor during an actual fermentation. It was found that the local gas holdup, bubble size and the bubble velocity changed only slightly along the length of the riser. et
cerevisiae
Suh et al. (1992) conducted a comparative study of a 0.05 m3 bubble column and a 1.2 m3 concentric tube ALR for the production of xanthan gum with Xanthomonas campestris in synthetic media. They studied the and mass transfer, as well as xanthan productivity and hydrodynamics quality during the fermentation. The ALR did not perform as well as the bubble column in terms of either mass transfer or xanthan productivity and quality. This was attributed to a lack of oxygen supply in the downcomer, with an high residence accompanying time due to low circulation velocity (especially at higher xanthan concentrations). It should be noted that the ALR used in this study had a AD/A, of ca. 1. Using an ALR with AD/A, < 1 would help reduce residence time in the downcomer. Several of the studies summarized in Table 2. Table
2.
Organism Reactors
Organism
Suspended
butyricum
Sorangium
cellulosum
Recombinant Escherichia
Triqonella
Growth
focused
Studies
Reactor
Volume
Tm?e
(m’)
on
growth
kinetics
are
Airlift
in
Reference
CT
0.05 1.50
Giinzel
CT
0.04 0.05
Hopf
CT
0.06
Kracke-Helm (1991)
CT
0.009
Bonnarme (1990)
CT
0.0028
Stasinopoulos & Seviour (1992)
CT
0.009
Rodriguez-Mendiola (1991)
Coli
Phanerochaete chysosporium
Attached
have
Cultures
Clostridium
Acremonium
which
persicinum
et
et
(1991)
al.
al.
(1990) et
al.
& Jeffries
Cultures foenum-graceum
Immobilized
Cell
Penicillium
chrysogenum
CT
0.250 0.320
Keshavarz (1990)
Nitrobacter
agilis
EL
0.0024 0.0034
Wijffels
et
al.
(1990)
mesenteroides
CT
0.0025
El-Say-ed
et
al.
(1990)
Leuconostoc Bacterial Thiobacillus
Cultures et
al.
Leachinq ferrooxidans
CT
Couillard (1991)
& Mercier
K3
Applications of airlift gas-liquid-solid
reactors in biotechnology
3225
Recent studies have also been conducted examining ALRs as potential reactors for treating various wastes. Hiippe et al. (1990) used a twoeffluents from a coal tar stage pilot plant to biologically treat refinery. The first stage was a modified concentric tube ALR (V = 1.5 coal dust entered the process. The effluent from the first m') where stage passed through sedimentation and crossflow filtration units before entering the second stage. The second stage was a 0.160 m3 EL-ALR with the biomass immobilized on sand particles. Aromatic substances which passed through the first stage were subsequently eliminated by the immobilized biomass in the second stage. Tyagi et al. (1990) used a 0.023 m3 laboratory-scale EL-ALR and a 1.15 m3 pilot-scale EL-ALR to mesophilic and thermophilic aerobic digestion of primary and study secondary municipal sludges. The pilot ALR, using air for aeration, gave comparable results to conventional aerobic sludge digestion systems which use pure oxygen for aeration. the pilot ALR was able to Also, achieve thermophilic (53°C) by means of autothermal temperatures A cost analysis showed that the autoheated ALR digester would heating. represent a significant savings in both capital and operating costs compared to a conventional two-stage aerobic digester using pure oxygen to achieve the required oxygen transfer rate. Several recent studies have indicated that hydrodynamic damage may occur mammalian and insect to cells with a high sensitivity to shear (e.g. (Tramper et al. 1986, 1988; cells) due to bubble-cell interactions Handa-Corrigan et al. 1989; Kunas and Papoutsakis, 1990; Jijbses et al. Bubble breakup appears 1991; Bavarian et al. 1991; Papoutsakis, 1991). related to gas to be the primary mechanism involved in cell damage has not been examined in ALRs sparging. The effect of this phenomena per se, however, studies with bubble columns (Tramper et al. 1988; Handa-Corrigan et al. 1989) indicate that greater aspect ratios (heightincrease the retention of cell to-diameter ratio) and reactor heights viability. Further studies are needed of this phenomena in ALRs, as well as potential methods of manipulating the design of the gas-liquid sensitive cells in a degassed separator and/or entrapping the shear with downcomer to minimize bubble-cell the damage associated interactions. RECOMMENDATIONS Before universal design and operation correlations can be developed for ALR design and operation the following information must be obtained: influence of the gas-liquid 1. More information is needed on the separator on ALR behaviour. The hydrodynamic regime and flow patterns at the exit and at the of the riser, in the gas-liquid separator, The flow patterns in these entrance of the downcomer must be examined. allow the regions understanding will are very complex their and and dispersion coefficients. A development of local friction factors better understanding of this region would also allow better manipulation and flexibility of the ALR during operation. 2. Abetter understanding is needed of hydrodynamic flow regimes, flow regime transitions, as well as bubble coalescence and redispersion in all the different sections of the reactor. 3. The bulk of the current data for friction factors, mixing, andmass Much of this transfer has been presented in terms of a global approach. Methods and data has limited utility for reactor design and scale-up. techniques to measure local sectional coefficients must be developed. A method of determining coefficients has been local mass transfer no method of measuring presented (Merchuk and Siegel, 1988). However, local k,a values, reliable data for local dispersion nor sufficient coefficients currently exists. 4. More information is needed on the contribution of the downcomer to airlift reactor behaviour. This is especially true with regard to twosparger systems where supplementary gas is injected into the downcomer. 5. In general, data for heat transfer, gas dispersion, and liquid mixing in essentially twoand lacking or three-phase ALRs is unavailable. development of These data for the are essential
M. H. SIEGEL
3226
K3
and C. W. ROBINSON
sophisticated design and scale-up .models. 6. Much more work is needed on the scale-up of ALRs statements can be made about the scale-up process. where kinetic energy input from the short reactors, gas sparger design will be significant, geometrically may not give similar results during scale-up.
before conclusive For example, in gas injection and similar designs
This represents a partial list of the areas of knowledge where gaps still need to be filled. However, the greatest gap in ALR knowledge which still exists is a thorough understanding of the transport phenomena in reactors fermentation these under actual conditions. available data for ALRs is for water or Almost all the currently synthetic simulation media. These data must be checked against actual process conditions.
NOMENCLATURE J-k
uG VL
K, HD
A K AP, n H k,a z g al b, V Vbt, t2 L
WS
P D, Greek Y : eff P 0 2
superficial liquid velocity, m/s superficial gas velocity, m/s linear liquid velocity, m/s friction coefficient for fluid flow, dispersion height, m cross-sectional area, m2 consistency factor, power law rheological model, Pa-s" frictional pressure drop, Pa.s" flow behaviour index, power law rheological model, height, m volumetric mass transfer coefficient for oxygen, s-l axial coordinate position acceleration of gravity, m/s2 proportionality constant in Equation (l), (m/s>2'3 exponent in Equation (11, volume, m3 bubble terminal rise velocity, m/s time required for complete bubble disengagement in the gas-liquid separator section, s mean hydraulic residence time in the gas-liquid separator, s characteristic length of flow path on the gas-liquid the separator, m width of the gas-liquid separator, m aeration power (isothermal gas expansion), kW liquid-phase molecular diffusivity, m'/s Letters shear rate, s-l fractional gas holdup, effective viscosity of pseudoplastic density, kg/m3 interfacial tension, N/m shear stress, N/m'
Subscripts d D R G L T B
dispersion downcomer riser gas phase liquid phase total bottom section
of
airlift
liquid,
Pa.s
Applications
Ia
of airlift
gas--liquid-solid
reactors in biotechnology
3227
REFERENCES Bavarian, F, L. S. Chalmers Microscopic Fan and J. J. (1991). visualization of insect cell-bubble interactions. I: rising bubbles, air-medium interface, Biotechnol. Prog., 1, 140and the foam layer. 150. University of Waterloo, Canada. Belle, R. A. PhD thesis, (1981). Belle, R. A., C. W. Robinson and M. Moo-Young (1984). Liquid circulation and mixing of airlift contactors. Can. J. Chem. characteristics Engng., 62, 573-577. Blenke, H. (1979). Loop reactors. Adv. Biochem. Engng., 13, 121-215. Selective production of Bonnarme, P. and T. W. Jeffries (1990). extracellular chysosporium in an peroxidases from Phanerochaete airlift bioreactor. J. Ferm. Bioengng., 70, 158-163. A fluid dynamic model for airlift loop reactors. Calvo, E. G. (1989). Chem. Engng. Sci., 44, 321-323. Chakravarty, M., H. D. Signh, J. N. Baruah, M. S. Iyengar (1974). Liquid 17-22. velocity in a gas-lift column. Indian. Chem. Engng., 16, Liquid circulation in Chisti, M. Y., B. Halard and M. Moo-Young (1988). airlift reactors. Chem. Engng. Sci., 43, 451-457. Couillard, D. and G. Mercier (1991). OptiGm residence time (in CSTR and airlift reactor) for bacterial leaching sewage sludge. Wat. Res., 25, 211-218. Bubble driven fluid de Nevers, N. (1968).
14 222-226. A.-H. M. M., W. El-Sayed, Production of dextransucrase in
calcium-alginate
Biotechnol. Fan, of
L. S., a draft
by
beads:
Bioengng., S. J. tube
M.
of
metals
from
anaerobic
A.I.Ch.E.
circulations.
J.,
and R. W. Coughlin (1990). Mahmoud, Leuconostoc mesenteroides immobilized
1.
and
Batch
fed-batch
36, 338-345. and A. Matsuura
Hwang gas-liquid-solid
A (1984). spouted bed.
fermentations.
Hydrodynamic
behaviour
Chem.
Sci.,
Engng.
39,
1677-1688. Fields, P. R. and N. K. H. Slater NKH Tracer dispersion in (1983). laboratory air-lift reactor. Chem. Engng. Sci., 38, 647-653. Hold-up and liquid circulation Freedman, W. and J. F. Davidson (1969).
a
in bubble columns. Trans. Instn. Chem. Engrs., 47, T251-T262. Frijhlich, S., M. Lotz, T. Korte, A. Liibbert, K. Scmgerl and M. Seekamp airlift loop Characterization of a plant tower (1991). pilot mixing properties of the gas bioreactor: II. Evaluation of global phase during yeast cultivation. Biotechnol. Bioengng., 37, 910-917. Frbhlich, S ., M. Lotz, B. Larson, A. Liibbert, K. Schiigerl and M. Seekamp (1991). Characterization of a pilot plant bioreactor: III. Evaluation of local properties phase during yeast cultivation and in model
Bioengng., Hopf, in
38,
N. W., S. air-lift
Biotechnol., Hiippe, P., H. effluents
Engng. Giinzel, of
Yonseland and
34, xke
from
a
W.-D. Deckwer (1990). stirred-tank bioreactors.
350-353. and D. coal
tar
C.
Hempel
refinery
2 m3.
loop gas
Biotechnol.
from
Appl.
Deckwer glycerol by
Microbial.
Ambruticin
Biological immobilized
(1991).
Microbial.
treatment biomass.
Fermentative
Clostridium Biotechnol., 36,
Handa-Corrigan, A., A. N. Emery and R. liquid interfaces on the growth mechanisms of cell damage by bubbles.
S production
APP~.
(1990). using
Technol., 13, 73-79. B., S. Yonsel and W.-D. of
tower dispersed
56-64.
1,3_propanediol
scale
airlift of the media.
production
butyricum 289-294.
Effect Spier (1989). mammalian suspended
E. of
Enzyme
Microb.
of
Chem.
up
to
a
of gascells:
Technol.,
11,
230-235. holdup and liquid Hsu, Y. C. and M. P. Dudukovic (1980). Gas Engng. Sci., 35, 135-141. recriculation in gas-lift reactors. Chem. Lethal events during gas JBbses, I., D. Martens and J. Tramper (1991). Biotechnol. Bioengng., 2, 484-490. sparging in animal cell culture.
Liquid circulation Jones, A. G. (1985). Chem. Engng. Sci., 9, 449-462. Joshi, J. B., V. V. Ranade, S. D. Gharat loop reactors (Review), Can. J. Chem.
in
a draft-tubes
and S. Engng.,
bubble
S. Lele (1990). 68, 705-741.
column. Sparged
3228
M. H. SIEGEL and C. W.
ROBINSON
K3
Keshavarz, T., R. Eglin, E. Walker, C. Bucker G. Holt, A. T. Bull, and M. D. The large-scale immobilization of Penicillium Lilly (1990). chrysogenum: Batch and continuous operation in an air-lift reactor. Biotechnol. Bioengng., 3, 763-770. Koide, K., S. Iwamoto, Y. Takasak, S. Matsuura, E. Takahashi, and M. Kimura (1984). Liquid circulation, gas holdup and pressure drop in bubble column with draught tube. J. Chem. Engng. Japan, 17, 611-618. Rinas, 3. Hitzmann and K. Schiigerl (1991). Kracke-Helm, H. A., U. Cultivation of recombinant E. coli and production of fusion protein in 60-L bubble column and airlift tower loop reactors. Enzyme Microb. 13, 554-564. Technol., Kubota, H., Y. Hosono and K. Fujie (1978). Characteristic evaluations of ICI air-lift type deep shaft aerator. J. Chem. Engng. Japan, 11, 319-325. mechanisms of Kunas, K. T. and E. T. Papoutsakis (1990). Damage suspended animal cells in agitated bioreactors with and without bubble entrainment. Biotechnol. Bioengng., 36, 476-483. Merchuk Pickett and J. C. Cameron, M. Bulmer, A. M. Ladwa, N., A. Influence of draft tube clearance on pressure drop and gas (1988). International airlift reactor. In: 2nd holdup in a 250 litre Conference on Bioreactor Fluid Dynamics (R. King, ed.) pp. 395-413, Elsevier Ltd., London. Lee, C. H., S. A. Patel, L. A. Glasgow and L. E. Erickson LE (1987). L_iquid circulation in airlift fermentors. In: BiotechnologyProcesses: Scale-up and Mixing (C. S. Ho and J. Y. Oldshue, eds.), pp. 50-59, AIChE Press, New York. Lefrancois, M.L., C. C. Mariller and J.C. Mejane (1955) Effectionnements aux procedes de cultures forgiques et de fermetations industrielles. Brevet d'rnvention, France, No. 1102200. Novel bioreactor systems and Margaritis, A. and J. B. Wallace (1984). their applications. Biotechnology, May, 447. Hydrodynamics and hold-up in air-lift reactors. Merchuk, J.C. (1986). In: Encyclopedia of Fluid Mechanics (N. P. Cheremisinoff, ed.) Vol 3, 1485-1511, Gulf Publishing, Huston. PP. Shear effects on suspended cells. Adv. Biochem. Merchuk, J. C. (1991). Engng., 44, 65-95. Air-lift reactors in chemical Merchuk, J. C. and M. H. Siegel (1988). and biological technology. J. Chem. Tech. Biotechnol., 41, 105-120. Local hold-up and liquid velocity Merchuk, 3. C. and Y. Stein (1981). in air-lift reactors. A.I.Ch.E. J., 27, 377-388. On the role of the gas-liquid Merchuk, J.C. and R. Yunger (1990). of air lift reactors in the mixing process. Chem. Engng. separator 2973-2975. Sci., 45, construction, Airlift fermenters: Onken, U. and P. Weiland (1983). Biotechnological Processes (A. Advances behavior, and uses. In: Mizrahi, ed.), Vol. 1, pp. 67-95, Alan R Liss, New York. Media additives for protecting freely Papoutsakis, E. T. (1991). damage. aeration agitation and suspended animal cells against TIBTECH, !3, 3 6-324. The specific interfacial area PopoviC, M. and C. W. Robinson (1987). column. II. in external-circulation-loop airlifts and a bubble Engng. Sci., 42, Carboxy-methyl cellulose/sulphate solution. Chem. 2825-2832. External-circulation-Ioop Popovic, M. and C. W. Robinson (1988). circulating velocity in of the liqiud airlift bioreactors: Study Biotechnol. Bioengng., 32, highly viscious non-Newtonian liquids. 301-312. studies of transfer Mass Popovic, M. and C. W. Robinson (1989). 393A.I.Ch.E. J., 35, external-loop airlifts and a bubble column. 405. Rodriguez-Mendiola, M. A., A. Stafford, R. Cresswell and C. Arias-Castro Enzyme Microb. Technol., 13, 697-702. (1991). Siegel, M. H., J. C. Merchuk and K. Schiigerl (1986). Air-lift reactor analysis: Interrelationships between riser, downcomer, and gas-liquid including gas recirculation effects. A.I.Ch.E. J., separator behavior, 1585-1596. 32,
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gas-liquid-solid
reactors
in biotechnology
3229
and J. C. Merchuk (1988). Air-lift reactors: Siegel, M. H., M. Hallaile Design, and applications. In: Advances in Biotechnological operation, Processes (A. Mizrahi, ed.), Vol. 7, pp_ 79-124, Alan R. Liss, New York. Merchuk transfer in a Mass Siegel, M. H. and J. C. (1988). rectangular air-lift reactor: Effects of geometry and gas recirculation. Biotechnol. Bioengng., 32, 1128-1137. Siegel, M. H. and J. C. Merchuk (1991). Hydrodynamics in rectangular air-lift reactors: Scale-up and the influence of gas-liquid separator design. Can. J. Chem. Engng., 69, 465-473. Gaslift fermenters: Smart, N. J. (1984) Theory and practice. Laboratory Practice, July. Seviour Stasinopoulos, S. J. and R. J. (1992) a Exopolysaccharide production by Acremonium persicinum in stirred-tank and air-lift fermentors. Appl. Microbial. Biotechnol., 36, 465-468. Xanthan production in Suh, I.-S., A. Schumpe and W.-D. Deckwer (1992). bubble column and air-lift reactors. Biotechnol. Bioengng., 2, 85-94. Tramper, J., J. B. Williams and D. Joustra (1986). Shear sensitivity of insect cells in suspension. Enzyme Microb. Technol., 8, 33-36. Bubble-column Tramper, J., D. Smit, J. Straatman and J. M. Vlak (1988). Bioprocess Engineering, 3, design for growth of fragile insect cells. 37-41. (1990). Mesophilic and Tyagi, R. D., F. T. Tran and T. J. Agbebavi thermophilic digestion of municipal sludge in an. airlift U-shape bioreactor. Biological Wastes, 31, 251-266. Verlaan, P., J. Tramper, K. Van't Riet and K. Ch. A. M. Luyben (1986). an airlift-loop bioreactor with external A hydrodynamic model for Chem. Engng. J., 33, B43-B53. loop. Verlaan, P, J. Tramper and K. van't Riet (1986a). Hydrodynamics and bioreactor with two and three axial dispersion in an airlift-loop International Conference on Bioreactor Fluid phase flow. Proceedings: Dynamics. Cambridge, England, April 15-17, pp. 93-107. One Dimensional Two Phase Flow, McGraw-Hill, New Wallis, G. B. (1969). York. Influence of draft diameter on operation Weiland, P. (1984). tube behaviour of airlift loop reactors. Ger. Chem. Engng., 1, 374-385. Wijffels, R. H., C. D. de Gooijer, S. Kortekaas and J. Tramper (1991). consumption of Nitrobacter agilis cells Growth and substrate Biotechnol. immobilized in carrageenan: Part 2. Model evaluation. Bioengng., 38, 232-240. Ollis Airlift Young, M. A., R. G. Carbonell, and D. F. (1991). J., bioreactors: Analysis of local two-phase hydrodynamics. A.I.Ch.E. 403-428. 37, Zuber, N., and J. A. Findlay (1965). Average volumetric concentration in two phase systems. Trans. A.S.M.E. J. Heat Transfer, m, 453-468.
Applications
of
Airlift
Gas-Liquid-Solid
M.H. Department
of
SIEGEL
and
Reactors
C.W.
Chemical Engineering, Ontario, Canada
in
ooo9-25cw92 $5.oo+o.M) 1992 Pcrgamon Press Ltd
Biotechnology
ROBINSON
N2L
University 3Gl
of
Waterloo
ABSTRACT Airlift reactors are pneumatically agitated reactors with circulation The possibility in a defined cyclic pattern through a loop of conduits. during fermentation of providing the required aeration and agitation flourishing of with has brought a processes, low energy input, Due to the mildness and airlift reactors. industrial interest in uniformity of turbulence in airlift reactors compared to conventional such as stirred tank reactors and bubble columns, particular reactors, interest is being focused on airlift reactors for use with various cell This paper presents an overview of the current state of cultures. knowledge in airlift reactor design and operation with a focus on Emphasis is placed on a discussion of external-loop airlift reactors. macroscale liquid circulation velocity because it is this parameter which distinguishes airlift reactors from other pneumatically agitated In addition, specific areas where such as bubble columns. reactors, design, reactor gaps presently exist in the knowledge of airlift and scale-up are identified and recommendations are made for operation, future research.
KEYWORDS Airlift, bioreactor, liquid velocity, gas
pneumatically holdup, mass
agitated transfer
reactor,
loop
reactor,
INTRODUCTION Since the airlift reactor was first patented for use as a bioreactor by significant advances have been reported Lefrancois (1955) in 1955, many and many articles have been published describing airlift reactor (ALR) operation, and performance. much of the published data design, However, consistent due to the wide variations in reactor geometries, are not experimental techniques, While the andphysio-chemical systems studied. volume of the data is substantial, much work remains to be done to facilitate a better understanding of ALRs and exploit their potential. With few exceptions, basic transport phenomena studies of ALR behaviour well as synthetic have been conducted on water-air systems, as "representative" fermentation media (usually of Newtonian nature). While many papers have been published discussing various applications of ALRs, very little data has been published on transport phenomena actual have during fermentations. these studies The bulk of concentrated on the growth kinetics during the fermentation process in the ALR. Furthermore, due to the costs and complexities involved in large-scale studies of actual fermentations, these studies usually have been conducted on small bench-scale ALR fermenters which do not provide 3215
32 16
M.
the basic successful
hydrodynamic scale-up of
H.
SIEGEL
and ALRs.
and C. W. ROIIINS~N
mass
transfer
K3
information
critical
for
Therefore, it is not surprising that the biochemical industries hesitate to select ALRs for commercial application. The stirred tank reactor remains the work-horse of the biochemical industry due to its (STR) relatively "off-the-shelf" convenience and its generally well defined characteristics. improved performance and scale-up Reports of fermentation performance on the bench-scale have not convinced the biochemical industry to make widespread use of ALRs due to the remaining question of whether or* not these reactors will exhibit similar performances after scale-up. This paper reviews the current status of knowledge on ALRs, focusing on concepts which can be used for design and scale-up of these reactors. Particular attention will be paid to external-loop ALRs due to their distinct nature as loop bioreactors. Recommendations will also be presented as to the many areas of knowledge which remain to be addressed before ALRs will achieve widespread commercial acceptance. What
are
Airlift
Reactors?
Airlift reactors are pneumatic reactors. They are different from the other commonly used pneumatic reactor, the bubble column, in that ALRs are comprised of four distinct zones, each with its own distinct flow pattern, which divide the reactor into separate upward and downwardtwophase flow regions. The zones, or channels, enable macroscale liquid circulation around the loop. The first zone, in which the gas is sparged, is denoted the "riser", as the gas-liquid dispersion travels upward in co-current, two-phase flow. This section has the higher fractional gas holdup (~~~1 and is where most of the gas-liquid mass transfer takes place. The liquid leaving the top of the riser enters a gas disengagement zone, separator", where, depending the "gas-liquid on its specific design, some or most of the dispersed gas is removed. The gas-free liquid (or a dispersion of lesser holdup, &,,) then flows into the "downcomer" and travels to the bottom of the device, through the "base", where it re-enters the riser. Thus, the liquid phase circulates continuously around the loop. While retaining some of the characteristics of conventional bubble columns, the macroscale liquid circulation exhibited by loop bioreactors is a unique feature. This circulation is an effect caused by the difference in the fractional gas holdup (AeG) that exists between the riser (EGR) and the downcomer (E,~). In turn, this creates a hydrostatic pressure difference between the bottom of the riser and the bottom of the downcomer which acts as the driving force for the fluid circulation. Internal-Loop
Airlifts
External-Loop
;
?G
Concentric
Fig.
1.
TG Tubes
Types
3
TG
Split Vessel
of
airlift
Airli!is
reactors
Applications of airlift gas-liquid--solid
K3
reactors in biotechnology
3217
ALRs
are based on their physical commonly divided into two types structure (Fig. 1). Internal-loop ALRs (IL-ALR) are baffled vessels where baffles are placed within a bubble column to provide the distinct flow channels of the loop. Concentric tube ALRs and split vessel ALRs are examples of IL-ALRs. External-loop ALRs (EL-ALR) are constructed which separate conduits attached at the top and bottom by of are horizontal conduits to form the circulation loop. One fundamental difference between the EL-ALRs and IL-ALRs is the design of the gasIn IL-ALRs, the gas-liquid separator is usually liquid separator. simply an unbaffled extension above the riser and downcomer, allowing for little gas disengagement. In the EL-ALR the gas-liquid separator has a clear region of horizontal flow, either a closed horizontal connection pipe or open reservoir between the riser and downcomer conduits, which allows for either partial or total gas disengagement. The subsequent differences in the operating behaviour of EL-ALRs and ILALRs implies the important role played by the gas-liquid separator in overall reactor performance. However, except for a few mixing studies 1983; Weiland, 1984; Verlaan et al. 1986a; Merchuk (Fields and Slater, and Yunger, 1990) and hydrodynamic and mass transfer studies (Siegel et Siegel and Merchuk, 1991) the influence of the gasal. 1986; 1988, liquid separator has been largely ignored. will profoundly influence the overall The extent of gas disengagement behaviour of ALRs as can be clearly seen by comparing gas holdup, liquid and mass transfer in EL-ALRs and IL-ALRs. Since circulation velocity, the driving force for liquid circulation is the difference in the mean riser and downcomer hydrostatic pressure between the density or EL-ALRs with near total gas disengagement in the horizontal sections, will have much greater liquid circulation gas-liquid separator The consequences of this velocities. due to higher riser liquid velocity It follows that 1983; Merchuk, 1986). ALRs than IL-ALRs due to lower riser results in less gas-liquid interfacial
is reduced gas holdup in the riser (Belle, 1981; Onken and Weiland, mass transfer is also less in ELand downcomer gas holdups which area for mass transfer.
The modelling and scale-up of ALRs is complicated by the interaction of liquid velocity, and principle parameters of holdup and the gas After the initial consequently mass transfer and mixing intensity. rate is usually the are set the influent gas flow geometric parameters At any specific gas flow rate (and sole adjustable operation variable. for a given physio-chemical composition of the fermentation broth) there will be a given AsG between the riser and downcomer which will dictate In order to change the the liquid circulation velocity around the loop. in the ALR the influent gas flow rate must be mass transfer or mixing velocity. changed resulting in a new A&, and liquid circulation Alternatively, in EL--ALRs, a low-pressure-drop throttling valve may be installed in one of the horizontal conduits linking the riser and the downcomer the liquid velbcity independently of the gas to regulate All the geometric factors sparging rate (Popovi& and Robinson, 1987). which influence the pressure drop around the loop (e.g. reactor height, cross-sectional areas ratio of downcomer/riser aspect ratio, (A,/&) r subsequently base diameter, and gas-liquid separator configuration) spite of many design influence ALR operation. Therefore, in the correlations in the literature for liquid circulation velocity, gas holdup, mixing, and mass transfer, are as yet no general there
correlations which are applicable literature in the are type substantially larger data base scale ALRs correlations
during actual be available.
Advantaqes
of
Airlift
The current correlations to all ALRs. until a specific. Not and system of transport phenomena data on large
fermentations
is
established
will
generalized
Reactors
Pneumatically agitated reactors, several advantages over STRs.
such Their
as
ALRs
simple
and
bubble
construction
columns, is
a
offer primary
3218
M.
H. SIEGEL
and C.
W. ROBINSON
K3
advantage. Since there are no moving mechanical parts needed for agitation, there is a reduced danger of contamination through seals or the need for complicated shaft bearings, seals or magnetically-driven agitators. The vertical orientation of these reactors, as well as the lack of internals, facilitate easier cleaning and sterilization. The injected gas serves the dual function of aeration and agitation. This promotes efficiency in the overall energy balance, eliminating the need for a separate expenditure of energy for agitation. When compared with bubble columns, ALRs have the additional advantages of loop reactors, such as increased heat and mass transfer capacity and a reduction of energy consumption for mixing. Heat removal and control is improved by the slender reactor configuration, providing a favourable wall-area-to-volume ratio, as well as high liquid velocity and turbulence at the heat exchange contact surface (Blenke, 1979). In biological processes, the primary advantage of ALRs over bubble columns and STRs is related to the shear stress imposed by the turbulent field on the cells or pellets suspended in the medium. One of the most important aspects of the flow in ALRs is the homogeneous field of shear, which is relatively constant throughout the reactor. In ALRs, fluid motion is induced by differences in the mean densities between the riser and downcomer sections of the reactor. Therefore, there is an overall directionality of liquid flow, even if random movements may be superimposed on it. In contrast, in bubble columns and STRs the energy source inducing fluid motion is focal. The shear forces in bubble columns will be greatest adjacent to the gas sparger and dissipate with distance from the sparger. In STRs, a region of very high shear exists near the agitator, which decreases with increasing distance from the agitator. The lack of uniformity in the field of shear in bubble columns and STRs exposes the organisms to varying shear stresses and environments as they pass through the reactor which can adversely affect the organism. Numerous studies have been conducted investigating the shear effects on microorganisms and cells in an effort to quantify the level of shear various organisms will tolerate (as reviewed by Merchuk, 1991). ALRs have many potential applications in chemical as well as biological processes as either twoor In ALRs, three-phase reactors. the fluidization of solids is not a direct consequence of the bubbling of but rather due to the liquid circulation within the reactor. gas, Therefore, these reactors offer the possibility of very simple and highly effective solids fluidization. This indicates a high potential for application in three-phase processes where gas, liquid, and solids must be brought into contact. solid phase may be one of the The reactants or a catalyst. the ALR is an attractive option for Thus, slurry reactors. DESIGN
AND
OPERATING
CHARACTERISTICS
As discussed below, the rate of liquid circulation, expressed as the linear liquid velocity in the riser (vLR = uJE~~) can be shown from an = (E,, - E,,)~~'. For stable energy balance to be proportional to A.E,'*~ unidirectional liquid circulation, E== always must be greater than &co, and obviously the maximum liquid circulation rate is obtained when E,~ = 0, i.e., when in the upper gas all the dispersed gas is removed disengagement application to aerobic section. However, for fermentations, a detrimental effect on the fermentation kinetics may occur if oxygen the cells are zero dissolved exposed to low or concentration. This may occur in a gas-free downcomer (or even one where E,, is relatively small) if the liquid residence time therein is depleted by sufficiently such oxygen is long that the dissolved microbial uptake (respiration) before the liquid reaches the bottom of the downcomer. Particularly in tall loop bioreactors, i.e., one of industrial scale, it may be necessary to either provide for gas carry-
K3
Applications of airlift gas--liquid-solid
reactors in biotechnology
over from the riser into the downcomer, or to separately the downcomer (at a point above the bottom) such that maximum potential liquid circulation rate may not practice.
3219
sparge gas Thus, E,, > 0. be achievable
into the in
While in the first instance the liquid circulation rate is an effect (i.e., a dependent variable), it in turn becomes the cause of the gas holdup, mass transfer and liquid-phase mixing characteristics exhibited by loop bioreactors compared to bubble columns in which macroscale for optimum design and scaleliquid circulation is absent. Therefore, it is essential to be able to predict the liquid up of loop bioreactors, (usually in terms of the superficial liquid velocity circulation rate in riser, liquid physicochemical properties the phase the ULR ) I non-Newtonian Newtonian vs. (viscosity, interfacial tension, density; rheological and the bioreactor geometry (dispersion characteristics) height, H,; downcomer-to-riser cross-sectional area ratio, A,/A,; the type of gas disengagement section, i.e., the geometry of the riserdowncomer interconnections which influence the frictional resistance to the circulating liquid flow). Thus, it is not surprising that there is no correlation of uLR (or J&, bioreactor which is external-loop etc.) applicable to all %.a, fluid all fermentation systems (i.e., geometries for types of several correlations which can serve as useful However, properties). guidelines for design and scale-up are summarized and briefly discussed in the following sections_ Circulatinq
Liquid
Velocitv
Numerous investigators have developed Newtonian, Low Viscosity Systems. models of which demonstrate the theoretical the fluid hydrodynamics circulation pressure drop, and relationship between the gas hold-up, liquid velocity in the different types of airlift reactors (de Nevers, Freedman and Davidson, 1969; Chakravarty et al., 1974; Kubota et 1968; al., 1978; Merchuk and Stein, 1981; Jones, Hsu and Dudukovic, 1980; et al., 1984; Koide et al., 1984; Verlaan et al., 1986: Lee 1985; Fan 1990; 1987; Calvo, 1989; Joshi et al., et al., Chisti et al., 1988; These models were derived using either an energy Young et al., 1991). balance on the gas input due to the isothermal expansion of the sparged and/or a steady state mechanical macroscopic momentum balances, gas, energy balance, usually in conjunction with the Zuber and Findlay (1965) drift-flux model. total A key parameter in these models is the frictional resistance to fluid flow in the circulating liquid loop, Newtonian including the flow reversals at the top and bottom. For systems, friction coefficients (K,) have been estimated by applying standard relationships for single-phase flow in pipes and bends (Verlaan et al., 1986), two-phase flow (Joshi et al., 1990), or by treating K, as estimated for a particular airlift design by an adjustable parameter While all these models have fit data fitting (Chisti et al., 1988). they have some of the published experimental data to varying degrees, the limitation of empiricism for friction factors, slip velocities, and/or gas which present overall recirculation rates. Also, models circulation well defined loss sometimes on very coefficients, not systems, do not consider the effect of gas-liquid separator design with its consequent effect on Furthermore, experimental gas holdup. verification of these models generally has been restricted to benchor pilot-scale airlifts, and to the air-water systems. Belle liquid
et al. (1984) velocity in
derived the riser V LR
However,
they
did
not
estimate
the
=
following
(3%
KfT,
%,
relat
ionship
for
the
(1)
1 Km)1'3
rather
they
1.inear
correlated
their
air-
M. H.
3220
water
data
empirically
SIEGEL
according V LR
=
and C. W. ROBINSON
K3
to
(2)
a,(A, / A,jb' uG~R/~
where for both internaland external-loop contactors b, - 0.75. For the external-loop case, a, = 1.55 and for the internal-loop a, = 0.66 (m/sj2". Comparison of the values of a, for the two different types of designs shows that the circulating liquid velocity in the external-loop was more than double that in the internal-loop contactor contactor (concentric cylinder draft tube). This reflects the fact that bubble disengagement from the upflowing gas-liquid dispersion at the top of the riser in the former apparatus is considerably more extensive, such that (external loop) < E,, (internal loop). It also should be noted that E i: the internal loop case, a considerable fraction of the total gas holdup in the downcomer was comprised of motionless or near-motionless bubbles, i.e., having a buoyancy force equal to the opposing drag force exerted by the downflowing liquid. these stagnant bubbles In practice, would contribute little or nothing to the gas-liquid oxygen transfer in the overall contactor, process due to eventual depletion or neardepletion of their oxygen content. The model of Verlaan et al. (1986) was developed for the case of &GR 5 On this basis, 0.10 and for E,, = 0. they were able to estimate KLT using as previously had been only single-phase (liquid) flow relationships, Wallis Application of their required an shown by (1969). model interactive calculation procedure from which for specified contactor geometry and gas sparging rate the hydrodynamic parameters uLR, ecR and The two-phase drift-flux model of Zuber and E,, could be predicted. Flndlay (1965) was used in the gas-holdup estimation to account for the effective slip velocity between the bubbles and the liquid which arises from the non-uniformity of the holdup and velocity profiles in the Using two different sizes of external-loop airlifts, radial direction. they obtained good agreement between predicted and experimental values For the smaller contactor (V = 0.165 m', H = 3.23 m, K,, = 1.8), of ULD. for 0.005 1. uGR 5 0.17 m/s; in the they observed 0.14 5 u,, _< 2.1 m/s larger vessel (V = 0.6 ma, H = 10.5 m, K,, = 4.75), 0.1 < uLR 1.8 m/s for u,,(V = 0.6;;;3)/u,,(V = 0.165 0.005 F u,, < 0.075. m/s. At UC;, = 0.05 m/s, whereas from Equation (1) the corresponding ratio of = 1.067; m3) et al. Although the two airlifts used by Verlaan (H,,'K,,)= = 1.073. (gas disengagements) (1986) had different designs for the top sections the agreement with the interconnecting the risers and the downcomers, prediction from Equation (1) suggests that on scale-up the effect of be accounted for by (HD/KfT)1'3. increased height on uIR may reasonably Bello (1981) Chisti et al. (1988) extended the modelling of and low-viscosity liquids, i.e., they considered only the case of Newtonian, could ignore the negligibly small contribution of wall friction losses Only the losses in the top and bottom sections arising from the to K:,. change on fluid flow direction were considered; furthermore, in the case the top and bottom flow reversal losses of external-loop contactors, were considered to be equal. For these contactors, they then obtained ULF7
= {2gH,
(A&,)
/Kfs[
(1-E,,)2)
+
(AD/A,)'
(l/
(1 -E,,J2)
I}'.'
(3)
(3) requires a priori knowledge of both E,, and Application of Equation Chisti (which are system specific), as well as an estimate of K,,. %D et al. (1988) obtained experimental values of uLR for the air-water contactor (V system in an internal-loop (draft tube; annulus sparged) = 1.46 A pulse (V = 0.234 m3). m3) and a split-cylinder airlift was used to measure vLD injection of concentrated HISO, in the downcomer the by means of two pH electrodes separated by a known distance: Apparently, their measurement method for E,, and E,, was not described. own results were compared to the predictions using Equation (3) by be an adjustable parameter evaluated by data taking K,, to to be
K3
Applications
of airlift
gas-liquid--solid
reactors in biotechnology
3221
regression. They also compared the predictions of Equation (3) to the results of several previous investigators of internaland external-loop airlifts et al., 1986). Observed values of (including those of Verlaan the uLR generally agreed with the predictions from Equation (3) to within 30% for both internaland externalloop airlifts. Joshi et al. recently provided an extensive review of the (1990) hydrodynamics of external-loop reactors. In addition, they developed a model for the steady-state circulating liquid superficial velocity (u,,) by equating in differential form the driving force for liquid circulation resisting (i.e. with the forces (dAP,/dz). EGR %D) Homogeneous gas-liquid flow was assumed in downcomer and the heterogeneous flow in the riser. The model (solved by applying a 6-step was used to analyze the effect of various design iterative procedure) parameters on the hydrodynamic and mass transfer performance for a range air-water of essentially for reactor volumes (10 1. V 3 1000 m3, dispersions. They showed that for the case of sparging gas only in the riser there is only one (stable) steady-state value of uLR. On the other two possible values of hand, when gas is also sparged in the downcomer, They also uLR can exist, only one of which (the higher value) is stable. axial location for the downcomer showed that there was a critical sparger, which depended on the airlift geometry and on the relative If gas were to rates of gas sparging in the downcomer and the riser. no stable liquid circulation could be be sparged above this location, established. As a result of their simulation studies for Newtonian, low viscosity is a possibility systems, Joshi et al. (1990) concluded that 'I... there height to diameter ratio of selecting optimum values of reactor volume, "However, the optimum values are different and area ratio" (i.e. AD/A,). liquid circulation rate, for different design objectives such as overall effective interfacial area, mass transfer rate and extent of mixing. consumption per unit These values also depend upon the level of power dissipated by the isothermal expansion of the ;;:;;:I; ,',ise. the power While an extensive which is directly proportionalto uGR). simulation $tudy of the type performed by Joshi et al. (1990) has not been conducted for external-loop airlift contactors for the case of viscous, non-Newtonian results obtained for the latter, as systems, summarized below, indicate that there is no single optimum condition for non-Newtonian external-loop airlift design and operation with viscous, mycelial microorganisms or fermentation systems, eg. cultures of extracellular polysaccharides gum) fermentations. (e.g. xanthan Young et al. (1991) developed a differential, two-fluid hydrodynamic model for two-phase flow in airlift risers using point equations of continuity and motion. Macroscopic energy balance equations were used to describe flow in the downcomcer and gas-liquid separator. By using a differential separated-flow type model to describe flow in the riser along the length of the they incorporated changes in flow phenomena riser. The model has the advantage that the only empirical parameters They compared are those related to friction effects in the reactor. their own experimental results and those of Merchuk and Stein (1981) to predicted model values and found an agreement of approximately 10%. It should be noted that both the reactors of Merchuk and Stein (1981) and Young et al. (1991) used straightening vanes, in the downcomer entrance of the former to achieve more and the riser of the latter, entrance uniform liquid velocity profiles. Using the air-water system to investigate the performances of three 1.09-1.23 and 1.03relatively large airlift contactors (v = 0.16-0.23, 1.88 m3), Siecrel and Merchuk (1991) hiahliahted the sicnificant effect of the gas disengagement section. Conslderlngthat a gas bubble leaving the riser will completely disengage from the liquid phase in a time period t, (related to the terminal rise velocity of the bubble) and time of the gas-liquid dispersion in the defining the mean residence separator as t,, for a rectangular separator Siegel and Merchuk showed CES
47:13/14-E
3222
M.
H.
SIEGEL
and C.
W.
K3
ROBINSON
that
h/ % = (UU)/Vb~)(AD&Ws) They defined circulating studied was
the term A,/L,W, as the "disengagement liquid velocity in the downcomer of the correlated in the form
(4)
ratio", smallest
DR. The contactor
UIJJ = 0.45(P/V$"*33/DRo.*5
In Equation theoretical
(5), P/V,) prediction
=
uGI, such that of Bello (1981)
uLD - uGR0-33,in and Bello et
(5)
agreement al.
with
the
(1984).
By means of using a movable vertical baffle in the gas-liquid separator, Siegel and Merchuk (1991) were able to vary DR such that in a given split-vessel ALR they could achieve a considerable variation in E,,. At low values of t,, high values of E,, (up to 0.12, depending on uGR) were obtained, producing a uLR similar to that of concentric tube IL-ALRs. At higher values of tz, nearly complete gas disengagement was achieved producing a maximum U,, (similar to that of EL-ALRs). f&G, 5 0.02), A high level of disengagement, or essentially complete even gas disengagement can be achieved in practice with moderately-sized disengagement sections for low viscosity liquids, as in the case of the studies by Siegel and Merchuk However, it is unlikely that (1991). nearly complete removal of all the bubbles exiting at the top of the riser can be achieved in the case of viscous fermentation broths. In these latter systems, the bubble size distribution is binodal. The large bubble fraction likely would disengage fairly readily in the gasliquid separator. However, small fraction, which the bubble can comprise ca. 15 percent of the total gas holdups, likely would not disengage during any practical residence time Studies on the (t2) effect of gas separator conditions for viscous, non-Newtonian systems should be conducted. Viscous, non-Newtonian Systems. Popovic andRobinson (1987, 1988, 1989) made extensive investigations of the hydrodynamic and mass transfer behaviours of external-loop airlift contactors, using pseudoplastic Their solutions of various types of carboxymethyl cellulose (CMC) . airlift contactors also could be operated in the bubble column mode (A,/A,=O) by closing the butterfly valves installed in the top and bottom horizontal piping sections joining the riser and the downcomer. The empirical correlations developed by them are listed in Table 1, and were obtained for churn-turbulent to slug flow dispersion conditions in the riser. liquids As shown by Equation (l.l), Table 1, for non-Newtonian the dependency of uLR on uGa (- u~~"-~*) is nearly identical to that non-viscous, Newtonian predicted and verified by Bello (1981) for liquids, namely uLR - uGR0-33. As expected, uLR decreases with increasing liquid-phase viscosity (- q,fL-".3g). The effective viscosity exerts a moderate On the other hand, ?leff exerts a strong influence on uLR. influence on the mass transfer capabilities of an EL-ALR (Equation 1.3, Table 1). Depending on the actual value of qeff, k,a for viscous, nonNewtonian broths in a specified EL-ALR can be up to lo-fold lower than in the case of non-viscous, Newtonian fermentation broths. K,a also decreases with increasing AD/AR, i.e. increasing uLR in the riser where co-current upward flow of gas and liquid exists (see Equation 1.1, Table Thus, k,a always is greater in the corresponding bubble column 1). (A,/A,=O) than in an EL-ALR.
K3
Applications of airlift gas-liquid-solid
Table
reactors in biotechnology
Hydrodynamic and Mass Parameter Correlations of Robinson for Non-Newtonian Airlifts External-Loop Columns.
1.
3223
Transfer Popovic and Systems in or Bubble
Ref. Year
No.
Circulating
1.1
liquid ULR =
1.2
Riser
gas
Volumetric
k,a, = 1.4
0.005
=
O-02
I
0, rleff
Perforated
0.111, 5
uGR0-65 [l
transfer
uGRO.=
interfacial a =
A,/A,
0.50
plate
u,:-~~
(AD/A,) o-g7 fl,Ct-o-39
1989
0.456
mass
Specific
0.23
1988
holdup
EGR = 1.3
velocity
233
0.25,
[l +
+
(A,/A,)]-1-06 T&~-'-~'~
(A,/A,) J -'*" Hetf DLom5 p,l.03 CS-'." 1987
area
u,,'-'~ [l
+
(A,/A,)]-1.52 f7,ft-0-327
0.444
Pa.s; y = 5000 u=(a) sparger; 0.03 5 u G(R)-< 0.26
APPLICATIONS
1989
coefficient
OF
AIRLIFT
m/s
REACTORS
the application of production-scale ALRs in the At the present time, biochemical industries is limited due to the lingering questions related ALRs are less flexible Furthermore, to the scale-up of these reactors. geometric requirements than STRs. Once the t0 changing process parameters of the ALR have been selected for a given process at the time the gas flow rate is the principle, if not sole, adjustable of design, the ALR is less adaptable to parameter during operation. Therefore, liquid velocities, need different other processes which might gas mixing intensities andmasstransfer characteristics than distributions, conventional STRs where the aeration and agitation can be independently The influence of A,/A, (the ratio of downcomer/riser crosscontrolled. sectional areas) has received much attention as a design parameter that Other geometric parameters significantly affects reactor performance. which have been shown to strongly influence ALR performance are gas(Siegel and Merchuk, 1991) and bottom liquid separator configuration While these parameters have been shown clearance (Ladwa et al. 1988). and pilot-scale reactors using air-water to be adjustable in faboratorythey remain largely untested during actual systems and synthetic media, process operations and on production-scale reactors. The applications of ALRs have been reviewed previously by Onken and Margaritis and Wallace (1984), Smart (1984) and Siegel Weiland (1983), and pilot-scale applications of et al. (1988). Recently, many benchALRs have been studied for a variety of microorganism and cell cultures. Many of these studies have been conducted on small bench-scale ALRs focusing on growth kinetics rather than transport phenomena during the fermentation. little work has been conducted to optimize Furthermore, ALR design and operation during actual fermentations. Consequently, little basic hydrodynamic andmass transfer information is available for optimum reactor design and scale-up. What follows is a brief synopsis of some of the more recent works discussing the application of ALRs.
M. H. SIEGEL and C. W. ROBINSON
3224 Frdhlich
EC3
al. (1991, 1991a) examined the cultivation of Saccharomyces m3 in a 4 m3 concentric tube pilot plant ALR and a 0.08 They laboratory ALR using both batch and continuous modes of operation. examined the global axial mixing (Frijhlich et al. 19911, and local gas bubble diameter and bubble velocity (Frohlich et al. 1991a) holdup, during the yeast cultivation and in model media. This work represents the first report of the measurement of local properties of the dispersed gas phase in a pilot-scale reactor during an actual fermentation. It was found that the local gas holdup, bubble size and the bubble velocity changed only slightly along the length of the riser. et
cerevisiae
Suh et al. (1992) conducted a comparative study of a 0.05 m3 bubble column and a 1.2 m3 concentric tube ALR for the production of xanthan gum with Xanthomonas campestris in synthetic media. They studied the and mass transfer, as well as xanthan productivity and hydrodynamics quality during the fermentation. The ALR did not perform as well as the bubble column in terms of either mass transfer or xanthan productivity and quality. This was attributed to a lack of oxygen supply in the downcomer, with an high residence accompanying time due to low circulation velocity (especially at higher xanthan concentrations). It should be noted that the ALR used in this study had a AD/A, of ca. 1. Using an ALR with AD/A, < 1 would help reduce residence time in the downcomer. Several of the studies summarized in Table 2. Table
2.
Organism Reactors
Organism
Suspended
butyricum
Sorangium
cellulosum
Recombinant Escherichia
Triqonella
Growth
focused
Studies
Reactor
Volume
Tm?e
(m’)
on
growth
kinetics
are
Airlift
in
Reference
CT
0.05 1.50
Giinzel
CT
0.04 0.05
Hopf
CT
0.06
Kracke-Helm (1991)
CT
0.009
Bonnarme (1990)
CT
0.0028
Stasinopoulos & Seviour (1992)
CT
0.009
Rodriguez-Mendiola (1991)
Coli
Phanerochaete chysosporium
Attached
have
Cultures
Clostridium
Acremonium
which
persicinum
et
et
(1991)
al.
al.
(1990) et
al.
& Jeffries
Cultures foenum-graceum
Immobilized
Cell
Penicillium
chrysogenum
CT
0.250 0.320
Keshavarz (1990)
Nitrobacter
agilis
EL
0.0024 0.0034
Wijffels
et
al.
(1990)
mesenteroides
CT
0.0025
El-Say-ed
et
al.
(1990)
Leuconostoc Bacterial Thiobacillus
Cultures et
al.
Leachinq ferrooxidans
CT
Couillard (1991)
& Mercier
K3
Applications of airlift gas-liquid-solid
reactors in biotechnology
3225
Recent studies have also been conducted examining ALRs as potential reactors for treating various wastes. Hiippe et al. (1990) used a twoeffluents from a coal tar stage pilot plant to biologically treat refinery. The first stage was a modified concentric tube ALR (V = 1.5 coal dust entered the process. The effluent from the first m') where stage passed through sedimentation and crossflow filtration units before entering the second stage. The second stage was a 0.160 m3 EL-ALR with the biomass immobilized on sand particles. Aromatic substances which passed through the first stage were subsequently eliminated by the immobilized biomass in the second stage. Tyagi et al. (1990) used a 0.023 m3 laboratory-scale EL-ALR and a 1.15 m3 pilot-scale EL-ALR to mesophilic and thermophilic aerobic digestion of primary and study secondary municipal sludges. The pilot ALR, using air for aeration, gave comparable results to conventional aerobic sludge digestion systems which use pure oxygen for aeration. the pilot ALR was able to Also, achieve thermophilic (53°C) by means of autothermal temperatures A cost analysis showed that the autoheated ALR digester would heating. represent a significant savings in both capital and operating costs compared to a conventional two-stage aerobic digester using pure oxygen to achieve the required oxygen transfer rate. Several recent studies have indicated that hydrodynamic damage may occur mammalian and insect to cells with a high sensitivity to shear (e.g. (Tramper et al. 1986, 1988; cells) due to bubble-cell interactions Handa-Corrigan et al. 1989; Kunas and Papoutsakis, 1990; Jijbses et al. Bubble breakup appears 1991; Bavarian et al. 1991; Papoutsakis, 1991). related to gas to be the primary mechanism involved in cell damage has not been examined in ALRs sparging. The effect of this phenomena per se, however, studies with bubble columns (Tramper et al. 1988; Handa-Corrigan et al. 1989) indicate that greater aspect ratios (heightincrease the retention of cell to-diameter ratio) and reactor heights viability. Further studies are needed of this phenomena in ALRs, as well as potential methods of manipulating the design of the gas-liquid sensitive cells in a degassed separator and/or entrapping the shear with downcomer to minimize bubble-cell the damage associated interactions. RECOMMENDATIONS Before universal design and operation correlations can be developed for ALR design and operation the following information must be obtained: influence of the gas-liquid 1. More information is needed on the separator on ALR behaviour. The hydrodynamic regime and flow patterns at the exit and at the of the riser, in the gas-liquid separator, The flow patterns in these entrance of the downcomer must be examined. allow the regions understanding will are very complex their and and dispersion coefficients. A development of local friction factors better understanding of this region would also allow better manipulation and flexibility of the ALR during operation. 2. Abetter understanding is needed of hydrodynamic flow regimes, flow regime transitions, as well as bubble coalescence and redispersion in all the different sections of the reactor. 3. The bulk of the current data for friction factors, mixing, andmass Much of this transfer has been presented in terms of a global approach. Methods and data has limited utility for reactor design and scale-up. techniques to measure local sectional coefficients must be developed. A method of determining coefficients has been local mass transfer no method of measuring presented (Merchuk and Siegel, 1988). However, local k,a values, reliable data for local dispersion nor sufficient coefficients currently exists. 4. More information is needed on the contribution of the downcomer to airlift reactor behaviour. This is especially true with regard to twosparger systems where supplementary gas is injected into the downcomer. 5. In general, data for heat transfer, gas dispersion, and liquid mixing in essentially twoand lacking or three-phase ALRs is unavailable. development of These data for the are essential
M. H. SIEGEL
3226
K3
and C. W. ROBINSON
sophisticated design and scale-up .models. 6. Much more work is needed on the scale-up of ALRs statements can be made about the scale-up process. where kinetic energy input from the short reactors, gas sparger design will be significant, geometrically may not give similar results during scale-up.
before conclusive For example, in gas injection and similar designs
This represents a partial list of the areas of knowledge where gaps still need to be filled. However, the greatest gap in ALR knowledge which still exists is a thorough understanding of the transport phenomena in reactors fermentation these under actual conditions. available data for ALRs is for water or Almost all the currently synthetic simulation media. These data must be checked against actual process conditions.
NOMENCLATURE J-k
uG VL
K, HD
A K AP, n H k,a z g al b, V Vbt, t2 L
WS
P D, Greek Y : eff P 0 2
superficial liquid velocity, m/s superficial gas velocity, m/s linear liquid velocity, m/s friction coefficient for fluid flow, dispersion height, m cross-sectional area, m2 consistency factor, power law rheological model, Pa-s" frictional pressure drop, Pa.s" flow behaviour index, power law rheological model, height, m volumetric mass transfer coefficient for oxygen, s-l axial coordinate position acceleration of gravity, m/s2 proportionality constant in Equation (l), (m/s>2'3 exponent in Equation (11, volume, m3 bubble terminal rise velocity, m/s time required for complete bubble disengagement in the gas-liquid separator section, s mean hydraulic residence time in the gas-liquid separator, s characteristic length of flow path on the gas-liquid the separator, m width of the gas-liquid separator, m aeration power (isothermal gas expansion), kW liquid-phase molecular diffusivity, m'/s Letters shear rate, s-l fractional gas holdup, effective viscosity of pseudoplastic density, kg/m3 interfacial tension, N/m shear stress, N/m'
Subscripts d D R G L T B
dispersion downcomer riser gas phase liquid phase total bottom section
of
airlift
liquid,
Pa.s
Applications
Ia
of airlift
gas--liquid-solid
reactors in biotechnology
3227
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