Bubble-column and airlift photobioreactors for algal culture

16 Pages·2000·0.31 MB·English

Bubble-Column and Airlift Photobioreactors for
Algal Culture
AsterioS´anchezMir´on,FranciscoGarc´ıaCamacho, AntonioContrerasG´omez,EmilioMolinaGrima,
andYusufChisti
Dept.ofChemicalEngineering,UniversityofAlmer´ıa,E-04071Almer´ıa,Spain
Bubblecolumnsandairlift photobioreactorscan be useful for culturingphototrophic
organisms requiring light as a nutrient. Light a¤ailabilityin bubble columns and airlift
de¤ices is influenced by aeration rate, gas holdup, and the liquid ¤elocity (mixing and
turbulence). The photosynthetically generated oxygen also needs to be remo¤ed, as ex-
cessi¤edissol¤edoxygensuppressesphotosynthesis.Oxygenremo¤alcapacityisgo¤erned
by the magnitude of the o¤erall gas(cid:93)liquid mass-transfer coefficient, k a . This work
L L
characterizes the rele¤ant hydrodynamic and mass-transfer parameters in three air-
agitated reactors: bubble column, split-cylinder airlift de¤ice and concentric draft-tube
sparged airlift ¤essel. The reactors are then e¤aluated for culture of the microalga
Phaeodactylum tricornutum. All reactors were about 0.06 m3 in working ¤olume, and
the working aspect ratio was about 10. Data were obtainedin tap water for a base-line
comparisonand in Mediterraneanseawater, as a potentialmediumfor algal culture. A
theoreticalrelationshipwas de¤elopedand pro¤edbetween k a and the aerationrate.
L L
In addition,a methodbasedon mechanisticrelationshipswas pro¤edfor predictingthe
liquidcirculation¤elocityandk a in airlift reactors. Existingcorrelationsappliedsat-
L L
isfactorilyto gas holdupandk a dataobtainedin the bubblecolumn. Aqueoussolu-
L L
tion of sodium chloride (0.15 M) closely resembled seawater in terms of its hydrody-
namics and oxygen transfer beha¤ior. Under the conditions tested, all three reactors
attained a biomass concentrationof about 4 kg(cid:63)my3 after(cid:59)260 h. The mean maxi-
mumspecificgrowth rate was 0.022 hy1 in all casesat a powerinputof 109 W(cid:63)my3.
Introduction
Airlift and bubble-column bioreactors are simple devices sential nutrient for phototrophic culture and the need for
that have gained wide acceptance in gas(cid:93)liquid contacting sufficient illumination significantly affects the design of an
applications in bioprocessing, the chemical process industry, outdoor culture facility (cid:14)Tredici, 1999; S´anchez Mir´on et al.,
andtreatmentofwastewater. Substantial knowledgeexistson 1999.. Formostcommercialprocessing, outdoorillumination
gas(cid:93)liquid hydrodynamics and mass transfer in bubble (cid:14)sunlight. appears to be the only viable option.
columnsandairlift bioreactors, ascomprehensivelydiscussed Atpresentbubblecolumnsandairliftreactorsarenotused
in majortreatise (cid:14)Chisti and Moo-Young,1987; Chisti, 1989, as photobioreactors, except for investigational purposes;
1998, 1999a, b; Deckwer, 1992; Joshi et al., 1990; Merchuk however, because of the significant potential advantages of
andGluz,1999..Withfewexceptions(cid:14)Contrerasetal.,1998a; thesesystems(cid:14)S´anchezMir´onetal.,1999.relativetoconven-
Garc´ıa Camacho et al., 1999; Matthijs et al., 1996; S´achez tional tubular loopsolarharvesters (cid:14)Tredici, 1999.,there isa
Mir´onetal., 1999;Silvaetal., 1987;Suzukietal., 1995.,ear- needtofurtherdeveloptheairliftandbubble-columndevices
lierworkwiththesereactorsfocusedonnonphototrophicap- as photobioreactors. Such systems have already shown
plications. Unlike in conventional bioreactors, light is an es- promising performance in outdoor culture of microalgae.
Data suggest that a single vertical tubular photobioreactor
(cid:14)bubble-column or airlift design. cannot exceed about 0.2 m
in diameter or light availability will be reduced severely
CorrespondenceconcerningthisarticleshouldbeaddressedtoY.Chisti.
1872 September2000 Vol.46,No.9 AIChEJournal
(cid:14)S´anchez Mir´onetal., 1999..Inaddition, the height ofasin- Substantial experimental evidence affirms that
gle device is limited to about 4 m for structural reasons and
to reduce mutual shading of reactors in a multicolumn facil- k
ity that would be necessary for any commercial-scale opera- Lsconstantsz, (cid:14)3.
d
tion (cid:14)S´anchez Mir´on et al., 1999.. B
Further restrictions on acceptable aeration rate are posed
by considerations of shear sensitivity (cid:14)Chisti, 1999b; Contr- irrespective of the flow regime and the type of fluid (cid:14)Chisti
eras et al., 1998a; Silva et al., 1987. and light penetration andMoo-Young,1987;Chisti,1989,1998..Inaddition,based
(cid:14)S´anchez Mir´onetal.,1999..Acertain minimalaerationrate ontheory, the gasholdup isnecessarily related (cid:14)Chisti, 1989.
is essential so that the cells do not stagnate for long in the withthesuperficial gasvelocity U andthemeanbubblerise
G
dimly lit interior of the reactor (cid:14)S´anchez Mir´on et al., 1999.. velocity U , as follows:
b
At the same time, there is an upper limit on the acceptable
level of turbulence, because hydrodynamic forces affect cer- U
tain algal cells, as reviewed recently (cid:14)Chisti, 1999b.. Also, in (cid:101)s G. (cid:14)4.
U
seawater, excessively high aeration rates generate persistent b
microbubbles that accumulate over time, thus reducing light
penetration over time even at a fixed aeration rate (cid:14)S´anchez Consequently, Eq. 2 can be expressed as
Mir´on et al., 1999.. In addition to mixing the culture, aera-
tionaidsinremovingthephotosynthetically producedoxygen 6zU 6zU
k a s G s G . (cid:14)5.
from the broth. Accumulation of oxygen inhibits photosyn- L L (cid:15) U / U yU
thesis. Similarly, good gas(cid:93)liquid mass transfer is necessary U 1y G b G
b U
for efficient transfer of carbon dioxide that is the carbon b
source in photosynthetic cultures.
This article evaluates and compares airlift and bubble-col- Equation 5 is obtained by substitution of Eq. 3 and Eq. 4 in
umndevices,mainlyintermsofhydrodynamicsandtransport Eq.2.Foragivenfluidandflowregime,themeanvelocityof
phenomena, in anticipation of a more extensive use of these bubble rise depends (cid:14)Clift et al., 1978. only on the diameter
systems in producing microalgae. The focus is on fractional ofthe bubble,
gas holdup, liquid circulation velocity, and the overall
gas(cid:93)liquidoxygenmass-transfer coefficientandtheinterrela- U sf(cid:14)d .. (cid:14)6.
b B
tionships among these variables in regimes relevant to algal
culture. Data are alsoreported onaculture ofthe microalga
For otherwise fixed conditions, the bubble size is controlled
Phaeodactylumtricornutum,whichisapotentialsourceofcer-
bythespecificenergyinput E inareactor(cid:14)Chisti,1989.and,
tain omega-3 polyunsaturated fatty acids of therapeutic sig-
for a bubble column, we have
nificance.
d AEkA(cid:14)gU .k. (cid:14)7.
Theoretical Developments B G
This section details the development ofanoveltheoretical The exponent k is usually around y0.4 (cid:14)Bhavaraju et al.,
equation that links the overall volumetric gas(cid:93)liquid mass-
1978; Calderbank, 1958.. Thus, Eq. 5 can be written as fol-
transfer coefficient k a with gasholdup and the superficial
L L lows:
aeration velocity, orthe principal operational variable in air-
lift and bubble-column reactors. The experimental data are
6zU
discussed laterintermsofthetheoretically derivedequation. k a s G , (cid:14)8.
In a batch bubble column, the specific gas(cid:93)liquid interfa- L L cUGkyUG
cial area a , the overall gas holdup (cid:101), and the mean bubble
L
diameter d are related (cid:14)Calderbank, 1958; Chisti, 1989. by or
B
the equation:
6z
k a s . (cid:14)9.
L L cUyy1
6(cid:101) G
a s . (cid:14)1.
L d (cid:14)1y(cid:101).
B The parameter c is close to unity in the bubble flow regime;
thus,
Equation 1 is based on fundamental principles, as discussed
(cid:70)
in depth elsewhere (cid:14)Chisti, 1989.. Multiplying both sides of k a s , (cid:14)10.
Eq.1bythe mass-transfer coefficient k produces the equa- L L Uyy1
L G
tion
where (cid:70)s6z. Equation 10 is dimensionally consistent when
6k (cid:101) the product cUGy is taken to be dimensionless; that is, c has
k a s L . (cid:14)2. the units myysy. The parameters c and y may take other
L L d (cid:14)1y(cid:101).
B values, depending on the fluid and the flow regime.
AIChE Journal September2000 Vol.46,No.9 1873
Figure 1. Reactors: (a) vessel dimensions and air sparger details; (b) location of dissolved oxygen (DO) and pH
electrodes.
Alldimensionsinmm.
1874 September2000 Vol.46,No.9 AIChEJournal
Equation 10 is derived for bubble columns, but a similar superficialgasvelocitybasedontheentirecross-sectionalarea
relationshipcanbeshowntoholdforairliftbioreactors.Thus, of the reactor tube. The specific energy input per unit mass
the overall volumetric gas(cid:93)liquid mass-transfer coefficient was obtained with the equation:
k a in an airlift device consists ofcontributions ofthe riser
L L
and the downcomer zones (cid:14)Chisti, 1989, 1998., as follows: P
Es G sgU . (cid:14)15.
(cid:114)V G
A (cid:14)k a . qA (cid:14)k a . L L
k a s r L L r d L L d (cid:14)11.
L L ArqAd All measurements were at 22(cid:34)2(cid:56)C.
where A and A are the cross-sectional areas of the riser
r d Gas holdup
andthedowncomerzones,respectively. Thesubscripts r and
d denote the riser and the downcomer zones. Even when (cid:101) The overall gas holdup in airlift reactors was measured by
d
(cid:47)0, (cid:14)k a . (cid:60)(cid:14)k a . (cid:14)Chisti, 1989, 1998. and, generally, the volume expansion method. Inverted manometers were
L L d L L r
A FA . Consequently, in an airlift device, used tomeasure theseparate holdup valuesfortheriser and
d r
the downcomer zones. The overall holdup in the bubble col-
umn was measured manometrically. Both the volume expan-
A (cid:14)k a .
k a f r L L r. (cid:14)12. sionandthemanometricmethodshavebeendescribedinde-
L L ArqAd tail and used widely (cid:14)Chisti, 1989.. The holdup was calcu-
lated using the equation:
Now,following the logicofEqs. 2(cid:93)9, we obtain
(cid:68)h
(cid:70) (cid:101)s m (cid:14)16.
k a s a , (cid:14)13. ht
L L Uyy1
G
where h istheverticaldistancebetweenthemanometertaps,
t
where (cid:70) s6zAr(cid:14)A qA .. Note that in an airlift reactor, and (cid:68)h is the manometer reading. The location of the
a r r d m
U is the bubble rise velocity relative to the liquid and not manometertapsisshowninFigure1forthevariousreactors.
b
relative to wall of reactor. Equations 10 and 13 are used to
interpret the kLaL data obtained in this work. Liquid circulation 'elocity
Asmallamountofconcentrated sulfuric acidwasaddedto
Materials and Methods
the reactor to reduce the pH to around 4, and the reactor
Reactors and fluids was bubbled with air (cid:14)U (cid:59)0.02 m(cid:63)sy1. for at least 30 min
G
Measurements were made in a bubble column, a split-cyl- prior to measurements. This removed most of the buffering
inder airlift device, and a concentric draft-tube airlift vessel due to dissolved carbonates and bicarbonates. The pH was
sparged in the draft tube. All vessels were made of 3.3-mm- raisedtoaroundpH5byaddingsodiumhydroxide(cid:14)12M..A
thick transparent poly(cid:14)methyl methyacrylate., except for the measuredamountofconcentratedacidtracerwasnowadded
lower 0.25-m sections, which were made of stainless steel to the reactor instantaneously. Additions were made on the
(cid:14)Figure 1.. The vessels were 0.193 m in internal diameter. surface ofthedispersion atthecenterofthevesselcrosssec-
The riser-to-downcomer cross-sectional area ratio was unity tion.Thetracersignalwasfollowedbytwoidentical pHelec-
forthe split cylinder and 1.24forthe draft-tube airlift vessel. trodes positioned in the downcomer, as shown in Figure 1.
The internal diameter of the draft tube was 0.144 m. The Theplacement ofelectrodes attempted tominimizeentrance
drafttubeandthebafflewerelocated0.091and0.096mfrom effects, but maintained a sufficient vertical distance between
the bottoms of the reactors, respectively. The gas-free liquid the probes so that measurement inaccuracies were mini-
height was about 2 m in all cases. Other geometric details, mized. ThesignalfromthepHelectrodes wasexpressed asa
including those of the air spargers for the various reactors, normalized concentration of the Hq ion and plotted against
are noted in Figure 1. Tap water and Mediterranean seawa- time(cid:14)Figure 2..The dimensionless normalized concentration
terwere theliquids used. Theseawater contained about36.6 was defined as
kg(cid:63)my3 total dissolved solids, almost all as inorganic salts.
Theionicstrengthofseawater, estimatedusingtheLangelier wHqxywHqx
equation (cid:14)Snoeyink and Jenkins, 1980., was 0.92. wHqxNormalizedswHqx(cid:96)ywHqxoo, (cid:14)17.
In all cases, the specific power input in the reactors was
calculated (cid:14)Chisti, 1989; Chisti and Moo-Young, 1987, 1989.
where wHqx is the instantaneous molar concentration and
using the equation:
subscripts o and (cid:96) denote initial and final equilibrium val-
ues, respectively.
P
Gs(cid:114) gU , (cid:14)14. Thetracerresponsesignaldisplayed thedampenedoscilla-
VL L G torypatternthatischaracteristicofairliftreactors(cid:14)Chistiand
Moo-Young,1987;Chisti, 1989..Themeanlinear flowveloc-
where P is the power input due to aeration, V is the cul- ityintheriser(cid:93)downcomerloopwascalculatedfromthetime
G L
turevolume, g isthegravitationalacceleration,andU isthe interval between adjacent tracer peaks (cid:14)Figure 2. of a given
G
AIChE Journal September2000 Vol.46,No.9 1875
V qV U (cid:15) 1 1 /
V s Lr Lds Lr q . (cid:14)22.
Loop 2 2 (cid:14)1y(cid:101). (cid:14)1y(cid:101).
r d
Gas–liquid mass-transfer coefficient
The well-known dynamic gassing-in and gassing-out meth-
ods (cid:14)Chisti, 1989, 1999a. were used to measure the k a .
L L
Two independent measurements were made simultaneously
usingtwodissolvedoxygen(cid:14)DO.electrodes, locatedasnoted
in Figure 1. Both probes provided identical k a values,
L L
henceconfirmingtheassumedwell-mixed liquidphase. Mea-
surements were made during absorption and desorption of
oxygen. For absorption, the fluid was deaerated by bubbling
withnitrogenuntiltheDOconcentrationhaddeclined tobe-
low5%ofairsaturation. Thenitrogenflowwasthenstopped
Figure 2. Normalizedtracerconcentrationprofilesfrom
and the bubbles were allowed to leave the liquid. A preset
the upperandlower pHelectrodes.
flowofairwasnowestablished, andtheincrease inDOcon-
Timeintervalsusedinestimatingthemeanloopvelocityand
centration was followed with time until the concentration
thelinearvelocityinthedowncomerareshown.
reached almost 100% of the air saturation value. The k a
L L
was calculated as the slope ofthe linear equation:
pHelectrode andthegeometry-determined lengthofthecir-
culation path. The linear liquid velocity V in the down-
comerwascalculated fromthetimeintervalLbdetweenthecor- (cid:15)C(cid:85)yC /
ln o sk a (cid:14)tyt ., (cid:14)23.
responding peaks (cid:14)such as the second peak. of the two elec- C(cid:85)yC L L o
trodes (cid:14)Figure 2. and the known vertical distance between
them.
The measured linear velocity V in the downcomer was
Ld where C(cid:85) is the saturation concentration of DO, C is the
related(cid:14)Chisti,1989.tothesuperficial velocityintheriserby o
initial concentration of DO at time t when a hydrodynamic
the continuity relationship: o
steady state has been reestablished upon commencement of
aeration,andC istheDOconcentrationatanytime t(cid:14)Chisti,
U A sV A (cid:14)1y(cid:101).sV A (cid:14)1y(cid:101)., (cid:14)18.
Lr r Lr r r Ld d d 1989, 1999a.. Both the absorption and desorption methods
gave identical values of k a under identical hydrodynamic
where V is the linear liquid velocity in the riser. In the L L
Lr conditions. Only the k a values measured during oxygen
split-cylinder reactor,becausethecross-sectionalareasofthe L L
absorption are reported here.
riser and downcomer zones were identical, that is, A sA ,
r d
Eq. 18 simplified to
Algal culture
U sV (cid:14)1y(cid:101).sV (cid:14)1y(cid:101).. (cid:14)19.
Lr Lr r Ld d
Phaeodactylum tricornutum UTEX 640 was the microalga
used. The culture was obtained from the collection of the
Because thesuperficial liquidvelocity U inthedowncomer
isV (cid:14)1y(cid:101).,U andU valueswereiLddentical, irrespective University of Texas, Austin. The inoculum for the photo-
Ld d Lr Ld bioreactors was grown indoors under artificial light (cid:14)230
of the gas holdup; thus, the mean superficial velocity of the
riser(cid:93)downcomer loop was the same as U . For the draft- (cid:109)E(cid:63)my2sy1 light flux at the vessels’surface. in a 20-L bub-
Lr ble column. The medium was prepared in seawater as previ-
tube reactor, the equivalent ofEq. 19 was
ously detailed (cid:14)S´anchez Mir´on et al., 1999..
Outdoor cultures were carried out ‘‘batchwise,’’simultane-
A
U sV (cid:14)1y(cid:101).sV d(cid:14)1y(cid:101).. (cid:14)20. ously in all reactors, during August 5(cid:93)16, 1999. The mean
Lr Lr r Ld Ar d outdoor irradiance during this period was 200(cid:34)69 (cid:109)E
(cid:63)my2(cid:63)sy1at8:00h,risingtoameandailyvalueof1,056(cid:34)278
For both airlift reactors, the mean linear velocity V (cid:109)E(cid:63)my2(cid:63)sy1 at noon.The reactors were located in Almer´ıa
through the riser(cid:93)downcomer loop was related to the supLeoorp- (cid:14)36(cid:56) 50(cid:88) N, 2(cid:56) 27(cid:88) W., Spain. The biomass concentration at
ficial liquid velocity in the riser, as follows: inoculation was about 0.07 g(cid:63)Ly1. The aeration rate during
culture was constant at a U value of 0.011 m(cid:63)sy1, corre-
G
V qV 1 (cid:15) U U / sponding to a specific power input of 109 W(cid:63)my3. The tem-
V s Lr Lds Lr q Ld . (cid:14)21. perature was controlled at 20(cid:56)C by circulating chilled water
Loop 2 2 (cid:14)1y(cid:101). (cid:14)1y(cid:101).
r d through a jacket that surrounded the lower steel portion of
the reactors. Seawater was added fromtime totime tomake
Forthesplit-cylinder vessel,because ULr and ULd wereiden- upthelosses. Other aspects oftheculture methodologyhave
tical, Eq. 21 simplified to been detailed previously (cid:14)S´anchez Mir´on et al., 1999..
1876 September2000 Vol.46,No.9 AIChEJournal
Figure 3. Comparison of the measured gas holdup in Figure 4. Comparisonofthemeasuredrisergasholdup
the bubble column with the correlations of in the draft-tube airlift vessel with published
Chisti(1989):(a)tapwater;(b)sea-orsaltwa- data:(a) tapwater;(b) sea-or saltwater.
ter.
churn turbulent regime. As expected for media containing a
Results and Discussion large amount of dissolved salts, the flow transition occurs at
slightly greater power input (cid:14)Chisti, 1989. in seawater com-
Gas holdup
pared to tap water (cid:14)Figure 3.. Dissolved ions inhibit bubble
Numerousgasholdup correlations areavailable forbubble coalescence (cid:14)Akita and Yoshida, 1973; Chisti, 1989: Deck-
columns (cid:14)Akita and Yoshida, 1973; Chisti, 1989; Deckwer, wer, 1992; Hikita et al., 1981., hence postponing flow transi-
1992. and airlift bioreactors (cid:14)Akita et al., 1994; Chisti, 1989, tiontohighervaluesofgasholdup.AsshowninFigure3,the
1998,1999a;Kawaseetal.,1995;Miyaharaetal.,1986..While correlation developed for 0.15 M sodium chloride solution
there tends to be a general agreement among the different (cid:14)Chisti, 1989. is quite satisfactory for seawater, because the
correlations for bubble columns, this consistency is lacking effect of dissolved ions on gas holdup is marginal once the
for airlift reactors (cid:14)Chisti, 1989, 1998.. In the latter, the ionic strength is 0.2 orgreater.
holdup is influenced by the induced liquid circulation rate In the two airlift reactors, the gas holdup values agreed
thatdependsonthegeometryoftheflowpath,thegas(cid:93)liquid less well with published data. Thus, as shown in Figures 4
separating ability ofthe head zone ofthe reactor (cid:14)Chisti and and 5, the riser holdup in tap water was consistently low in
Moo-Young, 1993., and also the height of the airlift column comparison with the equation:
(cid:14)Chisti, 1989, 1998.. As shown in Figure 3, the gas holdup
datainthebubblecolumnagreedcloselywithequationspub-
lished for tap water and salt solutions (cid:14)Chisti, 1989., thus (cid:101) (cid:108) (cid:15) gd2(cid:114) /1r8(cid:15) gd3(cid:114)2 /1r12(cid:15) U (cid:101) /
cthoenfsilrimghintgdetvhieatiaocncuorfacthyeodfatthaefrmomeasthuerecmoernretsla.tiIonnsFifgourrsepe3-, (cid:14)1yr(cid:101).4s’gd (cid:115)r L (cid:109)r2 L UGry1yLr(cid:101)r ,
r r L r
cific power input values greater than about 400 W(cid:63)my3 is
(cid:14)24.
because of the change in flow regime from bubble flow to
AIChE Journal September2000 Vol.46,No.9 1877
uidcirculation ongasholdup. Also,Eq.25doesnotconsider
effects of surface tension and dissolved ions. Moreover, the
‘‘theoretical’’foundation of the equation is questionable be-
cause the assumption of isotropic turbulence in bubble
columns and airlift reactors is not valid, even at high-energy
inputs in waterlike media (cid:14)Chisti, 1998; L¨ubbert and Larson,
1990;L¨ubbert et al., 1990;Okada et al., 1993.. AlthoughEq.
25is intended alsoforviscousnon-Newtonian media, the as-
sumption of isotropic turbulence in such systems is even less
realistic.
Another equation forriser gas holdup in internal-loop air-
lift reactors is that ofMiyahara et al. (cid:14)1986.:
’
0.4 Fr
(cid:101)s , (cid:14)26.
r (cid:15) U /
1q0.4’Fr 1q Lr
U
Gr
wheretheFroudenumber Fr isbasedonthediameterofthe
sparger hole, that is,
U2
Frs Gr. (cid:14)27.
gd
o
Equation26wasdevelopedforavarietyofNewtonianfluids,
but not for media containing large quantities of dissolved
salts. However, as shown in Figure 6 for tap water only, Eq.
26 consistently and substantially overpredicts holdup values.
ThecomparisonsinFigures 3(cid:93)6clearly demonstrate that the
gasholdupdataindifferent airlift devicesarenotwellcorre-
lated with equations developed without considering the un-
derlying mechanics. Numerouscorrelations forgasholdup in
airlift devices have taken the general form (cid:101)spUq (cid:14)Chisti
G
Figure 5. Comparisonofthemeasuredrisergasholdup andMoo-Young,1987;Chisti,1989,1998;MerchukandGluz,
in the split-cylinder airlift vessel with pub- 1999., but these equations are suited only to specific combi-
lished data: (a) tap water; (b) sea- or saltwa- nationsoffluidproperties andreactorgeometry,because the
ter. parameters p and q are sensitive to these factors.
Asuperior approach tocorrelating gasholdup inairlift re-
actors is the use of the drift-flux model-type equations in
established byAkitaetal. (cid:14)1994..InEq.24,theparameter (cid:108) combination with a mechanistic relationship for liquid circu-
is0.20fornonelectrolytesand0.25forelectrolytes(cid:14)Akitaand lation velocity in two-phase flow (cid:14)Chisti, 1989, 1998.. The
Yoshida, 1973.. The riser diameter d in Eq. 24 has no im- drift-flux relationship for gas(cid:93)liquid flow in vertical conduits
r
pact on the value of gas holdup. For both airlift devices, takes the form:
agreement with Eq.24improvedgreatlyinseawater (cid:14)Figures
4 and 5.. In addition, the riser holdup in tap water did not U
correlate well with the equation: (cid:101)rs(cid:97)(cid:14)U qGUr .q(cid:98), (cid:14)28.
Lr Gr
(cid:101) (cid:14)U rn.(cid:14)nq2.r(cid:14)2(cid:14)nq1.. where the parameters (cid:97) and (cid:98) have physical meanings
r s Gr ,
1y(cid:101)r 2(cid:14)3nq5.r(cid:14)nq1.(cid:15) gnK /1r(cid:14)2(cid:14)nq1..(cid:15)1qAd/3(cid:14)nq2.r(cid:14)4(cid:14)nq1.. (cid:14)tiCvheisctai,se1s98in9,cl1u9d9i8n.g.AbostshhoawirnlifitnrFeiagcutroers7afonrdtwthoertewporefsleunidtas-,
(cid:114)L Ar the riser gas holdup correlates well with Eq. 28. For the two
(cid:14)25. fluids (cid:14)Figure 7., a mean (cid:98) value of 0.30 m(cid:63)sy1 was in the
expected range for bubble rise velocities. The parameter (cid:97)
differedforthetworeactors(cid:14)Figure7.becausetheshapesof
where, for Newtonian fluids, the flow index n is unity and the flowchannels were quitedifferent forthe twocases. The
theconsistency index K isreplaced byviscosity.Equation25 riser of the draft-tube airlift had a circular cross section,
wasdevelopedbyKawase etal.(cid:14)1995.usingtheoretical prin- whereasinthesplit-cylinderdevicetherisercrosssectionwas
ciples and assuming isotropic turbulence. Limited usefulness asemicircle. InviewofthefitinFigure7,theinduced liquid
of Eq. 25 is apparently due to its disregard for effects of liq- circulation rate clearly needs to be taken into account for
1878 September2000 Vol.46,No.9 AIChEJournal
Figure 6. Comparisonofthemeasuredrisergasholdup Figure 7. PlotsofEq.28fortworepresentativecasesin
in the two airlift vessels with Eq. 26 of Miya- the airliftvessels.
haraet al.
where the parameter a isaconstant (cid:14)Chisti, 1989..However,
predictinggasholdup.Incircularchannelsan (cid:97)-valueofunity as was recently pointed out (cid:14)Contreras et al., 1998b., Eq. 29
implies a flat radial velocity profile. In view of the high (cid:97)- disregards the fact that gas holdup in the downcomer re-
value (cid:14)Figure 7. in the split-cylinder device, the radial veloc- mainszerountilafiniteholdupvaluehasbeenestablished in
ity profile was apparently parabolic in the semicircular chan- theriser(cid:14)Wengeetal.,1996..Consequently,abettercorrela-
nel. tionbetweenriseranddowncomergasholdupshasbeenpro-
Unfortunately,inanairliftdevice,theinducedliquidcircu- posed (cid:14)Contreras et al., 1998b. in the form:
lationvelocityisnotusuallyknown apriori,andthereforefor
predictive purposes, the theoretical Eq. 28 needs to be used
incombinationwithotherequations(cid:14)Chisti,1989,1998..The (cid:101)sa(cid:101)yb. (cid:14)30.
d r
needtopredict gasholduparises mainlybecause oftheneed
toknowthegas(cid:93)liquidmass-transfercoefficientthatdepends
on holdup. As shown in the following subsection on the ef- The meanings of parameters a and b in Eq. 30 have been
fect of liquid circulation velocity on mass transfer, Eq. 28 in discussed elsewhere (cid:14)Contrerasetal., 1998b..Representative
combinationwithawell-knownmechanistic modelforthein- dataintapwaterandseawater areshowninFigure8plotted
ducedliquidcirculation velocity(cid:14)Chisti,1989.,providedare- accordingtoEqs.29and 30.Clearly, Eq.30providesasupe-
liable method for predicting the overall volumetric mass- rior fit of the data, all of which displays a distinct nonzero
transfer coefficient (cid:14)k a . values in airlift reactors. x-intercept. Similar behavior was seen for the draft-tube air-
L L
With few exceptions (cid:14)Ganzeveld et al., 1995;Wengeet al., lift device.
1996.,the relationship between the riser and the downcomer In the two airlift reactors, the overall holdup (cid:101) was mea-
gas holdups has been generally expressed (cid:14)Contreras et al., suredbyheightdisplacement, wheretheholdupvaluesinthe
1998b. in the form: riser (cid:14)(cid:101). and downcomer (cid:14)(cid:101). were determined manometri-
r d
cally. In addition to the direct measurements, a value of the
(cid:101)dsa(cid:101)r, (cid:14)29. overall holdup also could be calculated using the measured
AIChE Journal September2000 Vol.46,No.9 1879
Figure 9. Comparison of the measured overall holdup
withvaluescalculatedaccordingtoEq.31for
two cases in airlift reactors; diagonals repre-
sentanexactagreement.
(cid:14)Chisti, 1989.. As shown in Figure 9 for two representative
cases,excellentagreementisobservedbetweenthemeasured
overall holdup and the values calculated with Eq. 31. This
confirmsinternalconsistencyofmanometricallymeasuredgas
holdup values in the riser and downcomerwhile alsovalidat-
ing Eq. 31.
The various gas holdup values in the three reactors used
arecomparedinFigure10forthetwofluids.Generally,fora
given specific power input, the riser gas holdup in the two
airlift vessels iscomparable tothe overallholdup inthe bub-
ble column; however, the downcomer gas holdup is signifi-
cantly less than the holdup in the riser (cid:14)Figure 10.. Conse-
quently, the overall holdup of the airlift vessels is somewhat
lowerthaninthebubblecolumn.Incomparisonwiththeriser
holdup, the holdup in the downcomer is much lower at the
lower power input values than at higher ones (cid:14)Figure 10..
Figure 8. Relationship between riser and downcomer
Thisisbecauseunderconditionsoflow-powerinputthemean
gasholdups.
bubble size in the riser is bigger, and bigger bubbles are less
Split-cylinder data are shown for tap water and seawater
plottedaccordingtoEq.29andEq.30. readily dragged into the downcomer with the flow.
Gas–liquid mass transfer
riser and downcomer gas holdups (cid:14)Chisti, 1989., as follows:
Much more information exists on gas(cid:93)liquid mass transfer
A (cid:101)qA (cid:101) in bubble columns than exists in airlift devices (cid:14)Akita and
(cid:101)s r r d d. (cid:14)31. Yoshida, 1973; Chisti, 1989, 1998, 1999a; Deckwer, 1992.. In
A qA
r d addition, comparedtoairlift reactors, fewer factorsinfluence
k a values in bubble columns and, for a given fluid, the
L L
Equation 31 is based on geometric reasoning and it is an ex- k a data obtained in different columns generally compare
L L
act relationship for the kinds of airlift reactors used here well irrespective of the column aspect ratio (cid:14)Chisti, 1989., so
1880 September2000 Vol.46,No.9 AIChEJournal
Figure 11. Comparisonofthemeasured k a inbubble
Figure 10. Comparisonof variousgasholdup valuesin L L
column with the published correlations: (a)
the three reactors for various values of spe-
tapwater;(b) sea-or saltwater.
cificpowerinput:(a)tapwater;(b)seawater.
surface tension (cid:115), the viscosities of the gas (cid:14)(cid:109) . and the
longasthecolumndiameter exceeds about0.1m,aswasthe G
liquid (cid:14)(cid:109) . phases, the density (cid:114) of the liquid phase, and
case here. Because of this consistency, accuracy of the mass- L L
the diffusivity D ofoxygenin the liquid.
transfer measurements may be demonstrated by comparing L
2.TheonerecommendedbyHeijnenandVan’tRiet(cid:14)1984.:
dataobtainedinabubblecolumnwithotherwell-established
reference data before applying the same measurement
methodtoairliftreactors, orbeforenewdataareinterpreted kLaLs0.32UG0.7. (cid:14)33.
inanovelway.AsshowninFigure11forthebubblecolumn,
the measured k a data agreed remarkably well with some 3. That established by Chisti (cid:14)1989.:
L L
of the well-known correlations in both fluids. The correla-
tions used in the comparison were as follows.
(cid:15) P /0.86
1. That of Hikita et al. (cid:14)1981.: k a s2.39(cid:61)10y4 G . (cid:14)34.
L L V
L
14.9g(cid:102)(cid:15)U (cid:109) /1.76(cid:15) (cid:109)4g /y0.248(cid:15) (cid:109) /0.243
k a s G L L G For the comparison in Figure 11, Eq. 34 was expressed in
L L U (cid:115) (cid:114) (cid:115)3 (cid:109)
G L L terms ofthe superficial gas velocity.
(cid:15) (cid:109) /y0.604 Having validated the kLaL data, let us see how the theo-
(cid:61) L , (cid:14)32. retically developed Eq. 10 and Eq. 13 fare in correlating the
(cid:114) D
L L measurements. As shown in Figure 12, for all six combina-
tions of reactors and fluids, the k a data correlated excep-
L L
whereas (cid:102) was 1.0 for tap water. For seawater the (cid:102)-value tionally well with equations of the same general form as Eq.
was1.2,asrecommendedbyHikitaetal.(cid:14)Chisti,1999a..The 10. Use of Eq. 10 is preferred to purely empirical relation-
othervariables inEq.32aregravitationalacceleration g,the ships ofthetype k a saUb thathavebeenusedcommonly
L L G
AIChE Journal September2000 Vol.46,No.9 1881