Effect of Contaminants on the Gas Holdup and Mixing in Internal Airlift Reactors Equipped with ...

16 Pages·2015·3 MB·English

HindawiPublishingCorporation
InternationalJournalofChemicalEngineering
Volume2012,ArticleID569463,15pages
doi:10.1155/2012/569463
Research Article
Effect of Contaminants on the Gas Holdup and Mixing in Internal
Airlift Reactors Equipped with Microbubble Generator
SuryaK.PallapothuandAdelM.AlTaweel
MultiphaseMixingandSeparationsResearchLaboratory,DepartmentofProcessEngineeringandAppliedSciences,
DalhousieUniversity,Halifax,NS,CanadaB3J2X4
CorrespondenceshouldbeaddressedtoAdelM.AlTaweel,[email protected]
Received6June2012;Accepted27August2012
AcademicEditor:DiegoGo´mez-D´ıaz
Copyright©2012S.K.PallapothuandA.M.AlTaweel.ThisisanopenaccessarticledistributedundertheCreativeCommons
AttributionLicense,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkis
properlycited.
Theimpactofcontaminantsonthegasholdupandmixingcharacteristicsencounteredininternalairliftreactorswasinvestigated
usinga200Lpilotscaleunitequippedwithatwo-phasetransonicspargercapableofgeneratingmicrobubbles.Smalldosages
of a cationic surfactant (0–50ppm of sodium dodecyl sulfonate (SDS)) were used to simulate the coalescence-retarding effect
encounteredinmostindustrialstreamsandresultedintheformationofbubblesthatvariedinsizebetween280and1,900μm.Gas
holdupsashighas0.14wereachievedintheriserunderhomogeneousflowregimewhenslowlycoalescentsystemswereaerated
attherelativelylowsuperficialvelocityof0.02ms−1,whereasliquidcirculationvelocitiesashighas1.3ms−1 wereachievedin
conjunctionwithrapidlycoalescentsystemsatthesamesuperficialvelocity.Thisexcellenthydrodynamicperformancerepresents
a5-foldimprovementintherisergasholdupandupto8-foldenhancementintheliquidcirculationvelocityandisexpectedto
yieldgoodmixingandmasstransferperformanceatlowenergydissipationrates.
1.Introduction appliedtoslowlycoalescentindustrialstreams.Forexample,
whereassomeinvestigatorsreportthatthevolumetricmass
Virtually all process streams encountered in the chemi- transfercoefficientispositivelyimpactedbythepresenceof
cal/biochemical/process industries contain varying concen- surfactants,othersreportanegativeimpact.
trations of amphiphilic materials (such as alcohols, surfac- Asignificantpartofthefactorshamperingthedevelop-
tants, organic acids, electrolytes, amines, glycols, proteins, ment of a systematic and comprehensive understanding of
phenols, and finely divided particles) that are introduced howcontaminantsimpactgas/liquidcontactinginindustrial
as reactants, as impurities in the feed and recycle streams, situations stems from the strong interaction between the
or formed as products and/or byproducts. Although it various factors affecting it [1]. Consequently, the influence
is well known that the presence of such materials can of the aforementioned contaminants on the specific inter-
significantly impact gas/liquid contacting operations, the phase area of contact, a, and the liquid side mass transfer
manner and magnitude by which these changes take place coefficient, k , could be significantly different. Although
L
are still controversial. Consequently, much of the practices there is no clear understanding of the mechanisms by
prevalenttodayingas/liquidcontactingarebasedoninfor- which amphiphilic materials impact k , there is a general
L
mation and observations obtained using relatively clean recognition that it is adversely affected in the presence of
systems where the equilibrium between bubble breakage amphiphilicmaterials[2–8]Meansbywhichlargeinterfacial
and coalescence is quickly approached, a situation that area of contact can be generated between the phases are
does not truly reflect what is happening in most industrial therefore promising avenues by which high k a values can
L
situations. The fact that there is no definite agreement on be achieved in industrial systems. This is usually achieved
evensomebasicfundamentalconceptsresultedingas/liquid by increasing the energy input to the system, an approach
contactingoperationsnotachievingtheirfullpotentialwhen that can adversely affect the energetic performance of gas
2 InternationalJournalofChemicalEngineering
liquidcontactingoperations.Industrialpracticehashowever Table1:CharacteristicsoftheinternalloopALR.
proventhatlargeinterfacialareaofcontactcanbeachieved
Characteristic Scale Units
without large energy expenditures by focusing energy dis-
sipation within a small portion of the contactor’s volume Reactordiameter 0.3 m
(wherebyverysmallbubblesareformed)andrelyingonthe Riserdiameter 0.23 m
coalescence retardation characteristic inherent to industrial Riserheight 2.3 m
streams in order to maintain relatively large interfacial area Reactorheight 2.75 m
inotherpartsofthecontactor[9,10]. Gasseparatordiameter 0.5 m
A similar situation exists in the case of bubble columns Bottomclearance 0.2 m
and airlift reactors (ALRs). As was shown by Bordel et al. Totalreactorvolume 200 L
[11], the assumption that the entire column is in dynamic
Riservolume 95 L
equilibriumisingeneralnotvalid;furthermore,thecolumn
Downcomertoriserarearatio 1.2 —
length it takes to approach a state of dynamic equilibrium
Separatortoriserarearatio 4.7 —
between bubble breakage and coalescence in the column
depends mainly on the difference between the initial mean Reactorheighttodiameterratio 9 —
bubble diameter and that achieved at equilibrium. Their
results, as well as those obtained by many other computa-
Attempts have been recently undertaken to overcome
tional fluid dynamics simulations of gas/liquid contacting
this barrier, and the potential for using microbubbles to
in bubble columns [12, 13], show that this phenomenon is
enhance CO absorption/stripping was investigated by Al-
particularly significant in the case of homogeneous flow of 2
Mashhadani et al. [26] where a bubble cloud (with a
slowlycoalescentgas/liquiddispersionswheretheinfluence
Sauter mean diameter, d , of 550μm) was generated using
of the initial conditions that are dictated by the type and 32
a two-chamber microporous ceramic diffuser with fluidic
configuration of the sparger plays a more pronounced role
oscillation.Themasstransfercoefficientobtainedusingthe
[14–16].
microbubbles generated by that device (k a = 0.0015s−1)
Several investigators experimentally studied the impact L
wasabout20%higherthanthatachievedusingfinebubble
of soluble organic amphiphilic compounds on gas liquid
spargers(d =1,300μm)atthesamegasflowrate.Themain
32
contacting in bubble columns with a particular emphasis
reasonbehindsuchlimitedvolumetricmasstransfervalues
on surfactants and alcohols [5, 7, 9, 15, 17–19]. On the
istherelativelylowthroughputsthatcanbehandledbysuch
otherhand,mostoftheinvestigationsinvolvingALRfocused
units;hence,manyspargerswillbeneededinordertohandle
on alcohols [7, 16, 19, 20] while relatively few assessed the
the large volumetric gas flow rates typically associated with
impactofsurfactants[7,21–23].
suchoperation.
Several attempts were made to improve gas/liquid con- The objective of this study is to investigate the impact
tactinginairliftreactorsbyusingspargerscapableofgener- of using microbubbles on the hydrodynamic performance
ating microbubbles [24–27] but the extent of mass transfer of airlift reactors using rapid and slowly coalescent systems
improvementachievedwassomewhatlimited.Bredwelland that simulate what happens in industrial streams. The
Worden explored the potential for using microbubbles to microbubble generator (sparger) tested is one that was
overcome mass transfer limitations in fermentation opera- developedtohandletherelativelylargegasflowratesneeded
tions and achieved more than 6-fold increase in the value in the oil and gas industry and can provide an additional
of k a by replacing the stainless steel frit sparger by a process control parameter in the case of ALR by virtue
L
microbubblegenerator.Althoughthepowerneededtodrive of its ability to uninterruptedly change the size of bubbles
the microbubble generator is very small (0.01kW/m3 of generated.
the fermentation capacity), the maximum mass transfer
coefficientachievedwaslessthan0.025s−1.Inasimilarfash- 2.Experimental
ion, significant improvement in the decomposition rate of
organiccompoundswasachievedwhenozonewasdispersed The effect surfactants have on the hydrodynamics of ALR
in the form of microbubbles [28, 29], but the maximum was investigated using the experimental setup depicted in
volumetricmasstransfercoefficientachievedinanimproved Figure1.A200Lpilotscaleinternalloopairliftreactor,the
versionoftheirreactorwasverylow(k a<0.002s−1)mainly characteristics of which are given in Table 1, was used in
L
becauseoftheverysmallgasflowratesthatcouldbehandled an arrangement that could be operated in both batch and
usingthemicrobubblegeneratorinvestigated[25].Likewise, continuous modes. However, the batch operational mode
the mass transfer rates achieved by Chu et al. [30] were (achieved by recirculating some of the liquid present in
relativelylow(k a < 0.006s−1).Onecanthereforeconclude the reactor through the transonic sparger) was adopted
L
that although the use of microbubbles represents a very throughoutthisinvestigationinordertosimplifytestingand
promising avenue for increasing interphase mass transfer, shortentimerequirements.
it has been successfully used only in situations where the The geometric proportions used in the present investi-
amount of gases to be transferred is relatively small (e.g., gationwerecommonlyusedinmanypreviousinvestigators
using ozone to disinfect water and oxidize small residual andfoundtoresultinadequategas/liquidphaseseparation
organiccontaminants). when conventional bubbles were used. However, the size of
InternationalJournalofChemicalEngineering 3
DO probe
T To drain
Pump 2
C
MFC 1
Tap water
P
7 P
3
P
6
P
5
P
4
FM 1 MFC 1
P2 PRCF Compressed air
or
nitrogen
P
1 Pump 1
Pump 3
MFC: mass flow controller Pump 1: high pressure
P: pressure transducer Pump 2: centrifugal pump
C: oxygen concentration CF: carbon filter
T: temperature PR: pressure regulator
Figure1:Schematicrepresentationoftheexperimentalsetup.
the separation section was found to be not very effective in Asimilarapproachwasusedfordeterminingtheaverage
separatingmicrobubbles. gasholdupinthedowncomer.
Adataacquisition/controlsystem(NationalInstruments The “Power Spectral Density Function” of the pressure
data acquisition board with 16 analog input channels and signals was analyzed off-line and used to identify the flow
2 analog output channels with a sampling rate of 100k regimes in a fashion similar to that used by Vial et al. [31]
samples/s)wasusedtomonitortheexperimentalparameters andbyGourichetal.[32].
(pressure,temperature,oxygenconcentration,conductivity) The liquid circulation velocity in the ALR was deter-
andtocontrolthesuperficialgasvelocity.Pressuresampling mined using the conductivity tracer technique in which
ports were located at the inlet and outlet of the riser and 25mL of a 3M NaCl solution was injected from the top of
downcomerregionsandconnectedtofastresponsepressure thecolumn20–30mmabovetheconductivitymeterplaced
transducers. in the reactor. The conductivity of the solution passing
Theaveragepressurevaluesmeasuredattheselocations the probe was continuously monitored and sampled at
were used to determine gas holdup in the riser using the the rate of 100 samples per second to ensure significance
followingexpression: of high-frequency signals (typical results are shown in
Figure 2). The resulting data file was then analyzed off-
(P −P )
∅ =1− 4 5 , (1) line using Labview’s harmonic analysis capabilities, thereby
Riser
ρ gH
L Riser achieving high reproducibility in estimating the cycle time
where ∅ gas holdup in the riser, P mean pressure at andcirculationvelocity.Theanalysisignoredthefirstpeak,
therisereRnisterrance(Pa),P meanpressure4atriseroutlet(Pa), whichwasfoundtobestronglyaffectedbyslightvariations
5
ρ liquid density (kg/m3), g acceleration of gravity (ms−2), intracerinjection,andincorporated3to8cyclesdepending
L
H heightofthedrafttube(m). onthemixingintensity.
Riser
4 InternationalJournalofChemicalEngineering
0.03
0.0275
0.025
0.0225
e
d
u
plit 0.02
m
A
0.0175
0.015
0.0125
0.01
0 100 200 300
Time Figure 3: Schematic representation of the two-phase transonic
sparger and its use to generate fine bubble dispersions (U =
Figure2:Temporalvariationofconductivity(timeinseconds). G
0.01ms−1).
Since the distance travelled by the tracer is more than
standingwave.Theuseofamodifiedversionofthissparger
twice the length of the riser, the following equation is used
reportedlydoubledthecapacityofthebioreactorsoperated
to obtain conservative estimates of the liquid circulation
by “Royal DSM” at their plant in The Netherlands [35],
velocity:
whereasanearlierfixed-throatversionyieldedgasholdupsas
2∗H highasεG = 0.45atasuperficialvelocityof0.03ms−1 [36].
UCirc= Riser, (2) Using this sparger and a slightly modified geometry of the
t
Circ presentsetup,interphasemasstransfercoefficientsashighas
where t average circulation time (s), U average liquid 0.13s−1wereachievedatlowsuperficialvelocitiesinaslowly
Circ Circ
circulationvelocity(ms−1). coalescentsalt-containingsystem[37].
The 25mm diameter transonic sparger used in this Theaforementionedbubblebreakage/coalescencemech-
investigation(Figure3)isascaledupversionofthesparger anisms resulted in the gas dispersion performance of the
previouslyusedtoproducefinegas/liquiddispersions,with transonicspargerbeinginfluencedby[33]
bubble sizes as small as 35μm and interfacial area as high
(i)pressuredropacrossthesparger,
as 3,500m2/m3, being achieved in the presence of SDS
[33]. It focuses energy dissipation rate within the throat (ii)gastoliquidmassflowratiointhesparger,
of an annular venturi where a very high-speed two-phase (iii) coalescencetendenciesofthesystem.
jet is formed. The divergent section of the sparger helps in
For the case of aqueous SDS solutions, the size of bubbles
recovering the substantial kinetic energy of the two-phase
formed by dual-phase transonic sparger (which varied
jet and minimizes excessive mixing at the point where the
between 35 and 1,700μm) was thus estimated using the
spargedgas/liquidmixtureisintroduced.
followingcorrelationdevelopedbyAlTaweeletal.[33]:
At relatively low velocities, the gas phase is dispersed
(cid:2) (cid:3)
into small bubbles as a result of the very high local energy Q 0.70
dissipation rates prevalent in the venturi throat. The fine d32=1.57×103Cs−0.4ΔPSparger−0.35QL−0.26 QG , (3)
bubbles thus formed tend to coalesce as they migrate into L
thelowerenergydissipationregionsprevalentinthesparger’s whered sautermeanbubblediameter,(μm),C surfactant
32 S
divergentsectionaswellasintheALRrisersection.However, concentration,SDS(ppm),ΔP pressuredropacrossthe
Sparger
the rate at which these bubbles coalesce is strongly affected sparger(Pa),Q gasvolumetricflowrate(m3s−1),Q liquid
G L
by the interfacial characteristics of the gas/liquid system volumetricflowrate(m3s−1).
used (a factor that is strongly affected by the presence of One of the advantages of using this sparger is the
contaminants). On the other hand, when the operating abilitytoadjustthebubblersizeofthegasliquiddispersion
conditions are such that the velocity of the two-phase flow introducedintotheALRwithoutinterruptingtheoperation.
withinthethroatislargerthanitssonicvelocity,astanding This can be achieved by adjusting the gas to liquid ratio
shock wave is formed in the divergent section in a fashion and/or the pressure drop across the sparger (the latter
similar to that present in the DeLaval nozzle [34]. Very can be achieved by turning the spindle which moves the
fine bubbles are thus generated as the gas liquid dispersion conicalspargercoreupwardsand/ordownwards,resultingin
passes through the high energy dissipation region in the alteringthecross-sectionalareaofthethroat).
InternationalJournalofChemicalEngineering 5
In the present investigation, the liquid mass flow rate 2500
was kept constant throughout the experiments (W =
L
0.06kg/s),whereasthegasmassflowratewasvariedbetween
1.3∗10−4 and 1∗10−3kg/s. The pressure drop across the
2000
spargerwasmaintainedatthedesiredvaluebyadjustingthe
cross-sectional area of annual channel in the throat. Most
of the experiments were performed at two pressure drops
across the sparger (140kPa, 280kPa) but several systematic m) 1500
µ
investigations of the effect of bubble size were conducted e (
usingfivedifferentpressuredropsacrossthesparger(70,140, e siz
210,280,350kPa)ataconstantliquidtogasmassflowratio bl
ub 1000
of150. B
Theeffectoftheoperatingconditionsonthesizeofthe
bubbles generated by the twin fluid transonic sparger in
the present investigation (predicted using (3)) is shown in 500
Figure 4. The size of the smaller bubbles predicted to have
been achieved in the present investigation compares well
withthoseobtainedbyAl-Mashhadanietal.(d =550μm)
32 0
and those obtained by Kawashima et al. using a 50ppm 3- 0 100 200 300 400
pentanolsolution(189–358μm). Pressure drop across sparger (kPa)
The operational characteristics of the unit were inves-
CS=0ppm CS=10ppm
tigated using the system air/tap water to which minute
CS=2ppm CS=20ppm
quantities(0to50ppm)ofsodiumdodecylsulfonate(SDS) CS=5ppm CS=50ppm
surfactantwereintroducedinordertosimulatetheslowcoa-
lescence behavior encountered in most industrial streams. Figure 4: Effect of sparger pressure drop on the predicted size
This system was selected because it is nonvolatile and of bubbles generated by the dual-fluid transonic sparger (U =
G
nontoxicanddoesnotdegradequickly.Itsstatic/equilibrium 0.008ms−1,W /W =150).
L G
interfacialcharacteristicsarewellknown,anditiscommonly
used to test the effect of interfacial properties on the
performanceofgas/liquidcontactors[7,21,33,38,39].
Atotalof82runswereconductedinordertodetermine frequencyrange(Figure5(b)).Thisdramaticdifferencecan
the effect of various operating conditions on the hydrody- be attributed to inefficient dispersive action of the sparger
namic performance of the ALR. The range of experimental when it is operated using air only (and the consequent
conditionsinvestigatedisgiveninTable2. formation of a wide range of bubble sizes, mostly coarse
ones), whereas a narrowly sized fine bubble dispersion was
observed to occur when the sparger is operated using an
3.ResultsandDiscussion air-waterdispersion(particularlyathighpressuredropsand
elevatedsurfactantconcentrations).
3.1. Flow Regime. The power spectrum density function As can be seen from Figure 6, both the characteristic
(PSDF) of the pressure signal in both the riser and down- frequencyandtheintensityofthepressurefluctuationswere
comer regions was found to provide a good indication of found to vary with the operating conditions. The intensity
the flow conditions in their respective regions. However, of the pressure fluctuations appear to be a function of the
since the maximum superficial gas velocity used in this bubblesizewiththeintensityincreasingasthesizeofbubbles
investigation is much lower than that at which transi- formedbythespargerbecomeslargerduetothehighergas
tion has been reported (typically ≥0.05ms−1 particularly flowratesprocessed(3).Similarly,thefactthattheintensity
in coalescence-retarding systems), the exploratory results of the pressure fluctuations in the downcomer is higher
reported here apply only to the homogeneous flow regime. than the corresponding values in the riser (Figure 6(a))
This is confirmed by the absence of any sudden changes in can be attributed to the need for the bubbles suspended
thevariationofgasholdupwithU (e.g.,Figure6). in the downcomer to become larger than those in the
G
The frequency, intensity, and breadth of the pressure riser before they can move against the downwards moving
differential signal spectrum were found to be strongly liquidinthedowncomer.However,noattemptsweremade
affected by the flow conditions. For example, very strong to analyze these results in depth because of the absence
pressure fluctuations were observed to occur when the of detailed information concerning the size distribution of
transonicspargerwasoperatedasasingle-phasedistributor bubbles formed. It is however interesting to note that the
even in the presence of surfactants (Figure 5(a)), while the characteristic frequencies obtained by Vial et al. [31] at
signalspectrumwasalsofoundtobeverybroad.Conversely, highersuperficialgasvelocities(0.07to0.12ms−1)arelower
the pressure fluctuations obtained when the dual-phase than those obtained in the present investigation (0.3 to
transonicspargerwasoperatedatapressuredropof350kPa 5Hz) whereas the intensity of their pressure fluctuations
were found to be weak and bounded within a very narrow (0.0009 to 0.6kPa) is within the range observed in the
6 InternationalJournalofChemicalEngineering
Table2:Rangeofexperimentalconditionsinvestigated.
Superficialgasvelocity 0.0025–0.020 ms−1
SDSconcentration 0–50 ppm
Equilibriumsurfacetension 72.0–44.1 mN/m
Temperature 25±2 ◦C
Gasholdupintheriser(±4%) 0.007–0.14 —
Gasholdupinthedowncomer(±4%) 0–0.11 —
Liquidcirculationvelocity(±4%) 0.57–1.3 ms−1
Mixingtime(±7%) 18–40 s
Pressuredropacrosssparger 70–350 kPa
MassFlowrateofwaterthroughthesparger 0.06 kg/s
Gastoliquidmassflowratiointhesparger 0.0021–0.017 —
0.04 0.0006
0.035
0.0005
0.03
0.0004
ude 0.025 ude
mplit 0.02 mplit 0.0003
A 0.015 A 0.0002
0.01
0.0001
0.005
0 0
1 2 4 6 8 10 12 14 16 18 20 1 2 4 6 8 10 12 14 16 18 20
Frequency (Hz) Frequency (Hz)
(a) Singlephasesparger (b) Dual-Fluidoperation(ΔPS=350kPa)
Figure5:EffectofspargeroperationalmodeonthePSDFofthepressureintheriser(U =0.005ms−1,C =50ppm).
G S
present investigation. This variation could be attributed to Inrapidlycoalescentmedia,thebubblesizeintroducedat
the different geometrical designs and sparger types used in thebaseoftheALRchangesrapidlyasthebubblesascendin
the two investigations, as well as differences in the systems the riser in an attempt to approach quasisteady conditions
investigated. [11, 15, 16]. Consequently, the size of bubbles formed at
Due to the increasing tendency to form finer bubbles, thespargerhaslittleinfluenceonthatprevailingintheriser
thepresenceofsurfactantswasfoundtoreducethepressure in rapidly coalescent media, whereas the size of bubbles
fluctuation intensity and frequency in both the riser and prevailingthroughouttheriserremainsclosetothatformed
downcomerregionswiththistrendincreasingasthesurfac- at the sparger in the case of slowly coalescent media. The
tantconcentrationincreased. relative importance of sparger design was thus found to
depend strongly on the interfacial properties of the liquid
phase[14,16,39–42].
3.2.GasHoldup. Hydrodynamicsandmasstransferratesare Similarobservationswereobtainedinthepresentinves-
the most important parameters used in assessing the per- tigationwhere,forrapidlycoalescentsystem,thegasholdup
formance of air lift reactors. These parameters are sensitive intheriserwasfoundtoincreasewithincreasinggasvelocity
to the reactor design configuration as well as operational inanearlylinearfashionwithoutsignificantlyincreasinggas
variationssuchassuperficialgasvelocity,thesizeofbubbles holdup in the downcomer (Figure 7). This observation is
introduced at the base of the riser, and the bulk/interfacial verysimilartothatreportedwhenperforatedplatespargers
properties of the system. However, ALRs suffer from the areusedandcanbeattributedtotheprevalenceofrelatively
limited number of parameters that can be used to control largebubbleswhichtendtodisengageeasilyintheseparator
theirperformance.Thus,whereasthesuperficialgasvelocity section.Thegasholdupintheriseranddowncomersections
canbeeasilymodifiedinconventionalALRs,itisnotpossible increased only slightly when the pressure drop across the
tochangethebubblesizewithoutstoppingtheoperationin sparger was increased in order to introduce finer bubbles
order to change the sparger. The use of transonic spargers at the bottom of the riser. This limited sensitivity to the
thus provides the opportunity to change the bubble size size of bubbles can be attributed to the relatively fast rate
withoutdisruptingtheoperation. at which the dynamic equilibrium between the breakage
InternationalJournalofChemicalEngineering 7
1 100
0.1
a)
P
k
y (
ntensit 0.01 Hz)
n i y (
fluctuatio 0.001 Frequenc 10
e
ur
ess
Pr
0.0001
0.00001 1
0 0.01 0.02 0.03 0 0.01 0.02 0.03
Superficial gas velocity (m/s) Superficial gas velocity (m/s)
CS=0ppm, riser CS=0ppm, downcomer CS=0ppm
CS=10ppm, riser CS=10ppm, downcomer CS=10ppm
CS=50ppm, riser CS=50ppm, downcomer CS=50ppm
(a) Fluctuationintensity (b) Characteristicfrequency(riser)
Figure6:Effectofsuperficialgasvelocityandsurfactantconcentrationontheintensityandcharacteristicfrequencyofpressurefluctuations
(ΔP =280kPa).
S
and coalescence processes is approached in tap water. riserincreasessignificantlyassmallerbubblesareintroduced
Consequently,thedifferenceingasholdupbetweentheriser intotheALRwiththeeffectbeingmostpronouncedathigher
and downcomer was found to increase significantly with surfactant concentration. The gas holdup achieved in the
increasingsuperficialgasvelocity. slowlycoalescentaqueoussolutionscontaining50ppmSDS
Although transonic spargers are not ideally suited for isconsequently4-foldlargerthanthatobtainedintherapidly
the case of rapidly coalescent systems, the results depicted coalescentair/watersystem.
in Figure 8 clearly show that, in the case of the air/water This can be mainly attributed to the ability of the
system, they are capable of generating gas holdups that transonic sparger to form smaller bubbles in the case of
are comparable to the values obtained using plates with slowlycoalescentsystems,withthefinebubblesformedatthe
0.6mmperforations[40]buthigherthanthoseinducedby spargertiptendingtomaintaintheirsizeforalongerperiod
perforated plate spargers equipped with 1mm perforations as they ascend through the surfactant-containing liquid
orlarger. present in the riser. This is clearly evident from the results
Theimpactofcoalescenceretardationonthegasholdup depicted in Figure 9 where the gas holdup in the riser was
performance of ALR was found to be very pronounced foundtobeverysensitivetothesizeofbubblesintroducedat
whentransonicspargersareusedforaerationpurposes.The thebottomoftheriserwiththecoalescencetendenciesofthe
visualappearanceofthedispersionthusacquireda“milky” liquidpresentintheriserplayingasecondaryrole.
appearance due to the formation of extrafine bubble with Furthermore, for the case at hand (i.e., SDS surfactant
the optical density of the gas/liquid dispersion increasing andtransonicsparger),thereisnoindicationoftheexistence
with increasing gas flow rate and/or pressure drop across of a critical concentration beyond which no changes in gas
thesparger.Thenoisemadebythedisengagingbubblesalso holdup take place. This can be mainly attributed to the
changedfromthatofavigoroussplashingsound(inthecase ability of the transonic sparger to form smaller bubbles in
oftapwater)tothemoreuniformhissreportedbyNicoland the case of slowly coalescent systems, with the fine bubbles
Davidson[38]forthecaseofsurfactant-contaminatedwater. formed by the sparger tending to maintain their size for
However,someofthefinebubblespresentintheriserwere a longer period as they ascend through the surfactant-
entrained in liquid circulating in the downcomer thereby containingliquidpresentintheriser.Thisisclearlyevident
contributingtowardsincreasinggasholdupinthatregion. fromtheresultsdepictedinFigure10wherethegasholdup
The effect of coalescence retardation on the gas holdup in the riser was found to be very sensitive to the size of
intheriserisdepictedinFigure9whichclearlyshowsthat, bubbles (predicted using (3)) introduced at the bottom
foranyparticularsurfactantconcentration,gasholdupinthe of the riser, with the coalescence tendencies of the liquid
8 InternationalJournalofChemicalEngineering
0.05 0.16
0.14
0.04
0.12
p 0.1
u
0.03 old
holdup er gas h 0.08
as Ris 0.06
G
0.02
0.04
0.01 0.02
0
0 0.005 0.01 0.015 0.02 0.025
0 Superficial gas velocity (m/s)
0 0.01 0.02 0.03
Superficial gas velocity (m/s) CS=50ppm CS=5ppm
CS=20ppm CS=2ppm
Riser (ΔPS=280kPa) Riser (ΔPS=140kPa) CS=10ppm CS=0ppm
Downcomer (ΔPS=280kPa) Downcomer (ΔPS=140kPa)
Figure9:Effectofcoalescenceretardationgasholdupintheriser
Figure7:Effectofsuperficialgasvelocityontherisergasholdup (transonicsparger,ΔPS=280kPa).
forarapidlycoalescentsystem(C =0ppm).
S
0.07
0.08
0.06
0.07
0.05
0.06
p
u
d 0.04
ol p
h u
gas old 0.05
Riser 0.03 er gas h 0.04
Ris
0.02
0.03
0.01
0.02
0
0 0.01 0.02
0.01
Superficial gas velocity (m/s) 100 1000 10000
Predicted Sauter mean bubble diameter (µm)
3 mm perforated plate, Merchuk et al. [40]
1.2 mm perforated plate, Merchuk et al. [40] CS=0ppm CS=10ppm
0.6 mm perforated plate, Merchuk et al. [40] CS=2ppm CS=20ppm
3 mm single orifice sparger, Tsuge et al. [50] CS=5ppm CS=50ppm
Dynamic sparger
F28ig0ukrPea)8.:Effectofspargerdesignongasholdupintheriser(ΔPS= F0.i0g0u8rme 1s−01:,EWffLe/cWt oGf=bu1b5b0)le. size on the riser gas holdup (UG =
InternationalJournalofChemicalEngineering 9
presentintheriserplayingasecondaryrole.Unfortunately, 0.16
the fine bubbles generated by the transonic sparger have
reducedgas/liquidslipvelocitiesandcanthusbemoreeasily 0.14
reentrainedintothedowncomerstreamtherebyreducingthe
difference in gas holdup between the riser and downcomer
0.12
sections.
The effect of surfactant concentration on the difference
p 0.1
inriseranddowncomergasholdups(Figure11)suggeststhat u
d
thegasholdupdifferenceishighestfortherapidlycoalescent ol
h
air/water system and decreases quickly in the presence of as 0.08
g
surfactants. Although such a conclusion is expected to be er
generally true, the extent to which it occurs is strongly Ris 0.06
influenced by the hydrodynamic conditions prevalent in
the gas disengagement section. Greater attention should 0.04
therefore be paid to the design of the gas disengagement
sectionwheneverfinebubblesareusedtopromotegas/liquid 0.02
contacting in order to ensure high levels of gas/liquid
separationandtheinducementofelevatedliquidcirculation 0
velocities. 0 0.01 0.02
However,evenwiththeinefficientseparatorusedinthe Superficial gas velocity (m/s)
present investigation, the riser gas holdups obtained at the
lowsuperficialvelocitiestestedarequitehighincomparison 1 mm perforated plate, Al Masry and Dukkan [21], 50 ppm octanol
to those typically reported in the literature. Furthermore, 1 mm perforated plate, Al Masry and Dukkan [21], 50 ppm
polypropylene glycol
theycouldhavebeenhigherif
1 mm perforated plate, Muthukumar and Velan [44], 1000 ppm
benzoic acid
(i)longerriserswereused(whichwouldprovidegreater
1 mm perforated plate, Muthukumar and Velan [44], 250 ppm
opportunity for the fine bubbles to coalesce into
isoamyl alcohol
largeronesthatcanbemoreeasilyseparated),or 1 mm perforated plate, Muthukumar and Velan [44], 2500 ppm
(ii) a more effective separator design (which can elim- propanol
1 mm perforated plate, Wachi et al. [43], 1000 ppm ethanol
inate most of the bubble reentrainment in the
1 mm perforated plate, Wachi et al. [43], 5000 ppm glycerol
downcomer)wasused.
0.5 mm perforated plate, Snape et al. [39], 800 ppm MgSO4
A comparative evaluation of the transonic sparger with 0.5 mm perforated plate, Snape et al. [43], 800 ppm CaCl2
0.5 mm perforated plate, Snape et al. [39], 800 ppm NaCl
other sparger designs is complicated by the need to utilize
Dynamic sparger, SDS 50 ppm
liquids with identical coalescence retarding characteristics.
Thistaskishoweverhinderedbythepaucityofinvestigations Figure11:Effectofspargertypeandcontaminantsongasholdup
intheriser.
in which ALR performance is systematically investigated
using liquids of different interfacial characteristics (hence
differentcoalescenceretardationrates).Figure11compares
the riser gas holdups obtained in the present investigation
withthoseobtainedinthepresenceofvariouscontaminants ALR from bubble columns and significantly affects the gas
such as salts [39], alcohols [21, 43], and a series of other holdups achievable in the riser and downcomer regions,
compounds[44].Itclearlyshowstheadvantageofusingthe theaxialandradialmixingintensities,themixingtime,the
transonicspargerwhichiscapableofgeneratingfinebubbles overall mass transfer coefficients, as well as the ability to
inslowlycoalescingsystemswherebytheholdupsobtainedin suspend catalyst particles. It is driven by difference in gas
thisinvestigationareabout5-foldhigherthanthoseobtained holdup between the riser and the downcomer zones [45],
inthepresenceof50ppmofalcohol(using1mmperforated anditsmagnitudeisdeterminedbythebalancebetweenthe
platesparger)andareslightlyhigherthanthoseobtainedin drivingforceandtheoverallhydrodynamicresistanceinthe
the presence of 250ppm of isoamyl alcohol or 1,000ppm variouspartsofthereactor.
ofbenzoicacid.TheholdupsrecentlyreportedbyMoraveji AsshowninFigure12,theliquidcirculationvelocityfor
et al. [7] using 5ppm SDS solution are similar to those therapidlycoalescentair/watersystemwasfoundtoincrease
reported by Muthukumar and Velan [44] and are therefore with increasing superficial gas velocity, reaching a velocity
notdepictedinFigure11.Ontheotherhand,usingasingle of 1.3ms−1 at U = 0.02ms−1 when high pressure drops
G
orificenozzle(3.5mm)byEl-Azheretal.[20]resultedingas are applied across the sparger to generate fine bubbles. The
holdupsbelowεG = 0.03forthesuperficialvelocitiestested circulationvelocityishoweverreducedwhenthesomewhat
inthiswork(UG≤0.02ms−1). coarser bubbles are generated at the lower pressure drop
of 140kPa. However, it is important to note that, in all
3.3. Circulation Velocity. The liquid circulation velocity is experimentsconductedusingtherapidlycoalescentair/water
the main hydrodynamic parameter which differentiates system, virtually complete disengagement of bubbles took
10 InternationalJournalofChemicalEngineering
1.4
0.85
1.3
ocity (m/s) 11..12 elocity (m/s) 0.8
vel 1 n v
ation 0.9 ulatio 0.75
circul d circ
d 0.8 ui
Liqui Liq 0.7
0.7
0.6
0.65
0.5 0 100 200 300 400
0 0.01 0.02
Pressure drop across sparger (kPa)
Superficial gas velocity (m/s)
ΔPS=140kPa CS=0ppm CS=10ppm
ΔPS=280kPa CS=2ppm CS=20ppm
CS=5ppm CS=50ppm
Figure 12: Effect of superficial Gas velocity on liquid circulation
ratesinrapidlycoalescentsystems(C =0ppm,W =0.06kg/s). Figure13:Effectofsurfactantconcentrationonliquidcirculation
S L velocity(U =0.008ms−1,W /W =150).
G L G
place in the separator design adopted in the present inves-
tigationandfewbubbleswereobservedtobeentrainedinto the gas holdup in the riser (Figure 9), they also can be
thedownwardrecirculatingstream(Figure5). moreeasilyentrainedinthedowncomerstream.Depending
The presence of trace quantities of surfactants in the on the relative magnitude of the two, the net pressure
process stream was found to result in the formation of difference between the riser and downcomer regions can
smaller bubbles by the transonic sparger. These bubbles be reduced. This observation is similar to that reported
are usually much smaller than those prevalent under the by several investigators [7, 16, 21, 42] who observed a
dynamic equilibrium that could prevail in the riser and significant reduction in circulation velocity in the presence
will therefore tend to undergo net coalesce as they ascend ofamphiphiliccontaminants.
in it. The rate at which coalescence takes place will be Figure 13 clearly shows that an increase in the pressure
dictatedbythenatureandconcentrationofthecontaminant drop across the transonic sparger had a significant effect
(which determines the coalescence retardation effect) as on liquid circulation velocities in the case of the rapidly
well as by the hydrodynamic conditions in the riser. Taken coalescent air/water system but the impact is much less
alone, this phenomenon is expected to result in increasing in the case of slowly coalescent systems. This could be
the gas holdup in the riser section at increasing surfactant attributed to the fact that finer bubbles are generated at
concentrations with the consequent increase in circulation higherpressuredropsacrossthesparger(Figure4)andtothe
rate. However, counteracting that effect is the tendency of subsequent impact such a parameter has on the differential
smallbubblespresentatthetopoftherisertobeentrained gas holdups between the riser and downcomer regions.
intothedowncomerflow,resultinginsignificantlyincreasing This interpretation is confirmed by the results obtained
thegasholdupinthatregion.Thebalancebetweenthosetwo concerning the effect of bubble size on liquid circulation
opposingtrendsismainlycontrolledbythedesignofthegas velocitythatarepresentedinFigure14.
disengagementunitanditsabilitytoseparatesmallbubbles Theimpactthatbubblesizehasontheliquidcirculating
from the recirculating stream. This task is usually rendered velocitywasdeterminedusingtheresultsobtainedatasingle
moredifficultthesmallerthebubblesare. superficialvelocityinwhichthepredictedsizeofthebubbles
In the presence of surfactants, the effect of superficial generated by the sparger was manipulated by changing the
gas velocity on the value of the circulation velocity was surfactant concentration and/or the pressure drop across
found to depict the same trends as shown in Figure 12. it. The results depicted in Figure 14 clearly show that the
However, the magnitude of the induced liquid circulation behavior of the air/water system is significantly different
was much lower (Figure 13). This phenomenon can be fromthatofsurfactant-containingsystems.
attributed to the fact that whereas the extrafine bubbles In the former case, the liquid circulation velocity could
generatedathighsurfactantconcentrationstendtoincrease be increased by around 10% by introducing finer bubbles