Cylinder Airlift Reactor

13 Pages·2016·0.88 MB·English

Iranian Journal of Chemical Engineering
Vol. 11, No. 3 (Summer 2014), IAChE

Investigation of Gas Hold-up and Bubble Behavior in a Split-
Cylinder Airlift Reactor: Pseudo-Plastic Non-Newtonian Fluids

M. Keshavarz Moraveji, S.E. Mousavi
Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran

Abstract
In this research, the effect of carboxy methyl cellulose (CMC) addition into pure water
as pseudo-plastic non-Newtonian fluid and its concentration on bubble diameter and
gas hold-up were investigated. For this purpose, four different concentrations of CMC
(0.05, 0.1, 0.15 and 0.2%w/v) as the non-Newtonian fluid and five different superficial
gas velocities (0.2, 0.4, 0.6, 0.8 and 1 cm/s) as the gas phase were examined in an airlift
reactor. Bubble size distribution in the airlift reactor was measured by photography and
picture analysis at various concentrations of CMC and various velocities of gas.
Increasing in gas velocity created a wider bubble size distribution and thereby an
increase in bubble diameter and gas hold-up in both riser and down-comer. However,
the bubbles diameter in pure water was larger than those of the CMC solutions (in the
riser and down-comer), but CMC concentration enhancement increased bubbles
diameter and gas hold-up in the down-comer. Bubbles diameter expansion in the riser
by CMC concentration enhancement took place from concentrations of 0.05 to0.15%
(w/v) and then it suddenly decreased. Furthermore, gas hold-up decreased from
concentrations of 0.05 to 0.15%(w/v) and increased at concentration of 0.2%(w/v).The
gas hold-up increases (more than that in the concentrations of 0.1 and 0.15 %) when
bubbles diameter decreases in concentration of 0.2%. The overall gas hold-up trend
was similar to the gas hold-up in the riser.

Keywords: Airlift Reactor, Bubble Diameter, Gas Hold-up, Non-Newtonian, Pseudo
Plastic

1. Introduction∗ [2, 3]. This difference and therefore, the rate
Airlift reactors are the best types of two of induced liquid circulation can be enhanced
phase contactors in different processes such greatly by installing some form of a gas–
as wastewater treatment, animal cell culture liquid separator at the top of the reactor so
and aerobic fermentation (production of that the liquid returning to the down-comer
enzymes, antibiotics, proteins, biomass and zone is mostly free of the entrained gas
other biotechnology products) [1]. bubbles. Notwithstanding the beneficial
Liquid circulation in an airlift reactor is effect of gas–liquid separators, airlift reactors
caused by the difference in gas hold-up are commonly operated without them as only
between the riser and the down-comer zones a small difference between the gas hold-up in
the riser and down-comer zones is sufficient

∗ Corresponding author: [email protected] to generate liquid circulation. In most
3
Investigation of Gas Hold-up and Bubble Behavior in a Split-Cylinder Airlift Reactor:
Pseudo-Plastic Non-Newtonian Fluids
applications the only controlled variable in Reynolds number.
an airlift reactor is the gas flow rate, or Shi et al. [8] evaluated the average shear rate
superficial velocity of the gas in the riser for non-Newtonian liquids in an external
channel. Gas velocity in turn affects gas loop airlift reactor by analogy with the down-
hold-up, the rate of induced liquid comer liquid velocity of Newtonian liquids,
circulation, and the gas–liquid mass transfer and proposed an empirical quadratic equation
rate. Gas–liquid hydrodynamics and mass for the effective shear rate. They found that
transfer in airlift reactors have been studied effective shear rate is lower in airlift reactors
extensively as reviewed elsewhere [4]. than in bubble columns.
With the developments in mineral recovery, Li et al. [9] studied the influence of non-
food processing, biomedical engineering, Newtonian fluid on the hydrodynamics and
biochemical engineering and waste-water mass transfer using a wide range CMC
treatment, situations increasingly arise where concentration of 1–4% in an internal airlift
the liquid behaves as non-Newtonian in reactor. However, this work was limited to
nature. Although many industrial liquids experimental determination of kla and the
which comprise solutions of low molecular reactor used a single-hole sparger, which is
weight can be considered Newtonian-like not usually adopted in industrial reactors. Al-
fluids, an increasing number of solutions Masry [10] investigated the contributions of
with high molecular weights and internal pressure drop due to wall frictional losses to
structure are being used that have non- the total gas holdup of two phase viscous
Newtonian behavior such as variable non-Newtonian systems (xanthan gum and
viscosity and memory effects [5]. Polymer carboxymethyl cellulose) in a circulating
solutions and melts, liquid crystals, gels, bubble column. Pressure drop due to wall
suspensions, emulsions, micellar solutions, shear stress was found to significantly
slurries and foams enter into this non- contribute to the total gas holdup by 10-70%.
Newtonian category [6]. Hence, there is a Kilonzo and Margaritis [11] have reviewed
need to study airlift reactor using non- the effects of non-Newtonian fermentation
Newtonian. According to the literature, the broth viscosities on gas–liquid mixing and
majority of research has been conducted into oxygen mass transfer characteristics in airlift
the airlift reactors with Newtonian fluids and bioreactors.
there are few researches on non-Newtonian Mouza et al. [12] studied the effect of liquid
or high viscosity liquids though there are properties (surface tension and viscosity) on
numerous applied non-Newtonian systems. the performance of bubble columns. They
Metkin and Sokolov [7] derived the wall also developed a correlation for gas holdup in
shear stress for a pseudoplastic non- the homogeneous regime.
Newtonian gas-liquid dispersion as a Khamadieva and Böhm [13] investigated the
function of the Reynolds number and effect of liquid viscosity on mass transfer at
rheological parameters. The authors the wall of packed and un-packed bubble
identified the transition from laminar to columns using Newtonian and non-
turbulent flow condition using the critical Newtonian liquids.
4 Iranian Journal of Chemical Engineering, Vol. 11, No. 3
Keshavarz Moraveji, Mousavi
Cerri et al. [14] evaluated oxygen transfer in column.
three internal-loop airlift reactors of different In another research, Anastasiou et al. [16]
working volumes and similar geometric studied the performance of a bubble column
configuration utilizing eight Newtonian and equipped with metal porous sparger when the
five non-Newtonian fluids. They reported liquid phase is a shear-thinning non-
that the effects of the superficial gas velocity Newtonian fluid. Their aim was to formulate
and liquid viscosity had opposite effects on a generalized correlation that can be applied
the volumetric oxygen transfer coefficient for predicting the average gas of these kinds
and the viscosity effect on oxygen mass of bubble column.
transfer cannot be neglected. In this research, the effects of carboxy
Deng et al. [15] studied hydrodynamics and methyl cellulose (CMC) as pseudo-plastic
mass transfer in a 5m internal-loop airlift non-Newtonian fluid as criteria of fungal
reactor with non-Newtonian CMC solution. fermentation broth and its concentration
They found that increase in aeration velocity onthe bubble diameter and gas hold-up in the
or CMC concentration led to a wider bubble riser and down-comer of split cylinde rairlift
size distribution and an increase in the bubble reactor were investigated.
Sauter diameter. The volumetric mass
transfer coefficient increased with an 2. Experiment
increase in aeration velocity and a decrease 2-1. Materials
in CMC concentration. CMC was purchased from Merck Company
Velez Cordero and Zenit [5] measured mean (Germany) and its various concentrations
rise velocity of bubbles warm ascending in (0.05, 0.1, 0.15 and 0.2 %w/v) were locally
shear-thinning fluids in a rectangular bubble prepared. Table 1 shows the physical
column. They found that the mean rise properties of the liquid phase used in this
velocity of the bubbles was larger than that work. The liquid densities were measured
of an individual bubble, in accordance with using a balance hydrometer and none were
previous studies. Furthermore, they found found to be significantly different from that
that the appearance of clusters produced a of water because the concentration of these
dramatic increase of the agitation within the polysaccharides is very low.

Table 1. Physical properties of the aqueous solutions of carboxymetyl cellulose.
CMC (%w/v) Density (kg/m3) Surface tension(mN/m)
0% (tap water) 998.2 72.75
0.05% 1000.5 73.00
0.10% 1001.2 74.00
0.15% 1001.3 74.50
0.20% 1001.5 75.50
Iranian Journal of Chemical Engineering, Vol. 11, No. 3 5
Investigation of Gas Hold-up and Bubble Behavior in a Split-Cylinder Airlift Reactor:
Pseudo-Plastic Non-Newtonian Fluids
The CMC solutions were assumed to be 2-2. Equipment
pseudo-plastic and were characterized by the The split-cylinder airlift reactor used in this
Ostwald-de-Waele power law model, research is schematically shown in Fig. 1.
(cid:2028) (cid:3404) (cid:1837)(cid:2011)(cid:3041), where τ, γ, K and n are shear The reactor includes a glass column with 1.3
m height and 0.136 m diameter. A
stress, average shear rate, power law fluid
rectangular Plexiglas baffle with 0.129 m
consistency index and flow behaviour index,
width, 1.0 m height and 0.005 m thickness
respectively.
was inserted in the glass column to divide the
The effective viscosity was defined as,
cross section into riser and down-comer
(cid:2020) (cid:3404) (cid:1837)(cid:2011)(cid:3041)(cid:2879)(cid:2869). The K and n values of the
(cid:3032)(cid:3033)(cid:3033)
zones (with the riser area of 86.115 cm2 and
CMC solutions and viscosity of the other
down-comer area of 40.299 cm2 and the ratio
liquids (μ ) were determined with the help of
L
of 2.136 riser per down-comer). The baffle
a Brook field cone plate viscometer (Model
was also located at 0.1 m from the bottom of
RVTDV-ICP) from the respective flow
the reactor. The gas free liquid height in the
curves. The values of the power law
column was about 1.23 m for all
constants K and n were obtained by
experiments. The gas sparger was a 0.015 m
regression of the viscometer results. Each
diameter sintered ceramic ball located at the
CMC solution was used within a day to
bottom of the riser. The volumetric flow rate
ensure that viscosity had not been reduced by
of air in the riser zone was controlled using a
biodegradation.
regulating valve and a calibrated rotameter.
In order to calculate the effective viscosity of
The inverted U-tube manometers were used
a non-Newtonian fluid in the airlift reactor
for hold-up measurement. All experiments
the effective average shear rate in the airlift
were carried out at ambient conditions
reactor, the relation for the shear rate
(atmospheric pressure and (25±0.5) °C).
proposed by Nishikawa et al. [17] for bubble

columns for this purpose was used.
2-3. Measurement methods
According to Nishikawa et al. [17] the
The steady‐state bubble diameter size was
relation is:
determined with photographic technique by a

digital camera (Panasonic, model: DMC-
(cid:2011) (cid:3404) 5000 (cid:1847)
(cid:3028)(cid:3049)(cid:3032) (cid:3034) FS20 with resolution of 10M pixels). The
moving average method [18] was used to

count number of bubbles which was more
Where (cid:2011) is average shear rate and U is
(cid:3028)(cid:3049)(cid:3032) G
than 300 bubbles that were randomly chosen
the superficial velocity (m/s), respectively.
in 10 pictures. Then, bubble diameters were
Surface tension (σL) reported in Table 1 was
measured by WINDIG software (version
measured using a ring tensiometer (model
2.5).
K10ST, KRUSS GmbH, Hamburg,
In spherical bubbles d is equal to d (so
1 2
Germany). Compressed and oil-free air was
d =d =d ), while elliptical bubbles were
v 1 2
used as the gas phase in all experiments. The
measured according to the maximum and
air superficial velocity was varied from 0.2 to
minimum diameters of bubbles as follows:
1ms−1.
6 Iranian Journal of Chemical Engineering, Vol. 11, No. 3
Keshaavarz Moravejji, Mousavi

Figure 1. Schematic diiagram of the split-cylinder airlift reactorr.

technique [220] by meaasuring the differentiall
(cid:1856) (cid:3404) (cid:3119)(cid:3495)(cid:3495)(cid:1856)(cid:2870)(cid:1856)
(cid:3049) (cid:2869) (cid:2870) (1) ppressure bbetween twwo inverteed U-tubee
mmanometerss (Fig. 1). Thhe gas holdd-up (ε) wass

ccalculated as:
Where d , d andd d are the maximmum,
1 2 v
minimuum and equivalennt diameeters,
ρ dh
respectiively. εε= L M (3))
ρ −ρ dz
The avverage of bbubbles diaameter (d ) is L GG
avee
calculatted as:
wwhere, ρ iss the densitty of fluidss, ρ is thee
L G
ddensity of aair, dh is thhe manomeeter readingg
∑(cid:3036)(cid:2880)(cid:3015)(cid:1856)(cid:2871) M
(cid:3036)(cid:2880)(cid:2869) (cid:3036)
(cid:1856) (cid:3404) aand dz is thhe space beetween the manometerr
(cid:3028)(cid:3049)(cid:3032) ∑(cid:3036)(cid:2880)(cid:3015)(cid:1856)(cid:2870) (2)
(cid:3036)(cid:2880)(cid:2869) (cid:3036)
taps.

d is buubble diameeter and I iss the numbeer of
i 33. Results aand discussiion
bubbless.
33-1. Bubble ddiameter meeasurement
The vollume expannsion methood was appplied AAs shown in Table 11, the surfaace tensionn
to meassure the oveerall gas holld-up duringg the inncreased bby increasinng the CMMC solutionn
steady sstate condittion. The ovverall gas hhold- cconcentratioon. With ddecreasing tthe surfacee
up (ε ) wwas recordeed by visuaal measuremments
g tension, bubbble volumme will inccrease [21]..
using thhe volume eexpansion mmethod [19]..
CCMC moleccules accummulate on the bubblee
The gass hold-up inn the riser annd down-coomer
ssurface and increase thhe required energy forr
sectionss was deteermined wiith manommetric
bbubble breaakup whenn the bubbles movee
Iranian JJournal of Chhemical Enginneering, Vol. 11, No. 3 77
Investigatioon of Gas Holld-up and Bubbble Behavior in a Split-Cyllinder Airlift RReactor:
PPseudo-Plastiic Non-Newtonnian Fluids
within thee continuouus phase. FFurthermoree,
the densities of solutions increasse with CMC
concentratiions. This increases thhe buoyanccy
force. Therrefore, liquiid phase maass increasees
with the ggrowth of bubble andd drag forcce
enhances, consequenttly the groowth time oof
bubble is extended [22]. Whhen bubblees
coalescencce rate inccreases andd instabilitty
occurs upp to a criitical pointt, instabilitty
reaches aa maximuum amounnt. At thiis
condition, bubbles bbreakup raate suddenlly
rises to ovvercome thhe bubbles coalescencce
Figuure 2. Averagge of bubbles diameter withh different
[23, 24]. AAs shown inn Fig. 2 bubbbles breakuup
aerattion velocity aand CMC conncentration in tthe riser.
and criticaal point in thhe riser wass obtained aat

concentratiion of 0.155 %. Thereffore, bubblees
breakup raate increaseed and bubbble diameteer
decreased in this conccentration. AAs illustrateed
in Fig. 3,, bubble diiameter inccreases witth
concentratiion enhanccement in the downn-
comer beccause the buubbles diammeter cannoot
reach a maximum amount. Since thhe
buoyancy fforce decreaases in commparison witth
the drag force for smaller buubbles, thesse
bubbles caan import the down--comer. Thhe
bubbles avverage is shown in Figs. 2 and 3 aas
plotted byy MATLAAB softwaare (versioon

7.6.0.324).. The bubblees diameterr increased iin

the riser ((Fig. 2) andd down-commer (Fig. 33) Figuure 3. Averagge of bubbles diameter withh different
aerattion velocity aand CMC conncentration in the down
when aerattion velocityy increased.
comeer.
The CMMC concenntration eenhancemennt

increases thhe viscosityy of solutionn. Thereforee,
the bbubble diammeter initiallly reduces and then
the drag fforce of fluuid aroundd the bubblle
rises. The turbbulence in the riser ddecreases
obstructs tthe bubble rrising [25]. This causees and the bubblle size disstribution increases
surface teension to increase for CMC wheen the soluution viscossity increasses [20].
solutions against waater and laarge bubblees Figss. 4 and 5 shhow the inffluence of CCMC and
break. Thee bubble diiameter inccreases wheen its concentratiion enhanccement on bubble
surface tennsion of ssolution inccreases. Thhe diammeter in the riser and downn-comer,
surface ttension, vviscosity aand densitty resppectively. AAs illustrateed in these figures,
increase wwhen CMC is added to water. Soo, the bubble diameter redduced fromm 1.8 to
8 Iranian JJournal of Chhemical Engiineering, Vol. 11, No. 3
Keshaavarz Moravejji, Mousavi
1.5mm and 2.3 too 1.2mm inn the riser and 00.15% (w/v)). The bubbble diameterr was aboutt
down-coomer, resspectively when CCMC 11.8 and 1.6mmm in the rriser and doown-comer,,
concenttration andd thereby density and rrespectivelyy, in the highh aeration vvelocities.
viscositty increasedd. Around 110% of bubbbles TThe numberr of bubbless which aree smaller orr
in pure water havee the same ssize while aabout bbigger thann the averaage diametter size iss
20 and 330% of bubbbles in the riser and doown- aalmost equaal in the rriser. They also havee
comer rrespectivelyy have the same sizee for vvarious diaameters (frrom 1.2 too 2.3mm)..
CMC soolutions. FFurthermoree, there arre not muuch biggerr
Figs. 6 and 7 shoow the efffect of aeraation bbubbles in the downn-comer (from 1.4 too
velocityy on bubblee diameter iin the riserr and 22mm).
down-coomer for CMC cooncentrationn of

Figurre 4. Percentagges of bubbles with the samme diameter inn the riser withh different
CMC conceentration at thee Ug of 0.4cmm/s.

Figure 5. Percentages oof bubbles witth the same diiameter in the down-comer with different
CMC conceentration at thee Ug of 0.4cmm/s.
Iranian JJournal of Chhemical Enginneering, Vol. 11, No. 3 99
Investigatioon of Gas Holld-up and Bubbble Behavior in a Split-Cyllinder Airlift RReactor:
PPseudo-Plastiic Non-Newtonnian Fluids

FFigure 6. Perccentages of buubbles with thhe same diameeter in the riserr with differennt Ug at the
CMC conceentration of 0.15 (w/v).

Figure 7. Perrcentages of bbubbles with thhe same diameter in the dowwn-comer witth different
Ug at the CMC cconcentration of 0.15 (w/v).

In accordaance with Denget. aal [15], ouur indeependent off aeration veelocity.
results shoow that bubbble diametter increasees Furtthermore, bbubble diammeter increeases by
by increassing the ggas velocitty, althouggh incrreasing the CMC conccentration. DDeng et.
Yoshimotoo et al. [266] found thaat increasinng al [[15] reporteed that thee increase iin CMC
bubble diameter is appproximatelly conccentration (liquid viscosity) leed to a
10 Iranian JJournal of Chhemical Engiineering, Vol. 11, No. 3
Keshaavarz Moravejji, Mousavi
decreasee in turbulence intennsity, whichh in rreactor. Moreover, it ccan producee the largerr
turn deecreased thhe bubble breakup and bbubbles. Therefore, thee gas hold-uup increasess
coalesceence. Furtheer, the bubbble breakup was [27].
more seensitive to liiquid viscossity than buubble FFigs. 8 and 10 also shoow that the ggas hold-upp
coalesceence, thus tthe bubble ssize distribuution rreduces bby increeasing thhe CMCC
shifted to larger size with an increase in cconcentratioon. Since thhe big bubbles in thee
CMC cooncentrationn [15]. vviscous soluutions havee short residdence time,,
thhe gas holdd-up decreaases. The ggas hold-upp
3-2. Gass hold-up inncreases (more thhan that in thee
Figs. 88 and 9 show gas hhold-up veersus cconcentratioons of 0.1 and 0.15 %) whenn
superficcial gas vvelocity (UU ) for varrious bbubbles diammeter decreeases in cooncentrationn
g
CMC soolutions in the riser annd down-comer, oof 0.2 %. HHwang andd Cheng [228] showedd
respectiively. Fig. 10 also shoows overalll gas thhat the gass hold-up inn the downn-comer forr
hold-upp versus ssuperficial gas veloocity. 00.8 wt% CMMC solutionn was higheer than thatt
Accordiing to theese figuress, gas hold-up ffor 0.5 wt%% CMC sollution. Frannsolet et al..
increaseed by increeasing the superficial gas sshowed thatt the gas holld-up was cclose for thee
velocityy. This resuult is in agreeement withh the 44 wt% and 5 wt% xannthan gum solutionsinn
literaturre [15]. UU less thann 8 cm/s [29]. They reeported twoo
g
Furthermmore, the ggas hold-up in the riserr and mmechanismss for this result: onee was thatt
overall gas hold-uup linearly increased with ddecreasing tthe bubble iincreases velocity thatt
U while it initiallly rose sliightly and then rresulted in llonger gas holdup, annd the otherr
g
increaseed sharply against gass hold-up inn the is, increasinng the bubbble increases velocityy
down-coomer. The gas velocitty enhancemment wwith an incrrease in bubbble diameteer.
increasees the smalll bubbles ddispersion inn the

Figure 8. Gas hold-up vversus superficial gas veloccity in the riserr.

Iranian JJournal of Chhemical Enginneering, Vol. 11, No. 3 11
Investigatioon of Gas Holld-up and Bubbble Behavior in a Split-Cyllinder Airlift RReactor:
PPseudo-Plastiic Non-Newtonnian Fluids

Figuure 9. Gas holld-up versus suuperficial gas velocity in thhe down-comeer.

Figure 10. OOverall gas hoold-up versus ssuperficial gass velocity.

Deng et all. [15] showwed that thee gas hold-uup currrent researrch, since the gas aeration
for the 00.40 wt% and 0.45 wt% CMC veloocity was inn the limitedd range, thee number
solutions wwere quite ssimilar in UU less than 6 of bbubbles thaat move intto the dowwn-comer
g
cm/s. Thee bubble rrise velocitty reductioon willl be few. Fiig. 11 showws a comparrison for
(with liquuid viscosiity incremment) causees gas hold-up in the riser annd down-comer.The
longer bubbble residence time andd gas hold-uup gas hold-up diffference in tthe riser annd down-
increment. Further, the bubbble velocitty commer creates the drivinng force foor liquid
enhancemeent (withh bubblee diameteer circulation veloocity.
reduction) increases tthe liquid vviscosity duue Theerefore gas hold-up risses with suuperficial
to the bubbble break upp reduction. gas velocity inccrement in the riser annd down-
The bubblle diameterr increases with CMC commer. A similar trend is oobserved for overall
concentratiion increment in the down-comeer gas hold-up, ass well. Howwever gas hoold-up in
and gas hoold-up increeases. Accoording to thhe the riser decreaased with Fiigure increaasing the
12 Iranian JJournal of Chhemical Engiineering, Vol. 11, No. 3