Balamurugan.pdf

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AENHANCEMENT IN GAS HOLDUP IN BUBBLE COLUMNS

THROUGH USE OF VIBRATING INTERNALSV. Balamurugan, D. Subbarao and Shantanu Roy*

1. Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India

Including internals in bubble columns is known to enhance the gas holdup. In this paper, a method to achieve this objective substantially has been

proposed via the use of vibrating helical spring internals. Experimental observations on effect of vibrating internals such as vibrating helical springson gas holdup in bubble columns are presented. Effects of superficial gas velocity, H/D ratio (height of the static liquid to column diameter ratio),volume fraction of helical springs, and thickness of the helical spring wires on hydrodynamics parameters are studied. Increase in gas holdup upto 135% is observed by using vibrating helical spring internals in bubble columns compared to bubble columns without internals. This methodoffers a simple, cost-effective, and easy way to enhance gas holdup even at high gas fluxes. It has been reported that this enhancement stemsfrom the fact that the vibrating springs breakup the gas into fine bubbles, which effectively reduces their rise velocity and enhances their averageresidence time in the liquid column.

On sait quinclure descomposantesinternesdans lescolonnesa bullesaccrot levolumemort. Dansce document, une methodepour atteindreen

substance cet objectif a ete proposee par lutilisation de composantes internes de ressorts helicodaux vibrants. Des observations experimentales

sur leffet des composantes internes vibrantes comme les ressorts helicodaux vibrants sur le volume mort dans les colonnes a bulles ont ete

presentees. Les effets sur la vitesse superficielle du gaz, le rapport H/D (rapport de hauteur du liquide statique au diametre de la colonne), la

fraction du volume des ressorts helicodaux et lepaisseur des fils des ressorts helicodaux sur les parametres hydrodynamiques sont etudies. Une

augmentation du volume mort jusqua 135 % est observee au moyen des composantes internes de ressorts helicodaux vibrants dans les colonnes

a bulles, en comparaison avec les colonnes a bulles sans composantes internes. Cette methode offre une facon simple et economique daccrotre

le volume mort meme lors de flux de gaz eleves. On a rapporte que cet accroissement decoule du fait que les ressorts vibrants divisent le gaz enfines bulles, ce qui reduit de maniere efficace leur vitesse dascension et accrot leur temps de sejour moyen dans la colonne de liquide.

Keywords: bubble column, gas holdup, volume fraction, vibrating internals, springs, churn-turbulent flow

INTRODUCTION

Bubble columns are generally cylindrical vessels where gas

is sparged into a pool of liquid or liquidsolid suspension in

the form of bubbles. Bubble columns serve as multiphase

reactors and contactors in chemical, petrochemical, biochemical

processes, effluent/waste-water treatment process, and metallur-gical industries. As reactors, bubble columns are encountered in a

wide range of applications in chemical processes involving oxida-

tion, chlorination, alkylation, polymerization, and hydrogenation

reactions (Shah et al., 1982; Fan, 1990). Bubble columns are

also used in processes such as hydrotreating and conversion of

petroleum residues (Lunin et al., 1985), coal gasification, and

coal liquefaction (Tarmy et al., 1984). Other processes that uti-

lize bubble columns are gas conversion processes involving the

production of liquid fuels from synthesis gas, for example, Fisher

Tropsch process (Kolbel and Ralek, 1980), synthesis of methanol

(e.g., LPMEOHTM technology for liquid-phase methanol synthe-

sis from Air Products and Chemicals), and other synthetic fuels.

In a world increasingly interested in bio-fuels and fuels from

renewable sources, bubble columns are thus expected to play an

increasingly dominant role as process equipment.

As separation units, bubble columns are utilized in the form of

bubble cap tray towers. Bubble columns are used as fermenters,in wastewater treatment and in variety of metallurgical opera-

tions such as leaching of metal ores. Bubble columns are preferred

over other types of reactors due to their simplicity in operation,

compactness, low operating cost, maintenance cost, lack of mov-

Author to whom correspondence may be addressed.E-mail address: [email protected]. J. Chem. Eng. 9999:1, 2010

2010 Canadian Society for Chemical EngineeringDOI 10.1002/cjce.20362Published online in Wiley InterScience(www.interscience.wiley.com)

| VOLUME 9999, 2010 | | THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING | 1 |


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Aing parts and therefore reduced wear and tear, high durability of

catalyst, and good heat and mass transfer characteristics.

The gas holdup and consequently the gasliquid interfacial area

is the single-most important variable governing the operation of

bubbles columns as process vessels. The gas holdup drives the

liquid circulation and the multiphase turbulence field, and thus

determines to a large extent the gasliquid interfacial area and

associated mass transfer rates as well. Provided that the reactions

are not all kinetically controlled, whenever inter-phase and intra-

phase transport are governing phenomena, having optimal gas

holdup is the first priority for smooth and economically viableoperation. The numerous reviews that have appeared suggest that

the bottleneck in further improvement in bubble column perfor-

mance comes because of our inability to control and enhance the

gasliquid interfacial area and the gas holdup at high gas fluxes

(e.g., JoshiQ1 and Shah, 1981; Shah et al., 1982; Deckwer, 1992;

Koide, 1996; Wild et al., 2003). Thus, if we are able to enhance

these two physical parameters, we will be successful in enhanc-

ing volumetric mass transport rate between the gas and liquid

phases, and hence enhance all those reactions which are lim-

ited by the interfacial transport rate. Unfortunately, this wish

is not easily realizable going forward, because the use of bet-

ter designed spargers, distributors as well as various kinds of

internals has already been attempted. These modifications have

provided marginal improvement in gas holdup and interfacial area

but have not been game-changers in bubble column technology.

With a broader perspective, the need to enhance gas holdup and

interfacial area is in line with the global objective of process

intensification that is likely to define the chemical and refining

industry of the future (Dautzenberg and Mukherjee, 2001), There-

fore, improved gasliquid contacting in bubble columns is not

only a future need for these reactors alone but also a demand of

the process industry of the future, in general.

One proposed method of improving gasliquid contacting in

bubble columns has been by introducing appropriately designed

internals (Fair et al., 1962; Patil et al., 1984; Yamashita, 1987a;

Pradhan et al., 1991). These internals (e.g., horizontal baffles, ver-

tical rods, helical coils, KATAPAK-S, Sulzer

SMV type) improvegasliquid contacting by intensifying the process parameters

through enhanced phase holdups, higher specific gasliquid inter-

facial area, better heat and mass transfer characteristics, all at

higher throughputs. The last point is of particular importance,

because while it is possible to improve holdup when gas flow

rates are small, as soon as the gas superficial velocity increases,

the complex multiphase turbulence takes over and widespread

bubble coalescence leads to the formation of larger bubbles. These

are more buoyant, have a higher rise velocity, and hence the effec-

tive holdup of gas in the vessel tends to level off. In other words,

the further enhancement of gas holdup is very marginal and cer-

tainly not commensurate with the imposed increase in gas flow

rate. The internals that have been considered in the past include

heat exchanger tubes, inserted into the reactor for cooling/heatingthe system and maintaining isothermal conditions, especially for

highly exothermic/endothermic reactions. In some cases, bubble

columns are also sectionalized using baffles to reduce back mix-

ing (Shah et al., 1982; Deckwer and Schumpe, 1993; Pandit and

Doshi, 2005). Even though these internals improve the bubble

column performance, they are too expensive and occupy space

inside the reactor (high volume fraction of internals). This consti-

tutes the valuable real estate in the reactor, that is, the volume

that would have been otherwise used for the desired reaction.

Perhaps the first reported instance for process intensification

in bubble columns using some sort of vibrational mode was the

work of Baird and Garstang (1967), wherein they reported that

pulsing the inlet air flow leads to enhancement of mass transfer. In

recent years, some researchers have reported vibration of bubble

columns as a possible means of improving the gasliquid contact-

ing. Vibrating or shaken bubble columns have been shown to be

advantageous over conventional bubble columns by reducing the

mean bubble size (Ellenberger and Krishna, 2003a; Ellenberger

et al., 2003), and enhancement of gas for the same overall oper-

ating conditions. Vibrations in the liquid phase at a frequency

of 40120 Hz have been achieved by a vibration exciter (Ellen-

berger and Krishna, 2003b) and by an oscillating plate connectedto the vibration exciter. These vibrations are generated by exter-

nally controlled vibration excitement devices. Consequently, an

external energy input is required to generate vibration inside the

bubble columns and also mechanical difficulties associated with

the moving parts are themain drawbacks of these vibrating bubble

columns.

In summary, a wish list of what one expects from internals

in bubble columns can be listed as follows:

(i) low volume fraction of internals;

(ii) high surface area of internals;

(iii) no external energy input;

(iv) high-throughput operation;

(v) minimum maintenance cost of internals and other bubblecolumn hardware.

In this paper, one way of addressing some of the points raised in

the above wish list is presented. Specifically, use of simple springs

as internals in bubble columns is proposed, with the underly-

ing idea that vibrations in the springs caused by gas flow can be

harnessed to modify the gasliquid contacting in the columns.

The objective of the work is to experimentally study the effect of

vibrating helical spring internals on gas holdup.

EXPERIMENTAL

A schematic diagram of the facility used in the present study isshown in Figure 1a. A bubble column made of Perspex was

designed, fabricated, and erected for the measurements. The bub-

ble column has an internal diameter of 15 cm and a height of

125 cm. Gas is sparged into a pool of liquid in the column through

a gas distributor, which is a 1 cm thick perforated plate made of

Perspex with 126 holes of 0.2 cm diameter each on a 1-cm pitch.

The internals employed in this work can be characterized by

their volume fraction and surface area of contact with the fluid

around them. The internals investigated in this work are essen-

tially straight or coiled structures positioned vertically along the

vertical axis of the bubble column, and fixed at both ends. In one

geometrical limit, they are straight vertical rods while in the other,

they are coiled springs. In all that follows, the term internal is

used to describe in general any such structure.For a given total volume occupied by internals, the surface area

of the internals increases with decrease in diameter or thickness

of the internals. However, there is a limit on thickness/diameter

at which mechanical strength of the internal declines to such an

extent that it cannot maintain its own structural integrity. Further

decrease can make the internals collapse unless supported oth-

erwise. As the internals thickness is reduced (they being fixed

at both ends), depending on their effective stiffness constant and

damping coefficient, they can vibrate notably when subjected to a

time-varying force field. The amount of vibration (amplitude and

frequency) rendered is a function of the nature of the time-varying

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AFigure 1. Schematic diagram of bubble column with spring internals. a: Set-up; (b) helical spring holderisometric view; (c) helical spring internalholdertop view.

force field, and modulating it for enhancing the bubble column

reactor performance is desirable.

In this work, the vibrating internals used are helical springs

of different diameters and thickness. Vertical rod internals are

also employed in the present study for the comparison purposes.

Details of helical springs and vertical rods internals used in the

study are given in Table 1.

The helical spring internals are held in position with the help

of holder as shown in Figure 1b,c. The spring holder consists of

two circular rings, one at the base and other at the top, held in

position by three tie rods. The circular rings are fitted with wire

mesh and the helical springs are supported at the nodal points

of the wire mesh at the top and bottom. The number of circular

rotation of the springs is kept constant, 105 rotation for all types

springs. Similar to spring holder, rod holder consists of a skeleton

of two circular rings supported by four tie rods. The circular rings

are provided with supporting holders (2 cm diameter and 2 cm

height) for location of a rod at the centre r/R = 0 and symmetric

location of eight more rods at r/R = 0.5. Top and front views of

the internals holders are shown in Figure 2.

In order to estimate the spring constant in the current work,

a spring is suspended from a support as shown in Figure 3 and

its unstretched (no-load) length is noted. Some known weight is

added to one end of the spring. The spring stretches and comes

into equilibrium at a length x beyond the springs unstretched

length. The experiment is repeated for different loads. By mea-

suring and plotting the spring force, F (=mg, where m is the

mass loaded), against the extension in the spring, x, a straight-line

graph with slope k. In the present analysis, the damping factor of

the spring has been ignored.

The natural frequency of vibration of the springs is determined

by hanging a known weight and pulling the weights down and

releasing them. A springs vertical motion in cycles per second

is measured and the same experiment is repeated for different

weights and the average natural frequency is determined. Spring

constant and natural frequency of the springs employed in the

present study are also listed in Table 1.

Gas Holdup MeasurementSaturated air is passed into a pool of water in the column through

a calibrated orifice meter. The system is operated at ambient con-

Table 1. Details of internals used

Diameter of Wire thickness Number of Volume fraction Spring constant NaturalType of internals internals (cm) (cm) internals of internals (N/m) frequency (Hz)

Springs (S1005) 1.0 0.05 39 2.29 103 20.82 1.15

Springs(S19005)

1.9 0.05 1 5.57 105 0.68 1.41.9 0.05 9 5.01 104

1.9 0.05 21 1.70 104

1.9 0.05 41 2.28 103

Springs(S4005)

4.0 0.05 14 2.25 103 0.41 0.634.0 0.05 21 3.37 103

Springs(S1901)

1.9 0.1 11 2.26 103 11.13 2.151.9 0.1 41 8.42 103

Springs (S401) 4.0 0.1 19 8.35 103 1.98 0.65Rods(R19)

1.9 1.9 1 0.0161.9 1.9 9 0.144



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AFigure 2. Photographs of internals. (a) Top view of column with springs. (b) Top view of rod frame. (c) Front view of spring internals. (d) Front view ofrod internals.ditions. Saturated air from the blower is passed into a pool ofwater in the column through a calibrated orifice meter. Height ofliquid dispersion to diameter ratio (H/D) is varied from 2 to 5.Superficial gas velocity used in the present study ranged from 3.6to 54.2 cm/s. At the higher gas velocities, there is some ambigu-ity in determining exactly the height of the expanded bed becausethe disengaging bubbles cause liquid splashing at the top, so thatFigure 3. Schematic characterization of springs.

the top surface of the liquid is highly dynamic. To overcome this

ambiguity, average dispersion height is taken into consideration,

which is measured by noting the dispersion height at three differ-

ent locations (located on the circumference of the column) and

taking the average of the time averages at these locations. The

time-averaged gas holdup (g) is estimated by noting the height

of dispersion Hd as a function gas flow rate for each clear liquid

height H using the following equation:

g =Hd H

Hd(1)

RESULTS AND DISCUSSION

Experiments are carried out for different internals and different

operating parameters (superficial gas velocity and H/D ratio) in

bubble columns, and the representative data of bubble columns

withoutand with internals are presented for discussion.

Gas Holdup in Bubble Columns Without InternalsThe gas holdup in bubble columns withoutinternals is measured

to benchmark the gas holdup of the bubble columns with inter-

nals. Here, the effect of superficial gas velocity and H/D ratio(height of the static liquid to columndiameter ratio) on gas holdup

is investigated. In the homogeneous bubbling regime, the size of

the bubbles is entirely dictated by the sparger design and physical

properties of the gasliquid phases. In contrast, in the heteroge-

neous regime the role of sparger design diminishes depending

upon the column height. The total height of the liquid in the col-

umn is divided into two regions, the sparger region and the bulk

region (Thorat et al., 1998). The size of the bubbles in the bubbly

regime is totally dictated by the sparger (primary bubble size) and

mainly depends on the sparger design. In the sparger (gas distrib-

utor) region, the size of the bubbles changes with respect to height



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AFigure 4. Effect of superficial gas velocity on gas holdup for bubblecolumn without internals.

depending upon the coalescence nature of the liquid phase (sur-

face tension of liquid), the level of turbulence, and bulk motion

(Thorat et al., 1998). At the end of the sparger region the bubble

attains an equilibrium size referred by Thorat et al. (1998) as the

secondary bubble size. The equilibrium bubble size is governed

by the breaking forces due to bulk motion (turbulent and vis-cous stresses) and the stabilizing force due to surface tension. The

height of the sparger region depends upon the difference between

the primary and secondary bubble size, the coalescence nature of

the liquid phase and liquid circulation in heterogeneous or churn-

turbulent regime. The relative proportion of the sparger region in

the total column height decides the effect of H/D ratio on gas

holdup. If the sparger region is small, the effect of H/D ratio on

gas holdup is minimum, and vice versa (Thorat et al., 1998).

In Figures 4 and 5, the effect of the superficial gas velocity

and H/D ratio on gas holdup with internals is presented. The

measured average gas holdup increases steadily with superficial

gas velocity for all H/D ratios. With increase in H/D ratio gas

holdup decreases to a limiting value, beyond which H/D ratio hasnegligible effect on gas holdup. Observation indicates forH/D 3,

there is no perceivable effect on gas holdup. Thus, the role of the

sparger region above H/D 3 is minimal.

For comparison, the range of experimental data of Thorat et al.

(1998) is shown on the same graph (Figure 4). It is clear that our

experimental data also fall within the acceptable range. The exper-

iments carried out in this work are repeated several times and the

Figure 5. Effect ofH/D ratio on gas holdup for bubble columns withoutinternals.

Figure 6. Effect of superficial gas velocity and H/D ratio on gas holdupfor bubble column with vertical rod internals (number of rods = 9,diameter of each rod = 1.9 cm, total volume fraction of ninerods = 0.144).

maximum relative error is observed to be 5%. The moderate devi-

ation in the gas holdup of the present study to the literature data

of Thorat et al. (1998) can be ascribed to the fact that the column

diameter used by Thorat et al. (1998) was 38.2 cm.

Gas Holdup in Bubble Columns With Internals

Gas holdup in bubble columns with vertical rodinternals (R19)Effect of superficial gas velocity and H/D ratio on gas holdup

for vertical rod internals (number of rods = 9, diameter of

rod = 1.9cm, total volume fraction of nine rods = 0.144) in bubble

columns is shown in Figure 6. Similar to bubble columns without

internals, the gas holdup increases with superficial gas velocity

and decreases with H/D ratio to a limiting value for H/D 3.

The data for various H/D ratios fall within the error range. The

same observations regarding relative insensitivity of the measured

holdup to H/D ratio were made with different numbers of rod

internals. This fact was essential to establish so that data for aver-

age gas holdup using volume displacement method (Equation 1)

can be taken with confidence.

Effect of volume fraction of vertical rod internals on gas holdup

is shown in Figure 7. Clearly, the gas holdup increases substan-

tially with increase in volume fraction of rod internals. A single

vertical rod does not have much effect on gas holdup, when

compared to a bubble column without internals. However, the

enhancement in gas holdup is significant for a higher number of

internals (e.g., nine vertical rods), whence the internals signifi-

cantly reduce the cross-sectional area available for flow, resulting

in higher interstitial gas velocity and promoting radial mixing and

circulation of bubbles. This observation of higher gas holdup forinternals due to the reduction in the cross-sectional area for flow

was also reported by Yamashita (1987b) and Pradhan et al. (1991,

1993).

Gas holdup in bubble columns with helical springinternalsExperiments are carried out for different helical spring internals

and different operating parameters (superficial gas velocity and

H/D ratio) in bubble columns and the representative data of bub-

ble columns with helical spring internals and the amount of

vibration of springs inside the bubble columns are presented here.



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AFigure 7. Effect of volume fraction of internals on gas holdup for bubblecolumn with rod internals (H/D ratio = 3, diameter of each rod = 1.9cm).

Peak-to-peak displacement and frequency of vibrating spring

internals. The motion of any simple harmonic oscillator (as the

helical springs have been reasonably assumed to be) is char-

acterized by two quantities: the amplitude and the time period

(or frequency). The peak-to-peak displacement (two times ofamplitude), the maximum upward (compressive) and downward

movement (expansive) of the springs from a specific reference

point, and thefrequency of the vibrating springs (number of cycles

per second) in the bubble columns are analysed by photographic

imaging. Figure 8 shows the pictorial representation of peak-to-

peak displacement of the helical spring internals from a specific

reference point. A reference point is a marking made to the one

edge coil of the springs which is as close as to the wall. A cam-

era (Silicon Graphics model: INDY CMNB0060) is focused at the

reference point and used to measure the movement of the springs

inside the column, and is interfaced with a computer. The camera

acquires 30 frames per second, and IRISTM software is loaded for

image processing. The camera records the vibration of the springs

with respect to time for various superficial gas velocities. The timeinterval of the entire observation is 20 s. A frame-by-frame anal-

ysis is done using the IRISTM software to determine displacement

and the frequency of the vibrating springs. The liquid height used

for this study is 75 cm (H/D ratio = 5).

When the bubble column is in operation, the springs are sub-

jected to an external forcing function generated by the gas flow.

Note that in stationary liquid the springs would not oscillate, and

this periodic motion is brought about by the chaotic and ran-

dom motion of the bubbles in the bubble column. The bubble

motion involves a multitude of frequencies and this spectrum of

frequencies serves to induce a multiple harmonics in the springs.

In turn, these harmonics induce periodic motions (at various fre-

quencies) in the liquid and indeed the two-phase (gasliquid)mixture. Thus, clearly, the coupling is two-way: gas flow induces

vibrations in the springs at multiple frequencies, which in turn

modulates the flow field in the liquid, and through the drag and

other interphase forces, the flow field, and configuration of the

gas phase. Earlier authors (e.g., Krishna et al., 2000; Krishna and

Ellenberger, 2002; Ellenberger et al., 2005) have also highlighted

the notion of external frequencies modulating the flow field; how-

ever, in their case the external vibration source affects the flow

field, but the flow field does not change the vibrations in any

way (one-way coupling). In the present work, the coupling is

two-way.

In addition to theabove effects, the spring internals play a role of

physically chopping up the bubbles because of direct mechanical

interaction with the gas phase as the bubbles make their way

up the column. Note that this is in addition to the modulation

of spring oscillations by the gas phase (gas bubbles), since that

effect would have occurred even if the bubbles were rigid entities.

The physical disintegration and redispersion of the bubbles by the

internals is a complex phenomenon which clearly is an undertone

to the present work.

The effect of the external force (superficial gas velocity) on

the amplitude and frequency of the springs of the type S19005

(springs made of 0.05cm wire thickness and 1.9 cm diameter)

is shown in Figure 9. This is typical for the behaviour of the

spring internals in bubble columns studied in this work. The

graph shows that the amplitude increases slowly in the super-

ficial gas velocity range of 010 cm/s, following which there isa sharp increase of the amplitude in the range of 1036 cm/s.

Further increase of the superficial gas velocity (beyond 36 cm/s)

Figure 8. Pictorial representation of peak-to-peak displacement of springs.



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AFigure 9. Effect of superficial gas velocity on vibration displacement andfrequency of the springs (number of springs = 41, diameter ofsprings = 1.9 cm, wire thickness = 0.05cm).

has no significant effect on the amplitude of the vibrating springs

inside the bubble column.

On the other hand, the frequencies of the oscillating spring are

very high at the bubbly regime (gas velocities below 6 cm/s) then

drop sharply in the superficial gas velocity range of 615 cm/s,

then remain almost constant up to superficial gas velocity36 cm/s,

and then increase slowly with further increase in the superfi-

cial gas velocity. While clearly there is a multitude of frequencies

involved in the process and the physics is quite complex and war-

rants a separate study under more controlled conditions of gas

flow than what exist in a bubble column, one may still attempt to

rationalize the observations of Figure 9 as follows.

At low gas superficial velocities, the bubbling at the orifice is

a rapid process and each bubble is small and all bubbles are of

fairly uniform size with minimal coalescence and redispersion.

The kinetic energy associated with the gas flow is low, thus when

this gas interacts with the springs they can only cause a small dis-

placement in them and consequently low amplitude (Figure 9).

Further, since the bubbles are rising in well-defined layers witha fairly uniform characteristic rise time, successive layers of bub-

bles interact at regular intervals with the springs resulting in a

high frequency of vibration. Above a gas superficial velocity of

6 cm/s, we believe (based on visual observations and photogra-

phy, though admittedly these images are not clear and it is hard

to see through the wall) that coalescence sets into the bubble

ensemble, and large bubbles begin to be formed as a result of

this coalescence. This initiates larger displacements in the springs

because of the presence of these large bubbles, which is shown in

the sudden and steep rise in amplitude starting from about super-

ficial gas velocity 10 cm/s. It seems that the range 610cm/s is a

transition regime in which this begins to occur. Associated with

the formation of large bubbles, smaller bubbles are lost from the

system so that the upward motion of bubbles is not as regularas it is at lower gas velocities. In other words, if one focuses atten-

tion on a single point along the spring internal, one does not see

bubbles arriving at such a regular interval as one observe in

bubbly regime. This causes a rapid drop in frequency as shown

in Figure 9.

Subsequently, as the typical characteristics of churn-turbulent

regime properly establish themselves beyond a superficial gas

velocity of 10 cm/s, the amplitude rises steadily as more and more

large bubbles are formed and more kinetic energy is associated

with the gas. We feel that in this regime in a bubble column

with internals, there is no preference for any frequency and all

Figure 10. Effect of superficial gas velocity and H/D ratio on gas holdupfor bubble column with spring internals (number of springs = 41,diameter of springs = 1.9 cm, wire thickness = 0.05 cm, volume fractionof springs = 2.28 103).

frequencies are equally preferred in the gasliquid dispersion. In

this white-noise scenario, the springs are perhaps forced by all

frequencies uniformly so that the net frequency that is observable

with the experimental techniques employed in this work limits usto observation of a fairly constant frequency. Note that it is in this

region (discussed later) that the gas holdup tends to peak under

all flow conditions.

Beyond 36 cm/s, we would observe a fairly uniform gasliquid

dispersion, which is stabilized on its own (based on the forces

of interaction between the gas and the liquid phases), as well as

the physical constraints put by the vessel dimensions. Thus, the

amplitude, which is basically governed by the large bubbles in

these conditions, levels off. Further increase in gas velocities adds

to more bubbles of smaller sizes, hence a marginal and steady

increase in frequency is observed.

Effect of superficial gas velocity andH/D ratioof helical spring

internals. Figure 10 shows the effect of superficial gas velocityand H/D ratio on gas holdup for 41 helical spring internals of

the type S19005. Gas holdup increases with superficial gas veloc-

ity and decreases with H/D ratio. The gas holdup is seen to be

higher at H/D ratio of 2 and decreases by 1012% as H/D ratio is

increased from 3 to 5. This is due to the steady-state bubble size

obtained at higher H/D ratio (>3) due to coalescence/dispersion

(Thorat et al., 1998). Similar effect of superficial gas velocity and

H/D ratio on gas holdup is observed for other helical springs dif-

fering in thickness of wire, helical spring diameter, and volume

fraction of helical spring internals.

Effect of volume fraction of helical spring internals. Effect

of volume fraction of helical spring internals (i.e., number

of springs) for S19005 on gas holdup for H/D ratio equal

to 3 is shown in Figure 11. The numbers of helical springsused are 1 (volume fraction = 5.57 105), 9 (volume frac-

tion = 5.0 104), 21 (volume fraction = 1.7 104) and 41

(volume fraction = 2.28 103). For comparison, data on bub-

ble columns without internals are also included (where volume

fraction of spring internals is zero). Gas holdup increases with

increase in volume fraction of helical spring internals. For larger

number of springs, it seems to go through a maximum around

20 cm/s gas superficial velocity, while the other trends seem to be

monotonically increasing.

At low volume fraction of internals, the main effect brought in

by the springs is the acoustic vibration effect causing enhance-



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AFigure 11. Effect of volume fraction of internals on gas holdup forbubble column with spring internals (H/D ratio = 3, diameter ofsprings = 1.9 cm, wire thickness = 0.05 cm, volume fraction of 1spring = 5.57 105, volume fraction of 9 springs = 5.0 104, volumefraction of 21 springs = 1.7 104, and volume fraction of 41springs = 2.28 103).

ment of gas holdup because of the vibrations. This effect has

been discussed by some earlier authors, notably in the paper of

Ellenberger et al. (2005). Following the theory reported in that

work, it can be argued that this enhancement of holdup stems

from the Bjerknes force. The Bjerknes force arises because the

existing gradient in pressure field interacts with volume pulsa-

tions in bubbles, which in turn are created when bubbles are

subjected to an acoustic pressure field. Ellenberger et al. (2005)

argue that along with drag, buoyancy and Bjerknes force lead to

formation of standing pressure waves, at the antinodes of which

the bubbles tend to accumulate leading to the enhancement of

holdup.

One may speculate here that similar phenomena occur in the

present case as well. However, the situation is far more complex

because unlike the case of Ellenberger et al. (2005), where in a

given experiment the excitation frequency was kept constant, in

the present case there is a spectrum of frequencies emanating fromthe complex interactions of the gas bubbles and the springs. Thus,

one expect many antinodes to form based on the harmonics, or

rather a diffuse region in the bubble column where enhancement

of holdup occurs.

As the volume fraction of the internals is increased, the above

effect is enhanced. At higher volume fractions, say for the case

41 springs, in addition to the effect discussed earlier, an effect of

steric hindrance also becomes important. Because of the physi-

cal obstruction of the rising bubbles and the ensuing chopping

action, the larger bubbles are broken into small bubbles due

the vibrating springs and their close packed configuration. The

springs do not allow the larger bubbles to escape freely and breaks

them into smaller entities.

The increase (or enhancement) in the gas holdup for 41 helicalsprings (p = 2.285 10

3) is as much as 135% at a gas velocity of

7 cm/s when compared to the bubble columns without internals,

at the same gas superficial velocity. Similar effect on gas holdup

for various volume fractions of helical spring internals is observed

for S4005 and S1901 type helical spring internals. The enhance-

ment of gas holdup is calculated by gas holdup of bubble column

with helical spring internals divided by the gas holdup of bubble

column without internals. The enhancement of the gas holdup

for bubble column with spring internals (S19005, 41 springs) is

2.3 times than the bubble column without internals at superficial

gas velocity 14 cm/s as shown in Figure 12. The enhancement

Figure 12. Effect of superficial gas velocity on percentage enhancementin gas holdup for bubble column with spring internals (number ofsprings = 41, diameter of springs = 1.9 cm, wire thickness = 0.05cm,volume fraction of springs = 2.28 103).

of gas holdup increases rapidly up to superficial gas velocity of

14 cm/s and then decreases gradually at higher superficial gas

velocity. In line with our earlier arguments, around 1214 cm/s

the formation of larger bubbles by coalescence is just initiated,

and all bubbles are of small size and occupy the entire column.Visual observation of the column under these conditions reveals a

froth-like gasliquid suspension (even though it was ensured that

the system is clean prior to the experiment, hence negating the

possibility of any stray surfactants), almost occupying the entire

column. As the velocity is further increased, the steady-state coa-

lescence rate picks up (even with the spring internals present)

and larger bubbles are formed. These bubbles rise quicker, thus

the enhancement is gas holdup is reduced. In summary, one may

argue that the presence of the spring internals effectively delays

the transition from bubbly to churn-turbulent flow regime (i.e.,

the transition occurs now at a higher gas superficial velocity), and

also enhances the average gas holdup at any given gas superficial

velocity.Effect of diameter of helical spring internals. The earlier

sub-section described the effect of overall volume fraction of the

springs on the gas holdup, without any direct reference to the

Figure 13. Comparison of gas holdup for bubble column with differentdiameters of spring internals and nearly equal volumefractions = 2.3 103 (wire thickness = 0.05 cm, H/D ratio = 3).



9/19

Author Proof

AFigure 14. Comparison of gas holdup for bubble column with differentwire thickness of spring internals and nearly equal volumefractions = 2.3 103 (diameter of helical springs = 1.9cm, H/Dratio = 3).

geometry (diameter, pitch, and wire thickness) of the springs.

Figure 13 compares the gas holdup observed with helical springsmade of different spring diameters (wire thickness of springs

being constant at 0.05 cm and H/D ratio of unexpanded bed

being fixed at 3). Experimental observations indicate that gas

holdup is maximum for 1.9 cm diameter helical springs when

compared to the 4 and 1 cm diameter helical springs. The 4 cm

helical springs provide enough space for the larger bubbles to

escape without breakage while that is not the case in bubble col-

umn with 1.9cm springs. The 1 cm helical springs are too rigid

(spring constant = 20.82N/m) and do not vibrate as compared to

1.9 cm helical springs. Higher gas holdup for 1.9 cm springs is due

to lower stiffness (spring constant = 0.68 N/m) of 1.9 cm springs

over 4 cm (spring constant = 0.41 N/m) and 1 cm springs. Simi-

lar effect of maximum gas holdup with good bubble breakage isobserved for springs of type S1901 as compared to S401. These

observations also corroborate our hypothesis that both the vibra-

tion in the springs as well as the steric hindrance offered by them

to the bubbles determine the average bubble size and hence the

final gas holdup.

Effect of thickness of helical spring wires. Figure 14 com-

pares the gas holdup as a function of wire thickness (for equal

spring diameter and H/D ratio of 3). Gas holdup is higher for

0.05 cm wire diameter springs than the 0.1 cm wire diameter

Figure 15. Comparison of gas holdup for bubble column with helicalspring internals with vertical rod internal internals (H/D ratio = 3, totalvolume fraction of internals = 2.3 103).

springs, since 0.05 cm wire diameter helical springs vibrates bet-

ter due to their lower spring constant. The 0.1 cm thickness wire

is too rigid (spring constant = 20.82N/m), that is higher spring

constant and vibrates less than the 0.05 cm thickness wire. Vibra-

tion of helical springs breaks the larger bubbles into smaller ones

and increases the gas holdup.

It is clear from the above observations that the stiffness of the

springs as well as their physical dimensions (diameter, thickness

of wire) plays an important role in determining the local hydrody-

namics and the gas holdup. Also the packing or volume fraction

(number) of springs play an equally, if not more, important role

in providing steric limitations to the bubbles and causing them to

disintegrate, thus limiting their size.

Comparison of helical spring and vertical rod internals. Fig-

ure 15 compares gas holdup for helical springs with vertical rod

internals. Volume fraction of one vertical rod and helical springs

(S19005, 41 springs) used are 16 103 and 2.25 103, respec-tively (Table 1). The maximum possible number of helical springs

that can be placed/positioned in the spring holder is 41. It can be

seen that even a single thin rod has a volume fraction of 16 103

and results in a much lower enhancement in holdup compared to

any of the spring internals. Reasons for this are twofold. First,

in terms of providing steric hindrance to the passage of bubbles

and hence limiting their size, the rods are practically ineffec-

tive because they do not change the cross-sectional area through

which bubbles rise, appreciably. In terms of vibrations, too, the

Table 2. Comparison of gas holdup for bubble columns with internals

Refs.Uog = 7cm/s Uog = 18 cm/s Uog = 35cm/s

g,i g % increase g,i g % increase g,i g % increase

Patil et al. (1984) 0.11 0.10 10.0Yamashita (1987a) 0.18 0.13 38.5 0.27 0.22 22.7 0.35 0.3 16.7Pradhan et al. (1991) 0.14 0.11 27.3PradhanQ2 et al. (1994) 0.15 0.11 36.4Urseanu et al. (2001) 0.18 0.13 38.5 0.25 0.24 4.2 0.35 0.34 2.9Birrer and Bohm (2004) 0.18 0.16 12.5 0.29 0.23 26.1Present work (S19005, 41 Springs) 0.33 0.14 135.7 0.52 0.24 116.7 0.53 0.33 60.6

g,i, gas holdup with internals; g, gas holdup without internals.

Bold and underlined values indicate maximum gas holdup achieved for the respective superficial gas velocity using different internals by the authors.



10/19

Author Proof

Asingle rod is too stiff and hence its motion is essentially un-

modulated by the passing gas (whose kinetic energy is too low to

induce any vibrations in the rod). Thus, clearly the springs occupy

less space inside the bubble columns and also lead to higher gas

holdup than vertical rods.

The message is that internals of regular shape, like rods,

enhance gas holdup only to a limited extent while the use of

potentially vibrating internals (like springs) would significantly

enhance their potential. This is shown through a comparison of

the literaturegas holdup with the present work in Table 2. It can be

seen clearly from the table that the present helical spring internalsare far better choice of internals compared to any other inter-

nals used in the literature. The increase of gas holdup is around

135% for 7 cm/s, while the maximum reported gas holdup at this

superficial gas velocity is only 38.5%.

CONCLUSIONS

One of the holy grails of bubble column research has been to

obtain high gas holdups at high gas fluxes. In conventional bubble

columns (either empty or with conventional internals like rods),

these two requirements are contradictory. This is because as gas

flux (superficial gas velocity) is increased, it leads to coalescence

forming larger bubbles which have in general a lower residence

time and higher rise velocity. Consequently, while the increase

of gas flow leads to increase in holdup, the enhancement is not

proportional and indeed it tends to level off at high values of gas

flux.

The experimental results presented in this paper indicate that

the vibrating structures like springs have a strong potential for use

as internals in industrial bubble columns. The results indicated

that at superficial gas velocities of around 12 cm/s, the percent-

age enhancement of gas holdup is around 230%. Even at higher

throughput of gas, the percentage enhancement of gas holdup

is more than 150% for superficial gas velocity 40 cm/s. Indus-

trially, the current trend is to use bubble columns at higher gas

throughputs, so the presented results indicate a promising option

for industrial practitioners to harness high gas holdups are highgas fluxes.

It is also clear that the geometry, material of construction, and

properties of the spring internals have a marked effect on their

role in holdup enhancement. The experimental observations indi-

cate that gas holdup is maximum for 1.9 cm diameter helical

springs compared to the 4 and 1 cm diameter helical springs. Gas

holdup is higher for 0.05cm wire diameter springs than the 0.1 cm

wire diameter springs, all other parameters being constant. This

point clearly needs further investigation and is a topic of ongoing

research in our group. The helical spring internals satisfies the

wish list as one expects from internals in bubble columns are low

volume fraction of internals, high surface area of internals, no

external energy input, high-throughput operation, and minimum

maintenance cost of internals and other bubble column hardware.Fact is that in spite of its good points, such fine structures like

springs (which seem so promising in cold flow experiments) will

either be inadequate for purposes of heat transfer (for instance, it

is difficult to envision that the tubes which carry a heat transfer

fluid to be so fine that they can also serves the cause of being

vibrating structures). However, the point we wish to highlight

through our contribution is the importance of vibrating struc-

tures in enhancing gas holdup, not necessarily springs. Any other

vibrating or flexible structure (not necessarily a simple harmonic

oscillator like a spring) would also serve to enhance the gas

holdup, albeit our understanding of structures whose oscillation

frequencies may have multiple modes may be more limited as

compared to a humble spring. An alternate suggestion may be to

have microtubes which candouble up as springs, much as thecon-

temporarily prevalent idea of microheat exchangers. These would

be designed to provide the same heat transfer area offered by the

conventional bubble column heat exchanger tubes by adjusting

the number of (vibrating) microtubes.

To make such concepts operational and realizable, the material

of construction of the vibrating microtubes would be very impor-

tant. Clearly, this is going to be a challenge for material scientists,

since the microstructures should have corrosion and erosion resis-tance offered by the sometimes harsh environment of a industrial

bubble column.

NOMENCLATURE

D column diameter (m)

F force exerted by a spring (N)

H static liquid height (m)

Hd liquid dispersion height (m)

k spring constant (N/m)

r radial coordinate

R radius of the rod holder (m)

Uogsuperficial gas velocity (m/s)x displacement of the end of the spring from its equilibrium

position (m)

g gas holdup

END NOTESPlease refer to Table 1 for listing of various internals used.

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3336 (1962).



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Author Proof

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2010.



12/19

Author Proof

AQ1: Please add Joshi and Shah (1981) in the reference list.

Q2: Please add Pradhan et al. (1994), Urseanu et al. (2001),

Birrer and Bohm (2004) in the reference list.

Q3: Please cite the following references in the list: Deckwer

and Schumpe (1987), Krishna et al. (1997), Saxena (1995),

Yamashita (1985).

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Softproofing for advanced Adobe Acrobat Users - NOTES toolNOTE: ACROBAT READER FROM THE INTERNET DOES NOT CONTAIN THE NOTES TOOL USED IN THIS PROCEDURE.

Acrobat annotation tools can be very useful for indicating changes to the PDF proof of your article.By using Acrobat annotation tools, a full digital pathway can be maintained for your page proofs.

The NOTES annotation tool can be used with either Adobe Acrobat 4.0, 5.0 or 6.0. Otherannotation tools are also available in Acrobat 4.0, but this instruction sheet will concentrateon how to use the NOTES tool. Acrobat Reader, the free Internet download software from Adobe,DOES NOT contain the NOTES tool. In order to softproof using the NOTES tool you must havethe full software suite Adobe Acrobat 4.0, 5.0 or 6.0 installed on your computer.

Steps for Softproofing using Adobe Acrobat NOTES tool:

1. Open the PDF page proof of your article using either Adobe Acrobat 4.0, 5.0 or 6.0. Proof

your article on-screen or print a copy for markup of changes.

2. Go to File/Preferences/Annotations (in Acrobat 4.0) or Document/Add a Comment (in Acrobat6.0 and enter your name into the default user or author field. Also, set the font size at 9 or 10point.

3. When you have decided on the corrections to your article, select the NOTES tool from theAcrobat toolbox and click in the margin next to the text to be changed.

4. Enter your corrections into the NOTES text box window. Be sure to clearly indicate where thecorrection is to be placed and what text it will effect. If necessary to avoid confusion, you canuse your TEXT SELECTION tool to copy the text to be corrected and paste it into the NOTEStext box window. At this point, you can type the corrections directly into the NOTES text

box window. DO NOT correc t the text by typing direc tly on the PDF page.

5. Go through your entire article using the NOTES tool as described in Step 4.

6. When you have completed the corrections to your article, go to File/Export/Annotations (inAcrobat 4.0) or Document/Add a Comment (in Acrobat 6.0).

7. When c losing your article PDF be sure NOT to save changes to original file.

8. To make changes to a NOTES file you have exported, simply re-open the original PDFproof file, go to File/Import/Notes and import the NOTES file you saved. Make changes and re-export NOTES file keeping the same file name.

9. When complete, attach your NOTES file to a reply e-mail message. Be sure to include yourname, the date, and the title of the journal your article will be printed in.

Balamurugan.pdf

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