Studies on gas holdup in a bubble column using porous spargers with additives

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2008; 3: 417–424Published online in Wiley InterScience(www.interscience.wiley.com) DOI:10.1002/apj.137

Research ArticleStudies on gas holdup in a bubble columnusing porous spargers with additives

Abhishek Jha, B. Raj Mohan, S. Chakraborty and B. C. Meikap*

Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721302, India

Received 20 March 2008; Revised 14 April 2008; Accepted 15 April 2008

ABSTRACT: High gas holdup and uniform fine bubble generation is very essential in froth floatation cell of mineral-based process industries. In this work, to generate uniform fine bubbles a bubble column with porous sparger havebeen used. The bubble column was characterized to study the effect of gas flow rate and concentration of additivesolution on the performance of bubble column with porous spargers. Various types of spargers namely poly par sparger,refractory brick sparger and ceramic sparger were used to produce air bubbles in the liquid phase water and the gasholdup is estimated. The variation of gas holdup with respect to gas flow rate for different spargers and for differentconditions is presented in this paper. It has been found that the gas holdup with addition of surfactants was in the rangeof 25–40%. The behavior of gas holdup is consistent with the existing theory and shows improvement over previousresults. 2008 Curtin University of Technology and John Wiley & Sons, Ltd.

KEYWORDS: bubble column; fine bubble; filter sparger; gas holdup; porous sparger; polypar sparger; surfactant

INTRODUCTION

Bubble columns are of great importance in chemical andbiochemical process industries where gas–liquid oper-ations like distillation, fractionation, humidification,gas–liquid reactions, etc. are carried out and reported byMeikap et al .[1] They are widely used because of theirsimplicity in construction and operation, low operatingcosts and high energy efficiency. In these processes, agas holdup is an important design parameter, becausegreater gas holdup implies greater interfacial area avail-able for mass transfer. Moreover considering the appli-cations in the mineral beneficiation, greater gas holdup indicates greater bubble population which means ahigher probability of the particles getting attached tothe bubbles that leads to greater amount of mineralbeing separated from its ore by means of this frothflotation method. The bubble size distribution and gasholdup in gas–liquid dispersions largely depend uponthe column geometry, operating conditions, physico-chemical properties of the two phases and type ofgas sparger and reported by Mouza et al .,[2] Colellaet al .,[3] Tao,[4] Zahradnik et al .,[5] Polli et al .,[6] Ruz-icka, et al .,[7] Ruzicka et al . and Parthsarthy.[8,9]

The two main approaches proposed to analyze thegas holdup and bubble size are the computational fluid

*Correspondence to: B. C. Meikap, Department of Chemical Engi-neering, Indian Institute of Technology, Kharagpur 721302, India.E-mail: [email protected];

dynamics model and the classical chemical engineeringapproach (flow regimes, global and local gas holdupetc.). The aim of this work is to improve the experimen-tal knowledge of bubble behavior in bubble columns.[10]

Bubble behavior has a direct bearing on hydrodynam-ics, mass transfer and reactor performance. The designand scale-up of bubble column has been based so faron empirical methods.[11] It is generally accepted thattwo regimes can be distinguished based on the gasflow rate: the homogenous (bubbly flow regime) andthe heterogeneous (churn turbulent flow) regime. Thehomogeneous bubbly flow regime encountered at lowgas velocities and characterized by a narrow bubble sizedistribution and radially uniform gas holdup; and theheterogeneous (churn turbulent flow) regime observedby Kazakis et al .[12] at higher gas velocities and char-acterized by the appearance of large bubbles, formedby coalescence of the small bubbles and bearing ahigher rise velocity hence leading to relatively lowergas holdup values depending on the type of the gas dis-tributor and the properties of the liquid phase.[11] Bothregimes can be obtained in the same equipment by vary-ing the gas input flow rate.

EXPERIMENTAL SETUP AND TECHNIQUE

The experimental setup consists of a perspex columnof height 1.45 m and the diameter is 0.116 m with aporous sparger fitted at the bottom. For experimental

2008 Curtin University of Technology and John Wiley & Sons, Ltd.

418 A. JHA ET AL. Asia-Pacific Journal of Chemical Engineering

purpose three different types of spargers are used.These are polypar sparger (acrylic fiber), refractorybricks material and ceramic with porosity 23, 20 and16% respectively. The first one is made up of apolymer named as polypar with a height of 0.15 mand diameter of 0.04 m, the rest two are 0.13 m heightand 0.04 m diameter and 0.06 m height and 0.12 mdiameter respectively. A Compressor has been used tosuck the atmospheric air, which is sent to the poroussparger for producing bubbles in the bubble columncontaining plain water. A rotameter of range: 0–50l/min is fitted along the air line to the sparger fromthe compressor to regulate the air flow rate. A paperscale was attached to the column. The experimental setup is presented in Fig. 1.

Water and dilute soap solutions are used for gasholdup experimentation in the bubble column. Thesoap solution is prepared by using 0.5 g of surfactant(detergent) in 1 l of tap water and diluted to variousconcentrations as per the requirement. The physicalproperties of different surfactant solution presented inTable 1. Type of surfactant employed was short-chainalkyl naphthalene sulphonate type anionic surfactant.

The bubble column was first filled with 5 l of plainwater and the initial level of the water was noted. Thegas (air) flow rate to the first sparger (polypar sparger)was varied from 15 to 50 l/min and final level of thewater in the bubble column was noted for each gas flow

rate at steady state. The gas holdup for each gas flowrate was estimated by subtracting the initial level ofthe water to from the final level of water in the bubblecolumn. In a similar way the gas holdup for 6, 7 and 8l of plain water in the bubble column was estimated.

The surface tension of the plain water is reduced byadding the prepared surfactant solution (detergent) indifferent volumes 30, 60, 90, 120 and 150 ml to thebubble column and made up 5 l using plain water.The gas holdup for different gas flow rates at differentdilutions of the surfactant solution in the bubble columnwas estimated. Exactly 36, 72, 108, 144, 180 ml of thesurfactant solutions are added to the bubble columnand made up to 6 l for each set of experiments.The gas holdup for different gas flow rate for eachof the above-mentioned dilutions of the surfactantwas estimated. Further 42, 84,126,168 and 210 ml ofsurfactant solution was added to the bubble column andthe total volume was made up to 7 l. The gas holdupfor each flow rate was for the above-mentioned dilutionswas noted. The above procedure both with pure waterand surfactant solution was repeated for different typeof sparger namely poly par sprger, refractory bricksparger and ceramic sparger. A photographic view ofthe above sparger are shown in Fig. 2. Experimentswere conducted for other type sparger of higher radiusfollowing the same procedure.

Figure 1. Schematic diagram of the experimental set up. This figureis available in colour online at www.apjChemEng.com.

Table 1. Physical properties of surfactant solution.

Surfactantdose (Vol(%)) 0.5 1 1.5 2 2.5 3 3.5 4

Density (g/m3) 999 998 996 995 993 990 988 985Viscosity Pa s (kgm/s) 0.2028 0.2067 0.2171 0.2247 0.2286 0.2324 0.2356 0.2432Surface tension × 103, N/m 53 48 42 40 38 36 34 33

2008 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2008; 3: 417–424DOI: 10.1002/apj

Asia-Pacific Journal of Chemical Engineering STUDIES ON GAS HOLDUP IN A BUBBLE COLUMN 419

Figure 2. Photographs of (a) Ceramic sparger (b) Refractory bricksparger (c) Polypar sparger. This figure is available in colour online atwww.apjChemEng.com.

RESULTS AND DISCUSSION

Effect of superficial gas velocity on the gasholdup with water without surfactant

The effect of gas velocity on the gas holdup inbubble column, fitted with polypar sparger, containingmeasured volume water in column is presented inFig. 3. It gives the clear understanding of the variationof gas holdup with respect to the change in the gas flowrate. Figure 3 reveals that the holdup increases almostlinearly with the increase in the gas velocity from 0.02to 0.08 m/s. A maximum holdup of 42% is attained forthe maximum initial height of 0.76 m of water. As thevolume of the water in the bubble column is increased,the holdup is also found to increase. The rate of increasein the gas holdup is observed to be steady upto 0.07 m/sgas velocity, after which the rate decreases. This trend isfound common in all the initial levels of the liquid in thebubble column. This may be because of turbulent flowdevelopment resulting in churn turbulent regime wherethere is comparatively less increase in gas holdup. With8 l of water in the bubble column, the holdup is foundto increase very much and linearly when compared toother volumes. Moreover there is no fall in the rate ofincrease in the hold up also. Thus, the turbulent regimemay not be developed in greater liquid volume for thegiven gas velocity of 0.08 m/s. In greater liquid volume,greater energy is required for the development of theturbulent flow regime.

Effect of gas flow rate on gas holdup withaddition of surfactant solution in the bubblecolumn

Figure 4 shows the effect superficial gas velocity on gasholdup for a bubble volume for a particular surfactantdose. It is very clear from this figure that at 3%

Figure 3. Effect of superficial gas velocity on gas holdupat different initial height for poly par sparger. This figure isavailable in colour online at www.apjChemEng.com.

of surfactant dose, the gas holdup is more when thebubble volume increased from 5 to 7 l. Experimentsconducted with addition of the prepared surfactantsolutions making it to the 5 l volume (5l) of the bubblecolumn and polypar sparger as the sparger are presented



Figure 4. Effect of superficial gas velocity on gasholdup at different bubble column volume for polypar sparger. This figure is available in colour online atwww.apjChemEng.com.

in Figs 4 and 5 for refractory sparger. When an input of30 ml of the prepared solution to the bubble column andmade up to 5 l, the gas holdup increased by an averageof 4–6% for all gas flow rates when compared to plainwater of 5 l. Thus, the addition of surfactant reducesthe surface tension of the water and promotes morenumber of bubbles getting formed. Further increase inthe concentration of the surfactant by the addition of 60,90, 120 and 160 ml and made up to 5 l, the gas holdupincreases by 6% per 30 ml of addition of solution.This increase in gas holdup due to increased bubbleformation by coalescence. It was observed that at highervolume of solution, first the foam forms which fills theentire column but afterwards it settles down. Figure 5,reveals that for decreasing gas flow rate gas holdup isnot the same as with the increasing volume. It was alsoobserved that the holdup increase rate reduces when thegas flow rate goes above 0.07 m/s. This may be due toturbulent flow development resulting in churn turbulentregime where there is comparatively less increase ingas holdup. The gas holdup increases more rapidly forthe initial sets of surfactant solutions than with the

Figure 5. Effect of superficial gas velocity on gas holdupat different surfactant concentrations for refractory bricksparger. This figure is available in colour online atwww.apjChemEng.com.

subsequent dosage of solution. This may be due to theeasy breakage of the bubbles caused by low surfacetension and thus giving way for the gas to pass throughthe column quickly.

Experiments carried out using prepared solution in6 l of the water and surfactant solution and theresults are presented in Fig. 6. For the 36 ml of theprepared solution being made up to 6 l in the bubble,the increase in the gas flow rate increases the gasholdup steadily from 25 to 43%. When 72 ml ofsurfactant solution is made up to 6 l in the bubblecolumn, the gas holdup showed a 24% differencefrom the previous holdup values. And further forevery increase in the concentration of the surfactantby addition 36 ml of the surfactant solution, the gasholdup increases by 25%. From the Fig. 6, it is clearthat for the same concentration of the surfactant solutionthe increase in gas holdup remains independent of theinitial volume of liquid in the bubble column for thispolypar sparger. A slight higher value of gas holdup isobserved for decreasing gas flow rates that may be dueinertia.



Figure 6. Effect of superficial gas velocity on gasholdup at different surfactant concentrations for ceramicsparger. This figure is available in colour online atwww.apjChemEng.com.

Figure 7 represents the effect of gas flow rate onthe gas holdup for ceramic sparger for 3% by volumeof surfactant dose. The gas holdup was found toincrease by 4% with increase of volume of liquid inthe column. This effect remains almost same at higherflow rates resulting in a difference of 3% with 7 lsolution. The gas holdup tends to shows nonlinearbehavior for 5, 6 and 7 l of water volume in thebubble column. This may be due to transition regimebetween bubbly flow regimes and churn turbulent flowregime. A highest of 45% of gas holdup is achievedin the bubble column for the given maximum gasvelocity from 0.02 to 0.08 m/s with 3 vol% of surfactantsolution.

Effect of gas velocity on gas holdup (withwater only) for refectory sparger

To study the effect of gas flow rate on the gasholdup in bubble column with respect to differenttypes of spargers experiments are conducted on the

Figure 7. Effect of gas flow rate on gas holdup in abubble column at different liquid volumes for ceramicsparger. This figure is available in colour online atwww.apjChemEng.com.

bubble column for the same volume and concentration.Figure 8 reveals the effect of gas rate on gas holdupfor different volumes of water using refractory spargeras the diffuser in the bubble column. The gas holdupincreases with increase in the liquid (water) volume forthe given gas velocity (0.02 to 0.08 m/s). There was amarked difference in the gas holdup for each volumeof the liquid for this filter type sparger. A maximum of27, 29, 31 and 33.5% of gas holdup was observed for5, 6, 7 and 8 l of water volume respectively at 0.08 m/sof gas rate. This may be due to increased number ofbubbles being present with larger residence time inthe water column (greater volume). For 8 l volumeof bubble column, the increase in gas holdup is prettyrapid when compared to the polypar sparger where asthe gas holdup is less than that in the case of polyparsparger. Moreover the increase in the gas holdup seemsto be steady, which is different from the polypar spargerholdup trend hence the entire range of operation is inbubbly flow regime which is characterized by uniformbubble size.



Figure 8. Effect of superficial gas velocity on gasholdup at different initial height for refractory bricksparger. This figure is available in colour online atwww.apjChemEng.com.

Effect of gas velocity on gas holdup atdifferent liquid volume of bubble column at 3vol% concentrations of surfactants

The results on the effect of gas holdup for 5 and 7 lof volume of water at 3% concentrations of surfactantin the bubble column with refractory brick sparger arepresented in Fig. 9. It can be observed from Fig. 9 thatthe gas holdup increased by 0.75% for a lower gasvelocity. As the gas velocity increases from 0.02 to0.08 m/s, the gas holdup increases from 26 to 31%.

Effect of gas velocity on gas holdup forvarious type of sparger and different liquidvolumes of bubble column at 3 vol%concentrations of surfactants

Figure 10 shows a typical comparison of various type ofsparger performance and their gas holdup values underdifferent superficial gas velocity. The observations of

Figure 9. Effect of gas flow rate on gas holdup in abubble column at different liquid volumes for refractorybrick sparger. This figure is available in colour online atwww.apjChemEng.com.

Fig. 10 is for 5 and 7 l of the volume, the solution in thebubble column respectively, the gas holdup increaseswith increase gas velocity for all types of sparger.Results indicate that the poly par sparger and ceramicsparger gives very good holdup than that of refractorysparger. The relationship between the gas holdup andgas flow rate are almost linear at low holdups. Thisrepresents that the entire region of operation was inthe bubbly regime. The gas holdup was least in thissparger as the gas holdup with pure water is least andalso the increase in gas holdup upon addition of solutionis less. The increase of gas holdup was very close tolinear values compared to all other spargers. The gasholdup increased by 10%. It can be concluded thatwith increase of concentration of surfactant solution gasholdup increases. This effect remains the same at higherflow rates resulting in a difference of 7–10% increasein solution. The gas holdup was least in this spargeras the gas holdup with pure water is also least andthe increase in gas holdup upon addition of solutionis also less. The liquid level was most stable and thereis almost no difference in increasing and decreasing gas



Figure 10. Effect of gas flow rate on gas holdup atdifferent volumes of water in the bubble column forvarious sparger. This figure is available in colour onlineat www.apjChemEng.com.

flow rates. Moreover the increase of gas holdup is alsoconstant with increase of the solution. The behavior ofthis sparger can be predicted most accurately becauseof its linearity in relationship between gas flow rate andgas holdup for different concentrations of surfactant.

Comparison of gas holdup experimentallyfound out and reported by various researchers

To compare data obtained in the present study anattempt has been made by for better fit of the data. Thegas holdup correlation as reported by Kazakis et al .[12]

given in Eqn (1) have been used to compare the gasholdup.

εG = 0.2

[Fr0.8Ar0.2Eo1.6

(ds

dc

)0.9 (dp

ds

)0.03]2/5

(1)

where, dS is diameter of sparger dP is the diameter ofpore and dC is dimeter of column and following are

Figure 11. Comparison of the experimental gas holdupand predicted values reported in the literature. This figure isavailable in colour online at www.apjChemEng.com.

dimensionless numbers Froude (Fr), Archimedes(Ar)and Eotvos (Eo) numbers defined as follows.

Fr = U 2GS

dcg(2)

Ar = d3Cρ2

Lg

µ2L

(3)

Eo = d2CρLg

σL(4)

Where ρL, µL, σL are the density, viscosity andsurface tension of the liquid respectively.

The experimentally observed values have been com-pared with the values reported in the literature[2,12–14]

and presented in Fig. 11. It has been found that most ofthe experimental values at lower superficial gas velocityare in good agreement with that reported in the literaturecited values within ±20%.

CONCLUSION

The gas holdup increased almost linearly with gasflow rate for all three spargers investigated. The gasholdup in pure water was maximum for polypar sparger



and minimum with refractory type sparger with greaterradius. The gas holdup in case of additive solution iscomparable for polypar sparger and ceramic spargerwith smaller radius. This is because the rate of increaseof gas holdup upon increase of concentration is higherwith filter sparger. The rate of increase of gas holdupfor a fixed concentration of solution is independent ofthe level of water in the column and dependent upontype of sparger. The stability of liquid level varies morein case of Polypar sparger and less in refractory spargermaking it more stable in predicting gas holdsup. The gasholdup increased more linearly with filter type spargerwith greater radius but at the cost of lowest gas holdup.Thus it can be said for polypar types of sparger that forsuperficial gas velocity of 5–6.67 cm/s the turbulentregime occurs. A comparison of the present experimen-tal data has been compared with the correlations anddata available in the literature and found in good agree-ment at lower superficial gas velocities.

REFERENCES

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[4] D. Tao. Sep. Sci. Technol., 2004; 39(4), 741–760.[5] J. Zahradnik, M. Fialova, M. Ruzicka, J. Drahos, F. Kastanek,

N.H. Thomas. Chem. Eng. Sci., 1997; 52, 3811–3826.[6] M. Polli, M. Di Stanislao, R. Bagatin, E. AbuBakr, M. Masi.

Chem. Eng. Sci., 2002; 57, 197–205.[7] M.C. Ruzicka, J. Drahos, P.C. Mena, J.A. Teixeira. Chem.

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1030–1034.[10] G. Vatai, M.N. Tekic. Chem. Eng. Sci., 1987; 42(1), 166–169.[11] B.C. Meikap, G. Kundu, M.N. Biswas. Can. J. Chem. Eng.,

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Sci., 2007; 62(12), 3092–3103.[13] M. Kaji, T. Sawai, K. Mori, M. Lguchi. Behaviours of Bubble

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Studies on gas holdup in a bubble column using porous spargers with additives