Fundação para a Ciência e a
Tecnologia (FCT); EC

Amaral, A. L.

Ferreira, E. C.

Fialová, Maria

Freitas, Catarina

Teixeira, J. A.

Zahradnik, Jindrich

Ferreira, Eugénio C.

Universidade do Minho: RepositoriUM

INTRODUCTIONCHARACTERIZATION OF BUBBLES IN A BUBBLECOLUMN BY IMAGE ANALYSISC. FREITAS , A. L. AMARAL ,M. FIALOVÁ , E. C. FERREIRA , J. ZAHRADNIK , J. TEIXEIRACentro de Engenharia Biológica-IBQF, Universidade do Minho, 4710-057 BRAGA, PORTUGALInstitute of Chemical Process Fundamentals, 165 02 Prague 6 - Suchdol, CZECH REPUBLIC1 1 2 2 1121SummaryThe aim of this work was to study the influence of airflow rate, the distributing plate orifice diameter and the presence of a coalescence inhibitor on the size and shapeof bubbles in a rectangular bubble column.In recent years there has been considerable interest in the use of pneumaticreactors for biological reactions. Bubble columns are characterized by the fact thatthe gas is dispersed in the continuous liquid phase in bubble form. Hence, bubblesize, rising velocity and distribution are important parameters thatSeveral methods for particle size measurement are available. They are generallydivided into two categories, depending upon whether or not a sample is removed.The most simple characterization method is the photographic technique that,however, demands for photographs analyses. With the exponential increase of theprocessing capabilities of computers, as well as their price reduction, imageanalyses has become a very important tool, being a routine in cellular biologystudies.Several parameters can be used to statistically describe the diameter of abubble. The most commonly used parameter to describe the diameter of a particleis the equivalent diameter, based on the projected area of the bubbles.influence theperformance of the reactor and, consequently have to be calculated.Bubble characteristics are determined by the operation conditions of the reactorand parameters as the distributing plate and the properties of the liquid phase aredeterminant.MATERIAL AND METHODSGas-phase:Liquid-phase:AirWater and aqueous solution of 10g/l ethanolThree-phase systemExperimental conditionsAirflow rate:Distributing plate orifice diameter:100, 125, 150, 175 and 200 l/hr0.5 mm, 1.0 mm and 1.6 mmFig. 1 - Rectangular bubble columnBubble size shapeandcharacterizations was done byrecording images,witha video camera .After digitalisation, imageswere analysed using animage analysis system basedon a MATLAB developedsoftware.at 30 cmfrom the distributor plate,Development of a hydrodynamic model in order to estimate the gas holdup and the liquid velocity in the riser and the downcomer of three-phase internal- andexternal-loop airlift reactorsRESULTS AND DISCUSSIONThis work was supported by FCT (Fundação para a Ciência e a Tecnologia), under project PRAXIS/2/2.1/BIO/1061/95 andgrant GGP XXI/BD/2937/96 awarded to Carla Maria Duarte de Freitas and by EC, under INCO-COPERNICUS contract NºERB IC15-CT98-0904.REFERENCESAtenas, M., Clark, M. and Lazarova, V. (1998) Characterization of bubbles in a rectangular air-liftbioreactor, Part. Part. Syst. Charact., 15, 191-199.Amaral, A. L., Baptiste, C., Pons, M. N., Nicolau, A., Lima, N., Ferreira, E. C., Mota, M. And Vivier, H.(1999) Semi-automated recognition of protozoa by image analysis. Biotechnol. Tech., 13, 111-118.Equivalent diameter of bubblesshape of the bubbleswas obtained from the calculated values of theprojected area.The was obtained by comparison of the ratio between itssuperficial area and volume with the one of a sphere.By the determination of the lengths of the perpendicular axes, the superficial area andthe volume of each bubble were calculated.CONCLUSIONSIn the range of the studied airflow rates, bubble diameter slightlyincreased.An increase of bubble size was also observed with the increase of thedistributing plate orifice diameter, being this variation morepronounced when the orifice diameter increased from 1.0 mm to 1.6mm.The presence of ethanol did not lead to any significant change onbubble diameter.Comparison of the ratio between the superficial area and the volumewith the one of a sphere, showed that ellipsoidal bubbles arepredominant.Fig. 2 - Image of bubbles obtained bydigitalization of video camera recordingsFig. 3 - Image of bubbles obtained byimage analysis treatment of Fig. 2Fig. 4 - Mean diameter of bubbles andstandard deviation vs. the orificediameter of the distributing plate, fordifferent airflow rates ( - water; -ethanolsolution) 100 l/hr35790 1 2do (mm)db(mm)150 l/hr35790 1 2do (mm)db(mm)200 l/hr35790 1 2do (mm)db(mm)0.5 mm0510100 150 200Airflow rate (l/hr)db(mm) Fig. 5 - Mean diameter of bubbles vs.theairflow rate, for three orifice diameters ofthe distributing plate ( - water; - ethanolsolution)1.0 mm0510100 150 200Airflow rate (l/hr)db(mm)1.6 mm0510100 150 200Airflow rate (l/hr)db(mm)100 125 150 175200<3.23.2-3.4>3.4020406080100Frequency(%)Airflow rate (l/hr)db (mm)Fig. 6 - Bubble shape distribution based on shape factor (A*r/V)SHAPE FACTOR:(A*r/V)>3.4 bubble is an ellipsoid3.2<(A*r/V)<3.4 bubble is an intermediate(A*r/V)<3.2 bubble is a sphere

Characterization of bubbles in a bubble column by image analysis

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Characterization of bubbles in a bubble column by image analysis

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