Hydrodynamics of a Pulsed Sieve Plate Extraction Column

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Hydrodynamics of a Pulsed Sieve PlateExtraction ColumnNitin Somkuwara, Nitin Kolhea & Virendra Rathoda

a Department of Chemical Engineering, Institute of ChemicalTechnology, N.P. Marg, Matunga, Mumbai 400019, IndiaPublished online: 28 May 2014.

To cite this article: Nitin Somkuwar, Nitin Kolhe & Virendra Rathod (2014) Hydrodynamicsof a Pulsed Sieve Plate Extraction Column, Indian Chemical Engineer, 56:3, 235-257, DOI:10.1080/00194506.2014.910707

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Hydrodynamics of a Pulsed Sieve Plate ExtractionColumn

Nitin Somkuwar, Nitin Kolhe and Virendra Rathod*Department of Chemical Engineering, Institute of Chemical Technology, N.P. Marg, Matunga,Mumbai 400019, India

Abstract: Hydrodynamic characteristics, such as drop size, dispersed phase hold-up andflooding have been studied experimentally in a pulsed sieve plate extraction column of0.076 m diameter. Geometrical parameters like perforation diameter, plate spacing andfractional free area (FFA) of sieve plate and operating parameters, such as pulse velocityand superficial velocities of dispersed and continuous phase, were studied using kerosene-water system. The drop size was reduced with decrease in perforation diameter, platespacing, FFA and pulse velocity. Dispersed phase hold-up was found to be a function ofdrop size and superficial velocity of dispersed phase. At pulse velocity of 0.025m/s theparabolic behaviour with maxima at the transition from mixer-settler to dispersion regimeof flooding curve has been noticed for all the different geometrical conditions. Themaximum mass transfer interface together with utmost throughput was obtained atperforation diameter of 0.003 m, plate spacing of 0.05m and FFA of 0.2. The optimaloperating parameters were pulse velocity of 0.025 m/s and superficial velocity of 0.006 m/susing organic to aqueous ratio (O/A) of 1:1. The drop size attained at these conditions was0.0015 m with dispersed phase hold-up of 18%. The correlations developed can be used forthe prediction of hold-up and drop size.

Keywords: Pulsed sieve plate extraction column, Hydrodynamics, Drop size distribution,Dispersed phase hold-up, Flooding characteristics.

1. IntroductionLiquid–liquid extraction is used for separation processes in different process industries.Diversified liquid-liquid extraction equipments like mixer-settlers, columns, centrifugal

*Author for Correspondence. Email: [email protected]

INDIAN CHEMICAL ENGINEER © 2014 Indian Institute of Chemical EngineersVol. 56 No. 3 September 2014, pp. 235–257Print ISSN: 0019-4506, Online ISSN: 0975-007X, http://dx.doi.org/10.1080/00194506.2014.910707

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http://dx.doi.org/10.1080/00194506.2014.910707

extractors have been developed as per the process requirement. The search for improvedextraction efficiencies has led to the development of a class of liquid-liquid contactors in whichmechanical agitation is provided to increase the interfacial area between the two liquid phases.One of these contactors is the pulsed sieve plate extraction column (PSPEC), which requiresexternal energy input in the form of pulsing motion usually sinusoidal superimposed oncounter-current flow of the liquid phases. This pulsation provides high turbulence which assistsin droplets breackdown. Consequently, the large interfacial area is obtained to get high masstransfer coefficient. Wiegandt and Von Berg [1] have shown that under pulsing conditions, theheight of a packed column required to achieve the given degree of extraction is reduced by afactor of three. Since the pneumatic pulsing unit can be placed remote from the actualprocessing area, the pulsed columns have a clear advantage over other mechanical contactors,while processing corrosive or radioactive solutions.

The design of any solvent extraction column necessitates the knowledge of cross-section andheight of the column to achieve the desired separation with given duty. The cross-section isdetermined by the hydrodynamic characteristics like drop size, dispersed phase hold-up and themaximum throughput from flooding curve. The latter requires the information of axial dispersionand the rate ofmass transfer between phases. All these characteristics aremainly dependent on thedrop size of dispersed phase. Extensive work has been done and various correlations are availablein the literature related to these hydrodynamic parameters. However, it is difficult to choose theone correlation which is precisely fitted amongst the range of correlations. Thus, for designaccuracy, it is crucial to produce the comprehensive experimental data of hydrodynamiccharacteristics for the desired system and correlate this with the available empirical correlations.Hence, this work consists of evaluation of best conditions for the hydrodynamic parameters bythorough experimental investigation on the basis of various geometrical and operating conditionsand examination of results with numerous available correlations.

2. Previous WorkIn PSPEC, the hydrodynamic characteristics like dispersed phase hold-up, flooding, axialdispersion and ultimately the mass transfer coefficient strongly depend on dispersed phase dropsize. Many investigators have studied the effect of various geometrical and operating conditionson drop size in PSPEC. Kagan et al. [2], Boyadzhiev and Spassov [3], Lorenz et al. [4] andSrinivasulu et al. [5] have studied the effect of pulse velocity on drop size and found that, with anincrease in the pulse velocity the drop size decreases along the height of the column. The individualeffect of pulse amplitude (A) and pulse frequency (f) was also observed byKhemongkorn et al. [6].It was found that the drop size was reduced with increase in A and f. The influence of otheroperating parameters like continuous and dispersed phase flow rates on drop size have beenexamined by Boyadzhiev and Spassov [3], and Srinivasulu et al. [5]. Negligible effects of both thecontinuous and dispersed phase flow rates have been observed in their study.Kagan et al. [2] haveobserved that the phase ratio has no effect on drop size. Khemongkorn et al. [6] have also foundinsignificant effect of dispersed phase superficial velocity on drop size.

The geometrical conditions have also a considerable effect on drop size in PSPEC. Misek[7], Khemongkorn et al. [6], and Boyadzhiev and Spassov [3] have focused on perforationdiameter of sieve plate and revealed that perforation diameter influencing more on drop size

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INDIAN CHEMICAL ENGINEER Vol. 56 No. 3 September 2014

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as compared to plate spacing and fractional free area (FFA). The effect of plate spacing andFFA was observed by Miyauchi and Oya [8] and Srinivasulu et al. [5] and found that the dropsize increases notably with FFA than that of other geometrical parameters. Khemongkornet al. [6] and Kleczek et al. [9] have focused on the effect of mass transfer direction andproposed correlations for drop size prediction. It was observed that the drop size obtainedwas smaller for mass transfer direction from dispersed to continuous phase than that fromcontinuous to dispersed phase. Usman et al. [10] also reported that pulse velocity have moreinfluence on drop size distribution. However, the uniform drop size distribution was found inthe top portion of the column.

Dispersed phase hold-up is also one of the important parameters of column from designpoint of view. Hold-up is a measured quantity of dispersed phase that contacted with thecontinuous phase during the counter-current flow. Larger hold-up helps to achieve the greaterefficiency of the column, and together with the drop size it gives maximum interfacial area forthe mass transfer. Sehmel and Babb [11], Pietzsch and Blass [12], and Venkatnarsaiah andVerma [13] have studied the effect of pulse velocity extensively and revealed that at low pulsevelocity the larger drops accumulate beneath the sieve plate in the mixer-settler regime andhold-up obtained is higher. With increase in pulse velocity in this regime the hold-updecreases and obtained minimum value at transition pulse velocity. Further increase in pulsevelocity results in increase in hold-up. Cohen and Beyer [14], and Lorenz et al. [4] have alsoobserved that, the dispersed phase hold-up increases with increase in pulse velocity. Cohenand Beyer [14], Sehmel and Babb [11], and Khemongkorn et al. [6] also found an increment inhold-up values with increase in A and f separately.

Miyauchi and Oya [8], Mishra and Dutta [15], Lorenz et al. [4], and Venkatnarsaiah andVerma [13] studied scrupulously the effect of geometrical conditions of PSPEC on dispersedphase hold-up. In this study, it was observed that increase in plate spacing, perforation diameterand FFA results in reduction of shear forces acting on droplets. Hence, the drop size obtainedwas greater, and ultimately, due to high slip velocity of large size drops, the hold-up wasreduced. Kumar and Hartland [16] also studied the physical properties of 14 different liquid–liquid systems, along with the operating and geometrical properties of the systems. This studyshowed that column diameter has insignificant effect on the hold-up and perforation diametershows its effect only in the mixer-settler regime of operation. Hold-up is independent of columndiameter over the range of 0.025–0.213 m. Most of the investigators also concluded that, theeffect of plate spacing and FFA has greater effect on the slip velocity rather than perforationdiameter, while calculating the hold-up and proposed the correlation for slip velocity. Theinfluence of continuous and dispersed phase superficial velocity on hold-up was investigated byCohen and Beyer [14], Sehmel and Babb [11], Miyauchi and Oya [8], Mishra and Dutta [15],Bell and Babb [17], Lorenz et al. [4], and Venkatnarsaiah and Verma [13]. Prominent effect ofdispersed phase superficial velocity on hold-up as compared to all other geometrical andoperating conditions has been observed in each work. On the other hand, the consequence ofcontinuous phase superficial velocity on hold-up was negligible. Khemongkorn et al. [6] havealso studied the effect of mass transfer direction on hold-up. At low pulse velocity the masstransfer influence was nil and the hold-up obtained was of same order as in case of no masstransfer. Miyauchi and Oya [8] have proposed the separate empirical correlation for hold-upconsidering mass transfer.

Hydrodynamics of a Pulsed Sieve Plate Extraction Column 237


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Thornton [18] considered the behaviour of pulsed column under high pulsations like spraycolumn. The slip velocity (Vslip) relation was differentiated to obtain the flooding capacity withrespect to continuous and dispersed phase velocities. A correlation for predicting thecharacteristic velocity (Vo) was developed using experimental data obtained on five liquid–liquid systems. Kagan et al. [2], Berger and Walker [19] and Lorenz et al. [4] have studied theeffect of operating parameters, that is, pulse velocity and phase ratio on throughput in PSPEC.It was observed that, in mixer-settler regime, as pulse velocity increases capacity of the columnincreases and flooding occurs at higher flow rates. Once the column reaches its maximumcapacity, further increase in pulse velocity resulted in decrease in its capacity. Thus, the well-known parabolic behaviour with maxima at the transition from mixer-settler to dispersionregime has been noticed. In the same study, the individual effect of A and f have also beenexamined. As there was no significant effect of A and f separately, the product Af has beensuggested to study the flooding behaviour. Kagan et al. [2], and Berger and Walker [19]studied the effect of phase flow ratio (Vd/Vc) on throughput. It was found that increase indispersed phase flow rate increases the resistance to the flow of continuous phase through thecolumn cross-section which causes flooding of column at greater phase ratio of Vd/Vc. Hence,with increase in phase flow ratio, the column capacity reduces. The effect of plate spacing wasfocused by Tribess and Brunello [20]. Increase in plate spacing resulted in reduction of thefrequency of shear force application on droplets. Hence, an increase in mean drop diameter wasobserved, with greater rising velocity. Thus, the throughput was increased with plate spacing.They have also studied the effect of mass transfer from dispersed to continuous phase on theflooding behaviour of PSPEC for the system toluene–acetone–water. In the presence of masstransfer, the maximum flow rate was achieved without flooding. Thus, flooding in the columnwas delayed. Yadav and Patwardhan [21] have scrutinized the information available inliterature, gathered the experimental data from previous work and checked the fitting ofvarious correlations reported in literature for drop size, hold-up and flooding.

Taking into consideration the comprehensive previous work it was noticed that extensiveexperimental study with all geometrical and operating conditions have to be accomplished forall hydrodynamic characteristics like drop size, dispersed phase hold-up and flooding. Sautermean diameter decides the value of other hydrodynamic properties like hold-up, throughput,axial dispersion and eventuallymass transfer coefficient. So, it was crucial to study drop size anddrop size distribution scrupulously. Altogether, to achieve greater efficiency with high capacity,it was necessary to study the drop size, hold-up and flooding characteristics thoroughly.Consequently, the objectives of the present work were the systematic study of all hydrodynamiccharacteristics at different geometrical and operating parameters and development of thecorrelations based on the results for design of PSPEC.

3. Experimental3.1. Experimental SetupFig. 1 shows the schematic of PSPEC. The glass column consists of 0.076 m internal diameterand 1.05 m length. Material of construction used for plate cartridge was SS-316. Thickness ofthe sieve plates was 0.0016 m. To study the consequence of perforation size, two differentperforation diameters of 0.0015 m and 0.003 m with triangular pitch were used. To determine

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the effect of plate spacing, the sieve plates were stacked at a distance of 0.05 m and 0.1 m. Twodifferent types of sieve plates of FFA of 0.2 and 0.3 were employed separately. To achieve thecomplete phase separation the glass disengaging sections of 0.15 m diameter and 0.2 m lengthwere fitted at the top and bottom ends of PSPEC. The perforated liquid distributors were

Fig. 1. Schematic of experimental set up of pulsed sieve plate extraction column. 1. Column shell(Diameter = 0.076 m, Length = 1 m), 2. Plate cartridge, 3. Liquid disengaging section, 4.Liquid distributor for aqueous phase inlet, 5. Liquid distributor for organic phase inlet, 6.Glass Square box filled with continuous phase, 7. Organic phase outlet, 8. Aqueous phaseoutlet, 9. Pulse leg, 10. Rota meter, 11. Ball valves, 12. Centrifugal pump, 13. Storage tank, 14.Solenoid valve, 15. Electronic timer, 16. Pressure regulator, 17. Compressor, 18. Jack leg.



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provided to feed continuous and dispersed phases in the column. The superficial velocities ofcontinuous and dispersed phase were varied with the help of rotameters. Jackleg was used tocontrol the constant level of phase interface at the top of the column. The pneumatic pulsationwas provided to the liquid pool in the column. A pulse leg of 0.05 m diameter and 1.8 mlength was fitted to the bottom disengaging section. The desired magnitude of pulsation wasobtained by controlling compressed air flow rate with pressure regulating valve. Solenoidvalve (3/2 way) was used for constant interruption of air flow rate. The pulse frequency wasvaried by on/off control electronic timer. The square boxes were provided outside the columnat three different heights (top, middle and bottom) and filled with water to eliminate thecurvature effect of glass column to get an accurate drop size of dispersed phase in the columnby photographic method. The inlet and outlet flow rates of both the phases were controlledby using ball valves.

3.2. Experimental ProcedureThe PSPEC was initially filled with water. The pneumatic pulsation was started and the pulseamplitude was varied by compressed air flow control. The amplitude was adjusted bymeasuring the water level displacement in the pulse leg with the help of scale. The desiredpulse frequency was achieved by solenoid valve. The water and kerosene flow was startedthrough the respective liquid distributors. The superficial velocities of continuous aqueousphase (water) and dispersed organic phase (kerosene) in the column were adjusted fordifferent set of operating conditions. The liquid interface was maintained steady for eachexperimental run.

3.2.1. Drop Size Measurement Method

Photographic technique was used to measure the drop size in the active length of the column.The drop size was measured by taking photographs at three different sections starting frombottom, middle and top as explained in the experimental setup. The bottom section consistedof the first three plates, middle section from 10th to 12th plates and top section from 17th to19th plates from the bottom. To eliminate the distortions caused by the curvature of thecolumn, the cross-section of the column under consideration was surrounded by a rectangularbox of acrylic sheets and filled with the continuous phase liquid. A number of videos weretaken from these three sections of the vertical column. Camera used in the measurement ofdrop size was Canon SX20 IS. Most of the photographs were taken on apparature value (AV)programme with SM (super micro) mode by adjusting the micro-scale approximately 0.05 mwith respect to distance between the lens and focus the object. The shutter speed was adjustedat 2500 to 3000 and frames at f/7.5 to f/8. The videos were converted into approximately 500images in to the convertor (magic video and K-Multimedia Player) and finally by making thePDF of these images analysis of drop size was done.

3.2.2. Dispersed Phase Hold-up Measurement Method

Integral dispersed phase hold-up over the entire length of the column was measured by thedisplacement method. This involves measurement of interface level at the top of the columnduring the operation. Subsequently, the operation of the column was interrupted by closing

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all inlets and outlets. After complete collection of the dispersed phase from the top of thecolumn, the level of the separation layer was measured again. The volume of the dispersedphase, which was present over the entire active length of the column during operation, wasdetermined from the difference between the levels at the interface.

3.2.3. Flooding Measurement Method

The superficial velocities of both the phases were adjusted with the help of rotameters. Thepulse velocity was increased from the minimum value. Flooding was observed at differentpulse velocities.

4. Results and Discussion4.1. Drop SizeIn PSPEC, the Sauter mean diameter and drop size distribution influence the terminal risevelocity of drop, dispersed phase hold-up, the residence time of the dispersed phase, axialdispersion and throughputs. Moreover, the interfacial area available for mass transferdepends on drop size and the corresponding dispersed hold-up and plays a very importantrole in continuous and dispersed phase mass transfer coefficient. Hence, the knowledge ofdrop size and drop size distribution has prime importance in a design of column. In a PSPEC,drop size and drop size distribution depend basically upon the drop breakage and dropcoalescence. The breakage of drop was obtained enormously by agitation produced by thepulsing motion (up stroke and down stroke) of liquid pool in the column, which causes thejetting of the dispersed phase drops through the perforation of horizontal plates and hittingon the wall of sieve plates and the column shell. The drop size depends on the geometricalconditions like perforation diameter, FFA and plate spacing as well as operating parameters,such as pulse velocity and superficial velocities of continuous and dispersed phase. The effectof these parameters have been studied comprehensively and discussed herein.

4.1.1. Effect of Phase Reversal on Drop Size

Phase reversal gives rise to bigger drop size as compared to the aqueous phase as thecontinuous phase. From Fig. 2, it could be seen that at the same geometrical conditions andsame set of operating parameters, when the aqueous phase was dispersed phase the drop sizeobtained was relatively very high because of plate wetability characteristic. From visualobservations also it was noticed that, the aqueous phase drop coalescence takes place to forma bigger drop. So, the phase reversal gives less interfacial area for mass transfer compared toorganic phase as dispersed phase. Therefore, for whole study, organic phase was used as thedispersed one.

4.1.2. Effect of Pulse Velocity on Drop Size

Fig. 3, shows the Sauter mean diameter as the function of pulse velocity. The effect of pulsevelocity (Af) on Sauter mean drop diameter was studied with different geometrical conditionsat constant continuous and dispersed phase superficial velocities. In this study, three flowregimes have been observed depending on the intensity of pulse velocity at constant phase flow



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rates. At low pulse velocity range of 0.008–0.015 m/s, mixer-settler regime was observed. At theend of the each pulse cycle, the separation of light and heavy phases into two distinct layers oneach plate of the column have been observed and performed as a series of mixers and settlers.At every pulse cycle, the drop formation of the light kerosene phase during upstroke and thedrop coalescence during down stroke took place. In this regime the distribution of drop swarm

Fig. 2. Effect of phase reversal on the drop size.

Fig. 3. Effect of pulse velocity on Sauter mean drop diameter.

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was also sluggish, which was also reported by Lorenz et al. [4]. At low pulse velocity, the shearforce applied on the drops was also low. All these aspects resulted in formation of relativelylarge drop size in this low range of pulse velocity.

In the pulse velocity range of 0.015–0.03 m/s, the dispersion regime was observed. Theagitation intensity was found to be increased with the pulse velocity in the column. Similarly,the shear force applied on the dispersed phase drops, while pushing through the perforationsof the plates, was enhanced. Also, there was much less possibility of coalescence of dispersedphase drops in this regime due to high turbulence. Because of above pulse velocity of 0.03 m/sdue to very high energy input, the related shear forces on drops were excessive. Also, the dropbreakage occurred intensely due to collision of drops on the wall of sieve plates and thecolumn. At high pulse velocity, the greater oscillatory Reynolds number prevents hindranceof drops beneath the sieve plate of the column. The entire dispersed phase drops were seen tospread throughout the active length of the column. The smallest drop size of 0.001 m wasobtained with smooth working of column in this emulsion regime. This observation was inagreement with Srininivasulu et al. [5], Khemongkorn et al. [6], Luo et al. [22] and Prabhakaret al. [23].

4.1.3. Effect of Geometrical Parameters on Drop Size

From Fig. 3, it has been also observed that, along with pulse velocity the plate geometry likeperforation diameter and plate spacing also have considerable effect on Sauter mean dropdiameter. The substantial decrease in the drop size was found with 0.0015 m perforation sizeas compared to 0.003 m. However, with increase in pulse velocity in both cases the drop sizedifference was reduced and with the pulse velocity of 0.025 m/s and onwards there was littlechange in the drop size at both perforation diameters. It has been revealed that, the resistanceoffered by smaller orifice was relatively greater in case of 0.0015 m perforation diameterplate. Hence, the drop size was smaller that is, 0.0019 m as compared to 0.0027 m atperforation diameter 0.003 m at low pulse velocity. However, an influence of pulse velocityon drop size was high relative to perforation diameter. Consequently, the drop sizes beyondthe pulse velocity of 0.025 m/s for all perforation diameter plates were in close proximity.Lorenz et al. [4] have found that the constant drop size was achieved sooner at a smallerperforation diameter. Khemongkorn et al. [6] have observed smaller drops with decrease inthe perforation diameter because of increase in shear forces on drops. Misek [7] has alsoproposed that the Sauter mean drop diameter was proportional to perforation diameter, d0.4.Whereas in industrial operations the pulsed columns can be operated smoothly up to 0.025 m/spulse velocity. Hence, from maintenance point of view, as well as to avoid chances of blockageof 0.0015 m perforation diameter sieve plate, the 0.003 m perforation diameter has beenrecommended.

At constant pulse velocity, the plate spacing also plays an important role in deciding thedrop size in PSPEC. From Fig. 3, it was observed that at the plate spacing of 0.05 m the Sautermean drop diameter was slightly smaller than that of 0.1 m. There was no significant effect onthe drop size for the pulse velocity in the range of 0.008–0.035 m/s. So, plate spacing has also aslight effect on Sauter mean diameter as compared to perforation diameter. Miyauchi and Oya[8] have reported the same results that the drop size increases with the plate spacing. Also,



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Srinivasulu et al. [5] has proposed in their correlation for drop size that the Sauter mean dropdiameter is proportional to plate spacing, h0.34.

4.1.4. Drop Size as a Function of Column Height

Drop size was measured at three different cross-sections along the axis of PSPEC as shown inthe schematic representation of PSPEC in Fig. 1. Maximum break-up of drops occurred atthe first two to three plates from the bottom of the column, as the organic light phase layerspread beneath these plates and then jetted through the orifices of the sieve plates. Maximumshear force was applied on the droplets by orifices of sieve plates, as the drop size at bottomsection of column was greater than the perforation diameter of plates. It has been observedthat an average reduction in the drop size was approximately 70% from the bottom sectionplate (3rd from bottom) to middle plate (10th from bottom). Since, the dispersed phasedroplets of smaller size moved up through the middle section of the column, the shear forcerecognized on these drops was low as compared to that of the bottom section. Hence,the reduction in the drop size was marginal as compared to the bottom half section of thecolumn. Nearly 30% average reduction in the drop size has been observed from the middlesection to the top section plate (18th from bottom). From Fig. 4, it could be clearly seen thatthe drop size profile is the function of active length of the column. Lorenz et al. [4], Joneset al. [24], and Srinivasulu et al. [5] have also shown that the majority of drop breakagewas observed to occur during passage through the first two or three plates just above thedistributor.

Fig. 4. Drop size as function of column height.

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4.1.5. Drop Size and Drop Size Distribution

The drop size distribution depends upon the breakage and coalescence of the drops at each sieveplate of the column. It strongly depends on the range of pulse velocity. Generally, coalescenceof drops takes place at low pulse velocity and the mass transfer coefficients decreases as theinterfacial area decreases. Breakage of drops facilitates increase of the interfacial area and thus,increases the mass transfer coefficients. Typical drop size distribution throughout the length ofcolumn at three different sections, starting from bottom to the top, specifically at 3rd, 10th and18th number of plates with four different pulse velocities have been shown in Fig. 5. Inparticular, the effect of different pulse velocity on drop size distribution was studied and it wasfound that, the drop size distribution narrowed down with increase in pulse velocity from 0.01to 0.035 m/s. At low pulse velocity, the oscillatory Reynolds number (i.e. Reo =DAf/ν) was less,so the turbulence was low. Flow developed due to oscillation of pulse velocity was characterizedby dimensionless number Reo. As a result, large size of droplets was formed. Due to low pulsevelocity, the coalescence of some of the drops also occurred. This causes the formation ofgreater size drops in the column. Hence, high drop size distribution was observed at low pulsevelocity. Same results were also observed by Srinivasulu et al. [5] using kerosene–water systemfor 0.76 m diameter column. As the pulse velocity increases, the energy input to the pool of thecontinuous phase in the column increases. While forced through the perforations due to highturbulence, the shear forces on droplets also increased proportionately with the pulse velocity.As a result, the greater size droplets become unstable and the drop size reduced subsequently.Eventually, the uniform size droplets were formed at high pulse velocity. It can be easilyobserved from Fig. 5, that the effect of pulse velocity on the drop size distribution was minimumat the 18th plate from bottom because of high pulse velocity (Af= 0.035 m/s), which means that,deviation in the drop size decreases with increase in pulse velocity as production of morehomogenous drops takes place. Lorenz et al. [4] have also established that with increase in pulseintensity, the drop size moved to the smaller diameter and also the drop size distribution turnsout to the thinner. Kagan et al. [2] attributed that at high pulse frequency the occurrence ofshear force on dispersed drops was repeated in less interval of time and resulted in the smallersize drops. Also the break-up of greater size drops had taken place due to turbulence.

As the dispersed phase drops passes above the first 2–3 plates from bottom, the repeatedbreak-up at plate perforations, as well as horizontal plates and column walls, occurred. Dropsize distribution decreased as the dispersed phase flows upward in the column. At all pulsevelocities studied, with an increase in height of column from bottom to top, the drop sizedistribution was shifted towards the narrow range of the drops.

It has been observed that the drop size distribution was also dependant on thegeometrical conditions. The drop size distribution was unimodal from second bottom plateand uniform at low pulse velocities for 0.0015 m perforation diameter plates. As theperforation diameter was small, the orifice velocity was very high, as well as resistance offeredby small flow area to the drops were also high. It has caused the high shear force on dispersedphase drops. As the orifice velocity was high, the turbulence maintained throughout thelength of the column was greater even at low pulse velocity. Hence, the coalescence rate wasalso very low at the bottom section of the column. With plate spacing of 0.05 m the repeatedbreak-up of drops was observed. Therefore, the rate of coalescence was also low and frequent



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application of shear force was realized by drops. Thus, the drop size distribution was thinnerat low plate spacing. Lorenz et al. [4] also revealed that the uniform drop size distribution wasdependent on plate geometry and it could be attained quickly at smaller perforation diameterof the sieve plates. They have also reported that the drop size distribution was the function ofnumber of sieve plates and it was independent of plate spacing in the upper section ofthe column. Moreover, it was observed that Sauter mean drop diameter and drop sizedistribution were independent of the operating parameters like continuous and dispersed

Fig. 5. Drop size distribution along the column height (from bottom to top).

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phase superficial velocity. The same results have been reported by Lorenz et al. [4] andSrinivasulu et al. [5].

4.1.6. Model Predictions for Sauter Mean Diameter

All of the experimental drop size data reported in this study was correlated on the basis of alloperating parameters of PSPEC like continuous and dispersed phase superficial velocities andpulse velocity and geometrical parameters, such as perforation diameter and plate spacing.Linear regressions were performed to obtain the intercepts and correlation coefficients andmodel has been formulated. The correlation developed for the Sauter mean diameter is,

d32 ¼ 0:137d0:16h0:18V�0:07d V 0:02

c Af �0:55 ð1ÞFrom Fig. 6 it was found that, the correlation developed was fitted with 15% deviation fromthe experimental values of Sauter mean diameter. It could be concluded from the operatingparameters that, with an increase in pulse velocity, the shear force on the drops increasesresulting in the smaller size of drops. It has also been established that the perforation diameterand plate spacing have little role in reducing the Sauter mean diameter. Continuous anddispersed phase superficial velocities have an insignificant part in drop size.

4.2. Dispersed Phase Hold-upThe design of PSPEC requires the information of cross-section to accommodate the specifiedflows in the column. The knowledge of dispersed phase hold-up was required to determine the

Fig. 6. Plot of Sauter mean diameter computed from correlating equation versus experimental values.



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cross-section of column that corresponds to the flooding of the column. Hold-up means thetotal amount of dispersed phase that contacted with the continuous phase in the active lengthof the column. It was a function of drop size formed during pulsing motion in the column andthe dispersed phase flow rate. The interfacial area available for mass transfer depends onhold-up besides drop diameter. Also, any counter-current column has limit of maximumhold-up beyond which flooding occurs. Hence, one of the important control objectives toachieve during column operation was to maintain a high value of dispersed phase hold-up,while keeping away from flooding point for its efficient performance. Consequently, the hold-up study was carried out extensively.

4.2.1. Effect of Pulse Velocity on Dispersed Phase Hold-up

From Fig. 7, three different regimes like mixer-settler, dispersion and emulsion regime havebeen observed with increase in pulse velocity in PSPEC. In case of mixer-settler regime beloweach of the horizontal sieve plate, dispersed phase layer was formed which gave slightlyhigher value of hold-up, but the dispersed phase was not distributed in the continuous phase.So, in mixer-settler regime, hold-up decreases to some extent with increase in the pulsevelocity, which corresponds to the transition from mixer-settler to the dispersion regime.Further increase in pulse velocity gave the higher concentration of dispersed phase drops inthe neighborhood of the plates relative to the average concentration of the drops at everysection created by sieve plates. It was observed that, the hold-up increased in this regime withadditional pulse velocity. This is due to uniform distribution of dispersed phase drops in thecontinuous phase throughout the active length of the column. The drop size in this regimewas smallest and the hold-up obtained was highest as a result of low rising velocity of smallsize droplets. In the present work, the dispersed phase hold-up was measured for the pulse

Fig. 7. Effect of dispersed phase superficial velocity on hold-up at different pulse velocity.

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velocity ranging from 0.015–0.031 m/s and around 10% increase in dispersed phase hold-uphas been observed at constant throughput. The maximum value of hold-up obtained was 21%for a pulse velocity of 0.031 m/s and dispersed phase superficial velocity of 0.006 m/s.

Below the pulse velocity of 0.02 m/s the hold-up obtained was less and beyond 0.025 m/spulse velocity, the steady state operation would be difficult. Hence, for industrial scale, thepreferable pulse velocity range of 0.02–0.025 m/s has been selected. Lorenz et al. [4] have alsoobtained the results in the same line.

4.2.2. Effect of Dispersed and Continuous Phase Superficial Velocity on Hold-up

The vital operating parameters next to the pulse velocity in PSPEC are dispersed andcontinuous phase superficial velocities. From the experimental results shown in Fig. 7, it hasbeen observed that the dispersed phase superficial velocity was strongly influencing thedispersed phase hold-up. Rapid increase in the hold-up was obtained with the dispersed phaseflow rate. However, the continuous phase superficial velocity has negligible effect on the hold-up as shown in Fig. 8 at constant pulse velocity and dispersed phase throughput with samegeometry. In the range of continuous phase superficial velocity of 0.002–0.005 m/s, thedispersed phase hold-up was 13%. From this it can be concluded that the hold-up increaseswith phase ratio Vd/Vc. Lorenz et al. [4] have also obtained the same results for three differentsystems. They have revealed that drop size is independent of throughput. Hence, the highervalues of hold-up was only the effect of throughput enhancement up to flooding point withdirect inducement of dispersed phase by increase in Vd. Other investigators, Sehmel and Babb[11], Bell and Babb [17], Mishra and Dutta [15], Khemongkorn et al. [6] and Venkatanarasaiahand Verma [13] have also obtained the same results.

Fig. 8. Effect of continuous phase superficial velocity on hold-up.



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4.2.3. Effect of Plate Spacing on Hold-up

As shown in Fig. 9, dispersed phase hold-up was measured at different plate spacing of 0.05m and 0.1 m. It was observed that hold-up reduces with increase in plate spacing in thePSPEC. The visual observation concluded that close plate spacing helped in breaking thedrops as they pass through more number of plates and higher restriction was created inthe direction of the flow. Hence, the shear force realized on the dispersed phase drops wasmore and small size of drops was formed with maximum number of plates (obstacles). As aresult of this rise, velocity of dispersed phase drops decreases and residence time of the dropsincreases. Eventually dispersed phase hold-up increases. This finding was also in line with theobservation of various authors like Miyauchi and Oya [8], Mishra and Dutta [15], Kumarand Hartland [16], Venkatanarasaiah and Verma [13].

4.2.4. Effect of Perforation Diameter on Hold-up

The sieve plate of two different perforation diameters of 0.0015 and 0.003 m was studiedseparately for dispersed phase hold-up measurement. It was observed from Fig. 9, that thehold-up decreases with increase in the perforation size. The shear force exerted on drops at0.0015 m perforation diameter was greater than that of 0.003 m perforation size plates. Thereduction in drop size took place throughout the active length of the column started at thefirst plate from the bottom. Dispersed phase hold-up was increased as a result of highresidence time of smaller size droplets. The hold-up does not change according to perforationdiameter, once the drop size becomes smaller than perforation size. Hence, the effect ofperforation size was less as compared to other geometrical parameters like plate spacing andFFA. The same results were observed by Mishra and Dutta [15]. They have also studied theeffect of perforation size, plate spacing and FFA of the multiorifice sieve plates on 0.0765 m

Fig. 9. Effect of plate spacing and perforation diameter on hold-up.

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column diameter. It was concluded that hold-up decreases with increase in any of thesegeometrical properties. Venkatanarasaiah and Verma [13] have also studied the effect ofgeometrical parameters using the kerosene-water system with n-butyric acid and benzoic acidas solute. They have carried out experiments on the geometrical conditions of sieve plate andobtained similar results.

4.2.5. Effect of Fractional Free Area on Hold-up

Two different sieve plates of 0.2 and 0.3 FFA with 0.003 m perforation diameter and 0.05 mplate spacing were used to study the effect of free area on hold-up. The reduction in dispersedphase hold-up was observed with increase in the FFA of the plate, as shown in Fig. 10.Increase in the FFA resulted in decrease in restriction of drops to travel in the direction offlow. Hence, less shear force was realized by drops at greater FFA of 0.3 as compared to theFFA of 0.2. At dispersed phase superficial velocities above 0.005 m/s, the significant increasein hold-up was observed with high FFA. Mishra and Dutta [15], Lorenz et al. [4], Kumar andHartland [16], and Venkatanarasaiah and Verma [13] have also obtained the similar trend forfree area of the plate.

4.2.6. Model Predictions for Dispersed Phase Hold-up

The dispersed phase hold-up data was also correlated with the model formulated form linearregression method as follows,

/ ¼ 8:98d�0:12h�0:16a�0:2V 0:92d V 0:05

c Af 0:7 ð2ÞFrom Fig. 11 it was observed that, the correlation developed was fitted with 20% deviationfrom the experimental values of dispersed phase hold-up. The dispersed phase superficial

Fig. 10. Effect of fractional free area on hold-up.



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velocity was highly influencing the dispersed phase hold-up. Pulse velocity also plays animportant role to obtain greater dispersed phase hold-up. Negligible effect of the continuousphase superficial velocity was found on dispersed phase hold-up. Perforation diameter, platespacing and FFA are inversely proportional to the dispersed phase hold-up.

4.3. Flooding CharacteristicsPSPEC was operated in a counter-current mode in which aqueous phase was fed at the top ofthe column and organic phase was dispersed by charging from bottom of the column. Theliquid phases in the column were displaced in converse direction with the help of energy inputin the form of pulse velocity. The pulse velocity helps to generate agitation inside the columnthat leads to the breakage of lighter or organic phase as it was forced to pass through theperforated plates. Thus, formation of small drops throughout the length of the column hadtaken place. Pulse velocity was the only parameter that has decided the flow regime and theflooding inside the column. Flooding was the limitation of the operation and the columncross-section area can be measured by the flooding throughput. Flooding was mainlydepending on the operating pulse velocity and phase flow rates. Flooding curve was aparabolic curve which starts at the origin of the co-ordinates and it forms by different flowrates of the phases, insufficient pulsation and higher pulsation depending on regime ofoperation.

4.3.1. Effect of Pulse Velocity on Flooding Curve

When the column was operated below the minimum pulse velocity that is, insufficient pulsevelocity zone, the flooding occurred in the mixer-settler regime of operation as shown in Fig. 12.

Fig. 11. Plot of dispersed phase hold-up computed from correlating equation versus experimentalvalues.

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Flooding commenced at the bottom plate because of insufficient pulses available to push theorganic phase, through perforations. In mixer-settler regime of operation, the dispersed phasedrops were larger in size and form a settled layer beneath the next plate before the arrival of thenext pulse. Hence, the dispersed phase accumulated into the column and finally went out fromthe outlet of the continuous phase. In this regime, with increase in pulse velocity the capacity ofthe column decreased and operation was shifted to the dispersion regime. In this regime, thedispersed phase was evenly distributed throughout the continuous phase with the increase inpulse velocity in the column. It was also observed that the rise velocity of the drops was lesscompared to the upcoming flow of the continous phase. Hence, drops have startedaccumulating in the disenging section of the column and finally passed out through the outletof the continuous phase. This was the flooding due to excessive pulsing in the emulsion regimeof operation. Thornton [18], McAllister et al. [25], Smoot and Babb, [26], Kagan et al. [2],Berger and Walter [19], Lorenz et al. [4] and Tribess and Brunello [20] have also observed thesame results. Kagan et al. [2] have also used the kerosene-water system and found that, inmixer-settler regime as the pulse velocity increases, the capacity of the column increases.However, flooding occurs with further increase in the pulse velocity in dispersion regime athigher flow rates, which decreases the capacity of the column.

Fig. 12. Effect of pulse velocity on throughput at different geometry.



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4.3.2. Effect of Plate Spacing on Throughput

From Fig. 12, it was observed that with increase in plate spacing the throughput increased.Increase in plate spacing from 0.05 m to 0.1 m resulted in reduction of the number of platesfrom 20 to 10 in 1 m length column. Hence, the occurrence of the application of the shearforce on droplets was reduced. Thus, the drop size was greater at plate spacing of 0.1 mcompared to 0.05 m. Consequently, with further increase in rise velocity more number ofdrops easily reached the top of the column resulting in higher throughput. It was concludedthat the throughput increases with an increase in the plate spacing. This was also consistentwith the result observed by McAllister et al. [25], Smoot and Babb [26], Kagan et al. [2].Tribess and Brunello [20] reported that with increase in plate spacing, the number of platesreduces and the shear force on the drops also reduces, which results in an increase in the meandrop diameter with greater throughput.

4.3.3. Effect of Perforation Diameter on Throughput

Decrease in perforation size reduced the throughput slightly as shown in Fig. 12. However,the effect of perforation size was less compared to plate spacing. The perforation diameterused for this study was 0.0015 and 0.003 m. Since, the dominance of pulse velocity was higherthan that of different perforation diameters; the comparable sizes of drops were formed.Thus, no substantial difference in throughput occurred. Lorenz et al. [4] have also explainedthe effect of perforation size on flooding curve by considering the 0.004 m and 0.006 mperforation sizes. They have observed that, maximum throughput was obtained at 0.006 mperforation diameter. This effect was also explained on the basis of drop size formation andits rise velocity with two different perforation sizes.

4.3.4. Effect of Fractional Free Area on Throughput

It was observed that, the throughput increases with FFA in PSPEC. At high FFA the cross-sectional area, available for both phases was greater. So, at upstroke and down stroke theshear force realized by drops was low. From Fig. 12, it was found that for FFA of 0.2 and 0.3the maximum throughput obtained was 0.016 and 0.02 m at pulse velocity of 0.025 m/s. Itwas also observed that, beyond pulse velocity of 0.025 m/s the reduction in throughput wassignificant with FFA of 0.2 as compared to 0.3 due to greater turbulence generated in case ofFFA of 0.2. Berger and Walter [19] and Lorenz et al. [4] have also studied the effect of freearea and concluded that with increase in FFA, maximum throughput increased. They havealso found the effect of physical properties on flooding curve with increasing free area usingthree different systems and concluded that flooding increased with the interfacial tension dueto increase in the drop size.

4.3.5. Effect of Phase Ratio on Throughput

From Fig. 13, it was observed that with increase in phase ratio (O/A) from 1:6 to 1:1,throughput decreases. Since the dispersed phase flow rate increases, the dispersed phase hold-up increases and consequently the drop population throughout the length of columnincreases. In the presence of greater number of drops, the flow area available for continuous

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phase flow was reduced. So, the resistance to the continuous phase flow increased andultimately the overall throughput increased at low O/A ratio. Berger and Walter [19] andLorenz et al. [4] have also observed that the flooding curve decreases with increase in O/Aratio. They have also studied flooding for the different systems and finally concluded that theflooding curve was function of interfacial tension, density and visocity of the system. Kaganet al. [2] also found that the throughput decreased with increase in the phase ratio Vd/Vc forkerosene-water system.

5. ConclusionsHydrodynamic characteristics in a PSPEC have been studied scrupulously at differentgeometrical and operating parameters. From this comprehensive work it has been observedthat, all hydrodynamic properties that is, drop size, dispersed phase hold-up and floodingcharacteristics are influenced by column sieve plate geometry like perforation diameter, platespacing and FFA. Pulse velocity was the vital operating parameter in hydrodynamics ofPSPEC and notably influences the Sauter mean diameter and consequently the dispersedphase hold-up and throughput in the column. Dispersed phase superficial velocity affect hold-up and throughput substantially. However, insignificant effect of continuous phase superficialvelocity has been found on drop size.

For design of PSPEC the preferable geometrical conditions for kerosene-water systemwere perforation diameter of 0.003 m, plate spacing of 0.05 m and FFA of 0.2 with theoperating parameters like pulse velocity of 0.025 m/s and superficial velocity of 0.006 m/susing organic to aqueous ratio (O/A) of 1:1. The drop size obtained at these conditions was0.0015 m with dispersed phase hold-up of 18%. The correlations developed in the work willhelp in predicating the values, which will further helpful in a design of the column.

Fig. 13. Effect of phase ratio on throughput at different geometry.



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NomenclatureA amplitude of pulsation (m)Af pulse velocity (m/s)D column diameter (m)d perforation diameter (m)d32 Sauter mean diameter (m)f frequency of pulsation (Hz)h plate spacing (m)Reo oscillatory Reynolds numberVc superficial velocity of the continuous phase (m/s)Vd superficial velocity of the dispersed phase (m/s)Vo characteristics velocity of drops (m/s)Vslip slip velocity (m/s)

Greek Symbolsα fractional free areaϕ dispersed phase hold-upν kinematic viscosity (m2/s)

AcknowledgementThe authors would like to acknowledge Department of Atomic Energy (DAE) for the financial support.

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Hydrodynamics of a Pulsed Sieve Plate Extraction Column