bubble diameter and effective interfacial area in a novel hybrid rotating and reciprocating...

INTERNATIONAL JOURNAL OF CHEMICAL

REACTOR ENGINEERING

Volume 10 2012 Article A12

Bubble Diameter and Effective InterfacialArea in a Novel Hybrid Rotating and

Reciprocating Perforated Plate BubbleColumn

Dhanasekaran S∗ Karunanithi T†

∗Annamalai University, Tamil Nadu, India, [email protected]†Annamalai University, Tamil Nadu, India, [email protected]

ISSN 1542-6580DOI: 10.1515/1542-6580.2914Copyright c©2012 De Gruyter. All rights reserved.

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Bubble Diameter and Effective Interfacial Area in aNovel Hybrid Rotating and Reciprocating Perforated

Plate Bubble Column∗

Dhanasekaran S and Karunanithi T

Abstract

This investigation reports on the experimental and theoretical investigationcarried out to evaluate the bubble diameter and effective interfacial area in a novelHybrid Rotating and Reciprocating Perforated Plate Bubble Column. Air-watersystem is used in this investigation. Countercurrent mode is employed. The ef-fects of agitation level, superficial gas velocity and superficial liquid velocity onthe bubble size distribution are studied. The mean bubble diameter is predictedusing photographic technique. A simple correlation is developed for the determi-nation of mean bubble diameter. It is found that the mean bubble diameter valuesfor hybrid column are 1.8 to 2.5 times smaller when compared with conventionalreciprocating plate column. The interfacial area is calculated based on the exper-imental results of the gas holdup and bubble diameter. Effects of agitation level,superficial gas velocity, superficial liquid velocity and plate free area on the in-terfacial area have been investigated. Correlations are developed for the determi-nation of interfacial area for both mixer-settler and emulsion regions. It could benoted that the interfacial area for the hybrid column is 3 to 6 times higher in bothmixer-settler region and emulsion region than that of conventional reciprocatingplate column which is quite large.

KEYWORDS: gas holdup, bubble diameter, interfacial area, hybrid rotating andreciprocating perforated plate bubble olumn, air-water system

∗Corresponding author. Dhanasekaran S.: Department of Chemical Engineering, FEAT, Anna-malai University, Annamalai Nagar- 608002.Tamil Nadu, India, Tel.: +91 4144 237168, Fax.:+91 4144 239737, Mob.: +91 994 262 6198, [email protected], [email protected]. Karunanithi: Department of Chemical Engineering, FEAT,Annamalai University, AnnamalaiNagar- 608002, Tamil Nadu, India., [email protected].


1 INTRODUCTION

Gas - liquid mass transfer is the process of transporting a solute from one of the phases into the other. These industrial processes range from the manufacture of gasoline and petrochemicals to sewage treatment and the making of pharmaceuticals. The absorption of lean CO2 in caustic alkali solution, the absorption of lean NH3 in an aqueous solution of H2SO4 and H3P04, the absorption of lean H2 S in aqueous caustic alkali solution, the absorption of lean SO2 in aqueous alkali solution are a few examples of gas – liquid mass transfer operations. In any case, whether followed by a chemical reaction or not, the gas must first be dissolved in the liquid. Thus, solute transfer is the most fundamental step in improving the mass transfer rate or the overall reaction rate. The mass transfer rate is influenced, in particular, by the design of the gas – liquid contactor.

Process equipment for gas – liquid mass transfer have a mature technological base. Nowadays, one of the major challenges that the chemical industry has to deal with is to create innovative processes with less pollution, improved chemistry and high energy efficiency (Guangwen et al., 2008) or the process equipment for gas – liquid contacting systems should be designed to achieve the appropriate transfer operations with minimum energy requirement and operating cost.

Knowledge of the specific interfacial area, a, is of essential importance for designing the gas – liquid contacting equipment. In order to increase the overall mass transfer rate between the phases, it is necessary to increase the liquid film mass transfer coefficient and/or interfacial area (Veljkovic and Skala, 1988). This is connected with extra cost usually as external energy input. An increase in mass transfer rate can be achieved in a more suitable way by increasing interfacial area rather than liquid film mass transfer coefficient. In that case, the construction of a gas-liquid contactor with a high specific interfacial area at relatively low capital and operating costs is the main design goal ((Veljkovic and Skala, 1988).

In general bubble columns and mechanically agitated gas – liquid reactors like stirred tank reactors, rotating disk reactors, reciprocating plate columns, pulsating plate columns, liquid pulsating column, etc., are extensively used for gas-liquid mass transfer operation in chemical and biological processes (Deckwer et

1S and T: Novel Hybrid Bubble Column

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al., 1974; Miyanami et al., 1978; Gagnon et al., 1998; Paglianti et al., 2000; Verma, 2002; Palma and Giudici, 2003; Garcia and Gomez, 2005; Stella et al., 2006; Tang et al., 2009).

The stirred tank reactor is still the most commonly employed type of bioreactor in fermentation technology. With the introduction of external mechanical agitation, it provides highly efficient contact and hence improves greatly the mass transfer between gas and liquid phases. Unfortunately, a single stirred vessel provides no more than one equilibrium stage (Deckwer et al., 1974; Lounes and Thibault, 1993).

Rather than stirred tank reactors, the most common gas – liquid contact device is the bubble column (Lounes and Thibault, 1996). It is frequently applied in industry for various processes as inexpensive, simple gas – liquid contactors (Lee et al., 2000). When these are operated, countercurrently offers the possibility of multistage performance (Deckwer et al., 1974; Lounes and Thibault, 1993). They are particularly attractive for the applications in biotechnology as bioreactors where strong mixing action has to be avoided. The main advantage of the bubble column is the absence of moving parts which facilitate the operation under sterile conditions. An additional advantage of a tall bubble column is the increased solubility of gas at the base of the column (liquid exit). This is due to the higher hydrostatic pressure in accordance with Henry’s law. But it is surprising to observe relatively high axial dispersions in these widely used columns. This high mixing is mainly due to the motion and recirculation of the liquid induced by the raising bubbles in the columns with no restrictions to inhibit both the flow of gas as well as liquid. Various solutions have been proposed to reduce the axial dispersion coefficient, including the use of partitions, plates and packing elements (Karunanithi, 1983; Lounes and Thibault, 1996).

An advancement of bubble column is Karr’s Reciprocating Plate Column (Karr, 1959). This column has found wide acceptance in chemical, waste water treatment, hydrometallurgical and pharmaceutical industries (Karr et al., 1987; Baird and Rama Rao, 1988; 1998; Lounes and Thibault, 1993; 1996; Veljkovic and Skala, 1986; 1988; 1989; Yang et al., 1986a; 1986b; Rama Rao et al., 1983; Rama Rao and Baird, 1986; 1988; 2003; Gomaa and Taweel, 2005; Gomaa et al., 1991; Harikrishnan et al., 1994; Stella et al., 2006; 2008). The advantages of the reciprocating plate column used as a reciprocating plate bioreactor are the maximum

2 International Journal of Chemical Reactor Engineering Vol. 10 [2012], Article A12


oxygen mass transfer to satisfy the oxygen demand of microorganisms, low energy dissipation and the reduced back – mixing (Lounes and Thibault, 1993). This Reciprocating Plate Column combines the favorable properties of bubble column and contactors with the external.

Though studies on reciprocating plate column and its application are quite abundant as a gas-liquid contactor, there has been a greater interest in studying the effects of rotary and reciprocatary flow dynamics in gas-liquid mass transfer.

The internal geometry of the reciprocating plate column is modified by Dhanasekaran and Karunanithi (2007) by including the rotational action along with reciprocation action simultaneously and a hybrid form is reported by combining the effects of bubble columns, stirred tanks and the reciprocating plate columns using a bevel gear arrangement (Dhanasekaran and Vijayagopal, 2007; Dhanasekaran et al., 2008; Dhanasekaran, 2008; Dhanasekaran and Karunanithi, 2010a; 2010b; 2012). The response of this novel hybrid column is found to be similar to that of the reciprocating plate column showing mixer-settler, transition, and emulsion regions. The hybrid arrangement ensures that the attainment of minimum agitation level falls early when compared to the values of earlier investigations done in reciprocating plate column. The lower level of agitation obtained in this novel hybrid column is mainly due to the introduction of rotational effect along with the reciprocation in this column. Since the emulsion region starts early, the gas holdup obtained is high at low-power consumptions. When these systems are used as bioreactors, higher holdup at low agitation level ensures lesser shear effects and minimum rupture on the cell.

Moreover, the conventional fermentor contains only the rotational action of the paddles (rotors of impellers). It is a promising alternative investigation to replace the conventional fermentor by adding reciprocation effect along with the existing rotation action of paddles. While reciprocation action is introduced with perforated plates, it results in reduced bubble size and uniform bubble size distribution. Reduced bubble size provides more contact with liquid phase; hence the best mass transfer can be achieved. Though the rotation action increases the residence time of gas bubbles from bottom to top of the fermentor by vortex movement of gas bubbles, reciprocation action still enhances the stagnation time of gas bubbles by zigzag movement along the axis.




The increased stagnation time of gas bubbles in results more solubility of gaseous solute in liquid phase. Hence, this novel hybrid bubble column can effectively be used where the best oxygen mass transfer is required, i.e., production of a single cell (yeast), where the microbes require more dissolved oxygen. Also, it could be suitable for continuous biological treatment of effluents (aerobic condition) to achieve greater BOD and COD reduction.

Performances of this hybrid column based on gas holdup, axial mixing and volumetric mass transfer coefficient are studied (Dhanasekaran and Vijayagopal, 2007; Dhanasekaran et al., 2008; Dhanasekaran, 2008; Dhanasekaran and Karunanithi, 2010a; 2010b; 2012) and the continuation of the same is an attempt made to study the performance of this hybrid column based on bubble diameter and effective interfacial area in this investigation.

This investigation reports the effects of agitation level, superficial gas and liquid velocities on the bubble size distribution and the effects of agitation level, superficial gas velocity, superficial liquid velocity and plate free area on the interfacial area. Correlations are developed for both bubble diameter and effective interfacial area. 2 EXPERIMENTAL SET-UP The novel hybrid rotating and reciprocating perforated plate bubble column is designed and fabricated indigenously. Figure 1 shows the photographical representation of the novel hybrid rotating and reciprocating perforated plate bubble column. The experimental setup and its ancillary connections are shown schematically in Figure 2. The flanged acrylic column (15) has 10.0 cm internal diameter and 120 cm height. This is equipped with a central acrylic drive shaft (14) containing circular perforated plastic plates (16) with equal plate spacing. The diameter of the shaft is 1.0cm and the thickness of the plastic plate is 0.2 cm. Copper sleeves are used to set the plate spacing. The central acrylic drive shaft and circular perforated plates together comprise the plate stack. The height of the plate stack is 100 cm. The stack contains plates of uniform perforation. A D.C motor (13) is provided at the top of the column. The shaft (29) of the D.C motor is attached to a bevel gear arrangement (11). The bevel gear arrangement consists of a vertical gear (25),



Figure1: Photographic Representation of Experimental Setup- Novel Hybrid Rotating and Reciprocating Perforated Plate Bubble Column Eccentric (27), connecting rod (12), horizontal gear (26), ball bearing and milling (28). The vertical and horizontal gears are in the ratio of 1:1. The hybrid rotating and reciprocating action is brought about by a bevel gear arrangement. The reciprocating action (up and down movement) of the plate stack is caused by attaching the central shaft with milling on the top. It is connected to an eccentric arrangement on the vertical gear. The rotating action is caused by the horizontal gear. The horizontal gear has a shaft with inner milling which meshes with milling on the central shaft. Thus, when the vertical gear rotates, the horizontal gear drives the inner shaft to rotate. The inner shaft is made to move up and down, since the inner shaft is connected to an eccentric on the vertical shaft. Figure 3 shows the three different views of bevel arrangements.

The amplitude(A) is half of the full reciprocation stroke which




Figure 2: Experimental Setup—Hybrid Rotating and Reciprocating Perforated Plate Bubble Column.

(a) Left side view (b) Front view (c) Right side view

Figure 3: Photographic Representation of Bevel Gear Arrangements Could be adjusted upto 1 cm. The drive frequency (f) could be maintained upto 3.0 s-1. A frequency controller (24) is used to control the speed. An RPM indicator (10) and a sensor (9) are used to note the drive frequency. Agitation level (Af) is the product of amplitude (A) and frequency (f). Water is pumped to the overhead tank (4) from the sump (1) by using the pump (2) through the water inflow line (3). The level in the overhead tank is maintained by the overflow line (5). Water is supplied from the overhead tank to the top of the plate stack through a globe valve (6) and rotameter (8). Two solenoid valves (7) in the water line, one at the inlet and another at the outlet, are used to shut-off during holdup measurements. A globe valve (30) is



provided in the water outlet line. It is used to maintain the height of the water level in the column. Air is fed to the column from the air compressor (22) through the solenoid valve (17), needle valve (18), pressure regulator (19) and rotameter (20). Solenoid valve is used for shut-off during holdup measurements and the needle valve is used to set the airflow rate. Air leaves the top of the column at atmospheric pressure. 3 METHODS 3.1 BUBBLE DIAMETER Air-water system is used in this study. The range of experimental variables investigated for bubble diameter is given in Table 1. The water level in the overhead tank (4) is maintained continuously by circulating the water from the sump (1) through pump (2). The globe valve (6) in the water inlet line is opened and fixed to a desired level of water flow rate. The inlet water flow rate is noted (QL) using the rotameter (8).The column is initially filled with water. The desired height of water level in the column is maintained in a steady state by adjusting the globe valve (30) provided in the water outlet line. The height of the water level in the column is noted. This is termed as the height of the ungassed column and is represented by the notation ‘Ho’. The needle valve (18) in the air inlet line is opened and fixed to a desired level of air flow rate. The inlet air flow rate is noted (QG). The increase in the height of the water level in the column is noted. This is termed as the height of the gassed column and is represented by the notation ‘H’. The three solenoid valves at water inlet (7), water outlet (7) and air inlet (17) are used for rapid shut-off during the holdup measurements. If these three valves are closed simultaneously, then the height of the ungassed column should be similar to that of the height maintained previously by the adjusted water outlet flow. This ensures that the experiments are carried out under steady state countercurrent conditions (It takes approximately 1.5 to 2 hours for each reading). The heights of the gassed and ungassed columns are noted without agitation. For this fixed air and water flow rate, plate spacing and perforation diameter, the experiments are carried out under steady state countercurrent conditions. The agitation is initiated by the




Table: 1 Range of Experimental Variables Investigated for Bubble Diameter and Interfacial Area S.No Variables Range 1. QG (lpm) 10, 15, 20, 25 2. UG (cm/s) 2.12, 3.18, 4.24, 5.31 3. QL (lph) 40, 50, 60, 70, 80 4. UL (cm/s) 0.14, 0.18, 0.21, 0.25, 0.28 5. A (cm) 1 cm. 6. f (s-1) 0 – 3.0 7. Af, (cm/s)) 0, 0.37, 0.73, 1.20, 1.48, 1.66, 1.92, 2.18, 2.75, 3.08 cm/s 8. np 10, 20. 9. Sp (cm) 4.5, 2.25 cm. 10. dp (cm) 0.8, 0.6, 0.4 cm

variable speed DC motor (13) for this fixed condition. The agitation level is varied, namely, 0.37, 0.73, 1.20, 1.48, 1.66, 1.92, 2.12, 2.50 and 2.75 cm/s for each set i.e., uG = 5.31cm/s, uL = 0.28 cm/s, Sp = 2.25 cm, np = 10, dp = 0.8 cm. The column section is photographed at all the desired levels of agitation. Figure 4 shows the photograph taken at an agitation level of 2.75 cm/s, superficial gas velocity of 5.31cm/s, superficial liquid velocity of 0.28cm/s, plate spacing of 2.25 cm, number of plates of 10and plate perforation diameter of 0.8 cm respectively. Table 2 represents the experimental data obtained at the above condition.

The procedure is repeated for another two different superficial liquid velocities, namely, 0.21, 0.14 cm/s and for another set of two different superficial gas velocities, namely, 4.24, 3.18 cm/s. The same procedure is repeated for the plate spacing of 2.25 cm and perforation diameter of 0.6 and 0.4 cm respectively.

The photographs are then transferred into a transparent sheet. From the transparent sheet, the number of bubbles in the photograph is counted with respect to their size using the profile projector. The influence of the axial position (height) on bubble diameter is assumed to be negligible. Also, it is assumed that the bubble size distribution obtained from the projected surface is the same as that in the column. The mean bubble diameter is calculated by using the profile projector. From the photographs, the bubbles are counted approximately according to their sizes and the mean bubble diameter is calculated from the following Equation 1. The bubble size and bubble size distributions are



presented in Table 3 for uG = 5.31 cm/s, uL = 0.28 cm/s, Sp = 4.5 cm, np = 10, dp = 0.8 cm respectively. Table: 2 Bubble Size Distribution (Projected), uG = 5.31cm/s, uL = 0.28cm/s, Sp = 4.5 cm, np = 10, dp = 0.8 cm

Af cm

Bubble size mm

Number of bubbles In the photograph

0.37 2 50 3 46 4 25

0.73 2 60 3 25 4 20 5 17

1.2 2 30 3 40 4 25 5 25

1.48 2 60 3 35 4 10 5 18

1.66 2 15 3 40 4 54 5 40

1.92 2 30 3 46 4 24 5 27

2.12 2 50 3 46 4 25

2.5 2 45 3 63 4 24

2.75 2 50 3 60 4 25

Figure: 4 Bubble Diameter Photograph. Af = 2.75 cm/s, uG = 5.31cm/s, uL = 0.28cm/s, Sp = 2.25 cm, np = 10, dp = 0.8 cm




n3

i ii=1

B n2

i ii=1

n dd = ---- (1)

n d

Table No: 3 Photographic Determination of Bubble Diameter, uG = 5.31cm/s, uL = 0.28cm/s, Sp = 4.5 cm, np = 10, dp = 0.8 cm

Af cm/s

Bubble size mm

Number of

bubbles

nidi2 nidi3 Bubble diameter mm

3i i

B 2i i

Σn dd =

Σn d

0.37 2 50 200 400 3 46 414 1242 4 25 480 1420 nidi2= 1094 nidi3= 3562 3.2 0.73 2 60 240 480 3 25 225 675 4 20 320 1280 5 17 425 2125 nidi2= 1210 nidi3= 4560 3.7 1.2 2 30 120 240 3 40 360 1080 4 25 400 1600 5 25 625 3125 nidi2= 1505 nidi3= 6045 4.0 1.48 2 60 540 1620 3 35 560 2240 4 10 250 1250 5 18 648 3388 nidi2= 1998 nidi3= 8998 4.5 1.66 2 15 60 120 3 40 360 1080 4 54 864 3456 5 40 1000 5000 nidi2= 2284 nidi3= 9656 4.2 1.98 2 30 120 240 3 46 414 1242 4 24 384 1536 5 27 675 3375 nidi2= 1593 nidi3= 6393 4.0 2.12 2 50 200 400 3 46 414 1242 4 25 480 1920 nidi2= 1094 nidi3= 3562 3.3 2.5 2 45 180 360 3 63 567 1701 4 24 384 1536 nidi2= 1881 nidi3= 5847 3.1 2.75 2 50 200 400 3 60 540 1620 4 25 400 1600 nidi2= 1140 nidi3= 3620 3.1



3.2 INTERFACIAL AREA The interfacial area (per unit volume of the column) is calculated based on the experimental results of the gas holdup and bubble diameter by using Equations 2 and 3.

B

6εa = ---- (2)

d

Where, the gas holdup oH - Hε = ----(3)

H

4 RESULTS AND DISCUSSION 4.1 BUBBLE DIAMETER In the absence of agitation due to hybrid action, the bubble size distribution is non-uniform. The size ranges from 6 to 15 mm approximately. The bubbles are irregular in shape and non-spherical. Even though small sized bubbles are formed, they are disturbed due to coalescence. While increasing the agitation level, the bubble size starts to decrease due to the continuous breakup. At high agitation level, the bubble sizes are smaller and bubble size distribution is also uniform. The agitation level plays a predominant role in the bubble size distribution. The effect of superficial gas and liquid velocities on the bubble size distribution is significantly less. The same is reported by Rama Rao et al. (1983). Yang et al. (1986a) report that the effect of gas flow rate is to cause a decrease in bubble size with increasing the agitation level. The data reported by the authors, indicate that the mean bubble diameter remains constant upto an agitation level of 10 cm/s. At higher levels of agitation, the bubble diameter is constant. Veljkovic and Skala (1986) report that the dispersion of bubbles is enhanced by increasing the agitation level. Rama Rao and Baird (1988) report that in the absence of agitation, the distribution is very broad. It should be noted that these authors have carried out experiments in reciprocating plate column. The hybrid action reduces the dependence of bubble diameter on the phase velocities still. Table 4 represents the experimental




prediction of interfacial area for the condition of uL =0.14cm/s, dp = 0.8cm, Sp = 4.5cm, np = 10 respectively. Table No: 4 Experimental determination of interfacial area, uL =0.14cm/s, dp = 0.8cm, Sp = 4.5cm, np = 10

uG cm/s

Af cm/s

dB cm

a cm-1

2.12 0.37 0.1878 0.3486 3.23264 2.12 0.73 0.1837 0.3191 3.45411 2.12 1.20 0.1753 0.2991 3.51618 2.12 1.40 0.1667 0.2932 3.41136 2.12 1.60 0.1833 0.2868 3.83506 2.12 1.92 0.1919 0.2814 4.09166 2.12 2.18 0.2157 0.2768 4.67568 2.12 2.50 0.2310 0.2719 5.09729 2.12 2.75 0.2529 0.2686 5.65012 2.12 3.08 0.2727 0.2646 6.18290 3.18 0.37 0.2195 0.3674 3.58430 3.18 0.73 0.2118 0.3364 3.77800 3.18. 1.20 0.2078 0.3153 3.95405 3.18 1.40 0.1960 0.3091 3.80501 3.18 1.60 0.2079 0.3023 4.12641 3.18 1.92 0.2308 0.2966 4.66840 3.18 2.18 0.2453 0.2918 5.04429 3.18. 2.50 0.2661 0.2866 5.57033 3.18 2.75 0.2920 0.2831 6.18870 3.18 3.08 0.3103 0.2790 6.67416 4.24 0.37 0.2523 0.3814 3.96867 4.24 0.73 0.2453 0.3492 4.21494 4.24 1.20 0.2381 0.3273 4.36430 4.24 1.40 0.2308 0.3208 4.31612 4.24 1.60 0.2453 0.3138 4.69000 4.24 1.92 0.2661 0.3079 5.18484 4.24 2.18 0.2793 0.3029 5.53263 4.24 2.50 0.2982 0.2975 6.01314 4.24 2.75 0.3220 0.2939 6.57401 4.24 3.08 0.3388 0.2896 7.01967 5.31 0.37 0.2793 0.3928 4.26672 5.31 0.73 0.2661 0.3596 4.44052 5.31 1.20 0.2593 0.3371 4.61586 5.31 1.40 0.2381 0.3304 4.32427 5.31 1.60 0.2661 0.3231 4.94101 5.31 1.92 0.2523 0.3171 4.77423 5.31 2.18 0.2982 0.3119 5.73672 5.31 2.50 0.3103 0.3064 6.07674 5.31 2.75 0.3277 0.3026 6.49750 5.31 3.08 0.3596 0.2982 7.23582

4.2 INTERFACIAL AREA Since gas holdup is an essential design and operating parameter frequently used to describe the interfacial area, details of effects of gas holdup on this hybrid column are very important.



4.2.1 GAS HOLDUP For a given plate spacing and superficial velocities of gas and liquid phase at low agitation level, the gas holdup decreases with an increasing agitation level (mixer-settler region) and reaches a minimum (transition region); thereafter the gas holdup increases significantly with increasing agitation level (Emulsion region). Dhanasekaran and Karunanithi (2012) describe the ranges of mixer-settler region, transition region and emulsion region for this hybrid column.

In reciprocating plate columns, the same is reported by Rama Rao et al. (1983), Veljkovic and Skala (1986), Rama Rao and Baird (1988) and Sundaresan and Varma (1990a) for air-water system under countercurrent condition. Yang et al. (1986a) report that gas holdup shows no immediate increase until the agitation level reaches a critical value for air-water system under cocurrent operating condition. For air-water system under semi-batch condition, Lounes and Thibault (1993) report that the gas holdup decreases with frequency at low agitation levels until a minimum gas holdup is reached but Aleksic et al. (2002) report contradictorily, i.e., gas holdup increases with increasing the vibrating speed in reciprocating plate column with raschig rings placed in interplate space, under semi-batch operating condition using air-water system. In this investigation, when the agitation is zero, visual observation shows that the gas phase is dispersed more and more as large and uneven sized gas bubbles under the space below the plates. As agitation starts from the zero level (mild agitation level), the clustered gas bubbles below the plates move along with the plate during the upward stroke. During the successive downward stroke, the gas bubbles pass through the perforation and as usual cluster at the space below the plate of one which is above. At this moment, it is possible to see the formation of separate discrete layers of gas and liquid phases in the interplate space.

At low level of agitation, the perforated plates act as a piston to lift the bubbles continuously in the upward direction. Due to this piston action of the plates at low agitation level, the thickness of the clustered gas bubbles below the plate space is reduced, i.e., the stagnation gas bubbles are reduced in the column. Visual observation ensures that at low agitation levels, passing of gas bubbles through the perforation in the upward direction is predominant than the passage of liquid through the perforation in




the downward direction. This is the exact reason why the gas holdup goes on decreasing with an increasing agitation at low levels. As agitation increases slowly, at a particular level of agitation, the number of gas bubbles in the column is less. This is identified as the minimum gas holdup region or transition region. Beyond this level, while increasing the agitation, the gas holdup starts to increase. At this stage, the gas phase is dispersed into small bubbles of approximately uniform size and the formed bubbles thus have a tendency to coalesce into large ones. During the upward stroke, the down coming liquid phase through the perforation is predominant than the movement of gas bubbles in the upward direction unlike at low agitation level. The down coming liquid acts as a barrier and prevents the frequent movement of gas bubbles in the upward direction. Due to this, the stagnation of bubbles starts to increase inside the column. Consequently, the gas holdup goes on increasing and uniform dispersion of gas bubbles is resulted in the liquid phase. In this investigation, the gas holdup is found to be the minimum in the range of agitation levels of 1.3 to 1.5 cm/s. Table 5 shows the range of agitation level at minimum gas holdup region obtained from the earlier investigations done in reciprocating plate columns. It also shows the comparison of range of variables investigated in the reciprocating plate columns.

In reciprocating plate bubble column, under countercurrent condition, Rama Rao et al. (1983) report that at low vibrating speeds the dispersed phase holdup decreases with an increase in the vibrating speed in the mixer-settler (up to 6 cm/s) and reaches a minimum in the transition region (about 6 cm/s); thereafter holdup increases with an increase in the vibrating speed (above 6 cm/s) in emulsion region. The same is reported by Veljkovic and Skala (1986) but the minimum gas holdup falls at agitation levels in the of range of 2.4 cm /s to 3.18 cm /s and Rama Rao and Baird (1988) report that gas holdup increases slowly with agitation till a level of about 6 cm/s is reached; beyond this level, holdup increases significantly with agitation. Sundaresan and Varma (1990a) report that the minimum gas holdup falls at an agitation level of 5 cm/s. Yang et al. (1986a) report that in reciprocating plate column for air water system under cocurrent condition, the critical agitation value falls in between the range of 20 to 40cm/s. In reciprocating plate column for air water system under semi-batch condition, Lounes and Thibault (1993) report that the gas



holdup decreases up to the agitation level of approximately 5 to 7 cm/s in the reciprocating plate gas liquid column. The hybrid arrangement ensures that the attainment of minimum agitation level falls early when compared to the values obtained by other researchers. The lower level of agitation obtained in this study is mainly due to the introduction of rotational effect along with the reciprocation in this column. This rotational effect hinders the axial movement of the bubbles. The bubbles move in a rotational manner along the axis and this result in increased residence time. This increased residence time results in more stagnation of gas in the column. Visual observation shows that at low agitation levels, the gas bubbles move faster from plate to plate through the perforation in the upward direction. While increasing the agitation level, bubbles are retained in the upper space between the plates. During the upward movement of the plate stack, the down coming liquid flow hinders the upward movement of the clustered bubbles around the plates. It increases the stagnation of gas within the column and result in an increased gas holdup. During the successive downward stroke, the bubbles clustered at the bottom plates get uniformly dispersed into the liquid. This is clear evidence for the complete intimatacy and contact between gas and liquid. It indicates that introduction of rotational effect increases the dispersion of gas into the liquid phase at the top of the column, where the real counter current operation exists. This enhances the rate of mass transfer of gas into the liquid. For all the superficial gas velocities considered in the present study, the critical agitation level at minimum gas holdup remains almost constant in the range of 1.3 to 1.5 cm/s. When the liquid phase velocity is increased, the gas holdup in mixer-settler region is flattened. 4.2.2 INTERFACIAL AREA The influence of the operating conditions on the interfacial area is shown in Figures 5 and 6. The idea of minimum agitation level as in the case of gas hold up can be utilized again. With an increase of agitation level, the interfacial area increases slowly up to the transition region and then it increases significantly with increasing agitation level. When the flow regime is changed from the mixer-settler to an emulsion regime, the interfacial area raises strongly. The effect of superficial gas velocity is also found to be significant on interfacial area.




Table: 5 Comparison of Critical Agitation Level at Minimum Gas Holdup with Earlier Investigations Done in Reciprocating Plate Column

Authors Column diameter

DC cm

Column height

HC cm

Range of agitation

level Af

cm/s

Range of superficial gas velocity

uG cm/s

Range of superficial

liquid velocity uL

cm/s

Number of plates

np

Plate spacing

Sp cm

Perforation diameter

dP cm

Range of agitation level

at minimum gas holdup cm/s

RamaRao et al (1983) countercurrent Air-water system Reciprocating Plate Column

9.3 100

Amplitude: 1.4 – 6.35 Frequency: 0.75 – 4.0

Af=1.05-25.4

1.24– 3.72 0.04– 5.10 10 5.6 0.3,0.5, 0.65, 0.8

6

Veljkovic and Skala (1986) countercurrent Air-water system Reciprocating Plate Column

2.54 200

Amplitude: 0.1 – 1.0

Frequency: 1 – 10

Af= 0.1-10

Upto 13.5 0.5 – 2.0 33, 65 2.5, 5.0 0.7, 0.5 2.4 – 3.18

RamaRao and Baird (1988) countercurrent Air-water system Reciprocating Plate Column

5.08 245

Amplitude: 4.5

Frequency: 0 – 5

Af=0-22.5

0 – 0.99 0 – 3.95 54 2.7, 5.2, 7.75

0.14 6

Sundaresan and Varma (1990a) countercurrent Air-water and CO2 - water system Reciprocating Plate Column

9.3 100

Amplitude: 1.4 – 5.0

Frequency: 0.75 – 3.0

Af=1.05-15.0

0.6 – 4.6 0.4 – 3.7 9 5.6 0.3, 0.5, 0.65 5

Yang et al. (1986a) cocurrent Air-water system Reciprocating Plate Column

5.08 396

Amplitude: 0 – 1.88

Frequency: 0 – 5.0

Af= 0-9.4

0 – 14.3 0 - 6 84 2.54 Fractional open area :

0.53 20 - 40

Mustapha and Thibault (1993) semi-batch Air-water system Reciprocating Plate Column

10.16 126 Af= 1.25 –

5.0

0 - 4 - 18 5.0 0.635 5 – 7

Present investigation Hybrid column Air-water system

10 120 Af=0 - 3.1 2 – 5.4 0.14 – 0.3 10. 20 4.5, 2.25 0.8, 0.6, 0.4. 1.3 – 1.5


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It increases with increasing superficial gas velocity. Yang et al. (1986a) report that the interfacial area is a function of superficial gas and liquid velocities and agitation level. Veljkovic and Skala (1988) report that the interfacial area dependent on both agitation and aeration intensities and independent of the liquid flow rate. Sundaresan and Varma (1990b) report that the interfacial area increases with an increase in the flow rates of either of the phases. An increase in the liquid rate increases the interfacial area and it is influenced by plate geometry. An increase in the plate free area adversely affects the interfacial area by increasing the bubble diameter and reducing the gas holdup.

Figure: 5 Effect of Agitation level on Interfacial Area uL = 0.14cm/s, dp= 0.8cm, Sp= 4.5cm, np= 10. 5 CORRELATION In order to effectively use the experimental data obtained, it is necessary that correlations should be developed for the parameters investigated. However, it should be emphasized that these correlations can be applied only in the range of process conditions under which the experiments are carried out. In this present investigation, the performance of the novel hybrid rotating and reciprocating perforated plate bubble column has been investigated. The hydrodynamic parameters are influenced by the following variables: (1) Flow conditions - superficial gas and liquid




velocities, (2) Fluid properties - densities of gas and liquid, liquid phase viscosity and surface tension of the liquid and (3) Geometrical properties - plate perforation diameter, plate spacing and agitation level.

Figure: 6 Effect of Superficial Gas Velocity on Interfacial Area uL=0.14cm/s, dp= 0.8cm, Sp= 4.5cm, np= 10. 5.1 CORRELATION FOR BUBBLE DIAMETER The following equation expresses the functional relationship between the bubble diameter and other variables such as perforation diameter, superficial gas velocity, difference in densities, spacing between the plates and agitation level.

B 1 p G pd = f ( d , u , Δρ, S , Af) ---- (4) By using the dimensionless analysis, the following simple

correlation 5 is arrived which theoretically relates the variables such as agitation level, superficial gas velocity, plate spacing and perforation diameter with the mean bubble diameter.

1.03 0.13p G

B pp

S ud = 0.059 d ---- (5)

d Af

Regression coefficient = 0.90 The experimental reading indicates that the mean bubble is

independent of perforation diameter in the range of 0.4 to 0.8cm. Also, it is found that the mean bubble diameter is independent of the plate spacing in the range of 2.25 to 4.54 cm. As can be seen from the



earlier investigation in reciprocating plate column, almost all the works are carried out in this range only. Hence the above correlation 5 is modified and can be used with 95% accuracy.

0.13

GB

ud = 2.778 ---- (6)

Af

The correlation 6 infers that an increase in Af decreases the dB, and an increase in UG increases the dB. The correlation 6 for the prediction of mean bubble diameter for the hybrid column is compared with the experimental values. Table 6 represents the same. Figure 7 shows the comparison of mean bubble diameter predicated experimentally with correlation values. Most of the correlation values fit with the experimental values within the reasonable accuracy. Table: 6 Comparison of Bubble Diameter Predicted Experimentally with the Correlation Developed for Hybrid Column

uL cm/s

Af cm/s

uG cm/s

Sp cm/s

dP cm

dB cm Exp

dB cm

Correlation 13.003.1

.059.0

Af

u

d

Sdd G

p

ppB

dB cm

Correlation 13.0

778.2

Af

ud GB

%error

0.28 0.37 5.31 4.5 0.8 0.32 0.3953 0.39276 -22.7 0.28 0.73 5.31 4.5 0.8 0.37 0.3619 0.35955 2.82 0.28 1.2 5.31 4.5 0.8 0.4 0.3393 0.33706 15.74 0.28 1.48 5.31 4.5 0.8 0.45 0.3301 0.32799 27.11 0.28 1.66 5.31 4.5 0.8 0.42 0.3252 0.32313 23.06 0.28 1.98 5.31 4.5 0.8 0.4 0.3179 0.31581 21.05 0.28 2.12 5.31 4.5 0.8 0.33 0.3151 0.31302 5.15 0.28 2.5 5.31 4.5 0.8 0.31 0.3084 0.30638 1.17 0.28 2.75 5.31 4.5 0.8 0.31 0.3046 0.30261 2.38 0.28 2.12 5.31 4.5 0.6 0.293 0.3178 0.31302 -6.83 0.28 2.5 5.31 4.5 0.6 0.328 0.3111 0.30638 6.59 0.28 2.75 5.31 4.5 0.6 0.278 0.3072 0.30261 -8.85 0.28 2.12 5.31 4.5 0.4 0.311 0.3217 0.31302 -0.65 0.28 2.5 5.31 4.5 0.4 0.279 0.3149 0.30638 -9.81 0.28 2.75 5.31 4.5 0.4 0.297 0.3110 0.30261 -1.89 0.28 2.12 4.24 4.5 0.8 0.283 0.3060 0.30399 23.09 0.28 2.5 4.24 4.5 0.8 0.306 0.2995 0.29755 7.16 0.28 2.75 4.24 4.5 0.8 0.274 0.2958 0.29388 -5.44 0.28 2.12 3.18 4.5 0.8 0.32 0.2948 0.29284 -7.42 0.28 2.5 3.18 4.5 0.8 0.3 0.2885 0.28663 2.76 0.28 2.75 3.18 4.5 0.8 0.328 0.2850 0.28310 -7.26 0.21 2.12 5.31 4.5 0.8 0.426 0.3151 0.31302 8.49 0.21 2.5 5.31 4.5 0.8 3.27 0.3084 0.30638 4.46 0.21 2.75 5.31 4.5 0.8 3.22 0.3046 0.30261 13.69 0.14 2.12 5.31 4.5 0.8 4.55 0.3151 0.31302 26.52 0.14 2.5 5.31 4.5 0.8 3.0 0.3084 0.30638 6.31 0.14 2.75 5.31 4.5 0.8 3.21 0.3046 0.30261 6.02 0.28 2.12 5.31 2.25 0.8 0.407 0.1543 0.31302 31.20 0.28 2.5 5.31 2.25 0.8 0.33 0.1510 0.30638 -2.13 0.28 2.75 5.31 2.25 0.8 0.287 0.1492 0.30261 5.73

Average error 5.58




Few values show higher deviation, which might be the reason of experimental error; also, the photograph method is adopted to determine the bubble diameters in this investigation. Though this method can only give the bubble size to the nearby the column wall, this experimental method is very simple and effective. As the column is cylindrical, elimination of the effect of refraction on bubble sizes is important. And a quadrate tank could be installed outside the cylindrical column for clear optical observation. These are the reasons for the deviation of a few readings between experimental and correlation. An error analysis is made and the average error is found to be 5.6%. The correlation 6 for the determination of mean bubble diameter for this hybrid column can be used with 94 % accuracy within the range of variables investigated in this present study. Table 1 shows the range of variables investigated for mean bubble diameter. In Table 7, mean bubble diameter predicted by experimental and the correlation 6 developed for hybrid column is compared with the bubble diameter predicted by the correlations of Rama Rao et al. (1983) & Sundaresan and Varma (1990a) for reciprocating plate column. There is clear evidence that the bubbles distribution is narrow for hybrid column leading nearly uniform size of bubbles, whereas in reciprocating plate column the bubble sizes are greatly influenced by the gas flow rate and agitation level leading to wide distributions in size. Also, bubble diameter for hybrid column is smaller when compared to that of reciprocating plate column under identical conditions. The mean bubble diameter values obtained by the correlation developed by Rama Rao et al. (1983) for reciprocating plate column depends only on the agitation level. From the mean bubble diameter values by the correlation developed by Sundaresan and Varma (1990a) for reciprocating plate column, it is found that the mean bubble diameter values for hybrid column are 1.8 to 2.5 times smaller which is quite small. 5.2 CORRELATION FOR INTERFACIAL AREA The following equation expresses the functional relationship between the interfacial area and other variables

2 L L L G L p Pa = f ( Af, ρ , σ , μ , u , u , S , d ) ---- (7)



By using the dimensionless analysis, the following correlation is obtained.

Figure: 7 Comparison of Mean Bubble Diameter Predicted Experimentally with the Correlation Developed for Hybrid Column.

b1 b2 b3 b4 b52G P P L L P L L

p 3L L P L L

u S d . u . ρ d . u . ρAfa. d = f ---- (8)

u u d μ σ

Regression analysis has been used to evaluate the constant and coefficients of the above correlation 8. For mixer-settler region

0.29 0.05 -0.98 3.86 1.002G P P L L P L L

P L L P L L

u S d . u . ρ d . u . ρ2.74 Afa = ---- (9)

d u u d μ σ

i.e., 0.29 0.05 -0.98

3.86 1.00G PRe We

P L L P

u S2.74 Afa = N N

d u u d

Regression coefficient = 0.95. For emulsion region

0.19 0.69 2.50 0.003 0.742G P P L L P L L

P L L P L L

u S d . u . ρ d . u . ρ2.27 Afa = ---- (10)

d u u d μ σ




i.e., 0.19 0.69 2.50

0.003 0.74G PRe We

P L L P

u S2.27 Afa = N N

d u u d

Regression coefficient = 0.96. In mixer-settler region, the Liquid Reynolds number (NRe) is more predominant in enhancing the interfacial area based on the exponent value of 3.86 in Equation 9. It is inferred that the UL is the only parameter to enhance the interfacial area to a great extent because the other parameters, namely, dP. ρL and μL are constant, where as a conflict of interest is shown in the emulsion region that is the NRe is near negligible. In emulsion region, the ratio of Af/UL and SP/dP shows a significant interest in increasing the interfacial area based on the exponent value of 0.69 and 2.5 respectively in Equation 10 than the mixer-settler region. The significance of the ratio of UG/UL and the Weber number (NWe) in both regions shows equal significance. Figure 8 shows the comparison of interfacial area predicated experimentally with the values by correlations 9 and 10 developed for the hybrid column. The correlation values fit with the experimental values with reasonable accuracy. The comparison is presented in Table 8. Based on the error analysis, the average error between the experimental and correlation values is found to be 4.1%. The correlations 9 and 10 developed for the prediction of interfacial area can be used with 95% accuracy for this hybrid column for the range of variables investigated in this study. The range of variables for interfacial area is given in Table 1 for this study. The experimental and correlation values of interfacial area for this hybrid column is compared with the investigations done in the reciprocating plate column by Yang et al. (1986a) and Sundaresan and Varma (1990b). The comparison is presented in Table 9. This comparison gives the evidence that the interfacial area is 3 to 12 times higher than the values by Yang et al. (1986a) and 3 to 6 times higher than the values by Sundaresan and Varma (1990b).



Table 7: Comparison of Experimental and Correlation Values of Mean Bubble Diameter for Hybrid Column with Earlier Investigations in Reciprocating Plate Column, Rama Rao et al. (1983) & Sundaresan and Varma (1990a).

uL cm/s

ud cm/s

Af cm/s

dp cm

Sp cm

s

dB (cm) Exp

dB (cm) Corr

0.13

GB

ud =2.778

Af

dB (cm) RPC,

RamaRao et al., (1983)

-1.2

Bd =0.45 Af

dB (cm) RPC

Sundaresan and Varma (1990a)

-0.33 0.06 -0.206 -0.1 0.21B G Ld =0.65 Af u u d s

0.28 5.31 0.37 0.8 4.5 0.2 0.32 0.39 1.48 0.94 0.28 5.31 0.73 0.8 4.5 0.2 0.37 0.36 0.65 0.75 0.28 5.31 1.2 0.8 4.5 0.2 0.4 0.34 0.37 0.64 0.28 5.31 1.48 0.8 4.5 0.2 0.45 0.33 0.28 0.59 0.28 5.31 1.66 0.8 4.5 0.2 0.42 0.32 0.24 0.57 0.28 5.31 1.98 0.8 4.5 0.2 0.4 0.32 0.20 0.54 0.28 5.31 2.12 0.8 4.5 0.2 0.33 0.31 0.18 0.53 0.28 5.31 2.5 0.8 4.5 0.2 0.31 0.31 0.15 0.50 0.28 5.31 2.75 0.8 4.5 0.2 0.31 0.30 0.13 0.48 0.28 5.31 2.12 0.6 4.5 0.24 0.293 0.31 0.18 0.56 0.28 5.31 2.5 0.6 4.5 0.24 0.328 0.31 0.14 0.53 0.28 5.31 2.75 0.6 4.5 0.24 0.278 0.30 0.13 0.52 0.28 5.31 2.12 0.4 4.5 0.11 0.311 0.31 0.18 0.50 0.28 5.31 2.5 0.4 4.5 0.11 0.279 0.31 0.14 0.47 0.28 5.31 2.75 0.4 4.5 0.11 0.297 0.30 0.13 0.46 0.28 4.24 2.12 0.8 4.5 0.2 0.283 0.30 0.18 0.52 0.28 4.24 2.5 0.8 4.5 0.2 0.306 0.30 0.14 0.49 0.28 4.24 2.75 0.8 4.5 0.2 0.274 0.29 0.13 0.48 0.28 3.18 2.12 0.8 4.5 0.2 0.32 0.29 0.18 0.51 0.28 3.18 2.5 0.8 4.5 0.2 0.3 0.29 0.14 0.48 0.28 3.18 2.75 0.8 4.5 0.2 0.328 0.28 0.13 0.47 0.21 5.31 2.12 0.8 4.5 0.2 0.426 0.31 0.18 0.56 0.21 5.31 2.5 0.8 4.5 0.2 0.327 0.31 0.14 0.53 0.21 5.31 2.75 0.8 4.5 0.2 0.322 0.30 0.13 0.51 0.14 5.31 2.12 0.8 4.5 0.2 0.455 0.31 0.18 0.61 0.14 5.31 2.5 0.8 4.5 0.2 0.3 0.31 0.14 0.58 0.14 5.31 2.75 0.8 4.5 0.2 0.321 0.30 0.13 0.56 0.28 5.31 2.12 0.8 2.25 0.2 0.407 0.31 0.18 0.53 0.28 5.31 2.5 0.8 2.25 0.2 0.33 0.31 0.14 0.50 0.28 5.31 2.75 0.8 2.25 0.2 0.287 0.30 0.13 0.48


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Figure: 8 Comparison of Interfacial Area Predicted Experimentally with the Developed Correlation. uL= 0.14 cm/s, dp = 0.8cm, Sp = 4.5cm, np = 10 6 CONCLUSION In the absence of agitation due to hybrid action, the bubble size distribution is large. While increasing the agitation level, the bubble size starts to decrease due to the continuous breakup and a narrow distribution of size is attained. When hybrid columns are used as reactors, bubbles of uniform size ensure higher average conversion. The effect of superficial gas and liquid velocities on the bubble size distribution is significantly less. The experimental readings indicate that the mean bubble is independent of perforation diameter and plate spacing. Correlation is developed for the determination of mean bubble diameter. This correlation can be used for the determination of mean bubble diameter for this hybrid column with 94 % accuracy within the range of variables investigated in this present study.

0.13

GB

ud = 2.778

Af

It is found that the mean bubble diameter values for hybrid column are 1.8 to 2.5 times smaller when compared with



reciprocating plate column. Smaller size leads to larger interfacial area of contact resulting in higher mass transfer.

Table No: 8 Comparison of Interfacial Area Predicted Experimentally with the Correlation. uL = 0.14 cm/s, dp = 0.8 cm, Sp = 4.5 cm, np = 10.

uG cm/s

Af cm/s

dB cm

Bda

6

(cm2/cm3)

a correlation

(cm2/cm3)

%error

2.12 0.37 0.1878 0.3486 3.2326 3.2735 -1.2671 2.12 0.73 0.1837 0.3191 3.4541 3.3867 1.9506 2.12 1.20 0.1753 0.2991 3.5161 3.4719 1.2576 2.12 1.40 0.1667 0.2932 3.4113 3.4988 -2.5638 2.12 1.60 0.1833 0.2868 3.8350 3.5418 7.6462 2.12 1.92 0.1919 0.2814 4.0916 4.0166 1.8337 2.12 2.18 0.2157 0.2768 4.6756 4.3844 6.2278 2.12 2.50 0.2310 0.2719 5.0972 4.8190 5.4584 2.12 2.75 0.2529 0.2686 5.6501 5.1466 8.9109 2.12 3.08 0.2727 0.2646 6.1829 5.5652 9.9897 3.18 0.37 0.2195 0.3674 3.5843 3.6820 -2.7277 3.18 0.73 0.2118 0.3364 3.7780 3.8093 -0.8292 3.18. 1.20 0.2078 0.3153 3.9540 3.9051 1.2361 3.18 1.40 0.1960 0.3091 3.8050 3.9353 -3.4265 3.18 1.60 0.2079 0.3023 4.1264 3.8254 7.2931 3.18 1.92 0.2308 0.2966 4.6684 4.3382 7.0711 3.18 2.18 0.2453 0.2918 5.0442 4.7356 6.1194 3.18. 2.50 0.2661 0.2866 5.5703 5.2049 6.5586 3.18 2.75 0.2920 0.2831 6.1887 5.5587 10.1783 3.18 3.08 0.3103 0.2790 6.6741 6.0109 9.9374 4.24 0.37 0.2523 0.3814 3.9686 4.0024 -0.8507 4.24 0.73 0.2453 0.3492 4.2149 4.1407 1.7598 4.24 1.20 0.2381 0.3273 4.3643 4.2449 2.7346 4.24 1.40 0.2308 0.3208 4.3161 4.2777 0.8880 4.24 1.60 0.2453 0.3138 4.6900 4.0403 13.8511 4.24 1.92 0.2661 0.3079 5.1848 4.5820 11.6265 4.24 2.18 0.2793 0.3029 5.5326 5.0016 9.5970 4.24 2.50 0.2982 0.2975 6.0131 5.4974 8.5767 4.24 2.75 0.3220 0.2939 6.5740 5.8710 10.6923 4.24 3.08 0.3388 0.2896 7.0196 6.3486 9.5594 5.31 0.37 0.2793 0.3928 4.2667 4.2723 -0.1317 5.31 0.73 0.2661 0.3596 4.4405 4.4199 0.4622 5.31 1.20 0.2593 0.3371 4.6158 4.5312 1.8339 5.31 1.40 0.2381 0.3304 4.3242 4.5662 -5.5963 5.31 1.60 0.2661 0.3231 4.9410 4.2168 14.6555 5.31 1.92 0.2523 0.3171 4.7742 4.7821 -0.1666 5.31 2.18 0.2982 0.3119 5.7367 5.2201 9.0046 5.31 2.50 0.3103 0.3064 6.0767 5.7375 5.5817 5.31 2.75 0.3277 0.3026 6.4975 6.1275 5.6936 5.31 3.08 0.3596 0.2982 7.2358 6.6259 8.4285

Average Error 4.7264




Table: 9 Comparison of experimental and correlation values of interfacial area for hybrid column with earlier investigations in Reciprocating plate columns, Yang et al., (1986a) and Sundaresan and Varma (1990b)

uG cm/s

Af cm/s

Interfacial area

Present investigation Experimental

cm-1

Interfacial area Present investigation

Correlation cm-1

For mixer-settler region 00.1286.398.005.029.0

....74.2

L

LLP

L

LLP

P

P

LL

d

P

udud

d

S

u

Af

u

u

da

For emulsion region

74.02003.050.269.019.0....27.2

L

LLP

L

LLP

P

P

LL

G

P

udud

d

S

u

Af

uda

Interfacial area Yang et al., (1986a)

cm-1

59.0133.091.0333.0 Afuua LG

Interfacial area Sundaresan and Varma (1990b)

cm-1

02.03.076.02.063.0 sdUUAfa cd

2.12 0.37 3.2115 3.2735 0.2817 0.6224 2.12 0.73 3.4316 3.3867 0.4214 0.7130 2.12 1.20 3.4932 3.4719 0.5659 0.7876 2.12 1.40 3.3891 3.4988 0.6201 0.8122 2.12 1.60 3.7919 3.5418 0.6712 0.8342 2.12 1.92 4.0650 4.0166 0.7478 0.8652 2.12 2.18 4.6452 4.3844 0.8063 0.8874 2.12 2.50 5.0641 4.8190 0.8746 0.9121 2.12 2.75 5.6133 5.1466 0.9254 0.9296 2.12 3.08 6.1426 5.5652 0.9898 0.9510 3.18 0.37 3.5609 3.6820 0.4074 0.8471 3.18 0.73 3.7534 3.8093 0.6095 0.9704 3.18 1.20 3.9283 3.9051 0.8185 1.0718 3.18 1.40 3.7802 3.9353 0.8968 1.1054 3.18 1.60 4.0799 3.8254 0.9708 1.1353 3.18 1.92 4.6380 4.3382 1.0816 1.1775 3.18 2.18 5.0114 4.7356 1.1662 1.2077 3.18 2.50 5.5340 5.2049 1.2649 1.2413 3.18 2.75 6.1484 5.5587 1.3384 1.2652 3.18 3.08 6.6307 6.0109 1.4315 1.2942 4.24 0.37 3.9428 4.0024 0.5293 1.0541 4.24 0.73 4.1875 4.1407 0.7920 1.2075 4.24 1.20 4.3358 4.2449 1.0634 1.3338 4.24 1.40 4.2880 4.2777 1.1652 1.3755




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Table 9: Continued

uG cm/s

Af cm/s

Interfacial area

Present investigation Experimental

cm-1

Interfacial area Present investigation

Correlation cm-1

For mixer-settler region 00.1286.398.005.029.0

....74.2

L

LLP

L

LLP

P

P

LL

d

P

udud

d

S

u

Af

u

u

da

For emulsion region

74.02003.050.269.019.0....27.2

L

LLP

L

LLP

P

P

LL

G

P

udud

d

S

u

Af

uda

Interfacial area Yang et al., (1986a)

cm-1

59.0133.091.0333.0 Afuua LG

Interfacial area Sundaresan and Varma (1990b)

cm-1

.02.03.076.02.063.0 sdUUAfa cd

4.24 1.60 4.6372 4.0403 1.2613 1.4127 4.24 1.92 5.1510 4.5820 1.4053 1.4652 4.24 2.18 5.4966 5.0016 1.5152 1.5029 4.24 2.50 5.9740 5.4974 1.6434 1.5447 4.24 2.75 6.5312 5.8710 1.7390 1.5744 4.24 3.08 6.9739 6.3486 1.8599 1.6105 5.31 0.37 4.2389 4.2723 0.6496 1.2507 5.31 0.73 4.4116 4.4199 0.9719 1.4328 5.31 1.20 4.5858 4.5312 1.3051 1.5825 5.31 1.40 4.2961 4.5662 1.4301 1.6321 5.31 1.60 4.8854 4.2168 1.5479 1.6763 5.31 1.92 4.7431 4.7821 1.7246 1.7385 5.31 2.18 5.6993 5.2201 1.8596 1.7832 5.31 2.50 6.0371 5.7375 2.0169 1.8328 5.31 2.75 6.4552 6.1275 2.1342 1.8680 5.31 3.08 7.1887 6.6259 2.2825 1.9109


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With an increase of agitation level, the interfacial area increases slowly upto the transition region and then it increases significantly with increasing agitation level. When the flow regime is changed from that of a mixer-settler to an emulsion regime, the interfacial area raises strongly. The effect of superficial gas velocity is also found to be significant on interfacial area. It increases with an increasing superficial gas velocity.

Correlations have been developed for the determination of interfacial area for both mixer-settler and emulsion regions. These correlations for interfacial area can be used with 95% accuracy for this hybrid column within the range of variables investigated. For mixer-settler region:

0.29 0.05 -0.98

3.86 1.00G PRe We

P L L P

u S2.74 Afa = N N

d u u d

For emulsion region:

0.19 0.69 2.50

0.003 0.74G PRe We

P L L P

u S2.27 Afa = N N

d u u d

It should be noted that the interfacial area for the hybrid column is 3 to 6 times higher in both mixer-settler region and emulsion region than that of reciprocating plate column. To summarize,

Bubbles are of nearly uniform size. Bubbles generated in hybrid column are nearly 50% smaller

in size compared to reciprocating plate columns. Interfacial area for hybrid column is 3 – 6 times larger than

reciprocating plate columns leading to higher mass transfer rate.



NOMENCLATURE a = interfacial area, cm2/cm3 A = amplitude of reciprocation (half of the full stroke), cm Af = agitation level, cm/s b1, b2, b3, b4, b5 = constants of the dimensionless equation dB = diameter of the bubble, cm dp = diameter of the perforation in the plate, cm f = frequency, s-1 f1, f2, f3 = coefficients of the dimensionless equation, HC = height of the column, cm H = height of the gassed column, cm Ho = height of the ungassed column, cm HR = height to diameter ratio of the column np = number of plates Qd = gas flow rate, lpm QC = liquid flow rate, lph s = fractional free area of the perforated plate Sp = space between the plates, cm uG = superficial gas velocity, cm/s uL = superficial liquid velocity, cm/s GREEK SYMBOLS L = surface tension of the liquid phase.gm/s2 = gas holdup, fraction L = density of liquid, gm/cm3 L = liquid phase viscosity, gm/cm.s




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