2.1. Materials
In the present study, aqueous solutions of cetyltrimethyl ammonium bromide (CTAB) was employed as liquid phase, whereas air were used as gas phase. The concentration of CTAB varied from 50 mg/l to 3000 mg/l. Experiments were also carried out with silica nanoparticles to alter the stability of the microbubbles. The nanoparticles of weight 0.5 gm, 1 gm, 1.5 gm, 2gm, 2.5 gm and 3 gm are mixed in 250 mg/l aqueous concentration of surfactants. The densities of the solution were measured with a specific gravity bottle. The various physicochemical properties of liquid are presented in Table 1.
Table 1
Density and viscosity for different concentration of CTAB
Concentration of CTAB (mg/l)
|
Density (kg/m3)
|
Viscosity (mpas)
|
50
|
999.73
|
1.291
|
100
|
999.78
|
1.293
|
200
|
999.88
|
1.310
|
250
|
999.91
|
1.311
|
500
|
1000.18
|
1.313
|
750
|
1000.43
|
1.315
|
1000
|
1000.68
|
1.318
|
3000
|
1002.68
|
1.338
|
2.2. Methods
2.2.1. Generation of microbubbles
Microbubble can be generation by several methods (Parmar and Majumder, 2013). In the present work microbubble dispersion was generated by the method similar to Seeba (1985). The surfactant is mixed in 250 ml of water. The solution of surfactant and water with and without nano-particles were mixed in high speed stirrer (approx. 18000 rpm; Model- Jaipan JX 4) to generate microbubble. The blade within the stirrer rotate at about 18000 rpm producing a high sheared within the gas-liquid/gas-liquid-solid mixture, for about 60 seconds, which creates microbubble dispersion. The schematic of experimental set up is shown in figure 1. All the experiments were carried out at room temperature.
2.2.2. Estimation of microbubble gas fraction
The efficiency of any gas-liquid reaction is highly governed interfacial area (Kantarci et al., 2005). The interfacial area in turn depends on the gas fraction of the microbubble suspension. Gas fraction is affected by various parameters like phase velocities, bubble diameter, physical and chemical properties of liquid, operating temperature and pressure. Microbubbles dispersion gives a high gas fraction, which is significant for many process industries. An increase in gas holdup will increase gas-liquid interfacial area. After generation of microbubble it is passed to the measuring cylinder. With the passage of time the microbubbles disengage and the level of clear liquid increases in the cylinder. The final gas fraction of the microbubble dispersion (εmd) is calculated as
where H represent total height of microbubble dispersion initially in the cylinder and h denotes final height of clear liquid in the cylinder.
2.2.3. Determination of stability of microbubble dispersion
The half-life of the microbubble dispersion is widely used as an indicator for analysing the stability (Amiri and Sadeghialiabadi, 2014; Parmar and Majumder, 2015; Ruby and Majumder, 2018). The half-life of microbubble dispersion is the time taken to drain half of initial liquid volume of microbubble dispersion. Freshly prepared microbubble dispersion from the mixer are poured into the measuring cylinder and the volume of the drained liquid below the dispersion are recorded and measured as function of time. The variation of liquid drainage volume with time for 500 mg/l of CTAB is shown in Figure 2. From the plot of liquid drainage volume and drainage time the half-life of dispersion can be calculated. Figure 3 shown a typical demonstration for the estimation of half-life from the liquid drainage in 500 mg/l of CTAB without containing nanoparticles.
2.2.4. Estimation of the microbubble rise velocity
The microbubble due to its small size possess low rise velocity. Though the value of rise velocity is very important for determination the bubble residence time and Reynolds number of bubbles. In the present work, the rise velocity is determined by tracking the level of clear liquid interface in the cylinder. It was assumed that microbubble dispersion contains equal sized and spherical bubbles and there is no coalescence and breakup of microbubbles. As microbubble dispersion is poured in measuring cylinder the liquid flows downward and bubbles rises due to buoyancy, leaving the clear liquid interface. This rise in interface with respect to the time is recorded. The rise velocity (Vmd) of microbubble dispersion can be calculated as
Where Hi is interface height at ti and Hj is the interface height corresponding tj.
2.2.5. Determination of the average microbubble size.
The size microbubble dispersion is vital parameter to govern the interfacial area and mass transfer characteristic of bubbles. So, determination of microbubble size become very crucial for its implication in process intensification unit. If it is assumed that the all bubbles moves with same velocity equal to the velocity of clear liquid interface. Since, the value of rise velocity of microbubble dispersion is very less and it can be assumed that the microbubbles rises with a velocity equal to the terminal velocity of bubble. Then the microbubble size (dmb) in the dispersion can be calculated as per the relation:
where μl denotes the viscosity of the liquid or slurry, ρl represents the density of microbubble-liquid mixture, ρg is the density of air and g is the gravitational acceleration.