Number of Equilibrium Stages With and Without Forward Mixing and Backmixing

CHAPTER 4: RESULTS AND DISCUSSION

4.2. Mass Transfer Experimental Results

4.2.8. Number of Equilibrium Stages With and Without Forward Mixing and Backmixing

mixing, the McCabe Thiele method was used for the stepping off of the stages. The equilibrium line was plotted using equilibrium data for the acetone-toluene-water system and linear relation was established with a gradient of 0.832 (Lisa, et al., 2003 and Saien, et al., 2006). The actual operating line was plotted using the acetone concentrations from the gas chromatograph analysis for the extract binary sample (assuming no carrier present), raffinate binary sample (assuming no solvent present) and binary samples along the length of the extraction column. An uncertainty of ± 2 % was established in the compositions using the gas chromatograph detector. The measured number of equilibrium stages with backmixing was then determined by stepping off between the actual operating line and the equilibrium line, while the number of equilibrium stages without backmixing was established by stepping off between the ideal operating line and the equilibrium line and using Equation (2.45). Refer to Appendix C6 for detailed sample calculations regarding the determination of the ideal number of equilibrium stages.

The stepping off of the equilibrium stages using the McCabe Thiele technique is depicted in Figure 4.14 below, which is for a tray spacing of 100 mm at a solvent to feed ratio of 1:2, for the lowest agitation level of 1.25 mm/s investigated.

Figure 4.14: Stepping off stages by the McCabe Thiele method for the acetone-toluene-water system, for h = 100 mm, S/F =1:2 and af = 1.25 mm/s.

0 1 2 3 4 5 6

0 2 4 6 8

Continuous Phase Acetone Concentration (%)

Dispersed Phase Acetone Concentration (%)

Equilibrium Line

Operating Line

Ideal Operating Line

90 Figure 4.14 indicates a fairly linear operating line for the lowest agitation level investigated, thus indicating negligible backmixing occurring in the dispersed phase, as compared to Figure 4.15 which illustrates a nonlinear operating line for the highest agitation level investigated, indicating the occurrence of backmixing as well as forward mixing in the dispersed phase. This result was also deduced by Rathilal (2010) through his investigations on vibrating plate extraction columns.

Backmixing in the dispersed phase is due to a decrease in the accumulation of the dispersed phase under each perforated plate as the agitation level is increased, therefore the dispersed phase droplets re-enters the previous stage it came from. Forward mixing is the case where the dispersed phase droplets move through the downcomers fixed on the perforated plates above, thus allowing the droplets to pass a stage and circulate within the next stage. Rathilal (2010), also explains that forward mixing is also experienced by very fine dispersed phase droplets which bypass stages and thus have no residence time in that stage.

The stepping off of the equilibrium stages using the McCabe Thiele technique is depicted in Figure 4.15 below, which is for a tray spacing of 100 mm at a solvent to feed ratio of 1:2, for the highest agitation level of 7.5 mm/s investigated.

0 1 2 3 4 5 6

0 2 4 6 8

Continuous Phase Acetone Concentration (%)

Dispersed Phase Acetone Concentration (%)

Equilibrium Line Operating Line Ideal Operating Line

Figure 4.15: Stepping off stages by the McCabe Thiele method for the acetone-toluene-water system, for h = 100 mm, S/F = 1:2 and af = 7.5 mm/s.

91 Thus from a comparison between Figure 4.14 and Figure 4.15 it can be gathered that the backmixing in the dispersed phase increases as the agitation level increases, consequently a higher number of equilibrium stages without backmixing is obtained, as shown in Table 4.3 below. Please refer to Appendix D 2 for the stepping of stages using the McCabe Thiele method for the other tray spacings, solvent to feed ratios and agitation levels investigated, where Appendix D 2.1 contains the stepping off plots for h = 100 mm, Appendix D 2.2 for h = 150 mm and Appendix D 2.3 for h = 200 mm.

Table 4.3 below contains the number of equilibrium stages with and without backmixing obtained from stepping off using the McCabe Thiele method, for the different tray spacings, solvent to feed ratios and agitation levels investigated. These results indicate fairly similar real and ideal values for the number of equilibrium stages, but in some cases the number of stages without backmixing is higher than the number of stages with backmixing and this is attributed to backmixing occurring in the dispersed phase.

Table 4.3: Number of equilibrium stages with and without backmixing from stepping off.

af (mm/s)

h = 100 mm h = 150 mm h = 200 mm

S/F = 1:1 S/F = 2:1 S/F = 1:2 S/F = 1:1 S/F = 2:1 S/F = 1:2 S/F = 1:1 S/F = 2:1 S/F = 1:2 Nox

With Backmixing