Microfiltration of neutralization float

Cassano et al. (2000 & 2001) conducted trials that showed the feasibility of using a microfiltration technique such as

ultrafiltration to filter out all the unwanted macro and micro

particles from process floats. In theory, this technique will succeed in the separation of the bulk of un-wanted macro and micro

particles found in the process float from the enzyme process. This will in effect drastically lower the occurrence of ‘dirty pelts’ which usually occurs as a result of solids accumulating in the process float. To achieve that, a suitable membrane with the right cut-off size had to be chosen to perform this separation.

To do this, the molecular weight range of proteins in the float and the enzyme needed to be determined. This was done by Edmonds (2008a) using an SDS-PAGE (gel electrophoresis) technique at various buffer concentrations and conditions. The results of the experimental work are shown in Figure 5.11. For the trial run which resulted in Figure 5.11 seen below, a gradient gel of 4-14% Polyacrylamide (PA) was used (detailed methodology on the way this was prepared may be seen in appendix 8). Recycled float

results. Table 5.1 shows the contents of each lane of the gel of figure 5.11.

Table 5.1: Lane contents of the gel electrophoresis (SDS-PAGE) of figure 5.11

Lane Content of gel of Figure 5.11

1 Recycle float (cycle 1) 1:2 dil.

2 Recycle float (cycle 1) 1:4 dil.

3 Recycle float (cycle 1) 1:8 dil.

4 Recycle float (cycle 1) 1:16 dil.

5 Recycle float (cycle 2) 1:2 dil.

6 Recycle float (cycle 2) 1:4 dil.

7 Recycle float (cycle 2) 1:8 dil.

8 Recycle float (cycle 2) 1:16 dil.

9 Protein Marker

10 Pure enzyme solution

10 9 8 7 1 2 3 4 5 6 Å 200,000 Da Å 116,250 Da Å 66,200 Da Å 96,400 Da Å 21,500 Da Å 31,000 Da Å 45,000 Da A B Å 14,400 Da Å 6,500 Da

Figure 5.11: Shows an SDS-PAGE analysis of process float with lane numbers stated along with the size indicators of the marker proteins.

Looking at Figure 5.11 the largest molecular weight band on lane 10 was 31,000 Daltons (marked B). A faint band was also found in the lanes of float sample of the first cycle (marked A). This

suggests the presence of enzymes which have a molecular weight of approximately 31,000 Daltons as there can only be enzyme particles found in the pure enzyme solution. With that, should a

membrane filtration process be adopted a filter with a slightly larger than 31,000 Daltons cut-off may be trialled.

5.3.2.1 Mass balance analysis of microfiltration of neutralization float

The forward mass balance for the study of microfiltration the neutralization float may be constructed in a similar way as was done with the forward mass balance for the coarse filtration study. The difference between this study and for coarse filtration is the characteristics of the filter used. Given the nature of the use of a microfiltration technique, solids retention of up to 60% of

suspended (insoluble) solids may be feasibly attained. As for dissolved solids, Vinovations (2009) report being able to retain 25% of retentate concentration using reverse osmosis with a

particle size cut-off of approximately 1000 Daltons. To increase the processing speed, a compromise of solids retention of about 40% of the retentate concentration may be sufficient and this was applied to the forward mass balance.

Figure 5.12 below compares the standard configuration of zone 2 with the system which has microfiltration of the neutralization float using the mass balances. A marked improvement can be seen in the reduction of solids in the effluent stream of E4.

0 0.005 0.01 0.015 0.02 0.025 1 2 3 4 5 6 7 8 9 10

Tria l Run Numbe r

C onc e nt ra tion o f TD S in fr e e wa te r of s tr e a m E 4 ( k g TD S /k g fr e e w a te r E4 ) Standard zone 2

Added microfiltration to zone 2

Figure 5.12: Graph showing the comparison between the TDS accumulation in float of the standard zone 2 configurations and the

microfiltration modification to zone 2.

To ensure that the desired concentration is achieved, a cross-flow configuration as seen in Figure 5.13 would have to be employed. To achieve their retentate concentration of 25%, Vinovations (2009) reported the usage of a similar cross-flow system.

Microfiltration Float containing

enzyme

Figure 5.13: Cross-flow filtration system that could be used to achieve optimum microfiltration results.

However, some disadvantages of this modification include the added processing time needed in order to retrieve as much enzyme as possible. To achieve the desired retentate (containing recovered enzyme) concentration, multiple runs will be needed through the cross-flow configuration seen in Figure 5.13 to optimize the

enzyme recovery. This will add to total processing time. In addition because the enzyme will pass through the membrane (its size being smaller than the molecular weight cut off) its concentration

will be constant throughout the float in the cross-flow system and will result in unwanted enzyme wastage. The fact that 40%

retentate remains after filtration (in the compromised scenario) means that up to 40% of enzyme will be discarded. This could compromises the original intention of this modification to save as much enzyme as possible (even in the best case scenario as described in literature, up to a quarter of enzymes will be discarded). To further increase enzyme retention would be to further increase the processing time and cut back on original time and energy savings in this area as detailed in chapter 3.

Based on the results of the forward mass balance study, the option of a microfiltration recovery system of enzyme for zone 2 was not considered any further. The minimal savings of enzyme achieved and the potential of processing time savings lost through microfiltration does not justify this modification to zone 2.