Float separation before neutralization stage

One of the key problems with the standard configuration of zone 2 is the dilution of the enzymes that occurs during the

neutralization process. To remedy this, an investigation was made into a modification that first separates the float of the enzyme treatment stage before filtration and recycling to the enzyme

a) Reduce loss of enzyme activity as the enzyme is now

recycled before dilution and being subjected to a drop of pH through the neutralization stage. With that, less enzyme top-up is needed in following trial runs.

b) Reduced use of acid to deactivate the enzyme still remaining on slats as with the drained skins, the pH buffering

capacity will be lowered.

This modification to zone 2 would be able to achieve its optimal results if the float of the enzyme treatment process is drained fully and the neutralization process conducted in the presence of as little moisture as possible (i.e. concentrated acid is used to neutralize the remaining enzyme found on the slats which only contains entrained water surrounding it). This would mean that an optimal amount of acid may be used and freshwater use reduced. However, this sudden pH change to the pelts may cause undesired damage to the surface of the sheep slats. A comprise would be to still drain all the float used at the enzyme treatment stage but use acid diluted in a stream of freshwater of the same volumetric ratio as other solvents streams. Zone 2 of the LASRA enzyme treatment process with this alternative of modification applied is seen in Figure 5.14 below;

To the pickle NP acid Fresh enzyme ET From Wash 2

Recycled float from previous run

If final run in a block – Waste. Otherwise recycle to next run

Waste S3 S4 P2 P4 P3 E3 E4

Figure 5.14: Schematic diagram showing the draining of process float being done before the neutralization process

A forward mass balance was also used to investigate the benefits of this modification to zone 2. It was constructed in a similar fashion as the ones used in previous two modification alternatives. The only difference with this configuration of zone 2 is that the effluent stream E3 was now present and this was calculated using the equations shown in section 5.2.

0.01 0.015 0.02 0.025 o n o f T D S i n fr e e w a te r 4 (k g T D S /k g fr e e w a te r E4 )

Standard zone 2 configuration

Float separation before neutralization stage

Figure 5.15 shows the results of a forward mass balance study done comparing the standard configuration of zone 2 with the system with one which has the float drained before the

neutralization process (i.e. at the enzyme treatment stage). Figure 5.15 shows a reduced amount of solids at stream P4 and hence a cleaner float for up to 10 recycling trials investigated than the standard configuration. The rate of accumulation of solids was also significantly lowered as the float is recycled from one trial run to another.

One of the possible disadvantages with this modification is that there may be an increase of enzyme being discarded. From initial testing with mass balances, it can be seen that the concentration of enzymes in water associated with the skin going from the enzyme treatment process to the neutralization process is higher with this modification as compared with the standard

configuration. This loss is in proportion to the ratio of water associated with the skins to overall water found in the stage. Percentage enzyme loss may be calculated using equation 5.27 below; % 100 % × + = i i i E in Water P in Water P in Water loss enzyme (Equation 5.27)

*Where ‘i’ denotes stream number. For the standard configuration, ‘i’ is the number ‘4’ while for the modified configuration; ‘i’ is the number ‘3’

Using equation 5.27, it was found that the enzyme losses were 40% and 60.0% for the standard (values used were from run 1) and modified configuration (values were from run 9) of zone 2 respectively for 1 process run.

Although the enzyme loss in the standard configuration is lower than that of the modified configuration, only half of the recycled float of the standard configuration may be used at a given time (i.e. only up to half the concentration of recycled enzyme may be used at a given time). Due to the dilution that occurs by adding the neutralisation liquor, approximately twice as much process float is available to recycle into the next batch. With each successive recycle of float, there will be added losses of enzyme being ‘left behind’ in unused float as well as the burden of having to handle and store process floats. In this regard, the modified configuration is in actual fact an improvement over the standard recycle configuration. In a series of process runs where the

process float is recycled, the modified configuration of zone 2 may save approximately 10% more enzyme over the LASRA process configuration. This is because the losses from the LASRA process zone 2 configuration over a series of recycle runs is 70% (if the dilution losses are considered), while its still 60% for the modified configuration.

With this modification to Zone 2 of the LASRA enzyme treatment process, it may also be possible to recycle the float of the

neutralization process to incur further savings on freshwater. As the concentration of acid used in the neutralization process is quite low (~0.5%), a recycle of neutralization float would require a top-up of acid to the required strength. However, this option was