(789d) Secondary Nucleation of Lactose By Fluid Shear Forces

Lactose has many uses in the pharmaceutical and food industries. It is produced by crystallisation from whey or whey permeates produced by the dairy industry. Understanding the mechanisms of lactose crystallization is critical to control crystal size, and thus maximise profitability. To do this the nucleation rate must be controlled both at the start of the crystallisation process and during the growth phase. Nucleation can occur as a primary event or by secondary mechanisms. If an evaporative crystallisation process is being used, then secondary nucleation is the dominant mechanism. If a batch cooling crystalliser is being used without the addition of any seeds, then there will be a primary nucleation event followed by the growth of those crystals. In this case, in order to maximise the size of the crystals produced, secondary nucleation is to be avoided. Secondary nucleation mechanisms involve initial breeding, polycrystalline breeding, macroabrasion, dendritic, fluid shear and contact.

This project focused on the effect of fluid shear forces on secondary nucleation under two different levels of absolute supersaturation (8.4 g and 10.7g lactose/100g water). The experimental methodology used circulated supersaturated solutions through a glass tube with immobilized crystals inside for 15 minutes at constant temperature (30^oC). The pressure drop across the tube was measured by a differential pressure transducer and this was used to estimate the shear rate on the crystals. The sheared solution was incubated for two hours, allowing the nuclei to grow. A Malvern Mastersizer 2000 was used to estimate the number of nuclei.

Blank runs, using an agitator in a beaker when growing the nuclei showed that agitation induced both primary and secondary nucleation. This lead to the modification of using rolling tubes for the growth phase. Repeated blank runs showed that some primary nucleation caused by the action of pumping the supersaturated solution through clean glass tubes at similar fluid shear rates did cause some nucleation but at rates significantly below those found when crystals were present.

It was found that the fluid shear forces did induce secondary nucleation.  The nucleation rate increased proportionally with absolute supersaturation solution level and shear rate from 200 to 4000  with the supersaturated solution of 8.4g lactose/100g water. There was no increase in shearing effect on nucleation rate detected when the shear rate was over 4000 . At the supersaturated solution of 10.7g lactose/100g water, there was no increase in shearing effect at shear rates above 3000 .