Evaporative crystallization of alpha-lactose monohydrate : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical Engineering at Massey University, Manawatu, New Zealand.
Evaporative crystallization has been used by Fonterra Cooperative Group (New
Zealand) for producing lactose. It represents an important step during lactose
manufacturing where control over crystal size can be obtained, a critical parameter
governing the yield and end use. The art of operating these crystallizers has been
developed by observation and not from scientific principles. This project was
undertaken to understand the mechanisms controlling the crystal size in evaporative
A review of the existing literature showed that secondary nucleation is the major
source of nuclei in industrial crystallizers. Based on the review, attrition, contact and
fluid shear induced nucleation were identified as the probable secondary nucleation
mechanisms in the studied system. Experimental investigation on each of the three
mechanisms was carried out separately on a laboratory scale.
It was found that the crystal size had the most significant effect on attrition, followed
by impeller speed, which together implies that the crystal collision energy intensity is
the dominant factor producing new fragments. Contact nucleation was also found to
be controlled by crystal-impeller collisions. It was found that at the studied
supersaturation there exists a minimum kinetic energy of contact below which
secondary nucleation would not occur. This threshold value was used as the basis to
assess the contribution of various mechanisms at the industrial scale. Shear
nucleation was found to be independent of shear above 5000 s-1.
A mathematical model describing the operation of the industrial crystallizer was
formulated. Sensitivity analysis was conducted by simulating the model for a range
of operational and kinetic parameter values. It was found that the crystal size is
affected most by secondary nucleation. The volume weighted mean size
approximately halved with a 5.5 times increase in the secondary nucleation rate.
The model was refined to accommodate size dependent growth rate and growth rate
dispersion. The kinetic parameters were fitted to match the measured size distribution
from the industrial crystallizer. A range of simulations were conducted for various
theoretical and empirical models and compared to that of plant measurements. Based
on the results it was proposed that the majority of secondary nucleation is expected to
occur in the pump and the boiling zone.