Modelling industrial lactose crystallization

Abstract

Lactose is currently crystallized industrially from a concentrated permeate. A major improvement to the lactose crystallization process would be the control of final product crystal size and purity. The aim of this thesis is to produce a crystallization model that simulates the product crystal size and concentration-time profiles of an industrial process.A literature review of lactose research resulted in lactose property relationships and revealed major inconsistencies. The main correlations produced by the compilation of literature values were for lactose solubility, alpha lactose solubility depression factor, mutarotation kinetics and alpha lactose equilibrium fraction in water over a range of temperatures. Other correlations generated were lactose solution density, refractive index, viscosity and vapour pressure over a range of temperatures and lactose concentrations in water. Lactose crystal growth rates reported were converted to common units. The values found ranged over one order of magnitude. An additional and important piece of information from the literature review was that lactose crystals show growth rate dispersion (GRD).The difficulty in defining the growth rate of a crystal population with growth rate dispersion led to the development of a crystallization tool named 'Common History' seed. Common History seed comprises of a group of crystals that have all been created by a single nucleation and all crystals have thence been bom at the same time and grown in the same conditions for the same length of time. The size distribution of Common History seed crystals is identical to the distribution of growth rates, using the assumption that the Common History seed crystals have a constant relative growth rate. This property provides a method for the simple determination of the growth rate distribution. About twenty batch crystallizations in water and Simulated Whey Ultra- Filtrate (SWUF) were performed using Common History seeds. The assumption that Common History seed crystals have a constant relative growth rate compared to each other was verified by all experiments with zero nucleation.Crystallization experiments investigated the effect of supersaturation, temperature and salt presence on the growth rate of standard lactose crystals. The standard lactose crystals were taken as a Common History seed (number 3). The median growth rate of these crystals was proportional to the square of the alpha lactose supersaturation. The proportionality constant, the median growth rate constant, increased linearly with temperature in the temperature range investigated. Salt presence increased the growth rate of these standard lactose crystals up to salt concentrations used in SWUF but further salt increases appeared to have no significant effect. Other crystals were compared to these standard crystals and showed equivalent temperature, supersaturation and salt effects.Lactose crystallization studies in SWUF closely modelled results in industrial solutions. The study of mutarotation rate, lactose solubility and equilibrium alpha fraction in SWUF showed that only mutarotation rates were significantly greater than the literature values in water. The batch crystallization experiments in permeate performed in laboratory and industrial scale equipment showed that all lactose crystallization properties in permeate are consistent with the correlations developed for SWUF. No serious scale-up problems were found.Impurity analysis of crystals grown in industrial permeate solutions revealed entrapped mother liquor, an entrapped form of solid calcium phosphate and adsorbed riboflavin. Inclusions may be a result of calcium phosphate particles acting as nuclei or initially rapid growth causing a less structured initial crystal. Riboflavin adsorption on the crystal exterior was found to be diminished at lower temperatures.Miscellaneous studies in water solutions produced additional data. Lactose nucleation studies found secondary metastability limits and nucleation rates. However, further nucleation data in permeate is needed to more accurately model industrial results. Dissolution studies revealed that the dissolution rate is apparently mass transfer diffusion limited and that all crystals have an uniform dissolution rate. The combination of the lactose crystallization properties, the growth kinetics for SWUF and nucleation rate data produced a computer model simulating batch industrial lactose crystallization. This computer model, written in Visual Basic for Excel, allows the users to calculate the results from a set temperature profile or calculating an optimum temperature profile minimising nucleation. The computer model gave a reasonable fit to the experimental industrial data which indicates that it will be useful for simulating industrial processes.