Clear as crystal
15 Jan 2000
Crystallisation is a vital part of many chemical processes, particularly in the fine chemicals and pharmaceutical industries. New research by Narayan Tavare of the University of Bradford's school of chemical engineering could help operators control their evaporators more accurately to 'tailor' the final product.
Crystallisers generally configured as shown above work by 'supersaturating' a solution. There are four mechanisms for this cooling, evaporation, dilution and chemical reaction which may be used alone, in series or in parallel. However, when the crystallisers are characterised for design and control purposes, only one mode of action is considered. Tavare, by contrast, has analysed the action of a crystalliser that works by both dilution and cooling.
Tavare used a commercial compound which is made in a methanol solvent. To crystallise this, the solution is cooled and stirred while water is added, which decreases its solubility in methanol until supersaturation is reached.
If the initial addition rate of water is slow, with slow stirring and a large number of 'seed' crystals, the solute tends to deposit on the seeds, giving large, coarse crystals, says Tavare. Conversely, increasing the stirrer speed, fast addition and few seed crystals increases the nucleation effects, producing large numbers of small crystals.
