TOWARDS ON-SCALE CRYSTALLIZER DESIGN USING COMPARTMENTAL-MODELS

In this paper the importance of on-scale crystalliser design is outlin ed. An on-scale approach is specifically required for the analysis and optimisation tasks in design. The need for this approach is a direct consequence of the nonlinear dependency of most physical processes in crystallisation on the degree of saturation, the energy dissipation, t he crystal size and its distribution. The hydrodynamics in a crystalli ser vessel are typically such, that these process variables are distri buted non-uniformly throughout the vessel. The conventional, geometric ally lumped description of the physical processes inside a crystallise r vessel, i.e. nucleation; growth, dissolution, attrition,;breakage, a gglomeration and particle segregation, has therefore never proven-to b e reliable for scale-up purposes. Furthermore, as the interactions bet ween these processes lead to an intricate dynamic behaviour, models de scribing the effect of changes in time of process variables on the pro duct quality are essential. Compartmental modelling, a well known tech nique in reactor engineering and applied within crystallisation since a number of years, facilitates on-scale/design since it allows a natur al separation of kinetic and hydrodynamic mechanisms. The-resulting dy namic models (order of 10(4) equations) can be easily tackled with sta ndard DAE solvers. Here we will focus upon the need for a proper physi cal description of the aforementioned crystallisation mechanisms. Firs t of all, a brief description of the dependencies of these mechanisms upon local supersaturation or undersaturation, local energy dissipatio n and crystal size is given. Depending on the type of crystallisation process, suspension crystallisation or precipitation, the dependencies necessary to be included in the compartmental model, in order to desc ribe their overall effect are discussed. The next step is deriving the geometric structure of a compartmental model for a certain scale crys talliser and material, for which two methodologies will be presented. Finally, the approach will be illustrated for evaporative crystallisat ion of ammonium sulphate from water in 0.15 and 18.5 m(3) FC (Forced C irculation) and 0.022 and 1.1 m(3) DTB (Draft Tube Baffle) crystallise rs, using size dependent nucleation, growth, dissolution, attrition an d segregation models. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.