Dynamics of continuous precipitation under non-stoichiometric conditions

Abstract

Continuous precipitation from ionic solution is investigated here under conditions of non-stoichiometric operation, with a two-step model for growth. Prior studies of crystallization or precipitation have all been carried out for stoichiometric solutions. It is shown that, when supersaturation is generated by a chemical reaction such as double decomposition, and the mass transfer step for growth offers a significant resistance, maximum productivity is achieved at non-stoichiometric reactant concentrations. The effect is particularly pronounced at high supersaturations which are often attained in reaction-crystallization, and two- to three-fold improvements in production may be obtained compared to operation with stoichiometric reactant feed streams. The stability under non-stoichiometric conditions is also studied and it is found that over a certain range of the ratio of reactant concentrations the system can be unstable. Under certain conditions instability is also found to arise as the mass transfer resistance for crystal growth is reduced, indicating that low agitation speeds may facilitate stable operation. As in prior studies of crystallizer dynamics instability is found to manifest itself in the form of limit cycle oscillations.