Feedback Control of Multicomponent Salt Crystallization

A closed-loop strategy is developed for controlling batch cooling multicomponent crystallization. The strategy represents the sequential application of two established feedback control techniques: direct nucleation control followed by supersaturation control. Experimental results show that such a control scheme produces larger crystals (compared to linear cooling crystallization with the same batch time). In using this scheme to control the crystallization of a double salt from a solution containing sodium nitrate and sodium sulfate, we demonstrate the application of supersaturation control to a multicomponent salt crystallization—which requires knowledge of the solubility as a function of temperature, the ability to monitor concentrations in a multicomponent solution, and an appropriate expression for the driving force for crystallization of a salt. In this paper, a methodology for rapidly identifying the solubility of a solute in a multicomponent solution is presented and a new expression for supersaturation—termed the molar supersaturation—is advanced as a measure of the driving force for crystallization of salts.