Effect of feed location on particle size distribution in production of crystals in a classified bed crystallizer

摘要

Industries like sodium chloride manufacturing require the crystalline products that they produce be of a stable particle size. In Japan, sodium chloride is manufactured by continuous crystallization using multi-effect crystallizers, with concentrated seawater (20 wt.% sodium chloride) being used as the raw material. In general, mixing-type crystallizers produce crystals with a mean particle size of 400 μm. When using a classified bed crystallizer, however, crystals with a particle size of as much as 1000 μm can be produced. The main problem with these processes is a cyclic swing in the size of the particles produced by the crystallizer. This is a result of several factors. If a dynamic control method could be applied to the size of the particles being produced by the crystallizer, these cyclic swings could be prevented and the degree of noncompliance reduced. To that end, an adjustment to the operation of the crystallizer would be necessary, based on the monitoring of the size of the crystals being produced, but the actual number of ways in which control can be applied is limited, because the production rate must remain constant in the case of a multi-effect crystallizer. In previous studies, we examined various methods of applying dynamic control to the size of the crystals being produced by a large crystallizer [1, 2].