Production of large crystals by continuous ripening in a stirrer tank

\u3cp\u3eA new bulk crystallisation process is described in which the supersaturation in the crystalliser is maintained by feeding very small subcritical crystals. Supercritical crystals that are present in the crystalliser grow at the expense of the dissolving subcritical crystals. The process is described by means of a mathematical model. With this model the influences of crystal concentration, production rate, and size of the feed crystals upon the average product crystal size are calculated. The model predicts an increase of the mean size of the product crystals with a decrease of feed crystal size for small feed crystals. The size of the product crystals also increases with an increase in crystal concentration in the crystalliser. The process has been studied experimentally for the growth of ice crystals from aqueous sucrose solutions. The theoretically predicted effects of crystal concentration and feed crystal size on product crystal size are confirmed by the experiments.\u3c/p\u3

Nucleation and growth kinetics for the crystallization of ice from dextrose solutions in a continuous stirred tank crystallizer with supercooled feed

\u3cp\u3eThe crystallization of ice in aqueous dextrose solutions is studied in an adiabatic continuous stirred tank crystallizer with a supercooled feed stream. The effective diameter of the ice crystals was determined for various values of mean crystal residence time, feed supercooling, magma density, stirring rate, and dextrose concentration. For all process conditions the supercooling was measured at 9‐12 different locations in the crystallizer. These local supercoolings were averaged algebraically to yield the bulksupercooling. From the experimental results growth and nucleation rates have been calculated. By comparing the experimental growth rates to growth rates calculated by means of a mathematical model kinetics for the inbuilding of water molecules into the ice lattice have been determined. The growth rate appears to be directly proportional to the interface supercooling. The rate constant decreases exponentially with increasing weight percentage of dextrose in the solution. The nucleation rate was found to be directly proportional to total crystal surface per unit volume of suspension and proportional to the bulksupercooling to the power 2.1. Nucleation is believed to occur by breakage of dendrites from the surface of parent crystals.\u3c/p\u3