Nucleation induced by high frequency sound for the production of sugar refinery seed crystals.

Abstract

The crystallisation process is the second oldest unit operation in existence and a sine qua non in the production of crystalline sugar. It comprises two distinct phases viz. nucleation and the growth of the subsequent crystal. The former is conventionally achieved by either shock- or replaced by slurry-seeding, and is not a very well understood process in the sugar industry. It is generally accepted then that the adopted industrial nucleation processes have been derived more out of experience than from scientific fundamentals. The use of sound waves has the potential to create repeatable numbers of seed crystals from supersaturated sucrose solutions, that can be grown to a robust size in the absence of evaporation, for a controlled full seeding technique. This study describes benchtop-scale trials performed at the Sugar Milling Research Institute NPC (SMRI) that explored the usage of sound waves for seed crystal formation. The range of suitable operating conditions which could enable this process to produce seed for conventional panboiling was investigated by calculation. Using this range of operating conditions as a specification, an extensive benchtop scale laboratory investigation identified the equipment and operating conditions that could be able to produce acceptable seed crystals in a continuous process. As part of the investigation it was been necessary to develop appropriate techniques for measuring crystal sizes, size distributions and number densities. The results of the laboratory tests show that acceptable seed crystal sizes, with an acceptable crystal size distribution, can be achieved by growth at constant temperature. During the growth phase the nuclei exhaust the available supersaturation of the mother liquor from which they have been nucleated. The crystal number densities obtained are high enough to achieve a range of acceptable densities by the addition of extra mother liquor after nucleation if desired. It was concluded that the process is technically feasible for refinery seed production. Various classical crystal growth theories were modelled on Matlab®. The results predicted for the empirical nth-order kinetics with concentration-based driving force, compared well with the experimental results for growth of crystals obtained from sound induced secondary nucleation. Potential benefits of the full continuous seed production system include: Addressing the skills shortage associated with batch panboiling (a skill recognised by the Agricultural Sector Educational and Training Authority (AGRISETA) of South Africa to be the only unique skill in the sugar industry). Poor recoveries of sucrose have been experienced during panboiling due to diminishing skills in this area. A new full continuous seed production system without evaporative crystallisation involved will make batch seed pans obsolete and eliminate the pan skills issue. Steam savings from elimination of the slurry nuclei conditioning phase in panboiling. Elimination of this phase would also improve the capacity utilisation of pans. Improved sugar quality through narrower crystal size distributions. This will also improve drying and conditioning operations. Improved sugar recovery in centrifuges as the narrower size distribution will reduce the amount of smaller crystals escaping through screen perforations into the lower value molasses. Elimination of multi-storey crystallisation plants due to eliminating the need to discharge large quantities of seed hydraulically from pans in short times. The proposed seed system could lead to a ground floor crystallisation operation with large structural savings for new factories.