CFD modelling of a novel clarifier design for use in sugar cane juice clarification.

Abstract

The purpose of clarification in the sugar industry is to remove soluble, insoluble and colloidal matter from cane juice. Efficient clarification is required to produce high quality sugar and to prevent entrainment of solids in downstream equipment. The objective of this study is to produce a Computational Fluid Dynamics (CFD) model of the Magra Ultrasep clarifier. This was accomplished by: • Modelling the hydrodynamics of a laboratory scale clarifier In the Fluent CFD program. • Incorporating the flocculation process into the CFD model. • Performing experiments on a pilot scale clarifier to obtain parameter values for the flocculation model. The hydrodynamic model of the clarifier showed the presence of a recirculation zone above the baffle plate. Particle injections using Fluent's discrete phase modelling option determined that particles within the size range of IOOj..Lm to 4mm would circulate in this region, forming the bed of floc particles required for the Magra Ultrasep to work efficiently. The flocculation process in Fluent was represented using three different solid phases of different particle sizes. Small and medium sized particles were allowed to combine to form larger particles by changing the volume fractions according to three rate equations. A fibre glass laboratory scale model was set up at Maidstone Sugar Mill and fed the same sugar cane juice that enters the Rapi-Dorr clarifiers. The experimental results were then fed into a simplified flocculation model in MATLAB. An overall rate constant (k) of 5kg.m-3.s-) for the flocculation kinetic equation satisfied the experimental result.