IMPROVED ROBERTS EVAPORATOR PERFORMANCE THROUGH CIRCULATION MODELLING WITH CFD

The design of Roberts evaporators in the Australian sugar industry is currently based on parameters evolved from experience with a limited amount of experimentation into some of the critical design criteria. Numerous configurations of juice inlets and outlets, calandrias and downcomers exist in the industry. Experimental and computational fluid dynamics (CFD) modelling studies were undertaken to identify design modifications to Roberts evaporator vessels that could lead to an increase in throughput capacity. Residence time and heat transfer trials were conducted on an existing evaporator set to provide data for fluid dynamics modelling. A CFD model of the first vessel of the set was developed. In addition, two alternative evaporator configurations were explored with CFD. The following conclusions are drawn from the experimental and modelling work. The optimal flow pattern of juice inside an evaporator vessel is plug flow, with no internal recirculation, stagnant regions or bypassing. The calculated benefits from plug flow on the throughput capacity of an effect range from 4% for the No. 1 effect to above 30% for the final effect. The evaporator vessels, which typically have three peripheral juice inlets and a single central juice outlet, showed significant mixing and recirculation of the juice, as well as short-circuiting of juice from the feed inlet to the outlet. This was seen in both the experimental data and the CFD model predictions. CFD modelling of alternative evaporator configurations has indicated that the preferred configuration is a vessel with fully distributed feed and a single central outlet.