Development and verification of an Aspen Plus® model of a sugarcane biorefinery.
Guest, Kylan Leigh.
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This dissertation describes a steady-state mass and energy balance model of a raw sugar mill which has been developed using the Aspen Plus® software. The Aspen Plus® model was designed to replicate an existing MATLAB model of a ‘generic’ South African sugar mill with the aim of facilitating future expansion to different products which cannot be easily handled in MATLAB. The first step was to create the entire sugar mill flowsheet in Aspen Plus®. This involved deciding how best to model the complex processes such as multiple-effect evaporators and pans. Key factors in simulating sugar mills are modelling the boiling point elevation and crystallisation. The UNIQUAC thermodynamic model with coefficients regressed by Starzak (2015) was used in Aspen Plus® to predict the vapour-liquid equilibria in sugarcane juice solutions. Also, a solid-liquid equilibria model was developed in order to accurately handle the crystallisation and dissolution of sucrose. A dynamic tool (proportional integral controller) was used to solve the material balance of the evaporator station. Microsoft Excel® has been incorporated into the Aspen Plus® model to iteratively solve this dynamic tool. Initially, the Aspen Plus® model was verified for a cane throughput of 244 t/h against the results of the existing MATLAB model. The stream results showed a good comparison between Aspen Plus®, MATLAB and real sugar mill data. Different scenarios have been tested in Aspen Plus® and compared to the MATLAB model. Cane throughputs of 230 and 270 t/h were simulated and the results compared favourably between the two models. The energy requirement of the sugar mill was calculated for different flow rates of imbibition (water used for juice extraction). A portion of an intermediate stream (a potential biorefinery feedstock) was diverted after the clarifiers and the effects on the rest of the sugar mill were quantified. Finally, different cane purities were simulated in order to assess the effects on syrup purity, molasses purity and boiling house recovery.