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Design, implementation, and simulation of control systems for extractive and recovery distillation columns using Aspen Plus and Aspen Dynamics

Jayasinghe, Ashen (2018) Design, implementation, and simulation of control systems for extractive and recovery distillation columns using Aspen Plus and Aspen Dynamics. Honours thesis, Murdoch University.

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In practice, distillation may be carried out by either of two methods. The first method is based on the production of vapor by boiling the liquid mixture to be separated and condensing the vapour without allowing any liquid to return to the still. Then, there is no reflux. The second method is based upon the return of part of the condensate to the still under such conditions that this returning liquid is brought into the intimate contact with the vapours on their way to the condenser. Either of these two methods may be conducted as a continuous or as a batch process, but the study of dynamics and control of the process is one of most important part of each process.

Distillation is one of the commonly used separation technique in the chemical industries. The separation is based on differences in “volatilities” (tendencies to vaporize) among various chemical components. In a distillation column the more volatile, or lighter, components are removed from the top of the column, and the less volatile, or heavier, components are removed from the lower part of the column. Further Aspen Plus makes it easy to build and run the process simulation model by providing with a comprehensive system of the online process modelling. Process simulation allows one to predict the behaviour of a process by using basic engineering relationships, such as mass and energy balances, and phase and chemical equilibrium. Process simulation enables one to run many cases, conduct „what if‟ analysis and perform sensitivity analysis and optimization runs. With simulation one can design better plants and increase the profitability of the existing plants. Process simulation is helpful throughout the entire life of a process, from research and development through process design to production.

This thesis studies the dynamics and control of distillation columns using Aspen Plus. In this thesis, simulation studies of the distillation column are presented. Steady-state simulations are being performed using Aspen Plus followed by Aspen Dynamic simulation. In the steady state simulation, it was tried to see the effect of changing the flow rate of the extractive distillation. And finding the optimum flow rate in the distillation column. Controllers are then implemented for controlling sump level, reflux level and feed flow rate. Furthermore, two strategies were used for controlling the purity of distillate product controlling the distillation column tray temperature where the maximum change of temperature is observed due to reboiler heat change and the purity of the product by using composition controller.

The case study was an example taken from Aspen Plus (version 8.4v). In the example, there are two main streams enters the distillation column and phenol will be the stream one, and methyl cyclone hexane (MCH) and toluene mixer will enter the distillation column as the second stream. MCH has been distilled from the top of the column and the phenol and toluene the bottom product. With the latest Aspen Plus and Aspen Dynamics version V10 with operating under Windows 10, because of that, we will come across few compatibility issues in Aspen Dynamics mainly when it comes to MATLAB. Moreover, due to incompatibility MATLAB and Simulink were not tested for this process.

In this study, Methyl Cyclo Hexane (MCH) been separated from Toluene by using Phenol as the third component in an extractive distillation column. And in Aspen Dynamics new controllers been developed to control the product Methyl Cyclo Hexane (MCH) purity by making adjusting the flow rate level of the Phenol. DMC controllers were tried to implement in the process to replaces the PI controller but fail attempt.

All the PI controllers have been auto-tuned in Aspen Dynamics using it tool of the faceplates. Which given the best possible controller parameters to for the process. Therefore, all the controller’s other was able to reach its set-point expect the composition controller. The controllers were helping to achieve the maximum purity of the distillate stream.

All the obtained results have been discussed and the Important guidelines been outlined and explained in the overall simulation. Most of the objective been achieved in this thesis.

Item Type: Thesis (Honours)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor(s): Vu, Linh
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