Process Integration Applied to the Design and Operation of Distillation Columns
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Process integration of distillation columns with focus on multi-effect distillation is studied in this thesis. Columns are integrated by using different pressures, so that the condensing duty of one column can be used to boil a second column. This system of multi-effect integration is applied to a complex distillation system; a prefractionator arrangement. The resulting multi-effect prefractionator arrangement has a much lower energy requirement than a conventional distillation arrangement and it is shown that the savings can be in excess of 70 %. The emphasis of the work is on the energy savings of such arrangement, looked at mainly in terms of minimum vapour flow requirements. Energy savings are considered in terms of both design and operational issues. The first part of the thesis deals primarily with minimum vapour flow expressions. It is shown how minimum vapour flow of multi-effect arrangements can be visualised in a Vmin-diagram. These diagrams can be used to find the minimum energy requirement for different multi-effect configurations and can further be used to analyse and understand the multi-effect prefractionator arrangement. Having shown the potential savings of the multi-effect prefractionator arrangement it is interesting to look at how such integrated arrangements can be operated in practice. A method of self-optimising control is used as a systematic tool to find the variables that should be controlled in order to keep the system close to its optimal operating point. Dynamic simulations are used to verify the suggested control structure. The final part of the thesis looks at an industrial case study to see if the tools and a multi-effect prefractionator arrangement can be implemented for a real separation task.