Optimal retrofit and process design of distillation plants for energy saving and process intensification

This work is divided in two principal sections. The first concerns the application of more convenient structures to intensive energy consuming plants built before the energy crisis. Two case studies are considered. The first regards the implementation of a preflash device (drum or column) on a crude distillation unit. The limitations of both apparatuses are considered together with the energy saving realized. Particular emphasis is given to the evaluation of the best structure that matches the scope in the energy plant reduction and the respect of the production specifications. The second case considered regards a light ends distillation plant. Different separation sequences derived from the plant configuration are considered. The possibility to reach a reduction of the energy consumption is considered together with the maximum reusage of the plant apparatus, that allows to minimize the capital cost investment. The total annual cost is associated to the required heat exchanger area for the condensers and the reboilers and to the column section diameter. The best solution, that satisfy the scope of the retrofit work minimizing at the same time the employing of new equipment and the energy consumption, is then identified. In the second section of the work the column sequence design is considered. The starting point was the separation of a four components mixture considered with five different feed compositions. All the possible simple column sequences were first analyzed using different sets of heuristic rules and after with more rigorous evaluation. The different solutions are compared to put in evidence the limitations of the heuristic rules, which anyway remain a good tool for a first screening of the most promising structures. The best distillation sequence for each feed composition case is also considered for the implementation of a divided wall column to perform a three component separation. The resulting “hybrid” sequences are made by a simple column that follows or precedes a divided wall column. This type of column was modeled using the modified Underwood-Fenske-Gilliland method to obtain the first design parameters to be utilized as the input to a more rigorous simulation performed with the Aspen Plus simulation package. The different hybrid configurations are then compared with respect to the best simple column structures from which are derived. In the last part of the work a new method to map the space of distillation columns with less than n-1 columns is proposed. Up to now this configuration space was never predicted in a systematic way and only heuristic rules are available to predict only a few of all the possible configurations. Including more distillation column configurations in the research space increases the possibility to identify the sequence that satisfies the research scope. The generation method is presented for a four components mixture but is absolutely general and can be applied for any number of components.