Integrated framework for synthesising energy-efficient distillation column sequence

Abstract

This thesis presents and describes the development and application of an integrated framework for the synthesis of energy-efficient distillation column sequences. The framework is generic and applicable to various types of distillation columns. It is unique in the sense that it integrates distillation column sequencing, selection and design with the graphical representation of the driving force method. The existing driving force method was improved to include the effect of feed composition and also several concepts from existing methods, which can improve the capability of the method in finding optimal solutions that are feasible, economical, energy-efficient and material-efficient. The framework consists of five stages: 1) energy analysis of the existing sequence, 2) determination of the driving force sequence, 3) design of the driving force sequence, 4) feasibility, energy intensity and material intensity analyses and 5) economic analysis. In Stage 1, an existing distillation column sequence was simulated using the Aspen HYSYS process simulator to obtain its energy usage. In Stage 2, the graph of the improved driving force method was used to determine an energy-efficient distillation column sequence, which was also simulated to obtain energy usage. Then, by using a similar graph, suitable unit operations (flash columns, ordinary distillation columns, or extractive distillation columns) for the sequence were selected and designed in Stage 3. This post-design driving force sequence was also simulated for the same purpose as in Stage 1. The analyses began in Stage 4, where the feasibility, energy intensity and material intensity of the distillation column sequences obtained in Stages 1, 2 and 3 were compared. Feasibility was determined based on the reflux ratio range, distillation column height and product purity whilst energy and material intensities were based on mass, water and energy indexes. Finally, in Stage 5, an economic comparison based on capital, operation and total annual costs was employed. The framework was successfully tested on five different case studies with different objectives to test and verify the methodologies used in the framework. The application of the overall framework showed that energy savings of up to 32.94% could be achieved whilst operating within the feasible range. The energy and material intensities were also reduced by up to 59.31%, indicating lesser amount of energy and material used for the framework’s sequence. The capital and operation costs were also reduced, as much as 35.05% and 30.88%, respectively, which led to 31.71% lower total annual cost, compared with the sequences obtained by previous studies.