Synthesis and Optimisation of Demethaniser Flowsheets for NGL Recovery

Synthesis and Optimisation of Demethaniser Flowsheets for NGL Recovery

Muneeb Nawaz¹, Megan Jobson², Adrian Finn¹

1. Costain - Energy & Process, Costain House, Styal Road, Manchester, M22 5WN

2. Centre for Process Integration, School of Chemical Engineering and Analytical Science, University of Manchester, Manchester, United Kingdom

A demethaniser process is characterised by interactions between the complex distillation column and other flowsheet units, including the turbo-expander, flash units, multistream exchangers and refrigeration system. This research presents a systematic approach for demethaniser flowsheet synthesis to generate cost-effective designs.

A demethaniser column has many degrees of freedom, including the operating pressure, multiple feeds, the number and duty of side reboilers and the flow rate of the external reflux stream. The additional feed and side reboiler streams enhance the efficiency of the process, but complicate process modelling. The number of design variables is also augmented by additional degrees of freedom such as the location and the order of feeds, the number of stages and the reflux ratio in the column. The complexity of the demethaniser column precludes the use of the Fenske–Underwood–Gilliland shortcut design method. A semi-rigorous boundary value method is proposed for the design of complex demethaniser columns for application within an optimisation framework for process synthesis and evaluation.

A simplified flowsheet simulation model based on shortcut models is developed that is able to account for various configurations and inter-connections in the demethaniser process. A methodology accounting for heat integration in multistream exchangers is proposed. A generalised superstructure has been proposed for demethaniser flowsheet synthesis that includes various structural combinations in addition to the operational parameters. A stochastic optimisation technique, simulated annealing, is applied to optimise the superstructure and generate energy-efficient and cost-effective flowsheets. The application of the developed synthesis methodology is illustrated by a case study of relevance to natural gas processing. The results allow insights to be obtained into the important trade-offs and interactions and indicate that the synthesis methodology can be employed as a tool for quantitative evaluation of preliminary designs as well as to facilitate evaluation, selection and optimisation of licensed demethaniser flowsheets.