(118f) 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.