(131b) Process Design and Optimization of Complex Demethanizer Flowsheets

Despite recent success of membrane and pressure swing adsorption in low-throughput applications, low temperature processing remains the most important route for the separation and purification of gas mixtures, especially when high recoveries are required. A low temperature gas separation system involves interactions between the separation system, refrigeration system and the heat recovery system. When a design problem dealing with low temperature separations is approached in a logical way, the number of alternatives to be studied is generally very large. The assessment of all these possible flowsheets with numerous options is a time consuming task with a lot of simulations required to select the economically best option. Therefore, a systematic approach for synthesis is required to generate effective and economic design with minimal time and effort.

This aim of this research is to extend an existing platform for the design of heat integrated separation systems to address the demethanizer subsystem; which is characterized by complex interactions between the distillation column and other flowsheet components, including turbo expander, flash units, multistream exchangers and external refrigeration. The number of design variables in the case of demethanizer is augmented by additional degrees of freedom compared to a simple column such as, e.g., the location and the order of feeds, the number and duty of side reboilers, the flowrate of external reflux stream and the minimum reflux ratio for the complex column.

This work presents an approach to screen the various options and narrow the design options to viable schemes. A methodology is being developed for the design of a demethanizer system, ensuring that promising design options are identified at an early stage and all major design options are considered. A new design method based on the boundary value method is employed for generating the column design, verifying the separation performance and calculating energy requirements. Case studies illustrate that the optimization framework allows energy-efficient and cost-effective gas separation flowsheets to be developed.