Vapor Recompression C3 Splitter: Design and Control

Abstract

Vapor recompression (VRC) based heat integration is usually employed for purifying a propylene-propane mixture in a C3 splitter. A fixed speed drive or a variable speed drive compressor may be employed for overhead vapor recompression. In this work, economic controllable designs for the two compressor types in a VRC C3 splitter are obtained via rigorous steady state and pressure driven closed loop dynamic simulations. A simple dual ended decentralized composition control system is shown to provide effective column regulation. The economic controllable design results indicate that when a fixed speed drive compressor is used, significant pressure drop must be provided across the compressor suction throttling valve for sufficient rangeability towards worst-case disturbance rejection. Accordingly, the column pressure and necessary compression ratio are higher for an overall more expensive process compared to a variable speed drive compressor. The case study highlights the importance for considering control at the conceptual design stage for designing a robust operable process.