Design and Control of a Vapour Recompression C3 Splitter

Abstract

Vapor recompression (VRC) based heat integration is usually employed for purifying a propylene-propane mixture in a C3 splitter. A fixed-speed drive (FSD) or a variable-speed drive (VSD) 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. Results indicate that for FSD compression, 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 VSD compressor. The VSD compressor design corresponds to the lowest condenser temperature with water as coolant and the reboiler temperature driving force at the minimum constraint. In the FSD compressor design, the condenser temperature must be increased sufficiently above the minimum for an acceptable compression ratio. The case study highlights the importance for considering control at the conceptual design stage for designing a robust operable process.