Pressure control of a continuous pilot scale supercritical fluid extraction (SFE) process

Pressure control for a pilot scale continuous supercritical fluid extraction (SFE) process, has been developed for consistent extractions, overall efficiency and process safety reasons. The Internal-Model-Control (IMC) framework (specifically Skogestad's IMC method) was employed for the design of a Proportional-Integral (PI) controller using both the actual pilot process and a hydrodynamic model of the process. Controller performance was assessed for set point changes and disturbance rejection. The best controllers lead to limited overshoot (less than 1 MPa), a response time of 200 s (within +/- 1% of set point) and little oscillatory behaviour. The design and performance results of the controllers generated using the hydrodynamic model were consistent with those of the actual process. Tests at alternate pressure conditions confirmed that the controller design and performance is pressure dependent owing to the nonlinear character of SFE processes.