One of the key parameters in supercritical CO2 extraction (SFE) is pressure control. There are two aspects that make this possible, a good automatic back pressure regulator and accurate PID control of this regulator.
On the market there are a number for reputable companies that offer highly capable back pressure regulators that work with a variety of applications. While developing our systems we evaluated a number of these regulators and found Premier Industries were able to offer a high-quality product that met our control requirements.
Having a good automatic back pressure regulator, we began developing a control system with PID (proportional, integral, and derivative) control parameters aiming to achieve +/-2 bar control we desire for a stable extraction.
If you have observed a pressure curve while compressing CO2 from bottle pressure (55 bar / 800 psi) to supercritical pressure you will have observed a change in the rate of compression. The rate of compression between 55 bar and 73 bar in the liquid phase is different once it changes into the supercritical phase. With a standard PID control loop the system will usually overshoot the pressure due to the change in rate of compression (dependent on the desired conditions) as the PID loop builds and creates error. It takes a small amount to compensate for this and stabilise on the desired pressure.
Accurately controlling this pressure in a timely manner is key to a good and stable extraction. Fluctuations result in density changes in the CO2 and thus solubility changes during the extraction.
We have spent a significant amount of time compensating for this rate of change and developing a robust solution that offers a superior level of control while retaining flexibility in the system to make changes as and when required. This is known as adaptive pressure control (APC™)! It is key to our quick and stable pressure control within our extraction systems allowing us to achieve +/-2 bar of the setpoint which enables us to perform accurate and consistent extractions.