A supercritical fluid is any substance at a temperature and pressure above its critical point. Such fluids can diffuse through solids like a gas and dissolve materials like a liquid. Near the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be “fine‐tuned”. Supercritical fluids are often suitable substitutes for organic solvents in a range of industrial and laboratory processes.
Carbon dioxide is one of the many commonly used supercritical fluids. It is relatively simple to exceed its critical point (31 °C, 1057 psi). Applications that involve supercritical fluids include extractions, nano‐particle and nano‐structured film formation, supercritical drying, carbon capture and storage, as well as enhanced oil recovery studies. Parr has provided systems at one time or another for all the aforementioned applications.
Water is another substance that is often used in its supercritical condition (374 °C, 3185 psi). Its excellent thermal conductivity properties make it the fluid of choice in pressurized nuclear reactors for electricity generation. The extremely aggressive and reactive nature of supercritical water makes it an excellent choice for the oxidative destruction of some hazardous waste materials.
The corrosive nature of supercritical water would prohibit the use of T316 stainless steel.
The batch supercritical extraction vessel pictured above is constructed of Hastelloy C-276 and designed for use to 6000 psi (410 bar) at 400 °C. This 600 mL vessel was equipped with a liquid CO2 pump capable of delivering up to 24 mL/min.
The supercritical fluid extraction system pictured and diagrammed above incorporates a 1.2 liter vessel rated for use at 3000 psig (200 bar) at temperatures to 300 °C. The system includes an automated inlet valve and an air-piloted back pressure regulator which is used to facilitate a controlled pressure release at the end of the test. The vessel is heated with a 1500W flexible mantle heater.