We are developing a method to process substances into microparticles within a closed loop thermosyphon using carbon dioxide as the working fluid. Thermosyphons have been used for various purposes including building heating, electronics cooling, and supercritical fluid extraction. In this study, the thermosyphon consists of stainless steel tubing and vessels arranged in a closed loop with two temperature zones. The two temperature zones create low (~0.3 g/cm³) and high (~0.6 g/cm³) CO₂ density regions within the thermosyphon.

To operate the system, the substance is loaded into the cold temperature zone where it is dissolved by high density carbon dioxide. Carbon dioxide and the dissolved substance are then transported through a nozzle to the low CO₂ density region where the substance precipitates in a vessel. The carbon dioxide is then recycled to the high CO₂ density region. Flow within the loop is driven by buoyant forces which result from the CO₂ density differential. This method allows for substances to be processed without mechanical energy, rapid depressurization, or organic solvents.

A closed loop thermosyphon system was designed and developed to process naphthalene using CO₂. At 83.8 bar, the solubility of naphthalene in carbon dioxide is ~2.5 wt% at 32°C and ~0.15 wt% at 50°C. The decreased solubility of naphthalene at the higher temperature has been observed to be sufficient to precipitate naphthalene in the low CO₂ density region. Effectiveness of system conditions (temperatures in each zone) and design features are assessed through high speed and high magnification visualization of processes in the precipitation vessel and through measurements of mass flow rates. Resulting particles and materials are analyzed with a light microscope or SEM.