(529e) The Relevance of Solvent Condensation During Rapid Expansion of Supercritical Solutions

The advantages of using supercritical fluids as solvents in chemical processing have been demonstrated for a wide spectrum of processes such as extraction, synthesis, and crystallization. These novel methods take advantage of the unique characteristics of supercritical fluids such as gas-like diffusivity and pressure dependent solvent strength. Such properties offer unique processing capabilities not feasible with conventional liquid solvents.

In the field of recrystallization, rapid expansion of supercritical solutions (RESS) has received much attention due to the unparalleled ability to generate nanoparticles of a wide range of materials with exceptional control of the particle size and purity, while processing at relatively mild conditions without detrimental effects on the final product. Since the solvents utilized (e.g. CO2) are typically gases at ambient temperature and pressure, another benefit is the ability to produce neat product free of residual solvents. Pharmaceutical compounds are a good example of a type of materials that can benefit from RESS as many such compounds tend to be thermally labile and elimination of toxic solvents from processing is greatly desired.

Understanding of the particle formation mechanism during RESS has significantly evolved over the past two decades. Mathematical models of fluid expansion and the resulting solute nucleation and particle growth have increased in complexity and account for more of the phenomena occurring within the expanding jet. Nevertheless, due to the complex nature of such expansion processes an accurate mathematical representation does not yet exist. Currently the predicted particle size is typically significantly smaller than that experimentally obtained.

To date, as a result of added complexity, models do not account for a significant aspect of RESS, in particular the condensation of solvent during expansion. As a result, processing conditions considered in such models lie outside of the relevant range for many processes of interest.

In the present paper the impact of CO2 condensation on the particle growth is examined. In this work the recrystallization of cyclotrimethylene trinitramine (RDX) from CO2 was investigated. The pre-expansion temperature and pressure were studied in the range from 50 to 90oC and around 100 to 350 bar respectively. The effect of the discharge pressure was studied in the range from atmospheric pressure to around 60 bar. Within the span of the experimental conditions both liquid and solid-phase solvent condensate was encountered. Based on empirical data as well calculations particle growth mechanisms are proposed.