Influence of expansion conditions on the characteristics of cholesterol crystals analyzed by statistical design

The rapid expansion of supercritical solutions process (RESS) was applied to the crystallization of cholesterol. The influence of selected process parameters on the product characteristics (crystal structure, particle size and shape and particle size distribution (PSD)) was analyzed through a statistical design. A two-level four-factor full experimental design (DOE) was used to evaluate the significance of the main effects and interactions. The selected parameters focused on nozzle and collection conditions. A swirl nozzle was tested for the first time in the RESS-process. Whatever the conditions, cholesterol was precipitated as acicular particles whit sizes below 5 μm. Among the product characteristics, the total surface area to volume ratio of the crystallized systems was the most sensitive response to the process parameters. DOE analysis showed that collection chamber temperature, nozzle type and nozzle diameter had a relatively high and similar effect, whereas the nozzle temperature only slightly influenced the response. A significant interaction between the nozzle type and nozzle temperature factors was found, that cannot be enlightened by the one-factor-at-a-time route.