Energy analysis of supercritical carbon dioxide extraction processes

Supercritical fluids are being used to develop new extraction and cleaning systems. In this work, common cycles used in extraction cycles are analyzed in detail with respect to energy requirements and exergy losses. First, cycles that operate between the saturation region and supercritical region are considered. From the exergy analysis for pump cycles, it can be seen that there is an optimum extraction pressure that provides a minimum in exergy loss for a given extraction temperature and separation pressure. The minimum exergy losses are described by parametric equations in terms of extraction temperature, pressure and separation pressure. A similar set of equations is provided in terms of extraction temperature, density and separation pressure. Compressor cycles are also analyzed and it was found that selection between pump and compressor cycles can be made by determination of equal exergy loss points. These points are presented graphically and also in terms of parametric equations for extraction pressure or extraction density in terms of extraction temperature and separation pressure. Cycles that operate solely in the supercritical region were studied. The cycle proposed by de Swaan Arons (4th International Symposium on Supercritical Fluids, May 11-15, 1997, Japan) was found to provide very low exergy losses for a given change in extraction and separation density. It was also found that, while supercritical region cycles can provide lower exergy losses than cycles operating between the saturation and supercritical region, for some cases of high density difference, pump or compressor cycles can be competitive with the supercritical region cycles.