Supercritical and near-critical CO₂ processing

Abstract

CO₂ is the supercritical fluid of choice because its critical temperature is readily accessible (∼304 K), because it is a relatively benign material itself (nonflammable, relatively low toxicity), and because it is naturally abundant and hence inexpensive. Unfortunately, CO₂ is a feeble solvent, making it difficult to use for processes where substantial solubility of high molecular weight and/or polar compounds is required. Although there are several compounds whose critical temperatures are sufficiently mild (<100°C) to allow easy access to the supercritical regime, most have received little attention owing to various process or safety issues. From its inception, research into the use of CO₂ as a process solvent has been motivated by the desire to create process schemes that are greener than their predecessors. Whereas the bulk of CO₂-based applications in the food industry to date have targeted extractions, there is the possibility for other CO₂-based technologies to cross over from polymer science (foaming, additive blending), catalysis (hydrogenation), and biotechnology (enzymatic reactions). In this chapter, the physical properties of CO₂, environmental advantages and disadvantages to the use of CO₂, process design, and the economic viability of supercritical CO₂ use are discussed. Current and potential applications are also highlighted. Much of the most exciting work in CO₂ technology since 1990 has involved the design of auxiliaries (surfactants, chelating agents), which may find use in food processing as well.