Gas-expanded liquids

Although gas-expanded liquids have been known in some form or other for some time, the realization in the research community of their potential in a wide range of applications has only happened in the past few years. Of the references cited in this review, 60% date from 2000 onward. This is despite the fact that the use of CO2 as a viscosity-reducing agent for crude oil has been investigated since 1951 (see references within ref 219). Nevertheless, the creativity of the research community is now fully engaged, and there are already many examples of facilitated separations, reactions, materials processing, and particle formation. In virtually every application, the GXLs represent an enabling tool for effecting significant improvements in one or more process performance measures such as reaction rate, transport rate, product selectivity, product quality, and environmental friendliness. New research directions include cleaning, in situ acid-promoted reactions, and multifunctional CO2-aided continuous catalytic reaction systems that integrate reaction and separation. Successful development of new technologies will require multi-scale interdisciplinary research collaborations involving scientists and engineers. There are gray areas between expanded liquids and supercritical fluid mixtures that have not been discussed in this paper. For example, the CO2/coating material mixtures that are used in CO2-based spray coating systems are usually described as being supercritical, but they may not be supercritical in comparison to the true critical point of the mixture. CO2 /liquid mixtures of all kinds, whether expanded, supercritical, or both, will continue to be applied in new areas, including many that have not yet been imagined. Industrialization of expanded liquids has started in applications requiring viscosity reduction (e.g., crude oil recovery). Future commercial applications that exploit viscosity reduction will be most likely in polymer processing and particle formation. We plan occasional updates of this review in accord with the new living review" format supported by this journal. Preprints /reprints of relevant work and/or private communications are most welcome and will be incorporated with fitting acknowledgment. © 2007 American Chemical Society."