New trends in extraction-process intensification: Hybrid and sequential green technologies

The multidisciplinary "Green Extraction" approach has recently been expanded by merging the concepts of Green Chemistry and the Circular Economy. This approach considers waste from different production chains to be a valuable resource for other sectors, and can significantly improve the economic and environmental sustainability of current processes on a global scale. The goal is to produce high-value products from residual biomass in an inherently safe and environmentally friendly way, which is referred to as being "benign-by-design". In this framework, the application of enabling technologies can ideally maximize heat and mass transfer, leading to significant improvements in yield/conversion rates, energy savings, and lower production costs. However, this trend is becoming mandatory and requires even more effort. New solutions are needed to meet increasing demand, and two or more enabling technologies can be combined in sequential or simultaneous systems to dramatically increase process intensification and applicability. These combinations can significantly improve mixing and heating, as well as biomass pretreatment and metabolite dissolution. This literature survey focuses on both hybrid and coupled green-extraction technologies over a five-year period and highlights the differences between the two strategies, their achievements and technology readiness levels (TRLs). The study underlines the potential offered by synergistic combinations of green technologies for the extraction of bioactive compounds from biomass. The information gathered provides an outlook as to the state-of-the-art, and highlights the challenges that must be overcome and the opportunities that exist for future extraction research.