Solvents play a vital role in chemical processes such as solubilizing reactants, facilitating product/catalyst separation, increasing reaction rates, enhancing solubilities of gaseous reactants (such as O2, CO, H2) and providing heat capacity to manage the heat of reaction. However, solvent use can also cause adverse environmental impacts by increasing the carbon footprint and/or emitting harmful vapors. This chapter highlights such roles of solvents in multiphase catalytic processes such as hydroformylation, carbonylation, hydrogenation and oxidation through industrial examples. It also discusses how the pressure-tunable properties of supercritical fluids (SCFs) and gas-expanded liquids (GXLs) can be harnessed to develop greener chemical technologies through efficient feedstock utilization, process intensification, enhanced process safety and reduced use of volatile organic solvents. Emerging feedstocks, such as plant-based biomass, shale gas and sequestered CO2 offer excellent opportunities for using such tunable solvents.

The principles of process intensification and the positive impacts for process safety, economics, and the exploitation of novel chemistry are described. The nexus between process intensification and sustainability is explained. The role of novel solvents such as ionic liquids in process intensification is discussed. The principles of liquid–liquid contact and phase separation are described, followed by a review of current engineering technologies for liquid–liquid processes which embrace the principles of process intensification. A brief overview of state-of-the-art mixing technology for liquid–liquid systems and for mixer settlers precedes a summary of current column contactor types and rotary contactors. Established designs of column contactors are briefly reviewed. The chapter includes some description of industrial coalescence equipment, showing how the design of coalescence equipment has been improved to enhance performance. A final section dealing with recent oscillatory baffled contactor technology is included, demonstrating how they meet the criteria for process intensification.