Continuous Hydrogenation of Cinnamaldehyde: Gas–Liquid–Liquid–Solid Helical Coil Reactor

The advantages and challenges of using the gas–liquid–liquid–solid (GLLS) hydrogenation system are discussed in this work using the case of selective hydrogenation of cinnamaldehyde to cinnamyl alcohol (an important ingredient in the perfume and flavoring industry). The four phases in this system include gas (hydrogen)-liquid (organic, reactant + solvent)-liquid (aqueous KOH)-solid (5% Pt/C catalyst). The addition of second liquid phase, i.e., aqueous KOH significantly increases selectivity toward cinnamyl alcohol compared to the conventional three-phase hydrogenation. The four-phase GLLS reactions were carried out and optimized in a continuous helical coil reactor. The role of key aspects such as gas solubility, kinetics, flow hydrodynamics, axial dispersion, and mass transfer on the performance of a continuous GLLS reactor is presented and discussed in this work. The presented results and discussion will be useful for addressing conflicting demands like long residence time, low axial dispersion, and high mass transfer. The experimental studies and results of the developed mathematical model indicate that the continuous GLLS helical coil reactor outperforms the batch operation. The production rates (kg day–1) of cinnamyl alcohol achieved in continuous operation were at least double in comparison to batch operation, with 32% less consumption of precious catalyst (per kg of product). The presented results will open up new opportunities for enhancing selectivity and overall performance of hydrogenations via introducing a second immiscible liquid phase and designing continuous tubular reactors for the same.