Process Intensification Using Reactive Distillation: Conversion of Crude Glycerol to Propanediol

Combining reaction engineering and distillation into a single unit operation presents some interesting challenges, yet, reduces the operational footprint as well as the operating and capital costs of manufacturing. Operational and design challenges include mismatched temperatures between the separating components and the reacting components, unfavorable volatilities, and slow reaction rates generally due to poor reactant/catalyst/product mass transfer rates. Reactive distillation is a likely pathway towards process intensification to minimize overall energy consumption, particularly since distillation accounts for 20 % of total energy used within the U.S. manufacturing sector. Increased reflux ratios typically yield higher product purities but at the cost of reboiler heat duties.

In this work, AspenPlus was used to study the feasibility of utilizing a Reactive Distillation column for the catalytic hydrogenolysis of glycerol using isopropanol as a donor molecule for hydrogen. In short, isopropanol was converted to acetic acid, yielding a hydrogen atom that would be available for the conversion of glycerol to 1,2-propanediol and 1,3-propanediol. To perform the simulation, a series of preliminary tests were performed using a CSTR to generate initial parameters for the reaction and a shortcut column to generate initial distillation column operating conditions. Finally, the column design was implemented with the RADFRAC model employing a partial condenser. Important operating parameters such as column pressure, Distillate to feed ratio, reflux ratio, feed temperature, feed flowrate, feed stage, reaction zone, changes in feed composition molar ratio and liquid holdup where analyzed for optimum values to see how the system responds to changes in these variables. Using the Non-Random Two Liquid activity coefficient model, the number of trays, conversion, and product selectivities were estimated. It was found that the column was energy efficient with a reflux ratio value of 0.42 , condenser temperature of 62 ^oC with a boilup ratio of 2.7 which corresponds to a reboiler temperature of 209 ^oC for a glycerol conversion of 96 %. While this work is focused at providing economic avenues for the biodiesel industry, it can also be extended to other industrially-relevant processes (i.e. mild hydrodesulfurization of crude oils, manufacturing of surfactants and detergents, and lubricants).