J Scott Paren-Interfacial Mass Transfer in Gas-Liquid Reacto.pptVIP

J.S. Parent Interfacial Mass Transfer in Gas-Liquid Reactors Consider a metal-catalyzed hydrogenation of a non-volatile olefin (liquid-phase). A semi-batch process is used, wherein hydrogen is replenished to maintain the reactor pressure. An autoclave is charged with the olefin solution, pressurized with hydrogen and allowed to equilibrate at the reaction temperature. Interfacial Mass Transfer in Gas-Liquid Reactors Hydrogenation in a gas-liquid contactor involves: 1. Physical adsorption of H2 across the gas-liquid interface into the bulk liquid phase. 2. Catalytic hydrogenation within the liquid phase. If r1 r2, then the overall rate is that of the catalytic process. Kinetic Control, [H2]?[H2]eq If r1 r2, then the rate equals the rate of interfacial mass transfer. Mass Transfer Limited, [H2]?0 Quantifying Interfacial Mass Transfer In all gas-liquid contactors, mass transfer takes place under forced convection, meaning the gas and liquid phases are mixed. Physical adsorption brings the system towards an equilibrium condition. It is known from experiment that the farther the system is from an equilibrium condition, the faster the rate of mass transfer. Define the rate of H2 transfer across a gas-liquid interface, FH2: where, FH2 = interfacial transfer rate: mole/s kl = convective mass transfer coefficient: m/s A = gas-liquid interfacial area: m2 [H2]* = equilibrium H2 concentration of liquid: mole/m3 [H2] = H2 concentration of liquid: mole/m3 Measuring klA in a Stirred-Tank Contactor It is difficult to determine both kl and A by experiment, as measuring interfacial area can be tedious if not impossible in some cases. However, the product kl A is more accessible. Mass transfer in a stirred tank can be quantified quite easily: Equilibrate the system at low pressure under static conditions Raise the reactor pressure several bar At t=0, start the agitator and measure the amount of H2 required to maintain constant pressure as a