Gas-liquid interfacial mass transfer in trickle-bed reactors at elevated pressures

A phenomenological description and a semiempirical two-zone model are proposed for the gas-liquid interfacial areas and the volumetric liquid-side mass-transfer coefficients in cocurrent downflow trickle-bed reactors operated at elevated pressure. Gas-liquid interfacial areas, a, and volumetric liquid-side mass-transfer coefficients, k(L)a, are measured in the trickle flow regime at high nitrogen pressure (0.3-3.2 MPa). Use is made of diethanolamine carbamation in aqueous viscous and organic model solutions in which fast and slow absorptions of carbon dioxide occur. In order to extract genuine mass-transfer parameters, a rigorous thermodynamic model is established to account for liquid and gas nonidealities. The influence of pressure, gas and liquid superficial velocities, liquid viscosity, and packing size on the gas-liquid interfacial mass transfer is examined. At constant gas and liquid superficial velocities, increasing the reactor pressure improves the gas-liquid interfacial mass transfer at the expense of increased two-phase pressure drop and gas holdup. At high pressure, the gas-liquid flow may be viewed as a two-zone flow pattern: (i) a liquid-free gas continuous phase which delineates a macroscopic gas-liquid interface (ii) and a gas-liquid film emulsion comprised of tiny bubbles which form in the films and delineate a microscopic gas-liquid interface. Taylor's theory of fluid-fluid sheared emulsions is used to quantify the microscopic interface via the effect of pressure on the size of bubbles in the trickling film. A bubble Sauter diameter is related to viscous shear stress and surface tension force, the two competing forces that determine bubble size. The model is also extended to estimate volumetric gas-liquid mass-transfer coefficients under high-pressure conditions.