Gas−Liquid Interfacial Mass Transfer in Trickle-Bed Reactors at Elevated Pressures

Abstract

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, kLa, 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.