Data on design and operation of trickle beds at elevated pressures are scarce. In this study the influence of the gas density on the liquid holdup, the pressure drop, and the transition between trickle and pulse flow has been investigated in a tricklebed reactor operating up to 7.5 MPa and with nitrogen or helium as the gas phase. Gas-liquid Part 1: Gas-Liquid Interfacial Areas IntroductionGas-liquid mass transfer processes in trickle-flow columns can be performed either in the cocurrent or countercurrent operation. From a hydrodynamic point of view, the cocurrent operation is preferable because it has no limitations in the gas and the liquid throughput. In cocurrent operation, only one equilibrium stage can be reached. The cocurrent gas-liquid trickle-flow operation is suitable when the transferred component is removed by a chemical reaction as is the case in threephase catalytic trickle-bed reactors. In this application, the transfer rate of the gaseous reaction component to the liquid bulk can play an important role on the overall conversion rate, especially when the intrinsic rate of reaction is relatively fast.Several published studies have dealt with gas-liquid mass transfer in the cocurrent downflow operation with packings typically used in absorption and desorption processes, such as AIChE JournalDecember 1991 saddles and rings, as well as for relatively small packings, dp< 4 mm with cylindrical or spherical geometry as generally used in catalytic trickle-bed reactors (see, for example, Gianetto and Silveston, 1986). From these studies, it can be concluded that the gas-liquid mass transfer rate, especially the gas-liquid interfacial area, depends strongly on the hydrodynamic flow pattern. In the trickle-flow regime, resistances for mass transfer are larger than the resistances in the spray-, pulse-and bubbleflow regime. Further, investigations on gas-liquid mass transfer at pressures above atmospheric conditions are hardly available. In industrial practice, the catalytic trickle beds always operate at elevated pressures to increase the concentration of the gaseous component in the liquid phase. In previous studies (Wammes et al., 1990a,b), we have shown that pressure has a strong influence on the hydrodynamics in a trickle-flow col- Vol. 37, No. 12 1849umn. The operating region for trickle flow becomes larger at higher pressures. In contrast to atmospheric trickle-flow conditions, the liquid holdup depends strongly on the gas velocity at elevated pressures. In this study, we investigate whether or not pressure also has an influence on the specific gas-liquid interfacial areas in the cocurrent trickle-flow operation. Greenfield (1978, 1979) and Levec et al. 1986Levec et al. , 1988 found experimentally that the liquid holdup and the pressure drop can exhibit hysteresis. Because the gas-liquid interfacial area is likely to be related to the holdup and pressure drop, we also pay attention to possible hysteresis phenomena for the interfacial areas.The gas-liquid interfacial area can be determined by usi...