The pressure drop across a packed-bed collector of low-concentration stratospheric gases

Abstract

Packed beds of molecular-sieve pellets have found application in the high-altitude collection of certain gases present in the atmosphere in low concentrations. To facilitate the design of such low-pressure gas samplers, a theory is developed establishing practical relations between pressure drop, gas flow rate, and various gas and packing characteristics. Pressure-drop vs flow-rate relations are confirmed experimentally over the range of ambient pressures of 0.0013-0.79 atm, and for conditions of flow that may be either viscous or inertial, compressible or incompressible, or with or without molecular slip. The theory shows that, for a given set of experimental conditions, there exists a critical gas velocity below which a pressure drop and its associated velocity are possible, if at all, at only a single ambient pressure. Above the critical velocity, however, a given pressure drop-velocity combination may occur, if at all, at either one or two ambient pressures. Methods are described for correlating experimental data in terms of dimensionless groups and, on the basis of relatively few measurements near atmospheric conditions, for estimating pressure drop over a broad range of ambient pressures and gas velocities. The theory derived is thought to be applicable, under appropriate conditions, to a variety of porous media including fibrous filters.