Page 34 - gas transport in porous media

CHAPTER 3

VAPOR TRANSPORT PROCESSES

CLIFFORD K. HO
Sandia National Laboratories, P. O. Box 5800, Albuquerque, NM 87185, USA

3.1 OVERVIEW

In this chapter, important processes associated with vapor in porous media are investi-
gated. The term vapor is used here to identify the gaseous phase of a substance that can
be condensed under standard temperatures and pressures. With this definition, com-
poundssuchaswater, alcohol, andbenzenecanexistasvapors, butcompoundssuchas
nitrogen, helium, and hydrogen exist as gases. The first section describes the partition-
ing of a vapor with its liquid phase, either as a single component or in a mixture of other
components. The second section describes a phenomenon known as vapor-pressure
lowering, which reduces the ability of compounds to partition into the vapor phase
when strong capillarity exists. The third section describes the rate of evaporation and
provides sample calculations. Finally, the fourth section describes a process known
as enhanced vapor diffusion, which allows condensable vapors to apparently diffuse
through unsaturated porous media at high rates relative to Fickian diffusion.

3.2 PHASE PARTITIONING
Vapor-phase compounds can partition into mobile liquid phases and immobile solid
phases in porous media. The mobility of various compounds in porous media depends,
in part, on the ability for those compounds to partition into a mobile phase. In addition,
the partitioning between vapor and solid phases can cause retardation of transport.
This section describes the various mechanisms of vapor-phase partitioning between
the liquid and gas phases. A separate chapter is devoted to solid–vapor partitioning.

3.2.1 Single-Component Liquid/Vapor Partitioning
The partitioning of a single-component liquid into its vapor phase is dictated by
the vapor pressure of the liquid, which is determined by the temperature of the
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C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 27–46.
© 2006 Springer.