Page 50 - gas transport in porous media

43
Chapter 3: Vapor Transport Processes
variation with temperature), there are other situations that can cause isothermal vapor
concentration gradients (e.g., spatial variability of a liquid species, the existence of
brine, etc.). The next section discusses experimental studies that were performed to
verify the results of both the analytical and numerical studies.

3.5.3 Experimental Studies
Experiments were performed to confirm the postulated processes of enhanced vapor
diffusion. Experiments were conducted at both the pore scale and at a larger porous-
media scale (containing a network of many pores).
The pore scale experiments performed by Silverman (1998) consisted of a small
fabricated single-pore test cell (Figure 3.10). The column on the left contained pure
water, and the column on the right contained a saturated brine solution with lithium
chloride to reduce the saturated vapor pressure of water in the right column. The top
of each column was covered by septa through which the different liquids could be
introduced. Tests were conducted with and without the liquid island present, and the
drawdown of liquid in the left column was used to evaluate the mass flow of water
through the pore.
Results of the single-pore tests are shown in Figure 3.11. The plot clearly shows
that the drawdown (and, hence, mass flow through the pore) was significantly greater
when a liquid island was present in the pore. In addition, the drawdown rate increased
as the length of the liquid island increased. Because the water vapor condenses on
the upstream side and then evaporates on the downstream side of the liquid island, a
longer liquid island produces a shorter effective pathway for the transport of water
molecules. Additional tests were performed with a two-pore test cell, and results
also confirmed that the presence of a liquid island within the pores augmented the
transport of water from one column to another. Measured enhancement of vapor
diffusion compared to diffusion through an open pore was up to a factor of three for
a single pore. Additional enhancement is expected for a larger network of pores.

25 mm

Figure 3.10. Pore-scale test cell with liquid island (Silverman, 1998)