(230e) Capillary Disconnect during Evaporation in Porous Media: Visualization of Transition from Stage-1 to Stage-2 Evaporation Regime

Ayo Rufai, John
Crawshaw and Geoffrey Maitland

Abstract

Evaporative
drying of porous media is a complex process involving mass transport in the
liquid and vapour phase on the pore scale. Most evaporation experiments
available in the literature on artificial porous media have focused on single
capillaries or sand packs. We have carried out, for the first time, evaporation
studies on a 2D micro-model based on a thin section of a carbonate rock,
sucrosic dolomite. This allows direct visual observation of pore scale
processes in a network of pores. We saturated our micromodel with NaCl
solutions at concentrations from 0 wt% (deionised water) to 36 wt% (saturated
brine) and passed dry air across a vertical channel in front of the micromodel
matrix for evaporative drying.

For
de-ionised water we observed (see figure 1) the three classical periods of
evaporation expected from earlier drying experiments in porous media: the
constant rate period (CRP) in which liquid remains connected to the matrix
surface, the falling rate period (FRP) and the receding front period (RFP), in
which the capillary connection is broken and water transport becomes dominated
by vapour diffusion. However, when brine was dried in the micro-model we
observed that the length of the CRP decreased with increasing brine
concentration and became almost non-existent for 36 wt% NaCl solution
(saturated brine), as is also shown in Figure 1.

In
the experiments with brine, the dry area of the matrix (equivalent to the mass
lost by evaporation) became linear with the square root of time after the short
CRP, as is shown in Figure 2. However, this is unlikely to be due to capillary
disconnection from the surface of the matrix as is usually the case for
de-ionised water drying, as the salt crystals continued to be deposited almost
exclusively in the channel above the matrix (see Figure 2 inset). We propose
that this behaviour is due to a combination of salt deposition in the vertical
channel and at the matrix surface greatly impeding hydraulic connectivity to
the evaporating surface as well as the high viscosity of the saturated brine
increasing the viscous resistance to flow.

Figure
1. Evaporation as a function of liquid saturation for de-ionised water (blue)
and 36 wt% NaCl brine (green). The end of the constant rate period for the
de-ionised water plot is indicated with the dotted lines.

Figure
2. Plot of pore dry area as a function of for 36
wt% NaCl brine (Upper inset is a plot of pore dry area versus time for
early time (CRP) evaporation. Lower inset is an image of salt deposition during
the falling rate period).