Micromodel observations of evaporative drying and salt deposition in porous media

Abstract

Most evaporation experiments using artificial porous media have focused on single capillaries or sand packs. We have carried out, for the first time, evaporation studies on a 2.5D micromodel based on a thin section of a sucrosic dolomite rock. This allowed direct visual observation of pore-scale processes in a network of pores. NaCl solutions from 0 wt. % (de-ionized water) to 36 wt. % (saturated brine) were evaporated by passing dry air through a channel in front of the micromodel matrix. For de-ionized water, we observed the three classical periods of evaporation: the constant rate period (CRP) in which liquid remains connected to the matrix surface, the falling rate period, and the receding front period, in which the capillary connection is broken and water transport becomes dominated by vapour diffusion. However, when brine was dried in the micromodel, we observed that the length of the CRP decreased with increasing brine concentration and became almost non-existent for the saturated brine. In the experiments with brine, the mass lost by evaporation became linear with the square root of time after the short CRP. However, this is unlikely to be due to capillary disconnection from the surface of the matrix, as salt crystals continued to be deposited in the channel above the matrix. We propose that this is due to salt deposition at the matrix surface progressively impeding hydraulic connectivity to the evaporating surface.

Effect of Wettability Changes on Evaporation Rate and the Permeability Impairment due to Salt Deposition

“…The claim that the CRP is negligible even for deionized water drying was made over thirty-five years ago 3 . This claim has however been disproved by many studies including this present study 1,[3][4][5][6][7][8][9][10][11][12] . The falling rate period and the receding rate period are often combined together as stage-2 evaporation 6,[13][14][15] .…”

“…Colour-based thresholding was then used to segment the dry pore areas followed by its area measurement. The procedure for estimating the dry pore area from the images using ImageJ and some examples of this was highlighted in our recent paper 12 . The quantitative data presented in this study are subject to uncertainty in dry pore area measurements.…”

“…Studies based on these simple models, such as the capillary bundle model 78 , have been shown to be very useful and they often act as a vital link to understanding the transport phenomena in real porous media 1,7,50,[79][80] . In this study, we have increased the complexity of the artificial porous media beyond bundles of capillaries by working with micromodels that are based on a thin section of a sucrosic dolomite carbonate rock 12,81 . The use of micromodels also afforded us direct visual observation and imaging using imaging and optical spectroscopy in real time 82 .…”

Capillary disconnect during drying in model porous media at different wettability

“…This is followed by a transition period characterized by a large drop in evaporation rate, known as the falling rate period (FRP). The late stage of evaporation is characterized by a very slow evaporation rate limited by vapour diffusion through the porous media, this stage is known as the falling rate period (FRP)Materials and Methods [1][2][3][4][5][6].…”

“…Drying here is said to be controlled by external demand such as temperature, the flow rate of drying gas, relative humidity, the surface area of the evaporating surface and so on. During this period, the porous media behaves as though it is fully saturated because the mass transport through capillarity is sufficient to sustain evaporative demand at the surface [3,5]. Simply, the constant rate period is dominated by capillarity due to hydraulically connected pores [7].…”