Pore-scale dynamics of salt precipitation in drying porous media

Mansoureh Norouzi Rad, Nima Shokri, and Muhammad Sahimi

Phys. Rev. E 88, 032404 – Published 9 September 2013

Abstract

We study the pore-scale dynamics of salt precipitation in three-dimensional drying porous media, utilizing high resolution x-ray microtomography and scanning electron microscopy. Our results illustrate that the salt precipitation patterns in drying porous media are nonuniform, manifesting the influence of the spatial distribution of pore sizes on the dynamics of salt crystallization and formation of discrete efflorescence. Results reveal that during stage-1 evaporation from saline porous media, the salt precipitation rate initially increases which is followed by a constant precipitation rate. This non-linear behaviour is attributed to the preferential liquid vaporization and salt precipitation in finer pores located at the surface of the porous medium contributing in evaporation according to the pore sizes. We also show that, contrary to common practice, the macroscopic convection-diffusion equation cannot provide accurate predictions for the dynamics of salt precipitation, at least at the early stages, due to the microscale heterogeneity of evaporation sites at the surface that results in salt precipitation exclusively in the finer pores.

Received 3 December 2012

DOI:https://doi.org/10.1103/PhysRevE.88.032404

Authors & Affiliations

Mansoureh Norouzi Rad¹, Nima Shokri^1,*, and Muhammad Sahimi²

¹School of Chemical Engineering and Analytical Science, University of Manchester, Manchester M13 9PL, United Kingdom
²Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1211, USA

^*E-mail address: nima.shokri $@$ manchester.ac.uk

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Vol. 88, Iss. 3 — September 2013

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Images

Figure 1
(a) A typical horizontal cross section above the surface, taken at the beginning of the experiment, illustrating the liquid [blue (dark gray)] and solid [brown (medium gray)] phases. (b) The same cross section after 17 h, showing the distribution of the solid [brown (medium gray)] and air (white) phases. (c) The image constructed by comparing the solid phases in (a) and (b) with the difference [orange (light gray)], corresponding to the precipitated salt. (d) A typical vertical close-up of the surface of the sand column saturated with 4 M NaCl solution, constructed by the same method as in (c) and applied to all cross sections. The spectrum of orange to yellow (medium to light gray) indicates the addition of precipitated salt in each scan. Brighter colors indicate longer times.Reuse & Permissions
Figure 2
Cumulative mass of liquid water removed by evaporation from the sand columns, initially saturated with various NaCl solutions. The notably lower water evaporation from the 4.0 M solution might be due to the lower atmospheric demand. Although the evaporative demand was nearly constant during each round of the experiment, the relative humidity and temperature inside the x-ray chamber could not be controlled precisely. The inset shows the near surface (the top 0.5 mm) water saturation, confirming the presence of liquid at the surface during the entire course of the experiments.Reuse & Permissions
Figure 3
Patterns and distribution of precipitated salt at the surface of the sand column at various times, saturated with the 4 M NaCl solution. The color scale indicates the height of the precipitated salt. Black, gray, and white indicate, respectively, the regions filled with the liquid, sand grains, and the air phase. All other colors correspond to the precipitated salt and the color spectrum indicates the thickness of the precipitated salt such that the closer the color is to orange (darker color), the thicker is the salt crust.Reuse & Permissions
Figure 4
(a) Cumulative precipitated salt as a function of the cumulative mass of liquid water lost by evaporation. The legend indicates the initial salt concentration in each evaporating sand column. (b) Time dependence of the cumulative precipitated salt in each sand column. The precipitation rate (the slope of the curves) changes with time until it becomes nearly constant in the cases of the columns saturated with 3.5 M and 4 M solutions. The transition time to a constant precipitation rate is the time at which all the evaporation spots at the surface turn into precipitation spots. Solid lines correspond to the precipitated salt at the surface over time predicted by convection. The vertical dash lines indicate the onset of NaCl precipitation at the surface predicted by the numerical solution of the CDE which is in a reasonable agreement with the experimental data.Reuse & Permissions
Figure 5
The SEM images, showing the structure of the precipitated salt at the surface of the sand column, saturated with a 3.5 M NaCl solution after (left) 18 and (right) 168 h with magnification factors of (a) and (b) 150, (c) and (d) 500 and (e) and (f) 2000.Reuse & Permissions

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