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Evaporation from a Porous Medium in the Presence of Salt Crystallization

  • Marc Prat
Conference paper
Part of the NATO Science for Peace and Security Series C: Environmental Security book series (NAPSC)

Abstract

The interplay between salt transport, crystallization and evaporation from a porous medium is a topic rich in interesting open problems. This is illustrated through the consideration of a few of them from experiments with model porous media. We notably discuss the factors controlling the localization of crystallization spots at the evaporative surface of a porous medium and the impact of surface crystallization on evaporation kinetics.

Keywords

Porous Medium Salt Concentration Evaporation Process Evaporation Flux Salt Transport 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

19.1 Introduction

Evaporation in a presence of dissolved salt is a key process in several applications such as the injection of CO2 in saline aquifers, the soil salinization problem and the preservation of our cultural heritage (frescos, statues, monuments, etc). The latter application is directly related to the damages that can be induced in a porous material or at its surface by the salt crystallization, i.e. [1]. The damage problem is generally studied in relation with the so-called crystallization pressure, e.g. [2, 3], which is the key concept in order to evaluate the stresses generated within the pores by the crystallization. According to the current theory, the crystallization pressure notably depends on the local supersaturation, that is the amount of dissolved salt in excess compared to the amount corresponding to the thermodynamic equilibrium between the crystal and the solution. It is therefore crucial to predict the salt concentration transient distribution within the system during the evaporation process in order to be able to address the associated poremechanical problem or more modestly to predict where the damages are likely to be generated in the materials. Even in the absence of coupling with poremechanical effects, the study is made complicated by the various couplings existing between evaporation, salt transport and crystallization. The main objective of the present article is to illustrate the interplay between different phenomena (in the absence of structural damages). We indeed believe that it is crucial to develop a much better understanding of the trio: evaporation/transport/crystallization. A few obvious basic questions are: Is evaporation in the presence of salt faster, identical or slower than for pure water? How does the effect of salt on evaporation depend on the initial salt concentration? Does the salt crystallize at the surface of the porous medium or within the porous medium? These are the main points that will be addressed in what follows. The idea is to give an overview of some key results. Details can be found in the references. Also, we do not discuss in details evaporation with pure water, see [4] in the present book for an introduction to evaporation in porous media. The results we discuss were obtained with sodium chloride, which can be considered as a simple salt, notably because it forms an anhydrous crystal.

19.2 Efflorescence/Subflorescence

Salt crystals appear in a solution when the dissolved salt concentration C reaches the crystallization concentration C sat . The latter is generally greater than the saturation concentration corresponding to the thermodynamic equilibrium between the crystal and the solution. Depending on various factors, the crystallisation concentration can be reached within the porous medium, leading to formation of subflorescence or at the surface of the porous medium, which leads to formation of efflorescence (Fig. 19.1). In classical drying experiments, the advection effect induced in the porous medium by the evaporation process is sufficient to lead to a concentration build-up at the surface of the porous medium, e.g. [6, 7], and thus to efflorescence. Subflorescence can form at later stages when the liquid is not connected anymore to the outer open surface of the porous medium or more generally when evaporation takes place not at the surface but inside the porous medium.
Fig. 19.1

Efflorescence and subflorescence in a model porous medium [5]. Subflorescence refers to crystallized salt structures inside the porous medium whereas efflorescence refers to crystallized salt structures at the surface of the porous medium

19.3 Cauliflower Efflorescence and Crusty Efflorescence

As reported in [8] and illustrated in Fig. 19.2, two main types of efflorescence are distinguished: cauliflower, also referred to as patchy, and crusty. The crusty efflorescence forms a crust covering entirely the surface (the surface of the underlying porous surface is not visible where the crust is present) whereas the cauliflower efflorescence is discrete, characterized by well individualized salt structures. As can be seen from Fig. 19.2a a significant fraction of the surface of the porous medium remains free of efflorescence when the efflorescence is patchy. The factors controlling which type of efflorescence is going to form are a subject of investigation, [8, 9]. Patchy efflorescence will be favoured when the pores are relatively big and the evaporation rate is small whereas a crust is expected when the pores are smaller and/or the evaporation rate is high. Also, it should be mentioned that distinguishing only two types of efflorescence is a simplification. It is likely that the typology of efflorescence is richer, e.g. [10]. In our experiments, these two main types can be clearly distinguished, notably through their impact on evaporation (see Sect. 19.6 below).
Fig. 19.2

Example of cauliflower (or patchy) efflorescence (a) and crusty efflorescence (b)

19.4 Efflorescence Is Porous. Efflorescence Growth Mechanism

Efflorescence is porous. This has been clearly established for the patchy efflorescence using simple capillary invasion experiments (see Fig. 19.3) and X-ray microtomography [11]. The dissolved salt is transported through the pore network of the efflorescence structures up to their upper regions where the evaporation fluxes inducing the transport are higher. This leads to the preferential crystallization in the upper regions of the salt structures. As a result the growth of efflorescence occurs from above by successive deposition of salt crystals from above. This is therefore an evaporation flux controlled growth, which has some obvious but not yet fully studied connections with classical Laplacian growth processes (the vapour partial pressure is governed by a Laplacian operator in the diffusive limit), see for example [12] for an introduction to Laplacian growth.
Fig. 19.3

Patchy efflorescence is porous. A red dyed aqueous solution put in contact with a dry efflorescence rises by capillary forces up to the top of efflorescence where higher evaporation fluxes induce a preferential deposition of the dye

The case of the crusty efflorescence is much less clear. The model proposed in [9] for explaining qualitatively the transition from patchy to crusty as the mean pore size of the underlying porous medium is decreased is based on the assumption that the crusty efflorescence is porous. It has been also seen that the vapour can be transported through a salt crust [13]. However, the details of the crust formation and crust growth are a widely open subject. Further experimental investigations are needed.

19.5 Factors Affecting the Localisation of Cauliflower Efflorescence

One striking feature of the patchy efflorescence is the formation of discrete crystallization spots at the porous surface (see Fig. 19.2a). This is interpreted as the signature of the existence of local maxima in the salt concentration field at the porous surface prior and up to the formation of first crystals. The existence of spatial fluctuations in the salt concentration field is explained by the spatial fluctuations in the velocity field existing in the liquid phase. Because the advection effect is significant, the spatial fluctuations in the velocity field induce fluctuations in the concentration field. The factors affecting the salt concentration distribution and thus the localization of crystallization spots at the surface are therefore intimately related to the factors affecting the velocity distribution within the pores. As discussed in [9, 14] and [15], those factors are the disordered nature of a porous medium (= random distribution of pore sizes), the evaporation flux distribution at the surface, e.g. [14], and the possible Darcy’ scale heterogeneities of the medium, e.g. [15]. The more detailed analysis reported in [9] leads to distinguish the impact of surface disorder and internal disorder. The surface disorder refers to the spatial fluctuations in the pore opening at the surface, which induce fluctuation in the evaporation rate at the surface of each meniscus, and thus velocity fluctuations in the adjacent pores. The internal disorder refers to the fluctuations in the pore size within the porous medium, which induce fluctuations in the velocity field, and thus in the salt concentration field.

As discussed in [9] and [15], salt concentration gradients are important only in a region of size ξ(t) adjacent to the evaporative surface. This is a consequence of the advection effect on the salt transport, see also [6]. As a result only the internal disorder of this region of size ξ(t) is important. The disorder located further away from the evaporative surface can generally be ignored.

The next question is why the patchy efflorescence continues to grow under the form of well individualized salt structures. As discussed in [9] and [14], this is explained by the screening of the porous surface free of efflorescence located between the already growing efflorescence structures and the redirection of dissolved salt present in the porous medium toward the growing salt structures. The screening means that the evaporation flux at the porous medium surface between the growing efflorescence structures tends to zero, which “kills” the advection effect and thus the salt concentration build-up at the surface.

19.6 Impact of Efflorescence Type on Evaporation

As reported in [8], the type of efflorescence, i.e. crusty or patchy, has a great impact on evaporation. The patchy efflorescence does not affect significantly the evaporation process and can even enhance the evaporation rate compared to pure water. By contrast the crusty efflorescence can greatly affect the evaporation process and even blocks the evaporation. It has also been observed [8] that the interplay between drying and the efflorescence formation leads to a non-monotonous variation of the drying rate with the initial salt concentration when the efflorescence is patchy but not when the efflorescence is crusty. This has to do with a porous medium “coffee ring” effect due to evaporation fluxes higher at the periphery of the sample, see [8] for more details. The ring effect is illustrated in Fig. 19.4.
Fig. 19.4

Top view of the evaporative surface of a porous sample and illustration of the efflorescence ring effect. Efflorescence first forms at the periphery of the porous sample owing to evaporation fluxes greater at the periphery

19.7 Conclusion

Evaporation from a porous medium in the presence of dissolved salt is a particularly interesting problem because of the complex interplay between evaporation, salt transport and crystallisation. This field is widely open and many questions are still to be answered. The detailed understanding of the crusty efflorescence growth and the patchy/crusty transition are just two examples of interesting questions. A more advanced understanding of the transport/crystallization problem is needed not only because the crystallization can greatly affect the evaporation but also because the salt concentrations that can be reached in the solution in contact with growing crystals has a direct impact on the crystallisation pressure, and therefore the possible associated poromechanical effects.

As for evaporation with pure water, e.g. [4], studies in recent years have mainly focussed on microporous materials (pore size equal or greater than 1 μm). Very little is in fact known for the systems involving nanopores (∼ pore sizes lower than 100 nm).

Notes

Acknowledgement

I am very thankful to the students and colleagues I have worked with over the years on the “salt problem”: S. Ben Nasrallah, P. Duru, H. Eloukabi, F. Hidri, M. Marcoux, N. Sghaier, S. Veran-Tissoires. Their contribution is greatly appreciated.

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Copyright information

© Springer Science+Business Media Dordrecht 2014

Authors and Affiliations

  1. 1.Institut National Polytechnique de Toulouse (INPT), Institut de Mécanique des Fluides de Toulouse (IMFT), UMR 5502 CNRS-UPSToulouseFrance

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