The Influence of Surface Features in the Salt Dissolution Process
Authors: Durie R.W. , Jessen F.W. 
Published: December 2007

The dissolution of salt in the development of salt cavities is controlled by free-convection boundary layer flow along the salt surfaces. It is the purpose of this paper to expand upon results published previously, and primarily to illustrate the influence of surface features on the boundary layer and hence the dissolution process in the development of underground salt cavities. General dissolution equations are presented that apply to a surface inclined to the vertical. A salt surface inclined so as to be overlain by water is dissolved by boundary layer flow, while an inverted surface is dissolved by a combination of boundary layer and cellular flow. Detailed investigation of the boundary layer was limited to laminar free-convection flow. Truly laminar free-convection boundary layer flow may be encountered only in laboratory solution cavities, but the boundary layer will control dissolution even where cavity dimensions are such that free convection turbulence will occur. The factors found to significantly affect the laminar boundary layer will affect dissolution with a turbulent boundary layer.

Introduction

Underground salt cavities have been used extensively for the storage of LPG products, and the use of these cavities is expanding each year. The increasing requirements for such cavities suggest the need for a better understanding of the mechanisms involved, for the purpose of the control of cavity geometry and the most effective use of the injected water. A previous paper outlined a quantitative approach to the determination of salt dissolution rates in the laminar free-convection dissolution of salt. It is the purpose of this paper to expand upon the results previously published and to point out significant factors in the application of these results to the dissolution of field cavities. The dissolution of salt in laboratory scale cavities is controlled by the free-convection boundary layer, which is the region of flow along the salt surface caused by the difference in buoyancy forces resulting from the salt dissolution. The transfer of salt into solution at the salt face causes an increase in salinity, and hence a greater density, in the water at the salt surface. The region of higher density tends to fall through the lighter, less saline water and a flowing boundary layer is the result. The water remains stationary at the contact with the salt surface, so the resulting velocity profile is such that the influence of gravity is opposed by a viscous shear force at the salt surface. For salt to be transferred into solution from a solid surface to a boundary layer of fluid, it must diffuse through a stationary film of water at the salt contact. According to Fick's first law, the rate of mass transfer by diffusion is the product of the diffusion coefficient and the concentration gradient, with diffusion transfer in the direction of decreasing salinity. Once the salt has passed into the boundary layer, it will be transferred by a combination of molecular diffusion and convection. The convection transfer will in turn affect the rate of dissolution by influencing the gradient of the salt concentration at the surface.In the previous investigation by the authors to determine the basic mechanism of the salt dissolution process, an equation was developed that agreed very well with the experimental rates of free-convection salt dissolution from a smooth vertical surface. The technique was taken from the work of Eckert in the analogous heat transfer system and will be referred to as the integral method. The rate of salt transfer from a vertical section of height H was found to be:

(1)

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