Cake filtration and mechanical gas dewatering are well-known processes for solid-liquid separation. While cake geometry is usually assumed to be homogeneous during process design, filter cake height can certainly vary locally on industrial scale filtration equipment. E.g., unsteady and non-uniform slurry feed conditions often lead to irregular filter cake height, especially on large filter areas. In case of belt filters, these inhomogeneities can be worsened by the separation of vacuum chambers and the resulting pressure field. Even though much more extreme, a widely studied example of irregular cake geometry is cracking of filter cakes. However obvious the negative influence of irregular filter cake geometry on gas dewatering may seem, there has not yet been established a way to predict or quantify their relationship.

In this study, a two-dimensional model describing mechanical gas dewatering of isotropic filter cakes based on the law of conservation of mass and two-phase Darcian flow. A numeric scheme adopted from Kueper et al. (1991) involves grid-centered finite differences and Newton-Raphson iteration handling the non-linearity of the underlying system of PDEs. Owing to the two-dimensional approach, locally different cake heights can easily be incorporated.

By determining the overall cake saturation against time, predictions for the dewatering kinetics based on capillary pressure and relative permeability functions are possible. Simulated saturation fields give an insight into the transient distribution of pore liquid during dewatering. The volume flow of gas produced can be calculated as well.

As a result, inhomogeneous cake height can affect gas dewatering negatively. Compared to homogeneous filter cakes, both the saturation decrease and the gas volume flow at a constant gas differential pressure are influenced. This can be explained by nonuniform flow leading to different residence time distributions of gas within the filter cake.

Experimental data of filter cakes with exemplary cake geometries is used for validating the model. Comparison of gas velocity and liquid saturation between experiment and model prediction looks promising. Going forward, this work helps to describe the impact of irregular cake geometries occurring in industrial processes.