Factors influencing backwashing operation in the liquid phase after cake filtration

Abstract

The separation of solids from process streams takes place in almost every process plant. In particular, cake-forming filtration in the liquid environment is advancing as a widespread method of separating low-concentration suspensions from their solids content and offers energy advantages over other separation principles. In addition to the actual separation performance, the filter fabrics must be regularly replaced or regenerated. The reason for this is the growing resistance because of the increasing filter cake thicknesses. Considering the reduction of disposable products, the regeneration of the fabrics is an interesting possibility to reduce costs and to guarantee a stable process control. This can be achieved by means of filter cake discharge. In the course of this regeneration, the filter cake should be discharged as completely as possible so that the fabric reaches the required initial filtrate flow of the previous filtration cycle again.Investigations concerning cake discharge have shown that the regeneration step can be divided into different discharge mechanisms (complete, partial, resuspended) which result in different cleaning qualities. In the context of this study, backwashing processes with different backwashing parameters (pressure, cake thickness) as well as different particle shapes and their influence on the regeneration quality of filter fabrics are presented and discussed.The focus of this work is the investigation of the backwashing process “cake discharge” as a subsequent and necessary step in cake-forming filtration. Within the scope of this study, a test setup was designed which allows the variation of different process variables and tissue-particle combinations. The experimental setup is kept simple and allows easy reproduction of experiments. The aim is the integrated consideration of the influencing variables on the regeneration quality. The variables cake thickness, particle shape and backwash pressure were investigated within the scope of this study.A significant influence of backwash quality on the examined multifilament filter fabric for particle systems ≤20 µm can be attributed to the particle shape and filter cake thickness. Here, a good regeneration quality can be achieved after exceeding a defined filter cake resistance. This depends significantly on the specific filter cake resistance and thus determines the required cake thickness. Particle systems which, due to their shape and size, have a lower specific cake resistance therefore require a higher cake thickness for a good cleaning result. As a rule of thumb, the absolute filter cake resistance can be estimated with ≥4 ∙ 1010 m−1. A possible influence of the backwash pressure was investigated and can be denied in the range of 0.1–2 bar. The necessary backwash volume in l/m2 was constant over this pressure range. Only the regeneration time decreased, which, relative to the filtration time, is considered insignificant.