Process design of horizontal vacuum belt (HVB), drum (DF) or hyperbaric disk filters (HBF) is often based on laboratory tests and data evaluation according to VDI guideline 2762. For a suspension sample, the transient cumulative filtrate mass is recorded for filter cake formation and subsequent, often uninterrupted mechanical deliquoring via gas purge for defined operating conditions, i.e., pressure difference and temperature.

Filter cake formation starts when the first particles deposit on the filter media. Particles may adhere on the filter media as they come in contact by random (Brownian) motion or as they experience a directed force field, i.e., gravity resulting to sedimentation and by a directed flow of the liquid through the fluid-permeable filter media driven by an externally applied pressure difference – vacuum or over-pressure – and/or the hydrostatic head of the suspension itself.

By definition, filter cake formation is finished when the entire supernatant suspension has just vanished but the cake saturation is still at S=1. Moreover, subsequent liquid removal from the filter cake is immediately accompanied by a reduction of S. Traditionally, the surface of the supernatant suspension is observed optically by naked-eye through a top inspection window and, it is related to the end of cake formation.

In addition of measuring the cumulative filtrate mass by a scale, a laser distance meter is positioned above the filter cell that is capable of measuring the transient distance to the surface of the supernatant suspension through the inspection window and - once the suspension has vanished - the filter cake, respectively. The geometry of the filter cell together with the transient distance allows to calculate the transient cumulate volume and subsequently the filtrate mass passing through the filter media. While the filtrate mass experiences an accumulation in the downstream filter cell assembly resulting in a skewed recording of the filtrate mass by the scale, the laser measurement is perfectly synchronized. The laser measurement is used to also identify the end of cake formation which interferes with a distinct distance peak that is, however, not fully conclusive yet.

Three markedly different aqueous suspensions were investigated, i.e., limestone, cellulose, and talcum, respectively, that gave concordant insights: The filtrate that exits the filter cake arrives at the scale with some time delay that is almost constant. Based on the optical measurement data, the resulting...