Deriving cake detachment versus cake area load in a jet pulsed filter by a mechanistic model

Abstract

Dust particles in gas streams are frequently removed through cake filtration on rigid or flexible filter media. The cake is mostly removed by jet pulses. There patchy cleaning is a typical phenomenon, which means that a patch of a filter cake is torn off by the pressure pulse completely whereas the rest of the cake remains intact on the filter. Pilot plant and large-scale filters are usually divided in compartments where only one compartment is cleaned at a time. Often only some of the compartments, i.e. only part of the total filter area is cleaned at the end of one filter cycle. In this study, a model is proposed that mirrors this behaviour from the start up of the filtration process with a clean filter medium through division of the filter in model areas. Each model area represents one cake of uniform thickness and a fraction of this cake is removed altogether at the end of a filter cycle, provided this cake is exposed to a jet pulse. In order to gain a true detachment function of the pulsed cake (i.e. the removed cake fraction versus cake thickness), the single compartments are treated separately in this study, so that an intrinsic detachment function of the filter cake can be decided from pilot plant filter experiments. These genuine cake detachment functions are typical of one filter operating point only, although the pulse reservoir pressure, the duration of a pulse and the cleaning system are identical. This implies that the cleaning effect of a jet pulse on the filter depends on the cake mass and its distribution on the filter but also cake compaction and cohesive effects could be liable for the discrepancies in the cake detachment functions determined. A by-product of the model application is an estimate of the total solid hold up on the filter cloth, which is in excellent agreement with the experiments presented here. Also the distribution of the solid load and gas velocity over the filter area are decided as well as the residence time distribution of the solid in the filter cake. These are key factors for modelling the filter as a chemical gas solid reactor. The model application can give an estimate, to which extent the solid concentration in the gas of jet pulsed filters is increased during filter cleaning due to disintegration of removed cake. This phenomenon is known as redeposition or reentrainment of detached cake.