Filter Cake Dislodgement in Gas Cleaning

In process engineering, filtration is often applied to separate small particles from gases. While the gas flows through a flexible, porous filter media, the particles are collected on the filter surface where they form a filter cake. While particles accumulate on the filter surface, flow resistance increases and intermittent filter cleaning is required to continue operation. Cleaning is often accomplished by initiating short termed high pressure air pulses in the opposite gas flow direction. Thereby the filter media is accelerated together with the deposited filter cake. At some point the filter cake is dislodged from the filter media and it breaks. Overall filter cleaning is, however, ineffective, when these filter cake patches or particles are not withdrawn from the filter but are re- deposited on the filter media once particle separation continues right after the air cleaning pulse. (Particle removal from the filter unit is generally accomplished by gravitational sedimentation of the filter cake patches and their withdrawal at the bottom of the filter unit.) Filter cake sedimentation is enhanced when cake patches settle like a curtain, i.e., when the filter cake patches are rather large and aligned vertically. Filter cleaning is investigated to understand filter cake dislodgement and its breakage, dependent upon duration and pressure of the cleaning pulse and the properties of the filter cake. Special attention is paid to the velocity of these cake patches and their positions after the cleaning pulse was initiated. Experiments are performed where filter cakes are produced from different gas-solid mixtures and under different conditions. These filter cakes are exposed to reverse air jet pulses and the resulting filter cake patches are measured optically in terms of size and location. Patterns are impressed locally on filter media by e.g. annealing to change cake formation locally. During subsequent cleaning cake breakage is likely to occur along these patterns where the filter cake interconnectivity is weaker. Existing mathematical models are applied and expanded, and simulations are performed to mimic the experimental situation and verify the models. Triggered by model simulations, experiments are performed with parameters where cake dislodgement is ensured, and the resulting cake patches are expected to have a rather uniform and large size and the distance from the filter media is constant. This information is used to visualize the optimization potential in filter design and operation.

Filter Cake Dislodgement in Gas Cleaning During many technical but also naturally occurring processes, e.g., wood burning, erosion of soil or cement production fine particles are air borne. These particles represent a valuable product or they pose a hazard which are subject of removal. Particle removal is accomplished effectively by sucking the particle-laden gas through a porous media, generally termed filter media, where the solid particles are withheld forming a filter cake. This growing filter cake results in an increasing flow resistance and thus the filter is subject of periodic cleaning. Periodic cleaning is accomplished by reversing the regular gas flow for a short time period. Thereby the filter cake is detached from the filter media and small filter cake fragments are formed. Subsequently, for a successful overall particle removal process, these cake fragments must settle by gravity. This short reverse gas flow is also termed reverse pressure pulse which must be sufficiently strong to guarantee that the filter cake is actually detached from the filter media. However, if the pulse is too strong, the filter media can be damaged and the cake fragments become very small and thus the subsequent sedimentation is hampered. On the other hand, too much energy is wasted when the pressure pulse lasts too long and the cake fragments are propelled to neighboring filter elements instead of being removed. The cake cleaning measure is supported additionally by the filter media that deforms elastically under pressure exposure. Thereby the filter cake fragments are basically ejected when the filter media reaches its limit of elasticity. By means of a synchronized stereo high-speed camera system, the process of filter cleaning is observed experimentally starting from a filter cake formed on a circularly fixed, elastic filter media sample: When the pressure pulse rises very rapidly, both, cake and filter media move with the same speed and only once the elasticity limit of the filter media is reached, the filter cake disconnects apparently from the filter media. Thereby many particularly small cake fragments are formed that move away from the filter media. When, however, the pressure pulse increases more slowly, filter cake liberates from the filter cake already before the elasticity limit of the filter media is reached, namely at a critical pressure level. Under these conditions the cake fragments are significantly larger which makes them settle faster. Practical experience is in line with this experimental and theoretical investigation suggesting that filter cake cleaning is more efficient when the maximum pressure level is lightly above the critical one and the pressure increase rate is not too high to allow the filter cake to detach already before the limit of the elasticity of the filter media is reached.