Filter cake, liquid entrained

Filtration A process in which a slurry of solid particles suspended in a liquid passes through a porous medium. Most of the liquid passes through the medium (e g., a filter) to form the filtrate, and the solids and some entrained liquid are retained on the filter to form the filter cake. Filtration may also be used to separate solids or liquids from gases. 

Figure 7. Estimated total adipic acid losses due to chemical degradation, coprecipitation or occlusion in the solids, and liquid entrained in the filter cake vs. percent oxidation. Key a, total loss of adipic acid b, loss of adipic acid due to chemical degradation c, loss of adipic acid due to coprecipitation or occlusion in the solids and d, loss of adipic acid in liquid entrained in the filter cake solids.

The washing analysis presented here is for a steady state, completely mixed system where the wash water and the water entrained by the solid matrix are in equilibrium When filter cakes are washed, the operation is batch, the system is not well mixed because flow is close to plug flow, and the operation is not at equilibrium since the entrained fluid has to diffuse into the wash liquid. This case is analyzed by Harrison et al. 20031. 

Separation Efficiency. Similarly to other unit operations in chemical engineering, filtration is never complete. Some solids may leave in the liquid stream, and Some liquid will be entrained with the separated solids. As emphasis on the separation efficiency of solids or liquid varies with application, the two are usually measured separately. Separation of solids is measured by total or fractional recovery, i.e., how much of the incoming solids is collected by the filter. Separation of liquid usually is measured in how much of it has been left in the filtration cake for a surface filter, i.e., moisture content, or in the concentrated slurry for a filler-thickenef. i.e., solids concentration. 

The second mixer is charged with the spent leaf from the first mixer and with the filtrate from the previous batch in the third mixer. The leaf is extracted for several more hours, and the contents of the mixer are then discharged to a second filter. The filtrate, which contains 1% of the leaf fed to the second mixer, is pumped to the same holding tank that received the filtrate from the first mixer, and the solid cake—spent leaf and entrained liquid—is sent to the third mixer. The entrained liquid mass is 15% of the mass of liquid charged to the second mixer. 

The contents of the filtrate holding tank are filtered to remove the carried-over spent leaf, and the wet cake is pressed to recover entrained liquid, which is combined with the filtrate. A negligible amount of liquid remmns in the wet cake. The filtrate, which contains D, E, and W, is pumped to an extraction unit (another mixer). 

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