|That crack (lower left) could become a ravine (or a highway).|
Photo credit: peach
Here's a fairly detailed description of what happens when you wash a cake of product with some fresh solvent on large scale :
In almost all cases, the requirement for maximum wash efficiency is to develop a mechanism, which removes the required amount of mother liquor with a minimum of washing liquor. Basically there are only two mechanisms, and in practice usually both are present:
• Displacement of the mother liquor by washing liquor
• Dilution of the mother liquor through mixing with washing liquors
Displacement is by far the most efficient mechanism and under ideal conditions would cause the washing liquor to act as a piston driving the mother liquor out of the cake without any dilution. This is clearly utopia although a high degree of “piston or plug-flow effect” can be achieved with a careful preparation of the cake and a careful application of the wash liquor. The trick lies above all in the careful application of the washing liquor to the saturated face of the filter cake.What happens if you got the chance to force your rinse through the cake with pressure?
It must be clear, but is often ignored, that washing at too high a pressure differential only leads to poor efficiency. Too high a pressure almost always leads to bypassing. Although the ideal cake has an even structure, no pinholes, and no cracks and lies firmly bedded against the retaining walls, we are not living in an ideal world and to some degree all cakes will have these faults.
Forcing liquid through the cake at unnecessarily high pressures only makes matters worse. A pinhole will become a hole, a crack a ravine and if the cake does not bed down perfectly, the washing liquid will make a highway of the gap. [emphasis CJ's] In addition, high pressure differentials reduce the contact time. In all cases therefore the optimum pressure differential is the lowest possible.You didn't know that washing the filter cake in a Buchner funnel had so much behind it, did you? Leave it to the chemical engineers to understand this stuff.
 Barry A. Perlmutter, "A Treatise of Filter Cake Washing Mechanisms In Pressure and Vacuum Filtration Systems."