In a washing-type plate-and-frame filter used for solid–liquid separation, what is the typical ratio of the washing rate to the final (end-of-cycle) filtration rate under comparable operating conditions?

Introduction / Context:
In chemical engineering practice, plate-and-frame filters are frequently operated in a sequence that includes initial filtration, washing of the filter cake to remove mother liquor, and then air-drying or blowdown. A common rule-of-thumb concerns the recommended washing rate relative to the final filtration rate near the end of the cycle. This ratio helps ensure that fresh wash liquid effectively displaces interstitial liquor within the porous cake without excessive channeling or breakthrough.

Given Data / Assumptions:

• Washing is performed after cake formation using the same hydraulic path.
• “Final filtration rate” refers to the filtrate rate towards the end of cake build-up (highest resistance period).
• Comparable viscosity and temperature of the filtrate and wash liquid.

Concept / Approach:
Washing effectiveness depends on achieving sufficient superficial velocity through the cake to minimize axial dispersion and bypassing. Empirical design practice typically recommends a modestly higher washing rate than the terminal filtration rate so the wash front advances uniformly, improves displacement, and reduces residual solute in the cake pores.

Step-by-Step Solution:

Verification / Alternative check:
Operating manuals and classical filtration heuristics indicate 1.2–1.3 times as a practical range that balances improved displacement with pump energy and mechanical constraints of the filter pack.

Why Other Options Are Wrong:

• 0.25 or 0.50: Too low; insufficient velocity causes poor displacement and channeling.
• 0.75: Still below final rate; rarely adequate for effective displacement washing.

Common Pitfalls:
Assuming much higher rates always improve washing; excessive flow can fluidize or crack the cake, lowering washing efficiency. Temperature and viscosity matching between filtrate and wash also matter.

Final Answer:
1.25