Difficulty: Medium
Correct Answer: Filter medium resistance increases (e.g., due to blinding or fines migration).
Explanation:
Introduction / Context:
After a cake forms, a wash step is often used to displace mother liquor. Engineers must anticipate how hydraulic resistance evolves during washing to size pumps and set wash times. While detailed behavior depends on cake compressibility and chemistry, field practice observes that resistances rarely decrease spontaneously during washing.
Given Data / Assumptions:
Concept / Approach:
Hydraulic resistance is the sum of the medium resistance and cake resistance. Washing can drive fines deeper into the medium and alter pore structures, effectively increasing the medium resistance. Compressible cakes often densify under sustained pressure, raising cake resistance. Therefore, statements that “all resistances remain constant” or “resistances decrease” are generally not reliable. The safest generalisation is that resistances tend to increase or, at best, remain similar; among the provided options, the most defensible trend is an increase in medium resistance via blinding/migration during washing.
Step-by-Step Solution:
Verification / Alternative check:
Pilot washing often shows higher pressure drop for a given wash flux compared to initial filtration, indicating increased effective resistance.
Why Other Options Are Wrong:
Common Pitfalls:
Assuming cleaner liquor automatically implies lower resistance; structure and fouling dominate hydraulics.
Final Answer:
Filter medium resistance increases (e.g., due to blinding or fines migration).
Discussion & Comments