Cake washing behavior — resistance trends:\nDuring the washing of a formed filter cake under steady conditions, which statement best reflects what typically happens?

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:

  • Existing cake on medium; washing at roughly constant pressure/temperature.
  • Possibility of fines migration and partial blinding.
  • Cake is not dissolving significantly during wash.


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:

Identify components: R_total = R_medium + R_cake.Anticipate fines migration → R_medium increases.Select option (b) as the best general statement.


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:

  • (a) Unrealistic for compressible cakes and fouling media.
  • (c) Decrease is uncommon absent active cleaning or dissolution.
  • (d) Cake resistance usually increases under sustained pressure.


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).

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