Question

The filter aid is a granular or fibrous material and is used to

• A. Increase the cake resistance
• B. Increase the cake porosity
• C. Increase the pressure drop across the cake
• D. Increase the filter medium resistance

Hint:

Filter aids (e.g., diatomaceous earth) increase the porosity of the filter cake, improving filtration rates by allowing easier passage of the filtrate.

Answer 1

The Correct Option is B

Step 1: Understand the role of a filter aid in filtration.
A filter aid is a granular or fibrous material (e.g., diatomaceous earth, perlite) added during filtration to improve the process. In filtration, a slurry is passed through a filter medium, forming a cake of solids on the medium while the liquid (filtrate) passes through. The filter aid is typically added to the slurry or pre-coated on the filter medium to enhance filtration performance.
Step 2: Analyze the purpose of a filter aid.
The primary purpose of a filter aid is to improve the filtration process by:
Increasing cake porosity: Filter aids are highly porous materials. When mixed with the slurry or pre-coated on the filter medium, they form a more porous filter cake. A more porous cake allows the filtrate to flow through more easily, preventing clogging and maintaining a higher filtration rate.
Preventing blinding: Filter aids help prevent fine particles from clogging (blinding) the filter medium, which would otherwise reduce flow.
Filter aids do not aim to increase resistance or pressure drop; rather, they aim to facilitate flow by making the cake less dense and more permeable.
Step 3: Evaluate the options.
(1) Increase the cake resistance: Incorrect, as filter aids reduce cake resistance by increasing porosity, making it easier for the filtrate to pass through. Incorrect.
(2) Increase the cake porosity: Correct, as filter aids are used to create a more porous cake, improving flow and filtration efficiency. Correct.
(3) Increase the pressure drop across the cake: Incorrect, as filter aids typically reduce the pressure drop by making the cake more permeable, not less. Incorrect.
(4) Increase the filter medium resistance: Incorrect, as filter aids are used to protect the filter medium and reduce its resistance to flow by preventing clogging. Incorrect.
Step 4: Select the correct answer.
The filter aid is used to increase the cake porosity, matching option (2).