Air Pollution MCQ with answer and solution

Concept

Brownian diffusion capture in fabric filters

Formula

Pe = d_f × u / D

Step-by-step solution

1. Particulate collection in fabric filters operates via several physical mechanisms depending on particle size. 2. Large particles (> 1 μm) are collected primarily by inertial impaction and direct interception. 3. For very fine submicron particles (< 0.1 μm), molecular bombardment creates significant random Brownian motion. 4. This Brownian diffusion brings the fine particles into contact with filter fibers where they are captured. Answer: C

Explanation

Very small submicron particles with diameters below 0.1 micrometers exhibit significant Brownian motion due to continuous collisions with surrounding gas molecules. This erratic random motion causes them to diffuse across streamlines and collide with filter fibers, making diffusion the primary capture mechanism for ultrafine dust in fabric filters.

Why the other options are wrong

A) Inertial impaction dominates for heavier particles larger than 1 to 2 micrometers that fail to follow curving streamlines. B) Gravitational settling is negligible for particles under 1 micrometer inside normal filter velocities. D) Sieving only occurs after a thick dust cake has accumulated and fails to capture isolated ultrafine particles in a clean medium.

Shortcut

< 0.1 μm = Brownian diffusion; > 1 μm = Inertial impaction; Cake layer = Sieving.

Common mistake

Believing mechanical sieving captures ultrafine particles when clean fabric pores are orders of magnitude larger.