Module 3: Characteristics of Particles - Collection Mechanisms - Practice Problems

Features

Instructions:: Work these problems on a sheet of paper and check your answers against those provided below.

#1

Assume that an industrial process generates the particle size distribution given in Figure 7. The Figure has been divided into five different zones, four of which are labeled (a) through (d). Determine in which zone (or zones) each of the following particle collection mechanisms is usually highly efficient for particulate matter.

Inertial impaction
Electrostatic attraction
Gravitational settling
Brownian diffusion
No collection mechanisms are highly efficient

: Answer: i. Zones c and d; Inertial impaction is generally highly efficient for particles greater than 10 micrometers. Impaction also occurs for particles between 0.3 and 10 micrometer; however, the efficiency becomes progressively less as the diameter decreases.; Answer: ii. Zones c and d; Electrostatic attraction is also generally highly efficient for particles greater than 10 micrometers. The efficiency drops off as the diameter decreases below 10 micrometers.; Answer: iii. Zone d; Gravitational settling is only efficient for larger particles, which have higher terminal settling rates.; Answer: iv. Zone a; Brownian diffusion removes particles less than 0.3 micrometers and is highly efficient for particles between 0.01 and 0.1 micrometers.; Answer: v. Zone b; Collection efficiency for particles in the 0.1 to 0.5 micrometer range tends to be very low.

The shaded zone from 0.5 to 10 micrometers represents a gray area where the collection efficiencies for electrostatic attraction and impaction usually drop off. The zones (a), (c), and (d) represent particle size ranges where one or more collection mechanism operate at high efficiency.

#2

Select the collection mechanism that would most likely be used to collect the particles described in items (i) through (iv) below.

Collection Mechanisms

Impaction/Interception
Brownian diffusion
Gravitational settling
Electrostatic attraction

Very large particles (> 100 micrometers) and the accumulated clusters of particulate matter collected on some control devices
Particles with lower resistivity
PM_2.5
Supercoarse particles moving at high velocity

: Answer:

#3: Match the particle collection mechanism with its proper description.

: Answer:

#4: Name the particle size range that has the lowest collection efficiency even when all the collection mechanisms are acting together.

: Answer: 0.1 - 0.5 micrometers

#5

Use the data below to calculate the terminal settling velocities of spherical particles having the specified Stokes diameters and Cunningham slip correction factors. Assume that Equation 3 for calculating the terminal settling velocity applies in these cases.

Stokes particle diameter = 0.25 micrometer, C_c = 1.73
Stokes particle diameter = 25 micrometer, C_c = 1.0

: Answer: i. 0.00033 cm/sec; Solution:; For a 0.25 particle,; Answer: ii. 1.89 cm/sec; Solution:; For a 25 particle,

#6

Two wet scrubbers are described below. Using the data provided, determine which situation should exhibit better inertial impaction efficiency. Assume a gas viscosity of 1.8 10^-4 gm/cmsec. For both Scrubbers #1 and #2, the water droplets are moving in the same direction as the gas stream.