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Module 3: Characteristics of Particles - Collection Mechanisms
Introduction
Inertial Impaction and
Interception
Brownian Diffusion
Gravitational Settling
Electrostatic Attraction
Thermophoresis and
Diffusiophoresis
Combined Effect of Collection
Mechanisms
Practice
Problems
Objectives
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Describe the six main particle collection mechanisms used in
particulate control systems including factors that influence their
collection efficiency.
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Identify the particle size ranges where the particle collection
mechanisms are most efficient.
- Calculate the terminal settling velocity of particles.
When sunlight streams into a quiet room, particles of many different shapes and sizes can be seen, some appear to float while others slowly settle to the floor. All of these small particles are denser than the room air, but they do not settle very fast. The solid, liquid, and fibrous particles formed in air pollution sources behave in a manner that is very similar to standard household dusts and other familiar particles. What we instinctively understand about these everyday particles can be applied in many respects to the particles from air pollution sources.
There are, however, two major differences between industrially generated particles and those in more familiar settings. The industrial particles are much smaller than most household particles. Also, some industrial particles have complex chemical compositions and include compounds and elements that are known to be toxic.
Emission testing devices and air pollution control systems apply forces to the particles in order to remove them from the gas stream. These forces are basically the "tools" that can be used for separating particles from the gas stream.
All of these collection mechanism forces are strongly dependent on particle size:
- Inertial impaction and interception
- Brownian diffusion
- Gravitational settling
- Electrostatic attraction
- Thermophoresis
- Diffusiophoresis
Inertial Impaction and Interception
Due to inertia, a particle moving in a gas stream can strike slowly moving or stationary obstacles (targets) in its path. As the gas stream deflects around the obstacle, the particle continues toward the object and impacts it. The obstacle may be a water droplet as shown in the Figure below.
Two primary factors affect the probability of an impaction occurring: (1) aerodynamic particle size and (2) the difference in velocity between the particle and the obstacle. Larger particles are collected more easily than smaller particles due to their greater inertia. Also, collection efficiency increases as the difference in velocity between the particle in the gas stream and the obstacle (or target) increases.
Inertial impaction is analogous to a small car riding down an interstate highway at 65 mph and approaching a merge lane where a slowly moving truck is entering. If the car is unable to get into the passing lane to go around the merging truck, there could be an "impaction" incident. Larger cars will have more difficulty going around the truck than smaller cars. Also, the faster the car is going relative to the truck, the more probable is an impaction.
The efficiency of impaction is directly proportional to the impaction parameter shown in Equation 1. As the value of this parameter increases, the efficiency of inertial impaction increases. This parameter is related to the square of the Stokes particle diameter and the difference in velocity between the particle and the target droplet.
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Where:
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Where:
The Cunningham slip correction factor (also called Cunningham's correction factor) accounts for molecular slip. Molecular slip occurs when the size of the particle is of the same magnitude as the distance between gas molecules. The particle no longer moves as a continuum in the gas, but as a particle among discrete gas molecules thereby reducing the drag force. For particles in air with actual diameters of 1.0 micrometer and less, the Cunningham correction factor is significant (see Appendix B, Cunningham Slip Correction Factors for Air).
Inertial impaction occurs when obstacles (e.g. water droplets) are directly in the path of the particle moving in the gas stream. Sometimes the obstacle or target is offset slightly from the direct path of the moving particle. In this instance, as the particle approaches the edge of the obstacle, the obstacle may collect the particle through a process called interception. Interception is illustrated in Figure 2.
Inertial impaction and interception are usually highly efficient for particles larger than 10 micrometers. They become progressively less effective as the size decreases. Impaction is not efficient for particles less than 0.3 micrometers due to their low inertia.
Brownian diffusion becomes the dominant collection mechanism for particles less than 0.3 micrometer and is especially significant for particles in the 0.01 to 0.1 micrometer size range.
Very small particles in a gas stream deflect slightly when gas molecules strike them. Transfer of kinetic energy from the rapidly moving gas molecule to the small particle causes this deflection, called Brownian diffusion. These small particles are captured when they impact a target (e.g. liquid droplet) as a result of this random movement.
Diffusivity is a measure of the extent to which molecular collisions influence very small particles, causing them to move in a random manner across the direction of gas flow. The diffusion coefficient in the equation below represents the diffusivity of a particle at certain gas stream conditions.
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Where:
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Where:
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#1
- Based on Equation 2, what is the relationship between the rate of Brownian diffusion and particle size?
Particles in still air have two forces acting on them; (1) a gravitational force downward and (2) the air resistance (or drag) force upward. When particles begin to fall, they quickly reach a terminal settling velocity, which represents the constant velocity of a falling particle when the gravitational force downward is balanced by the air resistance (or drag) force upward. The terminal settling velocity can usually be expressed using Equation 3.
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Where:
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Where:
Note: Equation 3 is applicable for particles less than 80 micrometers in size (aerodynamic diameter) and having a Reynolds number [NRe(p)] less than 2.0 and a low velocity. Refer to the lesson on Flow Characteristics in Module 2 for more information about Reynolds numbers.
Example Problem 1.
Calculating the Terminal Settling Velocity
Calculate the terminal settling velocity for the following particles
in still air having a gas temperature of 20°C. Use an air
viscosity of 0.000182 gm/cm
sec and the slip correction
factors (Cc) provided. Assume in all cases that the
Reynolds numbers are less than 2.0.
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80-micrometer particle with a density of 1.0 gm/cm3,
Cc = 1.0 (Calculate the velocity in terms of ft/min.)
-
8.0-micrometer particle with a density of 1.0
gm/cm3,
Cc = 1.0 (Calculate the velocity in terms of cm/sec.)
-
0.8-micrometer particle with a density of 1.0
gm/cm3,
Cc = 1.05 (Calculate the velocity in terms of cm/sec.)
Solution:
Since the particles are in still air, the particle diameters are not greater than 80 micrometers, and the NRe(p) < 2, the following equation can be applied. Also, because the particles have a density of 1gm/cm3, the aerodynamic diameter is equivalent to the Stokes diameter.
Part i
For an 80-micrometer particle,
Part ii
For an 8.0-micrometer particle, vt = 0.191 cm/sec since the particle diameter has changed by a factor of 10 and the diameter is raised to the power of 2.
Part iii
For a 0.8-micrometer particle, the equation will be used again since the Cunningham slip correction factor has changed.
This problem illustrates the large difference in settling rates for particles in this common size range. Particles of 0.8 micrometer and less have essentially negligible settling rates. The terminal settling velocities calculated indicate that even 80-micrometer particles settle at the relatively slow velocity of approximately 37.7 ft/min. A 0.8-micrometer particle settles at the extremely slow rate of 0.002 cm/sec. Obviously, very large control devices would be necessary if the gravitational force were the only tool available for the removal of particulate matter.
Due to the low settling velocities of essentially all particles less than 80 micrometers (aerodynamic diameter), gravitational settling of particles is not used for the initial separation of particles from the gas stream. However, this does not mean that gravitational settling is unimportant. Settling by gravity plays an important role in the removal of large clumps of dust when fabric filter bags and electrostatic precipitator collection plates are cleaned.
In air pollution control, electrostatic precipitators (ESPs) use electrostatic attraction for particulate collection. Electrostatic attraction of particles is accomplished by establishing a strong electrical field and creating unipolar ions. The particles passing through the electrical field are charged by the ions being driven along the electrical field lines. Several parameters dictate the effectiveness of electrostatic attraction including the particle size, gas flow rate, and resistivity.
The particles will eventually reach a maximum or saturation charge, which is a function of the particle area. The saturation charge occurs when the localized field created by the already captured ions is sufficiently strong to deflect the electrical field lines. Particles can also be charged by diffusion of ions in the gas stream. The strength of the electrical charges imposed on the particles by both mechanisms is particle size dependent.
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#2
- A small and a larger particle pass through a strong electrical field. Which particle is more strongly affected by electrostatic attraction and migrates faster toward a collection plate?
Figure 4 provides a typical curve showing particle collection efficiency as a function of the particle size. The particles in this case are fly ash.
Electrostatic precipitators are designed based on the flow rate of the flue gas to be treated. Flow rates that exceed design specifications may result in increased penetration of particulate matter. Flow rates that are too low may allow particulate matter to drop out of the gas stream near the inlet of the ESP and lead to buildup of particulate matter in the ducts.
Resistivity is a measure of the ability of the particle to conduct electricity and is expressed in units of ohm-cm. Particles with low resistivity have a greater ability to conduct electricity (and higher electrostatic attraction) than particles with high resistivity. The following factors influence resistivity:
- Chemical composition of the gas stream
- Chemical composition of the particle
- Gas stream temperature
Figure 5 shows a resistivity curve for particulate matter in a particular gas stream. For this example a gas stream temperature of 300°F to 400°F will provide poor collection efficiency. Generally, the preferred resistivity range is 108 to 1010 ohms-cm.
Thermophoresis and Diffusiophoresis
Thermophoresis and diffusiophoresis are two relatively small forces. Thermophoresis is particle movement caused by thermal differences on two sides of the particle. Gas molecules at higher temperatures have greater kinetic energy than those at lower temperatures. Therefore, when the particle collides with a gas molecule from the hotter side, the particle receives more kinetic energy than when it collides with a gas molecule from the cooler side. Accordingly, particles tend to be deflected toward the colder area.
Diffusiophoresis is particle movement caused by concentration differences on two sides of the particle. When there is a strong difference in the concentration of gas molecules on two sides of the particle, there is a difference in the number of molecular collisions, which causes an imbalance in the total kinetic energies of the gas molecules. Gas molecules in the high concentration area striking a particle transmit more kinetic energy to the particle than molecules in the lower concentration area. Therefore, particles tend to move toward the area of lower concentration.
Diffusiophoresis can be important when the evaporation or condensation of water is involved since these conditions create substantial concentration gradients. The normal differences in pollutant concentration are not sufficient to cause significant particle movement.
Combined Effect of Collection Mechanisms
Due to the combined action of the various collection mechanisms described previously, the performance of particulate control devices often has the Particle Size - Collection Efficiency Relationship shown in Figure 6. Above 100 micrometers, particles are collected with very high efficiency by inertial impaction, electrostatic attraction, and even settling due to gravity.
Efficiency remains high throughout the range of 10 to 100 micrometers due to the inertial and/or electrostatic forces (depending on type of collector). Both inertial and electrostatic forces are approximately proportional to the square of the particle diameter. For particles less than 10 micrometers, the limits of inertial forces and electrostatic forces begin to become apparent, and the efficiency drops. Efficiency due to these collection mechanisms reaches negligible levels between 3 and 0.3 micrometers depending on the factors such as gas velocities (inertial forces) and electrical field strengths (electrostatic attraction).
Below 0.3 micrometer, Brownian diffusion begins to become effective. Accordingly, the overall efficiency curve begins to rise in this very small size range.
The result of these various collection mechanisms is a potentially low collection efficiency for particles in the 0.1 to 0.5 micrometer range. In many control devices, none of the collection mechanisms are highly efficient for particles in this range. These particles can be classified as "difficult-to-control" due to the inherent limitations of the collection mechanisms.
This relationship, which can be seen in a number of studies of actual sources, indicates that stationary sources generating high concentrations of particles in the 0.1 to 0.5 micrometer range may be an especially challenging control problem.
The extent to which this gap in particulate control capability actually exists varies substantially among the types of particulate control systems. The gap is most noticeable in wet scrubbers and electrostatic precipitators. Fabric filters generally have a very minimal decrease in overall efficiency in this range due to the multiple collection mechanisms inherently present. Cyclonic collectors are generally inefficient for particles less than 1 to 3 micrometers. Therefore, the left side of the efficiency curve in the Figure is irrelevant.
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#3
- Can you think of any ways to improve the collection efficiency of particles in the difficult-to-collect range of 0.1 to 0.5 micrometers?
Practice Problems
Particle Formation
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Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.