Chapter 21 - Ventilation And Pollution Control

Question 1

What is an environmental pollutant?

Answer 1

Environmental pollution is any discharge of material or energy into land, water or air that may damage the earth’s ecological balance or lower the quality of life.

Question 2

What are the four general approaches to pollution control?

Answer 2

the 4 general approaches to pollution control are - intermittent reduction of industrial activities during periods of high air pollution conditions - wider dispersion of pollutants using such devices as taller smokestacks - reduction of pollutants industrial emissions. - change of an industrial process or activity in order to reduce overall pollution

Question 3

What is the preferred approach to pollution control?

Answer 3

Most effective and preferred approach is to change the process or activity to produce less pollution.

Question 4

What organization sets rules for handling dangerous materials on the workplace?

Answer 4

Joint federal and provincial workplace hazardous materials information system also sets of rules for handling dangerous materials.

Question 5

What types of equipment are used to control pollutants?

Answer 5

Types of control equipment for pollutants maybe broadly classified in several types: filters electrostatic precipitators cyclones mechanical collectors scrubbers adsorbers burners (including incinerators, afterburners, and catalytic combustion) ultraviolet light treatment (for sewage effluent).

Question 6

Describe the primary treatment of waste water

Answer 6

Primary wastewater treatment involves such physical techniques as screening out large debris, skimming off floating debris, and settling out suspended material in tanks called primary clarifiers

Question 7

Describe the secondary treatment of waste water

Answer 7

If there is acid effluent, it must be neutralized with chemicals such as lime before flowing into storage with primary effluents. This is done because secondary treatment must work in a neutral pH environment (that is, not acid or alkali). The effluents flow through a lift station to a mix tank. In the mix tank, the pH is readjusted with chemicals. Temperature is also controlled, Various products are added to supply the chemicals needed in secondary treatment

Question 8

What does advanced treatment of waste water remove?

Answer 8

Advanced treatment of waste may involve biological, chemical, or physical methods. These methods remove: • nutrients that promote excessive growth of algae in water -industrial pollutants such as heavy metals and non-biodegradable organic materials.

Question 9

Why are extremely small air-borne particles very dangerous?

Answer 9

Extremely small particles are the most dangerous because they can penetrate deeply into the lungs.

Question 10

What four techniques are used to remove particulate matter from air?

Answer 10

cyclone separator-This causes air emissions to whirl around, forcing heavy particles to the outside and ultimately to removal below. wet scrubber—This washes particulates out of the exhaust with a water spray. electrostatic precipitator-This electrically charges the particles and attracts them toward a charged plate to remove them. baghouse—This operates like a vacuum cleaner, trapping particles in fabric filters placed in the exhaust stream.

Question 11

What is the size of particulates in oil smoke?

Answer 11

0.05 to 1 microns

Question 12

What size particulates will cloth collectors remove?

Answer 12

.001 to 80 microns

Question 13

How is Sulphur dioxide removed from boiler flue gas?

Answer 13

Sulphur dioxide may be removed from exhaust gases using devices called scrubbers. Limestone scrubbers have a removal efficiency of up to 90% of Sulphur dioxide.

Question 14

What are sources of nitrogen oxide pollution?

Answer 14

Nitric acid mfg., high temperature oxidation using air, motor vehicles, nitration

Question 15

What mechanisms are used to collect solid pollutants from air?

Answer 15

In general, there are six mechanisms for collecting solid pollutants from air (or other gaseous fluids). They use the effects of gravitational, centrifugal, inertial, and electrostatic forces, direct interception, and diffusion.

Question 16

What special problems are present with wet particulate collectors?

Answer 16

Wet collectors present special problems: • As they eliminate air pollution, they can create water pollution, Ways to dispose of the resultant contaminated liquid or sludge must be found. • They are subject to corrosion. -They may freeze in cold weather

Question 17

Describe the operating principles of gravity settling chambers

Answer 17

In gravity settling chamber, a gas stream containing particulates enters a large chamber through a relatively small inlet duct. The gas travels swiftly through the narrow duct but slows down when it enters the large chamber. Gravity causes the particles to fall to the bottom of the chamber, This is a dry collector.

Question 18

What are the advantages and disadvantages of gravity settling chambers?

Answer 18

The advantages of gravity settling chambers are: -very low energy costs • low maintenance costs • low installation costs • excellent reliability • high efficiency even when individual chambers are full, The disadvantages are: -Settling chambers are very large. They are not used very often because of their space requirements, • They may be used only for coarse particulates.

Question 19

Describe the operating principles of gravity spray towers

Answer 19

In a gravity spray tower, as particles ride upward on a gas stream, they are trapped by much larger drops of liquid (usually water) sprayed from above, and fall to the bottom. This is a wet collector that uses gravity, They work well with larger particles (down to 2 microns) particular liquid ones.

Question 20

Describe the operating principles of a cyclone separator

Answer 20

Cyclones rely on inertial and centrifugal forces as well as gravity. A gas stream rushes into a conical (cone shaped) chamber and the design of the equipment causes the gas to spin. The shape of the spinning gas is called a vortex. Centrifugal force throws the particulates outward to the walls of the chamber, separating them from the gas stream.

Question 21

What is the advantage of a multi-clone collector in comparison to a single large capacity unit?

Answer 21

Efficiency is also affected by variable dust densities (loads): the more dust in the air, the less efficient the cyclone.

Question 22

Describe the operating principles of centrifugal spray scrubbers

Answer 22

In a centrifugal spray scrubber, the dirty air enters through a tangential duct, spins upward, and meets a series of sprays located around the lower part of the cylinder. The particulates are caught on the droplets in the sprays and washed out of the stream or hurled against the cylinder wall and washed down and out.

Question 23

Describe the operating principles of a cloth filter (baghouse) system

Answer 23

In operation, the gas stream carrying particles passes through a woven fabric that catches the particles.

Question 24

What are the applications for baghouse filters?

Answer 24

They are used to trap particulate emissions from many industrial installations such as Portland cement kilns, iron foundry cupolas, primary steel-production furnaces.

Question 25

Are baghouse filters usually regarded as primary or secondary filters?

Answer 25

Usually, a baghouse is regarded as a secondary separator. No matter how well designed a cyclone is, small particles always escape to the atmosphere. Baghouses act as secondary separators.

Question 26

What are the advantages and the disadvantages of the baghouse system?

Answer 26

The advantage of a baghouse is that it can remove close to 100% of particles as fine as 0.4 microns. -The bags are a fire hazard when used with combustibles such as flyash from a power boiler. Even with carefully chosen fabrics, very hot gases must be cooled before they are filtered -The bags are affected by humidity and by chemicals. They must be replaced after, at most, five years.

Question 27

Describe three types of baghouse systems

Answer 27

• unit collectors • single wall -double wall.

Question 28

Describe two methods of reconditioning the bags

Answer 28

As bags become clogged, airflow diminishes and the bags must be cleaned. This is called reconditioning. Many baghouses are divided into sections that can be cleaned separately to avoid shut-down.

Question 29

Describe the operating principles of an electrostatic precipitator

Answer 29

Electrostatic precipitators capture particles through the use of electrodes. The negative electrode in older types of precipitator is usually a small diameter metal wire. These can erode and be blown about by fan draft, causing short circuits and downtime. Most modern precipitators have schedule 20 mild steel pipe or tubing with a short wire electrode. The positive electrode is an electrically grounded metal plate. In operation, a strong electric charge from the negative electrode sets up a one-directional electric field. The particles passing through the field pick up the charge and are drawn to the positively charged collecting surface.

Question 30

How do wet and dry electrostatic precipitators collect particulate matter?

Answer 30

In wet electrostatic precipitators, particles are removed by washing them off the electrode plate.

Question 31

Describe the operating principles of dry and wet dynamic precipitators

Answer 31

In dry dynamic precipitators, the particle-laden gas stream is driven against a series of fan blades shaped to catch the particles. The particles are deposited in a hopper. The cleaned air continues around, up, and out. In wet dynamic precipitators, a film of water containing particles is constantly sprayed on a moving surface such as the blades of a fan.

Question 32

Describe the operating principles of a Venturi scrubber

Answer 32

In a venturi scrubber, the gas stream is pushed (usually by a fan) through a venturi tube. This is a short tube with a constricted middle section that increases the velocity of the gas. A coarse spray of water is injected at the throat of the tube through radial jets. The spray is atomized by the rapidly moving gas, It forms droplets about 50 microns wide. These droplets collide with and trap the dust particles in the gas. A cyclone separator usually follows a venturi scrubber,

Question 33

What are the advantages and disadvantages of Venturi scrubbers?

Answer 33

Venturi scrubbers can remove 97 to 99% of particles as fine as 0.5 microns. However, the disadvantages are: -resulting waste water must be treated -noise -high power requirements.

Question 34

Describe the operating principles of s wetted filter scrubber

Answer 34

Wetted filter scrubbers pass the gas and liquid through a medium (sometimes called packing) with small openings. A filtering process occurs and the particulate sticks temporarily to the filter. A series of filters or packings is used. This process is used when there is a small amount of particulate.