ENGINEERED SYSTEMS FOR AIR POLLUTION CONTROL
• The principles involved in the natural atmospheric cleansing processes.
3-1 ATMOSPHERIC CLEANSING PROCESSES
1- Dispersion.
2- Gravitational settling.
3- Flocculation.
4- Absorption (involving washout and scavenging).
5- Rainout.
6- Adsorption
1- Dispersion of pollutants by wind currents lessens the concentrations of pollutants in any one place.
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2- Gravitational settling
Particles smaller than 0.1 μm can be settled out by flocculation. Two particles bump together to form a unit, and the process is repeated until a small flock (زغب من القطن أو الصوف) particle is formed that will be large enough and heavy enough to settle out.
3- The natural absorption process, in this process particulates or gaseous pollutants are collected in rain or mist, then settle out with that moisture. This phenomenon, known as washout or scavenging. SO2 gas, which is simply dissolved into rain, falls with the droplets as SO2. However, SO2 may also react with rainwater to form H2SO3 (sulfurous acid) or H2SO4 (sulfuric acid) mists
4- Rainfall through uncontaminated air has a PH of 5.6 to 6.0. As noted earlier, acid rain runoff can cause extensive erosion of some surfaces (notably limestone) and can change the PH in streams and rivers, thereby influencing the species of algae which predominate in those streams.
5- Rainout rainout occurs within clouds when submicron particulates serve as condensation nuclei around which drops of water may form.
6- Adsorption occurs primarily in the friction layer of the atmosphere, the layer closest to the earth’s surface. In this phenomenon, gaseous, liquid, or solid contaminants are attracted (generally electrostatically) to a surface, where they are concentrated and retained.
• Effects of air pollution:
• Clothing is soiled.
• Particles are deposited on buildings and other surfaces.
• Plants are damaged.
• Visibility is reduced.
• Human respiratory problems are increased.
2 APPROACHES TO CONTAMINANT CONTROL
• There are two broad approaches to the control of particulate and gaseous contaminants— dilution of the contaminants in the atmosphere and control of the contaminants at their source.
1- Dilution , dilution of contaminants in the atmosphere can be accomplished through the use of tall stacks.
• Control at Source
• Control of pollutants at their source can be accomplished by several different means:
• 1- substituting an alternative power source (i.e., hydraulic, geothermal, or solar energy for fossil-fuel-derived energy)
• 2- The traditional fossil fuel can be replaced by another fuel of lower air pollution potential.
• 3- Coal or natural gas can be refined to desulfured, liquefied natural gas (LNG) or to liquid petroleum gas (LPG).
4- The proper use of existing equipment by the proper operation and maintenance of equipment.
5- The regular, competent automotive inspection and maintenance .
6- Industry can decrease emissions from stationary sources by proper operation and maintenance of equipment.
7- NOx emissions from ore- and gas-fired boilers have been reduced by modification of combustion conditions.
8- Changing the process being used . For example, replacing open-hearth furnaces with controlled basic oxygen furnaces or electric furnaces.
9- Controlling emissions of air pollutants at their sources is to install control equipment
10- Control devices are usually designed to control gaseous pollutants and particulates from stationary sources .
Control Devices for Particulate Contaminants
• Gravitational settling chamber.
• Centrifugal collectors.
• Wet collectors.
• Electrostatic precipitators.
• Fabric filters.
Particle characteristics as
1- Size distribution.
2- shape, density, stickiness, hygroscopicity.
3- Electrical properties.
4- Carrier gas properties.
Table 3-2 Industrial process and control summary
• 3-3 GRAVITATIONAL SETTLING CHAMBERS
• (3-1)
• (3-2)
Example 3-1: Designing a settling chamber Calculate the minimum size of the particle that will be removed with 100 percent efficiency from a settling chamber under the following conditions.
Air: Horizontal velocity is 0.3 m/s.
Temperature is 77°C.
Particle: Specific gravity is 2.0.
Chamber: Length is 7.5 m., Height is 1.5 m.
dp = 4.81x10-5 m
dp =48.1 µm
• 3-4 CENTRIFUGAL COLLECTORS
• Two general types of centrifugal collectors-cyclones and dynamic-precipitators are commonly used.
• Cyclones
• where Fc = centrifugal force, N
• Mp = particulate mass, kg
• centrifugal acceleration where vi equals particle velocity and R equals radius of the cyclone, m/s2 .
Determination of collection efficiency
The size of this particle is determined by the following equation based on work by Lapple.
where d50 = diameter of the particle that is collected with 50 percent efficiency, m
µ = gas viscosity, kg/m.s
b = width of cyclone inlet, m
Ne = number of effective turns within the cyclone
vi = inlet gas velocity, m/s
ρp = density of the particulate matter, kg/m3
Example 3-2: Determining particle removal efficiency in cyclones. An air stream with a flow rate of 7 m3/s is passed through a cyclone of standard proportions. The diameter of the cyclone is 2.0 m, and the air temperature is 77°C.
(a) Determine the removal efficiency for a particle with a density of 1.5 g/cm3 and a diameter of 10µm.
(b) Determine the collection efficiency based on the above if a bank of 64 cyclones with diameters of 24 cm are used instead of the single large unit
• Cyclone collectors are:
1- Relatively inexpensive to construct and operate.
2- They can handle large volumes of gases at temperatures up to 980°C.
3- Pressure drops across these units are generally low and range from 2.5 to 20 cm of water.
4- Cyclones have been used successfully at feed and grain mills, cotton gins, cement plants, fertilizer plants, petroleum refineries, asphalt mixing plants, and other applications involving large quantities of gas containing relatively large particles.
Dynamic Precipitators
Dynamic precipitators are compact units that impart a centrifugal force to the particulate by the action of rotating vanes, a force about seven times that of a conventional cyclone of the same capacity.
3-5 WET COLLECTORS
Wet collectors, or scrubbers, remove particulate matter from gas streams by incorporating the particles into liquid droplets directly on contact.
Table 3-3 Average pressure drops from well-designed collectors of several different types
Three of the wet collectors most commonly used for control of particulate matter are:
1- The spray tower.
2- The wet cyclone scrubber.
3- The venturi scrubber.
Spray Towers are:
1- Low-cost scrubbers that can be used to remove both gaseous and particulate contaminants.
2- Effective in removing
particles in excess of
10 μm.
Wet Cyclone Scrubbers
In a simple wet cyclone scrubber, high-pressure spray nozzles located in various places within the cyclone chamber generate.
For droplets of 100 μm, efficiency approaches 100 percent, and 90 to 98 percent removal is achieved for droplets between 5 and 50 µm.
Venturi Scrubbers
1- Venturi scrubbers are most efficient for removing particulate matter in the size range of 0.5 to 5 µm, that makes them especially effective for the removal of submicron particulates associated with smoke and fumes.
2- At velocities from 60 to 180 m/s, the contaminated gas passes through a duct that has a venturi-shaped throat section.
3-6 FABRIC FILTERS (BAGHOUSE FILTERS)
Filter bags are capable of removing most particles as small as 0.5 μm and will remove substantial quantities of particles as small as 0.1 μm. Filter bags ranging from 1.8 to 9 m long.
Cleaning fabric filters
1- Intermittent cleaning. Because intermittent cleaning means the unit must be shut down to prevent the discharge of raw gases directly into the atmosphere, that method is seldom used.
2- Periodic cleaning. In periodic cleaning portions of the filter device are shut down and cleaned for brief intervals, while the rest remains operational.
3- Continuously cleaning. In a continuous, automatic cleaning operation, cleaning of some parts of the filter occurs at all times. Continuously cleaned filters that use a traveling blow ring or reverse-air jets leave little filter cake in place at any time.
The problems associated with the use of fabric filters:
• The possibility of explosion or fire exists if sparks are discharged in a baghouse area where organic dusts are being filtered.
• Space limitations may prohibit use of baghouses large enough to handle heavy loads.
• There is always a slight possibility of rupture or other adverse effects because of temperatures too high for the fabric medium or because of the moisture, acidity, or alkalinity content of the particulate-laden gas stream. Judicious fabric choice can minimize these problems
Table 3-4 Summary of data on the common filter media used in industrial baghouses
Example 3-3: A fabric filter is to be constructed using bags that are 0.3 m in diameter and 6.0 m long. The baghouse is to receive 10 m3/s of air, and the appropriate filtering velocity has been determined to be 2.0 m/min. Determine the number of bags required for a continuously cleaned operation.
SOLUTION
• Determine the cloth area required.
2. The area of one bag is:
3. The total number of bags is
3-7 ELECTROSTATIC PRECIPITATORS (ESP)
Electrostatic precipitators can be classified as:
1- Low-voltage two-stage units, operate at 6000 to 12,000 V and are employed mainly in conjunction with air-conditioning systems for hospitals and commercial installations. They are used mainly to collect liquid particles and are not generally recommended for control of solid or sticky material.
2- High- voltage single-stage units, operate in the 30,000- to 100,000-V range and are used at large industrial plants such as coal-fired utility boilers.
Four basic steps are required in the operation of a high-voltage single-stage electrostatic precipitator such as the one pictured in Fig. 3-12:
(1) electrical charging of the particulates,
(2) collection of charged particles on a grounded surface,
(3) neutralization of the charge at the collector, and
(4) removal of the particulate for disposal.
1- The electrical charge is imparted to the particulate by passing the particles through a high-voltage direct-current corona.
2- The high-voltage field ionizes the gas molecules in the air stream, which in turn become attached to the particulate matter and give them a negative charge.
3- After being charged, the negative particles move toward the positive electrodes and are collected there.
4- Their charge is neutralized at the moment of collection, and they can be removed from the collection surface by rapping, washing, or plain gravity.
Advantages of electrostatic precipitators
1- They are extremely efficient (99 percent or higher) for a wide range of particle sizes; even submicron-size particles can be collected.
2- They can handle large volumes of gas-25 to 1000 m3/s.
3- They have low pressure drops, and can operate continuously with little maintenance.
4- They can be used to collect acid or tar mists, but they cannot be used with explosive materials.
Disadvantages of electrostatic precipitators
• Initial installation cost is high.
• Electrostatic precipitators generally require a great deal of space for an industrial operation.
• Higher efficiency levels nay be attained by increasing the surface area, but increases must be substantial in higher ranges.
• Electrostatic precipitation systems only operate at peak efficiency within a limited temperature range, and they may use excessive power if buildup of collected material causes “spark over”.
• They may also become inefficient if buildup of collected material suppresses the corona discharge from the negative electrode.
The size-efficiency relationship for an electrostatic percipitator is a curvilinear unction similar to that for a cyclo ne.
(3-5)
Example 3-4: An electrostatic precipitator is to be constructed to remove fly-ash particles from stack gases flowing at 10 m3/s. Analysis of a similar system shows that the drift velocity can be taken as: w = 3.0 x l05 dp m/s
Determine the plate area required to collect a 0.5 µm particle with
(a) 90 percent efficiency (b) 99 percent efficiency
SOLUTION
I. Using Eq. (3-6)
w = 3.0 x l05 dp m/s
= 0.15 m/s
2. Using Eq. (3-5) for 90 percent efficiency
CONTROL DEVICES FOR GASEOUS CONTAMINANTS
The principal gases of concern in air pollution control are:
1- The sulfur oxides (SOx).
2- Carbon oxides (especially CO).
3- Nitrogen oxides (NOx).
4- Organic and inorganic acid gases.
5- Hydrocarbons (HC).
Major treatment processes for control of these and other gaseous emissions include:
1-adsorption,
2- absorption,
3- condensation, and
4- combustion.
3-8 ADSORPTION
The pollution control process of gas adsorption involves passing a stream of effluent gas through a porous solid material (the adsorbent) contained in an adsorption bed. The surfaces of the porous solid material attract and hold the gas (the adsorbate) by either physical or chemical adsorption.
Table 3- 5 Comparison between Physical adsorption and chemical adsorption
Adsorbents
1- Alumina, bauxite, and silica gel have a higher affinity for water a polar vapor, they are used as drying agents.
2- Activated charcoal preferentially adsorbs nonpolar organic compounds such as lower paraffin hydrocarbons.
3- The most common adsorbent, activated carbon, is prepared by carbonizing wood, fruit pits, or coconut shells at very high temperatures and treating the substance with steam to burn away part of the carbon material and create a large internal pore structure that provides an exceptionally large internal surface area—an area from l0 to 106 m2/kg.
Table 3-6 Types of adsorbents
Adsorption Equipment
1- Multiple fixed-bed adsorber
The adsorbent, often activated carbon, is arranged on beds or trays in layers 1.3 cm thick in thin-bed adsorbers.
2- Moving bed adsorber
In this unit, the adsorption bed, activated carbon, is contained in a rotating drum.
3- Fluidized bed adsorber contains a shallow, floating bed of adsorbent. The air flows upward, expanding the bed and suspending or fluidizing the adsorbent.
In systems which rely on physical adsorption, regeneration of an adsorbent can be accomplished by use of super- heated steam or circulating hot air.
Organic vapors include those discharged by the following industrial processes:
1- Dry cleaning, degreasing, paint spraying, tank dipping, solvent extracting, and metal-foil coating.
2- Emissions from plastics, chemical, pharmaceutical, rubber, linoleum, and transparent- wrap manufacturing processes may also be controlled by adsorption.
3-9 ABSORPTION
• Absorption, or scrubbing, involves bringing contaminated effluent gas (the absorbate or solute) into contact with a liquid absorbent (the solvent) so that one or more constituents of the effluent gas are removed, treated, or modified by the liquid absorbent.
• Liquid absorbents may utilize either chemical (reactive) or physical (nonreactive) change to remove pollutants.
• A reactive liquid absorbent (water and limestone) may be used to remove sulfur dioxide from flue gases.
• (water) H2O+ (limestone) CaCO3 →(calcium hydroxide) Ca(OH)2 + CO2
Ca(OH)2 + SO2 + CO2 → CaSO4 (calcium sulfate) + H2O + CO2
Calcium sulfate can be scrubbed from the gas stream by the addition of more water.
• Absorption has been used primarily in the control of gases such as sulfur dioxide, oxides of nitrogen, hydrogen sulfide, hydrogen chloride, chlorine, ammonia, and some light hydrocarbons.
• Absorbent (Solvent)
• characteristics of the solvent
1-Quite soluble
2- Low freezing point and be low in toxicity.
3- Nonvolatile, nonflammable, and chemically stable.
4- Relatively inexpensive, readily available, and noncorrosive.
• The leading alkali absorbents are sodium and ammonia.
• Alkaline earth compounds being used as solvents are magnesium oxide (MgO), calcium oxide (CaO), and calcium carbonate (CaCO3).
• Absorption Units
• 1- Spray towers removal gaseous contaminant concentrations. Spray towers are also effective for dual removal of particulate and gaseous contaminants.
Relatively inexpensive
to install and operate.
2- Plate or tray towers
3- Bubble-cap tray column
4- Packed towers
5- Venturi scrubbers
3-10 CONDENSATION
Surface and contact condensers.
1- Surface condenser
2- Contact condenser
• Less expensive
• More flexible
• more efficient in
removing organic
compounds
Table 3-7 Representative applications of condensers in air pollution control
3-11 COMBUSTION
Convert the air contaminants (usually hydrocarbons or carbon monoxide) to innocuous carbon dioxide and water.
Four basic elements: oxygen, temperature, turbulence, and time.
1-Direct-Flame Combustion
Used in petrochemical plants and refineries.
2-Thermal Combustion
3- Catalytic Combustion
Thermal incineration may require residence times 20 to 50 times greater than catalytic incineration.
• Most catalytic systems that can convert CO to CO2 operate at a minimum temperature of 200 to 220°C.
• A Pd(II)/Cu(II) catalyst has been developed that can oxidize CO to CO2 at ambient temperature.
3-12 AUTOMOTIVE EMISSION CONTROL
Emissions from gasoline-powered vehicles come from these sources:
1- 20 percent coming from the crankcase.
2- 15 percent from fuel tank and carburetor.
3- 65 percent from tail pipe.
• The crankcase, fuel tank, and carburetor are the major sources of hydrocarbon emissions.
• Almost all the carbon monoxide and oxides of nitrogen comes from the tail pipe.
1- Positive Crankcase Ventilation (PCV) Systems
• These systems close off the vent to the atmosphere and recycle the blow-by back into the engine intake, an operation regulated by the (PCV) valve.
• Incorporation of this simple, inexpensive device has reduced crankcase hydrocarbon emissions to negligible levels.
2- Adsorption Canisters
The direct control of hydrocarbon emissions from fuel tanks was the installation of a canister filled with activated charcoal that adsorbs hydrocarbon emissions. The adsorbed vapors are then desorbed and fed back to the intake manifold during high-power operating conditions
3- Catalytic Converters
1-Oxidizing catalytic systems accelerate the completion of the oxidation of carbon monoxide and hydrocarbons so that CO is converted to CO2 and HC is converted to CO2 and water vapor. Metals such as platinum and palladium are used as catalysts.
• These catalysts can be poisoned by lead, sulfur, and phosphorus, only unleaded gasoline should be used in cars equipped with catalytic converters.
2- Reducing catalytic conversion systems, generally using rhodium and ruthenium, accelerate the reduction of NO to N2.
3- A three-way catalytic converter system using platinum and rhodium. This system is capable of promoting reactions among the air contaminants at close to stoichiometric conditions. Here the HC and CO react with the oxygen in the NO to form H2O, CO2, N2, and O2.
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