study guide
EHST 3600: Air Pollution 1
GENERAL CONTROL OF GASES AND VAPORS
EHST 3600: Air Pollution
LAST SESSION REVIEW
• Mechanisms of PM generation
• Vapor condensation
• Chemical reaction
• Mechanical processes
• Liquid spray
• Partial combustion of fossil
fuels
• Particle diameter
• Physical characteristic
• Behavior
• Particle size distribution
• Mean, median and modal diameter
• Terminal or settling velocity
• Hood design
• Flange
LAST SESSION REVIEW
• Mechanisms of PM collection
• Gravitational settling
• Centrifugal impaction
• Inertial impaction
• Direct interception
• Diffusion
• Electrostatic effects
• Particulate control equipment
• Gravity settling chamber
• Cyclone separators
• Wet collectors
• Air filtration
• Electrostatic precipitators
SESSION OBJECTIVES
• To describe air pollution control measures for gases and vapors
• To describe the use of incineration for solid waste management
SESSION OUTLINE
• Adsorption
• Absorption
• Combustion
• Formation and control of carbon monoxide
• Incineration or afterburning
• Incineration of waste
ADSORPTION
• A separation process based on the ability of certain solids to remove gaseous (or liquid) components preferentially from a flow stream
• Pollutant gas or vapor molecules present in a waste stream collect on the surface of the solid material.
• Adsorbent – solid adsorbing medium
• Adsorbate – gas or vapor adsorbed
http://www.dictionary.com/browse/adsorption
• Uses
• Dehumidification
• Removal of objectionable odors and
pollutants from industrial gases
• Recovery of valuable solvent vapors for air and other gases
EHST 3600: Air Pollution 2
ADSORPTION IS USEFUL WHEN…
1. The pollutant gas is noncombustible or difficult to burn.
2. The pollutant is sufficiently valuable to warrant recovery.
3. The pollutant is in very dilute concentration in the exhaust system.
TYPES OF ADSORPTION
• Physical adsorption • Gas molecules adhere to the surface of the solid
adsorbent as a result of intermolecular attractive forces (van der Waals) between them.
• Exothermic (heat liberated depends upon the magnitude of the attractive forces)
• Advantage 1: reversible; adsorbed gas is ready desorbed without a change in chemical composition
• By lowering the pressure of the adsorbate in the gas stream
• By raising the temperature (most commonly used)
• Directly proportional to the amount of solid surface available
• Advantage 2: Adsorption rate is quite rapid.
https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/waals.htm
van der Waals
TYPES OF ADSORPTION
• Chemisorption
• Results from a chemical interaction
between the adsorbate and the adsorbing
medium
• Bonding force is much stronger and heat
liberated is much larger than those for
physical adsorption
• Irreversible
• Only a monomolecular layer of adsorbate appears on the adsorbent
http://www.chem.qmul.ac.uk/surfaces/scc/scat2_5.htm
GOOD PROPERTIES OF ADSORBENT
• Large surface area per unit weight
• Internal pores of the solid
• Effective pore diameter larger than the molecular diameter of the gas of interest
• Chemical specificity
• Pressure drop through the bed held to a tolerable level
• Adsorbent particles not easily carried away by the flow stream
• Solid adsorbent meeting certain standards of strength and hardness
EXAMPLES OF ADSORBENTS
Activated carbon Silica gel
Activated alumina
Fuller’s earth
Molecular sieves
ACTIVATED CARBON
• Made by the carbonization of coal, wood, fruit pits, and coconut shells
• Made active by treatment with hot air or steam
• Available in pelleted or granular form
• Particularly useful for the recovery of solvent vapors
• May be desorbed
EHST 3600: Air Pollution 3
ACTIVATED CARBON FIBER
• Cloth form
• Felt form
SILICA GEL
• Granular product made from the gel precipitated by sulfuric acid treatment of a sodium silicate solution
• Used primarily to dehydrate air and other gases
• Limited to temperatures below 250°C (500°F)
• May be desorbed
ACTIVATED ALUMINA
• Another dehydrating agent
• Reactivated by heating to
175°C to 325°C (350°F to 600°F)
FULLER’S EARTH
• Natural clays used in the
petroleum industry, and with
vegetable and animal oils
• Adsorbed organic material
can be removed by washing
and burning.
SYNTHETIC ZEOLITES • Crystalline metal aluminosilicates called molecular
sieves
• Advantage: selectivity or specificity that is attained by tailor-making their crystalline structure so that they will adsorb only certain molecules
• Pores are uniform in molecular dimensions
• Depending on pore size, gaseous molecules will be readily adsorbed, slowly adsorbed, or completely excluded.
• Also made to separate molecules of different dipole moment and hydrocarbons which differ in their degree of unsaturation
• Can be regenerated by heating or elution (solvent extraction)
MOLECULAR SIEVES
• Developed for the control of pollutants (e.g. SO2, NOx and Hg emissions)
• Specific applications: Exhaust gases
from sulfuric and nitric acid plants
• Regenerated by a heated purge gas when the sieves are exhausted
EHST 3600: Air Pollution 4
GENERAL REQUIREMENTS IN THE DESIGN OR SELECTION OF
ADSORPTION EQUIPMENT
1. Provision of sufficient dwell time
2. Pretreatment of the gas stream to remove nonadsorbable matter that would impair the operation of the adsorption bed
3. Pretreatment to remove high concentrations of competing gases by other more effective processes to prevent overburdening the adsorption system
4. Good distribution of flow through the bed
5. Provision for renewing or regenerating the adsorbent bed after it has reached saturation
THE ADSORPTION WAVE
• Adsorption zone
• The region between
positions 1 and 2
• Adsorption wave
• Adsorption phenomenon in
a stationary bed wherein the
concentration profile thru
the adsorption zone is s-
shaped or wave-like
• Breakthrough point or break
point
• Condition of a rapid change
in C at the bed exit
inlet
Figure 6-5. Passage of an adsorption wave through a fixed bed of depth, Db
ADSORPTION WAVE
Figure 6-6. General features of an adsorption wave
REGENERATION OF AN ADSORPTION BED
When breakthrough is reached in an adsorption bed,
it is necessary to switch the
flow to another unsaturated bed and then proceed with
regeneration of the saturated bed.
Cold regeneration vs
Hot regeneration
ABSORPTION
http://www.diffen.com/difference/Absorption_v s_Adsorption
• Basic chemical
engineering unit operation which in the air
pollution control field is
referred to as scrubbing
• Widely used for control of SO2, H2S and light HC
• Absorber units are
designed to provide large
liquid surface area with a minimum of gas pressure
drop
Absorption vs Adsorption
PACKED TOWER
• Packing materials
• Designed to increase the liquid- film surface area
• Various geometric shapes with unique surface areas and associated gas
pressure drop
Figure 6-8. (a) Basic arrangement of a counterflow packed absorption tower, and (b) shapes of conventional packing
EHST 3600: Air Pollution 5
HOW DO YOU CHOOSE THE LIQUID ABSORBENT?
• Pollutant solute must diffuse out of one phase and into the other through the gas-liquid interface.
• No appreciable diffusional resistance occurs at the actual interface.
The solubility of the pollutant gas normally determines the liquid
that is chosen (i.e. liquid in which
the gas is highly soluble).
COMBUSTION OR INCINERATION
• Alters the chemical form of the
pollutants oxidation to specific end products
• Another technique: prevention of the initial formation of the pollutant
by controlling variables that promote its formation (related to
the chemical kinetics of formation
of the pollutants)
Used when not feasible to remove the
required amount of a specific pollutant
from an exhaust stream by adsorption
and absorption
Combustion plays a dual role…
COMBUSTION
• Rapid oxidation of substance (i.e. fuels)
with the evolution of heat
• A process with the potential to emit
both uncombusted gases and particles
THE COMBUSTION PROCESS
• Process in which a fuel (hydrocarbons) is burned or combusted to produce the final products CO2 and water
• Gas fuel: methane or other gaseous hydrocarbons
• Liquid fuel: fuel oil or liquid hydrocarbon waste stream
• Solid fuel: coal or solid waste
• Elements other than H and C
• Cannot be oxidized to CO2 and water
• Emitted as pollutants (gas, vapor, liquid, solid)
3 COMPONENTS IN THE COMBUSTION PROCESS
• Fuel
• A solid, liquid or gaseous substance containing energy rick C-C and C-H bonds among others, which are broken up during the combustion process, releasing heat
• Oxidant
• A substance which aids in the combustion process by breaking the chemical bonds, allowing the release of heat
• Oxygen – most common oxidant (21% by volume in ambient air)
• Diluent
• A substance that does not take part in the combustion process but acts as a carrier of the fuel or the oxidant
• Nitrogen – most common diluent (78% by volume in ambient air)
• Acts as a heat sink to absorb a substantial part of the heat released during the combustion process
• A substantial constituent of the gas emitted from the combustion process
COMBUSTION PRODUCTS
• Reaction products of the combustion process
• Generally made up of the following:
• Oxidized products of the C-C and H-C bonds in the fuel (CO2,
water)
• Oxidized products of other substances (usually impurities) in the fuel (sulfur, chlorine and metals), which end up as gases, vapors and/or
particulates in the gases exiting the combustor
• Incomplete products of combustion (unburned hydrocarbons, CO,
secondary H-C componds)
• Diluents (excess oxygen, nitrogen in the air)
Fuel + Oxidant Combustion Products
EHST 3600: Air Pollution 6
EXCESS AIR IN THE COMBUSTION PROCESS
• Nearly all combustion systems (except automobiles) are operated with an excess of air in order to ensure complete combustion of the fuel.
• Excess air – part of the air which is injected that is over and above the stoichiometric amount
• Stoichiometric amount – quantitative relationship between reactants and products in a chemical reaction
Excess Air (%) = Excess Air Theoretical Air
(100)
CARBON MONOXIDE
• Intermediate product of the chemical reaction between carbonaceous fuels and oxygen
• Occurs as a final product of the combustion
process when an insufficient quantity of oxygen is provided
• Occurs for one of two reasons:
1. Poor mixing of the fuel and air in the reaction zone
2. CO originating in high-temperature regions of the combustion zone
CONTROL OF CARBON MONOXIDE
• Control of CO formation
• Proper design, installation,
operation and maintenance of
combustion equipment
• Scrubbing with specific solutions
(e.g. copper ammonium formate)
INCINERATION OR AFTERBURNING
• A combustion process used to remove combustible air pollutants (gases, vapors, or odors)
• Frequently used in situations where the volume flow rate of waste gas from a process is large but the level of contaminant gas is small
WHY USE INCINERATION FOR OBJECTIONABLE GASEOUS WASTES?
1. Almost all highly odorous pollutants are combustible or are changed chemically to less odorous substance when heated sufficiently in the presence of oxygen (mercaptans, cyanide gases, hydrogen sulfide).
2. Organic aerosols that cause visible plumes are effectively destroyed by afterburning (emitted by coffee roasters, meat smoke houses, enamel baking ovens).
3. Certain organic gases and vapors become involved in smog reactions.
4. Some industries (e.g. refineries) produce large quantities of highly combustible waste gases and otherwise dangerous organic materials (burning in stack flares or specially designed furnaces).
ADVANTAGES OF AFTERBURNING
1. Essentially complete destruction of all combustible pollutants when equipment is properly designed and operated
2. Capability of adapting the equipment to moderate changes in effluent flow rate and concentration
3. A control effectiveness which is relatively insensitive to the specific gaseous pollutant
4. Absence of performance deterioration (of a thermal-
type unit)
5. Possibility of economical waste heat recovery
EHST 3600: Air Pollution 7
DISADVANTAGES OF AFTERBURNING
1. Reasonably high capital and operating costs
2. The necessity of providing collection and ducting equipment in some instances, which adds
significantly to the cost
3. The possibility of introducing special pollution problems when atoms other than C, H, and O are present in the hydrocarbon (e.g. chlorine, nitrogen, sulfur)
FACTORS TO KNOW IN DESIGNING AN INCINERATION PROCESS
1. Chemical composition of the contaminants and their concentration level
2. Inlet waste gas temperature
3. Volume rate of waste gas to be handled
4. Permissible emission levels for the pollutants
TYPES OF INCINERATION
• Direct flame incineration
• Thermal recuperative incineration
• Catalytic incineration
DIRECT FLAME INCINERATION
• A method by which waste gases are burned directly in a combustor
with the aid of an additional auxiliary fuel such as natural gas
• Goal: To raise the gas stream to the desired temperature and hold it
at that condition for sufficient residence time to thermally destruct
and oxidize the pollutants within the gas stream
• Requires the knowledge of the explosive or flammability limits of both
the waste materials and the fuel gas in mixtures with air
• Disadvantage: Flame temperature in the range of 2500°F are possible.
DIRECT FLAME INCINERATION
• Flare
• An open-ended combustor, usually aimed vertically upward
• Used in petrochemical plants and refineries)
• Primarily for waste gaseous fuels
which cannot be disposed of conveniently by other means
• Requires a pilot to assure continuous burning
• Problem: Occasional smoky or sooty
appearance
RECUPERATIVE THERMAL INCINERATION
• Used when the concentration of combustible pollutants is quite low
• Comprised of combustion chamber and one or more means of recovering part of the energy from the incinerator
Fig. 6-24
• Primary energy recovery
• Accomplished by preheating the waste gas stream and/or the auxiliary air
• Secondary energy recovery
• Accomplished by utilizing the heat in the exit gases to produce steam or hot water for use in other on-site processes
EHST 3600: Air Pollution 8
CATALYTIC INCINERATION
• Use of catalyst that accelerates the rate of a chemical reaction without undergoing a chemical change itself
• Residence times required are much less than those for thermal units
• Catalyst lowers energy requirements of the oxidation process
• Waste-gas stream does not have
to be heated as high as in thermal incineration
Fig. 6-27
CATALYTIC INCINERATION
• Catalytic properties
• Relatively inexpensive
• Long lasting
• Able to function at the required temperature
• Capable of being formed into a variety of shapes
• Ribbons, rods, beads,
pellets, etc.
INCINERATION OF WASTE
• Municipal solid waste incineration
• Primary goal: To reduce the volume
of the final waste to be disposed in a
solid waste landfill
• Typically reduced by 70-90%
• Hazardous and medical waste
incineration
• Primary goal: To provide permanent
destruction of many of the hazardous components present in the waste
MUNICIPAL SOLID WASTE INCINERATION
Fig. 6-33
HAZARDOUS WASTE INCINERATION
• Hazardous waste
• May cause an adverse effect on human health or the
environment
• may be ignitable, corrosive, reactive, toxic, or on a specific list of wastes defined by EPA as hazardous waste
• Consists of liquids, solids, sludges, powders and slurries
HAZARDOUS WASTE INCINERATORS
1. Liquid waste incinerator
2. Rotary kiln/ afterburner incinerator
3. Fixed hearth incinerator
4. Fluidized bed combustion chamber
EHST 3600: Air Pollution 9
HAZARDOUS WASTE INCINERATION
HAZARDOUS WASTE INCINERATION
HAZARDOUS WASTE INCINERATION
Fluidized bed combustion chamber
SUMMARY
• Adsorption
• Absorption
• Combustion
• Formation and control of carbon monoxide
• Incineration or afterburning
• Direct flame, thermal recuperative, catalytic
• Incineration of waste
• Municipal waste incineration
• Hazardous and medical waste incineration