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