ENVIRONMENTAL ENGINEERING 343

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1 Philadelphia University Faculty of Engineering Department of Civil Engineering First Semester, 2013/2014 ENVIRONMENTAL ENGINEERING 343 Lecture 12: Air pollution 3 SOURCE CONTROL TECHNOLOGY Air quality management sets the tools to control air pollutant emissions. Control measurements describes the equipment, processes or actions used to reduce air pollution. The extent of pollution reduction varies among technologies and measures. The selection of control technologies depends on environmental, engineering, economic factors and pollutant type. 1

2 OBJECTIVES OF CONTROL EQUIPMENT o Prevention of nuisance o Prevention of physical damage to Environment o Elimination of health hazards o Recovery of valuable waste product o Minimization of economic losses o Improvement of product quality CONTROL OF POLLUTANTS FROM STATIONARY SOURCES The most common method for controlling pollutants is the addition of add-on control devices to recover or destroy a pollutant Air Pollution control- at source-equipments for control of air pollution- I- For Gaseous pollutants 1.1 Absorption, 1.2 Adsorption, 1.3 Condensation, and 1.3 Incineration (combustion) II- For particulate matter 2.1 Settling chambers- 2.2 Fabric filters 2.3 Scrubbers 2.4 Cyclones 2.5 Electrostatic precipitators 2

3 1.1 ABSORPTION The removal of one or more selected components from a gas mixture by absorption is probably the most important operation in the control of gaseous pollutant emissions. Absorption is a process in which a gaseous pollutant is dissolved in a liquid. As the gas stream passes through the liquid, the liquid absorbs the gas, in much the same way that sugar is absorbed in a glass of water when stirred. Absorbers are often referred to as scrubbers, and there are various types of absorption equipment. The principal p types of gas absorption equipment include spray towers, packed columns, spray chambers, and venture scrubbers. In general, absorbers can achieve removal efficiencies grater than 95 percent. One potential problem with absorption is the generation of waste-water, which converts an air pollution problem to a water pollution problem. 1.1 ABSORPTION o Effluent gas passed through absorbers (scrubbers), which contain liquid absorbent. o Efficiency depends on o 1. Amount of surface contact between gas and liquid 2. Contact time 3. Conc. of absorbing medium 4. Speed of reaction between the absorbent and gases Absorbents used to remove SO 2, H 2 S, SO 3, F and oxides of nitrogen. 3

4 Equipments using principles of absorption for removal of gaseous pollutants o Spray tower/ wet scrubber Gaseous pollutants SO 2 H 2 S HF NOX Common absorbents used in solution form Dimethylaniline, ammonium sulphite, ammonium sulphate, sodium sulphide, calcium sulphite, alkaline li water, NaOH and phenol mix (3:2), tripottasium phosphate, sodium alamine, sodium thioarsenate, soda ash Water, NaOH Water, aqueous nitric acid 4

5 1.2 ADSORPTION When a gas or vapor is brought into contact with a solid, part of it is taken up by the solid. The molecules that disappear from the gas either enter the inside of the solid, or remain on the outside attached to the surface. The former phenomenon is termed absorption (or dissolution) and the latter adsorption. The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they have enormous surface areas per unit weight. Activated carbon is the universal standard for purification and removal of trace organic contaminants from liquid and vapor streams. Carbon adsorption systems are either regenerative or nonregenerative. - Regenerative system usually contains more than one carbon bed. As one bed actively removes pollutants, another bed is being regenerated for future use. - Non-regenerative systems have thinner beds of activated carbon. In a non-regenerative adsorber, the spent carbon is disposed of when it becomes saturated with the pollutant. Equipments using principles of adsorption for removal of gaseous pollutants o Packed tower 5

6 1.2 ADSORPTION o Surface phenomenon, require large solid surface o Adsorption towers use adsorbents to remove the impurities from the gas stream. o The impurities bind either physically or chemically to the adsorbing material. o The impurities can be recovered by regenerating the adsorbent. o Adsorption towers can remove low concentrations of impurities from the flue gas stream. Advantages of Adsorption Towers o Very low concentrations of pollutants can be removed. o Energy consumption is low. o Do not need much maintenance. o Economically valuable material can be recovered during regeneration. 6

7 Gaseous pollutants SO 2 H 2 S HF NOX Organic solvent vapours Adsorbents used in solid form Pulverized limestone or dolomite, alkalized alumina Iron oxide Lump limestone, porous sodium fluoride pellets Silica gel Activated carbon 1.3 INCINERATION Incineration, also known as combustion, is most used to control the emissions of organic compounds from process industries. This control technique refers to the rapid oxidation of a substance through the combination of oxygen with a combustible material in the presence of heat. When combustion is complete, the gaseous stream is converted to carbon dioxide and water vapor. Equipment used dto control waste gases by combustion can be divided in three categories: - Direct combustion or flaring, - Thermal incineration and - Catalytic incineration. 7

8 Fume incinerators o Used to control effluent gases, fumes and odors from refineries, burning waste, cracking gases and chemical plants Method is expensive when 1. Fuel values of gaseous discharge low 2. Moisture content of discharge is high 3. Exhaust volume is extremely large 8

9 Factors considered in design of incinerators are 1. Sufficient air for combustion reaction 2. Adequate temp. 3. Adequate retention time o Incineration equipment, single combustion chamber. o Combustion chamber proportioned that gas velocity, gas flow patterns, established produce adequate retention time. o Avg. retention time s at temp. 650 C and higher. o When conc. of combustible portion of gas stream below flammable range, catalytic combustion process used CATALYTIC INCINERATORS Catalytic incinerators are very similar to thermal incinerators. The main difference is that after passing through the flame area, the gases pass over a catalyst bed. A catalyst promotes oxidation at lower temperatures, thereby reducing fuel costs. Destruction efficiencies greater than 95 percent are possible using a catalytic incinerator. Catalytic incinerator 9

10 1.4 CONDENSATION Condensation is the process of converting a gas or vapor to liquid. Any gas can be reduced to a liquid by lowering its temperature and/or increasing its pressure. Condensers are typically used as pretreatment devices. They can be used ahead of absorbers, adsorbers, and incinerators to reduce the total gas volume to be treated by more expensive control equipment. Condensers used for pollution control are contact condensers and surface condensers. In a contact condenser, the gas comes into contact with cold liquid. In a surface condenser, the gas contacts a cooled surface in which cooled liquid or gas is circulated, such as the outside of the tube. Removal efficiencies of condensers typically range from 50 percent to more than 95 percent, depending on design and applications. II-PARTICULATE CONTROL EQUIPMENT 1. Gravitational settling chambers 2. Fbi Fabric filters 3. Scrubbers 4. Cyclone separator 5. Electrostatic precipitators 10

11 2.1 SETTLING CHAMBERS/ GRAVITATIONAL SETTLING CHAMBER Settling chambers use the force of gravity to remove solid particles. The gas stream enters a chamber where the velocity of the gas is reduced. Large particles drop out of the gas and are recollected in hoppers. Because settling chambers are effective in removing only larger particles, they are used in conjunction with a more efficient control device. 2.1 SETTLING CHAMBERS/ GRAVITATIONAL SETTLING CHAMBER o Used to remove particles with size greater than 50 μm. o Velocity of flue gas reduced in large chamber. o Particles settle under gravitational force. Application o Industrial application is limited. o Used widely for removal of large solid particulates o Sometimes used in process industry, food and metallurgical industry. o Used as pre-cleaners for high efficiency collectors. 11

12 2.1 Settling Chambers/ Gravitational settling chamber Advantages o Low initial cost. o Easy to design. o Low pressure drop. o Low maintenance cost. o Dry and continuous disposal of solid particulates. Disadvantages o Require large space. o Less collection efficiency. o Only larger size particles can be collected. 2.2 CYCLONES The general principle of inertia separation is that the particulate-laden gas is forced to change direction. As gas changes direction, the inertia of the particles causes them to continue in the original direction and be separated from the gas stream. The walls of the cyclone narrow toward the bottom of the unit, allowing the particles to be collected in a hopper. The cleaner air leaves the cyclone through the top of the chamber, flowing upward in a spiral vortex, formed within a downward moving spiral. 12

13 2.2 Cyclone separator o Cyclones are efficient in removing large particles but are not as efficient with smaller particles. For this reason, they are used with other particulate control devices. o Centrifugal force is utilized to separate the particulate matter. o Design factor having greatest effect on collection efficiency is cyclone diameter. o Smaller dia, higher is efficiency, because centrifugal action increase with decreasing radius of rotation. o Used mostly in industries. o It can remove 10 to 50 μm particle size. o Cyclone efficiencies > 90 % with particle dia of 10 μ o > 95 % with particle dia 20 μ. Efficiency o Conventional efficiency o High efficiency- smaller body dia to create greater separating force. o Increase collection efficiency, if increase in dust particle size, dust particle density, gas inlet velocity, inlet dust loading, cyclone body length (no of gas revolutions) o Decrease collection efficiency due to increase in gas viscosity, cyclone dia, gas outlet dia, inlet width, and inlet area Operating problems o Erosion o Corrosion o Material build up 13

14 Advantages o Low initial cost. o Require less floor area. o Simple construction and maintenance. o Can handle large volume of gas at high temp. o No moving parts Disadvantages o Requires large head room. o Less efficiency for smaller particles (<10μm). o Sensitive to variable dust load and flow rate. 2.3 ELECTROSTATIC PRECIPITATORS (ESPS) An ESP is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates. The ESP places electrical charges on the particles, causing them to be attracted to oppositely charged metal plates located in the precipitator. The particles are removed from the plates by "rapping" and collected in a hopper located below the unit. The removal efficiencies for ESPs are highly variable; however, for very small particles alone, the removal efficiency is about 99 percent. Electrostatic precipitators are not only used in utility applications but also other industries (for other exhaust gas particles) such as cement (dust), pulp & paper (salt cake & lime dust), petrochemicals (sulfuric acid mist), and steel (dust & fumes). 14

15 ELECTROSTATIC PRECIPITATORS (ESPS) Figure: Electrostatic precipitator components Electrostatic precipitators o Works on the principle of electrical charging of particulate Matter (-ve) and collecting it in a (+ve) charged surface. o 99% efficiency. o Can remove particle size range of 0.1 μm to 1 μm. 15

16 Advantages o High collection efficiency. o Particles may be collected dry or wet. o Can be operated at high temp. ( c). o Maintenance is normal. o Few moving parts. Disadvantages o High initial cost. o Require high voltage. o Collection efficiency reduce with time. o Space requirement is more. o Possible of explosion during collection of combustible gases or particulates. 16

17 2.4 Fabric filters or cloth filters o Flue gas is allowed to pass through a woven fabric, which filters out particulate matter. o Small particles are retained on the fabric. o Consists of numerous vertical bags mm dia and 2-10 m long. o Remove particles up to 1 μm. o Its efficiency up to 99%. 17

18 Factors affecting efficiency Efficiency decrease due to o Excessive filter ratio:- ratio of carrier gas vs gross filter area o Improper selection of filter media:- temp. resistance, resistance to chemical attack and abrasion resistance taken into consideration Filter cleaning o Rapping o Shaking o Back wash o Pulse jet. 18

19 Fabric Max. operatin g temp. ( C) Acid resistance Alkali resistance Abrasion resistance Tensile strength Kg/cm 2 Cotton 82 Poor Good Very good 4920 Wool 93 Very good Poor Fair to 1755 good Nylon 93 Poor to Excellent Excellent 5625 fair Dacron 135 Good Good Very good 5625 Polypropyl py ene 93 Excellent Excellent Excellent 7730 Fiber glass 290 Fair to good Fair to good Fair 14,060 Physical properties of bag filters Advantages o Higher collection efficiency for smaller than 10 μm particle size. o Performance decrease ease becomes es visible, giving g prewarning. o Normal power consumption. Disadvantages o High temp. gases need to be cooled. o High maintenance and fabric replacement cost. o Large size equipment. o Fabric is liable to chemical attack. 19

20 2.5 Venturi scrubber/ sprayer tower Packed wet scrubbers 20

21 Cyclone scrubber Mechanical scrubbers 21

22 CHOICE OF EQUIPMENT 1. Particulate size 2. Particulate loading 3. Efficiency required 4. Properties of carrier gas 1. Composition 6. Humidity 2. Temp. 7. Combustibility 3. Pressure 8. Reactivity 4. Viscosity 9. Toxicity 5. Density 10. Electrical property 5. Flow characteristics of carrier gas 1. Flow rate 2. Variation in flow rate 6. Specific property of contaminant 1. Composition i 6T 6.Toxicity i 2. Contaminant phase 7. Hygroscopicity 3. Solubility 8. Agglomerating characteristics 4. Combustibility 9. Electrical and sonic properties 5. Reactivity 10. Catalyst poisoning 7. Allowable pressure drop 8. Contaminate disposal 9. Capital and operating cost of equipment 10. Ease of maintenance and reliability 22

23 ECONOMICAL ASPECTS 1. Cyclones:- cheap to install, power consumption moderate, maintenance cost normal. 2. Filters:- expensive to install, power consumption moderate. Maintenance cost high. 3. Electrostatic precipitators:- most expensive regarding installation, power consumption moderate to low as pressure drops. Maintenance cost moderate 4. Scrubbers :- installation cost moderate, maintenance cost not high, high rate of power consumption. 23