AIR POLLUTION CONTROL

ATMOSPHERIC CLEANSING PROCESSESNatural removal mechanisms in atmosphere

Mechanism Details Dispersion -done by wind current-lessens the conc at one placeGravitational settling-removing particulates those >20 micronFlocculation -settle out particles

APPROACHES TO CONTAMINANT CONTROLDilutionAccomplished using tall stacksPenetrate the inversion layer and disperse the contaminantReduce the ground-level concMeans of spreading air contaminantDiluting air contaminant to harmful level and less noticeable near the sourceOnly short-term control measure

2) Control at sourceLong-term control, desirable and effectivePrevent the contaminant from existUse alternative power source in case of contaminants associated with combustion processessAltered high contaminant fuels to desulfured, LNG or LPG

Proper use of existence equipment competent inspective and maintenanceChanging the process being usedInstall control equipment either destroy, counteract, collect or mask pollutants

CONTROL DEVICES FOR PARTICULATE CONTAMINANTS FROM STATIONARY SOURCESCan be divided into five major groups:Gravitational settling chambersCentrifugal collectorsWet collectorsElectrostatic precipitatorsFabric filters1) Depends on particle characteristics size distribution, shape, density, stickiness, electrical properties2) Depends on carrier gas properties flow rate and particle conc3) Depends on economic considerations cost of installation, operating and maintenanceSELECTION

1) Gravitational settling chambers Settling chamberBaffled chamber

Provide enlarged areas to minimize horizontal velocities and allow time for vertical velocity to carry particle to the floorUsual velocity 0.5 to 2.5 m/s (best result -obtained at uniform flow of

Simple in design and operationRemove particle >50 micronEmploy gravity to separate particlesRequire a large space for installationLow efficiency for small particlesAdvantages Disadvantages

2) Centrifugal collectorsEmploy centrifugal force which can be generatedMuch smaller particles can be removed compared to gravity settling chamber Advantages

2.1) Cyclones

Diameter of exit, De = D/2L1L2h

bL3Length of cone, L2 = 2DLength of cylinder, L1 = 2DHeight of entrance, h = D/2Diameter of dust exit, Dd = D/4Width of entrance, b = D/4Length of exit duct, L3 = D/8W = bVi = i

Empirical efficiency for standard dimension cyclone

Centrifugal force generated by spinning of gasMagnitude depends on particle mass, gas velocity within cyclone and cyclone diameterSolid particles are thrown to the walls as gas spirals upwardSeparation efficiency depends on centrifugal force exerted on particles

Large-diameter cyclone remove particles 40 to 50 micron in diameter23 cm or less diameter cyclone high efficiency for particles of 15 to 20 micronSmaller diameter cycloneIncrease the centrifugal forceReduce distance of particle travelling before collectionProblems with equalizing gas flow to each coneAbrasion of tube high velocityPlugging of heavily loaded tubesAdvantages Disadvantages

TypesAdvantagesDisadvantagesCyclonesHave no moving parts Have low collection efficiency for respirable particulates Can be used as precleaners to remove coarser particulates and reduce load on more efficient dust collectors Suffer decreased efficiency if gas viscosity or gas density increases Can be designed to remove a specific size range of particles Are susceptible to erosion Have drastically reduced efficiency due to reduction in airflow rate Cannot process sticky dust

MulticyclonesHave no moving parts Have low collection efficiency for respirable particulates Are more efficient than single-cyclone separators Are prone to plugging due to smaller diameter tubes Have low pressure drop when used as a precleaner Improper gas distribution may result in dirty gas bypassing several tubes Cannot process sticky dust For a given gas volume, occupy more space than single-cyclone separators Normally have higher pressure drop than single-cyclone separators

2.2) Dynamic precipitatorsCentrifugal force Generated by the action of rotating vanesSeven times higher than conventional cycloneServe as both exhaust fan and dust collectorWidely used in ceramics, food, pharmaceuticalCannot handle wet, fibrous materialRequires higher power inputAdvantages Disadvantages

3) Wet collectorsIncorporating particles into liquid droplets by impingement or interception during gravitational settlingEfficiency depends on energy consumed in the air-to-water contactDirectly proportional to pressure dropApplying this theory high efficiency is expected in venturi-typeLow- remove medium to coarse size particlesHigh remove fine particles

Provide efficient, low-cost solutions to air pollution problemsCan handle hot and moist gasesHigh or fluctuating pressure dropHigh maintenance cost if collecting corrosive materialsNot recommended for use where high plume rise is importantDisposal problems of the particulates require other treatmentAdvantages Disadvantages

3.1) Spray tower

Low-cost scrubberRemove both gaseous and particulate contaminantsCause little pressure lossAble to handle large volumes of gasesEffective in moving particles in excess of 10 micron

3.2) Wet cyclone scrubber

High-pressure spray nozzles generate a fine spray that intercepts the small particles entrained in the swirling gasesEfficiency100% - particles of 100 micron90-98% - particles 5 to 50 micronHigher compared to spray towerParticle removal depends onLiquid flow rateLiquid droplets and particle size

3.3) Venturi scrubber

Most efficient in removing submicron particles of 0.5 to 5 micron (smoke and fumes)Efficient in removing particulate and gaseous contaminantHigh power cost high inlet gas velocityRemoval efficiency depends on particle size and head loss

Advantages Disadvantage

4) Fabric filters (baghouse filters)

Gas stream with particulate contaminant passes through a woven or felted fabricWays of how small particles are retainedDirect interceptionInertial impactionDiffusionElectrostatic attractionGravitational settling

Filter bags usedTubular or envelope-shapedCapable of retaining particles < 0.5 micron (+ substantial quantities of < 0.1 micron)Ranging from 1.8 to 9 m longUpper ends are closed and lower ends are attached to inlet manifoldAfter the pressure drop increase, can be cleaned intermittently, periodically or continuously

Unit must be shut down seldom usedPortions are cleaned while the rest in operationUsing mechanical shaker, reverse-air flow, pulse jets

DisadvantagesPossibility of explosion or fire if sparks are discharged in baghouse where organic dust are being filteredSpace limitation for heavy loadsPossibility of ruptureif temperature too high for the fabric mediumbecause of moisture, acidity or alkalinity content of the particulate

The design based on filtering rates/air-to-cloth ratiosFiltering rates range from 0.5 to 5 m/min depends on dust loading, fabric material and method of cleaningAdvantagesHigh collection efficiency over a broad range of particle sizeExtreme flexibility in design

Advantages (continue)Able to handle large volumes of gases at relatively high speed, reasonable operating pressure drop and power requirementAbility to handle a diversity of solid materialsApplication in high-volume operation such as cement kiln, foundries, steel furnaces and grain-handling plants

5) Electrostatic precipitators (ESP)

Can be classified as low-voltage two-stage or high-voltage single-stage unitsLow-voltage unitOperate at 6000 to 12 000 VEmployed mainly in conjunction with air-conditioning systems for hospitals and commercial installationUsed mainly to collect liquid particlesNot recommended for control of solid or sticky material

Its precipitators have a separate ionizing zone located ahead of the collection platesHave design capacities approaching 10m3/s with air velocity of 0.5 m/sHigh-voltageOperate at 30 000 to 100 000 V rangeUsed at large industrial plant such as coal-fired utility boilersRequire 4 basic steps in in the operationElectrical charging of the particulatesCollection of charged particles on a grounded surfaceNeutralization of the charge at the collectorRemoval of the particulate for disposal

Advantages Have wide application=>99% removal efficiency for a wide range of particle sizesCan handle large volumes of gas 25 to 1000 m3/sLow pressure dropsCan operate continuously with little maintenanceCan be used to collect acid or tar mists

DisadvantagesCannot be used with explosive materialsInitial installation cost is highRequire a great deal of space for industrial operationOnly operate at peak efficiency within a limited temperature rangeMay use excessive power if buildup of collected material causes spark overInefficient if buildup suppresses the corona discharge from negative electrode

CONTROL DEVICES FOR GASEOUS CONTAMINANTS FROM STATIONARY SOURCEGas of concern in air pollution control SOx, CO, NOx, organic and inorganic acid gases & HCMajor treatment processesAdsorptionAbsorptionCondensationCombustion

1) AdsorptionPassing a stream of effluent gas through a porous solid material (adsorbent)Adsorption of adsorbate on adsorbent can be either by physical or chemicalPhysical adsorptionCondensation of gases and vapors on solid above dew pointDepends upon van der Waals forceThe higher the boiling point, the greater the amount adsorbed

Directly proportional to the amount of solid surface availableAccompanied by capillary condensation within the poresSmall amount of heat is liberatedProcess relatively rapid and readily reversibleLowering pressure/increasing temperature adsorbed gas can be desorbed

Chemical adsorption/chemisorptionGas molecule forms a chemical bond with the adsorbentGas is strongly held to the solid surface by valence forcesA slower process since displacement of atoms must occur in moleculesLiberates greater amount of heat and require more energy

Results in formation of single layer of molecules on solid surfaceProcess is irreversible chemical nature of adsorbate have been alteredAmount of gas adsorbed depends on pressure and temperature

1.1) Adsorbents Key characteristics of solid adsorbentspreferential affinity for specific substancesAlumina, bauxite, silica gel affinity for polar such as waterActivated charcoal nonpolar organic compoundsMolecular sieve (tailor-made) SO2Surface-to-volume ratiosProvided by internal poresCan be increased by activating some adsorbentActivated carbon treating with steamActivated alumina reactivated by heating to 175 to 325oC

Quiz 2 List 2 advantages of applying centrifugal collectors in air pollution controlList 2 disadvantages of dynamic precipitators3 types of wet collectors used for control of particulate

1.2) Adsorption equipmentCan be designed with fixed, moving or fluidized bedsFixed bedVertical or horizontal cylindrical shellAdsorbent used is activated carbon in layers of 1.3 cm thick (thin) or >1.3 cm (deep)

Gambar fixed bed

Moving bedAdsorbent is contained in a rotating drum

Fluidized adsorbentContains a shallow, floating bed of adsorbent

Highly efficient until a breakpoint saturated adsorbents2 types of adsorbersRegenerativeIf depends on physical adsorption, can be accomplishe using superheated steam or circulating hot airCool the bed before reuseOther ways burning, pressure reduction and chemical treatmentNonregenerative - costlyDepends on gas properties easily desorbed or not

Application Adsorb organic vapors from dry cleaning, degreasing, solvent extractingRecovery economically feasible when concentrations of organic vapors is high

2) AbsorptionContaminated effluent gas (absorbate or solute) is brought into contact with liquid absorbent (solvent)Utilize chemical (reactive) or physical (nonreactive) change to remove pollutantsReactive absorbent - water and limestoneNonreactive water

Amount of gas absorbed depends on properties of gas and solventPressure of gas above the solutionTemperature of the systemPacking used turbulence, flow rate and typeApplicationControl of SO2, H2S, Cl2Remove of HC

2.1) Absorbent (solvent)Good solubility of solvent ifChemically similar to the soluteLow viscosityHave low freezing pointLow in toxicityRelatively nonvolatileNonflammableChemically stable

Application in SO2 absorption processAlkalies (sodium and ammonia) Sodium not volatile and have no fume problemAmmonia by product (ammonium sulfate) is more desirableAlkaline earths (calcium and magnesium)Compound being used MgO, CaO, CaCO3

2.2) Absorption unitsDesigned to provide intimate contact between gas and liqOptimum diffusion of the gas in solutionSelection depends on number of scrubbers to be usedSingle - removes both particulate and gaseousTwo separate

2.2.1) Spray towers

Can handle large volumes of gasLittle pressure dropHigh efficiency of removal low conc of gaseous contaminantApplicable for dual removalChance of absorption depends on droplet size and turbulenceInexpensive to install and operateAdvantages

Have less gas-liquid interfacial areaLess effective in removing gaseous contaminantDisadvantages

2.2.2) Plate or tray towers

Plate or tray towersContain horizontal trays or plates

Bubble-cap trayContain cap at which gas are diverted downward and discharged as small bubbles from slots

Provide large liquid-gas interfacial areasUsually spaced 0.3 to 0.9 m apart

2.2.3) Packed towers

Packing is used to increase the contact time between vapor and liquidMaterials of packingHas large surface-to-volume ratioHas large void ratio to minimize the resistance of gas flowLightweight and unbreakableCountercurrent flow gas from bottom, liquid from topHighly efficient for gaseous removalDisadvantage easily clogged if gas of high particulate is introduced

2.2.4) Venturi scrubber

Cocurrent unit gas/particulate and absorption solutionAre brought into contact in or near the venturi throatMoved together into an entrainment separatorSeparated by centrifugal force of the liquid droplets

3) Condensation2 basic types of condensation surface and contact condensersKey role is physical adsorptionCooling medium air or waterVapor and cooling medium is separated by a metal wallVapor and cooling medium are brought into direct contactLess expensive and more flexibleMore efficient in removing organic compoundsDisadvantage create water pollution problem

Gambar surface and contact condenser

Application depends upon Amount and type of coolant used Waste liquid disposal problems that will resultAmount of compound to be recoveredUsed in conjunction with afterburners, absorbers or adsorption unit

4) CombustionBasic for important air-pollution control processConvert the air contaminants to innocuous CO2 and water4 basic elements for efficient combustion to occurOxygenTemperatureTurbulencetimeDetermine the end product obtainedMust be keep at ignition temKeep O2 well mixed with the combustible subsProvided by baffles or injection nozzlesFor sufficient burningDone by increasing stack height

4.1) Direct-flame combustionWaste gases are burned directly in a combustorBurn by own if sufficient heat value and O2 contentFrequently used in petrochemical plants and refineriesFlares is usually open-ended combustion unitsSafe means of disposing highly combustible waste gases (not ideal)

Disadvantages of flaresBurn at high temp and long periods cause formation of oxides of nitrogenProduce visible smoke or sootWaste large amount of heat energyEconomical when the waste gas contribute >50% of the total heating value for incineration

4.2) Thermal combustionApplied when combustible gaseous pollutant is too lowThe waste gas is preheated using heat exchanger by utilizing the heat produced by thermal incineratorTemp of operation depends on nature of the waste gas

AdvantagesWell-designed unit can produce odorless steam plumesClean stream of hot air produced can be used as a heat source for other operation

4.3) Catalytic combustionUsed when combustible materials is lowConsist of preheating and catalytic sectionEfficiency depends on Contaminant concentrationTemperature of gas streamO2 concentrationContact timeType of catalyst

4.4) Automotive emission controlIndirect controlAbolition of the internal combustion engineUse of electric-powered vehiclesIncreased the utilization of mass transitSources of emissions from gasoline-powered vehicle20% from crankcase15% from fuel tank and carburetor65% from tail pipe

Flocculation-larger part act as receptor for smaller part part bump together to form a unit process repeated***