Air pollution control methods and equipments

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Prevent the formation of pollutants or minimize their emission.Source correction method: Efficient design and development of processFor existing plants its difficult to apply new designsRaw material change, operational change, modification or replacement of process equipment, effective operation of existing equip.Effluent gas cleaning techniques:Involves many chemical engineering unit operationsSource correction methodsRaw material changes: Use purer grade of Raw material. ex: use of low sulphur fuel in place of high sulphur one Burning of natural gas(limited availability) in place of coal Coal gasification: easy to remove unwanted comp from gas Gas can be produced by destructive distillationcoke is by-product Gasification of coke residues of the carbonization process using steam, CO2, O2 and Air. The main prod are H2, CO2, CO and undecomposed steam.

*Destructive distillation is the chemical process involving the decomposition of feedstock by heating to a high temperature; the term generally applies to processing of organic material in the absence of air

Source correction methodsProcess changes: New or modified tech offer important ways of lowering atm pollutants emissions Petroleum ind: volatile sub are recovered by condensation, non-condensable gases are recycled for additional rxns.EX: H2S recycled and used in claus process to recover the SRotary kilns: source for dust generationdust control: adjusting the operating conditions, reduction gas velocities, modification of rate and location of the feed introduction, use of dense curtain of light weight chain at the discharge end.Smelting and Paper ind: sulphurous material Controlled by: major process changes- hydro metallurgical separations of ores, use of no sulphides in paper makingSteel Ind: new process changes are proposed to lower sulphurous emissions during combustion.S contained fuel lime stone air molten iron bathCombustibles partially oxidesed into CO and comes to topS retained in molten iron bath and forms slog with limestoneOther Examples: Reduction of the formation of nitric oxides in combustion chambers by low excess air combustion in two stages, flue gas recirculation and water injectionWashing of coal before pulverization to reduce the fly ash emissions

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Source correction methodsEquipment modification or replacement: Unburnt CO and HC in the cylinder of an automobile engine can be burnt by injecting the air into hot exhaust manifold of the engine.In refineries: HC released into the atm from storage tanks due to temp changes, evaporation, displacement during filling. Losses can be minimized by designing the tank with floating roof covers or pressurising the tank.Replacement of open hearth furnace by oxygen furnace in steel industry

Pollution emission can be reduced by: proper equip maintenance, housekeeping, and cleanliness in the facilities and premises. By Minimizing leakages.

Cleaning of gaseous effluents Cleaning tech are required where emissions of pollutants can not be prevented. Economical to install the control equip at source where the pollutants are present in the smallest vol.2 types: Particulate control type, gases and odours control type. wet absorption, dry adsorptionGases pollutants Diffusion chemical alteration of the pollutants *

Particulate emission controlPartly the diffusion and partly on nature and properties of the PMParticles sizes range form 100m to 0.1 m. Choice of collection devices: Physical and Chemical characterstics of particulates, particle size and conc of gas, vol of particulates to be handled, and temp and humidity of gaseous medium. Collection efficiency

Particulate control equipmentMechanism: Gravitational settling, centrifugal impaction, inertial impaction, direct interception, diffusion and electro static precipitation Equipments: gravitational settling chambers, fabric filters, electrostatic precipitators,cyclone separators, wet collectors (scrubbers).Low efficiency for small size particles high efficiency Gravitational settling chambers: Used for particles of size >50 mOffer low pre drop, simple maintenance.Generally used as pre cleaners *

*Horizontal flow settling chamberHoward settling chamber

Turbulent flow in settling chamber

- = *Mass conc of particle entering the control volMass conc of particle leaving the control volRate at which the particles are deposited with in the vol

*Reverse flow cyclone separatorTo avoid short circuiting

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Because of a centrifugal force of mpv2 / r, the particle will follow a path outward across the flow streamlines. Its velocity vector will have a tangential component (v) and a radial component (vr).The velocity of the spinning gas is assumed to have only a tangential component, u , with ur = 0.Tangential gas flows of this type u rn = constant.For an ideal fluid flow in such a vortex flow n = 1, although in real flows the value of n may range downward to 0.5.*

laminar Flow Cyclone Separatorsflow cyclone does not have laminar flow in the sense of the laminar flow settling chamber, but rather a frictionless flow in which the streamlines follow the contours of the cyclone. u r =

p-f~pv=u

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The distance traveled in the direction in a time interval dt

The distance the particle moves in the r-direction in time dt

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The differential equation describing the particle's trajectory. If the particle enters the device at r = r3 and hits the outer wall at = f*

collection efficiency of a cyclonethe cyclone has an angle fs =2Ne.All particles that enter the cyclone at rr3 hit the wall over 0 f f fs If the entering particle concentration all gas velocity are uniform across the cross section,*

Particulate emission controlFilters Packed filters: fibers are loosely packed inside an enclosure Fabric filters: fibers are woven into a thin layer of fabric (natural, synthetic, metal or glass).Packed filters Particulate matter is trapped on to the fibers: Inertial impaction, direct interception and diffusion. Inertial impaction Particles have high inertia

Collision factor c = 2y0 / dfAdherence factor aCollection efficiency impact = c a

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Particulate emission controlDirect interception Particles have less inertia and almost follow the stream line around the obstruction If the particle centre is travelling on a streamline which is closer than the radius of the particle, the particle will touch the fiber and deposited

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Particulate emission controlPacked filters:Diffusion Particles of submicron range (0.001-0.05 micro meter)

Estimate for combined efficiency of collection

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Particulate emission controlFabric Filters Open spaces in woven fabrics are usually many times the size of individual particles.After short period of operation, the particles from a bridge across the cloth openings and form a particle layer.99% efficient Cleaning and maintenance is required to prevent a deterioration of efficiencyFabric and fiber characteristicsWoven fabrics: Long range repeating pattern and have high porosity in the direction of gas flowOpen space must be bridged by impaction and interception Felted fabrics: Randomly oriented fibers compressed into a mat and needled to loosely woven backing material. Require less area for given loading More expensiveCan not be used in high humidity service Choice of fiber Based on operating temp and corrosivenessEx: Cotton Silicon coated glass fiber cloth

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Fabric Filter Systems Consist of tubular bag bag house

Cleaning: shaking the bags, increasing the air pre that deform sufficiently to dislodge the accumulated dust.Adv:Very high efficiencies, retention of finest particle, collection of particles in dry form, low pre dropDisadv:Large size, high construction cost, at low temp *Orientation of bags

More number of bags

Electrostatic PrecipitatorsUsed in power plants, cement and paper mills, oil refineries. Particle suspended in gas stream are charged electrically, and separated under the influence of the electrical field.

Periodic cleaning is necessary.

* corona discharge occurs close to the negative electrode.

Voltage 50kV Wire and Pipe Precipitator

-ve ions and electrons charge the PM

Electrostatic PrecipitatorsParallel plate precipitator is frequently used in collecting liquid particles. vertical plates exposed to horizontal gas flow.

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*Collection Efficiency

Wet Scrubbers More efficient than dry collectors Disad: Corrosion and liquid waste disposal Basic principle is to provide contact between the scrubbing liquid and particulates Wetted surface, individual droplets Mechanisms: inertial impaction, interception >0.3m and diffusion

Wet Scrubbers Centrifugal Scrubbers:Collection efficiency is more than spray scrubbers Simple centrifugal Scrubber: by inserting bank of nozzles inside a conventional cyclone.Spray acts on the particles in outer vertex.

*Gas is introduced tangentially at bottom

Water drops are injected into the flow stream from sprays directed outward from a center.

The droplets caught in the spinning gas stream and thrown upward towards the wall by centrifugal force.

During their motion the droplets collide with the particle and capture them.Another Version

Wet Scrubbers Packed Beds and Plate ColumnsThe packing allows the gas stream to take a curved path through the pore spaces Particle captured by inertial impaction Bcz of high surface area of contact its efficient for fine particles by diffusion Packing materials: Raschig rings, Saddles, Coke or broken stone.Plugging problems can be reduced by using sprays to wash the packing or using low density spheres.

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*Impingement Baffle plate scrubberGas jet impinged the baffle

Particles are collected by inertial impaction

Superficial gas velocity 5 times>the gas vel in normal cols

High efficiency for small particles(2 to 3 m) Suitable for PM is sticky, flammable or highly corrosive The particles are impacted against the slow moving droplets.Mechanisms: Inertial impaction, diffusion, condensation and agglomeration *Venturi Scrubber

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Control of gaseous emissions Sorption of pollutants, Chemical alteration of a pollutants Absorption by Liquids Concentration profiles

Selection of scrubbing medium: High gas solubility, high selectivity, low vapor pre, non-corrosiveness, easy regeneration, low costs, low viscosity, nontoxicity, nonflammability and chemical stability. Ex: SO2 NaOH; Nitrogen oxides ammonium bicarbonate, H2S NaOH, NH3 H2SO4

Gas absorption equipment: Packed towers, tray or plate towers, venturi scrubbers.

Venturi scrubbers: gas liquid flow in the same direction.Liq is introduced at the throat of the venturi and dispersion of the liq is accomplished by high vel gas stream.Extremely efficient but high energy required

Basic Design Considerations:

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Adsorption by Solids Adsorption is a surface phenomenon. Attractive forces may be physical or chemical.Physical adsorption: gases material condenses upon the surface of the solid.The adsorbed material can be removed by reducing the pre or by increasing the temp. Chemical adsorption: chemical interaction between the solid and adsorbed material. Heat of rxn is very high.Uses: field of odour controlCollecting of valuable organic substances.Rate of adsorption: conc of material around the adsorbent, surface area of the adsorbent, pore vol of the adsorbent, chemical nature of adsorbent surface, and temp.*

Adsorbents: activated carbon, activated alumina, silica gel. Stages of adsorption: Diffusion of pollutant from bulk gas phase to the external surface of the solid Diffusion of gas molecule into pore of the solid Adsorption on active sites in the pore (faster)Mode of operation: batchwise or continuous

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Batch AdsorptionBased on material balance and equilibrium dataEquilibrium data q=f(c)Material BalanceqfM + cfS = qM + cSPlot q vs c*

*Fixed Bed AdsorptionGas pass through the packed bed at a const flow rate.Mass transfer resistance, unsteady state processThe conc in fluid phase and solid phase change with time and with position as adsorption proceeds

*q0 saturated bed concBreak throughCb is the outlet conc in the fluid Equilibrium time te : Time required to reach the rear end of the MT zone to top of the bed

Effect of mass transfer on shape curve Construction of Breakthrough curve reffer to Treybal page no 634

*CbC0 is feed conc

*Lb length of sat bed0-a-a-d-g-c-b-0 used abs capacityAdgfea unused abs capacityCde-> cdgc=adeaAmt of adsorbent required to fill the bed length L-Ls remains effectively unused LUB About tb and te

ts is the stoichiometric timeStoichiometric Front

*L=ustsus velocity of Stoichiometric Front

Design calculations using LUBRun the test column at desired gas flow rate.Determine the value of LUB by finding tb and te from the break through curveFind usLUB=L-Ls, usts-ustbCompute the staichiometric height needed to produce desired adsorption capacity. (Ls=ustb;) i.e. decide how long you want to run the adsorption process Make the actual column longer by amount of the LUB, to account unused adsorption capacity under actual conditions.

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Combustion Can be used when gas streams have no recovery valuesIf the waste gas contain sufficient combustible materials incineration Combustion application destruction of odours, toxic sub, reactive materials, prevention of explosion hazards, reduction of pollutants in the oxidized form. Products of combustion process odourless, colourless, harmless(CO2, H2O).High mol wt HC cause smoke prob.inorganic material releases SO2 and oxides of N2.Ensure complete combustion For complete combustion time temperature and turbulence (3Ts).Methods: Direct combustion, thermal incineration, catalytic oxidation. flaring: highly combustible streams can be eliminated Flaring is not efficient: gas stream contain high amt of inorganic material- S, CL, F (Pretreat)

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Unsaturated HC with low H-C ratio produces smoke.

*Steam injection through the jets surrounding the flare tipAdv: increase turbulence Add heat to the combustion processSets up a water gas rxn (CO & H2)

Steam Injection Type Flare Preheating is required if the inert gases are present to ensure minimum fume energy concentration is not reached.

Combustion Thermal incineration:For diluted gases pre heating is required. waste gas is preheated over an auxiliary fuel fired burner Temp 500 to 800 0.3 to 0.7 sec residence time to allow complete oxidation Turbulence Cost involved in the form of fuel requiredRecovering the waste gas heat

*Thermal Incinerator

Regenerative Heat RecoveryOxidiser followed by HTEX

Effectiveness of heat exchanger *

Combustion Catalytic Oxidation Useful when TI is not economical (fuel cost)The combustion takes place on a catalyst usually at low temp (Comp with TI)Temp necessary to initiate rxn depend upon the type of pollutant Catalyst used are platinum and palladium or their alloysCatalyst is coated onto sustainable ele metal ribbons, ceramic rods, alumina pellets

*1% excess O2 is used complete combustion and maintain cat in active state

Recommended for PM and metallic sub free gases (avoid poison)

Periodic cleaning is required heated to req temp to support the catalyst combustionCatalyst Combustion Unit

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