air pollution 2

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SYLLABUS � Air pollution control equipments viz. settling chambers, inertial separators, cyclones,

multiple cyclones, bag house filters, scrubbers or wet collectors, electrostatic precipitators, � Advantages and disadvantages of control equipments. � Air pollution abatement technologies including vehicular emissions. Air Pollution: In general, the actions of people are the primary cause of pollution and as the population increases, the pollution problems also increase proportionately. The first significant change in man’s effect on nature came with his discovery of fire. Prehistoric man built a fire in his cave for cooking, heating, and to provide light. The problem of air pollution came into existence at this time and kept on aggravating with time. Air pollution is the modification of the natural characteristics of the atmosphere by a chemical, particulate matter, or biological agent. Air pollution is the appearance of air contaminants in the atmosphere that can create a harmful environment to human health or welfare, animal or plant life, or property. Air pollution is the presence in the air of substances which pose a potential threat to human health and/or the environment. According to the Bureau of Indian Standards (BIS), IS – 4167 (1966): Air pollution is the presence in ambient atmospheres of substances, generally resulting from the activity of man, in sufficient concentration, present for a sufficient time and under circumstances such as to interfere with comfort, health or welfare of persons or with reasonable use or enjoyment of property. According to American Medical Association: Air pollution is the excessive concentration of foreign matter in the air which adversely affects the well being of the individual or causes damage to property. Ambient Air means outdoor surrounding air. Indoor air means air inside the room of a building. Much of the works and problems associated in today’s world are because of the small set of six primary pollutants present in the air called “Criteria pollutant” by USA, European Union and WHO, these are : CO, NO2, O3, SO2, PM10 and Pb. PM10: Particulate Matter with an aerodynamic diameter less than 10 µm. RSPM/RPM: Respirable Particulate Matter, the particles which can find its way to lungs through inhalation, particles having dia less than 10 µm i.e. PM10. PM2.5: Particulate Matter with an aerodynamic diameter less than 2.5 µm. Human hair has approximate diameter of 100 µm. Q. NAAQS is the acronym for…………….?

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Q. What is the permissible level of RSPM in ambient air for residential area? Q. What is the permissible level of CO in ambient air of sensitive area? Q. What is the permissible level of Lead (Pb) in ambient air of industrial area? Q. What is the permissible level of NH3 in Residential, Commercial & Industrial area?

National Ambient Air Quality Standards

Pollutants Time-

weighted average

Concentration in Ambient Air

Method of measurement Industrial Areas

Residential, Rural &

other Areas

Sensitive Areas

Sulphur Dioxide (SO2)

Annual Average*

80 µg/m3 60 µg/m3 15 µg/m3 - Improved West and Geake Method - Ultraviolet Fluorescence

24 hours** 120 µg/m3 80 µg/m3 30 µg/m3 Oxides of Nitrogen as (NO2)

Annual Average*

80 µg/m3 60 µg/m3 15 µg/m3 - Jacob & Hochheiser Modified (Na-

Arsenite) Method 24 hours** 120 µg/m3 80 µg/m3 30 µg/m3 - Gas Phase Chemiluminescence

Suspended Particulate Matter (SPM)

Annual Average*

360 µg/m3 140 µg/m3 70 µg/m3 - High Volume Sampling, (Average flow rate not less than 1.1 m3/minute).

24 hours** 500 µg/m3 200 µg/m3 100 µg/m3 Respirable Particulate Matter (RPM) (dia < 10 µm)

Annual Average*

120 µg/m3 60 µg/m3 50 µg/m3 - Respirable particulate matter sampler

24 hours** 150 µg/m3 100 µg/m3 75 µg/m3 Lead (Pb)

Annual Average*

1.0 µg/m3 0.75 µg/m3 0.50 µg/m3 - AAS Method after sampling using EPM 2000 or equivalent Filter paper

24 hours** 1.5 µg/m3 1.00 µg/m3 0.75 µg/m3 . Ammonia Annual

Average* 0.1 mg/ m3 0.1 mg/ m3 0.1 mg/m3

.

24 hours** 0.4 mg/ m3 0.4 mg/m3 0.4 mg/m3 . Carbon Monoxide (CO)

8 hours** 5.0 mg/m3 2.0 mg/m3 1.0 mg/ m3 - Non Dispersive Infra Red (NDIR) 1 hour 10.0 mg/m3 4.0 mg/m3 2.0 mg/m3 - Spectroscopy

* Annual Arithmetic mean of minimum 104 measurements in a year taken twice a week 24 hourly at uniform interval. ** 24 hourly/8 hourly values should be met 98% of the time in a year. However, 2% of the time, it may exceed but not on two

consecutive days.

Primary and secondary pollutants: Primary pollutants are those which are emitted directly from identifiable sources. Particulates, SO2, Oxides of nitrogen, CO, radioactive elements, halogens and organic compounds, fumes, carbon, pollens, bacteria etc. are the different primary pollutants.

Secondary pollutants are those which are formed in the air formed in the air to interaction of primary air pollutants among themselves or by reaction with normal atmospheric constituents like sunlight, water vapour etc. with or without photo activation. Eg. Ozone, Formaldehyde, CH2O, PAN (Peroxy Acetyl Nitrate) – CH3(CO)O2.NO2, Smog, Photochemical smog etc.

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Collection Efficiency: It is defined as the quantity of particulates collected from the gas from the total quantity of particulates present in the gas. It is generally expressed as percentage by symbol η (nita).

η = � �� x100 In particulate collection systems, the efficiency of collection varies with particle size. This variation of efficiency is often expressed in the form of fractional efficiency. It is the efficiency with which particles of a specified size range are collected. The overall efficiency i.e. total efficiency ηT can be calculated over n number of size fraction as:

ηT = ∑ �� %

Where, mi = mass fraction of ith size particle ni = fractional efficiency of ith size particle M = total amount entering the collector. Example: 1. The following table shows the size distribution of a dust sample and the fractional efficiency of removal in a gas cleaning equipment. Calculate the overall collector efficiency.

Dust size µ Weight of dust (gm), !" Fractional efficiency ηi (%)

< 5 2 1 5 to 50 63 70 51 to 70 20 80 > 70 65 100

Solution:

ηT = ∑ �� %

∑ !"#"$%&' = ( (')* +1) + (

-.')* +70) + (

(*')* +80) + (

-)')* +100)

= 0.0133 + 29.4 + 10.67 + 43.34 = 83.42 M = 2 + 63 + 20 + 65 = 150 gm

ηT = 1..3(')* = 0.5560, therefore ηT = 55.60 %

PARTI CULATE CONTROL EQUIPMENTS 1. Gravitational settling 2. Inertial separator 3. Cyclone separators (multiple 4. Fabric filters/ bag house filters5. Scrubber/ wet collectors6. Electro static precipitator

The gravitational settling chambers and cyclone separators will not generally achieve high efficiencies for removing small size particles. For most of the practical applications, only fabric filters, electrostatic precipitators and high enrigorous air pollution control regulations. Among all control devices the particulate emissions.

GRAVITATIONAL SETTLING CHAMBERS It is simplest of all the pollution control devices. The gravitational force acting on the particles is used to separate the particles from the gas stream by allowing it to sufficiently stay in the settling chamber by reducing the velocity of flow by providing large areabased on the principle of stoke’s law

The terminal settling velocity can be calculated by using the stoke’s law:

Vt = terminal settling velocityg = acceleration due to gravity, 9.8 ρp = density of particle, kg/mρg = density of gas, kg/m3 d7 = diameter of the particle, m

CULATE CONTROL EQUIPMENTS

separators (multiple cyclones) bag house filters

Scrubber/ wet collectors c precipitator

The gravitational settling chambers and cyclone separators will not generally achieve high efficiencies for removing small size particles. For most of the practical applications, only fabric filters, electrostatic precipitators and high energy scrubbers are capable of meeting the rigorous air pollution control regulations.

control devices electrostatic precipitators are the most efficient for controlling

GRAVITATIONAL SETTLING CHAMBERS

of all the pollution control devices. The gravitational force acting on the particles is used to separate the particles from the gas stream by allowing it to sufficiently stay in the settling chamber by reducing the velocity of flow by providing large area. The design of the chamber is based on the principle of stoke’s law.


V = g �ρ7 :ρ��d7(18µ�

= terminal settling velocity, m/s = acceleration due to gravity, 9.8 m/s2

, kg/m3

, m

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The gravitational settling chambers and cyclone separators will not generally achieve high efficiencies for removing small size particles. For most of the practical applications, only

ergy scrubbers are capable of meeting the

electrostatic precipitators are the most efficient for controlling

GRAVITATIONAL SETTLING CHAMBERS

of all the pollution control devices. The gravitational force acting on the particles is used to separate the particles from the gas stream by allowing it to sufficiently stay in the settling

. The design of the chamber is


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μ� = kinematic viscosity of gas, m2/s Simplified terminal velocity = 30,000 ρ7d7( (by putting the values of each components)

Q. Compute the terminal settling velocity in air of a sphere with diameter 1.0 µm. Consider density of the particle as 2000 kg/m3 and density of gas as 1.20 kg/m3 at a given particular temperature. Consider kinematic viscosity of the gas as 1.51 x 10-5 m2/s. (Ans: 7.21 x 10-5 m/s) Laminar flow: Reynold’s number < 2300 Turbulent flow: Reynold’s number > 2300

Reynold’s number, Re = <=>?@

v = velocity inside the chamber, velocity of particles is considered same as the velocity of gas stream.

Dh = hydraulic diameter of the flow ρ = density of the fluid, for dry air it is 1.205 kg/m3 at 20°C. µ = Kinematic viscosity of the fluid, for dry air it is 1.51 x 10-5 m2/s.

The hydraulic diameter, Dh, is a commonly used term when handling flow in noncircular tubes and channels. Using this term one can calculate many things in the same way as for a round tube.

D = 4xAreaPerimeter Where, Area is the cross sectional area and Perimeter is the wetted perimeter of the cross-section of the flow.

For a round tube, this is: D = 3KL�M(L� = 2 r = Diameter

The minimum particle size that can be removed with 100% efficiency can be found from the equation:

d7, �� = O 18μ�QnWLg(ρ7 −ρ�) Where, d7, �� = minimum particle size that can be removed with 100% efficiency μ� = viscosity of gas Q = volumetric flow rate of the gas, m3/s n = number of trays including bottom of the chamber W = width of the chamber L = length of the chamber ρp = density of particle, kg/m3 ρg = density of gas, kg/m3

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The above equation is used only as approximation of collection efficiency of the settling chamber, as several other factors cause the deviations in the efficiency. These include hindered settling at high particle concentrations, non-uniform gas velocity over the settling height and width, particle re-entrainment, and turbulence. As a general rule, chamber velocities below 3 m/s are satisfactory for avoiding re-entrainment of most particles. Although the efficiency relationship is based on laminar flow conditions within the unit, it is practically impossible to achieve laminar conditions without a very large particle size or an inordinately large number of trays combined with an awkward shape of chamber. Hence, the flow in the settling chamber will probably be turbulent rather than laminar. For turbulent flow, the following theoretical equation can be used for calculating the collection efficiency by following equation:

η = 1 : exp�WXYZ[\] �

Where, Vt = terminal settling velocity and other terms implies as usual discussed earlier. Advantages of gravity settling chambers: � Low initial cost � Simple construction, operation and maintenance � Continuous disposal of solid particles � Can be constructed of any material according to the temperature and pressure requirements. Disadvantages of gravity settling chambers: � Aberration problem of collector walls. � Re-entrainment of particles (settled particles again enter into the flow) for higher velocity,

hence velocity of entering gas generally to be kept below 3 m/s. � Large volume of gas to be handled has problem for a smaller dia collector. � The efficiency is better for the particles above size 50 µm diameter, particles having dia < 50

µm has lesser collection efficiency. � It removes only particulate matters, gaseous impurities are not removed. Q. A multi tray settling chamber having 8 trays including the bottom surface handles 6 m3/s of air at 20°C. The trays are spaced 0.25 m apart and the chamber is to be 1 m wide and 4 m long.

i. What is the minimum particle size of density 2000 kg/m3 that can be collected with 100% efficiency?

ii. What will be the efficiency of the settling chamber if 50µm particles are to be removed? Assume the flow to be laminar inside the chamber and no dust to be present initially on trays. μ� at 20°C = 1.81 x 10-5 kg/ms. (Ans: i. 56 µm, ii. 80%)

Hint: ^_` =a �b�b,cdXe

( where, ̂_` is the % efficiency for a particular dia particle.

Q. In the above example, estimate whether the laminar flow assumption is justified? If not, what is the collection efficiency for 56 and 50 µm particle? (Ans: Not justified, 63%, 55%)

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Determination of Length for a flow for having 100 % collection efficiency Collection efficiency, η = Ration of the space (area) where dust is collected and total area available for collection. η =

�.f�.gh ,

Where, Y is the thickness of the collected dust. ∆ H is the space between the trays. W is the width of the chamber. i\i = fj , Y = ji\i Where, L = length of the chamber Vt = terminal settling velocity of the particles V = Q velocity.

η = �.f�.gh =

ji\igh (by putting the value of Y)

= ji\.��l (by putting V =

lm = l�gh� )

L = l�.i\ .� , for η = 1, i.e 100%

Time interval for cleaning/removing dust m`n = C < . Q n = pqrs#"tt"!q, u = pqrs#"tvrqv Kg/sec = kg/m3 x m3/sec C < =mass volume concentration of dust i.e. kg/m3, Q = m3/s

η = �ww��w�� ww��x��y��.��

= z b{n

Therefore, t = z b|n

t = z|}c~.l

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CYCLONE It is also called as inertial separators, centrifugal collectors. When used in multiple numbers simultaneously in series to handle large volume of air it’s called multiple cyclones. Its application: Cement manufacturing unit, food and grain processing, beverage processing, mineral processing, paper and textile industries, utensils making industries, buffing units and in other units where disintegration process is involved. Efficiency: It has 70% efficiency for 15 to 40 µm particles. Advantages of cyclones:

� Low initial cost � Simple construction and operation � Low pressure drop and low maintenance � Continuous disposal of solid particles � Can be constructed of any material according to the temperature and pressure

requirements. Disadvantages of cyclone:

� Aberration problem of collector walls. � In high efficient cyclone, dia is less, so plugging problem and it needs frequent cleaning. � Re-entrainment of particles (settled particles again enter into the flow) � Large volume of gas to be handled has problem for a smaller dia collector.

Erosion problem of cyclone: Impingement or rubbing of wall is more at the just opposite wall of inlet and is more at the bottom where dust is collected. For prevention increase the diameter of the cyclone cylinder but keep outlet diameter same. Use the abrasion resistant wall. Cyclone Consists of:

i. Tangential gas inlet ii. An axial gas outlet iii. Main body having vertical cylinder with tapering bottom iv. Conical shaped lower section with an axial dust outlet

Cyclone separators utilize a centrifugal force generated by a spinning gas stream to separate the particulate matter from the carrier gas. The centrifugal force on particles in a spinning gas stream is much greater than gravity, therefore cyclones are effective in removal of much smaller particles than gravitational settling chambers, and require much less space to handle the same gas volume.

A typical conventional standard cyclone has vertical cylinder with conical bottom and is fitted with a tangential inlet located near the top and an outlet at the bottom of the cone for discharging separated particles. The gas outlet pipe is extended into the cylinder to prevent direct escaping of dust laden gases entering through tangential inlet to outlet pipe.

In operation, the particles laden gas upon entering the cyclone cylinder receives a rotating

motion. The vortex so formed develops a centrifugal force, which acts to throw the particles

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radially towards the wall. The gas spirals downward to the bottom of the cone, and at the bottom the gas flow reverses to form an inner vortex which leaves through the outlet pipe from where the gas is sent to the atmosphere though stack attached to it having adequate height to further dissipate if any suspended materials left into it.

Working Principle: In a cyclone, the inertial separating force is the radial component of the simple centrifugal force and is a function of the tangential velocity. The centrifugal force is expressed as Fc, where

F� = !��(r

Where, m = mass of the particle �� = tangential velocity of the particle (and gas) as the particles move at the same tangential velocity at which gas is moving r = radius of rotation

Separation factor (S): It is the ratio of the centrifugal force (mv2/r) and the gravitational force (mg). It is a dimensionless factor.

S = ��(�r

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Where, S = Separation factor �� = tangential velocity of the particle/gas g = 9.8 m/s2 r = radius of rotation Separation factor varies from 5 in large and low velocity unit to 2500 in small and high pressure units. Higher the separation factor better is the efficiency of the cyclone. But till present no direct correlation has been established between separation factor and collection efficiency because of the factors such as re-entrainment, bounce, and particle interactions. However collection efficiency is a function of the separation factor. The most satisfactory expression for cyclone performance is still the empirical one. Lapple correlated collection efficiency in terms of the cut size diameter, which is dia of those particles which can be collected with 50% efficiency i.e. d0.5.

d*.) = O 9��2��V(�` − ��) Where, d*.) = Cut size diameter, i.e. particle size for which collection efficiency is 50% �� = Viscosity of gas/air b = width of inlet duct �� = effective number of turns made in traversing the cyclone =

�� (2�' + �(), �'&�( are the length of the cylinder and cone, respectively. V = gas inlet velocity. The efficiency of collection of various particle sizes (η) can be determined from Lapple’s empirical expression and graph below:

Q. What is Cyclone dust separator? Show the proportions of a standard conventional cyclone with schematic diagramme. Define its working principle with advantages and disadvantages of using it. For which types of Industries you will recommend Cyclone separators?

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GENERALISED CYCLONE DESIGN CONFIGURATIONS:

Symbol Nomenclature Conventional type High Efficiency D2 Cyclone Diameter 1.0 1.0 H Entrance Height 0.5 0.5 B Entrance Width 0.25 0.2 L3 Exit length 0.625 0.5 De Exit diameter 0.5 0.5 L1 Cylinder Height 2.0 1.5 L2 Conical Height 2.0 2.0 Dd Dust exit diameter 0.25 0.375

Q. A conventional cyclone with diameter 1.0 m handles 3.0 m3/s of air carrying particles with density of 2000 kg/m3. For Ne = 6, determine the size of the particle for which collection efficiency would be 50%. (Ans: 4.7 µm)

Q. Determine the efficiency of a “standard” cyclone having the following characteristics for

particles 10 µm in diameter with a density of 800 kg/m3. Cyclone barrel diameter = 0.50 m Gas flow rate = 4.0 m3/s

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Gas temperature = 20°C. Ans: dynamic viscosity of air, µ at 20°C = 1.81 x 10-5 kg/ms (from the air temperature and viscosity table) For standard cyclone, B= 0.25 x 0.50m = 0.13 m H= 0.50 x 0.50m= 0.25m L1 = L2 = 2 x 0.50m= 1.0 m N = π/H * (2L1 + L2) = π/0.25 * (2 + 1) = 37.7 D0.5 = [(9 µ B) / (2πNV (ρp – ρa))]

1/2 = x µm, Therefore, d/d50 = 10 µm / x µm=? From the graph of d/d0.5 vs. efficiency, efficiency can be read. MULTIPLE CYCLONES Smaller cyclones operating together in parallel are collectively called as multiple cyclones. These are also called as multi-cyclones, tubular cyclones, tubular collectors and multiple collectors. Care to avoid channeling of dirty gas in any particular cyclone. Hopper should be designed taking the centrifugal action of small diameters of the cyclones. Since velocity and rate of dust collection is higher, continuous storage bin emptying is required. It has good abrasion resistance, is compact, efficient in collecting heavy particles and has convenient inlet and outlet arrangement. Cyclone in series

Efficiency given by: η = ηp+ ηs(100-ηp) Where η = efficiency of the combination of both cyclones ηp = efficiency of the primary cyclone ηs = efficiency of the secondary cyclone

� Secondary cyclone serves to remove particles which are not collected in the first cyclone left due to accidental entrainment in the vortex and entrainment due to eddy .

� Advantageous when large degree of dust collection required. Even if primary is plugged still secondary can work. A primary large diameter can be used to collect coarse material which would otherwise clog the smaller passages of efficient cyclones.

� Generally installed before electrostatic precipitator. High overall efficiencies obtained in case particles can be removed in cyclones.

Efficiency Ranges of Cyclones Particle size range, µm

Efficiency Range, η, i.e. % weight collected Conventional High Efficiency

< 5 < 50 50 to 80 5 to 20 50 to 80 80 to 95 15 to 40 70 to 90 95 to 99

> 40 95 to 99 95 to 99

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BAG HOUSE FILTERS BAG FILTERS: Used in Cement Industry, Floor Mills, Ingot making, building materials dust removal, grain processing, soap powders, dry chemical recovery, Brick works, ceramics industry, chalk and lime plants, foundries, talc dust recovery, dry food processing, metals dust recovery, fertiliser dust removal, etc. One of the oldest and most widely used methods of separating particulates from a carrier gas is “filtration”. Material of filter: Woven or felted textile material. Heat resistant materials are used: e.g. Fabric glass (resist upto 150°C), Teflon (resists upto 500°C). These are in the shape of sleeve or tube, with dia varying between 12.5 to 30 cm and length 2 to 6 m based upon the type of industry and need. Fiber used in making bags is “Crossed mesh”. Spacing 100 to 200 µm. Thickness of fiber ≥ 500 µm. Filter should be: 1. Durable 2. Chemical resistant 3. Robust to resist rupture 4. Temperature tolerant HEPA: High Efficiency Particulate Air Filter used for military and nuclear reactor container vessels. Material of very fine glass fiber. Fiber dia. < 1 µm held together by organic binders. Efficiency 99.97 % for ≥ 0.3 µm particles. Air Cloth Ratio (ACR) = It is an important parameter for the selection of material to be filtered. = (Vol. of gas flow rate filtered) / (Surface area of the filter) = (m3/s) / m2 = m/s = Also called Filtering Velocity = Superficial Velocity. Range = 5 to 75 m/s High ACR: for more flow rate less surface area. Low ACR: more surface area of filter and less gas flow rate. MECHANISM FOR PARTICLE SEPERATION BY FILTER:

1) Inertial Impaction 2) Direct Interception 3) Diffusion

The particles in the dust laden air follow the laminar path as long as they reach the filter element. When the filter element comes in the way the particles are unable to follow the air path and get attached to the filter element and the air gets clean. Larger the particle more the mechanism is effective. Smaller particles after reaching near to the fiber get attracted and adhere because of Vander Wall’s forces. Small particles are retained on the fabric, initially through interception and

electrostatic attraction; and latter particles more efficiently. BAGHOUSE: A closed chamber containing several vertically hangUpper ends of the bags are closed, and lower ends are attached to a hopper where the inlet of flue gas is also located. The upward moving gas drops out particulate matter in these bags, which settle down into the hopper, and clis provided with an automatic shaking device for cleaning the bags having deposited dusts. In some cases the cleaning is done by introducing air flow as in pulse jet filters and reverse air flfilters.

CLASSIFICATION OF BAG FILTERS

1. On the basis of cleaning time: a. Intermittent type:

electrostatic attraction; and latter on, when a dust mat is formed, the fabric starts collecting

A closed chamber containing several vertically hanging fabric cylindrical bags. Upper ends of the bags are closed, and lower ends are attached to a hopper where the inlet of flue gas is also located. The upward moving gas drops out particulate matter in these bags, which settle down into the hopper, and cleaner gas goes out through the fabric filters. The hanging bags is provided with an automatic shaking device for cleaning the bags having deposited dusts. In some cases the cleaning is done by introducing air flow as in pulse jet filters and reverse air fl

Installation example of Fabric Filter

CLASSIFICATION OF BAG FILTERS

On the basis of cleaning time: a. Intermittent type: Single chamber, suitable for small size particles and handling small

volume of flow operations.

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when a dust mat is formed, the fabric starts collecting

ing fabric cylindrical bags. Upper ends of the bags are closed, and lower ends are attached to a hopper where the inlet of flue gas is also located. The upward moving gas drops out particulate matter in these bags, which

eaner gas goes out through the fabric filters. The hanging bags is provided with an automatic shaking device for cleaning the bags having deposited dusts. In some cases the cleaning is done by introducing air flow as in pulse jet filters and reverse air flow

Single chamber, suitable for small size particles and handling small

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b. Continuous type: Compartmentalized, for handling large volume operation.

2. On the basis of cleaning mechanism: a. Shaking: Bags suspended with hooks. Hooks give oscillation. This oscillation provides enough energy to dislodge the dust. The purpose of oscillation is to provide enough energy to dislodge the dust. It is a violent/vigorous movement. Cleaning is violent that’s why life of bag is not so much. It requires replacement after certain time interval. Its for mild dusts. Generally used for small scale operation. b. Reverse air flow: Iron/steel ring sewed along the length of bag to avoid total collapsing. c. Pulse jet: It’s also violent/ vigorous. High air pressure air is introduced from the top through Venturi. Short waves are created and bags inflate along the length. This helps in dislodging the particulate matters deposited on the surface of the bag. Advantages of pulse jet

� We can do filtration and cleaning of bags at the same time. � We need not to shut down any system. � All the bags are cleaned simultaneously. � Highest volume of air flow ratio

BAG FILTERS Darcy’s relation for pressure drop ∆P (Important for estimation of design parameters)

� Even after cleaning constant mass of collected dust always remain on the filter. I.e. “Equilibrium dust content of filter”

� Equilibrium dust content depends upon i. Type of filter ii. Size of dust iii. Type & timing of cleaning method.

∆P = ∆Pf + ∆Pd = K1V + K2V.Cma = (K1 + K2 Cma) V = (K1 + K2 Cma) Q/A Where, ∆P = Pressure drop across dust loaded filter ∆Pf = Pressure drop across the filter when it is clean ∆Pd = Pressure drop across the filter with dust Cma = Mass area concentration of dust. Kg/m2 of the bag. K1 = Constant for filter element, Ranges 12,000 < K1 < 1,20,000, Unit: Newton.Sec/m3 K2 = Constant for filter element, Ranges 10,000 < K1 < 1,30,000, Unit: Sec-1 Cma = more the thickness of dust more is Cma . = more the timing of cleaning more is Cma . Cmv = mass volume concentration of the dust, Unit: kg/m3

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Cma = Ratio of the mass collected when the filter is cleaned to the area of filter = (Q . Cmv . t ) / Af Where, Af = Area of filter t = time since the filter is cleaned. Just after cleaning, Cma = 0, thus ∆P = (K1 + K2 Cma) Q/A = K1Q/A therfore, K1 = ∆P.A / Q Before cleaning Bag: K2 = [(∆P.A / Q) – K1 ] / Cma

Power required to overcome the pressure drop = Q. ∆P watt, where ∆P = pressure drop in Newton/m2, Q = Volumetric flow rate of air in m3/s. Q. A filter has K1 = 30,000 N.s/m3 and K2 = 75,000 s-1. The filter area is 8000 m2, Q is 120 m3/s and Cmv is 0.02 kg/m3. What is the pressure drop through the filter immediately after cleaning and after 3 hrs of operation? (Ans: 450 N/m2, after 3 hrs. 4095 N/m2) Q. A filter has 1000 m2 of face area and treats 10 m3/s of air carrying dust with a concentration Cmv equal to 0.001 kg/m3. Assume K1 = 20,000 N.s/m3, and K2 = 25,000 S-1. If the filter must be cleaned when ∆P = 2000 N/m2, after what period of time cleaning must occur? Q. In a test for measuring K1 & K2, the following data were obtained: ∆P after cleaning = 400 N/m2 ∆P before cleaning = 2100 N/m2 Flow rate = 0.5 m3/s Mass collected = 55 kg Filter area = 40 m2 Determine K1 & K2. Advantages of the fabric filters:

� High collection efficiency for all particle sizes especially for particles smaller than 10 µm � Retention of finest particles possible � Simple construction and operation � Nominal power consumption � Dry disposal of collected materials

Disadvantages of fabric filters: � Larger size of equipment � High initial installation cost � High maintenance and fabric replacement cost � Humid gases makes the filter blind and blocks the passage of air through it � High temperature gases or corrosive gases destroys the fabric

ELECTROSTATIC PRECIPITATOR

Its application: ESP is widely used in thermal power plants, pulp and paper industries, cement industry, mining and metallurgical industries, iron and steel plants, chemical industries, etc. ESP has many advantages over other types of particulate collection devices. Precipitators are unique in that the collecting force is applied only to the material being collectethe total gas stream. ESP has the ability to handle very large gas flow rate (Q) and can remove very small particles at high efficiency. They can operate over a wide range of conditions, temperatures up to 800°C and pressure of 50 atmospheres.

ELECTROSTATIC PRECIPITATOR (ESP)

Installation example of ESP

idely used in thermal power plants, pulp and paper industries, cement d metallurgical industries, iron and steel plants, chemical industries, etc.

ESP has many advantages over other types of particulate collection devices. Precipitators are unique in that the collecting force is applied only to the material being collecte

ESP has the ability to handle very large gas flow rate (Q) and can remove very small particles at high efficiency. They can operate over a wide range of conditions, temperatures up to 800°C and pressure of 50 atmospheres.

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(ESP)

idely used in thermal power plants, pulp and paper industries, cement d metallurgical industries, iron and steel plants, chemical industries, etc.

ESP has many advantages over other types of particulate collection devices. Precipitators are unique in that the collecting force is applied only to the material being collected and not to

ESP has the ability to handle very large gas flow rate (Q) and can remove very small particles at high efficiency. They can operate over a wide range of conditions,

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Dry collection of particles from hot gas streams can be obtained by electrostatic precipitation of the particles. The ESP is usually constructed of alternating plates and wires. A large direct current potential (30 to 75 kV) is established between the plates and wires. This results in the creation of an ion field between the wire and plate. As the particle laden gas stream passes between the wire and the plate, ions attach to the particles, giving them a net negative charge. The particles then migrate toward the positively charged plate where they stick. The plates are rapped at frequent intervals and the agglomerated sheet of particles falls to a hopper. Unlike the bag house, the gas flow between the plates is not stopped during cleaning. The gas velocity through the ESP is kept low (less than 1.5 m/s) to allow particle migration. Thus, the terminal settling velocity of the sheet is sufficient to carry it to the hopper before it exits the precipitator.

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Particles in air may have a positive, negative or no charge because air is a poor conductor of electricity. Introduction of a high energy electrical field in a gas result in the formation of large concentration of charged ions. This charge is given to particle in electrically active region i.e. the Corona region, which is an active glow zone near negative discharge electrodes. As the particle laden gas stream passes through this corona region it acquires a net negative charge. The particles then migrate toward the positively charged plates where they stick. The plates are rapped at frequent intervals and the accumulated sheet of particles falls to a collecting hopper below. Advantages of ESP:

� High collection efficiency (>99% collection efficiency can be achieved) � Even more efficient for smaller size particles, <0.1 µm can also be removed � Low maintenance & low operating cost � Low pressure drop � Can handle large volume of air � Can handle air having high temperature up to 800° C � Treatment is continuous � Time taken for treatment is less

Disadvantages of ESP: � High initial cost � Sensitive to variable dust load and flow rate. � High initial installation cost. � High voltage requires personnel safety for the unit operator. � Efficiency decreases with time. � Require huge space. � It is effective only for particulates separation and the gases can’t be removed by ESP.

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� Indian coal has high ash content with low sulphur content; to improve conductivity SO2 is injected.

� High electrical resistance of fly ash by Indian coal makes it less efficient.

FLY ASH RESISTIVITY IN ESP (Fly ash: Fly ash is the particulate matter carried in the flue gases from furnaces burning fossil fuels.) ESPs are often used to collect fly ash. The strongest force holding fly ash to the collection plate is the electrostatic force caused by the flow of current through the fly ash. The fly ash acts as a resistor to resist the flow of current through it. If the resistivity is too high there is an insulating effect and thus the collection efficiency decreases. The presence of SO2 in the gas stream reduces the resistivity of the fly ash. This improves particle collection efficiency. However, to limit the SO2 emission in the flue gases coal having low sulphur has always been a choice. Thus for improving the collection efficiency SO2 is injected in the ESP. ESP Efficiency proposed by Deustch η = 1 – e [- ( A. w ) / Q ] - Equation ( ESP -1) Where, η = Collection efficiency A = collection area of plates, m2 w = migration velocity of particles, m/s Q = air flow rate, m3/s Thus, w = - Q / A ln (1- η ) A = - Q / A ln (1 - η ) The migration velocity of the particles (w), is a function of the electrostatic force. Which is defined by the following equation: w = q Ep C / 3 π dp µ Where, q = charge in Coulomb Ep = collection field intensity, volts/m dp = diameter of the particle, m µ = dynamic viscosity of the air, Pa.s C = Cunningham correction factor. Q. Determine the collection efficiency of the ESP having length 6.10 m, Height 7.32 m, number of passages = 5, for the particles having a drift velocity of 0.184 m/s. Given gas flow rate = 19.73 m3/s. Solution: Area of single plate = 7.32 x 6.10 m2 = 44.65 m2 Five passages means four plates, each plate with two surfaces so total surfaces = 8, therefore total surface area = 8 x 44.65 = 357.2 m2 η = 1 – e [- ( A. w ) / Q ] = 1 – e [ - (357.2) (0.184) / 19.73 ] = 0.964 = 96.4 % Q. An electrostatic precipitator with 6000 m2 of collector plate area is 97 % efficient in treating 200 m3/s of flue gas from a 200 MW power plant. How large would the plate area have to be to increase the efficiency to 98 % and 99%. Solution: w = - Q / A ln (1- η ) = - (200 m3/s) / (6000 m2) ln ( 1 – 0.97 ) = 0.117 m/s

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to achieve 98% efficiency, the area required, A98 A98 = Q / w ln ( 1 – η ) = - (200 m3/s) / (0.117 m/s) ln ( 1 – 0.98 ) = 6690 m2 To achieve 99 % efficiency, the area required, A99 A99 = Q / w ln ( 1 – η ) = - (200 m3/s) / (0.117 m/s) ln ( 1 – 0.99 ) = 7880 m2 Thus we can see, if η = 97 % has area of plates 6000 m2, the area required to increase efficiency to 98 %, area = 6690 m2, for 99%, area = 7880 m2 (i.e. 6690 + 1190, for 1 % increase it required 1190 m2).

� These calculations suggest, the additional percent efficiency enhancement demands greater area.

Q. A plate type electrostatic precipitator for use in a cement plant for removing dust particles consists of 10 equal channels. The spacing between the plates is 0.15 m, and the plates are 2 m high and 2 m long. The unit handles 10,000 m3/hr of gas. What is the efficiency of collection? What should be the length of plates for achieving 99% collection efficiency if other conditions are same. For cement plants generally value of w i.e. particle migration velocity consider 0.10 m/s.

SCRUBBERS OR WET COLLECTORS It is based on the principle of Henry’s law and Brownian diffusion. Molar fraction of the gas dissolved in liquid is directly proportional to the partial pressure of gas in the air. The different stages which help in scrubbing the gases involve impingement, interception, Diffusion and Condensation. These are devices which utilize a liquid to assist in removal of particulates from the carrier gas stream. Generally, water is used as the scrubbing liquid. In a wet collector, the dust is agglomerated with water and then separated from the gas together with water. Impingement: When gas containing dust is swept through an area containing liquid droplets, dust particles will impinge upon the droplets and if they adhere they will be collected by them. In general, the efficiency of collection is more when the liquid droplet is approximately 100 to 300 times the size of the dust particle, in order to increase the number of inelastic collision. Interception: Particles that move with gas stream may not impinge on the droplets but can be captured because they push against the droplet and adhere there. This is known as interception. Diffusion: Diffusion of the gases into the liquid medium is proportional to their partial pressure. Condensation: Condensation of the liquid medium vapours on the particulates increases the size and weight of particles. Consequently, it helps in the easy removal of particulates.

The overall efficiency of a scrubber is however dependent to a great extent upon impingement and diffusion process. Flue gas is made to push up against down falling water current. The particulate matter and gases mixes up with the water droplets and thus is separated from the flue gases. The particulate

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matters and gases dissolved in the water are further treated before finally discharging out from the industry. Types of Scrubbers: 1. Spray towers 2. Venturi scrubbers 3. Cyclone scrubbers 4. Packed scrubbers 5. Mechanical scrubbers Spray Towers: It is the simplest type of gas scrubber. It can be either round or rectangular. Gas is passed against the direction of falling drops of liquid usually water from a top spray nozzles. Units of this type are used to remove generally coarse dusts having dia in range of > 1 to 2 µm where high efficiency is not required. It helps in removing fly ash. Venturi Scrubbers: It is a high energy wet scrubber with good efficiency. In this the particulate laden stream is directed through a venturi tube at throat velocity of 60 to 100 m/sec. Water sprays are introduced just ahead of the venturi throat. The faster the gas passes through the venturi, the higher is the efficiency. The dust laden flue gas enters the scrubber and is accelerated to a high velocity while passing through the converging section and approaching the throat section. The velocity of the flue gas is the maximum at and in the throat section of the scrubber. Venturies are the most frequently used scrubbers. They are used generally for small gas rates that contain predominantly small size particles. It is also suited for variable and intermittent gas flows. Venturi scrubber among the other wet scrubbers has comparatively greater cleaning efficiency. Advantages of Scrubbers: � Low initial cost � Moderately high collection efficiency for smaller size particles � Can be used for treating high temperature exhaust gases � It removes both particulates as well as gaseous matters � There is no particle re-entrainment and plugging problem Disadvantages of Scrubbers: � High power consumption for achieving higher efficiency � Corrosion and abrasion problem of inner walls � Waste water generated requires its treatment before being finally discharged

Venturi Scrubber

Schematic di

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iagram of spray tower

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AIR POLLUTION ABATEMENT TECHNOLOGIES

Long range strategy for air pollution control involves control of pollutants at their sources in following ways: 1) Use devices to remove all or parts of the pollutant from the gases discharged to atmosphere. 2) Change the raw material in pollution producing processes. 3) Change the operation of process so as to decrease the pollutant emitted. 1. Applicable to all emissions

A. Decrease or eliminate production of emissions � Change specification of process � Change design of product � Change process temperature, pressure, or cycle � Change specification of material � Change the product

B. Confine the emissions � Enclose the source of emissions � Capture the emission in industrial exhaust scheme � Prevent drafts

C. Separate the contaminant from effluent gas scheme � Scrub with liquid

2. Applicable specifically to gaseous emissions

A. Decrease or eliminate gas or vapor production � Change the process � Change from liquid or gaseous to solid material � Changes to process that does not require gaseous or liquid material

B. Burn the contaminant to CO2 and H2O � Incinerator � Catalytic burners

C. Adsorb the contaminant � Activated carbon

3. Applicable specifically to particulate matter

A. Decrease or eliminate particulate matter production � Change process � Change from solid to gaseous material � Change dry to wet solid material � Change particle size of solid material � Changes to process that does not require particulate material

B. Separate the contaminant from effluent gas stream � Gravity separator � Centrifugal separators � Filters � Electrostatic precipitator

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Alternate control strategies involve in the form of: 1. Stringent pollution prevention acts and rules 2. Effective implementation of the pollution prevention laws by regular inspection and

monitoring by pollution control authority 3. Taxes and polluters to pay policy 4. Subsidies for industries which uses pollution control devices 5. Fines to polluting industries, etc.

The other measures for pollution prevention include:

1. Dispersion/dilution: By winds, through long chimneys (stacks) 2. Proper site selection for the industrial establishments. 3. Natural settling of the particles: Generally for particle > 20 µ m dia. 4. Absorption: The gaseous as well as particulate pollutants from the air get collected in the

rain or mist, and may settle out with that moisture. This phenomenon takes place below the cloud level. When falling raindrops absorb pollutants it is also known as washout or scavenging. The process however is not effective for particles having dia < 1 µ m.

5. Rainout: Process involving precipitation above the cloud level, where submicron particles present in the atmosphere in the clouds, serve as condensation nuclei, around which drops of water may form, and fall out as raindrops. This phenomenon helps in increased rainfall and fog formation in urban areas.

6. Adsorption: On natural surfaces, such as soils, rocks, leaves, grass, buildings and other surfaces.

7. Controlling pollution at the source through PCD- Pollution Control Devices. 8. Technology improvement: Eco-friendly technologies. 9. Alternative fuel uses, substitution of fossil fuels by alternative sources of energy, etc.

CONTROLLING VEHICULAR EMISSIONS � Main contribution: Carbon Monoxide CO, Hydrocarbons HC, Nitrogen Oxides NOx . � An ideal IC Engine (Internal Combustion Engine) burns the fuel completely to CO2,

water and Nitrogen, if the Stoichiometric mixture of Air and Fuel is 14.7. � Air / Fuel ratio = 14.7, means 14.7 parts air and one part of fuel (petrol/diesel). � Vehicles having petrol engines during acceleration, high speed, at start, idling have high

fuel content mixture also called Rich mixture, which encourages production of CO, unburnt HC since there is not enough oxygen for complete combustion.

� Lean mixture: where the air provided is more than the required. � Since Diesel engines run with very lean mixtures, emission of HC and CO is inherently

very low. However, because high compression ratios create high temperature, therefore, NOx emissions are relatively high.

� Use of cleaner fuel with less lead and sulphur content minimizes the pollution. � Use of catalytic convertor: made of metals like platinum, palladium etc. which helps in

oxidising CO & HC into their final end product of CO2 and also helps to reduce NO into Nitrogen. These noble metals are highly active and resist sulphur poisoning.

� Proper tuning of engine and carburetor prevents vehicular pollution.

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Emissions from gasoline powered vehicles are generally classified as: A. Exhaust emission B. Crank case emission C. Evaporative emissions

A. Exhaust Emission: These are the emissions after burning of the fuel which is emitted as

exhaust. The important exhaust emissions from a gasoline engine are carbon monoxide, unburnt hydrocarbons, nitrogen oxides and particulates. These emissions vary with air-fuel ratio, spark timing and engine operating conditions.

B. Crank Case Emission: It consists of engine blowby which leaks past the piston mainly during the compression stroke, and of oil vapours generated into the crank-case. The quality of blowby depends on engine design and operating conditions. Worn out piston rings and cylinder liner may greatly increase blowby. About 20 o 40% of the car’s total hydrocarbon emissions are sent into the atmosphere from the crankcase. These emissions are called crankcase blowby. All vehicles now are required to have Positive Crankcase Ventilation (PCV) valve to eliminate blowby emission.

C. Evaporative Emission: This includes evaporation from Fuel tank and Carburetor. Evaporation of volatile hydrocarbons from the fuel tank is controlled by placing an activated charcoal adsorber in the tank vent line. Thus, as the gasoline expands during warm weather and forces vapour out of vent where the HC is trapped on the activated carbon.

During engine operation, the HC vapours generated in the carburetor are vented internally to the engine intake system. After the engine is shut off, the gasoline in the float bowl continues to evaporate because of the high temperature in the engine compartment. This phenomenon is called “Hot Soak”. These losses may be controlled by the use of an activated carbon adsorption system called “Canister” or by venting the vapours to the crankcase. The modern fuel injection systems do not have carburetors and thus avoid evaporation losses.

Exhaust Treatment Devices: The basic technique is to promote oxidation of HC and CO after emission from the engine. Exhaust gases are either treated by introducing additional air supply and by providing sufficient volume to ensure adequate reaction time OR by use of catalytic converters. Exhaust Gas Recycling (EGR) System: The exhaust recirculation systems utilize up to 15% of the exhaust stream to recycle into the intake manifold with the fresh air-fuel mixture. The exhaust gas dilutes the fresh charge and thus lowers the flame temperature and consequently the nitrogen oxides. The fuel-air equivalence ratio has a marked influence on the effectiveness of exhaust recycling. The exhaust gas recycling reduces volumetric efficiency and hence maximum power output available from an engine of given size. It also adversely affects the vehicle drivability. Four strokes in an ignition engine are: 1. Fuel intake stroke 2. Compression stroke 3. Power stroke, and 4. Exhaust stroke

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Flash Point: The flash point is defined as the minimum temperature at which the given oil evolves just sufficient vapour to form inflammable mixture with air. Fire Point: it is the minimum temperature at which the oil vapours will continue to burn instead of just flashing. For the same product the fire point is higher than the flash point. Smoke Point: It is the maximum flame height in millimeters to which a kerosene will burn without smoking in a standard apparatus. Good quality kerosene shows smoke point of 20 to 25 mm. Octane number: Maximum power is derived from gasoline when it burns silently and relatively slowly. Under certain engine conditions the combustion may begin smoothly and then the whole of the unburnt fuel may burn rapidly with the formation of pressure waves. This leads to knocking of the gasoline engine. Much of the generated power is wasted and the engine life is shortened when knocking occurs. A good gasoline should resist knocking. The anti-knock quality of fuel is measured in a standard engine in terms of the relative performance of two standard fuels and then expressed as its octane number. By definition, the octane number of a gasoline is equal to the percentage by volume of iso-octane (2,2,4-trimethyl pentane) in a mixture of n-Heptane and iso-octane having the same knocking tendency as the sample being tested. n-Heptane is of poor anti-knock quality and is assigned zero octane number. On the other hand iso-octane has excellent anti-knock quality and is assigned an octane number of 100. If a gasoline matches with a 80/20 blend of iso-octane and n-Heptane, its octane number is 80. The octane rating of commercial gasoline is not entirely due to the hydrocarbons only. Certain additives are put into the gasoline to raise its octane number. The most effective and common additive is Tetra Ethyl Led (TEL). Other anti-knock agents are Tetra Methyl Lead (TML). Cetane number: There is a time lag i.e. delay period between the injection of diesel fuel into hot compressed air and its ignition. If the delay period is unduly large there will be accumulation of too much fuel in the cylinder, which will eventually burn with an undue rapidity. A very rapid rise in the cylinder pressure will follow and cause a “diesel knock”. The ignition quality of diesel fuel is measured in a standard engine by matching against blends of two reference fuels and expressed in terms of Cetane number. A n-paraffin (n-hexadecane or Cetane, C16H34) is given 100 cetane number and an aromatic (α-methyl naphthalene) is given zero cetane number. If the given fuel matches with a 40/60 blend of cetane and α-methyl naphthalene it is assigned a cetane number of 40. High speed engines above 1500 rpm need high cetane number - 45 to 50 fuels. A low speed engine is not so demanding since there is more time available for the combustion. A cetane number of 25 to 30 is sufficient for low speed engines.

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EXERCISE Q. Draw a schematic diagram of gravity settling chamber having five numbers of trays. Also

show the direction of the entrance of dirty gas and exit of clean gas from the chamber. Q. Using the cyclone of standard settling proportions with diameter 2.0 m and following

relationships: b = D/4, H = 0.5 D, Lb = 2D. Find (i) Number of turns in the cyclone (ii) Cutsize diameter of the particle, (Given, Viscosity of air = 1.55 x 10-5 m2/s, density of

particle = 3000 kg/m3, Flow of air = 1.0 m3/s)

Q. Calculate the number of cyclones required to treat a flow of 60 m3/sec with an inlet velocity of 15 m/sec. The diameter of cyclone is 1.8 m. (Ans: 10 cyclones)

Q. What is re-entrainment problem in air pollution control? How this problem is caused in

gravity settling chamber & cyclone? Suggest the remedial measures. Q. Justify the following statements:

(i) The laminar flow based settling chamber provides a very awkward design of chamber. (ii) Cyclone of small diameter can achieve better collection efficiency associated with some

inherent operation & technical problems. (iii) Cyclones as compared to gravity settlers are more effective in removing the smaller size

particles. Q. Write down two basic assumptions in the derivation of collection efficiency of turbulent

based settling chambers. Q. A settling chamber is 3 m wide, 10 m long and has 10 trays, including the bottom surface.

The tray spacing is 15 cm from centre to centre. At what interval will it be necessary to clean the trays of the chamber if maximum allowable dust thickness is to be 3 cm. Assume that mass-concentration of 0.35 kg/m3 and overall collection efficiency of 0.95. A stream of 1.5 m3/s of standard air containing particles of specific gravity of 1.5 flows through settling chamber.

Q. An air stream with flow rate of 7.0 m3/s is passed through a cyclone of standard proportions.

Determine the removal efficiency for particles with density of 1.5 gm/cm3 with diameters of 5 µm and 25 µm for the given parameters: (i) Gas inlet width = 0.5 m (ii) Gas inlet height = 1.0 m (iii) No. of turns = 5 (iv) Viscosity of air = 2.07 x 10-5 kg/m.s

Q. The diameter, number of turns and inlet gas velocity can’t be varied too much to achieve

higher cyclone efficiency. Justify with explanation. Q. Explain Air-Cloth-Ratio. What is fabric blind and how is it prevented?

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Q. A filter is to be designed to treat 25 m3/s of standard air with Cmv = 0.008 kg/m3. For this dust and for the type of filter proposed, it has been found that K1 = 40,000 N.s/m3 and K2 = 65,000 s-1. For filtering velocity of 1.0 cm/s, find

(i) How much surface area of filter is required? (ii) What is pressure drop through the filter immediately after cleaning? (iii) After what period of time must cleaning occur, if the filter must be cleaned when ∆P =

2000 N/m2. (iv) Find the number of bags required, if each bag is 30 cm diameter and 4 m length.

Q. What are advantages of using pulse jet bag filters? Q. A filter has 1000 m2 of surface area and treats 10 m3/s of air carrying dust with a

concentration Cmv equal to 0.008 kg/m3. Assume K1 = 30,000 N.S/m3 and K2 = 40,000 s-1. If the filter must be cleaned when ∆P = 2000 N/m2, after what period of time must cleaning occur? Also find the number of bags required, if each bag is 30 cm diameter and 4 meter long. Also compute the power requirement to overcome the pressure drop.

Q. Design a parallel type electrostatic precipitator with 10 channels to handle 10,000 m3/hr of

gas for efficiency of 90% and 99% respectively. Assume velocity of gas entering the plates to be kept as 0.1 m/sec, spacing between plates be 0.15 m and height of each plate as 2m.

Q. What is bag house? Describe the working principal of a bag house with the advantages and

disadvantages of using it. A dusty gas from a fertilizer plant is flowing at the rate of 15 m3/sec at 400oC. What type of cloth do you recommend? If the design filtering velocity must be limited to 0.01 m/sec, what is the cloth area needed?

Q. What is Cyclone dust separator? Show the proportions of a standard cyclone with schematic

diagramme. Define its working principle with advantages and disadvantages of using it. For which types of Industries you will recommend Cyclone separators?

Q. Write notes on:

(i) Design parameters of cyclones (ii) Ethanol as bio-fuel (iii) Transesterification (iv) PCV system in vehicles (v) Fuel Injection system (vi) Henry’s law (vii) EGR system (viii) Filtering velocity of a fabric filter

Q. Write a detail note one Electrostatic precipitator, including diagrams. Q. What is stoke’s law? On its basis derive the formula used to find out the settling velocity and

diameter of a particle to be separated in a separating device?

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Q. Write in detail about fuel cells, its advantages. What are fuel reformers? Q. Describe various types of scrubbers and their advantages? Q. Explain Biofuels with emphasis on Ethanol and Biodiesel and their manufacturing processes. Q. Describe two and four stroke engines? Why two stroke engines pollute more than four stroke

engines? Q. In a scrubber, stream no. 1 is the recirculation liquid flow stream back to the scrubber and it

is 30 gallons/minute (gpm). Liquid being withdrawn for treatment and disposal (stream 4) from the recirculation tank is 12 gpm. Assume that inlet gas stream (stream2) is completely dry and that the outlet stream (stream 6) has 3.20 gpm of moisture evaporated in the scrubber. Stream 3 is also the outlet from the scrubber which is being added into recirculation tank but how much stream 3 is added to recirculation tank is not known. The water being added to the scrubber through recirculation tank is through stream no. 5. On the basis of mass balance, make the diagram of the process and find out the amount of water continually added to the wet scrubber through stream no. 5 in order to keep the unit running?

References: 1. Bela G. Liptak, “Environmental Engineers’ handbook”, Air Pollution, Volume 2, Chilton Book Company,

Pennsylvania, 1974. 2. C.S. Rao, “Environmental Pollution Control Engineering”, Wiley Eastern Limited, New Delhi, 1991. 3. M N Rao, and H V N Rao, “Air Pollution”, TMH Publishing Co. Ltd., New Delhi, 1989. 4. Mackenzie L. Davis, and David A. Cornwell, “Introduction to Environmental Engineering”, McGraw-Hill

International Edition, Singapore, 1998. 5. Martin Crawford, “Air Pollution Control Theory”, Tata McGraw Hill Edition, New Delhi, 1976. 6. Samir Sarkar, “Fuels and Combustion”, Orient Longman, Mumbai, 1990.

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