Air Quality: Definition, Characteristics PDF

Air Quality: Definition, Characteristics Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Air pollution  Classification of air pollutants  Particulate matter (PM)  Health effects of particulate matter  Gaseous air pollutant...

Air Quality: Definition, Characteristics Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Air pollution  Classification of air pollutants  Particulate matter (PM)  Health effects of particulate matter  Gaseous air pollutants, their properties and significance  Air quality monitoring  Ambient air pollution monitoring  Stack monitoring  Layers of the atmosphere Department of Chemical Engineering, TIET November 22, 2024  Air pollution Air pollution may be defined as the presence of one or more contaminants in the air in such quantities and for such durations which may be or tend to be injurious to human, animal or plant life, or property, or which unreasonably interferes with the comfortable usage of air. Air quality is affected by various economic and industrial activities which alter the composition of air and affect the environment locally, regionally and globally. It is estimated that anthropogenic sources have changed the composition of global air by less than 0.01%. However, this change has adversely affected the climate of the earth. Both natural and/or anthropogenic activities introduce air pollutants which can be solid (large or sub-molecular), liquid or gas into the atmosphere that pose problem to human health and other life forms on earth. These air pollutants include CO, SOx, NOx, SPM, CO2, ozone, photochemical smog, etc.  Fuel C, H, S, N, Pb, Hg, ash  + Air N 2 +O 2  Main cause of air pollution  is Combustion  CO 2 , CO, NO X , SO X , Pb, Hg, SPM, RSPM, PM 10 ,VOCs  During combustion, elements Departmentin the fuel getEngineering, of Chemical burned inTIET air to form various November 22, 2024 air pollutants.  Classification of air pollutants Natural contaminants: Natural fog, pollen grain, bacteria, volcanic eruption, wind blown dust, lightning generated fires. Particulate (aerosols): Dust, smoke, fog, mists, fume. Gases and odor: SOx, NOx, CO, CO2, halogen compounds, hydrocarbons, radioactive compounds. Department of Chemical Engineering, TIET November 22, 2024 Contd.. Major classes Subclasses Typical members of subclasses Particulates Solid Dust, smoke, fumes, fly ash Liquid Mist, spray Gases Organic Hydrocarbons Hexane, benzene, ethylene, methane, butane, butadiene Aldehydes and Formaldehyde, acetone ketones Chlorinated hydrocarbons, alcohols Other organics Inorganic Oxides of carbon Carbon monoxide, carbon dioxide Oxides of sulphur Sulphur dioxide, sulphur trioxide Oxides of nitrogen Nitrogen dioxide, nitric oxide Other inorganics Hydrogen sulphide, hydrogen fluoride, ammonia Department of Chemical Engineering, TIET November 22, 2024  Particulate matter (PM) PM is a complex mixture variable in size (0.01-100 μm), composition [metals, nitrates, sulfate, polynuclear aromatic hydrocarbons (PAH), volatile organic compound (VOC), etc.] and concentration. Toxicity and penetration depends on the composition and size of the particles. Solid or liquid particles with sizes from 0.005 to 100 μm, general term is aerosols. Dust originates from grinding or crushing. Fumes are solid particles formed when vapors condense. Smoke describes particles released in combustion processes. Smog used to describe air pollution particles. Department of Chemical Engineering, TIET November 22, 2024  Health effects of particulate matter Impact depends on particle size, shape and composition Large particles trapped in nose Particles > 10 μm removed in tracheobronchial system Particles < 0.5 μm reach lungs but are exhaled with air Particles 2-4 μm most effectively deposited in lungs Inhalable PM includes both fine and coarse particles. Coarse particles  aggravation of respiratory conditions, such as asthma. Fine particles  increased hospital admissions and emergency room visits for heart and lung disease  increased respiratory symptoms and disease  decreased lung function  premature death Other effects of particulate matter Decreased visibility Damage to paints and building materials Department of Chemical Engineering, TIET November 22, 2024  Gaseous air pollutants, their properties and significance Name Formul Properties of Significance as air a importance pollutant Sulfur SO2 Colorless gas, intense acrid Damage to vegetation, dioxide odor, forms H2SO3 in water building materials, respiratory system Sulfur SO3 Soluble in water to form H2SO4 Highly corrosive trioxide Hydroge H2S Rotten egg odor at low Extremely toxic n sulfide concentrations, odorless at high concentrations Nitrous N2O Colorless; used as aerosol Relatively inert; not a oxide carrier gas combustion product Nitric oxide NO Colorless; sometimes used as Produced during combustion an aesthetic and high-temperature oxidation; oxidizes in air to NO2 Nitrogen NO2 Brown or orange gas Component of dioxide photochemical Department of Chemical Engineering, TIET smog November 22, 2024 formation; toxic at high  Contd.. Name Formul Properties of Significance as air a importance pollutant Carbon CO Colorless and odorless Product of incomplete monoxide combustion; toxic at high concentration Carbon CO2 Colorless and odorless Product of complete dioxide combustion of organic compounds; implicated in global climate change Ozone O3 Very reactive Damage to vegetation and materials; produced in photochemical smog Hydrocarbo CxHy Many different Emitted from automobile ns compounds crankcase and exhaust Hydrogen HF Colorless, acrid, Product of aluminum smelting; fluoride very reactive causes reactive fluorosis in Department of Chemical Engineering, TIET November 22, 2024 cattle; toxic  Air quality monitoring Sr. No. Items to be monitored Unit 1 Suspended particulate matter (SPM) g/m3 2 Repairable particulate matter (RPM) g/m3 3 Oxides of nitrogen (NOx) g/m3 4 Sulphur dioxide (SO2) g/m3 5 Carbon monoxide (CO) mg/m3 Coal, Coke: SPM, CO2 Stack Air Furnace oil: SPM, CO2, SO2, NOx, CO, HC SPM (RSPM: 10 µ) Ambient SO2, NO2, CO, NH3 Department of Chemical Engineering, TIET November 22, 2024  Contd.. Ambient  Particulates  RDS (Respirable dust sampler)  HVS (High volume sampler)  Gases SO2, NO2, CO, NH3 Sampler Wet chemistry analysis Stack SPM: 1. Velocity monitor 2. Temperature indicator Stack velocity monitor 3. Sampler CO2, SO2, NOx, CO, HC Flue gas analyzer CS2, H2S, Cl2, HCl (mist) Collection of sample by handy stack sampler, Department followedof Chemical Engineering, TIET by wet chemistry analysisNovember 22, 2024  Layers of the atmosphere The atmosphere is generally divided into  Lower atmosphere, and  Upper atmosphere The lower atmosphere is generally considered to extend up to the top of the “stratosphere”, an altitude of about 50 km. Study of lower atmosphere is generally considered to extend up to the top of the “Stratosphere”, an altitude of about 50 km. Study of lower atmosphere is known as “Meteorology”. Study of the upper atmosphere is called “Aeronomy”. The earth’s atmosphere is characterized by variations of temperature and pressure with height. In fact, the variation of the average temperature profile with altitude is the basis for distinguishing the layers of the atmosphere. Department of Chemical Engineering, TIET November 22, 2024 Temperature profile of the atmosphere Department of Chemical Engineering, TIET November 22, 2024 Thank You Department of Chemical Engineering, TIET November 22, 2024  Meteorological parameters & air pollution Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Introduction  A brief history  What is atmosphere?  Boundaries of the atmosphere Earth’s atmosphere,  Horizontal atmospheric motion Principles of  Equatorial heating and polar cooling meteorology and  Atmospheric circulation The effect of the earth's rotation  Influence of the ground and the sea  Vertical atmospheric motion  Air density change with temperature  Air density change with pressure  Meteorological parameters  Conclusions Department of Chemical Engineering, TIET November 22, 2024  Introduction Learning objectives To make the student aware of dispersion phenomenon of air pollutants covering diffusion and advection, meteorological components, stability of atmosphere and corresponding plume shapes.  Meteorology depends greatly on the Earth's axis turn, which is responsible for air flow both in the northern and southern hemisphere.  Warm air rises at the solar-heated equator while cool air sinks to the poles due to equatorial heating and polar cooling.  Both horizontal and vertical motion in the atmosphere aid in understanding air pollution dispersion.  Measurements of meteorological parameters assist in the evaluation, modeling, and forecasting of air quality Department of Chemical Engineering, TIET November 22, 2024  Contd.. Role of atmosphere in source-sink relationship  Today, we will discuss about meteorological parameters and air pollution (meteorology of air pollution). Contd..  As you know, we have discussed like emissions sources, and its atmospheric transformations or formations of air pollutants, then their dispersion or diffusion their ultimate fate.  But as you know, even if the emissions are same, the concentrations of air pollutants in ambient air may be different at different places.  So, what is the difference? Basically the meteorological parameters govern all these emission sources, how much they will turn into air quality.  So, we will discuss about the impact of meteorology on air pollution means meteorological parameters and air pollution. How do they interact with each other? How these meteorological parameter  influence the air quality we will discuss around about the different places. metrological parameters roll into the air pollution, the fate of the air pollutants or decision of air pollution dispersion and the air pollution the total, the net worth of the air pollution in that sense. Contd..  Meteorology is the study of the atmosphere and the motions within it on short time scales (minutes to weeks).  Meteorology focuses on the atmospheric variables related to weather forecasting, at current or near-future conditions.  The manner air pollutants are dispersed and transported through the troposphere is determined by weather patterns (air quality forecasting, AQI). Contd..  Air movements influence the fate of air pollutants. So, any study of air pollution should include a study of the local weather patterns (meteorology).  If the air is calm and pollutants cannot disperse, then the concentration of the air pollutants will build up.  On the other hand, when strong, turbulent winds blow, pollutants disperse quickly, resulting in lower air pollutant concentrations. Contd.. Meteorological data helps:  Identify the sources of pollutants.  Predict air pollution events such as inversions and high-pollutant concentration days.  Simulate and predict air quality using computer models.  A brief history  The ancient Greeks contributed some of the earliest known observations and hypotheses on atmospheric phenomena, which led to the discovery of meteorology.  In 340, before Christian era (BCE), Aristotle produced Meteorological, the first summary on Schematic representation of the general circulation of atmospheric science atmosphere knowledge and usage of Earth’s orbit around the Sun. During this movement the Earth is rotated around an axis which has an angle of 23 ½ from its vertical axis The rotation axis is constant during the year. The result is that in June, when the northern hemisphere is turned towards the Sun, there is more direct light and longer duration of the day there. This results in warmer weather, compared to December when the northern hemisphere is turned away from the Sun Contd.. Hadley circulation  Scientists observed and created ideas on global atmospheric motion and regional scale meteorological phenomena in the 18th and 19th centuries.  In 1735, the planetary-scale George Hadley proposed a single cell circulation system in each hemisphere, with a low-pressure area around the equator and a high-pressure area around the Idealized general air circulation patterns, poles. drawn at the equinox. High-pressure zones are common around 300 latitude, producing clear skies and subsidence inversions  The air that moves along the surface of the Earth to close a Hadley cell is affected by Coriolis forces, causing it to curve toward the west if it is moving toward the equator or toward the east if it is moving toward the poles.  The resulting winds between roughly 30° and 60° are known as the westerlies,  and between the 30° and the equator they are the trade winds.  Near the equator, there is little wind since the air is mostly rising. That band is called the equatorial doldrums.  Similarly, the surface air is relatively calm around 30° latitude, forming a band called the horse latitudes. Contd..  The Earth is divided into six zones at different latitudes, the in the northern and three in the southern hemisphere which are:  0-300 Latitude, Hadley cell  30-600 Latitude, Ferrel cell  60-900 Latitude, Polar cell  The movement of air masses leads to the formation of zones of high pressure at latitudes 300 and 900 and, correspondingly, zones of low pressure at the Equator and at latitude 600  Based on the Coriolis effect (1835), the global circulation has three major Ideal temperature and surface pressure circulation cells in each hemisphere (Polar, distribution the rotation Earth with the three cell model Ferrell, and Hadley).  In 1904, established modern synoptic  The sun's rays heat the earth near the equator to a greater extent than at the poles. If the rotation of the earth were discounted, the heated air at the equator would rise, and cool air from the poles would move in to take its place. This would set up two theoretical cells, involving only longitudinal motion  However, the west-to-east rotation of the earth must be taken into account, since it has a profound effect on air currents, deflecting the winds to the right in the northern hemisphere and to the left in the southern hemisphere. The effect of the earth's rotation on wind velocity and direction is called the Coriolis force , and this  Thus, air movement on the global scale is not simply in longitudinal directions for the dual effect of heat differential between poles and equator and of the rotation of the earth along its axis establishes a more complicated pattern of air circulation.  The general global circulation pattern is composed of three cells of air movement in each hemisphere.  It is under this dual influence of thermal convection and the Coriolis force that high-and low-pressure areas, cold or warm fronts are formed.  One of the primary elements influencing air mass movement on this scale is the distribution of land and water masses over the surface of the earth. The great variance between conductive capacities of land and ocean masses accounts for the development of many of our weather systems. Over land masses, atmospheric temperature rises rapidly in the presence of solar radiation (day), then drops with equal rapidity in its absence (night), since landmasses quickly reradiate hear into the atmosphere. Conversely, air temperature over water rises and falls glory slowly, since heat energy received by water penetrates to deeper layers than het absorbed by land and is reradiated in lesser amount  At the equinox, the equator is directly under the sun, and the air there will be heated, become buoyant, and rise. As that air approaches the top of the troposphere (roughly 10 to 12 km), it begins to turn, some heading north and some south.  An eighteenth century meteorologist, George Hadley, postulated that the air would continue to the poles before descending. In actuality, it descends at a latitude of about 30° and then returns to the equator, forming what is known as a Hadley cell.  In similar fashion, though not as distinct, there are other cells, linked as in a chain, between 30° and 60° latitude, and between 60° and the poles. The descending air at 30° creates a persistent high-pressure zone with corresponding lack of clouds or rainfall that is a contributing factor to the creation of the world’s great deserts.  The deserts of southern California, the southwestern United States, the Sahara, the Chilean desert, the Kalahari in South Africa, and the great deserts in Australia, are all located at roughly 30° latitude for this reason.  Conversely, rising air near the equator and near 60° latitude tends to be moist after passing over oceans. As it cools, the moisture condenses, causing clouds Contd..  At the onset of World War II, the technological advances came to the forefront.  Radar development (1935), as well as increased upper-air surveillance from weather balloons and aviation.  In 1937, introduce the methods for analysing the upper-level atmospheric wave structures. Schematic representation of the general circulation of atmosphere Contd..  Based on earlier mathematical atmospheric modeling efforts, in 1950 produced the first computer which generate the weather forecasts.  Weather satellites (1960), Doppler radar (1990), and other technological innovations have continued to shape our understanding and forecasting of atmospheric processes.  What is atmosphere  Earth's atmosphere forms a very thin layer surrounding the globe.  95% of this air mass is within 20 km of the earth's surface.  This 20 km depth contains the air, we breathe as well as the pollutants we emit.  This layer, called the troposphere, is where we have our weather and air pollution  Vertical Division of the Atmosphere – Temperature Change The main characteristics of the troposphere are:  Continuous and uniform decrease of the temperature with height. The rate of temperature decrease is close to 6.50C/km.  The wind velocity increases with height since at the lower heights there is the effect of the Earth’s surface friction. The maximum velocity is observed at the upper layer of the troposphere.  Almost the whole quantity of water in all three phases (solid, liquid and vapor) occurs in the troposphere with the maximum concentration occurring at the lower layers.  The weather phenomena occur inside the troposphere. Atmosphere’s layers  Boundaries of the atmosphere  The lower boundary of the atmosphere has a perfectly well- defined but quite uneven, the surface of the land and the oceans.  Its upper boundary is not as well- defined, the atmosphere simply becomes thinner and thinner with increasing height until it is as thin as outer space Contd..  If the atmosphere were peeled off the earth and its edges stitched together, it would have an approximate thickness of 20 miles and a diameter of 16,000 miles.  This large width and small depth mean that most of the motions in the atmosphere must be horizontal.  they really play a significant role in the atmospheric motions or atmospheric dispersion of air pollutants and all other metrological parameters, transportation, temperature, humidity, etc.  Horizontal atmospheric motion  The horizontal movement of the atmosphere (e.g., winds) is driven mostly by uneven heating of the earth's surface.  The horizontal movement is also affected by the rotation of the earth (i.e., the Coriolis force) as well as the influence of the ground and the sea.  Equatorial heating and polar cooling  The average annual solar heat transfer to the earth's surface near the equator is 2.4 times that at the poles.  The atmosphere moves in response to this difference in heating, and in so doing transports heat from the tropics to the Poles.  Equatorial heating and polar cooling  Transports of heat primarily depends upon three processes:  Sensible heat flux: It is the process where heat energy is transferred from the Earth's surface to the atmosphere by conduction and convection.  This energy is then moved from the tropics to the poles by advection, creating atmospheric circulation.  Atmospheric circulation moves warm tropical air to the polar regions and cold air from the poles  A brief history  Based on the Coriolis effect (1835), the global circulation has three major circulation cells in each hemisphere (Polar, Ferrell, and Hadley).  In 1904, established modern synoptic meteorology (large-scale weather analysis taken at simultaneous time periods).  Equatorial heating and polar cooling  Latent heat flux: It moves energy globally when solid andliquid water is converted into vapor.  This vapor is often moved by atmospheric circulation vertically and horizontally to cooler locations where it is condensed as rain or is deposited as snow releasing the heat energy stored within it.  Equatorial heating and polar cooling Surface heat flux  The large quantities of radiation energy are transferred into the Earth's tropical oceans.  The energy enters these water bodies at the surface when absorbed radiation is converted into heat energy  Horizontal transfer of this heat energy from the equator to the poles is accomplished by ocean currents.  Equatorial heating and polar cooling Significance to air pollution  Air pollutants in the air circulate in the same way as air in the troposphere does.  Air movement is caused by solar radiation and the irregular shape of the earth and its surface which causes unequal absorption of heat by the earth's surface and atmosphere.  The effect of the earth's rotation  If the Earth did not rotate and remained stationary, the atmosphere would circulate between the poles (high pressure areas) and the equator (a low-pressure area) in a simple back-and-forth (backward and forward) pattern.  But because the Earth rotates, circulating air is deflected.  The effect of the earth's rotation  Earth rotates on its axis, circulating air is deflected toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. This deflection is called the Coriolis effect  The Coriolis effect is responsible for many large-scale weather patterns on the earth. The effect of the earth's rotation Significance to air pollution Air movement around low-pressure fronts in the Northern Hemisphere is counter clockwise and vertical winds are upward, where condensation and precipitation take place.  The effect of the earth's rotation Significance to air pollution  High-pressure systems bring sunny and calm weather.  Anticyclones (high-pressure) are weather patterns of high stability, in which dispersion of pollutants is poor, and are often precursors to air pollution episodes.  The high-pressure area indicates a region of stable air, where pollutants build up and do not disperse.  Influence of the ground and the sea  Major mountain range like Himalayas is the major barrier to horizontal winds.  Different climates on one side than on the other side of the mountains.  Even smaller mountains and valleys can strongly influence wind direction.  Influence of the ground and the sea  The surface of the ground heats and cools rapidly from day to night and from summer to winter.  Solid ground is not mixed by the wind or convection currents, so heat cannot mix up and down.  So, solid surface temperature changes more rapidly than that of water bodies. Influence of the ground and the sea  The surface of oceans and lakes heats and cools slowly, mostly because their surface layers are mixed by the winds and by natural convection currents, thus mixing heat up and down.  Influence of the ground and the sea Example:  The summer sun warms the air above India more than the air over the surrounding oceans, which causes strong upward motion of the air over India.  Moist air from over the surrounding warm oceans flows inward to fill the low-pressure region caused by this rising air.  This moist air rises, cools, and forms the monsoon rains.  Vertical atmospheric motion  Vertical and horizontal motions in the atmosphere interact, the horizontal flows are driven by rising air at the equator and sinking air at the Poles.  In the atmosphere any parcel of air that is less dense than the air that surrounds, it will rise by buoyancy.  Vertical Atmospheric Motion  If any parcel denser than the surrounding air will sink by negative buoyancy.  Most vertical motions in the atmosphere are caused by changes in air density.  Air density change with temperature The density of any part of the atmosphere is given almost Air density change with temperature  Density is inversely proportional to temperature.  When temperature increases, with pressure constant, density decreases.  Air density change with temperature Example: Temperature inversion in atmosphere  A layer of cool air at the earth surface is overlain by a layer of warmer air. (Under normal conditions air temperature usually decreases with height.)  As a result, convection caused by air heating from below is limited to levels below the inversion  Diffusion of dust, smoke, and other air pollutants is likewise limited.  Air density change with temperature  Density is directly proportional to pressure.  As pressure increases, with constant temperature, density increases.  Thus, Air density will decrease by about 1% for a decrease of 10 hPa in pressure or 3 °C increase in temperature.  Meteorological parameters The main meteorological factors that affect dispersion are wind direction, wind speed and atmospheric turbulence (which is closely linked with the concept of stability).  Conclusion  Meteorology depends greatly on the Earth's axis turn, which is responsible for air flow both in the northern and southern hemisphere.  Warm air rises at the solar-heated equator while cool air sinks to the poles due to Equatorial Heating and Polar Cooling  Both horizontal and vertical motion in the atmosphere aid in understanding air pollution dispersion.  Measurements of meteorological parameters assist in the evaluation, modeling, and forecasting of air quality Department of Chemical Engineering, TIET November 22, 2024 Thank You Department of Chemical Engineering, TIET November 22, 2024  Atmospheric stability and plume behavior Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Atmospheric stability  Atmospheric stability classification  Dispersion coefficient  Plume  Plume behaviour/Types of plume  Looping  Fanning  coning  Lofting  Fumigating  trapping Department of Chemical Engineering, TIET November 22, 2024 Meteorological parameters Department of Chemical Engineering, TIET November 22, 2024  Environmental lapse rate (ELR)  Air movement in the atmosphere is strongly influenced by atmospheric stability.  In the troposphere, the temp of ambient air usually decreases with an FIGURE- The dry adiabatic lapse rate ( ) is a increase in altitude. This rate of temp d constant 10°C/km, but the saturated adiabatic change is called “lapse rate” (or lapse rate ( ) varies with temperature. In the s environmental lapse rate or ambient troposphere, ( ) is approximately 6°C/km. s lapse rate, ∧)  There are a number of factors, such as wind speeds, sunlight, and geographical features, that cause the actual ambient lapse rate in the real atmosphere  Differences between the ambient lapse rate and the adiabatic lapse rate determine the stability of the atmosphere.  The types of ALR are: Department of Chemical Engineering, TIET November 22, 2024   Atmospheric Stability  Atmospheric stability is associated with the atmospheric temperature profile. It refers to the vertical movement of air i.e. tendency of air to rise or to resist vertical motion.  Atmospheric stability determines whether an air Stable, neutral and unstable systems. parcel in the atmosphere will rise, sink, or be neutral.  Atmospheric stability can be classified based on:  Lapse Rate  Classification Scheme Department of Chemical Engineering, TIET November 22, 2024  Contd..  First, consider the different physical situations shown in Figure (a),  a marble is at rest in a bowl. Any small displacement of the marble results in a restoring force - a force that moves the marble back towards its initial position. Such a system is said to be stable,  In Figure (b), the marble is on a flat surface. No Stable, neutral and unstable systems force in any direction is generated by a displacement, and the system is described as neutral.  Finally, in Figure (c), the marble is poised on the top of the upturned bowl. Now any displacement results in a force away from the initial position, and the system is unstable. Department of Chemical Engineering, TIET November 22, 2024  These ideas can be applied to air parcels in the  Contd.. Relation between actual lapse rate, adiabatic lapse rate, and atmospheric stability The relation of the temperature versus height for an air volume for (a) stable and (b) unstable conditions Department of Chemical Engineering, TIET November 22, 2024  Atmospheric stability depends on the variation of temperature with height/altitude.  In general stability refers to the tendency of air to rise or to resist vertical motion. Thus, it determines atmospheric status in terms of whether or not air will rise, sink, or be neutral.  As the air parcel rises, it will expand and cool adiabatically to its dew point at which clouds are formed.  If an air parcel is warmer than its surrounding environment, then it will be less dense than its surroundings and will rise like a hot air balloon.  If an air parcel is cooler than its surrounding environment, then it will be denser than its environment and will sink.  If an air parcel has the same temperature as its surrounding environment, then the parcel will not move and remain in suspension in the atmosphere. Department of Chemical Engineering, TIET November 22, 2024  Stability classification based on lapse rate Classification Lapse rate Absolute stability DALR>SALR>ELR Absolute ELR>DALR>SALR instability Conditional DALR>ELR>SALR stability Wet neutral DALR>SALR=ELR Dry neutral DALR=ELR>SALR Extreme stability ELR negative Figure: shows a summary of how lapse rates determine atmospheric stability. Department of Chemical Engineering, TIET November 22, 2024  How to determine atmospheric Stability?  Stability is dependent upon the Environmental Lapse Rate (ELR), the Dry Adiabatic Lapse Rates (DALR), and Saturated Adiabatic Lapse Rates (SALR).  The measured ELR can be compared with both the DALR and SALR to determine the atmospheric stability conditions. Department of Chemical Engineering, TIET November 22, 2024  Specific stability conditions Absolute Stability: DALR>SALR>ELR  In this case, both plots are on the left or "cool" side of the ELR. That is, both are on the stable side. This is called absolute stability  Air parcel of any relative humidity (RH) will cool faster than the surrounding environment and will not rise. Air would tend to sink and create clear skies Department of Chemical Engineering, TIET November 22, 2024 Contd.. Absolute Instability: ELR > DALR > SALR.  In this instance, both parcel lines are on the right or "warm side" of the ELR.  This means that air parcel of any RH will cool more slowly than the environment and therefore always be warmer than the surrounding environment.  Air parcels will be buoyant and rise like hot air balloons. Department of Chemical Engineering, TIET November 22, 2024  Contd.. Conditional Stability: DALR > ELR > SALR  The SALR is on the "warm" side and the DALR is on the "cool" side of the ELR.  This means that saturated parcels will be unstable and dry parcels will be stable. Department of Chemical Engineering, TIET November 22, 2024 Contd.. Wet Neutral: DALR > (SALR = ELR)  The ELR matches the SALR. Saturated parcels will thus be neutral.  Dry parcels, being on the left or "cool" side of the ELR will be stable.  Contd.. Dry Neutral: (DALR = ELR) > SALR  The ELR now matches the DALR meaning that dry parcels will be neutral.  Note that saturated parcels, being on the right or "warm" side of the ELR, will be unstable.  Contd.. Extreme stability (inversion): ELR is negative  This indicates that the layering of the lower atmosphere is such that warmer air lies on top of cooler air, called temperature inversion.  Both parcel lines are on the same, "cool" side of the ELR. This is a special form of Absolute Stability.  In this situation, no air parcel may rise. This often has negative effects upon pollution dispersal. Contd.. The relationships between the environmental lapse rate (ambient lapse rate) and the dry adiabatic lapse rate essentially determine the stability of the air and the speed with which pollutants will disperse. These relationships are i. Super Adiabatic lapse rate ii. Sub-adiabatic lapse rate iii. Neutral iv. Negative/Inverse lapse rate or Inversion  Smoke stack Plumes and Adiabatic Lapse  The Rates atmospheric temperature profile affects the dispersion of pollutants from a smokestack, as shown in Figure.  If a smokestack were to emit pollutants into a neutrally stable atmosphere, we might expect the plume to be relatively symmetrical, as shown in Figure a. The term used to describe this plume is coning.  In Figure b, the atmosphere is very unstable, and there is rapid vertical air movement, both up and down, producing a looping plume.  In Figure c, a fanning plume results when a stable atmosphere greatly restricts the dispersion of the plume in the vertical direction, although it still spreads FIGURE Effect of atmospheric lapse rates and horizontally. stack heights on plume behavior. The dashed line is the dry adiabatic lapse rate for reference. Contd..  In Figure d, when a stack is under an inversion layer, emissions move downward much more easily than upward. The resulting fumigation can lead to greatly elevated downwind, ground level concentrations.  When the stack is above an inversion layer, as in Figure e, mixing in the upward direction is uninhibited, but downward motion is greatly restricted by the inversion’s stable air. Such lofting helps keep the pollution high above the ground, reducing exposure to people living downwind. In fact, a common approach to air pollution control in the past has been to build taller and taller stacks to emit pollutants above inversions. An unfortunate consequence of this approach, however, has been that pollutants released from tall stacks are able to travel great distances and may cause unexpected effects, such as acid deposition, hundreds of miles from the source. Thank You Department of Chemical Engineering, TIET November 22, 2024  Air Pollution Meteorology Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Learning objectives  Air pollution and meteorology  Adiabatic lapse rate  Atmospheric stability  Pasquill stability types  Temperature inversion  Plume behavior  Lapse rates and dispersion Department of Chemical Engineering, TIET November 22, 2024  Learning Objectives To make the student aware of dispersion phenomenon of air pollutants covering diffusion and advection, meteorological components, stability of atmosphere and corresponding plume shapes. Dispersion = Advection Trasport  + Dilution Diffusion  The ecological crisis - A philosophical perspective Department of Chemical Engineering, TIET November 22, 2024 Contd.. Fick's law of diffusion Diffusion occurs in response to a concentration gradient expressed as the change in concentration due to a change in position. The flux “J” is driven by the negative gradient in the direction of increasing x. δc J = -D δx δc J= mass flux, D= Diffusivity coefficient, = Concentration gradient δx Department of Chemical Engineering, TIET November 22, 2024 Contd.. Diffusion of pollutants occur due to turbulence, which further depends upon many factors:  Ambient temperature  Temperature of emissions  Roughness factors  Wind velocity  Wind direction  Humidity  Stability Department of Chemical Engineering, TIET November 22, 2024 Contd.. Air Pollutant Cycle Department of Chemical Engineering, TIET November 22, 2024  Air pollution and meteorology  Air Quality often depends on the dynamics of the atmosphere, the study of which is called “meteorology”. Lapse Rates  The ease with which pollutants can disperse in the atmosphere is largely determined by the rate of change of air temperature with altitude.  In the troposphere, the temp of ambient air usually decreases with an increase in altitude. This rate of temp change is called “lapse rate” (or environmental lapse rate or ambient lapse rate, ∧) Department of Chemical Engineering, TIET November 22, 2024 Contd.. A specific parcel of air whose temperature is greater than that of the ambient air tends to rise until it reaches a level at which its own temperature and density equals to that of the atmosphere that surround it. Thus, a parcel of artificially heated air (e.g. automobile exhaust, stack gas) rises, expands, becomes lighter and cools. The rate at which the temperature of the parcel decreases (i.e. lapse rate) may be considerably different from the ambient lapse rate (lambda, ∧) of the air. The lapse rate for the rising parcel of air may be determined theoretically. For this calculation, the cooling process within rising parcel of air is assumed to be “adiabatic” (i.e., Occurring without the addition or loss of heat). This is called “adiabatic lapse rate” (gamma, Γ). Department of Chemical Engineering, TIET November 22, 2024  Adiabatic lapse rate  Important characteristic of atmosphere is ability to resist vertical motion: stability  Affects ability to disperse pollutants  When small volume of air is displaced upward - Encounters lower pressure - Expands to lower temperature - Assume no heat transfers to surrounding atmosphere  Γ-=Called 0.976adiabatic °C/100 mexpansion = 9.76 °C /km = 5.4 °F /1000 ft  Dry adiabatic lapse rate. (often taken as 1 °C /100 m)  In moist atmosphere, because of the release of latent heat of vaporization, a saturated parcel cools on rising at a slower rate than a dry parcel.  So, Γdry > Γwet Department of Chemical Engineering, TIET November 22, 2024  Atmospheric Stability  Unstable atmosphere  ∧>Γ Department of Chemical Engineering, TIET November 22, 2024  Contd..  Stable Atmosphere  ∧ Γwet , a moist atmosphere is inherently less stable than a dry atmosphere. Thus a stable situation with reference to Γdry may actually be unstable for upward displacement of a saturated air parcel. Department of Chemical Engineering, TIET November 22, 2024 Contd.. Radiation Inversion: Arises from unequal cooling rates of the earth and the air above the earth. Usually a nocturnal phenomenon that breaks up easily with the rays of the morning sun. Radiation inversion prompts the formation of fog and simultaneously traps gases and particulates, creating a concentration of pollutants. Subsidence Inversion: Usually associated with a high pressure system. As the high pressure air descends, it is compressed and heated, forming a blanket of warm air over the cooler air below and thus creating an inversion that prevents further vertical movement of air. Department of Chemical Engineering, TIET November 22, 2024  Plume behavior (page-496) Lapse rates and dispersion By comparing the ambient lapse rate to the adiabatic lapse rate, it may be possible to predict what will happen to gases emitted from a stack. Plume: Pocket of polluted smoke Plume types are important because they help us understand under what conditions there will be higher concentrations of contaminants at ground level. Shapes of plumes depend upon atmospheric stability conditions. Degree of stability is a measure of the ability of the atmosphere to disperse pollutants. Department of Chemical Engineering, TIET November 22, 2024 Contd.. Department of Chemical Engineering, TIET November 22, 2024 Contd.. Figure: Effect of lapse rate on plume behaviou (a) looping, (b) neutral, (c) coning (d) fanning (e) lofting, (f) fumigating and (g) trapping Department of Chemical Engineering, TIET November 22, 2024  Lapse Rates and Dispersion By comparing the ambient lapse rate to the adiabatic lapse rate, it possible to predict what will happen to gases emitted from a stack. In the following examples, the dry adiabatic lapse rate is used, but prediction of plume patterns is more likely to be accurate if the moisture content of the stack gas is taken into account when ambient and adiabatic lapse rates are compared. When the ambient lapse rate is super adiabatic (greater than the adiabatic), the turbulence of the air itself causes the atmosphere to serve as an effective vehicle of dispersion. As indicated in Fig.a, the resultant plume is designated "looping" plume. In this highly unstable atmosphere, the stream of emitted pollutants undergoes rapid mixing, and any wind causes large eddies which may carry the entire plume down to the ground, causing high concentrations close to the stack before dispersion is complete. In areas where conditions make looping plumes likely, higher stacks may be Department of Chemical Engineering, TIET November 22, 2024 needed to prevent premature contact with the ground.  Lapse Rates and Dispersion When the ambient lapse rate is equal to or very near the dry adiabatic lapse rate, the plume issuing from a single chimney or smoke stack tends to rise directly into the atmosphere until it reaches air of density similar to that of the plume itself. This type emission called a neutral plume, is seen in Fig. b. However, this neutral plume tends to "cone" (Fig. c) when wind velocity is greater than 20 miles/h and when clouds cover blocks solar radiation by day and terrestrial radiation by night. When the ambient lapse rate is subadiabatic (less than the dry adiabatic) the atmosphere is slightly stable. Under such conditions, there is limited vertical mixing, and the probability of air pollution problems in the area is increased. The typical plume in such a situation is said to be coning." since it assumes cone like shape about the plume line, as shown in Fig. c. While the dispersion rate is faster for a looping plume, the distance at which a coning plume Department of Chemical Engineering, TIET November 22, 2024 first reaches the ground is greater.  Lapse Rates and Dispersion When the ambient lapse rate is equal to or very near the dry adiabatic lapse rate, the plume issuing from a single chimney or smoke stack tends to rise directly into the atmosphere until it reaches air of density similar to that of the plume itself. This type emission called a neutral plume, is seen in Fig. b. However, this neutral plume tends to "cone" (Fig. c) when wind velocity is greater than 20 miles/h and when clouds cover blocks solar radiation by day and terrestrial radiation by night. When the ambient lapse rate is subadiabatic (less than the dry adiabatic) the atmosphere is slightly stable. Under such conditions, there is limited vertical mixing, and the probability of air pollution problems in the area is increased. The typical plume in such a situation is said to be coning." since it assumes cone like shape about the plume line, as shown in Fig. c. While the dispersion rate is faster for a looping plume, the distance at which a coning plume Department of Chemical Engineering, TIET November 22, 2024 first reaches the ground is greater.  Lapse Rates and Dispersion When the lapse rate is negative, as in the presence of an inversion, the dispersion of stack gas is minimal, because of lack of turbulence. In the extremely stable air, a plume spreads horizontally, with little vertical mixing, and is said to "fanning" (Fig. d), and in flat country such a plume may be visible for miles downwind of its source. In areas where radiation inversions are common, construction of stacks high enough to allow for discharge of emissions above inversion layer is recommended. This solution is not practical for subsidence inversions, since they usually extend to much greater heights. Extenuating circumstances can often alleviate or aggravate the pollution possibilities accompanying negative lapse rate conditions. For instance, when the lapse rate is superadiabatic above the emission source and inversion conditions exist below the source, the plume is said to be lofting. As shown in (Fig e) lofting plume has minimal downward mixing, and the pollutants are dispersed downwind without any significant ground-level concentration. As long as stack height remains above the inversion, lofting will continue, but lofting a transitional situation. If the inversion grows past the stack height, lofting will change to fanning. Department of Chemical Engineering, TIET November 22, 2024  Lapse Rates and Dispersion When an inversion layer occurs a short distance above the plume source and superadiabatic conditions prevail below the stack, the plume is said to be "fumigating (Fig -f). Fumigating often begins when a fanning plume breaks up looping plume, as when morning sun breaks up a radiation in version and the superadiabatic conditions below the inversion act to move the plume into a vigorously looping pattern. Fumigating can cause high ground-level concentrations of air contaminants, though these usually last only a relatively short period of time. Similar to the conditions which provoke the "fumigating" plume are the conditions which create a "trapping effect. Here an inversion layer prevails both above and below the emission source. This results in the coming of the plume below the source and above the lower inversion, as seen in Fig. g. Department of Chemical Engineering, TIET November 22, 2024 Contd.. Department of Chemical Engineering, TIET November 22, 2024  Lapse Rates and Dispersion Fumigating plumes can lead to greatly elevated down-wind, ground level concentration Lofting plumes are helpful in terms of exposure to people at ground level. Thus a common approach to air pollution control has been to build taller stacks to emit pollutant, above inversion layers. However, pollutants released from tall stacks can travel long distances, so that effects such as acid deposition can be felt hundreds of miles from the source. Department of Chemical Engineering, TIET November 22, 2024 Thank You Department of Chemical Engineering, TIET November 22, 2024  The point-source Gaussian plume model Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Learning objectives  Mathematical model  What is air quality model?  What is deterministic approach?  Deterministic based air quality monitoring (AQM)  Gaussian plume dispersion model  Pasquill stability types  Stack design Department of Chemical Engineering, TIET November 22, 2024  Learning Objectives To make the student aware of the air quality model, its definition, types and description of Gaussian based air quality model for point source along with its application. Concept of mathematical modelling applied to air pollution Re-entrainment Department of Chemical Engineering, TIET November 22, 2024  Mathematical Model Modeling is the representation of a physical system by mathematical equations. A mathematical model of a real process is mathematical description which combines experimental facts and establishes relationship among the process variables. Uses of Models Mathematical models can be useful in all phases of chemical engineering, from research and development to plant operations, and even in economic studies. Department of Chemical Engineering, TIET November 22, 2024  What is air quality model? A mathematical relationship between emissions and air quality that incorporates the transport, dispersion and transformation of compounds emitted into the air. Department of Chemical Engineering, TIET November 22, 2024  What is deterministic approach? The deterministic mathematical models calculate the pollutant concentrations from emission inventory and meteorological variables according to the solution of various equations that represent the relevant physical processes. Deterministic modelling is the traditional approach for the prediction of air pollutant concentrations. Figure 1: The instantaneous plume boundary and a time- averaged plume envelope Department of Chemical Engineering, TIET November 22, 2024 Contd.. A stack emitting plume of contaminated air. through the stack passes the three axes. X axis is the wind direction, Y axis transverse to the wind (crosswind) Z axis passes vertically upward through the stack. Figure 2: Coordinate system showing Gaussian distribution in the horizontal and vertical directions Department of Chemical Engineering, TIET November 22, 2024 Dispersion coefficient The horizontal (y) and vertical (z) dispersion coefficient are a function of 1. Downwind position, x 2. Atmospheric stability conditions Dispersion coefficient  Deterministic based AQM The deterministic based air quality model is developed by relating the rate of change of pollutant concentration in terms of average wind and turbulent diffusion which, in turn, is derived from the mass conservation principle. δc  δc δc δc  δ δc δ δc δ δc = - u +v + w + Kx + Ky + Kz + Q + R-----(1) δt  δx δy δz  δx δx δy δy δz δz where C = pollutant concentration; t = time; x, y, z = position of the receptor relative to the source; u, v, w =wind speed coordinates in x, y and z directions; K x, Ky, Kz = coefficients of turbulent diffusion in x, y and z directions; Q = source strength; R = sink (changes caused by chemical reaction).  The above diffusion equation (1) is derived in several ways under different set of assumptions for development of air quality models. It relates dispersion in the “x” (downwind) direction as a function of variables in all directions of a three dimensional spare.  Gaussian model is one of the mostly used air quality model based on ‘deterministic principle’. It assumes that the plume ofhas Department a Gaussian Chemical Engineering,concentration TIET distribution November 22, 2024 in both the “z” (vertical) and “y” (horizontal) directions, as shown in Fig. 2  Gaussian plume dispersion model Assumptions  Steady-state conditions, which imply that the rate of emission from the point source is constant.  Homogeneous flow, which implies that the wind speed is constant both in time and with height (wind direction shear is not considered).  Pollutant is conservative and no gravity fallout.  Perfect reflection of the plume at the underlying surface, i.e. no ground absorption.  The turbulent diffusion in the x-direction is neglected relative to advection in the transport direction , which implies that the model should be applied for average wind speeds of more than 1 m/s (> 1 m/s).  The coordinate system is directed with its x-axis into the direction of the flow, and the v (lateral) and w (vertical) components of the time averaged wind vector are set to zero.  The terrain underlying the plume is flat.  All variables are ensemble averaged, which implies long-term averaging with stationary conditions. Department of Chemical Engineering, TIET November 22, 2024  Contd.. The concentration ‘C’ of a gas or aerosol (50 µm) from gas stream. It provides enlarged areas to minimize horizontal velocities and allow time for the vertical velocity to carry the particle to the floor. The usual velocity through settling chambers is between 0.5 to 2.5 m/s. Advantage: Low pressure loss, simplicity of design and maintenance. Disadvantage: Requires larger space and efficiency is low. Only larger sized particles are separated out. Department of Chemical Engineering, TIET November 22, 2024  Gravitational Settling Gravitation settling chamber Department of Chemical Engineering, TIET November 22, 2024  Contd. Design of a gravitational settling chamber If we assume that Stokes law applies, we can derive a formula for calculating the minimum diameter of a particle collected at 100% theoretical efficiency in a chamber of length L. vt v Where vt= terminal settling velocity, m/s = h           1 H L Where, g=gravitational constant, m/s2; ρp= density of vt =   g ρ p - ρa d p2           2  particle, kg/m3; ρa=density air, kg/m3; dp= diameter of 9 μa particle, m; µa= viscosity of air, kg/m.s; H = height of settling chamber, m; vh= horizontal flow-through velocity, Department m/s; and of Chemical L= length Engineering, of settling TIET chamber, November m. 22, 2024  Contd. Solving for dp gives an equation that predicts the largest-size particle that can be removed with 100% efficiency from a settling chamber of given dimension.   dp =  18 μ a H v h            3   gL ρ -ρ p a    All particles larger than dp will also be removed with 100% efficiency, while the efficiency for smaller particles is the ratio of their settling velocities to the settling velocity of the dp particle. Department of Chemical Engineering, TIET November 22, 2024  Contd. B. Cyclone Separators A cyclone separator consists of a cylindrical shell, conical base, dust hopper and an inlet where the dust-laden gas enters tangentially. Under the influence of the centrifugal force generated by the spinning gas, the solid particles are thrown to the wall of the cyclone as the gas spirals upward at the inside of the cone. The particles slide down the walls of the cone and into the hopper. The operating efficiency of a cyclone depends on the magnitude of the centrifugal force exerted on the particles. The greater the centrifugal force, the greater the spreading efficiency. The magnitude of the centrifugal force generated Department of Chemical Engineering, TIET November 22, 2024 depends on particle mass, gas velocity within the cyclone, and cyclone  Contd. Where, Fc=centrifugal force, N; Mp=particulate mass, kg; vi equals particle velocity, m/s; and R equals radius of the vi2 Fc = M p     4  cyclone, m. From this equation, it can be seen that the R centrifugal force on the particles, and thus the collection efficiency of the cyclone collector can be increased by decreasing R. Large-diameter cyclone have good collection efficiencies for particles 40 to 50 µm in diameter. Advantage: Relatively inexpensive, simple to design and maintain; requires less floor area; low to moderate pressure loss. Disadvantage: Requires much head room; collection efficiency is low for smaller particles, quite sensitive to variable dust loading and flow rates. Department of Chemical Engineering, TIET November 22, 2024  BY MECHANICAL SEPARATION …contd. C. Fabric Filters In a fabric filter system, the particulate-laden gas stream passes through a woven or felted fabric that filters out the particulate matter and allows the gas to pass through. Small particles are initially retained on the fabric by direct interception, inertial impaction, diffusion, electrostatic attraction, and gravitational settling. After a dust mat has formed on the fabric, more efficient collection of submicron particle is accomplished by sieving. Filter bags usually tubular or envelope-shaped, are capable of removing most particles as small as 0.5µm and will remove substantial quantity of particles as small as 0.1 µm. Filter bags ranging from 1.8 to 9 m long, can be utilized in a bag house filter arrangement. As particulates build up on the inside surface of the bags, the pressure drop Department of Chemical Engineering, TIET November 22, 2024 increases. Before the pressure drop becomes too severe, the bag must be  FABRIC FILTERS..contd. Fabric and Fibre Characteristics Fabric filter may be classified according to filtering media: woven fabric or felt cloth. Woven fabrics have a definite long range repeating pattern and have considerable porosity in the direction of gas flow. These open spaces must be bridged by impaction of interception to form a true filtering surface. Felted cloth consists of randomly oriented fibres, compressed into a mat and needled to some loosely woven backing material to improve mechanical strength. The choice of fabric fibre is based primarily on operating temperature and the corrosiveness or abrasiveness of the particle. Cotton is the least expensive fibre, and is preferably used in low temperature dust collection service. Silicon coated glass fibre cloth is commonly employed in high temperature applications. The glass fibre must be lubricated to prevent Department abrasion. of Chemical All fibre Engineering, TIETmay be applied November 22, to the 2024  FABRIC FILTERS..contd. Fabric Filter System Fabric filter systems typically consist of a tubular bag or an envelope, suspended or mounted in such manner that the collected particles fall into hopper when dislodged from fabric. The structure in which the bags are hanged is known as a bag-house. Advantage: Fabric filters can give high efficiency, and can even remove very small particles in dry state. Disadvantage: High temperature gasses need to be cooled. The flue gasses must be dry to avoid condensation and clogging. The fabric is liable to chemical attacks. Typical bag-house Department of Chemical Engineering, TIET November 22, 2024  BY MECHANIAL SEPARATION …contd. D. Electrostatic Precipitator The electrostatic precipitator is one of the most widely used device for controlling particulate emission at industrial installations ranging from power plants, cement and paper mills to oil refineries. Electrostatic precipitator is a physical process by which particles suspended in gas stream are charged electrically and, under the influence of the electrical field, separated from the gas stream. The precipitator system consists of a positively charged collecting surface and a high-voltage discharge electrode wire suspended from an insulator at the top and held in passion by weight at the bottom. At a very high DC voltage, of the order of 50kV, a corona discharge occurs close to the negative electrode, setting up an electric field between the emitted and the grounded surface. Department of Chemical Engineering, TIET November 22, 2024  BY MECHANIAL SEPARATION …contd. D. Electrostatic Precipitator …contd. The particle laden gas enters near the bottom and flows upward. The gas close to the negative electrode is, thus, ionized upon passing through the corona. As the negative ions and electrons migrate toward the grounded surface, they in turn charge the passing particles. The electrostatic field then draws the particles to the collector surface where they are deposited. Periodically, the collected particles must be removed from the Advantages: collecting surface. This is done by rapping or vibrating the collector to dislodge the  Maintenance is nominal, useless corrosive and adhesive materials are present in flue gases. particles.  They can be operated at high temperature up to 300-450 C. o Disadvantages:  Higher initial cost.  Sensitive to variable dust loading and flow rates.  They use high voltage, and hence may pose risk to personal safety of the staff.  Collection efficiency reduces with time. Department of Chemical Engineering, TIET November 22, 2024  PARTICULATE EMISSION CONTROL BY WET GAS SCRUBBING Wet scrubber removes particulate matter from gas streams by incorporating the particles into liquid droplets directly on contact. The basic function of a wet scrubber is to provide contact between the scrubbing liquid, usually water and, the particulate to be collected. This contact can be achieved in a variety of ways as the particles are confronted with so-called impaction target, which can be wetted surface as in packed scrubbers or individual droplets as in spray scrubbers. The basic collection mechanism is the same as in filters: inertial impaction, interception and diffusion. Generally, impaction and interception are the predominant mechanism for particles of diameter above 3 µm, and for particles of diameter below 0.3 µm diffusion begins to prevail. There are many scrubber designs presently available where the contact between the scrubbing liquid and the particles is achieved in a variety of ways. The major types are: (A) plate scrubber, (B) packed-bed scrubber, (C) spray scrubber, (D) venturi scrubber, (E) cyclone scrubber, (F) impingement-entrainment scrubber, Department of Chemical Engineering, TIET November 22, 2024 and (G) fluidized-bed scrubber.  BY WET GAS SCRUBBING …contd. A. Plate Scrubber It contains a vertical tower containing one or more horizontal plates (trays). Gas enters the bottom of the tower and must pass through perforations in each plate as it flows countercurrent to the descending water stream. Collection efficiency increases as the diameter of the perforations decreases. A cut diameter, that collected with 50% efficiency, of about µm aerodynamic diameter can be achieved with 3.2-mm- diameter holes in a sieve plate. Department of Chemical Engineering, TIET November 22, 2024  BY WET GAS SCRUBBING …contd. B. Packed-bed Scrubber Operates similarly to packed-bed gas absorber. Collection efficiency increases as packing size decreases. A cut diameter of 1.5 µm aerodynamic diameter can be attained in columns packed with 2.5 cm elements. Packed–bed scrubber Department of Chemical Engineering, TIET November 22, 2024  BY WET GAS SCRUBBING …contd. C. Spray Scrubber Particles are collected by liquid drops that have been atomized by spray nozzles. Horizontal and vertical gas flows are used, as well as spray introduced co-current, countercurrent, or cross-flow to the gas. Collection efficiency depends on droplet size, gas velocity, liquid/gas ratio, and droplet trajectories. For droplets falling at their terminal velocity, the optimum droplet diameter for fine-particle collection lies in the range of 100 to 500 µm. Gravitational settling scrubbers can achieve cut diameters of about 2.0 µm. The liquid/gas ratio is in the range of 0.001 to 0.01 m 3/m3 of gas treated. Department of Chemical Engineering, TIET November 22, 2024  BY WET GAS SCRUBBING..contd. D. Venturi Scrubber A moving gas stream is used to atomize liquids into droplets. High gas velocities (60 to 120 m/s) lead to high relative velocities between gas and particles and promote collection. E. Cyclone Scrubber Drops can be introduced into the gas stream of a cyclone to collect particles. The spray can be directed outward from a central manifold or inward from the collector wall. Department of Chemical Engineering, TIET November 22, 2024  BY WET GAS SCRUBBING..contd. F. Impingement-Entrainment Scrubber The gas is forced to impinge on a liquid surface to reach a gas exit. Some of the liquid atomizes into drops that are entrained by the gas. The gas exit is designed so as to minimize the loss of entrained droplets. G. Fluidized-bed Scrubber A zone of fluidized packing is provided where gas and liquid can mix intimately. Gas passes upward through the packing, while liquid is sprayed up from the bottom and/or flows down over the top of the fluidized layer of packing. Department of Chemical Engineering, TIET November 22, 2024 Thank You Department of Chemical Engineering, TIET November 22, 2024  DESIGN OF CYCLONES Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Introduction  Principle of cyclones  Construction and operation of cyclones Department of Chemical Engineering, TIET November 22, 2024  Cyclones Principle The particles are removed by the application of a centrifugal force. The polluted gas stream is forced into a vortex. the motion of the gas exerts a centrifugal force on the particles, and they get deposited on the inner surface of the cyclones. Department of Chemical Engineering, TIET November 22, 2024  Cyclones Department of Chemical Engineering, TIET November 22, 2024 Conventional reverse-flow cyclone Department of Chemical Engineering, TIET November 22, 2024 Dimensions of a Standard Cyclone Department of Chemical Engineering, TIET November 22, 2024  Generalized cyclone design configurations Nomenclature Conventional High efficiency Cyclone diameter, D 1.0 1.0 Height of entrance, h = 1/2D 0.5 0.5 Width of Entrance, b =1/4D 0.25 0.2 Exit length, S 0.625 0.5 Diameter of exit, De =1/2D 0.5 0.5 Length of cylinder L1 = 2D 2.0 1.5 Length of cone L2 = 2D - - Diameter of dust exit, Dd, ¼ D 0.25 0.375 Overall height 4.0 4.0 Department of Chemical Engineering, TIET November 22, 2024 Changes in performance of cyclone with changing variables Department of Chemical Engineering, TIET November 22, 2024  Construction and Operation The gas enters through the inlet, and is forced into a spiral.  At the bottom, the gas reverses direction and flows upwards.  To prevent particles in the incoming stream from contaminating the clean gas, a vortex finder is provided to separate them. the cleaned gas flows out through the vortex finder. Department of Chemical Engineering, TIET November 22, 2024  Design of Cyclones Theory 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 can be expressed by Fc. mv2 Fc          1 r Where, m=mass of the particle, vӨ = tangential velocity of the particle at radius r, and r=radius of rotation. The separation factor is given by S. v2 S          2  gr The separation factor varies from 5 in large, low velocity units to 2500 in small, high pressure units. Higher the separation factor better is the performance of the cyclone. Department of Chemical Engineering, TIET November 22, 2024  Design of cyclones In the cyclone, the gas, in addition to moving in a circular path, also moves radially inwards between the inlet on the periphery and the exit on the axis. Since the tangential velocities of the particle and the gas are the same, the relative velocity between the gas and particle is simply equal to the radial velocity of the gas. This results in a drag force on the particle towards the centre, and the equilibrium radius of rotation of the particle can be obtained by balancing the radial drag force and the centrifugal force: 2  v 3 g d p vr  6 3  d p  p - g r  3  Where, dp=particle diameter, VӨ = tangential velocity, and vr=radial velocity of the gas at radius r. Arranging the above equation, for vr vr    d 2p  p -  g v 2          4   18 g r Department of Chemical Engineering, TIET November 22, 2024  Design of cyclones The tangential velocity of the particle in the vortex has been found experimentally to be inversely proportional to the radius of rotation according to equation, v r n = Constant           5  Where, n is the exponent and dimensionless. For an ideal gas, n=1. The real values observed are between 0.5 to 1, depending upon the radius of the cyclone body and gas temperature. VӨ can be related to the tangential velocity at the inlet to the ncyclone V as: D v  vi             6  Өi  2r  Where, D=diameter of the cyclone. v may be taken as the velocity of the gas through the inlet pipe, i.e., Ɵi Department of Chemical Engineering, TIET November 22, 2024  Design of cyclones Q v i          7  Ai Where, Q=gas volumetric flow rate and Ai = cross-sectional area of the inlet. Substituting in equation (6), Therefore, n Q  D v              8  Now, from equation (4) Ai  2r  vr    d 2p  p -  g  Q  2   D  2n    1           9  18 g  Ai   2  r 2n 1 Generally, the collection efficiency is an increasing function of V r. The relationship cannot be defined explicitly, principally because of factors such as re-entrainment, bounce, and particle interactions. However, it is clear from Eq. (9) that as the diameter of the particle increases or as the cross-sectional area of the inlet decreases, Vr will increase appreciably, resulting Department in an increase of Chemical in TIET Engineering, efficiency but with November 22, a higher 2024  Design of cyclones The most satisfactory expression for cyclone performance is still the empirical one. Lapple correlated collection efficiency in terms of the cut size dpe which is the size of those particle that are collected with 50% efficiency. Particle larger than d pe will have collection efficiency greater than 50% while the smaller particles will be collected with lesser efficiency. The cut size is given by:9 b g d pe             10   2 N evi  p   g  Where, dpe=collection efficiency, b=inlet width, vi = gas inlet velocity and Ne = effective number of turns a gas makes in traversing the cyclone (5 to 10 in most cases). By plotting the ratio of the actual particle size (dp) to the cut size (dpc), generalized curve of collection efficiency may be obtained. Such cures is given in Fig. Department of Chemical Engineering, TIET November 22, 2024 Lapple’s correlation for cyclone efficiency Department of Chemical Engineering, TIET November 22, 2024  Pressure drop Pressure drop: The pressure drop may be estimated according to the following equation, K  g vi2 ab  p         11 2 De2 Where, K=a constant, which averages 13 and ranges from 7.5 to 18.4; ΔP= pressure drop; a, b and De=cyclone dimensions, vi=inlet gas velocity and Ƿg=gas density. Since the pressure drop is proportional to the square of the inlet velocity, it is obvious that high velocities causes not only re-entrainment but al excessive pressure drop. Department of Chemical Engineering, TIET November 22, 2024 Thank You Department of Chemical Engineering, TIET November 22, 2024  DESIGN OF FABRIC FILTER Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Design of fabric filter Department of Chemical Engineering, TIET November 22, 2024  Fabric Filters  Well known and accepted method for separating dry particles from a gas stream  Many different types of fabrics, different ways of configuring bags in a baghouse and different ways of flowing the air through the bags.  There are 3 common types of baghouse based on cleaning method  Reverse-air  Shaker  Pulse-jet Typical industrial baghouse of the shake-deflate design Department of Chemical Engineering, TIET November 22, 2024 Typical industrial baghouse of the pulse-jet design Department of Chemical Engineering, TIET November 22, 2024  Fabric Filters  Gas filtration: similarly, filter cloth - made of fibers (with holes) of 50-150 m - can remove sub-micron particles.  As more and more particles are caught on fibers through impaction, interception, or Brownian motion), the pores become smaller and smaller and eventually particles bridge over and seal the pores.  Hence, the "filter" actually consists of two parts: fabric (constant thickness) and dust cake (thickens w/time). The primary role of the filter medium is to provide structural support.  DESIGN OF FABRIC FILTER Fabric filtration is a physical separation process in which a gas containing solids passes through a porous fabric medium, which retains the solids. Pressure drop and air-to-cloth ratio are the major design parameters in bag- house design. Higher pressure drop implies that more energy is required to pull the gas through the system. Air-to-cloth ratio, also referred to as the face velocity, is the volume flow of gas received by a bag-house divided by the total area of the filtering cloth. It is expressed as acfm/ft2. The air-to-cloth ratio determines the unit size and thus, capital cost (CFM- Cubic Feet per Minute). Higher air-to-cloth ratio means less fabric, therefore less capital cost. However, higher ratio can lead to high pressure drop thus requiring higher energy. Also, more frequent bag cleanings may be required, thus increasing the downtime. Department of Chemical Engineering, TIET November 22, 2024  DESIGN OF FABRIC FILTER Fabric filters are classified by their cleaning method or the direction of gas flow and hence the location of the dust deposit. Pressure Drop: The pressure drop is the sum of the pressure drop across the filter housing and across the dust-laden fabric.  The pressure drop across the housing is proportional to the square of the gas-flow rate due to turbulence.  The pressure drop across the dust-laden fabric is the sum of the pressure drop across the clean fabric and the pressure drop across the dust cake. Successful design of a fabric filter depends on key design variables  Filter bag material  Fabric cleaning method  Air-to-cloth ratio  Baghouse configuration (i.e., forced or induced draft)  Materials of construction Department of Chemical Engineering, TIET November 22, 2024  DESIGN OF FABRIC FILTER P K1v  K 2vw          1 Where, v=the filtration velocity; K1=the flow resistance of the clean fabric; K2=the specific resistance of the dust deposit; w=the fabric dust areal density; K1 is related to Frazier permeability, which is the flow through a fabric in acfm/ft2 of fabric when the pressure drop across the fabric is 0.5 in water gauge (w.g.) as follows   K1 Pa S m-1  24590  cfm            2  Frazier permeability  at 0.5 in w.g.   ft 2    Department of Chemical Engineering, TIET November 22, 2024  DESIGN OF FABRIC FILTER Evaluation of specific resistance K2 : The dust collected on a membrane filter and K2 should be calculated from the increase in pressure drop (ΔP2- ΔP1) with filter weight gain (M2- M1) as follows: A  P2  P1  K 2            3 v  M 2  M1  Where, A is the surface area of the membrane filter, M1 and M2 is initial filter weight and final filter weight respectively. Department of Chemical Engineering, TIET November 22, 2024  Problem A fabric filter is to be constructed using bags that are 0.1 m in diameter and 5.0 m long. The bag house is to receive 5 m3/s of air. Filtering velocity is 2.0 m/min. Determine the number of bags required for a continuous removal of particulate matter. We know, total area of filter (At) = Qg/u Qg = flow rate and u = filtering velocity Area of single bags (Ab) = (Cylinder area of the side surface) D = diameter of a bag and H = height of the bag (length) Total no of bags (N) = Department of Chemical Engineering, TIET November 22, 2024 Thank You Department of Chemical Engineering, TIET November 22, 2024  PARTICULATE EMISSION CONTROL BY ELECTROSTATIC PRECIPITATION Department of Chemical Engineering Dr. Haripada Bhunia  Presentation Outline  Introduction  Electrostatic precipitator (ESP)  Applications of ESP  Advantages and disadvantages  Requirements of ESP process  Steps in ESP  Principle of ESP  Requirements of ESP process  Types of ESP  Operational issues Department of Chemical Engineering, TIET November 22, 2024  Introduction A device that removes suspended dust particles from a gas or exhaust by applying a high-voltage electrostatic charge and collecting the particles on charged plates Electrostatic precipitation is a method of dust collection that uses electrostatic forces, and consists of discharge wires (negative plates) and collecting plates (positive plates) A high voltage is applied to the discharge wires to form an electrical field between the wires and the collecting plates and also ionizes the air around the discharge wires to supply ions. When air that contains an aerosol (dust, mist, etc.) flows between the collecting plates and the discharge wires, the aerosol particles in the air are charged by the ions. The Coulomb force caused by the electric field causes the charged particles to be collected on the collecting plates, and the gas is purified. An electrostatic precipitator (ESP) is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates. The particles are givenDepartment an electrical charge of Chemical by forcing Engineering, TIETthem November to pass 22, through 2024 a corona, a region in which gaseous ions flow.  Electrostatic precipitator The electrostatic precipitator is one of the most widely used collection devices for particulates. An electrostatic precipitator (ESP) is a particulate collection device that removes particles from a flowing gaseous stream (such as air) using the force of an induced electrostatic charge. ESP can be operated at high temperature and pressures, and its power requirement is low. For these reasons the electrostatic precipitation is often the preferred method of collection where high efficiency is required with small particles. ESPs are highly efficient filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream. Department of Chemical Engineering, TIET November 22, 2024  Contd..  In the electrostatic precipitation process the basic force which acts to separate the particles from the gas is electrostatic attraction.  The particles are given an electrical charge by forcing them to pass through a corona, a region in which gaseous ions flow.  The electrical field that forces the charged particles to the walls comes from electrodes maintained at high voltage in the center of the flow lane.  Control of emissions from the industrial sources has served the threefold purpose of  Recovery of the for economic reason  Removal of abrasive dusts to reduce wear of fan component  Removal of objectionable natter from gases being discharged into Department of Chemical Engineering, TIET November 22, 2024 the atmosphere  Working The working principle of the electrostatic precipitator is quite simple. It has two sets of electrodes one is positive, and anther is negative. The negative electrodes are in the form of rod or wire mesh. Positive electrodes are in the form of plates. The positive plates and negative electrodes are placed vertically in the electrostatic precipitator alternatively one after another. The negative electrodes are connected to a negative terminal of high voltage DC source, and positive plates are connected to the positive terminal of the DC source. The positive terminal of the DC source may be grounded to get stronger negativity in the negative electrodes. The distance between each negative electrode and positive plate and the DC voltage applied across them are so adjusted that the voltage gradient between each negative electrode and adjacent positive plate becomes quite high to ionize the medium between these. Department of Chemical Engineering, TIET November 22, 2024  Applications of ESP Pulp and paper mills, non-ferrous metal industry, chemical industry, public buildings and areas Cement recovery furnace, steel plant for cleaning blast furnace gas. Removing tars from coke oven, sulphuric acid (pyrite raw material), phosphoric acid plant Petroleum industry for recovery of catalyst, carbon black, thermal power plant. Department of Chemical Engineering, TIET November 22, 2024  Advantages and disadvantages Advantages Disadvantages High collection efficiency High initial cost Low maintenance and operating costs More space requirement Handles large volume of high temperature Possible explosion hazards gas Negligible treatment time Production of poisonous gas Easy cleaning Department of Chemical Engineering, TIET November 22, 2024  Requirements of ESP process Source of high voltage Discharge and collecting electrode Inlet and outlet for gas A means for disposal of collected material Cleaning system, outer casing Department of Chemical Engineering, TIET November 22, 2024  Steps in ESP Generation of electric field high voltage direct current 20-80 kV Generation of electric charges Transfer of electric charge to a dust particle Movement of the charged dust particles in an electric field to the collection electrodes Adhesion of the charge dust particles to the surface of the collection electrode Dislodging of dust layer from collection electrode Collection of dust layer in a hopper Removal of the dust from t