CBB3024 Environmental Studies

Environmental Studies

P. BalasubramanianDepartment of Chemical Engineering

Universiti Teknologi PETRONASMalaysia

Recap

• Equipment design

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Outcome

• To apply a method to control the atmosphericemissions

• To design a water treatment method

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Content• Atmospheric Emissions• Water treatment methods• Environmental Impact Assessment(EIA)

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Atmospheric Emissions• There are many types of emissions to

atmosphere• These can be characterized as

– particulate (solid or liquid)– vapor and– Gaseous emissions

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Atmospheric Emissions• Industrial emissions:

– PM10– PM2.5– Ozone– VOC– SOx and NOx– CO and CO2

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Atmospheric Emissions• Industrial emissions of major concern are as

follows:• PM10:

– Particulate material less than 10 μm diameter isformed as a byproduct of combustion processesthrough incomplete combustion and through thereaction between gaseous pollutants in theatmosphere.

• PM2.5:– Particulate material less than 2.5 μm diameter

forms in the same way as PM10, but it canpenetrate deeper into the respiratory system thanPM10.

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Atmospheric Emissions• O3:

– Ozone is a very reactive compound present in theupper atmosphere and the lower atmosphere.

– Ozone is danger to human health and contributesto the formation of other pollutants.

• VOCs:– A VOC is any compound of carbon, excluding

carbon monoxide, carbon dioxide, carbonic acid,metal carbides or carbonates and ammoniumcarbonate.

– VOCs are major components in the formation ofsmog.

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Atmospheric Emissions• Sox:

– Oxides of sulfur (SO2 and SO3) are formed in thecombustion of fuels containing sulfur and as abyproduct of chemicals production.

• NOx:– Oxides of nitrogen (principally NO and NO2) are

formed in combustion processes and as abyproduct of chemicals production.

• CO:– Carbon monoxide is formed by the incomplete

combustion of fuel and as a byproduct ofchemicals production.

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Atmospheric Emissions• CO2:

– Carbon dioxide is formed principally by thecombustion of fuel but also as a byproduct ofchemicals production.

• There are four main problems associated withatmospheric emissions:– Urban smog– Acid rain– Ozone layer destruction– The Greenhouse effect

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Atmospheric Emissions• Urban smog:

– Urban smog is commonly found in modern citiesespecially where air is trapped in a basin.

• It is observable as a brownish colored air.• The formation of urban smog is through complex

photochemical reactions that can becharacterized by:

VOCs + NOx + O2 −−−→ O3 + otherphotchemical pollutants

• Photochemical pollutants such as ozone,aldehydes and peroxynitrates such asperoxyethanoyl (or peroxyacetyl) nitrate (PAN)are formed.

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Atmospheric Emissions• Ozone and other photochemical pollutants have

harmful effects on living organisms and onbuilding structures

• High levels of these pollutants can causebreathing difficulties and bring on asthma attacksin humans.

• Warm weather and still air exacerbate theproblem.

• In addition to VOCs and NOx , the problem ofurban smog is made worse by particulateemissions and carbon monoxide from incompletecombustion of fuel.

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Atmospheric Emissions• Acid rain:

– Natural precipitation is naturally acidic witha pH often in the range of 5 to 6 caused bycarbonic acid from dissolved carbon dioxideand sulfurous and sulfuric acids fromnatural emissions of SOx and H2S.

– Human activity can reduce the pH verysignificantly down to the range 2 to 4 inextreme cases, mainly caused by emissionsof oxides of sulfur.

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Atmospheric Emissions• Problems associated with acid rain include:

– damage to plant life, particularly in forests– acidification of water, leading to dead lakes and

streams, loss of aquatic life and possible damageto human water supply

– corrosion of buildings, particularly those made ofmarble and sandstone.

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Atmospheric Emissions• Ozone layer destruction:

– The upper atmosphere contains a layer rich in ozone.– Ozone in the lower levels of the atmosphere is

harmful– Ozone in the upper levels of the atmosphere is

essential as it absorbs considerable amounts ofultraviolet light that would otherwise reach theearth’s surface

– The destruction of ozone is catalyzed by oxides ofnitrogen in the upper atmosphere

– The result of ozone layer destruction is increasedultraviolet light reaching the Earth, potentiallyincreasing skin cancer and endangering polar species

– This is a global effect that requires global solutions5/1/2014 15

Atmospheric Emissions• The Greenhouse effect:

– Gases such as CO2, CH4 and H2O are present in lowconcentrations in the earth’s atmosphere

– These gases reduce the earth’s emissivity and reflectsome of the heat radiated by the earth

– The effect is to create a “blanket” to keep the earthwarmer than it would otherwise be.

– The problem arises mainly from burning fossil fuelsand clearing forests by burning.

• Results:– Global temperatures increase– Leading to melting of the polar ice caps and glaciers– Rising sea levels, desertification of areas, thawing of

permafrost, increased weather disruptions andchanges to ocean currents

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Atmospheric Emissions• Sources of atmospheric pollution:

– Incomplete combustion or fuel ash from furnaces,boilers and thermal oxidizers

– Incomplete combustion in flares– Solids drying operations– Kilns used for high temperature treatment of

solids– Metal manufacture– Crushing and grinding operations for solids– Any solids handling operation open to the

atmosphere, and so on.5/1/2014 17

Atmospheric Emissions• Vapor emissions are even more difficult as they

have even more sources such as:– Condenser vents– Venting of pipes and vessels– Inert gas purging of pipes and vessels– Process purges to atmosphere– Drying operations– Incomplete combustion of fuel in furnaces, boilers and

thermal oxidizers– Incomplete combustion in flares– Application of solvent-based surface coatings– Open operations such as filters, mixing vessels, and so

on, leading to evaporation of volatile organic materials

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Atmospheric Emissions– drum emptying and filling, leading to evaporation

of volatile organic materials– spillages of volatile organic material– process plant ventilation of buildings processing

volatile organic materials– storage tank loading and cleaning– road, rail and barge tank loading and cleaning– fugitive emissions through gaskets and shaft seals– fugitive emissions from sewers and effluent

treatment– fugitive emissions from process sampling points,

and so on.5/1/2014 19

Atmospheric Emissions• In larger plants, significant reductions in VOC emissions

can usually be made– by controlling major sources, such as tank venting,

condensers and purges, and– by inspection and maintenance of gaskets and shaft seals.

• The largest volume of atmospheric emissions fromprocess plants occurs from combustion producinggaseous emissions. Such emissions are created from:– furnaces, boilers and thermal oxidizers– gas turbine exhausts– flares– process operations where coke needs to be removed from

catalysts (e.g. fluid catalytic cracking regeneration inrefineries), and so on

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Example 1• A storage tank with a vent to atmosphere is to

be filled at 25◦C with a mixture containingbenzene with a mole fraction of 0.2 andtoluene with a mole fraction of 0.8.Estimate the concentration of benzene andtoluene in the tank vent:a) at 25◦Cb) corrected to standard conditions of 0◦C and

1 atm

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Example 1

• Assume that the mixture of benzene andtoluene obeys Raoult’s Law and the molarmass in kilograms occupies 22.4 m3 in thevapor phase at standard conditions. The molarmasses of benzene and toluene are 78 and 92respectively. The vapor pressures of benzeneand toluene at 25◦C are 0.126 bar and 0.0376bar respectively.

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Example 1: Solution (a)

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Example 1: Solution (b)

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Example 2• An airstream contains acetone vapor. Estimate

the following.a) The outlet concentration that can be achieved by

cooling to 35◦C at 1 atm pressure (cooling watertemperature).

b) The outlet concentration that can be achieved bycooling to −20◦C at 1 atm pressure.

c) The outlet temperature to which the air streammust be cooled to obtain an outlet concentrationof 20 mg·m−3.

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Example 2• The concentrations should be quoted at

standard conditions of 0◦C and 1 atm. It can beassumed that the molar mass in kilogramsoccupies 22.4 m3 at standard conditions. Themolar mass of acetone can be taken to be 58.The vapor pressure of acetone is given by:

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Example 2: Solution

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Example 2: Solution

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Control of Solid Particulate Emissions to Atmosphere

• The selection of equipment for the treatmentof solid particle emissions to atmospheredepends on a number of factors– size distribution of the particles to be separated– particle loading– gas throughput– permissible pressure drop– temperature

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• There is a wide range of equipment availablefor the control of emissions of solid particles:

• Gravity settlers– Gravity settlers use to collect coarse particles and

may be use as prefilters.– Only particles in excess of 100 μm can reasonably

be removed.

• Inertial collectors– Only particles > 50 μm can be reasonably

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• Cyclones– Particles size >5 μm.

• Scrubbers– Scrubbers are designed to contact a liquid with the

particle-laden gas and entrain the particles with theliquid.

– They offer the obvious advantage that they can beused to remove gaseous as well as particulatepollutants.

• Electrostatic precipitators.– Electrostatic precipitators are used where collection of

fine particles at a high efficiency coupled with a low-pressure drop is necessary.

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Table 1 Methods of control of emissions of solid particles.

Control of VOC Emissions to Atmosphere• The hierarchy appropriate for control of VOC

emissions:– Eliminate or reduce VOC emissions at source– Recover the VOC for reuse– Recover the VOC for treatment and disposal– Treatment and disposal of the VOC-laden gas

stream

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Control of VOC Emissions to Atmosphere

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• Tank loading can be a significant source ofVOC emissions from atmospheric storagetanks used for the storage of organic liquids(Figure 25.1)

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• There are a number of ways in which such VOCemissions can be prevented from being omitted fromstorage tanks.

• Figure 25.2 shows a simple technique involving abalancing line.

• The atmospheric storage tank in Figure 25.2 is fittedwith a vacuum/pressure relief system to guard againstover-pressure or under-pressure of the storage tank.

• Now as the atmospheric storage tank is emptied andthe road tanker filled in Figure 25.2, the displacedvapor from the road tanker is pushed back into theatmospheric storage tank.

• In this way, displaced vapor from the road tanker isprevented from being released to the atmosphere.

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• The methods of VOC recovery are:– Condensation– Membranes– Absorption– Adsorption

• Once VOCs have been minimized at source andrecovery possibilities have been exhausted, then anyresidual VOC needs to be destroyed using:– flares– thermal oxidation– catalytic thermal oxidation– gas turbines– bioscrubbers– biofiltration

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Control of sulfur emissions• Sources of atmospheric sulfur emissions from

the process industries are:– chemical production: sulfuric acid production,

sulfonation reactions, and so on– smelting processes: production of copper– fuel processing operations: fuel desulfurization– combustion of fuel: steam generation.

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Control of sulfur emissions• Atmospheric sulfur emissions can be

minimized at source by improving the processyields and desulfurization of fuels prior tocombustion.

• Sulfur can be removed from emissions eitheras SO2 or H2S.

• Removal of the H2S can be by:– physical absorption– chemical absorption– gasification of material followed by H2S removal– partial oxidation to elemental sulfur

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Control of sulfur emissions• Removal of SO2 can be by:

– absorption using sodium hydroxide– wet limestone scrubbing– Wellman–Lord process– or many other methods

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Control of oxides of nitrogen emissions• NOx (principally NO and NO2) are produced in

chemical production, metal and mineralprocessing and combustion of fuels.

• NOx formed in combustion processes is bythree mechanisms (fuel, thermal and promptNO).

• Nox emissions can be minimized at source byincreasing process yields in chemicalproduction, switching to a fuel with lowernitrogen content or minimizing NOx formationin combustion processes.

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Control of oxides of nitrogen emissions

• After minimizing the production of NOx ,removal by either oxidation or reduction canbe considered.

• NOx can be absorbed in hydrogen peroxide,which forms nitric acid.

• Reduction is usually carried out usingammonia.

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Control of oxides of nitrogen emissions• Flue gas emissions can be minimized at source

by:– increased energy efficiency at the point of use– increased energy efficiency of the utility system– improvements to combustion processes– changing fuel

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Example 3

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Control of combustion emissions• The major emissions from the combustion of

fuel are CO2, SOx, NOx and particulates.• The products of combustion are best

minimized by making the process efficient inits use of energy through efficient heatrecovery and avoiding unnecessary thermaloxidation of waste through minimization ofprocess waste.

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Summary• Flue gas emissions can be minimized at source

by:– increased energy efficiency at the point of use

(e.g. better heat integration)– increased energy efficiency of the utility system

(e.g. increased cogeneration)– improvements to combustion processes (e.g. low

NOx burners)– changing fuel (e.g. changing from fuel oil to

natural gas)

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Summary• Treatment of SOx: The various techniques that

can be considered for the treatment of SOx are:– absorption into NaOH, CaCO3, water, and so on– oxidation and conversion to H2SO4

– reduction by conversion to H2S and then sulfur

• Treatment of Nox: The techniques used fortreatment of NOx are:– absorption into an NO-complexing agent or oxidizing

agent– reduction using selective non-catalytic or catalytic

processes

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Summary• Treatment of particulates: These include

– scrubbers– inertial collectors– cyclones– bag filters– electrostatic precipitators

• Treatment of CO2: If CO2 needs to be separated fromthe other combustion products for the purpose ofreducing greenhouse gas emissions, there are fourways in which this can, in principle, be brought about:– absorption (for example, using amine)– desorption (for example, using alumina, zeolite, etc.)– membrane– cryogenic separation

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Conclusion

• In this lecture, different types of atmospheric emissions are discussed.

• The control of VOC, SOx, NOx, and combustion emissions are briefly presented.

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References

• R. Smith, Chemical Process: Design andIntegration, Wiley, 2005.

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Documents

CBB3024 Environmental Studies