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Units of measurement
In the study of environmental engineering, it isquite common to encounter both extremely large
quantities or extremely small ones, such as toxic
substancemaybe expressed in parts per billion
(ppb), or the rate of energy use maybemeasured in thousands of billions of watts
(terawatts)
Quite often, it is the concentration of somesubstance/sin air or water that is of interest.
In either medium, concentration maybe based on
mass, volume or a combination of both.
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Materials Balance Diagram
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Law of Conservation of Mass
When chemical reactions take place,matter is either created nor destroyed(though in nuclear reactions, mass is converted to
energy).
What this concept allows us to do is track
materials (pollutants) from one place to
another with mass balance equation.
Input
Rate
Output
Rate
Decay
Rate
Accumulation
Rate
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First Step in a Mass balance Analysis
Define a particular region in space that isto be analyzed.
A region maybe a simple mixing tank to
entire coal-fired power plant, a lake, airbasin, or the globe itself.
By picturing an imaginary around and
Identify the flow of materials across theboundary as well as the accumulation of
the materials within the region.
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Conservative Substances
Under steady-state conservative systems, there isno radioactive decay, bacterial decomposition, orchemical reaction occurring, thus, the decay rate iszero. Pollutants enter and leave the region at thesame rate.
Conservative substances areDissolved solid in a body ofwater. Heavy metals in soils.
Carbon dioxide in air.
Input
Rate
Output
Rate
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Sample of Nonconservative Pollutant
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First Law of Thermodynamics
The use of the First Law ofThermodynamicsto write energybalance equations that will help us analyze energy flows.
Energy can be neither created nor destroyed.
Energy can change forms in any given process.
To apply the first law, it is necessary to define the systembeing studied, much as was done in the analysis of massflows.
Systemsin which both energy and matter can flow across theboundary are referred to as open systems, while, those in whichenergy is allowed to flow but not of the matter, are called closedsystems.
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Energy Balance Equation System
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Second Law of Thermodynamics
Waste heat (Qc) is in any kinds of reaction.
When work is done, there is always be
some inefficiency; that is, some portion ofthe energy put into the process will end up
as waste heat.
Note: It is impossible to device a machine that
can convert heat to work with 100% efficiency
(page 23).
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Conductive heat transfer is usually associatedwith solids, the rate of heat transfer in a solid is
proportion to the thermal conductivity of the
material.
Convective heat transfer occurs when a fluid atone temperature comes in contact with a
substance at another temperature.
Radiation is transported by electromagneticwaves and does not require a medium to carry
the energy.
Example, radio waves, X-rays and gamma rays.
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Environmental Chemistry
Every pollution problem has a chemicalbasis. such as, greenhouse effect, ozone
depletion, toxic wastes, groundwater
contamination, air pollution, and acid rainrequires at least a rudimentary
understanding of some basic chemical
concepts.
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Basic Requirements for an Environment Engineer
An environmental engineer who mustdesign an emission control system or a
waste treatment plant must be grounded in
chemical principles and the techniques ofchemical engineering.
An essential chemical principles required
to understand the nature of the pollution
problems and the engineering approaches
to their solutions.
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StoichiometryStoichiometryis the branch of chemistry and chemical
engineering that deals with the quantities of substances thatenter into, and are produced by, chemical reactions.
Stoichiometryprovides the quantitative relationshipbetween reactants and products in a chemical reaction.
example, when methane unites with oxygen in completecombustion..
Such as 16g of methane require 64g of oxygen. At thesame time 44g of carbon dioxide and 36g of water areformed as reaction productions.
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Enthalpy in Chemical Systems
The use of conservation of mass to balancechemical equations, we can use conservation of
energy to learn about heat absorbed or released
during chemical reactions.
Since energy must be conserved, we should beable to track it from the beginning to end.
The change of enthalpyduring a constant
pressure reactionis equal to the heat absorbedby the system.
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The first law of thermodynamic, the energy in thereactants on the left side of the equation, plus
any heat added to the system, should be equal
to the energy contained in the reaction products
on the right side plus any work done during thereaction.
U1+ Q = U2+ W
where: U1 = internal energy of the chemical system at the beginning
U2= internal energy at the end
Q = heat absorbed during the reaction
W = work done by the system during the reaction
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Chemical Equilibriumreaction where, the ratesof reaction are the same (that is, products are
being formed on the right at the same rate as
they are being formed on the left).
Example: aA + bB cC + dD
H2O H+ + OH
Endothermic reactionheat is absorbed
Exothermic reactionheat is liberated