stack design done by eng. mohamed abdelrhaman. content definition of the stack applications of stack...
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Stack Design
Done by
Eng. Mohamed AbdElRhaman
Content
• Definition of the stack
• Applications of stack
• Dispersion Model
• Selection of stack design
• Conclusion
Definition
• Stack is a system used in chemical industry that acts as an exhaust to release
gaseous waste streams.
• Stacks are the highest construction in any factory and also higher than the
surrounding buildings
Applications of stack
• Furnaces– Discharge of flue gas
• Absorbers– discharge of clean gas
• Desalination industry– discharge of non-condensable gases
• Wastewater treatment
• Cooling tower in nuclear reactors
Difference Between Stacks and Flares
• Flares: A control devices that burn hazardous materials to prevent their release into the environment; may operate continuously or intermittently, usually on top of a stack.
• Stacks: only the part that acts as exhaust.
Dispersion Model
• Dispersion model describes the airborne transport of toxic materials away from the source into the plant or community
• Dispersion model is affected by the followings– Wind speed– Atmospheric stability– Ground conditions (buildings, water, trees)– Height of release (height of the stack)– Momentum and Buoyancy of initial material release
Dispersion Model
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Dispersion Model
Nomenclature • Ci is the concentration of the chemical species
involved in the model• Ux, Uy and Uz are wind velocities in the x-, y- and
z-directions• Kx, Ky and Kz are diffusion coefficients• Es are the emission sources• ki
1 and ki2 are deposition coefficients (for dry
deposition and wet deposition, respectively)• Q(Ci) represents chemical reactions.• Hr effective height
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Assumptions 1. Steady state conditions2. Uy = Uz = 0 (wind velocity in x-direction only).3. Transport by bulk motion in the x-direction << diffusion in the x-direction (Kx = 0).4. There is no deposition in the system 5. There is no reaction in the system
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Atmospheric Stability
Surface wind speedDaytime incoming solar radiationNighttime cloud cover
m/smi/hStrongModerateSlight >50% <50%
<2 <5AA – BBEF
2 – 35 – 7A – BBCEF
3 – 57 – 11BB – CCDE
5 – 611 – 13CC – DDDD
>6 >13CDDDD
Note: Class D applies to heavily overcast skies, at any windspeed day or night
Stability classDefinition
Stability classDefinition
Avery unstableDneutral
BunstableEslightly stable
Cslightly
unstableFstable
Atmospheric stability
• Dispersion coefficient calculations
Classσy σz
A0.22x(1+0.0001x)-1/20.20x
B0.16x(1+0.0001x)-1/20.12x
C0.11x(1+0.0001x)-1/20.08x(1+0.0002x)-1/2
D0..08x(1+0.0001x)-1/20.06x(1+0.0015x)-1/2
E0.06x(1+0.0001x)-1/20.03x(1+0.0003x)-1/2
F0.04x(1+0.0001x)-1/20.016x(1+0.0001x)-1/2
• (a) NOx and (b) SO2 concentration distributions at ground level. C is in g/m3, w is number of grids in x-direction and m is number of grids in y-direction.
Selection of stack design
• Design parameters – Stack height (construction parameter)
– Stack diameter (construction parameter)
– Exhaust gas velocity (operating parameter)
– Exhaust gas temperature (operating parameter)
hHH r
x
h
x
s
U
Q
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dVh
21
62.2029.0
Effective height Stack height Plume rise
Where Qh is the heat emission form the stack
asph TTcmQ .
Where Ts is the stack gas temperatureTa is the atmospheric temperature
Conclusion
• The selection of stack design starts by dispersion model.
• Environmental regulation and constrains around the stack controls the concentrations of pollutant and specifies the effective height.
• Stack’s height and diameter are fixed and the control of the effective height happens in the plume rise segment.
• Changing the plume rise height happens by changing the following:– Stack gas exit velocity– Stack gas temperature
00.02
0.040.060.08
0.10.12
0.140.16
0 0.2 0.4 0.6 0.8 1
distance km
con
cen
trat
ion
Hr1= 60 m Hr2 = 80 m Hr3 = 100 m
Loading = 1000g/m3Ux = 10 m/sStability class D
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