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

Nomenclature C i is the concentration of the chemical species involved in the model U x, U y and U z are wind velocities in the x-, y- and z-directions K x, K y and K z are diffusion coefficients E s are the emission sources k i 1 and k i 2 are deposition coefficients (for dry deposition and wet deposition, respectively) Q(C i ) represents chemical reactions. H r effective height

Assumptions 1.Steady state conditions 2.U y = U z = 0 (wind velocity in x-direction only). 3.Transport by bulk motion in the x-direction << diffusion in the x-direction (K x = 0). 4.There is no deposition in the system 5.There is no reaction in the system

Atmospheric Stability Nighttime cloud coverDaytime incoming solar radiationSurface wind speed < 50%> 50%SlightModerateStrongmi/hm/s FEB A – B A< 5< 2 FECB A – B5 – 72 – 3 EDC B – C B 7 – 113 – 5 DDD C – D C 11 – 135 – 6 DDDDC> 13> 6 Note: Class D applies to heavily overcast skies, at any windspeed day or night

DefinitionStability classDefinitionStability class neutralDvery unstableA slightly stableEunstableB stableFslightly unstableC

Atmospheric stability Dispersion coefficient calculations σzσz σyσy Class 0.20x0.22x( x) -1/2 A 0.12x0.16x( x) -1/2 B 0.08x( x) -1/2 0.11x( x) -1/2 C 0.06x( x) -1/ x( x) -1/2 D 0.03x( x) -1/2 0.06x( x) -1/2 E 0.016x( x) -1/2 0.04x( x) -1/2 F

(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)

Effective heightStack heightPlume rise Where Q h is the heat emission form the stack Where T s is the stack gas temperature T a 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

Loading = 1000g/m3 U x = 10 m/s Stability class D