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

Dispersion Model

iiiis

iz

iy

ix

iz

iy

ix

i

CQCkkE

z

CK

zy

CK

yx

CK

x

z

CU

y

CU

x

CU

t

C

21

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

iiiis

iz

iy

ix

iz

iy

ix

i

CQCkkE

z

CK

zy

CK

yx

CK

x

z

CU

y

CU

x

CU

t

C

21

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

si

zi

yi

x Ez

CK

zy

CK

yx

CU

0y

C ,

0 y

C 0,y

0C ,0

y

x

BC

0z

C ,z

0z

C ,

rHz

22

2

444

4,,

rz

xr

z

x

y

x HzxK

UHz

xK

UxK

yU

zy

si eee

KKx

EzyxC

x

yy U

xK

22

x

zz U

xK

22

2

2

2

2

2

2

222

2,, z

r

z

r

y

HzHzy

zyx

si eee

U

EzyxC

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

U

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