1. Flashing Liquids
Source Models

2. Flashing Liquids Adiabatic Flashing Adiabatic Flashing through hole
Isothermal Flashing through hole
Liquid pool boiling

3. Flashing Liquids
We have considered source models in terms of liquids leaking through a hole or pipe and vapors leaking through a hole or pipe.
For liquids stored under pressure above their normal boiling points, we need to consider flashing.

4. Adiabatic Flashing
Liquids stored under pressure above normal boiling point.
Large release of pressure (i.e. ruptured vessel).
Energy to vaporize comes from liquid

5. Adiabatic Flashing Excess energy in superheated liquid
Separate variables

6. Adiabatic Flashing
Cp,l & hv are functions of T. If you assume they are constant at an average value

7. Adiabatic Flashing Determining the fraction of liquid vaporized
Substitute back in
average between Tb and T1

8. Adiabatic Flashing
Design equation for fraction vaporized

9. Flashing Liquids Adiabatic Flashing Adiabatic Flashing through hole
Isothermal Flashing through hole
Liquid pool boiling

10. Adiabatic Flashing through holes
Liquids stored above saturation pressure

11. Adiabatic Flashing through holes
If L < 10 cm, assume incompressible liquid is flowing.
If L>10 cm, assume choked flow with P2=Psat.
Then design equation becomes:
Where Psat is at ambient conditions

12. Flashing Liquids Adiabatic Flashing Adiabatic Flashing through hole
Isothermal Flashing through hole
Liquid pool boiling

13. Isothermal Flashing through a hole
For liquids stored at saturation pressure, P1=Psat.
Assume choked two-phase mass flow
v is specific volume (1/density)

14. Isothermal Flashing through a hole
The two-phase specific volume is
vfg is difference in specific volume between liquid (fluid) and vapor (gas)
vf is the liquid (fluid) specific volume
fv is the mass fraction of vapor

15. Isothermal Flashing through a hole
Differentiate with respect to pressure
From before we determined

16. Isothermal Flashing through a hole
All vapor formed is from liquid
Substitute in

17. Isothermal Flashing through a hole
Now substituted dfv into dv/dP relationship
Clausius-Clapyron equation give dT/dP

18. Isothermal Flashing through a hole
Substitute in the inverse of the Clausius-Clapyron relationship
Substitute into final relationship

19. Isothermal flashing through holes
Reduce to get design equation for vapor mass flow rate flashing through a hole
When flashing at or near Psat small droplets of liquid are entrained with the vapor. Typically design assumption is that liquid mass is the same as the mass of the vapor formed from flashing

20. Flashing Liquids Adiabatic Flashing Adiabatic Flashing through hole
Isothermal Flashing through hole
Liquid pool boiling

21. Liquid Pool Boiling or Evaporating
Use same relationship derived previously for evaporation
Where K the mass transfer coefficient is estimated from

22. ChE 258 Chemical Process Safety In Class Problem
Calculate the mass flux (kg/m2s) of sulfur dioxide that is leaking from a storage tank that holds liquid sulfur dioxide at its vapor pressure at 25°C
Vapor pressure=0.39x106Pa
Heat of vaporization=3.56x105J/kg
vfg=0.09m3/kg
Heat capacity=1.36x103J/kgK

23. Solution
Use relationship derived in class
Flux is

24. Solution cont.
Substitute in values

25. Solution cont
Finish reducing the units

26. Solution continued
If we assume that entrained liquid droplets are being carried out with the flashing liquid then
Total flux

27. ChE 258 Chemical Process Safety In Class Problem
Calculate the mass flux (kg/m2s) of sulfur dioxide that is leaking from a storage tank that holds liquid sulfur dioxide at 300 psia and at 25°C. The wall thickness is 15 cm.
Vapor pressure at 25 °C =0.39x106Pa
Heat of vaporization=3.56x105J/kg
Heat capacity=1.36x103J/kgK
Liquid density=1.455gm/cm3

28. Solution
Use relationship derived in class
Flux is

29. Solution continued
Get common units

30. Solution cont.
Substitute in values

31. Solution Continued
C0 has value of 0.61 for sharp edges, 1.0 for worst case
Approximately 10 times greater than when stored at saturation pressure