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Flashing Liquids

Flashing Liquids. Source Models. Flashing Liquids. Adiabatic Flashing Adiabatic Flashing through hole Isothermal Flashing through hole Liquid pool boiling. Flashing Liquids.

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Flashing Liquids

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  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 258Chemical Process SafetyIn 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 258Chemical Process SafetyIn 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

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