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Wednesday, March 17, 2010

Liquid in tank truck or storage tank may spill on the ground. Wind blowing, solar and ground surface heating will results evaporation of spilled liquid. The evaporation rate is one of the key information required during design phase for consequence analysis and preparation of emergency handling measures. What is the quick way to estimate evaporation rate ?



Evaporation rate from spilled liquid subjects to several factors such as :
  • spilled liquid volatility
  • spilled liquid properties
  • spilled liquid surface area expose to wind blowing
  • spilled liquid temperature
  • wind speed
Evaporation Rate
Evaporation rate of spilled liquid can be estimated using following equation

E = (0.284 U0.78.MW2/3.A.VP) / (82.05 T)

where
E = Evaporation rate (lb / min)
U = Wind speed above spilled liquid surface (m/s)
MW = Spilled liquid molecular weight
A = Spilled liquid surface area (ft2)
VP = Spilled liquid vapor pressure at liquid temperature (mmHg)
T = Spilled liquid temperature (K)

Spilled Liquid Surface Area
Spilled liquid surface area is normally fix by the containment itself. Liquid spilled from tank into diked area, the spilled liquid surface area will be the diked area itself  unless the spilled quantity is so small where it will not covers entire diked area. To estimate spilled liquid surface area without any containment, it is assumed that the spilled liquid will have 10mm (or 0.033 ft) thickness. Therefore,

Spilled liquid surface area (ft2)

A = V / 0.033

where
V = Spilled liquid volume (ft3)


Example
Methanol with vapor pressure (VP) of 125 mmHg and MW of 32.04 stored at 25 degC in a 25,000 US gal tank (10 ft diameter). Overfilled methanol results spillage of 1,000 US gal of methanol spread in the diked area (100ft x 40 ft) which contain 8 identical tanks. Assuming wind speed is 1.5 m/s, estimate evaporation rate.

Spilled volume,
V = 1000 US gal 
V = 1000 /  7.48051948 ft3

[1 ft3 = 7.48051948 US gallons]

Spilled area (without containment),
Awc = V / 0.033
Awc(1000 /  7.48051948) / 0.033
Awc = 4,050 ft2

Diked area,
Adike = Total dike area - tank area
Adike = 100x40 - 8 x (PI x 10^2)/4
Adike = 3371.7 ft2

Since Awc is larger than Adike, entire dike area will be occupied by spilled liquid.


Inputs

U =1.5 m/s
MW =32.04
A =3371.7 ft2
VP =125 mmHg
T =25 + 273.15 = 298.15 K

Evaporation rate
E = (0.284 U0.78.MW2/3.A.VP) / (82.05 T)
E = (0.284x1.50.78.(32.04)2/3.3371.7.125) / (82.05x298.15)
E =   67.7 lb / min

Above method is a quick method in determining evaporation rate.


Ref : "Risk management program guidance for offsite Consequence Analysis", US EPA, Apr 1999

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posted by Webworm, 3:49 PM

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