Chemical & Process Technology: Quick Estimate Evaporation Rate From Spilled Liquid

Wednesday, March 17, 2010

Quick Estimate Evaporation Rate From Spilled Liquid

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 U^0.78.MW^2/3.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),

A_wc = V / 0.033

A_wc = (1000 / 7.48051948) / 0.033

A_wc = 4,050 ft²

Diked area,

A_dike = Total dike area - tank area

A_dike = 100x40 - 8 x (PI x 10^2)/4

A_dike = 3371.7 ft²

Since A_wc is larger than A_dike, 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 U^0.78.MW^2/3.A.VP) / (82.05 T)

E = (0.284x1.5^0.78.(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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