7 replies to this topic

[mbeychok]

Chris:

The flash point is the lowest temperature at which a liquid gives off enough vapour to form a flammable air-vapour mixture near its surface. The lower the flash point, the greater the fire hazard. The flash point is an approximate value and should not be taken as a sharp dividing line between safe and hazardous conditions. The flash point is determined by a variety of test methods which give different results. Two types of methods are abbreviated as OC (open cup) and CC (closed cup).

I hope that the above definition clarifies it for you.

[mbeychok]

I have seen this equation for estimating the flash point of petroleum products (which are hydrocarbon mixtures), but I don't know where it originated or how reliable it is:

Flash point = 0.77(ASTM 5% in °F - 150 °F)

The ASTM 5% refers to the 5 volume % temperature of the ASTM distillation of the petroleum product .

[gvdlans]

For estimating the flash point, you can use following rule of thumb:

The flash point occurs at about the temperature at which the liquid has a vapor pressure equal to the LFL partial pressure.

For example, ethanol has a LFL of 3.3 volume%. The LFL partial pressure is therefore 0.033 atm. The temperature that produces a vapor pressure of 0.033 atm is 11°C, which is the predicted flash point. This is close to the reported flash point of 13°C (see for example NFPA 325).

You can use Antoine equation to find temperature that produces LFL partial pressure.

The above is based on Gooding, Charles H., "Estimating Flash Point and Lower Explosive Limit," Chemical Engineering, December 12, 1983.

Same principle can be used to estimate flash point for mixtures. Then you also have to estimate the LFL of the mixture (e.g. using LeChatelier's Law).

[gvdlans]

Chris,

The LFL partial pressure is LFL (in volume %)/100*atmospheric pressure. So for the example of ethanol with LFL of 3.3 vol%, the LFL partial pressure is 3.3/100 * 1 atm = 0.033 atm or 3.3/100 * 1.013 bar = 0.0334 bar or 3.3/100 * 101.3 kPa = 3.34 kPa etc.

[ChemEngg]

Dear Jedi,

Can you provide the correlation for LFL of Hydrocarbon mixtures.
Can you provide a sample calculation for Temperature with LFL of the mixture.

Thanks