How explosive is benzene compared to TNT?

Question

Benzene is explosive within the explosive limits of minimum $\pu{1.2 \%}$ and maximum of $\pu{7.8 \%}$ in air. Source: Wikipedia More accurate, benzene itself is not explosive, but the fuel-air mixer or, fuel–air explosive (FAE), is. This fuel-air explosive is used, because of its power in Thermobaric weapons https://en.wikipedia.org/wiki/Thermobaric_weapon Benzene does burn and could be a fuel in a fuel-air explosive. So it would be, more specific, a benzene-air explosion.

I would like to know what is the explosive power of benzene measured in TNT-equivalent. Benzene can explode like in China in 2005 which was 'only' 100 ton benzene.

The reason why I want to know is that there was a huge explosion in Beirut on August 4th 2020, with the equivalent of 1.1 kiloton TNT. In my country, the Netherlands, benzene is transported in huge ships of 2000 ton. This could explode, spreading the benzene gas over the area, which is heaver than air. Benzene is highly carcinogen. I would like to know how a possible explosion could be compare with the explosion in Beirut. Benzene burns very easy, so may be only a fraction of benzene would explode, the rest would burn. I just want to know what we, as citizens, might expect when things goes seriously wrong.

An approach could be a list with the relative effectiveness of benzene compared to TNT. All kind of benzene derivatives are mentioned in that list but not benzene itself. I could not find a list in which benzene itself is mentioned.

So my (much to simple) calculation is:

• 1 ton TNT: $\pu{4184 \times 10^{12} J}$ see: TNT equivalent
• Energy [(Heat of combustion) / mol]: $\pu{3267.6 kJ/mol}$ see: Benzene
• Energy / kg: $\pu{41831 \times 10^{6}J/kg}$ ($\pu{\frac{3267.6 ~ kJ/mol}{78114 ~ g/mol} \times 1000 g/kg}$)
• Mass: $\pu{2000 kg}$
• Total energy: $\pu{83662 \times 10^{12} J}$ ($\pu{41831 \times 10^6 J/kg \times 2000 kg}$)
• TNT-equivalent: $\pu{~ 20 kiloton}$ TNT ($\pu{\frac{83662 \times 10^{12}~ J}{4182 \times 10^{12}~ J/ton}}$ TNT)

My questions are:

Could all benzene explode at once? Because benzene might be mixid in air within the explosive limits range, but this will be only for a limited ammount of benzene. But because the explosion the rest of the benzene will be blown into the air and possible then be again be within the explosive limites ranges. So causing a chain-reaction. The explosion in Jilin in China took 1 hour so that seems to be the case.

If not is it possible to calculate what is the maximum of benzene that could explode?

Note: I have very limited chemical knowledge. I can passive read chemical formulae and make very simple calculations like above.

Answer 1

All flammable organic liquids could, in principle, create large fuel-air explosions but the conditions are very hard to achieve accidentally

The first important point to note is that benzene should not be a particular worry. There are plenty of other, widely used, chemicals and mixtures that could, in principle, cause large explosions. Standard automotive petrol (gasoline) for example is present in most vehicles (non-diesel ones anyway) and is transported in 40 tonne tankers on most roads. Fires and explosions happen, but rarely, and they are never known to be of the thermobaric type you are worried about. Most motor vehicles have 40kg or so of fuel that is just as flammable as benzene but after accidents they rarely catch fire or explode and never do so with the power of a fuel air thermobaric explosion (the hollywood trope that all crashed cars explode is pure nonsense).

But big fuel air explosions are known to be possible and have been developed into some very effective military weapons. These are known as thermobaric bombs and the largest are just about the biggest non-nuclear explosions possible in existing military arsenals. But a quick read of how the weapons work (see this wikipedia entry) suggests that they require some fairly notable technology to get right. Largely this is because the fuel has to be dispersed at just the right concentration in the air to get a detonation or deflagration rather than a fiery but unimpressive "phut". It isn't easy to achieve this by accident with a large volume of fuel.

Bad maintenance and safety procedures in chemical and oil plants can lead to accidental fuel-air explosions. The Buncefield explosion in a UK oil storage depot was caused by one. It was a huge explosion probably involving about 300 tons of fuel (but that is only about 0.1% of the fuel stored in the depot a lot of which caught fire but didn't explode). Another famous UK example is the Flixborough disaster caused by a fuel-air explosion caused by a leaky pipe in a cyclohexane plant. In both incidents poor management and engineering contrived to make a dangerous event possible and standards were rewritten to prevent future mistakes. That such event are rare should be of significant comfort if you are worried about the dangers of transporting tankers full of benzene. It is worth noting that none of these (very bad0 accidents came close to involving the majority of the flammable compounds stored nearby. It is hard to get the right fuel-air mixture with very large volumes of fuel, thank goodness.

A more common form of fuel air explosion might also put the rare chemical plant incidents in perspective. This is explosions caused in flour mills and grain elevators. a mixture of finely divided flour or grain dust is explosive and plants that handle flour and grain have to be specially designed to avoid dangerous dust explosions. These are far more common that explosions in chemical plants. While this may not be much comfort, it does suggest that worrying about benzene is not that big a real world risk.

Answer 2

Could all benzene explode at once?

No, this is highly unlikely. While liquid benzene is flammable, it is not explosive. In the worst case, there will be a large fire, but chances of spontaneous combustion as compared to Trinitrotoluene are very, very low.

The explosion in Jilin in China took 1 hour so that seems to be the case.

According to the same Wikipedia article:

The accident site is a nitration unit for aniline equipment. T-102 tower was jammed up and was not handled properly, hence the blasts

Also, from China Daily:

The channel of one nitration tower of the benzene production branch got blocked at noon. A worker then bungled an attempt to unblock it.

These kinds of accidents in production plants are different from explosion of storage units: most likely there were other factors involved and it was not only benzene that was involved in the expolosion, but other chemicals as well, such as other nitrated benzene derivatives which are more powerful explosives than Benzene.

Finally, I would want to clear up your concepts of lower explosive limits and upper explosive limits^[1]:

The lower explosive limit (LEL) is the minimum concentration of a specific combustible gas required to fire combustion when in contact with oxygen (air). If the concentration of the gas is below the LEL value, the mix between the gas itself and the air is too weak to spark. The upper explosive limit (UEL) is the maximum level of concentration of the gas that will burn when mixed with oxygen; when the gas concentration is above the UEL value for the gas/vapor, the mix is too “fat” to ignite or explode.

It is possible for Benzene vapours to explode when the air is saturated with them to a concentration that is between LEL and UEL; here, the oxygen in the air helps the already flammable benzene catch fire spontaneously, causing an explosion. However, there are several safety guidelines with reference to benzene transportation and shipping^{[2], [3]}. Most notably, Benzene is transported in pressurized tanks and is subject to the requisite regulations for Hazard Class 3 liquids, which is why the accumulation of significant concentration of benzene vapours in the air (enough to cause a combustion) is unlikely.