Two Letters Re: Refurbishing Dead Gasoline

Here are some additional thoughts in regard to the letter, “Refurbishing Dead Gasoline”, from my perspective as an oil refinery chemist:
Gasoline is a complex mixture of hydrocarbons, and there are many different flammable materials that can be blended to achieve the desired specifications.
In regard to vapor pressure of U.S. gasoline blends, a mixture resulting in about 15 psi Reid vapor pressure is ideal for winter conditions, and a mixture resulting in about 7 psi Reid vapor pressure is ideal for summer conditions.
The gasoline blend should exhibit enough vapor pressure for ignition to occur while not over-pressuring and causing vapor lock.
Butane is superior to propane as a gasoline additive mainly because it has a higher octane value and a lower Reid vapor pressure, giving it properties that more closely resemble those in the desired gasoline blend.  Reid vapor pressure is about 50 psi for pure butane and about 150 psi for pure propane.  Although some winter gasoline blends may contain as much as 10 percent or more by volume of butane, much less propane would be needed to achieve the same vapor pressure in the gasoline blend.  Keep in mind that vapor pressure does not blend linearly – one-third the amount of propane would not give you the same vapor pressure as butane. 
In addition, propane has more value as a petrochemical precursor than butane so refiners typically blend butane to add vapor pressure to gasoline while selling propane as a separate product.
Pure ethanol or isopropyl alcohol (distilled with no water), acetone, paint thinner, or other flammable chemicals added in small amounts can also help add vapor pressure without overly affecting the other qualities of the gasoline blend.  
Without a gauge for measurement, I would recommend adding just enough propane or butane to hear a bit of vapor release when opening the container, but not enough to bulge the container.  A little butane or propane will go a long way in restoring the vapor pressure of old gasoline.  Ventilation and the absence of ignition sources is absolutely essential when mixing, of course.
Thanks for your blog.  It’s still the best out there. – Michael S.


Mr. Rawles;
First off, I need to state that I am a physical chemist who works with mostly inorganic chemistry, so I know far less than a petroleum chemist would, but I suspect that the fuel industry uses butane over propane for three major reasons.

First, we could approximate that gasoline will obey Raoult’s Law, and a heavier hydrocarbon (like butane) will “self-distill” out of the rest of the gasoline more slowly than a lighter one (like propane).  If they are trying to keep the gas viable as long as possible, using butane would be a better choice, of course.  In very cold climates, refineries might add some propane to keep the hydrocarbon vapor pressure as high enough for cold starts, but that is pure speculation on my part.

Second, there is a significant market for propane, less so for butane.  It makes economic sense for the refinery to use the butane that they would have a harder time selling.
Third, for storage concerns, propane vapor will effuse out of plastic containers even faster than butane, per Graham’s Law of Effusion, so if one must try to refresh gasoline using propane, it should be done at the point of use.

Personally, given the danger of working with gasoline, and given this idea likely won’t be of much use until after all the Hospitals, with their high-tech burn care, have ceased working, I would not try this.  Frankly, I would never have thought of this and it seems to be a very clever idea, but I think the dangers outweigh the benefits.  I think we would be better served learning to exploit fuels that will be available for the long-term: wood, coke, peat and coal.  Burning oil is a silly thing to do anyway (given all else that oil is good for). – The Tennessee chemist who belongs in Idaho

JWR Replies: I concur. It is much safer to use “dead” gasoline as-is, and simply get engines started with the aid of ether-based starting fluid. (Although even that has its own set of hazards.)