An interesting insight …

presscall · 11 Aug 2022

Over on CPL contributor ’MYN’ offered this insight (literally a visual image of them) into the additives in pump gasoline that gum up or leave coke deposits in stove and lantern generators: (I asked his permission to reproduce his post here)

Hello all,
I'd just share a simple test result on the reactivity of an unleaded gasoline. The gasoline used for this was Shell FuelSave 95 (RON 95).
I had earlier mixed a medium-diluted solution of 'sulfo-chromic' acid(a little chromic acid in sulfuric acid) with the said gasoline in a capped plastic container. The acid was diluted to such that there won't be an uncontrolled exothermic reaction. The concentrated form would have attacked the plastic as well.
After mixing, the gasoline-acid mixture was vigorously agitated in the capped bottle, periodically over a time of several days. The mixture was then allowed to settle and separate out within the bottle.
This is what it looks like now:-

The dark liquid at the bottom is the acid mixture while the upper portion is the gasoline.
The acid part changed colour from a very dark red to one that is still dark, predominantly red but with a little blue in it now.
The gasoline was a pale straw coloured when fresh. The colour did not change too much from before, although I believe its very slightly darker than before.
However, in between the two liquids, there's now a scum-like semi solid that's neither soluble in the acid nor the gasoline. Possibly, gummy formation.
I believe this is one of the possible stuffs that would cause trouble in a pressure lantern or stove's generator when unleaded gasoline is being used as the fuel. There had clearly been a chemical reaction.
If the same test is used using light naphtha instead of gasoline, there'd hardly be any reaction and there'd be no visible signs of any changes. Light naphtha isn't reactive to this particular acid mix.
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Following that I asked, “What makes this experiment legitimately mimic what occurs when heat does the ‘separating’ and not the added chemical? I don’t doubt it does, but would like to hear your explanation.”

MYN replied,

Ok, during normal lantern or stove operations, we all know that heat is the primary necessity for vaporization of the fuel. We are also well aware that light naphtha, Coleman fuel or similar fuels don't decompose easily even when heated to their vaporization temperatures in the generators of liquid fueled lanterns and stoves. Modern unleaded gasolines or petrols on the other hand, quickly leaves a nasty, gummy deposit within, when used in such manner. Some of these, would actually be cycled back into the fount during operation due to thermal expansion. Eventually or ultimately, we'd see substantial coking in the generators. That pretty much gives an indication that the fuel had undergone extensive thermal cracking and pyrolysis, with the more volatile newly formed products getting vaporized and burned normally. Unfortunately, there're also some a lot less volatile newly formed products in the process. These tend to accumulate in the generators to be further cracked down to black carbon in the end.
Besides that, modern gasoline contains substantial proportions of oxygenated hydrocarbons as well. These include ETBE, MTBE and other alkyl-tertiary-alkyl ethers and sometimes ethanol and other alcohols. Their presence would encourage some oxidative breakdown of the heated gasoline.
In addition, there is also a fair amount of olefins in gasoline. The presence of olefins are the main cause of gums. The whole mixture would undergo some extents of oxidation within the generator itself despite the absence of atmospheric oxygen.
The source of oxygen is already within the molecular structures of the said oxygenated hydrocabons. Of course, heat would greatly catalyze and hasten the decomposition.
Visually, we'd see the gasoline being quickly 'transformed' into a darker and often, more viscous liquid which might have a different odour from the fresh fuel.
The earlier experiment does not completely mimic what happens in the heated generators. For one, it cannot cause the gasoline to crack like what heat does to it.
However, it does cause an accelerated oxidation process to occur at lower temperatures. A mixture of chromic and sulphuric acids, even in diluted form, is a strong oxidizer. In the diluted form, it only attacks the more reactive components in the gasoline. The first to go would be the alcohols, followed by the other more 'polar' oxygenated hydrocarbons which are somewhat more water-soluble than the non-polar hydrocarbons. These oxygenated compounds also tend to get hydrolyzed in the presence of the acid. The alkanes or light paraffins within the gasoline are the most chemically unreactive. They don't tend to decompose with the presence of this diluted acid mixture.
Alkenes or olefins and aromatics, despite being non-polar, are still chemically reactive in the presence of a strong acid like sulphuric. That's due to them being unsaturated and having less stable molecular bondings as compared to light paraffins/alkanes.
The reaction with dilute sulphuric acid does nothing to mimic anything that happens in an operating stove or lantern generator. The main intention was to hasten the degradation of the reactive components. Here, some of the more reactive non-polar stuffs would co-incidentally react to form something more polar and easier to separate out with water. Ketones and aromatic sulfonic acids are some of the possible outcomes. A lot of them are water soluble.
If enough of those undesirable components can be easily separated out, the treated gasoline would probably be a little more friendly for liquid-fueled lanterns and stoves. Its not going to turn unleaded gasoline into something comparable to 'pure' light naphtha. Its might, possibly, remove just enough of the said undesirable compounds to make gasoline a useable option for such applications.
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An interesting insight …

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An interesting insight …

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