Challenging My Understanding of Gasoline

[ekpolk]

Last Friday, I decided to attempt another of my simplistic science experiments. I was about to feed a five-gallon jug of gasoline into my wife's Avalon. To promote good mixing, I had just dosed it up with 1.5 oz of TC-W3 2C oil. When I had filled the can, about two months ago, I had added recommended doses of both Sta-bil and FP+.

So, before dumping it into the car, I carefully poured out eight (8) ounces of this mixture into a cylindrical, graduated glass beaker. Then I set the beaker outside. I was wanting to see how quickly evaporation would eliminate this cup of sacrificial fuel. How surprised I would be.

Within several hours, seven of the eight ounces were gone. Originally, the fuel had a very light blue-green tint, from the TC-W3. With only one ounce left, the color was a much greener. It was more like a faded grass color, well short of emerald green.

Anyway, what's strange is that this last ounce of liquid has remained for several days. Evaporation is almost non-existent. It has gone down maybe the width of the meniscus curve on the top of the liquid. The liquid is still very clear, and very low vis (it has not thickened at all, at least not observably so). It has a stronger gasoline stench, but that's about the only very noticeable difference between the poured fresh gas, and this stuff.

I had expected that all of the gasoline would quickly evaporate, but obviously, I was wrong. I'm certainly no chemical or petro engineer, so I'd be interested in hearing an expert's take on this. BTW, the fuel involved is Shell 87 octane, pumped in the late summer in Pensacola FL.

 

[mstrjon32]

I'm a little surprised that so much evaporated in the first place... I would have expected very little to evaporate.

 

[StevieC]

Originally Posted By: mstrjon32
I'm a little surprised that so much evaporated in the first place... I would have expected very little to evaporate.
+1

 

[Pablo]

How hot was it?

Gas sits for a long while before it gets a bunch of rain water in it here.

All the lower boiling point goodies left first, then the alcohol, then you are stuck with cogeners, oils, higher boiling fraction......

 

[ionbeam22]

Higher boiling fractions, additives, alcohols + gums.

 

[Audi Junkie]

Originally Posted By: ionbeam22
Higher boiling fractions, additives, alcohols + gums.


Sounds like my weekend.

 

[ekpolk]

Originally Posted By: Pablo
How hot was it?

Gas sits for a long while before it gets a bunch of rain water in it here.

All the lower boiling point goodies left first, then the alcohol, then you are stuck with cogeners, oils, higher boiling fraction......

I left the beaker out front on a patio table for the "first phase" of this experiment. As I said, the first seven of eight ounces disappeared very, very quickly -- in a matter of hours. It was in the mid-80s F for temp, with breezy light wind. Since then, I've moved the beaker in or out depending upon the weather (didn't want to get any rain in the sample). As of today, we're down to probably 6/10ths of an ounce, still smells as skanky as can be, is crystal clear, no "thickness" detectable by eye, and it's now a deepening olive green color.

======================================================
Originally Posted By: ionbeam22
Higher boiling fractions, additives, alcohols + gums.


I figured as much. I'm just very surprised at how dramatic a difference there is between the stuff that evaporates quickly, and the stuff that seems to want to persist. I had figured, incorrectly it seems, that the whole body of fluid would disappear at a somewhat uniform rate, leaving behind some solid (or semi-solid) residue. Not so!

 

[jhenle]

Would have been interesting if you had set another beaker up at the same time, with gasoline straight from the pump.

 

[mechtech2]

Maybe some animal drank it.
That rate of evaporization is hard for me to totally believe, just yet.

 

[Lyondellic]

This is not all that surprising. Everyone knows that gasoline is a blend of various components, but few know that it has a published boiling point somewhere around 85°F at atmospheric pressure (14.7 psia). The initial and final boiling points for each component do vary and will increase/decrease as pressure is increased/decreased. Surface area also plays a role, which partially explains why the heavier components are taking longer to dissipate in the beaker.

Think about the last time you spilled a little gasoline while topping off your lawn mower's fuel tank. Even when the engine was cold, it does not take long for small gasoline spills to dissipate.

 

[ekpolk]

Originally Posted By: mechtech2
Maybe some animal drank it.
That rate of evaporization is hard for me to totally believe, just yet.


Well, based upon climatic factors, you should get even more volatile gasoline than I do. Try this for yourself, and see what happens before you declare what you do or don't believe. . .

 

[ekpolk]

Originally Posted By: Lyondellic
This is not all that surprising. Everyone knows that gasoline is a blend of various components, but few know that it has a published boiling point somewhere around 85°F at atmospheric pressure (14.7 psia). . . .


That would seem to explain what I'm seeing.

I wonder how long the olive green stuff will remain -- and what, if anything, will remain on the glass when the evaporation is complete.

 

[labman]

Originally Posted By: Lyondellic
This is not all that surprising. Everyone knows that gasoline is a blend of various components, but few know that it has a published boiling point somewhere around 85°F at atmospheric pressure (14.7 psia). The initial and final boiling points for each component do vary and will increase/decrease as pressure is increased/decreased. Surface area also plays a role, which partially explains why the heavier components are taking longer to dissipate in the beaker.

Think about the last time you spilled a little gasoline while topping off your lawn mower's fuel tank. Even when the engine was cold, it does not take long for small gasoline spills to dissipate.


85 F? What happens on a hot day?

 

[ionbeam22]

Originally Posted By: ekpolk
I had figured, incorrectly it seems, that the whole body of fluid would disappear at a somewhat uniform rate, leaving behind some solid (or semi-solid) residue. Not so!


No, in fact there are tests for the presence of solid residue in fuel and its not allowed. Ditto for filterable matter.

You have to remember though, that gasoline is burned as a fine mist, and most these residual compounds have interesting behaviour when in the form of thin films/mists. Or if they have been volitilaised by other lighter components.

All that will continue to happen is that you will eventually get a mix of varnish (from olefins that have oxidised) and various non-volatile components (lubricating oils + detergents)

 

[Lyondellic]

Originally Posted By: labman
Originally Posted By: Lyondellic
This is not all that surprising. Everyone knows that gasoline is a blend of various components, but few know that it has a published boiling point somewhere around 85°F at atmospheric pressure (14.7 psia). The initial and final boiling points for each component do vary and will increase/decrease as pressure is increased/decreased. Surface area also plays a role, which partially explains why the heavier components are taking longer to dissipate in the beaker.

Think about the last time you spilled a little gasoline while topping off your lawn mower's fuel tank. Even when the engine was cold, it does not take long for small gasoline spills to dissipate.


85 F? What happens on a hot day?


That would depend on the blend to some degree. For example, C4 hydrocarbons (BB's) are added to winter blend gasoline to increase vapor pressure. BB's have initial/final boiling points well below the ambient temperature range at 14.7 psia in most of the United States. The heavier components components have final boiling points around 375°F to 400°F, which is the range for C5 hydrocarbons (basically Naphtha).

I suspect that one of the previous replies was correct when they stated that the residual material in the beaker is most likely ethanol, fuel additives and a small quantity of entrained C5+ material. I can ask one of the chemists in the Lab at the refinery where I work for their opinion on Monday.

 

[labman]

Without a fractionating column, most of a solution of different boiling liquids will boil away once it reaches the boiling point of the mixture. By the way, octane is a C8. It comes in both straight chain and various isomers. Normal octane boils at 126 C, higher than water.

 

[ekpolk]

Originally Posted By: Lyondellic
. . .
I suspect that one of the previous replies was correct when they stated that the residual material in the beaker is most likely ethanol, fuel additives and a small quantity of entrained C5+ material. I can ask one of the chemists in the Lab at the refinery where I work for their opinion on Monday.


That would be cool, thanks!

EDIT: "BBs" -- I'm sorry, remember, it's been many years since I slogged through my last chemistry class (the dim recesses of the early 80s. . .). I remember basic concepts, but that's about it. Is this a reference to some form of butane?

 

[Shannow]

Originally Posted By: ekpolk
Anyway, what's strange is that this last ounce of liquid has remained for several days. Evaporation is almost non-existent. It has gone down maybe the width of the meniscus curve on the top of the liquid. The liquid is still very clear, and very low vis (it has not thickened at all, at least not observably so). It has a stronger gasoline stench, but that's about the only very noticeable difference between the poured fresh gas, and this stuff.

I had expected that all of the gasoline would quickly evaporate, but obviously, I was wrong. I'm certainly no chemical or petro engineer, so I'd be interested in hearing an expert's take on this. BTW, the fuel involved is Shell 87 octane, pumped in the late summer in Pensacola FL.

It's how the petrol companies stopped the 100MPG carburettors from functioning...

(well at least that's what I've read, LOL)

 

[StevieC]

Originally Posted By: Shannow
It's how the petrol companies stopped the 100MPG carburettors from functioning...

(well at least that's what I've read, LOL)


So it's true then...

 

[severach]

I've spilled small amounts of E10 and it all evaporates away leaving nothing but a surface pattern in the dust. When I evaporate gasoline treated with MMO and PEA spills leave an oil stain.

A lot of UCL went into that gasoline.

To understand vapor pressure: Why is summer fuel more expensive than winter fuel?

Be sure to read The Oil Drum article.