It's That Time Again (or Refining 101: Winter Gasoline)
Posted by Robert Rapier on September 11, 2007 - 10:00am
Every year in late summer, you will start hearing references in the media about the conversion to winter gasoline, such as the following (originally in the Bradenton Herald, but the link is long dead):
Motorists can thank a mild hurricane season in the Atlantic for the lower gas prices, according to the American Automobile Association.Other factors include the end of the summer driving season and a cheaper winter fuel mix.
Gas stations sell a special, more expensive fuel blend during the summer to cut down on smog during hot months. Stations nationwide will start selling a less-expensive winter fuel blend Friday, which could lead to even lower prices, analysts said.
So what does this mean, and why does it make winter gasoline less expensive?
A Primer on Gasoline Blending
Gasoline is composed of many different hydrocarbons. Crude oil enters a refinery, and is processed through various units before being blended into gasoline. A refinery may have a fluid catalytic cracker (FCC), an alkylate unit, and a reformer, each of which produces gasoline blending components. Alkylate gasoline, for example, is valuable because it has a very high octane, and can be used to produce high-octane (and higher value) blends. Light straight run gasoline is the least processed stream. It is cheap to produce, but it has a low octane. The person specifying the gasoline blends has to mix all of the components together to meet the product specifications.
There are two very important (although not the only) specifications that need to be met for each gasoline blend. The gasoline needs to have the proper octane, and it needs to have the proper Reid vapor pressure, or RVP. While the octane of a particular grade is constant throughout the year, the RVP spec changes as cooler weather sets in.
The RVP is the vapor pressure of the gasoline blend when the temperature is 100 degrees F. Normal atmospheric pressure varies, but is usually around 14.7 lbs per square inch (psi). Atmospheric pressure is caused by the weight of the air over our heads. If a liquid has a vapor pressure of greater than local atmospheric pressure, that liquid boils. For example, when you heat a pot of water, the vapor pressure increases until it reaches atmospheric pressure. At that point, the water begins to boil.
In the summer, when temperatures can exceed 100 degrees F in many locations, it is important that the RVP of gasoline is well below 14.7. Otherwise, it can pressure up your gas tanks and gas cans, and it can boil in open containers. Gas that is boiled off ends up in the atmosphere, and contributes to air pollution. Therefore, the EPA has declared that summer gasoline blends may not exceed 7.8 psi in some locations, and 9.0 psi in others.
A typical summer gasoline blend might consist of 40% FCC gas, 25% straight run gas, 15% alkylate, 18% reformate, and 2% butane. The RVP of the gasoline blend depends on how much of each component is in the blend, and what the RVP is of each component. Butane is a relatively inexpensive ingredient in gasoline, but it has the highest vapor pressure at around 52 psi.
In a gasoline blend, each component contributes a fraction to the overall RVP. In the case of butane, if there is 10% butane in the blend, it will contribute around 5.2 psi (10% of 52 psi) to the overall blend. (In reality, it is slightly more complicated than this, because some components interact with each other which can affect the expected RVP). This means that in the summer, the butane fraction must be very low in the gasoline, or the overall RVP of the blend will be too high. That is the primary difference between winter and summer gasoline blends.
Why Prices Fall in the Fall
Winter gasoline blends are phased in as the weather gets cooler. September 15th is the date of the first increase in RVP, and in some areas the allowed RVP eventually increases to 15 psi. This has two implications for gasoline prices every fall. First, as noted, butane is a cheaper blending component than most of the other ingredients. That makes fall and winter gasoline cheaper to produce. But butane is also abundant, so that means that gasoline supplies effectively increase as the RVP requirement increases. Not only that, but this all takes place after summer driving season, when demand typically falls off.
These factors normally combine each year to reduce gasoline prices in the fall (even in non-election years). The RVP is stepped back down to summer levels starting in the spring, and this usually causes prices to increase. But lest you think of buying cheap winter gasoline and storing it until spring or summer, remember that it will pressure up as the weather heats up, and the contained butane will start to vaporize out of the mix.
And that's why gasoline prices generally fall back in the fall, and spring forward in the spring.
ED: This post originally ran September 15, 2006.
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http://science.reddit.com/info/2o6cd/comments
if you are so inclined...
Thanks for the refresher Robert!
Any guesses on tomorrow's numbers?
However, a note about gasoline outside of the U.S. - my local source of oil retailing information (an independent dealer of heating oil, raps oil, biodiesel, gasoline, etc.) has never heard of summer/winter gasoline, and still assuming we both understood each other adequately after 10 minutes of discussion (mistakes crop up easily where basic assumptions may be quite different), there is no such difference in Germany.
Admittedly, the weather extremes are considerably narrower, but the retailer felt that different formulations for different weather was not necessary, and was still quite puzzled at the idea as I left.
This may be more a matter of custom than anything else. And for the record, Germany remains a huge heating oil market, so the refinery switch is essentially the same, but not the blending.
Perhaps, in a market where more of the cost per unit is tax, the difference realised is so much less in percentage terms that there's less market advantage in saving a few cents per litre from changing the blend contrasted with the complexity costs from managing the seasonal variation
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Jaymax (cornucomer-doomopian)
that little difference is a big margin. There are also reasons to change RVP in winter in terms of driveability. Summer gas has so little vapor in cold temps it's hard to get your car to start.
What portion of the reduction is passed on to consumers?
If it's all of it, then there's no margin in it at all, just market share protection necessities - which is where the high tax element in europe might push the issue towards redundant.
I can't speak to the technical stuff except to say that cars in the UK are harder to start in winter - my old MG used to border on the impossible...
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Jaymax (cornucomer-doomopian)
as little as possible of course!
but refining margins tend to be much lower in fall.
Right now the US gasoline crack is just $5/bbl down from near $30 in April/May. US gasoline is down 40+ cts/gallon ($17/bbl) at the wholesale level while crude is up $10/bbl
So the consumer is defn getting a bit of a break at the expense of the refiners. This is one reason not to expect much more drop off in gasoline prices this fall. They can't fall much without refiners cutting runs.
It's real. If you look at gasoline specs you see one called "drive ability index" Has inputs of RVP and front end distillation temperatures IIRC. Basically measures whether you can get things started.
I can't speak to the point about different formulations for hot or cold, as my only direct source of knowledge was completely unaware of it. A dimly recalled web search from years ago was also fruitless, though the mechanic for my BMW bike in the U.S. always maintained that Euopean gasoline and motor oil was refined and formulated to a much higher standard than common in the U.S.
However, as for the idea of costs being buried under taxes - absolutely no way. Prices here can shift 5 euro cents a liter in a day (or something like 25 US cents a gallon) - those shifts have absolutely nothing to do with taxes. However, they do seem fairly directly tied to the euro/dollar exchange rate, and the price of crude. Not perfectly, and as always, the price goes up like a rocket, and falls like a feather, but the correlation is pretty clear, as is the causation.
Nice to see even Europeans can believe in a myth of exceptionalism. European gas was cheaper and easier to make in my day. Most European refineries couldn't meet US RFG specs even back then.
Now Euro diesel was better stuff. Europeans demanded 45+ cetane while we lived with 40 in the US to be able to bury all the LCO from FCC units into the pool.
Which all then still leaves the question, why, when Summer/Winter gas is significant and known in the US, is Summer/Winter petrol unmentioned (or non-applicable?) in Europe?
still curious...
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Jaymax (cornucomer-doomopian)
Robert, if you were going to store some gasoline...which is better...summer or winter?
The way I read it is, that if you store Winter into summer...all the butane would boil off?
But it wouldn't be as bad storing or using summer blend in winter?
Is that right?
I believe that the concern with summer fuel in winter is with use: it may be harder to start your car/mower/boat/etc when using the summer fuel in winter time, as the colder temperatures and lower RVP would conspire to make for difficult starting because the fuel would remain a liquid in the colder engine.
As for diesel: summer diesel can become viscus in cold fuel lines. In that case you won't be going anywhere.
From a mechanics perspective, storing gasoline for longer than 3 or 4 months is a bad idea because it turns brown and gums up a fuel system. You can use "stay-bil" to extend this a few more months but it is costly.
Two notes about storage, and a warning -
1. Gasoline can most certainly be stored for longer than 3-4 months before having the problems you describe. At least if it was Amoco's best (gold, if I recall their naming correctly), from the mid 1990s - don't know about today, of course. My bike was left in the U.S., and when visiting, I would use the left over gas - cranked with no problem even after a year (however, I did drain the carbs before leaving - and when I originally bought the bike, the fuel had gummed, since it hadn't been ridden for a while).
2. The dealer I know was also unconcerned about storage times for gasoline or diesel, and was somewhat skeptical about my past experiences. Of course, he was also unfamilar with the idea of storing gasoline or diesel for any amount of time. (although heating oil may be stored for months in a home tank between two winters).
The warning is simple - don't store gasoline anywhere where people live, or in a structure which you care about, or with anything valuable. Gasoline is very dangerous, a fact which seems almost impossible to get across to most people. I am certain that in a shortage situation, a number of people will suffer horribly when they attempt to hoard gasoline.
Good advice!
I stored 20 gal using Sta-Bil for a year with no problems except I had to filter it because of some particulates that formed during storage.
This year I'm using PRI-G whose website claims 10 year storage.
http://www.priproducts.com/
I'll give an update then (if any of us are still around)
:)
there's a big difference between storing 10,000,000 gallons is a storage tank with a floating roof and storing a gallon in a fuel tank. But you do have to watch the gum formation. US gasoline is pretty heavily hydrotreated these days to get the sulfur out so the bad actors (double bonds = olefins) are gone. In the old days, olefinic blends were welcome in the blends as they have good octane and gave good lead response (IIRC).
you store summer grade in winter to deliver in spring/summer. That's when gasoline prices bottom and generally the futures will allow you some "carry" to hedge yourself. Refiners will make you summer spec in winter if they have surplus capacity and you pay a small premium. Or you can blend it yourself if you have the right components.
Robert,
What do you think of Rive Technology's new zeolite material that it claims will dramtically increase the yield of gasoline from a barrel of oil? -- by increasing the pore size from 1 nm to 2-10 nm.
How do they get butane to stay in liquid form in gasoline. I think its boiling temperature is -0.5C.
A boiling point of -0.5C means that a 100% butane mixture will boil at that temperature (at 101.3 kPa pressure).
My understanding as to why a mixture of butane and heavier hydrocarbons will not boil at -0.5C is boiling point elevation, much as solutes in water raises its boiling point.
Boiling happens because the molecules of butane evaporate faster from the surface than the gaseous butane condenses to rejoin the liquid, causing bubbles to grow in the fluid. When you have smaller concentrations of butane, the evaporation flux (measured by the partial pressure of the butane in the mix) reduces proportionately to the decrease in concentration as butane is less abundant in the liquid. However, the condensation flux (measured by the partial pressure of the gas bubbles) stays the same in the gas bubbles as it is mostly butane gas. At -0.5C and 100% butane concentration, the fluxes are balanced and boiling can occur if heat is added. At lower concentrations, higher temperatures are needed to generate more partial pressure.
While butane might not boil at -0.5C when mixed with heavier hydrocarbons, it will still readily escape by surface evaporation at rates roughly comparable to water evaporation @ 99C, so it should be kept in a closed container.
Thanks Yartrebo. A good explanation, understandable to someone who knows nothing about it.
Summer RVP in Los Angeles is down around 4-5 PSI IIRC. The variations across the world are pretty extreme.
I'm trying to picture Tucson gasoline. A hundred and twenty degrees in the summer and half a mile of altitude. I can't see holding on to butane even in a eutectic mixture.
RobertInSantaBarbara
I haven`t escaped from reality. I have a daypass.
Does anybody know how the change in formula affects fuel efficiency?
Butane also has lower volumetric energy density. This implies, to me, that winter gasoline may costs less but it also has lower Miles per Gallon.
Is this true ? And how strong is the effect ?
Thanks,
Alan