Understanding Hydrocarbon-Based Fuel

Saturday, Apr 3, 2010

Background

Fuel supplies are essential for many aspects of modern society. Complex supply chains rely on hydrocarbon-fueled trucks, trains and planes to deliver food and other supplies in near real-time. Natural gas is is used to heat homes and fuel generators that supply approximately one-sixth of all electricity produced in the US. Large-scale food production is only possible with diesel-fueled farm equipment and synthetic nitrate fertilizers, made from natural gas.

It is not hard to imagine that anything more than a brief blip in fuel supplies would result in TEOTWAWKI. We have seen the result of Hurricanes Katrina and Ike on fuel supplies and prices. Some of us are old enough to remember the Arab oil embargo of 1973. Much motor fuel is transported by long pipelines which cross active earthquake faults and are also vulnerable to “man-caused disasters”. I once asked an oil company employee responsible for fuel supply logistics, “how vulnerable is the system to disruption?”. She replied they could handle one hiccup, but two sequential events, or one large event would bring the system down. In my opinion, the question is not “if” we will see fuel supply disruptions, but rather “when” and “how severe”.

For the “prepper”, fuel has specific utility beyond the normal. It is essential to have enough motor fuel for your G.O.O.D. plan. Fuel is needed to power generators when the grid is down and to run your rototiller or other farm equipment. In a “Mad Max” scenario, fuel can be used as a currency for barter or exchange. This article is a very basic primer on modern fuels, with an emphasis on gasoline (a.k.a. petrol). It includes information that would help the user improve the selection, use and storage of gasoline.

Crude Fractions

Most hydrocarbon fuel is derived from “crude” oil, which is a mixture of hundreds or thousands of different carbon-based chemicals. These different chemicals are separated, transformed and blended to form the the final fuel products which we purchase. Crude “fractions” are generally categorized by the number of carbon atoms per molecule, and mostly separated by their boiling properties. Approximate numbers of carbon atoms of crude factions are listed below:

C1 - Methane, natural gas
C2 - Ethane
C3 - Propane
C4 - Butane
C5-C10 Naphtha, Gasoline
C10-C16- Kerosene, Jet fuel
C10 C20 - Diesel, Gasoil
C20+ Heating, Bunker (Ship) Fuel, Cracker Feed

Most modern refineries have process units which take longer hydrocarbons, crack them into smaller molecules, separate them into the fractions above and blend them into the final product. This allows refiners to increase the fuel (distillate) yield above what is possible from separation alone. These “cracked” molecules have more unsaturated chemical bonds and are more reactive than “straight run” (non-reacted) components.

Other Components

Two other gasoline components are often included in gasoline, and are not considered cracked or straight run. These components are ethanol and alkylate. Either can run alone in a gasoline engine with little or no modification, but they have very different burning and storage properties. Understanding the difference can help the prepper obtain the right fuel for purpose.

Ethanol can be “created” by fermenting plant products (carbohydrates). Ethanol has two carbon atoms, but also contains an oxygen atom. This gives the molecule polarity, which makes it possible to blend with water in any concentration. As a gasoline component, it will absorb water easily without phase separation (think alcohol proof vs. oil and vinegar dressing). However, it is thermodynamically impossible to purify alcohol to more than 97% purity from water mixtures with distillation alone.

The extra oxygen atom in ethanol also means it produces less energy on combustion, since it is already partially oxidized. Ethanol blended gasoline cannot even be pumped through pipelines because its water absorption properties can corrode the pipeline. Ethanol is transported separately by truck and blended at the terminal. It’s use as a fuel, on a commercial scale, is rarely economical without government subsidy, and also competes with food production with normal sources of carbohydrates.

Alkylate is a refiner's name for 2,2,4 trimethyl pentane. It is the only major gasoline / petrol component in traditional refineries created by combining smaller molecules, and can also be run in a gasoline engine, with little or no modification. (A friend of mine in a British refinery recounted the experience of discovering a hose running under the refinery fence, connected to the alkylate tank, where operator(s) were helping themselves to the company’s product). It would probably be difficult to start an engine with pure alkylate fuel on a cold day, but that could be solved by a shot of ether before engaging the ignition.

Don't Be Fooled by Octane Numbers

When you visit the gasoline pump, you are usually offered three different “grades” of gasoline, differentiated by the fuel’s octane number. Idiomatically, octane is often associated with energy (e.g. high octane energy drink), but often the exact opposite is true. There are different scales of octane (RON, MON, Average/AKI) used in different countries, just like Celsius and Fahrenheit, but overall octane is simply a measure of the fuel’s propensity to burn without applying a spark. Higher octane gasoline is less likely to auto-ignite and sells at a higher price, simply because it uses more expensive components.

The disconnect between energy and octane can be shown by examining the properties of ethanol and alkylate. Alkylate has an octane measurement (MON) of 100, by definition. Ethanol, on the same octane scale, has an octane measurement of 102. However, the energy content per volume (using properties from Wikipedia) of the two fuels is very different. Ethanol has an energy of combustion of 23.5 kJ/m3. Alkylate has a value of 32.9 kJ/m3, 40% more than ethanol! The alkylate fuel would move you 14 miles compared to only 10 for the ethanol fuel. Unfortunately you can’t buy a tank of alkylate, but you can avoid ethanol blends when you want a fixed volume of fuel to last as long as possible.

Most gasoline in the US is transported by pipeline, especially for the population-dense East coast. This is done by sequentially sending and segregating alternate batches of low and high octane gasoline from the Gulf Coast. Medium-grade gasoline is a product which is only blended at the pipeline terminus. The consumer can save themselves the blending premium by doing their own blending in their own gas tank. Using an approximate “linear blending by volume” rule, 5 gallons of 93 octane mixed with 10 gallons of 87 octane fuel will yield 15 gallons of approximate 89 octane fuel. When I looked at the pump this morning, that is a savings of approximately $0.07/gal for 89 octane fuel. Of course, you can save yourself the entire octane premium by using low octane fuel, if your engine will take it, and you may even go further on the same tank.

Gasoline Degradation

Over time, gasoline can degrade and become unusable. There are three main mechanisms that make this happen, unequal vaporization, water absorption and gum formation.

There are two main seasonal formulations of gasoline in North America. A higher volatility fuel is sold in the winter in order to help vehicles start in cold weather. This fuel usually contains additional amounts of butane, sometimes up to 10%. In general, butane has less energy density than other gasoline components, and the winter fuel blend will generally result in less gas mileage than the summer blend.

The winter blend is also more susceptible to unequal vaporization, where the lighter components will evaporate more quickly than the heavier components, especially in warm weather. The summer blend of gasoline is normally sold between May and September, although the laws vary by location. The summer blend will result in less pressure buildup in closed container and less loss by evaporation. They only potential negative of the summer blend is not enough volatile components in really cold weather. The user can get around this problem by starting the engine with the winter fuel, and then adding the summer blend. Or the other option already mentioned is to use a squirt of ether before ignition. Avoiding swings in temperature will minimize the unequal evaporation.

Absorption of water into gasoline is best avoided by minimizing contact of the fuel with air. An impermeable metal container is best. The other method, mentioned above, to reduce water absorption in fuel, is to avoid any blends that contain ethanol.

Lastly, gum formation is caused by the oxidation of unsaturated hydrocarbons. Cracked gasoline, one of the largest components in the final fuel, is the main source for these reactive components. Formation of gums is caused when these components come in contact with oxygen. This can be avoided (again) by minimizing contact with air by storing the fuel in an impermeable, metal storage container. For long-term storage, oxidizing scavenging chemicals contained in stabilizing additives will help avoid gum formation.

Ideality

Current gasoline products are not designed for long-term storage. It is in the oil company’s best interest to minimize inventory and speed the time to market. Unfortunately, that means there is no incentive to make fuel with longer shelf life. It also makes society more sensitive to supply disruptions.

However, the technology exists to make fuel with essentially an infinite shelf life. A mixture of alkylate and butane would probably meet all vehicle requirements and most government requirements (except for oxygenates). Such a fuel would also probably have an “infinite” shelf life. When I was a kid, my parents purchased an old home built before the civil war. In the basement was a kerosene storage tank that was still about half full of fuel. In 2004 I took my children to see the home that I grew up in. The current owners had had the tank drained, and found the kerosene still usable! Wouldn’t it be nice if we could make gasoline that would last for 70+years. Maybe if we get enough of us together, we could convince a smaller oil company to cook us up a special batch of survival gasoline :-)

Summary

• Octane is not a measure of energy content, pay only for the octane that you need to avoid engine knocking.
• You can blend your own medium octane gasoline at lower price by just mixing appropriate ratios of low and high octane grades at the pump.
• Ethanol as an additive results in fuel with lower energy content, and more susceptibility to water absorption - avoid it if possible.
• There are two main seasonal formulations of gasoline. The winter formulation contains a higher amount of light components (butane), which will boil off more quickly at high temperatures.
• When storing fuel for the long-term, use a non-permeable material (metal). Store in a location that has constant, cool temperature.
• Use gasoline stabilizer, in addition to the steps above, to avoid gum formation in gasoline stored for long-term.


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