Turning petroleum into diesel fuel is called refining. Refining is the process of converting crude oil into high-value products. The most important are transportation fuels: gasoline, jet fuel and diesel fuel. The products include liquefied petroleum gas, heating fuel, lubricating oil, wax and asphalt.
The world’s first large-scale refinery was built in Romania in 1856 at the site of the Ploiesti oil fields. The refinery technology in the mid-19th century was based on distillation. Distillation has been known since the 4th century BC when Aristotle first wrote about it.
Then it was used mostly for purifying water, and centuries later for making alcoholic beverages. It is the origin of the word still, as in a bootlegger’s still.
Distillation is a physical process, not a chemical one. Distilling separates two or more components in a liquid based on a physical property, usually the boiling point. For example, wine can be distilled into brandy to make a stronger after-dinner drink. By evaporating the flavored alcohol in a dry, white wine and then condensing that vapor back into a liquid, it becomes the raw material that is aged in barrels to make cognac.
At modern oil refineries, fractional distillation remains the major process used to produce diesel. It separates crude oil into its component parts, or fractions. Given the popular images of destructive oil well fires, it may seem odd to boil petroleum, but that is exactly how it is done.
Petroleum is heated by high pressure steam to about 1110 degrees F, until it boils continuously. This causes most of the petroleum to turn into different gases, each with different boiling points. These vapors are funneled into a distillation tower that can be up to 150 feet tall.
As the gases rise in the tower, they also cool. Once the vapor rises to a height high enough to cool it to its boiling point, it condenses back into a liquid. Collection points at different heights in the tower siphon off the various liquid fractions. The lower the boiling point, the higher in the tower the distilled product condenses and is collected.
From crude oil, propane and butane boil first and are collected at the highest points in the tower. Since gasoline has a higher boiling point than propane, it condenses a little lower in the tower. Kerosene and diesel have the next higher boiling points and they are tapped off at even a lower point in the distillation tower.
Separation techniques alone cannot meet the demand for higher quality diesel required by today’s computer-controlled common rail engines, so additional methods are needed to squeeze the most useable amount of diesel out of a barrel of crude.
A chemical process called upgrading is used to remove undesirable components found in fuel through use of a catalyst. The most common catalyst used in refining diesel fuel is hydrogen. This hydroprocessing technique of mixing diesel with hydrogen removes sulfur from the fuel. It is an important step, necessary to meet the stringent emissions requirements imposed by the International Maritime Organization on new yacht engines, which require the use of ultra low-sulfur diesel fuel.
Another technique used by refineries to yield more diesel is known as conversion, which literally changes the molecular structure of petroleum. Converting larger, heavier hydrocarbon molecules by breaking them into smaller, lighter molecules is known as cracking. Cracking is a key source of diesel fuel. Vacuum gas oil, a heavy leftover product from the crude oil distillation process, is chemically cracked to produce diesel.
Combining current advances in chemistry, engineering and metallurgy at newer refineries yields an average of 12 gallons of diesel from a 42 gallon barrel of crude oil. For every gallon of diesel bunkered aboard a yacht in the United States, 43 percent of the cost of that gallon is used to pay for extracting the crude oil from the ground and shipping it to the refinery. Seventeen percent of the cost is associated with the refining process, while distribution and marketing comprise 19 percent. The remaining 21 percent pays the taxes levied by federal, state and local governments.
In the next 15 years, worldwide demand for diesel fuel will reach 34 million barrels per day. This increase is being driven by the industrialization of emerging countries. This demand will help develop more efficient refinery techniques and continue to spur the growth of biodiesel alternatives. And diesel fuel will continue to remain the primary power source for yachts for many years into the future.
Capt. Jeff Werner has been in yachting for almost 25 years. Contact him through MyDieselDoctor.com.