by Caoimhan » Wed 15 Jun 2005, 14:09:36
$this->bbcode_second_pass_quote('lorenzo', 'V')ery interesting, Graeme. I have been wondering whether this trend towards diesel won't cause problems. As I understand it, a barrel of oil can be broken up into several fractions, but they're more or less fixed. So a certain amount of energy may be wasted by trying to get more diesel out of a barrel.
Or maybe it will just make gasoline cheaper?
This is only partially true. Here is a section of an article from this site:
http://www.dit.ie/DIT/science/chemistry ... oleum.html$this->bbcode_second_pass_quote('', 'C')rude oil is rarely used in its raw form but must instead be processed into its various products. Aside from contaminant minerals such as sulfur and small amounts of trace metals--which are removed during refining--petroleum is composed of hydrocarbons, essentially varying combinations of carbon and hydrogen atoms; any hydrocarbon can be converted into any other given the appropriate application of energy, chemistry, and technology. The smaller the molecule and the lower the ratio of carbon to hydrogen, the lighter the hydrocarbon, the lower the evaporation temperature, and, usually, the more valuable the product.
Every crude oil contains a mix of these different hydrocarbons, and the two tasks of a refinery are to separate them out into usable products and to convert the less desirable hydrocarbons into more valuable ones.
The tall metal towers that characterize petroleum refineries are distillation, or fractionating, towers. Distillation is the primary method used to refine petroleum. When the heated crude oil is fed into the lower part of a tower, the lighter oil portions, or fractions, vaporize. Losing temperature as they rise, they condense into liquids, which flow downward into the higher temperatures and are revaporized. This process continues until the various fractions have achieved the appropriate degrees of purity. The lighter fractions, like butane, gasoline, and kerosene, are tapped off from the top; heavier fractions, like fuel and diesel oils, are taken from below.
At more complex refineries the less valuable products of distillation are refined once again through various conversion processes, broadly referred to as "cracking." Through the application of vacuum, heat, and catalysts, larger, heavier molecules are broken down into lighter ones. Thermal cracking, for instance, uses heat and pressure, while catalytic cracking employs a finely powdered catalyst, and hydrocracking involves the addition of hydrogen to produce compounds with lower carbon to hydrogen ratios, such as gasoline. Other processes produce high-octane products for blending with fuels, remove undesirable constituents, or make special petroleum compounds, including lubricants.
Petroleum products are usually distributed from the refinery in the form in which they are to be used. Depending on the geographical location, customer demand, and seasonal needs, refiners can substantially alter their production. In winter, for example, less gasoline and more heating oil is produced.
The chief refinery products are liquefied petroleum gas (LPG); gasoline and jet fuel; petroleum solvents; kerosene; the so-called middle distillates, including heating oil and diesel fuel (known as gasoil outside the United States); residual fuel oil; and asphalts (bitumens), the heaviest fractions. In the United States, with its high demand for gasoline, refineries typically upgrade their products much more than in other areas of the world, where the heavy end products, like residual fuel oil, are used in industry and power generation.
It's important to note that #2 Diesel is the middle-distillate, but #1 "Winter" Diesel is kerosine, a light-distillate. Also, please note that currently, additional cracking of middle-distillates is done to increase the concentrations of light hydrocarbons. A greater reliance on middle-distillate grade fuels could mean less cracking, and more efficient refining.