Much like a simple still, in a distilling column, liquid is heated to a vapor and lifted upward to be distilled again into separate substances. This is the beginning of the refining process. Distilling exploits the characteristic of the chemicals in crude oil to boil at different temperatures, a phenomenon that engineers chart along distillation curves. Unlike a still, a distilling column contains a set of trays that allow heated vapors to rise and collect at different levels, separating out the various liquids derived from crude oil. The top of the column is cooler than the bottom, so as liquids vaporize and rise, they condense again, collecting onto their respective trays. Butane and other light products rise to the top of the column, while straight-run gasoline, naphtha, kerosene, diesel, and heavy gas oil gather on the trays, leaving straight run residue at the base of the column. Liquids are considered “heavy” or “light” based on their specific gravity, which is determined based on its weight and density compared to that of water.
Because there is more demand for some distilled products like gasoline, refiners have an incentive to convert heavy liquids into lighter liquids. The term cracking comes from the process of breaking up long hydrocarbon molecules into smaller, more useful molecules. The cracking process converts heavy straight run liquids into gasoline. There are multiple versions of the cracking process, and refiners use the process extensively. Cracking is a highly controlled process, so cracking units exist separate from distillation columns. The most common type of cracking is “cat cracking,” named for the use of catalysts, substances added to a chemical reaction to speed up the process.
The process of reforming was developed to raise both the quality and volume of gasoline produced by refiners. Using a catalyst again, after a series of reforming processes, substances are converted into aromatics and isomers, which have much higher octane numbers than the paraffins and napthenes produced by other refinery processes. Most simply, reforming rearranges the naphtha hydrocarbons to create gasoline molecules. The reforming process produces reformate, which is needed to increase the octane for today’s cleaner burning fuels. Interestingly, hydrogen is also produced by the catalytic reforming process – this hydrogen is then used in other refining processes such as hydrotreating.
Crude oil naturally contains contaminants such as sulfur, nitrogen, and heavy metals, which are undesirable in motor fuels. The treating process, primarily hydrotreating, removes these chemicals by binding them with hydrogen, absorbing them in separate columns, or adding acids to remove them. The recovered molecules are then sold to other industries. Refineries that process sour crudes produce more sulfur than refineries that process sweet crudes. Following the treatment, blending, and cooling processes, the liquids finally look like the fuels and products you’re familiar with: gasoline, lubricants, kerosene, jet fuel, diesel fuel, heating oil, and petrochemical feedstocks that are needed to create the plastics and other products you use every day.
The last major step of the refining process is blending various streams into finished petroleum products. The various grades of motor fuels are blends of different streams or “fractions” such as reformate, alkylate, catalytically cracked gasoline, etc. Refineries blend compounds obtained either from their internal refining process operations as noted above, or externally, to make gasoline that meets specifications for acceptable motor vehicle performance. A typical refinery may produce as many as 8 to 15 different streams of hydrocarbons that they then must mix into motor fuels. Refiners might also mix in additives like octane enhancers, metal deactivators, anti-oxidants, anti-knock agents, rust inhibitors, or detergents into their hydrocarbon streams. Blending can take place at the refinery along the pipelines and tanks that house processed fuel or even at off-site locations or on ships or terminals once the fuel has left the refinery gate
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