by ROCKMAN » Tue 07 Feb 2017, 07:53:31
Z - "And I'm not sure how well refineries could adapt their output." Some flexibility but there are limits. For instance we produce more distillate then our domestic market demands so we export a good bit. One of the reasons the US is the largest exporter of refined oil products.
From the EIA: "In 2015, refineries in the United States produced an average of about 19 gallons of motor gasoline and 12 gallons of ultra-low sulfur distillate fuel oil (includes diesel fuel and heating oil) from one 42-gallon barrel of crude oil. Many other petroleum products are also refined from crude oil. Refinery yields of individual products vary from month to month as refiners focus operations to meet demand for different products and work to maximize profits."
IOW the diesel yield is a function of the composition of the blended oil processed by a refinery. And while blends are designed with specific yield targets all oil contains some distillate fuel oil (diesel fuel and heating oil). For those not familiar with the refining process:
Separation - Modern separation involves piping crude oil through hot furnaces. The resulting liquids and vapors are discharged into distillation units. Inside the distillation units, the liquids and vapors separate into petroleum components called fractions according to their weight and boiling point. Heavy fractions are on the bottom and light fractions are on the top. The lightest fractions, including gasoline and liquid petroleum gas (LPG), vaporize and rise to the top of the distillation tower, where they condense back to liquids. Medium weight liquids, including kerosene and diesel oil distillates, stay in the middle of the distillation tower. Heavier liquids, called gas oils, separate lower down in the distillation tower, while the heaviest fractions with the highest boiling points settle at the bottom of the tower.
Conversion - After distillation, heavy, lower-value distillation fractions can be processed further into lighter, higher-value products such as gasoline. This is where fractions from the distillation units are transformed into streams (intermediate components) that eventually become finished products. The most widely used conversion method is called cracking because it uses heat and pressure to crack heavy hydrocarbon molecules into lighter ones. A cracking unit consists of one or more tall, thick-walled, rocket-shaped reactors and a network of furnaces, heat exchangers, and other vessels. Cracking is not the only form of crude oil conversion. Other refinery processes rearrange molecules to add value rather than splitting molecules. Alkylation, for example, makes gasoline components by combining some of the gaseous byproducts of cracking. The process, which essentially is cracking in reverse, takes place in a series of large, horizontal vessels and tall, skinny towers. Reforming uses heat, moderate pressure, and catalysts to turn naphtha, a light, relatively low-value fraction, into high-octane gasoline components.
Treatment- The finishing touches occur during the final treatment. To make gasoline, refinery technicians carefully combine a variety of streams from the processing units. Octane level, vapor pressure ratings, and other special considerations determine the gasoline blend.