Blog 7
Write a blog post comparing catalytic cracking and catalytic hydrocracking processes with respect to feedstocks, process objectives, and products.
After all of the various physical separations have occurred to a crude oil, such as distillation, deasphalting, and dewaxing, there is a need to now change the composition of the crude oil using chemistry, breaking and creating bonds. The yields of this product after just undergoing the physical changes does not meet the demand required so further chemical separations must be pursued. The earliest discovered method for chemical separation is known as thermal cracking which uses brutal heat, heating the temperatures until the compounds crack and the chemical bonds are broken.
However the thermal cracking processes could not meet the demand for quality. This process delivered gasoline with a low octane number which was only acceptable for automobiles back in the day. Engines now have higher compression ratios and require a higher octane number in gasoline. They need a gasoline that does not ignite spontaneously with pressure when pressurized with air. This lead to the introduction of catalytic cracking.
Most catalytic conversion processes were developed right before and during Second World War for making higher quantities of better fuels with higher octane numbers. In catalytic cracking the reactive species are carbo cations that are produced on catalyst surfaces. Carbo cations go through isomerization reactions very quickly providing the opportunity to create isoparrafins. Almost all gasoline production in the U.S. is done through catalytic means.
There are a few different forms of catalytic cracking such as Houdry catalytic cracking, Thermafor catalytic cracking (TCC), and fluid catalytic cracking (FCC), however they are not all equally efficient. Fluid catalytic cracking is the most popular process and is the heart of the refinery. Catalytic cracking had a very flexible range of feedstocks that can be used from the gas oil boiling range all the way up to light vacuum gas oil. Cracking products after being fractionated can be separated into products such as gas, gasoline, light cycle oil (LCO) and heavy cycle oil (HCO).
For heavier aromatic feedstock materials such as heavy vacuum gas oil or vacuum distillation residue hydrogen must be introduced so that we can convert these heavy fractions without rejecting large quantities of Carbon. This is known as hydro cracking, which has the principal objective of upgrading products by decreasing the molecular weight and boiling point of heavy oils to produce products of saturated hydrocarbons, such as diesel and jet fuel. The hydrocracking process has two dimensions: Hydrogenation of aromatic rings and cracking of aliphatic compounds. Hydrocracking provides high yields of valuable distillates without producing low-grade byproducts such as heavy oils, gas, or coke, as experienced in carbon rejection processes such as coking. This method is less flexible with its feedstock range and also more costly than catalytic cracking.