Petroleum refining has drastically changed throughout history based off of advances in distillation technologies and demand for specific types of fuels. Today, petroleum refinery processes consist of mainly Catalytic Cracking (introduced in 1937) and Hydrocracking (introduced in 1960). [1] The reason refineries use these processes in tangent with one another is because of their abilities to process different feedstocks, their similar refining objectives, and ultimately their final products.
Out of these two processes used today, Catalytic Cracking is the most popular refining method which produces 35 to 45% of United States naphtha production. The main feedstock that a Fluid Catalytic Cracker (FCC) uses to produce these distillate products are paraffinic atmospheric and vacuum gas oils. [1] The reason that a catalytic cracking process was even invented was because of the low octane gasoline yields and high possibility for coke formation in the distillation columns that occurred in thermal cracking. The catalysts that are used in the cracking processes can be separated into three classes, those being; acid-treated natural aluminosilicates, amorphorous synthetic silica-alumina combinations, and crystalline synthetic catalysts (zeolites). In regards to today’s FCC process, they most commonly use the zeolite class catalyst to break apart the long chained feedstock while utilizing a regenerator, a reactor, and fractionator. All of these units increase the thermal efficiency and allow for the main objectives, which are to increase high octane gasoline yield, to lower coke yield, to increase isobutene production, and to allow for higher conversions without over cracking, to be reached. [1] The final products following the FCC process are Gas, Gasoline, LCO, HCO, and Decant Oil.
The purpose of Hydrocracking is to work along with Catalytic Cracking and allow for all types of hydrocarbons to be refined into light to middle distillates. Hydrocracking uses feed stocks such as aromatic cycle oils and coker distillate (feedstocks that aren’t used in FCC). Before hydrocracking can be performed, all feeds have to be hydro treated in order to remove metallic salts, oxygen, organic nitrogen compounds, and sulfur to prevent catalyst poisoning. [1] Once the feedstock is treated, hydrocracking can be done in a single or two stage process. In a single-stage process, a single catalyst is used to convert the feed into gasoline and lighter products, and in a two-stage process multiple catalysts are used to recycle the reactor bottoms back into the reactor to further refine the heavier hydrocarbons to produce the desired yield of distillates. The ultimate objectives of Hydrocracking it to improve the gasoline boiling-range, to improve gasoline pool octane quality, to produce less coke, and to re-use the heavier by-products from Catalytic Cracking to produce a useable fuel. [1] In the grand scheme of things, the final product in Hydrocracking can be dependent on what fuel is needed. Most of the time hydrocracking produces products similar to those formed from Catalytic Cracking including gasoline, jet fuels, and diesel. [1]
Resources:
1. Gary, J. H., & Handwerk, G. E. (2007). Petroleum refining: technology and economics. New York: M. Dekker.