In order to produce larger amount and high octane contain gasoline, catalytic cracking was developed during World War II. As moving alone with the time, the improvements of this process increase the thermal efficiency of the process. Fluid Catalytic Process (FCC) which was introduced in 1942 has the highest thermal efficiency among the catalytic cracking processes. Catalytic hydrocracking has shorter history than catalytic cracking and it was started in 1958. It is also known as a hydrogen addition process, but Fluid Catalytic Process is known as carbon rejection process. These two processes also have differences in feedstock, process objectives and their products.
For catalytic cracking, it used acid catalytic. Straight run atmospheric gas oil (AGO) and light vacuum gas oil (LVGO) are the typical feedstock for catalytic cracking. Compare to catalytic cracking, hydrocracking use metal catalytic on acid support and has a wider range of feedstock. It can process more aromatic feedstock which resists cracking such as light cycle oil (LCO). It can also process heavy vacuum residue under the extreme condition such as high hydrogen pressures. Those extreme conditions prevent the process from shut down which due to extensive coking on catalyst.
As we learned from lesson 7, the process objectives of two processes are also different. For hydrocracking, its main objective is to decrease both molecular weight and boiling point of heavy oils and produces saturated hydrocarbon from highly aromatic feedstock such as light cycle oil. Since the product of hydrocracking has low sulfur and nitrogen content, it also contributes to limit the sulfur emission and aromatic hydrocarbon in motor fuels. The main objectives of catalytic cracking are to increase the yield of gasoline and number of octane in the gasoline. At the same time, it also lower the yield of coke and achieve higher conversion, but prevent the over cracking.
From the products, we could also see the differences between these two processes. The products of catalytic cracking include light gas, gasoline with high octane component, light cycle oil, heavy cycle oil, slurry oil, and coke by-product. As we talked about the objective of catalytic cracking previously, the gasoline with high octane component is the main product of catalytic cracking. For hydrocracking, diesel, jet fuel, and gasoline with extreme low sulfur component are the main products. The process uses metal catalyst and hydrotreating to remove heteroatoms such as sulfur and nitrogen. The products of hydrocracking contain low sulfur and nitrogen component which is more environmental friendly. At the same time, hydrocracking produces high yield of valuable distillates without some undesirable byproduct such as heavy oils, gas and coke.
References:
1. Class website lesson 7
https://www.e-education.psu.edu/fsc432/content/lesson-7-catalytic-conversion processes-part-1