In petroleum refining there is a strong need to “crack” heavy, long chain alkane feedstocks into lighter, shorter chain alkane feedstocks. This can be done in a variety of ways. As mentioned in earlier lessons, one of these ways is through the process of thermal cracking. Another way this can be achieved is through the process of catalytic cracking, which will be the focus of this blog post.
Compared to thermal cracking, catalytic cracking occurs at lower temperatures and pressures, is more selective and flexible, and incorporates a catalyst. Catalytic cracking processes have evolved over the years, and are an exemplary display of chemical engineering. The most recent catalytic cracking technique was developed in 1942 and is called Fluid Catalytic Cracking. Even more recent is the addition of Catalytic Hydrocracking in refineries, which was developed by Chevron in 1958. These two most recent developments are useful in their own way yet very different in many others.
From a feedstock stand point, both catalytic cracking and catalytic hydrocracking use very different compounds. One of hydrocracking’s main advantages over catalytic cracking is its ability to cope with a much wider range of feedstocks. Hydrocracking processes are able to handle the upgrading of heavier crude oil fractions such as heavy vacuum gas oil and vacuum distillation residue. The heaviest fractions of crude oil, heavy vacuum gas oil and vacuum distillation residue, may not be easily processed by catalytic cracking because of potential problems with coking on the catalysts. For this reason, hydrocracking is able to handle much heavier feedstocks than catalytic cracking.
As for the processes themselves, there are many differences as well. The basis of catalytic cracking is carbon rejection, while hydrocracking is a hydrogen addition process. Catalyst cracking uses an acid catalyst, while hydrocracking uses a metal catalyst on acid support. Another differnce is that catalyst cracking is an endothermic process while hydrocracking is an exothermic process.
There are two main processes associated with hydrocracking, and they are; hydrotreating and hydrocracking. Hydrotreating is for the removal of heteroatoms, while hydrocracking is for the increase of the H/C ratio of the hydrocarbons and to decrease their molecular weight. This is done by hydrogenation and cracking, respectively. Hydrocracking is a very versatile process and can be adjusted according to its wide range of feedstocks.
In catalytic cracking, the process is a little different, and has evolved over time. The first process was the McAfee process which was a batch reaction process that involved a lewis acid to be incorporated in the batch. The next process was the first commercial process called the Houdry process which consisted of a continuous feedstock flow with multiple fixed-bed reactors. The incorporation of the reactor was what allowed the process to be used commercially. The process which followed the Houdry process was the Thermafore Catalytic Cracking process which adopted the use of moving-bed catalysts. Finally the last an most recent process is the afore mentioned Fluidized Catalytic Cracking which uses a fluidized bed catalyst. All of the processes were adapted and modified to increase the thermal efficiency of the process and have been increasing in order of appearance.
The last difference between hydrocracking and catalytic cracking is the products which they produce. The products of catalytic cracking can be described using the acronym PIANO, to represent the Paraffins, Iso-paraffins, Aromatics, Naphthenes, and Olefins produced in catalytic cracking. Catalytic cracking’s most important product is high octane gasoline which is a direct result of the branching alkanes produced in the process. As for hydrocracking, it provides a sizable amount of the diesel fuel production. This is due to straight-run light gas oil being a preferred stock for FCC to produce gasoline as the principal product. Catalytic cracking produces more gas and more coke than hydrocracking, but the liquid yield is higher for hydrocracking. Hydrocracking is more desireable in many areas when compare to catalytic cracking, but cost is not one of them as it is much more expensive to run.
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