Catalytic cracking process

#In the commercial cracking of gas oils to produce hydrocarbons boiling in the motor fuel range, there are a wide variety of reactions which take place, some of which are desirable, and some of which are undesirable. It is believed that the refiner today strikes a compromise between the desirable reactions and the undesirable reactions which are taking place, and the basis of such compromise varies from situation to situation and from refiner to refiner. It is simple to state that a refiner desires to make the maximum amount of desirable products, and yet to accomplish this in an economically favorable manner. However, when the above simple statement is buttressed against economic reality, compromises must be made. Thus, for example, a process which yields the maximum amount of gasoline from a barrel of crude oil is not necessarily the most economically attractive one at any given location, for the simple reason that such a process might require a catalyst which must be subjected to more frequent regenerations, or said process might simply bear too heavy an economic penalty as compared to a process which produces slightly less gasoline from a given barrel of crude, but at cheaper cost. Thus, the typical refiner of today is constantly compromising between the most effective manner of carrying out a desired reaction and the practicality of such desired reaction based on such ever-changing factors as the nature and type of the crude being used, the particular market value of the various products which can be obtained from the crude, the cost and nature of the catalyst which is being used, the availability and constraints caused by existing equipment, and quite recently, by restraints imposed by the Environmental Protection Agency with respect to certain emission standards. It also appears quite obvious that these various compromises will become even more troublesome in the future due to the dependency upon foreign crude and the fact that the most desirable feed for a particular operation may not be available or may be available at too high a cost, such that careful balances must be struck between obtaining the maximum amount of useful products from a barrel of crude so as to minimize our dependency upon foreign crude and accomplishing the same as to optimize the economic aspects of the operation and comply with the various regulations with respect to pollution primarily with respect to the nature and extent of contaminants which are released to the atmosphere.

#In general, commercial operations with regard to the cracking of gas oils can be placed into two broad categories which are commonly referred to in the art as "cracking" and "hydrocracking". Both these processes have been well defined in the art together with the various reasons why one is preferable over the other in a given situation. It is not the purpose of this invention to discuss the relative merits of cracking and hydrocracking. However, it is important to note that hydrocracking requires the presence of a hydrogenation/dehydrogenation component in a mixture with a cracking catalyst in order for it to function, whereas catalytic cracking does not. In fact, it is well known in the art that the presence of a hydrogenation/dehydrogenation component in a cracking process is detrimental to said process. Thus one of the major advantages of hydrocracking is that because of the presence of a hydrogenation/dehydrogenation component a very small amount of coke is deposited on the catalyst such that the catalyst remains active for prolonged periods of time and does not require regeneration except as long intervals. On the other hand, catalytic cracking inherently results in the deposition of significant coke on a catalyst thereby resulting in a decline of activity of said catalyst which must be compensated for by frequent regeneration of the same by burning off the coke at elevated temperatures in a regenerator. It is also well known in the art that if a hydrogenation/dehydrogenation component is present in a cracking operation, then undesirable increases in coke and hydrogen will result. Thus, the very material or substance which when employed in a hydrocracking operation effectively controls and minimizes the amount of coke which is formed (i.e., the H/D component) accomplishes the reverse result in a cracking reaction. At first blush, this would appear to be contradictory until it is remembered that conditions employed in a hydrocracker are drastically different than the conditions which are employed in a commercial cracking unit. The reason why a hydrogenation component is desirable and necessary in a hydrocracking operation is that such is carried out at conditions wherein the thermodynamics of the system favor desirable reactions. Thus, by way of considerable over-simplification, hydrocracking is carried out at conditions such that the hydrogenation component is made to work to produce desirable reactions. On the other hand, a commercial catalytic cracker involves conditions such that if a hydrogenation component were present the action of the hydrogenation component favors the undesirable reactions, which results in the production of increased coke and increased hydrogen.

#If one were to merely consider a catalytic cracking operation from the point of view of theory only, it can well be stated that the presence of a hydrogenation component would be completely undesirable and something to be avoided since the economic penalty posed by increased coke and increased hydrogen production is too high a premium to pay for any conceivable advantage which might be gained by changing the product distribution.

#FIG. 1. Catalyst Section of an FCC Unit

#FIG. 2. Section Through Regenerator

#FIG. 3. A Control Scheme, FCC Unit

#A kinetic predictive model of an FCC process, with limited constraint or with no contraint on the CO.sub.2 /CO ratio, is employed to generate optimal operating conditions for a particular feedstock in said process. A computation is made for the control of catalyst inventory in said process so as to provide oxidation activity at the level required to produce the required CO.sub.2 /CO ratio, and the process is operated at or near optimum conditions in accordance with the computed values.

#This invention contemplates the use of a correlative model in place of a kinetic predictive model, although this is believed to be less desirable than a kinetic predictive model.

#This invention also provides a method for control of an FCC process which utilizes a CO oxidation catalyst, wherein data on feedstock and catalyst activity are fed to an executive optimizer which in turn provides signal means to control means whereby the process is optimally operated.