Riser catalytic crackers

Coupled Simulation of a Fluidized Bed (or Riser) Catalytic Cracker and Regenerator... 

Figure 7.7b shows the essential features of a refinery catalytic cracker. Large molar mass hydrocarbon molecules are made to crack into smaller hydrocarbon molecules in the presence of a solid catalyst. The liquid hydrocarbon feed is atomized as it enters the catalytic cracking reactor and is mixed with the catalyst particles being carried by a flow of steam or light hydrocarbon gas. The mixture is carried up the riser and the reaction is essentially complete at the top of the riser. However, the reaction is accompanied by the deposition of carbon (coke) on the surface of the catalyst. The catalyst is separated from the gaseous products at the top of the reactor. The gaseous products leave the reactor... 

In a typical fluid catalytic cracker, catalyst particles are continuously circulated from one portion of the operation to another. Figure 9 shows a schematic flow diagram of a typical unit W. Hot gas oil feed (500 -700°F) is mixed with 1250 F catalyst at the base of the riser in which the oil and catalyst residence times (from a few seconds to 1 min.) and the ratio of catalyst to the amount of oil is controlled to obtain the desired level of conversion for the product slate demand. The products are then removed from the separator while the catalyst drops back into the stripper. In the stripper adsorbed liquid hydrocarbons are steam stripped from the catalyst particles before the catalyst particles are transferred to the regenerator. 

A number of different types of laboratory scale units have been developed to simulate commercial catalytic crackers. These include fixed bed (MAT), fluidized bed, and riser units.(1,2,3) In particular, for simulating commercial riser FCC units which process residue, a riser pilot plant is the preferred choice. 

Bed Preparation. To begin the process, solid particles are placed or injected onto a bed in a holding unit. This is called a packed or a fixed bed, until fluid is applied. The bed is typically in a reactor, boiler furnace, or another part of a processing unit, such as the catalytic riser of a refinery s fluid catalytic cracker. There are different kinds of beds, the most common being stationary or bubbling beds or more complex circulating beds. 

The great technical advantage of the fluid catalytic cracker is that the long hydrocarbon chains, typically light fuel (or gas) oil, come into contact with the solid catalyst particles in a fluidized, rather than a stationary, hed environment. That means that the total surface area between catalyst and oil particles is much larger than possible if the catalyst were just fixed to the floor or sides of the reactor. In the cracker s catalytic riser, tiny catalyst particles are completely surrounded by the oil particles and swim in a fluid stream together for a few crucial fractions of a second. 

Modern technology is considerably different, particularly since the introduction, in the early sixties, of synthetic crystalline zeolite catalysts. These were so active that the cracking mainly or entirely took place in the riser, so that the reaction vessel caused overcracking into undesired light gases and coke. A recent version of a catalytic cracker is shown in Fig. 13.2.2-1. The catalyst is completely entrained in the riser-reactor, to reduce the contact time. The former reactor vessel is now essentially reduced to a vessel containing cyclones and a stripping section. 

The understanding of fluidized-beds is far from satisfactory, particularly regarding the fluid mechanics. The value of various models for fluidized-beds, however, lies in providing the framework within which each specific application can be considered. This chapter examines fluidization characteristics, the role that gas bubbles play, and the rationale behind the two-phase theory, which naturally leads to the models based on the two-phase theory. A full section will be devoted to the catalytic cracker, in particular the riser cracker, since it represents the most important application of fluidized-beds to catalytic reactions. This chapter starts with the understanding that the intrinsic rate is essentially the same as the global rate in fluidized-beds, unless the catalyst is deactivated. 

SO that the reaction vessel caused over-cracking into undesired light gases and coke. In recent versions of the catalytic cracker, the catalyst is completely entrained in the riser-reactor to reduce the contact time. 

For a given catalyst and feedstock, catalytic coke yield is a direct function of conversion. However, an optimum riser temperature will minimize coke yield. For a typical cat cracker, this temperature is... 

It should be noted that, at this point, it is unlikely that pillared clays will replace zeolites for fluidized catalytic cracking, the reason being the hydrothermal instability of the clays at the conditions typically used in a modem riser cracker. Nevertheless, there is ongoing interest in clays and pillared clays as shape-selective catalysts for other, more specific reactions or separations. 

It takes less than one second for a catalyst to crack a long hydrocarbon molecule chain in the catalytic riser of a fluid catalyst cracker at the heart of a modern refinery. 

As a last example I want to discuss the catalytic cracking of oil in a riser cracker. Weekman (49. 50) proposed a simple scheme ... 

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