Fluidized bed catalytic cracker

Solids of group A have small particle diameters (% 0.1 mm) or low bulk densities this class includes catalysts used, for example, in the fluidized-bed catalytic cracker. As the gas velocity u increases beyond the minimum fluidization point, the bed of such a solid first expands uniformly until bubble formation sets in at u = //mb > mr. The bubbles grow by coalescence but break up again after passing a certain size. At a considerable height above the gas distributor grid, a dynamic equilibrium is formed between bubble growth... 

A part of the bottom section of a fluidized bed catalytic cracker was simulated in 2D using the Eulerian-Eulerian gas-solid flow model presented in Section 13.7 and the 10-lump kinetic model of Jacob et al. [1976] introduced in Section 13.8.1. The simulations aimed at illustrating to what extent a model of this type is capable of describing the details and dynamics of the complex gas-solid flow and species fields. 

It is of interest to note that the catalytic activity (1 — y) decays exponentially with time according to Eq. 5.77 if the bulk-fluid concentrations are constant. In well-mixed fluidized-beds, concentrations can be considered to be uniform everywhere, approaching CSTR behavior. This exponential decay of catalytic activity has been found (Weekman 1970) satisfactory for the simulation of a fluidized-bed catalytic cracker. 

Figure 2.8 A fluidized-bed reactor allows the catalyst to be continuously withdrawn and regenerated as with the refinery catalytic cracker.

The Snamprogetti fluidized-bed process uses a chromium catalyst in equipment that is similar to a refinery catalytic cracker (1960s cat cracker technology). The dehydrogenation reaction takes place in one vessel with active catalyst deactivated catalyst flows to a second vessel, which is used for regeneration. This process has been commercialized in Russia for over 25 years in the production of butenes, isobutylene, and isopentenes. 

At very high gas velocities the particles are carried out of the top of the bed. This is known as fast fluidization and is a type of pneumatic conveying. Fast fluidization has been used in catalytic crackers in order to circulate the catalyst particles the gas velocity is also high enough to break down any agglomerates of solids thus improving performance. 

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. 

The second reaction vessel in a catalytic cracker is called the regenerator. The solid catalyst from the reactor is combined with a compressed air stream from an air blower, and the solid and gas phases flow upward into a bed of fluidized solid catalyst. The early designs used a bubbling bed reactor in which the velocity in the bed is slightly above the minimum fluidization velocity. More recent designs use a transport fluidized-bed reactor. A typical air-to-oil weight ratio is 0.54. 

Fluidized beds are used for both catalytic and noncatalytic reactions. In the catalytic category, there are fluidized catalytic crackers of petroleum, acrylonitrile production from propylene and ammonia, and the chlorination of olefins to alkyl chlorides. Noncatalytic reactions include fluidized combustion of coal and calcination of lime. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

Other catalytic reactions carried out in fluidized-bed reactors are the oxidation of naphthalene to phthalic anhydride [2, 6, 80] the ammoxidation of isobutane to mcthacrylonitrilc [2] the synthesis of maleic anhydride from the naphtha cracker C4 fraction (Mitsubishi process [81, 82]) or from n-butane (ALMA process [83], [84]) the reaction of acetylene with acetic acid to vinyl acetate [2] the oxychlorination of ethylene to 1,2-di-chloroethane [2, 6, 85, 86] the chlorination of methane [2], the reaction of phenol with methanol to cresol and 2,6-xylenol [2, 87] the reaction of methanol to gasoline... 

Gas-solid Fluidized beds Trash incinerators Catalytic crackers Coal gasification Reformers Ore processing... 

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