Page 266 - Modeling of Chemical Kinetics and Reactor Design

236 Modeling of Chemical Kinetics and Reactor Design

DCC uses heavy VGO as feedstock and has the same features has the
FCC but with the following differences: special catalyst, high catalyst-
to-oil ratio, higher steam injection rate, operating temperature, residence
time, and lower operating pressure.
In the DCC unit, the hydrocarbon feed is dispersed with steam and
cracked using a hot solid catalyst in a riser, and enters a fluidized bed
reactor. A known injection system is employed to achieve the desired
temperature and catalyst-to-oil contacting. This maximizes the selective
catalytic reactions. The vaporized oil and catalyst flow up the riser to
the reactor where the reaction conditions can be varied to complete
the cracking process. The cyclones that are located in the top of the
reactor effect the separation of the catalyst and the hydrocarbon vapor
products. The steam and reaction products are discharged from the
reactor vapor line and enter the main fractionator where further
processing ensure the separation of the stream into valuable products.
The formed coke on the catalyst particles during cracking reduces
its activity and selectivity. The spent catalyst passes into a stripping
zone where steam is used to displace the entrained and adsorbed
hydrocarbons, which leave the reactor with the products. Stripped
catalyst particles are transported into the regenerator where the particles
are contacted with air under controlled conditions. The regeneration
process is the same as in the FCC unit. The DCC has been shown to
produce polymer grade propylene from heavy gas oils, and it produces
three and a half times more propylene and less than half the gasoline
than a conventional FCC unit. Figure 4-11 illustrates a DCC unit and
Figure 4-12 represents a typical DDC plant that produces propylene,
which is integrated to a petrochemical complex. Table 4-1 compares
the operating variables of the deep catalytic cracking (DCC), fluidized
catalytic cracking (FCC) and steam cracking (SC) units.
Another classification involves the number of phases in the reaction
system. This classification influences the number and importance of
mass and energy transfer processes in the design. Consider a stirred
mixture of two liquid reactants A and B, and a catalyst consisting of
small particles of a solid added to increase the reaction rate. A mass
transfer resistance occurs between the bulk liquid and the surface of
the catalyst particles. This is because the small particles tend to move
with the liquid. Consequently, there is a layer of stagnant fluid that
surrounds each particle. This results in reactants A and B transferring
through this layer by diffusion in order to reach the catalyst surface.
The diffusion resistance gives a difference in concentration between