Page 265 - Modeling of Chemical Kinetics and Reactor Design

Industrial and Laboratory Reactors 235

the collision of catalyst particles with each other and the reactor
walls. These fines can escape capture in the cyclones because the
removal efficiency of cyclones for particles of uniform density de-
creases with decreasing particle size. The catalyst fines captured in
the reactor cyclones are transferred to the catalyst regenerator. Here,
the fines are carried with the exhausted air and combustion products
known as fuel gas. Most of the fine catalyst particles that are entrained
in the flue gas are first captured in a two-stage cyclone within the
regenerator vessel and then returned to the catalyst bed. Coker [1]
illustrated the design of a cyclone separator, which is an economical
device for removing particulate solids from a fluid system. Cyclone
separators have been successfully employed in catalytic cracker oper-
ations. Designing cyclones often requires a balance between the
desired collection efficiency, pressure drop, space limitations, and
installation cost.
The advantages of the fluidized bed are:

• Savings in operating expenses due to heat recovery in the reaction-
regeneration steps.
• Rapid mixing of reactants-solids and high heat transfer rates.
• Easy to control both the heat transfer and the fluid flow system.

The disadvantages are:

• Backmixing due to particle distribution in dense and dilute phases.
• Inefficient contacting due to solids movement and the bypassing
of solids by bubbles.
• Possible channeling, slugging, and attrition of catalyst.
• Possible agglomeration and sintering of fine particles in the dilute
phase under certain conditions (e.g., high temperature).

The advantages of the ease of catalyst replacement or regen-
eration are offset by the high cost of the reactor and catalyst regenera-
tion equipment.

DEEP CATALYTIC CRACKING UNIT

An improved FCC unit is the deep catalytic cracking unit (DCC)
that is designed to produce gasoline from vacuum gas oil (VGO). The