Page 152 - Fluid Catalytic Cracking Handbook

Chemistry of FCC Reactions 127

Thermal cracking is a function of temperature and time. The reaction
occurs when hydrocarbons in the absence of a catalyst are exposed to high
temperatures in the range of 800°F to 1,200°F (425°C to 650°C).
The initial step in the chemistry of thermal cracking is the formation
of free radicals. They are formed upon splitting the C-C bond. A free
radical is an uncharged molecule with an unpaired electron. The
rupturing produces two uncharged species that share a pair of elec-
trons. Equation 4-1 shows formation of a free radical when a paraffin
molecule is thermally cracked.

R 2 V "D f* I */"* t>
IV,, 7 IV V- T V- ' .IV
| ' (4-0
H H H H

Free radicals are extremely reactive and short-lived. They can
undergo alpha scission, beta scission, and polymerization. (Alpha-
scission is a break one carbon away from the free radical; beta-
scission, two carbons away.)
Beta-scission produces an olefin (ethylene) and a primary free
radical (Equation 4-2), which has two fewer carbon atoms [1]:

i
1
J\ """"' V.'..il.^ —~ VvlT'} "•"*— V-, — Lisy ""' " °~~~ Vx S~M.-y T IT'iV--- ~™ V_-ilo \T" "jiii. )
The newly formed primary free radical can further undergo beta-
scission to yield more ethylene.
Alpha-scission is not favored thermodynamicaily but does occur.
Alpha-scission produces a methyl radical, which can extract a
hydrogen atom from a neutral hydrocarbon molecule. The hydrogen
extraction produces methane and a secondary or tertiary free radical
(Equation 4-3).

(4-3)
-» CH 4 + R-CH2-CH 2-CH 2-CH 2-'CH-CH 2-CH 3

This radical can undergo beta-scission. The products will be an
alpha-olefin and a primary free radical (Equation 4-4).