Page 83 - The engineering of chemical reactions

Petroleum Refining 67

cracking. However, now we want primarily to isomerize rather than crack the hydrocarbons
because the naphtha feed has the desired molecular weight. Therefore, in catalytic reforming
the temperature is lower and HZ is added to suppress coke formation, and thus regeneration
needs to be done much less frequently.
Modern catalytic reformers operate as either fixed beds in series, in which reactants
flow through several tanks filled with catalyst, or as moving beds, in which catalyst flows
slowly down a tube as the reactants move up. A major concern is the minimization of the
formation of cracking products such as coke that poison the catalyst. Catalysts inevitably
deactivate by coke formation, and this must be burned off by reacting with air. In fixed-
bed reactors this is done by varying the flow through the tanks such that one reactor is
periodically in “swing” where the coke is burned off. In the moving-bed reactors some of
the catalyst is continuously withdrawn from the bottom and the carbon is oxidized before
it is reinserted in the top of the reactor.
Reactions in catalytic reforming can be represented as ,

n-C~Hi6 + 2,2,3-trimethylbutane
n--C+Hi6 -+ toluene + 3H2
n-C7Hifj + 7c + 8Hz
These can also be written as shown in Figure 2-15.
The first reaction is the isomerization from a zero-octane molecule to an alkane with
100 octane; the second is the dehydrocyclization of heptane to toluene with 120 octane,
while the third is the undesired formation of coke. To reduce the rate of cracking and
coke formation, the reactor is run with a high partial pressure of Hz that promotes the
reverse reactions, especially the coke removal reaction. Modem catalytic reforming reactors
operate at 500 to 550°C in typically a 20: 1 mole excess of Hz at pressures of 20-50 atm.
These reactions are fairly endothermic, and interstage heating between fixed-bed reactors
or periodic withdrawal and heating of feed are used to maintain the desired temperatures
as reaction proceeds. These reactors are sketched in Figure 2-16.

Al kylation

The final refinery reactor on our list joins back together molecules that were cracked too
much in the previous processes. The process can be thought of as adding an alkyl group to
an alkane or aromatic, and the process is therefore called alkylation.

Figure 2-15 Reactions in catalytic reforming of
n-heptane to 2,2,3-trimethylbutane and toluene.