Page 81 - The engineering of chemical reactions

Petroleum Refining 65

TABLE 2-5
Operating Conditions of the FCC Reactor
Reactor Regenerator
feed gas oil air
products alkanes, olefins, H2 co2
temperature 550°C 650°C
pressure 2 atm 2 atm
gas residence time tl set several set
catalyst residence time tl set several set
flow pattern - PFTR (riser) CSTR
heat absorbed generated

catalysts, but fortunately the lighter naphtha and gas oil feeds to these reactors contain
negligible S, N, and metals.
The other problem with this petroleum fraction is that it is deficient in hydrogen. We
need C:H = 7:16 for 2,2,3-trimethylbutane (TMB), but this ratio is greater than 1:l for
many heavy feeds. Therefore, we need to add hydrogen in the refining process, and we
could describe them generically as the reaction

and the need to add Hz to obtain usable products gives the prefix “hydra” to process names
such as hydroprocessing, hydrotreating, hydrocracking, hydrorefining, hydrodesulfuriza-
tion, etc.
These cracking and H-addition processes also require catalysts, and a major engi-
neering achievement of the 1970s was the development of hydroprocessing catalysts, in
particular “cobalt molybdate” on alumina catalysts. The active catalysts are metal sulfides,
which are resistant to sulfur poisoning. One of the major tasks was the design of porous
pellet catalysts with wide pore structures that are not rapidly poisoned by heavy metals.
Modern processes operate with fixed beds with a 20: 1 to 50: 1 H2:HC ratio at pressures
typically 50 atm at 550°C. Since the reactants must be operated below their boiling points,
the catalyst is a solid, and the Hz is a gas, all these reactors involve three phases in which
the catalyst is stationary, the gas moves upward through the reactor, and the liquid fraction
flows down the reactor. This reactor type is called a trickle bed, and it is a very important
chemical engineering unit in refining heavy crude feedstocks and turning them into the
molecular weight suitable for gasoline. The flow of gas and liquid through a trickle bed
reactor is shown in Figure 2-14.

Catalytic reforming

The catalytic cracking and hydroprocessing reactors produce a large fraction of hydrocar-
bons with a molecular weight suitable for gasoline, C5 to Cs. However, these products have
a low octane number. In the spark ignition engine some isomers ignite appropriately, while
some tend to ignite before the piston has reached the top of the cylinder, and this causes the
engine to knock. We will discuss hydrocarbon combustion reactions in Chapter 10 because
they are free-radical chain reactions. Highly branched alkanes and aromatics are superior in
this to linear alkanes, and hydrocarbons are compared by their octane number. The molecule