Page 88 - The engineering of chemical reactions

7 2 Reaction Rates, the Batch Reactor, and the Real World

Ethylene can be oxidized to EO over a silver-on-alumina catalyst in l-in-diameter
tubes approximately 20 ft long. A modem EO plant produces 200 tons/day, with a typical
reactor consisting of 1000 tubes with an EO selectivity of 80% with a 4:l C2H4:02 ratio
at approximately 50% conversion of Oz. EO formation is mildly exothermic, while the
competing complete combustion reaction
C2H4 + 302 -+ 2C02 + 2H20
is extremely exothermic. The selectivity decreases rapidly if the temperature increases above
250°C, and heat release will then increase strongly (and catastrophically). Therefore, it is
essential to thermostat the tubes in a configuration that looks like a large heat exchanger in
which boiling organic hydrocarbon on the shell side circulates to a heat exchanger, where it
condenses to carry the heat generated on the shell side to a steam generator. An EO reactor
configuration is shown in Figure 2-18.
We will consider ethylene oxide reactors in more detail in later chapters, because the
cooling of these reactors is a major design consideration.

Polyethylene terephthalate

The most important polyester is polyethylene terephthalate (PET), which has many uses,
from fabrics to milk bottles. The polymerization reaction between ethylene glycol (EG) and
terephthalic acid (TPA),
EG + TPA -+ PET + H20
is carried out in a multistage reactor.
The bifunctional acid is terephthalic acid (TPA), which is made by oxidizing para-
xylene in the reaction
CH3C6H4CH3 + 302 -+ HOOCC&COOH + 2H20
This is another partial oxidation process that is usually carried out in several reactors in
series with both air and HNOs as oxidants. The student might guess that this reaction is
far from the only one, and much CO, COZ, alcohols, aldehydes, and other acids are also
produced that must be separated from the TPA before it is reacted to form PET.
para-Xylene is a product of petroleum refining, particularly in the catalytic reformer
that produces ortho, metu, andparu isomers in nearly equal amounts. Only theparu isomer
is useful for other than fuel, and the mixture of xylenes is isomerized into primarily pm-u
xylene in a reactor filled with a zeolite, called ZSMS.

H2O - - HOC,H,OH
C2H6

Figure 2-18 Flow sheet of process to produce ethylene glycol from e&me.