Page 85 - The engineering of chemical reactions

Polyester from Refinery Products and Natural Gas 69

important polymer for fabrics that uses ethylene in its synthesis. In this section we will
describe the processes by which components of natural gas and crude oil are turned into
these products. Natural gas sells for about $O.lO/lb, while clothing sells for about $20/lb,
and the job of the chemical engineer is to turn these reactants into products as efficiently
and cheaply as possible. [What is the price per pound of gasoline if one gallon weighs
8 pounds?]
However, in contrast to fuels, petrochemicals intermediates must be produced at
extremely high purities. For example, CO at ppm levels will poison polyethylene catalysts,
and acetylene in ethylene at this level will produce a crosslinked polymer that will have
unsatisfactory properties. Therefore, the chemical engineer must produce these intermedi-
ates with extremely high purities, and this requires both careful attention to minor reactor
products and to efficient separation of them from the desired product. These factors are also
important in the economics of petrochemicals.
A flow sheet of the steps in forming polyester is as follows:

ethane + ethylene + ethylene oxide + ethylene glycol +
polyethylene terephthalate
crude + naphtha --+ xylenes + p-xylene -+ terephthalic acid +
In stick notation we write these reactions as shown in Figure 2-17. We described the steps
by which crude oil is converted into aromatics such as p-xylene previously. Here we discuss
the production of ethylene glycol from ethane.
Natural gas is primarily methane, but it contains 5-10% of natural gas liquids,
primarily ethane, propane, and butane. These are available at essentially the cost of natural
gas heating except for the cost of condensing these less volatile components from methane.
Natural gas liquids are a major source of chemicals, along with petroleum, and we will
describe a number of similar examples in this book.

Ethylene from ethane
Ethylene is made primarily by steam cracking of ethane and higher alkanes. These alkanes
undergo dehydrogenation and cracking reactions. A higher-molecular-weight fraction of
alkanes, C4 to CT, is called naphtha, which in steam cracking reacts as

FigureZ17 Reaction steps topreparepoly- O H
ethylene terephthalate from ethane and ---WE-P\/
naphtha. C2H6 0 O H
ethylene
oxide ethylene \
glycol 0 0
-O-C,H,-0-Go:-
[ 1 n
CooH /’ polyethylene terephthalate
$4+QOH
octane p-xylene terephthalic
acid