Page 87 - The engineering of chemical reactions

Polyester from Refinery Products and Natural Gas 71
Polyethylene

Ethylene is used to produce many products, the largest volume of which is polyethylene (PE)

nC2H4 --+ KH2)2n
which is produced by free-radical homogeneous reactions (low-density PE) and also by a
titanium catalyst (high-density PE). We will consider these processes further in Chapter 11.

Ethanol _

Ethylene also readily reacts with Hz0 to make ethanol,
C2H4 + H20 -+ C2H50H
in liquid acids such as sulfuric acid at temperatures up to 2OO’C. The water molecule can
be thought of as adding across the double bond.
This is the most economical process to produce ethanol, but laws prohibit drinking
“synthetic” ethanol; so beverages are made much more expensively by fermentation of
sugar or carbohydrates. Recent laws also mandate the addition of 10% ethanol in gasoline in
cities during the winter, supposedly to reduce pollution. However, grain processors lobbied
to require “renewable resources”; so fermentation is required to produce this fuel alcohol.

Ethylene glycol

Ethylene glycol (EG) has two OH groups; so it will polymerize as a linear polymer in
polyesters, polyurethanes, or polyethers. Ethylene glycol is also water soluble and has a
low melting point; so it is used in antifreeze.
Ethylene glycol was formerly made by adding water to acetylene,

C2H2 + 2H20 + HOC2H40H
which can be thought of as adding two water molecules across the triple bond in acetylene.
However, acetylene is expensive to produce, and EG is now made from ethylene in a
two-stage process using an ethylene oxide intermediate.

Ethylene oxide

Ethylene oxide was formerly made in a two-stage process by first adding HOC1 to ethylene
and then removing HCl. However, in the 1960s Scientific Design, Union Carbide, and
Shell Oil developed a one-step direct oxidation process that has largely replaced the old
chlorohydrin process.
To make ethylene glycol, ethylene is first oxidized to ethylene oxide (EO)
C2H4 + iO2 -+ C2H40

which is then reacted with water in acid to form EG,
C2H40 + H20 -+ HOC2H40H