Page 77 - The engineering of chemical reactions

Petroleum Refining 61
Energy for lighting has an interesting history as compared to energy for power and
for chemicals. Campfires using solid fuel were replaced by candles and then by lamps that
burned liquid oil from animal fats and then whale oil to produce portable light sources. As
whale oil was depleted (as were the whales), lamps supplied by pipeline distribution systems
were developed for large cities. These were fueled first by “town gas” (a mixture of CO and
Hz), and then these distribution networks were replaced by today’s natural gas lines. Now
these same pipeline distribution systems are used mostly for heating. Finally, the generation
of electricity from coal, hydroelectricity, and nuclear fuel has made the electric light the
only significant source of lighting. Power and electricity generation have become topics in
mechanical engineering rather than in chemical engineering, although all but nuclear power
require combustion and control of resultant pollution, which mechanical engineers are not
equipped to handle (in the opinion of chemical engineers).
It was discovered in the late nineteenth century that coal can be incompletely burned
to yield a gas consisting primarily of CO and Hz, and many people were undoubtedly
asphyxiated and killed by explosions before these processes were harnessed successfully.
We will see later that the use of a CO + H2 mixture (now called synthesis gas) for the
production of chemicals has had an important role in chemical synthesis (it was very
important for explosives and synthetic fuels in both World Wars), and it is now one of the
most promising routes to convert natural gas and coal into liquid diesel fuel and methanol.
We will describe these processes in more detail in later chapters.

Cracking
Initially the petroleum (tar) that oozed from the ground in eastern Pennsylvania could
be burned as easily as whale oil and animal fats, although much of it was too heavy to
bum without processing. Processing was done initially by pyrolyzing the oil in retorts and
extracting the volatile components when the oil molecules cracked into smaller ones, leaving
tar at the bottom. (Many people were killed as this process evolved before they learned about
the volatility and flammability of the various hydrocarbon fractions produced.) Retorts
(batch reactors) were soon replaced by continuous cracking units (stills), in which the crude
oil was passed through heated tubes. One problem in these processes is that some of the
hydrocarbon is cracked down to methane, which cannot be liquefied easily. A much more
serious problem was that some also turns to tar and coke, a black solid mass (mess) that
coats and eventually plugs the tube furnace.
Reactions in hydrocarbon cracking may be represented as
hydrocarbons + smaller hydrocarbons + coke
The hydrocarbons in crude oil are alkanes, olefins, aromatics, polyaromatics, and
organic compounds containing S, N, 0, and heavy metals. Since there are many isomers
of all of these types of molecules, the reactions implied by the preceding equations rapidly
approach infinity. A representative reaction of these might be the cracking of hexadecane
(number 3 heating oil) into octane and octene (components in gasoline),

n--C16&4 -+ n-C&3 + ?d8Hl6
and
n-C16H34 + 16C, + 17Hz