Page 86 - The engineering of chemical reactions

70 Reaction Rates, the Batch Reactor, and the Real World

naphtha -+ olefins + H2 + smaller alkanes
When ethane is used as a feedstock, the major reaction is simple dehydrogenation,
Cd6 -+ C2H4 + Hz
In modern steam cracking processes using ethane as a feedstock approximately 80%
ethylene selectivity is attained at approximately 40% ethane conversion. This reaction is
highly endothermic, and more than 25 kcal/mole of ethane converted must be supplied in
the reactor.
Equilibrium in the reactions shown is unfavorable except at high temperatures, and
modem plants operate at temperatures of 850-900°C. At these temperatures reaction takes
place homogeneously, but it is far from elementary and proceeds by a chain reaction
involving many free-radical intermediates, which we will consider in Chapter 10. Reaction
is roughly first order in ethane,

In-free radical reactions many products are formed in small quantities such as propylene,
methane, butadiene, acetylene, and benzene. A major cost of an olefin plant is the separation
of these byproducts from ethylene and also separation of unreacted ethane, which is recycled
and fed back to the reactor.
Another major cost in producing ethylene results from the formation of carbon on the
walls of the reactor. This occurs through reactions such as

CzH4 + 2C, + 2H2
although ethane and all hydrocarbons can also decompose to carbon. Carbon formation
from pure alkane feed would plug the reactor very quickly, and in olefin plants a large
excess of water is added to suppress the cracking of large hydrocarbons into smaller ones.
The process is called steam cracking because water is added to suppress coke formation.
Water does not react directly with the hydrocarbon, but it reacts with carbon in the reaction
C, +H20 + CO+H2
a process called steam gasification. The addition of steam reduces the rate of coke formation
such that the tubes of the reactor only slowly develop significant pressure drops because
of carbon buildup. In an olefin plant the process must be shut down approximately once a
month for a day or so while air is blown through the tubes to oxidize the carbon,
c, + 02 -+ co2
a very exothermic gas-solid reaction.
Steam-cracking reactors typically consist of several steel tubes, perhaps 100 m long
and 4 in. in diameter in a tube furnace with reactants and steam fed through the several
tubes in parallel. The ceramic lined furnace is heated by burning natural gas at the walls
to heat the tubes to 900°C by radiation. The reactor is fed by ethane and steam in a ratio
of 1:l to I:3 at just above atmospheric pressure. The residence time in a typical reactor is
approximately 1 set, and each tube produces approximately 100 tons/day of ethylene. We
will return to olefins and steam cracking in Chapter 4.