Page 82 - The engineering of chemical reactions

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

+ products Figure 2-14 Sketch of a trickle bed reactor used for
\”
heavy Iaasl I hydroprocessing of the residual oil fraction of crude oil
oil into tde boilingyange used for gasoline and diesel fuel.
(liquid)

2,2,3-trimethyl butane is arbitrarily given an octane number of 100, and n-heptane is given
an octane number of 0. [For unknown reasons, the molecule 2,2,3-trimethyl butane is called
“isooctane” even though it is actually a heptane.]
An arbitrary mixture of hydrocarbons is compared to a mixture of these two com-
pounds, with its octane number that equal to the appropriate mixture of these standard
compounds. Some molecules and their octane ratings are indicated in Table 2-6. Aromatics
have a high octane number (toluene is 120), and some compounds such as tetraeihyl
lead have a strong octane enhancement when added to other mixtures (blending octane
number). Oxygenates such as ethanol and ethers (MTBE) have fairly high octane numbers
and supposedly produce less pollution, either alone or blended with hydrocarbons.
The products from the catalytic cracker and from the hydroprocessor contain too
many linear isomers and cyclic aliphatics, and the isomerization of these linear alkanes to
branched alkanes and dehydrogenation of cyclics to aromatics would enhance the octane
enormously. In the 1950s it was found that a catalyst consisting of Pt on y - A1203 would
almost miraculously achieve very large octane enhancements without cracking products
undesirably. This process is called catalytic reforming, because it “reforms” the skeleton of
molecules without cracking C-C bonds into smaller molecules. Reforming also produces
high yields of aromatics that have high octane and are needed as petrochemical feedstocks.
This process is similar to catalytic cracking in some ways. Because of the higher
pressure required, it uses fixed beds rather than fluidized beds that are now used for catalytic

TABLE 2-6
Octane Ratings of Hydrocarbons

Molecule Research octane number (RON)
n-heptane 0 (defined)
2,2,3-trimethylbutane 100 (defined)
toluene 120
benzene 110
2.methylhexane 42
ethanol 106
methanol 118
methyl-t-butyl ether (MTBE) 135