(1) Nondestructive hydrogenation processes are extensively used in the United States. The product
of nondestructive hydrogenation has a boiling range little changed from the boiling range of the charge stock.
The important changes are:
(a) Napthenes are converted to aromatic hydrocarbons which are highly desirable for high
performance gasolines.
(b) Olefins are converted to paraffins which are less susceptible to oxidation, reducing the danger
of gum formation in the product.
(c) Sulfur has been reduced.
(2) Destructive hydrogenation can be described as hydrogenation accompanied by cracking. As
cracking occurs, hydrogen adds itself to the chains where the rupture occurs. The product of destructive
hydrogenation may be lighter than the material charged to the process.
g. Dehydrogenation. Dehydrogenation is the reverse of hydrogenation. In this process, hydrogen is
removed form adjacent carbon atoms of an organic compound with resultant formation of a double bond.
Commercial processes now in operation are both catalytic and thermal, and are used principally to dehydrogenate
saturated gaseous hydrocarbons to produce starting material for the alkylation process.
h. Isomerization. Isomerization processes have been developed by the petroleum industry to convert
straight-chain hydrocarbons to the valuable branched-chain hydrocarbons which increase the antiknock properties
of gasolines. This conversion is brought about in the presence of a catalyst, usually a moderate temperatures and
pressures. In petroleum refining, the isomerization process is applied principally to butane and pentane. The
object of isomerizing butane is to obtain isobutane for alkylation and other uses; that of pentane isomerization, to
obtain isopentane for blending into gasoline.
i. Reforming. Reforming is a cracking process employed for the upgrading of stocks, with low octane
number. It may be either thermal or catalytic and may be mild or severe, depending on the desired end.
(1) Mild reforming is applied to stocks with a boiling range identical or very similar to gasoline. The
application of high temperature converts some paraffins to olefins, changes some straight-chain molecules to
branch-chain molecules, breaks alkane sidechains from alkan-naphthenes to form aromatic hydrocarbons, and
may isomeize some compounds.
(2) Severe reforming is often applied to heavy gasolines with a boiling range of 200-400ƒF. Enough
of the heavier material is cracked to give a finished product with a normal gasoline boiling range. The reactions
are the same as in mild reforming except that there is more cracking.
(3) In both thermal reforming and catalytic reforming, the reactions described above take place. In
catalytic reforming, the order of the reactions is changed since the catalyst promotes certain reactions and does not
affect others so readily. Thus, the dehydrogenation of naphthenes is much more rapid in catalytic reforming than
it is in thermal. Also, hydrogen may be recycled in catalytic reforming to bring about a higher degree of sulfur
reduction and to convert olefins to paraffins. Catalytically reformed gasoline is thus usually much more saturated
(with hydrogen) than is thermally reformed gasoline. Thermal reforming has been largely displaced by the
catalytic process.
20-7
QM5200
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