Page 43 - Chemical and process design handbook

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HYDROGENATION

In its simplest interpretation, hydrogenation is the addition of hydrogen to
a chemical compound. Generally, the process involves elevated tempera-
ture and relatively high pressure in the presence of a catalyst.
Hydrogenation yields many useful chemicals, and its use has increased
phenomenally, particularly in the petroleum refining industry. Besides satu-
rating double bonds, hydrogenation can be used to eliminate other elements
from a molecule. These elements include oxygen, nitrogen, halogens, and
particularly sulfur. Cracking (thermal decomposition) in the presence of
hydrogen is particularly effective in desulfurizing high-boiling petroleum
fractions, thereby producing lower-boiling and higher-quality products.
Although occasionally hydrogen for a reaction is provided by donor
solvents and a few older reactions use hydrogen generated by acid or alkali
acting upon a metal, gaseous hydrogen is the usual hydrogenating agent.
Hydrogenation is generally carried out in the presence of a catalyst
and under elevated temperature and pressure. Noble metals, nickel, cop-
per, and various metal oxide combinations are the common catalysts.
Nickel, prepared in finely divided form by reduction of nickel oxide in
a stream of hydrogen gas at about 300°C, was introduced by 1897 as a cat-
alyst for the reaction of hydrogen with unsaturated organic substances to
be conducted at about 175°C. Nickel proved to be one of the most suc-
cessful catalysts for such reactions. The unsaturated organic substances
that are hydrogenated are usually those containing a double bond, but
those containing a triple bond also may be hydrogenated. Platinum black,
palladium black, copper metal, copper oxide, nickel oxide, aluminum,
and other materials have subsequently been developed as hydrogenation
catalysts. Temperatures and pressures have been increased in many
instances to improve yields of desired product. The hydrogenation of
methyl ester to fatty alcohol and methanol, for example, occurs at about 290
to 315°C and 3000 psi (20.7 MPa). In the hydrotreating of liquid hydrocar-
bon fuels to improve quality, the reaction may take place in fixed-bed
reactors at pressures ranging from 100 to 3000 psi (690 kPa to 20.7 MPa).

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