Page 45 - Chemical and process design handbook

Speight_Part 1_H 11/7/01 3:03 PM Page 1.31

HYDROGENATION 1.31
for cooling offers more nearly isothermal operation but does not give con-
venient heat recovery.
Reaction vessels are usually of two types: one in which the contents are
agitated or stirred in some way and the other in which the reactor and con-
tents are stationary. The first is used with materials such as solids or liquids
that need to be brought into intimate contact with the catalyst and the hydro-
gen. The second type is used where the substance may have sufficient vapor
pressure at the temperature of operation so that a gas-phase as well as a liq-
uid-phase reaction is possible. It is also most frequently used in continuous
operation where larger quantities of material need to be processed than can
be done conveniently with batch methods.
In hydrogenation processes, heating of the ingoing materials is best
accomplished by heat exchange with the outgoing materials and adding
additional heat by means of high-pressure pipe coils. A pipe coil is the only
convenient and efficient method of heating, for the reactor is usually so
large that heating it is very difficult. It is usually better practice to add all
the heat needed to the materials before they enter the reactor and then sim-
ply have the reactor properly insulated thermally. Hydrogenation reactions
are usually exothermic, so that once the process is started, the problem may
be one of heat removal. This is accomplished by allowing the heat of reac-
tion to flow into the ingoing materials by heat exchange in the reactor, or,
if it is still in excess, by recycling and cooling in heat exchangers the
proper portion of the material to maintain the desired temperature.

See Dehydrogenation.