Fixed bed reactors resemble multitube heat exchangers,
with the catalyst packed in vertical tubes held in a tubesheet
at top and bottom. Uniform packing of catalyst within the tubes
is important to ensure uniform pressure drop, flow, and residence
time through each tube. Reaction heat is removed by generating
steam on the shell side of the reactor, or by flowing some other
heat transfer fluid through it. However, temperature control is
more difficult in a fixed bed than in a fluid
bed reactor because localized hot spots can develop in the
tubes. The tendency to develop hot spots can be minimized by packing
the reactor tubes with active catalyst and inert diluent mixtures
in proportions that vary along the length of the tubes, so that
there is low catalyst activity at the inlet, but the activity steadily
increases to a maximum at the outlet. Another remedy is to pack
the tubes with catalysts having a progressively higher loading
of cupric chloride so as to provide an activity gradient along
the length of the tubes. Multiple reactors in series are also used
in fixed bed oxychlorination, primarily to control heat release
by staging the air or oxygen feed. Each successive reactor may
also contain catalyst with a progressively higher loading of cupric
chloride. These methods of staging the air or oxygen feed and of
grading the catalyst activity work to flatten the temperature profile
and allow improved temperature control. Compared with the fluid
bed process, fixed bed oxychlorination generally
operates at higher temperatures (230-300°C) and gauge pressures
(150-1400 kPa -- 22-203 psig).
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