20-03-2012, 11:56 AM
Multiphase reactors revisited
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Introduction
Processes based upon multiphase reactions occur in
a broad range of application areas and form the basis for
manufacture of a large variety of intermediate and
consumer end-products. Some examples of multiphase
reactor technology uses include: (1) the upgrading and
conversion of petroleum feed stocks and intermediates;
(2) the conversion of coal-derived chemicals or synthesis
gas into fuels, hydrocarbons, and oxygenates; (3) the
manufacture of bulk commodity chemicals that serve as
monomers and other basic building blocks for higher
chemicals and polymers; (4) the manufacture of pharmaceuticals
or chemicals that are used in "ne and specialty
chemical markets as drugs or pharmaceuticals; and (5)
the conversion of undesired chemical or petroleum processing
by-products into environmentally acceptable or
recyclable products. An overview of the chemistry and
process technology of these various application areas is
provided in the monograph of Weissermel and Arpe
(1993).
Fixed beds with two-phase 6ow
Packed-bed reactors processing gas and liquid reactants
can operate in downward cocurrent two-phase #ow
(trickle-bed reactors } TBR), in upward cocurrent #ow
(packed-bubble columns } PBC) and in countercurrent
#ow. The three modes of operation are illustrated in
Figure 2 and the processes recently investigated in these
reactor types are listed in Table 2.
Reactors with moving catalyst
3.1. Bubble columns and slurry bubble columns
Bubble columns and slurry bubble columns are used
extensively in a variety of processes for hydrogenation,
oxidation, chlorination, hydroformylation, cell growth,
bioremediation, etc. Recently they have been identi"ed as
reactors of choice for gas conversion (e.g. liquid phase
methanol synthesis, Fischer}Tropsch synthesis, etc.) due
to their excellent heat transfer characteristics. Fig. 5 schematically
represents a typical bubble column reactor
(minus the internals needed for heat transfer). Gas is
sparged at the bottom of the column and the resulting
buoyancy driven #ow creates strong liquid recirculation.
Thus, as long as the liquid super"cial velocity is an order
of magnitude smaller than that of the gas, it is the gas
super"cial velocity that is the dominant variable which
drives the #uid dynamics of the whole system, and
whether the liquid is processed batch-wise or #ows
cocurrently or countercurrently to the #ow of the gas is
immaterial from the #uid dynamics point of view.