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Encyclopedia of Physical Science and Technology EN014A-654 July 28, 2001 16:35

42 Reactors in Process Engineering

a CSTR with recycle unless a separation or other process Speed and angle dictate the retention time in the kiln.
is being performed on the recycle stream, since the CSTR Gas is passed through the tube countercurrent to the solid
is already well mixed. reactant. The kiln is operated at high temperatures with
three or four heating zones depending on whether a wet or
dry feed is used. These zones are drying, heating, reaction,
2. Classification
and soaking. Bed depth is controlled at any location in the
The recycle reactor is used to reach an operating condi- tube with the use of a ring dam.
tion between the theoretical boundaries predicted by the
continuous stirred tank reactor and the plug flow reactor.
2. Classification
The rotary kiln is a continuous countercurrent heteroge-
3. Applications
neous reactor. Solids traveling down the kiln are in plug
The recycle reactor is used to control the reaction kinet- flow, as are the gases passing upward.
ics of multiple reaction systems. By controlling the con-
centration present in the reactor, one can shift selectiv-
3. Applications
ity toward a more desired product for nonlinear reaction
kinetics. The most common reactor of this type is the lime kiln.
This is a noncatalytic reaction where gas reacts with cal-
P. Roasting Furnace cium carbonate moving down the kiln. Other reactions
performed in the rotary kiln include calcination, oxida-
1. Description
tion, and chloridization.
Roastingfurnacesareinaclassofreactorsusedbythemet- Use of rotary kilns for hazardous waste incinera-
allurgical industry in a preparatory step for the conversion tion is becoming more common for disposal of chlo-
of ores to metals. There are three widely used roasted fur- rinated hydrocarbons such as polychlorinated biphenyls
naces: multiple hearth, fluidized bed, and flash roasters. In (PCBs). Flow in these kilns is cocurrent. Major ad-
the multiple hearth configuration hot gases pass over beds vantages include high temperature, long residence time,
of ore concentrate. The flash roaster injects pulverized ore and flexibility to process gas, liquid, solid, or drummed
with air into a hot combustion chamber. The fluidized bed wastes.
roaster operates as described in a separate heading.
R. Slurry Tank
2. Classification
1. Description
All of these roasting furnace reactors operate continu-
ously. They are noncatalytic gas–solid heterogeneous re- The slurry tank is a three-phase reactor where gas is bub-
actors. The multiple hearth has characteristics similar to bled up through a liquid–solid mixture. The slurry tank
plug flow operation. The flash roaster approaches CSTR, has the advantage of uniform temperature throughout the
and the third option is a fluidized bed configuration. mixture. This temperature control is extremely important
for highly exothermic reactions. Another advantage of the
3. Applications slurry tank is the low intraparticle diffusion resistance for
this contacting pattern. As a disadvantage, low mass trans-
Roasting furnaces are used to react sulfides to produce
fer rates occur in liquids when compared with gases, re-
metal oxides, which can be converted to metals in the next
quiring that small solid particles be used. These particles
process step. The sulfides are used as a reducing agent
can clog screens in the effluent stream used to keep solids
in nonferrous metallurgy for the recovery of metals. The
in the tank, thus making catalyst retention difficult.
process has been used for metals such as copper, lead,
zinc, nickel, magnesium, tin, antimony, and titanium.
2. Classification
Q. Rotary Kilns The slurry tank, when well mixed, can be considered a
continuous-stirred tank reactor for both the gas phase and
1. Description
the liquid phase. When the solid is retained in the reaction
The rotary kiln is a long tube that is positioned at an angle vessel, it behaves in a batch mode; however, catalyst can
near horizontal and is rotated. The angle and the rotation be removed and regenerated easily in a slurry tank, so
allow solid reactants to work their way down the tube. activity can be maintained.