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140 3. Heterogeneous Processes and Reactor Analysis

which could be under pressure, with the flo ution system allo w distrib wing liquid or gas to
flow with a specified rate. Retention screens on the inlet and outlet prevent the solids from
escaping into the process loop. The low porosity of beds of powdered media restricts their
use to thin layers, usually as a “precoat” on a filter medium. In practical applications, the
particle size used is in the range 0.25–3 mm.
ix The fed-bed operation is usually a semicontinuous process. When the medium gets
spent, the f ed-bed operation is stopped and the material is replaced with a fresh batch. In ix
fixed-bed operation, the determination of the medium being spent is usually based on the
breakpoint, which is the point at which the e xit concentration of the solutes being removed
starts to increase sharply to some predetermined leel (typically below 10%). If interrup- v
tions in the process to replace the adsorbent or the ion exchange media are not desirable,
multiple fixed beds can be connected in parallel. While one set is in operation, the other is
,
filled with a fresh medium or after ref is on standby illing, .
Catalysis

Fixed- or packed-bed reactors refer to two-phase systems in which the reacting fluid flows
through a tube filled with stationary catalyst particles or pellets (Smith, 1981). As in the
ed bed is the most frequently used oper- ix case of ion-exchange and adsorption processes, f
f,
ation for catalysis (Froment and Bischof 1990; Schmidt, 2005). Some examples in the
chemical industry are steam reforming, the synthesis of sulfuric acid, ammonia, and
methanol, and petroleum refining processes such as catalytic reforming, isomerization,
f, and hydrocracking (Froment and Bischof 1990).
In the case where a rapid removal or addition of heat is needed, it may not be possible to
use a single fixed bed or large diameter. In this case, the reactor can be built up of a number
of tubes, containing the catalyst particles and encased in a single body (Smith, 1981). Then,
the heat exchange can be easily done by circulating a fluid in the space between the tubes.
f
If eficient contacting in the reactor is of primary importance, then the fed-bed reac- ix
enspiel,
tor is preferred to a fluidized-bed reactor (Le 1962). Other advantages of f ed ix
v
wing: operations are the follo
• the flow regimes approach plug flo, so high conersion can be achie w v v ed
• pressure drop is lo w
• there is better radial mixing and channeling is not encountered owing to the high holdup,
• high catalyst load per unit volume of reactor is possible.

All types of catalytic reactors with the catalyst in a fixed bed have some common draw-
backs, which are characteristic of stationary beds (Mukhlyono v et al ., 1979). First, only
comparatively large-grain catalysts, not less that 4 mm in diameter can be used in a f , ilter-
ing bed, since smaller particles cause increased pressure drop. Second, the area of the inner
surface of large particles is utilized poorly and this results in a decrease in the utilization
v
er
(capacity) of the catalyst. Moreo the particles of a stationary bed tend to sinter and
,
cake, which results in an increased pressure drop, uneand lo v en distrib ution of the gas, wer
catalyst acti. Finally porous catalyst pellets exhibit low heat conductivity and as a
vity
,
result the rate of heat transfer from the bed to the heat exchanger surface is very lo . w
v Intensive heat remoal and a uniform temperature distribution oer the cross-section of a v
stationary bed cannot, therefore, be achieThe poor conditions of heat transfer within ed. v