Catalytic Fixed-Bed Reactors
GERHART EIGENBERGER, Universit€at Stuttgart, Stuttgart, Germany
WILHELM RUPPEL, retired from BASF SE, Ludwigshafen, Germany
1.
Fixed-Bed Reactors with Gas-Phase
Reactions . . . . . . . . . . . . . . . . . . . . . .
2.
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1. Fixed-Bed Reactors with
Gas-Phase Reactions
The core part of any fixed-bed reactor is the solid
catalyst where the reaction takes place. A large
variety of catalyst structures are applied in practice. One class of structures, used in randomly
packed beds, consists of catalyst pellets of different shapes. A second class comprises regularly arranged structures like monoliths with
flow channels of different shape.
With regard to application and design, it is
convenient to differentiate between fixed-bed
reactors for adiabatic and nonadiabatic operation. Since temperature control is one of the
most important means of influencing a chemical reaction, adiabatic reactors are used primarily where the adiabatic temperature change
during the reaction is small or where there is
only one major reaction pathway. In these cases
no adverse effects on selectivity or yield due to
the adiabatic temperature development are
expected. In adiabatic reactors the catalyst is
present in the form of a fixed bed which is
surrounded by an outer insulating jacket (Fig. 1,
left).
If the reaction temperature must be maintained within a specified range, multistage adiabatic reactors can be used, whereby between
each stage the temperature can be influenced by
heat exchange or by cold/hot gas injection.
Reactions with a large heat of reaction and
reactions that are very temperature sensitive are
usually carried out in reactors in which heat of
reaction is provided to or removed from the
fixed bed via a circulating heat-transfer
# 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10.1002/14356007
Fixed-Bed Reactors for Liquid-Phase
Reactions . . . . . . . . . . . . . . . . . . . . . .
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medium. Since in most cases the task of the
heat-transfer cycle is to maintain the temperature in the fixed bed within a specific narrow
range, this concept is frequently described as
“isothermal fixed-bed reactor”. The most common arrangement for isothermal reactor operation is the multitubular fixed-bed reactor, in
which the catalyst is arranged in the tubes, and
the heat carrier circulates externally around the
tubes (Fig. 1, right). Since isothermal reaction
control does not necessarily provide optimum
selectivity or yield, heat-exchange sections
with changing temperatures of the heat carrier
can be designed to establish an optimal temperature profile along the flow path.
Since the reactor feed must be heated to the
ignition temperature of the catalytic reaction
before reaction starts, the hot reactor effluent is
often used to heat the cold reactor feed. This
causes a thermal feedback which results in socalled autothermal reactor concepts.
Fixed-bed reactors for industrial syntheses
are generally operated in a stationary mode
(i.e., under constant operating conditions)
over prolonged production runs. Design therefore concentrates on achieving optimum stationary operation. Unstationary operation,
however, is unavoidable during startup and
shutdown as well as during load change or in
the case of automatic control actions. In particular, fixed-bed reactors with a strongly exothermic reaction exhibit an, at times, surprising
dynamic behavior which can affect operational
safety.
Contrary to the above-mentioned stationary
operation concepts, a deliberately unstationary,
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Catalytic Fixed-Bed Reactors
Figure 1. Adiabatic fixed-bed reactor (left) and multitubular fixed-bed reactor (right)
mostly periodic operating mode has proved to
be of advantage in a number of special cases.
This applies to periodic flow reversal or to
periodic feed cycling.
In a production plant the reactor can be
regarded as the central apparatus. However,
compared to the remaining parts of the plant
for preparing the feed and for separating and
working up the products, often it is by no
means the largest and most cost-intensive
component. In many cases the achievable
conversion in the reactor is limited for
thermodynamic (equilibrium) and kinetic reasons (selectivity). It is then necessary to separate the reactor effluent into products and
unconverted feed components (see Fig. 2),
which are recycled to the feed. This recycling
procedure involves costs
Figure 2. Fixed-bed reactor with product separation and recycle. a) Fixed-bed reactor, b) Feed preheater, c) Product cooler,
d) Recycle compressor, e) Separation column
Catalytic Fixed-Bed Reactors
For product separation
For recycle compression
For repeated heating and cooling of the
circulating reactants to the reaction temperature and back to the temperature of the
separating device
Due to loss of product, resulting from the
need to remove part of the recycle as a
bleed stream to limit accumulation of inert
substances or harmful byproducts in the
recycle loop.
To minimize these costs, improvements
should aim at increasing the product yield
per pass and at decreasing the amount of inert
substances in the reaction mixture.
A reaction process as depicted in Figure 2
follows the classical unit operation concept of
chemical engineering, according to which a
clear division into preparation (mixing and
preheating) of feed components, chemical
reaction, product separation, and cooling is
achieved in different units. A more recent
development under the heading of “integrated”
or “multifunctional” reactor concepts contrasts
the unit operation concept. Its aim is to provide
optimal reaction conditions in the reaction unit
by incorporating optimal heat and addition or
removal of reaction components at the reaction
site.
2. Fixed-Bed Reactors for
Liquid-Phase Reactions
Usually fixed-bed reactors are either operated
with gas-phase reactions or in a trickle-bed
mode, whereby a liquid reactant trickles
through the bed from top to bottom, while a
gaseous reactant flows upward. Here, fixed-bed
reactors with reactants in the liquid phase will
be considered. In contrast to trickle beds, such
reactors are generally operated in an upflow
mode to ensure that the catalyst is completely
soaked with liquid and to avoid gas-filled portions of the packed bed during startup or load
change. In some cases a gaseous reactant is
added to the bottom feed and is consumed in the
reaction or a gaseous product is formed during
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the reaction and is carried out with the liquid
flow.
The main advantages over the traditionally
applied stirred-tank reactors or slurry bubble
columns with suspended catalysts are the wellspecified residence time with minimum backmixing and the fixed catalyst bed, which avoids
separation of the slurry catalyst from the effluent and its recycling. Liquid-phase fixed-bed
reactions are usually operated at elevated pressure (> 100 bar) to keep volatile components in
solution. Only in case of weakly soluble gases
as reactants is an additional gas feed at the
bottom of the reactor applied.
Compared to gas-phase reactions in packed
beds, liquid-phase reactions are characterized
by diffusivities in the fluid phase being three
orders of magnitude lower than in the gas phase
and densities being two to three orders of
magnitude higher. This implies that transport
resistances in the liquid-filled catalyst pores
have a substantially larger impact on conversion and selectivity than in gas-phase reactions
and that flow velocities through the packing are
usually two to three orders of magnitude lower,
although the convective heat and mass flux are
in the same range (order of several kilograms
per square meter of reactor cross section and
second).
Because of the substantial increase of the
pore transport resistance, a heterogeneously
catalyzed reaction would preferably be carried
out in the gas phase at elevated temperature
and/or reduced pressure unless thermal stability
of the reactants or other considerations
excludes this option. Typical examples of liquid-phase reactions in packed beds are selective
hydrogenations of carbon triple or double
bonds or aminations of alcohols.
Compared to fixed-bed reactors with gasphase reactions, liquid-phase reactors contain
a substantially larger amount of potentially
decomposable material and are often operated
under high pressure. This must be considered in
layout and operation of respective reactors. The
reactor layout is primarily dictated by safety
considerations which are closely related to the
large hold-up of potentially decomposable
liquids.