Solid Catalyzed Gas Phase Reactor Selection

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-RXT-805

Solid Catalyzed Gas Phase Reactor
Selection

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Process Engineering Guide:

Solid Catalyzed Gas Phase
Reactor Selection

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

ADIABATIC REACTORS

2

4.1
4.2
4.3
4.4

Single Bed Reactors
Divided Bed Reactors
Moving Bed Reactors
Radial Flow Reactors

3
4
4
4

5

NON ADIABATIC REACTORS

5

5.1
5.2
5.3
5.4
5.5
5.6
5.7

Tubular Reactor with External Heating/Cooling
Tube Cooled Reactors
Autothermal Reactors
Hot/Cold Shot Reactors
Divided Bed Reactors with Intercooling
Radial Flow Reactors with Intercooling
Fluid Bed Reactors

5
6
7
8
9
9
9

6

NOTES ON USING REACTOR SELECTION
GUIDE (TABLE 1)

10


TABLE
1

REACTOR SELECTION GUIDE

10

FIGURES

1

TUBULAR REACTOR: EXAMPLE OF CATALYST IN ANNULAR
TUBES COOLED BY STEAM RAISING

5

2

AUTOTHERMAL REACTOR: CATALYST BED COOLED BY INFLOWING
GAS IN TUBES
7

3

COLD SHOT CONVERTER: FIXED ADIABATIC BEDS WITH
INTERBED QUENCH GAS MIXING

8


0

INTRODUCTION/PURPOSE

Reactors can be categorized in many different ways. For the purposes of this
Process Engineering Guide, a very simple categorization based on thermal
characteristics is used as follows:
•
•

Adiabatic
Non adiabatic

These two generic descriptions are further subdivided by type to reflect the many
variants to be found in industry. This Guide provides brief notes on the typical
intrinsic characteristics of the various generic reactor types together with a
selection guide to help determine the most appropriate type(s) for any given
application.
Where the choice of reactor type interacts with the rest of the flowsheet, either
via changes to conversion, selectivity or heat recovery, it will be necessary to
optimize the design of the complete process.

1

SCOPE

This Process Engineering Guide sets out the selection factors and criteria to
enable process developers to choose the most appropriate type of reactor for
solid-catalyzed gas phase reactions. It does not cover the detailed reactor design
nor the modeling of the reactions or the reactor.
2

FIELD OF APPLICATION

This Guide is applicable to GBH Enterprises process engineering community
worldwide and to development scientists working in conjunction with that process
engineering community.

3

DEFINITIONS

For the purposes of this Guide no specific definitions apply.


4

ADIABATIC REACTORS

Adiabatic reactors are probably the commonest class of reactor where, other
than external heat loss/gain through the vessel wall, there is no other heat
addition or removal within the reactor vessel (although clearly there may well be
elsewhere in the flowsheet up or downstream of the reactor).
Adiabatic reactors are suitable for reactions with mild to moderate
thermochemistry (typical temperature change across the reactor no greater than
50°C) and moderate variation of reaction rate with temperature (including byproduct formation rates). There can be control problems with kinetically limited
reactions where the reaction rate more than doubles for a 20° increase in
absolute temperature.
The catalyst is typically in the form of pellets or particles with characteristic size
in the range 2 - 6 mm to avoid excessive pressure drop. However, this can lead
to a significant diffusion limitation, the effects of which can be ameliorated in part
by use of ring or multi holed catalyst pellets.
Adiabatic reactors can be further sub-divided as follows:
•
•
•
•

Single Bed
Divided Bed
Moving Bed
Radial Flow

Single bed reactors should be used wherever practicable since these will always
offer the lowest vessel cost per unit volume of catalyst. Divided bed designs
should only be considered where the crush strength of the catalyst is significantly
lower than the imposed stresses. Moving bed designs tend to be mechanically
complex but can offer benefits where catalyst regeneration is a problem. Radial
flow reactors can be used beneficially where gas volumes are large and/or where
pressure drop is an issue.


4.1

Single Bed Reactors

A reactor having a single undivided catalyst bed offers simplicity of design and
construction, ease of charging and discharging, low cost and potential for good
flow distribution.
Optimum bed aspect ratio is decided by an economic tradeoff between pressure
drop and vessel costs. If this gives an aspect ratio of less than 1, gas distribution
will be difficult and a radial flow reactor should be considered. The optimum will
vary if a full bore closure is required. In general, on large diameter vessels, full
bore closures should not be provided. Adequate access for catalyst charging,
discharging and inspection purposes can be gained via a 24" (or greater)
manway.
Note:
A larger diameter access should be considered if access to the vessel requires
the use of breathing apparatus.
On large diameter vessels, a full bore closure can increase the vessel cost
significantly whilst the joint can prove troublesome to seal.
Imposed stresses on the catalyst can be quite high in certain conditions. Janssen
(ref. Janssen, H A, "Versuche über Getreidedruck in Silozellen", Z. Verein
deutscher Ingenieure, 39, No. 35, pp 1045-1049 (1895)) analysis can be used to
predict initial stresses quite accurately to compare with catalyst crush strength
results. In deciding between a single and multiple bed design, it is necessary to
consider all phases of the catalyst's life and to be aware of any sudden
reductions in catalyst strength which may occur during, for example, any catalyst
preconditioning process.
Where there is a risk of significant exotherms being generated in the catalyst bed
which would result in either an uneconomic set of design conditions or severe
damage to the pressure shell, it may be necessary to install either a heat shield
or a removable cartridge to provide a thermal barrier between the catalyst and
the vessel shell.


4.2

Divided Bed Reactors

Reactors having a divided catalyst bed with intermediate bed supports should
only be considered where catalyst strength is low and significantly less than the
imposed stresses imposed on the catalyst particles in service, or to overcome the
effects of channelling when very high conversions are needed. The option of a
single reactor shell with a divided bed should be compared to the provision of
several smaller single bed reactors in series to determine the optimum capital
cost. Examples are Cumene.
The internal structures necessary for the intermediate bed supports add
significantly to the cost of the vessel and restricts access for maintenance,
charging and discharging. In addition, the presence of any internal support pillars
tends to make uniform flow distribution difficult to achieve with inevitable
consequences on reactor performance.

4.3

Moving Bed Reactors

Several patented designs of moving bed reactors are available.
These designs are mechanically very complicated with numerous valves, seals
and mechanical conveying systems.
Moving bed systems should only be considered when the catalyst requires
frequent regeneration (typical time between regenerations lies in the range of a
few hours to a few days) or where very complex off line regeneration conditions
are required.
Catalyst attrition is inevitable with the attendant problems of dust generation. In
some cases, the catalyst bed may become locked and fail to move at all.
Any moving bed reactor design should always be compared to fluid bed design
(see 5.7) or parallel fixed bed reactors with off line regeneration.
Due to the inherent problems associated with sealing such a system, the
occupational hygiene issues associated with any such design should be
considered carefully.


4.4

Radial Flow Reactors

Several proprietary designs of radial flow reactors are available offering a
significantly reduced pressure drop for a given flowrate and catalyst volume.
Such designs are attractive where there are large quantities of recycle gas in a
synthesis loop and output is limited by pressure drop or where the volume of gas
is large relative to catalyst volume.
The design allows the use of smaller catalyst pellets (typically down to 2 mm)
which can make radial flow reactors suitable for diffusion limited reactions.
Radial flow reactors can often be retrofitted into an existing pressure vessel shell
to uprate production.
The design lends itself to the provision of internal heating/cooling (pseudo
isothermal).
Gas flow and reactor volume beyond which this type of reactor becomes
economically viable can be calculated.

5

NON ADIABATIC REACTORS

Non adiabatic reactors cover a very wide range of reactor designs which can be
subdivided into the following broad categories
•
•
•
•
•
•
•

Tubular with external heating or cooling
Tube cooled reactors
Autothermal reactors
Cold shot
Divided bed with intercooling
Radial flow with heating/cooling
Fluid Bed


5.1

Tubular Reactor with External Heating/Cooling

Tubular reactors with external heating/cooling are suitable for strongly
endo/exothermic reactions, reactions where there is a strong dependency of
reaction rate on temperature (especially for by-products) and where near
isothermal operation is required, reactions where there is a significant risk of
thermal runaway, where there are specific heating, cooling or temperature control
requirements during start up, shut down, regeneration or pre or post treatment, or
where reaction thermochemistry remains uncertain (see also example in Figure
1).

Heating or cooling media include:
•
•
•
•

Boiling/condensing water
Molten salt
Hot oil
External firing (heating only)

FIGURE 1

TUBULAR REACTOR: EXAMPLE OF CATALYST IN ANNULAR
TUBES COOLED BY STEAM RAISING

Maximum permissible radial temperature variation or overall heat transfer
requirements dictate tube diameter. The typical working range of tube diameters
is NPS 1½ to NPS 4. Optimum tube diameter can be readily obtained given a
kinetic model combined with heat transfer.

To avoid wall effects and poor flow distribution, tube diameter should be at least
10 times the catalyst pellet diameter.
Generally tubular reactors are complicated and expensive compared to adiabatic
systems. Reactor diameter will be significantly larger for a given catalyst volume
and pressure drop (typically only 40 - 50 % of vessel cross sectional area can be
usefully used for catalyst).
It is important to ensure even flow distribution through tubes. This is best
achieved by checking that the pressure drop through tubes at the packing stage
is within specified tolerances. The use of orifice plates or similar devices to
artificially increase the pressure drop through catalyst filled tubes is
recommended to ensure good distribution if the pressure drop is too low.
Charging/discharging time can be significant for this type of reactor (especially if
this requires an inert atmosphere) and will always be significantly greater than for
a corresponding adiabatic reactor of the same catalyst volume.
Consideration has to be given to the implications of any leak resulting from failure
of a tube.
The main control variable is the heating/cooling medium temperature.
External firing is a special case for extremely endothermic reactions (e.g. steam
reforming of methane).
Designs are generally complex and limited to a limited number of specialist
manufacturers (e.g. salt cooled systems, lack of verified heat transfer correlations
for boiling of water on the outside of vertical tubes).
The choice of heating/cooling medium can raise issues in its own right (e.g.
flammability of some oils, solidification of molten salt systems). For liquid cooled
tubular reactors attention should be paid to the shell-side coolant flow distribution
to avoid dead zones which may give rise to reaction hot spots. For critical
systems the variation of heat transfer due to velocities and to the temperature
dependence of the physical properties needs to be considered. Provision for the
venting of inerts from under the top tube sheet should not be overlooked.
Possible catalyst life issues can arise depending on deactivation mechanism.


5.2

Tube Cooled Reactors

Tube cooled reactors employ a catalyst packed around the outside of tubes
through which cooling the medium flows.
The issues are similar to those for tubular reactors with external heating/cooling
except:
(a)

Use for less strongly exothermic or temperature sensitive reactions than
tubular heated/cooled reactors.

(b)

The larger mean distance and greater variation in path length between the
catalyst and cooling surface leads to larger variations in the mean catalyst
temperature and hence the reaction rate than for tubular reactors.

(c)

The catalyst is packed on the shell side which makes charging and
discharging potentially complex and uniform flow distribution difficult to
achieve.

(d)

Designs can result in smaller reactor vessel diameters for a given catalyst
volume and pressure drop than for tubular reactors but catalyst handling
and flow distribution problems should not be underestimated.

Although less serious than in the case of a tubular reactor with external heating
or cooling, consideration has to be given to implications of any leak resulting from
failure of a tube and in particular how this will be detected.

5.3

Autothermal Reactors

Autothermal reactors are a specialist subset of tube cooled designs where the
heating/cooling medium is feed/product gas (see Figure 2). Use for fairly
exothermic reactions (e.g. MeOH synthesis).


FIGURE 2

AUTOTHERMAL REACTORS: CATALYST BED
COOLED BY INFLOWING

A limited number of variables are available for control.
Where the catalyst is on the shell side, the catalyst packing and handling issues
described in 5.2 apply.
Possible catalyst life issues can arise depending on deactivation mechanism.
The potential exists for the reactor to 'switch off’, (stable, unstable and meta
stable operating states may exist).
Autothermal reactors are not recommended when the heating medium is the
product gas or when the product is not stable at the reactor exit temperature e.g.
NOx, HCN.
There is no danger in the event of tube failure other than the loss of conversion
but this may be difficult to detect on line.
Usually better gas phase heat transfer can be achieved than to an atmospheric
pressure gas stream because operation is often at high pressure.


A separate start up heater is usually required.
Typical examples of the use of autothermal reactors are methanol and ammonia
synthesis, gas heated reformer.

5.4

Hot/Cold Shot Reactors

Such reactors are usually configured as cold shot (quench) converter but hot
shot design is theoretically possible.
Cold shot reactors are generally used in similar applications to autothermal
reactors.
They are normally used for exothermic reactions although they can theoretically
be used for endothermic reactions, but most of the advantages are lost.
Cold shot reactors are used to avoid having to heat all of the feed gas to the
reaction initiation temperature.
The reaction is "quenched" and approach to equilibrium composition is altered by
injecting a quantity of cold (or hot) feed gas.
They are generally configured with divided beds with inter bed gas mixing - see
Figure 3. Lozenge arrangements are now no longer favoured.
Mixing of fresh feed gas with gas already in the reactor is critical to success.
Cold shot reactors are generally simpler and cheaper than tube cooled reactors.
Potentially slightly less than optimal catalyst use.
Several variables are available for control (more than with tube cooled design).
As with tube cooled reactors and some adiabatic reactors, cold shot reactors
usually require a separate start up heater.


FIGURE 3

5.5

COLD SHOT CONVERTER: FIXED ADIABATIC BEDS
WITH INTERBED QUENCH GAS MIXING

Divided Bed Reactors With Intercooling

Such reactors comprise divided beds of catalyst followed by patten type heating
or cooling coils.
They are generally mechanically complex, usually with many penetrations of the
pressure shell.
Consideration should be given to consequences of tube failure.
Examples of the use of divided bed reactors with intercooling include sulfuric acid
converter and some aromatics reactors.

5.6

Radial Flow Reactors With Intercooling

See 4.4 for general features of the design.


The design lends itself to the provision of heating/cooling coils along the axis of
the vessel, across which the gas flows between passes through the catalyst.
Radial flow reactors avoid many of the catalyst handling problems associated
with tubular reactors.

5.7

Fluid Bed Reactors

Fluid bed reactors are suitable for very highly exothermic reactions.
They require very small catalyst particles to ensure fluidization.
Significant problems with attrition and dust generation are inevitable.
Fluid bed reactors are suitable for strongly diffusion limited reactions.
They are inherently prone to gas by-passing as bubbles flow up through the
catalyst bed.
They are suitable for heavily coking reactions with short time between
regenerations.
Good heat transfer is possible within the bed.
They are generally suitable for very fast non-equilibrium limited reactions.
Fluid bed reactors should be considered when one of the reactants is in the solid
state (e.g. coke).

6

NOTES ON USING REACTOR SELECTION GUIDE (TABLE 1)

Table 1 is intended to assist in selecting the most appropriate type(s) of reactor
for a given application. For most applications, there are a number of possible
alternatives and Table 1 is intended only to assist in narrowing the field and,
hopefully, stimulating consideration of some of the issues involved.
For each of the characteristics or parameters listed, those reactor types marked
√√ are particularly suitable for the given application. Any marked X are not
considered suitable. After working through the table, a subset of reactor types
suitable for the given application should begin to emerge.

GBHE advice should be sought before making a final decision.
TABLE 1

REACTOR SELECTION GUIDE


KEY:
√√
√
X
†

Reactor type particularly suitable for this application
Reactor type suitable for this application
Reactor type unsuitable for this application or unlikely to be cost effective
Reactor type not necessarily ideal but often no alternative available

Notes:
1

Heat transfer limitations require small diameter tubes.

2

Upper operating temperature limits set by shell side conditions. Typical
limits; water 240 -260°C (@40 - 60 bara); molten salt, 400°C +; oil,
typically 300 - 350°C.

3

Only used for very endothermic reactions requiring very high process
temperatures (e.g. methane steam reforming, ethylene oxide etc.).

4

Usually requires additional heat removal.

5

Stable reaction defined typically as one where the rate of the main or any
significant side reaction (including coking) doubles/halves for a change in
temperature of 20° or more.

6

Unstable reaction defined typically as one where the rate of the main or
any significant side reaction (including coking) doubles/halves for a
change in temperature of 20° or less.

7

For reactions requiring very short (millisecond) contact times,
consideration should be given to using gauze type reactors (e.g. ammonia
oxidation) or shock tubes.

8

This type of reactor can be used but the catalyst volume is rarely sufficient
to justify this type of system.

9

These reactors are particularly suitable because they offer the opportunity
to control conditions to maximize the approach to equilibrium.

10

Very close approach to equilibrium is not possible with this type of reactor
due to gas bypassing through the bed via bubbles.


11

Small particle size catalyst (typically mean particle diameter 2 - 6 mm) are
suitable for strongly diffusion limited systems. Penalty is high pressure
drop.

12

Large catalyst pellets suitable for non diffusion limited systems or where
cost of pressure drop is high.

13

Maximum catalyst particle diameter is dictated by tube diameter. For good
packing and to avoid bypassing, the tube diameter should be 10 particle
diameters or greater.

14

Consider reactors in parallel to avoid loss of availability.

15

Upper limit of regeneration conditions are defined by water side.

16

Typical maximum vessel diameter in the range 10 - 14 feet for offsite
fabrication. This can limit the maximum output from a single reactor.
Adiabatic reactor design gives greater catalyst volume for an equivalent
vessel diameter.


Solid Catalyzed Gas Phase Reactor Selection

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