Novel Reactor Technology

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-RXT-811

Novel Reactor Technology

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Process Engineering Guide:

Novel Reactor Technology

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

FUEL CELLS

3

4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8

Introduction
Examples of Fuel Cells
PAFC (Phosphoric Acid Fuel Cell)
AFC (Alkaline Fuel Cell)
DMFC (Direct Methanol Fuel Cell)
SPFC (Solid Polymer Fuel Cell)
PCFC (Proton Conducting Fuel Cell)
MCFC (Molten Carbonate Fuel Cells) and SOFC
(Solid Oxide Fuel Cells)

3
5
5
5
6
6
6

5

ULTRASONIC REACTORS

7

5.1
5.2
5.3

Introduction
Recommendation
Design of Sonoreactor

7
7
8

7


6

THIN FILM REACTORS

10

6.1 Introduction
6.2 Equipment

7

10
10

ELECTRON BEAM REACTORS

11

8

MEMBRANE REACTORS

12

8.1
8.2

12

8.3
8.4
8.5

Introduction
Circumstances Suggesting Consideration of Membrane
Reactors
Example of a Membrane Reactor
Modeling Membrane Reactors
A Typical Analysis of a Membrane Reactor

12
13
14
15

9

SHOCK TUBE REACTORS

17

10

REACTIVE DISTILLATION

17

10.1
10.2

Introduction
Example of Reactive Distillation

17
18

11

OTHER NOVEL REACTOR TECHNOLOGY

20

11.1
11.2
11.3

Heterogenous Catalytic Reactors
Liquid and Liquid/Gas Phase Reactors
Reactor/Separation Combinations

20
22
24


FIGURES

1

SONOREACTOR PILOT PLANT LOOP RIG

9

2

CROSS SECTION OF SPINNING CONE REACTOR

11

3

REACTIVE DISTILLATION SCHEME

19


0

INTRODUCTION/PURPOSE

This Guide is one in a series on Reactor Technology produced by GBH
Enterprises.
In other Reactor Technology Guides, the design or analysis of conventional
reactors are discussed. While, hopefully, these approaches would result in the
specification of economically viable new processes, or in the improvement of the
running costs of existing ones, it is to be expected that step changes in
commercial performance will result from radical changes in the process, e.g.
substitution of one diluent for another, or from novel reactor technology.

1

SCOPE

This Guide contains brief notes on novel reactor technologies which range from
devices which are already being exploited, albeit in a limited way, to others
whose potential is clear but which require further considerable development.
The aim of this Guide is to make the user aware of a range of novel technologies;
it does not give detailed information on their design.

2

FIELD OF APPLICATION

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

3

DEFINITIONS

For the purposes of this Guide, no special definitions apply.


4

FUEL CELLS

4.1

Introduction

To date, the incentive for developing fuel cell technology has centered on the
potential for producing electrical energy from fuel at greater efficiencies than can
be achieved using "more conventional" heat and power systems. However, the
capital cost of these units is sufficiently high that applications are limited largely
to specific areas, e.g. space technology, military equipment, hospitals etc. For
example, the cost of a phosphoric acid fuel cell will have to fall by at least a factor
of four in order to compete head on with gas turbine combined cycle power
generation. The attractiveness of fuel cells for power generation is enhanced if
the waste heat that is inevitably produced can usefully be used for
domestic/commercial heating or further power generation (i.e. CHP).
There are various types of fuel cell under development, each of which is
generally known by the name of the electrolyte used in the cell.
A useful introductory survey of fuel cell types is provided by Linden in “Handbook
of Batteries and Fuel Cells”. It is generally accepted that seven major types of
fuel cells are under active development but only two, the phosphoric acid fuel cell
(PAFC), and alkaline fuel cell (AFC), are close to commercialization. However,
both the molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) are
under intensive development, predominately in the USA and Japan. Brief
comments on the seven fuel cell types are given in 4.3 to 4.8 (inclusive).
Alkaline fuel cells (AFC) were used for the space shuttle and have been
developed for transport applications, but they are not sufficiently durable for
widespread stationary power generation. Of the other types of cell, most
development has been on phosphoric acid fuel cells (PAFC) for stationary power
generation. Molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC)
are potentially more efficient than PAFCs, but are at least ten years behind in
development.
In addition to the use of fuel cells specifically for heat and power generation,
other areas of application can ultimately be envisaged within the chemical
industry whereby the exothermic nature of "traditional" reactions is harnessed to
generate electrical energy. In his review Prof. Steele (IC 16653) indicates the
potential of "second generation" type of fuel cells, principally those utilizing a
solid oxide electrolyte, for undertaking traditional oxidation or partial
oxidation reactions. The following processes are cited as being suitable, in
principle, for use in ceramic fuel cells:

Fuel cells of this type can also be used to undertake chemical reactions which
are not possible in more conventional reactor systems, e.g. selective partial
oxidations. This can be achieved by chemical activation of the catalytic surface
through operation only slightly above the equilibrium potential.
The first of the reactions cited above is described in some detail within Prof.
Steele's report. The electrolyte used within ceramic fuel cells is normally
composed of a solid solution of zirconia and yttria which permits migration of
oxygen ions. Selective catalysts laid down on either side of the thin solid oxide
electrolyte provide the anode/cathode over which the reactant/air pass. Various
potential designs of the overall cell are discussed in the report.
There are other potential applications for fuel cells within the chemical industry,
especially where hydrogen is present as a waste byproduct, e.g. some
chlorine/caustic manufacturing sites. The economics of such applications are
regularly reviewed to assess their potential. GBH Enterprises have made a
study of the markets and opportunities for fuel cells and should be contacted to
discuss the economic evaluations made as a result of this investigation.

4.2

Examples of Fuel Cells

A fuel cell is a device for converting the chemical energy of a fuel directly to
electrical energy. In principle, a fuel cell can convert primary energy to electricity
with greater efficiency than can be achieved with the conventional steam turbine
or other forms of heat engines. However, it will be seen that achieving these high
efficiencies in practical engineering systems can be difficult and so, to date,
commercial applications have been limited.


4.3

PAFC (Phosphoric Acid Fuel Cell)

The PAFC has a planar configuration incorporating two gas diffusion electrodes
and the immobilized phosphoric acid electrolyte. The system usually operates at
around 200°C at 7 bar with hydrogen as fuel. For hydrocarbon fuel sources an
external reformer is thus required which inevitably limits the overall efficiency.
Forty-six 40 kW PAFC units have undergone an extensive field test program
equivalent to 300,000 hours of cumulative operating experience, and a 4.5 MW
plant has been successfully operated by Tokyo Electric Power Company. The
International Fuel Cell company (a consortium of United Technologies,
Toshiba and Bechtel Corporation) are now offering 200 kW, and 11 MW PAFC
units for sale at special rates (~$1,800/kW) to allow further evaluation of these
units in a variety of locations. However, the value of electrical generation
efficiency is unlikely to exceed 40% using natural gas as a feedstock, and so the
market penetration of these units will remain very small, except possibly for some
special CHP applications and for selected strategic situations requiring
uninterruptable power supplies.

4.4

AFC (Alkaline Fuel Cell)

This system has been extensively developed for space applications and for
limited terrestrial applications, particularly in the military sector. A planar
configuration is usually employed for the stack which incorporates aqueous KOH
as the electrolyte. For terrestrial applications, AFC units are usually operated in
the temperature range 60-80°C and can attain up to 65% efficiency using pure
H2. Civil applications are considered to be limited by the requirement
that both fuel and oxidant gases need to be rigorously scrubbed of oxides of
carbon and so pure hydrogen supplies are needed. Moreover, high catalyst
loadings are usually specified, which is a further disadvantage of the AFC
system. These drawbacks are contested by the ELENCO consortium which is
committed to developing hydrogen-air fuel cells for large city vehicles (buses,
etc.) and for specific market niches where the environmental advantages of
fuel cells can allow a higher cost per kW.


4.5

DMFC (Direct Methanol Fuel Cell)

This fuel cell uses an acid electrolyte (usually H2SO4) in which the methanol fuel
is dissolved. The cathode is of the air diffusion type and the anode (usually
carbon activated by a noble metal catalyst) is immersed in the H2SO4 electrolyte.
This system was under active development for transport applications by Shell
and Esso for more than a decade (until around 1980) and more recently Hitachi
has commenced investigations into this system. Details of the Shell program are
available (“Power Sources for Electric Vehicles” ed B D McNicol and D A J
Rand). An advantage of DMFCs is the prospect of producing compact systems
ranging in size from a few watts up to several kilowatts for ambient temperature
operation. However, the performance of DMFC systems to date has been
disappointing due to the rapid poisoning of the anode electrocatalysts by the
methanol oxidation products. This, and the need for high noble metal catalyst
loading, will probably ensure that this system does not emerge from the
development laboratories until a major breakthrough is made relating to the
anode electrocatalyst.

4.6

SPFC (Solid Polymer Fuel Cell)

These systems incorporate a planar configuration in which the polymer is
sandwiched between the two porous electrode structures. Most development
work has been based on the NAFION perfluorosulfonic acid polymeric ion
exchange membrane originally manufactured for the chlor-alkali industry. This
material is relatively expensive and so SPFC systems have been developed,
essentially for space and defense applications (e.g. submarine propulsion
units). It is believed that considerable operating experience has been
accumulated for small units but little information is in the public domain. It should
be noted that SPFC units can also be used in the electrolysis mode for the
production of hydrogen and oxygen and relevant investigation will also benefit
the associated fuel cell technology.
Relatively high noble metal catalyst loadings also contribute to make this system
expensive and so the recent announcement by Ballard Corporation (Canada) of
a new low cost unfluorinated polymer is very significant. However, the system still
operates on pure hydrogen and oxygen and so civil applications are still unlikely.


4.7

PCFC (Proton Conducting Fuel Cell)

Central to the development of this system is the availability of a solid state proton
conducting membrane that can operate at intermediate temperatures (300400°C). In this range of temperatures it is hoped that the improved electrode
kinetics will avoid the need for noble metal catalysts and yet provide a
satisfactory fuel/electricity conversion efficiency. Many hydrated salts including
hydronium beta Al2O3 exhibit useful conductivities for H3O+ ion but the
transport of water through the membrane and sensitivity to changes in the partial pressure of H2O
make it unlikely that these compounds can be developed into engineering materials.

Non-hydrated inorganic proton conductors such as a-zirconium hydrogen
phosphate are unlikely to exhibit high H+ conductivities due to the propensity of
protons to form relatively stable hydrogen bonds with oxygen ions in the
structure. Indeed it is noteworthy that the high temperature protonic transport in
SrCe(Yb)O3, is believed to be associated with the migration of oxygen ions. The
writer considers that the search for stable solid state proton electrolytes
is unlikely to be very rewarding, but progress in this area can be followed by
consulting the proceedings of the International Conference on Protonic
Electrolytes which is held every two years.

4.8

MCFC (Molten Carbonate Fuel Cells) and SOFC (Solid Oxide Fuel
Cells)

Both these high temperature systems are considered to be contenders for the
second generation of fuel cells, with multi-kW demonstration units becoming
available in the mid 1990’s. In situ reforming (and direct electrochemical
oxidation with SOFC systems) promise higher fuel electricity conversion
efficiencies together with high grade heat for industrial CHP applications.
Moreover, operation at high temperatures does not require noble
metal electrocatalysts.
However, both systems face major scale-up uncertainties and the reader is
referred to section 5 Prof. Steele’s report for a discussion of current performance
and development problems associated with both high temperature systems.


5

ULTRASONIC REACTORS

5.1

Introduction

Ultrasonics are well known in the lab for intensive contacting of immiscible liquids
and solids in low and moderate viscosity liquids and for enhancing certain
reaction rates and yields. However, the laboratory equipment does not lend itself
to scale-up, and until recently (with the Harwell sonoreactor) there have been no
recommendable production-scale sonoreactors available.

5.2

Recommendation

The AERE Harwell reactor (Figure 1) should be considered where there is a
need to:
(a)

Make a liquid-liquid or solid-liquid dispersion of small, uniform drop or
particle size.

(b)

Enhance the rate of a liquid-liquid or solid-liquid reaction or dissolution.

(c)

Promote reactions of the type reported to be successfully enhanced.

(d)

Generate local heating.

(e)

Break up friable particles or lyse microorganisms, degass a liquid.

The reactor is new and its performance on a new process would need to be
proven by experiment. The construction seems to be robust, but so far, of
course, reliability is somewhat untested over long timescales. Given simple
acoustic shielding, the ultrasound does not present a safety problem.
The design should be done with care to avoid energy losses and to ensure that
the energy and, hence, the cavitation field responsible for the effects are well
distributed across the reactor. Arbitrary addition of transducers to a vessel or
pipe is not recommended.
The use of ultrasound for processing high viscosity liquids is not recommended.


5.3

Design of Sonoreactor

Figure 1 shows a version of the Harwell sonoreactor. It is recommended that any
new applications use an existing Harwell design, or that Harwell carry out a new
design. The energy is transmitted to the fluid via several transducers fixed
around a pipe section (6" NPSin the current model), designed and tuned to give
cavitation across the whole section. Several banks of transducers can be used in
series if necessary.
The reactor can be batch (or batch loop) or continuous. In continuous mode the
process fluids pass through the active zone, in which the cavitation occurs. The
surrounding system can provide once-through or recirculating loop flow as
desired. Residence time in the active zone will be established by experiment.


FIGURE 1

SONOREACTOR PILOT PLANT LOOP RIG


6

THIN FILM REACTORS

6.1

Introduction

Thin film reactors are used:
(a)

For rapid heat transfer, for example with rapid, exothermic or endothermic
reactions or heat-sensitive materials (liquids, slurries, or solids).

(b)

For rapid reactions with vapor release.

The design methods for film thickness, residence time distribution and heat flux
are not well established but have been partially tested. Heat transfer can be via
the surface behind the film, using a jacket or induction heating, or by evaporation
from the free surface, in which case very high fluxes are achievable. The
residence time distribution in the film can be calculated from theory for laminar
films.
With low viscosities on centrifugal devices the film can become quasi-turbulent
(i.e. some circulation within the film) which provides a nearer approach to plug
flow.
6.2 Equipment
Falling-film and scraped- or wiped-film devices (e.g. Luwa) are available
commercially. They provide moderately thin films, with somewhat uncontrolled
liquid flow patterns.
Thinner, faster moving films can be obtained with centrifugal film reactors, often
spinning discs, cones or cylinders. The Alfa-Laval Centritherm is a nested
spinning cone device with steam jackets on the cones, intended for evaporation
of sensitive foods, but possibly adaptable to reaction use.
Some reactor technology vendors have developed semi-tech scale spinning
cone, cylinder and packed-bed reactors and obtained some design data and
experience with, for example, cumene hydroperoxide cleavage,
polycondensation, polymer devolatilization, and solid-phase reactions.


FIGURE 2 CROSS SECTION OF SPINNING CONE REACTOR

7

ELECTRON BEAM REACTORS

At least one attempt has been made to develop an industrial scale process
making use of electron beams as chemical reaction activators. In 1970 the Ebara
Corporation tested a flue gas scrubbing process in which the removal of NOx and
SOx was promoted by firing a beam of electrons into the flue gas. The electronic
activation affects primarily the rate of NO oxidation by creating OH radicals and
possible oxygen atoms. The oxidation of NO is slow at low NO partial pressures,
being second order in NO and, very unusually, having a reverse temperature
dependence, that is proceeding more slowly at higher temperatures.
Furthermore, NO2 (brown) is photochemically dissociated to NO and O2. The
idea for electron activation sprang from consideration of the reactions producing
photochemical smogs (such as those which plagued the Tees Valley in the
’sixties).


It is necessary to add ammonia to produce a consistently recoverable solid
product consisting of a mixture of ammonium sulfate and ammonium nitrate. This
is recovered using bag filters or electrostatic precipitators.
Since 1975 about ninety five publications have appeared in the literature (mainly
in "Radiation Physics and Chemistry") and at least two US Research
organizations have conducted evaluations. A pilot demonstration unit has been
built and operated in the USA, 1985; a pilot unit treating 10,000 Nm3/hr of sinter
plant exhaust operated in Japan 1978/8. The US plant used two 80 kW beam
generators, which are described as electron accelerator/scanner packages. The
equipment resembles a TV cathode-ray system (although much higher
powered) firing a scanning electron beam through a window of thin "1 - mil"
Titanium foil. Protection against secondary radiation such as X-rays needs to be
a feature of the design. The power requirement is claimed to be 2 to 8 kW per
1000 actual cubic feet per minute of flue gas (1 Mrad dosage = 10 J/g of gas).

8

MEMBRANE REACTORS

8.1

Introduction

The term "membrane reactors" is applied to a variety of reactors which use a
semi-permeable membrane directly within a reactor system. This membrane
selectively favors transport of some reactant or product species over others (to
which it is preferably impermeable) and either directly influences the course of
the reaction, or enables the reaction system to exist. The term "membrane
reactor" does not include applications of semi-permeable membranes as
a separate unit operation (e.g. a recycle loop).
A broad classification of membrane reactors would be:
(a)

Inorganic membranes, e.g. metals, glasses.

(b)

Organic polymeric membranes e.g. polyethersulfones.

(c)

Liquid membranes, e.g. hydrocarbon oils.

The rarity of such reactors suggests that the membrane costs, and added reactor
complexity compounded by the mass transport resistance inherent in the
membrane, have rarely been justified by the advantages afforded by such a
system, It may also be associated with a relatively low level of awareness of their
possibilities.

8.2

Circumstances Suggesting Consideration of Membrane Reactors

The following are examples of circumstances which prompt consideration of
membrane reactors:
(a)

Equilibrium limited reaction producing a diffusive product, particularly
hydrogen, or water.

(b)

One permeable reactant needs to be kept at low concentration throughout
the reaction to avoid side reactions (applies to some analytical techniques;
can be true in supplying microbial metabolites, high concentrations being
wasted or even toxic).

(c)

It is desirable to separate the catalyst or enzyme from the reaction system,
either simply to retain it, or to prevent some component of the reaction
system from adversely affecting performance or life.

(d)

One reactant is much more dilute than the other and by avoiding simple
mixing of the two, downstream product separation can be vastly cheaper
(e.g. sulfuric acid in a hydrocarbon liquid droplet to recover ammonia from
a dilute effluent stream, an example of a liquid membrane).

(e)

Ion selectivity is required to favor a desired electrode reaction or to avoid
mixing of an electrode product with the source electrolyte.

It is worth noting that living cells are far more sophisticated membrane reactors
than anything yet devised by man, combining functions such as ion selectivity,
active transport of ions and molecules against concentration gradients, the
electrical mediation of multiple interconnected and nested biochemical cycles, pH
regulation, not to mention self-replication! However, as an example, consider a
simple membrane permeable to one species - hydrogen.

8.3

Example of a Membrane Reactor

Equilibrium - Limited Reactions Producing a Diffusive Product, Especially
Hydrogen.
While a cynical interpretation is that it is a phenomenon searching for a use, very
much research effort and patent lawyer time has been invested in the ability
specifically of hydrogen to diffuse through certain precious metals, including
silver, platinum and palladium (and its more malleable alloys with ruthenium) at
moderate to high temperatures.

Other work has attempted to utilize the relative diffusivities of, say, hydrogen and
hydrocarbon species through semi-permeable glasses and polymeric materials.
(Because of temperature limitations the polymeric materials are less studied for
direct mediation in hydrogen producing or consuming reactions, but of course
exceedingly important in separate hydrogen separation processes).
Two papers (K Mohan et al, AlChE Journal 32 (1986) p 2083 and N Itoh, AlChE
Journal 33 (1987) p 1576) have presented the results of mathematical modeling
of cyclohexane dehydrogenation reactors utilizing porous glass and palladium
alloy respectively to overcome the equilibrium limitation to full conversion to
benzene.
One of the earliest reactions studied was that of hydrocarbon steam reforming.
This is an endothermic equilibrium limited reaction:

Over a nickel catalyst the atoms of C, H and O are all brought into an equilibrium
distribution amongst the species CH4, CO, CO2, H2O and H2. Reaction (2) has a
much smaller heat of reaction than reaction (1) so that high temperatures favor
CO and H2: temperatures of the order of 1000°C are needed to effect near
complete decomposition of methane. Such temperatures can be attained by
internal combustion (using air or oxygen). In practice, conventional hydrogen
plants settle for about 4% residual methane in the hydrogen product
after shift and CO2 removal, or use additional separation.
It is reasonably clear that by removing H2 from the equilibrating mixture (while
continuing to supply heat) the reaction can be driven to higher conversions at any
temperature than if the H2 is left in the mixture, in which case it increasingly
drives the back-reactions as its concentration increases. In the limit at a modest
temperature sufficient to allow catalyst activity, very high conversion is possible
to produce H2 at a very low pressure (at least in the final stages). In practice Claydown considerations intervene. The use of precious metal membranes has
long been thought about as a means of removing H2 directly from the reaction.
One such study and the results of experiments are described in M Oertal et al,
Chem Eng Technol, 10 (1987) p 248. Other studies further integrate the
separation and reactor functions by using Pd-Ru alloy as a catalytically active
membrane.

8.4

Modeling Membrane Reactors

The modeling of membrane reactors involves adding to a basic reaction model,
catering as necessary for concentration and temperature variables through
kinetic and thermodynamic models, the transport of at least one of the reactant or
product species. A well designed membrane reactor will not be so limited in
either transport rate or reaction rate that the two phenomena can easily be
separated, except possibly where the sole purpose is one of reactant
concentration control. In general, the model should simultaneously address both
phenomena in order to achieve accurate simulation.
Two strategies suggest themselves. One is to add to a known reaction scheme a
consecutive reaction yielding a non-interfering product (for example a liquid
phase product in a gaseous reaction system), and to search for any available
analytical results or set such a scheme up in general reactor simulation software.
The second is to devise a specific model for the particular system being studied.
Although useful analytical results are unlikely to be available without
simplification of the problem as presented, some such results are available from
related problems such as bubbling fluidized bed reactors, where gas circulation
between unreactive bubble phase and reactive catalyst phase should be
combined with a reaction model and either transport or reaction rate or both can
control the final conversion. Under certain circumstances fairly simple compound
exponential decay expressions result, containing potential control by transport or
reaction in a summed decay exponent.
The appended model due to Itoh gives an example of the model form obtained in
the relatively simple case of H2 alone permeating a plug flow reactor wall. Note
that the root partial pressure dependence of H2 transport rate is specific to H2
transport through a particular metal membrane (experimentally verified). It also
probably applies to H2 transport through other metal membranes in which
transport is effected as protons. Therefore the "metal film limited" process
involves a proton concentration gradient in the metal, and a modified equilibrium
constant analogous to a Henry’s constant at each surface:

at both inlet surface and exit surface.


The reaction modeled is cyclohexane dehydrogenation to benzene.
8.5

A Typical Analysis of a Membrane Reactor


Where the Ui and Vi (i = C, H, B and A) are the flow rates of component i in the
reaction and separation sides, respectively. VR is the gross volume of the
reaction section. PTr and PTs are the total pressures in the reaction and
separation sides, respectively. An expression for the disappearance rate of
cyclohexane, rC, can be expressed by:


Where p i is the partial pressure of component i in the reaction side. Equations (3)
to (8) can be numerically integrated by standard techniques selected from the
Nag routines, or published methods such as that due to Runge-Kutta-Gill.


NOMENCLATURE:
k
KB
Kp
L
Pi
P Tr
P Ts
rC
T
U A°
U I°
V I°
αH

=
=
=
=
=
=
=
=
=
=
=
=
=

apparent rate constant, eq’ n (8), mol m-3 Pa-1 s-1
adsorption equilibrium constant of benzene, Pa-1
equilibrium constant, Pa3
dimensionless length of reactor
partial pressure of component i on reaction side, Pa
total pressure on the reaction side, Pa
total pressure on the separation side, Pa
dehydrogenation rate of cyclohexane, mol m-3 s-1
absolute temperature, K
flow rate of argon at reaction-side inlet, mol s-1
flow rate of component i at reaction-side inlet, mol s-1
flow rate of component i at separation-side inlet, mol s-1
permeation rate constant for hydrogen, mol s-1

Subscripts:
A
B
C
H
i

=
=
=
=
=

argon
benzene
cyclohexane
hydrogen
component i

9

SHOCK TUBE REACTORS

Consideration was given in the late 70’s to their application to naphtha cracking.
Maximum conventional process gas temperatures of 850°C or so, were limited by
the metallurgy of the cracking furnace tubes. The chemistry suggested that
improved yield of ethylene was available at higher temperatures and consequent
lower residence times. At temperatures of about 1100°C, a few milliseconds were
required, and the shock tube offered these conditions followed by a partial
quench. In addition, the size of the shock wave reactor would be very
much smaller than the conventional furnace.


10

REACTIVE DISTILLATION

10.1

Introduction

This is an operating technique applicable to equilibrium chemical reactions where
the products have different volatilities. The reaction is carried out in a distillation
column so that the products are separated from each other, thus allowing the
reaction to proceed to completion as far as the reaction kinetics will allow.
Advantage has been taken of this principle for many years in batch operation of a
reaction pot with overhead distillation, but exploitation by reaction and separation
together in a continuous distillation column is more recent.
A North American Company has reduced the costs of its ethyl acetate process by
60% compared with previous technology, and it is exploiting reactive distillation in
other processes.
A quantitative treatment of reactive distillation is being developed by Professor M
F Doherty and co-workers at the University of Massachusetts at Amherst. This
treatment is not yet ready for general use, so the objectives of this Clause are
limited to alerting GBHE's technologists to the principles and to encourage
appropriate experimental development.
10.2

Example of Reactive Distillation

Ethyl acetate may be produced as follows:


Batch process:
In a batch process, consisting of a kettle reactor attached to a distillation column,
the ethyl acetate will necessarily appear in the first distillate fraction as the
EtOH/EtAc azeotrope. At atmospheric pressure this limits the conversion of
ethanol to a maximum of about 54%. Also, further processing is needed to
separate the acetate from the azeotrope.

Continuous process:
By carrying out the reaction in a reactive distillation column and by using excess
acid, the EtOH/EtAc azeotrope may be avoided and pure acetate is produced. By
virtue of their relative volatilities, the two products, water and acetate, are
separated - the water descending and the acetate rising in the form of EtOH/EtAc
azeotrope. As it rises, the azeotrope is contacted with acid and its ethanol is
reacted to extinction. The absence of water in this part of the column prevents
back reaction.

Figure 3

illustrates the process as an outline flowsheet.


FIGURE 3

REACTIVE DISTILLATION SCHEME


11

OTHER NOVEL REACTOR TECHNOLOGY

This note lists other types of reactor which are either unusual or novel, or
sometimes both. Proprietary reactors which use well established technology, but
are marketed as being novel, are not included.
Some of this technology is promoted by particular companies that own the
patents, while other technology just exists as patents, which may be in force or
may have expired. Many of the ideas have been around for a long time and are
technologies looking for a problem.

11.1
11.1.1

Heterogeneous Catalytic Reactors
Magnetically Stabilized Fluidized Bed Reactor (MSFBR)

The MSFBR consists of a fluidized bed but with magnetically susceptible
particles stabilized by an externally applied electric field. The magnetic supports
can be coated with an enzyme or catalyst. The reactor has an advantage over a
normal fluidized bed in that it has better mass transfer and can sustain high gas
velocities without particle elutriation. Also, the bed can deal with a feed
containing dust that is not magnetically susceptible.
Of interest when:
•
•
•
•

There is a need for heterogeneous catalysis.
There are suspended particles in the fluid stream.
No heat transfer is required.
Testing has been undertaken for effluent processing and reactions using
enzyme catalysts.

Being promoted by EA Technology.

11.1.2

Flow Cycling Fixed Bed Catalytic Reactor

This is a single bed gas/solid exothermic reactor where the gas flow direction is
periodically reversed to maintain a high temperature zone in the middle of the
reactor bed and lower temperatures at the inlet and outlet. Choice of particle size
and bed dimensions and cycle time enable appropriate relationships between
bed heat capacity, reaction rate and heat transfer. The temperature reduction
between the middle of the bed and the outlet enables better equilibrium
conversion to be obtained.

This reactor has advantages when:
•
•
•

The reaction is exothermic and uses a heterogeneous catalyst.
The reaction is equilibrium limited by product formation.
Heat recovery is not important either because
o Small scale of plant - can’t afford complication
o or exotherm is small or reaction is at low temperature

The reactor has been commercialized for gaseous waste treatment for removal
of VOCs, N0x and S02.
11.1.3

Plate Reactors

The catalyst is coated onto plates so that the reactor has a dual function as a
plate heat exchanger. Of interest when:
The Catalyst is highly active.
The reaction is highly exothermic or endothermic i.e. tubular reactor is currently
used.
Small scale is appropriate.
The Catalyst is regenerable or has a long life.
11.1.4

Regeneratively Heated Reactor - Houdry Catofin Process

Uses two packed beds. One bed is heated and cleaned up using clean fuel and
air while the other bed does the catalytic reaction. Beds are switched after a time
interval. Used for endothermic reactions which give carbon laydown or fouling.
Considered as alternative for GHR on some Ammonia plant flowsheets.

11.1.5

Concurrent Downflow Contactor - Patented 1976 - Birmingham
University

Of advantage for thermally neutral reactions where heat exchange isn’t needed –
avoids circulation loop. Must have complete conversion of inlet gas. Can be used
for gas liquid reactions over fixed bed catalysts.


11.1.6

Non-Catalytic Packed Bed Reactor

Homogeneous gas phase reaction proceeds within a packed bed of balls etc. It
allows better distribution of the gases and avoids diffusion flames. Used for
incineration to give low N0x combustion and more certain combustion. It can be
controlled over a wide range of combustion conditions.
It has the advantage that it is not classed as an incinerator by the Environmental
Agency, although if will achieve the same effect.
Has been considered as an option instead of using an incinerator. General
application for burning low calorific value gas.
Possible process application is HCN. It is commercially available from
Thermatrix.

11.1.7

Catalytically Coated Surface Reactor

Catalyst is applied to a stirrer in a stirred tank reactor or to a static mixer in a plug
flow reactor to provide initiation of a reaction.
Of interest when:
•
•

Only a small amount of catalyst is needed - e.g. for initiation
No catalyst is required in the product - e.g. polymers

Further development could be to use an electrically heated stirrer or static mixer
to maintain the catalyst hot without having to heat up all the reacting liquid.
Was considered by GBHE. Possible applications are liquid phase
hydrogenations.


11.1.8

Rapid Cycling Regeneration Reactor

Each active catalyst molecule only turns over one molecule of reactant before
being regenerated. The catalyst in this case is sometimes called a
’cataloreactant’. The surface layer of catalyst will be transformed from one state
to another by performing the reaction and will then be regenerated by using
another reactant. This can be done by moving the solid from one reaction vessel
to another and back again, or by having two or more fixed catalyst beds in which
the feeds are cyclically switched. Most academic work is done using the former
system, but this is very expensive to build at full scale, so full scale plants are
likely to be cycling fixed beds.
The technology is applicable to reactions, where the selectivity would be low if
the feeds were mixed together over the catalyst bed. The most typical application
is oxidation reactions. It is only likely to be economic, where existing routes to a
product are not viable or are very expensive.
This type of reactor has been evaluated by GBHE for oxidation of HCI to chlorine
and for the production of aniline.

11.1.9

Hot Spot Reactors/Catalytic Partial Oxidation Reactor

Reactants mixed together within a catalyst bed or immediately upstream of a
catalyst bed can give different products than reactants being premixed and
undergoing homogeneous reaction before entering a catalyst bed.
Of interest for:
•
•

Exothermic reactions involving combination of homogeneous and
heterogeneous reaction.
When high selectivity is difficult to achieve at present.


11.2
11.2.1

Liquid and Liquid/Gas Phase Reactors
Porous Wall Rate Controlled Reactor

The reaction rate between two liquid or a liquid and gas reactants is controlled by
using a porous metal barrier to control the rate of mixing between reactants. It
has an advantage over stirred pot reactors and other mixing reactors that the
concentration of the reactant that passes through the porous barrier is always
low in the reacting mixture.
Of interest when:
•
•
•
•

Two liquid or one liquid and one gaseous reactant.
There is a need to avoid high local temperatures, or a need to maintain
one
component in excess to avoid by-product formation.
Total exotherm can be accommodated in one (or two) adiabatic reactors.

(Not clear what the advantage is over an intensive reactor mixer).

11.2.2

Pulsatile Plug Flow Reactors

Used to give plug flow without using long thin pipes by pulsating the flow. Plug
flow is most important for product quality where solids are produced and it is
important to minimize by-product formation by achieving a narrow residence time
distribution.
Two versions are available:
Mackley Oscillatory tube

AEA Fluidic Plug Flow Mixer
This has been commercialized, but maximum size to date is 250ml.


11.2.3

Vortex Mixer

This is a static mixer which uses the pressure drop in the fluids to give intensive
mixing. It is claimed to give low local shear compared to other intensive mixers,
but it is not clear why this should be so.
Of interest when:
•
•
•
•

11.2.4

Mixing liquids or liquids and gases.
Fast reactions when short mixing times are needed.
Rapid dispersion of small volumes are needed to avoid byproduct
formation.
Rapid uniform mixing is needed for instance in precipitation reactions.

Gas Lift Loop Reactor

The recycle in a liquid/gas phase reactor is achieved by utilizing the lift from the
gas, which could be a reactant or product.
Applicable when:
•
•
•

11.3
11.3.1

Liquid/gas phase reactors.
Kinetics are slow, so large liquid volumes are needed.
Liquid recycle is needed.

Reactor/Separation Combinations
Exothermic Catalytic Condensing Reactor

This is a multi-functional reactor for a gaseous exothermic reaction, with a fixed
catalyst bed that uses a cold surface to condense reaction product inside the
reactor. The total heat from the reaction exotherm is taken out at the product
condensation temperature, so the scope for heat recovery is limited. The heat
transfer to the cold surface is by natural convection. This is
applicable when:

•
•
•

11.3.2

The reaction is exothermic and uses a fixed bed catalyst.
The reaction is equilibrium limited by product formation.
Heat recovery from gas is not important either because:
o The plant is a small scale one.
o The reaction temperature is within, say, 80° C of the condensation
temperature.

Adsorbent Reactors

Reaction products removed by adsorbent to give higher conversion.
Alternatives are:
•

Equilibrium-Limited Periodic Separating Reactor
o a fixed bed using pressure swing.

•

Gas/ solid/solid trickle flow adsorbent reactor
o the absorbent is circulated through a regeneration process.

Applicable when:
•
•

Low conversions with current technology and
Difficult to separate products with current technology.

11.3.3 Supercritical Fluid Reactors
Reaction takes place at supercritical conditions of reactants or reactant medium.
Applicable when:
•
•
•

Reaction takes place in inert liquid medium.
Preparation of reactants is difficult.
Difficult to separate products from reactants or reaction medium.


11.3.4

Ceramic Electrochemical Reactors

Oxygen can diffuse through ceramic oxides at high temperature to give low
oxygen concentrations when it comes into contact with another reactant. This
means that the other reactant is always in excess and avoids formation of
combustion or other byproducts. Of interest for:
•
•
•

•

Oxidation or dehydrogenation reactions using oxygen as a reactant.
Current technology gives poor selectivity.
Reaction is endothermic or mildly exothermic.
Reaction can operate at greater than 750°C.

Originally developed for solid oxide fuel cells.

11.3.5

Microwave Heating of Reactors

The use of energy derived from electricity as a continuous heat source is
generally uneconomic at the scale at which some processes work. However, the
use of electrical derived heating can be a good option for intermittent use.
Reactor vessels or the catalyst itself in fixed bed reactors can be heated using
microwaves. This could give the following benefits:
•
•
•
•

an alternative to the use of separate start-up heaters.
to avoid condensing steam on catalysts during start-up.
supplying heat evenly during catalyst regeneration so that heat does not
have to be supplied by the regeneration fluid stream.
to evenly preheat furnace boxes or maintain them at temperature during
temporary shut-downs.

By tuning the microwave frequency, the heat can be directed specifically at the
catalyst support or at the active species. Microwave units up to 1-2MW are
available depending on microwave frequency.


11.3.6

Induced Field Catalytic Reactors

By putting a catalytic reactor in an electric field, polar reactants may be made to
line up with the catalyst at a constant predetermined orientation. This will enable
a highly selective reaction or reaction where it would otherwise not occur.
Applicable for:
•
•
•

11.3.7

Liquid phase reaction
Polar reactants
Kinetics very slow using current technology.

Rotating Cone Calciner

Applicable when:
•
•
•

Small throughputs required.
Low residence time required.
Low residence time distribution required.


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