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Industrial Electrochemical Synthesis Processes Recent Developments in Reactor Design.

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Industrial Electrochemical Synthesis
Processes: Recent Developments in
Reactor Design
Part I: The Scope of Electrochemical
K. Scott
Department of Chemical and Process Engineering, University
of Newcastle-upon-Tyne, Newcastle-upon-Tyne, NE 1 7RU, UK
This review considers the design methods and recent developments in reactor
selection for industrial electrochemical synthesis. The scope of electrosynthesis is
reviewed, together with the reactor technology required to provide commercially
viable operation. Improvements in reaction engineering techniques as applied to
electrochemical synthesis, both inorganic and organic, will continue to provide
developments in this valuable and important sector of the process industries.
Part I provides a classification of electrochemical synthesis reactions in terms
of direct and indirect processes, and reviews the application of electrosynthesis in
the production of organic and inorganic chemicals.
Part I I considers the important design factors in the development of
electrochemical processes and their limitations. These include aspects of
productivity, energy requirements, mass transport, electrode and membrane
Part III The final part of this review describes the design of electrochemical
reactors used f o r both organic and inorganic synthesis, to ensure that the
electrochemical cell is an efficient and economic unit for industrial use. The
designs include reactors which use either two-dimensional or three-dimensional
electrodes. Recent examples of the use of such reactors are described. A brief
overview of reaction engineering principles is included in the context of both
single-phase and two-phase fluid systems.
Electrochemistry and its applications play an important role in the industrial and
commercial world. The applications are diverse and ever increasing and cut across
many d i s c i p l i n e s . A n overall perspective o f the s c o p e o f industrial
electrochemistry is summarised as follows:
- Inorganic electrosynthesis,
- Extraction and production of metals,
- Organic electrosynthesis,
- Metal finishing and processing,
- Water purification and effluent treatment,
Developments in Chemical Engineering and Mineral Processing,Vol. 1, No 2/3,page 71
K. Scott
- Energy generation,
- Corrosion,
- Sensors and monitors.
The field of inorganic electrosynthesis is well established and has a high
profile, for example, in excess of 4,000 ton per hour of chlorine is produced by
the chlor-alkali industry world-wide, in conjunction with sodium hydroxide and
hydrogen. This high tonnage bulk chemical operation has changed radically over
the past decade, and has resulted in many advances and improvements in cell
design, electrodes and materials. These developments have had an overall positive
effect on electrochemical process technology in general.
Many other inorganic processes are in operation, and currently there is
somewhat of a revival in this technology, due to: (i) demand for on-site production
of relatively small quantities of inorganic chemicals as reagents, erc.; and (ii) the
current emphasis on reducing effluent emissions and recycling materials.
The second aspect comes under the category of water purification and effluent
treatment which will be the focus of a subsequent review to be published in this
The production of metals, notably aluminium, from their ores by molten salt
electrolysis’ is a major aspect of the metal extraction and production industry.
Secondary to these operations is metal production from aqueous electrolytes,
particularly copper and zinc, i.e. hydrometallurgical processes. Cell designs for
these two areas of operation are quite different, with the former not surprisingly
being quite unique due to the high temperatures of operation (around l,OOO°C).
Cell designs for hydrometallurgical processes and other electrochemical syntheses
however, possess a number of similarities. This is evidenced by electrowinning
and electrorefining operations,2 where the only major difference is the use of
either soluble anodes of the metal to be refined or insoluble anodes.
This review of electrochemical technology and engineering is in the context of
chemical synthesis, but particular mention should also be made of the potentially
lucrative area of organic electrosynthesis. Growth in this area has never quite
matched predictions, and some might say that with the coming demise of lead
tetra-alkyl processes (anti-knock petrol additives) there will be no net growth.
However, the number of individual processes with an electrochemical organic
synthesis step is steadily increasing. In this context electrochemistry must be seen
to be no different from any other method of synthesis, and it is an effective means
of performing a ‘chemical’ reaction. Its adoption must rely on performance and
economic comparisons with the alternatives, and not be prejudiced by lack of
knowledge of the subject area and the reluctance to turn to a ‘less traditional
technology’. This can only come about if electrochemistry, and its applications for
synthesis reactions, receives sufficient exposure in industrial chemistry to remove
the unmerited label of a black art.
This review will provide an overview of industrial electrochemical processes,
both inorganic and organic, and discuss some of the recent developments in
reactor selection and design. The areas of application of electrochemical synthesis
are therefore reviewed in conjunction with the associated cells. The methods of
electrosynthesis fall into several categories, these are outlined with the special
requirements of engineering design and scale-up. The scope of this review must
be limited by journal size and therefore a number of aspects are omitted, some
intentionally! Amongst these is the area of molten salt electrolysis, and aspects of
the chlor-alkali industry. The areas of energy generation (batteries and fuel cells),
corrosion and protection, sensors and monitors are not discussed.
Industrial Electrochemical Synthesis Processes
Categories of Electrochemical Processes for Synthesis
Electrochemical processes can be divided into two general categories, either
inorganic or organic synthesis. The basic reason for this division is that inorganic
reactions generally involve highly ionisable salts in high concentrations, in
aqueous electrolytes or molten salts. Whereas, organic reactions frequently
involve either weakly ionisable species, or species which have limited or low
solubility in aqueous electrolyte. This division means that certain aspects which
are very important in, say, organic synthesis are less so in inorganic synthesis. A
notable and perhaps obvious alternative differentiation is by the conductivity of
the reacting solution, which is usually high for inorganic synthesis and low for
organic systems. The process energy usage may then become an over-riding
concern in organic synthesis, with the reactor design in particular focusing on this
aspect. However, with the manufacture of bulk (low value) chemicals by
electrolysis, the consumption of energy is the overall concern.
Electrosynthesis has long been established as a method of inorganic chemical
manufacture. Such processes are well known and involve relatively
straightforward reactions. This is usually achieved by good design of the reaction
chemistry, rather than being inherent in the chemistry. The selectivity of these
reactions is often high, whereas with organic electrosynthesis a range of products
can often result from one compound. This also makes product work-up more
difficult, due to both the removal of the organic species from aqueous electrolyte
and further processing required to separate the organic species. However this is
not a feature peculiar to electro-organic synthesis but is applicable to organic
synthesis in general. One of the advantages of performing organic synthesis
electrochemically can be an improved selectivity over catalytic or chemical
One area where inorganic electrochemistry and organic synthesis overlap is for
processes which utilise inorganic oxidising agents, reducing agents, nitrating
agents, etc. Considerable interest is now being shown in the electrochemical
regeneration and recycling of a range of such mediating agents for organic
Classifications of electrochemical processes
Electrochemical reactions are surface processes, they are instigated by a suitable
charge transfer at a fluid-solid interface. This solid electrode may be either metal,
carbon, semiconductor or conducting polymer. Although the physical chemistry of
the charge transfer is complex, it is possible and useful to present a simple picture
of the interfacial processes. This will allow a classification of electrochemical
Two electrodes become charged when they are placed in an ionic conducting
solution and connected externally. Thus locally at the electrode-solution interface,
there is a large potential difference occurring over a molecular scale of a few
nanometres. A simple model of this situation' consists of a double layer,
comprised of a plane of closest approach (i.hp.) and a diffuse layer or outer layer
(see Figure 1). These electrostatic interactions determine the distribution of
potential difference, which itself constitutes the driving force for electrochemical
reaction when a current flows through the circuit. An increase in magnitude in the
potential applied between the two electrodes increases this potential difference
between electrode and solution, i.e. it is the driving force for reaction. The
mathematical description of the relationship between the electrode-solution
K. Scott
Figure 1 Model of the interfacial region between electrode and electrolyte.
potential difference (measured as an electrode potential) and the current (or
current density) is referred to as the electrode kinetics.
Considering the electrode-solution interface and the processes which occur,
this provides a means of classifying the electrochemical process. A normal
electrode process (outer Helmholtz plane) is when a reactant A never comes into
direct contact with the electrode surface (see Figure 2a). When a reactant is in
contact with the surface, i.e. strongly adsorbed, then an inner Helmholtz plane
charge transfer mechanism is operative and this is referred to as electrocatalysis
(see Figure 2b).
The reactions are usually with an intermediate adsorbed charged state:
Any chemical steps are catalysed by interactions with the surface of the
electrocatalyst, rather than by electrochemical steps. Electrocatalysis is the
analogue of classical heterogeneous catalysis, in that materials can be selected to
effectively overcome a slow rate by providing an alternative reaction pathway.
This essentially enables high current densities to be used at potentials closer to the
reversible potential. Just as important is the ability of the electrocatalyst to change
the reaction selectivity.
Industrial Electrochemical Synthesis Processes
(Normal outer helmholtz plane
electrode process)
(Redox layer serves as electron transfer mediator)
Figure 2 Categories of electrochemical processes: (a) outer Helmholtz plane
electrode reaction; (b) heterogeneous electrocatalysis, inner Helmholtz plane
charge transfer; (c) heterogeneous redox catalysis.
K. Scott
Redox catalysis
Redox catalysis3 is a special case of indirect electrochemical reaction in which
electron transfer is indirect through a redox system and not direct at the electrode
surface (see Figure 2c). This is a two step process which may be seen from an
engineering stand-point as chemical, but mechanistically it is electrochemical. If
the redox agent is locally regenerated, in low concentrations in a region close to
the electrode, this is homogeneous redox catalysis. However, if the redox system
is fixed to the electrode surface, this is referred to as heterogeneous redox
catalysis. The redox system (Figure 2c) serves as an electron transfer mediator:
It also provides a new surface for adsorptive interactions. The advantage of
this latter redox process is the elimination of the redox species from the
electrolyte, with potentially simpler process operations in product recovery, cell
operation and effluent management. There are many applications of redox
electrocatalysis in electrochemical synthesis, although these are not widely
adopted in an industrial context. One specific example is that of lead dioxide in
anodic oxidation^.^
The adoption of redox mediators is becoming increasingly important in
electrochemical synthesis and a large range of couples have been used (see
Table 1). Indirect electrochemical reactions can be classified as either in-cell or
ex-cell processes as shown in Figure 3.
In the ex-cell process (see Figure 3b), reaction between mediator and substrate
occurs in a chemical reactor separate from the cell. Following reaction, electrolyte
containing used mediator and the second product phase are separated, and the
electrolyte phase is returned to the cell for redox regeneration. This is often
preferred in order to minimise contact of (say) the organic phase with the
electrode. Such contact can result in electrode fouling and de-activation when a
two-phase medium is ‘reacted’ in-cell.
‘In-cell’ processes (see Figure 3a) also occur when the mediator and reactant
phases are soluble in the electrolyte phase. Generally, in both the in-cell and
ex-cell cases, if reaction is slow then the cell must include a separator between the
anode and cathode to prevent the counter-electrode causing electrochemical
back-reaction of the redox agent. A strategy which has been used to try and
eliminate the need for a cell separator (and thus reduce the cost) is to introduce a
phase-transfer agent into the emulsion phase, this selectively removes the active
redox agent after re-generation. This has been applied in the anodic oxidation of
Ce(II1) to Ce(1V) using DEHPA in k e r ~ s e n e . ~
The Scope of Electrochemical Synthesis Processes
This section provides an overview of industrial electrochemical synthesis
processes, included are those syntheses which have undergone significant process
development. Over recent years there have been a number of reviews of
electrochemical processes and several text books partially or almost entirely
devoted to this task.’’ ** 6-11 This apparent wealth of information is recommended
reading for those in search of greater detail.
Industrial Electrochemical Synthesis Processes
Table 1 Redox couplesfor indirect electrosynthesis.
cr3+/cr207 2Ni(OH)/NiOOH
Figure 3 Homogeneous redox catalysisfor indirect electrochemical reactions:
(a) in-cell process; (b) ex-cell process.
K. Scott
This overview will attempt to give a sufficient flavour of electrochemical
synthesis to demonstrate the, as yet, untapped potential of this technology. This
potential is clearly only tenable when electrochemistry is seen to reduce overall
process costs. Electrochemistry can achieve this overall goal, in competition with
chemical, catalytic and biochemical processes, due to its many inherent
characteristics, such as:
- Mild conditions of operation, e.g. low temperature and pressure,
- Improved selectivity of existing reactions,
- Availability of novel chemical transformations,
- Reduction in the number of synthesis steps,
- Improved management of potential pollutants,
- Avoidance of aggressive and hazardous reagents,
- Use of alternative feedstocks.
The syntheses cited in this section (and throughout this article) which have
been commercially adopted, have all benefited from one or more of these
Organic electrosynthesis
A wide range of electro-organic synthesis reactions have been demonstrated in
laboratories around the world. These include hydrogenation, oxidation,
substitution, reduction and oxidative coupling, and cleavage, cyclization and
polymerisation. New electrochemical analogues to organic chemical reactions
involving electron transfer continue to be developed, the vast majority of which
use aprotic electrolyte media. Amongst other factors, this offers a wider electrode
potential range for transformation than aqueous-based media. However,
aqueous-based electrosynthesis often results in less complex electrochemistry,
better pH control (through the splitting of water), and less problems due to solvent
loss. This has seen aqueous-based electrolytes take a predominant role in
industrial electro-organic synthesis.
The types of electrochemical reactions used extensively in industrial synthesis
- Reduction of carbon-carbon double bonds,
- Reduction of carbonyl groups,
- Reduction of nitrile or nitro groups,
- Reductive coupling,
- Reductive cleavage,
- Oxidation of hydrocarbons,
- Oxidation of functional groups,
- Oxidative coupling,
- Oxidative substitution,
- Electrochemical fluorination,
- Indirect oxidation or reduction.
From this potentially vast catalogue of syntheses there are approximately sixty
commercial electro-organic syntheses in operation'' (see Table 2), with about
twice that number having reached the pilot scale. The majority of these are small
tonnage operations, with the notable exception of the production of adiponitrile
from acrylonitrile (electro-hydrodimerisation).
The economics of electrochemical processes are now well understood and it is
accepted that only in the case of large tonnage low-value chemicals is the cost of
electrical energy a major factor. Even then the improved performance of
electrosynthesis can outweigh the cost of supplying the clean electrode reagenL6
Industrial Electrochemical Synthesis Processes
Table 2 A selection of commercial electro-organic syntheses.
1. Hydrodirnerization
CI+ C H ~
Two phase aqueous
phosphate buffer
+ actylonitrile.
Undivided bipolar
parallel plate cell.
Cd cathode, steel
anode, i=2000 Am-2
Pb cathode inH,SO
medium. Divided ce?l.
Pb cathode in H,SO,
medium. Filterpress
divided cell.
Pb cathode in acid
sulphate medium,
Divided cell including
high-area cathode
i=5-20 rnA crn-2.
Pb cathode in aqueous
Pb cathode in acid
Pb cathode in H2S04
Dioxan as cosolvent
Divided filterpress cell.
Pb cathode in H2S04.
heterocycles, eg.
3. Hydrogenationof
Reduction of
carboxylic acids
K. Scott
5 . Cathodic cleavages
Acidic CH3CN/H20.
Pb cathode. Divided
filter-press cell.
Current Density = 1500
A m-2.
H,O - no electrolyte.
Product precipitates.
Zn cathode.100 A m-2.
Divided FM21 cell.
Pb cathode in acid.
Divided cell.
Roughened Ag cathode
in aqueous acid.
Nitrogroup reduction
Pb cathode in acid
Stronger acid.
membrane cell
and conditions
Simons process. Ni
anode in liquid HF.
NaBr/CH OH in narrow
gap, bipolar C disc cell.
i=1500 A m2.
NaBrlCH30H. C cylinder
Base generation
8. Fluorinations
Perfluorination of RCOOH,
9. Methoxylations
anode in steel pipe
cathode 1 mm gap.
i=lOOO A m-2.
Undivided bipolar
parallel plate reactor.
Industrial Electrochemical Synthesis Processes
10. Oxidation of
aromatic hydrocarbons
Indirect via Cr@-,
Pb anode in divided
parallel plate reactor
CH30H/1% KF.
Undivided C disc cell.
i=400-1000 A m-*
pipe cell.
PbO, anode in H2SO4/
Pb shot anode.
Indirect via Cr2072-
Pt anode in methanol.
Acid partially
neutralized but no
electrolyte added.
Indirect oxidation via
Br2. Acid precipitated
as Ca2+ salt
11. Oxidation of
methyl aromatics
C tube in
12. Kolbe coupling
of half esters
13. Sugar Chemistry
Glucose---* Gluconic
Glucose ---*
Pb cathode in aqueous
K. Scott
Table 3 Inorganic electrochemical processes.
Al, Na, Mg, Li
Cu, Zn, Cu, Ni, Cr, Pb
Cd, Mn, TI, Ga, In, Ag, Au
Manganese dioxide
Water electrolysis (H2,02)
Hydrogen peroxide
Chromic acid
Cuprous oxide
Potassium stannate
Chlorine dioxide
Molten salt electrowinning
Electrowinning or refining
Noble metal oxide anode, brine electrolyte
Noble metal oxide anode, brine electrolyte
P n i , PbO, anodes, chlorate electrolyte
m i anode, conc. H SO,
DSAR, aqueous
Ni, monel anode, Kh4n04 electrolyte
Carbon anode, KF/2HF eutectic
C, Pb, Ti anodes, MnS04
Ni on steel, KOH
Carbon cathodes, NaOH
Vitreous carbon anode, conc. aqu. HBF4
C, Pt/Ti, PbO,, aqu. NaBr
Lead anode, Cr(II1) in H2S04
Copper, aqu. NaCl
Anodic dissolution
DSAR, carbon cathode, sodium chlorate and HCI
There are a number of important features of the syntheses cited in Table 2
which indicate the versatility of electrochemical processing. These include the
wide range of electrolyte solutions and electrode materials used, most of which
are chosen by experimentation at the laboratory stage, although operational
difficulties on scale-up often necessitate material re-evaluation. Reactions can be
performed with two-phase gas-liquid media and two-phase liquid-liquid media
using either direct or indirect electrochemical paths. This can be one of the initial
choices made in the selection of a specific route to a chemical. It is not the
intention here to provide detailed discussion of individual syntheses, but merely
to create an awareness of their scope. Later sections will provide more details of
specific processes as required.
Inorganic processes
The scale of operation of the chlor-alkali industry and metal- winning processes
makes inorganic processes the dominant area in electrosynthesis. Developments
in these technologically advanced areas have produced valuable spin-offs for
electrosynthesis as a whole, notably i n electrode and membrane material
availability, and for cell design. The introduction of dimensionally stable coated
metal anodes, and resilient cation-exchange membranes, have made many
syntheses economically viable, and there is now a resurgence of interest in
inorganic processes.
Table 3 gives an indication of the range of processes in operation, some of
which have been developed for wastewater and effluent management (the subject
of a subsequent review to be published in this journal). Among the chemicals
listed in Table 3, several are non-chlorinating bleaching agents (peroxide, ozone,
persulphate) used to a small extent in place of chlorine, as is chlorine dioxide. A
Industrial Electrochemical Synthesis Processes
novel electrochemical route for producing high-purity chlorine dioxide and
co-generating sodium hydroxide has recently been evaluated.' Important features
include the use of a general purpose electrolyser (Electrocell AB), Nafion
cation-exchange membranes, and a fixed-bed cathode of graphite particles.
The use of industrial electrochemical synthesis is relatively large in spite of a lack
of real awareness of the benefits it affords, and the developed design methods
available to the process engineering sector. Electrochemical synthesis has often
been used as the last choice, as for example in the chlor-alkali industry, rather than
as an important first choice to be considered with catalysis and chemical
synthesis. However, once an electrochemical process is established it is hard to
dislodge, and this is testimony to what may appear to be a somewhat personal
Received: 4 November 1991; Accepted: 15 November 1992.
Part II: Design Factors for
Electrochemical Synthesis
The majority of the process design factors in electrosynthesis are common to
synthesis in general, and the engineering and reactor design principles are equally
applicable. However, the necessary supply of electrochemical reagent (i.e. the
electron) at a surface can impose certain unique design aspects, some of which are
discussed here.
Regardless of the type of synthesis, electrochemistry requires at least two
associated reactions at two separate surfaces to effect the flow of electrons. This
universal heterogeneous nature of electrochemistry as performed in the liquid
phase, imposes certain limitations and specific requirements for design. These
requirements can be categorised under the following headings:
- Productivity,
- Cell voltage,
- Temperature control,
- Hydrodynamics and mass transport,
- Reactor operation factors,
- Electrode, membrane and other materials,
- CelVreactor design.
None of these items can be considered entirely as a separate entity,
considerable interaction exists between the controlling parameters of these
Production Capacity
Electrochemical cells
achieved, and they are
Production capacity is
current per unit area of
as :
are limited by the production capacity which can be
also restricted by a number of important design criteria.
related directly to the applied current density (i.e. the
electrode surface, i = UA), and is conveniently expressed
Capacity = [i(CE)]/[nF 1001 mol m-2 s-'
where CE is the current efficiency of the reaction and is usually set a target
approaching 100% to maximise performance; n is the number of electrons
required to achieve the species transformation, often providing the key to success
or failure of commercial electrosynthesis. The capacity of a cell is often expressed
in mol per unit time per unit volume, obtained by multiplying equation (1) by the
active electrode area per unit active volume (6).The active volume is the solution
volume where reaction changes the composition. This is the space time yield
(STY) of the electrochemical reactor, defined as:
STY = [oi(CE)l/[nF 1001 mol m-3 s-'
Developments in Chemical Engineering and Mineral Processing, Vol. 1, No 2/3,page 84
Industrial Electrochemical Synthesis Processes
The area per unit volume available thus restricts the output from the reactor.
There are a number of inter-related factors which limit 6,and therefore the
production capacity. Interacting with this parameter is the current density, which
itself is limited by several factors, not the least of which are reaction selectivity
and efficiency. Values of current density used i n synthesis vary from
approximately 100 to 10,000 A m-2 (based on the superficial area of the
electrode), this is testimony to the range of production capacity available to
Energy Requirements
Multiple electron transfer reactions will be large energy users, and will often
become less competitive when compared to catalytic routes. For example, in the
anodic oxidation of benzene to p-benzoquinone (a six electron process), energ
estimates for the overall process prescribe approximately 75% for cell operation.?I .
In comparison, the process energy requirements of a chemical route are about 60%
lower than that of the electrochemical route. The significance of electron demand
per mol of species is clear.
It is worth stressing that the cell energy requirements for p-benzoquinone
production (36,000 kWh t-') are not representative of electrochemical processes
in general. For example, cell energy consumption in chlor-alkali cells or
adiponitrile cells are approximately 2,500 kWh t-'. Factors which primarily
govern energy requirements are the operating current density and cell resistance.
These two factors determine the cell voltage (E,) which is dependent upon a
number of contributions associated with the electrochemical process, and the cell
internal resistances (Re). The relationship can be conveniently written as:
The cell energy requirement is given by:
Energy = [nF E,]/[(CE)(3.6 x lo6)] kWh mol-'
The current density determines the degree of electrode polarisation, and thus
the magnitude of overpotentials (q). Clearly a higher current density or production
capacity gives rise both to a larger resistance and polarisation losses for a given
system (see Figure 4). To make greater use of a given electrode area and to
increase the capacity, a greater energy requirement is demanded. The result is the
familiar economic 'trade-off' for industrial operation between decreasing capital
cost and increasing energy cost, with increasing capacity or current density
resulting in an economic minimum cost (see Figure 5). Reductions in energy
costs, or more specifically the cell energy requirement, at a fixed current density
can be achieved in the following ways.
(1) Using highly conducting electrolyte solutions. Practical constraints can arise
due to limitations in material properties, and from specific interactions between
electrode and electrolyte for effective synthesis. In general, highly concentrated
aqueous solutions and molten salts are suitable materials.
(2) Minimising the space between the electrodes (the inter-electrode gap). There
are practical limits imposed on this procedure due to several factors, such as: (i) a
need to avoid contact between opposite electrodes, and hence short circuiting;
K. Scott
S E PA R A T 0 R
E, +
Figure 4 Relative contributions to cell voltage (Ecell).(EA:anode potential; E,:
cathode potential; iR: voltage terms due to resistance).
Figure 5 Determination of optimum current density for process operation.
(ii) the requirements of gas evolution reactions (the most commonly occurring
reaction at one electrode in a cell), which can generate a substantial rise in internal
resistance due to gas bubble hold-up; (iii) the need for good mass transport
characteristics to the electrode, often induced by relatively high flow velocities,
which in turn can demand significant amounts of energy for pumping.
Industrial Electrochemical Synthesis Processes
Figure 6 Electrode designs with gas release.
Solid Polymer
Electrolyte (S.P!E.)
Figure 7 Principle of solid polymer electrolyte cell.
The gas bubble problem can be tackled by using open electrode structures and
designing for gas release at the back surface(s) of the electrode, as occurs in the
chlor-alkali cells (see Figure 6).
(3) Using solid polymer electrolytes (SPE).','' as shown in Figure 7. The
technology is at a reasonably developed stage for inorganic synthesis with
K. Scott
applications in water electrolysis (CJB Developments Ltd) and in the generation
of ozone:
3H20 - 6e-
+ 0, + 6H’
For water electrolysis, at a nominal cell voltage of 2V, the current carrying
capacity can be increased from 2,500to 10,000 A m-2 using SPE.
For organic electrochemistry, the technology is at an early stage of
development. A number of organic syntheses have been studied, including the
methoxylation of furan, alkoxylation of N-alkyl-amides, and the oxidation of
alcohols to aldehydes, ketones and acids. l 3
The procedure requires good ionic conductivity of the polymeric-based
ion-exchange membranes, and is particularly suited to poorly conducting
organic-based electrolytes. Product selectivity benefits can also be realised due to
the effect of electro-osmosis.
(4) Modifying the overall cell reaction chemistry. Here the counter-electrode
reaction is replaced by a thermodynamically more favourable reaction, and in
particular by a reaction which is not ‘driven’ but is ‘spontaneous’. A typical
example is the reduction of oxygen using so-called gas diffusion fuel cell
electrodes, these have been suggested for chlor-alkali electrolysis and for ozone
generation. A reduction of approximately 1.2 V is theoretically possible when
these electrodes are used to replace the hydrogen evolution cathodic process. Fuel
cell electrodes have also been suggested for reacting gaseous species other than
0 2 and H2 in electrochemical liquid-phase synthesis. Examples include the
epoxidation of propylene, the oxidation of propylene to acrolein and acrylic
acid,I5 and in the production of methyl ethyl ketone.16 These oxygen reduction
electrodes are easily manufactured [e.g. ref.151, and can be purchased from
commercial organisation such as the Electrosynthesis Company.
Temperature Control
The supply of electrical energy to an electrochemical cell usually gives rise to the
generation of excess heat, due primarily to sensible heating of the electrolyte (and
heat of reactions). The design of the reactor must consider the required heat
transfer characteristics in order to limit, control or capitalise on the resulting
temperature changes. The cell reactor type and its mode of operation play a
significant role; a particularly good example is in molten salt electrolysis, e.g.
aluminium electrowinning. Temperatures in excess of 900°C are used and
electrolytic heat generation must balance the heat losses in order to maintain a
stable melt, among other factors.
The role of heat transfer has important implications in the performance of
electrochemical reactors, for example:
(1) The attainment of good reaction selectivity may be limited to a small
temperature range and thus appropriate heat removal may be required.
(2) Heat generation may result in partial or continuous non-steady state operation.
In the case of continuous reactors this applies to start-up, and shut down for direct
(3) With continuous flow cells, or cells operated in batch recycle, there may be
limitations in reactant conversion or applied current density due to restrictions in
Industrial Electrochemical Synthesis Processes
operating temperature. Estimation of the temperature(s) of operation can be
achieved through an approximate adiabatic heat balance as illustrated in Figure 8.
(4)Temperature contro! is generally via external heat exchangers, and is not built
into the reactors which have a small volumetric hold-up. The use of a flowing
electrolyte is thus convenient.
( 5 ) Operating temperatures above ambient due to Joule heating, can offer
increases in electrolyte conductivity, mass transport rate and kinetic rates.
Reactors which require little, or no, external heat exchange are an advantage. The
reactor for the production of adiponitrile (to be discussed later) is an example of
this design philosophy, where a temperature of approximately 5OoC is maintained.
Figure 9 Rate processes at an electrode surface.
K. Scott
Hydrodynamics and Mass Transfer
The heterogeneous nature of electrochemistry makes intraphase mass transport
important in cell operation and design, In the simplest case, an electrochemical
reaction can be viewed as a succession of three rate steps (see Figure 9), as
(1) Mass transport of reactant to the surface.
(2) Electrochemical transformation.
(3) Mass transport of product from the surface.
Therefore, both the ‘kinetics’ of electron transfer and the mass transport
influence the overall rate. With increasing electrode polarisation, the electrode can
become starved of reactant and the rate of mass transport governs and limits the
overall reaction rate (see Figure 10). The mass transport of species is partly
determined by the hydrodynamics of electrolyte flow in the vicinity of the
electrode, i.e. the convective movement of species. In addition, two other modes
of mass transport exist, namely:
Diffusion: the movement of species under a concentration gradient, brought about
by electrode reaction.
Migration: the movement of species under a potential field. This is essentially the
mechanism of charge transfer i n an electrolyte and the means by which
electrolysis occurs. If the reacting species is not in a high concentration (i.e.
excess supporting electrolyte), then this contribution to mass transfer is often
In general, mass transfer is controlled by the selection of the cell or reactor. It
can be varied by selection of suitable operating parameters, such as electrolyte
velocity or electrode rotation.
Reactor Operational Factors
Electrochemical reactors can be operated in either batch or continuous mode, with
the scale of operation usually being the deciding factor. The type of reaction can
also be a contributory factor, e.g. electrowinning from aqueous solutions. The
-Mixed control
Industrial Electrochemical Synthesis Processes
need to supply current to the cells gives rise to two other factors. First, whether
to adopt a monopolar electrical connection (see Figure l l a ) with all electrodes
buzzed to the power supply, or to adopt a bipolar connection with only the end
electrodes of each cell bank electrically buzzed (see Figure llb ) . Bipolar
connections involve lower costs due to the simplicity of the connections and the
lower cell currents used (at higher voltage) which can lead to lower costs for the
DC power supply. These advantages must be balanced against power losses
associated with leakage currents (i.e. current which ‘bypasses’ the electrodes and
flows solely through the electrolyte), and the cost of fabrication of the anodes and
cathodes into suitable composite bipoles.
Second, whether or not to attempt potentiostatic control of the reactor, i.e. a
fixed electrode potential, to give improved selectivity. In general, unless it is a
small scale synthesis, the practical difficulties of control via electrode potential
measurement, and the cost of the power supplies relative to DC supplies, make
operation based on a fixed value of current the appropriate method.
The number of individual cells used in any reactor module allows a choice of
the electrolyte feed arrangement which can be either by series or parallel hydraulic
flow. The cell mainfolding can also be either external (see Figure 12a) or internal
(Figure 12b). Several factors such as gas disengagement, inter-electrode gap and
Figure 11 Alternative ways of electrically connecting cell electrode assemblies:
(a) monopolar; (b) bipolar.
K. Scott
degree of conversion will affect the selection. Designs using both approaches are
discussed later.
The selection of materials for a cell or reactor is an important design aspect for
electrochemical processes. This is due mainly to the necessity of selecting suitable
electrodes for the process which satisfy several criteria, namely:
(1) Chemical stability, this applies to both on load and open circuit operation.
(2) Physical stability.
(3) Availability in a suitable fabricated form.
(4)High electrical conductivity.
( 5 ) Low cost.
(6) Low corrosion rate.
(7) High selectivity and efficiency.
(8) Non-polluting.
Many of these criteria are conflicting, notably low cost with good selectivity
and suitable fabrication. The emergence of coated titanium electrodes using
platinum, ruthenium and iridium oxides, for example, met this challenge and led
to the introduction of a range of alternative versatile materials. This has led
c c
Figure 12 Alternative ways of hydraulically connecting cells: (a)external
manifold; ( 6 ) internal manifold.
Industrial Electrochemical Synthesis Processes
Table 4 Common electrode materials.
(a) H2 evolution
(b) 0 2 reduction
(c) Other reactions
(a) 0 2 evolution
(b) Cl2 evolution
(c) Other reactions
Steel, Ni, Ni- coatings, precious
metal coating.
Dispersed Pt on high area carbon.
High H2 overpotential metals, e.g.
Hg, Pb, Cd.
Other metals e.g. Ni, Cu, Ag, steels,
stainless steels, Hastelloy (Ni-Mo-Fe
or Ni-Mo-Cr), graphite, other carbons,
conducting ceramics, e.g. TiOx
(Ti&, Ti509), Raney Ni, Pt/Pt,
P d C for electro-hydrogenation.
IrOz-coated Ti.
Pb02 on Ti or carbon.
Pb in H2S04.
Steel in a neutral and basic medium.
Ni and Spinels in basic medium.
RuO2-based coatings on Ti(DSA).
Other oxides based on c0304 and PdO2.
Pt, P o i , Ir/Ti, Pt-Ir/Ti and other substrates.
PbO2 on Ti, Nb or C.
Fe or Pb in acid sulphate media.
Ni and spinels in basic media.
Conducting ceramics, e.g. TiOx(Ti407,Tis09)
indirectly to a ‘catalogue’ of commonly used materials (see Table 4), from which
suitable materials for electrosynthesis can be selected. These electrodes can often
be supplied as a package with the cell assembly. Within this package, the ability
to ‘separate’ the chemistry associated with anodic and cathodic reactions can also
be provided by the introduction of cell separators. These separators can take many
forms but the trend is away from microporous media towards ion-selective
materials. This is evidenced by the steady replacement of diaphragm cells with
membrane cells in the chlor-alkali industry. However, in water electrolysis where
the separator must simply separate the gases H2 and 0 2 , the use of porous asbestos
or similar material is adequate. Membrane selection and performance is also
important i n electrochemical wastewater treatment processes and is discussed in
a subsequent review.
A major problem with electrochemical synthesis is the use of heavy metals as
electrodes, particularly as cathodes in electro-organic synthesis where a high
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hydrogen overpotential is required to minimise hydrogen evolution. Ideally,
operation should (and can be) designed to avoid corrosion, however, this is not
always desirable for reaction efficiency. For example, the synthesis of adiponitrile
‘encourages’ the cadmium cathodes to corrode in order to maintain a clean,
non-fouled surface. Without this corrosion reaction, the efficiency falls off
markedly with time due to contamination from trace metal impurities and
The engineering of electrochemical reactors embodies all the design principles
associated with the catalytic and chemical counterparts, with only a few additional
factors associated with the supply of energy. These principles are well established
and readily applied to electrochemical reactor design. This practice has seen a
range of reactors emerge which have transformed the electrochemical cell from
laboratory-based models to commercial reactors. The final part of this review will
consider these reactor designs.
Part 111. Reactor Cell Designs for
Electrochemical Synthesis
Historically, a large number of cell designs have been used in electrosynthesis,
and these were usually the result of R&D programmes focused on a specific
reaction with specific materials. A number of these designs have been radically
improved over recent years to give greater versatility and enhanced performance
in terms of energy consumption and throughput, and they have found a wide range
of applications. Several commercial general-purpose electrochemical cells are
now available which can meet the requirements of many syntheses. The marketing
of such units has taken into account me scale-up requirements of users, and has
resulted in a range of sizes. These cells, and others used in industry, are discussed
in this paper.
Design of Electrochemical Reactors
In principle, an electrochemical cell is a simple device requiring a container for
an electrolyte and two immersed electrodes. Early cell and reactor designs
effectively mimicked this concept and took the form of tank electrolysers operated
as either batch or semi-batch devices. This design has the advantage of being
simple and robust and allowing inspection of the cell contents and the electrodes.
Electrowinning and electrorefining cells are the classic examples of tank
electrolysers where the increase in size of the cathodes, and their eventual removal
and replacement, makes operation difficult in any other form. Similarly, the
construction and operational challenges of high temperature molten-salt cells
made the simple tank concept the natural choice. Outside these areas, the tank
electrolyser concept is restricted to a few processes, water electrolysis probably
being the best known. The tank concept makes for easy overhead collection of
product gases, and no penalty is paid for the lack of control of mass transport. This
latter feature together with low space-time yield and poor suitability for scale-up,
has tended to favour the adoption of flow cells using either parallel-plate
electrodes, three-dimensional electrodes, or rotating electrodes.
Parallel-plate flow cells
A good cell design must satisfy a number of requirements, including:
- High productivity,
- Good mass transport,
- Good temperature control,
- Low electrical resistance,
- Ease of operation,
- Safety in operation,
- Provision for cell separators,
- Ability to deal with gaseous products and reactants,
- Minimum cost.
There are practical (and economic) factors which limit the size of individual
electrodes to areas of a few square metres, thus electrochemical reactors are
modular in design. Each module (see Figure 13) consists of a set of alternating
Developments in Chemical Engineering and Mineral Processing, Vol. 1, No 2/3,page 95
K. Scott
anodes and cathodes, with separators if necessary, contained within a suitable
housing with hydraulic manifolds for electrolyte supply and removal.
Scale-up is then achieved by networking the modules, both hydraulically and
electrically, into suitable series and parallel combinations. This allows flexible
operation as individual modules can be isolated to allow for maintenance, and
electrode or separator replacement. The parallel-plate flow cell is an appropriate
Ca fholyfe
Figure 13 Schematic of a parallel-plate module.
Figure 14 Typical parallel plate flow cell concept.
Industrial Electrochemical Synthesis Processes
Table 5 Parallel-plateflow electrolysers.
Monsanto EHD
A s h EHD
DuPont ESE
BASF Pilot Plant
Ionics Chemomat
Expanded metal
Stainless steel
Ni. Hastelloy
carbon steel
Pb (1% Ag)
Pb (Sb)
design which meets many of the above requirements. Scale-up is achieved in three
(1) Increasing individual electrode size from 0.1 m2 up to 2 m2 (for example).
(2) Increasing the number of cell units in each module (10 to 100 cells in one
module is not unusual).
(3) Increasing the number of modules.
Crucial factors in stage (1) include changes in the mass transport and potential
distribution, gas voidage distribution on increasing the electrode length, as well
as maintaining good flow distribution. A crucial factor in stage (2) is to maintain
a uniform distribution of flow between the cells.
A typical parallel-plate flow cell is shown in Figure 14. The concept shown for
a membrane separator, is one of forming individual cells with an alternating
arrangement of ‘electrode/cell frame/membrane/cell frame/electrode’, and sealing
using suitable gaskets and compression. The inter-electrode gap is small (0.5 to
2.5 mm) to minimise IR losses, it is determined by the cell frame structure which
usually must accommodate external pipework for electrolyte flow.
A variety of designs based on the above concept have evolved, some of which
are listed in Table 5. These units have been designed by leading companies in the
area of electrosynthesis, and often in order to meet the requirements of specific
reactions, All adopt bipolar electrical connection and external hydraulic
connections, except the DuPont Cell which uses an internal hydraulic connection.
Development costs of such cells are high and companies on the verge of adopting
electrosynthesis should consider a number of commercially available general
purpose flow electrolysers now on the market (detailed in Table 6). These designs
can be adapted for a wide range of electrode materials and configurations. Among
the variations in the design concept are the use of either internal or external flow
manifolding, series or parallel flow, and bipolar or monopolar electrical
connections (see Figures 11 and 12).
The design concepts of three commercially available cells are shown in
Figure 15. The DEM, dished, electrode, membrane cell (Figure 15a) uses shaped
electrodes to provide a low inter-electrode gap, whilst allowing adequate space in
the cell frame for pipe manifolding. The DEM cell has been used for both organic
and inor anic synthesis. A recent example of the latter is the production of sodium
bromate$0 by the anodic oxidation of sodium bromide via the reactions:
6Br- - 6e- -+ 3Br2
Chemical: 6NaOH + 3Br2 + 3NaOBr + 3NaBr + 3H20
3NaOBr -+ NaBr03 + 2NaBr
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Table 6 Commercial jlow electrolysers.
Electrical connection
Electrode materials
Metals, flat Plate metals,
plate, lantern carbons, threeblade
promotors and
dished electrode
Bipolar and
Plate metals,
Interelectrode gap (mm)
Electrode area (m2)
Cell material
Performance of the DEM cell after scale-up from a 0.05 m2 unit to a 1.0 m2
pilot cell at a current density of 2,000 A m-2, was virtually unchanged.
Performance criteria included an efficiency of 95-97%, cell voltage of 3.3 to
3.35 V, and power consumption 3.7 kWh kg-’. The anode material in this system
was a RuO, coated titanium.
The ICI FM2SP cells (Figure 15b) arose from ICI’s experience and activities
which chlor-alkali membrane cells, and is thus custom-made to handle gases.17
The cell has been used for both organic synthesis ,1i18 e.g. Kolbe synthesis, and
for inorganic synthesis, e.g. oxidation of dinitrogen tetroxide to dinitrogen
pentoxide in anhydrous nitric acid.lg The concept of production of N205 is shown
in Figure 16, where both the anode and the cathode reactions are coupled in the
synthesis. The ability of electrochemical cells to handle harsh chemical
environments is further demonstrated by this process, which also produces 100%
nitric acid. The unique lantern blade electrode design gives an extended surface
area, low effective inter-electrode gap, and good gas release. This reactor contains
no cell frames, and the electrolyte compartments are formed by the sealing
The electrocell unit (Figure 1%) is a well engineered multi-purpose unit
available in a wide range of sizes for scale-up. It adopts an internal manifolding
arrangement with monopolar electrical connections. It has been used in a number
of electrosyntheses, including the cathodic reduction of oxalic acid to glyoxylic
acid2’ and the manufacture of epichlorohydrin from alkyl chloride.22
Other reactor designs
There are many cell designs described in the open literature, of which a number
have been successful at the commercial level.’ These can be categorised as
- Monopolar planar-electrode tank cells,
- Thin-film bipolar cells,
- ‘Swiss roll’ cell,
- Rotating electrode cells,
- Three-dimensional electrode cells.
Industrial Electrochemical Synthesis Processes
Membrane configuration
Undivided configuration
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frame. inner
frame, outer
frame. inner
Figure 15 Commercial parallel-plate electrochemical reactors: ( a ) the DEM
reactor; (b) the ICI FM2ISP; ( c ) the Electrocell reactor.
Industrial Electrochemical Synthesis Processes
Figure 16 Concept of the production of dinitrogen pentoxide.
Table 7 Cell designs used in electrosynthesis.
Monopolar planar electrode tank cells:
Kryschenkoz3 L ~ D u c ~ ~MacMullinzS
Olefin (gas)
Olefin oxide
Fluorinated HC
Lead dioxide
on Ni grid
Thin-film bipolar cells:
BASF capillary Monsanto cell Hoechst2’
Configuration Disc
Rotating electrode cell:
Udupa32*33 Engineering34
Cylinder or
Many of these cells were designed for specific reactions, especially the first
category. Table 7 lists some of the prominent designs used in electro-organic
Thin-film bipolar cells: When electrolyte conductivity is low, a small
inter-electrode gap is advantageous in order to minimise IR losses. Thin-film
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Figure 17 Thin f i l m bipolar cells: (a) capillary gap cell; (b) concentric gap cell.
bipolar cells (as shown in Figure 17) have been designed to satisfy this
requirement. Bipolar cells are an assembly of parallel, planar electrodes separated
by insulating spacers and installed in a vessel. Electrolyte flow between electrodes
may either be by natural convection or forced circulation. Extreme simplicity of
construction is possible, the cell is merely a stack of alternating electrodes and
spacers, without the complex hydraulic system of a plate-and-frame design.
Electrical connection is only made to the two end ‘feeder’ electrodes.
In general, bipolar thin-film cells are the preferred design for undivided cell
processes, whereas plate-and-frame cells are more commonly used in processes
requiring a separator. Some of the advantages of undivided cells over divided cells
- Single electrolyte system,
- Elimination of the IR drop of one electrolyte,
- Simple cell construction and low cost,
- Higher production per unit cell volume, i.e. higher space-time yield,
- Lower power usage.
A key feature of the bipolar stack cell is that a single planar electrode serves
as both the anode and cathode. Where different materials are needed for these
electrodes, a variety of schemes have been proposed for production of composite
electrodes. This requirement can be one of the major limitations in the application
of this type of cell. Other disadvantages include the problem of providing a
uniform distribution of flow, and the possibility of excessive current bypass
around the edges of the electrodes, although this can be partly reduced with
suitable insulated attachments to the electrode ends. Table 7 lists cell designs
which are classified as thin-film bipolar cells with inter-electrode spaces of 0.1 to
1 mm.
Another commercial design is the ‘thin film’ SNPE ‘pencil sharpener cell’ (see
Figure 18) used i n the electro-carboxylation of chloroethyldiphenylether to
f e n ~ p r o f e nA
. ~sacrificial
magnesium anode is consumed during the reaction in
an aprotic solvent (DMF), and conveniently solves the problem of a suitable
Industrial Electrochemical Synthesis Processes
Figure 19 Schematic of the Monsanto adiponitrile cell.
anode reaction. In this design, the sharpened anode is separated from the
lead-coated stainless steel cathode by a plastic mesh giving an inter-electrode gap
of a few millimetres.
The cell design used in the Monsanto process for the production of adiponitrile
is shown in Figure 19, and can also be classified as a thin-film bipolar cell. It
provides a clear demonstration of the success of the technology. With an
inter-electrode gap of <2 mm, a large bank of approximately 100 bipolar
cadmium-coated steel composite bipolar electrodes are fitted into a polypropylene
housing within a cylindrical vessel. Current bypass is minimised by using plastic
end extensions attached to the electrodes. Further details of the cell performance
are discussed in the following section.
‘Swiss roll’ cell: The Swiss roll cell as shown in Figure 20 is a novel high
surface-area device designed on the principle of spiral-wound membrane units. A
sandwich of alternating ‘anode/spacer/membrane/spacer/cathode’is rolled around
a cylindrical core which is the current feeder for one electrode. The cell housing
is designed to give electrical contact only to the opposite electrode. Electrolyte
flows axially along the roll. The electrodes used in the cell must be flexible and
are usually foils, which because of problems of changing electrode potential along
the length of the roll limits the current density to 10-50 A m-*. The
cartridge-orientated design means that for maintenance the unit must be
withdrawn from its holder. The unit must be protected from particulate material
which could block the flow channel and lead to electrical shorting. The Swiss roll
cell has been proposed for effluent treatment applications and has seen
commercial and near-commercial application” in organic electrosynthesis.
Rotating electrode cells: By rotating an electrode, mass transport can be enhanced
without significantly affecting the residence time of electrolyte in the cell. Two
principle configurations have been employed, these are rotating discs and rotating
cylinders as shown in Figure 21(a) and (b). Table 7 lists the principal designs used
in electrosynthesis.
Figure 20 The ‘Swiss roll’ cell.
Industrial Electrochemical Synthesis Processes
The Udupa cell is used extensively in India for the manufacture of a range of
organic chemicals on a small scale, e.g. p-aminophenol, hydrozoanisole,
o-toluidine. Units are essentially batch tank electrolysers fitted with a number of
rotating electrode elements and appropriate anodekathode separators.
The Cumberland Engineering cell utilises the rotation of a stack of parallel
bipolar disc electrodes and was devised to cope with electrolyses beset with
problem of insoluble precipitates, notably magnesium hydroxide formed during
hypochlorite production from sea water. The concept could also be applied to
organic electrosynthesis. The major disadvantages are those often associated with
rotating equipment, i.e. reliability, maintenance and additional electrical
connections to the rotating electrodes.
The pump cell (Figure 21a) is similar in principle to the Cumberland cell and
has been used commercially in metal powder production, as has the Eco cell
(Figure 21b).
Three-dimensional electrodes: Three-dimensional electrodes are used in cell
designs where a high surface area per unit volume is required, thus providing
Figure 21 Cells with rotating electrodes: ( a )pump cell with rotating disc;
(6) Eco-cell with rotating cylinder.
K. Scott
acceptable space time yields when either the reactant concentrations or the current
densities are low. These electrodes are essentially the electrochemical analogue of
heterogeneous catalytic reactors, and are available in equivalent forms, i. e. fixed
bed, porous bed, fluidised slurry, or circulating beds. Although often considered
for electrosynthesis, their major area of application is in wastewater and effluent
treatment, both for inorganic chemical and organic chemical removal or recycling.
However, a fixed bed of lead shot was used in the now, ill-fated Nalco tetra-alkl
lead process.37 Perhaps this technology will re-emerge when organic
electrosynthesis in non-aqueous solvents becomes more prominent, probably due
to the application of sacrificial anodes.30
To close this section, consider a recent application in organic electrosynthesis
of a cell design proposed for inorganic wastewater treatment. This cell utilises the
enhanced mass transport effect at an electrode, induced by the provision of an
inert fluidised bed of particles for the production of D-arabinase from a soluble
salt of D-gluconic acid.38 The electrode material used was carbon.
Reaction Engineering of Electrosynthesis
The similarities i n behaviour between catalytic/chemical reactors and
electrochemical reactors have over the last decade become more obvious in terms
of the design and engineering principles. A ma'or aspect is consideration of the
transport processes in electrochemical systemsA9. The phenomena of heat, mass
and momentum transfer associated with chemical reaction engineering, are
accompanied by the phenomena of charge transfer associated with the flow of
current. The driving force for both charge transfer and for the movement of ions
by migration is the potential field. The migration of ions to an electrode can result
in a consumption or inter-conversion of species. This causes concentration
differences, and inevitably diffusional mass transfer which can be augmented by
convective flux. Thus the transfer of species in electrochemical systems is a
combined process of migration, diffusion and convection, the mathematical
description of which is well e~tablished.~'
The transport equations describing total ionic flux enable the variation of
concentration and potential to be obtained in particular regions, especially near
electrodes and membranes. The mathematical solutions for this type of problem
have been obtained for many applications, they result in profiles for so called
primary, secondary or tertiary4' current or potential distributions over electrode
surfaces. The primary distribution obtained from solution of the Laplace equation,
allows for purely geometric factors. However, secondary and tertiary distributions
introduce the phenomena of electrode polarisation and kinetics, and mass transfer.
The determination of potential distributions is important in many areas of
electrochemical system design, particularly with textured, rough or particulate
electrodes. However, there are other areas, particularly in electrosynthesis, where
the cell engineering can almost eliminate geometric factors and a more pragmatic
design approach is possible.
Basic principles of reaction engineering as applied to electrosynthesis
In Part I1 of this review, certain design concepts of significance in electrochemical
synthesis were discussed, including the interaction between electrochemical
kinetics and mass transfer. In this section these concepts are considered within the
framework of a simplified view or model of an electrochemical reactor, based on
Industrial Electrochemical Synthesis Processes
established reaction engineering principles. This model subdivides the
electrochemical system into two parts: a bulk electrolyte region and a mass
transfer region bounded by an electrode interface. This is the picture of a single
reaction surface which can be extended to introduce the second electrode and, if
required, mass transport through a membrane phase.
The constituent parts which form the basis of the model can be summarised as
- Hydrodynamic description of the bulk electrolyte phase; plug flow, well mixed,
dispersion, etc.
- Interfacial mass transfer processes. Qpically the convenient use of the mass
transfer film coefficient, which can be modified to allow for a greater contribution
of migrational transport, through the use of an approximate transport number.
- Interfacial phenomena at an electrode surface, i.e. the combined processes of
electron transfer, chemical interactions and adsorption/desorption effects.
There are several ways in which these features can interact and enable
formulation of models for intrinsically different reaction systems. First, the
process by which the electrochemical reaction is carried out, this can be:
(1) Electrochemical reaction,
(2) Electrocatalysis,
(3) Indirect electrosynthesis,
(4) Homogeneous redox catalysis,
( 5 ) Heterogeneous redox catalysis.
Second, the significance of chemical reaction(s) in relation to their speed and
their role relative to the electrochemical reactions. Three general categories can
thus be identified.
(1) Fast chemical/electrochemical reaction: where all intermediate surface
processes are fast compared to mass transfer.
(2) Intermediate reaction: electrochemical, chemical and mass transfer rates are
(3) Slow reaction: chemical reaction is slow compared to mass transport.
It is important here to distinguish between chemical reactions which involve
short lived intermediates, radicals, etc., instigated by electron transfer, or
relatively stable species with 'finite' reaction rates. Thus a classification of
coupled electrochemical/chemicalprocesses within the second category is that of
simultaneous chemical reaction and diffusion (or intraphase mass transport).
Many electrochemical syntheses follow this type of mechanism and include a
range of indirect electrochemical syntheses, e.g. oxidation of halide to
hypochlorite, chlorate and bromate; epoxidation of ropylene to propylene oxide
with electro-generated bromine (or ~ h l o r i n e ) ; ~and
the preparation of
This latter example is the formation of a
pharmaceutical intermediate by the reactions:
+ ne-
The desired product (B) couples chemically with the reactant (A) to form an
unwanted by-product, and demonstrates the significance of the relative rates of
mass transport and chemical reaction. Analysis of this situation is achieved by
K. Scott
Table 8 Equations for electrochemical reactor models.
(1)Reaction rate model.
(2)Electrolyte phase, materialhate balances.
(3)Current balance.
(4)Charge balance.
(5)Voltage balance.
(6)Mass balance.
solving the ‘Fickian’ Second Law of Diffusion with coupled chemical reaction.
Taking the above category of reaction as a separate case, the mathematical
representation of a reactor model is structured around the generalised equations of
Table 8. This is discussed in the next section.
The electrochemical reactor model
For either a continuous reactor or a batch reactor, the model utilises bulk
electrolyte rate balances for each component of interest where the net rate of
material input, r‘ (by flow, reaction and mass transport) is equal to the
accumulation of tiat species (dNj/dt), therefore:
dNj/dt = rj
The reaction rate model combines mathematical descriptions of
electrochemical kinetics, mass transport and adsorption processes at the electrode
of interest. Electrochemical kinetics can be described in a number of ways, but
often the Butler Volmer or high field ‘Tafel’ approximations4’ are applicable.
These approximations express the variation of electrochemical reaction rate with
the current density, and the electrode potential (E) in an exponential form, for
i = i, [exp{-anFq/RT]
+ exp{(1 - a)nFq/RT)]
The exchange current density (i, ) which is of significance in electrocatalysis,
and the transfer coefficient (a)are the experimentally determined rate parameters.
Adsorption phenomena, although often ignored in many rate models, can play
an important role in the reaction system. This is shown by the commercial
electro-organic syntheses for the production of sorbitol and glyoxylic acid. In
both cases the electrochemical reduction of oxalic acid and glucose respectively,
involves adsorption of these species. Their rates of reaction have been xmodelled
using the familiar heterogeneous-catalysis ‘Langmuir-Hinshelwood’
(i/nF) = klCj/(l - kzCj)
(7 1
where kl and k2 are electrochemical rate constants. Equations of this type
manifest themselves in the asymptotic rise of current density with the reactant
concentration as shown in Figure 22 for the reduction of a ~ e t o p h e n o n e . ~ ~
Electrochemical processes are governed by a coulombic charge balance (by
virtue of Faraday’s law) which links the history of the applied current to the
transformation of electrochemically active species:
Industrial Electrochemical Synthesis Processes
Figure 22 Variation of current density with reactant concentrationfor adsorption
controlled reaction.
A current balance over any time interval, is a statement that the sum of the
partial currents or current densities is equal to the total applied current:
The voltage balance has been discussed in Part I1 of this review, and the final
equation in Table 8 is a statement of the conservation of mass.
An example of the application of the generalised model is the cathodic
reduction of nitrobenzene to p-aminophenol, a commercial electro-organic
synthesis." For this case, the reaction rate model is shown in Figure 23, in which
nitrobenzene is cathodically reduced to an intermediate phenylhydroxylamine
which chemically rearranges in acid to the product p-aminophenol. The
intermediate can also be reduced cathodically to the by-product aniline. This
system has been modelled both in a batch reactor as a generalised case of
simultaneous diffusion and chemical r e a ~ t i o n ? and
~ also as a category three
system with slow chemical reaction.46 The performance of this system is shown
in Figures 24 and 25. Several performance trends are readily apparent, including:
(i) high temperature, high mass transport, low current density, or electrode
potential, all favour p-aminophenol production; and (ii) the intermediate
K. Scott
Bulk Solution
Figure 23 Reaction rate model for p-arninophenof production.
phenylhydroxylamine goes through a maximum concentration, with a resulting
fall in selectivity of p-aminophenol, with reaction time.
Multiphase reaction systems
Many electrochemical processes involve the presence of several phases or involve
phase transformations. These processes include metal deposition, gas evolution,
film formation, batteries and fuel cells, corrosion and electrolysis with emulsions
and reactive gases. The mechanisms by which these processes occur, the transport
processes involved, and the interaction with the electrical double layer, are not
well understood. Often only empirical models of such phenomena are available
for use in design cases. An important area is that associated with mass transfer
behaviour at electrodes either when gases are evolved, or when sparged with
non-reactive gases, or exposed to dispersed organics. It is known that the presence
of this second phase can result in a considerable enhancement of mass transport,
above that achieved with single-phase systems.47
A second important aspect of multiphase reaction systems is when a reactant
(or product) is centred in the second dispersed phase. For the reaction of a gaseous
species in an electrolyte phase, two possible reaction models might exist: (i) the
direct electrochemical reaction of the dissolved gas at the electrode (see
Figure 26), involving successive mass transport processes prior to reaction; and
(ii) mediated homogeneous reaction, in which interphase mass transport is
followed by successive diffusion and simultmeous reaction.
Industrial Electrochemical Synthesis Processes
The second model has been applied to the roduction of propylene oxide from
propylene using electrogenerated bromine!
This synthesis nearly reached
commercial operation in the 1970s.
Electrochemical synthesis involving two-phase liquid/liquid electrolyte
requires special design considerations for the interaction between mass transport,
phase equilibria and electrochemically initiated reaction. The second dispersed
organic phase generally acts as a source of reactant, but also as a sink for product
which can serve to protect this species from further reaction. For efficient
synthesis, the reactant must undergo fast transport from the organic phase to the
electrode. Similarly, the product must be rapidly transported back into the
dispersed phase.
c - p -aminophenol
Figure 24 Product distributions during the batch synthesis of p-aminophenol.
Values of potential E on figure. kfl = 6.0 x 1@ exp(-13.55 E ) m s-I;
k2 = 1.7 x 10-4 s-I; kp = 2.9 x 1@ exp(-13.55 E ) m s-'; kL = 1.3 x 10-4 m s-'.
K. Scott
Figure 25 Variation of by-product aniline with current density during
nitrobenzene reduction.
Mass transfer Temp
(m s-')
1.5 x
4.4 x lo4
kc (s-l)
1.4 x lo4
2.3 x lod
3.1 x
7.1 x lo4
2-3 mol dm-3 sulphuric acid on copper electrodes
4 -1
(-) model with kc = 1.7 x 10 s , kL = 1.3 x
The classification of two-phase electrosynthesis tends to distinguish between
direct and indirect electrochemical reaction. In principal both are limiting cases of
a general situation where chemical and electrochemical reaction can proceed in
two phases, with appropriate mass interchange of the species. This scheme is
depicted in Figure 27 for a single reaction between species Q and X to form
products B and Y. The total current is the sum of that in the aqueous phase,
through contact with the organic phase, and through mass transport enhancement
due to a direct supply of organic solute from the dispersed phase at the electrode
Industrial Electrochemical Synthesis Processes
Figure 26 Models for gas, liquid electrochemical reactions: (a) interphase
(direct) reaction; (b) intraphase (indirect) reaction.
surface. The behaviour of the system can be represented by the set of equations
for batch operation:
d[( 1 - e)Cj]/dt = r. - kL.a(1 - e)(Cj - Cje)
d(e C,d)/dt = kLja(l - ej(Cj - Cje)
Figure 27 Mass transport processes in electrolysis with organic dispersions.
A; +
K. Scott
A = acrylonitrile, (AN)
C = radical dianion D2J = radical dianion T2R = radical anion P B = radical dianion A*D = adiponitrile
P = propionitrile
T = 1,3,6 - tricyanohexane
Figure 28 Reaction scheme for the synthesis of adiponitrile.
where kLja is the specific interphase mass-transfer coefficient (s-’) between the
aqueous and organic phases. Cje is the equilibiium interfacial composition related
to the droplet phase concentration by the distribution coefficient (mj):
Cje = Cjd/mj
(1 1)
This model has been applied to the synthesis of a d i p ~ n i t r i l e based
, ~ ~ on the
reaction scheme depicted in Figure 28. The significance of interphase and
intraphase mass transport on the synthesis were successfully determined using the
Electrochemical synthesis is well established in many process industries.
However, in terms of capacity, the manufacture of inorganic chemicals such as
chlorine, caustic soda and metals, is the dominant area. Extended applications of
electrochemical synthesis in industry is occurring at a moderate rate in both
inorganic and organic chemical production. In the latter case, the syntheses are
mainly focused on fine chemical and pharmaceutical intermediate manufacture,
where the cost of electrical energy is not significant in comparison to improved
selectivity and new, simpler reaction pathways. There is also a rapid growth in the
use and integration of electrochemistry in biochemical systems.
A factor which will continue to make industry turn to electrochemistry is the
emerging availability of ‘off-the-shelf’ reaction units. This is somewhat analogous
to multipurpose batch-stirred reaction vessels, the mainstay of industry for
small-scale chemical manufacture. Another factor which will strengthen this
position is a greater integration and development of chemical engineering
principles into the design of electrochemical synthesis units. If electrochemistry
can become more cost effective in its use of electrical energy, then a number of
industrial sectors will be persuaded to use this valuable form of synthesis. This
can be achieved i n a number of ways, in addition to using improved
electrocatalysts and minimum cell resistance, including: (i) application of zero
cell-gap technology and solid polymer electrolytes; (ii) ‘fuel cell electrode’, e.g.
Industrial Electrochemical Synthesis Processes
in oxygen reduction cathodes; (iii) use of both cathode and anode reactions in dual
synthesis systems.
The third approach, although already used in inorganic synthesis (e.g.
chlor-alkali), is not significant in organic synthesis systems. However, a few pilot
examples are being evaluated, e.g. the paired electrochemical synthesis of sorbitol
and gluconic acid in undivided flow cells,49 and the paired electro-oxidation of
toluene to benzaldehyde by Mn3+ and OH- free radical^,^' as generated in the
anodic and cathodic reactions, respectively.
Using one cell to either synthesise two products, or to use two feeds to form
a common product, radically reduces both operating and capital costs.
Overall the prospects for electrochemical synthesis are good, especially with
the current interest in environmental management. This will be the subject of a
second review paper.
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Received 4 November; Accepted: 15 November 1992.
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