Note: Descriptions are shown in the official language in which they were submitted.
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~ ELECTROOE~MICAL CELL AND PRO OESS
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Technical Field
The invention relates to electrochemical
cells~ particularly for carrying out electrochemical
reactions involving a gaseous reactant or in which gas
is supplied ~or another purpose such as purging or
sweeping a~a~ a product of reaction~ or as a ~uffering
agent or to inhihit unwanted reactions.
The invention is more specifically, but not
exclusively~ concerned with an electrochemical cell
for electro-organic synth,esis, ~or example the electro- ~-
chemical oxidation of unsaturated and poly-unsaturated ¦:
hydrocarbons, The electrochemical production of ¦ :
propylene oxide is particularly interesting, and will
; 15 be discussed in greater detail by way of example.
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I Background Art ~
~ In the electrochemical production of propy- .
lene oxide, propylene is converted to propylene halohydrin :
by reaction with halo~en ~enerated in.situ by the anodic
oxidation of th.e halide salt of an alkali metal in aqueous
solution. T~e propyléne haloh.ydrin is converted to ~
propylene oxide by reaction ~ith the hydroxyl group at .
th.e caihode from which hydrogen is liberated. The !~
general scheme of reaction ~en sodium bromide is used !i
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as the electrol~te is;
Anode 2 Br ~ r2
C3~6 + Br2 + H-2 ~ C3~6BrOH ~ HBr
Cathode 2 --t H2 ~~ 2 OH - 2e
~ C~3H6~rOH + O~ C3~60 ~ H20 ~ Br
Overall C3~6 + H20 ~ C3 6 2
In principle, only water, propylene and
electrical energy are consumed in the formation of
propylene oxide and hydrogen. The halide electrolyte,
sodium bromide, is continuously oxidized and re-
generated within tne cell for further use, although
losses o ~romine may be caused by the formation of
hypobromite and bromine gas.
The advantages of this electrochemical route,
which o~viates the production of waste calcium chloride
- encountered in the conventional chemical process,
have long been recognized but attempts to implement it
have not proved to be very effectiven
French Patent Specification NoO 1,375,973 and
2Q ~. German Auslegeschrift No. 1,258~B56 proposed the use
of diapnragm cells in which propylene halohydrin is
generated a~ a porous anode and passes through the
diaphragm into an alkaline cathoIyte in a porous
cathode where it is saponified to propylene oxide.
~owever, these cells are complex and the efficiency low.
U.S Patent Specification No. 3,3q4,059
proposed carrying out the haloh~drin process in a non~
divided cell, prefera~ly a flowing mercur~ cathode cell, ~;`
in which propylene was simply bu~bled into the electro-
lyte. Again~ the performance ~as poor and F. Beck
(IUPAC XXIVth International Congress, ~am~urg, 1273,
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Vol.5, "Ap~lied Electrochemistr~ pages 111~l361 has
claimed an improved per~ormance using a capillary gap
cell. In this, propylene dispersed in a dilute
NaBr electrolyte is supplied through a central hole
5 in a pile of electrode discs and flows radiall~ out-
wards ~et~een -the discs. The gap between the elec-
trode discs was made small CQ.2 to 0.5 mm) to enable
low bromide concentrations to ~e handled with low
ohmic losses, A current efficiency of 70~ or just
lQ a~ove and an energy consumption of Q.~3 - 0.30 kwh/
gmol propylene oxide are reported for a small
capillary ~ap cell, ~ut scaling up this cell for
industrial production would involve difficulties.
Fleischmann et al, (,Symposium on Electro-
15 chemical Engineering I, Newcastle 1971, Editor J.D.
Thorntonl have studied the synthesis of propylene
oxide using a bipolar packed bed cell. The cell
consisted of a packed bed made up o~ a mixture of
conducting and non-conducting particles. The
2Q conducting particles become bipolar by using dilute
electrolyte in the cell and applying sufficient voltage
gradient between the contact electrodes so as to over-
come the resistance drop in the electrolyte. Usin~ ?
glass coated with graphite as the conducting particles
~5 and glass ~eads as the non-conducting particles, all
particles having a diameter of about 0.05 cm, the
energy consumption of,such a cell was found to be high,
in the rangP of 2.5 - 3 kwh/gmol propylene oxide.
A bipolar rod flow cell was used b~ '~ing et al.
3Q ~Trans. Inst. Chem. Eng.~ 53, 1~75) for the production
of propylene oxide. The cell consisted of vertical
rows of electrically-conductive rods, separated from
one another ~ a small gap. The electrolyte was
fed to the top rods~ flowed down~rards over the vertical
35 rows and was collected from the ~ottom rods for re-
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circula-tion. The gaseous reactant/ prop~lene, was
passed up the space between the vertical rows, in
continuous contact with the electrolyte film. The
current efficiency oE this cell was of the order of
70% and the energy consumption is estimated in the
range Q.35 - 0 ~ kwh/gmol propylene oxide.
R.E.W. Jansson et a:L. have developed a
bipolar electrochemical pump cell for which an energy
yield below 0.2 kwh/gmol of propylene oxide is claimed
(Journal of App. Electrochemistry, 7, (1977~, 437-443)
for trial experiments on a laboratory scale using
a cathode rotatiny at 3000 rpm with an electrode gap
of 0.25 mm. However~ the structure is not easily
scalea-up for industrial production.
Various other cell structures designed to
provide a gas supply to the electrolyte are also known.
For example, in the electrowinning of metals such as
copper~ it i5 well known to supply a gas through bubble
tubes situated below the electrodes in order to agitate
the electrolyte (e.g. see U.S. Patent Specification No.
2Q 3,875,041 and the earlier patents referred to therein~.
Another sug~estion made in U.S. Patent Specification No.
3,259,049 was to provid~ electrolyte agitation in an
electroplating tank using a hollow, flat manifold which
is placed in the electrolyte~ under the electrodes, and
has a perforated upper surface for bubbling gas up into
the electrolyte~ tne gas being supplied to the manifold
via a gas flow tube. In contrast to the fixed bubble-
tube arrangements, the entire manifold structure was made
removable to facilitate periodic cleaning to remove
fragments which ma~ block the perforations in the
manifold.
Disclosure of Invention
! 35 An object of the invention is to provide an
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electrochemical cell specificall~ (but not exclusively)
for the production of propylerle oxide and which can he
designed to meet up to the follo~ing requirements better
than the previously proposed cells:
1) Simplicity of the mech.ani.cal construction;
2) Good heat and mass transfer characteristics;
3~ Simplicit~ of operation and continuous operation;
4) Good gas-liquid contact; and
5) Good mixing of anolyte and catholyte products,
According to the invention~ in its simplest
form, an electrocnemical cell comprises electrodes
disposed over a perforated generall~ horizontal plate, an
èlectrolyte inlet and an electrolyte outlet spaced
apart on opposite sides of the electrodes across the
perforated plate, and a cell housing which is divided l,
by the perforated plate into an upper chamber and a ¦:
lo~er chamber. The lower cham~er is a gas supply
- cham~er fro~ whicil~ in use, gas passes up through
perfora~ions in the plate and bubbles through electro-
lyte on the plate to collect in the upper chamber.
The electrolyte inlet and electrolyte outlet
of the cell are each advantageously formed by a weir.
The top o~ tne inlet weir is higher than the top of the .
outlet weir which in turn is higher than the top of the
electrodes. Hence, by controlling the supply of fresh
electrolyte to the inlet weir, the electrolyte is made to
flow over the inlet weir and bet~een the electrodes as
it passes across ~he perforated plate, while spent or
reacted electrolyte flows out over the outlet weir at a
chosen rate. These weirs may be formed by upstanding ¦ :
plates integral with or fixed on the perfora,ed plate.
The electrodes~ advantageously a bipolar array
o~ vertical plate-like electrodes'disposed in spaced .
; parallel relationship to provide channels between the
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electrol~te inlet and outlet~ ma~ xest on thé per~oxated
plate which~ in this instance~ ~ill be made of electxic~
ally insulating mater~al. The perforations in the plate
can be arranged in rows each spaced about mid~way between
adjacent electrodes. Perforations in the form of gen~
erally circular bores having a diameter of about l mm have
been found sat;sfactory, but perforations of other form
and size can be used.
The bottom of the lo~er chamber of the cell
lQ housing can serve as a receptacle for a pool of spent
or reacted electrolyte which flows via a do~ncomer tube
leading from the aforementioned outlet weir into the
pool from which electrol~te is removed via an autlet and
may be rec~cled. Fxesh electrolyte can be supplied to
the aforementioned inlet weir via an incomer tube which
extends down through the upper chamber of the cell housing
into the electrolyte retained by the inlet weir.
The upper and lower chambers may be formed by
upper and lower sections of a box-like cell housing, these
2Q sections ~eing separated by the aforesaid plate which is
perforated only in the region under the electrodes. A
rectangular enclosure for the electrodes may thus be
formed ~y the side walls of the upper housing section
fitting against upstanding plates forming the inlet and
outlet weirs. These side walls can carr~ inset terminal
electrodes of the electrode array.
The invention also concerns an electrochemical
reactor formed ~y stacking several cells according to the
invention in a column whereby the gas-collection chamber
3Q of each cell (except the top one) forms the gas-supply
chamber for the cell above. In other words, the per-
forated plate of each cell ~except the lowest one~ forms
the top of the gas~collection chamber of the cell belo~.
With this arxangement, in operation, gas passes up
through the cells from the ~ottom of the column to the
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top, bubbling through the electrol~te in each cell.
Pre~erably, tne electrolyte outlets and inlets of the
successive cells are connecte~ in cascade 50 that the
electrolyte flows down the column from one cell to the
next, the electrol~te flowing across the perforated
plate of each cell ~rom the lnlet to the outlet and
then down to the inlet of the cell below. Each cell
of such a reactor can have the aforementioned preferred
features of a single cell unit, such as the electrolyte
lQ inlets and outlets being formed by weirs.
Another aspect of the invention is a method
of carrying out an electroc;~emical process or reaction
using a cell according to the invention, this method
comprising passing gas up-through the perforations in
the plate so that it bubbles into the electrolyte on
the plate. Depending on the reaction, the method may
be operated continuously~ i.e. continuously supplying
gas~ electrolyte and electric current, or discontinuously,
i.e. with an intermittent supply of gas, electrolyte and/
or current.
~ et another aspect of the invention is a method
of carr~ing out an electrochemical process or reaction
using a reactor formed b~ a column of cells according
to the invention! this method comprising flowing
electrolyte down the column from one cell to the next
and across the p2rforated plate o-f each cell, and passing
gas up through the perforations in the successive plates
so that the gas bubbles through the electrolyte on each
plate. This method may also be operated continuousl~
3Q or discontinuousl~.
The gas ma~ be a reactant or a mixture of
reac~ants, or may serve another purpose~ for instance an
inert purge ~as such as nitrogen ma~ be used to s~eep
awa~ a product of reaction, or a buffering agent such as
C2 or NH3 ma~ be used to control the prl of the electro-
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lyte, for example to inhibit unwanted reactions.
In addition to the preparation of propyleneoxide, the cell accordillg to the invention could be
used in the electro-synthesis of other products such as
the formation of butylene oxide from butene.
Another important application is the electro-
chemical treatment of some effluent gases. Generally
speaking, many applications concern the situation where
one or more of the reactants is a gas and where mixing
of anolyte and catholyte is advantageous or inconsequen-
tial.
Brief Description of Drawings
Embodiments of the invention are shown, by way
of example, in the accompanying drawings~ in which:
Fig, 1 is a cross-section of a cell along the
line I-I of Fig. 2;
Fig. 2 is a cross-section along the line II-II '
20 of Fig. l; j~
Fi~. 3 is a sectional view of the cell of Figs.
1 and 2 along the line III-III of Fig. l;
Fig. 4 is a cut-away view similar to Fig. 1,
of part of a column formed of several cells connected
in cascade.
Best modes for carrying out the Invention - ¦
The cell of Figs. 1 to 3 comprises a generally
rectangular box-like housing composed of an upper section
1 and a lower section 2. A plate 3, fixed between
30 - flanges -34 oE the upper and lower sections 1,2 divides
the housing into an upper chamber 4 and a lower chamber
5. The joints ~etween the flanges 34 and the plate 3
`' are sealed by gaskets 6.
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The upper chamber 4 has an electrolyte inlet
section 7~ an electrode section 8 and an electrol~te
outlet section 9. The inlet section 7 comprises an
incomer tube 10 which passes through the top 35 of the
upper section 1 and extends down to near the plate 3,
between an upstanding plate 11 and three side walls 36
~of the section 1. The plate Ll~ which is integral with
the plate 3, extends across the width of the chamber 4
and forms an inlet weir which, i.n use, holds a pool of
electrolyte at a level 12, this electrolyte being del- .
ivered via the incomer tube lO.
The electrolyte outlet section 9 comprises an
outlet weir plate l3 which also extends across the
width of the chamber 4, but is forrned by one wall of an
enlarged square end 14 of a downcomer tube 15 which
passes through a hole 16 in the plate 3. The square
end 14 is fitted in a corresponding square recess
defined by the side walls 36 of the upper section 1 and .
an upstanding plate l7 integral with the plate 3. The
top of the outlet weir plate 13, is lower than the top of
the inlet weir plate ll and, in use, it maintains :
electrolyte in the electrode section 8 at a level l8.
In the electrode section 8 are disposed seven
electrodes lq in the form of plates held in spaced '
parallel relationship in vertical grooves 20 in the .
plates 11 and 17. The electrodes l9 are disposed between .
two terminal electrodes 21 which are inset
in the side walls 36 through which pass current leads 22.
The plate 11 at one end of the spaced electrodes, and
the plates 17 and 13 at the other end define, with the .
side walls 36 of the section l, an electrolyte receptacle :
whose' ~ottom is formed b~ a perforated central part of
the plate 3, The perforations in the plate 3, indicated
at 23~ are in the for~ of circular holes or bores having
35 a diameter of a~out lmm, arranged in eight rows each of :
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seven equall~spaced holes disposed mid~wa~ between
the adjacent electrodes 19 or 19 and 21.
The upper housing section 1 also comprises,
in its top 35, a gas outlet pipe 24 for the removal of
gas from the upper chamber 4.
In the lower chamber 5, the downcomer tube
15 extends near to the bottom, below the level of an
upstanding wall 3Q which forms a trap or weir holding
a pool of outgoing electrolyte at a level 31. In the
10 bottom of the lower section 2 is an outlet pipe 32 for
removing the electrol~te which has flowed over the weir
wall 3Q. The lower housing section 2 also has a gas
inlet pipe 33 for delivering gas into the lower chamber 5.
Electrolyte in the bottom of the chamber 5 prevents this
15 gas from escaping via the outlet pipe 32 or the down-
comer tube 15, so that the gas passes up through the
perforations 23 in the plate 3 and bubbles into the
electrolyte ~etween the electrodes 19, and 19 and 21
in the electrode section 8.
To operate the cell, the electrolyte outlet
ipe 32 is connected to the incomer tube 10 by an
electrolyte circulating system, and the gas outlet pipe
24 is connected to the inlet pipe 33 by a gas circula~
ting s~stem. The electrolyte is circulated at a chosen .
25 rate so that fresh electrolyte from the incomer tube 10
flows over the inlet weir, namely the plate 11, across
the electrode section 8, i.e. through the parallel channels
defined between the electrodes 19, and 19 and 21, and out
over the outlet weir plate 13. The gas is also cir-
3Q culated at a chosen rate, which can be adjusted
independently of the electrolyte flow rate, It passes :
from,the lowe:r chamber 5 up through the perforations 23 .
and bubbles.t~rough the electrolyte between the electrodes
19, and 1~ and 21 into the upper or gas-collection chamber
4. When all of th.e flows have been set up, current is
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s~pplied to the electrodes 19 and 21. In some
instances~ i.e. when the gas is a reactant, current is
supplied to the electrodes as the gas is supplied/ and
the operation is advanta~eously continuous, i.e with
constant electrolyte and gas flowrates. In other
instances, however, it may be advantageous to operate
discontinuo~lsly, i.e. with an intermittent flow of
~electrolyte or gas or both, with current supply during
the appropriate phase The product of the electro-
chemical reaction may be taken off as a gas and removedfrom the gas stream before recirculating, or may be :
taken of~ dissolved in the electrolyte, in which case it
is removed from the electrolyte before recycling. For
the production of propylene oxide, the product will
partition itself between the electrol~te and the gas
phase and may therefore advantageously be removed
through the gas outlet pipe 24 and separated by conden-
sation.
Industrial applicability
A ce~l as shown in Figs. 1 to 3 was used for
the production of propylene oxide. The electrodes were
plates of graphite each 5.3 cm high, 8.3 ~ lo~g and
O.3 cm thick and spaced apart by a distænce ~ 2~out 4 mm.
The electrolyte, 5 litres of 0.1M or Q.2M NaBr solution, .
was flowed at a constant rate, in the range from about 20
to 45 cm3/sec. Propylene gas was also circulated, using
a supply of fresh propylene at a constant rate in the
range from about 5 to 40 cm3/sec. Before supplying a
constant current (at from 1 to 2A and a constant voltage
from 25 to 4QV), the propylene was circulated Ior several
minutes to remo~e air from the cell housing and to saturate
the electrolyte solution. Operation ~as carried out at
ambient temperature and atmospheric.pressure and the pH of
the electrol~te ~as main~ained between about ll and 12 by
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adding HBr solution. Gas and li~uid samples were
checked at 1/2 hourly intervals. In some instances,
a foaming agent ("Decon", Trademark~ was added with a
view to promoting rapid mass transport of the reactants
to the electrodes, and to increase the solubility of
propylene. The results showed a high current efficiency,
about 8~%, and a low energy consumption, 0.2 to 0.3
kwh/gmol of propylene oxide when operating at low
temperature, low current and lo~ gas flowrate using
la dilute NaBr. For the epoxidation of l-butene using the
same cell, an energy consumption of 0.26 kwh/gmol of
butylene oxide was achieved at a current efficiency
approaching that obtained with propylene oxide. These
performances may be improved b~ optimizing the cell
dimensions and process conditions and, possibly, by
operating at elevated pressures.
~ s shown in Fig. 4~ several cells similar to
that of Figs. 1 to 3 can be stacked in a column with the
electrolyte flow system connected in cascade. In Fig.
2Q 4~ the same parts are designated by the same references
as before, some parts of the intermediate cells being
dèsignated by double references. The upper section 1
of the housing of the top cell and the lower section 2
of the housing of the bottom cell are exactly the
same as the upper and lower sections 1 and 2 of Figs. 1
and 2. Howe~er~ in the reactor column, the perforated
plate 3 forming the bottom of one cell also forms the
top of the gas collection chamber 4 of the cell below
and its perforations 23 act as the ~as outlet for tha-t
3Q chamber; the downcomer tube 15 for the discharge of
electrolyte ~:rom one cell forms the incomer tube 10
of the cell helow; the gas collection chamber 4 of each
cell (except the top onel forms the gas suPpl~ cham~er 5
for the cell above; and so forth.
In operation of this reactor column~ gas is
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supplied at the bottom of the column, via the inlet
pipe 33, passes up through the successive cells,
bubbling up through the electrolyte in each electrode
section 8, and is removed from the top of the column
via the outlet pipe 24. Electrolyte is supplied
at the top 35 of the column via the top incomer tube
10 and, as indicated by the arrow, cascades down from
one cell to the next, flowing across the electrode
section 8 of each cell, and is removed from the hottom
of the column via the outlet pipe 32. ~s before,
current is supplied to the electrodes of each cell and
the operation may be continuous or discontinuous.
Many variations may be made to the described
embodiments. Various electrode materials can be used,
depending on the reaction: in particular, dimen-
sionally-stable metal electrodes will be preferred for
some reactions. Also~ the electrodes need not be
bipolar, In some instances, spaced parallel electrodes
can be disposed transverse to the general direction
2Q of flow of electrolyte across the perforated plate.
Various shapes and sizes of perforations can be pro~ided
in this plate and, instead of being disposed between the -
adjacent electrodes, for certain reactions these per-
forations could lead into porous or foraminous electrodes
disposed on the perforated plate. Instead of the
preferred electrolyte inlet and outlet weirs, other means
could be provided to enable a flow of the electrol~te
generally across the perforated plate, while maintaining
a given electrolyte level.
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