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The behaviour of bipolar packed-bed electrodes
THE BEHAVIOUR ‘OF BIPOLAR PACKED-BED ELECTRODES F. GOODRIDGE, C. J. H. KING* and A. R. WRIGHT Department of Chemical Engineering, University of Newcastle upon Tyne, NE1 7RU, England (Received2 June 1976; and in final form1 August 1976) Abatraet-A novel cell design consisting of a packed bed of bipolar particles is described. Reactions investigated are the production of hypobromite and the epoxidation of styrene. Results are,interpreted in terms of energy consumption and space time yields. A model of the bed is shown to predict observed energy consumption.
NOMENCLATURE
A
A+
A:*
L N R
s
v,
K r,T
reactionarea anodiqreaction area cathodic reaction area voltage gradient through bed energy consumption length ofcylinders no. of rnds per unit volume radius of cyliadeas active area per unit volume equivalent cell voltage thrashold voltage space time yield
(cm’) g:i (V cm-‘)
(kWh kg mole-‘) (cm) (cm-? $-I) (V) (kg mole s-l rne3)
E
distance. deEned by Fig. 3 voidage
IZjiension-
6
angle &efinedby Fig. 3
(Radians)
x
although for the purpose of comparison, some of the experiments reported were done using mixed beads. A third method of particle isolation in wtiich no mechanical means of separation is employed, is by fluidisation[4]. Work on this type of electrode will be presented in a later paper. Two reactions have been investigated so far. The first is the production of hypobroinite with a view to its use in epoxidation reactions. The second is M example of such an epoxidatian, namely the formation of styrene oxide. The overall scheme for the reactions can be written as follows. At the anodic face: 2Br- - 2e+Br, Brl + Hz04
(1)
H* + Br- + HOBr
(2)
At the cathodic face: 2H20 + 2e + Hz + 20H1 lNTRODUCTION In recent years considerable work has been done on particulate electrodes. These consist of individual electronically conducting particles, and can be in the form of packed or fluid&d beds, with either system being capable of operating in a monopolar or bipolar manner[l]. The present paper deals with packed bed systems of bipolar particles. So far bipolar packed beds have not been used industrially but the system has certain advantages. For example, it is capable of dealing with poorly conducting electrolytes, often a feature of organic synthesis. Quite clearly this system can only be used where intimate mixing of catholyte and anolyte is either an advantage or at least not deleterious. For a bipolar bed to work, the electronically conducting particles must be isolated from each other, One way of achieving this is by the use of a mixture of conducting and non-conducting beads[2,3]. An alternative technique is to use a packed bed of rods isolated from each other by means of Nylon rings (Fig. 1). Most of the results given in the present paper were obtained with this system, * Now with Monsanto Company, Pensacola, Florida, U.S.A.
(3)
If styrene is present then bromide reacts with this in the anode region to form the bromohydrin.
(41
Applied
field
Fig. 1. Schematic of arraqwmentof rods and insulating hlgS.
F.
348
&3DRIDGE,
C. J.
H. KINGAND A. k. WRIG~
Due to the intimate mixing of anolyte and catholyte, the bromohydrin is saponified by the hydroxyl ions generated according to reaction (3).
The organic species were determined by means of gas liquid chromatography, using a 2 m long column of 5% Apiezon 2’ on Chromosorb G. A temperature programme of 80”-140°C was used. Product identification was confirmed on a 4m long column of 5% Carbowax 20M on Chromosorb A, using the same temperature programme as above. Further details on experimentation and analysis can be found in[S].
2 EXPERIMENTAL
2.1. Cell and associated
3 THEORETICAL
equipment
The cell C, (Fig. 2) was made from P.T.F.E. and fitted with two graphite plate current feeders E, (14 cm’ in area) placed as shown. Two forms of packing material were used: mixtures of glass and graphite coated[3], beads (0.05 cm in dia) and a stacked array of graphite rods (Fig. i), 3.5 cm in length, each fitted with two Nylon O-rings of 1 mm wall thickness. The rods were obtained from British Acheson Ltd type A.G.S.R. and cut to the required length. The electrolyte flow circuit was arranged for a single pass through the cell (hypobromite production) or recirculation (styrene oxide production). Power to operate the cell was obtained from a variety of voltage stabihsed dc units. Extreme values of current and voltage used (not at the same time) were 25A and 200 V respectively. All the results quoted in the present paper were obtained using identical flow rates (3 cm3 s-i). 2.2. Chemical analysis The electrolyte was analysed for total bromine generated (in the form of hypobromite and brotnate) by standard iodometric titration in an acid medium. Under the conditions of operation, bromate would only be formed by chemical reaction, and then only if there was significant heat evolution in the cell. In the presence of styrene, no bromate or unreacted hypobromite was detected.
For a bipolar bed electrode where the reaction takes place at either end of each particle, the reaction areas per unit volume of bed, A+ and A_, for the anodic and cathodic reactions (Fig. 31,may be related to the applied voltage gradient, and further related to the power consumption of the bed. This treatment was first applied by Fleischmann et nI[6] to a bipolar system of conducting and non-conducting spheres. One simplifying assumption made was that the reaction was mass transfer limited over the whole of the reaction areas; and could be approximated by the shape shown dotted in Fig. 3, hence the ul over the active particle surface would be uniform. It is interesting to note that in the present system the reaction is not diffusion limited and yet the current diitribution over the projected area of the particle surface is nevertheless uniform. This has been confirmed experimentaIly[TJ by the use of a cylinder machined from a laminated block of graphite and Perspex sheets. The multi-sectioned particle (Fig. 4) enabled the current flowing in each section to be monitored externally with an ammeter. In the present situation the system has two degrees of freedom for coping with increases in Farad& current. Firstly by increasing the extent of active areas
+
I
Fig. 2. Schematic of flow apparatus for the Bipolar Packed Bed cell. C-cell; F-flowmeter; H-cooler; G-pH elantrade; R-reservoir; T-thermometer; P-PalrIP; V-valve; E-feeder electrode.
-E
Fig.
V.cm-’
3. Electraactive area on a bipolar cylindrical electrode.
Behaviour of bipolar packed-bed electrodes
349
If there are N rods per unit volume then S = ZNRLcos-l
& [
1
where S=NA.
(vii)
Now N=
solid volume l--E volume of a rod =a’
(viii)
Where Qis the voidage in the bed
L-l-
,?(I -E)COS-lvo
;,
XR
2RE
(ix)
Now S can be tonsidered as the number of bipolar cells per unit length in order to give the same performance for a plane parallel configuration as that of the packed bed. We can therefore define an equivalent cell voltage V, by
Applied field
E
xRE
” = s = 2(1 - E)COS-’[1/,/2REj
(‘)
Fig. 4. Sedioned particle.
shown in Fig. 3 and secondly by raising the electrode potential and hence the current density. At this stage of the experimental work, indications are that for lower values of applied voltage the systems copes by an increase in active area, followed at higher voltages by an increase in cd. For this reason we can expect Fleischmann et ars model to apply outside mass transfer controlled conditions at lower applied voltages. We will now apply Fleischmann et nPs[6] treatment to the present system of conducting cylinders of length Land radius R, immersed in an electrolyte parallel to the feeder electrodes. If a voltage gradient E is applied to the cell (Fig. 3) then above a threshold voltage V, the cylinder begins to function as a bipolar electrode. The active area A, where it is assumed that A = A+ = A_, is given by: A = 2RBL but 0 = cos-’ (1 - x/R).
(ii)
A = 2RLcos-’
(iii)
Hell.% (1 - x/R).
Hence V, can be regarded as the voltage across each of the S cells that would be necessary to achieve this performance, For a %-electron process, and looo/, current efliciency E, the power required to produce 1 kg-mole of product at a “cell voltage” V, is given by: EC= 55.5 V,. Thus from (x) 21.8 zRE Ec = (1 - c)cos-~ [Vo/2REj
(4
Differentiating (xi) with respect to E and equating the condition for whi& E, is a minimum:
to zero gives
Em,,,= 0.77 V,/R
(xii)
hence 18 v,
E”‘”= (1 c
In the treatment above it has been assumed that the whole of the current flows through the conducting cylinders. In practice it has been found[5] that for a given set of experimental conditions (electrode material, geometry and electrolyte compositions) the proportion of the non-Faradaic by-pass current through the electrolyte is a function of applied voltage. The calculated plot of E, vs applied voltage (Fig. 7) incorporates experimentally determined by-pass currents. Additionally, the effect of feeder area on S has also been included.
Now
(iv)
x = R - V,/2E. Hence
1
vo
2RE .
A= 2RLcos-’
[
(VI
4 IWSULTS AND DISCUSSION The results are interpreted in terms of two costsensitive parameters: energy consumption E, already considered in the theoretical section and the reciprocal of space time yield l/Y,,.
F. &ODRIDCE,
C. J.
H
7
u. b
IS0
I.0 -
i
KING AND A. R. WRIGHT
Looking at Fig. 5, it can be seen that E, passes through a minimum as predicted by theory. The scatter, however, is too great to make any more detailed consideration worth while. One of the reasons for this experimental uncertainty is the already mentioned maldistribution of conducting particles resulting in a spectrum of effective particle size. 4.2. Rod electrodes Results discussed so far $rompted the development of a structure, ie packed rods, which have a large electroactive area yet avoid the shorting. out of individual packing pieces. The remaindti of the paper concentrates on results obtained with this system.
OS-
THE PRODUCTION OF HYPOBROMIII! (i) Energy conswnpzion 40
20
60
E (Vcm“) Fig. 5. Plot of energy consumption against applied field, for mixed beads.
4.1. Bead electrodes Early results which were confined to the production of hypobromite used mixtures of conducting and nonconducting beads as bed materials. Figurea 5 and 6 show the variation of the values of ECand l/Ys, with voltage gradient, E, across the bed for 0.05cm dia spheres, and a ratio of 1:l between non-conducting and conducting beads. The main problem expe&nced with this type of packing was the difficulty of achieving a sufficiently uniform mixture of the two types of beads. In order to isolate more of the conducting beads, the above ratio was increased[3]. This improved values of EC but understandably increased the respective vallles of l/Y,. Larger beads (ClUcm dia) gave similar values for E,, but resulted in a considerable increase in i/Y,,. These later experiments were carried out in a larger cell (7cm x 7cm x 8.5cm) in order to reduce wall effects.
Figure 7 shows calculated and experimental energy consumptions as a function of the applied field, the rods being 0.5 cm dia with a voidage of 0.4. Using the model described in Section 3, curve I has been calculated for this rod size, assuming a current efficiencies of looO/,and using observed by-pass currents. The initial linear portion corresponds to the operation of the feeder plates alone. At a value of 1.8 V across the particle (ie 3.6 V on the abscissa), corresponding to the decomposition voltage of NaBr, the bed becomes active with a resultant decrease in EC. Beyond the minimum, there is relatively little increase in active area with fiedd strength and. hence EC rises. On the whole the experimental curve II follows that predicted by the model and exhibits a distinct minimum. The divergence between curves I and .I1 beyond Em,,,,could be ascribed to a reduction in current efficiency as a consequence of additional reaftiona. Figure 8 shows experimental energy consumptioK.l$ as a function of rod dia. It can be seen that p decreases with increase in rod size. According to (xiii) I$“‘” should, however, be independent of R. The explanation can be found in a change in l, since the
3-
lIw’MBr.
1
5rlo-sMor-
0
ICI-’M ar-
i
52
1”
a
VI
“E
20 E.
&’
Fig. 6. Plot of reciprocal of space time yield against applied field, for mixed beads.
Fig. 7. Influence of applied
field on the energy consumption for hypobromite production on 0.47cm dia rods.
Behaviour of
ZxlUIM,
t
.
Nab
L
bipolarpacked-bed
351
electrodes
0.94
1.20
”
Fig. 8. Influence of applied field on energy consumption for different rd sizes-2 x IO-‘M NaBr. 0
thickness of the riylon spaqr rem&s the same for all rod sizes. &itiougb the acti@ vzit@ion ‘in c is not sufficient to ac&unt for the whole’ df the observed change, ttqre, .is also as a,,result of the variation in 6 a significant change in Ce:cje@w af by-w current. Taking I&,,, values from Fig. 8 and plot& them against l# (Fig. 9) not only gives a sttaigbt line as predicted by (xii), but, also results in a value of 1.9 V for v,, in good :agreement with that estimath from polarisation curves.
Fi&e 40 Sotis the tapid decreasq ia required cell size with inqeaqe b appiii voltage and &ncenttation of pkectrolyte. Clearly the actual conditions of operation will depend on the relative magnitude of capital and power costs. THE PRODUCTION
OF STYRENE OXIDE
The overall reactions have already been given in (4) and (5). The system is a difficult one to operate
i
5
IO E,
km-’
Fig: 10. Variation of reciprocal of space time yield with applied field for 0.47cm dia rods.
since-ih the absence of anv organic solvent both stvrene and styrene oxide to&llypa&vate the graphiie. For this reason the work described below employed dioxane as a co-solvent with water. The equipment was identical with that used for hypobromite product&, but in the case of styrene oxide was operated with recirculation of electrolyte over a number of hOIll?%
(i) Energy cumumption
Figure 11 is a plot of fi, against applied field. As expected, the shape is similar to that for hypobromite production and the value of E,,,, is the same. The steepness of the curve beyond Emi, can be explained by the fact that analysis of styrene and styrene oxide
E
0 4s
I
IO
5 E,
Vcm-’
cm-’
Fig. 9. Variation of E,,, with rod size.
Fig. 11. Energy consumption for styrene oxide production as a function of applied field.
F. GOODIUDGB, C. J. H. KINGANDA. R. W~UOHT
352
Styrene dibromide was detected and was found to increase with increasing styrene and bromide concen-
-L IO s
CONCLUSIONS
II)
“E
(1) The use of insulated rods instead of mixtures of conducting and nonconducting spheres for bipolar packed bed electrodes represents a significant improvement in performance. (2) The resulting electrode system is shown to exhibit attractive qualities for industrial electro-synthesis, namely Iow energy consumption and high space time yields.
0’
'0 _-
5
0
I
2 Time.
h
Fig. 12. Variation of the reciprocal of space time yield for styrene oxide production with time.
REFERENCES
1. F. Goodridge and C. J. H. King, Technique OJ’Electrooqpnic Synthesis (Edited by N. L. Weinberg) Ch.
indicated the disappearance of the starting material. This could not be accounted for by the production of brominated products or the desttiotion of styrene oxide. At this stage we can but speculate that a new electrode process is occurring namely the anodie oxidation of styrene[S] with a resultant polymerk product or even C02. (ii) Space time yield
From the plot of l/Y,, against voltage (Fig. 12) it is clear that the performance is below that for hypebroniite produation. The reason for this can be found in the relatively low conductivity of the electrolyte due to the presence of dioxane, resulting in lower cell currents and hence space. time yields.
2. In Volume V, (I) of the series TecIiniqua try, p. 98. John Wiley, New York (1974).
of Chemis-
2. ibidi. 111. 3. 0. B. Osiiadq Ph.D. Thesis, Univasity of Newcastle uuon Tvne. Emland (1973. 4. F: Goo&i&e, 5. J. h. King and A. R. Wright, Proceed&p ef 25th. Meerh of Int. Sot. /br Eiecrro-
&n~&, -Brighton,Engl&d~(Sepkanber1974). 5. A. R. Wright, Ph.D. Thesis, University of Newcastle upon Tync, Edand (1975). 6. M. Fleisbmann, J. W. Oldfield and C. L. K. Tennakeon, Symposium of Electrochemical EngineorM, Volume 1. p. 1.53. The Institution of Chemical Engineers Symposium Skria No. 37, London (1973). 7. C. J. H. King and A. R. Wright, To be pubfished. 8. M. Katz, P. RienIctschneider and H. Wet&, Eiectroc/Urn.Acta 17, 1595(1972).
NOMENCLATURE
A
A+
A:*
L N R
s
v,
K r,T
reactionarea anodiqreaction area cathodic reaction area voltage gradient through bed energy consumption length ofcylinders no. of rnds per unit volume radius of cyliadeas active area per unit volume equivalent cell voltage thrashold voltage space time yield
(cm’) g:i (V cm-‘)
(kWh kg mole-‘) (cm) (cm-? $-I) (V) (kg mole s-l rne3)
E
distance. deEned by Fig. 3 voidage
IZjiension-
6
angle &efinedby Fig. 3
(Radians)
x
although for the purpose of comparison, some of the experiments reported were done using mixed beads. A third method of particle isolation in wtiich no mechanical means of separation is employed, is by fluidisation[4]. Work on this type of electrode will be presented in a later paper. Two reactions have been investigated so far. The first is the production of hypobroinite with a view to its use in epoxidation reactions. The second is M example of such an epoxidatian, namely the formation of styrene oxide. The overall scheme for the reactions can be written as follows. At the anodic face: 2Br- - 2e+Br, Brl + Hz04
(1)
H* + Br- + HOBr
(2)
At the cathodic face: 2H20 + 2e + Hz + 20H1 lNTRODUCTION In recent years considerable work has been done on particulate electrodes. These consist of individual electronically conducting particles, and can be in the form of packed or fluid&d beds, with either system being capable of operating in a monopolar or bipolar manner[l]. The present paper deals with packed bed systems of bipolar particles. So far bipolar packed beds have not been used industrially but the system has certain advantages. For example, it is capable of dealing with poorly conducting electrolytes, often a feature of organic synthesis. Quite clearly this system can only be used where intimate mixing of catholyte and anolyte is either an advantage or at least not deleterious. For a bipolar bed to work, the electronically conducting particles must be isolated from each other, One way of achieving this is by the use of a mixture of conducting and non-conducting beads[2,3]. An alternative technique is to use a packed bed of rods isolated from each other by means of Nylon rings (Fig. 1). Most of the results given in the present paper were obtained with this system, * Now with Monsanto Company, Pensacola, Florida, U.S.A.
(3)
If styrene is present then bromide reacts with this in the anode region to form the bromohydrin.
(41
Applied
field
Fig. 1. Schematic of arraqwmentof rods and insulating hlgS.
F.
348
&3DRIDGE,
C. J.
H. KINGAND A. k. WRIG~
Due to the intimate mixing of anolyte and catholyte, the bromohydrin is saponified by the hydroxyl ions generated according to reaction (3).
The organic species were determined by means of gas liquid chromatography, using a 2 m long column of 5% Apiezon 2’ on Chromosorb G. A temperature programme of 80”-140°C was used. Product identification was confirmed on a 4m long column of 5% Carbowax 20M on Chromosorb A, using the same temperature programme as above. Further details on experimentation and analysis can be found in[S].
2 EXPERIMENTAL
2.1. Cell and associated
3 THEORETICAL
equipment
The cell C, (Fig. 2) was made from P.T.F.E. and fitted with two graphite plate current feeders E, (14 cm’ in area) placed as shown. Two forms of packing material were used: mixtures of glass and graphite coated[3], beads (0.05 cm in dia) and a stacked array of graphite rods (Fig. i), 3.5 cm in length, each fitted with two Nylon O-rings of 1 mm wall thickness. The rods were obtained from British Acheson Ltd type A.G.S.R. and cut to the required length. The electrolyte flow circuit was arranged for a single pass through the cell (hypobromite production) or recirculation (styrene oxide production). Power to operate the cell was obtained from a variety of voltage stabihsed dc units. Extreme values of current and voltage used (not at the same time) were 25A and 200 V respectively. All the results quoted in the present paper were obtained using identical flow rates (3 cm3 s-i). 2.2. Chemical analysis The electrolyte was analysed for total bromine generated (in the form of hypobromite and brotnate) by standard iodometric titration in an acid medium. Under the conditions of operation, bromate would only be formed by chemical reaction, and then only if there was significant heat evolution in the cell. In the presence of styrene, no bromate or unreacted hypobromite was detected.
For a bipolar bed electrode where the reaction takes place at either end of each particle, the reaction areas per unit volume of bed, A+ and A_, for the anodic and cathodic reactions (Fig. 31,may be related to the applied voltage gradient, and further related to the power consumption of the bed. This treatment was first applied by Fleischmann et nI[6] to a bipolar system of conducting and non-conducting spheres. One simplifying assumption made was that the reaction was mass transfer limited over the whole of the reaction areas; and could be approximated by the shape shown dotted in Fig. 3, hence the ul over the active particle surface would be uniform. It is interesting to note that in the present system the reaction is not diffusion limited and yet the current diitribution over the projected area of the particle surface is nevertheless uniform. This has been confirmed experimentaIly[TJ by the use of a cylinder machined from a laminated block of graphite and Perspex sheets. The multi-sectioned particle (Fig. 4) enabled the current flowing in each section to be monitored externally with an ammeter. In the present situation the system has two degrees of freedom for coping with increases in Farad& current. Firstly by increasing the extent of active areas
+
I
Fig. 2. Schematic of flow apparatus for the Bipolar Packed Bed cell. C-cell; F-flowmeter; H-cooler; G-pH elantrade; R-reservoir; T-thermometer; P-PalrIP; V-valve; E-feeder electrode.
-E
Fig.
V.cm-’
3. Electraactive area on a bipolar cylindrical electrode.
Behaviour of bipolar packed-bed electrodes
349
If there are N rods per unit volume then S = ZNRLcos-l
& [
1
where S=NA.
(vii)
Now N=
solid volume l--E volume of a rod =a’
(viii)
Where Qis the voidage in the bed
L-l-
,?(I -E)COS-lvo
;,
XR
2RE
(ix)
Now S can be tonsidered as the number of bipolar cells per unit length in order to give the same performance for a plane parallel configuration as that of the packed bed. We can therefore define an equivalent cell voltage V, by
Applied field
E
xRE
” = s = 2(1 - E)COS-’[1/,/2REj
(‘)
Fig. 4. Sedioned particle.
shown in Fig. 3 and secondly by raising the electrode potential and hence the current density. At this stage of the experimental work, indications are that for lower values of applied voltage the systems copes by an increase in active area, followed at higher voltages by an increase in cd. For this reason we can expect Fleischmann et ars model to apply outside mass transfer controlled conditions at lower applied voltages. We will now apply Fleischmann et nPs[6] treatment to the present system of conducting cylinders of length Land radius R, immersed in an electrolyte parallel to the feeder electrodes. If a voltage gradient E is applied to the cell (Fig. 3) then above a threshold voltage V, the cylinder begins to function as a bipolar electrode. The active area A, where it is assumed that A = A+ = A_, is given by: A = 2RBL but 0 = cos-’ (1 - x/R).
(ii)
A = 2RLcos-’
(iii)
Hell.% (1 - x/R).
Hence V, can be regarded as the voltage across each of the S cells that would be necessary to achieve this performance, For a %-electron process, and looo/, current efliciency E, the power required to produce 1 kg-mole of product at a “cell voltage” V, is given by: EC= 55.5 V,. Thus from (x) 21.8 zRE Ec = (1 - c)cos-~ [Vo/2REj
(4
Differentiating (xi) with respect to E and equating the condition for whi& E, is a minimum:
to zero gives
Em,,,= 0.77 V,/R
(xii)
hence 18 v,
E”‘”= (1 c
In the treatment above it has been assumed that the whole of the current flows through the conducting cylinders. In practice it has been found[5] that for a given set of experimental conditions (electrode material, geometry and electrolyte compositions) the proportion of the non-Faradaic by-pass current through the electrolyte is a function of applied voltage. The calculated plot of E, vs applied voltage (Fig. 7) incorporates experimentally determined by-pass currents. Additionally, the effect of feeder area on S has also been included.
Now
(iv)
x = R - V,/2E. Hence
1
vo
2RE .
A= 2RLcos-’
[
(VI
4 IWSULTS AND DISCUSSION The results are interpreted in terms of two costsensitive parameters: energy consumption E, already considered in the theoretical section and the reciprocal of space time yield l/Y,,.
F. &ODRIDCE,
C. J.
H
7
u. b
IS0
I.0 -
i
KING AND A. R. WRIGHT
Looking at Fig. 5, it can be seen that E, passes through a minimum as predicted by theory. The scatter, however, is too great to make any more detailed consideration worth while. One of the reasons for this experimental uncertainty is the already mentioned maldistribution of conducting particles resulting in a spectrum of effective particle size. 4.2. Rod electrodes Results discussed so far $rompted the development of a structure, ie packed rods, which have a large electroactive area yet avoid the shorting. out of individual packing pieces. The remaindti of the paper concentrates on results obtained with this system.
OS-
THE PRODUCTION OF HYPOBROMIII! (i) Energy conswnpzion 40
20
60
E (Vcm“) Fig. 5. Plot of energy consumption against applied field, for mixed beads.
4.1. Bead electrodes Early results which were confined to the production of hypobromite used mixtures of conducting and nonconducting beads as bed materials. Figurea 5 and 6 show the variation of the values of ECand l/Ys, with voltage gradient, E, across the bed for 0.05cm dia spheres, and a ratio of 1:l between non-conducting and conducting beads. The main problem expe&nced with this type of packing was the difficulty of achieving a sufficiently uniform mixture of the two types of beads. In order to isolate more of the conducting beads, the above ratio was increased[3]. This improved values of EC but understandably increased the respective vallles of l/Y,. Larger beads (ClUcm dia) gave similar values for E,, but resulted in a considerable increase in i/Y,,. These later experiments were carried out in a larger cell (7cm x 7cm x 8.5cm) in order to reduce wall effects.
Figure 7 shows calculated and experimental energy consumptions as a function of the applied field, the rods being 0.5 cm dia with a voidage of 0.4. Using the model described in Section 3, curve I has been calculated for this rod size, assuming a current efficiencies of looO/,and using observed by-pass currents. The initial linear portion corresponds to the operation of the feeder plates alone. At a value of 1.8 V across the particle (ie 3.6 V on the abscissa), corresponding to the decomposition voltage of NaBr, the bed becomes active with a resultant decrease in EC. Beyond the minimum, there is relatively little increase in active area with fiedd strength and. hence EC rises. On the whole the experimental curve II follows that predicted by the model and exhibits a distinct minimum. The divergence between curves I and .I1 beyond Em,,,,could be ascribed to a reduction in current efficiency as a consequence of additional reaftiona. Figure 8 shows experimental energy consumptioK.l$ as a function of rod dia. It can be seen that p decreases with increase in rod size. According to (xiii) I$“‘” should, however, be independent of R. The explanation can be found in a change in l, since the
3-
lIw’MBr.
1
5rlo-sMor-
0
ICI-’M ar-
i
52
1”
a
VI
“E
20 E.
&’
Fig. 6. Plot of reciprocal of space time yield against applied field, for mixed beads.
Fig. 7. Influence of applied
field on the energy consumption for hypobromite production on 0.47cm dia rods.
Behaviour of
ZxlUIM,
t
.
Nab
L
bipolarpacked-bed
351
electrodes
0.94
1.20
”
Fig. 8. Influence of applied field on energy consumption for different rd sizes-2 x IO-‘M NaBr. 0
thickness of the riylon spaqr rem&s the same for all rod sizes. &itiougb the acti@ vzit@ion ‘in c is not sufficient to ac&unt for the whole’ df the observed change, ttqre, .is also as a,,result of the variation in 6 a significant change in Ce:cje@w af by-w current. Taking I&,,, values from Fig. 8 and plot& them against l# (Fig. 9) not only gives a sttaigbt line as predicted by (xii), but, also results in a value of 1.9 V for v,, in good :agreement with that estimath from polarisation curves.
Fi&e 40 Sotis the tapid decreasq ia required cell size with inqeaqe b appiii voltage and &ncenttation of pkectrolyte. Clearly the actual conditions of operation will depend on the relative magnitude of capital and power costs. THE PRODUCTION
OF STYRENE OXIDE
The overall reactions have already been given in (4) and (5). The system is a difficult one to operate
i
5
IO E,
km-’
Fig: 10. Variation of reciprocal of space time yield with applied field for 0.47cm dia rods.
since-ih the absence of anv organic solvent both stvrene and styrene oxide to&llypa&vate the graphiie. For this reason the work described below employed dioxane as a co-solvent with water. The equipment was identical with that used for hypobromite product&, but in the case of styrene oxide was operated with recirculation of electrolyte over a number of hOIll?%
(i) Energy cumumption
Figure 11 is a plot of fi, against applied field. As expected, the shape is similar to that for hypobromite production and the value of E,,,, is the same. The steepness of the curve beyond Emi, can be explained by the fact that analysis of styrene and styrene oxide
E
0 4s
I
IO
5 E,
Vcm-’
cm-’
Fig. 9. Variation of E,,, with rod size.
Fig. 11. Energy consumption for styrene oxide production as a function of applied field.
F. GOODIUDGB, C. J. H. KINGANDA. R. W~UOHT
352
Styrene dibromide was detected and was found to increase with increasing styrene and bromide concen-
-L IO s
CONCLUSIONS
II)
“E
(1) The use of insulated rods instead of mixtures of conducting and nonconducting spheres for bipolar packed bed electrodes represents a significant improvement in performance. (2) The resulting electrode system is shown to exhibit attractive qualities for industrial electro-synthesis, namely Iow energy consumption and high space time yields.
0’
'0 _-
5
0
I
2 Time.
h
Fig. 12. Variation of the reciprocal of space time yield for styrene oxide production with time.
REFERENCES
1. F. Goodridge and C. J. H. King, Technique OJ’Electrooqpnic Synthesis (Edited by N. L. Weinberg) Ch.
indicated the disappearance of the starting material. This could not be accounted for by the production of brominated products or the desttiotion of styrene oxide. At this stage we can but speculate that a new electrode process is occurring namely the anodie oxidation of styrene[S] with a resultant polymerk product or even C02. (ii) Space time yield
From the plot of l/Y,, against voltage (Fig. 12) it is clear that the performance is below that for hypebroniite produation. The reason for this can be found in the relatively low conductivity of the electrolyte due to the presence of dioxane, resulting in lower cell currents and hence space. time yields.
2. In Volume V, (I) of the series TecIiniqua try, p. 98. John Wiley, New York (1974).
of Chemis-
2. ibidi. 111. 3. 0. B. Osiiadq Ph.D. Thesis, Univasity of Newcastle uuon Tvne. Emland (1973. 4. F: Goo&i&e, 5. J. h. King and A. R. Wright, Proceed&p ef 25th. Meerh of Int. Sot. /br Eiecrro-
&n~&, -Brighton,Engl&d~(Sepkanber1974). 5. A. R. Wright, Ph.D. Thesis, University of Newcastle upon Tync, Edand (1975). 6. M. Fleisbmann, J. W. Oldfield and C. L. K. Tennakeon, Symposium of Electrochemical EngineorM, Volume 1. p. 1.53. The Institution of Chemical Engineers Symposium Skria No. 37, London (1973). 7. C. J. H. King and A. R. Wright, To be pubfished. 8. M. Katz, P. RienIctschneider and H. Wet&, Eiectroc/Urn.Acta 17, 1595(1972).