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Hydrazine synthesis in an electrochemical reactor
Pergamon Press.
Chemical Engineering Science, 197 1, Vol. 26, pp. 2087-2098.
Printed in Great Britain
Hydrazine synthesis in an electrochemical reactor P. L. SPEDDING Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand
J. D. THORNTON Department of Chemical Engineering, University, Newcastle upon Tyne, England (First received
14 December 1970: in revisedform
30 March 197 1)
Abstract-Hydrazine was synthesised in two types of gas phase electrochemical reactors, an a.c. barrier parallel electrode cross gas flow reactor, and a d.c. parallel electrode-parallel gas flow reactor. Energy yields of up to 5 g/kWhr were achieved. Optimum operating conditions were in the low power density, pressure and gas residence time regions, and with the use of a pulsed discharge of low active residence time. Of the two reactors the parallel electrode-parallel gas flow type gave the better energy yields. Other design criteria were the use of bonded dielectrics and as large a reactor volume as practicable provided that the gap distance did not exceed the Paschen minimum point. The results are consistent with the mechanism of hydrazine being first formed and then degraded in the discharge. INTRODUCTION
activating properties of the nondisruptive electrical discharge have been known for a long time and a great deal of effort has been directed towards the development of a commercial method of chemical synthesis using this route. The present scanty knowledge of the processes within this type of discharge, coupled with its complicated nature have restricted the application of the technique to virtually one commercial product, namely ozone. However, recent successful application of liquid-phase electrochemical techniques for synthesising organic polymer intermediates on a large scale has given impetus to the current resurgence of interest in gas discharge synthesis [ 11. Chemicals which show promise of commercial production by this route are those which are more reactive than their raw materials and are difficult and expensive to produce by conventional processes. Hydrazine is an obvious candidate for synthesis by the gas discharge route as the present Raschig method of production is inefficient and results in a product costing in excess of f1200 per ton [2]. The production of hydrazine from ammonia
THE CHEMICAL
using the electrical discharge was first demonstrated by Besson [3]. Subsequent detailed investigations showed that both the electrical energy yield and the percentage conversion obtained were low[4]. More recently, substantial improvements in the yields have been obtained under certain circumstances; by a reduction of atomic hydrogen concentration in the discharge through recombination[5], by a reduction in residence time of the hydrazine in the discharge [6.7], and by the removal of hydrazine in liquid adsorbents@]. A mechanism for the synthesis has been proposed based on these scattered data in which it was suggested that hydrazine was first formed in the discharge and then degraded by back-reaction. While certain general trends in available data lend considerable weight to this proposal, certain anomalies do exist. These latter indicate that both operating conditions and the actual design of reactor can be extremely important [9-l 11. These aspects were particularly noticeable when a reduction in residence time was attempted by modifying the waveform characteristic of the discharge to give short activating pulses. Energy yields of hydrazine were observed to increase considerably with these discharge
2087
P. L. SPEDDING
and J. D. THORNTON
modifications. Preliminary reviews of the general field along these lines have been presented recently but more detailed investigations are required to help ascertain the effect on the hydrazine yield of discharge parameters, reactor design, and reactor geometry [ 12, 131. In this work the operating characteristics of both the parallel discharge-cross gas flow and parallel discharge-parallel gas flow types of reactors were investigated. These data, together with previously reported work on the co-axial reactor [ 141, have enabled many of the effects of design variables on yields to be ascertained for the case of the hydrazine reaction.
TOR 4
EXPERIMENTAL
The apparatus consisted of a discharge reactor set between a measuring and an analysing section of a flowing gas system. Commercially pure ammonia was dried and fed to the reactor via reducing valves and a calibrated rotameter. Hydrazine formed in the discharge was absorbed in a liquid, in this case ethylene glycol, in an absorption train and the product hydrazine was determined using the spectrophotometric method developed by Watt and Chrisp[ 151. Provision also was made to take gas samples. Pressure control of the system was achieved by means of a Cartesian manostat which was located before the absorption train and vacuum pump. A one kW, 2.5 MHz generator provided a.c. power for the parallel electrode-cross gas flow barrier discharge reactor, which was designated the type A reactor. An 8 kW. d.c. generator was used for the parallel electrode-parallel gas flow reactor, which was designated the type B reactor. Measurement of the discharge power was achieved by suitable combination of an oscilloscope trace and the appropriate ammeter and voltmeter readings [ 121. Both discharge reactors used were of the parallel electrode type. These reactors are illustrated schematically in Fig. 1 -and essential geometric details and operating data are given in Table 1. The parallel electrode-cross gas flow type reactor employed a flat circular silica barrier set between steel Rogowski[ 161 elec-
REACTOR
B
Fig. 1. Schematic diagrams of the electrochemical reactors used. CO = cooling oil; E = earthed electrode; Gf = gas inlet; GO = gas outlet; HT = high tension electrode; S = silica dielectric barrier.
trodes of effective diameter 3-l cm. The incoming gas was fed through the centre of the inner earthed electrode and passed radially through the discharge gap. Electrode cooling was provided by circulating oil within the electrodes. With this design it was possible to alter the electrode gap, the barrier thickness and the size and design of the upper electrode. However, it was not possible to include an absorbing liquid spray. The parallel electrode-parallel gas flow type reactor consisted of a pair of porous circular steel electrodes set in a cylindrical glass tube. The tube had a centre cylindrical segment of its wall made of porous glass to enable gas to be withdrawn from the inter-electrode region of the reactor. D.C. power was used with this latter type of reactor and current stabilization was achieved by incorporating a suitable resistive load in the power circuit. No provision was made to use an in situ absorbant spray with this reactor. During the experimental work care was taken to ensure that only one variable affecting the
2088
Hydrazine synthesis in an electrochemical
reactor
Table 1. Reactor geometries and operation
Reactor type Parallel -cross Al Parallel -cross A2 Parallel -cross A3 Parallel -cross A4 Parallel -cross A5 Parallel -cross A6 Parallel -cross A7 Parallel -cross A8 Parallel -cross A9 Parallel -cross A 10 Parallel -cross A I 1 Parallel -cross A 12 Parallel -cross A 13 Parallel -parallel B 1 Parallel -parallel B2
Discharge width/or Flow area diameter (cm*) (cm)
Gap (cm)
Discharge volume (cm?
Barrier (cm)
variable
3.100
0.100
0.755
0.317
variable
3.100
0.100
0.755
0.317
variable
3.100
0.075
0.565
0.317
variable
3.100
0.025
0.189
0.317
variable
3.100
0.200
1.510
0.317
variable
3.100
0.500
3.774
0.317
variable
3.100
10lO
7.548
0.317
variable
3.100
I.500
1I.322
0.317
variable
3.100
2.000
14.0%
0.317
variable
3.100
2400
18.114
0.317
variable
2Wo
0.025
0.079
0,317
variable
0.950
0.025
0.161
0.317
3.100
0.100
0.755
0.158
2.54
1.800
6.300
16.08
0
2.54
I.800
6.300
16.08
0
variable
-
reaction was changed at any one time. The work was based on the underlying assumption that the residence time of the gas in the discharge was the important variable affecting reaction yield. It was decided to use the A type reactor because it enabled small gap distances to be achieved readily. This in turn, would lead to low physical residence times in the reactor. The B type reactor was used to determine the effect of surfaces and discharge pulsing on the course of the reaction. RESULTS
The results for the type A reactors are given in Figs. 2-9. The type Al reactor had the high
tension electrode mechanically pressed into contact with the dielectric barrier. All other type A reactors had the high tension electrode bonded to the dielectric barrier. Figures 2-4 detail the effect of physical residence time, pressure and power density on product yield for the type Al reactor. Figures 5-8 give the effect of physical residence time, pressure, power density and gap distance on product energy yield for the bonded electrode design. Figure 9 details the effect of alteration of electrode crosssectional area on the power density-energy yield relationship for the bonded electrode. The power density-energy yield relation for the B type reactors is given in Fig. 10. The
2089
P. L. SPEDDING
and J. D. THORNTON
r -Residence
0.11
20 71-
Time
0409 ac
1M
40
60
60
Pww
[knsity
watts
I
140
120
cc
Fig. 2. Variation of hydrazine energy yield with power density and pressure for the parallel electrodecross gas fiow Al reactor, with mechanical contact between the high tension electrode and dielectric, and operating at constant gas residence time. The discharge extinguishing point is shown as a broken line.
P-Pressure
90mm
2 -
05E 3 .r >
-
h P I! w
f-R&ma
-
SIC
Time
” .E I 4 1 0.1 -
I >
_ -
0.040
QOS-
0.03
1 20
60
40 IT-
Rwer
Density
1W
80 wtts
/
120
140
cc
Fig. 3. Variation of hydrazine energy yield with power density and residence time for the parallel electrodecross gas flow Al reactor, with mechanical contact between the high tension electrode and dielectric, and operating at a constant gas pressure.
2090
Hydrazine synthesis in an electrochemical
P -
Ressurr
reactor
90mm
IT-Power
Drnsity
i
0.03
4
0.10
005
Time
r-Residcnc~
MS
PK
Fig. 4. The effect of residence time on hydrazine energy yield for the parallel electrode-cross gas flow Al reactor, with mechanical contact between the high tension electrode and dielectric, and operating at constant gas pressure.
type Bl reactor employed straight d.c. power while the type B2 reactor had a pulsed d.c. discharge. The pulsed d.c. discharge had an activating period of 13 psec followed by a 100 psec ‘off’ period during which product was swept out of the reaction zone. DISCUSSION
The Al type reactor employed mechanical contact between the electrode and the dielectric plate. Further, the electrode gap was small in order to allow short residence times to be achieved in the reactor. Figures 2-4 show that the product energy yield increased with decreasing power density, residence time and pressure between 20 and 90 mm Hg; that is the product yields follow a similar relation to that found for the same reaction in a co-axial reactor design [14]. However, the absolute magnitude of the yields obtained in this work was about an order of magnitude smaller than those obtained for the co-axial reactor. Possibly the lower energy
yields obtained with the parallel electrodecross flow design compared to the co-axial reactor were caused by the particular geometrical aspect ratio of the reactor which was used. In this case the gap distance was purposely kept small to ensure a low physical residence time in the reaction zone and that true crossflow conditions were obtained. However, the resulting long path that the product molecules had to pass along between the electrodes meant that they were exposed to a greater possibility of degradation by the discharge particles passing between the two electrodes. Another possible explanation for the observed lower yields with the parallel electrode-cross flow reactor is that its larger electrode area led to product decomposition. This is unlikely because surfaces are reported to promote hydrazine production by removing atomic hydrogen (Eq. 1) which degrades hydrazine (Eq. 2).
209 1 C.E.S. Vol. 26 No. 12-I
2H =f%
H, .
(1)
P. L. SPEDDING
and J. D. THORNTON
t-
Reidencm Time 0909 ICC
P - Pressure
O.ll
I
I
I
I
20
40
60
80
w-
Powor
Density
I
I
im wtts
I
120
I
10
cc
Fig. 5. Variation of hydrazine energy yield with power density and pressure for the parallel electrodecross gas flow A2 reactor, with bonded high tension electrode and dielectric, and operating at constant gas residence time. The broken line gives data for the non-bonded A 1 reactor as a comparison.
2H + N,HI -
2NH,.
(2)
It is obvious that this particular geometrical aspect ratio of the parallel electrode-cross flow reactor is not well suited to the hydrazine or similar reactions where optimum product energy yields are in the low power density region. This type of reactor would be better suited to reactions such as methane cracking and polymerization where product energy yield increases with power density. The relation between energy yield and power density shows two distinct regions, the usual semi-logarithm form of Eq. (3) in the lower power density region and the log-log relation of Eq. (4) at power densities above 65 W/cm3; Y= YOexp-_
(3)
Y = Y&-”
(4)
where Y is the energy yield, g/kWhr, m is the power density, W/cm3, and the other symbols are appropriate constants. The latter region has not been reported previously but may be related to the fact that the high power density region in which it occurs is not able to be reached with other reactor designs such as the co-axial configuration. Again from inspection of Figs. 2-4 the energy yield decreased with rise in pressure between 20 and 90 mm Hg but thereafter increased steadily up to 250 mm Hg where it remained constant to 300 mm Hg. In the region of constant energy yields the effect of residence time appeared to become unimportant as shown in Fig. 6. The reason for these effects of pressure on energy yield are obscure. These results are at variance with the data of Andersen et a1.[9]; the only reported work at pressures beyond 100 mm Hg. However, the probe type electrodes
2092
Hydrazine synthesis in an electrochemical
reactor
lb \ t
f <
P-Pmsrurr mm 260-270
c
T-Raidum Thn 0403--0034
\
E m
Hg se
\
o5-
¶.
P
e
w
I >
01
0
loo
I
200
I
I
300
4W
Density
Tl--PSWW
I
1
500 watts/
SW cc
Fig. 6. The energy yield power density relation for the parallel electrode-cross without electrode-dielectric bonding. ----- Reactor A 1 -Reactor A2 l r = 0.003 set 0 r = 0.018 set X r = 0.034 sec.
used by these workers almost certainly permitted variable by-passing of the discharge to occur. Another point is that thermal degradation of hydrazine takes place in the pressure region above 100 mm Hg, unless effective electrode and gas cooling is achieved. Andersen et al. [9] did not report on any steps that were taken to overcome thermal effects in their apparatus so it can be assumed that it occurred. For these two reasons their results are not generally applicable to conditions outside those they employed and therefore cannot be compared to the present work. The maximum energy yield which was obtainable for a given operating pressure before the discharge was extinguished is shown as a dotted line on Fig. 2. Consequently, in order to obtain the maximum energy yield, the discharge needs to be operated at low pressures because of its
I
700
gas flow A reactor with and
inability to operate in the optimum power density region below 20 W/cm3 at pressures above 50 mm Hg. Whether the effect of discharge volume, which will be mentioned later, will materially alter this is not clear. The energy yield increased markedly with a reduction in residence time. This is in line with other work reported on this system and has been fully discussed elsewhere [ 12, 141. It was noted during experiments that poor mechanical contact between the high tension electrode and its dielectric led to erratic results being obtained. In order to overcome this difficulty the electrode was bonded directly on to its dielectric barrier using a silver plating technique. Energy yields with this modification (reactor A2) increased about three fold as shown in Figs. 5 and 6. With bonded electrodes it was essential to use good electrode cooling especially
2093
P. L. SPEDDING
and J. D. THORNTON
P-
Prrsur~
‘r-
Ruidanca
Time
Dmsity
watts EC I
W-RWW
65mm 001 see
Fig. 7. The effect of gap distance on the energy yield power density relation at constant pressure and gas residence time for the parallel electrode-cross gas flow bonded electrode reactors A2 and AS-A 10.
s-
-e
P -
L-
65mm
Pmssur*
f -Rsddsncs
E
05 TT-
rim
UC
I
1
I
100
200
300
Poww
Dsnsity
6
watts/cc
Fig. 8. The effect of small gap distances on the energy yield power density relation at constant pressure and short gas residence times for the parallel electrode-cross gas flow bonded reactors. --------- Reactor A3 Reactor A4.
2094
Hydrazine synthesis in an electrochemical
P -Pressure
Id
reactor
66mm
1
I
266
166 7T-
466
366
Paver
Dursity
666
I
600
I
766
watts / CC
Fig. 9. The effect of alteration of the design of the upper electrode at short gas residence times on the energy yield power density relation for the parallel electrode-cross gas flow bonded reactor. Reactor A 12 used the doughnut upper electrode design. Reactor A4 ---Reactor Al I ---------Reactor A 12.
in the pressure region above 100 mm Hg. The reason for the increase in yield with the bonded electrode was that it led to a reduction in the total resistive load of the reactor. The effect of gap distance on energy field, presented in Figs. 7 and 8, is in many ways similar to the variation of electrode size for coaxial reactors [ 141. This similarity may be linked to the change in reactor volume which occurs..As the gap distance was decreased the energy yield rose for a given power density. However, the minimum operating power density increased sharply, consequently the actual maximum energy yield obtainable was reduced. This follows from Paschen’s law which predicts that the minimum power of a discharge will decrease to a minimum value as the gap distance is increased. Consequently, better energy yields will be obtained as the gap distance is increased up to a particular value. Beyond this point the energy yield will remain substantially constant or fall slowly. There are a number of objectionable characteristics of the parallel electrode-cross gas flow
reactor when used with the hydrazine reaction. One is that it has a variable flow area for gas flow in a radial direction which results in a falling residence time across the electrode surface. The other is that the product is exposed to discharge particles passing between the electrodes as it goes through the reactor. Both these aspects increase the possibility of hydrazine degradation in the discharge. In a naive attempt to minimize these adverse effects the design of the top electrode was altered to give a reduction in gas residence time in the discharge. Both a reduction in the size of the top electrode (reactor Al 1) and the use of a doughnut shaped design (reactor A 12) resulted in a fall in energy yield as shown in Fig. 9. The data did not follow the pattern of other changes in reactor volume such as with the alteration of gap distance. Therefore, the effect of altering electrode area or gap distance appears to be complex and is not simply a case of increased reactor volume. Comparison of the results of reactors A 13 and A2 showed that there was no effect on energy yield by altering the discharge barrier thick-
2095
P. L. SPEDDING
and J. D. THORNTON
P - Pressun
7f-
Poww Dursity
1Omm
watts/cc
Fig. 10. The effect of gas and active residence time on the energy yield power density relation for the parallel electrode-parallel gas flow B type reactor. _ Residence time B 1 +rO = 1’120sec B2 7rr= 8.462 set 7. = 1.120 set - - - - Residence time B 1 7g = 0.526 set B2 70 = 0.526 set 7, = 0.634 set --------- Residence time B2 r8 = 1.120 set 7. = 0.146 sec.
ness. This is in agreement with the work of Smith[ 171 on the oxidation of sulphur dioxide in the parallel electrode-cross gas flow reactor. The B type reactor enabled the effect to be gauged of subjecting the reactant to the discharge under parallel gas flow conditions. This could not be achieved without the use of porous surfaces and the d.c. type discharge. A substantial increase in energy yield was obtained when the activated gas was withdrawn through the positive electrode. This confirms the findings of others[5] that hydrazine was preferentially formed in the positive column of the discharge. There was no advantage ,in withdrawing the activated gas through the porous silica wall of the reactor. For these reasons the B reactor was
operated with product withdrawal through the porous metal positive electrode. Figure 10, which details the results of the B type reactor, shows that there must have been a surface effect due to intimate contact between the discharge products and the porous metal of the electrode because the energy yield was substantially larger than that for the corresponding d.c. parallel electrode-cross flow reactor [ 141. Further, the increase in energy yield with fall in power density for the B type reactor was much greater than with any other reactor design[ 18, 191. The same was true of the pronounced effect of residence time on energy yield. All these aspects point to an additional influence upon the reaction which could well be the catalytic effect of the electrode surface. The exact mechanism
2096
Hydrazine synthesis in an electrochemical reactor
is, of course, uncertain but the abstraction of hydrogen by Eq. (1) could well account for the increased yield of hydrazine obtained. The d.c. pulsing data of the B2 type reactor implied that the active residence time (based on activating pulsing time) was of major importance in increasing the energy yield. The gas residence time of the reactant in the inter-electrode gap played a relatively minor role by comparison. Indications were that the energy yield could have been increased to a maximum value by subjecting the reactant gas to more than one activating pulse while it was resident in the interelectrode gap. This has since been confirmed to be the case by other workers[20]. If these data are compared with the pulsed d.c. data for the parallel electrode-cross gas flow reactor [ 141, it should be realized that, because of the streamlined conditions in that reactor, twice the theoretical displacement volume of reactant was passed through the reactor in the non-discharge period to ensure complete removal of all product before the next activating pulse was commenced. However, this method, while ensuring all product was removed from the reactor, virtually meant that by-passing of the reactor took place and the overall energy yield was correspondingly reduced. No such by-passing occurred in this work.
not go beyond the Paschen curve voltage minimum. The electrode construction was best achieved by bonding directly onto the dielectric which should be of sufficient thickness to ensure adequate mechanical strength. The a.c. single barrier parallel electrodecross gas flow reactor was not the ideal reactor design for the hydrazine synthesis and should only be used for reactions where the product energy yield rises with power density and residence time. Products which are subject to degradation in the discharge, such as hydrazine, should be produced in the parallel electrodeparallel gas flow reactor where the product is less subject to discharge degradation. Furthermore, the parallel gas flow reactor apparently removed discharge phenomena or products responsible for back reactions through surface reactions within the porous surface of the electrodes. Discharge pulsing increased energy yields. Indications were that the pulsing should be at such a frequency that more than a single 10 psec pulse will activate the gas while it is within the reactor. Acknowledgements-
Financial support for this work was received from S.R.C. We are indebted to D. Savage who conducted some of the experimental work.
NOTATION CONCLUSIONS
constant of Eq. (4) constant of Eq. (3) cm3/W pressure, mm Hg Y energy yield, gm/kW hr Y,, constant of Eqs. (3) and (4), g/kW hr a
Ideal operating conditions for the a.c. single barrier parallel electrode-cross gas flow reactor for the hydrazine synthesis were found to be as low a power density and residence time as practical and an operating pressure of 20 mm Hg. A more practical operating pressure of 500 mm Hg led to a drop in energy yield and electrode cooling was essential to avoid thermal degradation of product. It was found that the reactor volume was an important operational variable and should be as large as practicable with the restriction that the gap distance should
b P
Greek symbols g power density, W/cm3
T residence time, set Subscripts a
g
activated; gas
REFERENCES
111BAlZER M. M. and ANDERSON J. D., J. Electrbchem. Sot. 1964 lllil5,223,226. [2l Br. them. Engng 1966 11795. [31 BESSON A., Compt. rend. 191 I 1521850.
2097
P. L. SPEDDING
and J. D. THORNTON
141 KOENIG A. eraf.,Z. Phys. Chem. 1929 139A211; 1929 144A213. [51 DEVINS J. C. and BURTON M.,J. Am. them. Sot. 195476 2618. [61 OUCH1 K., J. Efecrrochem. Soc.Jnnan 1949 17285: 1952 20 164,168,378. [7] JOGARAO A. and SHASTRI B. S.‘R., J. Sci. Ind. Res. 1959 18B 38. 181 I.C.I. Brit. Pat.. 948.772: 958.776-8: 966.406: 1964. i9j ANDERSEN W. HI, ZWOLINSK B: J., and PARLIN R. B., Ind. Engng Chem. 1959 51529. [lo] RATHSACK H. A.,Z. Phys. Chem. 1960 214 101. [I 11 SKOROKHODOV I. I. et& Russ. J. Phys. Chem. 1961 35 503. [ 121 SPEDDING P. L., Inst. Chem. Engrs. Ind. Fell, Rept. 5 1969. [13] THORNTONJ. D.,Adu.Chem. 196980372. [I41 THORNTON J. D., CHARLTON W. D. and SPEDDING P. L., Adu. Chem. 1969 80 165. [ 151 WATT G. W. and CHRISP J. D., Anal. Chem. 1952 24 2006. [I61 ROGOWSKI W., Arch. Elekt. 1923 12 I ; 1926 16 1. [17] SMITH C. R., Ph.D. Thesis Univ. London 1964. 1181 SPEDDING P. L., J. Chem. Engng.Fuel Sci. N’cle 1967 3 25. [19] SPEDDING P. L., Engng Aspecfs ofPlasma Reactors Symp. Denver 1970. [20] BROWN B. A., CHRISTOPOULOS J. C., HOWARTH C. R. and THORNTON J. D., Engilg.Aspects of Plasma Reactors Symp. Denver 1970. R&m&- On synthttise I’hydrazine dans deux types de reactems tlectrochimiques a phase gazeuse: un reacteur a Ccoulement gazeux transversal muni d’une electrode parallele a courant alternatif, et un reacteur a tcoulement gazeux parallele muni dune electrode pat-allele a courant continu. On obtient des rendements d’energie allant jusqu’a Sg/Kwh. Les conditions optimum doperation sont atteintes dans le cas oh la densitt d’energie, la pression et le temps de sejour du gaz sont bas et quand on utilise une decharge pulsee. Celui des deux rdacteurs qui donne les meilleurs gains d’energie est celui a Ccoulement gazeux parallele et a electrode parallele. De plus, il est egalement avantageux d’utiliser un dielectrique et un volume de reaction aussi grand que possible, mais la distance entre les electrodes ne doit toutefois jamais depasser le point minimum de Paschen. Les resultats sont compatibles quand le mecanisme de I’hydrazine est forme en premier et ensuite degrade dans la decharge. Zusammenfassung - Hydrazin wurde nach dem elektrischen Entladungsprozess in zwei Typen von Reaktoren synthetisiert, einem Querstromungsreaktor mit Wechselstrom-Sperrparallelelektrode und einem Parallelstriimungsreaktor mit Gleichstromparallelelektrode. Es konnten Energieausbeuten bis zu 5 g/Kwh erzielt werden. Die optimalen Betriebsbedingungen waren in den niedrigen Leistungsdichte-, Druck-, und Gasverweilseitbereichen, sowie bei Verwendung einer Impulsentladung von niedriger aktiver Verweilzeit. Von den beiden Reaktoren gab der Typ mit Parallelelektrode- Parallelgasstrijmung die besseren Energieausbeuten. Andere Merkmale des Autbaus betrafen die Verwendung verklebter Dielektrika und einem grosstmoglichen Reaktorvolumen, vorausgesetzt, dass der Liickenabstand den Paschenschen Minimalpunkt nicht iiberschritt. Die Ergebnisse sind vereinbar mit dem Mechanismus wonach Hydrazin in der Entladung erst gebildet und dann abgebaut wird.
2098
Chemical Engineering Science, 197 1, Vol. 26, pp. 2087-2098.
Printed in Great Britain
Hydrazine synthesis in an electrochemical reactor P. L. SPEDDING Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand
J. D. THORNTON Department of Chemical Engineering, University, Newcastle upon Tyne, England (First received
14 December 1970: in revisedform
30 March 197 1)
Abstract-Hydrazine was synthesised in two types of gas phase electrochemical reactors, an a.c. barrier parallel electrode cross gas flow reactor, and a d.c. parallel electrode-parallel gas flow reactor. Energy yields of up to 5 g/kWhr were achieved. Optimum operating conditions were in the low power density, pressure and gas residence time regions, and with the use of a pulsed discharge of low active residence time. Of the two reactors the parallel electrode-parallel gas flow type gave the better energy yields. Other design criteria were the use of bonded dielectrics and as large a reactor volume as practicable provided that the gap distance did not exceed the Paschen minimum point. The results are consistent with the mechanism of hydrazine being first formed and then degraded in the discharge. INTRODUCTION
activating properties of the nondisruptive electrical discharge have been known for a long time and a great deal of effort has been directed towards the development of a commercial method of chemical synthesis using this route. The present scanty knowledge of the processes within this type of discharge, coupled with its complicated nature have restricted the application of the technique to virtually one commercial product, namely ozone. However, recent successful application of liquid-phase electrochemical techniques for synthesising organic polymer intermediates on a large scale has given impetus to the current resurgence of interest in gas discharge synthesis [ 11. Chemicals which show promise of commercial production by this route are those which are more reactive than their raw materials and are difficult and expensive to produce by conventional processes. Hydrazine is an obvious candidate for synthesis by the gas discharge route as the present Raschig method of production is inefficient and results in a product costing in excess of f1200 per ton [2]. The production of hydrazine from ammonia
THE CHEMICAL
using the electrical discharge was first demonstrated by Besson [3]. Subsequent detailed investigations showed that both the electrical energy yield and the percentage conversion obtained were low[4]. More recently, substantial improvements in the yields have been obtained under certain circumstances; by a reduction of atomic hydrogen concentration in the discharge through recombination[5], by a reduction in residence time of the hydrazine in the discharge [6.7], and by the removal of hydrazine in liquid adsorbents@]. A mechanism for the synthesis has been proposed based on these scattered data in which it was suggested that hydrazine was first formed in the discharge and then degraded by back-reaction. While certain general trends in available data lend considerable weight to this proposal, certain anomalies do exist. These latter indicate that both operating conditions and the actual design of reactor can be extremely important [9-l 11. These aspects were particularly noticeable when a reduction in residence time was attempted by modifying the waveform characteristic of the discharge to give short activating pulses. Energy yields of hydrazine were observed to increase considerably with these discharge
2087
P. L. SPEDDING
and J. D. THORNTON
modifications. Preliminary reviews of the general field along these lines have been presented recently but more detailed investigations are required to help ascertain the effect on the hydrazine yield of discharge parameters, reactor design, and reactor geometry [ 12, 131. In this work the operating characteristics of both the parallel discharge-cross gas flow and parallel discharge-parallel gas flow types of reactors were investigated. These data, together with previously reported work on the co-axial reactor [ 141, have enabled many of the effects of design variables on yields to be ascertained for the case of the hydrazine reaction.
TOR 4
EXPERIMENTAL
The apparatus consisted of a discharge reactor set between a measuring and an analysing section of a flowing gas system. Commercially pure ammonia was dried and fed to the reactor via reducing valves and a calibrated rotameter. Hydrazine formed in the discharge was absorbed in a liquid, in this case ethylene glycol, in an absorption train and the product hydrazine was determined using the spectrophotometric method developed by Watt and Chrisp[ 151. Provision also was made to take gas samples. Pressure control of the system was achieved by means of a Cartesian manostat which was located before the absorption train and vacuum pump. A one kW, 2.5 MHz generator provided a.c. power for the parallel electrode-cross gas flow barrier discharge reactor, which was designated the type A reactor. An 8 kW. d.c. generator was used for the parallel electrode-parallel gas flow reactor, which was designated the type B reactor. Measurement of the discharge power was achieved by suitable combination of an oscilloscope trace and the appropriate ammeter and voltmeter readings [ 121. Both discharge reactors used were of the parallel electrode type. These reactors are illustrated schematically in Fig. 1 -and essential geometric details and operating data are given in Table 1. The parallel electrode-cross gas flow type reactor employed a flat circular silica barrier set between steel Rogowski[ 161 elec-
REACTOR
B
Fig. 1. Schematic diagrams of the electrochemical reactors used. CO = cooling oil; E = earthed electrode; Gf = gas inlet; GO = gas outlet; HT = high tension electrode; S = silica dielectric barrier.
trodes of effective diameter 3-l cm. The incoming gas was fed through the centre of the inner earthed electrode and passed radially through the discharge gap. Electrode cooling was provided by circulating oil within the electrodes. With this design it was possible to alter the electrode gap, the barrier thickness and the size and design of the upper electrode. However, it was not possible to include an absorbing liquid spray. The parallel electrode-parallel gas flow type reactor consisted of a pair of porous circular steel electrodes set in a cylindrical glass tube. The tube had a centre cylindrical segment of its wall made of porous glass to enable gas to be withdrawn from the inter-electrode region of the reactor. D.C. power was used with this latter type of reactor and current stabilization was achieved by incorporating a suitable resistive load in the power circuit. No provision was made to use an in situ absorbant spray with this reactor. During the experimental work care was taken to ensure that only one variable affecting the
2088
Hydrazine synthesis in an electrochemical
reactor
Table 1. Reactor geometries and operation
Reactor type Parallel -cross Al Parallel -cross A2 Parallel -cross A3 Parallel -cross A4 Parallel -cross A5 Parallel -cross A6 Parallel -cross A7 Parallel -cross A8 Parallel -cross A9 Parallel -cross A 10 Parallel -cross A I 1 Parallel -cross A 12 Parallel -cross A 13 Parallel -parallel B 1 Parallel -parallel B2
Discharge width/or Flow area diameter (cm*) (cm)
Gap (cm)
Discharge volume (cm?
Barrier (cm)
variable
3.100
0.100
0.755
0.317
variable
3.100
0.100
0.755
0.317
variable
3.100
0.075
0.565
0.317
variable
3.100
0.025
0.189
0.317
variable
3.100
0.200
1.510
0.317
variable
3.100
0.500
3.774
0.317
variable
3.100
10lO
7.548
0.317
variable
3.100
I.500
1I.322
0.317
variable
3.100
2.000
14.0%
0.317
variable
3.100
2400
18.114
0.317
variable
2Wo
0.025
0.079
0,317
variable
0.950
0.025
0.161
0.317
3.100
0.100
0.755
0.158
2.54
1.800
6.300
16.08
0
2.54
I.800
6.300
16.08
0
variable
-
reaction was changed at any one time. The work was based on the underlying assumption that the residence time of the gas in the discharge was the important variable affecting reaction yield. It was decided to use the A type reactor because it enabled small gap distances to be achieved readily. This in turn, would lead to low physical residence times in the reactor. The B type reactor was used to determine the effect of surfaces and discharge pulsing on the course of the reaction. RESULTS
The results for the type A reactors are given in Figs. 2-9. The type Al reactor had the high
tension electrode mechanically pressed into contact with the dielectric barrier. All other type A reactors had the high tension electrode bonded to the dielectric barrier. Figures 2-4 detail the effect of physical residence time, pressure and power density on product yield for the type Al reactor. Figures 5-8 give the effect of physical residence time, pressure, power density and gap distance on product energy yield for the bonded electrode design. Figure 9 details the effect of alteration of electrode crosssectional area on the power density-energy yield relationship for the bonded electrode. The power density-energy yield relation for the B type reactors is given in Fig. 10. The
2089
P. L. SPEDDING
and J. D. THORNTON
r -Residence
0.11
20 71-
Time
0409 ac
1M
40
60
60
Pww
[knsity
watts
I
140
120
cc
Fig. 2. Variation of hydrazine energy yield with power density and pressure for the parallel electrodecross gas fiow Al reactor, with mechanical contact between the high tension electrode and dielectric, and operating at constant gas residence time. The discharge extinguishing point is shown as a broken line.
P-Pressure
90mm
2 -
05E 3 .r >
-
h P I! w
f-R&ma
-
SIC
Time
” .E I 4 1 0.1 -
I >
_ -
0.040
QOS-
0.03
1 20
60
40 IT-
Rwer
Density
1W
80 wtts
/
120
140
cc
Fig. 3. Variation of hydrazine energy yield with power density and residence time for the parallel electrodecross gas flow Al reactor, with mechanical contact between the high tension electrode and dielectric, and operating at a constant gas pressure.
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Hydrazine synthesis in an electrochemical
P -
Ressurr
reactor
90mm
IT-Power
Drnsity
i
0.03
4
0.10
005
Time
r-Residcnc~
MS
PK
Fig. 4. The effect of residence time on hydrazine energy yield for the parallel electrode-cross gas flow Al reactor, with mechanical contact between the high tension electrode and dielectric, and operating at constant gas pressure.
type Bl reactor employed straight d.c. power while the type B2 reactor had a pulsed d.c. discharge. The pulsed d.c. discharge had an activating period of 13 psec followed by a 100 psec ‘off’ period during which product was swept out of the reaction zone. DISCUSSION
The Al type reactor employed mechanical contact between the electrode and the dielectric plate. Further, the electrode gap was small in order to allow short residence times to be achieved in the reactor. Figures 2-4 show that the product energy yield increased with decreasing power density, residence time and pressure between 20 and 90 mm Hg; that is the product yields follow a similar relation to that found for the same reaction in a co-axial reactor design [14]. However, the absolute magnitude of the yields obtained in this work was about an order of magnitude smaller than those obtained for the co-axial reactor. Possibly the lower energy
yields obtained with the parallel electrodecross flow design compared to the co-axial reactor were caused by the particular geometrical aspect ratio of the reactor which was used. In this case the gap distance was purposely kept small to ensure a low physical residence time in the reaction zone and that true crossflow conditions were obtained. However, the resulting long path that the product molecules had to pass along between the electrodes meant that they were exposed to a greater possibility of degradation by the discharge particles passing between the two electrodes. Another possible explanation for the observed lower yields with the parallel electrode-cross flow reactor is that its larger electrode area led to product decomposition. This is unlikely because surfaces are reported to promote hydrazine production by removing atomic hydrogen (Eq. 1) which degrades hydrazine (Eq. 2).
209 1 C.E.S. Vol. 26 No. 12-I
2H =f%
H, .
(1)
P. L. SPEDDING
and J. D. THORNTON
t-
Reidencm Time 0909 ICC
P - Pressure
O.ll
I
I
I
I
20
40
60
80
w-
Powor
Density
I
I
im wtts
I
120
I
10
cc
Fig. 5. Variation of hydrazine energy yield with power density and pressure for the parallel electrodecross gas flow A2 reactor, with bonded high tension electrode and dielectric, and operating at constant gas residence time. The broken line gives data for the non-bonded A 1 reactor as a comparison.
2H + N,HI -
2NH,.
(2)
It is obvious that this particular geometrical aspect ratio of the parallel electrode-cross flow reactor is not well suited to the hydrazine or similar reactions where optimum product energy yields are in the low power density region. This type of reactor would be better suited to reactions such as methane cracking and polymerization where product energy yield increases with power density. The relation between energy yield and power density shows two distinct regions, the usual semi-logarithm form of Eq. (3) in the lower power density region and the log-log relation of Eq. (4) at power densities above 65 W/cm3; Y= YOexp-_
(3)
Y = Y&-”
(4)
where Y is the energy yield, g/kWhr, m is the power density, W/cm3, and the other symbols are appropriate constants. The latter region has not been reported previously but may be related to the fact that the high power density region in which it occurs is not able to be reached with other reactor designs such as the co-axial configuration. Again from inspection of Figs. 2-4 the energy yield decreased with rise in pressure between 20 and 90 mm Hg but thereafter increased steadily up to 250 mm Hg where it remained constant to 300 mm Hg. In the region of constant energy yields the effect of residence time appeared to become unimportant as shown in Fig. 6. The reason for these effects of pressure on energy yield are obscure. These results are at variance with the data of Andersen et a1.[9]; the only reported work at pressures beyond 100 mm Hg. However, the probe type electrodes
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Hydrazine synthesis in an electrochemical
reactor
lb \ t
f <
P-Pmsrurr mm 260-270
c
T-Raidum Thn 0403--0034
\
E m
Hg se
\
o5-
¶.
P
e
w
I >
01
0
loo
I
200
I
I
300
4W
Density
Tl--PSWW
I
1
500 watts/
SW cc
Fig. 6. The energy yield power density relation for the parallel electrode-cross without electrode-dielectric bonding. ----- Reactor A 1 -Reactor A2 l r = 0.003 set 0 r = 0.018 set X r = 0.034 sec.
used by these workers almost certainly permitted variable by-passing of the discharge to occur. Another point is that thermal degradation of hydrazine takes place in the pressure region above 100 mm Hg, unless effective electrode and gas cooling is achieved. Andersen et al. [9] did not report on any steps that were taken to overcome thermal effects in their apparatus so it can be assumed that it occurred. For these two reasons their results are not generally applicable to conditions outside those they employed and therefore cannot be compared to the present work. The maximum energy yield which was obtainable for a given operating pressure before the discharge was extinguished is shown as a dotted line on Fig. 2. Consequently, in order to obtain the maximum energy yield, the discharge needs to be operated at low pressures because of its
I
700
gas flow A reactor with and
inability to operate in the optimum power density region below 20 W/cm3 at pressures above 50 mm Hg. Whether the effect of discharge volume, which will be mentioned later, will materially alter this is not clear. The energy yield increased markedly with a reduction in residence time. This is in line with other work reported on this system and has been fully discussed elsewhere [ 12, 141. It was noted during experiments that poor mechanical contact between the high tension electrode and its dielectric led to erratic results being obtained. In order to overcome this difficulty the electrode was bonded directly on to its dielectric barrier using a silver plating technique. Energy yields with this modification (reactor A2) increased about three fold as shown in Figs. 5 and 6. With bonded electrodes it was essential to use good electrode cooling especially
2093
P. L. SPEDDING
and J. D. THORNTON
P-
Prrsur~
‘r-
Ruidanca
Time
Dmsity
watts EC I
W-RWW
65mm 001 see
Fig. 7. The effect of gap distance on the energy yield power density relation at constant pressure and gas residence time for the parallel electrode-cross gas flow bonded electrode reactors A2 and AS-A 10.
s-
-e
P -
L-
65mm
Pmssur*
f -Rsddsncs
E
05 TT-
rim
UC
I
1
I
100
200
300
Poww
Dsnsity
6
watts/cc
Fig. 8. The effect of small gap distances on the energy yield power density relation at constant pressure and short gas residence times for the parallel electrode-cross gas flow bonded reactors. --------- Reactor A3 Reactor A4.
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Hydrazine synthesis in an electrochemical
P -Pressure
Id
reactor
66mm
1
I
266
166 7T-
466
366
Paver
Dursity
666
I
600
I
766
watts / CC
Fig. 9. The effect of alteration of the design of the upper electrode at short gas residence times on the energy yield power density relation for the parallel electrode-cross gas flow bonded reactor. Reactor A 12 used the doughnut upper electrode design. Reactor A4 ---Reactor Al I ---------Reactor A 12.
in the pressure region above 100 mm Hg. The reason for the increase in yield with the bonded electrode was that it led to a reduction in the total resistive load of the reactor. The effect of gap distance on energy field, presented in Figs. 7 and 8, is in many ways similar to the variation of electrode size for coaxial reactors [ 141. This similarity may be linked to the change in reactor volume which occurs..As the gap distance was decreased the energy yield rose for a given power density. However, the minimum operating power density increased sharply, consequently the actual maximum energy yield obtainable was reduced. This follows from Paschen’s law which predicts that the minimum power of a discharge will decrease to a minimum value as the gap distance is increased. Consequently, better energy yields will be obtained as the gap distance is increased up to a particular value. Beyond this point the energy yield will remain substantially constant or fall slowly. There are a number of objectionable characteristics of the parallel electrode-cross gas flow
reactor when used with the hydrazine reaction. One is that it has a variable flow area for gas flow in a radial direction which results in a falling residence time across the electrode surface. The other is that the product is exposed to discharge particles passing between the electrodes as it goes through the reactor. Both these aspects increase the possibility of hydrazine degradation in the discharge. In a naive attempt to minimize these adverse effects the design of the top electrode was altered to give a reduction in gas residence time in the discharge. Both a reduction in the size of the top electrode (reactor Al 1) and the use of a doughnut shaped design (reactor A 12) resulted in a fall in energy yield as shown in Fig. 9. The data did not follow the pattern of other changes in reactor volume such as with the alteration of gap distance. Therefore, the effect of altering electrode area or gap distance appears to be complex and is not simply a case of increased reactor volume. Comparison of the results of reactors A 13 and A2 showed that there was no effect on energy yield by altering the discharge barrier thick-
2095
P. L. SPEDDING
and J. D. THORNTON
P - Pressun
7f-
Poww Dursity
1Omm
watts/cc
Fig. 10. The effect of gas and active residence time on the energy yield power density relation for the parallel electrode-parallel gas flow B type reactor. _ Residence time B 1 +rO = 1’120sec B2 7rr= 8.462 set 7. = 1.120 set - - - - Residence time B 1 7g = 0.526 set B2 70 = 0.526 set 7, = 0.634 set --------- Residence time B2 r8 = 1.120 set 7. = 0.146 sec.
ness. This is in agreement with the work of Smith[ 171 on the oxidation of sulphur dioxide in the parallel electrode-cross gas flow reactor. The B type reactor enabled the effect to be gauged of subjecting the reactant to the discharge under parallel gas flow conditions. This could not be achieved without the use of porous surfaces and the d.c. type discharge. A substantial increase in energy yield was obtained when the activated gas was withdrawn through the positive electrode. This confirms the findings of others[5] that hydrazine was preferentially formed in the positive column of the discharge. There was no advantage ,in withdrawing the activated gas through the porous silica wall of the reactor. For these reasons the B reactor was
operated with product withdrawal through the porous metal positive electrode. Figure 10, which details the results of the B type reactor, shows that there must have been a surface effect due to intimate contact between the discharge products and the porous metal of the electrode because the energy yield was substantially larger than that for the corresponding d.c. parallel electrode-cross flow reactor [ 141. Further, the increase in energy yield with fall in power density for the B type reactor was much greater than with any other reactor design[ 18, 191. The same was true of the pronounced effect of residence time on energy yield. All these aspects point to an additional influence upon the reaction which could well be the catalytic effect of the electrode surface. The exact mechanism
2096
Hydrazine synthesis in an electrochemical reactor
is, of course, uncertain but the abstraction of hydrogen by Eq. (1) could well account for the increased yield of hydrazine obtained. The d.c. pulsing data of the B2 type reactor implied that the active residence time (based on activating pulsing time) was of major importance in increasing the energy yield. The gas residence time of the reactant in the inter-electrode gap played a relatively minor role by comparison. Indications were that the energy yield could have been increased to a maximum value by subjecting the reactant gas to more than one activating pulse while it was resident in the interelectrode gap. This has since been confirmed to be the case by other workers[20]. If these data are compared with the pulsed d.c. data for the parallel electrode-cross gas flow reactor [ 141, it should be realized that, because of the streamlined conditions in that reactor, twice the theoretical displacement volume of reactant was passed through the reactor in the non-discharge period to ensure complete removal of all product before the next activating pulse was commenced. However, this method, while ensuring all product was removed from the reactor, virtually meant that by-passing of the reactor took place and the overall energy yield was correspondingly reduced. No such by-passing occurred in this work.
not go beyond the Paschen curve voltage minimum. The electrode construction was best achieved by bonding directly onto the dielectric which should be of sufficient thickness to ensure adequate mechanical strength. The a.c. single barrier parallel electrodecross gas flow reactor was not the ideal reactor design for the hydrazine synthesis and should only be used for reactions where the product energy yield rises with power density and residence time. Products which are subject to degradation in the discharge, such as hydrazine, should be produced in the parallel electrodeparallel gas flow reactor where the product is less subject to discharge degradation. Furthermore, the parallel gas flow reactor apparently removed discharge phenomena or products responsible for back reactions through surface reactions within the porous surface of the electrodes. Discharge pulsing increased energy yields. Indications were that the pulsing should be at such a frequency that more than a single 10 psec pulse will activate the gas while it is within the reactor. Acknowledgements-
Financial support for this work was received from S.R.C. We are indebted to D. Savage who conducted some of the experimental work.
NOTATION CONCLUSIONS
constant of Eq. (4) constant of Eq. (3) cm3/W pressure, mm Hg Y energy yield, gm/kW hr Y,, constant of Eqs. (3) and (4), g/kW hr a
Ideal operating conditions for the a.c. single barrier parallel electrode-cross gas flow reactor for the hydrazine synthesis were found to be as low a power density and residence time as practical and an operating pressure of 20 mm Hg. A more practical operating pressure of 500 mm Hg led to a drop in energy yield and electrode cooling was essential to avoid thermal degradation of product. It was found that the reactor volume was an important operational variable and should be as large as practicable with the restriction that the gap distance should
b P
Greek symbols g power density, W/cm3
T residence time, set Subscripts a
g
activated; gas
REFERENCES
111BAlZER M. M. and ANDERSON J. D., J. Electrbchem. Sot. 1964 lllil5,223,226. [2l Br. them. Engng 1966 11795. [31 BESSON A., Compt. rend. 191 I 1521850.
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P. L. SPEDDING
and J. D. THORNTON
141 KOENIG A. eraf.,Z. Phys. Chem. 1929 139A211; 1929 144A213. [51 DEVINS J. C. and BURTON M.,J. Am. them. Sot. 195476 2618. [61 OUCH1 K., J. Efecrrochem. Soc.Jnnan 1949 17285: 1952 20 164,168,378. [7] JOGARAO A. and SHASTRI B. S.‘R., J. Sci. Ind. Res. 1959 18B 38. 181 I.C.I. Brit. Pat.. 948.772: 958.776-8: 966.406: 1964. i9j ANDERSEN W. HI, ZWOLINSK B: J., and PARLIN R. B., Ind. Engng Chem. 1959 51529. [lo] RATHSACK H. A.,Z. Phys. Chem. 1960 214 101. [I 11 SKOROKHODOV I. I. et& Russ. J. Phys. Chem. 1961 35 503. [ 121 SPEDDING P. L., Inst. Chem. Engrs. Ind. Fell, Rept. 5 1969. [13] THORNTONJ. D.,Adu.Chem. 196980372. [I41 THORNTON J. D., CHARLTON W. D. and SPEDDING P. L., Adu. Chem. 1969 80 165. [ 151 WATT G. W. and CHRISP J. D., Anal. Chem. 1952 24 2006. [I61 ROGOWSKI W., Arch. Elekt. 1923 12 I ; 1926 16 1. [17] SMITH C. R., Ph.D. Thesis Univ. London 1964. 1181 SPEDDING P. L., J. Chem. Engng.Fuel Sci. N’cle 1967 3 25. [19] SPEDDING P. L., Engng Aspecfs ofPlasma Reactors Symp. Denver 1970. [20] BROWN B. A., CHRISTOPOULOS J. C., HOWARTH C. R. and THORNTON J. D., Engilg.Aspects of Plasma Reactors Symp. Denver 1970. R&m&- On synthttise I’hydrazine dans deux types de reactems tlectrochimiques a phase gazeuse: un reacteur a Ccoulement gazeux transversal muni d’une electrode parallele a courant alternatif, et un reacteur a tcoulement gazeux parallele muni dune electrode pat-allele a courant continu. On obtient des rendements d’energie allant jusqu’a Sg/Kwh. Les conditions optimum doperation sont atteintes dans le cas oh la densitt d’energie, la pression et le temps de sejour du gaz sont bas et quand on utilise une decharge pulsee. Celui des deux rdacteurs qui donne les meilleurs gains d’energie est celui a Ccoulement gazeux parallele et a electrode parallele. De plus, il est egalement avantageux d’utiliser un dielectrique et un volume de reaction aussi grand que possible, mais la distance entre les electrodes ne doit toutefois jamais depasser le point minimum de Paschen. Les resultats sont compatibles quand le mecanisme de I’hydrazine est forme en premier et ensuite degrade dans la decharge. Zusammenfassung - Hydrazin wurde nach dem elektrischen Entladungsprozess in zwei Typen von Reaktoren synthetisiert, einem Querstromungsreaktor mit Wechselstrom-Sperrparallelelektrode und einem Parallelstriimungsreaktor mit Gleichstromparallelelektrode. Es konnten Energieausbeuten bis zu 5 g/Kwh erzielt werden. Die optimalen Betriebsbedingungen waren in den niedrigen Leistungsdichte-, Druck-, und Gasverweilseitbereichen, sowie bei Verwendung einer Impulsentladung von niedriger aktiver Verweilzeit. Von den beiden Reaktoren gab der Typ mit Parallelelektrode- Parallelgasstrijmung die besseren Energieausbeuten. Andere Merkmale des Autbaus betrafen die Verwendung verklebter Dielektrika und einem grosstmoglichen Reaktorvolumen, vorausgesetzt, dass der Liickenabstand den Paschenschen Minimalpunkt nicht iiberschritt. Die Ergebnisse sind vereinbar mit dem Mechanismus wonach Hydrazin in der Entladung erst gebildet und dann abgebaut wird.
2098