Chemical Reactions in Electrical Discharges

36 A Viewpoint on Electrical Discharge Devices

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch036

and Their Application as Chemical Reactors JAMES E . F L I N N and W I L L I A M M . G O L D B E R G E R Columbus Laboratories, Battelle Memorial Institute, Columbus, Ohio

Discharge devices of many types are available for study and use in connection with chemical reactions of commercial significance. In such studies it is the reaction rather than the device which usually receives the major attention of the experimenter. Consequently, any attempt to compare results from more than one study is usually frustrated through lack of an adequate knowledge of the similarities and differences among the large number of devices in use. This paper attempts to show in a limited, way that such interrelationships do exist—e.g., between a low pressure glow discharge and a high pressure high temperature plasma arc. Also factors of importance in selecting and evaluating devices for process use are discussed.

s p h e r e are many long-established and well-known chemical processes based on electrical discharge phenomena. The subject of discharge chemistry certainly is not new. However, the relatively recent development of new types of reliable discharge devices has stimulated interest in their use as chemical reactors. The extent of research in discharge chemistry is rapidly increasing and it is of value, therefore, to establish bases for comparing the results of these various investigators. For the most part, the ultimate objective of this research is the production of new and useful products or the production of known products at less cost. Thus, the essential background of information already developed by the physicist, electrochemist, chemical and electrical engineer must be brought together in a manner that is readily understood by each. The papers presented in this volume reflect this diversity of interest in discharge chemistry. This paper attempts to provide a perspective showing the interrelationships existing among a few types of discharge devices of interest in 441 Blaustein; Chemical Reactions in Electrical Discharges Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

442

CHEMICAL REACTIONS IN ELECTRICAL DISCHARGES

discharge chemistry and chemical processing research.

In doing so no

new theories are presented, nor are existing theories discussed.

Rather

phenomenological descriptions of various discharge modes are used to achieve the desired purpose. Depending on the chemical system of interest and the ultimate objective of the study or process, there will probably be one mode of discharge and one type of discharge device that will be best suited. Thus, advanced consideration of both the differences and similarities of various


discharge modes and devices can prove extremely valuable.

Discharge Devices Some of the more common names for a variety of electrical discharge devices are listed in Table I. These names and others not listed, when conceived, were meant to be descriptive: (1) of the physical aspect of a given device, (2) of the plasma environment formed in a device, or (3) of an individual who was closely associated with the study of a particular discharge device or phenomena. Thus, one can readily visualize a glow discharge as a type of discharge whose plasma environment "glows"; an ozonizer as one in which ozone is produced; a brush cathode discharge (10)

as one whose cathode has the appearance of a wire brush; etc.

This

method of identifying specific discharges or devices has led to confusion in the literature. It is, for instance, not uncommon to see the words "glow discharge" used to describe the phenomena observable in a number of distinctly different discharge devices. Table I.

Names for a Variety of Electrical Discharge Devices

Induction plasma Plasma jet (or torch) Silent Ozonizer ozonator Electrodeless Electronic torch Penning Townsend Wood-Bonhoeffer tube

Spark Glow Arc Ring Corona Semi-corona Microwave Brush cathode plasma Radio frequency

It will take some time before this collection of terms will fade from usage and a more rational basis for identifying different discharge devices and phenomena is developed. In the meantime these terms will serve, albit imprecisely, their intended purpose. In developing such a basis the interrelationships existing among the various devices will have to be defined more clearly than is presently possible. Because of this situation a problem arises with respect to a symposium such as this on reactions in electrical discharges.

The question is

Blaustein; Chemical Reactions in Electrical Discharges Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

36.

FLiNN

Electrical Discharge Devices

A N D GOLDBERGER

asked: what is an electrical discharge?

443

Is an arc or a plasma jet to be

considered in the same vein as a glow or corona discharge? A n examination of the numerous types of electrical discharges mentioned in this collection of symposium papers revealed that each almost without exception has the three elements illustrated in -Figure 1. That is, they are sustained by a source of electrical power ( 1 ), this power being delivered by means of a coupling mechanism (2), to a plasma environment (3), associated with the particular device. This simplified picture


is rapidly obfuscated when one realizes that the number of combinations of power sources, coupling mechanisms, and plasma environments can be quite large, not to mention the variety of device geometries and modes of operation that are possible. POWER SOURCE

COUPLING MECHANISM

D.C. A.C. R.F.

resistive capacitive inductive

PLASMA ENVIRONMENT current level pressure gas flow, temperature, etc. boundaries solid, liq., phases electrical fields magnetic fields.

microwave

(2)

(1)

Figure 1.

(3)

Elements of an electrical discharge device

Perhaps the most meaningful of the three elements in Figure 1 with respect to a classification scheme for discharge devices is the method by which electrical energy is coupled into a given plasma environment. Kunkel (9) has discussed such a scheme. The mode of coupling is resistive for those devices which have electrodes in direct contact with the ionized gas or plasma environment. In these devices the electrical field necessary to sustain the plasma is caused by positive and negative charge accumulations both within and at the boundaries (walls, electrodes, etc.) of the plasma region. A finite potential difference at the gas-electrode boundary always exists as a consequence of the accumulated charges.

This potential supports a number

of collision processes (ionization, excitation, electron emission or collection, etc. ) which act to sustain the discharge. Inductively or capacitively coupled discharges on the other hand do not have electrodes in direct contact with the gaseous plasma region and hence are frequently referred to—sometimes without adequate distinction—as "electrodeless" discharges.

The electric field is generated


444

CHEMICAL

R E A C T I O N S IN E L E C T R I C A L

DISCHARGES


(induced) in the inductively coupled discharge by a changing magnetic field. Electrons are generated entirely within the discharge device in contrast to resistively coupled discharges where electrons flowing in the external circuitry enter and leave at the electrode boundaries. Thus, current flowing in an inductively coupled discharge flows in closed loops. This situation with inductive discharges is analogous to that of a transformer. That is, the plasma environment acts as a one-turn (short circuited ) secondary coil. Capacitively coupled discharges—also known as polarization discharges—are characterized by the fact that the electrodes through which the electrical power is delivered are physically separated from the plasma region by a non-conducting material—i.e., & dielectric barrier. This barrier material is in direct contact with both the electrodes on one side and the plasma region on the other. In contrast to inductive discharges the electrical field in the plasma region of a capacitive discharge is caused by oscillating electrostatic charges at the dielectric barrier surfaces covering each electrode rather than by a changing magnetic field in the plasma region. T o obtain such coupling a high voltage oscillating power supply is needed to allow a displacement current to be passed through the dielectric barrier material (4). This general way of classifying electrical discharge devices, while convenient, is not absolute. Discharges having combinations, both inherent and purposeful, of these coupling mechanisms are known (14). However, such a categorization is useful as a first step in understanding the interrelationships and differences among the various types. For a given power source, mode of coupling, and device geometry, a plasma environment can be generated. The properties of this environment are readily altered by any number of externally controlled variables. These include the gas pressure; the gas flow rate, direction, and inlet temperature; the frequency, current, voltage level, and duration of the input power; the presence or absence of a magnetic field, solid or liquid phases, and others. If a chemical reaction is occurring in the plasma environment, it follows that manipulation of any of these variables is apt to affect such a reaction through the changes that would occur in the plasma. These changes include the electron and gas temperatures, degree of gas ionization or excitation, electrical field strengths, gas density, volume of plasma relative to the volume of the device, and other factors which directly or indirectly determine the course of a chemical reaction. Since very little is known about the true nature of chemical reactions involving ions, electrons, radicals, photons, etc., it is not possible to predict a priori what would be expected from one type of device or another under specified conditions. For that matter, in most studies of chemical reactions in electrical discharges, very little is known or reported


36.

FLiNN

A N D GOLDBERGER


about the true state of the plasma environment—i.e., strengths, geometry, gas flow, temperatures, etc.

445

electrical field

Thus, meaningful com-

parisons of experimental results for the same reaction in different devices often cannot be made. Yet we know that the same reaction can be carried out in discharges differing greatly with respect to the state of the plasma environment. Let us examine just how radically the plasma environment of a given device can be altered by manipulation of a few selected externally con-


trolled variables. Figure 2 shows two routes by which one can, in a device of more or less fixed geometry, alter the plasma environment to form a low intensity arc roughly at atmospheric pressure beginning with a glow discharge at a much lower pressure.

Since each device in the

series can be initiated by resistive coupling to a d.c. power supply, the transitions from one type of discharge to another can be made (in principle) by varying only the current, pressure, and possibly the electrode spacing.

Other transitions from the low intensity arc are possible and

will be briefly discussed. One of them (plasma jet) involves the introduction of yet another variable, namely, gas flow. Low Intensity Arc Discharge (high pressure)

Glow Discharge (high pressure) Increasing Pressure Glow Discharge (low pressure)

Arc Discharge (low pressure)

Increasing Current

Figure 2.

Two routes of transitions from glow to arc

The physical aspects of each of these type discharges have been described in detail in the literature. Our only purpose here is to emphasize the interrelationships among these devices which in themselves are usually studied as separate entities. The low pressure glow discharge (a few cm. of H g ) can be formed by a transition from a Townsend discharge (5), as is typically shown by means of a V - I plot of the type shown in Figure 3. It is characterized by a high potential drop at the cathode of several hundred volts. Because of this high potential drop, ions are accelerated toward the cathode at high velocities. Thus, the emission of electrons from the cathode is largely caused by positive-ion bombardment in this type of discharge. As with


446

CHEMICAL

REACTIONS

IN

ELECTRICAL

DISCHARGES


most discharges at low pressure the electron temperature is much higher than that of the heavier ions or neutral gas atoms, which themselves are at a moderate temperature level of 3 0 0 ° - 5 0 0 ° K . (even lower if the discharge is externally cooled). This is because at these pressures the electron has more time between collisions ( a longer mean free path ) to gain energy from the electrical field. Likewise, the moderate temperatures are because of the fewer elastic collisions which occur between electrons and neutral gas atoms at the lower gas densities prevailing.

v

Figure 3. Typical voltage-current diagram for electrical discharges As a consequence of the high electron temperatures, ionization will be low, on the order of 10" percent of the neutral particle density. The current is typically at the milliamp to hundreds of milliamps level. Because of the low gas densities, electron-ion recombination at the discharge boundaries predominates rather than in the gas volume. The discharge will be diffuse—i.e., will fill the device with its glow. 4

Now it is possible, in raising the pressure of a glow discharge to the atmospheric level and above, to maintain the essential characteristics associated with the low pressure glow. These so-called high pressure glows (7, 13) differ from low pressure glows in that the positive column is no longer diffuse but rather is constricted; appearing as a streamer between the electrodes. A discussion of some parameters causing constriction is given by Phelps (11). The temperature of the ions and neutral gas in the streamer is much higher and the electron temperature much lower relative to the low pressure glow. The high potential drop at the cathode, however, is essentially preserved as is the process of emission of electrons by ion-bombardment of the cathode. By increasing the current flow through this high pressure glow a point is reached where a transition to a low intensity arc discharge can be made to occur. This transition is characterized by a considerable re-


36.

FLiNN

A N D GOLDBERGER


447

duction in the potential drop at the cathode and an increase in the current density along the discharge length. Typical operating current levels for this type of discharge are in the 1 to 20 ampere range. Other essential changes in the glow to arc transition are compared in Tables II and III. Notable in these tables are the decreased potential drops both at the cathode and along the positive column for the arc type discharge.

This

implies that different and perhaps more efficient electron emission and ionization processes are operating in these regions.

In contrast to the


glow discharge the mechanism for electron emission from either a low or high pressure arc cathode is believed to be thermionic for electrodes which are not readily vaporizable and by field emission for vaporizable electrodes (3).

T h e temperature necessary to cause thermionic emission

is still the result of ion-bombardment of the cathode; however, the bombardment occurs at much higher current densities than in the glow discharge regions.

Similarly, there is considerable thermal ionization in

addition to ionization by electron impact in the positive column of the arc as a result of the much higher temperatures.

The electron, ion, and

neutral particle temperatures in arcs at atmospheric pressure are nearly equal owing to the greater frequency of collisions at the prevailing particle densities. Temperatures of the low intensity arc column at atmospheric pressure are roughly in the 4 0 0 0 ° to 7 0 0 0 ° K . range. Table II. Comparison of Glow and A r c Discharges at One Atmosphere in A i r with Tungsten Electrodes 0

Glow Power input, watts Discharge current, amps Cathode drop, volts Positive column gradient, volts/cm. Discharge length, mm.

60 0.1 425 350 5

Arc 60 0.( 45 —90 5

See Reference 7.

a

Table III. Comparison of Glow and A r c Discharges at One Atmosphere in Hydrogen with Carbon Electrodes"

Current, amps Positive column gradient, volts/cm. Change in total voltage across discharge, volts Appearance of discharge

1

Glow

Arc

2 530

2 280

+96 less intense, striated

highly luminous, homogeneous along axis

See Reference 13.



448

CHEMICAL

REACTIONS

IN

ELECTRICAL

DISCHARGES

The alternate route shown in Figure 2 to an arc at atmospheric pressure is via transition from a low pressure glow to a low pressure arc. This low pressure transition is likewise made by an increase in the discharge current (Figure 3). Cobine (3) discusses this transition in detail. The cathode emission and voltage phenomena discussed for the high pressure glow to arc transition are in general the same here. However, the luminous positive column for a low pressure arc is similar to the low pressure glow, differing only in the higher degree of ionization that usually prevails. Electron temperatures are much higher than the neutral gas temperature which itself is moderate. Ionization is by electron impact. The low pressure arc is also diffuse, and electron loss by recombination at the discharge boundaries predominates. From the low intensity arc at near atmospheric pressure additional transitions can be made—e.g., either to a plasma jet or a high intensity arc. The term "jet" ("torch" is sometimes used) is again descriptive, in that a stream of glowing hot plasma issues from these devices. Until now, little mention has been made of gas flow in connection with the glow or arc, because gas flow was not necessarily needed for electrode cooling and, therefore, the maintenance of the discharge. In the plasma jet, however, the flow of gas is an essential element. It serves to cool the discharge at the boundaries of the device, inducing a thermal pinch effect which is not present in the arc or glow. This thermal pinch is a constriction of the plasma toward the axis of the device that occurs because of the boundary cooling. The net result is an increase in current density, degree of ionization, and plasma temperature in the axial plasma core. Further increases in current to the discharge of fixed geometry results in higher current densities and plasma temperature. At even higher currents (hundreds of amperes) the magnetic field of force induced by the electron flow begins to exert a second pinch effect causing even further increases in current density and gas temperatures (so that temperatures up to 2 0 , 0 0 0 ° K . become possible). This magnetic pinch effect is not peculiar to the plasma jet device but occurs in other plasmas where the current becomes sufficiently high. The flow of gas acting to cool the boundaries of the plasma jet, combined with magnetohydrodynamic ( M H D ) forces, act to force the core of hot plasma gas out of the discharge region—in most designs usually through a hole created in the cathode. This jet of gas carries away by convection to the surroundings much of the heat generated in the discharge. Other heat transfer mechanisms operating are radiation and transfer by convection/conduction to and through the device boundary walls. The transition from a low intensity to a high intensity arc is simpler and can be achieved primarily by increasing the current flow to an arc. As shown in Figure 3, one manifestation of the transition is an abrupt


36.

FLiNN


A N D GOLDBERGER

change in the V - I characteristic

449

from a negative to a positive slope.

Accompanying this change is an increase in the rate of vaporization of the anode, and (at

sufficiently high currents),

the appearance

of a high

velocity jet of superheated vapor issuing at an angle from the anode crater. This jet or "tail flame" is the means by which heat is convected away.

Operating currents for high intensity arcs can range upwards to

several thousand amperes. Thus, we see that by manipulation of pressure, current, and gas flow


that various transitions from a glow type plasma environment to a high intensity arc can be made. New phenomena occur as these variables are manipulated, namely, thermal ionization, thermionic emission, thermal and magnetic pinch effects, etc.

These phenomena change the nature of

the plasma environment, generally toward higher degrees of ionization, and higher gas temperatures. W i t h reference to chemical reactions in such devices, the mechanism by which a given product is produced would certainly be a function of the specific plasma environment established.

Not enough is known at

present about reaction mechanisms or the control of plasma environments to select the best device for a desired product. However, some general comments regarding the problems of selection and use of plasma devices for chemical processing applications can be made.

Chemical Processing The possibilities of extremely rapid processing and the production of useful new products stimulates interest in the development of plasma reactors for the chemical process industries.

Categories for potential

near-term application of the discharge reactor include ( 8 ) : Highly pyrolysis.

endothermic

reactions

such

as

acetylene

formation

Reactions that are extremely slow at ordinary temperatures; reactions of this type involve high melting point solids. Reactions dependent on excited species.

by

many

The step of evaluating the technical and economic merit of a proposed process requires selection of the type of equipment and processing conditions to be employed.

However, too little is known about

the

factors controlling the plasma reaction to apply kinetic theory to aid in the selection. As a result, reaction studies have been empirical. However, as this series of papers shows, there is extensive research being undertaken.

What is needed is a means of correlating the results reported

for the various discharge reactor systems used so as to guide future research and to obtain answers for the many questions yet unresolved.


450

CHEMICAL

REACTIONS

IN

ELECTRICAL

DISCHARGES

A major factor in the economics of any electrothermal chemical process will be the operating pressure on the process.

Generally it is

easiest and less costly to operate a process at atmospheric pressure or slightly higher. A first consideration in the development of a process should be the possibility of conducting the discharge reaction at atmospheric pressure. The higher the pressure, the more thermally stable the product must be to withstand the plasma environment. As for production of high endothermic compounds such as acetylene or H C N , major atten-


tion has been given to the plasma jet which can operate at pressures well above atmospheric and at extremely high throughput.

Conversely, for

production of compounds of low thermal stability or for generation of excited species that rapidly recombine and revert to less excited states, the low pressure plasma environment may be the only possible one of practical interest. It is one thing to demonstrate conclusively in the laboratory that a desired product can be made at high yield by plasma processing. It is by far another to design the discharge reactor system for commercial operation. If this latter phase of a development is to be done in a systematic way, information is needed relative to the factors controlling the rate of the reaction and the influence of the geometry of the device on the results. The research results included in this compilation of papers relate to studies of chemical synthesis and reaction mechanism and also to studies concerned more with reaction kinetics, the latter being the most meaningful at the process development stage.

The kinetic data

required for process reactor design will differ from those needed to elucidate the reaction mechanism. Design data will be concerned mainly with the effect of the process variables, such as flow rates, power input, pressure, and reactor geometry on the conversion and yield.

Attempts

will be made to determine if the process rate is controlled by chemical kinetics or by mass or heat transfer factors. The work of Ruppel, Mossbauer, and Bienstock (12)

on the water-

gas shift reaction in a corona discharge is a good example of a process kinetic study which provides reactor design data. The experimental data were correlated empirically to show the dependence of hydrogen yield as a function of pressure, flow rate, wall temperature, and input power. Thornton, Charleton, and Spedding (15)

show the importance of reactor

geometry in accomplishing hydrazine synthesis. Use of enthalpy rather than temperature has been shown to be a means for correlating process rate data for the high pressure, high flow reactor systems in which extreme temperature gradients exist (I, 6).

The influence of the kinetics

of quenching the plasma mixture on the product yield is of extreme importance as illustrated again by the paper of Bronfin

(2).


36.

FLiNN

A N D GOLDBERGER


451

Too often in the literature the details of a device used in a kinetic study are only vaguely recorded.

Discharge parameters, common to all

devices, need to be identified and measured in the course of such studies. In this way the results of various investigators can be properly compared and used to advantage in the selection of a device for chemical processing. Phelps (11)

has discussed some parameters of particular importance.

However, to date very little correlation of these parameters against chemi


cal reaction results in discharges has been reported.

Summary Despite apparent gross physical differences between a low pressure, low temperature d.c. glow discharge and a high pressure, high tempera ture d.c. plasma jet, it was shown that the two extremes are interrelated. A n understanding of the interrelationships between various types of devices appears to be important from the standpoint of selecting the best device for either research or process applications. Greater effort needs to be exerted in identifying the specific type device being used for a given chemical reaction study. Research is needed which compares the results obtained with the same chemical reaction in different types of devices.

Literature Cited (1) Ammann, P. R., Timmons, R. S., Krukonis, V., "Abstracts of Papers," 153rd Meeting, ACS, April 9-14, 1967, I101. (2) Bronfin, B. R., ADVAN. CHEM. SER. 80, 423 (1969). (3) Cobine, J. D . , "Gaseous Conductors," Dover, 1958. (4) Coffman, J. Α., Browne, W . Α., Sci. Am. 91-97 (June 1965). (5) Francis, G . , "Handbuch der Physik," Springer-Verlag, Berlin, 1956. (6) Freeman, M . P., ADVAN. CHEM. SER. 80, 406 (1969). (7) Bamblin, W . Α., et al., Brit. J. Appl. Phys. 5, 36 (January 1954). (8) Goldberger, W . M . , Chem. Eng. (March 14, 1966). (9) Kunkel, W . B., "Plasma Physics in Theory and Application," Chap. 10, McGraw-Hill, New York, 1966. (10) Persson, Karl-Birger, J. Appl. Phys. 36 (10), 3086 (1965). (11) Phelps, Α. V., ADVAN. CHEM. SER. 80, 18 (1969). (12) Ruppel, T . C., Mossbauer, P. F., Bienstock, D . , ADVAN. CHEM. SER. 80, 214 (1969). (13) Suits, C. G., Phys. Rev. 53, 609 (1938). (14) Thornton, J. D . , Chem. Processing 12 (2), S6 (1966). (15) Thornton, J. D . , Charlton, W . D . , Spedding, P. L . , ADVAN. CHEM. SER. 80, 165 (1969). RECEIVED

October 9, 1967.


Chemical Reactions in Electrical Discharges

Recommend Documents