plasma chemical and process engineering

Plasma Chemical and

Process Engineering

Potential for chemical synthesis is high but still unrealized

Fioure 7.

A . High-intensity combustion reactor

FLAME

CARBO8 ELECVRO5E * -

CARBON $ELECTRODE

FRfMlXEO FUEL OXYGEN 4- SEED

B. Electrically augmented flame

C. Porous-disk-augmented flame reactor

D. Diaphragm shock tube reactor-pressure distribution

---

48 INDUSTRIAL AND ENGINEERING CHEMISTRY

VINCENT J. IBBERSON MEREDITH W. THRING

whole new branch of engineering is becoming A established which is concerned with the applications of plasma physics and chemistry and can be termed plasma engineering (44)--i.e., the use of partially ionized gases for industrial purposes. If a plasma is defined as a quasineutral gas ionized to the extent of at least 0.1% of its molecules, there is a range of methods (3, 27) available for its production which involve high energy inputs to a gas. These include high-temperature combustion (with or without seeding), detonation and shock waves, different types of electric discharge such as

the glow, the high intensity dc arc and the radio-frequency induction discharge. Plasma engineering, which in the present context covers the temperature range 2500 to 50,000°K, can be divided into three branches:

(1) Plasma chemical engineering, which utilizes the reactions occurring in ionized gases for the production of known and possibly new chemicals. The range of possibilities for the manufacture of endothermic compounds or elements produced by endothermic reactions is being gradually widened.

(2) Plasma process engineering. Here the plasma

E. High-intensity arc reactor G. PZasma jet reactor constricted arc

. , . . . .

F. Plasma je t reactor H. R.f. discharge reactor

VOL, 6 1 NO. 1 1 NOVEMBER 1 9 6 9 49

torch is proving to be versatile for a number of processes requiring thermal treatment, with or without chemical reaction. In particular, it supersedes the combustion torch when an inert atmosphere and temperatures above 2500°C are essential.

(3) Plasma power engineering--i.e., the generation of electricity from plasma which includes the diode generator, plasma electrolyte fuel cells, and magnetohydrodynamic energy conversion (MHD) (9). This is a subject in itself, although the basic techniques of plasma generation are applicable, and chemical reactions are utilized.

Suffice it to say that the MHD method, researched intensively during the past 10 years, has been proved as an engineering proposition, and the decision on economic feasibility is imminent. The plasma reactions required here are not specific so long as a sufficient degree of ionization is attained and the various methods-combustion, electric discharge, shock, etc.-have been tried. An electron concentration of about 2 X l O I 4 electrons/cc is required giving a conductivity (u) of about 10 mho/m, which, in conjunction with high gas velocity ( v ) , provides a sufficiently high power density P = uv2B2K (1 - K ) to ensure efficient Lorentz force (braking action) of the magnetic field ( B ) on the gas. The basic difficulty is in obtaining simultaneously high u and u.

It is the authors’ belief that a more sophisticated cycle will be developed in the next few years to overcome this difficulty, and that ground power generation in very large units of 1000 mW or more will be feasible both for fossil and fissile fuels within the next 30 years. However, this is outside the scope of the present review.

The emphasis in this paper is on the use of plasma for chemical production, on the one hand, and for metallurgical and ceramic processes on the other. These will be discussed in turn after the most important methods for generation of “low temperature” plasma have been described. The applications of plasma in fusion research, propulsion systems, space simulation, and to provide high temperatures for experimental purposes are not included.

PRODUCTION OF PLASMAS In the present context, a “low temperature” plasma

is taken to be a gas with the order of 0.1% or more of its molecules ionized. The main methods available for generation of such plasmas are: combustion, shock and detonation waves, and electric discharges. The nature of the processes may be either chemical or physical- e.g., in combustion flames a nonequilibrium ionization occurs caused by chemical reactions ( i .e . , chenii-ionization), whereas in burnt combustion gases seeded with

an alkali metal, normal thermal ionization occurs due predominantly to removal of electrons from alkali atoms. In the case of pressure waves, shock waves are basically physical, but detonation waves are driven by chemical reaction. The various electric discharges depend on physical processes. In addition to these three main groups of methods, various combinations may have their own advantages-e.g., electrically augmented flames (8), shock or explosive discharges, combustion-driven shock waves, and the electrohydraulic effect (7). These principles of plasma generation are illustrated in the diagrams of Figure 1, A-H.

Combustion Plasmas Two distinct regimes of ionization occur in combustion

and are referred to as nonequilibrium ionization occurring in the flame reaction zone and thermal ionization as in the burnt gases.

In the case of nonequilibrium ionization the electron temperature is likely to be several orders of magnitude higher than the equilibrium value. This is considered to be due to a free radical reaction mechanism and is termed chemi-ionization. A proposed mechanism (20) depends on CHO radicals as follows :

H.vO CH + 0 = CHO+ + e-

CHO+ + H20 = H30+ + C O I.€ = -34 kcal

Flame ionization can be attributed to nonequilibrium concentrations of C H and 0 which eventually result in H 8 0 + ions, the most abundant ionic species present. This type of ionization implies electron concentrations of the order of lOI3/cc and is little affected by the addition of alkali; therefore, this plasma is too low in conductivity for MHD power generation, but may be useful as a potential source of free radicals or of intermediate combustion products.

An equilibrium-type plasma is attained in the combustion products which have been seeded with a low ionization potential material--e.g., cesium at 3.98 eV (9) . I n this case, provided high intensity combustion is used, the ionization is sufficient for MHD interactions. The combustion conditions required are that the oxidant be oxygen or air preheated to 2000°C in, for instance, a hot valve pebble-bed regenerator, and that the oxidant be seeded to the extent of about 1% of potassium. This gives a plasma of temperature 2500-3000’K and electrical conductivity of the order of 1 mho/cm. Higher temperatures may be obtained by other fuel/oxidant corn- binations--e.g., hydrogen/fluorine at 64,000 cal/mol HF and aluminum/oxygen at 388,000 cal/mole A1203.

Combustion flames with pure oxygen may be extended to industrial temperatures of 3000’K, but above this the high intensity electrical plasma processes will be


Figure 2. Electromagnetic T-shock tube

developed to a greater extent. Shock wave reactors that are either pressure, electrically or electromagneti- cally driven may be suitable for special cases.

From the point of view of chemical production and processing, the ranges of enthalpy levels and possible products from combustion or partial combustion are useful, but this range is greatly extended by the other methods of plasma formation.

Shock Wave Plasmas The shock tube is an important tool for a wide range

of chemical and physical studies ( I S ) and is a convenient generator of slugs of plasma for short times of the order of a few hundred microseconds. The principle is illustrated in the diagram of Figure 1D. A high-pressure driver gas is expanded suddenly via a diaphragm into a low-pressure driven gas. The resultant shock produces temperatures of the order of thousands of degrees Kelvin and, therefore, appreciable ionization. For an ideal gas the temperature ratio across the shock is :

where AS = entropy change, so that for a pressure ratio of 200 : 1 and y = 1.4, the shock Mach number is 13.1 and the shock wave temperature is 11,700"K. In the case of a real gas, the temperature would be less- e.g., nitrogen reaches only 6000°K at Mach 13.

In the limit of an infinite initial pressure ratio (Le . , driver/driven gas) the maximum obtainable Mach number depends on a high ratio of sonic velocities in the driver/driven gases and low y in the driver gas. Ac- cordingly, the performance is improved by using light driver gases such as helium and hydrogen. Further improvement is obtained by heating the driver gas and thus increasing its sonic velocity. The density-driven shock tube then becomes thermal-driven and Mach numbers of the order of 60 are possible.

The energy source may be combustion or an electric discharge, thus combining two of the methods of plasma generation. In the simple electric shock tube, electrical energy is rapidly discharged into the gas, generating a high-temperature plasma in the electrode region, which acts as the driver gas, and a diaphragm is unnecessary. In the electromagnetic-(EM)-type shock tube, a still further acceleration is provided by the action of a magnetic field on the discharge current. These latter types are capable of extremely high Mach number shocks, up to several hundred, and temperatures of over 1 00,00O"K, and thus generate highly ionized plasmas as shown in Figure 2.

Finally in this section, the electrohydraulic effect (EHE) is another electric discharge-shock combination (7). A high-voltage dc discharge (about 20,000 V) in a liquid generates a plasma bubble instantaneously, and thus an intense shock wave. This, accompanied by chemical effects, has a variety of possibilities such as the shaping of metals and the shattering of rocks. In water, a plasma bubble of about 1-cm diameter is formed in about 1 psec at conditions of up to 30,000"K and 20,000 atm pressure. Relatively low energy inputs result in high power and, therefore, conversion into intense mechanical energy.

Electric Discharges The third method for plasma production involves the

release of electrical energy into a carrier gas-such as argon, helium, nitrogen, hydrogen-by means of an electric discharge of which there are several types (3, 4, 27, 24). The glow and microwave discharge, for instance, produce "cold" plasmas (low gas temperature, high electron temperature) while the dc arc and the r.f. discharge generate hot (approximately equilibrium-type) plasmas.

This kind of discharge operates under a variety of conditions, the most familiar application being in lighting-fluorescent, neon, sodium, and mercury lamps for example. The glow is maintained by electrons produced by bombardment of the cathode with particles from the gas. The discharge may be ac or dc and occurs a t a pressure of a few millimeters of mercury. Under special conditions of high voltage and low pressure, penetrating beams of energetic ions and electrons can be produced which have great potential for applications-such as crystal growing, melting, chemical processing, vapor forming. The operating conditions for the high-voltage, low-pressure regime are 0.1 to 100 N/m2 and 1-100 kV.

The arc is a discharge of low voltage and high current, differing from the glow type in that the cathode potential drop is only about 10 V although

Glow discharge ( 7 7).

Arc discharge.

VOL. 6 1 NO. 1 1 NOVEMBER 1 9 6 9 51

C A T H O D E ASSEMBLY

F U R N A C E - H E A D

- P L A S M A

d + v e

Figure 3. Rotating wall plasma generator ( 7 7 )

the current density is high (5). The differences are due to the electron emission mechanism at the cathode which for the arc is mainly thermionic. Electric arcs may be of low or high intensity, and also may be divided into the direct and the transferred-arc types.

The low-intensity arc is relatively well known, attaining a temperature of approximately 5000-60OO0C. If the anode material is heated to its boiling point, the discharge changes to a Beck-type arc of high intensity and temperature of about 9000-10,OOO"C. The arc voltage increases with increasing current compared with the falling voltage-rising current characteristics of the low intensity arc. The Beck arc has been used as a light source, and for metallurgical purposes, by employing a composite anode. Rapid evaporation of the required material from the anode results in a high-temperature jet at a velocity of 10-100 m/sec (70). Figure 1E shows the principle of a special high-intensity arc reactor used for the study of the carbon/hydrogen system.

An interesting example of the high-intensity arc is the rotating-wall arc plasma generator. This has been developed at the National Physical Laboratory (45),

among others, and uses the principle of arc stabilization by rotation of a cylinder round the arc column (Figure 3). Plasma streaming is utilized to carry powders into the discharge for processing purposes, and the large plasma volume allows long residence times. Discharges of up to 500 A have been operated in this furnace, and it is claimed that larger volumes of plasma can be generated than in other arc reactors. Also better control of temperature gradient at the plasma boundary is obtained by variation of rate of rotation of the wall. The furnace appears to be promising as a chemical reactor in that the desired products can be removed through controlled temperature regions.

The modern plasma jet is a development of the arc discharge using both thermal and magnetic pinch effects to increase further the current density and temperatures attainable.

The plasma jet is essentially a high-current arc producing a plasma which passes through a nozzle. The cathode is often of thoriated tungsten and the anode of water-cooled copper, as illustrated in Figure 4 and in Figure l F , G. Typical voltage current characteristics for such a jet with constricted arc are presented in Figure 5 for different plasma gases (23) . I t is evident that above 350 A, the voltage is insensitive to the current. The large variation in arc voltage for different gases is attributed to differences in thermal conductivity. The cooling system both keeps the electrodes from evaporating and provides the thermal pinch effect because of constriction of the arc by cooling its periphery as it passes through the water-cooled nozzle. Thus the current density and temperature are increased, which results in a self-induced magnetic field in such a direction as to cause further confinement of the arc- i .e . , magnetic pinch.

The dc plasma jet is a versatile tool capable of temperatures up to 50,000"K and velocities in the subsonic to the Mach 20 range. Power inputs vary from less than

Plasma jet generator.

REAc'ANTs SUPEKSONIC NOZZLE

CARRIER GAS

/-----/

DlKhARGE

-WATER-COOLED THORiATiO COPPER ANODE IUNGSTEN CATHODf dc POWER

SUPPLY

Figure 4. Plasma-jet-constricted arc with supersonic nozzle

52 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

Figure 5. Current-voltage characteristics for plasma generator, chamber pressure 7 atm (35)

1 kW up to several megawatts. Stabilization of the arc is often obtained by tangential injection of the carrier gas to form a vortex flow, or by rotation of the arc caused by confinement in a magnetic field, with consequent improvement in the energy transfer from arc to working gas.

The plasma jet is used as a heat source for processes such as welding, cutting, spheroidizing and spraying, and to accomplish chemical reactions at superflame temperature (2). The reactor may be so designed that there is no direct contact of the reactants with the electric arc. Particularly in the chemical reaction applications, the fast heating of reactants and quenching of products are of fundamental importance.

The temperature of the working gas in a plasma system corresponds to the heat balance equation relating power input (2) to the enthalpy of the plasma H ( T ) and the heat radiated and conducted to the water- cooled anode. In arcs at atmospheric pressure, heat transfer is mainly by radiation and increases as T4 for a grey-body arc

Z = H ( T ) w + L

L = rate of heat flow to anode (cal/sec) If w is very small, all of 2 goes to the walls of the reactor. In normal high intensity reactors L can be as low as 5% of Z, if T is relatively low, but the highest temperature obtainable is that when L is nearly equal to Z. Here the scale effect is relevant: for a constant arc length and gas velocity, the wall heat loss will increase as the diameter, but with constant velocity the heat capacity of the gas will increase as d2 so that the larger the arc, the greater the temperature achievable.

The electron temperature may be considerably higher than that of the ions or molecules because the applied electric field directly heats the electrons which heat the heavy particles by collision. The heavy ions will be at a slightly higher temperature than the neutral particles, and any solid or liquid drops will be at a lower temperature, again because of thermal radiation to the walls.

w = flow rate of gas ( g mol/sec) (2)

R.f. plasma torch. The radio-frequency induction- heated plasma generator (38) produces temperatures up to 20,000°K, using frequencies in the range 1-25 Mc/sec (Figure 1H). The energy of the high-frequency field causes a high degree of dissociation and excitation in the working gas, and by allowing recombination among the various species, the absorbed energy is given off in the form of heat and light. The gas flows into one end of a quartz tube surrounded at the other end by a flat or cylindrical water-cooled copper coil. The discharge is initiated by various means such as a pilot arc or an inductively-heated graphite rod. A pilot discharge appears because of the emission of thermal electrons, and this can couple with the r.f. field to give the full discharge. A 3-40 Mc/sec oscillation with a power of the order of 2 kW/ml of discharge volume produces a single turn discharge in the gas.

The heat balance of Equation 2 in this case has to include a coupling efficiency factor, E, to allow for the degree of coupling, which under normal conditions approximates to 0.5. The actual electric power input to the stabilized plasma therefore becomes ZE where Z is the ref. generator power. Stabilization of the arc is attained by vortex gas flow, or magnetic confinement, by providing a work coil with a reverse turn at the downstream end. The flux from this turn repels the plasma carrying the induced current from the other turns. The tube radius ( r ) required is highly influenced by the skin effect peculiar to ref. induction heating. The energy (80-90%) is dissipated in a thin layer, 6, so that r should not be a lot greater than 6 and r /6 Q: 1.75-e.g., the skin thickness for argon is about 2.5 mm at 5 Mc/sec, and a suitable tube radius is 12 mm.

Advantages of the ref. torch are longer residence times due to lower gas velocities and absence of electrodes. The latter allows the use of erosive gases and facilitates introduction of solids for processes such as crystal growing.

A combination of combustion and discharge plasmas occurs in the case of electrically augmented flames (Figure lB , C) where the proportions of chemical and physical energy input are comparable. Karlowitz (26) has described an electrically boosted burner operating on distributed discharger in turbulent-seeded combustion products. On work carried out at Imperial College (8), normal flame temperatures can be increased appreciably--e.g., for a ratio of electrical to chemical energy of about 3.5, the flame temperature of ethylene/ air was increased to 4500°K from 2250'K. Reactors found suitahle for combustion/discharge studies were plasma jet configurations modified to facilitate introduction of inflammable mixtures as in Figure l F , G, C. One of the main difficulties is to spread the discharge

VOL. 6 1 NO. 1 1 NOVEMBER 1 9 6 9 53

Figure 6. Enthalpy of pure gases at plasma temperatures

( a ) Hydrogen ( I atm), ( b ) helium ( I atm), ( c ) argon ( I atm), and ( d ) argon (variouspressures, log scale)

evenly through the flame, as it tends to channel along a thin arc. The most promising solutions appear to be feeding the fuel/oxidant through discharges either held stationary or rotated by a magnetic field at a rate of about l o 5 rpm. An interesting porous cavity reactor was tried (Figure 1C) in which flame blow-off tendency was reduced and energy flux increased from 3 kW/cm2 (plasma jet) to 5.7 kW/cm2, the chemical reaction providing 15 and 40y0 of the total energy, respectively.

Useful effects of discharge/flame systems were diminu- tion of burner blow-off, increase in throughput, control of carbon formation, and enhancement of heat transfer and ionization. The augmented flame system therefore has applications in chemical production and processing where relatively large enthalpy levels are necessary, where it is desirable to provide part of the energy chemi- cally, or where oxygenated products of partial combustion are required.

For engineering purposes, therefore, the generation of plasmas (as summarized in Figure 1A + H ) amounts

to raising the working gas to high enthalpy levels. These enthalpy levels may be required to achieve electrical conductivity of the gas as in MHD power generation, to supply the energy of activation for chemical reactions, or to reach high temperatures for thermal processing. These processes are indicated in the graphs-Figure 6, for the monatomic gases argon and helium ( b , c, d) and a diatomic gas, hydrogen ( u ) . The graphs show the enthalpy per gram molecule required to produce any given temperature in these gases.

PLASMA CHEMICAL ENGINEERING The chemical engineering applications of plasma

include the use of the various discharges, combustion, and shock or detonation waves to accomplish desirable chemical reactions at temperatures up to 50,000”K. At these temperatures thermal excitation and ionization activation processes initiate very fast reactions between electrons, ions, and neutrals, often with a free radical mechanism. A plasma “equilibrium” is produced


which can be frozen out by rapid cooling to minimize back reactions. Several metastable compounds of industrial importance have been synthesized in this way-for example, NOz, (CN)z, HCN, and CzHz (7-3, 5, 27, 24, 37, 47). A second type of product is completely unstable such that the quenching does not freeze an equilibrium composition because the highly reactive species recombine. This occurs in the case of the preparation of some organometallic compounds and the polymerization of organic radicals.

Because of the complexity and unfamiliarity of plasma reactions, predictability and therefore systematic design of plasma reactors are difficult. The normal kinetics change, as the temperature rises, such that the Arrhenius- type equation does not hold when E,,t/RT ,< 5 (33). Also, the chemical properties of atoms are not dependent to the same extent on the degree of completion of the outer electron shell-for instance, the noble gases with completely filled outer shells can undergo such reactions as oxidation because of the higher energy levels of some of the electrons.

However, broadly speaking, the law of mass action can be applied to show that high temperatures push equilibrium in the direction of endothermic reactions- ;.e., either toward the formation of endothermic compounds or the decomposition of exothermic compounds. Pressure affects the degree of disequilibrium between the different particle temperature-for example, low pressures favor high-electron temperature and concentration because the electron recombination rate is reduced. High pressures also influence the equilibria in the direction of the formation of more complex molecules. In some cases, approximate plasma equilibrium composi- tions can be calculated from thermodynamics as has been done by Baddour and Iwasyk for the carbon/ hydrogen system at temperatures above 2800'K (2). Their results are plotted in Figure 7.

In designing a plasma system for a specific reaction, therefore, the important parameters include : temperature and pressure in the reactor, residence time of the reactants, rate of mixing between plasma and reactants, and rate of quenching of the products. Methods of quenching available with approximate cooling rates include (1) direct spraying with a cold inert fluid-e.g., water, liquid nitrogen - 100"K/psec; (2) heat transfer to a cold surface - 1 'K/Msec; (3) adiabatic expansion of the gases - up to 1 'K/psec; (4) passage through a fluidized bed - 100°K/psec; and (5) expansion through a supersonic nozzle at about 100"K/psec (3, 76, 37). Thus the latter is a practical way of attaining a high quench rate with good control, although it should be coupled to a device for utilization of the kinetic energy

Figure 7. Equilibrium diagram for system C f HZ at 7 atm pressure ( 75)

Figure 8. Diagram of plasma chemical process

of the resultant gas. An MHD power generator could be proposed for this function.

The quench part of the system should be used as a reactor in itself-that is, it should be so controlled that the equilibrium composition is frozen in or the recombination reactions to desired products are favored. An additional technique available is to react independent reactants with the plasma species for the desired reactions-e.g., by spraying in a reactive liquid or solid or fluidizing a solid with the plasma gases.

A plasma reaction process, therefore, may be repre- sented by the generalized line diagram of Figure 8 showing the constituent operations.

VOL. 6 1 NO. 1 1 NOVEMBER 1 9 6 9 55

The main group of reactions which have been studied with the possibility of using some form of plasma process as a commercial reaction system are syntheses of endothermic compounds (inorganic such as nitrogen compounds and organic such as acetylene and halocarbons) and preparation of pure elements. The reacting system has to be efficiently raised to the energy level to activate the required reactions and the use of a catalyst may decrease the energy of activation and increase the selec- tivity of reaction.

Nitrogen Compounds Since introduction of the Birkland-Eyde process for

the manufacture of nitric oxide in a low-intensity arc, a number of reactions for nitrogen fixation have been investigated in recent years. These include the systems H/N, C/H/N/O, air/water, and reactions for the production of NO, HCN, (CN)z and metal nitrides, and nitrogen fluorides (6).

Nitrogen oxides (3, 27, 24, 40). The attractive possibility of NO, for example, from oxygen and nitrogen appears to require reaction temperatures of the order of 3000-5000°K. I t has, however, met with limited success-a major difficulty being to suppress reformation of the elements on cooling. A further difficulty is the large energy of dissociation of the nitrogen molecule which has a binding energy of 7.4 eV. Actually, an electron energy of the order of 17 eV is needed to obtain a fast rate of reaction, and this is approximately the energy required to ionize the nitrogen molecule. There- fore, the resultant mechanism can probably be repre- sented by the reactions

Nz+ + e + 0 2 --t NzOz + e + 2 NO

I n contrast to previously reported low yields of about 20/, in both dc and r.f. discharges, yields of up to 12yo NO have been claimed at the Research Institute of Tem- ple University (40). I n a 15-kW nitrogen plasma jet with stoichiometric oxygen feed, the improved design features were: (1) a high concentration of reactive species was attained by the confined nitrogen jet, and (2) very fast cooling of the products was obtained by expanding into a large vacuum chamber.

At this stage, however, the plasma process is not competitive with the Ostwald process using the oxidation of ammonia.

Hydrogen cyanide. Hydrogen cyanide has been successfully synthesized from the elements by feeding hydrogen into a nitrogen plasma reactor incorporating a consumable graphite cathode (78).

2 C + H z + Nz + 2 HCN - 60.2 kcal

Over 50% of the carbon input was converted to HCN with acetylene as the main by-product.

Other reactant combinations tried and yields were :

(1) carbon + NH3 -t I 39% HCN + 18% CzHz

Nz plasma jet + { (2) CHd + Nz -+ 90% carbon conversion to HCN + CzHz

(3) CHd + NH, -j I 60-7570 carbon conversion

Freeman (12) also treated methane in a nitrogen plasma jet, determining the production of HCN as a function of nitrogen enthalpy, CHg/Nz ratio, and gas flow rate. He found that carbon condensation was suppressed at pressures less than 0.5 atm. This allows the possibility of high equilibrium concentrations of certain products to be extended to lower temperatures. In the course of studying the decomposition of NHs (and also CHg/Nz system), it was concluded by Freeman and Scriven (73) that intimate mixing of reactants is one of the key problems in plasma jet reactions. In this connection Grey and Jacobs (76) found, on mixing cold helium with an argon plasma jet, that radial temperature and concentration gradients indicated only partial mixing until four nozzle diameters downstream of the nozzle. At this position temperatures were already considerably reduced.

Another endothermic compound of con- siderable industrial interest which has been synthesized from the elements is cyanogen (28).

2 C + Nz -+ (CN)z - 71 kcal

As for HCN, at Temple University, the reactor was a nitrogen plasma jet with a consumable graphite cathode working at power inputs of about 10 kW. Conversions of up to 15y0 on carbon input were obtained, and the unconverted carbon was collected as soot. Fast quenching reduced the yield appreciably, indicating too short a reaction time or catalytic decomposition by the copper cooling funnel. Electrical power is the important cost item in most plasma processes and in this case was estimated at $1.40/lb of cyanogen.

A nitrogen plasma jet is an obvious possibility for the high temperature formation of nitrides. TiN, MgaNZ, and WN have been synthesized by feeding in the respective metals. Yields of 3oY0, 4oY0,

AUTHORS Vincent J. Ibberson is Senior Lecturer i n the Depart- ment of Chemistry and Chemical Technology, Borough Poly- technic, London, S.E. 7 , England. Professor Meredith W. Thring is Head of the Mechanical Engineering Department, Queen M a r y College, University of London, London, E. 7 , England.

Cyanogen.

Metal nitrides.


and 25Q/, were obtained, respectively, by Stokes and Knipe (41). S4N4 has also been made, and Harmisch et al. (79) have prepared TiN by adding titanium tetrachloride to an ammonia or nitrogen/hydrogen arc :

Tic14 + H + NHE - TiN + 4 HCl

Hydrazine. Another nitrogen compound showing great promise is hydrazine. Appreciable yields have been obtained (from ammonia) a t Newcastle University (43) in a silent electrical discharge. Reactors were designed for variable residence times or for use with a liquid absorbent for the hydrazine (which is also equiva- lent to reducing residence time). Both direct reduction of residence time and use of an absorbent-e.g., ethylene glycol-in a spray reactor increased yields substantially. The best yield obtained was 15 g of hydrazinelkwh which makes it one of the few chemicals from plasma which is already economically comparable with present industrial production. In the plasma jet, a small yield of hydrazine has been obtained from the elements :

NZ + 2Hz = NzH4 - 22.75 kcal (25)

Carbon Compounds Carbon plays a notable role in the present plasma

chemical engineering scene in that consumable carbon electrodes are convenient to use and the temperatures necessary for vaporization of the carbon are easily reached (>40OO0C). The production of HCN and (CN)z has already been noted, and other examples are the formation of acetylene and some halocarbons. Another potential use of carbon is as a reducing agent- e.g., to accomplish carbothermic reductions of metal oxides at high arc temperatures. Here composite electrodes can be used. Otherwise, carbon as a reactant can be injected in powder form (e.g., coal) or as a gas in the form of, for instance, methane.

Carbon itself can be prepared by cracking hydrocarbons in a plasma jet and it has been studied in connection with the manufacture of carbon black (36). Both methane and liquid hydrocarbons have been tried as feedstock.

Industrially, acetylene is made in two stages by first forming calcium carbide from lime and coke in a low intensity arc:

Acetylene.

CaO + C + CaCz + CO - 111 kcal

and then reacting the carbide with water at room temperature :

CaCz + HzO + CZHZ + CaO

At high temperatures, the stability of hydrocarbons increases in the order: alkanes, alkenes, alkynes, and

the equilibrium concentration of radicals-e.g., CH3, CHZ, CH-increases. Therefore, the high temperature treatment of hydrocarbons favors formation of some alkenes and alkynes either directly or by recombination of such radicals as CH and CH2. Thus acetylene can be formed from methane at temperatures over 2000"K, and subsequent rapid quenching of the equilibrium composition: 2CH4 --t CzHz + 3H2 - 95 kcal. Leutner and Stokes (29) introduced methane at right angles into an argon plasma jet and obtained 80% yield of CzH2. Residence times of the methane in the jet were of the order of 0.5 msec and the average temperature of the jet was calculated to be 12000°K.

One alternative method is to feed powdered carbon or a consumable carbon cathode into a hydrogen plasma jet:

2 C + Hz + CzHz - 54 kcal

Yields of about 34% have been obtained at Temple University (25), and at Sheffield University (35), by injecting powdered coal into an argon/hydrogen arc at a power input of 8 kW, 40% of the carbon in the coal was converted to C ~ H Z . Baddour and Iwasyk (2) designed a special high-intensity arc reactor for a study of the C/H system (Figure 1) and achieved a yield of %yo acetylene in the quenched gas. This compared with a theoretical yield of 34% in accordance with a reaction mechanism postulating C2H radicals as an important intermediate. Their equilibrium calculations are given in Figure 7.

These and other researches show a keen interest in acetylene synthesis, and in the U. K. the use of methane is relevant to the present North Sea gas potential. The indications are that this process is already becoming competitive in that the yields appear to be higher than present industrial processes, including the pyrolysis of hydrocarbons (about 16% conversion of methane), in addition to which separation of acetylene from side products is easier. A modified arc process has been claimed by one company to have costs competitive with those of the calcium carbide hydrolysis process.

I n this case a range of halocarbons has been successfully synthesized using carbon as both anode and cathode in a high intensity arc (46), e.g.,

Halocarbons.

graphite

(charcoal) electrodes C + CaFz - CF,

CaClz + CaFz --+ CFsC1, CFzClz

Kanaan and Margrave (25) carried out an interesting aromatic synthesis from carbon tetrachloride vapor in an argon plasma jet:

V O L 6 1 NO. 1 1 NOVEMBER 1 9 6 9 57

A free radical mechanism was proposed in which chlorine atoms were split off from the tetrachloride with subsequent polymerization of species such as CC1 to give the hexachlorobenzene.

Uranium carbide. The reactor for this synthesis (22) was designed so that the carbide was formed from carbon and uranium dioxide by reaction at the anode- ; .e. , the composite anode was made from a stoichiometric mixture of the two reactants and was maintained at the required conditions by a high intensity arc :

UOz + 3 C + U C + 2 CO - 179 kcal

At arc temperatures of 4000-5000°K and 1-2 mm Hg pressure, the carbide was formed as a liquid sphere on the electrode face. As its melting point is 2773°K it is purified of lower boiling impurities during the process, and the uranium carbide spheres drop from the anode and freeze as they reach the bottom of the reactor.

Reduction Reactions For some reductions, carbon is again useful as in the

carbothermic reduction of metal oxides at high arc temperatures. Again, composite anodes are effective : magnesium oxide, for example, has been treated by incorporating it in the anode with carbon and obtaining good reduction, although some reformation of the oxide occurred on cooling:

MgO + C + M g + CO

The reduction of aluminum oxide by hydrogen has been tried by Grosse et al. (77). In the argon plasma reactor, powdered A1203 was fed in as a suspension in hydrogen or methane with residence times of the order of 5-20 msec. The high heat of formation of A1203 (390 kcal/mol) requires rapid quenching to prevent recombination of the elements, but even though fast quenching was achieved, yields were only about 2y0.

The reduction of silicon tetrachloride, as a further example, gives a useful starting compound for semi- conductor materials :

Sic14 + Hz + SiHC13 + HC1

Oxidation Reactions A particularly interesting type of oxidation reaction

combines combustion and electrical aspects of plasma. I t is the partial oxidation of paraffin hydrocarbons such as methane, ethane, and propane to form a range of more reactive oxygenated compounds such as aldehydes and alcohols. The oxygen/hydrocarbon mixture is fed into the discharge reactor designed for a very low

residence time to prevent degradation of the initial products formed.

The oxidation of some metals and metallic halides can be conveniently carried out in an r.f. plasma torch as in the case of Ti02 formed from a mixture of Tic14 and oxygen (34). A mixture of finely divided nickel and nickel oxide has been obtained from various nickel compounds fed into an argon/oxygen plasma jet (39).

Ozone has been produced industrially for many years in corona-type discharges and more recently by injection of liquid oxygen into a plasma jet (42).

Summary for Chemical Reactors The foregoing description indicates the range and

potential of plasma chemical engineering with regard to types of reaction and range of reactors. The reactions of benzene may be quoted as an example of the effect of different discharges because, generally speaking, benzene gives degradation or condensation products ac- cording to the nature of the discharge and the electrodes. With arc discharges and explosive arcs, acetylene, ethylene, and carbon are common products, whereas with the high-pressure, high-voltage arc, phenyl acetylene, diphenyl, and anthracene are produced. Some of these- e.g., diphenyl and diphenylene isomers-also occur in the r.f. electrodeless discharge.

In general it can be stated that there is great potential in the use of plasma reactors (high-intensity arc, plasma jet, and r.f. torch) for chemical reactions-particularly of the endothermic type. A few industrially important reactions are already economically feasible. The “cold” plasma reactors using glow and microwave discharges will also be increasingly used, particularly where less drastic conditions are desirable. Electrical energy is easily the most important cost factor; therefore, the efficiency of these processes will depend on the ratio of electrical energy absorbed in the reaction to the energy lost as sensible heat in the products.

The scope is further widened in that new alloys and compounds now become possible because novel reactor conditions can be created. It is largely a question of understanding the high-temperature kinetics, when almost any thermodynamically feasible molecule will become possible.

PLASMA PROCESS ENGINEERING In addition to the use of plasma generators for the

realization of new and known chemical reactions, there is a wide range of processes for which these versatile devices are highly suitable. The electrically augmented combustion flame, the plasma jet, and the high-intensity arc are being increasingly used as torches for various


Figure 9. R.f. discharge reactor for polymer coating of materials

thermal processes involving physical and chemical changes (30). These devices have the advantages of inert gas atmospheres (where desirable), higher temperatures, and high rates of heat transfer compared with conventional torches and furnaces. The novel electrohydraulic reactor combining electrical and shock effects has many unique process applications now under development (7). The glow discharge similarly has a much greater scope than was previously realized (I 7).

Cutting An almost unlimited range of materials that are

difficult to cut by conventional means is within the scope of the plasma torch, so that a clean cut, suitable for immediate welding, is obtained (32). Refractory oxides, carbides, and nitrides can be cut and rewelded; concrete can be drilled and fused. The direct plasma jet is good for nonconductors such as stones and asbestos, and the transferred arc device, with a voltage difference between the water-cooled anode and the workpiece, is valuable for conductors. The power to the transferred arc an be separately controlled for efficient operation.

Spraying and Coating Plasma spraying is another process with a wide field

of applications-that is, in terms of materials: organic, inorganic, refractories, metals ; and types of surfacing. Apart from the increased temperature range, a less porous surface is obtained compared with conventional flame spraying.

Coatings have been applied to improve resistance to heat, abrasion, and erosion by feeding materials into the arc as a powder, rod, or wire. Impinging on the target,

the molten particles coalesce to form a high-density and well-bonded layer. The process is capable of fine control, important variables being particle size and melting point, plasma velocity, and thermodynamic properties of the plasma and the feed material.

There are a number of techniques available for plasma spraying such as spray forming in which the substance is sprayed into a mold or onto a mandrel to produce the required shape. Many materials have been successfully treated--e.g., powdered tungsten, molybdenum, and hafnium carbide. The hard-facing technique deposits a hard coating (tungsten carbide for example) on a soft surface such as stainless steel. A promising and probably important application being studied at Newcastle University (75) is the plastic coating of materials in an r.f. discharge reactor as indicated in Figure 9.

Some coatings undergo chemical change in the plasma reactor, as in the case of a Be0 spray in a nitrogen jet which formed a complex containing BeN and Be-0-N. A further application is vapor-phase alloying in which elements are mixed as vapors in the arc reactor and condensed as alloys; carbon and tungsten have been reacted in this manner producing apparently new materials. These applications hint at the enormous potential for formation of new compounds and its importance to high temperature chemistry.

As the materials problem is so severe in MHD power generation ducts-Le., a high temperature, oxidizing, alkali-containing environment-a resistant plasma coating process might be the answer.

Spheroidization Particle spheroidization occurs when a feed material is

raised to its melting point in a plasma device. On volatilization a very fine particle size (<I p) is produced. Spheres of refractories of the order of 100-150 p have been made in a plasma jet at 2000-30OO0C and jet velocity of up to 200 m/sec (37) . The equipment is basically a powder feeder, plasma generator, and a chamber where the quenching fluid is admitted at different points. The spheres produced may be separated into sizes by a cyclone.

Crystal Growth This process is particularly apt for the induction type

of plasma torch as described by Reed (38), because of the high temperatures (10,000-20,000°K in this case), and the choice of inert or reactive atmospheres. Fur- thermore, there is no central electrode ; therefore, powder introduction is simpler than with dc torches and the relatively large cross section, slow-moving plasma flame envelopes the growing crystal uniformly. Sap- phire, stabilized zirconia, and niobium crystals were

VOL. 6 1 NO. 1 1 NOVEMBER 1 9 6 9 59

Figure 70. Modifcation of induction-coupled plasma torch for crystal growth (72)

grown in the special torch shown in Figure 10. Other examples in this field are the growth of sapphire crystals by the Thermal Syndicate process of controlled oxidation in a flame of aluminum chloride, and the Linde Co. process for sapphire and ruby crystal growth. The flexibility of the r.f torch thus ensures its application to a variety of gas-solid reactions.

An interesting example of crystal formation in an electric arc is that of rubies (27). This was achieved in a reactor containing two horizontal carbon electrodes, and alumina powder (containing 2.5% Cr203) was fed into the arc via a third vertical electrode.

Metallurgical Processes The r.f. induction plasma generator is used for zone

melting and floating zone refining to produce pure high-melting point metals. Also ac and dc arcs in controlled atmospheres are highly appropriate for such metals as molybdenum, which may be fused using the ore as consumable electrode. I n an electric furnace, alloying of metals in the vapor phase has been carried out with both titanium and zirconium in conjunction with various alloying agents-e.g., Mo, W (32). The initial reactants were the carbides or oxides of these elements in the form of composite electrodes.

Figure 77 . Electrohydraulic system

The formation of metal suspensions in solutions is managed by striking an arc across two electrodes of the metal placed in the liquid (70). The potential across the electrodes is about 25 V. Vaporization of the metal takes place in the arc, and rapid quenching in the liquid gives the required suspension. Here again, many possibilities are envisaged for chemical reactors in which, for instance, a spark discharge would create mists of reactant in a liquid or molten second reactant. Convenient rapid cooling is inherent in the system, thus ensuring minimum residence time in the high temperature zone.

Applications of Electrohydraulics

The formation of metal suspensions of the previous section and the electrohydraulic effect are both arc-in- liquid processes, but the latter depends essentially on the shock effect. I t has its own series of very promising processing possibilities.

An electrohydraulic processing system is relatively simple in design, consisting essentially of a dc power supply which charges a capacitor bank, electrode system, and a tank containing the fluid and material to be processed (7) (Figure 11).

The switching device to regulate the frequency of discharge is designed to handle high currents at rapid repetition rates. The optimum fluid has a high resis- tivity during the preionization period of the discharge and high conductivity after the arc is initiated. This gives maximum voltage retention before discharge and, subsequently, minimum gap ionization time. If the material to be processed has to be separated from the discharge, a flexible diaphragm is used.

Applications depend on the fact that material sub- jected to the electrohydraulic effect experiences various physical and chemical actions. Cleaning of surfaces, splitting, deformation, and comminution of solids have been successfully effected, and chemical attack is possible by newly formed compounds or highly reactive species.

The only extensively used process at the moment is metal forming-especially as applied to difficult metals such as niobium, titanium, and inconel. Otherwise,

60 I N D U S T R I A L A N D ENGINEERING CHEMISTRY

there are numerous cleaning applications : in elec- trolytic refining to remove deposits from electrodes and to remove impurities such as soil resins from manufac- tured metal parts. Size reduction may be effected on hard or brittle materials-the hard rock mining of copper and the comminution of coal are being considered, although they are not yet commercially feasible. Many other applications have been suggested, such as the extraction of protein from plants by combined solvent and electrohydraulic action and the very effective killing of microbes in distilled water. The latter appears to be caused by the shock wave forcing the chemical species through the cell walls, resulting in a “chemical” kill.

The EHE, therefore, is extremely versatile because of the combined plasma-shock wave action, and although apparently not yet tried, it could be considered as a technique for accomplishing chemical reactions, a t the same time posing novel problems in reactor design.

CONCLUSIONS

The high-intensity arc, the plasma jet, and the ref. induction discharge reactors have already been proved as unique means of producing chemicals and processing materials u p to 50,000°C. Several reactions for chemical production appear to be commercially feasible-e.g., C2H2 formation-and a number of processes are being developed industrially using the glow discharge, the three high-intensity discharges referred to, and the electrohydraulic effect.

Apart from the high temperatures, there are advantages of wide choice of plasma gas (inert or active), high rate of heat transfer (caused by recombination on solid surfaces), easy containment because of the low temperatures near the walls, flexibility in that reactants can be injected before the discharge or after, and finally relatively low capital cost.

A large variety of chemical reactions remains to be studied in plasma reactors. In aromatic chemistry, for example, reactions such as the air oxidation of benzene to phenol and the catalytic vapor-phase oxidation of naphthalene to phthalic anhydride have been suggested. These are really partial combustion reactions. Com- plex species such as amino acids can be synthesized, as well as highly unstable species such as free radicals. In addition, new reactions will be discovered and new plasma processing methods will be evolved.

Present and future studies of well-defined plasma systems will lead to more thermodynamic and kinetic data and better predictability, so that systematic design methods can be employed to replace the present relatively arbitrary ones. Control of the recombination reactions to favor desired products is a basic difficulty to be overcome.

Otherwise outstanding problems include the attain- ment of a higher thermal efficiency-Le., the ratio of electric power absorbed in work to the power input. Sec- ondly there is the overall problem of feeding megawatts of electricity continuously into a small industrial appli- ance with control of containing wall temperature, degree of ionization, and supply and removal of reacting gases and solids. The scale effects may well ease these problems on a large scale, provided the gas flow pattern can be controlled.

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plasma chemical and process engineering

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