Presented as a state-of-the art review of applied plasma chemistry
Chemical The Future of the Chemical Industry F. B. VURZEL L. S. POLAK he ever-growing use of operating conditions charT acterized by extremes of temperatures, velocities, and reaction time is a main trend in the development of today’s chemical processing and technology. T h e development of economical and radically new technological methods for obtaining chemical products and producing new materials with specific properties, requires the study of physical and chemical processes proceeding a t tem~ reaction times ranging peratures of l o 3 to 1 . 5 ~ 1 0OK, from to sec, and pressures ranging from a fraction of a millimeter of mercury to tens of atmospheres. T h e problems mentioned above have resulted in investigation of chemical processes in a low-temperature plasma-a new trend of physical chemistry, “plasma chemistry (48).” The low-temperature plasma is characterized by partial or complete ionization of atoms and molecules; plasma naturally is quasi-neutral. Obtaining this plasma (which is viewed by chemists as high-temperature) has been expedited in the last few years by the development of different types of engines in rocketry and space technology as well as by the studies conducted in such fields as nuclear synthesis, gas dynamics with chemical reactions, gas-discharge techniques, and plasma metallurgy. Because of this, the technology of chemical reactions in a plasma was raised to a substantially new technological level in contrast to the previous 30-60 years when the first rather timid and 8
INDUSTRIAL A N D ENGINEERING CHEMISTRY
technically imperfect attempts were undertaken in this field (87). At present, the low-temperature plasma affords the possibility of conducting chemical processes at temperatures up to 20,00O0K,under pressures ranging from to lo4 atm, and under both equilibrium and nonequilibrium conditions. Low-temperature plasma can be used in chemical reactions as :
1. a source of extremely concentrated specific energy, in other words, of high heat content at high temperatures; 2. a source of positive and negative ions, potential precursors for ion and ion-molecule reactions ; 3. a source of luminous high-intensity radiation for photochemical reactions. Low-temperature plasma may be generated in dc and ac plasmatrons (plasma generators) using 50 Hz with . and typical characefficiency u p to ~ 9 5 7 ~Designs teristics of plasmatrons are described in detail in many works-eg., ( 3 ) . Plasma may be also generated using RF and microwave frequencies, as well as in glow and corona discharges, in adiabatical pistons, shock tubes, and lasers. T h e typical construction of the dc plasmatron, widely used in laboratory research, and the designs of a n induction RF and a inicrowave plasma gcnerator are
Figure 7a. Schematic diagram of a dcplasmatron. 7-Cathode, %-Electrical insulator, 3Constrictor, 4-Anode, 5Inlet gas, 6-Exit gas-plasma jet Figure 7b. Induction plasma torch. 7-Carbon rod, 2Metallic head, 3-Plasma gas, 4-Quartz tube, 5-Coils, 6Fireball Figure 7c. Schematic diagram of a microwave plasrnatron. 7-Wave guide, 2-Inlet gas, 3-Discharge tube
shown in Figure 1. T h e cathode of the dc plasmatron may be made of tungsten, molybdenum, carbon, or copper (in the last case it must be cooled). T h e anode is made of the copper. Plasma gas (Ar, H2, N2, air, 0 2 , CH4, He, Cl2, etc.), or the reaction gas mixture is fed around the cathode, passes through the arc, and flows through the nozzle-shaped anode as a luminous jet of plasma. T h e arc column may be stabilized with three basic techniques-magnetic, aerodynamic, and wall stabilization. R F and microwave plasmatrons may be electrodeless. A quartz tube or a special metal tube surrounded with the R F coil is used in a n RF plasma generator. Plasma gas may be introduced into the tube tangentially to provide axial stabilization. I n the microwave plasmatron the discharge tube passes directly through a broad face of a wave guide. These plasmatrons are energized by vacuum tubes generators with a capacity of a few kW or a hundred kW and a frequency of 0.1 to 3000
MHz. Presently of particular industrial interest are the d c and ac electrode plasmatrons and plasma generators with high-intensity arcs as the source of low-temperature plasmas. T h e conventional definition of low-temperature plasma implies the temperature interval of 103l o 6 "K (0.1-10 eV) whereas the conception of high-tem-
perature plasma implies higher temperatures. From the classical chemistry point of view low-temperature plasma provides conditions for high-temperature chemical reactions to occur. From the chemical standpoint it will be more correct to define low-temperature plasma using the conception of the bond energy. We do not dwell here upon the so-called cold plasma, etc. Note that the temperature is characteristic only of a molecule's mean energy. T h e energy of each molecule and molecule's number in any energy interval is given by the distribution which is well known and either equilibrium or nonequilibrium. Plasma temperature should be high to ensure complete dissociation of reagent molecules, but the new compounds were produced in marked quantities at threshold temperatures determined by thermodynamic and kinetic factors. T h e quenching seems to play a significant, if not deciding, part in the technology of quasi-equilibrium-type plasma chemical processes. T h e character of chemical conversions, occurring a t equilibrium temperatures in the order of several thousand degrees, is largely determined by thermodynamic properties of substances taking part in a reaction a t one stage or another. Given reliable thermodynamic constants it should be possible to determine, in most cases, optimal temperature conditions for reactions, values of product yields expected, and energy indices of the process. VOL. 6 2
NO. 6 J U N E 1 9 7 0
9
At the same time, the course of the reaction usually does not depend solely on the thermodynamic properties of a reacting system. Before reaching a n equilibrium state, which is determined by the thermodynamics of the reaction, the system undergoes a series of intermediate states. T h e rate a t which the system goes through these stages is determined by the kinetics of the process. T h e rate of achieving equilibrium energy distribution according to degrees of freedom is determined by physical kinetics, and the rate of achieving equilibrium chemical composition is determined by chemical kinetics. I n this case, the plasma chemical reactions are characterized by the strong reciprocal interdependence of the physical and chemical kinetics. T h e terminal rate of setting u p equilibrium energy distribution can limit the possible use of classical methods of chemical kinetics, based on a n assumption of the MaxwellBoltzmann energy distribution in the reacting system. But in the cases when the methods of chemical kinetics are thought to be applicable, the studies on chemical kinetics of the system present difficulties in that the rather high velocities of chemical reactions a t the temperatures under consideration can depend rather considerably on the velocity of physical processes, such as the diffusion-molecular and turbulent transfer, and the microscopic mixing of the reacting system components. T h e investigation of a plasma chemical process suggests, in general, study of the elementary act of colEact E b o n d , thermodynamics, physical lisions at k T and chemical kinetics of the process as well as the gas dynamics of mixing flows of reacting substances, and finally the complexity with which the above factors act on each other. T h e difficulty of the task put is obvious. Therefore, it is necessary to adopt simplification of some side of the problem, differentiation of individual factors, and their mutual effects. Special theoretical and experimental investigation of the problem of quenching, which plays a deciding role in most quasi-equilibrium plasma chemical processes, has a dominant role in its technology. There are two types of reactions in a plasma jet where the composition of the products depends on quenching conditions. The first type includes reactions which produce a number of intermediate products, some of which are desired frozen. An example of such a reaction is the conversion of methane into acetylene, described in detail below. Of prime importance in such reactions are quenching rate and the moment when the temperature starts to decrease. Reactions of the second type are peculiar in that the desired compounds are final products of a reaction which proceeds only a t high temperatures and in which the product molecules are sufficiently stable a t room temperature. T h e purpose of quenching, in this case, is to cool the products of reaction as soon as possible so that undesirable decomposition reactions, within the intermediate range of temperatures, are impeded. These types of reactions include, for example, thermal formation of nitric oxide in air. I n this case, it is nec-
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10
I N D U S T R l A L A N D E N G I N E E R I N G CHEMISTRY
essary to ensure the required sate of quenching, which, however, should not start too early in the reaction sequence when equilibrium compositions of N O are not reached. The selection of quenching rate greatly affects product yield in plasma chemical reactions--e.g., in the consec delay version of methane to acetylene even a 2 . in quenching would result in a decrease in the acetylene concentration from 15.5% to 10% (76). The decrease in the quenching rate of gaseous products in the synthesis of nitrogen oxides from l o s "/sec-107 O/sec leads to a decrease in the nitrogen oxide product concentration from 7.6y0 to 6.4% (82). Moreover, it is necessary not only to provide an average quenching rate in a certain temperature range but to observe a definite law di"/dt = f ( T ) (82). T h e violation of this law in any temperature range cannot be offset by an increase in the quenching rate in another range. Thus, the process must be optimized in regards to products yield and the quenching rate, and a higher level of economic efficiency may be achieved if there is the heat utilization during and after the quenching. Consideration of the main methods of quenching (72, 82) indicates the method most extensively employed is the cooling of hot gases (pe of chemicd reaction
Dissociation of CO
!
Decomposition of nitrous oxide
Oxidation of hydrogen Oxidation of CO Cracking of methane
Decomposition of ethylene
ReferConditions and results ences -__ low-frequency discharge of the same power Hf without electrode discharge in the air a t frequency of 27.5 Mc under 110 mm Hg. Content of N O in plasma at 375 W is 2% Glow discharge in the air, zone of positive column. Pressure is 50-300 mm Hg. Relationships between N O content in gas and i, ip are given. Maximum concentration of NO is up to 5-6y0 Glow discharge in the air, cathodic area. Pressure is 50400 mm Hg. Relationships between NO content and i are given. Initial section of the curve is the same (29). Then h-0content increases linearly up to 8y0 with i growth of the initial section Glow discharge in mixtures of N Z and 0 2 , cathodic area. The effect of the Nz-and-02 ratio on NO content in gas was studied. Maximum content of h-0 when h - 2 : 0 2 = 1 : l ; then 1 : 4 a n d 4 : l . Pulse discharge in the air under pressures of 20-500 m m Hg. Power in pulse is 20 M W when pulse duration is 2 msec. N O content, up to 2.5y0 Microwave discharge in the air. Relationships between N O yield and p = P Z / E 2 7 are given Maximum yield of N O 0.8 mol/kW Glow discharge in COI under pressures of 30-100 mrn Hg. Degree of dissociation depends on pressure, amounting to 407, under 50 mm Hg, and increases with increasing in current density. Silent discharge in COZ. De(87) gree of dissociation amounts to 307, Microwave in C O under 1-8 (651 mm Hg. When power in discharge was 400 W, layers nf carbon black deposited on the discharging tube walls were noted; COn and O2 were producrd. When power in discharge was 120 hr,(2302 was deposited instead of the carbon black layers Silent discharqe in NzOunder (87) pressure of 200-800 mm Hg. Degree of N z O decomposition is maximum under 200 mm Hg (-90Y0)
Glow discharge
(87)
Glow discharge Glow discharge in methane under pressure of 40-50 mm Hg. Degree of conversion in C Z H Zis up to SOYc,content of CzHz-9Ojo. Energy consumption-I 3 kW/m3 of CzHz Glow discharge in ethylene under pressure of 0.7-1.7 m m Hg Mechanism and kinetics are studied
(87) (87)
(9)
Refer-
Type of chemical reaction
Conditions and results
Glow discharge in mixtures of C H I with COZand CHI with HzO under pressure of 50 m m Hg. Degree of methane conversion is 98%. When current densities are high, CO and H Z are produced, when low current densities-C?Hz, CO, and Hz Production of hydrocar- Glow discharge in mixtures of CO and Hz under pressure bons from carbon oxof 10 mm Hg. Yields of ide and hydrogen CzHz and CHI are low Microwave discharge in mixture of CO:H2 under pressure of 10-50 mm Hg yields CH4, CZHZand HzO during stay in zone of discharge approximately for 1 min. Degree of conversion of CO to CHt is 8OY0 Synthesis of hydrocyanic Glow discharge in mixtures of N z and CH4 under pressure acid in mixtures of of 10-15 mm Hg. Convernitrogen with sion degree of methane to methane hydrocyanic acid depends on current density and on relationship of mixture components. Degree of conversion is 8OY0when methane concentration is 15%. CZHZis produced simultaneously with HCN. CzHz: HCN ratio varies according to current density Silent discharge in acetone Decomposition of vapors under pressure up to acetone 100 mm Hg. COP, CO, C2H2,CZH6, H Z are produced Silent discharge in benzene Decomposition of vapors H z , CH4, CZHZ, benzene CZH4, and diphenyl are produced Glow discharge in oxygen Production of ozone under pressure of 0.5 mm Hg Glow discharge in mixture of Reduction of tetraTic14 with Hn. The prodchloride titanium ucts, containing goy0 T i by hydrogen (purity 99.6%) and 10% of lower chlorides were produced Reduction of zirconium Glow discharge under 3-4 mm Hg in mixtures of halide halides with hydrogen. ZrCla, ZrBra, ZrIa, ZrFz were produced Reduction of trichloride Discharge in mixtures of BC1, boron with HZunder pressure of 30-200 mm Hg. Optimum yield was achieved when ratio BC13:Hz = 1:5. Purity of boron was 99.97c Production of hydrides Discharge in H Zinside a reaction tube, the walls of which are coated with corresponding elements (phosphorus, sulfur, arsenic) Production of boranes Discharge in mixture of HZ with BCla taken in the ratio 12: 1, under 20 mm Hg. The quenching of the products was performed in liquid nitrogen. Compounds of composition B2Hs, BloH16, BZH16 were produced Production of oxygenDischarge between copper and-fluoride comelectrodes in mixture of pounds 0z:Fz = 1 : 2 under 12 m m Hg (2000 V, 25 mA). Dark red liquid 03Fz was pro-
ences
Tyke of chemical reaction
References
Conditions and results duced at 77"K, and decomposed into O Z F Za t 115OK Discharge in mixture of 02:Fz = 2: 1 under 5-15 mm Hg (840-130 V, 4.5 mA). Dark red 04Fz was deposited on the walls a t T < 90°K, a t T 90-110'K. OaFz is decomposed into 03Fz and
Conversion of methane with carbon dioxide and water vapor
(39)
-
0 2
Discharge in mixture of (65, 72) Xe:Fz = 1 : 2 ; under 2-15 mm Hg, between copper electrodes (V 1100-2800 V, i-10-30 ma). Reactor was cooled to 78'K. Consumption of mixture of Xe with FZ was 136 cm3/hr Practically all reagents were fully converted to XeF4 Discharge in mixture of Production of hexa(65, 72) Xe:F2 = 1:3,under2-15 fluoride xenon mm Hg, in a reactor under conditions described in (65) Electrodeless discharge, 1 mm (44) Preparation of BzC14 from BC13 Hg Synthesis of GezCl6 Microwave discharge in (88) GeC14, pressure, 0.1 m m H g ; rate of GeC14, 30 mmol/hr; yields of GezC16, 250 mg/hr Microwave discharge in a (27) Synthesis of SF6C1 mixture of SF6 and C ~ Z80 , W ; conversion up to 25% Microwave discharge in a mix- (27) Synthesis of SF40 ture of 0 2 and SF6, SzF10, or SFbCI, conversion into SFaO 60% when a mixture of 0 2 and S2Flois used Excitation of SF4 Microwave discharge in Nz (92) SFa; pressure, 4 mm Hg, yield of SF.5, 80%; conversion of SF4, 5% Production of chlorine Microwave discharge, mixture (4) by oxidizing hydrogen of HCl 0 2 under 10-50 chloride mm Hg; power in plasma is 200-300 W, 50% conversion of HCl occurred under 20 m m Hg Arc between carbon electrodes (3) Production of fluoroin CF4, CFsC1, CzF5Cl organic compounds under 1-50 m m Hg. C Z F ~ , CZF6, CSFR were found in the products Silent discharge in the mixProduction of amines (38) tures of ammonia with hydrocarbons (saturated, unsaturated, and cyclic aliphatic) Dimerization of carCorona discharge (20) bonic acids Microwave discharge, 100 W, (95) Pyrolysis of organic compounds tube temp. 185 OK, pressure 20 mm Hp, - residence time 0.03 sec Aliphatic compounds Low conversion (1-3%), large (95) number of products Aromatic compounds High yields (15-60'%), small (95) number of products
Production of compounds of inert gases. Production of tetrafluoride xenon
-
+
+
V-power in discharge V-gas consumption P-pressure i - c u r r e n t density
2-impedance E-field intensity in plasma r--stay time in discharge zone
Below, we list some processes in nonequilibrium lowtemperature plasma which are of commercial interest: 1. Oxidation of nitrogen contained in air in microwave plasma VOL. 6 2
NO. 6
JUNE 1 9 7 0
21
2. 3. 4.
Production of C2F4 in glow discharge Decomposition of nitrous oxide in silent discharge Synthesis of hydrocyanic acid in a nitrogenmethane mixture 5. Reduction of tetrachloride titanium by means of hydrogen 6. Reduction of zirconium halides 7. Production of boranes (B2H6, B l O H 1 6 , etc.) 8. Production of hydrazine Plasma chemical technology is the technology of tomorrow, but today we have only a vague idea of the truly boundless possibilities for utilization of low- temperature plasmas in chemical and other branches of the industry.
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