C . A.
PAPP
PLASMAS and PHOTOCHEMISTRY Radiation at high intensity, controlled waue length may provide the breakthrough needed by photochemical PYocesses
ontrolled energy at high levels-this is the unique contribution of plasma technology. The extremely high temperatures available have already been put to work. But the high temperature effect is only one way of using the energy of a plasma. A more sophisticated approach is production of radiation of specific wave length, suited to the job in hand. Such an approach is particularly suited to photochemistry. Equipment potentially fitted for such a job has been developed by Plasmadyne Corp. of Santa Ana, Calif. Their Vortex Stabilized Radiation Source is essentially a plasma jet enclosed in a transparent envelope (patent applied for by D. Van Ornum, J. W.Winzeler, and A. Miller, assigned to Plasmadyne). By choice of the working gas and of the operating conditions, intense radiation can be produced within the range of wave lengths useful for photochemistry. Power level can be several times that of available lamps-an advanced prototype unit has operated for 15 minutes at an input of 100 kw. .4t this point, the large amounts of ozone generated in the surrounding atmosphere required shutdown until a proper ventilating system could be installed. The limit of power input is not yet known. A 50 kw. argon source is now available, but the implications for higher powered systems drawn from the work may have to be reassessed as the research finds new paths or meets roadblocks. Meanwhile, however, the chemist may be interested in an advance look at a technique which promises:
C
-More intense radiation than is provided by any other light source. -Spectra tailored to specific needs by choice of arc gases. -Operation without discoloration of the transparent envelope due to reactions with usual arc metals. Equipment
The sketch of the unit shows the rather simple concept. For work with the more expensive rare gases, a recirculation system is necessary, and is now under development. Of considerable importance in photochemistry is the window, shown here as fused silica. Lithium fluoride, magnesium fluoride, or sapphire could be used 48
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
to take advantage of their different absorption characteristics. The vortex of relatively cool gases around the plasma reduces infrared irradiation and lowers the is relatively working temperature of the envelope-it cool to the touch. Experimental Data
With photochemical applications still in the development stage, few data are available. A few exploratory studies have been made, simply to prove that such reactions are possible. So far, the equipment has done the following : -Polymerized a number of solid rocket propellant binders of undisclosed composition. -Formed a thin polymer skin on Penton monomer. -Degraded certain materials by exposure to intense ultraviolet. -Produced ozone. -Given a good suntan to unwary lab workers. Working Gases
The Vortex Stabilized Radiation Source has operated with argon, nitrogen, and neon. Argon produces intense radiation in the ultraviolet region. The neon spectra lies largely in the infrared, at the power levels applied so far. Work on xenon and xenon-argon mixtures are proposed, but must await the recirculation equipment. A large portion of the xenon spectrum lies in the visible wave lengths. I t is possible, and is hoped, that a suitable mixture of argon and xenon will effectively simulate sunlight. These working gases have a n advantage in commonthey do not react with envelope materials even a t high temperatures. Envelope darkening does not occur, and lamp life is increased.
C. A . Pa$j is the Director of the Plasma Chemistry Laboratory, Plasmadyne Corj., Santa Ana, Calif.
AUTHOR
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Arc plasma dimensions Inputpower Arc chap& pressure Plasma tempaature Total radiant flux Efficiency UV. output (2000 to 4000 A,)
10 mm. X 3 mm. 24.8 kw. 17 a m . 7000' K. 7.68 kw. 30.9% 2.6 kw.
lengths, since wave length can be changed by selection of one of several gases. Spacefactm in reactor design may be partly solved by the more intense source. Reactions hampered by low quantum yield may become p r s c W with use of the high intensity source. A reaction of immediate interest will be the announced Toyo Rayon process for making caprolactam, the Nylon6 monomer. This process a t least stretches the capabilities of available lamps to their utmost. Intensity is the most important requirement here. An attractive aspect of photochemistry is the promotion of very specific reactions without catalysts. Where streams contain catalyst poisons, photochemistry may provide very simple processes. For example, the known photochemical decomposition of hydrogen sulfide could probably be carried out without separation from the natural gas stream, if the source were intense. Conversion of sulfur dioxide to sulfur and the trioxide would be a route to removal of another troublemaker. It is difficult a t this time to pinpoint the reactions most likely to be practical. The equipment has shown its potential in polymerization; certainly this area deserves close attention. Many industrially important monomers absorb strongly within the range emitted by this source. Organic oxidation reactions by photo methods are particularly interesting, because they are characterized by high yields of single products, with few side reactions. Again catalysts are not needed. Perhaps a n obvious application of a n intense lamp which simulates sunlight lies in the artificial aging of materials. Also, because the light is so intense, very slow reactions may reach practical speeds. Photocopying methods may benefit from such a speed-up. And a n interesting possibility is the use of such a source to pump a laser device. Fulun Equlpmenl Design
Methods of focusing and further concentrating the light beam are being considered. An ingenious arcimaging furnace is now on the drawing boards. With a reflector and a conimating lens of magnesium fluoride, it is hoped to produce spectra to the Lyman alpha range. VOL 55
NO. 4
APRIL 1963
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