CURRENT RESEARCH
Photochemical Ozone Formation in Cyclohexene-Nitrogen Dioxide-Air Mixtures Chi-Hung Shen', George S. Springer* I,and Donald H. Ste,dman2 Department of Mechanical Engineering', and Department of Chemistry2, University of Michigan, Ann Arbor, Mich. 48104 Experiments were performed investigating the formation of ozone in cyclohexene-nitrogen dioxide-air mixtures flowing inside a 9-m long and 15-cm i.d. Pyrex tube irradiated by ultraviolet fluorescent lamps. The ozone concentration along the tunnel was measured with the initial cyclohexene concentration varying from 0.5 to 50 ppm and the initial nitrogen dioxide concentration from 0.6 to 10 ppm. Measurements were made a t relative humidities of 50 and 100%;the relative humidity did not affect the ozone formation in the mixture. The data obtained were compared with data reported by previous investigators, and on the basis of the present and existing data, expressions were developed for predicting the maximum possible ozone concentration in different types of hydrocarbon-nitrogen dioxide-air mixtures.
vertical with their openings about 4 cm deep inside the tunnel. The plungers inside the syringes were removed, and one end of a 60-cm long, 0.376-cm i.d. polyethylene intramedic tubing was inserted into each syringe. The other end of the tubing was connected directly to the ozone analyzer. The flow rate through the sampling probe was about 6 cm3 s-l. This is only -13% of the flow rate in the tunnel which was 46 cm3 s-l (see below). For the 6 cm3 s-l sampling flow rate, the residence time in the sampling tube was 2 ms. During this period only a negligible amount of ozone (-0.1%) is expected to be lost in the sampling tube due to dark reaction of ozone and nitric oxide. The mixing chamber, made from a 30.4-cm long, 15.2-cm i.d. stainless steel tube, was connected to the upstream end of the irradiation tunnel. The open end of the chamber was covered by a stainless steel plate connected to the gas supply
Ozone is formed when a mixture of air, nitrogen oxides, and certain types of hydrocarbons is. irradiated by sunlight. However, the mechanism by which ozone is produced in photochemical reactions is not yet clearly understood, and the influence of many important parameters on the amount of ozone generated is not yet well established. The objective of this investigation was to study the effects of mixture composition and relative humidity on ozone formation in different hydrocarbon-nitrogen dioxide-air mixtures. To this end, the amounts of ozone formed in irradiated mixtures of cyclohexene, nitrogen dioxide, and air were measured under a wide range of experimental conditions. The results were then extended to other types of hydrocarbons by combining the present data with data reported by previous investigators.
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Experimental Apparatus The experiment was performed in a flow-type apparatus which consisted of the test section, the gas supply system, and the measuring instruments (Figure 1). The test section had three major components: the irradiation tunnel, the gas mixing chamber, and the lighting system. The photochemical reactions were generated in the irradiation tunnel made from six 1.52-m long, 15.2-cm i.d. Pyrex tubes joined together and arranged horizontally in a straight line. Two openings were made in the wall of each tube on top of the tubes for inserting the sampling probes. The first opening was 25 cm from the entrance of the tunnel. The distance between any two successive openings was 76 cm. The sampling probes were 26 gauge (0.02 cm i.d. and 1.27 cm long) hypodermic needles connected to 9-cm long and 0.48-cm i.d. syringes (Figure 2). The syringes were mounted in ground glass joints welded to the tunnel. The needles were
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Schematic of sampling probe Volume 11, Number 2, February 1977 151
line and a mercury manometer. The gas-air mixture inside the chamber was stirred by a fan. Six screens, made of 0.08-cm thick perforated stainless steel plates and serving as flow straighteners, were placed downstream of the fan. The downstream end of the tunnel was connected to an exhaust vent. The temperature of the mixture was measured by thermocouples both near the exhaust vent and in the mixing chamber. The ultraviolet radiation was produced by 48 F40BL (General Electric) fluorescent lamps arranged in six clusters along the tunnel. Each cluster contained eight lamps placed around the tunnel symmetrically on a 31.7-cm diameter circle. The lamps in each cluster were mounted on two semicircular metal shells covered on the inside with aluminum foil. To minimize the temperature rise of the gas-air mixture inside the tunnel, room air was blown through the annulus between the irradiation tunnel and the shell in the direction opposite to the flow inside the tunnel. The temperature of the cooling air was monitored by thermocouples placed inside the annulus. The maximum temperature rise of the cooling air was 7 “C. The temperature rise of the gas-air mixture in the tunnel (i.e., the temperature difference between the gas-air mixture in the mixing chamber and at the exhaust port) was always less than 4 “C. The air used in the experiments was supplied from an airconditioned room kept a t constant temperature (20 “C) and constant relative humidity (75%).The air was compressed by an oil-free, diaphragm-type compressor and then passed through four traps connected in series: 0.45 kg of activated carbon in a Plexiglas container; 1.5-m long, 1.9-cm i.d. tygon tubing filled with anhydrous calcium chloride; 0.45 kg of “indicating” anhydrous calcium sulfate packed in a glass jar; and a Gelman-type A glass fiber filter. After these four stages of purification, the air was branched into two streams. One stream was designated as “dry air”, and the other stream passed through a humidifier where it was saturated with water vapor. The two streams were then joined together. The relative humidity of the test air was controlled by adjusting the flow rates of the two streams of air. The test gases were prediluted with pure dry nitrogen to approximately 200 ppm by the manufacturers. The flow rates of each gas were regulated by stainless steel needle valves and were measured by flow meters. Beyond the flow meters the air and gas lines were joined. The mixture then passed through a Gelman-type A glass fiber filter and entered the mixing chamber. The pressure in the mixing chamber was nearly atmospheric. The pressure drop along the tunnel was less than 0.7 mN mP2so that the pressure throughout the tunnel was practically atmospheric during the tests. The flow rate in the tunnel was kept at 46 cm3 s-l. The average velocity corresponding to this flow rate is 0.26 cm s-l, and the Reynolds number is approximately 24. A t such low Reynolds numbers the flow was expected to be laminar in the tunnel. The concentration of ozone in the tunnel was measured by a Thermo Electron Corp. Model 12A chemiluminescent analyzer: The total intensity of the ultraviolet light inside the irradiation tunnel was monitored by a silicon Schottky PIN-5 photodiode inserted about 30 cm deep inside the downstream end of the tunnel. The spectral distribution of the ultraviolet radiation was measured with a CGA/McPherson monochromator and an RCA photomultiplier tube 1P28-A. The outputs of the photodiode and the monochromator were not calibrated and hence did not provide directly the light intensity and the spectral distribution. These measurements were performed only to ascertain that the light intensity and spectral distribution remained constant throughout the tests. A measure of the total light intensity was obtained as described in the next section. 152
Environmental Science 8. Technology
Experimental Procedure Before the start of the tests the system was “cleaned” as follows. The ultraviolet lamps were turned on, and 50% relative humidity air (without any of the test gases) was fed through the tunnel continuously until the ozone meter reading was practically zero (
