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Rates of the air oxidation of parts-per-million concentratio

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Rates of the air oxidation of parts-per-million concentrations of nitric oxide were studied homogeneouslyat atmospheric pressure and ambient temperatures in a constant-volume batch reactor. The initialconcentration of nitric oxide was varied from 2 to 75 p.p.m., while the oxygen concentration ranged from3 to 25 volume %. The initial order of the oxidation reaction in the absence of nitrogen dioxide wasdetermined to be 1.0 for oxygen. From initial rate data at 26.5oC, a third-order rate constant of (1.297 ?.051)x104 (liter)2/(g. mole)2(sec.) was obtained.INTRODUCTIONPollution of the air in metropolitan areas is an increasing problem. Nitric oxide and unburnedhydrocarbons, high-compression, internal-combustion engines and plants, are of major concern in airpollution. Nitric oxide reacts in the atmosphere with molecular oxygen to form nitrogen dioxide,which oxidizes hydrocarbons photochemically to ketones, aldehydes, and alcohols. Thesecompounds, nitrogen oxides and oxidized hydrocarbons, are the constituents of called “smog”and are detrimental to the health of both the human and plant population. The basic chemicalreactions which occur in these atmospheres, however, are not well understood. Because of the smallamount of quantitative work formed by which has been carried out on the air oxidation of nitric oxideindustrial in parts-per-million concentrations, rates of reaction of nitric oxide with oxygenwere measured, and a mechanism is proposed.The mechanism of the oxidation of nitric oxide to nitrogen dioxide has been a subject ofcontroversy ever since Raschig (20) found that the reaction was third order. He reported that theoxidation of nitric oxide was second order in nitric oxide and first order in oxygen. A number ofother investigators (3-5, 1 6 , 2 4 , 2 8 , 3 3 ) after Raschig also studied the system. Although theygenerally agreed that at total pressures below 50 mm. of mercury the reaction was second orderin nitric oxide and first order in oxygen, different mechanisms were proposed which involved NO3,(NO)2, or N2O3 as intermediates or an actual three-body collision.Some investigators in studies with total pressures of 1 to 50 mm. of mercury noticed adeparture from third-order kinetics as the reaction neared completion or the total pressure wasreduced. Smith (26) found that the third-order rate constant increased as the total pressure wasreduced to near 1 mm. of mercury. Hasche and Patrick (10) found an increase in the third-order rateconstant as the reaction neared completion in several runs at 0?. Treacy and Daniels (29) reportedan increase in the third-order rate constant during any given run.Recently two papers have been added to the literature on the oxidation of nitric oxide at lowconcentrations and atmospheric pressure. Glasson and Tuesday (8), using long-pathspectrophotometry, studied the air oxidation of nitric oxide at concentrations between 2 and 50 ppm.They reported an initial third-order rate constant that was comparable to the initial rate constantsobtained by previous investigators at high concentrations but low total pressures. The addition ofnitrogen dioxide had no effect on the oxidation rate. Altshuller et al. (2), using chemical techniques,studied the air oxidation of 3 ppm of nitric oxide. They obtained a value for the third-order rateconstant which was approximately 200% of that reported by Glasson and Tuesday.All investigators have reported a negative temperature coefficient for the initialtermolecular oxidation of nitric oxide. Trotman-Dickenson (30) correlated the results of severalinvestigators and obtained an activation energy of -1.5 kcal. per gram mole. Rice (22) presenteda theoretical consideration of the possibility of a “negative activation energy.” Gershinowitz andEyring (7), using statistical techniques, calculated the rate constant for the oxidation reactionassuming a three-body reaction. Their method predicted the negative temperature dependence.In consideration of the apparent inconsistencies in the reported rate and mechanism forthe oxidation reaction at low concentrations, the oxidation of nitric oxide was studied in the partsper-million range and at atmospheric pressure. The initial concentration of nitric oxide wasvaried from 2 to 75ppm and the oxygen concentration from 3 to 25% by volume. The effect ofnitrogen dioxide on the initial rate was studied, and the change in the initial rate constant in theabsence of nitrogen dioxide over the ambient temperature range of (17- 37) degree celcius was measured.EXPERIMENTALReaction Gases. Commercial-grade oxygen (purity of 99.5%) and a high-purity dry nitrogen (purityof 99.995%) were dried before use in the equipment by passage of the gases through a 4-foot lengthof copper tubing having an internal diameter of 1/4 inch and a packing of 13X molecular sieve sizedto 14/20 mesh. The column was held at constant temperature in a dry ice-acetone bath.Nitrogen dioxide was obtained from a mixture furnished by Matheson which containednominally 1500 ppm of nitrogen dioxide in nitrogen. Nitric oxide was similarly obtained in amixture containing nominally 1000 ppm of nitric oxide in nitrogen. The impurities in bothmixtures were less than 10 ppm of water, 20 ppm of oxygen, and 20 ppm of hydrogen. Oxides ofnitrogen were determined by the phenol-disulfonic acid method (ASTM D 1608-60). The mixturecontaining nitrogen dioxide was found to have 1627 ppm of nitrogen dioxide with a standard deviationof 70 ppm. The other mixture was found to contain 871 ppm of nitric oxide with a standard deviationof 41 ppm.Apparatus. The reaction gases were passed from the shipping cylinders into hold tanks where themixtures of nitrogen dioxide and nitric oxide with oxygen were prepared. A given mixture was thentransferred in this completely closed system to the reactor mounted in a constant-temperaturebay. Temperature control was maintained by a Fisher thermo-regulator located in the air bathsurrounding the reactor, and the temperature was measured with copper-constantanthermocouples that had been calibrated against a platinum resistance thermometer certified bythe National Bureau of Standards. Temperature fluctuations during any given run were less than?.05oC.The reactor was a borosilicate glass vessel. In the preparation of reaction mixtures, the reactorwas evacuated from the top through borosilicate glass tubing, and then gases were added from thehold tanks by way of borosilicate glass capillary tubing connected to the side of the reactor. Sampleswere removed through a 3 mm stopcock placed on the side of the reactor and 90?from the gas inlet.Because the reactor was mounted in a bay, it was completely shielded from outside light as well asfrom Nichrome heaters placed under the copper base upon which it was seated.The gases in the reactor were analyzed for nitrogen dioxide by gas chromatography (19). A20-foot column of 10% SF-96 on Fluoropak 80 was used to separate nitrogen dioxide from nitrogen,oxygen, and nitric oxide. The column was operated at room temperature and a gauge pressure ofapproximately 15 psi. An electron-capture detector was used in the determination of the smallquantities of nitrogen dioxide in 0. 5-cc. samples. The retention time for nitrogen, oxygen, and nitricoxide was 120 seconds, and nitrogen dioxide was eluted from the column in 148 seconds. Theanalysis for nitrogen dioxide in the range from 3 to 75 ppm was accurate to an average deviation of3.4% with a standard deviation of 1.3 ppm.Ideal gas behavior was assumed in the calculation of the composition of the initial reactionmixture. An error of less than 0.5% for the concentrations would be expected from that assumptionat the prevailing temperatures and pressures used in the oxidation study. At the low concentrationsof nitrogen dioxide that were used in this study, the association product, nitrogen tetroxide, wasunimportant. Therefore, the concentration of nitric oxide as a function of time was obtained bysubtraction of the concentration of nitrogen dioxide from the initial concentration of nitric oxide.Within experimental accuracy the concentrations of oxygen and the nitrogen diluent remainedconstant during any given oxidation test.Tests were carried out to determine whether any absorption of reactant gases occurred in theApiezon N stopcock grease. All tests showed that the Apiezon N grease did not affect the reactantgases in any way.Mixing of Initial Reaction Mixture. Before any oxidation tests were carried out, the time requiredfor the gas mixture to become homogeneous after injection into the reactor was checked. A mixtureof nitrogen and oxygen was added to the reactor until the pressure was 740 mm of mercury. Thenitrogen dioxide-nitrogen mixture containing 1627 ppm of nitrogen dioxide was added to the reactor,until the total pressure was 760 mm. of mercury, which gave a concentration of nitrogen dioxide ofapproximately 50 ppm. Thirty seconds after this addition, a sample was taken from the reactor. Threemore samples were taken from the reactor at 5-minute intervals and analyzed for nitrogen dioxidewith the chromatograph. The concentrations of nitrogen dioxide in the four samples were thesame within the experimental accuracy of 3.4%, which indicated almost immediate mixing of thegases in the reactor.Procedure. The calibration runs for the use of the gas chromatograph in the determination of nitrogendioxide have been described by Morrison (18). In the oxidation runs a hold tank for nitric oxide wasevacuated below 0.05 mm of mercury. Then the Matheson mixture of nitric oxide and nitrogen and aquantity of nitrogen were added to this hold tank. In the process the Matheson mixture was diluted toa concentration of nitric oxide between 150 and 300 ppm so that subsequent to fivefold dilution in thereactor the nitric oxide would be at the desired initial concentration. Where tests were to be made withvery low initial concentrations of nitric oxide, the mixture in the hold tank as diluted by partialevacuation and addition of nitrogen. The pressure in the hold tank was read on a mercury manometerand a McLeod gauge. A cathetometer was used to read the mercury level. No pressure change less than 5cm of mercury was ever read nor were more than three dilutions required. When an initial quantity ofnitrogen dioxide was desired, the mixture was made up the same way in a hold tank for nitrogen dioxide.After the gas mixtures were prepared in the hold tanks, the reaction mixture was prepared. Thereactor was pumped to less than 0.05 mm of mercury. Nitric oxide, nitrogen dioxide, and nitrogen wereadded from the hold tanks to the reactor in such a way as to obtain the desired initial quantities. Enoughvacuum was left so that when oxygen was added, the desired initial concentration of oxygen would beobtained with a final total pressure of 1 atm. Again no pressure changes less than 5 cm of mercury wereread with the cathetometer for the addition of nitric oxide, nitrogen, or nitrogen dioxide to the reactor.Oxygen was then added to the reactor, and the timer was started at the end of the oxygen addition, whichrequired approximately 20 seconds. Samples were then taken from the reactor and analyzed with thechromatograph. During any given test the overall pressure in the reactor never decreased more than 3 mmof mercury as a result of removal of samples. The maximum error in the initial concentration of nitric oxidedue to error in manometer measurement was 4% if three dilutions in the hold tank were required or 2% ifonly one dilution was needed. For most of the runs only one dilution was used. The maximum error in theinitial concentration of nitrogen dioxide from error in manometer measurement was 3%. The maximumerror in the measurement of the concentration of oxygen was 1.5%.Analysis of Experimental Data. Plots of the concentration of nitric oxide as a function of time wereprepared. Smoothed values of the concentration were read from those plots at different intervals oftime.Image transcription textRESULTS Only parts of the experimental data are presented in this section. Figure 1 shows the experimentalconcentrations of NO as a function of time for a typical test at the nominal temperature 26 C at atmosphericpressure. The initial NO concentration was 0.05 mol/m and initial oxygen concentration was 110 … Show more… Show more(1) Determine the rate law for the oxidation of nitric oxide which shall include the reactionorders and the temperature dependence of the rate constant (Arrhenius equation) usingbatch experimental data from the literature provided. Briefly explain the methodologyof the experiment for the relevant parameters studied in this assignment.Engineering & TechnologyChemical EngineeringLKCFES UECL 15100

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