454
Energy & Fuels 1988,2, 454-457
Correlation of Benzene Production with Soot Yield Measurements As Determined from Fuel Pyrolyses? R. D. Kern,* C. H. Wu, J. N. Yong, K. M. Pamidimukkala, and H. J. Singh Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 Received September 15, 1987. Revised Manuscript Received January 25, 1988
Benzene growth profiles have been obtained by dynamic analysis of the reflected shock zone with time-of-flight (TOF) mass spectrometry for various fueb: allene, 1,2- and l,&butadiene, vinylacetylene, pyridine, and acetylene. Modeling was employed to extend the TOF benzene profiles recorded to reaction times that were comparable to those employed by the laser extinction (LEX) technique to obtain soot yields. In those systems where comparable mixtures have been investigated by LEX, it is apparent that the relative magnitudes of TOF benzene production are of the same order as the soot yields determined by LEX; namely, allene > 1,3-butadiene > vinylacetylene N acetylene > pyridine.
Introduction The rate of production of soot in pyrolytic reaction systems has been studied in shock tubes by using a variety of non-intrusive analytical techniques: laser extinction (LEX),l+ static analysis of the product distribution from single-pulse shock tubes (SPST),’+ and dynamic analysis of the reflected shock zone by time-of-flight mass spectrometry (TOF).1° The data reduction process often involves measurement of a changing bulk quantity; e.g., attenuation of a He-Ne laser beam due to absorption by high molecular weight gas-phase species and discrete soot particles via LEX or deficiencies in the carbon atom mass balance via SPST or TOF. The concentrations of the various polycyclic hydrocarbons formed in the preparticle soot chemistry phase are extremely low9 and are below the detectability limit of the TOF technique,l0 which is about 10-lomol ~ m - ~ The . nondetected hydrocarbons constitute the “missing” mass. The ultimate goal of work in this area is to write a complete chemical mechanism for soot formation. This formidable task has been attempted for acety1ene.l’ Some 180 species and 600 reactions were considered in an effort to model the soot yield obtained by LEX. Both the calculated and experimental yields were very low ( 300 absorbed 632.8nm radiation. The summation of these high molecular weight concentrations converted to carbon atoms/cm3 and divided by the input carbon atom concentration yielded the computed soot yield. The effort herein is to develop a correlation between a readily observable molecular species whose presence is diagnostic of subsequent soot formation and the bulk observables of laser extinction and mass balance deficiency. Experimental Techniques Employed LEX has been utilized behind incident and reflected shock waves during various observation times ranging from 0.5 t o 2.5 In order to compare the relative sooting tendencies of fuels, a total carbon atom concentration of 2 x 1017 atoms cm-3 was chosen for such fuels as ethylbenzene, toluene, benzene, pyridine, allene, 1,3-butadiene, vinylacetylene, and acetylene. Comparison ‘Presented a t the Symposium on Advances in Soot Chemistry, 194th National Meeting of the American Chemical Society, New Orleans, LA, August 31-September 4, 1987.
0887-0624/88/2502-0454$01.50/0
of relative sooting tendencies was made at the same carbon atom concentration and the same no-reaction temperature. The total pressure range was also the same, 2-3 atm. The soot yield vs the no-reaction shock zone temperature curves are bell-shaped. Aromatic compounds produced the greatest amount of beam attenuation or soot yield while acetylene exhibited the least. The early LEX ~ o r k l reported -~ values for the absolute soot yield on the order of 80-90% conversion of aromatic fuels to soot. It was subsequently realized that these values were too high in part due to uncertainty of the literature value for the refractive index of soot and to light absorption by preparticle species: It was also known that a significant amount of soot formation occurred in the accompanying cooling wave.’,’ For these and other reasons, the ordinate for soot yield plob was taken to be E(m) x soot yield with the value of E(m)left unspecified pending resolution of the uncertain tie^.**^ This adjustment cast the LEX results as a measurement of relative rather than absolute soot yields. The thrust of the mass balance deficiency procedure was t o add up a l l of the carbon-containing products detected and subtract that from the total carbon atoms in the original fuel. Aromatic compounds showed the greatest deficiencies (with the notable exception of pyridines) and acetylene the least. There was good agreement with regard to the relative sooting tendencies of fuels investigated with LEX14 and the mass deficiences obtained by SPSTg and TOF.1° The TOF method offen the advantage of recording the product distribution a t selected times during the observation period. Concentration profiles of various observed species are constructed within the m l e range 12-300 during typical observation times
(1) Graham, S. C.; Homer, J. B.; Rosenfeld, J. L. J. h o c . R. SOC. London, A 1975,344,259-285. (2)Graham, S. C.; Homer, J. B.; Rosenfeld, J. L. J. Mod. Deu. Shock Tube Res., h o c . Znt. Shock Tubes Symp., 10th 1975,621-631. (3)Wang, T. S.; Matula, R. A,; Farmer, R. C. Symp. (Znt)Combust., [Boc.] 1981,I S t h , 1149-1158. (4)Frenklach, M.;Taki, S.; Matula, R. A. Combust. Flame 1983,49, 275-288. (5)Frenklach, M.; Taki, S.; Durgaprasad, M. B.; Matula, R. A. Comb u t . Fhme 1983,54,81-101.Frenklach, M.; Clary, D. W.; Ramachandra, M. K. NASA Contractor Report 174880; p 29. (6)Rawlins, W. T.; Schertzer, S. P.; Tanzawa, T.; Kreck, R. H. Presented at Symposium on Combustion of Synthetic Fuels, 183rd National Meeting of the American Chemical Society. - . Las Vems, - . NV.. March 28April 2;1982. (7) Vaughn, S. N. Ph.D. Thesis, Kansas State University, 1980. (8)Vauehn. S.N.: Lester. T. W.: Merklin. J. F. Int. SvmD. - . Shock Tubes W&es .1982,i 3 t h , 866-868. ’ (9)Colket, M. B.Znt. Symp. Shock Tubes Waves 1986,15th,311-317. (10)Kem, R. D.; Singh, H. J.; Esslinger, M. A.; Winkeler, P. W. Symp. (Int.) Combust., [Proc.] 1982,ISth, 1351-1358. (11)Frenklach, M.;Clary, D. W.; Gardiner, W. C.; Stein, S. E. Symp. (Int.) Combust., [Proc.] 1984,20th,887-901.
0 1988 American Chemical Society
Energy & Fuels, Vol. 2, No. 4, 1988 455
Correlation of Benzene Production with Soot Yield
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