Langmuir 1988,4,891-898 be independently overthrown. An example for the present instance is the generalized acid-base theory, which Neumann and his associates2 would have to overthrow if the equation of state were to be given ultimate acceptance. Second, if the advocate of a new hypothesis cannot overthrow the concepts and results in the rest of scientific knowledge that conflict with the hypothesis, then he must concede that the hypothesis is not valid. Our purpose in this paper has been to develop and expound the modern theory of surface tension components. Until recently, we had thought that an equation of state could (at least as an approximation that was useful in a limited range of cases) coexist with the theory of surface
891
tension components. Reference 2 closes the door on this coexistence. Spelt et al. have brought forward their equation of state, and some data taken with a very limited number of liquids on apolar or monopolar solids, as being “in direct conflict with the theory of surface tension components”. We have, above, laid out a number of ways in which this equation of state is in conflict with accepted theory, in regard to intermolecular forces in general, and with a large number of experiments in surface chemistry. On the basis of these arguments, we conclude that this “equation of state”, and Spelt’s experiments, do not raise any serious challenge to the theory of surface tension components.
Soot Oxidation in Fibrous Filters. 1. Deposit Structure and Reaction Mechanisms Chiao Lint and Sheldon K. Friedlander” Department of Chemical Engineering, University of California, Los Angeles, Los Angeles, California 90024 Received July 10, 1987. In Final Form: February 12, 1988 Soot generated in an acetylene flame was collected in quartz and glass fiber filters and oxidized by placing the loaded filters in a tube furnace at 1atm and temperatures of 400 and 525 “C.The rate of soot oxidation was about 2 orders of magnitude higher on glass filters than on quartz filters. Elementary spheroids present in the soot flame were about 20 nm in diameter. When the soot loading was low, the spheroids deposited as single particles on the fiber surface, and the soot/fiber contact was good. At higher loadings the spheroids agglomerated in the gas and grew further on the fibers to form clusters as large as a few microns in size; most of the particles were not in contact with the fiber surface. The oxidation rate of soot on glass fiber filters was proportional to the total soot loading even when most of the elementary soot particles were not in contact with the fiber surface. EDX analysis of partially oxidized soot particles showed that sodium from glass fibers was transported to the soot and catalyzed the surface reaction. Because the sodium species uniformly covered the soot surface after a short induction period, the rate per unit soot surface area is independent of the filter loading when the loading is 3.1 pg/cm2 or less.
Introduction Fibrous filters are widely used for the high-efficiency, low-pressure drop filtration of small particles from gases. The fundamental filtration mechanisms’ are well understood, and good estimates can be made of the efficiency of filtration of spherical particles when the filters are clean, before significant buildup of deposits has occurred. Although the filtration removal mechanisms are well understood, chemical reactions of particles in filters have not been carefully studied. Despite this, combustion in filters has been used to measure the mass of different fractions of the carbon-containing components of ambient aerosols.2-6 Fibrous filters may offer an alternative to current diesel trap technologies for the removal of soot from exhaust gases followed by the oxidation of the deposited particles in situ.- The cellular ceramic traps now in use generally have a collection efficiency of 34.5-98.8% depending on the operating conditions.1° The fibrous filter is lightweight and highly efficient (>99%), and the fibers can be made of materials resistant to high temperatures and corrosion. Moreover, we have found that catalytic agents can be incorporated into the fibers.
The goal of this research was to elucidate the kinetics of reactions of small particles deposited in fibrous filters, which we call ”immobilized aerosol reactors”. Soot was chosen for study because of its practical importance in atmospheric pollution. Soot is made up of aggregated carbon particles with varying ratios of organic compounds to carbon. Soot formation in flames has been extensively studied.”-’* The (1) Friedlander, S. K. Smoke, Dust and Haze; Wiley-Interscience: New York, 1977. (2) Novakov, T. In Particulate Carbon: Atmospheric Life Cycle; Wolff, G. T., Klimish, R. L., Eds.; Plenum: New York, 1982; p 19. (3) Mack, E. S.;Chu, L.4. In Particulate Carbon: Atmospheric Life Cycle; Wolff, G. T., Klimish, R. L., Eds.; Plenum: New York, 1982;p 131. (4)Huntzicker, J. J.; Johnson, R. L.; Shah, J. J.; Cary, R. A. In Particulate Carbon: Atmospheric Life Cycle; Wolff, G. T., Klimish, R. L., Eds.; Plenum: New York, 1982; p 131. (5) Cadle, S. H.; Groblicki, P. J. In Particulate Carbon: Atmospheric Life Cycle; Wolff, G. T., Klimish, R. L., Eds.; Plenum: New York, 1982; p 131.
(6) Howitt, J. S.;Montierth, M. R. SAE Paper 810114 1981. (7) Oser, P.; Thomas, U. SAE Paper 830087 1983. (8) Enga, B. E.; Buchman, M. F.; Lichtenstein, I. E. SAE Paper 820184 1982. (9) Murphy, M. J.; Hillenbrand, L. J.; Trayser, D. A. Interagency EnergylEnvironment R and D Program Report; EPA-600/7-79-232b, 1979. . (10) Miller, P. R.; Scholl,J.; Bagley, S.;Leddy, D.; Johnson, J. H. SAE Paper 830457 1983. (11)Haynes, B. S.;Wagner, H. Gg. B o g . Energy Combust. Sci. 1981, 7, 229. ~~
*Author to whom correspondence should be addressed. Current address: PTD, Intel Corporation, Hillsboro, Oregon 97124.
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892 Langmuir, Vol. 4, No. 4, 1988
Table I. Nominal Composition of EPM 2000 Glass Fiber Filters (Whatman Corp.) component SiOz NazO Kz0 CaO + MgO B203
content, % 60.9 10.0
2.9 3.0 10.7
component A1203 BaO Fez03 ZnO F2
content, % 5.8 5.0 0.1 1.0 0.6
fundamental colloidal units are spheroids that collide as a result of Brownian motion to form chains or clusters. The morphology of soot agglomerates has been characterized by using concepts of fractal analysis.'"" The spheroids are constructed of small platelets of graphite with the layer planes oriented parallel to the surface (turbostratic structure).le Wood and Sancier,19McKee,m and Wen21 have reviewed the catalytic gasification of graphitelcoal char. Alkali metal compounds were found to be catalytically active in the oxidation of carbon, although the basic reaction mechanism is not fully understood. Since the structure of graphite/coal char is similar to that of soot, the results of those studies may be applicable to the oxidation of soot. There is an important difference between the use of alkali metal compounds in coal-gasification research and in the catalytic oxidation of immobilized soot. In coalgasification research, the catalyst is well mixed and impregnated with carbon particles that may range in size from ca. 20 nm to about 2 mm. In the case of deposition on a catalytic surface, only the first soot particles to deposit are in direct contact with the surface. After the surface is partially covered, incoming particles deposit on top of the earlier deposits and have no contact with the collecting (catalyst) surface. The course of the reaction is influenced by the particle deposition pattern; both particle transport and deposition and chemical reaction mechanisms are important to understanding the overall process, as shown in this paper. Experimental Section Filters. Two types of fibrous filters were used in this study. The glass fiber filter is made by Whatman (EPM2000) and, as will be shown, is catalytic to the oxidation of soot. These filters are made of borosilicate glass. The nominal composition provided by the manufacturer is listed in Table I. The other type of filter made of quartz fibers is manufactured by Pallflex (2500QAO). Since the quartz filter is relatively inert toward the oxidation of soot and is temperature resistant (>lo00 "C), it was used as the substrate to capture the soot and measure uncatalyzed rates of oxidation. The glass and quartz fiber filters were studied with a scanning electron microscope (SEM)(Cambridge, Stereoscan 250) to determine the fiber size. These fdters are 47 mm in diameter and 0.4-0.7 mm thick. Soot deposits over the central 35 mm of the filter. New filters (before preheating) contain 0.69-2.1 pg/cm2 of carbon as organic or other carbonaceous substances, determined from a superficial cross (12) Smyth, K. C.; Miller, J. H.; Dorfman, R. C.; Mallard, W. G.; Santoro, R. J. Combust. Flame 1986,62, 157. (13) Sentoro, R. J.; Miller, J. H. Langmuir 1987, 3, 244. (14) Calcote, H. F. Combust. Flame 1981, 42, 215. (15) Mandelbrot, B. B. The Fractal Geometry of Nature; W. H. Freeman: New York, 1983. (16) Samson, R. J.; Mulholland, G. W.; Gentry, J. W. Langmuir 1987, a"" 0
a,
AIL.
(17) Meakin, P. J. Colloid Interface Sci. 1984, 102, 505. (18) Lahaye, J.; Prado, G . In Particulate Carbon: Formation During Combustion; Siegla, D. C., Smith, G . W., Eds.; Plenum: New York, 1981; p
33.
(19) Wood, B. J.; Sancier, K. M. Catal. Reu.-Sci. Eng. 1984, 26, 233. (20) McKee, D. W. Chem. Phys. Carbon 1981,16, 1. (21) Wen, W. Y. Cat. Rev.-Sci. Eng. 1980, 22, 1.
Quartz lube
r
light pipe
Laser I l n e filler
Figure 1. Schematic diagram of the experimental system for monitoring the oxidation rate of soot in fibrous filters. About 15 L/m of airs flows through the quartz tube of which 2 L/m passes through the filter. sectional area of the filter. Most of the carbon is removed by preheating in a furnace a t 360 OC for 1 h. Soot Generation. Soot was generated by burning acetylene incompletely in a rich laminar diffusion flame. The burner was constructed of a bed of 252 stainless steel hypodermic tubes in the center 1.641. diameter of the burner surrounded with a 0.5-in. ring of sheath air. This geometry was utilized to ensure homogeneous combustion conditions. The sooty flame (ca. 1040 O C by in situ thermocouple measurement) was sampled from the center of the flame at about 5 cm above the burner by using a 3/8-in.-o.d. probe and then cooled by mixing with dilution air to about 48 "C. The stream was split in half, and soot was deposited on substrates by drawing the gas through two 47-mm fibrous filters a t a flow rate of 15 L/m each for about 1.5 min. The final soot loading of the fiiter was determined by the flame condition when the collection took place. The higher the fuel to air ratio, the higher the soot concentration in the flame, and the higher the deposited soot loading. Soot Deposition in Filters. The structure of the soot deposits in glass fiber filters at various loadings was investigated by using a JEOL-100CX scanning-transmission electron microscope (STEM). Small pieces of filter corresponding to a few fiber layers were picked off the surface of a soot-loaded filter with a pair of sharp tweezers and glued onto 3-mm TEM grids (Pelco, 7GG400). The sample grids were sputter coated with platinum to make them conductive and were studied in the STEM. Reaction System. The reaction and monitoring system is adapted from the system of Novakov2 used for the thermogravimetric analysis of carbonaceous materials; it consisted of a tube furnace, gas-monitoring instruments, and an optical measurement system. The measurement system was constructed and calibrated to provide quantitative information on the reaction rate without monitoring the reaction gas. The reaction system (Figure 1) consisted of a three-zone high-temperature tube furnace (Lindberg, type 54357) and a 2.4-in.-0.d. quartz furnace tube. A flow of 15 L/m of dry filtered air was passed through the tube to provide a controlled environment during the measurements. All the samples were studied a t fixed temperature a t 1 atm. Before each test, the furnace was preheated and the furnace tube filled with nitrogen. The sootloaded filter was mounted in a glass assembly and slid into the furnace tube. The filter was heated for about 7 min before the nitrogen atmosphere was replaced with air to ensure that the temperature was constant throughout the run. The air flow rate through the filter was 2 L/min, corresponding to a superficial velocity of 3.5 cm/s. Reactions were studied a t temperatures of 400 or 525 "C, and soot loadings ranged from 0.5 to 12.6 pg/cm2. Calibration of Optical Detection System. The progress of the reaction was monitored optically. A 0.5-mW helium-neon laser (Hughes 3224H-C) directed a beam along the axis of the furnace tube, a t the center of the soot-laden filter. Downstream from the filter, a 6-mm quartz light pipe directed the transmitted beam to a photodiode outside the furnace. An optical filter (Oriel, 5274) removed all the signals with wavelength other than 632.8 nm. The light intensity was measured with the photodiode (UDT, Pin lOD), amplified (UDT, lOlC transimpedence amplifier, voltage gain = IO5), and recorded (Linear, oscillograph recorder, Model 585). Since the transmitted light intensity is related to the soot
Soot Oxidation in Fibrous Filters so 70
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Langmuir, Vol. 4, No. 4,1988 893
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Equivalent Voltage. Volt
Figure 2. Calibration curve relating transmitted light intensity to soot loading for a Wbatman EPM 2ooo fdter. The m e d r a m through the points represents the best fit. Table 11. Comparison of Optical Measurements with Carbon Analyzer Results loading after reaction initial loading, rg/cm' carbon analyzer opticd Mm Mopt/McA Mo MCA 5.5 11.6 12.8 18.6 28.2 38.5 52.5
2.30 6.70 4.25 9.35 2.50 26.6 26.1
2.70 5.72 3.95 8.21 0.32 15.3 4.99
1.17 0.85 0.93 0.88 0.13 0.57 0.19
loading of the filter, a correlation can be prepared between light transmission and soot loading. To perform the calibration, 44 sets of soot samples were collected. Each set consisted of a g h fiber filter and a quartz fiber filter with the same soot loading. The absolute carbon loading was measured with a Dohrmann carbon analyzer (DC-MA). The volatile organic, residual organic, and elemental carbons were volatW/oxidized a t 115,510, and 825 O C , respectively, followed by reduction to methane, which was measured with a flameionization detector. Optical transmission through the glass fiber filters was measured. The amplified voltage output of the photodiode, together with the soot loading from the carbon analyzer, was used to construct a calibration curve (Figure 2). Beer's law is applicable only over a small region of the curve (W3 ,tg/cm2). The apparently linear region at higher loadings is actually a non-Beer's law region in which optical saturation is occurring. The curve was used to convert the furnace test results to reaction rates of soot in EPM 2ooo glaas fiber filters. For qumk fiber fdtern, the m e is slightly different because of differences in the absorbency of quartz and other characteristics. Experiments were conducted to test the accuracy of the correlation between optical transmission and soot loading. Samples with different initial soot loadings were reacted in the system with their transient loading monitored optically. The reaction was stopped at an arbitrary point by removing the sample filter from the furnace and cooling. The residual soot loading of the filters was measured with the Dohrmann carbon analyzer. This value of the carbon loading, assumed to be the true one, was compared with the loading indicated hy the recorded voltage before the rnn was stopped. As shown in Table I1 for initial loadings below 18.6 rg/cmz, this method is reasonably accurate (within 17%). For higher loadings,large errors may result. The errors may be related to the formation of a thick layer of soot depmit a t the front end of the fdkr, which results in inhomogeneous oxidation of the soot deposit. Hence application of the method was limited to soot loadings less than 18.6 pg/cm2.
Figurr 3. W E \ l phuit, 01 g l : filrrr ~ filter inrnplr w i t h a ,001 loatliiil: u i O . , T ~g m?. :\I t h h loadmy, most of t he .wt IS present in thr Iomi of siiigle ,phernrds a d the w i t Fiher contart IS rmd. Analysis of Elemental Composition. A scanning electron microscope (SEMI equipped with an energy dispersive X-ray (EDXJ analyzerz2was used for elemental analysis. Samples anal@ included soot in quartz fiber filters, u n r e a d snnt in glass fiber filters, and partially reacted soot in glass fiber filters. l h e samples were picked off the surface of a filter with tweezers and affixed M standard SEM sample mounts (Pelco) with doublecoated tape OM, type 665). The samples were then coeted with carbon by evaporation u, make them electrically conductive for SEM study. This process was carried out with care to avoid altering the composition of the samples. Afwr the sample was loaded in the SEM. cIusters of soot on fibers near the edge of [he Pample were selected for composition analysis. The thinnest region oi the sample was chosen for analysis so that no fibers were buried in or hidden beneath the soot cluster under examination.
Results a n d D i s c u s s i o n Structure of t h e Particle Deposit. I n aerosol filtration, three deposition mechanisms are important under different conditions: diffusion, interception, a n d impacti0n.l In this study, impaction WBS not important because of t h e small size (244), values of k for a number of Na-quartz filter samples fall on a single line on the Arrhenius diagram with an apparent activation energy of 22.2 kcal/mol. Since the rate is not limited by sodium supply, it is probable that the reaction is kinetically controlled. For samples with low catalyst to soot ratios, the higher activation energies indicate the reaction is probably influenced not only by the kinetics but also by the catalyst supply and/or diffusion. The reaction rate was proportional to the oxygen partial pressure for both catalyzed and noncatalyzed soot oxidation. The overall rate equation follows the shrinking core model and can be written as dM/dt = -Aond 2 N P ~exp(-E,/RT), , where the preexponential factor, Ao, and the activation energy, E,, are functions ofthe sodium/soot ratio for glass fiber and Na-quartz filters. The catalyst loading of glass fiber filters is equivalent to about 400 pg/cm2 of Na2C03,even though the true Na 0 content is 740 pg/cm2. Values of the preexponential factor range from 8.8 X lo5to 2.35 X lo8 pg/(s.m %-Torr),while the corresponding activation energies range from 21.2 f 0.7 to 32.6 f 1.1 kcal/mol. For noncatalyzed soot oxidation, the apparent activation energy is 32.5 f 1.0 kcal/mol and A. is 3.8 X lo7 pg/(s.m2.Torr).
Introduction Soot is a colloidal form of elemental carbon containing some fraction of organic materials. The ignition temperature of diesel soot, as reported by Enga et al.,l is about 665 "C. Catalysts can be used to oxidize soot a t a lower temperature and a faster rate. Examples are metal salt/oxides of Na, K, Li, Cs, Pb, Pt, Mn, Cr, Co, and Cu, which catalyze the oxidation/gasification of graphite/coal char / ~ o o t . ~ - ~ In a study of the oxidation of soot in fibrous filters,1° we found that alkali metal compounds in glass fibers catalyze the oxidation of deposited soot. The alkali metal compounds appeared to move from the fibers over the soot surface where they catalyze the reaction. As a result, soot particles are rapidly oxidized even though many of them are not in direct contact with the fiber surface. This increases the total oxidation rate of the deposited soot. The rate of reaction depends on temperature, soot loading, and the density of deposition on individual fibers. The de-
* Author to whom correspondence should be addressed. 'Current address: PTD,Intel Corporation, Hillsboro, Oregon 97124. 0743-7463/88/2404-0898$01.50/0
pendence on soot loading probably results from the insufficient supply of catalyst from the smaller, heavily loaded fibers. Since oxygen is the other reactant, its partial pressure must also play a role in the reaction. The object of this study was to gain a better understanding of how these parameters influence the reaction and to derive an expression for the rate of reaction of soot deposited in filters. Experimental Section In a previous study,'O the rate of soot oxidation was measured with an optical measurement system consisting of a He-Ne laser, a photodetector, and a high-temperature tube furnace. The (1) Enga, B. E.; Buchman, Em. F.; Lichtenstein, I. E. SAE Paper 820184 1982. ( 2 ) Wood, B. J.; Sancier, K. M. Catal. Rev.-Sci. Eng. 1984, 26, 233. (3) Otto, K.; Lehman, C.; Bartosiewicz, L.; Shelef, M. Carbon 1982,20, 243. (4) McKee, D. W. Carbon 1970, 8, 623. (5) McKee, D. W.; Chatterji, D. Carbon 1975, 13, 381. (6) McKee, D. W. Fuel 1982, 62, 170. (7) Amariglio, H.; Duval, X.Carbon 1966, 4, 323. (8) Wen, W. Y. Catal. Rev.-Sci. Eng. 1980, 22, 1. (9) Cox, J. L. Clean Fuels from Coal; Symposium 11, 1975; p 271. (10) Lin, C.; Friedlander, S. K. Langmuir, preceding paper in this issue.
0 1988 American Chemical Society
