Energy & Fuels 1988,2,743-750 elimination to produce the olefin. One can postulate a similar mechanism involving a Lewis site. The fact that olefins did not form when neat TBA was injected indicates that this reaction occurs on the surface and not in the gas phase in the FIMS instrument.
Conclusions An investigation of surface acidity studies using the temperature-programmed desorption of tert-butylamine has shown that the primary constituent of the @-peakis isobutylene. Isobutylene is formed only a t high temperatures, possibly via a Hofmann-type elimination initiated by the surface acid sites. Because this reaction should occur a t lower temperatures for stronger acid sites, the 8-peak temperature should correlate inversely with acid site strength. This conclusion is supported by the inverse correlation of &peak temperature with electronegativity. Apparently the low temperaure a-peak is caused by the desorption of physically adsorbed or weakly hydrogenbonded amine while the species that reacts and desorbs to produce the @peak is bonded more strongly via H+ transfer from a Brransted site or electron-pair donation to a Lewis site. Finally, some comments comparing TBA TPD with the much more established T P D of ammonia appear to be in order. It should be clear from the results of this communication that the prospects for actually titrating acid sites with TBA are remote. Such experiments are conducted routinely in NH3 TPD12-14since the desorption process is (12)Hidalgo, C. V.;Itoh, H.; Hattori, T.; Niwa, M.; Murakami, Y. J. Catal. 1984,85,362-369.
743
relatively clean and involves no decomposition of the molecule. Using the theory developed by Cvetanovic and Amenomiya,15 it is sometimes possible to calculate desorption activation energies from NH3 T P D experiments and thereby obtain a quantitative measure of acid site strength. In TBA T P D the interpretation of &peak maximum temperature as a measure of acid strength is only quaIitative. Steric considerations are also important in some situations. The size of the ammonia molecule (critical diameter = 3.8 A) permits its use as a probe for characterizing acidity in zeolites. TBA would not be suitable for such applications. As stated in the introduction, the advantages claimed for TBA T P D are primarily along lines of experimental convenience and the fact that desorption temperatures are generally low. TBA may be the preferred choice for use with temperature-sensitive catalysts.
Acknowledgment. This work is jointly sponsored by the U.S. Department of Energy (Grant DE-FG2284PC70812) and the Amoco Oil Co. We are grateful for both the financial support and consultations provided by these organizations. Some of the TPD data in Table I were taken from the University of Wyoming Master of Science Theses by T. R. King (Dec, 1986) and J. A. King (May, 1987). Registry No. TBA, 75-64-9; SOz, 7631-86-9; AZO3,1344-28-1; Cr203, 1308-38-9; Ni, 7440-02-0; Mo, 7439-98-7; P, 7723-14-0. (13)Post, J. G.;van &off, J. H. C. Zeolites 1984,4,9-14. (14)Corma, A,; Fornes, V.; Melo, F. V.; Herrero, J., Zeolites 1967,7, 559-563. (15)Cvetanovic, R. J.; Amenomiya, Y. Adv. Catal. 1967,17,103-149.
Oxidation Kinetics of Carbon Blacks over 1300-1700 Kt W. Felder,* S. Madronich, and D. B. Olson AeroChem Research Laboratories, Inc., Princeton, New Jersey 08542 Received September 15, 1987. Revised Manuscript Received July 19,1988
-
The oxidation kinetics of two carbon blacks were measured over 1300-1700 K in oxygen at partial pressures from 0.02 to 60 kPa. Raven 16, a furnace black (primary spherule diameter 60 nm), and Conductex SC, a conductive furnace black (primary spherule diameter = 20 nm), were entrained in nitrogen/oxygen flows and passed through a heated tubular reactor (a modified high-temperature fast-flow reactor, HTFFR) a t subatmospheric pressure. The yield of product COX(=CO C02) was measured gas chromatographically a t the HTFFR exit as a function of reaction time and oxygen concentration a t each temperature. The oxidation kinetics of both carbon blacks are similar; they react by external surface burning and exhibit single overall reaction orders between 0.5 and 0.8 in [O,]. An activation energy of ~ 1 7 kJ/mol 0 is observed. At an oxygen partial pressure of =20 kPa the fraction of reactive collisions (number of COXmolecules produced per 02/surface collision) rises from to =lo4 over the 1300-1700 K range. The observed reactivity of the carbon blacks with O2is smaller than that previously measured for soot and other carbon blacks. It is speculated that sulfur impurity may be the cause of this reduced reactivity.
+
Introduction The oxidation of carbonaceous particulate matter is of wide practical concern in power generation and in pollution reduction. Typically, in the fuel-rich portions of com'Presented at the Symposium on Advances in Soot Chemistry, 194th National Meeting of the American Chemical Society, New Orleans, LA, August 30-September 4, 1987.
bustion flames, OH radicals can be the major oxidizing species.lg However, in regions removed from the primary (1)Neoh, K.G.;Howard, J. B.; Sarofii, A. F. In Particulate Carbon: Formation During Combustion; Siegla, D . C., Smith, G. W., Eds.; Plenum: New York, 1981;p 261. (2) Page,F. M.; Ates, F. In Evaporation and Combustion of Fuels; Zung, J. T., Ed.; Advances in Chemistry 166;American Chemical Society: Washington, DC, 1978;p 190.
0887-0624/88/2502-0743$01.50/0 0 1988 American Chemical Society
Felder et al.
744 Energy & Fuels, Vol. 2, No. 6, 1988 PHOTOMULTIPLIER
combustion zone, where soot is present and OH concentrations are essentially negligible (regions in which particles spend the major portion of their lifetimes within a device), oxidation by excess oxygen is important. The work described here utilized an entrained flow reactor (a modified high-temperature fast-flow reactor, HTFFR) to determine the reactivity of two carbon blacks with O2 in the 1300-1700 K range. A technique was developed for feeding particles to the HTFFR and assuring that particle sizes lie below specified limits. A wide range of oxygen concentration was investigated while independent control of temperature, total pressure, and flow velocity (particle residence time) was maintained.
THERMOCOUPLE (SHIELDED)
GAS ANALYSIS
HEATER ELECTRODE CONNECTION
-dm/dt = wA (g s-l) (1) where m is the mass of carbon per unit volume, A is the surface area of carbon per unit volume, and w is a “reactivitycoefficient” relating the reacting surface area to the rate of change of m. The reactivity coefficient is implicitly a function of [O,] and temperature. For the small particles used in these experiments (see below), it is reasonable to assume that equilibration between the gas and surface temperatures is rapid even for high reaction rates. Ayling and Smith‘ showed that over the temperature range covered in this work, surface temperatures of =6 hm diam coal particles deviated only a few percent from the bulk gas temperature while they were being oxidized in an entrained flow reactor. The number of moles of carbon consumed per unit volume is equal to the number of moles of COXproduced per unit volume. In terms of mass -dm/dt = 12 d[CO,]/dt where [COX]is the molar concentration of COX.For monodisperse spherical particles that oxidize at their surfaces, evolving COXat the expense of particle radius, the surface area is a function of reaction time. This is the “shrinking sphere” model for particle oxidation? and it was used in obtaining the results reported here. According to this model, we use m = n(4/3)uPp A = n(4?rr2) which, upon substitution into eq 1, yield - 4 d dr = Re(4t?) dt
Integration of this equation gives rt = ro - Ret / p or m, = n ( ( 4 / 3 ) ? r ~ ) (-r 0Ret/pI3 mo = n((4/3)apro3)
In the above equations, n refers to the carbon particle number density, r refers to the particle radius, p refers to the particle density, and the subscripts “0” and “t” refer to times zero (initial, unoxidized condition) and t . We have replaced w in eq 1 with Re to emphasize the use of the shrinking sphere model in which reactivity is dependent only on external surface area. (3)Fenimore, C. P.;Jones, G. W. J. Phys. Chem. 1967, 71, 593. (4) Ayling, A. B.; Smith, I. W. Combwt. Flame 1972, 18, 173. (5) Laurendeau, N.M. B o g . Energy Combust. Sci. 1978,4, 221.
GAS
PARTICLE COLLECTION FILTER
PtlRh RESISTANCE HEATER WIRE
ZIRCAR INSULATION ALUMINA REACTOR
Methods and Apparatus Kinetic Parameters. The primary measurement in thie work is the number of moles of carbon converted to COX(=CO + Cod. Collected gas samples are analyzed gas chromatographically to determine the amount of COXevolved after the particle and oxidizer mixture has traversed the flow tube reactor at a temperature, 2’. The reaction time, t = x / u , where x is the length of the hot reaction zone and u is the gas flow velocity, is determined from the HTFFR temperature and preasure and the totalgas flow. Phenomenologically, the mass of carbon converted to COXper unit reaction volume per unit time is given by
0
WATER-COOLED VACUUM HOUSING
t? PARTICLE-LADEN FLOW FROM SETTLING CHAMBER
Figure 1. HTFFR for particle oxidation kinetics studies.
When all of the available carbon has been oxidized, the concentration of COXwill be at its maximum and given by [COXl, = m o w For conveniencein interpreting the experimental measurements of [COX],we use the burnoff, u, at time t : ut = (mo- mt)/mo (la) From this we obtain du/dt = -(l/mo) dm/dt = (l/[CO,],) d[CO,]/dt or U t = [COXlt/~C~Xl, Substitutingthe shrinking sphere expression for m into eq la then gives the integrated equations used for interpreting the data
[COxlt/[CO,l, = 1 - (1- ( R , t / p r ~ ) ) ~ (2a) which can be further reduced in terms of measured quantities by substituting the measured specific surface area for the unoxidized particles, So (cm2/g) 4?rr,2/((4/3)?rrtp)= 3/rop. Equation 2a becomes ut =
Ut
= 1 - (1- ( R , S O ~ / ~ ) ) ~
(2b)
Equation 2 assumes, in addition to external surface oxidation and equal gas and particle temperatures, that the oxidation rate is chemically controlled (i.e., oxygen diffusion to the reacting surface is rapid compared to reaction) and that the surface reactivity is a function only of available external area. The assumptions of surface reaction and chemical control are shown in the work described below to be valid. The possible change in surface reactivity with oxidation was not addressed; thus, the R, values refer to the external surface area. High-Temperature Fast-Flow Reactor (HTFFR). The HTFFR described by Fontijn and Felder6was modified for this work. The modified reactor is shown schematically in Figure 1. Optical observation ports in the reaction tube were eliminated, allowing the isothermal zone of the reactor to include between 50 and 70 cm of the overall 90-cm length of the tube, depending on the flow conditions used in the experiments. The reaction tube (two different tubes were used, a 4.5-cm4.d. mullite tube and a 2.5-cm4.d. 998 alumina tube) is resistively heated in three separately controlled zones of =30 cm each with 0.127-cm-diameter Pt/40% Rh resistance wire. The upstream zone serves mainly as a preheat region for the gases and particles entering at the base of the reactor; the heating currents applied to the two downstream zones are adjusted to give temperature profiles (measured with an axially traversing shielded thermocouple’ before and after each (6) Fontijn, A.; Felder, W. In Reactive Intermediates in the Gas Phase: Generation and Monitoring;Setser, D. W., Ed.; Academic: New York, 1979;Chapter 2. (7) Fontijn, A.; Felder, W. J. Phys. Chem. 1979, 83, 24.
Energy & Fuels, Vol. 2, No. 6,1988 746
Oxidation Kinetics of Carbon Blacks TO HTFFR
FLANGE
TO MOTOR
RTICLE TAKEOFF TUBE AEROSOL PYREX CYLINDER
MAIN G A S and OXIDIZER FEED ‘0’ RING S E A L OUTLET F U N N E L
G A S IN
SEAL TO SETTLING C H A M BE R
Figure 2. Tumbling bed particle feeder.
experimental run) that are constant within f 4 % over the downstream 50-70 cm of the reaction tube. At the exit of the reaction tube, an unheated Zircar (an alumina/zirconia insulating material) tube, whose i.d. matches that of the reaction tube, leads the gas flow from the reaction tube to the pump line (5-cm i.d.). The Zircar tube has a port =3 cm downstream of the reaction tube exit, through which a sampling probe (0.8-cm4.d. alumina tube) is inserted to withdraw gas samples for analysis. Downstream of the Zircar tube, there are optical observation ports in the pump line so that a HeNe laser (Spectra Physics 133, =l mW) beam crosses the particle-laden flow; the scattered laser light is detected (Hamamatsu R242 photomultiplier/Keithley417 picoammeter) perpendicular to the beam. Downstream of the optical ports, a 10-cm-diameterfilter support and paper or Fiberglas filter are mounted in a bypass line so that the entire flow can be routed through it to collect partially oxidized particles for subsequent surface area measurements. Particle Feed System. Particle feed to the reactor at subatmospheric pressure needs to be stable and reproducible over long periods. Particle aggregates with diameters 210 Mm are removed from the flow by settling before entering the reaction tube. This ensures that mass transport limitations (diffusionas discussed below) do not affect the kinetic measurements. Particle feed stability and reproducibility are achieved by using the tumbling-bed feeder shown in Figure 2. The bed is held in a lO-cm-i.d., 40 cm long Pyrex tube, sealed by flanges at each end. A shaft in the center of the end flange is connected to a variable-speed motor that rotates the tube at 230 rpm. A t the other end, a rotating vacuum feedthrough is mounted on the flange. The entire assembly is supported =30° from horizontal (motor end up) on two bearings. The particle-bed charge consists of 90 % (by weight) silica sand and 10% carbon black. The charge partially covers the feedthrough flange and fills about half of the remainder of the tube. Nitrogen diluent is admitted to the bed by a 0.5-cm4.d. tube penetrating =4 cm into the particle bed. A concentric 0.35-cm4.d. particle takeoff tube is mounted inside the gas admission tube and extends -35 cm into the Pyrex bed tube so that the open end of the particle takeoff tube is above the bed at the motor end of the Pyrex tube. The gas admission tube and particle takeoff tube remain fured on the centerline while the Pyrex tube rotates. The nitrogen fed through the bed raises a carbon black aerosol,some of which enters the particle takeoff tube. From there it flows =30 cm to the “settling chamber” (see below) before entering the reactor. The gas flow is sufficiently small that sand particles do not leave the bed. The bed pressure is 4 . 5 times that of the settling chamber, which is at essentially the reactor pressure. The tumbling motion of the bed and the presence of the sand effectively prevent caking of the carbon black particles. Heating the walls of the Pyrex tube continuously (to =325 K) with a lamp improves the feed rate stability. Feed rates (measured by collecting and weighing particles at the bed outlet) of 1-100 mg/min (104-104 mol of C/s) are achieved. The particle feed, monitored at the HTFFR outlet, has excellent short-term stability (f5% variation in the light scattering signal is typical) and is reproducible based on measurements of the amount of COX produced by complete oxidation, CO,,, (&lo% is typical). Large particle aggregates are eliminated in a vertically oriented, large volume cylindrical (15cm-i.d., 60 cm long) “settling chamber” between the particle feeder and the reaction tube (cf. Figure 3).
1 / \i /
SLOT TO REMOVE S E T T L E D PARTICLES
SUPPORT and SPACING STUD ( 1 of 4) S E T T L E D PARTICLES PARTICLE TAKEOFF TUBE FROM PARTICLE FEEDER
Figure 3. Settling chamber.
The particle-laden flow from the particle feeder enters the base of the settling chamber through the 0.35-cm4.d. particle takeoff tube (a small diameter is used for rapid gas flow),where it encounters a conic section mounted about 1 cm above a tapered outlet from the particle takeoff. In a distance of =llcm, the gas flow expands from 0.35 cm to the settling chamber diameter of =15 cm through an annulus -1 cm wide formed from the (point down) conical section and an upright funnel. The particle-laden flow then further expands along a second cone that tapers inward from the wall of the settling chamber to an apex =35 cm downstream of the annulus,where smooth, slow gas flow fills the settling chamber tube. At -2 cm above the apex of the upper cone, a collimated white-light beam traverses the settling chamber to monitor the particle feed rate stability by measuring the opacity of the gas/particlemixture. No absolute calibration of the opacity Rgainst the particle mass flow rate was made; however, the measurement does provide a good qualitative check on the feed rate and is used as an indicator of deviations in the mass feed rate. The exit of the settling chamber is an inverted funnel (=15-cm base diameter, tapering to a 2.5-cm-diameter outlet over =11 cm) to gather the slowly flowing gas/particle mixture and direct it into a 15-cm-long, 2.5-cm-i.d. Cu tube leading into the HTFFR reaction tube. The coupling to the reaction tube allows radial addition of the bulk of the diluent gas (N,) and oxidizer to the flow. Typically, the gas flow velocity in the settling chamber is sufficiently slow ( ~ 0 . 5cm/s) that particle aggregates with aerodynamic diameters 24 wm settle out of the flow; at the highest flow speeds used ( 1 2 cm/s), particle aggregates up to =10 wm could have passed through the settling chamber. This range of particle sizes is below the size at which bulk diffusion affects the oxidation rate (see below). Diagnostics. The COXproduced by the 02/carbon black reaction is measured gas chromatographically by using a 0.3-mlong, 5A molecular sieve (Supelco) column with a differential thermal conductivity bridge detector (GowMac). Gases are sampled into a 1.2-cm-i.d. copper coil with a volume of 1.03 X lo3 cm3. Sampled gas is admitted to the coil until the pressure reaches 50-60% of the reactor pressure (sample pressures range from 3 to 30 kPa). A slow sampling rate is used to maintain the reactor pressure within 5% of its set value during the sampling period (3-5 min). After sampling, the coil is isolated from the reactor with a valve, and the sample is compressed with He to the GC column entrance pressure (-130 kPa) and injected into the GC column. Nitrogen and unreacted oxygen elute within 5 min at room temperature, after which the column is heated to 460 K at a rate of -20 K/min. Reaction product CO elutes about 2 min after heating begins, followed by the CO, product about 12 min later. After the sample is injected into the GC column, the sampling coil is isolated from both the GC apparatus and the reactor and evacuated to prepare for the next sample. The GC detector is calibrated periodically by syringe injection of known amounts of CO, CO,, mixtures of the two, and mixtures of one or both with air and measuring the areas under the eluted peak signals. In addition, the coil sampling system has been tested by flowing CO/C02 through the HTFFR and taking a sample as done in the experiments. Such tests show an exact correspondence
Felder et al.
746 Energy & Fuels, Vol. 2, No. 6, 1988
Table I. Physical and Chemical Properties of Carbon Blacks' anal., w t W mean particle BET (N,) surface diam, nm area, m2/g volatile5 metal content ash
trade name Raven 16 (R16) Conductex SC (CSC)
61 20
25 (2gb) 220 (1906)
0.9 1.5
0.098 0.075
O.lC
0.08C
sulfur 1.67 0.85
Data supplied by manufacturer. Measured in this work. Metallic impurities (wt W )are as follows: ZMn, Mg, Al, and Ti, 0.007 (R16),
0.008 (CSC);Fe, 0.045 (R16), 0.005 (CSC); Na, 0.021 (R16), 0.047 (CSC);Ca, 0.025 (R16), 0.016 (CSC). 1 .o 1.0
-
?
r"" -I
w 0.2
.
0 14.5
"
"
$1
.
'
15.0
.I_
,0.0.00
15.5
i0.o
LOG ([021,cm-3)
Figure 4. Laser-scatteringmeasurement of CSC oxidation at 1650 K. [02]= 2.8 X 10l6(arrow), t b = 380 ms.
between the measured CO or C02 collected and the amounts added to the reactor flow. The COXmole fraction in the sample is determined from the GC peak iweas. The flow rate of carbon entering the reactor is determined from the measured COXat 100% burnoff, COX,,. The mass flow of carbon to the reactor indicated by these measurements is 0.05-5.0 mg/min (10-7-10-6 mol of C/s) or about 5 % of the output of the particle generator. The remaining carbon black particles are collected in the settling chamber. The molar flow of O2in the experiments reported here mol/s and always exceeded the ranged from 5 X lo6 to 5 X molar "carbon" flow by a factor of at least 100; for measurements of [COX],,, the oxygen flow was 103-104 times in excess of the "carbon flow". A second method was also used to obtain kinetic data. In this method, the [02] required to consume all of the input carbon in exactly the residence time, t , is measured by using laser light scattering at the reactor outlet, cf. Figure 1. The [Odin the reactor is progressively increased while the scattered light intensity is recorded, and a plot is made of relative (to [O,]= 0) scattered light intensity against log [02](similar to the method used by Page and Ate& The scattered light intensity decreases as the particles are consumed, eventually going to zero at 100% burnoff. In their work, Page and Ates assumed that scattering intensity (Z) 0: r8 and the reactivity coefficient was determined from the slopes of log I against t . The method used here is similar, except that no assumption is made about the functional dependence of scattering intensity on particle radius. It is assumed that particle number density remains constant, and that scattering intensity varies monotonicallywith particle size. Extrapolating the linear portion of the scattering intensity curve to zero gives the value of [02]at which the input carbon is consumed in time, t b , the burnup time as shown in Figure 4. The burnup time is simply related to the phenomenological reactivity coefficient for surface or diffusion-limited reaction. The reactivity coefficient values obtained from the scattered-lightmethod were identical with those obtained by using the GC method. Partially oxidized particles are collected on the inline filter for surface area measurements, which yield information on the physical mechanism of the oxidation process (see below). An adsorption analyzer (Quantachrome MS-8) is used for Nz adsorption at 77 K. The N2 results are analyzed by using the one-point BET method.8 Specific surface areas are measured as a function of fractional burnoff, u, from u = 0 (particles that have traversed the reactor with [O,]= 0) to as high a value of u as possible. As the fractional burnoff increases, the collection of a suitable mass of partially oxidized sample becomes more time-consuming. The highest value of u for which samples have been analyzed is 0.7. (8)Lowell, S. Introduction to Powder Surface Area; Wiley-Interscience: New York, 1979.
0.5
.
0.8
-
0.4
.
0.2
1
0 0
0
R18 CSC
-
-
1400
K
\ \1
1880 K
' 0.5 BURNOFF. u
1
Figure 5. Variation of carbon black surface area with burnoff.
Materials. The carbon blacks, Raven 16 (R16),a lampblack, and Conductex SC (CSC),a conducting black, were chosen to provide a wide difference in initial specific surface area and iron impurity concentration. Both have "high" sulfur concentrations. The blacks were donated by Columbian Chemical Corp., Tulsa, OK; their properties are summarized in Table I.
Results Specific Surface Area Measurements. The interaction between O2and the carbon black particles can occur between two extreme mode~:~+'J~ (1)reaction on an external nonporous surface; (2) reaction inside a completely porous mass. In the first extreme, a particle of constant density, p , is oxidized and its radius decreases with burnoff. In the second extreme, a porous particle of constant radius is oxidized internally and its density decreases with burnoff. High intrinsic reactivity surfaces tend to behave like case 1,with little or no penetration of oxidant to pores. Low reactivity favors case 2 behavior. Carbon black (and soot) oxidation has been treated previously as case l.3J1J2 As the particles oxidize, internal structure can develop (e.g., due to some areas of the surface being more reactive than others5J3or to changes in particle aggregate s t r u ~ t u r e s ) , ~ J ~ after which oxidation rates can be controlled by transport of oxidizer to the reactive site within pores. The measurements of the surface areas of partially oxidized carbon black particles extracted from the HTFFR indicate that the present observations are essentially case 1. The nitrogen surface area measurements show the changes in external surface areas available for reaction as a function of u,and the results indicate that these areas increase in a manner consistent with a constant density (case 1 above) burning. Representative surface area measurement data for R16 and CSC are presented in (9)Mulcahy, M. F. R.; Smith, I. W. Pure Appl. Chem. 1969,19, 81. (10)Essenhigh, R. H.In Chemistry of Coal Utilization, 2nd Supplementary Volume; M. A. Elliott, Ed.; Wiley: New York, 1981;Chapter 19. (11)Park, C.; Appelton, J. P. Combust. Flame 1973,20,369. (12)Lee, K.B.;Thring, M. W.; Beer, J. M. Combust. Flame 1962,6, 137. (13)Laine, N. R.;Vastoh, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67, 2030. (14)Neoh, K.G.;Howard, J. B.; Sarofm,A. F. Twentieth Symposium (International)on Combustion: The Combustion Institute: Pittsburgh. - . PA, 1984;p 951. (15)Szekely, G.A,, Jr.; Faeth, G. M. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982;p 1077.
Energy & Fuels, Vol. 2, No. 6, 1988 747
Oxidation Kinetics of Carbon Blacks
/:
i-l.o)
a. 4;,
. ,116,
.
1300 K
-1.4
I
-1.6 16
15
17
18
LOG (~021, cm-3)
-l.O
I
14
I 15
16
17
LOG ([02].
Figure 6. Burnoff of R16 at 1300 K (0,A,0 ) 300-ms residence time; ( 0 )800-msresidence time. Solid lines are NSC16 predictions.
Figure 7. Burnoff of CSC at 1650 K (0) 50-ms residence time; (0)100-msresidence time; (A)300-ms residence time. Solid lines are NSCIs predictions.
Figure 5 as a function of burnoff. The solid lines in Figure 5 correspond, respectively, to constant density and constant radius particle oxidation. From the definition of u m, = mo(l - u )
sured burnoff depends only on reactor temperature, [02], and residence time. The GC data were analyzed to extract Re by rearranging eq 2b.
Re = ( 3 / S o t ) ( l- ( 1 - u ) " ~ )
for a spherical particle
m = p(4/37r?)
The results are plotted in logarithmic form in Figure 8. For the laser-scattering measurements, u = 1 and Re = 3/Sotb
r?pt = ro3po(l- 4 and thus
r, = ro(l - u)lI3 for a constant density particle and Pt
= P o 0 - u)
for a constant radius particle. The measured specific surface area, S (m2/g), is related to the above quantities by S = (47rf)/(p(4/3)7r?) = 3 / p r Thus
So/S, = ( 1 - u)I/3 p = constant
So/S, = (1 - u ) r = constant The agreement of the measured specific surface area ratios with the prediction of constant density oxidation is apparent in Figure 5. In a more quantitative test, a fit was made to log (So/S,) againt log (1- u ) . The slope of the fit was found to be 0.32 f 0.08, indicating the validity of the constant density assumption. It is also clear that, a t least up to -70% burnoff, no "additional" s,urface is liberated by the breakup of aggregates. With the support of these surface area measurements, the present results have been interpreted as the oxidation of nonporous spherical particles (assuminga monodisperse size distribution) that react a t constant density and with no change in particle number density. Oxidation Rates. Oxidation rates were measured from 1300 to 1700 K for R16 and from 1400 to 1700 K for CSC, over more than three decades of oxygen partial pressure (0.02-60 kPa) and a t total pressures (0, N2) from 20 to 60 kPa. Particle residence times were varied from 50 to 800 ms. The majority of the data were obtained by using the GC method; additional data were obtained by using the laser scattering method. Representative burnoff data from the GC measurements, plotted against [O,],are shown in Figures 6 and 7 . There is no discernible total pressure effect on the measured [CO,] yields over the approximately factor of 2-3 variation in total pressure covered at each temperature investigated and no gas velocity effects over a factor of 6 at any temperature, nor were there any effecta of changing the reaction tube diameter from 2.5 to 4.5 cm. The mea-
+
where tb is the residence time for complete burnoff a t the [O,] determined from plots such as Figure 4. These observed surface oxidation rates are chemically controlled; i.e., the surface reaction rate is slow compared to diffusion of oxidant to the surface of the particle. This can be seen by comparing the calculated diffusion-limited rate with the observed rate. The reactivity coefficient for diffusion-controlled reaction on a spherical particle of radius r is given by5
R,D = ($'Da/r)(Co- CJ
(3)
where R,, = diffusion-controlled reactivity constant based on external surface area, g/(cm2 s). $' = (Mc/Mov),where v is the molar stoichiometric coefficient for the gas and M J M 0 is the molecular weight ratio of carbon to the reactant gas. For the present studies, C '/,02 CO, and v = 1/2, with $' = (12/32)2= 3/4. D, = binary diffusion coefficients of O2into N2 (cm2/s). C = mass density of the gas a t the particle surface, C,, and in the free system, Co (g/cm3). Thus, diffusion control (small values of R,D) is favored by high pressure (low diffusion rates), large particle size, and high temperature (high surface reaction rates). For pure diffusion control, i.e., when the surface reaction rate is infinitely rapid, C, 0. Figure 9 shows the range of Re, calculated from eq 3 with C, = 0 at the temperature and pressure extremes used in the present work (60 kPa and 1700 K)compared to the experimentally measured values. For the nominal particle diameters of the carbon blacks, 010-100 nm, diffusion-controlled rates are large and the reaction rate is controlled by surface chemistry processes. Figure 9 shows that even if aggregates as large as 100 pm were present in the reactor (and the settling chamber ensures that they are not), bulk diffusion rates would still be 010 times larger than the observed reaction rates under the present experimental conditions. On this basis, mass transfer to the carbon black particles does not significantly affect the observed measurements. Alternatively, we can determine the value of C, by calculating the concentration at the surface of an imaginary sphere bounding the carbon black particle. The radius of the sphere is chosen to be r* = r A, where r is the particle radius and h is the mean free path of 0, in N2 a t the reaction T and P. Oxygen molecules within the boundary strike the particle surface and react with probability E.
+
-
+
-
Felder et al.
748 Energy & Fuels, Vol. 2, No. 6,1988
-4
1
R16 1300 K
-
1410K
-5
-5 -6
-6
-
-7
... 1400 K
1580 K
9
-6
-6
-7 1
I
-4 -
.
.
.
,
.
.
:
Rl6
. 1470 K
9
1
-4
..
1 csc
-
. 1650 K
-
.
.
-7
-
-
t 14
1 15
16
17
16
LOG ( ~ 0 2 1 ,cm-3)
-4
-5
-6
t -4
-
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-
-6
-
14.5
.
.
.
15.5
.
.
16.5
.
t
17.5
16.5
LOG (1021,“3)
Figure 8. Carbon black oxidation by 02: (0)GC data; ( 0 )laser-scattering data;
-9
0.5
9
a
-0.5
1.00
c)
-1.5 -2.5
O
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4
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NSC predidion.16
f
PARTICLE DlAM
2 2
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I)
g
0.06
c ” c
f
A 15
16
17
16
LOG (LO2], cm-3)
0.00
0.85
3
5
7
9
REACTION PROBABILITY.
Figure 9. Comparison of diffusion-limited reactivity with observed values at 1700 K.
Figure 10. Effect of particle size on surface [02] for large reaction probability at 60 kPa and 1700 K.
Solving the one-dimensional-diffusion problem and equating the flux of 0, a t the particle surface with that a t r* allow the ratio of free stream to “surface” [O,]to be calculated: [02](r*)/[02](freestream) = 1/(1 + A)
The surface to free stream [O,]ratio at 1700 K and 60 kPa (most favorable conditions for diffusion control) is plotted in Figure 10 as a function of for particle sizes up to 100 pm. In order to discern an effect on the ratio, values chosen for $. had to exceed the experimental values by factors of 10-100. Even at these large reaction probabilities, the correction to the surface [O,]indicated is insignificant for particles 5-nm diameter, oxygen penetration was calculated to be between 74% and 99% of what it would have been for a nonporous particle between 20- and 100nm diameter. For comparison pore diameters of 1-2 nm reduce penetration to as low as 23% for a 100-nm particle. Thus aggregate penetration is relatively unimpeded for O2 (compared to pore penetration). Under the present experimental conditions where (1)lower temperatures have generally been used than in the Neoh et al. work and (2) reactivity is substantially lower than predicted by NSC kinetics,'6 the interpretation that the carbon blacks oxidize as nonporous particles (or aggregates) with effectiveness ~~~
1300
[OZI range,
for the present measurements, the value of [O,]used in calculating rate data is that of the free stream. In the absence of diffusion effects, the slopes of the plots in Figure 8 give the apparent reaction order in [02]. The reaction orders, n, lie between 0.6 and 0.8. Table I1 contains a summary listing of the least-squares parameters for the present experiments in the form log Re = log Ro n log [O,].Rois a fitting constant with units of g/[cm2 s (~m-~)~].
~~
1400
1 T -i-l
~
(16)Nagle, J.; Strickland-Conatable, R. F. Proceedings of the Fifth
Conference on Carbon; Macmillan: New York, 1962; Vol. 1, p 164. (17)Blyholder, G.;Binford, J. S., Jr.; Eying, H. J. phys. Chem. 1958, 62, 263.
(18)Thiele, E. W. Znd. Eng. Chem. 1939,2, 916.
.
.LTB
-5.
ro
. (c)
.-9)-
(soot) \
\
0
5 0
6 3
8 5
6 9
7 5
1 0 4 1 ~ .K - I
Figure 11. Temperature dependence of carbon black oxidation at several [O,]: (0) R16;(0) CSC. Key: NSC, ref 16, and PA, ref 11 (extrapolated);LTB,ref 12;RA,ref 19 for isotropic (iso) and pyrolytic (pyro) graphites.
factors near unity is reinforced. In Figure 11,the Re values are compared with previous studies of soot and carbon black oxidation. Parts a and b of Figure 11show the temperature dependences at [O,] = 1 X 10l8 and 1 X 10l6 (roughly 20 and 2 kPa), respectively, compared with the extrapolated results of NSC16 and Park and Appelton" (PA) on carbon black (a furnace black and a high-purity channel black were studied) oxidation in a shock tube (the PA and NSC results are identical). Over this practically important range of [02], oxidation of R16 and CSC is significantly slower than that of previously studied carbon blacks. Figure l l c displays the results of Lee, Thring, and Beer12 (LTB) on soot oxidation in an 02-rich flame ([O,]~ 3 . X5 1017),PA, NSC, and the present measurements. Again, our results show a smaller reactivity for R16 and CSC. An "activation energy" of ~ 1 7 0kJ/mol is consistent with all of the measurements. The present results, a t low [02](=l X IOz4), are compared with those of Rosner and Allendorflg (RA) and NSC in Figure lld. Here, at low temperatures, RA's measurements on isotropic and pyrolytic graphite bracket those predicted by NSC and those measured in the present work. The maxima in the reactivity coefficient (19) Rosner, D. E.; Allendorf, H. D. AIAA J. 1968, 6, 650.
Energy & Fuels 1988,2,750-756
750
i
t
5 :
csc
(b) CSC (1850 K )
5 30
.
R16 (1880 K)
R16 ( 1 4 0 0 K )
CBC ( 1 4 1 0 K )
I5
18
17
18
LOG ([02], cm-3)
Figure 12. Dependence of carbon black reaction probability on [O,]:RA,ref 19 for pyrolytic (pyro) graphite. values predicted by NSC and observed by RA may be present for R16, but are not suggested by the CSC data. Figure 12 displays the probability of reaction, 5, per O,-surface collision calculated by using kinetic theory to determine the number of collisions per second per unit surface area and the measured Re values. The RA data on pyrolytic graphite over a range of low [O,] at =1500 K are shown for comparison in Figure 12a. Figure 12b shows 5 for R16 and CSC for most of the wide range of [O,] covered in this work a t the extremes of the temperature ranges investigated. The low reactivity of these carbon
blacks translates into collision efficiencies as low as -1 X lo* a t [O,] = 1 X 10l8 cm-3 and as high as -4 X loe3 a t [O,] = 1X 1015~ m - ~These . values are comparable to those obtained for graphitic carbons. Thus,the present resulta indicate low reactivity for R16 and CSC for [O,] values of practical interest. The complex [O,] dependence of Re required by NSC kinetics is not observed over the temperature range studied, despite extremely wide variations in [02].The data show no significant reactivity differences that can be attributed to metallic content. We speculate that the high sulfur content in these carbon blacks may be the cause of their low reactivity and the failure of the two-site model to describe their oxidation kinetics. This sulfur content (mole fraction =40 ppm) may be sufficient to poison potential metallic catalytic sites (on a molar basis, sulfur is -20 and 10 times more abundant than metah in R16 and CSC, respectively) as well as to interfere with active sites in both carbon blacks. The furnace black studied by PA" had a sulfur cohtent similar to that of R16, but the higher temperature regime they studied may have allowed this impurity to be volatilized (at least from the surface) and thus not to interfere with the oxidation. If correct, this speculation suggests that it is important to avoid sulfur contamination where carbonaceous burnout is desired. Significant further study is required to verify this speculation. Acknowledgment. This work was sponsored by the
U.S. Army Research Office, under Contract No. DAAG 29-83-C-0023; the content of the information does not necessarily reflect the position or policy of the U.S. Government, and no official endorsement should be inferred. We acknowledge helpful discussions with Drs. N. M. Laurendeau (Purdue), S. J. Harris (GM), H. F. Calcote (AeroChem), and D. M. Mann (ARO).
Interaction between Potassium Carbonate and Carbon Substrate at Subgasification Temperatures. Migration of Potassium into the Carbon Matrix M. Matsukata,* T. Fujikawa, E. Kikuchi, and Y. Morita Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 160, Japan Received April 27, 1988. Revised Manuscript Received August 2, 1988 The interaction between potassium carbonate and carbon substrate in an inert atmosphere was investigated by using carbon black with an amorphous structure and graphite. Qualitative and quantitative changes of potassium species on carbon in the course of heat treatment were monitored by means of a temperature-programmed reaction, extraction of potassium with a HC1 solution, Auger electron spectroscopy, and electron probe microanalysis. Potassium carbonate impregnated on carbon black decomposed to give potassium oxide and COz in the temperature range 470-900 K. We found that potassium species migrated into the carbon matrix in the temperature range 670-900 K. At higher temperatures potassium oxide remaining on the surface was reduced by reaction with carbon. Although metallic potassium on graphite was last due to evaporation, no pronounced loss of potassium was observed from carbon black containing leas than 5 w t % of potassium. The migration of potassium into bulk carbon was not observed on graphite. Introduction The catalytic gasification of carbonaceous materials is one of the potential routes to produce industrially useful gases such as hydrogen, methane, and syngas. Many in0887-0624/88/2502-0750$01.50/0
vestigatorsl-' have reported that alkali-metal carbonate is catalytically effective, whereas the catalytic structure and (1) Wen, W. H.Catal. Rev.-,%
Eng. 1980, 22, 1-28.
@ 1988 American Chemical Society
