Ind. Eng. Chem. Res. 1987,26, 1707-1713
c, Figure 2) shows small peaks corresponding to the compound Ca12A114033.
Conclusions Programmed thermal reduction of fresh catalyst shows three signals in the 250-950 OC range. The first two (400 and 490 "C) correspond to only one species and the origin of both signals has been attributed to the water freed by the support retarding the reaction rate. When support water is eliminated before carrying out the reduction, there appears only one signal at 400 "C which coincides with that of unsupported NiO. The last signal in the TPR diagram (660 "C) has been attributed to NiO having interacted with the support. When the catalyst is heated previously in nitrogen stream, the reduction peak appears at 680 OC. From the DTA and X-ray diffraction experiments, it has been concluded that transformation of boehmite into yalumina occurs at 460 "C. Catalyst activation by varying the gas mixture evidences that the smallest crystallite size is obtained by using hydrogen in the heating and activation stages. Sintering for 20 h at 800 "C leads to an increase of NiO crystallite size from 415 to 490 A and to formation of a calcium and aluminium compound whose stoichiometry is Ca12A114033. Acknowledgment We gratefully acknowledge technical assistance by Lic Norbert0 Firpo for chemical analysis and Ing. Marcos Garazi for metallic area. We express thanks to Nestor Bernava for TPR measurements. We acknowledge the financial aid received from CONICET.
Nomenclature B = corrected sample line broadening
1707
b = corrected reference line broadening Co = inlet hydrogen concentration, ~ m o l / c m ~ D = average crystallite size K* = Monti's characteristic number ki = thermal conductivity of component i k = thermal conductivity of the gaseous mixture K = constant of the Scherrer formula So = amount of reducible species, pmol V* = total flow rate, cm3 (NTP)/s X i= molar fraction of component i Greek Symbols
0 = breadth of pure diffraction profile X = wavelength of X-rays B = angle between incident and diffracted beams Registry No. CH,, 74-82-8; AlzO3, 1344-28-1; NiO, 1313-99-1; Ni, 7440-02-0; Ca12A114033, 12005-57-1.
Literature Cited Bandrowski, J.; Bickling, C. R.; Yang, K. H.; Hougen, 0. A. Chem. Eng. Sei. 1962, 17, 379. Bartholomew, C. H.; Farrauto, R. J. Catal. 1976, 45, 41. Bartholomew, C. H.; Pannell, R. B. J. Catal. 1980, 65, 390. Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J. Catal. 1980,65, 335-347. Butt, J. B.; Wachter, C. K.; Billimoria, R. M. Chem. Eng. Sci. 1978, 33, 1321. Holm, V. C. F.; Clark, A. J. Catal. 1968, 11, 305. Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed.; Wiley: New York, 1973. Lo Jacono, M.; Schiavello, M.; Cimino, A. J. Phys. Chem. 1971, 75(8), 1044. Monti, D. A. M.; Baiker, A. J. Catal. 1983,83, 323. Roman, A.; Delmon, B. J. Catal. 1973, 30, 333. Rostrup-Nielsen, J. R. J. Catal. 1984, 85, 31. Zielifisky, J. J. Catal. 1982, 76, 157.
Received f o r review September 30, 1985 Revised manuscript received February 19, 1987 Accepted March 30, 1987
Catalytic Deactivation on Methane Steam Reforming Cata1ysts. 2. Kinetic Study Miriam E. Agnelli,t Esther N. Ponzi,*+and Avedis A. Yeramiant Centro de Investigacidn y Desarrollo en Procesos Cataliticos (CINDECA), Facultad de Ciencias Exactas, U.N.L.P.-CONICET, 1900 La Plata, Argentina
T h e kinetics of methane steam reforming reaction over an alumina-supported nickel catalyst was investigated at a temperature range of 640-740 O C in a flow reactor at atmospheric pressure. The experiments were performed varying the inlet concentration of methane, hydrogen, and water. A kinetic scheme of the Houghen-Watson type was satisfactorily proposed assuming the dissociative adsorption of CH4 as the rate-limiting step, but this kinetic scheme can be easily replaced by a first-order kinetics (rCH4 = k p c H a ) for engineering purposes. Catalyst activation with H2 and N2 mixtures or with the reactant mixture results in the same extent of reaction. Catalyst deactivation due to carbon deposition is a serious problem in the steam reforming process. This deposition may act in three ways: (1) fouling the metal surface, (2) blocking the catalyst pores and voids, or (3) disintegrating the catalyst support. The various ways of deactivation depend on the type of deposited carbon: whisker-like carbon produces loss of the catalytic activity blocking the pores, whereas encapsulating carbon produces deactivation by fouling the metal surface. Whisker-like Members of CONICET's Scientific a n d Technological Research staff.
0888-5885/87/2626-1707$01.50/0
carbon also breaks the structure of the catalyst support, which causes pressure increase in the reformer furnace tubes (Rostrup-Nielsen, 1975). To our knowledge, no studies have so far attempted to postulate an adequate model for deactivation in the steam reforming conditions. Such a system can be modeled by using the separable kinetic concept (Butt et al., 1978) which allows us to express the instantaneous rate of the reaction equation as the product of terms containing concentration (C), temand activity ( a ) dependences: perature (T), r = r l ( C , T)r&) 0 1987 American Chemical Society
1708 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987
A kinetic study in a system that involves deactivation must include a study of the main reaction and of the deactivation reaction as well. The kinetic study of the main reaction without deactivation (namely, rl(C, T ) )may be performed in two ways depending on how rapidly the system deactivates itself. One way is to extrapolate the data to zero time on stream, but certain precautions should be taken upon evaluation of kinetics when there is rapid carbon deposition. Kinetic evaluation depends on fixed bed reactor stabilization speed, which determines the time a t which the first samples can be taken in relation to the total process time. Sample taking as close as possible to the zero time on stream makes the kinetic evaluation more precise. Besides, some catalysts need a period of activation that may overlap the deactivation process and mask it. The other way of performing the kinetic study, when the system allows it, is to choose operating conditions in which the rate of deactivation is negligible. In this part of the study, the kinetics of the methane steam reforming reaction will be presented under such operating conditions that the activity remains constant over a long period of time. Then, in a further study, we will aim at finding a model representing deactivation due to carbon deposition. The deactivation rate will be measured through the changes of the extent of reaction with time on stream of the main reaction and will be related to the operation conditions. The reaction scheme is (Hyman, 1968; Grover, 1970) CHI + HzO e CO + 3H2
CO
+ HZO s Cot + H,
(1)
(2)
Kinetic Study. The kinetics of methane steam reforming on a nickel catalyst has been studied before for various catalysts prepared in various ways. Most authors agree on a kinetics of first order with respect to methane. Akers and Camp (1955) studied the kinetics of the reaction on a kieselguhr-supported nickel catalyst over a temperature range of 340-640 "C. They found no dependence on other reactants. The catalyst particle used in that study was somewhat large (4 = 3.2 mm) for a moderately active catalyst. Also, the apparent activation energy found was lower than in other cases at 8.8 kcal/mol. These facts suggest that those authors apparently worked in the pore diffusion limitation zone at least for part of the temperature range. On the other hand, Bodrov et al. (1964, 1967, 1968) found that there is a certain correlation with the partial pressures of CO, HzO, and H,, their influence varies with temperature: (a) for 400 "C
< T < 500 "C r = KPCHJPH~
(b) for 500 "C < T < 600 " C r = k p ~ ~ , / p ~ ?with ' E , = 36.2 kcal/g-mol
(c) for 700 "C r=
< T < 900 "C kpCHl
1 + A ( P H , o / P H+ ~ )BPCO
with E , = 19.4 kcal/g-mol where E, is the activation energy. Ross and Steel (1973) confirmed that the rate of reaction was first order with respect to methane and that it was
u2
,-
I ........ i
E1 Figure 1. Schematic diagram of experimental apparatus: 1,2 , 3 = capillary flow meters, 4 = reactor, 5 = refrigerant, 6 = water collector, 7 = bubbler, and 8 = chromatograph.
inhibited by water in the 500-700 "C temperature range. In this case, too, the low activation energy found (6.9 kcal/g-mol) would suggest that they worked under control diffusion conditions. Munster and Grabke (1981) found an expression similar to the one given by Akers and Camp but only in those cases in which the relationship pH20/pH2was greater than 0.1. For values lower than 0.1, there was dependence on that relationship. The temperature range in that case was 700-900 "C, and the activation energy was 43 kcal/g-mol. In contrast, Udrea et al. (1983) proposed a reversible kinetic expression of the Langmuir-Hinshelwood type for experiences performed at 700 "C.
Experimental Section Figure 1shows a schematic diagram of the experimental apparatus which consists of a stainless steel reactor of 1-cm diameter and 30-cm length heated by a 600-W electric furnace. The furance has three heating sections each connected to a temperature controller, which allows us to carry out the reaction under isothermal conditions. The temperature is measured axially by a Chromel-Alumel thermocouple which can slide inside a 2.2-mm stainless steel thermowell located at the reactor axis. Methane (99.99%), hydrogen (99.995%), and nitrogen (99.99%) are fed through flow-controller valves. Nitrogen is used for dilution to obtain H20/CH4relationships greater than 1.0 and to prevent carbon deposition. Hydrogen is fed for the same purpose and to avoid catalyst oxidation. The gaseous mixture (CH,, H,, N,) then passes through a bubbler containing water. The bubbler is surrounded by a shell through which water circulates from a thermostat. Thus, water temperature can be varied and so can the amount of steam carried by the gaseous mixture which, in such conditions, does not reach saturation. Therefore, the amount of steam in the mixture was determined independently as a functon of volumetric flow and shell temperature. This was done by weighing the amount of water
Table I. Physical Properties of the Reduced and Sintered Catalyst real density, g/cm3 2.80 specific area, m2/g 36 apparent density, g/cm3 1.02 metal surface area, m2/g 2.2 total porosity 0.49
adsorbed on a bed of molecular sieves through which the wet gaseous mixture circulated for a certain time. The composition of the gases entering and leaving the reactor was determined by taking samples with a syringe. The samples were then analyzed by gas chromatography. A thermal conductivity detector was utilized, and CH,, H2, COz, and N2 were separated on a Porapack Q column ( L = 3.20 m). The amount of CO leaving the reactor was measured by separating it from the other compounds in a 5A molecular sieve column (L = 2.50 m). The analysis was performed by using each column alternatively as working and reference column and He (99.995%) as gas carrier (40 cm3/min) a t 40 "C (Wilhite et al., 1968). CO was analyzed only periodically in order to check the mass balance. It should be remarked that the volume of H2 in the sample injected into the gas chromatograph should not exceed 0.2 cm3,for greater amounts in the prescence of He as carrier produce two peaks. If amounts as large as 0.7 cm3 are used, a negative peak appears. Catalyst. The catalyst was crushed down to a particle size of 0.420-0.297 mm to avoid diffusional effects and was pretreated by heating it with a mixture of Hz (10 vol %) and N2from room temperature to 805 "C and by leaving it at that temperature for 20 h to cause sintering of the active phase. Thus, a constant metallic area was kept during the kinetic determinations. An increment of 18% in the crystallite size of the active phase was observed in relation to the crystallite size measured before pretreatment. Physical properties of pretreated catalyst are shown in Table I. They were determined on a sample of the catalyst batch used for kinetic determinations.
Procedure and Experimental Results A catalyst sample of 200 mg diluted with 80 mg of crushed quartz of the same particle size was loaded in the reactor. The bed length was 0.3 cm. Catalyst dilution in this proportion ensures that the error due to canalization will be lower than 5% (Mears, 1971). Once loaded, the catalyst was reduced for 15 h at reaction temperature by a 10% gaseous mixture of Hzand NP Runs were performed at 642,707, and 738 "C, the inlet concentration of CHI, H20, and H2 varying in each case. The results were expressed as extent of reaction per mole of methane in the feed. Variations in catalyst activity were noticed after each temperature change. At the start of the run, an increase in activity for a maximum period of 40 h was observed after which a steady-state condition was reached. This effect has also been reported by Bodrov et al. (1964,1967,1968) and Ross and Steel (1973). Some of the data obtained at each temperature were checked using another catalyst sample, but the results obtained were not used in the kinetic model fitting and are available in Agnelli et al. (1985). Dependence of the extent of the methane steam reforming reaction upon the partial pressures of methane, hydrogen, and water was investigated. Methane partial pressure was varied from 0.040 to 0.110 atm keeping pH = 0.026 atm and p H 0 = 0.136 atm constant. Water partid pressure was varied between 0.08 and 0.200 atm at pu2 = 0.025 atm and p C H , = 0.042 atm, and hydrogen partial pressure was varied from 0.025 to 0.120 atm at p H l O = 0.133
XI
'
"'-
p H2 :0.026 a l m pH20'0.136 a t m
tR
-*----.--4---
0.5
:0 . 2 5 5
( 7 1
x
*-- --*-
-8.1
:
738 C'
------.---.--------*-
I
-- -_--
707
OC
1 - 6 4 2 C'
.-----+--A-*-
I
I
I
p H 2 :0.025 a t m pCH4=0.042 a t m
os/-
-------
-D-----,-
a- 1;707'C
-*- - ----c-,- - __ L
-4-
I
I
I : 6 4 2 "C p H ~ O[ a t m l
1
0.2
0.1
Figure 3. Dependence of the methane steam reforming reaction upon water partial pressure at 642, 707, and 738 O C .
0.5
_____ _-----------.---*
*
,--*--+-*-----*-
e
a
*-- .----0--
I
f
0.05
0.1
1: 707
OC
r
:642 o c
p H~ [ a t m i
0.15
T
I
*
Figure 4. Dependence of the methane steam reforming reaction upon hydrogen partial pressure at 642, 707, and 738 O C .
atm and pcH4= 0.040 atm. In each case, the extreme points of the partial pressure interval were replicated. Figures 2-4 show the results thus obtained at the various temperatures. From Figure 2 it can be seen that the extent of the first reaction remains almost constant when methane partial pressure varies, whereas the other variables remain constant at 642 and 707 "C. It can be concluded that the reaction is of first order with respect to methane, since in this case the extent of reaction does not depend on the inlet concentration. At 738 "C, the extent of reaction decreases as methane partial pressure increases. This fact would not invalidate the assumption of first-order reaction, since in kinetics of the Houghen-Watson type the variables that appear in the denominator depend on the extent of
1710 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 Table 11. Experimental Conditions of the Data Presented in Agnelli et al. (1985) pCH, variation, atm p H z , atm PHzOP atm T, O C 0.040-0.110 0.110 0.135 707 0.025 0.080 707 0.030-0.060 0.040-0.080 0.025 0.215 707 0.030 0.080 642 0.040-0.070 0.040-0.090 0.025 0.205 738 Table 111. Experimental Conditions of the Data Presented in Agnelli et al. (1985) P H ~ Ovariation, atm PCH,, atm PHz' atm T, O C 0.080.200 0.040 0.110 707 0.040 0.080 738 0.080-0.200 Table IV. Equilibrium Constants and Their Equivalent Relationship with Reactor Outlet Conditions for Reactions 1 and 2
T,O C
KI, atm2
KZ
Q1,m.w
642 707 738
2.074 14.69 34.68
1.99 1.494 1.315
0.201 0.715 0.700
atm2 X X
lo-'
Qz,min
Qz,max
0.640 0.291 0.810
1.88 1.43 1.26
reaction and on the inlet methane partial pressure. As in this case, the extent of reaction is greater, its value being more affected by the inlet methane concentration. From Figure 3 it can be concluded that the extent of reaction decreases as water partial pressure increases. The opposite conclusion can be drawn from Figure 4, since the extent of reaction increases as hydrogen partial pressure increases at 642 and 707 "C, whereas at 738 "C the variation of hydrogen partial pressure has no effect on the extent of reaction. Other experimental determinations were made at the conditions described in Table I1 and 111. These results are presented elsewhere (Agnelli et al., 1985). The same conclusions mentioned above can be drawn from those data. It should be remarked that the available data, that is, those shown in Figures 2-4 and in Agnelli et al. (1985), were used for the kinetic model fitting. In order to check whether reactions 1and 2 were near equilibrium, the ratio of the partial pressures of products to reactants for both reactions was calculated and compared to the equilibrium constants. Table IV shows the values for the equilibrium constants ( K , and K2)and the ratio of the partial pressures at the reactor outlet for reactions 1and 2, named Q1and Q2,respectively. In the case of Q2, the minimum and maximum values found among all the data for each temperature have been shown, and in the case of Q1,only the maximum value has been presented. From the above, it can be stated that reaction 1 is far from equilibrium, whereas reaction 2 is close to it. It must also be remarked that after having worked at 738 "C,runs were performed at 642 "C, and it was noticed that the extent of reactions 1and 2 was remarkably higher ( X , = 0.35) than those previously determined ( X , = 0.12) but they declined to the formerly obtained values, although slowly, with time on stream.
Influence of Reduction and Oxidation Treatment As it was stated above, the reduction previous to every run for kinetic measurements was performed with an H2 and N2 mixture (10% vol) at reaction temperature. Some authors (Bartholomew and Farrauto, 1976; Bartholomew et al., 1980) consider that there is dependence of the reduction conditions on catalyst activity. To find out whether the method of catalyst activation had any influence on catalyst activity, some runs were carried out
Table V. Influence of Reduction and Oxidation Treatment 10IPCH,, PHz, PH20, 103tR, atm atm atm (g/min)/cm3 XI lo1& Section A" 0.42 0.26 0.135 0.255 0.101 0.895 0.47 0.57 0.149 0.255 0.110 0.832 0.46 0.43 0.146 0.255 0.104 0.898 0.45 0.35 0.143 0.255 0.100 0.921 0.48 0.34 0.154 0.255 0.114 0.884 0.42 0.27 0.134 0.254 0.100 0.953 0.43 0.30 0.138 0.254 0.116 0.936 0.43 0.31 0.135 0.261 0.100 0.841 0.43 0.31 0.135 0.261 0.958 0.940 0.43 0.28 0.134 0.261 0.097 0.843 0.42 0.44 0.41 0.40 0.40
0.224 0.313 0.499 0.321 0.285
Section Bb 0.134 0.255 0.141 0.255 0.133 0.255 0.130 0.254 0.129 0.254
0.104 0.101 0.099 0.097 0.100
0.830 0.909 0.802 0.872 0.897
"Reduced by H2 and Nz mixture previously oxidized with an 02-N2mixture. Reduced by reaction mixture previously oxidized with an 02-N2 mixture.
reducing the catalyst directly with the reaction mixture as it is done industrially. As a previous step, the catalyst had to be oxidized with an O2 and N2 mixture. Some references (Unmuth et al., 1980) indicated that Ni on the surface results in a more highly dispersed metal than that found upon reduction of initial oxide when Ni is subjected to an oxidation-reduction sequence. Therefore, in order to investigate catalyst activity dependence on oxidation-reduction conditions, the catalyst was oxidized at T,, = 400 "C in an O2 and N2 mixture (10% vol). Oxidation progress was followed up by means of a thermal conductivity detector. Reactor inlet and outlet streams circulated through the channels of the thermal conductivity detector, which quantifies O2 consumption during oxidation. Maximum temperature was fixed at 400 " C to avoid formation of nickel aluminates. The catalyst was reduced under the kinetic study conditions, and the extent of the methane steam reforming was measured at 642 "C.The data thus obtained are shown in the first part of Table V. As can be seen, there is no difference between these data and those obtained before oxidation during kinetic measurements at 642 "C. Once it was confirmed that the oxdiation-reduction cycle has no effect on activity, the catalyst was oxidized and then reduced but now directly with the reaction mixture at the reaction temperature. The extents of reaction obtained are shown in the second part of Table V, and they allow us to conclude that catalytic activity did not depend on reducing feed.
Discussion Model discrimination and parameter evaluation were based on the integral reactor treatment, and the Marquardt iteration technique was employed. Initially, obtention of optimal parameter values for each kinetic expression requires a two-variable fitting, XI and X 2(extents of reactions 1and 2). However, the available data did not allow us to fit X 2 ,since it was very close to the equilibrium extent of reaction. This problem was overcome assuming a linear variation of X 2 with the residence time between X 2 = 0 and its value at the reactor outlet. In order to determine the validity of this assumption, the kinetic models were fitted for extreme cases: (1)X 2 = 0 and (2) X 2 equal to its outlet value throughout the reactor, which shows a very low sensitivity of the kinetic constants with respect to X2.
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1711 Table VI. Proposed Models and Their Rate Expressions model
-
exDression
kpCHI
I
= (1
+
K Z+
KaPco)"
n = 1-3 kpCH,
I1 = (1
+ K:z
+ KaPco)n
n = 5-7
IV
r =
kpCH,
mechanism
CH, + nS C(SJ + ZHZ H,O + S s O(S) + Hz C(S,) + O(S) a C O W + nS
cow a co + s co + ow .= cop+ s
-
+ nS C(S,J + 4HS 4HS a 2H, + 45 H20+ S a O(S) + Hz C(S,.) + O W t C O W + (n-4)S CH,
cow I co + s co + O(S) co, + s f
s
CO,(S) a co, + CH, + 3s C(S) + 2HdS) H,(S) a H,+ S H,O + S a O(S) + H2
-
CO(S) I co + s
cow + o w
I GO,
+ 2s
V
Several models of the Houghen-Watson type were proposed assuming that the rate-determining step was the dissociative adsorption of methane. Previous studies demonstrated that this adsorption occurs without the formation of intermediate hydrogenated species (Martin and Imelik, 1974). Although all the models were analyzed supposing different controlling steps, all their rate expressions had to be rejected because they did not agree with the experimental tendencies observed when varying the reactant partial pressure. Table VI shows the proposed models and their expressions. Recent studies have determined that in the steam reforming reaction more than one active site is involved. Rostrup-Nielsen (1984a) working on a sulfur-passivated
nickel catalyst found that an ensemble of three nickel atoms was required. On the other hand, Martin and Imelik (1974) on the basis of adsorption experiments up to 500 "C determined that methane was adsorbed following the reaction CHI + 7Ni s CNi, 4HNi
+
For the above reasons, the influence of the involved active sites (n) was analyzed in models I and 11. The constant values for each temperature and the standard error for each fitting are shown in Table VII. As has been pointed out elsewhere (Kittrell et al., 1965), an important problem in the application of nonlinear regression for parameter estimation is determination of the initial values. In this case, at the lower temperature the parameters were selected by linearization, since the extent of reaction is low enough to allow the differential reactor treatment. The constant values obtained from nonlinear regression of the lower temperature data were then used as initial values for the temperature 707 "C and so on. Plotting the logarithm of the constant against the reciprocal of temperature, we can see that the kinetic constants follow Arrhenius' law in all cases. In contrast, the pseudoadsorption constants do not always follow van't Hoff s law. Model V presents a negative constant (KB,at 707 "C), and once the standard error of this constant is considered, it still does not yield a positive value interval within an acceptable limit of confidence. Therefore, this model was rejected. From the model analysis which takes into account carbon monoxide adsorption, it can be seen that the constants follow van't Hoff s law in all cases. The adsorption heat value of each model lies between the limit values found for carbon monoxide adsorption over transition metal (Sakai et al., 19851, but the correlation coefficient for the adsorption heat evaluation was better for model I, with n = 2 and 3, and for model 11,with n = 5-7. The rest of the
Table VII. Results of Parameter Estimation for the Proposed Models 1O2k,gmol/(g of model T,O C catalyst-minsatm KA KB,atm-' Kc, atm-' KD,atm-' KE,atm-' std error I 642 1.104f 0.034 0.235f 0.019 131.72f 0.001 0.777X lo-* n = 1 707 2.933f 0.071 0.142f 0.017 2.69 f 1.66 0.322 X lo-' 10.949f 0.709 0.239f 0.041 17.62f 4.36 738 0.365 X lo-' I 642 1.047f 0.049 0.103f 0.011 145.19f 19.9 0.930X 3.039 f 0.080 0.073f 0.008 2.786f 1.215 n = 2 707 0.330X lo-' 0.364X lo-' 738 10.833f 0.624 0.098f 0.014 7.836 f 1.48 I 642 1.048f 0.031 0.055f 0.004 51.62f 7.91 0.898X 2.57f 0.90 3.062f 0.086 0.047f 0.004 n = 3 707 0.328X lo-' 10.331f 0.628 0.056f 0.009 4.44f 0.93 0.366X lo-' 738 I1 642 0.938f 0.033 0.024 f 0.003 41.42f 5.37 0.770X 3.013f 0.080 0.026f 0.003 0.329X lo-' 1.29f 0.43 n = 5 707 10.192f 0.542 0.032f 0.004 2.49 f 0.45 0.366X IO-' 738 0.038 iz 0.002 26.13 f 2.61 I1 642 1.197f 0.028 0.580X 3.049 f 0.084 0.022f 0.002 0.329X lo-' 1.34f 0.39 n = 6 707 2.26 f 0.39 738 0.368X lo-' 10.513f 0.539 0.028f 0.003 I1 642 0.895f 0.030 0.013f0.002 30.01f 4.41 0.880X lo-* 2.962f 0.073 0.018f 0.002 0.67f 0.27 n = 7 707 0.329X lo-' 9.904f 0.468 0.021f 0.003 1.58f 0.28 0.366 X lo-' 738 0.672X I11 642 1.187f 0.036 0.316f 0.024 194.017f 0.001 0.534 0.620 0.326X lo-' 4.369f 2.250 1.080 1.515 3.052f 0.088 0.166f 0.024 707 0.339X lo-' 13.297f 0.831 0.129f 0.044 17.167f 4.423 80.938 20.141 738 0.057 f 0.020 0.562 X 1.251f 0.031 0.140& 0.007 74.282f 7.684 IV 642 0.302 0.108 0.324X lo-' 3.278f 0.103 0.083f 0.008 2.287 f 1.360 707 0.365X lo-' 8.069f 1.440 0.008f 0.018 10.903f 0.590 0,099f 0.013 738 0.286f 0.114 0.141X lo-' V 642 0.739f 0.022 171.070f 6.916 3.777f 0.542 0.356 X IO-' -2.501f 1.839 3.432f 0.149 707 4.822f 0.870 0.368X IO-' 22.746f 5.344 13.056f 1.096 738
*
**
*
1712 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 Table VIII. Correlation Coefficient ( r , )for the Linear Regression of In K A vs. Temperature Reciprocal for Models 1 and I1
model I
n
r, 0.19 0.35 0.13
1 2 3
model I1
n 5
6 7
r,. 0.88 0.72 0.99
models yielded very poor correlation coefficients. Model I11 takes into account carbon dioxide adsorption. Its constant does not agree with van’t Hoff s law. Model IV considers hydrogen adsorption but the correlation of the logarithm of its constant with the reciprocal of temperature presents a very low coefficient to ensure that van’t Hoff s law is followed. For the above stated reasons, models I11 and IV were rejected. The other models (I and 11) show very similar deviation values between measured and calculated extent of reaction, so model discrimination is impossible in these conditions. However, some considerations about steam adsorption can still be made. The results reported elsewhere and reviewed by Rostrup-Nielsen (1984b) concluded that catalyst support plays a very important role in water adsorption. Thus, the step written HzO + S e O(S) + H2 (3) consists of the following steps: H 2 0 support F? HzO - support
+
H 2 0 - support
+ O(S) + H2
(4)
(5)
The support enhances adsorption of steam which is then adsorbed on the nickel surface. Since steam is also adsorbed directly on the nickel surface as in (3), the constant which appears in the models that have taken into consideration steam adsorption cannot be considered a true equilibrium constant. In fact, it only reflects a steady-state condition reached by the processes involved in (3)-(5). So KA can be written as
where K4 = equilibrium constant of reaction 4, k5 = direct kinetic constant of reaction 5, k-3 = reverse kinetic constant of reaction 3. As stated above, K A cannot be considered an equilibrium constant and in consequence does not follow van’t Hoff s law. But, since K A is a relationship between constants which individually should have exponential dependence upon temperature reciprocal, KA should have the same dependence. The linear regression of In KA with the reciprocal of temperature was well correlated for model I1 with n = 7, as it can be seen in Table VIII. As mentioned above, the use of model I1 (n = 7) is recommended to predict the extent of methane steam reforming reaction for this temperature range. Figures 2-4 show the predicted values (in dotted lines) for the extent of reaction with model 11, n = 7, under the respective experimental conditions. The activation energy calculated from the slope of Arrhenius plot is 44 kcal/g-mol. As already mentioned, many authors proposed firstorder kinetics for the methane steam reforming reaction. Fitting our data to an expression of first order with respect to methane, we obtained the constants shown in Table IX. As it can be seen, the error in the extent of reaction prediction using this last rate equation is very small.
Table IX. Results of Parameter Estimation for a First-Order Expression 102k,g-mol/(gof model T,“C catalvst.min.atm) std error r = kpCHl 642 0.720 i 0.015 0.185 X lo-’ 707 2.409 =k 0.038 0.448 X lo-’ 738 6.971 f 0.108 0.476 X lo-’
Conclusions The kinetics of the methane steam reaction is first order with respect to methane partial pressure. The dependence of that kinetics on water partial pressure is the reverse of that on hydrogen partial pressure: the former acts as inhibitor, whereas the latter acts as promoter. The equation which best fits the results in the 640-740 “C temperature range is kPCH~
However, the influence of the reactants and products (except CHJ is sufficiently low as to allow the use of a first-order kinetic equation to predict the extent of reaction introducing a negligible error. Acknowledgment We acknowledge the cooperation of NBstor Bernava and the financial aid received from CONICET. Nomenclature a = activity c = concentration k = kinetic constant K1,K 2 = equilibrium constants of reactions 1and 2, respectively KA/KF = pseudoadsorption constants pcHl = methane partial pressure pHp = hydrogen partial pressure ~ H =~ water O partial pressure Q1, Q2 = products to reactant partial pressure ratio of reactions 1 and 2, respectively r = rate of reaction T = temperature tR = residence time defined as mass of catalyst/volumetric flow
XI, X , = extents of reactions 1 and 2, respectively Greek Symbols AT = axial temperature difference 4 = particle diameter Registry No. CHI, 74-82-8; H2, 1333-74-0; NiO, 1313-99-1; A1203,
1344-28-1.
Literature Cited Agnelli, M.; Ponzi, E. N.; Yeramian, A. A. “CINDECA”, Internal Publication No, 40,1985; Centro de Investigacidn y Desarrollo en Procesos Cataliticos, La Plata, Argentina. Akers, W.; Camp, D. D. AIChE J . 1955, 1, 141. Bartholomew, C. H.; Farrauto, R. J . Catal. 1976, 45, 41-53. Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J . Catal. 1980, 65, 335-347. Bodrov, I. M.; Apelbaum, L. 0.;Tempkin, M. I. Kinet. Katal. 1964, 5(4), 696-705. Bodrov, I. M.; Apelbaum, L. 0.;Temkin, M. I. Kinet. Katal. 1967, 8(4),821-828. Bodrov, I. M.; Apelbaum, L. 0.; Temkin, M. I. Kinet. Katal. 1968, 9(5), 1065-1071. Butt, J. B.; Watcher, C. C.; Billimoria, R. Chem. Eng. Sci. 1978, 33, 1321-1329. Grover, S. S. Chim. Ind., Genie Chim. 1970, 103, 93. Hyman, M. H. Hydrocarbon Process. 1968, 47, 131.
I n d . Eng. Chem. Res. 1987,26, 1713-1716 Kittrell, J. R.; Mezaki, R.; Watson, C. C. Znd. Eng. Chem. 1965, 57(12), 19. Martin, G. A.; Imelik, B. Surface Sci. 1974, 42, 157-172. Mears, D. E. Znd. Eng. Chem., Process Des. Dev. 1971, 10(4), 541. Munster, P.; Grabke, H. J. J. Catal. 1981, 72, 279-287. Ross, J. R: H.; Steel, M. C. F. J. Chem. SOC.,Faraday Trans. 1 1973, 69, 1-10. Rostrup-Nielsen, J. R. Steam Reforming Catalysis; Danish Technical: Copenhagen, 1975. Rostrup-Nielsen, J. R. J . Catal. 1984a, 85, 31-43. Rostrup-Nielsen, J. R. Catal. Sci. Technol. 198413, 5, 1.
1713
Sakai, N., Chida, T.; Tadaki, T.; Shimoiizaka, J. J . Chem. Eng. Jpn. 1985, 18(3), 199-204. Udrea, M.; Moroianu, L.; Pop, T.; Musca, T. Proc. 5th Znt. Symp., Heterogeneous Catal. Varna, 1983, 2, 1. Unmuth, E. E.; Schwartz, L. H.; Butt, J. B. J . Catal. 1980, 61, 242-255. Wilhite, W. F.; Hollis, 0. L. J . Gas Chromatogr. 1968, 6, 84-88.
Received for review September 30, 1985 Revised manuscript received February 19, 1987 Accepted March 30, 1987
Calorimetric Determination of the Heat of Combustion of Spent Green River Shale at 978 K Stephen C. Mraw* and Charles F. Keweshan Exxon Research and Engineering Company, Clinton Township, Annandale, New Jersey 08801
A Calvet-type calorimeter has been used to measure heats of combustion of spent Colorado oil shales. For Green River shale, the samples were members of a sink-float series spanning oil yields from 87 to 340 L*tonne-l. Shale samples (3&200 mg) are dropped into the calorimeter at high temperature, and a peak in the thermopile signal records the total enthalpy change of the sample between room temperature and the final temperature. Duplicate samples from the above sink-float series were first retorted at 773 K and then dropped separately into nitrogen and oxygen at 978 K. The resulting heats are subtracted to give the heat of combustion, and the results are compared to values from classical bomb calorimetry. The agreement shows that the heats of combustion of the organic component are well understood but that questions remain on the reactions of the mineral components. Two important considerations for the future production of oil from shale are solids handling and heat management. For a resource yielding about 90 Latonne-’ (1Latonne-’ = 4.174 U.S.gabton-’), 2 tonne of rock must be handled for each barrel of oil produced, and large quantities of heat must be supplied or removed for the high-temperature processes involved. Proper reactor design and efficient process operations require accurate enthalpy data for retorting raw shale and combusting spent shale. It is part of our program in high-temperature calorimetry to develop novel techniques for oil shale measurements and to provide accurate enthalpy data for shales of commercial interest. In previous papers, we have described a new method for determining the enthalpy of vaporization of organic materials at high temperatures (Mraw and Keweshan, 1984), demonstrated the application of this method to oil shale systems (Mraw and Keweshan, 1985),and given a complete set of results for the heat of retorting raw Colorado Green River and Australian Rundle oil shales to 773 K (Mraw and Keweshan, 1986). In the present paper we present new results for the heat of combustion of spent Green River shales at 978 K.
Experimental Method Our apparatus is the high-temperature Calvet-type calorimeter described previously (Mraw and Kleppa, 1984). Briefly, it consists of a central inconel block surrounded by a furnace designed for operation from room temperature to 1273 K. Within the block are two symmetrically mounted wells which serve as sample chambers. Each well is surrounded by a multijunction thermopile which monitors the temperature difference between the well and the block. When any process within the sample chamber absorbs or liberates heat, the thermopile signal observed is
* To whom
correspondence should be addressed.
proportional to the heat flow between the chamber and the block. The twin thermopiles are wired in opposition so that a difference reading is obtained for the left vs. right sample chambers. Most of the experiments to be described are “drop” experiments. A quartz or ceramic tube within the sample chamber is continuously swept with gas (nitrogen or oxygen, depending on the experiment), and a sample is dropped from room temperature into the receiving tube at the high temperature of the calorimeter. A peak in the thermopile signal records the total heat necessary to bring the sample from room temperature to its final state at high temperature. The area of the peak is directly proportional to the total heat. However, when all processes requiring heat can be completed well within the time constant of the calorimeter (e.g., within 2-3 min), the calorimeter thermopile responds “ballistically”,and the peak height (as well as the more traditional peak area) is directly proportional to the total heat transferred to the sample (Mraw and Keweshan, 1984). This experiment for a complex material such as oil shale gives an additional opportunity to study the physical chemistry of the associated processes. When a sample is dropped into high temperature, some processes may be completed within the response time of the calorimeter, and other processes may take longer. The peakheight value will accurately represent the heat for the rapid processes, while the peak-area value will represent the total heat for all of the processes until the calorimeter returns to equilibrium (about 1.5-2.5 h). Although comparison of the peak-height and peak-area values for each run nominally distinguishes rapid vs. slower processes, this can sometimes allow a distinction between organic and inorganic contributions (Mraw and Keweshan, 1986). Calibration of the calorimetric sensitivity, i.e., the microvolt signal produced by a given amount of joules, is interspersed repeatedly among the actual shale runs. Experiments which produce a known amount of heat, ei-
0888-588518712626-1713$01.50/0 0 1987 American Chemical Society
