ARNOLDE. REIF
778 when net flow is prevented. fractional difference
Vol. 56
factor which depends on the structure of the double layer. I n general, Cu = Co(l QoF X) = Co(1 (C11 - C,)/C,l = Q (29) X)/(1 - Q F ) where QoF is the electro-osmotic Surface conductance is considered to arise from term and X represents the other two. F is an the increase of electrolyte concentration and integral depending on the viscosity and potential altered ionic mobility near the wall, and an electro- distribution assumed in the double layer. It can osmotic term, due to the motion of the liquid be evaluated explicitly from the formulas of the carrying a surplus of charges of one sign. To evalu- authors mentioned. ate surface conductance theoretically, it is necesThe relative surface conductance takes the form sary to make a detailed analysis of the electric (C11 - CO)/CO = &OF ;t x (32) double layer. This has been done for single capillaries by B i k e ~ - m a nReichardtlo~~~ ,~~ and Kan- It must be measured with a constant pressure e k ~ .All~ three ~ authors obtained fundamentally difference. If it were measured with zero flow, the same expression for the electro-osmotic term,45 it would be which reduces to the Smoluchowski expression4' (CI - CO)/CO = & d F - 1) x (33) when a number of more or less customary assumpThis quantity would have a very different value tions are made and might even be negative. Still other values of (Cll - Co)/C0 = r 2 ~ 2 / 1 6 ~ 2 6 r ~ ~ o (30) surface conductance could be measured, correwhere 6 is the average thickness of the double layer, sponding with the variety of possible conductances and KO is the ordinary specific conductivity of the previously discussed. Perhaps if these possibilities liquid. The similgrity to equation 12will be noted. had been realized, there might be fewer discrepIf we define QO = Ce2/CoCz2, equation 30 becomes ancies between surface conductance values measured a t different laboratories. (GI - CO)/CO= &o(r/86) (31) The relation between Q and Qois Comparison of equations 29 and 31 shows that the electro-osmotic term of surface conductance differs Qo = Q(1 X)/(1 - &F) from the electro-osmotic conductance effect by a It follows that Q0 < (1 X)/(1 - Q F ) < (1 (42) J. J. Bikerman, 2.physik. Chew., 8163, 378 (1932) w/(1 - F ) , which amounts to an explicit restric(43) H. Reichardt, ibid., A l 6 6 , 433 (1933). tion on relative values of s", surface conductance, (44) 9. Kaneko, J . Chem. SOC.J a p a n , 66, 600 (1935). and capillary radius. (45) Numerical errors must be corrected in Reichardt's formula 15a43 and Kaneko's formula 35. Bikerman's formula 37 omits the electroThe author is grateful to Dr. A. S. Coolidge, of osmotic term from conductance in the expression for streaming potenHarvard University, for reading the original tial, but ite presence is clearly required by formula 22 of the present manuscript and pointing out the relation of Onpaper. sager's work to this paper. (46) Reference 37, formula 43.
I
We have for the
+
+
+
+
+
+
+
-
A STUDY O F THE REACTIONS OF CARBON MONOXIDE WITH COKE' BYARNOLDE. RE IF^ Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh I S , Pennsylvania Received October 8, 1961
The reaction between carbon monoxide and a degassed high temperature coke has been invest,igated in the temperature range from 700 to 1000° and in the pressure range from 1 to 63 cm. The equations CO e (CO) and CO (CO) + COz C (1)where (CO) represents a molecule chemically bonded to the carbon surface are pro osed for the initial reactions observed under these conditions. Rate constants and temperature coefficients that described tiese equations have been evaluated. The data indicate that the tendency for carbon dioxide formed during this reaction to revert t o carbon monoxide increases as the carbon dioxide-carbon monoxide equilibrium composition is approached.
+
Introduction The trend in use of liquid or gaseous fuels in preference to solid fuels has given impetus to research directed toward gasification of solid fuels.%4 The gasification of carbonaceous fuels by oxygen, carbon dioxide or steam has long been known to be retarded by carbon monoxide and hydrogen, products (1) Abstracted from a dissertation by the author, Coal Research Laboratory Fellow in the Department of Chemistry, Carnegie Institute of Technology, submitted in partial fulfillment of the requirements for the degree of Doctor of Science, June, 1950. (2) McArdle Memorial Laboratory, Medical School, University of Wisconsin, Madison, Wisconsin. (3) J. J. Morgan, "Chemistry of Coal Utilization," H. H. Lowry, Editor, John Wiley and Sons, h a . , New York, N. Y., 1945, pp. 1693-
1709. ( 4 ) B, J , C , vnn der Hoeven, %'bid', pp, 1586-1636,
+
of the gasification reactions. However, lack of agreement exists between various authorities as to the mechanism of the retardation reactions. This mechanism is usually studied by flowing carbon dbxide-carbon monoxide mixtures, O r steam-hydrogen mixtures, through a bed of heated carbonaceous fuel under controlled conditions. Another approach is to study the direct interaction Of carbon monoxide Or Of hydrogen with such materials. While the reaction between hydrogen and low-ash carbons has been carefully investigated by Barrer and Rideal,6,6 there is little information on (5) R. M. Barrer, Proc. Roy. Boc. ( L o n d o n ) , A149, 253 (1935); J . Chsm. Soc., 1256 (1936); 1261 (1936). (6) R. M. Barrer and E. K,Rideall Proc, Roy, 8001(London), AX491 231 (isar3,
.
June, 1952
A STUDY OF REACTIONS OF CARBON MONOXIDE WITH COKE
the direct interaction of carbon monoxide and carbonaceous fuels at elevated temperatures. Previous investigators7-10 have studied this reaction as a subsidiary to the carbon dioxide-carbon reaction. The present paper describes result,s obtained in a study of the reaction between carbon monoxide aiid a degassed high temperature coke at temperatures yanging from 700 to 1000” and pressures from 1 to 63 em. of mercury. The experimental procedure involved continuous recirculation of carbon monoxide over an externally heated coke sample. Equations are proposed for the observed reactions aiid the three rate constants, which describe the rate at which these reactions proceed, have been evaluated. Experimental
779
tempcrature of the reaction chamber was held within =!=0.5” for thc duration of the experimmt. Variation in temperature along the reaction chamber was reduced to + l o by means of t,wo manually controlled end heaters, which were culibrated to give tcniperature uniformity a t different temperatures.
Apparatus.-Figure 1 is a schematic diagram of tmhe apparatus. Circulation was maintained in the closed cycle ABC of capillary t)ubing both by the thermal convection of gases passing through the reaction chamber ( l ) , which was situated in a high temperat>urefurnace (2), and by means of a Pyrex pump (3 ). Circulation minimized differences in gas composition along the flow path resulting from reaction. The gases passed through a gas buret ( 4 ) and returned to the reactmionchamber ( l ) ,with intermediate connections to a precision manometer ( 5 ) and a gas sampling device (6). The reaction loop ABC was served by equipment for vacuum production and low pressure measurement (7), gas storage ( 8 ) and gas purification (9).
SECTION A-A‘
SECTION 8-8’
Fig. 2.-Diagram
Fig. 1.-Schematic
diagram of the apparatus.
Details of the reaction chamber are shown in Fig. 2. It consisted of a quartz chamber E, holding roughly 30 g. of coke. Quartz-Pyrex graded seals a t D and D’ joined the quartz assembly to the Pyrex capillary of the reaction loop CC’. The leads, N and N’, of a platinum-platinum 10% rhodium thermocouple led into the refractory casing G and the junction a t F was shielded by cement. The leads N and N’ split a t I and ran out of t.he assembly through lead-soldered joints M and M’ inside thin cylinders of Kovar metal, attached to the Pyrex by Kovar-Pyrex graded seals at L and L’. Platinum resistance wire was wound on the quartz reaction chamber, HH‘, acted as one arm of an AC bridge which formed part of a thyratron thermostat. The output of a thyratron controlled by the AC bridge was used to vary the impedance of a saturable reactor, which was connected in series with the Globar heating elements of the furnace (2) and with the power supply. By this means, the over-all (7) W. E. J. Broom and M. W. Travers, Proc. Rou. SOC.(London), A186, 512 (1932). (8) J. Ctadsby, F. J. Long, P. Sleightholm and K. W. Sykes,i b i d . , A198, 357 (1948). (9) A. F. Semechkova and D. A. Frank-Kemenetrky, kcta Physicothirn. U.R. 8. S.,it, 879 (1940). (10) H, 9, Taylar and H. A. Neville, J , dm4 Chrm, Soc.,43, 2065 (leal),
\ N’ of the reaction chamber.
The pump (3) was constructed from Pyrex precision tubing. Poppet valves ground a t the bottom of both piston and cylinder permitted only upward flow of gas. To the inside of the piston was sealed an iron nail encased in Pyrex tubing, whereby the piston could be raised when current activated a solenoid concentric to piston and cylinder. A contact breaker served to cut off the current and drop the piston periodically, a pumping speed of approximately 150 cc./ minute being obtained with three breaks of contact a second. The “gas flow buret” (4) was designed to permit continuous circulation of gas in the reaction loop ABC. It contained a const.ant mass of mercury, such that when the mercury fell to the lowest mark of the graduated bulbs, it rose in the buret head to a height which just permitted free passage of gas through i t . The volume of mercury in the buret head could be varied by known amounts by running mercury into or out of the graduated bulbs. The precision manometer ( 5 ) was fitted with a mirror scale and electrical contacts for zero adjustment, which enabled pressure readings reproducible to zkO.1 mm. t o be taken. The sampling device (6) enabled 85y0of the gas in the reaction loop to be transferred into a sampling tube in two operations of filling and emptying its bulb. Vacuum production ( 7 ) was accomplished by a mercury diffusion pump backed by a single stage Welch Duo-Seal vacuum pump. The combination gave a vacuum of better than 10-6 cm., as measured by a McLeod gage designed t o cover the range 10-6 to 7 cm. Gas storage (8) was provided by three 5-liter flasks, each connected to a mercury manometer. Materials.-Helium and nitrogen were bought already purified in sealed 1-liter flmkR from the Matheson Company,
ARNOLD E. REIF
780
Inc., East Rutherford, New Jersey. These flasks wcre sealed to the apparatus as desired and the seal broken in the usual manner with nail and magnet. Carbon monoxide was purified in section (9) of the apparatus. Carbon monoxide from a tank was condensed in a%rapfilled with glass spheres and cooled in liquid nitrogen. The trap was then permitted to warm up, the first half of the distillate being rejected, while the second half was collected: analysis showed this to be 99.9% carbon monoxide. To prevent excessive pressure building up during distillation, a mercury safety valve was incorporated, which would release pressure a t 110 cm. A high-temperature coke was used having an analysis as shown in Table I.
TABLE I ANALYSIS OF COKE Ultimate analysis (dry, ash-free basis)
Proximate analysis
Fixed carbon Volatile matter Moisture Ash Total
Carbon 89.1 Oxygen Hydrogen 0 . 6 Nitrogen 0 . 3 Sulfur 10.0 - Total 100.0
Ash softening Temperature, C.
96.9 Initial deformation, 0.5 1207 0 . 8 Softening tempera1.1 ture, 1430 0 . 7 Fluid temperature] 1450 100.0
The coke was ground to under 8 mesh, U. S. standard sieves, and only the 8-12 mesh fraction was used. The Ramp> was initially evacuated and heated for three days at 1000 The sample did not have to be replaced during all reported experiments since coke was not consumed. There was no visible deposit of carbon on the coke when removed from the furnace after the experiments. Experimental Accuracy.-The reaction investigated was small in extent and slow in rate, hence accurate measurements and careful calibration of dead space volumes were necessary. The dead space volume of the reaction chamber was calibrated with helium at different temperatures, and since the walls of the reaction chamber were permeable to helium a t high temperatures, a correction for helium leak rate was applied. The effective volume of the entire reaction loop was calculated as the sum of the component parts of the system. The reproducibility of adsorption experiments was det,ermined by performing several duplicate experiments at each temperature investigated. The standard deviation of m, the decrease in volume of gas per gram carbon due to adsorption, was found toobe 8.4, 7.1, 7.1 and 14.8% at 1000, 900, 800 and 700 , respectively. Errors in the analysis of the carbon dioxide formed were not of prime importance since the major reaction was the formation of an oxygen complex on the carbon surface. Experimental Procedure.-Experiments consisted in admitting a measured quantity of carbon monoxide to the coke
.
f
50
100
150
2
Time (min.). Fig, 3.-Decrease in volume of gas with time (temperature 1000°, initial carbon monoxide pressure 56.40 cm.).
Vol. 56
sample, measuring subsequent pressure changes and finally analyzing the gas in contact with the coke. The coke saniple was thoroughly degassed by evacuation at a temperature 100-200° higher than that to be used in each experiment. After the temperature of the reaction chamber had been adjusted to the desired value, the sample under vacuum was isolated by means of stopcocks A and C (see Fig. 1). Carbon monoxide gas was then admitted into the reaction loop through stopcock B a t a pressure approximately known from the reading of a manometer in the gas storage section (8) of the apparatus. The mass of the gas contained within stopcocks ABC, in terms of the ratio p V/Y,was then determined as the mean of three values for three different settings of the gas buret volume (4). The experiment proper started with the admission of carbon monoxide into the reaction chamber through stopcocks A and C. The pressure inside the reaction loop was then increased to the desired value by adjusting the gas buret volume and the first pressure reading was taken. Readings were cont,inued a t suitable time intervals until the end of the experiment. Immediately after the final pressure reading, the gas in the reaction tube was sampled and analyzed for caibo~idioxide in an Orsat gas analysis apparatus.
Experimental Results Adsorption Experiments.-Figure 3 shows the change with time of the volume of gasin the reaction loop, per gram of carbon in the reaction chamber for a typical experiment. Similar curves were obtained for all experiments involving the interaction of carbon monoxide and the degassed coke sample. From such data the decrease in volume of gas ( m ) per gram of carbon was calculated as the difference between volume of gas the initial (MI)and final (Mz)
where 29.015 was the gram weight of carbon in the reaction chamber. Knowing m and the percentage of carbon dioxide in the gas at the end of the experiment, the following quantities could be calculated % carbon dioxide carbon dioxide formed, n = X 100
-
oxygen adsorbed m n CC.NTP/g. c (4) total caibon monoxide reacted = m n CC.NTP/g. C (5) At each of the four temperatures investigated, two types of experiments were carried out: experiments in which the initial pressure of carbon monoxide was nearly constant and the time of reaction was varied and ex eriments in which the time of reaction was held constant w h e the initial pressure of carbon monoxide was varied. Results for the first type of experiment were corrected from t8heactual initial pressures t o a constant initial carbon monoxide pressure of 60.0 cm. by means of corrections based on slopes in Fig. 5 . Correction factors for m were nearly constant a t 0.00050 cc./g. C/cm., while for n correction factors had the values 0.00016, 0.00036, 0.00016 and 0.00009 a t 700, 800,900 and 1000°, respectively. The actual initial pressures which were corrected to the standard pressure of 60.0 cm. were usually within 6% of this value. P , T h e results, corrected to constant initial pressure, are plotted in Fig. 4. The curves labeled '0' give the amount of oxygen adsorbed on the carbon surface, without making any assumptions as to the chemical state of this bound oxygen. The quantity of oxygen adsorbed on the carbon surface approaches a constant value at 700 and 800°, while at 900 and 1000° adsorption continues even after 90 minutes. Experiments at constant time of reaction are shown in Fig. 5. I t is seen that oxygen adsorption approaches saturation a t an initial carbon monoxide pressure of about 30 cm. The amount of carbon dioxide formed is directly proportional to the initial carbon monoxide pressure. Figure 6 shows that the ratio of carbon dioxide formed to oxygen adsorbed is a linear function of the time of reaction and also directly proportional to initial carbon monoxide pressure. The data relating the dependence of oxygen adsorbed on
+
t
A STUDYOF REACTIONS OF CARBON MONOXIDE WITH COKE
Julie, 1952
I
I
coz 0
10
I
I
I
I
20
30
40
50
I
60
70
80
90 0
700°C.
b IO
20
30
40
50
60
coz
70
80
I
TIM E
Pig. 4.-Varistion
78 1
(min.1,
of total carbon nionoxide reacted, oxygen adsorbed and carbon dioxide formed with time of reaction (initial carbon inonoxide pressure 60.0 om.).
700°C.
c
PRESSURE
Fig. B.--Variation
I
’
(cm ) I
of total carbon inonoxide reacted, oxygen adsorbed and carbon dioxide formed with iiritial carbon monoxide pressure (time of reaction 90 ininutcs).
782
ARSOLDE. REIF
VOl. 66
/ /
0
10
IOOO~C.
,
I
I
I
I
I
I
20
30
40
50
60
70
80
TIME
Fig. 6.-Variation
t
/
.
!
**
IO0O"C. I
10
90
*
I
30
20
,
40
PRESSURE
Cminl.
A
,
I
1
50
60
70
C c m ).
of the ratio of carbon dioxide formed to oxygen adsorbed with time (initial carbon monoxide pressuw 60.0 cm.) and with initial carbon monoxide pressure (time of reaction 90 minutes).
the initial carbon monoxide pressure a t constant time ot reaction was treated using Langmuir's adsorpt,ion isotherni
where s is the gram molecular quantity of gas adsorbed per grain of adsorbent, SO is t,he value of s requircd to form a rnonolayer, p is the pressure of adsorbate gas (carbon monoxide) and KL is the Langinuir adsorption equilibrium constant. Rewriting equation (6) in the form P l S = (1ISOKL) -I- (p/sa) (7) it is possible to evaluate SO and KL from a plot of p / s against p , as shown in Fig. 7 . Figure 8 indicates that the values of SO thus obtained increase exponentially with rixe in temperature, and that the maximum quantity of carbon dioxide was formed a t around 800". 2500
/I
I
3
700'C.
1500
-
& 1000 0 LT 4
3110'A 2110"-
0
10
20
30
PRESSURE
Fig. 7.-Langmuir
40
pco
50
60
10
l"10-
CmJ.
-
adsorption isotherm plot. r;
The equilibrium composition of the carbon dioxide-carbon monoxide-carbon system was estimated from data by Rossini, et aE.,11 and Austin and Day,12 as correlated by Wu.13 The equilibrium percentage of carbon dioxide is 36.0, 10.5, 2.6 and 0.6% a t 700,800, 900 and 1000". A comparison of these figures with the percentage of carbon dioxide obtained in cxperiments a t an initial carbon monoxide pressure of 60.00 cm. and after a reaction period of 90 minutes indicates. that the ratio of carbon dioxide formed to carbon dioxide a t equilibrium was 0.02, 0.17, 0.44 and 0.60 a t the respective temperatures. Thus, the equilibrium composition was approached far more rapidly a t higher than a t lower teniperatures. Desorption Experiments.-Figure 9 shows the rate a t which pressure dropped when the system was pumped with
Fig. 9.-Change of pressure with time of evacuation over coke surface saturated with carbon monoxide.
(11) F. D . Rossini, et ol., J . Research NaLl. BUT. StUndaTd8 R.P. 1951, 82, 143 (1945). (12) J. B. Austin and N. J. Day, Ind. Eng. Chem., 93, 23 (1941). (13) P. C. Wu, "The Kinetics of the Reaction of Carbon with Carbon Dioxide," 9c.D. Thesis, Massachusetts Institute of Technology, 1949.
A series of experiments was carried out a t 1000, 900 and 800°, in which the carbon surface was saturated with carbon monoxide for 120 minutes. After desorption periods of varying length, carbon monoxide was again admitted for 90 minutes and the fresh adsorption measured. The amount
" 5810''W
9
wg
3x10.'2.10"-
1m.*-
\"\, I
I
700.C.
TIME
,
Cminl.
.
of carbon monoxide readsorbed was assumed equal to t,hat desorbed during the evacuation peri0.d. A small correction was applied to compensate for incomplete saturation of the carbon surface during t,he first adsorption period. Assuming that the desorption process was a first order reaction expressed by the equation
Figure 10 gives a plot of logloK L against 1/T from which the non-exponential factor KOLwas evaluated as lo2.*atm.-l and the heat of reaction EL as 8.0 kcal., with a standard deviation of 0.51 kcal.
01
(CO) --+-CO t8hedesorption rate is given by the expression dz/dt = (Y(U - 2)
I50
(8) 4
X CI
(9)
where‘5qO) represents an adsorbed carbon monoxide inolecule, (Y is a rate constant, “$’ is t,he quant’ity of carbon monoxide desorbed in time “t” and “a” is the saturation sorption. Values of ‘‘a” were determined from sets of four experiments a t each temperature investigated. The values obtained were 3.32 X 10-3, 2.76 X lo-’ and 7.78 X lo-‘, inin.-’, a t 1000, 9bO and SOO”, respectively. Surface Area Determination.--A quartz bulb holding approximately 25 g. of the same coke sample was attached to the reaction loop and degassed for 18 hours a t 230”. Duplicate nitrogen adsorption experiments were performed at 77.4”K., after dead space calibration with helium under identical conditions. The B.E.T. monolayer adsorption was calculated a t 0.096 and 0.093 cc./g. coke, corresponding to a mean surface area of 0.38 sq. meter/g. coke, assuming a surface area of 89.6 sq. meter/qillimole nitrogen.14 This value for the B.E.T. monolayer adsorption is of the same order as that for the Langmuir monolayer adsorption SO of carbon monoxide, which averaged 0.070 cc./g. coke for the four temperatures investigat,ed. These estimates are in marked contrast to those obtained by Gadsby, et a1.,8 in their study of coconut shell charcoal. They found that the maximum quantity of carbon monoxide adsorbed in experiments at 850”, namely, 0.34 cc./g. carbon, was equivalent to only 0.5% of the surface of the carbon as estimated from a n adsorption isotherm with water vapor at room temperature. Such differences with previous experience are believed due to differences in characteristics of “carbons.” The present investigation was carried out with a high-temperature coke, il fuel having surface characteristics markedly different from either an active carbon or a graphite.
Theoretical The following equations are proposed to account for the reaction found between the degassed coke and carbon monoxide
2
n
100
-800
=I
-
Ln
-8 50
-7%
-100
-050 N
XI
m
-
-900
-9,5c
I
080
Fig. lO.-Arrhenius
005
090
(10)
kZ
+ ( C O ) -3-coz + c
To evaluate the rate constant k 3 for the formation of carbon dioxide, it may be assumed that the reactions represented by equations (10) and (11) occur exclusively on the carbon surface. At equilibrium, the rate of formation of (CO) will equal its rate of desorption (kiPco
+ k4Pcoz + ksPooJ (1 - k o ) = (kz + k3Pco)ko
+ co
cos +( 0 ) + co
(12)
nC0= wcoeco - ( I C+, k 5 ) ~ c (1 o - eCo) (17) Since K L = (kl/ka), and uiith ka < kl and Pcoz
(13)
T w o types of oxygen complex a.re postulated in cquations (12) and (13), since no evidence is given in this paper, to show whether the complex (CO), formed ~7hen carbon monoxide is adsorbed on carbon, is equivalent to the complex (0),formed when carbon dioxide reacts with carbon. Assuming that equation (10) holds, the ratio k l / k z is equal to the Langmuir adsorption equilibrium constant KL. The temperature dependence of an equilibrium constant may be expressed in the form K L = KOLe - ( E L / R T )
and rate of formation of carbon dioxide is given by
comparatively negligible
k5
(141 S. W. Benson and D. A. Ellis,
where OCO is the fraction of active surface covered by (CO), whence
(11)
The following reactions are assumed to take place only after appreciable quantities of carbon dioxide have been formed ki c + COI + (CO)
100
(15)
co I_ ( C O ) GO
095
plot of the variation of KL, k3 and k: with temperature.
ki
(1950).
783
A STUDY OF REACTIONS OF CARBON MONOXIDE WITH COKE
June, 1952
(14)
J. A m . Chem. Soc., 72, 2005
and IiCOz
= k3pCO8CO
(19)
If equation (19) holds, Rcol should be proportional to PCO at constant Bco. This is shown by Fig. 5 . Values of OCO may be assumed constant for each temperature since the quantity of oxygen adsorbed after 90 minutes increased only at 900 and lOOO”, and even at these temperatures continued adsorption was slow. Estimates of ka may be calculated by means of equations (18) and (lo), and the values are given in Table 11.
ARNOLDE. REIF
784 TABLEI1 EVALUATION OF RATECONSTANT k3 Temp., OC.
1000 900 800 700
Rcor/,Pc_q, eco
0.032 .930 .894 .SO6
cc. min.
atm. - 1
g. - 1
0.0270 .0146 .0114 .0044
k8,
6. mole min.
1.437 .779
-1
x x
atin. - 1 g. - 1
10-8 10-8
.632 X
.271 X
A plot of loglok3 against 1/T is also shown in Fig. 10 from which ko3 was evaluated as 10-5.6g. mole min.-l g.-I and Es as 13.2 kcal. with a standard deviation of 0.19 kcal. The values of the rate constant "CY" for the desorption reaction (equation (8)) were converted to the conventional rate constant by means of the equation kp = asoeco
.
VOl. 66
was dependent on previous adsorption of carbon monoxide on the coke surface. At 700 and 800", the maximum quantities of carbon dioxide formed at the end of a 90-minute reaction period were only 2 and 17'%, respectively, of the equilibrium composition values, and adsorption had practically ceased a t the end of this period. On the other hand, at 900 and lOOO", respectively, 44 and 60% of the equilibrium carbon dioxide composition was attained within 90 minutes, and adsorption was still appreciable a t the end of this time interval. From these data it is concluded that the initial reaction a t all temperatures investigated was the adsorption of carbon monoxide on the degassed coke surface. This initial reaction is represented by the equations
(20)
where so is the monoIayer adsorption of carbon monoxide in g. mole g.-I and OCO is recorded i n Table 11. A plot of log,, k2 versus 1/T is also shown in Fig. 10. The factor, koz, was evaluated as g. mole min.-' and Ez as 33.6 kcal., with a standard deviation of 0.15 kcal. From a knowledge of kz and K L , k , was evaluated. The non-exponential factor kol was loo.* g. mole min.-l atm. g.-I and the energy of activation E1 mas 41.6 kcal., with a standard deviation of 0.53 kcal. Discussion In estimating rate constants, the concentration of carbon dioxide was assumed to be negligible. Had this assumption been unwarranted, either of the reactions, expressed by equations (12) or (13), could account for oxygen complex chemisorbed on the carbon surface. Evidence that carbon monoxide mas indeed directly adsorbed when admitted to a degassed coke surface was provided by the initial, relatively rapid decrease in pressure, which fell off with time. I n the absence of carbon dioxide in the carbon monoxide initially admitted, carbon dioxide formation
ka co + (CO) --+
GO,
+10-~,6 c
-
e _ R13.200 Tg.molemia.-lalm.-lg.-l
3 -
In the course of a 90-minute reaction period, the extent to which carbon dioxide, formed in the course of the reaction, reverted to carbon monoxide was probably small at 700 and 800". However, a t the higher temperatures of 900 and lOOO", the carbon monoxide-carbon dioxide equilibrium was more nearly attained and t,he slonr rates of oxygen adsorption found at the end of the 90-minute period is partly attributed to the reactions expressed by equations (12) and (13). Acknowledgment.-The author is deeply indebted to Dr. H. H. Lowry, Director of the Coal Research Laboratory, for his direction of the work and to Dr. J. P. Fugassi, Professor of Chemistry, for his good advice. Thanks are due to Dr. A. A. Orning of the Coal Research Laboratory and to Dr. J. T. Ihmmer of the Mellon Institute for aid in the design of equipment.
1
