Hydrogenation of Carbon Dioxide on Nickel-Kieselguhr Catalyst J. N. DEW', R. R . WHITE, AND C. 31. SLIEPCEVICH Department of Chemical and illktallurgical Eiagineering, Cnizersity of Michigun, A n n Arbor, Wich.
I
XCREASISG demand for natural gas as a fuel and chemical raiT- material, coupled with limited gas reserves, has focused considerable interest on utilizing the t,remendous coal reserves as a possible source of gaseous fuel. An attractive process for the production of gaseous fucl from coal involvcs the preparation of a synthesis gas, containing carbon monoxide and carbon dioxide, by the reaction of coal with oxygen and superheated steam, and the subsequent catalytic hydrogenation of the gas to methane and other hydrocarbons. Studies on the rates of hydrogenatiori of carbon monoxide and carbon dioxide are needed to evaluate the process and t o design the equipment properly. This paper presenk a portion of a systematic study that includes the investigation of a nickel-kieselguhr cat.alyst, over a \Tide variety of reaction conditions. The rates of react'ion between carbon monoxide and hydrogen at atmospheric pressure were st,udied by Akcrs ( I ) . This study was extended by Pursleg ( 7 ) to include pressures greater than atmospheric. The spt,hesis of methane from carbon dioxide and hydrogen at, 1 atmosphere was investigated by Binder ( 2 ) . These studies have been supplemented by the work of Doerner ( 5 ) :n h o investigated the diffusion and adsorption of hydrogen in the siinie catalyst,. I n the present investigation, the reactions between hydrogen and carbon dioxide that produce methane, carbon monoxide, and water were studied a t pressures to 30 atmospheres. The effects of pressure, composition, and temperat,ure on the rates of methane and carbon monoxide formation were det,ermined. Initial reaction rates were calculated from experimental data obtained in a differential reaction system, and t'hese rate data were correlated by rate equations developed from the theory of heterogeneous catalysis. The most probable mechanisms for the reactions were determined on the basis of t,hese equations.
a water cooled condenser and condensate trap, through a backpressure regulator, and viere metered in a wet-test meter and vented t o the atmosphere. A detailed description of this equipment has been reported previously ( d J 7 ) . All gases were commercial grades and were supplied in conimercial 1A cylinders. Carbon dioxide was obtained from the Pure Carbonic Co. and contained a maximum impurity of 0,5yo air. Electrolytic hydrogen \vas supplied by tlie Bird Co. and contained less than 0.1% oxygen as the only impurity. The catalyst used in this investigation was supplied by thc HarshaIT Chemical Co. and was designated as Reduced Kiclrcl 88, Lot KO.219-1-11. The catalyst was the same nicliel-liieselguhr type used by Akers ( I ) , Binder (Z), Doerner (51, and L'ursley ( 7 ) . The catalyst, pellets were approximately i,'3 X i / 3 inch cylinders and had an average weight of 0.0495 gram. Their hulk density in t,he reactor vias 94 pounds per cubic foot. An ai,proximate annlysis of t,he catalyst was nickel, 59.4: silica, 18.2; carbon, 5.0; sulfur, 0.0670; and traces of iron and aluminuni. OI'ERATISG PROCEDURE
X run was made by passing a feed gas of a definite composition over the catalyst bed a t a constant pressure, ternperaturc, and flow rate. Feed and product samples were collect,ed and operat'ing conditions were recorded after steady-state conditions xere reached. A change in operating conditions started a r u n The pressure in the reactor was set at, the desired value. The approxiniate temperature desired at t,he catalyst bed was obtained I)y placing the proper pressure on the reactor jacket containing the hoiling Dowthcrm. The feed rate and feed compositions u-ere adjusted by observiug t,he pressure drop across the calibrated orifices. An analysis of the feed gas on the Orsat apparatus verified t h e feed composition. Reaction \vas observed immediately following tlie clitinge in tlie operating conditions by a change in the temperature of the catalyst bed. The catalyst temperature was then regulated by manual adjustment of the pressure on the boiling Dowtherni until the reaction attained equilibrium a t the desired iemperature. Steady-state conditions were continued approximately 1 hour before the data were recorded and gas samplcq n-crc taken. Thc folloTving data were recorded for each run:
EQUIPMENT AND MATERIALS
The equipinent was designed for the study of gaseous reactions at preesures to 30 atmospheres and temperatures to 750' F. The flow control system consisted of t\vo parallel feed systems, each capable of controlling the flax rates of a gas from a feed manifold. Each feed gas floxed from a manifold through two pressure-reducing valves, a pressure controller, a needle valve, and an orifice in series. The gas streams were then mixed and passed through a back-pressure regulator which maintained a pressure on the feed system greater than 30 atmospheres. Thus the pressure in the reactor could be varied without affecting t,he flow control system. The mixed gases passed into the reactor and donnn-ard through inch seamless the catalyst bcd. The reaction tube was a 36 X steel pipe. A ":isinch thermox-ell was centered in t,he reaction tube and ran its length. The temperature in the catalyst bed was measured by a 1/8-inch pipe-type thermocouple. A perforated steel plate attached to the thcrmowell supported the catalyst bed. -44-inch pipe jacketed the reaction tube and contained pressurized boiling Don-t,herm rl-hich controlled the temperat,ure in the reactor. After leaving the reactor, the product gases flowed through 1
Run number Date of measurement' Time of measurement Pressure in reactor Temperature of Dowtherm boiling in reactor jacket Temperature in catalyst bed Orsat analysis of feed gas Orsat analysis oi product gas Infrared analysis of product gas Wet-test meter data (t,emperature, barometric ptwsuw, pressure differential, and time per revolution) The data were collected on a daily schedule, followed !,y the reactivation of the catalyst by partial oxidation. This oxidation
Preaent address, Continental Oil Co., Ponca City, Okln
140
I N D U S T R I A L A N D E N G I N E E R I N G CHEMIS,TRY
January 1955
\vas accomplished by passing carbon dioxide (containing 0.5% air) continuously over the catalyst between each daily series of runs. L~ETHODSOF ANALYSIS. The concentrations of methane and carbon monoxide in the product gases were determined by a Haird Associates Model B infrared spectrophotometer and were recorded on a dry basis. The carbon dioxide content of the atmosphere within the instrument made the spectrophotometer unsuitable for accurate carbon dioxide analysis. Analyses for carbon dioxide were obtained from duplicate samples by absorption in potassium hydroxide in a conventional Orsat apparatus. PRELIMISARY TESTS. Prior to obtaining the experimental data, preliminary tests were made to determine the extent of the noncatalyzed or homogeneous reaction and the catalytic activity of copper which surrounded the catalyst during the experimental runs. These tests were conducted simultaneously, and it was concluded that these effects were negligible in the range of operating conditions encountered ( 4 ) . An 8-day test on the stability of the catalyst was conducted a t a pressure of 1.5 atmospheres and a reactor jacket temperature of 395" F. with a feed composition of 25% carbon dioxide and 75% hydrogen. The catalyst activity decreased as an exponential function of time, following a short induction period in which the activity increased. iifter several days of synthesis, the catalyst could be considered relativelv stable for tests conducted over a short time.
r
I
-m
'0
141
25
50
75
100 125 150 17.5 pHg+ POCe) (ATM 1
225
200
Figure 2
2 2ol
,
/< 4I
I
In
-
I75
EFFECT OF PRESSURE ON REACTION RATE Cog
1
+ 4HB-0Ct4 + 2+0
REACTION RATE CORREIATION COP
+ 4Ho
C b
+ 2kO
TEMPERATURE: 596OF:
30
Figure 3 25 n
2-
20
e \
tN
2 0"
-
15
n IO
5
0 0
5
10
15
20
25
30
PCOz (ATM.)
Figure 1
The test was started with fresh catalyst which had received no form of pretreatment. The initial activation of the catalyst was probably the result of the reduction of oxides which had formed following the original preparation and reduction of the catalyst. The subsequent deactivation is apparently thermal and not the result of sulfur poisoning since the feed contained no knoFn sulfur and the analysis of the catalyst a t the end of the test showed no increase in sulfur content. An effect similar to that of synthesis time on catalyst stability could be obtained more rapidly by a treatment in which the catalyst was reduced by hydrogen a t an elevated temperature and pressure. It is recommended that the maximum hydrogen pressure available be used during this reduction period. The catalyst deactivates rapidly a t high temperatures and it is important
to reduce the catalyst a t a tempcrature greatei than any teniperature expected during synthesis and to collect the experimental data in the order of increasing temperature. The stability of the catalyst a t a specific temperature can be improved further by maintaining the catalyst in an inert atmosphere a t that temperature for several days before starting the collection of data. ildditional exploratory data showed that the catalyst deactivation accumulated over several days of synthesis and became noticeable. In order to obtain reproducible data over several weeks of synthesis, it was found necessary to reactivate the catalyst each day by partial oxidation. This ovidation !vas accomplished by passing carbon dioxide (containing 0.5% air) continuously over the catalyst a t a rate of about 0.5 cubic foot per hour betneen each group of runs. These exploratory data indicated that the catalyst deactivation resulted from two simultaneous actions, thermal and carbon deposition. The thermal deactivation could be minimized by thermal treatments, while the deactivation I esulting from carbon deposition could be eliminated by partial oxidation. EXPERIMENTAL DATA
The experimental data were obtained from a catalyst bed, onepellet deep, formed by 25 Xi-88 pellets. The catalyst bed was preceded by a 3l/2-inch layer and followed -I, i~ ll/p-inch layer of copper pellets ( I / a X '/a inch cylinders). These copper pellets m-ere included in order to eliminate entrance and exit effects on the flow pattern of the gas in the catalyst bed and to improve the heat transfer charactelistics of the reactor. Prior t o the collection of experimental data the catalyst had received a stabilizing treatment TI hich consisted of reducing the
INDUSTRIAL A N D ENGINEERING CHEMISTRY
142
catalyst over several hundred hours of operation. These runs were conducted a t 596" F. It was observed that the level of catalyst activity could kip maintained constant for over 200 hours and that the order of data collection did not affect the results. Several random check points on the activity of the catalyst xere used Typical cxperimentd data are presented ia Table I.
TABLE I. TYPICAL EXPERIMEXTAL DATA (Complete experimental d a t a are available, 4)
xun No.
Temperature, F. Jacket Catalyst
7114
Reactor
pressure, Atm. 2.1 2.1 2.2 2.1 2.1 2.1 10.1
COz
Hz
COz
Hz
CH4
10.0
810
596
ploduct Rate S t d . C;
Composition (Dry Basis), Feed Product
10.0 9.9 10.0 10.1 10.0 10.0 9.9 20.2 20.0 20.0 20.1 20.1 20.0 20.1 20.1
Cb
Reaction R a t e , Mole/(Hr.) (Lb Catalyst)
Ft/Hr.
CHI
9.78 9.90 9.52 9.55 9.65 10.0s 9.99 9.87 9.48 9.29 9.13 9.20 9.19 9.51 9.94 9.94 9.63 9.43 9.36 9.14 9.14 9.15 9.20
0.015 0.028 0,037 0.041 0.035 0.024 0.029 0.046 0.060 0.074 0.089 0.093 0.089 0.074 0.049 0,032 0,059 0.090 0 122 0.141 0.148 0.148 0.133
Vol. 47, No. 1
CO 0: 004 0.009 0.014 0.015
...
0:009 0.004 ,..
o:oi4 0.018 0.024
EVALUATION OF D.AT.4
+
For tho reaction, C>02 4Hz = CHq 2H1,0, the generalized rate equation has 0 : 009 the following form in the case 0,009 of molecular adsorption without dissociation in which a surface reaction mechanism is rate coiltrolling and all other resistances are negligiblc (4):
+
0: 004 ,..
where 1' is the reaction rate, moles nic4mnc lormed per iinit of catalyst k is the reaction coilslant lor a specific catalyst, pcol, p ~etc., ~ are , partial pressures of carbon dioside, hrdrogen, etc. Ii1, Kg, etc., are adsorption equilibrium conytanta for the specific catalyst K is the cheiiiical equilibrium constant for thc reaction z is the total numher of molecular units 01 carbon dioxide in surface mechanism reacting up to and including rate-wnirolling step y is the total number of molecular units of hydrogen i n surface mechanism reacting up to and including rate-controlling step n is the number of active sites involved in rate-controlling step
TEMPERATURE ("E) o.6 800 750 700 650 600
550
500
p - ~ - - - - r - - - T - - ~
EFFECT OF 1EMPERATLRE ON REACTION RATE cop t 4N*-=-CR4+ 2v20
04 01
80 85 90 95 RECIPROCAL TEMPERATURE
100
x lo4 ( IPR)
105
Figure 4
catalyst in a hydrogen atmosphere for 50 hours, passing synthesis gas (carbon dioxide and hydrogen) over the catalyst for 300 hours, reducing the catalyst a n additional 1600 hours, and maintaining the catalyst in an atmosphere of carbon dioxide for 400 hours. Data were collected at the folloaing experimental conditions: Temp.,
O
F.
Feed Compn., % COz
Pressure, Atm.
Oo2
Two independent series of experimental runs were conducted to investigate the effect of the order of data collection on the precision of the experimental data and to test the stability of'the
-\d --
0 O l b FEED COM'?GITION 2 0 % ; 0 ~
2.1-30 2.1-30 2.1 10 2.1 10, 20, 30
0
LEGEND P = 3 0 ATM
0
P: I O A T M
O
P = 2 1 ATM
___
80 85 RECIPROCAL TEMPERATURE X
Figure 5
104 (
__
i
I /"Q)
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1955
0 P=x)
Am.
EFFECT OF FEED COMPOSITION ON REACTION RATE
+
CO, 4Hg-CH, TEMPERATURE:
143
If it is assumed that the reaction takes place between four absorbed molecules of hydrogen, y, and a n absorbed molecule of carbon dioxide, x, on the catalyst surface, Equation 2 becomes
t 2H,O 541 E
(3) Various other value5 were assumed for the constants, 2 , y, and n, in Equation 2. However, it was found that the values used in Equation 3 fitted the experimental data the best (4). CORRELATION O F DATA
Equation 3 can be written for const,ant pressure as follows: FEED COMPOSITION
%COz)
Figure 6
Experimental runs in which methane was added to the reactants indicated that the effect of the products, or the reverse reaction, is negligible. The resistance to mass transfer of the components between the gas stream and the surface of the catalyst was shown to be negligible by varying the mass velocity through the reactor while holding all other conditions constant. These tests showed that the reaction rate was independent of the mass velocity for the range of mass velocities encountered in this work. To determine experimentally the magnitude of the resistance due to diffusion of the reactants into the pores of the catalyst would require a t least two sizes of catalyst pellets of identical composition and porosity. However, only one pellet size was available. By the methods outlined by Wheeler ( 8 ) , it was calculated, using experimental measurements of the pore volume and surface area (a), that 70 to 100% of the catalyst surface is available to the reactants. I n the present treatment of the experimental data, the effect of diffusion, if any, has been included in the reaction constantR since diffusion is an inherent property of the catalyst pellets. I n this work the experimental data have been limited to initial reaction rates, ro, calculated from small conversions obtained in a differential reactor. Equation 1 can be simplified as follows.
where P i s the total pressure
I 2okC€l
5.kO
600 650 O I!?' 750 CATALYST TEMPERATURE (dl
'
Figure 8
Also, for constant composition, Equation 3 becomes
where 1: is the mole fraction of carbon dioxide and (1 - 5) is the mole fraction of hydrogen. The experimental data were plotted according to Equations 4 and 5-that
is
PCO.#H*:
(
)
-
'/'
versus pCo2. The best straight lines
were drawn through the experimental points, and the average values for k , K I , and K , were determined from the slopes and intercepts of these lines. For example, a t 596" F, the following average values were obtained: k = 7, K , = 1.8, and KZ = 0.24. Figure 1 is a typical plot. The slopes of the lines drawn through the points of constant pressure were slightly different for the experimental data of 541' and 596' F. As shown in Table 11, the calculated values of K1 and Ke show a n apparent pressure dependency. 'The experimental data obtained a t 672' and 747' F. were not sufficient to determine the effect of pressure on the adsorption constants. Equation 3 may also be written
INDUSTRIAL AND ENGINEERING CHEMISTRY
144
The experimental data may he presented on one straight line when plotted in accordance with Equation 6. All of the data for 596" F. are shown in Figure 2 plotted according to Equation 6 and with the ratio of adsorption constant, K1/K2, equal to 7.5 (as determined from Figure 1).
T.4BLE
Pressui'e, Atlll.
11.
.bSORP'TIOh' COSSTAXlX
Temperature, $41' K1 KI 2.26 2.26 2.17 2.10 2.01 1.91
0
10 15 20 2: 30
F.
Temperature, 596' F.
Ki
K2
...
0.12 0.19 0.20 0.213 0.23 0.24
... ...
1.96
0.22
,..
1.82
0.27
1.73
0.29
...
Vol. 47, No. 1
in Figure 8. Two values of feed composition, representing the observed variation, are given for each temperature. LOT-PRESSURE D . 4 ~ s . The experimental data obtained a t 2.1 atmospheres, the lowest pressure investigated, could be better correlated with values of x, y, and 1~ of Equation 2 equal to l!Z, 2, and 3, respectively. These values correspond to a surface mechanism in which a dissociated carbon dioxide moleculc reacts n i t h two adsorbed moleculeE of hydrogen. The adsorption constant for dissociated carbon dioside is negligible with respect to the adsorption constant for hydrogen, The initial reaction rate for this mechanism is represented by Equation 7 .
...
(7) Equation 7 may be expressed as a function of the total pressure, P, and the mole fraction of carbon dioxide, x,
The effect of pressure on the initial E,FFECT OF PRESSURE. rate of methane formation from the stoichiomctric fccd composition, 20y0carbon dioxide and 80% hydrogen, is shown in Figure 3. The curves through the experimental points are plotted according to Equation 5 and the values of the constants in Figure 4. The effect of temperature on the EFFECT OF TEUPERATURE. The constants, k and K1, in Equation 7 can he obtained 131initial rate of nietmlianeformation for the stoichiometric feed applying the conditions for a maximum point to Equation 8. composition is shown in Figure 5, EFFECT OF FEEU COUPOBITIOX. The effect of k(S - x)P6/2 [[I f ''lP(l - jZ) + 3K1p(1 - z)x"'] feed composition 011 the initial rate of methane for2x1'2 - = 0 (9) dx mation is shonn in Figure 6 for 541' F. The [l KlP(1 - X ) i curves for the individual isobars have been plot,ted Thus, for the mole fraction of carbon dioxide at the maximum according to Equat,ion 3 and the constants given in Table 11. reaction rate for a specific catalyst temperature, The effect of feed coniposit,ion on the init,ial rate of methane formation is shown in Figure 7 for 747" F. The curves are plotted according to Equation 3 and the constants of k , IC,, and K , equal to 12, 1.45: a n d 0.5, respect'ively. The rate data collected a t 2.1 atmospheres were plotted versus Figurrs 6 and i ?how t,hat t,he composition of the fced misture the mole fracbion of carbon dioxide in the reaction mixture. that produces t,he maximum initial reaction rate is a function of Curves Tvere draxm through the experimental points, and the the temperature. The mole fraction of carbon dioxide required mole fraction of carbon dioxide a t t,he maximum rate was selccted to produce the niasinium rate decreases as the temperature infor each temperature. These mole fractions were substituted crcz~ses. The ratio of the hydrogen adsorption constant t,o the into Equation 10 and the values of K 1 xere calculated. The carbon dioxide conptant, KlIK,, decreases Iyith an increase in values for K , and the corresponding maximum reaction rates temperature (Figure 4). The relative amount of carbon dioxide n-ere then substituted int'o Equation 8 and values for k were calrequired in the gas stream to produce a specific concentration on culated. The values for k and Ii, were found to be 0.33 and 1.5, thc cat,nlyst surface ie reduced as the temperatwe increases. 0.39 and 1.2, 0.39 and 0.87, and 0.56 and 0.67 for the temperaThe relationship bet.n-een the feed composition that, produces tures of %lo,5(36", 6i2", and 737" F., respectively. The logat,he maximum initial reaction rate and t,he temperature is show1 rithm of these constants are shon-n as linear functions of the rcciprocals of the absolute temperature in Figure 9. The failure of the reaction const,ant, k , to lie on the curve a t TEMPERATURE 800 750 700 650 600 550 500 the temperature of 672" F. is attributed to thermal deactivation. The data at 672" F. and 2.1 atmospheres were obt,ained following REACTION RATE CONSTANTS COp+4H2 -CH4+2H& several days of synthesis a t 747' F. The corresponding adsorp-
flilu
+
I
(OF,)
PRESSURE:
2.1 ATM.
u m
-EFFECT OF FEED COMPOSITION ON REACTION RATE
Coet 4 H E - C H 4 t 2 H 2 0
0.21
OS
I1
I
I
;
,
I 1
I ~
8.0 8.5 9.0 9.5 10.0 10.5 RECIPROCAL TEMPERATURE X IO* ( 1PR.I
Figure 9
0 FEED COMPOSITION (%COe)
Figure 10
'INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1955
tioii constant, K,. d o c ~ snot tlcviate from linearity. This linearity indicates that deactivatioii affects the amount of active surfacse area available but doe? uot affect the adsorption properties of the surface. This result is in :rgrcement with the results obtained by hfaurrr (6), who found that activity changes in an alumina-silica catalyst during the dehydration of 1-butanol had no effect on the adsorption constant for 1-butanol. The reaction constant, IC, arcounted for the change in activity. The cffert of feed compo8ition on the initial rate of methane formation at 2.1 atmospheres is shown in Figure 10. The curves have been plotted to Equation 7 and the constants given in Figure 9. CARBON WOSOXIDEF o R v CTIOY. Carbon monoxide is formed during the hydrogenation of cttrbon dioxide through the reverse 1 1 2 = CO H,O. Low converwater shift reaction, CO, sions t o carbon monoxide were TEMPERATURE ('E) encountered 800 750 700 650 600 550 I I 1 , throughout the REACTION RATE CONSTANTS range of reacC o t + H2 -CO + H2O tion conditions PRESSURE: 2.1 A T M investigated in t h e present 8 0 1, work. T h e largest effect of the reverse reaction on the driving potential, (pC02PH2) -
+
201 -1
( E S p )
145
The constants were evaluated for the pressure of 2.1 atmosphcrcs. The value of the adsorption constant for carbon dioxide was found to be negligible with respect to value for the adsorution _ _ constant of hydrogen. The logarithms of k and K1 are shown as linear functions of the reriprocal of the absolute temperature in Figure 11. I
-
+
0 0
2 o
-
-.
L#
-I
20- ,F'7?0 AT*67Z°F. 0 T * 596-F.
T-T-
15----
a
i
l
1
l
EFFECT OF FEED COMPOSITION ON REACTION RATE C02+& co+qo PRESSURE. 2 I ATM
I re: k PCdfPH2 -r- ,I (ItKIPH,)*
I
i
l
I
!
1
re-
sulted from rea c t i o n contlitions of fG.570 a r b o n dioxide, 13y0 hydrogen, 2% water, 1 % I c a r b o n monO.'' 8.0 sir 9L 25 Ih, oxide, and 0.5% RECIPROCAL TEMPERATURE X IO4 ( I / O R ) methane a t a Figure 11 temperature of 747" F. and u pressure of 2.1 atmospheres. For these conditions the driving potential is equal to
0
FEED COMPOSITION (%COS)
Figure 12
rios. The effect of feed composition thc initial rate of c-arboii monoside formation is shown in Figure 12. The curves arc plottod according to Equation 11 v:rlues of k and K , shown in Figure 11. OF PRT:SSIIRE.A correlation of the data over the range U I W was obtained by including the absorption term for carbon dioxide in the denominator of 1Squation 11. The effect ol pressui'(?on the initial ratc of carbon monoxide formation for a feed composition of 25% carbon dioside and 75% hydrogen and a temperature of 717' F. is shown in Figure 13. The curve is plottctl rtccording to 1Squatiou 11 and v:iluee of k , Kl, rind K s .35, 0.9, and 0.2, rcspcct,ively. OF TE:MIJERATVRE. The effect of temperature 011 the initial rate of carbon monoxi+ format,ion from the stoichiometric feed coinposition of 50% Icarhoii dioxide and 50% hydrogen is shown in Figurc 14 for thc prcssurc of 2. L utniospheres. 011
~7
-G
25-
0 '
4
.
ON REACTION RATE C q f HeCO t k 0 .
20
The effect of the reverse reaction is small, but it is not negligible. The quality of the rate data on the formation of carbon monoxide is not comparable to that of the data on the rate of methane formation, hut it is still satisfactory for correlation purposes. The generalized rate equation applied to the initial reaction hetueen hydrogen and carbon dioxide, Equation 2, was fitted by the procedure described previously to a series of experimental data collccted a t temperatures of 596 ', 672 ', and 747' F. The initial rate of carbon monoxide formation a t 541' F. could not be measured N ith any degree of acc'uracy. Thefie data were correlated according t o a surface mechanism in which a dissociated molerule of carbon dioxide reacts with a molecule of hydrogen in the presence of adsorbed molecular hydrogen and carbon dioxide. The adsorption constant for dissoriated carbon dioxide is negligible with respect to the constants for niolecularly adsorbed h, drogen and carbon dioxide. The initial reaction rate for this mechanism is represented hy Equation I 1
9
.IO------
Figure 13 TEMPERATURE EXPLORATION
series of tests was obtained over a range of temperatures from 380' to 1230" F. These tests were conducted to determine the existence of a threshold temperature and t o explore the effect of high temperatures and thernial deactivation on the rates of methane and carbon monoxide formation. ,411 additional
I N D U S T R I A LA N D E N G I N E E R I N G C H E M I S T R Y '
146
Vol. 41, No. 1
the effect of lower total pressure since in either case the etrective partial pressures of the reactants a t the reaction sites will be lowered. COh C LU SION S
The initial rate of methan? formation from carbon dioxide and hydrogen on a nickel-kieselguhr catalyst can be correlated by the eauation
1
I
1
I
I
I
over a range of reaction conditions of 540" t o 790" F., 2 t o 30 atmospheres, and 5 t o 90% carbon dioxide feed mixtures. The initial rate of methane formation at 2 atmospheres can be correlated more closely by the equation,
ON REACTION RATE
co, t H2-co 0 .I
FEED
0.08
t H&
COMPOSITION: 50%CQ 2.1 ATM.
PRESSURE:
The initial rate of carbon monoxide formation can be correlated by the equation,
0.06
kpCOz"2PH2
=
1'0
(I
+ K P H 2 + KLPCo,)Z
The feed composition that produces the maximum initial rate of reaction is a function of temperature. The mole percentage of carbon dioxide in the feed mixture decreases as the temperature increaees. 50 U
8 I
I
I
li feed gas containing 20% carbon dioxide and SO% hydrogen and a preseure of 30 atmospheres was employed throughout this test. Steady-state conditions were maintained approximately 2 hours following each change in reaction conditions. The temperature was increased incrementally until the maximum rate in methane formation had been covere'd. The temperature was then decreased incrementally to observe the thermal deactivation resulting from the high temperatures. These data are presented in a graphical form in Figure 15. It may be observed that a threshold temperature does not exist. Even though the concentration of methane in the product gas could not be detected by infrared analysis, it was possible to detect the existence of a reaction a t the minimum temperature, 380" F., by a slight rise in the temperature of the catalyst bed. The maximum rate of methane formation was obtained a t an approximate temperature of 950 O F.; however, because of the large thermal dcactivation that was evident, the significance of this maximum is doubtful. A change in the selectivity of the catalyst was noted from the reaction rates obtained following reaction a t the higher temperatures. The catalyst had deactivated with respect to methane formation while the carbon monoxide formation had increased slightly. Figures 3 and 13 sho.rv that a decrease in pressure has a similar effect on reaction rate. The resistance to diffusion of the reactant3 into the catalyst pores consumes some of the driving potentlal for the over-all reaction. This resistance may become appreciable for small catalyst pores. It is known that Pintering a t excessive temperatures destroys the internal surface of a catalyst. Therefore, it is conceivable that the diameters of the catalyst pores decreased in size during the exposure to the higher temperatures. The effect of smallcr pores on rate of reaction will be somewhat equivalent to
5
1
40
EFFECT OF TEMPERATURE AND DEACTVATION ON REACTION RATE FEED COMPOSITION: 20% Cop PRESSU;-iTM 0 COp f ~
3M)
400
H
500
~
~ f C2 k H O I --
600 700 800 900 lox) CATALYST TEMPERATURE (T)
,
I
-
I
1100 1200 I=
Figure 15
The selectivity of the catalyst is affected by thermal deactivation. T h r rate of methane formation is decreased and the rate of carbon monoxide formation is increased following the exposure of the catalyst to high temperatures. I t is indicated that the adsorption equilibrium constant; are independent of the catalyst activity. LITERATURE CITED
Akere, W. W.,and Thite, R. It., Chenz. Eng. P t . o g ~ . ,44, 553 (1945). Binder, G. G., and White, It. I t . , Ibid.. 4 6 , 563 (1950). Clarke, B. J., Ph.D. thesis, Univ. of Michigan, .Inn Arbor, in progress. Dew, J. X., Ph.D. thesis, Univ. of Michigan, -4nn Arbor, 1953. Doerner, W. A,, Sc.D. thesis, Univ. of Michigan, Ann Arbor. 1951. Naurer. J. F., and Sliepcevich, C. ll,,C k e m . Eng. PTOQT., &inposium Ser. 4 8 , No. IV. 31 (1952). Pursley, J. A., Ph.D. thesis, Gniv. of hlichigan, Ann Arbor, 1951; Pursley, J. A , , White, R. R., and Sliepcevich, C. X., Chem. Eng. P r o g ~ .Symposium , Ser., 4 8 , N o . I\', 51 (195%). Wheeler, A , "Advances in Catalysis." Val. 111, Academic Press, NewYork, 1951. RXCEIVED for review May 11, 1954.
L%CCEI'rrED September 8, 1954. Preiicnted before the Fourth Chemical and Petroleum Engineering Conferenoe of the Chemical Institute of Canada, Montreal, March 1-2, 1954. T h e research work constitutes a portion of an extensive program of fundamental studies on catalysis which is being sponsored by the Xichipan Gas
Association.
