Carbon Dioxide Methanation on a Ruthenium Catalyst Peter J. Lunde”’ and Frank L. Kester’ Hamilton Standard. Division of United Aircraft Corporation. Windsor Locks, Connecticut 06096
An empirical expression was determined for the rate of methanation of carbon dioxide with hydrogen from 200 to 350” on a 0.5% ruthenium on alumina catalyst according to 4H2(g) C02(g) F ~ :2H20(g) 4- CH4(g) 43 kcal. Feed ratios (H2:C02) of 2:l and just under 4 : l were investigated by using partly reacted feed gases to reduce reaction rates as necessary to keep the test reactor isothermal. Sixtyeight tests were conducted using a small (0.5 ir?. diameter X 1.75 in. length) reactor filled with commercially available catalyst held at a constant temperature and atmospheric pressure. A complete description of the experimental apparatus is given. Theoretical equilibrium compositions for the feed mixture from 200 to 800” are DreSented.
+
+
Introduction The catalytic hydrogenation of carbon dioxide to methane 4H,(g)
+ CO,(g) === CH,(g) + 2H20(g)+ 43kcal
(1)
is often called the Sabatier reaction, after the Belgian chemist who investigated the hydrogenation of hydrocarbons using a nickel catalyst (Sabatier, 1902). The Sabatier reaction is becoming of commercial interest for the manufacture of substitute natural gas from the products of fuel gasification. The reverse reaction, of course, is called steam re-formation and is a commercial method for hydrogen manufacture. This paper developed from work performed under NASA Contract No. NAS 9-9844 to investigate the Sabatier reaction as a step in reclaiming oxygen within closed cycle life support systems. Carbon dioxide from the cabin atmosphere is thus combined with hydrogen to produce methane and water vapor which is electrolyzed to provide oxygen for the cabin plus one-half the hydrogen required for the Sabatier reaction. The rest of the hydrogen is provided from the electrolysis of stored water, which produces breathing oxygen as a by-product, reducing the proportion of available carbon dioxide which must be reacted and assuring excess carbon dioxide in the feed mixture. The Sabatier reaction is a reversible, highly exothermic reaction which proceeds a t a useful rate at the low temperatures required for high yields only when a catalyst is used. Dew, White, and Sliepcevitch (1955) studied this reaction using a nickel catalyst. Karn, et al. (1965), investigated a ruthenium catalyst for hydrogenation of carbon monoxide and carbon dioxide a t high pressure. This paper examines the kinetics of the Sabatier reaction using a ruthenium catalyst, and derives from experimental data a correlation describing the kinetics of this catalysis in the 400 to 700°F temperature range a t 1 atm pressure. Thermodynamics. Equilibrium compositions for hydrogen and carbon dioxide mixtures a t 1 atm are shown in Figure 1, which was prepared with the aid of a computer program developed by United Aircraft Research Laboratories using free energies from Wagman, et al. (1945), to predict all possible reactions. Carbon and carbon monoxide are possible products, as well as methane and water vapor. The Sabatier reaction represents actual equilibra for molar feed ratios (H2:COz) of over 3.5:1 a t temperatures from 400 to 700°F. Low temperatures favor high con-
’
Present address, The Center for the Environment & Man, Inc., Hartford, Conn. 061 20. Present address, Institute of Gas Technology, Chicago, Ill 60616.
versions. At 700°F and a feed ratio (H2:C02) of 3.5:l the equilibrium conversion of H2 is only 9070, while a t 400°F it is about 99%. As the feed ratio falls below 3.5:1, carbon becomes thermodynamically stable a t increasingly higher temperatures. At 3:1, carbon deposition is possible only below 500°F while a t 2:l it is stable below 1100°F. Carbon monoxide formation is thermodynamically possible above 700”F, where the reaction encounters the wellknown “water-gas shift.”
+
+
CO, 111 ==+ CO H,O This reaction does not cause a limitation in maximum operating temperature because any carbon monoxide formed is quickly converted to other products downstream in the reactor’s subsequent 400-700°F temperature zone which is necessary for a practical yield.
Experimental Section Catalyst Selection. Thompson (1964-1967 conducted a Sabatier catalyst screening program for the U. S. Air Force. Four catalysts were experimentally evaluated: (1) nickel (80% Ni and NiO) (on Kieselguhr); (2) 0.5% ruthenium (on alumina); (3) 0.570 rhodium (on alumina); (4) 0.57” cobalt (on alumina). Ruthenium and nickel were found to be the most active catalysts for promoting the Sabatier reaction. Nickel, however, presented the following operating problems: ( 1) slow deterioration over the test period. attributed to sulfur poisoning; (2) reactor startup in hydrogen was advisable to assure reduction of nickel to its most active form; (3) carbon deposition was reported at 650 to 700°F. Ruthenium had none of these problems, was somewhat more active than the nickel as a catalyst, and had a potential for even more activity if heavier loadings of the m e t d on the substrate was used. Consequently a 0.5% ruthenium catalyst on ys in. x in. cylindrical alumina pellets was selected for further investigation. The prepared catalyst, Englehard type “E,” was purchased from Englehard Minerals and Chemicals Corp., Newark, N. J. The manufacturer furnished no lot number or other specific information but did disclose that the catalyst performed within the limits of their internal specifications. Superficial examination of the pellets indicated the ruthenium did not penetrate more than 0.1 mm into the alumina, indicating that pore diffusion was not likely to be important in the performance of this catalyst. The bulk density of the pellets was measured as 1.0 g/cc. Approach. The ruthenium catalyst is relatively new, and there are little published quantitative data from
v8
Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 1 , 1974
27
Figure 1. Thermodynamic equilibria for various mixtures of carbon dioxide and hydrogen. which the kinetics can be determined. Consequently, an experimental apparatus was designed and a program to acquire rate data from a small isothermal reactor was begun. Steady-state conversions were determined from chromatographic analyses of the test reactor inlet and outlet streams. Mass flow to the reactor was held steady for runs a t several temperatures, giving data for calculation of the reaction activation energy, to describe the temperature dependence of the reaction rate. Additional runs were made a t two bath temperatures to allow more precise determination of the basic reaction rate constant. Feed flow ratios (H2:COz) from 2 : l to nearly 4:l were investigated. Temperatures of 400 to 700°F were selected 28
Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 1, 1974
for activation energy data accumulation since a t temperatures over 700°F the reaction proceeds rapidly and is complicated by carbon monoxide formation, while 400°F is low enough to allow virtually complete conversion of the feed in a practical reactor. Apparatus. A simplified schematic for the experimental apparatus is shown in Figure 2. Details of the experimental reactor are shown in Figure 3. Hydrogen and carbon dioxide were fed continuously to the experimental apparatus, using commercial laminar flow meters with electronic readouts for flow measurement, The test reactor, a tube filled with catalyst and held isothermal by immersion in a molten salt bath, was made small so that the conversion of unreacted feed was
COhTESSFRS
I A.
T
Figure 2. Apparatus for collection of reaction rate data. Solenoid valves are shown actuated for sampling at reactor inlet.
Figure 3. Test reactor for rate data collection.
low but measurable a t the lowest operating temperature. At high temperatures part of the feed was passed through a large “supply reactor” operating a t 650”F, providing a partly reacted feed for the test reactor in order to reduce the reaction rate and thus the reaction heat. Sampling. All samples except the inlet feed were fed to a Bendix process chromatograph at very low flow (Figure 2). The inlet feed sample was taken by actuating two three-way valves which directed the entire feed stream through the chromatograph sampling valve. A sample could be taken in this manner without affecting the feed flow rate. When other samples were being taken the
pressure drop of the chromatograph sampling valve (about 0.6-0.8 psi) was simulated in the feed line with a precision metering valve set to maintain a constant pressure upstream of the reactor so that there was no transient when the feed sampling valves were actuated. Heating tape and heated and insulated boxes for valves were necessary throughout the sampling system to prevent water from condensing in the sample lines. The process gas chromatograph analyzed feed and effluent gases using samples of equal volume for analysis. Peaks were automatically gated, attenuated, and recorded. Peak heights were then manually measured and logged as raw composition data. Components analyzed were HzO, COz, Hz, CO, CH4, Nz, Ar, and 0 2 , but the last three were not present in significant quantities. Carbon monoxide was present in products of reaction above 650”F, but only in trace amounts. A typical chromatogram is shown in Figure 4. Each analysis took 13 min and was always repeated before data were recorded. The chromatograph was calibrated by using pure COz, HS, and CH4 a t several pressures in the 0-1 atm range. The chromatographic peak heights then corresponded to partial pressures of the calibrated constituents. Water was calibrated indirectly using Sabatier reactor effluent, in which the partial pressure of water vapor was necessarily exactly twice t h a t of the methane which was already calibrated. After the chromatograph was calibrated, the hydrogen peak signal became erratic during the data collection phase. Successful gas analyses were continued by taking the correct hydrogen partial pressure as equal to the difference between analysis pressure and the sum of the other constituents as determined from their peak heights and calibration curves. The accuracy of this method was confirmed later in this work after the electronic malfunction responsible was repaired. Test Reactor. The reactor used for the actual kinetic study (Figure 3) was made from 0.5-in. stainless steel tubing (0.43 in. i.d. X 1.75 in.). The catalyst charge of 3.58 g (about 80 ?& in. X ys in. cylindrical alumina pellets coated with 0.5% ruthenium) filled the 4.15-cc reactor tube. The entire reactor assembly consisting of a feed preheating coil, thermocouples, and sample tubes was submerged in an oven-heated molten salt bath to keep the reInd. Eng. Chem.. Process Des. Develop., Vol. 13, No. 1, 1974
29
then the temperature dependence of the reaction rate constant can be described by the general Arrhenius relationship
k f " ( T )= k e x p ( - E , / R T ) The equilibrium constant is derived thermodynamically from data by Wagman, et al. (1945), according to methods described by Pitzer and Brewer (1961), using heat capacit y data summarized from Bureau of Mines Bulletins (1949, 1950, 1960) as
K , ( T ) = exp[(1.0/1.98'i)(j6,000/Th' f 34,633/?'h 16.4 In
Tb
+ 0.00557Tk) -I- 33.1651
(5)
where TI, is the temperature in "K. The final form of the rate expression is then
-d[P,.,,]/dt
=
k exp(-E,,/RT) x
TIME
Figure 1.Typical chromatograph.
actor isothermal. Thermocouples were installed in the inlet stream (T2), outlet stream (T4), at the center of the reactor (T3), and on the reactor wall (T5). Samples could be taken from the feed (after preheat) (S4), effluent (S5), and externally from the feed before entering the reactor. Experimental Operation. Experimental work confirmed the manufacturer's claim that the ruthenium catalyst need not be reduced with hydrogen before use. Exposure to oxygen when hot, however, destroyed the catalyst activity. Reaction rates were low at low temperatures (400500°F) and the reactor wall and center thermocouples then agreed within k1"F. At higher temperatures when the reaction rate became high it was reduced so that the temperature differential was held below 20°F by partly reacting the inlet feed before it entered the differential reactor. Using this technique good experimental data were taken from 400 to 700°F in a single reactor with a constant feed rate.
where k (the rate constant) and E , (the activation energy) and n (catalyst coefficient) are constants to be determined from the experimental data. Since the modification of gas-phase kinetics from which this expression is derived has no theoretical justification, care should be taken that use is restricted to the range over which experimental data were collected. To find the constants eq 6 is rewritten and integrated over the reactor length
where T is the contact time, usually expressed as space velocity, S ,= 1 / ~ Performing . the integration on the left side and rearranging gives
Discussion Since the mechanism for ruthenium catalysis is unknown, gas-phase type kinetics are proposed for the reaction using the technique followed by Dew, et al. (1955) 4H,(g)
+ CO,(g) 72HHLO(g)+ CH,(g) I? ,
where k , and h , represent reaction rate constants for the forward and reverse reaction, respectively. Thus
exp( I') Evaluation of exp(Y) can be made with a digital computer by using small values of dPcclz ( i e . , Uco,) and stepping through the reactor using a trial value for n
-d[P,y,.l/dt = ~ ~ ' ~ ~ ~ ~-~fi,"[Pttl~iI'"[P~it~In ~ ~ , l " ~ ~(2)~ ~ where n is an empirical constant equal to 1 for pure gasphase kinetics. Dew, et a!. (1955). found n = 1h for a pressure of2 atm, and n = 1for 30 atm. When equilibrium is achieved, -d[Pco,]/dt = 0, and eq 1becomes
K,.
=2,1,,,
~ I ' " i
s,
l'
1' =/J,,,,*,,
QCO,
[P,I~Oli[P, t l : l /
~ ~ ~ ~ ~ 1 2 1 ~ ~ (3) , l ~ 1 4
Ind. Eng. Chem., Process
Des. Develop., Vol. 13, No. 1 , 1974
(5)
[P,.021"[P,21"1 - ~~~~~I,,lf~~~tI~OI"li/K,"~T~ ( i = CO,. H L ,CH,. H,O )
where after each increment new Pi are calculated = hi//?,. =
and the empirical exponent applied to the exponential coefficients cancels so that the equilibrium constant, K,, is defined as in classical thermodynamics. Koting the K e n = k f n / k r n ,ifeq 2 is rewritten
30
exp(Y) =
P I , "
....,
F i g u r e 6. Selection of t h e catalyst coefficient
the rate constant for each individual run shown in Tables I and 11. Rate constants from Table I1 were further processed to obtain an average reaction rate constant. The lower inlet flow ratios of H2:COz in each series were within the range for which carbon deposition was thermodynamically stable (Figure 1 ) . No evidence for such deposition was observed in these tests in performance degradation or after post-test catalyst examination.
Data Correlation Activation Energy. The values of Y shown in Table I used a catalyst coefficient of 0.225 and were calculated by a digital computer program according to eq 7. A leastsquares fit was made according to eq 8 to determine the activation energy E,. The plot of eq 8 for these runs is shown as Figure 5. An activation energy of 30,320 Btu/lb mol of COz was determined from the slope (-E,,") of the line in Figure 5. The reaction rate constants tabulated in Table I were calculated for each run using this activation energy. An arithmetic average of these values for rate constant for the usable runs was 0.15% X 1O1O. which cornInd. Eng. Chem., Process Des. Develop., Vol. 13, No. 1 , 1974
31
Table I. Activation Energy Series SUMARV OF EXPERIMENTAL UATA AFTER PRELIMINARY PKOCtSSING 3.58 GRAMS CATALVST U S E 0 I N 4.15 E L REACTOR I N L E T / O U T L E T P A R T I A L PRESSURES SUM TO I N L E T / W T L E T TOTAL PRESSURE SPACt VELOCITY CALCULATED AT REACTOR TEMP AN0 PRESSURE I N L t T FLOW i C U B I C F T I H R ) MEASURE0 AT 19 P S I A AN0 73 DEG F lkPUT, OATA OF 7 - 2 4 - 7 2 CATdLYST COEFFLCIENT= O.ZZ500E
TEST NUULLIER
ABSCISSA XINPJ
ORDINATE VINPI
Il/OEG R I
* ~ i e c o o.coi15i * ~ Z O . O O o.ociizo 5~0.10 521.0~ +521.10 521.40 521130 523.30 524.00 525.00 525.10
3.3544 4.0732 3.9713 4.2306 5.0496 4.7937 5.5539 5.4613 5.6478 6.1293 5.9929
c.ooi120 D.OOI~O~ 0.001085 0.001076 0.001036
G.COI042 o.ocioi3 c.ooo98n C.000995
528.10 5 ~ 9 53Q.70 531.0C 532.~0 533.~0 1534.120 $38.50 539L50 540.1~ 540.20 540.80 541.10 541.10 542.00
C.000943 ~c.000945 2 ~ O.COO923 C.OOOd99 c.000~99 C.COOUBC c1.0~0~62 C.CCllb3 D.OC1133 0.001124 0.001111 o.001099 C.OCIOSU 0.001058 o.001053 542.10 F.OG1036 543.00 C.001047 544.00 O.OC1029 544.:10 C.OG1020 544.20 O.GG1012 544.36 0.001OC6 545.00 c.co102i 545..10 c.0~1012 545.30 0.001006 546.10 0.000999 C.CCO969 547.00 548.~0 C.OOO~MO.
6.6091 6.6274 7.006% 7.~669 7.3220 7.7087
6.8607 3.3132 3.9248 4.Cl67 4.2528 4.4~18 5.1035 5.C846 5.~928 5.4771 4.9328 5.5029 5.5874 5.Y222 5.98b2 5.5024 5.6042 5.7672 5.9260 6.4725 6.4450
OC
DELTA= 0.1COOOE-03
REACTOR TEMP. IOEG F)
t A C T I N = C.30320E
hALL TEMP (OEG F I
4 ~ 4 .
433. 433. 443. 462. 469. 5c5. 500. 527. 552. 545.
REACTION RATE COkSTANl ~ . 1 3 39tic 404. 4 3 3 . c . 1 5 4 9 ~ IC IC 433. 0.1399t 443. 0 . 1 5 c ~ t i o 4 6 2 . 0.24C3E 1 c 469. c . 1 6 4 2 ~ i c 505. c . 1 9 ~ 3 t i c 5C0. 0.1863t 1 C 520. c . 1 4 6 9 ~ IC 543. c . 1 6 2 3 ~ i c 537. 0 . 1 5 7 3 k 1C
587. 589. 612. b39.
600.
596.
b23. 652. 652. b77. 7c0. 4C0. 423. 430. 440. 450. 485. 485. 490. 505. 495. 512. 520. 528. 534. 519. 528. 534. 541. 572. 5bo.
64C.
67c. 694.
400. 423. 43C. 440. 45c.
480. 480. 485. 498. 491.
505. 514. 521. 525. 512. 519. 524. 532. 558. 555.
C.1325E 1 0 c.1387~ i c C . l s 5 3 E 1: 0.13C5t 1C c . i j 7 b t IC c . 1 5 ~ 1 ~i c c . 3 6 4 ~ ~i c 0.1395t I C IC C.162Ct C.155LF IC C.1623E 1C c.171it CI IC 0.1695t 0.1663E 1 C c . 1 5 4 c t 10 0.1762t I C C.liC7t 1C 0 . 1 6 1 3 t 10 C.1545k I C 0.19C3E L C C.184bE IC, 0 . 1 4 4 1 ~ IC c . 1 3 ~ 5 t CI 0.1485t IC 10 C.1563t I C C.17C8k c . 1 9 7 7 t IC
SPACE VELOCITY Il/HRI 2854. 3005. 5585. 5649. 1766. 6175. 6337. 5828. 5.~~ 5874. 5835.
5426. 5125. 515:. 5235. ~ b 6 4764.
95
IYLFT FLJW ICFH) 0.2117 c.2117 0.4112 c.4169 C.4172 c.4239 0.4243 C.4256 1 0. . 4 2 3 1 c.4231 C.4231
IULET CG2
'IUTLET GO2
INLET H2
14131
IATM)
IAlMl
0.2036 ?.20c2 c.1978 c.1911 C.2047 ".2015 c . 7 ~ 3 4 r,.1993 C.1-86 ?.1305 c.2~36 c.197~ r . 2 ~ 5 5 r.1899 r.1714 ;.I575 c . 1 7 1 ~ "1645 r.1799 2.1573 C.1'9O r.lbOC
c.4227 c.4223
C.4223 C.4223 0~ . 4. 2 2 3 0.4223 0.4223 C.4352 C.4^63 C.4963 0.4963 n.4~63 . 0.4?63 C.4363 c . 4 ~ 7 C.4C7C
3.8~33 "'.809i 3.~022 c.8~35 Z.8683
?.e:?& 0.805n 3.6RlL c.6818 0.6826 '.hPTC
c.1477 c.1243 ?.I255 l.1229 2.0714 ~."714 c.c714 r.3893
1 . 1 1 8 1 :.53C5 ? . c y 8 3 c.43.52 *.?916 r . 4 2 9 0 ,.3846 '1.42'2 1 . ~ 5 9 4r.18c5 ?.059i n.ib05 " . ~ 5 a z c.ie.5 Q . 3 8 7 4 C.6175 2.3415 *.3374 P.6654 C.34322.3393 C.6633 '.3435 3.3388 C.6633 r.3435 '.33a2 c.6633 c.3455 ".3379 l.6~13 '.3455 7.338C " 6 6 1 3 c~. 3 4 ~ 4 ?.3359 c . 6 6 3 4 r.3414 -.333h n.6634 C.33'5 1 . 3 2 6 4 C.5117 C.3JLZ 7 . 3 2 4 1 ' . 5 1 2 L
4855. 53C2. 4956. 4967. 5316. 5062. 51~2 5192. 5276. 5324. 4 7 4 1 . C.4?77 4 7 8 2 . C.427" 4 8 1 5 . 0.4C77 7 4 8 1 6 . C.4?7' ".3" 4 3 3 9 . C.4"7i' C.3 4 5 7 7 . c . 4 ~ 7 r~ 4624. 0.4073 c . 3 3 ~ 2 462b. n . 4 2 7 ~ c . 3 3 ~ 2 4 6 6 8 . r.49711 C.3216 4 7 2 5 . C.477C C.3275 4177. c.4~7': r.2647
OUTLET
INLtT
H2 (ATMI
HZO (ATMI
c d i e a ~9 . r
c.7828 r , r 0.7893 3.r r . 7 8 7 0 1.r C . @ ~ 2 7? . c r.7767 r.p 0.7~22 r.p P.6757 ? . l P h C ' r.6274 3 . r q p 7 c . 5 8 9 ~ n,ror;8
0.5079
:.pq%
c.2197 r.21~7 C.2IY7 C.4f71 "2197 c . 3 i n r c.2965 0.287E C.3CCL C . 2 5 Q P P.3059 c.1256 5 . 5 ~ 2 3 0 . 1 2 ~ 1 r.5r23 ~ . i 2 e e *.5r2? C.6578 C.C r.hLf.3 P . p C.hA.77 C,' C . 6 3 S i :.C r.63br c.r '.6117 ~ . r r.6121 r.r r . C i L e
1.r
C.59'5 C.'-7t2
".r O.IC51
r~~..4J-~-.IC81 c81
rei -.322" r.3209 C.3182 1.3125 0.2581
r.4476 - . i ~ i h
C.4427 3.44211
0.1577
r.3622 c.3722 C.359'. p.3CC6 r.ir2i
543 r.1543 c.1541 ".I587 ?.I583 p.39~1
3lJTLET
INLET
3JTLET CH4 (ATN) 3.0058 > . r 7 ? r n.? 0.0110 c . r i c 7 9.0 3 p r.ri37 n.1 ?.cq69 r.rz25 r.? c.c112 ? . ~ ? ? 2 c.0 0.~111 r . c 4 5 3 0.0 3.022~ C.1494 0.0517 0.0714 p . 1 4 ~ r ~. ~ 4 7 5 c . 0 7 2 3 r . 1 7 3 0 c.0476 ?.')E61 n . l h l E *.04?0 C.C&l& h2O IATMl
CH4
IATMl : . ~ 1 1 5 n.0
0.2932 1.2778 r.2865
r.ic9c
-.PI??
p.9
".PI47 ?.pie5 ?.r336 ^.r333 z.r73r, ".7477 "1314 ".1'~1 1.157r r.175~
*.9
ri.1~90 n.1C90 n.3217 r . l d 9 P r . ? 8 5 9 0.1499 0 . 4 1 7 8 n.1513 C . 4 3 8 C C.1579 z.5441 c . 2 5 2 6 r.5L5n r.2526 ?.5482 r . 2 5 2 6 C . r r s 5 C.0 r . p l " 9 @.0
c.1973
c.0 r.n *.n
0.1457 0.1380 0.1424 C.1600 c.19a 3.2397 0.2245 r.2735 c.2740 c.2156
0.@02?0.0-54 CLCfa @.PO73 0.9093 3.0168 O.C>-66
3.~165 C.9 ?.?236 C.q595 @.e728 ~",?~5~-~C_.~~~8~~ c.1552 c.n796 c.0552 c.0~~5 1.9
0.C997 ?.in16 c~._i:x. ?.?2"6 P.0779 0.1088 ".757R C.O783.-C,1-283 n . 4 3 2 ~ r.2C24 c.2215
r.2-1r
P.9787 7.n782
-.z:L~ c . ? l 8 2
'Not processed to cbtain activation energy-inlet flow low. tNo: processed to obtain activation energy-flow ratio too nigh. f Kot processed to obtain activation energy-too near equilibnum.
Table 11. Reaction R a t e Series S U M A R Y OF EXPERIMENTAL U A I A AFTER PRELIMINARY PROCtSSlNG 3.58 GRAMS CATALYST USED I N 4.15 EL REACTOR I N A E T / D U T L E T P A R T I A L PRESSURES SUM TO I N L E T l W T L E T TOTAL PKESSURE SPACt VELOCITY CALCULATED AT REACTOR TEMP AND PRESSURE l N L t T FLOW ( C U B I C F T l H R l MEASURED AT 19 P S I A AN0 7 3 OEG F I h P U 7 DATA OF 7 - 2 4 - 7 2 CAT1LYST C U E F F I C I E N T TEST NUMBER
ABSCISSA XINPJ ll/OEG R I C.GC093 @.COO935 2.000942 C.COJ947 O.CGO954 @.Coo954 0.C00950 C.CO3945 o.Co0943 C.COO925
0.22500E
UROlNATf YINPI
~~
26C.CO 561.00 562.C0 563.00 564..CC 565.Co 566.GO 567.'CC 568.00 56P.CO 570.00 571.00 572.00 573.00 *,574.co 575.CC 576.00 577.00 578.C0 579.C0 58C.00 581-00
C.001009
O.COlC13 c.001014 C.001021 0.licioib C.CClOlb 0 . ~ ~ i o i 3 o.001012 0.001011 3.601011 C.001010 3.CGlOCY
* ~ o ~t r o c ~ s e for d
7.4153 7.i887 7.C268 6.9666 6.9101 6.7169 7.C887 6.8884
6.5717 7.2110 5.9441 5.7676 5.5660 5,6628 6.2543 5.6175 5.7556 5.5894 5.6253 5.8M01 5.7625 5.8508
OC
DELTA= 0 . 1 0 0 0 0 E - 0 3
REACTDR TEMP. (DEG F I 624.610. 602. 596. 588. 588. 593. 5YM. 601. 617. 531. 527. 526. 519. 524. 524. 527. 528. 529. 529. 530. 531.
~
kALL REACTION TEMP RATE (DEG F4 COhSTANT 589. C . 2 1 5 7 t 1C 588. C . Z O b 8 k 1 C IC 583. 0.19586 5 8 1 . C.2OCCE I C 1C 578. C.2132k 5 7 9 . C.1740E 1 0 579. C.2355E I C 58C. 0 . 1 8 O I E 1C 580. c . 1 8 7 8 t i c 583. C . 2 0 4 b t IC 520. C . 1 8 5 6 t 1 0 520. 0 . 1 6 5 t t 1 0 519. c . 1 3 7 5 ~ I O 520. C . 1 6 9 2 t 1C 5 2 n . c . 2 8 ~ 4 I~C 5 2 0 . 0.1494E it 520. c . l b 3 6 t 1c 520. c . 1 3 6 r t IC 520. C . 1 3 9 2 t 1C 521. 0.1796E 1 C 1C 520. C.1572E 1C 52C. C . 1 7 t C t
reaction Fate determinatim-poor
EACTIN= 0 . 3 0 3 2 0 E SPACE VELOCITY ll/HR) ~5483. 4909. 4721. 4587. 4361. 4566. 4548. 5'222. 5234. 5872. 5670. 4971. 4932. 4552. 4358. 4231. 4347. 4581. 4754. 5058. 5382. 5644.
INLET FLOW ICFHI 0.41167 C.4C67 0.4'59 0.4059 0.4959 0.4321 0.4321 0.4321 0.4321 C.4321 C.4311 C.4311 c.4311 C.4311
c.4311 C.4311 0.4082 c.4082 C.4067
C.4oH2 0.4082 0.45511
05 IVLtT COZ IATMI r.3617
6.3237 0.3151 C.3C17 p.!G55 0.17Q8 C.lqSr C.2143 c.2zsP C.26:: C.26?3 C.?220 p.2:'i: C.1924 0.1755 C.16C4 0.2828 p.3~33
C.3C89 C.3223 C.3316 C.3317
OUTLET INLET C02 H2 IATMI lAlM1 3.299P ? . 6 6 8 6 '1.2943 ? . . L d 4 8 C.2923 : , . 4 1 4 2 7.2824 6.3468 3 . 2 7 2 1 C.2367 2 . 1 5 0 9 C.3C28 " . l b 5 4 3.3599 C.16CS C . 4 9 9 0 -.186r c.wi2 ?.2C27 C.7482 ^..2436 C.7514 '.2?76 ?.5567 7.1887 c.51,43 2.180775.4154 m.1584 c . 3 4 r 4 1.1523 C.2454 2.2755 0 . 2 6 ~ 2 ?.29~3 c.37~9 9.30?3 0 . 4 3 5 0 7.5442 ".310b 7.3195 5.6270 1.3253 3.67bP
OUTLCT H2 IATHI C a Z C
r.24i2 ".221" C.18tC P.12te C. 2 " 1 4
lNLt1 e20
1UTLET H2il
INLET
CH4
CH4
IATM)
IATMl
IATMI
IATMI
".r
3JTLEl
~~~Z5~'~1_..??,l_Z_qs
:a1315 ?.lPh5
?.31R4 r.33'2
r.24C':
r.35a7
C.1663 3.1563 *.0944 >.I664 C.1217 C.l-B.lz ".3214 7 . 4 * ? 1 P.1638 ?.2"48 .35W n.434r c . 1un3 n.22>& n.4556 ? . I 7 7 4 c . 1 9 6 4 ".3248 C.1894 C.3713 3.1981 O.3372 P.098R ?.j_6_8_4 r . 3 6 2 2 c . 1 4 5 ~ 0.3rch r.9717 c.1521 r . 4 5 1 5 r . c 2 . 7 3 5 1 C.? -?.>17.5. r.h5tl p.p " . r 7 3 7 C.9 3.n368 C.4R2C C . 1 5 6 7 '.2142 '.C77S C.IL6-3 r.4459 -.2r52 p . 7 ~ 7 9 c . 1 ~ 3 ~c . 1 2 ~ r.3571 r . 2 6 9 2 C.3116 C.1361 ?.l-?. c.3965 r . i h i r 0.1999 r.2555 r . 3 3 v C.eZ2C.3 - . 3 8 9 5 y . 4 l H 6 C.1984 C.713.lc.173~ c.2179 p.3063 *.343r r.1554 r.3153 :.22a8 n . z 6 ~ ~ . 1 1 4 9c . 1 3 4 5 V.7255 C 6.-r C.3778 C . 1 8 r 6 @.@79B" . I 6 3 4 3.0478 C . r 5 6 6 C.0993 "."9P3 1.0175 0."473 C.5453 l . r 3 R 6 3.C337 2 . r 6 7 5 '.9 C.5837 ? , e @
analyses.
pares with 0.15664 X 1O1O determined by extrapolation to the ordinate of the line in Figure 5 according t o eq 8. Reaction Rate. Table I1 summarizes the processing of the reaction rate runs using a catalyst coefficient of 0.225 and the activation energy of 30,320 Btu/lb mol of COz de32 Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. l , 1974
~
termined in Table I. The average rate constant for the usable runs was 0.1769 x 10-lo. This is the preferred value since these runs covered a wider range of compositions than the activation energy series. Catalyst Coefficient. Values of the catalyst coefficient
the average rate of reaction using eq 6 for comparison with the actual rate calculated from the change in compositions across the reactor. Examination of Table I11 shows that eq 6 predicts the measured reaction rate with a standard deviation of about 20%.
102
Conclusions The Sabatier reaction ?HA CO 2HL0 CH, + h e a t was examined theoretically and experimentally using a 0.5% ruthenium catalyst and the reaction rate was correlated over the experimental range with a standard deviation of the error of 20% by
+
+
dt
>
60 FEXL\T CJ\?IRSP?\
3 ':
90
IC0
1SlJ I T E P
Figure 7. Use of t h e rate constant series to evaluate t h e catalyst coefficient.
of 0.125 to 0.25 were found to provide reasonable fits of the experimental data when analyzed as in Tables I and
11. Figure 6 summarizes the values of activation energy, rate constant, and the changes in fit to eq 8 which resulted for catalyst coefficients of 0.125 to 0.30, expressed as the standard deviation of the points used in fitting eq 8. The standard deviation of the fit to eq 8 limits the value of n to 0.25 or less. The agreement between the rate constant values determined from the intercept of eq 8 and from the average values of the individual experimental runs is best a t n = 0.22,5, and this was selected as the best value for n. This choice was confirmed by examination of the reaction rate runs for the change in calculated rate constant as inlet conversions varied (Figure 7). For high values of n, the rate constant tends to rise with increasing conversion of the feed. For low n values, the converse is true. The least effect is seen for n = 0.225. When n = 0.25 (not shown) the result is almost equivalent. Evaluation of the Correlation. The previous discussion established n = 0.225, E , = 30,320 Btu/lb mol of C O z , and i: = 0.1769 x These values are to be used in eq 6 t o predict reaction rate over the experimental temperature range. To evaluate the precision of such a prediction, Table I11 (available as supplementary material) was prepared in which the experimental data were used to predict
I L
LAP
where K,(T) = exp[(l.O/l.987)(56,OOO/T1,~ + 34,633/Tk 16.4 In T k + 0.00557 Tk) 33.1651. Sixty-two usable experimental runs were made in a 0.5 in. diameter X 1.75 in. length isothermal reactor a t 1 atm with from 0 to 85% conversion in the inlet feed gases and feed flow ratios (Hz:COz) of 1.9 to 3.9. and temperatures from 400-680°F. Correlations were made which established n = 0.225; E, = +30,320 Btu/lb mol of C O z ; and k = 0.1769 X 10lo hr-1 ,trn-0.125.
+
Literature Cited Dew, J . M . , White, R. R . . Sliepcevitch. C . M..ind Eng. Chern.. 47. 140 ( 1955). Karn, F. S , Shultz. J . F.. Anderson, R . E., ind. Eng. Chem.. Prod. Res Develop.. 4, 265 (1965). Pitzer, K . S.. Brewer, i.."Thermodynamics, Lewis and Randall," 2nd ed. McGraw-Hill. New Y o r k , N . Y , 1961, pp 165-156. Sabatier, P., Sendersen. J. B . , A c a d . Scr.. 134. 689 (1902). Thompson, Edward B. J r . . Technical Documentary Report No. FDLTDR-64-22, Parts I-V. 19'64-1967. On sale from Office of Technical Services. Deparment of Commerce, Washington. D . C. a s A D 608 4 1 1 . A D 4 6 5 4 2 2 . A D 4 7 6 5 7 1 , A D 4 8 1 984.AD816508. U. S. Bureau of Mines Bulletins 476 (1949):477 (1950):584 (1960). Wagman, D . D , Kilpatrick, J. E . . Taylor. W. J.. Pitzer, K . S . . Rossini. F. D., J . Res. N a i Bur S f d . . R P 1634. 34. 143 (1945)
Hc.ccii.c~!/or W L > L P U J a n u a r y 15. 1973 Accrlpff'd August 8. 1973
Supplementary Material Available. T a b l e I11 a n d Appendix A will appear following these pages in t h e microfilm edition of this volume of t h e journal. Photocopies of t h e supplementary m a terial from this paper only or microfiche (105 x 148 m m . ' 2 0 ~reduction. negatives) containing all of t h e supplementary material for t h e papers in this issue may he obtained from t h e Journals Department. American Chemical Society, 1155 16th St,. N.W.. Washington, D. C . 20036. Remit check or money order for $3.00 for photocopy or S2.00 for microfiche, referring to code n u m b e r
PROC-74-27.
Ind. Eng. Chem., Process Des. Develop., Vol. 1 3 , No. 1 , 1974 33
