Ind. Eng. Chem. Process Des. Dev. 1986, 25, 487-494
Carbon Deactivation of Fischer-Tropsch Ruthenium Catalyst Suryanarayana Mukkavllllt and Charles V. Wlttmann Department of Chemical Engineering, Iliinois Institute of Technology, Chicago, Illinois 606 16
Lawrence L. Tavlarldes' Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 132 10
Carbon deactivation of a 0.5 wt % Ru/y-Al,O, surface-impregnated catalyst was studied by using a Berty continuous-stirred,gas-soli reactor (CSGSR)-gaschromatograph setup. The experimental variables were as follows: temperature, 473-573 K; pressure, 2-6 atm; weight hourly space velocity, 0.85, 16.5 h-'; H,/CO feed ratio, 3 and 2; and synthesis time, 0.5-5 h. Carbon deposited in a synthesis run was measured by integrating the methane evolution profile during catalyst reduction at 723 K in H,. Significant amounts of carbon were deposited, increasing to several monolayers during 5-h synthesis periods. The methanation rate decreased as the synthesis continued, while the seiectivii for C,-C, hydrocarbons showed a maximum during the initiil stages of deactivation. The kinetic data could be correlated by assuming both hydrogen-assisted CO dissociation and hydrogenation of surface carbon to be rate determining. The turnover numbers for methanation (NCH,) and carbon deposition (NCIRU) are given by eq 3 and 4.
partially hydrogenated CH, fragments. Support for the mechanism of direct hydrogenation has come mainly from rate modeling (Ollis and Vannice, 1975; Vannice, 1977; and Dautzenberg, 1977). Some of the questions unanswered by this mechanism, such as why alcohols can initiate synthesis but cannot contribute to chain growth by dehydrocondensation, are discussed by Ponec (1978). Recent evidence obtained in pulse reactor experiments (Gikis et al., 1977; Rabo et al., 1978), infrared studies (Ekerdt and Bell, 1979; King, 1980; Kellner and Bell, 1981~)and isotope substitution experiments (Sachtler et al., 1979; Biloen et al., 1979; Kellner and Bell, 1981b; Winslow and Bell, 1983) supports the mechanism in which surface carbon is an intermediate in FTS over Ru. For example, carbon was deposited from 13C0and subsequent exposure to l2C0 and H2 resulted in the abundant production of 13CH4and hydrocarbons containing several 13C atoms (Biloen et al., 1979). Surface carbon formed by CO dissociation is not only an important intermediate in FTS but also a precursor to less reactive forms of carbon that deactivate FTS catalysts. Everson et al. (1978) observed a marked decline in the BET surface areas of the used catalyst to as low as 40% of the fresh catalyst value concurrent with a significant drop in methanation activity. Dixit and Tavlarides (1983) observed a 24% drop in H2 adsorption for a Ru/y-A1203 catalyst with deactivation. Temperature-programmed surface reaction (TPSR) and electron spin resonance (ESR) studies by Gikis et al. (1977) on Ru/A120, indicate the presence of three forms of surface carbon: (a) a weakly sorbed CO species, (b) a more tightly sorbed active amorphous carbon, and (c) a graphitic carbon species that is essentially unreactive toward oxygen. McCarty and Wise (1979) found two forms of surface carbon (a and /3) on Ni/A1,03; the reactivity of the acarbon toward H2 is about 10000 times the reactivity of the 0-carbon at 550 K. McCarty and Wise (1979) and Zagli et al. (1979) observed a slow deactivation of the active carbon species. Recently, Biloen et al. (1983) concluded that only a small fraction of the total carbon overlayer belongs to the re-
I. Introduction The characteristics of ruthenium catalysts for the synthesis of methane and higher molecular weight hydrocarbons from carbon monoxide and hydrogen have been discussed in a number of recent publications. The prime motivation for such interest in ruthenium is its specific activity for methane synthesis and its high selectivity for the formation of straight-chain hydrocarbons at higher pressures. Much of the research to date has been aimed at elucidating the Fischer-Tropsch synthesis (FTS) mechanism. Only recently has the problem of deactivation of FTS catalysts by carbon deposition been addressed. This aspect of the reaction is of prime importance in regards tQ the selectivity of hydrocarbons produced and the lifetime or usefulness of the catalyst in commercial operation. The objective of this study is to model the deactivation of a commercially available and well-characterized ruthenium FTS catalyst on the basis of intrinsic kinetic data for the rates of synthesis and coke formation. The emphasis is on the development of useful rate models based on existing FTS mechanisms. Rate data are obtained over a wide range of conversions (3-98% for CO) to be of practical use. Rate models for synthesis and coke deposition describing catalyst deactivation as a function of the carbon deposited on the catalyst, instead of "time on stream", will be developed. Thus, these rate models would be of particular use in the design and optimization of FTS catalytic reactors. 11. Literature Review A review of the literature indicates that the principal questions in the current understanding of the mechanism of FTS are (i) whether synthesis is initiated by the direct hydrogenation of CO to form oxygenated intermediates or by the dissociation of the adsorbed CO to carbon which is subsequently hydrogenated and (ii) whether chain propagation occurs by repeated CO insertion into the adsorbed alkyl-type intermediate or by polymerization of Present address: Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY 13210. 0196-4305/86/1125-0487$01.50/0
0
1986
American Chemical Society
488
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986
PH
R
mI1
W
r-------
-
TO
Inj 0 He
TO VENT
1 TO
Inj A He
4+
FGl
EP1
%Z
BP2
?
Figure 1. Experimental setup. BP, back-pressure regulator; D, drain; E, H20 + O2eliminator; FCI, mass flow controller; H, heater; PF, particle filter; PG, pressure gauge; PH, preheater; R, reactor; RM, rotameter; SH, safety head; SV, GC sampling valve; T, C2+ + H 2 0 trap; TCI, temperature controller; VS, solenoid valve; WM, wet-test meter.
action intermediates but that these species are linked to the total carbon reservoir in a reversible fashion. Thus, there is strong evidence to support an FTS mechanism in which CO dissociatesto form surface carbon, which is hydrogenated to form CH, ( x = &3) intermediates. The rate-determining step is usually taken as the further hydrogenation of one of these CH, species, leading to the deactivation of the FTS catalyst. At present, no rate model for FTS can describe carbon formation and synthesis rates as a function of the surface carbon. The present research is aimed at developing rate models based on existing FTS mechanisms to describe the carbon deactivation of a Ru FTS catalyst. These objectives are accomplished by the following experimental studies to obtain intrinsic kinetic data and measure carbon formed on the catalysic.
111. Experimental System A Berty continuous-stirred, gas-solid reactor (CSGSR) with an on-line gas chromatograph (Figure 1)was used to obtain kinetic data. As shown by Berty (1974) and by Mahoney (1974), the Berty CSGSR has low dead volume and can be operated at high enough internal circulation rates to eliminate interparticle transport resistances. The feed system includes valve arrangements to switch between
argon, hydrogen, and CO + H2 feed instantaneously using solenoid valves. The gas flow rates were controlled by using a Tylan mass flow controller, FC 360. Product gasses passed through a heated (200 "C) six-port Varian sampling valve, an activated charcoal-drierite packed column maintained at 0 "C to remove C2+hydrocarbons and water, another six-port Varian sampling valve, back-pressure regulators, and a wet-test meter before being vented. The first of the sample valves was used to analyze C1-C4 hydrocarbons on a '/8 in. X 6 ft Porapak-Q column using FID. The second sample valve was used to analyze CO, C02,and CH4 on a '/8 in. X 6 ft Carbosieve-S column using TCD. Product analysis was performed on a Varian 1860GC with a multilinear temperature programmer. The peak areas were measured by using a Hewlett-Packard 3390A integrator. The catalyst bed temperature was controlled, using a Chromel-Alumel Type K thermocouple connected to a Barber-Colman 520 temperature controller, to within *l "C of the set point. Gas-phase reactor temperatures were monitored by using another Chromel-Alumel Type K thermocouple. Materials. High-purity Ar(99.995%), H2 (99.99%), and premixed and analyzed CO + H2 feed, obtained from Matheson, were passed through Matheson high-pressure particulate filters, Model 6183, to eliminate particulates
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986
Table I. Methanation Rate and Coke Deposited vs. Temperature‘ run no. run time, min T,K PCO,atm 9 30 523 0.59 30 0.58 15 523 30 0.56 24 523 523 30 0.56 25 60 0.66 523 16 0.71 523 11 90 0.67 523 90 17 180 0.68 523 19 0.74 300 22 523 0.74 300 523 23 30 0.056 573 26 0.050 30 27 573 300 0.077 573 31 0.098 300 573 32 30 1.0 473 33
PH~ atm ,
NcH,, s-’
2.33 2.34 2.41 2.53 2.35 2.52 2.52 2.35 2.50 2.52 0.66 0.63 0.89 1.07 2.97
0.0641 0.0597 0.0622 0.0660 0.0515 0.0428 0.0440 0.0298 0.0227 0.0247 0.255 0.250 0.211 0.211 0.0015
489
coke/Ru 0.203 0.210 0.208 0.265 0.527 0.776 0.743 0.876 1.302 1.230 0.254 0.278 0.668 0.693 0.157
“Experimental conditions: P = 4 atm, H2/C0 feed = 3, WHSV = 0.85 h-I.
larger than 7 pm. The gases were further purified by using AIRCO Model 98 high-pressure oxygen and water eliminator to reduce O2 to 0.1 ppm and H 2 0 to 0.5 ppm. The GC carrier gas, He, was passed through a Supelco Carrier Gas Purifier to remove O2 and H20. Air (total hydrocarbons as CH4
