Ind. Eng. Chem. Res. 1994,33, 1363-1372
1363
Effect of Oxidizing and Reducing Gas Atmospheres on the Iron-Catalyzed Formation of Filamentous Carbon from Methanol William L. Holstein DuPont Central Research and Development, P.O. Box 80304, Wilmington, Delaware 19880-0304
Iron-catalyzed formation of filamentous carbon from methanol was studied a t 500-600 OC by exposing iron wire to methanol partial pressures of 1.33-300 kPa in the presence of added partial pressures of water vapor, carbon dioxide, and hydrogen. Filamentous carbon formed from feedstreams containing only methanol, and this process was accompanied by metal dusting corrosion. Addition of a sufficient water vapor partial pressure prevented both carbon deposition and corrosion. The role of water vapor is viewed as being to maintain the iron surface in an oxide state, which is inactive for the catalytic formation of filamentous carbon. Carbon dioxide was found to be a much less effective oxidant than water vapor, while hydrogen did not prevent filamentous carbon formation. Carbon deposition from methanol occurs under conditions where it would not be expected from the equilibrium products of its gas-phase decomposition (CO, C02, H20, H2, and CH4). Introduction
It has long been known that iron and iron-based alloys corrode in hydrocarbon or carbon monoxide environments at elevated temperature (Hoyt and Caughey, 1959; Eberle and Wylie, 1959; Lefrancois and Hoyt, 1963). This corrosion process, often referred to as "metal dusting" (Prange, 19591, is associated with the deposition of carbon. A similar phenomenon occurs on iron catalyst particles (Boellaard et al., 1985). For both cases, the deposited carbon is filamentous in nature, and the process is often referred to as "filamentous carbon formation." The carbon deposition process is also catalyzed by many other metals. Baker (19891, Baker and Harris (19781, and Rodriguez (1993) have written informative reviews of the filament growth process, and a review by Grabke and Wolf (1987) discusses the more complex corrosion process. Filamentous carbon formation leads to corrosion of reactor walls and coking of catalysts. The high surface area of the corroded metal particles can further catalyze unwanted reactions, leading to yield loss. The carbon fibers serve as a source for collection of tars and pyrolytic carbon from thermal decomposition of hydrocarbons. The filamentary carbon can reduce heat-transfer coefficients, increase pressure drops, and, in extreme cases, even lead to complete plugging of the reactor or failure of the reactor walls (Colton,1981). The initiation of filamentous carbon formation from bulk metal surfaces is not well understood, and materials often exhibit an induction period (Baker and Harris, 1979). Once corrosion has begun, carbon deposition can proceed at remarkably high rates. Filament growth rates as high as 75 nm/s (16.5 pg/(cm2.s)) at 600 "C have been reported (Rostrup-Nielsen and Trimm, 1977).
Due to their industrial importance, filamentous carbon formation has been studied primarily in association with thermal cracking (Albright and Marek, 1988) and steam reforming (Rostrup-Nielsen, 1975). The deposition process can also occur for other high-temperature reactions involving carbon monoxide or hydrocarbons, such as the synthesis of hydrogen cyanide (Satterfield, 1980). In addition, the carbon deposition process is an area of concern for gas-cooled nuclear reactors (Robertson, 1990). The deposition process has also been studied in association with the controlled fabrication of carbon fibers (Tibbetts, 1983; Bradley et al., 1985; Endo, 1988). The mechanism of carbon filament growth has been proposed to be the catalyzed decomposition of the
hydrocarbon on the leading surface of a metal particle, leading to adsorbed carbon atoms which then diffuse on the surface or through the bulk of the metal particle to the metal-carbon filament interface, where precipitation of carbon occurs. The driving force for the diffusion process has been proposed to be a concentration gradient (NishiyamaandTamai, 1974;Rostrup-Niehenand Trimm, 1977; Audier and Coulon, 1985)or a temperature gradient (Baker et al., 1972). Due to the limited temperature gradients that are possible across small particles under conditions where filament growth is observed (X0.1K), concentration gradients represent a more likely driving force than temperature gradients (Holstein and Boudart, 1983). Mechanistic studies have indicated that the filament formation process is catalyzed by reduced metals and possibly some metal carbides. The role of carbides has been debated, particularly in association with ironcatalyzed filament growth. Some studies have concluded that bulk carbides (Kocket al., 1985; Sacco and Caulmare, 1982) or surface carbides (Alstrup, 1988; Bianchini and Lund, 1989) are the active catalyst. Other studies suggest that iron carbides play an essential role in the creation of small iron particles (Sacco et al., 1984,1989; Stewart, et al. 19851,and still other studies indicate that they are only an inactive side product (Baker et al., 1982a,b). Some metals that do not form carbides, including platinum (Fryer and Paal, 1973) and ruthenium (Baker and Chludzinski, 19861,have been shown to be active catalysts while other elements,such as vanadium and molybdenum, have been shown to be active in the reduced metallic state but not as carbides (Baker et al., 1983). In spite of the widespread disagreement on the role of carbides, there is common agreement that metal oxides, including oxides of iron, do not catalyze the carbon deposition process. Transition metal oxides are similarly inactive for gasification of carbon by water vapor and carbon dioxide, and their inactivity has been postulated as being related to their lack of activity for reactions involving the breakage or formation of carbon-carbon bonds (Holstein and Boudart, 1982). The solubility of carbon in several metal oxides, including FeO and Fe304, has been measured to be below 0.01 ppm even at 1000 OC (Wolf and Grabke, 1985), and this low solubility might also be expected to prevent carbon filament growth from occurring through a bulk diffusion mechanism. The inactivity of metal oxides for the formation of filamentous carbon and, correspondingly, for metal dusting
Q888-5885J94/2633-1363$Q4.5Q/Q 0 1994 American Chemical Society
1364 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994
corrosion has lead to the development of oxide coatings for the protection of otherwise active reactor walls (Baker and Chludzinski, 1980; Graff and Albright, 1982; Szechy et al., 1992),but the long-term stability of such protective coatings under actual process conditions is uncertain. Iron and iron-based alloys are important materials of construction, and many reactions are carried out under oxidizing conditions near the boundary of the iron-iron oxide equilibrium. While reduced iron is a potent catalyst for filamentous carbon formation, the reaction can be avoided if the surface can be maintained in an oxide state. This phenomenon has been shown to minimize the usefulness of iron as a catalyst for the reduction of carbon dioxide by hydrogen to carbon and water vapor (Manning and Reid, 1977; Sacco and Reid, 1979). The iron-catalyzed formation of filamentous carbon has been studied from carbon monoxide (Renshaw et al., 1970; Jablonski et al., 1992) and a variety of saturated and unsaturated hydrocarbons (Robertson, 1970;Baker et al., 1973, 1983; Baird et al., 1974; Baker and Harris, 1979). Partially oxygenated hydrocarbons have been less widely studied two studies report on the formation of filamentous carbon from acetone (Evans et al., 1973; Stewart et al., 1985) and one reports on the formation from methanol (Boellard et al., 1985). Methanol is used or under consideration for several reactions that are carried out at moderately high temperatures of 400-600 "C. These include the formation of methanol through the partial oxidation of methane (Pichai and Klier, 19861,its oxidativedehydration to formaldehyde (Garibyan and Margolis, 1989),its direct dehydrogenation to formaldehyde (Halasz, 1989;Wiesgickl et al., 1990),its reaction with ammonia to form methyl amines (Hermann et al., 19881, its reaction with toluene to form p-xylene (Kaeding et al., 1981; D'Amore et al., 19911, and its conversion to diethyl ether, ethylene, and higher hydrocarbons (Chang, 1983). Water is a common byproduct for many of these reactions. Water and methanol do not react, and to the extent that water vapor partial pressures are not so high as to limit equilibrium conversions for these reactions, additional water vapor can often be added to the feedstream without serious adverse affects. As described further below, high partial pressures of water vapor serve to maintain iron surfaces in an oxide state inactive for filamentous carbon formation. Thus, the addition of water vapor to the feedstream can conceivably be used to prevent corrosion of iron and iron-based alloys in the presence of methanol at elevated temperatures. The intent of this study was to examine in more detail the role of water vapor in suppressing filamentous carbon formation on iron surfaces and to quantify the required water vapor partial pressure for which methanol could be handled safely in the presence of iron. In the course of the work, carbon dioxide was also investigated as an alternative oxidizing agent and hydrogen as an agent for carbon gasification to methane.
CH,OH(g) zCO(g) + 2Hz(g)
2
CH30H(g) C(fi1) + Hz(g) + HzO(g)
K4
CH,(g) = C(C)+ 2 H&)
(4)
2C(C) + CO,(g)
(5)
2CO(g)
KB
CO(g) + H&) = C(C)+ HzO(g) K7
CO(g) + HzO(g) = CO&)
+ 3H&)
K8
= CH,(g)
+ H&)
+ H&)
(6)
(7)
(8)
with equilibrium constants K4-K8. Only three of these reactions are linearly independent. Filamentous Carbon and Graphite. Filamentous carbon, C(fil),is a term that encompasses metal-catalyzed filamenbshaped carbons with a wide range of morphologies and properties (Baker and Harris, 1979). All are thermodynamically unstable with respect to decomposition into crystalline, graphitic carbon, C(c) KO
C(fi1) = C(C)
(9)
but the kinetics of the reaction are slow below 1000 "C, and C(fi1) is metastable. As a result of its positive free energy of formation relative to graphite of 4-10 kJ/mol when formed around 500-600 "C, filamentouscarbon does not form from gases such as CO and CH4 (reactions 4-6) even under some conditions where graphite is thermodynamically stable (Rostrup-Nielsen, 1972; de Bokx et al., 1985). Metals can catalyze the conversion of disordered carbons to more graphitic forms at elevated temperatures (Holstein et al., 1982), and similarly, metal-catalyzed formation of filamentous carbon at higher temperatures leads to more graphitic filamentous carbon and a lower free energy of formation (Rostrup-Nielsen, 1972). F e - F e a Equilibrium. Both water vapor and carbon dioxide can act as oxidizingagents, converting iron to iron oxide through the reactions xFe(c) + H,O(g) = Fe,O(c) Kii
Methanol Decomposition. At temperatures greater than 500 "C and low to moderate pressures, methanol is thermodynamically unstable with respect to its decomposition to formaldehyde, carbon monoxide and hydrogen, and hydrogen, water vapor, and carbon
(3)
When catalyzed by metals, reaction 3 results in the formation of filamentous carbon, C(fil), the properties of which we describe further later. C-H-0 Equilibrium. For any C-H-0 gas atmosphere a t thermodynamic equilibrium a t temperatures around 500-600 "C and pressures below 1 MPa, the only stable species in appreciable concentrations are CO(g), COz(g), HzO(g), Hz(g), CHdg), and graphite C(c). These are related through the chemical reactions
Kio
Thermodynamic Considerations
(2)
xFe(c) + CO,(g) = Fe,O(c)
+ H,(g)
(10)
+ CO(g)
(11)
The thermodynamically stable phases of iron in HzO-Hz atmospheres depend on the ratio of the partial pressures of water vapor, PHB, and hydrogen, PH?,and they are plotted as a function of temperature in Figure 1. Metallic iron (a-Fe below 910 "C)is stable at low values of PH*o/ PH*.Below 571 "C, a-Fe is convertedto magnetite (FesO4) ~ and finally to hematite (Fez031 as P H ~ o I PisHincreased,
Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 1365 TPC s.olOO 700 600
*:5
possibly, iron carbide. However, the corrosion process does not occur for any of the iron oxide phases. We are therefore interested in the process conditions for which these oxide phases are kinetically or thermodynamically stable. To the extent that equilibrium relationshipsapply, we need only be concerned with the equilibrium between a-Fe and FesOd below 571 "C, while from 571 "C to 723 "C, the critical equilibrium is that existing between a-Fe and "FeO".
30OI
4 O :
4.5 A.
. L
Experimental Section
-1.5 I
' 1.o
I
1.2
t
1
1.4
1
I
1.s
,
1.8
1000 K/T
Figure 1. Fe-0 phase diagram as a function of temperature and PHPIPHI.
while above 571 "C, iron is first converted to d s t i t e (FeOo.947, or "FeO"), followed by magnetite and hematite (Rau, 1972; Barin and Knacke, 1973). All of these iron oxide phases are inactive for filamentouscarbon formation, and we need only concern ourselveswith the phase forming ~ . for eqs 10 and 11, at the lowest value of P H ~ o / P HThus 2 = 0.75 for temperatures below 571 "C and 0.947 for temperatures above 571 "C. Methanol decomposition can result in the reduction of iron oxide to iron, 2Fe,O(c)
+ CH,OH(g) KIP = 2Fe,(c) + 2H,O(g) + CO(g) (12)
Reaction 12 is a linear combination of reactions 2 and 10. The reducing potential of methanol (eq 12) must be counterbalanced by the oxidizing potential of water vapor or carbon dioxide (eqs 10 and 11)in order to maintain the iron surface as iron oxide. Fe-C System. In the absence of carbon, ferrite (a-Fe) is the thermodynamically stable phase of iron below 910 "C and austentite (y-Fe) from 910 to 1390 "C. The presence of carbon stabilizes y-Fe so that it can exist at temperatures as low as 723 "C. The solubility of carbon is much greater in the face-centered-cubicy-Fe phase than in the body-centered-cubic a-Fe phase, 3.61 atom % vs 0.095atom % at 723 "C (Hansen, 1958). Baker et al. (1987) have demonstrated that both a-Fe and y-Fe can catalyze the formation of filamentous carbon. Cementite (Fe3C) is the most commonly formed iron carbide phase. It is thermodynamically unstable with respect to decomposition into iron (dissolved with carbon) and graphite. However, since the kinetics of graphite formation are slow, FesC is commonly formed as a metastable phase. Other metastable iron carbide phases (Fe2C and FesC,) have also been identified in association with the formation of filamentous carbon (Kock et al., 1985; Audier et al., 1983). In summary, previous work indicates that iron can corrode through the formation of filamentous carbon whenever the chemical state of iron is elemental iron or,
A 25-cm length of 50-pm-diameter iron wire was coiled and introduced into the reactor prior to each run. Experiments were carried out in two systems. Experimenta at atmospheric pressure (101 kPa) were carried out in a quartz reactor. Water vapor and methanol vapor were transported to the reactor by bubbling helium carrier gas streams through individual two-stage bubblers containing the respective liquids. Partial pressures of methanol and water vapor were controlled by controlling the temperatures of the baths and the flow rates of the helium through the bubblers, as well as the flow rate of a third stream of helium which bypassed both bubblers. A few runs were carried out with hydrogen or carbon dioxide carrier gases rather than helium. Experiments at 500kPa total pressure were carried out in a stainless steel reactor. Flow rates of liquid water and methanol to an evaporator were individually controlled. In some runs, helium was mixed with the gases. The water-methanol-helium feedstream was introduced into the reactor. Total flow rates for both systems were 250 cm3(STP)/min. A scanning electron (SEM) micrograph of the iron wire prior to use is shown in Figure 2a. Energy dispersiveX-ray analysis with a primary beam energy of 20 keV and a windowless detector indicated only the presence of iron. Two pretreatment methods were used for the wires. Some wires were pretreated in 250 cm3(STP)/min hydrogen at 550 "C for 0.5 h. This pretreatment resulted in no noticeable change in the wire. The other wires were pretreated in dry air for 0.5 h at 550 "C, which resulted in the oxidation of the iron surface (Figure2b), presumably to Fe203, which is the thermodynamically stable phase for these conditions (Figure 1). X-ray microanalysis confirmed the presence of an oxide phase. Further pretreatment of the oxide surface by reduction in hydrogen at 550 "C for 0.5 h resulted in ita conversion back to metallic iron (Figure 2c). The surface structure in this figure indicates that about the upper 1-2 pm of the surface had been oxidized during the 0.5-h oxidation in dry air. For most runs, following the pretreatment procedure, the iron wire was exposed for 2 h to a gas atmosphere containing fixed feedstream partial pressures Poi of methanol, water vapor, and helium. In some runs, methanol was used in combination with other gases. For the low-pressure (101 kPa) experiments, the furnace was opened and cooled rapidly to room temperature in flowing helium at the completion of the run. In the high-pressure (500kPa) experiments, the sample was cooled to room temperature in helium over the course of several hours. Following each run, the iron wire was studied visually, as well as by scanning electron microscopy (SEM) and X-ray microanalysis. The presence of carbon deposition was evident from visual examination, but the SEM analysis provided additional insight into structural changes in the iron, the chemical state of its top 1pm, and the nature of the deposited carbon. A few samples were studied by scanning transmission electron microscopy.
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1366 Ind. Eng. Chem. Res., Vol. 33, No.5, 1994 1
Figure 3. .;EM m i c r w r a p t >
