Nickel catalysts methanation reactions

The steam reformer process involves the reaction of methane and high temperature steam in the presence of a nickel catalyst. The reactions are... 

The two predominant methods oPmaking synthesis gas are steam reforming and partial oxidation. Both are quite simple. The steam reforming method involves passing methane or naphtha plus steam over a nickel catalyst. The reaction, if methane is the feedstock, is ... 

Carhon monoxide and hydrogen react to form CH in the presence of a nickel catalyst. Methane also is formed by reaction of magnesium methyl iodide in anhydrous ether (Grignard s rcagentl with substances containing the hydroxyl group. See also Grignard Reactions. Methyl iodide (bromide, chloride) is preferably made by reaction of methyl alcohol and phosphorus iodide (bromide, chloride)... 

After desulphurisation, a process which is necessary for the protection of the catalysts, the natural gas which is mainly methane is reacted with steam over a nickel catalyst. The reaction is overall endothermic, and so, in accordance with the laws of chemical equilibrium, as high a temperature as possible is required. The reactor, known as the primary reformer, is a collection of vertical metal tubes suspended in a furnace, and the exit gases around 800°C are unreacted methane 9 per cent, steam, oxides of carbon and hydrogen. The principal reactions occurring simultaneously are ... 

Graphite does not react with hydrogen at ordinary temperatures. It reacts in the 1000 -1500°C range to form methane (CH4). The reaction is accelerated in the presence of a platinum catalyst. With nickel catalyst, the reaction begins at approximately 500°C.I 1... 

This reaction is an undesirable side reaction in the manufacture of hydrogen but utilised as a means of removing traces of carbon monoxide left at the end of the second stage reaction. The gases are passed over a nickel catalyst at 450 K when traces of carbon monoxide form methane. (Methane does not poison the catalyst in the Haber process -carbon monoxide Joes.)... 

Fischer-Tropsch Process. The Hterature on the hydrogenation of carbon monoxide dates back to 1902 when the synthesis of methane from synthesis gas over a nickel catalyst was reported (17). In 1923, F. Fischer and H. Tropsch reported the formation of a mixture of organic compounds they called synthol by reaction of synthesis gas over alkalized iron turnings at 10—15 MPa (99—150 atm) and 400—450°C (18). This mixture contained mostly oxygenated compounds, but also contained a small amount of alkanes and alkenes. Further study of the reaction at 0.7 MPa (6.9 atm) revealed that low pressure favored olefinic and paraffinic hydrocarbons and minimized oxygenates, but at this pressure the reaction rate was very low. Because of their pioneering work on catalytic hydrocarbon synthesis, this class of reactions became known as the Fischer-Tropsch (FT) synthesis. 

Direct hydrohquefaction of biomass or wastes can be achieved by direct hydrogenation of wood chips on treatment at 10,132 kPa and 340 to 350°C with water and Raney nickel catalyst (45). The wood is completely converted to an oily Hquid, methane, and other hydrocarbon gases. Batch reaction times of 4 hours give oil yields of about 35 wt % of the feed the oil contains about 12 wt % oxygen and has a heating value of about 37.2 MJ /kg (16,000 Btu/lb). Distillation yields a significant fraction that boils in the same range as diesel fuel and is completely miscible with it. 

Methanation of the clean desulfurized main gas (less than 1 ppm total sulfur) is accompHshed in the presence of a nickel catalyst at temperatures from 260—400°C and pressure range of 2—2.8 MPa (300—400 psi). Equations and reaction enthalpies are given in Table 4. 

Thermodynamically, the formation of methane is favored at low temperatures. The equilibrium constant is 10 at 300 K and is 10 ° at 1000 K (113). High temperatures and catalysts ate needed to achieve appreciable rates of carbon gasification, however. This reaction was studied in the range 820—1020 K, and it was found that nickel catalysts speed the reaction by three to four orders of magnitude (114). The Hterature for the carbon-hydrogen reaction has been surveyed (115). 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

Kinetics. Extensive studies of the kinetics of methane synthesis were reported by White and co-workers (10,11, 12, 13, 14, 15). They studied the reaction between CO and hydrogen over a reduced nickel catalyst on kieselguhr at 1 atm and 300°-350°C (10). They correlated the rate of methane formation by the equation ... 

The synthesis of methane from C02 and hydrogen was studied by Binder and White (11) over a reduced nickel catalyst (Harshaw Ni-88). The surface reaction between the C02 and hydrogen appeared to be rate controlling. The rate of reaction can be correlated by either of the following rather awkward equations ... 

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

For the methanation reaction in the process of converting coal to a high Btu gas, various catalyst compositions were evaluated in order to determine the optimum type catalyst. From this study, a series of catalysts were developed for studying the effect of nickel content on catalyst activity. This series included both silica- and alumina-based catalysts, and the nickel content was varied (Table I). 

The catalysts were reduced with 100% H2 at 371 °C and an inlet space velocity of 1000/hr. Because of the carbon-forming potential of a dry gas recycle composition and the cost of reheating the recycle if the water produced by the methanation reaction is removed, a wet gas recycle composition was used. The catalyst loading, gas composition, and test conditions for these tests are listed in Table II, and the effects of nickel content are compared in Table III. 

The methanation process commonly operates at pressures up to 30 atm, and, with the nickel catalyst which is almost universally used for the process, the inlet temperature is about 300°C ( 570°F). Almost complete conversion of the oxides of carbon occurs giving a product synthesis gas containing less than 5 ppm CO + C02. The temperature rise for the exothermic methanation reactions is typically 35 °C (63°F). 

Nickel catalysts were used in most of the methanation catalytic studies they have a rather wide range of operating temperatures, approximately 260°-538°C. Operation of the catalytic reactors at 482°-538°C will ultimately result in carbon deposition and rapid deactivation of the catalysts (10). Reactions below 260°C will usually result in formation of nickel carbonyl and also in rapid deactivation of the catalysts. The best operating range for most fixed-bed nickel catalysts is 288°-482 °C. Several schemes have been proposed to limit the maximum temperature in adiabatic catalytic reactors to 482°C, and IGT has developed a cold-gas recycle process that utilizes a series of fixed-bed adiabatic catalytic reactors to maintain this temperature control. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

Microwave-induced, catalytic gas-phase reactions have primary been pursued by Wan [63, 64], Wan et al. [65] have used pulsed-microwave radiation (millisecond high-energy pulses) to study the reaction of methane in the absence of oxygen. The reaction was performed by use of a series of nickel catalysts. The structure of the products seemed to be function of both the catalyst and the power and frequency of microwave pulses. A Ni/Si02 catalyst has been reported to produce 93% ethyne, whereas under the same irradiation conditions a Ni powder catalyst produced 83% ethene and 8.5 % ethane, but no ethyne. 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

Steam, at high temperatures (975-1375 K) is mixed with methane gas in a reactor with a Ni-based catalyst at pressures of 3-25 bar to yield carbon monoxide (CO) and hydrogen (H ). Steam reforming is the process by which methane and other hydrocarbons in natural gas are converted into hydrogen and carbon monoxide by reaction with steam over a nickel catalyst on a ceramic support. The hydrogen and carbon monoxide are used as initial material for other industrial processes. 

Because some refining processes require a minimum of carbon dioxide in the product gas, the oxides are reacted with hydrogen in a methanation step. This reaction takes place in the methanator over a nickel catalyst at elevated temperatures. 

In addition to the activity, other important requirements for the catalyst are the capability to start the reaction rapidly without the necessity for previous reduction with hydrogen and to perform effectively with intermittent operation these are essential properties for the catalyst in reformers, especially for portable and small-scale stationary fuel cell applications. In this respect, Dias and Assaf [61] focused on the potential of Pd, Pt and Ir to promote fast and intermittent ignition of methane ATR in Ni/y-Al203. They concluded that the three metals are very good promoters of the reduction of the nickel catalyst with methane, but the lower cost of palladium makes this metal more suitable than Pt and Ir for small fuel cells. 

Unless there is a viable market for methane (eg as a fuel gas) the methane (both that from the coal gasifiers as well as that formed in the downstream FT process) is reformed using nickel catalyst at high temperatures to produce more CO and Hp which is then recycled to the FT reactors (see Figure 1). The CH/reforming reaction is typically ... 

Within the reaction parameters used, the nickel catalyst is highly selective towards carbonylation. With the exception of trace a-mounts of methane formed, no other hydrogenation product is found. This is in contrast with cobalt whose carbonylation catalytic activity is enhanced by hydrogen but generally associated with aldehyde formation and homologation. 

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