Reactor methanation

Synthetic Natural Gas. Another potentially very large appHcation of coal gasification is the production of synthetic natural gas (SNG). The syngas produced from coal gasification is shifted to produce a H2-to-CO ratio of approximately 3 to 1. The carbon dioxide produced during shifting is removed, and CO and H2 react to produce methane (CH, or SNG, and water in a methanation reactor. 

The following reactions can occur simultaneously within a methanation reactor. 

Carbon Laydown. The potential for carbon laydown is readily estimated from the thermodynamics of Reactions 4 and 5. The areas where carbon laydown, according to these reactions, is thermodynamically possible were developed by Gruber (36). It is readily seen that carbon laydown via Reaction 4 is thermodynamically favorable at the reactor inlet for practically any commercially conceivable feed gas composition. As noted by Gruber (36), carbon laydown is thermodynamically unfavorable at the reactor outlet for practically all commercially conceivable methanator conditions. The methanation reactor will therefore, in practice, have two zones—the first is a finite zone between the inlet and some way down the catalyst bed where carbon laydown is thermodynamically possible, and the second zone is the balance of the reactor. 

Four pilot plant experiments were conducted at 300 psig and up to 475°C maximum temperature in a 3.07-in. i.d. adiabatic hot gas recycle methanation reactor. Two catalysts were used parallel plates coated with Raney nickel and precipitated nickel pellets. Pressure drop across the parallel plates was about 1/15 that across the bed of pellets. Fresh feed gas containing 75% H2 and 24% CO was fed at up to 3000/hr space velocity. CO concentrations in the product gas ranged from less than 0.1% to 4%. Best performance was achieved with the Raney-nickel-coated plates which yielded 32 mscf CHh/lb Raney nickel during 2307 hrs of operation. Carbon and iron deposition and nickel carbide formation were suspected causes of catalyst deactivation. 

Figure 6. Details of thermowell of the HYGAS methanation reactors...

IGT selected Harshaw Ni-0104T nickel-on-kieselghur catalyst formed in 4 X y in. cylindrical pellets for the initial catalyst charge to the methanation section of the HYGAS pilot plant. This selection was based on high activity over a range of temperatures (274°-516°C) and space velocities. Catalyst activity life tests were conducted for 1420 hrs without deterioration (Table I) consequently, we felt that suitable longevity could be obtained in the pilot-plant methanation reactors. 

To further reduce the carbon monoxide, a preferential oxidation reactor or a carbon monoxide selective methanation reactor is used. The term selective oxidation is also used for preferential oxi-... 

The increase in efficiency between the first- and second-generation reactors was attributed to less water in the feed and lower operating temperatures. Reactor models indicated that the major source of heat loss was by thermal conduction. The selective methanation reactor lowered the carbon monoxide levels to below 100 ppm, but at the cost of some efficiency. The lower efficiency was attributed to slightly higher operating temperatures and to hydrogen consumption by the methanation process. Typical methane levels in the product stream were 5-6.2%. ... 

This basic approach is really divided into several distinct categories. Two of these, Davison s method and Marshall s method, provide suitable modal reduction for the state-space representation of the methanation reactor to a 12th-order model. Comparisons of the models and discussion of additional model reduction are presented in the next section. 

Table VII shows that, for the methanation reactor model, the dynamic response of the gas temperatures and CO and C02 concentrations should be much faster (by two orders of magnitude) than the response of the catalyst and thermal well temperatures. This prediction is verified in the dynamic responses shown in Figs. 18 and 19 and the previous analysis of the thermal and concentration wave velocities.

Materials with High-temperature Stability. As discussed in an earlier section, catalysts designed for methanation of synthesis gas have, in particular, to be able to withstand the extreme conditions encountered in the first methanation reactor (see Figures 2 and 3), i.e., they must have high activities at low temperatures and yet also have high stabilities at higher temperatures. Pre-... 

Since the methanation reaction is strongly exothermic, a sharp temperature rise can be measured across the reaction zone in the catalyst bed. Most methanation reactors are designed with a number of thermocouples that monitor the position of the exotherm. A strong indicator of the amount and rate of methanation catalyst deactivation is the position of the temperature profile in the catalyst bed and its rate of movement over time. A record of the temperature profile should be kept to detect any movement during the first one to two years of operation. An estimate of future life can then be made. ... 

The product gas of the methanation section contains mainly CHi, Hj, HjO, and CO2. Removing H1O from this stream results in SNG as the final product, which leaves the system at high pressure. The heat released from the hydrogasifier product gas, and the heat generated in the methanation reactors, are used to generate superheated steam (40 bar and 540°C), which enters a steam turbine. A fraction of partly expanded steam is used to dry the biomass, while the remaining part of the steam is used for power generation. 

Coke was characterized by Temperature-Programmed-Oxidation (TPO). These experiments were carried out using a modified unit. The CO2 produced during the coke burning is converted to CBU, in a methanator reactor. A H2 stream is fed to this reactor, which is loaded with a Ni catalyst, in order to quantitatively convert CO2 into CH4. This compound is then continously monitored by a flame ionization detector (FID). With this configuration the sensitivity and resolution of the classical TPO technique is greatly improved. Typically, 10 mg... 

Currently, the application of adsorption-based processes to reaction systems are of considerable interest. Hydrogen production from hydrocarbons and dehydrogenation are important industrial reactions, for example, the catalytic steam-methane reactor (SMR) ... 

Figure 10.6. Schematic diagram of methanation reactor showing catalyst coating on the inside of the reactor tube wall.

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