Adiabatic methanator

A rigorous dusty gas model and two simplified models have been used to simulate industrial steam reformers and methanators. The basic principles for the solution of both the nonadiabatic steam reformer and the adiabatic methanator are given. The details of developing solution algorithms from the models are left to the reader as a serious and extensive project. 

The reverse process of methanation was realized in 1979 in the test facility ADAM ( Anlage mit drei adiabaten Methanisierungsreaktoren - Facility with three adiabatic methanation reactors ). A three-stage methanation reaction including a dilution of the product gas by feedback of already retransformed gas was chosen to mitigate the large heat release, to minimize soot formation, and to maximize CH4 output. The feed gas was synthesis gas at a rate of 600 Nm /h in ADAM-I and 96(X) Nm /h in ADAM-II [6]. 

ADAM Facility with Three Adiabatic Methanation Reactors (Anlage mit drei adiabaten Methanisierungsreaktoren)... 

A natural gas having the volumetric composition of 90% methane, 8% ethane, and 2% nitrogen at 1 atm and 25°C is used as fuel in a power plant. To ensure complete combustion 75% excess air is also supplied at 1 atm and 25°C. Calculate (i) the lower and higher heating values of the fuel at 25°C and (ii) the theoretical maximum temperature in the boiler assuming adiabatic operation and gaseous state for all the products. 

The chapter by Haynes et al. describes the pilot work using Raney nickel catalysts with gas recycle for reactor temperature control. Gas recycle provides dilution of the carbon oxides in the feed gas to the methanator, hence simulating methanation of dilute CO-containing gases which under adiabatic conditions gives a permissible temperature rise. This and the next two papers basically treat this approach, the hallmark of first-generation methanation processes. 

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. 

The heats of these reactions (2, 3) (Figure 1) indicate that all the reactions are exothermic over the cited range of conditions. For example, the heat liberated under typical reaction conditions for the conversion of CO to methane is 52,730 cal/mole CO that for carbon dioxide is 43,680 cal/mole. Such high heats of reaction cannot be absorbed by the process stream in an adiabatic reactor unless the CO and/or C02 conversion is limited to less than about 2.5 moles/100 moles feed gas. Since... 

Figure 12. Schematic flow sheet for methanation system using equilibrium-controlled adiabatic reactors...

The basic advantages of this process are (a) elimination of a mechanical device (recycle gas compressor) for controlling the adiabatic temperature rise, (b) combination of CO shift with methanation, (c) significant increase in byproduct steam recovery, and (d) significant capital advantages. 

Interesting features of this process include the potential for one-stage methanation to completion without the need for gas recycle. This feature was cited by Chem Systems, but, according to Rheinpruessen-Koppers work on the Fischer-Tropsch (52, 53), gas recycle was necessary with high H2 CO ratios. Drawbacks include such factors as catalyst attrition (48, 50), and low volume productivities of the methanator (less than one-tenth that reported for fixed bed adiabatic reactors) (48, 50, 52, 53, 61). 

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. 

The catalyst in an isothermal tube-wall reactor (experiment TWR-6 in Ref. 2) deactivated much more slowly than did the catalyst in the best test (experiment HGR-14) in an adiabatic HGR reactor (0.009 vs. 0.0291 %/mscf/lb), and it also produced much more methane (177 vs. 32 mscf/lb catalyst). This indicates that adiabatic operation of a metha-nation catalyst between 300° and 400°C is not as efficient as isothermal operation at higher temperature ( 400°C). 

The scheme of commercial methane synthesis includes a multistage reaction system and recycle of product gas. Adiabatic reactors connected with waste heat boilers are used to remove the heat in the form of high pressure steam. In designing the pilot plants, major emphasis was placed on the design of the catalytic reactor system. Thermodynamic parameters (composition of feed gas, temperature, temperature rise, pressure, etc.) as well as hydrodynamic parameters (bed depth, linear velocity, catalyst pellet size, etc.) are identical to those in a commercial methana-tion plant. This permits direct upscaling of test results to commercial size reactors because radial gradients are not present in an adiabatic shift reactor. 

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. 

This you cannot do in an adiabatic reactor unless you go to extremely high mixing ratios of fresh feed and recycle gas. In summary, it is a question of selectivity, which is the reason for using the isothermal reactor for Fischer-Tropsch. An adiabatic reactor with a waste heat boiler is cheaper than an isothermal feactor, and hence it is used for methanation. 

It is also well known that there exist different extinction modes in the presence of radiative heat loss (RHL) from the stretched premixed flame (e.g.. Refs. [8-13]). When RHL is included, the radiative flames can behave differently from the adiabatic ones, both qualitatively and quantitatively. Figure 6.3.1 shows the computed maximum flame temperature as a function of the stretch rate xfor lean counterflow methane/air flames of equivalence ratio (j) = 0.455, with and without RHL. The stretch rate in this case is defined as the negative maximum of the local axial-velocity gradient ahead of the thermal mixing layer. For the lean methane/air flames,... 

K for lean methane/air = 0.455) flames, with and without RHL, respectively, indicated as nonadiabatic and adiabatic. Symbol indicates the adiabatic flame temperature Tad-... 

For the adiabatic condition in which RHL is suppressed, the flame response exhibits the conventional upper and middle branches of the characteristic ignition-extinction curve, with the upper branch representing the physically realistic solutions. It can be noted that the effective Le of this lean methane/air mixture is sub-unity. It can be seen from Figure 6.3.1 that, with increasing stretch rate, first increases owing to the nonequidiffusion effects (S > 0), and then decreases as the extinction state is approached, owing to incomplete reaction. Furthermore, is also expected to degenerate to the adiabatic flame temperature, when v = 0. 

A 12 in. ID gas pipeline carries methane (MW = 16) at a rate of 20,000 scfm. The gas enters the line at a pressure of 500psia, and a compressor station is located every 100 mi to boost the pressure back up to 500psia. The pipeline is isothermal at 70°F, and the compressors are adiabatic with an efficiency of 65%. What is the required horsepower for each compressor Assume ideal gas. 

RM [Ralph M. Parsons] A process for methanating synthesis gas, i.e. converting a mixture of carbon monoxide and hydrogen to mainly methane and carbon dioxide. Six adiabatic reactors are used in series, and steam is injected at the inlet. Under development by the R. M. Parsons Company in 1975. 

What is the LFL in air of a mixture of 80 % (molar) methane and 20 % propane by Le Chatelier s rule What is the adiabatic flame temperature of this system ... 

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