Steam Reformers Heat Balance

One of the major technical problems that had to be overcome to integrate the POLYBED system with the steam reformer was the variation in tail gas flow and composition. Because of the cyclic nature of the process, tail gas is rejected by the POLYBED unit during blowdown and purge with significant flow and composition variations. The fluctuations would have made it impossible to use the tail gas for fuel and a sophisticated system was developed to balance tail gas heating value. This buffer/mixing tank system has proven to be very reliable in holding heat input variation to 1% (2). ... 

MRG [Methane Rich Gas] A catalytic steam-reforming system, similar to the classic syngas reaction of steam with a hydrocarbon mixture, but yielding hydrogen, methane, and carbon monoxide in different proportions. The system is thermodynamically balanced, requiring no heat other than that required to raise the reactants to the operating temperature. Developed by the Japan Gasoline Company. 

Montedison Low-Pressure Process. The Montedison low-pressure process [940], [1036], [1128], [1129] involves a split flow to two primary reformers. About 65% of the feed-steam mixture flows conventionally through the radiant tubes of a fired primary reformer followed by a secondary reformer. The balance of the feed-steam mixture passes through the tubes of a vertical exchanger reformer. This exchanger reformer has a tube sheet for the catalyst tubes at the mixed feed inlet. There is no tube sheet at the bottom of the tubes, where the reformed gas mixes directly with the secondary reformer effluent. The combined streams flow on the shell side to heat the reformer tubes in a manner similar to that described for the M. W. Kellogg KRES reformer, see Sections 4.1.1.8 and 5.1.4.3). The process air flow is stoichiometric. Synthesis is performed at 60 bar in a proprietary three-bed indirectly cooled converter with am-... 

Chiyoda Process [1137], In this process the traditional fired primary reformer is also replaced by an exchanger reformer and the heat balance requires excess air in the secondary reformer with the consequence of a cryogenic unit as final step in the makeup gas preparation to remove the surplus of nitrogen. Additionally, gas turbines are proposed as drivers for the process air compressor and synthesis gas compressor with the hot exhaust being used for steam generation and feed gas preheating. 

Chapter 10 contains a literature survey of the basic fluidized bed reactor designs, principles of operation and modeling. The classical two- and three phase fluidized bed models for bubbling beds are defined based on heat and species mass balances. The fluid dynamic models are based on kinetic theory of granular flow. A reactive flow simulation of a particular sorption enhanced steam reforming process is assessed. 

Dryout of the insulation in the reformer furnace of Methanol 1 commenced in late August 1985 and methanol was synthesised on October 12. On October 17th the first gasoline was produced from the MTG plant. Only minor startup problems were encountered - a few valves operating unsatisfactorily, the odd steam leak and initial control system and heat balancing problems. 

The balance between heat input through the reformer tubes and the heat consumption in the endothermic reforming reaction is the central problem in steam reforming. 

The reformer takes an input flow rate of methane and computes the hydrogen output. The reformer module balances energy by combusting the reformate stream with air and exchanging the heat released to the catalyst reactor. Parameters on the reformer are the steam-to-carbon ratio and the outlet temperature of the exhaust products from the internal burner. The temperature at which the equilibrium reforming occurs depends on these parameters. Figure 1 shows the variation in thermal efficiency of the reformer with temperature and steam-to-carbon ratio. The minimum steam-to-carbon ratio is 2 however, reformers are often operated with excess steam to improve the efficiency and prevent coking problems. 

If steam is added to the fuel and the oxidant, it is possible to heat balance the exothermal partial oxidation reaction with the endothermal reforming reaction. The reaction is then said to be autothermal, meaning that no external heat source is required what raises the efficiency. 

Partial oxidation is able to convert methane and other hydrocarbons, catalyzed or non-catalyzed, at temperatures between 1100 and 1500 °C. Despite its lower efficiency compared with steam reforming, partial oxidation provides, on the other hand, exothermicity and a greater selectivity for synthesis gas production as well as advantages for certain applications such as compactness, rapid startup or load change, lower overall cost. If steam is added to the fuel and the oxidant, it is possible to heat balance the exothermal partial oxidation reaction with the endothermal reforming reaction, meaning that no external heat source (autothermal) is required. 

The steam-hydrocarbon reforming process is highly developed and will operate for months or even years without interruption, except for normal outages scheduled for boiler inspection, routine maintenance, and other attention which is placed on a definable schedule. The heat balance and utilization are well engineered ordinarily so that there is little waste, and what heat is unused on the furnace side of the reformer is subsequently recovered for use to generate steam. 

As the enthalpy balance of the overall reaction is almost neutral, the carbonation reaction, in addition to CO2 removal from the gaseous phase, provides the heat required for the steam reforming reaction, allowing for the use of adiabatic reactors, or with very limited heat duties. 

The heat balance over the steam reformer itself is thus almost unchanged when real gas properties are used. The reason is the moderate pressure and the high temperature, but at a higher pressure or for mixtures close to the dew point the difference increases. 

By use of the expression for the one-dimensional heat transfer coefficient in Equation (3.26) it is now possible to establish the heat transfer balance for the one-dimensional model for a tubular reformer tube considering only steam reforming of methane. The equation is established from the two-dimensional model in Equation (3.15), the boundary condition in Equation (3.17) and the derived heat transfer coefficient for the one-dimensional model in Equation (3.26) as ... 

Because partial oxidation reactions take place faster compared with steam reforming, the heat balance of autothermal reforming is only neutral on an overall basis. This leads to local overheating, similar to partial oxidation (see Section 4.2). 

Alternative processes. The most common alternative is to carry out the Haber process but provide the heat for the process internally rather than externally. The extra oxygen used to burn the fuel feedstock is provided in a pure state to ensure that a suitable balance between nitrogen (from the air) and hydrogen is maintained. This is known as the partial oxidation process. Although this process may appear to be more thermally efficient than the steam reforming process it does in fact use about 10% more fuel for an equivalent amount of ammonia. 

Figure 6.2.32 shows results of simulations of the steam reforming process carried out by Froment and coworkers based on kinetic data and the respective heat and mass balances. 

The outlet methane concentration bias in the objective function was heavily weighted (to drive it toward zero, since the objective function is the sum of the weighted squares biases) and therefore explains the good agreement between calculated and observed values. The process air flow measurement bias (difference between measured and calculated) was a parameter, so the calculated nitrogen composition is precisely equal to the measured value. The air flow measurement bias was 4.4% of the measured value at the solution. The calculated outlet temperature is surprisingly close to the measured value. The heat balance around the high pressure steam drum required only a 1.2% heat loss to close in this case. That balance is of course affected by numerous other measurements, so the calculated secondary reformer outlet enthalpy can only be said to be part of the overall consistent set of information. 

Since catalytic partial oxidation seems to be appropriate for Fischer-Tropsch syntheses, it has also been proposed that the heat required for methane steam reforming could be balanced by the exothermic oxidation of methane. In this way, a quasi-homogeneous, one-dimensional model has been developed from the study of partial oxidation of methane in the presence of steam over a 5% Ru-supported on y-Al203, which according to authors can help in pilot reactor design, materials, and further scale-up. 

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