Reformer high flux

One example is the formation of carbon in high flux reformers [389] operating far from thermodynamic carbon limits. It means that methane may decompose to carbon instead of reacting with steam to form the required syngas in spite of no potential for carbon in the equilibrated gas. This is of course not possible in a closed system, but in an open system carbon may be stable in a steady state and the accumulation of carbon may continue [389], This risk may be assessed by the so-called criteria of actual gas, which for the methane decomposition reaction as in Equation (5.5) can be written as ... 

Steam reforming is an important proeess to generate hydrogen for sueh uses as ammonia synthesis beeause of the high endothermie heat reaetion and its rapidity. High heat fluxes with a direet-fired furnaee are required. Although many steps of reaetions are possible, the typieal reaetion steps are as follows ... 

Heat flux is defined as heat input per unit of time per square unit of inside tube surface. A low heat flux provides extra catalyst volume and lower tubewall temperatures. This increases the reforming reaction conversion and increases tube life. A high heat flux reverses these effect, but reduces the number of tubes. The flux is highest at the zone of maximum heat release and then drops to a relatively low value at the tube outlet. 

Higher heat fluxes require a modified ring shape to sustain the reforming reaction conversion. A dual charge of catalyst may also be used. The tube s top half has a high-activity catalyst to prevent carbon formation in the maximum flux zone. The bottom half may be a more conven-... 

Intermediate Duty catalysts are for feeds with a significant content of components from ethanes up to liquid petroleum gas (LPG). The heavier feedstock increases the tendency for catalyst deactivation through carbon laydown and requires a special catalyst in the top 30% to 50% of the reformer tubes. This tendency also occurs when light feeds are run at low steam-to-carbon ratios and/or at a high heat flux. 

In many modern top-fired reformers the heat flux calculated for the inner tube wall surface is around 60 000 W/m2, although in some designs it cam be as high as 75 000 W/m2. The maximum heat flux may be 1.4 to 2 times higher. In side-fired and terraced-wall furnaces, where the mean fluxes are generally in the range of 60 000-85 000 W/m2, the difference between mean and maximum flux is much smaller, as shown in Figure 37 [444],... 

Bottom-fired furnaces are not very common in modern ammonia plants. They have a rather constant heat flux profile along the tube with high metal temperatures on the outlet side. Examples are the Exxon reformer and the old Chemico round furnaces. 

New tube materials allow the design for much higher exit temperatures and heat fluxes, in particular when applying a side wall fired reformer furnace to ensure better control of the maximum tube wall temperature and optimum use of the high alloy material. Thinner tube walls made possible by the use of the new materials reduce the risk for creep due to faster relaxation of stresses at start and stop of the reformer (14). 

Reformer tube heating with a high-temperature nuclear reactor is performed with helium, typically at 950 °C, as the heat source. The aim of reaching a heat flux density similar to that of the conventional method can be achieved by employing a helium-heated counterflow heat exchanger (see Fig. 2-11). Helium under pressure shows excellent heat transfer properties. Furthermore, precautions must be taken to minimize the effects of asymmetry or hot gas streaks in the helium flow as well as a non-uniform process gas flow. The materials of a helium-heated steam reformer should be selected such that the... 

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