Primary and secondary reforming

Steam-Reforming Natural Gas. Natural gas is the single most common raw material for the manufacture of ammonia. A typical flow sheet for a high capacity single-train ammonia plant is iadicated ia Figure 12. The important process steps are feedstock purification, primary and secondary reforming, shift conversion, carbon dioxide removal, synthesis gas purification, ammonia synthesis, and recovery. 

Ammonia production from natural gas includes the following processes desulfurization of the feedstock primary and secondary reforming carbon monoxide shift conversion and removal of carbon dioxide, which can be used for urea manufacture methanation and ammonia synthesis. Catalysts used in the process may include cobalt, molybdenum, nickel, iron oxide/chromium oxide, copper oxide/zinc oxide, and iron. 

Typical analysis of effluent from primary and secondary reformers... 

N2-H2 mixture (with small amounts of Ar and CH4) [33]. The amount of air added to the secondary reformer is adjusted to give the desirable H2/N2 ratio (which is close to 3 for the NH3 synthesis). The secondary reformer is similar to the autothermal reformer described in the previous section. The pressure at the outlet of the secondary reformer is in the range 2.5-3.5 MPa. The outlet temperatures from the primary and secondary reformers are 750-850°C and 950-1050°C, respectively. 

In 2001 Hyprotech and Synetix announced an ammonia plant simulation that can be used for modeling, on-line monitoring and optimization of the plant. The simulation includes Synetix reactor models, customized thermodynamic data and information to simulate the performance of a range of catalysts. The reactor models in the simulation include Primary and Secondary Reformers, High Temperature Shift converter, Low Temperature Shift Converter, Methanator and Ammonia Synthesis Converter80. 

A standard primary and secondary reforming front end of the plant will operate at proven process conditions. Scale-up of the reforming section is straight-forward when using a top-fired box reformer with a cold outlet manifold system. The size of the reformer is much smaller than reformers currently designed for large-scale methanol projects218. 

In the primary and secondary reformer the following steam reforming reaction takes place CH4 + H20 CO + 3H2... 

Description The gas feedstock is compressed (if required), desulfurized (1) and sent to a saturator (2) where process steam is generated. All process condensate is reused in the saturator resulting in a lower water requirement. The mixture of natural gas and steam is preheated and sent to the primary reformer (3). Exit gas from the primary reformer goes directly to an oxygen-blown secondary reformer (4). The oxygen amount and the balance between primary and secondary reformer are adjusted so that an almost stoichiometric synthesis gas with a low inert content is obtained. The primary reformer is relatively small and the reforming section operates at about 35 kg/cm2g. 

A table listing the thermodynamic equilibrium methane concentration in the outlet of the primary and secondary reformer over a wide range of operating pressures, outlet temperatures and S/C ratios can be found in [417] (see also [424]). 

The upper part of Fig. 1 shows the synthesis gas plant which is fed from the right-hand side with methane (stream l) and air (stream 2) for a combustion process to match the heat requirements of the synthesis gas process. The combustion process delivers the exhaust gas (stream 3). The synthesis gas is produced by methane, water vapor and air (streams U, 5 6) in a primary and secondary reformer and a converter (units REF1, REF2 and CON). The raw gas (stream 28) passes the gas conditioning (SEPl) which has been detailed in Fig. 3 and the synthesis gas (stream 29) enters the ammonia plant shown in the lower part of Fig. 1. The ammonia... 

Basically two different layouts of the ammonia synthesis loop exist, depending upon the quality of the make-up synthesis gas feeding the loop. In most cases the make-up gas also contains (apart from H2 and N2 in proper ratios) some inerts (CH4, Ar and minor traces of rare gases), which have to be purged from the loop to avoid build-up of inerts. (Inerts containing loop layout). This layout is primarily used in plants where the synthesis gas is generated from reformer based frond-ends with primary and secondary reforming. 

Formation of hydrogen. The next three stages (Figure 3.7) are concerned with the production of hydrogen. These are primary and secondary reforming and the so-called shift reaction. The primary reforming reactions are... 

Pressure drop calculations in the primary and secondary reformer models are based on the Ergun relationship, as presented in Reference 25. Both the laminar flow and turbulent flow terms are included. The natural parameter that arises from the Ergun equation, which can be updated with measured pressure drop information, is the TURBULENT DP COEF term in the models of both the primary and secondary reformers. This term affects only the pressure drop, whereas another term, the bed void fraction, which might also have been used as the parameter to update with measure pressure drop, also affects all the reaction rates. The bed void fraction affects the amount of catalyst in a fixed volume reformer tube, and is not an appropriate parameter to use in this case. The void fractions of typical packed beds are shown in Figure 5.70 of Reference (26). Void fractions of 0.4 to 0.6 are typical, and can be determined for specific catalysts sizes and shapes from vendor specification sheets, by measurement, or, with more difficulty, by calculation. 

Most new large methanol plants are built in areas where low cost natural gas is available. If the synthesis gas is produced by the conventional one-step process, with a steam reformer only, the module will always be around 3 with natural gas as feed. Process studies (Soegaard-Andersen, 1989) have shown that for capacities above 1000-1500 t/d it is economical to adjust the module to the optimum value by adding an oxygen-fired autothermal reformer downstream the steam reformer as shown in Figure 12. This two-step reforming process (Dybkjaer et al., 1985) is similar to the well-known process lay-out with a primary and secondary reformer used in ammonia plants. 

Ammonia synthesis gas made by primary and secondary reforming of hydrocarbons with partial bypassing of primary reformer. C. L. Winter (Humphreys Glasgow Ltd). GB 2126208 (1984) US 4613492 (1986). 

The key elements of the front end are the primary reformer, which can be described as a set of catalyst-fiDed tubes being arranged in a fired box, and the secondary (autothermal) reformer, where air is fed to the process for further heating and simultaneous introduction of nitrogen. Maximum temperatures of more than 1000 °C are reached. It has to be noted, however, that there is a variety of different front end processes available from different licensors. These include processes with deviating duty allocation for primary and secondary reformer as well as systems with process gas heated reformers and different solutions for carbon dioxide removal and synthesis gas purification. ... 

The design of secondary reformers is discussed in [90, 92, 127]. Special designs where primary and secondary reforming or primary reforming and non-catalytic partial oxidation are combined in one unit are suggested in [128-130, 964]. 

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