Hydrogen secondary reforming

MPa (300—400 psig), using a Ni-based catalyst. Temperatures up to 1000°C and pressures up to 3.79 MPa (550 psia) are used in an autothermal-type reformer, or secondary reformer, when the hydrogen is used for ammonia, or in some cases methanol, production. 

Under these conditions the issuing gases contain some 9% of unreacted methane sufficient air is injected via a compressor to give a final composition of 1 3 N2 H2 and the air bums in the hydrogen thereby heating the gas to 1100°C in the secondary reformer ... 

For producing hydrogen for ammonia synthesis, however, further treatment steps are needed. First, the required amount of nitrogen for ammonia must he obtained from atmospheric air. This is done hy partially oxidizing unreacted methane in the exit gas mixture from the first reactor in another reactor (secondary reforming). 

The main reaction occurring in the secondary reformer is the partial oxidation of methane with a limited amount of air. The product is a mixture of hydrogen, carhon dioxide, carhon monoxide, plus nitrogen, which does not react under these conditions. The reaction is represented as follows ... 

Figure 8.21 shows the scheme for producing ammonia. First, the natural gas is desulfurized and then steam-reformed in the primary reformer into a mixture of unreacted methane (10-13 %), CO, CO2, and FI2 that is then combined with air, which contains the necessary nitrogen for the ammonia process, to react in a secondary reformer. Here the oxygen reacts with hydrogen and methane in strongly... 

The source of nitrogen for the synthesis gas has always been air, either supplied directly from a liquid-air separation plant or by burning a small amount of the hydrogen with air in the H2 gas. The need for air separation plants has been eliminated in modern ammonia plants by use of secondary reforming, where residual methane from the primary reformer is adiabatically reformed with sufficient air to produce a 3 1 mole ratio hydrogen-nitrogen synthesis gas. 

In ammonia plants, the secondary reformer is included to decrease further the proportion of methane in the final gas and also to introduce the required amount of nitrogen for ammonia synthesis. The bed temperature is maintained at 1000 °C and this is achieved by adding air to the gas stream, the oxygen of the air reacting with the hydrogen of the gas stream to form water. The reactor consists of a packed bed and no additional heating is required. The exit gas contains less than 0.1% CH4. The catalyst for this reactor does not require to have very high activity but it must be stable under these reaction conditions. 

If the hydrogen is made by steam reforming, air is introduced at the secondary reformer stage to provide nitrogen for the ammonia reaction. The... 

In the secondary reformer, air is introduced to supply the nitrogen required for the 3 1 hydrogen H2 and nitrogen N2 synthesis gas. The heat of combustion of the partially reformed gas supplies the energy to reform the remaining hydrocarbon feed. The reformed product steam is employed to generate steam and to preheat the natural gas feed. 

The secondary reformer vessel is a refractory-lined vessel that has an oxygen burner in its top neck and a fixed catalyst bed. Installation of a secondary reformer usually requires significant changes to the CO2 removal system Hydrogen purity can be increased up to 98%. The economics generally depend on a reliable source of low-cost oxygen172. 

In an ammonia plant (Figure 4.2), the synthesis gas from the reformer furnace is fed into a secondary reformer vessel, where air is added through a burner to create outlet vessel temperatures of -1,800° F (980° C). The outlet of the secondary reformer vessel is cooled in a quench steam generator and sent to a shift converter this is followed by a carbon dioxide removal system such as the one in a hydrogen plant. The purified nitrogen from the air added in the secondary reformer vessel and hydrogen synthesis gas is fed to a methanator to convert residual oxides of carbon back to methane (which is inert in the ammonia conversion) the gas is then compressed to -3,000 psia (2,070 kPa). The compressed synthesis gas is fed to an ammonia converter vessel. As the synthesis gas passes over catalyst beds, ammonia is formed. The ammonia product is then cooled and refrigerated to separate out impurities. 

The front end section of hydrogen, methanol, and ammonia plants is shown in Figure 4.4. The secondary reformer is used only in an ammonia plant. The feed gas is desulfurized in carbon steel equipment. When the metai temperature exceeds 800 to 850° F (425 to 455°C), lCr-V Mo(2) or IViCr-VfeMo are used to avoid long-term deterioration of the mechanical properties by graphitization. Preheat coils in the top of the reformer furnace are usually 21/4Cr-1Mo up to 1,200°F (650° C) metal Temperature and type 304H... 

The secondary reformer in an ammonia plant is a carbon steel vessel with a dual layer refractory lining. Internal temperatures reach -2,000°F (1,090°C) from burning as a result of air added through a burner at the top of the vessel to the feed gas (hydrogen, carbon monoxide, carbon dioxide, and steam). The burner is a refractory-lined device that is subject to failure if not carefully designed. Quench steam generators have refractory-lined inlet channels and tube sheets. Tubes are often made of carbon steel because the heat transfer from the steam on the outside of the tube is markedly better than that from the synthesis gas inside the tube. As a result, the metal temperature closely approaches the temperature of the steam. The inlet ends of the tubes are protected from the inlet gas by ferrules, usually made of type 310 (UNS S31000) SS with insulation between the ferrule and the tube. The tube material should be selected... 

Key features are the high reforming pressure (up to 41 bar) to save compression energy, use of Uhde s proprietary reformer design [1084] with rigid connection of the reformer tubes to the outlet header, also well proven in many installations for hydrogen and methanol service. Steam to carbon ratio is around 3 and methane slip from the secondary reformer is about 0.6 mol % (dry basis). The temperature of the mixed feed was raised to 580 °C and that of the process air to 600 °C. Shift conversion and methanation have a standard configuration, and for C02 removal BASF s aMDEA process is preferred, with the possibility of other process options, too. Synthesis is performed at about 180 bar in Uhde s proprietary converter concept with two catalyst beds in the first pressure vessel and the third catalyst bed in the second vessel. 

After desulfurization, steam is added and the mixture heated to 480 to 550°C before it is fed into the primary reformer. The gas leaving the primary reformer contains between 7 and 10% methane. This is removed in so-called secondary reformers in which the gas leaving the primary reformer is partially burnt with air in nickel catalyst-filled shaft furnaces (autothermal process), whereupon the temperature increases to ca. 1000°C. Under these conditions the methane reacts with the steam reducing the methane content in the synthesis gas to ca. 0.5 mole %. The quantity of air is adjusted to give the nitrogen to hydrogen ratio required for the stoichiometry of the ammonia synthesis. 

FIGURE 11.2 Outline of the main components of an ammonia synthesis plant using reforming and secondary reforming as the principal sources of hydrogen. Electrolysis of water is used to supplement this. 

Typically, the secondary reformer is a refractory-lined cylinder, the lower part of which is filled with a catalyst similar to that in the primary reformer. The upper part is the combustion chamber and the lower part continues the formation of carbon monoxide and hydrogen from the unbumed methane. The gases leave the secondary reformer at 950-1000°C, and are passed through a heat exchanger and cooled to 375°C. The intended reaction in this stage is the water-gas shift reaction ... 

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