Secondary steam reforming

Ammonia manufacture from natural gas by the steam reforming process is well documented. Briefly, raw synthesis gas consisting mainly of H2, N2, and C02 is produced by primary and secondary steam reforming and CO shift conversion this is followed by bulk C02 removal, elimination of residual CO and CO2 through methanation, and ammonia synthesis. These basic steps... 

Primary steam reforming Secondary steam reforming Carbon monoxide conversion Carbon monoxide methanation Ammonia synthesis Sulfuric acid synthesis Methanol synthesis Oxo synthesis Ethylene oxide Ethylene dichloride Vinylacetate Butadiene Maleic anhydride Phthalic anhydride Cyclohexane Styrene Hydrodealkylation Catalytic reforming Isomerization Polymerization (Hydro)desulfurization Hydrocracking... 

Figure I Reoxidation of secondary steam reforming catalyst bed. Profiles of a) nickel and (b) hydrogen concentrations as a function of time. 

During ammonia synthesis, the major reactions of production and purification of synthesis gas and the synthesis of ammonia, all are carried out over different catalysts. At least eight kinds of catalysts are used in the whole process, where natural gas or naphtha is used as feedstock and steam reforming is used to produce synthesis gas. These catalysts are Co-Mo hydrogenation catalyst, zinc oxide desulfurizer, primary- and secondary-steam reforming catalysts, high- and low-temperature shift catalysts, methanation catalyst and ammonia synthesis catalyst etc (Table 1.1). 

The eight kinds of catalysts may be roughly classified as protective catalysts and economic catalysts . Co-Mo hydrogenation catalyst and zinc oxide desulfurizer are the protective catalysts for the primary steam reforming catalysts. The high-temperature shift catalyst protects the low-temperature shift catalyst, and the methanation catalyst are the protective catalyst for ammonia synthesis catalyst. The catalysts for primary- and secondary-steam reforming, low-temperature shift and ammonia synthesis are responsible for the conversions of raw materials and the yield of products, and have direct effect on economic benefits of the whole plant, and are thus called as economic catalysts. The amount of catalysts used depends on the process and raw material. Table 1.2 represents the amount of the eight kinds of catalysts used in the different processes. The total volume of the catalysts is about 330 m in every plant, while there are only two kinds of catalysts with the volume of about 100-140 m when heavy oil or coal is used as raw material. Both shift... 

Hydrocarbon steam reforming catalysts are classified into natural gas steam reforming catalysts and light-oil steam reforming catalysts according to the feedstock, and primary- and secondary- steam reforming catalysts according to the processes. 

The equipment of deep cooling brings a series of convenience for operation sequences before and after the purification and excellent comprehensive effects on the whole plant. For example, due to the deep cooling purification, the requirements for the secondary steam reformer are decreased remarkably, and there are no strict requirements for the ratio of H2 to N2 and the content of CH4. The outlet temperature of the secondary steam reformer can be lower than the conventional operation... 

The secondary steam reformer of an ammonia plant has to be opened to replace the Ni/alumina catalyst. This requires the prior careful reoxidation of the pyrophoric Ni-catalyst. Simulate the reoxidation by means of an oxygen/nitrogen mixture containing 5 volume % oj gen. Use the general model of Section 4.2 to calculate the evolution with time of the concentration profiles of oxygen ( ) and Ni (. in the catalyst particle for various values of the rate parameters and for conversions based on 5 of up to 50 %. The initial Ni content of the catalyst (Cso) is 0.00624 kmoFkg cat. 

Secondary steam reforming Ethylene dichloride Isomerization... 

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. 

Primely and Secondary Reforming . The conversion of natural gas to synthesis gas in the reforming operation is represented by steam reforming ... 

Figure 8.3.1 is a typical process diagram for tlie production of ammonia by steam reforming. Tlie first step in tlie preparation of tlie synthesis gas is desulfurization of the hydrocarbon feed. Tliis is necessary because sulfur poisons tlie nickel catalyst (albeit reversibly) in tlie reformers, even at very low concentrations. Steam reforming of hydrocarbon feedstock is carried out in tlie priiiiiiry and secondary reformers. 

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... 

Steam reforming needs a secondary fuel to provide the energy supply necessary for the reaction that occurs and a catalysts to improve the kinetic of this process. In Equation (3), the primary fuel is partially oxidised by a limited amount of oxygen. Partial oxidation produces less H2 per fuel unit than stream reforming, but the kinetic reaction is faster, it requires smaller reactors and neither catalyst nor energy supply from a secondary fuel. 

Natural gas is reacted with steam on an Ni-based catalyst in a primary reformer to produce syngas at a residence time of several seconds, with an H2 CO ratio of 3 according to reaction (9.1). Reformed gas is obtained at about 930 °C and pressures of 15-30 bar. The CH4 conversion is typically 90-92% and the composition of the primary reformer outlet stream approaches that predicted by thermodynamic equilibrium for a CH4 H20 = 1 3 feed. A secondary autothermal reformer is placed just at the exit of the primary reformer in which the unconverted CH4 is reacted with O2 at the top of a refractory lined tube. The mixture is then equilibrated on an Ni catalyst located below the oxidation zone [21]. The main limit of the SR reaction is thermodynamics, which determines very high conversions only at temperatures above 900 °C. The catalyst activity is important but not decisive, with the heat transfer coefficient of the internal tube wall being the rate-limiting parameter [19, 20]. 

Composition of Some Industrial Steam Reforming Catalysts (NG = natural gas, HC = hydrocarbon, PR = prereforming, LPG = liquefied petroleum gas, SEC = secondary reforming)... 

Here we shall briefly summarize the effects of individual poisons on various catalytic reactions taking place on automotive catalysts. There are three main catalytic processes oxidation of carbon monoxide and hydrocarbons and reduction of nitric oxide. Among secondary reactions there are undesirable ones which may produce small amounts of unregulated emissions, such as NH3, S03 (6), HCN (76, 77), or H2S under certain operating conditions. Among other secondary processes which are important for overall performance, in particular of three-way catalysts, there are water-gas shift, hydrocarbon-steam reforming, and oxygen transfer reactions. Specific information on the effect of poisons on these secondary processes is scarce. 

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

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