Reforming secondary

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. 

Gas-Heated Reforming. Gas-heated reforming is an extension of the combined reforming concept where the primary reformer is replaced by a heat-transfer device in which heat for the primary reforming reaction is recovered from the secondary reformer effluent. Various mechanical designs have been proposed which are variants of a shell-and-tube heat exchanger (12,13). 

The ICI Treading Concept Methanol (LCM) process incorporating a GHR and oxygen blown secondary reformer is shown schematically in Figure 4. This process was commercialized in AustraUa in 1994 (14). 

Reforming is completed in a secondary reformer, where air is added both to elevate the temperature by partial combustion of the gas stream and to produce the 3 1 H2 N2 ratio downstream of the shift converter as is required for ammonia synthesis. The water gas shift converter then produces more H2 from carbon monoxide and water. A low temperature shift process using a zinc—chromium—copper oxide catalyst has replaced the earlier iron oxide-catalyzed high temperature system. The majority of the CO2 is then removed. 

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 ... 

Excess Nitrogen Removal. A number of low energy processes use excess air in the secondary reformer in order to reduce the primary reformer duty. The surplus nitrogen so introduced has to be removed later in the process. 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

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. 

Ammonia Plant 1. Where possible, use natural gas as the feedstock for the ammonia plant, to minimize air emissions. 2. Use hot process gas from the secondary reformer to heat the primary reformer tubes (the exchanger-reformer concept), thus reducing the need for natural gas. 

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. 

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 ... 

The reactor temperature can reach over 900°C in the secondary reformer due to the exothermic reaction heat. Typical analysis of the exit gas from the primary and the secondary reformers is shown in Table 5-1. 

The second step after secondary reforming is removing carbon monoxide, which poisons the catalyst used for ammonia synthesis. This is done in three further steps, shift conversion, carbon dioxide removal, and methanation of the remaining CO and CO2. 

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

The product gas mixture from the secondary reformer is cooled then subjected to shift conversion. 

Figure 5-2. The ICI process for producing synthesis gas and ammonia (1) desulfurization, (2) feed gas saturator, (3) primary reformer, (4) secondary reformer, (5) shift converter, (6) methanator, (7) ammonia reactor.

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... 

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. 

Figure 3.4 A secondary reformer converts the last of the methane. (Courtesy of Solutia Inc., Luling, LA)...

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

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. 

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