Reactions steam-generation systems

The heat of reaction in ammonia synthesis is 46.22 kJ/mol ammonia at standard temperature and pressure. It is of utmost importance to integrate the ammonia synthesis process with the steam generation system. The waste heat not used for preheating the converter feed gas and for preheating the inlet gas streams for each catalyst bed is most efficiently used for production of high pressure steam. Normally waste heat down to around 80-100°C can be utilised for these purposes. 

In a typical PAFC system, methane passes through a reformer with steam from the coolant loop of the water-cooled fuel cell. Heat for the reforming reaction is generated by combusting the depleted fuel. The reformed natural gas contains typically 60 percent H9, 20 percent CO, and 20 percent H9O. Because the platinum catalyst in the PAFC can tolerate only about 0.5 percent CO, this fuel mixture is passed through a water gas shift reactor before being fed to the fuel cell. 

Description N-butane and air are fed to a fluid-bed catalytic reactor (1) to produce maleic anhydride. The fluid-bed reactor eliminates hot spots and permits operation at close to the stoichiometric reaction mixture. This results in a greatly reduced air rate relative to fixed-bed processes and translates into savings in investment and compressor power, and large increases in steam generation. The fluid-bed system permits online catalyst addition/removal to adjust catalyst activity and reduces downtime for catalyst change out. 

The trend followed in newer plants is to increase conversion per pass with the result of higher ammonia outlet concentrations and lower outlet temperatures from the last bed. However, as optimum energy efficiency of the whole ammonia plant requires maximum high-pressure steam generation, part of the heat must be recovered before the reaction is completed in the reactor system. This can be accomplished [900], [901], [930], [931] by using three catalyst beds in separate pressure vessels with boilers after the second and the third vessel and an inlet - outlet heat exchanger for the first catalyst bed. 

Reactions R-4.5 and R-4.6 are main reactions to generate H2 and CO. Both reactions are endothermic and have high activation energies. This means that they are slow compared with reactions R-4.1 and R-4.2, and slow down as they proceed since the temperature decreases with extent of reaction. The reaction rates are proportional to the partial pressure of the steam in the system. 

The reactor and the hydrogen production system are connected by the helium gas loop. A chemical reactor causes the temperature fluctuation of the secondary helium gas by the fluctuation of the chemical reaction that occurs at the normal start-up and the shutdown operation as well as malfunction or accident of the hydrogen production system. If the temperature fluctuation is transferred to the reactor, the reactor will be stopped. Therefore, the control technology should be developed to mitigate the temperature fluctuation within an allowable range to keep reactor operation, using a thermal absorber. JAEA proposed to use a steam generator (SG) as the thermal absorber that is installed downstream the chemical reactor in the secondary helium gas loop. 

Figure 4.13 The steam reforming process by Howe-Baker Engineers. Inc. (1) hydrotreater, (2) desulfurizer, (3) reformer, (4) process steam generator, (5) water-gas shift reaction, (6) PSA purification system.

Successful operation of steam generator (SG) holds the key to achievement of high capacity factors. SG requires the high quality during manufacture and a sensitive leak detection system for sodium-water reaction detection and mitigation. PFR incident of failure of 40 tubes of superheater has led to redefinition of design basis leak for SG from the earlier considered incident of double ended rupture of one tube [6]. 

---