The reaction heat of ammonia synthesis is about 2.72 GJ/t of NH3. The utilization of this heat at the highest possible temperature for the production of high pressure (HP) steam can significantly improve the overall energy efficiency of the plant (Ullmann’s, 2011 p.204).
There is a trend in new ammonia plants to increase the conversion per pass which results in higher ammonia concentrations at the converter outlet and lower outlet temperatures from the last conversion bed. To produce HP steam, a part of the heat needs to be recovered before the reaction is completed in the synthesis converter system. This can be accomplished with the use of three catalyst beds in separate pressure vessels and the use of waste heat boilers after the second and the third vessel and an inlet-outlet heat exchanger for the first catalyst bed (Ullmann’s, 2011 p.205).
Several ammonia plants have installed an additional ammonia synthesis converter in combination with a HP steam waste heat boiler, downstream of the existing ammonia converter. The result is increased conversion per pass, reduced compression requirements due to the smaller recycle gas stream, and improved waste heat recovery (CDM, 2006).
The Topsoe S-250 system uses two radial flow converters placed in series with waste heat boilers between the converters and after the last converter (see Figure). This system compared to the S-200 series (employing one converter) is claimed to increase the conversion per pass and reduce the energy use. Similar energy savings and increase in the conversion per pass can also be achieved with the replacement of the S-200 with a three-bed radial flow converter with two internal heat exchangers (S-300) (UNIDO, 1979 p.181-182).
To enable the HP steam production, KBR has recognized the following modifications in the existing ammonia synthesis converter configurations. According to KBR, HP steam production can be achieved by removing the existing internal heat exchanger bundle inside at the top of the converter and incorporating a new specially engineered external feed/effluent heat exchanger in the scheme outside the ammonia converter. The energy savings are estimated at 0.8-1.3 GJ/t NH3 and will depend on the existing configuration (KBR, 2009).