The additional syngas is then mixed with the gas coming from the second stage of the existing syngas compressor and the combined gas is then passed through the OT synthesis. This process unit comprises of a gas / gas heat exchanger to provide the elevated converter inlet temperature, the actual OT synthesis converter, a steam gen-erator / boiler feed water preheater and a cooling train. The latter consists of a water cooler and a series of chillers, bringing the process gas temperature down to a level for separation of most of the generated ammonia by condensation.
Subsequently, the remaining process gas is passed on to the third stage of the synthesis gas compressor for further compression up to the pressure level of the ammonia synthesis loop. Essentially, the flowrate and composition of the gas fed to the synthesis loop is the same as in the original plant.
The Uhde Dual-Pressure Process has been suc-cessfully installed already for a revamp in a plant in Slovakia  and for two new plants , being the two largest single-train ammonia plants in the world.
One item particular to this plant is that the purge gas from the loop is not sent to a hydrogen recovery unit because it is already used for other purposes in the existing complex. This feature is maintained also for the revamp calculations. Since it is done like that for all investigated concepts, it does not affect the results of the study. Technically, there would be no difficulty to add a unit separating the hydrogen from the purge gas and returning it to the synthesis loop.
For each process concept, the steam system is adjusted to match steam production from waste heat with steam consumption of process and turbines. Same as in the existing plant, also after the revamp, some MP steam has to be imported from outside battery limits.
Table 1 gives an overview which main equipment items have to be replaced or significantly modified for the three concepts. This list forms the basis for the capital cost assessment.
Results of the Economical Comparison
Operating Cost (OPEX)
The following streams entering or leaving the plant (see also Fig. 1) are associated with dedicated energy contents resp. cost data:
• Import: Feed gas, Fuel gas, MP steam, Electric energy.
• Export: Purge gas stream from ammonia synthesis.
Table 2 contains a representation of these streams for each revamp concept in terms of their energy content:
• Feed, fuel and purge gas export are repre-sented by their lower heating value (LHV).
• MP steam is represented by the fuel energy required for its production in a boiler.
• The imported electrical energy is shown as the fuel energy required for generating it via a steam cycle with an overall efficiency of 30 % (1 kWh electrical energy corresponds to 12000 kJ natural gas).
The overall consumption figures are shown in the bottom line of the table. Of course, the air separation unit with its motor-driven compressor is always included in the figures.
The consumption figures are relatively high compared to the values achieved by newly built ammonia plants. It is worth to mention that the study deals with an old plant and that the focus of the study is on capacity increase, not energy optimization. Certainly it would be possible to additionally improve the energy efficiency of the plant, but that would lead to additional cost. Since the changes would be about the same for all three concepts, they would not add value to the target of comparing the revamp concepts and thus are left out of consideration.
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