Ammonia Converter Efficiency
- Precise determination of the H2/N2 ratio of the converter feed
- Steam/carbon ratio
- Methane slippage
- Inert gas buildup
- Catalyst activity
Raw gases are usually produced in a primary-secondary stream reformed process according to the reaction:
CH4 + H2O
-> CO + 3H2
CO + H2O
-> CO2 + H2
Air is admitted in the Secondary Reformer step to give an exit gas with a three to one H2/N2 ratio.
On the pilot tests, this stream was monitored by the MGA, not only to control the ratio, but also to measure methane, which along with argon from the air, enters the synthesis loop as an inert gas.
The gas then goes to a two-stage catalytic Shift Converter, where the carbon monoxide is converted to carbon dioxide to produce more hydrogen according to the reaction:
CO + H2O
-> CO2 + H2
In the next stage, the carbon dioxide is removed in the Absorber Section to prevent poisoning the ammonia synthesis catalyst. The absorbent is generally an ethanolamine or hot carbonate. From the
Carbon Dioxide Absorber, the partially purified synthesis gas then goes to a Methanator, where the residual carbon dioxide, not removed during the absorber stage is converted to methane by:
CO2 + 4H2
-> CH4 + 2H20
From the Methanator, small amounts of methane and water enter the synthesis loop as inert gases, but at this stage of the process they are not poisonous to the catalyst. However, it should be noted that
the water is removed from the process stream before the gas enters the conversion stage. This exit gas from the Methanator is now the “make-up” gas for the synthesis loop, and the composition of the gas is
critical at this stage of the process. During the pilot tests, the process stream was monitored by the MGA at the Methanator exit stream.
From the Methanator, the “make-up” gas goes into the conversion-separation loop where it is mixed with recycle gas from the Ammonia Converter, compressed, fed to the Separator to extract the processed ammonia, then finally fed to the Ammonia Converter. The composition of the gas after the ammonia has been removed at the Ammonia Converter inlet stream was monitored at the Converter feed stream.
At the outlet of the Ammonia Converter, the effluent containing ammonia is analyzed, and the ammonia content is maximized by “trimming” the hydrogen-nitrogen ratio. Depending on the compressor loads, this can be achieved by either varying the gas or air at the reformers.
Thus, while this example is a vast simplification of the overall flow, and many of the finer details of plant operation have been omitted, a single MGA can monitor the four critical stages of the ammonia synthesis process once every 10 seconds, and repeat the entire cycle for all four processes once every 40 seconds.
In summary, it can be concluded that in addition to increased ammonia production, the MGA permits increased efficiency in the operation of the Ammonia Converter, thus decreasing the recycle compressor loading, and yielding a significant savings in energy. This is due primarily to the fact that in the ammonia synthesis process, the conversion per pass is relatively low, and the recycle rates are relatively high;
thus efficient operation of the compressors is of paramount importance to the unit.
With a four-stream application, a complete analysis of all process gases of interest can be accomplished in 40 seconds with the MGA