PROCESS FOR THE PREPARATION OF AMMONIA
Field of the Invention The present invention relates to certain improvements in the preparation of ammonia from an ammonia synthesis gas containing hydrogen and nitrogen. In a more particular way, this invention relates to the synthesis of ammonia in a high production capacity, in at least two ammonia synthesis reactors configured in synthesis cycles in parallel with a combined compression step of synthesis recycle gas . BACKGROUND OF THE INVENTION The synthesis of ammonia is conventionally carried out by passing a process stream of synthesis gas containing nitrogen and hydrogen, in essentially stoichiometric quantities, for the formation of ammonia, through a catalyst configured in one or more beds in a reactor. Thermodynamically, the reaction of hydrogen and nitrogen to obtain ammonia requires that the reaction be carried out at elevated temperature and pressure, usually more than
100 atmospheres, and at temperatures between 300 ° C and 600 ° C. Under the above process conditions, the equilibrium concentration of the ammonia in a stoichiometric synthesis gas composition is less than 20 percent by volume in the synthesis gas stream from the reactor, and the stream has to be recycled to the reactor together with fresh synthesis gas, in order to provide a reasonable yield of the ammonia product. Prior to recycling, the stream is cooled to separate the ammonia from hydrogen, nitrogen, and unreacted inert diluents present in the synthesis gas, and a fraction of the recycle gas is vented to vent the inert products. The separated gas is then passed to a compression stage, where it is recycled to the reaction. The expenses for compressing and recycling the synthesis gas are an important factor in the economy of ammonia production in general, and in particular when the production capacity of the resistant ammonia synthesis cycles has to be increased. Accordingly, it is the general object of this invention to provide improvements in the preparation of ammonia by utilizing a combined compression step in the recirculation of the synthesis gas from at least two ammonia cycles. SUMMARY OF THE INVENTION This invention is a process for the preparation of ammonia from a synthesis gas containing hydrogen and nitrogen, which comprises the steps of: (a) mixing with the synthesis gas, the recycle gas from a ammonia separation step (f) and (g);
(b) pressurizing the mixed gas from step (a); (c) dividing the gas from step (b) into a first filler gas stream and a second filler gas stream; (d) introducing the first inflow gas stream into a first ammonia reactor, and removing a first stream from the ammonia-rich process; (e) introducing the second stream of ammonia filling gas into a second ammonia reactor, and removing a second stream from the ammonia-rich process; (f) separating the ammonia from the first process stream, and recycling the remainder of the process stream to step (a); (g) separating the ammonia from the second process stream, and recycling the remainder of the process stream to step (a); (h) combining the ammonia separated from step (f) and step (g), and removing a stream of ammonia product. The above process may comprise one or more of the additional steps of: before step (a), pressurizing the ammonia synthesis gas; purify the pressurized ammonia synthesis gas by removing water and carbon dioxide; and further pressurizing the purified ammonia synthesis gas. As mentioned hereinabove, the invention is particularly useful when a higher production capacity of an existing ammonia cycle is required. When the existing cycle is modernized, an additional ammonia synthesis cycle is connected in parallel with the existing cycle. As an advantage of the invention, the existing synthesis gas recirculation compressor, optionally after the modification, recycles both streams of recycle synthesis gas from the existing cycle and from the connected additional cycle. In this way, only two links will be required in the synthesis cycle, upstream and downstream of the recirculation compressor. Detailed Description of the Invention Reference is made to the drawings, in which the single figure represents a simplified flow scheme of the process of the invention. The synthesis gas of ammonia 2 is compressed in a first stage of compression of synthesis gas 4 at 65.8 kg / cm2. The compressed gas is passed to a molecular sieve unit 6 to remove H20 and C02. The dry and purified synthesis gas 8 is then passed to a second synthesis gas compression stage 10, where it is first compressed to 137.69 kg / cm 2, and then mixed with the recycle gas 20 provided by the gas combination of recycle 22 from a second separator 32 and the recycle gas 24 from a first separator 26. The recycle gas 22 and 24 is heated in one or more heat exchangers (not shown) prior to their combination. The mixture of synthesis gas and combined recycle gas is further compressed to 148.54 kg / cm2 in a third compression stage 11. 88.1 percent of the synthesis gas 12 leaving the synthesis gas compression stage 11, passed in stream 14 to a hot gas heat exchanger (not shown), and to a first ammonia synthesis reactor 16 which is indirectly cooled, and adapted for a radial gas flow, as described in, Patent of the United States of North America No. 4,181,701. The effluent gas 18 from the reactor 16 is cooled in a conventional heat exchanger train (not shown), and 1591 STPD of liquid ammonia is separated in the gas separator 26, before the separated unconverted gas 24 is mixed with the gas. unconverted gas from stream 22. The remaining 11.9 percent of the synthesis gas leaving the synthesis gas compression stage 11 is passed in stream 30 to a second indirectly cooled radial flow ammonia synthesis reactor. The eluent gas 29 from the reactor 28 is cooled in a series of heat exchangers (not shown), and 218 STPD of liquid ammonia is separated in the gas separator 32. The unconverted gas leaving the separator 32 is separated in a small purge gas stream 34 and the unconverted gas stream 22. The gas stream 22 is mixed with the unconverted synthesis gas from the separator 26, and passes to the recycle part of the synthesis gas compression 11, as described above. Alternatively, purge gas stream 34 can be taken from stream 24 as shown by the dotted line of the Figure. A portion of the hydrogen contained in the purge gas stream 34 is recovered in a conventional hydrogen recovery unit 36, and recycled in stream 38 to the synthesis gas stream 8. The liquid ammonia product 39 from the separator 26, is combined with the liquid ammonia product 40 from the separator 32, and withdrawn in the product stream 42. The recycle stream 38 from the hydrogen recovery unit 36, may alternatively be recycled to stream 2, or in an alternative way (in the case of a two-stage recovery unit), it can be recycled to stream 8 and stream 2, as shown by the dashed line in the Figure. The composition of the currents described above is summarized in the following table.