Isothermal ammonia converter

Details Isothermal ammonia converter Isothermal ammonia converter

1998-10-12

2001-01-09

Langel, Wayne (Department: 1754)

Chemistry of inorganic compounds

Nitrogen or compound thereof

Ammonia or ammonium hydroxide

C422S148000, C423S360000

Reexamination Certificate

active

06171570

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an isothermal ammonia converter, and more particularly to an ammonia converter and method for converting ammonia synthesis loop purge gas containing nitrogen and hydrogen to form additional ammonia.
BACKGROUND OF THE INVENTION
Ammonia is commonly manufactured by reacting nitrogen and hydrogen in a synthesis loop including a compressor, an ammonia synthesis reactor and ammonia condensation and recovery. The unreacted synthesis gas mixture is typically recycled from the ammonia separator to the compressor and back to the reactor. Make-up synthesis gas is continuously added to the synthesis loop to provide fresh hydrogen and nitrogen. Because the synthesis gas contains argon, methane and other inert components, a purge stream is usually taken from the synthesis loop to avoid the excessive buildup of the inerts in the synthesis loop. The purge gas is typically processed in a hydrogen recovery unit, and a hydrogen-enriched stream is recycled to the synthesis loop. In some cases, the purge gas is used directly in the fuel system with or without any additional treatment or hydrogen recovery.
A significant technological advance in the manufacture of ammonia has been the use of a highly active synthesis catalyst comprising a platinum group metal such as ruthenium on a graphite-containing support as described in U.S. Pat. Nos. 4,055,628; 4,122,040; and 4,163,775; all of which are hereby incorporated herein by reference. Also, reactors have been designed to use this more active catalyst, particularly the catalytic reactor bed disclosed in U.S. Pat. No. 5,250,270 which is hereby incorporated herein by reference. Other ammonia synthesis reactors include those disclosed in U.S. Pat. Nos. 4,230,669; 4,696,799; and 4,735,780; and the like.
Ammonia synthesis schemes have also been developed based on the highly active catalyst. In U.S. Pat. No. 4,568,530, stoichiometrically hydrogen-lean synthesis gas is reacted in a synthesis reactor containing the highly active catalyst in the synthesis loop.
In U.S. Pat. No. 4,568,532, an ammonia synthesis reactor based on the highly active catalyst is installed in series in the synthesis loop downstream from a reactor containing the more conventional iron-based synthesis catalyst.
In U.S. Pat. No. 4,568,531, the purge gas removed from the primary synthesis loop is introduced into a second synthesis loop using the more active synthesis catalyst to produce additional ammonia from the purge stream. Another purge stream, significantly reduced in size, is taken from the second synthesis loop to avoid the excessive buildup of inerts. The second synthesis loop, like the primary synthesis loop, employs a recycle compressor to recycle synthesis gas to the active catalyst converters in the second synthesis loop.
It would be very desirable to convert hydrogen and nitrogen in the purge stream from a conventional ammonia synthesis loop into additional ammonia using a once-through reactor which does not require staged cooling and a synthesis gas recycle compressor.
SUMMARY OF THE INVENTION
The present invention is directed to an ammonia converter which can be used to convert ammonia synthesis loop purge gas containing nitrogen and hydrogen to form additional ammonia. The ammonia converter is a shell-and-tube reactor using a platinum group metal ammonia synthesis catalyst in the tubes which are maintained in essentially an isothermal condition by boiling water or another heat transfer fluid on the shell side. The ammonia converter allows the ammonia synthesis process to produce additional ammonia from the synthesis loop purge gas by passing the purge gas through the isothermal ammonia converter. The ammonia converter can be installed as a retrofit modification of an existing ammonia synthesis plant to pass the purge stream or a combination of purge streams from several plants through the isothermal ammonia converter on a once-through basis to form additional ammonia, and reduce the size of the purge gas stream which is either processed further in a hydrogen recovery unit or sent to the fuel system directly.
In one aspect, then, the present invention provides an ammonia converter for converting ammonia synthesis loop purge gas containing nitrogen and hydrogen to form additional ammonia. The ammonia converter is a shell and tube reactor having upright tubes. A source of feed gas contains nitrogen and hydrogen for supply to an inlet of the tubes. Ammonia synthesis catalyst in the tubes is adapted to convert the nitrogen and hydrogen to ammonia as the gas passes through the tubes. A source of saturated boiler feed water supplies boiling water to a shell-side of the reactor to maintain a substantially isothermal shell-side condition and remove heat from the tubes. A tube-side outlet is provided for recovering product gas having an increased ammonia content relative to the feed gas. The catalyst preferably comprises a platinum group metal such as ruthenium supported on graphite. The tubes are preferably sized for containing a catalyst volume, and present an area for heat transfer to the boiling water, to maintain the feed and product gases at a temperature in the range from 315° C. to 435° C. at a reaction pressure from 60 to 210 bar. The pressure of the shell-side boiling water is preferably from 60 to 150 bar. The feed gas preferably comprises synthesis loop purge gas having an ammonia content less than 4 mole percent, and the product gas preferably has an ammonia content from about 15 to about 40 mole percent. The converter can further include an ammonia separator for removing ammonia from the product gas to form an ammonia-lean stream, a hydrogen recovery unit for removing hydrogen from the ammonia-lean stream to form a nitrogen-rich stream, and a compressor for recycling a portion of the nitrogen-rich stream to the feed gas source.
In another aspect, the present invention provides a method for converting ammonia synthesis loop purge gas containing nitrogen and hydrogen to form additional ammonia. The method includes the steps of supplying the synthesis loop purge gas to the inlet of the tubes of the shell and tube reactor of the ammonia converter described above, operating the ammonia converter, and recovering ammonia from the product gas to form an ammonia-lean stream. The method can also include the step of preheating the synthesis purge gas in heat exchange with the product gas. The ammonia recovery step preferably includes cooling the product gas to condense ammonia and separating the liquid ammonia from the ammonia-lean stream. The method can also include the steps of supplying the ammonia-lean stream to a hydrogen recovery unit to form a nitrogen-rich stream and a hydrogen-rich stream, compressing a portion of the nitrogen-rich stream and recycling the compressed nitrogen-rich stream into the preheated synthesis loop purge gas, and recycling the hydrogen-rich stream to the synthesis loop.
In a further aspect of the invention, there is provided a method for retrofitting an ammonia plant having a synthesis loop and a purge gas loop. The retrofit method is particularly applicable to retrofitting an ammonia plant wherein fresh ammonia synthesis gas containing hydrogen and nitrogen is combined in the synthesis loop with first and second recycle streams to form a combined ammonia synthesis gas, the combined ammonia synthesis gas is reacted over ammonia synthesis catalyst to form a converted gas, and a purge gas stream and ammonia are removed from the converted gas to form the first recycle stream; and wherein the purge gas stream is processed in a hydrogen recovery unit to form a nitrogen-rich stream and a hydrogen-rich stream which is supplied to the synthesis loop as the second recycle stream. The retrofit method includes installing a shell and tube reactor having upright tubes containing ammonia synthesis catalyst for once-through conversion of nitrogen and hydrogen in a purge gas feed stream, including the purge gas stream from the synthesis loop, into additional ammonia in a reactor effluent stream. Boiler feed water i

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