The present invention relates to a process and an apparatus for the purification of hydrogen by cryogenic nitrogen wash and co-production of liquid methane.

Ammonia synthesis units are fed with a stoichiometric mixture of hydrogen and nitrogen. The hydrogen is produced from steam methane reforming, a partial oxidation reactor or coal gasification, for example. The hydrogen may contain impurities, such an argon, carbon monoxide, carbon dioxide or oxygen, which may act as catalyst poisons for the ammonia.

In the context of this document, a methane rich liquid contains at least 70% mol. methane. All the purities mentioned are molar purities.

The impure hydrogen produced in a steam methane reformer or a coal gasification is first treated to remove acid gases, then purified by adsorption to remove methanol, water or remaining carbon dioxide.

The hydrogen may then be treated by nitrogen washing in a cold box to produce the stochiometric mixture as described in GB-A-2192703.

When the hydrogen contains between 1 % and 20% methane, it may be advantageous to separate a liquid phase from the hydrogen, as shown in Figure 1 , before sending it to the liquid nitrogen wash column. In this way, the liquid phase can be separated to form a liquid rich in methane.

According to an aspect of the invention, there is provided a process for co- production of liquid methane and ammonia synthesis gas from a feed gas containing at least 70% mol. hydrogen and at least 1 % mol. methane comprising the steps of :

- sending the feed gas or a hydrogen enriched gas derived therefrom to a liquid nitrogen wash column,

- removing a mixture of nitrogen and hydrogen constituting an ammonia synthesis gas from the liquid nitrogen wash column,

- sending liquid from the bottom of the liquid nitrogen wash column to the top of a stripping column and,

- removing methane rich liquid from the bottom of the stripping column as a final product. The feed gas may contain at least 70% hydrogen. The feed gas may contain at most 99% hydrogen. It may contain at least 1 % methane. It may contain at most 20% methane. It may contain less than 4% carbon monoxide.

Other optional features include:

- the hydrogen enriched gas is derived from the feed gas by carrying out a partial condensation of the feed gas by cooling the feed gas in a heat exchanger and sending the partially condensed fluid to a phase separator, removing the hydrogen enriched gas from the phase separator and removing a methane enriched liquid from the phase separator.

- the methane enriched liquid is sent to an intermediate point of the stripping column.

- the refrigeration for the process is provided by a closed refrigeration circuit.

- gas from the top of the stripping column is removed as a fuel.

- liquefying gaseous nitrogen against at least one gas from the nitrogen wash column and/or the stripping column and/ or against a fluid of the closed refrigeration circuit.

- reboil for the stripping column is provided by the closed refrigeration circuit.

- gas at the top of the stripping column is cooled using a top condenser.

- cooling for the stripping column top condenser is provided by the closed refrigeration circuit.

- reboil for the stripping column is provided by the feed gas.

According to another aspect of the invention, there is provided an apparatus for co-production of liquid methane and ammonia synthesis gas from a feed gas containing at least 70% mol. hydrogen and at least 1 % mol. methane comprising a liquid nitrogen wash column, a stripping column, a heat exchanger, an expansion valve, a conduit for sending feed gas or a hydrogen enriched gas derived therefrom to a liquid nitrogen wash column, a conduit for sending liquid nitrogen to the top of the liquid nitrogen wash column, a conduit for removing a mixture of nitrogen and hydrogen constituting an ammonia synthesis gas from the liquid nitrogen wash column, a conduit for sending a bottom liquid from the liquid nitrogen wash column to the top of the stripping column via the expansion valve and a conduit for removing methane rich liquid from the bottom of the stripping column as a final product. Optionally the apparatus may comprise:

- a closed refrigeration circuit for supplying refrigeration to the heat exchanger and/or to the stripping column and/or a methane rich liquid subcooler.

- where the stripping column has a bottom reboiler, a conduit for sending a heating gas to the bottom reboiler, said heating gas being at least part of the feed gas or a gas circulating in the closed refrigeration circuit.

- means for liquefying gaseous nitrogen and sending the liquefied gaseous nitrogen to the liquid nitrogen wash column.

- the means for liquefying gaseous nitrogen uses refrigeration provided by the closed refrigeration circuit.

- the stripping column has a top condenser fed by the closed refrigeration circuit.

- a phase separator, a conduit for sending the feed gas to the phase separator, a conduit for sending the hydrogen enriched gas from the phase separator to the liquid nitrogen wash column, a conduit for removing a methane enriched liquid from the phase separator and sending the methane enriched liquid from the phase separator to an expansion valve and a conduit for sending expanded liquid from the expansion valve to an intermediate point of the stripping column.

The invention will now be described in greater detail with reference to the figures which show processes according to the invention.

Figure 1 shows a cold box 33 contains heat exchangers 3, 59, columns 1 1 , 23 and closed cooling circuit 43, 45, 49, 51 , 57, 55, 61 , 63, 65, 67, 69, 71 , 73, 75, 77, 79.

A feed gas 1 containing from 70% to 99% of hydrogen, from 1 % to 20% of methane, less than 4% of CO is cooled in an exchanger 3 to form partially condensed feed gas 2 which is sent to a phase separator 5. The gas 7 from the phase separator is sent to a liquid nitrogen wash column 1 1 and contains around 95% of H ₂ , 3% of CO, 1 % of N ₂ , 2% of CH ₄ and less than 1 % of Ar. The gas from the top of the column after being mixed with stream nitrogen 93 contains one part of hydrogen for three parts of nitrogen and can be used as an ammonia synthesis gas 99. The gas 99 warms in exchanger 3 and then in exchanger 59 as stream 101 , part of the warming for partial stream 103 being provided by exchanger 105 of a Rectisol™ unit. The two stream 99, 103 are then reunited to form stream 105.

The liquid nitrogen for the column 1 1 is supplied in gaseous form 89, cooled and liquefied in exchangers 59, 3 and part 91 of the liquid is sent via a valve 97 to the top of column 1 1 . The rest 93 may be mixed directly with gas 13 to ensure the composition is correct.

The liquid 9 from phase separator 5, containing between 70% and 90% methane, is expanded in valve 21 to a lower pressure and sent to an intermediate point of stripping column 23. The top of column 23 is fed by the bottom liquid 15 of the nitrogen wash column, after expansion in valve 17.

Gas 107 from the top of the stripping column is removed as fuel gas and warmed in exchangers 3, 59. The gas contains preferably around 70% CO, 15%f N ₂ ,

8% H ₂ , 2% CH ₄ and less than 1 % Ar.

Liquid methane 29 is produced at the bottom of column 23 and sent to exchanger 31 to be subcooled. The subcooled liquid methane is removed from the cold box 33 in liquid form and sent to storage tank 37 by pump 35. Boil off 39 from the storage is liquefied in exchanger 41 and sent back to the tank. The liquid in the tank may be used to feed a network of liquid methane conduits. The liquid methane may contain at least 70% mol. methane, preferably at least 90% mol. methane and traces of nitrogen and carbon monoxide (for example less than 0.5% carbon monoxide and less than 100ppm nitrogen).

Reboil for the column 23 may either be provided by at least part of the feed stream 95 or, as shown, by a closed refrigeration circuit including a stream 77 which flows into exchanger 25 to warm bottom liquid stream 27. The closed refrigeration circuit may contain nitrogen, carbon monoxide, methane or another industrial gas.

There is no mass transfer between the closed refrigeration circuit and the rest of the process.

The closed refrigeration circuit preferably supplies refrigeration to the heat exchanger 3 and/or to the heat exchanger 59, preferably to liquefy gaseous nitrogen for the wash column and/or to the stripping column 23 and/or a methane rich liquid subcooler 31 and/or to a cooler 41 for boil off gas 39 from the methane rich liquid storage 37.

The closed refrigeration circuit uses three compressors in series 61 , 65, 71 . Gas 57 warmed in exchangers 3, 59 is compressed in compressor 61 and cooled in cooler 63. It is then mixed with stream 87, also warmed in the two exchangers. The combined stream is compressed in compressor 65, cooled in cooler 67 and divided in two. One part 69 is sent to booster compressor 71 and the boosted stream 73 is cooled in exchanger 59, then partially in exchanger 3 and expanded in turbine 55, coupled to booster 71 . The expanded stream 73 is mixed with stream 51 to form stream 57.

The other part 75 is cooled in exchanger 59 and divided in two. One part 77 is used to cool bottom reboiler exchanger 25 and cooled in exchanger 3. The other part 79 is cooled in exchanger 3, expanded in valve 83 and mixed with part 79 of stream 77 after expansion in valve 81 . The combined stream 85 is cooled in exchanger 3 to form stream 87 sent back to the inlet of compressor 65, after cooling in exchanger 59.

Part of stream 77 forms stream 47 which is used in part (stream 49) to subcool the liquid methane in exchanger 31 and in part (stream 45 after expansion in valve 43) in exchanger 41 to cool the boil off gas 39. The streams 45, 49 are mixed to form stream 51 after the two cooling steps and stream 51 is added to expanded stream 73 to form stream 57.

Figure 2 differs from Figure 1 in that the feed for column 1 1 is sent directly to the column and there is no longer any phase separator 5. Thus all the feed for the column 23 is constituted by the bottom liquid from column 15 via valve 17.

To improve the methane purity, column 23 has a top condenser 24 fed by part 76 of the circuit stream 77 in which the top gas of column 23 is condensed. The vaporized stream 76 may then be mixed with stream 57 (not shown to simplify the figure). The stripping column 23 of Figure 1 could also have a top condenser.