1. Field of the Invention
This invention generally relates to the production of ammonia synthesis gas, and more particularly, to an improved cryogenic process employing liquid nitrogen washing.
2. Description of the Prior Art
In the production of ammonia, an ammonia synthesis gas containing hydrogen and nitrogen in approximately a 3:1 ratio is required. A hydrogen-rich gas stream is usually employed as a raw feed which must then be purified and combined with the required amount of nitrogen. The sources of such hydrogen-rich gas feeds vary widely, but typically the hydrogen gas feed will contain high boiling impurities such as CO.sub.2, higher boiling hydrocarbons, hydrogen sulfide, moisture, lower boiling hydrocarbons such as methane, carbon monoxide and nitrogen. Warm temperature processes which are known in the art are typically used to remove the higher boiling impurities such as CO.sub.2, the higher boiling hydrocarbons, H.sub.2 S, moisture and the like, and various cryogenic systems have been developed to effect the final purification by removing lower boiling impurities such as the lower boiling hydrocarbons (e.g., methane), carbon monoxide and the like.
U.S. Pat. No. 3,312,075 relates to a classic nitrogen wash system in which liquid nitrogen is produced and employed in a nitrogen wash tower for countercurrent contact of a cooled crude hydrogen-containing gas feed to remove carbon monoxide and other impurities. The vapors withdrawn from the upper portion of the nitrogen wash tower comprised the purified hydrogen stream and are combined with liquid nitrogen for additional refrigeration and then passed in indirect heat exchange for cooling of the feed gas streams, namely, the nitrogen gas stream and the crude hydrogen-containing gas feed. The process obtains the necessary cooling by an isentropic expansion of the hydrogen:nitrogen product stream. The impure nitrogen liquid stream withdrawn from the bottom portion of the nitrogen wash tower is passed through a pressure release valve to provide further refrigeration to the process. All of the non-hydrogen feed components are withdrawn via the waste nitrogen stream.
U.S. Pat. No. 3,327,487 is exemplary of multi-bed pressure swing absorption. This process employs switching exchangers to remove impurities other than carbon monoxide (by absorption) and expands crude hydrogen streams to obtain additional refrigeration. Crude (partially purified) hydrogen is reheated before recooling and washing with nitrogen, thereby providing heat inefficiencies in this system. Also, this process results in H.sub.2 losses, since the non-H.sub.2 components are separated as a single stream which contains as much as 20 wt. % of the H.sub.2 passed to the process. This process is very complex and extremely cross-integrated. In addition, without adequate surge volumes, product and/or waste flows could be cyclical, providing instabilities in the operation of such a process. The complexities of the process require cycling and precise control of timing of the varied cycles.
U.S. Pat. No. 3,508,413 employs sub-atmospheric flashes to obtain refrigeration, which increases the chance of dangerous air leakages into the system. Such air leakages are to be avoided due to the ever-present danger of explosions from the highly combustible gas mixtures. The gaseous product which is produced via the multi-phase flash and fractionating tower system does not achieve low levels of carbon monoxide in the hydrogen product stream, which contains about 2.5 percent of CO. This process cannot be successfully employed with high methane hydrogen feeds since CH.sub.4 would condense in the fractionating tower reboiler (line 13 in the patent's FIG. 1) and be transferred to the intermediate tower reflux (line 27), thereby contaminating the H.sub.2 overhead product. Again, refrigeration is provided by isentropic expansion of the hydrogen product stream.
U.S. Pat. No. 3,691,779 produces a high purity, 97-99.9 percent hydrogen product, and employs an isentropic expansion of a hydrogen product stream to provide a portion of the refrigeration. The impurities in the hydrogen containing gas feed are removed by a series of cooling and condensation stages having successively lower temperatures, and the process is therefore basically a pressure swing absorption system. As a result of the expansion and recycle of a portion of the hydrogen product stream to refrigerate and regenerate the pressure swing absorption beds, a portion (e.g., 10-20%) of the hydrogen is lost to waste gas.
U.S. Pat. No. 3,872,025 liquefies a portion of the H.sub.2 -rich feed gas to remove a portion of the carbon monoxide therefrom and then fractionates the remaining portion to provide a pure carbon monoxide stream which is then employed after pumping and expansion to provide refrigeration to the process.
U.S. Pat. No. 4,217,759 employs an isentropic expansion of the product hydrogen stream to obtain refrigeration and also employs a sub-atmospheric flash treatment, which raises the dangers discussed above for oxygen leakages into the system.