Hydrogen and Nitrogen Recovery from Ammonia Purge Gas

Abstract

An ammonia plant is disclosed, where ammonia purge gas (20), is sent to a cryogenic recovery unit, said recovery unit comprising means of cooling (102, 202, 302, 402, 502) and a high-pressure phase separator (103, 203, 303, 403, 503) operating at loop pressure; inside said unit the purge gas (20) is cooled to a cryogenic temperature, and a partial liquefaction of methane and argon is achieved; the high-pressure phase separator separates the cooled stream into a gaseous stream and a bottom liquid; the gaseous stream is reheated in a passage of a heat exchanger; the unit is then capable to export a gaseous stream (123, 223, 323, 423, 523) containing nitrogen and hydrogen at loop pressure, that can be reintroduced at the suction side of the circulator (4) of the loop.

Description

FIELD OF THE INVENTION[0001]The invention relates to the field of synthesis of ammonia from a make-up gas containing hydrogen and nitrogen. More in detail, the invention discloses a novel process and unit for recovery of hydrogen and nitrogen, and optionally of argon, from the purge gas which is taken from the high-pressure synthesis loop of an ammonia plant.PRIOR ART[0002]A known process to produce ammonia involves the catalytic reaction of a make-up synthesis gas comprising hydrogen (H2) and nitrogen (N2), in a high-pressure (HP) synthesis loop usually operating at around 80-300 bar pressure. The make-up syngas is produced in a front-end section, upstream the synthesis loop, by reforming a suitable hydrocarbon feed such as natural gas. For example, the hydrocarbon feed is desulphurized, then steam-reformed in a primary reformer, obtaining a first gas product containing CO, CO2 and H2 at a temperature around 800° C.; the first gas product is further reacted with air, enriched air o...

AI Technical Summary

Benefits of technology

The invention provides a recovery unit that recovers hydrogen and nitrogen from a syngas process at a high pressure, close to the loop pressure. This allows the recovered gases to be directly reintroduced into the loop without impacting the power requirements of the syngas compressor. The recovery unit can be connected to the suction of the circulator or across the circulator of the loop. Additionally, a small compressor can be used to re-compress the recovered stream for introduction in the loop. Compared to membrane-based systems, the recovered gases are recovered at a higher pressure, reducing the power and flow capacity requirements of the synthesis gas compressor. The invention also provides better recovery of nitrogen, approaching 75-80%, compared to membrane-based systems. Overall, the invention improves the efficiency and flexibility of the syngas process.

Problems solved by technology

A drawback of membrane-based systems however is that the hydrogen-rich gas is made available at a pressure much lower than that of the synthesis loop.

This means that the load and energy consumption of the compressor are increased or, in other words, the high pressure of the purge gas is lost in the HRU.

Another drawback is that membrane permeation is selective to H2, meaning that the N2 content of the purge is lost from the process and is discharged into the plant fuel system.

Images