Process for purification of a synthesis gas containing hydrogen and impurities

Abstract

A process for purification of a current of hydrogen synthesis gas (100), particularly in the front-end of an ammonia plant, wherein said gas contains hydrogen and minor amounts of carbon monoxide, carbon dioxide, water and impurities, said process including steps of methanation (13) of said current (100), converting residual amounts of carbon monoxide and carbon dioxide to methane and water, dehydration (14) of the gas to remove water, and then a cryogenic purification (15) such as liquid nitrogen wash, to remove methane and Argon; a corresponding plant and method for revamping an ammonia plant are also disclosed.

Claims

1. A process for purification of a current of hydrogen synthesis gas f or obtaining a make-up synthesis gas for the synthesis of ammonia, said current of hydrogen synthesis gas to be purified being composed of hydrogen and minor amounts of carbon monoxide, carbon dioxide, water and impurities and containing at least 90% (mol) hydrogen, said process including a step of cryogenic purification, and dehydration of syngas through molecular sieves prior to said cryogenic purification, the process comprising a step of methanation of said current of hydrogen synthesis gas to be purified, wherein carbon monoxide and carbon dioxide are converted into methane and water, prior to said steps of dehydration and cryogenic purification, said step of cryogenic purification comprising a liquid nitrogen wash; wherein hydrogen synthesis gas is washed with liquid nitrogen in a column and wherein said molecular sieves being regenerated by using tail gas discharged from said liquid nitrogen wash.

2. The process according to claim 1, wherein said current of hydrogen synthesis gas to be purified is obtained by reforming of a hydrocarbon source with oxygen, shift conversion of carbon monoxide to carbon dioxide, and removal of carbon dioxide.

3. The process according to claim 1, wherein a portion of the hydrogen synthesis gas, after said step of methanation and dehydration, bypasses said cryogenic purification.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a block scheme of a front end for generation of ammonia synthesis gas, according to an embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

(2) Referring to FIG. 1, a current of hydrogen syngas 100 is composed mostly of hydrogen (H2), and contains minor amounts of CO, CH4, CO2, N2, Argon and water. For example H2 is more than 90%, preferably more than 95%, CO and CH4 are around 1-2%, and the other components are preferably less than 1%.

(3) Said current 100 comes from previous treatments of shift and carbon dioxide removal, as will be explained below. Hence the CO and CO2 contained in the current 100 are residual amounts after the above treatment steps.

(4) Said current 100, according to the shown embodiment, is fed to a methanator 13 where CO and CO2 are converted to methane and water. The syngas 101 leaving the methanator 13 contains practically no CO and CO2, and a certain amount of methane, for example around 3% of methane.

(5) Said gas 101 is fed to a dehydration unit 14, to remove residual water. The dehydrated gas 102 leaving said unit 14 is sent to a cryogenic purification section, which in the example is embodied as liquid nitrogen washing (LNW) unit 15. In said unit 15, the gas 102 is washed with liquid nitrogen 103. The output of said unit 15 includes the purified syngas 104 and tail gas 105.

(6) Said purified syngas 104 can be used to produce ammonia, in a suitable synthesis loop. To this purpose, a suitable amount of gaseous nitrogen is added to the syngas in the washing unit 15, so that said syngas 104 contains hydrogen and nitrogen in the stoichiometric ratio 3:1 for the production of ammonia (NH3).

(7) The current 100 is generally obtained after previous steps of reforming, shift and CO2 removal.

(8) Referring to FIG. 1, and according to a preferred embodiment, a hydrocarbon source 106, for example natural gas, is reformed in a reforming section 10 with oxygen 107, obtaining a product gas 108 predominantly composed of hydrogen. Said gas 108 is shifted in a converter 11 to produce a shifted gas 109, and said shifted gas 109 is subject to carbon dioxide removal in a CDR section 12. The CO2-depleted gas leaving said section 12 forms the above mentioned hydrogen gas current 100, which is admitted to the methanator 13.

(9) In some embodiments, a portion of the syngas 102 may bypass the LNW unit 15 and join the stream 104, to reduce the size of said unit 15, provided that the impurities in the product as are within acceptable levels.

(10) A process according to FIG. 1 has been compared to a conventional process where the gas current leaving the CDR unit is sent directly to driers for removal of CO2 and water, and then to a LNW stage.

(11) For a given feed of natural gas, it has been noted that the process according to the invention delivers 4% less of synthesis gas, due to the consumption of the methanator 13.

(12) However, the water to be adsorbed is around compared to the prior art, and moreover water is much easier to capture in the mole sieves than CO2, which means that the molecular sieves are much smaller and less expensive; the regeneration can be carried out with tail gas from the same PSA which removes the carbon dioxide, without the use of fresh nitrogen or part of the purified gas; the nitrogen consumed in the LNW stage is 25% less; the tail gas released from the LNW has a higher heat value since it contains more methane and less nitrogen, hence more energy can be recuperated by using said tail gas as a fuel.

(13) The above positive effects over-compensate the consumption of hydrogen in the methanator.