Method for purification of a CO2 stream

Abstract

A process for removing hydrogen and methanol from a CO2 stream which contains hydrogen and methanol as contaminants, wherein hydrogen and methanol are removed by contacting the CO2 stream with a catalyst which oxidizes hydrogen to water and methanol to carbon dioxide, obtaining a purified CO2 stream.

Claims

1. A process for removing hydrogen and methanol from a CO.sub.2 stream which contains hydrogen and methanol as contaminants, wherein hydrogen and methanol are removed by contacting said CO.sub.2 stream with a catalyst which oxidizes hydrogen to water and methanol to carbon dioxide, obtaining a purified CO.sub.2 stream, wherein at least part of said purified CO.sub.2 stream, after removal of methanol and hydrogen, is fed into a urea synthesis process of an ammonia-urea integrated plant, and wherein said CO.sub.2 stream containing hydrogen and methanol is extracted from an ammonia section of said ammonia-urea integrated plant, wherein urea is produced at a urea synthesis pressure; said CO.sub.2 stream containing hydrogen and methanol is extracted at a pressure lower than said urea synthesis pressure; the CO.sub.2 stream containing hydrogen and methanol is compressed to a first pressure intermediate between said pressure of extraction and said urea synthesis pressure; and the removal of hydrogen and methanol is carried out at said first pressure; and at least part of the so obtained purified CO.sub.2 stream is further compressed to said urea synthesis pressure and is fed to said urea synthesis process.

2. The process according to claim 1, wherein said step of contacting the CO.sub.2 stream with said catalyst is performed in a single catalytic reactor.

3. The process according to claim 1, wherein said catalyst is a platinum-based or palladium-based catalyst.

4. The process according to claim 1, wherein the purified CO.sub.2 stream, after removal of hydrogen and methanol, contains no more than 10 ppm (vol) of hydrogen.

5. The process according to claim 1, wherein the purified CO.sub.2 stream, after removal of hydrogen and methanol, contains no more than 200 ppm (vol) of methanol.

6. The process according to claim 1, wherein said CO.sub.2 stream containing hydrogen and methanol is mixed with an amount of oxidant such as to provide at least an amount of oxygen required for oxidation of hydrogen to water and methanol to carbon dioxide.

7. The process according to claim 1, wherein oxidation of hydrogen to water and methanol to carbon dioxide is carried out at a pressure of at least 20 bar or greater.

8. The process according to claim 1, wherein said CO.sub.2 stream containing hydrogen and methanol is mixed with an oxidant in a sufficient amount to provide oxygen for oxidation of hydrogen and methanol, and further oxygen for use as passivating agent in the urea synthesis process.

9. The process according to claim 1, wherein the first compression is carried out in one or more first stage(s) of a multi-stage compressor and the second compression is carried out in one or more remaining stages of said compressor.

10. The process according to claim 1, wherein water obtained by oxidation of hydrogen is separated from the purified carbon dioxide stream by condensation in a separator at said first pressure.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1 shows a block scheme of an ammonia-urea integrated plant which can include an embodiment of the invention.

(2) FIG. 2 shows in greater detail the plant of FIG. 1 and the related gas purification section according to an embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

(3) FIG. 1 illustrates a block scheme of an ammonia-urea integrated plant 100 comprising an ammonia section 101 and a urea section 102.

(4) The ammonia section 101 comprises a front-end section 103 and a synthesis loop 106. The front-end section 103 essentially comprises a reforming section 104 for the conversion of a hydrocarbon feedstock 1 into a raw synthesis gas 2 and a purification section 105 for the purification of said raw gas into a make-up gas 3 and separation of a carbon dioxide stream 4. The synthesis loop 106 essentially comprises a reactor, a separator and a purge recovery unit (not shown) and provides an ammonia product 5.

(5) Said carbon dioxide stream 4 and ammonia 5 are fed to the urea section 102 at a suitable pressure, where they react to provide urea 6. In particular, carbon dioxide 4 is suitably compressed in a gas compressor 111 (shown in FIG. 2) to reach the synthesis pressure, for example around 150 bar.

(6) The urea section 102 can implement any of the known processes for the synthesis of urea, including e.g. the CO2 stripping process, the self-stripping process or another.

(7) According to FIG. 2, the purification section 105 of the front-end section includes a CO-shift converter 107, a CO2-removal unit 108, a catalytic converter 109 and a methanator 110. The unit 108 removes carbon dioxide from shifted gas 10 by a methanol washing, producing a CO2-depleted gas 11 and a CO2 stream 12 also containing some methanol and residual hydrogen.

(8) The CO2-depleted gas 11 is fed to the methanator 110 wherein residual carbon monoxide and hydrogen are converted into methane producing the make-up gas 3.

(9) The CO2 stream 12 is mixed with air 13 and sent to the converter 109 via the first two stages 111a and 111b of the gas compressor 111, an inter-stage cooler 115 and a separator 116. The resulting CO2 stream 12 enters the converter 109 under pressure.

(10) The so pressurized CO2 stream 12 reacts over a platinum or palladium-based catalyst contained in the converter 109, to provide a purified CO2 stream 14 containing water and carbon dioxide (i.e. oxidation products). Said purified stream 14 is withdrawn from the top of said catalytic reactor 109 and passed through a cooler 113, wherein at least some of the water is condensed. The resulting two phase stream 15 is passed through a phase separator 114, to obtain a gaseous stream of dry purified carbon dioxide 4 and a condensate 16. Said dry and purified carbon dioxide 4 is then delivered to the urea section 102 by the remaining stages 111c and 111d of the compressor 111, including a further intercooler 117 and separator 118.

(11) Hence it can be appreciated that the converter 109 is integrated with the multi-stage compressor 111, running at the intermediate pressure of delivery of the stage 111b.

(12) The CO2 stream 12 obtained from the washing unit 108 is purified from methanol and hydrogen, leading to a clean CO2 current 4 suitable to feed the urea synthesis.