PROCESS FOR PRODUCTION OF METHANOL

Abstract

The present application relates to a process comprising the steps ofproviding a purge stream from a synthesis section,preheating at least part of said purge stream,adding steam to the preheated purge stream to obtain a first mixed stream,passing the first mixed stream through a shift conversion step thereby obtaining a conversion product stream,passing at least part of the conversion product stream through a H.sub.2 separation step producing a H.sub.2 enriched stream and a H.sub.2 depleted waste stream, and returning at least part of said H.sub.2 enriched stream to and/or upstream the synthesis section.

Claims

1. A process comprising the steps of providing a purge stream from a synthesis section, preheating at least part of said purge stream, adding steam to the preheated purge stream to obtain a first mixed stream, passing the first mixed stream through a shift conversion step thereby obtaining a conversion product stream, p1 passing at least part of the conversion product stream through a H.sub.2 separation step producing a H.sub.2 enriched stream and a H.sub.2 depleted waste stream, and returning at least part of said H.sub.2 enriched stream to or upstream the synthesis section.

2. Process according to claim 1 comprising the step of cooling at least part of the conversion product stream before passing the cooled conversion product stream through a process gas separator thereby obtaining a condensate stream and a vapor stream, and passing at least part of the vapor stream to the H.sub.2 separation step.

3. Process according to claim 1, wherein the H.sub.2 depleted stream is used as fuel in fired heaters in the process unit and/or for power production.

4. Process according to claim 1, wherein the H.sub.2 separation step is carried out in a membrane unit or in a PSA.

5. Plant comprising a synthesis section and a purge gas treatment line, said purge gas treatment line comprising: preheater, steam addition means, shift conversion unit, and H.sub.2 separation means.

6. Plant according to claim 5 comprising a cooler and a process condensate separator upstream the H.sub.2 separation means.

7. Plant according to claim 5, wherein the plant is arranged to carry out a process comprising the steps of: providing a purge stream from a synthesis section, preheating at least part of said purge stream, adding steam to the preheated purge stream to obtain a first mixed stream, passing the first mixed stream through a shift conversion step thereby obtaining a conversion product stream, passing at least part of the conversion product stream through a H.sub.2 separation step producing a H.sub.2 enriched stream and a H.sub.2 depleted waste stream, and returning at least part of said H.sub.2 enriched stream to or upstream the synthesis section.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0049] In the following the invention will be further described with reference to the accompanying drawing. The drawing is exemplary and is not to be construed as limiting for the invention.

[0050] FIG. 1 shows a plant according to the present invention. Synthesis gas 1 is mixed with H.sub.2 rich recycle gas 2 and passed to the synthesis section 3. From the synthesis section 3 a product stream 4 and a purge stream 5 are withdrawn. The purge stream is heated in preheater 6 and mixed with process steam 7 to obtain a mixed stream 8. Stream 8 is passed to a shift conversion unit 9, where steam and CO react to H.sub.2 and CO.sub.2. The reacted gas 10 is cooled to below its dew point in cooler 11. The cooled stream 12 is passed to a process condensate separator 13. The vapor stream 14 from the condensate separator 13 is passed to a gas separation unit 15. From the gas separation unit a H.sub.2-enriched stream 16 and a H.sub.2-depleted waste gas stream 17 are withdrawn. The H.sub.2-enriched gas 16 may be compressed in the recycle compressor 18 to form the H.sub.2 enriched recycle stream 2, which is added to the synthesis gas 1 as described above. If the synthesis gas comes from a synthesis gas compressor, the H.sub.2- enriched gas 16 may also be added to the synthesis gas before this compressor. Alternatively or in combination, the synthesis gas and the H.sub.2-enriched gas may be added separately to the synthesis section at different points in the process.

Example

[0051] In a plant designed and operating according to the invention as described above and exemplified the accompanying FIG. 1, a total production of 1000 metric tons (MTPD) of methanol is produced when the plant is fed with 89090 Nm3/h of a synthesis gas containing 63.66 vol % H2, 1.63 vol % H2, 27,40 vol % CO, 4,42 vol % CO2, 0,36 vol % Ar, and 2,43 vol % CH4.

[0052] When a similar unit designed without the steps of treating the purge gas is fed with the same synthesis gas as the unit designed and operating according to the invention, a total production of 987.4 MTPD of methanol is obtained. I.e. the present process of treating the purge gas including heating, adding steam, converting CO and steam to CO.sub.2 and H.sub.2, cooling and separating condensate, all before admitting the gas to the gas separation unit, leads to an increased methanol production.