GB2158435A

GB2158435A - Methanol production process with stripping of carbon dioxide

Info

Publication number: GB2158435A
Application number: GB8510105A
Authority: GB
Inventors: Anthony Horton; Christopher Leslie Winter
Original assignee: Humphreys and Glasgow Ltd
Current assignee: Humphreys and Glasgow Ltd
Priority date: 1984-04-19
Filing date: 1985-04-19
Publication date: 1985-11-13
Also published as: GB2157688A; GB8510105D0; GB2157688B; GB2158435B; GB8510104D0

Abstract

A process for the production of methanol comprises absorbing carbon dioxide from a gas stream into a carbon dioxide-absorbing material, stripping the carbon dioxide out of the absorbing material using hydrogen to yield a synthesis gas stream containing carbon dioxide and hydrogen, and reacting together the carbon dioxide and the hydrogen from the synthesis gas stream to produce methanol.

Description

1 GB 2 158 435A 1

SPECIFICATION

Methanol production process with stripping The present invention relates to processes for the production of methanol from carbon diox ide utilizing a carbon dioxide stripping step.

It is known to produce methanol from car bon dioxide by reaction with hydrogen. Often this carbon dioxide is obtained from a C02 containing gas stream by a standard C02 absorption /desorption system using a physi cal solvent. Conventionally the carbon dioxide is recovered from such a solvent by reducing the pressure on the CO,-rich solution. Re compression of the recovered CO, is then necessary in order to bring the pressure of the gas up to that required for methanol synthe sis, which synthesis usually takes place in a synthesis -loop-.

It has now been found possible partially or totally to eliminate the need for such a sepa rate C02-COMpressor by stripping the C02 OUt of the absorbing material using the hydrogen to be reacted.

In accordance with the present invention there is provided a process for the production of methanol, which process comprises absorb ing carbon dioxide from a gas stream into a carbon dioxide-absorbing material, stripping the carbon dioxide out of the absorbing ma terial using hydrogen to yield a synthesis gas stream containing carbon dioxide and hydro gen, and reacting together the carbon dioxide and the hydrogen from the synthesis gas 100 stream to produce methanol.

Although a solid phase absorbing material can be used, a liquid phase material is pre ferred, desirably a physical solvent.

The absorber to be used will normally corn- 105 prise a tower containing a liquid/gas contact ing device up which flows the gas containing the carbon dioxide and down which flows the liquid physical solvent. The CO,-rich liquid is then passed to a stripping tower.

The-carbon dioxide-rich liquid may be passed directly to the stripping tower without reduction of its pressure or may indeed be pumped to a higher pressure before passing to said stripper.

Although the stripping pressure is not critical, in practice the stripper may be operated at one of two pressures. Either it may operate at substantially the same pressure as the ab- sorber, or it may operate at substantially the same pressure as the methanol synthesis loop.

It is also possible for the carbon dioxide to be stripped out using two or more strippers oper ating at different pressures.

The gas leaving the top of the carbon 125 dioxide absorber contains the hydrogen re quired for the methanol loop. If this gas contains unwanted impurities, the hydrogen can be recovered from the gas by using either a PSA,---Prism---or cryogenic unit or a combi- 130 nation of these. "Prism" is the trade name for a gas separation unit utilizing a gas permeable membrane to allow certain components through the membrane whilst retaining others behind the membrane.

In one embodiment of the present invention, it is this absorber gas or the recovered hydrogen therefrom which, after any necessary compression, is fed to the bottom of the stripper so as to strip out the carbon dioxide from the liquid.

In an alternative embodiment of the present invention, where the stripper is operated at the pressure of the methanol synthesis loop, the (predominantly hydrogen) recycle gas can be used to strip out the carbon dioxide from the liquid. These re-cycled gases, may be used alone or in combination with the hydrogen recovered from the absorber, in which case the re-cycle gases may be introduced into the stripper at a point commensurate with the small partial pressure of carbon dioxide in the re-cycle gas and/or the degree of carbon dioxide recovery required. 90 Where the stripper is at substantially the same pressure as the absorber, the mixture of carbon dioxide and hydrogen leaving the stripper is, if necessary, compressed up to the pressure of the methanol synthesis loop. 95 The present invention can be used wherever gas streams containing carbon dioxide and hydrogen are available. For example, natural gas is often found to contain significant amounts of carbon dioxide, which if the natural gas is to be reformed to produce hydrogen, usually have to be removed beforehand. Removal of this carbon dioxide by absorption can thus be utilized as the first step of the process of the present invention, with product hydrogen or any other convenient hydrogencontaining gas stream being used in the second step to strip out the absorbed carbon dioxide. In a particularly preferred process of the present invention, the use of hydrogen-stripping of carbon dioxide to yield a methanolsynthesis gas can be applied to the methanolproduction process described and claimed in the Applicants' co-pending application filed herewith under Agents' reference 29482 and entitled "Methanol production process using air".

The preferred process of that co-pending application may be described as follows:

Natural gas is desulphurised, mixed with steam, and preheated in a fired heater, after which heating the gas stream may leave the fired heater and pass through a catalyst bed to effect some reforming of the natural gas. The gas leaving the fired heater is passed to an air reformer where additional natural gas is reformed using heat released by the reaction of the oxygen contained in the air.

The stream from the air reformer passed through a boiler and then to a carbon monox- 2 GB 2 158 435A 2 ide-shift unit where most of the carbon monoxide is converted to carbon dioxide. Following the shift unit, the gas is fed to a conventional carbon dioxide-recovery unit, preferably of the physical-solvent type. The recovered carbon dioxide is compressed and joins a stream of hydrogen, the recovery of which is effected from the gas leaving the top of the C02 absorber by means of a PSA, "Prism" or a cryogenic unit. The residual gas from the hydrogen-recovery unit contains mainly nitrogen, together with some unreformed methane and unrecovered hydrogen, and may be used to fire the fired heater and/or to supply fuel for a gas turbine. The recovered hydrogen and 80 C02 are fed to a methanol synthesis loop. The hydrogen concentration in the methanol synthesis loop may be adjusted so as to give a high hydrogen partial pressure in order to achieve economic reaction rates.

By using the hydrogen to be reacted to strip out the carbon dioxide, the need for a separate carbon dioxide compressor is obviated. Thus in place of the aforementioned conven- tional carbon dioxide-recovery unit, a standard absorber and a pressure stripper are used, wherein the stripper is fed with the nonabsorbed, hydrogen-containing gas leaving the top of the absorber and/or with the hydrogen-containing methanol loop recycle gas.

Other features of this methanol production process using air are described in the aforementioned co-pending application.

Claims

1. A process for the production of methanol, which process comprises absorbing carbon dioxide from a gas stream into a carbon dioxide-absorbing material, stripping the carbon dioxide out of the absorbing material using hydrogen to yield a synthesis gas stream containing carbon dioxide and hydrogen, and reacting together the carbon dioxide and the hydrogen from the synthesis gas stream to produce methanol.

2. A process as claimed in claim 1 wherein at least some of the stripping is effected at substantially the same pressure as the pressure at which the carbon dioxide is absorbed.

3. A process as claimed in claim 1 or claim 2 wherein at least some of the stripping is effected at substantially the same pressure as the pressure at which the carbon dioxide is reacted with hydrogen.

4. A process as claimed in any one of the preceding claims wherein the methanol-producing reaction is effected in a synthesis loop using a hydrogen-containing re-cycle stream, and wherein the stripping of the carbon dioxide is effected using the said loop re-cycle stream.

5. A process as claimed in any one of the preceding claims wherein the carbon dioxide- containing gas Stream contains the hydrogen to be reacted to produce methanol.

6. A process as claimed in claim 5 wherein the hydrogen to be reacted is not absorbed by the absorbing material and is used in the stripping step.

7. A process as claimed in claim 6 wherein the non-absorbed hydrogen is puri fied prior to being used in the stripping step.

8. A process as claimed in any one of claims 5 to 7 wherein the carbon dioxide containing stream is obtained by air reforming a hydrocarbon feedstock to a mixture of hy drogen and carbon monoxide, and then con verting substantially all of the carbon monox ide into carbon dioxide.

9. A process as claimed in claim 8 wherein the hydrocarbon feedstock is natural gas.

10. A process as claimed in claim 1 sub stantially as hereinbefore described.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1985, 4235. Published at The Patent Office. 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.