GB2140801A - Methanol production process with gas separation - Google Patents

Abstract

A methanol production process in which the methanol synthesis gas comprises substantially a mixture of carbon dioxide and hydrogen, the said mixture being formed by the air-reforming of a hydrocarbon feedstock, characterised in that the synthesis gas is produced by co-permeating the said mixture through a gas permeable membrane whereby to separate the said mixture from at least a major proportion of the nitrogen present in the air used for the reforming.

Description

SPECIFICATION Methanol production process with gas separation The present invention relates to processes for the production of methanol which, although suitable for use in normal climatic conditions onshore, are particularly applicable to offshore or difficult climatic conditions.

Current state of the art processes for the large scale production of methanol utilise either tubular steam reformers or oxygen-fed partial oxidation plants, depending on the feedstock. These processes generally use natural gas and coal/heavy hydrocarbons, respectively, to form carbon monoxide which is then reacted with hydrogen to produce methanol.

The high cost of tubular steam reformers, and the high capital and energy requirements of air separation units (ASU), which usually supply oxygen to the partial oxidation plants, are considerable charges on processes using such units. Also, if one wishes to build a methanol plant on, say, an offshore platform, the physical size and hazard presented by an ASU or a tubular reformer are a considerable drawback to their use.

Furthermore, when methanol is made from natural gas using a tubular steam reformer, the oxygen which has to be added to the natural gas to make methanol is derived from water, and the energy required to split water is considerable. Also, the hydrogen from the water is not utilized and has to be purged from the methanol loop and burnt; i.e. such a process creates an excess of hydrogen.

In contrast with conventional teaching, it has now been found to be advantageous to produce the methanol not from carbon monoxide, but from carbon dioxide, particulary that formed from the carbon monoxide usually produced. If natural gas, optionally with steam is reformed solely by the addition of air, the process becomes hydrogen deficient, but this does allow the very expensive tubular steam reformer to be eliminated. Inasmuch as a tubular steam reformer represents up to 50% of the cost of a large methanol plant, this is a very large saving.

Naturally when air is used, nitrogen is introduced into the process gas stream. The present invention seeks to use air, but to eliminate the nitrogen which would otherwise cause an uneconomically-large partial pressure loss in the methanol synthesis loop. By using carbon dioxide, rather than carbon monoxide, to produce methanol, the separation of components, particularly nitrogen, out of the process gas stream is much easier.

In general, this process comprises using steam or process or gas turbine exhaust streams to pre-heat desulphurised natural gas, optionally adding steam, possibly also preheated natural gas feed, followed by air reforming of this feed to produce a crude synthesis gas. After passing through a boiler to reduce its temperature the crude gas is passed through a carbon monoxide-shift unit which converts nearly all of the carbon monoxide to carbon dioxide. It is then fed to a carbon dioxide-separation system. Although such a system could be a physical solvent-based system or a chemical system, such as the Benfield system, the avoidance of large and heavy CP2-separation systems is clearly desirable, particularly if lower running costs can be achieved thereby. It has now been found that the combining together of the separation of CO2 with the separation of hydrogen, i.e.

separation of both of these gases from the residual nitrogen and methane, by means of a gas permeable membrane used in a particular way yields an improved process.

In accordance with the present invention there is provided a methanol production process in which the methanol synthesis gas comprises substantially a mixture of carbon dioxide and hydrogen, the said mixture being formed by the air-reforming of a hydrocarbon feedstock, characterised in that the synthesis gas is produced by co-permeating the said mixture through a gas permeable membrane whereby to separate the said mixture from at least a major proportion of the nitrogen present in the air used for the reforming.

Generally the temperature of the feed to the air reformer is raised, but pre-heating as described above, using process gas streams, steam or gas turbine exhausts, can only reach a temperature of approximately 400"C. Also the air requirement in certain situations can be such that the power for the air compressor is a very considerable charge on the process.

Furthermore the overall process can sometimes be hydrogen deficient.

These drawbackd can be reduced or eliminated by the introduction of a fired heater situated upstream of the air reformer.

The introduction of such a fired heater allows the temperature of the feed to the air reformer to be raised considerably. It also allows some of the reforming to be effected outside the air reformer by pre-heating the feed to, say, 500-1100 C in the fired heater and then passing the gas, usually a steam/natural gas mixture, out of the fired heater and into an external bed of catalyst where the natural gas is reformed. Normally the energy of reforming is supplied solely the the cooling of the gases, but air could be introduced at this point to supply additional heat. A number of such reforming stages with pre/re-heat may be used.

By the means described above, the amount of energy which has to be supplied by the reaction of oxygen with the gases is substantially reduced, thus also reducing substantially the amount of air required for a given degree of reforming. Furthermore, because of the reduced amount of oxygen entering the process, the amount of water which reacts is increased, thereby increasing the hydrogen:carbon oxides ratio.

It may be desirable to increase the hydrogen:carbon oxides ratio to the extent needed to compensate for the lack of hydrogen recovery in the downstream hydrogen recovery unit, and also to ensure that the fuel stream has burning characteristics suitable for either a gas turbine or the fired heater.

Whilst the preferred process reduces the amount of air required, nevertheless the oxygen that is introduced with the air, albeit smaller in amount than if there were no preheat, reduces the amount of water needed to effect the reforming of the natural gas. This is an important consideration in an offshore situation.

When steam is added to the natural gas feed, the gas passing through the fired heater may also be made to pass through a bed of catalyst to reform some of the natural gas.

The stream leaving the fired heater (or the otherwise-heated feed stream, if no fired heater is used) is fed to an air reformer along with compressed air, which may in turn contain steam, preferably picked up in a saturator. In the air reformer, the oxygen of the air reacts with the gases of the other stream and then may pass through a catalyst bed to emerge from the air reformer at high temperatures. The reformed gases then pass through a boiler and thence to a carbon monoxide shift unit which may contain high and low temperature beds of catalyst. Alternatively or additionally selective catalytic oxidation by air or an oxygen-enriched air can be employed.

The shifted gases are then cooled to condense out water which may be fed back to a saturator, optionally slightly heated, then passed, possibly along with recycled gas, into a Monsanto "Prism" unit or similar permeable membrane device to effect the desired gas separation.

Unlike conventional practice, the permeable membrane device is used here in such a way that at least the amounts of hydrogen and carbon dioxide required for methanol synthesis pass through the membrane simultaneously.

By arranging the overall process such that the feed stream to the permeable membrane contains hydrogen and carbon oxides in the form of carbon dioxide, co-permeation of these two compounds is effected. This copermeation is highly advantageous in that the pressure at which the hydrogen and the carbon dioxide are recovered is the sum of their partial pressures on the permeate side of the membrane.

Because of this, the power necessary to raise the pressure of this H2/CO2 mixture, to methanol synthesis pressure, is reduced.

The non-permeate nitrogen / methane-rich stream is either used as fuel, say for a gas turbine inasmuch as it is still at high pressure, or for the fired heater, if used. It may also be heated and expanded to gain power for use anywhere in the process. Such heating could be affected by causing its hydrogen and oxides of carbon contents to react together in a methanation reactor with the addition of water, if necessary. Alternatively or additionally, the nitrogen/methane-rich stream may be fed to a methane-separation device, such as a cryogenic unit or molecular sieve device in order to obtain a reiatively methane-rich stream.This latter stream may be compressed and recycled back to the front-end of the process, thereby ameliorating the conditions necessary to reform the natural gas by increasing the concentration of methane leaving the reforming section. This may be carried out to the extent that the residual fuel value of the resulting nitrogen-rich stream may be disregarded. However, the pressure energy of the nitrogen in the feed to such a cryogenic unit would, upon its let-down, contribute significantly to the power required to drive the cryogenic unit. Easier commissioning of a plant is possible using a gas turbine for expanding the gases.

The hydrogen/CO2-rich stream from the permeable membrane device is, if necessary, compressed and passed into a methanol synthesis unit, such as a conventional loop.

By varying the air flow, the methanol loop purge hydrogen recovery, and/or the hydrogen recovered from the waste nitrogen stream, the overall process may be arranged such that the concentration of hydrogen in the loop is significantly higher than that required for stoichiometric reaction in order to obtain economic reaction rates.

In particular, it is possible to eliminate the need to recover any methane present in the reformed gases, by using sufficient air to raise the temp,erature of the reforming reaction to a level where the methane content of the gases leaving the reformer is very small.

Because some impurities will temd to enter the loop with the nitrogen/carbon dioxide, these must be purged from the loop. The purge stream may be either recycled to a suitable point upstream in the process; may be used directiy as fuel, or may be processed in a cryogenic, PSA or permeable membrane device, as appropriate.

The methanol produced by the process of the present invention will contain more water than current state of the art processes using such a synthesis loop, because the loop is fed with carbon dioxide rather than carbon monoxide. However, this is not a significant drawback since this water leaves the bottom of the conventional methanol distillation columns.

The steam raised in any boiler downstream of the reformer and any additional steam raised, e.g. by the methanol synthesis unit or in the exhaust gas of any gas turbine may be used e.g. to drive additional turbines and/or may be used to supply energy for a distillation unit which may be used to purify the methanol.

A major advantage of the present process is that the resulting plant will be relatively small and light-weight, both features being very important in offshore situations. Such a plant will also have a comparatively low water requirement.

The main, nitrogen-removal cryogenic unit, if used, could be arranged such that the nitrogen and methane are condensed out of the hydrogen first. The resulting liquid could then be removed from the gas, distilled to separate the nitrogen from the methane, and the methane stream re-combined with the hydrogen-rich gas stream so as to leave only two streams to be re-heated to ambient temperature. The nitrogen could either be expanded through a valve or through a rotating expander, as desired.

One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawing, which shows a single flowsheet for a methanol synthesis plant labelled Fig. 1, in which the dotted lines show some alternate flow schemes. (The methanol distillation unit is not shown).

In Fig. 1, item 1 is a fired heater: item 2 is an air compressor; item 3 is an air reformer; item 4 is a boiler with its associated steam drum; item 5 is a shift unit; item 6 is a cooler condenser; item 7 is the main "Prism" unit; item 8 is a methanol loop, item 9 is a loop purge gas separation device; item 10 is a cryogenic separation device, and item 11 is a gas turbine.

Desulphurized natural gas is mixed with steam and fed into fired heater 1. The heated mixture is passed into air reformer 3 together with air fed from compressor 2. The reformed mixture is fed via steam-raising boiler 4 to shift unit 5, and thence via cooler condenser 6 to the main "Prism" unit 7. The H2/CO2rich stream from the "Prism" unit 7 passes on to methanol loop 8, whilst the rejected stream is passed back as fuel to the fired heater 1.

The purge stream from the methanol loop 8 can either be used as a power source, or, as indicated in the alternative arrangement of Fig. 1., can be fed to a loop purge gas separation device 9. The recovered hydrogen and carbon dioxide are recycled to the main "Prism" unit 7 and/or to the methanol loop 8, as appropriate, whilst the methane/nitrogen/argon-containing stream is further separated by cryogenic separation device 10 into a methane-rich stream and a nitrogen/argonrich stream. The methane-rich stream is returned to join the main methane feed stream entering the fired heater 1, whilst the nitrogen/argon-rich stream is used for generating power before being vented. A gas turbine 11, being fed with some of the rejected stream from the "Prism" unit 7, drives the air compressor 2.

If a cryogenic unit is used to remove inerts from the methanol loop, then the methane/nitrogen/argon-containing stream from such a cryogenic unit may be fed to the nitrogen/ methane-separating cryogenic unit, if one is used, thus to allow any argon entering the loop to be rejected with the main nitrogen stream.

If a methane/nitrogen-cryogenic separation unit is used, the carbon dioxide in the feed will have to be removed either by means of liquid device, such as Benfield or Selexol unit, or-preferably by means of a methanation stage. This latter stage would also methanate the carbon monoxide, and thereby reduce the loss of fuel value by converting the monoxide, which otherwise would partially leave with the nitrogen stream, into methane. Thus monoxide would thereby be recovered and recycled.

Any fuel stream may be used to drive a gas turbine to power the various process compressors. The turbine exhaust gases may be used to superheat steam which in turn may be used in a steam turbine to supply additional power for compressor drives. The steam turbine may be wholly or partly of the pass-out variety, and any steam which is passed out may be used as process steam or to supply heat for distillation reboilers.

Of course, both the air and the natural gas may be saturated with water using saturators heated by means of heat given up process and/or flue gas steams. Also, although natural gas has been specifically described as the feedstock, other feedstocks, such as oil-based hydrocarbons like naphtha could also be used.

As is well known in the art, one or more gas permeable membranes may be used in series or in parallel with or without recycle.

Claims (5)

1. A methanol production process in which the methanol synthesis gas comprises substantially a mixture of carbon dioxide and hydrogen, the said mixture being formed by the air-reforming of a hydrocarbon feedstock, characterised in that the synthesis gas is produced by co-permeating the said mixture through a gas permeable membrane whereby to separate the said mixture from at least a major proportion of the nitrogen present in the air used for the reforming.

2. A process as claimed in claim 1 wherein the ratio of carbon dioxide to hydrogen in the synthesis gas is arranged to be slightly lower than the stoichiometric ratio for methanol production.

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

4. A process as claimed in claim 1 substantially as hereinbefore described with reference to the specific drawing.

5. An apparatus adapted to operate offshore and capable of producing methanol -in accordance with a process as claimed in any one of the preceding claims.