GB2157688A - Methanol production process using air - Google Patents

Abstract

A process for the production of methanol comprises: (a) reacting a hydrocarbon feedstock with air to yield a gas stream containing nitrogen, hydrogen and carbon monoxide, (b) converting substantially all of the carbon monoxide in the gas stream into carbon dioxide, (c) separating out from the gas stream the nitrogen therein, and (d) reacting together the hydrogen and the carbon dioxide of the gas stream to produce methanol. Preferably the feedstock is natural gas, and the gas separation step is effected in two stages: (i) using a carbon dioxide absorption/desorption system to separate the carbon dioxide from the nitrogen and hydrogen, and (ii) using preferential adsorption or diffusion to split the nitrogen and the hydrogen. A feed gas pre-heated can also be used with advantage.

Description

SPECIFICATION Methanol production process using air The present invention relates to processes for the production of methanol from a hydrocarbon feedstock using air for reforming purposes.

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 or heavy hydrocarbons, respectively, to form carbon monoxide which is then reacted with hydrogen to produce methanol. Air is generally not used in partial oxidation plants because of the presence therein of nitrogen. This nitrogen causes an uneconomicallylarge partial pressure loss in the methanol synthesis loop. Although it is possible to separate it out from the carbon monoxide reactant entering the loop, a good separation is not easy or cheap to obtain.

The high cost of tubular steam reformers on the one hand, and the high capital and energy requirements of air separation units (ASUs), which usually supply oxygen to the partial oxidation plants, on the other hand 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 hazards 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 utilised 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 formed from the carbon monoxide which is usually produced. If natural gas is mixed with steam and 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, unwanted nitrogen is introduced into the process gas stream, but the present invention seeks to eliminate this nitrogen after, rather than before, reforming of the feedstock. By using carbon dioxide, rather than carbon monoxide, to produce the methanol, the separation of components, particularly nitrogen, out of the process gas stream is much easier.

In accordance with the present invention there is provided a process for the production of methanol, which process comprises: (a) reacting a hydrocarbon feedstock with air to yield a gas stream containing nitrogen, hydrogen ar.d carbon monoxide, (b) converting substantially all of the carbon monoxide in the gas stream into carbon dioxide, (c) separating out from the gas stream the nitrogen therein, and (d) reacting together the hydrogen and the carbon dioxide of the gas stream to produce methanol.

The preferred process of the present invention will now be described, in which the feedstock is natural gas, desirably mixed with water in the form of steam. The natural gas and steam feeds are preheated using steam or process or gas turbine exhaust streams and then the natural gas is desulphurised. The steam is then added to the desulphurised natural gas, and this mixture is air reformed to produce a gas stream containing nitrogen, hydrogen and carbon monoxide, usually with some carbon dioxide. After passing through a boiler to reduce its temperature, this gas stream is passed through a carbon monoxide-shift unit which converts all or nearly all of the carbon monoxide to carbon dioxide. It is then fed to a gas separation step in order to remove the nitrogen therefrom. The hydrogen and the carbon dioxide remaining are then reacted together to form methanol.Although this separation step can be accomplished in one stage, e.g. using a "Prism"unit as described below, it is preferable to separate out the carbon dioxide first, before splitting the nitrogen off from the remaining hydrogen.

Accordingly the gas stream leaving the shift unit is fed first to a carbon dioxide-removal system.

Such a system is preferably a physical solventbased absorptionidesorption system, but can be a chemical system, such as the Benfield system. The carbon dioxide is thereafter recovered in any of the conventional ways, e.g. by reducing the pressure on the CO2- rich solution, and then compressing the recovered CO2, either alone or together with hydrogen, as described below. Special units such as the Hi-Gee unit can, of course, be used.

The gas stream leaving the CO2 absorber contains predominantly hydrogen and nitrogen, together with small amounts of unreformed methane, carbon monoxide, residual carbon dioxide and argon. This stream is passed to a hydrogen-separation unit, such as a pressure swing adsorption (PSA) unit which utilises molecular sieves, or a "Prism" unit, or a cryogenic unit.

"Prism" is the trade name for a gas separation unit utilising a gas permeable membrane to allow certain components through the membrane whilst retaining others behind the membrane.

The hydrogen-separation unit produces two streams; one containing the recovered hydrogen, and the other, a fuel stream, containing the unwanted nitrogen. The hydrogen is then compressed to the pressure of the methanol synthesis unit, usually a loop, and is reacted with the recovered carbon dioxide to produce methanol.

The concentration of hydrogen in the gases in the loop may be adjusted by not recovering all of the available CO2, so as to maintain excess of hydrogen in the loop gases. This is desirable because high hydrogen partial pressures are required to achieve economic reaction rates in the methanol loop.

The crude methanol from the loop will generally contain more water than current, state of the art processes. However, this is not a great disadvantage since this water preferentially passes out of the bottom of the conventional methanol distillation columns, i.e. the amount of water entering the vapour phase during distillation is relatively small.

If a "Prism" or a cryogenic unit is used to separate the hydrogen, then the impurities which enter the methanol synthesis loop with the hydrogen, and the much lesser amount of impurities which enter the loop with the CO2, will have to be purged from the loop. Such a purge may be recycled upstream of the shift unit to the CO2 absorber, or to the hydrogen recovery unit.

Also if a "Prism" unit is used, it is possible to recover the hydrogen either by increasing the amount of hydrogen recovered by ventilating the "Prism" unit with the natural gas feed, or by decreasing the partial pressure of the hydrogen on the recovered hydrogen side of the "Prism" unit.

For offshore installation, a "Prism" unit has the advantage of a comparitively light-weight separation unit. A "Prism" unit also has the further advantage in that approximately one half of the CO2 fed to it passes through with the hydrogen. Thus it can be used as a single stage gas separation unit.

Clearly, however, it is highly desirable to recover this unseparated carbon dioxide in a further separation stage using, e.g. an absorption'desorption system.

If, on the other hand, a cryogenic unit is used to separate the hydrogen, then this unit can also be used to separate unreacted methane from the nitrogen. This may be beneficial in that it would allow methane to be recovered without great loss of pressure. This recovered methane could, for example, be used to alter the burning characteristics of the rejected gas streams or could be re-cycled back to the front end of the process. By such means, the severe conditions needed to reduce the concentration of methane to low levels would be ameliorated, and reforming could be effected with less air. Furthermore, the expansion of the nitrogen in the feed to the cryogenic unit would significantly contribute to the refrigeration energy needed to drive the cryogenic unit, thus usefully utilising the pressure energy of the nitrogen.

The rejected nitrogen-containing 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 by process andior flue gas streams. Also, although natural gas has been specifically described as the feedstock, other feedstocks, such as oil-based hydrocarbons like naphtha could also be used. In certain cases, a conventional hydrocarbon reformer could with advantage be included in the process prior to the shift reactor.

Also the air feed to the process can advantageously be enriched.

So far, pre-heating of the feed gases to the air reformer has only been described as that achievable by the use of process gas streams, steam or gas turbine exhausts, i.e. to a temperature of approximately 400QC. Also the air requirements of the described processes of the invention can in certain situations be such that the power needed for the air compressor is a very considerable charge on the process. Furthermore the overall process can sometimes be hydrogen deficient.

It has been found that these drawbacks 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, i.e. the steam.nat- ural gas mixture, out of the fired heater and into an external bed of catalyst where the natural gas is partially reformed. Normally the energy of reforming is supplied solely by the cooling of the gases, but air could be introduced at this point to supply additional heat. A number of such reforming stages with preire-heat may be used.

By this means, 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 ration.

With this preferred arrangement it is 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 this preferred arrangement reduces the amount of air required, nevertheless the oxygen that is introduced with the air, albeit smaller in amount than if there were no pre-heat, reduces the amount of water needed to effect the reforming of the natural gas. This is an important consideration in an offshore situation.

Claims (9)

1. A process for the production of methanol, which process comprises: (a) reacting a hydrocarbon feedstock with air to yield a gas stream containing nitrogen, hydrogen and carbon monoxide, (b) converting substantially all of the carbon monoxide in the gas stream into carbon dioxide, (c) separating out from the gas stream the nitrogen therein, and (d) reacting together the hydrogen and the carbon dioxide of the gas stream to produce methanol.

2. A process as claimed in claim 1 wherein step (c) is effected in one stage using a gas permeable membrane.

3. A process as claimed in claim 1 wherein step (c) is effected in two stages, in the first of which the carbon dioxide is separated from the nitrogen and the hydrogen, and in the second of which the nitrogen is separated from the hydrogen.

4. A process as claimed in claim 3 wherein the second stage is effected cryogenically and wherein unreacted feedstock is also separated out for return to the process.

5. A process as claimed in any one of the preceding claims wherein the feedstock is pre-heated prior to step (a) to a temperature of at least 500 C.

6. A process as claimed in claim 4 wherein the pre-heated feedstock is partially catalytically reformed prior to step (a).

7. A process as claimed in any one of the preceding claims wherein the feedstock is natural gas.

8. A process as claimed in claim 6 wherein the natural gas is mixed with water.

9. A process as claimed in claim 1 substantially as hereinbefore described.