Methanol and hydrocarbons

Details Methanol and hydrocarbons Methanol and hydrocarbons

2000-06-19

2002-12-17

Parsa, Jafar (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Plural zones each having a fischer-tropsch reaction

C518S700000, C518S702000, C518S704000

Reexamination Certificate

active

06495610

ABSTRACT:

This invention relates to the production of methanol and higher hydrocarbons, i.e. hydrocarbons containing four or more carbon atoms. Higher hydrocarbons may be made from a synthesis gas containing hydrogen and carbon monoxide by the known art of the Fischer-Tropsch process. Likewise methanol is often produced from a synthesis gas containing hydrogen and carbon oxides.
Synthesis gas is normally produced by steam reforming a desulphurised hydrocarbon feedstock, especially natural gas. In this process a mixture of steam and a hydrocarbon feedstock is passed, at an elevated temperature and pressure, through externally heated tubes containing a suitable steam reforming catalyst. Catalysts employed are typically nickel on a suitable support, e.g. alumina, magnesia, zirconia, or calcium aluminate cement. The tubes are heated by a suitable gas, typically the product of combusting a suitable fuel. Typically the temperature is in the range 700 to 950° C. and the pressure is in the range 15 to 40, particularly 20 to 30, bar abs. The steam is normally present in an excess over that required for the reforming reaction: in order to reduce the risk of formation of carbon deposits on the reforming catalyst, the steam ratio is typically in the range 1.5 to 4, especially 2 to 3.5. By the term steam ratio is meant the number of gram moles of steam per gram atom of hydrocarbon carbon in the feedstock. The reformed gas will contain hydrogen, carbon monoxide, carbon dioxide, unreacted steam and methane: the precise composition will depend on a variety of factors including the pressure, temperature, and composition of the hydrocarbon/steam mixture. Normally the reformed gas is cooled, with heat recovery, to below the dew point of the steam therein to condense the unreacted steam which is then separated, leaving the residual reformed gas as the synthesis gas.
The Fischer-Tropsch process is often operated by passing the synthesis gas at an elevated temperature and pressure, for example 30 to 50, particularly 35 to 45, bar abs. through a reactor wherein it is contacted with a catalyst, usually an iron- or cobalt-containing composition: a mixture of hydrocarbons is formed together with water and the water and higher hydrocarbons are separated from the residual gas. Conventionally, part of the residual gas is recycled to the reactor as part of the synthesis gas feed, while the remainder of the residual gas is taken as a purge. For the production of hydrocarbons, a stoichiometric synthesis gas has a hydrogen to carbon monoxide molar ratio of about 2, but in order to achieve a high conversion of carbon monoxide, the reactor is often operated with a carbon monoxide-rich gas, i.e. having a hydrogen to carbon monoxide molar ratio below 2, for example having a hydrogen to carbon monoxide molar ratio in the range 1.4 to 1.8, especially 1.4 to 1.6.
Carbon dioxide is largely inert in the Fischer-Tropsch reaction, although some Fischer-Tropsch catalysts exert some activity for the shift and reverse shift reactions:
CO+H
2
O ⇄CO
2
+H
2
As a result of the presence of carbon dioxide in the synthesis gas fed to the Fischer-Tropsch reaction and possibly the production of carbon dioxide by the shift reaction by the Fischer-Tropsch catalyst, the purge gas from the Fischer-Tropsch stage contains carbon dioxide as well as some hydrogen and carbon monoxide.
Methanol is normally synthesised from a synthesis gas containing hydrogen and carbon oxides by passing the synthesis gas over a suitable catalyst at an elevated temperature and pressure. The synthesis gas is normally produced by steam reforming as described above. Normally a copper-containing catalyst is employed: suitable catalysts include compositions containing copper, zinc oxide, chromia and/or alumina and possibly other oxidic materials such as magnesia. The reaction is typically operated at temperatures above 200° C. and at pressures in the range 50 to 150, especially 70 to 120, bar abs. The methanol synthesis is generally effected in a loop wherein the synthesis gas as “make-up” gas is mixed with recycled gas, and the mixture fed to the synthesis reactor. The reacted gas from the synthesis reactor is cooled to condense methanol which is then separated and then the residual gas recycled as the recycle gas. To avoid a build-up of inerts, some of the residual gas is taken as a purge. In the methanol synthesis reaction, methanol is synthesised from both carbon monoxide and carbon dioxide. A gas that is stoichiometric for methanol synthesis has a “R” value of 2 where
R=([H
2
]−[CO
2
])/([CO]+[CO2])
where [H
2
], [CO
2
] and [CO] respectively are the molar proportions of hydrogen, carbon dioxide and carbon monoxide.
I have realised that the residual gas from the Fischer-Tropsch reaction may be used as some or all of the feed to a methanol synthesis process: in this way, use may be made of the carbon dioxide, as well as hydrogen and carbon monoxide, in the residual gas from the Fischer-Tropsch process.
Accordingly the present invention provides a process for the co-production of methanol and higher hydrocarbons by synthesising the hydrocarbons from a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide by the Fischer-Tropsch reaction, separating the higher hydrocarbons, and synthesising methanol from the residual gas.
Thus in the present invention, the residual gas from the Fischer-Tropsch stage is used for the synthesis of methanol. As a result the Fischer-Tropsch reaction may be operated on a “once-through” basis, rather than employing a recycle of part of the residual gas remaining after separation of higher hydrocarbon products from the reacted gas from the Fischer-Tropsch reaction.
The synthesis gas resulting from steam reforming of a hydrocarbon feedstock such as natural gas, as described above, will generally have a hydrogen to carbon monoxide ratio of at least 3, and often in the range 4 to 6. Also it generally has a “R” value above 2.5. In order to render the synthesis gas more suited to the Fischer-Tropsch reaction, some of the hydrogen may be separated, for example by a membrane system, in one or more stages, to give a synthesis gas having a hydrogen to carbon monoxide ratio below 2.5, and preferably below 2, e.g. in the range 1.4 to 1.8. When using a membrane separation system, a small amount of the carbon dioxide may also permeate through the membrane with hydrogen. Some or all of the separated hydrogen may be used as fuel, e.g. that combusted to heat the reformer tubes.
The process of the invention may also be employed where steps are taken to produce a reformed gas having a “R” value closer to 2. Thus processes are known for the production of methanol synthesis gas wherein the reformed gas is subjected to a stage of partial combustion with oxygen. This has the effect of removing some of the excess of hydrogen by forming water and hence decreasing the value of “R”. Since the partial combustion raises the temperature of the gas it is also possible to decrease the residual methane content of the synthesis gas and hence the amount of inerts fed to the synthesis step. In some known processes, the hot partially combusted gas is used as the heating medium for the reformer tubes.
Whether or not hydrogen is separated to adjust the hydrogen to carbon monoxide ratio to render the synthesis gas more amenable to the Fischer-Tropsch reaction, it will normally be necessary to compress the synthesis gas somewhat from the pressure at which the reforming was effected. Thus while the reforming is preferably effected at a pressure in the range 20 to 30 bar abs., the Fischer-Tropsch reaction is preferably effected at a pressure in the range 35 to 45 bar abs.
In a preferred process the synthesis gas is produced by reforming and separation of the excess of steam as described above and then passed through a membrane separation unit to remove some hydrogen, and then at least some of the non-permeate from the membrane separation unit is compressed and at least

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