Process for the production of hydrogen

Details Process for the production of hydrogen Process for the production of hydrogen

2001-03-23

2002-12-31

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Hydrogen or compound thereof

Elemental hydrogen

C423S437200, C252S373000

Reexamination Certificate

active

06500403

ABSTRACT:

This invention relates to hydrogen and in particular to the production of a hydrogen-containing gas stream from a carbonaceous feedstock. Such processes are well known and involve the steam reforming of a hydrocarbon feedstock, e.g. natural gas, or of a hydrocarbon derivative e.g. methanol, or the partial oxidation, using an oxygen-containing gas, e.g. substantially pure oxygen, air, or oxygen-enriched or oxygen-depleted air, of a hydrocarbon, or hydrocarbon derivative, feedstock or of a solid carbonaceous feedstock such as coal. Such gas generation processes produce a gas stream at a relatively high temperature, normally above 700° C., containing hydrogen, carbon monoxide, and steam, and usually also some carbon dioxide. The gas stream will normally contain some methane together with any inert gases, e.g. nitrogen, that were present in the reactants.
In order to increase the hydrogen content of the gas stream it is well known to subject the gas stream to the shift reaction
CO+H
2
O→CO
2
+H
2
by passage of the gas through a bed of a suitable catalyst.
The forward shift reaction equilibrium is favoured by low temperatures. However since the reaction is exothermic, unless steps are taken such as cooling the gas while in the catalyst bed, the temperature rise occurring if the feed contains a substantial amount of carbon monoxide is often such that low outlet temperatures can not achieved and/or the catalysts effective at low outlet temperatures are rapidly de-activated. For this reason the shift reaction is often carried out in two stages; the first stage (high temperature shift) employing a catalyst comprising iron oxide, e.g. an iron oxide/chromia catalyst, and, after some form of inter-bed cooling, the second stage (low temperature shift) employing a copper-containing catalyst.
In use, the iron oxide in the high temperature shift catalyst may be reduced to a state wherein the catalyst tends to catalyse the Fischer-Tropsch reaction forming hydrocarbons. Reduction of the iron oxide to such a state is thus desirably avoided. We have found that for high temperature shift using conventional iron oxide/chromia catalysts and conventional high temperature shift exit temperatures, e.g. in the range of about 350-500° C., the risk of hydrocarbon formation depends upon the molar ratio of carbon monoxide to carbon dioxide and the proportion of steam in the shift inlet gas. The risk of hydrocarbon formation increases as the carbon monoxide to carbon dioxide ratio increases: however provided sufficient steam is present, the risk may be minimised.
The gasification stage used to produce the shift inlet gas is normally operated at a pressure in the range 5 to 50 bar abs., and in particular in the range 10 to 40 bar abs. The temperature at which the gasification stage is effected will normally be in the range 700 to 1200° C., particularly 750 to 1100° C.
The carbon monoxide to carbon dioxide molar ratio and the proportion of steam will depend on the conditions employed in the gasification stage, i.e. the reforming or partial oxidation stage. Increasing the outlet temperature, increasing the pressure, and/or decreasing the steam to feedstock carbon ratio (i.e. moles of steam per g atom of feedstock carbon) employed in the gasification stage, all tend to increase the risk of hydrocarbon formation in the shift stage.
Generally to minimise risk of formation of hydrocarbons in a subsequent high temperature shift stage employing an iron oxide catalyst, it has generally been necessary to employ a gas mixture containing a substantial amount of steam (so that the steam to dry gas molar ratio is greater than about 0.5) and/or to employ gasification conditions such that the molar ratio of carbon monoxide to carbon dioxide in the gas stream is limited to no more than about 1.9.
Where the gasification process involves steam reforming, it is possible to operate with a sufficient excess of steam that such problems are avoided. However the generation of such an excess of steam is not energy efficient and, in the interests of economy, it is desirable to operate steam reforming processes at low steam to carbon ratios so that the reformed gas stream fed to the shift stage has a relatively low steam to dry gas molar ratio, particularly below 0.6. Indeed practical steam reforming processes generally give gas compositions having a steam to dry gas molar ratio in the range 0.2 to 0.6. Likewise, with partial oxidation processes, the carbon monoxide content of the gas stream is generally at a level at which hydrocarbon formation would present a problem. While these difficulties can be overcome by the injection of steam prior to the shift reaction, the amount of such injected steam is desirably minimised in the interests of economy. For each mole of carbon monoxide converted in the shift reaction a mole of steam is required but the amount of steam required to avoid the risk of hydrocarbon formation is generally much greater than that required simply to have a steam to carbon monoxide molar ratio of at least 1.0.
It has been proposed in U.S. Pat. No. 5,030,440 to overcome these problems by employing a preliminary shift stage at a temperature above 550° C. using an iron-free catalyst such as a calcium aluminate support impregnated with palladium.
We have devised an alternative process wherein the preliminary shift stage is effected at a lower temperature, thus enabling a greater amount of heat recovery to be effected before the shift stage.
It has been proposed in U.S. Pat. No. 4,861,745 to reduce the risk of hydrocarbon formation by including a small proportion of copper in the iron oxide-containing high temperature shift catalyst. However it has been found in practice that this is only partially effective: thus the presence of copper in the iron oxide-containing catalyst merely retards the rate at which the latter is reduced to a state in which the formation of hydrocarbons is catalysed.
In the present invention, the risk of hydrocarbon formation is decreased by contacting the gas stream with an iron-free, copper-containing, catalyst prior to contacting the gas with the iron-containing catalyst.
Accordingly the present invention provides a shift process wherein a gas stream containing carbon monoxide and steam is contacted with an iron oxide-containing catalyst, characterised in that, prior to contact with the iron oxide-containing catalyst, the gas stream is contacted with an iron-free, copper-containing, catalyst at an inlet temperature in the range 280-370° C.
Iron-free, copper-containing, catalysts are normally employed for the so-called “low-temperature” shift reaction which often follows a stage of high temperature shift reaction. Normally catalysts containing a substantial proportion of copper are not employed at temperatures above about 300° C. as the copper tends to sinter and so the catalyst loses activity. However, in the present invention, although some sintering and loss of activity will inevitably occur, the copper-containing catalyst is not required to effect shift to near equilibrium but only to effect some degree of shifting to modify the carbon monoxide to carbon dioxide ratio so that the problem of undue reduction of the subsequent iron oxide-containing catalyst and consequent Fischer-Tropsch reactions is avoided. Furthermore, although the inlet temperature to the preliminary shift stage is relatively low for a high temperature shift reaction, it is high for a shift reaction employing a catalyst containing a substantial proportion of copper, and this relatively high inlet temperature compensates for the loss of activity of the copper catalyst.
Copper-containing catalysts that may be employed include any of those well known for the methanol synthesis or the low temperature shift reaction. Typically the catalyst comprises the product of reducing pellets formed from a calcined composition of co-precipitated copper, zinc and aluminium and/or chromium compounds, e.g. oxides, hydroxides or basic carbonates. Often such catalysts contain more than 20% by weight of copper. Other components such

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