Compositions – Gaseous compositions – Carbon-oxide and hydrogen containing
Patent
1996-01-22
1998-02-03
Geist, Gary
Compositions
Gaseous compositions
Carbon-oxide and hydrogen containing
252372, 502337, C07C 102, C09K 300, B01J 3300
Patent
active
057140928
DESCRIPTION:
BRIEF SUMMARY
According to the present state of the art, mixtures of hydrogen and carbon monoxide are produced by reacting methane with steam, the so-called methane-steam reform process. If only hydrogen is desired, the carbon monoxide is allowed to react with steam to form carbon dioxide and hydrogen. The carbon dioxide formed is removed by dissolving under pressure in aqueous solutions or regenerable solid sorbents.
In addition to methane, other gaseous hydrocarbons or naphtha or other hydrocarbons that can be readily brought into the gaseous phase, can be employed for this process.
By radiation the reaction heat generated in the combustion reaction is transferred to the reaction mixture. The reaction mixture is passed through tubes of a high-grade alloy in which a suitable catalyst has been provided. The tubes are exposed to the radiation of the burners.
To enable the highly endothermic reaction between methane and steam to proceed, the required reaction heat has to be supplied to the reaction mixture at a high temperature, for instance 850.degree. C. (allothermic process). In general, the required heat is generated outside the reaction mixture through combustion of, for instance, methane. In order to transfer the so generated thermal energy to the reaction mixture, a reactor wall with a sufficiently high thermal conductivity has to be employed.
To prevent oxidation at the required high temperatures, costly (nickel-containing) alloys have to be used for the reactor wall. Through radiation the reaction heat generated in the combustion reaction is transferred to the reaction mixture. The reaction mixture is passed through tubes of a high-grade alloy in which a suitable catalyst has been provided. The tubes are exposed to the radiation of the burners. To save compression labor, the methane-steam reforming is often carried out at elevated pressure, for instance at 30 bar, which imposes even more stringent requirements on the oxidation resistance.
It is attractive to work with the lowest possible steam/methane ratio. Theoretically, a ratio of 1 is necessary to react methane with steam according to: a higher steam/methane ratio has to be used, viz. 2 to 3.
For the methane-steam reforming process a nickel catalyst is used. In order to obtain a sufficient thermal stability, the nickel is provided on a thermostable support, such as .alpha.-aluminum oxide or magnesium aluminate (spinel). To effect a sufficiently fast transport of thermal energy in the catalyst bed, relatively large catalyst bodies of different forms are used.
Although the methane-steam reform process has long been used with excellent results, it has a few major drawbacks. First of all, the fact applies that much thermal energy is generated in the flue gas and in the process gas of the plant, which has to be recovered for economic reasons. This leads to the production of much (high-pressure) steam, which cannot always be properly used. Another problem is that for a complete conversion of the methane supplied to the process at elevated pressure, a relatively high temperature has to be set, viz. between about 800.degree. and 1000.degree. C. At temperatures above 850.degree. C., the life of the pipes in which the catalyst is provided is shortened. It is also desirable to obtain a greater flexibility in the carbon monoxide/hydrogen ratio in the gas mixture produced.
The hydrogen content can be markedly increased by carrying out the so-called carbon monoxide shift conversion reaction in two steps, one at a high and one at a lower temperature: the shift reaction alone, and fairly much thermal energy has to be dissipated at a relatively low temperature in order to shift the equilibrium of this reaction to the right.
If it is desired to work with a H.sub.2 /CO ratio of less than three, as with methanol synthesis or the Fischer-Tropsch reaction, special measures have to be taken. The last drawback of the methane-steam reform process is that it can be implemented efficiently only on a small scale. In the case where hydrogen-oxygen fuel cells are used, it would be particu
REFERENCES:
patent: 5206202 (1993-04-01), Lachman
Geus John Wilhelm
van Looij Francine
Gastec N.V.
Geist Gary
Puttlitz, Jr. Karl J.
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