Hydrogen generation process using partial oxidation/steam reforming

Abstract

Partial oxidation / steam reformers (222) which use heat integrated steam cycles and steam to carbon ratios of at least about 4:1 to enable efficient operation at high pressures suitable for hydrogen purification unit operations such as membrane separation (234) and pressure swing adsorption.

Description

FIELD OF THE INVENTION[0001]This invention relates to processes for generating hydrogen involving the partial oxidation and reforming of fuel, especially to autothermal reforming processes. The hydrogen generators using the processes of this invention may find beneficial use in smaller-scale hydrogen plants.BACKGROUND TO THE INVENTION[0002]Hydrogen is used as a feedstock for many chemical processes and has been proposed as an alternative fuel especially for use in fuel cells in stationary and mobile facilities. Steam reforming of hydrocarbon-containing feedstock is a conventional source of hydrogen. Steam reforming of hydrocarbons is practiced in large-scale processes, often at a facility having refinery or chemical operations. Thus, for instance, the large-scale hydrogen plant will likely be able to draw upon the skills within the entire facility to operate sophisticated unit operations to enhance hydrogen production efficiency. An additional benefit of having a large scale hydroge...

AI Technical Summary

Benefits of technology

[0016]In accordance with the processes of this invention, an undue adverse effect from high pressure reforming is avoided by the use of a heat integrated steam cycle employing a ratio of steam to carbon in the hydrocarbon-containing feedstock above about 4:1. While these higher steam to carbon ratios are expected to favor the production of hydrogen in the partial oxidation / steam reforming, the adverse effect of pressure and of energy consumption required to vaporize the higher amounts of steam are reduced by using a heat integrated steam cycle. The heat integrated steam cycle takes advantage of the increased mass of effluent from the partial oxidation reformer to generate at least about 40, and preferably at least about 50, percent of the steam for supply to the reformer at a high temperature, e.g., at least about 300° C. or 350° C., preferably at least about 400° C., say 450° to 60° C.

[0025]In another preferred embodiment of the invention, the reforming effluent is cooled in at least two indirect heat exchanger stages, each with a feed containing liquid water. By having the vaporization occur in each indirect heat exchanger section, several advantages are obtained. For instance, the heat exchanger surface area can be more effectively used to recover large amounts of steam. The reforming effluent can be rapidly cooled, and the amount of steam being produced can be easily and quickly varied to accommodate changes in production rate. Where a water gas shift is used, heat exchanger stages may straddle the shift reactor and heat generated by the exothermic shift reaction would thus also be recovered as steam for cycling to the reformer.

Problems solved by technology

For instance, steam reforming generally uses very high temperatures, often in excess of 800° C., which in turn requires expensive materials of construction.

While the economics of large-scale steam reforming make attractive the shipping of hydrogen from such a large-scale reformer to the point of use, hydrogen, nevertheless, is difficult to store and distribute and has a low volumetric energy density compared to fuels such as gasoline.

There are a number of practical hurdles for such a smaller-scale hydrogen generator to overcome before it is commercially viable beyond overcoming the loss of economy of scale.

For instance, the smaller scale may not support sophisticated operating and technical staff and thus the hydrogen generator must be able to operate reliably with minimal operator support while still providing an economically acceptable hydrogen product meeting purity specifications.

Often smaller-scale hydrogen generators face problems that do not occur with large-scale hydrogen plants.

As sulfur compounds can poison catalysts and may be unacceptable in the product hydrogen, smaller-scale hydrogen generators must incur the expense to remove them.

But as a portion of the feed is oxidized in the reformer, efficiency penalties are taken that are not incurred by steam reforming.

All other things being equal, lower pressures and higher temperatures favor the production of hydrogen, but higher temperatures necessitate more consumption of fuel, thus are disadvantageous.

However, membrane and pressure swing adsorption systems typically require the gases fed to them to be at elevated pressure.

However such is not the case with smaller-scale partial oxidation / steam reforming units where it is desirable to operate at lower temperatures in order to avoid expensive metallurgy and reduce capital costs.

And it is not the case for stand alone hydrogen generators where opportunities to export steam do not exist.

However, additional operating and capital costs are entailed in employing such a compressor.

Moreover, membrane and pressure swing adsorption systems can be particularly disadvantages for a smaller-scale hydrogen generator due to loss of hydrogen.

The retentate, in the case of membranes, and the purge gas, in the case of pressure swing adsorption, contain unrecovered hydrogen and thus reduce the net hydrogen efficiency (NHE) (heating value of purified hydrogen recovered per unit heating value of hydrocarbon-containing feedstock to the generator).

This reduction in net hydrogen efficiency can be deleterious to achieving an economically-competitive smaller-scale hydrogen generator.

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