Hydrogen Specification
Just as the range of hydrogen consumers is wide, so are the demands on quality. Typical specifications are:
For silicon wafer production 99.9999%
For optic-fiber cable production 99.99%
In the refinery environment it is necessary to define the quality of the hydrogen product on a case-by-case basis. Many hydrocracking processes will accept a 98% H2 purity, which can be produced by the traditional shift, C02 removal, and metha — nation route. A number of processes do, however, require higher purities of 99% H2 or higher, in which case the final purification step will have to be pressure swing adsorption (PSA). The higher purity is achieved, however, at the cost of a lower hydrogen yield (about 85-90% instead of 98%) and the production of a relatively large quantity of low-pressure, low-Btu fuel gas that one may or may not be able to accommodate in the refinery-fuel gas balance. It is important in this context to review the hydrogen purity specification carefully with the hydrocracker requirements, since all too often a purity of >99.5% is specified on the basis of the economics of the conversion unit alone, or on the assumption that hydrogen will be generated from a steam reformer that can accommodate the PSA tail gas internally, without reference to the economics of the overall configuration. A careful review of site-specific parameters will produce different solutions. Thus, for instance, the Pernis plant uses the methanation route; others use PSA units (de Graaf etal. 1998; Kubeketal. 2002).