Hydrogen Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 121

Modern hydrogen plants typically use a PSA unit for hydrogen pu-
rification since PSA units are more efficient at higher pressures. The mini-
mum pressure for PSA operation is 150 to 200 psig. The optimum pressure
may be as high as 300 to 400 psig. In hydrogen plants the reformer outlet
pressure usually runs between 150 and 400 psig.
In most applications, the hydrogen product requires a much higher
pressure for refinery hydrotreating applications. Hydrogen plant reform-
ers typically operate at process gas outlet temperatures of up to 1,600°F. At
these temperatures, the outlet piping limits the reformer outlet pressure to
about 400 psig. This corresponds to a hydrogen product pressure of about
350 psig.
The reforming reaction equilibrium is favored by high temperature.
Reformer process-gas exit temperatures are typically 1,500°F to 1,600°F.
Lower temperatures give inadequate conversion and higher tempera-
tures increase metallurgical requirements, tubewall thickness and fuel
consumption. The reforming reaction rate becomes significant at about
1,000°F. An inlet temperature near this value is typically achieved by pre-
heating the reformer feed.
The hydrocarbon feed must contain sufficient steam to avoid carbon
formation on the catalyst. The steam-to-carbon ratio is defined as moles of
steam per mole of carbon in the hydrocarbon. The steam-to-carbon ratios
are about 3.0 for hydrocarbon feedstocks but lower values can be used for
some feedstocks. Carbon formation is more likely with heavier feedstocks.
An alkali-based catalyst can be used to repress carbon formation.
Heat flux is defined as heat input per unit of time per square unit of
inside tube surface. A low heat flux provides extra catalyst volume and
lower tubewall temperatures. This increases the reforming reaction con-
version and increases tube life. A high heat flux reverses these effect, but
reduces the number of tubes. The flux is highest at the zone of maximum
heat release and then drops to a relatively low value at the tube outlet.
The reformer pressure drop depends on the number of tubes, tube
diameter and catalyst selection. The typical pressure drop ranges from 40
to 60 psi. The reforming catalysts are made in a ring or modified ring form.
Nickel is the chief catalytic agent. Heavier feedstocks use an alkali pro-
moter is to suppress carbon formation.
Higher heat fluxes require a modified ring shape to sustain the re-
forming reaction conversion. A dual charge of catalyst may also be used.
The tube's top half has a high-activity catalyst to prevent carbon forma-
tion in the maximum flux zone. The bottom half may be a more conven-