The most common method of "on-purpose" hydrogen production is the steam reforming process. The main process step involves the reaction of steam with a hydrocarbon over a catalyst at around 750-8000C (1380-1470ºF) to form hydrogen and carbon oxides. However, there are several other steps to remove impurities and maximize hydrogen production. The main steps involved are as follows:

• Feedstock purification - removal of poisons such as sulphur and chloride to maximize the life of the downstream steam reforming and other catalysts
• Steam reforming - the main hydrogen-producing reaction
• Shift conversion - carbon monoxide reacts with steam to produce carbon dioxide and additional hydrogen. This often done in two stages: High Temperature Shift (HTS) and Low Temperature Shift (LTS).
• Product purification - in older designs, carbon dioxide is removed in a liquid absorption system and finally the product gas goes through a methanation step to remove residual traces of carbon oxides. In most new plants, a Pressure Swing Absorption unit (PSA) is used instead, producing 99.99% product hydrogen and an-off gas used in the fuel system.

Feedstock Purification

The catalysts used in the steam reforming process are poisoned by trace components in the hydrocarbon feed, particularly sulphur, chlorine and metal compounds. Sulphur is the commonest problem, the nature and level of sulphur species being dependent on the source, pre-treatment and molecular weight of the hydrocarbon. Chlorine compounds are less common and metal compounds are typically found in some heavier LPG and naphtha feeds.

The best way to remove sulphur compounds is to convert the organic sulphur species to H2S over a hydrodesulphurization catalyst. The HDS catalyst removes organic-sulphur compounds by reaction with hydrogen to convert the sulphur to H2S.

The next step is sulphur removal with an absorbent. The same catalyst similarly converts any organic-chloride species to give HCl and also acts as an absorbent for most problematic metal species. A second absorbent is used for chloride removal.

CO + 3H2 «=» CH4 +H2O
CO2 + 4 H2 «=» 2CH4 + 2H2O

Most catalysts are formulated on a calcium aluminate base with the active nickel incorporated in a NiO/MgO solid solution. This results in negligible nickel sintering during operation. Deactivation mostly results from solvent carry over from the upstream CO2 removal section.
 

Steam Reforming Catalysts

This is the heart of the hydrogen generation process. The main steam reforming reactions are:

CH4 + H2O «=» CO + 3 H2
CxHy + H2O «=» x CO + (x + 0.5y) H2
and
CO + H2O «=» CO2+H2

The reaction takes place across a nickel catalyst packed in tubes in a fired furnace. An excess of steam is used to promote the reforming reaction and avoid carbon deposition on the catalyst.
Some flowsheets may have pre or post-reforming in addition to the conventional steam reformer. Newer plants tend to be PSA-based, with typically only a High Temperature Shift bed rather than both High and Low Temperature Shift.

This selection considers the feedstock types and operating conditions. The selection is usually dictated by the heaviest hydrocarbon feed and may involve two or three catalyst types in order to obtain optimum performance. The main categories of catalyst are as follows.
Light duty reforming:

For light feeds (most natural gas, refinery off gas and pre-reformed feeds), a range of nickel based catalysts using either alpha-alumina or calcium aluminate support media.

Intermediate duty reforming

Intermediate duties comprise hydrocarbon feeds with a significant C2+ content up to LPG. The heavier feedstock increases the tendency for catalyst deactivation through carbon lay down and requires a specialist catalyst in the top 30-50 % of the reformer tubes. This tendency also occurs with light feeds run at low steam/carbon ratio and/or high heat flux.

Heavy duty reforming

Heavy duty primarily means naphtha feeds which have yet more tendency for carbon deposition. Highly specialized catalysts are required. The co-precipitated nickel based catalysts contain complex stabilized alkali phases and other promoters to maximize carbon gasification activity.

High Temperature Shift Conversion (HTS)

The high temperature shift section increases the hydrogen yield by driving the water-gas shift reaction, shown below, to the right.

CO + H2O «=» CO2 + H2

Most HTS reactors operate at about 350ºC (662ºF) inlet temperature and lower the CO level from 10-15 mol % (dry) to 1-2 mol % (dry). Ideally, the catalyst takes the reaction to equilibrium at as low a temperature as possible as this benefits the H2 make. The catalyst must retain a high activity and also resist breakage and poisoning to give an extended operational life.

The high activity allows enhanced performance through lower temperature operation with inlet temperatures as low as 290ºC (554ºF) leading to less CO slip and slower sintering. This allows new plants to be designed with smaller catalyst volumes saving both capital and inventory costs. Also vessels on existing plants can be short loaded to achieve the same performance as older catalyst types using less of the new catalyst formulation. The increased in-service strength provides more resistance to breakage and a lower tendency for increasing pressure drop. Thus, recovery is possible from upsets such as boiler leaks when deposition of condensate and solids would usually result in catalyst breakup.

Low Temperature Shift Conversion

Many newer flowsheets use a PSA unit downstream of the HTS vessel to produce high purity product hydrogen. Alternative flowsheets feature an LTS converter before the PSA unit or an LTS converter followed by CO2 removal and methanation. The LTS converter enables increased hydrogen yield by further moving the water-gas shift equilibrium in favour of H2.

The LTS reactor operates at about 190-210ºC (374-410ºF) inlet temperature and lowers the CO level from 1-2 mol % (dry) to 0.1-0.2 mol % (dry). As for the HTS catalyst, the LTS catalyst ideally takes the reaction to equilibrium at as low a temperature as possible to favour H2 make. Again, the catalyst must retain a high activity and also resist breakage and poisoning to give an extended operational life.
The copper based LTS catalyst has operational limits. Firstly, the inlet gas temperature must be above its dew-point by a reasonable margin as water condensation damages the catalyst. This limits the minimum inlet temperature to around 190ºC (374ºF). Secondly, the LTS catalyst is affected by traces of poisons such a sulphur and chloride which have little effect on, and pass through the upstream reforming and HTS sections at sub-ppm levels.

Methanation

In flowsheets featuring a methanation step, this removes traces of carbon oxides which may affect downstream H2 user plants. An exit specification in the order of 5ppm is usual. The methanation reactor converts any residual carbon oxides back to methane using a small part of the hydrogen product to effect the reactions -

CO + 3H2 «=» CH4 +H2O
CO2 + 4 H2 «=» 2CH4 + 2H2O