Catalysts for cost effective hydrogen production

Processes now used in the new generation of hydrogen plants make a wide choice of catalysts necessary in order to meet the requirements of particular systems

Leif Storgaard and Hans Christian Nielsen, Haldor Topsøe

Viewed : 8413

Article Summary

Hydrogen plants have traditionally experienced long catalyst lifetimes. In older plants, where purification of the hydrogen is taking place by methanation, the stress on the installed catalyst has been low due to ample design of the plants and to relatively mild operating conditions, ie high steam to carbon ratio.

During the past 10 years, the demand for hydrogen has been increasing and many new hydrogen plants have been constructed. The demand for more cost-effective hydrogen production has increased and the new generation of hydrogen plants is operated under more critical conditions (lower steam to carbon ratio) and a variety of hydrocarbon feeds is applied as feedstock.

Old hydrogen plants have been designed with CO2 removal and methanation for final purification of the hydrogen. A typical pro    cess layout is shown in Figure 1. The steam to carbon ratio is normally 4.0–6.0.

New hydrogen plants are designed with pressure swing absorption (PSA) for purification of the hydrogen. The offgas from the PSA unit can be used as fuel for the steam reformer. A Topsøe-designed designed hydrogen plant is shown in Figure 2. The steam to carbon ratio is 2.5.

Traditionally, the feedstock for the hydrogen plants has been natural gas, LPG or naphtha. However, at the refinery, there are normally various offgases available deriving from other units in the refinery. These offgases have in the past been used as fuel or been flared. In order to reduce cost for the hydrogen production, they are today widely used as feed for hydrogen production and LPG or naphtha is used only as make-up/backup.

This means that the catalysts in today’s hydrogen plant must be able to operate on various types of hydrocarbon feeds or a mixture thereof. Furthermore, the catalyst must cope with feedstocks containing olefins as well as various levels of sulphur and chlorine.

Purifying hydrocarbon feed
The first catalytic step in hydrogen production is purification of the hydrocarbon feed. The hydrocarbon feedstock may contain various levels of sulphur, chlorine or olefins, which have to be removed since these components will create problems for the downstream units.

For hydrogenation of organic sulphur and chlorine compounds into H2S and HCl and for saturation of the olefins, Topsøe has TK-250 catalyst. This catalyst is based on a high surface area alumina carrier impregnated with cobalt and molybdenum. Under normal operating conditions, the TK-250 only deactivates very slowly and lifetime will be many years. The offgases containing olefins may have to be treated in a special way.

The hydrogenation of the olefins is a strongly exothermic reaction. It is generally recommended to limit the operating temperature for TK-250 catalyst to 400°C in order to prevent possible hydrocracking or carbon formation. The inlet temperature, on the other hand, should preferably be higher than 280°C.

If the olefin content in the feed is high, the adiabatic temperature increase across the catalyst may be higher than the operating temperature range foreseen. In such a case, recycling of part of the hydrogenated gas can solve the problem.

This is illustrated in Figure 3. The flow diagram shows actual operating data from a hydrogen plant operating on refinery offgas. The recycle flow is controlled by the temperature measurements in the top part of the catalyst bed since the olefin content in the feed varies from nil to 25 per cent.

Another problem, which may arise when operating on various types of feedstocks, is a large variation in sulphur content in the feed. The TK-250 catalyst is most active in the sulphide state. If the feed for longer periods is essentially sulphur free, the sulphur on the catalyst will gradually be stripped off and the catalyst activity will decrease.

When switching to a feed with high sulphur content, the activity of the catalyst may not be sufficient for converting all the organic sulphur. This problem can be solved by adding small quantities of sulphur to the sulphur-free feed, which will ensure that the CoMo catalyst is kept in sulphide state.

The refinery offgases often contain chlorine and in many hydrogen plants it has been necessary to include a chlorine removal step. After the hydrogenation step, chlorine is efficiently picked up on a chlorine guard. HTG-1 catalyst is applicable for this application. It can pick up 18 per cent chlorine on weight basis.

The hydrogen sulphide is removed by zinc oxide for which HTZ-3 and HTZ-5 is recommended. Both catalysts consist of more than 99 per cent pure zinc oxide. HTZ-3 is the standard product for sulphur removal. It is especially suited for sulphur removal of hydrocarbon feedstocks containing more than 5ppm of sulphur.

It has a capacity of more than 500kg sulphur per m3 of catalyst. HTZ-5 has a higher porosity and higher surface area than HTZ-3. It is recommended for hydrocarbon feedstocks with low sulphur content or for low temperature application.

Some plants, however, experience problems with efficient desulphurisation of the feedstock. Reasons for this may be a large variation in sulphur content in the feed, low operating temperature or high CO2 content in the feed. CO2 will react with hydrogen on the TK-250 catalyst according to the reverse shift reaction.

The steam formed from this reaction has an impact on the equilibrium H2S leakage from the zinc oxide. For such cases, a Cu-based catalyst, type ST-101, has been developed, which efficiently removes all types of sulphur which may leak from the desulphurization section.

Add your rating:

Current Rating: 3

Your rate: