Jan-2006

Catalyst catastrophes in hydrogen plants

Processing mishaps can occur if catalyst is placed in abnormal conditions. These catastrophes include temperature runaways, the formation of toxic nickel carbonyl, steam-reforming disasters, unplanned exotherms and side reactions

John R Brightling, Peter V Broadhurst and Mike P Roberts
Johnson Matthey Catalysts

Article Summary

A catalyst normally performs the reaction for which it was designed without causing any problems for the plant operator. However, if subjected to abnormal conditions, whether during normal operation, start-up or shutdown, the catalyst may perform other unplanned reactions. These reactions can generate large amounts of heat, produce toxic materials or have other potentially dangerous effects. In the worst cases, there can be serious equipment damage and a threat to personnel.

There have been many lessons learned over the years from incidents in which catalysts being used in hydrogen plants and similar processes have demonstrated unexpected behaviour as a result of abnormal conditions that, in general, have been outside those specified in the operating procedures for the catalyst in question.

The incidents include temperature runaways, which have exceeded vessel design temperatures and even led to vessel failure; catastrophic failure of steam reformer tubes; formation of the extremely toxic nickel carbonyl; the effects of catalyst wetting; catastrophic carbon formation in a steam reformer; hydration exotherms; catalyst milling and reactor loading issues. In many cases, the analysis of these incidents indicates that many occur during infrequent procedures. An unfamiliar task assessment procedure and staff training in the prevention and handling of incidents should minimise incident frequency, especially at start-ups and shutdowns, which are becoming less frequent events as the reliability of plant systems improves.

Improvements in catalyst technology enable catalytic reactions to happen at ever-improving energy efficiencies, while also reducing their environmental impact. For the most part, catalysts work in a quiet, sustained way and perform the reactions required of them without issue. This, however, can lead operators into a false sense of security. If given the opportunity, catalysts can also perform other unplanned and undesirable side-reactions. Depending on the reactions, these can generate large amounts of heat, toxic materials or other phenomena, which present in the worst cases a danger to life, equipment or the environment.

It should be remembered that there are significant potential hazards associated with the commercial-scale operation of most heterogeneous catalyst systems. Considering the syngas industry, the catalytic stages of these processes operate between 200 and 900°C (392 and 1692°F) and at pressures of 20–300 bara (290–4350psia). Many of the process streams are flammable and explosive if mixed with air. Some materials are noxious, such as ammonia, amines and caustic compounds. Heat management uses high-pressure steam systems and, in certain reactors, fired heaters. Nitrogen is used to flush vessels free of air or hydrocarbon and is an asphyxiant.

Frequency of incidents
In addition to the safety, health and environmental implications of serious catalyst-related incidents, the plant operator can incur significant costs. The cumulative costs of preventable incidents in the industry are not known, but it is likely that they amount to as much as $10 billion.

Paradoxically, a contributing factor to serious incidents can be that plants, equipment and catalysts nowadays deliver incredible reliability. This means there are much longer periods of time between transient operations such as start-ups, shutdowns and the occurrence of serious problems, which makes dealing with them less frequent and more unfamiliar. It also presents the problem of retention of corporate knowledge that has been built up over many years when there is staff turnover due to retirement, promotion or job relocation.

All of the incidents described are based on real events, which have resulted in risk to personnel, major plant equipment damage or irrecoverable catalyst damage leading to a plant outage.1,2 Although most examples within the context of this discussion are taken from syngas operations, the lessons are more broadly applicable across a wide range of catalyst operations. However, each plant will have individual circumstances and the applicability of the lessons must be reviewed in light of local operational knowledge.

Steam-reforming disasters
For those unfamiliar with steam- reforming technology, it lies at the heart of most plants that make on-purpose hydrogen in the oil refinery, as well as those petrochemical facilities making H2 and H2/CO/CO2 mixtures for the production of ammonia, methanol and other petrochemicals. The catalysed, strongly endothermic reaction of hydrocarbon and steam produces a gas rich in H2 and CO with some CO2 and unreacted steam and methane. Thus, the steam reformer is designed to pass the process gas down many narrow catalyst-filled tubes, which are positioned inside a fired furnace. The furnace provides the heat to drive the reaction, and the narrow tubes provide a large surface area between this heat source and the process gas to enable rapid heat transfer into the reacting gas. The heat in the hot flue gas produced in the furnace is recovered by heating the incoming process streams (Figure 1).

The complex control and integration of both the process and furnace side of the steam reformer can lead to many potential issues that could compromise the operation. Examples of disasters that can occur include:
- Tube failure during start-up due to over-heating or thermal shock from steam condensate carry-over
- Catastrophic carbon formation causing pressure drop increase and widespread catalyst damage
- Catalyst wetting leading to widespread catalyst damage
- Nickel carbonyl formation.

Examples will be considered in these areas, but it should be appreciated that there are many other incidents that can occur.

Tube failure on start-up
Throughout the world, there are one or more steam reformers each year that experience the failure of a significant number, if not all, of their catalyst-filled tubes. In the last 12 months, Johnson Matthey Catalysts is aware of at least seven such incidents, including one complete reformer burndown.