Apr-2018

Catalyst passivation for safer, more efficient turnarounds

Eliminating inert entry for catalyst changeout in a hydrocracker. Since hydrotreating and hydrocracking were introduced to the refining industry in the mid-20th century, it has always been a struggle to safely remove the catalyst from the reactors.

ALVARO BARRUETO, ENAP Refinerías
IAN BAXTER and GARY WELCH, Cat Tech International

Article Summary

These catalysts typically contain a combination of molybdenum or tungsten with nickel or cobalt. The catalysts are manufactured as stable metal oxides. During the activation process, the metal oxides are converted to sulphides. These metal sulphides are very reactive and, when exposed to air, can spontaneously ignite. This not only creates a fire hazard but can also release toxic sulphur dioxide, making it difficult to remove them safely from reactors.

In the early days, refiners addressed this problem by performing an in situ regeneration or passivation of the catalyst by heating with steam/air or nitrogen/air under controlled conditions. There are two major problems with this solution. First, it is very time consuming and would keep the unit off stream for days. Secondly, it frequently resulted in the release of toxic sulphur oxides to the atmosphere. In situ practices were essentially outlawed by the clean air regulations of the 1970s and 1980s.

The industry solution to these regulations was to remove the catalyst under nitrogen. With air eliminated, the catalyst could be safely unloaded, packaged in hermetically sealed containers, and sent for disposal, reclamation or regeneration. This created a new industry for ‘catalyst handlers’. These professionals would enter reactors under nitrogen with breathing apparatus to vacuum or shovel out catalyst. Unfortunately, this is a very dangerous operation and has resulted in accidents and fatalities over the years. The dangers are even more severe with newer multi-bed reactors which are difficult to egress in an emergency. The equipment and procedures have improved to the point that incidents are rare, but there are still fatalities in the industry today.

History of catalyst passivation
Catalyst Passivation Technology has been used successfully to treat over 350 million kg of catalyst to date, with 151 reactors treated in 2017 alone. Its development history started in the mid-1980s in Japan where Kashima Engineering Company (KEC) and Softard Industries developed the technology for passivating self-heating catalysts so that they could be safely removed under air. This has the obvious advantages of eliminating the need for inert entry operations. The technology was applied primarily to resid desulphurisation units because of the challenges they afforded with multi-bed reactors and agglomerated catalyst. The treatment process involves the application of a proprietary mixture of organic compounds to a reactor system while under oil recirculation during the cooling and shutdown process. These compounds have the ability to coat all surfaces with which they come into contact. This includes reactor internals but most importantly the catalyst itself. This organic film retards oxygen penetration to the reactive metal sulphide surfaces and retards the dangerous and exothermic oxidative reactions. This is somewhat different to the conventional shutdown procedure which involves a hot hydrogen strip. The process oil is usually flushed from the unit and replaced with a lighter oil of prescribed viscosity and other parameters and is termed the ‘carrier oil’. Once the unit is flushed and the process oil replaced, it is put on oil recirculation followed by injection of the organic compound.

KEC and Softard optimised this technology through application to their native refinery and have now expanded throughout the Far East. Cat Tech International Ltd, a specialist catalyst handling company, licensed the technology for application in Europe, Asia, Africa, Australia and the Americas.

ENAP interest
ENAP Refinerías S.A. operates two refineries in Chile, Bio Bio and Aconcagua, with a combined distillation capacity of 220 000 b/d. The refineries supply 80% of domestic fuel requirements and also export part of their production to Peru. At the Bio Bio refinery is a 20000 b/d mild hydrocracker (MHC) unit containing about 247000 kg of fresh basis catalyst. It consists of four reactors in series flow, processing vacuum gasoil (see Figure 1).

Historically, the unit has experienced unloading difficulties due to agglomerated catalyst requiring hazardous inert confined space work to perform vacuum extraction. ENAP was looking for other options to reduce the risks associated with nitrogen and confined space entry. Cat Tech started talking to ENAP in 2015 about its catalyst passivation technology as a way to shorten the shutdown duration and make reactor unloading safer. In March 2016, ENAP elected to use passivation technology in a trial application on its smaller diesel hydrodesulphurisation (HDS) unit and, following its success, then applied it to their MHC in the same year.
 
Benefits of catalyst passivation
Although the opportunity to eliminate inert entry was central to developing the technology, there are many other advantages in catalyst passivation. These can be categorised in the following sections.

Safety: opportunity to eliminate inert entry
The self-heating or pyrophoric nature of the catalyst and dust is suppressed or eliminated by the passivation treatment. This allows the safe handling of catalyst in an air atmosphere.

Figure 2 illustrates the passivating effect of the treated catalyst. The thermogram in Figure 2 shows the heat released by catalysts as they are heated. The red line represents an untreated catalyst whereas the blue line is the same catalyst treated by the passivation process. As can be seen, when the untreated sample reaches about 120°C, an exotherm is observed. This is the reaction of the metal sulphides with air. As the temperature is further increased, a second exotherm occurs around 250°C. This is carbon and coke on the catalyst burning. The treated sample does not demonstrate a significant exotherm until 300°C. This demonstrates the dramatic stabilisation provided by the treatment. If catalyst is removed under air, it is noted that many reactors are made of austenitic stainless which may become sensitised and prone to the phenomena of polythionic acid stress corrosion cracking (PSCC). Formation of polythionic acid requires three things to be present: sulphide corrosion products, oxygen and moisture. In the case of unloading in air, NACE international standards1 provide guidelines for protection of the surfaces of austenitic stainless steel through the exclusion of water using dehumidified air.

Dust-free operation
Dust from catalyst handling operations typically contains iron and catalyst metal sulphides which are both toxic and pyrophoric in nature. These types of dust are a risk to people and the environment and present a challenge to everyone involved in catalyst handling. In particular, compounds of nickel are under increasing European regulatory scrutiny. With passivated catalyst, dust and fines adhere to the treated catalyst surfaces, resulting in a dust-free handling operation. The technology can therefore be applied as an effective risk control measure, contributing to a safer system of work.