Eliminating inert entry for catalyst changeout (TIA)
Since hydrocracking and hydrotreating catalyst were introduced to the refining industry in the mid-20th century, it has always been a struggle to safely remove catalyst from the reactors.
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The catalysts are typically manufactured as stable metal oxides; however, during their activation the metal oxides are converted to sulphides. These metal sulphides are very reactive and, when exposed to air, can spontaneously ignite. The industry addressed the problems by removing the catalyst under nitrogen: an inert atmosphere. With air eliminated, the catalyst can safely be removed, but confined space entry into a nitrogen filled atmosphere is inherently a dangerous operation. The dangers are amplified with multi-bed reactors that are difficult to egress in an emergency. Even today, fatalities occur in the industry despite improvements in both equipment and systems/procedures.
In response, Kashima Engineering (KEC) and Softard Industries in Japan developed a technology for passivating self-heating catalysts so they could safely be removed under air. This has the obvious advantage of eliminating what is commonly referred to as the most dangerous operation in refineries today: inert entry operations. It employs the application of a proprietary mixture of organic compounds to the catalyst during a modified reactor shutdown procedure. These compounds have the ability to coat all the catalyst surfaces and pores with an organic film that retards oxygen penetration to the reactive metal sulphide surfaces. This severely retards the dangerous and exothermic oxidative reactions. Figure 1 illustrates the passivating effect of the treated catalyst, showing the heat released from a CoMo sulphided catalyst as it is heated in air. The red line represents an untreated catalyst whereas the blue line is the same catalyst treated by the passivation technology. As can be seen when the untreated sample reaches about 120°C an exotherm is observed. This is the reaction of the metal sulphides reacting with air. As this temperature is further increased, a second exotherm occurs around 270°C. This is the carbon and coke on the catalyst burning. The treated catalyst does not demonstrate a significant exotherm until 300°C. This shows the stabilisation provided by the passivation technology.
A case study describes how ENAP Refinerías performed catalyst removal under an air atmosphere for a mild hydrocracking unit (MHC). The 20 000 b/d unit processes vacuum gasoil and consists of four fixed bed reactors in series containing around 247 000 kg of fresh basis catalyst. On 25 March 2016, MHC feed rate and unit temperatures were simultaneously reduced. The reduction in feed rate is necessary to avoid excess unit pressure drop whilst the unit is cooling down. At the same time the flow must be high enough to maintain good liquid distribution in the reactor for treatment of the catalyst. During this cooling stage, the normal process feed is displaced from the unit and replaced with a carrier oil of prescribed viscosity and other properties, in this case a middle distillate. Once the unit was fully flushed it was put on oil recirculation from fractionator bottoms back to feed surge drum. When reactor temperatures were around 250°C the passivation additive was injected over a two-hour period at the suction side of the feed drum pump and this was followed by eight hours under oil circulation to treat the catalyst bed whilst continuing to cool the reactors. Once the target temperature was achieved, in this case 130°C, the unit was de-oiled, depressurised and degassed according to normal procedures, followed by final cooling to entry temperature. This modified shutdown procedure is somewhat different to the conventional hot hydrogen strip shutdown method but can typically save 12-36 hours on reactor shutdown time alone. Reactor heads were removed and analysed for hydrocarbon vapour (LEL), CO, H2S and SO2. In all four reactors gas samples were taken and found to be LEL free. The reactors were quickly and sequentially turned over to air, enabling catalyst unload operations to be expedited in a non-immediately dangerous to life or health (IDLH) atmosphere. The catalyst was well passivated, showing no signs of reactivity and all dust and pyrophoric material eliminated. Catalyst removal was all completed in an air atmosphere, ~21% oxygen. In summary, the opportunity to eliminate the hazards associated with inert entry met with the refiner’s expectations. Alvaro Barrueto, maintenance engineer for Bio Bio refinery, ENAP said, “Catalyst passivation technology enabled us to apply a safer system of work, eliminating two of the major hazards associated with catalyst removal, nitrogen while unloading catalyst, and inert entry in confined spaces. The modified shutdown procedure allowed us to reduce the overall downtime and gave us access to the reactors sooner.”
Cat Tech licenses the technology for application in Europe, Asia, Australia, Africa and Americas and has many examples of commercial applications globally. In the last two years alone, over 200 reactors have been treated with the passivation technology.
This short case study originally appeared in PTQ's Technology In Action feature - Q1 2018 issue.
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