W. Groendijk and A. BUIJS, Shell Projects & Technology, Amsterdam, the Netherlands
An unease around potential high temperature and pressures during fresh catalyst and adsorbent startups—which can lead to situations where designed pressures and temperatures are exceeded—led the authors’ company to create the Catalyst Safety Assessment (CSA) methodology.
The objective of the CSA is to increase focus on risks during transient phases of startups with fresh catalysts and adsorbents in Shell’s facilities. Conducting the CSA—a team exercise involving cross-functional technical experts (the CSA team)—enables Shell to learn from upsets during startups and reduces the risk of safety incidents.
What is a CSA? In a CSA, two main items are evaluated:
The first risk item is assessed by reviewing the specific catalyst/adsorbent startup activation procedure and considering learnings from previous CSAs and industry learning from incidents (LFI).
The second risk item starts with the theoretical risk (none, low, medium or high) of exceeding the reactor’s design temperature and/or pressure due to the reaction between metal oxides and hydrocarbons. The theoretical risks are assessed using an Excel-based CSA screening tool, and that assessment is based on:
Metal oxide + Hydrocarbon → Metal + Carbon dioxide (CO2 ) + Water (Eq. 1)
An example of a Gibbs plot created in the CSA screening tool is provided in FIG. 1. A negative Gibbs free energy indicates that the reaction could occur spontaneously. The only question then is: At what temperature does this become an issue?
Systems with a high or medium risk typically have:
Based on the theoretical risk and increasing CSA knowledge, experiments are often done for medium- or high-risk systems to determine the temperature region where certain reactions will occur. Experiments are performed in the runaway laboratory at Shell’s Energy Transition Campus Amsterdam (ETCA). In this laboratory, several adiabatic calorimeters are available for static experiments with hydrocarbons and catalysts or adsorbents (FIG. 2).
When to perform a CSA. The authors’ company mandates a CSA for:
These checks are embedded in various Shell internal best practices and procedures.
Changes in catalyst manufacturing are included because these could, in specific cases, lead to changes to the final catalyst, such as composition, metal dispersion, metal oxide state or particle morphology, thereby making the catalyst more reactive than expected.
CSA outcomes. The outcomes of CSAs can be diverse and include:
LEARNINGS FROM COMPLETED CSAS
New catalyst application: The importance of startup hydrocarbon. Nickel catalysts can be used for many applications—e.g., ketone or benzene hydrogenation. Experiments show the formation of CO2 from the reaction between the loaded catalyst—containing nickel oxide (NiO)—and the startup liquid. Typical temperatures at which this reaction is observed are:
The reason for the difference is the varying polarity of the hydrocarbon. The learning from this is that metal oxides can react with different hydrocarbons at different temperatures to form CO2.
Heat of vapor adsorption. A commercial sulfur guard bed in the methane feed section of an ethylene oxide (EO) unit showed a large exotherm (40°C–650°C) after a process upset in an upstream distillation column. The causes of the exotherm were:
Experiments with two different sulfur adsorbents showed large differences in ethylene heat of adsorption (TABLE 1). This clearly showed that choosing another adsorbent could lead to a much more robust adsorbent operation, as the adsorption heat effects would be a lot smaller. Also, startup could be quicker with fewer heat effects.
The results increased the awareness and knowledge of adsorption effects, which resulted in better operational procedures. This incident was also shared within the EO/ethylene glycol industry, thereby raising overall safety.
Higher than expected temperatures in transients. The modeling of a deoxygenator reactor bed of a green hydrogen (H2) project showed temperature excursions above the adiabatic temperature rise during transient states.1 Owing to the nature of the plant’s operation (i.e., green H2 is only produced when wind or solar energy is available), the standard operating mode includes rapid changes in reactor inlet conditions (flow and concentration). When changing from low to high flow, the heat present in the hot part of the bed can add to the reaction heat, thereby leading to temperature excursions above the adiabatic temperature rise (FIG. 3).
Reactivity of biofeeds. Renewable diesel fuels can be produced from a wide array of biofeeds, such as vegetable oils and fats. The products are termed hydroprocessed esters and fatty acids (HEFA) or hydrotreated vegetable oil (HVO).
Experiments showed that the biofeeds show decomposition exotherms at the normal operating conditions of the reactor and its inlet and outlet lines, and that hydroprocessing catalysts lower the temperature at which exotherms or exobars occur (TABLE 2).
These observations led to new design and operating considerations for units with these feeds. Hot spots should be avoided in stagnant parts of the feed preheat train, and the preferred liquid for reactor startup with fresh catalyst is a finished product, either biobased or crude-oil based.
Takeaway. A catalyst or adsorbent can act as a reactant, especially when it contains metal oxides. An increased risk exists during a reactor startup when there is a combination of catalyst or adsorbent in a reactive state, and transient reactor operating conditions.
To prevent unit upsets during startup and catalyst activation, it is crucial to understand exactly which reactions can happen at which temperatures. Shell’s CSA process aims to provide a better understanding, thereby continuously increasing the safety of catalyst and adsorbent startups. HP
NOMENCLATURE
LITERATURE CITED
Willem Groendijk is the Global Catalyst Safety Assessment Lead, based in Shell Projects & Technology. He leads the CSA team and is responsible for activities such as ensuring good quality CSAs, enhancing CSA tools and sharing learnings, both within Shell and externally. Groendijk has a background in downstream technology support for chemicals and refining, having worked at both the Pernis refinery and chemicals complex, and at Shell Projects & Technology.
André Buijs is the Principal Technologist for carbonates technologies. He has 38 yr of experience with Shell Chemicals in various roles. As part of multiple Shell process safety groups, he is one of the initiators of the CSA concept.