PTQ Q2 2024 Issue

Co-processing renewable feeds in hydrodesulphurisation units: Part 1

Ionic modelling’s contribution to designing crucial modifications in HDS unit process schemes to minimise the risk to operations while maximising renewable feed utilisation

Cristian S Spica OLI Systems, Inc

T he call for the energy transition underscores the need to decarbonise our lifestyles. Numerous pathways have been identified to contribute to the decarboni - sation of the transportation sector. It is imperative to incor- porate existing assets into these initiatives to swiftly attain decarbonisation benefits in the transportation fuel sector. The refining industry has responded to the target of reducing fossil carbon emissions by integrating renewable feedstock components into refinery operations, aiming to lower the carbon intensity of the resulting fuels. Refiners are employing diverse strategies for processing renewa- ble feeds. Some are undertaking the construction of new units dedicated to renewable diesel or sustainable aviation fuel (SAF). Simultaneously, others are actively engaged in adapting their existing facilities – hydrotreaters or fluid cat - alytic crackers (FCCs) – to co-process a portion of renew- able feeds. The introduction of new feed components triggers entirely new reactions, resulting in the formation of new products that may introduce fresh challenges. Thus, before introducing even a minor number of new feedstocks into an existing facility, it is crucial to understand the potential implications and have a clear strategy for mitigating any associated risks. In such a scenario, having access to a robust tool becomes paramount, aiding in identifying the type and safe percentage of renewable feed that can be incorporated into the existing feed. Ionic modelling tools offer a solution by enabling the

identification of plant bottlenecks and the development of mitigation strategies. This includes material of construction selection, integrity operating window identification, corro - sion inhibition package definition, and wash water injection design. By accurately predicting corrosion and fouling risk well ahead of introducing renewable feedstock in the unit, these tools can validate the actual risks post-introduction. Co-processing impact on hydrotreatment units Hydrotreating units (HDTs) are key facilities in upgrading renewable feedstocks, such as vegetable oils (for example, palm, soybean, rapeseed, sunflower, corn, and jatropha) and alternative or waste-based oils, such as used cooking oil (UCO), waste cooking oils (WCOs), fatty acid methyl esters (FAMEs), free fatty acid (FFA), palm fatty acid distil - late (PFAD), palm oil mill effluent (POME), tall oil fatty acid (TOFA), into high-quality biodiesel and renewable diesel. While these feedstocks may vary in appearance and contain differing levels of impurities, such as alkalis and phosphorus, they are provided in the form of triglycerides (TG), which can be viewed as the condensation of glycer - ols and three carboxylic acids. The desired reaction is the deoxygenation of the glycerides and free fatty acids in the presence of hydrogen to form linear paraffins, according to the mechanism in Figure 1 . There are two pathways for the main reaction. The first favours high yields (increased return), while the second minimises hydrogen consumption (decreases costs). The first pathway (1) involves complete hydrogenation to form six moles of water, one mole of propane (C3 H 8), and three moles of normal paraffins with the same chain length as the fatty acid chains (n-C 18 and n-C 22 in the case of rapeseed oil) per mole of reacted triglyceride. This pathway is usually termed hydrodeoxygenation (HDO). The second pathway (2) involves a decarboxylation step, where three moles of carbon dioxide (CO 2), one mole of propane, and three moles of normal paraffins with a chain length that is one carbon atom shorter than the fatty acid chains (n-C 17 and n-C 21 in the case of rapeseed oil) are produced. Since both CO 2 and carbon monoxide (CO) are produced, two additional reactions need to be considered, as shown in Figure 1 (3). Hydrotreating catalysts are known to be

H–C–OO–C

1 Hydrogenolysis / Hydrogenation

TG + 12 H

3 CH + 6 HO + CH

TG + 3 H 3 CH + CH + 3 CO 2 Decarboxylation / Hydrogenation

3 Methanation / WGS

CO + 4 H CO + H

CH + 2 HO

CO + HO

Figure 1 Triglycerides reaction pathways

PTQ Q2 2024