May-2024

Co-processing renewable feeds in hydrodesulphurisation units

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

Article Summary

The call for the energy transition underscores the need  to decarbonise our lifestyles. Numerous pathways have been identified to contribute to the decarbonisation of the transportation sector. It is imperative to incorporate 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 renewable 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 catalytic crackers (FCCs) – to co-process a portion of renewable 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, corrosion 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 distillate (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 glycerols 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 (C3H8), and three moles of normal paraffins with the same chain length as the fatty acid chains (n-C18 and n-C22 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 (CO2), 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-C17 and n-C21 in the case of rapeseed oil) are produced.

Since both CO2 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 active for both reverse water gas shift (CO2 + H2 ∀ CO + H2O) and methanation (CO + 3H2 ∀ CH4 + H2O). The relative extent of these two reactions determines the observed distribution between CO, CO2, and methane (CH4).

If all triglycerides undergo the decarboxylation route, seven moles of H2 will be consumed, compared to the 16 moles of H2 consumed when all triglycerides are converted via the HDO route. In other words, there would be a 63% reduction in hydrogen (H2) consumption. However, if all the CO2 produced is shifted to CO, and all the CO formed is subsequently converted into CH4, a total of 19 moles of H2 will be consumed by the decarboxylation route, resulting in a 19% increase in H2 consumption. Indeed, co-processing biofeeds in an HDT (see Figure 2) can have various effects, both positive (pros) and negative (cons):

Pros:
•    Co-processing biofeeds can lead to the production of intermediate and linear paraffins, thereby improving product properties.
    ν    This increase in volume swell enhances the profitability of the hydroprocessing unit.
    ν    It boosts the Cetane number, facilitating easier blending into the commercial pool or reducing the need for additives.
    ν    Cold flow properties (CFP) are minimally impacted by biofeeds co-processing options.

Cons:
•    Upstream reactors uniform corrosion risk – FFAs corrosion.

•    Increased H2 consumption (lipids HDT reactions).

•    Reactors cycle length reduction and performance decrease:
    ν    Catalyst deactivation and  reactor pressure drops increase:
        ›    Production of CH4/CO/CO2/H2O by conversion of lipids followed by water gas shift (WGS) equilibrium and production of C3H8 by removal of the glycerol group.
        ›    CoMo catalyst active sites partial Inhibition due to CO.
        ›    H2 partial pressure decrease.
        ›    Catalyst active sites partial deactivation due to Ca and Mg phosphate.
        ›    Fouling issues (phospholipids, peroxides, olefins and di-olefins, metals).

•    Reactor effluent air cooler (REAC) – fouling, uniform corrosion and pitting corrosion risk by chlorides in the effluent reactor side:
    ν    Increased organic and inorganic chloride concentration in biogenic feedstock.
    ν    H2O increased production and increased relative humidity (RH):
        ›    HDO pathway.
        ›    Increased water quenching due to higher exothermicity because of the lipids HDT reaction.

•    Low-pressure (LP) separator/stripper overhead/amine section – wet CO2 corrosion risk:
    ν    CO2 production.

•    Compressor capacity limitations:
    ν    Light ends load increase.