Question
-
What FCC and hydrotreater modifications are needed to increase refinery coprocessing of renewable feedstocks?
Feb-2025
Answers
-
Darrell Rainer, darrel.rainer@ketjen.com, Ketjen
Modifications necessary to enable renewable feedstock integration within existing operations are dependent primarily on the differences in feed properties compared to conventional petroleum feedstocks. Renewable feedstock properties related to increased and new contaminant levels, thermal stability and reactivity, and high oxygen content are generally the main factors influencing operational, process, and equipment modifications. Based on chemical composition and feedstock properties, renewable feedstocks for coprocessing can be summarised in two categories.
The first category is fats, oils, and greases (FOG), which are triglycerides from edible oils, non-edible oils, used cooking oil (UCO), and animal fats. The second category is bio-oil derived from either pyrolysis or hydrothermal liquefaction of agricultural, forestry, municipal wastes, and other lignocellulosic materials.
Common to both FOG and bio-oil is the potential for high levels of various contaminants, particularly alkali and alkaline earth metals, phosphorous, chlorine, and silica. High levels of metals can create corrosion concerns and impact catalyst activity and lifetime. The most difficult of the FOG group to treat would be UCO with elevated chlorides, acidity, metals, and other contaminates.
Crude FOGs are the next most challenging, with elevated metals and contaminates. UCO, crude FOGs, and bio-oils will exhibit elevated contaminant metals and chlorides with attendant corrosion concerns that may require lining storage tanks and selection of corrosion-resistant metals for process equipment and piping. These materials are not commonly processed at this time. They are not compatible with fossil fuels and have stability concerns. They will have extremely high water (~20 wt%) and oxygen (~20 wt%) levels with extremely high metals and impurities. Since they do not compete with food, they are considered substantially more desirable as a renewable feedstock.
Since bio-oils are far less commonly used, this response will focus primarily on the FOG effect on hardware changes/additions.FCC unit
Shipping/receiving and storage: Typically, feedstocks are delivered by ship or truck and should be diverted to a separate storage tank. Time-dependent feed quality degradation is a greater concern for FOGs and bio-oils relative to conventional feedstocks (a storage life of three months is usually considered acceptable, depending on ambient temperature and specific feedstock characteristics). Ageing of FOGs can result in increased fatty acid content, leading to higher acidity and corrosion concerns, while ageing of bio-oils can result in gumming and fouling of process vessels and piping and potential phase separation due to high water content.
It is recommended that this feedstock be sent to an isolated feed nozzle. The licensor should be consulted regarding the need for any special modifications to the existing nozzle or the introduction of entirely new designs (see Figure 1).When considering bio-oils, a specially designed nozzle is necessary due to the high water content and miscibility issues. The feed nozzle design and operation should account for the water-to-steam volume expansion and low temperature mixing with the dispersion steam. Bio-oils have poor thermal stability due to the high oxygen content, particularly of pyrolytic sugars present in the oils. Removing water through thermal pretreatment is not feasible due to hydrolysis and decomposition of the pyrolytic sugars at temperatures above approximately 50°C. Decomposition of sugars present results in severe coking and gumming of process lines, heat exchangers, and feed nozzles. Without pretreatment operations for the stabilisation of the bio-oils by removing these sugars, the bio-oil will require separate feed nozzles to directly inject bio-oil into the FCC riser while minimising any preheating (see Figure 2).
FOG feeds are particularly susceptible to thermal degradation at typical FCC preheat conditions, which is above the smoke point of the FOG. Elevated temperatures above the smoke point can result in elevated total acid number (TAN), increasing corrosion concerns and dehydration reactions and resulting in biogenic carbon losses as gums and carbon deposits in feed lines, heat exchangers, and injectors. Considerations must be made for any process component in contact with these high-TAN renewable feedstocks, with improved corrosion-resistant properties where required.
One of the main objectives for coprocessing is to incorporate biogenic carbon into fuels to maximise biogenic hydrocarbon products; switching to a catalyst system that maximises hydrodeoxygenation would be preferred. However, maximising hydrodeoxygenation results in lowering the H/C ratio of the final product slate and results in higher coke and lower product value. To minimise the impact on the product slate and/or improve overall hydrocarbon yields, catalysts favouring decarboxylation will result in maximising deoxygenation while maintaining higher H/C ratios of the final products at the expense of lower biogenic carbon in the final hydrocarbon product compared to hydrodeoxygenation. Dehydration and decarbonylation will result in increased biogenic carbon rejection as coke and CO (see Figure 3).
In the product recovery section, changes in the water and process/chemical chemistry are necessary. The increased chlorides, CO and CO₂, oxygen-containing hydrocarbons, and lower pH will result in increased corrosion, emulsion, and foaming in the downstream units. This negative impact must be addressed in cooperation with the water process chemical provider.
HPC
When introducing renewable feedstocks to the hydrotreater, there are several factors to consider that might require some hardware modifications. First of all, the H₂ availability must be evaluated: biofeed processing requires additional H₂ consumption, so the first limitation to increase the biofeed intake is set by the MUG compressor capacity.The second operational aspect to evaluate is the maximum delta T allowed. This is generally set by the reactor design. However, this limit can be managed up to a certain extent (by recycling the product and diluting the feed, for example).
Additional flexibility is also provided by the quench, with more impact on subsequent beds than the top bed but also on overall T profile. There are also some hardware limitations that might require revamp and modifications. The first one is related to the high acidity of the biofeed that can lead to corrosion problems upstream and within the reactor. In general, there are some minor preventative actions available to mitigate this (such as pretreatment of the biofeed and N₂ blanketing in the biofeed tank). In more severe cases, changing metallurgy upstream might be the only option. A second hardware limitation can be the formation of salt downstream caused by the presence of Cl in the biofeed. In this case, wash water injection can be applied to mitigate this complication.
The last aspect to consider is the formation of certain byproducts upon biofeed coprocessing. It is recommended to confirm that the high-pressure separator has sufficient capacity to deal with the amounts of propane and water formed. On the gas-make side, a series of components will be formed (methane, CO, and CO₂) that will impact the recycle gas purity. For this reason, it is recommended to increase the purge rate compared to a conventional operation.
Jan-2025
SPONSORS