May-2024

FCC co-processing of biogenic and recyclable feedstocks: Part 2

Part 1 provided a segue into operational concerns ranging from WPO co-processing to maximising oxygen removal and flexible catalytic solutions for FCC operations.

Jon Strohm, Darrell Rainer, Oscar Oyola-Rivera and Clifford Avery
Ketjen

Article Summary

Emerging corrosion concerns surrounding the processing of fats, oils and greases (FOGs) are highly paraffinic. They will vaporise and crack easily, but some naphthenic acid (TAN) may be present, particularly in tallow or used cooking oil (UCO). TAN has been known to increase corrosion in distillation towers and hydrotreating units. For FCC units, it is generally accepted that the TAN compounds will crack and pose little harm. However, little has been documented on the feed section’s corrosion effects before the riser.

As a rule of thumb, the presence of TAN in co-processing feeds follows the order of bio-oils > animal tallow > vegetable oils. Many refiners have a maximum TAN specification to reduce the corrosion in the main column overhead (MC OVHD). A careful inspection of the MC OVHD is important during the turnaround after co-processing.

Some co-processing feeds may contain carboxylic acids and other oxygenated species. When these feeds crack, they will form smaller, oxygen-containing compounds. Most of these compounds will crack into the lighter boiling fractions (wet gas compressor [WGC] section). Elevated carbon dioxide (CO2), carbon monoxide (CO), and low levels of alcohols are commonly seen. Other compounds observed are esters, ketones, phenolics, and acids (acetic, formic, and propionic).

In most cases, the appearance of these compounds will be in low concentrations (wppm or wppb levels). Better analytical methods to measure them should be developed for co-processing. These acids may increase the corrosion concerns in the MC OVHD section. Elevated methyl acrylate gas has been observed in tallow-based feeds in general and UCO feedstocks in particular. Control of deoxygenation pathways, as discussed later, is crucial to minimising light oxygenate formation and the impact on these supporting operations.

Light molecular weight (MW) products from co-processing will primarily concentrate in the WGC section. The acids will react with ammonia, and the pH of the systems will decrease. Methyl acrylate will most likely form solids. Amine units will have problems with elevated CO and CO2 levels. In general, foaming issues may occur in high-pressure process units, and corrosion concerns are more common.

Many industrial water and process chemical suppliers have developed improved practices when co-processing. While these issues have been documented, some FCC WGC sections have recorded little to no problems. Process units with no problems have used highly refined or refined/bleached/deodorised (RBD) FOGs. While FCC units have co-processed up to 100% FOGs,1 trials seldom exceed 10% of the total feed and are commonly below 5%.

Catalyst’s role in FCC co-processing
FCC catalysts drive the conversion and selectivity for upgrading heavy feeds to value-added fuel and chemical products. Conventional FCC catalysts are formulated within the constraints of each unit to maximise product value and unit objective. Examples include optimisation for slurry conversion, metals tolerance, coke selectivity, and maximising gasoline and/or light olefins for a given feed, product targets, and operational constraints.

The same holds true for catalyst technologies for FCC co-processing of alternative feedstocks. In addition to the different metals in the alterative feeds, the differences in the hydrocarbon species present alter the conversion chemistry. To maximise product value within the unit constraints and drive towards the incorporation of renewable and circular carbon, the co-processing catalyst can be formulated and optimised for the specific feed chemistry.

Maximising deoxygenation
As previously stated, most units may only process small quantities of FOGs. Typically, these units rely on the base catalyst already in the unit without consideration of the change in the feed chemistry, resulting in impacts on unit operations and the product slate. Successfully processing higher concentrations of FOGs requires maximising oxygen removal.

The deoxygenation efficiency of oxygenated molecules in FOG is influenced by the surface chemistry of the catalyst components, porosity, fatty acid composition, and synergy between vacuum gas oil (VGO) and FOG. The ReNewFCC technologies for FOG co-processing in FCC are designed to maximise deoxygenation and drive it to increase the quality of the hydrocarbon product slate.

To understand this, one must first consider the reaction pathways for deoxygenation. The four main deoxygenation pathways are dehydration, decarbonylation, hydrodeoxygenation, and decarboxylation. The net effect of dehydration results in the formation of biogenic coke, resulting in higher losses of bio-carbon from useful products.

In the case of FOGs, the thermal decomposition of the triglyceride followed by dehydration can result in the formation of light unsaturated oxygenates such as aldehydes, ketones, and acrylates. These light oxygenates can undergo oligomerisation reactions, leading to gum and coke. In addition, these compounds will concentrate in the WGC section of the unit if not converted in the riser. Decarbonylation rejects biogenic carbon as CO and can increase bottoms or coke yields from both biogenic and conventional oil.

While hydrodeoxygenation is dominant in hydroprocessing units, it also occurs in FCC operations due to hydrogen donation from the conventional feed to the biogenic oil. Removal of oxygen as water from the system results in a net decrease in the hydrogen-to-carbon (H/C) ratio of the final hydrocarbon products. Not removing the oxygen as CO or CO₂ provides the advantage of higher biogenic carbon retention in the final products (though CO and CO₂ are preferable to oxygen in the products).

In terms of FCC product slate, the net result from hydrodeoxygenation can be a reduction in overall product value as the reduction in the H/C ratio leads to an increase in slurry and coke make from both biogenic and mineral feedstocks. From the perspective of FCC unit operations, deoxygenation of the biogenic oils through decarboxylation will maximise the H/C ratio of the final products for an improved FCC unit product slate with some loss of biogenic carbon as CO₂.

Modification of deoxygenation pathways
Catalytically influencing the various deoxygenation pathways can have a significant impact on FCC product valorisation when processing unconventional FCC feedstocks by optimising catalytic properties. Throughout the open literature, there have been mixed reports regarding the net impact of adding FOG to VGO feed under cracking test conditions. Mixed results ranging from increased coke and light cycle oil (LCO) make to reduced coke make during testing are largely due to differences in catalyst technology and balancing the catalytically active sites.

Baseline performance evaluation of Ketjen’s standard FCC catalyst technology for maximising gasoline demonstrates that when 20 wt% soybean oil is added to VGO, there is a net positive feed impact of the soybean oil (see Figure 1). The main feed effect is an increase in gasoline yields with a reduction in coke, liquefied petroleum gas (LPG), and dry gas. To achieve further valorisation of co-processing soybean oil and leverage the conversion chemistry, that catalyst can be further tailored.