Transforming refinery opportunities through FCC
Pursuing a sustainable path for recycled and renewable feedstocks.
Lucas Dorazio and James Fu
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The integration of recycled and renewable waste streams into chemical value chains is a challenging topic driven by the need to make circularity in the production of chemicals and fuels a reality. Increased circularity in manufacturing will come from both chemically recycling fossil carbon from waste streams such as plastics or tyres and inserting renewable feeds such as lipid-based oils or oils from biomass wastes into refining processes.
Using existing refinery processes for upgrading these materials will offer the opportunity to reduce implementation costs and allow for a gradual transition from conventional oils. However, the properties of oils derived from recycled and renewable wastes will vary considerably and, in some cases, depart significantly from the properties of conventional oils. Some feedstocks will be much more challenging to integrate into existing refinery assets.
One process available to many refiners is fluid catalytic cracking (FCC). For more than 80 years, the process has proven valuable for converting heavy, low-value fractions of conventional oil into high-value products. Inherent in the design and operation of the FCC is a flexibility that will be valuable for upgrading renewable and recycled feedstocks. The combination of continuous catalyst regeneration, periodic catalyst replacement, and flexible catalyst design will make the FCC process an attractive choice for introducing the new feedstocks into the refinery, particularly for the more challenging oxygenated feedstocks not suitable for other refinery processes.
As more refiners begin to explore the incorporation of renewable and recycled feedstocks into their refining networks, a detailed study of the chemistry associated with these different feedstocks and the implications it will have on refinery processes and needs for new catalytic materials to enable refiners to achieve their sustainability targets are discussed herewith.
Chemistry of sustainable feedstocks
Renewable and recycled feedstocks can be derived from many organic-based wastes. The chemistry of these different wastes can vary considerably and impact how easily different materials can be converted into desired products. On one end of the spectrum will be polyolefin pyrolysis oils and Fischer-Tropsch waxes, having characteristics similar to conventional oil, making them more easily integrated into existing refinery processes.
On the other extreme, highly oxygenated oils derived from biomass will behave much differently than conventional oils, potentially requiring significant pretreatment or pre-processing prior to introduction into traditional refinery processes. Between these extremes will exist many other possible feedstocks derived from different materials and prepared using different processes. Given this diverse spectrum of possible feedstocks, it is imperative to understand how the underlying feedstock chemistry will drive upgradeability so we can better predict how various oils will behave in the refinery.
As with conventional oils, the ultimate analysis of the feedstock gives insights into upgradability (see Figure 1). In principle, evaluating the ultimate analysis is the same as that used for conventional oils. It is the carbon and hydrogen that are needed to make products, and higher hydrogen is associated with more easily upgradable feedstocks. Heteroatoms, such as oxygen and nitrogen, are undesirable, and their presence in these new feedstocks will in some cases be dramatically higher than what is found in conventional oil.
Metals making up the ash are also undesirable. In the case of metals, not only could the concentration be higher than what is typically found in conventional oils, the metals will be different. For example, biomass is commonly associated with relatively high concentrations of alkali metals such as sodium and potassium. The implication of these new metals getting incorporated into these new feedstocks will be the need for demetallation processes or new passivating materials incorporated into catalysts.
As with conventional oils, more hydrogen-rich feedstocks will be converted into desired products more easily. In the case of fuels, the relative hydrogen to carbon of the fuel is roughly ‘2’. The lower the hydrogen content is below 2, the more hydrogen will either need to be added via hydrotreating or carbon rejected via catalytic cracking to produce fuels. For conventional oils, the hydrogen-to-carbon is typically 1.7.
In the case of feedstocks derived from some renewable and recycled materials, the relatively high concentration of heteroatoms has the potential to reduce the effective hydrogen content as heteroatoms are removed from the hydrocarbon. For example, oxygen removed by dehydration can produce water, which consumes hydrogen. To account for the impact of heteroatoms on hydrogen content, the ‘effective hydrogen index’ can be defined.1
The effective hydrogen index will vary considerably for different waste streams (vertical axis of Figure 2). On one of the spectra will be polyolefin waste plastics that structurally exist as extremely long-chain saturated aliphatic hydrocarbons with zero heteroatoms, resulting in an effective hydrogen index of 2.0, which is higher than conventional gasoil. Upgrading these materials will be relatively easy. On the other end of the spectrum will be oils derived from biomass with an absolute hydrogen to carbon of only ~1.5. However, due to its high content of oxygen, oils derived from these materials will have a very low effective hydrogen index of around 0.5, making upgrading challenging.
A more complete picture of upgradability can be obtained by also considering the Conradson Carbon Residue (concarbon). Conradson Carbon Residue is a laboratory test widely used in the refining industry to provide an indication of the coke-forming tendency of an oil. We can combine concarbon with the effective hydrogen index as a cross plot (see Figure 2) to give us a more complete picture of the upgradability of various oils. In Figure 2, more easily upgradable feedstocks will be those with higher effective hydrogen and lower concarbon content (upper left corner of cross plot). Conversely, more challenging feedstocks will be those with lower effective hydrogen and higher concarbon content (bottom right of cross plot). Thus, by knowing where a particular feedstock falls within this two-dimensional space, an assessment can be made of its upgradability.
For reference, conventional FCC feedstocks are included in Figure 2 as well. Refineries have been designed around the properties of conventional oils. The closer a renewable or recycled feedstock falls in Figure 2 relative to conventional gasoils, the more easily these feedstocks can be upgraded in existing refinery processes. The further a feedstock falls from conventional oils in Figure 2, the more challenging it will be to upgrade in existing refinery processes.
For the specific case of FCC units, as hydrogen content decreases and concarbon increases, the coke yield during cracking will inevitably increase. At some position in Figure 2, there exists a minimum effective hydrogen index and maximum concarbon content, where if the feedstock were fed undiluted into the FCC unit, the coke yield would exceed what the heat balance can tolerate. At this point, the feedstock must be diluted in conventional oil (‘co-processing’) to increase the hydrogen content and reduce the concarbon of the blend. The lower the effective hydrogen and higher the concarbon content, the more dilution with conventional oil is needed. For example, in the case of biomass pyrolysis oils that have not been hydrotreated, the maximum concentration of bio-oil in gasoil is typically ~10 wt%.
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