PTQ Q4 2023 Issue

Polyolen waste Py-oils

2.0

Conventional gasoil

1.8

Municipal solid waste & mixed plastic Py-oils

1.6

1.4

1.2

1.0

0.8

0.6

Upgradability

0.4

Biomass wastes Py-oils

0.2

0.0

Conradson carbon residue (CCR), wt%**

** CCR = Conradson carbon residue (ASTM D4530)

Greater tendency to form coke

Figure 2 Upgradability of various sustainable feedstocks defined in terms of effective hydrogen index and concarbon

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 spec- trum of possible feedstocks, it is imperative to understand how the underlying feedstock chemistry will drive upgrade - ability 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 associ - ated with more easily upgradable feedstocks. Heteroatoms, such as oxygen and nitrogen, are undesirable, and their pres - ence in these new feedstocks will in some cases be dramati- cally 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 incor- porated 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 het - eroatoms 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 hydrocar - bons with zero heteroatoms, resulting in an effective hydro - gen 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 (concar - bon). 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 con - carbon content (upper left corner of cross plot). Conversely, more challenging feedstocks will be those with lower effec- tive 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

PTQ Q4 2023