(a)
(b)
70
30
60
25
50
20
40
15
30
10
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5
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0
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LPG Naphtha LCO HCO Coke
CH
Dry gas
H
C
C = C
C = C
C =
Figure 7 Results from catalytic cracking pyrolysis oil derived from mixed plastic waste using max naphtha catalyst design shown as (a) overall yields and (b) breakdown of dry gas and LPG
to biomass solids, such as woody biomass or agricultural wastes, and oils extracted from plants such as vegetable oils. Both sources of biomass offer the potential to incorporate renewable carbon, but their chemistry will be quite different. Plant-based oils such as corn oil and soybean oil are extracted directly from the plant and consist of a mixture of triglycerides, which are large molecules comprised of glyc- erol binding together three long-chain fatty acids. Different plant oils will contain different mixtures of triglycerides, but all plant oils will contain long-chain fatty acids with an overall carbon number on the order of 50. 3 In general, the distribution of different molecules in plant-based oils will be extremely narrow compared to oils derived from solid biomass via pyrolysis or liquefaction. Further, while there are many different plant oils, they will all be similar in that they are comprised of a relatively short list of different tri- glycerides and, therefore, will behave similarly as a refin - ery feedstock. In Figure 2, the effective hydrogen index is lower than conventional gasoil due to the oxygen content. However, the combination of the nature of the glycerol backbone and the highly saturated fatty acids results in a very low concarbon associated with plant oils. While maybe not commercially practical, it is technically possible to feed these oils undiluted into an FCC unit. This was demonstrated in an experiment conducted in an ACE reactor comparing yields from the incremental replacement
of gasoil with soybean oil using the max naphtha catalyst design. Figure 8 summarises the yields where the conver- sion is held constant. As the content of soybean oil increases in the feed oil, the yield of LPG hydrocarbons decreases due to structural differences between the gasoil hydrocarbons and the triglycerides. Similarly, the yield of light cycle oil (LCO) increases, which results from an accumulation of less reactive fatty acid remnants. What is absent from the hydrocarbon products is the presence of oxygenates. All of the oxygen contained in the triglyceride is removed through various deoxygenation pathways to produce either water, carbon dioxide, or carbon monoxide. One key takeaway from this experiment is that deoxygenation is inevitable, which will largely be the case for all biomass-derived oils. While deoxygenation cannot be prevented, the design of the catalyst may allow directing the deoxygenation towards the production of water to preserve renewable carbon or towards carbon oxides to preserve hydrogen. Oils derived from biomass solids will be significantly more complex. While there are many possible sources of biomass wastes, they are all similar in that they are made up of the same three basic solid structural components: cellulose, hemicellulose, and lignin. What will vary between sources of biomass is the relative amounts of each component, where woody biomass will have higher lignin content than non- woody biomass such as grasses.
(a)
10 (b)
4
2
CO + CO H O
8
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-2
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DG LPG Gasoline
LCO Bottoms Coke
-6
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100% Gasoil
25% Soybean 50% Soybean 100% Soybean
100% Gasoil
25% Soybean 50% Soybean 100% Soybean
Figure 8 Product yields from catalytic cracking of soybean oil using max naphtha catalyst design shown as (a) overall hydrocarbon yields and (b) yields of deoxygenation products
40
PTQ Q4 2023
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