this section, we will cover the three challenges and their solutions. It is crucial for certification to retain the biogenic carbon from the renewable feedstock in the liquid jet fuel product and meet the freezing point requirement. The key differen - tiator between co-processing in a diesel unit vs a kerosene hydrotreater is the requirement to achieve the product jet freeze point. Thus, the first major challenge for kerosene co-processing is meeting the freeze point requirement and keeping the biogenic carbon in the jet fuel product. Jet A and Jet A1 fuels have stringent product specifications, which include a requirement for the freezing point to be below -40°C for Jet A and -47°C for Jet A1 (as outlined in ASTM D1655). Even modest additions of renewable feedstock can significantly increase the freezing point due to biogenic n-paraffins formed during oxygen removal. As illustrated in Figure 1 , adding just 1% renewable feedstock can result in an increase of the freezing point by more than 30°C, and adding 5% renewable feedstock can result in a deteriora - tion of the freezing point close to 50°C. To meet the cold flow properties requirements of Jet A and Jet A1 fuels, deep dewaxing of these biogenic n-paraffins is essential. Freezing point improvement can be obtained through hydrocracking, where the n-paraffins are cracked into smaller molecules. This reaction, unfortunately, results in high yield loss to gas and naphtha. A much more efficient pathway to lower the freezing point is through hydroisom - erisation. The n-paraffins are isomerised to iso-paraffins, which have much better cold flow properties without losing any biogenic carbons to the gas and naphtha (outlined in Figure 2 ). To meet the cold flow properties requirements of Jet A and Jet A1 fuels, deep dewaxing of these biogenic paraf - fins is essential. Fortunately, this process is made possible by Topsoe’s highly selective dewaxing catalyst, TK-930 D-wax, which effectively facilitates the necessary deep dewaxing and, at the same time, retains the biogenic car - bon in the jet fuel product. This catalyst has been designed to operate even at reactor pressure as low as 25 barg. To
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Figure 1 How the freezing point is affected by even small amounts of renewable feedstocks
this extent, a highly selective dewaxing catalyst and HDO selective grading serve as a drop-in replacement for exist - ing kerosene hydrotreaters. Figure 3 demonstrates how to maximise SAF yield in a kerosene hydrotreater with the right catalyst loading, with selective HDO and isomerisation catalyst. The second challenge is removing oxygen atoms from lipidic feedstocks, converting FFAs and triglycerides into n-paraffins. Two possible reaction routes are available: hydrodeoxygenation (HDO), leading to water formation, or decarboxylation (DCO), leading to CO2 formation (as illus - trated in Figure 4 ). The preferred route is HDO, as it max - imises the yield of the renewable fraction and retains the biogenic carbon in the jet fuel fraction rather than convert - ing it to CO2. Topsoe’s selective HDO catalyst can increase HDO selectivity to 97%, resulting in a renewable yield that is more than 2 vol% higher than when using traditional fos - sil catalysts. The third challenge is the contaminants of renewable fuel. High levels of impurities in renewable feeds (met - als, phosphorous, and unsaponifiables) can be reduced to
Number of carbon atoms: 14 Boiling point: 254 ˚C Melting point: 6 ˚C
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Figure 2 Hydroisomerisation as a more efficient pathway for lowering the freezing point
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PTQ Q2 2024
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