Decarbonisation Technology - November 2024 Issue

a bench-scale unit capable of holding more catalyst (up to 150 mL) to process larger amounts of feed. In this configuration, the catalyst dilution may not be sufficient to carry out the large heat of reaction generated, and an oil-heated double-walled tubular reactor becomes the preferred option. The choice of the feeding system for ethylene depends on the required operating pressure. For low-pressure operations, a simple mass flow meter can be employed, whereas for high- pressure operation, ethylene in a supercritical state can be fed with a syringe pump. Similarly, if recycling operations need to be simulated, longer-chain olefins can be co-fed as liquids. The homogeneous distribution of the liquid or gas feed is ensured through capillary distribution systems. Since the feed distribution directly affects the mass balance, it is important to ensure that the physical state within the capillaries is not subject to change due to evaporation or condensation in the distribution system. If the conversion is not complete, product sampling needs to be tuned to ensure that all the molecules in the complex product mixture are taken into account. Therefore, it is necessary to depressurise the downstream section of the reactor prior to the separator to favour the stripping of liquified light components such as propenes, butenes, and pentenes. Quantification of the C3-C5 components in the gas phase will improve the mass balance and minimise safety concerns. Finally, it is important to keep the temperature of the sampling section high to avoid clogging due to the possible formation of longer-chain alkanes with higher boiling points. The mass balance is closed by accurate online and offline quantification and characterisation of the gas and liquid phases, respectively. Offline simulated distillation (SIMDIST) is used to analyse the liquid phase, quantifying the yield and selectivity to the specific product range. The yields and selectivities are then reconciled with the online GC gas phase quantification. If a more detailed analysis of the molecules in the mixture is required, GC-MS, GCxGC, or hydrogenation GC can be used. These techniques can distinguish the olefins, paraffins, isomers, and aromatics, providing even more information about the fuel composition. The

1400

n-octane

Raw sample Hydrogenated sample

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n-decane

600

n-dodecane

400

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Retention time (min)

hydrogenation GC allows an easy quantification of the ratio between branched and unbranched molecules in the product mixture. An example of the power of hydrogenation GC as a technology with respect to a simple GC is reported in Figure 3 . Hydroprocessing (dewaxing) SAF can be produced from vegetable oils (or animal fats) by means of hydroprocessing. Fischer-Tropsch waxes can also be hydroprocessed to meet the desired SAF specifications. Best practices for laboratory testing for the conversion of vegetable and pyrolysis oil to fuels and chemicals are described elsewhere ( Innocenti, et al., 2023 ). This section focuses on the treatment of paraffins with hydrogen to increase the concentration of isomers, improving the cold-flow properties of the fuel. The fuel cuts obtainable by hydroisomerisation of renewables are naphtha (C5-C7), SAF (C8-C16-C18), and diesel (>C16-C18 ). Isomerisation catalysts are very sensitive to impurities, so a thorough characterisation of the feedstock is required. It is important to assess the concentrations of sulphur (S), nitrogen (N), and oxygen (O), which can deactivate/passivate the hydroprocessing catalyst. Additionally, Figure 3 Oligomerisation product sample measured by GC (dark green) and hydrogenation GC (light green). The overlay displays the simplification of the complex product composition, allowing for evaluation of the ratio of branched to unbranched products during catalyst screening