PTQ Q4 2023 Issue

Cold condenser

Hot condenser

0.8

100

0.7

0.6

0.5

0.4

0.3

0.2

0.1

14 15 16 12 13 1234567891011 0.0

0.0

14 15 16 12 13 1234567891011 Reactor number

Reactor number

O_in O_CO O_CO

O_HO_g O_HO_l

Figure 2 Comparison between the difference in oxygen recovery by keeping the condenser at low (left) and high (right) temperature

deactivation and even reactor clogging if present in high amounts. Free fatty acids (FFA) and Cl in the feedstock can also cause corrosion. It is, therefore, important to know the composition of trace elements in the feedstock before starting a pilot plant testing, as well as the total acid num- ber (TAN) and the bromine or iodine number (unsaturation degree). Standard hydroprocessing technologies can be applied to convert VOs to fuels if they can be tuned to overcome the new challenges posed by the different chemical make-up of renewable feedstocks. These challenges include higher O content, corrosion issues, and different types of impurities. High throughput units (16- or 24-fold) can host reactors with an inner diameter of up to 8 mm, while bench-scale reactors (4- or 8-fold) can be used with reactors up to 19 mm of inner diameter. The bench-scale units also allow testing guard bed material to eventually process feedstocks with high content of P or Si and evaluate materials based on their performance for P and Si uptake. If the iodine num- ber of the feedstock is high and clogging of the reactors is experienced, it is possible to optimise the reactor loading by exchanging the top layer of inert with a lower activity catalyst that will hydrogenate the unsaturation. To be converted to fuels, VOs need to be hydrodeoxy- genated and then dewaxed (hydroisomerisated) to convert the linear paraffins into branched ones. The first step in the hydrotreatment of VOs is the hydrogenation of all the unsaturation, as shown on the top panel of Figure 1. The FFAs are then undergoing HDO, DCN, or DCX to yield lin- ear paraffins. However, the last three reactions mentioned may also take place on the hydrogenated triglyceride or any di- or monoglyceride still present in the feed. HDO is the preferred and targeted reaction since the co-product

is H₂O, while DCN and DCX are unwanted reactions that co-produce CO and CO₂. All the co-products can cause possible issues down- stream of the reactor. H₂O together with HCl, formed from trace amounts of Cl in the feed, can promote corrosion. The possible corrosion issues raised by the presence of water in the liquid phase can be solved by keeping as much water as possible in the gas phase. Vaporising the water is also helpful for the oxygen balance that can be affected by the separation efficiency. The H₂O and organic phases can be separated only to the extent they did not emulsify, and the water is not micro-dispersed. When it is not possible to keep all the H₂O in the vapour phase, it is good practice to run an elemental analysis and/or Karl-Fischer titration of the organic sample to account for dispersed/emulsified water. The oxygen balance can be significantly improved, as shown in Figure 2 , by increasing all temperatures down- stream from the reactor. Additionally, it is recommended to introduce some inert gas after the reactor to dilute the concentration of H₂O in the gas phase and promote its evaporation. In a single-stage configuration, H₂O, CO, and CO₂ will enter the hydroisomerisation (HI) reactor, possibly deactivating the noble metal catalyst (left panel in Figure 3 ). The sour mode of operation should only be chosen if the HI catalyst can work in such conditions or if its resilience to such contaminants is under evaluation. If the water effect on the HI catalyst performance is under evaluation, the test should be run at a specific temperature that yields a product with a specific target parameter value (such as density, cloud point, or pour point). The target parameter must be routinely tested to ensure that the sys- tem is operating correctly. Afterwards, once the behaviour of the system is well characterised, it is possible to increase

PTQ Q4 2023