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Veg e table oil in feed , vol%
Veg e table oil in feed , v ol%
Figure 2 Total H₂ consumption
Figure 3 Expected exotherm
the unit used for this test² are more cost-effective than large-scale reactors. Avantium performed a test with 0-100% soybean oil, achieving close to 100% mass balance without having pressure drop issues (see Figure 1 ). The results were published,² the main conclusion being that the Flowrence 16-microreactor test unit proved reliable and consistent for predicting start-of-run performance, simulating the impact of co-processing renewable feedstocks on cycle length, hydrogen consumption and product yields, and predicting the impact in the commercial unit. Vegetable oils decompose according to various reac- tion routes, such as hydrodeoxygenation, decarboxyla- tion, and decarbonylation, producing water, CO₂ and CO, respectively: Hydrodeoxygenation: C₃H₅(C17H33COO)₃ + 15 H₂ C₃H₈ + 6 H₂O + 3 C18H38 Decarboxylation: C₃H₅(C17H33 COO) 3 + 6 H₂ C₃H₈ + 3 CO₂ + 3 C17H36 Decarbonylation: C₃H₅(C17H33 COO) 3 + 9 H₂ C 3H8 + 3 CO + 3 H₂O + 3 C17H36 Using the product yields from the test, the importance of each reaction route can be calculated, allowing the refiner to make a good assessment of the yields originating from fossil feed and vegetable oil. H₂S partial pressure affects the degree of hydrodeox - ygenation (HDO) vs decarboxylation.³ CO is known to inhibit desulphurisation activity. 4 A significantly higher temperature is usually required to achieve the same HDS performance, reducing cycle length. CO does not dis- solve in oil products and is mostly removed by purging some recycle gas. Increasing the gas purging can lower the CO partial pressure, but this inevitably comes with a cost. Evaluating the effects of purge gas rates and cycle duration can assist the refiner in optimising the unit with minimal expenses. Catalyst suppliers have developed catalysts with reduced susceptibility to inhibition. Several catalyst characteristics, such as metal composition, affect the sensitivity to CO and the reaction mechanisms. According to Bezergianni, 5 the HDS effectiveness of the NiMo catalyst remains unaffected
by the addition of waste cooking oil, while the CoMo cat - alyst is significantly impacted. This can pose a problem for low-pressure units that require the most active and stable performance of CoMo catalysts. Ni is said to exclusively promote the decarboxylation of fatty acids, while Mo pro - motes HDO.⁶ HDO is preferred over decarboxylation as the yields are higher and less CO is formed, thereby reducing the need for purging gas. Catalyst suppliers have devel- oped various catalyst systems to provide the best catalyst performance. Refiners should conduct tests on different catalyst sys - tems to determine the impact on unit operation and prof- itability. However, this approach is not straightforward as the impact of conditions on CO formation and purge gas rates must be assessed, which varies for each catalyst sys- tem due to its sensitivity to CO being dependent on catalyst quality. The HydroScope prediction model translates pilot plant test data into commercial performance for all hydro- treating applications, including optimising units to process vegetable oil. Proper assessment of the optimal catalyst system requires quantifying CO formation and its effect on catalyst inhibition and gas purge. Simulation of pilot plant test results The soybean oil test was conducted at various temper- atures, sufficiently high to ensure full conversion of the soybean oil and enable production with sulphur below 10 ppm (<10 ppm S). To avoid H₂ starvation, the H₂/oil was increased from 400 to 1,400 Nl/l, ensuring that H₂ consumption remained below 25% of the H₂ supply. The H₂ consumption measured closely matched the expected value (see Figure 2 ). Converting vegetable oils generates a significant amount of heat, and based on the reactions that occur, the resulting exotherm can be calculated as depicted in Figure 3 . At levels of 20% vegetable oil or higher, the exotherm exceeds 100°C, making it unrealistic to process 40% or more vegetable oil without special temperature control measures. Increasing the H₂/oil did not result in a significant increase in CO partial pressure during the test, and the HDS per - formance was hardly affected by the presence of soybean oil in the feed. The percentage of HDO vs decarboxylation was calculated using the CO₂, CO, and water yields. Based on these data, product yields can be calculated, including
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PTQ Q3 2023
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