Catalytic reforming in the biorefinery
Optimum production of hydrogen in the reformer enables profitable production of diesel range biofuels by hydrotreating residual oils.
RAFAEL LARRAZ MORA
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The transition towards a world economy without net CO2 emissions will require profound changes in energy consumption habits. In the long term (2050), the changes will completely alter the supply and demand system for energy products and the global economic system. Faced with this future scenario, the decarbonisation of transport will entail significant technical and economic challenges. Consequently, companies in the refining and petrochemical sector already face a double challenge in the 2020s: first, to reduce the carbon intensity of their own activities; second, they must cope with a reduction in demand for fossil fuels. This progressive reduction will be gradually replaced by an increase in electricity consumption. Despite this growing electrification, liquid fuels will continue to be needed, although with less intensity in CO2 emissions than at present. In fact, the contribution of advanced biofuels must be key in the fuel mix if global warming is to be limited to 2ºC (see Figure 1).1 In the same vein, the International Renewable Energy Agency (IRENA) agrees that the current quota and forecasts for the use of advanced biofuels are not enough to comply with the 2DS scenario. IRENA quantifies a reduction in fossil fuel needs of 64% in 2050 compared to current consumption.2
In the case of the European environment, the legal framework is given by Directive 2018/2001/EU on Renewable Energy (abbreviated as RED II). In 2030, the main magnitudes and objectives established by RED II can be summarised as:
• The minimum target for renewable energy consumption across the EU is 32%. In transport, at least 14% of this energy must be of renewable origin in 2030
• Establishes limits for biofuels from high impact raw materials. These biofuels (1G) can continue to be used but up to a limit, setting their maximum degree of incorporation until 2023 at the level of 2019, with a gradual reduction to 3.8% of the bioenergy consumed in transport
• Instead, the contribution of advanced biofuels should reach a minimum of 0.2% in 2022, 1% in 2025 and 3.6% in 2030, from a series of residual and inedible raw materials, collected in the RED II
• An additional list of residual oils and fats will be provisionally considered as feedstock for advanced biofuels during the energy transition period, reaching 1.7% bioenergy in 2030
• In parallel, electrification for transport must reach a level of 1.5% of the energy consumed in transport
Ultimately, 6.8% low emission energy must be achieved in transportation (electricity plus advanced biofuels), while the use of 1G biofuels is reduced.
Processing of residual fats and oils
From an environmental point of view, residual oils and fats are considered transitional raw materials, not entirely ideal for the production of advanced biofuels. However, used cooking oils and inedible fats are a step up from refined vegetable oils. They could actually lead to a significant reduction in greenhouse gas emissions when compared to raw materials such as palm oil, rapeseed, or soybean. Residual fats and oils can be transformed into biofuel through two processes: transesterification to give conventional biodiesel, used cooking oil methyl ester (UCOME); or hydrotreatment to give diesel range hydrocarbons (HVO).
The transesterification of oils and fats with methanol using alkaline catalysts constitutes the usual industrial process for obtaining biodiesel (methyl esters of fatty acids). Biodiesel has good properties in terms of cetane number, reduction of emissions, and smoke opacity and good lubricity. On the contrary, it has worse cold properties than mineral diesel, with high cloud points and filter plugging, which can lead to problems during ignition and engine malfunction in cold climates. For this reason, the biodiesel content in automotive diesel must be limited below levels that do not cause such problems. Regarding the production process, biodiesel is produced in oil reactions with methanol catalysed by NaOH or KOH in a homogeneous liquid medium and mild pressure and temperature conditions (60-65ºC). During the process, glycerol is generated and, to a lesser extent, other byproducts such as water, metallic salts of fatty acids (soaps), inorganic salts, and so on. The change from refined vegetable oils to residual fats would require a more severe oil refining system than usual, since residual fats contain water, metals, free fatty acids, or polymers of triglycerides to a greater extent than refined vegetable oils. These pretreatment processes constitute an additional cost, but not a particularly complicated challenge from a technical point of view.
The hydrotreating of oils and fats to produce HVO consists of the catalytic hydrogenation of triglycerides at high pressure and temperature, using commercial nickel-molybdenum or cobalt-molybdenum catalysts supported on alumina. Fossil gasoil hydrodesulphurisation fixed bed reactors are used under standard operating conditions for the refining industry (320-360ºC, 50-80 bar). Under these conditions, hydrodeoxygenation, hydrodecarboxylation, and hydrodecarbonylation reactions occur to produce linear alkanes of 15-18 carbon atoms (depending on the starting oil) and propane, water, carbon monoxide, and carbon dioxide as byproducts. Biopropane is consumed in the same refinery or it can be commercialised as bio-LPG within the refinery’s pool of biofuels.
This process has important advantages over other routes:
• The processes are easy to integrate into the process schemes of a crude refinery
• The products are C15-C18 chain saturated hydrocarbons. In other words, they fall within the family of typical diesel molecules, so they do not present compatibility problems and do not have a technical limit for addition to mineral diesel. However, if added in high concentrations, it may require an isomerisation step to improve its properties for cold ambient conditions
• It is possible to use existing diesel hydrodesulphurisation units, through oil and diesel co-processing,3,4 or install independent units (stand-alone) fully dedicated to oil treatment
• It allows a reduction of emissions greater than other processes for obtaining advanced biofuels5
Therefore, it constitutes a viable route for the production of advanced biofuels. In fact, the installed capacity in 2020 in Europe has reached 3.9 million t/y. The main European producer is the refining and chemical company Neste (Finland), developer of its own HVO process (NExBTL), with 2.6 million tons per year. Eni SpA (Italy) uses Ecofining technology from Honeywell UOP and has pioneered the transformation of a crude oil refinery into a biorefinery based on oil and fat processing.
Many refiners are considering the hydrotreating of oils and fats to produce HVO as a feasible way of adapting existing refineries’ facilities to the demands of energy transition. During the last five years, several refineries have been transformed as HVO producers; some of these projects are shown in Table 1. In the event that this strategy spreads throughout the industry, the role of other key units will have to be reviewed. The objective of this work is to analyse how the performance of one of these, catalytic reforming, is impacted and which operational rules should improve its performance.
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