Q What is needed to significantly increase production of bio-LPG, renewable marine diesel, and bio-aromatic hydrocarbons like BTX? A Andrew Layton, Principal Consultant, KBC, andrew. layton@kbc.global • Sources: Most bio-LPG and biodiesel biofuels now come from recycled fats and vegetable oils, which do not com- pete with food sources. Recycled fats are the cheapest and most accessible source, but they still require pretreatment to remove several impurities. This pretreatment results in a 10% yield loss as water and requires additional isomerisa- tion for diesel production. These factors increase the cost compared to conventional fuels, adding perhaps two extra steps. The use of other waste or cheaper streams, such as woody feed sources, introduces additional issues with transportation costs, even more treatment, and potentially pyrolysis/gasification/Fischer-Tropsch steps before reach - ing conventional fuel-type treatment. Thus, the cost of these extra three to four steps in transportation and chemi- cal treatment means the overall cost is several times higher than that of conventional fuels. Production from electrolysis, H₂, and then reforming/F- T type processes to get to hydrocarbons is equally costly, especially if it starts from CO₂. CO₂ capture is feasible, but making hydrocarbons starting from CO₂ is like climbing a steep energy ladder, which was so easily descended by hydrocarbons going to CO₂. • Scale-up: Apart from the bio-hydroprocessing route, most processes are still small and from smaller suppliers, requiring scale-up as well as managing solids – similar to recycling issues from multiple and contaminated sources. Several of these are best if they get together to provide a viable intermediate product. Existing refineries are designed to best manage or blend these products; thus, intermediate products need to be transported or built on-site if space permits. • Investment: These processes require substantial invest- ment. Money is available from bankers. However, due dili- gence is required for investors to assess the many small process start-up companies and identify those with viable business plans. Such due diligence often leads to turn- arounds in the process configuration as various profit /prac - ticality problems arise that were not predicted. Examples include basic changes in the source of H₂. A solid business plan is essential to secure investment, as the cost of the final product is high, even with gov - ernment support, and a firm customer base is vital. Some customers have already moved away from renewables in the short term. Additionally, some government support is disappearing or being delayed due to other economic drivers. Many obstacles exist to quickly replace the conventional fuel supply routes, especially as the cost is high now. A key factor is choosing companies with good business and tech- nology plans from cradle to product. In many cases, a robust plan may mean facilitating joint programmes between mul- tiple companies.
before liquid recycle must be employed at high ratios. Many renewables hydroprocessing technology providers tend to leave out recycle flows in simple process flow diagrams for marketing purposes, but they are implied. A Joris Mertens, Principal Consultant, KBC, joris. mertens@kbc.global Most vegetable oils and animal fats are triglycerides con- sisting of fatty acid structures containing 16, 18 and 20 carbon atoms (C 16, C18 , C 20), rich in oxygen and olefins. Hydrodeoxygenation of 100 tonnes of an oil/fat triglycer- ide containing only tristearin (saturated C 18 chains, C 18 : 0 ) will generate 25 Gcal/100 MMBTU of heat, five times more than the exotherm resulting from hydrotreating a similar amount of fossil diesel. Most of that heat (70%) is gener- ated during the cracking of triglycerides into fatty acids and propane rather than by oxygen removal from the fatty acid using hydrogen (hydrodeoxygenation). Oxygen removal from fatty acids through the removal of a carbon atom and production of CO₂ (decarboxylation) is even endothermic. The exotherm will further increase to 30 Gcal/120 MMBTY per 100 tonnes of feed if it contains pure unsaturated tri- linolein (C 18 : 2). With exotherms at least five times higher than those of conventional mid-distillate hydrotreating and higher than full conversion hydrocracking, design and operational precautions are needed to avoid temperature runaway. Special attention should also be paid to maximis- ing the recovery of the heat generated. As in conventional hydrotreating and hydrocracking, the hydrodeoxygenation (HDO) catalyst will be distributed over different catalyst beds, and gas quench is applied to control exotherms and allow catalyst grading. Also, as with conventional hydrotreating and hydrocracking, there is the need for efficient feed mixture distribution over the catalyst beds. HDO will use three-to-six catalyst beds in one or two reactors, depending on licensor preferences and the need for pretreatment catalyst to remove catalyst poisons. Providing an additional heat sink by recycling the (heavier fraction of the) liquid product is the most important addi- tional handle to avoid excessive HDO exotherms. The amount of liquid recycled will be higher than the fresh feed rate, possibly twice as high. Some licensors design the HDO section with a split feed routed to the first two or three catalyst beds, which distributes the heat generation more evenly. Similar to conventional hydrotreating/hydrocracking, heat generated is recovered from the effluent in the feed/effluent exchanger, the effluent of which can be used to generate medium-pressure steam or to preheat the isomerisation or cracker section feed. Some designs use the HDO effluent heat to preheat the downstream iso-dewaxing or cracker sections. The design of hydrotreaters/hydrocrackers can be made more energy efficient by using a hot high-pressure sepa - ration. In the case of triglyceride feeds, the HDO section is normally followed by a sulphur-intolerant iso-dewaxing step to improve the cold flow properties of the paraffinic product. Therefore, a hot high-pressure stripper will need to be used rather than a simple separator.
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PTQ Q4 2024
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