Gas-to-Liquids (GTL) and Coal-to-Liquids (CTL) processing are attractive routes for the production of clean liquid transportation fuels, in particular middle distillate fuels, as an alternative to crude oil refining. The industrially proven and economically viable route starts with conversion of coal or gas to synthesis gas (syngas; a mixture of CO and H2) followed by Fischer-Tropsch synthesis (FTS) to hydrocarbons and subsequent product workup, as the FTS is notoriously unselective - producing a product stream with a very wide hydrocarbon chain length distribution. In practice, FTS is being developed towards long hydrocarbon chain (wax) production - followed by hydrocracking to obtain a high yield of transportation fuels.
Hydrocracking of FTS wax (dewaxing) is generally performed using a bifunctional catalyst, containing a hydrogenation/dehydrogenation function and an acidic function. The presentation will focus on Pt noble metal (de)hydrogenation / zeolite solid acid catalysts.
Two approaches have been studied to improve the yield of transportation fuels in the dewaxing process. One focusses on the synthesis of zeolites with hierarchical pore systems, introducing mesopores next to the inherent micropores. This serves to decrease diffusion limitations and especially the secondary cracking that results from long residence time in the zeolite pores. The other focusses on process parameters. We have studied the presence of water in the feed and the total process pressure.
Water is the main byproduct of FTS, and leaving it in the feed stream to the dewaxing step allows evaluation of the integration of synthesis and hydroprocessing for the potential one-pot production of middle distillates with good cold flow properties. We found the presence of water to decrease the activity and increase the selectivity to linear products, both due to competitive adsorption on the acid sites.
Surprisingly, when using zeolites as a solid acid, we find lowering the total process pressure to atmospheric increases the diesel yield dramatically and makes the whole process much more economical in terms of CAPEX and energy efficient in terms of OPEX and CO2 footprint. This is in contradiction to previously reported results using amorphous silica-alumina (ASA) as a solid acid, where more diesel is formed at higher pressure.
Audience take away:
- Hydrocracking is more efficient at atmospheric pressure, contrary to what is usually assumed.
- Making synthetic fuels from biomass is now feasible using small-scale, stand-alone units that can be operated by semi-skilled workers. This will empower rural communities in underdeveloped countries to store renewable energy from solar and wind sources using their local biomass waste.
- The production of synthetic Jet Fuel will use less energy using these new process optimizations.