Improved Fischer-Tropsch Economics Enabled by Microchannel ...

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In the absence of contaminants, there are two types of catalyst deactivation in FT synthesis: 1) short-term deactivation caused by wax build-up and/or partial oxidation of the cobalt catalyst particles, and 2) longer term deactivation, which involves sintering, loss of pore structure and/or build-up of refractory carbon. The first is reversible; the second may be reversible under certain conditions, but could require replacing the catalyst. For short-term deactivation, in situ hydrotreating was shown to be effective at restoring catalyst activity. Figure 7 displays reactor performance data at the same conditions before and after regeneration. Conversion, Selectivity (%) 100% 90% 80% 70% 60% 50% 40% 30% 20% Regeneration CO Conversion CH4 Selectivity Temperature 370 350 330 310 290 270 250 230 210 Temperature (°C) segregated by sequential condensation. In Shell’s fixed bed design, catalyst is packed into small tubes inside a large diameter vessel with means to remove heat by boiling water around the tubes. To maintain a reasonable pressure drop, catalyst particles in conventional fixed bed reactors are relatively large, in the range of 1 mm. This often limits the rate of reaction and leads to higher than desired selectively to methane.[7] Microchannel FT reactor technology provides substantial techno-economic benefits over conventional fixed bed technology, including: 1) Microchannel FT has thin reaction channels, 1 to 2 orders of magnitude smaller characteristic dimensions in competing fixed bed designs. This attribute greatly improves heat and mass transfer, allowing optimal temperature control across the catalyst bed, and leads to far higher reactor productivity. Figure 8 shows reactor productivity, defined as barrels/day of FT product per ton of reactor mass compared to large-scale FT reactor systems. Both Shell data points are fixed bed designs, while Sasol Oryx uses slurry bed reactors. 10% 190 12 0% 0 500 1000 170 2000 2500 Time on Stream (hr) Figure 7. Microchannel FT in-situ regeneration shown effective at recovering performance Catalyst Integration. Placing catalyst in packed bed reactors can be a challenge for any technology. Conventional FT reactors have as many as 29,000 small (1”) diameter tubes 12m in length, which must be equally loaded and then unloaded after 2 to 5 years of operation.[6] These high aspect ratio (over 500:1 length to diameter) tubes are hung in very large, immobile reactors, but commercial integration techniques have been shown effective for loading and unloading the catalyst. Because microchannel technology has extremely small passages, many expect catalyst loading and unloading to be especially challenging, but this was found not to be the case. Catalyst loading, regeneration and unloading for microchannel FT reactors have been successfully demonstrated. Multiple operating and non-operating reactors with commercial length microchannels (aspect ratio 385:1 length to hydraulic diameter) have been repeatedly loaded and unloaded. A mechanical loading method is employed using commercially available material handling techniques and equipment. Comparison with Competing FT Technologies Due to improved volumetric and catalytic productivity, microchannel FT enables lower capital and operating costs compared to conventional FT reactor systems, including tubular, fixed-bed with cobalt catalyst, and slurry-bubble with cobalt or iron catalyst. Fixed Bed. In a conventional (tubular) fixed bed designs, all reaction products exit through a single outlet, leaving the catalyst behind in the reactor. The resulting products are 0 0 2,000 4,000 6,000 8,000 10,000 12,000 Scale of Single Reactor (bpd) Velocys © 2011 Page 4 of 7 Reactor Productivity (bpd/tonne) 10 8 6 4 2 300 bpd Microchannel Reactor Assembly Shell - Bintulu Shell - Pearl Sasol - Oryx Figure 8. Microchannel FT improves reactor productivity and achieves economy of scale at far lower capacity Microchannel FT also achieves approximately 10 times higher catalyst productivity, defined as kg/hr of synthesis gas processed per cubic meter of catalyst volume. Both capital and operating costs are thus reduced. 2) The basic building blocks of the microchannel FT reactor systems are components with thousands of parallel microchannels. These reactor components, which have fixed production capabilities, can be added or removed to match throughput requirements. When this modular design approach is combined with process intensification benefits discussed above, two advantages are realized: a. Microchannel FT realizes economies of scale at much smaller size (500 bpd) than conventional technology (10,000 bpd). This advantage allows microchannel FT to be feasible for BTL applications since it is not practical to transport the required biomass feedstock of 10,000 tons/day for a 10,000 bpd facility. The smaller economy of scale also permits feasibility for distributed and offshore GTL.
. Since the basic reactor modules are small, reactor and FT unit fabrication can take place at indoor shops, which speeds construction and installation. On the other hand, commercial-scale conventional reactors are large and must be placed in ‘stick built’ facilities that have relatively long construct timelines. 3) The modular approach of microchannel FT helps minimize downtime due to individual modules needing components or catalysts to be replaced. Conventional fixed bed systems require an entire train or system shutdown to make changes or repairs. Because microchannel FT reactors are smaller the impact of taking a one or a few out of operation does not materially impact the facilities overall productivity. Slurry Bed. In slurry bed reactors, a heavy hydrocarbon liquid is used to suspend the catalyst and the heavier products remain in the reactor while the light ones are removed as vapors overhead as shown in Figure 9. A portion of the liquid mixture is continuously removed to recover the heavy hydrocarbon products, while the carrier liquid and majority of the catalyst are recycled back to the reactor. Removing small particles of catalyst, known as fines, from the wax is a significant challenge, and sometimes wax products from slurry bed processes are grey, instead of white, due to the presence of catalyst fines. This lowers the value of FT wax because final separation for chemical uses can be expensive.[8] Compared to commercial slurry bed sytems, microchannel FT offers a compact equipment footprint and/or lower profile. Microchannel reactor assemblies are relatively small and can be situated horizontally versus conventional slurry bed FT reactors that are vertically oriented and can be more than 60m tall. Figure 10 compares the size of a microchannel reactor assembly with a ConocoPhillips demonstration plant of similar capacity. ConocoPhillips 400 bpd demo plant Velocys 300 bpd reactor assembly Vapor product Slurry Bubble Zone (small catalyst particles) Vapor Zone Liquid product Catalyst/ Wax Separator Figure 10. Small size and low profile eases installation and operation in offshore environments Commercial Applications Since conventional FT technologies are not economically viable at small scale, below 10,000 bpd, the current focus for FT planning and installation projects are large land-based natural gas fields, such as those in Qatar. Boiler Feed Water Catalyst Return Figure 9. Slurry bed reactor schematic - Syngas Slurry bed reactors do have operational advantages, including the ability to change the catalyst without shutting down the system. However, due to their relative complexity compared to fixed bed designs, slurry bed reactors are typically best suited for very large-scale operations. Furthermore, this complexity leads to poor process economics when slurry bed systems are scaled down for distributed synthetic fuel operations. Biomass-to-Liquids. Biomass feedstock materials are not easily aggregated and transportation costs to large BTL plants are high. Microchannel FT can overcome these challenges by economically operating small (500-2,000 bpd) plants that require about 500-2,000 tpd of biomass. These synthesis gas generation plus FT synthesis facilities can then be located near low-cost, biomass sources. The heavy hydrocarbon products can then be economically shipped to central upgrading (refining) facilities since the FT product density is much higher than that of biomass. The feedstock choices for FT are non-food biomass, such as forest residues, agricultural residue, municipal solid waste (MSW), and construction and demolition waste (CDW). Based on the U.S Department of Energy (DOE) estimates and data on waste sent to landfills, the domestic biomass supply is large enough to replace more than half of petroleum-based distillate fuel demand [11] as shown in Figure 11. Velocys © 2011 Page 5 of 7
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