Improved Fischer-Tropsch Economics Enabled by Microchannel ...

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methane reforming. The heat generated in exothermic reactions or required for endothermic reactions must be efficiently transferred across reactor walls to maintain an optimal and uniform temperature so as to achieve the highest catalytic activity and the longest catalyst life. Conventional reactor systems use massive hardware to manage the heat in such reactions. Microchannel Fouling. Claims by microchannel practitioners are often met with skepticism from industry, and this commonly includes concerns about the plugging or fouling of the thousands of small channels inside each microchannel devices. While this is a legitimate worry, experiments show that two interrelated strategies help mitigate the risk of plugging: 1) high wall shear, and 2) good flow distribution. Long duration microchannel vaporizer experiments were run with and without good flow distribution. For the devices with good flow distribution, no pressure drop increases were observed in runs ranging from 1,000 to 9,000 hours at both ambient and high pressure conditions. The lack of pressure drop increases held true even when the feed water was intentionally doped with low levels of dissolved solids. However, higher levels did cause fouling, as is the case for plate-frame and other types of narrow passage heat exchangers. The ability of microchannel devices to handle any amount of solids is attributed to high wall shear that sweeps particles out instead of allowing them to build-up. This was verified with post operational autopsies that did show some fouling in the headers and footers, but they were sufficiently large as not to affect pressure drop or heat transfer performance.[3] Manufacturing Techniques. Microchannel development efforts have gone beyond the process design methodology to include the manufacturing techniques for devices that commonly operate at elevated temperatures and pressures. The selected process, known as laminate fabrication, provides cost effectiveness, tight tolerances and design flexibility for microchannel reactors which commonly accommodate a complex suite of chemical unit operations in a single device. Laminate construction involves forming many parallel microchannels by interleaving (stacking) thin sheets of formed material (shims) with solid sheets (walls). The steps of this process are shown in Figure 1. Shim Feature Creation Stacking Microchannel FT Reactor Technology The microchannel FT reactor system is an example of “process intensification”, whereby the reactor volume to produce a given amount of product is reduced by an order of magnitude or more. This is possible due to the enhanced heat and mass transfer capabilities of microchannel architecture, in conjunction with a highly active cobaltbased catalyst.[4,5] As shown in Figure 2, each reactor block has thousands of process channels filled with FT catalyst interleaved with water filled coolant channels. Water CO + 2H 2 Water/Steam 0.1 – 10.0 mm 0.1 – 10.0 mm -(CH 2 )n- + H 2 O Figure 2. Microchannel FT reactor schematic Multiple microreactors are manifolded in parallel to achieve commercially significant production volumes. Modularity allows for installations to be arranged in multiple configurations that permit either a smaller footprint or lower profile than conventional FT reactors, which can be as large as 9 m in diameter and stand 60m tall. Experimental Results. The testing of an integrated microchannel FT pilot reactor, shown in Figure 3, was first completed in January of 2007. This reactor had 40 process and 425 coolant microchannels. Cross Flow Design Partial Boiling Water Coolant Process length ~ 0.6 m Process microchannels = 40 Coolant Length ~ 0.3 m Coolant microchannels = 425 Nominal Capacity = ~ 7 lit/day Process In Coolant In Coolant Out Process Out Figure 3. Pilot-scale microchannel FT reactor Joining Machining Figure 1. Laminate microchannel fabrication process The performance of a pilot reactor during a long duration run is shown in Figure 4. The two key numbers are the conversion of CO, and selectivity to methane. The steady state CO conversion was over 70%. The reactor operated steadily and had minimal change in conversion level during the 1,100 hour run. Velocys © 2011 Page 2 of 7
Conversion, Selectivity (%) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% CO Conversion CH4 Selectivity Temperature 370 350 330 310 290 270 250 230 210 190 Temperature (°C) 0% 0 200 400 600 800 1000 Time on stream (hr) Figure 4. Microchannel FT reactor demonstrated equivalent of 28 - 44 bpd for a full-scale reactor 170 Selectivity to methane, also known as methane make, was approximately 9%. Methane production is counterproductive and should be minimized. The 9% methane make is on par with competing fixed bed FT technologies. The reactor was operated at conditions that correlated to commercial scale reactor capacities of 28 to 44 bpd. The anticipated capacity for initial commercial rectors is 30 bpd. Process Scale-up. While several R&D groups around the world have shown the potential of microchannel reactors for process intensification, few have been successful in developing the technology for industrial scale applications. At Velocys this effort, begun in 1998, spanned several key scale-up steps, and recently culminated in fabricating a full scale reactor, the device shown in Figure 6. If operated, this reactor would have a nominal capacity of 30 bpd. The key scale-up steps are summarized in Table 2. Note, the Brazil GTL reactor has been built, accepted by the process fabricator and is expected to start-up in third quarter 2011. Table 2. Microchannel FT process scale-up Type/Units Size Year Run Time Catalyst N/A 1998- 1,200 hr development 2003 Full length with 24” long 2003 600 hr oil cooling Integrated pilot 12”x 24”x 2006 1,200 hr water cooled 0.5” 2/3 scale 19 bpd 2008 mfg. test fabrication test Güssing field demonstration 25 gal/day 2010 Now operating Full-scale fabrication test 30 bpd see below 2010 mfg. test Brazil GTL demonstration 6 bpd 2011 6 months planned Figure 5. Commercial microchannel manufacturing techniques validated No manufacturing scale-up is required beyond this stage. In fact, the first commercial full-scale reactors were ordered in late 2010 for delivery in mid-2011. FT Catalyst. The most common metals used to catalyze FT are cobalt and iron. Cobalt-based catalysts are known for their higher activity and longer life. For this reason, FT reactors using iron based catalyst must either be larger or require more frequent catalyst replacement than those using cobalt. So, nearly all fixed bed reactors technologies employ cobalt catalyst, including microchannel FT. Because catalyst replacement is easier in slurry bed reactors, they can use either iron or cobalt catalyst. All FT catalysts are sensitive to poisoning by sulfur-containing compounds, and every effort must be made to clean these from the syngas before entering the FT reactor. An FT catalyst developed by Oxford Catalysts Group enables microchannel reactors to achieve catalyst productivities that are orders of magnitude higher than for more conventional systems (see Figure 6). A demonstration carried out in 2008 in a nominal two-gallon per day microchannel reactor operated for more than 4,000 hours and achieved productivities of more than 1500 kg/m3/h. In contrast, conventional fixed-bed reactors typically operate at 100 kg/m3/h, while slurry-bed reactors operate at 200 kg/m3/h. Velocys © 2011 Page 3 of 7 Catalyst Productivity (kg/m 3 /hr) (kg/m 3 /hr) 1600 1400 1200 1000 800 600 400 200 0 Fixed Bed Slurry Bed Microchannel Velocys Figure 6. Microchannel FT achieves far higher catalyst productivity
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