(20a) Development of Iron-Based Fischer-Tropsch Reactor Particle Management and Gas-Solid Fluidized Bed Activation Technology

The slurry bed column reactor (SBCR) with gas-liquid-solid three phase has the advantages of small investment, uniform temperature and easy operation. Therefore, most of the iron based low temperature Fischer-Tropsch (LTFT) technologies choose SBCR. But at the same time, SBCR need to be matched with Fischer-Tropsch heavy wax filtration system and online catalyst replacement technology to achieve wax separation from catalyst particles, and online replacement of fine particle or deactivation catalyst to maintain the catalyst performance.

The traditional SBCR slurry filter technology mainly focuses on filter and filtration process, including filter type selection, filtering precision and blowback frequency. The filtration efficiency decline is mainly due to filter channel blocked by the catalyst fine particles during reactor operation, but the filter can't be replaced under the reaction condition, so the only effective way is to reduce fine particles. The traditional way is online replacement with intermittent operation to discharge part of catalyst including fine particles from the reactor. Our research group proposes new catalyst particles management ideas that based on slurry bed fluid mechanics study, the fine particles can be discharged from top of the reactor continuously, so the backmixing fines in the reactor bed and the filter were deceased. This technology can prolong the service life of filter more than 20%. Besides, for the discharged residue slurry out of the reactor, we can apply magnetic separation technology to separate the ferromagnetic iron catalyst form the discharged residue. It can not only save the investment of traditional wax filtration equipment but also increase the recovery rate of wax by more than 10%.

Traditional catalyst online replacement technologies can discharge the fine catalyst particle and some of the deactivated catalyst intermittently, then the new activated catalyst can be added to the reactor. So the fresh inactive catalyst needs to be activated to the active state which has the Fischer-Tropsch activity. Now, for the iron-based LTFT, the catalyst is activated in an independent SBCR reactor system with the same operating temperature and pressure as Fischer-Tropsch reactor, and some other specific activation process conditions. The activation process is intermittent operation mode about 5-7 day per time to fit the Fischer-Tropsch reactor discharge operation. The activated catalyst should be added to Fischer-Tropsch reactor quickly, and cannot be stored for long time.

If using gas-solid fluidized activation method, the active catalyst does not need to suspend as in SBCR reactor, so the offline catalyst activation is possible, active catalyst storage time can be extended, it is also possible to achieve catalyst online replacement continuously. As a result, the catalyst particle management will be more flexible and controllable. But Iron-based LTFT gas-solid fluidized bed activation research is little.

Our group study gas-solid fluidized bed catalyst activation technology. From the hydrodynamic view, the iron-based LTFT catalyst belongs to A-type particle, so the fluidization is easy. We completed the 100 mm diameter gas-solid fluidized bed reactor cold model test to investigates the state of flow characteristics and hydrodynamics character; According to the physical property characteristics of iron-based catalyst ( such as particle density, particle size distribution, Angle of repose, etc.), we study the catalyst fluidization velocity, ensure that the superficial gas velocity within the range of 0.1 to 0.3 m/s can fully fluidize the catalyst particle, we also design a new type of gas distributor which can guarantee the fluidization effect and avoid the dead zone.

Fundamentally, from the activation mass transfer view, in gas-solid phase, the resistance is smaller than in three-phase SBCR, so the gas-solid activation may shorten the activation time. The same as SBCR activation, gas atmosphere, temperature, pressure also can affect gas-solid reactor activation result. So we also did the fluidized activation process test in fluidized reactor loading 1.0-5.0 kg catalyst, A variety of activation atmosphere (pure H₂, CO and the mixed H₂ + CO) was carried on the system research, it was found that atmosphere has a great influence on catalyst activity and stability. When the H₂/CO ratio of activation fresh syngas is between 5/1 and 20/1, catalyst will get a comprehensive performance. Test results also show that the catalyst activation temperature and time should match, proper temperature not only ensure the performance of the catalyst, also reduce activation time, the reasonable temperature range is 250-270^oC, the corresponding activation time is 6 to 12 hours, while the existing slurry bed activation requires 24h. On the other hand, if the activation temperature exceeds 280^oC, catalyst will be sintered to some extent and catalyst performance is poorer. Compared with SBCR activated catalyst, Gas-solid fluidized activated catalyst almost has the same Fischer-Tropsch catalyst performance: the CO conversion and C₅⁺ hydrocarbon selectivity difference is within 2%, methane selectivity is within 0.5%, CO₂ selectivity within 1%. At present, the crystal structure transformation of catalyst under different activation process parameters is study, and the precise control of the activity will be achieved finally.

Above all, the new catalyst particles management is the core in NICE LTFT technology, and we have developed a SBCR reactor fine particle treatment process and gas-solid fluidized bed catalyst activation technology. Now we are planning to build a 20 tons/batch fluidized bed activation demo unit.

Keywords: Fischer-Tropsch technologies; Slurry bed column reactor; Gas solids fluidized bed; Flow behaviors