Optimized Gas Loop Design for Fischer-Tropsch Process

Optimized gas loop design for Fischer-Tropsch process

The Fischer-Tropsch (FT) process is used to convert a feedstock such as coal or biomass to syngas (CO + H₂), and then to syncrude, which consists of a vast array of hydrocarbons ranging from methane to C35-C40 hydrocarbons. The syncrude is further refined to yield gasoline, diesel or kerosene fractions. The process has been in use since the early 1930s, when the Germans used a cobalt catalyst-based fixed bed FT synthesis reactor to obtain transportation fuels from coal. The process has undergone considerable developments since then, and is operated under a variety of conditions.

Potential feeds to the FT process include natural gas, which would be reformed, or coal or biomass, which would be gasified, to yield syngas in both cases. The syngas produced is then processed in a Fischer-Tropsch reactor which yields the syncrude. The gas loop thus consists of the feed, the syngas generation technology and the FT reactor (Figure 1).

In general, it is desirable to generate a syncrude with a majority of its components in the diesel or gasoline fractions, while at the same time minimizing the production of methane. The composition of the syncrude produced, however, depends on the gas loop, its components and its design. The composition of the syngas is determined by the type of feed and the type of syngas production technology used, and different FT reactors favour specific syngas compositions to give the desirable output. In this work, we model the different technologies available in the gas loop and carry out studies to select optimized gas loops for each of the following metrics: (1) minimizing methane generation, (2) increasing diesel cut selectivity in the syncrude, and (3) increasing conversion efficiency with respect to the feed. The optimization is conducted at two levels, the selection of the appropriate combination of technologies for the units in the gas loop, and in the optimization of operating conditions for each process unit. The process technologies considered are steam reforming, partial oxidation and autothermal reforming for the reformer; fixed bed, fluidized bed and entrained flow reactors for the gasifier; and slurry phase, fluidized bed, turbulent fluidized bed and fixed bed reactors for the Fischer-Tropsch synthesis reactor. Reaction kinetics for the FT reactor are obtained from Wang et al. [2] for iron catalyst based processes, and from Yates and Satterfield [3] for cobalt catalyst based processes.

References

[1] A. de Klerk, ‘Fischer-Tropsch fuels refinery design' Energy Environ. Sci., 4 (2011) 1177-1205.

[2] Y-N. Wang, W-P. Ma, Y-J. Lu, J. Yang, Y-Y. Xu, H-W. Xiang, Y-W. Li, Y-L. Zhao and B-J. Zhang, ‘Kinetics modelling of Fischer–Tropsch synthesis over an industrial Fe–Cu–K catalyst', Fuel, 82 (2003) 195–213.

[2] I.C. Yates and C.N. Satterfield, ‘Intrinsic kinetics of the Fischer-Tropsch synthesis on a cobalt catalyst', Energy and Fuels, 5 (1991) 168-173.

 Figure 1.Gas loop for syncrude production [1]