(617da) Modeling Syngas Production for Miniaturized Gas-to-Liquids Application: Extending Operation Window By Process Improvement from Different Scales

The scale-down of gas-to-liquids (GTL) has practical significance in developing renewable energy systems and mitigating green-house gas (GHG) emissions. Examples include on-site conversion of associated gas to liquid fuels to prevent flaring at remote onshore/offshore oil fields, and utilization of biogas/biomass or other domestic waste to produce valuable chemicals in distributed plants. Economy of such small-scale applications can benefit from their use of cheap, â??wasteâ? feedstock, but is very likely restricted by the device investment. This dictates the reactor operation at high space velocity with respect to reactor volume, resulting in a non-equilibrium operating regime, which is distinct from large-scale processes.

Herein, we focus on miniaturized GTL using modular microchannel reactors for associated gas utilization. Both two major stages of the process, syngas production and the following Fischer-Tropsch synthesis, remain challenging during scale-down due to the intrinsic complexity of the multi-scale transport and heterogeneous reactions. The problems involved can be characterized as intrinsic chemistry/physics on the micro-scale, internal mass transfer on the meso-scale and external heat and mass transport and pressure drop control on the macro-scale [1]. We modeled catalytic combustion assisted syngas production in an integrated microchannel reactor. The aim is to illustrate the combined effects of process improvement from an integral perspective of the entire process, as well as to highlight co-optimization of the reactor and catalyst of such systems.

To account for the effects of multiscale improvement measures, we developed a hybrid modeling scheme that decoupled the governing equations at different scales in order to save computational time in parametric study. The macro-scale phenomena were treated in conventional CFD framework and coupled to the lower scales. On the meso-scale, the catalyst washcoat was modelled as isotropic porous media. The effective diffusivity was calculated by different models specific to the pore size distribution. We derived a meso-scale effectiveness factor submodel to describe internal mass transfer limitation under typical reaction conditions. On the micro-scale, detailed surface chemistry for methane reforming over Ni [2] and catalytic combustion over Pt [3] were used, which can be related to the metal dispersion of catalysts.

Numerical simulations on the full, heat-coupled processes in a modular microchannel reactor disclosed the reactor operation maps by varying fuel flowrate for different reactant GHSVs. It is found that the volumetric CO yield increases with GHSV before fuel breakthrough (incomplete conversion), which almost never happens because of the ultra-fast intrinsic kinetics. In contrast, GHG emission rate slowly increases with GHSV. For a specific GHSV, the GHG emission rate has a minimum ranging from 70% to 80% when the fuel flowrate is adjusted to some medium value. These observations suggest that extending operable GHSV and enhancing GHG emission control should be the primary optimization targets for process improvement from an economic point of view. At low GHSV, the reactor has a wide operation window with respect to material stability and methane conversion. With increasing GHSV, however, the operation window narrows down quickly, mainly due to hotspot formation, which prohibits further possible increase of GHSV. Multiscale measures prove efficient to enhance the thermal uniformity, for instance, by increasing/reducing the activity of reforming/combustion catalyst, using a thicker, more heat conductive wall, and shortening the reactor length. Still at high GHSV, increasing fuel flowrate shows minor effect on raising methane conversion, indicating the dominant role of internal mass transfer. Introducing a small amount of transport pores on the scale of 1 μm into the catalyst coating can significantly improve intra-coating transport and further increase volumetric CO yield in the operable GHSV range. The results here indicate that the degree of freedom in process improvement can be more than envisioned through purposefully tailoring catalyst and reactor characteristics from micro-, meso- to macro-scale.

References

[1] R. Guttel, and T. Turek, "Improvement of Fischer-Tropsch Synthesis through Structuring on Different Scales", Energy Technol., 4, pp.44-54, 2016.

[2] L. Maier, B. Schädel, K. Herrera Delgado, S. Tischer and O. Deutschmann, "Steam reforming of methane over nickel: development of a multi-step surface reaction mechanism", Top. Catal., 54, pp.845-858, 2011.

[3] O. Deutschmann, R. Schmidt, F. Behrendt and J. Warnat, "Numerical modeling of catalytic ignition", Proceedings of the Combustion Institute, pp.1747-1754, 1996.