*3.3. Computational Modeling for Fixed-Bed Reactors*

The OxEon Energy commercial-scale fixed-bed reactor prototype results were modeled according to previous reaction kinetics based on Willauer et al. but factored by 100 for the reverse water–gas shift (rWGS) reaction, 6.75 for the Fischer-Tropsch reaction, and 6 for the methanation reaction [17,24]. The same reaction kinetics were used to model both the single pass and recycling capability. The single pass of the feedstock over the reactor along with the recycle capability were modeled with commercial CFD software CFX© on the Department of Defense High Performance Computing Modernization Program servers. The number of nodes used in the model was 60,975 with 305,050 elements. The k-ε turbulence model was used with the scalable wall function, compressible gas flow, and finite rate with equilibrium chemical kinetics. The reactor was modeled as a packed bed of length 1.2573 m (49.5 inch) and inner diameter of 0.1082 m (4.26 inch), a bed porosity of 40%, and a permeability of 1.9 <sup>×</sup> 10−<sup>9</sup> m2/s. A total catalyst weight of 6090 g with 1888 g of active metal was used as the basis for the reaction kinetics. The model outlet pressures and imposed wall temperatures were variables used to converge on the reported pilot test results for outlet temperature and mass flows. For both of the reactor configurations, heat was removed from the reactor, which is similar to the porotype reactor configuration.

## *3.4. Laboratory-Scale Fixed-Bed Reactor Setup*

The laboratory-scale fixed-bed plug flow thermochemical reactor process requires 20 g of catalyst for evaluation in a 9 to 12" long stainless steel tube (3/8" ID, 1/2" OD). The catalyst bed occupies 8 to 10" length of the stainless steel tube. The catalyst quantities required for the evaluations were synthesized commercially by Water Star Inc. Water Star prepared two small-scale iron-based metal catalysts for evaluation on the laboratory scale that differed in wt.% metal loading by IWI methods. The first catalyst, 100:3.93:2.36:3.23 M2:Fe:Mn:K, has been studied extensively [17] and is referred to as Fe:M2-1 in Table. The second catalyst was prepared at a higher catalyst loading of 100:17:12:16.5 M2:Fe:Mn:K and is listed in Table 1 as Fe:M-2. Each small-scale batch consisted of 500 g to 1000 g of catalyst. The catalysts were recovered after the hydrogenation reactions, rinsed with hexanes, vacuum-dried, and then characterized by XRD and XPS.

In a typical CO2 hydrogenation experiment, 10 mL of mineral oil is added to the catalyst and the catalyst was reduced in situ by flowing 100 mL/min H2 at 300 ◦C and 20 bar for 18 h. Three mass flow controllers (Brooks Instruments, Hatfield, PA, USA) were used to control the flows of CO2, H2, and N2 into the reactor. Immediately following reduction, hydrogenation of CO2 was conducted at 20 bar and 300 ◦C with a H2/CO2 ratio of 3:1 and a 10 mL/min N2 internal standard at 20 bar and gas hourly space velocity (GHSV) of 9.6 <sup>×</sup> 10−<sup>4</sup> L/s-g. The GHSV is defined as standard liters per second of total CO2 and H2 flow divided by the grams of total elemental Fe, Mn, and K metals in the reactor. The effluent gases were passed through a cold trap, at 10 ◦C to condense the water vapor and any heavy liquid hydrocarbons formed in the reactor. The effluent gases were analyzed in real time using an inline gas chromatograph (GC) (Agilent Technologies, Fast RGA analyzer, Santa Clara, CA, USA. Hydrocarbons were separated using an HP-Al/S column (Agilent Technologies) 19091P-512, 25 μm × 320 μm × 8 μm) and detected with an FID detector. Fixed gases (H2, CO2, CO, N2) were separated on a Unibead IS column (4 ft, 60/80 mesh in UltiMetal, Agilent Technologies) and a 5Å molecular sieve column (8 ft, 60/80 mesh) and detected with a TCD detector. The GC was calibrated using a mixture of gases with known molar ratio (MESA Specialty Gas, Santa Ana, CA, USA). Time-on-stream (TOS) for the catalyst was 48 h.
