(388e) Zero-D Thermodynamic Model As a Simple Tool for Screening Chemical Reaction Candidates and Benchmarking of the Piston Reactor Technology

In the future, electricity derived from renewable energy resources (e.g., solar and wind) will become the most widespread, cheap, and accessible form of energy. This work addresses the potential of utilizing an electrical-driven piston reactor as an alternative technology to conventional reactors to produce value-added chemicals in a more compact, safe, and with less emissions. The reactor concept is based on repurposing conventional engines to be used as chemical reactors. Different than conventional engines, the exhaust gases would become valuable products. One of its essential advantages the ability to achieve high temperatures and pressures upon compression at very short time scales. Therefore, the reactions toward metastable species proceed at relatively high rates. Upon expansion, the fast quenching preserves further conversion of the valuable metastable intermediates, which can be harvested from the effluent stream. The short cycle durations also minimize the energy losses to the surrounding, thus achieving near the adiabatic operation.

The piston reactor technology is still at its early stage of development. So far, the most tested application is the production of synthesis gas by partial oxidation of methane [1-5]. Studies on the production of chemicals other than synthesis gas are limited. [6-8]. Part of this is the multidisciplinary nature and the lack of understanding of this reactor technology and its expected advantages. This work aims to illustrate these advantages by comparing the performance of the piston reactor versus a fixed bed reactor for two relevant industrial reactions. The partial methane oxidation (POX), and methane steam reforming (SRM). These are selected as representative case studies for highly exothermic and endothermic reaction systems to quantitatively analyze the potential of producing hydrogen and mechanical power as in the case of exothermic reactions. Besides benchmarking, this work demonstrates how a zero-D thermodynamic model can be used as a simple tool for screening chemical reaction candidates to guide essential but expensive experimental work.

A zero-dimensional model was developed to describe and simulate the thermodynamic processes inside a single-piston cylinder. The simulation consists of compression and expansion strokes, starting at the bottom dead center (BDC) at the inlet valve closing time equivalent to a crank angle of 180^o. A full rotation of the crankshaft is then carried out till the exhaust valve opening time, equivalent to a crank angle of -180^o. The governing equations constituting the model's basics are derived based on mass and energy balances dependent on time/crank angle. Therefore, the spatial gradients inside the piston chamber are neglected. The concentration changes due to chemical reactions during the compression-expansion strokes were calculated using time-dependent kinetic models from the literature. Inlet temperature, pressure, RPM, feed composition, and compression ratio are all investigated to elucidate a feasible operating window to carry out the proposed reactions. Similarly, a conventional plug flow reactor (PFR) model was developed to benchmark the piston reactor in terms of conversion and productivity.

A parametric study was carried out using the piston reactor model to identify the conditions needed to achieve hydrogen productivity similar to the PFR reactor model in the case of SMR. A case study of an industrial steam reformer was simulated using a conventional PFR model. The operating conditions were adopted from a publication by Said Elnasaie et al. [9]. It was for a top-fired steam reformer (consisting of 897 tubes) operating at 1130 K and 22 bar. A gas flow rate of 3.953 kg.mol/hr (methane equivalent) containing 20.22% CH₄, 72% H₂O%, 4.92 H₂%, 2.44% CO₂% 0.42% N₂ is typically processed in a single tube pass of 13.72 m length and 0.0978 m inner diameter. The catalyst bulk density for the aforementioned dimensioned was 1360 kg/m³. Numerical simulations were run using an SRM kinetic model developed by Xu and Froment [10]. Using the aforementioned operating conditions, CH₄% conversion of 87.12% per tube pass was reached, which is very close to that reported by Said Elnasaie et al. from an actual plant output (85.27%). Moreover, the model predicted a hydrogen productivity per tube obtained of 30.87 mol/ (m³. s). The geometrical parameters of the piston reactor were adopted from a modeling study by Goßler [11]. The total volume of the piston reactor was calculated to be 671.03 cm³. Figure 1 shows the effect of intake temperature on methane conversion and hydrogen production rate per reactor volume. By increasing the intake temperature, the methane conversion, hence hydrogen production is significantly increased. The latter shows good agreement with thermodynamic studies, suggesting that high temperatures favor the highly endothermic methanation reaction in steam reforming.

To achieve the same hydrogen production rate of 30.15 mol/ (m³. s) as that of PFR, the piston reactor needs to operate at T_intake = 820 K, N=1000 RPM, and only 14.9 kg/m3 of catalyst. This is more than 100 fewer catalyst amounts and 310K lower than the PFR case, which minimizes the preheating energy requirements. This effect is mainly due to the high in-cylinder temperatures that result from compression, which facilitate the methanation reaction to proceed faster. To further enhance the hydrogen productivity, it is possible to increase the engine speed, hence process more gas per piston cycle to achieve a higher production rate. As shown in figure 2, increasing the piston speed from 1000 RPM to 5000 RPM results in lower methane conversions, which was expected since the chemical species' residence time decreases at higher speeds, thus lowering the conversion. However, it is possible to achieve a 30.05 mol/ (m3. s) hydrogen production rate at a lower intake temperate of 690K than the base case scenario. It should be noted that by running the piston reactor at an intake temperature of 1130 K (similar to the PFR) and 5000 RPM, the hydrogen production rate achieved would be approximately five-folds higher compared to the PFR case. Another study was conducted to study the effect of increasing the catalyst weight from 1 to 10 g to further enhance hydrogen production as shown in Figure 2 (c) and (d). It can be observed that using a catalyst weight of 10 g at N = 5000 RPM in the range of the considered intake temperatures, higher methane conversions and hydrogen productivities are achieved compared to the base case scenario (1 g catalyst). In this scenario, a hydrogen production rate of 30.61 mol/ (m³. s) (similar to the PRF case) is achieved at 489 K (632 K lower compared to the PFR case).

Initial studies on the piston reactor as a future hydrogen production technology (via SRM) have shown great potential in contributing to much less catalyst utilization, energy minimization, compact design, and having a reactor that is driven by renewable electricity. Moreover, the combination of high in-cylinder temperatures and high piston speeds positively affects productivity compared to a conventional PFR reactor. A similar analysis is carried out for POX as potential reaction candidates to produce hydrogen and mechanical work. Full details and finding from these studies will be demonstrated in this work. Although a quick screening and benchmarking tool was developed and demonstrated in this work, two challenges are limiting the utilization of the findings from this work. The limited validity of the kinetic models, which are based on fixed-bed reactor technology for narrow operating window, and the use of heterogeneous catalyst, which is still a challenge to handle in the piston reactor. This work paves the way toward the exploration and development of this promising but less explored reactor technology.

Acknowledgment: This work was made possible by funding from the Qatar National Research Fund (QNRF) project number NPRP12S-0304-190222 and co-funding by Shell Global Solutions International B.V.

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