Andreas Jess

Exothermic reactions in a wall‐cooled fixed‐bed reactor are analyzed using one‐dimensional (1D) and two‐dimensional (2D) models. Radial temperature profiles from the 2D model provide insight into heat transfer and serve as a foundation for optimizing the 1D approach by adjusting the overall heat transfer coefficient. Heat transfer correlations are derived and implemented in the 1D model to improve the prediction of reactor performance across varying reaction parameters. Thermal runaway behavior is examined for different scenarios, enabling analysis of runaway conditions and reactor performance for stable operation. The optimized 1D model is compared with the 2D model to evaluate their accuracy and sensitivity to varying reaction parameters.

For PEM fuel cell operation, high-purity hydrogen gas containing only trace amounts of carbon monoxide is a prerequisite. The water–gas shift reaction (WGSR) is an industrially applied mature operation mode to convert CO with H₂O into CO₂ (making it easy to separate, if necessary) and H₂. Since the WGS reaction is an exothermic equilibrium reaction, low temperatures (below 200 °C) lead to full CO conversion. Thus, highly active ultra-low-temperature WGSR catalysts have to be applied. A homogeneous Ru SILP (supported ionic liquid phase) catalyst based on the precursor complex [Ru(CO)₃Cl₂]₂ has been identified to operate at such low temperature levels. However, in a hydrogen rich atmosphere, transition metal complexes are prone to form nanoparticles (NPs) when dissolved in ionic liquids (ILs). In this article, the behavior of an anionic SILP WGSR catalyst, i.e., [Ru(CO)₃Cl₃]⁻ dissolved in [BMMIM]Cl, in an H₂-rich CO environment is described. The data reveal that during the WGSR, Ru nanoparticles form in the catalyst when very low CO concentrations are reached. The Ru NPs formation has been confirmed by transmission electron microscopy imaging and X-ray diffraction (XRD).

Knowledge of the temperature distribution in fixed-bed reactors is of great importance for process control and optimization in many processes of the chemical industry. The state of the art is defined by invasive measuring methods for temperature measurement in such reactors. However, all invasive methods influence both the structure of the fixed-bed and the flow conditions and thus falsify the measurement results compared to real undisturbed conditions in a fixed-bed, above all with regard to the radial temperature profile. In addition, the temperature distribution in the entire reactor is usually derived from a few discrete measuring points based on a model, which sometimes results in large deviations from reality. In this article, electrical capacitance tomography (ECT) is explored as an alternative, non-invasive measurement method to monitor the temperature distribution inside fixed-bed reactors. The method exploits the temperature dependence of the effective bulk permittivity of the reactor filling. By way of an example, this dependence has been investigated in this work for two different hydrogenation catalysts. A temperature-resistant ECT sensor has been designed to perform measurements under the influence of temperature. The investigations show that, with the aid of ECT, it is possible to detect relative temperature changes in the region of interest with the materials examined.

A multitubular Fischer–Tropsch (FT) reactor with gas recycle and a cobalt‐based catalyst is simulated by a 2D fixed‐bed model that considers the complex kinetics, all relevant mass and heat transfer aspects, and also the influence of the changing molar flow rate by the FT reaction, of pressure drop, and of formation of steam, which decreases the reaction rate of FT synthesis. Optimal values of gas velocity and single‐tube diameter and length are determined by a technoeconomic analysis. The comparison of the data of the FT reactor modeled in this work with data of commercial reactors indicates that the used model reflects reality well and is reliable. A superficial gas velocity of 0.5 m s⁻¹, a tube diameter of 3 cm, and a length in a range of 12–20 m are appropriate for a high production rate of C₂₊‐hydrocarbons with minimal operating costs of the FT unit, even for a large range of the electricity price from 0.06 to 0.3 € kWh⁻¹.

A Fischer–Tropsch (FT) fixed-bed reactor was simulated with reactor models of different complexities to elucidate the impact of a pressure drop, a change in the total molar volume rate (induced by the reaction) along the tubes, and a change in the axial variation of the external radial heat transfer coefficient (external tube wall to cooling medium, here, boiling water) compared to disregarding these aspects. The reaction kinetics of CO conversion for cobalt as a catalyst were utilized, and the influence of inhibition of syngas (CO, H₂) conversion reaction rate by steam, inevitably formed during FT synthesis, was also investigated. The analysis of the behavior of the reactor (axial/radial temperature profiles, productivity regarding the hydrocarbons formed, and syngas conversion) clearly shows that, for accurate reactor modeling, the decline in the total molar flow from the reaction and the pressure drop should be considered; both effects change the gas velocity along the tubes and, thus, the residence time and syngas conversion compared to disregarding these aspects. Only in rare cases do both opposing effects cancel each other out. The inhibition of the reaction rate by steam should also be considered for cobalt as a catalyst if the final partial pressure of steam in the tubes exceeds about 5 bar. In contrast, the impact of an axially changing heat transfer coefficient is almost negligible compared to disregarding this effect.

Accumulation of wax inside the catalyst pores during transient cobalt-catalyzed Fischer-Tropsch synthesis (FTS) leads to unfavorable product distribution and low activity by imposing internal mass transfer limitations. The condensation of paraffin severely changes the apparent product stream that actually leaves the reactor before the catalyst pores are filled completely and the steady state is reached. Thus, the product distribution of the transient FTS is less complex than expected in comparison to the steady-state FTS and increasingly consists of hydrocarbons (HCs) with an average chain length in the range of kerosene (C9-C17). So, in order to prevent FTS from reaching a steady state, the pores are drained periodically by hydrogenolysis (HGL). The alternating HGL is realized by a switch from syngas (H2, CO) to pure hydrogen at a reaction temperature in the range of 210°C to 240°C. The alternating process leads to an improvement in kerosene selectivity of 48%, 37%, and 28% at 210°C, 220°C and 240°C, respectively. Furthermore, the influence of temperature on the hydrogenolysis of long-chain HCs was experimentally investigated. It was found that temperature affects methane selectivity severely. A high hydrogenolysis temperature is favorable as this leads to a severely decreased overall methane selectivity and, thus to a higher production rate of alkanes within the carbon number range of 9 to 17.

Fischer–Tropsch fixed bed synthesis is frequently conducted with a gas recycle and a CO conversion per pass of less than about 60%. In this paper, we have investigated the influence of a gas recycle on the performance of a multitubular reactor (tube length 12 m, diameter 3 cm) for different total CO conversions (X_CO,total) and superficial gas velocities (u_s). The impact of the axial distribution of the intrinsic activity of the Co catalyst in the tubes was also studied. X_CO,total is 95% for the optimal value of u_s of 1 m s⁻¹ combined with a constant activity coefficient C_a of 3 (which equals a Co content of the catalyst of 30 wt%); the conversion per pass is then 45%. Further improvement (X_{CO,per pass} = 47%) is possible by using tubes with two zones of equal length (6 m) and a suitable activity distribution (C_a,1 = 2.5, C_a,1 = 3.5). An activity grading (C_a,mean = 3) with C_a,initial = 2.2 until T_max of 240 °C is reached at z = 2.2 m and then a stepwise rise to C_a of 4 in the remaining 9.8 m leads to X_{CO,per pass} of 49%, the maximum for the given conditions.

The effect of axial distribution of the intrinsic catalytic activity on the performance of a multi-tubular fixed bed Fischer–Tropsch reactor was investigated. The reactor was modelled based on kinetic data for a cobalt catalyst. The influence of pore diffusion on the effective rate of CO conversion was considered. The maximum temperature was limited to 240 °C to avoid excessive methane formation. In each case, the cooling temperature was set to a value 5 K below the ignition temperature to ensure a sufficient safety distance to thermal runaway. At first, the optimal activity for the standard case of a uniform axial activity was determined. The activity should then be limited for the given conditions to a value corresponding to a Co content in the catalyst of 13 wt% to reach the highest CO conversion, here X_CO = 41%. For a reactor with two zones of equal length, but different catalytic activity, a mean value of the Co content of 31 wt% is optimal, 23 wt% in the front zone and 40 wt% in the rear part of the tubes. X_CO is then 51% compared to only 38% for uniform distribution and a Co content of 31 wt%.