Empty Reactor Research Articles

Methane pyrolysis for hydrogen production: Modeling of soot deposition by computational fluid dynamics and experimental validation

Methane pyrolysis is an attractive technology to reduce greenhouse gas emissions, since the carbonaceous part of the hydrocarbon is captured into a solid, easier to store than carbon dioxide. In this framework, computational fluid dynamics (CFD) is an excellent tool to simulate the catalyst deactivation and the complex phenomena arising on the carbon surface. We investigated thermal and catalytical pyrolysis of methane in a tubular quartz reactor with an internal diameter of 3.8cm. We modeled soot deposition and added a new surface mass balance that parametrize bed porosity as time-dependent variable. Our model predicts occlusion (clogging) time for empty and packed bed reactor: operating at 1373 K, an empty bed clogs in 15 d, which is coherent with industrial operations. The model well predicts methane conversion in the empty reactor even after 24h time on stream. When a packed bed is used, the experimental conversion follow the predicted activity decay, with deviations due to the size distribution of the carbon during experiments. Radiation is the main heat transfer mechanism (86% of the total heat absorbed by the system), compared to the others involved in this system, i.e., conduction, which is more than 50 times smaller, natural convection, more than 18 times smaller, and forced convection, more than 300 times smaller.

Flow Field Characterisation of Gaseous Flow in a Packed Bed by Particle Image Velocimetry

AbstractPacked beds with gaseous flow are used in many industrial processes with high energy and resource consumption. This study provides velocity data of the gas flow through a model packed bed of 40 mm spheres arranged in body centred cubic packing obtained by Particle Image Velocimetry to allow for validation of numerical calculations which are often used for process optimization. Results are presented exemplarily for a particle Reynolds number range from 200 to 500. The flow fields are discussed in terms of inlet conditions to the empty reactor, symmetry and height independent flow structure above the bed and measurements in the optically accessible interstices of the bed. The measurement of a sufficient number of snap-shots for each measurement condition and location allowed also for the determination of the turbulent kinetic energy. Above the bed, low frequency oscillations of the flow are observed, that increase with the Reynolds number. In the interior of the bed, fluctuations are much less pronounced. The appearing flow structures are not significantly influenced by different particle Reynolds numbers, but the velocity magnitude changes.

Plasma-based dry reforming of methane in a dielectric barrier discharge reactor: Importance of uniform (sub)micron packings/catalysts to enhance the performance

This study presents new insights on the effect of (sub)micrometer particle sized materials in plasma-based CO2-CH4 reforming by investigating the performance of SiO2 spheres (with/without supported metal) of varying particle sizes. (Sub)micron particles synthesized through the Stöber method were used instead of (sub)millimeter particles employed in previous studies. Increasing particle size (from 120 nm to 2390 nm) was found to first increase and then decrease conversion and energy yield, with optimal performance achieved using 740 nm 5 wt% Ni loaded SiO2, which improved CO2 and CH4 conversion, and energy yield to 44%, 55%, and 0.271 mmol/kJ, respectively, compared to 20%, 27%, and 0.116 mmol/kJ in an empty reactor at the same flow rate. This is the first to achieve significant performance improvement in a fully packed reactor, highlighting the importance of selecting a suitable particle size. The findings can offer guidance towards rational design of catalysts for plasma-based reactions.

On the Importance of Benchmarking the Gas‐Phase Pyrolysis Reaction in the Oxidative Dehydrogenation of Propane

AbstractThe oxidative dehydrogenation of propane (ODP) proceeds catalytically on a gas‐solid interface (heterogeneous reaction) and/or in the gas phase (homogeneous reaction) via a radical chain process. ODP may therefore combine interrelated contributions from the heterogeneous dehydrogenation and gas‐phase reactions, which can be initiated by a catalyst. This study demonstrates that relatively high propene and ethene selectivities (ca. 80 % and 10 %) and propane conversions (viz., 10 % at 500 °C) can be achieved with an empty quartz reactor, which is comparable to the performances of state‐of‐the‐art ODP catalysts (boron‐based or supported VOx). Optimization of the post‐catalytic volume of a h‐BN catalyst bed tested at 490 °C allows to increase the conversion of propane from 9 % to 15 % at a propene selectivity of 77 %, highlighting this parameter as an important variable for improving catalytic ODP performances.

Enhancement of CO2 splitting in a coaxial dielectric barrier discharge by pressure increase, packed bed and catalyst addition

CO2 splitting in a coaxial dielectric barrier discharge (DBD) is demonstrated. Empty as well as packed bed reactors with and without cerium oxide as a catalyst are investigated, but despite most other studies, the DBDs are operated also above atmospheric pressure, up to 2 bar. Argon is admixed since it reduces the breakdown voltage and thus, enables plasma operation at the elevated pressure with still moderate high voltage amplitudes of up to 13.5 kVpp. A detailed electrical characterisation of the discharge is correlated with the plasma-chemical CO2 dissociation. In particular, the discharge power, discharge voltage, minimum sustaining voltage as well as the cell and effective barrier capacites are determined as a function of pressure and gas composition. In combination with an equivalent circuit model the electrode area covered by the plasma is elucidated. Increasing the pressure requires a higher minimum sustaining voltage amplitude, but the optimum of power input for each given voltage amplitude depends on pressure and gas composion. Increasing the pressure from 1 to 2 bar enhances the CO2 conversion from 3.4 % to 8.9 % for nearly the same specific energy input (0.35 and 0.31 eV/particle) in the empty reactor and to 12.7 % with a packed bed in the discharge space. The energy efficiency increase from 6 % to 20 % is explained by a higher number of collisions between CO2 molecules and active plasma species leading to dissociation. Moreover, reactor performance can be effectively increased by adding glass spheres coated with ceria (CeO2).

A novel Static Mixer for photochemical reactions

Photochemical reactions have become a widely studied topic in the past decade, as they are employed in a wide range of processes and are recognized to be a fundamental tool in the field of sustainable chemistry. Moreover, in recent years there is an increasing interest in turning discontinuous photochemical processes into continuous ones, since continuous processes can ensure better results in terms of productivity and selectivity. In this work, a novel static mixer useful to reach a plug-flow behaviour inside a square-based reactor, while not obstructing the photon flux coming from the light sources, was designed. Through RANS based CFD simulations, the residence time distribution of molecules inside the reactor was evaluated. Moreover, the effect of both absorption and scattering phenomena, which play a major role in photochemical reactions, was assessed. Finally, a case study was considered and the photochemical degradation of 2-hydroxybenzoic acid (2-HBA) was simulated. The conversion obtained in the reactor equipped with SMs was compared to an empty reactor, finding that the degradation efficiency is doubled.

Impact of Nonzeolite-Catalyzed Formation of Formaldehyde on the Methanol-to-Hydrocarbons Conversion

Formaldehyde is an important intermediate that affects the catalyst performance in the methanol-to-hydrocarbons (MTH) conversion. In this study, photoelectron photoion coincidence spectroscopy was applied to elucidate the formation of this species in empty quartz and stainless steel reactors as well as over zeolite catalysts (ZSM-5 and BETA) and commonly used bed diluents (silicon carbide and quartz). The yields of formaldehyde in an empty stainless steel reactor and over the crude silicon carbide particles were found to be higher or comparable to those over the zeolite catalysts under similar reaction conditions. In the former two systems, formaldehyde is formed via methanol dehydrogenation, which is catalyzed by transition metals and yields hydrogen and carbon monoxide as the main byproducts. Thus produced formaldehyde is readily consumed in the MTH reaction, wherein its conversion is higher for the more acidic zeolites. The formaldehyde generated by the transition metal sites causes a decrease of the catalyst stability as well as a reduction of the propene-to-ethene ratio, as corroborated by catalytic tests exploring different contacting patterns between the ZSM-5 catalyst and stainless steel chips or silicon carbide particles. These results uncover an important role of methanol dehydrogenation in the MTH conversion, which is relevant for the laboratory testing of the zeolite catalysts and the industrial implementation of this technology.

Hydrogen Production through Water Vapors using Optimized Corona-DBD Hybrid Plasma Micro-Reactor

Hydrogen as secondary energy carrier is one of the promising routes to meet renewable and sustainable energy demand. Hydrogen production using water plasmolysis, so far, has been considered as energy intensive. However, recent developments in the field of microplasmas has reduced energy requirement of water plasmolysis on par with electrolysis. However, there’s still room for improvement as water vapors dissociation efficiencies are less than thermodynamic and kinetic limits. One of the ways to do that is to split microchannel in plasma into sub-micron channels by packing beads inside the reactor. The current study investigates the plasmolysis of water vapours (steam) and argon gas with (1) empty channel plasma reactor and (2) glass beads packed plasma reactor. A custom-made Corona-DBD hybrid microplasma reactor has been designed and employed for water steam plasmolysis. This study aims to investigate the feasibility of water vapors dissociating into hydrogen at a relatively small interelectrode gap at atmospheric pressure. The minimum breakdown voltage has been found to be 3.7 kVpk-pk at 5 mA current. The maximum hydrogen production rate was observed 23.9 g/kWh at optimized conditions for the glass beads packed plasma reactor. So, in current study glass beads packed plasma microreactor yielded 23.9g of H2/kWh which is significantly higher than any previously published study

Ozone Formation during Photocatalytic Oxidation of Nitric Oxides under UV Irradiation with the Use of Commercial TiO2 Photocatalytic Powders

The application of photocatalytic materials has been intensively researched in recent decades. The process of nitric oxide (NO) oxidation during photocatalysis has been observed to result in the formation of nitric dioxide (NO2). This is a significant factor of the photocatalysis process, as NO2 is more toxic than NO. However, it has been reported that ozone (O3) is also formed during the photocatalytic reaction. This study analyzed the formation and oxidationof O3 during the photocatalytic oxidation of NO under ultraviolet irradiation using commercial photocatalytic powders: AEROXIDE® TiO2 P25 by Evonik, KRONOClean® 7050 by KRONOS®, and KRONOClean® 7000 by KRONOS®. An NO concentration of 100 ppb was assumed in laboratory tests based on the average nitric oxide concentrations recorded by the monitoring station in Warsaw. A mix flow-type reactor was applied in the study, and the appropriateness of its application was verified using a numerical model. The developed model assumed an empty reactor without a photocatalytic material, as well as a reactor with a photocatalytic material at its bottom to verify the gas flow in the chamber. The analysis of the air purification performance of photocatalytic powders indicated a significant reduction of NO and NOx and typical NO2 formation. However, no significant formation of O3 was observed. This observation was verified by the oxidation of pure ozone in the process of photocatalysis. The results indicated the oxidation of ozone concentration during the photocatalytic reaction, but self-decomposition of a significant amount of the gas.

CO2 dissociation in a packed bed DBD reactor: effect of streamer discharge

Non-thermal plasma catalysis, as a special heterogeneous catalytic reaction, needs to consider both gas discharge and catalytic reaction. Packed bed dielectric barrier discharge (PB-DBD) is widely used in non-thermal plasma catalysis, but the exact control principle of gas discharge, especially streamer discharge, is not clear. In this study, therefore, the orderly arranged dielectric rods were packed in the discharge gap of PB-DBD, and the streamer discharge behaviors were controlled by adjusting their diameter(s), quantity(ies), location(s) and dielectric constant(s). Al2O3 and ZrO2 dielectric rods with dielectric constants of about 9 and 25 were used as packing material. Pure CO2 was used as reaction gas and discharge gas. Discharge images showed that stable and controllable streamer discharges can be formed between the dielectric rod and ground electrode. The intensity, width and length of the streamer discharge can be significantly changed by optimizing the dielectric constant, diameter, packing number and position of the dielectric rod, thereby affecting the CO2 conversion efficiency. Increasing dielectric constant and the distance between the dielectric rod and ground electrode can increase the intensity of streamer discharge, thus promoting the CO2 conversion efficiency. Compared with an empty reactor, after packing 24 ZrO2 dielectric rods with a diameter of 1 mm, the CO2 conversion and energy efficiency increased from 9.58% to 20.1% and from 1.67% to 2.89%, respectively. In short, this research has important implications for plasma catalysis. This study not only reveals the synergistic characteristics between streamer discharge and CO2 dissociation, but also provides an important idea for structural optimization of PB-DBD catalyst.

A PRELIMINARY EXPERIMENTAL INVESTIGATION INTO AMMONIA OXIDATION IN A FIXED-BED

This paper reports the findings of a preliminary experimental investigation into NH3 oxidation in a quartz fixed-bed reactor. The reactor was filled with quartz sand of a size fraction of 1.0 ~ 2.0 mm to a bed height of 430 mm. NH3 and O2 were diluted with Ar, mixed and passed through the reactor at a constant total gas flowrate of 818 mL·min-1. The experiments were carried out at various reaction temperatures between 700 and 1450 K, corresponding to residence times ranging from 0.68 to 2.07 s and equivalence ratios of 0.5, 1.0, and 1.5. The reactor exit stream was sampled and analysed using a gas chromatograph (Shimadzu 2014) and an electrochemical analyser (Testo 340) for the concentrations of N2, O2, NO, and NO2. A set of comparative experiments without bed material (considered as an ‘empty bed reactor’) at identical residence times were also performed in order to appreciate the effect of quartz surface on NH3 oxidation and NOx formation. It was revealed that the lowest temperature to initiate noticeable NH3 oxidation was reduced from 1300-1350 K in the empty bed reactor, to 1050-1350 K in the quartz sand filled fixed-bed reactor. The presence of quartz sand significantly reduced NOx formation during NH3 oxidation. Additionally, the peak NO concentrations and the temperature to reach the peak were reduced from 578 to 191 ppm and ~1350 to ~1050 K, respectively. It is speculated that the active radicals such as NH* produced from the heterogeneous decomposition reaction of NH3 on the quartz surface may have promoted NH3 oxidation and NOx reduction.

Plasma-assisted catalysis for ammonia synthesis in a dielectric barrier discharge reactor: key surface reaction steps and potential causes of low energy yield

Ammonia synthesis experiments were carried out in a coaxial dielectric barrier discharge (DBD) reactor packed with several different supports and metal catalysts. There was a marked increase in the reaction rate, over that obtained in an empty DBD plasma reactor, upon introduction of a packed bed of γ-Al2O3, Ru/γ-Al2O3, or SiO2 particles. The difference in the reaction rates over γ-Al2O3 and Ru/γ-Al2O3 was minimal. Complementary zero-dimensional plasma kinetic model analysis was also performed using inputs from experimental data. This kinetic analysis allowed for gas phase reactions, Eley–Rideal (E–R) reactions, and direct adsorption of radical species on the γ-Al2O3 surface. On the metal surface, dissociative adsorption of N2 and H2, and Langmuir–Hinshelwood reactions were also included. This analysis revealed that, under the conditions of our experiments, ammonia synthesis proceeds principally by the formation of reactive radicals in the gas phase, which then adsorb and participate in E–R reactions on both the metal and support material surfaces. This finding illustrates a challenge for substantially increasing the energy yield for plasma-assisted ammonia synthesis in typical DBD reactors containing packed catalyst beads.

Distribution of the Deposition Rates in an Industrial-Size PECVD Reactor Using HMDSO Precursor

The deposition rates of protective coatings resembling polydimethylsiloxane (PDMS) were measured with numerous sensors placed at different positions on the walls of a plasma-enhanced chemical vapor deposition (PECVD) reactor with a volume of approximately 5 m3. The plasma was maintained by an asymmetric capacitively coupled radiofrequency (RF) discharge using a generator with a frequency 40 kHz and an adjustable power of up to 8 kW. Hexamethyldisiloxane (HMDSO) was leaked into the reactor at 130 sccm with continuous pumping using roots pumps with a nominal pumping speed of 8800 m3 h−1 backed by rotary pumps with a nominal pumping speed of 1260 m3 h−1. Deposition rates were measured versus the discharge power in an empty reactor and a reactor loaded with samples. The highest deposition rate of approximately 15 nm min–1 was observed in an empty reactor close to the powered electrodes and the lowest of approximately 1 nm min–1 was observed close to the precursor inlet. The deposition rate was about an order of magnitude lower if the reactor was fully loaded with the samples, and the ratio between deposition rates in an empty reactor and loaded reactor was the largest far from the powered electrodes. The results were explained by the loss of plasma radicals on the surfaces of the materials facing the plasma and by the peculiarities of the gas-phase reactions typical for asymmetric RF discharges.

Spatially and temporally non-uniform plasmas: microdischarges from the perspective of molecules in a packed bed plasma reactor

Dielectric barrier discharges (DBDs) typically operate in the filamentary regime and thus exhibit great spatial and temporal non-uniformity. In order to optimize DBDs for various applications, such as in plasma catalysis, more fundamental insight is needed. Here, we consider how the millions of microdischarges, characteristic for a DBD, influence individual gas molecules. We use a Monte Carlo approach to determine the number of microdischarges to which a single molecule would be exposed, by means of particle tracing simulations through a full-scale packed bed DBD reactor, as well as an empty DBD reactor. We find that the fraction of microdischarges to which the molecules are exposed can be approximated as the microdischarge volume over the entire reactor gas volume. The use of this concept provides good agreement between a plasma-catalytic kinetics model and experiments for plasma-catalytic NH3 synthesis. We also show that the concept of the fraction of microdischarges indicates the efficiency by which the plasma power is transferred to the gas molecules. This generalised concept is also applicable for other spatially and temporally non-uniform plasmas.

Effects of packing particles on the partial discharge behavior and the electrical characterization of oxygen PBRs

Packed-bed reactors (PBRs) hold great promise for environmental applications, but a deeper understanding of the behavior of plasma discharge within PBRs is required. To this end, a partial-discharge alternative equivalent circuit for PBRs was established in this work. Dielectric particles (glass beads or glass sand) were used to place focus on the effects of the particle size and shape on the partial discharge behavior of the oxygen PBRs. Some electrical characterizations were explored (e.g. the effective dielectric capacitance, partial discharge coefficient, and corrected burning voltage) that may differ from long-standing interpretations. The findings indicate that the suppressive effect of surface discharge on filament discharge is stronger with the decrease of the particle size. For partial discharge, the effective dielectric capacitance is always less than the dielectric capacitance. The corrected burning voltage and partial discharge tendency increase with the decrease of the particle size. As compared to an empty reactor, the average electric field in the PBR was found to be improved by 3–4 times, and the ozone energy efficiency and production were promoted by more than 20% and 15%, respectively. The plasma processing capacity can therefore be improved by choosing a relatively large size or a complex, irregularly-shaped packing material that is suitable for the discharge gap.

Intensification of ozone generation and degradation of azo dye in non-thermal hybrid corona-DBD plasma micro-reactor

A novel corona-dielectric barrier discharge hybrid non-thermal plasma micro-reactor was investigated for ozone generation and pollutant degradation. It was hypothesized that creating a submicron channel inside the micro-reactor would increase ozone generation efficiency. Therefore, the plasma micro-reactor was packed with dielectric glass beads. The plasma micro-reactor operating conditions were investigated for ozone formation. The efficiency of the empty channel plasma reactor was compared with the packed-bed plasma reactor which was 24 % higher than the former at optimized conditions-5 kV peak voltage, 36 mA peak current, 26.02 kHz frequency, 1 Lmin−1 air flow rate, and 4 cm electrode length. The performance of the reactor was evaluated for the degradation of Reactive black-5. The plasma micro-reactor operating conditions and the dye operational parameters- pH of the solution, initial dye concentration, and volume of solution were also investigated to see the efficiency of reactive black-5 removal in the plasma reactor. The dye degradation efficiency increased by 50 % in the packed-bed plasma micro-reactor as compared to the empty channel reactor. In the empty channel reactor, 98 % reactive-black-5 was removed with 87 % COD reduction in 16 min, and in the packed-bed reactor, 98.3 % was removed with 89 % COD reduction in 8 min of the plasma treatment.

Minimizing carbon deposition in plasma-induced methane coupling with structured hydrogenation catalysts

The effect of temperature and hydrogen addition on undesired carbonaceous deposit formation during methane coupling was studied in DBD-plasma catalytic-wall reactors with Pd/Al2O3, using electrical power to drive the reaction. Experiments with thin catalyst layers allowed comparison of the performance of empty reactors and catalytic wall reactors without significantly influencing the plasma properties. The product distribution varies strongly in the temperature window between 25 and 200 °C. Minimal formation of deposits is found at an optimal temperature around 75 °C in the catalytic-wall reactors. The selectivity to deposits was c.a. 10% with only 9 mg of catalyst loading instead of 45% in the blank reactor, while decreasing methane conversion only mildly. Co-feeding H2 to an empty reactor causes a similar decrease in selectivity to deposits, but in this case methane conversion also decreased significantly. Suppression of deposits formation in the catalytic-wall reactor at 75 °C is due to catalytic hydrogenation of mainly acetylene to ethylene. In the empty reactor, H2 co-feed decreases conversion but does not change the product distribution. The catalytic-wall reactors can be regenerated with H2-plasma at room temperature, which produces more added-value hydrocarbons.

Experimental Study on Sugarcane Bagasse Pyrolysis in a Hermochemical Processes Pilot Plant

This paper investigates the sugarcane bagasse pyrolysis for bio-oil and biochar production in a pilot plant that was designed to perform the gasification process. After some modifications of changing the temperature of fluidized bed reactor and the catalytic reforming reactor, the fast pyrolysis process was performed at 500–600 °C using a fluidized bed reactor (air flow rate: 20–70 L.min-1) under atmospheric pressure, with sugarcane bagasse feeding (2–3.5 kg.h-1) with particles size less than 1.18 mm. Fluidization tests were performed with air feed (oxidative pyrolysis), to guarantee the fluidization is occurring, finding that the minimum fluidization rate was 22 L.min-1 (minimum velocity of gas: 6.3 cm.s-1). The residence time was 7 seconds (ratio between empty reactor volume by fluidizing gas flow). The highest bio-oil yield of 7.43 wt. % was obtained at 500 °C, with a bagasse flow rate of 3.5 kg.h-1 and 25.93 L.min-1 of air flow rate, and this low yield can be explained by the inefficient heat exchanger used. On the other hand, biochar yield achieved 35.88 wt. % using a temperature of 570 °C, 3.1 kg.h-1 of bagasse flow rate, and 28.26 L.min-1 of air flow rate. Considering this pilot plant has never operated in a pyrolysis mode, these results demonstrate that pyrolysis can be applied to this pilot plant, being versatile for different types of processes with a great potential for biochar and bio-oil production besides the high gas production.

The Potential Use of Core-Shell Structured Spheres in a Packed-Bed DBD Plasma Reactor for CO2 Conversion

This work proposes to use core-shell structured spheres to evaluate whether it allows to individually optimize bulk and surface effects of a packing material, in order to optimize conversion and energy efficiency. Different core-shell materials have been prepared by spray coating, using dense spheres (as core) and powders (as shell) of SiO2, Al2O3, and BaTiO3. The materials are investigated for their performance in CO2 dissociation and compared against a benchmark consisting of a packed-bed reactor with the pure dense spheres, as well as an empty reactor. The results in terms of CO2 conversion and energy efficiency show various interactions between the core and shell material, depending on their combination. Al2O3 was found as the best core material under the applied conditions here, followed by BaTiO3 and SiO2, in agreement with their behaviour for the pure spheres. Applying a thin shell layer on the cores showed equal performance between the different shell materials. Increasing the layer thickness shifts this behaviour, and strong combination effects were observed depending on the specific material. Therefore, this method of core-shell spheres has the potential to allow tuning of the packing properties more closely to the application by designing an optimal combination of core and shell.

Energy efficiency improvement in nitric oxide reduction by packed DBD plasma: optimization and modeling using response surface methodology(RSM).

The non-thermal plasma (NTP) is a superior proposed method for nitric oxide removal because of operation at atmospheric pressure and ambient temperature. The energy consumption is the main challenge of using this technology. The packed plasma reactor with dielectric materials has been extensively investigated; it has higher energy efficiency. In this study, the energy efficiency and the other effectiveness factors in nitric oxide removal by NTP reactor packed with ceramic and glass beads optimized and modeled using Response Surface Methodology. The findings showed the maximum energy efficiency was 132.69g/J in the optimal conditions of initial concentration, gas flowrate, and duty cycle(voltage) equal to 1050 ppm, 2.5 L/min, and 9%(22KV), respectively in the packed reactor with ceramic beads by 1.7 times than the empty reactor. It is concluded that the use of ceramic beads as a dielectric material in the discharge space significantly increased energy efficiency in the removal of nitric oxide.