Packed Bed Material Research Articles

Effect of Porosity on Combustion Performance in Packed Bed Porous Media

This study investigates the effect of packed bed material porosity and air-to-fuel ratio on the combustion stabilization of a premixed gaseous mixture. An experimental work was carried out in a single-layer concept of a packed bed on a constant cross-sectional area tubular burner. Two types of materials, Alumina (Al2O3) and Zirconia (ZrO2), with different porosities, namely 0.36, 0.4, 0.44, and 0.46, were tested. The results showed that porosity has a significant effect on the position of the reaction zones. As porosity decreases, the reaction zone moves downstream of the packed bed. The excess air ratio does not affect the position of the reaction zone but has an impact on the temperature distribution inside the porous medium. The packed bed material affects the volume of the reaction zone and the temperature distribution inside the porous media, where Zirconia has a reaction zone volume higher than Alumina. The concentration of NOx was reduced with increasing porosity. Zirconia media exhibits a lower level of NOx emission compared to Alumina. For an excess air ratio of 1.6, the maximum NOx values were 22.5 and 17.5 ppm for Alumina and Zirconia, respectively.

Electrical characterization and imaging of discharge morphology in a small-scale packed bed dielectric barrier discharge

Packed bed dielectric barrier discharges (DBDs) exhibit an improved energy efficiency and selectivity in nonthermal plasma based gas conversion. They enable the direct interaction between plasma and catalyst. In this contribution a compact coaxial DBD reactor enabling the end-on imaging of the discharge with and without packed beds is studied. The discharge morphology is correlated with electrical measurements such as voltage-charge (V-Q) plots. The studies are performed for different packed bed materials, binary gas compositions of argon and carbon dioxide, voltage amplitudes, average powers, and pressures. The analysis points outs the role of parasitic capacitances and parasitic discharges as often overlooked aspects. The introduction of the packed bed material into the coaxial barrier discharge arrangement increases the total capacitance, but the barrier of the outer glass tube mostly determines the maximum effective dielectric capacitance. The choice of the packed bed material determines the voltage threshold and the average discharge power. The investigations leads to a revision of the equivalent circuit for packed bed barrier discharge reactors, which also accounts the properties of different filling materials.

Effect of operation conditions on particulate matter removal by a packed-bed wet scrubber for a small-scale biofuel boiler

In 2013 the EU’s Clean Air Policy Package was established, aiming to reduce air pollution to half by 2030 compared to the level in 2005. Small-scale (<500 kW)biofuel boilers play a key role in particulate matter emission, and exposure to particulate matter even in the short term can cause different diseases. With the aim of reducing particulate matter emission in Europe, this study presents an approach to improve the removal of particulate matter emitted by small-scale boilers. A biofuel combustion boiler was equipped with a packed-bed wet scrubber, and the flue gas emitted through combustion was cleaned through the wet scrubber using a saltwater mixture. The performance of a packed-bed wet scrubber was investigated under different operating conditions. The effect of the salt concentration of the absorption solution, the temperature of the absorption solution fed to the absorber, and the height of the packed-bed material on the particle collection efficiency were measured. The operating conditions were selected based on the results obtained in a previous computational fluid dynamic simulation study. The results obtained in the present study show that an absorption solution temperature of 30 °C and an absorption solution concentration of 75 % with a full height of the packed-bed material lead to the best performance in the system. Totally keeping the absorption solution temperature as low as possible, increasing the absorption solution concentration, and raising the packed-bed material height could improve the particle collection efficiency by enhancing the effect of the diffusiophoresis and thermophoresis forces and the contact time between the flue gas and solution.

Thermo-Physical Characterization of Waste-Glass-Induced Packed Bed Material as Thermal Energy Storage Device for Compressed Air Energy Storage System

Abstract The article presents the preparation and testing of packed bed (PB) material to be used as a thermal energy storage (TES) device. The proposed TES device will be used to store the high thermal energy attained during air compression in a compressed air energy storage (CAES) system. The article examines the utilization of mortar-based admixture by incorporating waste glass powder (WGP), graphite powder (GP), and waste glass sand (WGS). The selection of these constituents as a primary ingredient for the PB material has been made based on their availability, cost, and sustainability. The thermo-physical assessment of samples with different proportions of aggregates outlined two categories of PB: the first category of PB with low volumetric heat capacity (CP) for short/quick TES and the second category of PB with high CP for large/longer TES. The study also showcases the importance of GP in enhancing the CP of mortar-based TES devices as a result of high porosity.

Non-Oxidative Ethane Dehydrogenation in a Packed-Bed DBD Plasma Reactor

Plasma-assisted conversion of ethane (C2H6) can produce value-added chemical building blocks using green electricity. Here we employ a simple packed-bed coaxial dielectric barrier discharge (DBD) reactor to convert C2H6 at mild operating conditions unattainable by conventional thermocatalysis. Ethylene (C2H4), acetylene (C2H2), and methane (CH4) are the main products along with small fractions of C3 and C4 hydrocarbons. Interestingly, the C2H4 selectivity is primarily correlated to C2H6 conversion, dominated by electron dissociation and recombination reactions irrespective of the dielectric properties of the packed bed material (SiO2, Al2O3, ZrO2, TiO2, and BaTiO3), packing material size, supplied power, and C2H6 concentration. While a distortion of the electric field and discharge propagation results in varying dissipated power as materials change, the C2H4 energy yield remains constant. The particle size appears to affect conversion mainly due to pressure alterations. Pd/SiO2 catalyst can change the selectivity, favoring saturated species by expending hydrogen.

Treatment of the insensitive munitions compound, 3-nitro-1,2,4-triazol-5-one (NTO), in flow-through columns packed with zero-valent iron.

The need for effective technologies to remediate the insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) is emerging due to the increasing use by the US Army and environmental concerns about the toxicity and aqueous mobility of NTO. Reductive treatment is essential for the complete degradation of NTO to environmentally safe products. The objective of this study is to investigate the feasibility of applying zero-valent iron (ZVI) in a continuous-flow packed bed reactor as an effective NTO remediation technology. The ZVI-packed columns treated an acidic influent (pH 3.0) or a circumneutral influent (pH 6.0) for 6months (ca. 11,000 pore volumes, PVs). Both columns effectively reduced NTO to the amine product, 3-amino-1,2,4-triazol-5-one (ATO). The column treating the pH-3.0 influent exhibited prolonged longevity in reducing NTO, treating 11-fold more PVs than the column treating pH-6.0 influent until the breakthrough point (defined as when 85% of NTO was removed). The exhausted columns (defined as when only 10% of NTO was removed) regained the NTO reducing capacity by reactivation using 1M HCl, fully removing NTO. After the experiment, solid-phase analysis of the packed-bed material showed that ZVI was oxidized to iron (oxyhydr)oxide minerals such as magnetite, lepidocrocite, and goethite during NTO treatment. This is the first report on the reduction of NTO and the concomitant oxidation of ZVI in continuous-flow column experiments. The evidence indicates that treatment in a ZVI-packed bed reactor is an effective approach for the removal of NTO.

Multi-objective optimization for assisting the design of fixed-type packed bed reactors for chemical heat storage

• Multi-objective optimization applied on packed bed reactor for chemical heat storage. • Simultaneous maximization of heat storage density, rate and exergy efficiency. • Optimal design of thickness, reaction condition and fabrication of composite material. • High thermal contact conductance is recommended when utilizing enhanced composites. • Limitations on the packed bed’s thickness for achieving high exergy efficiency. The enhancement of heat transfer in packed bed reactors is essential for the practical application of chemical heat storage technology. This work presents the first attempt to utilize a multi-objective optimization algorithm for the design of packed bed reactors comprising heat-transfer-enhanced materials as an alternative to the insertion of metallic fins. A numerical model for the simulation of heat transfer with chemical reaction in a flat packed bed reactor comprising a composite of Ca(OH) 2 (reagent) and a silicon-impregnated silicon carbide foam (thermal conductivity enhancer) is developed. The genetic algorithm applied to the model aims to optimize four design parameters: the effective thermal conductivity of the packed bed material, its thickness, the thermal contact conductance, and the heat supply temperature. As a result, it determines the optimal trade-off between heat storage density, average heat storage rate and exergy efficiency. Under the assumptions of the model, the Pareto solution indicates that the optimal foam porosity is the interval 70-80%, while the optimal thickness is the interval 20-70 mm. The analysis suggests the necessity to improve the thermal contact conductance in packed bed reactors and, for achieving higher exergy efficiency, thinner (modular) packed beds result better performing than thicker ones.

Proposal of a thermocline molten salt storage tank for district heating and cooling

Thermal Energy storage is one of the critical components in district heating and cooling (DHC) facilities when based on intermittent energy sources (as solar). Currently, DHC energy storage is based on water tanks. Nevertheless, water storage cannot be used with medium temperature processes as double effect absorption chillers or other services requiring temperatures well above 100 °C. Current thermal storage at higher temperature may be based on a variety of technologies. Among then, molten nitrate salts are widely used for heat storage in concentrated solar plants (CSP), limited by the melting temperature of available commercial solar salts and their solidification risk. Such commercially available salts are not applicable to district heating and cooling as they are solid in the expected operational temperature range. Other storage media may be solid materials, either by the use of packed-bed materials, concrete or phase change materials. In this paper, an innovative molten salt mixture with a melting temperature below 150 °C is used. The availability of a storage medium in a temperature range from 150 up to 250 °C might increase the solar share in DHC installations, integrating concentrated solar technologies. Such storage allows to apply solar technologies to supply heat to services for DHC that now can be only provided by boilers. In this communication, a molten salt storage tank operating in thermocline mode will be proposed in the framework of the WEDISTRICT project [1]. The project seeks to deliver the highest possible share of renewable sources for the energy needs of a district demand, maximizing performance by the application of advanced absorption chillers, that requires temperature feeds well above 100 °C.

Numerical modeling of temperature-dependent effective thermal conductivity of two-phase porous system using square and circular geometries

Two-phase porous materials are extensively employed in industries and their thermal characteristics decide on the overall efficiency of the system. Several dependent parameters such as geometry, its structure, heat transfer mechanism, and contact ratio, strongly influence the effective thermal conductivity (ETC) of the two-phase system. There are various literatures on analytical and experimental prediction of ETC of two-phase porous materials such as packed beds, foams, and sponges. However, numerical studies are limited with the consideration of few influential parameters. This present study has attempted to investigate the thermal behavior of two-phase porous materials numerically using square and circular geometries with the consideration of all primary and secondary criterions at different pressures and elevated temperatures. The numerical results are correlated against the experimental data of different porous insulation materials and packed bed materials. Further, the square and circular models are assessed for their stability in predicting the ETC at higher temperatures. Even at elevated temperatures, the present models hold better accuracy and consistency in prediction.

Numerical modelling and optimization of packed-bed thermal-energy storage system

In this work, numerical modelling (including convection and radiation heat transfer) of Packed-bed storage systems (PBSS) is carried out and the effect of various system parameters affecting the efficiency and temperature is studied. One dimensional forms of the time-dependent, coupled partial differential equations for energy conservation of the heat transfer fluid (HTF) and the packing materials are solved using the finite difference method to design a PBSS for a given mass flow rate for 0.125 MWh (0.45 GJ). The packed-bed material is assumed to be spherical with an average diameter, and the mass flow rate, void fraction, packed bed material diameter, heat transfer fluid (HTF), and the packed-bed materials are varied for optimal efficiency during charging (i.e. when the temperature of HTF is greater than that of the packed bed). The results are compared with the analytical solutions for validation. Present solutions (considering only convection) are found to be matching reasonably well with the analytical data. With the validated model, the radiation heat transfer effect on the packed-bed is modelled (along with the convection) and void fraction, mass flow rate of HTF, a diameter of the packed-bed material, different HTF, various packed-bed materials are compared for maximum efficiency. From the present work, a PBSS design with void fraction =0.45, mass flow rate =1.8 kg/s, the average diameter of the packed-bed material = 0.02 m, water as HTF, and brick as packed-bed material is found to give the maximum efficiency along the length of the packed-bed.

Review of Design and Modeling of Regenerative Heat Exchangers

Heat regenerators are simple devices for heat transfer, but their proper design is rather difficult. Their design is based on differential equations that need to be solved. This is one of the reasons why these devices are not widely used. There are several methods for solving them that were developed. However, due to the time demands of calculation, these models did not spread too much. With the development of computer technology, the situation changed, and these methods are now relatively easy to apply, as the calculation does not take a lot of time. Another problem arises when selecting a suitable method for calculating the heat transfer coefficient and pressure drop. Their choice depends on the type of packed bed material, and not all available computational equations also provide adequate accuracy. This paper describes the so-called open Willmott methods and provides a basic overview of equations for calculating the regenerative heat exchanger with a fixed bed. Based on the mentioned computational equations, it is possible to create a tailor-made calculation procedure of regenerative heat exchangers. Since no software was found on the market to design regenerative heat exchangers, it had to be created. An example of software implementation is described at the end of the article. The impulse to create this article was also to broaden the awareness of regenerative heat exchangers, to provide designers with an overview of suitable calculation methods and, thus, to extend the interest and use of this type of heat exchanger.

The Benefit of Using Multiple Thin Tanks Versus a Short Big Tank for Thermal Storage in Ceramic-Sphere Packed Bed With Airflow

Abstract To make a better thermal storage system that uses air as the heat transfer fluid flowing through a packed-bed of ceramic spheres (Al2O3) as thermal storage materials, the present work studied cases using multiple small-diameter thin tanks to replace a large-diameter big tank while keeping the same total volume and the same air flow rate. Performance analysis of thermal storage has been conducted for comparison and optimization. The long flow passage and faster flow velocity of air in the small-diameter tanks was found to significantly benefit thermal storage performance compared with that of a short big tank. It resulted in a longer duration of discharge of high-temperature airflow, if the same operation time is applied to the two situations. The faster airflow enhances the heat transfer between air and thermal storage material, although it incurs larger pressure loss. Overall, the energy storage efficiency of using several thin tanks can be significantly better than that of using a big short tank if the height-to-diameter ratio in the multiple thin tanks is properly optimized. The optimization methodology and results are of great significance to the development of thermal storage systems that use air as heat transfer fluid and rocks or ceramic spheres as the packed-bed material for thermal storage.

Carbon gel-supported Fe-graphene disks: Synthesis, adsorption of aqueous Cr(VI) and Pb(II) and the removal mechanism

Carbon gel-supported Fe-graphene (Fe-G) disks were synthesized via sol-gel polymerization of resorcinol-formaldehyde (RF), followed by carbonization of the polymeric mass. Fe(III)-graphene oxide were in situ doped in the gel at the incipience of polymerization. The Fe(III) served as a crosslinker between graphene oxide and RF matrix as well as the polymerization catalyst. The prepared Fe-G/RF(C) disks were used as an efficient adsorbent for aqueous Cr(VI) and Pb(II). Various analytical techniques were used to characterize the physico-chemical properties of the prepared materials, including surface morphology, surface area and point of zero charge. The compressive strength of the Fe-G/RF(C) disks was measured to be significantly high (0.12MPa). The maximum adsorption capacities of Fe-G/RF(C) for Cr(VI) and Pb(II) were determined to be 108 and 172mg/g at the optimized solution pH. Significant adsorption of the metals was attributed to the large surface area (∼537m2/g) and microporosity (94%) in the material, high chemical reactivity of graphene, and amenability of the material surface to protonation by pH-adjustment. The synthesized amorphous carbon gel-supported Fe-graphene disks can be effectively used as packed bed materials for the adsorptive removal of toxic metal ions present in industrial aqueous effluents under flow or dynamic conditions.

Improving the pollutant removal efficiency of packed-bed plasma reactors incorporating ferroelectric components

In this work we have studied the plasma removal of air contaminants such as methane, chloroform, toluene and acetone in two parallel plate packed-bed dielectric barrier discharge (DBD) reactors of different sizes. Removal and energy efficiencies have been determined as a function of the residence time of the contaminated air within the reactor, the kind of packed-bed material (ferroelectrics or classical dielectric materials), the frequency and the incorporation of a ferroelectric plate onto the active electrode together with the inter-electrode ferroelectric pellets filling the gap. Results at low frequency with the small reactor and the ferroelectric plate showed an enhancement in energy efficiency (e.g., it was multiplied by a factor of six and three for toluene and chloroform, respectively) and in removal yield (e.g., it increased from 22% to 52% for chloroform and from 15% to 21% for methane). Such enhancements have been attributed to the higher energy of plasma electrons and a lower reactor capacitance found for this plate-modified configuration. A careful analysis of reaction efficiencies and electron energy distributions for the different investigated conditions and the simulation of the electric field at the necks between ferroelectric/dielectric pellets complete the present study. Overall, the obtained results prove the critical role of the barrier architecture and operating conditions for an enhanced performance of pollution removal processes using DBD systems.

Thermal and thermo-hydraulic performance investigation of double-pass packed bed solar air heaters under external recycle

In the present investigation, two types (Type A and Type B) of the double-pass packed bed solar air heater under external recycle are investigated theoretically. In Type A, the porous media is considered in the upper channel, whereas in Type B, the porous media is considered in the lower channel. Iron scraps are used as a packed bed material (porous media) to strengthen the convective heat transfer coefficient for air flowing through the packed bed. The mathematical model for these two air heaters operating under forced convection mode is presented. The results revealed that the thermal and thermo-hydraulic efficiencies of Type A are higher as compared to Type B. In order to validate the models, the theoretical results obtained from the conventional model of Type B are compared with the theoretical results obtained from the previous investigation and showed that good agreement is achieved.

Thermal and thermohydraulic performance evaluation of a novel type double pass packed bed solar air heater under external recycle using an analytical and RSM (response surface methodology) combined approach

In the present study, an analytical and RSM (response surface methodology) combined approach has been applied to investigate the thermal and thermohydraulic performances of a novel type double pass packed bed solar air heater under external recycle with wire mesh screen as a packed bed material. An analytical model describing the various temperatures and heat transfer characteristics of such a double pass packed bed solar air heater under external recycle has been developed and employed to study the effects of mass flow rate, recycle ratio and varying channel depth between the top (upper and lower) glass covers for a fixed 95% porosity of the packed material on its thermal and thermohydraulic performances. The analytical model employs an iterative solution procedure to solve the governing energy balance equations describing the complex heat and mass transfer involved. Furthermore, RSM is then applied for developing mathematical models based on simulation results obtained from analytical study. The effect of parameters and their interactions on the responses are studied using RSM. The results obtained from RSM revealed that proposed mathematical model is significant and good agreement is achieved with reasonable accuracy.

Production of hydrogen and syngas via pyrolysis of bagasse in a dual bed reactor

Pyrolysis of bagasse followed by thermal cracking of tar was carried out at atmospheric pressure using a dual bed reactor. The first bed was used for the pyrolysis and the second bed was used for thermal cracking of tar. Iron fillings were used as the packed bed material in the second bed. The effects of reaction time (20 to 40 min), reactor temperature (600 to 900 °C) and packed bed height (40–100 mm) on the product (char, tar and gas) yield and gas (H2, CO, CO2, CH4, CnHm) composition were studied. Over the ranges of the experimental conditions used, the operating conditions were optimized for pyrolysis temperature around 850 °C, a reaction time of 30 min and packed bed height of 100 mm, thus we could obtain a gas richer in hydrogen and carbon monoxide and poorer in carbon dioxide and hydrocarbons. It was observed that compared with single bed process, dual bed process increased the gas yield from 0.397 to 0.750 m3/kg and decreased the tar yield from 0.445 to 0.268 g/g while the heating value of the product gas remained almost constant (10–11 MJ/m3).

シルト質内湾での現地特性を活かした海岸侵食対策の検討

In many coasts of Thailand, serious coastal erosion has occurred and breakwaters for preventing it have been constructed. Especially since many coasts in the gulf of Thailand consist of very fine bed materials, wave dissipation breakwaters made of bamboo piles and detached breakwaters made of geotextile bags which were packed bed materials have been constructed instead of concrete block breakwaters. However, these arrangement plans were based on empirical techniques. This research examines the validity of the effect evaluation method of the coastal erosion prevention using a numerical prediction model. Moreover, the comparison of the bamboo pile breakwater and the geotextile detached breakwater shows that the former can acquire sufficient effect for the erosion prevention with construction costs cheaper a little than the latter.

Transient dynamic response of solar diffusion driven desalination

The dynamic response for the solar diffusion driven desalination (DDD) process has been investigated. The heat and mass transfer within the direct contact evaporator is analyzed by one-dimensional conservation equations. The heat and mass transport models account for the transient variations within the packed bed due to time varying inlet water and air temperatures and humidity. The conservation equations are solved numerically using a finite difference scheme to predict water, air/vapor mixture and packed bed temperatures and humidity ratio within the evaporator and the condenser. The system dynamic response for the packed bed materials, wettability, and liquid hold-up, sudden increase–decrease in the inlet water temperature is investigated. It is found that the response time for the outlet water to reach the steady state temperature in the evaporator is shorter for lower liquid hold-ups. The response time is not sensitive to the packing material. Higher wettability does not affect the response time, but it does result in improved heat transfer with a higher exit air temperature and lower exit water temperature. The steady state temperature in the evaporator is not dependent on the heat capacity of the packing material, and liquid hold up. A delayed operating mode for the solar DDD is introduced to enhance the fresh water production rate. It is found that increasing the initial saline water temperature and decreasing the initial water volume in the seawater tank results in a higher fresh water production rate.

Multistep flow synthesis of vinyl azides and their use in the copper-catalyzed Huisgen-type cycloaddition under inductive-heating conditions.

The multistep flow synthesis of vinyl azides and their application in the synthesis of vinyltriazoles is reported. The synthesis relies on a stable polymer-bound equivalent of iodine azide that serves to carry out 1,2-functionalization of alkenes in a telescope flow protocol. The intermediate 2-iodo azides are subjected to a DBU-mediated polymer-supported elimination step yielding vinyl azides in good yield. The third step involves the formation of vinyl triazoles by a copper-catalyzed Huisgen-"click" cycloaddition. The required heat is generated by electromagnetic induction based on copper. Copper serves both as heatable as well as catalytically active packed-bed material inside the flow reactor.