Gas-solid Contact Research Articles

Hydrodynamics and catalytic reaction inside a novel multi-regime riser

Hydrodynamics and performance of a novel multi-regime riser reactor, combined with a dense-phase turbulent bottom region and a dilute upper region, were studied in this work. Systematic research into gas–solids flow patterns indicated that, increased solids concentration, improved solids distribution and intensified gas–solids fluctuation over the conventional riser were achieved in the diameter-enlarged bottom section of this novel riser. Subsequently, catalytic ozone decomposition has been introduced to investigate the reactor performance. Compared with the conventional riser, where the reaction mainly occurred near the wall, notable ozone decomposition in the column center was representative for the multi-regime riser, consequently leading to a more uniform radial ozone concentration profile. Although ozone conversion was enhanced in the multi-regime riser for increased solids concentration and prolonged gas residence time, axial ozone concentration profiles in both riser reactors still deviated obviously from the plug-flow behavior. However, resulting from the improved gas–solids flow structure and strengthened inter-phase mass transfer behavior over the conventional riser, higher gas–solids contact efficiency was obtained for this novel riser.

Flow regime identification in a novel circulating-turbulent fluidized bed

Abstract Whether flow dynamics in a novel circulating-turbulent fluidized bed (C-TFB) belongs to a new flow regime or just a transition state is controversial. In order to resolve this confusion, transient fluctuation signal processing and wavelet analysis have been used to identify the flow regimes in five specially designed flow systems, which have been proven to be effective tools to extract detailed information of flow features in different flow regimes. Furthermore, the fuzzy system identification method, which was used to process wavelet energy data of various scales obtained by wavelet analysis, was employed to make a quantitative distinction among the flow regimes. The results showed that the flow regime in the C-TFB is a novel flow regime, which is different from traditional flow regimes such as bubbling, turbulent, fast fluidization (FF) and dense suspension up-flow (DSU) regimes. The new regime has some unique characteristics including intensive gas–solids interaction, high gas–solids contact efficiency, two peaks of probability density distribution (PDD) curves in the center of the fluidized bed, broader spectrum of solids holdup for PDD curves, a decreasing extent of gas–solids separation extent and unique radial detail wavelet energy percentage profiles. Quantitative analysis by the fuzzy system identification method further confirmed that the flow behavior in the C-TFB can be described as a novel flow regime.

Experimental Study and Modeling on Effects of a New Multilayer Baffle in a Turbulent Fluid Catalytic Cracking Regenerator

A new multilayer baffle for intensifying the catalyst regeneration process of a dense bed fluid catalytic cracking (FCC) regenerator was proposed and its effects on hydrodynamics and gas/solids mixing in fluidized beds of FCC particles were investigated experimentally in a large cold fluidized bed model. Experimental results show that, due to scale-up effects and serious gas bypassing, the new baffle was more effective in improving gas–solids contact as indicated by the weaker pressure fluctuations, increased bed expansion, and reduced freeboard particle carryover measured in the baffled fluidized bed (BFB) of this study. Compared with baffle-free fluidized bed (FFB), the new baffle can reduce internal gas circulation flux by 89–96%, indicating stronger suppression on gas/solids backmixing. An established baffled FCC regenerator model shows that, in a regenerator with this new baffle, cleaner regenerated catalysts can be obtained with less air consumption and catalyst inventory. Moreover, the modeled resu...

Performance Comparison of Moving and Fluidized bed Sorption Systems for an Energy-efficient Solid Sorbent-Based Carbon Capture Process

Abstract As a future post-combustion CCS technology, the solid sorbent-based process is extensively researched recently because lower energy penalty is expected thereof compared to that of the conventional amine scrubbing process. The type of gas-solid contactor of solid sorbent-based process has considerable impact on overall performance but systematic comparison of the different designs has not been reported yet. In this work, various sorption systems with different gas-solid contact types including i) moving-, ii) bubbling-, and iii) fast fluidized-bed using absorbents of amine supported sorbent are established. Mathematical models of each sorption system are developed from first principles and compared in terms of economy indicators. From the results, an optimal sorption system is discussed and a research direction of process design is suggested to improve the process economy of the solid sorbent-based carbon capture processes.

CO2 Sequestration by Direct Dry Gas-Solid Contact of Serpentinite Mining Residues: A Solution for Industrial CO2 Emission

Direct dry gas-solid carbonation is a simple approach towards mineral carbon dioxide sequestration. The route theoretically implies the direct reaction of CO₂ with silicates of Calcium and Magnesium in dry condition to form stable, insoluble metal carbonates. The mining regions of southern Quebec have a large deposit of serpentinite residues. The current study examines the suitability of serpentinite mining residues to use as the feedstock material for mineral carbonation. The focus of the present work is to assess the CO₂ removal efficiency of the residue from a simulated flue gas mixture of a typical cement plant (18 Vol% CO₂). This approach avoids the requirement of separate CO₂ capture and pre-concentration prior to mineral carbonation. The reaction parameters considered are temperature, pressure and time. The optimization of parameters is carried out for the maximum CO₂ removal efficiency (%) from the feed gas. Operating condition for CO₂ removal is optimized at 258 °C, 5.6 barg (pCO₂ ≈ 1) for 310 minutes with a removal efficiency of 37%. Preliminary analysis of reacted solid indicates carbonation is null at optimum condition, nevertheless, CO₂ is depleted from the feed gas might be due to a reversible adsorption. The study also checks the importance of pre-treatment options such as grinding, magnetic separation and heat treatment on CO₂ removal. A separate optimization study is carried out for magnetic separation of serpentinite residue and the separation parameters are optimized at an initial pulp density of 40% and magnetic intensity of 7.5*10⁻³ T with about 70% of iron oxide removal from the initial feed.

Operating and Peak Pressure Drops in Conical Spouted Beds Equipped with Draft Tubes of Different Configuration

Operating and peak pressure drops have been studied in conical spouted beds equipped with draft tubes of different configuration using materials of different size. The draft tubes used are nonporous and open-sided ones. Based on an experimental design, the factors of greater influence on both pressure drops have been determined and general correlations valid for fine and coarse materials have been proposed for predicting the operating pressure drop and peak pressure drop with each type of draft tube. The results show that conical spouted beds equipped with draft tubes give way to lower operating and peak pressure drops than those without draft tube. A comparison of draft tube systems reveals that nonporous tubes give way to the lowest values of both pressure drops, but the solid circulation rates and the gas–solid contact efficiency are poorer than those with open-sided draft tube systems.

Experimental Studies on a Newly Developed Mixed-Flow Dryer

Mixed-flow dryers are of great importance in worldwide agriculture for the drying of grain, corn, and rice. Unfavorable dryer designs can result in uneven particle and air flow distributions and, thereby, cause inhomogeneous gas–solids contact and drying conditions. As a consequence, the grain drying can locally be very uneven with high fluctuations of the moisture distribution over the dryer cross section. The main reasons are design and construction of the dryer apparatus and the discharge device. A new mixed-flow dryer design has been developed that promises more homogeneous drying, higher energy efficiency, and increased product quality. Firstly, the new dryer design was proved with respect to particle flow. For this purpose, a new test dryer was constructed. A series of particle flow experiments was performed using colored tracer particles. The flow of the tracer particles was observed through a transparent acrylic front wall by image analysis. Based on a comparison with the traditional design, the advantages and disadvantages of the new design were evaluated. The experimental investigations were accompanied by numerical simulations of the particle flow pattern using the discrete element method. The effects of design properties and different air duct arrangements were studied. The present results show that we are at the beginning of a new development concerning the optimization of mixed-flow drying apparatuses.

CO2 capture performance of HKUST-1 in a sound assisted fluidized bed

Among the CCS technologies, adsorption processes are attractive due to their low energy requirements, stimulating recent research to find suitable and highly specific adsorbents for removing CO2 from flue gas. Much attention has been focused on metal–organic frameworks (MOF), a new class of microporous materials that have potential applications in separation processes. As regards the handling of such fine materials, sound-assisted fluidization has been indicated as one of the best technological option to improve the gas–solid contact by promoting a smooth fluidization regime. The present work is focused on the CO2 capture by sound assisted fluidized bed of a specific MOF, HKUST-1. Tests have been performed in a laboratory scale experimental set-up at ambient temperature and pressure, pointing out the effect of sound parameters (intensity and frequency) and CO2 partial pressure. Effectiveness of CO2 adsorption has been assessed in terms of the moles of CO2 adsorbed per unit mass of adsorbent, the breakthrough time, the adsorption rate and the fraction of bed utilized at breakpoint. The results show the capability of the sound in promoting a more efficient adsorption process. Finally experimental tests have been carried out to find a possible regeneration strategy of the sorbent. The stability of the material has been assessed performing different chemico-physical characterizations (BET, XRD, TG, FT-IR and granulometric distribution) on a sample of HKUST-1 subjected to 10 CO2 adsorption/desorption cycles.

The design of cyclonic pre-heaters in suspension cement kilns

Cement manufacturing consumes two main types of energy: fuel and electricity. On average, energy costs represent 40% of the total production costs per ton of cement. The challenge is to reduce the consumption of energy to about 3000 MJ/ton clinker without the consumption of massive additional amounts of electricity, which is normally associated with additional fuel-saving measures. This can only be achieved by implementing sound thermal energy optimization measures. Energy-efficient suspension cement kilns are now widely applied and use a cascade of cyclonic pre-heaters of moist particulate feedstock, with heat transfer from hot kiln exhaust gas to particles being a function of heat transfer coefficient, temperature difference and gas–solid contact mode and time. The gas–solid contact mode and time depend on particle movement in cyclones, which has previously been studied by positron emission particle tracking. Heat transfer is governed by Nusselt–Reynolds equations, with gas velocity and properties being a function of the temperature profile along the cascade of cyclones. A stepwise approach of the design thus combines changing hydrodynamics and heat transfer along the successive cyclones, with the overall thermal balance of the cascade as control. This approach leads to major design recommendations, as developed in the present paper.

Influence of ring-type internals on the solids residence time distribution in the fuel reactor of a dual circulating fluidized bed system for chemical looping combustion

The intensification of gas-solids contact in the fuel reactor of a chemical looping combustion system is enhanced with the installation of ring-type internals. This can be a key issue for achieving the necessary fuel conversion rates. Wedged rings, previously designed and tested, were found to increase the particle concentration in the counter current section of the fuel reactor and hence, to achieve a more homogeneous particles concentration along this zone. The present work investigates the effect of the mentioned internals on the residence time distribution of particles in the fuel reactor of a dual circulating fluidized bed system for chemical looping. The study was carried out in a cold flow model especially designed for the fluid-dynamic analysis of the system equipped with a recently developed residence time measurement device based on the detection of ferromagnetic tracer particles through inductance measurement. Ring internals proved the positive effect on the particles residence time, the residence time distribution is more symmetric and shows lower dispersion, the flow pattern is more plug-flow-like, these effects are intensified with the reduction of the aperture ratio of the rings. On the other hand, the upward particle transport in the counter-current zone of the fuel reactor also increases with the installation of the rings, increasing the bypass flow of solids through the fuel reactor's return loop (internal circulation). For high internal circulation rates the solids residence time distribution of the fuel reactor is dominated by the bypass effect. The findings may be used for focused design improvement of the investigated fluidized bed system.

CO2 adsorption on fine activated carbon in a sound assisted fluidized bed: Effect of sound intensity and frequency, CO2 partial pressure and fluidization velocity

Among all the CCS strategies, post-combustion capture provides a near-term solution for stationary fossil fuel-fired power plants, eliminating the need for substantial modifications to existing combustion processes and facilities. In this respect, adsorption using solid sorbents has the potential, in terms of energy saving, to complement or replace the current absorption technology. Therefore, the design of highly specific CO2 adsorbents materials is requested. In this framework, great interest is focused on nanomaterials, whose chemico-physical properties can be tuned at the molecular level. As regards the handling of such materials, sound-assisted fluidization is one of the best technological options to improve the gas–solid contact by promoting a smooth fluidization regime. The present work is focused on the CO2 capture by sound-assisted fluidized bed of fine activated carbon. Tests have been performed in a laboratory scale experimental set-up at ambient temperature and pressure, pointing out the effect of CO2 partial pressure, superficial gas velocity, sound intensity and frequency. Effectiveness of CO2 adsorption has been assessed in terms of the moles of CO2 adsorbed per unit mass of adsorbent, the breakthrough time and the fraction of bed utilized at breakpoint. The results show, on one hand, the capability of the sound in enhancing the adsorption process and, on the other hand, confirm that sound assisted fluidization of fine solid sorbents is a valid alternative to the fixed bed technology, which require also an additional previous step of pelletization.

Experimental Investigation on Flow Behaviors in a Novel In Situ Gasification Chemical Looping Combustion Apparatus

A novel cold reactor apparatus for in situ gasification chemical looping combustion (iG-CLC) is proposed and investigated. It is mainly comprised of a circulating fluidized bed (CFB) riser as the fuel reactor and a cross-flow moving bed as the air reactor. The direct hydraulic link between the two reactors brings inherent simplicity and stabilization of the whole system. The CFB fuel reactor provides favorable gas–solids contacts over the whole reactor height. The realization of high solids flux operation conditions greatly enhances the solids holdups, and the gas–solids contacts and reactions in the riser. The moving bed air reactor has advantages in terms of having a low pressure drop, continuous solids flow, and large gas–solids contact area except for the risk of plugging and particle leakage caused by excessive high cross-flow gas velocity. Independent pressure adjustments between the two reactors could control the gas flow direction and restrain the gas bypassing to ensure high CO2 concentration wit...

Numerical Modelling of the Moving Bed Inside the COREX® Melter-Gasifier

The COREX®-process is the first commercially operating smelting reduction process for the production of pig iron. For this reason, it can be seen as an alternative to the common industrial iron-making route via the blast furnace. For both, the COREX® melter-gasifier (MG) and the blast furnace, good gas-solid contact is essential for the stable and efficient operation of the process. Due to the very harsh conditions inside the MG, the possibilities for measurement of process conditions are very limited. Thus, detailed numerical models are required to gain additional understanding of the physical and chemical processes occurring within the MG.

Conversion of Woody Biomass Materials by Chemical Looping Process—Kinetics, Light Tar Cracking, and Moving Bed Reactor Behavior

In recent years, chemical looping has evolved into a promising technique for carbonaceous fuel conversion. The chemical looping technology using biomass material as feedstock could provide a process that is environmentally attractive and sustainable. The focus of this study is to examine the kinetics of biomass decomposition and to evaluate the feasibility of using biomass as a renewable source for chemical looping processes. A series of thermogravimetric experiments were carried out under inert and reactive gas environments to investigate the kinetics of the devolatilization and gasification stages of biomass decomposition. Since tar derived from biomass pyrolysis is a major concern for many biomass conversion techniques, the cracking of biomass derived light tar using iron oxide-based composites was also investigated in a fixed bed reactor. Furthermore, a bench scale moving bed reactor was used to study the feasibility of biomass chemical looping in both cocurrent and countercurrent gas–solid contact modes.

Assessment of technological solutions for improving chemical looping combustion of solid fuels with CO2 capture

The chemical looping combustion of solid fuels by in situ gasification (iG-CLC) has great potential to reduce the economical and energetic cost of CO2 capture for generating energy from coal. Previous studies have highlighted that a high CO2 capture rate can be reached, but incomplete combustion is predicted by theoretical models or obtained during experimental work. In this paper, a mathematical model for the fuel reactor and carbon stripper, validated through experimental results, is adapted to evaluate the relevance of several technological improvements in order to increase the combustion efficiency of the iG-CLC process. The technological options evaluated include increasing the gas–solid contact in the fuel reactor, incorporating a secondary fuel reactor, re-circulating exhaust gases to the fuel reactor or the carbon stripper, or feeding coal into the carbon stripper instead in the fuel reactor. Model simulations showed that the use of a secondary fuel reactor has the major impact by reducing the unburnt compounds in the CO2 stream. The origin of the unburnt compounds is determined from a thorough analysis of the results obtained during the evaluation of these technological options. Thus, a new arrangement of the iG-CLC reactors is proposed, one which would minimise the presence of unburnt compounds. In this concept, exhaust gases from the fuel reactor are sent to the carbon stripper. The oxygen demand for this concept is predicted to be ΩT=0.9%.

Enhancement of CO2 capture at Ca-looping conditions by high-intensity acoustic fields

The Ca-Looping (CaL) technology, based on a dual gas-fluidized bed system of CaO/CaCO3 particles operated at high temperature, is a viable technological process for highly efficient pre-combustion and post-combustion CO2 capture. In this paper we show a lab-scale experimental study on the carbonation/decarbonation of a fluidized bed of CaO particles at CaL conditions as affected by the application of a high-intensity acoustic field. The results obtained demonstrate that both carbonation and decarbonation are remarkably enhanced for sound intensity levels above 140dB and frequencies of about 100Hz. Fine particles (of size smaller than dp∼100μm) are entrained in the oscillating gas flow induced by an acoustic field of such low frequency, which yields a strong agitation of the bed and improves the gas–solid contact efficiency. On the other hand, an intense convection of gas flow (acoustic streaming) is generated on the surface of larger particles unmovable by the sound wave, which promotes the heat/mass transfer at the gas–solid boundary in this case. Either of these mechanisms, whose relative importance will depend on the average particle size and sound frequency, will contribute to increase the carbonation and decarbonation rates of CaO fluidized beds in the CaL technology.

Enhanced Methanation over Aerogel NiCo/Al2O3Catalyst in a Magnetic Fluidized Bed

CO methanation reaction over a nanosized NiCo aerogel catalyst (NiCo) for synthetic natural gas production was investigated in a magnetic fluidized bed (MFB) reactor. The results indicated that the NiCo catalyst showed poor fluidization behavior, while the fluidization quality of the catalyst can be greatly improved by introducing an axial uniform magnetic field. The catalyst in the MFB reactor afforded higher conversion of CO and higher selectivity and yield of CH4 than those in the conventional fluidized bed, which was attributed to the MFB reactor improved gas-solid contract efficiency and inhibited axial back-mixing of the product gas. The catalytic performance in the MFB reactor was quite stable during a 100 h reaction. Such good stability is attributed to the superior property of the catalyst and the improved gas-solid contact efficiency in the MFB reactor. Furthermore, the particle agglomeration was inhibited when the catalyst was explored to a magnetic assisted fluidized bed. The combination of magnetic nanosized catalyst and MFB intensification as a promising route was developed for synthetic natural gas production.

Gas–solid flow behavior and contact efficiency in a circulating-turbulent fluidized bed

Abstract Circulating-turbulent fluidized bed (C-TFB) was characterized by high solid holdup, homogenous axial and radial flow structure, no net downflow of solids and high contact efficiency in this study. These flow dynamic properties were mainly represented by solid holdup profiles, the identification of flow regime and cluster dynamics in a riser, 100–150 mm in diameter and 10.06 m in height. To quantify the effect of novel diameter-expanding structure on flow dynamics, a parameter (contact efficiency) was introduced firstly. The effects of gas–solid interaction on flow performance were investigated by a fiber-optic probe to detect solid holdup. CO2 tracer injection and sampling system were used to characterize the flow structure and define the contact efficiency. The experimental results showed that flow regime in C-TFB is belonging to dense riser upflow (DRU) or dense-suspension upflow (DSU) regimes and transient region disappears in axial position. Flow structure is different from previous studies about traditional circulation fluidized bed (CFB) due to the effect of expanding structure and ring-feeder internal. A flow model based on the profiles of solid holdup and CO2 tracer concentration was proposed to account for the gas–solid contact efficiency in the reactor. The contact efficiency in C-TFB is much higher than that of high-density circulating fluidized bed (HDCFB), which means C-TFB reactor would exhibit better performance to optimize the product distribution under the same operating conditions.

Numerical Investigation of Airflow in a Swirling Fluidized Bed

A numerical investigation via commercial Computational Fluid Dynamic (CFD) package, FLUENT, was carried out to investigate the aerodynamic characteristics of air flow in a Swirling Fluidized Bed (SFB). The study focused on the distributor blade configurations whereby the effect of number of blades and blade inclination in radial plane were investigated. Twelve configurations were simulated at 30, 45 and 60 blades for four radial inclinations, 0, 10, 13.5 and 15°. Two criterions were taken as performance characteristics to evaluate each configuration namely the uniformity of tangential velocity distribution and pressure drop. High uniformity at the distributor outlet will ensure uniform solid-gas contact in the actual system while low pressure drop is vital to minimize power consumption in the system. From the results, it was evident that the 30 blades with 10E radial inclination of distributor blade in plenum chamber offer the best configuration for actual application.

Dry gas–solid carbonation in fluidized beds of Ca(OH)2 and nanosilica/Ca(OH)2 at ambient temperature and low CO2 pressure

This work presents a study on the chemisorption of CO2 by a bed of Ca(OH)2 powder subjected to the flow of a dry CO2/N2 gas mixture (1vol.% CO2) at ambient temperature and atmospheric pressure. The amount of CO2 and vapor water in the effluent gas from the fluidized bed is analyzed by means of FTIR spectrophotometry. The results obtained indicate that, even in an almost dry atmosphere (RH⪕0.01%), CO2 capture in the fluidized bed occurs by chemisorption on Ca(OH)2, as inferred from the rise of vapor water at the end of the fast carbonation phase. The use of nanosilica as an additive increases the gas–solids contact efficiency and, therefore, enhances CO2 chemisorption on Ca(OH)2 particles. This process is initially activated by free molecular water physisorbed on the material and becomes autocatalyzed by water produced from Ca(OH)2 carbonation. Accordingly, the addition of hydrophilic nanosilica, capable of retaining higher amounts of free molecular water, yields a further enhancement of the CO2 fast sorption capacity.