Gas-solid Contact Research Articles

Calculation of the heat flux near the liquid–gas–solid contact line

The study deals with the heat and mass transfer process near the dynamic three-phase liquid–gas–solid contact line. The evaporating sessile water droplets on a horizontal heated constantan foil are studied experimentally. The temperature of the bottom foil surface is measured by an infrared scanner. To measure the heat flux density for the inaccessible part of the boundary by temperature measurements obtained for the accessible part, the well-known heated thin foil technique is applied. In contrast to the usual approach, the heat conductivity along the foil is taken into account. To determine the heat flux value in the boundary region, inaccessible for measurements, the problem of temperature field distribution in the foil is solved. From the point of mathematics, it is classified as the Cauchy problem for the elliptic equation. According to calculation results, the maximum heat flux density occurs in the region of the contact line and it surpasses the average heat flux from the entire foil surface by the factor of 5−7. The average heat flux density in the wetted zone exceeds the average heat flux density from the entire foil surface by the factor of 3−5. This is explained by heat inflow from the foil periphery to the droplet due to the relatively high heat conductivity coefficient of foil material, and high evaporation rate in the contact line zone.

Fluidized bed coupled rotary reactor for nanoparticles coating via atomic layer deposition.

A fluidized bed coupled rotary reactor has been designed for coating on nanoparticles (NPs) via atomic layer deposition. It consists of five major parts: reaction chamber, dosing and fluidizing section, pumping section, rotary manipulator components, as well as a double-layer cartridge for the storage of particles. In the deposition procedure, continuous fluidization of particles enlarges and homogenizes the void fraction in the particle bed, while rotation enhances the gas-solid interactions to stabilize fluidization. The particle cartridge presented here enables both the fluidization and rotation acting on the particle bed, demonstrated by the analysis of pressure drop. Moreover, enlarged interstitials and intense gas-solid contact under sufficient fluidizing velocity and proper rotation speed facilitate the precursor delivery throughout the particle bed and consequently provide a fast coating process. The cartridge can ensure precursors flowing through the particle bed exclusively to achieve high utilization without static exposure operation. By optimizing superficial gas velocities and rotation speeds, minimum pulse time for complete coating has been shortened in experiment, and in situ mass spectrometry showed the precursor usage can reach 90%. Inductively coupled plasma-optical emission spectroscopy results suggested a saturated growth of nanoscale Al2O3 films on spherical SiO2 NPs. Finally, the uniformity and composition of the shells were characterized by high angle annular dark field-transmission electron microscopy and energy dispersive X-ray spectroscopy.

Fluidization and drying of biomass particles in a vibrating fluidized bed with pulsed gas flow

Fluidization of biomass particles in the absence of inert bed materials has been tested in a pulsed fluidized bed with vibration, with the pulsation frequency ranging from 0.33 to 6.67Hz. Intermittent fluidization at 0.33Hz and apparently ‘normal’ fluidization at 6.67Hz with regular bubble patterns were observed. Pulsation has proven to be effective in overcoming the bridging of irregular biomass particles induced by strong inter-particle forces. The vibration is only effective when the pulsation is inadequate, either at too low a frequency or too low in amplitude. Drying of biomass has been carried out to quantify the effectiveness of gas pulsation for fluidized bed dryers and torrefiers in terms of gas–solid contact efficiency and heat and mass transfer rates. The effects of gas flow rate, bed temperature, pulsation frequency and vibration intensity on drying performance have been systematically investigated. While higher temperature and gas flow rate are favored in drying, there exists an optimal range of pulsation frequency between 0.75Hz and 1.5Hz where gas–solid contact is enhanced in both the constant rate drying and falling rate drying periods.

Fluidized‐Bed Chemical Vapor Deposition of Silicon on Very Dense Tungsten Powder

AbstractA very dense tungsten powder was coated by silicon from silane using the fluidized‐bed chemical vapor deposition process. A reactor of reduced diameter was developed in order to decrease the weight of powders treated. Results show that the fluidization of this powder is possible in this reactor, but with non‐optimal gas solid contact induced by the high powder density. Important disturbances of the fluidized‐bed temperatures appeared in the presence of silane, due to a partial bed defluidization related to the cohesive nature of the deposit. These disturbances are clearly exacerbated by the exceptional density of the particles. This is probably why a powder agglomeration has been unavoidable when the bed temperature was too low and/or the silane inlet concentration too high. However, a set of operating parameters could be found, allowing a uniform and continuous coating of the whole powder surface.

Optimization of in Situ Gasification Chemical Looping Combustion through Experimental Investigations with a Cold Experimental System

In situ gasification chemical looping combustion (iG-CLC) is a promising coal combustion technology for implementing CO2 capture with a low energy penalty. A novel iG-CLC cold experimental system was developed in the authors’ previous work (Ind. Eng. Chem. Res. 2013, 52, 14208). It mainly consists of a high-flux circulating fluidized bed (HFCFB) riser as the fuel reactor and a cross-flow moving bed as the air reactor. As an extension of that work, in this study, we further optimized the iG-CLC system by redesignung the air reactor to enhance the carrying capacity of the gas flow and developing a two-stage separation system by adding a second-stage cyclone to the original first-stage inertial separator. Stability in operation and flexibility in adjusting operating parameters were achieved with the improved system. In the riser (fuel reactor), higher solids fluxes and solids holdups were achieved, which should enhance the gas–solid contact and promote the complicated heterogeneous reactions. In the moving bed (air reactor), the carrying capacity of the gas flow was significantly enhanced, which should lead to a great increase in the system power capacity. The confirmation of the ability to control the gas flow directions in the two reactors means that the gas bypassing between the two reactors can be restrained so as to ensure a high CO2 concentration in the exhaust of the fuel reactor. The high global separation efficiency and selective separation efficiency of the new two-stage separation system for fine particles indicate that a high combustion efficiency of coal can be achieved with a hot iG-CLC system.

Flow dynamics and contact efficiency in a novel fast-turbulent fluidized bed with ring-feeder internals

The flow dynamics in a novel fast-turbulent fluidized bed (FTFB) with middle-upper expanding structure and two different ring-feeder internals (mixed and vortex ring-feeder) were studied to achieve a reduction in gasoline olefin production. Compared with a conventional circulating fluidized bed, the novel FTFB displayed unique characteristics and advantages. A higher solids holdup and more uniform solids holdup distribution existed in the diameter-expanding region, especially for the FTFB with vortex ring-feeder structure. A probability density distribution analysis indicated that the novel fluidized bed could reduce gas–solids segregation and enhance gas–solids interaction. A constant carbon dioxide tracer system was used to simulate the reactant gas distribution. The gas–solids contact efficiency was defined according to the solid dispersibility and the amount of gas covering the solid surface. Novel FTFB risers, especially those with vortex ring-feeders, have a much higher gas–solids contact efficiency than that of traditional risers.

Investigation of fluidization behavior of high density particle in spouted bed using CFD–DEM coupling method

CFD–DEM coupling simulation method is used to study the fluidization behavior of particle with different densities in the spouted bed. The simulation results show that the particle spouting behavior is incoherent and periodic in the stable spouting state. The cycle periods of the whole bed and single particle are obtained from particle trajectory and compared with experimental results. The relationships between the minimum spouting gas velocity, the steady pressure drop and the particle density are obtained. It can be found that the gas velocity range of stable spouting state expands when the particle density increases. The dual, single and multiple dominant frequencies are found in the stable spouting process of high density particle. Through detailed analysis on the mechanism of spouting dominant frequency, the flow pattern map with different superficial gas velocities and particle densities is given and can be divided into five regions: fixed bed with internal spouting, transitional stable spouting state with dual dominant frequency, main stable spouting state with single dominant frequency, transitional stable spouting state with multiple dominant frequency, and unstable spouting state. The stable spouting gas velocity is influenced by the particle density, while the dominant frequency of the particle spouting process will keep invariant even if the particle density increases, as long as Ug/Ums is not changed. The influence of particle density on the particle residence time distribution in the spout region, which can be used to characterize the gas solid contact efficiency, is also discussed based on simulation results.

Effect of acoustic field on CO2 desorption in a fluidized bed of fine activated carbon

Adsorption using solid sorbents has the potential to complement or replace current absorption technology, because of its low energy requirements. Among the commercially available adsorbent materials, attention is focused on activated carbons because they are easily regenerable by reason of their low heat of adsorption. These sorbents are generally available in the form of fine powders. Sound-assisted fluidization can process large amounts of fine powders, promoting and enhancing CO2 capture on fine sorbents, because it maximizes gas–solid contact. Temperature swing adsorption (TSA), consisting of inducing sorbent regeneration and CO2 recovery by appropriate temperature increase and gas purge, is one of the most promising techniques. This study investigates the CO2 desorption process by TSA in a sound-assisted fluidized bed of fine activated carbon. Desorption tests were performed under ordinary and sound-assisted fluidization conditions to assess the capability of sound to promote and enhance the desorption efficiency in terms of CO2 recovery, CO2 purity, and desorption time. The results show that the application of sound results in higher desorption rates, CO2 recovery and purity. Regular and stable desorption profiles can be obtained under sound-assisted fluidization conditions. This stability makes it possible to successfully realize a cyclic adsorption/desorption process.

Effect of operating conditions on the CO2 recovery from a fine activated carbon by means of TSA in a fluidized bed assisted by acoustic fields

CO2 temperature swing adsorption (TSA), consisting in adsorbing the CO2 and, then, inducing the sorbent regeneration and CO2 recovery by a temperature increase and gas purge, is a promising strategy for CO2 capture. With reference to the sorbent, commercially available adsorbent materials are generally available in the form of fine powders. A sound-assisted fluidized bed is capable of fully exploiting the potential and properties of fine sorbents, due to large gas–solid contact efficiency, high rate of mass/heat transfer and low pressure drops. This work is focused on the CO2 desorption process by TSA in a sound-assisted fluidized bed of fine activated carbon. The effect of desorption temperatures and N2 purge flow rate on the regeneration efficiency has been assessed in terms of CO2 recovery level and purity and desorption time. Both of them positively affect the desorption process in terms of enhanced desorption kinetics. Increasing temperatures also yield higher CO2 purities, whereas, no remarkable dilution effect has been observed when increasing the N2 flow rate. Finally, the activated carbon keeps its performances over 16 adsorption/desorption cycles, due to the stability of the regeneration process under sound-assisted fluidization conditions.

Kinetic aspects on ferric arsenate formation in a fix bed gas-solid reaction system

The fixation of arsenic contained in gases produced during pyrometallurgical processes by using solid ferric oxide was studied in the range 873-1073 K under different oxygen potential and solid aggregates porosities. Arsenic fixation on solid iron oxides is described by the pore blocking model under the studied conditions. The solid product of the reaction has a molar volume 3 times larger than the solid reactant causing fast decreasing of the inter-granular spacing. The activation energies of arsenic fixation reaction are 34.96 and 35.46 kJ/mol for porosities of 0.88 and 0.74 respectively, and for porosity of 0.55 the activation energy was 26.88 kJ/mol. These values of activation energy show that intra-pellets diffusion has an effect only in samples with 0.55 porosity. Minor sintering of particles was detected. Industrial application of the concept demands a reaction system, which in is required better gas-solid contact for attaining larger conversions.

Effects of Operating Parameters on Hydrodynamic Behavior of Spout-fluid Beds without and with a Draft Tube

The spout-fluid bed (SFB) is one of many modifications to conventional spouted bed, which is achieved by introducing an auxiliary gas flow into the annular region of a spouted bed through a distributor. Therefore, the SFB has better gas-solid contact and mixing, and reduces the risks of particle agglomeration and dead zone. In addition, the gas-solid contact and the solids circulation rate can be controlled more easily and flexibly by installing a draft tube in the bed (DSFB). In this study, the effects of operating parameters on flow characteristics of the SFB and the DSFB were investigated and they were compared, using the same semi-cylindrical column and the same particles. As the results, the flow regime maps were recognized based on pressure fluctuation data and the DSFB had wide range of operating conditions for stable spouting, comparing with the SFB. The minimum spouting gas velocity for stable spouting state (spouting with aeration) decreased with increasing the auxiliary gas velocity. Also, longitudinal and radial distributions of particle velocity in the spout and the annulus were obtained in detail for the stable spouting state by means of the particle image velocimetry (PIV). Particle velocities in the spout and the annulus were increased with increasing the auxiliary gas velocity. Moreover, the distributions of particle velocities in the DSFB showed that particles in the draft tube were accelerated remarkably and particles in the annulus descended uniformly with small velocity, comparing with the SFB.

Role of Acoustic Fields in Promoting the Gas-Solid Contact in a Fluidized Bed of Fine Particles

The present work is a review presenting the main results obtained by our research group in the field of soundassisted fluidization of fine particles. Our aim is to highlight the role of acoustic fields in enhancing the gas-solid contact efficiency, with specific attention to the phenomenological mechanism upon which this technique is based. In particular, the first section presents the characterization of the fluidization behaviour of four different nanopowders in terms of pressure drops, bed expansion, and minimum fluidization velocity as affected by acoustic fields of different intensity and frequency. The fluidization of binary mixtures comprising two powders is also investigated under the application of different acoustic fields and varying the amounts of the two powders. The second section focuses on the study of the mixing process between two different nanopowders both from a “global/macroscopic” and “local/microscopic” point of view and highlighting the effect of mixture composition, primary particles density and sound intensity. The last section presents a promising application of sound-assisted fluidization, i.e. CO 2 capture by adsorption on a fine activated carbon, pointing out the effect of CO 2 partial pressure, superficial gas velocity, sound intensity and frequency on the adsorption efficiency.

A review of heat transfer models of nanoporous silica aerogel insulation material

In this paper, the development of effective thermal conductivity models for nanoporous silica aerogel insulation materials is summarized. In the first part, heat transfer characteristics of nanoscale gaseous/solid/radiative heat transfer inside aerogel insulation material are introduced. Then, the existing theoretical models, empirical models, and numerical methods for the thermal conductivity of each heat transfer mode are reviewed and analyzed. After that, the advances in effective thermal conductivity models for the nanoporous silica aerogel materials as well as their composite insulation materials after the addition of opacifiers and reinforced fibers are presented. Next, the authors previous work is taken as an illustration of a procedure for establishing an effective thermal conductivity model of an aerogel insulation material from nano- to macro-scale. A brief introduction to the application of molecular dynamics in the simulation of aerogel materials is also given. In the end, further researchneeds are indicatedfor the nanoporous silica aerogel insulation materials. These include: the coupled heat transfer behavior at the gas–solid contact interface, the influence of scale/interfacial effects on the thermal conductivity of nanoscale solid particles, and the effect of different influencing factors on the total heat transfer performance. Such studies could provide help in accurately predicting the thermal conductivity as well as optimizing the insulation performance for the aerogel material and could provide guide for the design of new type aerogel insulation material which can be used in industry such as aeronautics and astronautics, thermal insulation, building, chemical industry and so on.

Investigation of a Coupled Fuel Reactor for Coal-Fueled Chemical Looping Combustion

This work presents an experimental investigation and modeling study of a fuel reactor designed for in situ gasification chemical looping combustion (iG-CLC). The fuel reactor is designed to couple a bubbling fluidized-bed reactor and a downer reactor. The downer reactor can enhance gas–solid contact in the freeboard region and eliminate the unburned gases from the fuel reactor. The fuel reactor consists of an annular bubbling fluidized bed and a center-located riser. Cold-model experiments were conducted. The solids circulation rate and gas bypassing between the annular fluidized bed and the riser were studied. Experiments in a hot downer reactor were performed, and a two-dimensional model was developed and validated with experimental data. The verified model shows that a downer reactor with proper solids dispersion can eliminate more than 78% of the fuel gas exiting the fuel reactor with an overall solids dropping rate of 8 kg/m2·s.

CO2 capture performances of fine solid sorbents in a sound-assisted fluidized bed

Post-combustion CO2 capture processes have the greatest near-term potential for reducing greenhouse gas emissions because these processes can be retrofitted to existing units, thus providing a quicker solution to mitigate CO2 environmental impacts. Among all the post-combustion technologies, adsorption processes on solid sorbents are attractive due to their low energy requirements. Great interest is focused on ultra-fine materials, whose chemico-physical properties can be tuned at the molecular level. In this framework sound-assisted fluidization has already been proved to maximize the CO2 adsorption capacity of fine sorbents with respect to common technologies, due to the higher exploitation of the exposed surface. The aim of the present work is, therefore, to compare the adsorption performances of different materials (two activated carbons, two zeolites and a metal organic framework) under sound-assisted fluidization conditions (140dB–80Hz) in order to maximize the gas–solid contact efficiency and, in turn, minimize the limitations to the intrinsic adsorption capacity of the sorbents. All the tests were performed at ambient temperature and pressure with values of CO2 concentration typical of flue gases (5–10vol.%). The different behaviors exhibited by the materials were explained on the basis of their textural properties. In particular, the microporosity falling in the range of 8.3–12Å strongly affects the CO2 adsorption performances under the investigated operating conditions.

Mathematical modeling of waste plastic pyrolysis in conical spouted beds: Heat, mass, and momentum transport

A mathematical model has been developed for unsteady-state operation of waste plastic pyrolysis in spouted bed reactors. Such a model is not yet available in the literature. This model predicts radial distributions of solid conversion, yields of individual gas products, solid and gas temperatures and velocity profiles inside the bed. To prevent application of a bulk of empirical equations for determining the hydrodynamic parameters, the momentum balance equations have been included in the model. The results predicted from the model have been compared with those determined experimentally by other researchers for pyrolysis of polystyrene in a spouted bed. A satisfactory agreement has been observed with a mean relative deviation of 0.8%. The model predictions for a scaled-up spouted bed demonstrates that due to the existence of an excellent gas–solid contact in spouted beds the limitations imposed by heat and mass transfer are minimized. In addition, the bed is maintained under isothermal condition. The inlet gas temperature highly affects the model's results in comparison with mass of initial plastic. The amount of inert particles used can be reduced as less as a stable spouting is preserved. A critical value exists for power intensity of heating elements to keep the isothermal conditions of spouted bed that should be carefully considered in design purposes.

Using a manganese ore as catalyst for upgrading biomass derived gas

Secondary catalytic tar cleaning has been evidenced as a promising technology for upgrading gas derived from biomass gasification. When applying this technology downstream a biomass gasifier, the tar fraction in the raw gas can potentially be reduced and the content of hydrogen be increased. In this work, experiments have been conducted in a chemical-looping reforming (CLR) reactor. The present reactor system features a circulating fluidized bed as the reformer section, which offers a higher gas-solid contact time than a bubbling bed configuration previously tested. All experiments were performed using raw gas from the Chalmers 2–4 MWth biomass gasifier as feedstock to the reactor system. The catalyst inventory consisted of a natural manganese ore, and its activity was evaluated at three different temperature levels—800, 850, and 880 °C—and with an oxygen content of 2.2 %, corresponding to a theoretical air-to-fuel ratio of 0.06. Experimental results showed that the manganese ore exhibits catalytic activity with respect to tar conversion, and a tar reduction of as much as 72 % was achieved at 880 °C. Moreover, this material showed high activity towards hydrogen production and overall, an interesting upgrading capacity toward this producer gas. An H2/CO ratio of nearly 3 in the produced gas can make this material potentially interesting for application in an SNG system. Regarding the analysis of the physicochemical characteristics, the material showed signs of agglomeration with traces of sand most likely resulting from previous sieving during particle preparation. Though, a positive aspect is that this occurred without impacting the catalyst activity.

Gas bubble behaviors in a gas–solid fluidized bed with an arch agitator

The performance of gas–solid fluidized bed reactors (FBRs) strongly depends on the behavior of gas bubbles. Mechanical agitation is widely used in industrial FBRs to enhance gas–solid mixing; however, its influence on bubble behaviors has been poorly studied, especially in experimental study. Pressure fluctuations in a 300mm inner diameter plexiglas column fluidized bed of Geldart D particles with an arch agitator were measured under various gas velocities and agitation speeds. The bubble sizes were evaluated by incoherent analysis of pressure fluctuations. The effects of mechanical agitation on axial bubble growth, bubble sizes, regime transition velocity and fines elutriation were comprehensively investigated. Results show that mechanical agitation promotes bubble growth at 1.7 Umf but reduces the bubble sizes with increasing gas velocities. The axial maximum bubble size with increasing gas velocity occurs at 1.2m/s without agitation and 1.0m/s with agitation speeds of 14 and 21rpm. Interestingly, mechanical agitation leads to a small increase in minimum fluidizing velocity and a slight decrease in critical turbulent velocity. Furthermore, the entrainment rate is reduced with mechanical agitation, due to the decreasing bed height and small fines ejection at the bed surface. The arch agitator helps to reduce the channel flow, enhance the gas–solid contact efficiency and improve the fluidization quality.

Capturing CO2 from ambient air using a polyethyleneimine–silica adsorbent in fluidized beds

Carbon Capture and Storage (CCS) uses a combination of technologies to capture, transport and store carbon dioxide (CO2) emissions from large point sources such as coal or natural gas-fired power plants. Capturing CO2 from ambient air has been considered as a carbon-negative technology to mitigate anthropogenic CO2 emissions in the air. The performance of a mesoporous silica-supported polyethyleneimine (PEI)–silica adsorbent for CO2 capture from ambient air has been evaluated in a laboratory-scale Bubbling Fluidized Bed (BFB) reactor. The air capture tests lasted for between 4 and 14 days using 1kg of the PEI–silica adsorbent in the BFB reactor. Despite the low CO2 concentration in ambient air, nearly 100% CO2 capture efficiency has been achieved with a relatively short gas–solid contact time of 7.5s. The equilibrium CO2 adsorption capacity for air capture was found to be as high as 7.3wt%, which is amongst the highest values reported to date. A conceptual design is completed to evaluate the technological and economic feasibility of using PEI–silica adsorbent to capture CO2 from ambient air at a large scale of capturing 1Mt-CO2 per year. The proposed novel “PEI-CFB air capture system” mainly comprises a Circulating Fluidized Bed (CFB) adsorber and a BFB desorber with a CO2 capture capacity of 40t-CO2/day. Large pressure drop is required to drive the air through the CFB adsorber and also to suspend and circulate the solid adsorbents within the loop, resulting in higher electricity demand than other reported air capture systems. However, the Temperature Swing Adsorption (TSA) technology adopted for the regeneration strategy in the separate BFB desorber has resulted in much smaller thermal energy requirement. The total energy required is 6.6GJ/t-CO2 which is comparable to other reference air capture systems. By projecting a future scenario where decarbonization of large point energy sources has been largely implemented by integration of CCS technologies, the operating cost under this scenario is estimated to be $108/t-CO2 captured and $152/t-CO2 avoided with an avoided fraction of 0.71. Further research on the proposed 40t-CO2/day ‘PEI-CFB Air Capture System’ is still needed which should include the evaluation of the capital costs and the experimental investigation of air capture using a laboratory-scale CFB system with the PEI–silica adsorbent.

Performance of polyethyleneimine–silica adsorbent for post-combustion CO2 capture in a bubbling fluidized bed

The high performance of polyethyleneimine (PEI)-based solid adsorbent for CO2 capture has been well recognized in thermogravimetric analysis (TGA) and small-scale fixed bed reactors through the measurements of their equilibrium capacities but has not been really demonstrated on larger scales towards practical utilization. In the present study, a laboratory-scale bubbling fluidized bed reactor loaded with a few kg adsorbent is used to evaluate the adsorption performance of PEI–silica adsorbent under different working conditions including with/without the presence of moisture, different gas–solid contact times, initial bed temperatures, and CO2 partial pressures. The adsorption capacities have shown a clear degradation tendency under dry condition. However, they can be stabilized at a high level of 10.6–11.1% w/w over 60 cycles if moisture (ca. 8.8 vol%) is present in the gas flow during adsorption and desorption. Breakthrough capacities can be stabilized at the level of 7.6–8.2% w/w with the gas–solid contact time of 13 s. The adsorption capacities for the simulated flue gases containing 5% CO2 are only slightly lower than those for the simulated flue gases containing 15% CO2, indicating that the PEI–silica adsorbent is suitable for CO2 capture from flue gases of both coal-fired and natural gas-fired combined cycle power plants. The exothermal heat of adsorption is estimated by the energy balance in the fluidized bed reactor and found to be close (within 10%) to the measured value by TG-DSC. The regeneration heat for the as-prepared PEI–silica adsorbent is found to be 2360 kJ/kgCO2 assuming 75% recovery of sensible heat which is well below the values of 3900–4500 kJ/kgCO2 for a typical MEA scrubbing process with 90% recovery of sensible heat.