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The Calcium-looping (CaL) technology is based on the reversible carbonation reaction of CaO with CO2 and has been developing rapidly as a potential low cost process for post-combustion CO2 capture. One of the main challenges of CaL process, is the identification of CaO-based sorbents that can maintain their activity in multiple cycles and have proper fluidization. The present study focusses on the influence of Zr, Al and Ce additives in mono-, bi- (Zr/Ce, Zr/Al and Al/Ce) and tri-metallic (Zr/Al/Ce) configurations on the performance of synthetic CaO-based sorbents. The developed sorbents presented a substantially improved performance over multiple cycles compared to limestone-derived CaO, due to the formation of stable, mixed Ca-inert phases. Incorporation of Al, Al/Ce and Zr/Al/Ce in CaO structure led to the development of sorbents with small crystallites and porous morphology, resulting in improved stability with less than 25 % deactivation after 50 cycles. The enhanced stability of the Zr/Al/Ce-promoted sorbent was attributed to the formation of CaZrO3 and Ca3Al2O6 mixed phases in the CaO matrix, providing a porous and stable structure. The fluidizability of the most promising sorbents was also investigated. SiO2 nanoparticles were mechanically mixed with the sorbents in different fractions to improve their fluidization performance. A minimum amount of 15 wt% SiO2 was required in order to achieve a completely smooth and homogeneous fluidization without gas channeling, bubbles or even fine agglomerates. The highest bed expansion, accompanied by a high Richardson-Zaki index n (5.9) and low Umf (2.2 cm/s) were obtained with a Ca/Zr/Al/Ce +15 wt% SiO2.

Natural clinoptilolite (Clin) was modified with iron oxide using three different methods including precipitation, wet-impregnation and ion-exchange and then the modified adsorbent with highest As(V...

Novel magnetic adsorbents of Clinoptilolite/Fe3O4 (Clin/Fe3O4) nanocomposite powders and Alginate/Clinoptilolite/Fe3O4 (Alg/Clin/Fe3O4) nanocomposite granules were synthesized to efficient removal of the cationic crystal violet (CV) dye in single and CV/methylene blue (MB) binary systems. The resulting adsorbents were characterized by FTIR, XRD, SEM, EDX, dot mapping, and BET analysis. Adsorption tests showed that Clin/Fe3O4 and Alg/Clin/Fe3O4 presented an excellent high affinity to CV removal as 94.32% and 92.35%, respectively in the single system and as 84.19% and 80.23%, respectively in the binary system. Based on the findings, pH was the most effective variable in the elimination process and the highest yield achieved at pH 8 for both adsorbents. The equilibrium data followed the Langmuir model (R2 > 0.9) based which the maximum adsorption capacity (qmax) using Clin/Fe3O4 and Alg/Clin/Fe3O4 was determined as 44.662 mg/g and 16.528 mg/g, respectively in the single system and it was computed to be 15.797 mg/g and 11.476 mg/g, respectively in the binary system. The kinetic data fitted well to the pseudo-second-order model in both systems. The adsorption process was exothermic and thermodynamically spontaneous. Moreover, the adsorbents showed excellent recycling ability over the desorption tests, thus leading to a promising and green-based adsorbents for the removal of dyes. The electrical conductivity (EC) of MB, CV, and CV/MB solutions and also a real wastewater sampled from a dyeing unit were measured before and after treatment and the results showed that the EC of all samples was reduced.

The present investigation concentrated on the configuration of a novel synthetic adsorbent via the sol-gel auto-combustion method through incorporating ZrO2, Al2O3, and CeO2 simultaneously during the development of CaO adsorbent. The impacts of disparate calcium precursors and molar ratios of calcium precursors to promoters were also studied. The adsorbent developed from soluble calcium nitrate precursor showed the higher surface area and smaller CaO grain size accompanied by a more enhanced capture capacity and CaO conversion at the mild and realistic process conditions during the carbonation-calcination cycles. Among synthesized adsorbents with disparate the molar ratio of Ca/(Ze + Al + Ce) (calcium precursor: promoters), the prepared sample with the molar ratio of 20/1 (CN/20) indicated the most promising performance with the best morphological properties, higher sorption capacity, and CaO conversion in comparison with 15/1 (CN/15) and 30/1 (CN/30) during 18 carbonation-calcination multiple cycles under the realistic condition. The most highlighted result of this work was that cyclic performance of modified adsorbents is linearly proportional to their morphological properties, in that CN/20 with the largest free surface area of 31.3 m2/g captured 4.23 g CO2/gr adsorbent, 9.58 % and 11.6 % higher than that captured by CN/15 and CN/30, respectively, during 18 realistic carbonation/calcination cycles.

Hydrophilic (polar-P) and hydrophobic (apolar-A) SiO2 nanoparticles were used as assistant materials to improve the fluidizability of Al2O3 and TiO2 nanopowders, which are hard to fluidize normally. To decrease the strong electrostatic forces, binary mixtures prepared with SiO2(P) were fluidized in the presence of suitable alcohol vapors. Results showed that the amount of SiO2 nanoparticles, varying from 5 to 50 wt%, mostly had a beneficial effect on the fluidization quality of the binary mixtures in both cases. TiO2 and Al2O3 nanoparticles combined with 20 wt% SiO2(A) showed almost equal performance in terms of fluidization with the mixtures containing only 5 wt% SiO2(P). This behavior can be addressed by better material homogeneity of latter mixtures which led to a homogeneous, smooth and stable behavior with desirable bed expansion. By comparing the results obtained in this work with those available in the literature, it is proposed that physical mixing with SiO2(P) NPs to improve the flowability of Al2O3 and TiO2 hard-to-fluidize nanoparticles, may be comparatively more efficient than even some of the external methods such as acoustic field or mechanical vibration.

The adsorption capacity and fluidization behavior of silica-coated amine-functionalized multi-walled carbon nanotubes (MWCNTs) were studied in this work. The coupling of both studies is fundamental for the practical application of CO2 sorbents in fluidized beds. Sorbents were prepared by hydrolysis of tetraethylorthosilicate (TEOS) onto acid-treated multi-walled carbon nanotubes followed by impregnation with monoethanolamine (MEA). The effects of different loading amounts of MEA and TEOS on CO2 uptake were investigated using thermogravimetric analysis under a simulated flue gas composed of 15%CO2/85% N2. At the optimal loadings of TEOS (30 ml) and MEA (30 wt%) on MWCNTs, 2.67 wt% of CO2 was captured at 25 °C, while the adsorption capacity of the pristine MWCNTs before any modification was negligible. Adsorption/desorption tests repeated 25 times also showed that the prepared sorbent has excellent stability during the cyclic operation; 9.42% reduction of the uptake was reported. Fluidizability of the most promising sorbent was also assessed when adding hydrophobic silica nanoparticles (NPs). Maximum bed expansion of 3.9 was achieved in the presence of 7.5 wt% silica NPs which was comparable by the amount of 2.24 belonged to raw MWCNTs. Enhancement of fluidity may be attributed to the decreased van der Waals attraction forces between the particles of sorbent. Besides, the CO2 adsorption capacity of the sorbent mixed with 5 wt% silica NPs was surprisingly increased by approximately 8%, reaching ~3 wt% of CO2.

Natural clinoptilolite (Clin) was modified with iron oxide using three different methods including precipitation, wet-impregnation and ion-exchange and then the modified adsorbent with highest As(V) removal efficiency was encapsulated into Alginate by a simple cross-linking method to obtain Fe-Clin granules. The surface morphology and chemical composition of the Fe-Clin sorbents were characterized by scanning electron microscope and X-ray diffraction analysis. The selected Fe-Clin powders and granules possessed enhanced affinity towards the highly toxic arsenic pollutant in a very short time. Batch adsorption experiments showed that the Fe-Clin adsorbent can be widely used within a wide range of pH (2-9). In addition, to reach a high removal percentage (over 90%) of As(V), the optimum dosage of powder and granule shaped adsorbents was obtained as 0.1 and 0.6 g L

CaO-based materials have been identified as promising sorbents for highly efficient pre-combustion and post-combustion CO2 capture in fluidized beds operated at high temperatures by means of the Calcium Looping (CaL) process. However, Ca-based sorbents suffer from a decline of the capture capacity over multiple sorption/desorption cycles, mainly due to sintering, and from a markedly heterogeneous fluidization behavior due to the strength of interparticle attractive forces as compared to particle weight. The present study is focused on the development of novel synthetic CaO/Al2O3 sorbents for CO2 capture with enhanced CaL performance and fluidizability by dry mixing with flow conditioner nanopowders. The influence of initial precursors on the sorbents multicycle activity at realistic CaL conditions has been investigated. The formation of a stable Ca9Al6O18 mixed-phase during the preparation of the sorbents promotes the multicycle capture capacity. The type of Ca and Al precursors, either soluble or insoluble, can significantly affect the dispersion of this stabilizer (Ca9Al6O18) in the sorbent matrix and, consequently, may affect the carbonation activity of the materials. The sorbent prepared from soluble aluminum nitrate and calcium nitrate precursors by sol-gel method exhibits a very stable multicycle capture capacity with a capture capacity around 0.2 g of CO2/g of sorbent after 21 cycles keeping a 72% of its initial capture capacity. The fluidizability of this promising sorbent was also investigated as affected by the addition of three different flow conditioners. Fluidization experiments confirmed the positive effect of using hydrophilic alumina and hydrophobic silica nanoparticles on improving the fluidizability of the synthesized sorbents.

Hydrophilic silica nanoparticles were used to enhance the fluidization behavior of Ca(OH)2 powder and its CO2 adsorption. The strong electrostatic forces were decreased by fluidizing adsorbents modified by hydrophilic SiO2 in the presence of alcohol vapors. A Taguchi experimental design was used to identify the optimal characteristic parameters of the modified adsorbents to improve their fluidization. Four parameters, including SiO2 mass percentage, type of alcohol, sieved size of SiO2, and sieved size of Ca(OH)2 particles, were selected for conducting experiments in a fluidized bed. Analysis of variance showed that sieved size and mass percentage of SiO2 nanoparticles (accounting for 73.88% and 19.01% of the total contributions, respectively) were the most significant parameters determining fluidization quality of the modified adsorbents. Fluidization experiments confirmed the effectiveness of these two parameters. Based on bed expansion results, fluidization of Ca(OH)2 modified by hydrophilic silica nanoparticles in the presence of alcohols was considerably better than that modified by hydrophobic nanoparticles. CO2 adsorption tests, which were carried out by measuring the pH variation of pure water during adsorption, revealed that improved Ca(OH)2 fluidization enhanced the carbonation reaction.

A method is developed for hydrodynamics scale-up of gas-solid fluidized beds based on recurrence quantification analysis of nonlinear time series of pressure fluctuations. This method is an improvement of the previous method by including the entropy of pressure fluctuations to the list of scale-up parameters. Experiments were carried out at varying conditions, e. g., bed diameter (5, 9, 15, 40 and 80 cm ID), particle size (150, 300, 400 and 600 μm), bed height at aspect ratios (1, 1.5 and 2) and superficial gas velocities (ranging 0.1 to 1.7 m/s) to identify the main parameters that influence the dynamics and to develop a general interpretation of the analysis results. By investigation of the effect of operating parameters on entropy, a quantitative empirical correlation is proposed for including the entropy in the scale-up parameters. It was shown that this correlation improves the Glicksman’s method for the scale-up of fluidized beds.

In this study, TiO 2 nanotubes were synthesized via anodic electrochemical oxidation method and the effects of different parameters such as applied voltage, anodization time, electrolyte solution temperature and pH on the outer diameter and length of TiO 2 nanotubes in a NH 4 F/C 2 H 6 O 2 /H 2 O electrolyte system were investigated. Based on the experiments, length and outer diameter of the synthesized nanotubes were in the range of approximately 9–71 μm and 81–171 nm respectively which were found by scanning electron microscopy (SEM) analysis technique. Also a generalized theoretical model was developed to predict the morphology (length and outer diameter) of TiO 2 nanotubes prepared by anodization method. The collected experimental data based on classic design was used to obtain adjustable parameters in the presented model. Photocatalytic activity of different lengths TiO 2 nanotubes were evaluated in terms of the degradation of formic acid in aqueous solution under ultraviolet (UV) light irradiation. Results showed that the optimal tube length would be an important factor to achieve the good photocatalytic performance of TiO 2 nanotube arrays. The highest photocatalytic performance was found for the synthesized TiO 2 nanotubes with 46 μm length.

A comprehensive model was derived based on the balance between drag, collision, and gravity as separation and van der Waals and hydrogen bond as adhesion forces to estimate the equilibrium size of agglomerates formed during the fluidization of nanoparticles. Due to the approximately less than 9% of the total amount of forces, drag and collision forces were not considered in the final model. Also, the influence of using the vapor of different alcohols on the fluidization behavior of hydrophilic silica and alumina nanoparticles was studied by experiments. To justify the improving effect of using alcohols, the electrostatic repulsion force was added to the model for the first time. Methanol and 2-propanol were the most effective alcohols on fluidization improvement, and consequently the smallest size of agglomerates was estimated using physical properties of these two alcohols. The Richardson–Zaki (R-Z) analysis indicated that the fluidization degree of cohesive hydrophilic nanoparticles can be greatly impro...

This manuscript presents a general model for the fluidized bed carbonator reactor of the Calcium Looping (CaL) process based on bed hydrodynamics, kinetics and sorbent properties. The model proposed relies on the Kunii–Levenspiel theory for circulating fluidized beds in the fast fluidization regime and considers recent findings on the CO2 capture properties of CaO, including the effects of the diffusion-controlled carbonation stage and CaO modification by means of additives. By performing a sensitivity analysis, it is found that the solids inventory in the carbonator and superficial gas velocity are the main operational and hydrodynamic factors affecting the CO2 capture efficiency. Decay constant of the fraction of solids in the lean region and fraction of solids in the dense region are also identified as two important model parameters. Model predictions are shown to agree with experimental results retrieved from pilot-scale tests. The main novelty of the model is the consideration of a CaO/Al2O3 composite either as main CO2 sorbent used in the carbonator or as a make-up flow fed into the calciner. Using CaO/Al2O3 as the main CO2 sorbent leads to a 14.17% loss of capture efficiency after 100 cycles compared to 72.81% when using limestone derived CaO. It is demonstrated that feeding a very low ratio (~0.1) of the CaO/Al2O3 composite or a rather high ratio (~5) of limestone as makeup flow, leads to a significant improvement of the capture efficiency. The results obtained may be relevant for optimizing CaL operation conditions to be used in power plants.

Multi-scale analysis and non-linear analysis were combined to investigate the hydrodynamics of fluidized beds with and without horizontal tubes. Pressure fluctuations were measured and analyzed employing discrete wavelet analysis, recurrence plot analysis, and recurrence quantification analysis. A systematic procedure was followed to determine wavelet parameters. At low gas velocities, the energy of macro-structures reduces with the addition of the first tube and then increases with the addition of a second tube. However, there is no notable difference at high gas velocities. Determinism is high for the bed without tubes, which can be attributed to the periodic behavior of bubbles. Determinism decreases with the addition of tubes because the breakage of bubbles results in less periodic behavior. The three methods of analysis used in this study captured the effects of immersed tubes on the hydrodynamics of fluidized beds. Recurrence quantitative analysis was found to be a powerful and easy-to-use method that can capture the nonlinear characteristics of fluidized beds much more quickly than conventional methods of nonlinear analysis. This method can thus be effectively used for the online monitoring of hydrodynamic changes in fluidized beds.

This paper reports the development of nonlinear time series analysis technique based on recurrence quantification analysis (RQA) method to characterize the hydrodynamic of gas–solid fluidized beds and a comparison with the obtained results by wavelet transform (WT) analysis method is made. An experimental work has been carried out at varying conditions, e.g. bed diameter (5, 9, 15 cm ID), particle size (150, 300 and 600 μm), bed height at aspect ratios (1, 1.5 and 2) and superficial gas velocities (ranging 0.1–1.7 m/s). Both methods show that by using larger particles and higher aspect ratios, the contribution of macro structures increases in the system. By increasing the gas velocity, finer structures in the bed first lose their contribution and after passing a transition velocity (of about 0.3, 0.5 and 0.7 m/s for sands with mean diameters of 150, 300 and 600 μm respectively) their contribution increases again. While the frequency domain analysis is not sensitive to the effect of scale; the RQA method shows an increase in meso structure contribution by increasing of the bed diameter.

The dynamic features of an agglomerate bubbling fluidization of nanoparticles were investigated through the analysis of pressure fluctuations. Experiments were carried out in a lab-scale fluidized bed at ambient conditions using 10–15 nm silica nanoparticles without any surface modification. Pressure fluctuation signals were processed in both frequency and time-frequency domains to characterize the behavior of various scales of phenomena (i.e., macro-, meso-, and micro-structures) during fluidization. Due to the aggregation of nanoparticles, three separate broad peaks were observed in the frequency spectra of the pressure signals measured in the bubbling fluidized bed of nanoparticles. A non-intrusive method based on the decoupling of pressure fluctuations recorded simultaneously in the plenum and in the bed was used to determine the approximate size of the bubbles in the bed.

As a low-cost method, hydrophilic SiO2 nanoparticles (NPs) and fluid catalytic cracking (FCC) coarse particles were used as assistant materials to improve the fluidity of Al2O3 and TiO2 hard-to-fluidize nanopowders. To decrease the strong electrostatic forces between the hydrophilic nanopowders, prepared samples were fluidized in the presence of methanol vapor. Results revealed that the amount of SiO2 NPs, increased from 5 to 50 wt. %, has a beneficial effect on the fluidization quality of the binary (hard-to-fluidize NPs + SiO2) and ternary (hard-to-fluidize NPs + SiO2 + FCC) mixtures. However, the amount of FCC particles when it varied from 15 to 30 wt. % in the ternary mixtures should meet the optimal point, beyond which the fluidization quality was declined due to the segregation phenomenon. The laboratory results showed that the cost-effective ternary samples fluidized more homogeneously with higher bed expansions compared to the binary samples. In this regard, (Al2O3 + 20 wt. % SiO2) + 15 wt. % FCC and (TiO2 + 20 wt. % SiO2) + 15 wt. % FCC ternary samples were proposed as the alternatives of Al2O3 + 50 wt. % SiO2 and TiO2 + 50 wt. % SiO2 binary mixtures, respectively.

The effect of surface characteristics on the interaction between nanoparticles and their agglomeration in dense gas suspensions is still not fully understood. It is known that when the surface is covered with hydroxyl groups, the interaction between nanoparticles becomes substantially stronger than in the absence of these groups; this strengthening is typically attributed to the formation of capillary bridges between the particles. However, this work shows that part of the increase of the interaction is due to the direct hydrogen bonds formed between the surfaces of the polar particles. Dry nitrogen was used to fluidize polar (hydrophilic) and apolar (hydrophobic) SiO2, TiO2 and Al2O3 particles, with a size ranging from 13 to 21 nm. The dry polar particles showed smaller bed expansion and larger minimum fluidization velocity compared to their apolar counterparts, indicating stronger interparticle forces. The results show the importance of including the formation of hydrogen bonds in the modeling of the interaction between dry and polar nanoparticles.