Experimental investigation and CFD simulation of liquid–solid–solid dispersion in a stirred reactor

For understanding the monosodium aluminate hydrate crystallization from the supersaturated aluminate solution containing red mud as the leaching liquor of bauxite, the liquid–solid–solid dispersion of a simulant system, i.e. glycerite, red mud and sand, in a stirred reactor has been experimentally investigated as well as simulated using computational fluid dynamics model (CFD) for the first time. The computational model is based on the Eulerian multi-fluid model along with RNG k–ε turbulence model, where Syamlal–obrien-symmetric drag force model (Syamlal, 1987) of the inter-phase momentum transfer between two dispersed solid phases is taken into account. A good agreement is obtained between the experimental data of solid distributions and the simulation results in the flow fields of liquid–solid–solid as well as liquid–solid systems. The solid suspension qualities of both liquid–solid and liquid–solid–solid systems in the stirred reactors with and without draft tube were also studied in detail based on mixing time, the standard deviation of solid concentration proposed by Bohnet and Niesmak (1980), the flow pattern and power number. The influence of the interaction between two dispersed solid phases on the suspension of red mud is found significantly greater than that of sand. The holdup of sand below the impeller is considerably larger than that above the impeller and the red mud dispersion approaches homogeneous in the reactor. The mixing time of liquid–solid–solid suspension is longer than that of liquid–solid suspension under the same conditions, and the mixing times of both systems in the stirred reactor with draft tube are longer than that in the reactor without draft tube. Furthermore, the distributions of sand and red mud in the reactor with draft tube were found less homogeneous than those without draft tube in most cases.     

Experimental investigation on flow structures and mixing mechanisms of a gas–solid burner jet

The objective of this study was to investigate the flow structures and mixing mechanisms of a gas–solid two-phase jet from a burner employed in the utility boiler. Laboratory experiments were conducted in a 0.15 m internal diameter test facility. Gas–solid jet flow downstream of the burner exit was measured by a fiber optic probe. Local solid concentration and particle size distribution were obtained using the probe that was traversed over the cross-section of the jet. The effects of the side flow on the primary air gas–solid flow characteristics were also studied. The measurements showed the availability of fiber optic probe to investigate the gas–solid jet flow downstream the industrial burner nozzle with high solid concentration. Obvious slip between the gas phase and the solid phase in the gas–solid jet flow was found in the experiments. The side flow disturbed the solid concentration distribution in the jet, a more smooth solid concentration profile across the jet can be achieved after the side flow was introduced. The particle size distributions in the jet were similar with the distributions of the solid concentration, the particles with large diameters always appeared in the zones where the solid concentrations were high.     

Measurement of real-time flow structures in gas–liquid and gas–liquid–solid flow systems using electrical capacitance tomography (ECT)

The real-time cross-sectional distributions of the gas holdups in gas–liquid and gas–liquid–solid systems are measured using electrical capacitance tomography. For the gas–liquid system, air as the gas phase and both Norpar 15 (paraffin) and Paratherm as the liquid phases are used. Polystyrene beads whose permittivity is similar to that of Paratherm are used as the solid phase in the gas–liquid–solid system. The three-phase system is essentially a dielectrically two-phase system enabling measurement of the gas holdup in the gas–liquid–solid system independent of the other two phases. A new reconstruction algorithm based on a modified Hopfield dynamic neural network optimization technique developed by the authors is used to reconstruct the tomographic data to obtain the cross-sectional distribution of the gas holdup. The real-time flow structure and bubbles flow behavior in the two- and three-phase systems are discussed along with the effects of the gas velocity and the solid particles.     

Consolidation and characterization of highly dense single-phase Ta–Hf–C solid solution ceramics

Herein, Ta–Hf–C solid solution ceramics were consolidated from nano-scale Ta–Hf–C solid solution powders for the first time. Four different compositions (4TaC–1HfC, 1TaC–1HfC, 1TaC–3HfC, and 1TaC–4HfC) were prepared by hot-pressed sintering at 2100°C, 70 MPa pressure and a holding time of 30 minutes. The densification, formation of single-phase solid solution and mechanical properties of the samples were systemically investigated. Relative density >95% was achieved for all four compositions with some improvement when TaC content was increased. And the formation of single-phase Ta–Hf–C solid solution was strongly demonstrated by phase analysis and crystal measurement using XRD and TEM. A significant improvement of hardness up to ~30 GPa was achieved, which was much higher than that of pure TaC (18.9 GPa) and HfC (22.1 GPa), due to the high densification and solid solution strengthening mechanism.     

Wall-to-bed heat transfer coefficient in gas—liquid—solid fluidized beds

Wall to bed heat transfer has been studied in three-phase fluidized beds with a cocurrent up-flow of water and air. Six sizes of glass beads, two sizes of activated carbon beads and one size of alumina beads, varying in average diameter from 0.61 to 6.9 mm and in density from 1330 to 3550 kg/m³, were fluidized in a 95.6 mm diameter brass column heated by a steam jacket. Complementary heat transfer experiments have been performed also for a gas–liquid cocurrent column and liquid–solid fluidized beds. The wall-to-bed coefficient for heat transfer in the gas–liquid–solid fluidized bed is evaluated on the basis of the axial dispersion model concept. The ratio of the wall-to-bed heat transfer coefficient in the gas–liquid–solid fluidized bed to that in the liquid–solid fluidized bed operated at the same liquid flow rate is correlated in terms of the ratio of the velocity of gas to that of liquid and the properties of solid particles. A correlation equation for estimating the wall-to-bed heat transfer coefficient in the liquid–solid fluidized bed is also developed.     

Euler multiphase-CFD simulation on a bubble-driven gas–liquid–solid fluidized bed

An integrated flow model was developed to simulate the fluidization hydrodynamics in a new bubble-driven gas–liquid–solid fluidized bed using the computational fluid dynamic (CFD) method. The results showed that axial solids holdup is affected by grid size, bubble diameter, and the interphase drag models used in the simulation. Good agreements with experimental data could be obtained by adopting the following parameters: 5 mm grid, 1.2 mm bubble diameter, the Tomiyama gas–liquid model, the Schiller–Naumann liquid–solid model, and the Gidaspow gas–solid model. At full fluidization state, an internal circulation of particles flowing upward near the wall and downward in the centre is observed, which is in the opposite direction compared with the traditional core-annular flow structure in a gas–solid fluidized bed. The simulated results are very sensitive to bubble diameters. Using smaller bubble diameters would lead to excessive liquid bed expansions and more solid accumulated at the bottom due to a bigger gas–liquid drag force, while bigger bubble diameters would result in a higher solid bed height caused by a smaller gas–solid drag force. Considering the actual bubble distribution, population balance model (PBM) is employed to characterize the coalescence and break up of bubbles. The calculated bubble diameters grow up from 2–4 mm at the bottom to 5–10 mm at the upper section of the bed, which are comparable to those observed in experiments. The simulation results could provide valuable information for the design and optimization of this new type of fluidized system.     

SHS in the Nb–Si system: Influence of mechanical alloying

Influence of mechanical alloying (MA) on SHS in low-caloric Nb–2Si mixtures was explored as a function of MA time and Si content of green mixtures. The use of MA was found to extend the concentration limits for combustion synthesis in the Nb–Si system to the following range: between 43 wt % Si–57 wt % Nb and 10 wt % Si–90 wt % Nb. Due to MA, SHS in Nb–Si blends can be carried out in a mode of solid–solid or liquid–solid reaction.     

Influence of solid content on bioleaching of heavy metals from contaminated sediment by Thiobacillus spp

The bioleaching process is one of the promising methods for removing heavy metals from contaminated sediments. In this study the effects of sediment solid content on the performance of the bioleaching process using a mixed culture of two sulfur‐oxidizing bacteria were investigated. The results showed that the rate of pH reduction decreased with increasing sediment solid content because of the buffering capacity of sediment solids. It was found that there was a linear relationship between buffering capacity and sediment solid content. For different solid contents (10–100 gdm⁻³), 82–95% (w/w) of Cu; 58–70% (w/w) of Zn; 55–73% (w/w) of Mn; 33–72% (w/w) of Pb; 35–65% (w/w) of Ni and 9–20% (w/w) of Cr were leached from sediments in this bioleaching process. The rate of metal leaching was found to decrease with an increase in sediment solid content. The solubilization of heavy metal from sediments was well described by a solid content‐related kinetic equation. © 2000 Society of Chemical Industry     

Carnallite Decomposition into Magnesia,Hydrochloric Acid and Potassium Chloride: II. The Phase Diagram KCl–MgO

Differential thermal analysis and X-ray diffraction of 12 different compositions in the phase diagram KCl–MgO disclose that there exists an area of solid solution, extending from ca. 30–80 equiv. % of MgO/2 (10–55 wf. % MgO), and, depending on the composition, from ca. 759°–775°. Besides the solid solution solid MgO occurs in this area. No chemical reaction between the components takes place.     

Insight into Green Phase Transfer Catalysis

There is a need to develop green, clean and smart chemical technologies. Waste reduction through clever strategies and catalysis are at the heart of green chemistry. In the case of over 600 industrial phase transfer catalyzed (PTC) processes in several industries, mostly practised as liquid–liquid PTC, the catalyst is not recovered and disposed as a waste. Liquid–liquid PTC can be replaced by solid–liquid (S–L), liquid–liquid–liquid (L–L–L), solid (catalyst)–liquid–liquid, solid–liquid–omega liquid, gas–liquid–solid (G–L–S) and capsule membrane PTC to recover and reuse the catalyst and also to enhance selectivity, thereby advocating the realm of green PTC. Use of microwaves and ultrasound will also help in increasing rates. MILL–PTC and MISL–PTC are attractive techniques to enhance rate and selectivities under mild input of microwave irradiation (MI). In cascade-engineered PTC, several steps are combined in a reactor using the same catalyst and solvent, if used at all, without separation, or with partial replenishment of the reactants for all the steps to get substantial waste minimization. This paper provides a new insight into PTC for some reactions of industrial importance.     

HHT analysis of electrostatic fluctuation signals in dense-phase pneumatic conveying of pulverized coal at high pressure

Particle charging generated by particle-wall, particle–particle and particle–gas contacts in pneumatic transport pipelines contain rich information on gas–solid flow hydrodynamics. In this paper, experiments were performed in a dense phase pneumatic conveying system, and electrostatic fluctuation signals were detected by a ring-shape electrostatic sensor that is based on electrostatic induction theory. Power spectrum analysis and Hilbert–Huang transform (HHT) were applied to the electrostatic fluctuations, namely outputs of the electrostatic sensor, to extract and characterize the intrinsic features of dense-phase gas–solid flow. Results show that the dominant peak of the power spectrum of the electrostatic fluctuations moves toward higher frequency with the increasing gas superficial velocity. The Hilbert–Huang transform reveals the non-linear and non-stationary intrinsic nature of the electrostatic fluctuations. By using the non-linear and non-stationary signal processing method (HHT), non-linear inter-modulation characteristics in the dense phase gas–solid flow were analyzed, and the relations between the energy distribution transmissions in intrinsic mode functions (IMFs) with different orders and the flow characterization of the dense phase gas–solid flow, was investigated as well, which can represent the behavior, stability and regime transitions of the gas–solid flow in the dense-phase pneumatic conveying system at high pressure.     

Evolution of a metastable thin film with high-hardness wear resistance for advanced cutting tool application

The structure and elastic properties of rocksalt and wurtzite Ti–Al–N ternary solid solutions were investigated using first principles calculations. We also performed an experimental evaluation using the sputtering method to obtain the reliability. The phase transition of (Ti_1−xAl_x)N solid solutions was occurred at x = 0.5–0.75 for both calculations and experimental results. The bulk modulus of rocksalt and wurtzite solid solutions decreases with increasing Al content. On the other hand, shear moduli and Young's moduli gradually increase with Al content only in rocksalt solid solutions. The theoretical and experimental results indicate that the overall mechanical properties of rocksalt solid solutions are superior to those of wurtzite solid solutions. Therefore, controlling the crystal structure of the Ti–Al–N ternary metastable system was crucial for optimizing the material properties.     

An Ideal Solid Solution Model for C–S–H

A model for an ideal solid solution, developed by Nourtier‐Mazauric et al. [Oil & Gas Sci. Tech. Rev. IFP, 60 [2] (2005) 401], is applied to calcium–silicate–hydrate (C–S–H). Fitting the model to solubility data reported in the literature for C–S–H yields reasonable values for the compositions of the end‐members of the solid solution and for their equilibrium constants. This model will be useful in models of hydration kinetics of tricalcium silicate because it is easier to implement than other solid solution models, it clearly identifies the driving force for growth of the most favorable C–S–H composition, and it still allows the model to accurately capture variations in C–S–H composition as the aqueous solution changes significantly at early hydration times.     

Agitation requirements for complete solid suspension in an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller

Background: To develop a new type of solid–liquid apparatus, we have proposed the application of an agitation system with an impeller whose rotation alternates direction unsteadily, i.e., a forward–reverse rotating impeller. For an unbaffled agitated vessel fitted with this system, the suspension of solid particles in a liquid was studied using a disk turbine impeller with six flat blades. Results: The effects of the solid–liquid conditions and geometrical conditions of the apparatus on the minimum rotation rate and the corresponding impeller power consumption were evaluated experimentally for a completely suspended solid. The power consumption for a just suspended solid with this type of vessel was comparable with that for a baffled vessel with a unidirectionally rotating impeller, taking the liquid flow along the vessel bottom into consideration. Conclusion: Empirical relationships to predict the parameters of agitation requirements were found. A comparative investigation demonstrated the usefulness of the forward–reverse rotation mode of the impeller for off‐bottom suspension of solid particles. Copyright © 2007 Society of Chemical Industry     

Hydrodynamics of an inverse liquid–solid circulating conventional fluidized bed

An inverse liquid–solid circulating conventional fluidized bed (I-CCFB) is realized by injecting particles from the top of a conventional liquid–solid fluidized bed (0.076 m ID and 5.4 m height) that is operated in a newly developed circulating conventional fluidization regime located between the conventional and circulating fluidization regimes. The I-CCFB can achieve a higher solids holdup compared to both conventional and circulating liquid–solid fluidized beds. A new parameter, the bed intensification factor, is defined to quantify the increased solids holdup observed with external solids circulation. The Richardson–Zaki equation is shown to be applicable to the I-CCFB regime and can be used to correlate the slip velocity and solids holdup, both of which increase with the solids circulation rate. A new flow regime map is presented, including the I-CCFB and a variety of other liquid–solid fluidized beds.     

Synthesis of polyglactin by melt/solid polycondensation of glycolic/L‐lactic acids

Polyglactin was successfully synthesized by the melt/solid polycondensation of a mixture of glycolic acid (GA) and L ‐lactic acid (LA) mostly at a GA to LA monomer ratio of 90/10. In the polymerization procedure, a solid polycondensate was first prepared by melt‐polycondensation at 150–190 °C, mechanically crushed into particles of various sizes (150–180, 180–210, 210–250, 250–300 and 300–355 µm), and subjected to solid‐state post‐polycondensation at 170 °C for 10–20 h. The polyglactin finally obtained was a colourless solid. Catalyst screening revealed that the single use of methanesulfonic acid gave the highest molecular weight of the product. Starting from the crushed melt‐polycondensate with a diameter range of 180–250 µm, the highest number‐average molecular weight of attained was 80 000 Da. This process can afford a facile route to large‐scale synthesis of polyglactin with high molecular weight. Copyright © 2004 Society of Chemical Industry     

Synthesis and properties of (Hf1-xTax)C solid solution carbides

Thermodynamically stable (Hf_1–xTa_x)C (x?=?0.1–0.3) compositions were selected by First Principle Calculation and synthesized in nanopowders via high-energy ball milling and carbothermal reduction of commercial oxides at 1450?°C. The formation of a solid solution during powder synthesis was investigated. The solid solution carbide powders were sintered at 1900?°C by spark plasma sintering without a sintering aid. As a result, the (Hf_1–xTa_x)C solid solution carbides exhibited high densities, excellent hardness and fracture toughness (ρ: 98.7–100.0%, HVN: 19.69–19.98?GPa, K_IC: 5.09–5.15?MPa?m^1/2) compared with previously reported HfC and HfC–TaC solid solution carbides.     

Numerical simulations and validation of gas–solid flows in a fluidized-bed roaster based on the CFD-DPM model

In view of the complex gas–solid flow characteristics in a fluidized-bed roaster, the discrete phase model (DPM) provided by ANSYS software was used to numerically analyze the model using a coupled algorithm. The asymmetric flow phenomenon in the transition section at the top of the furnace was found to be unfavourable to the gas–solid flow, and an inverted U-shaped furnace structure was proposed to optimize the transition section at the top of the furnace. A large cold experimental setup was built to verify the model. The results showed that the air velocity is mainly in the axial upward direction; under the action of the gas, the solid particles and the air velocity are basically in the same direction. The main furnace and subfurnace connection section changes the movement of the gas–solid mixture, and its unreasonable structure leads to the asymmetric flow phenomenon of the gas–solid fluid at the top of the furnace. Compared with the previous furnace structure, the uniformity of gas–solid flow in the optimized ‘inverted U-shaped’ structure has been significantly improved. The cold experimental results are in good agreement with the numerical simulation results, which verifies the accuracy of the proposed model.     

Novel gel‐entrapped base catalysts for the Claisen–Schmidt reaction

Novel gel‐entrapped base catalysts (GEBCs) were prepared by entrapping aqueous solutions of bases in a gel matrix of agar agar. The bases used were NaOH, KOH, morpholine and piperidine. Ternary phase diagrams were constructed for the water–base–agar agar system to identify the various phases and especially the solid phase, useful as a solid base catalyst. The 10% NaOH solid gel was used to effect the Claisen–Schmidt reaction between benzaldehydes and acetophenones in ethanol under heterogeneous conditions to obtain 70–100% yield of the products. The solid GEBCs obtained using other bases were also used for the same reaction; however, the yields were lower. The catalyst needed no activation prior to use and could be recycled. Copyright © 2004 Society of Chemical Industry