Turbulence damping above the cloud height in suspensions of concentrated slurries in stirred tanks

Poor mixing in the clear liquid layer above the cloud height has been reported by several authors. This study uses LDV measurements to quantify turbulence above the cloud using a liquid level of 1.5 T to remove the barrier of a free surface at H = T. A D = T/3, down-pumping PBT was used at an off-bottom clearance of C = T/3. Three slurries were tested at impeller speeds 0.8, 1, and 1.2N_js. The change in turbulence was quantified using the normalized root mean square (RMS) of the fluctuating velocity summed and averaged over each radial traverse. A significant difference between the fluctuating values of the cloud height—minimum, average, and maximum—was observed. The turbulence decays until the maximum cloud height. Beyond that, it remains constant and near zero. The effects of both particle size and solids concentration prove to be important.     

The effect of impeller pumping and fluid rheology on solids suspension in a stirred vessel

Pumping velocities of different impellers were measured using laser Doppler velocimetry. It was found that there is a link between impeller pumping capacity and the 5 parameter in the Zwietering correlation. Measurements showed that N_js increases with impeller spacing for dual impeller configurations. A dual‐impeller configuration shows less sensitivity to solids loading change than a single‐impeller configuration, and the change in N_js is larger for small impeller spacing than for large impeller spacing. An increase in slurry cloud height and a reduction in N_js resulted at a small increase of non‐Newtonian viscosity.     

Liquid flow in impeller region of an unbaffled agitated vessel with an angularly oscillating impeller

The characteristics of a liquid flow were studied in the impeller region of an unbaffled agitated vessel with an angularly oscillating impeller whose unsteady rotation proceeds while periodically reversing its direction at a set angle. The measurement of the velocity of the liquid flow was performed by particle tracking velocimetry (PTV), abreast of that of the torque of the shaft to which the impeller was attached. When a disk turbine impeller with six flat blades was used with variations in operating conditions, such as the frequency and amplitude of impeller angular oscillation, a series of images obtained during one oscillation cycle were analyzed to characterize the internal and discharge streams inside and outside the impeller rotational region. Energy data were inferred on the basis of the circumferential and radial velocities of an internal flow. Results showed that although the total head provided to the liquid by the impeller blades is almost similar, independent of the amplitude of impeller angular oscillation, namely, the acceleration of its movement, the transformation of energy from the pressure head to the velocity head is more efficient at a larger amplitude. In addition, the discharge flow was characterized in terms of volumetric flow rates calculated from the radial and axial velocities. The operation at a smaller amplitude was shown to transform the flow more successfully from the radial direction to the upward and downward axial directions near the vessel wall.     

Oxygen mass transfer coefficient in bubble column slurry reactor with ultrafine suspended particles and neural network prediction

The gas–liquid volumetric mass transfer coefficient was determined by the dynamic oxygen absorption technique using a polarographic dissolved oxygen probe and the gas–liquid interfacial area was measured using dual‐tip conductivity probes in a bubble column slurry reactor at ambient temperature and normal pressure. The solid particles used were ultrafine hollow glass microspheres with a mean diameter of 8.624 µm. The effects of various axial locations (height–diameter ratio = 1–12), superficial gas velocity (u_G = 0.011–0.085 m/s) and solid concentration (ε_S = 0–30 wt.%) on the gas–liquid volumetric mass transfer coefficient k_La_L and liquid‐side mass transfer coefficient k_L were discussed in detail in the range of operating variables investigated. Empirical correlations by dimensional analysis were obtained and feed‐forward back propagation neural network models were employed to predict the gas–liquid volumetric mass transfer coefficient and liquid‐side mass transfer coefficient for an air–water–hollow glass microspheres system in a commercial‐scale bubble column slurry reactor. © 2012 Canadian Society for Chemical Engineering     

Liquid–solid mass transfer in a three-phase stationary catalytic basket reactor

Measurements of the liquid–solid mass transfer coefficient k_S were performed in a three-phase multiple impeller and stationary catalytic basket reactor, called a Robinson–Mahoney reactor. Local coefficients were determined using naphtol particle dissolution in water and n-heptane with or without gas flow. Experiments have shown that local coefficients k_{S loc} depend on the particle position in the basket, agitation speed, and liquid properties. A correlation linking k_S to liquid velocity was established in a reactor simulating hydrodynamics in a piece of catalytic basket. Liquid velocity values were validated using a particle image velocimetry technique in a Robinson–Mahoney reactor, using water. © 2005 American Institute of Chemical Engineers AIChE J, 2005     

Micromixing of Solid‐liquid Systems in a Stirred Tank with Double Impellers

The effect of solid particles on micromixing has been studied using the competitive iodide/iodate reaction system in stirred, multi‐impeller, solid‐liquid systems. The influences of particle size, impeller speed, solid holdup, feed position, and energy input have been investigated. The change of the segregation index with the power input was more distinguishable only for the 450–600 μm particles as compared with the large ones, at the same solid holdups. Also, for the small ones, cloud formation was observed at a particle concentration of 12.1 wt %. However, the influence of larger particles of 1–1.25 mm on micromixing was negligible, though both energy input and solid loading were increased. Besides, the optimal feed position was identified, and multiple feeds were also explored.     

Emulsion polymerization of methyl methacrylate in concentration of emulsifiers below their CMCs—polymerization rate,particle size,and particle-size distribution

The emulsion polymerization of methyl methacrylate (MMA) in concentration of emulsifiers below their critical micelle concentrations (CMCs) initiated by K₂S₂O₈ (KPS) was studied. It was observed that the initiator concentration has little effect on both polymerization rate and particle size. However, the polymerization rate is faster and particle size is smaller obviously when decreasing the ratio of the water/monomer or increasing the temperature of polymerization or the amount of the emulsifier. In the range of a 200–400 rpm stirring speed, the polymerization rate is almost unchanged although the particle size become larger with increase in the stirring speed. The monodisperse particle (size about 100–200 nm) can be obtained using this process. The mechanism of emulsion polymerization of MMA in the emulsifier concentration below its CMC is discussed. © 1995 John Wiley & Sons, Inc.     

A CORRELLATION FOR CYLINDER-TO-BED HEAT TRANSFER IN AERATED VIBRATED BEDS OF SMALL PARTICLES

ABSTRACT

Beds of alumina particles (d_p= 27 μm and 100 μm) were vibrated in the vertical direction at frequencies frdm 0–25 Hz and half-amplitudes from 0–4 mm. Air flow rate through a single-hole or multiple-holes bottom plate varied from 0 to 2 times the minimum fluidizing velocity. The contact heat transfer coefficients at resonance are much higher than those in packed beds and in vibrated fluidized beds (up to 1.2 times). The high heat transfer rates are due to enhanced particle mobility which reaches a maximum at the resonant point. A simple semi-empirical correlation is developed for contact heat transfer which is based on particle mobility. Heat transfer coefficients are correlated with frequency using amplitude, bed height and particle size as adjustable parameters. The correlation is found explain the observed trends in the data reasonably well over the range of parameters studied.     

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WH_U) and down-pumping (WH_D) modes. Glass beads of 50 ～ 150 μm diameter and density 2500 kg · m^?3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = P_G/P_U) than those with WH_D or WH_U as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WH_U has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.     

Experimental study of high-density gas-solids flow in a new coupled circulating fluidized bed

Experiments were carried out in a specially designed high-density coupled circulating fluidized bed system. Fluidized catalytic cracking (FCC) particles (ρ_p=1300 kg/m³, d_p=69 μm) were used. When the solids circulation flux is 400 kg/m²·s, the apparent solids holdup exceeds 20% near the top of the riser A, and the volumetric solids fraction (apparent solids holdup) is larger than 5.2% in the fully developed region of the downer. Hence, a high particle suspension density covers the entire coupled CFB system. Under the high-density conditions, the primary air rate had a small influence on the solids circulation flux, while the secondary air rate had an important effect on it. The results indicate a particle acceleration region and a fully developed region were identified along the downer from the pressure gradient profiles. In the fully developed region of the downer, the volumetric solids fraction increases with increasing solids circulation flux or decreasing superficial gas velocity U₁.     

Mass transfer into simulated fermentation media

The rates of oxygen mass transfer into a simulated fermentation medium, made up of 16 kg of paper pulp per m³ of aqueous sodium sulphite solution with a cupric ion catalyst, were determined in vessels of 0·187, 0·291 and 0·451 m dia., using flat-bladed turbine impellers, and the effect of varying impeller dimensions and operating speed were investigated. Above a critical impeller tip speed the volumetric mass transfer coefficients obtained at the same speed with different power inputs (produced by variations in the impeller blade dimensions) could be represented by the sum of two terms, one depending on the impeller speed, the diameters of the impeller and vessel, and the height of pulp suspension in the vessel, and the other function of the power input per unit volume and the air velocity through the vessel. At each impeller speed the increase in the volumetric mass transfer coefficient with power input was found to be greater below a certain power input per unit volume, and a correlation for this power per unit volume was based on it corresponding to a change in the rate of air recirculation through the impeller. The expression for the mass transfer coefficient provides a more accurate basis for scale-up than the use of simple rules, such as constant power per unit volume.     

Effect of particles on hydrodynamics and mass transfer in a slurry bubble column: Correlation of experimental data

The effects of particle concentration and size on hydrodynamics and mass transport in an air–water slurry bubble column were experimentally studied. When the particle concentration α_s increased from 0% to 20%, the averaged gas holdup decreased by ~30%, gas holdup of small bubbles and gas–liquid volumetric mass transfer coefficient decreased by up to 50%, while the gas holdup of large bubbles increased slightly. The overall effect of particle size was insignificant. A liquid turbulence attenuation model which could quantitatively describe the effects of particle concentration and size was first proposed. Semi-empirical correlations were obtained based on extensive experimental data in a wide range of operating conditions and corrected liquid properties. The gas holdup and mass transfer coefficient calculated by the correlations agreed with the experimental data from both two-phase and three-phase bubble columns, with a maximum error <25%.     

Hydrodynamic behaviour and oxygen transfer rate in a pilot plant fermenter. I. Influence of viscosity

The effects of varying impeller speed, aeration rate and viscosity on mixing time, power consumption and oxygen transfer rate were studied in the pilot plant fermenter. The rheological behaviour of erythromycin fermentation broth was simulated by colloidal starch solutions at apparent viscosities of 0.02–0.20 Pa/s. The volumetric oxygen transfer rate coefficient was determined by the sulphite and static method. The experimental results showed that at low viscosities, up to of 0.02 Pa/s, the suitable range of impeller speed is 250–300 min^?1 at aeration rate 0.6–0.8 VVM from the point of view of power input (2.6–2.8 kW/m³) and sufficient coefficient K_La (160–200 h^?1). At viscosities higher than 0.02 Pa/s (with pseudoplastical character) the suitable range of impeller speed is 300–350 min^?1 at the same aeration rate. Then the power input for mixing is 3.0–3.5 kW/m³ and the coefficient K_La ～ 50 h^?1.     

Hydrodynamic behaviour and oxygen transfer rate in a pilot plant fermenter: I. Influence of viscosity

The effects of varying impeller speed, aeration rate and viscosity on mixing time, power consumption and oxygen transfer rate were studied in the pilot plant fermenter. The rheological behaviour of erythromycin fermentation broth was simulated by colloidal starch solutions at apparent viscosities of 0.02–0.20 Pa/s. The volumetric oxygen transfer rate coefficient was determined by the sulphite and static method. The experimental results showed that at low viscosities, up to of 0.02 Pa/s, the suitable range of impeller speed is 250–300 min^?1 at aeration rate 0.6–0.8 VVM from the point of view of power input (2.6–2.8 kW/m³) and sufficient coefficient K_La (160–200 h^?1). At viscosities higher than 0.02 Pa/s (with pseudoplastical character) the suitable range of impeller speed is 300–350 min^?1 at the same aeration rate. Then the power input for mixing is 3.0–3.5 kW/m³ and the coefficient K_La ～ 50 h^?1.     

Critical impeller speed for solid suspension in multi-impeller three phase agitated contactors

The critical impeller speed for solid suspension in gas-liquid-solid systems has been measured in multi-impeller agitated contractors of 0.15 and 0.30 m and ID and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were used. Air, deionised water and sand particles were used as the gas, liquid and solid phases, respectively. The superficial gas velocity and solid loading were varied in the ranges 0–15 mm/s and 0.5 to 10% w/w, respectively. The effects of impeller type and its diameter, particle size and loading and gas flow rate were studied. Some measurements of gas hold-up and mixing time were also made in order to get some insight of the hydrodynamic behaviour of the reactor. The critical impeller speed for solid suspension in the presence of gas (n_isg) was found to be more than that in the absence of the gas and the increase of critical speed correlated well with the gas flow rate. The influence of particle—liquid parameters on solid suspension speed in the gassed system was similar to but relatively weaker than that in the ungassed condition.     

The effect of impeller-to-tank diameter ratio on draw down of solids

The effect of impeller-to-tank diameter ratio (D/T) on the draw down of solids was investigated using a mixed flow impeller (pitched blade turbine) and a narrow blade hydrofoil (LE-20) of D=T/2 and T/3. Operational conditions (impeller speed and power consumption) at which solids do not remain at the liquid surface for more than 2- were determined for a given solid type (polyethylene particles) at a given concentration (X=1%). Under selected conditions, liquid velocity values were obtained from LDA measurements and CFD simulations to better interpret the findings.Both upward and downward pumping modes were studied with the impeller mounted over a range of submergences, and different mechanisms of draw down were noted. For LDA measurements and CFD simulations, only the upward pumping PBT was considered. These were carried out under conditions where draw down did not occur through air ingesting vortices.When solids were drawn down without air being entrained from the surface, a smaller diameter impeller (D=T/3) was more energy efficient, although it required higher speeds to achieve draw down. Results from LDA measurements and CFD simulations showed that the discharge flow from a larger impeller has a stronger radial component of flow, and as a result the liquid is directed towards the vessel walls rather than the surface which supported this finding. The only exception to this was when solids were drawn down along with air through vortices. In this case, the power demand could be lower with a larger diameter impeller.     

Study of solid–liquid mixing in agitated tanks through electrical resistance tomography

Electrical resistance tomography (ERT), which is a non-intrusive flow visualization technique, was used to investigate the solid–liquid mixing in an agitated tank equipped with a top-entering axial-flow impeller. The signals obtained from eight ERT planes were utilized to reconstruct the tomograms by using the linear back projection algorithm. The ERT measurements were correlated to solid concentration profiles by which the degree of homogeneity was quantified. In this study, the effect of important parameters such as impeller type (Lightnin A100, A200, A310, and A320 impellers), impeller speed (250–800 rpm), impeller off-bottom clearance (T/5–T/2, where T is the tank diameter), particle size (210–1500 μm), and solid concentration (5–30 wt%) on the degree of homogeneity were explored. The results showed that the degree of homogeneity in the solid–liquid mixing was improved with increasing the impeller speed. However, after reaching the maximum achievable homogeneity, further increase in impeller speed was not beneficial but might be detrimental. Hence, the measurement of the optimal impeller speed as a function of operating conditions and design parameters has vital role in achieving maximum homogeneity in a solid–liquid mixing system.     

Effect of impeller clearance on liquid flow within an unbaffled vessel agitated with a forward–reverse rotating impeller

For an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller whose rotation proceeds with repeated acceleration, deceleration, and stop–reverse processes, liquid flow was studied through visualisation and measurement using particle tracking velocimetry (PTV). A disk turbine impeller with six flat blades was used with varied height settings. The impeller clearance and its forward–reverse rotation cycle characterised the impeller region flow: the radially outward flow in the deceleration process for the larger clearance relative to the vessel diameter of 1/3, and the axially downward flow in the acceleration process for the smaller clearance relative to the vessel diameter of 1/8. The flow patterns within the vessel resulting from the impeller's larger and smaller clearances were outlined, respectively, by double loops and a single loop of circulation, resembling the pattern produced by unidirectionally rotating turbine‐type impellers. The discharge flow was revealed to contain a comparable level of periodic circumferential velocity component, irrespective of the impeller clearance.     

AEROSOL DYNAMICS

ABSTRACT

Photodetector pulse heights from an ultrafine condensation nucleus counter increase monotonically with particle size in the ～ 2.7–15 nm diameter range. This relationship can be used to measure concentrations and size distributions of ultrafine aerosols. In this study, we investigated the sensitivity of size-dependent pulse heights to total particle concentration, absolute pressure (0.25–1 atmosphere), and particle composition (H₂SO₄, (NH₄)2SO₄, NaCl, and tungsten oxide). We found that pulse heights shifted significantly with pressure and slightly with concentration. Coincidence led to errors for concentrations exceeding 4 × 10³ cm^?3. Over the range of conditions investigated, however, the observed shifts in the pulse height voltage were independent of size. The pulse height method is particularly applicable to situations involving low ultrafine particle concentrations, such as are encountered in the remote troposphere.