Effect Of Impeller Diameter Research Articles

Volumetric mass transfer coefficient in the fermenter agitated by Rushton turbines of various diameters in viscous batch

In cases of aerobic fermentations, the apparatuses for their industrial performance are designed also from the viewpoint of sufficient oxygen supply to avoid oxygen limitation of biomass, i.e. to provide sufficient oxygen gas-liquid transfer rate. For biochemical processes with high respiration rate, the oxygen demand of which is also high, mechanically agitated vessels are usually used. In comparison with airlift reactors and bubble columns, these equipments enable to reach higher oxygen gas-liquid transfer rates, i.e., they reach higher values of volumetric mass transfer coefficient, kLa. Due to the hydrodynamic complexity of mechanically agitated gas-liquid systems, the kLa predictions from first principles are not reliable yet, and, therefore, experimental data are needed for rational design of fermenters. Most fermentation broths are of increased viscosity, for which the lack of reliable kLa data exists due to the limitations of most measurement methods. We recently suggested and verified the methodology to obtain reliable experimental kLa data in viscous batch even for high dissipated energies. We used the dynamic pressure method (DPM), the experimental set-up of which was modified for the measurement in viscous batch. In our previous work, we provided fairly large kLa database to establish suitable correlation shapes to describe kLa dependencies on process conditions in viscous liquids. Now, we focused on the effect of impeller diameter on transport characteristics because in cases of shear stress sensitive biomass this parameter strongly affects the fermentation efficiency. The measurements were conducted in multiple-impeller fermenters, both of laboratory and of pilot-plant scale, using viscous Newtonian batch under a wide range of experimental conditions (impeller frequencies, gas flow-rates, impeller diameters). Rushton turbines of various diameters were used. Based on the experimental data, the correlations were developed to predict kLa in industrial fermenters. Standard correlation kLa=0.0024(PTOT)0.86vs0.49 with SD 23%, based on gassed power input PTOT and superficial gas velocity vs, has low standard deviation. On the other hand, when the term of impeller tip speed (ND) is used instead of PTOT, predicted data have a higher standard deviation kLa=0.29(ND)2.15vs0.27 with SD 37%, but when this correlation is modified taking into account the D/T ratio, the standard deviation decreases significantly. The correlation kLa=1.14(ND)2.23vs0.27DT1.3with SD 25% suggested in this work can be used for the fairly accurate design of industrial fermenters. Both the experimental technique and the correlation shape are ready to be used to obtain the design tool for other batches with various viscosities.

In-line mixing states monitoring of suspensions using ultrasonic reflection technique

Based on the measurement of echo signal changes caused by different concentration distributions in the mixing process, a simple ultrasonic reflection technique is proposed for in-line monitoring of the mixing states of suspensions in an agitated tank in this study. The relation between the echo signals and the concentration of suspensions is studied, and the mixing process of suspensions is tracked by in-line measurement of ultrasonic echo signals using two ultrasonic sensors. Through the analysis of echo signals over time, the mixing states of suspensions are obtained, and the homogeneity of suspensions is quantified. With the proposed technique, the effects of impeller diameter and agitation speed on the mixing process are studied, and the optimal agitation speed and the minimum mixing time to achieve the maximum homogeneity are acquired under different operating conditions and design parameters. The proposed technique is stable and feasible and shows great potential for in-line monitoring of mixing states of suspensions.

CFD prediction of mean flow field and impeller capacity for pitched blade turbine

This work focused on exploring a computational fluid dynamics(CFD)method to predict the macro-mixing characteristics including the mean flow field and impeller capacity for a 45° down-pumping pitched blade turbine( PBT)in stirred tanks. Firstly, the three typical mean flow fields were investigated by virtue of three components of liquid velocity. Then the effects of impeller diameter(D)and off-bottom clearance(C)on both the mean flow field and three global macro-mixing parameters concerning impeller capacity were studied in detail. The changes of flow patterns with increasing C/D were predicted from these effects. The simulation results are consistent with the experimental results in published literature.

Effects of impeller diameter and rotational speed on performance of pump running in turbine mode

The major limitations of mini/micro hydropower schemes is the higher cost of small capacity hydro turbines. Also, it is very cumbersome, time consuming and expensive to develop the site specific turbines corresponding to local site conditions in mini/micro hydro range. In such plants, small centrifugal pumps can be used in turbine mode by running in the reverse direction. The efficiency of pump as turbines (PATs) is usually lower than the conventional hydro turbines; however, there may be substantial decrease in the capital cost of the plant.Hydropower plants usually runs at part load for several months in a year due to insufficient water availability for the power generation. The application range of PAT can be widened if its part load and/or maximum efficiency can be improved. In the present study, experimental investigations are carried out on centrifugal pump running in turbine mode to optimize its geometric and operational parameters e.g. impeller diameter and rotational speed. The experiments were performed in the wide range of rotational speeds varying from 900 to 1500rpm with original (∅ 250mm), 10% trimmed (∅ 225mm) and 20% trimmed (∅ 200mm) impellers. Impeller trimming led to improvement in efficiency at part load operating conditions. The performance of PAT was found better at the lower speeds than that at the rated speed. The effects of blade rounding were studied in all the cases and it led to 3–4% rise in efficiency at rated speed with the original impeller. The empirical correlation is also developed for prediction of efficiency in terms of impeller diameter and rotational speed in non-dimensional form.

Suspension of floating solids with up-pumping pitched blade impellers; mixing time and power characteristics

Floating solids suspension in water has been studied using up-pumping pitched blade turbines (PTU). Studies were performed to determine optimum geometry and hydrodynamic characteristics of agitated systems for floating solids suspension. The impact of floating solids concentrations and particle size on the mixing time, t m, the just completely suspended impeller speeds, N JS, and the power consumption were examined. The effects of impeller diameter and its off-bottom clearance as well as the impeller blade angle on mentioned parameters were also observed. The mixing time of suspended system was measured by a conductivity technique using the sodium chloride solution as tracer, whereas, power consumption was measured by the torque table. The efficiency of PTU impellers was compared to that of other, more conventional impellers. Obtained results point to the fact that the PTU may be a viable option if the floating slurry process requires the state of complete solids suspension and its longer time maintenance.

Stability of gas‐inducing type impellers

AbstractThe aim of this study is to understand the reasons for instability in the operation of a gas inducing impeller and to suggest the ways to eliminate the instability. Experiments were performed in a 1 m diameter vessel. The impeller clearance from the tank bottom was 0.52 m. The effects of impeller diameter, impeller blade width, blade thickness, impeller blade angle, shape of the impeller blade, impeller position with respect to the stator, impeller submergence and stator geometry on the instability were investigated. Phenomenological description has been provided for the occurrence of instability. An optimum design of a stator–impeller combination has been proposed for gas dispersion and stable operation.

Growth-independent breakage frequency of protein precipitates in turbulently agitated bioreactors

A model is provided to describe the influence of hydrodynamic conditions on the breakage of protein precipates in turbulent suspension. To examine the applicability of the model, experiments are performed to measure the breakage of soya protein precipitates in 0.29 m diameter vessel at impeller Reynolds numbers ranging from 3.6 × 10 4 to 5.6 × 10 5 and with initial protein concentrations of 0.35, 3.5 and 35 kg/m 3. An analysis of the experimental results based on the proposed model shows that the initial frequency of breakage of protein precipitates is a unique function of the energy dissipation rate in the vessel, encompassing both effects of impeller diameter and impeller speed. The model accurately decribes the influence of both energy dissipation rate and protein concentration on the initial breakage frequency of the aggregates.

Effect of impeller diameter on mean droplet diameter in circular loop reactor

AbstractExperiments in liquid‐liquid dispersion were performed using a circular loop reactor designed for suspension polymerization. In order to vary widely differences in physical properties such as the viscosity, density, and interfacial tension between the two liquids concerned, polystyrene pellets were dissolved into the dispersed phase which consisted of a styrene monomer liquid. The diameter distributions and mean diameters of droplets formed were measured by stepwise changes in the impeller diameter. From these results, the effect of the impeller diameter on the breakup of droplets was determined. Correlations relating the mean droplet diameter to the operational conditions were derived.

Mass transfer and hydrodynamic characteristics of gas inducing type of agitated contactors

AbstractPower consumption, rate of gas induction (Q), fractional gas hold‐up and mass transfer characteristics of 0.41, 0.57 and 1.0 m i.d. gas inducing type of agitated contactors were studied. The power consumption was measured by a torque table. The values of effective interfacial area (a), and liquid side mass transfer coefficient (kLa) were obtained by using chemical methods. Two types of impellers (pipe and flattened cylindrical) were employed. The impeller speed was varied from 3 to 11.7 r/s.The effects of impeller diameter, baffle position, orifice area on the impeller, number of impeller blades, position of impeller from the bottom and liquid submergence on the values of Q and a were investigated. Further, the effect of liquid viscosity and surface tension on the values of Q was studied.A satisfactory agreement was found between the observed and the predicted value of the minimum impeller speed at which gas induction occurs. The performance of the gas inducing type of agitated contactor has been compared with the conventional mechanically agitated contactor.