Specific Power Input Research Articles

Hydrodynamics and Mass Transfer in a Rotating Disk Supercritical Extraction Column

A novel rotating disk column (RDC) designed for high-pressure extraction was tested by extracting ethanol from aqueous solutions using supercritical carbon dioxide at 10 MPa and 313 K as a solvent. Mass-transfer efficiencies and hydraulic characteristics were measured as functions of a specific power input group (N3R5H-1D-2) and solvent-to-feed ratio. Agitation generally increased the value of the overall mass-transfer coefficient (Koda) and decreased the value of the height equivalent to a theoretical stage (HETS). The Koda values ranged from 0.006 to 0.015 s-1, and the HETS values ranged from 0.37 to 0.8 m. The dispersed-phase drop size decreased with the agitator rotor speed. The measured values were between 1.3 and 1.8 mm. Agitation and increasing solvent flow rate increased the dispersed-phase holdup from 0.04 to 0.2. The measured slip velocities ranged from 0.08 to 0.14 m s-1. The highest total throughput of the RDC column was approximately 74 m3 h-1 m-2.

Modeling brewers' yeast flocculation

Flocculation of yeast cells occurs during the fermentation of beer. Partway through the fermentation the cells become flocculent and start to form flocs. If the environmental conditions, such as medium composition and fluid velocities in the tank, are optimal, the flocs will grow in size large enough to settle. After settling of the main part of the yeast the green beer is left, containing only a small amount of yeast necessary for rest conversions during the next process step, the lagering. The physical process of flocculation is a dynamic equilibrium of floc formation and floc breakup resulting in a bimodal size distribution containing single cells and flocs. The floc size distribution and the single cell amount were measured under the different conditions that occur during full scale fermentation. Influences on flocculation such as floc strength, specific power input, and total number of yeast cells in suspension were studied. A flocculation model was developed, and the measured data used for validation. Yeast floc formation can be described with the collision theory assuming a constant collision efficiency. The breakup of flocs appears to occur mainly via two mechanisms, the splitting of flocs and the erosion of yeast cells from the floc surface. The splitting rate determines the average floc size and the erosion rate determines the number of single cells. Regarding the size of the flocs with respect to the scale of turbulence, only the viscous subrange needs to be considered. With the model, the floc size distribution and the number of single cells can be predicted at a certain point during the fermentation. For this, the bond strength between the cells, the fractal dimension of the yeast, the specific power input in the tank and the number of yeast cells that are in suspension in the tank have to be known. Copyright 1998 John Wiley & Sons, Inc.

Influence of the process parameters on the morphology and enzyme production of Aspergilli.

Several papers have been published dealing with various fungi to determine their morphology, enzyme production or process performance. However, no publication considered all of these aspects simultaneously. In the case of the production of xylanase by Aspergillus awamori the interrelationship of various key parameters are investigated. The influence of the reactor type (shake flasks, stirred tank and airlift tower loop reactor), the medium composition (semisynthetic and complex medium with wheat bran of different sizes, respectively as well as different concentrations of phosphate), and the specific power input (stirrer speed) on the growth, morphology, physiology, and productivity of the fungus are investigated. The results reveal a complex interrelationship which explains why the published results are contradictory. Without considering all of the relevant parameters, it is not possible to make general conclusions.

The Design of a Twin-screw Compressor Based on a New Rotor Profile

SUMMARY This paper describes the design of a high-efficiency twin-screw compressor with new rotor profiles. A well-proven mathematical model of the compression process within positive displacement machines was used to determine the optimum rotor size and speed, the volume ratio, and the oil injection position and jet diameter. In addition, modern design concepts such as an open suction port and early exposure of the discharge port were included, together with improved bearing and seal specification, to maximize the compressor efficiency. The prototype was tested and compared with the best compressors currently on the market. Its minimum measured specific power input at a delivery pressure of 8 bar (abs) was 5.6 kW m-3 min-1. This is less than the published value for any other equivalent compressor currently manufactured. Both the predicted advantages of the new rotor profile and the superiority of the design procedure were thereby confirmed.

Production of xylanase by Aspergillus awamori on complex medium in stirred tank and airlift tower loop reactors

Aspergillus awamori was cultivated on complex medium containing wheat bran as sole carbon source in the precultures as well as in the main cultures in stirred tank and airlift tower loop reactors. The decomposition of the wheat bran particles by α-amylase and xylanase produced by the fungus was investigated by monitoring the enzyme activities, the CO 2 production rate, the concentrations of reducing sugars, glucose, xylose, phosphate and ammonium as well as by applying transmission electron microscopy. By applying synthetic medium in the preculture, the time lag was longer, the growth faster, the xylanase activity higher in the main culture, than by using wheat bran in the preculture. With increasing the stirrer speed the intimate contact between the pellets and the wheat bran was reduced, which caused a decrease of the growth and the xylanase production. By applying ground wheat bran, filamentous mycelium was formed, the growth and the substrate uptake rates increased, but the xylanase production decreased due to the less intimate contact between the fungus and the wheat bran. In the airlift tower loop reactor filamentous mycelium was formed due to the low specific power input. In spite of the high medium viscosity, high xylanase productivity was obtained. With ground wheat bran the particles were completely covered by filamentous mycelium, medium viscosity strongly increased and as consequence the growth and xylanase production decreased.

Overall volumetric oxygen transfer coefficient and optimal geometry of airlift tube reactor

Abstract Absorption of oxygen from air into distilled water in a new type of airlift tube reactor has been investigated at the 1.3 1 scale. Results of this mass transfer study together with hydrodynamic results provide a means of optimizing the geometrical parameters of the reactor. The overall volumetric oxygen transfer coefficient ( K L a ) was measured as a function of the gas velocity in reaction tubes, the liquid hold-up in the reactor expressed in terms of liquid height, the cross-sectional area ratio of the recycle-to-reaction tubes ( A rec / A ), the liquid temperature and specific power input. The K L a was measured using a dynamic gassing method with polarographic measurements of dissolved oxygen.

Theoretical Prediction and Experimental Determination of Attrition Rates

When reasonably soluble substances (c*> 0.1mol l–1) are crystallized at a high specific power input and within their metastable zone, attrition is a primary source of nucleation. The study presented here is an attempt to quantify this attrition mechanism. Firstly, a physical model is introduced which relates the attrition resistance of a crystalline substance to its mechanical properties. In the second part, the emphasis is placed on the determination of the relevant mechanical properties, hardness, elastic constants, and fracture resistance. The physical model will finally be compared to experimental results. A good agreement is found between the experimentally determined and theoretically predicted attrition behaviour.

The effect of impact energy and the shape of crystals on their attrition rate

In an industrial crystallizer, contact between crystals and crystal-crystallizer parts increases the rate of production of potential nuclei. When brittle substances are crystallized at a low relative supersaturation and a high specific power input, attrition is the dominant nucleation mechanism. In order to predict nucleation rates caused by attrition, the following must be known: the volume of fragments produced, their particle size distribution as well as their growth behaviour. This paper considers the volume of attrition fragments produced from brittle crystals and demonstrates its relation to impact energy and particle shape. Two different types of impact experiments were performed, one by letting the crystals fall through an evacuated tube onto a glass plate, the second by impacting the crystals in a stirred vessel. The impact experiments demonstrate, that the volume removed from the particles is almost proportional to the impact energy. However, repeated contact of the same crystals does not give the same reduction of volume, which decreases significantly. The reason for this is the change of the crystals shape, which tend to round off after repeated impact. Crystal growth, on the other hand, leads to the redevelopment of edges and corners. From the experimental data presented it can be concluded that growth rates and attrition rates in industrial crystallizers are related. The nucleation rate due to attrition increases with increasing growth rate.

Effect of Gas Sparger Type on Operational Characteristics of a Bubble Column under Mechanical Foam Control

The effects of gas sparger types on the operational characteristics of a bubble column (BC) with a rotating-disk mechanical foam-breaker (MFRD), such as foam-breaking, foaming, flow and mass transfer characteristics, were studied. The required critical disk rotational speed, N c , and liquid hold-up in the foam, Φ L , reflected the foam-breaking behaviour of the MFRD and the foaming behaviour of the BC respectively, which changed depending on the gas sparger type. The dependence of the gas sparger type on the gas hold-up, e g , was similar to that observed for the volumetric mass transfer coefficient, k L a. A comparative investigation of k L a with various gas spargers, in terms of the specific power input, also demonstrated the usefulness of the porous sintered glass bead filter as a gas sparger.

Ultimate efficiency of a Penning neon plasma laser

The ultimate efficiency of a Penning neon plasma laser (λ = 585.3 nm) is shown to be less than 0.5%, relative to the energy deposited in the active medium. The optimal conditions are found for pumping by external sources acting for various durations and the threshold value of the specific input power delivered by nuclear pumping is calculated.

Studies of High Solidity Ratio Hydrofoil Impellers for Aerated Bioreactors. 4. Comparison of Impeller Types

This paper compares the performance of the Prochem Maxflo T and Lightnin A315 hydrofoil impellers, a Rushton disc turbine, and a mixed flow impeller in water, electrolyte solution, and viscous shear thinning sodium carboxymethylcellulose solution. Impeller performance was compared on the basis of (a) the reduction in power drawn on aeration, for operation at equal ungassed specific power inputs, (b) the power required to disperse gas at a given gas flow rate, (c) the gas holdup generated at a given gassed specific power input and superficial gas velocity, and (d) flow stability. The equipment, techniques, and model fluids used are described in parts 2 and 3 of this series of papers. Hydrofoil impellers can maintain a higher relative power than either the Rushton turbine or mixed flow impeller over a wide range of gas flow rates, and the overall fall in power drawn is much less (typically ∼30−50%, as compared with 50−65%). This is advantageous in terms of operating efficiency and implies a higher gas−liquid mass transfer potential since the latter is directly related to the gassed specific power input. Both classes of downward pumping impeller are prone to flow and associated torque fluctuations, while operation with the Rushton turbine is much more stable. The A315 is more energy efficient in dispersing gas than the Prochem. Although in this respect the efficiencies of the A315 and Rushton turbine are similar in water, the latter is superior at low and moderate gas flow rates in CMC solution. Gas holdup can be correlated with gassed specific power input and superficial gas velocity, and the holdups produced by the different impeller types in a given model fluid are broadly similar.

Hydrodynamic, physiological, and morphological characteristics of Fusarium moniliforme in geometrically dissimilar stirred bioreactors

The influence of two mixing geometries (at the same scale) with different flow energy distributions on the performance of the gibberellic acid fermentation and on the morphology of the producing fungus Fusarium moniliforme was investigated. Fermentations were performed using a turbine mixing system (TMS) and a counterflow mixing system (CMS), which were high and low power number mixing systems, respectively. Different agitator speed rate profiles were maintained to obtain equal specific power inputs to both mixing systems. Substantial differences in morphology and productivity of F. moniliforme were found. To investigate the causes of these differences, local values and spectra of the kinetic energy of flow fluctuations were measured during the fermentations using a stirring intensity measuring device (SIMD) and a frequency spectrum analyzer. Biomass and gibberellic acid concentrations were found to be higher in the TMS, where the energy distribution was less even, and Vi/here the main part of the energy was at small frequencies (large eddies). An automated image analysis method was used for quantitative characterization of F. moniliforme freely dispersed mycelia and clump morphology. A higher proportion of clumped mycelia with clumps of larger area, perimeter, and roughness was observed in the TMS. A correlation between the morphology and productivity was found, and TMS favored the development of more productive mycelia with longer and thinner hyphae. Introduced power was not a good parameter to characterize different impellers, even at a given scale.

Effects of the hydrodynamic environment and shear protectants on survival of erythrocytes in suspension

Survival of media-suspended porcine erythrocytes exposed to various hydrodynamic environments was investigated with and without such shear protectant additives as bovine serum albumin, dextran and the non-ionic surfactant Pluronic F68. Erythrocytes provided a model cell population with cells of a uniform size, metabolic state and shear tolerance. Because the cells were non-growing, any shear adaptation effects were avoided. Cell lysis was followed by microscopic counts or release of haemoglobin. The cells were susceptible to agitation damage in unaerated shake flasks agitated at 100 rpm or greater. Relative to additives-free operation, the presence of 0.1% (w/v) dextran or albumin prolonged cell survival, but Pluronic F68 actually enhanced cell lysis in flasks agitated at 100 rpm. The protective effect of the additives depended on the hydrodynamic conditions. The protective effect of albumin was demonstrated also in aerated conditions in a split-cylinder airlift bioreactor (aspect ratio of 8.8; riser-to-downcomer cross-sectional area ratio of 1.0; specific power input of 0.34 W m −3). Comparison of the cell lysis characteristics in the airlift device and the best case performance of the shake flask showed longer survival in the flask (100 rpm); however, the length of survival in the reactor (approx. 70 h) was sufficient for practical purposes. In all cases, the cell lysis pattern conformed initially to zero-order dependence in cell concentration, becoming first-order after varying degrees of exposure to hydrodynamic forces. Fatigue failure of cells was inferred.

Gas hold‐up and volumetric mass transfer coefficient in bubble columns under foam control

AbstractExperimental measurements of the gas hold‐up and volumetric mass transfer coefficient have been made for baffle columns (BCs) reacting various foaming liquids under mechanical and chemical foam control. The gas hold‐up and the volumetric mass transfer coefficient in a mechanical foam‐control system (BCs with rating‐disk mechanical foam‐breakers) were larger than those in a chemical foam‐control system (BCs with an antifoam agent added). Correlations for the gas hold‐up and the volumetric mass transfer coefficient in BCs under foam control are presented. Comparison of the volumetric mass transfer coefficient between the mechanical foam‐control system and the chemical foam‐control system in terms of the specific power input also demonstrated higher mass transfer performance and saving power requirements for the mechanical foam‐control system.

The characteristics of aerated 12- and 18-blade Rushton turbines at transitional Reynolds numbers

Rushton turbines with 6, 12 and 18 blades have been studied under unaerated and aerated conditions in water and 0.8% CMC solutions giving fully turbulent and transitional flow regimes, respectively. In water, as reported earlier, enhanced blade numbers give both a higher ratio of gassed to ungassed power, P g P , and a greater air handling capacity at an equal specific power input. However, in the transitional regime, it is found that these advantages no longer exist. This is because, at these Reynolds numbers, the size of the gas-filled cavities is greater and independent of air flow rate. Thus, P g P falls to values as low as 0.4, regardless of the number of blades. Comparison with radial flow Scaba 6SRGT impellers in which the flat blade of the Rushton is replaced with one parabolic in cross-section shows that changing the shape is always a superior way of improving impeller performance, compared to increasing the number of flat blades, especially if retrofitting is required.

Scale up of the mixer of a mixer-settler model used in a uranium solvent extraction process

Scale up relations were obtained for the mixer of a box type mixer-settler used in an uranium extraction process from chloridric leaches. Three box type mixers of different sizes and with the same geometry were used for batch and continuous-flow experiments. The correlations between the extraction rate and the specific power input,D/T ratio(=turbine diameter/mixer width) and residence time were experimentally determined. The results showed that the extraction rate increases with the power input at a constantD/T ratio equal to 1/3, remaining however, independent from the mixer size for a specific value of the power input. This behaviour was observed for power input values ranging from 100 to 750 W/m3.

Vehicle-and-trajectory optimization of nuclear electric spacecraft for lunar missions

A new combined vehicle-and-trajectory optimization problem is solved for a low-thrust nuclear-electricpropulsion spacecraft whose motion is governed by restricted three-body-problem dynamics for the Earth-moon system. The problem involves computing the optimal spacecraft sizing parameters and trajectory design variables that result in the maximum payload for a fixed-trip-time, planar transfer from circular low Earth orbit to circular low-lunar orbit. In particular, the optimal specific impulse and input power are computed. The detailed vehicleand-trajectory optimization approach is effective in solving a complex interactive problem that is important to both spacecraft and mission designers. Several numerical solutions are obtained for a wide range of trip times.

Aerobic thermophilic treatment of sewage sludge at pilot plant scale. 2. Technical solutions and process design

The performance of the ATS process depends essentially on the oxygen transfer efficiency. Improvement of the mass transfer capacity of a bioreactor allowed to reduce the incubation time necessary to attain sludge stabilization. It is important to use equipment with a high aeration efficiency such as an injector aeration system. The ratio between the total oxygen consumption and the organic matter degradation (ΔCOD) ranged between 0.4 and 0.8 in the pilot plant, whereas 1.23 was found in completely mixed bioreactors (Bomio, 1990). No significant improvement of the bacterial degradation efficiency was attained with a specific power input exceeding 6–8 kW m −3. A mean residence time of less than 1 d allowed organic matter removals up to 40% with specific power consumption of 10 kWh kg −1 COD oxidized. The sludge hygienization is one of the objectives and benefits of the thermophilic treatment: not only temperature but also the total solids content were important factors affecting inactivation of pathogens. The inactivation rate was promoted by the increase of temperature, while the residual colony forming units decreased with reducing the total solids content of sewage sludge. It is concluded that continuous operation mode would not affect the quality of the hygienization but could display the high degradation potential of the aerobic system.

Aerobic thermophilic treatment of sewage sludge at pilot plant scale. 1. Operating conditions

The aerobic thermophilic treatment process of sewage sludge was studied at different bioreactor scales in a pilot plant installation. Since, for a satisfactory sludge disinfection, the Swiss legislation requires minimal incubation times of all volume elements, the bioreactors were operated in repetitive batch mode (draw and fill). Different retention times and frequencies of the volume changes were applied in order to prove the capability of the particular operation modes in assuring high degradative potential. The main enzymatic activity involved during the aerobic treatment was proteolysis: the RQ values ranged between 0.8 and 0.9 depending on the applied operating conditions. Although not in a linear manner, the efficiency of the microflora decreased as the bioreactor scale increased, when this increase corresponded with a reduction of the specific power input. The sludge oxidation rates can be tuned by some process operating conditions such as the volume change frequency, the changed volume quantities and the retention times. It was possible to improve the microbial degradative efficiency by an increased frequency of the changes, while the mean retention time influenced in particular the ultimate product quality, described as residual organic matter content of the sludge. The microflora present was also satisfactorily active at mean hydraulic retention times of less than 10 h. The organic matter concentration of the inlet sewage sludge plays an important role: it influences the aerobic degradation process positively.

Modelling the increased gas capacity of self-inducing impellers

In many gas—liquid mixing processes occuring in stirred vessels, the rate of reaction is limited by mass transfer between the gas and liquid phases, e.g. acetic acid fermentation. An important aspect in the design of such devices is to increase the rate of reaction by improving the mass transfer performance. This study examines the effect of increasing the number of gas outlet orifices on each blade on the gas induction rate and mass transfer performance of a concave-bladed self-inducing impeller. Increasing the number of orifices is found to significantly increase the rate of gas induction, however, this effect diminishes as the number of orifices increases. Volumes of liquid per unit volume of gas per minute (VVMs) of up to 0.6 can be achieved using all four orifices on each blade at an impeller speed of 10 rps; this may be compared to a value of 0.25 obtained under similar conditions for a single orifice. Furthermore, this method of increasing the gas capacity is not at the expense of generating bigger bubbles, and thus large gas—liquid interfacial areas are maintained. This is reflected in experimental measurements of the mass transfer coefficient; the value of k L a for a given specific power input approximately doubles in moving from one to four orifices per blade. Mass transfer coefficients of up to 0.055s −1 (at a specific power input of 1.7 kW/m 3) are attainable for the coalescing air—water system. An existing model for gas-inducing impeller design (Evans., 1991a) is extended to include the effect of multiple orifices. The revised model gives acceptable predictions of the induced gas rate, to within 20% of the experimental data, over the full range of measurements.