Influence of operating variables on liquid circulation in a 10.5-m(3) jet loop bioreactor

A 10.5-m(3) concentric tube jet loop reactor was used to study the influence of the working liquid volume, mean superficial air velocity, operating pressure, downcomer aeration, liquid jet velocity, and two ratios of draft tube/reactor diameter (D(t)/D) on liquid circulation time (T(c)). The experiments were carried out in a water-air system with the use of the acid pulse method. Results showed that circulation time was independent of the working liquid volume over a certain minimum liquid level, whereas downcomer aeration and D(t)/D ratio appeared as amenable parameters to achieve a high degree of control over liquid circulation and mixing efficiency, and to optimize the overall reactor performance. Increasing the operating pressure caused a reduction of the liquid circulation rate. However, ionger residence times of the air bubbles and the higher mass transfer driving force that result at higher pressures improve oxygen utilization. The relationship between T(c) and air load was independent of the operating pressure, provided the correlation is given as a function of the mean superficial air velocity. Neither liquid circulation nor gas holdup were significantly affected by liquid jet velocity. (c) 1995 John Wiley & Sons, Inc.     

Visualization of High Speed Liquid Jet Impaction on a Moving Surface

Two apparatuses for examining liquid jet impingement on a high-speed moving surface are described: an air cannon device (for examining surface speeds between 0 and 25 m/sec) and a spinning disk device (for examining surface speeds between 15 and 100 m/sec). The air cannon linear traverse is a pneumatic energy-powered system that is designed to accelerate a metal rail surface mounted on top of a wooden projectile. A pressurized cylinder fitted with a solenoid valve rapidly releases pressurized air into the barrel, forcing the projectile down the cannon barrel. The projectile travels beneath a spray nozzle, which impinges a liquid jet onto its metal upper surface, and the projectile then hits a stopping mechanism. A camera records the jet impingement, and a pressure transducer records the spray nozzle backpressure. The spinning disk set-up consists of a steel disk that reaches speeds of 500 to 3,000 rpm via a variable frequency drive (VFD) motor. A spray system similar to that of the air cannon generates a liquid jet that impinges onto the spinning disc, and cameras placed at several optical access points record the jet impingement. Video recordings of jet impingement processes are recorded and examined to determine whether the outcome of impingement is splash, splatter, or deposition. The apparatuses are the first that involve the high speed impingement of low-Reynolds-number liquid jets on high speed moving surfaces. In addition to its rail industry applications, the described technique may be used for technical and industrial purposes such as steelmaking and may be relevant to high-speed 3D printing.     

Disruption of Candida utilis cells in high pressure flow devices*

The disruption of Candida utilis cells in suspensions subjected to different types of stress was investigated. Stresses caused by impingement of a high velocity jet of suspended cells against a stationary surface were found to be significantly more effective for disruption than either shear or normal stresses. The fraction of cells disrupted by impingement is a first order function of the number of passes through the disruptor and, over a prescribed range of operating pressures, is a power function of pressure. These results indicate that impingement is the predominant mechanism causing cells disruption in high pressure flow devices such as Manton–Gaulin homogenizers. The impingement results suggest that cells grown in cyclic batch culture are easier to disrupt than cells grown at a lower specific growth rate in continuous culture. In addition to determining the fraction of cells disrupted, the release of invertase activity was determined for the impingement experiments. The fraction of total invertase activity released was found to be somewhat greater than the fraction of cells disrupted.     

Thermal chip fabrication with arrays of sensors and heaters for micro-scale impingement cooling heat transfer analysis and measurements

The design and fabrication for a thermal chip with an array of temperature sensors and heaters for study of micro-jet impingement cooling heat transfer process are presented. This thermal chip can minimize the heat loss from the system to the ambient and provide a uniform heat flux along the wall, thus local heat transfer processes along the wall can be measured and obtained. The fabrication procedure presented can reach a chip yield of 100%, and every one of the sensors and heaters on the chip is in good condition. In addition, micro-jet impingement cooling experiments are performed to obtain the micro-scale local heat transfer Nusselt number along the wall. Flow visualization for the micro-impinging jet is also made. The experimental results indicate that both the micro-scale impinging jet flow structure and the heat transfer process along the wall is significantly different from the case of large-scale jet impingement cooling process.     

Molecular dynamics studies in nanoscale liquid structures: geometry and thermal effects on nanojet development

Conventional macroscopic jet theory relies heavily on experimental correlations which cannot be easily extended to the nanoscale regime. Moreover, the fluid dynamic effects at small length scales and their contribution to the development of nanoscale liquid structures are fundamentally different from their macroscopic counterparts. This coupled with the high spatial and temporal resolution requirements at nanoscale domains make molecular dynamics (MD) an excellent tool for studying such structures. In this study, the formation and breakup of nanojets (NJs) developing from high pressure into vacuum is investigated using MD based on non-Hamiltonian formulations. By ejecting the equilibrated argon atoms through various nozzle geometries and diameters, nanoscale jet flows were generated. The dependence of the jet structure on nozzle geometry and diameter is studied. The influence of geometry on NJ formation is also studied along with issues involved in the equilibration and thermostat coupling parameter. Various thermostats are compared to understand the role they play in MD simulations of liquid nanostructures. Tuning of the thermostat coupling parameter has also been discussed. The jet breakup phenomenon is analysed and a comparative study, vis-à-vis, well-established continuum and stochastic models, is attempted.     

Quantifying Cell Adhesion through Impingement of a Controlled Microjet

The impingement of a submerged, liquid jet onto a cell-covered surface allows assessing cell attachment on surfaces in a straightforward and quantitative manner and in real time, yielding valuable information on cell adhesion. However, this approach is insufficiently characterized for reliable and routine use. In this work, we both model and measure the shear stress exerted by the jet on the impingement surface in the micrometer-domain, and subsequently correlate this to jet-induced cell detachment. The measured and numerically calculated shear stress data are in good agreement with each other, and with previously published values. Real-time monitoring of the cell detachment reveals the creation of a circular cell-free area upon jet impingement, with two successive detachment regimes: 1), a dynamic regime, during which the cell-free area grows as a function of both the maximum shear stress exerted by the jet and the jet diameter; followed by 2), a stationary regime, with no further evolution of the cell-free area. For the latter regime, which is relevant for cell adhesion strength assessment, a relationship between the jet Reynolds number, the cell-free area, and the cell adhesion strength is proposed. To illustrate the capability of the technique, the adhesion strength of HeLa cervical cancer cells is determined ((34 ± 14) N/m²). Real-time visualization of cell detachment in the dynamic regime shows that cells detach either cell-by-cell or by collectively (for which intact parts of the monolayer detach as cell sheets). This process is dictated by the cell monolayer density, with a typical threshold of (1.8 ± 0.2) × 10⁹ cells/m², above which the collective behavior is mostly observed. The jet impingement method presents great promises for the field of tissue engineering, as the influence of both the shear stress and the surface characteristics on cell adhesion can be systematically studied.     

Hydrodynamics and performance in fluidized bed adsorption

The performance of fluidized bed adsorption is strongly influenced by the hydrodynamics of the fluidization process. Especially axial mixing in the liquid and solid phase may lead to reduced capacity and resolution. In this article axial mixing in the liquid phase of a classified fluidized bed based on porous glass granules is presented. Axial mixing was analyzed by measurements of residence time distributions in a fluidized bed, showing a reduction of mixing at increased ratio of bed height to diameter as well as at increased linear velocity of the liquid stream. These results were transferred to two real adsorption systems on two different scales: In a bench scale (up to 15 mL of adsorbent) the purification of monoclonal antibodies from hybridoma supernatant was performed with a cation exchanger, in a larger scale (up to 750 mL of matrix) the adsorption of bovine serum albumin (BSA) on the same matrix was investigated. The results showed an increase of capacity at increased bed height-to-diameter ratio; with regard to linear velocity a broad range of only slightly changed capacity was found. A shift from dispersion controlled to diffusion controlled adsorption at intermediate linear velocity was proposed by isolating the effect of pore diffusion from the effect of dispersion. (c) 1995 John Wiley & Sons, Inc.     

Hydrodynamics, mass transfer, and yeast culture performance of a column bioreactor with ejector

A bubble column fitted with an ejector has been tested for its physical and biological performance. The axial diffusion coefficient of the liquid phase in the presence of electrolytes and ethanol was measured by a stimulus-response technique with subsequent evaluation by means of a diffusion model. In contrast to ordinary bubble columns, the coefficient of axial mixing is inversely dependent on the superficial air velocity. The liquid velocity acts in an opposite direction to the backmixing flow in the column. The measurement of volumetric oxygen transfer coefficient in the presence of electrolytes and ethanol was performed using a dynamic gassing-in method adapted for a column. The data were correlated with the superficial air and liquid velocities, total power input, and power for aeration and mixing; the economy coefficient of oxygen transfer was used for finding an optimum ratio of power for aeration and pumping. Growth experiments with Candida utilis on ethanol confirmed some of the above results. Biomass productivity of 2.5 g L(-1) h(-1) testifies about a good transfer capability of the column. Columns fitted with pneumatic and/or hydraulic energy input may be promising for aerobic fermentations considering their mass transfer and mixing characteristics.     

Mixing time and oxygen transfer characteristics of double draft tube airlift fermentor

Despite the increasing importance of airlift fermentors, very little published information is available on how the geometric configurations of the draft tubes and the air-sparging system affect the mixing and oxygen transfer characteristics of the fermentor. A 14-L air-lift fermentor was designed and build with a fixed liquid height to diameter ratio of 1.5 utilizing four equally spaced air jets at the bottom. Two jet orifice sizes were used, 1.27 and 3.81 mm i.d., and for each jet size the following four geometric configurations were used: Single inner concentric draft tube, single outer concentric draft tube, two concentric draft tubes, and no draft tubes where the fermentor was operated as a shallow bubble column. It was found that the presence of draft tubes stabilized liquid circulation patterns and gave systemically higher mixing times than those obtained in the absence of draft tubes. In addition, the double draft tube geometry resulted in higher mixing times than the single draft tubes. For the power unit volume range 20 to about 250 W/m³ the larger 3.81-mm orifices gave systemically higher k_L a values than the smaller 1.27-mm i.d. orifices. At 200 W/m³ the use of a single outer draft tube with the 3.81-mm orifices resulted in 94% increase in k_L a values over that obtained with no draft tubes. However, the effect of draft tube geometry on k_L a values when the 1.27-mm orifices were used was not significant. The air bubble formation characteristics at the jet orifices were found to be different, which reflected the differences observed in mass transfer and mixing characteristics. The power economy for oxygen transfer was found to be depend strongly on the orifice size and less on the geometric configuration of draft tubes.     

Exploring high frequency temporal fluctuations in the terminal aneurysm of the basilar bifurcation

Cerebral aneurysms are a common cause of death and disability. Of all the cardiovascular diseases, aneurysms are perhaps the most strongly linked with the local fluid mechanic environment. Aside from early in vivo clinical work that hinted at the possibility of high-frequency intra-aneurysmal velocity oscillations, flow in cerebral aneurysms is most often assumed to be laminar. This work investigates, through the use of numerical simulations, the potential for disturbed flow to exist in the terminal aneurysm of the basilar bifurcation. The nature of the disturbed flow is explored using a series of four idealized basilar tip models, and the results supported by four patient specific terminal basilar tip aneurysms. All four idealized models demonstrated instability in the inflow jet through high frequency fluctuations in the velocity and the pressure at approximately 120?Hz. The instability arises through a breakdown of the inflow jet, which begins to oscillate upon entering the aneurysm. The wall shear stress undergoes similar high-frequency oscillations in both magnitude and direction. The neck and dome regions of the aneurysm present 180 deg changes in the direction of the wall shear stress, due to the formation of small recirculation zones near the shear layer of the jet (at the frequency of the inflow jet oscillation) and the oscillation of the impingement zone on the dome of the aneurysm, respectively. Similar results were observed in the patient-specific models, which showed high frequency fluctuations at approximately 112 Hz in two of the four models and oscillations in the magnitude and direction of the wall shear stress. These results demonstrate that there is potential for disturbed laminar unsteady flow in the terminal aneurysm of the basilar bifurcation. The instabilities appear similar to the first instability mode of a free round jet.     

The influence of vessel height and top-section size on the hydrodynamic characteristics of airlift fermentors

Fermentations of the yeast Saccharomyces cerevisiae were carried out in a 90 to 250-L working volume concentric tube airlift fermentor. Measurements of liquid circulation velocity, gas hold-up, and liquid mixing were made under varying conditions of gas flowrate, vessel height, and top-section size. Both liquid circulation velocity and mixing time increased with vessel height. Liquid velocity varied approximately in proportion to the square root of column height, supporting a theoretically based relationship. The effect of vessel height on gas hold-up was negligible. The height of the top-section had a significant effect on liquid mixing. Mixing time decreased with increasing size of the top-section up to a critical height. As the top-section was expanded beyond this height, little improvement in mixing was seen. This indicated the presence of a two-zone flow pattern in the top-section. Liquid velocity and gas hold-up were essentially independent of top-section height. (c) 1994 John Wiley & Sons, Inc.     

Macromixing hydrodynamic study in draft-tube airlift reactors using electrical resistance tomography

The present study summarizes results of mixing characteristics in a draft tube airlift bioreactor using ERT. This technique offers the possibility for noninvasive and nonintrusive visualization of flow fields in the bioreactor and has rarely been utilized previously to analyze operating parameters and mixing characteristics in this type of bioreactors. Several operating parameters and geometric characteristics were examined. In general, results showed that the increase in superficial gas velocity corresponds to an increase in energy applied and thus, to a decrease in mixing time. This generally corresponded to an increase in liquid circulation velocity and shear rate values. Bottom clearances and draft tube diameters affected flow resistance and frictional losses. The influence of sparger configurations on mixing time and liquid circulation velocity was significant due to their effect on gas distribution. However, the effect of sparger configuration on shear rate was not significant, with 20% reduction in shear rates using the cross-shaped sparger. Fluid viscosity showed a marked influence on both mixing times and circulation velocity especially in the coalescing media of sugar and xanthan gum (XG) solutions. Results from this work will help to develop a clear pattern for operation and mixing that can help to improve several industrial processes, especially the ones related to emerging fields of technology such as the biotechnology industry.     

Enzyme recovery during gas/liquid two-phase flow microfiltration of enzyme/yeast mixtures

The effect of a gas/liquid two-phase flow on the recovery of an enzyme was evaluated and compared with standard crossflow operation when confronted with the microfiltration of a high-fouling yeast suspension. Ceramic tubular and flat sheet membranes were used. At constant feed concentration (permeate recycling) and transmembrane pressure, the results obtained with the tubular membrane were dependent on the two-phase flow pattern. In comparison with single-phase flow performances at the same liquid velocity, the enzyme transmission was maintained at a high level with a bubble flow pattern but it decreased by 70% with a slug flow, whatever the flow rate ratio. Identical results were obtained with flat sheet membranes: for the highest flow rate ratio, the enzyme transmission was reduced by 70% even though the permeate flux was improved by 240%. During diafiltration experiments with the tubular membrane, it was found that a bubble flow pattern led to a 13% higher enzyme recovery compared to single-phase flow conditions, whereas with a slug flow the enzyme recovery was strongly reduced. With bubble flow conditions, energy consumption was minimal, confirming that this flow pattern was the most suitable for enzyme recovery.     

Studies on transfer processes in mixing vessels: power consumption of the modified Rushton turbine agitators in three-phase systems

In this work, the modified blade turbines, with the surface fraction of the perforations equal to 0.353, are used in the study of the relationship between the power consumption in three-phase systems and the superficial gas velocity, mixing intensity, agitator type, solid particles concentration, physical and rheological properties of the suspensions. The power consumption in three-phase mixing with modified turbines, single and double, is approximately 40% smaller than for the standard turbines at the same mixing intensity. The modified blade turbines are simple to manufacture and do not require any modification in electrical motor and drive assemblies.     

A formulation for calculating the translational velocity of a vortex ring or pair

Cephalopods, among other marine animals, use jet propulsion for swimming. A simple actuator is designed to loosely mimic pulsatile jet formation in squid and jellyfish. The actuator is comprised of a cavity with an oscillating diaphragm and an exit orifice. Periodic oscillation of the diaphragm results in the formation of an array of vortex rings and eventually could generate a periodic pulsatile jet. A general formulation for calculating the velocity of a steadily translating vortical structure in two-dimensional and axi-symmetric shear flows is presented. This technique is based on taking the variational derivative of an energetic function at its critical point. This technique is general, applicable to vortices in liquid and gas media, with no limitation on the relative size of the vortex core. The technique is then implemented to estimate the translational velocity of a vortex ring in a Helmholtz vortex ring generator.     

A simplified model of hydrodynamics in airlift loop reactors

As a function of the gas throughput the following parameters were measured in an external loop reactor with a riser diameter of 0.6 m and a gassed liquid height of 8.6 m: integral and local values of gas hold-up; liquid velocities; mixing times and axial dispersion coefficients of the liquid phase. The height of the reactor could be altered by reconstruction. Measurements were also carried out with lower heights than 8.6 m. Besides pure water, aqueous solutions of coalescing, non-coalescing and viscosity-increasing substances were used as model systems. With the results a general relationship between superficial gas velocity, gas hold-up and liquid velocity was established. This hydrodynamic model uses the relative velocity between gas and liquid phase as the fundamental parameter. The generally valid model consists of one term for the homogeneous and of two additional terms for the heterogeneous flow regime.     

Scale-up of enzymatic production of lactobionic acid using the rotary jet head system

Enzymatic oxidation of lactose to lactobionic acid (LBA) by a carbohydrate oxidase from Microdochium nivale was studied in a pilot-scale batch reactor of 600 L working volume using a rotary jet head (RJH) for mixing and mass transfer (Nordkvist et al., 2003, Chem Eng Sci 58:3877-3890). Both lactose and whey permeate were used as substrate, air was used as oxygen source, and catalase was added to eliminate the byproduct hydrogen peroxide. More than 98% conversion to LBA was achieved. Neither enzyme deactivation nor enzyme inhibition was observed under the experimental conditions. The dissolved oxygen tension (DOT) was constant throughout the tank for a given set of operating conditions, indicating that liquid mixing was sufficiently good to avoid oxygen gradients in the tank. However, at a given oxygen tension measured in the tank, the specific rate of reaction found in the RJH system was somewhat higher than previously obtained in a 1 L mechanically stirred tank reactor (Nordkvist et al., 2007, in this issue, pp. 694-707). This can be ascribed to a higher pressure in the recirculation loop which is part of the RJH system. Compared to mechanically stirred systems, high values of the volumetric mass transfer coefficient, k(L)a, were obtained when lactose was used as substrate, especially at low values of the specific power input and the superficial gas velocity. k(L)a was lower for experiments with whey permeate than with lactose due to addition of antifoam. The importance of mass transfer and of the saturation concentration of oxygen on the volumetric rate of reaction was demonstrated by simulations.     

Comparison of fish impingement by a thermal power station with fish populations in the Ems Estuary

On an annual basis >10 million individuals of c . 35 fish species are impinged at the EC20 unit of the Eems power station located on the Ems Estuary. The most abundant are: herring Clupea harengus , gobies ( Gobiidae ), Nilsson's pipefish Sygnathus rostellatus , three-spined stickleback Gasterosteus aculeatus and sprat Sprattus sprattus . The impingement shows a seasonal pattern which reflects the presence of the fishes in the estuary. The results are compared with other impingement studies at power stations on the Belgian and Dutch estuaries.
In 1995, the cooling water entrance was displaced from the shoreline to 300 m off the coast at the edge of the tidal channel. The effect of this relocation on the level of fish impingement, determined by comparing the ratio of fish density in the cooling water and in the estuary before (1992–1993) and after (1996–1997) the relocation of the intake canal is discussed.     

Unsteady Convection Flow and Heat Transfer over a Vertical Stretching Surface

This paper investigates the effect of thermal radiation on unsteady convection flow and heat transfer over a vertical permeable stretching surface in porous medium, where the effects of temperature dependent viscosity and thermal conductivity are also considered. By using a similarity transformation, the governing time-dependent boundary layer equations for momentum and thermal energy are first transformed into coupled, non-linear ordinary differential equations with variable coefficients. Numerical solutions to these equations subject to appropriate boundary conditions are obtained by the numerical shooting technique with fourth-fifth order Runge-Kutta scheme. Numerical results show that as viscosity variation parameter increases both the absolute value of the surface friction coefficient and the absolute value of the surface temperature gradient increase whereas the temperature decreases slightly. With the increase of viscosity variation parameter, the velocity decreases near the sheet surface but increases far away from the surface of the sheet in the boundary layer. The increase in permeability parameter leads to the decrease in both the temperature and the absolute value of the surface friction coefficient, and the increase in both the velocity and the absolute value of the surface temperature gradient.     

Improvement of mixing time,mass transfer,and power consumption in an external loop airlift photobioreactor for microalgae cultures

Longer mixing times and higher power consumption are common problems in the design of photobioreactors. In this study, a vertical triangular external airlift loop photobioreactor was designed, constructed and operated for microalgae production studies. Gas feeding was performed by two spargers: one at the bottom of the hypotenuse (downcomer) and another at the bottom of the vertical side (riser). This configuration provided more effective countercurrent liquid–gas flow in the hypotenuse. The mass transfer coefficient, gas hold-up, mixing time, circulation time, dimensionless mixing time, bubble size, and volumetric power consumption were measured and optimized using response surface methodology. Investigations were carried out on the performance of the riser (the vertical side), downcomer (the hypotenuse), and separator. The countercurrent flow in the hypotenuse provided sufficient contact between gas and liquid phases, and increased mixing and mass transfer rates, in contrast to the results of previous studies. The promising results of this geometry were shorter mixing time and a significant decrease in volumetric power consumption in comparison with other configurations for photobioreactors.