Scale-up of stirring as foam disruption (SAFD) to industrial scale

Foam disruption by agitation—the stirring as foam disruption (SAFD) technique—was scaled up to pilot and production scale using Rushton turbines and an up-pumping hydrofoil impeller, the Scaba 3SHP1. The dominating mechanism behind SAFD—foam entrainment—was also demonstrated at production scale. The mechanistic model for SAFD defines a fictitious liquid velocity generated by the (upper) impeller near the dispersion surface, which is correlated with complete foam disruption. This model proved to be scalable, thus enabling the model to be used for the design of SAFD applications. Axial upward pumping impellers appeared to be more effective with respect to SAFD than Rushton turbines, as demonstrated by retrofitting a 12,000 l bioreactor, i.e. the triple Rushton configuration was compared with a mixed impeller configuration from Scaba with a 20% lower ungassed power draw. The retrofitted impeller configuration allowed 10% more broth without risking excessive foaming. In this way a substantial increase in the volumetric productivity of the bioreactor was achieved. Design recommendations for the application of SAFD are given in this paper. Using these recommendations for the design of a 30,000 l scale bioreactor, almost foamless Escherichia coli fermentations were realised. Electronic Publication     

Influence of impeller type on power input in fermentation vessels

Prior investigations comparing radial flow Rushton impellers with axial flow hydrofoil impellers (Maxflo T and A315) were extended at the pilot scale. Six types of impellers (disk-style Rushton, Prochem Maxflo T hydrofoils of three diameters pumping downwards and A315 hydrofoils pumping upwards and downwards) were compared for qualitative differences in power number behavior with Reynolds' number, single versus double impeller power draw, gassed power reduction with aeration number and gas hold-up. Power measurements were obtained using watt transducers which, although limited in accuracy and prone to interferences, were able to provide useful qualitative monitoring results. Measurements were conducted using three model liquid systems: water, glycerol and Melojel (soluble starch). Apparent viscosities for actual Streptomyces cultivations were estimated using measured gassed power values and the experimental relationships obtained for gassed/ungassed power to aeration number and power number to Reynolds' number for the glycerol model system. Results confirmed the lower power number and lower shear environment for hydrofoil impellers, yet suggested useful trends for various process parameters and process fluids.     

Carbon monoxide mass transfer for syngas fermentation in a stirred tank reactor with dual impeller configurations

This study compares the power demand and gas-liquid volumetric mass transfer coefficient, kLa, in a stirred tank reactor (STR) (T = 0.211 m) using different impeller designs and schemes in a carbon monoxide-water system, which is applicable to synthesis gas (syngas) fermentation. Eleven different impeller schemes were tested over a range of operating conditions typically associated with the "after large cavity" region (ALC) of a Rushton-type turbine (D/T = 0.35). It is found that the dual Rushton-type impeller scheme exhibits the highest volumetric mass transfer rates for all operating conditions; however, it also displays the lowest mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) for all conditions due to its high power consumption. Dual impeller schemes with an axial flow impeller as the top impeller show improved mass transfer rates without dramatic increases in power draw. At high gas flow rates, dual impeller schemes with a lower concave impeller have kLa values similar to those of the Rushton-type dual impeller schemes but show improved mass transfer performance. It is believed that the mass transfer performance can be further enhanced for the bottom concave impeller schemes by operating at conditions beyond the ALC region defined for Rushton-type impellers because the concave impeller can handle higher gas flow rates prior to flooding.     

Effects of paddle impeller geometry on power input and mass transfer in small-scale animal cell culture vessels

Process scaleup for stirred-tank animal cell cultures such as suspension and microcarrier cultures often begins at the bench scale in small spinner vessels. In order to initiate process development under the proper conditions, it is essential to know the physical conditions under which the cells are grown. In this article, power inputs and surface oxygen transfer rates to culture medium in 500-mL Corning spinner vessels were determined as a function of the impeller geometry, impeller height, and agitation speed. The results obtained indicate that power dissipation dependency differs from literature correlations and may compromise scale up at constant power input from these vessels. These results are of general utility to researchers using small-scale vessels.     

A fluid dynamic study of the retrofitting of large agitated bioreactors: Turbulent flow

Studies were conducted in three 19-m(3) fermentors (14 m(3) working volume, aspect ratio = 3:1), one fitted with four Rushton turbines (D/T = 0.35), one with three Lightnin' A315 hydrofoil impellers (D/T = 0.46). The power drawn under the same aerated conditions relative to the unaerated ones was always greater with the hydrofoils, which gives them the potential for enhanced mass transfer rates under practical operating conditions. However, the power draw was also sensitive to the magnitude of the unaerated power. Indeed, at low unaerated specific power ( approximately 0.6 W-kg) and high air flow rates ( approximately 1vvm), the relative power draw with the hydrofoils could be even greater than 1. The hold-up with each of the impellers was broadly similar at the same aeration rate and power input, though the later had a much smaller impact in these large vessels than has been reported in the literature based on smaller scale work. As usual, repressed coalescence caused increased hold-up, and, with the hydrofoils, this increase was associated with a lower power draw. Because of the greater mechanical vibration of the reactors with the hydrofoils, vibration characteristics of the vessels were measured and they were very similar. The results showed that provided care is taken in the mechanical design of the system, such impellers can operate reliably in large-scale fermentations with the potential for enhanced biological performance. (c) 1994 John Wiley & Sons, Inc.     

Hong Kong’s worst “red tide”—causative factors reflected in a phytoplankton study at Port Shelter station in 1998

From March through April 1998, a massive “red tide” occurred in coastal waters of south China, including Hong Kong. The “red tide” rapidly killed various species of caged fish and affected coral fishes, killing a few of them, and caused great economic loss and ecological damage. Samples collected from a permanent station located in Port Shelter revealed a new dinoflagellate species, Karenia digitata which was suspected to be the causative species of this “red tide”. Species composition and abundance analysis revealed that an algal bloom persisted in Port Shelter during this entire period. Diatoms and dinoflagellates were the two main groups which dominated the phytoplankton and, in general, when there was an increase in the density of diatoms there was a decline in the density of dinoflagellates, and vice versa. The suspected “red tide” causative species together with other dinoflagellate species started to bloom in late February and reached their highest density on 18 March, when fish kills were first reported at Crooked Island, a semi-enclosed bay to the northeast of Hong Kong. During a 16-week period, dinoflagellate species dominated three times, and coincided with low wind speeds. Constant salinity and a continuing increase in sea surface water temperature suggested warm water intrusion into Hong Kong’s coastal waters during this “red tide” bloom period. Various nutrient elements, e.g. NH₄-N, total Kjeldahl nitrogen (TKN), and PO₄-P were high at the beginning of the bloom but experienced a sharp decrease thereafter. It is suggested that this early 1998 massive “red tide” in Hong Kong waters might have been triggered by a synchronous appearance of optimal climatic, nutritional and hydrographic conditions.     

“Harpoon” Model for Cell–Cell Adhesion and Recognition of Target Cells by the Natural Killer Cells

The mechanism of recognition by natural killer (NK) cells is still unknown. A dynamic model is formulated describing recognition or NK-sensitive target cells (TCs) by NK cells of NK-like cells. This model does not assume the presence of the specific NK-receptor(s) on the membrane of NK cells and corresponding specific ligands on the NK-sensitive TCs. We suggest: (1) the expression of various kinds of “non-NK receptors” and corresponding ligands (counter-receptors) on the plasma membrane of the same NK cell and, possibly, of TCs (e.g. LFA-1 and ICAM-1-ICAM3, CD2 and LFA-3; receptors for TNF and corresponding ligand etc.); (2) the presence of multiple disorders in the organization of “extracellular matrix-surface membrane-submembrane cytoskeleton” assembly of the NK-sensitive TCs; (3) non-specific primary linking of NK cell with TCs, which induces a transfer of vesicles or membrane fragments from the NK surface to the target cell surface (and perhaps vice versa). These processes may also permit the transfer of many types of receptor and counter-receptor molecules from the surface of one conjugated cell to another by vesicles or membrane fragments. After transferral through the intercellular cleft, the free receptors and counter-receptors will be localized on both cell surfaces at the contact region between conjugated cells. By this model the NK cell can “harpoon” the TC and enhance the binding forces between cells up to the critical level and then switch on killing mechanisms for the TC. By means of this “harpoon” model of cell recognition, it seems possible to explain the nature of the wide polymorphism of TCs which are sensitive to the effect of NK and NK-like cells. A mathematical model of the NK cell cytotoxic reaction is described. The model describes many nonlinear peculiarities of the cytotoxic process and predicts some new phenomena. We suggest new approaches of manipulation of cell membranes which can transform NK-resistant target cells in NK sensitive cells and vice versa.     

Power input measurements in a production scale bioreactor

Summary Power input measurements are carried out in a production bioreactor with a liquid volume up to 25 m³. The results show that the cavity formation principle is applicable to reactors at this scale. It can also be observed that empirical correlations are not useful to predict gassed power input accurately. It is found that at gas flow rates for normal production conditions (N_Q =0.1), the gassed power input is about 30–40 % of the non gassed power input.Nomenclature C_p specific heat J/kgK - D impeller diameter m - D_b1 impeller blade diameter m - d baffle diameter m - Fr Froude number - - g gravitation m/s² - h impeller clearance m - H liquid height m - N stirrer speed s^-1 - N_p power number - - N_Q gas flow (aeration) number - - N_Q ^* critical gasflow number for 3 cavity formation - - P_o ungassed power consumption W - P_g gassed power consumption W - Q gas flow rate (273 K, 10⁵ N/m²) m³/s - Re Reynolds number - - T tankdiameter m temperature K - t time s - V liquid volume m³ - ^Vtip impeller tip speed m/s - V_s impeller correlated superficial gas flow rate m/s - W impeller blade width m - density kg/m³     

Hydrology of winter–spring “red tides” in Bahía de Mazatlán, Sinaloa, México

“Red tide” events are frequent and periodical in Bahía de Mazatlán, Sinaloa, México. Yet, the ones observed from 4 February to 4 June 2000, showed some distinctive features: First, the dinoflagellates Prorocentrum balticum (85%), P. mexicanum (5%), and Ceratium furca (5%), dominated the composition of the blooms; Second, the average cell abundance by date was 1.3×10⁶ cells l⁻¹, with a range of 3.5×10³ to 24,500 × 10³ cells l⁻¹, well above previous records; Third, the temperature registered at 10–20 m deep was unusually cold (19 °C), below the normal average of 21.5 °C observed over the last 10 years. Salinity was high (35.9 psu) and showed very little influence on the water density gradient. A mean thermal stratification index (TSI), of 3.4, with a maximum of 7 °C, was observed throughout the period, in spite of a weak upwelling activity and intermittent strong NW winds which were unable to break up water column stratification. Temperature fluctuations at the surface and at the bottom layers showed a 30-day periodicity, suggesting some association with the lunar cycle. To explain the characteristics of the “red tides” registered in Bahía de Mazatlán during the winter–spring period of year 2000, it is proposed that the temperature and density stratification, stabilized further by internal waves that compensated for the weak upwelling activity and provided the necessary nutrients to the surface layer, favored the persistence and intensity of the harmful algal bloom events then observed.     

Alkaline Water Electrolysis at 25 A cm−2 with a Microfibrous Flow‐through Electrode

The generation of renewable electricity is variable, leading to periodic oversupply. Excess power can be converted to H₂ via water electrolysis, but the conversion cost is currently too high. One way to decrease the cost of electrolysis is to increase the maximum productivity of electrolyzers. This study investigates how nano‐ and microstructured porous electrodes can improve the productivity of H₂ generation in a zero‐gap, flow‐through alkaline water electrolyzer. Three nickel electrodes—foam, microfiber felt, and nanowire felt—are studied to examine the tradeoff between surface area and pore structure on the performance of alkaline electrolyzers. Although the nanowire felt with the highest surface area initially provides the highest performance, this performance quickly decreases as gas bubbles are trapped within the electrode. The open structure of the foam facilitates bubble removal, but its small surface area limits its maximum performance. The microfiber felt exhibits the best performance because it balances high surface area with the ability to remove bubbles. The microfiber felt maintains a maximum current density of 25 000 mA cm^?2 over 100 h without degradation, which corresponds to a hydrogen production rate 12.5‐ and 50‐times greater than conventional proton‐exchange membrane and alkaline electrolyzers, respectively.     

Hot Melt “Corner Point Method” for Attaching Large Plastic Sections to Glass Slides

We describe a fast method for firm attachment of large plastic sections to glass slides with EVA-copolymers, commonly known as hot melt sticks. Solid hot melt sticks dissolve slowly in xylene to form an adhesive gel within 6 hours. Small drops of hot melt gel are applied to the corners of the sections and surrounding slide surface at ambient or elevated temperatures. The gel sticks to both the plastic and the glass slides. The hot melt “corner point method” prevented detachment of sections in staining procedures. As an additional technique, we suggest the use of hot melt adhesive for attaching plastic specimen blocks to wooden blocks or metallic specimen holders.     

Flexibility versus “rigidity” of the functional architecture of AChE active center

Functional architecture of the AChE active center appears to be characterized by both structural “rigidity”, necessary to stabilize the catalytic triad as well as by flexibility in accommodating the different, high affinity AChE ligands. These seemingly conflicting structural properties of the active center are demonstrated through combination of structural methods with kinetic studies of the enzyme and its mutant derivatives with plethora of structurally diverse ligands and in particular with series of stereoselective covalent and noncovalent AChE ligands. Thus, steric perturbation of the acyl pocket precipitates in a pronounced stereoselectivity toward methylphosphonates by disrupting the stabilizing environment of the catalytic histidine rather than through steric exclusion demonstrating the functional importance of the “rigid” environment of the catalytic machinery. The acyl pocket, the cation-binding subsite (Trp86) and the peripheral anionic subsite were also found to be directly involved in HuAChE stereoselectivity toward charged chiral phosphonates, operating through differential positioning of the ligand cationic moiety within the active center. Residue Trp86 is also a part of the “hydrophobic patch” which seems flexible enough to accommodate the structurally diverse ligands like tacrine, galanthamine and the two diastereomers of huperzine A. Also, we have recently discovered further aspects of the role of both the unique structure and the flexibility of the “hydrophobic patch” in determining the reactivity and stereoselectivity of HuAChE toward certain carbamates including analogs of physostigmine. In these cases the ligands are accommodated mostly through hydrophobic interactions and their stereoselectivity delineates precisely the steric limits of the pocket. Hence, the HuAChE stereoselectivity provides a sensitive tool in the in depth exploration of the functional architecture of the active center. These studies suggest that the combination of “rigidity” and flexibility within the HuAChE gorge are an essential element of its molecular design.     

Homogeneous batch cultures of<Emphasis Type="Italic"> Aspergillus oryzae</Emphasis> by elimination of wall growth in the Variomixing bioreactor

A novel principle for mixing and aeration in stirred bioreactors, named Variomixing, was developed. Four baffles are rotated intermittently at a rotational speed slower or similar to the speed of a centrally placed axial flow impeller. Rotational speeds of the baffles and impeller of 5–10 and 500–600 rpm, respectively, results in the highly turbulent flow regime characteristic of conventional bioreactors with high mixing and mass transfer capacities. Stagnant zones around crevices and crannies in which wall growth may commence are avoided since the baffles are never completely at rest. Increasing the rotational speed of the baffles (5 s every 5 min), so that it follows the speed of the impeller (500–600 rpm), cancels the effect of the baffles and a deep vortex and high peripheral liquid flow rates at the reactor wall develop. The vortex ensures that also the head-space of the reactor wall is flushed and any deposits removed. The filamentous fungus Aspergillus oryzae has been grown in batch cultures in the Variomixing bioreactor. Compared to conventional laboratory-scale bioreactors, in which more than 30% of all biomass was found attached to walls, less than 2% of the total A. oryzae biomass was found on the walls in the Variomixing bioreactor.     

Protein recovery using gas–liquid dispersions

Two separation techniques, foam separation and colloidal gas aphrons (CGAs), both of which are based on gas–liquid dispersions, are compared as potential applications for protein recovery in downstream processing. The potential advantages of each method are described and the concentration and selectivity achieved with each method, for a range of proteins is discussed. The physical basis of foam separation is the preferential adsorption of surface active species at a gas–liquid interface, with surface inactive species remaining in bulk solution. When a solution containing surface active species is sparged with gas, a foam is produced at the surface: this foam can be collected, and upon collapse contains surface active species in a concentrated form. CGAs are microbubble dispersions (bubble diameters 10–100 μm) with high gas hold ups (>50%) and relatively high stability, which are formed by stirring a surfactant solution at speeds above a critical value (typically around 5000 rpm). It is expected that when proteins are brought into contact with aphrons, protein adsorbs to the surfactant through electrostatic and/or hydrophobic forces. The aphron phase can be separated easily from the bulk solution due to its buoyancy, thus allowing separation of protein in a concentrated form.     

Fusion with enucleated fibroblasts corrects “I-cell” defect

Fusions have been carried out between fibroblasts from patients with “I-cell” disease and enucleated human fibroblasts with a single lysosomal enzyme deficiency derived from patients with G_M1-gangliosidosis, Sandhoff disease and mannosidosis. Pure cytoplasts were obtained using cytochalasin B treatment followed by fluorescence activated cell sorting. After fusion with whole “I-cells”, the cybrid populations showed a restoration of deficient lysosomal enzyme activity and also the abnormal electrophoretic pattern characteristic for the residual hexosaminidase activity in “I-cells” was found to be corrected. The results described in this paper indicate that the defective post-translational modification, which is responsible for the multiple lysosomal enzyme deficiency, can be corrected by a factor that is stable for at least three days in enucleated cells. During this period the cytoplasmic factor can act without the need of de novo synthesis but the absence of correction in in vitro experiments shows that cellular integrity is required.     

Characterization of stirrers for screening studies of enzymatic biomass hydrolyses on a milliliter scale

The evaluation of mixing quality is an important factor for improving the geometry of stirred-tank reactors and impellers used in bioprocess engineering applications, such as the enzymatic hydrolysis of plant materials. Homogeneity depends on different factors, including the stirrer type and the reactor type (e.g., ratio of diameter/height, ratio of impeller tip diameter/reactor diameter) with or without baffles. This study compares two impellers for enzymatic hydrolysis of suspensions of biomass particles on a milliliter scale. Both impellers were derived from industrially relevant geometries, such as blade and grid stirrers, although the geometry of the second stirrer was slightly modified to an asymmetric shape. The stirrers were investigated with different stirrer–reactor configurations. This was done experimentally and with the aid of computational fluid dynamics. The flow field, mixing numbers, power characteristics and initial conversion rates of sugars were considered to compare the two stirrers. The simulated mixing numbers and power characteristics in baffled and unbaffled milliliter-scale reactors were found to be in good agreement with the measured mixing times and power consumption. The mixing numbers required to reach homogeneity were much higher for the symmetric impeller and remained at least twice as high as the mixing numbers required when using the asymmetric impeller. The highest initial sugar releases from milled corn stover suspensions were achieved with the asymmetric impeller shape. Regardless of the differences in the flow fields or mixing times, diverging enzymatic sugar releases could be confirmed for Newtonian media only.     

Turbulence, watermass stratification and harmful algal blooms: an alternative view and frontal zones as “pelagic seed banks”

Watermass stratification has been considered the essential physical condition that dinoflagellates require to bloom because of their relative inability, unlike diatoms, to tolerate the elevated shear-stress associated with water-column mixing, turbulence and high velocity, coastal currents. The swimming speeds of 71 flagellate taxa, with a focus on dinoflagellates, are compared to the turbulence fields and vertical velocities that develop during representative wind conditions, upwelling and at frontal zones. The results suggest that the classical stratification–dinoflagellate bloom paradigm needs revision. Tolerance of turbulence, growth within well-mixed watermasses and survival and dispersal while entrained within current systems are well developed capacities among dinoflagellates. Their secretion of mucous, often copious during blooms, is suggested to be an environmental engineering strategy to dampen turbulence. Biophysical tolerance of turbulence by dinoflagellates is often accompanied by high swimming speeds. Motility speeds of many species exceed in situ vertical current velocities; this also allows diel migrational patterns and other motility-based behavior to persist. Species belonging to “mixing-drift” life-form assemblages can increase their swimming speeds through chain formation, which helps to compensate for the increased turbulence and vertical water-column velocities of their habitats. The ability of dinoflagellate species to tolerate the vertical velocities of offshore, frontal zones, where abundant populations often develop, suggests that fronts may serve as “pelagic seed banks”, occurring as pelagic analogues of nearshore seed beds, from which seed stock is dispersed. The different ecologies associated with the hypothesized, “pelagic seed banks” of vegetative cells and the “seed beds” of resting stage cells deposited onto sediments are discussed. There is a contradiction in the stratification–HAB paradigm: the quiescent conditions of a stratified watermass, with its characteristic nutrient-poor conditions are expected to promote stasis of the population, rather than growth and blooms. The analyses suggest that dinoflagellate blooms do not preponderate in stratified watermasses because the bloom species are biophysically intolerant of the higher velocities and turbulence of more mixed watermasses. The watermass stratification that often accompanies flagellate blooms is probably a secondary, parallel event and less essential than some other factor(s) in triggering the observed bloom.     

Gas-liquid dispersion with dual Rushton turbine impellers

Aerated and unaerated power consumption and flow patterns in a 0.56 m diameter agitated vessel containing water with dual Rushton turbines have been studied. Under unaerated conditions with a liquid height-to-diameter ratio of 2, an impeller spacing of 2 to 3 times the impeller is required for each to draw an amount of power equal to a single impeller. For aerated conditions, if a similar spacing is used, equations for the flooding-loading transition and for power consumption for a single Rushton impeller can be extended relatively easily to dual systems. All results for this spacing are explained by reference to bulk flow patterns and gassed-filled cavity structures and the proportion of sparged gas flowing through the upper impeller is also estimated. Such a spacing is generally recommended since it maximizes the power draw and hence the potential for oxygen mass transfer. Data are presented for other spacings but the results do not fit in easily with single agitator studies because strong impeller-impeller flow pattern interactions occur.     

A portable flow chamber for in situ determination of benthic metabolism

1. Many stream ecologists are interested in determining the metabolic rates of benthic organisms, particularly those of production and respiration. It is often necessary to make these measurements on fresh material in the field at remote sites. Recirculating chambers are commonly used for this purpose.
2. A broad variety of recirculating chambers are described in the literature, but each design has inherent limitations. The most common are inability to control flow in the chamber and match it with external flow rates, and a lack of the power required to do this for extended periods. Alteration of spectral irradiance, temperature rise and elevated internal chamber pressures are additional limitations that have received little attention.
3. We have designed and constructed a flow chamber that eliminates some of these problems. The chamber utilizes a DC motor-driven propeller as an efficient recirculator (axial impeller), minimizing power requirements and it is constructed of UVB transparent acrylic to allow a full spectral complement of solar irradiance in the interior. Modular components allow the chamber to be taken apart quickly for cleaning and replacement of parts, making it more functional than some previous designs.
4. The axial impeller chamber was compared to a similar sized conventional chamber that had a small diameter return line and a high capacity centrifugal pump. The axial impeller chamber had less of a temperature rise during field incubations, lower power consumption and less internal pressure in the return line when producing equivalent water velocities.
5. The reported axial impeller design had relatively homogeneous velocity across the working section relative to other chambers and was capable of water velocities in excess of 1 m s^–1.     

The Effect of the Chopper on Granules from Wet High-Shear Granulation Using a PMA-1 Granulator

Chopper presence and then chopper speed was varied during wet high shear granulation of a placebo formulation using a PMA-1 granulator while also varying the impeller speed. The granules were extensively analyzed for differences due to the chopper. The effect of the chopper on the granules varied with impeller speed from no effect at a low impeller speed of 300 rpm to flow interruptions at an impeller speed of 700 rpm to minimal impact at very high impeller speeds as caking at the bowl perimeter obscured the effect of the chopper on the flow pattern. Differences in the granule flowability were minimal. However, it was concluded that the largest fraction of optimal granules would be obtained at an impeller speed of 700 rpm with the chopper at 1,000 rpm allowing balances between flow establishment, segregation, and centrifugal forces.