The role of interphase nitrogen transport in the dynamic measurement of the overall volumetric mass transfer cefficient in air-sparged systems

It has been shown both theoretically and experimentally that interphase nitrogen transport may have a significant influence on the rate of interphase oxygen transport, and thereby also on the value of the volumetric mass transfer coefficient of oxygen, k_la, determined in mechanically agitated bubble fermentors using the variants of dynamic method presented in the literature. The experiments were carried out in 1M KCI solution at five stirrer frequencies and two gas inlet levels. The gas interchanges were performed either without interrupting the aeration and agitation of the charge (A) or with the aeration and agitation of the charge turned on at the same time (B). The applied variants of the interchange were N₂→ O₂→, O₂→ N₂, N₂→ air, air→ N₂, O→ O₂, and O→ air. In the two last variants the oxygen dissolved in the charge was removed by reacting with sulfite ions. The k_la values calculated by allowing for the nitrogen transport for procedure A were approximately equal to the values obtained by disregarding the nitrogen transport, whereas those for procedure B were higher (up to 40%), than the values obtained disregarding the nitrogen transport.     

Dynamic measurement of the volumetric oxygen transfer coefficient in fermentation systems

A rapid and internally consistent technique has been developed to measure the volumetric oxygen transfer coefficient, k_La, in fermentation systems. The method consists of tracing the dissolved O₂ concentration of the fermentation broth during a short interruption of the aeration. The O₂ concentration trace thus obtained can be analyzed to determine the values of k_La. Additional experiments on prolonged O₂ starvation, carried out to find the limitation of the technique, suggest that O₂ uptake rate will vary if a prolonged (2–10 min.) O₂ starvation occurs.     

Gas holdup and overall volumetric oxygen transfer coefficient in airlift contactors

The two major types of airlift contactors, concentric-tube and external-loop, were investigated for their gas holdup (riser and downcomer) and overall mass transfer characteristics. Results obtained in batch charges of tap water and 0.15 kmol/m(3) NaCl solution are reported for external-loop airlift contactors having downcomer-to-riser cross-sectional area ratios, A(d)/A(r), ranging from 0.11 相似文献   

Measurement of the overall volumetric coefficient of heat transfer of a shaking flask

The overall volumetric coefficient of heat transfer (Ua) of a shaking flask was measured under various shaking conditions using three types of flask. Ua was significantly affected by flask weight (W_F), which could be attributed to the thickness of the flask wall, the rotational speed of the shaker (N), wind velocity (V_W), and the liquid volume in the flask (V_L). The limiting step of heat transfer seemed to be the heat radiation process from the surface of the flask to the surroundings. To predict the value of Ua, the following empirical equations were obtained for each type of flask by the least squares method:

• 1.(1) for deformed creased flask (M-type flask), Ua=17.4 W_F^−0.43V_L^−0.61N^0.12V_W^0.36
• 2.(2) for creased flask (N-type flask), Ua=9.2 W_F^−0.36V_L^−0.57N^0.17V_W^0.27
• 3.(3) for smooth flask (S-type flask), Ua=5.3 W_F^−0.26V_L^−0.62N^0.19V_W^0.25.

     

Enzyme mass-transfer coefficient in aqueous two-phase systems using static mixer extraction column

Recent technical advances in aqueous two-phase systems (ATPS) have made this a sound technique for the extraction of biomacromolecules. The extraction of alpha-amylase was investigated using aqueous two-phase systems formed by sodium sulphate-polyethylene glycol (PEG) in water in a 47-mm inner diameter spray column packed with three types of static mixers. The effects of dispersed-phase flow rate, phase composition, column height and diameter were studied. The extraction column was operated in a semi-batch manner. It was found that the hold-up and volumetric mass transfer coefficients increased with an increase in dispersed (PEG-rich) phase velocity and decreased with increasing phase composition. Empirical correlations were developed for fractional dispersed-phase hold-up and volumetric mass transfer coefficients.     

Hydrodynamic characteristics and overall volumetric oxygen transfer coefficient of a new multi-environment bioreactor

The hydrodynamic characteristics and the overall volumetric oxygen transfer coefficient of a new multi-environment bioreactor which is an integrated part of a wastewater treatment system, called BioCAST, were studied. This bioreactor contains several zones with different environmental conditions including aerobic, microaerophilic and anoxic, designed to increase the contaminant removal capacity of the treatment system. The multi-environment bioreactor is designed based on the concept of airlift reactors where liquid is circulated through the zones with different environmental conditions. The presence of openings between the aerobic zone and the adjacent oxygen-depleted microaerophilic zone changes the hydrodynamic properties of this bioreactor compared to the conventional airlift designs. The impact of operating and process parameters, notably the hydraulic retention time (HRT) and superficial gas velocity (U _G), on the hydrodynamics and mass transfer characteristics of the system was examined. The results showed that liquid circulation velocity (V _L), gas holdup (ε) and overall volumetric oxygen transfer coefficient ( $ k_{\text{L}} a_{\text{L}} $ ) increase with the increase of superficial gas velocity (U _G), while the mean circulation time (t _c) decreases with the increase of superficial gas velocity. The mean circulation time between the aerobic zone (riser) and microaerophilic zone (downcomer) is a stronger function of the superficial gas velocity for the smaller openings (1/2 in.) between the two zones, while for the larger opening (1 in.) the mean circulation time is almost independent of U _G for U _G ≥ 0.023 m/s. The smaller openings between the two zones provide higher mass transfer coefficient and better zone generation which will contribute to improved performance of the system during treatment operations.     

Dynamic measurement of the volumetric mass transfer coefficient in agitated vessels: Effect of the start-up period on the response of an oxygen electrode

The response of a polarographic oxygen electrode to a step change and to an exponential change in bulk oxygen concentration was studied theoretically and experimentally for the case where there is a significant liquid film resistance at the outerside of the membrane-covered electrode. The probe response has been described considering the start-up period of the concentration changes (the period of time that will elapse before the new concentration level is established and/or before the volumetric mass transfer coefficient k_La regains its steady-state value after the gas supply is opened to the fermentor). A linear change of the pertinent characteristics is assumed during this start-up period. It is shown that a substantial error could be introduced by neglecting the start-up period for cases frequently occurring in practice. In addition, the dependences of the probe response on the direct contact of bubbles with an electrode and on the fluid flow field around it were discussed.     

Determination of the overall volumetric mass transfer coefficient kLa,by a temperature dependent change of gas solubility

Summary The solubility of oxygen in the liquid phase of a bioreactor was changed by a ramp change of temperature, and k_La was determined from the resulting return to equilibrium of dissolved oxygen activity. The maximum k_La that can be measured by this method in a standard laboratory scale bioreactor is 145 h^–1 corresponding to a temperature change rate of 320°C h^–1.Nomenclature p Difference between p_G and p_L (% saturation) - T Ramp change of temperature (°C) - E Temperature-compensated output from the oxygen electrode (A) - E_u Uncompensated output from the oxygen electrode (A) - k_La Overall volumetric mass transfer coefficient (h^–1) - k_La_Tm Overall volumetric mass transfer coefficient at temperature T_m (h^–1) - P_G Dissolved oxygen activity in equilibrium with the gas phase (% saturation) - p_L Dissolved oxygen activity (% saturation) - p_Lm Dissolved oxygen activity at time t_m (% saturation) - t Time (h) - t_m Time of maximum p (h) - T Temperature (°C) - T_cal Calibration temperature of the oxygen electrode (°C) - T_m Final temperature after a temperature shift (°C) - T_n Temperature at time t_n     

A comparison of some methods of estimating volumetric mass-transfer coefficients in an external-loop airlift fermenter

Volumetric mass-transfer coefficients were measured in an 11-L external-loop airlift fermenter with deionized water, a fermentation medium, and during a fermentation. Both a Mackareth oxygen electrode and a novel rapid-response probe were used. When the conventional step-change dynamic method was used for water, the long, nonlinear response time of the Mackareth electrode made correction of its readings difficult; this problem did not occur when the rapid-response probe was used. A comparison was made with a method of mass-transfer coefficient determination which does not involve any assumptions about the gas residence time distribution. However, this method requires that the liquid phase is well-mixed and this requirement was not met in the airlift fermenter. Comparison of the present results with other K(L) a determinations for airlift fermenters showed that K(L) a in water depends on the active gas holdup, the value of K(L) a/epsilon at 20 degrees C being ca. 0.37 s(-1). Although higher gas holdups were obtained with the fermentation medium than for water, the values of K(L) a/epsilon were lower, ca. 0.22 s(-1) at 20 degrees C.     

The importance of bead size measurement in mass-transfer modelling with immobilised cells

The effective diffusivity of O₂ inside immobilised cell particles has been much discussed. Most reported estimates are based on fitting a mass-transfer reaction model to measured total oxygen uptake rates. The particle diameter has the largest single influence in such models, but its accurate measurement has probably recieved insufficient attention. We have studied sorbitol and glucose oxidation by cells of Gluconobacter suboxydans entrapped in calcium alginate gel beads. These beads were found to shrink rapidly in air, so that size measurement under water is essential. By comparison with rigid particles of similar known size, it was shown that measurement of the microscopic image gives a systematic underestimate. In consequence, the fitted oxygen diffusivity will be around 20% too low. Careful attention to size measurement gave good agreement between diffusivity estimates from beads with different mean sizes and cell loadings, with a best value of 2.51 × 10⁹ m²s^–1, 92% of the value for pure water. The estimated diffusivity is not significantly affected by a distribution of bead sizes with up to 10% standard deviation about the same mean.     

Determination of volumetric oxygen transfer coefficient by off-gas analysis

In various aerobic bioreactors including activated sludge aeration tanks, the volumetric mass transfer coefficient K_La is frequently used as an estimate of the rate of oxygen dissolution into the liquid phase. The K_La measurement in such bioreactors is widely applied with the aid of sodium sulfite (Na₂SO₃) as an oxygen-consuming substance used to maintain low dissolved oxygen concentration. In the present study, the effect of the addition of Na₂SO₃ on K_La, determined by an off-gas analysis, was investigated specifically from the viewpoint of variations in the size of air bubbles and the enhancement factor associated with the change in sulfite concentration. Experiments were conducted in a draft-tube bubble column, using a zirconia electrode oxygen analyzer for measurement of the O₂ mole fraction in the exhaust gas and a dual electrical resistivity probe for measurement of the bubble size. It was found that the increase in the specific gas-liquid interfacial area, resulting from bubble size reduction effected by Na₂SO₃ functioning as an electrolyte, is more pronounced than the enhancement of the absorption rate through the interface. The upper limit of Na₂SO₃ concentration for sustaining physical absorption, in the absence of any catalyst, ranges from 30 to 70 mol/m³, while that for preventing the average bubble size from decreasing is about 15 mol/m³. Furthermore, to secure a reliable K_La measurement, the K_La value should not exceed 50 h⁻¹ for the liquid depth of 3 m even when the limiting conditions are not exceeded. The off-gas analysis proposed in this study for K_La determination is expected to be extremely useful provided that the above conditions are fulfilled, since it only requires moderate addition of the sulfite as the oxygen-consuming substance and will not interrupt the reactor operation as long as oxygen uptake occurs in the system.     

Rendering volumetric data in molecular systems

A raster-based computer graphics method of imaging volumetric data (data sampled on a three-dimensional grid) has been added to an existing molecular rendering program. Any molecular property expressed as a 3D grid of scalar data values can be displayed around and inside of the molecular van der Waals surface as a collection of colored clouds and transparent surfaces. Different antical properties, such as color and opacity, are assigned to ranges of the molecular property and are rendered by a ray-tracing technique simulating the shading and shadowing of real objects, making the final images highly eadable. We have found useful applications in the visualization of crystallographic electron density, electrostatic potentials and protein active sites.     

The volumetric oxygen mass transfer coefficient in antibiotic biosynthesis liquids

This paper approaches the problem of oxygen mass transfer. This transfer is in antibiotic biosynthesis liquids produced by microorganisms belonging to the actinomycete and fungi classes, which exhibit a shear thinning non-Newtonian rheological behaviour. The volumetric oxygen mass transfer coefficients in these liquids (k_L a_b) change during biosynthesis processes. The change is mainly due to rheological parameter modifications, such as increasing the consistency index (K) and decreasing the flow behaviour index (n). The values of k_L a_b were 3.0–6.5 times lower than those recorded in water, and their decreasing depended on the k_L a values obtained without biological liquid and on the nature of fermentation broths, as well. Starting from experimental data, two correlations were established between k_L a_b and P/V,υ_SG and P/V,υ_SG, N, respectively. These correlations contain a dimensionless factor (η_am/η_g), which takes into account the rheological properties of the liquid phase and offers the possibility for a fast and sufficiently accurate estimation of k_L a_b. The empirical correlations developed in the paper correspond reasonably well with the relatively wide variety of experimental data, as in the model proposed by PEREZ and SANDALL , and allow for the comparison of the fermentation batches of the same or different microorganisms; also, they may be applied to the workings of design, scale-up, control and monitoring of bioreactors.     

Liquid phase volumetric mass transfer coefficient in dairy effluent stream

The effluent from a dairy cold storage plant was studied for the levels of dissolved oxygen saturation. The effluent to the aerobic digester mainly contains lactose, milk fat, protein and lactic acid. The study was conducted at five different temperatures in a two litre laboratory fermentor. The volumetric liquid phase mass transfer coefficient was correlated to temperature with an average absolute deviation of 6.2%.     

Determination of power consumption and volumetric oxygen transfer coefficient in bioreactors

A torque meter has been developed for determining the power consumption in a bench stirred tank. The device has been bonded in the stirrer shaft inside a commercial bench fermentor, in order to avoid frictional losses in the mechanical seal. Power consumption measurements in ungassed and gassed systems were obtained at different agitation and aeration conditions, for Newtonian and non-Newtonian fluids. Also, a "simple modified sulfite method" for volumetric oxygen transfer coefficient (k_La) determination was developed and the experimental data were correlated with the gassed power (P_g) by using well-known correlations presented in the literature.     

Dynamic measurement of oxidase activity based on oxygen consumption in open systems

Oxidases catalyze the oxidation of a variety of substrates with the concomitant reduction of molecular oxygen as a final electron acceptor. UV‐visible spectrophotometry is a simple and high‐throughput method commonly used to measure oxidase activities. However, drawbacks such as light scattering exist especially concerning the activity assessment of enzymes immobilized on supports. Monitoring of the universal cosubstrate O₂ circumvents these drawbacks. This study aimed at developing a methodology that allows activity measurement of many types of oxidases based on O₂ consumption applicable to various open systems. Dissolved oxygen in the reaction medium was monitored by an O₂ sensor and the reaction rate was deduced from the O₂ mass balance equation correcting for atmospheric diffusion. Common activity units (μM_product min^?1 or U/L) could be subsequently derived using calibration curves. The sensitivity of the method toward temperature, atmospheric pressure, and ionic strength variations was evaluated, and made it possible to define operating windows for the simplification of the proposed methodology.