Jorge M.T. Vasconcelos

Effect of contaminants on mass transfer coefficients in bubble column and airlift contactors

Abstract In this work, the effects of surface-active contaminants on mass transfer coefficients k L a and k L were studied in two different bubble contactors. The oxygen transfer coefficient, k L , was obtained from the volumetric oxygen transfer coefficient, k L a , since the specific interfacial area, a , could be determined from the fractional gas holdup, e , and the average bubble diameter, d 32 . Water at different heights and antifoam solutions of 0.5– 100 ppm were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and 0.85×10 −3 m 3 capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k L a and k L values over a span of almost 400%. A theoretical interpretation is proposed based on modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant cap model. Model results match experimental k L data within ±30%.

Bubble size in aerated stirred tanks

Abstract Local average bubble size in a dual turbine stirred tank is investigated. Results are compared with data from the literature, obtained under many different conditions, including different types and numbers of stirrers, different media and measuring systems. For Rushton turbines, bubble size increases from the stirrer tip along the discharge stream, soon to reach a value representative of the bulk of the tank, near the tank wall. This is common to coalescing and non-coalescing systems. Differences in d 32 cannot be clearly attributed to size of tank, number or type of stirrers or measuring method. Dispersion within each author’s data is at least as significant as the differences between authors. Bubble sizes in electrolyte solutions are smaller and more sensitive to power input than in water. Surfactant addition results in a further decrease in bubble size. Data may be correlated by d 32 = C ″( P g / V ) β with exponent β decreasing from (−0.52) near turbine to (−0.37) bulk for non-coalescing media and from (−0.24) near turbine to (−0.14) bulk for coalescing media. The effect of gas flowrate on d 32 is not detectable, except near the turbine for coalescing media. To correlate data under these conditions, the effect of gas loading must be included.

Gas-liquid mass transfer coefficient in stirred tanks interpreted through bubble contamination kinetics

Abstract Experimental data on the average mass transfer liquid film coefficient (kL) in an aerated stirred tank are presented. Liquid media used were tap water, electrolyte solutions and water with controlled addition of tensioactive material. Values of kL range from those expected for bubbles with a mobile surface to those expected for rigid bubbles. These data are quantitatively interpreted in terms of bubble contamination kinetics, using a stagnant cap model, according to which bubbles suddenly change from a mobile interface to a rigid condition when surface tension gradients, caused by surfactant accumulation, balance out shear stress.

Mixing in gas-liquid contactors agitated by multiple turbines

Abstract Liquid hydrodynamics in multiple-turbine aerated tanks was modelled in the loading regime by the compartments-in-series model with back flow streams. Back flows between compartments were assumed to be the sum of two components, which derive from mechanical and pneumatical mechanisms of flow generation. Liquid flows due to these mechanisms were correlated in terms of the operating variables N and QG. Correct simulation of gassed and ungassed mixing of a pulse input of tracer was achieved in all cases under study. The mixing model is robust to changes in geometrical parameters and operating conditions with tanks of 0.3 and 0.5 m diameter T, double- and triple-turbine agitators and turbine sizes T 3 and T 2 . A dimensionless mixing time correlation was deduced from mixing simulation by the model. The dimensionless gassed mixing time θ is obtained from θ0 in the ungassed situation through θ = θ 0 /∏ − KFl ef (K > 0) where Π is the gassed-to-ungassed power ratio, Fl ef = Fl Π is the effective gas flow number and K is a configurational constant. The correlation is general and it is found to represent the interactive combination of the mechanical and pneumatical mixing mechanisms. It was successfully tested not only in the same situations as the mixing model, but also with single-turbine agitators.

Experimental and modelling study of gas dispersion in a double turbine stirred tank

Abstract Gas dispersion in a double turbine stirred tank is experimentally characterised by measuring local gas holdups and local bubble size distributions throughout the tank, for three liquid media: tap water, aqueous sulphate solution and aqueous sulphate solution with PEG. For all these media, bubble coalescence generally prevails over breakage. Where average bubble size decreases, this can be attributed to the difference in slip velocity between different sized bubbles. Most of the coalescence takes place in the turbine discharge stream. A compartment model that takes into account the combined effect of bubble coalescence and breakage is used to simulate gas dispersion. The model predicts spatial distribution of gas holdup and of average bubble size, with average bubble size at the turbines as an input. Reasonable agreement between experiment and simulation is achieved with optimisation of two parameters, one affecting mainly the slip velocity, the other related mainly to the bubble coalescence/breakage balance. Different sets of parameters are required for each of the three liquid systems under study, but are independent of stirring/aeration conditions. The model only fails to simulate the smaller average bubble diameters at the bottom of the tank.

Alternative ways of applying the hydrogen peroxide steady state method of KLa measurement

Activated manganese dioxide can replace the enzyme catalase as catalyst of the hydrogen peroxide decomposition in the steady state peroxide feeding method for K L a determination in aeration equipment. The equivalence of both techniques was experimentally demonstrated in deionized water. Manganese dioxide is thus a reliable alternative for the application of the method in media like strong electrolyte solutions where catalase is quickly deactivated, but some limitations of its own have also to be considered with oxidizable fluids, e.g. polypropylene glycol.

Alternative compartment models of mixing in tall tanks agitated by multi-Rushton turbines

Liquid mixing in tall tanks agitated by multiple radial turbines can be modelled by a cascade of well-mixed compartments with backflows. Models have been proposed with different numbers of compartments per stage of agitation. In this paper, the five simplest models are compared in their ability to predict mixing times and concentration transients, and in their requirement for adjusted parameters. Examined models have from 1 to 4 compartments per agitation stage. The model which consists of 3 compartments per agitation stage is shown to be the best, since it predicts reasonably accurate turbulent mixing times and concentration transients, regardless of injection point position, without the need for any parameter adjustment. Compartments within an agitation stage are connected by the liquid circulation rate, known from the literature, while interchange of liquid between agitation stages was measured. The other models either require adjustment of one parameter for mixing time prediction or do not have a structure adequate to account for the effect of tracer injection point.

Effects of hydrocarbon additions on gas-liquid mass transfer coefficients in biphasic bioreactors

The effects of aliphatic hydrocarbons (n-hexadecane andn-dodecane) on the volumetric oxygen mass transfer coefficient (kLa) were studied in flat alveolar airlift reactor and continuous stirred tank reactors (CSTRs). In the flat alveolar airlift reactor, high aeration rates (>2 vvm) were required in order to obtain efficient organic-aqueous phase dispersion and reliablekLa measurements. Addition of 1% (v/v)n-hexadecane orn-dodecane increased thekla 1.55-and 1.33-fold, respectively, compared to the control (superficial velocity: 25.8×10−3 m/s, sparger orifice diameter: 0.5 mm). Analysis of the gas-liquid interfacial areaa and the liquid film mass transfer coefficientkL suggests that the observedkLa increase was a function of the medias liquid film mass transfer. Addition of 1% (v/v)n-hexadecane orn-dodecane to analogous setups using CSTRs led to akLa increase by a factor of 1.68 and 1.36, respectively (superficial velocity: 2.1×10−3 m/s, stirring rate: 250 rpm). These results propose that low-concentration addition of oxygen-vectors to aerobic microbial cultures has additional benefit relative to incubation in purely aqueous media.

Direct dynamic kLa measurement in viscous fermentation broths - the residual gas hold-up problem

Dynamic measurement of the volumetric oxygen transfer coefficient kLa and oxygen uptake rate OUR in viscous fermentation broths is distorted by a hold-up of small bubbles with very long residence times; these bubbles interact with the liquid and amplify the apparent capacity of the liquid for oxygen. Experimental results reveal that extremely large errors affect the apparent values of kLa and OUR as determined by conventional dynamic methods in respiring broths. Theory predicts that a simple correction procedure is applicable, if the residual gas hold-up of small bubbles is approximately permanent in the time scale of the dynamic experiment and if equilibrium with the liquid may be accepted. Experiments with a mycelial fermentation broth confirm the theoretical correction of measurements affected by up to a 400% error. Simulations suggest that the size of small bubbles consistent with the applicability of the correction procedure in this case was diameter dB⩽10−5 m. Residual gas hold-up was stable enough to be measured by a centrifugation technique.

Optimisation of agitation and aeration in fermenters

The problem of optimising agitation and aeration in a given fermenter is addressed. The objective function is total electric power consumed for agitation, compression and refrigeration. The major constraint considered is to ensure that the dissolved oxygen concentration is above the critical value. It is shown that it is possible to analytically calculate the optimal pair (air flowrate, stirrer speed) and that, at least for the industrial antibiotics fermentation used as case-study, the optimum lies within a window for satisfactory operation, limited by other possible constraints to the problem. Savings achievable by optimal operation as compared with current industrial procedure were found to be around 10% at pilot plant scale (0.26 m3) and 20% at full scale (85 m3).