A consistent dimensional analysis of gas–liquid mass transfer in an aerated stirred tank containing purely viscous fluids with shear-thinning properties

This paper deals with gas–liquid mass transfer in an aerated stirred tank containing Newtonian or shearthinning fluids. The aim is to demonstrate that, for a given mixing system, an unique dimensionless correlation gathering all the mass transfer rates (150 kla measurements) can be obtained if and only if the variability of the rheological material parameters is correctly considered when implementing the theory of similarity. More particularly, it is clearly illustrated that a too gross simplification in the relevant list of the parameters characterizing the dependence of apparent viscosity with shear rates leads to pitfalls when building the PI-space set. This is then a striking example showing that a robust predictive correlation can be established when the non-constancy of fluid physical properties ceases to be neglected

Critical agitation for microcarrier suspension in orbital shaken bioreactors : Experimental study and dimensional analysis

Orbital shaken bioreactors are widely used at the laboratory scale for the culture of animal cells in suspension mode. In the case of adherent-dependent cell lines,the choice of agitation conditions at which all particles are just-suspended (or attain complete suspension) has often to be determined.Indeed, with orbital shaken bioreactors,this choice results from the combination of two parameters : the orbital diameter and the agitation rate. That is why an experimental protocol for the determination of critical agitation conditions for microcarrier complete suspension has been developed in this paper.It consisted in a side-view visualization of blue-stained particles in shaken Erlenmeyer flasks and cylindrical flasks. 220 experiments representative of animal cell culture conditions have been carried out to study the effect of various operating conditions (bioreactor size and geometry, particle type, density and diameter,liquid viscosity, shaking diameter, filling ratio). Furthermore, a dimensional analysis has been performed, leading to a correlation relating a Froude number (in which the critical agitation Nc for complete particle suspension is sembedded) to four other dimensionless numbers. Then,the critical agitation conditions determined in this paper were analyzed and discussed with respect to the data available in the literature on the flow structure occurring inside the flask.Our findings revealed that high orbital shaking diameters and large cylindrical vessels should be promoted to get microcarriers into suspension at a minimized power dissipation per unit of volume

Understanding antral contraction in human stomach through comparison with soft elastic reactor

IntroductionHuman digestion, a major concern due to the rapid development of nutrition-related chronic diseases,rely apart from enzymatic reactions on homogenization of the gastric content by antral contractions.Although recent CFD studies made it possible to explore the influence of the rheologicalproperties of the gastric content on the efficiency of gastric mixing, this key step is yet far from beingfully understood.ObjectiveThis work aims at discussing the influences of contraction frequency and viscosity of the digestain human gastric mixing by comparison with the mixing efficiency of an innovative soft elastic reactor(SER), that induces mixing by vibration of its wall in a similar way as antral contraction waves(ACWs) promote stomach motility.MethodologyIn this view, the SER mixing curve, recently established by Delaplace et al. (2018) using a dimensionalanalysis approach, was considered in order to determine the flow regime under which thegastric mixing of foods (viscosity ranging from 10-3 to 1 Pa.s) was performedMain findingsIt was shown that depending on the viscosity of the SER/stomach content and the amplitude of penetration/ACWs, the number of strikes/contractions required to achieve homogenization was largelydifferent. Moreover, the operating points of SER and ACWs mixing were close, justifying the comparisonbetween both reactors.ConclusionBased on this, the level of mechanical solicitation provided by human peristalsis was shown to benot as high as expected, and the mixing performance of distal region, confined in laminar regime,was found limited and not only due to mechanical solicitations. Further efforts are needed to investigatethe role of other physiological processes such as gastric secretions and gastric emptying inmixing performance of intragastric fluid homogenized by the antral contraction

Liquids' atomization with two different nozzles: Modeling of the effects of some processing and formulation conditions by dimensional analysis

Liquid atomization is a largely widespread unit operation. The disintegration of a liquid into droplets depends on the nature of the nozzle, on the process parameters as well as physicochemical characteristics of the fluid. The aim of this work is to study the contribution of the process (liquid outlet speed and air pressure) and physicochemical (viscosity and surface tension) factors on the size distribution of droplets generated by single- and two-fluid flat spray nozzles. The obtained droplet median diameters which range between 77 and 594 mu m for the single-fluid nozzle and between 11 and 599 mu m for the two-fluid nozzle, are discussed in relation with operating conditions of atomization process. Dimensional analysis was performed as a modeling approach. Despite energy input for the droplet formation is known to be influenced by different origins according to single and two-fluid nozzles, it is shown that a unique correlation, with specific values of parameters for each nozzle type, gathers all the parameters affecting droplet size. In the range of process and formulation parameters tested, this correlation is validated and gives satisfactory agreement for the single- and two-fluid nozzles

When a collective outcome triggers a rare individual event: a mode of metastatic process in a cell population

A model of early metastatic process is based on the role of the protein PAI-1, which at high enough extracellular concentration promotes the transition of cancer cells to a state prone to migration. This transition is described at the single cell level as a bi-stable switch associated with a subcritical bifurcation. In a multilevel reaction-diffusion scenario, the microenvironment of the tumor is modified by the proliferating cell population so as to push the concentration of PAI-1 above the bifurcation threshold. The formulation in terms of partial differential equations fails to capture spatio-temporal heterogeneity. Cellular-automata and agent-based simulations of cell populations support the hypothesis that a randomly localized accumulation of PAI-1 can arise and trigger the escape of a few isolated cells. Far away from the primary tumor, these cells experience a reverse transition back to a proliferative state and could generate a secondary tumor. The proposed role of PAI-1 in controlling this metastatic cycle is candidate to explain its role in the progression of cancer

How dimensional analysis allows to go beyond Metzner–Otto concept for non-Newtonian fluids

The concept of Metzner and Otto (1957) was initially developed for correlating power measurements in stirred vessels for shear-thinning fluids in the laminar regime with regard to those obtained for Newtonian liquids. To get this overlap, Metzner and Otto postulated and determined an “effective shear rate” which was proportional to the rotational speed of the impeller Although it was not based on a strong theoretical background, it was rapidly admitted as a practical engineering approach and was extended for seeking out a “Newtonian correspondence” with non-Newtonian results (i.e. different classes of fluids). This was applied in a variety of tank processes even for predicting heat transfer or mixing time, which stretches far away from the frame initially envisaged by Metzner and Otto themselves. This paper aimed to show how dimensional analysis offers a theoretically founded framework to address this issue without the experimental determination of effective quantities. This work also aimed to enlarge the underlying questions to any process in which a variable material property exists and impacts the process. For that purpose, the pending questions of Metzner and Otto concept were first reminded (i.e. dependence of the Metzner-Otto constant to rheological parameters, physical meaning of the effective shear rate, etc). Then, the theoretical background underlying the dimensional analysis was described and, applied to the case of variable material properties (including non-Newtonian fluids), by introducing in particular the concept of material similarity. Last, two examples were proposed to demonstrate how the rigorous framework associated with the dimensional analysis is a powerful method to exceed the concept of Metzner & Otto and, can be adapted beyond the Ostwald-de Waele power law model to a wide range of non-Newtonian fluids in various processes, without being restricted to batch reactor and laminar regime

Modelling and optimisation of gas-liquid mass transfer in a microporous hollow fiber membrane aerated bioreactor used to produce surfactin

Aeration by a membrane contactor is a convenient method to produce surfactin, a bacterial surfactant compound, while avoiding foam to overflow as it is the case with most of aerated bioreactors equipped with gas sparger. This work helps improving knowledge on oxygen transfer in membrane-aerated bioreactors and optimizing the adjustment of culture aeration performances. In this work, oxygenation of a surfactin solution was studied in a bioreactor aerated by a microporous hollow fiber membrane contactor. First, a dimensional analysis was coupled in an innovative way with a fractional design of experiments, thus reducing greatly the number of experiments. Then, the analysis of the model helped to understand thoroughly the influence of the four main parameters, namely the liquid flow rate inside the fibers, the gas pressure outside the fibers, the liquid volume in the tank and the amount of surfactant in the bulk. Empirical process relationships were proposed to predict either the volumetric oxygen transfer coefficient (kLa) or the liquid-side oxygen transfer coefficient (kL) (with an average standard deviation < 11%). The liquid flow rate, the liquid volume and the gas pressure were found to be significantly influencing unlike the surface tension. The validity of the relationships with surfactin fermentations obtained at a larger scale was demonstrated

Graphite based heat exchangers for fouling control in dairy industry

Fouling of heat exchangers is a major problem in the dairy industry. Deposits indeed produce a thermally insulating layer over the surface of the heat exchanger that decreases the heat transfer toward fluids and increases the pressure drop. Additionally , fouling can seriously affect the quality of food products by favoring the development of harmful bacteria, and thus increase the costs and environmental impacts because thorough cleaning procedures have to be used. In this context, fouling control solut ions are thus required. The present work thus aimed at testin g graphite -based materials to heat exchangers for the dairy industry. The fouling behavior was analyzed for four commercial graphite plates, submitted to pasteurization conditions in a pilot pasteurizer

Atmospheric pressure plasma spraying of silane-based coatings targeting whey protein fouling and bacterial adhesion management

International audienc

Flow process and heating conditions modulate the characteristics of whey protein aggregates.

Whey protein fractal aggregates reveal different texturizing properties depending on their size. This studycharacterize the effect of three process parameters (flow regime, heating residence time (RTh) and heatingtemperature) on the size and shape of aggregates formed at a semi-industrial scale using a dynamic tubular heatexchanger, and identify the mechanisms involved in their formation. The study showed that physicochemicalparameters are not the unique levers to modulate agregates properties but process parameters are also efficient.Asymetrical-Flow-Field-Flow-Fractionation was used to highlight the significant increase of aggregate sizeproduced under transient regime conditions compared to laminar and turbulent regimes. Even larger aggregateswere obtained while increasing the heating temperature from 80 to 85 °C since the unfolding aggregation ofprotein was controlled by the aggregation step. Moreover, RTh showed no effect on aggregate formation. Thisstudy paves the way to the control of aggregate properties obtained in a continuous dynamic mode