Motion of a magnetic flow follower in two-phase flow: application to the study of airlift reactor hydrodynamics

A low-cost and simple magnetic particle tracer method was adapted to characterize the hydrodynamic behavior of an internal- and an external-loop airlift reactor (ALR). The residence time distribution of three magnetic particles differing in diameter (5.5, 11.0 and 21.2mm) and with a density very close to that of water was measured in individual reactor sections. The measured data were analyzed and used to determine the velocity of the liquid phase. Validation of the experimental results for liquid velocity was done by means of the data obtained by an independent reference method. Furthermore, analysis of the differences found in the settling velocity of the particle in single-liquid and gas-liquid phases was carried out, using a simplified 3D momentum transfer model. The model considering particle-bubble interaction forces resulting from changes in the liquid velocity field due to bubble motion was able to predict satisfactorily the increase in the particle settling velocity in the homogeneous bubbly regime. The effective drag coefficient in two-phase flow was found to be directly dependent on particle Reynolds number to the power of - 2 but independent of gas flow-rate for all particle diameters studied. Based on the experimental and theoretical investigations, the valid exact formulation of the effective buoyancy force necessary for the calculation of the correct particle settling velocity in two-phase flow was done. In addition, recommendations concerning the use of flow-following particles in internal-loop ALRs for liquid velocity measurements are presented.Fundação para a Ciência e a Tecnologia (FCT)University of Alcalá de Henare

Hydrodynamics of a three-phase airlift reactor with an enlarged separator : application to high cell density systems

The hydrodynamic study of a three-phase airlift reactor with an enlarged head zone was carried out. Effects of gas-solid-liquid separator design and riser to downcomer cross-sectional area using different lengths and diameters of the draft tube were investigated. Ca-alginate beads with entrapped yeast biomass at different loadings up to 21 % (v/v) were used in order to mimic the solid phase of conventional high cell density systems. The main hydrodynamic parameters – liquid circulation velocity, gas and solids holdups in all reactor sections have been measured and experimental values were satisfactorily predicted using an appropriate three-phase model. The results of this study may beapplied to suggest the optimal design (in terms of hydrodynamic behaviour) of a batch/continuous three-phase ALR for high cell density fermentations.On a mené une étude hydrodynamique d’un réacteur airlift triphasique ayant une zone de tête élargie. On a étudié les effets de la conception des séparateurs gaz-solide-liquide et de la section transversale entre la colonne montante et le déversoir avec différents diamètres et longueurs du tube d’aspiration. Des billes d’alginate de calcium avec de la biomasse de levure piégée à différents taux de charge jusqu’à des taux de charge de 21 % en volume ont été employées afin d’imiter la phase solide de systèmes à forte masse volumique de cellules. Les principaux paramètres hydrodynamiques – la vitesse de circulation du liquide, les rétentions de gaz et de solides – dans toutes les sections du réacteur ont été mesurés et les valeurs expérimentales sont prédites de manière satisfaisante à l’aide d’un modèle triphasique approprié. Les résultats de cette étude peuvent servir à éatblir la conception optimale (en termes de comportement hydrodynamique) d’un réacteur airlift triphasique discontinu/continu pour des fermentations à forte masse volumique de cellules.Marie Curie Fellowship of the European Community programme Improving Human Research Potential - MCFI-2001-01504

Study of hydrodynamics and mixing in an airlift reactor with an enlarged separator using magnetic tracer method

The magnetic tracer technique was used to obtain essential information on hydrodynamics, mixing and reactor design in a multiphase airlift reactor with an enlarged separator. The tracer method involving the use of a magnetic particle as a neutrally buoyant flowfollower allowed measurement of the residence time of the tagged particle in all reactor sections (riser, downcomer, separator and bottom sections). Knowing the settling velocity of the tagged particle, the liquid circulation velocity was also determined. The effects of the design of the enlarged separator (HDT/HC), of the riser to downcomer crosssectional area ratio (AD/AR) and of the liquid level in the head zone (HT) were investigated. The hydrodynamics of the ALR was affected by all design parameters. The results of experiments with different heights of draft tube demonstrated how easily various operating flow patterns can be achieved ranging from typical flow patterns for an internal-loop ALR with high downcomer gas holdup to those of an external-loop with low or nil one. On the base of histograms of the residence time of the tagged particle in the separator zone and visual observations, three flow patterns of the particle were described. The RTDs of the tracer particle were also used for assessment the mixing efficiency of reactor sections. The results showed that the best option for the design of ALR with an enlarged separator concerning mixing intensity is the use of reactor configuration with a dual separator and an area ratio AD/AR around 4.Marie Curie Fellowship of the European Community programm

Residence time of tagged magnetic particle in an internal-loop airlift reactor with an enlarged dual separator

European Community (EC) - Improving Human Research Potential - Marie Curie Fellowship - contract number MCFI-2001-01504

Behaviour of dual gas-liquid separator in an internal-loop airlift reactor : effect of top clearance

EU project Inco-Copernicu

Hydrodynamic considerations in three-phase internal-loop airlift bioreactors : effect of dual separator and draught tube design

Fundação para a Ciência e a Tecnologia (FCT

Continuous alcoholic fermentation in high cell density airlift bioreactor using flocculating yeast

European Community (EC) - Improving Human Research Potential - contract number MCFI-2001-01504

Performance fermentativa de uma estirpe recombinante de Saccharomyces cerevisiae consumidora de lactose e floculante

In recent years, there has been a growing interest in bioreactors utilizing immobilized or flocculating cells in continuous process in order to improve the bioprocess productivity. One of possible promising implementations of continuous flocculation yeast system is bioremediation of cheese whey by means of alcoholic fermentation of lactose. The aim of this work was to carry out a kinetic analysis of alcoholic fermentation of lactose using strain NCYC869-A3/T1, a recombinant Saccharomyces cerevisiae flocculent strain expressing both the LAC4 (coding for b-galactosidase) and LAC12 (lactose permease) genes of Kluyveromyces lactis. Fermentations were performed in a 600 mL bubble column bioreactor, with different initial lactose concentrations. The lactose was completely consumed in all the fermentations. The maximum specific growth rate was found to increase with initial lactose concentration, reaching its maximum at 20 g L-1 initial lactose (doubling time of about 2 h). At higher initial lactose concentrations, specific growth rate decreased, indicating that the effect of substrate inhibition had become significant. The maximum ethanol concentration produced increased linearly when the initial lactose concentration was increased between 5 and 200 g L-1. However, the ethanol yields obtained were low (45 60% of the theoretical value), probably because of the high aeration rates used. In shake-flask fermentations, in conditions of micro aeration, the yeast was unable to completely consume 200 g L-1 initial lactose, producing a maximum of 57 g L-1 ethanol (which is about the same concentration produced in the bioreactor from complete consumption of 200 g L-1 lactose). Probably, the yeast has low ethanol tolerance and the ethanol produced inhibits further lactose fermentation. Ethanol productivity increased with increasing initial lactose concentration up to 150 g L-1 (1.23 g L-1 h-1). Further increase in initial lactose to 200 g L-1 led to a slight decrease in ethanol productivity.Recentemente, tem havido um interesse crescente em biorectores que utilizam células imobilizadas ou floculantes em processos contínuos, para melhorar a productividade dos bioprocessos. Uma das possíveis e promissoras aplicações de sistemas contínuos com células de levedura floculante é a bioremediação do soro do queijo, através da fermentação alcoólica da lactose. O objectivo deste trabalho foi fazer uma análise cinética da fermentação alcoólica da lactose utilizando a estirpe NCYC869-A3/T1, uma estirpe recombinante de S. cerevisiae floculante que expressa os genes LAC4 (codifica a b-galactosidase) e LAC12 (permease da lactose) de Kluyveromyces lactis. As fermentações foram feitas numa coluna de bolhas de 600 mL, com diferentes concentrações iniciais de lactose. A lactose foi consumida completamente em todas as fermentações. A taxa específica de crescimento máxima ampliou com o aumento da concentração inicial de lactose, atingindo o valor máximo para uma concentração inicial de lactose de 20 g L-1 (tempo de duplicação de aproximadamente 2 h). Com concentrações iniciais de lactose mais elevadas, a taxa específica de crescimento diminuiu, indicando que o efeito da inibição pelo substrato se tornou significativo. A concentração máxima de etanol produzido ampliou linearmente com o aumento da concentração incial de lactose entre 5 e 200 g L-1. Contudo, os rendimentos em etanol obtidos foram baixos (45 60 % do valor teórico), provavelmente devido às elevadas taxas de arejamento utilizadas. Em fermentações realizadas em matrazes agitados, em condições de micro-arejamento, a levedura foi incapaz de consumir completamente uma concentração inicial de lactose de 200 g L-1, produzindo um máximo de 57 g L-1 de etanol (aproximadamente a mesma concentração produzida no biorector a partir do consumo completo de 200 g L-1 de lactose). Provavelmente, a levedura tem uma baixa tolerância ao etanol, e o etanol produzido inibiu a fermentação da lactose que restava. A productividade em etanol ampliou com o aumento da concentração inicial de lactose até 150 g L-1 (1.23 g L-1 h-1). O aumento da concentração inicial de lactose para 200 g L-1 conduziu a um ligeiro decréscimo da productividade em etanol.Fundação para a Ciência e a Tecnologia (FCT)

Hydrodynamic analysis of a continuous airlift bioreactor with flocculating high cell density system

One of the most common ways to improve the productivity of a fermentation process is the use of high cell density systems. In practice, such a system usually represents a threephase (gas-liquid-solid) dispersion operating in a continuous mode. The interest for these biosystems has been increasing because they seem to be a very promising alternative to the traditional batch fermentation with freely suspended cells. The cells are usually immobilised on a carrier or in a simpler and cheaper way, they are self-aggregated forming flocs. High cell density biosystems have many specific advantages: higher volumetric productivity, higher product concentration and substrate conversion, easy separation of biocatalyst (cells) from the liquid medium, utilization of the same biocatalyst (cells) for extended periods of process time and a minimised risk of contamination. A continuous airlift bioreactor (CALR) due to the advantageous combination of sufficient mixing, low shear stress and satisfactory flocs suspension at low power input is being often chosen for carrying out fermentations with high cell density. However, there is still a lack of reliable data on transport phenomena, which would allow bioreactor design and scale-up procedures to optimise a bioprocess performance at any bioreactor scale. In airlift bioreactors with a well-defined liquid circulation loop, the liquid velocity is the major hydrodynamic parameter, which considerably affects all physical phenomena. Most velocity measurement techniques are not suitable for use in fermentation processes (e.g. tagging of liquid elements with chemicals due to their interference with the exactly defined substrate pool and sterility problems, visual techniques as Laser Doppler Anemometry due to the opaqueness of the broth). The use of small flowfollowing particles with non-invasive detection of their movement is one of the promising methods. Detection techniques for opaque media include the use of radioactive counters, inductive coils and radio wave detectors. One of the attractive possibilities for a utilization of high cell density system is alcoholic fermentation of lactose from cheese whey using flocculating yeast. Cheese whey, as a by-product of dairy industry, represents a significant environmental problem due to very high values of BOD and COD. For this purpose, a flocculating recombinant strain of Sacharomyces cerevisae was developed enabling the hydrolysis of lactose to galactose and glucose, followed by sugar conversion into ethanol. The main goal of this study was to investigate the hydrodynamics of continuous airlift bioreactor during ethanolic fermentation using highly flocculating yeast. The magnetic particle-tracer method was used for hydrodynamic measurements. Different operation conditions (dilution and air flow rates and biomass concentration), bioreactor configuration and its scale (6 and 50 dm3) have been applied in order to assess their impact on bioreactor hydrodynamics and its operation and to study scale-up effects on the bioprocess. Measurements of liquid circulation velocity revealed one very important fact regarding to airlift bioreactor operation with high cell density system – the existence of a critical value of biomass concentration, at which a dramatic deceleration of net liquid flow appears when the biomass quantity increases (see Figure). Moreover, the magnitude of critical biomass concentration was found not to be dependent on gas flow rate.Marie Curie Fellowship of the European Community - Programme Improving Human Research Potential, Contract number HPMF-CT-2002-01643

Application of high cell density airlift bioreactors to bio-ethanol production – study on optimal bioreactor operation

In this paper a hydrodynamic and rheological analysis of a continuous airlift bioreactor with high cell density system is presented. A highly flocculating recombinant strain of Sacharomyces cerevisae containing genes for lactose transport (lactose permease) and hydrolysis (β-galactosidase) was exploited to ferment lactose from cheese whey to ethanol. The magnetic particle-tracer method was used to assess the effect of operational conditions (air flow rate, biomass concentration) on hydrodynamic behaviour of an the airlift bioreactor during the fermentation process. Measurements of liquid circulation velocity showed the existence of a critical value of biomass concentration at which a dramatic deceleration of net liquid flow appeared with increasing biomass quantity. Rheological analysis revealed dramatic changes in viscosity of the yeast floc suspension at the same biomass concentration of about 73g.dmˉ³ corresponding to 42.8 % v/v of solid fraction. These facts have a particular importance for the successful processing of high cell density airlift bioreactor as only a circulated flow regime will be favourable to keep the solid particles in suspension state and evenly distributed throughout the bioreactor.Marie Curie Fellowship of the European Community programme