Jean-Pierre Riba

An experimental study of the hydrodynamic characteristics of external loop airlift contactors

Global hydrodynamic characteristics - gas hold-up and liquid circulating velocity - were measured in two external loop airlift contactors. Geometrical parameters, such as the height of the airlift, the design of the phase separator, the gas sparger and operating parameters, were found to have significant effects on global hydrodynamic characteristics. In particular, circulation velocity was found to be dependent on liquid volume. To predict gas hold-up in the riser, a simple model, which gives good agreement with experimental data obtained with various airlift geometries, was proposed.

Production of extracellular lipase by Candida cylindracea CBS 6330

Abstract In this study we investigated the influences of aeration, substrate type and concentration on extracellular lipase production in a batch fermentor. The use of air enriched with pure oxygen is the most suitable for the lipase production. Additionally, we found that the presence of fats in the culture broth did not affect the value of the volumetric mass transfer coefficient of oxygen in our system. Olive oil or oleic acid was used as carbon sources. In both cases, the maximal specific rate of growth, μmax, was the same but the highest activity was obtained when 10 g/dm3 of olive oil were used as an initial substrate concentration.

Effect of gas liquid separator and liquid height on the global hydrodynamic parameters of an external loop airlift contactor

Abstract This study is devoted to the measurement of global hydrodynamic characteristics (gas hold-up and liquid circulating velocity) of an external loop airlift contactor filled with tap water, with different conditions for phase separation and for liquid height. It is shown that these two operating parameters have a great influence on the behaviour of the airlift reactor. These parameters generally are not specified for the use of the relations proposed in the literature, despite their importance. However, this study reveals that two openings in the top junction between the riser and the downcomer are able to generate doubling of the liquid circulation velocity and a decrease in the gas hold-up of about 30% for high gas throughputs. Furthermore, under these conditions, the liquid circulation velocity is multiplied by a factor of 2.7 with little change in the liquid height before aeration, and the gas hold-up is decreased about 27%.

A two-reservoir, hollow-fiber bioreactor for the study of mixed-population dynamics: design aspects and validation of the approach.

A two-reservoir, membrane bioreactor for carrying out studies of mixed-population dynamics in batch fermentations is presented. Mixing requirements and design aspects for the validity of the approach are given and discussed. Equations describing mixing times between the reservoirs are presented and compared to the experimental results. The validity of the approach is demonstrated by the study of an amensalistic-type interaction, the protein-mediated killer phenomenon between two Saccharomyces cerevisiae strains. The validation consisted in the comparison between the results obtained in actual mixed culture and the results obtained by keeping the strains separated. A good agreement was found which demonstrates the viability of the designed bioreactor.

Kinetic study and mathematical modelling of killer and sensitive S. cerevisiae strains growing in mixed culture

Abstract This paper presents a kinetic study of two yeasts growing in pure and mixed batch cultures. Two winemaking strains were used: S. cerevisiae K1 possessing the K2 killer character and S. cerevisiae 522D sensitive to the K2 killer toxin. Initially the kinetics of growth of the two strains were analysed in pure culture. In this case, the kinetic profiles of biomass production have shown that the growth rate of the K1 strain is slightly superior to the 522D strain. During the fermentation, the viability for both populations was higher than 90%. Fermentations in mixed culture with an initial percentage in killer strain of 5 and 10% with respect to the total population were carried out. The results showed a more important decrease in the percentage of total viable yeasts when the initial concentration of killer yeast increased. However, the kinetic profiles of total biomass (killer plus sensitive yeasts) were very similar for both fermentations. A mathematical model was proposed to simulate the microbial growth of the killer and sensitive strain developing in pure and mixed cultures. This mathematical model consists in three main reactions: the evolution of the killer toxin in the culture medium, the duplication and the mortality rates for each microbial population. The results of the simulation appeared in agreement with the experimental data.

A double optical-fiber probe to characterize gas-phase properties in gas-liquid contactors

A special double optical-fiber probe has been used in order to obtain the local characteristics of the gas phase in a gas-liquid reactor: bubbling frequency, size distribution and rising velocity. Using a particular treatment of the signals, it is possible to take into account the whole bubbles, even though they cannot initially be validated.

A new membrane tool for quantifying microorganism interaction dynamics. Application to yeast killer systems

The study of microbial interactions often requires fastidious and sometimes expensive analytical methods. A reactor specifically designed to carry out studies of interacting populations was developed. Its basic principle consists in keeping both strains of the mixed culture separate by the way of a porous membrane, allowing substrates and metabolites to pass through. As the different strains grow separately, they can be sampled and analyzed distinctly. The culture medium is shared by both strains, and interactions between them occur as if they were mixed in the same vessel. A specific application in winemaking killer yeast interactions is described. The membrane bioreactor made it possible to carry out mixed killer/sensitive fermentations with a double physiological characterization of both killer and sensitive strains.