C. Fonade

How slug flow can enhance the ultrafiltration flux in mineral tubular membranes

The study deals with the use of a gas-liquid two-phase flow to reduce tubular mineral membrane fouling by injecting air directly into the feed stream. The injected air is supposed to create complex hydrodynamic conditions inside the ultrafiltration module which destabilize the concentration layer over the membrane surface. The experimental study was carried out by filtering suspensions (bentonite and yeast) through an ultrafiltration tubular mineral membrane. A range of transmembrane pressures and various liquid and gas flow-rates were tested. Results related to the permeate flux showed an enhancement by a factor of 3, with a slug flow-structure for the two kinds of suspension (200% of flux increase). Furthermore, the applicability of such an unsteady technique was examined with a view to reduce energy consumption.

Yeast suspension filtration: Flux enhancement using an upward gas/liquid slug flow—application to continuous alcoholic fermentation with cell recycle

This study deals with the use of an upward gas/liquid slug flow to reduce tubular mineral membrane fouling. The injection of air into the feedstream is designed to create hydrodynamic conditions that destabilize the cake layer over the membrane surface inside the filtration module complex. Experimental study was carried out by filtering a biological suspension (yeast) through different tubular mineral membranes. The effects of operating parameters, including the nature of the membrane, liquid and gas flowrates, and transmembrane pressure, were examined. When external fouling was the main limiting phenomenon, flux enhancements of a factor of three could be achieved with gas sparging compared with single liquid phase crossflow filtration. The economic benefits of this unsteady technique have also been examined. To investigate the possibility of long-term operation of the two-phase flow principle, dense cell perfusion cultures of Saccharomyces cerevisiae were carried out in a fermentor coupled with an ultrafiltration module. The air injection allowed a high and stable flux to be maintained over 100 h of fermentation, with a final cell concentration of 150 g dry weight/L. At equal biomass level, a twofold gain in flux could be attained compared with classical steady crossflow filtration at half the cost.

How unsteady filtration conditions can improve the process efficiency during cell cultures in membrane bioreactors

Abstract Among processes developed to increase biological performances, membrane bioreactors have provided the best results. The membrane bioreactor combines a continuous fermentor and a crossflow filtration module enabling separation of cells from liquid media. Very high biomass concentrations have thus been reached and important bioconversion yields obtained. However the potentiality of this process is mainly limited by the rapid decline in permeate flux due to membrane fouling. In our laboratory, various technological solutions, based on unsteady hydrodynamics inside the tubular filters to limit the external fouling, have been developed and applied during cell cultures in membrane bioreactors. The biological model was alcoholic fermentation. The first kind of flow unsteadiness was based on an air injection at the membrane inlet to create a gas/liquid slug flow. For the same energy consumption, this process enabled a mean twofold gain in ultrafiltration flux with a lower efficiency for microfiltration due to pore blocking by cell debris. The impact of an unsteady jet generated by a pneumatically controlled valve was also evaluated. Although the strong physico–chemical affinity between the membrane material and the culture medium, a flux enhancement of 1.3 was achieved at the end of fermentation. It was also pointed out that when the formation of a cell cake layer was expected to be the main mechanism for flux decline, flow unsteadiness failed to disrupt a previously built-up deposit and for a maximal efficiency it had to be started at the very beginning of the filtration operation. After these feasibility studies on a relatively simple and well-known biological model, further applications on environmental problems were carried out. The interest of a gas/liquid slug flow as a means to increase both the permeate flux and the oxygen transfer rate was demonstrated during continuous phenol degradation by Ralstonia eutropha . The active biomass could be doubled without encountering oxygen depletion while the permeate flux was 75% higher. This led to the complete degradation of a high phenol load higher than 70 kg m −3 day −1 . Finally, a new biological treatment process combining a gas/liquid contactor (‘aero-ejector’) and a membrane bioreactor was developed in order to ensure total microbial degradation of pollutants which were initially present in industrial gaseous effluents. The ‘aero-ejector’ technology allowed the solubilisation of gaseous compounds (here ethanol) in a liquid phase before their degradation in the bioreactor itself. During aerobic cultures of Candida utilis , almost all injected ethanol was transferred and degraded over 350 h of culture.

Influence of the flow regime on the efficiency of a gas-liquid two-phase medium filtration

An easy technique, consisting in injecting air into the liquid stream, is proposed to enhance the permeate flux in crossflow filtration of a model fluid (i.e a bentonite suspension). The injected air promotes turbulence and increases the superficial crossflow velocity that leads to a regular disturbance of the boundary layer. A systematic study of different two-phase configurations points up that the slug flow seems the most appropriate regime. The resulting permeate rate is increased up to 140%, in comparison with the usual filtration processes.

Study of mass transfer in a novel gas–liquid contactor: the aero-ejector

Abstract This work has given the first insight into the mass transfer performances of a new contacting apparatus, the aero-ejector, which allows the processing of high gas-to-liquid ratios in a small sized device. Conventional liquid-side controlled oxygen transfer was tested which allows the quantitative comparison of this technology with existing technology. We have also demonstrated the ability of this device to solve an actual engineering problem, that is, the elimination of VOCs, assimilated here to ethanol.

Flux enhancement in crossflow filtration using an unsteady jet

This paper deals with the influence of a new type of unsteadiness in the flow on the permeate flux in crossflow filtration. A pneumatically controlled valve generates intermittent jets from the main flow leading to the formation of large vortices moving downstream along the tubular membrane. The experimental study was carried out by filtering a bentonite suspension through an ultrafiltration mineral membrane. Flux time measurements were taken under steady and unsteady operating conditions. The unsteadiness leads to a permeate flux more than two times higher than in the usual filtration processes.

Liquid mixing and phase hold-ups in gas producing fluidized bed bioreactors

A large number of biological processes are developed with fluidized beds as reactor. This technology allows a higher biomass retention and thus a better efficiency. Some anaerobic processes such as ethanol fermentation, anaerobic digestion or denitrification are characterized by an important gas production. Nevertheless, gas production is seldom taken into account in the description of liquid mixing. In this paper, we present an experimental study of mixing and phase hold-ups in gas producing fluidized beds. The properties of gas producing and classical gas injected three-phase fluidized beds are compared. For a same gas flow rate, gas hold-ups in gas generated systems appears to be much higher than in gas injected reactors. This is attributed to the gas generation mechanism, which provides smaller bubbles at lower velocities. The degree of axial mixing is also different when gas in generated in situ than when it is injected. The dispersed plug-flow model can be reasonably used to describe the phase mixing in the reactor. The nature of bubble population appeared to be the main origin of differences between the two modes of gas introduction.

Why and How Membrane Bioreactors with Unsteady Filtration Conditions Can Improve the Efficiency of Biological Processes

Abstract: A membrane bioreactor (MBR), an association of a bioreactor with a crossflow filtration unit, enables continuous processes with total cell retention within the reactor to be realized. Provided that high dilution rates can be applied and that inhibition processes are avoided, very high biomass concentrations can be reached, thereby improving the volumetric productivities. These membrane bioreactors have been successfully applied to various microbial bioconversion, such as alcoholic fermentation, solvents, organic acid production, starters, and wastewater treatment. On the basis of the biological reaction characteristics and bibliographic results, the potentialities and bottlenecks of this methodology are discussed. Depending on the application, it is shown how the performance of the membrane bioreactor can be enhanced by acting either on the biological reaction achievement, by controlling the balance between cell growth and death, or on the dilution rate, by increasing the permeate flux through the filtration unit. This discussion is based on results obtained in specific biological treatments applied to polluted liquid and gas.

Feasibility Study of a Compact Process for Biological Treatment of Highly Soluble VOCs Polluted Gaseous Effluent

Volatile organic compounds (VOCs), representing a wide range of products mainly generated by industrial activity, are involved in air pollution. This study deals with a new biological treatment process of gaseous effluent combining a gas/liquid contactor called an “aero‐ejector” and a membrane bioreactor. Combining these two innovative technologies enables a high elimination efficiency to be reached.We first focus on transfer phenomena characterization in a pilot installation on a laboratory scale, using a gaseous effluent polluted with a low ethanol concentration (7.1 × 10−3 kg·m−3). These experiments demonstrated the good transfer performances since 90% of the ethanol was absorbed in the liquid phase in one step. After this physical characterization, the biological aspect of the system was studied using the yeast Candida utilis as microorganism. During the experiment, no ethanol was measured in the fermentation broth nor in the outlet gas, confirming the efficiency of ethanol elimination by C. utilis. The experimental procedure emerging from the present study strongly validates the suitability of this process for ethanol removal from air.

Characterization of a new gas-liquid contactor for the biological treatment of gaseous industrial effluents

This paper deals with a new type of gas-liquid contactor for the biological treatment of volatile organic compounds (VOCs). The hydrodynamic and the mass transfer characteristics of this contactor are studied. With head losses quite adapted to the use of blowing engines, the mass transfer capacity is found to be much higher than the mass flowrates which are normally contained in the industrial effluents.