Stirring as foam disruption (SAFD) technique in fermentation processes

Abstract

Foam reduction through stimng was studied in 20 L bioreactors with artificial media and with an actual biotransformation process. For a given stirrer configuration and within a certain range of the broth mass, the foam height was correlated with the broth mass, i.e. the distance between upper stirrer and dispersion surface, and with the superficial gas velocity. Increasing the stirrer speed often resulted in reducing the foam height. A mechanistic model was developed for the Stirring As Foam Disruption (SAFD) technique, relating the foam height to the horizontal liquid velocity near the dispersion surface. The model illustrates the general applicability of the SAFD technique and points to foam entrainment as the major mechanism for the foam disruption.On a etudie la reduction du moussage par I'agitation dans des bioreacteurs de 20 1 avec des milieux artificiels et avec un procede de biotransformation reel. Pour une configuration d'agitateur donnee et dans une certaine gamme de masse de bouillon de culture, la hauteur de mousse a ete correlee a la masse de bouillon, soit la distance entre I'agitateur superieur et la surface de dispersion, ainsi qu'a la vitesse de gaz superficielle. Augmenter la vitesse de l'agitateur conduit souvent a la reduction de la hauteur de mousse. Un modele mecanistique a ete mis au point pour la technique de desintkgration de mousse par agitation (SAFD); ce modele relie la hauteur de mousse a la vitesse liquide horizontale pres de la surface de dispersion. Le modele illustre I'applicabilite generale de la technique SAFD et suggere que I'entrainement de la mousse est le mecanisme essentiel de la desintegration de la mousse.

Scale-up of stirring as foam disruption (SAFD) to industrial scale

“…Hoeks et al [6] demonstrated that for values of v L,dl above a certain critical value, v L,dl,0 , foam is virtually absent and thus foam disruption is complete. Furthermore, it was demonstrated that the SAFD technique works equally well with both artificial media and real fermentation broth and that v L,dl,0 does not depend on the medium [6].…”

“…This would be easiest by moving the upper impeller closer to the dispersion level of the broth. However, some impellers-particularly hydrofoil impellers-are more effective than others in foam disruption with respect to power draw [3,6,12]. The standard Rushton turbine (ratio of impeller diameter over tank diameter (D/T)=0.33) shows a rather poor performance with regard to SAFD [2].…”

“…A novel method for reducing the foam layer on broth by agitation was presented by Hoeks et al [6]. This new approach was called ''stirring as foam disruption'' (SAFD) and was developed at a laboratory scale with a small variety of commercially available impellers.…”

“…For this purpose, adequate scaleup of SAFD, including design parameters, has to be demonstrated. A mechanistic model for SAFD, relating the foam height to a hypothetical horizontal liquid velocity near the dispersion surface as an adequate design parameter, was previously developed [6]. Underlying the SAFD model are the discharge flow of the upper impeller under gassed conditions, Q L,g (see Appendix for definitions and nomenclature used in this paper), and an imaginary cylinder extending from the middle of the upper impeller to the gas-liquid dispersion surface, with a diameter of half the tank diameter (see Fig.…”

Performance Characteristics of a Stirred-Tank Reactor with a Mechanical Foam-Breaker Utilizing Shear Force

“…Both chemical and mechanical methods are available for foam control. Mechanical foam-control is preferable to chemical foam-control because it avoids problems such as the lowering of the mass transfer rate, reaction inhibition, and toxicity; moreover, adverse effects on separation and purification of products occur when foaming is controlled by adding antifoam agents (Yagi and Yoshida, 1974;Andrew, 1982;Koide et al, 1985;Hoeks et al, 1997). Numerous mechanical foam-breakers have been proposed.…”