Foaming: Consequences, prevention and destruction

Mesure des effets des particules solides sur les caracteristiques de fluidisation, de la retenue de gaz, et du coefficient de transfert de masse volumetrique (pour la phase liquide) dans une colonne a bulle contenant des particules en suspension. Le diametre et la densite des particules est similaire a celle que l'on peut avoir en biotechnologie avec des bacteries immobilisees. Basees sur des modeles de turbulence et sur une circulation induite par l'ascension de bulles de gaz, on obtient, par analyse dimensionnelle, la vitesse critique du gaz, la retenue de gaz et le coefficient de transfert de matiere volumetrique pour la phase liquide

Fluid dynamics and mass transfer in a bubble column with suspended particles

Hysteresis effects in suspended solid particles in bubble columns with and without draft tube

Unlimited foam formation and insufficient foam collapse can have serious effects in an aerated system such as a fermentation process. Mechanical foam breakers are used in foam control to avoid the drawbacks associated with the use of chemical antifoams and defoaming agents. In this paper, two new foam breakers consisting of a two-blade paddle with three slits, and a two-blade paddle with 168 thin needles have been tested. They gave significantly reduced critical speeds and power consumption for foam control in a stirred vessel, compared with some conventional foam breakers. The effects of various parameters, namely the physical properties of the foaming solution, type of gas sparger, surfactant concentration, and foam-breaker clearance above liquid level, have been investigated. The degree of difficulty of controlling dynamic foams generated continuously in a stirred vessel is found to be a function of the nature of the foam, ie its bubble size, its liquid holdup, and the degree of foamability of the system.



© 1999 Society of Chemical Industry

Performance characteristics of novel mechanical foam breakers in a stirred tank reactor

The necessity of suppressing foam formation arises in many industrial operations and in sewage treatment plants. Mechanical foam breakers are used for such a purpose when the use of chemical antifoams and defoaming agents is prohibited by the process. Two new foam breakers were used to control a dynamic foam head generated in a bubble column containing foamable liquid: a comb-type two-blade paddle with three slits, and a brush-type two-blade paddle with 168 needles. In comparison with more conventional foam breakers, both foam breaker designs gave significantly reduced critical speeds for foam control. Their performance was characterised under various experimental conditions including different foam-base liquid properties, gas sparger types, surfactant concentrations, and foam breaker clearances above the liquid surface level. The control of dynamic foams generated continuously in a bubble column was found to depend on the foam characteristics such as bubble size and liquid holdup, and the foamability of the base liquid. Foams generated in stirred tank reactors were found to be have different characteristics and hence different control requirements.

Mechanical suppression of the dynamic foam head in bubble column reactors

The first part of this paper presents a relationship for the minimum velocity of rotating installations for foam breaking. The derivation is based on equilibrium of inertia and surface forces. Inertia forces occur during the acceleration of foam bubbles and act mainly at the plateau borders. High and definite acceleration can be obtained with a deformer composed of a rotor and a stator. The surface force is due to the dynamic surface tension because surface-active solutions react to a rapid change in surface area by altering their surface tension. The theoretical relationship is compared with experimental results of minimum velocities needed to break foams produced from aqueous solutions of detergents. The equation presented here explains why measured minimum velocities often range between 10 and 20 m/s. The second part of the paper deals with condensation of continuously generated foam in a closed system. In the process of condensation, foam is not completely separated into liquid and gas phase but turns into foam with small bubbles and high density. The collapse of this condensed foam must be considered for the control of persistent foams in a closed system. The collapse of foams made of aqueous solutions of different surface-active agents has been investigated. Different highly surface-active agents show small variations in times of coalescence. A relationship for the lifetime is given, which is based on laminar flow along plateau borders. Recommendations are made with respect to the geometry of the foam breaker, scale-up and operating variables such as rotational speed of the foam breaker and gas flow rate.

Bodo Furchner

Papers

Foam breaking by high speed rotors

Mindestgasgeschwindigkeit zum Suspendieren von Feststoffteilchen im Dreiphasenbett

Mechanische Schaumzerstörung: Mechanische Schaumzerstörung