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

Abstract

Unlimited foam formation and insuf®cient 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 signi®cantly 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 dif®culty 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.

Inverted hollow spinning cone as a device for controlling foam and hold-up in pilot scale gassed agitated fermentation vessels

“…Mechanical "foam breakers", fitted to the agitator shaft, are contained within the vessel and offer no greater risk of contamination or failure than an agitator. Authors generally agree that mechanical foam breaking is based on application of shear (Goldberg and Rubin (1967), Deshpande and Barigou (1999) and Takesono et al (2002)). These shear forces are induced by suction, mechanical shear, turbulence and centrifugal forces.…”

“…The methods for mechanical foam breaking with rotating devices are: centrifugal basket, Goldberg and Rubin (1967), rotating devices, Deshpande and Barigou (1999) and Takesono et al (2002), upper impellers, Boon et al (2002), and inverted spinning cones, Cooke et al (2004). Deshpande and Barigou (1999) compared the performance of five rotating defoamers fitted to the same shaft as the agitator: a six-blade disc turbine, a six-blade pitchedblade turbine, a two-blade paddle, a two-blade paddle with three slits and a two-blade paddle with 168 needles. Takesono et al (2002) compared six rotating defoamers fitted to a different shaft from the agitator: a six-blade vaneddisc, a six-blade turbine, a two-blade paddle, a conical rotor, a fluid-impact dispersion apparatus (FIDA) and a rotating disc.…”

Improvement of foam breaking and oxygen-transfer performance in a stirred-tank fermenter

“…Numerous mechanical foam-control devices have been proposed [2,3]. Mechanical foam-control devices with rotating installations are the most common among these: impellers [13,14], conical dishes [2], centrifugal basket [10], and inverted hollow spinning cones [11,12]. Various kinds of impellers have been reported as foam-breaking impellers: a six-blade turbine [13,14], a six-blade vaned disk [14], a two-blade paddle [13,14], a two-blade paddle with three slits [13], and a two-blade paddle with 168 needles [13].…”

“…As a mechanical foam-control apparatus, a foam-breaking impeller has been mounted frequently on the same shaft as a six-blade turbine impeller [13,15]. The functions of liquid-mixing, gasliquid dispersion, and foam-control are thereby achieved simultaneously using one agitator.…”

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

“…In most of the proposed mechanical foam-breakers, the resultant secondary foams are returned to the area of primary foam generation and are thus accumulated in the reactor over its long lifetime. Examples include mechanical foam-breakers with rotating installations such as turbines, vaned disks, and paddles (Ng and Gutierrez, 1977;Deshpande and Barigou, 1999;Takesono et al, 2001Takesono et al, , 2002. In such devices, persistent secondary foams recirculate into the foam-breaker, but cannot be broken twice by the foam-breaker.…”