Chemical Engineering Science 60 (2005) 2231 – 2238 www.elsevier.com/locate/ces Inverted hollow spinning cone as a device for controlling foam and hold-up in pilot scale gassed agitated fermentation vessels Stuart M. Stocks a , Mike Cooke b, ∗ , Peter J. Heggs b a Novozymes A/S, SmZrmosevej 25, Bagsværd 2880, Denmark b The University of Manchester, P.O. Box 88, Manchester M60 1QD, UK Received in revised form 19 October 2004; accepted 1 November 2004 Abstract The effectiveness of an inverted hollow spinning cone (IHSC) for controlling foam and liquid hold-up is demonstrated in pilot scale equipment for a currently commercially exploited recombinant Bacillus fermentation with the characteristically challenging foaming and hold-up issues of this genus. The cone rotates on the existing agitator shaft at the normal production operating level. A series of 9 trials were conducted in 550-l pilot scale fermenters: 2 fermentations without foam control agent (FCA) addition, 4 controls with FCA addition on demand, and three with the cone as the sole method of level control. Batches running without FCA addition or the use of an IHSC lost 52 ± 15 kg of broth through the exhaust system. Batches with FCA addition on demand or the IHSC ended with a statistically indistinguishable filling of 308 ± 6 kg and 319 ± 13kg, respectively. Process performance in terms of enzyme titre was not affected. Such encouraging results indicate great potential savings in both fermentation and recovery costs since FCA addition can be completely avoided. A full-scale production evaluation is justified.  2004 Elsevier Ltd. All rights reserved. Keywords: Spinning cones; Gas–liquid hold-up; Bacillus fermentation; Degassing; Defoaming; Mechanically stirred tanks, vessels or reactors 1. Introduction Foaming in submerged fermentations has long been recognised as a challenge in aerated agitated fermentations, especially at the commercial scale. The common solution to the problem is two-fold; the addition of foam control agent (FCA), and the reduction of reactor filling. The addition of FCA is generally considered as something that should be minimised because it is expensive, it invariably reduces oxygen transfer capacity in the reactor and these oils or suspensions can be difficult to remove in later purification operations. More recently companies have been required to consider the environmental persistence of their raw ma- terials, products and by-products, and although usually ∗ Corresponding author. Fax: +44 161 200 4399. E-mail address: michael.cooke@manchester.ac.uk (M. Cooke). 0009-2509/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.11.016 considered non-toxic and food safe, when compared to other fermentation ingredients, the FCAs can score highly on this point. The reduction of the filling of the vessel has an obvious impact on process economy and is counter to the principle that filling is something that should be maximised. Foam control in fermentation is costly. Therefore a cost-efficient foam control system can significantly influence the process economics. The understanding of the mechanisms of foam formation and foam breaking is paramount for the effective prevention and control of the foam. The classification and characteriza- tion techniques of foams have been described by Bhakta and Ruckenstein (1996), Bikerman (1953, 1973), Kouloheris (1987), Pelton (1996), and Phillips et al. (1987). Bikerman (1953) defined foam as an “agglomeration of gas bubbles separated from each other by thin liquid films”. The walls of these liquid films have approximately plane parallel sides. These two-sided liquid films are called the lamellae. Foams