Antifoam and Mass Gas Transfer

Biotech processes require conditions for specific organisms to metabolise and flourish. In the main, the conditions in which organisms flourish are also the conditions in which foam production is encouraged. This can quickly lead to excessive foaming which can be problematic for producers.

These biotech processes require the following elements: agitation, temperature control, pH control, gas injection (often air or oxygen). Nearly all of these will generate foam in a reactor. Proteins are also frequently released by metabolising organisms, and these act to stabilise foams – thereby increasing the production rate.

Antifoams and de-foamers are chemical additives used to disperse and prevent the build-up of foam. Antifoams and de-foamers work in very similar ways, and for the purpose of this article we will treat them as being identical. Antifoam chemicals are surface acting molecules which generally act to increase surface tension and therefore increase the drainage rate of the foam. Antifoams work by having an effect on bubble surfaces to increase drainage in the bubble films which leads to film thinning followed by bubble rupture. This destabilises the foam causing collapse.

Antifoams may be added into the liquid phase or dropped or sprayed onto the foam, but generally the antifoam finds its way into the liquid phase below the foam. The antifoam also has an effect on gas bubble surfaces in the bulk media. This has the effect of destabilising small bubbles which combine together to form larger bubbles. While the total volume of gas is not affected by this, the surface area of the bubbles is reduced.

Gas divided into small bubbles has a much larger surface area than the same volume of gas separated into large bubbles. In most cases, gas is required to dissolve from the gas phase across the bubble films into the liquid phase in order to achieve good metabolism of the organism. So, the greater the surface area, the higher the dispersion of gas into the liquid. This requires a large bubble surface and hence a small bubble size.

The dispersion of gas in the liquid in biotechnological processes is termed ‘mass gas transfer’ and is an important variable. The addition of antifoam can have a harmful effect on mass gas transfer.

While antifoam in some cases can be metabolised by some organisms and have a positive effect on yield, the reduction of mass gas transfer can have a seriously negative effect on yield. For this reason, it is important not to add more antifoam than is required for the application. The presence of foam in a process demonstrates a strong likelihood of good mass gas transfer. But clearly, too much foam can destroy a process completely. The optimum result is to have foam level controlled at a reasonable level. If the antifoam addition is controlled by a closed-loop, responsive process, where it depends on the foam level that is actually present rather than being automatically dosed, then only the necessary amount of antifoam is added.

The effects of antifoams on biotech processes are currently not very well researched. This is partly because the formulation of antifoams is proprietary and guarded. There can be positive and negative effects due to a variety of mechanisms operating together. However, the effects on foam and surface tension with the subsequent result on mass gas transfer is a well-known if not well-understood effect.

Good foam control can give the best of both worlds by maintaining the foam at a safe level and at the same time ensuring good mass gas transfer.