A touch of FROTH

All fermentation processes produce foam, which causes problems within the food and pharmaceutical industries - fermenters have to be operated at reduced capacities because of the space the foam occupies.

Fermentation specialists have therefore tried to eliminate foaming by introducing relatively large quantities of chemical antifoaming agents, either with the batching of raw materials or during the fermentation itself. But in most fermentations, these added anti-foams (such as polypropylene glycol, silicones and vegetable oils) can reduce the efficiency of the process.

The basis of using an integrated separation system such as Domnick Hunter's Turbosep on the off gas is to accurately control - not eliminate - the foaming of the fermentation. This product allows the working capacity of the fermenter to be increased, while the amount of anti-foam added can be reduced.

The separation efficiency, even while recirculating separated foam back to the fermenter, is sufficient to reduce the levels of micro-organisms released to the atmosphere substantially.

STOPPING THE FROTH

The use of antifoam is the most common method of preventing foam formation. The effectiveness of the anti-foam depends on its type and when it is administered. Several techniques of varying sophistication are used throughout the fermentation industry to determine when antifoam should be added. These include visual determination followed by manual addition; automatic time-based addition; use of historical profiles of foam formation for particular processes; and the use of foam probes. However, none of these can guarantee that foam won't form. In fact, several authors on the subject of fermenter design state that `even if antifoam systems are in place and operating, sooner or later there will be a foam over.'

Other ways to prevent foam formation - and, particularly, the entry of foam into the offgas leg of the fermenter - include altering the process parameters. This might involve increasing the head space pressure by quickly restricting the flow of gas through the downstream control valve, or reducing the aeration rate to minimise the generation of foam.

Mechanical foam separation and `foam breaking' systems have also been used. These include an additional foam breaking paddle, installed on the impeller shaft. These are quite effective for light foams, but heavy foams would still require the addition of antifoam.

Cyclones are another foam-removing solution used on a large number of fermenters, but again there is a problem: cyclones are fundamentally designed to remove entrained aerosols, not foams. Their use in fermenters reduces their efficiency considerably, and `carry-over' will occur.

The philosophy behind the Turbosep is at first glance a cause of concern for fermentation scientists. Allowing a fermentation to foam is seen as promoting loss of control. But what if foaming can be used as a tool to improve the productivity of the fermentation using pro-active control? The time, duration and severity of foaming are all direct indicators to the state of the fermentation. Why not harness this information?

Not only has the control system on a fermenter to be successful, it also should be easy to operate and understand and, ideally, be fully automatic. The necessary control can be achieved using Turbosep in conjunction with the injection of antifoam via differential pressure. The concept provides a totally automated and optimised integrated control system which can be used to increase the overall productivity of the fermentation.

The foam/liquid mixture enters the Turbosep at the inlet. The airflow separates around the central exit tube and is directed through the static angled vanes which cause the air to spin. This air then falls onto an impinger plate, which forces the flow to change direction. The first separation of the liquid/foam and gas phase occurs at this point.

The separated liquid droplets coalesce into larger droplets as the liquid drains over the slope of the impinger plate. At this point the air flow changes direction again, and the inertia of the liquid droplets causes them to hit the inside surface of the outer wall where they drain down to the base of the Turbosep. During the separation, the centrifugal forces developed by the angled vanes increases the efficiency by separating the smaller droplets and foam.

The gas then spirals down between the vortex finder and the inside of the outer wall. The axial velocities generated at this point ensure that the separated foam does not re-entrain into the clean air.

At the base of the Turbosep is a vortex arrestor which halts the swirl on the gas stream. This prevents re-entrainment of the separated material and allows it to drain freely down the return leg back into the fermenter.

To maximise the potential of Turbosep, the addition of antifoam must be linked to the differential pressure (DP) generated across it as foam is separated and returned to the fermenter. Controlling the foaming of the fermentation using DP means that the separation system cannot be overloaded and therefore eliminates the possibility of carry over.

The increase in differential pressure is proportionate to the amount of foam entering Turbosep. Depending on the fermentation, the antifoam is injected at a predetermined differential pressure.

The clean operating differential pressure of Turbosep is typically around 10-15 mbar. As foam enters the off-gas leg, the differential pressure measured across Turbosep increases due to the increased resistance to flow through the connecting pipework, turbo blades and impinger. This rises to a pre-set level where the input of antifoam is triggered. Following antifoam injection, the foam entering the off-gas leg will reduce with a consequent reduction in the measured differential pressure. The fermentation itself - and not the operator - places demands on the requirement for antifoam, making the system self-regulating.

Mr Ridealgh is a product development manager at Domnick Hunter