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Understanding and controlling foam

22 December 2015

Optimised foam control and reduced process costs can be achieved through the use of a measurement solution developed specifically for the task, says Steve Gallagher.

Accurate and reliable measurement of foam can offer savings through improved control of control of anti-foaming agents, improved process control, increased yield, reduced product loss and reduction in equipment failure. In certain processes – especially the waste water sector – significant energy savings can also be made through improved foam control.

A foam measurement technique developed by Hycontrol uses level measuring technology developed for measuring foam levels and foam-liquid interfaces. Suitable for aqueous and non-aqueous liquids, the technology already has a proven track record in a range of applications and industries. 

Foaming affects almost every industrial sector – where foam may be an integral and important part of a process or where it is an unwanted side effect. Although it has the appearance of a simple material, in practice it is a very complex, dynamic material and its production involves a variety of physical, chemical and biological processes.

Most of the common foams are an unstable, two-phase medium of gas and liquid with a particular structure consisting of gas pockets trapped in a network of thin liquid films and plateau borders. 

Several conditions are needed to produce foam: there must be aeration (generated for example by mechanical agitation, mixing, stirring and sparging) and surface active components (surfactants) that reduce the surface tension. There is always a natural drainage along the thin films of liquid between the bubbles. Liquid gradually drains out from top to bottom, creating a density gradient through the column of foam. The foam at the top of the column collapses as the films become too thin to support the bubbles. Equilibrium develops between this material collapse at the top and the build-up of new foam from the liquid surface below.

This ongoing process limits the maximum height of the foam column. However, in some processes foam stabilising agents, such as proteins, reduce the drainage, resulting in much more stable foam. In these circumstances, the foam production rate can far exceed the dispersal rate and the foam can build up to a serious level. Proteins as long chain molecules have this effect by lying along the thin films between the bubbles, preventing drainage. The stability of such foams clearly has a large impact on their lifetime.

Additional factors, such as poor system design and leaking pumps, can exacerbate foaming problems. In all instances, in order to minimise the impact of foam, it needs to be effectively monitored, measured and controlled.

Controlling foam
There is a diverse set of chemical formulations that can be effective either to prevent foam forming or to destroy it once it has formed. In practice, most foam-dispersing chemicals can serve either role. Each anti-foam or de-foamer agent is specifically developed for individual applications and the world-wide market for these essential evils is worth billions of pounds per annum. Commonly used agents are insoluble oils, polydimethylsiloxanes and other silicones, certain alcohols, stearates and glycols.

The most universal characteristic of any de-foamer is the fact that it is surface active. Most are insoluble. However, some are water-soluble which only adds to the complexity. The latter type has a property known as inverse solubility. An increase in the temperature of an aqueous system in which the de-foamer is present causes a decrease in its solubility. At or above the cloud point (the initial effective temperature), the de-foamer separates from solution and acts as an effective de-foamer. Reduction of the system temperature below the cloud point enables the de-foamer to become solubilised again.

Insoluble de-foamers have to be formulated so that they will be dispersed as an emulsion. The surface-active nature of the material causes it to spread rapidly onto any air-liquid interface that it encounters. 

Anti-foamers and de-foamers operate by being absorbed into bubble surfaces in preference to the foam stabilising agent. They are then effective in increasing the drainage rate so that the bubbles drain quickly and then collapse. An effective de-foamer can disperse foam in a few seconds and the process can be dramatic to watch.

Foam generation can cause a variety of costly and time-consuming issues such as environmental pollution, potential product contamination, loss of product, and downtime and clean-up costs if foams spill over from the process. 

The key problem is being able to understand the characteristics of the foam and then measure its thickness and, in some applications, where the foam-liquid interface resides. Foam can be controlled through the addition of de-foamers, but without the above information their addition is typically done on an empirical basis, often based on historical experience. This can result in a cyclic or sine wave solution to the problem. De-foamers are added in quantities based on maximum demand and the foam subsides. It then develops again above acceptable levels and more de-foamer is added. This reactive, rather than proactive, approach is expensive and wasteful. In many cases, when the foam disappears and the problem subsides, the rate of de-foamer addition is not reduced, resulting in excess chemical usage, which can have substantial cost implications.

The effective measurement of foam thicknesses and foam-liquid interfaces presents a number of challenges. Results can be affected by a range of factors including constantly varying foam density and gradual coating or fouling of the measurement probes with residual product from the process chemicals.

Versatile foam control systems from Hycontrol have been developed to fulfil a variety of functions, for example measuring the thickness of foam in a process; detecting foam-liquid interfaces; or measuring liquid levels whilst ignoring the presence of any foam. The technology behind these systems originated from research into foam control during pharmaceutical fermentation and the measuring sensors and control equipment have been designed specifically for foam control. They are not simply modified level sensors. 

Food and beverage issues
In the food-processing and beverage industries, foam can be generated at various points in a production process. This is predominantly caused by the presence of surface-active substances such as proteins, fatty acids and sugars. Foam is a particular problem in alcohol distillation and in the production of deep-frozen foods, deep-frying oils and gelatine, as well as in fruit conservation and vegetable washing. Invariably, the resulting foam impairs product properties in many different ways and greatly disrupts the process flow. Silicone-based antifoam agents are the most common chemicals used to manage foam but product contamination is a major consideration.

Waste water issues
Bulking sludge and foam are two undesirable, complicated and unpredictable challenges for many waste water treatment plants (WWTP). The filaments causing bulking sludge tend to float and thereby produce large amounts of foam which can vary in depth and can extend throughout the biological circuit, as well as to anaerobic digesters and dewatering units.

The traditional approach is to continuously dose anti-foam into the process at a rate typically set to cope with foam during peak demand. However, this quantity is, mostly, far too high as in some systems there is little or no foam generation for most of the time. Alternatively, the addition of chemicals is done randomly, in far too large quantities and too late. In some cases, a WWTP can suffer for years without finding a sustainable solution to the problem. The chemicals used also add to the foaming problem and the cost of chemicals and energy through unnecessary pumping can become substantial. 

Pharmaceutical issues
Accurate foam control is essential in many pharmaceutical applications, especially during the fermentation process, used in the production of antibiotics, vaccines, steroids and other drugs.

These products often use natural organisms such as bacteria, algae or animal cells to produce the end product. Natural organisms require a healthy environment for growth and optimum yields. Products such as vaccines require very high-tech fermenters for efficient production and these usually involve stirring and air or gas mixing to operate. However, natural organisms can also create unwanted proteins as a by-product and these act as foam stabilisers, creating the perfect environment for stable foam production on top of the broth.

Stories of so-called ‘foam-overs’ abound in the pharmaceutical industry by which whole processes and equipment can be destroyed. A ruined batch can often represent hundreds of thousands of pounds in lost product before taking into account the costs of any equipment damage or clean-up costs. 

Typically antifoams are used to control the foam build up in fermenters but these are difficult to control and can have some unwanted side effects. Firstly, by reducing surface tension the gas bubble size increases, reducing the mass gas transfer into the broth. This can limit the yield of the process. Secondly, the excess use of some anti-foamers can contaminate to the end product and be very difficult to remove.

Steve Gallagher MSc, MBA, MIET, CEng is technical director of the Charis foam control range at Hycontrol in the UK.

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