WET News

18 WET NEWS JANUARY 2016 Foam reduction chemicals cause unnecessary environmental pollution. The challenge: Keeping foam under control INSIGHT SewAge tReAtmeNt Chemicals used to treat foam have a negative impact on the environment F oaming occurs in a wide spectrum of industries ranging from food pro- cessing to drug manufacture, from wastewater treatment to paper production and from brewing to paint manufacture. In all these processes, the chemicals used to treat the foam continue to have a nega- tive impact on our environment. This is a two edged sword because uncontrolled excessive foam released from plants can also cause environmental pollution. Now proven technology developments in foam measure- ment and predictive control are bringing important environ- mental benefits through the reduced use of anti-foamers and de-foamers. In parallel the accurate and reliable measure- ment of foam brings substantial savings to companies through improved process control, increased yield, reduced prod- uct loss and reduction in equipment failure. In some industry sectors significant energy savings can also be made be made through improved foam control. Hycontrol's patented meas- urement technique uses level measuring technology specifi- cally developed for measuring foam levels and foam-liquid interfaces. Suitable for both aqueous and non-aqueous liquids, it already has a proven track record in a wide range of applications and industries. By comparison, alternative, less- effective, solutions use existing level measuring technology 'adapted' to measure foam. These tend to be inherently unreliable, inaccurate and adversely affected by process product build-up on the measuring probes. Side effect The author, engineer and entre- preneur H James Harrington once said: "Measurement is the first step that leads to control and eventually to improvement. If you can't measure something, you can't understand it. If you can't understand it, you can't control it. If you can't control it, you can't improve it." This principle is certainly true of the challenges presented by industrial foaming and its subsequent control. Foaming affects almost every industrial sector: it may be an integral and important part of a process or it may be an unwanted side effect. Foaming has the appearance of a simple material, partly because we are all familiar with it, whether, for example, in the form of bath foam, bubbles in a milkshake or the head on a glass of beer. However, in practice foam is a very complex, dynamic material with its production involving 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 foam, it requires to be effectively monitored, measured and controlled. There is an extremely 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 although purists may differentiate between anti- foamers, chemicals introduced into a process to minimise foam and de-foamers used to reduce foam once it has formed.) 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, polydimethylsi- loxanes and other silicones, certain alcohols, stearates and glycols. The most universal charac- teristic of any de-foamer is the fact that it is surface active. Most are insoluble but some are water-soluble adding to the complexity. (The latter have 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 extremely effective de-foamer. Reduction of the system temperature below the cloud point enables the de-foamer to become solubi- lised again.) Insoluble de- foamers have to be formulated so that they will be dispersed as tiny droplets, i.e. as an emul- sion. The surface-active nature of the material causes it to spread very rapidly onto any air-liquid interface that it encounters. This is especially the case if that interface is already covered by the types of surface-active materials that tend to stabilise foams. Dramatic 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 expensive and time-consuming problems. These include environmental pollution, potential product contamination, loss of product through excessive foaming, downtime and clean-up costs if foams spill over from the process. Excess foam can severely limit product through- put in a process and can result in damage to equipment including pumps, filters and valves. Add into this the ongoing cost of de-foamers and it is clear how important it is to implement effective foam control. 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. An equilibrium develops between this material collapse at the top and the build-up of new foam from the liquid surface below. Stability This ongoing process limits the maximum height of the foam column. However, in some pro- cesses foam stabilising agents such as proteins reduce the drainage, resulting in much more stable foam. In these circumstances, the foam pro- duction rate can far exceed the dispersal rate and the foam can build up to a serious level. Proteins as long chain mole- cules have this effect by lying along the thin films between the bubbles, thereby preventing drainage. The stability of such foams clearly has a large impact on their life time. Additional factors such as poor system design and leaking pumps can exacerbate foaming problems. In all instances, in order to minimise the impact of THE CONCEPT • measurement technique use level measuring technology specifically developed for measuring foam levels and foam-liquid interfaces • De-foamers are added in quantities based on maximum demand and the foam subsides • Special measuring sensors and control equipment have been designed specifically for foam control • Individual sensors operate independently, and collectively to build up an accurate 3D picture of the process NEED TO KNOW 1 Companies can minimise the environmental impact of the chemicals, whilst achieving considerable savings 2 Foam is a very complex, dynamic material with its production involving physical, chemical and biological processes 3 Foam generation can cause environmental pollution, and potential product contamination 4 Significant energy savings can be made through improved foam control THE VERDICT According to Hycontrol, there is clear evidence that companies can minimise the environmental impact of these chemicals, whilst achieving considerable savings by actively controlling the addition of anti-foamers and de-foamers.

• Cover