Foaming occurs in a wide spectrum of industries, from food processing to drug manufacture, from waste water treatment to paper production and from brewing to paint manufacture. Generally, there is a need to manage a trade-off between the pollution arising from the foam itself, when released, and that occurring because of the chemicals used to treat it. In the following article, taken from a recent white paper, level measurement specialist HyControl explains why it believes technology developments in foam measurement and predictive control are bringing important environmental benefits through the reduced use of anti-foamers and de-foamers, and sets out the context around its own range of solutions in this area.
“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.”
H. James Harrington 1929
The above 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 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.
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, 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 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, 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 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 solubilized again.) Insoluble de-foamers have to be formulated so that they will be dispersed as tiny droplets, i.e. as an emulsion. 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 stabilize foams.
Anti-foamers and de-foamers operate by being absorbed into bubble surfaces in preference to the foam stabilizing 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.
Adding up the costs
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 throughput 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.
The key problem is being able to understand the characteristics of the foam and then measure its thickness and, in some applications, identify where the foam-liquid interface resides. The foam can be controlled through the addition of de-foamers, but without the above key information, their addition is typically done on an empirical basis, often based on historical experience. This can then 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 staccato, reactive rather than pro-active 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.
The effective and reliable measurement of foam thicknesses and foam-liquid interfaces presents a number of challenges. Results can be adversely 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
Traditional measurement techniques have been based on a range of existing level probe technologies adapted for foam measurement. In most applications, these fall short of providing an adequate and sustainable solution.
The Hycontrol Solution
Hycontrol’s highly versatile range of foam control systems have been specifically developed for measuring the thickness of foam in a process, detecting foam-liquid interfaces and measuring liquid levels, ignoring the presence of any foam. The patented technology behind these systems originated from detailed research into foam control during pharmaceutical fermentation. Hycontrol points out that the special measuring sensors and control equipment have been designed specifically for foam control and are not modified level sensors.
Aqueous liquid foams
The Hycontrol range of products for measuring foam in aqueous solutions is based on conductivity measurement technology.
The SureSense system is used for measuring the presence of foam in processes involving aqueous solutions. A typical system comprises a sensor installed, for instance, in the fermenter or mixing vessel and a controller connected via special cables. The probe is designed to sit above the liquid level and will detect the foam when it reaches a designated level. The level control output can be used to control the amount of de-foamer added to the process. It can be used for processes in which foam is unwanted and also those in which foam is a necessary part of the process.
The SureSense system also has the ability to detect the difference between foam and liquid if required. Therefore in circumstances where the liquid level unexpectedly rises and covers the sensor, an alarm can be activated.
The range of sensors has been certified for use in designated hazardous areas where an explosion risk is present. The sensors are classified as intrinsically safe and can therefore be used in Zone 0. This makes them suitable for use in environments such as sealed tanks and vessels where there is always a risk of explosions occurring for pro-longed periods.
The MultiSense system provides the ultimate in foam measurement and control in aqueous solutions, with measurement capabilities up to 6 metres. The standard measuring probe, constructed from 316 stainless and polyether ether ketone (PEEK) is designed to withstand temperatures up to 170⁰C and pressures up to 25 bar. The level sensor design features a unique array of small individual sensors, built into a single probe.
Each sensor or ‘section’ acts as a separate micro-sensor and can be specified in size for particular applications. The number of measuring sections varies from 5 to 24 depending on the length of probe required and the resolution. Special upper and lower sections, called terminators, ensure that the top and bottom sampling layers of the active range are flat and not curved. In addition, passive body and process fittings position the active part of the sensor in the desired measuring range. This design concept, which provides excellent resolution and accuracy, enables a number of separate parameters to be measured at the same time including height, volume, conductivity as well as the relative position of fluids in the vessel.
Although the individual sensors operate independently, they also work collectively to build up an accurate 3-D “picture” of the process. The data from all the sections is combined to give smooth continuous readings. Each sensor can measure a thin horizontal layer of material extending out from the probe to the vessel wall. This is not affected by other items in the vessel such as stirrer shafts. Each sensor measures the impedance of the fluid under test by passing a small current through the material. Interfaces between layers in the fluid are sensed by the rate of change of impedance. Foam is recognized by contrast with the liquid below. In this way the thickness and position of a layer of foam can be determined. The versatility of the multi-sensor system means the instrument can measure levels of foam and liquid, just foam or just liquid.
The Hycontrol sensor is designed to work reliably even in the presence of severe fouling. This is achieved via a special ‘guard electrode’ within the sensor, which disrupts the de-sensitising effect of the material build-up. As a result the main foam sensing electrode ignores the fouling and monitors only the active foam within the process.
Non-aqueous liquid foams
The Hycontrol range of products for measuring foam in non-aqueous solutions is based on dielectric measurement technology.
The DiFoam range of sensors and controllers are designed for the measurement and control of non-aqueous foams. They provide an efficient, reliable and cost effective solution to foam control in a diverse collection of industries where the production and control of foam causes a problem. They can also be used for level measurement of non-aqueous liquids.
The system, which comprises a sensor and a controller, was initially developed for the control of foam in a resin manufacturing process for yacht varnish. The technology has been further developed to become a useful tool in measuring and controlling foam in a variety of other applications including oils, solvents, hydrocarbons and products such as cocoa butter.
Predictive foam control
Hycontrol recognises that most foam reduction systems are reactive rather than proactive. Although their technology significantly improves foam control there are certain applications where installing sensors is not practical. To overcome this, they have developed their predictive off-line FTA Foam Tendency Analyser based on the tried and tested foam sensor technology.
Instead of measuring the foam created in the process, the FTA is used to predict in advance when foam will occur, allowing operators to take the appropriate action before any significant foam build up occurs.
The FTA continually samples the process liquid and creates foam inside a small test chamber which is then measured. The self-contained unit includes sampling pumps and an anti-foam pump to control the foam. If required the anti-foam pump can be replaced by an intelligent interface to an external process controller.
The Foam Tendency Analyser is particularly useful where there is no space for a normal sensor or in inaccessible locations such as underground sewers. Other applications have included foam control in large water features such as fountains, where the introduction of costly foaming agents such as liquid soap can prove very messy and also preventing outflows creating foam in rivers and streams.
Consideration must be given to the long-term, detrimental effect that the disposal and dispersal of foam reduction chemicals have on our health and the environment. Companies world-wide spend billions of pounds each year dealing with foam and the effects it has on their businesses. 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. The technology is now available to achieve this and there is no need for companies to continue with out-dated control systems, which only serve the interests of the chemical suppliers.