1. The Thermo Electron Foam Control System

Thermo Electron Foam Sensors and Controllers are designed for the control of foam in a wide variety of applications. These notes describe the types of application and the requirements which relate to Biotechnology. The Thermo Electron System provides these features:

Foam can be detected. But old style foam detectors are prone to fouling by condensation, surface growth, and even the media which they ought to be monitoring. They need constant attention, cleaning and recalibration to reduce these effects, but may still give false readings and cause overdosing of antifoaming agent. The result again is damaging to production.

Hygienic, steam sterilisable sensors
Wide range of sensors
Immune to fouling
No re-calibration or adjustment
Long term reliability and low maintenance
Precise, automatic control of anti-foam

Resulting Benefits :

Improved Vessel usage - Increased Production
Reduced Production Loss
Reduced Operator Intervention
Reduced Antifoam Usage means Lower costs

Some background information is provided, together with some explanation of the benefits of the Thermo Electron system.

2. Background

The word "Biotechnology" refers to a range of processes which use living systems to carry out a process or produce a product. They tend to be delicate systems which require a carefully controlled environment in order to maintain the health of an organism. These environments are frequently maintained within vessels, called fermenters, which are used to manufacture a wide range of products such as antibiotics, additives, foods and beers. Such vessels will vary from small 1 litre glass research versions, to large stainless steel tanks with a capacity of several hundred cubic metres. They may be called fermenters or bioreactors, or another name specific to a particular industry.

Various organisms will be contained within such fermenters including bacteria, fungi, actinomycetes, yeasts and cell cultures. Each has its own particular requirements for growth. Most of them need oxygen in the form of air dissolved into the medium. They will all require some form of gas transfer. This involves supplying gas (usually air) in the form of a stream of bubbles distributed throughout the medium. This is usually called gas sparging.

3. Why foam often occurs in Fermenters

Foam is a dispersion of air in liquid. It is generated when surface acting agents are present in liquid which is agitated. The surface acting agents or surfactants alter the surface tension of the liquid in such a way as to encourage the development of thin films. These films are prevented from draining and are hence stabilised by the presence of the surfactants which tend to accumulate in chains around the films. Thus, once bubbles of foam are created they do not disperse as they might in other circumstances. Some of the most successful molecules for stabilising foam are proteins.

In Biotechnology systems the conditions for foam production are always potentially present. Many organisms generate proteins which are released into the surrounding medium. In some cases the medium itself may contain foam stabilising agents. The sparging which is always present, together with the proteins, produce the sort of situation in which foam can develop quickly.

4. Why Foam is such a problem

Foam can be a disaster in fermenters because it frequently results in loss of production. There are various reasons for this. As the density of foam is so low compared to the liquid phase, it can be blown out of the vessel with the exit gas. In addition, any filters designed to prevent the escape of the organism in the vessel will be blocked by the moisture in the foam. This results in pressurisation of the vessel, and loss of gas flow or the opening of over pressure valves. Consequently the sterility of the whole system will be destroyed.

Sometimes overflow will occur with the foam being discharged into the area surrounding the fermenter. This may well represent a health hazard and certainly results in a large cleaning task.

There are occasions when foam builds up so rapidly that within a few seconds the entire liquid contents turn to foam and are lost before any remedial action can be taken.

5. Sensing Foam Is Difficult

The most effective way to control foam is to detect its presence as soon as it occurs, and then to do something about it. There is a widespread belief that foam cannot be sensed reliably. Certainly there is good reason for believing that it is very difficult to sense. The conditions found inside fermenters can be described as humid, warm and sticky. Also the nature of foam is such that it leaves sticky deposits over everything. Often, the organism being used will act with the product to produce a thick layer of conducting material which builds up over the sensors in the fermenter and particularly in the head space. It is this type of build-up or fouling which will cause other foam sensors to fail after a brief period. They give a false positive reading and exaggerated control action will then be taken, thereby disrupting the process.

These kinds of experience have convinced many C&I engineers and microbiologists that the sensing of foam cannot ever be done reliably. However, it was specifically to overcome these types of problems that the Thermo Electron Foam Sensor was designed. Because of its unique construction, it works reliably even in conditions of extreme fouling.

6. What's special about the Thermo Electron Foam Sensor?

The Thermo Electron Foam Sensor came out of a research programme carried out by Shell Research Ltd at a biological sciences research centre. This was part of a requirement to develop new sensors for Shell's various biotech facilities. After much development work, the result was a sensor which could operate reliably even with severe levels of fouling.

The sensor operates by measuring the electrical impedance of the material filling the headspace of the vessel. It is the unique dual electrode system which enables the sensor to withstand fouling. The upper guard electrode electrically isolates the area above the sensor, thereby preventing leakage currents from causing false triggers. Only foam build up beneath the sensor activates the controller enabling remedial action to be taken. This is truly a sensor which works consistently and reliably, and requires very little maintenance.

7. How Foam is Dispersed

Once the foam is sensed something has to be done to prevent it getting out of control. There are several feedback mechanisms which can be employed. The most common is to use a chemical additive to disperse the foam. This is generally called "antifoam", but may also be called "defoamer". (Strictly speaking antifoam is added before the start of the fermentation, and defoamer during the fermentation, but the terms are frequently used indiscriminately.)

There are many types and suppliers of antifoam. They can be effective, but when overdosing occurs there may be damaging results to the process. Some cultures will not tolerate antifoam at all. Also all antifoams tend to reduce the ability of gas to transfer into the liquid phase.

For these reasons, antifoaming agents have to be added in a well controlled way and in small doses. This is difficult to achieve without accurate sensing. The Thermo Electron Foam Controllers are designed to operate in this context.

The other main mechanism for control is the use of a mechanical foam breaker. This is a type of propeller which rotates in the fermenter headspace and acts as a centrifuge. It forces the foam against the sides of the vessel at high speed thereby breaking the structure. These generally prove ineffective, and can be expensive to run in large fermenters. They are sometimes controlled by sensors and sometimes run continuously.

8. What does control actually mean?

There are cases where the customer does not want any foam to be present in the process. More often however, a process can run quite effectively with a certain amount of foam, but not too much. In many biotech processes, foaming is an inherent part of the product and can only be eliminated by the addition of excessive antifoam. This would also increase the gas bubble size in the culture which reduces the gas transfer into the culture.

In many cases a better approach is to control the foam at a particular level in the fermenter. This can be achieved by adjusting the height of the sensor in the vessel to an appropriate level, provided that the control system is adjusted accordingly. In practise this works well.

9. How to investigate an application

It is important to clarify the requirements. Excessive foam is known to be a problem. It needs to be determined what aspect of the foam causes a problem, at which point in the system it is being generated, and what the critical parameters are. A useful starting place is to identify the current procedure. It is critical to get as much information as possible. It may be that the foam overflows and if kept below a certain level it would be acceptable. Alternatively, it may be that foam is interacting with other sensors and causing false readings.

There are three main parts to the control system:

a) Sensor b) Controller c) Actuator

All of these need to be investigated to develop a final control regime.

a) Sensor
The sensor is the most difficult part, but the Thermo Electron Sensor will do the job. It is necessary to fit the sensor in a manner which maintains sterility, and this will require a particular form of fitting. The most common fitting used for this is a 25mm fitting (sometimes called DN25 and originally developed by Ingold). This fitting can also be supplied welded to the probe (FP103 type sensor) or as a compression fitting.

b) Controller
The control system is important and can be implemented in a number of ways. There is a stand-alone controller, type FCR2 or FCW2 designed to operate with the Thermo Electron Sensor. This can be used to control a pump or valve to operate the process feedback.

c) Actuator
This is a control device such as a pump or valve which will inject an antifoam chemical where and when needed. In many cases such a pump or valve will already represent part of the dosing system, and the user may wish to retain the preferred method of dispersing the foam. They may well have been dispensing the antifoam manually.

10. Hazardous Areas / Intrinsically Safe Applications

There are occasional fermenter applications which involve a hazardous area (potentially explosive). These can be tackled by the use of zener barriers to safeguard the sensor against fault conditions. The sensor itself is a passive device and cannot generate sparks to cause an ignition, unless a piece of connected equipment becomes faulty. In such a situation, the zener barriers will prevent high power finding a way into the sensor.

Installations within hazardous areas have been successfully and safely handled on a number of occasions. Please contact Thermo Electron for more information on this.

11. Effluent Treatment

There are some bioreactors which are used for effluent treatment to deal with waste products. These will need to be organised rather differently from production fermenters. They may be open tanks which do not require sterility, but use robust bacteria. The main problem caused by foam in these cases is overflow, resulting in spillage. Such applications will need to be treated differently. Please contact the office for advice on these applications.

Detecting and Controlling Foam in Effluent Systems

1. Introduction

Foam is frequently generated as a by-product in industrial processes. This happens when fluids (usually water) containing surface acting molecules are agitated in air. The surface acting chemicals have the effect of stabilising the thin films produced between bubbles of air or other gases. This prevents drainage of the thin films resulting in rapid foam build up. This in turn causes the overflow of tanks or flumes with the resulting loss of contents.

Practical examples of these effects include effluent flumes, sewers, treatment tanks and biotreatment reactors. Bioreactors are particularly prone to this problem since air is passed through the medium to create a suitable environment for bacteria cultures to degrade the effluent.

2. Control Mechanisms

Foam generated in these circumstances is difficult to control. In general the main approach is to add quantities of antifoam chemicals to the process. This certainly results in the dispersal of the foam, but can become expensive and may also have a damaging effect on the cultures in the bioreactors. It is also somewhat ‘hit or miss’.

There are various methods of dispersing foam apart from adding chemicals. High pressure water jets can sometimes be used to break up the foam. In certain instances, process variables such as air flow can be regulated, offering some degree of control. A better approach is to sense the build up of foam and to use a closed loop control system to provide feedback to the process. This will result in a more controlled situation and a reduction in the cost of chemical additives. It very much depends on the particular application, as to which combination of methods will be the most efficient and cost effective.

Foam is notoriously difficult to sense reliably, but Thermo Electron Foam Sensors have been developed specifically for such applications.

3. How Foam can be Controlled

Where closed tanks are being considered the best approach is to install one or more Thermo Electron Sensors into the tank. These sensors need to be connected to a control unit which can be mounted some distance away if necessary. Some careful thought may need to be given to the position of the sensor within the tank. There are occasions where the foam is generated in a particularly awkward position, such as the inside of a machine or pump, where it is physically difficult to fix the sensor. In other cases it may be necessary for the sensor to move with the liquid levels, or to accommodate the collection of foam as it is blown about by the wind. Each tank will need individual assessment in order to site the sensor to the greatest advantage.

In these situations a helpful approach is to use a flowcell to simulate the effect of the process. A flowcell has been developed by Russell especially for foam sensing. It works by passing a sample of fluid from the process continuously through the cell. The foam which is generated in the flowcell can then be sensed and monitored as a simulation of the main process.

4. The Foam Flowcell - How it Works

The flowcell is designed to operate as a model of the process as far as foam build up is concerned. A sample will flow from the process into the cell and then return to the process. In this way the contents of the flowcell will always remain the same as that of the main process. Air is passed through the flowcell by means of a small sintered filter. The air flow is controlled precisely by flow valves and regulated to suit the application. The system is adjusted so that foam is generated in the flowcell before it appears in the main process. The flowcell models the behaviour of the foam in the main process, but in a slightly more sensitive way.

A foam sensor in the flowcell can then be used as an early warning device so that appropriate action can be taken before foam builds up significantly. This aspect is particularly helpful in applications where overflow and consequent spillage would cause considerable inconvenience, especially if on a large scale.

5. The Flowcell in Action - A Case Study

This system has been installed at ICI in Wilton, UK on the outflow of a detergent plant. There had been occasions when detergents had leaked into the effluent flumes at the plant, and had then passed over a weir into the river. Relatively small amounts of detergent could produce large amounts of foam because of the agitation created by the weir. This was difficult to sense due to the effects of wind and tide. The flowcell concept, applied in this situation, proved to be a very successful solution.

Sampling was carried out from the outflow above the weir and passed through the flowcell. The system was adjusted to ensure that foam always appeared in the flowcell just before it occurred in the river. The foam sensor was used to signal an alarm to the site control computer, and also to institute control functions which removed the danger of foam occurring downstream. Since its installation at ICI's Wilton site, the Thermo Electron foam sensor has worked reliably and consistently to prevent contamination of the river and to remove unsightly foam build up.

6. When to consider a Flowcell Model

This system is particularly useful in situations where direct sensing is difficult. Such problems may occur due to the scale of the process, or when complications such as wind or rapidly changing liquid levels make it difficult to site the sensors. It is more expensive than using a sensor and its control unit, but can be extremely effective where a simpler approach is inadequate.

Thermo Electron has supplied equipment to a diverse range of customers involved in the following industries:

Other Applications

Biotechnology
Fermenters
Pharmaceuticals
Food production
Vacuum cooling
Brewing
Water & Effluent Treatment
Paint Manufacture