CIP process efficiency: real-time monitoring and control — Part 2

The implementation of accurate process measurement in the CIP process enables food and beverage organisations to reduce waste and save energy, while minimising the production downtime needed for cleaning. In Part 1 we looked at the pressure and temperature requirements.

In the food and beverage industry, the cleaning of process equipment is critical to ensure the health and safety of the consumer, as well as maintain the quality of the product. Proper cleaning is essential for the production of high-quality food products, especially those with extended shelf life. As a result, cleaning-in-place (CIP) systems are commonly found in many dairy, processed food, beverage plants and breweries — replacing manual strip down and cleaning of process equipment.

In Part 1 of this article we examined the necessary conditions for effective cleaning of soil from food and beverage processing equipment, including the effective use of temperature and flow rate. We now need to look at the important monitoring of chemical composition of the cleaning wash cycles.

Concentration

Mechanical forces at the right temperature are not sufficient for many soils — the soils also need to be chemically attacked to help them leave the production surfaces.

Typically, there will be at least two cycles of chemical wash — usually an alkaline wash and an acid wash. Each wash is followed by a rinse cycle to clear out the remaining residue and chemicals.

Normally the alkaline wash takes place first — the alkaline chemicals help to break down the organic soil, such as proteins, fats and carbohydrates. The most commonly used chemical is caustic soda (NaOH), or a formulated mixture with NaOH and other additives to make it more effective or lower the sodium concentration in the waste. Sodium in wastewater poses an environmental problem because it is typically difficult or impossible for wastewater plants to remove. Potassium hydroxide (KOH) is sometimes used instead, but it requires a greater concentration and is significantly more costly than NaOH.

The caustic solution is usually used at a concentration of between 0.5 wt% and 2 wt%, although some foods may require a higher concentration. In most cases, however, a concentration that is too high can be counterproductive because it can induce crosslinking of proteins, making them harder to remove. In dairy applications, for example, 0.5 wt% has been found to be the most effective — dairy protein fouling is caused by protein crosslinking, and the right concentration of NaOH will break down the crosslinks, while too much can induce more crosslinking.

Similarly for breweries, the alkaline wash cycle breaks down the hop oils, tannins and resins that acid washing cannot. Mineral-based soils, however, as commonly found in milk or brewing, such as calcium oxalate, that lead to build-up of beerstone or milkstone, require an acid wash cycle to remove them.

The acid wash step is used to dissolve minerals, such as beerstone, water scale, calcium and magnesium carbonates, although it has some effect on organic soil as well. It is also more effective against bacteria than alkaline solutions. Typically nitric acid (HNO₃) or phosphoric acid (H₃PO₄) is used. As for the alkaline solution, there are also mixed formulas available. Nitric acid is typically used at a concentration of 0.5 wt% to 1 wt%. Having a solution that is too strong can attack some polymer materials and stainless steel.

Instruments for monitoring concentration and completion

Without a way of measuring the concentration of cleaning chemicals in a CIP system, a purely timing-based method tends to be used — assuming the whole system is up to the correct concentration and then running the wash through for a set period of time. To ensure cleaning occurs effectively, the set time for the wash normally includes a safety margin.

By installing conductivity sensors in the CIP return line, it is possible to know when the wash fluid is up to the correct concentration, and that ideal cleaning concentration has been reached — as well as when a rinse cycle has flushed the chemicals.

Conductivity sensors in the chemical storage vessels also confirm the correct concentration of the alkaline and acid cleaning solutions in storage, and whether the concentration needs adjustment after recycling.

Figure 1: Conductivity versus concentration at 25°C.¹

It should be pointed out, however, that conductivity is related to chemical concentration via a calibration curve (Figure 1), and such calibration curves relate to a specific temperature. As temperature increases, conductivity increases for the same concentration. Given that different cycles of a CIP process may occur at different temperatures, it is necessary to compensate the conductivity measurement with real-time temperature measurement collected by a temperature sensor. Temperature compensation coefficients are also required for the chemical cleaning agent being used (see Figure 2).

Figure 2: Temperature dependence of NaOH concentration.²

Optical sensors can also be used to detect suspended solids in the wash return and to detect when soil is no longer present.

Conductivity and optical instruments can be used to:

• detect when the alkaline or acid wash solution has achieved ideal concentration (conductivity);
• detect and adjust for fluctuations in the concentration of the wash solution due to soiling (conductivity);
• confirm the flow of soil in the waste return (optical);
• confirm the end of the wash cycle by detecting no further soil (optical) and normal chemical concentration (conductivity);
• confirm the end of a rinse cycle when all chemicals have been flushed (conductivity);
• monitor the correct concentration of wash chemicals in storage (conductivity).

Knowing when a cycle has fully begun (correct concentration), optimising the concentration throughout the wash and knowing as soon as cleaning has completed all serve to minimise the wastage of energy, chemicals, water and time, by using only as much of each as is necessary.

For interface detection and the measurement of chemical concentration at the elevated temperatures present in a CIP process, a sensor that is sanitary, robust and reliable is required. For this application, toroidal or inductive conductivity sensors are ideal. In addition, when connected to an appropriate smart transmitter, it is possible to convert and display the chemical concentration locally.

The measurement principle of such a conductivity sensor (Figure 3) is based on an inductive signal, whereby a generator (1) generates an alternating magnetic field in the primary coil (2) which induces a current in the medium (3). The strength of the induced current depends on the conductivity and thus the ion concentration of the medium. The current flow in the medium generates another magnetic field in the secondary coil (4). The resulting current induced in the coil is measured by the receiver (5) and processed to determine the conductivity. The benefits of inductive conductivity measurement are: There are no electrodes, and therefore no polarisation. They offer accurate measurement in media or solutions with a high degree of soiling and a tendency to deposition. There is complete galvanic separation of the measurement from the medium. Conductivity and optical sensors that come in contact with CIP cleaning solutions should be designed according to 3-A sanitary standards or EHEDG guidelines. They also need to be able to withstand contact with acid and alkaline cleaning chemicals without damage.

Figure 3: Inductive conductivity measurement.

PEEK (polyetheretherketone) is a common material used in the contact elements of instruments, being chemically, thermally and mechanically resistant to the cleaning chemicals. The inductive conductivity sensor is therefore considered to be a non-contact sensor, since the measurement coils are encased in the injection-moulded PEEK body. PEEK allows for a smooth surface finish (Ra <0.8 µm) and guarantees biological safety, as pathogens are unable to stick to the surface.

Contact time

The period of circulation depends on the degree of fouling and the type of equipment being cleaned. Typically, 20 minutes of caustic circulation is required for pipework and vessels.

In dairy processing, pasteurisers and UHT plants that suffer from higher levels of fouling may require up to 40 minutes of caustic circulation. Acid circulation is normally 10 minutes.

Of course the longer the contact time in each cycle of the CIP process (flush, alkaline wash, rinse, acid wash, rinse), the more pumping energy is used, the more heating energy is used and the more water and chemicals are used and need to be recycled. It is therefore essential that each stage of the CIP process is shortened to only as much time as is necessary to get the job done.

The contact time is dependent on correct temperature and flow rate, and on the right chemical balance during the wash cycles. Minimising the necessary contact time for effective cleaning therefore depends solely on optimising the flow rate, temperature and chemical concentrations — as well as detecting when cleaning has completed — as described in the above sections.

Other required measurements

CIP chemicals such as NaOH and HNO₃ are, of course, toxic and need to be stored carefully, and at the correct concentrations ready to use. In modern multi-use CIP systems, the recycling of filtered chemicals for re-use also does not result in all chemicals being returned, and so the storage vessels will need to be replenished from time to time.

Maintaining an accurate inventory as well as automating the filling and low level detection for chemical storage vessels are therefore also important aspects of managing a CIP system. Instrumentation that can be used to assist in the management of chemical inventory include:

• Conductivity and temperature sensors to detect concentration.
• Level limit switches to detect high and low level.

The choice of instruments will need to take into consideration their chemical compatibility with the chemicals they contact, as described previously.

Vibrating fork level switches designed for hygienic applications and with a protection class of IP69K are simple to commission (no calibration, specific know-how or tools are required for their set-up) and work with all types of liquid media found in the CIP process.  Such sensors can be used in areas where other measuring principles are not suitable due to conductivity, build-up, turbulence, flow conditions or air bubbles — all of which are found in the CIP process.

In vibrating fork level switches, a piezoelectric drive causes the tuning fork to vibrate at its resonant frequency. When the tuning fork is immersed in a liquid, its intrinsic frequency changes due to the change in density of the surrounding medium. The electronics system in the point level switch monitors the frequency and indicates whether the tuning fork is vibrating in air or is covered by liquid.

Level limit switches provide high levels of safety for the CIP process by ensuring reliable over-fill protection as well as avoiding pump damage by preventing dry running.

Figure 4: A typical vibrating fork level switch designed for sanitary applications.

Conclusion

Cleaning process equipment is a necessary and important part of food and beverage processing — for food safety and to maintain product quality. CIP systems, designed and operated correctly, can eliminate the need to dismantle equipment and manually clean it.

Operating CIP systems in the most cost-effective and efficient way requires accurate cleaning data at all steps of the process. The correct choice and application of process instruments can assist food and beverage operators to optimise their cleaning process, reducing chemical use and minimising energy consumption.

References

1. Tetra Pak, Cleaning in place: A guide to cleaning technology in the food industry.
2. op cit.