**2. Context**

Many biochemical industrial activities involve very complex physicochemical phenomena which enable products to be processed. These products often go through a variety of states during processing.

Processing uses energy from chemical reactions (e.g. enzymatic), thermal or mechanical energy, or even a combination of these different forms of energy. Physical modifications can also occur (incorporation of air in the matter…). All these forms of energy are often combined within the same process and it is difficult to quantify the contribution of each in the product processing phenomena.

Due to the complexity of the processes (several processing stages, multiplicity of forms of energy…) and the products (viscoelastic matter, visco-elasto-plastic…), associated with the legitimate concern of not interfering with the process, few measurements have been carried out during the various stages of processing. The temperature, pressure and flow are often monitored during processing even though they are not necessarily correlated to the desired properties of the product under development.

This is why we chose to develop acoustic sensors adapted to the constraints imposed by either the product or the process.

Among the essential parameters sought-after, rheological measurements are often determining in terms of the consumers' perception of the qualities of the end product such as texture, viscosity, elasticity.... Several techniques of investigation exist but the majority are laboratory applications and are difficult to adapt for in-line controls. The difficulty thus arises of using multiple techniques to obtain a more complete characterization of the process and the interpretation of the data obtained with these analysis techniques.

Furthermore, the quality control of the processed products also involves evaluating the performance of the process. The temperature, pressure and flow are of course part of the characteristics measured for the control and/or closed-loop control of the various stages of product processing. However, in the case of complex processes they are difficult to correlate to the final properties of the product.

The processes can also evolve over time. This is the case with heat exchangers for which the performance varies over time due to fouling. Only preventive maintenance leading to additional production costs can ensure stable performances of the process over time. The development of sensors integrated in the process to provide information on the evolution of the performance remains essential.

This work presents a selection of studies which have led to the development of low frequency acoustic sensors specifically adapted to monitor changes in the physical state of complex media and the process: fragile gel, highly heterogeneous or highly absorbent media, media with complex rheological behaviour...

Several cases were studied:

214 Acoustic Waves – From Microdevices to Helioseismology

frequency chosen to study the change in physical state of the media and to monitor the

Several approaches were used to optimise this technology: an analytical approach to determine the sensor's first vibratory mode which was consolidated by a numerical study,

• The physical or physico-chemical phenomenon linked to the transition phase and the sol-gel transition in the dairy field; the opaqueness and the fragility of this type of gel justifies the importance of quantifying the metrological parameters such as measurement accuracy, stability over time and mechanics of the sensor implemented. • The interaction of the sensor with its environment in a process. A bivariate study (sensor/propagation medium) was carried out in order to select the required geometry to ensure that the sensor is adapted to its environment: loaded metallic plates subjected to mechanical stress and heated to a temperature of around 100°C (plate heat exchanger) or strongly absorbent media (fermenting bread dough...). This part of the study consisted in finding a good compromise between the geometry of the sensor, its

Many biochemical industrial activities involve very complex physicochemical phenomena which enable products to be processed. These products often go through a variety of states

Processing uses energy from chemical reactions (e.g. enzymatic), thermal or mechanical energy, or even a combination of these different forms of energy. Physical modifications can also occur (incorporation of air in the matter…). All these forms of energy are often combined within the same process and it is difficult to quantify the contribution of each in

Due to the complexity of the processes (several processing stages, multiplicity of forms of energy…) and the products (viscoelastic matter, visco-elasto-plastic…), associated with the legitimate concern of not interfering with the process, few measurements have been carried out during the various stages of processing. The temperature, pressure and flow are often monitored during processing even though they are not necessarily correlated to the desired

This is why we chose to develop acoustic sensors adapted to the constraints imposed by

Among the essential parameters sought-after, rheological measurements are often determining in terms of the consumers' perception of the qualities of the end product such as texture, viscosity, elasticity.... Several techniques of investigation exist but the majority are laboratory applications and are difficult to adapt for in-line controls. The difficulty thus arises of using multiple techniques to obtain a more complete characterization of the process

Furthermore, the quality control of the processed products also involves evaluating the performance of the process. The temperature, pressure and flow are of course part of the characteristics measured for the control and/or closed-loop control of the various stages of product processing. However, in the case of complex processes they are difficult to correlate

and the interpretation of the data obtained with these analysis techniques.

evolution of the acoustic properties of products that are often heterogeneous.

location in the overall system and the required sensitivity.

then confirmed and validated experimentally. The application is based on two points:

**2. Context** 

during processing.

the product processing phenomena.

either the product or the process.

to the final properties of the product.

properties of the product under development.

	- continuous homogeneous medium: sol-gel transition,
	- complex heterogeneous medium: transition from a suspension of particles in a liquid to a cohesive visco-elasto-plastic solid.

Finally, the identification of the needs and constraints imposed by certain environments (temperature, hygiene, attenuation...) have led to the combination of these types of technology to monitor a process (e.g. fouling of plate heat exchangers, search for an optimum point in the kneading phase...). By taking into account the coupling of the sensor with its environment this technique can, in certain cases, exploit the noise emitted by the process itself, as in kneading for example.

In this work, we chose to illustrate the potential of low frequency acoustic methods on applications from the agri-foodstuffs sector. These same states can also be found in the pharmaceutical and cosmetics industries, the aviation industry, the medical field as well as in material chemistry.

The methodology implemented can be divided into several phases:

	- Analytical study
	- Numerical modelling
