Simulation inspires equipment and plant engineers

Tetra Pak Cheese and Powder Systems specialises in cheese-making equipment and provides proven solutions for evaporated and dried powder technologies. Tetra Pak CPS is part of Tetra Pak, a leading food processing and packaging solutions company with almost 22,000 employees based in over 85 countries.

Tetra Pak’s global reputation as an innovator is based, among other things, on the fact that the company invests around 4% of its yearly turnover in research and development and operates 11 R&D centres throughout the world. This deeply rooted culture of innovation has borne fruit: Tetra Pak currently holds more than 5100 individual patents for the packaging and processing of liquid and viscose foodstuffs.

The plant in the computer

Specialised engineering and simulation software solutions are important innovation tools. Today, both basic engineering (the determining of the key data of a process) and detailed engineering (the detailing of the pipelines and the individual components of a plant) are carried out via computer-aided-engineering (CAE) across all sectors. The result is a digital illustration of the plant, ideally as a 3D CAD model.

It is now possible to perform experiments that previously required elaborate laboratory or pilot plants by CAE as well - high-performance computers for the numerical treatment of powerful systems of equations are just as much a requirement as the further refined modelling of the relevant parameters.

But take care: even the best numerical simulation has to be critically scrutinised and interpreted. Put bluntly, intelligence has to be located in front of the computer, not just inside it.

CFD analysis optimises spray drying

CAE helps in the optimisation of equipment and plants - a decisive point to convince Tetra Pak to use these tools.  This is because their customers, who are operators in the food industry, place a great deal of value on continual innovation - the goal is always to produce better products with lower production costs in increasingly smaller plants with lower operating and service costs.

One of the tools Tetra Pak CPS relies on is STAR-CCM+ from CD-adapco. As Alfred Jongsma, Manager of Research & Development at Tetra Pak CPS, says: “We actively use computational fluid dynamics (CFD) for design optimisations, troubleshooting, and research and development projects.”

The company has had very good experience using this tool in a wide range of areas: investigating non-Newtonian fluid flow in distillation; minimising the loss of pressure in falling film evaporators; optimising the separation efficiency in separators; optimising the airflow in a spray dryer; and analysis of the recovery of waste heat at the dryer outlet, among many other applications.

Spray dryer: the goal is dry milk

The spray dryer takes advantage of the principle of surface enlargement in which liquid products, such as milk or whey in this case, are atomised into fine droplets. These are sprayed through a flow of hot air, drying them during their flight path. The temperature of the hot air can range between 150 and 250°C and the flow has only between 0.5 and 1 second to work on the product. The resulting particles fall downwards.

Low-output spray dryer (1000 kg/h). Typical plants have an output of 5 MW and a throughput of 5 t/h.

The operator needs to achieve the goal of a high-quality solid product retaining a small particle size with the smallest possible size distribution. The size of the spray dryer depends on the drying time (fall time) and the desired size of the particle. This has resulted in impressive dimensions as Alfred Jongsma describes: “A typical spray dryer has a volume of 700 m³ (diameter: 8.5 m; height: 20 m), processing 5 t milk or whey an hour and using 5 MW of power. The energy efficiency lies between 40 and 65% - which makes every step towards optimisation worthwhile.”

The significant parameters of a spray dryer are:

• the chemical composition and the initial concentration
• the temperature of the hot air flow
• the mixing properties of the droplets and air
• the flight path and the agglomeration properties

The quality of a simulation such as this relies on the quality of the underlying model - how realistic the relevant physical, chemical and technical processes are numerically described. The whole process should also be carried out as quickly as possible and should be easy to operate. STAR-CCM+ is capable of fulfilling all these requirements. Alfred Jongsma confirms that promise and reality match very well, noting, “I could not put a scale to it, but I can say I am very satisfied with the package itself, the workflow and the support that is given. There are, of course, limitations in what one can describe (in terms of physics) through CFD at the moment but I see a steady growth in capabilities of the STAR-CCM+ solution. The increased capabilities are made possible by the research efforts of numerous institutes around the world and it seems this trend is not coming to an end any time soon.”

Jongsma continues: “In general terms, I can state that by actively using CFD we have greatly improved our understanding of the complex physics of spray dryers. We now know more about the conditions that are favourable for spray dryer operation. The objective here was to optimise the spray dryer output and minimise fouling. Fouling limits the running time of a spray dryer; after several weeks it needs to be cleaned. It appeared that the factors that minimise fouling also favourably affect the capacity. The factors that were optimised were the airflow into the system and the way the liquid is introduced by high-pressure spray nozzles.”

The benefits of such results to Tetra Pak CPS can be clearly defined: improved production capacity and powder quality, and a longer period of time between necessary cleaning of the spray dryer.

The tool: STAR-CCM+

STAR-CCM+ is a CAE and CFD program, featuring an object-oriented, client-server architecture. One of the greatest challenges STAR-CCM+ was able to overcome was simulation with a ‘moving mesh’. An overset mesh (‘overlapping’ or ‘chimera’ mesh) allows the user to generate an individual mesh around each moving object, which can then be moved at will over a background mesh. The overset mesh capability is compatible with the full range of unstructured meshing options in STAR-CCM+. With no need to worry about interconnecting meshes or cell distortion, overset mesh allows engineers to make use of genuine moving object simulation.

Overset mesh can also be used for parametric studies, unsteady simulations, or even steady state simulations, providing an easy way to reposition or replace objects in order to study multiple design configurations.

Conclusion

The use of STAR-CCM+ at Tetra Pak CPS has shortened product development and realisation times and therefore also reduced costs. During research and development, it reduced the number of physical experiments required, making it easier to compare alternatives as well as to perform marginal analyses.

*Hans-Jürgen Bittermann Dipl.-Ing. is a professional engineer and freelance specialised journalist, based in Lambsheim, Germany.