Fundamental physics takes process to the limit

Fundamental to the whole issue of achieving cost savings whilst maintaining product quality is an understanding of the physical properties of the ingredients being processed and the way that they interact with each other when subject to heat, pressure, and the forces associated with mixing and flow, writes  Bogdan Dobraszczyk, technical specialist, Physical Sciences Laboratory, RSSL:

Reading, UK - The process industry is always under pressure to produce its goods more quickly and efficiently, perhaps even more so during a difficult economic period than when business is booming. Unfortunately, there is no simple way to speed the rate of production to counter a reduction in the rate of income.

That said, process engineers will always be looking for ways to use a little less energy, or to gain a little more throughput, and in some cases it might just be that pumping a little harder, stirring a little faster or heating a little less will have the desired effect.

Sadly, in other cases, these tweaks might result in disaster. After all, nothing is gained by increasing the speed of production if that is more than off-set by an increase in the amount of product that has to be rejected, or if reducing the heat means that materials no longer flow smoothly through pipelines.

Fundamental to the whole issue of achieving cost savings whilst maintaining product quality is an understanding of the physical properties of the ingredients being processed and the way that they interact with each other when subject to heat, pressure, and the forces associated with mixing and flow.

For whilst it might be obvious that materials behave differently at extremes of temperature, it is less obvious or predictable what difference a temperature change of one or two degrees might make. The same is true also of pumping speeds, mixing rates and so on. Many a production process has floundered on the mistaken assumption that modest changes to some machinery setting, or ingredient specification, will have negligible impact on the end result.

Understanding ingredients

There are a few keys parameters that need to be understood about any material in order for a process to operate at maximum efficiency. Clearly, its flow properties (rheological properties) are massively important. Rheology tells us about the flow behaviour of materials under various conditions of deformation, flow rate and temperature.

It is often possible to extrapolate from measurements made in the laboratory to conditions during processing and predict how that material will perform in the factory when conditions are changed, provided the rheological measurement conditions resemble those of the process being studied. Some materials flow more easily when deformed (thixotropic), whereas others harden (shear thickening).

Depending upon their behaviour under processing conditions, materials can be classified into rheological types. It is rare to find a material that fits a type exclusively. A material can exhibit different types of behaviour at the same time. Modern rheometers can characterise and quantify the above types of behaviour.

Some materials flow better when heated, some lose moisture and flow is reduced. Rheological tests are very sensitive to these changes, for example when a material goes through the glass transition, where it changes from a hard, glassy material to a soft, viscous material that can flow. Many materials show this kind of behaviour i.e. they remain solid but start to flow.

They have not melted, but have gone through the glass transition beyond which it is much easier to flow. Rheological testing picks up glass transitions much better than Differential Scanning Calorimetry (DSC), another technique commonly used to measure thermal transitions in materials.

Practical implications

The property of flow is not important only in terms of getting a liquid from A to B through a pipe in a process. It also impacts on the application of moulds and coatings, and the pressing of doughs and other materials that are rolled (see below).

The effects of temperature must also be understood, not just in terms of how a material behaves at different temperatures but also how quickly the material might gain or lose temperature as it moves to the next stage of the process. After all, there may be no point in plant A producing a liquid for use at point (or plant) B, if that liquid is going to cool to a solid before it arrives.

Particle size is another fundamental property that can have surprising consequences. It is counter-intuitive but dissolution studies show that some ’large’ size particles dissolve more quickly than smaller particles of the same material.

This observation could have a serious impact on the specification and preparation of ingredients, but also on the output that a process has to guarantee. In pharmaceutical production, for example, the dissolution of active ingredients will be key to ensuring that patients receive the correct dosage.

In this specific example, it may be possible to increase the rate of production and produce tablets that look right and contain the right amount of ingredient, but if the dosage is compromised then the end product cannot be considered acceptable.

Tweaking the process

Before making any adjustment to a proven process, or to the ingredients used in that process, it is always a good idea to test the likely outcome in the laboratory. The up front research is often less expensive than the retrospective trouble-shooting, and Reading Scientific Services Ltd (RSSL) has experience of both.

Many of RSSL’s projects are carried out for clients in the food industry, a notoriously difficult process environment to control due to natural variations in the starting materials. After all, there is no easy way to control the specification of something that is grown in a field, and natural variations can have a big impact on key physical parameters such as moisture content, oil or fat content, texture and particle size.

However, what RSSL can do is help companies to set the process conditions to cope with these natural variations, as well as to provide a quick check that ingredients are within the specification that the process requires. In one simple example, a food processor found it was having huge difficulty getting its flour and fat to mix properly for producing pastry. What should have been a simple process was proving impossible. In RSSL’s Physical Sciences laboratory the problem was ultimately shown to be due to a change in the initial hardness of the fat being used, and hence the specification of the fat was changed accordingly.

As well as dealing with ingredient differences, RSSL is often called on by companies to address differences between processes. This might be following introduction of new equipment, as was the case at a plant where dough was being shaped to specific dimensions. Here, the client had been ambitious to increase output, and to speed up rolling out of the dough. However, when the rollers operated at high speed the dough was always misshapen.

Quite simply, the flow property of the dough prevented it from rolling out to the desired shape. In this case, the specifications for the dough were changed so that the speed targets could be met, rather than simply having to advise that the rollers had to be slowed down.

Equipment/process issues also frequently arise when processors attempt to replicate the output of one factory in that of another. This is sometimes a cost-drive exercise, when companies are looking to move production to a ’cheaper’ production area, but equally it may just be driven by the need to achieve production volumes beyond the capacity of just one plant.

The challenge here is not merely to ensure that the product can be made in both plants, but that the same product can be made, i.e. one that tastes, stores, looks, chews, crunches, melts, dissolves etc in the same way as the ’original’. Again, using food as the example, the textural properties of the product can greatly affect the flavour characteristics.

Ease of use is also an issue that can be determined by invisible properties. For example, the way an ice-cream stores and serves is linked to the size and distribution of fat droplets, ice crystals and other particles (eg chocolate chips) in the product. Different processes may appear to create the same product as it is fed into the packaging, but those key parameters must be right for the product to meet the customer’s requirements.

End of line test

A big growth area right now for RSSL is in helping clients develop end of line tests that will demonstrate that by the time the product reaches the end user, it will indeed meet the user’s requirements. An appropriate test might be the moisture content of an item that is likely to dry out (cake icing) or that is likely to get softer (a cereal product). Texture analysis is also applicable in to the end-of-line testing regime, for example, hardness, compressibility, stickiness and spreadability.

Of course there are times when it simply is not possible to produce things more quickly without compromising on quality. What physical science studies can do, is show where the limits can safely be pushed, and where cost or time savings can be derived from a process.

It is often better to learn these lessons from some research in advance, rather than from trouble-shooting after a process has changed, but in either case, the fundamentals of physical science can ultimately be used to make our processes more efficient, and to make our products more consistent.