Microwave Processing – Beyond Food

Sam Kingman, Chris Dodds, Adam Buttress and Daniel Groszek discuss new technology that offers a high-temperature hybrid microwave reactor for large-scale industrial use

MICROWAVE technology is in nearly every home, where the microwave oven has been an essential kitchen appliance for decades. Less well known is how this technology is an integral part of many industrial manufacturing processes. Chances are that you have eaten, purchased or used something that has been through an industrial microwave process, with uses from the production of food and food ingredients, sterilisation, vulcanisation of rubber, to the processing of pharmaceutical reagents.

Microwave technology offers many benefits: heating rates orders of magnitude faster than other conventional sources; selectively heating one material contained within another; uniform heating throughout the entirety of a material (known as volumetric heating); excellent and instantaneous control over energy delivery; and the ability to be powered by sustainably-generated electricity.

There are, however, some significant challenges in the industrial use of microwave technology: poor heating homogeneity; expensive equipment due to bespoke designs; poorly-defined value propositions not supported by pilot-scale test work; and an inability to operate at temperatures above 250^oC due to issues arising from materials of construction. Due to these difficulties, the history of industrial microwave applications is littered with expensive, poorly-designed, and unreliable systems.

Torftech and the University of Nottingham, UK have developed a new system which solves these problems and can be deployed across the process industries, including in food processing, specialist chemical manufacture, demanding drying applications, and in regeneration and manufacture of catalysts.

The team has taken its combined experience in industrial scaleup of microwave processing and toroidal fluidised beds to offer a high-temperature hybrid microwave reactor for large-scale industrial use, offering unparalleled thermal process control. A pilot system is currently operational and a commercially-sized system is fully engineered. This article outlines how the technology works and provides a case study.

How does microwave heating work?

Microwave energy forms part of the electromagnetic spectrum in the range 300 MHz to 300 GHz. The use of electromagnetic energy in communications as well as industrial scientific and medical applications is highly regulated to prevent interference with other parts of the spectrum. Only certain frequencies can be used, with some small variation between countries. For example, your kitchen microwave at home generates microwave energy at a frequency of 2.45 GHz.

Materials in an applied electromagnetic field can respond in three ways: they conduct, insulate, or absorb energy (see Figure 1). Any material can be heated as long as it is not a perfect conductor or insulator. Insulators can be considered transparent to electromagnetic energy. Examples include quartz and Polytetraflouroethylene (PTFE), whereas conductors like copper and aluminium reflect the energy. In fact, the “microwave pipes” (called waveguides) which convey microwave energy from a generator to a process are commonly made from aluminium or stainless steel, with their inside surfaces dip-braised to improve conductivity.

Materials that absorb electromagnetic energy have properties that sit somewhere between those of an insulator and a conductor and are called dielectrics. These materials can be heated electromagnetically. The heating effect is derived from the atomic and/or molecular interaction of the material with the electric component of the applied field. The material absorbs the electromagnetic energy, which is then dissipated as heat.

Because microwave energy directly interacts with the process material, the heating effect in a correctly designed system can be uniform throughout the entire volume of a material. The processing benefits of this can be highly significant in terms of processing rate and overcoming diffusion limitations of conventional heat transfer mechanisms (ie conduction, convection and radiative heat transfer).  

The challenges of designing high temperature microwave processes

Developing systems for high temperature, continuous, and reliable microwave processing has proven incredibly challenging. This means the true benefits of microwave heating as a sustainable and controllable heating source for the process industries have not often been realised outside the laboratory.

For a microwave process to yield its full process benefit, several fundamental design challenges need to be overcome. The first is processing chamber (or “applicator”) design, ensuring electromagnetic fields propagate within it in an optimal pattern to ensure effective and efficient coupling of the electric field to the material to be heated. The ideal design maximises interaction of microwave energy and processed materials while also supporting appropriate presentation and transport of the material in either a batch or continuous manner.

The penetration depth of the microwave energy into the material must also be factored into the design. This is the depth within the material that the microwave power decays to 1/e of its surface value (approximately 37%). It is a function of not only the frequency of the microwave energy, but also the dielectric or microwave-adsorbing properties of the material. Bed cross-sections greater than the penetration depth invite poor treatment uniformities and this has limited effective scaleup of microwave technology in many cases.

Presenting the process material in the correct manner is the second significant challenge. The system needs to accommodate changing material dielectric properties with temperature, both as a function of composition (as a material dries, for example) and changes in density and morphology. Any successful system must be able to deal with these constantly-changing conditions.

Finally, materials of construction must be compatible with microwave energy (either conducting, insulating, or temperature resistant as required), whilst also supporting the other requirements of the applicator. There are also constraints to consider when feeding or discharging the applicator. Microwave ingress into parts of the system not designed for it must be avoided to ensure reliability, maintainability and safety.

These challenges are amplified when service temperatures increase. Thermal runaway can occur when process materials become exponentially more microwave absorbent with temperature, often seen when an organic material starts to carbonise (burn), as carbon is a strong microwave absorber. Heating rates of thousands of degrees Celsius per second can occur, with damage or complete failure of the processing system a likely outcome. Material handling systems, often using metallic moving parts, such as conveyors, chutes, scrapers, screw feeders, weir gates and baffles become areas of weakness in high temperature microwave processes. Differential thermal expansion of mixed materials of construction can also lead to system failure as temperatures rise.

Considering the food industry as an example, without good uniformity of microwave energy exposure, “hot-spots” can form, leading to overheating and burning of the product and other parts of the feed material being undercooked or heated. As a product dries (in roasting, for example) its dielectric properties change as it becomes less microwave absorbent, and the product morphology can also change. The materials of construction for the applicator must also all be food safe at the same time as meeting the microwave compatibility requirements.

Hybrid microwave toroidal fluidised bed reactor

It was recognised that the TORBED (TORoidal fluidised BED) technology, first invented in the 1980s for demanding high temperature mineral applications, could overcome many of the challenges that faced industrial microwave implementation. TORBED has been developed for a raft of applications and industries (Figure 2), with process temperatures ranging from cryogenic to 1,600⁰C and with installations handling in excess of 3,500,000 m³/h of process gas. In the food sector alone there are over 80 TORBED systems carrying out roasting, drying, snack and cereal expansion, and pasteurisation processes.

Torftech has developed the TORBED technology for process intensification. By pushing the design principles of fluidised beds with increased gas velocities and a highly engineered processor design, very high heat and mass transfer rates between solids and gases are achieved. This occurs alongside excellent mixing of the solid particles and subsequently precise control over the conditions in the processor. Combined, this creates the perfect system for presentation of a solid material to an energy source, including microwave power.

Torftech and the University of Nottingham have collaborated to develop TorWave, using Torftech’s existing fluidised bed
technology enhanced with a specially designed microwave applicator. This technology now offers the next level of process heating precision and control for particulate systems.

We set out to design a system where particles would be thoroughly mixed as they moved rapidly through areas of well-defined and concentrated microwave energy. This facilitates homogenous microwave processing without using moving parts, avoiding what is often a stumbling block for creating reliable and high temperature industrial microwave processors. The TorWave system design builds on this, based on five design pillars:

• Ensure product bed compactness and stability, to maintain good absorption of electromagnetic energy and reduce treatment uniformity issues related to penetration depth.
• Harness enhanced mixing ability of TORBED technology, to promote treatment uniformity, reducing the likelihood of overheating, thermal runaway, and problems related to penetration depth.
• Minimise plant footprint, to enable installation within existing industrial processing lines.
• Handle a wide range of material feeds and sizes and inherently scalable - whilst ensuring a high degree of control over the processing conditions.
• Minimal requirements for specialist hardware or complicated design features, in particular moving parts within any microwave active zone.

The system works by applying microwave energy to a quasi-triangular cross-section toroidal bed, formed between an internal cone and the external wall of the reactor. The bed of material is formed by a gas distributor at the base on the reactor. The gas is recirculated and can be heated in line (Figure 3 and Figure 4).

The design process for this system used a finite element method (FEM) to model the spatial distribution of the electric field within the applicator. The design team evaluated TorWave’s interaction with a simulated bed of material using a simplified model (Figure 5). The specific objective was, for a given bed of material, to optimise the interaction of the microwave energy with the bed to ensure the system coupled microwave power efficiently with the solids, minimising energy losses.

After building the TorWave we experimentally checked the heating pattern in the system using a microwave absorbent glass-carbon foam. We had excellent agreement between the measured and simulated performance with the models predicting the experimental performance almost exactly. At the design frequency (2.45 GHz), the efficiency of the system was set so that >98% of the microwave power added to the system was adsorbed in the load; experimentally this value was determined to be 98.7% of power being absorbed. This provided a very high degree of confidence that we can make very efficient use of the applied microwave energy and that on scaleup, the TorWave will achieve the level of performance and control required for a step change in process efficiency.

System performance

This case study looks at the processing of a sample of malt. This grain was selected because it has excellent fluidisation properties at this system scale, it absorbs microwave energy easily, and is representative of grain types that require real-world processing.

Thermal imaging of materials is invaluable in evaluating how well microwave systems are performing. An infra-red camera installed in the lid of the reactor is able to image the bed of material as it is processing in the TorWave unit, with the view shown and described in Figure 6.

The even and volumetric heating generated in the TorWave meant that the malt was uniformly processed, not just an even roast from grain to grain, but also throughout the individual grains themselves. This demonstrates a key benefit of the combination of microwave and conventional heating, in that it is possible to target any temperature profile required throughout the entire grain. This control over particulate processing is possible with any microwave absorbent material including many minerals, carbonaceous or biomasses, and damp, wet or moisture-containing materials, offering an unparalleled level of thermal process control. In Figure 7a, the malt was treated with 2 kW of microwave power for 60 s with an ambient temperature gas stream, while in Figure 7b, 5 kW of microwave power was used with a recirculating gas temperature of 100°C. Industrial systems, however, can easily be operated with many thousands of kW if required.
In Figure 7, note the white line denotes the position across the bed where the surface temperature coefficient of variation (CoV) was calculated. Measuring the bed surface temperatures across the bed gives a CoV of just 1.7% for microwave-only heating in Figure 7a). This shows that across the width of the bed there is very small variation in the temperature of the material; given the scalability of the technology this means that very high levels of temperature control are now possible exploiting the proven aspects of the TORBED technology. When coupled with the extremely fast heating rates possible in microwave adsorbent materials and the powerful selective heating nature of microwave energy this offers chemical engineers a whole new unit operation for processing particulate feedstocks. Crucially, such a unit operation is also underpinned by the use of electricity which can be provided from totally renewable sources.

TorWave also offers flexibility for solid-gas processes that benefit from preheating the process material prior to application of the microwave energy. This is significant, because this hybrid heating system unlocks a range of application areas in which microwave processing was previously considered to be unsuitable, such as processing of cement and concrete, glass and other amorphous materials, catalysts and ceramics. These areas often have materials which are poor microwave absorbers at room temperature, yet absorb strongly as the material temperature increases.

High rates of gas to solid heat transfer can also efficiently cool particles. This is especially useful in preventing thermal runaway of highly microwave-absorbent materials. This is particularly important where control over temperature is very important such as in food processing but also drying of high value materials where damage to the structure of the material would impact the quality or value of the material. It could also be used to keep a bulk material cool whilst selectively heating specific active sites, such as bound water or a metal on a catalyst substrate, and therefore is particularly important in catalysed chemical reactions or material drying applications.

Highly efficient mixing of the feed material, in a system with no moving parts, also lends itself to processes such as fast
pyrolysis of biomass and coals. Here there are requirements for material mixing, robust materials of construction, and high mass transfer rates are required. 

Future applications

The concept and design of TorWave results in a highly scalable technology. Currently, a large pilot scale system is operational and we are using it to carry out commercial testing programmes with food manufacturers, with a view to rapid deployment over multiple applications. It is clear that a 100 kW system, with a 1,000 mm diameter reactor could easily meet throughput requirements of over 1 t/h continuous processing. The engineering for this 100 kW system is complete and is now ready for industrial application and we are currently working with a number of industries to demonstrate the technical and commercial impacts possible. Due to the unique design, these units are also able to be repeated to allow much higher diameter beds with many hundreds of kilowatts of microwave power if required. With appropriate specification of system components, as well as careful selection of materials of construction used in the applicator, a microwave processing system which can operate reliably at temperatures in the range 350–1,000°C is now a reality.

The system unlocks the unique characteristics of microwave processing – volumetric and selective heating in one scaleable high-temperature unit. By harnessing the potential of microwave energy, there is now a path towards the development of new food products and routes to novel production of existing ones, with improved textures and flavours. Opportunities exist for exciting novel and high impact food processes where we can thoroughly roast without any burning, achieve excellent bloating and expansion for all manner of foodstuffs, and displace oil frying to produce lower-fat products.

In addition to a wide variety of food processes the team is also working on applications in many other sectors, for example processing of high value minerals and fine chemicals; recovery of rare earths from battery technology; pyrolysis; processing carbonaceous materials; manufacture and regeneration of catalysts; and specialist drying and recycling of contaminated and specialist materials. Having removed the barriers to industrial microwave processing, the technology now provides a new approach to a wide range of previously intractable problems.

In summary, chemical engineers now have a new unit operation in their toolbox capable of transformations impossible up until now. Based upon proven scientific and engineering principles it is easy to assess the value proposition for any particular application. As a 100% electric device well suited to being energised by renewables it also has the potential to play a large part in the zero carbon process future so desperately required.