As production efficiency becomes more critical, Mike Harrison, operations director at C-Tech Innovation, looks at the development and benefits of microwave heating equipment on a production scale
In the past decade, there has been a dramatic upsurge in the use of microwave heating to facilitate the chemical synthesis of new compounds. The increased uptake of this technology has been catalysed in part by the observation that reaction rates for the best cases could be accelerated 1,000-fold. New technology, in the form of commercially available single-mode microwave systems, as well as recent advances in multimode commercial laboratory units, has fuelled the introduction of this technique into pharmaceutical r&d synthesis chemistry laboratories.
For organic chemists that already use microwave reactors in the laboratory, the benefits of the technology are well proven. Microwave energy penetrates deeper into materials than conventional heating giving more volumetric heating, leading to faster heating rates and more uniform molecular temperature history. Therefore, heating a chemical reactor using microwaves generally offers enhanced reaction rates, higher yields and improved selectivity.
The application of microwaves to organic chemistry has been very successful for small-scale organic synthesis and for the optimisation of reaction conditions. There are currently many good commercial systems for microwave-assisted organic synthesis at the laboratory scale (milligrams) and it is now accepted as the standard tool for discovery chemistry. However, while other microwave heating applications such as powder sintering, drying and food processing, have been successfully scaled and are commonly used in industry, examples of the scale-up of microwave-assisted organic synthesis to production or pilot scale, are very rare indeed.
To fully realise the potential of microwave chemical synthesis, the equipment and systems that are capable of taking the many results and methods developed at laboratory scale need to be successfully scaled up to pilot and production scale, without any significant modification of process conditions. The development of genuine 'flow-through' reactors for pilot and production scale would provide this breakthrough.
C-Tech Innovation, based near Chester in North West England, is currently developing large-scale microwave heating systems in collaboration with end user manufacturers, testing pilot equipment on a variety of chemical systems. C-Tech, an independent technology development and multi-disciplinary consultancy company, has an interesting mix of people, skills and knowledge, including chemical engineers, chemists, mechanical engineers, material scientists and microwave/radio frequency (RF) engineers.
The firm's core expertise lies in the industrial application of a range of energy-related technologies, including novel process heating technologies (e.g. infrared, radio frequency, microwave and Ohmic heating). With almost four decades of experience in applying microwave (and radio frequency) heating technology to large-scale production, C-Tech offers a bespoke development service.
In the fields of microwave and RF processing, C-Tech has worked with many industrial companies to assist them in developing and applying novel energy solutions to conventional manufacturing processes. Example applications include food processing; rubber and plastics manufacturing; manufacturing of particulates; the drying and firing of ceramics; mineral processing; sterilisation and pasteurisation.
The company has recently been involved in the development of microwave reactor scale-up systems for microwave-assisted organic synthesis for leading chemical and pharmaceutical manufacturers. As a result of this work, C-Tech has a clear understanding of the requirements of pilot scale microwave reactor-scale up, believing that practical scale-up of microwave-assisted organic synthesis is possible with both continuous flow and batch reactors.
There are a number of considerations to be made in microwave reactor scale-up, including frequency selection. An appropriate frequency should be selected to suit the material to be heated. The ability of a material to absorb microwave or radio frequency energy is referred to as its dielectric loss and this varies with frequency, temperature and the molecular structure of the material (i.e. polar species, ionic conductivity, etc.), see Figure 1.
The choice of a suitable electromagnetic frequency to produce the heating required will depend upon whether the component to be heated is the solvent, particles within a solvent, a stationary catalyst bed or a mixture of these.
The material's loss factor also affects the penetration depth of the microwaves into the product. For example, microwaves will only penetrate a high loss material a small distance and therefore the advantages of uniformity of heating (full body heating) will be difficult to obtain. Lower loss materials, which heat to some extent, can be heated throughout their depth.
In addition to microwaves, a much wider range of electromagnetic frequencies is available which can used to heat products, ranging from infra-red at higher frequencies, through microwave and radio frequency, down to induction and Ohmic heating at low frequencies - see Figure 2. The use of a lower frequency (e.g. RF) can give a deeper penetration (provided the loss factor remains relatively constant).
However, the choice of microwave or radio frequency is also restricted by legislation. There are two main permitted microwave frequency operating bands, 896/915MHz and 2,450MHz, and three for radio frequency (RF): 40.68, 27.12 & 13.56 MHz. Table 1 compares the main features of the various available frequencies.
Higher powers are more easily achievable using the 900 MHz band than the 2,450 MHz band. Penetration depth will also often be greater at lower frequency, allowing the use of larger diameter vessels or channels. However, dielectric loss is dependent on the frequency, so knowing how the loss varies with frequency is essential before changing frequencies during scale-up.
In general, the 2,450MHz microwave frequency is used in domestic and low power applications and the 915MHz band is preferred for large-scale industrial units (40kW and above). Developments in microwave-assisted organic synthesis are likely to focus on the 2,450 MHz frequency band, due to equipment availability at smaller scales and there being no geographical restrictions on use. However, using lower frequencies such as 900 MHz microwave (if geographically allowed) or radio frequencies provide a range of potential benefits, including reduced power supply cost, improved energy efficiency, higher power and greater penetration depth.
Microwave power is transmitted to the material to be heated in a cavity designed to give the required electromagnetic field and energy density distribution with maximum transfer efficiency. These cavities can be either mono-mode or multi-mode.
Multi-mode cavities (such as in a domestic microwave oven, see Fig 3) are a flexible option, capable of processing a wide range of materials, geometries and sizes, but have the disadvantages of uneven heating, lower power densities and less predictable field distributions.
Single mode cavities need to be designed for a specific range of dielectric properties and may require retuning to maintain good performance, but have the advantages of higher efficiency and performance. The size of a single mode cavity is determined by the wavelength of microwaves used. Larger cavities are possible using lower frequencies or intermediate mode designs.
Factors such as temperature and throughput, together with the choice of frequency, influence the choice of reactor configuration.
Three basic configurations exist for chemical reactors: batch, semi-batch and continuous. Microwave-assisted processing is feasible for all three, however, batch processing, when the reactor dimensions exceed the microwave penetration depth, will give a broad time-temperature distribution during heating and so is likely to result in lower yields and selectivity than in a continuous reactor.
Semi-batch designs are also possible, giving improved time-temperature distributions and should be suitable for retrofitting to existing assets, but are likely to be inferior to continuous processes.
The existence of a defined penetration depth into a fluid being processed leads to a favoured configuration for microwave heating of a continuous, in-pipe design.
Many organic synthesis reactions involve the presence of solids, either as reagents, catalysts or products. The design of the microwave reactor (and the whole system) must take this into account and has traditionally favoured the use of batch or semi-batch reactors. Sedimentation of solids can be problematic for continuous flow systems, but careful design can mitigate these problems and allow the incorporation of a high solids content into such systems.
Some components in the microwave reactor must be microwave-transparent as well as being suitable for the reaction conditions: solvents, temperature and pressure. This limits the choice of materials in many cases to certain engineering polymers and some grades of glass.
With any industrial large scale heating process, safety is a critical factor. The safety of microwave processes can be assessed and controlled in the same manner as any other chemical process, through knowledge of the potential hazards and suitable risk assessments. Systems produced by C-Tech Innovation are well within recognised safe working limits.
Successful scale-up of microwave-assisted organic synthesis from the milligram lab scale to pilot and production scales is possible and requires a combination of skills in microwave engineering, process engineering and reaction chemistry.
C-Tech has already developed a prototype flow-through microwave chemical reactor system suitable for all common solvents, providing up to 25 bar operating pressures, temperatures of up to 250ºC, heating fluids at flow rates from 2kg/hr to 50kg/hr, although even larger flow rates are possible. This is being further developed.
Results obtained with the system on a typical 'esterification reaction' have shown good results with a 25%+ increase in isolated yield compared with the conventional batch process.