Reactor technology ᔡMicrowave flow chemistry:
Faster, cleaner, greener, safer
Continuous flow microwave chemistry is now a real alternative for industry, argues Yvonne Wharton of C-TechInnovation

M

icrowave chemistry has been known since the1980s and is now widely used in academia andindustry, particularly for the synthesis of small
quantities of new compounds, for example inpharmaceuticals. Whilst it is widely used on milligram scale,
however, not much work has been done on scaling up
microwave processes to production scales.
By tradition the chemical process and manufacturingindustry has relied upon batch methods of production. The
limitations of these methods have become increasingly
evident and companies are reporting problems with longprocess development times, high inventories of materials,
variable product quality and increased levels of wastage.
Therefore, companies are starting to look towards
alternative manufacturing methods to solve these problems.
Until recently, there has been very little choice of alternative
methods or equipment. Continuous flow microwave
chemistry overcomes these problems and goes a long way tomeeting the demands that most companies set for their newprocesses.

Design & benefits
To date microwave chemistry has been restricted in scale tothe laboratory, with equipment capable of producing
milligrams or a few grams of material. Scale-up has been
prevented by two major problems: the size limitations of thecavity and reactor and finding suitable materials to cope withthe required pressure and temperature while being
chemically compatible with the reaction mixture andtransparent to the chosen microwave frequency.
C-Tech Innovation’s design for a microwave flow reactor(Figure 1) overcomes these two stumbling blocks. Thedesign is a continuous flow-through system using a
travelling wave microwave applicator. This allows the use

Figure 1 - Microwave cavity

Quartz process tubeof relatively large bore process pipe-work. Large amountsof power can be coupled to the load using this design,
allowing the use of high flow rates and hence highthroughputs.
Investigation into materials and components found thatlarge bore quartz glass tubing is inert to most chemicals,
transparent to microwaves and can be obtained in grades
rated to >20 bar and >250 °C. A unique design wasdeveloped and patented for sealing the reactor tube andcontaining the microwave field.
Doing chemistry in flow can offer many benefits overdoing reactions in a traditional batch way and combining
this with microwave heating can enhance these benefits
further. Some of the key advantages of carrying out
chemistry in this way are that the reactions are:
ᔢ Faster: Microwave heating can produce substantiallyfaster reactions than conventional heating, due toselective heating, very rapid temperature rise, the removalof wall effects or simply the ease of operating at hightemperatures and pressures; there are hundreds ofliterature examples where the use of microwave
equipment has shortened reaction times from hours tominutes. In turn, this can substantially reduce the cost ofprocessing the required amounts of materials
ᔢ Cleaner: Rapid temperature rise and the absence of hotheat transfer surfaces can mean fewer undesired side
reactions, resulting in higher yields and lower impurity
levels. No steam or heat transfer fluids mean fewer leaks
and no fouling of heat transfer surfaces
ᔢ Greener: There are many literature examples of solventfree syntheses and many also show the potential for
reducing catalyst use. We have shown the ability toquantify the energy-saving potential of microwave
chemistry due to the shortened reaction times andreduced downstream work up as a result of the higher
yields and purity that are often achieved
ᔢ Safer: Microwave heating is a direct heating methodwith no need for heating jackets or heat exchangers
with large thermal mass. This means that turning off thepower immediately stops energy input to the system andthe temperature can be lowered quickly. Using acontinuous flow system also lowers the inventory of any
hazardous or unstable products and intermediates,making scale-up inherently safer than when using abatch approach

Process Improvements
Travelling wave
applicator

Microwave
sourcewww.specchemonline.com

Pressure and microwave
containment

A continuous flow microwave reactor was developed thatwas capable of producing >100 kg of product/working day
and was then trialled using a range of reactions that areapplicable to a variety of industrial sectors, with significant
reductions in reaction times and increases in yields. Examples
include the reduction in reaction time of a Suzuki couplingfrom two hours to one minute with no loss of yield, and adihydropyrimidine synthesis which gave a two-fold increasein yield and a reduction in reaction time from eight hours tofour minutes.1,2
November 2012 Speciality Chemicals Magazine ᔢ 27 Reactor technology
After demonstrating that it was possible to achieve a timeand materials saving with the reactor, we looked to see if it
there were any other energy and environmental benefits inusing microwave flow chemistry.
There is a real need to reduce energy demand in thechemicals industry as the industry contributes 4% of overall
emissions of CO2 from all industrial sectors using fossil fuelcombustion. While this may not seem like a significant
amount, when compared to other industries, the industry isa major industrial emitter.
The US and European chemicals industries have indicatedthat CO2 emissions have stabilised while production hasincreased, which shows that companies are increasingly
looking towards ways to make their production more
efficient and more environmentally friendly.
There is cause for concern that the rapidly expanding
chemicals industry in countries such as China is far morereliant on coal, which produces more CO2 per unit energy
than oil or gas. On the other hand, there has been a switchfrom coal to oil by the chemicals industry, which has helped
to stabilise CO2 emissions.
Most countries have programmes in place to achievesavings in energy use and governments are promoting
energy efficiency in all sectors. Because of this, the overall
energy efficiencies of these countries have improved. Energy
savings that can be achieved easily at no cost have been
estimated to be as high as 30% of the total energy use.
Once this low hanging fruit has been taken, companies
will have to start looking towards new innovations toincrease their energy efficiency further. The demand forchemicals is expected to increase and therefore the demandfor energy by the chemical industry will increase with it. If
CO2 emissions are to continue to fall, chemicals companies
will need to achieve greater efficiency gains.3,4The potential energy savings from using a continuous flow
microwave system approach in place of a batch process werequantified. In all cases such reactions showed significant
savings in energy and reduction in CO2 emissions, on
average 58%. In some cases, the switch gave a 90% saving
in energy consumption. This energy saving was brought
about by the faster reaction times possible, enabling larger
volumes of material to be processed per hour.
Even in the case of some reactions that did not show anincrease in energy efficiency over the batch process, overall it
could still be more efficient to carry out these reactions in
the microwave reactor. This generally leads to an increase inyield which would save energy in the downstream work-up
and reduce the volume of starting materials required.
The energy efficiency figures for the reactions carried out
in the microwave reactor were between 40% and 60%. We
anticipate that, when moving to a full-scale system, theenergy efficiency would be around 60%, giving an evengreater saving in energy over the batch process. Using
electricity as the energy source also makes it possible to use‘green’ energy tariffs or renewable energy sources on- or offsite.

Future developments
C-Tech is now continuing this research by embarking on a
two-year collaborative Eurostars R&D project called MiFlow,in partnership with Solvionic. This aims to develop a novel
microwave-assisted flow reactor to allow the scale-up ofreactions with minimal process development.
The reactor should reduce energy usage in trialledchemical transformations significantly (at least 30%) and also
give a large increase in energy efficiency (>50%). The ionicliquid processes being used to prove the reactor concept will
be tailored from a range of end-user specific processes
28  Speciality Chemicals Magazine November 2012

Microwave flow reactor

provided by Solvionic. The project will look to develop ahighly energy-efficient microwave flow reactor anddemonstrate its application using ionic liquid-basedprocesses.
There is a real need for better controlled processes of ionicliquid synthesis, since their uses in emerging applications likeelectrochemical energy storage requires a high degree ofpurity. Because batch production is not adapted to giving
colourless and high quality products, supporting solvents andwashing steps have to be used. The use of organic solventsin large quantities produces waste and is not in line with the12 Principles of Green Chemistry. In this context, the use of
microwave-assisted flow reactors is a very promisingalternative that will eliminate or minimise environmental
damage, while also reducing waste and energy
consumption.
The cost of ionic liquids is still high, mainly because theirproduction volumes are still very low compared to molecularorganic solvents. If high added value industrial applications
based on ionic liquids start, the production of larger volumes
will lead to costs falling. Moreover, the development ofcontinuous flow processes for ionic liquid production will
decrease production costs, thanks to better yields andreduction of waste and energy consumption associated withlow investment.

 References
1. Y. Wharton, IChemE
Symposium Series 157, EPIC
2011
2. J.E. Muir, Manufacturing
Chemist, 5 February 2010
3. Environmental Outlook for
the Chemicals Industry, OECD,
2001
4. S. Milmo, Chemistry World,October 2007, 62-64

 ContactYvonne Wharton
C-Tech Innovation Ltd.
Tel: +44 151 347 2900
E-mail: yvonne.wharton@
ctechinnovation.com
Website:www.ctechinnovation.com

Conclusions
Most of the significant reductions in energy consumption
have already been achieved by the large chemicals
manufacturers who can afford to invest in energy-saving
equipment and to employ specialists in energy management.
Even these companies, however, are finding that they cannot
make further energy savings without radically changing theway they produce chemicals.
Chemicals manufacturers will have to adopt newtechnologies if they are going to continue to reduce theirenergy consumption and environmental impact in an evergrowing market. Microwave flow chemistry offers one wayof contributing towards this. It also presents a convenient
way to scale up lab scale reactions to plant scale with lessprocess development.
With companies and governments ever more mindful ofthe safety implications of chemical production, microwave
flow chemistry also offers a safer way of heating large
volumes of flammable materials as there is a much smaller
inventory being heated at any one time when comparedwith batch chemistry.
www.specchemonline.com