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Technology turns orange peel into valuable chemicals

British researchers are attempting to industrialise a technology that uses microwaves to turn orange peel into valuable chemicals.

The team from the Biorenewables Development Centre (BDC) at York University is hoping to scale up a system that produces substances used in household and food products more cheaply than is currently possible.

‘Just in Brazil there is more than eight million tonnes of orange peel [produced every year],’ BDC communications manager, Maggie Smallwood, told The Engineer.

‘Half of the orange is thrown away and it’s a huge disposal problem. But actually, if you turn it on its head, the orange peel is actually a huge reservoir of useful chemicals.’

The technology, developed by academics from the university’s green chemistry group, uses microwaves to heat the peel in the absence of oxygen (pyrolysis) in a sealed vessel to generate high pressures.

This causes enhanced acid hydrolysis, a process where acid breaks down the cellulose molecules into sugars and which doesn’t occur without the microwave treatment.

This then produces chemicals including limonene (a fragrant substance used in cleaning products) and pectin (the gelling agent used to make jam).

‘Conventionally it’s not used in anything apart from jams because it’s difficult to get out,’ said BDC’s process development manager, Dr Mark Gronnow.

‘Typically they wash it with acid, then base, then acid again and that’s a very difficult process and that makes it expensive, but it’s a texturising agent so if you could make pectin cheaper it would be an interesting product for more chefs to use.

‘What we found was with the microwave system it just comes out without having to do all the acid-base washes.’

The BDC researchers have already produced a machine that can process 30kg an hour of peel and now hope to scale the technology up to be able to cope with the millions of tonnes currently wasted.

‘The difficulty in scaling up microwaves is they don’t penetrate,’ said Gronnow. ‘And then the other problem that we’re dealing with is the high pressure, the scaling up of flowing pressure vessels.’

The proposed solution, he added, was to use a system of multiple tubes that could expose an increased amount of peel to the microwaves and process them at a faster pace.

Pyrolysis systems are also being developed to produce cost-effective heat and power from landfill waste.

Readers' comments (4)

  • That really is a great bit of technology. My usual plea applies. Please please please ensure that once its developed and the machines go into production they are made here in the UK by a British firm and exported, rather than simply licencing to the highest bidder.

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  • Alternative Proposed Scale-up: (1) Forget microwaves. (2) Use solar energy focused sufficiently to rapidly heat the pre-treated flow in a pipe to a suitable pyrolysis temperature similar to the temperature in the microwave experiment.

    If algae can be converted to "bio-crude oil" at a 60% yield from slurry (and it can), then orange peels that have been macerated and made to a toothpaste consistency in slurry should do just as well. Maybe with only slightly different product yields.

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  • James Stewart is on the right track, but use solar-heated pipes filled with static mixer elements to improve heat transfer by 4-5X over ordinary pipe. Outflow from pipe goes to flash tank connected via an overhead condenser to a vacuum pump to remove volatile products while the stripped pulp falls to the base of the flash tank and is removed by a gear pump.

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  • In the work we have found microwave heating to be essential for the depolymerisation of the biomass. We have done a number of investigations mimicking temperature and heating rate in conventional equipment (as solar would be) to determine if microwave heating has benefits beyond simple rapid heating. We have proved in every case microwave heating to enhance breakdown and selectivity. We have developed a number of ideas as to why microwaves are capable of enhancing acid hydrolysis, we hope to publish these soon.

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