circular economy science waste

Guest post: An introduction to glycerol

A couple of months ago I wrote about Formula E and came across a technology I hadn’t heard of before: the glycerol generator, which uses a waste product to generate electricity. Neil Hollow, environmental biotechnology expert and author of No Oil in the Lamp, mentioned that he had worked with glycerol. It seemed like a good opportunity to find out more with a guest post:

Many years ago we went on a family holiday to York. Driving around the by-pass we happened to see a small scale manufacturer of biodiesel (from waste fat). This was when biofuels were seen as the saviour of the planet, even by green groups. I stopped and had a chat with the woman in charge. She asked me as a scientist what she should do with all the glycerol produced as part of the process. I hadn’t a clue and suggested burning it – not such a great idea with hindsight. This whole issue of waste glycerol disposal which Jeremy has asked me to write a guest blog on, raises some interesting ecological dilemmas.

Figure 1
Figure 1

Glycerol, also known as glycine, is a colourless viscous liquid with a boiling point of just under 300°C. Technically it’s an alcohol, with three alcohol groups on it. As a scientist I would prefer to call it propan-1,2,3-triol, as shown in a) in figure 1.

Glycerol is a growing waste issue, since it is a by-product of soap and biodiesel manufacture. Chemically when triglycerides (fats or oils) are reacted with an alkali, glycerol is produced along with 3 long chain fatty acids – see figure 2. (R is a chemists short hand for a chain of carbons and hydrogen atoms). The only difference chemically in making biodiesel is that you add an alcohol to this mix as well. This ends up bonded instead of the sodium to the fatty acid making an “ester”, which is chemically what biodiesel is. About 10% of the reaction volume of the biodiesel reaction mixture ends up as glycerol.

Figure 2

As biodiesel production has soared in recent years, so too has the production of glycerol. There are essentially two options for dealing with the glut: you can use the glycerol directly, or make it into something useful. Glycerol has many uses in itself. It’s used in food as a thickener and sweetener (E422). It has various pharmaceutical uses (my Grandparents had some in the bathroom cabinet). It’s used as a component of the e-liquid in E-cigarettes. It also now utilised by a company to produce electricity to power the Formula E racing cars. They have worked out a way to use it in diesel engines.

These above uses obviously mop a lot of glycerol up, but a more interesting proposition is to chemically modify the glycerol. Reducing it to a diol with 2 rather than 3 alcohol groups present (b) in the figure 1), produces another set of compounds – either propan-1,3-diol (PDO) or propan-1,2-diol. The latter is used as antifreeze on aircraft wings and as lubricant for condoms. It’s also used to dissolve foodstuffs such as vanilla essence in. Both diols can be used to make plastics, with PDO regarded as more useful.

Another issue is how you convert the glycerol to the diols. The traditional way is to use metal catalysts and loads of energy. However, the chemical industry is increasingly going biological. This way uses either bacteria or their enzymes to convert glycerol to the diol by fermentation. The advantages of enzymes are that they can be very specific and use a lot less energy.

I supervised a student project to try this since my bugs grew on glycerol (my idea was to make glycerol into electricity using bacteria). Unfortunately my bugs failed to make PDO from glycerol, possibly due the formation of a antibacterial intermediate called 3-HPA, or the presence of oxygen. However in itself 3-HPA has a number of potential uses, not least as a potential food preservative.

Plenty of other groups have succeeded in using bugs to make PDO. Dupont are making it from glucose in the form of corn syrup. This offers us the means of making plastics in a more environmentally friendly way and breaking our oil dependence. Most plastics however are not biodegradable and a biological route only solves some of the problems with our plastic addiction, a bit like electric cars viz à viz oil based vehicles.

In a circular economy, all waste is considered a potential input to another business or process. Glycerol is a good example of this. Thanks Neil.

4 comments

  1. WOW…I think it is always wonderful when one can use a “waste” product to create something else, as you stated in your conclusion. I often wonder WHY the garbage and trash we create are not burned in furnaces and then the heat byproduct could then be used to heat homes and businesses, rather than create hills of trash, that seem to be growing in Wisconsin. Is that possible? Too expensive?

  2. This is done incineration and is very controversial due to the release of unburnt compounds into the local environment. Its better to reuse the plastics to make new products.

  3. Landfill is the worst thing you can do with rubbish, with incineration close behind. It can be done, but nobody wants to live near an incinerator – with good reason!

    You can generate electricity from waste using gasification or anaerobic digestion however. That’s much cleaner, but hasn’t been done at scale until fairly recently. The first large plasma gasification plant in the UK was due to open this summer, so I’ve been waiting to hear it’s online before writing about it.

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