Thursday, November 29, 2012

It's Biology, So The Laws of Chemistry Don't Apply, Right?

It is becoming increasingly clear that more and more chemical production in the future will be bio-based in one way or the other. Most likely there will be multiple "bio" steps involved, such as using microbes to ferment a grown crop into something useful. For example, we currently see this in ethanol production, in which corn, sugar cane or some other sugar source is grown, feed into a reactor inoculated with yeasts which then turn the sugar into ethanol. At this point, traditional chemical operations take over to concentrate the ethanol to the desired level of purity.

While this new alchemy may seem magical, the laws of chemistry still apply. In particular, mass balances. If your output is going to have carbon in it, you need carbon as an input. Yes, the microbes already have carbon as part of their biochemical makeup, but they aren't going to sacrifice it for your efforts. Look upon them as a catalyst - they help the reaction proceed, but are not consumed by it.

So when I read articles like this:"Biodegradable Plastic Manufactured From Air And Bacteria", you can understand my frustration. The article discusses an improved process for making polyhydroxy alkanoates (PHAs), and while it is true that the bacteria use air in the process, some of which ends up in the polymer output, PHAs have quite a bit of carbon in them. PHA's are class of polymers with this generic structure:
In all cases, n is 1 or more. [*] The R group can be hydrogen, methyl, ethyl...which means that there is at least 1 carbon for every oxygen atom.

While biological processes are outside of the traditional areas of chemistry and we may not be perfectly comfortable with them, mass balances still apply. And that means atomic mass balances as well. So where is all that carbon in the polymer going to come from? Not only does the article not specify it, but it doesn't even point out that it is required. No, just air and bacteria. It's magic! This is the equivalent to saying that my car runs on air. It most certainly does need air, but it also needs a gasoline as well.

While this may seem like I'm being overly academic, the source of the carbon will have a major impact on how "green" this process is overall. Does the carbon come as waste from some existing process? If so, great! Is it something that needs to be grown especially for this process? That's not so great, particularly is the yield of that feed crop aren't high.

But just as importantly, we need to recognize that the new bio-based chemistries that will be sold to us in the future still have to follow some basic principles, ones that we as technical people already know quite well. We just need to have the confidence to apply them to these knew technologies, even to ones with which we are uncomfortable.

[*] n = 0 is certainly possible and is actually known as polylactic acid (PLA). For reasons that are not very logical, it is not considered a PHA.

1 comment:

Salomon T. said...

The conmpany's website says the carbon comes from either methane or co2.