Wednesday, September 26, 2012

"From Methane to Plastic to Methane - Without Waste" ?!?!?!?!?!?

Did anyone else notice that the New York Times yesterday attempted to overcome the Second Law of Thermodynamics?

That's right, they claimed that a new process could repeat the cycle illustrated here on the right WITHOUT WASTE!. Starting with methane, microbes would ferment it and produce the biodegradable polymer polyhydroxybutyrate (PHB), which could then be used as desired. After being used, the product would then be thrown into a digester which would then generate methane, that methane then being used to start the cycle all over again.

No waste, huh? Oh please!There is waste in every single step of that cycle. That it's not shown in the diagram doesn't mean it's not there. At the very least, the steps involved in forming the PHB into a usable product will require lots of energy to melt and pump it. If you've stood over a hot extruder that is melting and processing plastics, then you know there is a lot a waste heat given off.

But let's ignore all that and just focus on the mass balance in this cycle. Taking methane as the initial feed, it has the chemical formula CH4. The repeat unit in PHB (shown below) is C4H6O2. Right away, this doesn't look good. Not only is the C/H ratio off (it went from 1:4 to 2:3), but there is suddenly oxygen that wants to be in the mix. That means for every molecule of methane, 10 atoms of hydrogen have to be disposed of (hint: WASTE!) and 2 atoms of oxygen need to be called in from somewhere. I've limited knowledge of the biochemistry here, so it is possible that some of this hydrogen will be taken up by the microbes and used elsewhere, but the microbes also produce waste products of their own. I'll talk more about this again in a minute.
PHB exists in microbes as a source of stored energy. People store their excess calories as fat, microbes store it as PHB. But regardless of how it is stored, it is kept inside the cells which means you have to get it out of the little critters. And being a polymer, you can't just get it to diffuse out. Instead, you have to go in there and rip it out. Line up the bugler to play Taps, because it's the end of the line for the bugs. They lived a short happy life serving us well to make plastic. And while we are happy to get the plastic, you have all these dead microbe parts that you need to dispose of, and that is waste. Material waste. Material waste that isn't shown on the chart above, but still exists. Depending on the exact situation, the amount of PHB in a microbe can be as high as 75% (on a dry basis - meaning the water is recovered and reused), so that there is at least a 25% mass loss in this step.

Once you have the PHB, there is still more waste to come. While polymer processes that run continuously can be very efficient with material throughput, startups and shutdowns will lead to loss of material, although this is usually just a few percent or less. Consumer use of the products will also lead to some further waste as not all of it will end up being collected for the final digestion. Despite the ubiquity of recycling bins for aluminum cans, the national rate in the US is only 65% - we can't expect PHB to do any better, so that means are least a 35% loss of material.

Finally, in the digestion stage, the mass balance that was run above now needs to be reversed. To convert the PHB back to methane, the two atoms of oxygen need to disappear (WASTE) and 10 atoms of hydrogen need to be called in from somewhere.

So here is the bottom line on the waste:
  • Fermentation - 10 atoms of hydrogen per molecule of methane
  • Recovery of raw PHB - 25% mass loss from the dead microbes
  • Processing - minimal losses
  • Consumer use and recovery - At least a 35% loss
  • Digestion - 2 atoms of oxygen per molecule of methane

As you can see, the chart above has a number of missing inputs and output and therefor does not represent a mass balance. But more importantly, the question I have is this: why would you want to create a processing loop such as this? Once the PHB is made, keep it as a plastic and reprocess it. A tremendous amount of energy and matter has been used to make it and that should be respected. Degradation of polymers, particularly if it only to then serve as a feedstock for recreating that polymer anew is terribly inefficient, even if the concept flows well as a visual.

"Without waste"? Hardly. The New York Times should be better than this. Could you talk to a engineer or a chemist next time before you hype something, please?

It must be noted that it was the New York Times that proposed the waste-free title. Mango Materials makes no such claim on their website. Good for them.

Finally, many companies over the years have attempted to make PHB and other polyhydroxy alkanoates using a variety of microbes. The efforts have been only mildly successful. It is not a trivial task and I wish Mango Materials the best of luck in their efforts.


Unknown said...

I think they typically come in regular plastic use.
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