My original research was influenced by home brewing related sites. There appears to be a strong prejudice against HDPE there, but that is probably related to an obsession with taste. They really push metal.
Looking at scientific notes regarding HDPE it very easily could be used for the fermentation phase. The heat retention characteristics of HDPE actually would help reduce the energy needed from the heating bands.
The boiling, evaporation and cooling phases would be difficult though. The resistance to heat transfer would slow the transition to boiling temps (water in the first case ethanol in the second) and significantly increase cooling time.
The nitty gritty details.
(Forgive any mistakes as this is from memory) With a high grade steel 55 gal drum Generally you need to see
- 2 hours to bring to a boil (1st Heating phase)
- 4 hours at 100 degrees (Breakdown phase)
- 2 hours to reduce temperature (cooling phase)
- 36-48 hours at ~37 degrees (Fermentation phase)
- 1.5 hour to bring to an ethanol boil (2nd Heating phase)
- 3 hours at ~78 degrees (Evaporation phase)
The metal containers work well as you can use the boiling unit to get the mix to 78+ quickly as you can heat the bottom to hundreds of degrees via direct flame. However they would need more energy input during the longer fermentation phase due to radiant heat loss. This requires a intermittent burn (less desirable but greater flexibility in fuels) or heating bands (very precise temperature control but needs electricity).
The raw materials can also assuredly stay in the container for the entire process without worrying about heat degradation of the container. If you have 4 barrels like my kit suggests you can set up a pretty nice rotating 4 day schedule. 1 barrel in phase 1-3 , 2 in phase 4 and 1 in phase 5-6
As I said earlier HDPE would be great for the fermentation phase and storage. I still need to look at details of energy transfer before I completely dismiss HDPE as a one container solution. My biggest concern is that direct heat by flame is not an option.
Using only external heat for boiling and radiant heat for cooling and expecting that heat to move through the HDPE entirely would would increase the production time. I have found some numbers regarding heating bands from totally electrical brewing sites. To reach a boil in this container i think you are looking at 8-12 hours at 20,000 watts. So we are probably seeing about a 33-50% increase in the total production time.
The heating would probably require a finely tuned and insulated electrical heating cover for the bottom 2/3rds of the HDPE container. As it is insulated it would need to be removed for the cooling phase. Cooling would be even slower than heating and could not really be assisted unless the electrical cover was dual purpose (heating/cooling). While possible and desirable this is getting more complicated (compressors/radiators/etc). I suppose this unit could also be used to heating and cooling tents or such as I always like multi use equipment when putting anything in the project.
I think hdpe could work really well in a larger setup and I will certainly consider it when I work on the project equivalent of the T2k medium and more likely large still.
edit.
In case anyone is using my production timeline I realized I forgot phase 7 which is running the 96% pure alcohol through the zeolites. The is independent of the heating characteristics of the containers but would add another few hours to the total process. I always assumed it would from Jerrycan to Jerrycan given the smaller volumes.