Didn't want to get to the point where folks get undignified or anything. There was definitely some confusion and lack of context (or maybe irrelevant context?) on that thread beyond the PID debate.
Was just looking to politely comment about a couple of talking points used. My comments surely would have pulled the thread into another direction all together.
A really (really, really
) old timer once asked me to only talk about what I know. Hopefully I can articulate?The first thing to really stick out toward the end of that thread was this:
All fuel and alcohol distillation takes place by temperature. Methanol for example is one of the first things to come off at low temps, followed by other foreshots components, then comes your heads at slightly higher temperatures. Then hearts come off next at higher temperatures, followed by tails. The key being each of these come off at progressively higher temperatures.
Please have a read through this write up to gain a bit more clarity about Methanol:
This next comment is absolutely lacking anything substantive:
Big industrial continuous stills have multiple take-off points at different plate levels to take spirits at temperature. Each plate will have a different temperature. Same concept just handled differently.
Operationally, you can't compare temps on a batch system to temps on a continuous system. Batch system temps are ever changing. Batch temps have to change. No choice.
Continuous systems are tuned to run in steady state. This steady state tuning is entirely predicated upon the fact that an alcoholic(beer) mixture is continuously being fed into the system. At no point is any part of the system EVER depleted of alcohol as long as the system is being fed.
Steady state systems do very well with PIDs controlling various operations. But the one thing PIDs do not control as HDNB mentions is heat input to the system. PIDs (or the like) control pump speeds and coolant flow speeds.
As far as pulling product according to fractions are concerned again this concept is predicated upon a continuous feed of a reliably consistent alcoholic beer mixture. As an aside, product is taken off in the form of liquid (LM) rather than vapor (VM). Allowing vapor to be pulled off the system as product would surely add to the bad reputation of continuous systems not being able to make a good cut. Continuous systems can btw make a cut. Just not as good as a batch system.
But my point here is that measuring the temperature of vapor to make an assumption about collecting product at a specific plate level on a continuous system would be a flawed way to think. Much better off picking a plate level, draw off some product, taste it, then do the same on a different plate level. Once the sampling is complete,,,,pull product from the plate level that renders the desired outcome. Plate temps are nothing more than a by-product, a result of the function. Secondary data at best. Kinda like how the old timers did it.
Also,,, best not to confuse fuel production with spirits production as the OP alluded to in my first observation. Hopefully I don't have to explain why.
Then there is this:
All batch stills starts out with the boiler at a low temperature and works it's way up toward 100C/212F as it's depleted of alcohol. If YOU LEARN by logging where your hearts and tails come off the still, then you most certainly can use that temperature knowledge to your advantage as many of us do this. On a simple pot still for example you can set the target temp where your heads end/hearts start and get a clean transition. If you hold that temp the still will actually stop producing spirit (concentration of heads are taken). Then you can raise the target temp point and pull hearts. If you don't raise this target temperature the still will again slow down and stop producing and you have your hearts. Then when you raise the temperature again by raising your target temperature you will pull the tails. This is simply using the data you've logged in the past for your still to know where to set these target temps. You simply can not pull tails from a pot still if you don't allow the temperature to rise to where they come out!
You just do not need a PID to run this way. Nearly every operator running a little plated column knows that when they dial in their 100% variable SSR controller (or propane valve) and their coolant flow that they can make their still slow down at the end of a perceived cut point.
I mean you can run that way (with a PID),,,,but good grief its just not optimal to allow the PID to shut down the heat input and drop every bit of vapor out of suspension.
So you can certainly create set points if ya like. But if you've read the Methanol write up post above, you should be able to gather that the constituents contained within 100% infinitely miscible solutions very often have boiling points that are so very close (because of homogeneous miscibility) that you'd have to adjust your resolution beyond the capability of a $100.00 PID. In other words, the Amazon PID isn't capable of making a cut based on a predetermined temp setting. The only thing it does is stop your product flow and drop your vapor out of suspension.
Can you run your still with a PID? Sure.
That thing smeared like a gopher on the hi-way!
WHAT? 
