Ethanol recovery maths

soooo… I’m gonna try this on my own.

I’ve got a 100gal recovery Still that I’m assembling, and want an estimate on the recovery time to help with planning.

I asked the manufacturer and have an extremely hard time believing the “1 to 2hrs” they returned.

Knowns: 100gal EtOH. Vac of ~26" Hg to give boil temp of ~35C. heating using 60C water. aiming at 50kW electric. don’t have it in hand.

Ethanol suggests that the enthalpy of vaporization at 35C (308K) is about 41kJ/mol

based on math shown elsewhere I’m looking at 6500mol

suggesting I need 6500mol x 41kJ/mol = 266500kJ for vaporization &

15K * 112J/mol.K * 6500mol = about another 11,000kJ to get from 20C to 35C

call it 280,000kJ. which google tells me is about 80kWhrs.

suggesting that “an hr or 2” is not in fact out of the question!!!

@TheGratefulPhil: am I on the right track?


Correct. 100gal/hr is about 75-80kW of energy. I built a system that did this out of new and used parts.

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Thank you.

Hope the math helps others.

where did you source the required thermonuclear reactor?

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I’ll have to check after I get home, but this looks correct-ish but you should also consider overall Heat Transfer coefficient as stated in my previous post. These calculations are very ideal in nature, but definitely a good estimate and you did you due diligence. Nice work!

Thanks Phill!

I realize there are other bits I should be taking into account to approximate the real world conditions. I just have no handle on them at the moment.

I could do the math to figure out the heated surface inside the boiler, but the actual liquid contact surface is going to change over time, and as a biologist I tend to shut down when confronted with calculus :wink:

and even a 4hr recovery time would be spectacular as far as I’m concerned.

Commercial sized water heater! Then you gotta build the water pump circuit, a mechanical engineer helped with that. It was more complicated than expected. The plumbing costs were also an unanticipated cost.

yeah, I’ve got an engineer on the project. pretty sure he knows what he’s doing.

Sourcing a 50kW water heater is slowing me down. If you could share the pump you went with that too would be helpful.

I’m looking at grundfos 3-speed at about 1/6HP. because cheaping out on the pump seems dumb with the rest of the $$$$ we’re throwing around.

What information would I need to figure out appropriate pump sizing?

I imagine the heat exchange surface (moving target), delta-T (25C) and heat transfer fluid (plain old H2O) could be leveraged to required flow.

Read your write-up of the math for @Future’s (rather ridiculous) condensing 100gal per min with with 3 gal a min of 10C well water & can’t figure how to apply it here.

will read it again and see if it helps.

Okay i’m a little confused here about the setup. Will you clarify a few things?

  1. What is the temperature of the ethanol that you want to separate from the crude?
  2. What is the pressure of the ethanol you want to separate from the crude?
  3. What type of condenser are you going to use to condense your separated ethanol?

You have crude extract exiting the column at ??? Celsius, you want to heat it from ??? to 35 Celsius and then you want it to condense, right?

After I know the missing temperature, then we can get a more accurate number for what flow rate you will need for condensing the gaseous ethanol. Also you mentioned you can get a coolant that will be at -25 Celsius. Is that correct? If so, what coolant are you using?

If not, what coolant are you using and at what temperature?

After I know all this and what type of heat exchanger you are using, then we can start sizing pumps…this is because we will have to have a decently accurate flow rate calculation for coolant, given that we already know 100 gal/hr of ethanol.

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nope. sorry. my bad. too many questions…

let me try again. I’ve taken SoStupendous’ confirmation & your “looks close” as an answer to my first question.

Now I’m looking to size the pump for circulating the 50kW worth of hot water heater to boil my ethanol under 26in Hg (call it 35oC). I’m just going to pretend I have the condenser covered for now. if I’m wrong, I’ll have to dial back the heat input.

the reason I can’t generalize your condenser maths is because I’m trying to calculate the heat from water into ethanol/resin mixture in the tank, rather than the heat out of the ethanol and into the water. and while they are related, they are not the same.

Edit: check Phil’s answer to both questions. it’s the same equation. I’m an idiot, but I guess well :wink:

In a 100gal tank that is boiling towards dry(ish) (as an asymptote), the heat exchange surface gets smaller, but maybe that vanishes as the solvent volume is going toward zero too…?!?

only to rear its ugly head again as the resin gets more and more concentrated.


I’m looking to power the above vac assist still at 50kW, using 60C hot water. I can probably make a good guess at the surface area of the inside of the tank (or that may even come on the drawings being generated after it landed! They should be here soon.). From memory, it looks about 34" in diameter internally. Which I think makes 0.93m3 of surface area in contact with the solvent at 100gal. I’m assuming the surface area in contact with the gas phase makes a negligible contribution.

With surface area of the the heat exchanger, temp of the water heater, and applied thermal energy, I know I should be able to ball-park the required flow rate. which points at the right pump.

I tried. but running the “5000C water” example backwards just gave me a headache. not a flow rate :slight_smile:

Doing the math seems like a better approach than guessing, and spec’ing as a 3-speed to cover my ass, or going overboard and installing a speed controller.

Just for fun, and maybe to demonstrate how important it is to ask an engineer, the pump I was going to try before finding this forum, will do 4m3/hr at zero head in high, and 1.7m3/hr in low.

Was I close?!?

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Ahhhh, now I can size that pump!

So, if you ONLY need 50 kW of heat duty to warm your extract to the boiling point of ethanol you can use the following equation for the water used to heat:

Q = mc(Th-Tc)

We know Q = 50kW = 50 kJ/s = 50000 J/s
m = mass flow rate of water (g/s)
C = Heat capacity of water = 4.184 J/g*K
Th = 60C = 333 K
Tc = 35C = 308 K

m = 478 g/s. Let’s assume 500 g/s

Water’s density = 1 g/mL

478 mL/s = 1,720 L / hr of water flowing through your boiler.

You were basically right on the money with 1.7 m3/hr pump capacity but I would size on the higher end.

Determining your exchange area will be a much more difficult task especially considering the loss of ethanol you will experience


really? 1720l/hr?

and the pump I spec’d (months ago at this point) does that on low speed at zero head.

here’s the graph for the pump output. if the jacket inlet is at 1.2M (which is pretty close to real world) then “medium” speed is about right.

then I have “high” in my back pocket to make up for losses to the plumbing.

I don’t know that it counts as peer review if I guessed…


Edit: dammit, that looks to be the SAME equation you used for the 100gal/min cooling example. I’m going to claim the lower case C in the first equation threw me.

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Let me correct myself here (hard to keep track when typing on the phone)

You have to account for pump efficiency. You can estimate let’s say 60-70%. Let’s be conservative and assume 60%.

1720/.6 = 2867 L/hr = 2.87 m3/hr

That should be your low end, you will never find a pump with 100% efficiency. Hope that helps.

Just a heads up though, ANY equipment manufacturer should provide you accurate sizing if you give them inlet and outlet temperature, flow rates, heat capacities, etc.

But I understand the industry is new and this is no the general consensus


thanks @TheGratefulPhil!

I appreciate you making my wild guess look as if it was well considered.
That was not how I expected this to go down…

As a general rule, I don’t mind being proven wrong, because that almost always means I learned something. Which strikes me as the only decent reason to get up of a morning.

However, it is also very comforting that the pump I chose appears right on the money.

It is going to make ignoring the voices in my head a little harder… :slight_smile:

You are correct that the guys I purchased the still vessels from should have provided pump sizing for me, but after asking a half dozen times, I gave up and guessed.

hope this example helps others travelling the same path.


At this recovery rate, what’s the advantage of going with FFE and spending 2-3x as much??

footprint mainly. I believe.

there’s a little bit of energy savings, cause you’re not heating up so much stainless. but not much otherwise as near as I can tell.

I’ve got a FFE to setup in the next couple of weeks. might get to compare them at some point.