Selecting Chiller for Rotovap

easystephen · Oct 20, 2016

I recently bought the Across International 50L rotovap but the chiller that they bundle with it seemed underpowered. I wanted to share some calculations I did to figure out what power chiller I would actually need to get the 20L/hr throughput rate they spec. You can modify for whatever unit you might be considering.

Enthalpy change of condensation -38.56 kJ/mole
Density 0.79 g/ml
Molar mass 46.10 g/mole
Liters per mole 0.06 L/mole
Enthalpy change per liter -659.95 kJ/L
Condensation rate 20.00 L/hr
Time per liter 180.00 s/L
Cooling power needed -3.67 kW

So, in order to condense 20L/hr I need a chiller with 3.67 kW of cooling power at a temperature of around 20C less than the boiling point of ethanol at the pressure in the rotovap. So if I want to be able to distill down to 75 Torr I need 3.67 kW at around 0C. The specs on the chiller from AI say it has 2.8 kW at 0C which clearly doesn't cut it. In fact, 3.67 kW at 0C probably won't cut it either since the fluid warms up in the condensing coils as it pulls energy from the ethanol gas so we really want 3.67 kW at something like -10C.

Josh@Summit · Oct 20, 2016

You may need to upgrade to a larger chiller in the neighborhood of 5kW or more. The price of these varies, but expect them to be somewhere in the neighborhood of $2500 to $5000.

Josh@Summit · Oct 20, 2016

Take my last post back, 5kW (7.5hp) is a large chiller! I guess that makes sense for a 70L rotovap.

easystephen · Oct 20, 2016

I ended up buying the Julabo fpw55 which has a max of 5.5 kw, 4.1 kW at 0C and 2.2 kW at -20C; it's a nicely sized chiller for the application.

Predjee · Oct 20, 2016

How does a rotovap makes sure your sample does not boil down?

live resin · Oct 20, 2016

Predjee said:
How does a rotovap makes sure your sample does not boil down?

Rotary evaporation recovers solvent under vacuum. Higher vacuum=less heating=less degradation of product

easystephen · Oct 20, 2016

Predjee said:
How does a rotovap makes sure your sample does not boil down?

There's two ways to do this.

1) You can control the temperature and pressure in the rotovap and set these such that you don't go below the vapor pressure of any compounds you don't want to evaporate. For example, if you're distilling off ethanol and you don't want to remove any water, you can set the temperature to 65C and the pressure to 300 Torr, this will put you below the vapor pressure of ethanol but above the vapor pressure of water. You could do the same thing if you are trying to distill ethanol but no terpenes.

2) This is much less precise but the compounds will mostly boil off in order from highest volatility to lowest. So the components with the lowest boiling points will evaporate first. If you have a mixture of water and ethanol and you have the rotovap temp at 65C and the pressure at 50 Torr, you will boil off the ethanol azeotrope (190 proof) first and then the water. If you know how much of each component you have, you can fraction them off. It's not that precise though, and you will not get a perfect separation. When I've done this experiment I ended up with about 178 proof ethanol instead of 190.

montroller · Oct 20, 2016

Almost seems like it would be cheaper to buy an ice machine and a submersible pump at that point. I have been seriously wondering if these chillers even have a place in our labs at all lately. Our temperature ranges that we would normally use are 0c-40c unless you are trying to chill your solvent for extraction, in which case a chiller wouldn't even be better than a liquid nitrogen heat exchanger. It seems like everyone I hear about who got a chiller is disappointed in their performance and looking to upgrade.

Predjee · Oct 20, 2016

I a way it makes sense what you are writing stephan. But there are some things that i dont understand.

I understand that putting a solvent under vacuüm lowers its boiling point correct?
But what do you mean exactly by vapor pressure? The translator makes something weird of it.

When i make a extraction via a soxhlet, with ethanol and i take the captured extraction, can i place it in a rotovap to distilate the ethanol for re-use? And what will it leave me with? 1 flask of ethanol and 1 flask of what?

And would this be a odor less closed system?

easystephen · Oct 20, 2016

The vapor pressure is the pressure exerted by a compound when it's in the vapor phase. It changes with temperature so the hotter the material the higher the vapor pressure. If the air pressure in the rotovap is less than the vapor pressure, the material will evaporate.

The stuff you put in the rotovap will be a mix of ethanol, water, terpenes, cannabinoids and some heavier non-volatiles like chlorophyll. Let's say you set a temperature of 65C and a pressure of 100 Torr, the ethanol and water will be evaporated and condensed into a separate flask, leaving the concentrated, dark, sticky cannabis extract.

It's possible for there to be some smell at the end of the batch if the pressure is very low in the rotovap because you will boil some terpenes that can be pulled through the vacuum pump and into the room. If you keep the pressure around 200 Torr you shouldn't have much smell.

Thomas@LRL · Oct 20, 2016

We have a Lytron Kodiak RC045 at about 5.9kW for our 50L, have not begun processing yet though.

Predjee · Oct 20, 2016

Thanks Stephen! That helped a lot! But how do you decide which temperature to set and which amount of torr to use?

easystephen · Oct 22, 2016

Thomas@LRL said:
We have a Lytron Kodiak RC045 at about 5.9kW for our 50L, have not begun processing yet though.

It looks like that chiller only goes down to 5C. I think you might pay a penalty in condensation rate, but I'd be curious to hear your results.

easystephen · Oct 22, 2016

You just need to be somewhere under the ethanol curve. 60C and 100 Torr would be some reasonable settings.

Predjee · Oct 22, 2016

easystephen said:
View Image

You just need to be somewhere under the ethanol curve. 60C and 100 Torr would be some reasonable settings.

Thanks! so if u understand correctly, there is no exact science and you can play around to see what ever works for myself?

Predjee · Oct 22, 2016

Also, how do i read this table?

Cause for me, it looks like the boiling point of ethanol @ 100 torr is at ~37 degrees celsius?

(how can i edit my posts?)

easystephen · Oct 22, 2016

Right, so you need to have the ethanol above 37C for it to boil.

Predjee · Oct 22, 2016

But the solvent can never get hotter then its boilingpoint rite?

So why not put the settings on 37c @ 100 torr?

easystephen · Oct 22, 2016

Correct. Setting the rotovap bath hotter than the boiling temperature of the liquid allows for faster heat transfer which allows the liquid to boil faster. Remember you have to add the heat of vaporization energy even after the boiling temperature is reached in order to vaporize the liquid.

Thomas@LRL · Oct 22, 2016

easystephen said:
It looks like that chiller only goes down to 5C. I think you might pay a penalty in condensation rate, but I'd be curious to hear your results.

I agree, unfortunately neither were my choice and I have to document results before I can get funding to replace it

I'm still waiting on a WSLCB investigator for final approval though so its going to be a few weeks.

Selecting Chiller for Rotovap

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