noob question about reflux condenser/column still

[football29]

i think this question is mostly general to the theory of packed column distilling.

basically, how come in a CM still is the reflux condenser (or dephlegmator or whatever component facilitates a coolant flow that acts as a heatsink) is below the take off port?
I see if you have insufficient coolant flow, its more efficient to have the vapor have direct contact with the condenser. But if you have high enough flow cold coolant, could you put the condenser above the take off port.

Or to describe it another way, using the CCVM design, could you leave the cooling coil in the most raised position, and vary reflux by valving down the coolant flow into the coil, rather than physically moving the coil downwards?

I'm not trying to design a better still I am just trying to understand the theory of what is going on. It seems if you can drive the top of the still cold enough, a temperature gradient will form and the temperature drop will be seen lower and lower down the column as coolant flow increases. I hope that makes sense. I tried to also describe it in the attached pic, would the setup on the right work if you have enough flow of cold water?

[Tummydoc]

If the reflux condenser is above the vapor takeoff it's no longer a coolant management still, its a vapor management still. And in that case you need a valve to adjust the vapor output to the product condenser (reflux ratio) or a movable condenser to act as a valve (CCVM). In a coolant management system you adjust the amount of vapors allowed to pass through the reflux condenser (dephlagmator) to the product condenser by adjusting coolant flow, so the dephlagmator acts as your valve varying degree of reflux and vapor reaching the product condenser.

[Berserk]

I think I get what you're getting at, football29.

Correct me if I'm wrong: what you're discussing is the possibility to have a super cold condensor, located above the product take off, cool reflux to a point where the top of the gradient is so cold that no vapor is produced, and thus no vapor escapes out the product condensor.

The only problem is that for the reflux condensor to be able to cool anything vapor needs to reach it. If there's vapor reaching it, and it's above the product take off, vapor will go out the product take off as well as up to the reflux condensor.

I hope that makes sense. If I totally misunderstood what you meant, please correct me.

Cheers,
Berserk

[football29]

I hope that makes sense. If I totally misunderstood what you meant, please correct me.

Thank you! I think you got exactly what I'm asking, and the theory behind it. I guess it gets into the more detailed models of heat transfer. aka conduction, convection, radiation, etc.

I guess I was imagining a cold coil would "radiate cold" (Maybe its more accurate to say the vapors are radiating heat to the coils). But it sounds like the dominant form of heat transfer is condensed vapors (liquids) conducting heat to vapors, and the vapors must first reach the RC to condense. You can not induce condensation from vapors just getting "close" to a coil, they have to actually condense on a cold surface? Is that correct?


EDIT:
or another way I just thought of to rephrase the question. Could you leave the CCVM still in the 80/20 reflux position, partially overlapping the product takeoff port, but reduce the reflux rate by reducing the coolant flow via water valve?
Of course off the top of my head I realize why this would be worse: less repeatable, more variables to mess with. compared to visually moving the condensor to a very repeatable position, with no adjustments to water flow. SO my question is just based on the theory, WOULD changing coolant flow in CCVM accomplish the same changes as moving it (not would it be easier to control)

[Tummydoc]

No, reducing coolant flow in a ccvm would just vent alcohol vapor to the environment out the top of the still .

[still_stirrin]

Football,

If you used a “conventional” vapor managed reflux head, one that utilizes a physical valve such as a ball valve, butterfly valve, or gate valve, then you would control the reflux ratio by adjusting the valve. The reflux condenser (located AFTER the valve in the vapor path) must be sized to fully “knock down” or transfer all of the heat from the vapors rising up through the column. This would allow an infinite reflux ratio, or when 100% of the produced vapors are condensed and returned to the column for reflux.

The position of the valve is after a vapor line split so that when the valve is opened, some of the vapors will advance to the product condenser, which too must be sized to fully “knock down” all of the vapors produced, as in the case when the valve is fully opened and all of the vapors advance to the product outlet. This would be the case for zero reflux, as in a potstill operation.

The advantage of the coolant controlled vapor managed stillhead is that the reflux condenser and vapor splitting duties are controlled jointly by the reflux condenser coil. It acts as the valve and heat exchanger simultaneously. The disadvantage to this design is that it may be susceptible to coolant flow variations which would allow minute variations in the reflux ratio throughout the run.

But, considering the varying product in the boiler, where alcohol content is continuously changing, the minute variations in coolant flow to the CCVM “valve” are integrated in the product collection jar....and you’re worrying about something that simply doesn’t need to be worried about.

Bottomline, you need enough “condenser power” in BOTH the reflux condenser and the product condenser to fully handle the heat load introduced in the boiler. That is, you can’t count on the ambient condition to remove some of the produced heat for you, although heat will be lost to the environment during the process. If you do your heat balance equations, then you must assume all heat is transferred to the vapors and condensed along the path. Insulating the boiler and condenser column will help maintain this ideal condition, allowing better stacking in the column and better efficiency with the thermal processes.
ss