Batch Distillation and Alcohol Shifting: Exploiting Positive Feedback for Optimal ABV


The thoughts and ideas contained here have been formed through fundamental batch distilling with the distillation apparatus more widely known as “The Humper Thumper”. Furthermore, some of the concepts and theories went on to be tested by other members within the distilling community. Though much of the data is anecdotal, most of the participating testers also came to very similar conclusions.

Send Nothing Back

The first assertion being that, for optimal kettle behavior, the operator should never allow any previously distilled alcohol to return to the kettle for what should be obvious reasons. The alcohol has already been through mass transfer. It has already consumed a costly BTU expenditure. No need to generate additional heat to do what has already been done.

The second assertion is that not only do we not want alcohol returning to the kettle, but we also need to prevent (or greatly reduce) any water from returning to the kettle to only further dilute any remaining alcohol within the kettle. Further dilution will require an additional measure of heat that was previously not needed.

In other words, it takes more heat to liberate alcohol from a 3% mixture than it takes to liberate alcohol from a 10% mixture. Therefore, sending water back to the kettle only perpetuates inefficiency by using an additional, compounded amount of heat to liberate any remaining, usable alcohol contained within the primary kettle.

How Do We Do That?

Let’s start with a typical primary kettle and a good rule of thumb that if we strip beer into low wines, we will render approximately 1/3 of the total kettle charge as low wines. Therefore, we could say that once stripped, 100 gallons of beer would yield approximately 33.333 gallons of low wines.

Connected to the primary kettle, a concentric, inline retort would act as our first plate. We can refer to this retort as a distilling thumper that has a liquid level maximum fill volume that is the equivalent of the potential low wines yield of the primary kettle. So, in this case the thumper is sized to a working volume of about 33 gallons.

Then mounted to the retort, a 3 or 4 plate column with relatively deep liquid beds. To summarize this section, the distilling thumper volume and plate volume in the column needs to equal the total potential yield originally contained in the primary kettle.

The large bottom “plate” of the thumper acts as a liquid hold up device that prevents the return of any liquid to the primary kettle. The thumper does however have an overflow drain that does prevent complete flooding of the vessel should the operator become too complacent.

Shifting Alcohol via the Distillation Apparatus

Once the beer charge has been brought to running temperatures, the distillation apparatus is put into 100% reflux mode with the Dephlegmator. This prevents any alcohol from exiting the distillation apparatus by passing over to the product condenser.

Once heat and pressure are applied to the system, a mixture of water vapor and alcohol vapor will make its way toward the exit and the dephlegmator (reflux condenser) will condense the vapor back to liquid form and the resulting condensate will fall back onto the highest plate elevation.

This process happens many dozens, if not hundreds of times. Once the top plate fills with liquid it will then drain down to the next plate level and so forth and so on. All the while, forthcoming vapor from the primary kettle will continue to enter the distillation apparatus.

So, we have liquid draining downward as enriched vapor rises toward the top of the apparatus. At this point, it will be the lightest constituents within the column that must occupy the highest available space.

Ethanol is lighter than water, and with each phase change cycle of liquid to vapor and back to liquid, the water within the apparatus will be essentially walked back down the apparatus by virtue of overcrowding on the plates.

The lighter constituents must occupy the highest available space when heat is applied. And so, the column fills with alcohol as the water within is cycled back down. This behavior is positive feedback, meaning that at each plate level, the liquid will require less heat in order to get the alcohol to flash for the next cycle of phase change.

Each liquid bed on each plate level acts as a condenser for incoming vapor. And since the liquid bed is made up of mostly alcohol, the liquid bed will then determine the next forthcoming flash point of alcohol at that plate level. This is called temp gradient.

Ultimately, we hope to manipulate the gradient by so fully enriching the apparatus with alcohol as water gets walked down the apparatus and into the distilling thumper, and at the same time exploit positive feedback to optimally enrich the very top of the apparatus.

As previously mentioned, at no point during this phase of operation are we collecting any product. We are simply creating many, many phase change cycles to get the highest percentage of alcohol into the column section as possible.

More on Positive Feedback

This notion of this positive feedback type behavior can be confusing for beginners. Let’s simplify in a way that helps us understand better. So, let us think of negative feedback in order to understand positive feedback better. Specifically, let’s think in terms of electronics.

In electronics the definition of negative feedback is: “The return of part of an output signal to the input, which is out of phase with it, so that the amplifier gain is reduced, and the output is improved.” What does that mean? Some of you high school stagehands or old band mates likely remember this phenomenon?

If you put a live microphone next to a speaker, you can expect that the recirculation of sound frequency will recycle from the speaker to the microphone and back out from the speaker so rapidly until the speaker ultimately has a catastrophic failure due to excessive amplification.

The sound frequency gets caught in a vortex of never-ending circulation that continuously amplifies itself until the speaker eventually has a catastrophic failure. This is negative feedback.

Positive feedback is very similar in that with each distillation cycle, or amplification, mass transfer moves alcohol closer toward the top of the distillation apparatus.

And in doing so, each subsequent phase change cycle becomes easier because it requires less energy to initiate that phase change as the flash point requires less heat. In other words, the boiling point of the liquid on each plate is reduced as enriched vapor ascends to the top of the apparatus.

The good news is that ultimately there can be no catastrophic failure because ethanol has a limit to how much it can be amplified. Ethanol has a built-in safety mechanism really. Notwithstanding vacuum distillation, ethanol cannot be distilled to 100% purity with equipment that is otherwise used for the purposes of making spirits.

Certainly, high proof alcohol is very flammable. But it is the ethanol itself that puts the brakes on catastrophic failure as compared to our microphone and speaker demonstration.


So, essentially what we are doing is optimally heating our beer enough to shift all available alcohol into the distillation apparatus. During this time, we will initiate reflux to create an optimal environment to exploit positive feedback.

We are simply provoking phase change cycles while continuing to shift all alcohol from the primary kettle into the distillation apparatus. All the while, we will not drain any condensate (water or alcohol) back to the primary kettle by virtue of our deep liquid bed capability on our oversized first plate.

During this phase of operation, it is important to note that as soon as the liquid level in the distilling thumper fills to the point where liquid must return to the primary kettle, efficient behavior will incrementally reverse, and the system will then start a downward trend in purity.

Therefore, it is important that the operator execute the reflux period and pay close attention to when to start collecting product according to how long it will take the thumper to completely fill and return water and alcohol back to the primary kettle.

Nothing here is new here with respect to basic distilling with forced reflux. The difference that I have explained above, is that the distillation apparatus is specifically sized for the kettle charge volume. And during the 100% reflux period, the system is more able to exploit positive feedback behavior.

And in doing so the operator can expect very rapid collection speeds at stable, higher ABVs with a smaller plate count. This method explained however is not optimal for neutral spirits as flavor congeners do get entrained into the relatively small system because of the deeper liquid beds.

Just a reminder that high ABV does not necessarily equal a more neutral finished product. This system described likely performs best when the promotion of esterification is most desirable. But that is another discussion for another time.

Should you have any questions about selecting the right distillation apparatus for your needs, please don’t hesitate to contact the team at StillDragon North America.