What is a Reflux Column and How Does it Work?

Before we start, let’s first discuss some glossary terms many of us learned in 8th-grade science. 

Evaporation

Google says that the definition of evaporation is: “the process of turning liquid into vapor.” This is typically done by applying a measure of heat to the liquid. 

Phase Change

The next likely term we learned would have been the term: Phase Change. Evaporation is one form of phase change. Another example of phase change is when we take a liquid and freeze it until it becomes a solid—water into ice, for example. Here again, is Google’s result for the definition of phase change: “A phase change is when matter changes from one state (solid, liquid, gas, plasma) to another.”

Mass Transfer

With the concept of evaporation, or more specifically phase change, understood, we can move on to the next principle, known as mass transfer. And once again, here is Google’s search result for mass transfer: “the net movement of mass from one location (usually meaning stream, phase, fraction, or component) to another.”

Now with the review of phase change and mass transfer, we can now apply our understanding of how phase change and mass transfer are inextricably linked to distillation.

For the simplest, short-path distillation, only three-phase change cycles are required. From liquid to vapor (gas) and back to liquid as the finished distillate. The humble pot still is the closest example to this for distilled spirits. Make beer or wine, heat up until the liquid is hot enough to render forth vapor; pressure created by the heat then directs vapor through a basic pipe (mass transfer) toward a heat exchanger that will condense vapor back into liquid form to be collected as distillate. The resulting distillate will have a significantly higher alcohol concentration than the original alcohol content of the beer or wine in the kettle.

If we take the resulting distillate and run it through the pot again, the alcohol content would be even more concentrated. So, the concentration will increase each time we run the resulting distillate. The trouble with this plan, however is that distilling in this manner requires quite a bit of time and energy, particularly if optimal separation is one of the goals. 

So how do we hack this problem? How do we incorporate multiple phase change cycles into a single run? How do we optimize our ability to distill so that our resulting distillate has a very high concentration of alcohol? We would use the concept of forced reflux and create a tall column that has multiple plates or landings with which liquid can accumulate and be redistilled. Basically, a distillation system allows for multiple phase change cycles during a single run. 

Reflux

Further enhancing this process is a heat exchanger at the top of the column that will condense a percentage of vapor back into liquid form to be redistilled. This forced condensing at the top of the column sends back very high ABV. The liquid formed at the top of the column due to this forced condensing is referred to as reflux. At some point there will only be enough room for so much volume of liquid reflux, that some of the liquid will then be forced to return or drain to a lower portion of the column.

The return of this reflux will then enrich the material (liquid and vapor) by displacing the higher nonalcoholic content at the lower levels of the column. This will also affect the temperature gradient within the column. As this occurs, the heat needed at the top of the column to create additional phase change cycles will be some what reduced compared to the heat entering the base of the column where the ABV is less enriched. This is essentially known as a temp gradient. The result is higher ABV at the top of the column and a graduation of lower ABV down the length of the column toward the kettle.

Temperature Gradient

Here again we ask Google to explain temperature gradient: “the rate of change of temperature with displacement in a given direction (as with increase of height)”.

The temperature at any given point on the column during a distillation run can tell us the approximate purity of alcohol at that level. However, since liquid temps will always be cooler than vapor temps, it can be difficult to pinpoint purity levels in a column that has simultaneously mingled liquid and vapor. To help illustrate the notion of purity in liquid form vs vapor form, please see this ethanol phase diagram below.

After reviewing the liquid and vapor temps at different levels of alcohol concentration we can then visualize that the actual temperature within the column at a specific purity will likely be somewhere between the vapor temperature and liquid temperature shown on the diagram. This assumes that an RTD or temp probe located at the plate is influenced by both liquid and vapor. 

The above summary more or less characterizes what is happening inside of a reflux column though may be difficult to visualize?

Ready to run?

Ok so let’s fire the kettle and make some high proof spirits. 

First bring your kettle charge (beer or wine) to a boil. You’ll need to decide if a rapid heat up or a slow heat up is most beneficial. There are opinions for either choice. I am ambivalent. Once the top of your kettle is too hot to touch with your bare hand comfortably (scalding is 130°F) turn your cooling flow to your reflux condenser on and allow for full flow. Though not at all necessary for lower proof spirits, full flow to the reflux condenser will allow for the highest possible ABV. With a 10% beer or wine charge, your kettle should start making vapor at about 190° F. Once we observe any kind of liquid condensation on the viewing windows of our column, we will (if needed) reduce heat input enough to ensure that no alcoholic vapor can be pushed past the reflux condenser and exit the vapor outlet to the product condenser.

This is essentially called “100% reflux” mode. We will stay in this 100% reflux mode for as long as it takes to shift as much alcohol from the kettle into the column (via mass transfer) as possible. A good indicator that we have maximized the alcohol transfer is by monitoring the temperature of one or more plate or liquid landing levels. Once the temperature (especially at the top two or three plates) has stabilized, we can be confident that we have allowed as much alcohol to populate the column while simultaneously allowing the bulk of water to cascade downward back toward the kettle essentially. We execute this phase of the run while not taking (to the extent possible) a single drop of distillate. Again, this process allows for full enrichment of the column in order to maximize the ability to collect the highest possible ABV in our distillate. This is the essence of 100% reflux mode and is also known as bringing the column into equilibrium or steady state. 

Here is the Google search result for these glossary words. Equilibrium: “a state in which opposing forces are balanced. And here is steady state: “a state or condition of a system or process that does not change in time”.

Once you are satisfied that the column is fully enriched with alcohol, you are ready to start collecting distillate. The first drops we will take will be a Foreshot. The foreshot is characterized by a sharp smell of acetone. This distillate is essentially a very potent nerve toxin and should not at all be consumed. The next bit of distillate to come over will be Heads. This distillate may also have an odor of acetone as well as an additionally fruity aroma. You can collect Heads by slowly reducing the cooling media flow rate to your reflux condenser. Or you can increase the heat input. Or you can do both. Be mindful however that any adjustments to the system should be made incrementally so that you as the operator can learn to gauge the impact of the adjustment.

The system may very well take several minutes to reflect any adjustments made fully. The goal here for the heads cut is to draw product out of the system slowly so that we can ensure that our center or Hearts alcohol does not bleed into the Heads as we slowly collect. Heads distillate can have a sharp, acetone odor to it and is largely viewed as not good for consumption. Particularly if vodka production is the goal with your reflux column. Acetaldehyde and Ethyl Acetate are largely responsible for Heads aromas. Though depending on how efficient your column design is, there could also be some ethanol within the Heads constituents. This is why we draw heads off slowly so that we can minimize the amount of ethanol that may bleed off. Remember, it is ethanol that we are trying to collect.

At this point we have effectively made our fores cut and our heads cut. You will need to use your own sensory awareness to determine what the volumes should be. Temperatures do not dictate what constitutes acceptable tasting spirit. ABV does not determine what constitutes acceptable tasting spirit. Temperatures and ABV should only provide a guideline. The best placement for a temperature probe on a reflux column is at the highest possible location on the column. If we once again refer back to the ethanol phase diagram, we can see that there is a direct relationship between temperature and ABV.

At this point it is time to further reduce cooling flow to the reflux condenser or increase heat to the kettle in order to collect distillate in a timelier manner.  Or both. You will need to determine how fast that should be. Be mindful however that the goal here is to maintain a high ABV for the duration of the run. If your ABV becomes too low (particularly during the early stages of the run) you may have reduced cooling flow too much or increased heat too much? Adjust your heat input and or flow control on your reflux condenser accordingly and continue to monitor ABV and the temperature at the top of the column.

Once adjusted correctly, the duration of the run should be holding proof very consistently and the plate behavior should look very uniform from the top plate to the bottom plate.

Here is an example of good plate behavior:

Proof Gallon

With some basic math we can determine how many proof gallons of alcohol are in the kettle and compare that with how many proof gallons we have collected as distillate. This calculation should help you determine how much time you should spend collecting finished distillate.

Google says the definition of a proof gallon is: “one liquid gallon of spirits that is 50% alcohol at 60°F”.  A proof gallon is the metric by which the government taxes spirits by the way.

Well, that is all for now. Congratulations. You are one step closer to running a reflux column.

Contact us anytime here at StillDragon or log onto the global forum (stilldragon.org) with any questions. Good luck.