Here’s another Tuesday Tech Talk post! This time it’s about potstill column efficiency.
The goal of pot distillation can be twofold. The first goal (and often the first run) is to concentrate the alcohol percentage of the wash into low wines. For a future vodka run in Pure Mode, or for another taste rich potstill run.The second reason to use a potstill is to finish the low wines. The difference? Stripping is about speed and efficiency. Finishing is a slower more controlled process, where cuts for Heads, Hearts, and Tails are made.
Since the goal of stripping is to efficiently strip as fast as possible, and since the goal of finishing is to make perfect cuts, potstill column efficiency has to be about:
- The column being able to strip at the highest speeds;
- The column being able to harvest the right factions and associated tastes.
About autonomous pressure and overpressure
If you have read my posts on gas pressure inside the column (see: https://istillblog.wordpress.com/2015/09/14/about-gas-pressure-1/), it is easy to understand that we want to minimize autonomous pressure as well as overpressure.
Overpressure creates higher boiling points. Higher boiling points, for them to be achieved, require more energy input. In other words, overpressure is counter productive to stripping. And since column overpressure also creates gas hold up in the boiler, it does not create the right (serene) conditions for any potstill column to make perfect cuts either. Instead of the column being able to freely inhale the gases offered by the boiler, overpressure disturbs the gas bed from which the boiler is fed.
Any distillation system has autonomous pressure. We are not cooking in an open pot, but – instead – manipulate the gases upwards and sideways. The more we manipulate the gas stream, the more friction, autonomous (inherent to the system) pressure and unwanted reflux are created.
About column efficiency
An efficient potstill column design minimizes autonomous pressure. The less the gases are manipulated away from their normal vertical path upwards, the lower the autonomous pressure values are. And the lower the autonomous pressure, the more serene and efficient the potstill column can do its job.
An efficient potstill column does not create overpressure. In fact, for the best results, a potstill column should be unable to create overpressure. Not just from a safety point of view, but also from a stripping and finishing perspective overpressure is a no go. It simply hampers production rates (while stripping) and it hampers perfect cuts (while finishing).
About cooling efficiency
When we bring a wash or low wines charge to a boil, any size of liquid that vaporizes becomes 1200 times bigger in size, in content. One liter of liquids, while boiled away, will change into 1200 liters of gas.
Just imagine we won’t be cooling … can you imagine what this does to pressure build up? A lot, and that’s an understatement. Not cooling creates pressure. Cooling is essential for a column to work well, both at stripping and at finishing. In fact, it’s good cooling action, that collapses the gases back to liquid state, thus minimizing the volume by factor 1200 and creating a slight under pressure near the top of the column to gently draw new gases into the bottom of the column. Effective cooling is essential to creating a well working potstill column.
Here’s a false assumption I often hear: “I have a potstill and bridge to the cooler and the tube through the cooler are open to the air, so my system does not build up pressure!”
A false assumption because the horizontal bridge by definition manipulates the gases from a vertical to a horizontal movement. That creates more autonomous pressure in the system. As does the gas flow through the condensor.
But running a potstill that way, without cooling, also creates overpressure. Say, that one liter of liquid is boiled of per 5 minutes, that’s 1200 liters of gas or 240 liters of gas per minute, that’s not cooled down. Yes, it spits out of your potstill outlet, but there is a reason it races out. Right, overpressure.
Towards an optimized potstill column design
Learning from the above, an optimized potstill design has the following characteristics:
- A vertical gas path with minimal gas direction and gas speed manipulation to reach the lowest possible level of autonomous pressure inside the system;
- Insulation to minimize passive reflux on the inner column wall in order to maintain equal gas speeds throughout the column;
- Adequate cooling to counter any potential for overpressure while at the same time lowering the system’s overall autonomous pressure;
- A vent hole to prevent pressure levels to rise too high as a safe guard.
The iStill Solution
iStill’s potstill column is vertical. That way we don’t have to bend the gases. Also the gas trajectory does not get smaller when meeting the cooling head. Both design features minimize the column’s autonomous pressure.
The cooling capacity or our columns is around 200% of what’s needed. That way there is never a shortage of cooling, thus preventing overpressure while lowering the columns autonomous pressure.
Our potstill column also has a vent hole to counter any potential overpressure situation. And there is temperature control on the column cooler. In case of a lack of cooling water the heating system shuts down automatically.
Finally, our columns are insulated. This prevents the formation of passive reflux, while maintaining vapor speeds throughout the column.
Here are a few pictures of our potstill column. This one isn’t insulated yet. We’ll do that tomorrow. But tomorrow is Wednesday and this is Tech Talk Tuesday, so you’ll have to do with what we’ve got.
The iStill 2000 NextGen potstill column (uninsulated) …
From behind: water in, out, and CIP …
iStill 2000 NextGen with potstill column (uninsulated) …