About Gas Pressure (1)

Introduction

Another more technical post. This time about gas pressure. Is gas pressure important? Yes it is! Why? Because gas pressure is inherent to distilling. Without gas pressure no distillation.

And – even though it is inherent to distilling – it is something you want to be able to manage. If you (or your still) do not manage gas pressure, you can create overpressure situations. In certain types of distillation equipment at least. Let’s dive in deeper.

Two types of gas pressure

I distinguish between “autonomous pressure” and “overpressure”. “Autonomous pressure” is the pressure that’s part of the distillation process itsself. “Overpressure” refers to external factors that can cause gas pressures to rize above the autonomous level.

Autonomous gas pressure

Let me explain autonomous gas pressure first. Once a liquid (a beer or wine) is brought to a boil, most of the energy is then used to create gases. Since gases have a much bigger volume than the liquid they once were, pressure is created. Imagine, for example, that you boil off 1 cm3 of liquids in a second. The boiling action, in that same second, will expand that 1 cm3 of liquid into 1000 cm3 of gas. That’s a lot of gas. Like 3.6 m3 per hour! And that gas wants to go somewhere.

Where to? Well, normally, in a distillation device, the gasses enter the column, meet packing or plates or refluxed liquids, and then have to be bent to the product cooler. Usually, gas speeds are increased during, or just prior to cooling.

When compared to an open pot, that you may use to boil your eggs in, it is easy to imagine that gases in a distillation device meet resistance and drag is created. That resistance, inherent to distilling since we do not want to just create gases, but also want to cool them down to their liquid phase again, by definition creates pressure.

In short? The expansion of liquids to gases (x 1.000) in combination with distilling devices being mostly relatively closed systems (with quite some drag) creates pressure by definition. This is what I call autonomous pressure. Without it we wouldn’t be able to distill beer into more alcohol rich gases into more alcohol rich liquids.

Overpressure

Overpressure is (or can be) created when external influences add pressure. This pressure is then added to the autonomous gas pressure and creates more total pressure inside the system.

Overpressure is what we want to limit or at least control, when distilling, because it can potentially create dangerous situations.

Why uncontroled overpressure situations are bad? For two reasons:

  1. Limited overpressure hampers production speed and the overall efficiency of the distilling device;
  2. Excessive overpressure creates potentially dangerous situations.

The higher pressure in the column prevents rising gases to come over as fast as they would otherwise, hampering overall production speeds. Very high, unmanaged pressures can translate to augmented boiler pressure. And very high pressure will find a way out, creating potentially hazardous situations.

If we want to prevent high pressure build-up, we need to know what creates them. Basically, overpressure can be created in three situations:

  1. Too much power input;
  2. Too little cooling capacity;
  3. Stabilization in a closed system.

I will dive in deeper and explain how each of these causes work and have an effect on pressure in an ill designed still, but first I want to take you through what I call another “Distilling 101” rule. Just like there is a basic design for a well working boiler and column (please read my previous posts on boiler and column design), the total distillation system has to meet a basic design requirement as well. Two basic rules, actually.

Designing a distillation device 101

Distilling basically consists of two processes:

  1. Creating vapour from a liquid;
  2. Condensing that vapour from gas to liquid phase again.

The heating system of your still is responsible for vapour creation, while the cooling system collapses the gasses back to liquid state. A consecutive process, where first gases are formed and then collapsed back to liquid phase, right? Wrong! Very wrong, actually, but you wouldn’t be the only one thinking like this. We see most still builders make that mistake.

Vapour creation and vapours collapsing back to liquids is not a consecutive process that consists of two steps, but a continuous and intertwined process.

Imagine we create 2000 cm3 in gases per second, yet can only make 1000 cm3 of them collapse back to liquid state … do you see how we create overpressure right at that moment? I hope you do. It is essential.

Designing a distilling device 101? Here we go: the total capacity of vapour creation has to be balanced by the total capacity of vapour collapse (AKA cooling) during the production phase.

When this situation exists, there is minimal, autonomous gas pressure just above the liquid bath in the boiler. And that’s a good thing, because we want these gases to migrate up the column and towards the product cooler. The autonomous pressure helps achieve that.

So, autonomous vapour pressure is a good thing. A cooling system with the capacity to match power input during the actual run harvests that vapour pressure in such a way that no additional or “externally caused” overpressure is created.

Designing a distillation device 102

So, any still design should have a cooling system that matches the power input. And any still design, for it to be efficient, needs to have a power input that matches the cooling system. These are intertwined.

There’s another design rule. Let’s call it 102. Heating up the beer or wine (or low wines) in your boiler does not create vapour. Only when the pot contents boil, gases (in abundance) are formed. That’s when power in and cooling down capacity need to be in balance. But since no gases (in abundance) are formed during the heat-up phase, and since no actual distillation takes place during heat-up, “power in” during this first phase can be doubled, saving you time and shortening overall run time.

And that’s design rule 102: once vapour creation and collapse during the actual run are calculated and tested, and balance out well, then augment the total “power in” capacity by factor 2, so heat-up time is halfed.

That sounds like too simple, right? But it is important for two reasons:

  1. Without a clear distinction between heating up and the actual production process, the distiller runs a risk of overpowering the column and cooling system during that run. When power input is not limited after heat-up, the unwanted externally caused overpressure mentioned above is created;
  2. Without double power input during heat-up the run time goes up significantly.

As an example of the last … we have heard that some Chinese import stills, especially the double boiler ones, take 9 to 10 hours to heat up! Imagine that you want to do a run … and at the end of the work day you have to call your wife and tell her … you ain’t coming home just yet. Or imagine you run a bigger operation and now have to call in another shift manager and are facing double personnel costs!

These still importers might have mentioned that when you contacted them, right? Well, now that you know at least you can ask. And if you do, please ask for written confirmation and full refund when heat-up performance figures aren’t met. Most Craft Distillers get a shot at realizing their dream once, so do not screw up, please.

Enough about rules 101 and 102 for total still design. Back to what causes overpressure!

Too much power input

In a well designed distillation device vapour creation and vapour collapse are balanced during the production part of the run. During the heat-up phase, prior to the production phase, power input should be doubled.

This means that any well designed still has the power input to potentially overrun the column and cooling capacity. And that potentially creates overpressure situations: distillers that use too high power settings during the actual production phase of the run. Gases that are not cooled back to liquid state fast enough by definition create unwanted additional pressure.

Lack of cooling

Even in a well designed still there can be a lack of cooling water. Let’s say water pressure is gone, or your pump stopped working, or you rerun your cooling water but it got too hot … Remember, gases are still created by the power input system, but are now not cooled down sufficiently anymore. Gases that are no longer cooled down, potentially create overpressure in the total system.

Stabilization in a closed system

Stabilization is a technique where, at the beginning of the production process, all gases that rise up are cooled back to liquid state by a column cooler or dephlagmator. The column cooler refluxes the liquids down the column to assemble on the trays or plates or packing underneath.

This stabilization period, where no product is taken, but all product is sent down the column for redistillation, allows for the lighter alcohols, also called Heads, to compact nicely near the top of the column. Well compacted Heads give a better and bigger Hearts cut. And that’s why more and more distilling devices now have the capability of stabilization.

Great … but where does all the energy go to if no product is taken out of the system? Right, you got it, if you stabilize your column, using an already relatively closed system, you close it even more. We see more and more closed system still designs that promote stabilization, while stabilization should only be done in open, non-pressurized columns! Stabilization in a closed still design easily creates overpressure. If you stabilize, make sure your system is open, or at least have a pressure monitor gauge in place with an automated pressure release valve to vent out hazardous overpressure levels. “Venting out” means that the released overpressure is immediately vented outside of the building via a steel pipe. You don’t want alcohol rich vapours in your distilling hall.

So, let’s look at general still types and how prone they are to building up overpressure. And then I want to share how we design our stills and why we make the choices we do to make distilling easier, more efficient, and safer than some of the other manufacturers and designs out there.

Follow-up

But not now. This is a pretty long and technical post as it is. How different basic types of stills work, how they handle gas pressure, what the benefits and negatives are, and why we design the iStills the way we do … it will have to wait for another post. Gas Pressure (2) it will be called.

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One thought on “About Gas Pressure (1)

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