stills

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Bushman
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stills

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COMPOUND STILLS
This is an attempt at explaining the various types of compound stills and the components that make up a compound still. First we need to clear up some basic terms as they can be confusing as distillers/would be distillers use similar terms to mean the same thing.

Fractional Stills, Compound Stills, and Reflux Stills all used interchangeably throughout the Web to describe some type of still that separates and stacks vapor. Here we will define Fractional Stills as the part of the still between the boiler and the reflux condenser which has some design or material that allows for separation of the different ethanol vapors (more on this later). Compound Stills will be interchangeable with Reflux Stills in that it has the fractionating portion of the still but also has some type of reflux condenser on top that allows the vapor to be condensed and then fall back down the column for better separation.

Fractionating Column
The fractionating portion of the column can be built in many different ways but basically it’s any physical device that provides good contact between the vapor rising and the liquid present on the material.

First we will discuss different types of packing as a means of separation. Probably the most used at our level are copper or stainless mesh, SPP (Spiral Prismatic Packing), lava rock, and other material such as marbles that don’t seem to be as efficient.

Copper mesh
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Link to making your own copper mesh

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SPP

Making your own SPP

The other method of creating separation is through trays sometimes called plates. There are different designs in this area as well. I will mention two of the more common ones used at the hobby level but by no means is this list complete.

Bubble cap trays: A bubble cap has a riser fitted over each hole and a cap that covers the riser. The cap is mounted so that there is a space between the riser and cap to allow the vapor to pass. The way it works is vapor rises through and then goes downward due to the cap on top and then bubbling through the liquid on the tray.
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Sieve trays: Sieve trays are simply metal plates with holes in them sometimes called perforated plates. The size of hole is important, as vapor passes straight upward through the liquid on the plate. The advantage to this design over bubble cap is perforated plates are inexpensive to make and they drain themselves after distillation.
perferated tray copy.jpg
Before getting into more detail on how these work we need to discuss a couple of terms that affect these designs:

HETP: This refers to plate height or height equivalent to a theoretical plate (HTEP). Liquid wets the surface of the packing or tray and as the vapor rises it contacts the wetted surface this is where transfer occurs.

Weir: Defined as a barrier across something to alter its flow. In the design of the tray it ensures that there is always some liquid that is held-up on the tray.

Downcomer: This is a designed part of a tray and in our application usually a pipe that allows the liquid to fall through the pipe by gravity from one tray to the next one below. For this to work efficiently each downcomer is staggered from one side to the other.

Some people design their trays/plates to also include some packing material to improve separation. Packing would be considered a passive device that increases the contact area between vapor and liquid.

So the question comes up Packing vs Trays. Packed columns are called continuous contact columns and tray columns are referred to as staged-contact columns. In our application it’s mainly cost but with larger commercial stills trays are used in applications with liquid rates of 30m3 /m2–h and above or if solids are present. Packing is used where there is a lower liquid rate.

Reflux Condensers
There many different types of reflux condensers and depending on the diameter of your column and type of Reflux still you are running will determine the type and location of the condenser. Usually the condenser is located at the top of the column directly above the fractionating portion of the still or at the top of the column but offset. A lot depends on if you are running an LM (liquid management), VM (Vapor Management), or a CM (Cooling Management) system.

The reflux condensers purpose on a Compound Still is to condense the vapors so that the heavier liquid drops back into the packing. As the liquid drops down and the vapors rise up the column the heat evaporates the liquid, this process is what separates the ethanol and layers it in the column with the lighter vapors at the top. We call the process equilibrium. Thus the lighter heads are formed at the top and depending on the rate of takeoff will determine how well you separate the heads, hearts, and tails during collection.

Types of Reflux Condensers:
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Links to building Reflux condensers:
Double wound coil
Coldfinger
Bushman's CM Build
Crossflow Condenser

After the reflux condensers we should talk about valves. Several different types of valves are used depending on the type of still. We have needle valves used in an LM still and we have ball valves or gate valves used primarily for VM and CM stills. The valves are used for managing vapor take-off, liquid take-off, or controlling the amount of coolant to the various condensers. This is as far as I am going to take this discussion as you should find an appropriate link on the one you choose and its location by linking to the build design of your choice.

The last topic of discussion is product condensers. The product condenser in the reflux still is the final stage of condensing the vapor to liquid for collection. The following are a few images of different type product condensers:
liebig copy.jpg
Shotgun copy.jpg
Graham copy.jpg
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Links to Product condenser builds:
Liebig
Shotgun condenser

Links to different still builds:
Bokakob
Flute
Rad's Small scale Combo
Manu de Hanoi's VM
Dan's Flute build

Additional helpful build and design links:
Diagram & Plan Thread
Electric conversion
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