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Reflux Still Designs Explained: Part 1
30 Aug 2017
There can be a lot of debate among distillers over which type of moonshine still is the best. Although there are differences between them, each still was designed with a particular purpose in mind. The following is an excerpt from Rick's book, " The Joy of Home Distilling" from pages 67-76. In this section, he explains the different types of reflux stills and why he prefers the vapor management method, which is the style he used in designing Brewhaus' reflux columns. Below, he first details how reflux works in general, and then breaks down several of the most common reflux still designs.
What's a Column Still (Fractionating Still)?
Quite simply, as the vapors rise in the column, they will lose heat, partly due to the cooler air surrounding the column, but even more so due to the exchange of heat with the falling (refluxing) liquid. As the vapor temperature drops below the boiling point of a specific component, that particular fraction will condense and begin to fall back down your distillation column, while the remaining vapor will continue to rise. This continues all the way up the column, with each fraction pulling itself out of the rising vapor when the temperature drops below its boiling point. As the liquid falls, it will gain heat from the rising vapor while simultaneously cooling the rising vapor (heat exchange). The process is assisted by having material in the column, known as column packing, that will stop the refluxing liquid from simply collecting along the walls of the column while the vapor rises in the center of the column. Column packing pulls the liquid away from the walls of the column, slows the liquid’s descent, and essentially closes off parts of the path, forcing the vapor and liquid to utilize the same space. The falling liquid will continue to increase in temperature until it again reaches its boiling point, at which time it will turn back into vapor and begin to rise in the column. Each time that this happens, the fractions become more and more defined. Components with similar boiling points will often not separate on their first cycle of refluxing, so the more times that this takes place the purer each fraction becomes. The number of times that this takes place is called the reflux ratio, and it is relatively equal to a similar number of distillations with a pot distiller. Components with the highest boiling point (which are nearest the bottom of the column, as they are the first to fall below their boiling points) will usually not gain enough heat to reach their boiling point before falling back into the boiler. As water and alcohol make up an extremely large percentage of the total volume of your wash, this leaves plenty of room for all the other components, mostly by-products of fermentation, to separate and remain in the distillation column throughout the process.
The goal in a column still is for your column to be tall enough to separate all the components, leaving only a single fraction at the top of the column, which you will then capture and condense. This is why column height (relative to the diameter) is considered the most important factor in purity. If the column is too short, then you will not get sufficient separation and the resulting distillate will be of lower purity. If the column is too tall, then you will need to increase the speed of vaporization by adding more heat, which increases the cost of operating your still.
Diameter of the distillation column primarily affects the amount, or volume, of vapor that the column can effectively handle. Consider this at its most simple—your column has a defined and unchanging capacity (volume). It takes time for the separation to occur in the column, so if you feed vapor to it too quickly, it is unable to effectively perform this task. To increase the time that your column has to control x amount of vapor, you need to increase the capacity (volume) of your column. This can be done by increasing the height of your column, but most of us have limits on the reasonable workable height of the column. Ceilings often play a profound role in this. Also, as we are working with a cylinder, consider the volume calculation for a cylinder:
volume = π r 2h
Did you notice that the radius is squared? This means that increasing the diameter can have a far more profound effect on the column volume than increasing the height by a similar amount.
In fact, increasing from a 2-inch diameter to a 3-inch diameter (standard hobby still size) will increase your column volume by 2.25 times! To get this same increase in volume, you would need to increase a 36-inch column to 81 inches. That is nearly 7 feet! Just imagine what this means on a commercial scale. An optimal height to diameter ratio is 15:1 to 20:1. If you fall much below the bottom end of this range, you may have difficulty obtaining adequate separation, resulting in lower purity distillate. Increasing the height to diameter much above 20:1 will result in the need for added heat to get vapor to the top of the column, which is quite simply an added operating cost on top of the added cost of material to build the column....
What's a Forced Reflux Still?
....Probably the most well-known and still most effective forced reflux still designs is the “cooling tube” design. In this design, tubes run directly through the column and cold water is run through them. The reason that this design is so effective is because the cooling lines are in direct contact with the rising vapor. When the vapor comes into contact with these cold lines, it is instantly condensed, forcing it to fall down the column until if vaporizes and rises back up for another shot at getting past the cooling lines. This will take place several times until only the fraction with the lowest boiling point remains. The goal in this type of system is to force the maximum amount of vapor to come into contact with the cooling lines or to flow very near them, so that the “coolness” that radiates from the line cools the vapor. The speed of distillation, and therefore the reflux ratio, is controlled by the speed and temperature of the cooling water being run through the lines.
In the past, most of these designs used two cooling lines, one in the middle to lower portion of the column and another directly above it near the top of the column. The drawback to this design was that the lower cooling line affected mostly the higher boiling point fractions, such as water, which would not reach the upper portion of the column anyway. This lower cooling line was actually only of value in a system that was drastically overpowered and producing more vapor than the column could effectively control. However, this was changed a few years ago by Brewhaus America, when they raised the lower cooling line to be near the top of the column and subsequently began crossing the cooling lines in an “x” formation to increase the amount of vapor actually affected by the lines. They later increased the number of cooling lines in this formation to further improve efficiency. The result is that a very high portion of the rising vapor will either contact a cooling line or be affected by the cool temperature radiating from them. When this type of column is run properly, and the vapor does finally get past the cooling lines, it is usually of very high purity.
A system that is often marketed as an improvement on the cooling line design is one in which a cooling water jacket is placed over the outside of the column, allowing cold water to flow between the jacket and the distillation column. While visually more appealing, the system is far less effective than the internal cooling lines on anything but a very small diameter column, as very little vapor will be affected by the cold water. In fact, only the vapor at or very near the walls of the column will be forced to cool, leaving the majority of the vapor to rise to the condenser and exit the system. Increasing the lack of efficiency is the fact that the walls of the column are usually made of a considerably heavier gauge metal than that used to build cooling lines in the more traditional design, dramatically reducing the heat transfer rate. As a result, this is in essence just a very slight improvement on the simple column still, as little forced reflux actually takes place. The marketing for such designs often boasts increased distillation speed, but this is merely the result of a decreased reflux ratio, so the resulting distillate is usually of lower purity. Because the systems above are, to varying degrees, controlling the rising vapor, they are commonly known as vapor management stills.
External Valved Reflux
This is an extremely popular design that is widely used by people building their own columns. As the name suggests, the forcing of reflux takes place outside of the main column body. The column rises from the boiler with either a 90 degree elbow, or more commonly a tee fitting, at the top of the column. A thermometer is installed at the top to read the temperature of the vapor at the head (top) of the column. The vapor, having nowhere else to go, travels across the tee fitting to the reflux head. The reflux head consists of a tee fitting with a short rise on the upward facing section. This section houses a coil, usually made of copper, which has cold water running through it. The vapor will come into contact with the coil, and the refluxing liquid will then drip straight down into the lower portion of the tee fitting. This lower portion of the tee fitting will be capped with a needle valve installed in the cap. The needle valve allows the user to control the speed at which distillate is collected, and in doing so also control how much of the refluxing liquid is returned to the column. Leaving the valve open entirely collects all the distillate, with the only active reflux being that which is inherent in any simply column still, while closing the valve completely will return all refluxing liquid to the column. While it does give complete control over the reflux ratio, this type of system is not without its flaws.
The primary issue with such a design is that the lower portion of the tee fitting (where the condensate drips into) has a reservoir that holds the distillate to be collected. All rising vapor that reaches the top of the column will be condensed when it reaches the cooling coil, and that condensate will then drip into the reservoir. This means that even the vapor requiring further purification will be mixed with the other distillate in the reservoir, reducing the purity of the final distillate to at least some degree. This can be remedied by replacing the tee fitting with a 90 degree elbow so that this collection area is eliminated.
Internal Valved Reflux
While fundamentally the same as the external valved reflux still, the internal valved reflux still is a variation in which the forcing of reflux is brought inside the main column. The result is a better, sleeker appearance, but this does not substantially alter the performance or basic operation of the column. With the internal reflux design, you still control your distillation with a needle valve, which controls the speed at which you collect your distillate. Even the actual design inside of the column is altered very little, despite the substantial difference in appearance. In fact, aside from a very slight modification, that being that you must create the reservoir for the condensate to collect, you can essentially just move the reflux head of an external reflux still to the top of the column to create the internal valved reflux still. Because the design is fundamentally the same, it has the same benefits and drawbacks of the external valved reflux column.
Since the external reflux and internal reflux designs allow you to control your reflux and distillation through the speed at which you collect the refluxing liquid, these systems are generally known as liquid management stills.
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