Compound still management & Operation

Excerpt from "The Compleat Distiller" pg 55-57.

For associated images see the full version under "Recipes & Information"

Choosing the type of condenser is simple compared to the question of how to control reflux and
withdraw the product. There are three fundamental methods of accomplishing this:
• Liquid management. Divide and distribute the liquid condensate directly.
• Cooling management. Divide the vapor into two flows by managing the amount of cooling in the
reflux condenser.
• Vapor management. Divide the vapor into two flows mechanically.

Liquid management
The still head does two things: taps into the reflux stream and controls how much is withdrawn.
Neither the pot nor the fractionating still has this capability, because all the vapor arriving at the top is
removed. A compound still is able to produce excellent separation because only a small amount of the
vapor is collected. Large commercial operations solve this problem by collecting all reflux from the
condenser in a tank, and running two separately controlled lines from it, a small percentage to the
collection tank and the remainder back to the column.
A header tank is impractical for a small still, so the reflux dripping from the condenser is sampled
directly and product withdrawn through a valve that can control very small volumes of liquid. Getting
this process set up is touchy. You must have a good understanding of the rate at which reflux is being
produced, and a precision (needle-type) valve for controlling the withdrawal rate.

The simplest way of sampling the liquid is to offset the condenser, with an elbow. The vapor flows
upward through the elbow, and the reflux runs back down it past a sampling point. There are two ways
to construct the sampling point, and both are shown in the diagram. In one, a tube is set into the bottom
of the delivery/return elbow, and all the reflux from the condenser flows over the opening of the sampling tube. The other provides a separate return path for the reflux and samples from that. Both methods work well and the choice is entirely up to you. With either method, the reflux returned to the column has to drip onto the packing, rather than trickle quickly down the walls, so the re-entry point for reflux must be designed with care.

Fig. 4-12
Offset liquid management techniques

A more sophisticated method is to dispense with bypass tubes, as shown in this diagram. Vapor flows
through the holes in the central tube and is condensed. Reflux is prevented from passing back down the central tube by the top cap and collects in a small reservoir. The lowest holes in the central tube control the maximum depth of liquid in the reservoir. Product is removed from this reservoir by a sampling tube equipped with a valve to control the rate of flow. There are many ways to sample the liquid reflux from the condenser and control the extraction rate, but these two are the most widely used. You may be able to think up other, smarter ways of doing it, and we would encourage you to do so. The biggest problems you will have to solve are the accurate control of very low flow rates of liquid reflux, and the return of reflux to the center of the column.
 Fig. 4-13
Unfortunately, all these liquid management methods share the disadvantage that if the vapor delivery
rate changes, the reflux ratio changes, because the valve controls only the amount of material removed.
This is not usually a major problem with a well-controlled boiler, but can cause difficulty should the
composition of the vapor change, as it does towards the end of a run. The temperature of the product
can also be quite high, and since hot, concentrated ethanol has many similarities to hot gasoline, you
should treat it with caution! All of these difficulties: the delivery rate, the composition, and with the
temperature of the product, are neatly overcome by the other two types of still head.

Cooling management
If you reduce the amount of cooling water fed to a reflux condenser, at some point it can’t condense all the vapor, and some will begin to flow past the condenser. If you do this by design, then the vapor getting past the reflux condenser can be condensed by a second condenser and collected as product. Consider the operation of the unit in diagram 4-14. Initially, tap A is wide open and tap B fully closed. All the cooling
water flows through the reflux condenser, and 100% reflux occurs. After the column has achieved equilibrium, tap B is gradually opened. As more water flows through tap B, less water flows through the reflux condenser, and vapor begins to pass by without being condensed and returned to the column as reflux. This vapor reaches the product condenser, where it is condensed as product (at whatever temperature you choose!)
Fig. 4-14
We've found that turning just tap B offers more than enough control to reduce the reflux ratio to around
90%, but different stills may have different characteristics. The advantage of having two taps is that
control of the water flowing through condenser 1 can be reduced still further after tap B is wide open.
Closing tap A fully when tap B is wide open results in zero reflux, converting the system to a
fractionating still or, if the heat input is increased, to a pot still.
This option requires constant pressure cooling water if the reflux ratio is to be kept constant for any tap
setting. You can get this by using a small water pressure regulator or by making a small header tank.
Also, the piping for the cooling water has to be laid out carefully to prevent airlocks from forming.