Compound still management

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The still head does two things: taps into the reflux stream and controls how much is withdrawn.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 problem, 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header tank is impractical for a small still, so the reflux dripping from the condenser is sampled directly and product withdrawn through a valve that can control very small volumes of liquid.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.

Offset Distillation Still Head Reflux
"Sideswipe" connection "Bypass" connection

Offset liquid management techniques)

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.

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.

Offset liquid management techniques)

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