The Compleat Distiller

THE COMPLEAT DISTILLER 56
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.
Fig. 4-13
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.
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.