Chemical Engineering March 2023 - 29

AVN Corp.
FIGURE 17. A forced-circulation reboiler, like the
one shown here, has the best heat transfer, but is
the most complex
characteristic of laboratory and small
pilot columns. Highly accurate reflux
ratios, down to a value of about
0.5, can be achieved if the swinging
bucket moves quickly and is controlled
by a precise timer.
It is important that the magnet be
mounted next to the liquid dividing
head so that the swinging bucket
is pulled straight out and not to one
side of the other, which can bind
the hinge or slow the movement.
For reflux ratios of 1.0 and greater,
the off-time (distillate) should be
set at 2-3 seconds. Less than this
amount of time introduces error from
the switching time. More than this
amount of time allows the top tray,
or top of the packing, to begin to
dry out. Reflux ratios below 0.5 will
also tend to dry out the top of the
column. In addition, this requires an
electromagnet that will not overheat
when energized for most of a reflux
cycle. To resolve this problem, it is
possible to purchase reverse liquid
dividing heads, in which the distillate
position corresponds to the electromagnet
being de-energized. Some
electromagnets have been built with
cooling capability.
Another concern is polymerizable
chemicals. Vapor can condense,
uninhibited, on the swinging bucket
hinge, polymerize, then cause the
hinge to seize. In general, it is good
practice to check on a regular basis
that the swinging bucket is operating
properly.
Three-way valve. For metal columns
or larger glass columns, liquid
dividing heads are less practical, and
are difficult to obtain. An alternative
technique, which operates according
to the same principle as the liquid
dividing head, is based on a threeway
valve that can be actuated by
a solenoid (Figure 10). As with the
liquid dividing head, the reflux ratio
is controlled by a timer, which, in this
case, is connected to the three-way
valve. Either an updraft condenser,
as shown in the figure, or a downdraft
condenser, can be used. It is
important to balance the pressure
on each side of the three-way valve
so that the condensate flows at the
same rate in both the reflux and distillate
positions.
Reflux pumps. For a system similar
to what would be implemented in a
commercial-scale column, it is also
possible to use calibrated metering
pumps, shown schematically in Figure
11, or a pump with control valves
and flowmeters to control the reflux
ratio. A surge vessel with a liquidlevel
instrument is also required. This
is usually used to control the distillate
rate. The reflux rate would be controlled
at a fixed flowrate.
This system allows for a wide
range of reflux ratios and works well
with automated distillation columns.
The possibility of a pressure bump,
described earlier, is greatly reduced
with this system. However, it is more
expensive and requires additional effort
to set up.
Fixed reflux ratio. In some cases,
the operating temperature of the
column may be very high, making
it expensive and difficult to use the
equipment
discussed
previously.
In addition, the pressure may be
high, complicating the situation even
more. If the reflux ratio can be fixed
at a specific value, one possible solution
is to design a liquid distributor
in which the condensate is split at a
constant ratio, into the reflux and distillate.
This requires no moving parts
or seals, and would be relatively inexpensive.
The main concern would
be verifying that the distributor splits
the flow as expected, usually with
water tests, similar to what is used in
large-scale distillation columns.
Product takeoff
Removing sidestream products is
often an important aspect of smallerscale
distillation columns. Several
considerations are discussed here.
Liquid sidestream. The liquid dividing
heads described above can also
be used for liquid sidestreams. The
concepts are the same as for the
collection of distillate, except that the
pressure must be balanced between
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MARCH 2023
the vapor and liquid for the sidestream
make line. Otherwise, either
vapor will blow out the sidestream
make line or the sidestream liquid will
be prevented from flowing.
As an alternative, a special column
section can be used that collects liquid
in an annular cup. The cup must
be connected to a pump that can
then continually remove sidestream
product from the column.
One advantage to using a liquid
dividing head to collect a liquid sidestream
is that it provides an indication
of the liquid flowrate below the
sidestream takeoff point, since the
reflux timer controls the ratio of the
reflux and sidestream flowrates.
Vapor sidestream. Figure 12 is a
schematic diagram for an apparatus
to remove a vapor sidestream from
a small-scale distillation column. The
column section that allows vapor to
flow out of the column can be a repurposed
feed section or a relatively
simple glass piece made for this purpose.
The vapor is maintained above
the dew point by heat tapes prior to
flowing through a control valve. The
discharge from the valve flows into a
downdraft condenser, and the condensate
drops into a surge vessel. A
liquid-level instrument then controls
a pump connected to the bottom
of the surge vessel. The surge vessel
must operate at a lower pressure
than the column to provide pressure
drop across the valve.
An alternative design is to use
a flooded condenser that doubles
as a surge vessel. If the pump rate
is slow, the vapor will flow into the
condenser, and the condensate will
gradually fill the condenser, cover the
cooling surface, and slow or stop the
condensation and vapor flow. When
the pump rate is increased, the liquid
level will decrease and expose more
surface for condensation.
Low-flowrate vent condenser. For
some laboratory columns, the main
condenser may be a partial condenser,
in which case the vent stream
contains a small amount of condensable
material. Figure 13 shows an
apparatus that can be used to condense
and collect this material from
the vent stream, and accurately determine
the flowrate, even if the rate
is as low as 4-5 mL/h.
The device consists of a jacketed
vessel for coolant. A large-diameter
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