Chemical Engineering March 2023 - 28

AVN Corp.
FIGURE 14. The round-bottomed flask is used as a
reboiler, with a heating mantle
AVN Corp.
ambient temperature, glass vacuumjacketed
column sections may now
be acceptable, whereas they may
have experienced too much heat
loss at the original room temperature.
One of the main advantages of
using a heated enclosure is for
chemical systems with high melting
points. However, it is usually difficult
to heat tape or trace all the process
lines in a way that avoid all cold
spots. Also, during column startup,
the vapors initially rising in the column
are exposed to cold surfaces,
which can cause freezing to occur
within the column.
Figure 6 shows a " homemade "
Plexiglass enclosure for a 1-in. diameter
glass distillation column,
heated by two heat guns, with the
capability to operate at temperatures
up to 60°C. Figure 7 shows
an example of a larger, commercially
available enclosure.
FIGURE 15. This thermosyphon reboiler has three
arms wrapped in heat tape
Any given point in a column section
may not be exactly adiabatic, but the
average of the heat loss and gain for
that zone will be near zero.
In addition to the extra effort to
wrap the column sections with heat
tape and insulation, the process control
system for this type of a setup is
more complex due to the need for
thermocouples and heat-tape controllers,
as well as the required configuration
of the control system and
the connection of the control system
to the heat tapes.
Heated enclosure. Another means
of minimizing heat loss is to operate
the distillation column inside a heated
enclosure. By reducing the temperature
difference between the process
within the column and the external
28
If the column is manually operated
in a heated enclosure, the upper
temperature limit is determined by
human limitations. Also, any instruments
within the enclosure must be
able to operate at elevated temperatures.
Feed tanks and receivers may
need to be located near the column,
so this needs to be accounted for
regarding the size of the enclosure.
Vacuum vessel. For distillation
columns operating below ambient
temperature, a vacuum vessel can
be used (Figure 8). For this system,
the distillation column and associated
piping and instrumentation are
located inside of a vessel that can
be evacuated by use of a vacuum
pump. This eliminates heat gain by
convection and conduction, and radiation
effects are normally negligible
at these conditions. If that is not sufficient,
it is also possible to line the
vessel with multi-layer insulation.
This is the approach used in some
small cryogenic nitrogen plants.
The main difficulties with this system
have to do with the assembly
and mounting of the column within
the enclosure, sealing of the piping
and instrument line penetrations,
the necessity of complete automation
of the column, and sealing of
the vessel shell. In Figure 8, all the
penetrations are below the vessel
flange, so it can be removed to work
on the equipment.
This approach can also be used
with distillation columns operating at
very high temperature or with compounds
with high melting points, in
which heat taping or tracing may not
be practical. In this situation, prevention
of heat loss by radiation may
also be required.
Reflux control
Several approaches are available for
controlling the reflux ratio in smallerscale
distillation units, as discussed
in this section.
Liquid dividing head. The typical
way to control the reflux ratio for
small laboratory columns is by using
a liquid dividing head. Numerous
designs for this device exist, and the
main principle of operation is that
all of the condensate is directed to
the reflux return or to the distillateproducing
line for a specific period
of time (usually in the range of 2 to
3 seconds for the minimum time
period). Therefore, the reflux ratio
is determined by the ratio of time,
rather than by the ratio of flowrates,
as for a commercial column.
Figure 9 shows a liquid dividing
head with a swinging bucket. This
device would be located between
the top of the column and an updraft
condenser. The condensate drains
into the swinging bucket, whose
movement is controlled by an electromagnet
connected to a timer. When
the electromagnet is turned on, the
swinging bucket is pulled into the distillate
position, in which the condensate
is directed to the distillate line.
When the electromagnet is off (reflux
position), the condensate is allowed
to drain to the top of the column.
These devices are relatively simple,
with the only moving part being
the swinging bucket. No pumps or
flowmeters are required, which is
very beneficial at the low flowrates
AVN Corp.
FIGURE 16. In a kettle reboiler with a heating coil,
hot oil or steam flows through the coil
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2023
http://WWW.CHEMENGONLINE.COM
Chemical Engineering March 2023

Table of Contents for the Digital Edition of Chemical Engineering March 2023
