Chemical Engineering March 2023 - 26

Distillation
column(s)
AVN Corp.
Condensate
collection cup
Vacuum jacket
Cup drain tube
Swinger hinge
Swinger
Penetrations
AVN Corp.
FIGURE 8. Vacuum vessels, like the one
in the diagram, can be used for lowtemperature
distillation columns
FIGURE 9. This liquid dividing head reflux splitter is equipped with a swinging
bucket to collect distillate
jacket, glass bellows are incorporated
into the column wall to allow
the inside tube to expand relative to
the outside tube. Bellows can accommodate
reasonably high temperature,
but can crack if the column
temperatures become excessive.
Tapered ground-glass joints
are
common on glass column sections
and the vacuum jacket is often extended
to include the tapered joint.
However, this design shows much
heat loss at the joint, which is the
main source of heat loss in column
sections. Another concern about tapered
joints is that they can become
frozen, making it difficult to free the
column sections from each other;
Teflon sleeves are often used to help
prevent this. Vacuum grease is also
commonly used, although this can
contaminate the process if too much
is used. This is a common mistake,
and practice is required on applying
it correctly. The socketed nature of
these joints presents some potential
disadvantages: column modifications
usually require disassembling
the column; vacuum leaks are sometimes
hard to fix; and column alignment
is more difficult.
Vacuum jacketing allows for rapid
assembly of laboratory-scale distillation
columns. However, care must
be taken in their assembly, mounting
and use. These sections are relatively
delicate and are prone to breakage.
They must be carefully mounted so
that uniform pressure is applied by
the supporting column clamps, and
a sufficient number of clamps must
be used to minimize downward force
on any of the column sections.
Vacuum-jacketed glass sections
have the potential to implode, resulting
in glass shards being projected
in every direction. To prevent poten26
tial
injury, these sections must be
wrapped in plastic mesh or coated
with transparent plastic. More commonly,
the glass bellows can develop
cracks, allowing the vacuum, and its
insulation capability, to be lost. If a
vacuum-jacketed column section
feels hot during normal operation,
this indicates that the vacuum may
have been compromised.
For some chemical systems, solids
may build up in the trays. If not for
the silver coating, the column sections
can often be heated in ovens
used for glass annealing to burn out
the solids. However, this will destroy
the silvering and require that it be reapplied
by the supplier.
One possible alternative is to have
a non-silvered jacket that can be
evacuated with a vacuum pump.
This allows the column section to be
heated to a high temperature in an
oven to burn out polymer, where traditional
silvering would be destroyed
and thermal stresses on a thin-wall
vacuum jacket may lead to breakage.
To minimize radiant heat losses,
reflective nickel foil can be wrapped
around the jacket. Traditionally, glass
vacuum jackets are made with bellows
that allow for the expansion of
the hot inner process tube relative
to the cold jacket. If bellows are not
installed in the jacket for the non-silvered
alternative, the maximum operating
temperature for the column section
will be limited to 150°C or less.
Vacuum-jacketing works well
for
columns that will not need to get
too hot or have too many joints, as
well as those columns for which the
main purpose is to produce product,
usually for sample quantities.
A significant amount of heat loss
can occur from the joints in hot columns.
A rough guideline is that it
Distributor
Column
Reflux
seal
loop
Eletromagnet
contact location
Reflux tube
Distillate tube
Tapered joint
(connects to
ccondenser)
Rising vapor tube
Thermowell port
Distillate
product line
Rising vapor tube
Tapered joint
(connects to
tray or packing
section)
is not recommended
to validate simulation
models using vacuumjacketed
columns when
the temperature is
above 120°C, and the
column has more than
four joints, not counting
the connection to the
condenser. In this case,
one should consider the
solution discussed in the
next section.
Heat tape and insulation.
Metal columns
normally do not have jackets. Glass
columns can be purchased without
jackets to reduce the cost or
to allow the columns to be burned
out, as discussed earlier. Otherwise,
the amount of heat loss that would
occur from a vacuum-jacketed column
would be unacceptable due
to the high temperature and large
number of joints usually present with
a tall column. In these cases, the
use of heat tape and insulation is a
technique that allows a small-scale
distillate column to operate at nearadiabatic
conditions [1].
This technique can be used for unjacketed
glass and metal columns.
It involves wrapping each column
section with two layers of insulation
and wrapping heat tape between
the insulation layers. A thermocouple
should be inserted near one of
the heat tapes and another thermocouple
should be located within
the column to measure the process
temperature. If the process temperature
is not available, a thermocouple
can be placed on the outer wall of
AVN Corp.
Vent line
Updraft
ccondenser
Reflux and
distillate
vent line
3-way Valve
Condensate
splash plate
Distillate
surge
vesel
Distillate pump
FIGURE 10. The reflux splitter shown here has a
three-way valve to divert the distillate
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Chemical Engineering March 2023

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