Chemical Engineering March 2023 - 30

dip-tube extends halfway down the
vessel. This ensures that the vapors
entering from the top of the vessel
contacts the cooling surface but
leaves adequate volume in the bottom
for the condensate.
For operation at atmospheric
pressure, the condenser-collection
vessel can be simply drained. If the
column is operated under vacuum, a
lower receiver, which can be evacuated
by a vacuum pump, is connected
to the bottom of the upper
vessel, as shown in Figure 13.
Reboilers
The following describe different
types of reboilers for small-scale distillation
units.
Round-bottomed flask with heating
mantle. The round-bottomed
flask is the typical reboiler used for
laboratory-scale 1-in.- and 2-in.diameter
columns. Figure 14 shows
an example. They are simple, easy
to install and work well for batch
distillation columns, but can also be
used for continuous columns with
the addition of a port for liquid takeoff
(for tails).
However, the low surface-to-volume
ratio prevents the use of glass
round-bottomed flask reboilers for
most larger columns due to the lack
of sufficient heat-transfer surface
area. In addition, the reboiler residence
time may be too long, especially
for larger flasks, and promote
decomposition of materials for some
chemical systems.
For high heat input, the temperature
difference across the flask heating
surface can become too high
for the integrity of the glass. Boiling
chips are routinely used, due to lack
of agitation, which work reasonably
well, but bumping is possible, especially
under high vacuum. However,
an agitator can be added if an appropriate
joint is created to offset
the column connection, although
this adds additional opportunity for
vacuum leaks and agitators tend to
be expensive.
Thermosiphon. For continuous
columns, the thermosiphon reboiler
provides an improved surface-tovolume
ratio. It has no moving parts
but relies on natural convection.
They typically have two or three
arms that are wrapped in heat tape,
or possibly jacketed for hot oil. Fig30
ure
15 shows an example of a thermosiphon
reboiler with three arms.
These reboilers have reduced residence
times and avoid hot spots on
the heating surface. However, the
liquid level must be maintained within
a relatively narrow range, which requires
more attention during operation.
Also, they are not appropriate
for batch distillations, due to the low
holdup, and they do not work well
for high-vacuum operation because
of static liquid pressure on the boiling
point along the heating arms.
Kettle reboiler with heating coil.
This style reboiler is a further enhancement
with regard to heating
surface area. The design is relatively
simple and usually has a metal coil
located toward the bottom of the
kettle, shown schematically in Figure
16, through which hot oil (or steam)
flows. An electric immersion heater
could also be used. Often, this style
reboiler will have an agitator.
The heat transfer for the coils is
very good, with a low temperature
difference between the coil and process.
The temperature of the hot
oil at the coil inlet is set at a specific
value, which then becomes the
maximum for the heating coil, so decomposition
is minimized. This type
of reboiler also works well for batch
distillation if the heating coils are low
enough in the kettle.
However, a hot-oil system with
cooling capability is required (unless
electric immersion heaters are used).
The process chemicals must be
compatible with the metal coil. And
the coil must be sealed well where it
passes through the headplate.
Forced-circulation reboiler. In
terms of heat transfer and operating
conditions, probably the ultimate
system is the forced-circulation
reboiler (Figure 17). However, it is
also the most complex. It consists
of a vessel for vapor-liquid disengagement,
a circulation pump and
heat exchanger. Typically, the liquid
is pumped at a relatively high rate
through the heat exchanger and is
restricted from vaporizing until entering
the top of the vessel. This minimizes
the formation of deposits on
the heat-transfer surface.
The heat exchanger size can be arbitrarily
selected to suit the heat-duty
requirements, and can, therefore,
provide higher heat inputs than any
of the other reboiler styles discussed
earlier. The reboiler can be designed
to minimize liquid holdup in the vessel.
These systems work well in highvacuum
applications and can be
used for batch distillation columns.
Since a pump, heat exchanger
and interconnecting piping are required,
the process chemicals must
be compatible with the materials
of construction. The pump and its
seals must be suitable for the reboiler
operating conditions. A hot-oil
system or steam must be available.
The equipment cost and setup time
are higher than for the other types of
reboilers discussed.
Concluding remarks
As compared with commercial distillation
columns, the design and operation
of laboratory- and pilot-scale
columns are greatly influenced by
their purpose, the effect of scale on
engineering principles, the availability
of equipment, and the short-term
nature of projects.
n
Edited by Scott Jenkins
Reference
1. Nunley, R., Managing Heat Loss in Pilot Plant Operations
to Simulate Full Scale Operation, paper presented at the
AIChE National Spring Meeting, San Antonio, March, 2017.
Authors
Glenn Graham is a corporate fellow
at AVN Corp. (which recently
acquired Matric).
(Email:
glenn.
graham@AVNcorp.com; Phone:
304-552-6554). Graham is responsible
for providing technical
guidance and support for laboratory-
and pilot-scale research and
development projects,
including
ASPEN computer simulations, the
design and operation of laboratory- and pilot-scale equipment,
and experimental data analyses. He specializes in
distillation and related technologies. He joined AVN in
2013. Prior to joining AVN, Graham worked for Union Carbide
and Dow Chemical in their R&D separations groups,
also specializing in distillation and related technologies.
Graham holds B.S.Ch.E and M.S.Ch.E degrees from
Montana State University.
process
development,
Raymond Rooks is a principal
engineer at AVN Corp. (formerly
Matric). (Email: raymond.rooks@
AVNcorp.com; Phone: 304-7201037).
Rooks has more than 20
years' experience in the chemical
industry. He has held positions at
SimSci (Simulation Sciences),
Union Carbide/Dow, and Praxair/
Linde. He has broad experience in
including
simulation, experimental design, and experimental/laboratory
simulation,
especially in process
separations.·Rooks holds a Ph.D. in chemical engineering
from the University of Massachusetts at Amherst
and a· B. S.Ch.E. from the University of Oklahoma.
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