Chemical Engineering March 2023 - 24

AVN Corp.
Bubbler
N2
Vac Pump
Heated Bath
Heat Tape
AVN Corp.
FIGURE 3. The apparatus to sample high-melting-point materials under vacuum is shown here
FIGURE 2. This small-scale system uses a distillate-collection
design similar to a commercialscale
unit
Heat loss. Heat loss is a critical concern
for distillation columns. For columns
above ambient temperature,
wall condensation occurs; for columns
below ambient temperature,
wall boiling occurs. Wall condensation
and wall boiling are important
because they change the liquid-tovapor
ratio of the flows within the
column, and because they have an
effect similar to changing the reflux
ratio (although heat loss causes a
continuous variation in the flows and
ratios throughout the column). This
effect causes the liquid-to-vapor
ratio to be different than expected,
based on the apparent reflux ratio
(measured by the reflux-to-distillate
ratio at the top of the column) and
results in a separation that is different
than intended. In addition, for
packed columns, there could be significantly
different flows across the
column radius.
Column-wall effects. Column-wall
effects are more significant in smallscale
columns than commercialscale
units due to the enhanced
surface-to-volume ratio. This phenomenon
can increase the amount
of surface area for mass transfer
in a packed column, can provide
a path for liquid channeling, which
decreases the liquid available for vapor-liquid
interaction, and can lead
to axial dispersion. These effects
can cause the resulting separations
to be different than expected based
on the inherent characteristics of
a given packing. This is especially
important because the goal of the
smaller equipment is often to model
a larger column.
Vacuum leaks. Vacuum leaks are
a larger problem in small-scale col24
umns,
since the sealing-surfaceto-column-volume
ratio is larger,
so there are more opportunities for
leaks. Excessive leaks can prevent
the desired operating pressure from
being achieved, causing light components
to be swept from the condenser
to the vacuum system, and,
in severe cases at high vacuum, significantly
increase the vapor flowrate.
Some instrumentation and control
devices used for small-scale distillation
columns are similar to those
used for commercial-scale units,
such as pressure, differential pressure
and liquid-level instruments.
However, flowmeters, valves and
pumps must be able to work with
low flowrates at a wide range of process
conditions. It is often difficult to
accurately measure and control low
flowrates, and the small-diameter
tubing and orifices required can lead
to plugging problems.
Packing and trays have minimum
column diameters that will allow
them to generate representative
data that can be used for simulation
validation or scaleup. For random
packing, the rule-of-thumb is
that the column diameter should
be at least eight times the packingpiece
diameter. In some cases, this
ratio can be smaller. For example,
with Pro-Pak packing, this ratio is
about six. Structured packing has
similar constraints based on the
packing crimp size. For Oldershawstyle
trays, the minimum diameter is
about 19 mm. Below this diameter,
the hole pattern on the tray deck becomes
non-representative, and the
small diameter of the downcomer
causes problems with vapor-liquid
disengagement. Also, the liquid flow
suffers from surface tension effects.
For small seals and joints, the
choices are often limited to those
that are the correct size and can accommodate
the pressure, temperature
and chemical compatibility requirement
for both glass and metal.
The purpose of small-scale distillation
columns is usually for the
collection
of simulation-validation
or scaleup data. Additional instrumentation
is often required, such
as an enhanced number of column
thermocouples, for control or to
observe the column temperature
profile. More sample points may be
required, and sampling frequency is
normally higher than for a commercial
operation. Often, it is important
to use glass distillation columns so
that tray activity, process fluid color,
foaming issues or fouling problems
can be observed.
The following sections will provide
details about equipment and tools
that are used to address many of the
concerns discussed above.
High-vacuum operation
For distillation under vacuum, differences
between small- and largescale
distillation units include methods
of sample collection.
Distillate and sidestream recovery.
The traditional means of collecting
distillate
from a small-scale
Ice Trap
FIGURE 4. The photo shows a glass feed section
with vacuum jacket and silvering. (Courtesy of
Surplus Components, Granite Falls, Minn.)
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Chemical Engineering March 2023

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