Chemical Engineering March 2023 - 23

Cover Story
Pilot-Scale Distillation
Process Design
The inherent characteristics, requirements and special concerns of pilot- and laboratoryscale
distillation columns differ from those of commercial-scale equipment. The scale-related
differences of distillation assemblies are presented here
D
istillation processes at any scale
share many of the same theoretical
and practical concepts and
concerns. For example, pressure
has a similar effect on operating temperature
and component vapor-liquid equilibrium.
And a specific number of theoretical
stages, feed location, feed temperature,
reflux ratio, and distillate-to-feed ratio will
result in a similar component separation
for a given feed composition, column head
pressure and differential pressure.
Further, at any scale, vapor and liquid
contacting is generally accomplished by
trays, random packing or structured packing.
Reboilers are usually based on kettle,
thermosyphon, or pump-around designs
that employ steam, hot oil or electric elements
for their heat source to generate
vapor. Updraft or downdraft-style condensers
are required to condense all or part of
the overhead vapor stream. Vacuum or
pressure-control
systems
are
required
to operate a distillation column below or
above atmospheric pressure. Input sensors,
pumps and control values are required
on most distillation columns.
However, the scale of the equipment has
a major influence on its design, construction
and operation. Pilot- and laboratoryscale
distillation columns have inherent
aspects, requirements and concerns that
differ from commercial-scale columns. This
article highlights the specific concerns imposed
on the purpose, design and operation
of small-scale distillation equipment,
and discusses how these concerns manifest
themselves in the actual equipment
that is typically used. It also offers unique
ideas that have been presented for smallscale
equipment. Although this paper is
focused on the scale-related differences
of distillation columns, many of these concepts
apply to other unit operations.
Challenges of scale
The following list summarizes the main concepts
that affect the design of small-scale
distillation equipment:
* Certain engineering concepts require a
different emphasis or must be implemented
in a different manner
* Equipment design, type and availability is
different than for commercial-scale
* The main purpose of small-scale systems
is usually to collect data, but sample generation
is sometimes important
* Facilities in which the distillation equipment
is housed must have wide-ranging
capabilities to be able to accommodate
physical dimension (especially height) requirements,
and personal exposure, flammability
and reactivity concerns
* The life
of
the
small-scale
distillation
equipment setup is usually short-term
* Ease of modifying equipment or operation
is important
Surface-to-volume ratio is a key engineering
concept for
AVN Corp.
FIGURE 1. A double-bulb receiver, such as the one shown in
the diagram, receives distillate in a small-scale setup
certain aspects of
small-scale equipment, and is more of a
concern than for large-scale equipment.
This concern applies to heat loss, columnwall
effects related to mass transfer and
vacuum leaks.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2023
23
Glenn Graham
and Raymond
Rooks
AVN Corp.
IN BRIEF
CHALLENGES OF SCALE
HIGH-VACUUM
OPERATION
HEAT CONTROL
REFLUX CONTROL
DISTILLATE AND
SIDESTREAM PRODUCT
TAKEOFF
REBOILERS
CONCLUDING REMARKS

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Chemical Engineering March 2023

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