Chemical Engineering July 2023 - 39

Sopat
process, but not directly in
the flow of the product being
measured.
At-line. At-line measurements
are taken at a sampling
point in the process,
but
not
FIGURE 5. As the expanded polystyrene polymerization reaction progresses (left to right), monomer is consumed
and the polymer particles grow in size
other unwanted characteristics that
may impact product quality. In addition
to size measurements, inline microscopy
can provide detailed information
on the shape of the particles,
which can be useful in understanding
how the particles will behave
during downstream processing.
As with all inline technologies, inline
microscopy eliminates the need
for manual sampling and analysis,
which
reduces the potential for
sampling errors and ensures that
the data collected are representative
of the entire reaction mixture.
In many applications, the rotational
movement of the individual particles
in the process stream, combined
with high speed imaging, the IM
technology is considered a " 2.5D "
technology, as it does provide
shape information from different angles
of the particles, as well as their
sizes in different dimensions.
Measurement position
Standardization and clear communication
in process measurement
location and analysis is important
to avoid ambiguity and confusion.
In the context of process measurement
and analysis, a measurement
position refers to a specific point or
location where the measurement is
taken relative to the process. This
can be an exact physical location
in a process pipeline or vessel, or
even a virtual location in a computer
simulation or model.
The different use of the classification
inline, online, at-line, and offline
in the process analytical technology
(PAT) and automation communities
regularly causes ambiguity in communication.
There can be some overlap
among them, and the terms are
used inconsistently across different
industries or applications. Therefore,
the available definitions of the classifications
are summarized and merged
in several recent studies [8]. An updated
nomenclature targeting the intended
function of the analyzer rather
than its features was suggested by
Minnich [9]. Figure 1 shows schematic
diagrams from Ref. 9.
However, in general, they are used
to describe the timing and location
of process measurements and are
defined as follows:
Inline. Inline measurements are
taken while the process is running,
directly in the flow of the product
being measured. Inline measurement
devices are installed in pipes or other
process equipment and provide realtime
feedback on process parameters,
such as temperature, pressure,
or flowrate.
Online.
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Online
FIGURE 6. Engineers can adjust reaction conditions in real time to ensure
the desired product quality and yield
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measurements
are similar to inline
measurements in
that they are taken
while the process
is running, but they
are typically taken
from a separate location
that is connected
to the process.
For example,
an online pH sensor
might be installed in
a tank or vessel that
is part of a chemical
JULY 2023
necessarily
while
the process is running. The
sample may be collected
and transported to a laboratory
for analysis, or it may
be analyzed on-site using portable
instruments or benchtop equipment.
At-line measurements are useful
when inline or online measurements
are not feasible or practical.
Offline. Offline measurements are
taken outside of the process, typically
in a laboratory or other dedicated
analysis area. Samples are collected
and transported to the analysis location
for testing. Offline measurements
are often used for quality control and
troubleshooting purposes.
Bypass technologies
A bypass line is a common feature
in many process-measurement systems,
and it can be used to divert
a portion of the process flow away
from the main line to allow for measurement
or analysis. A setup like
this is recommended in terms of
process control to prevent a shutdown
or lost product if analyzers
require frequent cleaning, calibration
or validation.
A bypass might be an option
when the process media is too hot,
too cold or too corrosive for the
measurement instrument. In some
cases, the process conditions may
be too harsh for the measurement
instrument, and a bypass can be
used to divert a portion of the flow
to a separate location where the instrument
can be safely installed and
protected. On one hand, there are
some essential benefits of using a
bypass, such as the following measurement
situations:
If the measurement requires a
long or complex flow path. Some
measurement techniques require a
long or complex flow path to achieve
accurate results. A bypass can be
used to provide the necessary flow
path without affecting the main process
flow.
If the instrument requires frequent
maintenance or calibration.
A bypass can be used to iso39

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Chemical Engineering July 2023

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