Chemical Engineering June 2021 - 27

Reaction monitoring of fermentation
Traditional technologies
* pH
* Temperature
* DO = Dissolved Oxygen
With Raman technology
one probe measures the
concentration of:
* Glucose
* Glutamate
* Lactate
* Dextrin
* Glycerol
* Ethanol
Endress+Hauser
FIGURE 3. With a single Raman probe it is possible to monitor - in real time -
concentrations of glucose and metabolites during a fermentation process
parameters, such as polymer structure
and residual monomer content,
minimizes waste. Raman spectroscopy
can also be used to monitor
the production and processing of
synthesis gas (syngas; H2 + CO) to
generate ammonia or urea, and thus
significantly increase yields. New
possibilities are also opening up in
the life sciences sector, for example
in controlling cell growth in bioreactors.
Here, the precise determination
of the concentrations of glucose
and metabolites is important, among
other things (Figure 3). Thanks to inline
measurement, laboratory release
can be replaced by realtime release,
so batches no longer have to be held
back for days. The monitoring of a
distillation process is another important
application of Raman spectroscopy
(Figure 4).
Decisive further developments also
characterize ultraviolet/visible (UV/
VIS) and near infrared (NIR) spectrometry.
Here, the trend is moving
from online to inline measurement
due to technical innovations. With
inline spectrometers, measurement
times in the range of seconds can be
achieved, and thus advanced process
control can be realized. In addition,
the reduced complexity of the
instruments leads to a broader range
of applications. Until now, the sensitive
spectrometers had to be located
outside the process and connected
to the measuring probes via a fiberoptic
cable. Innovative inline spectrometers
now integrate the previously
separate measurement-probe
components, via fiber-optic cable
and sensor, into compact and robust
26
devices. This
enables them
to be directly
installed in the
process and
integrated into
the processcontrol
system
(See Table 2
for a comparison
of Raman
and NIR).
Now that
such devices
are easier to
use, even more
plant operators
can benefit from the high precision
of the spectrometers, for example,
in the color analysis of products. Unlike
photometers, which operate at
only one or multiple wavelengths,
these new process spectrometers
measure throughout the entire visible
range (UV/VIS wavelength ranges
from about 200 to 750 nm), and thus
the entire color spectrum is covered.
The wavelength range of NIR (about
700-3,000 nm) complements the
color spectrum, and is capable of exciting
molecular vibrations. From the
reflected spectra, information about
molecular composition can be determined.
NIR can be used for both
identification and quantification of
substances. NIR spectroscopy can
therefore be widely used in chemical
analysis, for example, to determine
water content in feedstocks or products
being dried, as well as for quality
control and process control.
There have also been further developments
in the measuring of
moisture in solids and the concentration
of liquid solutions. Time-domain
reflectometry with intelligent micromodule
elements (TRIME-TDR) is
increasingly being used for moisture
measurement. This special variant of
radar time-of-flight measurement is
a highly accurate and reliable online
" high-tech " measurement technique.
In this method, a pulse is generated
that travels along a conductor and
is reflected. The transit time of the
pulse is directly related to the water
content or moisture in a material. In
contrast to other technologies, the
conductivity has no impact on the
measurement. As a result, a high accuracy
is possible.
For concentration measurements
in liquid process media, new systems
are available for realtime online
measurement that work with surface
acoustic waves. These are highfrequency
sound waves that can be
compared in their physical behavior
to seismic waves, such as those that
occur in earthquakes. By evaluating
the transit time and wave amplitude,
acoustic parameters of the liquid,
such as the speed of sound, the
impedance and the density can be
determined, and from this information,
concentration can be derived
precisely and quickly. Because the
system has no moving parts, there is
no wear and maintenance is low.
If we consider the area of quality
monitoring of feed gases, tunable
diode-laser absorption spectroscopy
(TDLAS) is gaining in importance,
compared to alternative methods,
such as gas chromatography (GC).
Modern online TDLAS analyzers selectively
measure impurities, such
as ammonia, moisture or acetylene,
with high precision, even at concentrations
below 1 part per million
(ppm). The analyzers provide reliable
measurements within seconds, enabling
fast action. They help improve
process control, meet product specifications,
avoid catalyst poisoning
and minimize the need for flaring.
Costs versus benefits
The preceding overview of the advances
in PAT shows that its utility is
great in all steps of the production process,
from feedstock analytics, in-process
analytics and product analytics.
PAT devices can be used to monitor
feedstock purity and catalyst concentration,
and to adjust stoichiometry in
reaction vessels. Concentration and
state data from the process allow
an improved understanding of the
process via correlation analysis. The
rapidly available PAT readings allow
improved process control and even
realtime process control.
Thus, the use of PAT leads to a
higher degree of conversion and endpoint-controlled
processes. Faulty
batches and the associated waste
of unusable product are avoided
and byproducts reduced. Yield is increased,
product quality is improved,
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Chemical Engineering June 2021

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