Chemical Engineering January 2023 - 28

Hydrodesulfurization
Top view
Endress+Hauser
Side view
Probe 1
Probe 2
Probe 3
Probe 4
Flowmeter
FIGURE 2. Using a multipoint temperature probe (right), a petroleum refinery created an accurate temperature
profile for a catalyst bed (left) without causing obstruction problems
to improve placement options
* Identical sensors must be used in
each location, plus probe wall thickness
must also be uniform, to deliver
consistent and accurate readings
with similar response time. It is only
under these conditions that an accurate
and dynamic temperature profile
can be determined
Putting these requirements into
practice means balancing multiple
trade-offs, as follows:
* While a resistance temperature
detector (RTD) generally offers more
consistency and accuracy than a
thermocouple (TC), TCs tend to win
out in multipoint applications due to
their durability, fast response, and
ability to pack into compact packages
with minimal insulating material
* Probe materials must be corrosion
resistant. However stainless
steel and other exotic alloys frequently
have low thermal conductivity,
slowing response time. Adding
wall thickness for strength and
internal protective insulation makes
the problem worse, so careful probe
design is critical
* Probe locations must minimize internal
blockage, particularly in areas
near catalyst beds
* Constructing probes as thermowells
allows for sensor removal, but
also usually results in thicker walls
and internal gap. This
slower response
results
in
Endress+Hauser
Effective results
Building highly-effective probes requires
expertise and optimum component
selection, but the positive
results speak for themselves. A petroleum
refinery wanted to develop a
temperature profile across the catalyst
bed of a hydroprocessing unit
that routinely handled crudes with
significant hydrogen sulfide content
(Figure 2). In this aggressive environment,
corrosion of probes and wiring
28
is a constant issue, causing temperature
measurement inconsistency
and outright sensor failure.
After an examination of optimal
sensor placement, it was possible to
design probes to reach all strategic
measurement locations with minimal
catalyst bed blockage. Clever probe
design - using a hydrogen sulfideresistant
alloy in addition to internal
wire protection - made it possible
to deliver sensors that withstand the
harsh environment, while providing
consistent and directly comparable
readings, capable of generating a
true temperature profile necessary
for process modeling and analysis.
Sensor characteristics
The debate of TC versus RTD as an
ideal universal sensor technology
continues, but fortunately, there is
no need to make an either/or decision.
Each has its niches, depending
on process requirements, although
the parameters of the traditional
discussion are always evolving. The
two methods are conceptually different,
and they are worth a brief
look (Figure 3).
A TC works on the basis that a
wire subject to a temperature gradient
also creates a corresponding
voltage gradient, and different alloys
respond differently to the same temPressure
gage
Flowmeter
Flowmeter
perature gradient. Consequently, a
TC uses two dissimilar alloys, joined
at one end, as the sensor. Temperature
is calculated based on the voltage
difference, but to determine the
absolute temperature at the sensor,
there must be a known temperature
reference. This is where the terms
" sensing " (hot) junction and " reference "
(cold) junction, two ends of the
electrical circuit, are derived from.
TCs are easy to make, and can
be optimized using specific alloy
combinations (designated as types)
to cover different ranges. Generally,
TCs are valued for fast response and
durability. However, they often take
a back seat in accuracy and repeatability,
because any change in the
electrical characteristics of the wires
can cause a reading shift or drift.
Drift is typically the result of corrosion
at some point along the lengthy
electrical path.
An RTD operates on the basis that
various metals change electrical resistance
according to temperature.
Platinum is very good for this purpose,
and the sensor uses a tiny coil
of fine platinum wire as the sensor.
The circuit measures resistance, and
to avoid changes related to the lead
wire's resistance, multiple lead wires
- usually three or four - cancel out
this element electrically. RTDs are
valued for their accuracy and repeatability.
However, they are more fragile
and the protection they require slows
their response time.
New manufacturing techniques
have resulted in more robust RTDs
that do not require as much protective
insulation, providing faster
response times. And for TCs, improved
electrical insulation better
protects lead wires from corrosion,
resulting in less severe drift.
FIGURE 3. RTDs (left) and TCs (right) are different conceptually. TCs modulate voltage, whereas RTDs
modulate resistance. Both require specialized cables to send their measurement over any distance
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