Hydrocarbon Processing - December 2021 - 70

Process Controls, Instrumentation and Automation
TDLAS: One analyzer for both measurements. Tunablediode
laser absorption spectroscopy (TDLAS) takes an entirely
different approach to gas analysis than the technologies
just discussed. It applies the characteristics of gases to absorb
specific wavelengths of light depending on their chemical
composition. How this works has been summarized by the
Beer-Lambert law, and it has been applied in numerous ways
for industrial gas detection and analysis. TDLAS analyzers are
particularly well suited to the components found in natural
gas and refining applications, so they are seeing wider deployments
within the larger hydrocarbon industry.
For this specific application, the characteristics of H2
S and
O are both well understood. Using wavelengths between
1.8 µm-2.8 µm, it is possible to identify absorption peaks in
the recycle hydrogen stream caused by H2
S and H2
O, such that
they can be quantified simultaneously but separately. For example,
the peak caused by H2
O is easy to measure, providing
an accurate value of water content in the stream.
A TDLAS sensor (FIG. 2) is very simple and has no moving
parts outside of valves for the sample handling lines. The
gas sample flows into a tube driven by line pressure. Placing
the assembly in a heated enclosure ensures that the sample remains
fully in a gaseous state, preventing condensation of any
components. The sample's temperature and pressure are both
monitored for the final measurement calculations.
At one end of the tube is a mirror, and at the other end is a
sapphire glass window, behind which is a diode laser next to a
detector. The laser puts out a brief flash, sending its beam into
the tube where it is reflected to the detector by the mirror, effectively
doubling the travel distance through the sample gas.
Software scans the detector to check the most critical wavelengths
absorbed by H2
O and H2
S. The algorithm performs
its calculations, resulting in the final concentration values
(TABLE 1), and the sample can be exhausted from the tube.
This process happens in a matter of seconds, so measurement
intervals can be performed often to suit the requirements of
the application.
H2
Since the characteristics of the diode laser and detector are
very stable and the absorption characteristics of the subject
gases do not change, a TDLAS analyzer exhibits virtually no
drift and generally needs no calibration. With no moving parts,
gas bottles or delicate mechanisms, TDLAS analyzers are easy
to deploy and maintain in the field.
Financial effects and considerations. An SRR unit is expensive
to operate, but it yields high-value products. Therefore,
it is incumbent on operators to fulfill three objectives:
* Optimize the process to produce the highest possible
reformate yield and quality
* Extend the time between catalyst regeneration
* Maximize service life of the expensive catalyst.
Clearly, process availability is paramount, since having an
SRR unit down for regeneration or worse-catalyst replacement-results
in loss of revenue. Since control of both H2
and H2
O
S have a major impact on yield, catalyst life and overall
efficiency, the analyzer on the hydrogen recycle line plays a
critical role in ensuring the success of the SRR unit.
Simultaneously, the analyzer must deliver its value with
minimum cost. A TDLAS analyzer is generally not the lowest
cost option, at least based on initial purchase price; however,
its reduced operating costs quickly overcome the higher initial
investment. There are no consumables, and these devices can
run for years without requiring maintenance, recalibration or
replacement. Therefore, costs such as analyzer technicians, repair
or replacement of probes, as well as an inventory of spare
sensor heads are eliminated. The aggregate savings on such
tangible costs can easily amount to tens of thousands of dollars
per year for each analyzer.
There are also operational considerations. During an abnormal
process event, a liquid slug in a sample line can render
conventional moisture sensors inoperative for hours or days.
For this reason, many refineries have resorted to redundant
analyzers, with associated sample handling conditioning and
stream switching, which can double or triple the installed
costs. These installation complexity costs, combined with
consumables and maintenance, quickly negate lower initial
purchase costs. Moreover, the revenue gained from even a
single additional day of production, or an improvement in reformate
quality resulting from tighter process control, greatly
exceed the total cost of any analyzer deployment.
TDLAS analyzer technologies for H2
S and H2
O measurements
in hydrocarbon processing applications have proven
highly accurate and reliable over the long term, even in hostile
operating environments. The results of higher unit availability
with increased production revenues, combined with reduced
operating and maintenance costs, should clarify the decisionmaking
process when selecting an analyzer technology.
ALAN GARZA is the Product Marketing Manager for the
advanced analysis product lines at Endress+Hauser.
He began his career at Endress+Hauser as a Rotational
Engineer, where he developed multiple instrumentation
technologies. Mr. Garza was also part of the inside sales team,
where he championed gas analytics and developed as an
FIG. 2. A TDLAS sensor measures absorption of specific wavelengths
of light caused by the presence of specific chemical components.
70 DECEMBER 2021 | HydrocarbonProcessing.com
applications engineer. His background also includes business development and
operations management. He earned a BS degree in mechanical engineering
technology from the University of Houston.
http://www.HydrocarbonProcessing.com
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