Hydrocarbon Processing - December 2021 - 69

Process Controls, Instrumentation and Automation
Improved operation begins with careful monitoring.
The chemical composition of the hydrogen recycle gas is critical
to equipment and process efficiency. There are
two main areas of concern: sulfur and chloride.
Hydrogen sulfide (H2
S) and other sulfur comit
is capable of speciating a range of sulfur compounds. However,
it requires specialized gas consumables and tends to be mainTracking
the H2
pounds (carbonyl sulfide, dimethyl sulfide, tetrahydrothiophene
and various mercaptans) poison
the Pt/Re catalyst and increase coking, decreasing
hydrogen production and reformate yield. Tolerable
amounts can be as high as 1 ppm, but if the naphtha
feedstock carries sulfur compounds more than
this amount, it must pass through a hydrotreater unit to bring
these contaminant levels down. Recycled hydrogen must be
monitored for sulfur content to maintain process efficiency
and minimize regeneration cycles.
The process requires a small amount of H2
O and chloride
so the catalyst can perform hydrocracking, isomerization and
cyclization conversion reactions
much H2
efficiently. However, too
O allows formation of corrosive hydrochloric acid
(HCl) capable of reducing reformate yield and damaging the
equipment. For example, HCl reacts with any traces of ammonia,
creating ammonium chloride that can form deposits
in the hydrogen recycle compressor. Entrained HCl can also
cause corrosion farther downstream in the gas separator heat
exchangers. If any equipment requires repairs, the entire SRR
unit must be shut down, causing the loss of reformate production.
This incurs a hefty financial impact usually well beyond
$1 MM/d in addition to the repair costs.
Tracking the H2
termine when it is necessary to reduce the H2
S and H2
O level in recycled hydrogen gas can deO
level and dry
down the catalyst. The most effective and practical place to
monitor and measure H2
O is at the hydrogen recycle
return line, downstream of the compressor (FIG. 1). Finding
the best way to measure these components has provided many
challenges for SRR unit operators.
Monitoring H2S. A traditional method for measuring H2
S is
lead-acetate sensing tape, with a length of paper tape impregnated
with lead acetate inserted into the recycle hydrogen
stream. Over time, it reacts with H2
S to form lead sulfide, which
S conappears
as a black solid. The amount formed over a specific period
can be measured by an analyzer to indicate the H2
centration. The analyzer automatically advances the tape mechanically
to expose a fresh section to the gas stream for the
prescribed interval. The tape is a consumable item, and the
presence of lead acetate (CAS 6080-56-4) and lead sulfide on
the spent tape makes it a hazardous waste according to U.S. and
EU regulations under the Resource Conservation and Recovery
Act (RCRA) Code D002/D003 and EU 16 05 06, respectively,
necessitating regulated disposal. This method of analysis
can be accurate and require little or no calibration, but it is
maintenance intensive and requires special disposal methods.
A more sophisticated approach for measuring sulfur and its
compounds is a flame photometric detector analyzer. A sample
of the recycled hydrogen is mixed with air and a flammable
carrier gas and then burned as a reducing flame in an airless
oven. This creates chemiluminescence caused by excited sulfur,
which can be measured by a photomultiplier tube and flame
photometric detector. This approach can be very accurate, and
O level in recycled
hydrogen gas can determine when
it is necessary to reduce the H2
and dry down the catalyst.
O level
tenance intensive. It is also overkill for this application where a
simple H2
S measurement is sufficient for process control.
Monitoring water. Measuring H2
O in the stream calls for
entirely different technologies than those mentioned for
H2
S, usually necessitating a second analyzer. Traditional H2
O
sensors pass a sample of the recycle hydrogen over a surface
chilled to well below the dewpoint, causing water vapor to
condense on the surface.
A common element of these measurement approaches is
that the sensing surface must be heated and dried after each
reading to clear the deposit and then rechilled for the next measurement
cycle. Aluminum oxide probes can measure moisture
without being chilled but must still be heated to take a new
reading. This slows down the cycle time, which may result in
delayed detection of a changing situation. In addition, some
contaminants, if present in the gas stream, can also condense
and leave traces on the sensor. These traces tend to build up
over time, decreasing measurement accuracy.
TABLE 1. A TDLAS analyzer is designed for hydrocarbon
analysis of H2
S (upper table) and H2
Application data
H2S analysis
Target component (analyte)
H2S in semi-regenerative reformer
hydrogen recycled gas
Typical measurement ranges 0 ppm-50 ppm through
0 ppm-300 ppm
Typical repeatability
Measurement response time
Principle measurement
Validation
Target component
(analyte)
Typical measurement
ranges
Typical repeatability
Measurement response time
Principle measurement
Validation
Note: Analysis of H2
S and H2
± 2% of full scale
1 sec-60 sec
Differential TDLAS
(H2
balance
H2
O
H2O in semi-regenerative reformer
hydrogen recycled gas
0 ppm-50 ppm (control) and
50 ppm-500 ppm (trend)
± 1 ppm (control) and ± 10%
of reading (trend)
1 sec-60 sec
Non-differential TDLAS
Certified blend of H2
O in pure nitrogen
or integrated permeation system
O simultaneously requires optional accessory
Hydrocarbon Processing | DECEMBER 2021 69
S scrubber included)
Certified blend of H2
S in nitrogen
O (lower table)
Hydrocarbon Processing - December 2021

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