This document discusses troubleshooting issues that can occur in sulfur recovery units (SRUs). It begins with an overview of sulfur chemistry and the Claus process for converting hydrogen sulfide to elemental sulfur. Common problems that can cause conversion losses or pressure drops are then examined, such as carbon deposits, leaks, catalyst deactivation, and improper air-to-acid gas ratios. Specific case studies are presented on troubleshooting carbon deposits from hydrocarbon contamination and identifying leaks from declining steam production and rising pressure drops. The document emphasizes the importance of continuous monitoring and preventative maintenance to address problems in SRUs before catastrophic failures occur.

1. SRU Problems &Troubleshooting
By:Ahmed S. Omran
M.Sc.Chem. , AMIChemE, MAIChE, ACS Member
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2. Introduction
 Sulfur Chemistry, Physical properties and Safety.
 Importance of SRU troubleshooting.
What can goes wrong?
Conversion loss Vs Pressure Drop.
Problems & Troubleshooting(Case Studies)
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3. Sulfur Recovery Chemistry
• The Claus reaction to convert H2S into elemental sulfur requires the
presence of one mole of SO2 for each two moles of H2S:
(1) 2H2S + SO2 → 3Sx + 2H2O
•
To provide that ratio of components, the first step in the Claus process is
the combustion of one-third of the H2S in the feed gas:
(2) H2S + 1.5 O2 → SO2 + H2O
•
Combining equations (1) and (2), the overall process reaction is:
(3) 3H2S +1.5 O2 → 3Sx + 3H2O
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Sulfur forms over 30
solid allotropes, . -which are
different structural modifications
of an element- more than any other
element. Besides S8, S7, which is
more deeply yellow than
S8. Analysis of "elemental sulfur"
reveals an equilibrium mixture of
mainly S8, but with S7 and small
amounts of S6.

4. Our Scope
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7. Why
Troubleshooting
SRU?
• Pollution abatement has
become as important as
profitability.
• Environmental authorities
have shutdown entire
refineries because of sulfur
plant outage.
• Performance evaluation is
closely related to the
troubleshooting.
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8. Consequences!
Ammonia salts
plugging
off WHB/condenser
tubes
Missing
mesh pads
within
condenser
Main burner
damage
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Refractory
damage due to
overheat

9. What Can Go Wrong?
Pressure Drop?
CAUSE:
• Carbon deposits.
• Leaks in boiler/condensers.
• Plugged seal legs.
END RESULT(S):
Air deficiency or even blown
seal legs.
Inadequate conversion of H2S to
liquid sulfur?
CAUSE:
• Improper air-acid gas ratio.
• Loss of catalyst activity.
END RESULT(S):
Increased SO2 in the
incinerator stack.
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10. Inadequate conversion of H2S to
liquid sulfur?
1-Measuring Conversion(Mass Balance):
 Claus Reaction:
 The mandated Sulfur Recovery is 99.4%;
 SRU is designed for 99.6%.
Sulfur Recovery Level=
((Net Sfrom all streams entering the unit)-(S of incinerator emission and sour water)
(Net S from all streams entering the unit)
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11. 2-Measuring Conversion(colorimetric
tube):
Check H2S and SO2 in the final condenser
effluent.
How?
Dräger tubes are a simple and reasonably
accurate.
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12. Steps to calculate the
conversion(approx.):
1. Add the ppm of H2S + SO2.
2. Add 2,000 ppm to the
preceding(this allows for
COS,CS2,Sulfur vapors, and
entrained sulfur droplets).
3. Divide the total ppm of sulfur
as obtained above 300,000.
4. Express the result as percent.
5. Subtract the percent from
100%.
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13. Wrong Air Ratio
Air flow too high
• Easiest way to lose
conversion.
Symptoms:
-SO3 is formed in the
incinerator with a white
plume.
-Large amount of fuel is
required to maintain the
incinerator temperature.
Air flow too low
• Environmental issue.
• Symptoms:
-Yellowish plume in the
incinerator.
-A high incinerator
temperature coupled with low
incinerator fuel use.
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14. Combustion Air Control
For best conversion ,the ratio H2S/SO2 is 2:1(Supply sufficient air
to burn 1/3 H2S in the total feed).
This ration is measured in the tails gas from the tail gas coalescer.
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15. Reactor Problems
• Catalyst deactivation:
• Symptoms:
1. It is very to do much harm to the catalyst
without causing excessive pressure drop.
2.If you suspect reduced recovery due to lost
catalyst activity check the temperature rise
across the reactor(Outlet-inlet).
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16. Check the Temperature profile:
• This is a good profile • This temperature shift
means the effluent in the
first stage is not reaching
equilibrium.
• In the first reactor, sulfur
formation has decreased
30% and overall catalyst
effectiveness has declined.
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17. Low Reactor
Feed
Temperatures
Sulfur
Precipitation
on the Catalyst
Catalyst
Deactivation
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cause
and cause
may occur
and result
in
conversion
loss.

18. Troubleshooting
• Troubleshooting: Check the operation of the
reheat exchanger upstream of the reactor
with the reduced temperature rise.
• Solution: Raise the reactor inlet temperature
about 30⁰F (17 ⁰C);this will dissipate the
offending sulfur deposits after few days.
Question: If catalyst
activity has been
irreversibly lost, when
catalyst change may
be considered?
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19. COS and CS2
Presence of
hydrocarbons and
CO2 in acid gas
Formation of
COS and CS2 in the
reaction furnace.
Increase in SO2
emissions.
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cause
cause
results in
conversion
loss.

20. Troubleshooting
• Symptoms:
An increase in the SO2 emission
accompanied by lower than
normal 1st reactor inlet
temperature.
• Problem : COS and CS2 in the
reaction furnace.
• Solution : Destroy both by
operating the 1st reactor at outlet
temperature of 650F (343 C),so
these compounds are hydrolized
to H2S and CO2.
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21. Sulfur Fog/Demister Damage
Cause & Problem:
As the unit charge drop the unit converts a lower percentage of H2S to
sulfur.
Sulfur should condense on the walls of the tubes. However ,at low
tube-side gas velocities, the sulfur precipitates in the gas stream
itself; A sulfur fog formed.
Damage to the final condenser demister(or coalescer) may allow
entrained sulfur to escape to the incinerator.
-This can be extensively damaged from sulfur fires during start-up.
Symptoms:
The fog does not drop out of the end of the condenser, much of it
appears as SO2 in the incinerator.
Solution:
Avoid unnecessary unit charge drop. Monitor SO2 emission closely at
unit charge drop.
-Avoid oxygen deficiency during start-ups.
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22. When to change a catalyst?
A 4 years desired catalyst life in Barzan SRU.
Damage to catalyst and reduced conversion can
be due to many factors besides lost activity:
carbon deposits, leaking condenser tubes,
damaged support screens, sulfuric acid
formation, or operation at the sulfur dew point.
All these problems are invariably associated with
increasing pressure drop.
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23. In case of normal
pressure drop
through the
catalyst bed, Do
we need to change
catalyst during the
unit turnaround?
.
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With the adequate
instrumentation
available, a firm
decision can
obtained via a
vertical
temperature
profile through the
1st catalyst bed

24. When to change catalyst?
90%+ of the reaction
heat is released in top
6 inches(0.5 feet)
If the catalyst
activity
dropped, the
reaction is
shifted down
in the bed.
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25. Pressure Drop
It is of utmost importance to watch for high-
sulfur plant pressure drop.
Sulfur plants don’t suddenly plug without a
prior pressure drop increase.
However a foresight troubleshooting need
continuous data collection and analysis.
Using the capacity ratio parameter plotted
data can tell a trouble in Claus unit.
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26. Where
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27. Pressure drop
αThroughput2
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Carbon Deposits (Case Study)
What happened?
1.The data plotted in the
previous graph actually not
assembled until after the
catastrophic pressure rise.
2.The plant operators had
not noticed the increased
in the reaction furnace
pressure.
3.Only when they tried to
increase the acid gas
charge and ran short of air
blower capacity, did they
realize something was
amiss.
4.An abnormality had been
reported in the 30th day, a
quantity of hydrocarbon was
skimmed off the amine
regenerator reflux drum .When
the HC sample was drawn, it
bubbled in the sample container.

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Light Hydrocarbons had
accidentally entered the
amine regenerator,
along with the rich
amine.
The
Hydrocarbon
was stripped
overhead.
Some was condensed in
the reflux drum, the rest
remained as a vapor and
was charged, along with
H2S to the sulfur plant.

31. 10 times more air
needed to oxidize a mole
of propane than a mole
of H2S
The black carbon
deposited on the top of
first bed catalyst
resulting in high
pressure drop.
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32. How it can be determined that the increasing ∆P
is due carbon contamination on catalyst?
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1.SO2 Concentration in sulfur plant tail gas is very
low?
Low SO2 is a sign of insufficient air in the reaction
furnace.
2.Are light hydrocarbons accumulating in the amine
regenerator reflux drum?

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Catalyst has
already plugged
with carbon:
• Over a period of
time,SO2 react with
carbon in a slow
reaction at low
temperature.
• Maximizing reactor
inlet temperature and
SO2 levels will help.
• Significant (10%)
reductions in pressure
drop can take weeks.
• Shutting down and
catalyst change is
more practical.
Keep carbon
black from
forming in the
first place:
• This can be
achieved by
better control via
increasing the air
flow(Tail gas
H2S/SO2 ration
analyzer
automatically or
manually by
alerting the
operators).
Reliable way to
prevent:
• Liquid
hydrocarbons
must be
separated from
reach amine
upstream of
amine
regenerator.

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Leaks cause pressure drop (Case Study)
What happened?
1.Observing a high
pressure drop; the plant
operators had suspected a
plugged condenser sulfur
seal leg
2.They opened a drain on
the condenser with intent
of drawing off excess sulfur.
Steam not sulfur,
discharged from the drain.
4.Six days later ,the plant
shutdown with a giant leak
in the high-pressure boiler
tube sheet.

35. A tube leak in the HP steam boiler can
lead to a DISASTER!
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1.The high-pressure water will erode the
metal,and the flow of water into the hot gas
stream will rapidly increase
2.If the direct reheat line is open, sulfur
precipitates on the catalyst; stopping the gas
flow through the plant and can’t be
reestablished
Crash shutdown is the worst thing
that can happened for SRU! Sulfur
plant should be cleared of sulfur by
burning fuel gas instead of H2S before
a shutdown(either ESD or long-
period).Continue flue gas firing(for
longer than 24 hrs) until the molten
sulfur cease to flow from
sultrapsTM(and maintain condition
more 4 hrs to confirm sulfur purge).

36. How to identify boiler tube leaks
before its too late?
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1.The data plotted in the
capacity ratio plot should
be an early sign as a
gradual increase in
pressure drop will appear.
rise.
2.When this happens check
for low steam production
rate from the HP steam.
3.A low gas outlet
temperature from this
boiler.
4.Water(steam) leaks also reduce
conversion of H2S to sulfur. Claus
reaction shows that equilibrium
is shifted to the left as the water
partial pressure increases!
If both steam production
and outlet temperature
are low and pressure
drop is relatively high;
SHUT DOWN the plant.
There is a tube leak!

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38. Conclusion
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39. References
1. Lieberman, N. (1987). Troubleshooting natural
gas processing: Wellhead to transmission.
2. Kidnay, A. J., Parrish, W. R., & McCartney, D. G.
(2011). Fundamentals of natural gas
processing (Vol. 218). CRC Press.
3. Zachariah, Michael R., and Owen I. Smith.
"Experimental and numerical studies of sulfur
chemistry in H 2/O 2/SO 2 flames." Combustion
and flame69.2 (1987): 125-139.
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