SRU Problems &TroubleshootingBy:Ahmed S. Omran

M.Sc.Chem. , AMIChemE, MAIChE, ACS Member

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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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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.

Our Scope

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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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Consequences!

Ammonia salts pluggingoff WHB/condensertubes

Missing mesh padswithin condenser

Main burner damage

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Refractory damage due to overheat

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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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 S from all streams entering the unit)-(S of incinerator emission and sour water)

(Net S from all streams entering the unit)

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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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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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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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Combustion Air ControlFor 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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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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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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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.

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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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.

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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Sulfur Fog/Demister DamageCause & 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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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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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

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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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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Where

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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.

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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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.

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).

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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Conclusion

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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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