Feed Contamination With Sodium and Its Impact

5/21/2018 Feed Contamination With Sodium and Its Impact

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AIChE Paper Number 33a

Feed Contamination With Sodium And Its Impact OnEthylene Furnaces

Bala S. Devakottai *

Consulting Process Engineer

Equistar Chemical Company, LP, a LyondellBasell Company

Pinakin B. Mistry *

Consulting Materials Engineer

Lyondell Chemical Company, a LyondellBasell Company

Robert Pritchett

Principal Reliability Engineer


Prepared for Presentation at the 2011 Spring National Meeting

Chicago, Illinois

March 14, 2011

* Presenter

AIChE and EPC shall not be responsible for statements or opinions contained inpapers or printed in its publications.


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Feed Contamination with Sodium and Its Impact on EthyleneFurnaces

Bala S. Devakottai *

Consulting Process Engineer


Pinakin B. Mistry *

Consulting Materials Engineer


Robert Pritchett

Principal Reliability Engineer


Abstract

Sodium is a known contaminant in feedstock to ethylene furnaces. This paper

describes an incident involving the contamination of a natural gasoline feedstockwith sodium. The incident resulted in aggressive hot-salt corrosion whichimpacted the radiant tubes and convection tubes. Analysis of coke at differentlocations showed the presence of hexavalent chrome, which led to theimplementation of several health and safety precautions as well as environmentalmonitoring during the clean-up process. Metallurgical failure analysis andsubsequent repairs are also discussed.


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Introduction

LyondellBasell relies on stringent specifications on all feedstock brought into itsolefins plants to avoid process upsets and / or damage to equipment. Ethyleneplant feedstock contamination has been widely discussed in past Ethylene

Producers Conferences. This paper addresses sodium contamination issuesassociated with natural gasoline, the C5 and heavier bottoms product fromnatural gas liquids fractionation facilities. Natural gasoline is typically marketedas a gasoline blend stock. Because it contains mercaptan sulfur, natural gasolineis typically treated to convert the mercaptan sulfur to disulfides, a process called

sweetening, to pass a Doctor Sweet specification required by gasoline blenders.This process involves the addition of air and caustic to the gasoline, which isthen passed over a catalyst bed where the sulfur conversion takes place.Sodium contamination from caustic not completely removed from the treatednatural gasoline is a potential threat to furnace components. Another source ofsodium is brine contamination of natural gasoline stored in salt caverns. Thispaper discusses the impact of sodium contamination on the cracking furnace andsubsequent cleanup efforts during an incident at one of LyondellBasell ethyleneplants.

Symptoms of Damage

When feedstock is introduced into a cracking furnace after decoke, it is notunusual a see a spike in CO/CO2 due to the exposure of catalytic nickel surfaceof radiant tubes. This phenomenon is typical of a gas feed if a sulfiding agent

such as dimethyl sulfide (DMS) or dimethyl disulfide (DMDS) is not used topassivate the exposed radiant coil metal surface.

As feed was introduced to the furnace in one of LyondellBasells plants followinga regular decoke, the caustic tower inlet analyzer showed a sudden increase inconcentration of CO2as shown in Figure 1. This increased CO2level lasted daysbefore subsiding to the base level.


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1 Da

Figure 1: CO2spike in Caustic Tower Feed

While all furnaces did not exhibit this symptom, multiple furnaces showed anabnormal increase in CO2levels upon reintroduction of feed following decoke, asshown in Figure 2.

7 Da s

Figure 2: CO2spike in Caustic Tower Feed during Multiple Furnace Feed-in

Root Cause Analysis

A cross functional team was formed to gather data and perform a root causeanalysis of the cracking furnaces exhibiting this unusual behavior, develop a planto clean up any contaminated equipment and recommend corrective actions toprevent future recurrence.

Analysis of the data showed several uncharacteristic behaviors. Pre-sulfidingthese furnaces prior to reintroduction of feed did not change the CO2concentration. The CO2 increase was observed regardless of the feed type ethane, naphtha or condensate. Also, while the CO2concentration increased asfeed was introduced, there was no appreciable change in CO levels, as indicated


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by methanator temperatures. Furnace operation started to deteriorate in thesefurnaces.

The root cause analysis indicated one common factor among all of the furnaces they processed natural gasoline feed just prior to and during the initial incident.

Furnaces that did not see this feed did not exhibit high CO2behavior. Sodium innatural gasoline was suspected to be a causal factor for this abnormal behavior.

Evidence of Damage

When one of the affected furnaces experienced a radiant tube leak, a thoroughinspection of the furnace was performed to understand the extent of the damage.

Coke with green deposits were found on the TLE inlets, as shown in Figure 3.Figure 4 shows green and brown deposits on the coke found at the TLE inlet.

Figure 3: Green deposits on inlet tube sheet of TLE


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Figure 4: Green and brown deposits on coke from TLE inlet

Extent of Damage

The radiant coil failure at the bottom return bend is shown in Figure 5, whichwas not a typical radiant coil failure mechanism.

Figure 5: Radiant coil failure

A boroscope of the radiant coil and crossover piping was conducted to assess thedamage. Figure 6 shows the inside wall of the radiant coil and Figure 7 showsthe crossover piping from radiant section to convection section.


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Figure 6: Green Deposits in Radiant Coil

Figure 7: Green Deposits in Crossover Pipe

Radiant coil outlet piping bend showed large amounts metal deposits, as shown

in Figure 8.


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Figure 8: Deposits in radiant coil outlet pipe

Lab results

Table 1 shows the analysis of coke by XRD and XRF analysis.

Table 1: Analysis of TLE inlet coke


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The presence of a significant amount of sodium in coke alluded to sodium hotsalt corrosion. The absence of chloride confirmed that the sodium was not fromrine (sodium chloride), but most likely from caustic.

nd willisintegrate into Cr +3and Cr +6, both of which are known carcinogens.

t coil. Sulfur ine coke is presumed to have come from the feedstock or DMS.

iscussion of Hot Salt Corrosion

e presence of nickel catalysesthe reforming of steam and the water gas shift.

O + H2O CO2 + H2

These reactions increase CO2 formation dramatically.

ickelulfide eutectics at about 1175F, and corrodes and reduces the thickness.

in the formation of additional (Na2O) (Cr2O3), thuscreasing chromium loss.

sincehromium will only slowly migrate from the bulk alloy to the tube surfaces.

b

Since the formation of burkeite requires a very high temperature, this confirmedthe suspicion that the coke on the TLE inlet must have migrated from the radiantcoil. A literature survey revealed that Cr +4valency is quite unstable adThe presence of Ni, Fe and Cr in the coke, which are components of radiant coilmetallurgy, confirmed the damage to the inside surface of radianth

DSodium prevents formation of protective oxide film on the inside diameter of theradiant tubes. Lack of protective oxide film and th

C + H2O CO+ H2C + 2H2O CO2 + 2H2C

The interaction of Na with chromium leads directly to sodium chromate (Na2CrO4)which will peel off from tube walls. Presence of sulfur also forms Nickel-NSThe presence of vanadium in the feed forms vanadium pentoxide (V2O5) whichacts as a catalyst in these reactions. A complex reaction scheme involving thesethree compounds resultsinThe loss of chromium at the tube surface results in additional coke formation.The coking rate will be higher than normal for several furnace cycles,c


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Cleanup Effort and Recovery

The cleanup effort was time consuming due to the major extent of the damage.There are 6 convection passes, with 6 crossover pipes and multiple tubes in eachof the 6 radiant coils, as shown in Figure 9.

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Typical Furnace Layout

Burners

Fuel Gas(Wall)

Burners

Hydrocarbon Feed

BFW

1500 PSIG Steam

Radiant Coil (6/furnace)

Heater B(common st`ack

for 2 htrs)

HC Preheat

Mixed Preheat

BFW Preheat

Heater A

Steam

Drum

TLE(3/furnace)

Dilution

Steam

Deposits

Figure 9: Extent of Damage

Extra precautions had to be taken during the cleanup effort due to the presenceof hexavalent chrome. The investigation team developed several steps for thecleanup and restoration of the furnaces to restore them to their normal, cleancondition. The TLEs were filled with demineralized water and flushed severaltimes until the sodium concentration in the water samples was low. The waterfrom these flushes was handled as toxic waste and disposed of accordingly. TheTLEs were then hydroblasted using typical TLE cleaning practices.

The decoking drum area was barricaded and the atmosphere monitored for the

presence of chromium during decoking of the affected furnaces. Fortunately,there was no measurable presence of chromium in the air, and potential riskswere not found to be significant.

A plan was developed to water wash the convection tubes, crossover piping andradiant coil as a complete loop. Again, the water from this cleaning was sampled,handled as toxic waste and disposed of accordingly.


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Several radiant coil bottom return bends showed significant wall loss and leaks.The damage to the base metal also caused significant weldability issues. Thepicture below (Figure 10) shows a temporary repair to a radiant coil to enablethe water flush of the inside surface.

Figure 10: Weld overlay temporary weld repair for cleaning of inside coil

Radiography (X-ray) of the bottom return bends after the water flushes showedmetal deposits which could not be cleaned and significant wall losses. Many of

these bends and lower portions of the tubes could not be welded and had to bereplaced. In some furnaces, the entire radiant coil had to be replaced. Later,after multiple cycles, one of the furnaces had a convection tube leak which wastraced back to sodium damage. This was followed by more similar failures onother heaters. Repairs for this issue are ongoing.

Conclusion

As shown in this paper, hot salt corrosion due to sodium attack in multiplefurnaces can affect different components of the furnace. Extreme damage to theprotective chromium oxide layer in a radiant coil may not only lead to furnaceperformance deterioration but to a toxic form of hexavalent chromium as well.Care should be taken to deal with the possibility of hexavalent chrome whenhandling coke and cleaning the TLEs.


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Acknowledgements

This paper is the result of countless hours of investigation and cleanup effort byvarious groups of people. The authors would like to thank the people inproduction, environmental, maintenance, reliability, R&D and laboratory for their

diligent help in dealing with this issue.

References

1. Damaging Mechanisms in Ethylene Plant, Pinakin Mistry and Bala Devakottai ,20thEthylene Producers Conference, AIChE, 2008

2.An Overview of Self-Inflicted Contaminants Stephen De Haan, Vice President,Plant Performance Improvement, ABB Lummus Global 18thEthyleneProducers Conference, AIChE, 2008

3. Top 5 Contaminants in Ethylene Production Unit Feedstocks Mark Brayden,Dow Chemical, Dwight Hines, BASF Fina Petrochemicals, JamesGraham, ExxonMobil Chemical Company, Thomas Pickett, Shaw Stone &Webster Inc., 20thEthylene Producers Conference, AIChE, 2008

4.ASM Handbook Vol. 11, Failure Analysis and Prevention5.ASM Handbook Vol. 13, Corrosion6. LyondellBasell Internal Failure Analysis reports from Olefins Plants

Feed Contamination With Sodium and Its Impact

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