Lesson Learned during First Years of Operation of CaMau Ammonia Plant

Despite the fact that the plant was able to reach design capacity, it was operating at conditions deviating from design in a number o/places during the first years of operation. As part o/the recent turnaround, a task force was formed with tire objective to identify the rool causes for such deviations

and to investigate whether this operating mode posed any safety risk to the plant personnel and equipment.

Furthermore, various operational issues encountered over the first years of operation will be covered in the paper. The reformer in particular performed differently than expected, and post combustion

was taking place in the top o/the reformer section; the side effect of this turned Ollt to be quite significant for plant operation in general.

With assistance from the licensor, the different observations were assessed and a plan was developed to address the deviations. both during the turnaround and during the subsequent start-up.

The start-up after turnaround was carried out with assistance from the licensor. After start-up and subsequent trimming afthe plant. optimal performance was achieved, and a plant capacity of 107%

was demonstrated.

Mr. Dang Hoang Quan Petro Vietnam CaMau Fertilizer Company, Vietnam

Mrs. Janni Ostergaard Halder Topsoe AlS, Denmark

Introduction

C aMau fertilizer plant, belongs to Petro Vietnam CaMau Fertilizer Jointstock Company, (PVCFC) and is located in CaM au, Vietnam. The plant started

commissioning January 1, 2012, and had its first ammonia produced on February 22, 20 12. The plant was finally handed over by March 24, 2012.

The plant uses process licenses from Haldor Topsoe A/S (Denmark), Saipem (Italy) , and Toyo Corporation (Japan) for ammonia unit, urea

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unit and granulation unit respectively, and the EPe contractor who constructed the plant was Wuhuan (China).

Since the very first ammonia production on Feb­ruary 22, 20 12, the plant produced its name plate capacity of 1350 MTPD.

Year Ammonia [metric ton) 2012 345,607 2013 458,012 2014 459,702

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The plant has a scheduled turnaround every year, due to the connecting power plant's yearly maintenance shutdown.

During its lifetime, the plant achieved a high av­erage plant capacity, stable operation, and low energy consumption. Despite the overall good re­sults, the plant has faced some difficulties includ­ing operational challenges, failures of critical equipment, and lack of vital spare parts etc.

PVCFC has been working jointly with the EPC contractor, the licensors and the vendors to recti­fy these issues, and to identify root causes of the problems experienced Additionally, PVCFC de­cided in 2013 to enter into a Technical Service Agreement with Haldor Topsoe.

Based on the experience built during the first years of operation, the plant had the goal to en­sure optimal plant performance with respect to energy efficiency, production output and sus­tained results. Part of the agreement has been to ass ist with an expert team to supervise during the start-up of the plant fo llowing the scheduled turnaround in 2014 as well as to perform a com­plete plant optimization together with PVCFC.

Wi th PVCFC in charge of the entire turnaround, the agreed procedure was that PVCFC would perform the planned repair and maintenance ac­tivities. After that, the operators would start up the plant using the exact same operational prac­tice as usual. Topsoe had the role to observe and learn where any kind of optimization could be done and to give recommendations and training and jointly, together with PVCFC optimize the plant's performance. When PVCFC started up the reformer based on their operational experi­ence, it turned out to deviate from Standard Op­erating Procedures. This was partly the cause of some of the operat ional challenges described in this paper.

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Observations and lessons learned during plant optimization in CaMau ammonia plant.

For a long time, the fo llowing sections have op­erated with deviations from the design process flow diagrams, and needed further investigation during the plant optimization

• Low inlet temperature to feed purification section

• High reformer tube temperatures • High Methane leakage from reformer out­

let • High H2/N2 ratio in synthesis gas • Increasing pressure drop across High

Temperature Shift reactor (HTS) • High C02 leakage from absorber • High pressure in ammonia synthesis

Furthermore the plant had following equipment and machinery problems that required to be handled during the turnaround:

• Secondary reformer bottom support dome damaged

• Fai lure of rotating equipment

1. Low inlet temperature to feed purifica tion section

Shortly after the plant went into commercial op­eration, it was unable to increase the inlet tem­perature of the hydrogenation reactor to the de­sign temperature of 350°C (662°F). In the past three years, the inlet temperature has been be­tween 331 'C (628 ' F) and 345'C (653 ' F). To en­sure sufficient catalyst activity throughout the expected catalyst lifetime the inlet temperature is of high importance.

Operating the hydrogenation reactor at lower temperatures than design for a longer period may in the worst case cause organic sulfur break­through. FurthemlOre the lower temperature would have an impact on plant's efficiency.

The root cause of the low inlet temperature was found to be insufficient heat transfer from the

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waste heat section. This will be further described in section 2.

During the plant optimization, the inlet tempera­ture was increased to operate close to design temperature. However to keep operating on the safe side, Topsoe's hydrogenation catalyst TK-26l will be installed at the next catalyst re­placement. TK-26 1 is a highly active hydrogena­tion catalyst used to expand the safe operating window to lower operating temperatures.

2. High reformer tubes temperatures, and higher methane leakage at reformer outlet

Deviations from Standard Operating Procedures were observed when PVCFC started up the re­fonner. The main areas where a different practice was applied were:

• Reformer firing profile • Combustion air flow minimized • Reformer was operating manually with­

out the duty control in automatic mode.

To keep the inlet hydrogenator temperature be­low design temperature throughout the startup phase, the reformer was started up using only half of the burner rows (row 1-3), resulting in a very low flue gas temperature and an uneven heating of reformer tubes. .

The tube skin temperature, without correction for the radiant wall temperature, was measured close to the design value. The operators were con­cerned with the tube temperatures and therefore continued with low firing.

T opsoe recommended to use more burner rows during start-up, and to control fuel gas pressure on a dedicated burner row, rather than adjusting individual burners. Furthermore it was recom­mended to increase load simultaneously, to en­sure sufficient flow through coils and reformer tube when firing was increased.

The operators expressed concern regarding the firing profile recommended by Topsoe, mainly

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because they had observed high reformer tube temperature during normal operation previously. However after the new firing profile was imple­mented, the tube wall temperatures were checked, and found well below acceptable limits.

PVCFC attempted to optimize the energy con­sumption by minimizing the combustion air flow to achieve the lowest load on the flue gas fan, and thereby the lowest energy consumption dur­ing plant operation. The flue gas fan contributed significantly to the total energy consumption, and PVCFC perceived it to be a reasonable place to optimize.

Additionally, operators did not trust the online oxygen measurement in the flue gas, and the la­boratory was not able to verify the oxygen con­tent in the flue gas due to a lack of a sample suc­tion pump.

It turned out that the energy saved on operating at lower load on the flue gas fan led to higher pressure in the ammonia loop due to a higher me­thane leakage outlet the reforming section. The higher loop pressure increased energy consump­tion of the synthesis gas compressor.

In general, the low (manual) firing in the refonn­er led to low flue gas temperatures. The flue gas temperature was so low that it could not preheat the inlet gas to the feed purification section to the desired temperature. Furthermore, the lack of fir­ing lead to high methane leakage out of the pri­mary reformer, resulting in higher feed gas con­sumption and more inerts in the ammonia loop. Higher loop inerts results in higher loop pressure.

The low combustion air flow to the burners led to a sub stoichiometric combustion and post com­bustion below the reformer roof. This is difficult to control, releases less heat and reduces burner efficiency compared to a complete combustion. Furthermore, the post combustion is considered unsafe operation and led to high reformer tube temperatures in some areas.

Increase of combustion air flow to the burners will result in complete combustion and avoid-

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ance of post combustion under the reformer roof. indirectly, the low combustion air flow bad led to high local tube skin temperatures, and low flue gas temperature . By increasing the combustion air flow, lower tube skin temperatures were measured and the heat transfer from the waste heat section improved significantly. The im­proved heat transfer increased the inlet tempera­ture to the feed purification section.

It also turned out that the carbon number put in the DCS for SIC control was not corrected after the gas composition had changed. Therefore, the plant was not operating at the correct SIC ratio. This was corrected ensuring a sufficient steam flow to the reformer. By optimizing the steam flow to the reformer and the firing profile in the reformer, the methane leakage outlet the reform­er was lowered and design values were achieved.

3. Secondary reformer damage of the bottom support dome, and high Hv'Nl ratio in synthesis gas

During the turnaround in 2013, the reformed gas waste heat boiler No. 1 (E 208) was routinely opened to check the conditions of the refractory, as well as to inspect the catalyst support dome in the bottom of the secondary reformer. It was ob­served that some of the bricks which form the dome were damaged. Therefore, it was recom­mended to have a full inspection during the next planned turnaround in July 2014 by the vendor of the dome (Calderys) and Topsoe.

Following this recommendation, the catalysts and the ceramic inert balls were removed in or­der to check the dome properly. The dome was carefully taken out and every single brick was checked. There were many broken bricks, mak­ing the repair work of the dome more difficult than expected.

With the talented skills of the experts from the dome vendor and the highly appreciated support from the other Vietnamese ammonia plant at Phu My Fertilizer, the dome was repaired utilizing bricks from several sources. The damaged bricks were partly replaced by new spare bricks, and

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partly by used bricks from Phu My Fertilizer plant.

During plant optimization it was observed that the plant was operating with a H2fN2 ratio at 3.54. An optimum H2!N2 ratio for ammonia pro­duction is around 3, and it is recommended to keep the ratio a little below 3 rather than above, to minimize inerts in the ammonia loop.

4. Increasing pressure drop High Temperature Shift reactor (HTS)

In April 2013, after 15 months of operation the HTS reactor pressure drop (DP) was observed to be increasing. Data was sent to Topsoe for a per­formance evaluation, and in the following period the DP was closely monitored by PVCFC. Data was frequently collected and sent to Topsoe to be processed and evaluated. After two months with no signs of improvement, it was clear that prompt action needed to be taken in order to keep the plant running at 100% capacity.

Due to limited time and the fact that the reason for the increased DP was unknown, it was, to­gether with Topsoe, decided to plan for an in­spection of the top part of the catalyst bed. Prep­arations were made to enter the reactor under int::rt atmospht::re and t::xamine status of the reac­tor and also possibly skim the top part of the bed.

When entering the reactor, no evident reason for the increased pressure drop was seen. When the hold-down material was removed, the top cata­lyst particles were weaker than expected, and a significant amount of dust was also present.

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Samples were taken, both from the whole pellets and the dust. Then, skimming of the catalyst bed was commenced. Simulations made by Topsoe prior to the inspection showed that a short­loading of I meter would leave a sufficient amount of catalyst to ensure conversion and ena­ble operation until the next scheduled turna­round.

The samples were sent to Topsoe's laboratory for testing to establish the reason for the increase in pressure drop. The testing was done for the top pellets (sample I ), top dust (sample 2) and pellets at the level of 0.8 meters from the top layer (sample 3). A high temperature shift catalyst sample from the top of an HTS in an ammonia plant operating at similar conditions and for 2 years without any issues is shown as sample (4):

Sample Si AI K Na I (ppm) (ppm) I (ppm) (ppm)

1 730 720 360 13,470 2 5, 100 4,290 1,360 13,680 3 400 300 - 110 4 370 610 120 820

The chemical analyses revealed that the catalyst had been exposed to increased levels of silica, aluminum, potass ium and especially sodium. It was also clear that the level of silica, aluminum and potassium were considerably higher in the dust sample than in the catalyst samples. The root cause for the unexpected high content of sil­ica, aluminum, potassium and sodium is still un­der investigation, however it is assumed that dust from the broken dome containing above men­tioned minerals and were part of the explanation for the high pressure drop.

During the 2014 turnaround, the high tempera­ture shift catalyst was replaced. This time SK-201-2, the same catalyst which had been used for the first load, was used for the replacement charge together with a top layer of TK-20. TK-20 is a high void topping material that is de­signed to capture impurities from upstream units that can deactivate the high temperature shift cat­alyst such as sodium, silica, aluminum and potas-

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sium and thereby prolonging the catalyst life­time. During the initial 9 months of operation the pressure drop across the new loading has re­mained stable and 10% lower than with the pre­vious charge at start of run.

5. High C02 leakage from absorber

When the plant was operated at a capacity higher than the des ign capacity, it was observed that the quality of the CO, product and of the purified process gas did not meet the design specifica­tions. The following was found:

• High hydrogen concentration in the COz product, which is an indirect loss of ammonia production

• High COz concentration in the purified pro­cess gas, operating in the range of 800 -1200 ppmv, which actually reduced the ammonia production in the plant (since the design val­ue is 500 ppmv at 100% capacity)

During the turnaround, it was observed that the vortex breaker in the bottom of the COz absorber was broken and needed to be repaired. This seemed to be the root cause of the high Hz con­tent in the COz product.

In order to lower the hydrogen content in the C02 product the operating pressure of the high pressure (HP) flash vessel was lowered to flash more hydrogen in the flash gas stream. This, however, also resulted in additional loss of COz in the flash gas.

Obvious points for optimization were the high C02 slip from the absorber and the poor quality of product C02 with too high content of hydro­gen and nitrogen.

The lean solution flow and temperature was ad­justed to achieve optimum operating condition, and thereby decrease the COz content in the ab­sorber outlet. Furthennore, the purity of the lean solution was improved by increasing the COz stripper top temperature.

A vortex is fonned inside the absorber, and it is important to keep a sufficient level to prevent the

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vortex from reaching the bottom allowing vapor to flow out the bottom together with the liquid. The level was increased during optimization to ensure C02 product purity.

By making these adjustments, the C02 slip was reduced and the product purity came back to the design value.

6. Ammonia synthesis - High pressure in the synthesis loop and leakage observed in the hot heat exchanger

In 2014, it was noticed that there was no temper­ature difference between the inlet and the outlet of the second catalyst bed of the ammonia reac­tor. This indicated lack of reaction taking place in the second bed. At the same time, it was also found that the pressure of the synthesis loop was increasing. Furthennore the outlet temperature of first catalyst bed at 460°C was higher than the expected 390° and there was no significant tem­perature change through second catalyst bed, in­dicating only limited reaction in second catalyst bed.

Many efforts were made to find the root causes for this strange behavior. This included, among others, a check of the pipe sizes, increasing the recycle gas flow rate, and cleaning the inlet filter of the compressor recycle stage. However, none of these solved the problem.

The low temperature increase of the second bed was checked against the overall converter per­formance. It was concluded, that the low tem­perature increase was due to a faulty temperature measurement, and the converter was performing as expected.

Afterwards the ammonia content of both the hot and the cold stream in the hot heat exchanger (E 04503) was checked. This indicated possible leakage taking place between the two streams, which could partly explain the obselVed in­creased loop pressure. In view of the obselVations, the bottom manhole of the hot heat exchanger was opened for inspec­tion of the exchanger condition. During inspec­tion, it was found that the tube bundle was bent,

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and fi ve of the tightening nuts were either loose or blown-otT. The gap between the tube bundle and the shell , measured at the bottom manhole, was not even due to the transition of the tube bundle caused by thermal expansion. Please see Figure 2, Figure 3 and Figure 4.

The tube bundle was then forced back to its orig­inal position using the thermal method, and new packing materials were installed.

The heat exchanger was only given a provisional repair due to very limited access via the bottom man hole. The exchanger will be carefully in­spected again during the next scheduled turna­round.

Figure 2. Position of the gasket (I)

-'-

Figure 3. A detailed CUI of the gasket

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Figure 4. The loosen nuls werefound plugging Ihe holes

Another factor for the increased loop pressure turned out to be related to the refooner operation, where the high methane leakage resulted in more inerts in the ammonia synthesis loop.

Due to an improved fir ing profile, the inert level in the ammonia loop could be reduced to 8 mole % because of a lower methane leakage from the refooner. A higher purge gas flow was also rec­ommended, since there was capacity available for this in the hydrogen recovery unit. Before the plant optimization started, the syngas com­pressor was one of the plant 's bottlenecks. Due to the reduction of the inert content, the ammonia loop pressure could be decreased. This led to a more efficient operation, and increased the plant capac ity.

7. Failures of rotating equipment

7.1. Process air compressor: Broken impellers and thrust bearing housing.

In August, 2012, the process air compressor was tripped partially and operated at no load mode due to an upset in the plant. Unfortunately, the pressure of the steam header increased, due to the low steam consumption of the compressor, and the higher steam pressure increased the tur­bine speed resulting in a trip on overspeed.

After a process investigation, the compressor was restarted. During the "go to load" condi­tions, the compressor vibrations signals in­creased rapidly causing the compressor to trip aga in.

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Instrumentation was further inspected, and no failures were detected. It was decided to restart the compressor again. Each time the compres­sor was restarted, the turbine speed increased to approx. 3500 rpm (50% of nominal speed), and the compressor tripped due to high vibration level on the second rotor - stages 3 and 4.

After opening of the stages 3 and 4, damage on both impellers and broken impeller tension tie bolts were observed. Please refer to Figure 5 and Figure 6.

Figure 5. Damaged impeller slage 3

In addition to the impeller damage, the gear box inspection revealed cracks in the thrust bearing housing of stages 3 and 4. Furtheonore, the lo­cating shoulder of the bottom half of the thrust bearing housing in stage 4 was broken complete­ly, and the impeller labyrinth of stage 5 was also damaged - Please refer to Figure 7 and Figure 8.

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Figure 7. Broken thrUSf bearing housing sfage 4

Several parts of the compressor needed to be replaced, including the impellers of stages 3 and 4, the shaft (connecting stage 3 and stage 4), as well as the labyrinth seals of stages 3, 4 and 5.

The damage found on the locating shoulder casing was the most critical problem, as the Chinese compressor vendor needed minimum a year for a replacement, meaning a tempo­rary solution was required. So a creative so­lution was invented by the plant maintenance staff, subsequently approved by the Chinese compressor vendor, and finally executed in a Vietnamese workshop.

The Vietnamese workshop cut the lower cas­

ing using machining tools, and an annular ad­ditional support plate was added which was fixed on the casing by several bolts. See Fig­ure 9 for more details.

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Figure 9. The repair of the lower casing

A fier the repair was done, the repaired casing was able to run safely for nearly two years be­fore being replaced in the annual turnaround in July 2014.

The damage was inspected and reported by the compressor vendor Atlas Copco. Based on Atlas Copco's investigation, the root caus­es of the failures were:

• Malfunction of the start-up flap valves, during compressor trip. The opening was not as spec­ified in the logic control, which might cause the compressor to surge.

• Blow-off valves did not respond quickly enough during the first trip of the compressor, which caused the compressor to surge.

• The gap between the impeller back area on the balancing grove and the volute was 1.4 mm instead of 4.7 mm due to manu­facturing failure. This caused no issue during normal operation of the compres­

sor. However, an insufficient gap might cause the impellers to touch the volute due to vibration when the compressor trips.

7.2. High vibration of hydraulic turbine and subsequent energy loss

In February, 2014, the hydraulic turbine runn.ing the semi-lean pump in the C02 section was stopped due to high vibration of the axial bear­ing. The hydraulic turbine was then inspected in order to find the cause for the failure.

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After being repaired and checked several times, the problems have still not been solved. The pipeline strainers of the suction and dis­charge system were checked using a dynamic analysis method to simulate the operation for changing process conditions. However, no re­sults of these calculations have shown a way to resolve the problems.

Since the energy generated by the hydraulic tur­bine is 942 kW, the electricity cost increased when the hydraulic turbine was stopped by ap­proximately 1,800 USD per day, 54,260 USD per month or 651 , 110 USD per year.

The inlet pipeline has been modified by changing the isometric and by increas ing the pipe diame­ter, to ensure improve flow dynamics. These minimize the carbon dioxide released from the rich solution, containing a high carbon dioxide level. However, the vibration of the hydraulic turbine has not been reduced.

The root cause for this is still under investigation, but in the meanwhile CaMau are looking for a service company to provide required assistance and review the design to confirm that it is suited for the required operation.

7.3. The damage of the ammonia booster compressor

Several problems with the ammonia booster compressor led to a thorough inspection of the compressor.

The inspection showed that the compressor was damaged by the presence of a sticky jelly materi­al found in the lube oil filters and lube oil reser­voirs. Thjs caused insuffici ent supply of lube oil to the surface of the compression screws. The damage found on the compressor due to this is shown in Figures 9-11.

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Figure 10. The domage of the rotors (screw type)

Figure II . Cracks of the compressor cusing

The compressor was afterwards checked for me­chanical damage, but the root cause of the trip was not established. The damage to the compres­sor might not have been that severe, if the com­pressor had been carefully and properly inspect­ed after the compressor tripped the first time due to the high electric current.

Figure 12. Solid materials ins ide thejilter cartridges

The oil and the solid samples found on the inner side of the oil filter were sent to CPI Fluid Engi­neering for analyzing. The compositions of the oil were in consistence with the chemistry of the oil used for the com­pressor, and no contamination was detected.

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The solid sample was washed with solvent and analyzed for compositions. During the solvent washing, the sample separated into two different types of solid, a brown sludge fraction and a white, powdery, solid fraction. The conclusion of the analysis was that, the white, powdery, solid fraction contained approx­imately 70% inorganic material, mainly consist­ing of calcium carbonate, while the brown sludge fraction appears to be polyurethane or a similar polymer.

The solids were found almost everywhere in the oil system, including oil filters, oil reservoirs and even inside the compressor casing. They were strongly attached to the metal surface and not easily removed.

L~wQ,( Iji ~ 1AJv M;'~: )(04451 OI' LW.\\ \4~,-

Figure / 3. Solid materials removed f rom the lower oil fi lter

The root cause of the damage has not been de­tennined, but one of the things currently investi­gated is the non-continuous operation. The ammonia booster compressor operates only when liquid ammonia is pumped to the ammonia storage tank. During the non-operating time, the circulation of the oil stops periodically, causing a risk of settling of sludge material. This sludge material built up and formed a thick layer of sol­ids over the casing of the compressor until it pre­vented the rotor from freely moving and caused the damage

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8. Optimization of ammonia plant operation after start-up - the value added due to the presence of technical service specialists from Topsoe

In general, thorough optimization of every plant section increased plant capacity to 107 %. Train­ing of plant personnel during the optimization provided them with insights on how to reduce plant energy consumption and operating cost. Operational procedures were corrected and im­proved. This will help the plant to sustain better operational results in future, increasing plant profitabi lity.

The optimized operation has removed some of the bottlenecks the plant had been struggling with. First of all , by adjusting the combustion air and subsequently the firing in the reformer, the reformer tube skin temperatures were lowered and are no longer a limitation. This gave room for increasing the outlet reformer temperature and thereby a decrease in the methane leakage outlet the reformer.

Another bottleneck before plant optimization was the high synthesis loop pressure, restricting the synthesis compressor. The lower methane leak­age from the reforming section, a closer to opti­mum H21N2 ratio inlet the secondary reformer, and increased purge gas flow reduced the inert level, and thereby decreased the synthesis loop pressure.

Lessons learned

1. Operate the reformer during normal oper­ation with duty control in automatic mode and not manually.

2. Control the excess oxygen level in the flue gas properly, in order to get the right coil preheat temperatures and avoid un­safe operation with potential risk of post combustion.

3. Follow tube wall temperatures better to allow proper reformer firing.

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4. Firing in refonner and thereby methane leakage has a big impact on the ammonia loop as well , due to increased level of in­erts.

5. Importance of correcting the SIC calcula­tion based on the correct feed composi­tion.

6. Proper operating temperatures in C02 section are very important. The limitation of increasing the plant load is very much dependent upon the operation of the C02 removal section.

7. The ri sk of leaking between the hot and the cold stream of the hot heat exchanger affects the operation of the synthesis loop significantly.

8. Importance of proper inspection of rotat­ing equipment after every trip to prevent serious failure affecting the plant opera­tion for a longer period.

9. Importance of having critical spare parts ready, especially for a plant in a remote area like CaMau Fertilizer plant.

10. Temporary solutions can save the plant from a valuable product loss. The solu­tion used to have the broken casing re­paired was not only successful for the owner, but also for the compressor ven­dor and the contractor.

11 . It is important and beneficial to have plant optimization done by licensor ex­pert teams.

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