an exclusive experience of synthesis gas compressor's low

An Exclusive Experience of Synthesis Gas Compressor’s Low

Efficiency Problem Diagnostics The Synthesis Gas Compressor is installed at 1220 MTPD ammonia plant of Haldor Topsoe Design.

Starting from May 2007, a gradual decrease in polytropic efficiency of the compressor’s 4th stage urged operation and technical personnel to probe into the issue which started to hamper plant operation. Subsequent to meticulous investigation, the compressor was opened and its 4th stage

impeller eye was found blocked with Ammonium Carbamate deposits giving a new direction to the diagnostic proceedings. The paper describes the interesting interaction of certain anomalies such as leakage in the feed-effluent gas heat-exchanger downstream of the Methanator and the presence of

ammonia in the sour-gas recycle to the compressor which resulted in an unprecedented phenomenon inside the compressor. The paper also encompasses step by step investigation findings, data analysis,

application of simulation tools and corrective actions taken.

Ather Iqbal Fauji Fertilizer Company Ltd., Goth Machhi, Pakistan

Rehan Ahmed Fauji Fertilizer Company Ltd., Rawalpindi, Pakistan

Introduction auji Fertilizer Company (FFC) is the largest urea manufacturer in Pakistan, operating three ammonia-urea plants; two

at Goth Machhi and one at Mirpur Mathelo. The first plant (Plant-I) was commissioned in 1982 at Goth Machhi with design capacities of 1,000 and 1,725 metric tonnes ammonia and urea per day, respectively. The ammonia plant employed conventional Haldor Topsoe design, while the urea plant was based on Saipem (Snamprogetti) ammonia stripping technology. The plant was successfully revamped to 122.5% of design capacity in 1992. Plant-II was commissioned in March 1993 with design capacities of 1,100 and 1,925 metric tons ammonia and urea per day, respectively.

Plant-III at Mirpur Mathelo was acquired in 2002 and was similar in design to Plant-I; design capacities were 1,000 and 1,740 metric tons ammonia and urea per day, respectively. This plant was also successfully revamped to 125 % of design capacity in 2008.

Ammonia-I Plant

The ammonia plant being discussed in this paper is the one at Goth Machhi (also designated as Plant-I).

Process Description

Ammonia-I plant is a conventional Haldor Topsoe design of the late seventies, featuring high steam to carbon ratio (3.75), hot potassium carbonate system for carbon dioxide removal and ammonia synthesis loop operating at high pressure.

F

692017 AMMONIA TECHNICAL MANUAL

It consisted of desulfurization, conventional reforming and high and low temperature shift conversion sections in the front-end. The carbon dioxide removal section utilizes the Benfield technology from UOP, which was up-rated to Benfield Lo-Heat process in 2006. The synthesis loop operates at a very high pressure of 266 kg/cm2g (3783 psig) with Topsoe’s S-200 series basket in the ammonia reactor. This basket was later replaced with a new S-300 basket in 2009. All the major compressors i.e., process air, synthesis gas and ammonia refrigeration are centrifugal compressors driven by steam turbines. A simplified process flow diagram of the Ammonia-I plant is presented in Figure 1.

Desulfurization Natural Gas Feed

Ammonia Refrigeration

Ammonia Synthesis

Synthesis Gas Compression

Methanation Carbon-dioxide Removal

Low Temp. Shift Conversion

High Temp. Shift Conversion

Waste Heat Recovery

Secondary Reforming

Primary Reforming

Steam

Air

Fu

el

Ammonia product Pu

rges

Figure 1. Ammonia-I Process Flow Diagram

Synthesis Gas Compressor

For makeup of synthesis gas in the loop, a high pressure and high speed compressor is installed. Reformed gas from the front-end after passing through the shift conversion and CO2 removal section is passed through the Methanator and then routed to the suction of synthesis gas compressor. Synthesis gas from the Methanator to the compressor suction contains negligible amounts of carbon oxides (CO, CO2) which are otherwise considered major source of synthesis catalyst poisoning / deactivation. The normal operating

conditions at the compressor suction are 26 kg/cm2g (370 psig) and 10 oC (50 oF).

Figure 2. General layout of synthesis compressor

The four stage centrifugal compressor takes the suction from the knock out (K.O.) vessel after a chiller, installed as part of the debottlenecking (DBN) scheme downstream of the Methanation section. Synthesis gas from 4th stage is fed to the synthesis loop at a pressure of 258 kg/cm2g (3670 psig). The recycle stage coupled with the fourth stage in a single barrel type casing is used to boost the pressure of the recycle synthesis gas in the loop. A diaphragm in the high pressure casing separates the two gases i.e. make up & recycle and it is equipped with a high differential pressure trip for safety of the diaphragm.

Pressure, kg/cm2g

(Psig)

Temperature, oC

(oF)

Suction Discharge Suction Discharge

Stage 1 26 (370)

68 (967)

12 (54)

141 (285)

Stage 2 67 (953)

132 (1877)

45 (113)

155 (311)

Stage 3 131 (1863)

202 (2873)

37 (99)

103 (217)

Stage 4 201 (2859)

258 (3670)

39 (102)

82 (179)

Recycle 251 (3570)

266 (3783)

33 (91)

43 (109)

Table 1. Normal operating conditions of synthesis compressor

70 2017AMMONIA TECHNICAL MANUAL

Problem Background

To cope with the high plant load operation (135% of design), the synthesis compressor is usually running close to its rated speed of 13300 rpm with all stages running at higher pressures than at the original production rate. In May 2007, a change in the 3rd stage discharge pressure was observed by operation staff. The pressure increased from a normal 201 kg/cm2g (2859 psig) to 206 kg/cm2g (2930 psig). The change was so gradual that no one could notice until operation staff started to encounter difficulty in maintaining the compressor speed to support plant load. In the subsequent months till December 2007, the situation aggravated and 3rd stage pressure increased further to 211 kg/cm2g (3001 psig) at which safe operation of the compressor was not possible without substantial reduction in plant load. Increased pressure caused the 4th stage suction pressure safety valve to simmer due to operating pressure above 90% of its set value.

Subsequent to detailed investigation of the compressor’s problem and keeping in view the substantial energy / production loss, plant management was convinced to shutdown the compressor for inspection in December 2007.

Step-by-Step Investigation

Compressor individual stage pressures usually increase in the summer season when cooling water (CW) supply temperature increases and loop pressure increases due to limiting refrigeration. As a result, the synthesis compressor operates at its peak capacity with higher stage pressures.

Initially, the change in 3rd stage discharge pressure was attributed to usual seasonal changes. Data comparison of summer season conditions indicated its abnormal trend because past values were significantly lower at the same CW supply temperature and at similar operating load.

Further gradual elevation of the stage pressure in the subsequent period raised the alarm for the technical team to intervene and they undertook a comprehensive analysis that included data authentication and comparison, software evaluation and simulation of compressor train. Systematic investigation details are given below:

Data Validation

To authenticate the operating data, a rigorous field exercise was done. A complete map of pressure readings of synthesis compressor train was obtained at different operating conditions using portable digital gauges. The data sets confirmed that 3rd stage discharge pressure was actually running above the normal values.

Restriction between 3rd & 4th Stage

One of the suspected reasons could be a restriction or a high pressure drop between the 3rd & 4th stages which might have instigated the 3rd discharge pressure to rise above the normal value. However, pressure data profile indicated that 4th stage suction pressure too increased correspondingly. Pressure drop measured across the 3rd intercooler and knockout vessel was also normal. This led to the conclusion that there was no physical restriction in-between the two stages.

Bypassing from Recycle to 4th Stage

Along with the 3rd stage higher discharge pressure, there was corresponding flow reduction across the compressor. There was a possibility that the increased 4th stage suction pressure may owe to a leakage in the diaphragm between recycle and 4th stage. Consequently, high pressure leaking recycle gas was causing flow restriction of makeup gas to the 4th stage suction. Laboratory analyses were performed to see if there was any substantial change in the makeup


gas composition at the discharge of 4th stage, as a result of mixing of recycle stream having significant (2.6 mol%) ammonia contents from the loop. The analysis (repeated number of times) exhibited presence of ~0.2% ammonia in the 4th discharge (makeup gas to the synthesis loop). However, the makeup synthesis gas coming from the Methanator did not contain any ammonia. This further added to the suspicion that some quantity of recycle gas was bypassing to 4th stage.

Sample Location

H2

(mol%)

N2

(mol%)

Ar

(mol%)

CH4

(mol%)

NH3

(mol%)

Methanator outlet

73.83 24.83 0.20 1.14 ND

4th Stage Discharge

73.37 24.13 0.37 1.92 0.21

Table 2. Gas Analyses

A simulation of the compressor train was done and it was estimated that around 9,000 Nm³/hr (5,587 SCFM) gas might be bypassing to reproduce the lab results of 4th stage (balance was done on ammonia contents). However, bypassing of the recycle gas would have resulted in somewhat lower temperature at 4th stage discharge owing to mixing of low temperature recycle gas. There should also have been an indication of 4th stage efficiency increased because of less measured temperature gain across the stage at higher compression ratio. Both these parameters did not support the bypassing phenomenon and therefore attention was diverted to other possibilities. The reason for the presence of ammonia content in the makeup discharge was further investigated and it was attributed to sour gases (the gases exiting with seal oil from end seals of recycle stage containing significant ammonia contents) which is recovered in the first stage of the compressor.

Narrowing down the problem area

Polytropic efficiencies of the compressor stages are usually evaluated on regular basis and have normal variations of around 1-2% depending upon the accuracy of data & operating conditions, as given in Table 3.

1st Stage

2nd Stage

3rd Stage

4th Stage Recycle

74% 67% 68% 61% 60%

Table 3. Normal efficiencies of stages

During the abnormal behavior of 3rd stage high discharge pressure, a number of evaluations were performed. Substantial loss in the 4th stage efficiency along with reduction in compression ratio (1.20 against normal value of 1.29) was observed, and an efficiency plot revealed a steep downward trend. However, the efficiency of the 3rd stage and rest of the stages were within normal range (Table 4).

1st Stage

2nd Stage

3rd Stage

4th Stage Recycle

73% 67% 69% 43% 58%

Table 4: Efficiencies of stages evaluated in October 2007

Figure 3. 4th stage efficiency plot

Plausible Reasons

The above step evaluations instigated the investigation team to concentrate on 4th stage

40

45

50

55

60

65

Mar-07 May-07 Jul-07 Sep-07 Nov-07

4th Stage Efficiency %


instead of 3rd stage as originally conceived to be the problem area. Abnormally low polytropic efficiency of the 4th stage indicated that there is some issue within the stage or equipment around it. However, these simulations or evaluation of the compressor train could not point to any potential source of the problem. Symptomatic analysis revealed following:

• Discharge pressure of 4th stage gradually came down with hampered compression ratio.

• Stage work and temperature rise across the stage was higher despite lower compression ratio.

• Makeup flow was gradually going down and plant load got affected despite increase in suction (system) pressure.

Based on above evaluations and symptoms, plausible reasons for the stage efficiency loss that required further focus were:

• Any abnormal seal losses from the compressor may result in efficiency loss / lower stage work however; there was no physical evidence as seal gas flow was in the normal range.

• Change in the molecular weight of the gas can also alter stage efficiency but the normal analysis of makeup gas did not support it.

• Any erosion / physical change in the compressor impeller or inlet vanes may alter the compressor’s fluid dynamics resulting in efficiency loss. It should also result in other associated symptoms like imbalance or vibrations and these parameters were all healthy with no variation as evident from the vibration monitoring system.

• The temperature rise within adiabatic stage above usual values with hampered compression ratio is an indication of any kind of restriction or internal recycling of gas (within impeller). Temperature rise across the 4th stage was observed to be elevated

from 41 oC (106°F, normal value) to around 46 oC (115°F) along with decline in stage efficiency and compression ratio. Fouling / restriction within the 4th stage impeller / labyrinth seals seemed to be the most plausible reason for hampered stage efficiency as well as increased 3rd stage discharge pressure.

Plant Shutdown for Inspection of 4th & Recycle Stage Barrel

Now that the problem area was narrowed down but without any confirmed source of fouling / blockage in the stage internals, plant management was convinced to go for the unit shutdown for inspection of the respective stages. Shutting down of the plant for such an ambiguous phenomenon and unclear corrective action plan was a difficult decision. It also involved substantial production / revenue loss, so the investigating team undertook a comprehensive economic analysis on post shutdown condition. Payback for plant shutdown came out be in couple of months in terms of energy / production savings owing to improved conditions of the compressor after rotor / internal replacements. Refurbished rotor was prepared beforehand for a contingency. During the above investigation proceedings till December 2007, the compressor conditions further aggravated and based on safety of the machine, operational limitations and economic feasibility in terms of payback, a four day shutdown of ammonia plant was planned for inspection / internal replacement of 4th / recycle stage barrel. Major deciding factors to go for the shutdown are outlined below:

• Progressive deterioration of performance

• Operation close to PSV set point

• Production loss associated with load reduction


• Prospective gain in energy efficiency and hence higher yield in post shutdown situation

A new Direction to the Investigation

After removal of the barrel of 4th / recycle stage from its casing it was opened in the workshop. Following were the interesting observations:

• Second impeller of the stage was clogged with hard deposits (light yellowish gray) especially the eye area.

• Suction side labyrinth seal found choked with the same material and its teeth corroded.

• Deposition of material was also found on the face of the 2nd impeller.

• These deposits were not limited to impeller and were visible inside the casing as well

Figure 4. Deposition on 4th stage impeller

Figure 5. Deposition on 4th stage impeller eye

Figure 6. Light yellowish-gray deposition on in the casing

Figure 7. Deposition / erosion & corrosion on labyrinth

seal


Ammonium Carbamate Deposition and Source Identification

The deposited material collected from the compressor internals was analyzed and results revealed that the deposits were mainly ammonium carbamate.

Formation of ammonium carbamate in an ammonia plant is not very surprising. As combination of CO2 (present in the feed gas) and ammonia (present in several streams) under favorable reaction conditions can result in ammonium carbamate formation. However, finding this particular material in a compressor having no or little traces of reactants was a real surprise and it required the investigating team to adopt unconventional approach for the problem source identification.

Reaction for formation of carbamate is as below:

CO2 + 2NH3 NH2COONH4 (1)

This reaction is highly reversible. High pressure favors the reaction in forward direction and low temperature helps deposition of formed carbamate. This is the reason that carbamate deposition was present at only specific locations i.e. at suction eye and blades of 2nd impeller, with no traces at the 1st impeller.

The presence of ammonia at the compressor suction from sour gas (from oil seals) is expected during normal operation, and this was confirmed from lab analysis of makeup gas. Please refer to Table 2. However, the source of second reactant i.e. CO2 was yet to be identified. Detailed analysis of plant parameters indicated leakage in an exchanger at the front-end that may have caused abnormal CO2 contents to reach the compressor suction.

Figure 8. Methanation section layout

The Methanation Reactor is provided in the ammonia process scheme to remove carbon oxides to a very low level as these oxides are poisons to the downstream ammonia reactor catalyst. Ideally, there should be no or undetectable carbon oxides in gas routing to synthesis section. However, analysis data indicated that the oxides level in synthesis gas was gradually rising starting from normal values of less than 5ppm in May 2007 and approaching as high as 28ppm in September 2007.

Figure 9. Oxides in the synthesis gas to the compressor

Further investigation with repeat analyses revealed that oxides level at the exit of Methanator was normal. However, there was a significant pickup of oxides across the Feed-Effluent exchanger, downstream of Methanator. It confirmed that the exchanger was leaking and


causing elevated oxides level in the synthesis gas to the compressor suction

Feed-Effluent Gas Heat Exchanger Observations

During the shutdown for maintenance of synthesis compressor, the exchanger E-311 was opened for the first time to attend the leakage. There were visible marks on tube sheet from the leaking tubes weeping while few tubes found choked with Benfield solution carried with the inlet gas from CO2 removal section to the exchanger shell side.

Figure 10. Flow marks on E-311 tube sheet from leaking

tube

A total of 12 tubes were found leaking as confirmed by soap film test (SFT) and these were plugged. Before plugging of the tubes, boroscopic examination of the leaking tubes was done and one tube was found snapped. Two tubes including the one observed snapped were pulled out to investigate the failure cause. Both tubes were sheared off at shell’s gas inlet side of the exchanger. One tube was found snapped at inlet tube-sheet edge while second was snapped at 2nd baffle. The snapping of both tubes at the baffle location indicated high flow induced vibrations as a probable cause of failure.

Flow induced vibration was later confirmed by simulating exchanger, E-311 on the software. It was concluded that owing to higher than design flows (at higher plant loads 135% of design), vibration tendency of the exchanger tube bundle was greater. The exchanger, E-311 was later replaced in 2009 with improvements like reversal of fluids in shell & tube side (hot gas from Methanator in shell side while cold gas from CO2 removal section in tube side) and with a new thermal design to accommodate higher gas flows equivalent to 160% of plant loads. Oxides level in the synthesis gas to the compressor suction came down to normal values subsequent to exchanger maintenance and synthesis gas compressor 4th stage efficiency normalized with the new impeller and maintenance during the shutdown.

Conclusion Ammonia plant having complex process dynamics, can exhibit such phenomenons that are unprecedented. The whole proceedings of the investigation that lasted for months, imparted great opportunity of learning and sharing a unique experience. For a meaningful investigation, it is always beneficial to adopt unconventional approach of correlating various anomalies instead of focusing on one area of the problem. Additionally plant operators should be careful while targeting higher throughput, i.e., operation of the plant at higher loads, as some equipment may be running beyond their safe operation limits and issues like vibrations in exchangers may not be visible till a leak occurs. A careful and thorough safety review of higher loads operation of critical equipment is mandatory especially by the technology provider or designer.


Authors’ Introduction

Ather Iqbal is working as Manager Engineering (Process) at FFC, Mirpur Mathelo plant-site and had earlier supervised process engineering at Plant-1 & 2 of FFC Goth Machhi. He has over 22 years of experience mainly in process engineering with track record of accomplishment of several major projects including debottlenecking studies, Benfield Lo-heat commissioning, S-300 basket installation, New ARU for Ammonia-1, laying of new Natural Gas line. He has previous worked in offices of Haldor Topsoe in Copenhagen Denmark as the coordinator between FFC and Haldor Topsoe for FEED studies and various improvement projects. He holds a bachelor’s degree in chemical engineering from University of Engineering and Technology, Lahore, Pakistan. Rehan Ahmed is working as General Manager Technical & Engineering. He has over 30 years’ experience and is leading the various departments of FFC at Head Office in Rawalpindi. He has previously worked in the engineering department at FFC GM as Engineering Manager. His distinct contributions in various assignments include engineering and commissioning of DBN Urea-I plant, expansion projects – Plant-II at Goth Machhi, new grass-root project FFBL, Karachi, DBN FFBL, etc. He holds a bachelor’s degree in chemical engineering from University of Engineering & Technology, Lahore, Pakistan


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