Low Oxygen Test Urea

8/12/2019 Low Oxygen Test Urea

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Improve Safety Level Urea Plant of Shiraz PetrochemicalCompany by Lowering Passivation Air

AuthorsJan van der Werf, Jo Eijkenboom, Alex Scheerder, Stamicarbon bv, The Netherlands

A. Reza Foolad, Shiraz Petrochemical Company, I.R. Iran

ABSTRACTShiraz Petrochemical Company (SPC) operates a Stamicarbon CO 2 stripping urea plantsince 1983 with a design plant capacity of 1500 mtpd near Shiraz in I.R. Iran. Early2003 SPC installed the first Safurex stripper in the world after experience was gainedwith Safurex high efficiency trays in the urea reactor.

As the high pressure carbamate condenser was made of BC.05 stainless steel, SPCoffered the first opportunity to reduce the oxygen content in the CO 2 from normally 0.6vol.-% to 0.3 vol.-%. Stamicarbon and SPC agreed to perform a low oxygen test.The only reason to add oxygen is to passivate the stainless steels in the HP synthesissection in contact with the process medium. Having a Safurex HP Stripper and BC.05HPCC less oxygen is needed for passivation.Lowering the oxygen level in a urea plant leads to an improvement of the safety level.The ultimate goal is to operate an urea plant oxygen free, which is intrinsically safe. Apre-requisite is that all equipment must be in Safurex .Lowering the oxygen level also leads to numerous other benefits such as lower energyconsumption and lower investment figures. The low oxygen test at SPC is a milestoneto realize an oxygen free and thus intrinsically safe urea plant.This paper presents the results of the low oxygen test at SPC both from process as wellas from corrosion point of view.

The low oxygen test demonstrated no increase in corrosion in the HP synthesis section.Process benefits were evident; increased reactor conversion, lower steam consumptionin HP Stripper and lower ammonia emission.There are no limitations for the SPC plant to run the plant at 0.3 vol.-% oxygen.

Based on this result, Stamicarbon specifies an oxygen content of 0.3 vol.-% in the CO 2 feedstock in a Stamicarbon stripping plant when following pre-requisites are met:

HP Stripper in Safurex material quality HPCC / HP Poolcondenser in BC.05 material quality

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LIST OF CONTENTS

1. Introduction

2. Low Oxygen Test Procedure

3. Risk analysis

4. Process consequences

5. Corrosion consequences

6. Process safety

7. Conclusions

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2. Low Oxygen Test p rocedure

SPC operated the urea plant already at 0.45 vol.-% oxygen before the low oxygen teststarted. In order to judge the effect of lower oxygen content, base line readings weretaken from July 31 till August 6:

- Nickel content in the end product to monitor any increase in corrosion.- Essential process parameters to evaluate the effects on process conditions.

Amongst others: temperature in top of Urea reactor and steam consumption inHP Stripper.

On August 7 the oxygen was reduced to 0.3 vol.-% and the plant operated at this lowoxygen level till September 9 and was stopped for a planned turnaround. Due tomechanical problems in the compressor the CO 2 was kept constant (compressor speedand capacity did not change after reducing the oxygen content).In the TA a corrosion inspection was done in the HP synthesis section.

First the base line of the Nickel content in the end product at higher oxygen content(0.45 vol.-%) was established: 0.2 ppm. In case of active overall corrosion the Nickelcontent will increase significantly by a factor 5 and thus readily detectable.

After monitoring the Nickel content for 6 days, the oxygen content was reduced to 0.3vol.-%. The Nickel content did not change during the test, see figure 1. The Nickelcontent of about 0.2 ppm is constant throughout the test.

[Ni] content end product (ppm) Laboratory analysis

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Figure 1: Nickel content in end product (0.2 ppm). No change during Low Oxygen Test

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3. Risk analysis

As the proposed low oxygen test (0.3 vol%) is for the first time executed in aStamicarbon CO 2 stripping urea plant, the execution of a risk analysis before starting thetest seems appropriate.

3.1. Risk analysis for Safurex The risk to operate Safurex with an oxygen content of 0.3 vol% can be qualified as nonexisting based on the following facts:

3.1.1. Zero oxygen tests on laboratory scaleStamicarbon did perform numerous corrosion tests at nearly zero oxygen levels inlaboratory.

> 30 mm / yr 75 days183C25.22.2

? mm / yr 75 days183C316L-UG

0,05 mm / yr 75 days183CSAFUREX

SAFUREX

BC.01

BC.05

disappeared

> 30 mm / yr 75 days183C25.22.2

? mm / yr 75 days183C316L-UG

0,05 mm / yr 75 days183CSAFUREX

SAFUREX

BC.01

BC.05

disappeared

Figure 2: Stamicarbon laboratory tests in oxygen free carbamate

From Figure 2 one can conclude that Safurex can withstand very low oxygen contentscontrary to the normal austenitic stainless steels.

3.1.2. Flange face leakage of SPC's Safurex stripper Another evidence and experience was gained by incidence. In October 2003 as well asSeptember 2005 a flange face leak occurred in the Safurex stripper. Such a leakoccurs in case the gasket in heavily damaged. Such an occurrence normally results inheavy corrosion of the gasket serrated ring (BC.03 material quality) and flange faces(BC.05 material quality) due to crevice corrosion (oxygen depletion in the crevice), seefigure 3.In SPC's case the severeness of the corrosion could only be observed by the damageof the BC.03 serrated ring, see figure 4a. However the flange faces were intact, see

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figure 4b and after cleaning and gasket replacement the operation could directly restart

again. Not even machining was needed.

Figure 3: Flange faces after a leakage of the man way gasket in case of a BC.05 and Safurex

flange face. In the picture left the heavily corroded flange face (BC.05) is shown due to crevicecorrosion. On the right hand side a Safurex flange face is presented having no corrosion at allafter a gasket leak

Figure 4a: (Sept. 2005): heavily corroded Figure 4b: no corrosion of Safurex BC.03 gasket ring flange face

Not corrodedSafurexflange face

BC.05 Safurex

BC.05 Safurex

PTFEenvelope

BC.03serratedgasket ring

Corroded BC.05flange face

Not corroded Safurexflange face

Not corroded

3.1.3. Experiences in NH 3 stripping plantNo corrosion occurred in a plant test of Safurex parts in a NH 3 stripping plant at 0.25 to0.30 vol% oxygen levels and elevated temperatures around 200 oC.

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3.2. Risk analysis for Austenitic Stainless Steels

3.2.1. Conventional Urea plantsEvidence that the oxygen content can be reduced safely to 0.3 vol.-% is the fact that inthe old days conventional urea plants operated at oxygen levels of 0.3 vol.-%. Thesynthesis section of the conventional plants consisted of a reactor and mixer in BC.01quality. Process conditions were even more severe from corrosion point of view: 190 Cand 200 bar.

3.2.2. HP carbamate condenser in BC.05 At SPC the HP carbamate condenser is made from BC.05. After the HP stripper the HP carbamate condenser is the most critical when reducing theoxygen content. In general corrosion is more critical at higher temperatures and lowoxygen contents.

Figure 5: Temperature and partial oxygen content in the synthesis section

O x y g e n

p r e s s u r e

T e m p e r a

t u r e

Stripper Reactor HP CarbamateCondenser

TopBottom Bottom Top

Temperature

Reactor

HP Scrubber

HP CC

Stripper

O x y g e n

p r e s s u r e

T e m p e r a

t u r e

Stripper Reactor HP CarbamateCondenser

TopBottom Bottom Top

Temperature

Reactor

HP Scrubber

HP CC

Stripper

Figure 5 shows the temperature (red line) and partial oxygen content in the CO 2 streamthrough the HP synthesis section. In the top of the Stripper the highest temperatures incombination with the lowest partial oxygen content prevails, indicating the worsecorrosion conditions. In SPC the original stripper is replaced by a Safurex stripper,making it no longer sensitive to corrosion anymore even at low partial oxygenpressures. Milder corrosion conditions prevail in the other equipment. The top of theHPCC is the second worse location in the synthesis form corrosion point of view. InSPC the original BC.01 HP carbamate condenser is replaced by a new HP carbamatecondenser made from BC.05, a more corrosion resistant material quality. This meansthat also the HP carbamate condenser allows for a lower oxygen level for passivation.

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4. Process consequences

The reduction of the oxygen content first of all leads to more urea capacity as the airadded in the CO 2 compressor feed can now be replaced by CO 2. Some 2% additionalCO 2 in this way is sent to the synthesis section at constant speed of the CO 2 compressor.

The reduction of the oxygen content resulted in Increase in reactor top temperature (see figure 6) resulting in an increase in

reactor conversion This leads to a lower steam consumption in HP Stripper, see figure 7

These results are obtained at a synthesis pressure of 140 142 Bar.

Figure 6: Increase in reactor top temperature

Reactor Top Temperature (Deg C)TAG-number: T141202

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oxygen (vol %)Temperature (Deg C)Low Oxygen Testbaseline

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Figure 7: Reduction of steam consumption in HP Stripper

Steam Consumption Stripper (Tons/hr) TAG-number: FR41094

Reduced steam consumption per ton of end product of 40 kg was obtained.

4.1. Emission f iguresWhen less air is introduced in the HP synthesis the ammonia emission from the 4 barabsorber is reduced.In the SPC plant the throughput of the vent valve position of the 4 bar absorber wasreduced from 50 % to 30 %.

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oxygen (vol %)

Steam (Tons/hr)

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5. Corros ion consequences

The Nickel content during the low oxygen test did not change, indicating that there is noincrease of corrosion.

During the turnaround of the SPC plant, this observation was confirmed in the corrosioninspection of the HP equipment.

The SPC plant was also inspected by Stamicarbon in 2000. The findings during thisinspection can thus be compared. Based on the inspection in 2000 and 2005 thecondition of the HP equipment is excellent taking into account the on-stream time of theequipment.

5.1. HP Stripper (Safurex )The Safurex stripper, installed in 2002 has an effective on-stream time of 1097 days (3years). The average corrosion rate measured in the heat exchanger tubes of 0.05 mm/ywas far better than the corrosion rate of the former BC.05 stripper; 0.09 mm/y.The lining and overlay welding in both top and bottom channels showed no corrosion.This was confirmed by UT thickness measurements.

5.2. HPCC (BC.05)The HPCC was replaced in 1996 and has an effective on-stream time of 1956 days (5,4

years). The corrosion rate of the heat exchanger tubes in the HPCC did not changebefore and after the low oxygen test. During the inspection in 2000 as well in 2005 thecorrosion rate in the heat exchanger tubes nears to zero. Also the liners and overlaywelding in top and bottom channels showed no corrosion, as was confirmed by UTthickness measurements. This is a normal corrosion behavior for BC.05 carbamatecondenser in Stamicarbon stripping plants.

5.3. HP Reactor (BC.01)The HP reactor is still original, having a total effective on-stream time of 5277 days (14.5years). Typical in the top of the reactor the highest corrosion will take place. Based onUT measurements a corrosion rate in the top part is 0.06 mm/y and also the corrosionrate did not change due to the low oxygen test. A corrosion rate of 0.06 mm/y is low;normally a corrosion rate of 0.1 mm/y is found.

5.4. HP Scrubber (BC.01)The HP Scrubber is still the original, except for the tube bundle, which was replaced in1999. Effective on-stream time of the heat exchanger tubes mounts up to 1744 days(4.8 years). The corrosion rate measured in the tubes is 0.02 mm/y. This was alsomeasured in 2000. Again there is no change in corrosion rate observed before and afterthe low oxygen test.

5.5. HP Piping (BC.01) At random UT wall thickness measurements on all HP pipelines showed no significantcorrosion from process side.

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6. Process safety

The vapor stream in the synthesis with the highest risk with respect to flammability is theHP purge stream from the HP scrubber (highest hydrogen and oxygen concentrationunder high pressure and temperature).

When reducing the oxygen content in the synthesis section, the composition of the HPpurge stream will change (less oxygen), but also the total purge stream will reduce(ammonia purge will be lower).

Both effects will result in a situation (at 0.3 vol % oxygen) in which the composition ofthe HP purge stream is moved away from the explosion limits of the O 2 H 2 NH 3 mixture at the given circumstances. Thus the risk for explosions is reduced.

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7. Conclusions

The low oxygen test showed that the SPC plant can run safely with 0.3 vol.-% oxygen inthe CO 2 feedstock without any risk for higher corrosion rates in the synthesis section.Process benefits were clearly demonstrated; the reactor conversion increases, steamconsumption in the HP Stripper reduces and ammonia losses decreases.There became more room to increase the compressor capacity for higher plantproduction. The SPC compressor speed however was limited for mechanical reasons.Finally the plant runs safer at lower oxygen levels, since the composition of the HPpurge system moves away from the explosion limits. The ultimate goal is to have zerooxygen for an intrinsically safe plant. This can only be realized by having all equipmentin the synthesis section in Safurex .

However for existing urea plants Stamicarbon decided to set the oxygen content in theCO 2 feedstock at 0.3 vol.-% when following pre-requisites are met:

HP Stripper in Safurex (BE.06) HPCC / HP Poolcondenser in BC.05 material quality

Thus, all urea plants having a Safurex stripper and a BC.05 HP carbamate condenseror Poolcondenser have the possibility to increase the safety level and benefit fromprocess advantages by lowering the oxygen content to 0.3 vol.%.

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Low Oxygen Test Urea

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