Corrosion in Urea Reactor

Corrosion in Urea Reactor

(2004) Q. We are in the nitrogenous fertilizer business and producing urea, situated in Northern India. In Urea plant, we are having one urea reactor. In this reactor, mixing of liquid ammonia and gaseous CO2 takes place to convert carbamate which further dissociates to form urea having 33% concentration. This reactor is having carbon steel shell with internal lining of SS 316-L (urea grade). Lining is provided so as to protect the shell against highly corrosive urea & carbamate solution. Passivation air (0.3%) is also fed to urea reactor for passivation.
This reactor is of 40-m height and originally was having 10 Nos. of sieve trays (inter-spacing of 2.5-m). Feed to this reactor is liquid ammonia along with Ammonium Carbamate and gaseous CO2. In the year 1996, we carried out some modifications in this reactor for increasing conversion efficiency. As a part of modification, 4 Nos. of additional trays were installed in this reactor. We have been observing that corrosion has taken place in the urea reactor and liner thickness has reduced from original 7-mm to 5-mm. As a precautionary measure, we have increased the passivation air from 0.3% to 0.45% in steps, despite this, we are observing corrosion on the liner and subsequent reduction in the liner thickness.
We seek your valued opinion in the above case and suggest what can be the exact reason for the continuous corrosion in the liner in spite of increasing passivation oxygen into the urea reactor. In case, you need further information on this, please do contact us.
Ajai Dwivedi
fertilizer company - Lucknow, UP, India

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Corrosion Control

(2004) A. You may consider to electropolish the inside of the reactor.
K.de Boer
- Heerenveen, the Netherlands

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(2005) A. Reasons for corrosion may be attributed to
1.Oil content in liquid ammonia feed, as few grades of oil contains sulphur.
2.Chloride in passivation air-seldom present in atmosphere
3.Hydrogen or iron in carbon dioxide feed
4.Holding reactor contents for more than 24 hrs during shutdown - avoid this
5.Poor CO2 purity - less than 98.8%
6.Mole ratio less than 4.0 ; C/N ratio
7.Ammonia liquid feed exceeding critical temp 132.6 deg C
8.Excess passivation air as inert
Rule out these possibilities
K S John
- S India

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(2005) A. Holding reactor contents for more than 24 hrs during shutdown or low quality of CO2.
Muhammad Adnan
- Pakistan

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(2007) A. The rate of corrosion may be minimized as follows
- Ensure proper NH3/CO2 ratio
- ensure purity of CO2 (reduce hydrogen & organic matter slipping from CO2 removal section)
- Ensure water contents are within specified range in recycle load.
- After plant shutdown, ensure cut off NH3 feed with 10-15 min delay than CO2 feed.
- During start up introduce NH3 feed 10-15 min prior to CO2 feed.
Waqqar Ahmad
- Pakistan

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July 29, 2008 A. I personally feel that with the addition of sieve trays turbulence has crossed the limit to avoid corrosion.
Suggestions:
1) Use tube coil bundle instead of sieve trays
2) Use alloy of ss 316-l with Sandvik 2RE 69.
3) Adjust NH3/CO2 ratio in the range of 4.5/1.
Salman Islam Shahzad
- Lahore, Pakistan.

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January 13, 2009 A. Re. corrosion in reactor in Northern India
Is the liner thickness reduction locally or overall ?
In what time period the reduction took place ?
Is a continuous refreshment of liquid assured between the trays and the liner.
Increasing the oxygen content from 0.3 to 0.45 vol% does not help as at 0.3 vol% the liquid contains already sufficient oxygen.
Pay attention to depletion of oxygen due to dead spots
Mark Brouwer
- Maastricht, The Netherlands

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October 22, 2009 A. Being working on a stamicarbon technology in Urea equipments a simple cause is the corrosiveness of carbamate due to presence of IONIC AMMONIA which is more corrosive towards Mild steel; however SS can resist it but it will be more more costly.

Simple way is to adjust the N/C ratio 3.1 and second one is the passivation layer presence.