UREA REACTOR -DISCUSSION = ITS PROBLEM AND SOLUTION AND POSSIBLE QUESTIONS

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.

What can be the exact reason for the continuous corrosion in the liner in spite of increasing passivation oxygen into the urea reactor.----------------------------IFFCO -INDIA

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-di-oxide 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
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.
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.

Subject: Failure in HP. Reactor R-101, Urea 1 Plant

• Dear Specialists:

We have a Urea Production Plant at Brea Libya. In the 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. Please refer to the technical information below. You are kindly asked to evaluate and recommend your repair procedures of the failure. Below you will find the details.
And answer the following:
General Information:
Vessel Manufacture: VEW Kapfenberg, Austria.
Year of fabrication: 1977
Vessel construction: Single Wall
Liner material: 1.4435
Liner thickness: 7 mm
Shell Material: 1.6368
Shell thickness: 82 mm
Shell thickness on top dome: 60 mm in the drawing (recorded 70 mm on site by UT.)
Design & Operating Temperature: 183 c
Design Pressure: 162 kg / cm2
Operating Pressure: 140 to 142 kg / cm2
Inside Diameter: 2400 mm
Length: 30000 mm
No of trays: 8
Tray Material: 1.4435
Introduction:
During planned turnaround of Urea-1 Plant which started on June 8th, 2008, HP Reactor was offered for internal visual inspection, heavy corrosion of the liner was noted at one of the vertical welds of the top dome .
Investigation Actions:
The investigation actions were as follows:
1 The affected liner area was cleaned form Urea carbamate deposits.
2- The linear was found completely corroded at that area.
3- Bigger area of the liner was cut around the affected zone until clean surface achieved on the liner (see pictures 4, 5, &6).
4 Accumulation of urea carbamate deposits and further corrosion was noted on the carbon steel section. (see pictures 4, 5, &6).
5- Further grinding and cleaning of the carbon steel section was done in order to perform UT. and MT.
6- UT. Was done and the thickness recorded was 61 mm (the nominal is 70 mm) meaning that 9 mm were gone.
7- MT .was done, several cracks were noted spread mostly on the affected area. (See pictures 7, 8, 9 & 10).
8- In order to assess the depth of the cracks they were ground about 9 mm and the crack appearance still appear.
9- The thickness of the carbon steel section left after grinding the cracks is 60 mm Min. at the top area up to 40 cm under the top manway; and 49 mm Min. at the area underneath. .
10- After removal of the cracks final Fluorescent MT was carried-out after 48 hrs and revealed minute cracks 

 We seek your valued opinion in the above case and suggest:
1. What is the exact reason for damage and cracking in the Stainless Steel liner, and the Carbon Steel?
2. Is this cracking a Hydrogen Damage Crack or a S.C.C.?
3. After removal of the cracks final Fluorescent MT was carried-out after 48 hrs and revealed minute cracks. (See pictures 13 & 14). Can those minute cracks be accepted for safe operation of the plant for one year until we purchase a new reactor?
4. Is the remaining thickness acceptable for safe operation in the plant until we purchase a new reactor?
5. Finally, can we run the plant safely for one year?

Your urgent reply is highly appreciated.
Regards.
El-Mabruk Omar
NDT Inspector - Tripoli/ Libya