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Problem No. 63 Holding reactor content during a long shutdown

Every urea plant continuously fights again corrosion. The intermediate product ammonium carbamate is extremely corrosive under synthesis conditions. The applied materials of construction require oxygen to form a protective passive layer of chromium oxides. The ammonium carbamate solution will continuously dissolve the passive layer, therefore it is vital to continuously supply oxygen, typically in the form of air, to maintain the passive layer. During blocking-in conditions of the synthesis section it is not possible to add air and the oxygen present will be consumed as a result of the passive corrosion reactions, while at the same time the passive layer dissolves in the ammonium-carbamate solution. At a certain point, the oxygen content in the solution becomes too low to assure a passive layer. At that moment active corrosion will start with much higher corrosion rates than passive corrosion. The picture on the left side shows the passive layer (blue, brown, grey surface) and the picture on the right side shows active corrosion (a shiny silver surface). It is important to realise that once active corrosion starts it cannot be stopped, adding more oxygen at this stage, for example, will not work. Active corrosion will continue, leading to the risk that the protective layer will be severely damaged. The only way to solve this situation is to drain the synthesis section and re-passivate the surfaces.

Vijayan Ananthavijayan of Madras Fertilizers Limited in India starts this round table discussion: During a long shutdown, how many days can we can hold the reactor content in the 316L stainless steel-lined reactor? What conditions are required to maintain the reactor content?

Meer Salman Muddasar of Engro Fertilizer in Pakistan shares his valuable experience: I assume your reactor will be lined with SS-316L Urea Grade liner, and provided you have maintained the necess ary dosing of passivation air during plant operations, you should be able to hold the solution with a pressurised reactor and heated loop for 48 hours. After this period active corrosion initiates. This is a recommended time period we have been applying for the last 20 years without experiencing any material issues in our high pressure loop.

Mohammad Farooq of SABIC Agri-Nutrients Co in Saudi Arabia joins the discussion: I agree with the comments above.

Kashif Naseem of SABIC Agri-Nutrients Co in Saudi Arabia contributes to the discussion: The holding time of the solution in the reactor depends on the N/C ratio in the reactor and the licensors state periods from 24 to 72 hours.

Mark Brouwer from UreaKnowHow.com, the Netherlands adds his experience to the discussion: Please also take the following points into consideration:

• The temperatures should remain higher than 130°C to avoid any risk of crystallisation.
• The condition of the liner is also related to the frequency of stops and starts and blocking in situations.
• A blocking-in situation is always a period of higher corrosion rates as no fresh supply of oxygen is possible and therefore should be avoided whenever possible.

Meer Salman comes back: Too low temperatures are avoided by maintaining the high-pressure stripper top temperature, that not only keeps on adding certain heat into the high pressure loop, it also avoids crystallisation.

Kashif replies: Mark’s points are also very important during blocking-in conditions.

Sourav Dutta of Matix Fertilizer & Chemical Ltd in India shares his valuable experience: In addition to this topic of discussion, can you please clarify what actually happens after 48 hours of holding time? How does corrosion accelerate after this time period? Is it due to the decreasing concentration of oxygen that accelerate the corrosion rates?

Mark replies: During blocking in, the oxygen content will slowly reduce due to the normal unavoidable passive corrosion. At a certain point the oxygen concentration reaches the critical level and active corrosion starts with higher corrosion rates.

Prabhat Srivastava of Yara Fertilizer in India shares his valuable experiences: As per technology licensor recommendations, the high-pressure section can be bottled for up to 48 hours. However, there are situations where the high-pressure section solution has been held for up to 72 hours. If planning in advance to hold the high-pressure synthesis solution more than 48 hours:

• The operator must feed an excess amount of ammonia.
• Immediately bottle the high-pressure section and maintain the pressure higher than 100 kg/cm 2 .

Since there is no option for adding extra oxygen, there is the possibility that urea will partially crystallize at the bottom. To avoid this, ensure flushing of the CO 2 inlet line.

If for any reason the blockage time increases to more than 72 hours then, if possible, feed ammonia for half an hour and maintain the high-pressure synthesis pressure above 100 kg/cm 2 as this will help and excess ammonia will help prevent corrosion.

Shoaib Minhas of FFBL in Pakistan asks for clarification: Prabhat, can you please elaborate on point no. 4, how does ammonia help to reduce corrosion at pressures above 100 kg/ cm 2 after 72 hours? To my knowledge 72 hours is the maximum allowable time and once active corrosion starts it cannot be controlled or reduced. The only way is by total draining. Please correct me if am wrong.

Prabhat replies: Regarding point no. 4: This is not applicable at all times, and it is not the intention to hold for more than 72 hours every time. Sometimes when we have planned some activities it has taken longer than expected. When we feed excess ammonia the small part of unconverted ammonia will overflow, and the flow of ammonia will again make the passivation film. This is the thinking behind the idea, I don’t have any actual proof.

Pradeep Pednekar of RCF Ltd Thal in India shares his valuable experience: Holding the reactor content during a shutdown for 48 hours is a very safe and proven practice. In a Snamprogetti urea plant passivation is added to the high-pressure stripper by the passivation air compressor. Plant passivation air is also added to the suction line of the CO 2 compressor.

My question is whether it is possible to provide passivation via the passivation air compressor (which is possible with little modification of passivation air lines to reactor feed line) to extend the holding period of the solution during shutdown to some extent?

What will the exact effect be of providing passivation to the reactor metallurgy with this modification?

Mohammad replies: The holding time for the urea reactor and synthesis section varies between 48-72 hours. Beyond this period, the passivation layer is affected and once its starts breaking, it cannot be maintained as there is no liquid or gas flow in the synthesis loop. So, in my opinion, feeding oxygen or air will not solve the problem. Addition of ammonia is a second option which may make the media alkaline, but again it can decrease the temperature of the loop which will lead to crystallisation.

Raja Samar Abbas of Agritech Ltd in Pakistan contributes to the discussion: The most important parameter is NH 3 :CO 2 ratio of the reactor. Snamprogetti and TOYO urea processes with higher N/C ratios allow the reactor solution to be held, while Stamicarbon prefers to drain it. It is important that if you are shutting down normally and you are going to hold solution that you feed excess ammonia during shutdown and keep the loop pressure higher than 100 kg/ cm 2 and the temperature above 120°C to avoid local crystallisation in the reactor and condenser bottom. Periodic flushing is also useful to avoid crystallisation. With a N/C ratio of 4:0 at our plant we have experienced no problems with holding the plant for more than 72 hrs for the last 12 years.

TK Batra of National Fertilizers Limited in India shares his valuable experiences: In case hold up is expected to be for more than 24 hours, we keep on injecting ammonia for five minutes every eight hours. This proves to be quite successful.