A recent replacement of PT Pupuk Kujang's urea reactor at its plant in Cikampek, Indonesia, has increased the reactor's efficiency, reliability, and safety, extending the reactor life by at least 20 years.
The project scope included new high-efficiency reactor trays and optimization of operating conditions, which increased the reactor's efficiency. It also included a new stainless steel lining that better resists erosion and corrosion and can be easily repaired.
The plant solved timing and transportation problems associated with the 23-year-old urea reactor and titanium internal lining by replacing it with another reactor with stainless steel lining. This cost considerably less than alternative solutions.
Furthermore, a new lining leak-detection system permits the plant to spot problems at their inception more reliably and accurately.
Siirtec Nigi SPA, Milan, completed the revamp activities on a turnkey basis using FBM Hudson Italiana SPA as its subcontractor. Snamprogetti SPA, Milan, authorizes Siirtec Nigi to revamp urea plants using Snamprogetti technologies.
Plant history
PT Pupuk Kujang, a state-owned Indonesian urea producer, has operated its natural gas-based ammonia-urea plant at Cikampek, on the island of Java, Indonesia, since 1978.
The urea plant has a nameplate capacity of 1,725 tonnes/day. Toyo Engineering Corp. (TEC) of Japan originally designed the plant, using its TR-CI process, which is a "conventional total recycle" process, a popular and successful process at that time.
TEC designed more than 60 of them between 1960 and 1980. Italy's Montecatini SPA, the Netherland's Stamicarbon BV, and others designed about 200 more of these urea plants using similar processes. Many of them are still operating.
"Conventional total recycle" urea plants are the workhorses of the fertilizer industry. Compared with today's designs, however, they consume a high amount of energy. The installation of high-efficiency trays in the urea reactor, absent in the original TEC plants, increases carbon dioxide conversion and thus decreases the use of valuable steam.
Problem
After many years of continuous operation, the PT Pupuk Kujang urea reactor developed leaks on its internal titanium lining as a result of a combination of erosion and corrosion.
The reactor was a typical multi-layered type manufactured by Kobe Steel Ltd. of Japan. Its shell consisted of 13 layers of high strength carbon-steel sheets for a total thickness of 153 mm, while the heads were of solid carbon steel.
Its external diameter was 2.3 m, its total length was 34.8 m, and its weight was 364 tonnes. The design operating temperature was 200° C., and the design operating pressure was 250 bar.
The reaction mixture inside the reactor at this temperature, i.e., ammonium carbamate and urea in water solution, is extremely erosive and corrosive to the carbon-steel material of which the pressure-resistant body is made.
To overcome this problem, manufacturers have used various materials as linings in the past to protect the reactor; these include silver, lead, zirconium, and titanium.
PT Pupuk Kujang's titanium reactor lining was 3-5 mm thick. Nevertheless, the combined erosive and corrosive actions of the reaction fluids damaged this protection of the reactor's internal wall.
A reactor of this type has a typical operating life of 10-20 years, depending on how properly the urea plant is operated. Shorter operating lives are not uncommon.
Aggression to the lining particularly occurs at its welds, which are a point of chemical and metallurgical discontinuity. A serious leak can lead to the perforation of the pressure-resistant body of the reactor in a few days. Since the reactor operates at a high pressure, such perforation can cause a burst with deadly consequences.
A common way to detect leaks in the lining is to establish a monitoring system made by a network of weep holes. A weep hole is made by drilling the entire thickness of the pressure-resistant body, except for its internal lining. Typically there are 40 to 60 of them on an average reactor, properly located throughout the reactor internal surface.
In case of leak from the lining, the escaping gas or liquid goes through the gap existing between the lining and the inner carbon-steel layer of the reactor, until it finds a weep hole from where it exits to the reactor external surface. Here, various systems can detect the leak.
This detection system is efficient but has a weak point. Before reaching the reactor surface, the escaping gas or liquid already contacts the inner carbon steel layer of the reactor, thus corroding it. Often the leaks are small and difficult to detect; therefore they continue their detrimental action on the pressure-resistant body of the reactor for long time. When they are finally noticed, the extent of damage may be wide.
Leaks in titanium are difficult to repair because a high-quality weld in titanium must be performed at carefully controlled conditions by skilled specialists. This is not easily done inside a reactor during a short shutdown.
Alternatives
When leaks continue to appear despite repairs, the common solution is to replace the entire reactor with a new one. A new reactor, such as the one in operation at PT Pupuk Kujang, has a typical delivery time of 18 months and costs about $3 million.
This expensive option was not immediately available to the urea producer because the port and the road facilities used 22 years ago for the first installation no longer allowed the transportation of a 360-tonne reactor.
The cost associated to modify the transport infrastructure to allow receipt of a new reactor was more than $10 million.
Modifications would have included dredging the port because it was silted and checking the pier structure to ensure that it could withstand the loads of the reactor and the lifting crane. In addition, changes to the access roads during the past years made the transport of such an oversized and heavy load impossible.
Another option available to PT Pupuk Kujang was to shut down the plant and reinstall the lining in its reactor by inserting small titanium sheets through its 500-mm manhole. This approach would have required a huge amount of welding in nonoptimal conditions, with a substantial risk of leaks during the subsequent plant operation.
It also would have required a shutdown of about 2 months because this type of work on such a large reactor is time consuming. A 2-month shutdown corresponds to a high loss of production and revenues.
Solution
Siirtec Nigi devised a good solution to the leaks in the titanium liner and implemented it in collaboration with PT Pupuk Kujang. It found and modified a suitable vessel to replace the existing one (Fig. 1).
The revamp took about 9 months to complete and required no loss of plant production. The total cost to PT Pupuk Kujang was $2 million, of which $1.5 million was the value of Siirtec Nigi's contract.
PT Pupuk Kujang acquired a 23-year old urea reactor, identical to that installed at Cikampek. It lay idle in Palembang, a town in Sumatra, a different island of the Indonesian archipelago. It belonged to PT Pupuk Sriwidjaja, a major producer of urea in Indonesia. PT Pupuk Sriwidjaja had also recently replaced its urea reactor because of the same lining problems.
At Palembang, after placing the idle reactor on four saddles, Siirtec Nigi cut it halfway lengthwise. Each of its two parts weighed about 180 tonnes, just below the maximum limits allowed for their transportation to Cikampek. Workers stripped its titanium lining and inspected its body to verify pressure integrity.
After some repairs, which included repair of a hole in the inner surface of the bottom head caused by carbamate leakage through the lining, inspectors declared the reactor fit for use.
PT Pupuk Kujang transported the two parts of the reactor to the port of Palembang. From there, the reactor traveled to the commercial port of Tanjung Priok, near Jakarta, by barge.
From the port, the two parts traveled via highway to the Cikampek plant site where PT Pupuk Kujang placed them on motorized rollers.
Once in Cikampek, Siirtec Nigi fitted the reactor with a new lining made of 25Cr-22Ni-2Mo stainless steel. This special steel was developed in the 1960s to overcome corrosion problems in the urea plants.
Since the early 1980s, it has been used for urea stripper tubes and carbamate condensers. Today, 25Cr-22Ni-2Mo replaces AISI 316-L UG (urea grade) liners in urea reactors because it has a lower rate of corrosion.
It also has better weldability than titanium. That is, its welds are easier to make than titanium welds and more reliable even if made in less than optimal conditions.
Since the reactor was cut in the middle, the new 5 mm thick lining did not have to go through a small-size manhole like traditional relining work. The median cut allowed entry of wide-size courses of lining. The advantage was a dramatic reduction of lining welds (about 12 times less), which lowers the risk of leakage and shortened the execution time.
Unlike the original vessel, Siirtec Nigi protected all the lining welds with a cover strip of the same material. This arrangement creates a tubular-shaped gap between the lining weld and the cover strip.
Weep holes are drilled into this gap and are internally protected by a stainless steel tube welded to the lining (Fig. 2). Therefore, the gap collects leaks from the welds and discharges them outside the reactor through weep holes. This setup confines weld leaks to stainless-steel surfaces and thus prevents the highly corrosive leaked fluids from touching the carbon steel pressure-resistant body of the reactor.
Connecting the gaps in groups reduces the number of weep holes while maintaining the reliability of their monitoring.
The design provided similar protection for other parts of the reactor, such as the manhole, manhole cover, and thermocouples. Siirtec Nigi built this system of interconnected cover strips and weep holes according to Snamprogetti's proprietary design.
Siirtec Nigi also replaced the inlet and outlet nozzles, which were made of solid titanium, with solid stainless-steel nozzles.
Project closure
After installation of the new lining, while the reactor was horizontal, it received new Snamprogetti high-efficiency urea reactor trays. These are sieve-type trays, made with the same stainless steel of the lining and evenly spaced in the reactor.
The trays improve the mixing of the reaction fluids, thus increasing the carbon dioxide conversion and decreasing the quantity of carbamate leaving the reactor. These actions decrease the steam required to decompose the carbamate in the plant downstream and decrease the quantity of carbamate recycled. Since a steam turbine drives the pump that recycles carbamate, less recycling resulted in additional steam savings.
The effect of new trays was therefore a decrease in the overall consumption of steam by the plant.
After welding the two parts of the reactor together, inspectors used gamma rays to inspect the weld. An air-leak test and an ammonia-leak test verified the lining's tightness. Finally, the mechanical crew performed an hydraulic test on the complete vessel.
The project team performed the reactor repair work according to requirements of the US National Board of Boiler & Pressure Vessel Inspectors. The reactor vessel therefore received an "R" stamp and certificate of authorization.
On Feb. 8, 2001, Siirtec Nigi handed over the refurbished reactor project to PT Pupuk Kujang for lifting and installation in the place of the aged reactor. This 10-day activity took place during a planned annual shutdown, thus avoiding any plant-production loss.
Once the reactor was installed, PT Pupuk Kujang started up the plant and conducted a performance test under the supervision of Siirtec Nigi.
During the 3-day performance test, the plant measured its steam consumption. Compared to the steam measurement of the first performance test, done before the replacement of the reactor, the revamp saved 0.12 tonnes of steam/tonne of urea production. These savings met the contractual requirement of Siirtec Nigi.
Thus, the payback time relevant to the installation of reactor trays only was 6-7 months. On Mar. 12, 2001, the plant successfully completed its performance test. It slightly increased the volume of injected passivation air to reduce the corrosion rate of the stainless steel.
Operators were careful to keep the oxygen concentration away from the explosive range, and the relevant plant sections easily handled the quantity of additional inert gas.
To increase the process performance of the reactor further, the contractor defined new operating conditions for the vessel, decreasing its operating pressure and temperature and slightly increasing NH3/CO2 ratio.
The authors
Vincenzo Laganá is head of the ammonia-urea revamping department of Siirtec Nigi. Before joining Siirtec Nigi, he worked for Montecatini SPA and then for Snamprogetti SPA, where he contributed to the development of ammonia stripping urea technology, filing more than 40 patents. Laganá holds a chemical engineering degree from the Politecnico di Milano, Italy.
Sergio Milac is sales director of Siirtec Nigi. Among other duties, he is in charge of the ammonia-urea revamping business. He worked for Technip-Italy before joining Siirtec Nigi in 1997. Milac holds a chemical engineering degree from "La Sapienza" University of Rome, Italy.
Sunarko Kardjuni is senior staff of PT Pupuk Kujang, where he has worked since 1976. He holds a degree in mechanical engineering and a degree in management from the University of Indonesia, Jakarta.
