Duplex stainless steel refers to ferritic austenitic steel alloy. Such steels have a microstructure comprising ferritic and austenitic phases. The duplex steel alloy, to which the invention pertains, is characterized by a high content of Cr and N and a low content of Ni. Background references in this respect include WO 95/00674 and U.S. Pat. No. 7,347,903. The duplex steels described therein are highly corrosion resistant and can therefore be used, e.g., in the highly corrosive environment of a urea manufacturing plant.
Urea (NH2CONH2) can be produced from ammonia and carbon dioxide at elevated temperature (typically between 150° C. and 250° C.) and pressure (typically between 12 and 40 MPa) in the urea synthesis section of a urea plant. In this synthesis, two consecutive reaction steps can be considered to take place. In the first step, ammonium carbamate is formed, and in the next step, this ammonium carbamate is dehydrated so as to provide urea, The first step (i) is exothermic, and the second step can be represented as an endothermic equilibrium reaction (ii):2NH3+CO2→H2N—CO—ONH4  (i)H2N—CO—ONH4H2N—CO—NH2+H2O  (ii)
In a typical urea production plant, the foregoing reactions are conducted in a urea synthesis section so as to result in an aqueous solution comprising urea. In one or more subsequent concentration sections, this solution is concentrated to eventually yield urea in the form of a melt rather than a solution. This melt is further subjected to one or more finishing steps, such as prilling, granulation, pelletizing or compacting.
A frequently used process for the preparation of urea according to a stripping process is the carbon dioxide stripping process, as for example described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. In this process, the synthesis section is followed by one or more recovery sections. The synthesis section comprises a reactor, a stripper, a condenser and, preferably but not necessarily, a scrubber in which the operating pressure is in between 12 and 18 MPa, such as in between 13 and 16 MPa. In the synthesis section, the urea solution leaving the urea reactor is fed to a stripper in which a large amount of non-converted ammonia and carbon dioxide is separated from the aqueous urea solution.
Such a stripper can be a shell- and tube-heat exchanger in which the urea solution is fed to the top part at the tube side and a carbon dioxide feed, for use in urea synthesis, is added to the bottom part of the stripper. At the shell side, steam is added to heat the solution. The urea solution leaves the heat exchanger at the bottom part, while the vapor phase leaves the stripper at the top part. The vapor leaving said stripper contains ammonia, carbon dioxide, inert gases and a small amount of water.
Said vapor is condensed in a falling film type heat exchanger or a submerged type of condenser that can be a horizontal type or a vertical type. A horizontal type submerged heat exchanger is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The formed solution, which contains condensed ammonia, carbon dioxide, water and urea, is recirculated together with the non-condensed ammonia, carbon dioxide and inert vapor.
The processing conditions are highly corrosive, particularly due to the hot carbamate solution. In the past, this presented a problem in the sense that the urea manufacturing equipment, even though made from stainless steel, would corrode and be prone to early replacement.
This has been resolved, particularly by making the equipment, i.e. the relevant parts thereof subjected to the mentioned corrosive conditions, from a duplex steel described in WO 95/00674 (also known by the trademark of Safurex®). However, even though the foregoing reflects a major advancement in urea production, a particular problem exists in the stripper. A typical carbamate stripper comprises a plurality (several thousand) of tubes. Through the tubes, a liquid film runs downwards whilst stripping gas (typically CO2) runs upwards. Provisions are generally made to ensure that all tubes have the same load of liquid so as to have a flow of the liquid at the same speed. For, if the liquid does not flow through all of the tubes at the same speed, the efficiency of the stripper is reduced. These provisions comprise a liquid distributor, generally in the form of a cylinder with small holes in it.
It has been experienced that the liquid distributors need a relatively frequent replacement. Particularly, the size and shape of the holes changes with time, apparently as a result of corrosion, despite the fact that the liquid distributors are made from corrosion-resistant duplex steel as mentioned above. Thus, the affected distributors result in a different throughput of liquid in the stripper, as a result of which the desired equal loading of the stripper's tubes is less efficient.
It is therefore desired in the art to provide a corrosion resistant material that would provide the liquid distributors in the stripper with a better corrosion endurance.