Stamicarbon CO2-Stripping Urea Process of Duplex Stainless Steels

(Ref. 2) The production of urea is based on the following reaction mechanism:

2 NH₃ + CO₂ <====>  NH₂COONH₄

NH₂COONH₄ <===> CO(NH₂)₂ + H₂O

The process flow diagram of the Stamicarbon stripping process is shown in figure 1. The synthesis section consists of a urea reactor (c), a stripper for unconverted reactants (d), a high-pressure carbamate condenser (e) and a high-pressure off-gas scrubber (f). In the carbamate condenser (e) CO₂ and NH₃ are converted into ammonium carbamate. In the reactor the ammonium carbamate is partly converted into urea and water.

The urea synthesis process takes place at a pressure of 140 bar and a temperature of 185°C. The bulk of the unconverted carbamate decomposes in the stripper, where ammonia and carbon dioxide are stripped off. This stripping is effected by counter current contact between the urea solution and oxygen-containing carbon dioxide at synthesis pressure. The oxygen is needed to maintain a passive (corrosion resistant) layer on the stainless steels in the synthesis section. On leaving the stripper, gaseous ammonia and carbon dioxide are condensed in the high-pressure carbamate condenser (e) at synthesis pressure. Before being purged from the synthesis section, the inert gases, mainly oxygen and nitrogen, are washed in the high-pressure scrubber (f) with carbamate solution from the low-pressure recirculation section.

The urea synthesis solution is highly corrosive, the most aggressive component being ammonium carbamate. onsequently, materials of construction to be used here must meet high standards in terms of composition and quality. Awareness of the important factors in materials selection, equipment design, manufacture and inspection, technological design and proper plant operations, together with periodic corrosion inspections, are the key factors for safe operation for many years.

Other articles you might like;

• Radiographic Indications for Castings
• Hydrogen contribution in Welding
• Welding Tools and Supplies: Your Global Market
• URANUS 45N METALLURGY