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The present invention relates to novel improvements in preparing urea from gaseous ammonia and carbon dioxide using procedures in which the gaseous starting materials are mixed under pressure, heated and expanded, and ammonium carbamate is formed as an intermediate product.
Procedures of the type referred to above are generally carried out at relatively high, temperatures and pressures, e. g., 170-200 °C and 150-200 atmospheres, respectively. Under such conditions, it is known that the intermediate carbamate melt is extremely corrosive, particularly when the carbon dioxide used contains sulphur compounds, e. g., hydrogen sulphide. It has also been thought that oxygen increases the corrosive nature of the carbamate melt.
Accordingly, precautions have been taken in prior procedures (see, for example, German Patents 812,909 and 860,487) to remove sulphur compounds and oxygen from the carbon dioxide utilized. Additionally, it is conventional practice to carry out the synthesis in apparatus made of or coated with suitable corrosion-resistant materials. Thus, for example, lead and silver have been used to form or coat at least those portions of the apparatus which contact the carbamate melt at the high reaction pressures and temperatures. The use of chromium-nickel steels has also been proposed for this purpose. However, experience has shown that such steels are severely attacked. Consequently, with apparatus made of these steels, small amounts of a polyvalent metal or metal compound are also used (U. S. Patents 1,986,973 and 2,129,689).
A further technique for minimizing corrosion caused by reaction mixtures containing ammonia and carbon dioxide, as encountered in urea synthesis, is described in U. S. Patent 1,875,982. This technique involves adding to the reaction mixture, corrosion-inhibiting substances which are capable of forming negatively charged colloidal particles in the mixture.
The principal object of the present invention is to provide novel improvements in minimizing corrosion hi the synthesis of urea from gaseous NH3 and CO2.