GB488404A - Improvements in the manufacture of urea - Google Patents

Abstract

In the synthetic manufacture of urea by heating under pressure ammonium carbamate, obtained by reaction of gaseous carbon dioxide with liquid ammonia in the presence of an inert liquid, there is first prepared, in a closed vessel, by means of residuary gaseous carbon dioxide and ammonia, both derived from a preceding operation, ammonium carbamate in suspension in an inert liquid not miscible with water, e.g. a mineral oil, and this ammonium carbamate suspension is passed under pressure, at the same time as fresh carbon dioxide and ammonia, the latter advantageously in excess, into a high-pressure urea-formation autoclave heated to 150-250 DEG C., which is constructed of materials resistant to attack by the reaction mixture, e.g. tin-coated or lead-coated steel, nickel, aluminium or its alloys, or highchromium steels with or without nickel, cobalt, molybdenum, tungsten and silicon. The resulting mixture of inert liquid, residuary carbon dioxide and ammonia, urea and water may, after reduction of the pressure, be passed to the top of a fractionating column, heated at its base, preferably working under pressures of 4-12 atmospheres, from the top of which the ammonia and carbon dioxide leave whilst the urea solution and inert liquid collect at its base and are separated by difference of density. The urea solution may be directly crystallized, or first purified, e.g. by filtration in the presence of active carbon, if necessary after oxidation of ferrous compounds by a current of air or oxygen, and, when nickel oxide is present in the form of an ammoniacal complex, after passing the solution over aluminium cuttings. If desired, the separation of the inert liquid may be effected at the top of the autoclave before release of pressure, or at the top of the fractionating column after release, instead of at the base of the column.