Urea Production Ammonium carbamate decomposition Ammonia and CO

Urea Production

Ammonium carbamate decomposition: Ammonia and CO 2 are compressed and reacted at 100 -200 atms and 170 -190 deg. C. in an auto clave to form ammonium carbamate. Urea is formed by dehydration in a low pressure stripping operation. Reactions:

Major engg. Problem

Major Engineering problems Urea (a) Autoclave variables: The objects in this autoclave reaction is to produce the optimum economic yield. The conditions which affects this are temperature, pressure , ammonia/ CO 2 ratio and feed rate. Figure shows that Urea production rate can be varied as (i) increases with increase in pressure (ii) Increases with temperature to a maximum at 175 -180 deg. C. then falls off sharply. The operating pressure should be above the dissociation pressure for the carbamate. (dissociation pressure is 180 atm. at 190 deg. C. ) (iii) Uses no excess ammonia. Reasons for not operating at maximum temperature and pressure with no excess ammonia are

Major Engineering problems Urea (iii) Uses no excess ammonia. Reasons for not operating at maximum temperature and pressure with no excess ammonia are (1) increased pressure increases capital and operating costs and reaction equipment (2) increased temperature accelerates decomposition of urea. (3) above conditions produces corrosion rates. (b) Carbamate decomposition and recycle: Temperature should below 110 deg. C. to avoid the biuret formation. (C) Production of Granular Urea: Problem again is biuret formation. Vacuum drying of urea from 80% urea to 99% and spraying air to cool and solidify must be done just above melting point of urea and with minimum residence time.

(d) Heat dissipation in autoclave: The exothermic heat of reaction can be removed by coils, wall cooling or by adding excess reactants to provide sensible heat pick up. (e) Corrosion: This has been the major reason why ammonia-CO 2 process was slow to develop. High cost silver and tantalum liners are used in the autoclaves. Minimum temperature and pressure conditions with excess ammonia are desirable to reduce several corrosion rates. Under these conditions stainless steel can be used in autoclave. (f) Process design modifications. Off gas treatment and their effect on the remainder of process: (1) Hot recycle process (2) ‘Inventa’ process (3) ‘Chemico’ process (4) Solution recycle