Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 88.2 Applied processes and techniques8.2.1 UreaThe commercial synthesis of urea is achieved by the reaction of ammonia and carbon dioxide athigh pressure forming ammonium carbamate, which is then dehydrated by applying heat,forming urea and water:2 NH 3 + CO 2 X NH 2COONH 4AmmoniumcarbamateX CO(NH 2) 2Urea+ H 2 OBoth reactions take place in the liquid phase in the same reactor and are in equilibrium. Theyield depends on various operating parameters. The most typical production conditions aresummarised in Table 8.2. Reaction 1 is fast and exothermic and essentially goes to completionunder the industrial reaction conditions used. Reaction 2 is slower and endothermic and does notgo to completion. The conversion (on a CO 2 basis) is usually in the order of 50 – 80 %. Theconversion increases with increasing temperature and NH 3 /CO 2 ratio and decreases withincreasing H 2 O/CO 2 ratio.ParameterTable 8.2: Typical urea production parameters[121, German UBA, 2001]UnitPressure 140 – 250 barTemperature 180 – 210 ºCNH 3 /CO 2 ratio 2.8:1 – 4:1 molar ratioRetention time 20 – 30 minutesSeveral side reactions may occur in urea synthesis. The most relevant equilibrium reactions are• hydrolysis of urea: CO(NH 2 ) 2 + H 2 O d NH 2 COONH 4 d 2 NH 3 + CO 2• formation of biuret: 2 CO(NH 2 ) 2 d NH 2 CONHCONH 2 + NH 3• formation of isocyanic acid: CO(NH 2 ) 2 d NH 4 NCO d NH 3 + HNCO.The hydrolysis reaction is the reverse reaction of the urea formation and only takes place in thepresence of water. Acids or alkaline solutions can also accelerate the rate of hydrolysis. Inpractice, residence times of urea solutions with low NH 3 content at high temperatures must beminimised. Biuret must be limited in fertiliser urea (preferably maximum of 1.2 % EC), sincebiuret might cause crop damage, notably during foliage spraying. In technical urea (e.g. used inthe production of synthetic resins), the biuret content is generally up to 0.3 – 0.4 % or muchlower (even