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

Chapter 88.4.12 Process water treatmentDescriptionA 1000 tonnes/day urea plant generates approximately 500 m 3 process water per day. Theprincipal source of this water is the synthesis reaction, where 0.3 tonnes of water is formed pertonne of urea produced. The other sources of water are ejector steam, flush and seal water andthe steam used in the waste water treatment plant.Figure 8.4 shows an example for a process water treatment. Heated process water is fed to thetop of desorber 1, where NH 3 and CO 2 are removed by the gas flow from desorber 2 and thehydrolyser. The liquid leaving the bottom of desorber 1 is preheated and fed under pressure tothe top of the hydrolyser. Steam is fed into the base of the hydrolyser. Under these conditionsurea is decomposed and the gases are stripped countercurrently. The vapours go to desorber 1.The urea-free liquid stream leaving the hydrolyser is used to heat the hydrolyser feed stream andis fed, after expansion, to desorber 2 where LP steam countercurrently strips the remaining NH 3and CO 2 and the off-gases pass to desorber 1.The off-gases from desorber 1 are condensed in a cooled reflux condenser/separator. A part ofthe separated liquid is pumped back to desorber 1 and the remainder is returned to the the ureaprocess plant. Residual NH 3 in the separator off-gas is recovered in an atmospheric absorber andis also returned to the process.Exhaust gas toatmosphericscrubberRefluxcondenserCarbamatesolution backto processCarbamatefor refluxDesorber 1 Desorber 2HydrolyserLP steamSteam20 barTreated water tousers (e.g. boilerfeed water)Process waterFigure 8.4: Example for a process water treatment[52, infoMil, 2001, 125, Stamicarbon, 2003]Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 357