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

Chapter 88.4.9 Combined condensation and reaction in one vesselDescriptionBasically, with combined condensation and reaction in one vessel, two configurations arepossible:• stripper + combined condenser/pre-reactor + reactor (rather as revamp option)• stripper + combined condenser/reactor (rather for new plants).Pool reactor. The stripper off-gases are condensed in a horizontal submerged condenser, whichis an intrinsic part of the urea reactor. In this way, two complete process steps, i.e. condensationand dehydration can be combined in one vessel. The CO 2 stripper is left unchanged. However,the stripper off-gases along with the recycled carbamate solution and the ammonia feed areintroduced into the pool reactor. The liquid phase is thoroughly agitated by the gases from thestripper. The heat of condensation is used for aiding the dehydration reaction and for generatingsteam in the tube bundle. The pressure of the generated steam controls the rate of condensation.The part of the pool reactor that is equipped with the tube bundle is the condensing part of thereactor, while the other section can be called the reaction part. The pressure at the shell side ofthe reactor is 140 bar, the temperature at the reactor outlet is 185 °C [52, infoMil, 2001, 124,Stamicarbon, 2004].Pool condenser. It is also possible to install only a pool condenser. Basically, the poolcondenser is a horizontal vessel with a submerged U-tube bundle. It is fabricated from carbonsteel with internals and lining in stainless steel. Strip gases are condensed in a pool of liquid onthe shell side, with low pressure steam being generated on the tube side. Applying an adequateresidence time allows the reaction of ammonium carbamate to urea and water to proceed up to60 % of equilibrium. The condensation temperature on the shell side is high as a result of theformation of high boiling components of urea and water, resulting in a higher temperaturedifferential for the exchanger. The formation of gas bubbles ensures a high degree of turbulenceand provides a large area for mass and heat transfer. Both phenomena contribute to a higher heattransfer [52, infoMil, 2001, 124, Stamicarbon, 2004].Vertical submerged carbamate condenser (VSCC/ACES21). The stripped gases from thestripper are led to a vertical submerged carbamate condenser, where NH 3 and CO 2 condense toform ammonium carbamate and urea on its shell side. The condensation heat is recovered togenerate low pressure steam on the tube side. A packed bed for scrubbing is provided at the topof the carbamate condenser to absorb uncondensed NH 3 and CO 2 into recycled carbamatesolution (coming from the medium pressure section). Urea is partly formed in the carbamatecondenser and the reaction is completed in the reactor [122, Toyo, 2002, 123, Toyo, 2003].Technical advantages of the combined condenser/reactor are:• the heat transfer in the reactor/condenser bundle is much better than in falling filmcondensers• the baffles in the reactor prevent back-mixing• the inverse response of the synthesis loop is eliminated• the synthesis is much less sensitive to deviations in the N/C ratio.Technical advantages of the combined condenser/pre-reactor are:• the urea reactor volume is reduced, or the capacity of the reactor is increased accordingly• less heat exchange area• the horizontal arrangement offers a structural alternative• less stress corrosion• higher operational flexibility.352 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers