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

Chapter 2Process name Solvent/reagent + additives CO 2 in treated gas (ppm)Physical absorption systemsPurisol (NMP) N-methyl-2-pyrrolidone Less than 50Rectisol Methanol Less than 10Fluorsolv Propylene carbonate Function of pressureSelexol Polyethylene glycol dimethyl ether Function of pressureProcesses with chemical reagentsMEA Water/monoethanolamine (20 %) Less than 50PromotedMEAWater/MEA (25 – 30 %) + amine guard Less than 50Benfield Water/K 2 CO 3 (25 – 30 %) + DEA, etc. 500 – 1000Vetrocoke Water/K 2 CO 3 + As 2 O 3 + glycine 500 – 1000Catacarb Water/K 2 CO 3 (25 – 30 %) + additives 500 – 1000Lurgi Water/K 2 CO 3 (25 – 30 %) + additives 500 – 1000Carsol Water/K 2 CO 3 + additives 500 – 1000Flexsorb HP Water/K 2 CO 3 amine promoted 500 – 1000Alkazid Water/K 2 -methylaminopropionate To suitDGA Water/diglycolamine (60 %) Less than 100MDEA Water/methyl diethanolamine (40 %) + additives 100 – 500Hybrid systemsSulfinol Sulphones/DIPA Less than 100TEA-MEATriethanolamine/monoethanolaminewater/sulpholane/MDEALess than 50Table 2.4: Overview of some CO 2 removal processes[4, European Commission, 2000]2.2.3.6 MethanationThe small amounts of CO and CO 2 , remaining in the synthesis gas, can poison the ammoniasynthesis catalyst and must be removed usually by conversion to CH 4 by hydrogenation in themethanator:CO + 3H 2 f CH 4 + H 2 O H = -206 kJ/molCO 2 + 4H 2 f CH 4 + 2H 2 O H = -165 kJ/molThese reactions take place at a temperature of around 300 °C in a reactor filled with a nickelbased catalyst. The residual concentration of these carbon oxides is usually less than 10 ppmv.Methane is not involved in the synthesis reaction, but the water formed must be removed beforeentering the converter. This is achieved by cooling, followed by condensation downstream ofthe methanator and finally by condensation/absorption in the product ammonia either in the loopor in a make-up gas drying unit.2.2.3.7 CompressionModern ammonia plants use centrifugal compressors to pressurise the synthesis gas to therequired level (100 – 250 bar, 350 – 550 °C) for ammonia synthesis. Molecular sieves aresometimes used after the first compressor stage to remove the last traces of H 2 O, CO and CO 2from the synthesis gas. These compressors are usually driven by steam turbines, utilising steamproduced from the excess process heat. A small quantity of condensates is removed from thesynthesis gas during compression. These condensates still contain ammonia. Lubricating oilsfrom mechanical equipment, which are typically removed in oil/water separators, cancontaminate them.42 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers