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

Chapter 22.2.4.8 Liquid nitrogen washFor the final purification, normally liquid nitrogen at approximately -185 °C is used to removeresidual CO, CH 4 , and most of the argon. At the same time, nitrogen is added to the synthesisgas. To prevent blocking of the cryogenic unit, the traces of CO 2 and H 2 O are separated fromthe inlet gas by molecular sieve adsorption. The resulting synthesis gas is very pure and purgingin the synthesis loop is minimised. The rejected waste gas fraction serves as a fuel.2.2.4.9 Ammonia synthesisThe ammonia synthesis loop is identical to the synthesis for steam reforming, seeSection 2.2.3.8.2.2.4.10 Compression, steam and energy systemThe degree of energy integration is lower than in the conventional steam reforming process.Separate auxiliary boilers are necessary to provide steam for mechanical energy and powergeneration, because of the absence of hot reformer flue-gases. The NO x emitted in this processmainly originates from the flue-gas of the auxiliary boiler and fired preheaters. Centrifugalcompressors are used to compress air, oxygen needed in the gasifier, nitrogen used in the liquidnitrogen wash, ammonia used in the refrigeration system, and the make-up gas and recycle gasin the ammonia synthesis loop. Steam turbines and occasionally electrical energy are used todrive these compressors.2.2.5 Startup/shutdown and catalyst replacementStartup/shutdown operations, trip-conditions, leaks and fugitive sources cause periodicemissions. The initial startup is usually the most severe because of its duration. The normal ventpoints are the desulphuriser’s outlets, the high temperature shift reactor outlet, the CO 2 absorberinlet, the methanator inlet and outlet, the ammonia converter outlet and the purge from thesynthesis loop and refrigeration system. The pollutants comprise NO X , SO 2 , CO, H 2 , and naturalgas. Releases of NO x during synthesis gas flaring at startup or trips is estimated to be 10 – 20kg/hour as NO 2 [3, European Commission, 1997]. The ammonia plant is started up in stages.Nitrogen, heated by a primary reformer furnace is circulated and then steam is fed to thereformer from an auxiliary boiler. The feed gas is passed initially through the desulphuriser andthen to the reformer. The product gases are vented, subsequent parts of the process are thenstarted-up, with a subsequent venting of each stage. The converter in the synthesis loop isusually brought up to temperature using a fired startup heater. The full startup sequence maytake one or two days. Shutdown is a reverse of the startup procedure. Ammonia plants tend torun continuously for extended periods of time with only minor interruptions that require apartial shutdown. Shutdown due to technical failure happens on average 5.7 times per year.These procedures need atmospheric venting of large volumes of gases. The flows are usuallyless than half the full capacity flowrate. The main pollution control issue is how to dispose ofthese gases. One practice is to vent at a safe location. If loop purge gas is vented, it is usual tofirst remove the ammonia by scrubbing or other means. The alternative to venting at a safelocation is to flare the vent gases. The vent gases are strongly combustible as they containhydrogen, carbon monoxide, and methane. If not flared, spontaneous ignition may occur at thetop of the vent.All the catalysts in the plant require replacement at the end of their useful life. The life of eachcatalyst varies considerably with plant design [7, UK EA, 1999]. If zinc oxide is used as adesulphurisation guard bed, the resulting zinc sulphide will also need periodic disposal. Withessentially sulphur-free natural gas, the zinc oxide charge life may exceed 15 years. These solidsare normally removed from the site at a cost by a specialist contractor and taken for valuablemetal recovery and controlled final disposal.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 47