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

Chapter 22.4.25 Handling of startup, shutdown and abnormal operatingconditionsDescriptionDuring startup and shutdown higher emissions occur than during normal operation. The initialstartup is usually the most severe because of its duration. Ammonia plants are started up orshutdown in a controlled sequence, which results in reformed and synthesis gases to be ventedthrough various vents of the plant. It may also be necessary to vent natural gas during thestartup of the desulphurisation section (see also Section 2.2.5).Options to minimise emissions include the following:• minimisation of the startup and shutdown time by using interlocks and a logical operationalsequence• use of recycled inert gases for preheating• application of maximum allowed prudent preheat rates for equipment and catalysts• reduction of the low temperature shift catalyst with an inert gas carrier• taking the synloop into operation as quickly as possible• flaring of non treatable vent gases.Increased emissions can result from emergency shutdowns due to equipment failure, upsets inprocess conditions, human error, and force majeure. Proper risk analysis is carried out toprovide the necessary preventive or control sequence in case of emergency. Instrumentinterlocks, emergency backup equipment such as batteries, sparing of equipment, instrumentvoting systems, adequate system inventory, computer control, and scrubbers are all used toavoid or minimise emissions during an emergency. Proper shutdown procedures are establishedto prevent formation of the toxic nickel carbonyl from gases containing carbon monoxide.These gases are normally excluded from methanators containing nickel catalysts at anytemperature below 150 °C (below this temperature, nickel carbonyl is formed).Achieved environmental benefitsReduced emissions.Cross-media effectsNone believed likely.Operational dataNo information provided.ApplicabilityGenerally applicable.EconomicsNo information provided.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 89