Hydrocarbon Feed Purification for Ammonia Plants
By: Gerard B. Hawkins
Managing Director, CEO
Purification
• Need to remove poisons prior to entering downstream catalyst beds, including • Pre reformers • Primary reformers • HTS • LTS
• Note : no Secondary - poisons do not stick as temperature is too high
• Note that methanator is a purification step • Removes CO and CO2 which poisons
synthesis catalyst
Effect of Poisons Reformer catalyst poisoning
• Increased methane slip Low plant efficiency
• Hot tubes Tube life reduction or tube failure
• Carbon formation Increased pressure drop Increased methane slip and hot tubes
LT Shift catalyst poisoning • Reduced life • Premature plant S/D due to high CO slip
and high pressure drop
Poisons Limits • Steam reforming catalyst requirements
• Process gas feed to reformer (dry basis) Sulfur <0.05-0.1 ppm Poison Chlorides <0.05-0.1 ppm Poison As / V / Pb / Hg<5ppb Poison Olefins <1-2 % Carbon
Formation
• LT shift catalyst requirements
• Process gas feed to LT Shift (dry basis)
Sulfur <0.1 ppm Poison
Chlorides < 5 ppb Severe poison
Effect of Chlorides
Most common as HCl or highly mobile ion Remove to less than 5 ppb Accelerates sintering in catalyst metal
crystallites Deactivates reforming catalyst, most lethal
to LT Shift catalyst In process air added to secondary Weakens catalyst
Typical Configuration
Hydrocarbon Feed
Sulfur Removal
Hydrogenation
Typical Configuration Hydrocarbon Feed
Hydrogenation
Sulfur Removal
Chloride Removal
Hydrocarbon Feed
Hydrogenation Chloride Removal
Sulfur Removal
To Reformer