ANS:C - maintain H₂ : N₂ ratio at 3 :1.

In an ammonia plant, the purge off is essential to remove uncondensed ammonia vapor. Explanation:

1. Purpose of Purge Off: The purge off process in an ammonia plant is crucial for removing any uncondensed or excess ammonia vapor from various stages of the ammonia synthesis loop, including the synthesis reactor, high-pressure separator, and recycle gas compressor.
2. Preventing Accumulation: During the synthesis of ammonia from nitrogen and hydrogen, there may be instances where not all of the ammonia vaporizes or condenses. This uncondensed ammonia vapor can accumulate in the process equipment, leading to potential safety hazards and operational inefficiencies.
3. Safety Considerations: Ammonia is toxic and poses health risks if inhaled in high concentrations. Therefore, it's essential to remove any uncondensed ammonia vapor to maintain safe working conditions for personnel and prevent environmental pollution.
4. Process Efficiency: Removing uncondensed ammonia vapor through the purge off process helps maintain the proper balance of reactants and products in the synthesis loop. This ensures optimal process efficiency and product quality in the production of ammonia.
5. Inert Gas Concentration and Gas Ratios: While maintaining inert gas concentration and gas ratios are important considerations in ammonia plant operations, the primary purpose of the purge off is specifically to remove uncondensed ammonia vapor rather than inert gases or to adjust gas ratios.