Why purge the synthesis loop? In an ammonia plant, what is the primary purpose of a small purge from the synthesis gas recycle loop?

Introduction / Context:
Ammonia synthesis loops recycle unconverted hydrogen and nitrogen at high pressure over a catalyst bed. Inevitably, small amounts of inert gases (argon from air, methane from feed impurities, residual neon/helium traces) enter the loop. Because these species do not react to form NH3, they accumulate. A controlled purge is therefore required to stabilize loop composition and operating pressure drop.

Given Data / Assumptions:

• The loop includes compressor, converter, heat recovery, condenser/separator, and recycle circuitry.
• Upstream purification removes CO, CO2, H2O, and sulphur; remaining inerts are mainly Ar and CH4.
• Purge gas is often sent to a recovery unit (e.g., hydrogen recovery by PSA) to minimize losses.

Concept / Approach:
Without purging, inert gases build up, diluting reactive components, lowering partial pressures of H2 and N2, and reducing per-pass conversion. Accumulation can also increase compressor load and off-design operation. A small steady purge bleeds inerts from the loop, restoring the intended composition. The H2:N2 ratio is maintained by upstream blending and controls, not by purging reactants wholesale. Ammonia vapour is condensed and separated, not purged intentionally with the loop gas.

Step-by-Step Solution:

Verification / Alternative check:
Standard loop material balances show inert mole fraction approaching an asymptote determined by purge fraction; designers select a purge that balances hydrogen loss against acceptable inert loading.

Why Other Options Are Wrong:

• Removing “poisonous catalysts” is nonsensical; catalysts are fixed in beds, not in the gas stream.
• Exact 3:1 H2:N2 ratio is controlled by feed blending, not by purge alone.
• Uncondensed NH3 is minimized by refrigeration/absorption, not purged deliberately.
• CO2 and CO should be removed upstream; methanator protection does not rely on loop purging.

Common Pitfalls:
Confusing purge duty with ammonia condensation or with upstream purification; assuming purge is a crude control for feed ratios rather than an inert-management tool.

Final Answer:
To maintain the concentration of inert gases within acceptable limits