Hydrogen for nitrogenous fertilizers: In steam reforming of naphtha to produce hydrogen, what is the usual steam-to-carbon (S/C) molar ratio maintained in the reformer feed?

Introduction / Context:
Hydrogen is a key feedstock for ammonia synthesis in the nitrogenous fertilizer industry. Steam reforming converts hydrocarbons and steam into synthesis gas. The steam-to-carbon (S/C) ratio is a critical operating parameter affecting coke formation, equilibrium, and downstream shift conversion.

Given Data / Assumptions:

• Feedstock: naphtha (heavier than natural gas).
• Catalytic steam reforming process with downstream shift.
• Usual industrial practice, not extreme coking or water-limited cases.

Concept / Approach:
Adequate steam suppresses coking on the reforming catalyst and drives equilibria toward hydrogen and carbon oxides. For light feeds (natural gas), S/C around 3 is typical. For heavier feeds like naphtha, a slightly higher S/C—about 3.5—is commonly used to mitigate coking while maintaining energy efficiency.

Step-by-Step Solution:
Recognize heavier feed → higher coking tendency.Choose S/C to balance coke suppression and heat duty → ~3.5.Match to provided options: 3.5 : 1.

Verification / Alternative check:
Process design manuals list S/C ≈ 3.0 for methane and ≈ 3.5 for naphtha, consistent with field operation to protect nickel catalysts.

Why Other Options Are Wrong:
1.5 : 1 risks severe coking; 10 : 1 or 15 : 1 are unnecessarily high, penalizing energy and equipment size.

Common Pitfalls:

• Running too low S/C to save steam, which shortens catalyst life.
• Ignoring feed quality; heavier or sulfur-laden feeds may need higher S/C or pre-treatment.

Final Answer:
3.5 : 1