Difficulty: Easy
Correct Answer: About 6
Explanation:
Introduction / Context:
Design and operation of reformers depend on the elemental composition of the hydrocarbon feed. The carbon-to-hydrogen ratio influences steam-to-carbon targets, reforming severity, and downstream CO2 removal loads. Naphtha used in legacy ammonia/hydrogen plants can be approximated by paraffinic compositions similar to the empirical unit “CH2,” which simplifies quick estimates.
Given Data / Assumptions:
Concept / Approach:
For the CH2 basis, the molecular mass is 14, with 12 units from carbon and 2 from hydrogen. Thus, the carbon mass to hydrogen mass ratio is 12:2 = 6:1, i.e., C/H by weight ≈ 6. This approximation aligns well with typical light naphtha reformer design calculations used historically in fertilizer complexes when exact ultimate analyses are not provided.
Step-by-Step Solution:
Assume naphtha ≈ (CH2)n for estimation.Compute mass ratio: C/H = 12 / 2 = 6.Match to nearest option: “About 6.”Note that real feeds vary, but 6 is the accepted exam benchmark.
Verification / Alternative check:
Ultimate analyses of paraffinic naphthas typically yield carbon mass fractions around 84–86% and hydrogen around 14–16%, again consistent with a C/H mass ratio near 6.
Why Other Options Are Wrong:
2: far too hydrogen-rich for naphtha; fits lighter species than CH2 average.13 or 20: imply extremely carbon-rich feeds more akin to asphaltenic/heavy oils, not light naphtha for reforming.
Common Pitfalls:
Confusing mass ratio with atomic ratio; using C/H atomic ratio (~0.5 for CH2) would mislead if not converted to mass basis.
Final Answer:
About 6
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