In industrial ammonia (NH3) manufacture, when the hydrocarbon feed is fuel oil rather than natural gas, the make-up synthesis gas is chiefly generated by which process route?

Difficulty: Easy

Correct Answer: Partial oxidation

Explanation:


Introduction / Context:
Ammonia plants require a large stream of make-up synthesis gas containing hydrogen (and nitrogen after air addition). The route used to create this gas depends strongly on the hydrocarbon feed. Light feeds like natural gas are ideally handled by steam reforming. However, for heavier feeds such as fuel oil or vacuum residues, partial oxidation becomes the practical front-end. This question tests your ability to match the correct gas-generation technology to a heavy liquid feedstock.


Given Data / Assumptions:

  • Feedstock: heavy hydrocarbon (fuel oil), not methane-rich gas.
  • Target: produce synthesis gas suitable for downstream shift, CO2 removal, methanation, and ammonia synthesis.
  • Industrial constraints: coking tendency, heat balance, and reactor design for heavy feeds.


Concept / Approach:
Steam reforming excels with light hydrocarbons because they crack and mix with steam without severe coking at tube surfaces. Heavy liquids rapidly coke reformer tubes and burners, making conventional tubular steam reforming unsuitable. Partial oxidation (sub-stoichiometric combustion with oxygen) handles heavy feeds by generating a hot raw synthesis gas (H2, CO, CO2, H2O) in a refractory-lined vessel. The high temperature suppresses coke formation, and the downstream shift converts CO to more H2. Hence, for fuel oil, partial oxidation is the industry choice.


Step-by-Step Solution:
Identify feed as “heavy”—steam reforming is problematic due to coking.Recall partial oxidation uses O2 to partially burn the feed, yielding H2 + CO at very high temperature.Downstream: apply water–gas shift and CO2 removal to reach H2-rich gas for Haber–Bosch.Therefore, the correct selection is partial oxidation.


Verification / Alternative check:
Process lineups for oil-based ammonia plants specify POX (with an air separation unit for oxygen) rather than primary fired tubular steam reformers. Many designs may add a small steam addition (autothermal) but the core step remains partial oxidation for heavy feeds.


Why Other Options Are Wrong:
Steam reforming: ideal for methane/naphtha but not for fuel oil due to coking and heat transfer limits.Hydrocracking/hydrogenation: refining processes for upgrading oil, not for bulk synthesis-gas generation.


Common Pitfalls:
Assuming one universal front-end for all hydrocarbons; ignoring how feed quality dictates the gas-generation technology.


Final Answer:
Partial oxidation

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