The correct answer is C (Sweep through purging) because it is the least efficient inerting method in terms of inert gas consumption , as described in CCPS guidance on inerting and purging practices.

In sweep through purging , nitrogen is continuously introduced into the vessel while the existing gas mixture is displaced and vented. This method relies on dilution rather than compression or evacuation, meaning that large volumes of nitrogen are required to achieve the desired reduction in oxygen concentration. The inefficiency arises because complete mixing is assumed, and multiple vessel volume exchanges are needed.

In contrast, pressure purging (B) and vacuum purging (A) are significantly more efficient. Pressure purging uses repeated pressurization and venting cycles to reduce oxygen concentration, while vacuum purging removes gases before backfilling with nitrogen. The most efficient method is typically combined vacuum/pressure purging (D) , which minimizes nitrogen usage by leveraging both techniques.

Siphon purging (E) is not a commonly recognized CCPS inerting method for gas-phase systems.

CCPS emphasizes selecting inerting methods based on vessel design, pressure capability, safety requirements, and gas consumption efficiency. For large vessels, minimizing nitrogen usage is often important for both cost and operational practicality.