This work is a continuation of a previous one, which focuses on characterizing given process configurations by their SIMAR curves. SIMAR, which stands for System Intrinsic Maximum Recovery, is a hypothetical operating scenario in which an expander-based recovery system can draw refrigeration freely from the environment. SIMAR operation is approximated when the feed stream to the system is LNG, because LNG contains abundant refrigeration that it can be treated as unlimited.

Given an expander-based C2 recovery configuration, this paper describes a systematic approach leading to the optimal operating conditions of the system. The difficulties in the traditional search task are caused by the intricate interplay of many system parameters. Consequently, extensive experiences of process engineers are relied upon to overcome the challenges. SIMAR serves as a guide leading directly to a short list of several combinations of process parameters. The final decision should include an economic objective function, which balances the cost of providing external refrigeration versus increasing the recompression duty of the system.

Existing plant design data in the open domain will be presented as illustrations. The conclusions and recommendations of this work will be useful guidelines for future developments in the C2 recovery systems. In addition, the art of designing expander-based processes will be propagated to general process engineers with the help of SIMAR.