Effect of Natural Gas Composition on the Design of Natural Gas Liquid Demethanizers

The hydrocarbon composition of natural gas varies fairly significantly from location to location. The relative amounts of methane (C1) and ethane (C2) have a profound effect on the cryogenic high-pressure distillation column used to recover the C2 and heavier components as a bottoms product called “natural gas liquid” (NGL). This “demethanizer” column is a complex system that uses compression and expansion to achieve the required low temperatures (190 K) for achieving liquids at the operating pressure of 25 atm. Side-reboilers are used to cool the ambient-temperature natural-gas feed stream and to partially reboil the column. A multiunit heat exchanger network, multiple flash drums, and an expander are used to produce four different liquid and vapor feed streams to the column. This paper studies the effect of feed composition on the design of the demethanizer column. Natural gas feeds with C2 compositions ranging from 5 to 15 mol % are considered. Significant modifications in the design are required as the amount of NGL varies. For example, bypassing some of the feed around the side-reboilers is required when the C2 composition of the feed is high. High C2 feed compositions require more external refrigerant, and less power is generated in the expander. More NGL and less product gas is produced, so product compression work is lower.