Lessons learned

Demethanisers play a critical role in ethylene plants by separating methane from valuable C2+ hydrocarbons. Poor column performance can significantly increase energy consumption, restrict plant throughput, and negatively impact downstream unit reliability. The case study in this article presents a systematic troubleshooting and revamp approach for a demethaniser column that experienced chronic hydraulic limitations from the time of plant startup. Through two major internal retrofits, the project highlights the importance of balancing hydraulic capacity with mass transfer efficiency and demonstrates how targeted internal modifications can restore both throughput and separation performance.

Original column design and operating challenges

The demethaniser column (20-T-2401) was originally designed with 48 conventional trays, consisting of three single-pass trays in the upper section and 45 two-pass trays below. The upper column dia. was 3000 mm, expanding to 4000 mm at the bottom, with 600 mm tray spacing throughout. The column separates methane and lighter components overhead while recovering C2+ hydrocarbons in the bottoms product. The design capacity was based on an ethylene production rate of 125 tph, with a methane specification of 200 ppm in the bottoms product and a top recycle rate of 32 tph.

From startup, however, the column experienced flooding at approximately 95% of design capacity, preventing stable full-rate operation. To reduce hydraulic load, furnace severity was increased to alter cracked gas composition, but the column continued to exhibit severe constraints.

Gamma scan analysis confirmed the presence of internal hydraulic maldistribution within the tray section. During a scheduled shutdown in 2016, internal inspection showed that the trays were clean and installed according to original drawings, suggesting that the root cause was design-related rather than mechanical damage or fouling.

Following a series of technical reviews with the technology licensor and the tray supplier, the tray supplier developed an independent process simulation using the original design data provided by the licensor. The resulting load and physical property profiles closely matched those predicted by the original design model.

A detailed hydraulic assessment subsequently identified a critical restriction near the bottom of the column. Between the inclined partition baffle and the long sump baffle, the reboiler return stream was forced through a narrow opening where both liquid and entrained vapour competed for limited flow area. Because no dedicated vapour vent was provided, vapour interference impeded liquid drainage, resulting in liquid accumulation, localised foaming, and carryover into the bottom tray. This bottleneck was ultimately identified as the primary cause of flooding.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/21082026/lessons-learned/