Demethanizer Revamp
Demethanizer Revamp Improves Separation While Reducing Recycle Load
A distribution-focused internal retrofit restored fractionation performance and enabled operation above nameplate capacity.
CLIENT: Braskem Idesa | APPLICATION: Ethylene-plant demethanizer
Full technical paper. Download the AIChE paper “Revamping a Demethanizer for Improved Separation and Energy Performance: Lessons Learned from Two Internal Retrofits” for the detailed methodology, configuration drawings and operating-data comparisons.
130 t/h
post-revamp case evaluated
61–64 t/h
top recycle flow
~500 ppm
methane in bottoms
−75°C
design overhead temperature reached
Executive Summary
Braskem Idesa’s demethanizer had experienced persistent hydraulic and separation limitations. The original 48-tray configuration flooded before the ethylene plant could operate steadily at its 125 t/h nameplate rate. A first retrofit completed before GTI’s involvement reduced column pressure drop and enabled nameplate throughput, but the packed configuration did not deliver the expected fractionation performance. Top recycle flow increased, methane in the bottoms remained high and the additional recycle load increased demand on the cracked gas compressor.
GTI Solutions and Braskem Idesa initiated a detailed study in 2024 and implemented a second internal retrofit during the 2025 turnaround. The revamp addressed flashing-feed momentum, excessive packing-bed height, liquid distribution and reboiler-return vapor distribution as one integrated system. Post-revamp evaluation included a 130 t/h operating case, reduced top recycle flow from at least 85 t/h to 61–64 t/h and lowered methane in the bottoms from 1,600–1,750 ppm to approximately 500 ppm.
The Challenge
The original demethanizer was designed with 48 conventional trays. From plant start-up, hydraulic flooding and high pressure drop prevented stable operation at full production. Before GTI became involved, the technology licensor and incumbent internals supplier developed a 2018 retrofit that replaced most of the trays with three beds of high-capacity random packing while retaining two trays at the top.
The first retrofit successfully relieved the hydraulic constraint, but separation performance deteriorated. Methane slip to the bottoms increased, recycle to the cracked gas compressor rose substantially and downstream operating flexibility was reduced. The experience showed that adding hydraulic capacity alone could not correct deficiencies in phase distribution and mass-transfer performance.
The GTI review identified four interacting causes: the cold-box feed entered at approximately 90% vapor fraction and with excessive momentum; the lowest packing bed was approximately 8.9 m (29 ft) tall; the existing channel collector did not distribute reboiler-return vapor uniformly; and the seal pan and adjacent baffles obstructed more than 55% of the column cross-sectional area. A temperature gradient of less than 1°C across the bottom bed confirmed that the section was providing little effective fractionation.
GTI's Integrated Scope
GTI treated the project as a complete column-performance investigation, coordinating process diagnosis, internal equipment design and turnaround-ready mechanical modifications around the client’s operating objectives.
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Operating-data review and diagnostics. Reconciled plant data, reviewed temperature profiles and gamma-scan findings, and established the performance baseline.
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Feed and distribution engineering. Evaluated feed momentum and flashing behavior and redesigned the feed galleries, liquid distributors and vapor-distribution system.
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Packed-bed redesign. Reduced the effective height of the lowest bed through intermediate redistribution and selected G-SR™ high-performance random packing for the new lower beds.
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Mechanical engineering and equipment supply. Engineered new support arrangements, feed devices, distributors and the DCT-210 riser collector for installation without direct welding to the vessel shell.
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Post-start-up performance evaluation. Compared five operating scenarios and verified the achieved capacity, recycle, methane-slip and temperature-profile improvements.
The Engineered Solution
GTI redesigned the internal system to improve both vapor and liquid distribution. Beds 1 and 2 received new flashing-feed galleries and liquid distributors, with V-baffle diffusers added in Bed 2. The existing packing in these sections was inspected and reused.
The original 8.9 m bottom bed was divided into two shorter beds—Beds 3 and 4—with new liquid distributors and G-SR™ high-performance random packing. This reduced the distance over which maldistribution could develop and introduced intermediate redistribution to improve vapor–liquid contacting.
At the column bottom, the existing channel collector was replaced with a DCT-210 riser collector, and an obstructing upper guide baffle was removed. Cylindrical risers incorporating restriction plates promoted vapor flow across the full column cross-section. The mechanical support arrangement avoided direct welding to the pressure-retaining shell.
Turnaround Implementation
The new internal configuration was implemented during the 2025 turnaround. Reusing suitable packing in the upper beds limited unnecessary replacement, while the lower packed section and column-bottom distribution system were rebuilt to address the identified root causes.
New flashing-feed gallery and split-bed configuration
G-SR™ high-performance random packing and DCT-210 riser collector
The final arrangement combined targeted reuse with new mass-transfer equipment and distribution hardware. The non-welded support strategy reduced mechanical execution risk while allowing the required internal modifications to be completed within the existing vessel.
Verified Results
Operation above nameplate capacity was evaluated. The post-revamp evaluation included a 130 t/h case compared with the 125 t/h plant nameplate rate.
Top recycle flow decreased to 61–64 t/h. Before the 2025 revamp, recycle flow was at least 85 t/h, increasing load on the cracked gas compressor.
Methane in the bottoms fell to approximately 500 ppm. The pre-revamp range was approximately 1,600–1,750 ppm.
The column temperature profile improved. The bottom packed-section ΔT increased from approximately 1°C to 4°C, while the top-section ΔT increased from 16°C to 23°C.
The overhead temperature reached the −75°C design value. The improved temperature profile confirmed more effective vapor–liquid contacting throughout the column.
The result: higher production with improved fractionation, lower recycle load and substantially less methane slip.
Why the Revamp Succeeded
GTI did not treat the project as a packing replacement alone. The troubleshooting process connected operating data with feed behavior, packed-bed geometry, liquid distribution and column-bottom vapor flow. By addressing these interacting limitations as one system, the revamp converted available hydraulic capacity into measurable separation and energy-performance gains.
Engineering Lessons
Hydraulic capacity and separation efficiency must be evaluated together. The first retrofit reduced pressure drop, but the operating data showed that additional open area alone could not restore fractionation.
Distribution quality is fundamental to packed-column performance. The second revamp addressed feed momentum, liquid distribution, bed height and reboiler-return vapor flow as interconnected design variables.
Temperature profiles provide a practical performance diagnostic. The stronger post-revamp temperature gradients across the packed sections were consistent with improved vapor–liquid contacting and lower methane slip.
Mechanical execution should be integrated from the start. Equipment reuse, modular internals and non-welded supports helped translate the process design into a turnaround-ready retrofit.
Experiencing high recycle, methane slip or poor fractionation?
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