From bottleneck to breakthrough: revamping an LPG amine liquid-liquid extraction column

When a major Indian refinery's LPG treating unit became trapped at 31% of design capacity, Sulzer's systematic troubleshooting and targeted internals upgrade restored full performance and delivered measurable operational gains.

The challenge: operating at the limits

Mangalore Refinery and Petrochemicals Limited (MRPL) operate an LPG amine liquid-liquid extraction (LLE) column within their Crude Distillation Unit (CDU). The column's function is to remove hydrogen sulfide (H₂S) and other impurities from sour LPG using a lean methyldiethanolamine (MDEA) amine solvent. In this process, countercurrent contact between the immiscible sour LPG and aqueous amine phases enables effective mass Transfer. The treated (sweet) LPG, with significantly reduced H₂S content, is routed for downstream utilization, while the rich amine containing absorbed H₂S and other impurities is sent for regeneration to Amine Regeneration Unit (ARU).

In practice, however, the unit was far from delivering the capacity and product quality as compared to its design. Stable operation had become possible only at approximately 5.8 m³/hr of LPG — just 31% of the column's design capacity — and at amine flows limited to 55% of design capacity. Any attempt to push rates higher triggered a cascade of problems that made continued operation unsafe and economically unsustainable.

"The plant was forced to trade off between throughput and product quality. Neither could be achieved simultaneously within the existing configuration."

The symptoms were severe. Increasing LPG flow caused free amine to carry over into the downstream settler. LPG was ingressing into the rich amine stream, causing upsets in the ARU and triggering flaring events. Feed flow fluctuations made it impossible to maintain a stable interphase level, which amplified carryover further. Operators were forced to drain the settler boot approximately 48 times per day to manage the accumulation due to free amine carryover a significant operational burden that was not sustainable at scale.

Root cause analysis: identifying the underlying issues

Sulzer's engineering team applied a structured troubleshooting methodology, combining plant operating data with detailed hydraulic analysis of the existing column internals. The investigation revealed that the performance limitations were not simply a matter of insufficient capacity they were the result of multiple interacting internal design deficiencies.

Key findings included poor phase distribution, leading to maldistribution of both the continuous amine phase and the dispersed LPG droplets across the column cross-section. Excessive dispersed-phase velocity was generating unstable droplet populations that resisted coalescence. The existing tray geometry was unsuitable for the actual operating conditions, creating flooding tendencies at rates well below the design capacity. At the top of the column, insufficient residence time meant that amine droplets entrained in the LPG stream could not separate before reaching the settler explaining the chronic carryover issues.

These factors, acting together, had effectively capped the column's usable envelope far below its rated design. The challenge for the revamp was therefore not merely to repair individual items, but to redesign the hydraulic logic of the column as an integrated system.

The Sulzer solution: a precision revamp

Based on the diagnostic findings, Sulzer proposed a targeted set of internal upgrades designed to address each identified root cause without requiring any structural modifications to the column shell an important consideration for minimizing revamp cost and downtime.

• New DC coalescer at the column top
  A dedicated dispersed-phase coalescer was installed at the top of the column to improve phase separation and provide the residence time needed to prevent free amine from reaching the settler.
• New continuous phase distributor (ladder type)
  A new ladder-type distributor replaced the original configuration at the lean amine inlet, ensuring uniform distribution of the continuous phase across the full column cross-section.
• Modified tray geometry
  The existing sieve trays were redesigned to improve hydraulic performance at the actual operating conditions reducing flooding tendency, improving droplet generation, and expanding the stable operating envelope.
• New dispersed phase distributor (ladder type)
  A second ladder-type distributor was installed at the LPG inlet to ensure controlled, uniform introduction of the dispersed phase and eliminate the droplet instability observed at higher flow rates.

Performance after revamp: results that speak for themselves

Following installation of the new internals, the column's performance was validated through a formal Performance Guarantee Test Run (PGTR). The results confirmed that the revamp had not only resolved the operational problems but had restored and, in key metrics, exceeded the original design intent.

LPG throughput increased from 5.8 to 14.0 m³/hr a 141% improvement while treated LPG H₂S content was brought to below 6 ppmw, well within product specification. Free amine carryover was eliminated entirely, and LPG ingress into the rich amine stream ceased. Settler boot draining was reduced from 48 to just 2–3 times per day, representing a dramatic reduction in operational intervention.

An additional benefit emerged from the revised hydraulics: the amine-to-LPG ratio decreased from 0.19 to 0.11, meaning less rich amine is now returned to the regeneration unit. This directly reduces the reboiler energy demand in the ARU, delivering an ongoing energy saving that improves the overall plant efficiency and contributes to the refinery's sustainability objectives.

Beyond performance: operational and sustainability impact

The benefits of the revamp extend beyond the column itself. By eliminating LPG ingress into the rich amine, the project resolved the root cause of upstream ARU upsets and removed a significant source of unplanned flaring — a positive outcome both operationally and from an emissions standpoint. Amine losses are reduced, solvent costs are lower, and the reliability of the wider amine circuit has improved.

From a capital efficiency perspective, the revamp was achieved through targeted internal modifications rather than a full column replacement, delivering a fast return on investment with minimal disruption to plant operations during installation.

Smart revamps address root causes, not symptoms — and unlock both immediate performance gains and long-term operational value without the cost or downtime of full equipment replacement.

Conclusion

The MRPL LPG amine extraction column project demonstrates the value of a rigorous, data-driven revamp approach. By combining detailed hydraulic analysis with targeted upgrades to column internals — including a new DC coalescer, ladder distributors, and optimized sieve trays — Sulzer restored the column to design capacity, eliminated carryover and ingress issues, and generated measurable improvements in energy consumption and product quality.

For refineries experiencing similar constraints in LLE or amine treating operations, this project illustrates what is achievable through the right combination of diagnostic expertise and proprietary internal design capabilities.