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8 min read
May 14, 2026

AAK Zhangjiagang: 10 Duplex Stainless Steel Heat Exchangers for Organic Solvent Separation

By Qiangbin Chu
Sales Manager & Operations Lead, Suzhou Lmart Energy Equipment

How a global oleochemical leader upgraded its distillation train—and what the dual-material strategy revealed about corrosion in fatty acid service.

The Challenge No Datasheet Prepares You For

The global heat exchanger market is growing at 7.0% CAGR through 2030, projected to reach $33.52 billion. But raw market numbers obscure the real story happening at plant level: the shift from austenitic 316L toward duplex stainless steels and nickel alloys is accelerating, driven by plant engineers who've seen 316L fail in service that was supposed to be "mild."

Organic solvent separation—fatty acids, fatty alcohols, methyl esters—sits in a deceptively challenging zone. Temperatures are moderate (80–160°C). Pressures are low. But the combination of chloride traces, organic acid attack, and condensing vapor creates a corrosion environment that punishes under-specified materials quietly, over 18–36 months, until a tube leak forces a shutdown.

AAK, one of the world's largest producers of specialty vegetable fats and oleochemicals, knew this. When they engineered a new pre-distillation and solvent separation train for their Zhangjiagang, China facility, material selection was not an afterthought.

We manufactured all 10 heat exchangers in the package. Here's what we learned.

The Project: 10 Units, Two Material Strategies

The full package covered the pre-distillation and solvent separation sequence:

Unit	Type	Length × Dia	Weight	Shell Material	Tube Material
Predistiller (6-pass heating)	Shell & tube	8,800 × 2,600 mm	7,140 kg	Duplex 2205	904L
Predistiller Condenser	Shell & tube	3,530 × 900 mm	4,500 kg	Q345R	Duplex 2205
Condenser / Steam Generator #1	Shell & tube	3,750 × 1,560 mm	3,298 kg	Q345R	Duplex 2205
Condenser / Steam Generator #2	Shell & tube	4,300 × 1,660 mm	3,516 kg	Q345R	Duplex 2205
Falling Film Evaporator	Falling film	10,600 × 1,400 mm	6,447 kg	Q345R	904L
Reboiler	Shell & tube	4,500 × 950 mm	5,004 kg	Q345R	904L
Final Condenser	Shell & tube	2,600 × 800 mm	5,200 kg	Q345R	Duplex 2205
Support units (×3)	Various	—	—	Q345R	Duplex 2205 / 904L

Standard: GB 151-2014 throughout. Witness testing at our facility by AAK's inspection team.

Why Two Different Corrosion-Resistant Alloys?

The most common question we get: why not just specify Duplex 2205 for everything?

The answer is that different streams in a fatty acid distillation train have different dominant corrosion mechanisms.

Where Duplex 2205 Is the Right Answer

In the condensers and pre-distillation condenser, the primary attack vectors are:
- Chloride stress corrosion cracking (SCC): Fatty acid feedstocks carry trace chlorides (typically 20–150 ppm). At condensation temperatures, chloride concentrations at the tube wall spike. Duplex 2205, with its ferrite-austenite microstructure, has substantially higher resistance to chloride SCC compared to 316L. The critical pitting temperature (CPT) of Duplex 2205 is approximately 35°C in ASTM G48 testing—well above the operating range where 316L becomes vulnerable.
- Erosion-corrosion in vapor service: Condensing organic vapors create a two-phase (vapor + liquid) flow at the tube inlet. Duplex 2205's higher hardness (HB 290 vs HB 160 for 316L) gives measurably better erosion resistance in this regime.

Where 904L Is the Better Tube Material

In the falling film evaporator and reboiler, the dominant corrosion mechanism shifts:
- Organic acid attack: Concentrated fatty acids in the liquid phase attack the passive film on standard stainless steel at elevated temperatures. The organic acid pitting resistance equivalent (PREN) of 316L is insufficient here.
- Hot wall corrosion: The falling film evaporator operates with a heated wall (steam on shell side, falling liquid film on tube outer surface). At the liquid film–wall interface, local concentration effects amplify acid attack. 904L's 4.5% molybdenum content and 25% nickel give it the passive film stability needed for continuous service in this environment.
- The main predistiller: The 8,800mm vessel runs a 6-pass heating configuration. The shell sees concentrated fatty acid vapor; the tubes carry heating medium. Here we used Duplex 2205 shell (for overall structural integrity and chloride resistance) with 904L tubes (for superior organic acid resistance on the process-contact surface).

The Fabrication Challenges

Duplex 2205 Welding: Ferrite-Austenite Balance Is Everything

Duplex 2205 performs as designed only when post-weld ferrite content stays between 35–55%. Outside this band:
- Too much ferrite (>60%): Brittle sigma phase formation at service temperature; loss of toughness
- Too little ferrite (<30%): Loss of chloride SCC resistance; approaches austenitic behavior

For the main predistiller shell (Duplex 2205, 2,600mm diameter, 6-pass configuration), we qualified the welding procedure per GB/T 4334 corrosion testing and ASTM A923 embrittlement check. Heat input was controlled to ≤1.5 kJ/mm. Inter-pass temperature was held below 100°C. Ferrite count was verified by magnetic induction gauge on 100% of longitudinal and circumferential welds.

904L Tube-to-Tubesheet Joints

904L's higher nickel-molybdenum content makes it more prone to hot cracking in the tube-to-tubesheet weld zone if heat input is excessive. For the falling film evaporator (10,600mm, 1,400mm dia), tube-to-tubesheet joints were qualified with full-strength welds per GB 151 Class A requirements. Each joint was subjected to leak test at 1.5× design pressure plus helium mass spectrometry leak check on critical circuits.

Delivery and Witness Testing

The 10-unit package was delivered in two shipments coordinated with the AAK site construction schedule. Key QC milestones:

Hydrostatic testing: All units tested at 1.5× design pressure. Test records submitted in Chinese and English per GB 151 documentation requirements
Material certification: Mill certs for all Duplex 2205 and 904L plate, tube, and fitting stock traced to heat number, with NACE MR0175 compliance statements where required
Witness points: AAK's inspection team witnessed tubesheet drilling, tube expansion, and final hydrotest at our Zhangjiagang workshop
Dimensional verification: All nozzle locations and support saddle positions verified against certified drawings before shipment

Total delivery: on schedule. No post-delivery non-conformances reported within the 18-month warranty period.

What This Project Confirmed About Material Selection

1. Dual-material strategies are cost-optimal, not premium.
Specifying 904L for all 10 units would have added approximately 35–40% to material cost with no benefit in the condenser service (where Duplex 2205 is already adequate for chloride SCC resistance). The targeted approach—904L where organic acid attack dominates, Duplex 2205 where chloride and erosion matter—delivered necessary corrosion resistance without unnecessary cost.

2. GB151 compliance for complex packages requires integrated documentation.
With 10 units on one project, managing inspection hold points, material traceability, and test records across simultaneous production streams is a coordination challenge as much as a technical one. AAK's inspection team was at our facility for a combined 6 witness visits. Having all documentation in a unified system prevented the hold point overruns that often delay multi-unit packages.

3. Falling film evaporator tube material selection deserves special attention.
The falling film evaporator is often under-specified because its operating conditions look mild on paper (low pressure, moderate temperature). The thin-film liquid environment, however, creates a concentrated corrosion scenario that 316L cannot handle in fatty acid service beyond 2–3 years. 904L is the correct specification here, and the 6,447 kg unit in this project has performed to expectation.

FAQ

Q: Why not use Super Duplex 2507 instead of standard Duplex 2205 for the condensers?
A: In this application, 2507 is over-specified for chloride concentration levels below 500 ppm and temperatures below 100°C. Duplex 2205 provides adequate PREN (≥35) for the operating conditions here. 2507 would be warranted in seawater service, brine concentrators, or offshore environments where chloride levels are an order of magnitude higher.

Q: Can Duplex 2205 be used for the falling film evaporator tubes in similar services?
A: In services where chloride SCC is the dominant risk and organic acid concentration is low, yes. For fatty acid / oleochemical service with concentrated organic acids in the liquid film, 904L's higher molybdenum and nickel give it better passive film stability. We recommend process-specific corrosion analysis before making this substitution.

Q: What welding standard applies to Duplex 2205 in GB151 pressure vessels?
A: GB 150.4 governs welding procedure qualification. For Duplex 2205, additional requirements apply: ferrite measurement per GB/T 1954, corrosion testing per GB/T 4334 Method E (Strauss Test), and sigma phase check per ASTM A923. We hold qualified WPS/PQR covering all common Duplex 2205 joint configurations.

Q: What is your typical delivery time for a 10-unit Duplex 2205/904L heat exchanger package?
A: For a package of this complexity (custom tubesheet design, multiple material grades, witnessed testing), 16–22 weeks from final drawing approval. Schedule depends on material procurement lead time for Duplex 2205 plate and 904L tube (typically 6–8 weeks from mill order).

AAK Zhangjiagang: 10 Duplex Stainless Steel Heat Exchangers for Organic Solvent Separation

The Challenge No Datasheet Prepares You For

The Project: 10 Units, Two Material Strategies