Manufacturing 27-Ton Phthalic Anhydride Reactors: Engineering a DN3550 Multi-Tube Shell-and-Tube Vessel for a European Specialty Chemical Project

Asia-Pacific's petrochemical expansion is reshaping who can build what. As new PA synthesis capacities come online across China — the world's largest phthalic anhydride producer — project owners are discovering that the bottleneck is not the chemistry. It is finding fabricators capable of building the reactors.

A few years ago, Lmart was approached by the engineering procurement team of a leading European specialty chemical company for a PA synthesis project in China. The brief: multi-tube fixed-tube-sheet reactors, nominal diameter 3,550 mm, weight approximately 27 metric tons each, designed to handle the highly exothermic o-xylene oxidation reaction with molten salt bath cooling, operating at 350–420°C. The fabrication scope required demonstrated experience with heavy-wall carbon steel rolling, large-diameter circumferential weld alignment, mass tube bundle installation, post-weld heat treatment (PWHT) in a full-vessel furnace, and hydrostatic testing of a 27-ton assembly.

This article documents the key engineering and fabrication challenges of that project and how Lmart's manufacturing capabilities — built around ASME U stamp and PED certification — were applied to deliver reactors that met the client's exacting quality plan.

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Background: Why Phthalic Anhydride Reactors Are Hard to Build

Phthalic anhydride (PA) is produced by the catalytic oxidation of o-xylene or naphthalene in air. The reaction is intensely exothermic (ΔH ≈ −1,285 kJ/mol for o-xylene), which demands precision temperature control. The standard industrial solution is a multi-tube fixed-bed reactor in which thousands of reaction tubes (typically 25 mm OD, ~3–4 m long) are packed with V₂O₅-based catalyst and submerged in a circulating molten salt bath — a eutectic mixture of sodium nitrite and potassium nitrate — that absorbs and removes the reaction heat at tightly controlled temperature windows (typically 340–420°C on the salt side).

The reactor shell is essentially a large-diameter pressure vessel that simultaneously serves as:

1. A structural containment vessel for the molten salt bath (operating at near-atmospheric pressure, high temperature)
2. A process-side pressure vessel for the gas stream passing through the tubes (moderate gauge pressure, high temperature)
3. A heat transfer chassis — tube-to-tubesheet joints and tube pitch uniformity directly determine thermal performance

This combination of scale, temperature, and precision tube bundle geometry makes PA reactors one of the most technically demanding categories of shell-and-tube equipment. Standard fabricators who routinely build TEMA-class heat exchangers in the DN600–DN1200 range face fundamental capability gaps when the diameter jumps to DN3000+.

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Challenge 1: Heavy Plate Rolling and Circumferential Seam Alignment at DN3550

The Rolling Problem

For a nominal diameter of 3,550 mm, the shell course plates — in carbon steel with wall thicknesses typically in the 28–45 mm range for this service — must be rolled with dimensional accuracy that survives assembly. The critical specification for this project was a circumferential seam alignment (edge offset, or "hi-lo") tolerance of less than 1 mm over a 3,550 mm diameter.

This tolerance is not arbitrary. At large diameters, misalignment at circumferential seams concentrates stress at the weld root. Under cyclic thermal loading — every startup and shutdown cycles the reactor through a temperature swing of 350+°C — a 2–3 mm hi-lo that would be acceptable in a water service heat exchanger can initiate fatigue cracking in a thermal cycling application.

Lmart's Approach

Lmart's plate rolling capacity supports shell diameters up to approximately 5,000 mm and plate thicknesses consistent with the heavy-wall schedule specified here. The key process controls applied:

Pre-rolling inspection: Each plate was verified for flatness and edge squareness before entering the rolls. Edge deviations >0.5 mm were corrected by grinding before rolling.

Incremental rolling with template checks: Plates were rolled in multiple passes. After each pass, a precision radius template (fabricated to the nominal inside radius ± 0.2 mm) was applied at eight positions around the circumference to verify the developing curvature. Plates not meeting template contact criteria were re-rolled before proceeding.

Tack-weld and dimensional check cycle: Prior to full-seam welding, the rolled shell course was tack-welded, then measured by the metrology team using a calibrated inside diameter tape and diametrically opposite point measurement at four cross-sections. Ovality (difference between max and min diameter readings at any cross-section) was verified against the allowed tolerance before welding proceeded.

Circumferential seam fit-up: Longitudinal seams in adjacent courses were staggered by a minimum of 100 mm arc length. Circumferential seam fit-up used internal alignment clamps with continuous feeler gauge measurement to hold hi-lo below 1 mm before tack welding.

The result: shell roundness and seam alignment met the project quality plan at every stage inspection, verified by the client's third-party inspection agency.

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Challenge 2: Multi-Thousand Tube Bundle Installation

Scale of the Task

Large PA reactors in the 3,000–3,550 mm diameter range typically contain between 15,000 and 20,000 reaction tubes. For this project, the tube bundle comprised thousands of tubes, each requiring:

• Proper seating in tubesheet holes (controlled-clearance fit)
• Full-penetration or fillet weld at both tubesheets (per the applicable code and client weld specification)
• Expanded joint at both ends (roller expansion or hydraulic expansion per design)
• Dimensional verification (projection length, axial position)

The tube-to-tubesheet joint is the most safety-critical weld in the assembly. A failed joint under operating conditions allows process gas to enter the molten salt side, with potential for explosive reaction. The client's inspection plan called for 100% visual inspection of all tube-to-tubesheet welds and a defined sample rate of radiographic or dye-penetrant examination.

Assembly Sequence and Quality Controls

Tubesheet inspection first: Both tubesheets were dimensionally verified (hole pitch, hole diameter, perpendicularity) before tube insertion. Any hole with burrs or dimensional deviation was corrected.

Tube sorting and pre-marking: All tubes in a lot were measured for OD, wall thickness, and length before installation. Tubes outside the tolerance band were segregated. Each tube received a batch identity mark for traceability.

Guided insertion tooling: Tubes were inserted using alignment guides that prevented scoring of the tubesheet hole bore on entry. Insertion force was monitored; tubes requiring abnormal force were pulled and the hole examined.

Expansion procedure qualification: The expansion procedure (roller type, torque setting, or hydraulic mandrel pressure) was qualified on a mock-up tubesheet coupon before production. Hardness testing on the expanded zone confirmed the expansion was within the qualified range.

Welding sequence: Tube-to-tubesheet welding was performed by qualified welders using a rotary fixture where possible, with documented procedure qualification records (PQR) and welder performance qualification records (WQPR) per the applicable code.

100% weld inspection: Every tube-to-tubesheet weld was visually inspected. The random sample selected for NDE was examined by dye-penetrant testing (PT) to the criteria in the inspection plan. All indications were assessed and documented.

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Challenge 3: Post-Weld Heat Treatment (PWHT) of a Full DN3550 Assembly

Why PWHT Is Non-Negotiable for This Service

Carbon steel and low-alloy steel weldments accumulate residual stresses during welding. For a vessel operating at 350–420°C in continuous service with thermal cycling, unrelieved residual stresses contribute to stress corrosion cracking risk and accelerate fatigue crack initiation at weld toes. For heavy-wall carbon steel above a threshold thickness (typically 38 mm per ASME VIII Div. 1 UCS-56), PWHT is code-mandatory regardless of service. For this project, the combination of wall thickness and service temperature made PWHT a hard requirement.

The challenge at DN3550 and approximately 27 tons is purely physical: the vessel must fit inside a furnace large enough to heat it uniformly to the soak temperature (typically 595–650°C for carbon steel per ASME), hold it for the required duration (based on material and thickness), and cool it at the controlled rate specified by the code (typically ≤ 55°C/hour through the cooling zone above 400°C).

Furnace Capacity and Thermal Uniformity

Lmart operates furnace capacity capable of accepting large vessels in this weight and envelope class. For this project:

Furnace envelope verification: The vessel's maximum cross-section (approximately DN3550 + nozzle projections) was verified against the furnace interior dimensions with clearance margins meeting the code requirement for thermocouple access and heating element clearance.

Thermocouple placement: Multiple thermocouples were attached to the vessel surface at locations specified by the PWHT procedure: top of shell, bottom of shell, mid-length, each tubesheet face, and representative nozzle connections. The thermocouple placement plan was reviewed and accepted by the authorized inspection agency (AI) before PWHT.

Heating and cooling rate control: The furnace control system logged thermocouple readings throughout the cycle. The heating rate was controlled to avoid differential temperature exceeding 55°C between any two thermocouples on the vessel during the heating phase. The soak temperature band was maintained across all thermocouples during the entire hold period.

Post-PWHT verification: After PWHT, hardness testing was performed on weld zones and heat-affected zones per the procedure to confirm that the material properties met the required specifications.

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Challenge 4: Hydrostatic Testing and Dimensional Verification at 27 Tons

The Hydrostatic Test Challenge

Hydrostatic testing of a 27-ton vessel presents logistics that smaller equipment never requires. The test pressure for a typical PA reactor design — calculated per ASME VIII Div. 1 or PED per the project-specific design code — results in a shell-side (salt bath side) and tube-side (process gas side) test conducted separately, with different test pressures for each side.

At 27 tons, the vessel must be supported on purpose-built test cradles that:

1. Distribute the vessel weight plus the water weight (the shell volume, when filled with water for testing, adds significant mass)
2. Allow access to all nozzle connections for pressure application and venting
3. Allow rotation or repositioning if needed to ensure complete filling and air venting

Lmart's Testing Protocol

Cradle design: Purpose-designed saddle supports were fabricated for this project, sized to the vessel's shell OD and positioned per a stability calculation. The saddle width and contact angle were selected to limit bearing stress on the shell below the code-required limits.

Filling and venting procedure: The fill sequence was documented to ensure all high points in the vessel were vented before pressure was applied. A dedicated vent valve at the highest point was monitored during filling.

Pressure application and hold: Test pressure was applied in steps (typically 25%, 50%, 75%, 100% of test pressure) with inspection at each step. The hold period at full test pressure met the code requirement. During the hold, all external surfaces, nozzle welds, and flange faces were inspected for leakage.

Dimensional verification after testing: After hydrostatic testing, the vessel was re-measured for overall length, nozzle centerline positions, and flange face flatness. Comparison with the dimensional report from before PWHT confirmed that neither PWHT nor hydrostatic testing had introduced dimensional distortion beyond the acceptance criteria.

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Materials and Welding: Key Decisions

Shell and Head Material

The shell and dished heads were fabricated in carbon steel (material grade per ASME or EN designation per the applicable code), selected for:

• Sufficient elevated-temperature strength up to the design temperature
• Compatibility with molten salt (nitrate-nitrite eutectic) on the shell side
• PWHT response that achieves the target hardness and toughness combination
• Weldability with qualified procedures using low-hydrogen electrodes or submerged arc welding (SAW)

For the cylindrical shell courses, SAW was the primary welding process for longitudinal and circumferential seams, with SMAW or FCAW used for joint access locations. All weld procedures were qualified per the applicable code.

Tube Bundle Material

The reaction tubes in PA service are typically in carbon steel or low-alloy steel, depending on the design temperature and the client's corrosion allowance requirements. Tube material certification (mill certificates with heat and lot traceability) was required by the client's material control plan.

Dissimilar Joint Considerations

Where nozzle materials or ratings differed from the shell grade, weld procedure qualification addressed the dissimilar joint chemistry and post-weld mechanical properties.

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Quality System and Third-Party Inspection

For this project, the client's quality plan specified:

• Authorized Inspection Agency (AI) oversight throughout fabrication, as required by ASME U stamp program
• Client-nominated third-party inspector with hold points at: plate material receipt, weld procedure qualification, tubesheet fabrication, tube insertion completion, PWHT, hydrostatic test, and final dimensional inspection
• Full radiographic examination (RT) of all pressure-bearing butt welds above a defined thickness threshold
• Documented NCR process for any nonconformance identified during inspection

Lmart's ISO 9001-certified quality management system provided the documentation infrastructure: traveler documents, welder certifications, NDE records, heat treatment charts, test records, and dimensional reports were compiled into the final Data Book delivered to the client at project close.

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Project Outcome and Delivery

The PA synthesis reactors fabricated for this project were delivered on schedule, meeting all dimensional, mechanical, and non-destructive examination requirements of the client's inspection plan. The third-party inspection agency's final inspection report recorded zero outstanding nonconformances at the time of release.

Post-delivery feedback from the client's engineering team confirmed that the reactors were installed without field modification requirements — a measure of dimensional accuracy that matters when a 27-ton vessel must align with existing plant structural steel and interconnecting piping.

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Implications for the Asia-Pacific Chemical Reactor Market

China's position as the world's largest PA producer — accounting for more than half of global phthalic anhydride production capacity — means that capacity additions and technology upgrades in this market are continuous. European and North American chemical companies operating in China, as well as domestic Chinese chemical companies pursuing technology upgrades, require fabricators who can:

1. Meet international code requirements (ASME VIII, PED) with an active stamp program and audit-ready documentation
2. Handle vessel sizes (DN2500 to DN4000+) that exceed the capability of most workshop-level fabricators
3. Manage complex sub-operations — large furnace PWHT, multi-thousand tube bundle assembly, large-diameter rolling — within a single controlled manufacturing environment

The PA synthesis reactor project described in this article illustrates that these requirements can be met by a Chinese manufacturer with the right equipment infrastructure, certified quality system, and experienced engineering team — without the cost and logistics of sourcing from Western Europe or North America.

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Frequently Asked Questions

Q: What is the maximum reactor diameter Lmart can fabricate?
A: Lmart's rolling and handling capacity supports shell diameters up to approximately 5,000 mm. Furnace capacity for PWHT is matched to this envelope. For diameters above DN3000, we recommend early engagement to review specific dimensional requirements against current capacity availability.

Q: Can Lmart supply reactors to PED (European Pressure Equipment Directive) instead of ASME?
A: Yes. Lmart holds both ASME U stamp and PED certification. The applicable design code — ASME VIII Div. 1, EN 13445, or another recognized standard — is agreed with the client during the engineering phase. Both code paths share the same quality management infrastructure.

Q: What is the typical delivery schedule for a DN3550-class reactor?
A: Delivery schedule depends on engineering finalization, material procurement lead time, and the queue position in the fabrication schedule. For reactors of this complexity, total fabrication time from drawing release to final inspection is typically in the range of 6–10 months. Schedule risk is most commonly concentrated in heavy-wall plate procurement and furnace PWHT scheduling.

Q: How does Lmart handle tube-to-tubesheet joint qualification for very large tube counts?
A: Procedure qualification is conducted on representative mock-up coupons before production. The expansion procedure, welding procedure, and inspection method are all qualified on coupons that replicate the actual tubesheet material, hole pattern, and tube material. Production tube-to-tubesheet work does not begin until procedure qualification is accepted by the AI.

Q: What NDE methods are applied to PA reactor fabrications?
A: Depending on the design code and client specification, NDE typically includes: 100% RT or UT on pressure-bearing butt welds; 100% visual and PT or MT on tube-to-tubesheet welds; PT or MT on nozzle-to-shell welds; and dimensional verification at defined hold points. The NDE plan is prepared by Lmart and reviewed/approved by the client's inspector and the AI.

Q: Does Lmart have experience with molten salt service vessels beyond PA reactors?
A: Yes. Lmart has fabricated shell-and-tube heat exchangers and reactor vessels for other high-temperature services including industrial gas processes and thermal energy storage applications involving molten salt or salt-bath media. Material selection and joint design considerations for these services are handled through our engineering review process.

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Further Reading / 延伸阅读

• Custom Shell-and-Tube Heat Exchangers — Lmart Engineering Overview
• ASME U Stamp Fabrication Capabilities
• Pressure Vessel Fabrication for Chemical Process Industry
• 苏州利玛特能源装备 — 压力容器与换热器制造

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Lmart (苏州利玛特能源装备) is a certified manufacturer of custom shell-and-tube heat exchangers, pressure vessels, and modular process skids. ASME U stamp | PED 2014/68/EU | ISO 9001 | CCS. For project inquiries: www.jnlmart.net