Stainless Steel Coil Tube for Maleic Anhydride Reactor Units

Product Overview

The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a specialized heat-exchange tubing product engineered for high-temperature, high-pressure catalytic oxidation reactors used in maleic anhydride (MA) production units. These coil tubes are formed into helical or serpentine configurations and serve as the core heat-transfer elements within reactor vessels, where hydrocarbon feedstocks (such as naphtha or n-butane) are oxidized in the presence of vanadium pentoxide (V₂O₅)-based catalysts.

Based on extensive failure analysis studies of MA reactor coil tubes, the product is designed to withstand extreme thermal cycling, oxidative degradation, and mechanical fatigue under sustained operating conditions exceeding 550°C. The coil tube design provides a high surface-area-to-volume ratio, enabling efficient heat removal from the catalytic reaction zone and preventing hot-spot formation that could lead to catalyst deactivation or catastrophic tube failure.

Material & Properties

Base Alloy Composition

The coil tube is fabricated from austenitic stainless steel grades optimized for high-temperature oxidation resistance and thermal fatigue performance. The primary alloy specifications include:

Key Mechanical & Physical Properties

Specifications

Parameter Specification
Product Name Maleic Anhydride Reactor Coil Tube (Coiltube)
Item No. 1345
Material Grade 310S / 309S / HP-N (Customizable)
Outer Diameter (OD) 25 mm – 60 mm (typical: 32 mm, 38 mm, 45 mm)
Wall Thickness (WT) 1.5 mm – 4.0 mm
Coil Pitch 40 mm – 120 mm (customizable per reactor design)
Coil Diameter 200 mm – 800 mm
Tube Length (per coil) 15 m – 60 m
Number of Tubes per Reactor 800 – 3,500 (depending on reactor capacity)
Operating Temperature Range 400°C – 700°C (process gas side)
Maximum Peak Temperature ≤ 800°C (hot spot limit)
Operating Pressure 0.1 MPa – 0.5 MPa (gauge)
Heat Transfer Medium Boiling water / Molten salt (tubeside)
Surface Finish Mill finish / Polished interior (Ra ≤ 1.6 μm)
Welding Method Seamless / TIG welded (for formed coils)
Applicable Standards ASTM A313, ASTM A213, EN 10216-5, GB/T 12771

Manufacturing Process

Step 1: Raw Material Selection & Inspection

High-purity austenitic stainless steel billets or seamless pipe blanks are selected based on strict chemical composition requirements. Incoming materials undergo 100% spectroscopic analysis (OES) to verify Cr, Ni, Mo, Si, Mn, and carbon content within specified tolerances. Sulfur (S) and phosphorus (P) levels are controlled below 0.030% to minimize hot shortness during forming.

Step 2: Hot Rolling & Seamless Pipe Production

Seamless tubes are produced via the Mannesmann piercing process followed by hot rolling and cold drawing to achieve precise dimensional tolerances (±0.1 mm for OD, ±0.05 mm for wall thickness). The seamless construction eliminates the weld seam that would be a potential failure initiation site under thermal cycling.

Step 3: Heat Treatment (Solution Annealing)

Tubes are solution-annealed at 1050°C – 1100°C followed by rapid water quenching to achieve a fully austenitic microstructure free of carbide precipitation. This ensures maximum corrosion resistance and eliminates residual stresses from cold working.

Step 4: Coil Forming (Cold Bending)

Individual straight tubes are cold-formed into helical coil configurations using precision CNC coil bending machines. The forming process is carefully controlled to avoid:

Step 5: Stress Relief Annealing

After coil forming, the tubes undergo a stress-relief annealing cycle at 850°C – 950°C for 2–4 hours to remove residual bending stresses while maintaining dimensional accuracy. This step is critical for preventing stress-corrosion cracking (SCC) during service.

Step 6: Quality Inspection & Testing

Step 7: Packaging & Shipping

Coil tubes are individually wrapped in moisture-proof protective film, bundled with wooden or steel cradles to prevent deformation, and shipped with full traceability documentation including mill test certificates (MTC), heat numbers, and inspection reports.

Applications

Primary Application: Maleic Anhydride (MA) Production

The primary application of these coil tubes is in the tubular fixed-bed reactors used for the catalytic oxidation of hydrocarbon feedstocks to maleic anhydride. The reaction typically proceeds as follows:

C₄H₁₀ (n-butane) + 3.5 O₂ → C₄H₂O₃ (Maleic Anhydride) + 4H₂O

ΔH ≈ -1065 kJ/mol (highly exothermic)

The coil tubes function as the reactor wall through which heat is removed by a boiling-water system on the tubeside, maintaining the catalyst bed at optimal temperature (550°C – 650°C) for maximum selectivity while preventing thermal runaway.

Secondary Applications

Failure Analysis Insights & Design Advantages

Common Failure Modes Addressed in Design

Based on extensive field failure analysis of MA reactor coil tubes, the following degradation mechanisms have been identified and mitigated in the product design:

  1. Oxidative Scale Spalling: At temperatures above 700°C, thick Cr₂O₃ scales can spall due to thermal cycling, exposing fresh metal to re-oxidation. The high Cr (25%) content in 310S provides a stable, adherent oxide scale.
  2. Carburization: In hydrocarbon-containing atmospheres, carbon diffuses into the tube wall, forming chromium carbides (Cr₂₃C₆) that deplete the matrix of Cr, leading to accelerated oxidation. The high Ni content and controlled cooling rates minimize this effect.
  3. Thermal Fatigue Cracking: Repeated thermal cycling (daily start-up/shutdown or load variations) causes low-cycle fatigue at tube-to-header joints. Proper stress-relief annealing and generous wall thickness specifications address this.
  4. Creep Rupture: Sustained high-temperature operation can lead to creep deformation and eventual rupture. Material selection is based on verified creep rupture strength data exceeding design margins of 1.5×.
  5. Flow Maldistribution: Geometric imperfections in coil forming can cause uneven gas flow distribution across the tube bundle, creating localized hot spots. Precision manufacturing tolerances ensure uniform flow.

Unique Selling Points

Quality Certifications & Compliance

Product Certifications

Material Standards Compliance

Testing & Inspection Certifications