Fixed-bed olefin conversion reactors that react ethylene with isomerized butenes to produce propylene operate in a service where the normal hydrocarbon atmosphere is comparatively benign but the periodic steam-air decoking cycle determines the maximum allowable metal temperature and therefore the pressure-boundary alloy class. The reactor is typically designed to ASME BPVC Section VIII Division 1 for a pressure between 2.0 MPa and 3.5 MPa and a normal run shell temperature between 250 °C and 400 °C, while regeneration can take the bed support zone and outlet collector to 500 °C to 575 °C depending on the licensor procedure. The feed to the metathesis reactor is treated in guard beds to reduce oxygenates, nitriles, water, and sulfur species to low single-digit mg/kg levels because tungsten oxide-on-silica and rhenium oxide-on-alumina catalysts are highly sensitive to these poisons; this purification also removes many species that would otherwise drive acid gas corrosion. The dominant material selection variables are therefore oxidation resistance under cyclic oxygen exposure, carburization resistance under coke burn conditions, creep strength at the regeneration temperature, and resistance to chloride-induced damage from external insulation or shutdown wash water. A shell that meets the normal run condition but fails under regeneration can lose metal thickness by oxide spalling, develop subsurface carbides that reduce crack resistance, or crack at welds if the alloy is sensitized and exposed to moisture during a turnaround.

Why Does Regeneration Temperature Rather Than Normal Run Conditions Govern Reactor Shell Alloy?

Regeneration governs shell alloy selection because the coke burn front is exothermic and transient, with local catalyst bed temperatures commonly exceeding the average gas outlet temperature by 50 °C to 100 °C unless oxygen injection is staged and diluted with steam or nitrogen. Carbon steel pressure plate such as ASTM A516 Grade 70 can handle the normal run temperature but begins to oxidize at a rate that becomes life-limiting above about 430 °C to 450 °C, and cyclic oxide spalling can reduce wall thickness at a rate that is difficult to predict from isothermal test data. For a reactor that must be decoked at metal temperatures above 480 °C, low-alloy chromium-molybdenum plate specified to ASTM A387 Grade 11 Class 2 is the common minimum; it contains nominally 1.25 wt% chromium and 0.5 wt% molybdenum, and its chromium oxide scale reduces further oxidation and carburization when compared with carbon steel. Where the regeneration temperature approaches 575 °C or where the reactor is close to a furnace or receives hot regeneration gas, ASTM A387 Grade 22 Class 2 with 2.25 wt% chromium and 1.0 wt% molybdenum provides additional creep and oxidation margin, but it requires more demanding fabrication controls. The selection is not made only on oxide thickness; the allowable stress in ASME BPVC Section II Part D drops rapidly with temperature, and a carbon steel shell may require such a large wall thickness that the vessel weight, field postweld heat treatment, and weld conservation become economically impractical. In many olefin conversion units the regeneration outlet line and the reactor overhead nozzle are upgraded to 1.25Cr-0.5Mo even when the main shell is carbon steel because the outlet nozzle sees the highest gas velocity and the most severe thermal cycling. API RP 571 identifies oxidation as a thickness-loss mechanism that becomes significant when the scale is non-protective, and cyclic operation accelerates damage by spalling of the oxide layer.

Because catalyst support grids, distributor pipes, spargers, and thermowells inside the metathesis reactor operate at the highest local temperatures, they are embedded in catalyst or directly exposed to regeneration gas and are not heat-sinked as effectively as the shell wall. These components are usually cast or fabricated from stabilized austenitic stainless steel grades; ASTM A351 Grade CF8C is a common cast equivalent of wrought 347, with nominal 18.0 wt% to 21.0 wt% chromium, 9.0 wt% to 12.0 wt% nickel, and a niobium addition that stabilizes carbon and minimizes intergranular corrosion adjacent to welds. A 316L grid may survive normal run exposure below 450 °C, but it is generally excluded for load-bearing internals that see more than 540 °C during regeneration because the molybdenum-bearing grade can form sigma phase after 1,000 h to 10,000 h in the 600 °C to 750 °C range, and the resulting loss of fracture toughness can be detected only by destructive methods or instrumented impact testing. The microstructure of cast CF8C should have a ferrite number between 3 FN and 8 FN to avoid hot cracking during weld repair and to maintain a balance between thermal fatigue resistance and embrittlement resistance; a fully austenitic casting with less than 2 FN may crack during weld overlay or repair. Because batch-to-batch variance in cast CF8C ferrite number arises from casting temperature and cooling rate, the purchase specification should require a production qualification plate and a statistically valid sampling plan rather than relying on a single chemistry report. Supports should be designed with slotted holes or guided sliding surfaces to absorb differential expansion; a 2.5 m diameter grid can develop several millimetres of radial growth when the grid temperature is 50 °C higher than the shell during regeneration. Thermowell installation should conform to ASME PTC 19.3 TW for wake frequency calculation, and the material order should specify through-thickness ductility and grain size no coarser than ASTM E112 grain size 5 to improve ultrasonic inspectability.

When Chloride From Insulation or Wash Water Meets Austenitic Stainless Steel at Sustained Stress

When the reactor shell or its associated piping is made from an austenitic stainless steel such as ASTM A312 Grade TP321 or TP347, the external surfaces become susceptible to chloride-induced stress corrosion cracking if water and chloride concentrate under insulation at skin temperatures above about 60 °C. The tensile stress can originate from internal pressure, weld residual stress, or differential thermal expansion; the chloride source can be insulation containing leachable chloride, marine atmosphere, or wash water that is not chloride-free. The standard specification ASTM C795 is used for insulation that will contact austenitic stainless steel, and the purchaser should require a leachable chloride content below 10 ppm from the insulation supplier. In addition, the weather barrier should be continuous and sealed with a chloride-free caulking, and the use of aluminum foil directly on stainless steel should be evaluated for galvanic and local concentration effects. Hydrostatic testing and turnaround washing should use water with chloride content below 5 mg/L or the vessel should be drained and dried with oil-free compressed air having a dew point below -40 °C within 24 h. The same precaution applies to crevices at manway gaskets, thermowell bosses, and nozzle reinforcement pads; these locations can concentrate chloride by evaporation even when the bulk rinse water meets purity specifications. For new construction, a low-temperature shop practice of solvent cleaning and removal of all surface iron contamination is specified to prevent pitting initiation sites; passivation with nitric acid according to ASTM A967 is commonly used before first service. The specification should also require that any temporary shipping covers, flange protectors, or marking paints used on stainless steel be chloride-free, because residual chloride from these sources has been found in turnaround inspections at gasket seating areas.

After extended hydrocarbon exposure or regeneration, shutdown of a metathesis reactor leaves sulfur-containing scale or coke deposits on internal surfaces; if moisture and oxygen enter before the vessel is fully cooled and purged, polythionic acid can form and rapidly crack sensitized austenitic stainless steel. NACE SP0170 provides protection measures for austenitic stainless steels exposed to polythionic acid forming environments, including immediate dry nitrogen purging to keep oxygen out or alkaline washing with a 2 wt% to 5 wt% sodium carbonate solution followed by chloride-free water rinsing. Stabilized grades such as ASTM A240 Type 347H resist this damage because niobium preferentially ties up carbon and prevents chromium carbide precipitation at grain boundaries; however, the weld heat-affected zone in an unstabilized 304H or 316H shell can become sensitive after service in the 450 °C to 850 °C range. Published failure reports from shutdown surveys in hydroprocessing and olefin conversion units indicate that cracks initiate at weld toes and at attachment welds where the stainless surface remains stressed and moist. For this reason, solid stainless steel reactors and large stainless internals are often supplied in the solution-annealed and water-quenched condition, with low-carbon weld filler but not necessarily postweld heat treatment, because field solution annealing of a large reactor is difficult to perform uniformly. The shutdown procedure should also specify that air exposure of stainless internals is not permitted until the skin temperature is below 60 °C or until the polythionic acid neutralization has been completed.

Selecting Stabilized versus Low-Carbon Austenitic Grades for Catalyst Support Grids and Thermowells

Selecting between 321H, 347H, and 316L for reactor internals requires a simultaneous evaluation of creep strength, sigma-phase embrittlement, weldability, and field repairability. Type 321H contains titanium at 5×C to 0.70 wt% and Type 347H contains niobium at 10×C to 1.10 wt%; both grades avoid sensitization more effectively than 304H or 316H when exposed to welding or to long-term service at 425 °C to 815 °C. Titanium-stabilized 321H is more difficult to weld with gas tungsten arc welding because titanium oxide can form in the weld pool when the shielding gas contains more than 25 ppm oxygen or moisture, leading to porosity and reduced toughness. Type 347H generally has better high-temperature strength than 321H because niobium carbonitride precipitation is more stable, but it can be prone to hot cracking in fully austenitic weld deposits; therefore filler metal conforming to AWS A5.4 E347-15 or E347-16 should be specified with a ferrite number between 3 FN and 8 FN. Low-carbon 316L may be acceptable for non-load-bearing components below 450 °C, but it should not be used for support grids above 540 °C because sigma-phase formation can reduce impact energy below 27 J after long-term aging. Where the service includes frequent thermal cycles between 300 °C and 600 °C, the internal components should be designed by elastic-plastic finite element analysis rather than simple elastic stress classification, because thermal ratcheting damage accumulates in weld toes and cast surface notches. A purchase specification for CF8C cast grids should require a casting quality factor according to ASTM A703 and radiographic or penetrant inspection of all load-bearing sections to ASME BPVC Section V Article 2 and Article 6. The same specification should limit weld repair depth and require re-solution annealing if more than 20% of the casting cross-section is excavated for repair.

Weld Overlay Economics for Carbon Steel Reactor Shells Exposed to Cyclic Decoking

Carbon steel reactor shells lined with a stainless steel weld overlay can reduce capital cost relative to solid Cr-Mo or stainless construction when the normal run temperature is below 350 °C and only the internal surface requires protection during decoking. A common overlay configuration is applied by submerged arc strip cladding with a first layer of AWS ER309L and a second layer of AWS ER347, producing a total deposit thickness of 8 mm to 12 mm after machining. The outer surface of the second layer should have chromium content of at least 18 wt%, carbon content no greater than 0.04 wt%, and iron dilution from the carbon steel base should be controlled to below 15% per pass by limiting welding current and travel speed. The overlay must be inspected by shear-wave ultrasonic testing according to ASME BPVC Section V Article 4 to detect lack of fusion and by liquid penetrant testing according to ASME BPVC Section V Article 6 to detect surface cracks. However, the dissimilar-metal interface between the carbon steel shell and the austenitic overlay can become a site for hydrogen-induced disbonding if the process environment contributes hydrogen or if water vapor from decoking and hydrocarbon coke react to produce hydrogen at the surface. For this reason, the first regeneration cycle should be followed by a supplementary ultrasonic inspection of the overlay bond zone and by a hardness survey of the base metal heat-affected zone. The overlay approach is less suitable when the shell must be postweld heat treated after overlay application, because the thermal expansion mismatch between carbon steel and austenitic stainless steel can generate residual stress and accelerate disbonding during cooldown. In cases where a shell overlay is applied over a Cr-Mo base material, the weld procedure should be qualified to avoid reheat cracking in the coarse-grained heat-affected zone, and the composite vessel design should conform to ASME BPVC Section VIII Division 1 Part UCL for clad vessels.

For high-temperature reactor internals and feed-effluent exchanger tube sheets where carburization and oxidation resistance beyond Type 347H are required, Alloy 800H and Alloy 800HT are occasionally specified. These alloys are nickel-iron-chromium grades with chromium content of 19 wt% to 23 wt%, nickel content of 30 wt% to 35 wt%, and controlled carbon plus titanium or aluminium for high-temperature creep strength. The product form is typically plate, pipe, or fittings conforming to ASTM B409, ASTM B163, or ASTM B366, and the solution-annealed condition is required for service above 600 °C. In a metathesis reactor, Alloy 800H is usually reserved for components that are too thin for a cast CF8C design, such as thermowell protection tubes and catalyst unloading nozzles, because the material cost is significantly higher than stainless steel and because fillet welds between Alloy 800H and stainless steel require nickel-base filler metal such as AWS A5.14 ERNiCr-3. The high nickel content can also make the alloy more prone to metal dusting under certain carbon-rich regeneration gases; therefore the regeneration procedure should limit carbon monoxide concentration to below 2 vol% and maintain adequate steam partial pressure. Published bend-fatigue and creep-fatigue data for this specific cyclic decoking atmosphere is limited, so component life should be validated by a low-cycle fatigue test programme or by conservative design factors against the ASME BPVC Section II Part D allowable stresses.

Table 1 summarizes the alloy classes and limitations for the major reactor zones. The upper temperatures are engineering practice limits for this damage regime and must be checked against ASME BPVC Section II Part D allowable stresses for the specific design life.

Although the metathesis catalyst requires near-total removal of sulfur, a temporary sulfur breakthrough from an upstream dry bed or feed contamination can expose the reactor shell to organic sulfides and H2S at temperatures that are above the threshold for high-temperature sulfidic corrosion. Carbon steel can corrode at rates exceeding 0.25 mm/year under upset conditions, while chromium-containing steels form a mixed chromium sulfide/oxide scale that reduces further attack. A shell containing 1.25 wt% to 2.25 wt% chromium is therefore less vulnerable to sulfur upset events than a plain carbon steel shell. When sour service classification is imposed by the site or by local regulation, the materials specification should refer to NACE MR0175/ISO 15156 for carbon and low-alloy steels, with weld hardness limits and environmental limits established by the specific clause for the H2S partial pressure and pH. The selection of the reactor shell should not be based solely on the steady-state sulfur concentration; the upset condition and the regeneration atmosphere must be considered in the damage assessment. Low-alloy chromium steels can also suffer from temper embrittlement if the tramp element content is not controlled, so the purchase specification should require a J factor below 100 or an X factor below 15 ppm for shells operating above 450 °C.

On external flange bolting for the reactor manway and catalyst loading nozzle, the bolting is selected to retain preload at the maximum service temperature without exceeding creep limits. ASTM A193 Grade B16 stud bolts with ASTM A194 Grade 2HM nuts are common for flanges with design metal temperatures up to 480 °C; for regeneration temperatures above this limit, ASTM A193 Grade B8T Class 2 or precipitation-hardening grade ASTM A453 Grade 660 bolts may be required, but the higher coefficient of thermal expansion of stainless steel bolts must be considered when calculating bolt stress at operating temperature. The bolt material should be compatible with the flange material to avoid galvanic corrosion, and the use of nickel-based anti-seize compounds is generally acceptable if the compound does not contain graphite or sulfur above 2 wt%. For internals bolting made from high-temperature stainless steel, the design should include a minimum preload and locking arrangement that prevents loosening under cyclic thermal expansion, because loosened bolts in catalyst support grids have caused localized abrasion and catalyst attrition in fixed-bed units.

Fabrication of the metathesis reactor shell and internals should include weld procedure qualification to ASME BPVC Section IX, with impact testing for low-alloy steel welds at the minimum design metal temperature and hardness testing after postweld heat treatment. For Cr-Mo pressure vessels, the purchase specification typically invokes API RP 934-A for materials and fabrication of heavy-wall pressure vessels, including a maximum weld hardness of 237 HV10 and a minimum Charpy impact energy of 54 J at 0 °C for the base metal and weld metal. The stainless internals should be solution annealed after forming if cold work exceeds 10%, and welded joints in 347H should be inspected by liquid penetrant testing to ASME BPVC Section V Article 6 after final surface finishing. The final clean condition should be verified by white-cloth wipe testing and by measuring surface chloride on stainless steel using a conductivity extraction method with an acceptance criterion below 5 mg/m². The same fabrication specification should require a documented final cleaning procedure and a closure inspection before the catalyst loading nozzle is sealed, because embedded scale and chlorides introduced during factory fabrication or field assembly can initiate corrosion during the first regeneration cycle.

Refractory-lined carbon steel reactors are not typically used in fixed-bed metathesis service because spalled refractory particles can poison the catalyst and absorbed moisture can extend startup drying time beyond acceptable limits.

Table 2 provides the compliance inspection matrix for the damage mechanisms relevant to the reactor shell and internals. The inspection intervals should be risk-based and should include baseline thickness data taken before the first regeneration cycle.