Reformer tubes Tube materials

Tube wall temperature is an important parameter in the design and operation of steam reformers. The tubes are exposed to an extreme thermal environment. Creep of the tube material is inevitable, leading to failure of the tubes, which is exacerbated if the tube temperature is not adequately controlled. The effects of tube temperature on the strength of a tube are considered by use of the Larson-Miller parameter, P (Ridler and Twigg, 1996) ... 

The Primary Reformer is a steam-hydrocarbon reforming tubular furnace that is typically externally fired at 25 to 35 bar and 780°C to 820°C on the process side. The reformer tubes function under an external heat flux of 75,000 W/m2 and are subject to carburization, oxidation, over-heating, stress-corrosion cracking (SCC), sulfidation and thermal cycling. Previously SS 304, SS 310 and SS 347 were used as tube materials. However these materials developed cracks that very frequently led to premature tube failures (see Table 5.10)88. 

In the mid-1960 s HK 40 alloy (see Table 5.11) was developed and proved to be a good material for vertical reformer tubes. Consequently, plant capacities were extended to 600 tons per day with this tube material. Although this alloy had a design service life of 100,000 hours, overheating considerably reduced tube life. A 55°C excursion above the design temperature could lower tube service life to 1.4 years88. 

A high reformer exit temperature of 1616°F is made possible by high alloy tube materials such as Manaurite 36X and Paralloy. This leads to a reduction in methane slip, and an increase in the CO/CO ratio. Both effects enhance the plant s efficiency and result in a reduction of feedstock natural gas consumption of 3.5% over previously used reforming conditions. 

The secondary reformer in an ammonia plant is a carbon steel vessel with a dual layer refractory lining. Internal temperatures reach -2,000°F (1,090°C) from burning as a result of air added through a burner at the top of the vessel to the feed gas (hydrogen, carbon monoxide, carbon dioxide, and steam). The burner is a refractory-lined device that is subject to failure if not carefully designed. Quench steam generators have refractory-lined inlet channels and tube sheets. Tubes are often made of carbon steel because the heat transfer from the steam on the outside of the tube is markedly better than that from the synthesis gas inside the tube. As a result, the metal temperature closely approaches the temperature of the steam. The inlet ends of the tubes are protected from the inlet gas by ferrules, usually made of type 310 (UNS S31000) SS with insulation between the ferrule and the tube. The tube material should be selected... 

There are additional reasons for applying a higher S/C ratio. First, it prevents carbon deposition on the catalyst, which may not only increase the pressure drop but also reduce the catalyst activity. As the rate of the endothermic reforming reaction is lowerded this way, it can result in local overheating of the reformer tubes (hot bands) and premature failure of the tube walls. Second it provides necessary steam for the shift conversion (Section 4.2). Third, it reduces the risk of carburization of tube material. 

New tube materials allow the design for much higher exit temperatures and heat fluxes, in particular when applying a side wall fired reformer furnace to ensure better control of the maximum tube wall temperature and optimum use of the high alloy material. Thinner tube walls made possible by the use of the new materials reduce the risk for creep due to faster relaxation of stresses at start and stop of the reformer (14). 

Reformer tube heating with a high-temperature nuclear reactor is performed with helium, typically at 950 °C, as the heat source. The aim of reaching a heat flux density similar to that of the conventional method can be achieved by employing a helium-heated counterflow heat exchanger (see Fig. 2-11). Helium under pressure shows excellent heat transfer properties. Furthermore, precautions must be taken to minimize the effects of asymmetry or hot gas streaks in the helium flow as well as a non-uniform process gas flow. The materials of a helium-heated steam reformer should be selected such that the... 

The tubes containing the reformer catalyst are one of the most important elements of the reformer. They represent up to 30 7o of the total reformer cost and the maximum operating conditions for the process are established by the tube material [2]. The tubes are normally 108 mm outer diameter (O.D.) X 72 mm inner diameter (l.D.) (4,25 in. O.D. x 2.83 in. I.D.). Materials of construction are high nickel alloy such as HK 40 (25 Cr/20 Ni), Inconel 617, Inconel 800 and Supertherm [1]. Low pressure reformers use primarily HK 40 tube material. However, high pressure units require more expensive alloys like 25 Cr/35 Ni,Nb, known as HP with Nb, to withstand more severe pressure and temperatures [2]. 

Materials for catalyst tubes are selected in combination with the process conditions employed. Alloys with high chromium and nickel content are used for the reactor tubes in a steam-reforming furnace. The first centrifugally cast tubes such as HK 40 contained 25% Chromium and 25% Nickel. Today, tube material containing 25% Chromium and 35% nickel, niobium, and traces of zirconium and titanium are used (so called HP alloys) (50). The HP alloys are more expensive but allow a higher tube design temperature and have a better creep strength and oxidation as well as carburization resistance. 

The progress in steam reforming technology has resulted in less costly plants, in part because of better materials for reformer tubes, better control of carbon limits, and better catalysts and process concepts with high feedstock flexibility. This progress has been accompanied by a better understanding of the reaction mechanism, the mechanisms of carbon formation and sulphur poisoning, and the reasons for tube failure. 

The reformer tubes are one of the most important pieces of equipment in a hydrogen plant. These tubes are built from micro-alloyed materials in order to handle the extreme environment in which they are exposed to. The industry standard for reformer tube design is for 100,000 hour life. To ensure that the tubes will last the designed 100,000 hour life, the reformer tubewall... 

Regarding material selection for high-temperature piping and reformer tubes used in the reforming furnace, selection should be made with consideration given to economy because of the very expensive materials used. 

Reforming at higher pressure saves energy due to reduced power requirements for synthesis gas compression. Reduction of the steam to carbon ratio also saves energy due to savings in process steam consumption. In both cases, more severe conditions are required in the reformer to obtain the required high conversion of the feedstock. This has led to the requirement for better catalysts and better materials for the reformer tubes. 

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