Reformer tube

Petrochemical Heat Exchangers Reformer tubing Water and Gas supply piping Pipelines... 

Naphtha desulfurization is conducted in the vapor phase as described for natural gas. Raw naphtha is preheated and vaporized in a separate furnace. If the sulfur content of the naphtha is very high, after Co—Mo hydrotreating, the naphtha is condensed, H2S is stripped out, and the residual H2S is adsorbed on ZnO. The primary reformer operates at conditions similar to those used with natural gas feed. The nickel catalyst, however, requires a promoter such as potassium in order to avoid carbon deposition at the practical levels of steam-to-carbon ratios of 3.5—5.0. Deposition of carbon from hydrocarbons cracking on the particles of the catalyst reduces the activity of the catalyst for the reforming and results in local uneven heating of the reformer tubes because the firing heat is not removed by the reforming reaction. 

Reducing gas is generated from natural gas in a conventional steam reformer. The natural gas is preheated, desulfurized, mixed with steam, further heated, and reformed in catalyst-filled reformer tubes at 760°C. The reformed gas is cooled to 350°C in a waste heat boiler, passed through a shift converter to increase the content, mixed with clean recycled top gas, heated to 830°C in an indirect-fired heater, then injected into reactor 4. 

Ammonia Plant 1. Where possible, use natural gas as the feedstock for the ammonia plant, to minimize air emissions. 2. Use hot process gas from the secondary reformer to heat the primary reformer tubes (the exchanger-reformer concept), thus reducing the need for natural gas. 

Pretreated NG feedstock is mixed with steam (2.6 MPa), the resulting mixture is preheated to 500°C and introduced to the catalytic reforming reactor. In the reforming reactor, the steam-methane mixture is passed through externally heated reformer tubes filled with Ni catalyst, where it is converted to CO and H2 at 850-900°C according to the following equation ... 

Fewer reformer tubes per quantity of hydrogen and CO produced at equal heat loads per unit area... 

B. CFD Simulation of Reformer Tube Heat Transfer with... 

The effects of excessive temperatures on reformer tubes are in fact quite dramatic. Fig. 24 shows photographs of reformer tube banks with poor performance and tube over-heating. In Fig. 24a, the flame from the burner is visible in the top of the photograph. On several of the tubes clear evidence of hot bands... 

Fig. 24. Photographs of primary steam reformer tube banks showing high tube wall temperature features, (a) showing bands and hot patches and (b) showing an entire tube that has overheated.

From the Larson-Miller analysis, it is possible to derive more easily interpreted information relating to the effects of sustained high temperature on the life of a tube. A common rule of thumb is that a tube wall temperature increase of 20°C will shorten a tube life by over 50% from its design period of 10 years to less than 5 years. The cost of a typical reformer tube is USD 6000-7000. With typical reformer sizes in the order of 300-400 tubes and taking on-site expenditure into account, this puts the cost of a complete re-tube in the range... 

This study was carried out to simulate the 3D temperature field in and around the large steam reforming catalyst particles at the wall of a reformer tube, under various conditions (Dixon et al., 2003). We wanted to use this study with spherical catalyst particles to find an approach to incorporate thermal effects into the pellets, within reasonable constraints of computational effort and realism. This was our first look at the problem of bringing together CFD and heterogeneously catalyzed reactions. To have included species transport in the particles would have required a 3D diffusion-reaction model for each particle to be included in the flow simulation. The computational burden of this approach would have been very large. For the purposes of this first study, therefore, species transport was not incorporated in the model, and diffusion and mass transfer limitations were not directly represented. 

Hydrocarbon feedstocks for steam reformers include natural gas, refinery gas, propane, LPG and butane. Naphtha feedstocks with boiling points up to about 430°F can also be used. The ideal fuels for steam reformers are light hydrocarbons such as natural gas and refinery gas, although distillate fuels are also used. Residual fuels are not used since they contain metals that can damage reformer tubes. 

The reformer tubes typically operate at maximum temperatures of 1,600°F to 1,700°F and are designed for a minimum stress-to-rupture life of 100,000 operating hours. A 35/25 Ni/Cr alloy is used that is modified with niobium and microalloyed with trace elements such as titanium and zirconium. Smaller tube diameters provide better heat transfer and cooler walls. This reduces tube and fuel costs and increases tube life. But more tubes increases the pressure drop. The optimum inside tube diameter is 4 to 5 in. The wall thickness may be as low as 0.25 inch with a length of 40 to 45 ft. The lane spacing between tube rows must be enough to avoid flame impingement from the burners. Typical spacing is 6 to 8 feet. 

Special high-strength metallurgy is used in the construction of reformer tubes to help ensure longer life and better efficiency. 

We assume a one dimensional model for mass, heat, and momentum transfer. This means that the composition, heat, and pressure is uniform at any cross section of the catalyst bed in the reformer tubes. 

All hydrocarbons in the feed higher than methane are assumed to be instantly cracked into CH4, CO2, 112, and CO. Consequently the reaction system inside a reformer tube is described by the rate expressions of the kinetics of steam reforming in the methane reactions I, II, and III. 

The total amount of heat that is transferred to the reformer tubes is obtained from the flames and the furnace gas. For this heat transfer, we use the following assumptions. 

Radiative transfer from the furnace gases to the reformer tubes ... 

The radiation heat-transfer equation from the furnace gas to the reformer tubes has the form... 

The heat released by the furnace gas is transferred to the reformer tubes, the flue gas and 2% is lost to the surroundings. This heat balance can be written as... 

For the reformer we assume that the outer wall temperature profile of the reformer tubes decouples the heat-transfer equations of the furnace from those for the reformer tubes themselves. The profile is correct when the heat flux from the furnace to the reformer tube walls equals the heat flux from the tube walls to the reacting mixture. We must carry out sequential approximating iterations to find the outer wall temperature profile Tt,o that converges to the specific conditions by using the difference of fluxes to obtain a new temperature profile T) o for the outer wall and the sequence of calculations is then repeated. In other words, a T) o profile is assumed to be known and the flux Q from the furnace is computed from the equations (7.136) and (7.137), giving rise to a new Tt o-This profile is compared with the old temperature profile. We iterate until the temperature profiles become stationary, i.e., until convergence. 

Heated length of reformer tubes 13.72 m Shape Raschig18 rings... 

A short 1 m isothermal steam reformer tube was used for this test. The reformer (3) was run under the same operating conditions as reformer (2), but with a relatively large catalyst pellet of characteristic length 0.007619 to. For Plant (3) the exit methane conversion X, the CO2 yield X, and the equilibrium values Xe and Xe for methane and carbon dioxide, respectively, are as follows. 

Intermediate Duty catalysts are for feeds with a significant content of components from ethanes up to liquid petroleum gas (LPG). The heavier feedstock increases the tendency for catalyst deactivation through carbon laydown and requires a special catalyst in the top 30% to 50% of the reformer tubes. This tendency also occurs when light feeds are run at low steam-to-carbon ratios and/or at a high heat flux. 

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. 

To help evaluate the quality of the tubes, Quest Integrated has developed a tube inspection system using laser measurement for more accurate determination of tube diameter. The new system is called LOTIS (Laser Optical Tube Inspection System), and it provides rapid reformer tube diameter measurement with very high accuracy. The data from LOTIS can be used to estimate remaining tube life, determine appropriate operating adjustments or identify tubes that need to be replaced82. 

Conventional steam reformers are furnaces containing tubes filled with reforming catalyst. Radiation burners, which are usually installed at the top and the bottom of the furnace, generate the process heat (Fig. 1.4(a)). Figure 1.4(b) shows a schematic lateral temperature profile inside a single reformer tube. 

The impact of heat transfer limitations is illustrated with a generic example describing a best-case scenario (Fig. 1.5). The allowed maximum temperature of the reformer tube is assumed at 950 °C. Hence, the wall temperature and inlet temperature of the reaction gas are set to 950 °C. The reforming reaction is assumed to be instantaneous - that is, at each axial position conversion is set equal to the equilibrium conversion at the respective temperature Xequ(T). Dissipative effects (i.e.,... 

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