Reformer tubes Wall temperature

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) ... 

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... 

The overall effect of catalyst pellet geometry on heat transfer and reformer performance is shown in the simulation results presented in Table 1. The performance of the traditional Raschig ring (now infrequently used) and a modern 4-hole geometry is compared. The benefits of improved catalyst design in terms of tube wall temperature, methane conversion and pressure drop are self-evident. 

Reformers are fired to maintain a required process gas outlet temperature. Most modern reformers are top fired. In a top-fired reformer, the burners are located at the top of the furnace and fire downward. Process gas flows downward through catalyst-filled tubes. This flow of process gas and flue gas allows the highest flue gas temperature when the in-tube process gas temperature is lowest and the lowest flue gas temperature when the in-tube process gas temperature is highest. This results in tube-wall temperatures that are uniform over the tube s length and since the average tubewall temperature is lower this reduces tube cost and increases tube life. 

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. 

Their thermal efficiency is not very different and in a top-fired furnace can be as high as 95 %. The enthalpy difference between inlet and exit, often referred to as reformer duty, is made up of the heat required to raise the temperature to the level at the tube exit and the enthalpy of the reforming reaction. In a typical tubular steam reforming furnace, about 50% of the heat generated by combustion of fuel in the burners is transferred through the reformer tube walls and absorbed by the process gas (in a conventional ammonia plant primary reformer 60 % for reaction, 40% for temperature increase). 

Producing more active catalysts is still an important objective in order to be able to operate at lower tube wall temperature for the same heat flux and to build more compact steam reformers. 

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). 

The deposition of fission products on primary circuit surfaces and, in particular, on the reformer tube walls causes difficulties during maintenance and catalyst refilling procedures, if the activity is intolerably high. Cesium and silver isotopes released during reactor operation are of major concern. Particularly silver diffuses easily out of the fuel elements at operating temperature conditions into the coolant and migrates easily into metal surfaces and is difficult to remove in decontamination operations [32]. 

Schematic temperature and heat flux profiles for a top-fired and a sidewall-fired reformer for identical process outlet conditions are seen in Figure 3.5 below. The top-fired furnace has a high heat flux at the inlet, whereas the sidewall-fired furnace has a more equally distributed heat flux profile. The top-fired furnace has an almost flat tube temperature profile, whereas in a sidewall-fired furnace the tube-wall temperatures increase down the reformer. The terrace-wall fired reformer has profiles similar to the sidewall-fired reformer, whereas the bottom-fired reformer has a larger heat flux in the lower part of the reformer.

The constraints in designing tubular and convective reformers are hence primarily the mechanical material properties. The catalyst volume may then be considered a derived property. However, high catalyst activity is essential to ensure low tube-wall temperature. A higher... 

In fired reformers, outside tube-wall temperatures must be measured carefully [19] [131]. Apart from their impact on tube life, they are key variables in the evaluation of heat transfer coefficients. Measurements may be carried out using commercial IR cameras with different wavelengths, but in a pilot plant such measurements are best carried out using Pt/Rh thermocouples attached on the cold side of the tubes. In an industrial plant thermocouples must be embedded, but a gold cup pyrometer [131] can be used locally to obtain accurate measurements. 

A proper optimisation of a steam reformer must always be based on a furnace model, since the delivered heat flux profile is bounded by the furnace configuration and the flexibility of the burners. Seen from an exeigy point of view outer tube-wall temperature and heat flux profiles may decrease the exergy losses [335], but it should be checked if they can be provided by a furnace. 

Simulation of tubular steam reformers and a comparison with industrial data are shown in many references, such as [250], In most cases the simulations are based on measured outer tube-wall temperatures. In [181] a basic furnace model is used, whereas in [525] a radiation model similar to the one in Section 3.3.6 is used. In both cases catalyst effectiveness factor profiles are shown. Similar simulations using the combined two-dimensional fixed-bed reactor, and the furnace and catalyst particle models described in the previous chapters are shown below using the operating conditions and geometry for the simple steam reforming furnace in the hydrogen plant. Examples 1.3, 2.1 and 3.2. Similar to [181] and [525], the intrinsic kinetic expressions used are the Xu and Froment expressions [525] from Section 3.5.2, but with the parameters from [541]. 

As discussed earlier (Chapter 3), the effectiveness factor of the catalyst in the tubular reformer is small. Even the low activity of alkali-promoted catalysts usually results in satisfactory performance in industry. However, as outlined in Example 3.4, high catalyst activity means that the same performance can be attained at a lower tube-wall temperature and hence leaves room for longer tube life or operation at higher heat fluxes (i.e. smaller reformer). 

The desulfurized natural gas feed is mixed with steam and preheated to 500 °C before entering the reformer tubes. The heat for the reforming reaction is supplied by combustion of fuel in the furnace, which may contain up to 500 tubes with a length of 10 m and a diameter of 10 cm. Figure 6.2.31 shows axial profiles of the tube wall temperature and the heat flux. 

The balance between heat input through the reformer tube walls and the heat consumption in the endothermic reforming reaction is the central problem in steam reforming. The maximum allowed stress value in the tubes is strongly influenced by the maximum tube wall temperature... 

The heat input through the reformer tube wall results in radial temperature and concentration gradients in addition to the axial gradients. Therefore, a two-dimensional model is required when a detailed knowledge of the conversion and temperature profile is necessary as is the case when the operation of the steam reformer is critical with respect to carbon formation (Rostrup-Nielsen, 1984a). 

It is evident from the above and also from eq. (12) that lack of catalyst activity will cause high temperature both in the gas and in the tube. The advantage of high catalyst activity (and the disadvantage of an unsuitable flux profile) is illustrated in Figure 8 which shows calculated tube wall temperatures in a top fired reformer for various relative catalyst activities in the upper half of the reformer tube. It is seen that loss of activity in the upper part of the tube may lead to significant overheating of the tube. 

Figure 8. Tube Wall Temperatures in Top Fired Reformer at various Catalyst Activity Levels (TF) in the upper 50% of the Tube (Storgaard, 1991).

The retarding effect of sulphur is a dynamic phenomenon. This means that carbon may be formed at certain conditions in spite of sulphur passivation - although at strongly reduced rates and by a mechanism different from the formation of whisker carbon. Therefore, it was important to develop design criteria to make sure that the kinetic balance is in favour of no carbon formation at all positions in the reformer tube. This work was carried out mainly in the full-size monotube process demonstration plant (Dibbern et al., 1986). The influence of various process parameters (pressure, heat flux, sulphur content in feed, etc) was studied. It was demonstrated that the impact of variations in sulphur content in the feedstream on tube wall temperature and exit gas composition was completely reversible. 

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