Reformer Convective Heat Transfer Reformers

Convective Heat Transfer Reformers provide additional reforming capacity by using the heat contained in the primary reformer exit gases. Several designs are available, but not all have been commercialized. These units typically replace a portion or the entire duty of the waste heat boiler. So they significantly reduce the steam capability of the reformer. Potential increases in capacity of between 10% and 30% are possible. The modifications are capital intensive but relatively easy to implement170. 

Another type of steam-reforming reactor that is attracting increasing attention is known as gas heated reformers or heat exchange reformers. In such reformers, heat is transferred by convection and the heat source is a hot process gas from another reformer or a partial oxidation reactor. A number of different installations of heat exchange reformers can be envisaged. In Fig. 5, the installation of a heat exchange reformer either in series or in parallel to an auto-thermal reformer (ATR) is illustrated. 

In the CAR process, the natural gas feed is mixed with steam and introduced into the CAR reactor via a tube sheet to the catalyst-filled tubes in which reforming to synthesis gas takes place. The natural gas is partially converted, and the slip methane is allowed in the lower chamber where partial oxidation takes place. In this lower section, temperatures are about 1300-1400°C. The resulting hot synthesis gas then passes upward and supplies heat to the primary reforming reaction inside the catalyst tubes. An important element in the CAR reactor is the tube sheet, which acts as a feed stream distributor to the reformer tubes. In addition, there are enveloping tubes around the catalyst tubes, which constrict the flow of the autothermal product gas, thereby increasing the convective heat transfer coefficient. The CAR reactor, due to the high temperatures, is also jacketed with water. 

Also important is the effect of the size and shape of the catalysts [428] on heat transfer and consequently performance. Unlike the most processes carried out under substantially adiabatic conditions, the endothermic steam reforming reaction in the tubes of the primary reformer has to be supplied continuously with heat as the gas passes through the catalyst. The strong dependency of the reaction rate on the surface temperature of the catalyst clearly underlines the need for efficient heat transfer over the whole length and crosssection of the catalyst. However, the catalyst material itself is a very poor conductor and does not transfer heat to any significant extent. Therefore, the main mechanism of heat transfer from the inner tube wall to the gas is convection, and its efficiency will depend on how well the gas flow is distributed in the catalyst bed. It is thus evident that the geometry of the catalyst particles is important. 

The heated tube length in the reformer is 10 m and the external diameter of the tubes is 10.5 cm. If the rate of heat transfer (Q) from the combustion gases in the firebox to the reformer gases were accomplished entirely by convection. the following equation would apply ... 

Other types of more compact steam reformers are also used. In most cases, the heat transfer is then accomplished by convection with the reformed gas itself, a fine gas, or by a combination as illustrated in Fig. 4. In this case, about 80% of the heat is transferred to the process and export of steam from the plant can be reduced or avoided. 

In IPOX it is essential to operate the reactor in adiabatic mode as the exothermal heat has to be kept within the reactor for use in the reforming step. As the adiabatic operation hinders the radial heat transfer, gradients in this direction can be considered as negligible compared to the ones in the axial (z)-direction. Assuming that the convective effects are dominant over the diffusive ones in the z-direction, the spatial changes in the molar... 

These advantages promise substantial reduction in reformer size, coital costs, and operating conditions. It must be emphasized, however, that these benems may only be realized with reformers having higher heat transfer coefficients than conventional radiant or convective systems, e impact of lower mechanical strength for the foam remains to be addressed. 

However, the heat exchange is primarily by convection which generally leads to lower average heat fluxes (and hence bigger heat transfer surfaces) than in tubular reformers with radiant heat transfer... 

For heated and convective reformers a scale-up requires at least one full-size tube due to the interaction between complicated heat transfer, catalyst and in particular mechanical design. For an adiabatic reactor scale-up can in principle be carried out using the reactor diameter, but the basic experiments must be truly adiabatic and at industrial mass velocities, so also in this case a pilot plant can be used (refer to Section 3.5.1). For other equipment items, such as burners, scale-up will usually require a combination of flow experiments, CFD modelling and full-scale tests. 

The convective reformer is compact, but has larger heat transfer areas compared with a tubular reformer due to the smaller heat transfer coefficients in the cold end. A fair comparison should thus include the heat exchange areas in the waste heat section in the tubular reformer, but still the fired tubular reformer remains the most economical solution for large-scale reforming. The economy of scale is more advantageous for a... 

The heat transfer rate from the primary reformer fire box to the catalyst tubes is calculated along the tube using radiant and convective heat transfer relationships. Heat transfer is also calculated from die tube outer surface, across the tube metal, through the inside tube surface film to the bulk fluid, and finally from the bulk fluid to the catalyst particles. Measurements of die catalyst temperatures at several positions along the tube, as well as measured... 

At fuel manifold inlets, gaseous species concentrations are specified as equilibrium compositions of the town gas reformate at 650°C. Steam-to-carbon ratio is kept as 3.06 for this particular steady-state analysis. Both fuel and air gas manifold inlet conditions are summarized in Table 9.5. Mixed convective and radiative heat transfer boundary conditions are applied to the side surfaces of the stack to accurately model the heat exchange with the balance of plant components. Top and bottom surfaces, on the other hand, are assigned with... 

The reformer is a direct fired chemical reactor consisting of numerous tubes located in a firebox and filled with catalyst. Conversion of hydrocarbon and steam to an equilibrium mixture of hydrogen, carbon oxides and residual methane takes place inside the catalyst tubes. Heat for the highly endothermic reaction is provided by burners in the firebox. The heat is transferred to the catalyst filled reactor tubes by a combination of radiation and convection. 

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