Terrace wall

With a Terrace Wall or side-fired unit, the limit to capacity will generally not be apparent in the radiant section. The increase in firing is matched by an increase in heat transfer on the inside of the catalyst tube. So the actual tube temperatures show only a minor increase86. 

Figure 1.8 gives a simplified representation of the steam reforming section for natural gas, using a terrace wall ftirnace equipped with peripheral facilities (desulfurization, waste heat boilers, steam drum etc.). 

Today contractors and licensors use sophisticated computerized mathematical models which take into account the many variables involved in the physical, chemical, geometrical and mechanical properties of the system. ICI, for example, was one of the first to develop a very versatile and effective model of the primary reformer. The program REFORM [361], [430], [439] can simulate all major types of reformers (see below) top-fired, side-fired, terraced-wall, concentric round configurations, the exchanger reformers (GHR, for example), and so on. The program is based on reaction kinetics, correlations with experimental heat transfer data, pressure drop functions, advanced furnace calculation methods, and a kinetic model of carbon formation [419],... 

In many modern top-fired reformers the heat flux calculated for the inner tube wall surface is around 60 000 W/m2, although in some designs it cam be as high as 75 000 W/m2. The maximum heat flux may be 1.4 to 2 times higher. In side-fired and terraced-wall furnaces, where the mean fluxes are generally in the range of 60 000-85 000 W/m2, the difference between mean and maximum flux is much smaller, as shown in Figure 37 [444],... 

The terraced-wall type, developed by Foster Wheeler may be regarded as an intermediate between the side-fired and bottom-fired tubes. The reformer has inclined walls with several terraces on which upward firing burners are installed. This unique burner positioning makes it possible to adjust the heat flux in each zone. Figure 41 is a schematic drawing of the Foster Wheeler terraced-wall furnace [426],... 

FIG. 23 Schematic illustration of the formation of an SECM-induced dissolution pit with terraced walls. The UME induces dissolution from the area directly under it (a), resulting in a current surge. The flux of material from this area then rapidly declines and slow dissolution occurs from the edge of the resulting pit (b), resulting in expansion. Concurrently, the area directly under the center of the UME becomes increasingly undersaturated, until the critical value for the creation of fresh dissolution sites is attained, when there is a subsequent burst of material from the surface (c), producing a terraced structure within the pit. The overall process may then be repeated. 

Figure 2.10 Foster Wheeler terraced-wall reformer. Source [20],...

Diagram c in Fig. 114 gives the metal skin temperatures of the vertical tubes for a terrace wall furnace, as well as the temperatures reached by the wa s and supports. The top part of the figure shows the suspension of the coils by hangers, rod and counterweight Heating is provided in this case by combination burners running both on gas and fad oil atomized with steam, in the form of a line burner for the gas. and a tunnel burner for the fuel. 

Test furnace capable of simulating terrace wall-fired heaters. 

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.

If dT/dz becomes very high as in the upper part of top fired reformers or in the bottom of bottom fired or terrace wall fired reformers, or if the catalyst looses activity, e.g. by poisoning or ageing, then the reaction rate may not be able to follow the rate of heat input, and the temperatures of the gas and the tube wall and also the approach to equilibrium will increase. 

The Foster Wheeler reformer furnace is a Terrace-Wall furnace. The unique feature of this side-fired design is the burner location (Fig. 27 and 28). The burners are directed at the walls of the furnace, which radiate heat to the tubes. As with the top-fired reformer, the process gas enters the top and passes to the bottom. Unlike many top-fired furnaces, the terrace-walled furnace tubes have... 

Terraced -wall steam reformer. (Courtesy of Foster Wheeler Coiporatioa)... 

Figure 9.2. Schematic arrangement of a terrace wall fired furnace for the primary reformer in synthesis gas production. Reprinted from Catalyst Handbook, 2 ed., by kind permission ofM. Twigg.

---