Steam tube configuration

Other variables of importance in designing these tubular pyrolysis reactors include the mass velocity (or flow velocity) of the gaseous reaction mixture in the tubes, pressure, steam-to-hydrocarbon-feedstock ratio, heat flux through the tube wall, and tube configuration and spacing. Pressure drop in the reactor is of major importance, especially because of the extremely high flow velocities normally employed. 

Figure 16 shows a conventional reactor tube configuration as present in a steam reformer furnace. The preheated hydrocarbon feedstock passes through the catalyst tube in which it reacts producing an equilibrium mixture of hydrogen and carbon oxides the reformer effluent, at a temperature, which is in the range of 800-950°C, is then sent to the process gas boiler where steam is generated. 

In the ART regenerative catalyst tube configuration (Fig. 17), the effluent flows upward from the bottom to the exit of the catalyst tube inside the regenerative tube. With this arrangement, more than 25% of the heat required for the endothermic steam-reforming reaction can be provided by the reforming effluent. As a result, the amount of fuel is reduced and the temperature of the reformer effluent decreases. 

The shell/tube configuration of tubular PBRs depends on the nature of the catalytic reaction. For highly endothermic reactions such as catalytic steam reforming, the reactor geometry is similar to that of a fired furnace in which the catalyst-packed tubes are heated by the energy released by the combustion of a fuel on the shell side. Catalytic steam reforming involves the conversion of a hydrocarbon to a hydrogen-rich mixture in the presence of steam ... 

Water tube boiler design and construction provide for much greater capacity, pressure, and versatility than FT boilers because of the subdivision of pressure parts and the ability to rearrange boiler components into a wide variety of configurations. As a result, steam output may be from under 1,500 lb/hr to several million lb/hr. Designers have, over the years, developed WT boilers for many diverse industrial process applications. 

Most boiler plants with electrical power generating facilities employ surface condensers. These are shell-and-tube heat exchangers in either one-, two-, or four-pass configurations. Surface condensers typically receive cooling water on the tube-side and steam on the shell-side of the heat exchanger. The LP turbine steam generally is received at the top of the condenser and proceeds through the condenser in a downward flow, while the FW turbine exhaust steam enters at the side. 

Industrial steam reformers are usually fixed-bed reactors. Their performance is strongly affected by the heat transfer from the furnace to the catalyst tubes. We will model both top-fired and side-fired configurations. 

The constraints changed from one trial configuration of the reaction system to the next, but typically included things like the minimum coolant temperature to permit efficient utilization of the heat of reaction as process steam, the maximum allowable aldehyde concentration in the condensed crude product to avoid refining and product specification problems, and a prescribed reactor pressure drop to insure adequate flow distribution among the reactor tubes at a minimum energy cost. All of these are implicit constraints — they establish the maximum or minimum levels for certain response variables. Explicit constraints comprise the ranges for search variables. 

---