Reactor radial flow types

Figure 17.25. Catalytic reforming reactors of axial and radial flow types. The latter is favored because of lower pressure drop (Sukhanov, Petroleum Processing, Mir, Moscow, 1982). (a) Axial flow pattern, (b) Radial flow pattern.

Retrofitting features of the more efficient reactor types have been the principal thmst of older methanol plant modernization (17). Conversion of quench converters to radial flow improves mixing and distribution, while reducing pressure drop. Installing an additional converter on the synthesis loop purge or before the final stage of the synthesis gas compressor has been proposed as a debotdenecking measure. 

Catalysts intended for different appHcations may require their own unique types of reactor and operating conditions, but the key to designing a successful system is to use the same feedstock composition that is expected in the ultimate commercial installation and to impose so far as is possible the same operating conditions as will be used commercially (35). This usually means a reactor design involving a tubular or smaH-bed reactor of one type or another that can simulate either commercial multitubular reactors or commercial-size catalyst beds, including radial flow reactors. 

In a fixed bed reactor, gas phase reactions are generally carried out using a stationary bed of solid catalyst. In a typical reactor, suitable screens support the bed of catalyst particles, through which the gas phase flows. Gaseous reactants adsorb on the catalyst surface, reactions occur on this surface and reaction products desorb back to the gas phase. Two major types of fixed bed reactor are the conventional axial flow fixed bed reactor and the radial flow fixed bed reactor. These types are shown... 

FIGURE 13.1 Types of fixed bed reactors, (a) Axial flow fixed bed reactor Up or down flow, single or multi-stage, with or without inter-stage cooling, single or multi-tubular, (b) Radial flow fixed bed reactor Radially inward or outward flow, straight or reverse flow (direction of inlet and outlet is same or opposite to each other). 

Figure 17.21. Some recent designs of ammonia synthesis converters, (a) Principle of the autothermal ammonia synthesis reactor. Flow is downwards along the wall to keep it cool, up through tubes imbedded in the catalyst, down through the catalyst, through the effluent-influent exchanger and out. (b) Radial flow converter with capacities to l tons/day Haldor Topsoe Co., Hellerup, Denmark), (c) Horizontal three-bed converter and detail of the catalyst cartridge. Without the exchanger the dimensions are 8 x 85 ft, pressure 170 atm, capacity to 2000 tons/day (Pullman Kellogg), (d) Vessel sketch, typical temperature profile and typical data of the ICI quench-type converter. The process gas follows a path like that of part (a) of this figure. Quench is supplied at two points (Imperial Chemical Industries).

Another type of reactor has one or more annular beds of catalyst with radial flow of gas either inward or outward [5], as shown in Figure 3.12b. This type may be preferred when the diameter of an axial-flow reactor would be much greater than the required bed depth. By putting the same amount of catalyst in a narrower but longer reactor, the wall thickness can be reduced and the reactor cost decreased. This is particularly important for high-pressure reactions, such as the synthesis of ammonia. 

Cabaret et al. (2008) and Gagnon et al. (1998) concluded that better mixing and higher product conversion can be achieved if a close clearance impeller, such as the helical ribbon, is used in conjunction with a radial flow impeller such as the RT in a highly viscous system. The Rushton-type turbine provides proper gas dispersion, while the close clearance impeller attempts to contact most of the reactor volume and provides proper bulk mixing, shear distribution, lower apparent viscosity, and minimal stagnant zones (Tecante and Choplin, 1993). These effects also lead to higher reactor utilization and can decrease power requirements. 

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