Other Reactor Types

Other reactor types are also used for gas-liquid reactions, but they are not very common in fine chemicals manufacture. Spray towers and jet reactors are used when the liquid phase is to be dispersed. In spray towers the liquid is sprayed at the top of the reactor while the gas is flowing upward. The spray reactor is useful when a solid product, possibly suspended in the liquid, is formed, or if the gas-phase pressure drop must be minimized. In a jet reactor, the liquid is introduced to the reaction zone through a nozzle. The gas flows in, being sucked by the liquid. 

For a constant-volume batch reactor operated at constant T and pH, an exact solution can be obtained numerically (but not analytically) from the two-step mechanism in Section 10.2.1 for the concentrations of the four species S, E, ES, and P as functions of time t, without the assumptions of fast and slow steps. An approximate analytical solution, in the form of a rate law, can be obtained, applicable to this and other reactor types, by use of the stationary-state hypothesis (SSH). We consider these in turn. 

The small conversions needed in differential reactors require more accurate measurements of composition than the other reactor types. 

In conclusion, a fast and exothermal reaction such as this cannot be performed in a batch reactor. This reaction will also be studied in other reactor types in the following chapters. 

In a stirred tank, either liquid can be made continuous by charging that liquid first, starting the agitator, and introducing the liquid to be dispersed. For other reactor types, the choice of which phase is continuous and which is dispersed will depend on the physicochemical properties of the phases and operating conditions (such as temperature,... 

Continuously operated, fixed bed reactors are frequently used for kinetic measurements. Here the reactor is usually a cylindrical tube filled with catalyst particles. Feed of a known composition passes though the catalyst bed at a measured, constant flow rate. The temperature of the reactor wall is usually kept constant to facilitate an isothermal reactor operation. The main advantage of this reactor type is the wealth of experience with their operation and description. If heat and mass transfer resistances cannot be eliminated, they can usually be evaluated more accurately for packed bed reactors than for other reactor types. The reactor may be operated either at very low conversions as a differential reactor or at higher conversions as an integral reactor. 

There are numerous reactor types, but in this chapter the objective is to consider only a few common types. These are batch, continuous stirred tank, homogenous plug flow and fixed bed catalytic reactors. To size other reactor types and for a more thorough treatment of reactor design than presented here, the reader can consult books written on reactor design, such as Fogler [16], Smith [23], and Forment and Bischoff [31]. 

The idealized plug-flow and bateh reactors are the only two classes of reaetors in which all the atoms in the reaetors have the same residenee time. In all other reactor types, the various atoms in the feed spend different times inside the reactor that is, there is a distribution of residenee times of the material within the reactor. For example, eonsider the CSTR the feed introduced into a CSTR at any given time beeomes eompletely mixed with the material already in the reactor. In other words, some of the atoms entering the CSTR leave it almost immediately, because material is being continuously withdrawn from the reactor other atoms remain in the reactor almost forever because all the material is never removed from the reactor at one time. Many of the atoms, of eourse, leave the reactor after spending a period of time somewhere in the vieinity of the mean residence time. In any reactor, the distribution of residence times ean significantly affect its performance. 

Yield-shift reactors are particularly useful when modeling streams that contain pseudocomponents, solids with a particle size distribution, or processes that form small amounts of many byproducts. These can all be described easily in yield correlations but can be difficult to model with the other reactor types. 

Besides the developments described, stirred-tank reactors will stay as standard reactors, but the implementation of other reactor types (e. g., bubble columns) might be recommended compared with stirred tank reactors the retention time characteristics might be beneficial to substrate conversion and sty. 

This chapter is concerned primarily with the electrochemistry of BWR and PWR primary cooling circuits and the impact that electrochemistry has on the accumulation of damage due to SCC. The available space does not permit discussion of other worthy subjects, such as PC and CF, nor does it allow the author to explore the impact of electrochemistry on the coolants in other reactor types. In keeping with the encyclopedic nature of the article, the general principles are presented and the reader is referred to the published literature for detailed information on specific topics. 

For reactors in which the dispersion number varies significantly with changing hydraulic loading (i.e., less stable hydraulics) the effectiveness of the floe formation process is more significant than for other reactor types. 

As mentioned earlier, downdraft gasifiers are simpler to construct and operate than the other reactor types, but they have more exacting fuel requirements which include ... 

The discovery of solid catalysts led to a breakthrough of the chemical process industry. Today most commercial gas-phase catalytic processes are carried out in fixed packed bed reactors. A fixed packed bed reactor consists of a compact, immobile stack of catalyst pellets within a generally vertical vessel. On macroscopic scales the catalyst bed behaves as a porous media. The fixed beds are thus employed as continuous tubular reactors in which the reactive species in the mobile fluid (gas) phase are reacting over the catalyst surface (interior or exterior) in the stationary packed bed. Compared to other reactor types or designs utilizing heterogeneous catalysts, the fixed packed bed reactors are preferred because of simpler technology and ease of operation. 

In order to maximize the cross section for fission, which is greatest for low energy neutrons, the neutrons are slowed down or "moderated by a material (the moderator) that elastically scatters neutrons but has a small neutron capture cross section. In LWRs ordinary (but very pure) water serves the purpose of both moderation and cooling (in other reactor types the moderator may be a liquid like D2O, a solid material like graphite or absent and the cooling medium may be a gas or a metal like lead, mercury or sodium). 

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