Considerations for Reactors

Altliough some reactors (e.g., those u.sed in methanol and catalytic reforming units) are. spherical, most are vertically mounted ve.ssels with elliptical heads. There are fewer connections for the plant layout designer to be concerned with than there are with the dLstillation tower. Generally, connections are limited to inlet and outlet, maintenance access, unloading,. sample, and temperature. Internally, reaaors are furnished with bed supptms, screens, inlet baffles, outlet collectors, catalysts, and inert materials. Exhibit 9-3 depicts a typical reactor and its principal components. 

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Brasie,W.C., "Some Design Considerations for Reactors for Mass and Solution Polymerization . Paper presented at 63rd. National A.I.Ch.E. Meeting, St. Louis, (Feb. 1968). 

Qualitative Considerations for Reactor Choice, Scaleup and Optimization... 

Evidently, similar considerations for reactor residence time distribution can be made for any polymer made in a series of reactors where the conditions vary from one reactor to the other, such as the polyethylene pipe resins described earlier. 

Italian Approach to Severe Accidents" Consideration for Reactors of the Next Generation", by A. Ferreli, ANPA... 

Plasticity. A series of relaxation tests of INOR-8 at 1200 and 1300°F has indicated that creep will be an important design consideration for reactors operating in this temperature range. The rate at which the stress must be relaxed in order to maintain a constant elastic strain at 1300°F is shown in Fig. 1.3-14, and similar data for 1200°1 are presented in Fig. 13-15. The time lapse before the material becomes plastic is about 1 hr at 1300°F and about 10 hr at 1200°F. The time period during which the material behaves elastically becomes much longer at lower temperatures, and below some temperature, as yet undetermined, the metal will continue to behave elastically indefinitely. 

In 1994 Stamicarbon introduced a pool condenser in the synthesis section (see Figs. 4 and 5). This allowed a 34% decrease in reactor volume and a 45% decrease in carbamate heat-exchange area, thus reducing costs considerably for equipment, stmctural steel, and constmction. 

Among continuous reactors, the dominant system used to produce parasubstituted alkylphenols is a fixed-bed reactor holding a soHd acid catalyst. Figure 3 shows an example of this type of reactor. The phenol and alkene are premixed and heated or cooled to the desired feed temperature. This mix is fed to the reactor where it contacts the porous soHd, acid-impregnated catalyst. A key design consideration for this type of reactor is the removal of the heat of reaction. 

Porosity and Pore Size. The support porosity is the volume of the support occupied by void space and usually is described in units of cm /g. This value represents the maximum amount of Hquid that may be absorbed into the pore stmcture, which is an especially important consideration for deposition of metal salts or other active materials on the support surface by Hquid impregnation techniques. The concentration of active material to be used in the impregnating solution is deterrnined by the support porosity and the desired level of active material loading on the catalyst. If the porosity is too low, inefficient use of the support material and reactor volume may result. If the porosity is too high, the support body may not contain sufficient soHd material to provide the strength necessary to survive catalyst manufacture and handling. 

For reactors containing flammable liquids, where the reactor design pressure is insufficient to contain a deflagration, consideration should be given to providing an inert gas blanket (usually nitrogen). 

Size requirements are limited by packaging considerations for neutron irradiation. Typically, polyethylene or quartz containers are used to contain the sample in the reactor core. For example. Si wafers are cleaved into smaller pieces and dame sealed... 

This is the actual mental function required by the task that failed (see Figure 2.17). In the case study imder consideration the failure was at the Execute Action stage of the stepladder model, since the worker intended to operate the valve for reactor A, so there was no question of failure in the selection of actions. The connection with the task characteristics box indicates the fact that action is a fimction required by the task. 

Practical considerations of unloading play a major role in batch-mass reactors. For "low conversion reactors, a suitably sized dump line must be chosen, based on acceptable N2 dump pressure in the reactor, or discharge pump capability, as well as consideration for limiting reaction in the line between batches. 

When setting the conditions in chemical reactors, equilibrium conversion will be a major consideration for reversible reactions. The equilibrium constant Ka is only a function of temperature, and Equation 6.19 provides the quantitative relationship. However, pressure change and change in concentration can be used to shift the equilibrium by changing the activities in the equilibrium constant, as will be seen later. 

In this introductory chapter, we first consider what chemical kinetics and chemical reaction engineering (CRE) are about, and how they are interrelated. We then introduce some important aspects of kinetics and CRE, including the involvement of chemical stoichiometry, thermodynamics and equilibrium, and various other rate processes. Since the rate of reaction is of primary importance, we must pay attention to how it is defined, measured, and represented, and to the parameters that affect it. We also introduce some of the main considerations in reactor design, and parameters affecting reactor performance. These considerations lead to a plan of treatment for the following chapters. 

As a preliminary consideration for these two micromixing models, we may associate three time quantities with each element of fluid at any point in the reactor (Zwietering, 1959) its residence time, t, its age, ta, and its life expectancy in the reactor (i.e., time to reach the exit), te ... 

Since many important gas-phase catalytic reactions are reversible, we focus on the implications of this characteristic for reactor design. These stem from both equilibrium and kinetics considerations. 

In the quantitative development in Section 24.4 below, we assume the flow to be ideal, but more elaborate models are available for nonideal flow (Chapter 19 see also Kastanek et al., 1993, Chapter 5). Examples of types of tower reactors are illustrated schematically in Figure 24.1, and are discussed more fully below. An important consideration for the efficiency of gas-liquid contact is whether one phase (gas or liquid) is dispersed in the other as a continuous phase, or whether both phases are continuous. This is related to, and may be determined by, features of the overall reaction kinetics, such as rate-determining characteristics of mass transfer and intrinsic reaction. 

POX reactor exit temperatures vary widely. Noncatalytic processes for gasoline reforming require temperatures in excess of 1,000°C. These temperatures require the use of special materials and significant preheating and integration of process streams. The use of a catalyst can substantially reduce the operating temperature allowing the use of more common materials such as steel. Lower temperature conversion leads to less carbon monoxide (an important consideration for low temperature fuel cells), so that the shift reactor can be smaller. Lower temperature conversion will also increase system efficiency. 

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