Transient continuously stirred

Transient continuous stirred tank reactors Chapter 8, p. 137. 

X H kJ/mol. Since the sticking coefficient of H2 on Cu( 111) is not known, the prefactor for the sticking and the activation energy of the Cu(l 10) surface was used as areliable approximation. From the kinetic gas theory, a preexponential factor Aads = 9.2 x 10 (torr-s) was derived for our model. In our calculation we assumed 132 /xmol/gr active sites based on theexperimental results of Muhler et al. [6] for the Cu/Zn0/Al203 based on a H/ = H (2 Cu) = 1 1 stoichiometry. Under the conditons of perfect mixing gas phase condition is uniform throughout the bed. The reactor mass balance for a transient continuous stirred tank reactor was used in the form... 

For steady-state operation of a continuous stirred-tank reactor or continuous stirred-tank reactor cascade, there is no change in conditions with respect to time, and therefore the accumulation term is zero. Under transient conditions, the full form of the equation, involving all four terms, must be employed. 

The chemical reactor is the unif in which chemical reactions occur. Reactors can be operated in batch (no mass flow into or out of the reactor) or flow modes. Flow reactors operate between hmits of completely unmixed contents (the plug-flow tubular reactor or PFTR) and completely mixed contents (the continuous stirred tank reactor or CSTR). A flow reactor may be operated in steady state (no variables vary with time) or transient modes. The properties of continuous flow reactors wiU be the main subject of this course, and an alternate title of this book could be Continuous Chemical Reactors. The next two chapters will deal with the characteristics of these reactors operated isothermaUy. We can categorize chemical reactors as shown in Figure 2-8. 

The principal advantage of continuous reaction vessels is that they operate (after an initial transient period) under steady-state conditions that are conducive to the formation of a highly uniform and well-regulated product. In this section, we shall confine the discussion to continuous stirred-tank reactors (CSTRs). These reactors are characterized by isothermal, spatially uniform operation. 

In an ideal continuous stirred tank reactor, composition and temperature are uniform throughout just as in the ideal batch reactor. But this reactor also has a continuous feed of reactants and a continuous withdrawal of products and unconverted reactants, and the effluent composition and temperature are the same as those in the tank (Fig. 7-fb). A CSTR can be operated under transient conditions (due to variation in feed composition, temperature, cooling rate, etc., with time), or it can be operated under steady-state conditions. In this section we limit the discussion to isothermal conditions. This eliminates the need to consider energy balance equations, and due to the uniform composition the component material balances are simple ordinary differential equations with time as the independent variable ... 

Using the thus built kinetic model, both the dynamic and steady state cases of a continuous stirred tank reactor were simulated. We show here the simulation results concerning transient effects. The case considered is the switching of feeds with different H2S concentrations. The shapes of the trajectories and the variations of activities are generally comparable to the experiment results (20,21), although the latter were obtained under low pressure (fig. 2). It is known that the addition of H2S depresses the HDS activity. Simulation results refine the conclusion. They confirm the experimentally found phenomenon in (22) that, for catalysts with different compositions, the depression... 

Miro, E.E., D.R. Ardiles, E.A. Lombardo, and J.O. Petunchi, "Continuous-Stirred Tank Reactor (CSTR) Transient Studies in Heterogeneous Catalysts", J. Catal., 97,43,1986... 

The development of practical methods [56] for the systematic design of new oscillating reactions in continuous stirred tank reactors (CSTR) lead to the discovery of several dozens of different isothermal oscillating systems, including the CIMA reaction [57]. This reaction is one of the very few to also exhibit transient oscillatory behavior in batch conditions. This and the fact that it does not exhibit marked excitability character like the well-known Belousov-Zhabotinsky reaction [5], lead us to select the CIMA reaction for systematic research on stationary spatial structures in open spatial reactors [14]. 

Kinetics in a Continuously Stirred Photochemical Tank Reactor. Gregoire, F. Lavabre, D. Micheau, 1. C. Gimenez, M. Laplante, J.P. (Lab. Interact. Mol. React. Chim. Photochim., Univ. Paul Sabatier, F-31062 Toulouse, Fr.). J. Photochem. 1985, 28 (2), 261-71 (Eng.). The continuously stirred photochem. tank reactor (REPAC) is a new semiautomatic app. for photochem. measurements. From the kinetic anal, of steady-state regimes, this open system allows detn. of the quantum yields, the thermal-return rate consts. and the spectra of photoproducts. Moreover, the kinetic anal, of transient regimes affords further information on the mechanism of the photochem. process. The possibUities of the REPAC are shown using various photochromic compds. Theor. anal, of the kinetic rate equations in the REPAC shows that quite unusual behavior, such as unstable steady states or photochem. oscillations, can be exhibited. Nonhnear photochem. reaction schemes are likely to show such behavior. 

From these examples it is apparent that one needs to be cautious when using steady-state methods and continuation procedures near turning points. While the solutions may converge rapidly and even appear to be physically reasonable, there can be significant errors. Fortunately, a relatively simple time-stepping procedure can be used to identify the nonphysical solutions. Beginning from any of the solutions that are shown in Fig. 15.9 as shaded diamonds, a transient stirred-reactor model can be solved. If the initial solution (i.e., initial condition for the transient problem) is nonphysical, the transient procedure will march toward the physical solution. If the initial condition is the physical solution, the transient computational will remain stationary at the correct solution. 

In Section 16.5 the transient stirred reactor equations are left in terms of the enthalpy, and not the temperature. Use the continuity equation and the definition of enthalpy dh = Cpdt to continue manipulating the equations such that temperature emerges as a dependent variable. 

An attractive property of monolithic reactors is their flexibility of application in multiphase reactions. These can be classified according to operation in (semi)batch or continuous mode and as plug-flow or stirred-tank reactor or, according to the contacting mode, as co-, counter-, and crosscurrent. In view of the relatively high flow rates and fast responses in the monolith, transient operations also are among the possibilities. 

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