Reactor nonstationary

Batch (noncontinuous) reactors are characterized by nonstationary conditions, that is, there are composition and heat generation changes during operation. 

On the basis of the considered macroscopic flow pattern, the dominant circulation flows (/ c and Fc/2) subdivide the reactor into three parallel levels, where each level is then divided into Nc/3 equally sized compartments of equal volume Vc = Vr/Nc. Every compartment is modeled as a nonstationary ideal continuous stirred tank reactor, with a main inlet and outlet flow, which connects the given compartment with adjacent compartments on the same level, and secondary exchange flow rates accounting for the turbulent mixing with adjacent compartments laying on the upper and/or lower level (Fig. 7.3). 

For the description of nonstationary processes in a periodic reactor, the CBR is used. The equations for gas-phase and surface component concentrations are described by the ODE system. 

The diagrams of the direct synthesis of alkyl- and arylchlorosilanes differ mostly in the construction of the contact apparatus. At present there are two basic types of contact apparatuses known stationary reactors, in which contact mass is in the immobile layer, and nonstationary reactors, in which contact mass is mobile. 

At present the direct synthesis of organochlorosilanes is conducted in nonstationary reactors, in which the reactive mass is agitated, either with... 

For industrially relevant process the relaxation time is ca. 1-100 s. For construction of a kinetic model for nonstationary conditions, knowledge about the evolution of the concentrations of adsorbed species on the catalyst surface is needed. Under nonstationary conditions the changes of concentration fields in time, reactor space and catalyst surface (for heterogeneous catalysis) are interrelated by complex dependencies. Therefore, for experimental investigation under nonstationary conditions, knowledge about the gas and surface composition is required. 

The main drawback of kinetic models, based only on steady-state data, is associated with the fact, that start-up and transient regimes cannot be reliably modeled. Kinetic models for nonstationary conditions should be applied also for the processes in fluidized beds, reactions in riser (reactor) - regenerator units with catalyst circulation, as well as for various environmental applications of heterogeneous catalysis, when the composition of the treated gas changes continuously. 

Stoichiometry, for example, in the case of nonstationary modes of reactor operation, needs another fundamental equation. For the CSTR the balance of a component is written as... 

Characteristically this equation contains again the time t as a process variable. This term with the time t disappears and can be neglected in the case of stationarity (if t t) and when Cj q = c , that is, in a batch reactor. Therefore the stoichiometric balance equation of a discontinuous system can be formally applied to a nonstationary continuous stirred tank. Similarly, the stoichiometric equations of heterogeneous reactor systems with interfacial mass transfer can be derived (Budde, Bulle, and Riickauf, 1981). 

RFR is working perpetually in nonstationary state forced by the reversion of the flow through the reactor. It was revealed by numerical simulation and was confirmed by the experiments (Matros, 1985) that after each reversion of the flow reaction zone is formed. The reaction zone has much higher temperature than the rest of the bed and... 

If the reactor is not operating at steady state, the calculation of the multiplication constant is more difficult, since, in the differential equation, there is now a time-derivative term to be considered. A convenient method for treating this case involves the use of the quantity Ve (introduced in Sec. 4.6d), a fictitious number of neutrons per fission, which may be adjusted so that for any composition and configuration the fission sources can be made just to balance the losses. Thus we choose as the applicable differential equation for the nonstationary system... 

The nonstationary (transient) operation of chemical reactors is traditionally applied in kinetic research in order to reveal reaction mechanisms. Pulses and step changes can be introduced in continuous reactors, and concentration changes at the reactor outlet are monitored by on-line or off-line analysis. The method is applicable to both gas- and liquid-phase systems. Isotope exchange can be commenced wherein H2/D2 (hydrogen/deuterium) experiments can reveal the role of hydrogen in a catalytic process. Some examples of catalytic isomerizations are displayed in Figure 9.22. A reaction network for hydrocarbon isomerization is shown in Figure 9.22. 

Under nonstationary conditions, in particular at periodic change of input concentration, it is possible to obtain even more narrow MWD [42—44] than 1 -I- r, obtained for stationary process. However, such regime, similar to the regime of equal distribution of conversions, is hardly practicable on a commercial scale. More realistic is the isothermal regime (temperature in ail reactors is equal). For such a case, the conversion in the kth reactor is described by a known fimction [39]... 

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