Unsteady state reactors

The general material balance of Section 1.1 contains an accumulation term that enables its use for unsteady-state reactors. This term is used to solve steady-state design problems by the method of false transients. We turn now to solving real transients. The great majority of chemical reactors are designed for steady-state operation. However, even steady-state reactors must occasionally start up and shut down. Also, an understanding of process dynamics is necessary to design the control systems needed to handle upsets and to enable operation at steady states that would otherwise be unstable. 

Nauman (J ) has proposed an interesting treatment of RTD in unsteady-state reactors. Two washout functions are defined ... 

Unsteady-state reactor operation is traditionally considered to be related to the performance of catalytic processes which are characterized by quick loss in catalyst activity. For such processes as, for example, catalytic cracking (Section B.3.10) or dehydrogenation of alkanes (Section B.4.3), a sequence of reaction and regeneration stages is unavoidable and should be included into the design. 

After specifying the energy form, the catalyst and the phases in contact, the next task is to decide whether to conduct the reaction in a batch or continuous mode. In the batch mode, the reactants are charged to a stirred-tank reactor (STR) and allowed to react for a specified time. After completing the reaction, the reactor is emptied to obtain the products. This operating mode is unsteady state. Other unsteady-state reactors are (1) continuous addition of one or more of the reactants with no product withdrawal, and (2) all the reactants added at the beginning with continuous withdrawal of product. At steady-state, reactants flow into and products flow out continuously without a change in concentration and temperature in the reactor. 

The energy of the system at any instant in time, is the sum of the products of the number of moles of each species in the system multiplied by their respective energies. This term will be discussed in more detail when unsteady-state reactor operation is considered in Chapter 9. 

The performance of trickle-bed reactors may be affected by many factors, such as interphase mass transfer, intraparticle diffusion, axial dispersion and incomplete catalyst wetting. Therefore, knowledge about these influenced factors is important for their mathematical description by an unsteady-state reactor model. Until now, the literature analysis shows the experimental and theoretical understanding of trickle-bed reactors under unsteady-state-operation conditions has improved, but not considerably. The following studies are focused on the trickling regime under unsteady-state-operation conditions. 

Matros, Y. S., Bunimovich, G. A., Unsteady-state reactor operation, in Handbook of Heterogeneous Catalysis, Vol. 3, 1464-1479 (Editors G. Ertl, H. Kndzinger, J. Weitkamp), Wiley-VCH, 1998... 

The case of a steady- or unsteady-state reactor with a non-linear reaction rate is more complicated. For example, with a second-order reaction and isothermal conditions ... 

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