Unsteady-State Reactor Operation

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. 

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. 

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 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. 

It has also been recognized that micro-structured components, because of their low mass and thermal inertia, are able to offer short response times for unsteady state periodic operations. Micro-reactors have been used successfully for fluorination, oxidations and both heterogeneous [63-65] and homogeneous hydrogenations [66]. A review on gas-liquid micro-structured reactors has been published [67]. The very small material inventory when using micro devices offers another advantage, notably as a laboratory tool for screening applications, kinetics determination and process data acquisition, where the main concern is... 

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 use of the Ljungstroem air heater as SCR reactor is another strategy for NO abatement in power plants that is based on unsteady-state periodic operation [56]. Indeed, catalytic reactors in SCR technology require considerable volumes that are not always available when existing boilers have to be retrofitted. Moreover the high costs of traditional SCR installations favor alternative solutions requiring limited modifications of existing plants. 

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. 

Unsteady-State Direct Oxidation Process. Periodic iatermption of the feeds can be used to reduce the sharp temperature gradients associated with the conventional oxidation of ethylene over a silver catalyst (209). Steady and periodic operation of a packed-bed reactor has been iavestigated for the production of ethylene oxide (210). By periodically varyiag the inlet feed concentration of ethylene or oxygen, or both, considerable improvements ia the selectivity to ethylene oxide were claimed. 

A batch reactor is never a steady-state process operator, unlike a continuous process in which a steady state is defined as the state of a process in which there is no change with time of any condition of the process. This includes the amount and average composition of the material within the process, so that in a continuous process, there can be no accumulation or depletion. Notwithstanding an unsteady-operation where composition changes with... 

Silveston, P. L., Hudgins, R. R., Bogdashev, S., Vernijakovskaja, N and Matros, Yu. Sh., Modelling of a periodically operated packed bed S02 oxidation reactor at high conversion in Unsteady-state Processes in Catalysis (Matros, Yu. Sh., Ed.). VNU Science Press, Utrecht, 1990. 

The operation of the reactor is inherently unsteady-state for example, batch composition changes with respect to time. 

The general characteristics of a batch reactor (BR) are introduced in Chapter 2, in connection with its use in measuring rate of reaction. The essential picture (Figure 2.1) in a BR is that of a well-stirred, closed system that may undergo heat transfer, and be of constant or variable density. The operation is inherently unsteady-state, but at any given instant, the system is uniform in all its properties. 

A semibatch reactor is a variation of a batch reactor in which one reactant may be added intermittently or continuously to another contained as a batch in a vessel, or a product may be removed intermittently or continuously from the vessel as reaction proceeds. The reaction may be single-phase or multiphase. As in a batch reactor, the operation is inherently unsteady-state and usually characterized by a cycle of operation, although in a more complex manner. 

A semicontinuous reactor is a reactor for a multiphase reaction in which one phase flows continuously through a vessel containing a batch of another phase. The operation is thus unsteady-state with respect to the batch phase, and may be steady-state or unsteady-state with respect to the flowing phase, as in a fixed-bed catalytic reactor (Chapter 21) or a fixed-bed gas-solid reactor (Chapter 22), respectively. 

As in the case of a batch reactor for commercial operation, a CSTR is normally used for a liquid-phase reaction. In the laboratory, it may also be used for a gas-phase reaction for experimental measurements, particularly for a solid-catalyzed reaction, as in Figure 1.2. The operation is normally one of steady-state, except for startup, shutdown, and operational disturbances or upsets, in which cases unsteady-state operation has to be taken into account. 

Although there is steady flow of feed, the reactor is in unsteady-state operation during the time t, since the outlet concentration cA (and hence fA) is continuously changing (cA decreasing from cAo, and fA increasing from 0). 

It is not possible to predict a priori which of the possible stationary-states is actually attained, based on steady-state operating considerations. It may be done by integrating the material-balance equation in unsteady-state form, equation 14.3-2 or equivalent, with the given rate law incorporated. For this, the initial concentration of A in the reactor cA(f) at t= 0 must be known this is not necessarily the same as cAg. 

In comparing the TIS and DPF reactor models, we note that the former is generally easier to use for analysis of reactor performance, particulariy for nonlinear kinetics and unsteady-state operation. 

Remark 1. Eq.(9) can be used on a CSTR in unsteady state operation or in a semibatch reactor when there is no phase change. 

In the batch reactor, or BR, of Fig. 5.1 the reactants are initially charged into a container, are well mixed, and are left to react for a certain period. The resultant mixture is then discharged. This is an unsteady-state operation where composition changes with time however, at any instant the composition throughout the reactor is uniform. 

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