Reactor design transient behavior

Nowadays, the most common small-scale application of hydrogen is the use in residential or mobile fuel cell systems. Special requirements of this application are compact design, integrated CO-removal, high energetic efficiency, quick start-up and fast transient behavior. The proposed solutions comprise unit-operation-based concepts as well as multifunctional, micro-structured reactors. 

Altshuller, D., "Design Equations and Transient Behavior of the Countercurrent Moving-Bed Chromatographic Reactor,"... 

In all these cases, the correct design must grow from the equations of mass, energy, and momentum balance to which we now turn in the next few sections. From these we proceed to the design problem (Sec. 9.5) and hence to elementary considerations of optimal design (Sec. 9.6). The stability and sensitivity of a tubular reactor is a vast and fascinating subject. Since the steady state equations are ordinary differential equations, the equations describing the transient behavior are partial differential equations. This... 

The next task was to model the reformer itself to understand design issues and be able to predict performance of various reactor/catalyst types and transient behavior. However, upon trying to obtain kinetic rate expressions for the reforming reactions, it was found that very little information existed in the public domain. This led to the decision/need to develop reaction kinetics for catalytic partial oxidation and steam reforming at National Energy Technology Laboratory s (NETL s) onsite research facility. 

In general, it can be concluded that substantial progresses have been made in the experimental and theoretical analysis of trickle-bed reactors under unsteady-state conditions. But until now these results are not sufficient for a priori design and scale-up of a periodically operated trickle-bed reactor. The mathematical reactor models, which are now available are not detailed enough to simulate all of the main transient behavior observed. For solving this problem specific correlations for specific model parameters (e.g. Hquid holdup, mass transfer gas-solid and liquid-solid, intrinsic chemical kinetic, etc.) determined under dynamic conditions are required. The available correlations for important hydrodynamic, mass-and heat-transfer parameters for periodically operated trickle-bed reactors leave a lot to be desired. Indeed, work for unsteady-state conditions on a larger scale may also be necessary. 

The fifth question focuses on a particular fixed volume element in the reactor and whether it changes as a function of time. If it does not, then the reactor is said to operate at a stationary state. If there are time variations, then the reactor is operating under transient conditions. A nontrivial example of the transient situation is designed on purpose to observe how a chemically reactive system at equilibrium relaxes back to the equilibrium state after a small perturbation. This type of relaxation experiment can often yield informative kinetic behavior. 

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]. 

A RELAP5-3D model has been developed to analyze both steady-state and dynamic behavior of a reactor coupled to a parallel pair of closed-loop Brayton-cycle systems. Figures 2 and 3 contain schematics of the input model. A brief description of this model and some steady state and transient results are described. If the design effort had continued, it is judged that RELAP5-3D with the appropriate input procedure could have been used to answer mar of the Gas-Brayton design and operational questions. 

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