Unsteady-State Flows in Fixed-Bed Reactors

Imaging Unsteady-State Flows in Fixed-Bed Reactors 

The results presented in Fig. 27 represent an exciting advance because they demonstrate that the GERVAIS pulse sequence allows direct imaging of regions of liquid recirculation in both the z- and. vv-vclociiy fields. However, a word of caution is needed. Because GERVAIS is an EPI-based technique, it requires that variations in magnetic susceptibility within the sample be minimized. In this system, plastic spheres were used, which were naturally susceptibility-matched with water. If this 

---

A.2. Imaging Unsteady-State Flows in Fixed-Bed Reactors... 

Figure 2. Schemes of fixed-bed reactors operated under forced unsteady-state conditions (a) Reverse-flow reactor (b) Rotary reactor (c) Reactor system with periodic changes between the inlet and outlet ports in two fixed beds. The tables show positions of switching valves during two successive cycles C = valve closed O = valve open.

Gas-solid reactions are carried out on a commercial basis using fixed-bed, moving-bed, and fluidized-bed reactors. The fixed-bed reactor is an unsteady-state system as reactive gas is fed on a continuous basis through the reactor that is packed with a finite quantity of solid reactant. The solid is depleted and breakthrough of the gas reactant occurs after a certain reaction time. In the moving-bed reactor, both solid and gas are fed on a continuous basis and overall operation is steady state. The fluidized-bed reactor, where small solid particles are fluidized by upward flow of gas, also operates in a steady-state manner. Diffusional reaction resistances are reduced because of the small solid particles while solid backmixing reduces solid concentration gradients and promotes isothermal operation. 

Periodic flow reversal inducing forced unsteady-state conditions [339]. The flow to the reactor is continuously reversed before the steady state is attained. A dual hot-spot temperature profile, characterized by a considerably lower temperature than in the single hot spot that would develop in the traditional flow configuration, forms in exothermic oxidation reactions. An increase in selectivity and better reactor control (lower risk of runaway) is possible over fixed-bed reactor operations, but compared... 

The plot of consumption of reactant and production of products according to space time is shown in Figure 10. The unsteady period of this reaction was longer than 1 h. The effectiveness factor q, was assumed to be constant at steady state condition, the relative reaction-rate constants were calculated and are listed in Table I. The pressure drop was controlled below 1 psia. Different amounts (1.2 and 1.8 g) of catalyst were housed in dilTerent bed heights (20. 30 cm ). The conversion of reactant and yields of products for various residence times (or liquid flow rates) were obtained from the same catalytic bed (i.e. the same amount of the catalyst) at the steady state condition. The curve shapes of products in the slurry reactor arc different from those in the fixed-bed reactor, as shown in Figure 11. 

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. 

Several other examples for potential application of reverse-flow operated catalytic reactors are described in Ref. 9. Also, other potential techniques of forced unsteady-state operation which allow for combining chemical reaction and heat exchange in a fixed catalyst bed are discussed. One such technique is sequential switching between inlet and outlet ports of the reaction gas between two or more packed beds (Fig. 2(c)). In this case, the thermal wave travels continuously through a series of packed beds in one direction, as if along a closed ring. However, this operation is more complex and requires more catalyst than the reverse-flow operation. 

The partial derivative in temperature given by equation (20.62) may be compared with the derivative of the temperature in a reaction vessel of fixed volume, given by equation (13.45). The reader may notice that the partial derivative with respect to time of equation (20.62) depends on the enthalpy of each reaction, A//y, whereas the total derivative of equation (13.45) depends on the internal energy of (each) reaction, A(7y, a different quantity, although numerically similar (see Chapter 13, Section 13.8). The reason for this apparent anomaly is that the mass flow has been considered to be in an evolving steady state in the catalyst bed reactor, whereas the analysis of the reaction vessel allows for unsteady mass flow. 

---