Reactors with moving bed of catalysts

Figure 17.28. Reactors with moving beds of catalyst or solids for heat supply, (a) Pebble reactor for direct oxidation of atmospheric nitrogen two units in parallel, one being heated with combustion gases and the other used as the reactor (Ermenc, (1956). (b) Pebble heater which has been used for making ethylene from heavier hydrocarbons (Batchelder and Ingols, 1951). (c) Moving bed catalytic cracker and regenerator for 20,000 bpsd the reactor is 16 ft dia, catalyst circulation rate 2-7 Ibs/lb oil, attrition rate of catalyst 0.1-0.5 Ib/ton circulated, pressure drop across air lift line is about 2psi (L. Berg, in Othmer, 1956).

Another group of FUSO combines chemical reaction with separation of products. These methods can employ a reactor with circulating bed of catalyst [3,4] or by periodic changes of feed and product ports in a reactor with several fixed beds, known as simulated moving bed reactor [5-7]. Reaction and separation can include periodic pressure changes using the known separation technique of pressure-swing adsorption. 

FIG. 23-24 Reactors with moving catalysts, a) Transport fluidized type for the Sasol Fischer-Tropsch process, nonregenerating, (h) Esso type of stable fluidized bed reactor/regeuerator for cracldug petroleum oils, (c) UOP reformer with moving bed of platinum catalyst and continuous regeneration of a controlled quantity of catalyst, (d) Flow distribution in a fluidized bed the catalyst rains through the bubbles. 

This term embraces a wide class of potentially efficient techniques combining chemical reaction and separation in a catalytic reactor. If the reaction products are able to be adsorbed on the catalyst to different extents and for different lengths of time, these products can be separated from each other. This feature of catalytic processes can be used for enhancement of the reaction rate or selectivity, or for improvement of the quality of a desirable product. The process can be arranged in various ways, e.g. as a system with a fixed catalyst bed operated with periodic changes of the inlet composition or as various types of reactors with moving beds. To improve the separation, a mixture of catalyst and adsorbent can be loaded in the fixed bed reactor, or adsorbent can be fed into the reactor. 

The use of a fluidized-bed reactor has a number of advantages in the MTO process. The moving bed of catalyst allows the continuous movement of a portion of used catalyst to a separate regeneration vessel for removal of coke deposits by burning with air. Thus, a constant catalyst activity and product composition can be maintained in the MTO reactor. Figure 12.10 demonstrates the stability of a 90 day operation in the fluidized-bed MTO demonstration unit at the Norsk Hydro Research Center in Porsgrunn, Norway. A fluidized-bed reactor also allows for... 

We consider a reactor with a bed of solid catalyst moving in the direction opposite to the reacting fluid. The assumptions are that the reaction is irreversible and that adsorption equilibrium is maintained everywhere in the reactor. It is shown that discontinuous behavior may occur. The conditions necessary and sufficient for the development of the internal discontinuities are derived. We also develop a geometric construction useful in classification, analysis and prediction of discontinuous behavior. This construction is based on the study of the topological structure of the phase plane of the reactor and its modification, the input-output space. 

Catalytic reactions are carried out in reactors with a fixed, fluidized, or moving bed of catalyst. Although the chemical kinetics of the reaction obviously remains the same for all these reactors, the hydrodynamic features vary considerably. Because no complete description of these features is possible, it is convenient to postulate different situations and develop mathematical models to represent these situations for each type of reactor. It is also important to note that wherever solid catalysts are used, the question of catalyst deactivation cannot be ignored. Several books and reviews covering a variety of situations have been written, including those marked with an asterisk in the list of references. They are recommended for general reading. 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

We will develop the rest of this chapter assuming that the catalyst is in a sohd phase with the reactants and products in a gas or liquid phase. In Chapter 12 we will consider some of the more complex reactor types, called multiphase reactors, where each phase has a specific residence time. Examples are the riser reactor, the moving bed reactor, and the transport bed reactor. 

Reactors with moving solid phase Three-phase fluidized-bed (ebullated-bed) reactor Catalyst particles are fluidized by an upward liquid flow, whereas the gas phase rises in a dispersed bubble regime. A typical application of this reactor is the hydrogenation of residues. 

UOP s catalytic dehydrogenation processes typically make use of radial-flow adiabatic fixed-bed (or slowly moving bed) reactors with modified Pt-alumina catalysts. 

Moving bed reactor U sed for reactions with moderate rate of catalyst decay. 

In conventional fixed-bed reactors the fluid phases move on a packed bed of catalyst particles. Since the two phases are not miscible and both need to be in contact with the catalyst, different operation modes are applied to improve the contact between the fluid phases and the catalysts. For example, in a trickle-bed reactor both fluid phases are introduced from the top in a vertically positioned reactor. Unfortunately poor distribution of the fluid phases gives rise to poor performance, local hot spots and sintering of the catalyst. 

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