Reactors with moving catalyst

The mechanically stirred tank reactor is most commonly used in batch pi o-cesses. It is the workhorse of the fine-chemicals industry. The catalyst particles are suspended in the liquid, which is almost perfectly mixed by a mechanical agitator. It is also possible to apply a hollow stirrer that encompasses two functions, 

In bubble columns, agitation of the liquid phase, and hence suspension of the catalyst is effected by the gas flow. The gas is often recycled to cause more turbulence and thus better mixing. Circulation of the liquid is often required to obtain a more uniform suspension. This can either be induced by the gas flow (airlift loop reactor) or by use of an external pump. In the latter instance it is possible to return the slurry to the reactor at a high flow rate through an ejector (Venturi tube). The local under-pressure causes the gas to be drawn into the passing stream, thus affording very efficient mixing. This type of reactor is called a Jet-loop or Venturi reactor. 

Jet-loop reactors tend to replace stirred-tank reactors in recently built equipment for fine-chemical hydrogenation. The external heat exchanger on the liquid circu- 

The three-phase fluidized-bed reactor (ebulliated-bed reactor) differs from suspension reactors in the use of larger catalyst particles (0.1 to 5 mm) and the formation of a well-defined agitated catalyst bed. Whereas suspension reactors can operate in both batch and continuous mode with regard to the liquid phase (and catalyst), the ebulliated-bed reactor only operates in continuous mode, and hence is generally not the appropriate choice for tire production of fine chemicals. 

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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. 

Figure 5.4-10. Column reactors with moving catalyst.

Internal recycle reactors with moving catalyst bed... 

Figure 2. Reactors with moving catalyst particles (G = gas, L = liquid, dashed arrows indicate optional liquid flow for reactors operating in continuous mode).

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). 

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. 

Figure 4. Units combining reaction and separation (a) reactor with countercurrent moving bed, (b) simulated moving-bed reactor with four catalyst layers (bold lines show gas streams during a quarter of cycle), (c) pressure-swing adsorption reactor separator (on the left i given time dependency of pressure during a cycle)...

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. 

Many reactors fall in the classification of fluid-solid catalytic units where the catalyst may be retained in a fixed-bed position in the reactor with the reactant flowing through the catalyst bed, or the unit may be operated as a fluidized-bed reactor with the catalyst particles being suspended in the flowing fluid due to motion of the fluid. A third type of reactor is one in which the catalyst particles fall slowly through the fluid by gravity in the form of a so-called moving bed. 

Figure L2 Common reactor types with moving catalyst beds (a) fluid-bed reactor (b) bubble column with suspended catalyst (c) sparged stirred tank with suspended catalyst.

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).

Maleic anhydride is made by oxidation of benzene with air above 350°C (662°F) with V-Mo catalyst in a mnltitiibiilar reactor with 2-cm tubes. The heat-transfer medium is a eutectic of molten salt at 375°C (707°F). Even with small tubes, the heat transfer is so hmited that a peak temperature 100°C (212°F) above the shell side is developed and moves along the tubes. 

Jankowski et al (1978) discuss in detail the great variety of gradientless reactors proposed by several authors with a pictorial overview in their paper. All of these reactors can be placed in a few general categories (1) moving catalyst basket reactors, (2) external recycle reactors, and (3) internal recycle reactors. 

In the moving-bed process, oil is heated to up to 1,300"F and is passed under pressure through the reactor where it comes into contact with a catalyst flow in the form of beads or pellets. The cracked products then flow to a fractionating tower where the various compounds are separated and collected. The catalyst is regenerated in a continuous process where deposits of coke on the catalyst are burned off. 

Several processes based on non-precious metal also exist. Because of high catalyst deactivation rates with these catalyst systems, they all require some form of continuous regeneration. The Fluid Hydroforming process uses fluid solids techniques to move catalyst between reactor and regenerator TCR and Hyperforming use some form of a moving bed system. 

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

Of these, fixed-bed adiabatic reactors are the cheapest in terms of capital cost. Tubular reactors are more expensive than fixed-bed adiabatic reactors, with the highest capital costs associated with moving and fluidized beds. The choice of reactor configuration for reactions involving a solid supported catalyst is often dominated by the deactivation characteristics of the catalyst. 

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