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FIG. 3.10.1 Feed distribution systems. (a) Poor feed distribution in a fixed-catalyst bed causes poor conversion and poor yield. (b) Uniform feed flow improves both yield and conversion.

FIG. 3.10.1 Feed distribution systems. (a) Poor feed distribution in a fixed-catalyst bed causes poor conversion and poor yield. (b) Uniform feed flow improves both yield and conversion.

Improving Physical Mixing in the Reactor

Modifications to the reactor such as adding or improving baffles, installing a higher revolutions-per-minute motor on the agitator, or using a different mixer blade design (or multiple impellers) improves mixing. Pumped recirculation can be added or increased. Two fluids going through a pump, however, do not necessarily mix well, and an inline static mixer may be needed to ensure good contacting.

Distributing Feed Better

Part (a) in Figure 3.10.1 shows the importance of distributing feeds. The reactants enter at the top of a fixed-catalyst bed. Part of the feed short-circuits through the center of the reactor having inadequate time to convert to the product. Conversely, the feed closer to the walls remains in the reactor too long and overreacts creating by-products that become waste. Although the average residence time in the reactor is correct, inadequate feed distribution causes poor conversion and poor yield.

One solution is to add a distributor that causes the feed to move uniformly through all parts of the reactor (see part (b) in Figure 3.10.1). Some form of collector may also be necessary at the bottom to prevent the flow from necking down to the outlet.

Improving Methods of Adding Reactants

The purpose of this option is to make the reactant concentrations closer to the ideal before the feed enters the reactor. This change helps avoid the secondary reactions which form unwanted by-products. Part (a) in Figure 3.10.2 shows the wrong way to add reactants. The ideal concentration probably does not exist anywhere in this reactor. A consumable catalyst should be diluted in one of the feed streams (one which does not react in the presence of the catalyst). Part (b) in Figure 3.10.2 shows one approach of improving the addition of reactants using three inline static mixers.

Improve Catalysts

Searching for better catalysts should be an ongoing activity because of the significant effect a catalyst has on the reactor conversion and product mix. Changes in the chemical makeup of a catalyst, the method of preparation, or its physical characteristics (such as size, shape, and porosity) can lead to substantial improvements in the catalyst life and effectiveness (Nelson 1990).

Providing Separate Reactors for the Recycling Stream

Recycling by-product and waste streams is an excellent way of reducing waste, but often the ideal reactor conditions for converting recycling streams to usable products are different from conditions in the primary reactors. One solution is to provide a separate, smaller reactor for handling recycling and waste streams (see Figure 3.10.3). The temperatures, pressures, and concentrations can then be optimized in both reactors to take maximum advantage of reaction kinetics and equilibrium.

Catalyst I

Reactant A Reactant B

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