The Reactor

The reactor is the heart of every chemical plant, even though in terms of space requirements and investment it is often the smallest component of the plant. Because of its central function it fundamentally influences all other plant components consider, for example, the measures necessary to recycle unconverted starting material. Therefore, besides the choice of catalyst, the selection of the reactor type is the most important step in process development [Donati 1997]. 

With the aid of chemical reaction technology [Levenspiel 1980, Hofmann 1983], the theoretical basis of which was only developed in the mid-1900s, the reactor can be designed. To this end, the questions of  

In addition to the intense internal current of liquid continuously penetrating the polymer support inside the spinning rotor, an external flow can be diverted since the mode of action in the vessel is comparable to a rotary pump. In this way external flow cells can be circulated with reaction solutions from the centrifugal reactor without any additional pump. Hereby, the alterations of concentrations in the circulating solutions caused by consumption of reagents or liberation of molecules cleaved from the gel support can be measured directly by flow photometry (UV, IR, conductivity, etc.) and recorded continuously. On opening an outlet valve on the bottom, the reactor is emptied rapidly and completely by spinning off. 

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Figure 1.2 Process design starts with the reactor. The reactor design dictates the separation and recycle problem. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

Since process design starts with the reactor, the first decisions are those which lead to the choice of reactor. These decisions are among the most important in the whole design. Good reactor performance is of paramount importance in determining the economic viability of the overall design and fundamentally important to the environmental impact of the process. In addition to the desired products, reactors produce unwanted byproducts. These unwanted byproducts create environmental problems. As we shall discuss later in Chap. 10, the best solution to environmental problems is not elaborate treatment methods but not to produce waste in the first place. 

Having made a choice of the reaction path, we need to choose a reactor type and make some assessment of the conditions in the reactor. This allows assessment of reactor performance for the chosen reaction path in order for the design to proceed. 

Table 2.2 gives the compositions of the reactor feed and effluent streams. Calculate the conversion, selectivity, and reactor yield with respect to (a) the toluene feed and (b) the hydrogen feed. 

Reactor yield of benzene from toluene = ( luene fed to the reactor)... 

Because there are two feeds to this process, the reactor performance can be calculated with respect to both feeds. However, the principal concern is performance with respect to toluene, since it is much more expensive than hydrogen. 

In describing reactor performance, selectivity is usually a more meaningful parameter than reactor yield. Reactor yield is based on the reactant fed to the reactor rather than on that which is consumed. Clearly, part of the reactant fed might be material that has been recycled rather than fresh feed. Because of this, reactor yield takes no account of the ability to separate and recycle unconverted raw materials. Reactor yield is only a meaningful parameter when it is not possible for one reason or another to recycle unconverted raw material to the reactor inlet. By constrast, the yield of the overall process is an extremely important parameter when describing the performance of the overall plant, as will be discussed later. 

Cfeed = molar concentration of FEED in the reactor di, 0-2 = constants (order of reaction) for primary and secondary reactions... 

Maximum selectivity requires a minimum ratio rjr in Eq. (2.17). A high conversion in the reactor tends to decrease Cfeed- Thus... 

Again, it is difficult to select the initial setting of the reactor conversion with systems of reactions in series. A conversion of 50 percent for irreversible reactions or 50 percent of the equilibrium conversion for reversible reactions is as reasonable as can be guessed at this stage. 

In the second model (Fig. 2.16) the continuous well-stirred model, feed and product takeoff are continuous, and the reactor contents are assumed to he perfectly mixed. This leads to uniform composition and temperature throughout. Because of the perfect mixing, a fluid element can leave at the instant it enters the reactor or stay for an extended period. The residence time of individual fluid elements in the reactor varies. 

When more than one reactant is used, it is often desirable to use an excess of one of the reactants. It is sometimes desirable to feed an inert material to the reactor or to separate the product partway through the reaction before carrying out further reaction. Sometimes it is desirable to recycle unwanted byproducts to the reactor. Let us now examine these cases. 

Single reversible reactions. The maximum conversion in reversible reactions is limited by the equilibrium conversion, and conditions in the reactor are usually chosen to increase the equilibrium conversion. Le Chatelier s principle dictates the changes required to increase equilibrium conversion ... 

The equilibrium conversion can be increased by employing one reactant in excess (or removing the water formed, or both). b. Inerts concentration. Sometimes, an inert material is present in the reactor. This might be a solvent in a liquid-phase reaction or an inert gas in a gas-phase reaction. Consider the reaction system... 

Product removal during reaction. Sometimes the equilibrium conversion can be increased by removing the product (or one of the products) continuously from the reactor as the reaction progresses, e.g., by allowing it to vaporize from a liquid-phase reactor. Another way is to carry out the reaction in stages with intermediate separation of the products. As an example of intermediate separation, consider the production of sulfuric acid as illustrated in Fig. 2.4. Sulfur dioxide is oxidized to sulfur trioxide ... 

Multiple reactions in parallel producing byproducts. Once the reactor type is chosen to maximize selectivity, we are in a position to alter selectivity further in parallel reaction systems. Consider the parallel reaction system from Eq. (2.20). To maximize selectivity for this system, we minimize the ratio given by Eq. (2.21) ... 

For all reversible secondary reactions, deliberately feeding BYPRODUCT to the reactor inhibits its formation at the source by shifting the equihbrium of the secondary reaction. This is achieved in practice by separating and recycling BYPRODUCT rather than separating and disposing of it directly. 

An alternative way to improve selectivity for the reaction system in Eq. (2.27) is again to deliberately feed BYPRODUCT to the reactor to shift the equilibrium of the secondary reaction away from BYPRODUCT formation. 

An example of where recycling can be effective in improving selectivity is in the production of benzene from toluene. The series reaction is reversible. Hence recycling diphenyl to the reactor can be used to suppress its formation at the source. 

These polyethylbenzenes are recycled to the reactor to inhibit formation of fresh polyethylbenzenes. ... 

The choice of reactor temperature depends on many factors. Generally, the higher the rate of reaction, the smaller the reactor volume. Practical upper limits are set by safety considerations, materials-of-construction limitations, or maximum operating temperature for the catalyst. Whether the reaction system involves single or multiple reactions, and whether the reactions are reversible, also affects the choice of reactor temperature, as we shall now discuss. 

Temperature control. Let us now consider temperature control of the reactor. In the first instance, adiabatic operation of the reactor should be considered, since this leads to the simplest and cheapest reactor design. If adiabatic operation produces an unacceptable rise in temperature for exothermic reactions or an unacceptable fall in temperature for endothermic reactions, this can be dealt with in a number of ways ... 

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