Packed Bed Reactor Design

To optimize the performance of the fixed bed reactor operation several constructions of fixed bed reactors have been investigated over the years. Three of the most common reactor designs are  

The single bed reactor is simply a vessel of relatively large diameter, as sketched in Fig. 11.1. This simple reactor design is best suited for adiabatic processes and not applicable for very exothermic or endothermic processes. If the reaction is very endothermic, the temperature change may be such as to extinguish the reaction before the desired conversion is attained. Strongly exothermic reactions, on the other hand, can lead to a temperature rise that is prohibitive due to its unfavorable influence on the equilibrium conversion, the product selectivity, the catalyst stability, and in extreme cases unsafe operation. 

For endothermic reactions the problem can be solved by dividing the reactor into multiple stages, with intermediate heat exchangers, defining a multi-bed reactor. In exothermic processes, the intermediate cooling may be achieved by mean of heat exchangers or by injection of cold feed. A schematic illustration of a multi-bed reactor is shown in Fig. 11.2. 

With very exothermic reactions the number of beds would have to be uneconom-ically large to limit the temperature increase per bed. This problem has been solved by introducing the multi-tube reactor. A schematic illustration of a multi-tube reactor is shown in Fig. 11.3. A representative multi-tube reactor can contain hundreds or thousands of tubes with an inside diameter of a few centimeters [3]. The diameter is limited to this small size to avoid excessive temperature and hot spots. The multi-tube reactor is more common than the other two fixed bed designs because many of the important heterogeneous catalytic processes require effective heat transfer between the mobile fluid, catalyst bed and heating/cooling media. 

Typical for strongly exothermic processes is that at some location in the reactor an extreme temperature occur, frequently named the hot spot. In some processes with very strong exothermic reactions the hot spot temperature can raise beyond permissible limits. This phenomenon is called runaway. An important task in reactor design and operation is thus to limit the hot spot and avoid excessive sensitivity of the reactor performance to variations in the temperature. The value of the temperature at the hot spot is determined mainly by the reaction rate sensitivity to changes in temperature, the heat of reaction potential of the process, and the heat transfer potential of the heat exchanger units employed. A heat exchanger is characterized by the heat transfer coefficient and heat transfer areas. 

The selection of an appropriate fixed bed reactor design for a given process is performed assessing the main limitations of these reactors. The fixed packed bed reactors can be malfunctioning due to improper temperature control, pressure drop for processes with low tolerance, and deactivation of the catalyst. 

The single bed reactor is simply a vessel of relatively large diameter, as sketched in Fig 11.1. This simple reactor design is best suited for adiabatic 

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Figure 1. Packed bed reactor designs - examples of geometries and connections.

The problem of the optimal particle shape and size is crucial for packed bed reactor design. Generally, the larger the particle diameter, the cheaper the catalyst. This is not usually a significant factor in process design - more important are the internal and external diffusion effects, the pressure drop, the heat transfer to the reactor walls and a uniform fluid flow. 

Figure 7.7 Packed bed reactor design. From Ref. 19 with permission.

How much did computational fluid dynamics (CFD) enter into the actual design of the packed-bed reactors Perform a thorough literature survey and find a solid evidence of the nse of CFD tools in packed-bed reactor design. 

Different packed bed reactor designs are illustrated in Figure 6.8. The flow properties are of utmost importance for packed beds used in three-phase reactions. The most common operation policy is to allow the liquid to flow downward in the reactor. The gas phase can flow upwards or downwards, in a concurrent or a countercurrent flow. This reactor is called a trickle bed reactor. The name is indicative of flow conditions in the reactor, as the liquid flows downward in a laminar flow wetting the catalyst particles efficiently (trickling flow). It is also possible to allowboth the gas and the liquid to flow upward in the reactor (Figure 6.8). In this case, no trickling flow can develop, and the reactor is called a packed bed or n fixed bed reactor. 

There are two basic types of packed-bed reactors those in which the solid is a reactant and those in which the solid is a catalyst. Many e.xaniples of the first type can be found in the extractive metallurgical industries. In the chemical process industries, the designer normally meets the second type, catalytic reactors. Industrial packed-bed catalylic reactors range in size from units with small tubes (a few centimeters in diameter) to large-diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat transfer rates in large-diameter packed beds are poor and where high heat transfer rates are required, Jluidized beds should be considered. ... 

Another important challenge is to enhance the reliability of the design and scale up of multi-phase reactors, such as fluidized bed reactors and bubble-colunms. The design uncertainty caused by the complex flow in these reactors has often led to the choice of a reactor configuration that is more reliable but less efficient. An example is Mobil use a packed-bed reactor for the methanol to gasoline process in New Zealand, even though a... 

A great savings in enzyme consumption can be achieved by immobilizing the enzyme in the reactor (Fig. 12). In addition to the smaller amount of enzyme required, immobilization often increases the stability of the enzyme. Several designs of immobiliz-ed-enzyme reactors (lERs) have been reported, with open-tubular and packed-bed being the most popular. Open-tubular reactors offer low dispersion but have a relatively small surface area for enzyme attachment. Packed-bed reactors provide extremely high surface areas and improved mass transport at the cost of more dispersion. 

Cylinders have the advantage that they are cheap to manufacture. In addition to varying the shape, the distribution of the active material within the pellets can be varied, as illustrated in Figure 6.7. For packed-bed reactors, the size and shape of the pellets and the distribution of active material within the pellets can be varied through the length of the reactor to control the rate of heat release (for exothermic reactions) or heat input (for endothermic reactions). This involves creating different zones in the reactor, each with its own catalyst designs. 

Tubular reactors are normally used in the chemical industry for extremely large-scale processes. When filled with solid catalyst particles, such reactors are referred to as fixed or packed bed reactors. This section treats general design relationships for tubular reactors in... 

If the measuring points are chosen far enough away from the ends of the test vessel so that end effects are negligible, this expression may be used with confidence. Bischoff and Levenspiel (12) have presented design charts that permit one to locate monitoring stations so as to avoid end effects. For example, in a packed bed reactor with a tube to pellet diameter ratio of 15, where the packed bed is followed by an open tube, at least 8 pellet diameters are required between the measurement point and the open tube if errors below 1% are to be obtained. 

The design q>roblem can be approached at various levels of sophistication using different mathematical models of the packed bed. In cases of industrial interest, it is not possible to obtain closed form analytical solutions for any but the simplest of models under isothermal operating conditions. However, numerical procedures can be employed to predict effluent compositions on the basis of the various models. In the subsections that follow, we shall consider first the fundamental equations that must be obeyed by all packed bed reactors under various energy transfer constraints, and then discuss some of the simplest models of reactor behavior. These discussions are limited to pseudo steady-state operating conditions (i.e., the catalyst activity is presumed to be essentially constant for times that are long compared to the fluid residence time in the reactor). 

Packed-bed reactors, 21 333, 352, 354 Packed beds, 25 718 Packed catalytic tubular reactor design with external mass transfer resistance, 25 293-298 nonideal, 25 295... 

At these lower conversions, the previously described multi-bed down-flow reactor may not be the best design. At the higher space velocities it is more expedient to have the liquid in up-flow, and the reduced penalty from axial (particularly) forward mixing means also that the necessary conversion can frequently be achieved in a single catalyst bed. Thus, the reactor design is considerably simplified, with an associated reduction in the capital cost for a given catalyst volume. The development of the design for an up-flow-packed bed reactor for the decomposition of sodium hypochlorite is described elsewhere [6] and will not be considered in detail here. 

Various reactor types have been used as the foundation for microreactor designs, including coated wall reactors, packed-bed reactors, structured catalyst reactors, and membrane reactors. 

A single catalytic packed bed reactor is to be designed to treat 100 mol/s of reactant A and produce product R. Feed gas enters at 2.49 MPa and 300 K, the maximum allowable temperature is 900 K unless otherwise noted, the product stream is wanted at 300 K, and the thermodynamics and kinetics of the exothermic reaction are given to us in Fig. 19.11. Prepare a sketch showing the details of the system you plan to use ... 

The formation of phthalic anhydride is highly exothermic, and even with the most careful design the heat removal from packed bed reactors can become uncheckable, leading to temperature runaways, meltdowns, and even explosions. If the chief engineer of those reactors had been required to sit on the reactor during start up, there would be fewer chief engineers about. ... 

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