Fixed or packed beds

From the point of view of mass transfer there is much similarity between these two modes of operation. The following observations for fixed beds may also be applied to moving beds. 

An important characteristic of a fixed bed of particles is the specific outer surface area of the particles. When the particles all have the same diameter, the specific surface area a can be related to the void fraction e of the bed and the solids fraction e by the following equation  

The shape factor Y is 1 for for spheres, cubes and cylinders with height equal to diameter. The shape factor Y is greater than 1 for particles that are oblong or 

It is often assumed that the void fraction e is constant throughout the bed. However, there will always be a certain wall effect. This is illustrated in figure 4.8. Even in a random packing, the variations in void fraction increase toward the wall. In a layer with a thickness of half the particle diameter, adjacent to the wall, the void fraction is higher than average. 

The consequence is that there is a higher flow rate there. The relative effect of irregular distribution of flow become small when the vessel diameter is at least 20 times the particle diameter. 

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Fixed (or packed) bed operation occurs when the fluid velocity is low or the particle size is large so that fluidization does not occur. For such operation, Jakob (Heat Tran.sfer, vol. 2, Wiley, New York, 1957) gives... 

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... 

Fixed or packed bed reactors have many advantages relative to other types of heterogeneous catalytic reactors and, consequently, are em-... 

Fixed- or packed-bed reactors refer to two-phase systems in which the reacting fluid flows through a tube filled with stationary catalyst particles or pellets (Smith, 1981). As in the case of ion-exchange and adsorption processes, fixed bed is the most frequently used operation for catalysis (Froment and Bischoff, 1990 Schmidt, 2005). Some examples in the chemical industry are steam reforming, the synthesis of sulfuric acid, ammonia, and methanol, and petroleum refining processes such as catalytic reforming, isomerization, and hydrocracking (Froment and Bischoff, 1990). 

Adsorption in fixed beds is the most important method used for this process. A fixed or packed bed consists of a vertical cylindrical pipe filled or packed with the adsorbent particles. Adsorbers are mainly designed using laboratory data and the methods described in Section 12.3C. In this section the basic equations are described for isothermal adsorption so that the fundamentals involved in this process can be better understood. 

The most important fixed-bed designs are the nonisothermal, nonadiabatic, fixed- (or packed)-bed reactor (NINA-PBR) (also called the multitubular or heat-exchanger-type reactor), and the single or multistage adiabatic fixed-bed reactor (A-PBR), and it is important at the outset to note the difference between the approaches and the design of these two operational categories. 

Carbon should be prewetted prior to being placed in the test columns. Backwashing the carbon at low rates (2.5 m/hr) does not remove the air. Rates that would expand the bed 50 percent or 15-30 m/hr, are required. The liquid used for prewetting can either be water, if it is compatible with the liquid to be treated, or a batch of the liquid to be treated which has been purified previously. There are three types of carbon systems (1) fixed beds, (2) pulse beds, and (3) fluidized beds, and these can be used singly, in parallel, or in combination. The majority of systems are either fixed or pulse beds. The two basic types of adsorbers which can be designed to operate under pressure or at atmospheric pressure are the moving or pulse bed and the fixed bed. Either can be operated as packed or expanded beds. 

L Fixed-bed, or packed-bed, filters. These units are not cleaned when they become plugged with deposited dust particles but are broken up for disposal or simply abandoned. If they are constructed from fine granules (e.g., sand particles), they may be designed to give high collection efficiencies on fine dust particles. However, if such a filter is to have a reasonable operating life, it can be used only on a gas containing a low concentration of (Just particles. 

Fluidized beds, however, have some important disadvantages relative to packed (fixed or moving) bed... 

Packed bed regime (fixed or moving bed operation). Here the particle hold-up is typically in the region 0.5-0.7. The particle size suitable in the packed bed regime is usually larger than 1 mm because smaller particle sizes result in unacceptably high pressure drops. 

In conclusion, it should be stated that the present primitive level of understanding of rotary and ultrasonic defoaming may prevent design of significantly improved devices and realization of the true potential of these methods. Finally, we note that the use of fixed porous packed beds of appropriate wettability may have potential in either replacing or supplementing such devices in certain circumstances where the presence of antifoams is unacceptable. 

Figure 4-8 shows a continuous reactor used for bubbling gaseous reactants through a liquid catalyst. This reactor allows for close temperature control. The fixed-bed (packed-bed) reactor is a tubular reactor that is packed with solid catalyst particles. The catalyst of the reactor may be placed in one or more fixed beds (i.e., layers across the reactor) or may be distributed in a series of parallel long tubes. The latter type of fixed-bed reactor is widely used in industry (e.g., ammonia synthesis) and offers several advantages over other forms of fixed beds. 

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

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