Countercurrent flow, example

Use of Operating Curve Frequently, it is not possible to assume that = 0 as in Example 2, owing to diffusional resistance in the liquid phase or to the accumulation of solute in the hquid stream. When the back pressure cannot be neglected, it is necessary to supplement the equations with a material balance representing the operating line or curve. In view of the countercurrent flows into and from the differential section of packing shown in Fig. 14-3, a steady-state material balance leads to the fohowing equivalent relations ... 

In the field of heat transfer, a good example of this category of shortcut design method is the famous F correction factor to correct the log mean temperature difference of shell and tube heat exchangers for deviations from true countercurrent flow. For multipass heat exchangers, the assumptions are ... 

Effectiveness factors q are plotted against number of transfer units N with (Gj CPI jG2CPl) as parameter for a number of different configurations by Kays and London 251. Examples for countercurrent flow (based on equation 9.235) and an exchanger with one shell pass and two tube passes are plotted in Figures 9.85a and b respectively. 

The 2-D models also allow for different effects to be studied in depth. For example, the simulations show that, as mentioned above, countercurrent flow yields better performance than coflow due to water recycling. This is something that a 1-D... 

In multiphase reactors we frequently exploit the density differences between phases to produce relative motions between phases for better contacting and higher mass transfer rates. As an example, in trickle bed reactors (Chapter 12) liquids flow by gravity down a packed bed filled with catalyst, while gases are pumped up through the reactor in countercurrent flow so that they may react together on the catalyst surface. 

An example of an incredibly complex multiphase chemical reactor is iron ore refining in a blast furnace. As sketched in Figure 12-22, it involves gas, liquid, and solid phases in countercurrent flows with complex temperature profiles and heat generation and removal processes. 

Such equipment consists of a cylindrical shell into which the wet material is charged at one end and dry material leaves at the other end. Figure 9.9 shows some examples. Drying is accomplished by contact with hot gases in parallel or countercurrent flow or with heat transfer through heated tubes or double shells. Designs are available in which the tubes rotate with the shell or are fixed in space. 

Although the most useful extraction process is with countercurrent flow in a multistage battery, other modes have some application. Calculations may be performed analytically or graphically. On flowsketches like those of Example 14.1 and elsewhere, a single box represents an extraction stage that may be made up of an individual mixer and separator. The performance of differential contactors such as packed or spray towers is commonly described as the height equivalent to a theoretical stage (HETS) in ft or m. 

Calculate the log mean temperature difference. See Example 7.21 and use the equation for countercurrent flow. Thus,... 

Example 6.4 Wetted wall column with a ternary liquid mixture We have the experimental data on distillation of ethanol (1), feri-butanol (2), and water (3) in a wetted wall column reported by Krishna and Standard (1976). As Figure 6.2 shows, the column operates at total reflux with countercurrent flow. Therefore, at steady state, the compositions of the liquid and vapor phases at ary point in the column are equal to each other. The measured compositions of the phases at the bottom are... 

The countercurrent-flow fixed-bed operation is often used for gas-liquid reactions rather than gas-liquid-solid reactions. Examples of reactions using this type of reactor are given by Danckwerts.29 A comparison between a gas liquid-solid (catalytic) fixed-bed reactor and a gas-liquid-solid (inert) fixed-bed reactor is shown in Table 1-7. The major difference between packed-bed gas-liquid reactors and gas-liquid-solid catalytic reactors is in the nature and size of the packing used and the conditions of gas and liquid flow rates. The packed-bed gas-liquid reactors use nonporous, large-size packing, so that they can be operated at high gas and liquid flow rates without excessive pressure drop. The shape of... 

From a theoretical point of view, the cocurrent mode of operation is in many cases not the optimal one, and countercurrent operation, in which gas and liquid flow in opposite directions over the fixed catalyst, should be preferable, as will be discussed below. Countercurrent flow of gas and liquid through a random packing of catalyst particles is difficult or hardly obtainable under industrial conditions, however. One possibility for obtaining the desired countercurrent flow is to use fixed catalysts with a well-defined specific geometry instead of a random packing. An example of a reactor with such a structured catalyst is the internally finned monolithic reactor (IFMR), to be discussed below. 

Countercurrent flow of gas and liquid, in contrast to the cocurrent trickle flow now in use, can have important advantages. The incentives for countercurrent operation are illustrated by several examples in the field of oil hydroprocessing. 

Another possibility is the use of recycle in a single-stage cell, operating in countercurrent flow, as shown in Figure 19.12 (Hoffman, 2003). More complicated arrangements are shown, for example, in Gas Research Institute (1982). There is the potential here for sharp separations, as will be subsequently derived and explained. 

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