Sidestream Withdrawal

FIGURE 6.17 Minimum reflux designs for (a) Scenario 1 and (b) Scenario 2 in Example 6.2b. 

In all the systems considered so far, we have only been concerned with obtaining a relatively pure distillate or bottoms stream. However, in tamary systems, for example, it is often desirable to obtain three relatively pure products. The traditional approach for achieving this has been with a sequence of simple columns, although this has obvious disadvantages to it, because to achieve this separation at least two separate columns have to be erected (refer to Section 2.6.4) meaning an increase in both capital and operating costs. The removal of a sidestream product from the main column may however be more advisable in certain instances. 

FIGURE 6.18 A schematic representation of a sidestream colunm and accompanying CS breakdown with the side products located (a) above and (b) below the feed stream. 

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The effect of a sidestream withdrawal point is illustrated by Fig. 13-29. The material-balance equation for the column section below the sidestream is... 

Depth of packing without intermediate supports is limited by its deformability metal construction is limited to depths of 20-25 ft, and plastic to 10-15 ft. Intermediate supports and liquid redistributors are supplied for deeper beds and at sidestream withdrawal or feed points. Liquid redistributors usually are needed every 2 -3 tower diameters for Raschig rings and every 5-10 diameters for pall rings, but at least every 20 ft. 

The withdrawal of this sidestream introduces two new variables which may be specified such as the stage number + 1 of the sidestream withdrawal, and its flow rate W. Instead of kx and W, any other two product specifications may be made such as wr/dr, ws/ds, x s, x r, and Tw, xWr as shown in Table 9-5. Components r and s consist of any two components of the feed which are selected in the order of increasing volatility s is more volatile than r. In the following application of Box s complex method, it will be supposed that the additional specifications for the stream W are ws/ds, wr/dr. The complete set of additional purity specifications are taken to be... 

The method used to obtain approximate solutions to complex columns consists of an extension of the one proposed above for conventional columns. The equations used consist of a combination of the component-material balances and the equilibrium relationships. For the complex column with one feed shown in Fig. 9-2, the combination of the equilibrium relationships and the component-material balances are given by Eq. (1). For multiple feed plates and sidestream withdrawals, minor modifications of A, and / are required. 

The total distillate rate D and the sidestream withdrawal rate W are found by summing the d, s and the w/s, respectively, over all components. 

Examples of such complex distillation structures are thus columns that have more than one feed point and/or more than two product streams, like distributed material addition/removal columns, and thermally coupled columns. Obviously, as the complexity of the distillation structure increases, so does the design itself thereof. This chapter will, as an introduction to complex column design, treat the design of elementary complex columns such as distributed feed and sidestream withdrawal columns, and side rectifiers, and strippers, before discussing more intricate complex columns like fully thermally coupled columns (sometimes referred to as the Petlyuk and Kaibel columns) in the subsequent chapter. Despite... 

The sidestream column can be broken into a series of CSs in an analogous manner to Figure 6.1. Since sidestream withdrawal is essentially the reverse of multiple feed additions, the difference points obey the same linear mixing laws as shown in Figure 6.19. Therefore, all difference points and compositions are linked to each other via straight lines in composition space. 

Assuming that only liquid products are being drawn off, it is also possible to obtain a qualitative understanding of the effect of sidestream withdrawal on the reflux ratio. Each liquid withdrawal increases the net flow from one CS to the next, as the quantity V —L becomes larger. 

Both configurations can be divided into four CSs for the case of a ternary system where we wish to obtain relatively pure distillate, bottoms, and sidestream products. For the sake of consistency, the side unithas been numbered as CS4 while the internal CS is labeled as CS2 in both configurations. In the side-stripper unit, the liquid coming from CSi is divided into two streams one that is directed to the main column body and another one that is directed toward the side-stripping unit. The vapor flow in CSi is also the sum of the vapor flows from the side stripping unit and the main column. Similarly, the vapor flow from CS3 in the side rectifier is directed toward the main column and the rectifying unit, while the liquid flowrate in CS3 is the sum of the liquid flowrates in the main column and the rectifier. The location of the sidestream withdrawal and addition at the thermally coupled junction are assumed to take place at the same location. This assumption can be relaxed however, an additional CS would be created and this case will not be further discussed in this text... 

Interestingly, the placement for X in the thermally coupled arrangements is qualitatively similar to the sidestream withdrawal columns (refer to Figure 6.20). The difference point of the internal CS (X a), in both cases, has to lie between to product producing CSs (by mass balance see Figure 6.28) and therefore has to lie in positive composition space since real compositions are naturally forced to. Again, this means that the design is somewhat more constricted by potentially useful CPM... 

The divided-wall column has many degrees of freedom at the steady-state design stage. The number of stages in the four different sections of the column, the locations of the feed and sidestream withdrawal points, and the location of the wall are seven of the parameters that must be specified and are all fixed by the physical equipment at the time of construction. They cannot be changed during operation. The location of the wall fixes how the vapor splits between the two sides of the wall, so the vapor split is not adjustable during operation for control purposes. 

The bottoms composition can be controlled most quickly with vapor boilup. However, the flow rate of the sidestream could be used if there are not too many trays between the bottom and the sidestream withdrawal stage. In the numerical case studied, there are 25 trays between the sidestream and the bottom of the column. Therefore, we will control sidestream impurity with sidestream flow rate and bottoms impurity with vapor boilup. 

A variety of other configurations and modifications of the basic design shown in Figures 3-6 and 3zZ are possible. Valve trays (see Chapter lOi are popular. Downcomers can be chords of a circle as shown or circular pipes. Both partial and total condensers and a variety of reboilers are used. The column may have multiple feeds, sidestream withdrawals, intermediate reboilers or condensers, and so forth. The column also usually has a host of temperature, pressure, flow rate, and level measurement and control devices. Despite this variety, the operating principles are the same as for the sinple distillation column shown in Figure 3-6. 

If CMO is valid, the total vapor and liquid flow rates will be constant in each section of the column. The total flow rates can change at each feed stage or sidestream withdrawal stage. This behavior is illustrated for a conputer simulation for a saturated liquid feed in Figure 5-2 and is the same behavior we would expect for a binary system. For nonconstant molal overflow, the total flow rates will vary from section to section. This is also shown in Figure 5-2. Although both liquid and vapor flow rates may vary significantly, the ratio L/V will be much more constant. 

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