Column section side rectifiers

We note that sections can be grouped into columns in various ways. For example, at separation of ternary mixture 1,23 in a partially coupled sequence, the sections can be grouped into a column with side rectifier (i.e., sections 1,23 1,133... 

Besides sequences of simple columns, some types of distillation complexes, each of which can replace two adjacent simple columns, were examined in work (Ghnos Malone, 1988). The following complex columns and distillation complexes were examined column with side output above the feed cross-section, column with side rectifier, column with side stripping flowsheet with prefractionator, Petlyuk column top and side flows from the first column into the second one (Fig. 8.3a),... 

Fig. 5.146, the four column sections are rearranged to form a side-rectifier arrangement. ... 

The side-rectifier and side-stripper arrangements have some important degrees of freedom for optimization. In these arrangements, there are four column sections. For the side-rectifier, the degrees of freedom to be optimized are ... 

The partitioned thermally coupled prefractionator in Figure 11.14c can be simulated using the arrangement in Figure 11.14b as the basis of the simulation. However, like side-rectifiers and side-strippers, fully thermally coupled columns have some important degrees of freedom for optimization. In the fully thermally coupled column, there are six column sections (above and below the partition, above and below the feed in the prefractionator and above and below the sidestream from the main column side of the partition). The degrees of freedom to be optimized in partitioned columns are ... 

By eliminating the reboiler and condenser in the prefractionator column in Fig. 13-67a (the column containing sections 1 and 2) we obtain a thermally coupled system, also known as a Petlyuk system, shown in Fig. 13-67b [Petlyuk, Platonov, and Slavinskii, Int. Chem. Eng., 5, 555 (1965)]. Side stripper, side rectifier, and Petlyuk systems can also be built as divided wml columns, as explained in detail in the subsection below on thermally coupled systems. 

As will be shown later, the trays in the main column are mostly rectifying sections for the side products some of them are pumparound contact condensers. There is a wide variation in the industry of tray count between side draws, from 4 to 15, depending on the degree of rectification desired. 

By thermal coupling the heat is transferred by direct contact between vapour and liquid flows that connect sections of different columns. This is a major difference with heat integrated columns , where the heat exchange takes place by condenser/reboilers. Hence, thermal coupled columns have a more complex behaviour. Figure 11.21 illustrates two basic arrangements with side-columns. The first is the side-rectifier, derived irom a direct sequence. The second one is the side-stripper that corresponds to... 

The two internal degrees of fi eedom of each column have been specified through the appropriate split ratio and a single, reference reflux. Both Rm in the side stripper and Rm in the side rectifier have been chosen as the respective reference refluxes since they lie the closest to the thermally coupled sections. This choice is of course arbitrary, and one may equivalently choose any CS to serve as the reference CS. 

Notice that in both configurations that they are very closely related to simple columns. For the side stripper, for instance, the simple column has a feed of flowrate F and quality q, a bottoms product of flowrate B and composition x. One can show by mass balance that CS2 produces a distillate product, or a pseudo distillate, of flowrate A2 = V2 — L2 and composition Xa2. Thus, for all practical purposes the CS above and below the feed is just a simple column with one feed and two products. One can in a similar way deduce that the CS2 in the side rectifier acts as a conventional product producing stripping section with a flowrate of — A2 =L2 — V2 (since product flowrates have to be positive) and bottoms composition of X. ... 

FIGURE 633 Column section breakdown across the feed stage for (a) a side strippo and (b) a ade rectifier. 

We can turn from partially coupled sequences to completely coupled ones (to Petl5Tik columns). For this purpose, the reboilers and condensers connected with the sections, products of which are components with intermediate volatilities, should be excluded. It is possible if each excluded condenser or reboiler is replaced by flow of liquid or vapor from another section. To ensure this flow, it is necessary to supplement the network with an additional node (i.e., with one top and one bottom section). We note that inclusion of additional nodes is equivalent to inclusion of columns with distributed components into the sequence. For example, in the side rectifier 2,3 2 condenser can be replaced if network 1,23 1,133 2,3,... 

The RadFrac block can be used for the ordinary distillation column and also for the extractive distillation column as shown in the example in Section 3.1. It can also be used as strippers (with a reboiler but no condenser), rectifiers (with a condenser but no reboiler), absorbers (with neither), and more complex columns with side pmnp-around. In the following, several columns other than the ordinary distillation will be outlined. The RadFrac can also be used as a heterogeneous azeotropic distillation column with decanter replacing the reflux drum at top of this column. The vapor-liquid-liquid calculation can be performed inside the column if needed. 

Example benzene column with a rectifying section side stream... 

Figure 8.10 shows the flowsheet configuration of a column with a rectifying section liquid side draw. The multi-component feed comes from an upstream unit in the process. The benzene liquid side draw is the product stream, and has a purity specification in terms of benzene. The distillation removes n-pentane from the feed mixture and the heavies (toluene, naphthalene and biphenyl) are purged from the reboiler. A small overhead purge stream is coimected to the condenser for pressure rehef. 

Place 50 g. of anhydrous calcium chloride and 260 g. (323 ml.) of rectified spirit (95 per cent, ethyl alcohol) in a 1-litre narrow neck bottle, and cool the mixture to 8° or below by immersion in ice water. Introduce slowly 125 g. (155 ml.) of freshly distilled acetaldehyde, b.p. 20-22° (Section 111,65) down the sides of the bottle so that it forms a layer on the alcoholic solution. Close the bottle with a tightly fitting cork and shake vigorously for 3-4 minutes a considerable rise in temperature occurs so that the stopper must be held well down to prevent the volatilisation of the acetaldehyde. Allow the stoppered bottle to stand for 24-30 hours with intermittent shaking. (After 1-2 hours the mixture separates into two layers.) Separate the upper layer ca. 320 g.) and wash it three times with 80 ml. portions of water. Dry for several hours over 6 g. of anhydrous potassium carbonate and fractionate with an efficient column (compare Section 11,17). Collect the fraction, b.p. 101-104°, as pure acetal. The yield is 200 g. 

In the section below the feed, the more volatile components are stripped from the liquid and this is known as the stripping section. Above the feed, the concentration of the more volatile components is increased and this is called the enrichment, or more commonly, the rectifying section. Figure 11.1a shows a column producing two product streams, referred to as tops and bottoms, from a single feed. Columns are occasionally used with more than one feed, and with side streams withdrawn at points up the column, Figure ll.lt . This does not alter the basic operation, but complicates the analysis of the process, to some extent. 

To redistribute the stages in the remaining sections, a shortcut simulation is used to find out the required number of trays, the feed tray location and the minimum reflux ratio for each column in the sequence. To make use of the existing column with the same number of trays (24 trays) iterations are required to adjust the sum of the rectifying sections in each column equal to 24 (number of trays in the main column). Finally, the sequence of the simple columns is merged into a complex column. The main column is not changed, but the side strippers and pump arounds need to be relocated or adjusted. 

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