Thermally coupled columns

Various studies have compared the thermally coupled arrangement in Fig. 5.166 with a conventional arrangement using simple columns on a stand-alone basis. These studies show that the thermally coupled arrangement in Fig. 5.166 typically requires 30 percent less energy than a conventional arrangement using simple columns. The fully thermally coupled column in Fig. 5.166 also... 

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 ... 

Consider the use of a side-rectifier or side-stripper for the separation of a three-product mixture. Assume that thermally coupled columns operate at the same pressure. Also, assume the feed to be saturated liquid. Data for the operation of the two arrangements are given in Tables 21.9 and 21.10. 

Control Performance of Thermally Coupled Columns 63 Table 2. Minimum singular value and condition number for each arrangement... 

Disks, for digital computer, 555-56 Distillation batch binary, 108 degrees of freedom, 87-88 difficulties in modeling, 77 ideal binary, 70-74 modeling, 70-74 as a multicapacity process, 214 nonideal binary, 109 thermally coupled columns, 536 Distillation control adaptive, 442-43... 

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... 

Thermal integrated columns offer significant savings, typically between 20-30%. The schemes with prefractionator are particularly interesting. Thermal coupled columns are promising, but still in research, although divided-wall column has known some success recently. 

Complex columns are in a broad sense all columns that are not simple columns, like the column shown in Figure 1.1. Complex columns may have multiple feeds, side product streams, stream transfers between two column units (thermally coupled columns), simultaneous chemical reaction(s) within the column body, hybrid membrane-distillation columns, and so on. Each of these columns present unique opportunities for cost saving. Typical complex columns are shown in Figure 1.3a d. 

Due to the tremendous costs associated to distillative separations, many alternate schemes to the simple column shown above have been proposed over the past several years both to improve on some of its inherent costs. Traditionally, when purifying a multicomponent mixture, an entire series of distillation columns are used in series, and the way in which these columns are sequenced may make a tremendous difference in the eventual process costs. However, due to the large energy requirements of even the most optimal sequence, more complex column arrangements have been proposed and subsequently utilized. These arrangements include thermally coupled columns such as side rectifiers and strippers, the fully thermally coupled columns (often referred to as the Petlyuk and Kaibel columns). 

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... 

THERMALLY COUPLED COLUMNS SIDE RECTIFIERS AND STRIPPERS... 

The internal degrees of freedom, on the other hand, are concerned with the internal workings of the column. The columns we have dealt with up to this point only had one internal degree of freedom a single reflux ratio of a particular CS, which then allows one to infer all remaining column parameters. However, thermally coupled columns are fundamentally different from this point of view because there is a degree of freedom associated with each side unit. In other words, the designer may specify the reflux in the side unit s CS as well as the reflux of a CS in the main column independently of one another, which will subsequently specify the flows and refluxes in the remainder of the column. Thus, a three component thermally coupled system with one thermally coupled unit requires two refluxes to be specified. 

Furthermore, it should be noted that in thermally coupled columns, finding pr< le intersections is somewhat different from those we have become accustomed to thus far, because at a thermally coupled junction, there are not only two neighboring CSs, but three. Consequently, three profiles, not two. all have to intersect one another at the same point. A typical feasible design for a side-strippo configuration is shown in Figure 6.31. 

We have now demonstrated quick and relatively simple algebraic method to design thermally coupled columns at minimum reflux conditions. All relative... 

THERMALLY COUPLED COLUMNS SIDE RECTIFIERS AND STRIPPERS 199 TABLE 6 Summary of the Pseudo Simple Column at the Thermally Coupled Units... 

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