Difference point sidestream withdrawal columns

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 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. 

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. 

---