two-column variant, the first column serves
as a prefractionator instead of producing
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Figure 2 shows the towers arranged with individual reboilers and condensers but this isn’t required and alternative arrangements may be developed. If we thermally integrate the two towers by using a single reboiler and condenser, we arrive at an arrangement (Figure 3) often referred to as a “fully thermally coupled” or “Petlyuk” column, after the noted Russian academician who wrote extensively on the thermodynamics of separating multi-component mixtures. The logical next and last step on the way to the DWC is moving the prefractionator inside the second tower so the prefractionator and the main column are a single vessel (Figure 4).
Figure 5 illustrates operation of a DWC unit. A packed column is used for ease of illustration, as is the symmetry of the sections, which isn’t a design requirement. The feed enters on the prefractionator side of the tower. If the separation is to be sharp, then the entire amount of A and some fraction of B will go over the wall and up the column to the upper rectification section. The entire amount of C and the remaining portion of B will go under the wall to the lower stripping section. The location of the partition determines the ratio of vapor entering the two sides of the dividing wall section.
Figure 3. Integrated columns -- Fully thermally
Our discussion thus far has been about “sharp separations,” that is, ones where a stream, be it overhead, middle or bottom, largely consists of a single component — that’s where our experience lies. However, a DWC also can bring value to “non-sharp” applications, i.e., those with intermediate components between A and B or B and C; these components will distribute according to the vapor/liquid equilibrium that governs the separation.
To sum up the basics:
• A DWC is a distillation column that has a vertical partition wall in the central section.
• The column may contain either trays or packing. While it may be easier to visualize a DWC with packing, there’re examples of successful installations of both types. (The ExxonMobil application previously cited is a tray tower.)
• The feed side of the two compartments acts as the prefractionator and the product side as the main column.
• In the case of a sharp split, a DWC can be used to produce three pure products from a single tower.
• A DWC can handle more than three components. There may be components lighter than the light component A that’s in the overhead product, and components heavier than the heavy component C that’s in the bottom product.
Figure 4. Single shell -- The thermally coupled
We’ll give some general heuristics for when DWC installations should (and shouldn’t) be considered. However, different companies view capital, energy and risk differently and, thus, evaluate a project according to disparate sets of metrics; so, one firm might support an installation while another might not.