Consider Dividing Wall Columns
These distillation units often can provide both capital and energy savings.
two-column variant, the first column serves
as a prefractionator instead of producing
pure product.
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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.
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Figure 3. Integrated columns -- Fully thermally
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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.
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Figure 4. Single shell -- The thermally coupled
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Appropriate Applications
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.
