Put Your Column on the Map

Residue curve maps (RCMs) can be employed in a variety of ways, including system visualization, evluation of data, and process synthesis, modeling and troubleshooting

By Scott Barnicki

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With this background, we now can examine the practical use of RCMs.

Let’;s begin with a fairly common occurrence: operational problems with an azeotropic distillation column/decanter system. The system in question is designed to produce dried ethanol using benzene as the entrainer. Figure 4 depicts the steady-state material balance. The desired product is 99.9 wt.% ethanol, with less than 10 ppm benzene. The ternary ethanol/water/benzene azeotrope is decanted, with the organic layer refluxed to the column to provide entrainer flow.

The column has always been difficult to operate and often cannot meet the tight benzene specifications of the product ethanol. Two very different temperature profiles have been observed between on-spec and off-spec operation. The column operation is stable with either profile. However, the normal control action of increasing reflux or boilup (vapor from the reboiler) does not correct off-spec operation in most cases. Often the problem appears and disappears suddenly but seems to be related to fluctuations in feed composition. Is there an explanation for this apparent multiple steady-state behavior?

The number of saddles in a particular distillation region can have significant impact on design, operability and control. In a distillation region with one saddle (a “three-sided” region), all residue curves track toward the solitary saddle. However, in a “four-sided” region with two nonadjacent saddles, some residue curves tend to track toward one saddle, while others track toward the other saddle (the residue curves labeled A and B in Figure 1).

The ethanol-drying column operates in one of three such “four-sided” regions of the ethanol/water/ benzene system. As shown in Figure 5, column profile A, the major impurity in the ethanol product is benzene. For curve B, water is the major impurity. With sufficient stages below the feed, the stripping profile will generally follow very closely to one edge of the diagram (either the benzene- or water-free edge).

A column operated to simultaneously take both the high- and low-boiling nodes as products has very few degrees of freedom from a material balance standpoint. Small fluctuations in reflux, boilup or feed composition can result in feasible operation on many different residue curves that originate and terminate at these compositions but still meet material balance constraints. Increasing reflux further constrains the distillate composition. The bottoms composition will have to swing with column disturbances. Depending upon the direction this shift takes, the trace impurity profile in the ethanol product may move from water to benzene or vice versa. This is especially true if the control strategy involves maintaining a constant temperature at a point in the column prone to wide fluctuations in composition and temperature (i.e., the stripping section). The control system may have a difficult time compensating for disturbances.

Increasing the boilup does nothing to shift from a stable, yet undesirable, composition profile. Sufficiently high boilup can be used to drive enough benzene out of the ethanol product to meet specs. However, the purity will be higher than the 99.9-wt.% target, with concomitant high steam usage. The better approach is to ensure that the stripping profile stays on the benzene-free edge of the diagram (Profile B).

Generally, it is better not to try to closely approach the compositions of the high- and low-boiling nodes at once but instead to enable the distillate composition to “float” more freely with feed and reflux. Allowing reflux of both phases from the decanter can add an important degree of freedom that helps to dampen variations in the feed composition.

Columns such as this are often difficult to model with a process simulator. Simulation algorithms frequently rely on perturbation of a variable, such as reflux, reboil or distillate-to-feed ratio, while checking for convergence of column enthalpy and material balance. The simulator may be close to a feasible solution  — but successive iterations may appear to be far apart, as the unconverged solution swings between different composition and temperature profiles. As with column operation, over-constraint by specifying excessive staging or reflux can exacerbate simulation convergence problems. Often if the simulation is difficult to converge, then the column probably won’;t run well either.

It is tempting to add a few extra stages into a design when data are limited. However, overdesign of the rectifying section is not good practice for operation in a four-sided region. It also constrains the distillate composition and can lead to many of the same difficulties with column operation.

Conceptual design
How does the ethanol-drying column fit into an overall azeotropic distillation scheme for producing dry ethanol? A good place to start the design process is by examining the topology of the RCM. As illustrated in Figure 1, there are three binary and one ternary minimum-boiling azeotropes. Only one of the binary azeotropes and the ternary azeotrope are heterogeneous. Each pure component is a high-boiling node in one of the three distillation regions. The ternary is the low-boiling node in all three regions. Many other potential entrainers for the ethanol/water system, such as n-hexane, cyclohexane, heptane, toluene, ethyl butyl ether and dipropyl ether, exhibit qualitatively similar RCMs.

Because of the distillation boundary between Regions I and II, a single distillation from the indicated feed composition (Figure 4) cannot provide pure ethanol. Instead, obtaining both high purity and high recovery of ethanol requires a more complicated distillation sequence. Pure water is in the reachable composition space for any arbitrary column feedpoint in Region II. Pure ethanol is obtainable only in the composition space where ethanol is a high-boiling node (i.e., Region I). Exploitation of liquid/liquid equilibrium allows us to cross over the distillation boundary between Regions I, II and III to generate a benzene-rich stream. A convenient distillate composition in the two-phase LLE region is the ternary azeotrope, which is the low-boiling node in all distillation regions. The organic layer formed from the ternary azeotrope is in Region III, whereas the aqueous layer is in Region II. The organic layer can be mixed with the feed to produce a composition in Region I.

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