(216a) Optimal Design and Synthesis of Ternary Distillation Systems Involving Complex Column Configurations

The synthesis of the optimal separation sequence for the distillation of multicomponent mixtures is until today a challenging task [1] particularly if complex column configurations are involved. Indeed, the available short-cut methods used for the screening of different complex distillation column alternatives including single-feed columns with a side stream above or below the feed section, columns with side stripper / rectifier, and side stream columns with prefractionator are approximate and have many restrictions [2-4].

In this contribution, the evaluation of the above mentioned complex distillation designs for the separation of ternary mixtures is carried out at operating conditions with respect to both the total vapor flow rate provided by the column reboiler(s) [3-4]_, and the minimum total number of theoretical stages of the respective column (s), so that both relevant screening criteria [3] related to operating and capital costs, may be considered.

For this purpose, a new generalized design approach [5] for the determination of the minimum internal vapor / liquid flow rates at pinch condition for each column configuration, is applied. The new design approach assumes constant relative volatilities and constant molar flow rates within each column section. Furthermore, it is based on the analytical characterization of each mass transfer section as well as on the identification of all possible pairs of constant composition zones in a column including both those around the feed sections and those around the side-stream sections so that no conceptual splitting [3, 6] of a column with side stripper / rectifier in a sequence of simple columns is required.  Accordingly, Underwood’s equations may be applied to each pair of constant composition zones along with the corresponding separation taking place in the two respective column sections should be considered. For the calculation of the minimum total number of theoretical stages of a column at operating condition, optimization algorithms based on the analytical formulations of the distillation line, the Eigenfunction and the number of theoretical stages for each column section, have been elaborated with taking into account the mass balance at each feed section [7]. Furthermore, the analysis of sharp separation geometry for a column with a side stripper / rectifier allowed the determination of the optimal composition of the column side stream (feed of the side section) at operating conditions. Indeed, it was found that for a column with a side stripper, the optimal composition of the liquid side stream (feed of the side stripper) is located within the straight line section bound between the saddle pinch of the rectifying  (or middle) section and the unstable node pinch of the side stripping section. Likewise, for a column with a side rectifier, the optimal liquid composition corresponding to the vapor side stream (feed of the side rectifier) under prevailing thermodynamic equilibrium, is located within the straight line section bound between the saddle pinch of the stripping  (or middle) section and the stable node pinch of the side rectifying section.

As compared to the available short-cut methods, the new design methods require a minimum number of specifications, apply for arbitrary feed quality, are easy to use, and do not need any graphical support [8].

The design methods developed in this work for the investigated complex column configurations, have been applied for the calculation of design parameters for separation examples including ternary ideal behaving mixtures, as well as ternary azeotropic mixtures approximated by appropriate pseudo-ideal mixtures [7]. For all tested separation examples, the developed design algorithms have been successfully validated for each complex column configuration by comparing the relevant design parameters obtained in this work with those obtained by rigorous simulation using Radfrac (Aspen plus), so that a very good agreement between them could be established.

Finally, the new short-cut methods have been successfully, used for rapid and accurate screening of the above presented complex column configurations as well as the direct / indirect simple column sequences; allowing thereby the synthesis of the optimal complex column configuration / sequence for the distillation of both ideal behaving and azeotropic ternary mixtures.

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