Thermodynamics distillation sequencing

Koehler JP, Aguirre P, Blass E. Evolutionary thermodynamic synthesis of zeotropic distillation sequences. Gas Sep Purif 1992 6 153. 

Koehler, J., Aguirre, P., and Blass, E. Evolutionary Thermodynamic Synthesis of Zeotropic Distillation Sequences, Gas Separ. Purif. 6, 4153 (1992). 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

From the thermodynamic information given in Tables 1-3, the residue curve maps are drawn in Fig. 1 for each entrainer and the batch distillation task sequence required to perform the separation of the original components is deduced using published rules [5], The residue curve maps of the ternary... 

The design of RD is currently based on expensive and time-consuming sequences of laboratory and pilot-plant experiments, since there is no commercially available software adequately describing all relevant features of reactions (catalyst, kinetics, holdup) and distillation (VLE, thermodynamics, plate and packing behavior) as well as their combination in RD. There is also a need to improve catalysts and column internals for RD applications (1,51). Figures 8 and 9 show some examples of catalytic internals, applied for reactive distillation. 

A final important point to be made is that most of the steps in such sequences are reversible the overall sequence proceeds to product nearly always because the product is the thermodynamically most stable molecule in the sequence, or because the product is removed from the equilibria by a step which is irreversible under the conditions used. A nice example is the inter-relationship between 1,4-diketones and furans the latter can be synthesised by heating the former, in acid, under conditions which lead to the distillation of the furan (18.13.1.1), but in the reverse sense, furans are hydrolysed to 1,4-diketones by aqueous acid... 

For the development of the separation section we will examine the composition and the thermodynamic behaviour of the outgoing reaction mixture. This contains benzene, ethylbenzene and poly ethylbenzenes. The separation sequencing is simple because the mixture is zeotropic and the difference in the normal boiling points of components is large. A first distillation column takes off benzene for recycle, a second one separates... 

Knowledge of the equilibrium is a fundamental prerequisite for the design of non-reactive as well as reactive distillation processes. However, the equilibrium in reactive distillation systems is more complex since the chemical equilibrium is superimposed on the vapor-liquid equilibrium. Surprisingly, the combination of reaction and distillation might lead to the formation of reactive azeotropes. This phenomenon has been described theoretically [2] and experimentally [3] and adds new considerations to feasibility analysis in RD [4]. Such reactive azeotropes cause the same difficulties and limitations in reactive distillation as azeotropes do in conventional distillation. On the basis of thermodynamic methods it is well known that feasibility should be assessed at the limit of established physical and chemical equilibrium. Unfortunately, we mostly deal with systems in the kinetic regime caused by finite reaction rates, mass transfer limitations and/or slow side-reactions. This might lead to different column structures depending on the severity of the kinetic limitations [5], However, feasibility studies should identify new column sequences, for example fully reactive columns, non-reactive columns, and/or hybrid columns, that deserve more detailed evaluation. 

Later, these columns were independently rediscovered (Petlyuk, Platonov, Slavinskii, 1965 Platonov, Petlyuk, Zhvanetskiy, 1970) on the basis of theoretical analysis of thermodynamically reversible distillation because this distillation complex by its configuration coincides with the sequence of thermodynamically reversible distillation of three-component mixture (see Chapter 4), but in contrast to this sequence it contains regular adiabatic columns. The peculiarities of Petlyuk columns for multicomponent mixtures are (1) total number of sections is n(n - 1) instead of 2(n - 1) in regular separation sequences (2) it is sufficient to have one reboiler and one condenser (3) the lightest and the heaviest components are the key components in each two-section constituent of the complex and (4) n components of a set purity are products. 

However, while estimating expenditures by thermodynamic efficiency rj (Agrawal Fidkowski, 1999), as has to be expected, the region of preferability of Petlyuk columns occupies only a small part of the area of the concentration triangle, compared with sequences of simple columns and other distillation complexes. 

The information required to obtain a base-case process flow diagram is discussed and categorized into the six basic elements of the generic block flow process diagrana. The need to obtain reaction kinetics, thermodynamic data, and alternative separation methods is discussed in the context of building a base-case process. Special enphasis is placed on alternative distillation schemes and the sequencing of columns needed for such separations. 

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