Mass balances multicomponent distillation

If one or more unit operations have been given infeasible specifications, then the flowsheet will never converge. This problem also occurs with multicomponent distillation columns, particularly when purity specifications or flow rate specifications are used, or when nonadjacent key components are chosen. A quick manual mass balance around the column can usually determine whether the specifications are feasible. Remember that all the components in the feed must exit the column somewhere. The use of recovery specifications is usually more robust, but care is still needed to make sure that the reflux ratio and number of trays are greater than the minimum required. A similar problem is encountered in recycle loops if a component accumulates because of the separation specifications that have been set. Adding a purge stream usually solves this problem. 

Multicomponent rectification Consider a multicomponent mixture of m-species continuously fed into a distillation column (Acrivos and Amundson, 1955 Amundson, 1966 Ramkrishna and Amundson, 1985). Let and be the compositions of the ith species on the nth plate for the liquid phase and the vapor phase, respectively. Based on constant molal overflow of liquid with a downflow rate L and a vapor upward flow rate V, the steady-state mass balance for the ith species in the rectifying section above the nth plate leads to the equation... 

For single-stage multicomponent batch distillation, the stage mass balance may be applied successively to any two components, j and k, as follows ... 

Continuous distillation of multicomponent mixtures is also described by mass balances and phase equilibria ... 

In industry many of the distillation processes involve the separation of more than two components. The general principles of design of multicomponent distillation towers are the same in many respects as those described for binary systems. There is one mass balance for each component in the multicomponent mixture. Enthalpy or heat balances are made which are similar to those for the binary case. Equilibrium data are used to calculate boiling points and dew points. The concepts of minimum reflux and total reflux as limiting cases are also used. 

For example, in the case of mass balance, pumps can be neglected because they have only one input and one output and their mass balance is a trivial one. Even in the case of enthalpy balance, pumps are often neglected as the shaft work exerted on the pump is often negligible in comparison with the enthalpy changes of streams. Similarly, if only the multicomponent balance is set up around a distillation column, the column proper is usually merged with the condenser, boiler, or other supporting equipment. 

A rigorous model in batch distillation involves consideration of column dynamics along with the reboiler and condenser dynamics. Distefano presented detailed analysis of the characteristics of differential mass and energy balances associated with the complete dynamics of a multicomponent batch distillation column. Distefano s work forms the basis for almost all of the later work on rigorous modeling of batch distillation columns, and this model is presented below. 

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