Energy balances multicomponent distillation

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

Ranzi E, Rovaglio M, Faravelli T, Biardi G. Role of energy balances in dynamic simulation of multicomponent distillation columns. Computers Chem Eng 1988 ... 

Before energy balances can be used in the analysis or design of multicomponent flow processes, we must have data or correlations for mixture enthalpies as functions of composition. Such correlations can be developed from models for volumetric equations of state or from models for g in the latter case, we would need a form for the temperature-dependence of the parameters in the g model. In this section we discuss how an equation of state can be used to compute an enthalpy-concentration diagram for a two-phase equilibrium situation such diagrams contribute to the analysis or design of distillation columns. 

E. Ranzi, M. Rovaglio, T. Faravelli, G. Biardi, Role of Energy Balances in Dynamic Simulation of Multicomponent Distillation Columns, Comp. Chem. Eng., 1988,12, 783-786. 

The Ki, values for each species i and the enthalpies used in the energy balance equations for any stage ra are obtained from conventional approaches used in multistage distillation analysis. However, the species flux is expressed in terms of the sum of a convective component and a diffusive component. The diffusive component is modeled using the Maxwell-Stefan approach (Section 3.1.5.1) for this complex multicomponent system in a matrix framework. For an illustrative introduction, see Sender and Henley (1998). 

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. 

Note that the mole ftactions in each stage do not add up to unity, because the above solution is only a single step in the solution of multicomponent distillation problem. Assumptions 2 and 3 are only initial guesses that must be subsequently corrected from energy balances and bubble point calculations. 

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