Multicomponent distillation columns

The modeling of the column is accomplished by individual analysis of each stage and then the lumping together of all the stages for an overall and simultaneous solution. These solution equations are integrated wdth respect to time for the dynamic response of the system. 

The equations for the various stages will depend upon the actual conditions, but they will all have the general form of the following equations. For any stage n, an overall mass balance, energy balance, and a 

Normally, the hold-up in the vapor phase is neglected and only the liquid hold-up is considered this hold-up is usually constant. This gives the algebraic relation 

The energy balance states that the change in energy content on a stage is equal to the flow of energy in minus the flow of energj out. 

The change in heat content on any stage is usually small compared to heat flows and may be approximated as zero. This gives the algebraic relation 

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Fredenslund, A., Gmehling, J., Michelsen, M. L., Rasmussen, P. and Prausnitz, J. M. (1977a) Ind. Eng. Chem. Proc. Des. and Dev. 16, 450. Computerized design of multicomponent distillation columns using the UNIFAC group contribution method for calculation of activity coefficients. 

Single-stage flash distillation processes are used to make a coarse separation of the light components in a feed often as a preliminary step before a multicomponent distillation column, as in the distillation of crude oil. 

MCSTILL - Continuous Multicomponent Distillation Column System... 

Continuous Multicomponent Distillation Column 501 Gas Separation by Membrane Permeation 475 Transport of Heavy Metals in Water and Sediment 565 Residence Time Distribution Studies 381 Nitrification in a Fluidised Bed Reactor 547 Conversion of Nitrobenzene to Aniline 329 Non-Ideal Stirred-Tank Reactor 374 Oscillating Tank Reactor Behaviour 290 Oxidation Reaction in an Aerated Tank 250 Classic Streeter-Phelps Oxygen Sag Curves 569 Auto-Refrigerated Reactor 295 Batch Reactor of Luyben 253 Reversible Reaction with Temperature Effects 305 Reversible Reaction with Variable Heat Capacities 299 Reaction with Integrated Extraction of Inhibitory Product 280... 

Even if you were only half awake when you read the preeeding chapter, you should have recognized that the equations developed in the examples eonstituted parts of mathematical models. This chapter is devoted to more complete examples. We will start with simple systems and progress to more realistic and complex processes. The most complex example will be a nonideal, nonequimolal-overflow, multicomponent distillation column with a veiy large number of equations needed for a rigorous description of the system. 

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

S. Griiner, S. Schwarzkopf, I. Uslu, et ah, Nonlinear model predictive control of multicomponent distillation columns using wave models. Proceedings, jth International Symposium on Advanced Control of Chemical Processes. Vol. 1,... 

Problem 12. Sizing and Costing of Multicomponent Distillation Column for Biphenyl Recovery Unitf... 

The feed to a multicomponent distillation column is as follows (in order of... 

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. 

Two different approaches have evolved for the simulation and design of multicomponent distillation columns. The conventional approach is through the use of an equilibrium stage model together with methods for estimating the tray efficiency. This approach is discussed in Chapter 13. An alternative approach based on direct use of matrix models of multicomponent mass transfer is developed in Chapter 14. This nonequilibrium stage model is also applicable, with only minor modification, to gas absorption and liquid-liquid extraction and to operations in trayed or packed columns. 

Chan, H., Tray Efficiencies for Multicomponent Distillation Columns, Ph.D. Thesis in Chemical Engineering, The University of Texas at Austin, TX, 1983. 

Fredenslund. A., J.G. Gmehling, M.L. Michelsen, P. Rasmussen and J.M. Praus-nitz, Computerized Design of Multicomponent Distillation Columns Using the UNIFAC Group Contribution Method for Calculating Activity Coefficients. Ind Eng. Chem. Process Des. Dev., 16, 450-62 (1977). 

Fenske (1932) derived a rigorous solution for binary and multicomponent distillation at total reflux. The derivation assumes that the stages are equilibrium stages. Consider a multicomponent distillation column operating at total reflux. For an equilibrium partial reboiler, for any two components A and B,... 

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. 

Some of the general relationships that hold for both binary and multicomponent distillation columns are as follows... 

The method of calculation of packed columns is based on stepwise calculation of multicomponent distillation columns with variable molar overflow. The introduced modifications represent simply the presence of the reaction. 

Sugie, H., Benjamin, C. Y. L. (1970). On the Determination of Minimum Reflux Ratio for a Multicomponent Distillation Column with Any Number of Side-Cut Streams. Chem. Eng. Set., 25,1837-46. 

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