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Ideal stage or mass transfer...which model should be used?

The design and optimization of separation processes is carried out using process simulators, which utilize various calculation approaches. Two techniques that are widely used for modeling distillation are the ideal stage model and the mass transfer model.

Ideal Stage Models - The ideal stage model requires a minimum amount of data—only equilibrium relationships and enthalpy data for the heat balance. The assumptions of the ideal stage approach are: 1) that the vapor and liquid are both perfectly mixed so that the vapor and liquid leaving a stage are at the same composition as the material on the stage and 2) that thermodynamic equilibrium is obtained on each stage. The equilibrium assumption also means liquid and vapor leaving a stage are at the same temperature. Ideal stage models can also account for non-ideal column performance through the use of reaction kinetics as is done for amine sweetening columns. For most processes encountered in gas processing and other industries, the overall efficiencies are well established for proper operating conditions of the column. For systems that are unavailable, similar systems often exist to allow for efficiency estimation. If not, the mass transfer approach is available as an option.

Mass transfer models - For the end user, the notable feature made available via the mass transfer approach is the ability to model a column with the actual number of trays in the unit or the actual depth of packing. However, there are still several assumptions that are made in this approach that can have a significant impact on results. Two that are worth mentioning include the mixing model for trayed columns and the discretization of the packing depth for packed towers.

Application of the mass transfer model to random or structured packing requires the column height to be discretized into vertical segments or stages. For trayed columns, various mixing models can be used for the liquid and vapor phases. The most basic assumption is that of complete mixing in both the liquid and vapor phases. However, the concentration gradients that develop across operating trays can significantly impact the predictions made by this model since this gradient is the driving force for mass transfer. As the column diameter becomes larger, the perfectly mixed flow model is less applicable.

For modeling both liquid phase chemical reaction and mass transfer, the use of the Mass Transfer - Kinetic (MTK) technique may be considered. The enhancement factor describes the increased rate of absorption due to the effect of a chemical reaction. The material balance requires kinetic rate expressions for all chemical reactions occurring in the system. As with equations for a non-reacting system, an appropriate model for interface behavior must be used.

Mass transfer models require data necessary to calculate interphase mass and heat transfer coefficients and interfacial area based on correlations of the following transport and thermal properties: diffusivities, viscosities, densities, heat capacities, thermal conductivities, etc. Furthermore, mass transfer models require detailed information on the column internals. For trays, this includes information such as weir heights and fraction active area. For packing, this includes surface area per unit volume and void fraction.

If the simulator allows the user to select from various alternatives for these parameters, knowing the correct selection may be problematic. Further, the prediction of multicomponent mass transfer coefficients is of questionable accuracy. These facts prompt the recommendation that columns modeled with the mass transfer approach be checked against an ideal stage model with an expected efficiency until sufficient experience with the particular application is achieved.

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Ideal stage or mass transfer...which model should be used?

The design and optimization of separation processes is carried out using process simulators, which utilize various calculation approaches. Two techniques that are widely used for modeling distillation are the ideal stage model and the mass transfer model.

Ideal Stage Models - The ideal ...