Process
Ideal Stage or Mass Transfer...
Which Model Should Be Used?
Insight:
The design and optimization of separation processes is
carried out using process simulators, which utilize various calcula-
tion 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 va-
por leaving a stage are at the same temperature. Ideal stage mod-
els can also account for non-ideal column performance through the
use of reaction kinetics as is done for amine sweetening columns.
Obviously, the main disadvantage of the ideal stage
approach is just that—the use of ideal stages to model real trays
or packing depths. However, 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
on a tray 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 enhancement factor 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 reac-
tions 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 proper-
ties: 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 pack-
ing, 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.
CONCLUSIONS
When performed properly, both the ideal stage and mass
transfer approach as implemented in ProMax 4.0 can calculate ac-
curate results for a variety of separation processes with and without
reactions. The ideal stage approach can be used initially to deter-
mine appropriate equipment sizes and operating conditions. More
detailed studies can be performed using the ideal stage approach,
the mass transfer approach, or both. Although significant operat-
ing experience provides reasonable efficiency estimates for most
processes, the empiricism in scaling up from ideal to real stages or
ideal stages to real bed lengths can be a disadvantage when ac-
curate overall efficiencies or HETP’s are unavailable.
The mass transfer approach requires more equipment
design details and does not make use of overall efficiencies or
HETP’s. More detailed composition and temperature profiles are
produced by this method at the expense of longer calculation time.
The mass transfer approach may appear more predictive in nature,
but is not necessarily more accurate. It relies on more parameters
that must be estimated, as both require thermodynamic data to
model equilibrium—for the tray composition in the ideal stage
approach and for the interface composition in the mass transfer
approach. Many of these mass transfer parameters are of limited
accuracy but also may be of limited sensitivity in some systems.
Both techniques are useful tools in process simulation.
For more information about this study, see the full article at
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