BIPS binary interaction parameters

An essential part of a simulation package is the component data bank, which commonly contains more than a thousand chemical compounds. For chemicals not available in the data bank, process simulators provide a user-added component facility to include the required compounds in the simulation. A short overview of the data needed in the simulation and design of processes is given in Table 8.3. The properties required for the design of a chemical process depend upon the temperature, pressure, and concentration. The most crucial and least available physical property data are for phase equilibrium. The key parameters for the design of separation units that are based on equilibrium stages are called binary interaction parameters (BIPs). 

The predicted phase equilibrium is a strong function of the binary interaction parameters (BIPs). Process simulators have regression options to determine these parameters from experimental phase-equilibrium data. The fit gives a first-order approximation for the accuracy of the equation of state. This information should always be considered in estimating the accuracy of the simulatioa Additional simulations should be run with perturbed model parameters to get a feel for the uncertainty, and the user should realize that even this approach gives an optimistic approximation of the error introduced by the model. If BIPs are provided in the simulator and the user has no evidence that one equation of state is better than another, then a separate, conplete simulation should be performed for each of these equations of state. The difference between the simulations is a crude measure of the uncertainty introduced into the simulation by the uncertainty in the models. Again, the inferred uncertainty will be on the low side. 

The DME-water binary system exhibits two liquid phases when the DME concentration is in the 34% to 93% range [2]. However, upon addition of 7% or more alcohol, the mixture becomes conpletely miscible over the complete range of DME concentration. In order to ensure that this non-ideal behavior is simulated correctly, it is recommended that binary vapor-liquid equilibrium (VLE) data for the three pairs of components be used in order to regress binary interaction parameters (BIPs) for a UNIQUAC/UNIFAC thermodynamics model. If VLE data for the binary pairs are not used, then UNIFAC can be used to estimate BIPs, but these should be used only as preliminary estimates. As with all non-ideal systems, there is no substitute for designing separation equipment using data regressed from actual (experimental) VLE. 

For binary pairs without measured phase equilibrium, use the predictive UNIQUAC functional-group activity coefficients (UNIFAC) [19] property method to estimate the BIPs. There are two UNIFAC methods one for VLE and another for LEE. A predictive model requires that group interaction parameters be available for the various subgroups of a chemical s structure. 

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