UNIFAC

UNIFAC ( short for Universal Quasi Chemical Functional Group Activity Coefficients ) is a method for the estimation of activity coefficients, which is widely used in process engineering and technical chemistry.

UNIFAC is derived from UNIQUAC. UNIQUAC used substance-specific parameters that must be determined from experimental data. These parameters are predicted in UNIFAC so that no measured data longer needed.

• 2.1 Combinatorial share
• 2.2 Residual proportion

Principle

UNIFAC is a group contribution method based on the principle of combination of structural groups. This is in contrast to the normal view, the mixture of molecules. The groups are usually functional groups, for example, alcohol or carbonyl group, as well as smaller molecular fragments such as single atoms, but these small fragments are always considered taking account of their chemical environment. A few smaller molecules (such as water) are fully defined as a separate group.

Group contribution methods principle

The principle of the group contribution method essentially consists in that only the properties of some dozen structure groups must be known to require instead the properties of several million materials for the calculation of mixture properties. With these few structural groups as building blocks can be a very large number of molecules can be constructed.

UNIFAC group contributions

UNIFAC used to predict two types of group contributions:

• Additive contributions that very small groups ( subgroups ) are assigned. These are group volume and group surface.
• Interaction parameter between larger groups that include several similar sub-groups.

The group volumes and surfaces based on approximate van der Waals surfaces and volumes and are therefore constants in the model that have a physico-chemical background.

The interaction parameter can be adapted to the experimental activity coefficients and to phase equilibrium data which activity coefficients can be derived by non-linear optimization method. The interaction parameters are thus determined only empirically.

Current parameterization

The UNIFAC model allows the prediction of activity coefficients in mixtures with alkanes, alkenes, alkynes, alcohols, aromatics, esters, ethers, amines, carboxylic acids, organic fluorides, organic chlorides, organic bromides, organic iodides, thiols, sulfones, water, furfural, thiophenes, pyridines, morpholine, isocyanates, silanes, siloxanes, amides, and organic nitrates.

For these functional groups are present interaction parameter with at least one of the other groups. There are, however, not completely parameterizes all combinations, for example, an activity coefficient of a mixture of 3- methylthiophene and n- hexane are calculated, however, a prediction of activity coefficients in a mixture of thiophene with 3-hexene fails due to missing parameters.

UNIFAC can not be used to calculate the activity coefficients in the mixtures, salts, or, more generally, contain electrolytes or polymers. For these two classes of compounds, there are extensions of the UNIFAC model described here.

Equations

Detailed formulations can be found in the original article or in textbooks.

The calculation of the activity coefficients are additive over two terms:

Combinatorial share

Y c is called a combinatorial part and van der Waals surface is made ​​of (F) and volumes ( V) and mole fractions (x ) were calculated.

With

In addition to the mole fractions are van der Waals surfaces and volumes qi ri of the molecules needed. This can be determined from tabulated values ​​for the groups ( group surface Qk and Rk group volume ).

νk (i ) the frequency of the group K in the molecule, i

Residual fraction

? R is called the residual fraction is calculated and finally adapted from interaction parameters.

The remaining balance is calculated from group activity coefficients Γk.

The group activity coefficients in the mixture Γk and Γk pure substance ( i) on the relationship

Calculated.

Θ is the surface fraction

And X is the Gruppenmolenbruch.

The ψ parameter contains the adjustable parameters of the UNIFAC model.

Example calculation

History

UNIFAC was developed in the 1970s, with the emphasis was easier schedule to start alone on the prediction of vapor -liquid equilibria of mixtures essentially of organic substances and water mixtures. As for the model development activity coefficients were needed for a variety of mixtures ( the Dortmund Data Bank ) was begun with the construction of a fact database, which still exists today, albeit in a vastly enhanced and modified form.

Since activity coefficients also allow using simple thermodynamic relations solid-liquid equilibria ( solubility of solids in liquids ) and liquid-liquid equilibria (see miscibility gap ) to calculate the UNIFAC model in the eighties was also increasingly used for the calculation this phase equilibria used. The case outcrop model weaknesses of the original model led to the development of a specifically parameterized models for the calculation of liquid-liquid equilibria ( LLE UNIFAC ) and later became a model for the estimation of octanol - water partition coefficient. Secondly, the model is extended, for example to temperature-dependent interaction parameters:

These advanced models are referred to as modified UNIFAC models.

Importance

UNIFAC (especially the newer versions ) is now the most commonly used method for the estimation of mixture data. Forecasts are for example used in the process simulation, the currently prevailing methods for the design and optimization of chemical processes, plants and entire factories. UNIFAC is also used in the synthesis process in which, in very general terms, substances with specific properties for specific tasks are being sought. This task can be, for example the an entrainer for the azeotropic or extractive.

Recent Developments

UNIFAC is used in a number of research groups and developed. Current developments (selection) on target

• The calculation of electrolyte mixtures containing
• The estimation of viscosities
• The integration of UNIFAC in mixing rules for equations of state ( PSRK )
• Predicting UNIFAC interaction parameters
• The extension of the UNIFAC model to specific groups of substances
• The derivation of the UNIFAC model for prediction of excess enthalpies of vaporization and: group contribution models UNIVAP & EBGCM
• The revision of interaction parameters for existing groups and the addition of interaction parameters for new functional groups to improve the applicability and quality of the UNIFAC model ( UNIFAC Consortium ).