Joback method

Joback the method ( often referred to as Joback Reid Method) allows the prediction of eleven major thermodynamic properties of pure substances exclusively from the molecular structure.

• 2.1 strengths
• 2.2 weaknesses

Basics

Group contribution method

The Joback method is a group contribution method. This type of prediction methods used simple structural information of a chemical molecule such as a list of the functional groups, these groups has to parameters and calculated thermophysical and transport properties as a function of the sum of these group parameters.

Joback assumed that there are no interactions between the groups and therefore used exclusively additive group contributions and no contributions for interactions between groups. Other methods, such as UNIFAC, estimates the mixture properties such as activity coefficients, or the Benson method which estimates heat capacities, enthalpies and entropies of use, in addition to the simple purely additive contributions and interaction parameters. The great advantages of the restriction simple posts is the small number of necessary parameters ( one parameter for each group and property), while the number of interaction parameters greatly increases as the number of groups increases (1 for two groups of three for three, six for four, 45 for ten and even twice as much, when the interaction parameter is not symmetric).

Nine of the predicted from the Joback model properties are temperature- independent quantities, most are simply calculated from the sum of group contributions plus a summand.

Two of the properties are dependent on the temperature: The ideal -gas heat capacity and the dynamic viscosity of fluids. For the heat capacity of a cubic polynomial is used with four parameters, the liquid viscosity of a polynomial with only two parameters ( straight line). In both cases, the equation parameters are determined from group amounts.

History

The Joback method is an extension of the Lydersen method, and uses very similar groups, formulas and parameters for the properties that Lydersen already supported (critical temperature, critical pressure and critical volume ).

Joback extended the model for other characteristics, certain new parameters and modified slightly the defining equations of the old Lydersen model.

Model strengths and weaknesses

Strengthen

The popularity of Joback method derives essentially from a group list that is the same for all properties. This allows the prediction of all eleven properties supported from a single analysis of a chemical structural formula.

In addition, the groups of Joback model are very simple and can be used with only minor chemical knowledge.

Weaken

Recent developments of estimation have shown that the quality of the Joback method is limited. Also, the original authors have noted in their paper: "High accuracy is not Claimed, but the Proposed methods are Often as or more accurate than techniques in common use today. " (English as: " High accuracy is not claimed, however, are the proposed methods are often as accurate or more accurate than current conventional methods. " )

The list of groups covers many common components from only inadequate. In particular, aromatic substances can not be distinguished from normal ring components. This is a serious problem, since the properties of these different classes of components significantly.

The data base used the Joback and Reid to determine the parameter group was quite small and covered only a small number of different substances from. The best data base was for normal boiling points reached ( 438 components) and the worst for the enthalpy of fusion (155 components). Recent model developments through the use of factual databases such as the Dortmund Data Bank or the DIPPR database a much larger data base.

The formula that is used for the prediction of the normal boiling point, is a different problem. Joback assumed that the contribution of groups in a homologous series of alkanes, such as the remains constant. However, this is not a correct assumption. Instead of the constant posts a decrease in contributions with increasing number of groups must be used. The formula that chose Joback, leads to high variations in small and large molecules and provides an acceptable estimate for medium size components.

Formulas

In the following formulas represents a group contribution. are added for each occurrence of a group. If so happens, for example, a group three times, their contribution is added three times.

Group contributions

Example calculation

Acetone ( propanone ) is the simplest ketone and is divided according to the Joback method into three groups: two methyl groups (- CH3) and a keto group (C = O). Since the methyl group occurs twice, its contribution is added twice.