![]() ![]() Newer developments of estimation methods have shown that the quality of the Joback method is limited. Systematic errors of the Joback method (normal boiling point) The Joback method additionally uses a very simple and easy to assign group scheme, which makes the method usable for people with only basic chemical knowledge. This allows one to get all eleven supported properties from a single analysis of the molecular structure. The popularity and success of the Joback method mainly originates from the single group list for all properties. Model strengths and weaknesses Strengths Joback extended the range of supported properties, created new parameters and modified slightly the formulas of the old Lydersen method. The Joback method is an extension of the Lydersen method and uses very similar groups, formulas, and parameters for the three properties the Lydersen already supported ( critical temperature, critical pressure, critical volume). In both cases the equation parameters are calculated by group contributions. The heat-capacity polynomial uses 4 parameters, and the viscosity equation only 2. Two of the estimated properties are temperature-dependent: the ideal-gas heat capacity and the dynamic viscosity of liquids. Nine of the properties are single temperature-independent values, mostly estimated by a simple sum of group contribution plus an addend. The number of needed group-interaction parameters gets very high for an increasing number of groups (1 for two groups, 3 for three groups, 6 for four groups, 45 for ten groups and twice as much if the interactions are not symmetric). ![]() The big advantage of using only simple group parameters is the small number of needed parameters. Other group-contribution methods, especially methods like UNIFAC, which estimate mixture properties like activity coefficients, use both simple additive group parameters and group-interaction parameters. Joback assumes that there are no interactions between the groups, and therefore only uses additive contributions and no contributions for interactions between groups. These kinds of methods use basic structural information of a chemical molecule, like a list of simple functional groups, add parameters to these functional groups, and calculate thermophysical and transport properties as a function of the sum of group parameters. The Joback method is a group-contribution method. ![]()
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