Tecnologia

Joback method

Joback method
The Joback method[1] (often named Joback/Reid method) predicts eleven important and commonly used pure
component thermodynamic properties from molecular structure only.

Basic Principles
Group Contribution Method
The Joback method is a group contribution method. These kind of
methods use basic structural information of a chemical molecule like a
list ofsimple functional groups, adds parameters to these functional
groups, and calculates thermophysical and transport properties as a
function of the sum of group parameters.
Joback assumes that there are no interactions between the groups and
therefore only uses additive contributions and no contributions for
interactions between groups. Other group contribution methods,
especially methods likeUNIFAC, which estimate mixture properties
like activity coefficients, use both simple additive group parameters
and group interaction parameters. The big advantage of using only
Principle of a Group Contribution Method
simple group parameters is the small number of needed parameters.
The number of needed group interaction parameters gets very high for
an increasing number of groups (1 for twogroups, 3 for three groups, 6 for four groups, 45 for ten groups and twice
as much if the interactions are not symmetric.).
Nine of the properties are single temperature-independent values, mostly estimated by a simple sum of group
contribution plus an addend. Two of the estimated properties are temperature-dependent: the ideal gas heat capacity
and the dynamic viscosity of liquids. The heatcapacity polynomial uses four parameters and the viscosity equation
only 2. In both cases the equation parameters are calculated by group contributions.

History
The Joback method is an extension of the Lydersen method[2] and uses very similar groups, formulas, and
parameters for the three properties the Lydersen already supported (critical temperature, critical pressure, critical
volume).Joback extended the range of supported properties, created new parameters and modified slightly the formulas of the
old Lydersen method.

Model Strengths and Weaknesses
Strengths
The popularity and success of the Joback method mainly originates from the single group list for all properties. This
allows to get all eleven supported properties from a single analysis of the molecular structure.The Joback method additionally uses a very simple and easy to assign group scheme which makes the method usable
also for people with only basic chemical knowledge.

1

Joback method

2

Weaknesses
Newer developments of estimation methods[3][4] have shown that the
quality of the Joback method is limited. The original authors already
stated themselves in the original paper: “Highaccuracy is not claimed,
but the proposed method are often as or more accurate than techniques
in common use today.”.
The list of groups don't cover many common molecules sufficiently.
Especially aromatic compounds are not differentiated from normal ring
containing components. This is a severe problem because aromatic and
aliphatic components differ strongly.

Systematic Errors of the JobackMethod (Normal

Boiling Point)
The data base Joback and Reid used for obtaining the group parameters
was rather small and covered only a limited number of different
molecules. The best coverage has been achieved for normal boiling points (438 components) and the worst for heat
of fusion (155 components). Current developments which can use data banks like the Dortmund Data Bank or the
DIPPR database have a much broader coverage.

The formula used for the prediction of the normal boiling point shows another problem. Joback assumed a constant
contribution of added groups in homologous series like the alkanes. This doesn't describe the real behavior of the
normal boiling points correctly[5]. Instead of the constant contribution a decrease of the contribution with increasing
number...