Abstract:
Group contribution method is one of
the main methods for predicting the physicochemical properties of
organic compounds, the position of each group is not taken into
consideration by classical group contribution method yet.
In this work, atomic connectivity
group contribution (ACGC) method is developed for predicting critical
properties of organic compounds for the first time.
Herein, a new group defining
method, namely atomic adjacent group (AAG) method, is proposed to
describe the relationship between core atom and its adjacent atoms. For
distinguishing isomers effectively, the shape factor (SF) is used to
describe the effect of molecular shape on group, and atomic connectivity
factors (ACF) are defined for describing the position of each group in a
molecule. ACGC models using a general formula are developed for
predicting three properties of 710 organic compounds. The external
verification and internal verification methods, usually ignored by the
traditional group contribution method, were further utilized during the
modelling process. Compared with AAG model, ARE decreased by 6.82-42.57
% when SF was considered and, ARE decreased by 24.19-62.25 % when both
SF and ACF were applied as using the ACGC method. Accordingly, SF and
ACF are effective in improving the group contribution method and ACGC
method is accurate in calculating the properties of organic compounds.
KEYWORDS: Atomic connectivity group contribution (ACGC) method;
Group contribution (GC) method; Atomic connectivity factors (ACF);
Critical properties; Shape factor (SF)
Introduction
The
physical and transport properties of compounds play an indispensable
role in many chemical engineering applications. Critical properties
(critical temperature T c,critical pressureP c,critical volume V c)
are important properties of substances in chemistry and chemical
engineering. For example, when calculating thermodynamic properties and
transfer parameters of matter by corresponding state method, critical
properties of matter need to be used. When the corresponding state
method or state equation method is used to solve the PVT relation of
pure matter and mixtures1, 2, the knowledge of
critical properties is needed. Critical properties are essential
property parameters for understanding the phase behavior of pure
components and mixtures3. Critical
properties are also needed in
situations where
critical
phenomena are studied for high-pressure operations such as
supercritical
extraction4, 5 and oil drilling6. In
view of the importance of critical properties, a great deal of effort
has been expended on collecting, sorting and evaluating them bit by bit.
Unfortunately, data on experimental
critical properties of organic compounds are limited. Experimental
measurements of critical properties are sometimes laborious, expensive,
and even difficult to measure2. On the other hand,
with the increase of organic compound data, some experimental data
cannot be found from the database, so it becomes crucial to develop
mathematical models to provide reasonable estimates of these properties.
As a simple pen-and-paper
structure-property relationship
(SPR) method7-9,
group
contribution (GC) method is based on the principle of group
addition10. The group contribution method is widely
used to predict various thermodynamic
properties of organic compounds,
such as critical temperature11-20, critical
pressure13, 21-24 , critical
volume17, 25 and normal boiling
point10, 26. Among
GC approaches, Joback27, C-G (Constantinou and
Gani)28 and M-G (Marrero and Gani)29 methods are classical GC methods.
Joback27 is the first-order group technique and the
principal advantages of this method are the simplicity and generality.
And yet the combination of first-order groups in one molecule may result
in some isomeric molecules and thus the properties of these molecules
calculated by first-order group contribution method is the same. The
first-order groups are insufficient in describing the portions of the
molecular structure and in distinguishing isomers. Based on the
first-order group technique, Constantinou and Gani28developed the second-order group technique, which further considers the
influence of the first and second nearest neighbors of the group under
consideration. In C-G method, 12 first order level groups and 41 second
order level groups were used for about 300 compounds. Average relative
error (ARE) is in the range of 0.85-2.89 % for critical properties and
decreased by 22-48 % compared with first order level group models. In
order to better describe the molecular structure part and distinguish
isomers, Marrero and Gani29 developed a group
contribution (M-G) method at three levels. In M-G method, 124 first
order level groups, 79 second order level groups and 32 third order
level groups were used for about 800 compounds with critical properties.
The use of third level provided more structural information about
compounds and improved the accuracy and applicability of GC method. For
example, ARE for critical properties decreased by 5-12 % compared with
second order level group models. Hukkerikar et. al.30presented revised model parameters for M-G method
(M-G+) at three levels for 18 pure component
properties with about 12000 compounds. 130 first order level groups, 90
second order level groups and 31 third order level groups were used for
about 900 compounds with critical properties. The critical properties of
ARE are higher than MG method.
The hypothesis of classical GC method is that a group has an equal
contribution value at any molecule. The position of each group plays an
important role in the properties of compounds, while it is also not
considered by the traditional group contribution methods. In addition,
no uniform rule for group division was developed in the above classical
GC methods, which also brought inconvenience to the division of complex
compounds. Also, traditional group contribution methods usually do not
carry out external verification
and internal verification, so the stability of the model is not
verified. Sun and Sahinidis13 proposed a method to
identify functional groups in molecular structures and to verify the
stability of the model by dividing the training and test sets. A total
of 74 functional groups were used to describe the critical properties of
about 860 organic compounds. TheR 2 of critical temperature, critical pressure
and critical volume are 0.96, 0.95 and 0.99 respectively, and the ARE
are 2.02 %, 4.09 % and 3.10 %
respectively.
The aim of proposed atomic connectivity group contribution (ACGC) method
is to establish a general and simple group contribution method for
calculating phase transition properties of organic compounds. A general
formula containing group, shape factor and localization factor is used
to predict the different properties of organic compounds. The present
work is completed for the calculation of the critical properties. Based
on the relationship between core atom and its adjacent atoms,atomic
adjacent group (AAG) with unified dividing rules is defined. In order to
describe the influence of the molecular shape of compounds such as
branched chains and rings on the calculation of properties, the shape
factor (SF) is defined. The atomic connectivity factors (ACF) are
defined for describing the position of each group. The predictive
ability and stability of the model are verified by external and internal
validation.
Methodology