(4aq) A Property Based Approach for Simultaneous Process and Molecular Design

Chemmangattuvalappil, N., Auburn University
Eden, M. R., Auburn University

The selection of product/product mixtures that give the optimum performance of a process is a critical issue for a design engineer. The process performance is usually understood in terms of physical properties and on many occasions, the physical properties of the product rather than their chemical structure determine the suitability of a specific product as the input to the process. Molecular design algorithms generally require target properties to design the molecules. Therefore, for obtaining the optimal solution for this type of problem, it is necessary to have a methodology to represent the product performance in terms of measurable physical properties and identify the molecule/mixture that gives the property targets corresponding to the optimum process performance.

In my dissertation work, systematic algorithms have been developed for the design of molecules corresponding to the optimum performance of a process. The concept of property clustering has been extended to the formation of molecular clusters based on second and third order group contribution methods. An algebraic approach has been developed utilizing higher order groups built from first order groups subject to the constraints of overlapping. The advantage of using an algebraic approach is that it can handle any number of molecular groups or properties and can generate all possible compounds within the required range of properties. The most significant aspect of the aforementioned method is that both the application range and reliability of the molecular property clustering technique are considerably increased by incorporating second and third order estimation. A methodology has been developed for incorporating the property contribution predicted using combined group contribution and connectivity indices into the design framework in case the property contributions of any of the molecular groups of interest are not available in literature. For the design of simple monofunctional molecules, a modified visual approach has been used whereas for the design of more complicated structures and/or for treating more than three properties at a time, an algebraic method has been used.

The developed algorithm can be used as a tool for the solution of integrated process and product design problems. Since an algebraic approach for the solution of process design problems is already available, it is now possible to solve the integrated problem irrespective of the number of properties involved. The process design problem can be solved in terms of constitutive variables without committing to any component beforehand. The property targets estimated in the process design step along with the molecular groups will form the input to the molecular design algorithm. The algebraic equations formed using the molecular operators are solved simultaneously to generate the possible molecular structures that match the optimum process performance defined by the property targets. However, apart from the targets set by the process design, the molecule has to satisfy a number of other environmental and safety constraints in order to be used in an actual process. In addition, there are many properties which cannot be estimated through group contribution methods, because it is not always possible to find a correlation between the molecular groups and the properties. Similarly, not all possible atomic arrangements are represented in group contribution methods. So, there is a need for an efficient methodology for the design of molecules with more diverse property targets. In my dissertation, I have developed an algorithm that uses the concept of molecular signature descriptors for molecular design. The signature is a systematic coding system of atom types and the signature of a molecule can be obtained as a linear combination of its atomic signatures. It has been proven that any topological indices of molecules can be represented in terms of molecular signatures and it is possible to correlate the topological indices to the actual properties and biological activities. Here, the new algorithm utilizes molecular property operators based on signatures for solving the reverse problem of obtaining the molecular structures that satisfy the property targets estimated in the process design step. A new set of equations is employed to ensure that the molecule meets the safety and environmental constraints as well. The principles in graph theory are incorporated to avoid the generation of infeasible structures. The property models required to describe the target properties may be based on QSPR/QSAR (quantitative structure-property/activity relationship) models or in the form of group contribution methods. The accuracy of this method depends only on two factors, i.e. how well the actual property-topological index relationships are estimated and the height of atomic signatures used to describe the topological indices. Since, many topological indices can be used to describe each property, this algorithm generally provides reliable results. Finally, a framework has been developed to incorporate the design of process flow sheets into the molecular design problems. The recently developed computer aided flowsheet design techniques are utilized along with the molecular design techniques to obtain the required process performance.

As a future faculty candidate, my primary interest is in the area of process and product design. However, different tools used and developed in the research I have conducted so far have the ability to be applied in many related areas as well. Similarly, many of the methods I have developed in my dissertation can be applied in different area of chemical engineering and a number of multi-disciplinary fields.

The initial projects I will be working on are:

1. Extend the applicability of the developed algorithms into more rigorous industrial problems by including the effects of varying process conditions and incorporating more flowsheet properties. This approach will be helpful for the design of materials with good accuracy.

2. The iterative nature of the molecular design problems will be reduced into a systematic generation process by introducing three dimensional descriptors in the molecular design step to expand the application range of reverse problem formulations and by including constraints in the form of signatures that ensure the stability of the designed structures

My long term projects are:

1. The extension of the molecular signature concept into the exploration of biochemical reaction pathways

2. A property based approach to designing optimum molecules by designing the reaction networks using the concept of molecular signatures in process design