Exergetic Evaluation and Advanced Thermodynamics

Especially the transition from fossil feedstock to biobased raw materials will lead to new chemical processes and to corresponding challenges in chemical engineering. Three aspects will introduce specific challenges:

• The energy mix and thus any realistic estimate of energy costs are unavailable. Thus for cost estimation for a product from a process to be newly designed a more general measure of energy costs is seeked.
• There is a many possible production routes from biomass to any product, including pyrolysis or gasification, using only dedicated components from biomass like starch, sugar, or plant oil, and finally using all biomass components including cellulose and lignin in a biorefinery. This wide variety of options, for which little previous expertise exists, requires a tool for process evaluation, which allows reliably comparison with only little input information. At the same time, the tool should allow more detailed evaluation as more information becomes available during process development.
• Since the oxygen content of biomass is significantly higher than that of fossil resources, biobased products and intermediates will have a higher degree of functionalization. For such components, thermodynamic description is a challenge.

For the evaluation of process options exergy is being used, which allows first estimates of energy demand of a process based on information only including the chemical reactions taking place. As a next step of refinement, information that is more detailed can be introduced like estimates of temperature and pressure of process steps and expected compositions. This even allows a first evaluation of the energy demand for separation and purification. Exergy can also be used to evaluate the energy losses of individual pieces of equipment. For these exergy evaluations a MATLAB-based tool has been developed.

The design of completely new processes, especially in the area of downstream options for biobased chemicals, can be supported by cascaded option trees, which allow systematic development of processes while leading to a very clear graphical representation of the various options. The option trees can be applied on increasing levels of detail as process development proceeds.

To tackle the challenges in thermodynamic description of complex mixtures, in cooperation with Ass.Prof. Dr. Thomas Wallek from TU Graz new model concepts are developed, which allow taking into account the full three-dimensional information of molecular interactions. In this way simultaneous, multiple strong interactions like hydrogen bonds between two molecules can be accounted for, which of course strongly influence thermodynamic behavior.

Current projects are :

• Exergetic evaluation of process alternatives as basis for exergo-economic comparison of process routes 
• Cascaded Option Trees for biobased processes 
• Thermodynamic modelling of strongly non-ideal and complex components

Publications :

• P. Frenzel, R. Hillerbrand, A. Pfennig, 2014: Exergetical Evaluation of Biobased Synthesis Pathways. 
• P. Frenzel, R. Hillerbrand, A. Pfennig, 2014: Increase in energy and land use by a bio-based chemical industry.
• A. Bednarz, B. Rüngeler, A. Pfennig: Use of Cascaded Option Trees in Chemical-Engineering Process Development. Chem. Ing. Tech. 86(5) (2014) 611-620.
• T. Wallek, M. Pfleger, A. Pfennig, 2016: Discrete Modeling of Lattice Systems: The Concept of Shannon Entropy Applied to Strongly Interacting Systems. 

Contact(s) : Andreas Pfennig