Exergy Evaluation

Exergy is used as a general measure of energy to compare process options. Exergy has the advantage that it allows quantifying energy required by a process independent of the energy mix used. Also, exergy is closely related to the costs of energy carriers, i.e. industrial steam at different pressure levels, fuels, and electricity have a comparable price, if compared on the basis of exergetic value. Thus, exergy losses of a process characterize the associated energy costs. In chemical processes, a major contribution to exergy is the chemical exergy, i.e. that energy associated with the chemical bonds of the molecules. Thus regarding only the major reaction and the associated changes in chemical exergy already allows some fundamental insight into the energy requirements of a process. As more information like temperatures and pressures becomes available during process design, this information can be used again applying exergy and thus refining the results. The level of detail can be refined even to the level, where the exergy losses even of individual pieces of equipment can be included.

To perform exergy evaluation, a MATLAB-based simulation tool, ExergySim, has been developed, which allows exergetic evaluations. The advantage of this tool is that it can be used in a very general way, e.g. defining characteristic components in case the actual product is not exactly known at the start of the design process. In ExergySim also a shortcut method for estimating downstream exergy consumption has been developed.

ExergySim has been used to compare different routes from raw biomass to final products. Some insights gained are:

• The oxygen content of future intermediate products and intermediates is expected to be higher than today, because of the high oxygen content of biomass. Removing this oxygen requires additional exergy input.
• If higher oxygen content in the products is acceptable, efficient raw materials are starch and sugar, if the exergy of production of biomass is accounted for as well. If a low oxygen content is desired, plant oil is a good choice as feedstock.
• The overall energy required for biobased processes will be higher than that for today’s fossil-based chemistry.

Thus, overall exergy has proven to be a very versatile basis for comparing process options. The energy devaluation can be roughly estimated or evaluated accounting for even equipment details based on a unique and universal quantity. Thus, exergy can consistently be used throughout all steps of process development, where exergy describes the minimum energy input required for the process.

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.
• P. Frenzel, S. Fayyaz, R. Hillerbrand, A. Pfennig, 2013: Biomass as Feedstock in the Chemical Industry – An examination from an Exergetic Point of View.

Contact(s) : Andreas Pfennig