(112h) The Fluxmax Approach: Integrated Process Synthesis and Heat Integration Exemplified for the Production of Hydrogen Cyanide

Renewable energies and alternative feedstock aside, increasing energy efficiency through process intensification and optimal process design are key measures to mitigate global warming. The European chemical industry accounts for about 20% of total industrial energy consumption and the German society for chemical engineering and biotechnology (DECHEMA) estimates that up to 20-30 Mt of carbon dioxide emissions can be reduced by energy efficiency measures alone. Resource optimization of chemical processes is typically addressed in two consecutive steps: the optimization of single process units in a first step and subsequent energy integration of the overall process structure to identify the minimum utility requirements. This two-step procedure prevents, however, the identification of the globally-optimal process design because individual optimal process units are not necessarily optimal from the overall process perspective. Furthermore, optimizing resource efficiency can mean energy efficiency but also raw material efficiency which and setting the objectives at the process unit level may have negative side effects on the process scale.

In order to address this problem, we successfully developed the FluxMax approach [1] which is a single step procedure for process synthesis and simultaneous heat integration and comprises three steps: the discretization of the thermodynamic state space and modeling of elementary edges to obtain a superstructure and a subsequent network flow optimization in order to identify the optimal process pathway [2]. The advantage of the method is that heat integration is ensured using linear instead of nonlinear constraints as compared to the Duran-Grossmann method and the user can choose between direct and indirect heat integration [3,4]. The method is exemplified for the production of hydrogen cyanide. This case study is selected because competing reactor technologies with opposite characteristics exist: an endothermic and an exothermic reactor where the endothermic reaction requires substantial heating duties but provides superior product purity than the exothermic process. In addition, both chemical reactions yield hydrogen as a byproduct. Its recycling alternatives are thus included in the optimal process design.

Within this contribution, the novel method is introduced starting with the discretization, modeling of elementary transitions and the resulting superstructure up to the network flow optimization for the case study. It is illustrated how the simultaneous approach enables better decision making on the process level [2]. At last, the overall most resource-efficient process for the case study of HCN production is identified: total variable cost can be reduced by 67% for the resource-optimal process pathway.

[1] Liesche, G.; Schack, D.; Rätze, K.H.G. and Sundmacher, K. (2018). Thermodynamic Network Flow Approach for Chemical Process Synthesis. Computer Aided Chemical Engineering 43, 881-886

[2] Liesche, G.; Schack, D.; Sundmacher, K. (2019). The FluxMax Approach for Simultaneous Process Synthesis and Heat Integration: Production of Hydrogen Cyanide. AIChE J., DOI:10.1002/aic.16554.

[3] Duran, M.A.; Grossmann, I.E. (1986). Simultaneous Optimization and Heat Integration of Chemical Processes. AIChE J., 32, 123-138

[4] Schack, D.; Liesche, G.; Sundmacher, K. (2019). The FluxMax Approach: Simultaneous Flux Optimization and Heat Integration by Discretization of Thermodynamic State Space Illustrated on Methanol Synthesis Process. Computer and Chemical Engineering, under review