(718g) On a Systematic Approach to Simultaneously Integrate Water and Energy Utilities in Multi-Product Biorefineries

Recent negative prices of crude oil put more stress on green products to minimize their production cost and environmental footprint. Water, solvent, and energy, known as utility requirements (mass and energy), are the obvious contributors to both economic and environmental sustainability. Reuse/recycle can reduce the cost but, at the same time, increases the complexity of the network. Regeneration can save mass utilities and reduce the effluents, provided that the regeneration cost is lower than the utility savings. The optimal use of each utility is a complicated design problem, and numerous systematic approaches exist in the literature. However, existing methods consider only the simultaneous use of water and energy (or one main component and energy), handling any other substance as a contaminant. Moreover, multiple technological options result in different process designs. Different design configurations create different thermal profiles for the energy network and different effluents for the mass network. The higher the chemical/biological load of the discharged stream, the higher its treatment cost. In addition, intermediate storage is another degree of freedom, which has an impact on the thermal profile of the network and, thus, on energy consumption. As a result, degrees of freedom are not only the stream allocations, but also the mass and energy flows in the network, and the design of the auxiliary units. Decisions need to consider the cost for utilities, for treatment, and for investment in auxiliary units and, at the same time, to keep the complexity of the network low. The challenge is to coordinate all the individual problems into a single formulation.

This work introduces the concept of interacting superstructures for the integrated optimization of utility networks and process design. The method takes into account multiple components (contaminants) and does not pivot on a central component (e.g., water, H2). Alternative design options exist for regeneration and storage. The objective function evaluates the role of each component and balances the trade-offs between reuse, regeneration, and discharge. The problem is nonlinear by nature, but assumptions and piecewise linearization techniques are adopted to keep it linear. The optimization model is applied to design the utility network of a real-life biorefinery process. Results show that the utility network optimization (UNO) model managed to reduce up to 70% of the total annual cost, compared to the initial design, by reducing mainly the utility cost for both mass and energy. UNO identified that, in the particular case study, 62% of the thermal flows are a degree of freedom and managed to allocate mass and thermal flows properly. The UNO model finds practical application in grassroots and retrofit systems, exclusively for the design of the utility network but not for the optimization of the process or the supporting unit design. It can be used to analyze how the uncertainty of different costs affect the process and find flexible designs, able to absorb price fluctuations, simply by changing the mass flows. Future work will incorporate options for the energy system, including heat pumps and generation of different steam qualities. The integration of detailed treatment systems and the analysis of total site networks is also left as a future challenge.

Acknowledgments

Financial support from CIMV and the Marie Curie project RENESENG [FP7-607415] is gratefully acknowledged.