(666g) Total Site Integration As a Synthesis Tool to Select Biomass Valorization Paths and Schedule Multiple-Feedstock Operations

This paper presents a systematic approach to select and integrate product portfolios in multiple-feedstock biorefineries. Value chains include all potential routes from biomass to intermediate and end-products that may enter biorefineries. Once the problem requires minimum energy requirements to secure process sustainability, Total Site integration is called to minimize consumption of steam (heat transfer mean among processes). The use of Total Site Analysis to screen biorefinery paths has been recently presented by the authors (Kokossis et al., 2015) to detect promising process portfolios that optimally integrate each other in terms of energy use. The methodology introduced a modified transshipment model to evaluate energy synergies among candidate processes of value chains given the type and annual capacity of biomass feedstock. Once the majority of applications refer to lignocellulosic biorefineries, the analysis naturally extends to the assessment of changes in operation and efficiencies due to seasonal availability of each lignocellulosic biomass type. This paper extends previous concepts to assess the impacts of switching feedstocks on energy efficiencies and the selection of biorefinery portfolios.

Lignocellulosic biorefineries are based on the exploitation of the three fundamental chemical components: C5 and C6 sugars and lignin. Though products of value chains remain the same with changes in the type of lignocellulosic material – wheat straw, rice, poplar or birch, they all include sugars and lignin – changes appear in process capacities and energy consumption due to seasonal variations in capacities and yields in sugars and lignin for each biomass type. Accordingly, heat interactions and recovery among processes based on the valorization of each component change by switching feedstocks during the year.

The combinatorial process synthesis and integration problem is formulated by the mass balances along value chains (selection of paths) and energy balances at each interval of the proposed transshipment (evaluation of selected paths). The problem also assumes changes in biomass capacities at discrete time periods for each candidate type of biomass. Accordingly, the domain of flowrates along value chains is expanded by the indexes of biomass type and the time period that is applied. Large deviations in common chemicals flows and processes capacities at different periods are not industrially accepted. Likewise, the domain of heat flows in the transshipment model and utilities is also expanded by time and biomass index and limited for logic deviations. The MILP model combines selection of value chain paths with integration in transshipment model to evaluate energy savings at each period and thus, to synthesize the seasonal feedstocks plan and select the processes portfolio that minimize energy cost of the biorefinery site.

 

References

A.C. Kokossis, M. Tsakalova, K. Pyrgakis, Design of integrated biorefineries, Computers & Chemical Engineering, 2015, 81, 40–56