(548x) A Total Site Synthesis (TSS) Model for the Selection, Integration and Planning of Multiple-Process and Multiple-Feedstock Biorefineries

Authors: 
Pyrgakis, K. A., National Technical University of Athens
Kokossis, A. C., National Technical University of Athens
This paper presents a systematic approach to screen biorefinery value chain paths by combining process integration thermodynamics with mathematical programming. The incentives of this work originate in lignocellulosic biorefineries, where numerous candidate valorization paths (value chains) are based on the use of C5/C6 sugars and lignin intermediates. Once minimum energy consumption is always the case, energy integration should be applied (beyond energy targeting) to systematically evaluate the infinite process combinations that emerge through value chains and may get integrate across the biorefinery site. The work introduces systems representations (i) to map all synthesis routes along value chains simultaneously coping with biomass seasonality issues and (ii) to mathematically formulate Total Site energy integration of multiple-process sites considering processes as additional degrees of freedom. The process synthesis representation is based on the use of a bipartite graph that follows the properties of petri-nets, while the integration representation is constructed based on a properly extended transshipment model that accounts for both direct and indirect integration of involved processes.

As competitive paths branch along the value chains, critical questions raise about which process portfolios increase the biorefinery efficiencies. The candidate processes may get integrated in various ways changing energy efficiencies each time. The integration problem requires for energy complementary process combinations that optimally utilize generated steam and minimize energy cost. A Total Site heat Cascade is introduced describing heat exchange options among hot-cold streams serving with additional options for process-to-process integration. Heat contribution to the proposed cascade is adjusted by process selections made by the process synthesis representation. Once the problem applies for lignocellulosic biorefineries, additional challenges emerge related with seasonal availability of biomass varieties. Since biomass varieties feature with different intermediates (sugars and lignin) compositions, process and energy efficiencies differentiate at the use of multiple feedstocks over the year. The proposed process synthesis and integration model is used to make selections about processes and products portfolios and plan multiple-feedstock biorefinery operations so as to minimize annual energy cost of the under-construction biorefinery.

The proposed methodology has been applied in contexts of a real-life biorefinery case that involves 21 candidate processes and 6 biomass varieties (miscanthus, wheat, rise, barley, poplar, birch). The model has been used (i) to highlight energy promising biorefinery solutions including the production of xylitol, butanol and poly-urethanes by using miscanthus and wheat mixtures and resulting up to 24% steam cost savings from process-to-process integration and (ii) to examine preferences on biomass varieties that improve biorefinery efficiencies.