(268c) From Molecules to Modular Processes: Hydrothermal Liquefaction as a Case Study of Innovations in Renewable Fuel Production

Without substantial government subsidies, the economic viability of large-scale thermochemical conversions of biomass to sustainable transportation fuels is tenuous at best. A profitable biorefinery will require new approaches to modular unit operations and the valorization of all byproducts. The design of such processes requires us to work against the the fundamental truths of chemical engineering: namely that systems will seek out their highest entropic state, reactions and separations are disparate unit operations, and water and oil don’t mix. Together we will explore how today’s advances in fundamental science will accelerate tomorrow’s innovations.

For example, one of the advantages of Hydrothermal Liquefaction (HTL) over other thermochemical conversion techniques is the direct treatment of wet biomass without an energy-intensive pre-drying step. During HTL, the water’s dielectric constant decreases so drastically that it acts as both a reactant and an organic solvent. However, HTL suffers from low yields of a water-insoluble biocrude phase having a high oxygen content and low heating value, as well as an aqueous phase enriched in organics. Both the biocrude and aqueous phase (which are often difficult to separate) require expensive, time consuming, material- and energy-intensive post-processing. While the literature is replete with studies on the impacts of process severity (temperature, pressure, heating rate, time), pH, (co)solvents, catalysts and feedstocks on HTL product yield and selectivity, the approach to HTL research writ large has been to understand the impact of process conditions on product distributions and then “back out” reaction pathways. This understanding forms the basis for catalyst selection for in situ or downstream upgrading of biocrude. Overcoming barriers to widespread implementation of HTL for wet waste to biofuel conversions requires a radical new approach to process design. In our laboratory, we have adopted a new view of HTL as the creation of a supersaturated solution, enabled by the “un-ordering” of water’s hydrogen-bonding network. Through the design and construction of a new apparatus to measure the dielectric constant during HTL, we propose to tune the selectivity of the products and make separation and product recovery more efficient.

A complementary but often disparate literature explores management of the organic-laden process water (PW). Given the high amount of biomass-based carbon that partitions into the HTL process water (PW) we must find ways to capture and valorize the carbon to enable the commercialization of HTL as a waste management strategy. Capturing and utilizing the carbon present in the PW could represent the an impactful way to reduce the cost of biofuel production via HTL. Current approaches usually focus on anaerobic digestion; the relatively low concentration of carbon in the water requires a large reactor volume and the PW’s antimicrobial activity may limit the amount of PW that can be introduced to the microbial community at any one time. The carbon could be valorized through aqueous phase processing (APP), the subject of considerable effort in the literature of late. Others suggest liquid-liquid extraction (LLE) to recover organics from the aqueous phase. However, LLE is limited because of the wide range of polarities and solubilities of the aqueous phase compounds, many of which are water soluble. Organic recoveries are low even when using copious amounts of solvent. New developments such as separation of the aqueous and hydrochar/biocrude phase by centrifugation prior to LLE or solvent extraction via supercritical CO₂ may enhance product yields.

As a society, we find ourselves at the precipice of climactic ruin. To escape this fate, we must be willing to push the boundaries of conventionally accepted process wisdom to design new approaches to renewable fuel production. In this talk, we think beyond the current literature norms of “make and characterize a fuel” to envision integrated processes for holistic carbonaceous waste valorization that teeter on the edge of thermodynamic equilibrium.