IDR Improved HTL Bio-oil Separation by Gas Exsolvation (April 8, 2014) Prepared by: AJ Schmidt HTL Background and Importance of Bio-oil Separation Hydrothermal liquefaction is a conceptually simple process in which bio-oil is generated by heating biomass slurry to temperatures in the range of 300 to360 degrees C while the pressure is maintained above the vapor pressure of water (2000 to 3000 psig) to facilitate a condensed phase reaction medium. Compressed hot water has enhanced solvent properties that facilitate the formation of liquid oil products from biomass. HTL reactions involve fragmentation and condensation coupled with dehydration, decarbonylation, and decarboxylation. The major products are bio-oil, water with dissolved organics, gas (predominantly CO2), and solids (minerals and unconverted biomass). Challenges to the commercialization of HTL exist in the areas of pumping biomass slurries to high pressures, process heat integration, efficient separation of the bio-oil from the aqueous phase, and undesirable physico-chemical properties of the bio-oils such as high viscosity. To allow pumping to pressures of 3000 psig, the solids concentration of biomass slurries to the HTL process typically ranges from about 10 to 30 wt% (with the balance being water). Mass yields to bio-oil range from about 25 to 40 wt% (dry ash-free biomass basis). As an example, a 20 wt% slurry with a mass yield of 35% will produce an HTL product stream that is 7 wt% bio-oil with the balance consisting of an aqueous phase. For an economical HTL process, efficient separation of the bio-oil from the aqueous phase is critical. Current HTL Bio-oil/Water Separations Over the past 5 years, continuous bench-scale HTL testing has been conducted at PNNL with biomass slurries from agricultural residues (e.g. corn stover), forest residuals (e.g. pine), industrial/municipal sludges, and aquatic biomass sources (e.g. algae, kelp). While the bench-scale testing has been performed using continuous flow, product collection and bio-oil/water separations have been done batch-wise. For the test system, downstream of the HTL reactor, solids are removed; the product is collected in one of two alternating separators/collection vessels. The collection vessels facilitate degassing of the product, liquid/gas separation, and product collection at system pressure. When one collection vessel is full, it is valved offline and the product is diverted to the second collection vessel. The product in the valved-out collector is depressurized from 3000 psig to ambient pressure, and then removed. The removed product is settled under quiescent conditions, and the less polar bio-oil coalesces and separates from the aqueous phase. The bio-oil can then be recovered by decanting the aqueous phase (higher density bio-oil). When the bio-oil has a lower density then the aqueous phase, a separatory funnel is used. The gravity separation is a relatively slow process (10 to 60 minutes), and not efficient if oil emulsions form. While generally effective in a research setting, batch-wise gravity separation will present scale up challenges to pilot and commercial HTL applications. Invention Figure 1 provides an overview of the HTL configuration and shows the product collection vessels, which are bypassed in the subject invention. A more detailed sketch and an image of the invention in operation are shown in Figure 2. A continuous HTL bio-oil water separations process has been conceived and demonstrated in a proof of principle test with an algae feedstock (tetraselmis). For the invention, a biomass slurry feed is processed in an HTL reactor. The HTL product slurry is filtered (to remove solids/particulate) at HTL temperature and pressure (300 to 350 degrees C, at 2000 to 3000 psig). Next, the product is cooled to between 20 - 110 degrees C via heat exchange (40 degrees C in the test performed) and then flashed to atmospheric pressure. During the flashing, dissolved CO2 (which is more soluble in the bio-oil vs. the aqueous phase) exsolves and a bio-oil froth or foam is formed on top of the aqueous phase (see example, Figure 3). Nearly all of the produced bio-oil is present in the froth. With the proper use of float traps and expansion vessels, and effective separation of bio-oil is achieved. Figure 1. Continuous HTL System Configuration. Red dashed line show flow routing for invention (bypass of Jacketed Liquid Collections and Rapid Pressure letdown. Figure 2 Detailed schematic on rapid pressure letdown and bio-oil/water separation Figure 3Example of bio-oil foam formation during convention collection of product. The use of Bio-oil Separation via Gas Exsolvation Enhancement has advantages over other HTL bio-oil separations approach including the following: Increases HTL scalability: This invention eliminates labor intensive batch-wise, poorly scalable separation process. Eliminates batch-wise product collection and the need for high pressure nitrogen. In the current product collection approach, after emptying the product accumulator, it must be backfilled with nitrogen gas at 3000 psig to allow smooth transition when it is brought back on line (else, system pressure will drop and boiling will occur throughout the HTL reactor). Improves process safety. The liquid collection vessels represent the largest pressure vessels in the HTL system. Elimination of the need for these vessels significantly reduces the system volume at pressure. Increase quantity of oil recovered (speculative at this time) This invention takes advantage of the much higher solubility of CO2 in bio-oil vs. the aqueous phase. In a separation much like dissolved air flotation; the aqueous phase is essentially scrubbed of bio-oil constituents, which in turn are separated in a quasi-stable bio-oil foam. The foam is stable for a short time (15 sec to 2 minutes) and can be readily collapsed to a liquid. Keys Parameters for Implementation Effective/Efficient Solids Removal Before Flashing. Fine particulate will erode the pressure let down system (e.g., back pressure regulator), will promote emulsion formation, and will stabilize the bio-oil foam. Appropriate Geometry for Foam Routing and Foam Breaking. During flashing, the volume of bio-oil will expand by a factor of 10 to 50 (foaming). A route for this foam to separate from the liquid aqueous phase followed by a means to collapse the foam (e.g., condenser, demister pad) is essential for implementation of this approach (e.g. Figure 2). Potential enhancement: Flashing of product stream above 100C to enhance oil/organic recovery via steam stripping mechanism. Recycle of the gas stream (that exits the second float trap in Figure 2) back through the aqueous phase in the collector to promote additional oil separation/recovery. Attachments: Video 1: Video of separate bio-oil and aqueous collection (method demonstration). Video 2: Video of bio-oil foam during convention collection (while draining of bio-oil colllecter (Figure 1) at moderate pressure.