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Solvent Liquefaction

Solvent liquefaction converts biomass or other carbonaceous feedstocks in the presence of a solvent and elevated temperatures and pressures into primarily liquid or solubilized compounds, gas, and solids.1 In some manifestations, solvent liquefaction can process wet feedstock, thus omitting the energy-intensive drying of biomass prior to thermochemical reaction. Polar solvents are capable of selectively catalyzing the depolymerization and hydrolysis reactions to produce cellulosic sugars from biomass.2 Liquefaction is conducted at relatively modest temperatures (105-350°C) compared to pyrolysis. Product distribution can be tuned by selecting different solvent media and reaction conditions, which provides high flexibility to the process.3,4

Iowa State University conducts solvent liquefaction at several scales. These include microreactors (milliliter samples); autoclave reactors (500 mL samples); and continuous reactors (1 kg/h).

Microreactors fabricated from Swagelok fittings are rapidly heated by immersing them in a sand bath heated to reaction temperature.5 Although widely used by the research community, it is difficult to measure and control pressure in these batch reactors.  It is difficult to vigorously agitate the contents of these reactors during tests, which sometimes results in poor solubilization and mass transfer limitations in reaction rates.

[DIAMGRAM] Autoclave at Iowa State University
Figure 1. Batch in/batch out autoclave reactor system developed at Iowa State University

Stirred autoclave reactors are also commonly employed to study solvent liquefaction as batch processes. They can overcome the mass transfer limitations of microreactors and are more amenable to measuring and controlling pressure of the process, as described in one of our papers.  However, the large mass of steel associated with these pressure vessels results in very long heating and cooling times, which complicates the interpretation of kinetic data.  Experiments in autoclave reactors do not well simulate what occurs in continuous processes.

We have developed a “batch in/batch out” autoclave reactor system that overcomes many of the limitations of conventional autoclave reactors.  As shown in Fig. 1, a slurry of biomass and desired solvent for the process is sealed and over pressurized (relative to desired operating pressure) in a small autoclave vessel known as the slurry feeder where it is continuously stirred. Upon the start of an experiment, the slurry is rapidly discharged to a larger under-pressurized autoclave located below the slurry feeder that serves as the process reactor. This second autoclave is preheated to a temperature high enough to rapidly bring the slurry to the desired reaction temperature while the initial pressures of the two autoclaves are set to achieve the desired operating pressure upon slurry discharge to the reactor. Volatiles and gas released during reaction are discharged to an overheads condenser with non-condensable gases exiting through a pressure regulator set to the desired operating pressure. After desired residence time of the reaction mixture, the slurry is exhausted under pressure through a bottoms quench, which rapidly cools the products.

[CHART] Slurry times
Figure 2. Slurry heating and cooling times for the batch in/batch out autoclave reactor system

Figure 2 illustrates the rapid heating and cooling of the slurry that can be achieved with this new autoclave system. Three tests were conducted: zero minutes residence time (outlet from reactor was open at the start of the test); 10 minutes and 20 minutes. The desired reaction temperature of 200⁰C was achieved in less than 3 minutes compared to as long as 20 minutes in a conventional autoclave. Products discharged from the reactor were cooled to ambient in less than 10 minutes vs up to an hour in a conventional autoclave. This system has several advantages over a conventional autoclave system including (1) control of pressure; (2) continuous removal of volatile products; (3) rapid turn around of experiments; and (4) close simulation of a continuous solvent liquefaction system.

We have developed a 1 kg hr-1 continuous solvent liquefaction process development unit (PDU) to evaluate the thermal deconstruction of biomass in a hydrocarbon solvent.6 The PDU (Figure 3) was also designed to evaluate several unit operations critical to large-scale operations. Online solids removal was conducted with inline wire mesh barrier filters with separation efficiency of over 99%. Acetone injection was used to aid in solids removal, and an online recovery system was demonstrated with greater than 97% acetone recovery. Continuous online bio-oil fractionation was also demonstrated using a distillation column to separate approximately 93 wt% of the initial solvent from the biomass-derived products.

[ILLUSTRATION] Annotated solvent liquefaction system
Figure 3. Continuous solvent liquefaction process development unit at Iowa State University.
  1. Ghosh A, Haverly MR. Solvent Liquefaction. In: Brown RC, ed. Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power. 2nd ed. John Wiley and Sons; 2019:257-306.
  2. Mellmer MA, Martin Alonso D, Luterbacher JS, Gallo JMR, Dumesic JA. Effects of γ-valerolactone in hydrolysis of lignocellulosic biomass to monosaccharides. Green Chem. 2014;16(11):4659-4662. doi:10.1039/C4GC01768D
  3. Ghosh A, Bai X, Brown RC. Solubilized Carbohydrate Production by Acid-Catalyzed Depolymerization of Cellulose in Polar Aprotic Solvents. ChemistrySelect. 2018;3(17):4777-4785. doi:10.1002/slct.201800764
  4. Ghosh A, Brown RC, Bai X. Production of solubilized carbohydrate from cellulose using non-catalytic, supercritical depolymerization in polar aprotic solvents. Green Chem. 2016;18(4):1023-1031. doi:10.1039/C5GC02071A
  5. Ghosh A, Brown RC. Factors Influencing Cellulosic Sugar Production during Acid-Catalyzed Solvent Liquefaction in 1,4-Dioxane. ACS Sustain Chem Eng. 2019;7(21):18076-18084. doi:10.1021/acssuschemeng.9b05108
  6. Haverly MR, Schulz TC, Whitmer LE, et al. Continuous solvent liquefaction of biomass in a hydrocarbon solvent. Fuel. 2018;211(September 2017):291-300. doi:10.1016/j.fuel.2017.09.072