CARBON DIOXIDE ENHANCED HYDROTHERMAL LIQUEFACTION

Abstract

A process where a liquid or supercritical CO2 co-solvent is used in a hydrothermal liquefaction (HTL) process. The process improves yield of the HTL process.

Claims

1. A method of producing liquid biocrude comprising: providing an aqueous biomass slurry comprising biomass and water; adding liquid CO.sub.2 or supercritical CO.sub.2 to the slurry to form a slurry with CO.sub.2 cosolvent; heating and pressurizing the slurry; reacting the slurry in a HTL process in the presence of the CO.sub.2 cosolvent under conditions where the CO.sub.2 cosolvent is in the form of a liquid or is supercritical; forming a product mixture from the step of reacting; cooling the product mixture to form a cooled product mixture; subjecting the cooled product mixture to a separation process; and recovering a liquid product from the separation process.

2. The method of claim 1 wherein the biomass comprises sewage sludge, food waste, algal biomass, wet agricultural residues, or a combination of these materials.

3. The method of claim 1 wherein the method comprises a preheating step followed by a passing the preheated solution into a reactor where the step of reacting occurs.

4. The method of claim 3 wherein the liquid or supercritical CO.sub.2 is added to the slurry after the preheating step and before the reacting step.

5. The method of claim 4 wherein the pressure in the reactor is in the range of 5 to 25 MPa.

6. The method of claim 1 wherein separation process comprises a step of removing products in a stream of liquid CO.sub.2 or supercritical CO.sub.2; and wherein the stream of liquid CO.sub.2 or supercritical CO.sub.2 comes from the HTL process.

7. The method of claim 1 wherein the pressure in the reactor is at least 10 MPa, or in the range of 10 to 100 MPa, or 10 to 50 MPa, or 12 to 30 MPa, or 13 to 20 MPa, or 10 to 15 MPa.

8. The method of claim 1 where the step of reacting is conducted in the range of 300 to 370? C. or 350 to 370? C. or 300 to 330? C.

9. The method of claim 1 wherein the aqueous biomass slurry comprises at least 5 wt % biomass, or in the range of 5 to 50 wt % biomass, or 10 to 30 wt % biomass, or 15 to 25 wt % biomass.

10. The method of claim 1 wherein the mass of added liquid CO.sub.2 is at least 1.1 times to 4 times the mass of CO.sub.2 that is generated in the method.

11. The method of claim 1 wherein the separation process comprises a step of CO.sub.2 gas removal wherein at least 1% (or at least 3%, or at least 5%) of carbon in the added CO.sub.2 is present in the liquid product after the step of CO.sub.2 gas removal.

12. The method of claim 1 wherein the preheating is conducted in a vessel that is separate from a vessel where the slurry is reacted in the HTL process, and wherein the preheating is conducted to a temperature of at least 150? C. or at least 200? C.

13. The method of claim 1 wherein liquid CO.sub.2 is dispersed in the aqueous slurry.

14. The method of claim 1 wherein the mass ratio of the added liquid CO.sub.2 or supercritical CO.sub.2 to the slurry is at least 0.01 or at least 0.03 or at least 0.05 and is 0.5 or less or 0.3 or less, or 0.1 or less.

15. The method of claim 1 wherein the biocrude yield is at least 30%; or wherein the solids yield is 10% or less.

16. The method of claim 1 wherein, as compared to a process that does not add CO.sub.2 but is otherwise identical, the biocrude yield increases by at least 5% or at least 10% (for example, instead of 30 wt % yield, at least 33 wt % biocrude yield), or in the range of 5 to 20 to 5 to 15% increase. Likewise, the method can be characterized by any of these increases relative to an identical method conducted in the presence of an atmosphere of CO.sub.2 but no liquid or supercritical CO.sub.2.

17. The method of claim 1 wherein, as compared to a process that does not add CO.sub.2 but is otherwise identical, the solids yield decreases by at least 5% or at least 10%, or in the range of 5 to 30 to 5 to 20% decrease. Likewise, the method can be characterized by any of these decreases relative to an identical method conducted in the presence of an atmosphere of CO.sub.2 but no liquid or supercritical CO.sub.2.

18. The method of claim 3 wherein the aqueous slurry is mechanically stirred in a preheater section prior to passage into a HTL reactor.

19. A multi-phasic composition, comprising: a first phase comprising a mixture of biomass and liquid water (an aqueous biomass slurry); and a second phase comprising liquid CO.sub.2 or supercritical CO.sub.2; wherein the phases are in contact.

20-21. (canceled)

22. An HTL system, comprising: a subsystem comprising: a vessel comprising an aqueous biomass slurry; a pump adapted to transfer the slurry into a preheater; and a reactor connected to the preheater wherein the reactor is adapted to conduct a hydrothermal reaction; a container of liquid CO.sub.2 and a conduit or conduits adapted to carry liquid CO.sub.2 into the subsystem.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1 is a flow diagram of baseline HTL.

[0043] FIG. 2 is a flow diagram of carbon dioxide-enhanced HTL.

[0044] FIG. 3 is a pie chart showing normalized mass yields without CO.sub.2 (SS-1 experiment).

[0045] FIG. 4 is a pie chart showing normalized mass yields with CO.sub.2 (SS-2 experiment).

[0046] FIG. 5 is a pie chart showing normalized carbon yields without CO.sub.2 (SS-1 experiment).

[0047] FIG. 6 is a pie chart showing normalized carbon yields with CO.sub.2 (SS-2 experiment).

DETAILED DESCRIPTION OF THE INVENTION

[0048] The following description provides a specific description of a continuous HTL process; the invention encompasses broader ranges of conditions. An aqueous slurry of biomass is pumped at 2800 psi through a preheater and then into a reactor at nominally 375? C., then through a pressure-let down unit where it is depressurized and separated into gas and liquid components. The gas comprises predominantly carbon dioxide, with some light hydrocarbons. The liquid comprises a separable biocrude and aqueous phase, of which the biocrude is retained and the aqueous is a side stream. The solid byproduct is separated mid-process prior to the depressurization unit, or it is filtered from the liquid product after depressurization. The resultant biocrude is a mixture of organic compounds containing predominantly carbon and hydrogen, with some oxygen. This biocrude is nominally an energy carrier, that can be catalytically upgraded to remove the oxygen through hydrogenation and form a side-product aqueous phase and a hydrocarbon that can be distilled into fractions similar to various transportation fuels.

[0049] FIG. 1 shows a conceptual flowsheet of HTL and in FIG. 2, a loop is added as an example of CO.sub.2 enhanced HTL. The aqueous slurry can comprise a range of biomass materials, including sewage sludge, food waste, algal biomass, agricultural residues, forest residues, or similar materials or combination of materials. The process is thought to be solvolytic, and the water plays the part of solvent, reaction media, and viscosity reducer. The water content is nominally held to as low a mass percentage as results in a pumpable slurry to and through the process.

[0050] In FIG. 1, the slurry is pumped to nominally 2800 psi to a heater. The heater may comprise various approaches to adding heat to the slurry including direct heating as well as recovery of heat from other parts of the process. The slurry is fed to the reactor at less than 374? C. at which point it is maintained at a temperature in the nominal range of 350? C. to 370? C. to allow the solvolytic reaction to occur. In some forms, the heater and reactor are the same tubing system, and the heater is merely considered the adiabatic section. The pressurized, heated, and reacted slurry exits the reactor, is cooled and depressurized, and the gas and liquids are separated. The gas can be released, while the liquids are subjected to further separation into aqueous and biocrude fractions. A byproduct solid is collected either from in process filtration following the reactor unit operation, or during filtration of the liquid products.

[0051] In the baseline, CO.sub.2 is not available in meaningful amounts to participate in the chemistry occurring during the heat-up and the bulk of the reaction zone. As the baseline process generates a small amount of CO.sub.2 in the reaction of biomass, a small amount of it is present in the reactor section, particularly towards the end of the reaction. In this invention, liquid and/or supercritical CO.sub.2 is injected to participate as a co-solvent in the heater zone and/or throughout the reaction zone and/or prior to separations.

[0052] For this invention, the liquid CO.sub.2 can be fed concurrently with the biomass prior to heating unit operations. Nominally, it would be fed to the system via a different high-pressure pump than is used to pump the biomass and the pressurized biomass and pressurized CO.sub.2 would be fed into a single stream prior to entering the preheater at the beginning of the HTL process, which could be nominally 2800 psi and ambient temperature. In this case, the liquid CO.sub.2 would mix with the biomass slurry and have the initial solvent effects of increasing biomass homogeneity as well as diluting the slurry, which would reduce its viscosity and make pumping the material easier in the parts of the HTL process where the biomass has a high viscosity. As the mixture increases in temperature through the heating section, the CO.sub.2 transitions to supercritical, which further increases biomass homogeneity and reduces its density and yields different solvation effects. This viscosity modification of the feedstock by the co-solvent would also improve heat transfer, tend to solubilize compounds that would resist liquefaction, dilute and/or dissolve the reactive biomass that may result in plugging and solids formation during the heat up, and the additional solvent will reduce the tendency for inhomogeneity of the biomass. The heating phase of biomass from room temperature up to reaction temperature in HTL has been notorious for high viscosities, the potential for plugging and segregation, significant changes in rheology, and difficulty in transferring heat. The co-solvation by CO.sub.2 reduces barriers in all of these areas but without the negative implication in cost and solvent recovery that organic solvent HTL must deal with.

[0053] In the reactor, the added CO.sub.2 enables higher biocrude yield, and lower yield to byproduct solids. The dilutive impact of the solvent likely plays a similar role as it did in the heater section. Additionally, the acidic strength of CO.sub.2 will also participate in the reaction of biomass as it converts it into the biocrude product.

[0054] Finally, in the separations train, the CO.sub.2 will continue to benefit the process with its dilutive impact. If optional pressurized separation is practiced, reduced viscosity and density imparted by the dissolved CO.sub.2 in the biocrude will make filtration and settling of byproduct solids easier. This is practiced in situ during HTL, and will be realized when adding CO.sub.2 as a cosolvent to HTL without additional modification to in situ filtration of HTL solids. This same impact will result in easier liquid/liquid separations of the biocrude and aqueous phases, as the dissolved CO.sub.2 will create a larger density disparity between the two phases, resulting in faster and cleaner separation. However, this benefit will only be realized if traditional HTL configurations are modified to allow for pressurized liquid/liquid separation prior to reducing the pressure where CO.sub.2 is flashed.

[0055] If pressurized liquid/liquid separations is not practiced, CO.sub.2 would be flashed from the process stream at the point of depressurization and liquid/liquid separations would be performed after CO.sub.2 is flashed. The various streams (with the exception of the gas stream) would be relatively free of residual CO.sub.2 solvent thereby reducing biocrude and organic loss to the aqueous solvent, particularly when CO.sub.2 reduces the need for excess aqueous in the biomass feed.

[0056] As the HTL process generates some CO.sub.2 from the biomass, the solvent is self-replenishing and the HTL process is already designed with gas/liquid separation capability and does not require extensive modification besides a system for compressing byproduct gas to be used as liquid (or supercritical) solvent. This fact allows the process to be run at very high concentrations of CO.sub.2 co-solvent if the chemistry is beneficial, as negative impacts to process economics are very low compared to HTL with organic liquid cosolvents.

EXAMPLES

[0057] This invention was demonstrated in two continuous HTL experiments that were performed on subsequent days without and with carbon dioxide injection while maintaining similar reaction conditions and using the same batch of biomass feed. The experiment used an injection rate of 0.04:1 carbon dioxide to biomass volume ratio.

[0058] The feed comprised sewage sludge from a wastewater treatment plant at 0.75:1 ratio of primary to secondary sewage sludge. The feed was formatted by grinding the biomass in a ball mill with additional water to reduce particle size and diluting it to a pumpable slurry. This resulted in diluting it to ?17% weight percent solids and frozen until the experiments. Experiments were conducted on subsequent days in a continuous HTL system with identical configuration where the system was assembled, the baseline test was performed, the system was idled overnight, and then the test with carbon dioxide injection was performed while processing with the same operating conditions of the baseline. In, the second test (SS-2), liquid CO.sub.2 was injected with the feed prior to the heater similar to the leftmost injection point in FIG. 2. The conditions and results are captured in Table 1.

TABLE-US-00001 TABLE 1 Conditions tested for baseline HTL (SS-1) and CO.sub.2 Enhanced HTL (SS-2) SS-1 SS-2 (with Test (baseline) CO.sub.2) Biomass Feed Rate mL/h 4000 4000 Carbon dioxide liquid feed rate mL/h @ ~2900 psig, ~20? C. 0 140 Reactor Temperature ? C. 322 327 Pressure psig 2935 2943 TOS (continuous) hour 1.33 1.00 Total Solids in Feed wt % 18.9%.sup. 19.1%.sup. Ash in Dry Feed wt % 30.7%.sup. 30.3%.sup. Ash in Slurry Feed wt % 5.8% 5.8% AF Solids in Slurry Feed wt % 13.1%.sup. 13.3%.sup. Average Feed density g/ml @20? C. 1.04 1.04 Mass Yields (Dry, Ash Free, Normalized) Mass Balance (normalized) % 103% 105% Biocrude Yield, Mass g.sub.oil/g.sub.fd 34% 39% Biocrude Yield, Carbon % 48% 51% Aq Yield, Mass g.sub.aqu/g.sub.fd 37% 33% Aq Yield, Carbon % 27% 25% Solid Yield, Mass g.sub.solid/g.sub.fd 11% 8% Gas Yield, Mass g.sub.gas/g.sub.fd 18% 19%

[0059] In the SS-1 experiment, 4000 ml/h of biomass was fed to the HTL reactor in a baseline configuration and samples and measurements were taken normally. In the SS-2 experiment, the process was performed nearly identically except that liquid CO.sub.2 was injected at 140 ml/h and 2943 psi into the line with the biomass prior to the heater. The CO.sub.2 was obtained from a cylinder with a liquid dip-tube. The experiment simulated full first-pass recycle of CO.sub.2 gas back to the process as liquid CO.sub.2. The pressurized liquid CO.sub.2 injected into SS-2 represented approximately 65 L/h of CO.sub.2 gas, which is nominally similar to the 55 L/h of gas produced during SS-1, which was comprised 90.6% CO.sub.2 and the balance light hydrocarbons. Note that a purge of offgas shown in FIG. 2 could be required because both the liquid CO.sub.2 and the CO.sub.2 produced by reaction would be present at the completion of the reaction. This allows for much higher concentrations of liquid CO.sub.2 to be used as an HTL co-solvent as the process operates at a surplus of CO.sub.2.

[0060] This test demonstrated that applying the invention of HTL with co-solvent CO.sub.2 increased the biocrude yield and decreased the byproduct solids yield. Baseline results are shown in FIG. 3, which can be directly compared with the yields from a repeat of that test with CO.sub.2 injection shown in FIG. 4. Thus, CO.sub.2 injection results in increasing the mass yield of product biocrude from 33% to 39%, and decreased the solids yield from 11% to 8%. To refine this further and eliminate the potential impact of diluents in the product and byproduct streams, the carbon yields are captured below to show where the biomass carbon is partitioning. The baseline HTL is shown in FIG. 5, which can be compared directly to the test with injection in FIG. 6. The comparison of carbon yield confirms a positive increase in partitioning of the carbon to the product biocrude phase, and away from the solid phase. The aqueous and gas yields do not demonstrate a meaningful difference, particularly due to excess CO.sub.2 in the gas byproduct due to the injection.