Field: The invention relates to methods and systems for thermochemical processing of lignocellulosic biomass, in general, and to hydrothermal liquefaction (HTL) in particular.

Hydrothermal Liquefaction (HTL) processes provide efficient conversion of biomass including high value feedstocks such as wood and straw as well as low or negative value feedstocks such as sewage sludge and manure. HTL systems can be configured in many different ways but generally produce a primary product such as bio crude oil, a combustible gas side stream and a significant amount of conversion product recovered as an aqueous phase with soluble organic constituents including organic acids, alcohols, aldehydes, phenols, furfural derivatives and nitrogen heterocyclic components.

In some continuous process configurations, part of the aqueous product is perpetually recycled, along with some portion of the product oil stream, whereby aqueous organic solutes are eventually converted to oil, gas and char. See e.g. Jensen 2017. Other HTL processes where product oil is not recycled produce an aqueous wastewater fraction which is typically a portion of the aqueous product stream that is removed while the remainder is recycled. In still other HTL processes, the feed stream is subject to only one pass through a thermochemical reactor, with no recycling of aqueous product. In all cases, HTL aqueous product (i.e., wastewater) requires further processing because it is enriched in organic pollutants which act as inhibitors of various metabolic processes in nature. For review, see Leng 2018.

HTL wastewater is typically very high in chemical oxygen demand (COD). These high COD levels render classical aerobic biological wastewater treatment approaches to be prohibitively expensive. In a recent technoeconomic analysis the cost of classical wastewater treatment units for HTL wastewater were 2-3 times more costly than the HTL process plant itself. See Van Dyk et al, 2019. A variety of biological approaches to treatment and “valorization” of HTL wastewater streams have been reported. For review see Gu 2019. Anaerobic digestion to biomethane has been promising in theory but difficult to achieve in practice due to the inhibitory effect of HTL wastewater solutes. See e.g. Si 2018. Electrolytic conversion of HTL wastewater has been proposed, although this consumes energy without contributing any value to the HTL process. See WO2016169927. Dedicated thermochemical processing has also been proposed, with an aim of valorization, particularly including hydrothermal gasification of HTL wastewater. See Xu 2019, and see CN1066938. However, technoeconomic studies indicate that this approach will also prove prohibitively costly. See e.g. Zhu 2019.

Because of its high COD level, HTL wastewater can be efficiently treated using a thermochemical wet oxidation process where compressed air or oxygen is added to an aqueous feedstock under high pressure and temperature with or without added catalyst. Wet oxidation is widely used to treat problematic wastewater streams rich in bio-resistant organic pollutants. Residual COD remaining in the process stream after wet oxidation is typically readily digestible by ordinary anaerobic digestion. For reviews see Bhargava 2006 and see He 2007. The wet oxidation process is highly exothermic and the resulting process heat can be “harvested” to provide some of the energy required for the underlying endothermic HTL process.

In the case of hydrothermal carbonization (HTC), wet oxidation of the aqueous product stream as a means of simultaneously providing wastewater treatment and process heat is an integral component of the commercially available OxyPower HTC™ process produced by C- Green Technology AB, Solna, Sweden. See Hareskog 2018. In an HTC process, the biomass feed stream is subject to thermochemical treatment at low temperatures, compared with HTL, typically within the range 180 to 250° C. Whereas the aim in HTL is to produce liquid oil product, HTC aims to produce solid char. Residence times in HTC are much longer than with HTL, typically on the scale of hours compared with minutes. In the OxyPower HTC™ process, aqueous filtrate having low particulate content is subject to wet oxidation and the resulting flash steam is used to pre-heat the feed stream to a process temperature of 200° C.

A similar system for combined wastewater treatment and production of process heat for an HTL process would be highly advantageous. However, the simple system applied in the OxyPower HTC™ process is impractical for HTL, which is conducted at much higher temperatures, typically within the range 300 to 425° C. In a continuous HTL process, heat exchangers between the feed stream and the product stream can typically bring the feed stream to within 50 to 60° C of the process temperature. But flash steam from wet oxidation of the aqueous product stream can never provide the required final “top heating” to process temperature.

Summary

We provide here systems and methods for cost efficient integration of hydrothermal liquefaction and wet oxidation treatment of the resulting wastewater. By separating, such as filtering, the aqueous HTL product mixture at process temperature, before cooling down in the HTL heat exchanger, under pressure, problematic salts can be removed which precipitate in subcritical and supercritical water at process temperatures within the range 300 to 425° C. The de-salted aqueous product stream can, after separation of the oil and gas phases, be subject to continuous wet oxidation at temperature slightly above process temperature in a rapid process that can provide heat for the required top heating of the HTL feed stream within 10 minutes or less. Both HTL conversion of the feed stream and also wet oxidation of the aqueous product stream can be conducted in a simple continuous process in tubular reactors arranged co- or counter-current in direct thermal contact via a heat exchange system. Separate HTL and aqueous phase heat exchangers provide efficient pre-heating of both the feed stream and the aqueous product stream.

The invention relates in a first aspect to a hydrothermal liquefaction (HTL) system comprising: an inlet unit for aqueous slurries of biomass, a high pressure pumping system providing an outlet pressure of at least 100 bar in communication with the inlet unit, a continuous HTL reactor adapted to process biomass feedstocks at temperature within the range 300 to 425° C so as to produce an HTL product stream, an HTL heat exchanger, adapted to transfer heat from the HTL product stream to the HTL feed stream, a solid-liquid separation system adapted to operate under pressure at temperature within the range 200 to 425° C so as to separate solids from the HTL product stream, a separator adapted to recover a separate aqueous phase from the solid-liquid separated HTL product stream, a continuous wet oxidation reactor adapted to process at temperature within the range 300 to 425° C the aqueous phase obtained from the solid-liquid separated HTL product stream, and a wet oxidation heat exchanger adapted to transfer heat from the wet oxidation product stream to the wet oxidation feed stream, wherein the HTL reactor and the wet oxidation reactor are integrated by an arrangement co- or counter-current in direct thermal contact via a heat exchange system.

It is noted that the output of the solid-liquid separation system is a solid-liquid separated HTL product stream. An advantage of the present invention may be that an effective separation of solids from the HTL product stream, e.g. including precipitated salts, may be obtained, e.g. without inhibiting the operation of the wet oxidation reactor. Utilization of certain specific characteristics of water within the temperature range of 300 to 425 degrees Celsius in the HTL reactor, such as low viscosity and/or low density, allows an effective separation e.g. by filtration or other means.

In an embodiment of the invention, the high pressure pumping system is arranged to provide an outlet pressure of at least 100 bar and at most 400 bar, such as at least 100 bar and at most 350 bar.

In an advantageous embodiment of the invention, the HTL reactor and the wet oxidation reactor are each tubular reactors arranged within a heat exchange system by being surrounded by a series of heat transfer clamps made from a solid matrix of heat conducting material where each individual heat transfer clamp comprises support formations that position and support the tubular reactor.

In an advantageous embodiment of the invention, the HTL reactor and the wet oxidation reactor each comprise separate tubular sections into which the HTL and wet oxidation feed streams are divided.

In an advantageous embodiment of the invention, the series of heat transfer clamps forms a circular array.

In an advantageous embodiment of the invention, the inlet unit is in fluid communication with a stirred feed buffer tank.

In an advantageous embodiment of the invention, comprising a sparger for addition of air or oxygen rich gas to the wet oxidation feed stream. The invention relates in a second aspect to a hydrothermal liquefaction (HTL) system comprising: an inlet unit for aqueous slurries of biomass, a high pressure pumping system providing an outlet pressure of at least 100 bar in communication with the inlet unit, a continuous HTL reactor adapted to process biomass feedstocks at temperature within the range 300 to 425° C so as to produce an HTL product stream, an HTL heat exchanger, adapted to transfer heat from the HTL product stream to the HTL feed stream, a solid-liquid separation system adapted to operate under pressure at temperature within the range 200 to 425° C so as to separate solids from the HTL product stream, a separator adapted to recover a separate aqueous phase from the solid-liquid separated HTL product stream, a continuous wet oxidation reactor adapted to process at temperature within the range 300 to 425° C the aqueous phase obtained from the solid-liquid separated HTL product stream, a wet oxidation heat exchanger adapted to transfer heat from the wet oxidation product stream to the wet oxidation feed stream, and a heat transfer system whereby excess heat from the wet oxidation process can be provided to the HTL process via a fluid heat transfer medium.

It is noted that the output of the solid-liquid separation system is a solid-liquid separated HTL product stream.

In an advantageous embodiment of the invention, the heat transfer medium is high pressure steam, high pressure compressed water, hot oil or molten salt. In an advantageous embodiment of the invention, the solid- liquid separation system comprises a filtration system.

In an embodiment of the invention, the solid-liquid separation system is a filtration system.

A filtration system may e.g. be characterized by having one or more filter element(s) with a certain screen size or mesh size.

In an embodiment of the invention, the filtration system comprises a filter element having a screen size of no more than 10 micrometer, such as no more than 8 micrometer, such as no more than 5 micrometer.

In an embodiment of the invention the filtration system may have a filter rate of at least 98% removal of solids with a particles size of 5 micrometer or more.

In an embodiment of the invention the filtration system may have a filter rate of 100% removal of solids with a particles size of 18 micrometer or more.

In an advantageous embodiment of the invention, the solid-liquid separation system comprises a hydrocyclone.

In an embodiment of the invention, the solid-liquid separation system is a hydrocyclone.

In an advantageous embodiment of the invention, the solid-liquid separation system comprises a centrifuge.

In an embodiment of the invention, the solid-liquid separation system is a centrifuge. In an advantageous embodiment of the invention, the solid-liquid separation system is selected from the group consisting of a filtration system, a hydrocyclone, a centrifuge, or any combination thereof.

In an advantageous embodiment of the invention, the solids separated by the solid-liquid separation system comprise precipitated salts from the HTL product stream.

In an embodiment of the invention, the precipitated salts from the HTL product stream comprises divalent salts and trivalent salts. In an embodiment of the invention, the precipitated salts from the HTL product stream further comprises monovalent salts.

In embodiments of the invention, any embodiment of the first aspect of the invention may be combined with the second aspect of the invention, and vice versa.

The invention further relates in a third aspect to a method of hydrothermal liquefaction (HTL) comprising the steps of: providing an HTL biomass feed stream, conducting continuous HTL conversion of the HTL feed stream at temperature within the range 300 to 425° C in such manner as to produce an HTL product stream which is separated under pressure at temperature within the range 200 to 425° C so as to separate precipitates and which is subsequently separated into an aqueous product stream and an oil product stream, and conducting concurrent continuous or semi-continuous wet oxidation at temperature within the range 300 to 425° C of all or some portion of the HTL aqueous product stream, wherein heat produced by the wet oxidation process is used to heat the HTL feed stream.

In an advantageous embodiment of the invention, heat produced by the wet oxidation process is used to heat the HTL feed stream by integrating a separate HTL reactor and a wet oxidation reactor in a co- or counter-current arrangement whereby the reactors are in direct thermal contact via a heat exchange system.

In an advantageous embodiment of the invention, the wet oxidation process is conducted at a temperature higher than the temperature applied in the HTL process such that a DT (temperature difference) is maintained within the range 10 to 50° C.

In an advantageous embodiment of the invention, the wet oxidation process provides more than 60% COD removal within 10 minutes.

In an advantageous embodiment of the invention, the wet oxidation process provides more than 50% of the energy requirement for top heating the feed stream in the HTL process from the feed-in temperature to the process temperature.

As used herein the term “top heating” may refer to the extra heating to reach process temperature. It may also be referred to as “trim heating”, a term well-known for the person skilled in the art.

In an advantageous embodiment of the invention, HTL conversion is conducted with recirculation to the feed stream of some portion of product aqueous phase.

In an advantageous embodiment of the invention, the aqueous product phase subject to wet oxidation has COD levels within the range 60 to 120 g/L.

In an advantageous embodiment of the invention, the HTL product stream which is filtered under pressure at temperature within the range 200 to 425° C so as to separate precipitates and which is subsequently separated into an aqueous product stream and an oil product stream. In an advantageous embodiment of the invention, the system according to the invention or any of its embodiments is adapted to operate in accordance with the method according to the invention or any of its embodiments.

In an advantageous embodiment of the invention, the method of the invention or any of its embodiment is operated using the system of the invention or any of its embodiments.

Brief description of the drawings.

Figure 1 shows a schematic illustration of an HTL system according to an embodiment of the invention with HTL and wet oxidation reactors integrated, co- or counter-current.

Figure 2 shows a process flow scheme for an HTL system integrated with wet oxidation in a heat exchange system.

Figure 3 shows one embodiment of integration of HTL and wet oxidation reactors by arrangement co- or counter-current within a heat exchange system.

Figure 4 shows another embodiment of integration of HTL and wet oxidation reactors by arrangement co- or counter-current within a heat exchange system.

Figure 5 shows a manifold which distributes HTL and wet oxidation feed streams to an integrated arrangement in a heat exchange system.

Figure 6 shows one embodiment of an HTL system of the invention with external wet oxidation system and heat transfer loop. Detailed description of embodiments.

In order to efficiently “harvest” process heat from wet oxidation of wastewater in an HTL process, both the HTL process and the wet oxidation process can advantageously be conducted in direct thermal contact. At HTL process temperatures within the range 300 to 425° C, wet oxidation proceeds very quickly. By maintaining a slightly higher temperature in the wet oxidation process, resulting heat transfer to the HTL process can provide the heat required for top heating the HTL feed stream to process temperature within a few minutes. However, at the high temperatures used in HTL processes, the dielectric properties of water are dramatically altered. A significant consequence is that divalent and trivalent salts, and even some monovalent salts, become insoluble and precipitate. In order to conduct wet oxidation at these high temperatures, a variety of corrective measures are required to deal with precipitated salts, which would otherwise eventually clog reactors, transfer pipes, heat exchangers and other equipment. See e.g. Schubert 2010 a and Schubert 2010 b.

In integrating an HTL process with wet oxidation treatment of HTL wastewater, problems with precipitating salts in high temperature can be avoided by applying a solid liquid separation system, e.g. by a filtration system, to the HTL product stream under pressure at temperature within the range 300 to 425° C. As described in US9,404,063, which is hereby incorporated by reference in entirety, at these high temperatures, the oil and aqueous phases of the HTL product stream are effectively one liquid phase or at worst a low viscosity two- phase liquid from which solids can be readily recovered. Having already removed potentially troublesome salts by separation/filtration of the HTL product stream under reaction conditions, wet oxidation of the subsequently separated aqueous HTL product phase can proceed as a continuous reaction using the same kind of tubular reactor used for the HTL process itself. When referring to “separation/filtration”, “separated/filtrated” or similar, this intends to refer to a separation, which can be done by any applicable means, such as filtration by a filtration system, or by separation by a hydrocyclone or a centrifuge. In some embodiments, a combination of two or more of a filtration system, a hydrocyclone, a centrifuge, or any combination thereof may be applied. In some embodiments, a filtration system is the preferred solid-liquid separation system.

In some embodiments, the invention provides a hydrothermal liquefaction (HTL) system comprising: an inlet unit for aqueous slurries of biomass, a high pressure pumping system providing an outlet pressure of at least 100 bar in communication with the inlet unit, a continuous HTL reactor adapted to process biomass feedstocks at temperature within the range 300 to 425° C so as to produce an HTL product stream, an HTL heat exchanger, adapted to transfer heat from the HTL product stream to the HTL feed stream, a solid-liquid separation system adapted to operate under pressure at temperature within the range 200 to 425° C so as to separate solids from the HTL product stream, a separator adapted to recover a separate aqueous phase from the solid-liquid separated HTL product stream, a continuous wet oxidation reactor adapted to process at temperature within the range 300 to 425° C the aqueous phase obtained from the solid-liquid separated HTL product stream, and a wet oxidation heat exchanger adapted to transfer heat from the wet oxidation product stream to the wet oxidation feed stream, characterized in that the HTL reactor and the wet oxidation reactor are integrated by an arrangement co- or counter-current in direct thermal contact via a heat exchange system. In some embodiments the solid-liquid separation system is a filtration system. Alternatively, the solid-liquid separation system could e.g. be a hydrocyclone, or a centrifuge.

In some embodiments the HTL heat exchanger and the wet oxidation heat exchanger are combined into one unit. In some embodiments, the HTL heat exchanger and the wet oxidation heat exchanger are separate units. In some embodiments, the HTL reactor and the wet oxidation reactor are combined into one unit. In some embodiments, the HTL reactor and the wet oxidation reactor are separate units.

In some embodiments, the invention provides a method of hydrothermal liquefaction (HTL) comprising the steps of: providing an HTL biomass feed stream, conducting continuous HTL conversion of the HTL feed stream at temperature within the range 300 to 425° C in such manner as to produce an HTL product stream which is separated/filtered under pressure at temperature within the range 200 to 425° C so as to separate precipitates and which is subsequently separated into an aqueous product stream and an oil product stream, and conducting concurrent continuous or semi-continuous wet oxidation at temperature within the range 300 to 425° C of all or some portion of the HTL aqueous product stream, wherein heat produced by the wet oxidation process is used to heat the HTL feed stream.

Biomasses suitable for use with the systems and methods of the invention typically include aqueous slurries of sewage sludge, energy grasses, wood chips, cereal straws, corn stover, other lignocellulosic agricultural and forestry wastes, seaweeds, algae, plankton, and other feedstocks. HTL processes are typically conducted at the outer limit of pumpability of the slurry. In the case of sewage sludge, slurries as high as 15-20 wt. % dry matter (DM) content can typically be used, sometimes even as high as 35%. In the case of lignocellulosic substrates, slurries are typically within the range 14-18 wt. % DM.

It is typically advantageous that HTL feed stream slurries are stirred to maintain pumping efficiency. Thus in some embodiments, the inlet unit for an HTL system of the invention is in fluid communication with a stirred feed buffer tank.

In some embodiments, the high pressure pumping system for the HTL feed stream in an HTL system of the invention comprises high pressure pumps, which may include one or more positive displacement pumps such as piston pumps, membrane pumps, gear pumps, lobe pumps or hydraulic driven pumps, in which a pump chamber changes volume due to the admission of a hydraulic fluid. The high pressure pumping system may preferably be arranged to provide a pressure high enough to keep the water stream in liquid state, e.g. a pressure of at least 100 bar, depending on the specific process conditions applied. In some embodiments, an HTL system of the invention further comprises a high pressure pumping system for the wet oxidation feed stream.

Continuous HTL conversion is typically conducted using a system that pumps biomass slurry into a tubular reactor at process pressure, which is typically well over 100 bar. The temperature of the feed stream typically is increased during the conversion process from an initial feed-in temperature that can be achieved through use of heat exchangers that recover heat from the product stream to the steady-state process temperature. In using methods and systems of the invention, prior to its removal from the pressurized HTL reactor, the HTL product stream is separated/filtered at process temperature within the range 300 to 425° C, or in some embodiments, after slight cooling at a temperature within the range 200 to 425° C. Any suitable solid-liquid separation system may be used for this purpose, including but not limited to filtration system, such as the system described in US9,404,063, or hydrocyclones, or centrifuges.

In some embodiments, the HTL conversion process is conducted at a temperature within the range 300 to 425° and at a pressure within the range 85 to 300 bar. Residence time within the HTL reactor is typically controlled by the rate of pumping feed stream into and removal of product out from the reactor. Residence time is thus ultimately limited by the length and internal volume of the reactor, in the case of a tubular reactor. However, flow within the reactor can be oscillatory such that the feed stream moves back and forth within the reactor as it is gradually transported towards the output end. Removal of product stream from the reactor is typically achieved using a de-pressurization system that conserves process heat as much as possible. Suitable pumping systems include a variety of systems known in the art, including but not limited to any of the combined pumping and depressurization systems described in W02016/004958 which is hereby incorporated by reference in entirety. In some embodiments, HTL conversion is conducted according to the method described in Anastasakis (2018) with the additional feature of separation/filtration of the HTL product stream under pressure at process conditions.

After its removal from the HTL reactor, the HTL product steam is passed through a heat exchange system that simultaneously heats the incoming feed stream and cools the product stream. The cooled HTL product stream after heat exchange is separated into aqueous, oil and gaseous phases. In some embodiments, a three phase separator is applied to the HTL product stream after heat exchange.

In some embodiments, continuous HTL conversion is conducted with recirculation to the feed stream of some portion of product oil or aqueous phase product or both. The COD levels obtained in the aqueous product phase differ depending on the extent to which aqueous phase has been recirculated. Even in single-pass configurations, where the biomass feed stream has 15 to 20 wt. % dry matter content, COD content of the aqueous product stream typically has between 20 to 50 g/L COD. In processes where some portion of the aqueous product stream is recycled, aqueous COD levels typically fall within the range 60 to 120 g/L.

In some embodiments all or part of the HTL aqueous product stream is subject to continuous wet oxidation at temperature within the range 300 to 425° C and heat produced by the wet oxidation process is used to heat the HTL feed stream. The wet oxidation processes a feed stream of aqueous separated/filtered HTL product to a product stream of comparatively purified wastewater which can typically be readily consumed in anaerobic digestion.

The wet oxidation process may use in situ oxygen generation to supplement or replace compressed air. In some embodiments, the high pressure gas injector is a gas sparger providing good contact between the aqueous and gas phase. In some embodiments, applied oxygen pressure is within the range 100 to 300 bar, or 150 to 250 bar, or 60 to 200 bar. In some embodiments, COD levels of the aqueous product stream subject to wet oxidation is between 20 and 120 g/L, or between 23 and 70 g/L. In some embodiments the oxygen concentration in the injected gas stream is between 15% and 100% as a wt.% of the gas phase, or between 20-50%, or between 60-100%. As will be readily understood by one skilled in the art, oxygen can be generated using comparatively inexpensive systems such as membrane-based concentrators, or, alternatively via more costly systems that provide additional advantages, such as an alkaline electrolyzer or a solid oxide electrolysis cell. Electrolysis will provide both oxygen and a valuable side stream of hydrogen that can be utilized for various processes such as hydrotreatment of the product biocrude oil. In some embodiments, electrolytically generated oxygen will ideally be compressed to around 200 bar before injecting into the wet oxidizer, which can provide a desired preheating of the produced oxygen.

In some embodiments, the addition of air or an oxygen rich gas stream is performed using one or several injection points, such as gas spargers, to ensure the necessary amount of oxygen available for the reaction. The injection points may be located near the entry of the wet oxidation reactor or distributed over the length of said reactor. In some embodiments, two or more wet oxidation reactors placed in the system in a parallel or serial manner.

In some embodiments, the addition of air or an oxygen rich gas stream is performed in a manner where the said gas flow is controlled via a feedback loop (control system) to ensure a desired conversion rate and production of heat. This control system may also provide a means for avoiding overheating of the wet oxidation reactor. The feedback loop may be controlled by one or more sensors measuring temperature, pressure, or chemical characteristics of the reaction medium.

In some embodiments heat produced by the wet oxidation process is used to heat the HTL feed stream by integrating a separate HTL reactor and a wet oxidation reactor in a co- or counter-current arrangement whereby the reactors are in direct thermal contact via a heat exchange system. In some embodiments, this integration is achieved where both the HTL reactor and the wet oxidation reactor are tubular, or otherwise designed to provide some degree of tubular flow. Tubular reactors are typically the most technically simple and least costly available option.

The wet oxidation process can advantageously provide sufficient excess heat to increase the temperature of the HTL feed stream from the feed-in temperature achieved by heat exchangers to the steady state process temperature. In some embodiments, the wet oxidation process is conducted at a temperature higher than the temperature applied in the HTL process such that a DT (temperature difference) is maintained to drive heat transfer. In some embodiments, DT between the wet oxidation process and the HTL process is within the range 10 to 50° C, or between 15 and 70° C. At temperature within the range 300 to 425° C, wet oxidation can typically be conducted so as to provide more than 60% COD removal within 10 minutes. In some embodiments, the wet oxidation process achieves COD removal within 10 minutes, or within 15 minutes, or within 20 minutes between 50 and 99% or between 40 and 80% or between 60 and 85%. One skilled in the art will readily determine without undue experimentation an appropriate set of wet oxidation conditions such that the residence time for wet oxidation can be within 20 minutes, or within 15 minutes, or within 10 minutes, so that heat transfer to the HTL process can provide the required top heating quickly. In some embodiments, tubular wet oxidation and HTL reactors are integrated within a heat exchange system such that direct thermal contact is maintained for sufficient time to provide heat transfer for top heating the HTL feed stream followed by one or more continuing sections of HTL reactor which is not integrated with the wet oxidation reactor.

In embodiments using tubular reactors arranged co- or counter-current direct thermal contact, the wet oxidation and HTL reactors can typically have similar dimensions. In some embodiments, the wet oxidation reactor is arranged as a tube within a tube heat exchanger with the HTL reactor residing in the central tube and the wet oxidation reactor residing in the outer tube shell. This arrangement of the reactors is feasible since the aqueous HTL wastewater typically contains no particulate material and is thus less prone to clog the layer between the inner and outer tube. In such a setup the wall thickness of the inner tube may be reduced to improve heat transfer as the outer tube wall will carry the main part of the pressure load. Alternatively, in some embodiments, the residence time for the wet oxidation process is longer than the residence time for the HTL process. As the amount of HTL aqueous product is much less than the HTL feed flow, a longer residence time in wet oxidation can be readily obtained even where the HTL and wet oxidation reactors have the same inner volume.

In some embodiments, residence time in the wet oxidation process is between 5 and 30 minutes, or between 2 and 10 minutes, or between 3 and 15 minutes, or between 5 and 20 minutes, or between 12 and 60 minutes, or between 1 and 8 minutes. In some embodiments, residence time in the HTL process is between 5 and 120 minutes, or between 10 and 60 minutes, or between 12 and 40 minutes, or between 7 and 100 minutes. In some embodiments, the wet oxidation process is conducted at a temperature between 300 and 375° C, or between 330 and 425° C. In some embodiments, the HTL process is conducted at a temperature between 300 and 375° C, or between 330 and 425° C. In some embodiments, the wet oxidation process provides more than 50% of the energy requirement for top heating the feed stream in the HTL process from the feed-in temperature to the process temperature, or more than 60%, or more than 80%.

The HTL and wet oxidation reactors can ideally ensure a residence time of at least 300 seconds (5 minutes) up to 120 minutes. The diameter of the reactors is typically optimized to ensure good thermal transfer and typical dimensions are between 15 mm and 100 mm and the length between 5-500 m or between 10 m and 500 m or between 50 m and 500 m. The flowrate applied in an HTL process conducted in a tubular reactor is suitably in the range of 0.05 and 2 m/s.

Figure 1 describes one non-limiting example of methods and systems of the invention, any feature of which may be used in an HTL system or method of the invention in combination with any other system or feature described here or otherwise known in the art. As shown, a biomass feed stream is combined with some portion of aqueous phase HTL product in a feed mixer (A). The mixed feedstock and recycled aqueous phase slurry is pre-heated by being fed through a high pressure feed pump (B) such as, for example, any of the pump systems described in W02016/004958 into an HTL heat exchanger (C). The pre-heated feed stream is further pumped into the HTL component of an integrated reactor system (D) having separate HTL and wet oxidation reactors arranged co- or counter-current in direct thermal contact enhanced in some embodiments by heat exchangers such as any of those described in WO2021/024176, which is hereby expressly incorporated by reference in entirety. The output from the HTL component of an integrated HTL and wet oxidation reactor system (D) is fed through an online solid liquid separation system 0), such as a filtration system Q), which is adapted to separate solids including precipitated salts from the product stream under pressure at the HTL process temperature. Solids recovered from J are periodically blown through a valve to a blow down tank K. The separated/filtered HTL product stream is then fed through an HTL heat exchanger (C) to contribute to pre-heating of the HTL input feed stream, then released through a valve into a three phase separator (E) that separates a gaseous product phase, a product oil phase and an aqueous product phase. Some portion of the aqueous product phase separated by a three phase separator (E) is recycled to the HTL process by mixing with incoming feedstock in a feed mixer (A). Some portion of the aqueous product phase separated by a three phase separator (E) is pumped through a water phase feed pump (F) through a wet oxidation heat exchanger (G) to pre-heat the wet oxidation wastewater stream prior to its being aerated/oxygenated by an air/oxygen sparger (H). The pre-heated, aerated/oxygenated wastewater stream is further pumped into the wet oxidation component of an integrated reactor system (D) having separate HTL and wet oxidation reactors arranged co- or counter-current in direct thermal contact. The output from the wet oxidation component of an integrated HTL and wet oxidation reactor system (D) is fed through the wet oxidation heat exchanger (G) to pre-heat the wastewater stream prior to wet oxidation, then released through a valve into a water/gas separator (I) that separates a gaseous phase and a comparatively purified water phase. The rapid rate at which wet oxidation of the HTL wastewater stream can be achieved at HTL process temperatures is described in Table 1. This shows results of batch wet oxidations experiments which are explained in detail in example 1 conducted with HTL aqueous product. The underlying HTL process from which the aqueous samples were derived was conversion of dewatered sewage sludge at 16 wt. % DM at a temperature of 325° C for a residence time of approximately 60 minutes in a one-pass process without recirculation of aqueous phase. The HTL aqueous product had COD content 28.3 g/L, which is typical of levels achieved in a one pass HTL process. COD removal efficiency achieved by the wet oxidation process is shown as the mean of two replicate experiments for retention times between 2 and 10 minutes at oxygen initial pressure loading 90 bar for temperatures between 300 and 350° C. Also shown for each set of process conditions is the estimated energy released from the wet oxidation process in kJ/L, based on a yield of 435 kj per mole of 02 reacted. As shown, at 350° C, which is a typical HTL process temperature, and 90 bar initial oxygen pressure loading, 73% COD removal is achieved within 10 minutes.

The heat required for top heating the HTL feed stream to process temperature is described in Table 2. Inputs for the table are based on results from the Aarhus University HTL pilot plant and an assumption that the wet oxidation heat exchanger can be operated with high efficiency of 95%. Aqueous product mass is estimated to be 0.85 that of the feed stream slurry. The results in Table 2 refer to the process flow scheme shown in Figure 2. As shown in Figure 2, the initial feed stream is flow A to the HTL heat exchanger. Flow B is the feed- in to the integrated HTL/wet oxidation reactor. Flow C is the HTL product stream to the pressurized solid-liquid separation system, such as filtration system. Flow D is the separated/filtered HTL product stream to the HTL heat exchanger. Flow E is the cooled, separated/filtered HTL product stream to a three phase separator. Flow F is the aqueous product stream to the wet oxidation heat exchanger. Flow G is the wet oxidation feed-in to the integrated HTL/wet oxidation reactor. Flow H is the wet oxidation output stream to the wet oxidation heat exchanger. Flow I is the cooled wet oxidation output stream. For each of the flows shown in the process scheme in Figure 1, Table 2 shows the expected temperature and corresponding energy content for pure water in integrated HTL/wet oxidation processes at 325 and 350 °C with DT for the wet oxidation process of 25 and 20° C respectively. An actual biomass slurry has considerably less heat capacity than pure water. However, the net sum of chemical reactions in hydrothermal liquefaction is slightly endothermic. Thus, an estimate based on pure water provides a reasonable approximation of the energy required for the HTL process shown in the table, including top heating the HTL feed stream and maintaining DT for the wet oxidation process. As shown, the estimated energy requirement at 350 °C is 414.4 KJ/kg and at 325 °C is 300.6 KJ/kg.

The percentage of HTL energy requirement for a process at 350° C which can be provided by 10 minutes wet oxidation of aqueous product is shown in Table 3. The % COD removal efficiency from Table 1 for 28.3 g/L COD at 350° C is extrapolated to higher levels of COD associated with an HTL process where aqueous product is recirculated. The resulting energy release is expressed as a percentage of the energy requirement shown in Table 2.

Table 1. COD removal efficiency for wet oxidation treatment of HTL wastewater having 28.3 g/L COD.

Table 2. Energy requirement for HTL process. Table 3. Percentage of HTL energy requirement at 350° C provided by 10 minutes wet oxidation of aqueous product.

In some embodiments heat produced by the wet oxidation process is used to heat the HTL feed stream by conducting wet oxidation in a separate, external unit. In some embodiments, excess heat can then be provided to the HTL process via a fluid heat transfer medium such as high pressure steam, high pressure compressed water, hot oil or molten salt. After the wet oxidation process delivers excess heat to the HTL process, the product stream from the separate wet oxidation unit can further be used in an additional heat exchanger to preheat the wet oxidation feedstock stream to, for example, 250° C. In embodiments where heat transfer between the HTL and wet oxidation processes is made via an external medium or via an external heat sink, the dimensions of the wet oxidation reactor can be independent of the HTL reactor dimensions. In some embodiments, the wet oxidation reactor can be a fed- batch system.

Figure 3 describes one non-limiting example of integration of tubular wet oxidation and HTL reactors by an arrangement co- or counter-current in direct thermal contact via a heat exchange system. Shown is the heat exchange system for a tubular HTL reactor described in Figure 3 of WO2021/024176. The HTL feed stream is divided into tubular reactor sections (1) each of which is surrounded by heat transfer clamps (2) made from a solid matrix of heat conducting material. The individual heat transfer clamps (2) comprise support formations that position and support the tubular reactor sections. The tubular sections (1) can be arranged in different ways. For example, three sections corresponding to equivalent residence times can be aligned co- or counter current within the heat exchange system with three sections corresponding to later residence times. When this system is applied to integrated HTL and wet oxidation processes, as shown, the wet oxidation stream and the HTL stream are divided into sections aligned co- or counter within the heat exchange system such that a tubular section of the HTL reactor (3) is always adjacent to a tubular section of the wet oxidation reactor (4).

Figure 4 describes another non-limiting example of integration of tubular wet oxidation and HTL reactors by an arrangement co- or counter-current in direct thermal contact via a heat exchange system. Shown is the heat exchange system for a tubular HTL reactor described in Figure 6a of WO2021/024176. As with the system shown in Figure 3, Figure 4 shows tubular reactor sections (6) surrounded by heat transfer clamps (5) made from a solid matrix of heat conducting material where each individual heat transfer clamp comprises support formations that position and support the tubular reactor. In this case the geometry of the heat clamps provides a circular array. Here again, when this system is applied to integrated HTL and wet oxidation processes, as shown, the wet oxidation stream and the HTL stream are divided into different sections aligned co- or counter within the heat exchange system such that a tubular section of the wet oxidation reactor (7) is always adjacent to a tubular section of the HTL reactor (8).

Figure 5 shows one non-limiting example of how the HTL feed stream can be fed to the combined HTL and wet-oxidation reactor/heat exchanger shown in Figure 4. The relatively high viscosity preheated biomass suspension is distributed via the inlet connector (9) into the HTL reactor tube sections (14) via the distributor block/manifold (12). Viscous biomass slurries are advantageously pumped in an unrestricted direct flow path with minimized curvatures. The low viscosity preheated and oxygenated wet oxidation wastewater is easier to handle and can be fed to the side inlet (10) and distributed to the wet oxidation reactor tube sections (13) via the circular distributor channel (11). The wet oxidation feed stream may be fed co- or counter-current to the direction of the HTL flow.

Figure 6 describes another non-limiting example of methods and systems of the invention, any feature of which may be used in an HTL system or method of the invention in combination with any other system or feature described here or otherwise known in the art. As shown, a biomass feed stream is combined with some portion of aqueous phase HTL product in a feed mixer (A). The mixed feedstock and recycled aqueous phase slurry are pre heated by being fed through a high pressure feed pump (B) such as, for example, any of the pump systems described in WO2016/004958, into an HTL heat exchanger (C). The pre heated feed stream is further heated by being pumped into a co-current heat exchanger with molten salt (D). The input feed heated to process temperature is further pumped into an HTL reactor (E). Output of product from the HTL reactor (E) is fed through an online solid- liquid separation system (N), such as an online filtration system (N), which is adapted to separate solids including precipitated salts from the product stream under pressure at the HTL process temperature. Solids recovered from N are periodically blown through a valve to a blow down tank O. The separated/filtered HTL product stream is then fed through an HTL heat exchanger (C) to contribute to pre-heating of the HTL input feed stream, then released through a valve into a three phase separator (F) that separates a gaseous product phase, a product oil phase and an aqueous product phase. Some portion of the aqueous product phase separated by a three-phase separator (F) is recycled to the HTL process by mixing with incoming feedstock in a feed mixer (A). Some portion of the aqueous product phase separated by a three phase separator (F) is pumped through a water phase feed pump (G) through a wet oxidation heat exchanger (H) to pre-heat the wastewater stream prior to its being aerated/oxygenated by an air/oxygen sparger (I). The pre-heated, aerated/oxygenated wastewater stream is further pumped into a wet oxidation reactor with molten salt cooling (J). The molten salt cooling system for wet oxidation reactor (J) comprises a molten salt buffer tank (K) from which molten salt is pumped by molten salt pump (L) to a co-current heat exchanger with molten salt (D) to transfer heat from molten salt cooling to the HTL input stream. The output from the wet oxidation reactor with molten salt cooling (J) is fed through the wet oxidation heat exchanger (H) to pre-heat the wastewater stream prior to wet oxidation, then released through a valve into a water/gas separator (M) that separates a gaseous phase and a comparatively purified water phase.

As will be readily understood by one skilled in the art, different features of the different embodiments described here can be combined.

Examples.

1. Wet-oxidation treatment of HTL wastewater.

Experiments were performed at Aarhus University Department of Biological and Chemical Engineering and will be subsequently published as “Wet oxidation treatment of hydrothermal liquefaction of sewage sludge aqueous phase,” by Lars Bjorn Silva Thomsen, Konstantinos Anastasakis and Patrick Biller.

Hydrothermal liquefaction of dewatered sewage sludge was performed on the continuous HTL pilot-plant at Aarhus University as described in detail by Anastasakis (2018). HTL process conditions were 325° C, 43 L/hour, feed stream 16 wt. % DM content, residence time approximately 60 minutes. The HTL product was separated into an aqueous phase (AP) and an oil phase. The aqueous product was not recirculated such that the COD level of the resulting AP corresponds to values associated with a “one pass” process. Wet oxidation of HTL-AP was carried out in custom built Hy-Lok reactors. The reactors were constructed from 316 stainless steel ¾ inch Hy-lok pipes. The bottom of each reactor was capped and the top was connected with a ¾ to ½ inch reduction union following by a ½ to ¼ inch reducer. The top part of each reactor was connected to a union cross with a 1.5 m long 1/8 inch pipe. The union cross was connected to a pressure gauge, safety valve and quick connector for oxygen injection. Each reactor had a total internal volume of 20 mL - ¾ inch pipe with 10 cm in length.

Experiments were performed in duplicate using 5 mL of HTL-AP inserted into the reactors. The reactors were vented three times with 02 and afterwards pressurized with oxygen gas to pressure 90 bar.

The pressurized reactors were submerged into a pre-heated fluidized sand bath and continuously mixed in a shaker. Experiments was performed at two different temperatures (300, and 350°C) and two retention times (2 and 10 minutes). After the reaction time, the reactors were cooled to room temperature and the samples were collected and stored at 2°C.

HTL-AP samples and the samples after wet oxidation were analyzed for chemical oxygen demand (COD) and ammonium (NH ₄ ⁺ ) content using Merck Spectroquant cell tests (part numbers: COD-114541 and NH4+-114559). The initial HTL-AP material had COD content 28.3 g/L. The COD removal was calculated using: COD removal (%) = ((COD initial - COD finaiyCOD initial) ^* 100. The energy release (ER) was calculated based on the difference between the initial and final concentration of COD (mol) and the general material balance for wet oxidation process, where the heat value is estimated to be approximately 435 kj per mole of 02 reacted, as reported by Debellefontaine (2000). Table 1 shows results of the experiments. Shown are the average +/- standard deviation of COD removal % and energy release. As shown, at process temperature 350°C, 73% COD removal can be achieved by the wet oxidation process within 10 minutes residence time.

The embodiments and examples described are exemplative only and not intended to limit the scope of the invention as defined by the claims.

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