Depolymerisation of lignin-containing materials with supercritical fluids in vortex reactors

Abstract

Processes and apparatuses, in the form of passive mixing reactors, are provided for the depolymerisation and valorisation of lignin. The mixing of streams of supercritical or subcritical water with a lignin source is employed to precisely control mixing temperatures, pressures and reaction times. The separate fluid streams are mixed using precisely controlled geometries to ensure near instantaneous mixing. Uses of the passive mixing reactors and methods for the depolymerisation of lignin are also provided. Finally, products obtained by any of the processes, uses or methods are further provided.

Description

[0001] DEPOLYMERISATION OF LIGNIN-CONTAINING MATERIALS

[0002] FIELD OF THE INVENTION

[0003] The invention relates to the field of lignin valorisation, in particular to the conversion of lignin derived from natural sources into high-value smaller molecules useful in a range of industries, including the cosmetic, food and pharmaceutical industries.

[0004] BACKGROUND OF THE INVENTION

[0005] There exists an ongoing need for the valorisation ofcheap, abundant natural materials, especially those which are considered waste / side streams of other processes. Lignin is one such material. It is a naturally abundant biopolymer that binds fibres of cellulose and hemicellulose together in woody and agricultural materials and provides stiffness to plants by imparting rigidity to cell walls. After cellulose, lignin is in fact the second most abundant biopolymer on Earth.

[0006] Importantly, lignin is made up of a range of small molecule phenolic monomers, predominantly para-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol which are oxidatively coupled together to form a highly crosslinked, rigid polymer. The monomers of lignin are therefore a potential natural source of highly valuable phenolic / aromatic chemicals for a wide range of industries, including the food, cosmetic and pharmaceutical industries. One example of a phenolic monomer of lignin is vanillin, which is widely used in the food industry for artificial vanilla flavouring.

[0007] Perhaps the most abundant form of available lignin comes from the papermaking industry in the form of black liquor, a by-product material resulting from the Kraft process which involves the removal of lignin during the conversion of wood into wood pulp. However, owing to the difficulties in the depolymerisation of lignin on scales suitable for industrial exploitation, lignin is predominantly used as a combustion energy source.

[0008] A number of methods for the depolymerisation of lignin have been developed. WO 2015 / 075290 discloses the depolymerisation of lignin in black liquor but the process results in low yields of valuable phenolic monomers, requires long reaction times and employs sub-optimal batch processing. Matsumara etal. (Industrial & Engineering Chemistry Research, 2012, 51 , 11975-11988) investigated lignin depolymerisation using supercritical water under continuous processes on sub-second timescales. However, decomposed lignin was predominantly converted into char leading to reduced efficiency in generating valuable chemical products.

[0009] WO 2019 / 120956 discloses a continuous flow process for the depolymerisation of lignin in the form of black liquor using mixing of supercritical water at high temperatures and pressures. Whilst improvements over previous attempts were made, relatively high levels of char were still recorded (a minimum level of 7% char resulting from the lignin starting material). Further, the process was only demonstrated to produce the results described therein on small, non-industrially relevant scales (lignin concentrations of 0.3% w / w and lignin solution flow rates of 1.6 to 2.5 L / h).

[0010] A problem which exists in the prior art relates to the uneven distribution of heat and the uneven contact of lignin with supercritical or subcritical water which is used during the depolymerisation of lignin. Super critical or subcritical water is used as a way to instantaneously start the depolymerisation reaction, which is a critical requirement for efficient and economical application. Due to the inefficient mixing simultaneous overheating and underheating local spots of lignin particles occurs, resulting in ‘overreaction’ and the formation of low-value gaseous products, and incomplete depolymerisation, repolymerisation and formation of char, respectively. This inefficiency coupled with clogging problems made prior art processes completely non-scalable.

[0011] Clearly, there is still a need for developing more efficient, selective and fast processes for lignin valorisation suitable for large scale industrial exploitation and the generation of valuable phenolic / aromatic organic molecules.

[0012] SUMMARY OF THE INVENTION

[0013] The present invention provides a surprising development of the present technology, in particular regarding the valorisation of lignin feedstocks and the selective generation of high-value phenolic small molecules in high yields and on large scales.

[0014] In a first aspect of the invention there is provided a continuous flow process for the depolymerisation of lignin, the process comprising the steps:

[0015] i) providing a first fluid stream comprising a lignin solution or suspension comprising lignin;

[0016] ii) providing one or more additional fluid stream comprising supercritical water or subcritical water;

[0017] iii) bringing the first fluid stream into contact with the one or more additional fluid stream via passive mixing to generate a mixed fluid stream for a reaction time period in order to obtain a depolymerised lignin mixture comprising monomeric and / or oligomeric phenolic compounds; and

[0018] iv) isolating the monomeric and / or oligomeric phenolic compounds from the depolymerised lignin mixture;

[0019] wherein the passive mixing to generate a mixed fluid stream comprises turbulent mixing, wherein the turbulent mixing comprises generating at least one vortex; and Step iii) further comprises increasing the temperature of the lignin in the first fluid stream to a reaction temperature of at least about 300 °C within a heating time period of at most about 0.5 s. In a second aspect of the invention there is provided a passive mixing reactor suitable for the depolymerisation of lignin using the process according to the first aspect, comprising:

[0020] a first fluid stream inlet at a first end suitable for injection of a first fluid stream;

[0021] a fluid outlet at a second end suitable for ejection of a reaction mixture;

[0022] a central flow channel, comprising a longitudinal axis, and extending between the first end and the second end enabling fluid communication between the first fluid stream inlet and the fluid outlet; and one or more tangential segments arranged between the first end and the second end, each of the one or more tangential segments comprising one or more tangential fluid flow channels extending between a channel inlet and the central flow channel to provide fluid communication between the one or more tangential fluid stream inlets and the central flow channel, wherein each of the channel inlets is suitable for receiving one or more additional fluid stream for delivery to the central flow channel; wherein, in use, the one or more tangential fluid flow channels is oriented for generating turbulent mixing between the first fluid stream travelling along the central flow channel and the one or more additional fluid stream entering the central flow channel from the one or more tangential fluid flow channels; wherein each of the one or more tangential fluid flow channels is orientated such that, in use, injection of the one or more additional fluid stream through the one or more tangential fluid flow channel is capable of generating a vortex within the central flow channel on mixing with a first fluid stream injected through the first fluid stream inlet.

[0023] In a third aspect of the invention there is provided a use of the passive mixing reactor according to the second aspect in the process for the depolymerisation of lignin according to the first aspect.

[0024] In a fourth aspect of the invention there is provided a method for the depolymerisation of lignin, the method comprising:

[0025] providing the passive mixing reactor according to the second aspect;

[0026] providing a first fluid stream comprising a lignin solution or suspension comprising lignin through the central flow channel; and

[0027] providing supercritical water or subcritical water as one or more additional fluid streams through the one or more tangential fluid flow channels, to obtain a depolymerised lignin mixture comprising monomeric and / or oligomeric phenolic compounds

[0028] Within the scope of this disclosure it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description, drawings and clauses, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic vertical cross sectional representation of an embodiment of the passive mixing reactor viewed perpendicular to the longitudinal axis of the central flow channel showing two tangential segments. Thick arrows (solid and dotted line arrows) represent the directional flow of fluid streams.

[0030] Figure 2 shows a schematic vertical cross sectional representation of an embodiment of the passive mixing reactor viewed perpendicular to the longitudinal axis of the central flow channel showing one tangential segment. Thick arrows (solid and dotted line arrows) represent the directional flow of fluid streams.

[0031] Figure 3 shows a schematic horizontal cross sectional representation of the passive mixing reactor viewed along the longitudinal axis of the central flow channel, including one representation of an offset angle.

[0032] Figure 4 shows a schematic flow diagram of an embodiment of the process of the invention.

[0033] Figure 5 shows a schematic flow diagram of a separation / purification procedure of an embodiment of the process of the invention.

[0034] DETAILED DESCRIPTION OF THE INVENTION

[0035] Unless otherwise indicated, the practice of the present invention employs conventional techniques of chemistry, materials science and process engineering, which are within the capabilities of a person of ordinary skill in the art.

[0036] Prior to setting forth the invention, a number of definitions are provided that will assist in the understanding of the invention. All references cited herein are incorporated by reference in their entirety. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0037] As used herein, the term ‘comprising’ means any of the recited elements are necessarily included and other elements may optionally be included as well. ‘Consisting essentially of’ means any recited elements are necessarily included, elements that would materially affect the basic and novel characteristics of the listed elements are excluded, and other elements may optionally be included. ‘Consisting of’ means that all elements other than those listed are excluded. Embodiments defined by each of these terms are within the scope of this invention.

[0038] The term ‘fluid stream’ or ‘feed stream’ refers to a flowable substance, typically an aqueous liquid, in which the target substance is dissolved, suspended, emulsified, dispersed, or otherwise carried. Typically, the liquid contains water, or is predominantly water based, and may be in the form of: filtered water; deionised water; or more specifically any type of water between 1-20 MQ-cm resistivity; contaminated water; waste water; drinking water; seawater; and / or industrial or agricultural byproduct or runoff.

[0039] The term ‘passive mixing’ refers to the mixing of at least two fluids without the use of external mechanical forces or energy inputs, such as pumps, stirrers, or agitators, instead relying on the flow dynamics of said fluids. A preferred form of passive mixing in the context of the present disclosure is ‘turbulent mixing’, which as understood by the skilled person, refers to the mixing of at least two fluids as a result of chaotic and / or irregular fluid motion achieved at relatively high flow rates upon the combination of said fluids. A preferred form of turbulent mixing in the context of the present disclosure is ‘vortex mixing’, also understood by the skilled person, wherein the term ‘vortex’ assumes its commonly understood meaning in the art, i.e. the turbulent flow of a fluid revolving in a series of planes orthogonal to the directional flow axis. The term ‘vortex mixing’ refers to the mixing of at least two fluids through the creation of at least one vortex.

[0040] The term ‘lignin’ assumes its commonly understood meaning in the art, which is described in detail above.

[0041] The term ‘char’ assumes its commonly understood meaning in the art, which can be carbonisation of organic material, partially depolymerised organic material, repolymerised organic material, and / or any kind of organic material that is not soluble in ethyl acetate at room temperature. In the context of the present disclosure, char refers to organic material which has been heated to high temperatures in the absence of sufficient contact with supercritical or subcritical water and low levels of oxygen, resulting in the carbonisation of the organic material, inefficient depolymerisation and / or repolymerisation of depolymerised parts of lignin characterised by heavier molecular weight products being formed.

[0042] The term ‘supercritical water’ may assume its commonly understood meaning in the art. The water may be heated to a temperature of at least about 373 °C, and pressurised to at least about 220 bar. For example, the water may be heated to a temperature of above about 374 °C, and pressurised to above about 221 bar.

[0043] The term ‘subcritical water’ may assume its commonly understood meaning in the art. The water may be heated to a temperature between its normal boiling point and its critical point (i.e. between about 100 °C and 374 °C) and pressurised to a pressure high enough to maintain the water in a liquid physical state (typically between about 5 and 220 bar).

[0044] The term ‘heterogeneous catalyst’ assumes its commonly understood meaning in the art, which is a catalyst which exists in a different phase from that of the reaction medium. The term ‘capping agent’ assumes its commonly understood meaning in the art, specifically in the field of polymer chemistry; ‘capping agents’ are chemical species used to prevent or minimize polymerization of monomers or oligomers.

[0045] The term ‘total solids’ refers to the total amount of material in a defined mixture of material which assumes a solid phase under normal conditions of temperature and pressure in the absence of a solvent.

[0046] The term ‘depolymerisation of lignin’ refers to the conversion of the lignin macromolecule into phenolic oligomers or monomers present in the lignin macromolecular structure, or derivatives thereof. The term, ‘phenolic oligomers’, refers to compounds comprising at least two phenolic monomers (e.g. at least two of the phenolic monomers listed below) joined together. The oligomer is typically a tetramer, a trimer, or a dimer.

[0047] The term ‘depolymerisation of lignin’ may refer to the conversion of the lignin macromolecule into phenolic monomers. The term ‘phenolic monomer’ refers to compounds which present a single phenol functional group. Examples of phenolic monomers include phenol, guaiacol, alkyl guaiacols such as creosol, 5-methylguaiacol, 4-ethylguaiacol, 4-vinylguaiacol, guaiacyl acetone, eugenol, isoeugenol, 4-hydroxybenzaldehyde, vanillin, vanillic acid, homovanillic acid, homovanillyl alcohol, acetovanillone, propiovanillone, syringaldehyde, 3,4,5-trimethoxybenzyl alcohol, 4-hydroxyacetophenone, 4-hydroxybenzoic acid, syringol, acetosyringone, syringic acid, ferulic acid, caffeic acid, dehydrozingerone, sinapyl alcohol, coniferyl alcohol and p-coumaric acid.

[0048] The term ‘depolymerisation of lignin’ may refer to the conversion of the lignin macromolecule into at least guaiacol, creosol, vanillin, acetovanillone, homovanillic acid, 5-methyl guaiacol, acetosyringone, syringaldehyde and / or syringol. Typically, it refers to the conversion of the lignin macromolecule into at least syringol, vanillin and / or homovanillic acid. Furthermore, it may refer to the conversion of the lignin macromolecule in black liquor into at least syringol, syringaldehyde and vanillin. It may also refer to the conversion of the lignin macromolecule in Kraft lignin into at least vanillin and / or homovanillic acid.

[0049] As used herein, ‘biomass’ refers to a renewable energy source generally comprising hydrocarbonbased biological material derived from living organisms. The organisms may be plants, animals, fungi, etc. The term ‘biomass’ as used herein does not include fossil fuel sources.

[0050] The term “% w / w” refers to the weight percent, which is the weight of a particular component of a material, substance or composition expressed as a percentage of the total weight of that material, substance or composition or relative to a defined set of subcomponents.

[0051] As used herein, ‘lignocellulosic biomass’ refers to a biomass, typically a plant biomass, comprising cellulose and / or hemicellulose, and lignin. Lignocellulosic biomass can be obtained from a variety of sources, for example from: agricultural waste, including corn stover and sugarcane bagasse; or wood, including hardwoods (such as eucalyptus, aspen or birch wood) and softwoods (such as spruce, pine, fir, larch and hemlock wood).

[0052] A prominent example of lignocellulosic biomass fractionation is the pulping process. Different kinds of pulping processes are known, such as the Kraft process, the sulphite process, or organosolv pulping. These methods generally refer to the treatment of a lignocellulosic biomass with a cooking liquor to break the bonds that link lignin, hemicellulose, and cellulose, and result in the production as a byproduct of pulping liquors comprising lignin and hemicellulose, from which lignin can be precipitated.

[0053] The Kraft pulping process involves treatment of the lignocellulosic biomass with a mixture of water, sodium hydroxide, and sodium sulfide (the so called white liquor) at a temperature of above 100 °C, for example at a temperature between 140 and 180 °C. The white liquor solubilises lignin and hemicellulose to form the so called black liquor, which is separated from cellulose. Black liquor is thus a liquid lignin source obtainable from the treatment of a lignocellulosic biomass with the Kraft process.

[0054] The lignin in raw Kraft black liquor is usually concentrated through an evaporation process and then burnt in the recovery boiler at mills, thus providing the energy needed for further pulping. The raw black liquor can be employed as a lignin source in the process of the present invention. The raw black liquor can be employed directly (i.e. without any treatment or modification) in the process of the present invention. Thus, in an embodiment, where the lignin source is a black liquor, the black liquor is neither dried nor concentrated prior to subjecting it to Step iii) of the present invention, described below. However, surprisingly the inventors have found that black liquor may be employed as a lignin source with lignin concentrations higher than those typically generated as part of the Kraft process (typically 10% w / w total solids) and with lignin concentrations higher than processable by previously disclosed processes and methods for lignin depolymerisation using supercritical water. Thus, in embodiments, the lignin source in the form of a black liquor is dried or concentrated by the mill (typically to 40% w / w total solids) in order to increase the lignin concentration therein prior to subjecting it to Step iii) of the present invention. Surprisingly, the present invention is capable of reducing the generation of low value gaseous products, which typically form upon overheating of lignin in contact with supercritical water, typically on the outer side of suspended and / or agglomerated lignin particles, as well as reducing the formation of char (i.e. non-solubles), which typically forms upon underheating of lignin in the absence of contact with supercritical water and at low oxygen levels, typically within the bulk of suspended and / or agglomerated lignin particles. The reduction in wasteful overheating and / or underheating of lignin thus surprisingly results in improved efficiencies and increased yields of valuable monomers and oligomers.

[0055] Alternatively, the solubilized lignin in black liquor can be precipitated from the black liquor, for example by addition of any pH decreasing acidifying agent, such as hydrochloric acid, sulfuric acid and / or carbon dioxide, to the black liquor, and then isolated by means of filtration and / or centrifugation. The lignin obtained is referred to as Kraft lignin. Kraft lignin sources are lignin sources that consist of lignin, or comprise at least 80% w / w lignin, typically at least 90% w / w lignin, suitably at least 95% w / w lignin with respect to the total weight of the lignin source. The sulfite process on the other hand involves treatment of the lignocellulosic biomass with sulfite or bisulfite salt solutions such as aqueous solutions. In these salts the counterion is typically an alkaline earth metal (e.g. calcium, magnesium) or an alkali metal (e.g. sodium, potassium) or ammonium. The sulfite process is generally carried out at a temperature of above 100 °C, for example at a temperature of between 130 and 160 °C. The sulfite or bisulfite salt solution solubilizes lignin and hemicellulose to form the so-called brown liquor (also known as red liquor, thick liquor or sulfite liquor), which is separated from cellulose. Lignin can then be separated from hemicellulose by acidification to precipitate lignin, and the precipitated lignin is recovered by means of filtration and / or centrifugation. Brown liquor is thus a liquid lignin source obtainable from the treatment of a lignocellulosic biomass with the sulfite process.

[0056] The organosolv pulping process is also known as a means for fractionating biomass and recovering lignin. Organosolv pulping involves contacting a lignocellulosic biomass with an aqueous organic solvent at temperatures ranging from 140 to 220 °C to solubilize lignin and hemicellulose, the organic solvent then being separated from cellulose. Solvents used include acetone, methanol, ethanol, butanol, ethylene glycol, formic acid and acetic acid. Lignin can then be separated from hemicellulose by acidification to precipitate lignin, and the precipitated lignin is recovered by means of filtration and / or centrifugation. Such lignin is known as organosolv lignin.

[0057] Other methods of lignocellulosic biomass fractionation are known in the art and are described, for example, in US 2013 / 0145995 A1. Any of these methods may be used in conjunction with the present invention.

[0058] A first aspect of the invention provides a process for the depolymerisation of lignin, the process comprising Steps i) to iv).

[0059] In a preliminary step of the process of the present invention, lignin in the form of a lignin source is provided. In embodiments, the lignin source is treated by raising the temperature of the lignin in the lignin source above a temperature at which the lignin source is initially provided. The lignin source of the preliminary step of the present invention may comprise any form of lignin known in the art. Typically, the lignin is a black liquor or a solution or suspension of Kraft lignin or organosolv lignin. Other kinds of lignin known in the art are Brauns' lignin, cellulolytic enzyme lignin, dioxane acidolysis lignin, Klason lignin, periodate lignin, lignosulfates, lignosulfonates, and steam explosion lignin. Lignin can also be classified based on the biomass submitted to a fractionation process.

[0060] In embodiments, the lignin source is hardwood lignin, i.e. lignin obtained from the fractionation of hardwood. Examples of hardwoods are eucalyptus, aspen or birch wood. In other embodiments, the lignin source is softwood lignin, i.e. lignin obtained from the fractionation of softwood. Examples of softwoods are spruce, pine, fir, larch or hemlock wood.

[0061] In other embodiments, the lignin source is plant lignin, more particularly grass (Poaceae or Gramineae) lignin, i.e. lignin obtained from the fractionation of plant / grass material. Examples of plant material from which lignin can be obtained are the tobacco plant, cereal straw (e.g. wheat straw) or beet pulp.

[0062] In embodiments, the lignin in the lignin source is obtained from a mixture of different lignocellulosic biomasses, typically from a mixture of wood (softwood or hardwood) and plant material, in particular wood (softwood or hardwood) and grass.

[0063] In embodiments, the lignin source is black liquor obtained from wood and / or plant material.

[0064] In embodiments, the lignin is neither acylated nor alkylated prior to subjecting it to depolymerization.

[0065] In embodiments, the lignin source comprises at least about 1% w / w, at least about 2% w / w, at least about 3% w / w, at least about 4% w / w or at least about 5% w / w total solids with respect to the total weight of the lignin source.

[0066] In embodiments, the lignin source comprises at most about 99% w / w, at most about 90% w / w, at most about 80% w / w, at most about 60% w / w, at most about 40% w / w, at most about 20% w / w or at most about 10% w / w total solids with respect to the total weight of the lignin source.

[0067] In embodiments, the lignin source comprises between about 0.1% w / w and 99% w / w, between about 0.1% w / w and 90% w / w, between about 0.1% w / w and 80% w / w, between about 0.1% w / w and 60% w / w, between about 0.1% w / w and 40% w / w, between about 1% w / w and 99% w / w, between about 1% w / w and 90% w / w, between about 1% w / w and 80% w / w, between about 1% w / w and 60% w / w, between about 1% w / w and 40% w / w, between about 10% w / w and 80% w / w, between about 10% w / w and 60% w / w, between about 10% w / w and 40% w / w, between about 20% w / w and 80% w / w, between about 20% w / w and 60% w / w, or between about 20% w / w and 40% w / w total solids with respect to the total weight of the lignin source. In a preferred embodiment the lignin source comprises between about 10% w / w and 40% w / w total solids with respect to the total weight of the lignin source.

[0068] In embodiments, the lignin source comprises at least about 0.1% w / w, at least about 1% w / w, at least about 2% w / w, at least about 3% w / w, at least about 4% w / w, at least about 5% w / w, at least about 15% w / w, at least about 20% w / w, at least about 30% w / w, at least about 40% w / w, at least about 50% w / w, at least about 60% w / w, at least about 70% w / w, at least about 80% w / w, at least about 90% w / w, at least about 95% w / w, or at least about 99% w / w lignin with respect to the weight of the total solids of the lignin source. In embodiments the lignin source comprises between about 0.1% w / w and about 99% w / w, between about 0.1% w / w and about 90% w / w, between about 0.1% w / w and about 80% w / w, between about 0.1% w / w and about 60% w / w, between about 1% w / w and about 90% w / w, between about 1% w / w and about 80% w / w, between about 1% w / w and about 60% w / w, between about 15% w / w and about 80% w / w, between about 15% w / w and about 60% w / w, or between about 20% w / w and about 80% w / w lignin with respect to the weight of the total solids of the lignin source.

[0069] In embodiments, the lignin source comprises about 0.1% w / w, about 1% w / w, about 2% w / w, about 3% w / w, about 4% w / w, about 5% w / w, about 15% w / w, about 20% w / w, about 30% w / w, about 40% w / w, about 50% w / w, about 60% w / w, about 70% w / w, about 80% w / w, about 90% w / w, about 95% w / w, or about 99% w / w lignin with respect to the weight of the total solids of the lignin source.

[0070] In embodiments, the lignin source consists essentially of lignin.

[0071] In embodiments, the lignin source consists of lignin.

[0072] In embodiments, the lignin source comprises at most about 50% w / w, at most about 40% w / w, at most about 30% w / w, at most about 20% w / w, at most about 10% w / w, at most about 5% w / w, or at most about 1% w / w cellulose with respect to the weight of the total solids of the lignin source.

[0073] In embodiments, the lignin source comprises no cellulose and / or no hemicellulose.

[0074] The above amounts of lignin and / or cellulose and / or hemicellulose may be combined to arrive at combined embodiments. For instance, in a particular embodiment, the lignin source comprises at most 50% w / w cellulose and at least 50% w / w lignin, with respect to the weight of the total solids of the lignin source. As such, the lignin source may comprise lignocellulosic biomass.

[0075] Lignin sources comprising the above amounts of lignin and / or cellulose and / or hemicellulose are known in the art. These products are commercially available (e.g. from Sigma-Aldrich, CAS 8068-05-1) or can be prepared by methods well-known in the art, such as by fractionation of lignocellulosic biomass.

[0076] In Step i) of the first aspect of the invention, a first fluid stream (102) is provided comprising a lignin solution or suspension comprising lignin [at an initial temperature]. The lignin solution or suspension may be generated from the lignin source via the preliminary step as described herein. The lignin solution or suspension may comprise solutions or suspensions wherein the lignin is dissolved, partially dissolved or suspended.

[0077] In embodiments, the lignin solution or suspension (i.e. the lignin in the first fluid stream) is provided at a temperature substantially corresponding to about room temperature (i.e. about 20 °C to 25 °C). In preferred embodiments, the lignin solution or suspension is provided at about room temperature (i.e. about 20 °C to 25 °C). In other embodiments, the lignin solution or suspension is supplied at any temperature between about 5 and 95 °C, typically between about 10 and 40 °C. In further embodiments, the lignin solution or suspension is provided at a temperature of between about 40 and 80 °C, between about 45 and 75 °C, or between about 50 and 70 °C.

[0078] In an embodiment, the lignin in the lignin source is dissolved or suspended in a fluid wherein the fluid may comprise dioxane, acetic acid, THF, DMSO, DMF, an alcohol such as methanol or ethanol, water or a mixture thereof. Typically, the fluid comprises an alcohol such as methanol or ethanol, or water, or a mixture thereof. In a most preferred embodiment, the fluid comprises water. The degree of dissolution or suspension of the lignin in the fluid may be adjusted by means known by the person skilled in the art, such as by adjusting the temperature of the fluid, wherein a higher temperature provides higher dissolution. The choice of lignin source may also affect the solubility of the lignin in the solution or suspension.

[0079] Where the lignin source or lignin comprising the lignin source of Step i) is already in the form of a solution or suspension, it may be employed as the lignin solution or suspension of Step i) or it may be further processed (e.g. further diluted) to provide a new lignin solution or suspension. This may, for instance, be the case with black liquors.

[0080] In embodiments, a base is added to the lignin solution or suspension. The base is typically an alkali metal hydroxide or an alkali metal bicarbonate. Preferred alkali metals are sodium or potassium, suitably sodium. A preferred base is sodium hydroxide.

[0081] In a preferred embodiment, the lignin solution or suspension of the first aspect is an aqueous solution or suspension which comprises sodium hydroxide or sodium bicarbonate, suitably sodium hydroxide.

[0082] The addition of the base may be carried out after preparing the solution or suspension of the lignin source; alternatively, the base may be added to the dissolving or suspending fluid prior to dissolving or suspending the lignin source; or the base may be added to the lignin source prior to dissolving or suspending the lignin of the lignin source.

[0083] The concentration of base in the lignin solution or suspension is generally low and may depend on the specific conditions of the subsequent steps of the process of the first aspect. The concentration of the base may be at least about 0.01 M or at least about 0.1 M and may be at most about 0.5 M or at most about 2 M at a point in time when the lignin in the lignin solution or suspension reaches the temperature described in Step iii). The skilled person knows how to select the appropriate base concentrations in the solution or suspension of Step i) to arrive at the desired base concentration in Step iii), taking into account Step ii). In embodiments, the concentration of base in the lignin solution or suspension is between about 0.1 and 5 M. In embodiments, the concentration of lignin in the lignin solution or suspension is at least about 0.01% w / w, at least about 0.1% w / w, at least about 0.5% w / w, at least about 1% w / w, at least about 2% w / w, at least about 3% w / w, at least about 5% w / w, or at least about 10% w / w with respect to the total weight of the first fluid stream (102).

[0084] In embodiments, the concentration of lignin in the lignin solution or suspension is at most about 1% w / w, at most about 3% w / w, at most about 5% w / w, at most about 10% w / w, at most about 15% w / w, at most about 20% w / w, at most about 35% w / w, at most about 40% w / w, at most about 50% w / w or at most about 60% w / w with respect to the total weight of the first fluid stream (102).

[0085] In embodiments, the concentration of lignin in the lignin solution or suspension is between about 35% w / w and 0.2% w / w, between about 30% w / w and 0.5% w / w, between about 25% w / w and 1% w / w, between about 20% w / w and 2% w / w, between about 20% w / w and 5% w / w, between about 20% w / w and 10% w / w, or between about 15% w / w and 10% w / w with respect to the total weight of the first fluid stream (102).

[0086] In Step ii) of the first aspect of the invention, one or more additional fluid stream (104) is provided comprising supercritical water or subcritical water. The supercritical water or subcritical water may be generated by any suitable means known to the person skilled in the art.

[0087] In embodiments, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a temperature of at least about 350 °C, at least about 360 °C, at least about 370 °C, at least about 380 °C, at least about 390 °C, at least about 400 °C, at least about 420 °C, at least about 450 °C, at least about 470 °C, at least about 500 °C, at least about 550 °C, at least about 575 °C, at least about 600 °C or at least about 700 °C. In a preferred embodiment, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a temperature of about 450 °C, about 460 °C, about 470 °C, about 480 °C, about 490 °C, about 500 °C, about 520 °C, about 540 °C, or about 560 °C.

[0088] In embodiments, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a temperature of between about 350 and 700 °C, between about 375 and 650 °C, between about 400 and 600 °C, between about 425 and 550 °C, between about 450 and 500 °C, between about 460 and 490 °C, between about 460 and 480 °C, or between about 465 and 475 °C. In a preferred embodiment, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a temperature of between about 450 and 560 °C, more typically between about 460 and 480 °C. In a particularly preferred embodiment, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a temperature of about 470 °C. In embodiments, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a temperature of at least about 450 °C, or at least about 550 °C.

[0089] In embodiments, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a temperature of between about 450 and 500 °C.

[0090] In embodiments, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a pressure of at least about 200 bar, at least about 210 bar, at least about 220 bar, at least about 230 bar, at least about 240 bar, at least about 250 bar, at least about 260 bar, at least about 270 bar, at least about 280 bar, at least about 290 bar, or at least about 300 bar.

[0091] In embodiments, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a pressure of about 200 bar, about 210 bar, about 220 bar, about 230 bar, about 240 bar, about 250 bar, about 260 bar, about 270 bar, about 280 bar, about 290 bar, or about 300 bar.

[0092] In embodiments, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a pressure of at most about 250 bar, at most about 260 bar, at most about 270 bar, at most about 280 bar, at most about 290 bar, at most about 300 bar or at most about 325 bar.

[0093] In embodiments, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a pressure of between about 200 and 300 bar, between about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, between about 230 and 270 bar, between about 240 and 270 bar, between about 250 and 270 bar, or between about 220 and 260 bar. In a preferred embodiment, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a pressure of between about 250 and 270 bar. In a particularly preferred embodiment, the one or more additional fluid stream (104) comprising supercritical water or subcritical water is provided at a pressure of about 260 bar.

[0094] In embodiments, at least one, at least two or at least three additional fluid streams (104) comprising supercritical water or subcritical water are provided in Step ii).

[0095] In embodiments, at least one fluid stream (104) comprising supercritical water and at least one fluid stream (104) comprising subcritical water is provided in Step ii).

[0096] In embodiments, the first fluid stream (102) comprising a lignin solution or suspension comprising lignin is pressurised in a step between Step i) and Step iii) to a pressure substantially the same as the pressure of the one or more additional fluid stream comprising supercritical water or subcritical water, to enable sufficient mixing between the fluid streams. The skilled person understands that two fluid streams at substantially different pressures mix less well than the same two fluid streams at substantially the same pressure, particularly using short timescales. In preferred embodiments, the first fluid stream (102) is pressurised to a pressure sufficiently similar to that of the one or more additional fluid stream (104) to enable efficient mixing between the streams (102, 104). In embodiments, the first fluid stream (102) is pressurised to a pressure greater than or less than about 1%, about 2%, about 3%, about 4%, about 5%, about 10% or about 20% of the pressure of the one or more additional fluid stream (104). In a most preferred embodiment, the lignin is pressurised to the same pressure (a pressure greater than or less than 0.5%) of the pressure of the one or more additional fluid stream (104) comprising supercritical water or subcritical water.

[0097] In embodiments, the first fluid stream (102) is provided at a pressure of at least about 200 bar, at least about 210 bar, at least about 220 bar, at least about 230 bar, at least about 240 bar, at least about 250 bar, at least about 260 bar, at least about 270 bar, at least about 280 bar, at least about 290 bar, or at least about 300 bar.

[0098] In embodiments, the first fluid stream (102) is provided at a pressure of about 200 bar, about 210 bar, about 220 bar, about 230 bar, about 240 bar, about 250 bar, about 260 bar, about 270 bar, about 280 bar, about 290 bar, or about 300 bar.

[0099] In embodiments, the first fluid stream (102) is provided at a pressure of at most about 250 bar, at most about 260 bar, at most about 270 bar, at most about 280 bar, at most about 290 bar, at most about 300 bar or at most about 325 bar.

[0100] In embodiments, the first fluid stream (102) is provided at a pressure of between about 200 and 300 bar, between about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, between about 230 and 270 bar, between about 240 and 270 bar, between about 250 and 270 bar, or between about 220 and 260 bar. In a preferred embodiment, the first fluid stream (102) is provided at a pressure of between about 250 and 270 bar. In a particularly preferred embodiment, the first fluid stream (102) is provided at a pressure of about 260 bar.

[0101] In embodiments, the first fluid stream (102) and the one or more additional fluid stream (104) are both provided at a pressure of between about 200 and 300 bar, between about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, between about 230 and 270 bar, between about 240 and 270 bar, between about 250 and 270 bar, or between about 220 and 260 bar. In a preferred embodiment, the first fluid stream (102) and the one or more additional fluid stream (104) are both provided at a pressure of between about 250 and 270 bar. In a particularly preferred embodiment, the first fluid stream (102) and the one or more additional fluid stream (104) are both provided at a pressure of about 260 bar. In embodiments of the first aspect of the invention, the temperature of the lignin in the first fluid stream (102) comprising a lignin solution or suspension is increased in Step iii) through contact with the one or more additional fluid stream (104) comprising supercritical water orsubcritical water via passive mixing. The temperature increase is affected within a time period, also referred to herein as the heating time period. Typically, the heating time period is the time period extending between the time of first contact between the first fluid stream (102) [at an initial temperature] and the one or more additional fluid stream (104) comprising supercritical water or subcritical water, and the time when the lignin reaches a temperature, above the initial temperature, suitable to affect depolymerisation of the lignin, referred to herein as the reaction temperature.

[0102] In Step iii) of the first aspect of the invention, the first fluid stream (102) comprising a lignin solution or suspension comprising lignin [at an initial temperature] is brought into contact with the one or more additional fluid stream (104) comprising supercritical water or subcritical water via passive mixing to generate a mixed fluid stream [in order to bring the lignin to a temperature above the initial temperature] for a predefined time period (also known herein as the reaction time period) in order to obtain a depolymerised lignin mixture (106) comprising monomeric and / or oligomeric phenolic compounds.

[0103] In embodiments, the temperature of the lignin in the first fluid stream (102) is increased in Step iii) through contact with the one or more additional fluid stream via passive mixing.

[0104] The temperature increase is intended to be rapid, fast, over a short time period, or nearly instantaneous, i.e. is affected within a heating time period of at least about 1 ps, at least about 1 ns, at least about 1 ps, at least about 10 ps, at least about 50 ps, at least about 100 ps, at least about 200 ps, at least about 500 ps, at least about 750 ps, at least about 1 ms, at least about 2 ms, at least about 5 ms, at least about 10 ms, at least about 25 ms, at least about 50 ms, at least about 75 ms or at least about 100 ms.

[0105] In embodiments, the temperature of the lignin in the first fluid stream (102) is increased in Step iii) through contact with the one or more additional fluid stream via passive mixing, and the temperature increase is affected within a heating time period of at most about 0.5 s, at most about 1 s, at most about 1.5 s, at most about 2 s, at most about 2.5 s, at most about 5 s, at most about 7.5 s, at most about 10 s, at most about 20 s or at most about 30 s.

[0106] In embodiments, the temperature of the lignin in the first fluid stream (102) is increased in Step iii) through contact with the one or more additional fluid stream via passive mixing, and the temperature increase is affected within a heating time period of between about 1 ms and 30 s, between about 10 ms and 15 s, between about 20 ms and 10 s, between about 25 ms and 5 s, between about 30 ms and 3 s, between about 40 ms and 2 s, or between about 50 ms and 1.5 s. In embodiments, the reaction time period is at least about 10 ms, at least about 20 ms, at least about 25 ms, at least about 30 ms, at least about 40 ms, at least about 50 ms, at least about 250 ms, at least about 1 s, at least about 1.5 s or at least about 2 s.

[0107] In embodiments, the reaction time period is at most about 1 s, at most about 2 s, at most about 3 s, at most about 5 s, at most about 10 s, at most about 15 s, at most about 20 s, at most about 25 s or at most about 30s.

[0108] In embodiments, the reaction time period is between about 20 ms and 30 s, between about 25 ms and 25 s, between about 30 ms and 20 s, between about 30 ms and 10 s, between about 40 ms and 5 s, between about 50 ms and 2 s, between about 250 ms and 1.5 s, between about 500 ms and 1 .5 s, or between about 100 ms and 1 s. In a preferred embodiment, the reaction time period is between about 500 ms and 1.5 s.

[0109] In embodiments, the reaction time period is about 10 ms, about 20 ms, about 30 ms, about 40 ms, about 50 ms, about 0.1 s, about 0.2 s, about 0.3 s, about 0.4 s, about 0.5 s, about 1 s, about 1 .5 s, or about 2 s.

[0110] In embodiments, the duration of the reaction time period, also known as the reaction time, is controlled by the length of a reaction tube extending between the fluid outlet (203) of the passive mixing reactor (200) and a decompression valve (see below).

[0111] In embodiments, the temperature of the lignin in the first fluid stream (102) is increased in Step iii) through contact with the one or more additional fluid stream via passive mixing. In Step iii), the temperature the lignin in the first fluid stream (102) is increased to is also known as the reaction temperature. In certain embodiments, the temperature is increased to at least about 300 °C, at least about 310 °C, at least about 320 °C, at least about 330 °C, at least about 340 °C, at least about 350 °C, at least about 360 °C, at least about 370 °C, at least about 380 °C or at least about 390 °C. In another embodiment, the temperature is increased to at most about 400 °C, at most about 410 °C, at most about 420 °C, at most about 430 °C or at most about 440 °C. In a further embodiment, the temperature is increased to between about 300 and 430 °C, between about 310 and 430 °C, between about 320 and 430 °C, between about 330 and 430 °C, between about 340 and 430 °C, between about 350 and 430 °C, between about 360 and 420 °C, between about 370 and 410 °C, or between about 380 and 400 °C. In a preferred embodiment, the temperature is increased to between about 375 and 395 °C. In a particularly preferred embodiment, the temperature is increased to about 385 °C.

[0112] In embodiments, the temperature of the lignin in the first fluid stream is increased to a temperature of between about 300 and 430 °C (e.g. between about 350 and 430 °C) in Step iii) through contact with the one or more additional fluid stream via passive mixing. In embodiments, Step iii) further comprises, after the reaction time period, reducing the temperature of the mixed fluid stream. In embodiments, the overall temperature reduction is a result of the temperature being reduced once, twice or three times. In preferred embodiments, the overall temperature reduction is a result of the temperature being reduced once or twice.

[0113] In embodiments, after the reaction time period, the temperature of the mixed fluid stream, comprising the depolymerised lignin mixture ( / reaction mixture) (106) comprising monomeric and / or oligomeric phenolic compounds, is reduced by at least about 100 °C, at least about 125 °C, at least about 150 °C or at least about 175 °C. In embodiments, after the reaction time period, the temperature of the mixed fluid stream is reduced by at most about 275 °C, at most about 300 °C, at most about 325 °C, at most about 350 °C or at most about 400 °C. In embodiments, after the reaction time period, the temperature of the mixed fluid stream is reduced by between about 100 and 400 °C, between about 100 and 375 °C, between about 100 and 350 °C, between about 125 and 325 °C, between about 150 and 300 °C, or between about 175 and 275 °C.

[0114] In embodiments, after the reaction time period, the reduction in temperature of the mixed fluid stream is affected within a time of at most about 1 s, at most about 0.5 s, at most about 0.25 s, at most about 0.1 s, at most about 50 ms, at most about 30 ms, at most about 10 ms or at most about 1 ms.

[0115] In embodiments, after the reaction time period, the reduction in temperature of the mixed fluid stream is affected within a time of between about 0.5 ms and 1 s, between about 0.5 ms and 0.5 s, between about 0.5 ms and 0.25 s, between about 1 ms and 0.25 s, between about 1 ms and 0.1 s, or between about 5 ms and 50 ms.

[0116] In embodiments, Step iii) further comprises, after the reaction time period, reducing the pressure of the mixed fluid stream. In embodiments, the pressure is reduced by at least about 200 bar, at least about 210 bar, at least about 220 bar, at last about 230 bar or at least about 250 bar. In embodiments, the pressure is reduced by at most about 300 bar, at most about 290 bar, at most about 280 bar, at most about 270 bar, at most about 260 bar or at most about 250 bar. In embodiments, the pressure is reduced by between about 200 and 300 bar, about 210 and 290 bar, between about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, or between about 220 and 260 bar. In preferred embodiments, the pressure is reduced to atmospheric pressure.

[0117] It is understood that any and each combination of the above pressure and temperature ranges is also preferred. For example, reducing the temperature of the mixed fluid stream by between about 100 and 400 °C, between about 100 and 375 °C, between about 100 and 350 °C, between about 125 and 325 °C, between about 150 and 300 °C, or between about 175 and 275 °C in combination with reducing the pressure by between about 210 and 290 bar, between about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, or between about 220 and 260 bar after the reaction time period in Step iii) are also specific embodiments of the present invention. In embodiments, the reduction in pressure of the mixed fluid stream is affected within a time of at most about 1 s, at most about 0.5 s, at most about 0.25 s, at most about 0.1 s, at most about 50 ms, at most about 30 ms, at most about 10 ms, or at most about 1 ms. In embodiments, the reduction in the pressure of the mixed fluid stream is affected within a time of at least about 0.5 ms, at least about 1 ms, or at least about 5 ms.

[0118] In embodiments, the reduction in the pressure of the mixed fluid stream is affected within a time of between about 5 ms and 1 s, between about 1 ms and 1 s, between about 0.5 ms and 1 s, between about 0.5 ms and 0.5 s, between about 0.5 ms and 0.25 s, between about 0.5 ms and 0.1 s, or between about 0.5 ms and 50ms. In embodiments, the reduction in the pressure of the mixed fluid stream is affected within a time of between about 1 ms and 0.25 s. In embodiments, the reduction in the pressure of the mixed fluid stream is affected within a time of between about 1 ms and 0.1 s. In embodiments, the reduction in the pressure of the mixed fluid stream is affected within a time of between about 5 ms and 50 ms.

[0119] Typically, the degrees and times of cooling and decompression are carried out simultaneously. This can be achieved for instance by flash evaporation, or in continuous flow setups through the use of a high temperature decompression valve. These degrees and times of simultaneous cooling and decompression can be achieved through any means making use of the Joule-Thompson thermodynamic effect.

[0120] In embodiments, the decompressed and cooled depolymerised lignin mixture (106) is further cooled, typically to room temperature, and / or further decompressed, suitably to atmospheric pressure, after Step iii).

[0121] Typically, the steps of increasing the temperature of the lignin in the first fluid stream (102) in Step iii) through contact with the one or more additional fluid stream via passive mixing and / or lowering the temperature of the mixed fluid stream after the reaction time period are carried out as quickly as possible, as time can be an important factor in the depolymerization of lignin. Lignin may undergo several depolymerization-repolymerization processes on the second and sub-second timescales. Therefore, precise control over the reaction time period may be significant in the context of the present invention. The slower the target temperature, also referred to herein as the reaction temperature, is attained [i.e. the longer the heating time period], the more lignin in the mixed fluid stream may start to react before the target temperature and target pressure are attained, negatively impacting the yields of the desired lignin depolymerization products and the efficiency of the process.

[0122] In embodiments, the depolymerization process of the present invention is efficiently carried out by employing only water (in the presence or absence of base); in particular by employing only water as the dissolving or suspending medium for generating the first fluid stream (102). In particular embodiments, the depolymerization process of the present invention is carried out in the absence of acid. In another particular embodiment, the process is carried out in the absence of a non-base catalyst. More particularly, the process is carried out in the absence of a metal catalyst, thus providing a green lignin depolymerization process. In an embodiment, the metal catalyst comprises a transition metal; in another embodiment the metal catalyst comprises a noble metal; and in another embodiment the metal catalyst comprises Si, Al, Zr, Fe, Pt, Pd, Ni, La, Ce, Mo, Ru or Cu. In another particular embodiment, the depolymerization process is carried out in the absence of a biocatalyst. In another embodiment, the depolymerization process is carried out in the absence of an ionic liquid, such as those comprising an alkylimidazolium (e.g. methylimidazolium) moiety. In another embodiment, the depolymerization process is carried out in the absence of a capping agent. Capping agents are well-known in the field of the present invention as compounds which are able to prevent or minimize the repolymerization of depolymerised lignin. In embodiments wherein a base is used in Step i), the capping agent may be different to the base. In a particular embodiment, the capping agent is an ester. In another embodiment the capping agent is a silylating agent. In another embodiment, the capping agent is a phenol, more particularly a functional group including phenol, more particularly alkylated phenol. In another embodiment, the capping agent is a cresol, more particularly p-cresol.

[0123] Surprisingly, the inventors have found that the manner in which the first fluid stream (102), comprising a lignin solution or suspension comprising lignin, is brought into contact with the one or more additional fluid stream (104) (also referred to as the second fluid stream), comprising supercritical water or subcritical water, to generate a mixed fluid stream in Step iii), may have a substantial impact on the lignin depolymerisation process. Surprisingly, the inventors have found that passive mixing (including turbulent mixing) between the first fluid stream (102) and the one or more (second) fluid stream (104) to generate a mixed fluid stream wherein the mixed fluid stream is maintained under preferred conditions of temperature and pressure for the reaction time period as disclosed above, may substantially improve the yields of small molecule monomers and oligomers resulting from lignin depolymerisation compared to methods disclosed in the prior art. In a preferred embodiment, the passive mixing comprises turbulent mixing. In a particularly preferred embodiment, the turbulent mixing comprises vortex mixing, which provides the highest yields of monomers and oligomers.

[0124] Vortex mixing comprises generating at least one vortex when the first fluid stream (102) is brought into contact with the at least one additional fluid stream (104). In preferred embodiments, the vortex mixing comprises generating two or more vortices. In embodiments wherein two or more vortices are generated, any two adjacent vortices may be co-rotary vortices or counter- rotary vortices. In preferred embodiments, any two adjacent vortices are counter- rotary vortices.

[0125] Without wishing to be bound by theory, it is understood that turbulent mixing, in particular vortex mixing, surprisingly improves the efficiency of the depolymerisation of lignin at least by improving the material homogeneity of the mixed fluid stream. Vortex mixing may also result in more homogenous heat distribution in the mixed fluid stream which is significant especially when the streams are at different initial temperatures and given the very short reaction time period, also described as the reaction time. As such, the temperature of the lignin is raised to the desired reaction temperature in the mixed fluid stream more quickly and evenly, affording greater control over delivering heat to the lignin and the reaction times. Additionally, vortex mixing may break up, or significantly reduce in size, any lignin particles or particulate agglomerates from the first fluid stream (102). Lignin contained within the bulk lignin particles or agglomerates may experience lower localised temperatures than the average temperature of the mixed fluid stream and less or no exposure to supercritical water or subcritical water than lignin either dissolved in solution or at the surface of suspended particles or particulate agglomerates, leading to formation of char. Additionally, lignin dissolved in solution or at the surface of particles or particulate agglomerates may experience higher localised temperatures than the average temperature of the mixed fluid stream leading to the formation of low-value gaseous products. Turbulent mixing, particularly vortex mixing, may act to break up lignin particles or particulate agglomerates such that the variation in temperature and exposure to supercritical water or subcritical water experienced by the lignin in the mixed fluid stream is substantially reduced (i.e. the homogeneity of overall reaction conditions experienced by the lignin is greatly improved). This affords greater control over the reaction conditions, including temperature, pressure and reaction time leading to reduced formation of char and low-value gaseous products and concurrently leading to higher conversion of lignin to high-value phenolic monomers and oligomers. This surprising technical benefit allows the desired products to be obtained in yields substantially above those found in the prior art.

[0126] To achieve effective turbulent or vortex mixing between the first fluid stream (102) and the one or more additional fluid stream (104), the relative linear velocities and mass flow rates of the separate streams may be selected accordingly. For the purpose of explanation, the following discussion is limited to embodiments in which a single tangential segment (207) (see Fig. 2) is used to create a single mixing vortex. It is possible using the knowledge and abilities of the skilled person combined with the teaching described herein to apply the same principles to work embodiments wherein more than one tangential segment (207) is used to generate more than one mixing vortex. The embodiments described relate to continuous flow embodiments and may be combined with other embodiments described herein. In embodiments, the ratio of the linear flow velocity of the one or more additional fluid stream (104) (also referred to as the tangential linear flow velocity) to the linear flow velocity of the first fluid stream (102) through the central flow channel (205) (also referred to as the central linear flow velocity) is greater than about 1. Typically, the ratio is at least about 5, at least about 10, at least about 15, at least about 20, at least about 30, or at least about 50. Typically, the ratio is at most about 60, at most about 70, at most about 80, at most about 90 or at most about 100. In a more preferred embodiment, the ratio is about 20. In another more preferred embodiment, the ratio is about 17.5.

[0127] The minimum mass flow of each of the one or more additional fluid stream (104) must be sufficient to create effective high velocity turbulence or vortex mixing when combined with the first fluid stream (102) and may be relatively higher than the mass flow of said first fluid stream (102). In embodiments, the ratio of the mass flow of all of the one or more additional fluid stream (104) combined (also referred to as the tangential mass flow) to the mass flow of the first fluid stream (102) (also referred to as the central input mass flow) is at least about 1 :1 , at least about 5:1 , at least about 10:1, at least about 15:1 , or at least about 20:1. Typically, the ratio is at least about 1 :1 , at least about 2:1 , at least about 3:1 , at least about 4:1 , or at least about 5:1. In embodiments, the ratio is at most about 5:1 , at most about 10:1, at most about 15:1 , at most about 20:1 , or at most about 25:1. Typically, the ratio is at most about 5:1 or at most about 10:1.. In preferred embodiments the ratio is between about 1 :1 and 3:1 , between about 1 :1 and 5:1 , between about 1 :1 and 10:1 , between about 1 :1 and 15:1 , between about 5:1 and 10:1 , or between about 5:1 and 15:1. In embodiments, the ratio is about 1 :1 , about 2:1 , about 3:1 , about 4:1 , about 5:1 , about 6:1 or about 7:1. In a preferred embodiment, the ratio is about 2:1.

[0128] In embodiments wherein at least a second vortex is desired, the mass flow of the one or more additional fluid stream (104) must be high enough to induce formation of said second vortex. In embodiments wherein the second vortex is counter- rotary with respect to the first vortex, the mass flow used to generate the second vortex is higher than in embodiments wherein the second vortex is co-rotary with respect to the first vortex. The means to calculate such mass flows to generate adjacent counter- rotary vortices or adjacent co-rotary vortices are known to the skilled person using knowledge of the parameters used to generate the first vortex.

[0129] In embodiments, the ratio of the mass flow through each channel (208) (which may be referred to as a tangential fluid flow channel (208); see below) of a tangential segment (207), providing the one or more additional fluid stream (104), to the tangential fluid flow channel cross section area (116) is at least about 1 , at least about 2, at least about 3, at least about 4, at least about 5, at least about 10, at least about 20, at least about 30, at least about 40, at least about 50 or at least about 60, in units of kg per hr per mm2. In preferred embodiments, the ratio is about 2, about 3, about 4, about 5 or about 6, in units of kg per hr per mm2. In more preferred embodiments, the ratio is about 2, about 2.04, about 6, or about 6.37, in units of kg per hr per mm2.

[0130] In embodiments, the ratio of the mass flow through the first fluid stream inlet (201) to the central flow channel cross section area (119) is at least about 0.01 , at least about 1 , at least about 5, at least about 10, at least about 20, at least about 30, at least about 40, at least about 50, at least about 60, at least about 70, at least about 80, or at least about 90, in units of kg per hr per mm2. In preferred embodiments, the ratio is about 0.5, about 1 , about 1.5, about 2 or about 3, in units of kg per hr per mm2. In more preferred embodiments, the ratio is about 0.41 or about 1.27, in units of kg per hr per mm2.

[0131] In Step iv) of the first aspect of the invention, the monomeric and / or oligomeric compounds from the depolymerised lignin mixture (106) are isolated and optionally purified. In an embodiment, Step iv) comprises isolating from the depolymerised lignin mixture (106) a product mixture which has a higher amount of the oligomeric and / or monomeric phenolic compounds than the depolymerised lignin mixture (106). In an embodiment, the product mixture comprises not less than about 5%, not less than about 10%, not less than about 15%, not less than about 20%, not less than about 25%, not less than about 30%, not less than about 35%, not less than about 40%, not less than about 45%, or not less than about 50% by weight of oligomeric and / or monomeric phenolic compounds, or typically of monomeric phenolic compounds, with respect to the total weight of the isolated product mixture. In an embodiment, the isolated product mixture is an oil, and more particularly it is the first phenolic oil described hereinafter. Oligomeric and / or monomeric phenolic compounds in the isolated product mixture can generally be isolated or quantified by standard purification methods known to the skilled person, such as those described in the experimental section hereinafter.

[0132] In embodiments, if the depolymerised lignin mixture (106) does not comprise a first liquid phase and a first solid phase, an acid may be added to the depolymerised lignin mixture (106) to obtain a first liquid phase and a first solid phase (also known as the acidified product - see Fig. 5). In further embodiments, the first solid phase is separated from the first liquid phase, for example the first liquid phase and the first solid phase may be separated by filtering the acidified product. In yet further embodiments, the first solid phase and the first liquid phase are extracted with organic solvent to obtain first and second organic phases comprising the oligomeric and / or monomeric phenolic compounds respectively.

[0133] The acid is typically an organic acid, such as carbonic acid, or a mineral acid, such as sulphuric acid. The acid is typically added dropwise to the decompressed and cooled depolymerised lignin mixture (106). The acid is typically added to the decompressed and cooled depolymerised mixture until the pH of said depolymerised mixture is between 1 to 4, typically about 2.

[0134] In an embodiment, where the decompressed and cooled depolymerised lignin mixture (106) resulting from Step iii) already contains a first liquid phase and a first solid phase, the addition of acid in Step iv) may be omitted. The decompressed and cooled depolymerised lignin mixture (106) resulting from Step iii) typically already contains a first liquid phase and a first solid phase when the pH of the mixture is acidic, i.e. under pH 7, for instance, when it is at pH 6 or lower. This may be the case where a base is not used in the process.

[0135] The first liquid phase and the first solid phase are typically then separated, suitably by filtration or centrifugation followed by removal of the first liquid phase. In a preferred embodiment, in order to further purify the phenolic oligomers and / or phenolic monomers, the separated first liquid phase is extracted with an organic solvent, such as ethyl acetate or DCM to obtain a first organic phase and a final aqueous phase. It is the first organic phase which contains the phenolic oligomers and / or phenolic monomers. The first organic phase can be evaporated and / or dried to remove the organic solvent therefrom, which typically results in a light oil enriched with phenolic oligomers and / or phenolic monomers (herein also referred to as the first phenolic oil).

[0136] In a preferred embodiment, the extraction of the first liquid phase is repeated, for example once, twice or three times, or more than three times. The amount of phenolic monomers observed in the first extraction may be higher than that observed in subsequent extractions. In an embodiment, the separated first solid phase (cake) is washed with water, for example, once, twice or three or more times. The optionally washed separated first solid phase may be extracted with an organic solvent, such as ethyl acetate or DCM to obtain a second organic phase and a final solid phase. The extraction can be repeated, for example once, twice or three times, or more than three times. The second organic phase may be evaporated and / or dried to remove the organic solvent therefrom to obtain a second phenolic oil (also referred to as the heavy oil). The second phenolic oil may contain phenolic oligomers and / or phenolic monomers to a lesser degree and in a more impure form as compared to the first phenolic oil, but may nevertheless be further processed to isolate and optionally purify the phenolic oligomers and / or phenolic monomers, therein. The first phenolic oil may be combined with the second phenolic oil.

[0137] The further processing of the first solid phase is typically not carried out when no base has been added to the solution or suspension of the invention. The final solid phase typically contains char including residual lignin produced during the process of the invention.

[0138] The process of the first aspect may be a batch process or a continuous flow process. In a most preferred embodiment, the process of the first aspect is a continuous flow process, which is advantageously suited to the implementation of the process of the invention in an industrial setting. Flow rates of the first fluid stream (102) and the one or more additional fluid streams (104) can be adjusted by the person skilled in the art, such that the desired temperature increase, maintenance and / or decrease times are achieved within the timeframes mentioned hereinabove. Similarly, the temperature and pressure of the first fluid stream (102) and the one or more additional fluid streams (104) can be adjusted by the person skilled in the art.

[0139] In preferred continuous flow embodiments, the first fluid stream (102) is prepared in a tank or reservoir, with means for fluid contact between said tank or reservoir and a reactor (200). In embodiments, the one or more additional fluid streams (104) is supplied to the reactor from another tank or reservoir with means for fluid contact with the reactor. The supercritical water or subcritical water may initially not be in a supercritical or subcritical state in the water tank or reservoir; rather it may be present at a lower temperature and / or lower pressure initially and heated and pressurised to a supercritical or subcritical state as it is pumped towards the reactor (200), for instance by the use of heat exchangers or heaters. If necessary, at least a third or fourth or other pump may be used to pump the supercritical water or subcritical water towards and into the reactor.

[0140] A second aspect of the invention provides a passive mixing reactor suitable for the depolymerisation of lignin using the process as described herein, comprising:

[0141] a first fluid stream inlet at a first end suitable for injection of a first fluid stream;

[0142] a fluid outlet (203) at a second end (204) suitable for ejection of a reaction mixture; a central flow channel, comprising a longitudinal axis, and extending between the first end and the second end (204) enabling fluid communication between the first fluid stream inlet and the fluid outlet (203); and

[0143] one or more tangential segments arranged between the first end and the second end (204), each of the one or more tangential segments comprising one or more tangential fluid flow channels extending between a channel inlet and the central flow channel to provide fluid communication between the one or more tangential fluid stream inlets and the central flow channel, wherein each of the channel inlets is suitable for receiving one or more additional fluid stream for delivery to the central flow channel; wherein, in use,

[0144] the one or more tangential fluid flow channels is oriented for generating turbulent mixing between the first fluid stream travelling along the central flow channel and the one or more additional fluid stream entering the central flow channel from the one or more tangential fluid flow channels.

[0145] In embodiments, the passive mixing reactor (200) is followed by a reaction tube equipped with a means for affecting sharp cooling and decompression of the depolymerised lignin mixture (106) once it has been ejected through the fluid outlet (203) and passed through the reaction tube. In an embodiment, the means for affecting sharp cooling and decompression of the depolymerised lignin mixture (106) is a high temperature decompression valve.

[0146] In embodiments, the central flow channel (205) of the passive mixing reactor (200) has a length of at least about 3 mm, at least about 5 mm, at least about 10 mm, at least about 30 mm, at least about 50 mm, at least about 100 mm, at least about 300 mm, at least about 500 mm, or at least about 750 mm. In embodiments, the central flow channel (205) has a length of at most about 3000 mm, at most about 2500 mm, at most about 2000 mm, at most about 1500 mm, or at most about 1000 mm. In embodiments, the central flow channel (205) has a length of between about 3 mm and 3000 mm, between about 5 mm and 2500 mm, between about 10 mm and 2000 mm, between about 50 mm and 1500 mm, or between about 100 mm and 1000 mm. In embodiments, the central flow channel length is about 10 mm, about 20 mm, about 30 mm, about 40 mm, or about 50 mm. In an embodiment, the central flow channel length is about 30 mm. In embodiments, the central flow channel (205) has a generally circular cross section.

[0147] In embodiments, the central flow channel (205) of the passive mixing reactor (200) has a length of between about 10 mm and 50 mm, between about 20 mm and 40 mm, or between about 25 mm and 35 mm.

[0148] In embodiments, the central flow channel (205) has a diameter (119) of at least about 3 mm, at least about 4 mm, at least about 5 mm, at least about 10 mm, at least about 20 mm, at least about 30 mm, at least about 50 mm, at least about 70 mm, or at least about 100 mm. In embodiments, the central flow channel (205) has a diameter (119) of at most about 200 mm, at most about 150 mm, at most about 140 mm, at most about 130 mm, or at most about 120 mm. In embodiments, the central flow channel (205) has a diameter (119) of between about 5 mm and 200 mm, between about 50 mm and 150 mm, between about 70 mm and 200 mm, between about 70 mm and 140 mm, between about 100 mm and 150 mm or between about 30 mm and 140 mm. In embodiments, the central flow channel (205) has a diameter (119) of about 3 mm, about 4 mm, about 5 mm, about 10 mm, about 20 mm, about 30 mm, about 50 mm, about 70 mm, or about 100 mm.

[0149] In embodiments, the central flow channel (205) of the passive mixing reactor (200) has a diameter of between about 1 mm and 10 mm, between about 2 mm and 9 mm, between about 3 mm and 8 mm between about 4 mm and 7 mm, or between about 4 mm and 6 mm.

[0150] In embodiments, the one or more tangential fluid flow channel (208) has a diameter (116) of at least about 0.5 mm, at least about 1 mm, at least about 2 mm, at least about 3 mm, at least about 5 mm, at least about 10 mm, at least about 15 mm, at least about 20 mm, or at least about 35 mm. In embodiments, the one or more tangential fluid flow channel (208) has a diameter (116) of at most about 60 mm, at most about 50 mm, or at most about 40 mm. In embodiments, the one or more tangential fluid flow channel (208) has a diameter (116) of about 0.5 mm, about 1 mm, about 2 mm, about 3 mm, about 5 mm, about 10 mm, about 15 mm, about 20 mm, or about 35 mm. In embodiments, the one or more tangential fluid flow channel (208) has the same diameter as the channel inlet.

[0151] In embodiments, the one or more tangential fluid flow channel (208) has a diameter (116) of between about 0.5 mm and 1.5 mm, between about 0.6 mm and 1.4 mm, between about 0.7 mm and 1.3 mm between about 0.8 mm and 1.2 mm, or between about 0.9 mm and 1.1 mm.

[0152] In embodiments, the one or more tangential fluid flow channel (208) and / or the channel inlet has a generally circular cross section.

[0153] In embodiments, the combined cross-sectional area of all of the one or more tangential fluid flow channels (208) in all tangential segments is at most about half, at most about a quarter, at most about a fifth, or at most about a tenth of the cross-sectional area of the central flow channel (205).

[0154] In embodiments, each of the one or more tangential segments (207) comprises one tangential fluid flow channel (208). In preferred embodiments, the one or more tangential segments (207) comprises more than one tangential fluid flow channel (208). In embodiments, the one or more tangential segments (207) comprises two tangential fluid flow channels (208). In more preferred embodiments, each of the one or more tangential segments (207) comprises at least two, at least three, at least four, at least five, or at least six tangential fluid flow channels (208).

[0155] In embodiments, the passive mixing reactor (200) comprises one tangential segment (207). In embodiments, the passive mixing reactor (200) comprises more than one tangential segment (207). In embodiments, the passive mixing reactor (200) comprises two or more tangential segments (207), or three or more tangential segments (207).

[0156] In embodiments, each tangential segment (207) is used for injecting an additional fluid stream (104) into the central flow channel (205). Where a tangential segment (207) comprises more than one tangential fluid flow channel (208), the additional fluid stream (104) is injected using all of the more than one tangential fluid flow channels (208). In embodiments, each tangential fluid flow channel (208) is orientated such that, in use, injection of an additional fluid stream (104) through each tangential segment (207) generates a vortex within the central flow channel (205) on mixing with a first fluid stream (102). In embodiments, each tangential fluid flow channel (208) is orientated such that, in use, injection of an additional fluid stream (104) through the at least one tangential fluid flow channel (208) generates a vortex within the central flow channel (205) on mixing with a first fluid stream (102).

[0157] In embodiments, any two tangential segments (207) are spaced apart sufficiently such that any two vortices formed by the injection of additional fluid streams (104) into the central flow channel (205) through the tangential segments (207) and mixed with the first fluid stream (102) are sufficiently separated in space to enable the desired vortex mixing effect. For example, if two adjacent counterrotary vortices are desired, sufficient spacing between the corresponding adjacent tangential segments (207) may enable efficient reversal of the rotation direction of the mixed fluid stream, resulting from formation of a first vortex, to generate a second vortex. In embodiments, a ratio of the distance (212) between any two adjacent tangential segments (207), as measured between any two corresponding points in each of the two adjacent tangential segments (207), to the diameter (119) of the central flow channel (205) is at least about 0.1 :1 , at least about 0.2:1 , at least about 0.5:1 , at least about 1 :1 , at least about 1.5:1, or at least about 2:1. In embodiments, the ratio is at most about 2:1 , at most about 2.5:1 , at most about 3:1 , at most about 3.5:1 , at most about 4:1 or at most about 5:1. In embodiments, the ratio is between about 0.1 :1 and 4:1 , between about 0.2:1 and 3.5:1 , between about 0.5:1 and 3:1 , between about 1 :1 and 2.5:1 , between about 1.5:1 and 2:1 , or between about 2.5:1 and 3:1. In preferred embodiments the ratio is about 1 .5:1 , 2:1 , 2.5:1 , 2.7:1 , 3:1 , 3.5 or 4:1.

[0158] In embodiments, each of the one or more tangential fluid flow channels (208) intersects the central flow channel (205) predominantly orthogonally with respect to the longitudinal axis (206) of the central flow channel (205). Each of the one or more tangential fluid flow channels (208) may be characterised by a pitch angle measured relative to a central flow axis (214) of the tangential fluid flow channel (208) at the point of intersection with the central flow channel (205) wherein the pitch angle is the angle between a line defined by the central flow axis (214) of the tangential fluid flow channel (208) and the longitudinal axis (206) of the central flow channel (205) extending between the first end (202) of the passive mixing reactor (200) and the tangential fluid flow channel (208).

[0159] Variation of the pitch angle enables alteration of the interaction between one more additional fluid streams (104) and the first fluid stream (102) in the central flow channel (205) at the point of intersection. This enables tuning the degree of turbulence upon mixing in the central flow channel (205). In embodiments, the pitch angle is about 90°. In other embodiments, the pitch angle is greater than about 90°. In preferred embodiments, the pitch angle is less than about 90°. In embodiments, the pitch angle at most about 85°, at most about 80°°, at most about 75°, at most about 70°, at most about 65°, at most about 60°, at most about 55°, at most about 50°, at most about 45°, at most about 40°, at most about 35°, at most 30° degrees, at most about 25°, at most about 20°, at most about 15°, at most about 10°, or at most about 5°. In embodiments, the pitch angle is between about 5° and 85°, between about 10° and 80°, between about 15° and 75°, between about 20° and 70°, between about 25° and 65°, or between about 30° and 60°. In embodiments where the pitch angle is less than about 90° the tangential fluid flow channel (208) has a flow component colinear with the flow direction of the central flow channel (205). In embodiments where the pitch angle is greater than about 90° the tangential fluid flow channel (208) has a flow component anti-linear with the flow direction of the central flow channel (205).

[0160] In embodiments, each tangential fluid flow channel (208) has its own unique pitch angle such that a first tangential fluid flow channel (208) has a different pitch angle to a second tangential fluid flow channel (208) within the same tangential segment (207). In preferred embodiments, each tangential fluid flow channel (208) within the same tangential segment (207) has the same pitch angle, which may facilitate well-defined vortex formation.

[0161] In embodiments, wherein the one or more tangential segments (207) comprises at least two tangential fluid flow channels (208), each tangential fluid flow channel (208) is characterised by an offset angle (215) relative to an adjacent tangential fluid flow channel (208) within the same tangential segment (207). The offset angle (215) is a radial angle in a plane orthogonal to the longitudinal axis (206) of the central flow channel (205); optionally wherein the angle (215) is measured in a clockwise direction when viewed along the longitudinal axis (206) of the central flow channel (205) from the first end (202) I first fluid stream inlet (201). The offset angle (215) may be between about 0° and 180°, between about 0° and 90°, or between about 90° and 180°. The offset angle (215) may be between about 20° and 40°, between about 35° and 55°, between about 50° and 70°, between about 50° and 80°, between about 80° and 100°, between about 110° and 130°, or between about 170° and 180°. The offset angle (215) between adjacent tangential fluid flow channels (208) may be about 30°, about 45°, about 60°, about 72°, about 90°, about 120°, or about 180°. In embodiments, the offset angle (215) of each tangential fluid flow channel (208) in the same tangential segment (207) is unique and has a magnitude which is either the same or different to the offset angle (215) of each other tangential fluid flow channel (208) in the same tangential segment (207).

[0162] In embodiments, the one or more tangential fluid flow channels (208) within each one tangential segment (207) are configured to generate, in use, one vortex within the central flow channel (205) upon mixing of the one or more additional fluid stream (104) with the first fluid stream (102). In further embodiments, at least a first tangential segment (207) and a second tangential segment (207) each comprise at least one tangential fluid flow channel (208), wherein, in use, the at least one tangential fluid flow channels (208) of the first tangential segment (207) are configured to generate a first vortex within the central flow channel (205), and the at least one tangential fluid flow channels (208) of the second tangential segment (207) are configured to generate a second vortex within the central flow channel (205).

[0163] In embodiments, each of the one or more tangential fluid flow channels (208) is characterised by a tangential angle measured relative to a radial axis (217) of the central flow channel (205) at the point of intersection with the central flow channel (205) and extending between said radial axis (217) and the central flow axis (214) of the tangential fluid flow channel (208). When the tangential fluid flow channel (208) is oriented in a generally clockwise direction when viewed along the longitudinal axis (206) of the central flow channel (205) from the first end (202), the tangential angle is between about 0° (radially aligned) and 90° (tangentially aligned); and when the tangential fluid flow channel (208) is oriented in a generally counter-clockwise direction when viewed along the longitudinal axis (206) of the central flow channel (205) from the first end (202), the tangential angle is between about 0° (radially aligned) and -90° (tangentially aligned). In embodiments, the tangential angle is at least about 1°, at least about 10°, at least about 20°, at least about 30°, at least about 40°, or at least about 50°. In further embodiments, the tangential angle is less than about 90°, less than about 80°, less than about 70°, less than about 60°, or less than about 50°. In embodiments, the tangential angle is between about 1° and 89°, between about 5° and 85°, between about 10° and 80°, between about 20° and 70°, between about 30° and 60°, or between about 40° and 50°. In embodiments, the tangential angle is at least about -1°, at least about -10°, at least about -20°, at least about -30°, at least about -40°, or at least about -50°. In embodiments, the tangential angle is less than about -90°, less than about -80°, less than about -70°, less than about -60°, or less than about -50°. In embodiments, the tangential angle is between about -1° and -89°, between about -5° and -85°, between about -10° and -80°, between about -20° and -70°, between about -30° and -60°, or between about -40° and -50°.

[0164] In embodiments, the passive mixing reactor (200) comprises at least a first and a second tangential segment (207), and wherein the at least one tangential fluid flow channel (208) of the first tangential segment (207) is oriented at a negative (counter-clockwise) tangential angle, and the at least one tangential fluid flow channel (208) of the second tangential segment (207) is oriented at a negative (counter-clockwise) tangential angle. In embodiments, the passive mixing reactor (200) comprises at least a first and a second tangential segment (207), and wherein the at least one tangential fluid flow channel (208) of the first tangential segment (207) is oriented at a positive (clockwise) tangential angle, and the at least one tangential fluid flow channel (208) of the second tangential segment (207) is oriented at a positive (clockwise) tangential angle. In embodiments, the passive mixing reactor (200) comprises at least a first and a second tangential segment (207), and wherein the at least one tangential fluid flow channel (208) of the first tangential segment (207) is oriented at a negative (counter-clockwise) tangential angle, and the at least one tangential fluid flow channel (208) of the second tangential segment (207) is oriented at a positive (clockwise) tangential angle, or vice versa. In embodiments, the passive mixing reactor (200) comprises at least a first and a second tangential segment (207), and wherein the at least one tangential fluid flow channels (208) of the first tangential segment (207) and the at least one tangential fluid flow channels (208) of the second tangential segment (207) are orientated, in use, for generation of two co-rotary vortices or two counter- rotary vortices.

[0165] In a third aspect of the invention there is provided a use (300) of the passive mixing reactor (200) as described herein in the process as described herein. For example, in embodiments of the use (300), the first fluid stream (102) comprises a lignin solution or suspension comprising lignin. In further embodiments, each additional fluid stream (104) comprises supercritical water or subcritical water.

[0166] In a fourth aspect of the invention there is provided a method for the depolymerisation of lignin, the method comprising:

[0167] providing the passive mixing reactor as described herein;

[0168] providing a first fluid stream comprising a lignin solution or suspension comprising

[0169] lignin through the central flow channel; and

[0170] providing supercritical water or subcritical water as one or more additional fluid streams through the one or more tangential fluid flow channels, to obtain a depolymerised lignin mixture comprising monomeric and / or oligomeric phenolic compounds.

[0171] In embodiments of the fourth aspect, the method (400) further comprises performing the steps of the process as described herein.

[0172] In a fifth aspect of the invention there is provided a product (500) comprising at least one phenolic monomer or oligomer obtained according to the process as described herein, the use (300) as described herein, or the method (400) as described herein.

[0173] The product (500) may be an oil, which may comprise between about 5% w / w and 50% w / w phenolic monomers with respect to the total weight of the product (500), or in particular, of the oil. The product (500) may comprise phenolic oligomers. In another embodiment, said weight of phenolic monomers is between about 5% w / w and 25% w / w, or between about 5% w / w and 20% w / w with respect to the total weight of the product (500). In another embodiment said weight of phenolic monomers is between about 10% w / w and 50%, between about 10% w / w and 25% w / w, or between about 10% w / w and 20% w / w with respect to the total weight of the product (500). In another embodiment said weight of phenolic monomers is between about 15% w / w and 50% w / w, between about 15% w / w and 25% w / w, or between 15% w / w and 20% w / w with respect to the total weight of the product (500). The expression "weight of phenolic monomers" refers to the sum of all phenolic monomers in the product (500), in particular in the oil. In an embodiment, the product (500), or in particular the oil, may comprise one, two, three, four or five or more than five phenolic monomers selected from at least guaiacol, creosol, vanillin, acetovanillone, homovanillic acid, syringol and syringaldehyde, and the combined weight of said two, three, four or five or more than five phenolic monomers with respect to the total weight of the product (500), or in particular the oil, is any of those specified above. In another embodiment, any of the phenolic oligomers and / or phenolic monomers is isolated from the phenolic oil by standard purification techniques known to the skilled person. The present invention also refers to the use (501) of the product (500), which may involve the use of the oil or any mixture of the monomers or any isolated monomers, in cosmetic or food products.

[0174] The invention is further illustrated by the following non-limiting examples.

[0175] EXAMPLES

[0176] Although particular embodiments of the invention have been disclosed herein in detail, this has been done by way of example and for the purposes of illustration only. The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims, which follow. It is contemplated by the inventors that various substitutions, alterations, and modifications may be made to the invention without departing from the spirit and scope of the invention as defined by the claims.

[0177] Example 1

[0178] Materials

[0179] Black liquor was provided by an industrial partner located in Finland in the form of a solution containing 40% w / w total solids (wherein the solution contained 13% w / w lignin) which was shipped by air and upon arrival stored in a container in oxygen free atmosphere in a freezer. Distilled water was used as the reaction medium to run the experiments.

[0180] Method

[0181] The depolymerisation was carried out using the following general reaction conditions (i.e. the conditions following mixing of the first fluid stream comprising a lignin solution or suspension comprising lignin and the one or more additional fluid stream comprising supercritical water or subcritical water):

[0182] • Reaction temperature: 385 °C

[0183] • Reaction pressure: 260 bar (both lignin and water streams are provided at this pressure in Experiments 1 to 3 below)

[0184] • Reaction time: 0.5 - 1.5 seconds

[0185] • Lignin source (in the form of black liquor) total solids concentration: 10 - 40% w / w

[0186] • Ratio of the mass flow of all supercritical water streams to the first fluid stream: 2:1

[0187] • Flow rate of first fluid stream: 8 kg / hr

[0188] • Flow rate of all supercritical water stream: 16 kg / hr

[0189] • Experiment duration: 30-60 minutes The reaction conditions were chosen based on best performing prior art conditions published in academic journals (Abad-Fernandez et al., “Kraft lignin depolymerisation in sub- and supercritical water using ultrafast continuous reactors. Optimization and reaction kinetics, j. of supercritical fluids 165 (2020) 104940). Pressurised water at the reaction pressure was pre-heated by three 10 kW electric heaters with temperature control and monitoring at each stage of the heating, reaching 470 °C prior to entering the mixing chamber in Experiments 1 to 3 below.

[0190] Four different mixing chambers were tested:

[0191] • Prior art horizontal % inch tee from Swagelok #SS-400-3 (M01) (see Abad-Fernandez et al., 2020, in particular Fig. 2)

[0192] • Prior art vertical % inch tee from Swagelok #SS-400-3 (M06) (variation of Abad-Fernandez et al., 2020 proposed by authors attempting to reduce clogging)

[0193] • Nova single vortex reactor (M10) (Figure 2)

[0194] • Nova double vortex reactor (M11) (Figure 1) (counter- rotary)

[0195] Vortex Reactor Parameters M10 M11 Central flow channel diameter (119) 5 mm 5 mm Number of tangential segments (207) 1 2 Tangential fluid flow channel diameter (116) 1 mm 1 mm Number of tangential fluid flow channels per segment (207) 5 5

[0196] Pitch Angle 90 ° 90° Tangential Segment 1 : Tangential Angle 90° 90° Tangential Segment 2: Tangential Angle NA -90° Tangential Segment 1 : Offset Angle 72 ° 72 ° Tangential Segment 2: Offset Angle NA 72 ° Distance between the segments (212) NA 13.5 mm Central flow channel length 30 mm 30 mm

[0197]

[0198] The reaction time was controlled by the length of a reaction tube between the mixing chamber and decompression valve. The reaction flow is set by variable frequency drives which operate the high pressure pumps. The reaction pressure was controlled by a high temperature needle valve. The reaction was stopped by rapid decompression immediately after the high temperature needle valve.

[0199] After the reaction time, the pressure and temperature of the mixed fluid stream were immediately reduced to 0 bar and 105 °C, respectively. The temperature was then further reduced to 40 °C.

[0200] Once the reaction was stopped, the depolymerised mixture was processed to recover products. Firstly, the mixture was acidified by concentrated sulphuric acid to reach pH 2. At this pH the mixture split into dispersed solids / precipitant and liquid phases. The mixture was then separated by centrifugation (60 min, 6000 rpm). The liquid phase was recovered carefully by pipette, the solids were washed by diluted sulphuric acid (pH 2) three times and recovered each time by centrifugation as above. Once separated, both solids and liquid were extracted by ethyl acetate three times. The ethyl acetate then was evaporated to obtain recovered products. The product extracted from the liquid using ethyl acetate was called the light oil. The product extracted from the solids using ethyl acetate was called the heavy oil. The residual liquid phase was called the aqueous residue. The residual unextracted solids were called the non-solubles / char.

[0201] Results

[0202] The first experiment was designed to create a base line for a scaled up process based on prior art conditions and then to test the novel vortex mixing solution that offers improved methods for lignin depolymerisation. This step is critical toward industrialising lignin biomass depolymerisation as prior methods are not suitable for industrial scale up.

[0203] For the first set of experiments (Experiment 1), the reaction conditions were fixed at prior art best performing conditions (reaction temperature: 385 °C; reaction pressure: 260 bar; reaction time: 0.5 seconds). Prior art lignin concentration was set very low at 0.3% w / w due to significant clogging issues within the tubing and reactor of the experimental set up. Such low biomass concentrations are not feasible for commercial use and makes the process economically unviable at industrial scales. Minimum lignin concentration suitable for industrial scale up was established at 3% w / w, which corresponds to 10% w / w black liquor total solids concentration. Black liquor was diluted from 40% w / w total solids down to 10% w / w total solids using distilled water. Compared to the prior art, the process was scaled up three times by the flow rate of biomass and ten times by biomass concentration. The total mass flow of lignin was therefore scaled by thirty times compared to the prior art.

[0204] Four mixing chambers were tested using the parameters listed in Table 1 , whilst keeping all other reaction parameters constant. Table 2 summarises the results of the experiments as conversion efficiency, the combined mass of recovered phenolic monomers and oligomers as a percentage of the total mass of lignin starting material used. As can be seen, the nova vortex reactors exhibit substantially improved conversion efficiencies compared to reactors described in the prior art and the use of a double vortex results in substantially improved performance compared to the single vortex.

[0205] Table 1. Experimental parameters for experiments testing various depolymerisation reactors.

[0206] Experiment Reaction Reaction Reaction Lignin Lignin Tangential Number temperature pressure time (biomass) (biomass) mass flow concentration flow rate

[0207] 1 385 °C 260 bar 0.5 s 10% w / w 8 kg / h 16 kg / h 2 385 °C 260 bar 0.5 s 40% w / w 8 kg / h 16 kg / h 3 385 °C 260 bar 1.5 s 40% w / w 8 kg / h 16 kg / h

[0208]

[0209] Table 2. Results of Experiments: conversion efficiencies given in total mass of recovered phenolic monomers and oligomers.

[0210] Experiment Horizontal1Z> tee Vertical1Z> tee Nova Single Nova Double Number Swagelok (M01) Swagelok (M06) Vortex (M10) Vortex (M11) 1 70% 71% 83% 91%

[0211] 2 36% 42% 61% 70%

[0212] 3 58% 63% 87% >99%

[0213]

[0214] The second set of experiments explored higher concentrations of lignin (see Table 1). The design of the vortex mixing chambers was chosen not only to improve mixing but also to avoid clogging which may occur at high solids concentrations. The first experiment above demonstrated that no clogging occurs under said reaction conditions and hence higher concentrations of lignin in the black liquor were investigated. Increasing lignin concentration has a direct impact on the economy of the process allowing more processing of biomass per unit of time at the same cost. Such scaling is not possible in any of the reported prior art. As the lignin concentration was increased by four times in Experiment 2 versus Experiment 1 , the total scaling factor compared to the prior art for Experiment 2 was 120 times (i.e. four times the scaling factor of Experiment 1 , which was 30). In other words, in the same period of time 120 times more lignin was processed in Experiment 2 compared to in the prior art.

[0215] In Experiment 2, the four mixing chambers were tested again using a higher concentration of total solids in the lignin source, whilst keeping all other reaction parameters constant, as shown in Table 1. As can be seen in Table 2, both horizontal and vertical prior art mixing reactors resulted in 36% and 42% conversion rates respectively. The single vortex reactor (M10) exhibited improved conversion of 61% and the dual vortex reactor (M11) achieved a conversion efficiency of 70%. The results showed that no clogging occurred in the case of both vortex reactors. State of the art mixing chambers exhibit a high chance of clogging; one in three experiments had to be terminated for system cleaning. The reduced depolymerisation efficiency of both vortex reactors relative to Experiment 1 was likely due to insufficient reaction time whilst the state of the art reactors suffer both from insufficient mixing and insufficient reaction time.

[0216] Experiment 3 explored the impact of longer reaction times on the depolymerisation of lignin at the same lignin concentration as used in Experiment 2. As prior art experiments only worked with very low concentrations of lignin (at least forty times lower), previously reported reaction times were likely insufficient to process higher amounts of lignin.

[0217] In respect of Experiment 3, the four mixing reactors were tested using the conditions outlined in Table 1. As can be seen in Table 2, both horizontal and vertical prior art mixing chambers resulted in slightly better conversion rates of 58% and 63%, respectively. Single and dual vortex reactors offered improved conversion of 87% and >99%, respectively. Since the same black liquor concentration was used in Experiment 3 as in Experiment 2, similar clogging issues were observed for the state of the art reactors whilst no clogging was observed for vortex reactors.

[0218] Conclusion

[0219] The efficiency of lignin depolymerisation is substantially improved over prior art methods using vortex mixing to mix a fluid stream comprising a lignin source with supercritical water at high temperature and pressure. Not only is the overall efficiency of the process improved, but the process may also be scaled to use higher lignin concentrations to make the process economically viable and suitable for industrial exploitation.

[0220] Embodiments of the invention are additionally described by the following set of clauses:

[0221] 1. A process for the depolymerisation of lignin, the process comprising the steps:

[0222] i) providing a first fluid stream comprising a lignin solution or suspension comprising lignin;

[0223] ii) providing one or more additional fluid stream comprising supercritical water or subcritical water;

[0224] iii) bringing the first fluid stream into contact with the one or more additional fluid stream via passive mixing to generate a mixed fluid stream for a reaction time period in order to obtain a depolymerised lignin mixture comprising monomeric and / or oligomeric phenolic compounds; and

[0225] iv) isolating the monomeric and / or oligomeric phenolic compounds from the depolymerised lignin mixture.

[0226] la. The process according to clause 1 , wherein the reaction time period is between about 20 ms and 30 s, between about 25 ms and 25 s, between about 30 ms and 20 s, between about 30 ms and 10 s, between about 40 ms and 5 s, between about 50 ms and 2 s, between about 250 ms and 1 .5 s, between about 500 ms and 1.5 s, or between about 100 ms and 1 s.

[0227] lb. The process according to clause 1 or clause 1a, wherein the reaction time period is between about 500 ms and 1.5 s.

[0228] lc. The process according to any preceding clause, wherein the reaction time period is about 10 ms, about 20 ms, about 30 ms, about 40 ms, about 50 ms, about 0.1 s, about 0.2 s, about 0.3 s, about 0.4 s, about 0.5 s, about 1 s, about 1.5 s, or about 2 s.

[0229] ld. The process according to any preceding clause, wherein the process comprises an additional step between Step i) and Step iii) comprising bringing the first fluid stream to a pressure substantially the same as the pressure of the one or more additional fluid stream comprising supercritical water or subcritical water to enable mixing between the fluid streams.

[0230] le. The process according to any preceding clause, wherein more than one additional fluid stream comprising supercritical water or subcritical water is provided.

[0231] lf. The process according to any preceding clause, wherein the passive mixing comprises turbulent mixing.

[0232] 2. The process according to clause 1f, wherein the turbulent mixing comprises generating at least one vortex.

[0233] 3. The process according to clause 2, wherein the turbulent mixing comprises generating one vortex.

[0234] 3a. The process according to clause 2, wherein the turbulent mixing comprises generating two or more vortices.

[0235] 3b. The process according to clause 2, wherein the turbulent mixing comprises generating two vortices.

[0236] 4. The process according to clause 3a or clause 3b, wherein any two adjacent vortices are either co-rotary vortices or counter- rotary vortices.

[0237] 4a. The process according to clause 4, wherein any two adjacent vortices are counter- rotary vortices.

[0238] 5. The process according to any preceding clause, wherein the process comprises a preliminary step prior to Step i) of preparing the lignin solution or suspension from a lignin source comprising lignin.

[0239] 6. The process according to clause 5, wherein the lignin source of the preliminary step comprises at most about 99% w / w, at most about 90% w / w, at most about 80% w / w, at most about 60% w / w, at most about 40% w / w, at most about 20% w / w or at most about 10% w / w total solids with respect to the total weight of the lignin source.

[0240] 6a. The process according to clause 5 or clause 6, wherein the lignin source of the preliminary step comprises at most about 40% w / w total solids with respect to the total weight of the lignin source.

[0241] 7. The process according to any of clauses 5 to 6a, wherein the lignin source of the preliminary step comprises at least about 1 % w / w, at least about 2% w / w, at least about 3% w / w, at least about 4% w / w, at least about 5% w / w, at least about 10% w / w, at least about 20% w / w total solids with respect to the total weight of the lignin source.

[0242] 7a. The process according to any of clauses 5 to 7, wherein the lignin source of the preliminary step comprises between about 0.1% w / w and 99% w / w, between about 0.1% w / w and 90% w / w, between about 0.1% w / w and 80% w / w, between about 0.1% w / w and 60% w / w, between about 0.1% w / w and 40% w / w, between about 1% w / w and 99% w / w, between about 1% w / w and 90% w / w, between about 1 % w / w and 80% w / w, between about 1 % w / w and 60% w / w, between about 1 % w / w and 40% w / w, between about 10% w / w and 80% w / w, between about 10% w / w and 60% w / w, between about 10% w / w and 40% w / w, between about 20% w / w and 80% w / w, between about 20% w / w and 60% w / w, or between about 20% w / w and 40% w / w total solids with respect to the total weight of the lignin source.

[0243] 7b. The process according to any of clauses 5 to 7a, wherein the lignin source of the preliminary step comprises between about 10% w / w and 40% w / w total solids with respect to the total weight of the lignin source.

[0244] 8. The process according to any of clauses 5 to 7b, wherein the lignin source of the preliminary step comprises at least about 1% w / w, at least about 2% w / w, at least about 3% w / w, or at least about 5% w / w lignin with respect to the weight of the total solids of the lignin source.

[0245] 9. The process according to any of clauses 5 to 8, wherein the lignin source of the preliminary step comprises at least about 10% w / w or at least about 20% w / w lignin with respect to the weight of the total solids of the lignin source.

[0246] 9a. The process according to any of clauses 5 to 9, wherein the preliminary step prior to Step i) further comprises increasing the temperature of the lignin in the lignin source.

[0247] 10. The process according to any preceding clause, wherein the lignin solution or suspension of Step i) comprises a base.

[0248] 11. The process according to clause 10, wherein the base is or comprises an alkali metal hydroxide or an alkali metal carbonate.

[0249] 12. The process according to any preceding clause, wherein the concentration of lignin in the solution or suspension of Step i) is between about 40% w / w and 0.1% w / w with respect to the total weight of the first fluid stream.

[0250] 12a. The process according to any preceding clause, wherein the concentration of lignin in the solution or suspension of Step i) is between about 35% w / w and 0.2% w / w, between about 30% w / w and 0.5% w / w, between about 25% w / w and 1% w / w, between about 20% w / w and 2% w / w, between about 20% w / w and 5% w / w, or between about 15% w / w and 10% w / w with respect to the total weight of the first fluid stream.

[0251] 13. The process according to any preceding clause, wherein the concentration of lignin in the solution or suspension of Step i) is between about 20% w / w% and 10% w / w with respect to the total weight of the first fluid stream.

[0252] 14. The process according to any preceding clause, wherein the temperature of the lignin in the first fluid stream is increased in Step iii) through contact with the one or more additional fluid stream via passive mixing.

[0253] 14a. The process according to any preceding clause, wherein the temperature of the lignin in the first fluid stream is increased to a temperature of between about 300 and 430 °C in Step iii) through contact with the one or more additional fluid stream via passive mixing.

[0254] 15. The process according to any preceding clause, wherein the temperature of the lignin in the first fluid stream is increased to a temperature of between about 300 and 430 °C, between about 310 and 430 °C, between about 320 and 430 °C, between about 330 and 430 °C, between about 340 and 430 °C, between about 350 and 430 °C, between about 360 and 420 °C, between about 370 and 410 °C, or between about 380 and 400 °C in Step iii) through contact with the one or more additional fluid stream via passive mixing.

[0255] 15a. The process according to any preceding clause, wherein the temperature of the lignin in the first fluid stream is increased to a temperature of between about 375 and 395 °C in Step iii) through contact with the one or more additional fluid stream via passive mixing.

[0256] 15b. The process according to any preceding clause, wherein the temperature of the lignin in the first fluid stream is increased to a temperature of about 385 °C in Step iii) through contact with the one or more additional fluid stream via passive mixing.

[0257] 16. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a temperature of at least about 350 °C.

[0258] 17. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a temperature of at least about 350 °C, at least about 360 °C, at least about 370 °C, at least about 380 °C, at least about 390 °C, at least about 400 °C, at least about 420 °C, at least about 450 °C, at least about 470 °C, at least about 500 °C, at least about 550 °C, at least about 575 °C, at least about 600 °C or at least about 700 °C.

[0259] 17a. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a temperature of between about 350 and 700 °C, between about 375 and 650 °C, between about 400 and 600 °C, between about 425 and 550 °C, between about 450 and 500 °C, between about 460 and 490 °C, between about 460 and 480 °C, or between about 465 and 475 °C.

[0260] 17b. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a temperature of between about 450 and 500 °C.

[0261] 17c. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a temperature of between about 460 and 480 °C.

[0262] 17d. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a temperature of about 470 °C.

[0263] 17e. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a temperature of about 450 °C, about 460 °C, about 470 °C, about 480 °C, about 490 °C, about 500 °C, about 520 °C, about 540 °C, or about 560 °C.

[0264] 17f. The process according to any preceding clause, wherein the one or more additional fluid stream comprising subcritical water of Step ii) is provided at a temperature of at least about 450 °C or at least about 550 °C.

[0265] 18. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a pressure of at least about 200 bar.

[0266] 18a. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a pressure of at least about 210 bar, at least about 220 bar, at least about 230 bar, at least about 240 bar, at least about 250 bar, at least about 260 bar, at least about 270 bar, at least about 280 bar, at least about 290 bar, or at least about 300 bar. 18b. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a pressure of between about 200 and 300 bar, between about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, between about 230 and 270 bar , between about 240 and 270 bar, between about 250 and 270 bar, or between about 220 and 260 bar.

[0267] 18c. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a pressure of between about 250 and 270 bar.

[0268] 18d. The process according to any preceding clause, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a pressure of about 260 bar.

[0269] 18e. The process according to any preceding clause, wherein the first fluid stream of Step i) is provided at a pressure of at least about 200 bar.

[0270] 18f. The process according to any preceding clause, wherein the first fluid stream of Step i) is provided at a pressure of at least about 210 bar, at least about 220 bar, at least about 230 bar, at least about 240 bar, at least about 250 bar, at least about 260 bar, at least about 270 bar, at least about 280 bar, at least about 290 bar, or at least about 300 bar.

[0271] 18g. The process according to any preceding clause, wherein the first fluid stream of Step i) is provided at a pressure of between about 200 and 300 bar, between about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, between about 230 and 270 bar, between about 240 and 270 bar, between about 250 and 270 bar, or between about 220 and 260 bar.

[0272] 18h. The process according to any preceding clause, wherein the first fluid stream of Step i) is provided at a pressure of between about 250 and 270 bar.

[0273] 18i. The process according to any preceding clause, wherein the first fluid stream of Step i) is provided at a pressure of about 260 bar.

[0274] 18j. The process according to any preceding clause, wherein the first fluid stream of Step i) and the one or more additional fluid stream of Step ii) are both provided at a pressure of between about 200 and 300 bar, between about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, between about 230 and 270 bar, between about 240 and 270 bar, between about 250 and 270 bar, or between about 220 and 260 bar. 18k. The process according to any preceding clause, wherein the first fluid stream of Step i) and the one or more additional fluid stream of Step ii) are both provided at a pressure of between about 250 and 270 bar.

[0275] 181. The process according to any preceding clause, wherein the first fluid stream of Step i) and the one or more additional fluid stream of Step ii) are both provided at a pressure of about 260 bar.

[0276] 19. The process according to any of clauses 14 to 181, wherein the temperature increase is affected within a heating time period of at least about 1 ms.

[0277] 19a. The process according to any of clauses 14 to 19, wherein the temperature increase is affected within a heating time period of at least about 2 ms, at least about 5 ms, at least about 10 ms, at least about 25 ms, at least about 50 ms, at least about 75 ms, or at least about 100 ms.

[0278] 19b. The process according to any of clauses 14 to 19a, wherein the temperature increase is affected within a heating time period of at most about 0.5 s, at most about 1 s, at most about 1.5 s, at most about 2 s, at most about 2.5 s, at most about 5 s, at most about 7.5 s, at most about 10 s, at most about 20 s or at most about 30 s.

[0279] 19c. The process according to any of clauses 14 to 19b, wherein the temperature increase is affected within a heating time period of between about 1 ms and 30 s, between about 10 ms and 15 s, between about 20 ms and 10 s, between about 25 ms and 5 s, between about 30 ms and 3 s, between about 40 ms and 2 s, or between about 50 ms and 1.5 s.

[0280] 19d. The process according to any of clauses 14 to 19c, wherein the temperature increase is affected within a heating time period of between about 500 ms and 1.5 s.

[0281] 20. The process according to any preceding clause, wherein Step iii) further comprises, after the predefined heating time period, reducing the temperature of the mixed fluid stream.

[0282] 20a. The process according to clause 20, wherein reducing the temperature of the mixed fluid stream comprises reducing the temperature by between about 100 and 350 °C.

[0283] 20b. The process according to clause 20 or clause 20a, wherein reducing the temperature of the mixed fluid stream comprises reducing the temperature by between about 125 and 325 °C, between about 150 and 300 °C, or between about 175 and 275 °C.

[0284] 21. The process according to any of clauses 20 to 20b, wherein reducing the temperature of the mixed fluid stream is affected within a time of at most about 1 s, at most about 0.5 s, at most about 0.25 s, or at most about 0.1 s. 21a. The process according to any of clauses 20 to 21 , wherein reducing the temperature of the mixed fluid stream is affected within a time of at most about 50 ms, at most about 30 ms, at most about 10 ms, or at most about 1 ms.

[0285] 22. The process according to any of clauses 20 to 21a, wherein reducing the temperature of the mixed fluid stream is affected within a time of between about 0.5 ms and 1 s, between about 0.5 ms and 0.5 s, between about 0.5 ms and 0.25 s, between about 1 ms and 0.25 s, between about 1 ms and 0.1 s, or between about 5 ms and 50 ms.

[0286] 23. The process according to any preceding clause, wherein Step iii) further comprises, after the reaction time period, reducing the pressure of the mixed fluid stream.

[0287] 23a. The process according to clause 23, wherein reducing the pressure of the mixed fluid stream comprises reducing the pressure by between about 200 and 300 bar.

[0288] 23b. The process according to clause 23 or clause 23a, wherein reducing the pressure of the mixed fluid stream comprises reducing the pressure by between about 200 and 300 bar, about 210 and 290 bar, between about 220 and 280 bar, between about 220 and 270 bar, or between about 220 and 260 bar.

[0289] 23c. The process according to any of clauses 23 to 23b, wherein reducing the pressure of the mixed fluid stream is affected within a time of at most about 1 s, at most about 0.5 s, at most about 0.25 s, or at most about 0.1 s.

[0290] 23d. The process according to any of clauses 23 to 23c, wherein reducing the pressure of the mixed fluid stream is affected within a time of at most about 50 ms, at most about 30 ms, at most about 10 ms, or at most about 1 ms.

[0291] 23e. The process according to any of clauses 23 to 23d, wherein reducing the pressure of the mixed fluid stream is affected within a time of between about 5 ms and 1 s, between about 1 ms and 1 s, between about 0.5 ms and 1 s, between about 0.5 ms and 0.5 s, between about 0.5 ms and 0.25 s, between about 0.5 ms and 0.1 s, or between about 0.5 ms and 50ms.

[0292] 24. The process according to any preceding clause, wherein a heterogeneous catalyst is not used during Steps i) to iii).

[0293] 25. The process according to any preceding clause, wherein a capping agent is not used during Steps i) to iii). 26. The process according to any preceding clause, wherein the process is a continuous flow process.

[0294] 27. The process according to any preceding clause, wherein the first fluid stream of Step i) comprises a black liquor or a solution or suspension of Kraft lignin.

[0295] 28. The process according to any preceding clause, wherein Step iv) further comprises:

[0296] if the depolymerised lignin mixture of Step iii) does not comprise a first liquid phase and a first solid phase, adding an acid to the depolymerised lignin mixture to obtain a first liquid phase and a first solid phase.

[0297] 28a. The process according to clause 28, further comprising:

[0298] separating the first solid phase from the first liquid phase, and

[0299] extracting the first liquid phase with an organic solvent to obtain a final organic phase comprising the oligomeric and monomeric phenolic compounds.

[0300] 29. A passive mixing reactor suitable for the depolymerisation of lignin using the process according to any of clauses 1 to 28a, comprising:

[0301] a first fluid stream inlet at a first end suitable for injection of a first fluid stream;

[0302] a fluid outlet at a second end suitable for ejection of a reaction mixture;

[0303] a central flow channel, comprising a longitudinal axis, and extending between the first end and the second end enabling fluid communication between the first fluid stream inlet and the fluid outlet; and

[0304] one or more tangential segments arranged between the first end and the second end, each of the one or more tangential segments comprising one or more tangential fluid flow channels extending between a channel inlet and the central flow channel to provide fluid communication between the one or more tangential fluid stream inlets and the central flow channel, wherein each of the channel inlets is suitable for receiving one or more additional fluid stream for delivery to the central flow channel; wherein, in use,

[0305] the one or more tangential fluid flow channels is oriented for generating turbulent mixing between the first fluid stream travelling along the central flow channel and the one or more additional fluid stream entering the central flow channel from the one or more tangential fluid flow channels.

[0306] 30. The passive mixing reactor according to clause 29, wherein the central flow channel has a length of at least about 3 mm, at least about 5 mm, at least about 10 mm, at least about 30 mm, at least about 50 mm, at least about 100 mm, at least about 300 mm, at least about 500 mm, or at least about 750 mm. 31. The passive mixing reactor according to clause 29 or clause 30, wherein the central flow channel has a length of at most about 3000 mm, at most about 2500 mm, at most about 2000 mm, at most about 1500 mm, or at most about 1000 mm.

[0307] 32. The passive mixing reactor according to any of clauses 29 to 31 , wherein the central flow channel has a length of between about 3 mm and 3000 mm, between about 5 mm and 2500 mm, between about 10 mm and 2000 mm, between about 50 mm and 1500 mm, or between about 100 mm and 1000 mm.

[0308] 33. The passive mixing reactor according to any of clauses 29 to 32, wherein the central flow channel has a generally circular cross section.

[0309] 33a. The passive mixing reactor according to any of clauses 29 to 33, wherein the one or more tangential fluid flow channels has a generally circular cross section.

[0310] 34. The passive mixing reactor according to any of clauses 29 to 33a, wherein the central flow channel has a diameter of at least about 3 mm, at least about 4 mm, at least about 5 mm, at least about 10 mm, at least about 20 mm, at least about 30 mm, at least about 50 mm, at least about 70 mm, or at least about 100 mm.

[0311] 35. The passive mixing reactor according to any of clauses 29 to 34, wherein the central flow channel has a diameter of at most about 200 mm, at most about 150 mm, at most about 140 mm, at most about 130 mm, or at most about 120 mm.

[0312] 36. The passive mixing reactor according to any of clauses 29 to 35, wherein each of the one or more tangential fluid flow channels has a diameter of at least about 0.5 mm, at least about 1 mm, at least about 2 mm, at least about 3 mm, at least about 5 mm, at least about 10 mm, at least about 15 mm, at least about 20 mm, or at least about 35 mm.

[0313] 37. The passive mixing reactor according to any of clauses 29 to 36, wherein each of the tangential fluid flow channels has a diameter of at most about 60 mm, at most about 50 mm, or at most about 40 mm.

[0314] 38. The passive mixing reactor according to any of clauses 29 to 37, wherein the combined cross-sectional area of all of the one or more tangential fluid flow channels is at most about half the cross-sectional area of the central flow channel, at most about a quarter of the cross-sectional area of the central flow channel, at most about a fifth of the cross-sectional area of the central flow channel, or at most about a tenth of the cross-sectional area of the central flow channel. 39. The passive mixing reactor according to any of clauses 29 to 38, wherein each of the one or more tangential segments comprises one tangential fluid flow channel.

[0315] 39a. The passive mixing reactor according to any of clauses 29 to 38, wherein each of the one or more tangential segments comprises two tangential fluid flow channels.

[0316] 39b. The passive mixing reactor according to any of clauses 29 to 38, wherein each of the one or more tangential segments comprises at least two tangential fluid flow channels, at least three tangential fluid flow channels, at least four tangential fluid flow channels, at least five tangential fluid flow channels, or at least six tangential fluid flow channels.

[0317] 40. The passive mixing reactor according to any of clauses 29 to 39b, wherein each tangential fluid flow channel is orientated such that, in use, injection of one or more additional fluid through at least one tangential fluid flow channel is capable of generating a vortex within the central flow channel on mixing with a first fluid stream injected through the first fluid stream inlet.

[0318] 41. The passive mixing reactor according to any of clauses 29 to 40, comprising one tangential segment.

[0319] 42. The passive mixing reactor according to any of clauses 29 to 40, comprising two tangential segments.

[0320] 43. The passive mixing reactor according to any of clauses 29 to 40, comprising two or more tangential segments, or three or more tangential segments.

[0321] 44. The passive mixing reactor according to clause 42 or clause 43, wherein a ratio of the distance between any two adjacent tangential segments, as measured between any two corresponding points in each of the two adjacent tangential segments, to the diameter of the central flow channel is at least about 0.1 :1 , at least about 0.2:1 , at least about 0.5:1 , at least about 1 :1 , at least about 1 .5:1 , or at least about 2:1.

[0322] 44a. The passive mixing reactor according to any of clauses 42 to 44, wherein the ratio of the distance between any two adjacent tangential segments and the diameter of the central flow channel is at most about 4:1 , at most about 3.5:1 , at most about 3:1 , at most about 2.5:1 , or at most about 2:1.

[0323] 44b. The passive mixing reactor according to any of clauses 42 to 44a, wherein the ratio of the distance between any two adjacent tangential segments and the diameter of the central flow channel is between about 0.1 :1 and 4:1 , between about 0.2:1 and 3.5:1 , between about 0.5:1 and 3:1 , between about 1 :1 and 2.5:1 , between about 1 .5:1 and 2:1 , or between about 2.5:1 and 3:1. 45. The passive mixing reactor according to any of clauses 29 to 44b, wherein each of the one or more tangential fluid flow channels intersects the central flow channel predominantly orthogonally with respect to the longitudinal axis of the central flow channel.

[0324] 46. The passive mixing reactor according to any of clauses 29 to 45, wherein each of the one or more tangential fluid flow channels is characterised by a pitch angle measured relative to a central flow axis of the tangential fluid flow channel at the point of intersection with the central flow channel.

[0325] 47. The passive mixing reactor according to clause 46, wherein the pitch angle of the tangential fluid flow channel is the angle between a line defined by a central flow axis of the tangential fluid flow channel and the longitudinal axis of the central flow channel extending between the first end of the passive mixing reactor and the tangential fluid flow channel.

[0326] 48. The passive mixing reactor according to clause 46 or clause 47, wherein the pitch angle is about 90°.

[0327] 48a. The passive mixing reactor according to clause 46 or clause 47, wherein the pitch angle is less than about 90°.

[0328] 48b. The passive mixing reactor according to clause 46 or clause 47, wherein the pitch angle is greater than about 90°.

[0329] 49. The passive mixing reactor according to any of clauses 46 to 48a, wherein the pitch angle is at most about 85°, at most about 80°, at most about 75°, at most about 70°, at most about 65°, at most about 60°, at most about 55°, at most about 50°, at most about 45°, at most about 40°, at most about 35°, at most about 30°, at most about 25°, at most about 20°, at most about 15°, at most about 10°, or at most about 5°.

[0330] 49a. The passive mixing reactor according to any of clauses 46 to 48a, wherein the pitch angle is between about 5° and 85°, between about 10° and 80°, between about 15° and 75°, between about 20° and 70°, between about 25° and 65°, or between about 30° and 60°.

[0331] 50. The passive mixing reactor according to any of clauses 46 to 49a, wherein a first tangential fluid flow channel has a different pitch angle to a second tangential fluid flow channel within the same tangential segment.

[0332] 51. The passive mixing reactor according to any of clauses 44 to 49a, wherein each tangential fluid flow channel within the same tangential segment has the same pitch angle. 52. The passive mixing reactor according to any of clauses 29 to 51 , wherein the one or more tangential segment comprises at least two tangential fluid flow channels, and wherein each tangential fluid flow channel is characterised by an offset angle relative to an adjacent tangential fluid flow channel.

[0333] 53. The passive mixing reactor according to clause 52, wherein the offset angle is a radial angle in a plane orthogonal to the longitudinal axis of the central flow channel; optionally wherein the angle is measured in a clockwise direction when viewed along the longitudinal axis of the central flow channel from the first end I first fluid stream inlet.

[0334] 54. The passive mixing reactor according to clause 52 or clause 53, wherein the offset angle between adjacent tangential fluid flow channels is between about 0° and 180°, e.g. between about 0° and 90°, or between about 90° and 180°.

[0335] 55. The passive mixing reactor according to any of clauses 52 to 54, wherein the offset angle between adjacent tangential fluid flow channels is between about 20° and 40°, between about 35° and 55°, between about 50° and 70°, between about 80° and 100°, between about 110° and 130°, or between about 170° and about 180°.

[0336] 56. The passive mixing reactor according to any of clauses 52 to 55, wherein the offset angle between adjacent tangential fluid flow channels is about 30°, about 45°, about 60°, about 72 °, about 90°, about 120°, or about 180°.

[0337] 57. The passive mixing reactor according to any of clauses 52 to 56, wherein the offset angle of each tangential fluid flow channel in the same tangential segment is unique and has a magnitude which is either the same or different to the offset angle of each other tangential fluid flow channel in the same tangential segment.

[0338] 58. The passive mixing reactor according to any of clauses 52 to 57, wherein the magnitude of the offset angle of each tangential fluid flow channel in the same tangential segment is the same.

[0339] 59. The passive mixing reactor according to any of clauses 29 to 58, wherein the one or more tangential fluid flow channels within each one tangential segment are configured to generate, in use, at least one vortex within the central flow channel upon injection of the one or more additional fluid stream into the first fluid stream.

[0340] 59a. The passive mixing reactor according to any of clauses 29 to 59, wherein the one or more tangential fluid flow channels within each one tangential segment are configured to generate, in use, one vortex within the central flow channel upon injection of the one or more additional fluid stream into the first fluid stream. 60. The passive mixing reactor according to any of clauses 29 to 59a, comprising at least a first tangential segment and a second tangential segment, wherein the first tangential segment and the second tangential segment each comprise at least one tangential fluid flow channel, and wherein, in use, the at least one tangential fluid flow channels of the first tangential segment are configured to generate a first vortex within the central flow channel, and the at least one tangential fluid flow channels of the second tangential segment are configured to generate a second vortex within the central flow channel.

[0341] 61. The passive mixing reactor according to any of clauses 29 to 60, wherein the one or more tangential fluid flow channels of the one or more tangential segment intersect the central flow channel at a tangential angle relative to a radial axis of the central flow channel at the point of intersection, wherein when the tangential fluid flow channel is oriented in a generally clockwise direction when viewed along the longitudinal axis of the central flow channel from the first end I first fluid stream inlet, the tangential angle is between about 0° (radially aligned) and 90° (tangentially aligned); and when the tangential fluid flow channel is oriented in a generally counter-clockwise direction when viewed along the longitudinal axis of the central flow channel from the first end I first fluid stream inlet, the tangential angle is between about 0° (radially aligned) and -90° (tangentially aligned).

[0342] 62. The passive mixing reactor according to any of clauses 29 to 61 , wherein the one or more tangential fluid flow channels of the one or more tangential segment intersect the central flow channel at a tangential angle of at least about 1°, at least about 10°, at least about 20°, at least about 30°, at least about 40°, or at least about 50°.

[0343] 63. The passive mixing reactor according to any of clauses 29 to 62, wherein the one or more tangential fluid flow channels of the one or more tangential segment intersect the central flow channel at a tangential angle of less than about 90°, less than about 80°, less than about 70°, less than about 60°, or less than about 50°.

[0344] 64. The passive mixing reactor according to any of clauses 29 to 63, wherein the one or more tangential fluid flow channels of the one or more tangential segment intersect the central flow channel at a tangential angle of between about 1° and 89°, between about 5° and 85°, between about 10° and 80°, between about 20° and 70°, between about 30° and 60°, or between about 40° and 50°.

[0345] 65. The passive mixing reactor according to any of clauses 29 to 64, wherein the one or more tangential fluid flow channels of the one or more tangential segment intersect the central flow channel at a tangential angle of at least about -1°, at least about -10°, at least about -20°, at least about -30°, at least about -40°, or at least about -50°.

[0346] 66. The passive mixing reactor according to any of clauses 29 to 65, wherein the one or more tangential fluid flow channels of the one or more tangential segment intersect the central flow channel at a tangential angle of less than about -90°, less than about -80°, less than about -70°, less than about -60°, or less than about -50°.

[0347] 67. The passive mixing reactor according to any of clauses 29 to 66, wherein the one or more tangential fluid flow channels of the one or more tangential segment intersect the central flow channel at a tangential angle of between about -1° and -89°, between about -5° and -85°, between about -10° and -80°, between about -20° and -70°, between about -30° and -60°, or between about -40° and -50°.

[0348] 68. The passive mixing reactor according to any of clauses 29 to 67, comprising at least a first and a second tangential segment, and wherein the at least one tangential fluid flow channels of the first tangential segment are oriented at a negative (counter-clockwise) tangential angle, and the at least one tangential fluid flow channels of the second tangential segment are oriented at a negative (counterclockwise) tangential angle.

[0349] 69. The passive mixing reactor according to any of clauses 29 to 67, comprising at least a first and a second tangential segment, and wherein the at least one tangential fluid flow channels of the first tangential segment are oriented at a positive (clockwise) tangential angle, and the at least one tangential fluid flow channels of the second tangential segment are oriented at a positive (clockwise) tangential angle.

[0350] 70. The passive mixing reactor according to any of clauses 29 to 67, comprising at least a first and a second tangential segment, and wherein the at least one tangential fluid flow channels of the first tangential segment are oriented at a negative (counter-clockwise) tangential angle, and the at least one tangential fluid flow channels of the second tangential segment are oriented at a positive (clockwise) tangential angle.

[0351] 71. The passive mixing reactor according to any of clauses 29 to 70, comprising at least a first and a second tangential segment, and wherein the at least one tangential fluid flow channels of the first tangential segment and the at least one tangential fluid flow channels of the second tangential segment are orientated, in use, for generation of two co-rotary vortices.

[0352] 72. The passive mixing reactor according to any of clauses 29 to 70, comprising at least a first and a second tangential segment, and wherein the at least one tangential fluid flow channels of the first tangential segment and the at least one tangential fluid flow channels of the second tangential segment are orientated, in use, for generation of two counter- rotary vortices.

[0353] 73. Use of the passive mixing reactor according to any of clauses 29 to 72 in the process for the depolymerisation of lignin according to any of clauses 1 to 28a. 74. The use according to clause 73, wherein the first fluid stream comprises a lignin solution or suspension comprising lignin.

[0354] 75. The use according to clause 73 or clause 74, wherein each of the one or more additional fluid stream comprises supercritical water or subcritical water.

[0355] 76. A method for the depolymerisation of lignin, the method comprising:

[0356] providing the passive mixing reactor according to any of clauses 29 to 72;

[0357] providing a first fluid stream comprising a lignin solution or suspension comprising lignin through the central flow channel; and

[0358] providing supercritical water or subcritical water as one or more additional fluid streams through the one or more tangential fluid flow channels, to obtain a depolymerised lignin mixture comprising monomeric and / or oligomeric phenolic compounds.

[0359] 77. The method according to clause 76, further comprising performing the steps of any of clauses 1 to 28a.

[0360] 78. A product comprising at least one phenolic monomer or oligomer obtained according to the process of any of clauses 1 to 28a, the use of any of clauses 73 to 75, or the method of clause 76 or clause 77.

Claims

1. CLAIMS1. A process for the depolymerisation of lignin, the process comprising the steps:3.i) providing a first fluid stream comprising a lignin solution or suspension comprising lignin;4.ii) providing one or more additional fluid stream comprising supercritical water or subcritical water;5.iii) bringing the first fluid stream into contact with the one or more additional fluid stream via passive mixing to generate a mixed fluid stream for a reaction time period in order to obtain a depolymerised lignin mixture comprising monomeric and / or oligomeric phenolic compounds; and6.iv) isolating the monomeric and / or oligomeric phenolic compounds from the depolymerised lignin mixture.

2. The process according to claim 1 , wherein the reaction time period is between about 20 ms and 30 s, between about 25 ms and 25 s, between about 30 ms and 20 s, between about 30 ms and 10 s, between about 40 ms and 5 s, between about 50 ms and 2 s, between about 250 ms and 1.5 s, between about 500 ms and 1.5 s, or between about 100 ms and 1 s, preferably between about 500 ms and 1 .5 s.

3. The process according to claim 1 or claim 2, wherein the process comprises an additional step between Step i) and Step iii) comprising bringing the first fluid stream to a pressure substantially the same as the pressure of the one or more additional fluid stream comprising supercritical water or subcritical water to enable mixing between the fluid streams.

4. The process according to any preceding claim, wherein more than one additional fluid stream comprising supercritical water or subcritical water is provided.

5. The process according to any preceding claim, wherein the passive mixing comprises turbulent mixing; preferably wherein the turbulent mixing comprises generating at least one vortex.

6. The process according to claim 5, wherein the turbulent mixing comprises generating two or more vortices; preferably wherein any two adjacent vortices are either co-rotary vortices or counter- rotary vortices; more preferably wherein any two adjacent vortices are counter- rotary vortices.

7. The process according to any preceding claim, wherein the process comprises a preliminary step prior to Step i) of preparing the lignin solution or suspension from a lignin source comprising lignin.

8. The process according to any preceding claim, wherein the one or more additional fluid stream comprising supercritical water or subcritical water of Step ii) is provided at a temperature of between about 350 and 700 °C and / or at a pressure of between about 200 and 300 bar; preferably at a temperature of between about 425 and 550 °C and / or at a pressure of between about 220 and 280 bar; more preferably at a temperature of between about 465 and 475 °C and / or at a pressure of between about 250 and 270 bar.

9. The process according to any preceding claim, wherein the temperature of the lignin in the first fluid stream is increased in Step (iii) through contact with the one or more additional fluid stream via passive mixing; preferably wherein the temperature of the lignin in the first fluid stream is increased to a temperature of between about 300 and 430 °C; more preferably wherein the temperature of the lignin in the first fluid stream is increased to a temperature of between about 370 and 410 °C.

10. The process according to claim 9, wherein the temperature increase is affected within a heating time period of at least about 1 ms, at least about 2 ms, at least about 5 ms, at least about 10 ms, at least about 25 ms, at least about 50 ms, at least about 75 ms, or at least about 100 ms; and / or at most about 30 s, at most about 20 s, at most about 10 s, at most about 7.5 s, at most about 5 s, at most about 2.5 ms, at most about 2 ms, at most about 1 .5 s, at most about 1 s, or at most about 0.5 s.

11. The process according to any preceding claim, wherein Step iii) further comprises, afterthe reaction time period, reducing the temperature and / or pressure of the mixed fluid stream; preferably wherein reducing the temperature of the mixed fluid stream comprises reducing the temperature by between about 100 and 350 °C and / or wherein reducing the pressure of the mixed fluid stream comprises reducing the pressure by between about 200 and 300 bar.

12. The process according to any preceding claim, wherein the process is a continuous flow process.

13. A passive mixing reactor suitable for the depolymerisation of lignin using the process according to any of claims 1 to 12, comprising:18.a first fluid stream inlet at a first end suitable for injection of a first fluid stream;19.a fluid outlet at a second end suitable for ejection of a reaction mixture;20.a central flow channel, comprising a longitudinal axis, and extending between the first end and the second end enabling fluid communication between the first fluid stream inlet and the fluid outlet; and one or more tangential segments arranged between the first end and the second end, each of the one or more tangential segments comprising one or more tangential fluid flow channels extending between a channel inlet and the central flow channel to provide fluid communication between the one or more tangential fluid stream inlets and the central flow channel, wherein each of the channel inlets is suitable for receiving one or more additional fluid stream for delivery to the central flow channel; wherein, in use,21.the one or more tangential fluid flow channels is oriented for generating turbulent mixing between the first fluid stream travelling along the central flow channel and the one or more additional fluid stream entering the central flow channel from the one or more tangential fluid flow channels.

14. The passive mixing reactor according to claim 13, wherein the central flow channel has a length of at least about 3 mm, at least about 5 mm, at least about 10 mm, at least about 30 mm, at least about 50 mm, at least about 100 mm, at least about 300 mm, at least about 500 mm, or at least about 750 mm; and / or at most about 3000 mm, at most about 2500 mm, at most about 2000 mm, at most about 1500 mm, or at most about 1000 mm.

15. The passive mixing reactor according to claim 13 or claim 14, wherein each of the one or more tangential fluid stream inlets has a diameter of at least about 0.5 mm, at least about 1 mm, at least about 2 mm, at least about 3 mm, at least about 5 mm, at least about 10 mm, at least about 15 mm, at least about 20 mm, or at least about 35 mm; and / or at most about 60 mm, at most about 50 mm, or at most about 40 mm.

16. The passive mixing reactor according to any of claims 13 to 15, wherein the combined cross-sectional area of all of the tangential fluid flow channels is at most about half the cross-sectional area of the central flow channel, at most about a quarter of the cross-sectional area of the central flow channel, at most about a fifth of the cross-sectional area of the central flow channel, or at most about a tenth of the cross-sectional area of the central flow channel.

17. The passive mixing reactor according to any of claims 13 to 16, wherein each of the one or more tangential fluid flow channels is orientated such that, in use, injection of one or more additional fluid through at least one tangential fluid flow channel is capable of generating a vortex within the central flow channel on mixing with a first fluid stream injected through the first fluid stream inlet.

18. The passive mixing reactor according to any of claims 13 to 17, wherein each of the one or more tangential fluid flow channels is characterised by a pitch angle measured relative to a central flow axis of the tangential fluid flow channel at the point of intersection with the central flow channel; preferably wherein the pitch angle of the tangential fluid flow channel is the angle between a line defined by a central flow axis of the tangential fluid flow channel and the longitudinal axis of thecentral flow channel extending between the first end of the passive mixing reactor and the tangential fluid flow channel.

19. The passive mixing reactor according to claim 18, wherein the pitch angle is about 90°; or at most about 85°, at most about 80°, at most about 75°, at most about 70°, at most about 65°, at most about 60°, at most about 55°, at most about 50°, at most about 45°, at most about 40°, at most about 35°, at most about 30°, at most about 25°, at most about 20°, at most about 15°, at most about 10°, or at most about 5°.

20. The passive mixing reactor according to any of claims 13 to 19, wherein the one or more tangential fluid flow channels within each of the one or more tangential segments are configured to generate, in use, at least one vortex within the central flow channel upon injection of the one or more additional fluid stream into the first fluid stream.

21. The passive mixing reactor according to any of claims 13 to 20, wherein the one or more tangential fluid flow channels of the one or more tangential segments intersect the central flow channel at a tangential angle relative to a radial axis of the central flow channel at the point of intersection, wherein when the tangential fluid flow channel is oriented in a generally clockwise direction when viewed along the longitudinal axis of the central flow channel from the first end I first fluid stream inlet, the tangential angle is between about 0° (radially aligned) and 90° (tangentially aligned); and when the tangential fluid flow channel is oriented in a generally counter-clockwise direction when viewed along the longitudinal axis of the central flow channel from the first end I first fluid stream inlet, the tangential angle is between about 0° (radially aligned) and -90° (tangentially aligned).

22. The passive mixing reactor according to any of claims 13 to 21 , wherein the one or more tangential fluid flow channels of the one or more tangential segments intersect the central flow channel at a tangential angle of between about 1° and 89°, between about 5° and 85°, between about 10° and 80°, between about 20° and 70°, between about 30° and 60°, between about 40° and 50°, between about -10and -89°, between about -5° and -85°, between about -10° and -80°, between about -20° and -70°, between about -30° and -60°, or between about -40° and -50°.

23. Use of the passive mixing reactor according to any of claims 13 to 22 in the process for the depolymerisation of lignin according to any of claims 1 to 12.

24. A method for the depolymerisation of lignin, the method comprising:33.providing the passive mixing reactor according to any of claims 13 to 22;34.providing a first fluid stream comprising a lignin solution or suspension comprising lignin through the central flow channel; and providing supercritical water or subcritical water as one or more additional fluid streams through the one or more tangential fluid flow channels, to obtain a depolymerised lignin mixture comprising monomeric and / or oligomeric phenolic compounds.

25. A product comprising at least one phenolic monomer or oligomer obtained according to the process of any of claims 1 to 12, the use of claim 23, or the method of claim 24.

Images