*****

Summary of the invention

The object of the present invention is a process and the relative plant for producing levulinic acid from cellulose derived from biomasses subjected to a treatment with subcritical water and subsequent delignification in a cavitational reactor.

Field of the invention

The present invention relates to the field of the production of levulinic acid which can be used as a preparation intermediate for other processes such as the production of biopolymers.

Levulinic acid is obtained from the conversion of cellulose derived from biomass or vegetable scraps such as artichoke, wheat straw, rice straw, tomato plant stem, tomato peels or even legume pods.

Background art

Numerous processes are known for extracting cellulose from biomass, including from vegetable scraps. Some are mentioned here for the purposes of providing information .

Patent US 2014/315259 discloses a conversion process by fermentation and subsequent distillation of biomasses comprising, among others, corn, wheat, milo, rice, barley, sugar cane, sugar beet, tubers or topinambur, in order to obtain ethanol or other products, including also levulinic acid, where said biomasses are previously treated by dry milling and liquefaction aimed at producing a mixture of fermentable sugars subsequently sent for fermentation.

Patent WO 2011/039635 instead discloses a conversion process of biomass from agricultural waste by means of subsequent processing of the biomass so as to carry out a hydrolysis at a temperature between 400°C and 550°C, with subsequent vaporisation of the cellulose and its separation from the lignin-based residue.

Patent WO 2020/234761 discloses a process of extracting cellulose by means of deep eutectic solvents (DES) , which solvents consist of two or more components with at least one donor element and a hydrogen bond acceptor element .

The process disclosed in patent WO 2012/112529 involves a treatment of the biomass in which the cellulose is subjected to a conversion aimed at modifying the molecular structure, for example by radiation treatment, in order to reduce the molecular weight of the cellulose or to reduce the degree of crystallinity of the charge.

The process disclosed in patent ES 2212866 discloses a series of physical operations aimed at removing the superficial layers of thistle scraps, so as to separate the innermost part from the hardest waste.

Patent US 8383888 also discloses a method for processing cellulose extracted from particular raw materials in order to proceed with fermentation for the production of alcohols.

Patent EP 3080304 discloses a process for producing sugars from biomass by mixing said biomass with an alkyl sulfonic acid of generic formula R-SO3H in which the tail R corresponds to a C1-C3 linear or branched alkyl group.

The process disclosed in patent CN 104074086 instead discloses the extraction of cellulose by a preliminary leaching treatment of the biomass with an alkaline solution, lasting about 2-5 hours, followed by storage in a stirred container at a temperature between 60- 200°C, followed by filtration (or centrifugation) with subsequent addition of activated carbons and by mixing lasting between 100-500 min followed by an addition of ethanol, corresponding to 1-5 times the extract volume, to finalise the extraction of cellulose.

Processes are also known for extracting cellulose from biomasses which use a technology such as cavitation (hydrodynamic or ultrasonic) , and are characterised by the continuous formation of tiny bubbles (micro bubbles) of vapour within a fluid.

For example in patent EP 2712364 the biomass, previously treated with an organic or inorganic swelling agent is sent for mechanical comminution treatments, among which cavitation is also mentioned and in which the operating parameters of the mechanical treatment largely depend on the choice of swelling agent .

Similarly, the process disclosed in patent WO 2018/013501 discloses a process for producing cellulose from biomass in which an organic or inorganic solvent is used in advance which, depending on the case, may be either an organic oxygenate component such as an aliphatic alcohol comprised between Ch-Chs, an aromatic alcohol, a ketone, an aldehyde or an ether, or even an unoxygenated alkane, an olefin, or an aromatic hydrocarbon so as to operate the fractionation between cellulose and residue (consisting of lignin, hemicellulose and solvent) and in which the cellulose thus fractionated is sent for a subsequent mechanical treatment, also including cavitation, for the production of nanocellulose.

Cellulose extraction processes in which a cavitational reactor is used are also known, however in all these processes the cellulose is previously mixed with an acid or a swelling agent; among the processes of this type, patents US 2012/111322, CN 102899950, WO 2015/079414, US 2012/125547, KR 102073523 can be mentioned .

It is clear that in all the patents mentioned, the extraction of cellulose either occurs through the use of solvents which promote the separation, for example, from lignin, but which in return require solvent separation and recovery operations, or which in any case provide for cellulose treatments with technologies such as fermentation which are extremely slow (from an industrial point of view) .

As regards the conversion of cellulose into levulinic acid, numerous patent documents are known which disclose processes in which cellulose is sent to a conversion reactor after treatment with mineral acids. In some of these patent documents several reaction phases are included with different concentrations of acid, so as to obviate the problem of the duration of extraction times.

In the patent field, reference is also made to the use of a microwave reactor mainly for reducing processing times or limiting the occurrence of parallel reactions which would decrease the yield of levulinic acid. Among the patents generally disclosed, numerous can be mentioned: EP 0873294, EP 0365665, EP 3184518, EP 3047029, EP 2928856, EP 2872495, EP 2825519, EP

2537841, EP 1723099, EP 3665147, EP 28002551, EP

1851186, EP 2780433, US 9174909, US 2010/312006, US 5892107, WO 2020214835, US 2018/194711, US 2015/238923, WO 2013/135965, US 2011/160479, CN 113214196, CN 104292195, CN 104292194, CN 104311411, IT FI20090210, US 2840605.

However, the problem of the duration of the extraction and conversion times to levulinic acid remains unresolved, as well as the same yield in this product.

Object of the invention The object of the present invention is to provide a process and a plant for producing levulinic acid from cellulose extracted from biomass which overcomes the limits of the known processes and which allows a higher yield in extracted cellulose, a higher purity of the levulinic acid produced and processing times compatible with industrial requirements.

Proposed solution

The solution disclosed herein is a process, and a related plant, in which the cellulose separation occurs using subcritical water, and in which the cellulose thus produced is treated with acids for conversion to levulinic acid at high temperature.

List of figures

A better understanding of the invention will be had with the following detailed description and with reference to the accompanying figures which illustrate, merely by way of non-limiting example, a preferred embodiment .

In the drawings: figure 1 depicts a block diagram of the process according to the invention; figure 2 shows the block diagram of figure 1, with the numbering of the flows and equipment of the plant according to the invention.

The invention is a process and plant for producing levulinic acid, usable as an intermediate for producing biopolymers, where said levulinic acid is produced starting from an organic residue containing cellulose which is separated from the biomass using subcritical water and then stabilised by means of cavitation.

Process

With reference to the block diagram of figure 1, the process according to the invention provides the following phases:

• Crushing (A) , or comminution, of the raw material;

• Extraction of the biomass with subcritical water (B) , with separation of the non-fibrous part, consisting of primary and secondary metabolites and water, an organic residue, consisting of cellulose, hemicellulose and lignin;

• Mixing (D) the organic residue from the cellulose separation phase (B) with an alkaline aqueous solution;

• Delignification by means of cavitation (E) of the flow consisting of the organic residue and previously- contained alkaline solution;

• Refinement (F) of the flow from the delignification phase by cavitation (E) with separation of an organic phase, containing cellulose, from an aqueous phase containing lignin;

• Conversion into levulinic acid (H) of the organic flow containing the cellulose, said flow being added with strong acids adapted to favour the conversion;

• Washing (M) the effluent from the reactor aimed at removing excess strong acids before the recovery of levulinic acid;

• Separation (I) of the levulinic acid from the inert residue.

Optionally, there may be a concentration phase (C) of the extract and relative separation of the dilution water from the organic components; such a phase allows a recovery of the primary and secondary metabolites and their use or sale.

In addition to the concentration phase (C) , there may be a recovery phase (J) of the water deriving, in fact, from the concentration of the extract, so as to optimise the consumption of utilities and, therefore, to substantially operate a closed cycle of the subcritical water, providing for the physiological reintegration thereof.

Depending on the biomass to be treated, a rehydration phase (K) may be provided downstream of the comminution (A) and before the extraction phase (B) .

Furthermore, there may be a further washing phase with water (L) downstream of the refining phase (F) for removing the salts obtained from the flow containing the cellulose and produced during the refining process. Furthermore, there is a separation phase (N) of the aqueous flow, consisting of water and lignin and coming from the refining phase (F) , in an organic flow mainly consisting of lignin and in a flow consisting of water recyclable to the dissolution process after reintegration of the alkaline solution.

Finally, a further purification phase, or clarification (G) , of the aqueous flow from the separation phase (N) may be present with separation of the organic residue from the water; from this phase purified water, recyclable in the mixing phase (D) after reintegration of the hydroxides (NaOH, KOH) , and an organic phase, consisting of lignin tails, is separated.

Plant

Therefore, to carry out the above process, the plant comprises the following units:

• At least one crusher (101) of the raw material;

• At least one subcritical water extraction reactor (102) with production of an extract, consisting of primary and secondary metabolites and water, and an organic residue, consisting of cellulose, hemicellulose and lignin;

• At least one mixing tank (104) of the organic residue from the cellulose separation phase with an alkaline aqueous solution;

• At least one cavitational reactor (105) for the stabilisation the flow consisting of organic residue and previously obtained alkaline solution;

• At least one separator (106) for the refining (F) of the flow from the cavitation reactor (E) ;

• At least one levulinic acid conversion reactor of the organic residue from the separator (106) ;

• At least one tank (109) for washing the effluent from the reactor (108) to remove the excess acid used in the conversion;

• At least one separator (110) of the levulinic acid from the non-cellulosic inert residue.

The plant is also provided with means for the handling, storage and control of various flows.

Optionally, there may be at least one expansion bubble (103) for the concentration of the extract and the relative separation of the dilution water from the organic components.

Further, there may be at least one separator (107) for separating the aqueous phase, coming from the separator (106) , with separation of the organic residue, consisting mainly of lignin, from the water.

Additionally there may be a further separator (111) for the clarification of the water coming from the separator (107) with production of purified water and separation of organic waste.

Biomasses

According to the invention, the biomasses which can be used in the present process are artichoke scraps, wheat straw, rice straw, tomato plant stem and tomato peels, legume pods in general, or mixtures of said raw materials .

The choice of one biomass rather than another implies the possibility of having a greater or lesser yield in the extraction of cellulose, a yield proportional to both the initial content of cellulose in the biomass, and dependent on the state of the biomass. In examples 1 and 2 the results of the tests carried out on two different types of biomass for the process object of the present invention are reported.

The results show a considerable advantage in the exploitation of a biomass consisting of artichoke scraps which results in a higher yield in cellulose and, therefore, a higher yield in levulinic acid.

This preferred embodiment, however, is not intended to be limiting of the inventive concept which may of course be extended to the biomasses indicated above.

Detailed description of the invention

With reference to the figures, the process phases are disclosed below.

Crushing

According to the invention, the process includes a first crushing phase (A) which occurs by means of at least one crusher (101) of the incoming biomass (1) , necessary to reduce the size of said biomass to a size suitable for the subsequent processing phases.

In a preferred, but non-limiting, embodiment, the size to which the biomass is reduced is in the range of 1-6 mm, said size being dependent on the type of biomass introduced into the process; even more preferably it is in the range of 2-5 mm.

Optionally, depending on the type of biomass treated, the flow (3) exiting the crusher can be sent to a rehydration phase (K) downstream of the comminution (A) and before the extraction phase (B) ; from this phase a flow (3' ) of crushed and rehydrated biomass will exit to be directed to the extractor. Separation of the cellulose

The crushed biomass (3) , and possibly rehydrated (3' ) , exiting said crusher (101) of phase (A) is sent to at least one reactor (102) together with a flow of water (5) , so as to operate an extraction with subcritical water (B) of the primary metabolites (mainly carbohydrates such as soluble fibres, inulin and mineral salts) and secondary metabolites (mainly polyphenols and bitter substances for liquorice) from a residue consisting mainly of cellulose and insoluble residue .

In the preferred, but non-limiting, embodiment disclosed, the water used for extraction is under subcritical conditions, preferably in a pressure range of 3-7 bar (a) and in a temperature range of 130-170°C, compatible with the need to work with water in the liquid phase; more preferably the water is sent at 5 bar (a) and 150°C.

Under these conditions, the water not only allows to solubilise the non-fibrous components, such as the primary and secondary metabolites, but also to cause a swelling of the fibres which favours the subsequent processing phases of the cellulose.

According to the invention, the volume ratio of biomass (3, 3' ) entering the extraction reactor with subcritical water (5) is between 1:4-1:10, even more preferably between 1:5-1 :7. The extraction can occur in continuous or discontinuous mode; in the preferred embodiment disclosed, it occurs discontinuously with a duration of the extraction cycle of at least 15 minutes. Two flows are thus obtained from the extraction reactor (102) of the separation phase with subcritical water (B) :

• an extract (9) consisting of water and metabolites (primary and secondary) ;

• an insoluble organic residue (7) consisting of lignin, hemicellulose and cellulose.

Dissolution of the residue

The insoluble residue (7) exiting the extractor (102) of the separation phase with subcritical water (B) and consisting of insoluble residue (lignin) with an enriched percentage of cellulose, is mixed in at least one tank (104) with an alkaline solution (15) so as to favour the dissolution of the lignin.

According to the invention the alkaline solution (15) is preferably, but not limited to, sodium hydroxide (NaOH) , or potassium hydroxide (KOH) ; said solution is sent in a concentration range of 5-15%, even more preferably 6-12%.

According to the invention the dilution volume ratio of the organic residue (7) with said alkaline solution (15) ( solid/liquid) is between 1: 8-1:16, also depending on the type of cavitational reactor used in the subsequent treatment phase.

Delignification (Cavitational Reactor)

The flow consisting of organic residue mixed with the alkaline solution (17) exiting the tank (104) is sent to a delignification phase (E) .

According to the invention said delignification phase (E) comprises at least one cavitational reactor (105) , whose function is to destructure the lignocellulosic material and progressively dissolve the lignin.

In this sense, either an ultrasonic cavitational reactor (105' ) , or high power sonicator, or even a hydrodynamic cavitational reactor (105' ' ) can be used. The extent of the dilution ratio between residue and alkaline solution will depend on the choice of one type rather than the other.

In the preferred embodiment disclosed, the cavitational reactor is a high power sonicator; advantageously this choice allows to operate with a lower residual dilut ion/alkaline solution ratio (about 1:8- 1:10) with respect to what would be the case using a hydrodynamic cavitational reactor (about 1:15) . Refinement

The effluent (19) from the cavitational reactor (105) is sent to a refining phase (F) , said phase comprising at least one separator (106) where a separation is carried out between the aqueous phase (23) , consisting of alkaline solution, lignin and salts thereof, and a solid fraction (21) mainly containing cellulose.

According to the invention, said separator (106) can be either a centrifuge (106' ) or a simple decanting tank (106' ' ) : the choice of one technology rather than another obviously implies differences in the duration of this phase which will result faster with a centrifuge and slower with a decanter.

Furthermore, the choice of centrifugation rather than decantation will have obvious repercussions on the volumes in play and, therefore, on the flow rates flowing within the process. Conversion

The organic flow (21) containing cellulose from the separator (106) of the refining phase (F) constitutes the charge to be converted into levulinic acid.

The dry residue of the organic phase entering the conversion reactor reaches, depending on the type of biomass used, a cellulose yield in the range 20-95%.

Optionally, a phase of removing the salts (L) with water may be further present upstream of said conversion phase (H) ; however, two flows will separate from the possible salt removal phase (L) : an aqueous flow (51) containing the salts recovered from the organic charge, and an organic flow cleaned of the salts (21' ) and destined for the conversion phase (H) into levulinic acid.

Alternatively to the salt removal phase (L) , the flow (21) destined for the conversion phase (H) can be recirculated in a loop in the cavitational reactor, allowing a further delignification phase and a solubilisation of the salts, possibly present in the organic flow, in the aqueous phase.

According to the invention, said organic flow (21, 21' ) , consisting of cellulose, hemicellulose and lignin, is added with strong acids (33) and sent to a conversion phase (H) .

The input charge to the conversion phase (H) is then suspended with strong acid (33) in a 1:10 ratio and sent to the conversion reactor (108) .

According to the invention, the strong acids usable for the conversion phase are hydrochloric acid, hydrobromic acid, sulphuric acid, but also mono-di-sulf onic acids, and polysulfonic acids and p-toluenesulf onic acid (pTsOH) .

In a preferred, but non-limiting embodiment, the acid used is preferably p-toluenesulf onic acid (pTsOH) . According to the invention, said conversion phase (H) preferably occurs, but is not limited to occurring, in a microwave reactor (108) .

Said reactor is loaded and heated at 225°C for a time interval between 1-5 minutes, depending on the type of strong acid used to acidify the charge.

The yield and purity of levulinic acid are closely linked to both the amount of cellulose present in the input charge, the type of acid used and the reaction time within the reactor.

Once the set time has expired, the reactor (108) is unloaded .

Reactor effluent wash

The effluent (35) from the reactor (108) is directed to a washing phase (M) adapted to remove the excess acids used .

In order to favour the removal of excess acid, an alkaline solution (47) may be used in phase (M) .

In a preferred, but non-limiting embodiment, said washing phase (M) occurs place in a tank (109) from which an organic flow containing the levulinic acid (37) and a flow consisting of the excess acid removed (39) from the effluent coming from the conversion reactor .

Separation of levulinic acid

The thus purified flow (37) is directed to a separation phase (I) in which there is at least one levulinic acid separator (110) thus produced from the unreacted mass. In a preferred, but non-limiting, embodiment, said separator (110) of the separation phase (I) is a filter .

The levulinic acid (41) thus recovered can thus be crystallised or sent for further processing, for example for the production of biopolymers.

The unreacted phase (43) , comprising only trace amounts of residue, constitutes a waste of the process.

As mentioned, the following apparatuses may be present, relating to optional process optimisation phases: Extract concentration

The extract (9) exiting the reactor (102) of the subcritical water separation phase (B) , and consisting of primary and secondary metabolites and water, is sent to a concentration phase of the extract which, in a preferred, but non-limiting, embodiment of the invention is carried out by means of at least one expansion bubble (103) .

This operation allows to separate the organic phase (11) comprising primary and secondary metabolites, from the vapour (13) .

The removed vapour (13) can be sent to a recovery phase (J) where it is condensed in water, added to new reintegration water and brought to the pressure and temperature conditions suitable for extraction, thus acting as a closed cycle within the process, said phase J being provided with means for condensation, storage, movement and control of the evolving currents.

The metabolites (11) recovered in the concentration phase can be used as intermediates for other preparations

Separation

The aqueous flow (23) , consisting of water and lignin, exiting the separator (106) is sent to a separation process (N) aimed at favouring the separation between the aqueous phase, intended for recovery and recycling to the process, and the organic component consisting mainly of lignin.

According to the invention, said separation phase (N) comprises at least one separator (107) from which two flows will exit: one consisting of water (27) and a solid phase (25) mainly consisting of lignin which is sent to storage or other treatments.

Further, to promote greater recovery of the lignin, the aqueous flow (23) entering the separation phase (N) can be acidified with strong acids (45) until the lignin itself precipitates. Clarification (Filtration) According to the invention, the aqueous flow (27) coming from the separator (107) of the separation phase (N) can be sent to a further clarification phase (G) comprising at least one separator (107) ; in the preferred embodiment disclosed, said separator (111) is a two-phase solid/liquid drum separator.

Clarified water (29) exits from the separator (111) from the clarification phase (G) , which is recyclable to the dissolution phase of the residue (D) after reintegration of the alkaline solution, and organic waste (31) .

A first advantage of the invention is found in the extraction using subcritical water which allows to have no solvent entrainments in the subsequent processing phases and therefore to not need solvent separation and recovery phases.

Further, the absence of solvent reduces processing costs .

Furthermore, choosing to operate with cavitational reactors and with a microwave conversion reactor allows to reduce processing times.

A second advantage consists in the choice to use p- toluenesulf onic acid (pTsOH) for the conversion of cellulose into levulinic acid; in fact, despite being a strong acid, this is not particularly aggressive for machinery, allowing it to operate for many more processing cycles with respect to those that can be carried out using strong acids such as hydrochloric or sulphuric acid.

A second advantage consists in the choice to use p- toluenesulf onic acid (pTsOH) for the conversion of cellulose to levulinic acid; in fact, although the use of p-toluenesulf onic acid (pTsOH) entails slightly less than the use, for example, of hydrochloric acid, the lower aggressiveness in terms of corrosion of the machinery entails the possibility of operating many more processing cycles without having to restore the machines, unlike what occurs when using strong acids such as hydrochloric or sulphuric acid.

The yields are also visible in examples 1 and 2.

Finally, the process according to the invention allows to obtain a high purity in levulinic acid produced, generally higher than 80%. Experimental examples and tests

Below are the results of 4 experimental tests carried out respectively on a biomass derived from artichoke scraps and wheat straw and in which the extraction of levulinic acid occurs either by hydrochloric acid or by p-toluenesulf onic acid.

The results confirm what has been disclosed above with regard to the yield of levulinic acid, as well as the extremely low reaction times, which are compatible with industrial practice.

Example 1: Artichoke extraction

Example 2: Wheat straw extraction