WO2015063033A1

WO2015063033A1 - Process for the separation of levulinic acid

Info

Publication number: WO2015063033A1
Application number: PCT/EP2014/073010
Authority: WO
Inventors: Pierre Louis Woestenborghs
Original assignee: Dsm Ip Assets B.V.
Priority date: 2013-11-01
Filing date: 2014-10-27
Publication date: 2015-05-07

Abstract

The invention relates to a process for the separation of levulinic acid from a biomass hydrolysate, said process comprising optionally subjecting the biomass hydrolysate to a solid-liquid separation to yield a solid fraction and a liquid fraction; optionally subjecting the biomass hydrolysate or the liquid fraction to a solvent- solvent extraction to yield an organic phase comprising levulinic acid, and an aqueous phase, and recovering said organic phase; subjecting the biomass hydrolysate or the liquid fraction or the organic phase to a distillation to yield a distillate comprising levulinic acid, and a distillation residue; adding a base to said distillation residue; optionally cooling the distillation residue after adding the base; and recovering the distillation residue. The process advantageously gives a flowable (less viscous) distillation residue even after cooling, making it easier to discharge the residue and use it e.g. as black liquor. Also, because it is no longer required to add levulinic acid to the residue in order to keep it flowable, levulinic acid yields are higher.

Description

PROCESS FOR THE SEPARATION OF LEVULINIC ACID

Field of the invention

The present invention relates to a process for the separation of levulinic from a biomass hydrolysate.

Background of the invention

Levulinic acid is a starting molecule for the synthesis of esters known as fuel additive and is known to be useful as plasticizers and solvents. Levulinic acid can be used to synthesize methyl tetrahydrofuran (MTHF) or can be used as a solvent. Other applications of levulinic acid are for example the synthesis of delta-amino levulinic acid used as herbicides and pesticides, diphenolic acid used to synthesize polycarbonates and succinic acid used to make polyesters. Levulinic acid can also be used to produce gamma valerolactone (5- methylbutyrolactone), which in turn can be used for production of adipic acid (1 ,6- hexanedioic acid).

Levulinic acid can be produced from furfuryl alcohol. It is also possible to produce levulinic acid by acid hydrolysis of biomass although this is not commercially practiced.

Levulinic acid can also be produced by acid hydrolysis of lignocellulosic feedstocks. Production of levulinic acid by acid hydrolysis of biomass is described e.g. in US2010312006 US5,608,105 and US4,897,497, and US6,054,61 1. Levulinic acid can be used for the synthesis of esters known as fuel additive and known to be useful as plasticisers and solvents. Levulinic acid can be used to synthesize methyl tetrahydrofuran (MTHF), used as a solvent. Other applications of levulinic acid are for the synthesis of delta-amino levulinic acid used as herbicides and pesticides, diphenolic acid used to synthesize polycarbonates and succinic acid used to make polyesters. Levulinic acid can also be used to produce gamma- valerolactone (5-methylbutyrolactone), which in turn can be used for production of adipic acid (1 ,6-hexanedioic acid). Adipic acid is an important precursor for inter alia the production of polyamides such as Nylon 6,6. The most important process to produce adipic acid is based on oil and starts from benzene. A disadvantage of this process is that it is based on fossil derived oil. Another disadvantage is the evolution of NOx during the oxidation step, which either is vented to the air, which is highly undesirable as it is a greenhouse gas, or its catalytically destroyed, which is an expensive process. New processes for the production of adipic acid have been developed based on butadiene. However, such processes are also environmentally unfavourable. A third production route to produce adipic acid involves the use of levulinic acid via the following reaction sequence: levulinic acid; gamma-valerolactone; methylpentenoate; dimethyladipate, adipic acid. It is highly desirable to produce polymers such as nylon-6,6 from renewable resources. Such a process reduces the total amount of C02 emitted for its production and thus helps to mitigate against global warming. In addition, it helps to slow down the depletion of fossil resources. Levulinic acid may be produced from agricultural waste products or waste from the paper industry or municipal waste and therefore constitutes a renewable source of a C-5 fragment.

After acid hydrolysis of biomass, the levulinic acid is separated from the biomass hydrolysate. One well-known separation technique is distillation. However, it is known that tar, made during the production of bio-products by acid hydrolysis, may cause problems in subsequent distillation. Tar, for example from lignin, is well known in the production of levulinic acid and other compounds from biomass The formation of tar is discussed e.g. by WO2010/138957, which describes the formation of a hard dark distillation residue consisting of solid tar (referred to as "char"), and by US2009/0281338, wherein a dark brown residue is formed.

Tar formation and accumulation during distillation is also described e.g. by Hayes et al. (The Biofine Process - Production of Levulinic Acid, Furfuraldehyde, and Formic Acid from Lignocellulosic Feedstocks. D. J. Hayes, S. Fitzpatrick, M. H. B. Hayes, J. R. H. Ross, in Biorefineries-lndustrial Processes and Products, Status Quo and Future Directions, B. Kamm, PR. Gruber, M. Kamm, eds., Wiley-VCH, Weinheim, Germany, 2010, p139-164) who report the formation of acid-resistant tar during the distillation of levulinic acid and 2,5(hydroxymethyl)furfuraldehyde obtained by acid hydrolysis of lignocellulosic feedstocks. They described the tar as the cross-reacted and coalesced product from intermediates of the conversion process, and report that these intermediates tend to cross-react and coalesce to form an acid-resistant tar which incorporates many insoluble residues such as humins.

Tar can be soluble or insoluble. Char (sometimes also referred to as "coke") always refers to insoluble (particulate) tar, whereas soluble tar is usually referred to as humins.

In the context of the invention, humins shall refer to soluble tar, whereas char shall refer to insoluble tar.

Yang and Sen (Chem. Sus. Chem. 2010, vol. 3, 597-603) report the formation of humins during production of fuels from carbohydrates such as fructose. They speculate that humins are formed by acid-catalyzed dehydration. According to US7,896,944 the molecular weight of humins ranges from 2.5 to 300 kDa. Of course, the solubility of humins may depend on the solvent in which it is present. Generally, humins are soluble in water: after solids removal, the resulting liquid fraction of a biomass hydrolysate is usually yellowish- browish colour, which is mostly caused by the (soluble) humins.

Solid tar (char) can be removed from the biomass hydrolysate by sold-liquid separation such as filtration. For example in US2010312006 insoluble char particles are isolated from a biomass hydrolysate at high temperature and pressurized conditions. Sen et al (Energy Environ. Sci., 2012, 5, 9690) remove insoluble materials (lignin, char) from the liquid product mixture using a pressure filter system.

Due to their solubility, however, humins cannot be removed by solid/liquid separation step, and will be present, together with the levulinic acid, in the liquid fraction after solid/liquid filtration of the biomass hydrolysate.

The inventors have found that when levulinic acid is isolated using distillation, a distillation residue is formed which is very viscous and darkly coloured, and which was found difficult to discharge. In biomass hydrolysate processes, such dark colour is usually indicative of tar.

It is believed that humins, being high-boilers, will accumulate in the distillation, which will render the distillation residue more and more viscous and dark-coloured, to almost black and solid. It then becomes more difficult to process the distillation unit in terms of flowing and pumping. Moreover, it tends to stick very strongly to the distillation units and will be very difficult to remove. To make things worse, tar can also be formed during the distillation process itself: products such as HMF and furfuraldehyde can degrade to form humins and tar.

The inventors have realized that when a certain amount of levulinic acid ends up in the distillation residue, the residue stays flowable even after cooling. This levulinic acid acts as a solvent for the distillation residue.

When such distillation residue is discarded from the distillation, the levulinic acid is discarded with it, signifying a loss of levulinic acid. The inventors found that the amount of levulinic in the residue, in order to keep the residue flowable can be up to 20% w/w relative the total amount of levulinic acid (i.e. prior to the distillation step).

Thus, the formation of a viscous distillation residue is a serious problem in the isolation of levulinic acid.

Summary of the invention The present invention provides an improved process for the separation of levulinic acid from a biomass hydrolysate, said process comprising:

- optionally subjecting the biomass hydrolysate to a solid-liquid separation to yield a solid fraction and a liquid fraction;

- optionally subjecting the biomass hydrolysate or the liquid fraction to a solvent- solvent extraction to yield an organic phase comprising levulinic acid, and an aqueous phase, and recovering said organic phase;

- subjecting the biomass hydrolysate or the liquid fraction or the organic phase to a distillation to yield a distillate comprising levulinic acid, and a distillation residue; - adding a base to said distillation residue;

- optionally cooling the distillation residue after adding the base; and

- recovering the distillation residue.

Adding a base to a distillation residue renders the residue flowable (less viscous) even after cooling, making it easier to discharge the residue and use it e.g. as black liquor. Also, because it is no longer required to add levulinic acid to the residue in order to keep it flowable, levulinic acid yields are higher.

Detailed description of the invention In a first aspect the invention provides a process for the separation of levulinic acid from a biomass hydrolysate, said process comprising:

- subjecting the biomass hydrolysate or the liquid fraction or the organic phase to a distillation to yield a distillate comprising levulinic acid, and a distillation residue;

- adding a base to said distillation residue;

- optionally cooling the distillation residue after adding the base; and

- recovering the distillation residue.

The inventors have found that adding a base to a distillation residue renders the residue flowable (less viscous) even after cooling, making it easier to discharge the residue and use it e.g. as black liquor. Also, because it is no longer required to add levulinic acid to the residue in order to keep it flowable, levulinic acid yields are higher. In the process of the invention levulinic acid, a composition comprising levulinic acid is subjected to distillation. Such composition may be a biomass hydrolysate, or an organic phase obtained after solvent-extraction, or a liquid fraction obtained after a solid-liquid separation, or a distillation residue obtained after a preceding distillation. Typically part of the levulinic acid will end up in the distillate and part of the levulinic acid will end up in the distillation residue.

The biomass hydrolysate is obtained by acid hydrolysis of biomass under conditions such that it results in the formation of levulinic acid, and optionally in the formation of tar and/or humins. In acid hydrolysis of biomass, C6 carbohydrates such as glucose are converted to levulinic acid and formic acid. Suitable biomass comprises cellulose and/or hemicellulose, preferably the biomass comprises lignocellulose.

Suitable acids in the acid hydrolysis process are sulphuric acid, hydrochloric acid, and phosphoric acid. A preferred acid is sulphuric acid, preferably diluted sulphuric acid, for example at a concentration between 1 .5 - 3%. The temperature in the acid hydrolysis process may depend on the source of carbohydrates, and may range between 150-250°C, preferably between 170-240°C, more preferably between 190-230°C, even more preferably between 200 and 220°C. Said process may comprise one, two, or more stages. The pressure may also depend on the source of carbohydrates, as well as on the temperature, and may be anywhere between 1 and 50 bar, preferably between 5 and 40 bar, even more preferably between 10 and 30 bar. Suitable reactors include plugflow reactors, backmix reactors, and CSTR reactors. Different reactors for different stages may be used.

The skilled person will understand that the reaction time in the process to convert a carbohydrate to a bio-based product depends on the reaction temperature, the pressure, as well as the source of carbohydrate and the concentration of the acid. At higher reaction temperatures the reaction time may be shorter in order to obtain the desired bio-based product, whereas at lower reaction temperatures the reaction time may be longer in order to obtain the desired bio-based product. Likewise, at lower pressure, the reaction time may be longer whereas at higher pressure the reaction time may be shorter. The skilled person may therefore, without undue burden, establish suitable conditions with respect to temperature, reaction time, and pressure in order to obtain the desired bio-based product. The reaction time may vary between one second and one day, preferably between 10 seconds and one hour, more preferably between 1 minute and 2 hours, more preferably between 10 and 60 minutes.

Suitable carbohydrates to be converted to levulinic acid in the acid hydrolysis reaction include sugar, such as glucose and fructose, disaccharides such as saccharose and lactose; polysaccharides such as cellulose and starch; wood; lumber processing side products such as saw dust, wood chippings and wood shavings; cellulosic material e.g. from lignocellulosic feedstock; grass; cereal; starch; algae; tree bark; hay; straw; leaves; paper pulp, and dung, particularly herbivore dung. Paper pulp, or simply pulp, is a lignocellulosic fibrous material obtained by prepared by chemically or mechanically separating cellulose from wood, fibre crops or waste paper. Pulp is rich in cellulose and other carbohydrates. The carbohydrates may be bound to a component, such as to lignin. Lignocellulosic feedstock typically has a fibrous nature and comprises a bran fraction that contains the majority of lignocellulosic (bran) fibers. As an example, corn fiber is a heterogeneous complex of carbohydrate polymers and lignin. It is primarily composed of the outer kernel covering or seed pericarp, along with 10-25% adherent starch. Carbohydrate analyses of corn fiber vary considerably according to the source of the material.

Typically, the conditions of the acid hydrolysis reaction are such that other compounds are also formed, such as 2,5(hydroxymethyl) furfuraldehyde, ethoxymethyl furfuraldehyde, furfuraldehyde, formic acid, acetic acid, angelica lactone, and/or valerolactone. Thus, the composition preferably also comprises 2,5(hydroxymethyl) furfuraldehyde, methoxymethyl furfuraldehyde, furfuraldehyde, formic acid, acetic acid, angelica lactone, and/or (gamma)valerolactone.

In an embodiment the process comprises:

- subjecting a biomass hydrolysate to distillation to yield a distillate comprising levulinic acid, and a distillation residue;

adding a base to the distillation residue;

optionally cooling the distillation residue; and

recovering the distillation residue.

The skilled person appreciates that the process of the invention can be carried out in several ways. An essential part of the invention is that the process of the invention includes at least one distillation step and that it includes adding a base to a distillation residue. The process may include additional steps such as a one or more solvent extraction steps, one or more solid-liquid separation steps, and one or more additional distillation steps, or a combination thereof. Such steps can be done before or after the distillation.

In an embodiment, prior to the distillation, the biomass hydrolysate is subjected to solid-liquid separation. This yields a solid fraction and a liquid fraction. The solid fraction will contain solids and preferably char. The liquid fraction will comprise levulinic acid and may also comprise humins and/or colour. The liquid fraction can be recovered and subjected to distillation. Examples of suitable solid-liquid separation techniques are filtration and centrifugation. Filtration is a simple, preferred method.

Therefore, in an embodiment the process of the invention includes:

- subjecting a biomass hydrolysate to solid-liquid separation to yield a solid fraction and a liquid fraction;

- recovering the liquid fraction and subjecting said liquid fraction to distillation to yield a distillate comprising levulinic acid, and a distillation residue;

- adding a base to said distillation residue;

- optionally cooling the distillation residue after adding the base; and

- recovering the distillation residue.

In an embodiment, prior to the distillation, the biomass hydrolysate, or the liquid fraction after solid-liquid separation, is subjected to a solvent-solvent extraction to yield an organic phase comprising levulinic acid, and an aqueous phase.

Therefore, in an embodiment the process of the invention comprises:

- subjecting a biomass hydrolysate to a solvent-solvent extraction to yield an organic phase comprising levulinic acid, and an aqueous phase;

- recovering said organic phase;

- subjecting said organic phase to distillation to yield a distillate comprising levulinic acid, and a distillation residue;

- adding a base to said distillation residue;

- optionally cooling the distillation residue after adding the base; and

- recovering the distillation residue.

In a further embodiment the process of the invention comprises:

- recovering the liquid fraction and subjecting said recovered liquid fraction to a solvent-solvent extraction to yield an organic phase comprising levulinic acid, and an aqueous phase;

- recovering the organic phase and subjecting said organic phase to distillation to yield a distillate comprising levulinic acid, and a distillation residue;

- adding a base to said distillation residue; - optionally cooling the distillation residue after adding the base; and

- recovering the distillation residue.

In the context of the invention, "extraction", "solvent extraction", and "solvent-solvent extraction" are understood to be the same. Extraction is a process for separating components (the solutes) of a liquid (the feed) by contact with a second liquid phase (the solvent). The two liquids must not be completely mutually miscible. The process takes advantage of differences in the chemical properties of the feed components, such as differences in polarity and hydrophobic/hydrophilic character to separate them (T.C. Frank, L.Dahuron, B.S. Holden, W.D. Prince, A.F. Seibert, L.C. Wilson, Liquid-liquid extraction and other liquid-liquid operations and equipment in Perry's Chemical Engineering Handbook, 8^th Edition, Section 15). Solvent extraction commonly (but not necessarily) takes place with an aqueous solution as one liquid and an organic solvent or mixture of solvents as the other. Numerous solvents with various properties are used in solvent extraction (Y.Marcus, Principles of Solubility and Solutions, in J. Rydberg, M. Cox, C. Musicas, G.R. Chopin (Editors), Solvent Extraction Principles and Practice, 2^nd Edition, Chapter 2, Marcel Dekker Inc., New York). Extraction capacity of a solvent can be adjusted by changing process parameters like temperature or pH. Two streams result from the extraction: an extraction phase, which is an organic phase comprising the levulinic acid, also referred to as extract, and an aqueous phase, also referred to as raffinate.

The aqueous phase may comprise salts, acid and other water-soluble components, and can be discarded.

The organic phase comprises levulinic acid and optionally colour and/or humins, and can be subjected to the distillation. Suitable solvents include alcohols, such as methanol, ethanol, propanol, butanol; ketones, such as for example methylbutylketone; ethers, such as for example anisole (methyl phenyl ether), 2,5,8-trioxanonane (diglyme), diethylether, tetrahydrofuran, 2-methyl-tetrahydrofuran, diphenylether, diisopropylether and the dimethylether of di-ethyleneglycol; esters, such as for example ethyl acetate, methyl acetate, dimethyl adipate and butyrolactone; amides, such as for example dimethylacetamide and N- methylpyrrolidone; sulfoxides and sulphones, such as for example dimethylsulphoxide, di- isopropylsulphone, sulfolane (tetrahydrothiophene-2,2-dioxide) 2-methylsulfolane and 2- methyl-4-ethylsulfolane. Other organic solvents may also advantageously be used such as DCM (dicholoromethane), DCE (dichloroethene), toluene, benzene, 2-Heptanone, Butyl acetate, 1 ,2-Dichloroethane, Methyl isobutyl ketone, Dichloromethane, Ethyl propionate, 2- Pentanone, Diethyl ether, t-Amyl alcohol, Butanol, Cyclohexanone, Ethyl acetate, Pyridine, Tetrahydrofuran, 2-Butanone, Acetone, Dioxane, Acetonitrile, Methanol, N,N- Dimethylformamide, Dimethyl sulfoxide, Formamide, Ethylene glycol, 2-ME-THF (2-methyl tetrahydrofuran), MTBE (methyl-ter-butylether), MiBK (methyl isobutylketone), HOAc (acetic acid), CPMe (cyclopentyl methylether), heptane, DMF (dimethyl formamide), NMP (N- methylpyrrolidone), 2-sec-butylphenol (SBP), 4-n-pentylphenol (NPP), 4-n-hexylphenol (NHP), THF (tetrahydrofuran), MTHF (methyl-tetrahydrofuran) and DEGDME (diethyleneglycol dimethylether).

In the process of the invention, a biomass hydrolysate, liquid fraction thereof, or the organic phase obtained after solvent-extraction, is subjected to distillation to yield a distillate and a distillation residue. Prior to distillation the biomass hydrolysate can be flashed to remove water and any light boilers. If the process includes a solvent extraction step prior to the distillation, the solvent that is used in the extraction step can be recovered as the top fraction. Such solvent can be re-used in the extraction. Levulinic acid can be recovered as a distillate. Conditions to conduct distillation are described among others in WO2013034763. The initial pressure of the distillation can be set at 500 mbars, and may decrease to 20 mbars to remove levulinic acid as distillate. The temperature at the bottom of the distillation column after removal of levulinic acid is usually 80°C or more.

The distillation residue may be dark in colour and may comprise humins. It may be very sticky and viscous, and it may be difficult to remove from the distillation unit.

In an embodiment, the amount of levulinic acid in the distillation residue is 20% w/w or less, relative to the total dry weight of the residue. The distillation residue will be essentially free of water as water has a higher boiling temperature than levulinic acid. The skilled person knows how to set the temperature, time and pressure of the distillation such that the amount of levulinic acid in the residue is 20% w/w or less.

If levulinic acid were separated without the addition of the base, it would become very viscous after cooling. Conducting the distillation such that a certain amount of levulinic acid ends up in the residue (e.g. higher than 20 % (w/w) relative to the residue) would be one way to keep the residue flowable, but this would mean a loss of levulinic acid.

In contrast, by adding base to the distillation residue according to the process of the invention, less or even no levulinic acid needs to be present in the residue. Preferably the amount of levulinic acid in the distillation residue is preferably 15% w/w or less, more preferably 10% w/w or less 5% w/w or less, 2% w/w or less, even more preferably 1 % w/w or less, most preferably the residue is free of levulinic acid.

Preferably at least 80% w/w of the levulinic acid is recovered as a distillate, relative to the amount of levulinic acid in the biomass hydrolysate, or liquid fraction thereof, of the organic phase obtained by a solvent-extraction, prior to distillation. Preferably at least 85%, at least 90%, more preferably at least 95%, at least 98%, even more at least 99% of the levulinic acid is recovered as a distillate. Most preferably essentially all levulinic acid is recovered as a distillate. The skilled person knows how to set the temperature, time and pressure of the distillation such at least 80% w/w of the levulinic acid is recovered as a distillate.

The biomass hydrolysate, or a liquid fraction thereof, or the organic phase after solvent-extraction may comprise colour. Such colour may be for example yellow to brown to black. Colour may be indicative of the presence of contaminants, particularly humins. Humins are a common problem in the production of bio-based products by acid hydrolysis of biomass, and although it may be difficult to define humins on molecular level, they are generally regarded as soluble, coloured, undesired components. Moreover, colour itself also presents a possible problem in a levulinic acid preparation, as colour is generally undesired. Levulinic acid itself is colourless, so most of the colour is believed to come from humins.

In an embodiment, at least 50% of the colour of the biomass hydrolysate, of a liquid fraction thereof, or of the organic phase after solvent-extraction is retained in the distillation residue.

In yet another embodiment, at least 50 % w/w the humins in the biomass hydrolysate, a liquid fraction thereof, or the organic phase after solvent-extraction is retained in the distillation residue.

In an embodiment, the process comprises two or more distillation steps, wherein the biomass hydrolysate, or liquid fraction thereof, or the organic phase after a solvent-extraction is subjected to a first distillation to yield a first distillate comprising solvent and a first distillation residue comprising levulinic acid and optionally humins and/or colour, recovering the first residue, and subjecting the first residue to a second distillation to yield a second distillate comprising levulinic acid and a second distillation residue, whereby the base is added to the second distillation residue.

Therefore, in an embodiment the process of the invention comprises:

- subjecting a biomass hydrolysate to a first distillation to yield a first distillate comprising solvent, and a first distillation residue comprising levulinic acid and optionally humins and/or colour;

- recovering the first residue and subjecting the first residue to a second distillation to yield a second distillate comprising levulinic acid and a second distillation residue;

- adding a base to the second distillation residue;

- optionally cooling the second distillation residue after adding the base; and

- recovering the distillation residue. In another embodiment the process of the invention comprises:

- recovering the liquid fraction and subjecting said liquid fraction to a first distillation to yield a first distillate comprising solvent, and first distillation residue comprising levulinic acid and optionally humins and/or colour;

- recovering the first residue and subjecting the first residue to a second distillation to yield a second distillate comprising levulinic acid and a second distillation residue; - adding a base to the second distillation residue;

- optionally cooling the second distillation residue after adding the base; and

- recovering the second distillation residue.

In another embodiment the process of the invention comprises:

- recovering said organic phase;

- subjecting said organic phase to a first distillation to yield a first distillate comprising solvent, and a first distillation residue comprising levulinic acid and optionally humins and/or colour;

- adding a base to the second distillation residue;

- optionally cooling the second distillation residue after adding the base; and

- recovering the second distillation residue.

In another embodiment the process of the invention comprises:

- recovering the organic phase and subjecting said organic phase to a first distillation to yield a first distillate comprising solvent, and a first distillation residue comprising levulinic acid and optionally humins and/or colour; - recovering the first residue and subjecting the first residue to a second distillation to yield a second distillate comprising levulinic acid and a second distillation residue;

- adding a base to the second distillation residue;

- optionally cooling the second distillation residue after adding the base; and

- recovering the distillation residue.

Most of the colour in the process of the invention is believed to come from humins, and humins are believed to form a substantial part of the distillation residue. The (amount of) colour can be quantitatively determined by absorption or extinction spectrometry. Any simple spectrophotometer is suitable which is able to measure wavelengths anywhere between 400 and 800 nanometres. Colour may be correlated with the amount of contaminants, particularly with (soluble) humins, and measuring the colour is a convenient means to quantify the amount of such contaminants.

Colour can be expressed as OD (optical density). A suitable wavelength to measure colour is 600 nm. For example, if the OD at 600 nm and 1 cm path length of the organic phase is 1.0, the OD in the retentate at 600 nm and 1 cm path length is preferably at least 0.5, more preferably at least 0.7, more preferably at least 0.8, 0.9, even more preferably at least 0.95; most preferably the OD in the retentate at 600 nm and 1 cm path length is 1 .0.

If the colour is too intense to be measured, the sample (permeate, retentate etc.) can be diluted with water or solvent before taking the OD. Alternatively, the shorter path length may be used, e.g. 1 mm. Suitable OD values (after optional dilution) may range between 0.1 and 2. The skilled person knows how to dilute samples prior to measuring the OD and to calculate the amount of humins, and how to correct for dilution and volume. If desired, a calibration curve can be made using a humins solution of known concentration. Alternatively, the OD can be used to quantify the efficiency of the distillation in relative terms. For example, if the OD of a biomass hydrolysate, at a certain wavelength and path length, corresponds to 1 .0, and the OD of the distillation corresponds to 0.2 (correcting for dilution and volume), 80% of the colour, and thus of the humins are retained in the distillate.

In the process of the invention a base is added to the distillation residue. It may be important to maintain the residue at elevated temperature after levulinic acid has been recovered. If the temperature drops too much, the distillation residue may become too viscous and it may be impossible to render the residue flowable even when base is added.

The base can be added as a solution, for example as a concentrated NaOH or KOH solution. Adding a concentrated basic solution has the advantage that the volume of the distillation residue remains limited. In an embodiment, the process comprises removing the distillation residue from the distillation prior to the step of adding the base. After removal it can subsequently be transferred to a separate reactor, and the base can subsequently be added to the recovered distillation residue. Alternatively, the distillation residue can be removed from the distillation and transferred to a reactor which already contains the base. Thus, in the context of the invention, "adding a base to the distillation residue" is understood to include both adding base to the distillation residue and adding distillation residue to the base, or combinations thereof. During the addition of the base the distillation residue is preferably stirred so achieve good mixing.

The step of adding the base is preferably done at a temperature of 50°C or more, more preferably at a temperature of 60°C or more, a temperature of 70°C or more, even more preferably at a temperature of 80°C or more, a temperature of 90°C or more, even more preferably at a temperature of 100°C or more.

It may be important to keep the distillation residue at elevated temperature prior to the step of adding the base, and preferably also during the step of adding the base, in order to avoid the residue from becoming too viscous or even becoming, preferably the distillation residue is kept at a temperature of 50°C or more, more preferably at a temperature of 60°C or more, a temperature of 70°C or more, even more preferably at a temperature of 80°C or more, a temperature of 90°C or more, even more preferably at a temperature of 100°C or more.

Preferably the temperature of the distillation residue between removal from the distillation unit and the step of adding the base does not drop below 50°C, more preferably not below 60°C, below 70°C, even more preferably below 80°C, below 90°C, even more preferably the temperature of the distillation residue between removal from the distillation unit and the step of adding the base does not drop below 100°C.

The base is added preferably in an amount to arrive at a final concentration of between 0.01 - 50 % (w/w), relative to the total weight of the distillation residue. More preferably the final amount of the base is between 0.1 and 50 wt%. The base is preferably added as a solution as this makes the handling easier. The concentration of the base in such a solution is not crucial. If the concentration is lower, simply more of the solution can be added to arrive at the suitable concentration; if the concentration is higher, simply less of the solution can be added to arrive at the suitable concentration.

After the base has been added, the distillation residue is optionally cooled, e.g. to ambient temperature. Once the base has been added, the residue does not have to be kept anymore at elevated temperature because it will stay flowable even at lower temperature. The base is preferably selected from NaOH and KOH, preferably it is NaOH. The use of NaOH is particularly advantageous since it allows the residue to be used as so-called "black liquor" in a paper mill. Black liquor is the term for spent cooking liquor from a paper mill using the Kraft process. Pulp is digested into pulp, whereby lignin, hemicelluloses and other extractives are removed, generating a cellulose fiber fraction. The black liquor is generally used to generate energy by burning.

Therefore, in an embodiment the process of the invention further comprises feeding the recovered distillation residue (i.e. after adding the base) to a paper plant, e.g. to provide energy.

In a further aspect the invention provides a distillation residue obtainable by the process of the process of the invention. Compared to distillation residues obtained in a process to separate levulinic acid without the step of adding a base, the distillation residue obtainable by the process of the invention has a lower viscosity at low temperature. For example, at a temperature of 25°C it is readily flowable. Said residue may be advantageously used as, or fed to a black liquor stream in a paper mill.

In another aspect the invention provides the use of the distillation residue obtainable by the process of the invention as energy source in a paper plant.

The invention will be further elucidated with reference to the following examples, without however being limited thereto.

EXAMPLE

A reaction mixture was obtained via acid hydrolysis of wood (sieved fraction less than 1 mm in size) in an aqueous environment. Reaction conditions: H₂S0₄, 4% w/w; temperature, 195°C; pressure, 20 bar (kept constant using nitrogen); time, 30 minutes.

Via a solid/liquid separation, using a 0.2 urn filter, char was separated. The resulting liquid fraction was extracted with a suitable organic solvent in order to extract the levulinic acid from the acidic water phase. Subsequently the extract was subjected to a distillation in order to remove the solvent and to recover the levulinic acid as pure product. The final levulinic acid distillation is performed at a pressure of 5 mbar until the bottom temperature reaches 150-160 °C. Pure levulinic acid was recovered as a distillate. The in the bottom remaining distillation residue was brown in colour. About one half of the distillation residue was allowed to cool to room temperature. The cooled residue was very viscous, not flowable, and therefore difficult to remove from the distillation column. The other half of the distillation residue was kept at elevated temperature (> 80°C), and diluted with two parts (on weight basis) of a 10 wt% NaOH solution under stirring while the distillation residue was kept at mentioned temperature. When this mixture was cooled down to room temperature it was still flowable and easily removed from the distillation column.

Claims

1 . A process for the separation of levulinic acid from a biomass hydrolysate, said process comprising:

- adding a base to said distillation residue;

- optionally cooling the distillation residue after adding the base; and

- recovering the distillation residue.

2. Process according to claim 1 wherein the amount of levulinic acid in the residue is 20% w/w or less, relative to the total dry weight of the residue.

3. Process according to claim 1 or 2 wherein at least 90% w/w of the levulinic acid is recovered as a distillate, relative to the amount of in the biomass hydrolysate prior to distillation.

4. Process according to any one of claim 1 -3 wherein the biomass hydrolysate comprises colour, and where at least 50% of said colour is retained in the distillation residue, as measured by absorption or extinction spectrophotometry at a wavelength of between 400 and 800 nm.

5. Process according to any one of claim 1 -4 wherein the biomass hydrolysate comprises humins, and wherein at least 50 % w/w of said humins are retained in the distillation residue.

6. Process according to any one of claim 1 -5 comprising two or more distillation steps, wherein the biomass hydrolysate or liquid fraction or organic phase is subjected to a first distillation to yield a first distillate comprising solvent and a first distillation residue comprising levulinic acid and optionally humins and/or colour, recovering the first residue, and subjecting the first residue to a second distillation to yield a second distillate comprising levulinic acid and a second distillation residue, whereby the base is added to the second distillation residue.

7. Process according to any one of claim 1 -6 comprising:

- removing the distillation residue from the distillation prior to adding the base.

8. Process according to any one of claim 1 -7 wherein the step of adding of the base is done at a temperature of 50°C or more.

9. Process according to any one of claim 1 -8 wherein the step of adding of the base is done at a temperature of 80°C or more.

10. Process according to any one of claim 1 -9 wherein said base is added in an amount to arrive at a final concentration of between 0.01 - 50 % (w/w), relative to the total weight of the distillation residue.

1 1 . Process according to any one of claim 1 -10 wherein the base is selected from NaOH and KOH.

12. Process according to any one of claim 1 -1 1 comprising:

- feeding the recovered distillation residue to a paper plant, e.g. to provide energy.

13. Use of the distillation residue obtainable by the process of any one of claims 1 -12 as energy source in a paper plant.