WO2017060722A1

WO2017060722A1 - Biomass processing method

Info

Publication number: WO2017060722A1
Application number: PCT/GB2016/053127
Authority: WO
Inventors: Edward Leslie Marshall; George Dunstan COOKE
Original assignee: Plaxica Limited
Priority date: 2015-10-08
Filing date: 2016-10-07
Publication date: 2017-04-13
Also published as: GB201517801D0

Abstract

A method of processing a feedstream comprising black or brown liquor which contains less than l%wt of hemicellulosic material, comprising: (a) contacting said feedstream with a C_3-8 alkyl alcohol at elevated temperature and acidic pH to produce a reaction mixture comprising a C_3-8 alkyl ester; (b) separating the reaction mixture into an aqueous phase and an organic phase comprising said C_3-8 alkyl ester; and (c) recovering said C_3-8 alkyl ester and/or lignin.

Description

BIOMASS PROCESSING METHOD

Field of the invention

The present invention relates to processing of lignocellulosic biomass. In particular, it relates to a method of processing specific black or brown liquor streams which have been obtained from biomass. Background of the invention

The three principal components of biomass are cellulose, lignin and hemicellulose, and they are present in almost all plant cell walls. The cellulosic material obtained from such biomass has a number of important industrial uses, notably in the production of paper from wood pulp. Accordingly a variety of processes have been developed for treating biomass to separate cellulosic material from other components of biomass, including the Kraft and sulfite processes. As well as producing cellulosic wood pulp, those processes also result in the production of by-products known as black liquor (Kraft process) or brown liquor (sulfite process) which typically contain hemicellulosic material together with lignin/lignin-derived products and inorganic chemicals. In recent times, demand for wood pulp containing higher cellulose content has been increasing, and processes for producing such forms of wood pulp (known as "dissolving pulp" or "dissolving cellulose") have been developed. Dissolving pulp finds use in the production of products such as rayon, viscose and cellophane. Typically in a process for producing dissolving pulp, an additional "pre-hydrolysis" stage is carried out in which lignocellulosic biomass is treated to remove hemicellulosic material and

lignin/lignin-derived products, prior to subjecting the remainder of the cellulosic solids to further pulping conditions, such as Kraft conditions (treatment with an aqueous solution of sodium hydroxide and sodium sulphide at elevated temperature) or sulfite conditions

(treatment with aqueous metal sulfite and/or bisulfite at elevated temperature).

Whilst the cellulosic material obtained from processes such as those outlined above is taken on and processed into various useful products, the hemicellulosic streams are often considered to be of little value and may be burnt or fed into a gasifier to recover their energy value. Our copending application PCT/GB2015/051081 claims a method of processing an aqueous hemicellulosic stream comprising contacting an aqueous hemicellulosic stream with a C3-8 alkyl alcohol at elevated temperature and acidic pH to produce a reaction mixture comprising a C3-8 alkyl ester and a hemicellulose-derived monosaccharide; and separating the reaction mixture into an aqueous phase comprising said hemicellulose-derived

monosaccharide and an organic phase comprising said C3-8 alkyl ester. Black and brown liquors containing hemicellulose may be used as feedstock in this process, as can other hemicellulosic streams.

However, in many biomass processing methods, the hot and strongly alkaline or acid conditions used can lead to rapid hemicellulose hydrolysis and destruction of the resulting sugars, which means that black and brown liquors are often obtained which contain no, or very low levels of, hemicellulosic material. These aqueous liquors contain both lignin and acetic acid, making their processing challenging. We have now found a process which enables the production of further useful products from such liquors. Specifically, we have found a process which allows biomass to be treated in a first, conventional, step, to produce a brown or, especially, a black liquor stream which contains no, or very low levels of, hemicellulosic material, which stream is then treated in a subsequent step which gives efficient lignin removal and yields useful products.

Summary of the invention The present invention provides a method of processing a feedstream comprising black or brown liquor which contains less than l%wt of hemicellulosic material, comprising:

(a) contacting said feedstream with a C3-8 alkyl alcohol at elevated temperature and acidic pH to produce a reaction mixture comprising a C3-8 alkyl ester;

(b) separating the reaction mixture into an aqueous phase and an organic phase comprising said C3-8 alkyl ester; and

(c) recovering said C3-8 alkyl ester and/or lignin.

Detailed description of the invention Black liquor is an aqueous waste product stream obtained from the Kraft process, which is a process for digesting biomass into delignified pulp, particularly wood into woodpulp. The biomass feedstock is treated under highly alkaline conditions to separate the cellulosic pulp from lignin, hemicellulose and other constituents of biomass. The

composition of the black liquor can vary widely: it will generally include lignin and organic acids, and may include hemicellulose or hemicellulose-derived sugars, see for example "Extraction of lignin and hemicelluloses from Kraft black liquor", Wallberg et al,

Desalination 2006, vol. 199, 413-414. Under typical mill operations, black liquor is evaporated to c. 65-85% solids and incinerated. As a result the term 'black liquor' is often used to describe a continuum of concentrations varying from 'light (or weak) black liquor' to 'strong black liquor' . Light black liquor, i.e. the material that is produced directly from the Kraft cook before being subjected to evaporation typically contains 10-20% wt solids, of which 60-75%) are organics and 25-40%) are inorganics, the organics comprising about 30- 50%) wt. lignin. Hemicellulose content (e.g. xylan, glucan, mannan, arabinan and galactan) may be <1% wt. In the Kraft process, typically wood chips are mixed with an aqueous solution of sodium hydroxide and sodium sulfide (a mixture known as "white liquor") in a pressurisable vessel known as a digester. The white liquor selectively attacks and dissolves the lignin, in the process becoming "black liquor" (also known as hydrolysate). The process converts the sodium salts into a mixture of sodium carbonate and sodium sulfate (the Kraft process is sometimes called the sulfate process). After a few hours the digester discharge - a mixture of pulp and black liquor (i.e. spent chemicals and wood waste) - is separated and the pulp is counter-current washed to remove any further traces of the black liquor, before being subjected to a bleaching process (which renders any residual lignin chromophores inert). The black liquor, containing lignin and other residues, is ca. 15%> wt. solids at this stage (often termed "light black liquor"). It is subsequently concentrated via a series of evaporators, to ca. 65-80%o solids - "heavy black liquor" - before being combusted in a recovery boiler.

Brown liquor is the corresponding aqueous waste product stream obtained from the sulfite process for digesting biomass into pulp, particularly wood into woodpulp. The biomass feedstock is treated under acidic conditions using various sulfite salts. In a preferred embodiment of the process of the invention, the feedstock is black liquor. Throughout this Specification, unless the context requires otherwise, any reference to a feedstock for the process of the invention should be understood to include a specific reference to black liquor.

Hemicellulose is one of the three principal components of biomass, together with cellulose and lignin. Hemicelluloses are the second most abundant biopolymer in the plant kingdom after cellulose and they constitute, in general, 15 - 35% of plant biomass. Whereas cellulose is a highly uniform linear polysaccharide (it is a l→4-P-linked polyglucan), the term hemicellulose defines a group of heterogeneous polysaccharides of comparatively low molecular weight, having a degree of polymerisation of from about 40 to about 600 (in many cases the degree of polymerisation is from about 80 to about 200). Most hemicelluloses are branched structures (see for example Ren and Sun, Cereal Straw as a Resource for

Sustainable Biomaterials and Biofiiels; Chemistry, Extractives, Lignins, Hemicelluloses and Cellulose, 2010, Chapter 4; also Girio et al, Bioresource Technology, 2010, 101 p4775- 4800).

Hemicelluloses have been classified into four groups: i) xyloglycans (xylans); ii) mannoglycans (mannans); iii) xyloglucans (XG); and iv) mixed-linkage β-glucans (Ren and Sun, Cereal Straw as a Resource for Sustainable Biomaterials and Biofuels; Chemistry, Extractives, Lignins, Hemicelluloses and Cellulose, 2010, Chapter 4).

Xylans comprise a P(l→4)-D-xylanopyranose backbone, and typically contain carbohydrate groups on the 2- or 3- position of backbone residues. Examples include glucuronoxylans (GX), arabino(glucurono)xylans (AGX), glucurono(arabino)xylans (GAX) and arabinoxylans (AX). Xylans are the most common hemicelluloses, and in particular are abundant in hardwood or annual plants.

Mannans have been categorised in two groups: i) galactomannans, which comprise a β(1→4) linked D-mannopyranose backbone; and ii) glucomannans, which have a backbone comprising D-mannopyranose and D-glucopyranose residues with β(1→4) linkages.

Mannans may have varying degrees of branching, with D-galactopyranose groups on the 6- position of the mannose backbone.

Xyloglucans (XG) comprise a P(l→4)-linked D-glucopyranose backbone with D- xylanopyranose residues at the 6-position of glucopyranose residues. There are two categories of xyloglucans, depending on the nature of the xylanopyranose-containing side- chains. Xyloglucans comprising two xylanopyranose units followed by two

glucanopyranose units are referred to as XXGG, and xyloglucans comprising three xylanopyranose units followed by one glucopyranose unit are referred to as XXXG.

Additional side-chains may also be present.

Mixed linkage β-glucans have a D-glucopyranose backbone with mixed β linkages (1→3, 1→4).

Throughout this specification and claims, the term "hemicellulosic material" should be understood to mean hemicellulose and sugars derived from hemicellulose. The present invention uses as feedstream a black or brown liquor which comprises less than l%wt hemicellulosic material, i.e. the sum of hemicellulose and sugars derived from hemicellulose in the feedstream is less than l%wt. Preferably the feedstream comprises less than 0.8%wt, less than 0.5%wt, or less than 0.25%wt hemicellulosic material. For example, feedstocks may be used which are free from hemicellulosic material.

Suitable feedstreams for the present invention are generally obtained by the processing of biomass by processes other than the organosolv process, for example by dissolving pulp processes such as those that may be operated at a Kraft pulp mill, or the sulfite process.

Generally lignin-containing black or brown liquor streams obtained by such processes are regarded as low value streams which are incinerated for energy and steam generation.

Accordingly, the invention is particularly valuable when carried out using a black or brown liquor stream which has been obtained by treatment of biomass by a method other than the organosolv process, i.e. by a method other than the fractionation of biomass into three separate streams, being a cellulosic stream, a black or brown liquor stream, and a lignin stream, using an organic solvent, particularly an alcohol, for example a C2-8, especially C3-8, alkyl alcohol. For example, the black or brown liquor stream may have been obtained by using one of the processes described in more detail below. It may for example have been obtained by the hydrolysis of wood chips using hot water under pressure, or using steam. Alternatively the black liquor may be produced from a Kraft dissolving pulp (speciality cellulose) mill. In a dissolving pulp process, high temperature steam or water treatment of wood chips is used to partially degrade the lignocellulosic structure to form a pre-hydrolysis liquor ("PHL") which contains some hemicellulosic material and some lignin. The PHL is then removed and the remaining material subject to standard Kraft cooking conditions to produce high purity cellulose and a black liquor stream.

The feedstream will generally contain lignin, and the organic phase produced in step (b) will contain lignin in addition to the C3-8 alkyl ester. Surprisingly, it appears to be the case that the presence of acid, for example added sulfuric acid, which catalyses the esterification of organic acids, particularly acetic acid, present in the feedstream, also enhances the extraction of lignin into the organic phase. This effect could not have been predicted, and the process of the invention provides a surprisingly efficient way of not only removing organic acid impurities ("wood acids") from the feedstream and converting them into valuable products (C3-8 alkyl esters) but also lignin from the feedstream.

Spent inorganic chemicals from initial processing of biomass may also be present in the feedstream (e.g. metal sulfate and/or carbonate, or metal sulfites). These will generally be extracted into the aqueous phase obtained in step (b) of the process of the invention. In one embodiment, the feedstream used in the process of the invention is black liquor. As discussed above, black liquor is a by-product of the Kraft process and is an aqueous mixture which may contain hemicellulosic material together with lignin/lignin- derived products and inorganic chemicals (e.g. sodium sulphate, sodium carbonate). Black liquor which contains less than l%wt hemicellulosic material is used in the process of the present invention. Typically, the light black liquor obtained following digestion of biomass (e.g. wood chips) and prior to any evaporation process contains approximately 15% by weight solid material, and is termed weak black liquor. Subsequent processing of the weak black liquor, for example by evaporating water, results in a substance which typically contains up to about 65 to 80% by weight solid material, which is termed heavy black liquor.

In another embodiment, the feedstream used in the process of the invention is brown liquor. As discussed above, brown liquor is a by-product of the sulfite process for making wood pulp, and is an aqueous mixture which may contain hemicellulosic material, lignin/lignin-derived products and inorganic chemicals. Brown liquor is also referred to as red liquor, thick liquor, spent liquor and sulfite liquor. Brown liquor which contains less than l%wt hemicellulosic material is used in the process of the present invention.

Many types of biomass are processed using methods which result in black or brown liquor. The biomass from which the feedstream for the present invention is obtained may for example comprise a hardwood, for example an Acer (maple), Populus (aspen), Betula (birch), Fagus (beech), Eucalyptus, Quercus (oak) Populus (poplar) or Liquidambar (sweetgum); or a softwood, for example an Abies (fir), Larix (larch), Picea (spruce) or Pinus (pine); or a grass, for example a Panicum (e.g. Panicum virgatum, switch grass), a Sorghum (e.g. sweet sorghum) or a Saccharum (e.g. sugar cane).

If desired, the feedstream may be pretreated before carrying out step (a). In one embodiment no pretreatment is carried out. In another embodiment, the feedstream is concentrated to increase its solids content. In any event, the feed to step (a) of the process of the invention must contain less than l%wt hemicellulosic material.

The process of the invention may be carried out batch wise, or as a continuous or semi-continuous process.

Constituents of biomass typically have a significant degree of acylation, and as the constituents of biomass are broken down, significant quantities of organic acids (e.g. acetic acid, formic acid) are formed. Under the process conditions of the present invention, a significant proportion of the organic acids and/or acyl moieties forming part of the feedstream are converted to the corresponding C3-8 alkyl ester, permitting separation of the alkyl ester from other impurities in organic and aqueous phases.

In one preferred embodiment the alkyl alcohol is a C3-6 alkyl alcohol. In one preferred embodiment the C3-8 alkyl alcohol contains only one hydroxyl group. Examples of preferred alkyl alcohols include i-propanol, n-butanol, n-pentanol, n-hexanol and 2- ethylhexanol.

In one preferred embodiment, the alkyl alcohol is a C4-8 alkyl alcohol containing only one hydroxy group. The use of a C4-8 alkyl alcohol containing only one hydroxy group results in the presence of a biphasic mixture in step (b), facilitating removal of acyl- containing species in step (b) due to preferential partitioning of the alkyl esters, plus any lignin present, into the organic phase. Most preferably the C3-8 alkyl alcohol is n-butanol.

Step (a) is carried out at acidic pH, i.e. at a pH of less than 7. Preferably step (a) is carried out at a pH of less than 5, less than 4, less than 3 or less than 2. If the feedstream is itself acidic, simply contacting the feedstream with C3-8 alkyl alcohol at elevated temperature may be sufficient to produce C3-8 alkyl esters. However, in some cases it may be preferred to add an acid in step (a) to catalyse the formation of the C3-8 alkyl esters. Some feedstocks may in fact be alkaline, and in this case sufficient acid needs to be added to neutralise alkali in the feedstock and then to acidify it. Where an acid is added in step (a), it may for example be a mineral acid, or an organic acid such as trifluoroacetic acid or methanesulfonic acid.

Alternatively a solid-state resin can be added as an acid catalyst. Preferably the acid added in step (a) is a mineral acid, more preferably a mineral acid selected from the group consisting of hydrochloric acid and sulfuric acid (e.g. concentrated hydrochloric acid or concentrated sulfuric acid). In one embodiment the acid is hydrochloric acid. In another embodiment the acid is sulfuric acid. In one embodiment concentrated sulfuric acid or concentrated hydrochloric acid is added, in a volume ratio in the range of from 0.1 : 100 to 3 : 100 relative to the total volume of solvent used in step (a).

Preferably step (a) is carried out at a temperature in the range of from 30 to 250°C; more preferably in the range of from 70 to 200°C; still more preferably in the range of from 90 to 170°C or from 100 to 190°C. Where the C3-8 alkyl alcohol used in step (a) is n-butanol, step (a) is for example carried out at a temperature in the range of from 95 to 150°C, or from 130 to 190°C; or for example in the range of from 100 to 125°C. The use of higher temperatures can reduce the reaction time leading to improved process economics. Heating under reflux may be preferred in many cases. Where step (a) is carried out batchwise, it may for example be carried out over a time period in the range of from 5 minutes to 3 hours; for example in the range of from 5 minutes to 1 hour.. In one preferred embodiment step (a) is carried out at a temperature in the range of from 70 to 200°C, and over a time period in the range of from 5 minutes up to 2 hours, for example from 5 minutes to 1 hour. However, in a further preferred embodiment, higher temperatures and lower reaction times may be used, for example step (a) may be carried out at a temperature in the range of 130 to 190°C for a time period of less than an hour, for example from 1 to 20 minutes.

Where an acid is added in step (a), the feedstream may be admixed with acid prior to, at the same time as, or after admixing with C3-8 alkyl alcohol. For example, C3-8 alkyl alcohol may be added to the feedstream followed by the acid, and the mixture then heated at elevated temperature. Alternatively the acid may be added to the feedstream and the mixture heated at elevated temperature, with the C3-8 alkyl alcohol then being added and the resulting mixture being heated at elevated temperature for a further period. Alternatively the acid may be added to the C3-8 alkyl alcohol, and the resulting mixture then added to the feedstream and the mixture heated at elevated temperature.

Step (a) is typically carried out at ambient pressure, or at higher or lower pressure if desired. In one embodiment, it is preferred to carry out the process at elevated pressure, for example at a pressure of up to 20barg.

Step (a) may for example be carried out by admixing the feedstream with an acid selected from the group consisting of hydrochloric acid and sulfuric acid and heating under reflux for an initial period; following optional cooling, the resulting reaction mixture is mixed with n-butanol and heated under reflux for a further period.

The conditions under which step (a) is carried out will determine whether the reaction mixture is a one-phase or a two-phase system. Generally, it may be preferred to carry out the reaction at a sufficiently high temperature for the reaction mixture to be a one-phase system. Cooling of a one-phase reaction mixture from step (a) will lead to a separation of the aqueous and organic phases, for separation in step (b).

In step (b), the products obtained from step (a) are separated into an aqueous phase comprising various impurities present in the original feedstream and an organic phase comprising a C3-8 alkyl ester.

As discussed above, where a C4-8 alkyl alcohol containing only one hydroxy group is used, a biphasic mixture results following step (a) without the need for additional organic solvent. However, if desired, additional organic solvent and/or water may be added prior to separation. In one preferred embodiment, where additional organic solvent is used in step (b) the additional organic solvent is a C4-8 alkyl alcohol containing only one hydroxy group, and the same type of alkyl alcohol is used in steps (a) and (b); more preferably the C4 -8 alkyl alcohol used in step (a) and additional organic solvent used in step (b) are both n-butanol.

In step (b) the organic and aqueous phases may be separated by routine techniques; for example by removing the bottom layer from a vessel through a bottom run-off valve, by decanting or siphoning off the top layer, or in a liquid-liquid phase separator. Step (b) may be carried out at ambient temperature, or at a higher or lower temperature if desired. In a preferred embodiment, step (b) is carried out at elevated temperature, preferably at a temperature in the range of from 30 to 95°C, more preferably in the range of from 40 to 90°C. Step (b) is typically carried out at ambient pressure, but it may be carried out at higher or lower pressure if desired.

In step (c), the C_3-8alkyl esters and/or lignin are recovered. In one embodiment, the C_3-8alkyl esters are recovered from the organic phase obtained in step (b). In another embodiment, lignin is recovered. And in a preferred embodiment, both the C_3-8alkyl esters and the lignin are recovered. In a further preferred embodiment, the C_{3 -}8 alcohol is also recovered, and may if desired be recycled back to step (a) of the process. Distillation provides a convenient way of separating and recovering liquid products. In one preferred recovery process, the organic phase is evaporated to produce a top stream comprising the esters together with some alcohol and water, and a bottom stream or residue which will mainly comprise lignin and alcohol. This bottom stream or residue may if desired be further treated by adding to another vessel containing water, and heated to remove any residual alcohol or esters, causing lignin to crystallise out. After such processing, the water-wet lignin is readily removed by filtration.

Additional routine processing steps may be carried out at any stage of the process, e.g. to add or remove solvent. By way of example, the feedstream may be concentrated to remove some water prior to carrying out step (a), provided that the feedstream to step (a) must always contain less than l%wt hemicellulosic material.

The following Example illustrates the invention. Example 1. Processing of Black Liquor containing less than l%wt hemicellulosic material

The feedstock black liquor contained less than 1% hemicellulosic material - analysis by IC showed less than O.Olmg/ml (the limit of detection) of monosaccharides. To a 250 mL round bottomed flask equipped with an overhead stirrer and reflux condenser was charged black liquor (100 mL /107.1 g) followed by concentrated sulphuric acid (5.0 mL / 9.2 g) and n- butanol (50 mL / 41.0 g). The biphasic mixture was stirred at 300 rpm and heated to reflux for 3 hours. The reaction mixture was then cooled to ambient temperature and the layers were separated. The organic phase (60 mL / 53.6 g) was dark brown / black and the lower aqueous phase (90 mL / 100.5 g) was pale brown.

The separated phases were analysed for organic acid content by HPLC and ester content by GC. The black liquor feed was also analysed for organic acid content by HPLC and lignin content by UV spectroscopy and the following results were obtained:

Black liquor aqueous layer organic layer

%w/v %w/v %w/v acetic acid 1.11 0.38 0.67

formic acid 0.63 0.22 0.18

glycolic acid 0.28 0.21 0.099

lactic acid 0.36 0.16 0.16

butyl acetate n/a 0.0069 0.98

butyl formate n/a 0.0060 0.92

butyl glycolate n/a not tested 0.11

butyl lactate n/a not tested 0.21

lignin 5.25 0.30 not tested

The separated organic layer (58.3 mL / 52.1 g) was charged to a 100 mL round bottomed flask equipped with a magnetic stirrer bar and set up to carry out a side-arm distillation. The mixture was stirred at 500 rpm and heated in an oil bath at 180°C and distillate was continuously removed until no further take-off was observed. The recovered biphasic distillate layers were then separated yielding a water white organic phase (47ml / 39. Og) and a water white lower aqueous phase (3.5 mL / 3.3 g).

50 mL demineralised water then was added to the flask residues and the mixture was stirred at 500 rpm and heated in an oil bath at 180°C. Distillate was continuously removed until no further take-off was observed. The recovered distillate (52.0 mL / 50.9 g) was monophasic. The separated phases from the organic layer distillation and the solids wash distillate were analysed for organic acid content by HPLC and ester content by GC and the following results were obtained: aqueous layer organic layer solids wash

%w/v %w/v %w/v

acetic acid 0.10 0.16 0.0011 formic acid 0.031 0.28 0.0002 glycolic acid 0.0007 0.00 0.0003 lactic acid 0.0014 0.00 0.0004 butyl acetate 0.024 0.12 0.031 butyl formate 0.029 0.072 0.042 butyl glycolate not tested 0.0026 not tested butyl lactate not tested 0.0069 not tested

The dark brown solid residues (lignin) were dried to constant weight (6.95 g) in an air bath at 60 °C.

The results show that the treatment of black liquor containing less than l%wt hemicellulosic material with C3 -8 alkyl alcohol at elevated temperature and acidic pH (i) produces the corresponding alkyl esters of the acids contained within the black liquor, and (ii) permits the independent recovery of the lignin and the alkyl esters.

Claims

1. A method of processing a feedstream comprising black or brown liquor which contains less than l%wt of hemicellulosic material, comprising:

(a) contacting said feedstream with a C3 -8 alkyl alcohol at elevated temperature and acidic pH to produce a reaction mixture comprising a C3-8 alkyl ester;

(c) recovering said C3-8 alkyl ester and/or lignin.

2. A process as claimed in claim 1, wherein the feedstream is black liquor.

3. A process as claimed in either claim 1 or claim 2, in which the feedstream contains less than 0.25%wt hemicellulosic material.

4. A process as claimed in any one of claims 1 to 3, wherein the C3-8 alkyl alcohol is a C3-6 alkyl alcohol.

5. A process as claimed in claim 4, wherein the C3 -8 alkyl alcohol is n-butanol.

6. A process as claimed in any one of claims 1 to 5, wherein step (a) is carried out at a temperature of from 95°C to 200°C.

7. A process as claimed in any one of claims 1 to 6, wherein in step (a) the feedstream is admixed with an acid.

8. A process as claimed in claim 7, wherein the acid is selected from the group consisting of hydrochloric acid and sulfuric acid.

9. A process as claimed in any one of claims 1 to 8, in which in step (c) the organic phase is evaporated to produce a top stream comprising C3-₈alkyl esters and a bottom stream or residue comprising lignin.

10. A process as claimed in any one of claims 1 to 9, in which lignin is recovered from the process.

11. A process as claimed in any one of claims 1 to 10, in which C3-salcohol is recovered from the process.

12. A process as claimed in claim 11, in which the recovered C3-salcohol is recycled to step (a) of the process.

13. A process as claimed in any one of claims 1 to 12, in which the alcohol is n-butanol, and wherein n-butyl acetate is recovered in step (c).