WO2009080737A2

WO2009080737A2 - A process for converting lignocellulose into sugars

Info

Publication number: WO2009080737A2
Application number: PCT/EP2008/067983
Authority: WO
Inventors: Evert Van Der Heide; Munro Mackay; Ting Zhang
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2007-12-21
Filing date: 2008-12-19
Publication date: 2009-07-02
Also published as: WO2009080737A3

Abstract

A process for converting lignocellulose into sugars comprising the following steps: (a) chemically pre-treating lignocellulose-comprising solid biomass by contacting the biomass with a cooking liquid comprising at least 40 vol% formic acid and at least 50 vol% organic acid, at a temperature in the range of from 90 to 180 °C, at a biomass to liquid ratio in the range of from 1:3 to 1:50 to obtain a pre-treated solid residue and spent cooking liquid; (b) separating the solid residue from the spent cooking liquid; (c) washing the solid residue with washing liquid comprising at least 40 vol% formic acid and at least 50 vol% organic acid to obtain washed solid residue and spent washing liquid; (d) separating the washed solid residue from the spent washing liquid; (e) supplying the spent washing liquid obtained in step (d) to step (a) to form part of the cooking liquid that is contacted with the biomass; (f) evaporating free organic acids from the washed solid residue to obtain evaporated washed solid residue; (g) subjecting the evaporated washed solid residue to enzymatic hydrolysis to obtain sugars.

Description

A PROCESS FOR CONVERTING LIGNOCELLULOSE INTO SUGARS

Field of the invention

The invention provides a process for converting lignocellulose into sugars. The process is particularly suitable for preparing sugars from biomass for fermentation into bio-ethanol. Background of the invention

Bio-ethanol is in practice mainly produced by fermentation of sugars derived from corn starch or sugar cane. There is, however, a strong drive to produce ethanol from lignocellulosic biomass, also referred to as cellulosic biomass, such as agricultural wastes, grasses, forestry wastes, and sugar processing residues. It is known to convert lignocellulosic biomass into sugars that can be fermented into ethanol by first pre-treating the lignocellulosic biomass to make the cellulose, and optionally also the hemicellulose, digestible for enzymes that are able to depolymerise the cellulose and hemicellulose into their monomeric sugars. The pre- treated biomass is then subjected to enzymatic hydrolysis using cellulase enzymes, and optionally also hemicellulase enzymes, to obtain the monomeric sugars.

Biomass pre-treatment processes that improve enzymatic digestibility are known in the art. Currently applied pre-treatment methods usually involve steam explosion and/or dilute acid pre-hydrolysis . In US 4,461,648 for example, steam explosion of lignocellulosic feedstock in the presence of sulphuric acid is disclosed. In WO 2006/034590, a continuous steam explosion pre-treatment process in the presence of acid with steam recovery is disclosed. An important disadvantage of steam explosion pre-treatment processes and dilute acid pre-hydrolysis processes is that lignin is not removed from the lignocellulosic biomass. The lignin present has a negative effect on the enzymatic hydrolysis and gives higher handling costs in the downstream processing, in particular in the distillation of ethanol from the fermented sugar stream. Another disadvantage of steam explosion processes is the high temperature and pressure required. Also in dilute acid pre-hydrolysis processes, high pre-treatment temperatures are required, typically above 160 ⁰C.

Alternative pre-treatment processes have been mentioned. In Lynd et al . , Microbial Cellulose Utilization: Fundamentals and Biotechnology, Microbiology and Molecular Biology Reviews, 66 (2002), p 506-577, for example, hydrothermal processes, organosolv processes using organic solvents in an aqueous medium, ammonia fibre explosion (AFEX) , and alkaline pre-treatments have been mentioned.

In the production of paper pulp, it is known to use organosolv processes for the removal of lignin and hemicellulose form lignocellulosic feedstock. In US 6,183,597, a process is disclosed for producing pulp from cellulose containing material using aqueous formic acid as solvent under reflux conditions. In WO 99/57364 for example is disclosed a process based on formic acid cooking for producing pulp from herbaceous plants and deciduous trees wherein a cooking liquid comprising formic acid, acetic acid and water is used. Similarly, in WO 97/26403, a process is disclosed for the production of pulp from herbaceous plants wherein a cooking liquid comprising formic acid and hydroperoxide is used. For pulp and paper production, free and bound formic acid has to be removed from the cooked biomass in order to avoid negative effects of the acid on paper quality. In WO 99/57364, it is disclosed to remove free formic acid from pulp by countercurrent washing with water followed by removal of bound formic acid by hydrolysis at a temperature of 50-95 ⁰C for 1 to 3 hours. In WO 99/10595 a similar procedure is disclosed for removing free and bound formic aid from formic acid cooked pulp. Part of the free formic acid is first removed with water washing to adjust the free formic acid content to a value between 3 and 20 wt% and then the bound formic acid is allowed to hydrolyse at a temperature in the range of from 50 to 95 ⁰C and a reaction time between 0.5 and 4 hours using free formic acid as catalyst .

An important disadvantage of using water as washing liquid is that an energy-intensive separation step is needed for recovery of formic acid from the spent washing water . Summary of the invention

It has now been found that, if lignocellulosic biomass is cooked with a liquid based on formic acid as pre-treatment for enzymatic hydrolysis of the biomass, the work-up for removal of formic acid from the cooked biomass can be done in a much simpler way than is usually done for cooked biomass that is used for pulp and paper production . It has been found that the cooked biomass can be washed with fresh cooking liquid to obtain (i) spent washing liquid that can be used as cooking liquid and (ii) washed, cooked biomass that is suitable, after simple evaporation of free organic acids, as feedstock for enzymatic hydrolysis.

Accordingly, the present invention provides a process for converting lignocellulose into sugars comprising the following steps: (a) chemically pre-treating lignocellulose-comprising solid biomass by contacting the biomass with a cooking liquid comprising at least 40 vol% formic acid and at least 50 vol% organic acid, at a temperature in the range of from 90 to 180 ⁰C, at a biomass to liquid ratio in the range of from 1:3 to 1:50 to obtain a pre-treated solid residue and spent cooking liquid;

(b) separating the solid residue from the spent cooking liquid; (c) washing the solid residue with washing liquid comprising at least 40 vol% formic acid and at least 50 vol% organic acid to obtain washed solid residue and spent washing liquid;

(d) separating the washed solid residue from the spent washing liquid;

(e) supplying the spent washing liquid obtained in step (d) to step (a) to form part of the cooking liquid that is contacted with the biomass;

(f) evaporating free organic acids from the washed solid residue to obtain evaporated washed solid residue;

(g) subjecting the evaporated washed solid residue to enzymatic hydrolysis to obtain sugars.

An advantage of the process according to the invention is that there is no need for a water-washing step and therefore no need to separate formic acid from water for formic acid recovery. Brief description of the drawings

The Figure shows a schematic diagram of the process according to the invention. Detailed description of the invention

In cooking step (a) of the process according to the invention, lignocellulose-comprising solid biomass is chemically pre-treated by contacting the biomass at a temperature in the range of from 90 to 180 ⁰C, preferably of from 100 to 150 ⁰C, more preferably of from 105 to 130 ⁰C, with a cooking liquid having a total organic acid content of at least 50 vol% and a formic acid content of at least 40 vol%.

The cooking liquid has a total organic acid content of at least 50 vol% and a formic acid content of at least 40 vol%. The organic acid content may comprise any suitable organic acid. Preferably, however, the organic acid comprises formic acid and acetic acid. More preferably, the cooking liquid comprises in the range of from 40 to 80 vol% of formic acid. More preferably, the cooking liquid comprises in the range of from 8 to 50 vol% acetic acid, yet more preferred in the range of from 10 to 40 vol% acetic acid, and in the range of from 5 to 35 vol% water, even more preferred in the range of from 10 to 25 vol% water, based on the total volume of cooking liquid. The biomass is contacted with the cooking liquid at a biomass-to-liquid ratio in the range of from 1:3 to 1:50, preferably of from 1:5 to 1:20. Reference herein to the biomass-to-liquid ratio is to the ratio of the volume of biomass to the volume of liquid.

The solid biomass is typically comminuted into pieces or particles of a small size before being contacted with the cooking liquid. Preferably the biomass is comminuted into pieces or particles with an average length of less than 3 cm, more preferably an average length in the range of from 0.5 to 2.5 cm.

Any suitable lignocellulose-containing solid biomass may be used in the process according to the invention.

Examples of suitable biomass are agricultural wastes such as corn stover, soybean stover, corn cobs, rice straw, rice hulls, oat hulls, corn fibre, cereal straw like wheat, barley, rye, and oat straw; grasses such as switch grass, miscanthus, cord grass, rye grass, reed canary grass; forestry biomass such as wood and saw dust; recycled paper or pulp fibres; sugar processing residues such as bagasse and beet pulp. Cereal straw, especially wheat straw, is particularly preferred.

In cooking step (a), at least part of the hemicellulose is hydrolysed and removed from the solid biomass. Also part of the lignin is dissolved or depolymerised and therewith removed from the solid biomass. Thus, the cellulose is made more accessible for subsequent enzymatic hydrolysis. Probably, the cellulose is also partly hydrolysed, thus further improving its enzymatic digestibility. After cooking step (a), typically 30 to 60 wt%, usually 40 to 50 wt%, of the dry content of the solid biomass is dissolved in the spent cooking liquid.

Water present in the solid biomass will also end up in the spent cooking liquid. In order to prevent large water built up in the spent cooking liquid, the solid biomass that is contacted with the cooking liquid preferably has a water content of at most 50 wt%, more preferably at most 30 wt%, even more preferably at most 10 wt%. The biomass will be contacted with the cooking liquid for a time period sufficient to achieve hemicellulose hydrolysis and partial delignification, typically in the range of from 0.3 to 10 hours, preferably in the range of from 0.5 to 5 hours. Typically, cooking step (a) is carried out at a pressure in the range of from 1 to 10 bar, preferably of from 1 to 3 bar.

Cooking step (a) may be carried out in any reactor configuration suitable for solid-liquid contact, for example a co-current, counter-current or flow-through configuration .

By carrying out cooking step (a), spent cooking liquid and pre-treated or cooked residue are obtained. In separation step (b), the pre-treated solid residue is separated from the spent cooking liquid. This may be done by any suitable technique known in the art, for example by pressing, filtration or centrifugation .

The spent cooking liquid obtained in separation step (b) will typically comprise dissolved lignin, hydrolysed hemicellulose and organic acids. Preferably, part of the spent cooking liquid obtained in separation step (b) is recycled to cooking step (a) to form part of the cooking liquid. More preferably, at least 50 wt% of the spent cooking liquid is recycled to step (a) , even more preferably in the range of from 50 to 80 wt% of the spent cooking liquid is recycled. In order to avoid a too large built-up of dissolved lignin and hydrolysed hemicelullose in the cooking liquid, not all spent cooking liquid is recycled to step (a) . A part of the spent cooking liquid is therefore withdrawn or purged from the process. Typically, a built-up of lignin and hydrolysed hemicellulose of up to 25 wt% can be allowed in the spent cooking liquid. Organic acids from the purged spent cooking liquid may be recovered and recycled to washing step (c) as part of the fresh washing liquid.

In order to remove hydrolysed hemicellulose and dissolved lignin, the separated pre-treated solid residue is, in washing step (c), washed with a washing liquid comprising at least 40 vol% formic acid and at least 50 vol% organic acid. The washing step is typically carried out at the temperature in the range of from ambient to 80 ⁰C, preferably of from 20 to 50 ⁰C. Washing step (c) may be carried out using any washing technique known in the art .

In washing step (c), washed solid residue and spent washing liquid are obtained. In separation step (d) , the washed solid residue is separated from the spent washing liquid. This may be done by any suitable technique known in the art, for example by pressing, filtration or centrifugation .

In step (e), the separated spent washing liquid is supplied to step (a) to form part of the cooking liquid that is contacted with the biomass. Typically, the amount of washing liquid required for step (c) is less than the amount of cooking liquid required for step (a) . Therefore, fresh cooking liquid (during start-up of the process) or recycled spent cooking liquid (during steady state operation of the process) will typically be added to the spent washing liquid to form the cooking liquid that is contacted with the biomass in step (a) . If fresh cooking liquid is added, the fresh cooking liquid preferably has the same composition as the washing liquid used in step (c) .

Preferably, the cooking liquid that is contacted with the biomass in step (a) essentially consists of spent washing liquid and spent cooking liquid. The fresh washing liquid, i.e. the washing liquid that is initially contacted with the solid residue has a total organic acid content of at least 50 vol%. Preferably, the total content of organic acids in the liquid is at least 65 vol%, more preferably in the range of from 75 to 90 vol%. At least 40 vol% of the fresh washing liquid is formic acid, preferably in the range of from 40 to 80 vol% is formic acid.

The fresh washing liquid may further comprise water and/or organic compounds known to be suitable organic solvents for organosolv processes. Examples of such solvents are methanol, ethanol, acetone, ethylene glycol, triethylene glycol, tetrahydrofurfuryl alcohol or furfural. Preferably, the fresh washing liquid comprises less than 20 vol%, more preferably less than 10 vol% of organic components other than organic acids . Preferably, the fresh solvent does not contain any inorganic acids such as sulphuric acid, phosphoric acid or hydrochloric acid. It is particularly preferred that the fresh washing liquid essentially consists of organic acids and water. The water content is preferably less than 35 vol%, more preferably less than 25%.

In particularly preferred embodiments, the fresh washing liquid comprises in the range of from 40 to

80 vol% formic acid, in the range of from 8 to 50 vol% acetic acid, even more preferred in the range of from 10 to 40 vol% acetic acid, and in the range of from 5 to 35 vol% water, even more preferred in the range of from 10 to 25 vol% water, based on the total volume of fresh washing liquid.

The washed solid residue obtained in step (e) is subjected to evaporation step (f) . In step (f), remaining free organic acids are evaporated from the washed solid residue to obtain evaporated washed solid residue. The evaporated organic acids are typically recovered and may be recycled to washing step (c) as part of the washing liquid. The evaporated washed solid residue will contain bound formic acid and optionally also bound acetic acid, typically in a concentration in the range of from 1 to 3 wt%.

Evaporation of free organic acids in step (f) is typically carried out at a temperature in the range of from ambient to 100 ⁰C under atmospheric or sub- atmospheric pressure. An inert stripping gas that can be easily separated from the free organic acids may be applied. Examples of suitable inert stripping gases are nitrogen and carbon dioxide. Typical evaporation times range from 0.5 to 5 hours.

The evaporated residue is subjected to enzymatic hydrolysis step (g) to obtain sugars. It is an advantage of the process of the invention that the bound organic acids do not need to be removed from the residue for the residue to be a suitable feedstock for enzymatic hydrolysis step (g) . The evaporated washed solid residue may be directly subjected to enzymatic hydrolysis step (g) . Alternatively, the evaporated washed solid residue is neutralised before being subjected to enzymatic hydrolysis. If the residue is neutralised, this may be done by washing the residue with an alkaline solution or by adding an alkaline solution to the evaporated washed solid residue to obtain a neutralised slurry. Such a slurry may be directly subjected to enzymatic hydrolysis step (g) .

Suitable enzymes and process conditions for enzymatic hydrolysis step (g) are known in the art, for example from WO2006/034590 or Lynd et al . , Microbial Cellulose Utilization: Fundamentals and Biotechnology, Microbiology and Molecular Biology Reviews, 66 (2002), p 506-577.

For enzymatic hydrolysis, a slurry of the evaporated solid residue in water is made. If needed, the pH of the slurry is adjusted to a pH within the range of optimum pH for the enzymes used, typically in the range of from 4.0 to 6.0. Any suitable alkaline solution may be used for pH adjustment, for example a solution of sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, or ammonium hydroxide. Cellulase enzymes, optionally together with hemicellulase enzymes, are added to the slurry of the evaporated residue, optionally after pH adjustment. Typically, the cellulase enzyme dosage is in the range of from 5 to 50 Filter Paper Units (FPU) per gram of cellulose in the evaporated residue.

Enzymatic hydrolysis is typically carried out at a temperature in the range of from 30 to 70 ⁰C for a period in the range of from 20 to 250 hours.

After enzymatic hydrolysis, the sugars obtained may be separated from the suspended solids using techniques known in the art, for example filtration or centrifugation .

The sugars thus-obtained may suitably be used as feed for the preparation of bio-chemicals or biofuels . The sugars may for example be converted into levulinic acid, into lipids, hydrogen, methane, or acetic acid by conversion processes using bacteria and into ethanol by fermentation. Such conversion processes are well-known in the art. The sugars are preferably fermented into ethanol in a subsequent fermentation step (h) . Fermentation of sugars into ethanol is well-known in the art. Any such fermentation processes known in the art may be used for step (h) . Sugars recovered from purged spent cooking liquid, typically sugars derived from hemicellulose hydrolysed in cooking step (a), may be co-fermented in fermentation step (h) .

The process according to the invention preferably comprises fermentation step (h) followed by an ethanol recovery step (i) for recovery of ethanol as product. Ethanol produced in fermentation step (h) may be recovered from the fermentation liquid by any suitable technique known in the art, usually by distillation. The ethanol product may be used in any suitable application, for example as gasoline component. Detailed description of the drawings

In the Figure is shown a process scheme of the process according to the invention. Comminuted solid biomass 1 and cooking liquid 2 are supplied to reactor 3. In reactor 3, cooking step (a) is carried out and a slurry 4 of cooked solid residue in spent cooking liquid is withdrawn from reactor 3 and supplied to separator 5. In separator 5, solid residue is separated from spent cooking liquid. Solid residue 6 is withdrawn and supplied to washing unit 7. Spent cooking liquid 8 is withdrawn from separator 5. A large part 9 of spent cooking liquid 8 is recycled to reactor 3 to form part of cooking liquid 2. A smaller part 10 of spent cooking liquid 8 is purged. Fresh washing liquid 11 is supplied to washing unit 7. Optionally organic acids removed from purged spent cooking liquid 10 may be recycled (not shown) to washing unit 7 as part of fresh washing liquid 11. A slurry 12 of washed solid residue in spent washing liquid is formed in washing step 7 and supplied to separator 13, in which slurry 12 is separated into spent washing liquid 14 and washed solid residue 15. Spent washing liquid 14 is recycled to reactor 3 to form part of cooking liquid 2.

Washed solid residue 15 is supplied to evaporator 16 for removal of free organic acids. Organic acids 17 are withdrawn from evaporator 13 and are recycled to washing unit 7 as part of fresh washing liquid 11. Evaporated solid residue 18, i.e. residue from which free organic acids are evaporated, and water 19 are supplied to enzymatic hydrolysis unit 20 for conversion into sugars, in particular monomeric sugars. Water 16 may comprise an alkaline salt for neutralising the acidic residue. In enzymatic hydrolysis unit 20 a sugar solution 21 is obtained. Sugar solution 21 may be further converted, for example into ethanol in a fermentation unit (not shown) . Example The process according to the invention is further illustrated by means of the following non-limiting simulated example.

In a process line-up as shown in the Figure, the relative volumes of the solid and liquid streams are as follows :

Two volumes of biomass 1 are cooked with ten volumes of cooking liquid 2 in reactor 3. In separator 5, solid residue 6 consisting of one volume of solid biomass and one volume of adhering spent cooking liquid is separated from spent cooking liquid 8 consisting of nine volumes of spent cooking liquid with one volume of biomass (mainly lignin and hemicellulose) dissolved in it . A stream 10 of three volumes of spent cooking liquid with 0.3 volumes dissolved biomass is purged; a stream 9 of six volumes of spent cooking liquid with 0.7 volumes of dissolved biomass is recycled to reactor 3 as part of cooking liquid 2. Four volumes of fresh washing liquid 11 are added to solid residue 6 (one volume solid; one volume liquid) in washing unit 7. In separator 13, four volumes of spent washing liquid 14 are separated from washed solid residue 15 (one volume solid; one volume liquid) and recycled to reactor 3 to form part of cooking liquid 2. From solid residue 15, one volume of organic acid 17 is evaporated in evaporator 16 and recycled to washing unit 7 as part of fresh washing liquid 11.

One volume of evaporated residue 18 is obtained for enzymatic hydrolysis in enzymatic hydrolysis unit 20.

Claims

C L A I M S

1. A process for converting lignocellulose into sugars comprising the following steps:

(a) chemically pre-treating lignocellulose-comprising solid biomass by contacting the biomass with a cooking liquid comprising at least 40 vol% formic acid and at least 50 vol% organic acid, at a temperature in the range of from 90 to 180 ⁰C, at a biomass to liquid ratio in the range of from 1:3 to 1:50 to obtain a pre-treated solid residue and spent cooking liquid; (b) separating the solid residue from the spent cooking liquid;

(c) washing the solid residue with washing liquid comprising at least 40 vol% formic acid and at least

50 vol% organic acid to obtain washed solid residue and spent washing liquid;

(d) separating the washed solid residue from the spent washing liquid;

2. A process according to claim 1, wherein part of the spent cooking liquid obtained in step (b) is supplied to step (a) to from part of the cooking liquid.

3. A process according to claim 2, wherein the cooking liquid essentially consists of spent washing liquid and spent cooking liquid.

4. A process according to any one of the preceding claims, wherein the washing liquid used in step (c) comprises formic acid, acetic acid and water, preferably essentially consists of formic acid, acetic acid and water.

5. A process according to claim 4, wherein the washing liquid used in step (c) comprises in the range of from 40 to 80 vol% formic acid, in the range of from 8 to 50 vol% acetic acid, preferably in the range of from 10 to 40 vol% acetic acid, and in the range of from 5 to 35 vol% water, preferably in the range of from 10 to 25 vol% water, based on the total volume of washing liquid.

6. A process according to any one of the preceding claims, wherein the evaporated washed solid residue is neutralised before being subjected to enzymatic hydrolysis .

7. A process according to any one of the preceding claims, wherein the temperature in step (a) is in the range of from 100 to 150 ⁰C, preferably of from 105 to 130 ⁰C.

8. A process according to any one of the preceding claims, wherein the temperature in washing step (c) is in the range of from ambient to 80 ⁰C, preferably of from 20 to 50 ⁰C.

9. A process according to any one of the preceding claims, further comprising a subsequent fermentation step (h) wherein the sugars are converted into ethanol.

10. A process according to claim 9 further comprising a ethanol recovery step (i) to obtain ethanol product.