WO2014060674A1

WO2014060674A1 - Method for producing ethanol from biomass, including recirculating an internal flow containing ethanol upstream from or within the pretreatment

Info

Publication number: WO2014060674A1
Application number: PCT/FR2013/052261
Authority: WO
Inventors: Caroline Aymard; Marcel Ropars; Pierre-Antoine Bouillon; Eszter Toth; Larissa PEROTTA
Original assignee: IFP Energies Nouvelles
Priority date: 2012-10-18
Filing date: 2013-09-25
Publication date: 2014-04-24
Also published as: FR2997094A1; FR2997094B1

Abstract

The invention relates to a method for producing ethanol from biomass, including the following steps: a) the biomass is pretreated in a pretreatment reactor by steam explosion so as to provide an effluent containing a pretreated substrate that contains cellulose; b) an enzymatic hydrolysis of the pretreated substrate resulting from step a) is carried out so as to provide a hydrolysate; c) an ethanol fermentation of the hexoses, and optionally of the pentoses, contained in the hydrolysate is carried out so as to produce a fermented effluent containing ethanol; and d) a step for extracting ethanol from the fermented effluent resulting from step c) is carried out. The method also includes a step e) in which at least one internal aqueous flow, containing ethanol resulting from ethanol fermentation, is recirculated upstream from or within the pretreatment reactor so as to recover an effluent containing ethanol, in vapor phase, that is output from said pretreatment reactor.

Description

PROCESS FOR PRODUCING ETHANOL FROM BIOMASS WITH RECYCLING OF AN INTERNAL FLOW COMPRISING ETHANOL UPSTREAM OR WITHIN THE DOMAIN PRETREATMENT OF THE INVENTION

The present invention is part of a process for producing so-called "second generation" ethanol from lignocellulosic biomass.

PRIOR ART

Lignocellulosic biomass is one of the most abundant renewable resources on earth. The substrates considered are very varied, since they concern both woody substrates (hardwood and softwood), agricultural by-products (straw) or lignocellulosic waste-generating industries (agro-food industries, paper mills).

Lignocellulosic biomass is composed of three main polymers: cellulose (35 to 50%), hemicellulose (20 to 30%) which is a polysaccharide essentially consisting of pentoses and hexoses and lignin (15 to 25%) which is a polymer of complex structure and high molecular weight, composed of aromatic alcohols connected by ether bonds.

These different molecules are responsible for the intrinsic properties of the plant wall and are organized in a complex entanglement.

Cellulose and possibly hemicelluloses are targets for enzymatic hydrolysis but are not directly accessible to enzymes. This is the reason why these substrates must undergo a pretreatment preceding the enzymatic hydrolysis step. The pretreatment aims at modifying the physical and physicochemical properties of the lignocellulosic material, with a view to improving the accessibility of the cellulose trapped within the matrix of lignin and hemicellulose. Many technologies to achieve this pretreatment exist. Examples include acid cooking, alkaline cooking, steam explosion, Organosolv processes, etc.

The effectiveness of pretreatment is measured both by the material balance at the end of pretreatment (recovery rate of sugars in the form of monomers or soluble oligomers or insoluble polymers) and also by the susceptibility to enzymatic hydrolysis of cellulosic and hemicellulosic residues.

The pretreatment by steam explosion is distinguished by its performance in terms of degradability of cellulose and its low dilution ratio.

Pretreatment by steam explosion is also known as "steam explosion", "steam gunning", "explosive relaxation", "steam pretreatment". The biomass is rapidly heated to high temperature (150 ° -250 ° C.) by injection of steam under pressure. Stopping treatment is usually done by sudden decompression, called detente or explosion, which destructures the lignocellulosic matrix. The residence times vary from 10 seconds to a few minutes, for pressures ranging from 10 to 50 bars. This technique has been implemented either discontinuously or continuously. Some technologies offer water injection to cool the environment before decompression.

Many configurations are possible for this type of pretreatment. The implementation may be batch type, or continuous type. The steam explosion may be preceded by an acid impregnation to promote the hydrolysis of hemicelluloses during cooking. When the steam explosion is applied to a previously acidified substrate, for example with H ₂ SO ₄ , it leads to a solubilization and almost complete hydrolysis of hemicelluloses in their monomers, limiting the degradation to furfural. In addition, the susceptibility of cellulose to enzymatic hydrolysis is improved. The use of an acidic catalyst makes it possible to reduce the temperature of the process (150 to 200 ° C. against 250 ° C. for the steam explosion without catalyst), and thus to minimize the formation of degradation compounds.

The steam explosion may be preceded by an acidic cooking step which aims to hydrolyze the hemicelluloses and to remove in a liquid solution in the form of monomeric sugars and / or oligomers.

The presence of ethanol in a biomass at the entrance of the steam explosion pretreatment has been little studied. Zahibi et al. [S. Zabihi, et al. "Pretreatment of wheat straw using steam, steam / acetic acid and steam / ethanol and its enzymatic hydrolysis for sugar production." Biosystems engineering 105 (2010): 288-97] have studied the impact of prior impregnation of straw with ethanol on a steam explosion pretreatment without the presence of acid. Impregnation with ethanol increases the efficiency of the steam explosion (increase of the sugar yield of the hydrolysis). The ethanol / water mixtures used for the impregnation are 25%, 50% and 75% by weight ethanol. The most concentrated solution showed the best results. In this paper, the effect of acetic acid has also been studied under the same conditions as ethanol, and a positive impact on enzymatic hydrolysis has also been observed. The operating parameters of the steam explosion can be adapted according to the type of biomass to be treated.

Important parameters for pretreatment include temperature, duration, presence of acid (type and concentration) and their effects are interchangeable to a certain extent. For example, a longer duration can compensate for a lower temperature.

Biochemical processes for producing ethanol from lignocellulosic biomass, referred to as "second generation processes", include at least the following steps:

• pretreatment of the substrate,

Enzymatic hydrolysis of the pretreated substrate,

· Ethanol fermentation of the hydrolyzate obtained, and

• separation / purification of the ethanol obtained after fermentation.

The economic validity of this type of ethanol production process is difficult to obtain even for operators with a large mobilizable resource. Several positions have a strong impact on the overall cost of operation of the units; we can mention in particular the accessibility of the plant resource and the energy implementation for the separation and purification of ethanol which are most often carried out by distillation. The optimization of this type of process necessarily involves an optimal valuation of all sugars, as well as a good management of the thermal integration of the process.

The pretreated substrate sees its accessibility to enzymes improved by pretreatment. The hydrolysis and fermentation steps may be carried out according to several arrangements known to those skilled in the art. They include: Separate (or Sequential) Hydrolysis and Fermentation (SHF), Simultaneous Saccharification and Fermentation (SSF), Presaccharification followed by Simultaneous Saccharification and Fermentation (PSSF), Hybrid Hydrolysis and Fermentation (HHF).

The yeast Saccharomyces cerevisiae is the most efficient organism for the production of ethanol. The natural strain uses only hexoses (glucose, mannose, and to a lesser degree, galactose). Some genetic modification work has led to strains that can also use pentoses (and especially xylose), but the performances (yield, productivity, final title) on pentoses remain behind the use of hexoses. It is known that the rate of consumption of pentoses is significantly lower than that of hexoses.

Several pathways are being studied for the industrial fermentation of pentoses in ethanol, either from microorganisms naturally fermenting pentoses, or by genetic modifications of microorganisms already performing for the use of hexoses. The introduction of pentose fermentation still allows other process configurations such as "Simultaneous Saccharification and Co-Fermentation" (SSCF). Finally, there is also the "Consolidated BioProcessing" (CBP) in which the enzymes are directly produced in the hydrolysis and fermentation reactor. However, no microorganism today has satisfactory performance for industrial implementation in the short term.

The extraction of ethanol produced by distillation is a particularly energy-intensive item. For balance sheet improvement economic development of ethanol production, it is advisable to consider reducing water volumes, in particular by means of recycling and / or consolidation of different streams that concentrate the alcohols and to integrate the entire heat chain. Indeed, the specific energy consumption to separate for example ethanol decreases sharply when the concentration in the initial flow (also called title) increases.

SUMMARY OF THE INVENTION

An object of the invention is therefore to provide a process for producing ethanol from biomass for which the energy and water budgets are optimized.

To this end, there is provided a method for producing ethanol from biomass comprising at least the following steps:

a) pretreating the biomass by steam explosion in a pretreatment reactor to provide an effluent containing a pretreated substrate;

b) the enzymatic hydrolysis of the pretreated substrate contained in the effluent resulting from step a) is carried out in the presence of cellulolytic and / or hemicellulolytic enzymes so as to produce a hydrolyzate containing solubilized sugars;

c) the solubilized sugars contained in the hydrolyzate from step b) are ethanol fermented in the presence of an alcoholic microorganism so as to produce a fermented effluent containing ethanol;

d) a step is carried out for extracting ethanol from the fermented effluent from step c).

The process according to the invention also comprises a step e) in which at least one aqueous internal flow comprising ethanol obtained from an ethyl fermentation is recycled upstream of or into the pretreatment reactor, so as to recover at the outlet of said pretreatment reactor a vapor phase effluent containing ethanol.

For the purposes of the present invention, the term "ethyl fermentation" denotes a process for the fermentation of sugars into ethanol only by means of microorganisms such as, for example, yeasts or bacteria.

The present invention describes a process for producing ethanol in which a stream comprising water and ethanol is recycled within the steam explosion pretreatment.

Thus, at least a portion of ethanol produced by ethyl fermentation is recycled to the pretreatment stage by steam explosion, so that the vapor effluents generated during the expansion of the pretreatment stage and containing ethanol can be sent to a distillation step, common or not with the main chain. The invention exploits the property of ethanol to azeotrope with water which, because of its lower boiling point than that of water, allows a favorable distribution of ethanol in the vapor phase. Thus, the ethanol which is sent to the extraction stage is already in vapor form, which makes it possible to reduce the energy cost of the separation by distillation, and therefore the overall energy cost of the process. The other part of the recycled ethanol that is not recovered after the expansion phase remains in the effluent at the end of the pretreatment, and contributes to increasing the title of the wine (s) to be distilled.

Moreover, the recycled internal stream can advantageously be easily and thermally integrated in the overall scheme of the process according to the invention. Thus, the internal flow, before being recycled, can be preheated by means of any hot flow produced by the process, such as for example during the extraction step which generally involves at least one distillation step.

Thus, thanks to the process according to the present invention it is possible to significantly improve the energy balance of the extraction step of the order of 5% to 50%.

Furthermore, the recycling according to the invention makes it possible to reuse part of the water in the pretreatment stage by steam explosion and thus goes in the direction of optimizing the water balance of the process.

According to one embodiment, the method further comprises the following steps: a solid / liquid separation of at least a portion of a vinasse produced in step d) is carried out so as to recover a

liquid fraction containing pentoses; and

an ethyl fermentation of the pentoses of the liquid fraction is carried out so as to provide said internal flow comprising

Ethanol.

According to another embodiment, the method further comprises the following steps:

- Solid / liquid separation of at least a portion of the effluent from step a) is carried out so as to recover a liquid fraction containing solubilized sugars; and

an ethyl fermentation of the sugars of the liquid fraction is carried out so as to provide said internal flow comprising ethanol. According to the invention, said liquid fraction comprises hexoses and / or pentoses.

- Solid / liquid separation of at least a portion of the effluent obtained in step a) to recover a pulp; washing said pulp so as to recover a liquid fraction containing solubilized sugars;

an ethyl fermentation is carried out of the solubilized sugars contained in the liquid fraction so as to supply the internal flow of the ethanol.

According to another embodiment of the process according to the invention, the internal flow containing ethanol originates from at least part of the fermented effluent produced in step c).

According to one embodiment of the process according to the invention, the internal flow is a liquid fraction obtained after solid / liquid separation of at least a part of the fermented effluent produced in step c).

According to one embodiment of the process according to the invention, the internal flow is a liquid fraction obtained after washing a cake obtained by solid / liquid separation of at least a part of the fermented effluent obtained in step c) .

According to another embodiment, the method comprises the following steps: - Solid / liquid separation of at least a portion of the hydrolyzate obtained in step b) is carried out so as to recover a pulp containing hexoses and optionally pentoses;

- Ethyl fermentation is carried out hexoses and optionally pentoses contained in the pulp so as to provide said internal flow comprising ethanol.

According to another embodiment, the method comprises the following steps:

- Solid / liquid separation of at least a portion of the hydrolyzate obtained in step b) is carried out so as to recover a liquid fraction containing hexoses and optionally pentoses;

- Ethyl fermentation of the hexoses and optionally pentoses contained in the liquid fraction to provide said internal flow comprising ethanol.

According to one embodiment, the method comprises the following steps:

- Solid / liquid separation of at least a portion of the hydrolyzate obtained in step b) is performed so as to recover a pulp;

the pulp is washed so as to recover a liquid fraction containing hexoses and possibly pentoses;

According to another embodiment of the process according to the invention, steps b) and c) are carried out in the same reactor.

According to another preferred embodiment, the internal flow containing ethanol is vaporized before the recycling step e).

The process according to the invention makes it possible to treat biomass resulting from resinous tree species, hardwoods or agricultural lignocellulosic waste.

In the context of the invention, the pretreatment with steam can be done in the presence of acid, the latter can be injected directly onto the biomass at the inlet of the pretreatment reactor. The acid can also be used under very dilute conditions to impregnate the biomass in the preparation phase. MS rate can be increased by solid / liquid separation after impregnation of the acid.

According to the invention, the cellulolytic and / or hemicellulolytic enzymes used during the hydrolysis step may be produced by a microorganism belonging to the genera Trichoderma, Aspergillus, Penicillium or Schizophyllum, or an anaerobic bacterium belonging to the genus Clostridium, for example.

According to the invention, the alcoholic microorganisms used in the conversion unit for carrying out the ethylic fermentation of hexoses are preferably chosen from yeasts and bacteria, possibly genetically modified.

When the alcoholic microorganism is a yeast, Saccharomyces cerevisiae is the one that is preferred. It is also possible to choose yeasts such as Schizosaccharomyces pombe or Saccharomyces uvarum or diastaticus. More thermophilic yeasts, such as Kluyveromyces fragilis (now often referred to as K. marxianus) are also of interest, especially when enzymatic hydrolysis and ethyl fermentation are carried out simultaneously (SSF method).

A genetically modified organism, such as, for example, a yeast of the Saccharomyces cerevisiae type such as TMB 3400 (Ohgren et al., J. of Biotech 126, 488-498, 2006) may also be used. This yeast makes it possible, in particular, to ferment in ethanol a part of the pentoses during the ethylic fermentation step of the hexoses, when the glucose is in a limiting concentration.

When the alcoholic microorganism is a bacterium, Zymomonas mobilis is preferred which has an efficient assimilation route. As for yeasts, species of Zymomonas mobilis have been modified to convert pentoses to ethanol, for example the strain Zymomonas mobilis 8bi described by Dutta et al (Biotechnol Prog., 2010, Vol.26, No. 1, pp. 64-72).

Yeasts, and preferably Saccharomyces cerevisiae are the microorganisms used very preferably for the conversion of hexoses. They have a better robustness, safety, and do not require sterility for the conduct of the process and installations. Yeasts of the genus Saccharomyces are able to ferment the only and unique hexoses (mainly glucose and mannose). These yeasts optimize the hexoses in ethanol and achieve mass conversion efficiencies of the order of 0.46 to 0.48, which is close to the maximum theoretical mass yield of 0.51. Only pentoses and some marginal carbon sources are not used by these wild yeasts. Modified strains of Saccharomyces have been developed to ferment pentoses, either in co-fermentation of hexoses or in a separate step.

The ethyl fermentation is preferably carried out at a temperature of between 30 ° C. and 40 ° C. and at a pH of between 3 and 6.5.

DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be clarified in the detailed description of particular embodiments of the invention, with reference to the drawings of the figures, in which:

Figure 1 is a schematic representation of a first embodiment of the process for producing ethanol according to the invention.

Figure 2 is a schematic representation of a second embodiment of the ethanol production process according to the invention.

Figure 3 is a schematic representation of a third embodiment of the ethanol production process according to the invention.

Figure 4 is a schematic representation of a fourth embodiment of the ethanol production process according to the invention.

Figure 5 is a schematic representation of a fifth embodiment of the ethanol production process according to the invention.

Generally, identical elements are denoted by the same references in the figures. DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, the term "pentoses" means soluble monomers and oligomers of sugars comprising 5 carbon atoms and under the term "hexoses" the monomers and soluble oligomers of sugars comprising 6 carbon atoms.

The abbreviation "MS" denotes the dry matter (solid and soluble) present in a medium and under the abbreviation "MES" the suspended matter (solids) present in a medium. The solids content (or "Total Solids") is determined according to the ASTM E1756-01 method which consists of a mass loss at 105 ° C.

With reference to FIG. 1, the cellulosic and / or lignocellulosic substrate is introduced via line 1 into a preparation unit 2 of the load. The water and / or steam necessary for driving the charge preparation are introduced into the preparation unit by a dedicated line (not shown). Reagents (e.g., acid) and other utilities are introduced through another dedicated line (not shown). The charge preparation comprises, for example, mechanical grinding, acid addition, adjustment of the "MS" by adding water in liquid form and / or steam before introduction into the steam explosion reactor.

The cellulosic or lignocellulosic substrate used in the process according to the present invention can be chosen from the most varied biomasses, but more particularly from resinous tree species (softwood species such as spruces or pines) or hardwood species (hardwood species such as eucalyptus trees). or lignocellulosic agricultural waste (wheat straw, rice, etc.) or dedicated crops (miscanthus, switchgrass).

The prepared substrate is introduced into the pre-treatment reactor 4, via line 3, which uses the steam explosion technique. The steam required for pretreatment is introduced through a pipe (not shown). The vapor phase generated at the expansion is extracted by the pipe 27.

The role of pretreatment is to make the cellulose accessible to enzymes, by destructuring the lignocellulosic matrix. When pretreatment by steam explosion, it is made accessible preferentially hemicellulose, which is found to a large extent in the liquid phase at the outlet of the pretreatment.

Typically, the operating conditions for pretreatment by steam explosion are as follows:

· T = 150 to 250 ° C

• P = 10 to 50 bar

• pressurization time = a few seconds to a few minutes

It should be noted that additional steps following the pretreatment step, for example of pH-setting or liquefaction, can be carried out in order to facilitate the implementation and the effectiveness of the enzymatic hydrolysis and subsequent ethyl fermentation steps.

The pretreated substrate preferably contains between 5% by weight and 80% by weight of MS, more preferably between 15% by weight and 60% by weight of MS and even more preferably between 20% by weight and 55% by weight of MS.

As shown in FIG. 1, the pretreated substrate is extracted via line 5 and sent to a solid / liquid separation unit 6. The solid / liquid separation unit 6 makes it possible to separate via line 18 a liquid stream containing sugars mainly from hemicellulose and a wet pulp which is sent via line 7 to a conversion unit 8 which converts the cellulose into ethanol. The sugars from 1 hemicellulose include pentoses and possibly hexoses depending on the type of biomass treated.

The conversion unit 8 from cellulose to ethanol comprises at least one enzymatic hydrolysis unit 9 and an ethyl fermentation unit 10. In FIG. 1, the units of enzymatic hydrolysis and ethyl fermentation are shown separately. however, it is possible in the context of this embodiment to have a single reactor in which hydrolysis and fermentation (SSF process) are carried out simultaneously.

The conditions of the enzymatic hydrolysis, mainly the solids content of the mixture to be hydrolysed and the amount of enzymes used, are chosen so that one obtains solubilization of the cellulose of between 20% and 99% and preferably between 30% and 95%. The water necessary to obtain the target MS level is added via a pipe (not shown). The desired level of MS is generally between 5% by weight and 45% by weight and preferably between 8% by weight and 40% by weight.

According to the invention, the enzymatic hydrolysis is preferably carried out at a pH of between 4 and 5.5 at a temperature of between 40 ° C. and 60 ° C.

The necessary additives, for example cellulolytic and / or hemicellulolytic enzymes, fermentation microorganisms (if the conversion reaction is in a single step), water, nutrients, chemical reagents such as soda and / or ammonia and / or potash, are introduced by a conduit (not shown) dedicated for this purpose.

As mentioned above, it is possible to carry out enzymatic hydrolysis and ethyl fermentation (hexoses or hexoses and pentoses) in one and the same operation designated by the term SSF. This operation is preferably carried out at a temperature of between 30 ° C. and 45 ° C., at a pH of between 4 and 6.

Optionally and depending on the fermentation microorganism (s) used (s), all or part of the pentoses can also be fermented into ethanol.

Preferably the alcoholic microorganism is a yeast. Even more preferably, the alcoholic microorganism belongs to the genus Saccharomyces.

At the end of the ethyl conversion step, a fermented wort comprising a solid residue and an ethyl wine is extracted via line 11 and is fed to a unit 12 for extraction / purification of ethanol contained in ethyl wine. . This extraction / purification step generally comprises at least one distillation step.

From the extraction / purification unit 12, at least: • an effluent containing ethanol is recovered via line 13; · A pulp, withdrawn via line 14, containing an insoluble residue which is partly composed of cellulose and hemicellulose which have not been hydrolysed and of lignin in mixing with a liquid fraction (vinasse) containing unfermented sugars (pentoses and possibly hexoses).

The pulp is transferred to a solid / liquid separation unit from which a wet cake comprising cellulose, hemicellulose which has not been hydrolysed and lignin is recovered via line 16. line 17, a clarified vinasse containing the unfermented sugars. The clarified vinasse contains, for example, unfermented pentoses (xylose, arabinose), even traces of galactose, which is the most difficult hexose to be metabolized by conventional yeasts, or even traces of glucose if the fermentation has been incomplete.

The wet cake can either be reintroduced into the process, or recycled to produce energy, for example by methanation or combustion, or as an agricultural supplement (spreading).

With reference to FIG. 1, a fraction of clarified vinasse, between 10% and 90% and preferably between 15% and 60% by weight, is sent via line 17 to an additional ethyl fermentation reactor 19.

As shown in FIG. 1, via line 18, all or part of the liquid containing solubilized sugars, possibly including pentoses, and coming from the solid / liquid separation 6 arranged upstream of the conversion unit 8, is also sent to the additional fermentation reactor 19. In the case where all the liquid is not sent to the fermentation reactor 19, the remaining part can be used for example for the production of enzymes or recovered in methane or in energy.

The reactor 19 is a pentose fermentation reactor and optionally residual hexoses in ethanol. The microorganisms used for the fermentation of pentoses can be bacteria, yeasts or fungi.

Among the bacteria, mention may be made of "wild" bacteria belonging to the genera Bacillus, Bacteroides, Thermoanaerobacter or Clostridium, recombinant bacteria of which the most interesting are Escherichia coli, Klebsiella oxytoca and especially Zymomonas mobilis.

Wild yeasts such as Pichia stipitis, Candida shehatae and Pachysolen tannophilus, identified as being the most effective for converting pentoses to ethanol, can also be used. It is possible to choose recombinant yeasts, especially Saccharomyces cerevisiae as mentioned above.

All or part of the ethyl wine produced by the fermentation reactor 19 is then transferred via line 20 to a processing unit 21.

The processing unit 21 may comprise, for example, a vaporization device and / or a unit for separating suspended matter (eg by centrifugation and / or by decantation and / or by membrane treatment (ultra-filtration, micro-filtration)) . The possible residues of the treatment can be removed by a pipe (not shown).

The effluent, in vapor or liquid form, coming from the treatment unit 21, and essentially containing water and ethanol (with a smaller amount of the compounds such as furfural, acetic acid) can be separated in two streams, as shown in Figure 1. One of the stream 24 is then sent to the pre-treatment reactor 4 and / or to the reactor for preparing the biomass 2. The other part of the stream 23 may optionally be recycled in the distillation unit 12.

An advantage of recycling the fraction 24 in the pretreatment reactor 4 and / or in the biomass preparation reactor 2 is that this recycle stream makes it possible to replace all or part of the makeup water required for these components. steps.

According to another embodiment of the process according to the invention, the internal stream containing water and ethanol is recycled in liquid form and / or in vapor form upstream or in the steam explosion pretreatment reactor. 4.

Typically, the amount of ethanol recycled in the steam explosion reactor 4 is adjusted so as to obtain an ethanol content in said reactor 4 of less than 15% by weight, and preferably less than 10% by weight. Acetic acid, furfural and the other compounds provided by the recycling have concentrations in the reactor 6 of less than 5%, or even less than 2% by weight.

Preferably, the processing unit 21 comprises at least one vaporization unit. The vaporization of the flow makes it possible to recycle only the volatile products, and thus to avoid the recycling of soluble products such as the residual sugars (hexoses and pentoses) which can degrade in the steam explosion reactor in furfural, 5 -HMF, or organic acids (formic, levulinic).

As shown in FIG. 1, the ethanol and optionally the other components mentioned above which are recycled in the pretreatment reactor 4 are recovered after the expansion phase, for the most part in gaseous form with the steam by the pipe. 27 and transferred to a distillation unit 28 from which two streams are separated; a stream 29 which contains mainly ethanol and a stream 30 mainly comprising water. The ethanol of the stream 29 is reintroduced into the extraction / separation unit 12.

According to an alternative embodiment not shown in FIG. 1, the gaseous phase containing water and ethanol recovered after expansion of the pretreatment reactor 4 is sent directly to the extraction / separation unit 12.

FIG. 2 represents another embodiment of the method according to the invention which differs from that presented in FIG. 1 by the fact that:

The conversion unit 8 comprises an enzymatic hydrolysis reactor 9, an ethyl fermentation reactor 10 separated from that of the enzymatic hydrolysis and a solid / liquid separation device 31 interposed between the said reactors 9 and 10;

The flow comprising the ethanol and the water which is recycled is obtained from a pulp, produced by solid / liquid separation of the hydrolyzate resulting from the hydrolysis step, which has undergone an ethyl fermentation step sugars C6 and optionally C5 contained in said pulp. With reference to FIG. 2, a liquid stream 33 which is sent into the fermentation reactor 10 and a pulp containing a solid residue impregnated with a juice containing C 6 sugars and optionally in the form of a pulp are extracted from the solid / liquid separation device 31. C5. The pulp, via line 32, is sent to a fermentation reactor 19 hexoses and possibly pentoses.

The fermented must from fermentation reactor 19 which comprises an ethyl wine mixed with the solid residue, is transferred via line 20 to a treatment unit 21 which comprises a solid / liquid separation device supplemented by heating means and / or or vaporization of the ethyl wine obtained after solid / liquid separation of the fermented must. All or part of the heated ethyl wine, or even in vapor form, is recycled in the pre-treatment reactor 4 and / or in the reactor for preparing biomass 2 via line 24, as in the implementation of FIG. solid residue extracted from the fermented must is either recycled in the process or discharged out of the process.

According to an alternative method of the method according to Figure 2, the pulp extracted from the pipe 32 is subjected to a washing step and solid / liquid separation to recover a juice containing hexoses (and optionally pentoses) and a wet cake. The wet cake is recycled in the process while the sweet juice is sent to the fermentation reactor 19 to produce an ethyl wine. The latter is then recycled to the pre-treatment reactor 4 and / or to the biomass preparation reactor 2, possibly after treatment in a unit 21, for example by heating or vaporization.

A third embodiment is shown in FIG. 3. This variant differs from that of FIG. 2 in the following way:

A solid / liquid separation unit 40 is disposed downstream of the conversion unit 8;

• the absence of an additional unit of ethyl fermentation. With reference to FIG. 3, a fraction of the fermented must extracted from the fermentation reactor 10 (or from the conversion unit 8 in the case where an SSF process is implemented) is sent to the solid / liquid separation unit. 40. From the solid / liquid separation unit 40 is separated a cake impregnated with an ethyl wine by the line 41 and a liquid fraction (or ethyl wine) by the line 42. The cake impregnated with ethyl wine is returned to the distillation unit 12 while the ethyl wine is treated in a treatment unit 21 (for example a vaporization unit) before being recycled in the pretreatment reactor 4 and / or in the reactor for preparing the biomass 2, as in the implementation of Figure 1.

According to an alternative embodiment of the method according to Figure 3, the solid / liquid separation unit 40 is completed by a cake washing unit. In this case, the fermented must is separated into a liquid fraction (ethyl wine) and a solid fraction containing a wet cake. The liquid fraction (or ethyl wine) is sent to the extraction / separation unit 12. As for the wet cake (or pulp), it is washed so as to recover a wash juice containing more than 70% by weight, preferably more than 85% by weight of the ethanol contained in the cake. The ethanol concentration of the wash juice is generally less than or equal to that of the liquid resulting from the solid / liquid separation. The wash juice is heated or vaporized in the treatment unit 21 before being recycled to the pre-treatment reactor 4 and / or to the biomass preparation reactor 2, as in the implementation of FIG. The washed solid residue can be reintroduced into the process, or recovered for example by combustion.

FIG. 4 shows another embodiment of the method according to the invention. This embodiment differs from that of FIG. 1 in that the effluent from the steam explosion pre-treatment reactor 4 is treated in a unit 50 consisting of a solid / liquid separation and washing unit. separating a washed wet cake (or pulp) and an effluent liquid containing essentially pentoses. The pulp is sent to the conversion unit 8 while the liquid effluent is fed through line 51 to the fermentation unit 19. This fermentation step, like the diagram of FIG. to transform by fermentation solubilized sugars into ethanol, sugars, which depending on the plant, will most often be pentoses. The ethyl wine, which is withdrawn from the reactor 19 by the line 20, is sent to the processing unit 21 for example to vaporize it. The ethyl vapor is extracted via line 22 and all or part of said steam is then recycled to the pre-treatment reactor 4 and / or to the biomass preparation reactor 2, as in the implementation of FIG.

FIG. 5 represents another embodiment of the method according to the invention which combines the embodiments of FIGS. 1 and 2. This embodiment implements a first solid / liquid separation step directly after the explosion step. with steam and a second solid / liquid separation step between the enzymatic hydrolysis and ethyl fermentation step.

With reference to FIG. 5, the effluent from the pre-treatment reactor is sent to a solid / liquid separation device 6 from which a wet cake is extracted via line 7 and a liquid fraction containing essentially pentoses by the line 18. As in the scheme of Figure 1, the liquid effluent is treated in a ethyl fermentation unit 19 pentoses ethanol. The effluent containing ethanol obtained at the outlet of the fermentation unit 19 is, for example, vaporized in the processing unit 21 before being recycled to the reactor for preparing the biomass 2 and / or in the reactor of pretreatment of steam explosion 4.

As for the wet cake extracted from the solid / liquid separation device 6, it is sent to the conversion unit 8 which comprises an enzymatic hydrolysis reactor 9, a fermentation reactor 10 and a solid / liquid separation device 31 disposed between the hydrolysis reactor 9 and the fermentation reactor 10. The hydrolyzate, derived from the enzymatic hydrolysis reactor, which contains a solid impregnated with a juice comprising essentially hexoses undergoes a solid / liquid separation step by means of the device 31 in order to extract via line 33 a liquid effluent closing off essentially hexoses and, via the line 32, a pulp containing a solid hydrolysis residue impregnated with hexose juice (with possibly pentoses). The pulp impregnated with hexose juice is transferred to a fermentation unit 60 which mainly converts hexoses to ethanol. The fermented must from the fermentation unit 60 is then sent to a unit 62 which successively comprises a solid / liquid separation device (optionally supplemented by washing means) and heating / evaporation means of the ethyl wine obtained after separation. solid / liquid. As shown in FIG. 5, a solid cake is extracted via line 63 while all or part of the ethyl wine is recycled to the biomass preparation reactor 2 and / or to the steam explosion pretreatment reactor 4 .

According to an alternative embodiment of the process described in FIG. 5, the streams 18 and 32 are fermented in the same ethyl fermentation reactor.

EXAMPLES

The following examples illustrate the invention without limiting its scope. Example 1 (not according to the invention)

In this example, a process for the production of ethanol from straw having the following characteristics is presented:

Load: Wheat straw of medium composition:

Wheat straw flow: 123.32 tonnes / hour Charge preparation and pretreatment:

The straw is crushed on a 50mm grid and then impregnated with sulfuric acid diluted to 0.8 g / L. The impregnation is followed by a solid / liquid separation and the impregnating liquor is recycled, the MS of the solid at the inlet of the steam explosion pre-treatment reactor is 30% by weight. The pretreatment by steam explosion is carried out at 202 ° C for 3 minutes, in a batch configuration. The medium is suddenly relaxed at a pressure of 1.3 atm. The steam consumption is 123509 kWh per hour. 12.8% weight of the mixture of the explosion reactor is recovered in the vapor phase at the expansion.

Conversion to ethanol:

The pre-treated effluent is sent to a solid / liquid separator so as to extract a liquid comprising 74% by weight of the pentoses.

The solid cake is sent to an SSF reactor whose solid concentration at the reactor inlet is 10% and which comprises fermentation microorganisms of the genus Saccharomyces cerevisiae. The conditions for carrying out the simultaneous hydrolysis and fermentation are chosen to obtain a yield of 33.8 g of ethanol per 100 g of cellulose introduced. The pentoses present are not converted by the fermentation microorganism chosen.

At the outlet of the SSF reactor, a fermented must is recovered which comprises 15.8 g of ethanol / kg of fermented must. This fermented must is then sent to the ethanol separation / recovery unit.

Separation and recovery of ethanol:

The ethanol is separated from the fermented must by distillation on two columns, the first in the presence of solids. A network of heat exchangers makes it possible to thermally integrate the separation process, in particular by using the condensers of the two columns for preheating the fermented must. The first column consumes 52,200 kWh per hour to the reboiler.

The reboiler of the second column is fed by an exchanger placed on the head flow of the first column. A fermented must / vinasse heat exchanger makes it possible to recover 36,900 kWh per hour of vinasse extracted from the first column.

A heat exchanger disposed upstream of the first column and downstream of the fermented must / vinasse exchanger makes it possible to heat the fermented must at a temperature of 120 ° C., the consumption of this exchanger is 12515 kWh per hour.

Thus, the net energy consumption of the ethanol recovery step is 64715 kWh per hour, or 16.55 MJ / kg of ethanol extracted. The extraction yield is 99.6%.

The process produces 112630 tonnes of ethanol annually with a specific consumption of 48.13 MJ / kg of ethanol for the pretreatment and distillation stages.

Example 2 (in accordance with the invention)

In this example, we present a process for producing ethanol from straw with the same characteristics as Example 1 by implementing the embodiment of Figure 3. The treated straw flow is the same as that of Example 1

Conversion to ethanol:

The effluent resulting from the pretreatment is sent to a solid / liquid separator from which a juice comprising 74% of the pentoses and a wet cake is extracted. The wet cake is sent to an SSF reactor whose solid concentration is 10% by weight and which comprises fermentation microorganisms of the genus Saccharomyces cerevisiae. The conditions for carrying out the simultaneous hydrolysis and fermentation are chosen to obtain a yield of 33.8 g of ethanol per 100 g of cellulose introduced. The pentoses present are not converted by the fermentation microorganism chosen.

At the end of this SSF conversion step, a fraction of the fermented must is, according to the invention, sent to the solid / liquid separation from which a wet cake and a cake are extracted. liquid stream containing ethanol. The solid cake is returned to the ethanol distillation unit. The liquid stream which contains water and ethanol (17.2 g / l) is completely sent to an exchanger to vaporize it before reintroduction into the pre-treatment reactor according to the invention.

The ethanol content of the fermented must which is sent for distillation is 15.8 g of ethanol / kg of fermented must.

Separation and recovery of ethanol:

The separation is done by distillation on two columns, the first distillation being conducted in the presence of solids. A network of heat exchangers makes it possible to integrate the separation process thermally, in particular by using the condensers of the two columns for the preheating of the fermented must. The first column consumes 43160 kWh per hour to the reboiler. The reboiler of the second column is fed by an exchanger placed on the head flow of the first column. A fermented must / vinasse heat exchanger makes it possible to recover 28060 kWh per hour of vinasse extracted from the first column.

The mixture comprising the preheated fermented must and the ethanol-containing vapor stream resulting from the relaxation of the pretreatment is sent to the first distillation column. This mixture has a temperature of 130 ° C. and an ethanol concentration of 16.8 g of ethanol / kg mixture.

The net energy consumption of the ethanol separation / recovery stage is 43160 kWh per hour, ie 11.77 MJ / kg of ethanol extracted, which represents a 29% increase in the specific consumption of the ethanol. separation / recovery step. The extraction yield is 99.6%.

The process produces 105610 tonnes of ethanol per year for a specific consumption of 45.45 MJ / kg of ethanol for the pretreatment and distillation stages. Compared to Example 1, this represents a loss in ethanol yield of 6% but for a 12% energy gain over the entire chain, which represents an energy gain relative to the ton of ethanol produced from 6%. Embodiment of the invention:

At the outlet of the SSF reactor, the fermented must is mixed with a stream coming from the washing of CO ₂ , then 38% of this stream is sent to a solid / liquid separator, from which a pulp at 16 is withdrawn, 8% MS and a liquid flow. The liquid flow from the solid / liquid separator has a flow rate of 158.3 tons / hour and an ethanol concentration of 17.2 g / l. This stream is vaporized at 17.5 bar and at a temperature of 310 ° C., and then injected into the steam explosion reactor. There is no additional addition of steam in this reactor.

The energy consumption to vaporize this flow is 123 500 kWh per hour. 12.8% by weight of the mixture of the steam explosion reactor is recovered in the form of vapor at the expansion.

Since ethanol is more volatile than water, the vapor stream is enriched with ethanol and contains 2.85% by weight of ethanol. The liquid effluent that is recovered after pretreatment contains 0.18% by weight of ethanol. Thus, 70% of the ethanol injected into the pretreatment reactor is recovered in the vapor phase at the outlet of the pretreatment stage. This vapor stream is directly mixed with the fermented wort preheated just before the first distillation column.

Example 3 (not in accordance with the invention)

In this example, a process for the production of ethanol from poplar with the following characteristics is presented:

Charge: Poplar of medium composition

Poplar flow rate: 100 tonnes / hour Charge preparation and pretreatment:

The poplar is reduced to chips and then introduced into a screw conveyor in which low pressure steam is successively introduced to heat the substrate to a temperature of about 80 ° C. and diluted H ₂ SO ₄ , with a acid / poplar ratio of 0.03 g H ₂ S0 ₄ per g MS.

At the outlet of the conveyor, the mixture having a DM level of 45% is introduced into a continuous steam explosion reactor. The pretreatment by steam explosion is carried out at 200 ° C for 5 minutes. The medium is suddenly relaxed at a pressure of 1.3 atm. The steam consumption is 67000 kWh per hour, of which 64% is used for pretreatment by steam explosion. 12.1% by weight of the mixture of the explosion reactor is recovered in the vapor phase at the expansion.

Conversion to ethanol:

The pretreated substrate is sent to a hydrolysis reactor where the enzymes and the dilution water (80660 kg / h of water) are introduced, then the mixture is sent to an SSF reactor where a yeast of the genus Saccharomyces is introduced. cerevisiae. The solids concentration at the inlet of the SSF reactor is 21%.

The conditions for carrying out the simultaneous hydrolysis and fermentation are chosen to obtain a yield of 20.2 g of ethanol per 100 g of introduced substrate (MS). The pentoses present are not converted by the fermentation microorganism chosen, in contrast hexoses from hemicellulose, and in particular the mannoses are converted into ethanol. The CO ₂ extracted is washed to recover the fraction of ethanol entrained in this stream. Thus, the ethanol content of the fermented must sent for distillation is 59.1 g of ethanol / kg of fermented must.

Separation and recovery of ethanol:

The ethanol is separated from the fermented must by distillation on two columns, the first in the presence of solids. A network of heat exchangers makes it possible to integrate the separation process thermally, in particular with the condensers of the two columns which serve for preheating the wine. The first column consumes 26475 kWh per hour to the reboiler. The reboiler of the second column is fed by a portion of the condenser of the first column. A fermented must / vinasse heat exchanger makes it possible to recover 9875 kWh per hour of vinasse extracted from the first column. A heat exchanger disposed upstream of the first column and downstream of the fermented must / vinasse exchanger makes it possible to heat the fermented must at 120 ° C., the consumption of this exchanger is 5475 kWh per hour. Thus, the net consumption of the ethanol recovery step is 31950 kWh per hour, or 5.71 MJ / kg of ethanol extracted. The extraction yield is 99.6%.

The process produces 161160 tonnes of ethanol annually with a specific consumption of 17.68 MJ / kg of ethanol for the pretreatment and distillation stages.

Example 4 (in accordance with the invention)

In this example, a process is described for producing ethanol from poplar with the same characteristics as Example 3, by implementing the embodiment of FIG. 3. The rate of treated poplar is identical to that of Example 3

Conversion to ethanol:

The pretreated substrate is sent to a pre-hydrolysis reactor where the enzymes and the dilution water (80025 kg / h of water) are introduced, and then the mixture is sent to an SSF reactor in the presence of the yeast Saccharomyces cerevisiae. . The solid concentration at the inlet of the SSF reactor is 21%. The conditions of implementation of the simultaneous hydrolysis and fermentation are adjusted to obtain a yield of 20.2 g of ethanol per g of introduced substrate (MS). The pentoses present are not converted by the fermentation microorganism chosen, in contrast hexoses from hemicellulose, and in particular the mannoses are converted into ethanol. The CO ₂ extracted is washed to recover the fraction of ethanol entrained in this stream. Thus, the ethanol content of the fermented must sent for distillation is 60.7 g of ethanol / kg of fermented must. Embodiment of the invention:

At the outlet of the SSF reactor, the fermented must is mixed with the flow coming from the washing of CO ₂ . 36% by weight of this mixture is sent to a solid / liquid separator which produces a cake containing 36% DM. Liquid effluent is also withdrawn from the solid / liquid separator at a flow rate of 52.5 tons / hour and has an ethanol concentration of 72.0 g / kg. This liquid effluent is vaporized at 17.5 bar and heated to 320 ° C. and then injected into the steam explosion reactor. There is no additional addition of steam in this reactor. The energy consumption to vaporize this flow is 38142 kWh per hour. 12.5% weight of the mixture of the explosion reactor is recovered in the vapor phase at the expansion. The vapor stream was enriched in ethanol and contained 8.07% by weight of ethanol. The pretreated substrate contains an ethanol residue of about 0.44% by weight. Thus, 71% by weight of the ethanol injected into the pre-treatment reactor is recovered in the vapor phase at the pre-treatment outlet. This vapor stream is recompressed at 3 bars and mixed with the preheated fermented wort just before the first distillation column.

Separation and recovery of ethanol:

The ethanol is separated from the fermented must by distillation on two columns, the first in the presence of solids. A network of heat exchangers makes it possible to thermally integrate the separation process, in particular by using the condenser of the second column for preheating the fermented must.

The condenser of the first column serves only to heat the reboiler of the second column. The first column consumes 20480 kWh per hour to the reboiler. A fermented must / vinasse heat exchanger makes it possible to recover 8750 kWh per hour of vinasse extracted from the first column.

The mixture comprising the preheated fermented must and the ethanol-containing vapor stream resulting from the relaxation of the pretreatment is sent to the first distillation column. This mixture typically has a temperature of 128 ° C and an ethanol concentration of 62.5 g ethanol / kg mixture. Thus, the net consumption of the ethanol recovery stage is 20 480 kWh per hour, or 3.66 MJ / kg of ethanol extracted, which represents a 36% gain on the specific consumption of the ethanol. separation step. The extraction yield is 99.6%.

The process produces 161160 tonnes of ethanol annually and has a specific consumption of 14.80 MJ / kg of ethanol for the pretreatment and distillation stages. This represents an energy gain per ton of ethanol produced of 16.3% and with a yield of ethanol that is preserved.

Example 5 (in accordance with the invention)

In this example, a process is described for producing ethanol from poplar with the same characteristics as in Example 3, by implementing the embodiment of FIG. 3. The flow rate of treated poplar is that of the example 3.

Conversion to ethanol:

The pretreated substrate is sent to a prehydrolysis reactor where the enzymes and the dilution water (80025 kg / h of water) are introduced and the mixture is then sent to an SSF reactor where the yeast Saccharomyces cerevisiae is introduced. The solids concentration at the inlet of the SSF reactor is 21%. The conditions for carrying out simultaneous hydrolysis and fermentation are chosen to obtain a yield of 20.2 g of ethanol per 100 g of introduced substrate (MS). The pentoses present are not converted by the fermentation microorganism chosen, in contrast hexoses from hemicellulose, and in particular the mannoses are converted into ethanol. The CO ₂ extracted is washed to recover the fraction of ethanol entrained in this stream. Thus, the ethanol content of the fermented must sent for distillation is 60.7 g of ethanol / kg of fermented must. Embodiment of the invention:

At the outlet of the SSF reactor, the fermented must is mixed with the flow coming from the washing of CO ₂ . 36% of this mixture is sent to a solid / liquid separator, which allows to release a cake with 36% of MS. The liquid effluent is withdrawn from the solid / liquid separator at a flow rate of 52.5 tons / hour and has an ethanol concentration of 72.0 g / l. This effluent is heated to 105 ° C by heat exchange coupled with the condenser of the first distillation column. The effluent is then vaporized at 17.5 bar and superheated to 320 ° C. The steam is then injected into the steam explosion reactor. There is no additional addition of steam in this reactor. The energy consumption to vaporize this flow is 34170 kWh per hour. 12.5% weight of the mixture of the explosion reactor is recovered in the vapor phase at the expansion. The vapor stream was enriched in ethanol and contained 8.07% by weight of ethanol. The pretreated substrate resulting from the treatment contains an ethanol residue of the order of 0.44% by weight. Thus, 71% of the ethanol injected into the pre-treatment reactor is recovered in the vapor phase at the pre-treatment outlet. This vapor stream is recompressed at 3 bar and directly mixed with the fermented wort preheated just before the first distillation column.

Separation and recovery of ethanol:

The separation is by distillation on two columns, the first in the presence of solids. A network of heat exchangers makes it possible to thermally integrate the separation process, in particular by using the condenser of the second column for preheating the fermented must.

Example 5 differs from Example 4 in that the condenser of the first column serves to heat the reboiler of the second column and to preheat the flow of recycled ethanol pretreatment. The first column consumes 19900 kWh per hour to the reboiler. A fermented must / vinasses heat exchanger thus makes it possible to recover 8720 kWh per hour of vinasse extracted from the first column.

The mixture composed of the preheated fermented wort and the ethanol-containing vapor stream resulting from the relaxation of the pretreatment is sent to the first distillation column. This mixture has a temperature of 128 ° C. and an ethanol concentration of 62.5 g of ethanol / kg of mixture.

The net consumption of the ethanol recovery step is 19900 kWh per hour, ie 3.55 MJ / kg of ethanol extracted, which represents a 38% gain on the specific consumption of the separation step. . The extraction yield is 99.6%.

The process produces 161160 tonnes of ethanol annually and has a specific consumption of 13.98 MJ / kg of ethanol for the pretreatment and distillation stages. This represents an energy gain per ton of ethanol produced of 20.9% and for a yield of ethanol that is conserved.

Thanks to the method according to the invention, the economy achieved on the single extraction step is substantial. As shown in the following examples, it can reach up to 50% on the extraction step, and up to 40% on the overall extraction and pretreatment consumption.

The various embodiments described above can be combined with each other insofar as the process involves recycling an aqueous stream containing ethanol that is derived from a flow internal to the process.

Claims

A process for producing ethanol from biomass comprising at least the following steps:

c) the solubilized sugars contained in the hydrolyzate from step b) are fermented in ethyl in the presence of an alcoholic microorganism so as to produce a fermented effluent containing ethanol; d) a step is carried out for extracting ethanol from the fermented effluent from step c),

characterized in that the process further comprises a step e) in which at least one aqueous internal stream comprising ethanol from an ethyl fermentation is recycled upstream of or into the pretreatment reactor, so as to recover at the outlet of said pretreatment reactor a vapor phase effluent containing ethanol.

The method of claim 1, wherein:

a solid / liquid separation of at least a portion of a vinasse produced in step d) is carried out so as to recover a liquid fraction containing pentoses; and

- Ethyl fermentation of the pentoses of the liquid fraction is carried out so as to provide said internal flow comprising ethanol.

3. Method according to one of the preceding claims, wherein: - Solid / liquid separation of at least a portion of the effluent from step a) is carried out so as to recover a liquid fraction containing solubilized sugars; and

an ethyl fermentation of the sugars of the liquid fraction is carried out so as to provide said internal flow comprising

Ethanol.

The method of claim 3, wherein said liquid fraction comprises hexoses and / or pentoses.

The method of claim 1, wherein:

- Solid / liquid separation of at least a portion of the effluent obtained in step a) to recover a pulp;

washing said pulp so as to recover a liquid fraction containing solubilized sugars;

6. The method of claim 1, wherein the internal flow containing ethanol is derived from at least a portion of the fermented mash produced in step c).

7. The method of claim 6, wherein the internal flow is a liquid fraction obtained after solid / liquid separation of at least a portion of the fermented must produced in step c).

8. The method of claim 6, wherein the internal flow is a liquid fraction obtained after washing a cake obtained by solid / liquid separation of at least a portion of the fermented effluent obtained in step c).

The method of claim 1, wherein:

- Solid / liquid separation of at least a portion of the hydrolyzate obtained in step b) is carried out so as to recover a pulp containing hexoses and optionally pentoses;

- Ethyl fermentation is carried out hexoses and optionally pentoses contained in the pulp so as to provide said inner flow comprising ethanol.

The method of claim 1, wherein:

12. Method according to one of claims 1 to 8, wherein steps b) and c) are carried out in the same reactor.

13. Method according to one of the preceding claims, wherein the internal flow containing ethanol is vaporized before the recycling step e).

14. Method according to one of the preceding claims, wherein the cellulolytic and / or hemicellulolytic enzymes are produced by a microorganism belonging to the genera Trichoderma, Aspergillus, Penicillium or Schizophyllum, or an anaerobic bacterium belonging to the genus Clostridium.

15. Method according to one of the preceding claims, wherein step c) is carried out in the presence of a microorganism selected from the genera Saccharomyces cerevisiae Schizosaccharomyces pombe, Saccharomyces uvarum, Saccharomyces diastaticus, Kluyveromyces fragilis and Zymomonas mobilis.