EP0432015A1 - Process and unit for producing a mixture of sugars containing at least 80% xylose from a lignocellulosic substrate - Google Patents

Abstract

A method and a unit for the continuous production of a mixture of sugars containing at least 80% by weight of xylose from a lignocellulosic substrate are described. The process comprises the following stages: the substrate is ground in 1, the impregnation zone 5 is continuously supplied with the substrate and an impregnation step is carried out in an acid medium under conditions such that the substrate recovered in 10 does not contain substantially no separate liquid phase and has a dry matter content of 30 to 70% by weight. A hydrolysis zone 11 is continuously supplied with an impregnated substrate in the presence of pressurized steam and a hydrolyzed substrate is recovered at 15 which contains substantially no separate liquid phase. It is extracted with water in 27 and a sugar solution containing at least 80% xylose is recovered. Application to the synthesis of xylitol as a sweetener. <IMAGE>

Description

The invention relates to a method and a unit for the continuous production of a mixture of sugars containing at least 80% by weight of xylose from a lignocellulosic substrate. It applies in particular to the transformation of xylose into xylitol which is a natural sweetener.

Most lignocellulosic substrates (wood, annual plants) have a heterogeneous composition in which there are usually three predominant fractions which are cellulose, hemicelluloses and lignin. This heterogeneity complicates their valuation. From this point of view, a separation of hemicelluloses (which have a variable sugar composition but are often rich in pentoses, in particular xylose) from cellulose (composed exclusively of glucose) is of great interest because it allows separate recovery of two types of constituents. This interest requires that the objective targeted is chemical use or fermentation since if most fermentations use glucose from the hydrolysis of cellulose, fewer are those that use, for example, the pentoses of hemicelluloses.

A so-called water vapor explosion treatment is already known in which the lignocellulosic substrate is subjected for a variable time to the action of water vapor under pressure at a temperature generally above 150 ° C. for example 150 at 250 ° C. This action ends with an explosive trigger. This treatment, formerly known for improving the digestibility of forages, also increases the susceptibility of lignocellulosic substrates to enzymatic hydrolysis (K. BUCHHOLZ, J. PULS, B. GADELMANN, MM DIETRICHS, Process Biochemistry, Dec / Jan. 1980 / 1981, pp 37-43).

It was found in patent FR 2580 669 that the addition of at least one acid during the steam explosion treatment in a batch chamber at high temperature for times of 2 to 5 minutes allows excellent separation of the hemicellulosic and cellulosic fractions and a very significant release of sugars constitutive of hemicelluloses, in particular pentoses such as xylose and arabinose without having undergone significant degradation. But one cannot operate continuously for reasons of corrosion. Furthermore, the fact of operating in a two-phase medium at a temperature of 150 to 250 ° C. has the drawback of degrading the pentoses obtained and of promoting the appearance of by-products such as furfural.

However, it is known from US Pat. No. 4,136,207, a device for continuous treatment with steam and under pressure up to 25 bar, for example of a lignocellulosic substrate previously ground or brought to a state of division specific to the starting material. .

At the recommended temperature and pressure levels (190-220 ° C), it is possible in particular to dissolve 50 to 60% of all the pentosans contained in the starting material. Only 10% of the pentoses passed in solution are in the form of monomers, the other 90% being in the form of oligomers with varying degrees of polymerization.

This device cannot be used in an acid medium and at high temperature, of the order of 200 ° C., because of the corrosion levels that the hydrolysis treatment in the presence of acid would generate.

One of the objects of the invention is to respond to the problems raised above.

It has in fact been found that with the device and the method according to the invention, corrosion of the various reaction vessels is minimized while obtaining an improved extraction yield, in particular in pentoses and particularly in xylose.

Furthermore, with a very limited investment cost due to the simplicity of the apparatus used, the device can operate continuously under severe temperature and pressure conditions, for example at a pressure which may be between 1 and 7 bar.

The invention therefore relates to a process for the continuous production of a mixture of sugars in solution containing at least 80% by weight of xylose from a lignocellulosic substrate comprising a step of grinding, a step of impregnation in aqueous solution of preferably acid, a hydrolysis step in the presence of water vapor, a dilution step in the presence of water, a step of extracting the mixture of sugars produced from the hydrolysed substrate, and a step of recovering xylose.

• a) on broye le dit substrat à une granulométrie adéquate et comprise entre 0,1 et 40 mm;
• b) on alimente en continu en substrat provenant de l'étape a), une zone d'imprégnation et on effectue l'étape d'imprégnation dans des conditions telles que l'on récupère un substrat imprégné en sortie de la zone d'imprégnation avec une teneur en matières sèches (de 30 à 70% en poids) telle qu'il ne contient sensiblement pas de phase liquide séparée;
• c) on alimente en continu en substrat imprégné résultant de l'étape b) une zone d'hydrolyse sous pression et on effectue l'étape d'hydrolyse dans des conditions de température, de pression et de débit de vapeur d'eau telles que l'on récupère un substrat au moins en partie hydrolysé ne contenant sensiblement pas de phase liquide séparée.

More specifically, the method comprises the combination of the following steps:

• a) the said substrate is ground to an adequate particle size and between 0.1 and 40 mm;
• b) continuously impregnated with substrate from step a), an impregnation zone and the impregnation step is carried out under conditions such that an impregnated substrate is recovered at the outlet of the impregnation zone with a dry matter content (from 30 to 70% by weight) such that it contains substantially no separate liquid phase;
• c) a continuous impregnated substrate resulting from step b) is supplied with a hydrolysis zone under pressure and the hydrolysis step is carried out under conditions of temperature, pressure and water vapor flow rate such that an at least partially hydrolyzed substrate is recovered which contains substantially no separate liquid phase.

The lignocellulosic substrate is generally wood, cobs and stalks of corn or straw. However, we prefer to use corn cobs.

The wood is usually debarked and cut in the form of chips while the stems are crushed and obtained at a particle size generally between 0.1 and 40 mm, preferably between 1 and 5 mm.

However, the straw is chopped and used in the form of strands. For reasons of convenience, the term grinding is used for the step of dividing to the appropriate dimension of each of these substrates. The dry matter content of the substrate is generally at least 50%, for example that of wood is approximately 50% while that of straw is approximately 80% and that of stalks approximately 80%.

The impregnation is generally carried out in an impregnation zone, supplied separately or not, with water and an acid. It could be a base if you wanted to do an alkaline treatment.

The water and acid supply rate is adjusted as a function of the supply rate of ground substrate, its initial dry matter content, its desired final content and its transfer speed in the impregnation zone. These water and acid flows are advantageously distributed annularly by suitable injection means known to those skilled in the art and generally located in the first quarter of this zone, upstream side.

The proportion of acid used relative to the dry matter content of the substrate is generally between 0.1 and 10%, advantageously between 2 and 5%.

Conventional organic or mineral acids are usually used. However, sulfuric acid or hydrochloric acid is preferred.

Advantageously, the dry matter content at the outlet of the impregnation zone is approximately 35 to 55%.

The impregnation is generally carried out at ambient temperature and pressure during a residence time which depends on the nature of the substrate used and which is conditioned by the speed of transfer into the impregnation zone and by the percentage of material. dries when entering and leaving this area. It is usually 1 to 60 mins and preferably from 10 to 40 minutes. Operating substantially in the absence of a separate liquid phase, the quantities of reactants and the size of the reactors are minimized.

The supply of substrate impregnated with the hydrolysis zone is generally carried out by an airlock controlled by servo and control means connected to a high level measurement in the airlock and to a timer controlling the opening or closing of the airlock valves. This supply is advantageously gravitational and the pressure is balanced by three-way valves connecting the airlock, either to the hydrolysis reactor described below and operating under pressure, or for example to the impregnation reactor operating substantially at pressure. atmospheric. The hydrolysis of the acidified substrate is generally carried out in the presence of medium-pressure water vapor, preferably 1 to 7 bar, advantageously 2 to 5 bar, and at a temperature of 120 to 170 ° C, advantageously of 120 to 150 ° C and preferably from 135 to 145 ° C. The residence time of the substrate in the hydrolysis zone is conditioned by the rate of transfer of the substrate in this zone. It is generally 10 minutes to 1 hour, preferably 20 to 40 minutes. It is all the better controlled as the hydrolysis is carried out in the absence of a separate liquid phase in the hydrolysis reactor.

The quantity of vapor introduced is a function of the dry matter content of the acidified substrate and the operating conditions in temperature and pressure of the hydrolysis reactor. It is introduced with a flow rate such that at the outlet of the hydrolysis zone, the impregnated and hydrolysed substrate contains substantially no separate liquid phase. The dry matter content is then generally between 25 and 55% by weight and preferably from 40 to 50% by weight.

According to another characteristic of the process, it is possible to carry out a pretreatment of the substrate before the impregnation step consisting of a prewash with water of the substrate in order to remove the tannins and the organic products capable of coloring the sugar solution.

The invention also relates to a unit for the continuous production, in particular of xylose, from lignocellulosic substrate. This unit includes means for grinding this substrate to an adequate size connected to means for supplying ground substrate. More specifically, the unit comprises in combination a reactor for impregnating this ground substrate, of substantially cylindrical shape having an inlet connected to the substrate supply means and an outlet, means for continuously transferring the substrate into the impregnation reactor and means for supplying a solution comprising water and at least one acid (or at least one base), said impregnation reactor comprising servo means connected to the substrate supply means , to the means for transferring this substrate into the impregnation reactor and to the means for supplying said solution and adapted to impregnate the substrate under conditions such that the latter is substantially free of separate liquid phase, the production unit being further characterized in that it comprises a continuous and pressurized hydrolysis reactor, of substantially cylindrical shape, made of stainless steel comprising at one of its ends emitted a sealed member for continuous supply of impregnated substrate connected to the outlet of the impregnation reactor and adapted to operate sometimes substantially at atmospheric pressure, sometimes under pressure, means for continuously transferring the impregnated substrate into the reactor hydrolysis, at least one means for supplying steam under pressure and at the other end a sealed member for discharging the hydrolyzed substrate adapted to operate sometimes substantially at atmospheric pressure, sometimes under pressure and suitable for supplying extraction means sugars below, said hydrolysis reactor comprising servo means connected to the substrate supply member and to the substrate discharge member, to the steam supply rate and to the speed of transfer to substrate, said servo means being adapted to hydrolyze the substrate impregnated, substantially in the absence of a separate liquid phase, said unit further comprising means for continuously extracting the sugars, containing the xylose from the non-hydrolysed substrate, connected to the discharge member and comprising means for supplying water and means for recovering the xylose produced connected to the extraction means.

The transfer means of the impregnation reactor can comprise either a single endless screw for conveying the stalks or a twin screw for the other substrates which are by nature less porous.

According to one embodiment, the hydrolysis reactor can be substantially horizontal or substantially inclined and the member for feeding the impregnated substrate and the member for discharging the hydrolyzed substrate at least in part are substantially vertical.

According to another characteristic of the unit, the sealed member for supplying impregnated substrate comprises an inlet valve and an outlet valve, means for level measurement inside the member, means for pressure balancing connected alternately to the pressurized hydrolysis reactor and to the atmosphere impregnation reactor and of the first means for controlling the level measurement means at the inlet and outlet valves of the supply member .

According to another characteristic of the unit, the sealed member for discharging the hydrolyzed substrate comprises an inlet valve and an outlet valve, second pressure balancing means connected alternately to the pressurized hydrolysis reactor and to the impregnation reactor for example which operates at atmospheric pressure and second means for controlling the inlet and outlet valves of the evacuation member to said second pressure balancing means.

According to another characteristic, the hydrolysis reactor can be tilted according to a downward slope of 0.1 to 1% directed towards the discharge member, so as to recover any condensates.

According to another embodiment, the means for continuously transferring the impregnated substrate into the hydrolysis reactor can comprise an endless screw. This endless screw can, according to a particularly advantageous embodiment, consist of a thick shaft with screw thread tightened in the first quarter, for example, of its length and of a smaller shaft with larger screw thread over the rest of its length. This configuration allows better distribution of the substrate and better regulation of the feed rate.

The process and the apparatus thus described operating continuously allow in particular by impregnation in acid medium to recover xylose with high yields. The fact of being able to control the operation of each reactor separately makes it possible to optimize the unit and to obtain an extraction yield of at least 50% of the xylose contained in the substrate and advantageously of at least 70%. In a basic medium, the apparatus makes it possible in particular to extract the lignin and to recover polymers based on easily hydrolyzable sugars.

The invention will be better understood from the attached figure schematically illustrating an advantageous embodiment of the device.

The corn cobs, after having been ground in a conventional type of grinder 1, at an appropriate particle size, are sent using an elevator 2, for example with discs, to an impregnation reactor 5 via a gravimetric doser 3 supplying as a substrate, the inlet 4 of this reactor. This substantially cylindrical-shaped reactor comprises an endless transfer screw 7 conveying the ground substrate to the outlet 10. This screw is controlled by a motor 6. The acid supply flow is ensured by conventional injectors 8a arranged substantially annularly around the reactor, via a line 8 and a metering pump while the water flow is provided by injectors 9a identical to the injectors 8, arranged substantially annularly around the reactor and connected to a supply line 9 via a pump 9b. These injectors distribute their respective fluid substantially radially by means of pumps 8b, 9b and are advantageously located at a distance from the upstream end corresponding to a quarter of the length of the reactor. First control and servo-control means make it possible to control the injection or supply flows in acid solution to the supply flow of the ground substrate, to its dry matter content and to the speed of transfer in the reactor. such that the dry matter content of the stems goes from about 90% to about 50% and that there is substantially no separate liquid phase at the outlet 10 from the impregnation zone. Once impregnated in an acid medium at room temperature and at a pressure substantially equal to atmospheric pressure, the stems are discharged by gravity and by an outlet perpendicular to the axis of the reactor 5 in a feed member 12 which is an airlock comprising an upper valve 18 called a guillotine valve in relation to the outlet 10 of the impregnation zone and a lower valve guillotine 19 which controls the entry of substrate into an acid hydrolysis reactor 11. The latter, of elongated shape , substantially cylindrical, and arranged in a substantially inclined downstream direction with a downward slope of approximately 0.5%, operates under pressure and at high temperature by means of the supply of water vapor injected substantially radially via a line 14. The entry airlock, arranged substantially vertically and substantially perpendicular to the axis of the reactor 11 comprises a level indicator 20 controlled by the first servo means 16 closing a valve 18 when the airlock is sufficiently filled, pressurizing the airlock in equilibrium with the internal pressure of the hydrolysis reactor via a 3-way valve 21 and finally l opening of the valve 19 so that the content of the airlock can flow into the hydrolysis reactor. The 3-way valve 21 ensures successive alternative pressurization and atmospheric pressure. The hydrolysis reactor 11 supplied with impregnated substrate ensures the transfer of the latter to the discharge member 15 by a transfer screw 13, set in motion by a motor 22. This screw advantageously comprises a first part 22a over a distance corresponding to about a quarter of its length, consisting of a shaft with a larger diameter than that which comes continuously on the second part 22b and with a pitch tighter than that of the second part.

The feed rates of impregnated substrate and of vapor and the speed of transfer into the hydrolysis zone are controlled by second control means 16 so that an at least partly hydrolyzed substrate is obtained which does not contain substantially no separate phase and a dry matter content lower than that which it had at the entry of the hydrolysis zone, for example 40 to 45%.

The pressure and the temperature of the hydrolysis reactor are regulated by sensors (not shown in the figure) and by the means 16 around a set value. When the setpoint value is exceeded, the steam supply is stopped. However, it is open when the setpoint is not reached. A solenoid valve 17 therefore controls the opening and closing of the steam supply 14 as a function of a signal delivered by the servo means 16 connected to the sensors.

The hydrolyzed substrate is then removed via an airlock 15 arranged vertically and substantially perpendicular to the axis of the hydrolysis reactor. This airlock comprises an upper guillotine valve 23 and a lower guillotine valve 24 operating using the second control means, the pressure balancing being effected by a 3-way valve 25 as already described in the case of the upper airlock. The hydrolyzed substrate flows into a buffer tank 26, from which this same hydrolyzed substrate is recovered to supply a diffuser 27 performing continuously, in the presence of diffusion water supplied by a line 28, a solid liquid extraction against the current , the diffusion water first meeting the hydrolyzed substrate whose content of soluble sugars is practically completely exhausted.

The non-hydrolyzed organic material (lignin, cellulose) is recovered at one end 29 of the extraction line while the mixture of sugars in solution containing at least 80% by weight of xylose is recovered at the other end 30. By known means and techniques, the mixture of sugars in solution is then concentrated, generally neutralized with lime, demineralized and discolored by passing, for example, through ion exchange resins. Purified xylose (purity greater than 95%) can be obtained by crystallization from this solution of discolored sugars.

The following example illustrates the invention and is in no way limiting.

During this test, 500 kg of corn cobs ground in the apparatus were treated continuously for 5 hours, the principle diagram of which is described above.

The composition of the roundups is as follows:

The particle size analysis of the roundups measured with different sieves gives the following weight distribution (%):

Refused to sieves 5 mm 3.15 mm 2.5 mm 1.3 mm 0.15 mm 0.1 mm 0.1 mm

Weight distribution (%) 16.8 26.8 12.0 24.6 14.8 4.2 0.8

The impregnation is carried out for approximately 2 minutes, at ambient temperature, at atmospheric pressure with water and concentrated sulfuric acid at 96% by weight (density = 1.84). During the whole experiment, 19 kg of acid and 380 kg of water were added.

The dry matter content at the exit from the impregnation zone is 50.6%. The concentration by weight of acid relative to the dry matter is 4.0%.

The operating conditions of the hydrolysis reactor are 140 ° C. and a pressure of the order of 4 bar with an average residence time of 30 min. The total quantity of product recovered at the outlet of the hydrolysis reactor is 1060 kg. The material content of the hydrolyzed substrate is 42.9%. The cobs having undergone the hydrolysis treatment are sent to a continuously operating diffuser in which the extraction of the sugars with water is carried out against the current. On the one hand, an aqueous solution of sugars is recovered and, on the other hand, stems exhausted in monomeric sugars which are drained on a belt filter, the filtrate then being added to the aqueous sugar solution, the final volume of which is 2200 liters. . The total sugar content is 61.8 gl-1 with respective concentrations of xylose, glucose, arabinose of 51.8 gl-1, 5.4 gl-1 and 4.5 gl-1, i.e. a xylose content of 83.9%. The aqueous solution is then evaporated under vacuum to reach a concentration of total sugars close to 150 g.l-1.

The evaporation temperature does not exceed 60 ° C. 920 l of a concentrated sugar solution are recovered, to which 10.1 kg of CaO are added. The precipitate of calcium sulphate obtained is separated by filtration on a band filter. The filtrate is then demineralized and discolored by passage over ion exchange resins and adsorbent resins.

Claims (10)

Process for the continuous production of a mixture of sugars in solution containing at least 80% xylose from a lignocellulosic substrate, comprising a grinding step, a step of impregnation in acidic aqueous solution, a hydrolysis step in presence of water vapor, a stage of dilution in the presence of water, a stage of extraction of the mixture of sugars produced from the hydrolysed substrate and a stage of recovery of xylose, characterized by the combination of the following stages: a) the said substrate is ground to an adequate size, b) continuously impregnated with substrate from step a), an impregnation zone and the impregnation step is carried out under conditions such that an impregnated substrate is recovered at the outlet of the impregnation zone with a dry matter content such that it contains substantially no separate liquid phase; c) a continuous impregnated substrate resulting from step b) is supplied with a hydrolysis zone under pressure and the hydrolysis step is carried out under conditions of temperature, pressure and water vapor flow rate such that an at least partially hydrolyzed substrate is recovered which contains substantially no separate phase. The method of claim 1 wherein the impregnation step is carried out with a proportion of acid relative to the amount of dry matter of said substrate of 0.1-10% at room temperature and at a pressure substantially equal to the atmospheric pressure for 1 minute to 60 minutes. Method according to one of claims 1 to 2 wherein the hydrolysis step is carried out at a temperature of 120 to 170 ° C at a pressure of 1 to 7 bar for 10 minutes to 1 hour. Method according to one of claims 1 to 3 wherein the hydrolysis step is carried out at a temperature of 135 to 150 ° C, under 2 to 5 bar for 20 to 40 minutes. Method according to one of claims 1 to 4 wherein the dry matter content at the outlet of the impregnation zone is 30 to 70% and advantageously 25 to 55%. Method according to one of claims 1 to 5 wherein the gravity hydrolysis zone is supplied with substrate. Continuous production unit, in particular xylose from a lignocellulosic substrate comprising means for grinding a substrate to an adequate size and means for supplying ground substrate, characterized in that the production unit comprises in combination , an impregnation reactor for this ground substrate of substantially cylindrical shape having an inlet connected to the substrate supply means and an outlet, means for continuously transferring the substrate into the impregnation reactor and means for supplying a solution comprising water and at least one acid or at least one base, said impregnation reactor comprising servo means connected to the substrate supply means, to the means for transferring this substrate into the reactor impregnation and means for supplying said solution and adapted to impregnate the substrate under conditions such that the latter is substantially free of phase li which is separate, the production unit being further characterized in that it comprises a continuous and pressurized hydrolysis reactor, of substantially cylindrical shape, made of stainless steel comprising at one of its ends a sealed member for supplying continuous impregnated substrate connected to the outlet of the impregnation reactor and adapted to operate sometimes at substantially atmospheric pressure, sometimes under pressure, means for continuously transferring the impregnated substrate into the hydrolysis reactor, at least one means for feeding in pressurized steam and at the other end a sealed member for discharging the hydrolyzed substrate adapted to operate sometimes substantially at atmospheric pressure, sometimes under pressure and adapted to supply means for extracting the sugars below, said reactor hydrolysis comprising servo means connected to the substrate supply member and to the substrate discharge member, to the steam supply rate and to the substrate transfer speed, said servo means being adapted to hydrolyze the impregnated substrate, substantially in the absence of a separate liquid phase, said unit further comprising means for continuously extracting sugars containing in particular the xylose from the non-hydrolysed substrate connected to the discharge member and comprising means for water supply and means for recovering the xylose product connected to the extraction means. Production unit according to claim 7 in which the hydrolysis reactor is inclined in a downward slope of 0.1 to 1% directed towards the discharge member. Production unit according to claim 7 or 8 wherein the hydrolysis reactor has an axis of symmetry and wherein the feed member and the substrate discharge member are perpendicular to the axis of symmetry. Production unit according to one of claims 7 to 9 in which a buffer tank is interposed between the substrate removal member and the sugar extraction means.

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