Process and unit for producing a mixture of sugars containing at least 80% xylose from a lignocellulosic substrate.

p0001The invention relates to a method and a production unit continuously a mixture of sugars containing at least 80% by weight of xylose from a lignocellulosic substrate. It applies in particular to the conversion of xylose to xylitol, a natural sweetener.

p0002Most lignocellulosic substrates (wood, annual plants) has a heterogeneous composition wherein one normally distinguishes three predominant fractions which are cellulose, hemicellulose and lignin. This heterogeneity complicates recovery. From this point of view, a separation of hemicellulose (which have a variable composition in sugars but are often rich in pentose, including xylose) cellulose (composed exclusively of glucose) is of great interest because it allows a separate valuation two types of constituents. This interest requires that the objective is to use chemical or fermentation as if most well fermentations using glucose derived from the hydrolysis of cellulose, fewer are those that use such as pentose hemicellulose.

p0003There are already known a processing said explosion to steam wherein the lignocellulosic substrate is subjected for a variable time to the action of steam under pressure at a temperature generally greater than 150 ° C eg 150 to 250 ° C. This action ended with an explosive release. This treatment formerly known for improving the digestibility of forages also increases the susceptibility of lignocellulosic substrates for enzymatic hydrolysis (K. BUCHHOLZ, PULS J., B. GADELMANN, MM Dietrichs, Process Biochemistry, December / January 1980 / 1981 pp 37-43).

p0004It was found in FR 2580 669 that the addition of at least one acid at the steam explosion treatment in a chamber in batches at high temperature for 2 to 5 minutes of time allowed excellent separation of hemicellulose and cellulose fractions and a very important release of the constituent sugars of hemicellulose, especially pentose such as xylose and arabinose without undergoing significant degradation. But can not operate continuously for corrosion reasons. Furthermore, the fact of operating in two-phase medium at a temperature of 150 to 250 ° C has the disadvantage of degrading the pentoses obtained and to promote the appearance of by-products such as furfural.

p0005However, it is known from US patent 4136 207, a continuous processing apparatus with steam and pressure up to 25 bar, for example, a previously ground lignocellulosic substrate or brought to a specific state division from material .

p0006The recommended temperature and pressure levels (190-220 ° C), it is possible to solubilize in particular 50 to 60% of all the pentosans contained in the starting material. Only 10% of pentoses are passed into solution in the form of monomers, the other 90% being in the form of oligomers with variable degrees of polymerization.

p0007This device can not be used in acid and high temperature environment of about 200 ° C, due to corrosion of levels that the hydrolysis treatment in the presence of acid generate.

p0008An object of the invention is to address the issues raised above.

p0009It was in fact found that with the device and method according to the invention, minimized corrosion of the various reaction vessels while obtaining an improved extraction yield including pentoses and particularly xylose.

p0010Furthermore, with a very limited investment cost because of the simplicity of the apparatus used, the apparatus can operate continuously under severe conditions of temperature and pressure for example at a pressure which can be between 1 and 7 bar.

p0011The invention is defined by the claims. The invention therefore relates to a continuous process for producing a mixture of sugars in solution containing at least 80% by weight of xylose from a lignocellulosic substrate comprising a crushing step, a step of impregnating an aqueous solution of Any acid, a hydrolysis step in the presence of steam, a dilution step in the presence of water, a step of extracting the product mixture of sugars from the hydrolysed substrate, and a xylose recovery step.

p0012More specifically, the method comprises combining the steps of:<ul><li>a) comminuting said substrate to a suitable particle size and between 0.1 and 40 mm;</li><li>b) continuously fed substrate from step a), an impregnation zone and the impregnation step is carried out under conditions such that a substrate impregnated is recovered at the output of the impregnation zone with a solids content (30 to 70% by weight) such that it does not contain a separate liquid phase;</li><li>c) continuously fed to the impregnated substrate from step b) a pressurized hydrolysis zone and the hydrolysis step is carried out under temperature, pressure and water vapor flow rate such that a substrate is recovered at least in part hydrolyzed containing no separated liquid phase.</li></ul>

p0013The lignocellulosic substrate is usually wood, stalks and stems of corn or straw. however preferred to use corn cobs.

p0014The timber is usually debarked and cut in chip form while cobs are ground and obtained at a particle size generally between 0.1 and 40 mm, preferably between 1 and 5 mm.

p0015The straw is chopped and used against the form of strands. For reasons of convenience, the term is used for grinding the division step to the appropriate size of each of these substrates. The dry matter content of the substrate is generally at least 50%, for example that of wood is about 50% while that of straw is about 80% while about 80% stalks.

p0016The impregnation is generally carried out in an impregnation zone, fed separately or not, with water and an acid. It could be a basis if we wanted to make an alkaline treatment.

p0017The water supply flow rate and acid is adjusted according to the feed rate of milled substrate from its initial dry matter content, to its desired final content and its transfer speed in the impregnation zone. These acid and water flow rates are advantageously distributed in a ring by suitable injection means known to the skilled in the art and located generally in the first quarter of this zone upstream side.

p0018The 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%.

p0019Organic acids or conventional minerals are usually used. however it is preferred sulfuric acid or hydrochloric acid.

p0020Advantageously, the solids content at the output of the impregnation zone is about 35 to 55%. The impregnation is generally carried out at a temperature and at ambient pressure during a residence time depending on the nature of the substrate used and which is determined by the transfer speed in the impregnation zone and the percentage of material dried at the inlet and the outlet of this zone. It is usually from 1 to 60 min and preferably from 10 to 40 minutes. Operating substantially no separated liquid phase, minimizing the amounts of reagents and the reactor size.

p0021The supply of substrate impregnated with the hydrolysis zone is generally carried out by a lock controlled via servo and control means connected to a level far above the airlock and a timer commanding the opening or closing the valves of the chamber. This supply is preferably gravity and pressure equalization is effected by three-way valves connecting the lock or to the hydrolysis reactor described below and operating under pressure, or for example by impregnation reactor operating substantially at the pressure atmospheric. The hydrolysis of the acidified substrate is generally conducted in the presence of water vapor medium pressure, preferably 1 to 7 bar, preferably 2 to 5 bar, and at a temperature of 120 to 170 ° C, preferably from 120 to 150 ° C and preferably from 135 to 145 ° C. The substrate residence time in the hydrolysis zone is determined by the transfer speed of the substrate in this area. It is usually 10 minutes to 1 hour preferably 20 to 40 minutes. It is even better controlled that the hydrolysis is performed in the absence of liquid phase separated in the hydrolysis reactor.

p0022The amount of steam introduced is a function of the solids content of the acidified substrate and operating conditions of temperature and pressure the hydrolysis reactor. It is introduced with a flow rate such that output of the hydrolysis zone, the impregnated and hydrolysed substrate contains substantially no separated liquid phase. The dry matter content is then generally between 25 and 55% by weight and preferably 40 to 50% by weight.

p0023According to another characteristic of the method, one can perform a pretreatment of the substrate prior to the impregnation step in a substrate of the water pre-wash in order to remove the tannins and organic products which may color the solution of sugars.

p0024The invention also relates to a production unit continuously include xylose from lignocellulosic substrate. This unit contains grinding means of this substrate to a suitable dimension connected to means for supplying ground substrate. More specifically, the unit comprises a combination of an impregnation reactor of this milled substrate of cylindrical form having an input connected to the substrate supply means and an outlet, continuous means for transferring the substrate in the reactor impregnation 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, the transfer means of the substrate in the impregnation reactor and means for supplying said solution and adapted to impregnate the substrate under conditions such that it has no separated liquid phase, the production unit being further characterized in that it comprises a hydrolysis reactor continuously and under pressure, cylindrical, stainless steel comprising at one of its ends a sealing member for continuously supplying impregnated substrate connected to the output of the reactor impregnation and adapted to operate sometimes at atmospheric pressure, sometimes under pressure, continuous transfer means of the impregnated substrate in the hydrolysis reactor, at least one pressurized steam supply means and at the other end a waterproof member for evacuating the hydrolysed substrate adapted to operate sometimes at atmospheric pressure, sometimes under pressure and adapted to supply means for extracting sugars below, said hydrolysis reactor comprising automatic control means connected to the substrate feed member and the discharge member of the substrate, the vapor feed rate and the speed of transfer substrate, said servo means being adapted to hydrolyse the impregnated substrate in the absence phase separated liquid, said unit further comprising means for extracting continuously sugars containing xylose of unhydrolyzed substrate, connected to the discharge element and comprising water supply means and means for recovering xylose product connected to the extraction means.

p0025The impregnation reactor transfer means may comprise an endless single screw for conveying the stalks is a twin-screw for the other substrates by less porous in nature.

p0026According to one embodiment, the hydrolysis reactor may be substantially horizontal or substantially inclined and power impregnated substrate and the discharge member body of the hydrolyzed substrate at least partly are substantially vertical.

p0027According to another feature of the unit, the sealed member impregnated substrate supply comprises an inlet valve and an outlet valve, level measuring means within the body, means for balancing pressure alternately connected to pressurized hydrolysis reactor and to the impregnation reactor and the atmosphere of the first servo-control means of level measurement means to the inlet and outlet valves of the feed member .

p0028According to another feature of the unit, the sealed body for evacuating the hydrolysed substrate having an input and an output valve valve, second pressure-balancing means alternately connected to the hydrolysis reactor under pressure and impregnation reactor which operates for example at atmospheric pressure and second control means of the inlet and outlet valves of the discharge member to said second pressure balancing means. According to another characteristic, the hydrolysis reactor can be inclined at a descending slope of 0.1 to 1% directed towards the discharge member, so as to recover any condensate.

p0029In another embodiment, the transfer means continuously impregnated substrate in the hydrolysis reactor may include a worm. This worm can, according to a particularly advantageous embodiment, consist of a thick tree, screw tightened in the first quarter, for example, its length and a smaller tree with no greater respect the rest of its length. This configuration enables better distribution of the substrate and to better regulate the power flow.

p0030The method and apparatus so described operating continuously it possible in particular by an acidic medium impregnation of recovering xylose in high yields. Being able to separately control the operation of each reactor optimizes the unit and to obtain an extraction yield of at least 50% of xylose contained in the substrate and preferably at least 70%. In basic medium, the apparatus allows from extracting lignin and retrieve based sugars easily hydrolysed polymer.

p0031The invention is best understood from the attached figure illustrating schematically an advantageous embodiment of the device.

p0032corn cobs, after being crushed in a mill 1 of the conventional type, appropriate particle size, are sent using a forklift 2 such disks in an impregnation reactor 5 via a gravimetric feeding 3 substrate entrance 4 of the reactor. This substantially cylindrical reactor comprising a screw transfer substrate 7 carrying the ground to the output 10. This screw is driven by a motor 6. The speed acid feed is provided by conventional injectors 8a arranged substantially annular 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, disposed substantially annularly around the reactor and connected to a feed line 9 via a pump 9b. These injectors distribute their respective fluid substantially radially through pumps 8b, 9b and are advantageously located at a distance from the upstream end corresponding to a quarter of the reactor length. First means of control and servo allow to slave the injection rates or acid solution supply to the feed rate of milled substrate, in its dry matter content and the transfer speed in the reactor such that the dry matter content of the cobs from approximately 90% to about 50% and that there is substantially no separated liquid phase at the outlet 10 of the impregnation zone. Once impregnated in an acidic medium at room temperature and at a pressure substantially equal to atmospheric pressure, the stalks are discharged by gravity and an outlet perpendicular to the axis of reactor 5 into a feed member 12 which is an airlock comprising a valve 18, said upper sash in relation to the outlet 10 of the impregnation zone and a lower guillotine valve 19 which controls the input substrate in an acid hydrolysis reactor 11. the latter, elongated substantially cylindrical and disposed substantially inclined toward the downstream with a downward slope of about 0.5%, operates under pressure and at elevated temperature due to the injected steam intake substantially radially via a line 14. the entry lock is arranged substantially vertically and substantially perpendicular to the axis of the reactor 11 comprises a level indicator 20 by commanding the first servo means 16, the closing of a valve 18 when the lock is sufficiently full, the pressurization of the balance in lock with the internal pressure of the hydrolysis reactor through a 3-way valve 21 and finally opening the valve 19 so that the contents of the chamber can flow in the hydrolysis reactor. The 3-way valve 21 ensures alternative successive pressurized and at atmospheric pressure. The hydrolysis reactor 11, supplied with substrate impregnated ensures the transfer thereof to the discharge member 15 by a screw conveyor 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, constituted by a larger diameter shaft than comes in continuity on the second portion 22b and a not closer than the second portion.

p0033The substrate feed rates impregnated and steam and the transfer speed in the hydrolysis zone are controlled by second servo means 16 so that the substrate is obtained at least in part substantially containing hydrolyzed no separate phase and a solids content less than he had at the entrance of the hydrolysis zone, for example 40 to 45%.

p0034The pressure and the temperature of the hydrolysis reactor are controlled by sensors (not shown in the figure) and by the means 16 around a set value. When the fixed setpoint is exceeded, the steam supply is stopped. For against, it is open when the set value is not reached. A solenoid valve 17 therefore controls the opening and closing of the steam supply 14 in accordance with a signal issued by the control means 16 connected to the sensors.

p0035The hydrolysed substrate is then discharged via a lock 15 arranged vertically and substantially perpendicular to the axis of the hydrolysis reactor. This lock includes an upper gate valve 23 and a lower slide gate 24 which operates with the second servo means, pressure equalization is effected by a 3-way valve 25 as has already been described in the case the upper lock chamber. The hydrolysed substrate flows into a buffer tank 26, from which the same hydrolysed substrate is recovered for feeding a diffuser 27 continuously effecting, in the presence of diffusion of water supplied through a line 28, a solid liquid extraction stream against , water diffusion first meeting hydrolyzed substrate, the soluble sugar content is almost completely exhausted.

p0036Non-hydrolysed organic material (lignin, cellulose) is recovered at one end 29 of the extraction line while the sugars in solution mixture containing at least 80% by weight of xylose is recovered at the other end 30. By known means and techniques, the solution of sugars mixture is then concentrated, usually neutralized with lime, demineralized and decolorized by passing, for example on ion exchange resins. The purified xylose (greater than 95% purity) can be obtained by crystallization from this discolored sugar solution.

p0037The following example illustrates the invention and is not limiting.

p0038During this test, it was treated continuously for 5 hours, 500 kg of crushed corn cobs in the apparatus whose block diagram is described.

p0039The composition of the raids is: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Humidity</entry><entry namest="col2" nameend="col2" align="char" char=",">9.0%</entry></row><row><entry namest="col1" nameend="col1" align="left">glucans</entry><entry namest="col2" nameend="col2" align="char" char=",">32.5%</entry></row><row><entry namest="col1" nameend="col1" align="left">xylan</entry><entry namest="col2" nameend="col2" align="char" char=",">28.8%</entry></row><row><entry namest="col1" nameend="col1" align="left">arabans</entry><entry namest="col2" nameend="col2" align="char" char=",">3.7%</entry></row><row><entry namest="col1" nameend="col1" align="left">lignin</entry><entry namest="col2" nameend="col2" align="char" char=",">13.4%</entry></row><row><entry namest="col1" nameend="col1" align="left">Ashes</entry><entry namest="col2" nameend="col2" align="char" char=",">2.0%</entry></row><row><entry namest="col1" nameend="col1" align="left">Other constituents (including acetyl groups, uronic acids ...)</entry><entry namest="col2" nameend="col2" align="char" char=",">10.6%</entry></row></tbody></tgroup></table></tables>

p0040Particle size analysis raids measured with different screen shows the distribution by weight (%) below: <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="8" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="19.68mm" /><colspec colnum="2" colname="col2" colwidth="19.68mm" /><colspec colnum="3" colname="col3" colwidth="19.68mm" /><colspec colnum="4" colname="col4" colwidth="19.68mm" /><colspec colnum="5" colname="col5" colwidth="19.68mm" /><colspec colnum="6" colname="col6" colwidth="19.68mm" /><colspec colnum="7" colname="col7" colwidth="19.68mm" /><colspec colnum="8" colname="col8" colwidth="19.68mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Refusal to mm sieve</entry><entry namest="col2" nameend="col2" align="left">5 mm</entry><entry namest="col3" nameend="col3" align="left">3.15 mm</entry><entry namest="col4" nameend="col4" align="left">2.5 mm</entry><entry namest="col5" nameend="col5" align="left">1.3 mm</entry><entry namest="col6" nameend="col6" align="left">0.15 mm</entry><entry namest="col7" nameend="col7" align="left">0.1 mm</entry><entry namest="col8" nameend="col8" align="left">0.1</entry></row><row><entry namest="col1" nameend="col1" align="left">Distribution by weight (%)</entry><entry namest="col2" nameend="col2" align="left">16.8</entry><entry namest="col3" nameend="col3" align="left">26.8</entry><entry namest="col4" nameend="col4" align="left">12.0</entry><entry namest="col5" nameend="col5" align="left">24.6</entry><entry namest="col6" nameend="col6" align="left">14.8</entry><entry namest="col7" nameend="col7" align="left">4.2</entry><entry namest="col8" nameend="col8" align="left">0.8</entry></row></tbody></tgroup></table></tables>

p0041The impregnation is carried out for about 2 minutes at room temperature, at atmospheric pressure with water and concentrated sulfuric acid 96 wt% (density = 1.84). Throughout the experiment, we added 19 kg of acid and 380 kg of water.

p0042The dry matter content at the outlet of the impregnation zone is 50.6%. The acid concentration by weight relative to the dry matter content is 4.0%.

p0043The operating conditions of the hydrolysis reactor are 140 ° C and a pressure of about 4 bar with an average residence time of 30 min. The total quantity of product recovered in hydrolysis reactor output is 1060 kg. The content in the substrate is hydrolyzed by 42.9%. Cobs having undergone the hydrolysis treatment are sent in a continuously operated diffuser wherein the water extraction of the sugars is carried out against the current. Is recovered on the one hand an aqueous solution of sugars and other cobs depleted sugar monomers which are drained on a belt filter, the filtrate being subsequently added to the aqueous sugar solution, the final volume is 2200 liters . The total sugar content is 61.8 gl-1 with respective concentrations of xylose, glucose, arabinose 51.8 gl-1, 5.4 and 4.5 gl-1 gl-1 is a xylose content 83.9%. The aqueous solution was then evaporated under vacuum to reach a neighboring total sugar concentration of 150 gl-1.

p0044The evaporation temperature does not exceed 60 ° C. Is recovered 920 l of a concentrated solution of sugar, to which is added 10.1 kg of CaO. The resulting calcium sulphate precipitate is separated by filtration belt filter. The filtrate is then demineralised and decolorized by passing over ion exchange resins and adsorbent resins.