METHOD FOR CLEANING A REACTOR FOR PROCESSING A LIGNOCELLULOSIC BIOMASS

Abstract

The present invention concerns a method for cleaning a reactor (4) for treating a lignocellulosic biomass (P), said method comprising the following steps: a) draining the reactor of the reaction mixture containing the biomass, b) filling the reactor with a basic aqueous solution (EB), c) draining the reactor, d) injecting steam (V) into the reactor, e) filling the reactor with an aqueous solution (E), f) draining the reactor.

Claims

1. A method for cleaning a reactor (4) for treating a lignocellulosic biomass (P), said method comprising: a) draining the reactor of the reaction mixture containing the biomass, b) filling the reactor with a basic aqueous solution (EB), c) draining the reactor, d) injecting steam (V) into the reactor, e) filling the reactor with an aqueous solution (E), f) draining the reactor.

2. The method as claimed in claim 1, characterized in that it comprises at least one step a′) of filling the reactor (4) with an aqueous solution (E), followed by a step a″) of draining the reactor, between step a) of draining the reaction mixture and step b) of filling with a basic aqueous solution (EB).

3. The method as claimed in claim 1, characterized in that it further comprises, after step a), introducing particles of solid abrasive material into the reactor (4), either separately or by introducing the particles into the basic solution (EB) or into the or one of the aqueous solution(s) (E).

4. The method as claimed in claim 1, characterized in that the basic aqueous solution (EB) and/or the or one of the aqueous solution(s) (E) is or are heated before introduction into the reactor (4), in particular to a temperature of at least 20° C., in particular of at least 60° C., and preferably at most 90° C.

5. The method as claimed in claim 1, characterized in that at least step b) of filling the reactor (4) with the basic aqueous solution (EB) takes place with the interior volume of the reactor at a temperature of at least 60° C., in particular of at least 80° C., in particular of at least 100° C.

6. The method as claimed in claim 1, characterized in that, during draining step c), the basic solution drained is recovered for recycling, with optional filtration to remove solid residues (C).

7. The method as claimed in claim 1, characterized in that, during draining step c), the basic solution drained is filtered for recovery of the solid residues (C) it contains, and in that these residues are treated, in particular by washing, for extraction of the basic solution from them for recycling thereof.

8. The method as claimed in claim 1, characterized in that the immersion of the reactor (4) with the basic solution (EB), when the reactor has been filled as per step b), before it is drained, has a duration of between 30 seconds and 4 days, in particular between 30 minutes and 10 h, in particular between 1 and 3 hours.

9. The method as claimed in claim 1, characterized in that the degree of filling of the reactor (4) with the biomass (EB) and/or with the or one of the aqueous solution(s) (E) is between 20 and 100%.

10. The method as claimed in claim 1, characterized in that the steam (V) in step d) is injected at a pressure of between 1.10.sup.5 and 20.10.sup.5 Pa, in particular between 12.10.sup.5 and 18.10.sup.5 Pa, in particular in the vicinity of 15.10.sup.5 Pa.

11. The method as claimed in claim 1, characterized in that steps e) and f) are repeated at least twice, in particular between 2 and 10 times.

12. A method for treating lignocellulosic biomass (P), comprising: preparing an impregnation liquor containing a catalyst, in particular an acidic catalyst, introducing the biomass into an impregnation reactor (3) for impregnation with the impregnation liquor, transferring the impregnated biomass to a pretreatment reactor (4) for pretreatment therein by cooking, enzymatically hydrolyzing the pretreated biomass, alcoholically fermenting the enzymatic hydrolysis must obtained, characterized in that the method is carried out continuously over all or part of said steps, and in that it pauses at the stage of pretreatment in the pretreatment reactor for the pretreatment reactor to be cleaned with the method as claimed in claim 1.

13. The method as claimed in claim 1, characterized in that it pauses for the pretreatment reactor (4) to be cleaned according to a given frequency and/or on exceedance of a threshold value by a physicochemical or rheological characteristic of the reaction mixture in the reactor.

14. A plant for implementing the method as claimed in claim 1, characterized in that it comprises a reactor (4) for pretreating lignocellulosic biomass, a tank (6) for preparing a basic aqueous solution (EB), which is in fluid connection with said reactor for bringing the basic solution (EB) from the tank into the reactor and for recycling all or part of the basic solution, withdrawn from the reactor when it is drained, from the reactor to said tank, with means, optionally, for filtering the solution withdrawn from the reactor before it is reintroduced into the preparation tank.

15. A plant for implementing the method as claimed in claim 12, characterized in that it comprises in succession a reactor (3) for impregnating the biomass (P), fed with impregnating solution by a tank (1) for preparing said solution, a reactor (4) for pretreating the impregnated biomass, which is fed with basic aqueous solution (EB) by a tank (6) for preparing said solution, an enzymatic hydrolysis reactor, and an alcoholic fermentation reactor, with all of the reactors being mounted in series, or at least two of them.

Description

LIST OF FIGURES

[0036] FIG. 1 represents the block diagram of the method for cleaning a biomass pretreatment reactor integrated into a biomass treatment method.

[0037] FIG. 2 represents a first phase of the cleaning method according to FIG. 1.

[0038] FIG. 3 represents a second phase of the cleaning method according to FIG. 1.

[0039] FIG. 4 represents a third phase of the cleaning method according to FIG. 1.

[0040] FIG. 5 represents a fourth phase of the cleaning method according to FIG. 1.

[0041] FIG. 6 represents a fifth phase of the cleaning method according to FIG. 1.

[0042] FIG. 7 represents a sixth phase of the cleaning method according to FIG. 1.

[0043] FIG. 8 represents a seventh phase of the cleaning method according to FIG. 1.

[0044] FIG. 9 represents a first way of recycling the basic solution used in the cleaning method according to FIG. 1.

[0045] FIG. 10 represents a second way of recycling the basic solution used in the cleaning method according to FIG. 1.

[0046] FIG. 11 represents a third way of recycling the basic solution used in the cleaning method according to FIG. 1.

DESCRIPTION OF THE EMBODIMENTS

[0047] The figures are highly schematic, with the same references corresponding to the same components from one figure to another. The equipment is shown substantially in the spatial position it occupies in the operational position.

[0048] The method for cleaning a biomass pretreatment reactor is illustrated here in the context of a biomass pretreatment method intended for producing alcohols, especially biofuel in the form of bioethanol, by the steps of the method according to FIG. 1, described rapidly below. One embodiment of this method is described in greater detail, for example, in patent WO 2018/015227, to which reference may be made as and when required.

[0049] It should be noted that the cleaning method according to the invention is equally applicable to any biomass treatment reactor, in particular but not solely for pretreatment, and, more generally, to any reactor intended for treatment of a lignocellulosic biomass, the purpose of the treatment being to modify a chemical, physical or rheological characteristic thereof, and being generally performed at elevated temperature.

[0050] The biomass treatment method taken here as an example and represented in FIG. 1 comprises a first step of impregnating the biomass in a vertical reactor 3, followed by a step of pretreating the impregnated biomass in a steam explosion reactor 4. These steps of the method are carried out continuously and are detailed below with the aid of the description of the equipment used for its implementation: [0051] A tank 1 is provided for preparing an impregnating liquor containing a chemical catalyst, which is made up from water E and catalyst A which are fed to the tank, the catalyst in the present instance being a strong acid such as concentrated sulfuric acid in aqueous phase, and this tank allowing the impregnation reactor 3 to be fed with a mixture of water E and chemical catalyst A,
A conical screw 2 (also referred to as a “plug screw” or “sealing screw”) for feeding fresh biomass (in this case the wheat straw P) into the impregnation reactor 3,
An impregnation liquor feed for the reactor, connecting the liquor preparation tank 1 and the impregnation reactor 3,
An impregnation reactor 3 equipped with two ascending transport screws (not shown) allowing the biomass to transit from the impregnation zone in the lower part of the reactor to the straining zone in the upper part of the reactor, and to take the impregnated and strained biomass at the reactor outlet at the top of the reactor. This impregnated and strained biomass is then sent for pretreatment via a feed opening out into a second conical screw 2′.
This second conical screw 2′ feeds impregnated biomass to the pretreatment reactor 4,
A pretreatment reactor 4 treats the impregnated biomass by steam explosion
A water circuit is provided for washing the conical screws 2,2′ of the impregnation reactor 3 and the pretreatment reactor 4, which are represented symbolically in FIG. 1 by inlets for water E at said screws
A steam separation means 5 is fed by the reactor 4 with biomass which has undergone steam explosion cooking, as for example a cyclone separation means, with steam V at the top outlet and the exploded biomass, also called must M, at the bottom outlet.

[0052] At this stage this must M has sufficient accessibility of the cellulose to enzymes in order to be treated by enzymatic hydrolysis for the production of 2G sugars. The conditions of the enzymatic hydrolysis and of the consecutive or simultaneous fermentation which follow this separation (not shown in FIG. 1) are suitable for the desired products and are known to those skilled in the art.

[0053] The use of the above-described pretreatment technique results in the deposition of different types of biomass (wheat straw here, but also miscanthus, poplar, etc.), which build up/adhere to the surface of at least one of the transport screws internal to the pretreatment reactor 4, and to the walls of the pretreatment reactor 4. These deposits are subject to cooking for longer times than the normal residence time of the biomass in the reactor, and are converted into a residue, which here may also be called “coke”. This “coke” can cause a variety of operational problems, such as plugging of the outlet orifice of the reactor 4, an increase in friction events on at least one of the internal transport screws on the wall of the chamber housing them, and a possible result of this is a reduction in performance of the pretreatment unit as a whole, as represented in FIG. 1.

[0054] Defining the composition of the “coke” has proven intricate, it being a residue with an appearance and composition evolving progressively over time: at the start of a production cycle, the material deposited is biomass, and so has essentially the same characteristics as the biomass which continues its way through the reactor 4 and toward the downstream steps. The deposit formed by adhesion on the inside wall of the reactor 4 will remain a much longer time than desired under the cooking conditions (temperature especially). The effect of the temperature affects the composition and morphology of the residue, which will become more and more a “baked” residue. The greater the extent to which the residue is “baked”, the more compact it is and the more it sticks to the walls of the reactor.

[0055] These “coke” deposits are cumulative: the longer the tool is continuously operated, the greater the deposition of the coke, and the more the “layers” of coke close to the wall will turn into a very hard solid. These deposits therefore provoke a fouling phenomenon, increasing the thickness of the walls and reducing the useful volume of the reactor. Depending on the configuration of the cooking reactor, and in particular the type of internals in place, certain elements may be seen to have their rotation affected, such as the screw, or at least the screws transporting the biomass in the reactor in the course of cooking. This effect is manifested in particular in a rise in the power of the motor driving the relevant screw in rotation.

[0056] Throughout production, another possibility is for a more or less hardened section of this residue to break off from the wall of the reactor, under the effect, for example, of the rotation of the screw or at least one of the screws in question, or of the passage of the biomass through the reactor: accordingly, particles with a density much greater than the bed of biomass undergoing cooking may be caused to detach themselves and be carried toward the outlet orifice of the reactor, and this may give rise to instances of plugging or operational problems downstream. Despite these detachments, the deposits are found to continue increasing over time during a given production cycle.

[0057] After stoppage, cooling and opening of the cooking reactor 4, it was found that the coke takes two forms: a hard form in direct contact with the inside walls of the reactor, and a more friable form covering the hard coke. The difference between these two cokes lies in their elemental compositions, as indicated in table 1 below:

TABLE-US-00001 Friable coke Hard coke Carbon content (%) 44.57 65.07 Hydrogen content (%) 5.85 4.67 Oxygen content (%) 34.63 24.58

[0058] It is found that the percentage of carbon in the hard coke is higher than that in the friable coke, while the opposite tendency is recorded for the oxygen content, and similar values for the hydrogen content. The conclusion is that the friable coke is in some way the precursor of the dense coke.

[0059] The invention involves halting the operation of the impregnation reactor 3 and pretreatment reactor 4, so as to carry out the chemical cleaning of the reactor 4 to extract this coke C. While this cleaning is a discontinuous step, it does not require opening of the reactor and mechanical cleaning of the reactor interior as was the case before. This cleaning according to the invention, detailed later on below, is therefore quicker, more economical and safer, as it limits the operational risks associated with the assembly and disassembly of the unit.

[0060] A working example of the method according to the invention and its variants are explained using the collective figures. It requires the following ancillary equipment, further to that already described, on the basis of FIG. 1:

A tank 6 for preparing a cleaning liquor EB containing a base. This tank 6 can be used to feed the pretreatment reactor 4 with basic solution at a certain concentration. It is fed with water E and with base B (for example from a base B in the form of a concentrated aqueous solution of KOH), the make-up of which is adjusted to give a liquor at the desired concentration of base and the desired pH. The concentration range of this basic solution is between 1 to 50 weight % of KOH and the pH of the solution is preferably greater than 8, preferably greater than 9 or 10, advantageously between 12 and 13. For example, the concentration may be 3 weight % KOH and the pH approximately 13.5 (a pH which will lower slightly when the solution has been introduced into the reactor, during the cleaning time).
A cleaning liquor feed to the reactor 4, connecting the cleaning liquor preparation tank 6 and the pretreatment reactor 4 for cleaning, by preheating it where appropriate with ad hoc equipment (resistance heaters surrounding the conduits, for example).

[0061] The exemplary embodiment of the method according to the invention runs as follows:

Sequence 1: Halt to production, as represented in FIG. 1 and in FIG. 3, with closure of the feed and the outlet of the pretreatment reactor 4 (shutoffs symbolized by a cross on the inlet and outlet conduits of the reactor), after which the reactor is drained, essentially drained of the biomass (FIG. 2 represents the biomass treatment method in production, before production is halted)
Sequence 2: Water bath by injection of water E heated to a certain temperature into the reactor 4, as shown in FIG. 3. The purpose of this first bath is to increase the pH of the residual biomass in the reactor so as to make the basic cleaning with the KOH solution more effective. The operating conditions are as follows: bath duration between 0 and 20 minutes, preferably less than or equal to 30 seconds; degree of filling of the reactor between 20 and 100% of its useful internal volume, for example 100%; the temperature of the preheated water before injection is between 20° C. (ambient temperature) to 90° C. (preheating), the water being preheated for example to a temperature of 80° C. [0062] Sequence 3: Draining of the water contained in the reactor 4, as shown in FIG. 4 (the water E evacuated from the reactor is liable to contain coke, hence the indication E+C in the figure),
Sequence 4: Filling of the reactor 4 with the basic solution EB, optionally preheated before being introduced into the reactor, from the preparation tank 6 for a certain duration, as shown in FIG. 5; the operating conditions are as follows: duration of reactor immersion between 30 seconds and 4 days, for example 2 hours; degree of filling of the reactor between 20 and 100% of its useful internal volume, for example 100%; the temperature of the basic solution before injection is between 20° C. (ambient temperature) and 90° C., the solution being preheated for example to a temperature of 80° C.
Sequence 5: Draining of the basic solution EB containing coke C, EB+C, from the reactor 4, as shown in FIG. 6,
Sequence 6: Heating of the reactor 4 by injection of steam V, as also shown in FIG. 6. The objective here is to detach the pieces of biomass residue C by a thermal action complementary to that of the basic solution. The operating conditions are as follows: pressure between 1 and 20 bar (1.10.sup.5 to 20.10.sup.5 Pa), for example 15 bar (15.10.sup.5 Pa), the duration at constant steam-heating temperature is between 30 seconds and 6 hours, being for example 2 hours,
Sequence 7: Water bath by injection of water E heated to a certain temperature into the reactor 4, as shown in FIG. 7. The operating conditions are as follows: bath duration from 0 to 20 minutes, for example at most 30 seconds; degree of filling of the reactor between 20 and 100% of its useful internal volume, for example 100%; the temperature of the preheated water before injection is between 20° C. (ambient temperature) to 90° C. (preheating), the water being preheated for example to a temperature of 80° C.
Sequence 8: Draining of the water E contained in the reactor 4, as shown in FIG. 8, then draining of the water E, optionally containing coke: E+C
Sequences 9 and 10: A repetition of sequences 7 and 8 under the same operating conditions, i.e., a water bath by injection of water E heated to a certain temperature into the reactor, then draining of the water E, optionally containing coke: E+C, which may be followed by at least one further repetition of sequences 7 and 8.

[0063] The frequency of the cleaning procedure may vary widely depending on the type and size of the pretreatment reactor 4 and/or on the type of biomass treated. For example, cleaning may be triggered when the torque of one of the conveying screws of the reactor increases by more than 15% relative to the torque observed at the start of production.

[0064] A variety of variants may be provided to the exemplary cleaning method described above, while remaining within the ambit of the invention, and some of them are detailed below (they may be alternative or cumulative):

The aqueous solutions E and the basic solution EB may be introduced into the reactor 4 at the ambient temperature, or may be preheated, to 60, 80 and even 90° C. It should be noted that, even if introduced at ambient, they will generally tend to heat up when in the reactor, as the latter has a certain thermal inertia (since production of pretreated biomass takes place at high temperatures).
The water bath sequences 2 and 3 before introduction of the basic liquor EB are optional and may therefore be omitted,
The basic liquor B exiting the reactor 4 contains residues of degraded biomass, the “coke” described above: EB+C. This liquor may be filtered to extract its coke C, and then recycled either to the basic liquor preparation tank 6 or to carry out sequence 2 (water bath) of the cleaning procedure, as shown in FIG. 9,
The coke C exiting the reactor 4 has a high base content. It may be washed with water E to extract its residual base, and then the washing water may be recycled either to the basic liquor preparation tank 6 or to carry out sequence 2 (water bath) of the cleaning procedure, as shown in FIG. 10. This is therefore a more extensive recycling than that proposed in FIG. 9.
The coke and also the basic liquor B+C exiting the reactor 4 may be sent directly into the basic liquor preparation tank 6, as shown in FIG. 11. The advantage is still to recover the residual base exiting the pretreatment reactor 4, as in the case of FIGS. 9 and 10, for preparing the basic liquor used to clean the reactor by immersion and to reduce the consumption of KOH. A filtration is then necessary before the basic solution B is injected into the pretreatment reactor 4.
The cleaning procedure may be applied in the case of a raw biomass treatment without a prior impregnation step (referred to then as autohydrolysis)
The interior volume of the reactor 4 may be immersed in the basic solution/liquor EB in the presence of a solid abrasive compatible with the mechanical operation of the reactor, to produce an auxiliary mechanical effect to the detachment of the coke from the walls of the reactor. The particles may be introduced into the reactor 4 either separately or in the basic liquor B.

[0065] Throughout the present text, the abbreviation “SC” denotes the solids content, which is measured according to the standard ASTM E1756-08(2015) “Standard Test Method for Determination of Total Solids in Biomass”.

EXAMPLES

Example 1: Not in Accordance with the Invention

[0066] This example was carried out with wheat straw as biomass. The characteristics and composition of the feedstock are as follows:

Solids content (SC): 91.07%
Biomass flow rate: 65 kg SC/h

[0067] The operating conditions for producing the pretreated biomass are as follows:

Impregnation for production and cleaning in the impregnation reactor 3:
H.sub.2SO.sub.4 acidic solution flow rate: 1.5 kg/h (pH close to 1)
Steam explosion of the impregnated biomass in the pretreatment reactor 4:
Residence time: 5 min
Duration of production: 72 hours
Prior-art mechanical cleaning:
Temperature decrease: 48 hours
Duration of opening and disassembly: 8 hours
Duration of cleaning: 8 hours
Duration of reassembly: 8 hours

[0068] The mass of coke C produced is 16 kg, occupying a volume of 0.012 m.sup.3 in the reactor 4, giving a reduction in reactor volume of 8.7% and a coke production rate of 222 g/h.

Example 2: in Accordance with the Invention

[0069] The impregnation and steam explosion are performed under the same conditions and with the same biomass as in example 1.

[0070] The halt in production is followed immediately by cleaning of the reactor 4 in accordance with the invention, under the following conditions:

Water bath at 80° C. with a degree of filling of 100%, then immediate draining of the reactor,
Immersion of the entirety of the useful internal volume (corresponding to a degree of filling of 100%) of the reactor 4 in the basic solution B containing 3% of KOH, corresponding to an initial pH before injection into the reactor of 13.6. Before being injected, the basic solution was heated at 80° C. for 2 hours. The reactor 4 is then drained,
Heating of the reactor 4 for 2 hours at 200° C. (no biomass present), by steam injection,
3 rinses with hot water at 80° C. with complete immersion of the internal volume of the reactor 4 (degree of filling: 100%), then immediate draining

[0071] The mass of coke recovered at the end of the cleaning procedure is 15 kg, giving a coke production rate of 208 g/h. The cleaning procedure thus allowed recovery of 94% of the coke formed in the reactor 4, so reducing its fouling rate by 94%.

[0072] The time needed for carrying out this chemical cleaning is markedly less than the time needed for the mechanical cleaning of comparative example 1. The cleaning method of the invention is also much less intrusive, as it does not require the reactor to be opened.