FI125991B - Method and apparatus for continuous precipitation of lignin from black liquor - Google Patents

Description

METHOD AND EQUIPMENT FOR THE CONTINUOUS PREPARATION OF LIGIN FROM BLACK LYGE

FIELD OF THE INVENTION AND BACKGROUND This invention relates to a process for the continuous precipitation of lignin from black liquor, wherein the pH of the black liquor is lowered to the lignin precipitation point and the precipitated lignin is separated from the black liquor. The invention also relates to an apparatus for the continuous precipitation of lignin from black liquor.

Lignin is the world's second most common biopolymer after cellulose. Chemically, it is a highly branched polyphenol, which is amorphous, irregular in structure, and tightly adhered to wood fibers, which is difficult to distinguish when treated with wood. In chemical pulping, lignin is separated from wood fibers in pulp cooking, whereupon it is dissolved in the cooking liquor. Black liquor from sulphate soup contains cooking chemicals, lignin dissolved in wood and other substances dissolved in wood. When lignin is soluble in sulphate soup, its structure also changes with respect to native lignin.

As an organic high-carbon biopolymer, lignin has a calorific value, and lignin remaining in the black liquor has traditionally been burned in a pulp mill soda recovery boiler to generate heat and electricity. Lignin can also be separated from black liquor and used in the manufacture of various chemicals. As synthesis methods evolve or new applications are found, the importance of lignin as a starting material for various products may increase, resulting in greater separation of lignin for further processing. As a result, various methods have been developed to separate lignin from black liquor.

A common method for lignin separation is precipitation. In the strongly alkaline pH range of black liquor, lignin remains dissolved. When the pH drops below 10, much of the lignin is precipitated by the neutralization of the phenolic hydroxyl groups it contains. Since lignin precipitation is based on the decreasing solubility of lignin as the pH decreases, a pH-lowering acidifying agent, such as sulfuric acid or carbon dioxide, is used as the precipitating chemical to be added to the black liquor. Since the black liquor from which lignin has been precipitated is returned to the chemical cycle of the pulp mill, the use of the acid introduces substances that disrupt the chemical balance of the cycle. As a result, carbon dioxide is a popular precipitating chemical that is gaseous introduced into black liquor. The precipitated lignin can be separated from the black liquor by filtration.

International Publication WO2009 / 104995 discloses a method for precipitating lignin, wherein the lignin is precipitated from black liquor, preferably using carbon dioxide. After precipitation, there is typically an aging, "aging" or "maturing" step, which seeks to increase the particle size of the precipitated lignin prior to separation by filtration, preferably by a pressure filter. After filtration, the lignin is washed with a filtrate from a subsequent resuspension and filtration step, the ion concentration and pH of which are adjusted as necessary.

Methods for precipitating and separating lignin by acidification of black liquor are also disclosed in International Publication Nos. WO2006 / 031175 and WO2006 / 038863.

In all three patents, attention has been paid to filtration and washing of the lignin obtained and to the recycling of various solutions. In addition, it can be deduced that a batch reactor is used for the precipitation of lignin.

International Publication WO2008 / 079072 discloses a method for precipitating lignin from a black liquor stream from an evaporation plant by lowering the pH to less than 10.5 with carbon dioxide. The precipitated lignin is washed at least in one step with a washing solution containing calcium or magnesium ions to displace the sodium to obtain a low sodium lignin.

U.S. Patent No. 26,230,40 discloses a method in which lignin is precipitated from black liquor by first passing flue gases upstream of the black liquor through two columns, followed by heating the acidified black liquor to a temperature above 75 ° C and gentle mixing by flowing through a tubular heater and above the vapor pressure corresponding to the temperature. It is clear from the patent specification that the black liquor is in fact passed through an externally heated coil (coil). When the so-treated black liquor is finally cooled, lignin is coagulated which is easy to filter. The patent specifically mentions the strength of the blend, expressed as a Reynolds number, affecting the size of the coagulated particles, and according to tests, Reynolds numbers greater than 2000 (equivalent to turbulence) resulted in such small particle sizes (1-7 µm) that it was virtually impossible to filter.

R. Alén, E.Sjöström, P.Vaskikari, "Carbon dioxide Precipitation of Lignin from Alkaline Pulping Liquors," Cellulose Chemistry Technol., 19, 537-541 (1985), discloses autoclaving of black liquor at 800 kPa (about 8 bar). , wherein the black liquor samples were impregnated with carbon dioxide so that the final pH after 60 minutes of treatment was 8.7. The lignin thus precipitated was separated by centrifugation. High pressure carbonation was found to increase the yield of black liquor. The apparatus used was a standard laboratory autoclave, the pressure of which is slowly lowered before the autoclave is opened and the samples taken, and can be compared to a batch process. This publication discusses factors influencing lignin precipitation and does not present an industrial process.

In J.F. Howell, R.W. Thring, “Recovery of Hardwood Lignin Using Generator Waste Acid. Statistical Analysis and Simulation ”Ind. Eng. Chem. Res., 39, 2534-2540 (2000), has been modeled on the basis of laboratory tests for lignin precipitation with waste acid from the production of chlorine dioxide. In the conclusions it has been found that it is advantageous for the filtration of the precipitated lignin to use as little mixing speed as possible in the process.

A disadvantage of the batch process is the large reaction tank it requires and the need to dehydrate the hydrogen sulphide that separates the black liquor during acidification. If carbon dioxide is used as the acidifying agent, which is advantageous for the chemical balance of the pulp mill, acidification takes a long time in a normal sized container. Reaction times to precipitate lignin are at least 30 minutes, followed by an aging step. In a batch process like this, the process conditions cannot be changed quickly.

Continuous processes have also been disclosed, as exemplified by the aforementioned U.S. Patent No. 2623040. However, to date, there is no method in which particles large enough for lignin separation, for example agglomerated particles, can be produced continuously with a short residence time. After acidification, whether with carbon dioxide or acid, there has traditionally been so-called. An "aging" or aging step, in which the particle size is allowed to grow to make it easier to separate the precipitated lignin from the black liquor for further processing. This is also consistent with the treatment in a tube heater after acidification disclosed in the aforementioned U.S. Patent No. 2623040. Such post-treatment steps require additional process tanks and other equipment, and the dwell times become long.

Summary of the Invention

It is an object of the invention to provide a process for continuously producing a suitable particle size lignin with a short residence time on an industrial scale. It is also an object of the invention to provide a continuous process for treating large quantities of black liquor continuously and at low operating costs and rapidly changing process conditions.

To accomplish this purpose, the process is essentially characterized by: - causing the black liquor to flow as a pressurized flow in the reactor with a residence time of less than 300 s, - introducing carbon dioxide as acidifying agent at one or more feed points to lower the pH of the black liquor, letting the pH suddenly releasing the pressure in the pressurized flow, and - separating the lignin particles from the black liquor.

The apparatus is essentially characterized in that the continuous reactor has a pressurized portion in which the black liquor is arranged to flow as a pressurized flow, the pressurized portion having one or more acidification feed points connected to a carbon dioxide source to a ends with a pressure release valve arranged to release the pressure of the pressurized flow.

The method provides for a short residence time in the reactor, preferably in a tubular reactor, of precipitated lignin which, by virtue of its particle size, is suitable for immediate separation by any conventional separation method and does not have to stand.

Preferred embodiments of the invention are set forth in the dependent claims and in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 illustrates the process as a process diagram, and Figures 2-4 show lignin samples precipitated by various methods.

DETAILED DESCRIPTION OF THE INVENTION In this specification, the following terms have the following meanings.

Lignin refers to native lignin (lignin stuck in wood material), lignin dissolved in solution or lignin precipitated, as the context requires. Em. lignins may not be chemically similar to each other.

Black liquor refers to an alkaline aqueous solution obtained in the cooking of sulphate pulp, to which lignin adhering to the wood material has been dissolved during the cooking of the wood material during the preparation of the chemical pulp from the wood material. This solution may be as such obtained from boiling or concentrated (obtained by evaporation).

Acidification refers to a decrease in the pH of a liquid, in this case black liquor, by adding an acidifying agent to the liquid.

An acidifying agent is any substance which, when added to a liquid in a solid, liquid or gaseous state, decreases its pH.

The lignin precipitation point is the pH of the black liquor at which lignin is precipitated. It may vary according to the desired lignin yield and has no absolute value.

Lignin particles refer to portions of the solid state lignin that can be separated from the liquid phase by the separation methods described below. The lignin particles may vary in size, and may be single particles formed by precipitation, or aggregates (agglomerates) of such single particles.

Separation of precipitated lignin means a separation process by which solid lignin particles can be separated from the liquid, such as filtration and centrifugation.

Figure 1 shows a process diagram of the method according to the invention. At the same time, it identifies the parts of the equipment implementing the method.

The apparatus includes a black liquor tank 1 with a stirrer, to which black liquor from pulp cooking is fed. In addition to dissolved lignin, black liquor contains cooking chemicals and other substances dissolved in wood from cooking. Black liquor contains eg. sodium, hydroxide, hydrogen sulfide and sulfide ions. However, the composition of the black liquor varies depending on the type of wood and the process. The black liquor can be taken into the black liquor tank from the black liquor obtained after cooking or from the black liquor evaporator at a suitable concentration. The black liquor can be part of the lignin precipitation and recovered after lignin separation, which is transported with the other black liquor through the evaporation plant for incineration in a boiler.

The black liquor tank 1 is connected via a pump P to a tubular reactor 2 having at least one acidification feed point 2a. The acidification feed point 2a is a tube in the wall of the reactor n 2 and is provided with a suitable feed device, such as a nozzle, for directing the flow of acidifying agent into the tubular reactor. The nozzle is wired to a source of acidifying agent, for example a reservoir containing an acidifying agent. In Figure 1, there are more than one such entry point 2a of acidifying agent spaced apart in the direction of flow defined by the tubular reactor 2. There is a static mixer 2b after each of the acidifying agent feed points 2a. In addition, the tubular reactor has at least one pH and temperature measuring point 2c and a flow meter 2d. These may also be more than one spaced apart, as in Figure 1.

The tubular reactor 2 has a pressurized section beginning at pump P and terminating in one or more pressure relief valves 3. With this section, black liquor flows as a pressurized flow. The pressure relief valves 3 are two in series in Figure 1 and are arranged to lower the pressure of the black liquor flowing in the tubular reactor 2 to atmospheric pressure. The valves in the figure are arranged to lower the pressure step by step, but the lowering can be done with one valve. However, the pressure change from the tubular reactor pressure to the atmospheric pressure is sudden, regardless of the number of valves. Preferably, the pipe has a larger diameter after the pressure relief valves 3. This results in a vigorous mixing operation immediately after the pressure drop as the black liquor discharges from the small opening to the open space.

The tubular reactor terminates in an aeration vessel 4 with a lower aerator 4a at its bottom for separating the hydrogen sulphide produced in the black liquor due to the lowering of the pH as a gas and leading to degassing (not shown) without being released into the ambient air. The aeration tank 4 also functions as an equalization tank, from which the black liquor containing the precipitated lignin can be taken for further treatment, i.e. separation of the lignin 6 through the pipe 5. The black liquor from which the lignin which has precipitated is separated off can be returned to the pulp mill for processing black liquor, for example in an evaporation plant.

The black liquor flows from the black liquor reservoir 1, pumped by pump P, along a pressurized portion of the tubular reactor 2 through which the acidifying agent is introduced into the black liquor at one or more acidifying feed points 2a, and the black liquor flow is stirred after each feeding point. As the pH drops to a lignin precipitation point (to a value below 11.5) as a result of the acidifying agent, lignin begins to precipitate in the black liquor in the pressurized portion of the tubular reactor 2. Good lignin yield is achieved when the pH drops to or below 10. Precipitated lignin ends up in the aeration tank 4 with the flow of black liquor.

The conditions used for the precipitation of lignin are detailed below. The black liquor is introduced into the tubular vector 2 at a temperature which is preferably between 60 and 80 ° C, more preferably between 65 and 75 ° C. The black liquor may already be ready at this temperature after cooking or evaporation so that it does not need to be heated or cooled. Also, due to the short residence time, the black liquor does not have sufficient time to cool in the tubular reactor. The process pressure for the pressurized portion of the tubular reactor 2 is preferably between 200 and 1000 kPa overpressure (about 2 to 10 bar), more preferably between 300 and 600 kPa. The overpressure in terms of precipitation may also be higher than 1000 kPa, but this will increase the reactor manufacturing cost.

The acidifying agent is carbon dioxide which is introduced gaseous under high pressure at feed point 2a by means of a suitable nozzle to the black liquor under pressure. The carbon dioxide may be introduced from a source of carbon dioxide, for example from a carbon dioxide reservoir, to a nozzle along a line having a regulating device, e.g. a valve, for controlling the flow and controlled by any actuator that can be connected to the regulating automation of the apparatus. After the feed point, there is a static mixer 2b, a structure located between the two tubes of the tubular reactor that directs the flow in a direction other than its main direction, causing intense high shear mixing and turbulence in the flowing black liquor and simultaneously carbon dioxide mixing and further dissolution. Known static mixers may be used.

Another alternative is to replace the static mixer 2b with the flow provided by the substance fed at the feed point 2a itself. When carbon dioxide is introduced at a sufficient pressure at the feed point 2a into the black liquor flow perpendicular to the direction of flow of the black liquor, it causes the flow to be mixed with high shear forces over its entire cross-sectional area and turbulence, and If necessary, carbon dioxide may be introduced from different sides of the feed point 2a, for example at intervals, through nozzles located at the periphery of the tubular reactor to enhance agitation. In the flow chart of Fig. 1, the combinations of the feed point 2a and the static mixer 2b can be replaced by the mixing provided by the feed of the acidifying agent at the feed point 2a. The mixing unit may also consist of a short residence time apparatus for providing high shear forces.

Turbulent mixing can also be obtained by a side stream 10 taken from the pressurized flow of reactor 2, which is fed back to the reactor via a booster pump 10a 10a. The acidifying agent can be fed to this side stream, whereby the side stream enters the reactor at the same feed point 2a as the acidifying agent, or the acidifying agent and the side stream are fed separately to the reactor 2. This type of combined feeder and agitator is known under the trademark TrumpJet®.

The solubility of carbon dioxide in water and aqueous liquids is enhanced under pressure, which makes it preferable to use a tubular reactor 2 in which the black liquor can be made to flow at a suitably high pressure.

The tubular reactor 2 has a pH and temperature measuring point 2c after each feed point 2a of the acidifying agent and subsequent static mixer 2b (if used), where sensors measuring the pH and temperature of the black liquor are located. By means of the measured pH, the amount of acidifying agent at the feeding point 2a can be adjusted, for example, to maintain the setpoint for the measuring point 2c. The feed point 2a may have a sensor for measuring the amount of acidifying agent introduced to monitor its consumption. In Figure 1, the control unit is designated by reference numeral 7. The figure also shows the communication lines 8 between the control unit 7 and the pH measuring sensors, and the communication lines 9 for transmitting control messages generated from the measurement data from the control unit 7 to actuator supply control actuators. This is a closed loop control circuit that can be used to perform feedback control in accordance with normal control principles.

If there are two or more acidifier feed points 2a in succession under the pressurized portion of the tubular reactor 2, the acidifier dosing may be uniformly distributed over the length of the tubular reactor such that new amounts of lignin are always precipitated from the black liquor after each feeding point 2a.

By using carbon dioxide as an acidifying agent, the pH of the black liquor in the tubular reactor is reduced to a level from 9 to 10, which provides a reasonably good lignin precipitation and yield. If an acid is used as an acidifying agent, the pH will be lowered and the lignin intake will be improved, but the pH of the black liquor returned to the process will also decrease. In order for the black liquor to be returned to the process, its pH must be raised to its original level by addition of sodium hydroxide, which in this case must be added more than to the process using carbon dioxide as an acidifying agent.

An essential part of the process is the rapid release of pressure at the end of the pressurized section of the tubular reactor 2 at the pressure release valves 3, i.e., a reduction of the pressure in the pressurized section to atmospheric pressure. This results in a strong blending effect in the black liquor, the "flash" blend, after which the lignin is finally precipitated and agglomerated into particles large enough to be easily separated from the black liquor. The rapid reduction in pressure at the end of the pressurized portion, i.e. the release of pressure, results in the simultaneous mixing of the black liquor and the agglomeration of the lignin particles.

If carbon dioxide has been used as an acidifying agent, carbon dioxide dissolved in the black liquor under pressure will also be suddenly released in gaseous form, enhancing black liquor mixing and agglomeration of lignin.

Due to the rapid increase in lignin particle size, the residence time of the black liquor in the tubular reactor 2 (with a pressurized portion from pump P to pressure relief valve 3) can be kept short, up to 300 s, e.g. 1 to 300 s, preferably 10 to 100 s. so that the aforementioned residence times are realized. The short dwell time allows rapid changes in process parameters due to black liquor quality variations.

The precipitated lignin particles form larger particles in the black liquor, the "agglomerates", which can be easily separated from the black liquor. The separation process is preferably pressure filtration. Pressure filtration makes the "cake" of lignin particles so dry from the filtrate that it is easier to wash off the black liquor trapped in the lignin, and the cake can be washed while it is in the pressure filter. due to the liquid separation, the amount of acid consumed is low, since the proportion of acid-consuming black liquor residues is minimized. In Figure 1, the pressure filter follows the aeration tank 4. Preferably, a concentration device, e.g. a centrifuge or other separator, is provided between the aeration tank and the pressure filter to increase the concentration of lignin particles in the black liquor. The separation step in which lignin is separated from black liquor is generally described by reference numeral 6.

The following table compares the fast pressurized (reactive tube) reactor of the invention (Fast continuous Precipitation, dwell time 50-100 s) with a batch reactor (Slow batch Precipitation) with a conventional slow dwell time (60 min) without aging and 13 days aging.

The starting material was the same black liquor (intermediate) and 100% CO2 was used for the precipitation. The precipitation temperature was about 68-70 ° C and the precipitation pH was 9.2-9.3. The batch reactor also ensured that CCh gas could not escape from the reactor.

The table shows that lignin intake is clearly better than batch processes, which is reflected in the reduced chemical cost / tonne of lignin. The treatment of black liquor requires the recovery of a smaller amount of black liquor and contains less lignin. As a whole, the load on the evaporator and the recovery boiler is thus reduced. The continuously operating short-lived tubular reactor also consumes much less energy for mixing than the slow batch reactor.

Figures 2-4 further show the preferred form of lignin particles to be achieved by the invention. Fig. 2 is a SEM image (2500x) of lignin precipitated with carbon dioxide by the process of the invention, Fig. 3 is a SEM image (2500x) of slow precipitated and overnight aging lignin, and Fig. 4 is a SEM image (2500x) of slowly precipitated , Aging 13 days.

The "rapid" precipitation (dwell time less than 300 s) of the invention surprisingly produces carbon dioxide as an acidifying agent, in which the precipitated small particles of only less than 2 µm in diameter form aggregates of dozens of particles, agglomerates which are easily separable from black liquor for example by filtration. When the acidifying agent is sodium bisulfite and the precipitation is slow and the precipitated lignin is aged, particle agglomerates are also obtained. Whereas the acidifying agent is carbon dioxide and the precipitation is slow and the precipitated lignin is stopped, lignin particles are obtained which have grown in size without agglomeration.

By visual inspection of the precipitated lignin samples, when carbon dioxide was used as an acidifying agent in the rapid process of the invention, the precipitated lignin is also lighter (light brown) than the slowly precipitated carbon dioxide (dark brown). Probably the higher brightness results from improved light scattering when small particles that optimally scatter light form agglomerates. This is advantageous if lignin is to be used in composites but does not want to add too much color.

The invention is not limited to the foregoing description, but may be used within the scope of the inventive idea of the claims.

Any carbonaceous substance may be used as the acidifying agent. This material is preferably pure carbon dioxide so that the pH reduction is as effective as possible in the amount of gas dissolved. Filtration methods other than pressure filtration may also be used, and a method other than filtration may be used as a separation method.

The apparatus of Figure 1 is most preferably located at a pulp mill because it can then be readily integrated into the chemical pulp manufacturing process. For example, the equipment may use a portion of the black liquor produced by the pulp soup as a raw material, and may recycle the black liquor after lignin separation to the black liquor treatment.

Claims (12)

A process for the continuous precipitation of lignin from black liquor, the method comprising causing the black liquor to flow as a pressurized stream, lowering the pH of the black liquor to the lignin precipitating point by introducing carbon dioxide into the black liquor stream as acidifying agent and separating - introducing carbon dioxide at one or more feed points (2a) to lower the pH of the black liquor, allowing the pH to decrease by the action of carbon dioxide on the lignin precipitation point under pressure, - abruptly releasing the pressure of the pressurized flow, and Method according to Claim 1, characterized in that after the carbon dioxide addition, the black liquor pressurized stream is stirred turbulently to mix the carbon dioxide with the flow. Method according to Claim 2, characterized in that the black liquor flow under pressure is mixed with a static mixer (2b). Method according to Claim 2, characterized in that the black liquor under pressure is mixed with the carbon dioxide introduced into the flow and / or by a side stream separated from the pressure by a back stream. Process according to one of Claims 1 to 4, characterized in that the pressurized flow of black liquor in the reactor (2) is at a pressure of 200 to 1000 kPa. Process according to one of Claims 1 to 5, characterized in that the lignin is separated from the black liquor by pressure filtration. An apparatus for continuously precipitating lignin from black liquor comprising a reactor and means for introducing an acidifying agent into a reactor, the reactor being a continuous reactor (2), wherein the black liquor is arranged to flow under pressure and having an acidifying feed point (2a) connected to a carbon dioxide source. , characterized in that the reactor (2) has a pressurized section in which the black liquor is arranged to flow, the pressurized section has one or more acidification feed points (2a) where carbon dioxide is introduced into the pressurized flow and the pressurized section terminates in a pressure release valve (3) pressure of the flow under pressure. Apparatus according to claim 7, characterized in that, after the acidifying agent feed point (2a), the reactor (2) has a static mixer (2b) arranged to cause turbulent flow mixing. Apparatus according to claim 7, characterized in that the carbon dioxide and / or the side stream separated from the pressurized flow is arranged to be introduced into the pressurized flow at the feed point (2a) at such a pressure that it causes turbulent mixing of the flow. Apparatus according to any one of claims 7 to 9, characterized in that it comprises a filter arranged to separate the lignin particles from the black liquor. Apparatus according to claim 10, characterized in that the filter is a pressure filter. Apparatus according to claim 10 or 11, characterized in that there is a concentration device before the filter, such as a centrifuge, arranged to increase the concentration of lignin particles in the black liquor.