Apparatus and methods for extracting lignin from non-wood black liquor

Abstract

Methods, apparatus and systems are disclosed for extracting lignin from a non-wood black liquor. The non-wood black liquor is heated to a first temperature to degrade hemicellulose contained therein. The liquor is then cooled to a second temperature. The pH of the cooled liquor is chemically reduced to precipitate out the lignin in the liquor. The liquor is filtered to separate the precipitated lignin from the liquor.

Description

APPARATUS AND METHODS FOR EXTRACTING LIGNIN FROM NON-WOOD BLACK LIQUORTECHNICAL FIELD

[0001] The present disclosure relates generally to lignin extraction, and more particularly to the extraction of lignin from non-wood black liquor.RELATED APPLICATION

[0002] This application claims priority from United States Patent Application No. 63 / 534,666 filed on August 25, 2023 entitled “APPARATUS AND METHODS FOR EXTRACTING LIGNIN FROM NON-WOOD BLACK LIQUOR”. For the purposes of the United States, this application claims the benefit under 35 U.S.C. §119 of United States Patent Application No. 63 / 534,666 filed on August 25, 2023 entitled “APPARATUS AND METHODS FOR EXTRACTING LIGNIN FROM NON-WOOD BLACK LIQUOR”. United States Patent Application No. 63 / 534,666 is incorporated herein by reference in its entirety for all purposes.BACKGROUND

[0003] Black liquor is a by-product of pulp production. It is a mixture of pulping residues, such as lignin and hemicellulose, and inorganic chemicals that are used to facilitate the pulping process. In traditional pulp production with wood-based feedstock, black liquor may be combusted in a recovery boiler to extract energy from the pulping residues and to recover the inorganic chemicals for future use in the pulp production process. In some cases, the recovery boiler creates a bottleneck in the black liquor processing. In such cases, it may be desirable to pre-treat the black liquor to precipitate lignin therefrom prior to combustion to improve processing efficiency.

[0004] T raditional techniques for extracting lignin from wood-based black liquor may involve precipitation and filtering of the lignin from the wood-based black liquor. The precipitated lignin can then be used as a valuable material for producing chemical products that reduce the use or generation of hazardous substances. Lignin extraction reduces the thermal value of the remaining lignin-lean black liquor. The reduced thermal value creates alower heat load on the recovery boiler, which can in turn be operated to process additional black liquor solids and support incremental pulp production.

[0005] Non-wood black liquor is a by-product of pulp production using non-wood feedstock such as straw. Conventional lignin extraction processes for black liquor are not viable for non-wood black liquor due to the high silica and hemicellulose content in non- wood feedstock (e.g., wheat straw). In particular, studies have shown that existing commercial wood-based processes are ineffective on non-wood black liquor due to the poor filterability of the lignin in non-wood black liquor. The elevated levels of silica present in non- wood black liquor will co-precipitate with the lignin, which impairs filtration and makes lignin removal difficult. In addition, non-wood black liquor usually also contains high levels of hemicellulose, especially arabinoxylans, compared to conventional wood-based black liquor. The elevated hemicellulose content can cause the formation of hydrogels which further impair the filterability and removal of precipitated lignin from solution. These issues generally result in an energy inefficient process for extracting lignin from non-wood black liquor.

[0006] Accordingly, there is a need for improved methods and processes for precipitating, filtering and / or separating lignin from non-wood black liquor. There is a need for more energy efficient processes that can overcome the filterability issues caused by the elevated silica and hemicellulose contents found in non-wood black liquor.SUMMARY OF THE DISCLOSURE

[0007] In general, the present specification describes apparatus, systems and processes for extracting lignin from non-wood black liquor.

[0008] One aspect of the invention relates to a method for extracting lignin from non- wood black liquor. The non-wood black liquor may have a dissolved solids concentration in the range of 5% to 30%. The method comprises the steps of heating the non-wood black liquor to a first temperature to degrade hemicellulose contained therein, followed by cooling the non-wood black liquor to a second temperature, followed by chemically reducing the pH of the cooled non-wood black liquor to precipitate out the lignin in the non-wood black liquor, followed by filtering the non-wood black liquor to separate the precipitated lignin from thenon-wood black liquor. The first temperature may be in the range of 150°C to 200°C. The second temperature may be in the range of 65°C to 95°C.

[0009] In some embodiments, the non-wood black liquor is oxygenated under pressure prior to cooling the non-wood black liquor. The non-wood black liquor may be oxygenated under a pressure between 1 bar to 10 bar. The non-wood black liquor may be oxygenated simultaneously with heating the non-wood black liquor to the first temperature. In some embodiments, the separated lignin is washed and the washed lignin is subsequently collected. The lignin may be washed with hot water.

[0010] In some embodiments, the non-wood black liquor is filtered using a filter press or a belt filter. In some embodiments, the non-wood black liquor is heated at the first temperature for a time period of between 5 minutes to 80 minutes. In some embodiments, the pH of the non-wood black liquor is chemically reduced to a value between 1 to 4 to precipitate out the lignin in the non-wood black liquor. The pH of the non-wood black liquor may be chemically reduced by adding a mineral acid to the non-wood black liquor.

[0011] In some embodiments, the non-wood black liquor is obtained from pulping non- wood feedstock that has been desilicated in a desilication process prior to pulping. In such embodiments, the non-wood black liquor may be cooled by indirectly preheating cooking chemicals used in the non-wood feedstock pulping process. The desilication process may comprise mechanically separating silica from the non-wood feedstock with a mechanical pulper. The desilication process may comprise chemically treating the non-wood feedstock with a chemical compound solution at a third temperature (e.g., between 50°C to 100°C) to selectively remove silica from the non-wood feedstock. The chemical compound solution may comprise sodium carbonate, sodium hydroxide or potassium hydroxide. The non-wood feedstock may comprise a straw-based feedstock.

[0012] Additional aspects of the present invention will be apparent in view of the description which follows.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Features and advantages of the embodiments of the present invention will become apparent from the following detailed description, taken with reference to the appended drawings in which:

[0014] FIG. 1 is a flowchart of a process for precipitating and extracting lignin from nonwood black liquor according to an example embodiment.

[0015] FIG. 2 is a flowchart of a supplementary process that can be adopted in conjunction with the FIG. 1 process to extract lignin from non-wood black liquor.DETAILED DESCRIPTION

[0016] The description, which follows, and the embodiments described therein, are provided by way of illustration of examples of particular embodiments of the principles of the present invention. These examples are provided for the purposes of explanation, and not limitation, of those principles and of the invention.

[0017] Aspects of the present invention relate to apparatus, systems and processes for precipitating and separating lignin from non-wood black liquor. For the purposes of facilitating the description, the term “non-wood” is used herein to refer to non-wood feedstock or agro-based fibers such as straw and / or agricultural residuals that remain after harvesting agricultural crops. The term “straw”, as used herein, includes but is not limited to wheat straw, barley straw, oat straw, flax straw, rice straw, hemp, bamboo, miscanthus, sorghum, switchgrass, ryegrass, com stover, bagasse, reed stems and banana tree. The term “non-wood black liquor”, as used herein, refers to the black liquor by-product produced from the process of manufacturing pulp from non-wood feedstock. As described in more detail elsewhere herein, non-wood black liquor usually has higher silica and hemicellulose content compared to wood-based black liquor, thereby making it more challenging to extract the lignin therefrom.

[0018] Referring now to FIG. 1 , shown therein is a flowchart of a process 10 for extracting lignin 15 from non-wood black liquor 17 according to one embodiment. Process 10 utilizes a combination of chemical and mechanical means to precipitate, filter and separate lignin 15 from non-wood black liquor 17. Dependent upon factors such as the composition of the non-wood feedstock and alkali addition rates, non-wood black liquor 17 collected from the pulping process may have a dissolved solids concentration that is typically in the range of 5% to 30%. In contrast to existing lignin precipitation and filtration systems which require the use of evaporators to increase the dissolved solids concentrationof the black liquor to between 30% and 50%, process 10 can extract lignin 15 from nonwood black liquor 17 having a dissolved solids concentration of less than 30%.

[0019] Process 10 begins with heating the non-wood black liquor 17 to an elevated temperature at heat treatment step 20. The heat treatment of non-wood black liquor 17, even with elevated levels of silica contained therein, can improve the filterability of the precipitated lignin 15 downstream. In some embodiments, step 20 is performed to raise the temperature of non-wood black liquor 17 from, for example, room temperature (e.g., 20°C to 25°C) to between 150°C to 200°C. In some embodiments, the non-wood black liquor 17 is heated for a time period in the range of 5 to 80 minutes. Since hemicellulose is solubilized at temperatures greater than 150°C, raising the temperature of non-wood black liquor 17 can help enhance the degradation of the hemicellulose contained therein. It can be desirable to degrade the hemicellulose in the non-wood black liquor 17 early on in the process 10 (i.e. , at step 20) since hemicellulose content impairs the filterability and removal of precipitated lignin from solution.

[0020] After heating the non-wood black liquor 17 to a desired temperature for a desired period of time at heat treatment step 20, process 10 proceeds to optional oxygenation step 30. At step 30, the heated non-wood black liquor 17 is oxygenated under pressure to further enhance the filterability of the precipitated lignin downstream. The oxygenation can help further degrade the hemicellulose remaining in the heated non-wood black liquor 17. The oxygenation can also help reduce the pH of the heated non-wood black liquor 17 to thereby improve lignin colloid agglomeration. The non-wood black liquor 17 may be oxygenated under pressure (e.g., between 1 bar to 10 bar).

[0021] In addition to improving the lignin filterability, oxygenation at step 30 also helps reduce the molecular weight of the organic acids in the non-wood black liquor 17 along with the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) levels to improve the treatability of the lignin-lean filtrate 18 obtained downstream.

[0022] In some embodiments, oxygenation step 30 may be performed before or simultaneously with heating step 20. In such embodiments, the oxygenation of the hemicellulose (i.e., an exothermic process) can help reduce the energy input required to heat the non-wood black liquor 17 in step 20.

[0023] Upon completion of heating step 20 and optional oxygenation step 30, process 10 proceeds to step 40. At step 40, the non-wood black liquor 17 is cooled to a temperature that is below the elevated temperature. The non-wood black liquor 17 may be cooled to, for example, a temperature that is typically in the range of 65°C to 95°C. The temperature is reduced at step 40 to facilitate the handling of the heat treated non-wood black liquor 17 in subsequent processing steps, but still usually maintained at a slightly elevated temperature (e.g., relative to room temperature) to encourage hemicellulose degradation. In some embodiments, the heat treated non-wood black liquor 17 is cooled by indirectly preheating cooking chemicals used in the non-wood feedstock pulping process. Cooling by indirectly preheating cooking chemicals improves the energy efficiency of the process.

[0024] After cooling the non-wood black liquor 17 to a desired temperature at step 40, process 10 proceeds to precipitation step 50. At precipitation step 50, the pH of the cooled non-wood black liquor 17 is reduced to precipitate out the lignin in the non-wood black liquor 17. In some embodiments, the pH of the non-wood black liquor 17 is reduced to a value between 1 to 4. In some embodiments, the pH of non-wood black liquor 17 is decreased in precipitation step 50 by adding a strong mineral acid, such as sulphuric acid, to the cooled non-wood black liquor 17. Other acids such as nitric acid, phosphoric acid and hydrochloric acid can also be used in precipitation step 50. In contrast to existing wood-based lignin precipitation processes which require the use of both carbon dioxide (CO2) and sulphuric acid for the precipitation and acid washing of the lignin, step 50 requires only a strong acid (e.g., sulphuric acid), thereby reducing the chemical usage required for lignin precipitation. A mixture of lignin-lean filtrate 18 and precipitated lignin 15 is obtained upon completion of step 50.

[0025] After completing precipitation step 50, process 10 proceeds to filtration step 60. In filtration step 60, the lignin-lean filtrate 18 and the precipitated lignin 15 are separated to complete the lignin extraction process. Filtration step 60 may be performed using a mechanical filter press, a mechanical belt filter, or the like. In contrast to existing lignin precipitation systems requiring the use of two or more filter presses for the separation and purification of the precipitated lignin, step 60 can be performed with the use of a single filter, thereby reducing the equipment required to separate and purify precipitated lignin. Filtration step 60 may optionally comprise washing the extracted lignin 15. The extracted lignin 15 may be washed using purified water and / or hot water.

[0026] The removal of lignin 15, along with heat treatment and oxygenation of the nonwood black liquor, significantly improves the treatability of the lignin-lean filtrate 18. The lignin-lean filtrate 18 obtained from process 10 has been effectively treated using aerobic treatment. The lignin-lean filtrate 18 can also be treated anaerobically to produce a biogas.

[0027] A wide range of variations and additions are possible within the scope of the present invention. These include supplementary processes that may be performed in conjunction with the main process 10 of extracting lignin 15 from non-wood black liquor 17 described herein.

[0028] For example, FIG. 2 depicts an additional exemplary process 80 that can be implemented prior to process 10 to assist with lignin separation. As depicted in FIG. 2, a desilication step 100 may be incorporated during the process of manufacturing pulp 92 from non-wood feedstock 90 to improve the filterability of the precipitated lignin in black liquor and to further enhance the efficiency of process 10. To the extent possible, it is desirable to minimize silica content in non-wood black liquor 17 since mixtures of lignin and silica can be difficult to manage in lignin extraction process 10. In particular, silica can co-precipitate with lignin during precipitation step 50, thereby impairing filterability and the purity of the final lignin product.

[0029] Performing desilication step 100 can significantly reduce the level of silica that is present in non-wood black liquor 17. As an example, non-wood feedstock 90 may be treated with hot water or a chemical solution (e.g., an alkaline solution) to pre-treat and separate some of the silica from the non-wood feedstock 90 during step 100. The treatment may be conducted for a period of 10 to 30 minutes or more. In some embodiments, the non- wood feedstock 90 is heated and chemically desilicated with compounds like sodium carbonate, sodium hydroxide, potassium hydroxide, or the like. Non-wood feedstock 90 may also be treated with hot water without the addition of alkaline chemicals in step 100. The compounds used in desilication step 100 can preferentially separate silica (over, for example, lignin) from the non-wood feedstock 90.

[0030] To preferentially separate silica from non-wood feedstock 90, desilication step 100 may be conducted under atmospheric pressure at temperatures in the range of 50°C to 100°C. If a strong alkali is used in step 100, then a relatively low reaction temperature (e.g., ~60 °C) may be sufficient to prioritize silica removal over delignification reactions. If a weakalkali or hot water is used in step 100, then a relatively high reaction temperature e.g., ~90 °C) may be required to prioritize silica removal over delignification reactions.

[0031] As another example, the non-wood feedstock 90 may be mechanically desilicated in step 100. In some embodiments, step 100 involves a combination of mechanical and chemical processes. In such embodiments, desilication step 100 may comprise mechanically pulping a stream of non-wood feedstock 90 after it has been heated and chemically reacted (e.g., in alkaline solution) to mechanically separate more silica from non-wood feedstock 90.

[0032] Upon completion of desilication step 100, a relatively large amount of silica will be dissolved in the alkaline desilication liquor and selectively removed from the stream of non-wood feedstock 90. The desilicated non-wood feedstock 90 may then be processed through a non-wood pulping process 200 to produce pulp 92. As a result of performing desilication step 100, the by-product non-wood black liquor 17 remaining from process 200 will have relatively low silica content. Such non-wood black liquor 17 can then be processed using process 10 of FIG. 1 to extract lignin 15.

[0033] The examples and corresponding diagrams used herein are for illustrative purposes only. Different configurations and terminology can be used without departing from the principles expressed herein.

[0034] Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the scope of the invention. The scope of the claims should not be limited by the illustrative embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole. For example, various features are described herein as being present in “some embodiments” or in “one embodiment”. Such features are not mandatory and may not be present in all embodiments. Embodiments of the invention may include zero, any one or any combination of two or more of such features. This is limited only to the extent that certain ones of such features are incompatible with other ones of such features in the sense that it would be impossible for a person of ordinary skill in the art to construct a practical embodiment that combines such incompatible features. Consequently, the description that “some embodiments” possess feature A and “some embodiments” possess feature B should be interpreted as an expressindication that the inventors also contemplate embodiments which combine features A and B (unless the description states otherwise or features A and B are fundamentally incompatible).

Claims

CLAIMS1. A method for extracting lignin from a non-wood black liquor, the method comprising: heating the non-wood black liquor to a first temperature to degrade hemicellulose contained therein; cooling the non-wood black liquor to a second temperature; chemically reducing the pH of the cooled non-wood black liquor to precipitate out the lignin in the non-wood black liquor; and filtering the non-wood black liquor to separate the precipitated lignin from the non-wood black liquor.

2. The method of claim 1 , or any other claim herein, comprising oxygenating the non- wood black liquor under pressure prior to cooling the non-wood black liquor.

3. The method of claim 1 , or any other claim here, comprising oxygenating the non- wood black liquor simultaneously with heating the non-wood black liquor to the first temperature.

4. The method of claim 2 or claim 3, or any other claim herein, comprising oxygenating the non-wood black liquor under a pressure between 1 bar to 10 bar.

5. The method of claim 1 , or any other claim herein, comprising washing the separated lignin and collecting the washed lignin.

6. The method of claim 5, or any other claim herein, wherein the lignin is washed with hot water.

7. The method of claim 1 , or any other claim herein, wherein the non-wood black liquor is filtered using a filter press or a belt filter.

8. The method of claim 1 , or any other claim herein, wherein the first temperature is between 150°C to 200°C.

9. The method of claim 1 , or any other claim herein, wherein the non-wood black liquor is heated at the first temperature for a time period of between 5 minutes to 80 minutes.

10. The method of claim 1 , or any other claim herein, wherein the pH of the non-wood black liquor is chemically reduced to a value between 1 to 4 to precipitate out the lignin in the non-wood black liquor.11 . The method of claim 10, or any other claim herein, wherein the pH of the non-wood black liquor is chemically reduced by adding a mineral acid to the non-wood black liquor.

12. The method of claim 1 , or any other claim herein, wherein the non-wood black liquor is obtained from pulping non-wood feedstock that has been desilicated in a desilication process prior to pulping.

13. The method of claim 12, or any other claim herein, wherein the desilication process comprises chemically treating the non-wood feedstock with a chemical compound solution at a third temperature to selectively remove silica from the non-wood feedstock.

14. The method of claim 13, or any other claim herein, wherein the third temperature is between 50°C to 100°C.

15. The method of claim 13, or any other claim herein, wherein the chemical compound solution comprises sodium carbonate, sodium hydroxide or potassium hydroxide.

16. The method of claim 12, or any other claim herein, wherein the desilication process comprises mechanically separating silica from the non-wood feedstock with a mechanical pulper.

17. The method of claim 12, wherein the non-wood black liquor is cooled by indirectly preheating cooking chemicals used in the non-wood feedstock pulping process.

18. The method of claim 12, wherein the non-wood feedstock comprises a straw-based feedstock.

19. The method of claim 1 , or any other claim herein, wherein the second temperature is in the range of 65°C to 95°C.

20. The method of claim 1 , or any other claim herein, wherein the non-wood black liquor has a dissolved solids concentration in the range of 5% to 30%. 21 . Apparatus having any new and inventive feature, combination of features, or subcombination of features as described herein.

22. Methods having any new and inventive steps, acts, combination of steps and / or acts or sub-combination of steps and / or acts as described herein.

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