Method for regenerating sodium hydroxide by black liquor auto-vaporization combined with catalytic reaction

Abstract

This invention belongs to the field of pulp and paper making, and provides a method for regenerating sodium hydroxide through spontaneous steam vaporization of black liquor combined with catalytic reaction, comprising the following steps: S1, mixing concentrated black liquor with a catalyst to obtain a mixed slurry; S2, burning diesel fuel, and injecting the mixed slurry into the reactor at a predetermined flow rate after the temperature inside the reactor reaches the first furnace temperature; S3, simultaneously increasing the air flow rate until the temperature inside the reactor reaches the second furnace temperature, then stopping the diesel fuel combustion; S4, maintaining the furnace temperature at the third furnace temperature, and continuously vaporizing and catalytically reacting for more than 1 hour; S5, discharging the molten product from the reactor, part of which undergoes a hydration reaction after being dissolved in hot water, and part undergoes an alkalization reaction, to obtain a sodium hydroxide mixed alkaline solution with a suitable causticization rate of 65-80%. This invention achieves sodium hydroxide regeneration in one step through spontaneous steam vaporization of black liquor and catalytic reaction.

Description

Technical Field

[0001] This invention belongs to the field of pulp and paper technology, and relates to the field of resource utilization of pulp black liquor, and in particular to a method for spontaneous steam vaporization of black liquor and catalytic reaction to regenerate sodium hydroxide. Background Technology

[0002] Currently, the pulp and paper industry still retains the traditional alkaline pulping process. During pulping, wood chips are cooked using alkaline solutions, resulting in a large amount of cooking waste liquor. This waste liquor is dark black and is called pulping black liquor. Black liquor contains a large amount of organic matter and sodium, posing a potential source of water pollution. The traditional method of black liquor treatment in the pulping industry is to first evaporate and concentrate the dilute black liquor to obtain concentrated black liquor with a solids concentration of 50%-70%, which is then sent to a black liquor combustion furnace to eliminate the organic matter that pollutes the water. Simultaneously, the sodium compounds are converted into molten sodium carbonate solid. The sodium carbonate solid is then dissolved in water to obtain a sodium carbonate aqueous solution, which is reacted with lime to regenerate sodium hydroxide, which is then recycled for cooking wood chips.

[0003] Traditional methods for treating pulping black liquor have the following drawbacks: 1) Complex production process and low efficiency. As mentioned above, the traditional concentration-combustion-lime alkalization process for treating black liquor involves many production steps, is complex, and has low production efficiency. 2) High energy consumption. In the black liquor treatment process, the evaporation and concentration of dilute black liquor consumes a large amount of heating steam, resulting in high energy consumption; evaporation energy consumption accounts for 21% of the total energy consumption of the pulping plant. On the other hand, the combustion process of black liquor consumes a certain amount of auxiliary fuel, increasing energy consumption. 3) Large emissions of carbon dioxide. In traditional alkali recovery reactions, a large amount of lime (20%-30%) is required. This lime is generally regenerated from the white mud produced during the calcination alkalization reaction. The regenerated lime reaction and fuel combustion emit large amounts of carbon dioxide.

[0004] To improve production efficiency and economic benefits while reducing energy consumption and carbon dioxide emissions, scientists have conducted various studies on the resource utilization of black liquor over the past fifty years, particularly on black liquor gasification and its applications, such as: 1) Black liquor gasification for hydrogen production: Using incomplete combustion of oxygen to gasify black liquor yields hydrogen with a volume accounting for 41% of the total gas volume; using externally heated steam to gasify black liquor yields hydrogen with a volume accounting for up to 67.1% of the total gas volume, showing broad development prospects. However, currently, there is no large-scale production and transportation demand for hydrogen, and hydrogen sales are still limited, not yet reaching the point where large-scale black liquor gasification for hydrogen production is feasible. 2) Black liquor gasification for synthesis gas and dimethyl ether (DME): Using black liquor gasification, black liquor is converted into carbon monoxide and hydrogen, which are then used as reactants to further synthesize dimethyl ether; thus, black liquor is converted into high-value-added dimethyl ether liquid fuel, while reducing carbon dioxide emissions from the black liquor combustion process. From the 1980s to the first decade of this century, black liquor gasification and its applications were extensively studied, and several pilot production plants were built. Production trials of the combined synthesis of dimethyl ether from black liquor gasification were completed. However, the high equipment requirements and complex production process of this method resulted in high production costs, making it difficult to maintain normal production and thus hindering its widespread application. 3) Black liquor gasification combined combustion power generation: Black liquor gasification converts black liquor into a mixture of hydrogen, carbon monoxide, and hydrocarbon molecules as fuel. This gasified fuel is then used to generate electricity in a recycle power generation system, thereby improving the power generation efficiency using black liquor as the primary fuel and indirectly reducing carbon dioxide emissions. However, research has found that when air is used as a combustion aid in the black liquor gasification process, the gasified gas still contains a large amount of nitrogen. This increases the compression power consumption in the high-pressure combustion chamber of the gas turbine, leading to a decrease in the external power supply efficiency of the combined air and black liquor gasification power generation system. Using oxygen as a combustion aid in the black liquor gasification process avoids the presence of large amounts of nitrogen in the gasified gas, thus reducing the compressor power consumption in the high-pressure combustion chamber of the gas turbine. However, the air separation oxygen production process consumes a significant amount of electrical energy, resulting in a reduced external power supply efficiency of only 28.5% for the combined black liquor gasification power generation. Therefore, compared to traditional black liquor combustion combined with steam turbine power generation, the power supply efficiency of oxygen-black liquor gasification combined cycle power generation is not significantly improved, and it has no practical production significance.

[0005] In summary, the market for hydrogen applications is limited, and the time is not yet ripe for large-scale black liquor gasification to produce hydrogen; the production cost of combining black liquor gasification with dimethyl ether production is high; and the external power supply efficiency of black liquor gasification combined with power generation has not been significantly improved, making it impractical for industrial applications. Therefore, there is an urgent need to conduct theoretical and technological research to find new methods and application pathways for black liquor gasification that meet the needs of modern pulping enterprises. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for regenerating sodium hydroxide using spontaneous steam vaporization combined with catalytic reaction of black liquor. This novel process, employing spontaneous steam vaporization combined with catalytic reaction to regenerate sodium hydroxide from black liquor produced by the caustic soda pulping process, achieves a one-step conversion of black liquor to sodium oxide within the vaporization and catalytic reaction furnace. Following hot water dissolution and a small-scale alkali reaction, a mixed alkali solution of sodium hydroxide with a causticization degree of 65-80%, suitable for wood chip cooking, is obtained. This solves the complex technical problems of traditional black liquor treatment and alkali recovery processes, significantly promoting improved pulping production efficiency and energy conservation and emission reduction, thus enhancing the economic benefits of pulping enterprises.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for regenerating sodium hydroxide through spontaneous steam vaporization of black liquor combined with a catalytic reaction includes the following steps:

[0009] S1. Pump the concentrated black liquor into the stirred storage tank, add the catalyst, and mechanically stir to mix evenly to obtain a mixed slurry of black liquor and catalyst.

[0010] S2. Start the diesel burner and supply air through the variable frequency blower to ensure complete combustion of diesel. When the temperature inside the reactor reaches the first furnace temperature, use a black liquor sprayer to spray the mixed slurry obtained in step S1 into the reactor at a predetermined flow rate.

[0011] S3. While the mixed slurry is being injected, the air flow rate is increased to ensure that the black liquor is stably burned and undergoes a gasification reaction in the reactor. When the temperature in the reactor reaches the second furnace temperature, the diesel burner is turned off.

[0012] S4. Within the predetermined black liquor flow rate range, maintain the temperature inside the reactor at the third furnace temperature and continuously gasify and catalyze for more than 1 hour.

[0013] S5. Using gravity or vacuum absorption equipment, the molten products accumulated in the inner cavity of the catalytic reaction layer are discharged from the reactor. During the dissolution process in hot water, the sodium oxide in the melt undergoes a complex reaction with water molecules to generate most of the sodium hydroxide. The small amount of calcium oxide in the melt can also undergo a complex reaction with water molecules to generate calcium hydroxide (slaked lime). Finally, the sodium carbonate component remaining in the melt dissolves in water and undergoes an alkalization reaction with calcium hydroxide to generate a small amount of sodium hydroxide product. Thus, a sodium hydroxide mixed alkaline solution with a causticization rate of 65-80% suitable for wood chip cooking is obtained.

[0014] Preferably, the catalyst is solid white mud or lime powder.

[0015] Preferably, the amount of catalyst added is 2%-5% of the weight of the concentrated black liquor fixative.

[0016] Preferably, the flow rate of each kilogram of diesel fuel corresponds to a flow rate of 28 to 32 cubic meters of air.

[0017] Preferably, the first furnace temperature is around 500°C.

[0018] Preferably, the second furnace temperature is around 650°C.

[0019] Preferably, the increase in airflow is achieved by increasing the airflow by 10 to 13 cubic meters of air per kilogram of the concentrated black liquid solids.

[0020] Preferably, in step S4, the furnace temperature is controlled by combining a temperature controller and a variable frequency blower to control the air flow rate based on the furnace temperature, thereby achieving furnace temperature control. The third furnace temperature is 830-950°C.

[0021] Preferably, the gasification reaction refers to the gasification reaction between the pyrolysis products generated by the cracking of concentrated black liquor under high temperature conditions, the water vapor generated by the concentrated black liquor in the high temperature reactor, and the carbon dioxide generated by the combustion of the concentrated black liquor.

[0022] Preferably, the reaction equation for the water vapor generated by the concentrated black liquor in the high-temperature reactor is as follows:

[0023] BL.H2O(l)→BL(s)+H2O(g) (1)

[0024] The reaction equation for the pyrolysis products generated by the pyrolysis of the concentrated black liquor under high temperature conditions is as follows:

[0025] Na-Lignin→Na2CO3+Na-O-C+CO↑+CO2↑+H2O↑+CH3OH↑ (2)

[0026] Cellulose→C+CO↑+CO2↑+H2O↑ (3)

[0027] Na-Acid→Na2CO3+CO↑+H2O↑ (4).

[0028] Preferably, the equation for the gasification reaction is as follows:

[0029] Na-O-C+H2O(g)→NaOH+CO+H2 (5)

[0030] C + H₂O(g) → CO + H₂ (6)

[0031] C + CO₂ → 2 CO (7)

[0032] C+2H2→CH4 (8).

[0033] Preferably, the catalytic reaction refers to the reaction in which sodium carbonate produced by the cracking of concentrated black liquor under high temperature conditions is converted into sodium oxide under the action of a catalyst, and then dissolved in water to form sodium hydroxide. The equation for the catalytic reaction is as follows:

[0034]

[0035] Preferably, the molten product comprises sodium oxide, sodium carbonate, and calcium oxide, and the equation for the complex reaction with water is:

[0036] Na₂O + H₂O → 2NaOH (10)

[0037] CaO + H₂O → Ca(OH)₂ (11)

[0038] The equation for the alkalization reaction is:

[0039] Na2CO3+Ca(OH)2→2NaOH+CaCO3(white mud) (12).

[0040] The principle of this invention is as follows:

[0041] (1) The principle of sodium carbonate production from black liquor cracking:

[0042] Because black liquor contains sodium organic acids such as sodium formate and sodium oxalate (Na-Acid), which decompose at high temperatures to produce sodium carbonate, carbon monoxide, and water.

[0043] (2) The principle of sodium lignin pyrolysis to form sodium carbonate complex:

[0044] During the pyrolysis of sodium lignin in black liquor, gaseous substances such as carbon monoxide, carbon dioxide, and water are decomposed. The remaining solid is a carbonized solid containing carbon and inorganic sodium ions. This solid is difficult to dissolve in water, making it difficult to separate carbon particles and sodium ions by water dissolution. Therefore, the solid product of sodium lignin pyrolysis is defined as a sodium-carbon complex.

[0045] (3) The principle of the gasification reaction between high-temperature water vapor and sodium carbonate complex:

[0046] Based on the principle that charcoal can undergo a gasification reaction with high-temperature water vapor, experiments have confirmed that sodium-carbon composites can react with water vapor to produce sodium hydroxide, hydrogen, carbon monoxide, and water.

[0047] (4) The principle of white mud or calcium oxide catalyzing the conversion of sodium carbonate to sodium hydroxide:

[0048] White clay is calcium carbonate powder. The principle behind the calcium carbonate-catalyzed sodium carbonate reaction is that calcium oxide catalyzes the sodium carbonate reaction. Calcium carbonate decomposes at high temperatures to produce calcium oxide. The principle behind the calcium oxide-catalyzed sodium carbonate reaction is that, at high temperatures, calcium oxide and sodium carbonate undergo a double displacement reaction to produce sodium oxide and calcium carbonate products. Then, at high temperatures, calcium carbonate molecules decompose into calcium oxide and carbon dioxide. This cycle of two reactions promotes the continuous conversion of sodium carbonate into sodium hydroxide. The reaction equation is as follows:

[0049] Na₂CO₃ + CaO → Na₂O + CaCO₃ (13)

[0050] CaCO3→CaO+CO2↑ (14)

[0051] Combining equations (9) and (10), we obtain the overall equation.

[0052]

[0053] In the water dissolution stage, the molten material (mainly sodium oxide, with small amounts of residual sodium carbonate and calcium oxide) is dissolved in hot water. First, solid sodium oxide reacts with water to form sodium hydroxide; simultaneously, the added calcium carbonate decomposes at high temperature to produce calcium oxide, or the previously added calcium oxide powder catalyst reacts with water to form slaked lime; then, the slaked lime and the residual sodium carbonate dissolved in water undergo an alkalization reaction, generating a small amount of new white mud and increasing the sodium hydroxide product, thus improving the causticization degree of the mixed alkali solution to a causticization rate of 65-80% suitable for wood chip cooking. The reaction equation is expressed as:

[0054] Na₂O + H₂O → 2NaOH (10)

[0055] CaO + H₂O → Ca(OH)₂ (11)

[0056] Na2CO3+Ca(OH)2→2NaOH+CaCO3(white mud) (12).

[0057] Compared with the prior art, the beneficial effects of the present invention are:

[0058] This invention proposes a novel approach to preparing a sodium hydroxide solution suitable for wood chip cooking by combining spontaneous steam vaporization of black liquor from caustic soda pulping with a catalytic reaction. This involves the production of a mixed melt of sodium oxide, calcium oxide, and sodium carbonate, followed by hot water dissolution and a small-scale alkalization reaction. This expands the application of black liquor vaporization in pulping. The invention enables the chemical conversion from black liquor to sodium oxide within a single reactor. Subsequently, hot water dissolution and a small-scale alkalization reaction yield an alkali solution with the appropriate causticization degree for wood chip cooking. This avoids the three production steps of black liquor combustion, lime-based alkali recovery, and lime regeneration in traditional black liquor treatment and alkali recovery processes. This significantly improves the production efficiency of black liquor treatment, reduces energy consumption in pulping black liquor treatment, reduces equipment investment and production costs for pulping companies, and enhances their economic benefits. Furthermore, the catalytic reaction process achieves a causticization degree of 65% to 80.7% for the produced mixed alkali, meeting the required causticization degree for wood chip cooking. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the process for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to the present invention.

[0060] Figure 2 This is a schematic diagram of the process flow of a device for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to the present invention.

[0061] Figure 3 This is a schematic diagram of the reaction process for preparing sodium hydroxide by the combined catalytic reaction of spontaneous steam vaporization of black liquor according to the present invention.

[0062] Figure 4 This is a schematic diagram of the reaction mechanism of the decomposition of sodium carbonate by white mud and calcium oxide catalyzed by the present invention. Detailed Implementation

[0063] In this invention, unless otherwise stated, scientific and technical terms used herein have the meanings commonly understood by those skilled in the art.

[0064] In this article, the term "black liquor" refers to the large amount of cooking waste liquor generated during the pulping process when wood chips are cooked with alkaline solution. This waste liquor is dark black in color and contains a large amount of sodium lignin and other organic acid sodium compounds.

[0065] In this article, the term "concentrated black liquor" refers to black liquor concentrate containing approximately 30%-50% water and 50%-70% oven-dried solids.

[0066] It should be noted that the composition of pulping black liquor varies depending on the raw materials of the pulping fiber and the cooking process. Generally, the oven-dry solids of black liquor contain approximately 38%-45% sodium lignin, 8%-18% sodium organic acids (including sodium formate, sodium oxalate, sodium phenylpropionate, and sodium hexenuronate, etc.), 5%-12% soluble sugars (including glucose and its oligomers produced by fiber dissolution, as well as various pentose sugars produced by hemicellulose hydrolysis), approximately 13%-20% fine cellulose, and 4%-10% inorganic components (including residual sodium hydroxide, sodium carbonate, and sodium chloride, etc.).

[0067] In this article, the term "black liquor gasification" refers to a chemical reaction process in which black liquor undergoes a cracking reaction under high temperature conditions using methods such as partial combustion gasification with added oxygen (or air) and steam gasification, producing a solid mixture of charcoal, sodium carbonate, and sodium hydroxide, while simultaneously generating a mixed fuel gas containing hydrogen, carbon monoxide, carbon dioxide, and other hydrocarbon gas molecules.

[0068] In this article, the term "spontaneous steam vaporization of black liquor" refers to the process of fully utilizing the water in concentrated black liquor. Under high-temperature conditions, the water in the concentrated black liquor evaporates to generate steam. Subsequently, the spontaneously generated steam is used as a vaporizing agent to promote the steaming reaction of black liquor, generating charcoal and sodium carbonate composites, as well as a solid mixture of sodium carbonate and sodium hydroxide. At the same time, a mixed fuel gas containing hydrogen, carbon monoxide, and other hydrocarbon gas molecules is generated.

[0069] In this article, the term "regenerated sodium hydroxide" refers to NaOH generated by converting sodium-containing organic and inorganic substances in black liquor through a special process.

[0070] In this article, the term "traditional black liquor alkali recovery" refers to the process of concentrating dilute black liquor to a solid content of 50%-70% using a multi-effect evaporator, followed by high-temperature combustion of the black liquor. The organic matter in the black liquor is converted into heat energy, while the inorganic matter melts into a molten material (mainly containing Na2CO3 and Na2S). This molten material is dissolved in water to form a green liquor. Lime milk (Ca(OH)2) is added to the green liquor to undergo a causticization reaction, producing a white liquor (a mixture of NaOH and Na2S) and white mud (CaCO3 precipitate). The white mud is then calcined to regenerate quicklime (CaO) for reuse in the causticization process, thus achieving calcium oxide recycling.

[0071] In this article, the term "white mud" refers to the calcium carbonate precipitate formed during the alkali recovery process in paper mills when the green liquor (sodium carbonate aqueous solution) produced after the combustion of black liquor undergoes an alkalization reaction with slaked lime. After filtration and separation, a solid resembling white mud is obtained, the main component of which is calcium carbonate.

[0072] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0073] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0074] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0075] like Figure 1 As shown, the present invention provides a method for regenerating sodium hydroxide through a combination of spontaneous steam vaporization and catalytic reaction in black liquor, comprising the following steps:

[0076] S1. Pump the concentrated black liquor with a solid content of 50%-70% into the stirred storage tank, add 2%-5% by weight of solid white mud or lime powder as a catalyst, and mechanically stir to mix evenly to obtain a mixed slurry of black liquor and catalyst.

[0077] S2. Start the diesel burner and supply air to the reactor at a flow rate of 28-32 cubic meters of air per kilogram of diesel fuel using a variable frequency blower to ensure complete combustion of the diesel fuel. When the temperature inside the reactor reaches about 500°C (first furnace temperature), use a black liquor sprayer to spray the mixed slurry obtained in step S1 into the reactor at a predetermined flow rate.

[0078] It should be noted here that the predetermined flow rate refers to the flow rate of concentrated black liquor during processing, which is determined based on production requirements and furnace size. This is because the actual processing capacity of concentrated black liquor is related to the size of the gasifier, and different gasifiers correspond to a certain flow rate of concentrated black liquor.

[0079] S3. At the same time as the mixed slurry is injected, the air flow rate is increased by 10-13 cubic meters per kilogram of concentrated black liquor solids to ensure stable combustion of the black liquor, increase the combustion and gasification reaction in the reactor, and further increase the furnace temperature. When the temperature in the reactor reaches 650℃ (second furnace temperature), the diesel burner is turned off to stop the combustion of diesel.

[0080] S4. Within the predetermined black liquor flow range, the temperature controller and variable frequency blower are combined to control the air flow by furnace temperature, so that the temperature inside the reactor rises to 900℃ and is controlled within the range of 830-950℃, and the continuous gasification and catalytic reaction in the furnace lasts for more than 1 hour.

[0081] It should be noted here that the black liquor undergoes four stages in the reactor: evaporation and drying, high-temperature pyrolysis, gasification, and catalytic reaction.

[0082] The first stage is the black liquor evaporation stage: in a high-temperature combustion furnace, concentrated black liquor containing 30%-50% water evaporates and dries, forming black liquor solid (commonly known as black ash).

[0083] BL.H2O(l)→BL(s)+H2O(g) (1)

[0084] The second stage is the high-temperature pyrolysis stage of black liquor: Inside the high-temperature furnace, the solid black liquor is heated to approximately 200-400℃. The sodium lignin (Na-Lignin), sodium acid (Na-Acid), and residual fine cellulose in the black liquor undergo pyrolysis reactions, producing sodium carbonate and sodium carbonate-carbonate complexes, as well as volatile gases such as CO, CO2, and CH3OH. The chemical reaction equations for the black liquor pyrolysis stage are as follows:

[0085] Na-Lignin→Na2CO3+Na-O-C+CO↑+CO2↑+H2O↑+CH3OH↑ (2)

[0086] Cellulose→C+CO↑+CO2↑+H2O↑ (3)

[0087] Na-Acid→Na2CO3+CO↑+H2O↑ (4)

[0088] The third stage is the gasification stage: In a high-temperature reactor, the steam generated in the first stage reacts with the carbon-oxygen-sodium complex gas produced by the black liquor pyrolysis in the second stage to produce sodium hydroxide, carbon monoxide, and hydrogen. Simultaneously, the charcoal produced by pyrolysis reacts with steam and carbon dioxide produced by combustion to generate hydrogen and carbon monoxide. Furthermore, the hydrogen generated in the gasification stage reacts with the charcoal to produce methane, achieving the goal of producing syngas from black liquor gasification (in this stage, without a catalyst, concentrated black liquor undergoes pyrolysis and gasification reactions in the gasification reactor to produce a mixed alkali of sodium hydroxide and sodium carbonate, with a causticization degree in the range of 48.6%-50.7%). The chemical reaction equations for the black liquor gasification stage are as follows:

[0089] Na-O-C+H2O(g)→NaOH+CO+H2 (5)

[0090] C + H₂O(g) → CO + H₂ (6)

[0091] C + CO₂ → 2 CO (7)

[0092] C + 2H₂ → CH₄ (8)

[0093] The fourth stage is the catalytic reaction stage: under the catalysis of white mud or calcium oxide, sodium carbonate is further converted into sodium hydroxide. Through hot water dissolution and supplementary alkalization reactions, the causticization degree of the mixed alkaline solution is increased to 65%-80.7%.

[0094] Na₂CO₃ + CaO → Na₂O + CaCO₃ (13)

[0095] CaCO3→CaO+CO2↑ (14)

[0096] Combining equations (13) and (14), we obtain the overall equation:

[0097]

[0098] S5. When the molten product accumulated in the inner cavity of the catalytic reaction layer reaches the discharge height, the molten product is discharged from the reactor using gravity or a vacuum absorption device. The molten material (mainly sodium oxide, with a small amount of residual sodium carbonate and calcium oxide) dissolves in hot water, undergoing the following three chemical reactions: First, solid sodium oxide reacts with water to form sodium hydroxide; simultaneously, calcium oxide reacts with water to form calcium hydroxide (slaked lime); then, the slaked lime and the residual sodium carbonate dissolved in water undergo an alkalization reaction, increasing the amount of sodium hydroxide product and improving the causticization degree of the mixed alkali solution to a causticization rate of 65-80% suitable for wood chip cooking; simultaneously, a small amount of new white mud is generated. The reaction equation is expressed as:

[0099] Na₂O + H₂O → 2NaOH (10)

[0100] CaO + H₂O → Ca(OH)₂ (11)

[0101] Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃ (white mud) (12)

[0102] A small-scale test of sodium hydroxide production based on the spontaneous steam vaporization combined with catalytic reaction of black liquor from caustic soda pulping has been successfully completed. Figure 2 As shown, this invention constructed a pilot-scale reaction device with a height of 4.5 meters and an outer diameter of 1.38 meters, equipped with auxiliary equipment such as a black liquor sprayer, a diesel burner, a blower, and an exhaust fan, capable of processing 500 kg of concentrated black liquor per hour. The following detailed description, in conjunction with embodiments, illustrates a method for regenerating sodium hydroxide through a combination of spontaneous steam vaporization and catalytic reaction of black liquor.

[0103] Example 1:

[0104] Weigh 2 kg of white mud catalyst and mix it evenly with concentrated black liquor containing 100 kg of solids. Inject the mixture into the reactor when the temperature reaches approximately 500°C. Increase the air flow rate until the furnace temperature reaches 650°C, then stop burning the fuel oil. Maintain the gasifier temperature at 850°C, continuously injecting the concentrated black liquor for 20 minutes, and then maintain the reaction temperature at 850°C for 60 minutes. Titration with 0.1M hydrochloric acid revealed a causticization degree of 67.3% in the resulting sodium hydroxide mixed alkaline solution.

[0105] Example 2:

[0106] Weigh 3 kg of white mud catalyst and mix it thoroughly with concentrated black liquor containing 100 kg of solids. Inject the mixture into the reactor at approximately 500°C, increasing the air flow until the furnace temperature reaches 650°C, at which point fuel combustion is stopped. Maintain the gasifier temperature at 900°C, continuously injecting the concentrated black liquor for 20 minutes, and then maintain the reaction temperature at 900°C for 60 minutes. Titration with 0.1M hydrochloric acid yields a sodium hydroxide mixed alkaline solution with a causticization degree of 72.3%.

[0107] Example 3:

[0108] Five kilograms of white mud catalyst were weighed and mixed evenly with a concentrated black liquor containing 100 kilograms of solids. This mixture was then injected into a reactor at approximately 500°C. The air flow was increased until the reactor temperature reached 850°C, at which point fuel combustion was stopped. The gasifier temperature was 900°C, and the concentrated black liquor was continuously injected for 20 minutes, maintaining the reaction temperature at 850°C for 60 minutes. Titration with 0.1M hydrochloric acid revealed a causticization degree of 80.7% in the resulting sodium hydroxide mixed alkaline solution.

[0109] Comparative Example 1:

[0110] Except for the absence of a catalyst, all other parameters in this comparative example were the same as in Example 1. The degree of causticization in the sodium hydroxide mixed alkaline solution obtained by titration with 0.1M hydrochloric acid reached 48.6%.

[0111] Comparative Example 2:

[0112] Except for the addition of 1 kg of catalyst, all other parameters in this comparative example are the same as those in Example 1. The degree of causticization in the sodium hydroxide mixed alkaline solution obtained by titration with 0.1 M hydrochloric acid reached 56.7%.

[0113] Comparative Example 3:

[0114] Except for the addition of 7 kg of catalyst, all other parameters in this comparative example are the same as those in Example 1. The degree of causticization in the sodium hydroxide mixed alkaline solution obtained by titration with 0.1 M hydrochloric acid reached 85.1%.

[0115] Through titration analysis of the solid products in Examples 1-3, it was confirmed that the obtained solid products contained both sodium carbonate and sodium hydroxide, with the sodium hydroxide content accounting for 67.3%-80.7% of the total alkali content. In other words, when the catalyst dosage is 2%-5%, the causticization degree of the sodium hydroxide mixed alkali obtained after spontaneous steam vaporization reaction meets the causticization degree required for cooking wood chips.

[0116] By comparing Example 1 and Comparative Example 1, it can be seen that when there is no catalyst, the sodium hydroxide mixed alkali obtained in the gasifier through spontaneous steam gasification reaction has a caustic degree of only 48.6%, which can be used for cooking wood chips, but the pulp hardness is relatively high, which affects the pulp quality to a certain extent.

[0117] By comparing Example 1 and Comparative Example 2, it can be seen that when the amount of catalyst is less than 2%, the catalytic reaction effect is poor, and the sodium hydroxide mixed alkali produced after gasification cannot reach 65% causticization degree.

[0118] A comparison of Example 1 and Comparative Example 3 shows that when the catalyst dosage exceeds 5%, the catalytic reaction effect is better, and the causticization degree exceeds 80%, which exceeds the causticization degree required for cooking wood chips and will affect the cooking effect. More importantly, increasing the dosage of white mud and lime powder catalyst will increase the viscosity of the concentrated black liquor, seriously affecting the operation of the black liquor spray gun. Therefore, a catalyst dosage of 2%-5% is selected.

[0119] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A method for black liquor auto-vaporization gasification combined catalytic reaction regeneration of sodium hydroxide, characterized in that, Includes the following steps: S1. Pump the concentrated black liquor into the stirred storage tank, add the catalyst, and mechanically stir to mix evenly to obtain a mixed slurry of black liquor and catalyst. The catalyst is one or more of white mud and lime powder; S2. Start the diesel burner and supply air through the variable frequency blower to ensure complete combustion of diesel. When the temperature inside the reactor reaches the first furnace temperature, use a black liquor sprayer to spray the mixed slurry obtained in step S1 into the reactor at a predetermined flow rate. S3. While the mixed slurry is being injected, the air flow rate is increased to ensure that the black liquor is stably burned and undergoes a gasification reaction in the reactor. When the temperature in the reactor reaches the second furnace temperature, the diesel burner is turned off. S4. Within the predetermined black liquor flow rate range, maintain the furnace temperature at the third furnace temperature and continuously gasify and catalytically react for more than 1 hour. S5. The molten product is discharged from the reactor. Part of the molten product undergoes a hydration reaction after being dissolved in hot water, while the other part undergoes an alkalization reaction to obtain a sodium hydroxide mixed alkali solution with a causticization rate of 65-80% suitable for wood chip cooking.

2. A method of black liquor auto-vaporization combined catalytic reaction and regeneration of sodium hydroxide according to claim 1, characterized in that, The amount of catalyst added is 2% to 5% of the weight of the black liquor fixative.

3. A method of black liquor auto-steam gasification combined catalytic reaction and regeneration of sodium hydroxide according to claim 1, characterized in that, Each kilogram of the diesel fuel corresponds to a flow rate of 28 to 32 cubic meters of air.

4. A method of black liquor auto-steam gasification combined catalytic reaction and regeneration of sodium hydroxide according to claim 1, characterized in that, The first furnace temperature is 500℃.

5. A method of black liquor auto-steam gasification combined catalytic reaction and regeneration of sodium hydroxide according to claim 1, characterized in that, The second furnace temperature is 650℃.

6. The method for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to claim 1, characterized in that, The increase in airflow is achieved by increasing the airflow by 10 to 13 cubic meters of air per kilogram of the concentrated black liquid solids.

7. The method for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to claim 1, characterized in that, In step S4, the furnace temperature is controlled by combining a temperature controller and a variable frequency blower to control the air flow rate based on the furnace temperature, thereby achieving furnace temperature control. The third furnace temperature is 830~950℃.

8. The method for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to claim 1, characterized in that, The gasification reaction refers to the gasification reaction between the pyrolysis products generated by the cracking of concentrated black liquor under high temperature conditions, the water vapor generated by the concentrated black liquor in the high temperature reactor, and the carbon dioxide generated by the combustion of the concentrated black liquor.

9. The method for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to claim 8, characterized in that, The equation for the reaction of water vapor generated in the concentrated black liquor in the high-temperature reactor is as follows: BL.H2O(l)→BL(s)+H2O(g))(1 The reaction equation for the pyrolysis products generated by the pyrolysis of the concentrated black liquor under high temperature conditions is as follows: Na-Lignin→Na2CO3+Na-O-C+CO↑+CO2↑+H2O↑+CH3OH↑(2) Cellulose→C+CO↑+CO2↑+H2O↑(3) Na-Acid→Na2CO3+CO↑+H2O↑(4).

10. The method for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to claim 9, characterized in that, The equation for the gasification reaction is as follows: Na-O-C+H2O(g)→NaOH+CO+H2(5) C + H₂O(g) → CO + H₂ (6) C + CO₂ → 2CO (7) C+2H2→CH4(8).

11. The method for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to claim 10, characterized in that, The catalytic reaction refers to the reaction in which sodium carbonate, produced by the cracking of concentrated black liquor under high temperature conditions, is converted into sodium oxide under the action of a catalyst, and then dissolved in water to form sodium hydroxide. The equation for the catalytic reaction is as follows: (9)。 12. The method for regenerating sodium hydroxide by spontaneous steam vaporization of black liquor combined with catalytic reaction according to claim 11, characterized in that, The molten products include sodium oxide, sodium carbonate, and calcium oxide, and the equation for the hydration reaction is: Na₂O + H₂O → 2NaOH (10) CaO + H₂O → Ca(OH)₂ (11) The equation for the alkalization reaction is: Na2CO3+Ca(OH)2→2NaOH+CaCO3(12)。

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