Abstract Mid-rich bleaching is a very promising bleaching technology. Medium-density pulp pumps and medium-concentration mixers are the key equipments for achieving medium-to-rich bleaching, and are directly related to the mixing quality of bleach (whether it is gaseous or liquid) and pulp. In this paper, the mixing mechanism of medium-concentration pulp and gaseous bleaching agent is discussed in detail, and the basic structure of the medium-consistency mixer is introduced. Key words Pulp Bleaching Mixing Mechanism The Mixing of Pulp with Gas Bleaching Chemicals at Medium Consistency Hou Qingxi Chen Kefu (South China University of Technology, 510641) ABSTRACT The medium-consistency (MC) bleaching technology has good prospects. MC pump and MC mixer are the The key equipment for MC bleaching process.They have direct effect on the mixing of pulp with bleaching chemicals(either gas or liquid state).This paper mainly discussed the mixing mechanism of pulp with gas bleaching chemicals at medium consistency.The basic structure of an MC Mixer was also described.KEYWORDS MC bleaching,MC mixer,MC pump
Due to environmental protection, economic benefits, and reduction of energy consumption, foreign countries have been studying and exploring the best way to satisfy the above three aspects in the slurry preparation process since the 1970s. In the late 1970s, the successful development of medium thick slurry pumps and medium-thicken mixers laid the foundation for reducing the discharge of bleaching plant wastewater, reducing energy consumption and equipment investment scale. The use of a high-efficiency mixer for concentrated chlorination and the replacement of part of the chlorine with chlorine dioxide into the chlorination section not only ensures pulp quality and bleach yield, but also significantly reduces the effectiveness of pulp with a certain degree of final whiteness. The total chlorine demand will reduce the organic chlorine content in bleaching wastewater. Passing a small amount of chlorine gas (about 3 to 5 kg/t) into the multi-stage bleached alkali extraction section can increase the efficiency of alkali extraction and allow more lignin to dissolve. According to reports, using the EO section can save 23% of the D-stage ClO2 consumption at the same final whiteness of the same pulp, and the entire bleaching station production capacity can be increased by 15%. The application of the EO section can significantly reduce the pulp hardness and reduce the number of bleaching stages, shortening Production Process. In recent years, with the development and application of bleaching agents such as ClO2, O2, O3, and H2O2, and the maturing of medium-density bleaching technology, elemental chlorine bleaching and chlorine-free bleaching of pulp have resulted in less contamination or no slurry preparation process. Pollution, and make energy saving and reducing consumption become a reality.
At present, Zhongnong technology has not been promoted in China. Due to the small scale of pulp mill production in China's paper industry, the average annual production capacity is less than 4,000t/a, and there are only 100 companies with an annual output of 30,000 tons or more. The bleaching process is still relatively backward and the pollution is serious. Therefore, it is of great practical significance to study and master the medium-concentration technology and develop the medium-concentration technology device suitable for the development of China's paper industry.
Medium-density bleaching generally refers to bleaching where the pulp concentration is in the range of 8% to 14%. The role of the medium-density mixer is to ensure that the pulp and bleach are thoroughly mixed and the bleach is quickly and evenly distributed on the pulp fibers. Since the mixing of pulp and liquid bleach is much easier than mixing with gaseous bleach, this paper mainly analyzes the mixing mechanism of pulp and gaseous bleach under medium and heavy conditions. 1 Mixing Mechanism of Pulp and Gaseous Bleach under Medium Concentration Pulp suspension is a multiphase fluid with solid fiber material. It has been determined that about 10% of air is contained in 10% pulp, and 12% of pulp fiber cells contain more than 40% water in the lumen and in the cell wall. In the mixing of gaseous bleaching agents with medium-consistency pulp, a gas-liquid mass transfer process must first be carried out so that the gaseous bleaching agent dissolves in the water in the surface layer of the fiber and then adsorbs to the cell wall and stores in the cell cavity by a certain means. The water exchanges quickly with the water, which has been dissolved in the gaseous bleaching agent, to achieve the mixing of the bleaching agent with the pulp fibers. 1.1 Mixing Mechanism Regardless of the type of material being mixed, and regardless of the structure of the mixing device used, the mixing mechanism of the material is convection diffusion, turbulent diffusion and molecular diffusion. In the mutual contact of materials, they are playing different roles.
Molecular diffusion is caused by the relative movement of molecules. In any system where two molecules are present, as long as the time is long enough, the molecules will mix with each other to form a homogeneous mixture on a molecular scale. Any mixing process eventually tends to be uniform, ie the molecules are uniformly distributed throughout the flow field. For the entire bleaching process, achieving uniform mixing on the molecular scale also ensures uniform contact of the bleach with residual lignin in the pulp and some coloured substances. However, due to the slow rate of molecular diffusion, it is often unable to adapt to the needs of the actual process. For example: a container with feature size L=5m, with a heater as the heat source. If the fluid in the container is stationary, heat is only distributed by the molecular heat to spread throughout the container. Since ΔT is the characteristic temperature difference, Tm is the time scale of the thermal conduction (diffusion) of the molecule. α is the thermal conductivity. Assuming a constant, the above equation can be further rewritten assuming that the fluid in the vessel is at room temperature and the thermal conductivity α=0.2cm2/s, so that the time of the diffusion process can be obtained. The number of scales tm is about 106s, that is, 300h.
Most of the fluid flow in the industry occurs in turbulent conditions, and fluids in the turbulent state tend to produce turbulence, which can break the fluid into a large number of small micelles, as shown in Figure 1. Molecular diffusion can allow different components to exchange through the boundaries of these micelles. The smaller the micelles, the larger the area that provides molecular diffusion, the larger the exchange boundary between the micelles, and the more conducive it is to diffusion. The reduction in the size of the micelles mainly depends on the high intensity and the small scale of the agitation. Molecular diffusion has created favorable conditions. figure 1
The main role of restlessness is to produce strong shear and velocity pulsations. If the mixing rate is determined by the diffusion distance of the molecules, that is, by the degree of elongation or division of the solute, then a strong shear will accelerate the mixing speed. As shown in Fig. 2, the square fluid element of length Lo is placed in a shear field with a shear rate so that it is drawn into a parallelogram. If it is specified, the thickness of the parallelogram is reduced to Lm to achieve the purpose of mixing, the elongation process The shear time experienced by the medium fluid unit is the main mixing time tm. When applying a small angle, it can be seen from Figure 2 that the mixing time is inversely proportional to the shear rate, ie the higher the shear rate, the mixing time The shorter. The turbulent transfer rate is usually several orders of magnitude larger than the diffusion rate caused by molecular diffusion, indicating that turbulent diffusion is more important than molecular diffusion. In addition, in the turbulent flow state, the shear stress of the fluid is expressed by the apparent viscosity μ, Ï is the fluid density, and l is the pulse length. It can be seen from the formula that the shear stress is proportional to the shear rate. In production practice, since pulp and bleach are mixed in the mixer and remain in the mixer for a very short time, although the pulp has been in a turbulent state and has a certain shear rate, it still needs to supply energy That is, a higher shear stress is produced by the mixer to shorten the mixing time.
The role of the speed pulsation is to make the flow rate of the mixed material in each point of the mixing field continuously change in size and direction, so that the mixed material has a uniform shearing effect and the mixing process is strengthened.
In the medium-consistency condition, the apparent viscosity of the pulp has changed nonlinearly with the increase of the concentration. Without the high shear force, the non-Newtonian fluid itself will lose the flow performance, showing obvious viscous characteristics. Fiber networks that are intertwined with each other can seriously impede the uniform contact of bleach with each fiber. Therefore, when the bleaching agent is mixed with the concentrated pulp, it is necessary to disperse the fiber network, that is, the pulp is fluidized, so that the pulp stream becomes a continuous medium composed of numerous individual fibers, and has similar flow characteristics, thereby ensuring the flow characteristics. The pulp is rapidly and evenly mixed with the bleaching agent in the mixing field.
It should be pointed out that due to the macroscopic nature of turbulence, it can only disperse the different components of the mixed field into small micelles of a certain size, the smallest limit size of which is still much larger than that of the molecule, and the general minimum micelle will also contain With millions of molecules, it cannot be expected that the size of the micelles in the turbulent flow will approach the molecular size, and further mixing will only rely on molecular diffusion. In order to describe this mixing process, the separation scale L is usually used to represent the extent to which the turbulence acts to disperse the material, and the separation strength I indicates that the molecular diffusion effects the material to a nearly uniform degree. The separation scale L and the separation strength I can be expressed by the following mathematical formulas. The middle R(r) is the correlation coefficient between the concentration values ​​r apart, δ2 is the variance of the fluid unit concentration a versus the average concentration, and δ20 is the variance of the concentration a versus the mean concentration when the fluid is not mixed and the mixing time is zero. From the above equation, it can be seen that when the unmixed time δ2=δ20, I=1; when the molecular level mixing is reached, the concentration of any fluid cell is the average concentration, δ=0, and I=0. The separation scale L decreases with the macroscopic mixing (turbulent diffusion, 0-a stage in Fig. 3), and gradually increases with the microscopic mixing (molecular diffusion, ab stage in Fig. 3). image 3
Turbulent flow accelerates the progress of molecular diffusion and increases the reaction rate. Still taking the heat conduction problem of the built-in heat source container mentioned above as an example, if the air in the container moves, even if the moving speed is as low as 0.1m/s or less, the turbulent heat conduction can spread the heat throughout the container, and the time required can be within the original container. The 300 h when the air is at rest is shortened to within 2 min. For the mixing of medium and thick pulp and bleaching agent, because the pulp is subjected to high shear forces in the turbulent state, the fibers are twisted and squeeze-expanded, part of the water in the fiber cell cavity can be “pumped†and bleached The agent is "pumped" in, speeding up the exchange with bleach. Mobility plays an important role in the mixing of pulp and bleach.
Convection diffusion refers to the diffusion caused by the movement of the main body. It is superimposed on the diffusion of molecules or turbulence, or the simultaneous existence of the three, and is an important aspect of the mixing process. Gaseous bleaching agents having a certain initial velocity and pressure are flushed into the fluidized medium-consistency pulp, and convection occurs, accelerating the exchange between different mixing speed zones, and the mixing speed of each mixing zone tends to be uniform. 1.2 Water Barriers Water in pulp generally exists in three states, either in the lumen of fiber cells, or in the fiber cell walls, or in the surrounding fibers. Figure 4 shows the volumetric comparison of wet fibers and their surrounding water at 3%, 12%, and 30% concentrations. For a 12% concentration of concentrated pulp, the sum of the volume of water in the cell cavities and the volume of water adsorbed in the cell wall is approximately 40% of the total volume of water. Obviously this part of the water impedes the direct contact of the bleaching agent with the fiber. Therefore, during the mixing process of the pulp and the bleaching agent, high-strength shearing action and the resulting localized intense velocity pulsation must be used to weaken the obstruction of this part of the water. The role, which is also a feature of the Zhongcong mixer. Figure 4
It can also be noted from FIG. 4 that as the pulp concentration increases, the volume of water surrounding the fibers is significantly reduced, and the concentration difference of the bleaching agent inside and outside the fibers is increased, which helps to increase the diffusion rate, on the other hand. It also reduces the amount of waste liquid after bleaching. 1.3 Mixture of medium and heavy pulp and gaseous bleaching The mixing of concentrated pulp and liquid bleach can be carried out with turbidity in the medium thick slurry pump. The mixing of medium-consistency pulp with gaseous bleaching agent, in order to ensure the mixing quality, is generally carried out by a medium thick slurry pump which pumps the pulp into the medium-concentration mixer and mixes it with the gaseous bleaching agent. The addition of gaseous bleach is at the pulp inlet end of the mixer. During the entire mixing process, the degree of convection, turbulence and molecular diffusion performed are the key factors that determine the mixing effect.
When gaseous bleach with a certain pressure is injected into the pulp flow path, convection diffusion occurs first, followed by turbulent diffusion and finally into the molecular diffusion stage. After the high-shear force of the turbulence generator in the medium-thick slurry pump, the medium-consistency pulp is in a fluidized state and the pulp stream is broken into smaller blocks. Because the gaseous bleaching agent is injected into the inlet end of the medium-concentration mixer under a certain pressure and a certain initial speed, it has been in a turbulent state. When it comes into contact with the pulp, it will interfere with each other, causing convection and causing extreme Stable vortex. The size of the pulp mass is further reduced, the gas is broken due to the jet, and bubbles of different sizes are formed.
Due to environmental protection, economic benefits, and reduction of energy consumption, foreign countries have been studying and exploring the best way to satisfy the above three aspects in the slurry preparation process since the 1970s. In the late 1970s, the successful development of medium thick slurry pumps and medium-thicken mixers laid the foundation for reducing the discharge of bleaching plant wastewater, reducing energy consumption and equipment investment scale. The use of a high-efficiency mixer for concentrated chlorination and the replacement of part of the chlorine with chlorine dioxide into the chlorination section not only ensures pulp quality and bleach yield, but also significantly reduces the effectiveness of pulp with a certain degree of final whiteness. The total chlorine demand will reduce the organic chlorine content in bleaching wastewater. Passing a small amount of chlorine gas (about 3 to 5 kg/t) into the multi-stage bleached alkali extraction section can increase the efficiency of alkali extraction and allow more lignin to dissolve. According to reports, using the EO section can save 23% of the D-stage ClO2 consumption at the same final whiteness of the same pulp, and the entire bleaching station production capacity can be increased by 15%. The application of the EO section can significantly reduce the pulp hardness and reduce the number of bleaching stages, shortening Production Process. In recent years, with the development and application of bleaching agents such as ClO2, O2, O3, and H2O2, and the maturing of medium-density bleaching technology, elemental chlorine bleaching and chlorine-free bleaching of pulp have resulted in less contamination or no slurry preparation process. Pollution, and make energy saving and reducing consumption become a reality.
At present, Zhongnong technology has not been promoted in China. Due to the small scale of pulp mill production in China's paper industry, the average annual production capacity is less than 4,000t/a, and there are only 100 companies with an annual output of 30,000 tons or more. The bleaching process is still relatively backward and the pollution is serious. Therefore, it is of great practical significance to study and master the medium-concentration technology and develop the medium-concentration technology device suitable for the development of China's paper industry.
Medium-density bleaching generally refers to bleaching where the pulp concentration is in the range of 8% to 14%. The role of the medium-density mixer is to ensure that the pulp and bleach are thoroughly mixed and the bleach is quickly and evenly distributed on the pulp fibers. Since the mixing of pulp and liquid bleach is much easier than mixing with gaseous bleach, this paper mainly analyzes the mixing mechanism of pulp and gaseous bleach under medium and heavy conditions. 1 Mixing Mechanism of Pulp and Gaseous Bleach under Medium Concentration Pulp suspension is a multiphase fluid with solid fiber material. It has been determined that about 10% of air is contained in 10% pulp, and 12% of pulp fiber cells contain more than 40% water in the lumen and in the cell wall. In the mixing of gaseous bleaching agents with medium-consistency pulp, a gas-liquid mass transfer process must first be carried out so that the gaseous bleaching agent dissolves in the water in the surface layer of the fiber and then adsorbs to the cell wall and stores in the cell cavity by a certain means. The water exchanges quickly with the water, which has been dissolved in the gaseous bleaching agent, to achieve the mixing of the bleaching agent with the pulp fibers. 1.1 Mixing Mechanism Regardless of the type of material being mixed, and regardless of the structure of the mixing device used, the mixing mechanism of the material is convection diffusion, turbulent diffusion and molecular diffusion. In the mutual contact of materials, they are playing different roles.
Molecular diffusion is caused by the relative movement of molecules. In any system where two molecules are present, as long as the time is long enough, the molecules will mix with each other to form a homogeneous mixture on a molecular scale. Any mixing process eventually tends to be uniform, ie the molecules are uniformly distributed throughout the flow field. For the entire bleaching process, achieving uniform mixing on the molecular scale also ensures uniform contact of the bleach with residual lignin in the pulp and some coloured substances. However, due to the slow rate of molecular diffusion, it is often unable to adapt to the needs of the actual process. For example: a container with feature size L=5m, with a heater as the heat source. If the fluid in the container is stationary, heat is only distributed by the molecular heat to spread throughout the container. Since ΔT is the characteristic temperature difference, Tm is the time scale of the thermal conduction (diffusion) of the molecule. α is the thermal conductivity. Assuming a constant, the above equation can be further rewritten assuming that the fluid in the vessel is at room temperature and the thermal conductivity α=0.2cm2/s, so that the time of the diffusion process can be obtained. The number of scales tm is about 106s, that is, 300h.
Most of the fluid flow in the industry occurs in turbulent conditions, and fluids in the turbulent state tend to produce turbulence, which can break the fluid into a large number of small micelles, as shown in Figure 1. Molecular diffusion can allow different components to exchange through the boundaries of these micelles. The smaller the micelles, the larger the area that provides molecular diffusion, the larger the exchange boundary between the micelles, and the more conducive it is to diffusion. The reduction in the size of the micelles mainly depends on the high intensity and the small scale of the agitation. Molecular diffusion has created favorable conditions. figure 1
The main role of restlessness is to produce strong shear and velocity pulsations. If the mixing rate is determined by the diffusion distance of the molecules, that is, by the degree of elongation or division of the solute, then a strong shear will accelerate the mixing speed. As shown in Fig. 2, the square fluid element of length Lo is placed in a shear field with a shear rate so that it is drawn into a parallelogram. If it is specified, the thickness of the parallelogram is reduced to Lm to achieve the purpose of mixing, the elongation process The shear time experienced by the medium fluid unit is the main mixing time tm. When applying a small angle, it can be seen from Figure 2 that the mixing time is inversely proportional to the shear rate, ie the higher the shear rate, the mixing time The shorter. The turbulent transfer rate is usually several orders of magnitude larger than the diffusion rate caused by molecular diffusion, indicating that turbulent diffusion is more important than molecular diffusion. In addition, in the turbulent flow state, the shear stress of the fluid is expressed by the apparent viscosity μ, Ï is the fluid density, and l is the pulse length. It can be seen from the formula that the shear stress is proportional to the shear rate. In production practice, since pulp and bleach are mixed in the mixer and remain in the mixer for a very short time, although the pulp has been in a turbulent state and has a certain shear rate, it still needs to supply energy That is, a higher shear stress is produced by the mixer to shorten the mixing time.
The role of the speed pulsation is to make the flow rate of the mixed material in each point of the mixing field continuously change in size and direction, so that the mixed material has a uniform shearing effect and the mixing process is strengthened.
In the medium-consistency condition, the apparent viscosity of the pulp has changed nonlinearly with the increase of the concentration. Without the high shear force, the non-Newtonian fluid itself will lose the flow performance, showing obvious viscous characteristics. Fiber networks that are intertwined with each other can seriously impede the uniform contact of bleach with each fiber. Therefore, when the bleaching agent is mixed with the concentrated pulp, it is necessary to disperse the fiber network, that is, the pulp is fluidized, so that the pulp stream becomes a continuous medium composed of numerous individual fibers, and has similar flow characteristics, thereby ensuring the flow characteristics. The pulp is rapidly and evenly mixed with the bleaching agent in the mixing field.
It should be pointed out that due to the macroscopic nature of turbulence, it can only disperse the different components of the mixed field into small micelles of a certain size, the smallest limit size of which is still much larger than that of the molecule, and the general minimum micelle will also contain With millions of molecules, it cannot be expected that the size of the micelles in the turbulent flow will approach the molecular size, and further mixing will only rely on molecular diffusion. In order to describe this mixing process, the separation scale L is usually used to represent the extent to which the turbulence acts to disperse the material, and the separation strength I indicates that the molecular diffusion effects the material to a nearly uniform degree. The separation scale L and the separation strength I can be expressed by the following mathematical formulas. The middle R(r) is the correlation coefficient between the concentration values ​​r apart, δ2 is the variance of the fluid unit concentration a versus the average concentration, and δ20 is the variance of the concentration a versus the mean concentration when the fluid is not mixed and the mixing time is zero. From the above equation, it can be seen that when the unmixed time δ2=δ20, I=1; when the molecular level mixing is reached, the concentration of any fluid cell is the average concentration, δ=0, and I=0. The separation scale L decreases with the macroscopic mixing (turbulent diffusion, 0-a stage in Fig. 3), and gradually increases with the microscopic mixing (molecular diffusion, ab stage in Fig. 3). image 3
Turbulent flow accelerates the progress of molecular diffusion and increases the reaction rate. Still taking the heat conduction problem of the built-in heat source container mentioned above as an example, if the air in the container moves, even if the moving speed is as low as 0.1m/s or less, the turbulent heat conduction can spread the heat throughout the container, and the time required can be within the original container. The 300 h when the air is at rest is shortened to within 2 min. For the mixing of medium and thick pulp and bleaching agent, because the pulp is subjected to high shear forces in the turbulent state, the fibers are twisted and squeeze-expanded, part of the water in the fiber cell cavity can be “pumped†and bleached The agent is "pumped" in, speeding up the exchange with bleach. Mobility plays an important role in the mixing of pulp and bleach.
Convection diffusion refers to the diffusion caused by the movement of the main body. It is superimposed on the diffusion of molecules or turbulence, or the simultaneous existence of the three, and is an important aspect of the mixing process. Gaseous bleaching agents having a certain initial velocity and pressure are flushed into the fluidized medium-consistency pulp, and convection occurs, accelerating the exchange between different mixing speed zones, and the mixing speed of each mixing zone tends to be uniform. 1.2 Water Barriers Water in pulp generally exists in three states, either in the lumen of fiber cells, or in the fiber cell walls, or in the surrounding fibers. Figure 4 shows the volumetric comparison of wet fibers and their surrounding water at 3%, 12%, and 30% concentrations. For a 12% concentration of concentrated pulp, the sum of the volume of water in the cell cavities and the volume of water adsorbed in the cell wall is approximately 40% of the total volume of water. Obviously this part of the water impedes the direct contact of the bleaching agent with the fiber. Therefore, during the mixing process of the pulp and the bleaching agent, high-strength shearing action and the resulting localized intense velocity pulsation must be used to weaken the obstruction of this part of the water. The role, which is also a feature of the Zhongcong mixer. Figure 4
It can also be noted from FIG. 4 that as the pulp concentration increases, the volume of water surrounding the fibers is significantly reduced, and the concentration difference of the bleaching agent inside and outside the fibers is increased, which helps to increase the diffusion rate, on the other hand. It also reduces the amount of waste liquid after bleaching. 1.3 Mixture of medium and heavy pulp and gaseous bleaching The mixing of concentrated pulp and liquid bleach can be carried out with turbidity in the medium thick slurry pump. The mixing of medium-consistency pulp with gaseous bleaching agent, in order to ensure the mixing quality, is generally carried out by a medium thick slurry pump which pumps the pulp into the medium-concentration mixer and mixes it with the gaseous bleaching agent. The addition of gaseous bleach is at the pulp inlet end of the mixer. During the entire mixing process, the degree of convection, turbulence and molecular diffusion performed are the key factors that determine the mixing effect.
When gaseous bleach with a certain pressure is injected into the pulp flow path, convection diffusion occurs first, followed by turbulent diffusion and finally into the molecular diffusion stage. After the high-shear force of the turbulence generator in the medium-thick slurry pump, the medium-consistency pulp is in a fluidized state and the pulp stream is broken into smaller blocks. Because the gaseous bleaching agent is injected into the inlet end of the medium-concentration mixer under a certain pressure and a certain initial speed, it has been in a turbulent state. When it comes into contact with the pulp, it will interfere with each other, causing convection and causing extreme Stable vortex. The size of the pulp mass is further reduced, the gas is broken due to the jet, and bubbles of different sizes are formed.
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