Method for calibrating rolling friction coefficient and surface energy in process of simulating ceramic membrane to filter lignin by DEM-CFD

Abstract

The invention provides a method for calibrating a rolling friction coefficient and surface energy in a process of simulating a ceramic membrane to filter lignin by DEM-CFD, develops a semi-analytical DEM-CFD coupling interface, and on the basis, takes movement and deposition behaviors of lignin particles in a process of filtering black liquor lignin by a single ceramic membrane pore as research objects, and provides a method for calibrating the rolling friction coefficient and the surface energy in the process of simulating the ceramic membrane to filter the black liquor lignin. The capturing mechanism of lignin particles in a ceramic membrane is researched, and it is determined that key factors influencing lignin particle deposition morphology, average coordination number, coordination number distribution and lignin particle deposition structure porosity are rolling friction coefficients and surface energy among lignin particles; the rolling friction coefficient and the surface energy between the lignin particles and the ceramic membrane have little influence on the ceramic membrane; the influence of the rolling friction coefficient among lignin particles and the surface energy parameters on the deposition morphology, coordination number and porosity of the lignin particles is comprehensively considered, calibration work of the rolling friction coefficient and the surface energy parameters is completed, and a powerful tool and accurate simulation parameters are provided for subsequent research of a membrane pollution mechanism.

Description

Technical Field

[0001] The present invention relates to the technical field of lignin separation, and particularly relates to a method for calibrating the rolling friction coefficient and surface energy during the process of filtering lignin by a ceramic membrane through DEM-CFD simulation. Background Art

[0002] Lignin is a representative organic substance in black liquor, and it is a renewable biomass energy with great utilization value. At the same time, it is the only non-petroleum resource in nature that can provide renewable aryl compounds. Nowadays, in the context of the forest-based circular bioeconomy, lignin has quite high commercial value. Therefore, separating lignin from black liquor efficiently and with low loss is of great significance for reducing environmental pollution and improving the utilization rate of waste.

[0003] With the development of membrane separation technology, ceramic membranes have become one of the most effective materials for separating lignin from black liquor due to their high temperature resistance and strong alkali resistance. However, the problem of ceramic membrane fouling has always existed and has become a technical problem restricting its development. Previous studies on separating lignin from black liquor by ceramic membranes mainly focused on macroscopic operation parameters during the filtration process, such as pressure drop, filtration flux, and filtration efficiency, etc. There are few studies on the movement and deposition behavior of lignin particles in the ceramic membrane filtration at the microscopic level, the membrane fouling mechanism, and the causes. The DEM-CFD (Discrete Element Method coupled Computational Fluid Dynamic) coupling method is an effective method for studying the movement and deposition behavior of lignin particles in the ceramic membrane filtration process, demonstrating membrane pore blockage, and revealing the membrane fouling mechanism. However, under microscopic flow, there are still great challenges in the dynamic study of the movement and deposition behavior of lignin particles in the ceramic membrane filtration process.

[0004] The rolling friction coefficient and surface energy parameters are two extremely important parameters in the DEM-CFD coupling solution. The former is the key to simplifying the shape of lignin particles, while the latter is the most crucial macroscopic characterization parameter reflecting the magnitude of van der Waals adhesion force. These two key factors have a great influence on the kinetic behavior of lignin particles in the ceramic membrane filtration. Therefore, whether the rolling friction coefficient and surface energy parameters can be correctly given is the key to whether the simulation conforms to reality, and at the same time, it also provides correct simulation parameters for the subsequent study of the membrane fouling mechanism during the process of filtering lignin from black liquor by a ceramic membrane. Summary of the Invention

[0005] The present invention provides a method for calibrating the rolling friction coefficient and surface energy in the process of DEM-CFD simulation of ceramic membrane filtration of lignin. Taking the movement and deposition behavior of lignin particles during the filtration of black liquor lignin through a single ceramic membrane pore as the research object, comprehensively considering the influence of the rolling friction coefficient and surface energy parameters on the deposition morphology, coordination number and porosity of lignin particles, the calibration work of the rolling friction coefficient and surface energy parameters is completed, providing a powerful tool and accurate simulation parameters for the subsequent research on the mechanism of membrane fouling.

[0006] A method for calibrating the rolling friction coefficient and surface energy in the process of DEM-CFD simulation of ceramic membrane filtration of lignin includes the following steps:

[0007] Step 1: Construct a semi-analytical DEM-CFD coupling interface for simulating the process of ceramic membrane filtration of lignin in black liquor based on the DEM-CFD coupling method.

[0008] Step 2: Simplify the shapes of lignin particles and ceramic particles into spherical shapes, simplify the porous ceramic membrane structure into a single ceramic membrane pore structure, and set the computational domain size and boundary conditions.

[0009] Step 3: Based on the physical model of a single ceramic membrane pore, use the semi-analytical DEM-CFD coupling interface to simulate the process of ceramic membrane filtration of lignin in black liquor, and calibrate the rolling friction coefficient and surface energy by analyzing the movement and deposition process of lignin particles in the ceramic membrane pores during the process of ceramic membrane filtration of lignin in black liquor. Among them, the rolling friction coefficient includes the rolling friction coefficient between lignin particles and the rolling friction coefficient and surface energy between lignin particles and the ceramic membrane, and the surface energy includes the surface energy between lignin particles and the surface energy between lignin particles and the ceramic membrane.

[0010] Optionally, the coupling between DEM and CFD of the semi-analytical DEM-CFD coupling interface adopts the Eulerian-Eulerian coupling method. The translation and rotation of lignin particles in DEM are solved by the explicit time integration method. The control equation of the fluid in CFD is solved by the SIMPLE algorithm under the pressure-based solver. The pressure term is discretized by the second-order format, and the discretization format of other terms except the pressure term is selected as the first-order upwind format. The drag model selects the Ergun / Wen&Yu drag model. The velocity inlet is adopted for the inlet of black liquor, and the pressure outlet is adopted for the outlet of black liquor.

[0011] Optionally, in Step 3, the calibration of the rolling friction coefficient and surface energy by analyzing the movement and deposition process of lignin particles in the ceramic membrane pores during the process of ceramic membrane filtration of lignin in black liquor includes:

[0012] Determine the deposition process and capture mechanism of lignin particles based on the movement and deposition process of lignin particles in the pores of ceramic membranes during the filtration of black liquor by ceramic membranes. Among them, the deposition process and capture mechanism of lignin particles include the initial deposition of lignin particles on the ceramic membrane, the formation of dendritic structures, and the final process of reaching equilibrium;

[0013] Based on the deposition process and capture mechanism of lignin particles, it is determined that the rolling friction coefficient and surface energy have an impact on the deposition of lignin particles in the pores of ceramic membranes, and the impact of the rolling friction coefficient and surface energy on the deposition of lignin particles in the pores of ceramic membranes is quantitatively characterized to determine the calibration values of the rolling friction coefficient and surface energy.

[0014] Furthermore, the determination of the deposition process and capture mechanism of lignin particles based on the movement and deposition process of lignin particles in the pores of ceramic membranes during the filtration of black liquor by ceramic membranes includes:

[0015] Set the rolling friction coefficient and surface energy between lignin particles, set the rolling friction coefficient and surface energy between lignin particles and ceramic membranes, simulate the movement and deposition process of lignin particles in the pores of ceramic membranes, and draw the change process of the deposition morphology of lignin particles over time according to the simulation result data;

[0016] Analyze the initial stage of the change process of the deposition morphology of lignin particles, and determine that in the initial stage, a small number of lignin particles come into contact with the membrane surface of the ceramic membrane and are directly captured by the ceramic membrane under the action of van der Waals adhesion force, while most of them flow out of the computational domain through the pores of the ceramic membrane with the fluid;

[0017] Continue to analyze the second stage of the change process of the deposition morphology of lignin particles, and determine that in the second stage, lignin particles are not only captured by the ceramic membrane, but also captured by the already deposited lignin particles, and the deposited lignin particles begin to extend outward to form dendritic structures;

[0018] Further analyze the third stage of the change process of the deposition morphology of lignin particles, and determine that on the one hand, the dendritic structures continue to capture lignin particles and grow continuously in the third stage, and on the other hand, they begin to rotate, bend, fold and break under the action of fluid force, increasing the contact area between the dendritic structures and the flowing lignin particles, and most of the lignin particles are captured by the dendritic structures;

[0019] Finally, stop injecting lignin particles, and the dendritic structures continue to rotate, bend, fold and break under the action of fluid force until they reach an equilibrium state.

[0020] Furthermore, the quantitative characterization includes the deposition morphology of lignin particles, the average coordination number, the coordination number, and the porosity of the deposition structure of lignin particles;

[0021] Correspondingly, based on the lignin particle deposition process and capture mechanism, it is determined that the rolling friction coefficient and surface energy have an impact on the deposition of lignin particles in the pores of the ceramic membrane, and the effects of the rolling friction coefficient and surface energy on the deposition of lignin particles in the pores of the ceramic membrane are quantitatively characterized to determine the calibration values of the rolling friction coefficient and surface energy, including:

[0022] According to the lignin particle deposition process and capture mechanism, it is known that the deposition morphology of lignin particles is related to the fluid force they receive, the rolling friction resistance and van der Waals adhesion force between the deposited lignin particles, and between the lignin particles and the ceramic membrane. The rolling friction resistance and van der Waals adhesion force are respectively related to the rolling friction coefficient and surface energy. Therefore, it is determined that the rolling friction coefficient and surface energy have an impact on the deposition of lignin particles in the pores of the ceramic membrane;

[0023] Under the condition of constant surface energy, different rolling friction coefficients are set to simulate the process of lignin filtration in black liquor by the ceramic membrane. According to the simulation result data, the deposition morphology of lignin particles under different rolling friction coefficients is plotted;

[0024] Under the condition of constant surface energy, different rolling friction coefficients are set to simulate the process of lignin filtration in black liquor by the ceramic membrane. According to the simulation result data, the curve of the average coordination number changing with the rolling friction coefficient, the influence of the rolling friction coefficient between lignin particles on the coordination number distribution, and the influence of the rolling friction coefficient between lignin particles and the ceramic membrane on the coordination number distribution are plotted;

[0025] Under the condition of constant surface energy, different rolling friction coefficients are set to simulate the process of lignin filtration in black liquor by the ceramic membrane. According to the simulation result data, the curve of the porosity of the lignin particle deposition structure changing with the rolling friction coefficient is plotted;

[0026] By analyzing the deposition morphology of lignin particles under different rolling friction coefficients, the curve of the average coordination number changing with the rolling friction coefficient, the influence of the rolling friction coefficient between lignin particles on the coordination number distribution, the influence of the rolling friction coefficient between lignin particles and the ceramic membrane on the coordination number distribution, and the curve of the porosity of the lignin particle deposition structure changing with the rolling friction coefficient, it is determined that the key factors affecting the deposition morphology of lignin particles, the average coordination number, the coordination number distribution, and the porosity of the lignin particle deposition structure are the rolling friction coefficient between lignin particles and the calibration value of the rolling friction coefficient between lignin particles;

[0027] Under the condition of constant rolling friction coefficient between lignin particles and between lignin particles and the ceramic membrane, different surface energies are set to simulate the process of lignin filtration in black liquor by the ceramic membrane. According to the simulation result data, the deposition morphology of lignin particles under different surface energies is plotted;

[0028] Under the condition that the rolling friction coefficients between lignin particles and between lignin particles and the ceramic membrane are constant, different surface energies are set to simulate the process of lignin filtration from black liquor by the ceramic membrane. According to the simulation result data, curves of the average coordination number varying with the surface energy between lignin particles, the influence of the surface energy between lignin particles on the coordination number distribution, and the influence of the surface energy between lignin particles and the ceramic membrane on the coordination number distribution are plotted.

[0029] Under the condition that the rolling friction coefficients between lignin particles and between lignin particles and the ceramic membrane are constant, different surface energies are set to simulate the process of lignin filtration from black liquor by the ceramic membrane. According to the simulation result data, a curve of the porosity of the lignin particle deposition structure varying with the surface energy is plotted.

[0030] By analyzing the deposition morphology of lignin particles at different surface energies, the curve of the average coordination number varying with the surface energy, the influence of the surface energy between lignin particles on the coordination number distribution, the influence of the surface energy between lignin particles and the ceramic membrane on the coordination number distribution, and the curve of the porosity of the lignin particle deposition structure varying with the surface energy, it is determined that the key factors affecting the deposition morphology of lignin particles, the average coordination number, the coordination number distribution, and the porosity of the lignin particle deposition structure are the surface energy between lignin particles and the calibration value of the surface energy between lignin particles.

[0031] After adopting the above technical solution, the present invention has at least the following beneficial effects:

[0032] (1) The capture mechanism of lignin particles in the ceramic membrane is studied, mainly including the initial deposition of lignin particles on the ceramic membrane, the formation of dendritic structures, and the process of approaching equilibrium. It is determined that the formation of dendritic structures has an important impact on the capture and deposition morphology of lignin particles.

[0033] (2) It is determined that the key factors affecting the deposition morphology of lignin particles, the average coordination number, the coordination number distribution, and the porosity of the lignin particle deposition structure are the rolling friction coefficients and surface energies between lignin particles, and the rolling friction coefficients and surface energies between lignin particles and the ceramic membrane have little influence on them.

[0034] (3) By reasonably specifying the rolling friction coefficient between lignin particles, spherical lignin particles can be used to replace non-spherical lignin particles. Considering the influence of the rolling friction coefficient on the deposition morphology of lignin particles, the average coordination number, the coordination number distribution, and the porosity of the lignin particle deposition structure, the calibration value of the rolling friction coefficient between lignin particles is determined, so that the simulation process can be closer to the actual lignin filtration process.

[0035] (4) Considering the influence of the surface energy between lignin particles on the deposition morphology, average coordination number, coordination number distribution of lignin particles, and the porosity of the lignin particle deposition structure, determine the calibration value of the surface energy between lignin particles, so that the simulation process can be closer to the real lignin filtration process. Description of the Drawings

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0037] Figure 1 It is a schematic diagram of the physical model of the pores of a single ceramic membrane and the size of the computational domain;

[0038] Figure 2 It is the process of the deposition morphology of lignin particles changing with time;

[0039] Figure 3 It is the influence of the rolling friction coefficient on the deposition morphology of lignin particles, where a is the rolling friction coefficient μ between lignin particles and the ceramic membrane p-m = 0.1, the deposition morphology of lignin particles under different rolling friction coefficients μ between lignin particles p-p b is the rolling friction coefficient μ between lignin particles and the ceramic membrane p-m = 1.0, the deposition morphology of lignin particles under different rolling friction coefficients μ between lignin particles p-p c is the rolling friction coefficient μ between lignin particles and the ceramic membrane p-m = 2.0, the deposition morphology of lignin particles under different rolling friction coefficients μ between lignin particles p-p d is the rolling friction coefficient μ between lignin particles and the ceramic membrane p-m = 3.0, the deposition morphology of lignin particles under different rolling friction coefficients μ between lignin particles p-p ;

[0040] Figure 4 It is the influence of the rolling friction coefficient on the average coordination number;

[0041] Figure 5 It is the influence of the rolling friction coefficient between lignin particles on the coordination number distribution, where a is the rolling friction coefficient μ between lignin particles and the ceramic membrane p-m = 0.1, the coordination number distribution under different rolling friction coefficients μ between lignin particles p-p b is the rolling friction coefficient μ between lignin particles and the ceramic membrane p-m = 1.0, the rolling friction coefficient μ between lignin particles under differentp-p The coordination number distribution under, where c is the rolling friction coefficient μ between the lignin particles and the ceramic membrane p-m The rolling friction coefficient μ between different lignin particles when = 2.0 p-p The coordination number distribution under, where d is the rolling friction coefficient μ between the lignin particles and the ceramic membrane p-m The rolling friction coefficient μ between different lignin particles when = 3.0 p-p The coordination number distribution;

[0042] Figure 6 The influence of the rolling friction coefficient between the lignin particles and the ceramic membrane on the coordination number distribution, where a is the rolling friction coefficient μ between the lignin particles p-p The rolling friction coefficient μ between different lignin particles and the ceramic membrane when = 0.1 p-m The coordination number distribution under, where b is the rolling friction coefficient μ between the lignin particles p-p The rolling friction coefficient μ between different lignin particles and the ceramic membrane when = 0.6 p-m The coordination number distribution under, where c is the rolling friction coefficient μ between the lignin particles p-p The rolling friction coefficient μ between different lignin particles and the ceramic membrane when = 1.2 p-m The coordination number distribution under, where d is the rolling friction coefficient μ between the lignin particles p-p The rolling friction coefficient μ between different lignin particles and the ceramic membrane when = 1.8 p-m The coordination number distribution under, where e is the rolling friction coefficient μ between the lignin particles p-p The rolling friction coefficient μ between different lignin particles and the ceramic membrane when = 2.4 p-m The coordination number distribution under, where f is the rolling friction coefficient μ between the lignin particles p-p The rolling friction coefficient μ between different lignin particles and the ceramic membrane when = 3.0 p-m The coordination number distribution;

[0043] Figure 7 The influence of the rolling friction coefficient on the porosity of the lignin particle deposition structure;

[0044] Figure 8 For the surface energy γ between the lignin particles and the ceramic membrane p-m = 0.5 J / m 2 The influence of the surface energy between the lignin particles on the deposition morphology of the lignin particles under the condition;

[0045] Figure 9 For the surface energy γ between the lignin particles p-p = 0.5 J / m 2 The influence of the surface energy between the lignin particles and the ceramic membrane on the deposition morphology of the lignin particles under the condition;

[0046] Figure 10The influence of surface energy on the average coordination number;

[0047] Figure 11 The influence of surface energy on the coordination number distribution, where a is the surface energy γ between the lignin particles and the ceramic membrane p-m = 0.5 J / m 2 The influence of the surface energy between lignin particles on the coordination number distribution under the condition, and b is the surface energy γ between lignin particles p-p = 0.5 J / m 2 The influence of the surface energy between the lignin particles and the ceramic membrane on the coordination number distribution under the condition;

[0048] Figure 12 The influence of surface energy on the porosity of the deposition structure of lignin particles. Specific implementation manners

[0049] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0050] The embodiments of the present disclosure provide a method for calibrating the rolling friction coefficient and surface energy in the process of simulating the filtration of lignin by a ceramic membrane using DEM-CFD, including the following steps:

[0051] 1. Construct a semi-analytical DEM-CFD coupling interface

[0052] In this embodiment, a semi-analytical DEM-CFD coupling interface for simulating the process of filtering lignin in black liquor by a ceramic membrane is constructed based on the DEM-CFD coupling method. In the DEM-CFD coupling calculation process, the coupling between DEM and CFD adopts the Eulerian-Eulerian coupling method, which not only considers the momentum exchange between the fluid phase and the particle phase, but also considers the influence of the particle phase on the fluid phase. The translation and rotation of particles in DEM are solved by the explicit time integration method, while the control equation of the fluid in CFD is solved by the SIMPLE algorithm under the pressure-based solver. The pressure term is discretized using the second-order format, and the discretization formats of other terms except the pressure term are all selected as the first-order upwind format. The drag model selects the Ergun / Wen&Yu drag model. The velocity inlet is used for the inlet of black liquor, and the pressure outlet is used for the outlet of black liquor.

[0053] 2. Physical model

[0054] (1) Physical model and calculation domain size setting

[0055] Through the microscopic characterization of the ceramic membrane structure and morphology, it can be found that the filtration channels in the ceramic membrane are formed by the accumulation of ceramic particles similar in shape to spheres. Such a porous ceramic membrane structure has too many membrane pores, which will make it impossible to characterize in detail the deposition morphology of particles in each membrane pore, bringing difficulties to the calibration of the rolling friction coefficient and surface energy. Therefore, the porous ceramic membrane structure is simplified to a single ceramic membrane pore structure. The simplified single ceramic membrane pore structure and the computational domain size are as Figure 1 shown. In addition, the shape of the ceramic particle sphere is also simplified accordingly. A sphere is used to replace the ceramic particle sphere similar in shape to a sphere, and the rolling friction coefficient between the lignin particle and the ceramic membrane is introduced in the solution settings. The computational domain is set in the range of -10μm < X < 10μm, -10μm < Y < 10μm, and -30μm < Z < 30μm. In order to enable the generated particles to fully enter the flow field, the particle generation plane is set at a distance of 2μm from the velocity inlet.

[0056] (2) Boundary condition setting

[0057] According to the characteristics of the ceramic membrane structure, the surface of the ceramic particle sphere is set as a no-slip boundary condition, and the other boundaries of the computational domain are set as symmetric boundary conditions. The total calculation duration is 2.2ms, and the lignin particles are only generated from the particle generation plane within 0 - 2ms, which ensures that all lignin particles in the computational domain have been deposited or flowed out of the computational domain at the end of the simulation. Since the particle size distribution of lignin particles in black liquor is very wide, and considering that it is a very time-consuming task to simulate the motion of tiny particles using the DEM-CFD coupling method, the diameter of the lignin particles is set to 1μm, which can not only reduce the calculation time but also well characterize the micron-sized particles with smaller particle sizes. The main parameters used in the simulation are shown in Table 1.

[0058] Table 1

[0059]

[0060] 3. Deposition process and capture mechanism of lignin particles

[0061] In order to study the kinetic behavior of lignin particles deposited on the ceramic membrane, taking the rolling friction coefficient μ p-p = 2.0 and surface energy γ p-p = 0.6J / m 2 between lignin particles, and the rolling friction coefficient μ p-m = 2.0 and surface energy γ p-m = 0.5J / m 2 as an example, the motion and deposition process of lignin particles in the ceramic membrane pores are simulated. Figure 2This is the change process of the deposition morphology of lignin particles over time. The arrows in the figure indicate the flow direction of the fluid. It can be seen from the figure that the deposition amount of lignin particles continuously increases with time.

[0062] In the initial stage of filtration (A - B), lignin particles gradually reach around the ceramic membrane under the carrying of the fluid. At this time, only a few lignin particles contact the membrane surface of the ceramic membrane and are directly captured by the ceramic membrane under the action of van der Waals adhesion force. The remaining majority of lignin particles pass through the membrane pores and flow out of the computational domain with the fluid. In this stage, lignin particles are mainly captured by the ceramic membrane, and the lignin particles are approximately evenly distributed on the upstream side surface of the ceramic membrane. The number of lignin particles captured by the ceramic membrane is not large.

[0063] As the filtration process proceeds (stage B - C), the deposited lignin particles begin to play a role in the particle capture process. At this time, lignin particles are not only captured by the ceramic membrane but also by the already deposited lignin particles. This also leads to a significant change in the morphology of the deposited lignin particles in this stage. The deposited lignin particles begin to extend outward to form dendritic structures (as shown by the black wireframe in Figure 2 ). In the B - C stage, since the deposited lignin particles start to participate in the capture of lignin particles, the capture efficiency of lignin particles increases.

[0064] As the filtration process further proceeds (C - E), the dendritic structures become more obvious and independent. On the one hand, the dendritic structures continue to capture lignin particles to continue growing. On the other hand, under the action of fluid force, they begin to rotate or bend, etc., increasing the contact area between the dendritic structures and the flowing lignin particles. As a result, at this time, the lignin particles are not completely deposited on the ceramic membrane, but most of them are captured by the dendritic structures, causing the number of captured lignin particles to increase significantly, and the pressure drop also begins to increase significantly, as shown by the pressure drop curve in the C - E stage in Figures 3 - 4 .

[0065] When the injection of lignin particles stops (E - F), the lignin particles on the dendritic structures cannot be replenished. At this time, the dendritic structures continue to bend and rotate under the action of fluid force until the final equilibrium state is reached.

[0066] 4. Influence of the rolling friction coefficient on the deposition of lignin particles in the pores of the ceramic membrane

[0067] The actual lignin particles have greater rolling friction resistance than the ideal spherical lignin particles due to their shape, thus limiting the movement between the lignin particles and between the lignin particles and the ceramic membrane. According to the lignin particle deposition process and capture mechanism, the deposition morphology of the lignin particles is related to the fluid force they are subjected to. When the rolling friction resistance between the deposited lignin particles and between the lignin particles and the ceramic membrane is not enough to resist the fluid force on the lignin particles, the lignin particles will move until a new force equilibrium state is reached again. The increase in the rolling friction coefficient will increase the rolling friction resistance and prevent the particles from rolling. Therefore, the rolling friction coefficient has a significant effect on the dynamic behavior of the lignin particles on the ceramic membrane. This part is constant in surface energy (γ p-p =0.6J / m 2 , γ p-m =1.0J / m 2 ) was used to quantitatively characterize the effect of rolling friction coefficient on the deposition of lignin particles.

[0068] (1) Effect of rolling friction coefficient on the deposition morphology of lignin particles

[0069] The effect of rolling friction coefficient on the deposition morphology of lignin particles is one of the criteria for calibrating the rolling friction coefficient. Figure 3 The deposition morphology of lignin particles under different rolling friction coefficients is given. It can be seen from the figure that regardless of the rolling friction coefficient μ between the lignin particles and the ceramic film, p-m For what value, the deposition morphology of lignin particles changes with the rolling friction coefficient μ between lignin particles. p-p The increase of the rolling friction coefficient between lignin particles shows a strong regularity. p-p ≤0.6), the lignin particles are deposited on the surface of the ceramic membrane in the form of agglomerates, and no dendritic structure is observed to be formed. The reason is that the rolling resistance is too small, which makes it easier for the lignin particles to roll. The dendritic structure formed in the early stage collapses to the surface of the ceramic membrane under the action of fluid force, causing the deposition morphology of the lignin particles to show agglomeration. With the increase of the rolling friction coefficient between the lignin particles, the rolling resistance between the lignin particles increases accordingly, and it is difficult for the lignin particles to rotate around the contact point. At this time, it can be observed that there is an obvious dendritic structure. As the rolling friction coefficient between the lignin particles increases, the dendritic structure becomes more and more obvious and independent, making the deposition morphology of the entire lignin particles look like a forest.

[0070] In addition, from Figure 3It can also be found that the rolling friction coefficient between lignin particles and the ceramic membrane has no obvious effect on the deposition morphology of lignin particles. Under the same rolling friction coefficient between lignin particles, the independence and length of the dendrite structures formed with different rolling friction coefficients between lignin particles and the ceramic membrane are not very different. The reason is that the number of contacts between lignin particles is much larger than the number of contacts between lignin particles and the ceramic membrane. Table 2 shows the proportion of the number of contacts between lignin particles and the ceramic membrane in the total number of contacts under different rolling friction coefficients. It can be seen that the maximum proportion of the number of contacts between lignin particles and the ceramic membrane in the total number of contacts is only 18.4%, which results in the insignificant effect of the rolling friction coefficient between lignin particles and the ceramic membrane on the deposition morphology of lignin particles.

[0071] Table 2

[0072]

[0073] (2) Influence of rolling friction coefficient on the coordination number of lignin particles

[0074] The coordination number (N) refers to the number of particles in contact with the central particle and is considered to be the most sensitive measurement parameter for the local microstructure of particles. Its size reflects the agglomeration properties of the packing structure. It is generally considered that the most reasonable minimum coordination number for 1μm particles is 2, as shown by the deposited particles at the end of the dendrite structure in the black dashed box in Figure 2 However, for the positions where the dendrite structures intersect and the particles overlap and accumulate, the local coordination number will become larger. Therefore, the average coordination number is usually used to characterize the agglomeration properties of particles.

[0075] From the study of the influence of the rolling friction coefficient on the deposition morphology of lignin particles, it can be seen that since the rolling friction coefficient has a significant impact on the equilibrium state of the dendrite structure, which in turn affects the final deposition structure of lignin particles, and the average coordination number is one of the important parameters characterizing the particle deposition structure, the influence of the rolling friction coefficient on the average coordination number of the lignin particle deposition structure was studied, and the rolling friction coefficient was calibrated from the perspective of the average coordination number. Figure 4The curve of the average coordination number varying with the rolling friction coefficient is given. It can be seen from the figure that the average coordination number decreases with the increase of the rolling friction coefficient between lignin particles. When the rolling friction coefficient between lignin particles increases from 0.1 to 3.0, the average coordination number decreases from 3.96 to 2.73. The decrease in the average coordination number indicates that with the increase of the rolling friction coefficient between lignin particles, the number of contacts between lignin particles gradually decreases, and the dendritic structure becomes more independent, which is also consistent with the variation law of the deposition morphology of lignin particles. In addition, it can also be found from the figure that the rolling friction coefficient between lignin particles and the ceramic membrane has little effect on the average coordination number, because the number of contacts between lignin particles and the ceramic membrane is too small. The average coordination number of 1μm particles is 2.13, and the existence of fluid force will increase the average coordination number. Therefore, the reasonable and acceptable minimum average coordination number is 2.13, that is, in terms of only meeting the requirements of the average coordination number, the rolling friction coefficients within the scope of this simulation can all meet the requirements of the average coordination number.

[0076] In addition, the distribution of the coordination number with respect to the rolling friction coefficient was also statistically analyzed. Figure 5 The influence of the rolling friction coefficient between lignin particles on the distribution of the coordination number is given. The increase in the rolling friction coefficient between lignin particles makes the dendritic structure formed by the deposition of lignin particles more independent, and the distribution range of the coordination number decreases with the increase of the rolling friction coefficient between lignin particles. For 1μm particles under only the action of gravity, the distribution range of their coordination numbers is mainly between 1 and 4. Due to the action of fluid force, the contacts between particles become closer, which will expand the distribution range of the coordination number.

[0077] Figure 6 The influence of the rolling friction coefficient between lignin particles and the ceramic membrane on the distribution of the coordination number is given. It can be seen from the figure that the rolling friction coefficient between lignin particles and the ceramic membrane has no obvious influence on the distribution of the coordination number. That is to say, the rolling friction coefficient between lignin particles is the main factor affecting the distribution of the coordination number.

[0078] (3) Influence of the rolling friction coefficient on the porosity of the lignin particle deposition structure

[0079] The porosity (ε) is another important parameter used to describe the properties of the particle packing structure, and its size reflects the compactness of the particle packing structure. The size of the porosity can be calculated by the following formula (3-1):

[0080]

[0081] where ε is the porosity, is the average coordination number, m and n are constants, which are 2.02, 87.38, and 25.81 respectively.

[0082] The influence of the rolling friction coefficient on the equilibrium state of the dendritic structure also affects the porosity of the lignin particle deposition structure. During the filtration process, the size of the filter cake porosity directly determines the permeability of the filter cake and has an important impact on the filtration performance. Therefore, taking porosity as a calibration standard, the influence of the rolling friction coefficient on the porosity of the lignin particle packing structure was studied. The change curve of the calculated porosity with the rolling friction coefficient is as Figure 7 shown. The size of the porosity is only related to the average coordination number. Since the rolling friction coefficient between the lignin particles and the ceramic membrane has no effect on the average coordination number, the rolling friction coefficient between the lignin particles and the ceramic membrane has no effect on the porosity either. When the rolling friction coefficient between the lignin particles increases from 0.1 to 3.0, the porosity increases from 0.65 to 0.73. This is because the increase in the rolling friction coefficient between the lignin particles increases the rolling resistance between the lignin particles, enhances the interaction between the lignin particles, and makes the dendritic structure more independent, resulting in an increase in porosity. In practice, the porosity of the filter cake formed by lignin particles is between 0.61 and 0.71. When the rolling friction coefficient between the lignin particles is in the range of 0.1 - 2.4, the porosity is between 0.65 and 0.72.

[0083] By analyzing the deposition morphology of lignin particles under different rolling friction coefficients, the change curve of the average coordination number with the rolling friction coefficient, the influence of the rolling friction coefficient between lignin particles on the coordination number distribution, the influence of the rolling friction coefficient between lignin particles and the ceramic membrane on the coordination number distribution, and the change curve of the porosity of the lignin particle deposition structure with the rolling friction coefficient, it is determined that setting the rolling friction coefficient between lignin particles and the rolling friction coefficient between lignin particles and the ceramic membrane between 0.1 and 2.4 during the simulation is more in line with the actual porosity situation;

[0084] 5. Influence of surface energy on the deposition of lignin particles in the pores of the ceramic membrane

[0085] The main reason for the influence of the rolling friction coefficient on the deposition of lignin particles is that an increase in the rolling friction coefficient will increase the rolling friction resistance of the particles. Different from this, the setting of the surface energy will increase the van der Waals adhesion force between lignin particles and between lignin particles and the ceramic membrane, enhance the ability of the dendritic structure to resist hydrodynamic forces, and thus affect the deposition process of lignin particles. Therefore, according to the calibration result of the rolling friction coefficient, the rolling friction coefficient between lignin particles and between lignin particles and the ceramic membrane is set to 2, and under the condition of a constant rolling friction coefficient, the influence of surface energy on the deposition of lignin particles is studied from three aspects: the deposition morphology of lignin particles, the coordination number, and the porosity.

[0086] (1) Influence of surface energy on the deposition morphology of lignin particles

[0087] The setting of surface energy will affect the equilibrium state of the dendritic structure, and then affect the deposition morphology of lignin particles. Therefore, the surface energy γ between lignin particles and ceramic membrane p-m =0.5J / m 2 Under the conditions, the effect of different surface energies between lignin particles on the deposition morphology of lignin particles was studied. The results are as follows Figure 8 As shown in the figure, it can be seen that the surface energy between lignin particles has a significant effect on the deposition morphology of lignin particles. The setting of surface energy can generate van der Waals adhesion between lignin particles, which brings certain resistance to the movement between particles. In the JKR model, the adhesion force and the torque caused by the adhesion force are related to γ 12 Therefore, the greater the surface energy between lignin particles, the greater the adhesion between particles. When the surface energy between lignin particles is small (γ p-p <0.2J / m 2 ), although lignin particles can be deposited on the membrane surface, due to the small surface energy, the adhesion force between lignin particles is not enough to resist the driving force generated by the fluid flow, causing the dendrite structure to bend and fall under the action of the fluid force, and finally reach the final equilibrium state on the downstream side of the ceramic membrane, showing the morphology of the dendrite structure growing along the direction of fluid flow. As the surface energy between lignin particles continues to increase (γ p-p =0.2-1.0J / m 2 ), the adhesion between lignin particles is also increasing, the ability of the dendritic structure to resist bending and lodging is increasing, and independent dendritic structures begin to grow on the upstream side of the ceramic membrane. In addition, the greater the surface energy between lignin particles, the greater the contact stiffness between the particles, making it more difficult for the particles to rotate around the contact points, and the dendritic structure is also more difficult to bend. This makes it difficult for the position of lignin particles in the dendritic structure to change, and the independence of the dendritic structure is also stronger, making the deposition morphology of the entire lignin particle look like a forest.

[0088] Figure 9 The surface energy γ between lignin particles is given by p-p =0.5J / m 2 Under the same conditions, the influence of the surface energy between lignin particles and ceramic membrane on the deposition morphology of lignin particles is not significant. The degree of agglomeration of particles, the length of dendritic structure and the countercurrent growth are similar. The reason is that the number of contacts between lignin particles and ceramic membrane is too small.

[0089] (2) Effect of surface energy on the coordination number of lignin particles

[0090] In addition to affecting the deposition morphology of lignin particles, the surface energy setting also has an impact on the coordination number of lignin particles. Therefore, in this section, the influence of surface energy on the average coordination number and the coordination number distribution of lignin particles was studied. Figure 10 The curve of the average coordination number varying with the surface energy is given. It can be seen from the figure that as the surface energy between lignin particles continuously increases, the average coordination number also increases continuously, and the increasing trend becomes slower and slower. When the surface energy between lignin particles increases from 0.01 J / m2 to 1.0 J / m2, the average coordination number increases from 2.03 to 3.69, which is contrary to the influence law of the rolling friction coefficient on the average coordination number. Although the increase in surface energy makes the dendritic structure become independent and reduces the average coordination number, the increase in the surface energy between lignin particles will increase the normal overlap amount between lignin particles, resulting in the gradual increase of the average coordination number with the increase of the surface energy between lignin particles. It has been previously studied the influence of the rolling friction coefficient on the coordination number of lignin particles, and the reasonable and acceptable minimum average coordination number of lignin particles with a particle size of 1 μm is 2.13. Therefore, simply from the aspect of meeting the requirement of the minimum average coordination number, the reasonable and acceptable minimum surface energy between lignin particles is 0.02 J / m2. In addition, when the surface energy between lignin particles is constant, the average coordination number fluctuates irregularly with the change of the surface energy between lignin particles and the ceramic membrane. This indicates that the surface energy between lignin particles and the ceramic membrane has no obvious influence on the average coordination number, and the reason is also that the contact number between lignin particles and the ceramic membrane is too small.

[0091] Figure 11 The influence of surface energy on the coordination number distribution is given. It can be seen from the figure that the surface energy between lignin particles and the ceramic membrane has no obvious influence on the coordination number distribution, and the surface energy between lignin particles is still the main factor affecting the coordination number distribution. As the surface energy between lignin particles gradually increases, the normal overlap amount between lignin particles also increases continuously, resulting in the gradual increase of the distribution range of the coordination number.

[0092] (3) Influence of surface energy on the porosity of the lignin particle deposition structure

[0093] Similarly, the surface energy setting also has an impact on the porosity of the lignin particle deposition structure. The curve of the porosity calculated by formula (3-1) varying with the surface energy is as Figure 12As shown, the surface energy between lignin particles and the ceramic membrane has no effect on the porosity. The porosity only gradually decreases with the increase of the surface energy between lignin particles. When the surface energy between lignin particles increases from 0.01 J / m2 to 1.0 J / m2, the porosity decreases from 0.91 to 0.66. This is because with the increase of the surface energy between lignin particles, the normal overlap amount between lignin particles also increases, resulting in the decrease of porosity. In practice, the porosity of the filter cake formed by lignin particles is between 0.61 and 0.71. When the surface energy between lignin particles is in the range of 0.2 - 1.0 J / m2, the porosity is between 0.66 and 0.71.

[0094] By analyzing the deposition morphology of lignin particles with different surface energies, the curve of the average coordination number varying with the surface energy, the influence of the surface energy between lignin particles on the coordination number distribution, the influence of the surface energy between lignin particles and the ceramic membrane on the coordination number distribution, and the curve of the porosity of the lignin particle deposition structure varying with the surface energy, it is determined that setting the surface energy between lignin particles and the surface energy between lignin particles and the ceramic membrane in the range of 0.2 - 1.0 J / m2 during the simulation process is more in line with the actual porosity situation.

[0095] Up to this point in the embodiments of the present invention, the technical solutions of the present invention have been described in combination with the preferred embodiments shown in the drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the protection scope of the present invention.

Claims

1. A method for calibrating rolling friction coefficient and surface energy in DEM-CFD simulation of ceramic membrane filtration of lignin, characterized in that: The steps include: Step 1: Based on the DEM-CFD coupling method, a semi-analytical DEM-CFD coupling interface is constructed to simulate the lignin process in black liquor filtered by ceramic membranes; Step 2: Simplify the shapes of lignin particles and ceramic particles into spheres, simplify the porous ceramic membrane structure into a single ceramic membrane pore structure, and set the calculation domain size and boundary conditions; Step three: Based on the physical model of the pores of a single ceramic membrane, the semi-analytical DEM-CFD coupling interface is used to simulate the process of ceramic membrane filtering lignin in black liquor. The rolling friction coefficient and surface energy are calibrated by analyzing the movement of lignin particles in the pores of the ceramic membrane and the deposition process during the process of ceramic membrane filtering lignin in black liquor. The rolling friction coefficient includes the rolling friction coefficient between lignin particles and the rolling friction coefficient surface energy between lignin particles and the ceramic membrane, and the surface energy includes the surface energy between lignin particles and the surface energy between lignin particles and the ceramic membrane.

2. The method for calibrating the rolling friction coefficient and surface energy in the DEM-CFD simulation of ceramic membrane filtration of lignin according to claim 1, characterized in that: The coupling between DEM and CFD of the semi-analytical DEM-CFD coupling interface adopts the Eulerian-Eulerian coupling method, the translation and rotation of lignin particles in DEM are solved by the displayed time integration method, the control equation of the fluid in CFD is solved by the SIMPLE algorithm under the pressure-based solver, the pressure term is discretized using the second-order format, and the discrete formats of other terms except the pressure term are all first-order upwind formats. The drag model selects the Ergun / Wen&Yu drag model, the velocity inlet is used for the inlet of black liquor, and the pressure outlet is used for the outlet of black liquor.

3. The method for calibrating the rolling friction coefficient and surface energy in the DEM-CFD simulation of ceramic membrane filtration of lignin according to claim 1, characterized in that: In the step three, the rolling friction coefficient and the surface energy are calibrated by analyzing the movement of lignin particles in the pores of the ceramic membrane and the deposition process during the lignin filtration in the black liquor by the ceramic membrane, including: Determine the lignin particle deposition process and capture mechanism according to the movement and deposition process of lignin particles in the pores of the ceramic membrane during the filtration of lignin in black liquor by the ceramic membrane, wherein the lignin particle deposition process and capture mechanism include the initial deposition of lignin particles on the ceramic membrane, the formation of dendritic structure and the final process of tending to equilibrium; According to the deposition process and capture mechanism of lignin particles, it is determined that the rolling friction coefficient and surface energy have an influence on the deposition of lignin particles in the pores of the ceramic membrane, and the influence of the rolling friction coefficient and surface energy on the deposition of lignin particles in the pores of the ceramic membrane is quantitatively characterized to determine the calibration values ​​of the rolling friction coefficient and surface energy.

4. The method for calibrating the rolling friction coefficient and surface energy in the DEM-CFD simulation of ceramic membrane filtration of lignin according to claim 3 is characterized in that: The method of determining the lignin particle deposition process and capture mechanism according to the movement and deposition process of lignin particles in the pores of the ceramic membrane during the lignin filtration process in black liquor by the ceramic membrane includes: Set the rolling friction coefficient and surface energy between lignin particles, set the rolling friction coefficient and surface energy between lignin particles and ceramic membrane, simulate the movement and deposition process of lignin particles in the pores of ceramic membrane, and plot the deposition morphology change process of lignin particles over time based on the simulation result data; The initial stage of the morphological change of lignin particle deposition was analyzed, and it was determined that in the initial stage, a small number of lignin particles contacted the ceramic membrane surface and were directly captured by the ceramic membrane under the action of van der Waals adhesion, while most of them passed through the ceramic membrane pores and flowed out of the calculation domain with the fluid. The second stage of the morphological change process of lignin particle deposition was further analyzed, and it was determined that in the second stage, the lignin particles were not only captured by the ceramic membrane, but also by the already deposited lignin particles, and the deposited lignin particles began to extend outward to form a dendritic structure. Further analysis of the third stage of the morphological changes in the deposition of lignin particles revealed that in the third stage, the dendritic structure continued to capture the continuous growth of lignin particles, and on the other hand, it began to rotate, bend, fold and break under the action of fluid force, increasing the contact area between the dendritic structure and the flowing lignin particles. Most of the lignin particles were captured by the dendritic structure. Finally, the injection of lignin particles is stopped, and the dendritic structure continues to rotate, bend, fold and break under the action of fluid force until it reaches a state of equilibrium.

5. The method for calibrating the rolling friction coefficient and surface energy in the DEM-CFD simulation of ceramic membrane filtration of lignin according to claim 4, characterized in that: The quantitative characterization includes the deposition morphology, average coordination number, coordination number and porosity of the deposition structure of the lignin particles; Accordingly, according to the deposition process and capture mechanism of lignin particles, it is determined that the rolling friction coefficient and surface energy have an influence on the deposition of lignin particles in the pores of the ceramic membrane, and the influence of the rolling friction coefficient and surface energy on the deposition of lignin particles in the pores of the ceramic membrane is quantitatively characterized to determine the calibration values ​​of the rolling friction coefficient and surface energy, including: According to the deposition process and capture mechanism of lignin particles, the deposition morphology of lignin particles is related to the fluid force, rolling friction resistance and van der Waals adhesion between the deposited lignin particles and between the lignin particles and the ceramic membrane. The rolling friction resistance and van der Waals adhesion are related to the rolling friction coefficient and surface energy, respectively. It is determined that the rolling friction coefficient and surface energy have an effect on the deposition of lignin particles in the pores of the ceramic membrane. Under the condition of constant surface energy, different rolling friction coefficients were set to simulate the process of ceramic membrane filtering lignin in black liquor, and the deposition morphology of lignin particles under different rolling friction coefficients was plotted based on the simulation results. Under the condition of constant surface energy, different rolling friction coefficients were set to simulate the process of ceramic membrane filtering lignin in black liquor. According to the simulation results, the curves of average coordination number changing with rolling friction coefficient, the influence of rolling friction coefficient between lignin particles on coordination number distribution, and the influence of rolling friction coefficient between lignin particles and ceramic membrane on coordination number distribution were drawn. Under the condition of constant surface energy, different rolling friction coefficients were set to simulate the process of ceramic membrane filtering lignin in black liquor, and the curve of the porosity of the lignin particle deposition structure changing with the rolling friction coefficient was drawn based on the simulation result data. By analyzing the deposition morphology of lignin particles under different rolling friction coefficients, the curve of the average coordination number changing with the rolling friction coefficient, the influence of the rolling friction coefficient between lignin particles on the coordination number distribution, the influence of the rolling friction coefficient between lignin particles and ceramic film on the coordination number distribution, and the curve of the porosity of the deposition structure of lignin particles changing with the rolling friction coefficient, it is determined that the key factors affecting the deposition morphology, average coordination number, coordination number distribution and porosity of the deposition structure of lignin particles are the rolling friction coefficient between lignin particles and the calibration value of the rolling friction coefficient between lignin particles. Under the condition that the rolling friction coefficient between lignin particles and the rolling friction coefficient between lignin particles and ceramic membrane are constant, different surface energies are set to simulate the process of ceramic membrane filtering lignin in black liquor, and the deposition morphology of lignin particles under different surface energies is plotted based on the simulation result data; Under the condition that the rolling friction coefficient between lignin particles and the rolling friction coefficient between lignin particles and ceramic membranes are constant, different surface energies are set to simulate the process of ceramic membrane filtering lignin in black liquor. According to the simulation results, the curves of average coordination number changing with the surface energy between lignin particles, the influence of surface energy between lignin particles on the distribution of coordination number, and the influence of surface energy between lignin particles and ceramic membrane on the distribution of coordination number are plotted; Under the condition that the rolling friction coefficient between lignin particles and the rolling friction coefficient between lignin particles and ceramic membranes are constant, different surface energies are set to simulate the process of ceramic membrane filtering lignin in black liquor, and a curve of the porosity of the lignin particle deposition structure changing with the surface energy is drawn based on the simulation result data; By analyzing the deposition morphology of lignin particles under different surface energies, the curve of the average coordination number varying with surface energy, the influence of surface energy between lignin particles on the coordination number distribution, the influence of surface energy between lignin particles and ceramic membrane on the coordination number distribution, and the curve of the porosity of the deposition structure of lignin particles varying with surface energy, it is determined that the key factors affecting the deposition morphology, average coordination number, coordination number distribution and porosity of the deposition structure of lignin particles are the surface energy between lignin particles and the calibrated value of the surface energy between lignin particles.

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