Calculation method and application of theoretical torque of rotor of scraper film evaporator

Abstract

The invention belongs to the technical field of fluid simulation and scraper type film evaporator optimization, and particularly relates to a calculation method and application of theoretical torque of a rotor of a scraper type film evaporator. According to the theoretical torque splitting calculation method based on the power characteristics, the total power consumption of the scraper type film evaporator is split into viscous torque and liquid mass load torque, contribution mechanisms of different operation parameters to power are disclosed, the optimization direction is accurately positioned, and the association between the dynamic characteristics of a flow field and the power is determined; the theoretical moment splitting method provided by the invention realizes accurate quantification of power consumption, provides a theoretical basis and a systematic solution for energy-saving design of the scraper film evaporator, and has engineering application value.

Description

Technical Field

[0001] The invention belongs to the technical field of fluid simulation and scraper film evaporator optimization, and in particular relates to a calculation method and application of a theoretical torque of a rotor of a scraper film evaporator. Background Art

[0002] As global energy demand continues to rise, energy consumption and environmental pollution issues are becoming increasingly severe. How to reduce energy consumption and emissions while improving energy efficiency has become a global focus. In the industrial field, thin film evaporators are key heat exchange equipment, and their energy efficiency is directly related to the energy saving and consumption reduction effect of the entire system. Therefore, optimizing the design and operation mode of thin film evaporators and reducing power consumption have become one of the core technologies to enhance industrial energy saving effects.

[0003] Scraped film evaporators are widely used in the chemical, food, pharmaceutical and other industries, especially when dealing with the evaporation of high-viscosity liquids, they show high thermal efficiency and thermal stability. The evaporator is mainly composed of a heating jacket and a scraper. The heating steam is passed into the jacket. The scraper is installed on a rotatable shaft. The gap between the scraper and the inner wall of the heating jacket is extremely small, generally between 0.5 and 1.5 mm. The preheated liquid enters the evaporator from the top along the tangential direction, and under the combined action of gravity and the rotating scraper, it is distributed on the inner wall to form a downward rotating film. During the descent process, the liquid continues to evaporate and concentrate, and is finally discharged from the bottom, and the secondary steam escapes from the top. However, due to its special structure, the energy efficiency and processing capacity of the scraped film evaporator are restricted by factors such as flow field distribution and power consumption.

[0004] At present, the research on the flow field of scraped film evaporator is mostly based on the in-line scraper blade structure, and is often simplified to only retain four rows of blades. Under this assumption, the classic flow field morphology theory is formed. Figure 1 As shown in the figure, the flow field of the in-line scraper film evaporator includes a liquid film and a liquid mass. The liquid film forms a thin layer on the wall 1, and relies on the scraper 2 to maintain uniformity. The liquid mass is the accumulation area of ​​the liquid in front of the scraper 2. The outer side of the wall 1 is the heating sleeve 3, and the scraper 2 is driven by the rotating shaft 4. The power consumption of the scraper film evaporator mainly comes from the shear force of the liquid film and the load of the liquid mass. The shear force of the liquid film is generated by the relative movement of the liquid and the wall, causing a certain viscous resistance; the load of the liquid mass comes from its own mass and gravity.

[0005] The existing technology has studied the energy efficiency optimization of scraped film evaporators. For example, the literature (Study on the scale-up of multi-stage agitators under laminar and transitional flow conditions: a computational fluid dynamics approach [J]. Chemical Engineering Science, 2002, 57(21): 4617-4632.) used computational fluid dynamics (CFD) technology to study three multi-stage agitators with different size ratios, obtained the corresponding power characteristic curves, and verified the reliability of the simulation results through experimental data. The literature (Numerical simulation of flow field characteristics of non-Newtonian fluid in scraped film evaporator [J]. Acta Physica Sinica, 2022, 71(05): 197-208.) conducted numerical simulation of non-Newtonian fluid in scraped film evaporator, explored the influence of scraper bending angle and speed on rotor torque, and obtained the influence of process parameters on power.

[0006] However, these existing technologies all obtain the torque of the rotor of the scraped film evaporator through simulation, and then multiply it by the angular velocity to obtain the power result. This method is difficult to explore the specific factors that affect the power, and thus it is difficult to guide production practice. Summary of the invention

[0007] The purpose of the present invention is to solve the problems existing in the prior art and to provide a method for calculating the theoretical torque of a rotor of a scraped film evaporator and its application.

[0008] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] A method for calculating the theoretical torque of the rotor of a scraped film evaporator uses the following formula:

[0010] ;

[0011] In the formula, Represents the theoretical torque of the rotor of the scraped film evaporator, in N·m, which can be converted to kg·m 2 ·S -2 ;

[0012] Represents the viscous torque of the scraped film evaporator rotor, in N·m; It is one of the key factors affecting power consumption and is related to the relative shear between fluid layers. Its size is closely related to the fluid viscosity, flow velocity gradient and liquid film area.

[0013] Represents the liquid mass load moment of the scraped film evaporator rotor, that is, the load moment generated by the mass of the liquid mass itself and the gravity, in N·m;

[0014] ;

[0015] In the formula, represents the moment arm, in mm;

[0016] Represents the dynamic viscosity of the material, the unit is Pa·s, which can be converted to kg·m -1 ·S -1 ;

[0017] Represents the distance between the heating wall of the scraped film evaporator and the central axis of the rotor, in mm;

[0018] Represents the thickness of the liquid film, in mm;

[0019] represents the circumferential contact angle, in degrees;

[0020] H represents the height of the scraped film evaporator, in mm;

[0021] Represents the shear rate of liquid film flow, in s -1 ;in, represents the velocity component of the fluid in the x direction, that is, the velocity parallel to the wall; y represents the vertical distance between the fluid data point and the wall; Describes the rate of change of fluid velocity in the direction perpendicular to the flow direction (y direction), reflecting the relative movement between fluid layers;

[0022] ;

[0023] In the formula, Represents the density of the material in kg / m 3 ;

[0024] Represents the total volume of material in the scraped film evaporator when the three-dimensional flow field is stable, in units of , the three-dimensional flow field is stable when the following conditions are met at the same time:

[0025] Condition 1: The residual curve (i.e., convergence curve) begins to show a continuous downward trend and eventually flattens to 10 -3 The magnitude of the simulation is always 10 -3 Magnitude;

[0026] Condition 2: The torque curve tends to be flat, and the fluctuation range is less than 5% within 1000 iteration steps;

[0027] Condition 3: The outlet flow curve starts to show an upward trend and eventually flattens out, that is, the difference between the outlet flow and the inlet flow setting is within 5% within 1000 iterations;

[0028] Represents the acceleration due to gravity, and its value is 9.81 m·s -2 ; The size of is directly related to the volume of the liquid mass (i.e. the mass of the liquid mass). When the volume of the liquid mass increases, As a result, the power increases.

[0029] As the preferred technical solution:

[0030] The above-mentioned method for calculating the theoretical torque of the rotor of a scraped film evaporator is as follows: The process of obtaining the value of is as follows: after obtaining the stable three-dimensional flow field of the scraped film evaporator through Fluent software simulation, the three-dimensional flow field map is extracted using CFD-Post post-processing software, and then the axial cross-sectional cloud map of the three-dimensional flow field map is obtained, and the grid with a volume fraction of 0.5 in the liquid part is screened out, and its distance from the heating wall is calculated, and the average value is taken, which is obtained. The value of .

[0031] The above-mentioned method for calculating the theoretical torque of the rotor of a scraped film evaporator is as follows: The process of obtaining the value of is as follows: first, the stable three-dimensional flow field of the scraped film evaporator is obtained by simulating with Fluent software, and then the three-dimensional flow field map is extracted by CFD-Post post-processing software. Then, the circumferential cross-sectional cloud map of the three-dimensional flow field map is obtained, and then the outermost circle with a width of The annular area (i.e., the liquid film part) is selected, and the grids with a volume fraction of 0.5 of the liquid part in the remaining area are selected. The least squares method is used to fit these grids around each liquid mass to obtain the circular boundary of each liquid mass. Finally, the distance between the center of the circular boundary of each liquid mass and the central axis of the rotor is calculated, and the average value is taken to obtain The value of .

[0032] The above-mentioned method for calculating the theoretical torque of the rotor of a scraped film evaporator is as follows: The value is obtained by simulating the stable three-dimensional flow field of the scraped film evaporator with Fluent software, and then extracting the average value of the dynamic viscosity at the interface between the liquid mass and the liquid film using CFD-Post post-processing software.

[0033] The above-mentioned method for calculating the theoretical torque of the rotor of a scraped film evaporator is as follows: The value of is obtained by simulating the stable three-dimensional flow field of the scraped film evaporator with Fluent software, and then extracting the central angle of the arc surface corresponding to the contact area between the liquid mass and the liquid film with CFD-Post post-processing software.

[0034] The above-mentioned method for calculating the theoretical torque of the rotor of a scraped film evaporator is as follows: The value of is obtained by simulating the stable three-dimensional flow field of the scraped film evaporator with Fluent software, and then extracting the average shear rate at the interface between the liquid mass and the liquid film using CFD-Post post-processing software.

[0035] The above-mentioned method for calculating the theoretical torque of the rotor of a scraped film evaporator is as follows: The process of obtaining the value of is as follows: using Fluent software simulation, during which the liquid holdup data of the scraped film evaporator is monitored in real time. After obtaining a stable three-dimensional flow field of the scraped film evaporator, the corresponding liquid holdup data is directly extracted from the data file exported by real-time monitoring, that is, The value of .

[0036] The present invention also provides a method for reducing the power of a scraped film evaporator, wherein the viscous torque of the scraped film evaporator rotor and the liquid mass load torque of the scraped film evaporator rotor are calculated respectively, and then the two are compared. If the viscous torque of the scraped film evaporator rotor is large, the scraper speed is reduced and / or the scraper gap is increased; if the viscous torque of the scraped film evaporator rotor is small or the two are equal, the feed amount is reduced;

[0037] The calculation adopts the calculation method of the theoretical torque of the rotor of a scraped film evaporator as described in any one of the above items.

[0038] Beneficial effects:

[0039] 1. The present invention proposes a theoretical torque splitting calculation method based on power characteristics. This method splits the total power consumption into viscous torque and liquid mass load torque, reveals the contribution mechanism of different operating parameters to power, accurately locates the optimization direction, and clarifies the relationship between flow field dynamic characteristics and power;

[0040] 2. The theoretical moment splitting method proposed in the present invention realizes the accurate quantification of power consumption, provides a theoretical basis and systematic solution for the energy-saving design and efficient operation of the scraped film evaporator, and has significant engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a schematic diagram of a scraped film evaporator ( Figure 1 a) and flow field diagram ( Figure 1 (middle b);

[0042] Figure 2 This is a quasi-slit flow field diagram;

[0043] Figure 3 The three-dimensional flow field diagram of the stable scraped film evaporator ( Figure 3a) and the circumferential cross-sectional cloud diagram of the stable scraped film evaporator three-dimensional flow field diagram ( Figure 3 (middle b);

[0044] Figure 4 The axial cross-section cloud diagram of the three-dimensional flow field diagram of a stable scraped-film evaporator and the schematic diagram of the calculation method of the liquid film thickness;

[0045] Figure 5 This is the fitting result of the liquid mass periphery data;

[0046] Figure 6 Schematic diagram of the contact area between the liquid mass and the liquid film;

[0047] Figure 7 a is the simulated torque of the rotor of the scraped film evaporator at different speeds. Figure 7 b in the figure is the theoretical torque and its components (viscous torque, liquid mass load) of the scraped film evaporator rotor at different speeds;

[0048] Figure 8 a is the simulated torque of the rotor of the scraper film evaporator under different scraper gaps, Figure 8 b in the figure is the theoretical torque and its components (viscous torque, liquid mass load) of the scraped film evaporator rotor under different scraper gaps;

[0049] Fig. 9 a is the simulated torque of the rotor of the scraped film evaporator under different feed rates, Fig. 9 b in the figure is the theoretical torque and its components (viscous torque, liquid mass load) of the scraped film evaporator rotor under different feed rates;

[0050] Among them, 1-wall, 2-scraper, 3-heating sleeve, 4-rotating shaft. DETAILED DESCRIPTION

[0051] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0052] The power consumption of the scraped film evaporator comes from the mechanical energy dissipation and load work during the fluid flow process. Its physical mechanism can be systematically analyzed based on the theory of fluid dynamics and rigid body mechanics. The power consumption splitting theory is based on the principle of energy conservation, which decomposes the total power into two parts: viscous shear power and liquid mass load power, which correspond to the energy dissipation of fluid viscosity effect and gravity effect respectively.

[0053] Shear rate of liquid film flow Characterizes the relative movement intensity between fluid layers, and its magnitude is affected by the scraper speed It is affected by the thickness of the liquid film; Figure 2 As shown in the simplified model, assuming that the velocity distribution in the liquid film is linear, then:

[0054] ;

[0055] in, Represents the shear rate of liquid film flow, in s -1 ; represents the velocity difference between the top and bottom of the liquid film (e.g. Figure 2 As shown, = , where represents the top speed, represents the bottom velocity), in m / s; Represents the gap height, in m;

[0056] Viscous shear power is the energy dissipated by the fluid near the wall due to the velocity gradient. Its theoretical basis is the Navier-Stokes equation and Newton's law of internal friction. Under laminar flow conditions, the shear stress is proportional to the shear rate of the liquid film flow, that is:

[0057] ;

[0058] in, Represents shear stress in kg·m -1 ·s -2 ; Represents the dynamic viscosity of the material, in Pa·s (can be converted to kg·m -1 ·s -1 );

[0059] The shear stress acts on the effective area of ​​the liquid film, which in turn forms the viscous moment term that constitutes the rotor of the scraped film evaporator:

[0060] ;

[0061] in, Represents the viscous torque of the scraped film evaporator rotor, in N·m; represents the moment arm (i.e., the distance between the center of mass of the liquid mass and the axis of rotation), in mm; Represents the contact area between the liquid mass and the liquid film, in m 2 ;

[0062] The viscous power corresponding to the viscous torque of the scraped film evaporator rotor is:

[0063] ;

[0064] in, Represents the viscous power corresponding to the viscous torque of the scraped film evaporator rotor, in W; Represents angular velocity in rad / s;

[0065] The liquid mass load power is generated by the moment work of gravity on the center of mass of the liquid mass. Its theoretical model is based on rigid body statics and the law of conservation of energy. For the mechanical analysis of gravity moment, the mass of the liquid mass is affected by gravity, so the liquid mass load moment term of the scraped film evaporator rotor is:

[0066] ;

[0067] in, Represents the liquid mass load torque of the scraped film evaporator rotor, in N·m; Represents the mass of the liquid mass, in kg; Represents the acceleration due to gravity, and its value is 9.81 m·s -2 ;

[0068] The load power corresponding to the liquid mass load torque of the scraped film evaporator rotor is:

[0069] ;

[0070] in, Represents the load power corresponding to the liquid mass load torque of the scraped film evaporator rotor, in W;

[0071] The mass of the liquid mass depends on the scraper gap and the flow field distribution. When the scraper gap decreases, the volume of the liquid mass in front of the scraper increases, resulting in an increase in the mass of the liquid mass. The scraper speed changes the dynamic accumulation morphology of the liquid mass through centrifugal force, which indirectly affects the instantaneous value of the mass of the liquid mass.

[0072] Based on the above physical mechanism, the total power of the scraped film evaporator can be expressed as the linear superposition of viscous power and load power:

[0073] ;

[0074] in, Represents the total power of the scraped film evaporator, in W;

[0075] This formula establishes the relationship between power consumption and flow field parameters. and operating parameters The theoretical rationality of the quantitative relationship is based on the following assumptions and boundary conditions:

[0076] a. Quasi-steady-state flow assumption, that is, the dynamic change frequency of the liquid film and liquid mass is much lower than the rotor rotation frequency, and the instantaneous power can be approximated to the steady-state value;

[0077] b. The center of mass equivalence assumption, that is, the gravity of the liquid mass is equivalent to the concentrated load at the center of mass, and the dynamic changes of the center of mass position with the flow field can be ignored;

[0078] c. The thickness of the liquid film is constrained by the scraper gap and the fluid flow rate, satisfying the mass conservation condition.

[0079] Based on the above research, the present invention proposes a method for calculating the theoretical torque of the rotor of a scraped film evaporator, using the following formula:

[0080] ;

[0081] In the formula, Represents the theoretical torque of the rotor of the scraped film evaporator, in N·m, which can be converted to kg·m 2 ·S -2 ;

[0082] Represents the viscous torque of the scraped film evaporator rotor, in N·m; It is one of the key factors affecting power consumption and is related to the relative shear between fluid layers. Its size is closely related to the fluid viscosity, flow velocity gradient and liquid film area.

[0083] Represents the liquid mass load moment of the scraped film evaporator rotor, that is, the load moment generated by the mass of the liquid mass itself and the gravity, in N·m;

[0084] ;

[0085] In the formula, represents the moment arm, in mm;

[0086] Represents the dynamic viscosity of the material, the unit is Pa·s, which can be converted to kg·m -1 ·S -1 ;

[0087] Represents the distance between the heating wall of the scraped film evaporator and the central axis of the rotor, in mm;

[0088] Represents the thickness of the liquid film, in mm;

[0089] represents the circumferential contact angle, in degrees;

[0090] H represents the height of the scraped film evaporator, in mm;

[0091] Represents the shear rate of liquid film flow, in s -1 ;in, represents the velocity component of the fluid in the x direction, that is, the velocity parallel to the wall; y represents the vertical distance between the fluid data point and the wall; Describes the rate of change of fluid velocity in the direction perpendicular to the flow direction (y direction), reflecting the relative movement between fluid layers;

[0092] ;

[0093] In the formula, Represents the density of the material in kg / m 3 ;

[0094] Represents the total volume of material in the scraped film evaporator when the three-dimensional flow field is stable, in units of , the three-dimensional flow field is stable when the following conditions are met at the same time:

[0095] Condition 1: The residual curve (i.e., convergence curve) begins to show a continuous downward trend and eventually flattens to 10 -3 The magnitude of the simulation is always 10 -3 Magnitude;

[0096] Condition 2: The torque curve tends to be flat, and the fluctuation range is less than 5% within 1000 iteration steps;

[0097] Condition 3: The outlet flow curve starts to show an upward trend and eventually flattens out, that is, the difference between the outlet flow and the inlet flow setting is within 5% within 1000 iterations;

[0098] Represents the acceleration due to gravity, and its value is 9.81 m·s -2 ; The size of is directly related to the volume of the liquid mass (i.e. the mass of the liquid mass). When the volume of the liquid mass increases, As a result, the power increases.

[0099] In order to prove the correctness of the calculation method of the theoretical torque of the rotor of the scraped film evaporator, the following steps are performed for verification:

[0100] S1. Modeling of the mesh model of scraped film evaporator (taking the in-line scraped film evaporator as an example);

[0101] S1.1. Import the geometric model of the scraper thin film evaporator into Fluent Meshing for model mesh division, and perform local mesh encryption on the scraper edge, near the wall and in the liquid film area to capture the high gradient flow characteristics; the relevant parameters of the geometric model are: the height of the scraper thin film evaporator is 150mm, the outer wall diameter is 250mm, the inner wall diameter is 200mm, the annular feed port width is 3mm, the number of in-line scrapers is 4, the scraper thickness is 3mm, and the scraper gap (i.e., the distance between the tip of the scraper blade and the wall) is 1mm;

[0102] S1.2. Use tetrahedral elements to fill the computational domain to ensure the mesh quality in complex geometric areas;

[0103] S1.3. By gradually refining the grid and comparing the changes of key parameters (such as wall shear stress and liquid film thickness), the independence of grid division is ensured, and finally a high-quality grid file (.msh format) is generated for subsequent numerical simulation calculations;

[0104] S2, simulation solution;

[0105] S2.1. Mesh model import: Import the generated high-quality mesh file into the discrete element simulation software Fluent to initialize the three-dimensional geometric model and discretized mesh data;

[0106] S2.2, Rotating reference frame setting: In order to accurately simulate the rotational motion in the scraped film evaporator, the single rotating reference frame (SRF) method is used, and the scraper speed (i.e. the rotation speed of the reference frame) is set to 90rpm, thereby activating the SRF motion model;

[0107] S2.3, Boundary condition definition: The material (low viscosity non-Newtonian fluid) enters the scraped film evaporator uniformly from the inlet at a certain mass flow rate of 0.025 kg / s. The wall boundary condition is set to an absolute velocity of 0, and the relative velocity of the scraper relative to the fluid domain is also set to 0, so as to improve the boundary condition definition and physical model constraints;

[0108] S2.4, Solver Configuration: The numerical calculation adopts the Pressure-Based Solver, which is particularly suitable for the simulation analysis of incompressible fluids and can efficiently and accurately handle the coupling problem between the pressure field and the velocity field. At the same time, the SSTk-ω turbulence model is used to improve the calculation accuracy;

[0109] The SSTk-ω model combines the advantages of the k-ε model and the k-ω model by using the hybrid function The k-ω model is used in the near-wall area, and the k-ε model is used in the far-field area. In the flow field of the scraped thin film evaporator, the interaction between the liquid film and the wall is the core issue. The SSTk-ω turbulence model uses the k-ω model in the near-wall area, which can accurately capture the low Reynolds number effect in the boundary layer, thereby providing reliable data for viscous power calculation. In front of the scraper, the liquid mass accumulation and flow separation phenomena are very significant. The SSTk-ω model can better predict the flow characteristics of the separation zone with the help of the shear stress transmission term, while the traditional k-ε model has low accuracy when dealing with separation flow. The SSTk-ω model combines the advantages of the k-ε model and the k-ω model, and can accurately simulate the near-wall flow, separation flow and wide range of Reynolds number flow, providing a reliable theoretical basis for power characteristic analysis and optimization design. Therefore, by comparing the advantages and disadvantages of each model, the SSTk-ω model is finally selected.

[0110] S2.5, spatial discretization settings: In terms of spatial discretization, the momentum equation adopts the second-order upwind scheme, and the high-order interpolation method is used to significantly improve the discretization accuracy, so as to more accurately capture the key characteristic areas in the flow field, such as the liquid film boundary layer and the liquid mass accumulation area;

[0111] S2.6, Convergence monitoring setting: To ensure the stability and reliability of the calculation results, the residual convergence standard is set to 1×10 -4 By real-time monitoring of the changing trends of key parameters such as outlet flow, scraper torque, and evaporator liquid holdup, until the simulation calculation is fully converged, a stable three-dimensional flow field of the scraped film evaporator is obtained (the value of the scraper torque at this time is 0.025N·m, which is the simulated torque of the rotor of the scraped film evaporator); the three-dimensional flow field is stable when the following conditions are met at the same time:

[0112] Condition 1: The residual curve (i.e., convergence curve) begins to show a continuous downward trend and eventually flattens to 10 -3 The magnitude of the simulation is always 10 -3 Magnitude;

[0113] Condition 2: The torque curve tends to be flat, and the fluctuation range is less than 5% within 1000 iteration steps;

[0114] Condition 3: The outlet flow curve starts to show an upward trend and eventually flattens out, that is, the difference between the outlet flow and the inlet flow setting is within 5% within 1000 iterations;

[0115] S2.7. Obtaining the moment arm The value of the film thickness of the liquid film The value of the dynamic viscosity of the material The value of circumferential contact angle The value of the shear rate of liquid film flow The value of the total volume of the material in the scraped film evaporator when the three-dimensional flow field is stable The value of, the distance between the heating wall of the scraped film evaporator and the central axis of the rotor The value of

[0116] The process of obtaining the value of is as follows: using CFD-Post post-processing software to extract the stable three-dimensional flow field diagram of the scraped film evaporator, and obtaining the circumferential cross-sectional cloud diagram of the three-dimensional flow field diagram (such as Figure 3 Then remove the outermost circle in the circumferential cross-section cloud diagram, and the width is The annular area (i.e., the liquid film part) is selected, and the grids with a volume fraction of 0.5 in the liquid part of the remaining area are selected. The least squares method is used to fit these grids around each liquid mass to obtain the circular boundary of each liquid mass (such as Figure 5 As shown in the figure), finally, the distance between the center of the circular boundary of each liquid mass and the central axis of the rotor is calculated, and the average value is taken to obtain value (120.2mm);

[0117] The process of obtaining the value of is as follows: using CFD-Post post-processing software to extract the stable three-dimensional flow field diagram of the scraped film evaporator, and then obtaining the axial cross-sectional cloud diagram of the three-dimensional flow field diagram (such as Figure 4 As shown in the figure), select the grid with a volume fraction of 0.5 in the liquid part, calculate its distance from the heated wall, and take the average value to get value (0.7762mm); Figure 4 In the figure, when the color on the left is orange, the value is close to 1, indicating the liquid part of the gas-liquid two-phase; when the color on the right is blue, the value is close to 0, indicating the gas phase; the middle transition color area represents the transition area of ​​the liquid-gas phase change, and the default value of 0.5 is the gas-liquid separation value point;

[0118] The value is obtained by extracting the average value of the dynamic viscosity at the interface between the liquid mass and the liquid film using CFD-Post post-processing software after obtaining a stable three-dimensional flow field of a scraped film evaporator. The value is 0.117 Pa·s;

[0119] The value of is obtained by extracting the center angle of the arc surface corresponding to the contact area between the liquid mass and the liquid film using CFD-Post post-processing software after obtaining a stable three-dimensional flow field of the scraped film evaporator. = (4.7-0.7212)° = 3.9788°;

[0120] The value is obtained by extracting the average shear rate at the interface between the liquid mass and the liquid film using CFD-Post post-processing software after obtaining a stable three-dimensional flow field of a scraped film evaporator. The value is 750s -1 ;

[0121] The process of obtaining the value of is as follows: after obtaining a stable three-dimensional flow field of a scraped film evaporator, the corresponding liquid holdup data is directly extracted from the data file exported by real-time monitoring, that is, The value (3.42 m 3 );

[0122] S3. Calculate the theoretical torque of the rotor of the scraped film evaporator. The calculation formula is as follows:

[0123] ;

[0124] ;

[0125] ;

[0126] ;

[0127] ;

[0128] ;

[0129] In the formula, Represents the theoretical torque of the rotor of the scraped film evaporator, in N·m; Represents the viscous torque of the scraped film evaporator rotor, in N·m; Represents the liquid mass load torque of the scraped film evaporator rotor, in N·m; Represents shear stress in kg·m -1 ·s -2 ; Represents the contact area between the liquid mass and the liquid film (e.g. Figure 6 As shown), the unit is m 2 ; represents the distance between the heating wall of the scraped film evaporator and the central axis of the rotor (1.25 m); Represents the height of the scraped film evaporator in m; Represents the mass of the liquid mass, in kg; Represents the acceleration due to gravity, and its value is 9.81 m·s -2 ; Represents the density of the material (low viscosity non-Newtonian fluid) (998.2kg / m 3); Represents the volume of the liquid mass, in m 3 ; Represents the volume of the liquid film, in m 3 .

[0130] Calculated =0.02705N·m; Compared with the simulated torque of the rotor of the scraped film evaporator, it can be seen that the relative error between the theoretical torque and the simulated torque is about 8.2% (no more than 20% is within the acceptable range), indicating that the calculation method has high accuracy.

[0131] The prior art obtains the rotor torque of the scraped film evaporator through simulation, and then multiplies it by the angular velocity to obtain the power result. It is difficult to explore the specific factors affecting the power according to this method. The present invention can overcome this defect, as described in detail as follows:

[0132] The simulated torque of the rotor of the scraped film evaporator obtained by the prior art method at different speeds (90 rpm, 120 rpm, 150 rpm) is as follows: Figure 7 As shown in a, it can be seen that as the speed increases, the simulated torque of the rotor of the scraped film evaporator gradually increases; the theoretical torque of the rotor of the scraped film evaporator obtained by the method of the present invention is as follows Figure 7 As shown in b, it can be seen that as the speed increases, the theoretical torque of the rotor of the scraped film evaporator gradually increases, and it can also be seen that when the speed changes, the viscous torque changes significantly, while the load torque changes slightly. Therefore, the speed mainly affects the change of the viscous torque term. If the viscosity of the material processed in actual production is high, the viscous torque can be affected by changing the speed, thereby achieving the purpose of reducing power.

[0133] The simulated torque of the rotor of the scraper film evaporator obtained by the prior art method under different scraper gaps (0.75mm, 1mm, 1.25mm) is as follows: Figure 8 As shown in a, it can be seen that as the scraper gap increases, the simulated torque of the rotor of the scraper film evaporator gradually decreases; the theoretical torque of the rotor of the scraper film evaporator obtained by the method of the present invention is as follows Figure 8 As shown in b, it can be seen that as the scraper gap increases, the theoretical torque of the rotor of the scraper film evaporator gradually decreases, and it can also be seen that when the scraper gap changes, the viscous torque changes significantly, while the load torque changes slightly. Therefore, the scraper gap mainly affects the change of the viscous torque term. If the viscosity of the material processed in actual production is high, the viscous torque can be affected by changing the scraper gap, thereby achieving the purpose of reducing power.

[0134] The simulated torque of the rotor of the scraped film evaporator obtained by the prior art method at different feed rates (0.15kg / s, 0.25kg / s, 0.35kg / s) is as follows: Fig. 9 As shown in a, it can be seen that as the feed amount increases, the simulated torque of the rotor of the scraped film evaporator gradually increases; the theoretical torque of the rotor of the scraped film evaporator obtained by the method of the present invention is as follows Fig. 9 As shown in b, it can be seen that with the increase of feed amount, the theoretical torque of the rotor of the scraped film evaporator gradually increases, and it can also be seen that when the feed amount changes, the load torque changes significantly, while the viscous torque changes less. Therefore, the feed amount mainly affects the change of the load torque term. If the volume of the material processed in actual production is large and affects the normal operation of the equipment, the load torque can be affected by changing the feed amount, thereby achieving the purpose of reducing power.

[0135] Based on the above research, the present invention proposes a method for reducing the power of a scraped film evaporator. After respectively calculating the viscous torque of the scraped film evaporator rotor and the liquid mass load torque of the scraped film evaporator rotor, the two are compared. If the viscous torque of the scraped film evaporator rotor is large, the scraper speed is reduced and / or the scraper gap is increased; otherwise, the feed amount is reduced.

[0136] The calculation adopts the method for calculating the theoretical torque of the rotor of a scraped film evaporator as described in any one of the above items.

[0137] The above research process not only proves the accuracy and reliability of the proposed theoretical torque calculation method, but also provides strong theoretical support for the industrial design and optimization of scraped film evaporators. By reasonably adjusting the scraper speed and scraper gap to affect the viscous torque term, and reasonably controlling the feed rate to affect the load torque term, the power consumption of the scraped film evaporator can be effectively reduced. Therefore, this method has broad application prospects and practical value.

Claims

1. A method for calculating the theoretical torque of a rotor of a scraped film evaporator, characterized in that: Use the following formula: ； In the formula, Represents the theoretical torque of the rotor of the scraped film evaporator, in N·m; Represents the viscous torque of the scraped film evaporator rotor, in N·m; Represents the liquid mass load torque of the scraped film evaporator rotor, in N·m; ； In the formula, represents the moment arm, in mm; Represents the dynamic viscosity of the material, in Pa·s; Represents the distance between the heating wall of the scraped film evaporator and the central axis of the rotor, in mm; Represents the thickness of the liquid film, in mm; represents the circumferential contact angle, in degrees; H represents the height of the scraped film evaporator, in mm; Represents the shear rate of liquid film flow, in s -1 ; ； In the formula, Represents the density of the material in kg / m 3 ; Represents the total volume of material in the scraped film evaporator when the three-dimensional flow field is stable, in units of , the three-dimensional flow field is stable when the following conditions are met at the same time: Condition 1: The residual curve begins to show a continuous downward trend and eventually flattens to 10 -3 The magnitude of the simulation is always 10 -3 Magnitude; Condition 2: The torque curve tends to be flat, and the fluctuation range is less than 5% within 1000 iteration steps; Condition 3: The outlet flow curve starts to show an upward trend and eventually flattens out, that is, the difference between the outlet flow and the inlet flow setting is kept within 5% within 1000 iteration steps; Represents the acceleration due to gravity, and its value is 9.81 m·s -2 .

2. The method for calculating the theoretical torque of the rotor of a scraped film evaporator according to claim 1, characterized in that: The process of obtaining the value of is as follows: after obtaining the stable three-dimensional flow field of the scraped film evaporator through simulation, extract the three-dimensional flow field map, obtain the axial cross-section cloud map of the three-dimensional flow field map, screen out the grid with a volume fraction of 0.5 in the liquid part, calculate its distance from the heating wall, and take the average value, which is obtained. The value of .

3. The method for calculating the theoretical torque of the rotor of a scraped film evaporator according to claim 2, characterized in that: The process of obtaining the value of is as follows: first, after obtaining the stable three-dimensional flow field of the scraped film evaporator through simulation, the three-dimensional flow field map is extracted, and then the circumferential section cloud map of the three-dimensional flow field map is obtained. Then, the width of the outermost circle in the circumferential section cloud map is removed. The annular area is selected, and the grids with a volume fraction of 0.5 in the liquid part of the remaining area are selected. The least squares method is used to fit these grids around each liquid mass to obtain the circular boundary of each liquid mass. Finally, the distance between the center of the circular boundary of each liquid mass and the central axis of the rotor is calculated, and the average value is taken to obtain The value of .

4. The method for calculating the theoretical torque of a rotor of a scraped film evaporator according to claim 1, characterized in that: The value of is obtained by simulating the stable three-dimensional flow field of the scraped film evaporator and extracting the average value of the dynamic viscosity at the interface between the liquid mass and the liquid film.

5. The method for calculating the theoretical torque of a rotor of a scraped film evaporator according to claim 1, characterized in that: The value of is obtained by simulating the stable three-dimensional flow field of the scraped film evaporator and extracting the center angle of the arc surface corresponding to the contact area between the liquid mass and the liquid film.

6. The method for calculating the theoretical torque of a rotor of a scraped film evaporator according to claim 1, characterized in that: The value of is obtained by simulating the stable three-dimensional flow field of the scraped film evaporator and calculating the average shear rate at the interface between the extract liquid mass and the liquid film.

7. The method for calculating the theoretical torque of a rotor of a scraped film evaporator according to claim 1, characterized in that: The process of obtaining the value of is as follows: using simulation, in this process, the liquid holdup data of the scraped film evaporator is monitored in real time. After obtaining a stable three-dimensional flow field of the scraped film evaporator, the corresponding liquid holdup data is directly extracted from the data file exported by real-time monitoring, that is, The value of .

8. Application of the method for calculating the theoretical torque of a rotor of a scraped film evaporator according to any one of claims 1 to 7, characterized in that: After calculating the viscous torque of the scraped film evaporator rotor and the liquid mass load torque of the scraped film evaporator rotor respectively, the two are compared. If the viscous torque of the scraped film evaporator rotor is large, the scraper speed is reduced and / or the scraper gap is increased; if the viscous torque of the scraped film evaporator rotor is small or the two are equal, the feed amount is reduced.

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