CN118821503A - A simulation method for flow characteristics in polyvinyl alcohol resin polymerization reactor - Google Patents

Abstract

The present application discloses a simulation method for flow characteristics in a polyvinyl alcohol resin polymerization kettle, which belongs to the field of chemical raw material processing technology. The method includes the following steps: obtaining multiple groups of different polymerization kettle-stirring paddle combinations and constructing corresponding three-dimensional simulation models; selecting material models and fluid models to solve the obtained polymerization kettle-stirring paddle combination for pressure-velocity coupling, based on the obtained polymerization kettle-stirring paddle combination with the best performance, calculating the steady-state flow field with only polymer as the medium, using the obtained steady-state flow field as the initial value for non-steady-state calculation, and then obtaining the residence time distribution of the material model; verifying the simulation results through experiments. Beneficial effects of the present application: The present application simulates the flow characteristics of the fluid in the polymerization kettle, designs a continuous stirring polymerization kettle and stirring paddle structure for the production of medium and high viscosity special PVA resins, which can achieve uniform stirring of materials and high heat transfer efficiency.

Description

A simulation method for flow characteristics in polyvinyl alcohol resin polymerization reactor

Technical Field

The present application relates to the technical field of chemical raw material processing, and in particular to a simulation method for flow characteristics in a polyvinyl alcohol resin polymerization kettle.

Background Art

At present, most polyvinyl alcohol production adopts the calcium carbide acetylene process route, which has high energy consumption, high pollution and low product quality. However, the ethylene-based special polyvinyl alcohol (PVA) resin has the advantages of good quality, high purity and low energy consumption. The ethylene-based polyvinyl alcohol device uses azo or tert-butyl peroxypivalate (BPV) as an initiator and methanol as a solvent. Vinyl acetate (VAC) and ethylene are polymerized in solution with the initiator to produce polyvinyl acetate. The polymerized liquid is subjected to an alcoholysis process to produce polyvinyl alcohol products.

The polymerization process of polyvinyl acetate is one of the keys to quality control, which determines the distribution of polymer molecular weight, uniformity of particle size, impurity content and quality stability. The stirred polymerization kettle is the place where the polymerization reaction takes place. In the synthesis process of high molecular polymers, there are often problems such as uneven mass and heat transfer caused by increased viscosity, difficult to control the reaction process, and uneven product quality. Therefore, the flow field characteristics of the polymerization kettle play a key role in product quality. There are many factors that affect the flow characteristics of the flow field in the polymerization kettle, and the existing technology lacks a method for accurately measuring the flow characteristics of the flow field in the polymerization kettle under the influence of multiple factors.

Summary of the invention

One of the purposes of the present application is to provide a method for simulating the flow characteristics in a polyvinyl alcohol resin polymerization kettle that can solve at least one of the defects in the above-mentioned background technology.

In order to achieve at least one of the above purposes, the technical solution adopted in the present application is: a simulation method for flow characteristics in a polyvinyl alcohol resin polymerization kettle, comprising the following steps:

S100: combining polymerization kettles of different structures and various types of stirring paddles to obtain multiple groups of different polymerization kettle-stirring paddle combinations and constructing corresponding three-dimensional simulation models;

S200: selecting a material model and a fluid model that meet the requirements, and performing pressure-velocity coupling solution on the obtained polymerization kettle-stirring paddle combination through an algorithm to obtain a polymerization kettle-stirring paddle combination with the best performance;

S300: Based on the obtained polymerization reactor-stirring blade combination with the best performance, the steady-state flow field with only the polymer as the medium is calculated, and the obtained steady-state flow field is used as the initial value for non-steady-state calculation, thereby obtaining the residence time distribution of the material model;

S400: Verify the simulation results through experiments.

Preferably, in step S200, the top of the polymerization kettle with the best performance is a standard elliptical head, and the bottom is a W-bottom elliptical head; the stirring paddle combination with the best performance includes a double-folded blade paddle and a curved-edge straight blade paddle; wherein the curved-edge straight blade paddle is located at the bottom of the polymerization kettle, the number of the double-folded blade paddle is at least one, and is arranged at intervals above the curved-edge straight blade paddle.

Preferably, in step S200, the material model adopts a single material model, and the fluid model adopts a κ-ε standard turbulence model; the blade motion of the agitator adopts multiple reference systems, and the pressure-velocity coupling solution is performed through the SIMPLE algorithm; wherein, the inlet boundary of the polymerization kettle is the inlet flow, the outlet boundary is the outlet pressure, and the wall is processed using a standard wall function.

Preferably, when performing step S300, a tracer is added to perform a simulation calculation of the residence time distribution; the tracer is instantaneously added at the inlet of the steady-state flow field after the calculation of the steady-state flow field converges; when performing a non-steady-state calculation, the tracer concentration at the outlet of the flow field is monitored; and the residence time distribution is calculated based on the duration from the addition of the tracer to the concentration approaching the set value.

Preferably, the residence time distribution is calculated by monitoring the duration from the addition of the tracer to the concentration approaching zero.

Preferably, from the time when the tracer is added to the time when the concentration approaches the set value, the tracer concentration is monitored once every Δt _i time interval to obtain the corresponding tracer concentration c _i ; the residence time distribution is represented by the residence time distribution density function E(t), and the specific calculation formula is as follows:

.

Preferably, in step S300, after obtaining the residence time distribution, the accuracy of the simulation result is determined by calculating the discreteness σ _t ² of the residence time distribution and the equivalent number of fully mixed reactors m. The specific calculation formula is as follows:

;

;

;

in, represents the average residence time.

Preferably, the factors affecting the flow characteristics in the polymerization kettle include the flow field distribution structure in the polymerization kettle, the rotation speed of the agitator, the flow rate of the fluid in the polymerization kettle and the viscosity of the polymer; then in step S400, experiments are carried out on all polymerization kettle-agitator combinations to verify the simulation results through four aspects: the flow field distribution structure in the polymerization kettle, the rotation speed of the agitator, the flow rate of the fluid in the polymerization kettle and the viscosity of the polymer.

Compared with the prior art, the beneficial effects of this application are:

This application simulates the flow characteristics of the fluid in the polymerization kettle and designs a continuous stirred polymerization kettle and stirring paddle structure suitable for the production of medium and high viscosity special PVA resins, which can achieve uniform stirring of materials and high heat transfer efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the overall workflow of this application.

FIG. 2 is a schematic diagram of the structure of a three-dimensional simulation model of a traditional polymerization reactor in this application.

FIG3 is a schematic diagram of the structure of a three-dimensional simulation model of the best polymerization reactor in this application.

FIG. 4 is a simulated cloud diagram of the flow field distribution of the polymerization kettle shown in FIG. 2 in the present application obtained through simulation.

FIG. 5 is a simulated cloud diagram of the flow field distribution of the polymerization kettle shown in FIG. 3 of the present application obtained through simulation.

FIG. 6 is a schematic diagram of the residence time distribution of the polymerization kettles shown in FIG. 2 and FIG. 3 in the present application at different rotation speeds.

FIG. 7 is a schematic diagram of the residence time distribution of the polymerization kettles shown in FIG. 2 and FIG. 3 in the present application at different flow rates.

FIG8 is a schematic diagram of the residence time distribution of the polymerization kettles shown in FIG2 and FIG3 in this application at different viscosities.

FIG. 9 is a residence time distribution curve diagram obtained by simulation and experiment of the polymerization reactor shown in FIG. 3 of the present application.

DETAILED DESCRIPTION

Below, the present application is further described in conjunction with specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

In the description of the present application, it should be noted that directional words, such as the terms "center", "lateral", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc., indicating directions and positional relationships are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of narrating the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and cannot be understood as limiting the specific scope of protection of the present application.

It should be noted that the terms "first", "second", etc. in the description and claims of the present application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

The terms "including" and "having" and any variations thereof in the specification and claims of the present application are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or apparatuses.

One of the preferred embodiments of the present application, as shown in FIG1 , is a method for simulating flow characteristics in a polyvinyl alcohol resin polymerization kettle, comprising the following steps:

S100: combining polymerization kettles of different structures and various types of stirring paddles to obtain multiple groups of different polymerization kettle-stirring paddle combinations and construct corresponding three-dimensional simulation models.

S200: Select a material model and a fluid model that meet the requirements, and use an algorithm to perform pressure-velocity coupling solution on the obtained polymerization kettle-stirring paddle combination to obtain a polymerization kettle-stirring paddle combination with optimal performance.

S300: Based on the best performance of the polymerization reactor-stirring blade combination, the steady-state flow field with only polymer as the medium is calculated, and the obtained steady-state flow field is used as the initial value for non-steady-state calculation, thereby obtaining the residence time distribution of the material model.

S400: Verify the simulation results through experiments.

It should be known that the purpose of the simulation of the fluid flow characteristics in the polymerization kettle in this application is to select the polymerization kettle-stirring paddle combination with the best performance. It should be known that there are many types of structures of polymerization kettles, and there are also many types of structures of stirring paddles. For a single-structure polymerization kettle, stirring paddles of different types and quantities can be adapted. Different polymerization kettles can produce different flow characteristics when corresponding to different stirring paddles, so the best polymerization kettle-stirring paddle combination is found by simulating the flow characteristics of the polymerization kettle. Thereby, a continuous stirred polymerization kettle and stirring paddle structure that meet the production of medium and high viscosity special PVA resins are designed, which can achieve uniform stirring of materials and have a high heat transfer efficiency. Of course, in order to ensure the accuracy of the simulation results, the accuracy of the simulation results can be verified by setting up a comparative experiment.

It should be known that there are many softwares for simulating flow characteristics in polymerization kettles. For example, Star-CD software can be used to simulate the residence time distribution of fluids in stirred polymerization kettles. For the establishment of three-dimensional simulation models of each polymerization kettle-stirring paddle combination, three-dimensional models of each polymerization kettle-stirring paddle combination can be first constructed by three-dimensional software; there are many types of three-dimensional software for three-dimensional model construction, such as Solidworks and Creo. Then, the three-dimensional model is meshed by the simulation software; the mesh type can be an unstructured tetrahedral mesh, and the number of meshes needs to meet the mesh independence requirements.

In this embodiment, the factors affecting the flow characteristics in the polymerization kettle include the flow field distribution structure in the polymerization kettle, the rotation speed of the stirring paddle, the flow rate of the fluid in the polymerization kettle, and the viscosity of the polymer. In step S200, based on the material used for polymerization (PVA resin), simulation is performed through four aspects such as the flow field distribution structure in the polymerization kettle, the rotation speed of the stirring paddle, the flow rate of the fluid in the polymerization kettle, and the viscosity of the polymer. The specific structure of the polymerization kettle-stirring paddle combination with the best performance is shown in Figure 3. Among them, the top of the polymerization kettle is a standard elliptical head (not shown), and the bottom is a W-bottom elliptical head. The blades of the stirring paddle combination corresponding to the polymerization kettle include a double-folded blade paddle and a curved straight blade paddle. The number of the curved straight blade paddle is one and is arranged at the bottom of the polymerization kettle; the number of the double-folded blade paddle is at least one, and is arranged above the curved straight blade paddle at intervals.

It should be known that, since the bottom of the polymerization kettle is a W-bottom elliptical head, the curved straight blade paddle is curved in a W shape, thereby ensuring that the curved straight blade paddle can also fully stir the bottom of the polymerization kettle. The specific number of double-folding blade paddles located above the curved straight blade paddle can be selected according to actual needs; one specific example is shown in FIG3, where there are three double-folding blade paddles, and adjacent double-folding blade paddles are equidistantly arranged in the axial direction, and the axial projections of adjacent double-folding blade paddles have an angle of 90°, thereby ensuring the dynamic balance stability of the double-folding blade paddle during the stirring process.

In this embodiment, the material model includes a single material model and a multi-material model; the single material model only needs to consider one material; correspondingly, the multi-material model needs to consider multiple materials. Since this application is only for the scenario of PVA resin polymerization, in step S200, a single material model is used for the material model, that is, a model that only considers PVA resin polymers.

At the same time, there are many types of fluid models, and the specific selection can be determined according to the actual needs of technicians in this field. In this embodiment, the κ-ε standard turbulence model is preferably used. The κ-ε standard turbulence model is one of the most widely used turbulence models in fluid mechanics. It uses two equations that describe the turbulent kinetic energy and the turbulent dissipation rate, respectively, to simulate turbulent motion under high Reynolds numbers; the specific principles and model structure of the κ-ε standard turbulence model are well-known technologies for technicians in this field, so they are not elaborated in detail here. Accordingly, in order to further improve the accuracy of the simulation results, the blade motion of the agitator adopts multiple reference systems, and the pressure-velocity coupling solution is performed by the SIMPLE algorithm. Among them, the inlet boundary of the polymerization kettle is the inlet flow rate, the outlet boundary is the outlet pressure, the wall is processed by the standard wall function, and the calculation residual is 1× ^10-4 .

In this embodiment, when performing step S300, in order to facilitate non-steady-state calculations to obtain residence time distribution, a tracer can be added to the fluid model. In the simulation, the tracer is a custom additional amount, and the residence time distribution can be calculated by monitoring the concentration change of the tracer. Specifically, the tracer is added instantaneously at the inlet of the steady-state flow field after the calculation of the steady-state flow field converges. By adding the tracer instantaneously, the influence of the addition time on the calculation of the residence time distribution can be reduced or avoided. When performing non-steady-state calculations, the tracer concentration at the outlet of the flow field is monitored; the residence time distribution is calculated based on the duration from the addition of the tracer to the concentration approaching the set value.

It should be known that after the tracer is added, the residence time distribution mainly reflects the concentration change of the tracer, so the concentration of the tracer can be measured repeatedly within a period of time after the tracer is added to the flow field to calculate the corresponding residence time distribution. The tracer measurement duration for calculating the residence time distribution can be selected according to the actual needs of those skilled in the art. For example, the duration from the addition of the tracer to the concentration being reduced to half can be selected to calculate the residence time distribution, and the duration from the addition of the tracer to the concentration approaching 0 can also be selected to calculate the residence time distribution. In order to ensure the calculation accuracy of the residence time distribution, it is necessary to obtain as much tracer concentration data as possible. Therefore, in the present embodiment, for the calculation of the residence time distribution, the duration from the addition of the tracer to the concentration approaching 0 can be preferably used as the calculation of the residence time distribution.

In this embodiment, the residence time distribution can be represented by the residence time distribution density function E(t). In the duration from the start of tracer addition to the concentration approaching the set value, the tracer concentration is monitored once every Δt _i time interval to obtain the corresponding tracer concentration c _i for the calculation of the residence time distribution density function E(t). The specific calculation formula is as follows:

.

It should be known that the specific value of the interval time Δt _i for monitoring the tracer concentration can be selected according to the actual needs of those skilled in the art, for example, 0.5 s, 1 s, 10 s, etc.

In this embodiment, after obtaining the residence time distribution, in order to further ensure the accuracy of the simulation results, the discreteness σ _t ² of the residence time distribution and the equivalent number of fully mixed reactors m can be calculated to determine the accuracy of the simulation results. The specific calculation formula is as follows:

.

.

.

in, represents the average residence time.

It should be known that the discreteness of the residence time distribution is used to characterize the strength of the divergence of the vector field at each point in space. The smaller the value of the discreteness σ _t ² of the residence time distribution, the more concentrated the residence time distribution calculated is. The equivalent fully mixed kettle number m is a parameter used to describe the degree of mixing in the polymerization kettle, which indicates the degree of closeness of the performance of the polymerization kettle to the ideal polymerization kettle. The larger the value of the equivalent fully mixed kettle number m, the higher the degree of mixing in the polymerization kettle, and the more uniform the residence time distribution of the material in the polymerization kettle, which is conducive to improving the selectivity and yield of the reaction.

In this embodiment, it can be known from the above content that the factors affecting the flow characteristics in the polymerization kettle mainly include the flow field distribution structure in the polymerization kettle, the rotation speed of the stirring paddle, the flow rate of the fluid in the polymerization kettle and the viscosity of the polymer. Therefore, in step S400, all polymerization kettle-stirring paddle combinations can be experimented from four aspects of the flow field distribution structure in the polymerization kettle, the rotation speed of the stirring paddle, the flow rate of the fluid in the polymerization kettle and the viscosity of the polymer to verify the simulation results. For ease of understanding, the experimental process will be described in detail below.

Specifically, in order to simplify the verification process, only the optimal polymerization kettle-stirring paddle combination shown in Figure 3 and the traditional polymerization kettle-stirring paddle combination shown in Figure 2 are used as an example for detailed description. In order to facilitate the description of subsequent contents, the traditional polymerization kettle-stirring paddle combination is defined as the original polymerization kettle, and the optimal polymerization kettle-stirring paddle combination is defined as the new polymerization kettle.

1. Define the structural parameters of the original polymerization kettle and the new polymerization kettle.

As for the original polymerization kettle, as shown in Figure 2, the inner diameter T of the polymerization kettle is 2891.5 mm, the straight section height is 9269 mm, the bottom is flat, the top is a standard elliptical head, the inlet and outlet pipe diameters d are 150 mm, and the liquid level height H in the polymerization kettle is 9100 mm. The offset frame stirring impeller FFKS is used, the impeller diameter D is 2132 mm, the diameter of the two outer frames is 408 mm, the width of the top and bottom support rods is 360 mm, and the diameter of the two internal support rods is 219 mm.

For the new polymerization kettle, as shown in Figure 3. The inner diameter T of the polymerization kettle is 2800mm, the straight section height is 4800mm, the top is a standard elliptical head, the bottom is a W-bottom elliptical head, the inlet and outlet pipe diameters d are 150mm, and the liquid level height H in the polymerization kettle is 4066mm; two circles of 12 groups of internal cooling tubes are evenly arranged circumferentially, a total of 24 groups, the outer diameter of the outer tube is 133mm, and the outer diameter of the inner tube is 114mm. Four layers of blades are installed, the top three layers are double folding blades CCJ, the blade diameter is 1550 mm, the bottom layer is a curved straight blade PTJ, the blade diameter is 1400mm, and the blade layer spacing is 950mm.

2. Define the experimental conditions.

Table 1 Parameters of simulation of original polymerization kettle and new polymerization kettle

As shown in Table 1 above, eight simulation conditions 1 to 8 are defined, and the original polymerization kettle and the new polymerization kettle each correspond to four conditions. Among them, the four conditions of the original polymerization kettle are: simulation condition 1, the speed of the blade is 18r/min, the flow rate is ^59.8m3 /h, and the viscosity of the material is 2.5Pa·s; simulation condition 2, the speed of the blade is 28r/min, the flow rate is ^59.8m3 /h, and the viscosity of the material is 2.5Pa·s; simulation condition 3, the speed of the blade is 28r/min, the flow rate is ^119.5m3 /h, and the viscosity of the material is 2.5Pa·s; simulation condition 7, the speed of the blade is 20r/min, the flow rate is 59.8m3/h, and the viscosity of the material is 10Pa·s. The four operating conditions of the new polymerization kettle are: simulation condition 4, the blade speed is 46r/min, the flow rate is ^22m3 /h, and the viscosity of the material is 2.5Pa·s; simulation condition 5, the blade speed is 69r/min, the flow rate is ^22m3 /h, and the viscosity of the material is 2.5Pa·s; simulation condition 6, the blade speed is 69r/min, the flow rate is ^44m3 /h, and the viscosity of the material is 2.5Pa·s; simulation condition 8, the blade speed is 63r/min, the flow rate is ^22m3 /h, and the viscosity of the material is 10Pa·s.

3. Experimental results and analysis.

(1) Results and analysis of flow field distribution experiments.

Under the working condition simulation condition 1 of the original polymerization kettle, the velocity vector field and velocity cloud map formed by the offset frame stirring are shown in Figure 4. It can be seen from the figure that the flow direction of the fluid is mainly circumferential rotation, and a part of it flows from the frame to the center of the axis. The flow velocity in the area outside the inner frame is faster, and the flow velocity in the central area of the kettle is smaller.

Under the simulated working condition 4 of the new polymerization reactor, the velocity vector field and velocity cloud map formed by the combined stirring paddle are shown in Figure 5. It can be seen from the figure that the outer edge blades of the top three layers of CCJ paddles discharge the flow upward, the blades at the center position discharge the fluid downward, and the bottom blades PTJ discharge the fluid outward along the edge, move upward after being blocked by the heat exchange tube, and return downward along the axis to form a local fluid circulation. The regional flow velocity between the top three layers of double-folded blade paddles CCJ is larger, and the regional flow velocity between the bottom curved straight blade paddle PTJ and the penultimate double-folded blade paddle CCJ is smaller.

By comparing the velocity size and distribution in the original polymerization kettle and the new polymerization kettle, it can be found that the original polymerization kettle is mainly dominated by radial flow, the new polymerization kettle has more axial circulation, and the area with low flow velocity in the new polymerization kettle is significantly less than that in the original polymerization kettle. Therefore, the flow field distribution state of the new polymerization kettle is better than that of the original polymerization kettle.

(2) The effect of the impeller speed on the residence time distribution.

The original polymerization kettle and the new polymerization kettle were respectively subjected to experimental simulation of residence time distribution by selecting different rotation speeds at a fixed flow rate, that is, the original polymerization kettle was selected to simulate the working conditions of working condition 1 and working condition 2, and the new polymerization kettle was selected to simulate the working conditions of working condition 4 and working condition 5. The results of the experiment are shown in Figure 6 and Table 2. It can be seen from Figure 6 that increasing the rotation speed of the original polymerization kettle advances the peak time, that is, the mixing rate is faster and the mixing time is shorter. It can be seen from the statistical table shown in Table 2 that with the increase of the rotation speed, the equivalent full mixed kettle number m of the original polymerization kettle decreases from 1.452 to 1.326, a decrease of 8.68%. Although the peak time of the new mixing kettle is also slightly advanced as the rotation speed increases, the equivalent full mixed kettle number m decreases from 1.051 to 1.037, only a decrease of 1.33%. It can be seen that the influence of rotation speed on the original polymerization kettle is far greater than that on the new polymerization kettle. This may be because the original polymerization kettle is mainly formed by radial flow during stirring. Increasing the rotation speed will enhance turbulence and strengthen back mixing in the kettle. The new polymerization kettle is mainly axial flow with a high degree of back mixing. Increasing the rotation speed will accelerate the liquid flow rate.

Table 2 Statistical table of residence time distribution of original polymerization kettle and new polymerization kettle

(3) The influence of fluid flow rate on residence time distribution.

The original polymerization kettle and the new polymerization kettle were respectively subjected to experimental simulation of residence time distribution by selecting different flow rates at a fixed speed, that is, the original polymerization kettle was selected to simulate working conditions 2 and 3, and the new polymerization kettle was selected to simulate working conditions 5 and 6. The results of the experiment are shown in Figure 7 and Table 2. It can be seen from Figure 7 that with a fixed stirring speed and an increase in the original flow rate, the peak of the residence time distribution density function gradually narrows. When the flow rate is small, the tailing of the curve is more obvious. It can be seen from the statistical table shown in Table 2 that with the increase in flow rate, the equivalent fully mixed kettle number m of the original polymerization kettle increased from 1.326 to 1.622, an increase of 22.3%. As the flow rate of the new mixing kettle increased, the equivalent fully mixed kettle number m decreased from 1.037 to 1.034, and the change was very small and basically negligible. It can be seen that the influence of flow rate on the original polymerization kettle is far greater than that on the new polymerization kettle. This may be because the backmixing degree of the original polymerization kettle is not high. After increasing the flow rate, under the action of radial flow, the backmixing degree of the material is further reduced, resulting in an increase in the equivalent fully mixed kettle number m. The new polymerization kettle is close to a fully mixed flow state, and the flow rate change has little effect on the mixing degree.

(4) Effect of material viscosity on residence time distribution.

The original polymerization kettle and the new polymerization kettle were respectively selected with different material viscosities at a fixed flow rate, and the experimental simulation of residence time distribution was carried out while keeping the unit power consumption relatively close; that is, the original polymerization kettle was selected to simulate the working conditions of working conditions 2 and 7, and the new polymerization kettle was selected to simulate the working conditions of working conditions 5 and 8. The results of the experiment are shown in Figure 8 and Table 2. It can be seen from Figure 8 that increasing the viscosity of the original polymerization kettle will delay the peak time, that is, the mixing rate is slower and the mixing time is longer. It can be seen from the statistical table shown in Table 2 that with the increase of viscosity, the equivalent full mixing kettle number m of the original polymerization kettle increased from 1.326 to 1.416, an increase of 6.79%. The new mixing kettle also has a slightly delayed peak time after the viscosity increases, but the equivalent full mixing kettle number m increases from 1.037 to 1.039, which only increases by 0.19%, which is a small change. It can be seen that the effect of viscosity on the original polymerization kettle is far greater than that on the new polymerization kettle. This may be because the original polymerization kettle is mainly radial flow, which increases viscosity, weakens fluid turbulence, and weakens backmixing in the kettle. The new polymerization kettle is mainly axial flow, with a high degree of backmixing, which increases viscosity and weakens the fluid flow rate.

In this embodiment, in order to further ensure the accuracy of the residence time distribution calculated by the simulation, in step S400, a tracer residence time distribution experiment can be performed based on the optimal polymerization reactor-agitation blade combination shown in Figure 3 and compared with the simulation results. The experimental process will be briefly described below with specific parameters.

Specifically, the parameters of the polymerization kettle and the stirring paddle are set: the inner diameter of the stirring tank of the polymerization kettle is T=0.5m, the ratio of the liquid level in the tank to the tank diameter is 0.7, and the inlet and outlet diameters are 0.02m; the stirring paddle is a double-bladed 45-degree inclined paddle with a blade diameter of 0.35m, the distance between the bottom double-bladed 45-degree inclined paddle and the bottom of the tank is 0.18m, the flow rate is 5.1L/min, and the stirring speed is 70r/min; the experimental simulation material is a mixed solution of water and syrup, with a viscosity of 0.146Pa·s and a density of 1280kg/m ³ ; the tracer concentration is monitored at the outlet of the polymerization kettle every 10s. Based on the above parameters, the tracer concentration data at every 10s interval can be obtained, and then the corresponding residence time distribution can be calculated. The residence time distribution obtained by the experiment is compared with the results of the simulation, and the residence time distribution curves of the experiment and the simulation can be obtained as shown in Figure 9. As can be seen from Figure 9, the simulation results are close to the experimental values, so it can be considered that the simulation results are closer to the actual situation.

The above describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited by the above embodiments, and the above embodiments and the specification only describe the principles of the present application. The present application may have various changes and improvements without departing from the spirit and scope of the present application, and these changes and improvements fall within the scope of the present application for which protection is sought. The scope of protection claimed by the present application is defined by the attached claims and their equivalents.

Claims (8)

1. A simulation method for the flow characteristics of a polyvinyl alcohol resin polymerization kettle is characterized by comprising the following steps:

s100: combining polymerization kettles with different structures and stirring paddles of various types to obtain a plurality of groups of different polymerization kettles-stirring paddles combinations and constructing corresponding three-dimensional simulation models;

S200: selecting a material model and a fluid model which meet the requirements, and carrying out pressure speed coupling solution on the obtained polymerization kettle-stirring paddle combination through an algorithm to obtain the polymerization kettle-stirring paddle combination with the optimal performance;

s300: based on the obtained polymerization kettle-stirring paddle combination with the optimal performance, calculating a steady-state flow field with only polymer as a medium, and performing unsteady-state calculation by taking the obtained steady-state flow field as an initial value to further obtain the residence time distribution of a material model;

s400: and verifying the simulation result through experiments.

2. The simulation method of the flow characteristics in the polyvinyl alcohol resin polymerizer according to claim 1, wherein in the step S200, the top of the polymerizer with the best performance is a standard elliptical head, and the bottom is a W-bottom elliptical head;

the stirring paddle combination with the optimal performance comprises a double-folded blade paddle and a bent-edge straight blade paddle; wherein, bent limit straight blade oar is located the bottom of polymeric kettle, and the quantity of double-folded blade oar is at least one, and the interval sets up in the top of bent limit straight blade oar.

3. The simulation method of flow characteristics in a polyvinyl alcohol resin polymerizer according to claim 1, wherein in step S200, a single material model is used as the material model, and a kappa-epsilon standard turbulence model is used as the fluid model;

The blade movement of the stirring paddle adopts a multiple reference system, and pressure velocity coupling solution is carried out through a SIMPLE algorithm; wherein, the inlet boundary of the polymerization kettle is inlet flow, the outlet boundary is outlet pressure, and the wall surface is processed by adopting a standard wall surface function.

4. A simulation method for flow characteristics in a polyvinyl alcohol resin polymerizer according to any one of claims 1 to 3, wherein in the step S300, a simulation calculation of residence time distribution is performed by adding a tracer;

The tracer is instantaneously added at the inlet of the steady-state flow field after the calculation and convergence of the steady-state flow field;

monitoring the concentration of the tracer at the outlet of the flow field when performing unsteady state calculation; the calculation of the residence time distribution is based on the duration of time the tracer is added from the addition to the concentration towards the set point.

5. The simulation method for flow characteristics in a polyvinyl alcohol resin polymerizer according to claim 4, wherein calculation of residence time distribution is performed by monitoring the duration of time for which the tracer is added to a concentration of 0.

6. The simulation method for the flow characteristics in the polyvinyl alcohol resin polymerization kettle according to claim 4, wherein the concentration of the tracer is monitored once every time delta t _i time from the beginning of the tracer to the duration of the concentration trend set value, and the corresponding concentration c _i of the tracer is obtained;

The residence time distribution is represented by a residence time distribution density function E (t), and the specific calculation formula is as follows:

。

7. The simulation method of flow characteristics in a polyvinyl alcohol resin polymerizer according to claim 6, wherein in step S300, after the residence time distribution is obtained, the accuracy of the simulation result is determined by calculating the dispersion σ _t ² of the residence time distribution and the equivalent total number m of the polymerizers, and the specific calculation formula is as follows:

；

；

；

wherein, Indicating the average residence time.

8. The method for simulating the flow characteristics in a polyvinyl alcohol resin polymerization vessel according to claim 1, wherein the factors affecting the flow characteristics in the polymerization vessel include a flow field distribution structure in the polymerization vessel, a rotational speed of a stirring paddle, a flow rate of a fluid in the polymerization vessel, and a viscosity of a polymer;

Then in step S400, experiments were performed on all the polymerizer-paddle combinations to verify the simulation results through four aspects of flow field distribution structure in the polymerizer, rotational speed of the paddles, flow rate of fluid in the polymerizer, and viscosity of the polymer.