CN116013439A - Method for on-line prediction of crystallinity and density of ethylene high-pressure polymerization product - Google Patents
Method for on-line prediction of crystallinity and density of ethylene high-pressure polymerization product Download PDFInfo
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Abstract
The invention provides a method for on-line prediction of crystallinity and density of an ethylene high-pressure polymerization product. The prediction method comprises the following steps: a) Constructing an ethylene high-pressure polymerization reactor mass conservation and energy conservation and momentum conservation equation based on real-time reaction conditions; b) Based on the reactor conservation equation and the reaction conditions, calculating the microstructure of all polymer molecules in the selected simulation volume through Monte Carlo simulation; c) Encoding the microstructure of the polymer molecules into a topological information recording mode which can be identified by a molecular dynamics simulation tool LAMMPS; d) Polymer molecular chains for molecular dynamics simulation are randomly selected from all polymer molecules, and topology information of microstructure is input into LAMMPS to predict crystallinity and density of the polymer on line. The method for the crystallinity and the density of the ethylene high-pressure polymerization product not only can accurately predict the crystallinity and the density of the polymer from the first principle, but also can correlate with real-time high-pressure polymerization reaction conditions to realize online prediction of the crystallinity and the density.
Description
Technical Field
The invention belongs to the field of high-pressure free radical polymerization, and particularly relates to a method for on-line prediction of crystallinity and density of an ethylene high-pressure polymerization product.
Background
The polymer product obtained by free radical polymerization of ethylene under high temperature and high pressure conditions is called low density polyethylene, also called high pressure polyethylene (LDPE). LDPE is the lightest variety in polyethylene resin, and the density is 0.910-0.925 g/cm 3 Between them. The LDPE has high branching degree and low crystallinity, so that the LDPE has lower density compared with HDPE and LLDPE, and also has good toughness and processability, and is widely used for film materials and packaging materials.
Crystallinity and density are important performance parameters of LDPE, and besides experimental measurement methods, density prediction models of LDPE are more common. For example, the empirically-related method described in Buchelli, a., et al, modeling fouling effects in LDPE tubular polymerization reactions.1. Modeling process determination, industrial and Engineering Chemistry Research,2005,44 (5), 1474-1479 predicts the density of polyethylene by temperature and pressure, but this method does not correlate the density with the microstructure of the polymer. There are also models relating density to short chain branch content, the method described in Rokudai, m.; okada, T., characterization of Low-Density Polyethylenes and Relationships among Melt Index, density, and Molecular Structural parameters. Nihon Reoroji Gakkaishi,1980,8 (4), 154-160. However, the method still belongs to an empirical model, the prediction accuracy is obviously affected by model parameters in the empirical correlation, and the applicability is limited. Yet another more complex method of calculating polymer density, the PC-SAFT model, is described in Gross, j; sadowski, G., perturbed-chain SAFT: an equation of state based on a perturbation theory for chain molecules. Industrial & Engineering Chemistry Research,2001,40 (4), 1244-1260. Although the model is a state equation model with a result close to that of a real system and successful application, the model is limited to density prediction and cannot predict the crystallinity of the polymer. Thus, there is still a lack of methods for simultaneously predicting the crystallinity and density of LDPE from the first principles of nature.
The microstructure of the polymer has important influence on the properties such as crystallinity, density and the like, and is directly controlled by the polymerization reaction conditions. If the relation between the polymerization condition, the microstructure and the product performance can be established, the microstructure of the polymer can be controlled by on-line regulating and controlling the polymerization condition, and the product which meets the expected polymer performance can be produced. CN102666608B proposes calculating the density of the current polymer in the high pressure reactor by means of a model, feeding back the density value as a controlled variable to a controller, and further regulating the operating parameters of the high pressure polymerization apparatus to achieve switching of the polymer product grades with different densities. The density prediction model in the method is only related to dynamics, thermodynamics and material balance, and the density prediction is still based on an empirical formula. Moreover, due to the loss of a large amount of molecular structure information, the performance prediction cannot be associated with the detailed microstructure of the polymer, and the crystallinity cannot be predicted, so that the precise control of the polymerization conditions on the product performance is difficult to realize.
Disclosure of Invention
In view of the foregoing, it is desirable to provide a method for online prediction of crystallinity and density of an ethylene high-pressure polymerization product, which not only can accurately predict the crystallinity and density of a polymer from the first principle, but also can correlate real-time high-pressure polymerization conditions to realize online prediction of crystallinity and density.
A method for on-line prediction of crystallinity and density of an ethylene high pressure polymerization product comprising the steps of:
a) Constructing an ethylene high-pressure polymerization reaction system mass conservation and energy conservation and momentum conservation equation based on real-time reaction conditions;
b) Calculating the microstructure of all polymer molecules in the selected simulation volume by Monte Carlo simulation based on the conservation equation and the reaction conditions constructed in the step a);
c) Encoding the microstructure of the polymer molecules into a topological information recording mode which can be identified by a molecular dynamics simulation tool LAMMPS;
d) Polymer molecular chains for molecular dynamics simulation are randomly selected from all polymer molecules, and topology information of microstructure is input into LAMMPS to predict crystallinity and density of a polymerization product on line.
In the step, the reaction conditions include reactor size, polymerization temperature, polymerization pressure, ethylene feed amount, initiator feed amount, telogen feed amount, and if the reactor uses a cooling water jacket for heat exchange, the temperature and flow rate of the circulating water for cooling are also included in the reaction conditions. Typically, tubular reactors use a cooling water jacket for heat removal, and a small portion of the tank reactors will also have a cooling water jacket.
In the step, the polymerization temperature and polymerization pressure used as the reaction conditions are temperature distribution data and pressure distribution data along the axial direction of the reactor. In high pressure polymerization reactors the temperature and pressure tend not to be constant but rather have a distribution along the axial distance of the reactor. The temperature distribution is generally represented as an incremental type or a multistage-first-increment-then-decrement type, and the pressure distribution is generally represented as a decremental type.
In the step, mass conservation is that in a reactor system, the related species comprise ethylene monomers, an initiator and a telogen which are injected into the reactor at any site, and the site of the injection of the species into the reactor comprises a reactor inlet, a reactor outlet and a side wall feed inlet; conservation of energy is conservation of heat in the reactor system, and comprises heat exchange of substances in the reactor, heat exchange of substances in the reactor and the wall surface of the reactor, and heat exchange of the wall surface of the reactor and cooling water in a cooling water jacket; conservation of momentum is conservation of pressure within the reactor system.
In the step, the Monte Carlo simulation method may be a classical Monte Carlo method or a series of novel Monte Carlo methods which are mutated and still maintain random sampling function. Monte Carlo simulation is a computational method known in the art for modeling the detailed molecular structure of polymers, and classical Monte Carlo methods are generally based on the Gillespie equation, which describes the probability of each reaction occurring at any simulation time in proportion to its reaction rate, as described, for example, in Gillespie, D.T., A general method for numerically simulating the stochastic time evolution of coupled chemical reactions. Journal of Computational Physics,1976,22 (4), 403-434. Because of the long computational duration of classical Monte Carlo methods, there are also some new Monte Carlo methods in the art that have been mutated to maintain random sampling for purposes of algorithm acceleration, computational resource conservation, etc., such as described in Gillespie, D.T., approximate accelerated stochastic simulation of chemically reacting systems.2001,115 (4), 1716-1733.
In the step, the microstructure information of the polymer obtained by Monte Carlo simulation comprises the main chain length, the number, the locus and the length of the long branched chains, the number, the locus and the length of the short branched chains of all polymers in the selected simulation volume, and the selected simulation volume is 1 multiplied by 10 -20 ～1×10 -18 m 3 . The simulation volume is an important model parameter that affects accuracy and timeliness of Monte Carlo simulation results. If the simulation volume is too small, the frequencies of different reaction types in the sample cannot accurately describe the real reaction process, which can interfere with the randomness of the sampling result, so that the Monte Carlo simulation result is distorted; conversely, if the simulation volume is too large, the calculation amount is increased seriously, and the simulation time is prolonged.
In the step, a topology information recording mode for encoding a polymer molecular microstructure belongs to a general language system based on a computer graph theory, and can be identified by an input interface of a molecular dynamics simulation tool LAMMPS.
In said step, said random selection from all polymer molecules is used for fractionationPolymer molecular chains simulated by sub-dynamics are specifically: the polymer molecular chains used for molecular dynamics simulation are screened out from all polymer molecules in an equilogarithmic spacing mode, so that the molecular weight of the selected polymer is uniformly distributed in the logarithmic space, and the number of the screened polymer molecular chains ranges from 10 to 50. The molecular weight (weight average molecular weight) of the ethylene high pressure polymerization product is usually 10 3 -10 6 The equilogarithmic molecular weight intervals were used as criteria for random screening in order to obtain representative polymer chains. The number of the polymer molecular chains screened for molecular dynamics simulation is not too small, otherwise, larger deviation between simulation results and real conditions can be caused; however, the number of the molecular chains is not too large, otherwise the calculation burden of the molecular dynamics simulation is seriously increased.
In the step, the molecular dynamics simulation uses a LAMMPS tool box, and adopts a coarse grain molecular dynamics simulation method, wherein the simulation force field is an OPLS-UA force field.
In the step, the temperature set by online prediction of the crystallinity and density of the polymer is 320-370K, and the pressure is 0.1MPa. The polymerization reactor is in a high temperature and high pressure state, and the polymerization product does not crystallize. The pressure of the polymer is usually reduced to near normal pressure during the post-treatment after the polymer exits the reactor, the temperature is gradually reduced, and the polymer begins to crystallize and fix during the process. It is believed that the structured polymer has an initial crystallization temperature of about 370K to about 380K, while the low density polyethylene has more branches and a relatively low crystallization temperature. Considering the time-consuming problem of the molecular dynamics simulation process, if the simulation temperature is set too high, the polymer is difficult to crystallize, and the simulation time will be significantly prolonged. A priori experience shows that the crystallization process of the polymer can be simulated in a limited time at 350K. Further lowering the temperature can rapidly calculate the crystallization state, but the temperature deviates from the actual crystallization process, resulting in a large deviation of the prediction result. Therefore, when the crystallinity and density are predicted online, the pressure of the set simulation scene can be set to be normal pressure, and the temperature is set to be 320-370K more proper.
In the method for on-line predicting the crystallinity and density of the ethylene high-pressure polymerization product, the crystallinity and density are obtained by inputting the topological information of the molecular structure of the polymer into a molecular dynamics simulation tool LAMMPS for calculation, and the method predicts from a first principle, so that the accuracy and universality of the crystallinity and density prediction method are ensured; the molecular structure information of the polymer is obtained from Monte Carlo simulation, and the Monte Carlo simulation is a conservation equation constructed based on real-time reaction conditions, so that timeliness of a prediction result is ensured, and online prediction of crystallinity and density is realized.
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FIG. 1 is a schematic diagram of an embodiment of the present invention for on-line prediction of crystallinity and density of ethylene high pressure polymerization products.
Detailed Description
The method for on-line prediction of crystallinity and density of ethylene high pressure polymerization products provided by the present invention is described in detail below in conjunction with FIG. 1.
As shown in fig. 1, a method for on-line predicting crystallinity and density of an ethylene high pressure polymerization product comprises the steps of:
s1, constructing an ethylene high-pressure polymerization reaction system mass conservation and energy conservation and momentum conservation equation based on real-time reaction conditions;
s2, calculating the microstructure of all polymer molecules in the selected simulation volume by Monte Carlo simulation based on a reaction system conservation equation and reaction conditions;
s3, encoding the microstructure of the polymer molecules into a topological information recording mode which can be identified by a molecular dynamics simulation tool LAMMPS;
s4, randomly screening polymer molecular chains for molecular dynamics simulation from all polymer molecules, inputting topological information of microstructure into LAMMPS, and predicting crystallinity and density of a polymerization product on line.
In the step, the reaction conditions include reactor size, polymerization temperature, polymerization pressure, ethylene feed amount, initiator feed amount, telogen feed amount, and if the reactor uses a cooling water jacket for heat exchange, the temperature and flow rate of the circulating water for cooling are also included in the reaction conditions.
In the step, the polymerization temperature and polymerization pressure used as the reaction conditions are temperature distribution data and pressure distribution data along the axial direction of the reactor.
In the step, mass conservation is that in a reactor system, the related species comprise ethylene monomers, an initiator and a telogen which are injected into the reactor at any site, and the site of the injection of the species into the reactor comprises a reactor inlet, a reactor outlet and a side wall feed inlet; conservation of energy is conservation of heat in the reactor system, and comprises heat exchange of substances in the reactor, heat exchange of substances in the reactor and the wall surface of the reactor, and heat exchange of the wall surface of the reactor and cooling water in a cooling water jacket; conservation of momentum is conservation of pressure within the reactor system.
In the step, the Monte Carlo simulation method may be a classical Monte Carlo method or a series of novel Monte Carlo methods which are mutated and still maintain random sampling function.
In the step, the microstructure information of the polymer obtained by Monte Carlo simulation comprises the main chain length, the number, the locus and the length of the long branched chains, the number, the locus and the length of the short branched chains of all polymers in the selected simulation volume, and the selected simulation volume is 1 multiplied by 10 -20 ～1×10 -18 m 3 A further preferred simulation volume is 1X 10 -19 ～1×10 -18 m 3 。
In the step, a topology information recording mode for encoding a polymer molecular microstructure belongs to a general language system based on a computer graph theory, and can be identified by an input interface of a molecular dynamics simulation tool LAMMPS.
In the step, the molecular weight of the polymer molecular chains used for molecular dynamics simulation is selected at random to be equal logarithmic interval (also called equal logarithmic interval), the number of the selected polymer molecular chains is 10-50, and the number of the polymer molecular chains is 20-30. In an alternative embodiment of the invention, it is assumed that the smallest of all polymer molecules has a molecular weight a and the largest has a molecular weight b. Assuming that the number of molecules used for molecular dynamics simulation is x, an equal-ratio array of length x is generated between [ a, b ], which contains x numbers. Based on each value in the array of ratios, the closest molecular weight molecules are selected from all polymer molecules and used as the molecules for molecular dynamics simulation.
In the step, the molecular dynamics simulation uses a LAMMPS tool box, and adopts a coarse grain molecular dynamics simulation method, wherein the simulation force field is an OPLS-UA force field.
In the step, the temperature set by online prediction of the crystallinity and density of the polymer is 320-370K, the pressure is 0.1MPa, and the more preferable temperature is 340-360K.
The invention is further illustrated by the following examples.
Example 1
According to the method for on-line prediction of crystallinity and density of ethylene high pressure polymerization product shown in fig. 1, on-line prediction of crystallinity and density is performed on the product of a certain ethylene high pressure polymerization industrial reactor. The high-pressure polymerization reactor is a tubular reactor, and has four reactor sections, and the total length of the reactor is 1760m. The inlet temperature of the first reactor zone was about 437K and the inlet pressure was about 230MPa. The initiator consists of three peroxides of different decomposition rates, injected at the start of each reactor zone. Ethylene and propylene (telogen) are injected only in the first reactor zone. Parameters of each reaction condition include reactor size, temperature distribution along the tube side, pressure distribution along the tube side, feeding amount of each species (ethylene, initiator and telogen) in each reactor zone, circulating water temperature and flow for cooling, which are provided by device real-time monitoring data.
The reactor was subjected to mass conservation, energy conservation, and momentum conservation calculations. Monte Carlo simulation was performed on the basis of conservation equations using a classical Monte Carlo model with a selected simulation volume of 1X 10 -18 m 3 . The microstructure of all polymer molecules in the simulation volume is obtained through calculation, and the microstructure comprises main chain, long-chain branched chains and short-chain branched chains.
A universal language system based on computer graph theory is adopted, and a polymer molecular microstructure obtained by Monte Carlo simulation is encoded into a topological information recording mode which can be identified by a molecular dynamics simulation tool LAMMPS and is used for inputting polymer molecules into the LAMMPS.
From all polymer molecules obtained by simulation, 20 chains were randomly screened in an equilogarithmic interval. The molecular dynamics simulation uses a LAMMPS tool box, topology information of the microstructure of the screened polymer chain is input into LAMMPS, a coarse grain molecular dynamics simulation method is adopted, a simulation force field is an OPLS-UA force field, the simulation temperature is 350K, the pressure is 0.1MPa, and the crystallinity and the density of the polymer are predicted on line. The calculation process is performed on some large super computing platform.
The crystallinity predicted was stabilized at 41.0% and the density was 0.924kg/m 3 The total calculated time period was 45 hours. The actual density of the polymerization product of the reactor provided by the factory was 0.921kg/m 3 The crystallinity was 43.1%. The relative error between the predicted value and the actual value of the crystallinity was-4.9%, and the relative error of the density was 0.3%.
Example 2
Example 2 differs from example 1 in that 5 chains were randomly screened from all polymer molecules obtained from the simulation in an equilogarithmic interval.
The crystallinity predicted was stabilized at 35.2% and the density was 0.911kg/m 3 The total calculated time period was 18 hours. The relative error between the predicted value and the actual value of the crystallinity was-18.3%, and the relative error of the density was 1.1%.
Example 3
Example 3 differs from example 1 in that 40 chains were randomly screened from all polymer molecules obtained from the simulation in an equilogarithmic interval.
The crystallinity predicted was stabilized at 45.3% and the density was 0.925kg/m 3 The total calculated time period was 103 hours. The relative error between the predicted value and the actual value of the crystallinity was 5.1%, and the relative error of the density was 0.3%.
The foregoing examples illustrate only a few embodiments of the invention and are described in detail herein without thereby limiting the scope of the invention. It will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the spirit of the invention.
Claims (10)
1. A method for on-line prediction of crystallinity and density of an ethylene high pressure polymerization product, comprising the steps of:
a) Constructing an ethylene high-pressure polymerization reaction system mass conservation and energy conservation and momentum conservation equation based on real-time reaction conditions;
b) Calculating the microstructure of all polymer molecules in the selected simulation volume by Monte Carlo simulation based on the conservation equation and the reaction conditions constructed in the step a);
c) Encoding the microstructure of the polymer molecules into a topological information recording mode which can be identified by a molecular dynamics simulation tool LAMMPS;
d) Polymer molecular chains for molecular dynamics simulation are randomly selected from all polymer molecules, and topology information of microstructure is input into LAMMPS to predict crystallinity and density of a polymerization product on line.
2. The method according to claim 1, wherein the reaction conditions include reactor size, polymerization temperature, polymerization pressure, ethylene feed, initiator feed, telogen feed, and if the reactor is heat exchanged with a cooling water jacket, the temperature and flow rate of the circulating water for cooling are also included in the reaction conditions.
3. The prediction method according to claim 2, wherein the polymerization temperature and the polymerization pressure used as the reaction conditions are temperature distribution data and pressure distribution data along the axial direction of the reactor.
4. The method of claim 1, wherein conservation of mass is conservation of species within the reactor system, the species involved comprising ethylene monomer, initiator and telogen injected into the reactor at any point comprising the reactor inlet, outlet, sidewall feed inlet; conservation of energy is conservation of heat in the reactor system, and comprises heat exchange of substances in the reactor, heat exchange of substances in the reactor and the wall surface of the reactor, and heat exchange of the wall surface of the reactor and cooling water in a cooling water jacket; conservation of momentum is conservation of pressure within the reactor system.
5. The predictive method of claim 1, wherein the Monte Carlo simulation method is a classical Monte Carlo method or a mutated series of novel Monte Carlo methods that retain random sampling function.
6. The method of claim 1, wherein the molecular weight microstructure information obtained from Monte Carlo simulation includes backbone chain length, number of long chain branches, sites, and length of short chain branches, number of sites, and length of short chain branches of all polymers in a selected simulation volume, the selected simulation volume being 1X 10 -20 ～1×10 -18 m 3 。
7. The prediction method according to claim 1, wherein in the step c), the topology information recording method for encoding the microstructure of the polymer molecule belongs to a general language system based on computer graph theory, and can be identified by an input interface of a molecular dynamics simulation tool LAMMPS.
8. The method according to claim 1, wherein the polymer molecular chains for molecular dynamics simulation are randomly selected from all polymer molecules, specifically: the polymer molecular chains used for molecular dynamics simulation are screened out from all polymer molecules in an equilogarithmic spacing mode, so that the molecular weights of the selected polymers are uniformly distributed in the logarithmic space, and the number of the screened polymer molecular chains is 10-50.
9. The prediction method according to claim 1, wherein the molecular dynamics simulation uses LAMMPS tool box, and the simulated force field is OPLS-UA force field by using a coarse grain molecular dynamics simulation method.
10. The method according to claim 1, wherein the temperature at which the on-line prediction of the crystallinity and density of the polymer is set is 320 to 370K and the pressure is 0.1MPa.
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