With the help of a simple reaction-diffusion model with constant striation thickness the influence of micromixing on free-radical polymerization was investigated for several test reactions with discontinuous prepolymerization and jerky addition of selected reactants. Monomer conversion or mean values of molar mass and chemical composition cannot be expected to be very sensitive to micromixing effects. If molar mass distributions are to be used, problems will arise from the fact that the distribution of the polymer accumulated during prepolymerization covers mixing influences occurring after reactant feed. The instantaneous molar mass distribution would be more suitable. Time-integral distributions of chemical composition or sequence length in combination with appropriate test reactions proved to be feasible indicators for the effects of micromixing as it becomes possible to separate the distribution of the prepolymer from that of the polymer which is formed after addition when micromixing is to be investigated. 相似文献
ACOMP allows comprehensive, model-independent, near realtime monitoring of many different types of polymerization reactions. It provides conversion kinetics, and the evolution of average molar mass, intrinsic viscosity and average composition distributions (for copolymers). Here, recent advances in ACOMP will be summarized, dealing with continuous reactors, copolymerization, ‘living’ type reactions (NMP, RAFT, ATRP, ROMP), polyelectrolytes, heterogeneous phase reactions, including free radical reactions in emulsions, and predictive control. In the case of emulsion polymerization, a new approach will be presented in which the evolution of the characteristics of both the soluble phase – monomer conversion, polymer molar mass and intrinsic viscosity- and the dispersed phase – particle size – are simultaneously monitored. NSF CBET 0623531, BoR ITRS 019B, NASA NCC3-946, TIMES, PolyRMC (Tulane Center for Polymer Reaction Monitoring and Characterization). 相似文献
Three different long‐chain branch (LCB) formation mechanisms for ethylene polymerization with metallocenes in solution polymerization semi‐batch and continuous stirred‐tank reactors are modeled to predict the microstructure of the resulting polymer. The three mechanisms are terminal branching, C–H bond activation, and intramolecular random incorporation. Selected polymerization parameters are varied to observe how each mechanism affects polymer microstructure. Increasing the ethylene concentration during semi‐batch polymerization reduces the LCB frequency of polymers made with the terminal branching and intramolecular mechanisms, but has no effect on those made with the C–H bond activation mechanism, which disagrees with most previous data published in the literature. The intramolecular mechanism predicts that LCB frequencies hardly depend on polymerization time or ethylene conversion, which also disagrees with the published experimental data for these systems. For continuous polymerization reactors, experimental data relating polydispersity to LCB frequency can be well described with the terminal branching mechanism, but both C–H bond activation and intramolecular models fail to describe this experimental relationship. Therefore, detailed simulations confirm that the terminal branching mechanism is indeed the most likely mechanism for LCB formation when ethylene is polymerized with single‐site coordination catalysts such as metallocenes in solution polymerization reactors. 相似文献
The present work presents phenomenological models to describe the coordination polymerization of β-myrcene using the Ziegler–Natta catalyst system composed by neodymium versatate (NdV3), diisobutylaluminum hydride (DIBAH), and dimethyldichlorosilane. The kinetic parameters required to simulate the reactions are estimated, and the amount of DIBAH used as a chain transfer agent (CTA) is obtained by a data reconciliation strategy since it can participate in side reactions. Several experiments are performed at different conditions to evaluate the impact of key operation variables on the control of monomer conversion and average molar masses. It is shown that the initial NdV3, β-myrcene, and DIBAH concentrations exert strong influences on the course of the polymerization. The kinetic mechanism of Coordinative Chain Transfer Polymerization (CCTP) fits well with the data of final average molar masses and monomer conversion, while the dynamic trajectories of these variables are fitted better by kinetic mechanisms of more conventional coordination polymerizations, considering site deactivation and termination by chain transfer. In all cases, the proposed models are able to predict the experimental data well after successful parameter estimation and reconciliation of CTA concentrations, indicating that the kinetic mechanism can be characterized by different kinetic regimes. 相似文献
Summary: This work deals with the development of a process for the radical copolymerization of acrylonitrile and styrene in a dispersed medium. This process was carried out in a continuous stirred tank reactor, in the presence of a stabilizing agent produced in situ during the polymerization. The continuous phase is a polyol. Besides all elementary chemical mechanisms related to the copolymerization and to the synthesis and grafting of the stabilizing agent, this process involves several complex physical phenomena. A tendency model of the whole process was developed, using the corresponding mass balances and thermodynamics. Its unknown parameters were identified by use of an evolutionary algorithm and experimental data resulting from an adapted experimental strategy. This model was then validated and allowed to predict monomers and transfer agent conversions, amounts of solids and average molar masses, versus the operating conditions. 相似文献
In order to implement nonlinear control, nonlinear system identification must be performed, however, there are open questions concerning this field of process control, for example, experimental planning, model structure selection, parameter estimation, and validation. Therefore, the study of nonlinear model identification is a relevant unsolved problem that needs to be handled for nonlinear control synthesis. This paper presents the use of bifurcation theory, dynamic and stability analysis for nonlinear identification, and control of polymerization reactors. Peroxide‐initiated styrene‐solution polymerization reactors (lumped‐distributed) are investigated: batch, continuous stirred‐tank reactor (CSTR), and tubular reactors. Open and closed loop analyses are carried out using jacket temperature and weight average molecular weight setpoints as the bifurcation parameters. Phenomenological mathematical models, neural network nonlinear models, and an experimental data from a polymerization unit are employed for validating the proposed methodology in order to implement confident nonlinear controllers. 相似文献
On the basis of quantum chemical modeling, a kinetic scheme of methyl methacrylate polymerization initiated by benzoyl peroxide in the presence of ferrocene was proposed. The process runs by mechanism, which includes the reactions of free radical polymerization, and the reactions leading to formation and operability of two type coordination active sites that are capable of converting into each other. On the basis of the proposed scheme, a kinetic model was developed. This model quantitatively described the following: the experimentally determined time dependences of the methyl methacrylate conversion, the conversion dependencies of the number‐average and weight‐average molar masses of poly(methyl methacrylate), the stereoregularity values of poly(methyl methacrylate), and the time dependencies of the methyl methacrylate conversion upon its polymerization on poly(methyl methacrylate) macroinitiators obtained in radical‐initiated polymerization in the presence of ferrocene. As a result of solving the inverse kinetic problem, the parameters of temperature dependences of the reaction rate coefficients of the proposed kinetic scheme were found. 相似文献
The regularities of methyl methacrylate and styrene (co)polymerization in the presence of catalytic systems based on a Ni(II) complex combined with zinc and an aryl halide have been studied. The effects of temperature and catalytic system components on conversion are established. The molecular masses of the polymers linearly increase with monomer conversion, thus suggesting the controlled character of the polymerization. Reactivity ratios are calculated for methyl methacrylate-styrene copolymerization (rMMA = 0.45, rstyrene = 1.70) in the presence of NiBr2(PPh3)2/Zn/PhI. The rate of copolymerization is shown to decrease with an increase in methyl methacrylate concentration. The scheme of the process is proposed based on an analysis of the experimental and literature data. 相似文献
The main factors determining molar mass characteristics of copolymers formed in nonterminating copolymerization under the conditions of chain transfer to solvent are studied theoretically. The dependences of the mean polymerization degrees on conversion and monomer feed composition for various values of reactivity ratios are obtained. The results obtained greately differ from those for homopolymerization. This is explained by the contribution of cross-propagation reactions. In particular, it is shown that at ≪ and ≪ the azeotropic copolymerization proceeds like the living one even if homopolymerization of each monomer is accompained by extensive chain transfer to solvent. 相似文献
A detailed dynamic mathematical model that describes the evolution of particle size distributions (PSDs) during emulsion copolymerization reactions in a continuous loop reactor was developed and compared with experimental data. The model is based on the assumption that two distinct particle populations exist: precursor particles and stable latex particles. Precursor particles are colloidally unstable and therefore may undergo coagulation with other precursors and be absorbed by stable latex particles. It is shown that the kinetic model is able to reproduce the rather complex dynamic behavior of the vinyl acetate/Veova10 emulsion copolymerization in a continuous loop reactor, including the development of oscillatory responses of PSDs during reaction start‐up. It is also shown that, for the studied polymerization system, oscillatory responses are obtained only when both particle populations are assumed to exist and when both coagulative and micellar particle nucleations are simultaneously considered. 相似文献
The effects of acrylonitrile (AN) water solubility on the limiting conversion and copolymer composition of the AN and AN/vinylidene chloride (VDC) suspension polymerization were investigated. It was found that AN dissolved in aqueous phase does not transfer back to oil phase in AN suspension homopolymerization but partially does in AN/VDC suspension copolymerization, and thus the limiting conversion is lowered and decreases with water/oil ratio increasing in both AN and AN/VDC suspension polymerization. For the continuous transport of AN in aqueous phase to oil phase during suspension polymerization, the composition distribution of AN/VDC copolymer prepared by suspension polymerization is narrower than that by bulk polymerization. The calculated composition of AN/VDC suspension copolymer with considering AN water solubility is consistent with the experimental data. 相似文献
In behalf of a detailed study on the course of copolymerization reactions, this paper describes an improved and generally applicable experimental method and an efficient computational procedure to match. The experimental method is based on quantitative gas chromatography, and permits frequent measurement of the monomer feed composition throughout (co)polymerization processes at pressures up to 40 kgf/cm2 ( = 38.7 atm). The given method is applied to the study of the radical copolymerization of ethylene with vinyl acetate in a series of kinetic experiments, at 62°C and 35 kgf/cm2 ( = 33.9 atm) in tert-butyl alcohol, in which 20–40% conversion is reached. Monomer feed composition and degree of conversion are entered into a computational procedure based on nonlinear least-squares methods applied to the integrated version of the copolymer equation. The experimental data, covering a region of ethylene molar feed fractions between 0.24 and 0.74 and copolymer concentrations up to 8 wt-%, are precisely consistent with the usual model. The respective reactivity ratios are r?e = 0.743 ± 0.005 and r?v = 1.515 ± 0.007. 相似文献
Multiscale mixing phenomena in stirred‐tank polymerization reactors are mainly caused by stir agitation, which performs a key function in macroscopic and microscopic flow fields. Both macroscopic and microscopic flow fields interact with each other and significantly affect the microstructure and product distribution of the resultant polymers. In this work, a computational fluid dynamics model combining the moment method used in the polymerization engineering field is implemented and validated using open data. Multiscale properties are characterized in terms of macroscopic mixing fields and the polymer microscopic structure of the atom transfer radical copolymerization system of methyl methacrylate and 2‐(trimethylsilyl) ethyl methacrylate. Agitation in a 3D stirred tank is also thoroughly studied by using the multiple reference frame approach, and the effects of several important parameters, such as impeller speed, impeller types, and feeding position, on the macroscopic and microscopic flow fields are investigated on the basis of the validated model. Interdependent relationships among agitation, multiscale flow fields, and polymerization are described clearly. The results highlight the function of stirring and provide useful guidelines for the scale‐up of stirred‐tank polymerization reactors.
Frontal copolymerization is a process in which a spatially localized reaction zone propagates into a mixture of two monomers, converting them into a copolymer. In the simplest case of free‐radical copolymerization, a mixture of monomers and initiator is placed into a test tube. Reaction is initiated at one end of the tube, and a self‐sustained thermal wave, in which chemical conversion occurs, develops and propagates through the tube. We develop a mathematical model of the frontal copolymerization process and analytically determine the structure of the polymerization wave, the propagation velocity, maximum temperature, and degree of conversion of the monomers. Specifically, we examine their dependence on reactivity ratios as well as other kinetic parameters, monomer feed composition, and exothermicity of the reactions. Our analytic results are in good quantitative agreement with both direct numerical simulations of the model and experimental data, which are also presented in the paper.
Dependence of front velocity on monomer feed composition for different heat release parameters. 相似文献