1,721,186 research outputs found
Modeling multiradicals in crosslinking MMA/EGDMA bulk copolymerization
No full textIn the present work, a kinetic model of crosslinking free-radical copolymerization based on multidimensional population balances and accounting for multiradicals is developed. The model is applied to the simulation of bulk copolymerization of methyl-methacrylate (MMA)/ethylene-glycol-dimethacrylate (EGDMA). A literature criterion proposed to elucidate the model type best suited for a given system (i.e., with or without MRs) is extended to the industrially relevant case of diffusion limited systems. Moreover, a master plot for the system under investigation is proposed: given the reaction recipe, the error on the gel point prediction employing the monoradical assumption is identified, thus allowing more conscious model selection. The relevance of active chains bearing multiple active sites (multiradicals) in the MMA/EGDMA system can be appreciated estimating the relative error on the gel point prediction vs. crosslinker content, when comparing the predictions of two models, one accounting and the other neglecting such species. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
A semi-parametric dynamic conditional correlation framework for risk forecasting
No full textWe develop a novel multivariate semi-parametric framework for joint portfolio Value-at-Risk (VaR) and Expected Shortfall (ES) forecasting. Unlike existing univariate semi-parametric approaches, the proposed framework explicitly models the dependence structure among portfolio asset returns through a dynamic conditional correlation (DCC) parameterization. To estimate the model, a two-step procedure based on the minimization of a strictly consistent VaR and ES joint loss function is employed. This procedure allows to simultaneously estimate the DCC parameters and the portfolio risk factors. The performance of the proposed model in risk forecasting on various probability levels is evaluated by means of a forecasting study on the components of the Dow Jones index for an out-of-sample period from December 2016 to September 2021. The empirical results support effectiveness of the proposed framework compared to a variety of existing approaches
Self-Diffusion of Small Molecules into Rubbery Polymers: A Lattice Free-Volume Theory
No full textIn the framework of the Free Volume Theory, a new equation was derived for the evaluation of self-diffusion coefficients of small molecules in polymers above the mixture glass transition temperature. The derivation of the equation turned out to be straightforward once the equivalence between the free-volume and the unoccupied volume given by Thermodynamic Lattice Theories is assumed. A parameter evaluation scheme is proposed, which is substantially simpler compared to the conventional Vrentas-Duda approach, even without losing generality. The key assumption is discussed and its consistency is verified from a numerical viewpoint. A comparison with experimental solvent self-diffusion coefficients for several solvent/polymer binary systems confirmed that the proposed theory presents good correlative ability over wide temperature and composition ranges. Moreover, the introduced thermodynamic foundation allows one to easily include the pressure effect too. In the frame of the proposed Lattice-Free Volume theory, the sizes of the polymer jumping units decrease with temperature and increase with pressure. Such behavior converges with theoretical expectations and opens the way for a predictive Free Volume Theory
Synthesis of bioresorbable polymers for medical applications
No full textBiodegradable polymers are very attractive materials because of the possibility to degrade them back naturally to their monomeric forms, with clear advantages for many applications. Degradable polyesters are the most widely used group of synthetic biodegradable polymers for medical applications, with many of its members already Food and Drug Administration approved and used as suture materials, implants, and drug delivery vehicles. This class of polymers has the ability to control molecular weight, and this ability is a key feature in tuning their mechanical properties and their degradation behaviors. In this chapter, we review the properties of most common types of poly(α-esters), starting from their monomers. We focus on the development of suitable mathematical models to describe both their polymerization kinetics, carried out by ring-opening polymerization, and their degradation kinetics, and we show how these models can be beneficial to understand and control the preparation and fate of these materials
Experimental and Modeling Study of Acrylamide Copolymerization with Quaternary Ammonium Salt in Aqueous Solution
No full textThe free-radical copolymerization of acrylamide with the cationic monomer DMAEA-Q in aqueous medium is investigated with special attention to its composition behavior, which reveals to be affected by the electrostatic interactions between the charges in the system. The reaction kinetics is determined by in situ 1H NMR experiments, showing a peculiar dependence of the copolymer composition upon initial monomer and electrolyte concentrations. A kinetic model simulating the evolution of copolymer composition as a function of conversion is developed, accounting for the nonconventional features of the system. Namely, a description of the electrostatic interactions based on the DLVO theory is introduced to define a functional dependence of the rate coefficients on the ionic strength. Secondary reactions are also included due to the acrylic nature of both monomers. The proposed model is applied to estimate the corresponding reactivity ratios and proves to exhibit the correct functionality with respect to monomer concentration and ionic strength. (Figure Presented)
Viscosity and drop size evolution during suspension polymerization
No full textAnnually, suspension polymerization produces kilotons of material with properties given by process conditions. The prediction of material properties requires a relevant description of processes on various scales from the molecular level to reactor design. The polymerization occurring on the molecular scale was described by a kinetic scheme of homopolymerization. The molecular level was connected to the meso-scale by the viscosity evolution inside a single monomer/polymer drop. The viscosity model follows the change in the reaction mixture composition and its predictions were validated by the rheology measurements. During the suspension polymerization, the viscosity evolution affects the dispersion breakage and coalescence on the meso-scale, which is closely connected to the flow conditions given by the reactor design and operation conditions. This complex problem was described by a coupled CFD-PBE model. The presented study proposes a modeling approach to control the suspension polymerization by stirring speed to obtain the desired drop size. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4229–4239, 2016
Process design and energy requirements for the capture of carbon dioxide from air
No full textA process to capture carbon dioxide from air to reduce its atmospheric concentration and to mitigate climate change is studied. It is based on the absorption of carbon dioxide in a sodium hydroxide solution, its precipitation as calcium carbonate, and its release as pure gas stream through oxy-fuel calcination. The process utilizes existing commercial technologies wherever possible, particularly in the case of the absorber, whose design is carried out in detail. The analysis allows deriving material and energy balances for the whole process and determining energy demands that can be used for a technical, economical, and environmental feasibility evaluation of the technology. In particular, it indicates that the real specific energy demand is larger than the heat released to emit the same amount of CO2 by the combustion of coal, and smaller than that of methane. (c) 2006 Elsevier B.V. All rights reserved
Particle state dependent radical desorption and its effect on the kinetics of emulsion polymerization
Full text linkRadical desorption from polymer particles is a kinetic event peculiar to the emulsion polymerization process. A careful modeling of this phenomenon is highly valuable in order to achieve accurate predictions of polymerization rate and average properties of molecular weight. In this work, radical desorption is described accounting for an aspect fully neglected in previous modeling literature. Specifically, particle state dependent desorption coefficients are used instead of a single average coefficient, and the corresponding rate expressions are developed and applied to the solution of the well-known Smith-Ewart equations. Parametric model simulations show that the higher level of detail introduced in the description of radical desorption improves the accuracy of the predicted values of the average number of radicals per particle, especially in the cases of high desorption rate and slow reactions in the aqueous phase. © 2014 American Chemical Society
Optimal Design Procedure of Dual-Reflux Pressure Swing Adsorption Units for Nonlinear Separations
Full text linkIn the frame of dual-reflux pressure swing adsorption processes, design strategies applicable to the complete separation of binary gas mixtures are available only in the case of linear adsorption isotherms. Therefore, in this work we propose a simple and efficient design procedure which enables the selection of suitable operating conditions when sharp separation is required and Langmuir adsorption isotherms are involved. Starting from the available strategy for the linear adsorption case, we adapt it to the nonlinear case tuning properly the pressure ratio. Then, the resulting design strategy is validated by application to selected study cases involving different values of key parameters such as different levels of nonlinearity of the isotherms, but also different values of the pressure ratio, feed composition, and selectivity
Solution of population balance equations by logarithmic shape preserving interpolation on finite elements
Full text linkA new numerical approach for solving population balance equations (PBE) is proposed and validated. The method employs a combination of basis functions, defined on finite elements, to approximate the sought distribution function. Similarly to other methods of the same family, the PBE are solved only in a finite number of values of the internal coordinate (grid points). The peculiarity of the method is the use of a logarithmic, shape-preserving interpolation (LSPI) procedure to estimate the values of the distribution in between grid points. The main advantages of the LSPI method compared to other approaches of the same category are: (i) the stability of the numerical approach (i.e., the absence of oscillations in the distribution function occurring when using “standard” cubic splines and a low number of elements), and (ii) the conceptual and implementation simplicity, as no mathematical manipulation of the PBE is required
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