Príamo A. Melo

Characterization of the residence time distribution in loop reactors

Abstract A mathematical model is developed to describe the residence time distribution (RTD) characteristics of the liquid-phase loop reactor, taking into account the capacitance of the recycling pump. Based on the transfer function of the reactor model, the dynamic behavior of the limiting cases is investigated in order to characterize the main modes of macromixing in the reactor vessel. The full mathematical model is then used to simulate the RTD of the loop reactor. The full model comprises a set of coupled parabolic partial and ordinary differential equations and is solved numerically. The numerical approach allows the implementation of a time domain-based parameter estimation procedure for evaluation of the RTD parameters of the reactor model. Tracer pulse experiments are then carried out in a lab-scale polymerization loop reactor in order to provide data for the parameter estimation procedure. It is shown that simulated reactor responses match quite well with experimental data. Besides, the estimation procedure employed is able to provide parameter estimates with high accuracy and low standard deviations. Therefore, the modeling approach presented here may be used for reliable estimation of the macromixing parameters and proper description of the RTD dynamics of loop reactors.

The bifurcation behavior of continuous free-radical solution loop polymerization reactors

The bifurcation behavior of continuous free-radical solution loop polymerization reactors is analyzed in this work. A mathematical model is developed in order to describe the impact of the recycling pump and other external reactor parts upon the process dynamics and stability. Stability analysis is performed using bifurcation theory and continuation methods. It is shown that under certain operational conditions as many as seven steady states are predicted for the loop polymerization reactor. Oscillatory behavior is observed for a wide range of process parameters and onset of oscillations is observed during the transition from operation without material recycling to operation with partial recirculation of the polymer solution. Besides, at certain constrained range of operation conditions, complex dynamics can be observed, including the onset of chaotic behavior. It is also shown that the thermal parameters of the reactor and recycling pump exert a profound effect upon the process stability. For this reason it is shown that oscillatory behavior is very unlikely to occur in actual industrial reactors.

Periodic oscillations in continuous free-radical solution polymerization reactors—a general approach

Abstract Previous studies regarding the bifurcation behavior of continuous stirred tank polymerization reactors have reported a multitude of different nonlinear responses. The nonlinear responses reported most often are steady-state multiplicity and self-sustained oscillatory behavior, which have been confirmed experimentally. Particularly, Freitas Filho, Biscaia, and Pinto (Chemical Engineering Science 49 (22) (1994) 3745–3755) used a generic model to describe the dynamic behavior of continuous free-radical solution polymerizations and were able to show that most reactors can present as many as five distinct steady-state solutions, regardless of the particular chemical system analyzed. The work of Freitas Filho, Biscaia, and Pinto is extended here to show that self-sustained oscillatory responses may also be considered to be generic reactor responses, regardless of the particular chemical system analyzed. The effect of both physico-chemical and operational reactor parameters on the topology of the branches of periodic solutions is also investigated. It is shown that most continuous solution polymerization systems may present regions where limit cycles are present and that the increase of the gel effect decreases the probability of observing sustained oscillatory responses.

Steady‐State Behavior of Slurry and Bulk Propylene Polymerization

This paper analyzes the steady‐state behavior of two typical processes to produce poly(propylene): the single‐reactor bulk polymerization process and the slurry polymerization process in a train of continuous stirred tank reactors. The investigation is performed using a commercial process simulator—SIMULPOL 3.0®, and aims at elucidating the general behavior of both processes when subject to perturbations of the input operation variables. Attention is given to the effect of the perturbations upon the polymer properties and to identification of process bottlenecks. Regarding the bulk polymerization process, unstable plant behavior was detected in the absence of a control policy. It was found that multiple steady‐states are possible for this process as a consequence of the recycling of material. As a matter of fact, the reactor recycle flow rate was found to be a fundamental parameter for the bulk process. Unstable behavior can be eliminated with the implementation of classical control strategies. #An earlier version of this paper was presented at ECOREP II, 2nd European Conference on Reaction Engineering of Polyolefins, Lyon, France, July 1–4, 2002.

Production of bone cement composites: effect of fillers, co-monomer and particles properties

Abstract - Artificial bone cements (BCs) based on poly(methyl methacrylate) (PMMA) powders and methyl methacrylate (MMA) liquid monomer also present in their formulation small amounts of other substances, including a chemical initiator compound and radiopaque agents. Because inadequate mixing of the recipe components during the manufacture of the bone cement may compromise the mechanical properties of the final pieces, new techniques to incorporate the fillers into the BC and their effect upon the mechanical properties of BC pieces were investigated in the present study. PMMA powder composites were produced in-situ in the reaction vessel by addition of X-ray contrasts to the reacting MMA mixture. It is shown that this can lead to much better mechanical properties of test pieces, when compared to standard bone cement formulations, because enhanced dispersion of the radiopaque agents can be achieved. Moreover, it is shown that the addition of hydroxyapatite (HA) and acrylic acid (AA) to the bone cement recipe can be beneficial for the mechanical performance of the final material. It is also shown that particle morphology can exert a tremendous effect upon the performance of test pieces, indicating that the suspension polymerization step should be carefully controlled when optimization of the bone cement formulation is desired.

In Situ Incorporation of Recycled Polymer in Suspension Polymerizations

Abstract Polymer materials find widespread use because of their characteristic low costs and wide range of end-use properties. However, the continuous accumulation of polymer materials in the environment constitutes a significant environmental problem. In order to promote the reutilization of used polymer materials, it is shown in this work that the in situ incorporation of recycled polystyrene into virgin polystyrene beads can be performed successfully during suspension polymerizations. The influence of increasing recycled polystyrene contents on the course of the polymerization is investigated. It is shown that the recycled polystyrene content does not affect the kinetics of the polymerization significantly, indicating that operation policies can be adapted to increasing amounts of recycled material with the help of common suspension polymerization models.

EFFECT OF PROCESS VARIABLES ON THE PREPARATION OF ARTIFICIAL BONE CEMENTS

The present work concerns the preparation of bone cements based on poly(methyl methacrylate) (PMMA), used mainly for prosthesis fixation and cavity filling for correction of human bone failures. A typical bone cement recipe contains methyl methacrylate, which polymerizes in situ during cement application. An inherent problem of this reaction is the large amount of heat released during the cement preparation, which may lead to irreparable damage of living tissues. Optimization of PMMA-based bone cement recipes is thus an important step towards safe and reliable clinical usage of these materials. Important process variables related to the reaction temperature profile and the mixing of the recipe constituents were studied in order to allow for the adequate production of bone cements. It is shown that the average molar mass and size of the PMMA particles used in the production of the bone cement, as well as incorporation of radiopaque contrast, co-monomers and fillers into the bone recipe play fundamental roles in the course of the polymerization reaction. Furthermore, the injection vessel geometry may interfere dramatically with the temperature profile and the time for its occurrence. Finally, it has been observed that the morphology of the PMMA particles strongly affects the mixing of the bone cement components.

Distribuição de peso molecular em poliolefinas: contribuição ao estudo da bimodalidade e da desconvolução

The deconvolution of molecular weight distributions (MWD) may be useful to obtain information about the polymerization kinetics and properties of catalytic systems. However, deconvolution techniques are normally based on steady-state assumptions and very little has been reported about the use of non-stationary approaches for deconvolution of MWDs. In spite of that, polymerization reactions are often performed in batch or semi-batch modes. For this reason, dynamic solutions are proposed here for simple kinetic models and are then used for deconvolution of actual MWD data. Deconvolution results obtained with dynamic models are compared to deconvolution results obtained with the standard stationary Flory- Schulz distributions. For coordination polymerizations, results show that dynamic MWD models are able to describe expe- rimental data with fewer catalytic sites, which indicates that the proper interpretation of the reaction dynamics may be of fundamental importance for kinetic characterization. On the other hand, reaction dynamics induced by modification of chain transfer agent concentrations seems to play a minor role on the shape of the MWD in free-radical polymerizations

Neural Networks Modeling of Dearomatization of Distillate Cuts with Furfural to Produce Lubricants

Abstract An important industrial stage in the solvent route production of API Group I lubricants is the dearomatizaton, a liquid-liquid extraction process. However, dearomatization feed is a complex mixture of components, a disadvantage for thermodynamic modeling approaches. As an alternative, we propose the use of artificial neural networks (ANN) to correlate compositions and crucial properties (e.g., specific gravity, viscosity index and refractive index) of raffinate and extract phases in liquid-liquid extraction. In order to improve the model reliability, data obtained in a pilot plant for dearomatization of three distillate cuts of Arab Light oil using furfural solvent were employed to fit and validate ANN models, evaluating different configurations. The dataset consists of six solvent/oil ratios at six different temperatures, resulting in a number of thirty-six experiments for each distillate cut. The ANN models fit suitably all investigated properties, for both fit and validate data sets, indicating the great potential of this approach as a predicting tool for reducing experimental efforts in characterization assays and also in predicting behaviour in industrial units.

ANÁLISE DINÂMICA E DE BIFURCAÇÃO EM PROCESSOS DE POLIMERIZAÇÃO

O polipropileno foi desenvolvido originalmente no Instituto Politécnico de Milão por Giulio Natta, que dividiu com Karl Ziegler o Prêmio Nobel de Química em 1963 por suas descobertas no campo da química e da tecnologia dos polímeros de alta massa molar, em particular pelo desenvolvimento da química do polipropileno [1]. Passados mais de meio século, a resina de polipropileno pode ser produzida hoje por diferentes processos industriais, incluindo processos de polimerização em lama a baixa pressão, de polimerização em massa a alta pressão e de polimerização em fase gasosa [1]. Muitos fenômenos intrigantes já foram observados em processos similares de polimerização, incluindo múltiplos estados estacionários, oscilações auto-sustentadas e toroidais e comportamento caótico. Em particular, respostas oscilatórias periódicas podem ser encontradas em faixas de condições operacionais e podem ser observadas tento em escala de laboratório quanto em escala industrial [2]. No caso da polimerização do propeno em massa, mais especificamente, diferentes aspectos dinâmicos e operacionais foram estudados na literatura com auxílio de modelos do processo [3], sendo que diferentes rotas de instabilidade operacional foram identificadas quando se utiliza um reator do tipo tanque agitado para conduzir a reação de polimerização do propeno [3-5].