Molecular weight distribution in disulfide polymers

The published data on molecular weight distribution in polysulfide are rather contradictory and cannot be correlated with the concept of the existence of thiol–disulfide interchange in these polymers. In the present work the viscosity and molecular weights of liquid thiokols have been investigated (the number-average molecular weight by iodometric titration and by ebulliometry and the weight-average molecular weight by the light-scattering method). When measuring the light scattering in Thiokol Solutions a considerable depolarization of the scattered light was observed. This fact must be taken into account in the molecular weight determination. It has been established that in polydisulfides with thiol endgroups thiol–disulfide interchange reactions take place not only during the synthesis, but also in the polymer itself (in bulk). These reactions lead to the formation of polymers with an equilibrium molecular weight distribution. A relation of log η = f(M _w)^½ has been found that can be used in determining the weight-average molecular weight of liquid Thiokols.     

Molecular weight distribution in living polymerization

An easily obtained parameter for characterizing polymers is the molecular weight distribution (MWD). Polymers with different MWD have different properties. The MWD can be easily influenced with controlled polymerization technologies. Examples of how these methods influence the properties of block and gradient copolymers in wetting and dispersing additives are discussed. These wetting and dispersing additives are used to make solvent-borne paints and aqueous pigment concentrates. An advantage of modulation of the MWD is the ability to fine tune-specific properties, such as the compatibility or the wetting activity of the polymer.     

Fuzzy control of a semi-batch copolymerization reactor

This study describes the development of a rule-based fuzzy controller for regulating the copolymer composition (F₁) in a semi-batch solution copolymerization s. The membership functions, fuzzy ranges for errors and the controller output are defined. A fuzzy rule base has been constructed relating error to controller output based on the operator's knowledge. The performance of the fuzzy controller has been evaluated by simulating the semi-batch copolymerization reactor by means of a mathematical model. The fuzzy controller developed at one operating condition has also been tested for other operating conditions.     

Integrated run-to-run and on-line model-based control of particle size distribution for a semi-batch precipitation reactor

The dynamics of particle size distribution (PSD) in a precipitation process are represented by a population balance (PB) and related differential–algebraic equations. The control of PSD is studied by using a closed-form solution of the PB. Batch-to-batch control and on-line single batch control strategies are investigated for controlling a semi-batch reactor. A systematic integration of the two strategies is shown to have a complementary effect on the control performance.     

Molecular weight measurement in weathered polymers

Gel permeation chromatography has been used to monitor the molecular weight changes that occur when polymers are degraded by photooxidation. Methods of sample preparation and procedures for conducting the chromatography measurements and for analyzing the results are discussed. Examples are given of applications to studies of polystyrene (PS), polypropylene (PP), and glass fiber-reinforced polypropylene (GFPP) exposed to ultraviolet (UV) irradiation in the laboratory. The results show that the degradation rate is fastest near the exposed surface, but in PS and PP, degradation is much faster at the unexposed surface than in the center of the molding, where the UV intensity is greater than at the unexposed surface. It is deduced that degradation at the center is slowed by a shortage of oxygen. © 1994 John Wiley & Sons, Inc.     

Behaviour of an elementary oxidation reaction in a semi-batch reactor

The dynamics of a non-isothermal bi-molecular gas-phase reaction in a semi-batch reactor is investigated. It is assumed that one of the reactants flows into a reactor containing the second. A reduced model is obtained by making a ‘pool-chemical’ approximation on the concentration of the reactant initially in the reactor. The region in parameter space in which oscillations are observable in the full transient model is estimated by determining the Hopf bifurcation locus in the reduced system. The contribution of the current work is its comparative study of the behaviour of the full system to that of the pool-chemical model. Although the reaction scheme is symmetric with respect to the reactants the regions of oscillatory behaviour are not identical because the reactants have different heat capacities.     

Dynamic optimization of a semi-batch reactor for polyurethane production

In this work, the dynamic optimization of a polyurethane copolymerization reactor is addressed. A kinetic-probabilistic model is used to describe the nonlinear step-growth polymerization of a mixture of low- and high-molecular-weight diols, and a low-molecular-weight diisocyanate. The dynamic optimization formulation gives rise to a highly complex and nonlinear differential-algebraic equation (DAE) system. The DAE optimization problem is solved using a simultaneous approach (SDO) wherein the differential and algebraic variables are fully discretized leading to a large-scale nonlinear programming (NLP) problem. The main reactor operation process control objective is the maximization of the molecular weight distribution (MWD) under a desired batch time, subject to a large set of operational constraints, while simultaneously avoiding the formation of polymer network (gel molecule). Typically, polyurethane formation is carried out using batch reactors. However, batch operation leads to attain relatively low MWD values and, if the process is not efficiently operated, there is always the possibility of obtaining a polymer network. In this work, it was found that process operation is greatly enhanced by the semi-batch addition of 1,4-butanediol and diamine, and the manipulation of the reactor temperature profile, allowing to obtain high molecular weights while avoiding the onset of the gelation point.     

Optimization and adaptive control of a semi-batch synthesis reactor

The optimization of a chemical synthesis aims at an adjustment of the operating conditions, i.e. pH, temperature and initial concentrations, in order to obtain maximum yield, minimum operating costs or similar. The optimization effort may be considerable so that, in many cases, its cost cannot be justified, especially for small volume products. This paper introduces a new strategy, namely to establish the optimum operating conditions with a relatively small effort. The new approach is illustrated by the synthesis of 2-furyl oxirane. In the first step, a simplified mechanistic process model has to be determined on the basis of easily accessible experimental data. Initial and final concentrations of all reacting components, reaction time and temperature must be known. A two-step evaluation yields the stoichiometric coefficients of all the occurring reactions as well as the parameters of the Arrhenius model. This so-called trend model is used in the optimization of the operating conditions. It is not only important to know the optimum operating conditions but also to maintain them as close as possible in practical operation. An adaptive controller is well suited to such an application and its use is discussed.     

CFD modeling of turbulent reacting flow in a semi-batch stirred-tank reactor

For the mixing-sensitive reactions,both chemical kinetics and mixing conditions of the reactants determine the distributions of products.The direct quadrature method of moments combining with the interaction by exchange with the mean micro-mixing model (DQMOM-IEM) has been validated for the chemical reacting flows in microreactors.Quite encouraging simulation results offer great promise,but the applicability of this method is needed to be explored furthermore,such as in stirred reactors.In this work,the two-environment DQMOM-IEM model was created with C language and used to customize Fluent through the user-defined functions.The mixing effects on the course of parallel competing chemical reactions carried out in a semi-batch single-phase stirred reactor were predicted.The simulation results show that the rising feed velocity enlarges the volume of reaction zone and maximize the yield of the by-product,which also indicates that the feed stream is more difficultly dispersed into the main stream and the zone surrounding feedpipe exit with high turbulent kinetic dissipation rate cannot be efficiently used.     

Ammonia removal by air stripping in a semi-batch jet loop reactor

Ammonia is very toxic chemical and it can be removed by air stripping at high pH. JLRs have found applications in wastewater treatment processes due to their high mass transfer rates. In JLRs, intrinsic high turbulence result in a very large air-liquid surface area for greater mass transfer. Therefore, in this study, ammonia removal by air stripping from synthetically prepared ammonia solution at the high pH in a semi-batch JLR due to its high mass transfer capabilities have been investigated. Investigated parameters in a JLR were initial ammonia concentration (10–500 mg/L), temperature (20–50 °C), air flow rate (5–50 L/min) and liquid circulation rate (35–50 L/min). While it was demonstrated that temperature and air flow rate have a significant effect on the ammonia removal, it was determined that initial ammonia concentration and liquid circulation rate have no significant effect on the ammonia removal. The overall volumetric mass transfer coefficients (K_La) have been calculated from obtained model and it was determined that increasing temperature and air flow rate have a very significant effect on K_La. It was concluded that JLR provides higher mass transfer capabilities than other type of reactors even if less air is given.     

半间歇搅拌釜式乙氧基化反应器的数学模拟

以KOH催化条件下乙二醇乙氧基化为例,建立了工业规模的、同时考虑动力学、气液平衡、气液传质、反应体积变化和惰性气体氮气影响等的半间歇搅拌釜式乙氧基化反应器的数学模型。通过数学模拟考察了环氧乙烷进料速率对反应器操作的影响,得到安全限制下适宜的环氧乙烷进料速率。在此基础上,模拟得到反应器中压力、环氧乙烷在气液相中的组成及乙氧基化产物随时间的分布,并同工业试验数据进行了比较。结果表明了模型的可靠性。     

半间歇聚合反应分子量分布周期操作的迭代学习控制

通过对半间歇聚合反应的引发剂进料实施周期操作,研究了这类操作方式对聚合物分子量分布的影响。研究结果显示,周期操作能改善聚合反应过程,对分子量分布有明显的加宽作用。对性能指标进行改进,以引发剂周期进料的占空比为控制变量,采用基于粒子群优化的迭代学习算法,对分子量分布进行了优化控制。仿真分析表明,在实际对象和模型存在不匹配的情况下,运用迭代粒子群算法,控制输入随着批次学习的进行而逐渐趋于最优解,聚合反应的分子量分布则不断逼近希望的分子量分布。实验结果验证了以周期操作方式对半间歇聚合过程分子量分布进行迭代优化控制的可行性。     

Molecular weight distribution in low-density polyethylene polymerization; impact of scission mechanisms in the case of a tubular reactor

It has been shown that scission kinetics strongly affects the molecular weight distribution (MWD) of low-density polyethylene (ldPE) in a continuous stirred tank reactor (CSTR). The present paper focuses on the effects of different chemical scission mechanisms, linear and topological scission, as well as mechanical scission on MWD in batch and tubular reactors. In contrast to the CSTR, a batch reactor MWD does not show bimodalities or long tails. The tubular reactor was modeled as an industrially representative system with four initiator injection points and a proper ‘cocktail’ of different initiators. Calculated MWD was compared to one experimentally determined with SEC-MALLS for a commercial tubular product and fair agreement was found. Typically, these MWDs are broad, but not bimodal. Sensitivity studies were performed as to scission kinetics and the effect of chain transfer agent (CTA). Both batch and tubular reactor were observed to be less sensitive to scission kinetics than a CSTR. In addition, alternative CTA injection strategies (downstream positions) were tested. These showed interesting behavior leading to very broad and bimodal MWD. The model allows following the MWD broadening along the tube. We conclude that batch and tubular ldPE reactors lead to completely different MWD behavior than a CSTR and that it is possible to manipulate it to a great extent.     

Two state estimators for the barium sulfate precipitation in a semi-batch reactor

The on-line determination of particle property distributions by direct measurements is often difficult, because the measurement equations are not invertible or because the inverse problem is ill-posed. If the process is observable, one can use state estimation techniques in order to reconstruct unmeasurable internal states of the process. This is discussed here for a semi-batch precipitation reactor. A square root unscented Kalman filter and state estimation by online minimisation are studied for the case of a measurable average particle size. Both estimators use a one-dimensional population balance model. The two approaches are compared in simulations.     

Chemical conversion of wood by treatment in a semi-batch reactor with subcritical water

Sugi (Cryptomeria japonica D. Don) wood meal was extracted with subcritical water in a semi-batch reactor. About 70–90% by weight of the sugi wood meal was converted into water-soluble compounds. The main components of the water-soluble compounds were monosaccharides and oligosaccharides, as a result of hydrolysis of cellulose and hemicellulose. White precipitates appeared in the solution after settling for 48 h. X-ray diffractometry clearly showed that precipitates had crystallized into cellulose II. High yield of total saccharides (including the precipitates) was obtained at 310–320 °C, 25 MPa and 65 g min⁻¹ water flow rate from a wood meal charge of 2.0 g, while the decomposition reaction was completed within about 20 min. Moreover, the yield increased to more than 60% when the wood meal was pretreated for improved wettability. It is shown that saccharides can be produced from sugi wood powder quickly and effectively by treatment in subcritical water.     

Molecular weight distribution in polyrecombination reactions

Synopsis It is demonstrated that the polymers obtained in batch polyrecombination reactions have a Schulz-Flory molecular weight distribution. Much wider distributions are obtained if the reaction is carried out in continuous stirred tank reactors. This article is a supplement to Dimerisation and oligomerisation by dehydrogenation as a general synthetic principle¹. Theoretically the inequality (46) can always be satisfied, the only condition is that 2a and thus 2N_V must be sufficiently large.Herrn Prof. Dr. Drs. h. c. G.V. Schulz zu seinem 75. Geburtstag gewidmet     

Effect of using batch or semi-batch reactor on the chemical composition distribution of styrene-maleic anhydride-acrylonitrile terpolymers

The influence of the reactor type on distribution of chemical composition of ST/AN/MA terpolymers obtained by free-radical polymerization is shown. When a batch reactor was used we have obtained polymers with broad chemical composition distribution. However, if a semi-batch reactor with appropriate addition protocol was used it can be possible to synthesize terpolymers with homogeneous chemical composition. Received: 26 November 1996/Revised: 30 January 1997/Accepted: 6 February 1997     

Molecular engineering of liquid crystal polymers by living polymerization

Summary This paper describes the synthesis of poly{11-[(4-cyano-4-biphenyl)oxy]undecanyl vinyl ether-co-2-[(4-cyano-4-biphenyl)oxy]ethyl vinyl ether} A/B {poly[(6-11)-co-(6-2)]A/B} where A/B refers to the molar ratio between the monomers 6-11 and 6-2, with number average molecular weights above 15, by living cationic polymerization. The phase behavior of the resulting copolymers was discussed as a function of copolymer composition. Within this range of molecular weights, in the first heating scan poly(6-11) exhibits a melting and an enantiotropic s_A phase, while poly(6-2) an inverse s_C phase. In the second and subsequent heating scans poly(6-11) displays an enantiotropic s_X (unidentified smectic phase) and a s_A mesophase, while poly(6-2) only a glass transition temperature. Depending on composition, poly[(6-11-co-6-2)]A/B either display an enantiotropic nematic, enantiotropic s_A or both enantiotropic nematic and anantiotropic s_A phases.