The viscoelastic behavior of microcellular plastics with varying cell size

In this study, the viscoelastic behavior of a microcellular plastic was investigated with particular emphasis on the effect of cell size. A poly(ethylene terephthalate) resin containing a polyolefin nucleating agent (CPET) was selected as the test material. In order to investigate the effect of cell size, microcellular CPET samples were produced having a nearly constant density of nucleated cells and a varying cell size. The viscoelastic behavior was investigated using a dynamic mechanical analyzer in tensile mode, and the relationship between viscoelastic behavior and cell size is discussed.     

Reduced distillation models via stage aggregation

A method for deriving computationally efficient reduced nonlinear distillation models is proposed, which extends the aggregated modeling method of Lévine and Rouchon (1991) to complex models. The column dynamics are approximated by a low number of slow dynamic aggregation stages connected by blocks of steady-state stages. This is achieved by simple manipulation of the left-hand sides of the differential equations. The algebraic equations resulting from the reduction procedure are replaced by interpolation in tables or polynomial approximations. The resulting reduced model approximates the original dynamic model very accurately, and for a realistic case study increases the simulation speed several times. This makes the reduced models interesting for real-time applications. The numerical properties of the models and possible improvements are discussed.     

Mixing and segregation in a liquid fluidized bed of particles with different size and density

Experimental studies have been carried out on fluidization of irregular particle mixtures of different size and density. The mixing and segregation phenomena could be interpreted on the basis of the diffusion model of Kennedy and Bretton. The dependence of computed particle dispersion coefficient on liquid velocity, particle density and size has been discussed.     

Propane oxidative dehydrogenation over vanadia catalysts supported on mesoporous silicas with varying pore structure and size

The structural characteristics and the performance of vanadia catalysts (0.7–8 wt.% V) supported on mesoporous (MCM-41, HMS, MCF, SBA-15), microporous (silicalite) and non-porous (SiO₂) silicas in oxidative dehydrogenation of propane were investigated. The structure of vanadia species, the redox and the acidic properties of the catalysts were studied using in situ Raman spectroscopy, TPD- NH₃ and H₂-TPR. The only vanadia species detected on the surface of HMS and MCM-41 for V loadings up to 8 wt.% were isolated monovanadates indicating high vanadia dispersion. Additional bands ascribed to V₂O₅ nanoparticles were evidenced in the case of SBA-15 and MCF supported catalysts while these bands were the only ones identified on the surface of the catalysts supported on silicalite and non-porous silica. The catalysts supported on mesoporous HMS and MCM-41 materials showed the best performance achieving high propane conversions (35–40%) with relatively high propene selectivities (35–47%). Lower activity due to the lower degree of vanadia dispersion, caused by the partial destruction of the pore structure was observed for the SBA-15 and MCF supported catalysts. The degree of dispersion of the V species on the catalyst surface and not the pore size and structure of the mesoporous support or the acidity/reducibility characteristics mainly determine the catalytic activity towards propene production. In addition, it was shown that the pore structure and size of the mesoporous supports did not have any significant effect in the turnover rates (TOF values) of propane conversion (and propene formation at low propane conversion, below ca. 10%). However, the highest propene yield (up to 19%) and stable catalytic behavior was attained for catalysts supported on HMS mesoporous silica, and especially for those combining framework mesoporosity and textural porosity (voids between primary nanoparticles).     

Mixing effects in stirred tank reactors: a comparison of models

The Two-Environment Model of Ng and Rippin and the Monte Carlo residual life time model of Kattan and Adler developed for studying the effect of arbitrary residence time distribution and intermediate degrees of segregation on chemical reactor performance were shown to be equivalent from the point of view of a physical argument and a comparison of conversions obtained for a second order chemical reaction with mixed feed. Conversions with mixed feed were compared for a single CSTR and a reactor with a 2-CSTR residence time distribution using identical values of the micro-mixing parameter for both of the models.The Monte Carlo Coalescnece Model of Spielman and Levenspiel, Macro-mixed feed model of Manning and the Two-Environment Model of Ng and Rippin were found to give nearly identical results for (1) a single second order reaction, (2) consecutive second order reactions, and (3) step-wise addition polymerization without termination taking place in a single CSTR with mixed feed. Identical values of the micro-mixing parameter for the respective models were used.The models of Manning , Ng and Rippin, and Villermaux and Zoulalian were extended to reactors with unmixed feed with special reference to a second order chemical reaction. Results calculated from these models were compared with those from the models of Spielman and Levenspiel and Kattan and Adler using identical values of the micro-mixing parameter. The models do not agree for unmixed feed.     

Calculation of the ignition sensitivity of dust clouds of varying size distributions

For a cloud of flammable dust in air to ignite, the temperature of the air must be a specific value that depends upon properties of the dust material and of the dust cloud.First, a theoretical treatment is used to explain variations in experimental ignition temperatures in terms of particle size. The theory modifies Cassel and Liebman's method to take account of residence time of dust in experimental furnaces or in hot air. It is shown that it is possible for the ignition temperature of mono-sized coal particles (about 50 μm diameter) to be minimal under a limited residence time.The theory is extended to deal with dust clouds with a distribution in particle size. It is shown that there exists a range of size distributions for which the possibility of ignition is at a maximum. The calculated results are presented in the form of Rosin—Rammler charts indicating the distribution most sensitive to ignition.     

Determination of optimal quick switching system with varying sample size for assuring mean life under Weibull distribution

Conventional sampling plans are applied to dispose an individual lot or isolated lot that requires a large sample size. Consequently, the inspection time and cost needed to make a decision under conventional plans are high. In order to reduce the sample size and to inspect the series of lots with minimum cost and time, special purpose plans are utilized. In this article, we propose one of the special purpose plans, namely, a quick switching sampling system that is also known as a two-plan sampling system. Through this sampling system, the Weibull-distributed mean life of the product is ensured based on time-truncated life tests. The proposed system is designed with the intention of minimizing the average sample number using two points on the operating characteristic curve approach. The optimal parameters of the proposed system are determined for different combinations of producer’s risk and consumer’s risk. Implementation of the proposed system is also explained and the performance of the proposed system is compared with other existing plans. In addition, the effects of misspecification of shape parameters on optimal parameters and the probability of acceptance are discussed.     

Increasing the efficiency of an air separation plant by varying the cycle stage durations

A mathematical model of a plant for oxygen-enriched air production by pressure swing adsorption is proposed. The model takes into account the experimentally determined actual dependence of the longitudinal dispersion coefficient on the gas velocity and the sorbent particle size. The relationship between the plant efficiency and the duration of each of the operating cycle stages is studied using an ad hoc computer program for numerically solving the model equations. It is shown that optimization of the durations of cycle stages increases the thermodynamic efficiency of the plant to 20–22% in comparison to industrial plants.     

Mixing and fast chemical reaction—IX: Comparison between models and experiments

Theoretical considerations given in earlier parts indicate that mixing on the molecular scale (micromixing) takes place by molecular diffusion into small, turbulence-free fluid elements, which are being gradually distorted by uniform shear. This mechanism, when applied to fast, consecutive, competitive chemical reactions, predicts that the product composition will be a function of the variables given in eqn (3). A pair of diazo coupling reactions has been employed to test these predictions. The additional experimental results contained in this part refer to three impeller types (Rushto turbine, marine propeller and axial flow turbine), nine feed locations and three volumetric feed ratios. Semi-batch operation was modelled by extending the earlier batch reactor model and also used experimentally. Taking reasonable estimates of the model parameters from the fluid mechanics literature, satisfactory predictions of the experimental results were obtained.     

Time‐varying multi‐regime models fitting by genetic algorithms

Many time series exhibit both nonlinearity and non‐stationarity. Though both features have been often taken into account separately, few attempts have been proposed for modelling them simultaneously. We consider threshold models, and present a general model allowing for different regimes both in time and in levels, where regime transitions may happen according to self‐exciting, or smoothly varying or piecewise linear threshold modelling. Since fitting such a model involves the choice of a large number of structural parameters, we propose a procedure based on genetic algorithms, evaluating models by means of a generalized identification criterion. The performance of the proposed procedure is illustrated with a simulation study and applications to some real data.     

Implications of particle size to transient stage of deep bed filtration

This study was aimed at investigating the effect of particle size, mostly in the submicron range, on break-through stage of filtration. Latex beads, with diameters ranging from 0.46- to 2.967-μm were filtered through filter grains of diameters 0.1-, 0.175- and 0.45-mm. Experimental conditions were chosen so as to obtain breakthrough curves. The experimental results showed that the initial efficiency follows the pattern reported by previous experimental and theoretical studies, i.e., lower efficiency for 0.825-μm particles which fall in the range of critical size. However, the particle removal during the transient stage increased with an increase in particle size for the range of sizes studied. This pattern is qualitatively confirmed by the theoretical predictions of Vigneswaran and Chang (1986) model. This study also provides experimental verification of the effect of the ratio of particle size and grain size at different stages of filtration.     

The stage efficiency in dynamic models of phase separation processes

In chemical engineering practice, deviation from a perfectly mixed equilibrium stage in phase separation processes is described by a stage efficiency. In dynamic models this parameter is in most cases meaningless, except when the deviation from equilibrium is caused by interfacial transport resistance. The present paper shows that when this resistance approaches zero, there is an eigenvalue related to the interfacial transport that goes to ? ∞. The response mode attached to this eigenvalue expresses the deviation from equilibrium.Approximationg this mode by zero, may be defined as a “pseudo-equilibrium”. In a dynamic sense, the stage efficiency has a proper meaning only in this case, but it should be emphasized that this state space reduction is an approximation valid only for moderate deviations from true equilibrium. Hence, the stage efficiency has a much more narrow range of sound application than is usually considered in the chemical engineering literature.     

Mixing of granular materials, part II: effect of particle size under periodic shear

Although the study of mixing of granular particles in the last years has shown notable advances, it continues being moderately understood. In this work, a low shear mixing device consisting of a box with two moving walls and three static walls was used again to study granular mixing as a function of particle size. The goal is to obtain a more in depth understanding of the internal behavior of granular material in the three dimensions when particle size is changed. Experiments at different particle size distributions and wall displacements were run. Results show that faster mixing is achieved with particles of high diameter. The particle size affected the granular movement at the three directions. The phenomenon of dilation varied proportionally with the particle size.     

A computational fluid dynamics study of supercritical antisolvent precipitation: Mixing effects on particle size

Supercritical fluids have been extensively used for particle production of many natural and pharmaceutical substances providing useful alternatives for pharmaceutical and nutraceutical particulate system formulation. Among the different methods, the gas or supercritical antisolvent (GAS or SAS) process and its variants, have received a considerable interest due to the wide range of materials that can be micronized. Controlling particle formation in order to nucleate small particles is a key issue in GAS and SAS processes and this is directly related to mixing at all scales. In this work, we focus on numerical simulation of the process, emphasizing mixing modeling. Different mixing devices characterized by different nozzles are analyzed, to get an insight into mixing dynamics and its influence on the final particle size distribution. Results show that mixing is determinant in obtaining small particles, and that mixing at the microscale is a significant parameter to account for in the proper design of precipitators. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Mixing of Polymers with Twin Screws

Blending of polymer melts in a twin-screw mixer with longitudinal circulation of liquid along one of the channels in the unit was investigated. Equations were obtained for determining the average shear rate and shear strain accumulated by the polymer melt, responsible for obtaining a mixture of the required quality. It was shown that the presence of a small radial gap separating the deep and shallow screw channels almost does not alter the pressure-flow characteristics of the unit but makes an important contribution to the value of the shear strain accumulated by the polymer melt.     

A comparison of nitrogen and mercury pore size distributions of silicas of varying pore volume

Although good correlation between the pore size distribution determined by nitrogen desorption isotherm and mercury penetration methods has been reported with low pore volume samples, large discrepancies in pore size distribution and in pore volume were found with high pore volume silicas and were shown to be a function of the pore volume of the silica. The mercury penetration method is believed-to compress these highly porous silicas and therefore reduces the pore volume and forms smaller pores.     

Mixing Agglomeration in a High‐Shear Mixer with a Stirred Mixing Vessel

This paper deals with the agglomeration process in a high‐shear mixer. High‐shear mixers rotate with a very high mixing tool speed such that not only a mixing effect, but also a grinding effect is achieved. The parameter study reported here was carried out to determine the parameters influencing mixing agglomeration. The results will help the user to decide which parameters have to be considered for an optimum mixing agglomeration. This article will highlight some of the findings obtained from the comprehensive parameter study.     

单喷雾射流在受限空间内的错流混合(英文)

In order to achieve uniform mixing between spray droplets and crossflow, cold-model experiment of a hollow-cone water spray in an air crossflow is investigated via a numerical simulation. The simulation cases are designed by using the orthogonal design method. The Eulerian-Lagrangian formulation is employed for modeling the droplets-crossflow two-phase flow while the realizable k-ε turbulence model is used to describe the turbulence. A new index, mixedness quality, is proposed to assess the overall mixing of the droplets in the crossflow. The simulation results demonstrate that the counter-rotating vortex pair (CVP) imposes a more significant impact on the spatial distribution than on the size distribution of the droplets. Pairs of CVP with smaller scales are preferable for achieving a better mixing. The influencing factors are listed in the following order in terms of the degree of their impact from the greatest to the least: the Sauter diameter of the initial droplets, the mixing tube diameter, the spray angle, the velocity of the inlet crossflow, and the vertical velocity of the initial droplets. A moderate droplet diameter, a smaller tube diameter, a moderate spray angle, a greater crossflow velocity and a moderate vertical velocity of the droplet are favorable for achieving a higher mixedness quality of the jet spray in a confined crossflow.     

内构件对于提升管中颗粒混合行为的影响

对加设了钝体式内构件的提升管内颗粒的轴径向混合为进行了研究。研究结果表明，内构件的存在并不能明显减少提升管内颗粒的轴向返混，这与提升管内稳定的微观两相结构密切相关，但颗粒的径向混合能力却可大大加强，而且在越高的气速和固含条件下，这种加强作用越明显，其原因是内构件的存在加强了边壁区颗粒的脉动，有利于破坏提升管边壁区的颗粒浓环，促进颗粒的径向交换与混合。     

含白炭黑胎面胶的混炼

以白炭黑作填充剂的轮胎具有滚动阻力低等优点。但是白炭黑在橡胶中不易分散。介绍了为提高含白炭黑的胎面胶的混炼质量，加速白炭黑在轮胎中的推广应用而采取的原材料、混炼工艺和混炼设备。