Dynamic modeling of multizone,multifeed high-pressure LDPE autoclaves

A comprehensive mathematical model is developed to simulate the dynamic behavior of multizone, multifeed high-pressure ethylene polymerization autoclaves. To describe the complex flow patterns occurring in low-density polyethylene (LDPE) autoclaves, a user-specified multisegment, multirecycle model representation of the actual multizone reactor is established. A general reaction mechanism is employed to represent the kinetics of ethylene polymerization. Dynamic mass, molar species, and energy balances are derived to predict the polymerization rate, monomer conversion, molecular weight developments (e.g., M_n, M_w, long- and short-chain branching), and temperature profile with respect to time and spatial position in the reactor. Detailed results on the start-up and grade transition of a four-zone autoclave reactor are presented and the effects of the macromixing parameters (e.g., number of segments per reaction zone and the total and side external recycle ratios) on the dynamic behavior of the reactor are investigated. It is shown that the model macromixing parameters can significantly affect the initiator consumption rate in a reaction zone. The present model is capable of predicting accurately the dynamic behavior of LDPE autoclaves and, thus, can be employed in the design, optimization, and control of these reactors. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2327–2348, 1999     

Kinetic modelling of non-enzymatic browning in honey and diluted honey systems subjected to isothermal and dynamic heating protocols

The kinetics of non-enzymatic browning (NEB) in honey and diluted honey systems was investigated. The effect of α_w (in the range of 0.54–0.99) on brown pigment formation in honey and its diluted solutions, differing in the concentration of reactant solutes was monitored upon heating at four temperatures (50, 60, 70 and 80 °C). The progression of the Maillard reaction was followed by spectrophotometric measurements at 420 nm (A₄₂₀) and the absorbance–time curves were fitted to the logistic model. The processing temperature and water activity, or the initial reactant concentration, had a significant impact on browning kinetics and the color change of honey. Secondary models, expressing the dependence of the best fitted primary model parameters on temperature and α_w, were further developed. In addition, an alternative method of expressing the temperature dependence of browning rate constants using the WLF kinetic formalism, based on the glass transition temperature, T_g, was applied. The main secondary models were validated by comparing the predicted model parameters with those obtained from isothermal experiments. Finally, the derived kinetic model was further evaluated against the observed browning responses of honey under dynamic heating conditions to examine the applicability of the developed model over fluctuating temperature–time protocols.     

Physico-chemical properties of whey protein isolate films containing oregano oil and their antimicrobial action against spoilage flora of fresh beef

Antimicrobial films were prepared by incorporating different levels of oregano oil (0.5%, 1.0%, and 1.5% w/w in the film forming solution) into sorbitol-plasticized whey protein isolate (WPI) films. The moisture uptake behavior and the water vapor permeability (WVP) were not affected by the addition of oregano oil at any of the concentrations used. A reduction of the glass transition temperature (∼10–20 °C), as determined by dynamic mechanical thermal analysis (DMTA), was caused by addition of oil into the protein matrix. A decrease of Young modulus (E) and maximum tensile strength (σ_max) accompanied with an increase in elongation at break (%EB) was observed with increasing oil concentration up to a level of 1.0% (w/w). Wrapping of beef cuts with the antimicrobial films resulted in smaller changes in total color difference (ΔΕ) and saturation difference (Δ_chroma) during refrigeration (5 °C, 12 days). The maximum specific growth rate (μ_max) of total flora (total viable count, TVC) and pseudomonads were significantly reduced (P < 0.05) by a factor of two with the use of antimicrobial films (1.5% w/w oil in the film forming solution), while the growth of lactic acid bacteria was completely inhibited. These results pointed to the effectiveness of oregano oil containing whey protein films to increase the shelf life of fresh beef.     

Impact of edible coatings and packaging on quality of white asparagus (Asparagus officinalis, L.) during cold storage

The effects of edible coatings and plastic packaging on quality aspects of refrigerated white asparagus spears were studied using two different experimental protocols. The first included four coating formulations based on carboxymethyl-cellulose and sucrose fatty acid esters, whey protein isolate alone and in combination with stearic acid, and pullulan and sucrose fatty acid esters, and an uncoated sample serving as a control. The second set consisted of four treatments; uncoated asparagus spears (control), coated with a carboxymethyl-cellulose formulation, packaged in plastic packaging and combination of coated and packaged asparagus spears. All products were stored at 4 °C and the quality parameters such as weight loss, texture, visual appearance, lignin and anthocyanins concentration, and colour were evaluated during their storage. Edible coatings exhibited a beneficial impact on the quality of asparagus by retarding moisture loss, reducing hardening in their basal part and slowing down the purple colour development. The plastic packaging had a remarkable influence in reducing weight loss and retarding hardening but its impact to the rest of the quality parameters was similar to that of the edible coatings. The combination of packaging and edible coating did not seem to offer any additional advantage on asparagus spears apart from the fact that the product had a brighter appearance at the middle part of the stem compared to the packaged spears alone.     

Subsecond voltammetric separation between dopamine and serotonin in the presence of ascorbate

Although voltammetry has proved an important tool for unraveling the dynamics of specific neurotransmitter molecules during the past decade, it has been very difficult to monitor more than one neurotransmitter simultaneously. In this work, we present a voltammetric methodology that allows discrimination between dopamine and serotonin, two important neurotransmitter molecules with very similar electrochemical properties, in the presence of high concentrations of ascorbate. We combined the application of a novel large-amplitude/high-frequency voltage excitation with signal processing techniques valid for the analysis of nonstationary and nonlinear phenomena. This allows us to minimize the contribution from capacitance and preserve the faradaic features of the voltammetric response providing us with excellent voltammetric detail. Using appropriate voltage excitation parameters and defining specific regions in the voltage space, so-called voltage windows, we can measure the concentrations of dopamine and serotonin separately or independently in mixed solutions even in the presence of high concentrations of ascorbate. Because of the enhanced voltammetric detail of this new technique, it is also possible to explore effects attributed to interfacial phenomena such as adsorption/desorption and electrode fouling.     

Large area crystallization of amorphous Si with overlapping high repetition rate laser pulses

This paper presents a pulsed laser crystallization technique, enabling large area crystallization of amorphous Si to produce grains having well-defined size and orientation. The method is developed by first determining the parameters influencing crystallization induced by single laser pulses of circular cross-sectional profile. In a second step, crystallization by overlapping round spots is examined. The experiments reveal three zones characterized by distinctly different crystallized morphologies following the laser irradiation. One of these zones corresponds to the regime of lateral crystal growth, wherein grains are driven towards the center of the spot by the radial temperature gradient. These findings are then applied to processing via line beam profiles that facilitate large area crystallization upon rapid translation of the specimen. Crystallization of extended areas hinges on the determination of the crystal growth length for a single spot. The pitch between successive pulses is then set on the basis of this information. It is shown that the pulse energy has only a weak effect on the crystal growth length.     

Compressive behavior of MWCNT/epoxy composite mats

Mats of vertically-aligned multiwall carbon nanotubes were grown in an thermal CVD reactor with simultaneous feed of the catalyst and carbon precursors. Mats were soaked into epoxy resin solutions without any prior chemical modification and then cured to produce composite plates of z-axis nano-reinforcement. Direct observations of the epoxy–CNT interactions at the nanoscale revealed that epoxy interacted naturally with the MWCNTs without affecting their physical characteristics, alignment, or the mat’s morphology. The compressive behavior of the pristine and composite mats was consistent with mechanical predictions accounting for an elastic regime followed by elastic instability and compaction. Strong evidence of reinforcement in the MWCNT/epoxy composites was indicated by increased strength, stiffness and toughness values with respect to the as-grown mats and pure polymer. The elastic instability strain of the composites was of the order of 0.4.     

Hierarchical weeping willow nano-tree growth and effect of branching on dye-sensitized solar cell efficiency

In this paper we have demonstrated the simple, low cost, low temperature, hydrothermal growth of weeping willow ZnO nano-trees with very long branches to realize high efficiency dye-sensitized solar cells (DSSCs). We also discuss the effects of branching on solar cell efficiency. By introducing branched growth on the backbone ZnO nanowires (NWs), the short circuit current density and the overall light conversion efficiency of the branched ZnO NW DSSCs increased to almost four times that for vertically grown ZnO NWs. The efficiency increase is attributed to the increase in surface area for higher dye loading and light harvesting and also to reduced charge recombination through direct conduction along the crystalline ZnO branches. As the length of the branches increased, the branches became flaccid and the increase in solar cell efficiency slowed down because the effective surface area increase was hindered by branch bundling during the drying process and subsequent decrease in the dye loading.     

Formation and behavior of composite CO2 hydrate particles in a high-pressure water tunnel facility

Sinking CO₂ composite particles consisting of seawater, liquid CO₂, and CO₂ hydrate were produced by a coaxial flow injector fed with liquid CO₂ and artificial seawater. The particles were injected into a high-pressure water tunnel facility to permit determination of their settling velocities and dissolution rates. Injections were performed at fixed pressures approximately equivalent to 1200-m, 1500-m, and 1800-m depths and at temperatures varying from approximately 2 to 5 °C. Immediately after injection, the cylindrical particles were observed to break away from the injector tip and often aggregated into sinking clusters. The seawater flow in the tunnel was then adjusted in a countercurrent flow mode to suspend the particles in an observation window so that images of the particles could be recorded for later analysis. The flow would often break or cause rearrangement of some of the clusters. Selected individual particles and some clusters were studied until they became too hydrodynamically unstable to follow. In general, the flow required to suspend clusters or individual particles decreased with time as the particles dissolved. For example, one particle was produced and observed for over 6 min at an average pressure of 15.022 MPa and an average temperature of 5.1 °C. Its sinking rate, determined from the flow required for stabilization, changed from 37.2 to 3.3 mm/s over this time. Particle sinking rates were compared to correlations from the literature for uniform cylindrical objects. Reasonable agreement was observed for short times; however, the observed decrease in sinking velocity with time was greater than that predicted by the correlations for longer times. Particle dissolution rates, based on changes in diameter, were also determined and varied from 5 to . A pseudo-homogeneous mass transfer model was used to predict single-particle dissolution rates. Good agreement was achieved between experimental dissolution data and the modeling results.     

Hot melt mixing and foaming of soluplus® and indomethacin

The hot melt mixing (HMM) process was used to dissolve 30 wt% of a model drug, indomethacin (INM), in Soluplus® a water soluble polymer excipient. Comprehensive characterization of the HMM‐prepared samples, using differential scanning calorimetry, X‐ray diffraction, Fourier Transform Infrared spectroscopy, and optical microscopy, strongly suggests that INM was in amorphous state, forming a solid solution with the polymer. Furthermore, to understand the impact of foaming on INM's release profile, the HMM product was foamed in a batch process using supercritical carbon dioxide (CO₂). Dissolution tests of HMM and reference samples were conducted in aqueous solutions with pH 7.4 and 1.2. In all cases INM's release showed strong pH‐dependency; faster release and a greater amount of INM was released at pH 7.4 than at pH 1.2. For pure INM and the physical mixture, the drug's ionizable character results in the observed pH‐dependency. While for the HMM samples it is also a consequence of theformation of hydrogen bonds between Soluplus® and INM which hinder polymer dissolution at pH 1.2. It was observed that the release rate of INM from different sample types at pH 7.4 decreased in the following sequence: foamed HMM > unfoamed HMM > crystalline INM > physical mixture. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers