Effects of variation in the palm stearin: Palm olein ratio on the crystallisation of a low-trans shortening

Changes in the rheological properties during crystallisation and in the crystal size and morphology of blends containing rapeseed oil with varying percentages of palm stearin (POs) and palm olein (POf) have been studied. The crystals formed from all three blends were studied by confocal laser scanning microscopy, light microscopy and environmental scanning electron microscopy, which revealed the development of clusters of 3–5 individual elementary “spherulites” in the early stages of crystallisation. The saturated triacylglycerol content of the solid crystals separated at the onset of crystallisation was much greater than that in the total fat. Fat blends with a higher content of palm stearin had a more rapid nucleation rate when observed by light microscopy, and this caused an earlier change in the rheological properties of the fat during crystallisation. Using a low torque amplitude (0.005 Pa, which was within the linear viscoelastic region of all samples studied) and a frequency of 1 Hz, the viscoelastic properties of melted fat during cooling were studied. All samples, prior to crystallisation, showed weak viscoelastic liquid behaviour (G″, loss modulus >G′, storage modulus). After crystallisation, a more “solid like” behaviour was observed (G′ similar to or greater than G″). The blend having the highest concentration of POs was found to have the earliest onset of crystallisation (27% w/w POs; 12 mins, 22% w/w POs; 13.5 mins, 17% w/w POs, 15 mins, respectively). However, there were no significant differences in the time to the point when G′ became greater than G′ among the three blends.     

XRD and EXAFS studies of crystallisation in films

This paper reports a detailed structural study on the nucleation of t-HfO₂ nanocrystals in thin films of 70SiO₂–30HfO₂ prepared by sol–gel route on v-SiO₂ substrates. Thermal treatment was performed at different temperatures ranging from 900 to 1200 °C for short (30 min) or long (24 h) time periods. Crystallisation and microstructure evolutions were traced by X-ray diffraction (XRD). The local structure around hafnium ions was determined from Hf L₃-edge extended X-ray absorption fine structure (EXAFS) measurements carried out at the BM08-GILDA Beamline of ESRF (France). XRD shows the nucleation of HfO₂ nanocrystals in the tetragonal phase after heat treatment at 1000 °C for 30 min, and a partial phase transformation to the monoclinic phase (m-HfO₂) starts after heat treatment at 1200 °C for 30 min. The lattice parameters as well as the average crystallites size and their distributions were determined as a function of the heat treatment. EXAFS results are in agreement with the XRD ones, with hafnium ions in the film heat treated at 1100 °C for 24 h are present in mixed phases.     

Determination of optimum blast furnace slag cooling rate for slag recycling in cement manufacture

Abstract

The crystallisation behaviour of molten blast furnace slag was observed in situ using the single hot thermocouple technique. Isothermal and non-isothermal experiments were conducted to construct the diagrams for time temperature transformation and continuous cooling transformation. The molten slag should be cooled at a minimum critical cooling rate of 10°C s^?1. During crystallisation, melilite is the main crystal phase and rankinite is the primary phase. The crystallisation mechanism of the melilite crystal phase involves one-dimensional direction growth with bulk nucleation, whereas the growth mechanism of the rankinite crystal phase is between the surface nucleation mechanism and the one-dimensional direction growth mechanism. The crystallisation activation energies of the melilite and rankinite crystal phases are 238.07±28.81 and 523.52±58.56 kJ mol^?1 respectively.     

Modelling and simulation of crystallisers under non-perfect micromixing conditions

A mathematical model, formulated in terms of interacting populations under perfect macromixing conditions, is presented and analysed for describing micromixing in crystallisation processes. The model consists of two population balance equations, describing the changes of fluid elements of solution and crystals, respectively. The kinetics of micromixing is described by the coalescence-dispersion model, and is characterised by a constant coefficient of coalescence of fluid elements as the parameter of the mixing intensity. The rates of nucleation and growth of crystals are expressed as the expected values over the randomly interacting fluid elements.A closed moment equation model is derived and used to analyse the effects of micromixing on the dynamic and steady-state properties of crystallisers, and on the properties of the crystalline product. The steady-state value of the mean concentration of fluid elements decreases, while the yield of crystallisers increases by increasing segregation level. In steady states, a crystalliser with segregated solution phase produces more mass of smaller crystals than the corresponding crystalliser with solution mixed perfectly on microlevel. In well mixed microlevel states, the mean size of the crystalline product increases with increasing mean residence time, while in segregated states this tendency reverses. In segregated states, higher standard deviation of concentration of fluid elements is induced with increasing residence time.     

Application of fractal theory for characterisation of crystalline deposits

The objective of this investigation was to study the mechanisms of mixed salt crystallisation fouling on heat transfer surfaces and to characterise the structure of the deposits formed on the heat transfer surfaces. The effects of various operating parameters such as solution composition and hydrodynamics of the system on crystallisation fouling of mixtures of calcium sulphate and calcium carbonate, which are the most common constituents of scales formed on heat transfer surfaces, were studied experimentally. After clarification of the effects of operating parameters on the deposition process, deposits on the heat transfer surfaces were collected for analysis. The crystalline samples were analysed using Scanning Electron Microscopy, X-Ray Diffraction and Ion Chromatography techniques. It was found that crystallisation fouling on heat transfer surfaces exhibits fractal geometry. Therefore, the concept of fractal theory was used to quantify the structure of the deposits by introducing a new quantity called the fractal dimension.     

Phosphorus Removal from Sidestreams by Crystallisation of Magnesium-Ammonium-Phosphate Using Seawater

In recent years, a number of biological-treatment processes have been developed for phosphorus removal, alone or in combination with nitrogen. In anaerobic/oxic and anaerobic/anoxic/oxic processes, it is necessary to lower the concentration of phosphorus in the sidestream from the sludge-disposal processes. To do this, the magnesium-ammonium-phosphate system has been developed in which phosphorus is removed in a side-stream process.
Alternative systems require chemicals such as magnesium chloride as the source of magnesium and sodium hydroxide for pH control, and are economically less viable than that described in this paper in which seawater is substituted for the chemicals.
Dissolved phosphorus removal of over 70% was achieved without pH control by feeding the sidestream which contained 50–111 mg/l dissolved phosphorus and had a pH greater than 7.77, together with a 9–10% flow of seawater into the reactor containing 4–13% of magnesium ammonium phosphate and which provided a retention of 29 mins.     

Nanocrystalline silicon solar cells from excimer laser crystallization of amorphous silicon

Excimer laser crystallized nanocrystalline silicon layers were used to fabricate solar cells. The laser crystallized layers are characterized with Raman spectroscopy for structural investigations and with atomic force microscopy to study surface modifications upon crystallization. The current–voltage characteristics of the devices completed with aluminium back contacts were investigated with air mass 1.5 G solar simulations. The resulting nanocrystalline solar cells show inferior performance compared with amorphous silicon devices. The degradation of open circuit voltages and short circuit currents with the increase of crystallization energy density is explained to be due to thermal modification of the p-type/intrinsic layer interface and band gap enhancement of fine-grained nanocrystalline layers of the devices along with increased surface and grain boundary recombination of carriers.     

Removal of iron fluorides from spent mixed acid pickling solutions by cooling precipitation at extreme temperatures

The precipitation crystallisation behaviour of spent mixed acid pickling solutions (HF/HNO₃) for stainless steel surface finishing was investigated. The main target was to extent the knowledge about the temperature dependency of these metal containing pickling acids at temperatures below 0 °C to induce precipitation by a temperature change instead of a salt supersaturation in the liquid.It was found, that ferric fluoride (FeF₃) precipitates at temperatures below −35 °C under the conditions investigated which provides new approaches for the regeneration of spent pickling solutions.     

Polymorphism in molecular crystals

Reports concerning polymorphism in molecular crystals published in the period 2007–2008 are reviewed. A general survey is given of structural and crystallisation studies in polymorphic systems, and selected advances are described in more detail. These include application of contemporary crystallographic techniques to obtain comprehensive structural information for established polymorphic compounds, multi-disciplinary experimental and simulation studies to probe the mechanisms of polymorph crystallisation, exploration of crystallisation confined within nanopores and characterisation of single crystals containing distinct polymorphic domains.     

Effects of composition on fat rheology and crystallisation

The crystallisation behaviour of three fat blends, comprising a commercial shortening, a blend of fats with a very low trans fatty acid content (“low-trans”) and a blend including hardened rapeseed oil with a relatively high trans fatty acid content (“high-trans”) was studied. Molten fats were lowered to a temperature of 31 °C and stirred for 0, 15, 30, 45 and 60 min. Samples were removed and their rheological properties studied, using a controlled stress rheometer, employing a frequency sweep procedure. Effects of the progressive crystallisation at 31 °C on the melting profile of fat samples removed from the stirred vessel and solidified at −20 °C were also studied by differential scanning calorimetry (DSC).