MODEL APPLICATION: HYDRATION AND GELATINIZATION DURING RICE PARBOILING

The rate of water absorption of a local rough rice variety at temperatures below and above the corresponding gelatinization temperature was investigated. A mathematical model that describes water diffusion and gelatinization within the grain, was used to simulate rice parboiling. The data were processed using a nonlinear regression procedure to calculate the diffusion coefficients and reaction rate constants in the temperature range 35-95°C. Both coefficients follow an Arrhenius temperature dependency with a break point at 77°C, where rapid gelatinization of rice begins. From the comparison between the activation energies for diffusion and reaction it is concluded that the parboiling process is limited by the reaction of starch with water below 77°C and by diffusion of water above 77°C.     

34 GHz e.p.r. study of vanadyl complexes in various asphaltenes: Statistical correlative model of the coordinating ligands

High precision 34 GHz e.p.r. measurements were performed to study the coordination of vanadyl complexes in various asphaltenes. The spin-Hamiltonian parameter analysis indicates that the asphaltenes can be classified into those which have g- and A-parameters nearly identical to those for the vanadyl tetraphenyl porphyrins doped in carbonaceous materials and those for which these parameters are nearly identical to those for the vanadyl etioporphyrins. Quantitative statistical analysis of the parameters for model compounds with coordination N₄, N₂S₂, NS₃, S₄, S₂O₂, N₂O₂ and O₄ shows that these parameters cannot be used to predict the coordination for various asphaltenes. It is found that g₀ is a better parameter than A₀ to predict the coordination, although it cannot resolve the N₄, S₄ and S₂O₂ coordinations. However, the spin-Hamiltonian parameter resolution was sufficient to distinguish between those vanadyl complexes present in porphyrins, humates and minerals. The importance of high precision e.p.r. measurement at 34 GH_z or higher frequencies for model compounds and chromatographically-separated fractions is discussed.     

Study of carbonaceous mesophase through the e.s.r. spectra of vanadyl chelates

The behaviour of vanadyl chelate complexes in Iranian heavy oil and ethylene tar pitch during the carbonization process has been investigated by the e.s.r. technique to obtain information on the mesophase transformation. Activation energies for rotation of chelates incorporated with aromatic lamellae, and ordering parameters, are estimated from correlation times and intensities of e.s.r. spectra. The molecular plane of the chelates incorporated into the mesophase was found to be oriented parallel to the applied field. The value of activation energy is higher for a mesophase of small size than for a larger one.     

Influence of carbonization conditions on the gasification of acacia and eucalyptus wood chars by carbon dioxide

Gasification rates of cubic shaped acacia and eucalyptus wood chars were measured thermogravimetrically in a carbon dioxide atmosphere at temperatures in the range 810–960 °C. The effects of wood species and carbonization conditions, such as temperature, heating rate and soaking time, were determined. Both reactivity and the activation energy for the gasification of wood chars were found to be strongly influenced by the carbonization conditions employed during their preparation and wood type. The reactivities of both the acacia and eucalyptus wood chars decreased with increasing preparation temperature; while the activation energy for their gasification increased. Slow carbonization (heating rate: 4 °C min⁻¹) led to the production of wood chars having lower reactivities and higher activation energies than those of the wood chars prepared under rapid carbonization (heating rate: 30 °C min⁻¹) at the same temperature. With increasing soaking time, at carbonization temperatures of 800 and 1000 °C, the reactivity of resulting wood chars was reduced. The results also show that the reactivities of acacia wood chars are higher than those of similarly prepared eucalyptus wood chars.     

A broad-line nuclear magnetic resonance investigation of polyacrylonitrile phase structure and chain conformation

The investigation of polyacrylonitrile (PAN) phase structure has been carried out in connection with its chain conformation by means of analysing the shape, width and the second moment of ¹H BL-n.m.r. lines. A complex structure of the n.m.r. lines within the range of temperatures between −180°C and 115°C is interpreted as resulting from the presence of a specific conformation in the iso- and heterotactic sequences of PAN. A complex shape of the lines in temperatures above 115°C may be explained by heterogeneity of phase structure or by the polymer's microstructure. The line width was found to narrow in the vicinity of 95°C and within the range of 120°C and 160°C, which was attributed to certain changes in the nature of molecular motion. Theoretical and experimental values of the second moment of ¹H BL-n.m.r. line for rigid lattice of PAN were compared. The n.m.r. spectra of two polyacrylonitrile samples heat-treated at 200°C and deuterated respectively, were studied to confirm the presence of specific chain conformation.     

Selective catalytic reduction of NOx in lean-burn engine exhaust over a Pt/V/MCM-41 catalyst

The activities of Pt supported on various metal-substituted MCM-41 (V-, Ti-, Fe-, Al-, Ga-, La-, Co-, Mo-, Ce-, and Zr-MCM-41) and V-impregnated MCM-41 were investigated for the reduction of NO by C₃H₆. Among these catalysts, Pt supported on V-impregnated MCM-41 showed the best activity. The maximum conversion of NO into N₂+N₂O over this Pt/V/MCM-41 catalyst (Pt=1 wt.%, V=3.8 wt.%) was 73%, and this maximum conversion was sustained over a temperature range of 70 °C from 270 to 340 °C. The high activity of Pt/V/MCM-41 over a broad temperature range resulted from two additional reactions besides the reaction occurring on usual supported Pt, the reaction of NO with surface carbonaceous materials, and the reaction of NO occurring on support V-impregnated MCM-41. The former additional reaction showed an oscillation characteristic, a phenomenon in which the concentrations of parts of reactant and product gases oscillate continuously. At low temperature, some water vapor injected into the reactant gas mixture promoted the reaction occurring on usual supported Pt, whereas at high temperature, it suppressed the additional reaction related to carbonaceous materials. Five-hundred parts per million of SO₂ added to the reactant gas mixture only slightly decreased the NO conversion of Pt/V/MCM-41.     

Activated carbons prepared from rice hull by one-step phosphoric acid activation

Activated carbons were prepared from rice hull by one-step phosphoric acid activation in this work. The evolution of pore structure and surface chemistry in the activation temperature range of 170–450 °C was investigated through various characterization techniques. The results showed that the development of porosity (extent of activation) was negligible at activation temperature below 300 °C, and rapid evolution occurred in 300–400 °C. Porous activated carbon with bimodel pore structure (pore < 1 nm and pore > 1 nm) and BET surface area as high as 1295 m²/g was obtained at 450 °C. The ash contents of samples prepared in this study were in the range of 5–21%. The ash contents of carbons prepared in this study initially decreased from 21.03% to 4.89% with the change of temperature from 170 to 300 °C, then increased to 8.72% at 450 °C. Boehm titration results suggested that low activation temperature (300 °C) benefits the formation of acidic surface groups. With the increase of activation temperature from 300 to 350 °C, the concentrations of strong, intermediate and weak acidic surface groups decreased from 2.23, 1.87, and 2.73 to 1.66, 1.32, and 2.16 mmol H⁺/g, respectively. Over 350 °C, the change of these groups were insignificant. FTIR results revealed the existence of carbonyl-containing, phosphorus-containing groups, and groups containing Si–O bond. The relative concentration of carbonyl-containing groups decreases with an increase in activation temperature, while that of phosphorus-containing groups follows the reverse trend. The content of Si–O decreased first, then slowly increased with the increase of activation temperature. Boehm titration and FTIR (Fourier transform infrared spectroscopy) results indicated that the surfaces of these carbons contain both temperature-sensitive and temperature-insensitive groups. The temperature-sensitive part consists mainly of carbonyl-containing groups, such as carboxylic groups, while the temperature-insensitive part is primarily phosphorus-containing groups and groups containing Si–O bond. This study demonstrated that carbon products with relative low ash content and high activation degree can be prepared from rice hull by H₃PO₄ activation at suitable temperature.     

Combustion of methane over palladium ZSM-5 and mordenite catalysts

The effect of Pd-loading on Pd-NaZSM-5 and Pd-NaMordenite catalysts prepared by ion exchange was studied for methane combustion with excess oxygen (1% CH₄, 18% O₂, balance N₂) in the temperature range 40–500°C. Fresh and calcined samples (3 h, 450°C) showed methane conversions proportional to Pd-loading on Pd-NaZSM-5 catalysts, while conversions decreased with Pd-loading on calcined Pd-NaMordenite catalysts. TOF (number of methane molecules converted per second per Pd²⁺ ion) for over exchanged Pd-NaZSM5-116 was low as compared to under exchanged Pd-NaZSM5-80 and Pd-NaZSM5-58 samples. Close TOF's were found for the last two samples at 330°C. TOF differences in Pd-NaMordenite catalysts demonstrate the heterogeneity of Pd⁺² sites due to structurally nonidentical locations of cations. TOF's appear to be related to Na/Pd ratios in both catalyst types. Apparent activation energies for Pd-NaZSM-5 materials are higher than those for Pd-NaMordenite catalysts.     

Kinetic aspects of the formation of lead zirconium titanate

The kinetics of the second calcination step in the formation of PZT solid solution (with perovskite ABO₃ lattice) has been investigated by using two different particle sizes of the B-site precursor (1.91 and 5.08 μm), the finer size being obtained by prolonged milling. In-situ analysis performed by high-temperature X-ray diffractometry in a non-isothermal mode (20–800 °C) revealed a reduction of the calcination temperature by 100 °C with a decrease in particle size of the precursor. In order to clarify the mechanism of the solid-state reaction to PZT, isothermal heat treatment of the mixtures was performed in the temperature range 540–700 °C. The activation energies for the fine and the coarse powders were estimated as 150 and 210 kJ mol⁻¹ respectively, and the reaction was found to follow the Jander model for diffusion-controlled solid-state reaction kinetics.     

Dielectric loss and phase transition of sodium potassium niobate ceramic investigated by impedance spectroscopy

The dielectric permittivity of Na_0.80K_0.20NbO₃ ceramic was investigated by impedance spectroscopy. The dielectric characterization was performed from room temperature to 800 °C, in the frequency range 5 Hz–13 MHz. The bulk permittivity was derived by the variation of the imaginary part of the impedance as a function of reciprocal angular frequency. The permittivity values as a function of temperature showed two maxima. The first maximum is very similar at 200 °C and the second one positioned at around 400 °C, which was associated to Curie’s temperature. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency dispersion was investigated in terms of dielectric loss. The Na_0.80K_0.20NbO₃ showed a dissipation factor between 5 and 40 over a frequency range from 1 to 10² kHz.     

A Survey of Rheological Properties of One-Component Epoxy Adhesives

Flow characteristics of seven commercially available one-component epoxy adhesive pastes were measured using a controlled shear stress rheometer and a controlled shear rate rheometer over a temperature range from 5°C to 60°C. Combining data obtained from both controlled rate and controlled stress experiments over a wide range of shear rates, we observed Newtonian flow (shear stress proportional to shear rate) at very low shear rates, a plateau “shear thinning” region at intermediate shear rates, and a second region of linear dependence of shear stress on shear rate at high shear rates. The adhesive pastes exhibited a very broad range of rheological behavior. Two flow parameters important to adhesive application technology, the plastic viscosity and the apparent yield stress, were measured for each adhesive. The plastic viscosity ranged from 11.6 to 329.5 Pa. s; the apparent yield stress ranged from 56.2 to 413 Pa. The temperature dependence of the rheological parameters of the epoxy adhesive pastes was also determined. The results are reported as the activation energies, E_η and E_σ, of plastic viscosity and apparent yield stress, respectively. The apparent yield stress of each adhesive paste was much less sensitive to changes in temperature than was the plastic viscosity. This suggests that the processing characteristics are likely to show qualitative as well as quantitative changes with temperature.     

Nuclear magnetic resonance studies of selectively deuterated aromatic liquid crystalline copolyesters

Deuterium n.m.r. has been used to follow the molecular dynamics of three polyester materials over a temperature range of −150°C to 150°C. Two of the materials are liquid crystalline copolymers, one of perdeuterated hydroxybenzoic acid and hydroxynaphthoic acid and the other of hydroxybenzoic acid, isophthalic acid and perdeuterated hydroquinone. The third sample is crystalline poly(ethylene terephthalate) (PET) in which the benzene rings are deuterated. At the lowest temperatures examined all three materials give n.m.r. lineshapes characteristic of little molecular motion. On heating the liquid crystalline materials, the onset of motion is observed, first in the form of 180^o flips and then unrestricted rotation about the polymer axes. The PET spectra show a small degree of 180^o ring flipping to be taking place above 100°C, but most of the motion is in the form of near random motion that comes to dominate at 150°C. The results confirm the relatively stiff rod-like nature of the liquid crystalline polymers in comparison to the more flexible PET.     

Kinetics of PdO combustion catalysis

The kinetics of the catalytic combustion of methane by supported palladium oxide catalysts (2 wt.-% Pd/La₂O₃·11A1₂O₃ and 5 wt.-%Pd/ γ-A1₂0₃ were examined for several oxygen partial pressure levels over the temperature range from 40–900°C using temperature-programmed reaction and slow ramp and hold temperature-time transient techniques. Combustion rates were measured by differential reaction in a fixed bed of powdered catalyst at lower temperatures (200–500°C). Also, by preparing the catalysts as thin (ca. 10 μm) coatings on an alumina tube and conducting the experiments with very high flows of dilute methane and oxygen in helium, the rate measurements were extended up to 900°C without significant contribution from gas phase reactions. The specific combustion activity of supported PdO shows a persistent hysteresis between 450 and 750°C, i.e., the rate of combustion between these temperature limits depends strongly on whether the catalyst is cooling from above 750°C or heating from below 450°C. This region is also notable for negative apparent activation energy in the rate of methane oxidation, i.e., the rate increases with decreasing temperature during reoxidation of the Pd metal and decreases with increasing temperature (especially with low oxygen partial pressure) prior to decomposition of the bulk oxide. Detailed time-temperature transient kinetic analyses were performed for supported PdO catalysts within the 450–750°C temperature range. The hysteresis in methane combustion rate is caused by a higher activation energy for reduction of oxygen chemisorbed on metallic Pd and by suppressed reoxidation of Pd metal relative to PdO decomposition.     

Synthesis, formation and characterization of ZnTiO3 ceramics

Zinc titanate (ZnTiO₃) powders of perovskite structure were synthesized by conventional solid state reaction using metal oxides. Powders of ZnO and TiO₂ in a molar ratio of 1:1 were mixed in a ball mill and then heated at temperatures from 700 to 1000 °C for various time periods in air. The crystallization temperature of ZnTiO₃ powder was 820 °C, activation energy for crystallization was 327.14 kJ/mol and for grain growth was 48.84 kJ/mol. A transition point was observed when the electrical resistivity was measured versus temperature. Like some ferroelectric materials, a PTCR behavior above the transition temperature was observed with Curie temperature of 5 °C.     

Reactions of coal with discharge-generated (excited) nitrogen species

Reaction between coal and nitrogen in a discharge furnishes hydrogen cyanide and smaller amounts of cyanogen, as well as some carbon monoxide and carbon dioxide; the cumulative yields of hydrogen cyanide and cyanogen depend upon rank. The apparent activation energies for formation of hydrogen cyanide and cyanogen are in the order of 2–4 kcal mol⁻¹ but increase sharply to ≈13–14 kcal mol⁻¹ at ≈200 °C (possibly coincident with incipient thermal decomposition of the coal). The cyanogen/hydrogen cyanide ratio increases with rank, and also depends upon the reaction temperature — falling with increasing temperature up to ≈200 °C, and thereafter rising progressively. Rate measurements and i.r. spectral changes accompanying the reaction suggest that hydrogen cyanide is mainly formed from non-aromatic carbon-hydrogen configurations in the coal, and that cyanogen derives for the most part from aromatic carbon — though some can also be generated from non-aromatic CH in competition with formation of hydrogen cyanide.     

Mechanochemical synthesis and thermal evolution of La–ZrO2 cubic solid solutions

The formation of cubic solid solutions in the system La₂O₃–ZrO₂ by mechanochemical activation of a mixture of the oxides (molar ratio ZrO₂ 82%–La₂O₃ 18%) is studied. After 6 h of activation at room temperature, a poorly crystalline cubic solid solution is formed, with ultimate crystallite sizes in the nanometer range. The mixtures activated during 1–3 h form the solid solution on subsequent heating at 1000 °C, while the non-activated mixture does not react, even after thermal treatment at 1200 °C. The solid solution obtained at room temperature undergoes partial structural ordering at temperatures between 800 and 1000 °C. Long time heating at temperatures of 1000 °C and above results in the formation of La₂Zr₂O₇ and rejection of the excess ZrO₂. Mechanochemical activation offers interesting possibilities for the synthesis of these materials at temperatures lower than those used in conventional processing, and for the control of their physicochemical and microstructural properties.     

Activity-loss characteristics of spores of Bacillus thuringiensis during spray drying

Survival of spores of Bacillus thuringiensis was determined under various processing conditions for spray drying. The results indicated that the viable spores number of B. thuringiensis decreased with increased inlet air temperature, outlet air temperature and atomising air pressure. When the inlet air temperature ranged from 170 °C to 250 °C, the outlet air temperature (75 °C) and atomising air pressure (0.15 MPa) were fixed, the pseudo-z value (one logarithmic cycle reduction) was 238.1 °C; when the outlet air temperature ranged from 65 °C to 95 °C, the inlet air temperature (200 °C) and atomising air pressure (0.15 MPa) were fixed, the pseudo-z value was 85.5 °C, the activation energy calculated according to the outlet air temperature was 59.96 kJ mol⁻¹; when the atomising air pressure ranged from 0.10 MPa to 0.25 MPa, the inlet air temperature (200 °C) and outlet air temperature (75 °C) were fixed, the pseudo-z value was 3.49 MPa, the variance analysis showed that the atomising air pressure has no significant influence to the spores. The diluting solutions between Tween-80 solution and phosphate buffer have significant influence on the plate count of spores.

The B. thuringiensis powder prepared by spray drying with inlet air temperature 250 °C, outlet air temperature 97 °C, the spores count of powder decreased obviously. But when the inlet air temperature of 155–165 °C, outlet air temperature of 66–70 °C were employed, the spores count of powder approaches to that of freeze drying powder. The spores count of oven drying powder was lower than that of the freeze drying powder, but close to the spray drying powder which inlet air temperature was 200 °C, outlet air temperature was 75 °C.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.     

Organo-soluble, segmented rigid-rod polyimide films: 2. Properties for microelectronic applications

The essential properties of polyimide films of importance in microelectronic applications are thermal and thermo-oxidative stability, dimensional stability, glass transition behaviour and the relative permittivity (dielectric constant ′). A segmented rigid-rod polyimide was synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in order to develop new materials for microelectronic applications. The thermal and thermo-oxidative stability were investigated by determining the thermal degradation activation energy in air (210 kJ mol⁻¹) and in nitrogen (303 kJ mol⁻¹). The thermal stability was further studied through thermogravimetry-mass spectroscopy. The coefficient of thermal expansion, which indicates the dimensional stability, was measured via a tension mode of a thermomechanical analyser and doubly extrapolated to zero stresses, and was 6.98 × 10⁻⁶°C⁻¹ for the BPDA-PFMB films. The glass transition temperature, measured thermomechanically, was found to be 287°C. The dielectric constant for the films, measured after ageing at 50% relative humidity for 48 h at 23°C, was between 2.8 and 2.9 in a frequency range from 0.1 kHz to 1 MHz. The temperature and frequency dependence of the dielectric behaviour is also discussed.     

Dynamic-mechanical behavior of polyethylenes and ethene-/α-olefin-copolymers. Part I. α′-Relaxation

Polyethylenes and polyethylene/α-olefin-copolymers covering a range in crystallinity between 12 and 85% were investigated by means of dynamic-mechanical measurements between −145 °C and their melting point. From the temperature and frequency dependence of the complex modulus α′-, α-, β- and γ-relaxations were analyzed. The α′-relaxation was discovered in all HDPE-, LDPE- and LLDPE-samples but not in plastomer- and elastomer-samples. The activation energies (30-140 kJ/mol) of this relaxation were found to decrease with increasing crystallinity. The α′-transition temperature at a fixed frequency rises with increasing degree of crystallinity and tends to reach the melting point when approaching the fully crystalline state. Thus, it is concluded that the α′-relaxation originates from the interface between crystal lamellae and amorphous interlamellar regions. By extrapolation of the storage modulus to the amorphous state the entanglement molar mass was calculated as 2300 g/mol for a completely amorphous polyethylene/α-olefin-copolymer.