Systematic front distortion and presence of consecutive fronts in a precipitation system

A new simple reaction-diffusion system is presented focusing on pattern formation phenomena as consecutive precipitation fronts and distortion of the precipitation front.The chemical system investigated here is based on the amphoteric property of aluminum hydroxide and exhibits two unique phenomena. Both the existence of consecutive precipitation fronts and distortion are reported for the first time. The precipitation patterns could be controlled by the pH field, and the distortion of the precipitation front can be practical for microtechnological applications of reaction-diffusion systems

Measurement of the times for pyrolysis and the thermal diffusivity of a pyrolysing particle of wood and also of the resulting char

Cubes and spheres of spruce wood have been prepared, with a fine thermocouple inserted to measure the temperature at their centre. Individual particles were immersed rapidly in a bed of sand (mean size ~ 0.2 mm), which was fluidised by nitrogen and held at a fixed temperature up to 700oC. The rising temperature measured at a particle’s centre yielded the effective value of the particle’s thermal diffusivity. The temperature response showed evidence of at least two endothermic decomposition reactions, which corresponded to the pyrolysis of fine particles of the wood in a thermogravimetric analyser (TGA). However, the wood undergoing thermal decomposition in a fluidised bed at 500oC revealed at least one exothermic step at the very end of heating. After being heated this way in a hot bed fluidised by nitrogen, the particles of char formed by spruce wood had very much the same size and shape as the original piece of wood before being heated. These new particles of “char”, formed whilst being heated in a hot fluidised bed, were cooled in a stream of nitrogen and returned to the fluidised bed for re-heating without any complications from pyrolysis. The rise in the char’s central temperature with time gave an unambiguous value for the thermal diffusivity of the char. It is clear that volatile matter leaving a particle of wood reduced the rate of heat transfer between a hot fluidised bed and the centre of a devolatilising particle. Also, the time for complete pyrolysis was proportional to the square of the characteristic size (r_0) of the spruce being heated. In addition, the time for pyrolysis was proportional to T_bed^( 3±1), so that for a cube of spruce tpyr = 2.9 0.3 1015 r_0^2/T_bed^3 in seconds. Photographic evidence confirmed that devolatilisation of particles of spruce larger than ≈ 2 mm in a fluidised bed follows a shrinking core model and is accordingly controlled by internal heat transfer

Using a flame ionisation detector to measure the rate and duration of pyrolysis of a biomass particle

Single cubes and spheres of spruce wood have been heated in beds of inert sand, fluidised by nitrogen and heated electrically to 500–700 °C. The release of volatile matter from these pyrolysing particles, submerged inside a cage in the bed, was monitored by continuously sampling the off-gas from the bed into a rapid, flame ionisation detector (FID). This instrument's output was shown to be proportional to the rate at which carbonaceous volatile species were produced by a wooden particle, when thermally decomposing in the hot fluidised bed. The FID's rapid response revealed that some volatiles were released in brief, explosive bursts (lasting ~ 1 s), probably after local build-ups of pressure inside the biomass. Interestingly, the production of volatiles continued after the centre of a decomposing particle had reached the bed's temperature. Thus, the FID provided good measurements of pyrolysis times. The measurements also indicated that volatiles appeared in a fluidised bed as a cloud of bubbles rising around a decomposing particle. The bubbles pushed away the hot sand and so markedly reduced the rate of heat transfer from the bed to a particle. This had unexpected consequences. In a hot bed (700 °C), the duration of pyrolysis for a cube of spruce was proportional to the length, L, of the cube's side, for L ≤ 7 mm. This means that external heat transfer, which included radiation from the bed, then controlled the rate of thermal decomposition. In a cooler bed (500 °C), the duration of pyrolysis depended on a mix of L and L2, indicating that control was then by both internal and external heat transfer. Thus, from 500 to 700 °C bubbles of volatiles increasingly inhibited external heat transfer from the bed to a pyrolysing particle. Also, at 700 °C, the bed's radiation was largely absorbed by the products of pyrolysis inside the bubbles. After a particle's centre had reached the bed's temperature, volatiles continued to appear slowly at a rate, probably controlled by chemical kinetics. The identity of the rate-determining step is discussed for spruce particles of different sizes and beds at various temperatures. However, it is clear that the FID, with its rapid response and sensitivity, revealed new details of the pyrolysis of small particles (2 – 7 mm) of wood in a fluidised bed. For example, the thermal decomposition of spruce involves at least two separate, endothermic reactions and a final, exothermic step

Measurement of the times for pyrolysis and the thermal diffusivity of a pyrolysing particle of wood and also of the resulting char

Cubes and spheres of spruce wood have been prepared, with a fine thermocouple inserted to measure the temperature at their centre. Individual particles were immersed rapidly in a bed of sand (mean size ∼0.2 mm), which was fluidised by nitrogen and held at a fixed temperature up to 700 °C. The rising temperature measured at a particle's centre yielded the effective value of the particle's thermal diffusivity. The temperature response showed evidence of at least two endothermic decomposition reactions, which corresponded to the pyrolysis of fine particles of the wood in a thermogravimetric analyser (TGA). However, the wood undergoing thermal decomposition in a fluidised bed at 500 °C revealed at least one exothermic step at the very end of heating. After being heated this way in a hot bed fluidised by nitrogen, the particles of char formed by spruce wood had very much the same size and shape as the original piece of wood before being heated. These new particles of “char”, formed whilst being heated in a hot fluidised bed, were cooled in a stream of nitrogen and returned to the fluidised bed for re-heating without any complications from pyrolysis. The rise in the char's central temperature with time gave an unambiguous value for the thermal diffusivity of the char. It is clear that volatile matter leaving a particle of wood reduced the rate of heat transfer between a hot fluidised bed and the centre of a devolatilising particle. Also, the time for complete pyrolysis was proportional to the square of the characteristic size (r0) of the spruce being heated. In addition, the time for pyrolysis was proportional to Tbed3±1, so that for a cube of spruce tpyr = 2.9 ± 0.3 × 1015 r02/Tbed3 in seconds. Photographic evidence confirmed that devolatilisation of particles of spruce larger than ≈ 2 mm in a fluidised bed follows a shrinking core model and is accordingly controlled by internal heat transfer

The Liesegang eyes phenomenon

Radially symmetric pattern formation phenomena were investigated experimentally and numerically in 2D using a new experimental arrangement in a precipitation system. We examined the dependence of the pattern structure and the evolution of the precipitation front on its curvature. Depending on the radius of the reaction medium, two types of precipitation patterns has been observed: (a) pattern with a continuously distributed precipitate and a well detectable precipitation-free domain (Liesegang eyes phenomenon); (b) pattern with a continuous precipitation zone and discrete ring formation. Results of our simulations are in a good agreement with the experimental observations

Stochastic cellular automata modeling of excitable systems

A stochastic cellular automaton is developed for modeling waves in excitable media. A scale of key features of excitation waves can be reproduced in the presented framework such as the shape, the propagation velocity, the curvature effect and spontaneous appearance of target patterns. Some well-understood phenomena such as waves originating from a point source, double spiral waves and waves around some obstacles of various geometries are simulated. We point out that unlike the deterministic approaches, the present model captures the curvature effect and the presence of target patterns without permanent excitation. Spontaneous appearance of patterns, which have been observed in a new experimental system and a chemical lens effect, which has been reported recently can also be easily reproduced. In all cases, the presented model results in a fast computer simulation