Alex C. Hoffmann

Dynamic properties of water/alcohol mixtures studied by computer simulation

We have studied mixtures of alcohol and water in an extensive series of 465 molecular-dynamics simulations with an aggregate length of 713 ns, in order to study excess properties of mixing, in particular the relation between mobility and viscosity. Methanol/water, ethanol/water, and 1-propanol/water mixtures were simulated using an alcohol content of 0–100 mass % in steps of 10%, using the OPLS (optimized potential for liquid simulations) force field for the alcohol molecules and the TIP4P (transferable intermolecular potential with four particles) water model. Computed densities and energies show very good agreement with experimental data for bulk simulations and the mixtures are satisfactory as well. The shear viscosity was computed using nonequilibrium molecular-dynamics simulations. Other properties studied include diffusion constants and rotational correlation times. We find the mobility to correlate well with the viscosity data, i.e., at intermediate alcohol concentrations the viscosity is maximal a...

Particle segregation in fluidised binary mixtures

The particle segregation in fluidised beds consisting of different types of binary mixtures is shown to be governed by the same particle transport processes. The segregation behaviour of both “different-density mixtures” and “equal-density mixtures”, two types of system which until now largely have been investigated and modelled separately, have been successfully simulated using the same technique, and assuming the same transport processes to be active in both types of system. Experimental results showing segregation profiles in both types of mixtures, but using particles very much coarser than those normally investigated, are presented. The significance of the particle transport parameters is discussed. A new empirical relation for the bubble wake angle in group B powders has been proposed, and it has been made likely that the rate of material interchange between the wake of a rising bubble and the surrounding bulk is independent of the minimum fluidisation velocity of the bed particles. A discussion of future research seen as useful for better prediction of particle mixing and segregation is provided.

Gas Cyclones and Swirl Tubes

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The effect of vibration on the fluidization behaviour of some cohesive powders

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Flow pattern in reverse-flow centrifugal separators

One bed of glass ballotini and several beds of potato starch were fluidized with air at varying fluidization velocities, while the distributor plate was being vibrated at different amplitudes and frequencies to study the effect on the fluidization quality. The degree of cohesiveness of the potato starch powders was altered by varying the moisture content. The cohesiveness was measured in a triaxial cell at different moisture contents and values of the unconfined yield strength are reported as functions of the preconsolidation stress. Results of the quality of fluidization are presented mainly in the form of three-dimensional plots showing the variation of the fluidization index as a function of the superficial fluidization velocity and a dimensionless vibration intensity number. Critical values of this vibration number and the fluidization velocity, beyond which the powders would fluidize well, were determined under the conditions investigated. The vibration intensity required to fluidize the powders well increased with increasing cohesiveness of the powder, as did, to a lesser extent, the required superficial gas velocity. The parameters affecting the quality of fluidization are discussed.

Segregation in a fluidised powder of a continuous size distribution

Experimental flow patterns, determined by Laser Doppler Anemometry (LDA) for two types of reverse-flow centrifugal separators, are presented. The flow patterns in (a) a conventional cylinder-on-cone cyclone with tangential inlet and (b) a swirl tube with vane-generated swirl and a cylindrical body are compared. The experimental data are also used to test the validity of the flow assumptions of some widely used cyclone separation models for each of the two devices. The measured flow patterns are also compared with the results of computational fluid dynamics (CFD) simulations wherever this was helpful for the interpretation. Although the data globally support the standard flow assumptions, some features were quite surprising; for instance, it appears that the surface delimiting the central region of upward flow is largely determined by the diameter of the body and not that of the vortex finder. The similarities and differences between the flow patterns in cyclones and swirl tubes are discussed, as is the suitability of using cyclone separation models for the prediction of swirl tube performance.

Stochastic models for transport in a fluidized bed

The state of mixing in a gas fluidised bed of sand of a continuous size distribution has been investigated at various fluidisation velocities. The results are shown mainly as axial concentration profiles of the individual size fractions obtained by sieving. It was found that the local size distribution of the bed material was strongly non-uniform axially due to segregation up to fluidisation velocities significantly higher than the minimum fluidisation velocity. Moreover, the segregated bed exhibited a pattern which can be described roughly as two superimposed layers, each of approximately constant powder composition axially. This is similar to what has been found in beds containing binary mixtures of powders. In contrast to binary systems, however, defluidisation of the bottom of the bed was not seen under any of the conditions investigated. An attempt at a qualitative, and in some measure quantitative, explanation of the obtained results has been made.

Improving the removal efficiency of industrial-scale cyclones for particles smaller than five micrometre

In this paper we study stochastic models for the transport of particles in a fluidized bed reactor and compute the associated residence time distribution (RTD). Our main model is basically a diffusion process in [0,A] with reflecting/absorbing boundary conditions, modified by allowing jumps to the origin as a result of transport of particles in the wake of rising fluidization bubbles. We study discrete time birth-death Markov chains as approximations to our diffusion model. For these we can compute the particle distribution inside the reactor as well as the RTD by simple and fast matrix calculations. It turns out that discretization of the reactor into a moderate number of segments already gives excellent numerical approximations to the continuous model. From the forward equation for the particle distribution in the discrete model we obtain in the diffusion limit a partial differential equation for the particle density p(t,x) \[ \frac{\partial}{\partial t} p(t,x) =\frac{1}{2} \frac{\partial^2}{\partial x^...

PET investigation of a fluidized particle: spatial and temporal resolution and short term motion

Abstract A new approach has been adopted to reduce the dust emission from gas cyclones by using the strong swirl present in the vortex finder of the cyclone. Preliminary calculations of the flow pattern in the vortex finder using computational fluid dynamics (CFD) indicate that it is possible to collect a certain fraction of the escaped particles utilizing the swirl present. The collection of the particles occurs in a so-called ‘Post Cyclone’ (PoC) in which the diameter of the vortex finder is broadened slightly. The particles in the effluent air from the cyclone enter the PoC region through a narrow slit where they are collected in a bleed stream. Different mechanisms such as impact on walls and agglomeration aid in the collection. Initial experiments performed with a cyclone of dimensions greater than the usual laboratory range (diameter = 0.4 m) indicate a reduction in emission of particles of 1–3 μm by around 30%. Experiments were conducted with different configurations of the PoC and also under different operating conditions in order to characterize the system. Flow patterns and collection efficiencies for the cyclone and the PoC, both individually and in combination, were calculated by CFD and compared with experimental data

The influence of horizontal internal baffles on the flow pattern in dense fluidized beds by X-ray investigation

The motion of a single particle in a fluidized bed has been followed with high temporal and spatial resolution using an ECAT EXACT HR+ PET camera. An account is given of the analysis of the output from the camera, and the calculation of the particle position. The particle position was determined with a precision of 1 mm once per ms. The scatter in the data was calculated using two methods, the variate method and a linear fit method. A comparison of the still bed with the fluidized bed did not reveal any fast particle movement in the latter. It is shown that optimal smoothing of the time series, without losing information about the particle movement, is obtained if the arithmetic averages of 10-50 data points are plotted against time.