ARR - Annual Report
A numerical simulation is presented of the FflteringIdewatering of structured suspensions. III the model the constitutive properties that are needed include:
- the membrane permeability as a function of the solidosity of the deposited particulates (this is obtained by analysing the experimental data of the initial stages of the flow for a wide range of solids concentrations);
- the effective drag coefficient which is found as a function of the solidosity from a least squares cell model; and
- the stiffness of the solids matrix due to the inter-particle repulsion. For this the double layer interaction is used, combined with recent advances in techniques of micromechanics, as well as improved estimates for the inter-particle distance in dense suspensions. The resulting stiffness function is valid over a wide range of solidosities and contains two easily measurable parameters.
Experimental data are presented for tests on anatase at initial solidosities in the range 0.07 < $u < 0.3. The filtrate collected from the slurry as a function of time is recorded and used for analysis of the initial stage of the experiment to obtain the membrane permeability as a function of solidosity. Then a nonlinear numerical simulation is presented using the aforementioned constitutive relations and the results of this are compared to the Volume vs Time curves. The comparison shows that alI experiments can be adequately described with the same set of constants for the suspension flow. Only two parameters need to be introduced: the effective thickness of the double layer (related to the <-potential) and one phenomenological constant that describes the effective inter-particle potential strength in a suspension.
With the aid of the simulation cake formation is studied; the evolution of various internal parameters, such as the solidosity, pressure, skeletal stress and tluid and particle velocities is presented.
This project focuses on the fluid dynamics of vertical gas-solid risers. Its principal objective is to produce data for evaluating theories elaborated by Professors Sundaresan and Jackson at Princeton. Thus in this report, we review Cornell activities in the area of gas-solid suspension flows.
At Cornell, we possess a unique facility with the ability to recycle - rather than discard - fluidization gases of adjustable composition to a vertical riser of 20cm diameter and 7m height. This allows us to simulate the fluid dynamics of industrial units (atmospheric and pressurized coal-burning circulating fluid beds, catalytic crackers) in a cold, atmospheric riser by matching the dimensionless parameters that govern the flow. The facility is equipped with capacitance, optical fiber and pressure instrumentation that records solid concentration profiles in the vertical and radial directions.
In the first year of the award, we have established that, under typical industrial conditions, dense gas-solid flows are nearly independent of gas density in the fully-developed region of the riser. This observation of a viscous flow regime suggests that particle clusters dominate the exchange of momentum between the two phases. It further suggests that extrapolations of flow behavior from atmospheric to pressurized conditions should be more straightforward than previously envisaged.
To inform closure of theories elaborated at Princeton, we have also carried out simultaneous measurements of pressure fluctuations and local wall volume fraction. Here we have shown that, because gas pressure reflects fluctuations originating throughout the vessel, they are not closely correlated with local solid volume fractions.
In addition, we have begun a study of cyclone performance under conditions of high gas density and solid loading, which have not yet received much attention despite their importance for a new generation of high efficiency coal gasifiers and other dense gas-solid processes operating at high pressures.
In 1996, we have also made progress in the area of instrumentation, which is often of interest to industry. In particular, we have designed an uncooled capacitance instrument capable of recording instantaneous solid volume fraction near the wall of an industrial vessel operating up to 950°C and 15 bar. In addition, we have completed a technology review of instrumentation for dense gas-solid suspensions to be presented at an upcoming IFPRI meeting.
This research work addresses the correlation between the material properties and the processing conditions to the final characteristics of powders and granular materials compacted at, low and medium pressures. This correlation is based on the study of the microstructural characteristics and evolution during the compaction process. The materials (powders: granules, binders and lubricants) selected for this study are representative: of those used mostly by pharmaceutical and household consumer companies.
The main objective of this study is focused on providing Guidelines to improve rationally and systematically the current compaction operations by helping in the optimal selection of particles, binder, lubricants as well as compaction pressures and compaction speeds.
Rutgers University offers a unique environment to conduct this investigation. This university provides first hand access to current research on fundamental aspects related to compaction such as granulation, milling, mixing and blending, within a coherent and collaborative effort with concentration on Pharmaceutical Manufacturing. Also, it provides the state-of the-art iii characterization techniques and computational facilities, and ad-hoc testing facilities such as the Compactor Simulator Laboratory.
A sensor with 12 sensing electrodes and 24 driven guard clcctrodes has been constructed. This provides an increase of 80% in the measured capacitances and enables narrower cross sectional slices to be imaged.
A series of experiments on both bubbling and fast fluidization flow regimes were conducted at University of Bradford. The tomographic data were compared with measurements taken with an existing mass flux probe as well as pressure transducer measurements. The results showed that for a bubbling fluidization the data obtained from ECT measurements agreed very well with the data obtained from the pressure drop measurements. A satisfactory correlation of the tomographic data was obtained for a fast fluidization flow regime.
A new method for setting the system measurement range has been incorporated into our Windows software to measure a mean solid concentration in the range 2 to 10%. This method allows more accurate measurement of low solid concentrations (up to 5% by volume).
To order to study the dynamic behaviour of a fluidized bed by using Deterministic Chaos Theory it is necessary to calculate statistical invariants from hundreds of thousands of data points. Our existing software has been modified to carry out such an analysis. The application of a singular value decomposition technique combined with the data from an ECT system is presented (Dyakowski et al, 1996).
AVS software has been used to visualize the movement of a bubble chain through a fluidized bed. In the future we intend to use this software to visualise slugging and turbulent flow regimes.
Summary
This project focuses on the fluid dynamics of vertical gas-solid risers. Its principal objective is to produce data for evaluating theories elaborated by Professors Sundaresan and Jackson at Princeton. Thus in this report, we review Cornell activities in the area of gas-solid suspension flows.
At Cornell, we possess a unique facility with the ability to recycle - rather than discard - fluidization gases of adjustable composition to a vertical riser of 20cm diameter and 7m height. This allows us to simulate the fluid dynamics of industrial units (atmospheric and pressurized coal-burning circulating fluid beds, catalytic crackers) in a cold, atmospheric riser by matching the dimensionless parameters that govern the flow. The facility is equipped with capacitance, optical fiber and pressure instrumentation that records solid concentration profiles in the vertical and radial directions.
In the first year of the award, we have established that, under typical industrial conditions, dense gas-solid flows are nearly independent of gas density in the fully-developed region of the riser. This observation of a viscous flow regime suggests that particle clusters dominate the exchange of momentum between the two phases. It further suggests that extrapolations of flow behavior from atmospheric to pressurized conditions should be more straightforward than previously envisaged.
To inform closure of theories elaborated at Princeton, we have also carried out simultaneous measurements of pressure fluctuations and local wall volume fraction. Here we have shown that, because gas pressure reflects fluctuations originating throughout the vessel, they are not closely correlated with local solid volume fractions.
In addition, we have begun a study of cyclone performance under conditions of high gas density and solid loading, which have not yet received much attention despite their importance for a new generation of high efficiency coal gasifiers and other dense gas-solid processes operating at high pressures.
In 1996, we have also made progress in the area of instrumentation, which is often of interest to industry. In particular, we have designed an uncooled capacitance instrument capable of recording instantaneous solid volume fraction near the wall of an industrial vessel operating up to 950°C and 15 bar. In addition, we have completed a technology review of instrumentation for dense gas-solid suspensions to be presented at an upcoming IFPRI meeting.
This research project aims at working out a method for describing the dispersibility of an agglomerated material in stirred vessels and to apply this method to the improvement of the redispersing properties. It is furthermore intended to establish a connection between the results from laboratory tests of the instant/dispersing properties and the large-scale dispersion of the product.
In the first year of the project, laboratory equipment for dispersing powders in stirred vessels was set up, and experiments were performed with agglomerated and milled skim milk powder (Nestle) in 10 1 and 50 1 vessels.
This report contains information required for an experimental set up of an agitated vessel at laboratory scale that enables the engineer to gain data suitable for the design of large-scale-vessels.
Mixing performance of the stirring system was investigated with and without baffles. Appropriate laboratory tests for describing the instant properties of the powder were also carried out, and a new testing method (dynamic wetting test) was developed.
The stirred-vessel experiment constructed permits investigation of the combined wetting/dispersing behaviour of powdered products under stirrer action. The results of the experiments indicated that there is an optimal point of operation (regarding specific power and feed rate), at which the total required stirring energy is minimal. The final degree of dispersion for skim milk powder depends upon the volume-specific stirring power.
It is difficult to establish a relation between the laboratory tests (static and dynamic wetting test) and the dispersing experiments in an agitated vessel. Since wetting tests do not take stirring power into account, they can not describe the processes taking place in the liquid. It is possible anyway, to derive some trends from these tests. The results of the laboratory tests with lecithin, for example, are in good agreement with the experiments conducted using the stirred vessel. The maximal possible feed rate per surface area, short of layer formation, could be determined using the dynamic wetting test. This feed rate was somewhat higher than the rate found in experiments with the agitated 10 1 vessel, the difference being caused by the uneven distribution of the powder across the liquid surface in the stirred vessel.
Use of surfactants improves the combined wetting/dispersing behaviour of powdered products, both in the wetting test and in the stirred vessel. In this case, lower values of specific power and energy are required. It has been shown that, preferably, a premix of surfactant and powder should be used.
The results gained so far enable the design of large-scale agitated vessels for milk powder and similar products.
Introduction
Our research, funded by IFPRI, has concentrated on developing physical models for the rapid flow of particles and gas, and exploring the consequences of these models for fully developed and developing flow of the two phases in both vertical and inclined ducts. Experimental studies have shown that solid particles transported by a gas in vertical pipes, such as those encountered in riser reactors, are distributed non-uniformly over the cross section (Bader et al., 1988). Consequently, neither quantitative nor qualitative features of the overall behavior can be represented correctly by one-dimensional flow models, which take into account the presence of the pipe walls only through empirically introduced friction factors. The origin of this segregation has been investigated by us and others over the last decade.
During the past twelve months, we have focused our efforts on understanding the various routes to formation of clusters in rapid flow of gas-particle mixtures. It was apparent from the reactions of some of the industrial representatives during the annual meeting in Nancy that the connection between this work and riser flow modeling is not entirely obvious. Therefore, we begin this report with a discussion of this connection (section 2), then highlight some of the results (section 3) and finally conclude with a summary of the anticipated course of research for the next year (section 4).
Following the previous work, calculation of gas-solid flow in a vertical duct was made by using LES (Large Eddy Simulation). The same models in LES as the single phase flow is applied to gas-solid flow under the conditions of small particle (50 micron meter) and dilute phase (solid volume fraction = 0.96 X 10^- 4). It was found that the importance of the inter-particle collision was recognized even at such low concentrations of particles, particularly for distributions of particle concentration and velocity fluctuation. As has been observed by many experimental workers, the turbulence modification due to the particles were found in the present simulation. That is, the intensity of gas turbulence is reduced by the presence of particles.
Executive Summary
This is the second Annual Report of the 1994-1997 IFPRI Project on “Reversibly Flocculated Suspensions” at K.U.Leuven. It is an extension of a similar project during the period 1991-1994. The project aims at understanding and predicting the flow properties of colloidal suspensions that are flocculated at rest but can be deflocculated during flow.
Two parts can be distinguished in this project. The first deals with describing the viscosity curves of weakly flocculated suspensions with controlled colloidal stability parameters. Reversibly flocculated suspensions often display time-dependent viscosities or “thixotropy”. It is the purpose of the second part of this project to attempt to identify and possibly quantify the role of the major factors governing thixotropy.
As far as the first part of the project is concerned, this second year was used to study suspensions based on two types of newly provided particles. They differ in particle size and in characteristics of the stabilizer layer. Firstly, stable suspensions of these particles have been studied in order to provide reference data with which to compare the data on reversibly flocculated systems with the same particles. Subsequently, flocculation was very gradually induced by changing the medium as well as the temperature. Gelation temperatures were determined as a function of particle volume fraction, using dynamic moduli. These data were used to determine interaction parameters, which should be useful in relating and predicting viscosity functions. The rheological data are being supplemented with small angle neutron scattering data on flowing systems. This provides information about the structure and the interaction parameters of the suspension. Preliminary results indicate the usefulness of this approach. More systematic data of this nature are being collected.
As for the second part, the available data have been extended in different ways. The scaling relations for sudden decreases in shear rate, i.e. for structure build-up, which had been suggested earlier have been confirmed by data on another system. In addition, the kinetics of structure breakdown after a sudden increase in shear rate have been studied. It is shown that, not surprisingly, breakdown follows somewhat different rules than build-up.
Still, a strain scaling seems to hold here as well and the relation of the characteristic times with the initial stress is in line with that for build-up. Build-up was now also followed after stopping the flow by using dynamic moduli. It has been found that this aspect of behaviour can be quite different in different materials. Some dispersions develop a three-dimensional particulate network nearly immediately after stopping the flow, others remain liquid-like while the moduli increase.
Finally, some thixotropic systems have been investigated by means of a dielectric technique. This should shed some light on the underlying structural behaviour. Transient dielectric data after stopping the flow are being generated. They are still under investigation.
Introduction
It is important to recall that the initial objectives of the research are to develop fundamental understanding and techniques to predict cornmunition behaviour from a universal test based on an experimental rig that reproduces single impact on a target in an air jet mill. The influence of the material properties on breakage in ultra-fine grinding are investigated. In Previous reports gave results for the impact of seven kinds of particles on a target and showed different types of behaviour. A classification was obtained (see report AR 27.03). A second rig has been built to study another impact configuration namely that of the impact between two jets of particles. Experiments are also performed with other types of mill and in particular an Alpine 100 AFG air jet mill. Morphological analysis is developed to study the shape of particles, debris and the action of the mills. In addition progress in the development of a model of an air jet mill is presented.
Part A
Part A of this report presents the results of the experiments performed with the single jet apparatus. Eight kinds of alumina particles impacted on a target are studied: three hydrargillites and five calcined hydrargillites. The influences of the material processing, the structure, and the calcination on the behaviour at impact are highlighted.
Part B
Part B presents the methods being developed for morphological analysis and gives results from the experimental impact rig and other methods of particle breakage.
Part C
Part C presents results towards the modelling of an air jet mill. In particular measurements and analysis of pseudo batch grinding experiments leading to the first determination of breakage and selection matrices for an air jet mill.