ARR - Annual Report
Future Goals.
For basic industrial control of shape, the enhanced image analysis feedback signal may be all that is required. One can envision using even simple proportional feedback control to adjust the impurity level to push the crystal shape back to the desired form once an excursion is detected. Even with the purposeful deletion of unclassifiable crystals, our sampling rate and image analysis is adequate to maintain control of the model system. These operational difficulties will only become less troublesome as the video cameras and imaging software continue to improve. We wish to push the analysis further, however. We understand the role of the impurity in the crystal structure. The essential understanding of the crystal structure comes from studies by Sherwood and coworkers [S, 71. With this microscopic understanding, we can model the dynamics of the transition of crystal shape in a distribution of crystals. We are constructing a population balance model with two internal coordinates that tracks crystal shape changes. This model is applicable to typical industrial crystallization processes. As far as we know, these results are the first demonstration of real time control of crystal shape. Although serious challenges remain, and the prototype process is somewhat idealistic, we hope these results inspire practitioners to think seriously about application of these principles in suitable industrial processes.
The goal of this research is to measure and regulate the shape and size of particles created by nucleation and growth processes in crystallizers.
In the final year of the grant, we implemented feedback control on a semi-batch crystallization in which an impurity free feed flows through the crystallizer and we regulate the flowrate of a habit modifier stream in order to maintain a desired shape.
At the 2000 IFPRI Annual meeting, we showed our first results in which without any prior knowledge of model parameters, a simple proportional-integral control algorithm is able to maintain a desired crystal shape and in doing so, determines the critical concentration of habit modifier required to maintain this shape. The prototypical system and process we selected is semi-batch crystallization of sodium chlorate (NaClOs). Sodium dithionate (NazS20s) is a habit modifier that influences the relative growth rates of 100 and i ii faces of the crystal. In the presence of at least 50 ppm sodium dithionate the growth of the iii faces is blocked by the impurity and the crystal shape changes from cubic to tetrahedral. Without impurity present, the 100 faces grow slower than the iii faces and the crystal shape changes from tetrahedral to cubic. The shape change is easy to detect with video images alone, though there are limitations with extracting useful numerical information from images for use as a signal for feedback control.
This prototypical process displays the following industrial characteristics.
- Particle shape is affected by unmeasured disturbance variables.
- Online sensing is available in the form of video images. The images are replete with bad data. Some particles are fused or broken; it is difficult to obtain representative samples; particle boundaries overlap each other; there are significant levels of process noise; and it is difficult to sample enough images to remove the effects of this noise through averaging. The standard image analysis software provides simple measures such as particle boxed area and aspect ratio; as we show later, these simple measures are inadequate signals for feedback control.
- We can manipulate a process variable that also influences particle shape. Through this feedback policy, we maintain the desired shape in the face of the unmeasured disturbances. The video images are processed in real time to produce the feedback signal that is used for control.
Accomplishments
- Developed and implemented a repeatable prototype process for illustrating online crystal shape control.
- Added a higher level functionality to the standard image analysis software and tailored it to detect transitions between cubic and tetrahedral crystals in a slurry.
- Implemented feedback control and maintained a desired shape in a semi-batch crystallization process.
As far as we know, these results are the first demonstration of real time control of crystal shape. Although serious challenges remain, and the prototype process is somewhat idealistic, we hope these results inspire practitioners to think seriously about application of these principles in suitable industrial processes.
The importance of explicitly considering interparticle interactions in the rheology of dense suspensions and particle slurries is well established, although the exact relationships between particle-level quantities and macroscopic rheology and stability are at best qualitative. Most of the understanding has been developed for low shear rheological (linear viscoclastic) properties and/or for dilute dispersions. This project has the goal of providing experimental evidence for the influence of interparticle surface forces and hydrodynamic forces (due to the presence of the solvent) on the moderate to high shear rheological properties and shear stability of dispersions that span the colloidal to particulate range (colloidal dispersions to slurries). Of particular interest is the shear thickening transition and dilatancy, and how that explicitly depends on the strategy used to stabilize the dispersion or slurry (i.e. steric, electrostatic, polymeric stabilization), as well as the hydrodynamic forces important at higher shear rates. The research to date has the following components:
- Systematically explore the influence of the basic methods of particle stabilization on the shear thickening and dilatancy of well-characterized colloidal dispersions and slurries of non-colloidal particles. In this report, in particular, the effect of weak electrostatic stabilization forces on determining the onset and severity of shear thickening is examined.
- Determine the effects of particle size and concentration on the onset and severity of shear thickening.
- Employ this experimental data to test simplified two-particle models for the onset of shear thickening that are derived from simulations and theory.
The non–Newtonian rheology
The non–Newtonian rheology is calculated numerically to second order in the volume fraction in steady simple shear flows for Brownian spheres in the presence of hydrodynamic and excluded volume interactions. Previous analytical and numerical results for the low–shear structure and rheology are confirmed, demonstrating that the viscosity shear thins proportional to Pe2, where Pe is the dimensionless shear rate or P ́eclet number, due to the decreasing contribution of Brownian forces to the viscosity.
Large Pe Limit
In the large Pe limit remnants of Brownian diffusion balance convection in a boundary–layer in the compressive region of the flow. In consequence, the viscosity shear thickens when this boundary–layer coincides with the near–contact lubrication regime of the hydrodynamic interaction.
Wakes Formation
Wakes are formed at large Pe in the extensional zone downstream from the reference particle, leading to broken symmetry in the pair correlation function. As a result of this asymmetry and that in the boundary–layer, finite normal stress differences are obtained as well as positive departures in the osmotic pressure from its equilibrium value.
Normal Stress Difference
The first normal stress difference changes from positive to negative values as Pe is increased when the hard–sphere limit is approached. This unusual effect is caused by the hydrodynamic lubrication forces that maintain particles in close proximity well into the extensional quadrant of the flow.
Conclusion
The study demonstrates that many of the non–Newtonian effects observed in concentrated suspensions by experiments and by Stokesian Dynamics simulations are present also in dilute suspensions.
EXECUTIVE SUMMARY
The objective of this work is systematic understanding of particle-particle nanorheology based on the single particle-particle contact of two atomically-smooth solid surfaces in molecularly-thin proximity. The main relevance is to understand the origins of suspension rheology, especially the origins of rheological anomalies that arise when interfacial films between two solid bodies are so thin that the intuition of what to expect based on bulk rheology no longer applies. Based on this understanding, we are seeking to develop new methods to control and manipulate the properties of their interfacial films.
Specifically, the dynamic mechanical properties of the resulting interfacial film are being studied directly with special emphasis on how they depend on both vibration frequency and strain rate. A homebuilt apparatus is employed to this purpose with the following unique properties:
- Surface-surface spacing are variable from thousands of Ångstroms to molecular contact. The surface force needed to produce this separation is measured while at the same time measuring shear nanorheology. The tip is imaged in situ directly during each experiment, therefore force can be normalized by area to produce stress.
- Oscillatory deformations with variable frequency in the range 0.01 to 103 rad-sec-1 can be applied, with deformations either in the shear direction or in the normal (pumping) direction.
- The amplitude of deformation can be varied from sub-Ångstrom to thousands of Ångstroms. The reason to use very small deformations is to produce a linear viscoelastic response (representative of the rest state, because this can be studied by methods of equilibrium statistical thermodynamics). The reason to use very large deformations is to produce strongly nonlinear deformations characteristic of very high shear rates.
To the best of our knowledge, no other instrument with these properties exists in any other laboratory in the world. We would like to take this opportunity to encourage IFPRI members to suggest new systems that would be interesting to study using these unique methods.
The main finding during Year I was to develop criteria with predictive power to understand whether opposed particles will move past one another with intermittent stick-slip motion or with smooth sliding. We found that stick-slip motion occurred only when thin films were deformed faster than their intrinsic relaxation time. The observation offered a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined film.
The main findings during Years II and III concerned methods to control suspension rheology with surface coatings. First, we found that because the interparticle potential is an equilibrium quantity, whereas nanorheological responses depend on rate-dependent processes, the interparticle potential failed to correlate directly with shear nanorheology when dealing with films that showed a viscoelastic response. We initiated studies concerning forces in a “tapping” mode – to hydrodynamics when particle surfaces come together and are pulled apart with fluid media in between. These studies focused on nonaqueous systems and some preliminary studies were undertaken in aqueous media.
Following on our new sensing strategies to enable on-line measurements in concentrated and, using different approaches, for dilute flowing mixtures reported in the second year of our work, we have since concentrated on the dilute flowing system using the Particle Gymnasium approach.
Over the last year extensive work has been conducted on design, fabrication and testing of a particle flow system, a tube sensor and its on-line data acquisition system. Significant results have been obtained in terms of the relationship between the sensor signal and particle size and shape. The benefits of the revised approach using a 'sensor tube' rather than an orifice (Coulter) have been confirmed.
An on-line data acquisition system was designed and assembled, including means of reducing signal noise without affecting the signal response, as we intended to use the response time for particle length estimation. An experimental apparatus for particle flow was designed and built for examining the effect of particle flow behaviour on the measurement signals. Most importantly, a new Pt wire sensor of a ring shape was designed and fabricated, which can be easily fitted via flanges into the pipeline for measurements. Special designs for adding particles into either a horizontal or vertical pipe flow system were also made to examine the effect on signal response.
Using the newly designed experimental apparatus, it is possible to carry out both dynamic tests (adding particles into to the flow system) and static tests (particles fixed on a string to achieve a specific particle radial position and orientation) tests. Results revealed that the change of the particle radial position, up to 83% of the tube diameter, did not affect the peak value of the signal, while particle orientation had a great effect on the peak, giving a 63% difference for a cylindrical particle of 2.2x5.35mm (diameter x length). Particles moving close to the electrodes produced some more complex voltage signals. Particles that rotate during measurements also yielded more complex voltage perturbations. Therefore it is possible to use the orientation effect for particle shape estimation. Experimental and modelling results have also shown that for cylindrical particles of constant orientation both the particle diameter and length could be estimated using the Particle Gymnasium sensor.
Further work is needed to confirm the above finding using particles of arbitrary shape and size. It is also necessary to examine whether we can recognise the particle shape, other than only non- spheroid, from the signal profiles. Other data analysis methods, such as neural networks, may be appropriate especially when the relationship between the profile and size and shape becomes too complicated to be described by simple equations and when particulates can be classified into different user-defined categories.
Work on concentrated systems as well as multiphase flows has also continued, as summarised at the Annual Meeting and in this report.
A revised forward programme has been discussed to focus upon microstructure sensing of concentrated flowing mixtures.
Executive Summary
Uniform anatase-type TiOl nanoparticles were prepared by Gel-Sol method, in which a condensed aqueous solution of Ti-triethanolamine (TEOA) complex is first aged at 100 “C for 24 h for the hydrolysis of the Ti-TEOA complex to Ti(OH)4 gel network, and then aged at 140 “C for 72 h for the nucleation and growth of the final product by gradual dissolution of the Ti(OH)4 gel. The Ti-TEOA complex was previously prepared by mixing titanium isopropoxide (TIPO) directly with TEOA in a dry box at a molar ratio 1:2. Typically, uniform TiOz particles of ca. 21 nm in mean diameter were obtained by aging an aqueous solution of 0.25 mol dm” TIP0 stabilized with 0.50 mol dmT3 TEOA (initial pH = 9.5) at 100 “C for 24 h, followed by aging at 140 “C for 72 h. Effect of pH on the particle size was remarkable, since the mean diameter of cuboidal particles increased from ca. 5 to 30 nm with increasing pH from 1 to 11.5. However, no TiO2 particles were obtained over pH 12 even after the 2nd aging for 72 h. If TIP0 is not stabilized by TEOA in advance, Ti(OH)4 flock is formed instantly on mixing TIP0 with water at room temperature, and it is completely converted to ill-defined polydispersed TiOl particle of ca. 9 nm in mean diameter after the 1 st aging at 100 “C for 24 h. From the reduction of the reaction rate with increasing pH, irrespective of the presence or absence of TEOA, the increasing particle size with pH is basically elucidated in terms of the reduction of the nucleation rate by the lowered concentration of precursor complexes to TiOZ particles with increasing pH. In addition, this pH effect on the final particle size of TiOz was pronounced by the presence of TEOA, since TEOA liberated after the 1st aging significantly lowered the solubility of the produced Ti(OH)4 gel by adsorption, and since this etTect of TEOA was 111 enhanced with increasing pH. The particle size was also varied systematically by adding different amounts of seeds. Dramatic increase in the formation of TiOz with the increasing amount of seeds provided us with information that the dissolution of the Ti(OH)4 gel is not the rate-determining step of the particle growth of TiOz, and that the particles are grown by deposition of monomeric solute and not by aggregative deposition of particulate matters such as hypothetical primary particles of TiO2. It was also found that the particle shape was changed from cuboidal form to rod-like one when the pH was increased to 11 S. This result was explained in terms of the adsorption of TEOA to the crystal planes parallel to the c-axis of the anatase crystals.
As an application of thus-prepared well defined TiOz nanoparticles, their catalytic performance as a photocatalyst for water photolysis was studied. For this purpose, a special reactor was designed for precise measurement of the quantum efficiency of the photo-excited electrons and holes. Using this reactor with Pt/TiOz catalysts prepared by the selective deposition of Pt onto TiOz particles precisely controlled in size and shape, we performed preliminary experiments for the effects of the particle size and shape of TiOz, Pt loading, pH, atmospheric pressure, and concentration of electrolyte on the quantum efficiency. As a consequence, we found significant effects of these factors. However, we also found that the most imminent issue to be resolved is the surface modification of the Ti02 particles to introduce separate electron and hole trapping centers for the prevention of the drastic recombination of photoelectrons and holes.
Present Annual Report - 2000
The present annual report-2000 described results on the third years’ task in the theme of “Mechanochemistry of Materials” approved by the IFPRI organization. The task covers development of novel material processes by means of mechanochemical treatment and its relation with a ball mill simulation work. The simulation work plays a significant role to elucidate mechanochemical phenomena of materials. The present report contains considerable findings on the relation of mechanochemical phenomena and information obtained from the simulation.
Report Composition
The report composes of three parts:
- Mechanochemical treatment of EP dust, forming soluble vanadium (V) compound in water.
- Mechanochemical treatment of fluorescent powder, accelerating its structure change.
- Dechlorination of PVC by its mechanochemical treatment with inorganic material such as CaO.
Findings
Regarding the first example, the yield of vanadium extracted by water leaching is well correlated with impact energy of balls in the mill calculated from the result obtained by the Particle Element Method (PEM). As for the second example, the dry grinding the EP dust enables us to form a water soluble vanadium compound. The well correlation between the V-yield and the impact energy of balls is obtained, suggesting that the ball impact energy plays a significant role to control the formation of vanadium compound.
The third one is dechlorination of polymers such as PVC (poly-vinyl chloride), PVDF (poly-vinylidene fluoride) and PTFE (poly-tetra fluoro ethane) by their mechanochemical treatment with inorganic material such as CaO. This work has been presented at the IFPRI AGM 2000 held at Scheveningen, Netherlands. The present report described only the dechlorination of PVC and its correlation with the impact energy of balls in the mill calculated from the result simulated by the PEM.
All the approximately same, the impact energy of balls in a mill is a significant key to control mechanochemical effect and reaction. In such sense, the computer simulation based on the PEM regarding the ball motion during milling is a quite useful tool for determining the optimum operational parameters, mill design with scaling-up.