Particle Formation

The silicotungstate anion-- [ SiW 1204OJ ‘4, referred to in this document as “STA”-- has a diameter of 1 nm and is used as a model particle to investigate the role of short range forces in controlling the colloidal stability of metal oxide nuclei formed in the early stages of precipitation reactions.

1) The role of counter ions

has been studied by investigated the interactions of STA in the presence of H+, Li+, and Na+.

2) By measuring the light scattered

from dilute STA suspensions, the second virial coefficient was determined in ranges of 0.3-5 M HCl, LiCl and N&l solutions where it was found to be a decreasing function of ionic strength. Only small differences appear with changes in cation type.

3) Osmotic stress techniques

indicate that the STA anions crystallize from solution at osmotic pressures that are weak functions of cation type. However, the degree of STA hydration is very sensitive to the cation. Osmotic pressures at crystallization lie between (2.5-3.5)x107 Pa. The largest hydrate of H4STA is 31 while for Li4STA and Na4STA, the largest hydrates are 25-26 and 13 waters per STA molecule respectively. With increasing osmotic pressure, the crystals dehydrate in a series of steps with the osmotic pressures required to induce a step change in hydration varying with temperature and cation type. To a first approximation, the pressures required to induce the first dehydration step vary as. Na>Li>=H.

4) By comparing the volume fractions

at crystallization, the osmotic pressures, and the second virial coefficients of STA particles with the values expected for hard spheres, we conclude that at crystallization, the particles have hard cores with diameters of 1.1-1.24 nm and feel an attraction of depth (0.3-0.8) kT,

5) Our analysis indicates

that the interactions of nanometer sized particles are sensitive to electrostatic, van der Waals, and hydration interactions. To a first approximation, hydration interactions screen van der Waals attractions and block aggregation into the primary minimum, However, the hydration interactions may also provide an attractive minimum in pair potential energy which facilitates aggregation of a reversible type. The hydration interactions are controlled by a competition for water by various species in solution. As a result, the degree of aggregation will be controlled by water activity. This conclusion implies that in precipitation reactions, the activity of water (or its chemical potential) will play a significant role in determining the colloidal stability of growing solid particles.

6) Working in collaboration

with Dr. F. van Swol, we are investigating the role of solvent chemical potential in controlling the state of aggregation of colloidal particles. The calculations of Frink and van Swol [4] and Kokkoli and van Swol demonstrate that these interactions can be understood in terms of the affinity of the solvent for the solid and the stacking of solvent molecules between the surfaces. As the chemical potential of the solvent is raised, solvent will begin to partition to wetting surfaces and thus force the surfaces apart. When submerged in pure solvent, the only way to increase the chemical potential of the solvent further is to apply a hydrostatic pressure to the liquid. With increasing hydrostatic pressure the clay swelling continues. While the current models deal only with nonionic solvents, extensions to ion containing solvents and charged surfaces are in progress which demonstrate that solvation interactions continue to play a major role when the surfaces are held at separations on the order of a few solvent molecules.

These models indicate that solvent activity plays an important role in determining the state of particle aggregation. With this concept in mind, the experimental portion of this contract is aimed at developing methods of characterizing the nature of solvation interactions and using solvent chemical potential to manipulate the state of aggregation of particles which model primary particles produced in homogeneous precipitation reactions. For this purpose we have chosen to work with silicotungstate anions [SiW 120a]-4 which are spherical and have a crystallographic diameter of 1 nm. These anions are highly soluble, and when they crystallize, the solids have many waters of hydration.

The chemical potential of the continuous phase is manipulated by two methods in these studies. In the first, the osmotic stress technique of Parsegian and coworkers is used [5]. Here STA solutions are equilibrated with a vapor of known humidity (water activity) and the degree of hydration is measured. In these experiments, the chemical potential of the continuous phase is set by the relative humidity of the water above the solution. The water in the solution equilibrates by increasing or decreasing the concentration of the STA particles and their counterions. At equilibrium, the osmotic pressure of the STA liquid or solid is determined from n: = -RT ln(p/po)/v where RT is the product of the ideal gas law constant (R) and absolute temperature (T), p is the vapor pressure of water above the STA, po is the saturated vapor pressure of pure water, and v is the molar volume of water (with p, po and v all at temperature T). The osmotic pressure (7~) is the pressure that would have to be applied to the STA liquid or solid to maintain the solid volume fraction if it were exposed to pure water.

The second method of controlling the activity of the water and the electrostatic screening ability of the continuous phase is by varying the added electrolyte concentration. Currently we have focused on using electrolytes to alter both the ionic strength and solvent activity. The intensities of the light scattered from dilute solutions of STA at fixed electrolyte concentration are used to determine the second virial coefficient in a concentration expansion of the suspension osmotic pressure. In the limit of small particles, the intensity of the light scattered from the suspension is independent of angle and can be written as:

-----------------------, dn/dc = refractive index increment, ho = incident wavelength in vacuum, no = solvent refractive index at h,, N, = Avogadro’s number 3 (6.02 x 1023), c = mass concentration, RQ = Rayleigh scattering intensity at 8,9 = angle, M, = weight-average molecular weight, and A, = second virial coefficient. In these experiments, as the ionic strength of the supporting electrolyte is increased, the electrostatic repulsions between the STA anions are screened. In addition, the activity of the continuous phase is reduced.

In attempts to investigate the role of the counterion in determining the magnitudes of hydration forces, the dehydration of STA crystals and the second virial coefficients of dilute STA suspensions have been measured with counterions of H+, Li’, and Na+. For this purpose, H&TA, LQSTA and Na&TA materials were synthesized and their composition confirmed with NMR and elemental analysis.

Executive Summary

A first order model of the dynamics of pyrogenous fumes, based upon experimental and theoretical studies of the kinetics of particle growth and the structural rearrangements that occur following coagulation, suggests that the growth of nonagglomerated particles in aerosol reactors is best accomplished by dropping the temperature rapidly to quench coalescence before significant agglomeration occurs. Because of their larger collision cross sections, agglomerate particles coagulate more rapidly than do spheres of equal mass. As a result, once agglomerates begin to form, particle growth accelerates dramatically. Although agglomerate particles can be densified, the temperature would have to be increased significantly to do this, instead of decreasing continuously as occurs in most practical reactors.

Notably, the model predictions suggest that, for a given cooling rate, a high initial temperature will more effectively limit neck growth than will a lower one. The rate of decrease of the coalescence rate following the onset of agglomeration depends on the cooling rate and on the initial growth temperature. If the initial operating temperature is low enough that the coalescence time is comparable to the coagulation time, the coalescence time will increase only slightly faster than that for coagulation. Strong neck formation and hard agglomerates can then be expected. On the other hand, if the initial operating temperature is much higher so that coalescence is initially very rapid, the transition will occur much more abruptly. Neck growth within the first agglomerates to form will be reduced, and agglomerates will be more amenable to dispersion. These inferences require experimental testing. The proposed continuation of this project will focus on experimental definition of the bound between dense particle growth and agglomerate formation, and on quantifying the quench rate that is required to inhibit the formation of sintered agglomerates.

The classical model of neck development during sintering has been extended beyond the early stage of neck growth. Neck growth predicted using our model deviates significantly from that of the early-stage sintering model, raising serious questions about efforts to model the evolution of particle structure with sintering rates based on the classical model. Although this means that the model developed in this report can only be assumed to provide a qualitative picture of the transition, importance of the coupling of coagulation with coalescence is clear. Attempts at quantitative prediction of the structural evolution of pyrogenous fumes based upon the classical sintering time scales are, however, premature. Trace oxygen contamination of the silicon we have studied led to anomalous sintering behavior. Oxidation of the silicon surface would be expected to dramatically reduce the rate of surface diffusion. Neck growth predictions based upon the transport properties of pure silicon, but with surface diffusion eliminated, agree well with experimental observations. Thus, future work on the structural evolution of pyrogenous fumes must take the reaction atmosphere into account.

This report focuses on the theoretical interpretation of the results of our experimental program, and on defining the direction for future work. Experimental investigations aimed at obtaining quantitative measurements of the sintering of model agglomerates and at validating the models of the coagulation kinetics of agglomerate particles are near completion, and analysis of those experimental results is underway.

This program consisted of two major portions: a two-year feasibility study and a three-year investigation into the fundamentals of effervescent atornization. A number of goals were proposed and met during each portion. They are listed in Tables 1 and 2. Those goals are also discussed in the following ten paragraphs.

The primary goal of the feasibility study

The primary goal of the feasibility study was to produce sub-50 µm droplets when spraying highly viscous (up to 100,000 cP) non-Newtonian fluids at process level throughputs (up to 1 kg/s). A secondary goal was to determine the spatial structure of the spray, in terms of how mean drop size and the width of the drop size distribution varied with axial and radial position throughout the spray. Work was conducted using a new type of nozzle developed at Purdue (an effervescent atomizer) because it was the only candidate likely to meet the sub-50 µm criterion. Fluids considered during the feasibility portion of the program consisted of glycerin/water/polymer and coal-water slurry/glycerin/polymer mixtures. Their consistency indices were as high as 41,500 and their flow behavior indices were as low as 0.27, Mean drop sizes as low as 28 µm were achieved when using air-liquid ratio values of less than 0.20, nozzle performance was shown to improve, i.e. mean drop size decreased, with increased throughput (in contrast to the behaviour of conventional atomizers) and polymer addition was demonstrated to have an adverse effect on the atomization of either single-phase or multi-phase feedstocks (although the extent of the increase left the mean drop size below the target value of 50 µm).

The fundamental investigation

The fundamental investigation had a number of goals We first focused on spray behavior in the transition region where the two-phase flow that exits the nozzle as discrete gas bubbles in a continuous liquid is transformed into a continuous gas stream containing discrete liquid drops. Our goal was to determine the mechanism(s) controlling effervescent atomization so that a model could be developed for future design usage. Single-pulse holography was employed to observe the liquid breakup phenomena characteristic of effervescent atomization. A qualitative explanation of the mechanisms responsible for effervescent atomization was provided, based on this data.

Investigation of interactions

We then broadened our efforts to include an investigation of the interactions between the spray and its surroundings. Our goal was to improve the already superior energy efficiency of effervescent atomizers by minimizing the impact of the major energy loss mechanisms. A computational study was performed to identify these major loss mechanisms and suggest methods for reducing their impact. We discovered that turbulent dissipation was the largest loss, followed by transformation of bulk kinetic energy into turbulent kinetic energy, and then entrainment of surrounding air. We concluded it was unlikely that turbulent dissipation and transformation of bulk kinetic energy into turbulent kinetic energy could be reduced, but that entrainment might be minimized.

Entrainment model development

We then focused on entrainment, with our efforts proceeding along two parallel paths. First, an entrainment model was developed for effervescent sprays that is based on the dimensional analysis performed by Ricou and Spalding [ 1961] in their work on entrainment by gas jets. This results in the entrainment number, which is the entrained gas flow rate normalized by jet momentum flux, axial distance, and entrained gas density.

Experimental apparatus

Second, experimental apparatus were built to measure both the entrained mass flow rate and momentum rate for a variety of sprays. The entrainment measurements showed that entrainment increases linearly with axial distance and that it increases with &-liquid-ratio (ALR) and viscosity. By measuring momentum rate, it was possible to calculate the entrainment number directly. Comparison of entrainment numbers showed that it increases with viscosity, nozzle orifice diameter, and the combined effects of surface tension and liquid density. A dimensional scaling by liquid density and orifice diameter collapsed the majority of the entrainment number curves onto a single line. The modified version of the entrainment predictive equation describes the entrainment behavior of effervescent sprays to within 25%.

Fluid rheological properties

Subsequent efforts were focused on the relationship between fluid rheological properties and spray mean drop size. Our goal was to identify the fluid rheological * property that resulted in reduced spray performance upon addition of polymer to a feedstock This work was motivated by our previous study into the influence of polymer addition on nozzle performance.

Study of viscoelasticity

One result of that study was the conclusion that spray mean drop size was independent of changes in either consistency index or flow behavior index for a power law fluid - it is fluid viscoelasticity that degrades nozzle performance. We then focused our efforts on a study of viscolelastic liquid effervescent atomization. Fluids were formulated using a Newtonian solvent into which were dissolved varying amounts of Poly(ethylene oxide). Six different polymer molecular weights were investigated between 12,000 and 900,000. Qualitative and quantitative drop size measurements were made for a range of fluid operating conditions. Liquid mass flow rate was held constant at 10 g/s while the air-liquid mass flow rate ratio varied from 2 to 10%.

Data analysis

Data show that the spray mean drop size decreases with increasing air-liquid ratio, that the addition of polymer increases mean drop size, and that spray mean drop size increases with polymer molecular weight and polymer concentration beyond a molecular weight of 35,000. Below 35,000 no significant variations in mean drop size were measured regardless of polymer concentration.

Holograms and ligament formation

Holograms of the near nozzle structure indicated that the presence of polymer in the fluid serves to delay the formation of ligaments from an annular sheet as the fluid exits the nozzle. Also the breakup length and diameter of the ligaments increases with the addition of polymer. These two phenomena explain the increase in mean drop size with the addition of polymer to a pure Newtonian solvent.

Modeling of spray formation

Modeling of the spray formation process was carried out in the spirit of previous effervescent spray models. The analysis correctly predicts an increase in mean drop size with increases in polymer molecular weight or polymer concentration. The model predicts excellent qualitative behavior of Sauter mean diameter (SMD) versus ALR. In addition the error between the experimental data and the model predictions was as low as 10%. The maximum disagreement was never more than 50%.

Theoretical analysis

Finally, a theoretical analysis was performed to describe the combined longitudinal- circumferential breakup of the annular liquid sheet present at the nozzle exit plane. The analysis considered only linear terms in the governing equations for momentum and mass conservation.

Conclusion of analysis

This analysis provided the correct scaling for ligament breakup length and number of ligaments formed from the annular sheet at the exit plane versus fluid viscosity, air-liquid ratio, and mass flow rate. However, the analysis did not provide the correct ligament breakup length and ligament number scaling versus surface tension, and predicted too few ligaments formed in the viscous case, as well as a fastest growing circumferential mode of order zero. Neutral stability plots indicated that the model was able to predict the combined circumferential-longitudinal breakup observed experimentally, thereby justifying a more sophisticated study that included the influence of non-linear effects. Finally, the neutral stability plots indicated that the present analysis can be used as a basis for derivation of drop size distribution functions from first principles. Such an analysis would couple classic instability theory, as presented here, with the discrete probability function (DPP) approach to incorporate contributions to the drop spectrum from all unstable modes. Fluctuations in fluid properties, such as viscosity, density and surface tension, due to inhomogeneities, as well as velocity variations due to turbulence could be incorporated. Consequently, the influence of these variations on the width of the drop size distribution could be evaluated.

Executive Summary

The primary project aims are to develop relationships which predict the wet massing behaviour of particulate solids granulated with binders by mechanical agitation, and to apply such findings in probing scale-up factors.

Work with model substrates: polymer binder systems has previously shown the critical estimate from surface free energy measurements, in determining the wet-massing and rheological character of the role of solid:liquid interfacial phenomena, particulate systems during granulation. This approach has been employed to predict the wet-massing behaviour of four representative powder substrates - two microcrystalline celluloses, calcium carbonate and griseofulvin - granulated with two aqueous polymer binders - polyvinylpyrrolidone and hydroxypropylmethylcellulose. The findings have been tested with a new mixer torque rheometer, which has been shown to provide data which can be directly related to the theological terms yield stress (T), kinematic viscosity (7) and degree of non-Newtonian rheological behaviour (n).

In general, the rheological behaviour of the various substrate:binder systems was consistent with the predictions made from surface free energy calculations, and followed similar patterns to those observed for model substrates. The spreading of substrate and binder components were critical factors in influencing the stability of the wet masses in the domains where, in industrial processes, many granules are prepared. Preliminary observations with mixed powder substrates suggest that topographical features of particulates also play an important role in determining rheological behaviour during granulation.

In the scale-up studies, a modified power number/Reynolds number relationship has been developed and successfully applied to large scale (up to 600L) mixer granulators. This approach has shown that, via measurements of wet mass rheology by mixer torque rheometry, a master surve for a specific formulation can be prepared using laboratory scale equipment which allows prediction of optimal granulation end-point conditions for large scale production equipment.

Executive Summary

Although compaction has been the object of considerable study, questions remain about compaction. These questions exist due to the difficulties with obtaining in-situ information about powder rearrangement/breakage during compaction. In general, compaction work has primarily studied stress-strain curves. Depending upon the relative density and the change of density with pressure, one tries to infer the mechanism(s) of compaction.

In this work, we address the development of in-situ techniques to tlrobe comtlaction mechanisms. Instead of simply measuring density variation and attempting to infer compaction mechanisms, we employ scattering to study compaction. In order to demonstrate the utility of these in-situ techniques, we have:

1. performed preliminary experiments to demonstrate the sensitivity of the technique to a dilute second phase (sensitive to l-5% depending upon the powder size, morphology, and electron density),
2. used compaction with ex-situ scattering to demonstrate how compaction mechanisms can be directly observed and
3. employed fluids with electron density matched to that of the solid phase (or one powder if a mixture) to selectively study the second powder.

In one example, we studied changes of a silica compact as it was compacted isostatically or by drying capillary pressure. By assessing the variation in the scattering intensity associated with different length scales as well as the change in the hydraulic radius with pressure/density, pores around the agglomerates were observed to disappear due to agglomerate breakage and compaction. This result could not have been obtained by simply measuring the stress-strain of the compact as is normally done in compaction studies. For another example, dibromomethane was impreganted into a silica-titania mixture and scattering was performed. This shows that contrast matching and small-angle x-ray scattering may be employed to selectively “illuminate” a few hard aggiomerates of a second material type during compaction.

Executive Summary

The role of short range interparticle forces in controlling the size distribution of submicron particles precipitated from solution has been investigated. The central hypothesis explored is that primary particles formed early in precipitation reactions are subject to short range forces that can be repulsive or attractive depending of the solvent chemical potential and the particle separation.

1. A model system was used to explore the role of short range forces in controlling the colloidal properties of sub-10 nm metal oxide particles. Criteria sought in looking for the particle were i) that it be on the order of the size of primary particles formed in precipitation reactions, ii) be readily available for investigation and iii) have a metal oxide composition. The particle chosen was the silicotungstate anion, SiW12O40,(STA) which is a sphere carrying four negative charges with a diameter of 1.1-1.2 nm. STA is readily soluble in water and is commercially available.
2. The solubility of the acid, lithium and sodium forms of STA was investigated as a function of supporting electrolyte concentration of HCl, LiCl and NaCl, respectively. Second virial coefficients of STA suspensions were measured by static light scattering.
3. The second virial coefficients were converted into an effective temperature, 7, by assuming the particles interact with an attractive pair potential with an extent which is a small fraction of the particle diameter. As T decreases, the strength of the interparticle attraction increases. A comparison was made between predicted and measured phase behavior where *c is plotted as a function particle concentration at the solubility limit. An excellent comparison was found suggesting the adhesive hard sphere model provides an adequate description of STA suspension thermodynamic properties.
4. These results demonstrate that as the supporting electrolyte concentration is increased, interparticle attractions increase. Detailed calculations suggest the attraction is stronger than can be reasonably attributed to van der Waals attractions. The conclusion is drawn that the salting out behavior seen in STA suspensions has an origin in the relative affinity of the solvent for the STA particles and the supporting electrolyte. We hypothesize that as the electrolyte concentration is increased, the water would rather hydrate the supporting ions than the STA particles resulting in a net interparticle attraction. This study clearly shows that the pair potential can be modulated by influencing the chemical potential of the solvent. In addition, these studies indicate that small particles feel weak attractions which will grow in magnitude as the solvent chemical potential is reduced. Note however, that this attraction does not bring particles into contact. Particles remain hydrated in the aggregated state.
5. The interactions of the STA particles were also investigated using osmotic techniques. Here STA crystals were equilibrated with nitrogen streams with different relative humidities of water. The dehydration properties of the STA crystals are very sensitive to the counterion. These studies indicate that the affinity of STA/counterion particles for water is high and that complete dehydration of STA does not occur at 25 C until the relative humidity is less that 0.05 STA crystals dehydrate in steps indicating that the pair potential is oscillatory in nature. If the relative humidity is converted to an osmotic pressure, 'IC (= -kT/vln(RH), where n is the osmotic pressure, v is the molecular volume of the solvent and RH is the relative humidity), one finds that at crystallization, the suspensions must be compressed to a pressure of near 300 atm if they are kept at a constant volume and exposed to pure water. Osmotic pressures of near 1000 atm are required to completely dehydrate the particles.
6. The dehydration experiments indicate that while STA crystals are heavily hydrated, the particle interactions are sensitive to counterion. From this result we conclude that oscillatory interactions arise from counterion hydration rather than particle hydration. Never-the-less, both the dilute solubility experiments and the crystal dehydration experiments indicate that the degree of aggregation (or the separation distance of the particles) can be controlled by alterations in the solvent chemical potential.
7. We conclude that in precipitation reactions, clusters or primary particles grow by molecular addition but do not aggregate because: i) van der Waals forces between small particles are weak, and ii) the hydrated state of the particle surface screens the van der Waals attractions. As the particles grow, the extent and strength of attractive forces increase and aggregation may occur. If the particles remain reactive, such flocculation can result in irreversible agglomerates. In addition, if over the course of the reaction, the solvent chemical potential is decreased, our results suggest that attractions will increase and may lead to aggregation and thus producing a broad particle size distribution.
8. Means of controlling the state of aggregation of sub 50 nm particles suggested by the preceding include control of the solvent chemical potential, and/or the adsorption of small stabilizing agents (such as citrate used in the control of particle size in the precipitation of gold from the reduction of auric acid).

A tutorial review of flame aerosol technology for manufacture of ceramic powders is presented. In the mid-20th century this field was driven by industrial research and development for production of commodities such as fumed silica and pigmentaty titania. With highly competitive market growth, inexpensive scale-up of existing units is required. In addition, the introduction of this technology to manufacture optical fibers and its potential for cheap synthesis of ultrafine particles (e.g. nanoparticles) has renewed research interest in flame aerosol reactors.

In this review, emphasis is placed in synthesis of particles with controlled size and crystallinity. After an overview of its history, the fundamentals of this technology are summarized, specific applications in the manufacture of fumed silica, pigmentary titania, alumina, composite and non-oxide powders are reviewed and finally research needs are highlighted. With major recent advances in process instrumentation and understanding in both combustion and aerosol science and engineering, this field is ready for a new leap forward.

During the 1995 Annual Meeting of IFPRI at Urbana, IL, USA, the enclosed statement was published and presented. It describes in a short form pressure agglomeration, its subdivision into low, medium, and high pressure techniques, the mechanisms of pressure agglomeration, as well as the author’s opinion as to where need exists for development and research.

To further describe the state-of-the-art of pressure agglomeration, excerpts from the author’s book entitled “Size Enlargement by Agglomeration” are submitted herewith.

It had been suggested to summarize recent research for this report. However, after obtaining “suitable references” from within IFPRI and from searching this author’s files it became clear that such published studies are very specific. They relate mostly to requirements in the pharmaceutical industry or to other specialized applications. They often draw conclusions which are not of general interest and sometimes even misinterpret the basics of the unit operation.

It is not the intent of this report to reconcile the results of such studies. This should be done in a scientific research environment during an interdisciplinary literature search which, again in the reporter’s opinion, is the first and foremost work in this field that should be sponsored by IFPRI. The result of such a study should be general and unified conclusions and a true statement as to what basic information is available. At the same time the many publications must be disregarded which try to explain the influence of specific materials, additives, process modifications, and parameters for very limited applications without connecting this work with a theory that is generally valid.

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.

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.