ARR - Annual Report

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. In this report, we review Cornell activities in the area of gas-solid suspension flows in 1997.

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.

By matching five dimensionless parameters, experiments employing plastic and glass powders fluidized with mixtures of sulfur hexafluoride, carbon dioxide, helium and air near ambient temperature and pressure achieved hydrodynamic similarity with generic high-temperature risers of variable scale operating at pressures of 1 and 8 at-m.

We interpreted our results in the upper riser using steady, fully-developed momentum balances for the gas and solid phases. This analysis showed that, for a wide range of experiments, two parameters capture the dependence of the pressure gradients upon the ratio of the mean gas and solid mass flow rates. The first is the ratio of the mean particle slip and superficial gas velocities. The second represents spatial correlations between the radial profiles of interstitial gas velocity and voidage. Variations of the first with dimensionless parameters indicated that our “atmospheric” and “pressurized” experiments conformed to distinct viscous and inertial regimes.

In 1997, we have also established that the descending velocity of particles clusters at the wall of a riser scales exclusively with the square root of the particle diameter and the gravitational acceleration. This observation showed that the dynamics of wall clusters is chiefly determined by inter-particle contacts. Because these clusters govern heat transfer at the wall, this conclusion has important consequences for modeling.

Zirconia (ZrO,) particles are one of the most important materials for refractory ceramics. However, commercially available ill-defined powders prepared by milling of calcined agglomerates or by gas-phase reaction of ZrCl, and 0, are normally difficult in preparation of crack-free compact for sintering or need high sintering temperatures above 1700 “C. On the other hand, the monosized amorphous powders of a high sinterability prepared by hydrolysis of the alkoxides are not free from the economical problem of their low productivity (< 0.2 mol dmm3 in final concentration).

The objectives of our project are to develop a new method for preparation of unagglomerated ultrafme crystalline spheres of ZrO, of a narrow size distribution with the mean diameter of the order of ten nanometers or less in large quantities on the basis of the gel-sol technique developed in our laboratory, to elucidate the formation mechanism, and to examine the sintering properties. The standard procedure is as follow.

• Zirconium (IV) n-propoxide (ZNP: Zr(OC,H,),) is mixed with triethanolamine (TEOA: N(CH,CH,OH),) at a molar ratio of ZNP:TEOA = 1:3 in a dry box filled with dry air to form a stable compound of Zr”’ against the exceedingly rapid hydrolysis of Zr4’.
• After aging the solution at room temperature for 24 h, doubly distilled water and NH, solution are added to the solution in order to make a solution of 0.5 mol drnm3 in Zr4’ and 1.0 mol dmw3 in ammonia. In this stage no reaction takes place.
• The resulting solution is then aged at 200 “C for 3 days without stirring in a preheated oil bath to nucleate and grow the zirconia particles.

From the UV spectra of the mixed compounds of ZNP and TEOA in n-propanol and FT-IR spectra in chloroform, it was found that the propoxy groups of ZNP were all replaced by ethoxy groups of TEOA. With the elevation of the internal temperature which took ca. 30 min to reach 200 + 1 “C, hydroxide gel was formed and the nucleation occurred on the gel network in 20 min. The nuclei were grown to fairly uniform particles of ca. 15 nm in mean diameter at the expense of the gel until, finally, the gel completely disappeared in 1 h. In this stage the particles were of a rough surface, but as they were further aged, they became spherical with a smooth surface due to the intra-particle recrystallization. When the aging was prolonged to 72 h, the particles were somewhat grown by Ostwald ripening. It is noteworthy that so-prepared particles were tetragonal ZrO, after 30 min as determined by X-ray diffractometry, even at the rather low temperature, 200 “C, while particles prepared at high pH (-13) in the absence of ammonia or those prepared in the presence of acetate ions at neutral pH (-7) were both monoclinic. Also, high-resolution transmission electron microscopy revealed that the produced particles were mostly single crystals.

From the analysis of growth mechanism it is concluded that the fairly uniform unagglomerated ultrafme particles are nucleated on the gel network of the hydroxide, and grown by dissolution of the gel without significant coagulation owing to the gel network holding each particle. In addition, extensive renucleation is inhibited due to the lowered supersaturation by the formation of the gel precursor.

The so-prepared powders (-15nm and -2.5nm) showed an excellent sinterability, as compared to power prepared by gas-phase reaction (-6Onm) or monosized amorphous powder by hydrolysis of zirconium butoxide at 50 “C (-250nm).

In the second year of the project, the laboratory equipment for dispersing powders in stirred vessels described in IFPRI annual report No. ARR 17-04 and the laboratory methods for measuring the wetting properties were used to investigate the dispersing of aspartame (supplied by DSM) and zeolites (supplied by Unilever Research) in the 10 1 scale. For comparing the pitched-blade turbine used so far with the Rushton type turbine, such a turbine was built and tested using wheat flour, instant tlour and other powders. Experiments were conducted with an unbaffled, partly or fully baffled vessel.

The dispersing behaviour of aspartame and zeolite differs considerably from the skim milk powder investigated during the first year of the project and required new methods for controlling dispersion/solution quality. Aspartame was dispersed in small concentrations (0.5 % wt./wt.). The dissolution process was controlled by simultaneous measurement of the size of the undissolved particles (using laser diffraction) and off-line photometric measurement of the aspartame concentration in the solution.

The main problem with aspartame was its low solubility (= 1 % wt./wt.) and slow rate of dissolution in water. For fast dissolution, a fine powder with accordingly large specific surface is required. This in turn requires that the stirrer be designed and operated in a way to immerge and disperse these fine particles as quickly as possible. The results show that using an unbaftled vessel is the best way to achieve this, but that the particle size distribution of the powder is of greater importance. If the powder is too fine, lumping occurs, diminishing the rate of dissolution.

Zeolites, which are insoluble in water, were used for producing slurries with 50 % wt./wt. solids. The main problem here was to immerge the powder during the second half of the mixing process, when the slurry concentration was already high. This problem had already been predicted by the results of wetting time measurements. Powder dispersion and avoidance of sedimentation, on the other hand, were easy to achieve. Again, an unbaffled vessel turned out to be better suited than a baftled one, where floating layers of unwetted or partly wetted powder prevented complete mixing. In the unbaffled vessel, vortex formation occured even at high solids concentration, aiding in immerging the powder. Additional wetting and immersion experiments indicate that a mixture of 20 % zeolite A4 and 80 % A24 may have much better properties than either of the pure powders.

Although the specific stirring power that can be achieved in an unbaffled vessel is inferior to the specific stirring power for a baffled one, the improved ability to submerge the powder with help of the vortex formed by the rotating liquid proved to be of decisive importance. For hard to disperse powders, however, this may be different. If such a material is to be dispersed, a partly baffled vessel might be the best solution.

Comparison of the pitched-blade turbine and the Rushton turbine showed that both perform equally well at immerging powders in unbaffled vessels, which can be seen by plotting the specific stirring power required for immediate immersion over the desired feed rate of the powder. If a fully baffled vessel is used, the Rushton turbine performs better. Further experiments will focus on partly baffled vessels, in which vortex formation occurs as in unbaffled vessels, while higher energy input may allow better dispersion of critical (lump-forming or gelatinizing) products.

While (agglomerated) skim milk powder could be easily dispersed in baffled vessels as well as in unbaffled ones (see annual report No. ARR 17-04), aspartame, zeolite and wheat flour require unbaftled vessels in which the powder is fed to a vortex. Currently, for scale-up from the laboratory scale to large-scale- vessels the concept of constant Froude numbers can be used as a first approach. For baffled vessels, this is not common as it leads to an impracticably high specific stirring power, but for unbaftled vessels, a reasonable design is easily achieved. Vessel operation is constrained regarding the liquid level, since gassing the liquid should usually be avoided. This problem can be solved by using speed-variable stirrers. The fact that unbaffled vessels can be much easier cleaned is an additional incentive to use this kind of design for industrial applications.

The results from the experiments conducted so far indicate, however, that a complete description of the immersion step would also preferably take into account vortex shape and surface flow pattern (surface shear), which depend on the stirrer type and operating variables, and also powder properties like static wetting time or dynamic wettability. Future work will therefore be directed towards characterizing the flow pattern caused by different stirrer types, characterizing the immersion capability and the dispersing efficiency of such vessels, and towards improving the measurement of dynamic powder wetting.

This report summarizes the activities conducted under IFPRI support from November 97 to November 98. The main target of our work is to understand, characterize, and quantify the behavior of powders in the early stages of compaction attendant to the intrinsic properties of the material and the processing conditions. The project involves theoretical, numerical, and experimental components. It is framed within a collaborative effort with concentration on pharmaceutical manufacturing, but the results are relevant to powder compaction in many other industries.

Topics Addressed

• Die pouring (Section 2)
• The reaccomodation process of Region I of the compaction curve (Section 3)
• The mechanics of regular aggregates as they obtain in pre-compaction structures (Section 4)
• The deformation process of Region II of the compaction curve (Section 5)
• An ongoing experimental program aimed at verifying our theoretical and computational results (Section F)

In the Introduction these topics are motivated in the context of the overall goals of the project.

This report summarizes the research activities for the project “Optimal Quality Control of Industrial Crystallizers,” ARR 32 - 01, during the period 1 September 1996 to 31 August 1997. The goal of this project is to develop on-line measurement technology and predictive crystallization models so that advanced on-line crystallizer control can be implemented to enhance precise control of crystal size and shape.

During the current year, we have demonstrated model identification and control strategies to improve filtration of a problematic photochemical of industrial interest. In order to analyze crystal shape as well as size, we have purchased and installed an Olympus BX60 microscope, image capturing hardware, and Image Pro Plus image analysis software. We have further developed our stochastic modelling capabilities so that models with new and complex crystallization mechanisms can be simulated quickly and accurately. We currently can simulate the following mechanisms:

• size dependent nucleation
• size dependent growth
• growth rate and nucleation rate fluctuations
• growth rate dispersion
• size dependent agglomeration

Plans for the next year are to test and commission the new image analysis equipment on a relatively simple system, and identify a model chemical system that is particularly relevant to industrial practice, for which improvements in particle size and shape measurement and control would provide large potential benefit. We will construct a flow cell and circulation loop to monitor the crystal size and shape in real time during crystallization experiments.

Introduction

High-velocity gas-particle flows in risers are accompanied by persistent fluctuations in pressure, velocities and particle concentration, and the presence of particle clusters. Risers are seldom used in isolation; instead, they are employed in conjunction with several other devices. For example, in circulating fluidized beds, they are used along with cyclones, standpipe, particle flow control device such as slide valve, etc.

Analysis of the power spectral density of temporal fluctuations (say, in gas pressure at some location in the riser) observed experimentally in the various components of the circulating fluidized beds including the riser often reveals both low and high frequency components [ 11. Our recent experiments on fluctuations in circulating fluidized beds suggest that the low frequency fluctuations in circulating fluidized beds are likely to be associated with the interaction between the various components of the system and have their origin in the standpipe. A manuscript based on these experiments is attached to this report as appendix A. The high frequency component of the fluctuations in the riser does not appear to be correlated with those observed elsewhere in the circulating fluidized bed [ 11, suggesting that it is associated with local phenomena.

This research work addresses the correlation between tht 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 relat,ed to compaction such as granulation, milling, mixing and blending, within a co- herent and collaborative effort with concentration on Pharmaceutical Manufacturing. Also, it provides the state-of the-art in characterization techniques and computational facilities, and ad-hoc testing facilities such as the Compactor Simulator Laboratory.

This report includes the current work since the beginning of the grant in October 1996 (some results were reported previously informally).

The aims of this Lancaster University - Bradford University collaborative project are to choose suitable particle compositions and ambient conditions, for work on the lack of reproducibility of powder flow behaviour; to measure force curves and investigate energy- dissipative contact processes; to clarify the role of ambient conditions (humidity) and of particle morphology; and to obtain complementary particle flow data. Our intention is thereby to assess how far such single-particle data are able to predict flow behaviour of real value to chemical engineers.

In the particle technology, it is fundamentally important to know the interaction and adhesive forces between particles and to find the correlation of those forces with the microscopic characteristics of particle surface, because these forces are the origin of many phenomena which particles exhibit. Most investigations on these forces reported so far have been carried out using macroscopic and statistical techniques. Hence it is impossible to clarify the relationship of these forces with the local and microscopic characteristics of the surfaces by which the interaction forces are influenced greatly.

There are two kinds of devices to obtain in-situ information of molecular level on the interaction forces between surfaces; Surface Force Apparatus (SFA) and Atomic Force Microscope(AFM). In the case of SFA, the interaction forces can be measured accurately, but transparent, smooth and large surfaces must be employed. The advantages of AFM are as follows:

• the interaction and adhesive forces between surfaces of any kind are possibly measured if the surfaces are able to be glued on the probe,
• the local microscopic features of the surface are obtained and they can be correlated with the interaction force, and
• the device is simple and easy to handle.

The aim of this research is to clarify on the molecular scale the interaction and adhesive forces between surfaces in various solutions and correlate those with the micro-structure of the surfaces, using an AFM. The following figure illustrates the whole map of experiments which we are going to do in this program.

In this report the experiments of the interaction and adhesive forces between SiOz and Mica surfaces in alcohol-water solutions and in dioxane-water solutions which are non- aqueous but miscible with water are reported, and the correlation of those forces with the microstructure on the surfaces is discussed. The effects of the addition of a surfactant, AOT, are also examined. The abstracts for these investigations are given below.