ARR - Annual Report

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.

The objective of this part of the report is to present the lastest results on cornmunition under the IFPRI grant that started in 1993. We recall that the objectives of the research are to develop fundamental understanding and techniques to predict communition behaviour from a universal test. An experimental rig was built up to reproduce single impact on a target in an air jet mill. The influence of the material properties on breakdown was investigated in ultra- fine grinding.

Section 1

Section 1 presents the results of the single particle impact experiments performed with the single jet apparatus. The influence of the size of particle is shown with glass beads and two polymers.

Section 2

Section 2 gives the results of the study of repeated impacts. Two approaches are used to compare the efficiency with a single impact.

Section 3

Section 3 studies the influence of the process of different hydrargillites on their behaviour at impact.

Morphological analysis tools are used in relation with the classical study (fragmentation profiles).

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.

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 have done so far and we are going to do in this program.

The objective of this work is systematic understanding of particle-particle nanorheology based on a single particle-particle contact: two atomically-smooth solid surfaces in molecularly-thin proximity (ultimately 1 nm or less) and the dynamic mechanical properties of the resulting inter-facial film studied directly as they depend on dynamic mechanical frequency and on strain rate.

This was the first year of a 3-year grant. Motivation is provided by the realization, upon surveying the literature, that progress in understanding fine powder applications is impeded by difficulties in separating the overall rheology of a macroscopic-sized sample into various mechanistic subprocesses. Much is known about inter-particle forces: van der Waals, electrostatic, hydrogen bonding, steric. The information obtained in rheology experiments has usually been interpreted in these terms. Yet these attempts have been largely unsuccessful, for systems of realistic makeup, owing to insufficient appreciation that the inter-facial rheology may dominate because it is the weakest link. This is a critical issue because particles are almost always lubricated by condensed films of moisture; films of condensed ambient gases are ubiquitous, whether intentionally or not.

A breakthrough was obtained as concerns rate criteria to observe stick-slip rather than smooth sliding. Complications in the traditional definitions of static and kinetic friction were analyzed as they pertain to lubricated sliding (for example, in the presence of condensed gases). We found that stick-slip motion occurred only when thin films were deformed faster than their intrinsic relaxation time. The observation offers a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined film.

The use of arrays of different types of sensors and the interpretation of signals derived from them provides a change in philosophy compared to traditional methods of particle characterisation and analysis. This change is necessary if measurements are to be made on line, in real process using sensor hardware that is inherently low cost, low maintenance and fault tolerant. This first report describes the objectives and deliverables of the overall programme of work, their significance, reviews the status and research development of new sensing methods that could be applied to flowing particulate systems.

Review (Part I)

In the review (Part I) attention is focused on concepts and technologies that might be used to enable measurement of the size and shape of particulates on-line in dilute and concentrated mixtures using multiple sensor arrays. The use of synergistic information from multiple sources in order to assist in the overall understanding of a phenomenon is deemed to be of considerable importance. However the application of such data fusion methods may rely upon the availability of a specific ‘model’ that describes the phenomena that are being investigated. Such a model is not always readily available, in which case more abstract methodologies based on multivariate statistical/chemometric analysis may be required.

The viability of using multiple sensors to gain more data (and hence more knowledge) about a particulate system is sensible, but the practical application is so system-specific that it is often not possible to make meaningful comments of a generalised nature. It is demonstrated that the two key factors include:

• translating the information that the user requires (or believes that they require) into a measurable parameter;
• defining the hierarchy of the measurements and/or analysis system so that it is sufficiently adaptable to tolerate loss of input from one or more sensor(s) or to allow new sensors inputs to be incorporated.

Some examples are given relating to the use of microelectrical resistance tomography applied to gas/paste mixtures.

Experimental Measurements (Part II)

Experimental measurements to establish the operational reliability of different sensing strategies to achieve the above goals are reported here (Part II). It is concluded that a system of continuous measurement using 4 multi-segment ultrasound sensors and an array of current/voltage electrodes can be utilised. For example, in the case of size measurement the electrical measurement system, current injection and voltage detection (from independent electrodes) will be made and interpreted using a hierarchy of models including combinations of direct parametric interpretation and particle timing information. The next stage is fabrication of such a sensor system. Useful contact has been made with several IFPRI members regarding the use of such a system.

It has hcen noticed that, the flows in risers of circula,ting fluidixed beds (CFI3s) is con- trolled by meso-scale structure of particles, that is clusters. The clusters largely changes fiow structure, consequently, they have considerable effect on the transport phenomena in risers of CFBs. Many experiments on the struct,ure of clusters have been performed, for example, Yerushalmi ct al. (1978), Rhodes ct, .al. (1992), Horio & Kuroki (1994), Hatano ct, al. (1994), zou et al. (1994), Li et al. (1995) and Tsukada ct al. (1997). Most of them intended to study the flows in the conditions of industriad applications, thus the conditions with respect, to the properties of gases aad solid particles are restrictfed to a small range. There are few experimental data on the structure of clusters in nonstandard conditions. For example, concerning the pressurised CFBs, it is difficult, to make observation. Therefore, it has been dcsircd to dcvclop a prediction method that is available for ra wide range of conditions.

Numerical simulation is a promising method to study these phenomena, because the presence of particles does not make the flow fields less acccssible, and it is easy to change parameters. Tsuo and Gidaspow (1990) calculated flow patterns in circulating fluid beds. They used a two-fluid model in which solid phase is rnodeled as a viscous fluid with constant effective viscosity, and predicted unstable flows with clusters. Our group proposed another simulation method for particle flows. We applied the Lagrangian approach coupled with direct simulation Monte-Carlo (DSMC) method, which was proposed for numerical simulations of rarcficd gas flows by Bird (1976), to the calculations of flow with clusters (Tanaka et al., 1995, 1996; Tsuji et al. 1997, 1998). The advantage of this method is that the properties of gas and particles can be easily taken into account. Tanaka, et, al. (1995) simulated two-dimensional flow under the same condition by Horio & Kuroki (1994) by using this method, and showed that the predicted structure and size of clusters are sirnilar to the experiments.

This project aims to produce numerical predictions that, are available to improve the understanding of the flows in CFB risers by using our method mentioned above. In this report, we present, a three-dimensional calculation that assumes the same conditions of gases and particles as Louge’s experiments (1997), in which riser flows of pressurised CFBs were studied by using high density gases. The effect, of the pressurised condition on the structure of clusters is discussed. Furthermore, correlations that appear in continuum rnodels are given.