Powder Flow

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 Theoretical and experimental studies have demonstrated that ac electric fields are effective in suppressing bubbles and promoting expansion of gas fluidized beds of fine powders (< 125 µm). In addition electric fields are effective in controlling elutriation of fines from the bed. A unified theory to explain bubble control and various bed phenomena was proposed and researched. Parametric studies of several variables affecting bubble control (temperature, particle size, particle material, fluidizing gas material, electric field strength, field frequency) were carried out over the last 5 years. A prototype 18 inch cold flow bed was built, tested, and instrumented during the 6th year.

Theoretical work included ac and dc interparticle force modeling, application of a perturbation theory for evaluating the bed elasticity modulus, and the utilization of a two-dimensional numerical code (K-Fix) incorporating electric fields to demonstrate real time bubble control. The use of correlations with electric fields represented an alternative approach to quantifying bubble control and bed expansion including the effects of bed voidage, superficial velocity, particle diameter, and electric field strength for glass spheres.

A total of five test facilities were constructed to verify bubble and elutriation control utilizing both rectangular and cylindrical beds. A radiant heater quartz bed with feedback temperature control was used to achieve temperatures up to 530 C for bubble control studies and to 500 C for elutriation control along with computer aided data acquisition. For our elutriation studies a unique electrode-from-below bed was designed to verify our theory of electric fields. A special sampling Faraday cage was designed to measure the charge of fines in the freeboard. A new real time laser monitoring system of particle concentration was developed allowing us to evaluate elutriation constants directly and with high accuracy. Previous researchers have emptied the bed and restarted experiments to achieve time dependent measurements.

For bubble control our studies confirm bed expansions up to 15% while maintaining control of bubbling up to 530 C for FCC Various equations have been developed such as the linear relationship between bed elasticity modulus “and electric field strength and an inverse relationship with field frequency. For elutriation control with fields, our experiments confirm a direct relationship with bubble control. Significant reductions in elutriation of fine particles up to 96 % were measured, The mechanism of retention of fines to parent particles in the bed with electric fields remains speculative.

Pneumatic conveying systems have been used by many industries to transport dry bulk particulate materials. The majority of installed systems are so called dilute phase systems where the particles that comprise the material are transported at low concentrations, suspended in the transport gas. Compared with other bulk handling systems they provide a simple means of enclosing the material and great flexibility in routing. The downside to this is that they are often less energy efficient and more hostile to the material being transported. The development of dense phase pneumatic conveying systems was aimed at reducing these disadvantages.

This review examines the state of the art of dense phase pneumatic conveying. The review is divided into five sections:

• Material capability for dense phase conveying.
• Conventional dense phase conveying systems.
• Special systems for low-velocity dense phase conveying.
• Modelling dense phase flow.
• Future research directions.

There has been a certain amount of confusion about the definition of dense phase conveying. The first section of the review examines the nature of dense phase flow, A gas-solids flow in a horizontal pipe is defined as being dense phase when the majority of particles are not suspended in the conveying gas. The possible modes of dense phase flow are identified, and the relationship between the shape of the operating envelope of the pneumatic conveying system and these modes of flow is presented. From this three classes of bulk material can be identified:

• Those only capable of dilute phase flow.
• Those capable of a moving-bed mode of dense phase flow.
• Those capable of a wave-like mode of dense phase flow.

Measurement of the gas diffusion properties (permeability and de-aeration) and cohesiveness of a bulk material provides a means of classifying materials according to the type of dense phase flow that they can achieve. This classification provides a means of assessing the suitability of the material for dense phase transport and the likely benefits in terms of system performance.

If a bulk material is naturally capable of dense phase flow then the selection of system components is similar to that for a dilute phase system. The second section of the review examines those systems that have most commonly employed for naturally dense phase capable bulk materials. Systems using pressure vessels as the solids feed device, blow tank systems, are probably the most common and feature throughout this section. Another important class of system employ so called ‘high pressure’ rotary valves. These permit a V IFPRI Report On Dense Phase Pneumatic Conveying Glasgow Caledonian University continuous solids feed while minimising the air leakage suffered by all rotary valves.

Although many of the feeders employed in these dense phase systems are capable of operating in systems with high pipeline pressure drops, the equation dense phase = high pressure is not always true.

Many users of dilute phase pneumatic conveying systems wish to obtain the benefits of conveying in a dense phase mode, but have materials that are not naturally capable of any mode of dense phase flow. The third section reviews the types of novel systems that have been developed to meet the demands of these applications. These systems may be classified as either:

• special feeders;
• special pipelines - air bypass and air injection system.

Special feeders attempt to introduce the solids into the pipeline in such a manner as to promote &nse phase flow. Special pipelines aim to maintain the correct conditions in the pipeline in order to sustain dense phase flow. From the previous discussion it is clear that the special pipeline systems have the best chance of achieving dense phase flow with a bulk material that is not naturally capable of such flow. When a bulk material is transported at a velocity below its natural minimum it will form a blockage. Most special pipelines permit operation below the natural minimum velocity by providing an automatic means of clearing pipeline blockages.

Air injection systems achieve this by injecting air into the blockage to clear the problem. Air bypass systems achieve this by providing an alternate route for the air flow. This directs air to the downstream end of the blockage to reduce the length of the blockage and hence aid its removal. Air bypass systems have the advantage of being less complex in construction, and do not change the total mass flow rate of gas in the pipeline.

The forth section reviews the types of model that have been developed to describe gas-solids flow in pipes and assesses them in terms of their suitability for predicting kzse phase flows. The aim of any model is to describe the behaviour of a physical process, in this case the model must be able to predict:

• Straight Pipe: A FALL in the pressure drop for a constant solids flow rate as the gas flow rate is increased over the range of velocities normally employed (2-30m/ s)
• Inclined Straigh Pipe: For constant gas and solids flow rates, a maximum pressure drop which occurs at an angle less than 90” (vertically up).
• Bends: A RISE in the pressure drop for a constant solids flow rate as the gas flow rate is increased over the range of velocities normally employed (2-30m/ s)

The modelling techniques developed may be classified as either:

• Inside describe the physical processes that occur inside the Pipe
• Outside describe the behaviour of system components, usually derived from experimental correlations

Outside models have the advantage of simplicity, but are dependent upon experimentally determined parameters. The quality of the experimental data limits the application of these models in terms of scale-up and the range of bulk materials that can be modelled.

The development of inside models provides a means of understanding the physical phenomena that occur during transport. The disadvantage of this type of model is the computational effort required to solve an industrial scale problem.

The final section suggests some areas for further investigation. The key to a successful outcome of this work is the strength of the links between the various strands of the programme (experimental - measurement - theoretical).

David J. Mason

Mixing of particulate materials is widely encountered in the process and related industries. So is segregation. They are competing processes; the one a deliberate act intended to increase the ho- mogeneity of a mixture of different components or sizes, and the other an involuntary process, occurring as a result of the fact that the various forces that may act on the individual components of a particulate mixture may cause them to move in different directions, or to different positions in a bulk, due to their different characteristics. Generally, the most important of these is particle size, although density, shape, surface roughness, resilience, electrostatic properties etc. all can play a role, as can process conditions.

The mechanisms that are mobilised in order to effect mixing are basically three: diffusion (or preferably dispersion), convection and shear. Diffusion with particulate materials only occurs over very short distances and, in order to mobilise the mechanism, the different components need to be brought into the vicinity of one another. This is generally accomplished by convection or shear, the latter being considered by many to be an idealised form of convection. Modern processes of mixing such as hybridisation using mechanical forces etc. are not covered in this review.

Twelve mechanisms have been identified which lead to segregation. Some of these can act within the mixer itself, while others are only encountered in subsequent handling operations. Wherever they are encountered, they reduce the homogeneity of mixtures. Since the three mix- ing mechanisms and several segregation mechanisms can act simultaneously, doubt has been expressed whether a mixing process can be modelled in a deterministic manner. The weight of opinion appears to be that heuristic experimentation must be accompanied by the development of algorithms to create an expert system in order to improve on the state of the art. Suggestions have been made that chaos theory may be applicable. Chemometrics may be another option.

The review includes a description of the classical approaches to characterising mixture quality and concludes that these have inadequate relevance to modern day needs. For the continuous monitoring of mixers and mixtures it is suggested that both the intensity of segregation (as indi- cated by the variance) AND the scale of segregation (as indicated by an autocorrelogram) be used. Such an approach would allow the continuous monitoring of mixing processes. Whatever method is chosen to monitor or evaluate the quality of a mixture, the sample size is critical. It will vary with the purpose for which the mixture is being created, and the demands made a on a mixer will very much depend on the homogeneity it can achieve in a sample of the chosen size. Optical probes offer the possibility of continuously monitoring mixtures, as do combinations of acoustic probes and chemometrics, but these may not be sufficient to determine the quality of a mixture at the microscope level.

The review concludes with a description of models proposed to describe mixing and segregation processes.

Research plans in these two areas at Telemark College and Telemark Technological R & D Centre are described in an appendix.

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.

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.

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.

This final report describes the method and results of discrete particle simulation which we have been developing under the support of IFPRI from 1994 to 1997. Calculations have been performed for dispersed gas-solid flows where particle concentrations are so high that particle-to-particle collision is essential. The method is based on calculation of trajectories of individual particles. In this report the method is described first, and then some results of a preliminary calculation are shown. Main calculations made in three years are classified into three cases.

• Calculations of the first case were made for comparison of our method with those by conventional numerical analysis based on a continuum model.
• The second case was made to investigate structure of gas-solid flows in detail.
• The third case concerns turbulence modification due to particles.

The most important aspect of our work is that the method uses very simple equations of motion and yet it succeeds in producing complicated phenomena in gas-solid flows. It is shown that particle clusters in a riser can be explained by repeated in-elastic collision. Since cluster generation was predicted by another numerical method by another group, their method is based on the model which regards the mixture of gas-solid as being consisted of two fluids. To compare both our method and their method, we performed the discrete particle simulation under the same conditions as their work. In these calculations we clarified similarity and dissimilarity of results between both methods. It is shown in the second case that the existence of such clusters causes large scale turbulent flow. In the third case, we extended our numerical analysis to analysis based on large eddy simulation.

In the first and second cases, the equations of gas motion based on inviscid gas, and thus fluid phase does not have any turbulence in the absence of particles. Thus, as long as we use the method based on inviscid gas, we are not able to predict turbulence phenomena near the wall. We used the large eddy simulation technique to consider the turbulence of gas phase. The empirical constant used in the present LES is the same as has been established in single phase flows. Like the first and second cases, we took into account the particle-particle collision. We found that the effects of particle-particle collision on velocity fluctuation and concentration of particles are significant even in dilute phase flows (volume fraction is the order of O(10^-1)). Most researchers consider that the particle-particle collision can be neglected at such a low concentration. Turbulence suppression due to particles was also observed in the calculation as in experiments.

Finally in this report the future work which should be made as an extension of the present work is briefly described. First, the importance of the work combining the discrete particle model and the continuum model is described. Second, the effects of inter-particle collision on flow structure should be made in more detail.

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.

The aims of this Lancaster University - Bradford University collaborative project arc 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.

The purpose of the force curve measurements (direct measurements of force as a function of separation) is to charactcrisc the interaction between pairs of individual particles, and between single particles and a flat “wall” surface. The details of particle shape and Gne-scale morphology are revealed by scanning force microscopy (“SFM”, available within the same experimental setup as is used for the force curve measurements). In the previous year’s report we described the results obtained up to 14 November 1997, including the design and construction of a system that allows us to control the relative humidity in a glove box within which the force curves are measured. WC also described how the work at Bradford enabled us to develop improvements to the Warren Spring - Bradford cohesion tester (WSBC) and to standardise the method.