SAR - Review

Abstract

Coating, or layering, is the process of applying a secondary layer of solid to a solid host substrate, for instance a core or carrier particle. The process is applied in many industries, like food and feed, pharmaceuticals, fuels or fertilisers, to design the functionality of the coated material. This report gives an overview of process technologies and their capabilities for the coating of core particles smaller than 200 µm in characteristic size.

Note to reader

Milling is an important technology which has a wide range of potential applications in many different fields. It can be applied in different ways to any type of compound. Its effects range from obtaining nano-particles to chemical synthesis, from the increase in reactivity to the forced formation of alloys.

In all cases, changes in the physical state, in the form of polymorphic modification between crystalline varieties, amorphization etc. may occur during milling. The accidental formation of these new states can dramatically affect the stability and expected performance of a product. On the contrary, milling can be used to induce voluntarily changes of state, which makes it possible to produce new solid forms, with interesting new capacities (for example, in pharmacy, to obtain an amorphization, which allows to improve dissolution properties of poorly soluble compounds). Milling is then a "green" route of synthesis that avoids the use of a solvent or a chemically destructive heating. The amorphization induced during co-milling can be a desirable intermediate of a chemical synthesis. These physical transformations occur in conditions that are far from equilibrium. Their fundamental understanding is a challenge for materials physics. At the present time, there is no universal theoretical framework for describing and predicting transformations induced by milling. Even a simple definition of the relevant control parameters is an open problem.

In this paper, I will consider essentially the issue of changes in physical states and will only briefly mention the other aspects: chemical synthesis, forced formation of alloys or co-crystals. Similarly I will not go into the details of the multiple technical aspects associated with the performance of the mills.

After some general considerations, I will give a rapid description of the classical thermodynamic conditions for obtaining changes in physical states (polymorphisms, phase transitions, amorphization and glass transitions). This is useful for making a comparison between the thermodynamically induced and the mechanically forced changes. I will then briefly introduce the main experimental techniques that are useful to identify and quantify the changes. The numerous examples described later will give an idea of the relevance of these different techniques, according to the types of compounds studied. I will then present typical examples of transformations for different classes of compounds (elemental compounds, minerals and oxides, metal compounds, molecular and macromolecular compounds). I will dwell more on the behavior of these organic compounds, which have an important practical interest in the fields of food, pharmacy, pigments, energetic materials and so on. My area of expertise essentially concerns these compounds. They have a high sensitivity to milling and temperature changes. They present a very rich polymorphism and are good glass formers with a glass transition that is well marked and close to room temperature. It is a favorable situation to investigate in detail the effect of changing milling conditions on the nature of the end products.

I will conclude with a presentation and discussion of the many theories which compete to describe the physical transformations under milling. They are usually based on thermal equilibrium considerations. However there are many shortcomings in these approaches. I will show how more recent non equilibrium approaches could provide a more universal framework of description.

A review of the literature on the atomization of high viscosity fluids is provided. The review starts with a brief presentation of various atomization models, which are used to predict the spray droplet sizes. Next, atomization results using different types of atomizers are reviewed. These include plain orifice, splash plate, swirl, and twin-fluid atomizers.

Generally, most studies show that the Sauter Mean Diameter of a spray (SMD) increases with viscosity. This increase can be compensated by increasing the energy input to the atomizer (such as increasing the atomizing gas pressure or velocity). However, even in the cases that SMD does not change significantly, the size distribution changes with viscosity. In sprays of high viscosity liquids, larger droplets take up a larger proportion of the spray size distribution, resulting in the deterioration of the spray quality. The primary atomization of high viscosity liquids generates longer liquid ligaments and filaments, which have to be atomized during a secondary atomization process. This later process is intimately related to the coflowing gases. The secondary atomization of the long ligaments can be expedited by having high velocity coflowing atomizing gas. The review ends with a case study for the prediction of the droplet size distribution using a detailed numerical modeling that considers the exact design of the atomizer.

Abstract

Bulk dry powder is commonly involved in industry. While it is necessary to have in-line sensors for real-time measurement and analysis of bulk dry powder, very few such sensors are commercially available for industrial application, or likely to see application in the near future. This article attempts to provide a comprehensive critical review of in-line sensor technologies for real-time measurement and characterisation of bulk dry powder properties, including application of sensors and associated analytical methods for measurement of such properties as particle size and particle size distribution, morphology, density, blend composition, moisture content, spatial moisture distribution, and for flow rate measurement. To measure a powder flow rate, concentration, concentration profile, velocity and velocity profile must be measured because in almost all cases, it involves in gas/powder multiphase flow measurement. The review of existing sensors is organised in terms of sensing principles or sensing technologies, such as optical (including Laser), capacitance, electrostatic inductance, electrodynamic, triboelectric, microwave, electromagnetic (EM), Coriolis, impact plate and acoustic, with analysis of the economics of implementing the technologies. Electrical capacitance tomography (ECT) is an emerging technology, which is particularly suitable for in-line real-time measurement of bulk dry powder flows and analysis of bulk dry powder properties. This article also reviews ECT technology, in particular the AC-based ECT system, and its applications dealing with bulk dry powder. A particular case study is the use of ECT for investigation of circulating fluidised beds for coal combustion and gasification, which demonstrated the advantages of ECT over conventional sensors for this purpose.

As a summary can be stated, that deliquoring of filter cakes is an complex issue. At

first the knowledge about the physical fundamentals of cake deliquoring is necessary

but in addition many random conditions have to be considered to find an economical,

effective and sustainable solution for the separation problem. This means, that not

only the whole process chain from slurry production to the final cake removal has to

be taken into account, but in addition one has to check which apparatus fulfills all

preconditions optimally. Recent developments for several cake filter apparatuses

have improved the state of the art remarkably especially with focus on the cake

deliquoring. Further ideas, which are investigated in recent and actual research

projects could lead to further progress. Theory provides many approaches from

relatively simple to highly sophisticated concepts for the explanation of the relevant

filter cake deliquoring phenomena. In this review the focus is not on an overview of

all these concepts but practically proved models will be taken to illustrate the

correlation of the relevant influencing parameters.

Mechanical size reduction methods have dominated the scene in nearly all industries involved with comminution since their beginning. However, the various challenges and limitations of mechanical methods have maintained widely open the opportunities for non-mechanical milling methods. This review analyzes critically several different approaches, including thermal shock, microwaves, lasers, pressure variation, high voltage pulses and ultrasound. While interest in some of them, namely thermal shock and pressure variation, has nearly vanished, others are intensively studied at present and seem to be on the brink of becoming industrially-available technologies. Examples of the later are microwaves and high voltage pulses. The large variety of materials and applications that involve size reduction and the nearly universal reliance on mechanical methods suggests that non-mechanical methods have found niche applications and that type of application with continue to grow. Examples of these are lithotripsy and laser ablation, which already find applications in narrow, but important fields.

New Classification Of Drying Processes Based Upon the Option Used for the Protection of Heat Sensitive Ingredients:

• A Low temperature
• B Short drying time (by high drying rate)
• C Even distribution of temperature in the particles
• D Equal residence time for all particles, if they are of equal size
• E Residence time adapted to the size of the particles, if they are of unequal size
• F Adapted dryer atmosphere, especially absence of oxygen

This review provides an overview of pore characterization and pore scale modeling techniques as applied in the earth sciences, and is intended to spark a cross-disciplinary discussion with fine particle and powder scientists and engineers in IFPRI. We think many of the problems faced in these disciplines are similar, and believe some of the new methods which have surfaced over the last few years in our quickly evolving field may prove useful for IFPRI members. At the same time, we also hope to learn from you, and we welcome you to contact us with comments, questions or ideas. Many of the techniques discussed in this review are rooted in the study of transport and degradation processes which occur in natural rock, topics with a wide variety of real-life consequences both in underground geological reservoirs and above ground (e.g. oil and gas production, environmental remediation of aquifers, building stone deterioration, carbon capture and storage). Throughout this work, we will also highlight some of the remaining challenges we face, together with possible solutions we see emerging.

The review focusses on progress in understanding, measuring and modelling the structures formed when single droplets dry. In the initial period of drying concentration gradients are formed, these establish the basis for subsequent transformation to the solid particle structure. The Péclet number is becoming established as a useful metric for evaluating the magnitude of these concentration gradients and therefore interpreting trends seen in the final particle structure. The structures formed after drying depend on the nature of the phase transition of the material being dried.

Three broad classes are discussed:

• colloidal suspensions
• crystallising materials
• skin forming materials

Recently significant progress has been made in the understanding of colloidal suspensions. Quantitative analysis and careful experimentation in the field has helped quantify and predict effects in model systems. These give insights that help the particle engineer design particles with the desired structure. That said there are still many open questions for this class of material, and micro-scale-modelling techniques are available and should be used to help address these questions.

Crystallisation and film forming systems have seen less progress. Some work has been done looking at alternative crystallisation models, which give some interesting insights into amorphous-crystalline transitions; however there is a lot of opportunity to extend these further. Film forming systems have received little fundamental activity in recent years, however researchers are beginning to characterise material properties and link these to behaviour to explain observed phenomena.

The whole area of materials characterisation in the non-equilibrium conditions seen during drying is a priority area for all material types, as the value of current modelling capability is limited by lack of reliable physico-chemical properties. The aerosol community has been active in developing new methods to address this need and their application should be extended to materials of industrial interest. Some of the recent developments in measurement techniques for free droplets has also been pushed by the aerosol community, beyond these methods there is still a need to extend and develop the state of the art in the area for better measurements of internal composition profiles. Drying of other particle systems is also discussed and key differences in the mechanisms driving the structure of dried thin films and sessile droplets are highlighted.

Particle Scale Simulation of Industrial Particle Flows

Particle scale simulation of industrial particle flows using DEM (Discrete Element Method) offers the opportunity for better understanding of the flow dynamics leading to improvements in equipment design and operation. These can potentially lead to large increases in equipment and process efficiency, throughput and/or product quality. Industrial applications can be characterized as large, involving complex particulate behaviour in typically complex geometries.

Importance of Particle Shape

The critical importance of particle shape on the behaviour of granular systems is demonstrated. Shape needs to be adequately represented in order to obtain quantitative predictive accuracy for these systems.

Exploration of Industrial Applications

We explore the breadth of industrial applications that are now possible with a series of case studies. The inclusion of cohesion, coupling to other physics such fluids, and its use in bubbly and reacting flow are becoming increasingly viable.

Challenges in Model Development

Challenges remain in developing models that balance the depth of the physics required with the computational expense that is affordable, and in the development of measurement and characterization processes to provide the expanding array of input data required.

Advancements in Computer Power

Steadily increasing computer power has seen model sizes grow from thousands of particles to many millions over the last decade which steadily increases the range of applications that can be modelled and the complexity of the physics that can be well represented.