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The Character is Key- Challenges for Bulk Solids Technology

| By Chemical Engineering

  • Characterization of bulk solids facilitates smooth production flows
  • Nanoparticles pose a special challenge
  • Bulk solids expertise needed for urban mining

Roughly 50% or more of substances used in the chemical industry are solids which need to be processed, handled and transported, so it is hardly surprising that exhibitors and visitors at ACHEMA have an interest in bulk solids technology. A learning curve is associated with the handling of powder and granulate.

Solid processing is relevant for the chemical, pharmaceutical and the food industry – and many more [Picture:]

Bulk solids are found in all sectors of industry including food, cosmetics, chemicals, automobile production, biomass processing and waste management. Particles with special property profiles are often the secret to success in new product development. Composite materials, fuel cells and catalysts are a few examples. In addition, special surface properties, particle size and particle distribution can be manipulated to create new active ingredients for cosmetics and pharmaceuticals.

The widespread use of pods and capsules for household coffee makers would be inconceivable without particle technology expertise. The same is true of powdered soup and prepared mixes. The trick is to prevent the ingredients from separating after packaging.  The dried product has a longer shelf life and is easier to transport than canned soup.

The basic principles behind many bulk solids processing techniques including screening, mixing and comminution have not changed for the past hundred years. That does not mean, however, that no further innovation is possible. Mechanical processing on display in Halls 5 and 6 at ACHEMA is currently undergoing a renaissance. Bulk solid property profiles are becoming increasingly complex and the quality standards are more and more demanding.  Particle size, for example, continues to decrease. 20 years ago the micrometer range tended to be the norm but nanotechnology has now become mainstream. This has created the need for finer distributions.

Bulk solids – a challenge for equipment operators

The properties of fluids are now well understood, but surprising things can happen during handling of powder, dust and granulate. This is something which equipment and plant operators are well aware of. What makes bulk solids handling so difficult? One bulk solid can be easily filled, but bridging can be a problem with a second product and a third substance rushes through the pipes like a liquid. There are products that constantly exhibit different behavior in identical filling trials or, even worse, stagnant zones form in the fourth trial (but not before), negating all of the previous results. Upstream handling of bulk solids can also have an influence. Particle compaction during conveying and filling is a factor that may need to be considered.

The ideal particle rarely exists. In most cases, minor variations in size and surface characteristics make it difficult to accurately predict filling and dosing behavior. Knowledge about properties such as bulk solid density, particle size, particle shape, moisture, etc. makes things easier. Precise characterization of the flow properties is also important. Getting that wrong can lead to disruption in the process flow. Most equipment manufacturers have a test and development center where they can run trials and investigations. The engineering teams identify industry-specific characteristics. Gentle handling is very important in the plastics industry to avoid problems like angel hair (hair-like strands caused by friction during material handling). Explosion protection is often mandatory at chemical plants. Hygienic material handling is the top priority in the food and pharmaceutical industries.

Nanoparticles: a lot to learn?

Nanoparticles create special challenges for equipment operators and bulk solids experts. These products are already being used on an industrial basis in cosmetics and detergents. High hopes are placed on them in the medical sector for drug delivery and even to release drugs only when they reach cancer cells in order to avoid therapeutic side effects.

The size, shape and surface properties of nanoparticles have a direct influence on product characteristics in items such as semiconductors and solar cells. Classification of particles < 20 nm is difficult using current techniques. Electrostatic forces and molecular interaction are the predominant factors rather than mass or density. A lot of research is still needed on nanoparticle characterization, properties, processing and classification. Scientists borrow knowledge from other disciplines which already have in-depth experience in handling large molecules and small particles. Chromatographic techniques used in protein purification are being applied to nanoparticles with promising results.

New optical techniques or combinations of different measurement methods make it possible to measure particle size and morphology online during ongoing production.

A window on the process

A few years ago, the general consensus was that bulk solids are unpredictable, but in the meantime this attitude has changed. With the increased computing power available today, simulation using techniques such as finite element analysis is an effective process optimization tool.

The advantage of methodologies like CFD (computational fluid dynamics) based simulation is that a good model enables engineering teams to safely investigate many process details and parameters. Simulation provides important information about local conditions in many different parts of the equipment. Filling operations in a silo at high and low load levels or flow conditions in a mixer can be optimized using the model. The list of parameters can include the location of the filling point, the size of the filter, the bulk solids involved and their temperature. Good solutions can also be developed for complex geometries and mounting configurations. This can reduce test and development costs.

Particle analysis also provides a detailed understanding of the process. Visualization has enormous advantages. What takes place at the interfaces and how that affects the process are now understood. As a result, properties such as the size, crystal morphology and shape of the particles can be controlled with greater precision. This is important for the development of personalized medicine where the active ingredients are tailored to the individual patient.

Understanding of bulk solids behavior increases equipment service life

At ACHEMA, the latest in bulk handling is presented in Hall 3

In bulk solids handling, the objective is often to maintain the material flow. The choice of the right gate, a change in the direction of flow from horizontal to vertical and detailed silo design keep the bulk solids moving. A lot can be accomplished by modification of the particle size distribution or the formation of agglomerate or micro-granulate. This includes the removal of fines, changing the comminution process or coating the surface with very fine-grained flow promoters.

A proper understanding of the product prevents early wear on the equipment. Wear normally depends more on system design than on the bulk solids. Design changes may not eliminate wear but they can reduce the deterioration rate. Changing the angle or speed of impact can increase equipment service life over the long term.

Equipment manufacturers need in-depth expertise

Requirements in the pharmaceutical industry have become increasingly demanding over the past 15 years. Dust-free transfer, for example from a container to a mixer, is a major priority. That is important in order to avoid cross-contamination, and factors such as increased handling of high-potency active ingredients have major employee safety implications. The situation is similar in the food and chemical industries. It is important that the environment is dust free to prevent cross-contamination with allergens and avoid human safety hazards and dangerous situations caused by airborne dust.

The proliferation of container types is a major headache for design teams. Raw bulk solids, intermediates and finished products are handled and transported in big bags, octabins, barrels, sacks, mobile containers and much more. The varying heights of feed and discharge connections are another problem. Equipment manufacturers may have to make special modifications or design changes, which can be expensive. The answer is to design versatility into the systems.

Precision, uniform dosing is often a built-in feature of transfer stations and that functionality is essential for high-quality production of many products. Selection of the right dosing method has to be a joint decision by users and manufacturers, and the choice becomes more difficult as the volumes being handled decrease. Minute amounts of vitamins have to be added with very high precision in the food industry. Hygiene requirements or major space constraints which often exist in the plastics industry are other factors that can make dosing more difficult.

Safety is the top priority

Explosion protection is an issue which most bulk solids technologies must address. In Germany alone, it is estimated that one dust explosion occurs every day. The outcome of these events may not always be tragic but they clearly show the need to take the issue seriously. The lower explosion limit (LEL) for many types of dust is 20 – 125 g/m³. The bulk density of the types of dust normally found in industrial environments is around 800 kg/m³. Given a dust layer of 1 mm and a room height of 3 meters, this would equate to 270 g/m³. Any air current which kicks up the dust would be enough to create a potentially explosive dust/air mixture. Effective explosion protection is mandatory in many applications.

The primary approach to explosion protection is to minimize or eliminate one or more of the elements (ignition energy, oxygen and dust) in the explosion triangle. It may be possible to find a non-combustible substitute for a combustible material. Nitrogen inertization can reduce the oxygen concentration. It is also important to eliminate ignition sources such as welding operations, hot surfaces, electromagnetic waves and hot gas. Once that has been done, the developers can look at ways of mitigating the effects of an explosion. One possibility is to design equipment which has explosion pressure relief, explosion suppression and limited explosion volume so that it is able to withstand the explosion pressure.

Organizational measures are also part of an effective explosion protection strategy. This can include simple rules such as a smoking ban, regular cleaning to remove dust, periodic grounding checks and confirmation that machines are still being used for the intended purpose. It is also important to record all of these measures in an explosion protection document. A documented explosion protection strategy provides sufficient evidence for the authorities and ensures that the explosion protection program is effective. User/supplier collaboration extends well beyond the supply of individual components. Delivery is preceded by the provision of detailed information on matters such as changes to applicable laws and regulations. In recent years, experience has shown that strict separation of fire protection and explosion protection during the storage of bulk solids is not expedient. For one thing, it is not always possible to determine whether it was the fire or the explosion that triggered an event. Moreover, the two types of protection have been shown to complement each other.

Urban Mining

While minute quantities are commonly handled in the pharmaceutical and cosmetics industries, material is handled on a totally different scale in metal recycling, at least at the start of the processing chain. The term urban mining refers to the recovery of valuable materials from cars, electrical appliances or even old waste disposal sites. One tonne of scrap PC motherboards contain 30 times more gold than one tonne of gold ore. 250,000 tonnes of rare earth metals are thought to be present as slag at German waste disposal sites. The recovery of re-usable material is an elaborate process and it is very energy-intensive. If the motherboard is not removed before the PC is put through the shredder, it is difficult or impossible to get at the important trace elements. 75% of the gold would be lost. Special comminution and sorting processes are needed to improve the situation.

The process sequence starts with mechanical processing based on conventional comminution and classification techniques which have a crucial impact on the subsequent process steps and on the feasibility of urban mining. A lot of research and development work lies ahead. One of the items that need to be addressed is the processing of finely dispersed polymetallic particle systems < 10 µm. Work remains to be done on sorting techniques for ultrafine particle systems with very fine grains (< 50 µm, mainly in the 0.1 µm – 10 µm range) and on energy-efficient comminution.

However even if a cost-effective process can be found, it is by no means certain that there will be a payback over the long term. Fluctuations in income generated from recycled materials tend to be in the double-digit range. A system which is currently generating a good return could quickly become a financial liability. In addition, a constant stream of new materials and composites makes recovery increasingly difficult. The systems of the future must have sufficient versatility to handle material streams which have varying composition. It is also important to look over the edge of the plate. Recycling always involves a long process chain which extends from consumers to recycling and metallurgy companies. The economic feasibility of the entire process depends on each individual link in the chain.

ACHEMA presents solutions

In order to design bulk solids handling systems and equipment which are not susceptible to material flow interruptions and production stoppages, developers need to have in-depth expertise and detailed knowledge of the material properties. Very small particles down to the nano-scale and very large volumes that need to be handled in urban mining call for new approaches for engineers and bulk solid handlers.  The ACHEMA exhibition and congress will present solutions for both.

ACHEMA is the world forum for chemical engineering, process engineering and biotechnology. Every three years the world’s major fair for the process industry attracts around 4,000 exhibitors from over 50 different countries to present new products, processes and services to 170,000 professionals from all over the world. The spectrum ranges from laboratory equipment, pumps and analytical devices to packaging machinery, boilers and stirrers through to safety technology, materials and software, thus covering the entire needs of the chemical, pharmaceutical and food production industries. The accompanying congress, featuring 800 scientific lectures and numerous guest and partner events, complements the wide range of themes of the exhibition. The next ACHEMA will take place from 15-19 June 2015 in Frankfurt am Main. More at