Bulk Solids on Their Way Through the Process Handling Bulk Solids? Always Keep Moving!

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The exciting thing about bulk solids remains their unpredictability — despite many theoretical models, sometimes it is only practice that decides whether you can make a product work well (or rather, get it moving).

In the process industry, 60 to 65 percent of all substances are said to be solids. These have to be moved in the best possible way from the silo vehicle, out of the silo, through the most diverse plant components to their final packaging. However, every industry understands something different by “best possible”: while the plastics industry attaches a lot of importance to gentle conveying, explosion protection is often mandatory at chemical sites, building materials place high demands on robustness, and in the food industry hygienic handling is the top priority. And anyway: what about energy efficiency?

Regardless of the industry: when it comes to internal material transport, the most important thing is a coherent overall concept. And this includes all the players. It covers processes used to refine a product, such as drying, crushing or mixing, the conveyor line, and storage. In the case of the latter, the situation is aggravated by the fact that sometimes a silo plays no role at all in the planning of a plant, as it contributes little to the actual value creation. A stagnant silo discharge can slow down the entire process. It is also often neglected that every bulk material has a history, and can change in state from static (e. g. in a silo or big bag) or dynamic (e. g. during filling).

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Given the abundance of these factors, it is therefore not easy to establish a clear relationship between certain parameters and the problems that occur in production when there are problems in the material flow.

In a functioning overall process, therefore, all influences — from the periphery, from the product infeed, along the conveying path to the discharge point — must be taken into account. Andreas Eibner from Azo also refers to the climatic, environmental and system conditions of the plant, which must also be known and evaluated when selecting the technical plant components and systems. “In this way, availability and performance in plants for raw material automation and logistics can be increased and discharge problems and losses in e. g. conveying or dosing capacities can be avoided,” Eibner says.

If the Material Handling is Right, the Process is Also Right

However, the reality is sometimes different. Sacks, big bags or other containers may not be completely emptied, the transition from the silo into the conveying line or into the silo vehicle may stall, or the products to be conveyed may even be destroyed. The consequences are sometimes fatal, as Hans Schneider, Vice President Technology and Innovation at Zeppelin Systems, confirms: “If there are leakage problems in the silo, blockages in the conveying line or malfunctions in the separator filter, the entire process comes to a standstill after a short time. So if the material handling is right, the quality, safety and productivity of the entire plant are also right.” For Schneider, the safe and preferably undisturbed handling of bulk materials is therefore a basic prerequisite for a stable production process and the key to a smooth internal material flow.

Nothing in Solids handling Works Without Mass Flow

When emptying from the silo or even from other containers, difficult-flowing, bridging, cohesive, fibrous or block-forming bulk materials are usually among the problematic raw materials.

It becomes even more difficult when the raw material or product has to remain within specifications. Schneider cites examples: “In the case of sensitive products, conveying over long distances can destroy the product, resulting in a reduction in quality. Pre-mixed semi-finished products segregate if conveyed improperly; here, too, quality reductions in the end product are the result.”

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At least with regard to the silo, visitors to our Bulk Solids Forum have long since known a solution to help solve emptying problems: Mass flow instead of core flow. If there is core flow, the product is always discharged from the centre above the outlet, but remains static at the edge, and dead zones form there. The consequences can be manifold: The product at the edge can solidify in such a way that in the end it can only be removed by digging. Segregation may occur, or the product may practically shoot through the flow zone into the outlet.

Incidentally, it is not just problematic from the point of view of material flow if a product stops flowing in the silo. Uneven emptying can lead not only to the formation of shafts, but also to dents in the silo. On occasion it can even endanger the structure of the silo.

Operators are therefore required to always ensure that there is mass flow in the silo. This is achieved by, on the one hand, knowing the bulk material properties (including bulk density and bulk strength) and, on the other hand, keeping an eye on the discharge device, the discharge size, the wall material and the inclination of the hopper. “By injecting air, for example, we can achieve improved flow behaviour,” says Schneider. “Alternatively, we use mechanical discharge aids such as vibrating floors, agitators, or fulling devices to maintain the product flow.”

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But a product in a silo can also be set in motion in other ways. A changed distribution of particle sizes or the creation of agglomerates or microgranulates can have a positive effect. The removal of fines, a different comminution process or coating the surfaces with very fine-grained flow aids (talcum) are also part of the repertoire of the bulk solids experts.

Keep Powder Flowing? Test, Test, and Test Again!

Speaking of experts: Many problems that occur when using powders in industrial processes can be traced back to a lack of knowledge about the behaviour of powdery bulk materials. The flow behaviour, for example, is primarily dependent on parameters specific to the powder — such as particle size, shape of the particles, porosity, density, roughness and type of material. “For a functional plant design, it is essential to know the raw material properties of the bulk materials used in order to be able to comprehensibly design the discharge and dosing behaviour, among other things,” confirms Eibner.

The Azo Group is one of the most experienced companies worldwide when it comes to automatic raw material handling. Raw material automation and logistics require solid knowledge of the properties of the products to be processed, plus a lot of experience in data evaluation and linking with regard to the design of the systems. Unknown material properties, procedures or processes are determined in a well-equipped raw material handling test centre and a professional bulk materials technology laboratory.

Do You Protect the Product or the Plant? Best to Do Both!

Pneumatic conveying is the best way to transport a product from A to B within a plant. This has a number of advantages, such as the use of closed pipes, small space requirements, flexibility during installation, and the fact that various receiving points such as silos and mixers can be reached from one starting point via pipe switches. Other advantages are continuity, low maintenance requirements and high adaptability. However, this is not possible without knowledge of the product properties.

For the Dutch company Dinnissen, this means that all products are first tested and checked in an in-house technical centre for their flowability, among other things. Based on these results, the type of materials, the surface finish and the diameter are chosen. “In the case of pipelines in pneumatic conveyors, for instance, deposits can form at non-smooth transitions, which can build up over a long distance until the pipeline is completely blocked,” explains Rieks Reyers, Sales Manager at Dinnissen Process Technology.

The relatively high energy consumption and the threat of wear and tear on walls and goods are also among the disadvantages. At least the former doesn’t have to be the case, however, according to Reyers: “Even older conveyor systems can be optimized at low cost, for example, by combining a variable-frequency fan drive with pressure monitoring.” However, abrasive bulk materials are not always to blame for wear. For example, faults in a pneumatic conveying line can cause the jet to be deflected and increase wear there. In this case, it’s more a case of turning the system around.

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But what can be done if the product or the system suffers? “When designing the pneumatic conveying system for these products, we make sure not to convey at too high speeds,” Schneider mentions one possibility. “In addition, friction on the pipe wall causes product abrasion and heat. This also has a negative impact on product quality. That’s why it’s important to design the system correctly and regulate the delivery gas volume flow and material feed.” In the ideal case, the plant design is perfectly matched to the bulk material or the product, and the quantity and quality of the end results are regularly put to the test. But what happens when the parameters change?

Retrofitting in Case of Changes in Raw Materials, Quantities or Requirements

If undesirable changes occur here or other adjustments become necessary, Zeppelin first refers back to the precise analysis of the bulk material. “From this, initial measures for material handling can be derived to ensure an optimal process,” says Schneider. Here, too, pilot plant trials are recommended in order to test and analyse the new bulk material, changed quantities or recipes under real conditions in different scenarios. “From these findings, we determine retrofit packages for upgrading the plant and install them to fit exactly. These can be adjustments to the silo outlet, changes to the cone, additional discharge aids or reparameterization of the pneumatic conveying system.”

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Incidentally, it is not only the hardware — i. e. pipelines, diverters, etc. — that has to be adapted when the process changes, but also the automation. According to Azo, this is all the easier if the process control system can be individually adapted to plant expansions, new recipes, raw materials and requirements. This is the only way to achieve high-performance raw material automation. And there is another aspect that is decisive for Eibner when it comes to changes: The rules of batch tracing must be followed without gaps. In combination with flexibly adaptable conveying and weighing systems, this is a guarantee for process- and cost-optimized plant designs. Or, as Eibner concludes: “Plant construction, process engineering and automation must be perfectly coordinated as holistic solutions”. So that bulk solids always stay in motion. (müh)

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