Particulate and granulate materials play a major role in many industries. How they are handled and processed has a decisive impact on the quality of the final product and on business performance. In the international market, German and European companies are among the mechanical process market and technology leaders. Increasing demand for continuous processing and transport of raw materials in the food processing, chemical and pharmaceutical industry create lucrative market opportunities for these companies. Chemical, pharmaceutical and food-processing plants handle powder and granulate on a daily basis. An impressive selection of material-handling systems with a proven track record are available including conveyors, separators, crushers, filters, mixers, screens and centrifuges. However, requirement profiles and expectations continue to evolve.

Learning From Nature

The skink (Scincus scincus) moves through sand as fast as a fish in water. Also known as the sand fish, the lizard is about 15-20 cm long and lives in the deserts of North Africa and the Middle East. Material handling and process engineers could learn a thing or two from this inconspicuous desert creature. The skink, which normally stays below the surface of the sand, shows us how to move with maximum energy efficiency in this environment. Researchers from the Department of Cellular Neurobionics at the RWTH Aachen, Germany, are convinced that what we learn from nature has relevance for industrial granulated material handling technology. The scientists were able to demonstrate, that the movements of the skink always have the same frequency. The animal creates vibration as it wriggles through the sand. The researchers discovered that the motion frequency is always 3 Hz (three movements per second). It seems reasonable to assume that the animals do this to save energy when they are moving from one place to another. The team constructed a model to confirm their theory. They moved an artificial motorized sand fish made of aluminum back and forth in the sand at different frequencies. The results showed that the minimum amount of energy was expended at exactly 3 Hz, because the surrounding sand was least compact at that frequency. Taking these observations into account, it is possible to determine the ideal transport frequencies for a wide range of granular materials with the aid of computer and mathematical models.

Pilot Studies Are Indispensable

How relevant are bionics to practical industrial applications? Manufacturers and users would undoubtedly like to have access to models which would allow them to predict the behavior of powder, granular material and bulk goods. Problems such as blockages, de-mixing, vibration and erratic flow (flushing) can occur, particularly when power and bulk goods are discharged from silos, funnel feeders, transport containers, etc. However, in many cases the only way of avoiding the problem is to conduct trials.

The German company Coperion Waeschle has set up an industrial-scale bulk goods test center. The specialists have experience with more than 10,000 different powdery and granulate materials. Characterization of the flow properties of powder and granulate material is necessary to optimize the design of material handling equipment. Granulate material can pose a number of different engineering challenges depending on the density, elasticity and grain shape. Powder can behave as a solid or a fluid in material handling systems. A good understanding of bulk material properties and mechanical parameters such as particle friction, adhesion and flow pressure is essential during the design of conveyors and storage silos in material handling applications. Coperion Waeschle points out that engineers can choose one of eight conveyor types ranging from lean-phase to stabilized slug conveying when they are designing a reliable, cost-effective bulk material handling system.

Dust

The following scenario is typical in the practical world of bulk material handling. A company in the plastic industry grinds up PMMA panels (acrylic glass). A fan is used to convey the granulated material to the bagging facility which is 80 m away. 6 kg of dust is produced per 1,000 kg of material that is conveyed, and it has to be separated out. In order to do this, a side channel compressor sucks in the granulated material and an Esta counterflow separator removes the dust. The material is put into the counterflow separator from above and falls downwards following the gravitation. At the same time air streams upwards from the bottom. The air takes the lighter particles and transports them to the top. Only 55 g of residual dust is left per metric ton of material being transported (55 ppm). Once the granulated material has been cleaned, it moves on to the filling station. A filter cyclone removes the dust from the air, and the unpressurised material falls from a bucket wheel feeder into a collection bag. This of course is only an example. There are many ways to ensure compliance with air pollution regulations. When the goods are delivered or shipped by truck, keeping the material under cover (silo trailers, containers canvas covers) can be very helpful. On continuous systems, enclosed or largely enclosed solutions (covered belt conveyors, bucket conveyors, screw conveyors, feed screws and pneumatic conveyors) help contain dust.

Particularly on pneumatic conveyor systems, the conveying air should be fed into a dust removal system or kept recirculating. When solids are being loaded into enclosed shipping containers, the displaced air must be contained and put through a dust removal system. Open transfer points must be kept moist as long as moisture does not affect downstream processing, shelf life or product quality. Otherwise the transfer stations must be encapsulated, and airborne dust must be routed to a dust removal system.

According to Glatt, docking equipment must be absolutely reliable to ensure that most of the dust is contained. This is necessary to ensure compliance with very stringent leak prevention requirements which apply to loading and discharging operations involving powder and granulate material. The company optimizes all of its docking elements to provide the necessary level of containment, for example by using silicone collars, inflatable radial and axial seals or patented conteinment valve systems. Other components such as positioning elements, ceiling feed-throughs, dust suction equipment, seepage containment and various types of cleaning equipment also help prevent the accumulation of dust.

It is important to keep in mind that around 70% of all dust in these applications is explosive in certain concentrations. Compliance with the ATEX 100a explosion protection directive is often an important consideration. When bulk material is loaded into silos and containers, there must be an unobstructed flow of displaced air to the outside. A filter can be mounted at the top of the silo to contain the dust particles. This type of system is very effective particularly in applications with fluctuating volume flows.

Lean Phase and Dense Phase -Conveyor Systems

There is an enormous diversity of techniques available for handling a large variety of bulk materials in different situations and operating environments. In addition to the volumes and distances involved, the physical configuration at the installation site is also an important factor. Screw and vibration conveyors are particularly suited to metering applications. Mechanical solutions such as belt conveyors and bucket conveyors are more suitable for longer distances. Ammag offers pneumatic systems as an alternative. These systems have a number of advantages:

• The line layout is very flexible. This is particularly helpful when the conveyors are being installed in existing production lines;
• The systems are enclosed and have no moving parts, so it is easy to maintain hygienic standards in the food processing industry;
• Minimal maintenance is required, because there are no moving parts.

There are two basic types of pneumatic conveyors: dense phase and lean phase. Compared to lean phase pneumatic conveyors which have been on the market for many years, dense phase technology is a relatively recent development. The solids-to-air fraction on dense phase systems is five to ten times higher than on lean phase conveyors (according to Gericke: lean phase conveyors up to 10 kg/kg conveying air, dense phase conveyors up to 80 kg/kg). The pipeline is densely filled, and the product moves along in the form of discrete plugs which occupy the entire cross-section of the pipe.

Because air volumes are 5-10 times lower, the material moves at a slower speed. Flow rates on lean phase systems are typically around 20 m/s, whereas dense phase conveyors normally operate at 2-3 m/s, and flow rates on some systems can be even lower than that. As a direct result, dense phase conveyors are gentler on the product and are less susceptible to wear, because minimal stress is placed on the pipes even if the system is handling highly abrasive materials.

Particle Shape as a Parameter in Pharma Production

Now that reliable, high-speed measurement technology has become available, the American Food and Drug Administration (FDA) has introduced new requirements as part of the Process Analytical Technology Initiative which increase the importance of solutions for determining particle shape in the pharmaceutical industry.

Particle size detection has become a routine operation. However, size is not the only important factor. The shape of the particle can also have a direct impact on product performance and the production process. Measurement of particle shape enhances the process and the understanding of the process, according to Malvern Instruments.

Automatic image analysis systems based on microscopy or laser diffraction technology are available to perform particle characterization (size and shape). In contrast to manual microscopy, automatic image analysis provides statistically valid data based on non-biased samples. Users can conduct a systematic analysis of particle shape and its effects. Image analysis generates quantity-based distributions, making it highly sensitive to small fine fractions or the presence of small amounts of foreign particles. The systems can capture an image of every single particle for visual detection and verification of agglomerates and foreign substances.

The following example shows just how sensitive a pharmaceutical process can be to the shape of the particles. One out of four batches of a pharmaceutical excipient caused problems during tablet pressing. Over time, the costs began to mount up, because tablet pressing is the last step in the production process, and the product already contains all of the expensive ingredients. The manufacturer was looking for a way of identifying bad batches earlier in the process, if possible at the raw material stage. Conventional microscopic analysis and other size characterisation techniques were unable to detect any difference between the four batches. Automatic image analysis was then conducted to asses the convexivity of the material in the four batches. This solved the problem. Convexivity, which defines the surface roughness or "serration" on the surface of the particles, was substantially lower in the bad batch compared to the other three batches.

CIP Wet Cleaning for Bulk Materials Handling Systems

Cleaning in place (CIP) is now standard on fluid media production and handling systems, but systems that handle bulk materials still lag behind in this respect. What requirements do subsystems such as bucket wheel conveyors, diverters, etc. have to meet?

According to Coperion Waeschle, the components must be made completely of stainless steel. Acid-resistant 1.4404 (AISI 316L) stainless steel is the most common choice because it provides good resistance against standard cleaning agents which normally contain low-concentration nitric acid and caustic soda. The parts must have a smooth surface and an average surface roughness Ra<0.8 µm. They must be free of dead space and gaps which can trap food residue or cleaning fluid. Any residue can promote the growth of microbial organisms. The seals represent a substantial engineering challenge. The following general rule applies to hygienic design of joints between two surfaces: the seal must be designed and compressed such that the sealing is flush mounted and there is a slight curvature which extends into the product chamber. There are no gaps with this design. DIN standard 11864 uses flange connections to illustrate aseptic joint design. It is better, however, to eliminate joints and seals altogether, while ensuring that the cleaning agent can still easily reach all areas of the handling system and that the system is easy to clean and dry.