By Donald L. Dunnington
Development and design of powder-handling equipment and systems for the process industries present engineers with two problems. First, the world’s process industries are very diverse, ranging from petroleum and petrochemicals to fine chemicals, food and beverages, pharmaceuticals, plastics, pigments, detergents, fertilizers and more.
Each industry, and seemingly every segment within an industry, has unique demands and constraints. And although all applications are based on chemical, thermal and mechanical processes, and employ the same engineering-unit operations to convert raw materials into finished products, material handling often requires industry-specific equipment or systems.
Second, it’s a problem when powder and other bulk solids materials behave badly, which they do with some frequency. Powder handling and the moving of bulk material — or, more critically, measuring and controlling its flow into a process — present equipment makers and process-system engineers with challenges not encountered with liquid systems.
Powder behavior can go from good to bad dependent on environment, how it’s handled and what’s done with it.
Behavior varies with material chemical and physical properties: particle size and shape, bulk density, compressibility, cohesive strength, moisture, oil content and the way it reacts to atmospheric or loading conditions. Flow behavior is influenced by whether a material is granular or a powder, pellet or flake. This in turn influences storage bin, hopper, feeder and conveying system design.
When systems aren’t engineered properly, and powders behave badly, everything from raw materials to intermediate and finished product can be compromised. A perfect world would deal only with easy-flowing materials such as plastic pellets — conveyed and fed by nearly any type of discharge device. But many process operations must deal with these challenging material types:
- Floodable materials such as fumed silica, gypsum and diatomaceous earth that flow like water, if not properly controlled during hopper discharge and feeding.
- Difficult-flowing materials such as fiberglass and rubber particles often require special feeder designs and flow-aid devices such as vibrators, mechanical agitators, air fluidizers or bin inserts.
- Cohesive materials such as titanium dioxide and pigments are prone to bridge in their hoppers and stick to anything they touch.
With so many variables threatening success, knowledgeable systems engineers need to make good decisions at each stage of any design process for handling bulk solids. Improper selection of components or engineered systems used for storage, feeding, blending and conveying can lead to erratic material behavior, ranging from flow stoppages to uncontrolled materials flooding.
Feeders at the heart
Equipment manufacturers have feeding solutions to address the many challenges present in dry bulk-solids handling. Feeder selection is guided by materials characteristics, required recipe precision and throughput. No one design or size meets the material characteristics or throughput requirements found in the broad range of typical processes.
Materials encountered, and feeding solutions from equipment makers such as K-Tron that address them, include the following:
Easy-flowing materials, such as plastic pellets, granules and flakes are most frequently metered with a single-screw feeder. A variety of screw sizes and geometries accommodate different material characteristics and feed rates. Bulk Solids Pump (BSP) feeders and vibratory feeders also deliver gentle, precise feeding of free-flowing materials. The positive-displacement action of BSP feeders works with exceptional accuracy.
Floodable materials, including the very fine, such as fumed silica, gypsum and diatomaceous earth, can behave like liquids if not properly controlled during hopper discharge and feeding. Here a feeder with interlocking twin screws is often best. Intermeshing screw flights act as a valve to control material flow.
Difficult-flowing materials, such as fiberglass and rubber particles, often require special feeder designs and flow-aid devices for better discharge control. The many metering options include screw and vibratory feeders as the most frequent choices.
Cohesive materials, such as titanium dioxide, pigments and APIs — susceptible to bridging inside hoppers and which can cake or accrete to equipment surfaces — are often best handled by a twin-screw feeder with hopper agitation, such as K-Tron’s Acti-Flow.
Fragile and friable materials, such as many food products and fibers, are gently fed by weigh-belt feeders, vibratory feeders and loss-in-weight belt feeders. Single-screw feeders are an option, when configured correctly.
Without consistent, dependable feeder accuracy and repeatability, recipe integrity — and hence the final product — are compromised. This directly impacts system reliability, product yield and the overall operation’s economic performance.
Volumetric or gravimetric
One of the first questions when specifying a feeding system is whether to feed volumetrically (discharge by volume) or gravimetrically (discharge by weight). Of all the choices faced, none has greater impact on process performance and financial success than this one.
Volumetric feeders hold bulk materials in a storage hopper and discharge discrete volumes of the material into the process per unit time. Except when employed in a gain-in-weight batching system, no weight information is available to control feeding. Volumetric feeders are suitable for materials with consistent bulk density and in processes that don’t require precise feeding control.
Gravimetric feeders (such as loss-in-weight feeders and weigh belt feeders), discharge material by weight per unit time. These more sophisticated and therefore complex systems integrate a feeder discharge system (the mechanical design) with a weight-sensing module (scale) and process controller. Monitoring and closed-loop control of feeding allows the system to maintain accurate material flow during the entire process run.
Although volumetric feeders are simpler and less expensive than their gravimetric counterpart, any changes in material density or flow behavior can create moment-to-moment discharge variations. Through measure and control of material flow by weight, gravimetric feeders are more precise and reliable. Weigh feeders are essential when precise recipe formulation is required. Only weigh feeders can assure moment-to-moment dosing accuracy and repeatability — even for materials that want to behave badly.
Dry bulk material transfer
Material characteristics and plant environment dictate the most appropriate conveying systems to move dry bulk solids, amidst a variety of options.
Pneumatic conveying is attractive for many applications. By applying a pressure differential inside a pipeline, air entrains the bulk solids, moving the stream from a higher to a lower pressure zone. Compressed air may be injected into one end of the pipeline to push materials through the conveying line, or a vacuum inducer can pull the material through the conveying line.
Air is the gas most used in pneumatic conveying systems. When materials are potentially flammable, or there is a risk of dust explosion, an inert gas such as nitrogen may be required. Food products that are subject to a loss of flavor when exposed to air may also require an inert gas.
Pneumatic conveying can be categorized as dilute-phase (low-pressure) or dense-phase (high-pressure) systems. Dilute-phase conveying systems tend to have a relatively large ratio of air-to-bulk solids and use positive or negative pressure to move the materials at relatively high velocity. Dense-phase systems are designed to push a denser concentration of bulk solids at relatively low velocities. Such systems push slower-moving slugs of product to minimize material degradation during transport.
Both vacuum- and pressure-conveying systems may employ the dilute-phase or dense-phase principle of operation. Pressure and vacuum each have their advantages, and choosing the best method depends on application. Often the two are combined in a single application.
Making the best choice may require a material test in a well-equipped test lab. K-Tron offers free testing and consultation at its test labs in Asia, Europe and the United States.
Problem materials and solutions
Problematic bulk materials are likely to create challenges when getting material into the convey line. Most pneumatic conveying systems are fed by a rotary airlock valve, which is itself a feeding device.
Materials that present conveying challenges include:
- Sticky materials such as Titanium Dioxide and many additives require blow-trough airlocks to empty rotor pockets. TiO2 sticks to everything and builds up in the
- Stranded or fibrous materials such as fiber glass, stringy regrind, thin flake and edge trim can lock up the airlock due to shearing. PET regrind is susceptible to angel hair or streamer generation if air temperature and velocity are too high.
- Materials that soften at a low temperature, such as wax powder, glue pellets, sugar, starch and milk whey can build up and freeze-up the airlock. Convey-line air temperatures must be kept low or build up can occur.
- Fine materials such as crumb rubber, polystyrene beads and acrylic powders can cause the airlock to squeal or freeze up. Rubbery materials can bounce in the convey line, causing additional friction loss.
- Degradable products such as fruits, raisins, shelled peanuts and corn may be cut or broken when fed with an airlock. Convey velocity and number of elbows used must be considered.
- For granite powder, steel shot or any extremely high bulk-density material, it can be hard to achieve air velocity fast enough for material pick-up.
- With abrasive materials such as steel shot, sand or filled pellets, the possibility of wear at elbows and receiver housing must be addressed.
- When using vacuum probes to pick up material into the convey line, the following concerns can give issue:
- Polystyrene flake regrind tends to interlock.
- Any odd shaped or large (1/4 inch or more) particle tends to interlock.
- Soft, tacky or compressible materials, such as crumb rubber and glue pellets will bridge.
- Powders that easily fluidize, such as fume silica, starch, calcium carbonate and soy protein, can flood the convey line. Material-to-air ratios for conveying extremely low bulk-density materials are difficult to calculate.
Some industry examples
From developing simple feeder and conveying systems to solving difficult material-handling challenges, application-specific industry knowledge is often required, as the following examples illustrate.
Plastics: Automating plastics production often requires high-accuracy gravimetric feeding systems integrated with pneumatic conveying and related bulk material-handling equipment. Feeding and pneumatic conveying solutions are specifically engineered for the industry, including powder and granular bulk material transport from railcar unloading to storage, conveying to feeding, blending, scaling, and sifting operations with integration to drying and dust-collection systems. Typical plastics processes that frequently deal with difficult dry bulk materials include resin manufacturing, compounding and extrusion.
Chemicals and Minerals: pneumatic conveying is used by the world’s chemical and mineral processors to transport and feed materials such as calcium carbonate, copper-hydroxide, talc, bentonite clay, kaolin clay, soda ash, limestone, sodium-benzoate, burnt wood chips, clay/carbon blend, gypsum, vanadium-pentoxide, dolocron, dicalcium-phosphate, silica, TiO2, lead oxide, ammonia-sulfate and boric acid.
Food and Pet Food: Moving ingredients in recipes for tortillas, bread and cake mixes, pasta, rice, snack food, cereal, candy, cookies and crackers, ice cream and pet food are just a few examples of food applications that have their share of material handling challenges. Sanitary design and cleanability are important factors in selecting feeding and conveying equipment, as well as any needs for gentle handling of fragile materials.
Pharmaceuticals and Nutraceuticals: High levels of product containment, sanitary design and clean-in-place capability are characteristic of the pharmaceutical industry. Delicate materials such as tablet granulations, active pharmaceutical ingredients (APIs), excipients and even finished capsules and tablets must be handled with care.
At end of day, development and design of powder-handling equipment and systems for the process industries involves complex processes and equipment made up of accessories, components and parts, dependent on applications that include weighing and batching, bulk unloading and storage, in-plant transfer, or loss-in-weight feeder refill. With so many components to choose from and so many critical decisions to be made in assembling a powder-handling solution, it’s hardly surprising that most processors choose to buy complete systems from equipment manufacturers.
Donald L. Dunnington is director, Internet and marketing communications for K-Tron.
K-Tron is a global leader in providing process automation systems and solutions for bulk material handling. It holds more than 100 patents for feeding, weighing, mechanical components and control technologies and their application to bulk solids handling in the Plastics, Chemical, Food, Pharmaceutical, Detergent and Nonwoven industries.