Process engineering: Pneumatic conveying moves ahead

Pneumatic conveying has long been a popular choice for moving bulk materials. However, the technology hasn't been standing still, and this can influence the choice between several types of pneumatic systems.

By Nick Basta

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Pneumatic conveying has long been a popular choice for moving bulk materials, either from storage facilities to a process unit, or between process units. However, pneumatic conveying technology itself hasn’t been standing still, and this can influence the choice between pneumatic and mechanical systems (such as conveyor belts or vibratory systems) and then among several types of pneumatic systems: dilute-phase, dense-phase or vacuum.

“Generally speaking, mechanical systems make sense with short, straight runs within a plant; they require less horsepower and sometimes can be a less expensive capital cost,” says Paul Solt, owner of Pneumatic Conveying Consulting, Allentown, Pa. “But they are usually a higher-maintenance choice, have problems with dust generation or contamination of process material, and are not as flexible in dealing with plant-floor configurations. Given sufficient capital, there is nothing that cannot be pneumatically conveyed,” he says.

Greg Steele, executive vice president for Dynamic Air, St. Paul, Minn., says, “Over the past 15-20 years, we’ve received a tremendous boost from EPA and OSHA. One by one, industries or applications where mechanical conveying has been the traditional choice have gone to pneumatic conveying. A decade or so ago, it was dry laundry detergent, then it was rubber processing and more recently pharmaceuticals. With pneumatic conveying, you reduce emissions and cut down on plant housekeeping requirements.”

Moreover, many new materials are more demanding to handle because of, for instance, reactivity or fire and health hazards; this also tips the scales toward pneumatic systems.

Because pneumatic systems run cleaner than mechanical ones and are less susceptible to product contamination, recycling materials is easier. A pneumatic system’s generally smaller product holdup also is an advantage for recycling. Some conveying systems are designed to handle different ingredients of a batch at different times, or must move the complete content of each batch together (which makes recycling of residues problematic). Mike Salvador, operations manager at Nu-Con Equipment, Chanhassen, Minn., says modern pneumatic systems need to be engineered with precise tolerances and should be capable of being disassembled, inspected and cleaned easily.

Entering another phase
No strict physical distinction marks the two main design options for pneumatic conveying: dilute-phase and dense-phase operation (vacuum conveying, which will be discussed later, can operate in either regime). Dilute-phase conveying is simply lifting and blowing powders or particles down a line, usually at higher gas velocities. Dense-phase conveying involves calibrating the line pressure based on the physical characteristics of the process material, trying to achieve full fluidization of material in the line, but moving it at a lower velocity.

“Dense-phase conveying was originally designed to meet the problems associated with the handling of abrasive materials,” says Mike Weyandt, sales manager for Nol-Tec Systems Inc., Lino Lakes, Minn. With the higher velocities found in dilute-phase conveying, particles can essentially sandblast holes into the elbows or connections of pneumatic systems, or can be degraded by impact with the walls of the conveying lines. “Many applications are now being handled with dense-phase systems because of process requirements that limit the amount of product degradation or separation allowable,” he says.
 
With dense-phase conveying, the designer strives to load the line with as much process material as possible and to run it at the most energy-efficient condition, which is just below the rate that would cause plugging. Over long distances or with high loadings, pressure variations can cause plugging and, to counter this, equipment vendors have developed various types of boosters that inject additional air as needed.

Dynamic Air, which has been offering booster technology for several years, is now introducing the DC-5 Air Saver unit . “Older boosters were simply put in place every 5 ft. or 10 ft. along a line and supplied a steady pressure,” Steele says. Some designers also experimented with adding pressure sensors and process controls to the boosters. “The DC-5 has a pressure-balancing valve that can regulate where pressure is applied automatically. In effect, the conveying line is constantly plugging and unplugging, which is the most efficient mode of operation.”

Meanwhile, Nol-Tec recently introduced its Air-Mizer injector, which is also designed to operate automatically to control conveying velocity in the line.

“The effectiveness of the dense-phase strategy is not guaranteed,” cautions Don McGlinchey, director of the Centre for Industrial Bulk Solids Handling at Glasgow Caledonian University in Scotland. “Much depends on the detail of the stressing mechanisms occurring during transport and the material’s response. A dilute-phase system can be fairly forgiving to changes in material specification, but these tend to be more critical in dense-phase systems, leading to many operating problems.”

Pulling a vacuum
Technically, the only difference between a pressurized system and a vacuum system is that the former pushes material through a line whereas the latter pulls it. However, the two systems pose distinct requirements for inlet feeding and outlet dispensing, and the physical properties of the material being conveyed or the process layout might make a difference.

“A vacuum system tends to make the most sense when it is dispensing to a single destination,” says Pneumatic Conveying’s Solt. If there are several delivery points, the designer can split the flow of a pressurized system with diverter valves, which is not as feasible with a vacuum system, he adds.
 “Vacuum systems are limited in capacity by the difference between atmospheric and vacuum pressures,” says Herman Purutyan, a vice president at Jenicke & Johanson, a materials-handling consulting company in Westford, Mass. “With a pressurized system, you have the design flexibility to raise the pressure as high as you need.”

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