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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There’s certainly no vacuum in terms of suppliers. For instance, IEDCO, Sewell, N.J., is now offering modular technology from Volkmann Vakuum Technik, Soest, Germany, to the North American market. IEDCO representatives say the technology is particularly suited to high-purity applications, such as pharmaceuticals or batch mixtures where there is concern about material left from a previous batch. A user can obtain dilute or dense-phase conveying by selecting from among 60 different vacuum generators; tangential or radial suction ports accommodate material properties such as dryness or stickiness, respectively, to control flow characteristics. Discharge valves have their pneumatic components outside the conveyor body, allowing for easy cleaning in place, according to IEDCO.

Cyclonaire Corp., York, Neb., addresses the pressure question both ways. Its HC Series system, thanks to a venturi tied to a blower, can use vacuum in one part and pressurized flow in another. A typical application would be vacuum unloading of, say, a railcar into a transfer vessel, according to the company. When the vessel fills, the system automatically cycles to a discharge mode, using the same blower to convey material along to its destination. Optional load cells allow the system to weigh and batch the process flow.

Because the pressurized side of the system uses relatively low air pressures, the system is well suited to handling abrasive materials. At cement-maker R-Con’s Plant No. 6 in Wichita, Kan., the HC system unloads railcars carrying cement into the plant, where more than 200 concrete blends are produced in different colors or compositions to handle architectural requirements. The system addressed one pressing problem — the buildup of dust in an unloading pit that had been caused by spillage from a screw-pump conveyor — while raising capacity and productivity at the plant. “Our railcars hold 80 to 84 tons each, and we unload them in about two hours,” says Rick Heise, production manager. “There’s no need to get down into the pit, no dust, nothing to clean up. When you time everything out, we’re getting the job done much faster.”

Coming down the pipe
Most experts in pneumatic conveying agree that technology changes in the field have been evolutionary rather than revolutionary. Nol-Tec’s Weyandt says the company has introduced new broken-bag detectors and rotary feeders, which help streamline operation at the feed end of a conveying system. Pelletron, Lancaster, Pa., now offers several types of engineered elbows to improve the abrasion resistance of a conveying line. Nu-Con has launched a compact rotary-valve tube selector that allows a conveying system to feed multiple lines or to converge multiple lines into one; the unit has inflatable seals on all ports to ensure airtight conveying.

System design might be poised for potentially significant changes, thanks to the emerging commercial use of a new software combination: the tying together of computational fluid dynamics (CFD) with so-called discrete element modeling (DEM).

CFD, which has been used in commercial applications in the chemical industry for more than a decade, models continuous fluid phases such as air flowing around a shape. “Our multiphase models have been used to solve particulate flow problems where the focus has been the effect of fluid mechanics, as opposed to particle dynamics,” says Ahmad Khadari, a business manager at Fluent Inc., Lebanon, N.H.

“Classical applications of this technology include cyclone separators, fluidized beds or classifiers.”
 However, to analyze a system from the perspective of the particles a “discontinuous” phase — another approach is needed, hence DEM. It originated, says John Favier, president of DEM Solutions, Edinburgh, Scotland, in geomechanics, where it was used to analyze a problem like the load of a building atop a gravel bed. “We’ve been constrained by the availability of computational power,” he says. “In the past, we were able to analyze thousands of particles in one system at a time; currently we can process hundreds of thousands, and very soon we will be doing millions.”

At that level of detail, modeling the interparticle effects of something like a dense-phase flow or a complex geometry within a conveying-system component becomes feasible. This can eliminate previously necessary simplifications and potentially provide more accurate results.

DEM Solutions is advancing with several efforts to have DEM address materials-handling problems in the commercial arena. Favier says the company has a project with John Deere, Moline, Ill., to analyze the flow of grain particles in its harvesting equipment. It also formed an alliance this fall with Fluent to combine its program, called EDEM , with Fluent’s CFD software.

Industry experts are taking a wait-and-see attitude. “DEM, or CFD for that matter, aren’t really ‘there’ yet,” says Jenicke & Johanson’s Purutyan. “A pneumatic conveying system can be very complex to model and, so far, all the simplifying assumptions you have to make limit the software’s predictability.” Instead, he recommends trying the actual process material in a test system and using the results for design.

Ted Knowlton, director of the Particulate Solids Research Institute (PSRI), Chicago, an industry-funded consortium to conduct research in materials handling, says the institute is evaluating several CFD packages and expects to have at least one in use within the coming year.

PSRI provides regular input about what is going on in CP’s PowderHandling Insider e-newsletter.

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