In-Line Filter Removes Contaminants From Pneumatic Trailer Air Supply
Oct 1, 1997 12:00 PM, Jennifer McGehee Marsh, PhD
CONTAMINATION of product is one of the biggest concerns for carriers that use pneumatic trailers to haul food, plastics, or other high-purity commodities. Microscopic particles carried by the conveying air can cause major contamination problems.
Extremely fine airborne pollutants can elude an air intake filter. Materials from the air intake filter and lubricating oil from a blower can enter the conveying air. All of these can result in product loss, silo cleanup costs, and downtime.
Moving air transports dry, free-flowing granular materials within the conveying line. The basic types of pneumatic conveying are dense and dilute phase. Dense phase uses air pressure to transfer slugs of ingredient from a pressure vessel to a storage vessel. The ingredient is emptied from a container into the pressure vessel. When the vessel is full, the inlet is sealed and the vessel is pressurized. The discharge valve is opened and the ingredient is pushed as a slug into the conveying line and a storage vessel.
Dilute phase also uses air pressure to transfer product to a storage vessel. The ingredient is emptied from a container into a surge hopper. It then flows through a rotary feeder into the conveying line, where the ingredient is suspended in a continuously flowing airstream. The rotary feeder ensures a continuous seal between the pressurized conveying line and the atmospheric storage hopper.
Dilute phase pneumatic conveying is the method most commonly used in the general process industry and on dry bulk trailers. Dilute phase conveying uses a relatively low pressure (less than 18 psi) and high air velocity (1000 cfm). The conveyed material occupies less than 40% of the cross sectional area of the conveying line.
Bulk conveying equipment includes a vacuum and a dense- or dilute-phase pneumatic conveyor. A bulk trailer equipped with a motor-driven blower acts as a self-c ontained pneumatic conveying system. The type of air mover is determined by the flow rate and pressure.
The environments of dry bulk trailers are extremely diverse and can contribute to contamination of plastics or foodgrade products. Dust concentration, particle size distribution, and the chemical composition of particulates are affected by local factors and environmental conditions. Airborne particulate matter is a location-dependent pollutant. Nonurban average annual concentrations of total suspended particulates range from 30-40 (mu)g/m3. In cities with heavy industry, airborne particulate matter can be as high as 100-150 (mu)g/m3. Ambient particulates are a mixture of particles from natural processes and anthropogenic sources. Pollutant concentration, size, and chemical composition also depend on the location of the dry bulk trailer, the industrial activities in the surrounding area, and weather conditions. For example, air contaminants may contain a high percentage of soot particles in areas with diesel engines. However, the primary sources of particles (direct emission) are windblown, metal shavings from the blower, industrial processes, motor vehicle emissions, and fuel combustion.
Coal and oil combustion contribute to most particulate emissions in industrial areas. Coal combustion particles consist primarily of carbon, silica, alumina, and iron oxide. Particles generated during oil combustion consist of carbon, cadmium, copper, cobalt, and nickel.
Secondary particles are formed by the atmospheric transformation of gases to particles. Sulfur-derived particulates are a major component of secondary particles. Most of atmospheric aerosol is distributed in a fine mode of 0.1-1.0 microns and a coarse mode, which is greater than five microns. These two modes generally have different physical and chemical characteristics and are produced by different sources.
Combustion particles, including motor vehicle emissions, typically are the fine mode. Particles generated by crushing, grinding, loading, and similar processes usually are larger than 15 microns. Particles ranging from 1-10 microns are suspended in air for appreciable periods by air currents and are not captured by an intake filter. An intake filter generally houses either a polyester or paper filter medium, which captures contaminants 10 microns and larger. The filter is positioned before the positive displacement (P/D) blower and protects it from airborne material. Particles larger than 10 microns settle fairly quickly and can be found only in air near their sources or under strong wind conditions. These particles typically will be captured by the intake filter.
In addition to airborne particulate matter, the P/D blower can be a source of contaminants. Wear and corrosion of a blower can cause particulate generation. Age and lobe wear, improper use and cooldown, high temperature, moisture, and corrosive gases can promote corrosion and other undesirable chemical reactions, such as scaling. Another source of contamination is the oil released during a catastrophic blower failure. Oil from a blower injected into the conveying line contaminates the product.
Which contaminants enter the blower of a pneumatic conveying system depend on the performance of the intake filter and the presence of an inline, high-pressure (24 psi rating) filter. An inline, high-pressure filter is mounted after the P/D blower and filters the pneumatic air before it mixes with the product. By installing an inline, high-pressure filter, 99.998% of particulates four microns and larger, and any hot oil or oil mist are removed from the airstream, leaving the load virtually free of contamination.5 In an 18-month study, filters from 15 dry bulk trailers operating in North America and equipped with P/D blowers that deliver up to 1000 cfm were analyzed. Each blower had an air intake filter and an inline, high-pressure filter to protect the bulk product from contamination during conveying. In most cases, there also was a discharge silencer. Particulates from the intake and inline, high-pressure filters on each trailer were analyzed and compared to typical airborne solid particles.
Methods To examine the size and chemical composition of particles entering a dry bulk conveying system, intake filters from the P/D blowers of four trailers, both intake and inline; high pressure filters from three trailers; and inline, high-pressure filters from 10 trailers were analyzed for particulates. Shavings captured in an inline, high-pressure filter assembly's endcap also underwent composition analysis.
Analysis of the shavings and different filter media was performed using X-ray fluorescence (XRF) spectrometry, which requires little or no sample preparation, and detects inorganic elements from sodium to lead. This type of qualitative analysis shows what elements are present in each sample. Some elements come from the filter material. The rest are contaminants picked up while the filter was in use.
Visual analysis of the filter media also revealed very large (30-3000 microns), dark, and magnetic particles on the filter surface and in the inline, high-pressure filter assembly's endcap. These particles were analyzed with XRF spectrometry.
Results and Discussion Calcium, chlorine, iron, titanium, silicon, sulfur, chromium, manganese, copper, zinc, nickel, cadmium, lead, tin, phosphorus, and potassium typically were found in filter samples from the 15 dry bulk trailers. Bromine, molybdenum, and zirconium also were present in numerous samples. A typical XRF readout indicated phosphorus, potassium, calcium, titanium, chromium, manganese, iron, nickel, copper, and zinc on an intake filter. The XRF of an inline, high-pressure filter from the same bulk trailer revealed chlorine, calcium, titanium, chromium, manganese, iron, nickel, copper, and zinc. Without the inline, high-pressure filter, these metals and other contaminants would have been blown into the product and conveyed into the silo.
XRF analysis also showed that the large, readily observable particles on the filter, as well as the particles accumulated in the endcap, primarily were iron shavings. Smaller particles consisted of chlorine, titanium, chromium, manganese, copper, zinc, nickel, cadmium, lead, strontium, zirconium, molybdenum, and tin.
Sources of this particle generation could be corrosion or oxidation of the mild steel and cast iron parts on the surface of the tractor-trailer, the intake filter housing, the piping and fittings between the intake filter housing and the silencer, the silencer, the piping and fittings between the silencer and blower inlet, the blower lobes, endplates, housing, and/or the blower bearings.
Analysis of both the intake and the inline, high-pressure filters from the same tractor-trailer showed substantially larger amounts of contaminants on the inline, high-pressure filter media than on the intake filter media. In some cases, contaminants that were not present on the intake filter were present on the inline, high-pressure filter. This is direct evidence that a significant source of product contamination comes from the P/D blower. Particles less than 10 microns that pass through the intake filter also contribute to the contamination. The inline, high-pressure filter captured these particles and protected the product from contamination.
Numerous samples contained significant concentrations of calcium, silicon, and sulfur caused by small particles that had accumulated on the larger iron particles. These contaminants could have come from airborne particulates-material used in the discharge silencer and/or exhaust from the tractor's diesel engine. The inline, high-pressure filter stopped these particles from contaminating the product.
Conclusions Material conveyed with a positive displacement blower can be contaminated in various ways. The intake filter allows small particles from ambient sources to pass into the conveyed material. In some cases, the filter element can break down and pass into the load. In the event of a catastrophic failure, lubricating oil will be injected from the P/D blower into the conveyed material. Most importantly, the deterioration of blower components produces very small particles that pass undetected into the conveyed material.
Knowing what must be removed from pneumatic air is essential. Cleanness of pneumatic conveying air is extremely important, especially in food and plastics because the air is in contact with the transported product.
No current legal standards impose acceptable contamination concentrations and particle size limits in food applications at the silo-loading point. However, contaminant standards are imposed by each manufacturer's quality assurance (QA) program in compliance with Food and Drug Administration recommendations. Most plastics manufacturers impose a similar QA program, and test product samples for contamination at the silo. Rejection of foodgrade or plastic products that are contaminated with shavings of iron, lead, and other metals is expensive for the shipper, carrier, and customer. Also consider the possibility of oil contamination resulting from the failure of a P/D blower. This results in wasted product and increased maintenance costs because the shipping container, air lines, and silo must be cleaned again before reuse.
An inline, high-pressure filter can provide additional protection by capturing contaminants four microns and larger, hot oil, and oil mist. Installed after the P/D blower and before the product pickup in a dilute-phase pressure system, the filter can ensure that product contacts the conveying air without becoming contaminated.
1 Cooper Industries, Gardner Denver Compressor Manual. 2 T G Pace, Ambient Particulate Baseline Conditions-Sources and Concentrations, Proc Specialty Conference of APCA, 1980. 3 R C Flagan, Fundamentals of Air Pollution Engineering, Prentice Hall, p 8, 1988. 4 R C Flagan, Fundamentals of Air Pollution Engineering, Prentice Hall, p 8, 1988. 5 T Ptak, Fine Particle Removal from Compressed Air of Pneumatic Conveying Systems, presented at the Powder & Bulk Solids Conference, May 93.
Jennifer McGehee Marsh is a professor in the biology department at the University of Louisville (502-852-6771). She has a PhD in environmental biology and teaches courses in environmental biology, geophysics, and environmental law. She also has a law degree and practices in the area of environmental defense.
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