The first part of this two-article series discussed conditions that are required to initiate fires and explosions inside of drying equipment and described potential sources of ignition that can trigger these events. This article will discuss how typical powder dryers operate, what hazards they present and strategies that can be used to manage the risk of fire and explosion inside of this equipment.
Types of Industrial Drying Equipment
|See the first part of this two-part article series, "Fire and Explosion Hazards of Powder-Drying Operations," to learn more about conditions necessary to ignite fires and create explosion hazards in powder-drying operations.|
Industrial drying equipment includes spray, pneumatic-conveying, fluid-bed, rotary, conveyor, tray and trough-type dryers. Depending on the dryer, they may be may be directly or indirectly fired.
In a direct-fired system, the source of heat is a combustion chamber that may burn flammable gases, oils or solid particulate fuels. These types of dryers present ignition risks from hot gases or particles entering the dryer. The hot gases or particles can be generated from the combustion process, by inefficient burning of solid particulate fuels, and by recirculation of the airstream, which may be contaminated with fines from the powders being dried.
In an indirect system, the heat source commonly is thermal transfer fluid, steam, hot water or combustion gases. In the case of thermal transfer fluid, it is usually heated in a separate circuit that consists of a combustion burner that transfers heat to the fluid through a heat exchanger arrangement. The heating medium may either pass through a heated jacket surrounding the material to be dried or through heat exchanger tubes over which the air passes prior to contacting the wet powders.
Spray Dryers. In a prototypical spray dryer (figure 1), the slurry of powder is fed into the atomizer and is dispersed into small, solid-laden liquid droplets. The spray is dried with either a cocurrent or countercurrent hot airstream. Customarily, the slurry is aqueous based, but it can be solvent based in some cases.
Dried powder exits the bottom of the spray dryer. From there, it typically enters a downstream fluid-bed dryer that serves as a second-stage dryer or powder cooler. Ultimately, the dried powder is conveyed to a cyclone to remove the air. Then, it is fed through an airlock to be packaged or further refined. Inside of these dryers, vapor (solvent-wet powders), a dust-cloud or dust-layer ignition hazards can be present, depending upon the design.
Pneumatic-Conveying Dryers. Sometimes referred to as ring or flash dryers, pneumatic-conveying dryers feed wet powder into a hot gas stream. From there, the wet feed is conveyed up through a vertical tube, where drying action takes place.
The dried powder is removed from the airstream through use of cyclones and, in some cases, dust-collection equipment. Inside of the dryer, there may be areas where combustible dust clouds are produced. In some cases, dried accumulations of dust can pose a self-ignition risk (figure 2).
Fluid-Bed Dryers. In a fluid-bed dryer (figure 3), water- or solvent-wet powder is fed into the drying chamber. There, it is exposed to a hot airstream passing through a perforated plate inside the dryer. The hot airstream aerates the wet powder and creates a dense, fluidized bed. The hot air passing through the dense fluid bed causes the liquid in the wet powder to evaporate.
Fluid-bed dryers may be designed to be continuous or batch. The wet powder is dried for a period of time, the duration of which is based on the temperature of the inlet air and the final desired moisture content. Dried powder generally is removed from the bottom of the dryer while some fine particulate may enter a dust-collector system with the drying air. Inside of fluid-bed dryers, there may be areas where combustible dust clouds are produced. Normally, these areas are above the fluidized bed. The density of the fluid bed is such that the fuel-to-air ratio is too high to present an explosion hazard; however, a fire hazard can exist inside the bed.
In the area above the fluid bed, socks or cartridges — part of the dust-collection system — are used to separate fine particles from the airstream. The socks or cartridges are pulsed periodically with high pressure air delivered to the inside of the filter media to remove dust accumulations. This cleaning feature can produce explosible dust clouds inside the dryer that can present a dust-explosion hazard.
In addition, if the fluid-bed dryer design is not adequate, there may be areas where dried powder can remain inside. When that occurs, there could be a self-heating hazard. Flammable vapor (from solvent-wet vapors) atmospheres also may be present.
Rotary Dryers. Rotary dryers (figure 4) sometimes have been used to dry powders, but the use of this type of dryer for powder-drying applications is uncommon. Rotary dryers may be directly or indirectly fired, and hot air may be cocurrent or countercurrent to powder flow. The powder enters the rotary drying chamber and travels from the front to the back due to the design of the chamber and gravity flow. Depending upon the design of the rotary dryer, the feed rate and the particle size of the powder, combustible dust clouds may exist inside.
Band Dryers. Band dryers (figure 5) are designed to handle a continuous flow of product. The product may enter the dryer on a conveyor or band-type belt. As the product travels from one end of the dryer to the other, moisture is removed by the drying air.
Band dryers may be directly or indirectly fired, and airflow may be cocurrent or countercurrent. Usually, combustible dust clouds are not generated inside of these types of dryers; however, a fire hazard can exist if the dried material remains in the oven or if some other upset condition occurs.
Tray Dryers. Inside of a tray dryer (figure 6), moisture or solvent is removed from the wet materials by passing hot air through the airspace between the trays. In some cases, these dryers can be operated under vacuum; in those instances, lower heating-air temperatures can be used.
Inside of a tray dryer, the primary fire or explosion hazards would be overheating of the product on the trays or accumulated dust spillage on ledges, floors or other areas inside the dryer. There also may be a risk of ignition if the product is packaged without adequate time to cool down. This risk would be enhanced for materials of low thermal conductivity. In instances where solvent-wet powders are being dried, there also may be a flammable vapor explosion hazard unless the dryer is designed to operate below the lower flammable limit of the solvent. Finally, handling of the dry powders may present a risk of ignition by a person if the minimum ignition energy (MIE) of the powder is less than 30 mJ. A dust cloud flash fire or explosion hazard would exist if the tray is dropped and a fine combustible dust cloud is produced in the vicinity of the person handling the material. Such a hazard also would exist when the tray contents are dumped in the receiving vessel or container.
Trough Dryers. In a typical trough dryer (figure 7), wet material enters the dryer and falls into a trough equipped with a rotating screw or screws. The walls of the dryer as well as the inside of the screw are heated, usually with a thermal transfer fluid. As the material travels through the dryer, contact with the inner walls of the dryer and screw results in evaporation of moisture from the powder.
These types of dryers are at risk for self-heating of powder if the powder is not cleaned out when shutting down. An explosion hazard would normally not exist in a trough dryer because suspended dust clouds can be prevented with a good design. These dryers can be operated under a reduced oxygen atmosphere. In addition, the evaporated water forms steam that might also serve to partially inert the atmosphere inside.
Mitigating Dryer Fires and Explosions Takes Diligence
In conclusion, where particulate solid materials are being dried in industrial process equipment, the potential for fires and explosions exists. An understanding of the conditions necessary for these events to be initiated is essential to effectively manage the risk.
It is important that reliable test data be available with regard to the powder’s explosibility, ignitability, thermal ignition and electrostatic properties. Where materials exhibit self-heating characteristics, measures should be taken to ensure that process drying temperatures do not approach temperatures where the material begins to self heat.
In addition, dryer design should ensure that residual material does not remain in the equipment after the completion of the drying operation. Monitoring of CO to detect the onset of combustion can be effective in preventing fires in drying systems. Good bonding and grounding practices also are essential where conductive components are present inside of the drying equipment. Explosion prevention and protection systems may need to be installed where suspensions of powders or dusts, above MEC, would be expected inside of the drying equipment. As an alternative, the equipment may be designed for containment. Expert advice should be considered to assist with the safe drying operations.
Control Strategies for Fire and Explosion Protection
The following strategies may be effective in the prevention or control of fires and explosions in drying equipment. The particular types of dryers that can benefit from these strategies are indicated in parentheses. In addition, the NFPA standards referenced at the end of the article provide more information regarding specific requirements for protecting ovens and dryers where combustible particulate solids are involved.
Prevention and Control of Fires and Explosions in Direct-Fired Dryers
Prevention and Control of Fires and Explosions Caused by Self-Heating (All Types)
In order to effectively manage self-heating hazards, a proper understanding of the self-heating behaviors of powders subjected to heat is essential. Effective measures to prevent or control the ignition of combustible particulate inside of drying equipment due to self-heating include the following:
Prevention and Control of Fires and Explosions Caused by Electrostatic Ignition (Spray, Pneumatic, Fluid Bed, Rotary)
It is important that measures be taken to avoid electrostatic discharges inside of drying equipment where flammable vapor or dust clouds can exist. Ignition of particulate solid materials inside of dryers will lead to explosions if suspended clouds are produced in the vicinity of an electrostatic discharge.
The minimum ignition energy (MIE) of vapors and gases is extremely low, and avoidance of any type of electrostatic discharge, including a brush discharge, is imperative. The MIE of a dust or powder cloud is a function of temperature. At higher temperatures, less energy is required to ignite the dust cloud. Electrostatic ignition properties of powders and dusts must be understood, and data must be available in order to effectively manage the risk. Examples of effective control measures to prevent or control electrostatic discharges inside of drying equipment include the following:
Prevention and Control of Fires and Explosions Caused by Friction (Rotary, Band, Trough)
Prevention of Fires and Explosions in Dryers A User Guide, J.A. Abbott, 2nd Ed., IChemE, Rugby, UK 1990.
Eckhoff, Rolf, K. Dust Explosions in the Process Industries, Third Edition.
NFPA 61, Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, 2013 Edition.
NFPA 68, Standard for Deflagration Venting, 2013 Edition.
NFPA 69, Standard on Explosion Prevention Systems, 2014 Edition.
NFPA 77, Recommended Practice on Static Electricity, 2014 Edition.
NFPA 86, Standard for Ovens and Furnaces, 2015 Edition.
NFPA 484, Standard for Combustible Metals, 2015 Edition.
NFPA 654, Standard for the Prevention of Fires and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, 2013 Edition.
NFPA 655, Standard for Prevention of Sulfur Fires and Explosions, 2012 Edition.
NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, 2012 Edition.