Vahid Ebadat, Ph.D., and Vladimir Stetsovsky, M.S.E.E.
Introduction
Surface contact, separation, and movement are essential features of powder handling and processing operations.
Furthermore, the majority of powders are not good conductors of electricity. Powder processing and handling operations, therefore, contain all the necessary factors for electrostatic charge generation and retention. Such electrostatic charging is commonly considered to be the product of frictional contact between two contacting materials. However, the use of the terms contact and separation may more accurately explain how static electricity is generated. When two materials make contact with each other, the balance of the surface electrons tend to be come disturbed. If the two materials are separated quickly, the displaced electrons may not be able to return to normal balance. This would leave one of the materials positively (+) charged and the other negatively (-) charged. The magnitude of the resulting electrostatic charge generation depends, amongst other factors, on the true contact area and the resistivity (conductivity) of the contacting surfaces.
The generation and accumulation of electrostatic charge on powder particles can cause issues such as powder agglomeration, adhesion to surfaces of processing equipment and containers, variations in packing density, flow problems and clogging of the hopper and conveying pipes, and could also result in dust cloud explosions and fires. In order to prevent/control such problems and hazards, it may be desired to control the electrostatic charge generation and/or accumulation on powder particles.
Electrostatic Charge Neutralizers (Eliminators)
Electrostatic charge neutralization (elimination) is one method to eliminate/reduce the accumulated electrostatic charge on materials. An electrostatic charge neutralizer ( eliminator ) causes the breakdown of the air molecules into positively and negatively charged ions (corona discharges). The ions are then attracted to any charged surface of opposite polarity.
There are two main types of electrostatic charge neutralizers:
A passive neutralizer consists of one or a series of electrically grounded conductive electrodes of small radius of curvature. The electrodes are mounted in the electric field associated with the electrostatic charge on the powder particles. In this case, corona discharges necessary for producing ions are created by the electrostatic charges to be neutralized. The intensity of the neutralizing ions depends on the magnitude of the electrostatic charge on the powder particles.
Passive neutralizers are relatively inexpensive and are very often simple to use. However, they are not
capable of completely eliminating the electrostatic charges from the surfaces. This is because in passive
eliminators the electric field producing the corona discharge is created by the charges to be neutralized and the discharge occurs only if the charge density on the powder particles is high enough.
• Active electrostatic charge neutralizers ( eliminators )
An active neutralizer consists of one or a series of conductive electrodes of small radius of curvature that are connected to a high-voltage power supply. In this case, the corona discharges (neutralizing ions) are created by an external power supply and therefore do not rely on the electrostatic charge on the powder particles to function.
Properly designed, positioned, and maintained active eliminators can therefore completely remove the problem charge from the powder particles. The high-voltage power supply is very often AC but on occasions when the polarity of the problem charge is known a DC power supply could be utilized.
For effective and safe electrostatic charge neutralization, the following points should be taken into consideration:
• While electrostatic charge neutralizers eliminate the electrostatic charge quickly, it is not instantaneous. Very highspeed processes, such as pneumatic conveying, may outpace the eliminator’s ability to neutralize the electrostatic charge effectively. The speed of the charged particles should be taken into consideration when selecting/designing
the neutralizer,
• Ionized air has little persistence; the ions “recombine” quickly and revert to normal air molecules. Effective neutralization requires quick delivery of the ions to the charged particles, and
• Special care should be taken to ensure that the electrostatic charge neutralization device does not become a source of ignition.
Chilworth Technology has extensive expertise and testing facilities for the design, modification, laboratory testing and installation of electrostatic charge neutralization systems for the elimination/control of processing/handling problems associated with the electrostatic charging of powders. Our recent projects have involved successful resolution of electrostatically caused problems such as powder agglomeration, packing density variations, flowability, and clogging of hoppers and conveying pipes.
Control of Unwanted Electrostatic Charges from your Powder
To effectively control unwanted electrostatic charges from powders, the following information would be required:
(a) Electrostatic charging rate ( Chargeability ) of the powder
(b) Conductivity ( Volume Resistivity ) of the powder
(c) Sensitivity of the powder to ignition by electrostatic discharges ( Minimum Ignition Energy )
(d) Material of construction of the powder processing/handling equipment (e.g. metal or plastic)
(e) Powder flow rate, and
(f) Process conditions such as temperature, pressure, presence of inert gas, etc.
The above information would be utilized to design/select the appropriate type of neutralizer ( passive or active, AC or DC ), magnitude and polarity of the required neutralizing current, required number of neutralizers, material of construction of the neutralizers, and location(s) where the neutralizer(s) should be mounted.
References:
? D.K. DAVIES and V. EBADAT, Electrostatic problems in powder handling. ImechE 1991 C418/028
? T. HORVATH and I. BERTA, Static Elimination. Research Studies Press, A Division of John Wiley & Sons, Ltd.
How Can Chilworth Technology, Inc. Help You?
Chilworth Technology has extensive laboratory testing facilities and a group of highly qualified electrostatic and Process Safety Specialists who can help you with all aspects of design, modification, laboratory testing, and installation of electrostatic charge neutralization systems for the elimination control of processing/handling problems associated with the electrostatic charging of powders. If you have any questions regarding this article or any other process safety issues, please contact Dr. Vahid Ebadat at https://edir.bulk-online.com/profile...technology.htm■
Electrostatic Charge Neutralization
Electrostatic Charge Neutralization for Powders
Vahid Ebadat, Ph.D., and Vladimir Stetsovsky, M.S.E.E.
Introduction
Surface contact, separation, and movement are essential features of powder handling and processing operations.
Furthermore, the majority of powders are not good conductors of electricity. Powder processing and handling operations, therefore, contain all the necessary factors for electrostatic charge generation and retention. Such electrostatic charging is commonly considered to be the product of frictional contact between two contacting materials. However, the use of the terms contact and separation may more accurately explain how static electricity is generated. When two materials make contact with each other, the balance of the surface electrons tend to be come disturbed. If the two materials are separated quickly, the displaced electrons may not be able to return to normal balance. This would leave one of the materials positively (+) charged and the other negatively (-) charged. The magnitude of the resulting electrostatic charge generation depends, amongst other factors, on the true contact area and the resistivity (conductivity) of the contacting surfaces.
The generation and accumulation of electrostatic charge on powder particles can cause issues such as powder agglomeration, adhesion to surfaces of processing equipment and containers, variations in packing density, flow problems and clogging of the hopper and conveying pipes, and could also result in dust cloud explosions and fires. In order to prevent/control such problems and hazards, it may be desired to control the electrostatic charge generation and/or accumulation on powder particles.
Electrostatic Charge Neutralizers (Eliminators)
Electrostatic charge neutralization (elimination) is one method to eliminate/reduce the accumulated electrostatic charge on materials. An electrostatic charge neutralizer ( eliminator ) causes the breakdown of the air molecules into positively and negatively charged ions (corona discharges). The ions are then attracted to any charged surface of opposite polarity.
There are two main types of electrostatic charge neutralizers:
• Passive electrostatic charge neutralizers ( eliminators )
A passive neutralizer consists of one or a series of electrically grounded conductive electrodes of small radius of curvature. The electrodes are mounted in the electric field associated with the electrostatic charge on the powder particles. In this case, corona discharges necessary for producing ions are created by the electrostatic charges to be neutralized. The intensity of the neutralizing ions depends on the magnitude of the electrostatic charge on the powder particles.
Passive neutralizers are relatively inexpensive and are very often simple to use. However, they are not
capable of completely eliminating the electrostatic charges from the surfaces. This is because in passive
eliminators the electric field producing the corona discharge is created by the charges to be neutralized and the discharge occurs only if the charge density on the powder particles is high enough.
• Active electrostatic charge neutralizers ( eliminators )
An active neutralizer consists of one or a series of conductive electrodes of small radius of curvature that are connected to a high-voltage power supply. In this case, the corona discharges (neutralizing ions) are created by an external power supply and therefore do not rely on the electrostatic charge on the powder particles to function.
Properly designed, positioned, and maintained active eliminators can therefore completely remove the problem charge from the powder particles. The high-voltage power supply is very often AC but on occasions when the polarity of the problem charge is known a DC power supply could be utilized.
For effective and safe electrostatic charge neutralization, the following points should be taken into consideration:
• While electrostatic charge neutralizers eliminate the electrostatic charge quickly, it is not instantaneous. Very highspeed processes, such as pneumatic conveying, may outpace the eliminator’s ability to neutralize the electrostatic charge effectively. The speed of the charged particles should be taken into consideration when selecting/designing
the neutralizer,
• Ionized air has little persistence; the ions “recombine” quickly and revert to normal air molecules. Effective neutralization requires quick delivery of the ions to the charged particles, and
• Special care should be taken to ensure that the electrostatic charge neutralization device does not become a source of ignition.
Chilworth Technology has extensive expertise and testing facilities for the design, modification, laboratory testing and installation of electrostatic charge neutralization systems for the elimination/control of processing/handling problems associated with the electrostatic charging of powders. Our recent projects have involved successful resolution of electrostatically caused problems such as powder agglomeration, packing density variations, flowability, and clogging of hoppers and conveying pipes.
Control of Unwanted Electrostatic Charges from your Powder
To effectively control unwanted electrostatic charges from powders, the following information would be required:
(a) Electrostatic charging rate ( Chargeability ) of the powder
(b) Conductivity ( Volume Resistivity ) of the powder
(c) Sensitivity of the powder to ignition by electrostatic discharges ( Minimum Ignition Energy )
(d) Material of construction of the powder processing/handling equipment (e.g. metal or plastic)
(e) Powder flow rate, and
(f) Process conditions such as temperature, pressure, presence of inert gas, etc.
The above information would be utilized to design/select the appropriate type of neutralizer ( passive or active, AC or DC ), magnitude and polarity of the required neutralizing current, required number of neutralizers, material of construction of the neutralizers, and location(s) where the neutralizer(s) should be mounted.
References:
? D.K. DAVIES and V. EBADAT, Electrostatic problems in powder handling. ImechE 1991 C418/028
? T. HORVATH and I. BERTA, Static Elimination. Research Studies Press, A Division of John Wiley & Sons, Ltd.
How Can Chilworth Technology, Inc. Help You?
Chilworth Technology has extensive laboratory testing facilities and a group of highly qualified electrostatic and Process Safety Specialists who can help you with all aspects of design, modification, laboratory testing, and installation of electrostatic charge neutralization systems for the elimination control of processing/handling problems associated with the electrostatic charging of powders. If you have any questions regarding this article or any other process safety issues, please contact Dr. Vahid Ebadat at https://edir.bulk-online.com/profile...technology.htm ■