The separation of particulates in a fluid can be efficiently accomplished using electrical forces, the main criterion being the fluid must have insulating properties so an electric field can be superimposed across it. Electrostatic separation is done by initially charging the particulates or contaminants and then moving and concentrating them at the electric field boundary.
Electrical separation was first demonstrated by Hohlfield in 1824 by clearing a smoked-filled jar with an electrified point. In 1884, Oliver Lodge attempted to collect lead fume using an elementary form of electrostatic precipitator, energized from a Voss machine driven by a steam engine. It was not until the early 1900’s that satisfactory means of electrical energization enabled various workers to install plants to collect sulphuric acid mists. During the 1920’s, the use of electrostatic precipitators became firmly established across all industries for the collection of both solid and liquid particulates.
In industrial applications, there are generally two arrangements:
The horizontal-flow plate type—where a series of parallel plates, spaced up to 400 mm, form the gas passages and the discharge electrodes are insulated and hanging centrally between them.
The tube type precipitator—where the gases pass upwards through vertical tubes measuring up to 250 mm dia., with the discharge electrode taking the form of a coaxial element.
Because of its higher potential separation efficiency, having several stages/precipitator fields in series, the plate precipitator has largely replaced the tube type for dry particulates. Nowadays precipitators capable of treating 1,000 m3/s of gas, removing at least 99.8% of the particulates from power plants are required to meet current legislation. These units are energized from silicon rectifiers having microprocessor-based thyristor control devices to optimize maximum voltage, and hence performance, at all times.
Where precipitators are used for mist/droplet collection, the vertical flow unit is ideally suited since the collected product is self-draining. Efficiencies in excess of 99% can be obtained from a single tube length.
The discharge electrode energized by a high negative potential, up to 70 kV depending on plant configuration, causes local ionization of the gas molecules as a result of high electrical stress on the sharp-edged electrode. The positive ions are immediately collected by the electrode while the negative ions escape into the space charge region, moving towards the positively-charged collector. As the gas-borne particles pass through the space charge area, they receive a negative saturation charge in around 0.1 second and, as charged particles, move under the influence of the field to be collected on earthed plates.
In order for the process to continue effectively, particulates which arrive at the collectors need to be periodically removed. Typically this is done by simple impact rapping, which shears the agglomerated product, which then falls into the hoppers. The frequency/intensity of the rapping blows is regulated by the thickness and type of deposition. To maximize corona production, the discharge electrodes are similarly cleaned to maintain their sharp edges, and hence their emission characteristic.
For gases which are close to dew point or dusts which are particularly adhesive, wet-type precipitators are used where the deposited material is water-washed from the electrode system in the form of a slurry.
For particles having a diameter in excess of 2 microns, the charge is acquired by ionic collision, whereas for particles less than 0.2 microns, the charge is acquired by diffusion processes. A typical fractional efficiency relationship, [McEvoy (1986)] is illustrated in Figure 1. The dip in the 0.5 micron range follows classical charging theory; then once diffusion charging becomes effective, efficiency begins to recover.
In spite of precipitators being widely used throughout industry for almost a century, the size of unit to meet a specific duty still cannot be derived from first principles. Sizing is based on evaluated precipitation constants, as determined from similar processes. Initial work by Deutsch (1922) has developed an exponential relationship between efficiency and contact time, i.e., plant size. More recently, because of the higher efficiencies now required, to correctly size the plant it is necessary to modify the Deutsch relationship. The equation currently used is that of Matts and Ohnfeldt (1963), incorporating the precipitation constant wk, which is derived/calculated from practical measurements.
ni = a factor ~0.5 ,
= Gas Flow m3/s
A = collector plate area m2
wk = Precipitation constant for the particulate m/s.
Because of the above exponential relationship, it is important that gas distribution throughout the plant is uniform. In practice, a 15% RMS standard of deviation is accepted as the norm [Lloyd (1988)]. Operating velocities are typically in the range 1.0–2.0 m/s.
The most important physical property of the particle, with regard to precipitation, is its electrical resistivity. As resistivity increases above 109 ohm-m, the precipitation constant wk decreases, resulting in increasing plant size requirements. A typical relationship between relative plant size and electrical resistivity is shown in Figure 2.
Efficiency problems of existing plant can be overcome when handling particulates having resistivities > 1010 ohm-m resulting from fuel changes. This can be achieved by chemical conditioning methods, such as sulphur trioxide and ammonia injection, or by modifying the energizing wave form to intermittent DC/pulse modulation or by pulse charging techniques.
In summary, the advantages of electrostatic separation are:
Separation efficiencies of 99.9% + are attainable.
High efficiency possible for particles of 0.01 micron.
Designs available for temperatures up to 850°C.
Operational pressure range, minus 0.2 to plus 20 bar.
Low overall pressure loss, typically < 20 mm wg.
Low power requirement < 500 W/m3/s for 99.5% efficiency.
Material can be recovered in its original form.
Equipment has low maintenance and high reliability.
With the empirical nature of precipitation and the large number of factors which affect sizing, it is recommended that anyone contemplating using this technique approach recognized manufacturers and authorities for technical advice.
In the hydrocarbon industry, electric coalescers, although slightly different in form to electrostatic precipitators, are used to remove suspended liquid droplets from many feed stock and product lines. With coalescers, however, friction and contact rather than ionic particle charging occurs. In moving to the field boundary, increased concentration and reduced surface tension results in particle coalescence. When sufficient mass is achieved at the boundary, it will fall out of the mainstream fluid under the influence of gravity. Again, field contact time is important to achieve effective separation. While electrostatic precipitators are usually DC-energized, both AC and DC energization can be found with coalescers, which are often applied for desalination of crude oil and hydrofiners on diesel and similar fuels.
Darby, K. (1971) The use of electrostatic forces for the separation of suspended materials in gases and liquids. Symposium on Electrochemical Engineering. University of Newcastle upon Tyne. April.
Deutsch, W. (1922) Ann d physik. 68: 335.
Lloyd, D. A. (1988) Electrostatic Precipitator Handbook. Adam Hilger. Bristol & Philadelphia.
Matts, S. and Ohnfeldt, P. O. (1963) Flakt Review. 64.
McEvoy, L. M. et al. (1986) The collection of fine particulates in power plant electrostatic precipitators. EPA/EPRI Symposium, New Orleans, February.
Rose, H. E. and Wood, A. J. (1956) An Introduction to Electrostatic Precipitation in Theory and Practice. Constable & Co. London.
White, H. J. (1963) Industrial Electrostatic Precipitation. Addison Wesley and Pergamon Press, New York.