Page 60 - Industrial Plants

submicronic and ultrafine particles, for both electrostatic

“A new wet electro-filter technology precipitators (ESP) and wet scrubbers (WS). A new wet
called “WES” has been developed electro-filter technology called “WES” has been
in recent years and is being tested developed in recent years and it is being tested on a
on a pilot scale for the abatement of pilot scale for the abatement of submicronic particles.
submicronic particles

Electrostatic precipitators (ESPs) have been widely WES as a combination of ESP

used since the middle of the 20 century. In the ESP, and WS
th
the dust suspended in the gas is electrically charged as
the effect of a high-intensity electric field. The charged Since 2013, the Environmental Division of Boldrocchi
particles deviate towards the collection electrodes, as S.r.l. is performing intensive research and development
the effect of the electric field. The greater the surface activities to develop innovative and high-effective
charge induced on the particles, the greater the electric technologies for the removal of submicronic and
force that moves the particle towards the collector, ultrafine dust. The company is developing a Wet
hence increasing the probability of capture. Oils, Electrostatic Scrubber (WES) in partnership with the
particles with electrical resistivities and densities University of Naples Federico II.
outside of the optimal range, as well as particles finer The WES synergically uses the main points of strength
than 500 nm are hard to capture in an ESP. of ESP and WS, i.e. the effective use of the electric field
Spray towers, or “Wet Scrubbers” (WS) represent the to move particles regardless of their low inertia and the
most versatile strategy for the combined abatement of availability of a distributed collector that is unaffected
particulates, oils and acids. The particle capture, in by the physical nature of the particles. In a WES unit,
WS, is based on the hydrodynamic interception of the gas enters first a section called Particle Charging
particles against the droplets, which allow high particle Unit (PCU), where it is exposed to an optimised corona
abatement efficiency for particles having enough discharge at medium electric potential (15-20 kV). Low
inertia, like those in the micrometric range or for current intensity micro-discharges, of the order of nano
nanometric particles subjected to an appreciable or microamperes, increase the surface charge of the
Brownian motion. Particles that impact the liquid particles that are passing through the PCU. The
droplets, coming from the sprays, are most likely particles charging is optimised to achieve the desired
trapped in the liquid phase, hence cleaning the gas. particles charging in a compact unit, with a residence
The most important parameters for WS design are the time that is 4-5 times smaller than the ESP.
drops size and velocity and their volumetric density, The charged particles leaving the PCU enters a contact
which determines the probability that particles in the chamber, where they meet charged droplets that are
gas come into contact with the droplets. As a generated by electrified sprays (Droplets Charging Unit,
consequence, the design of the contact chamber and DCU). Further to the directional interception and the
the spray properties are thus critical parameters for WS inertial impact, another phenomenon contributes to the
design. capture of the particles in the WES: if particles and
The capture efficiency decreases significantly for droplets have opposite charges, they are electrically

Fi gure 2 - Block
Diagram of a closed-
loop configuration of
a horizontal cross-
flow closed-loop WES
unit

INDUSTRIAL PLANTS - September 2021
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