Electrostatic Precipitator
❑ Can be used to clean the flue gases from the largest power plants as well as in small household air cleaners.
❑ Electrostatic precipitators can treat large volumetric flow rates of gas at relatively low pressure drops with
very high removal efficiencies. However, electrostatic precipitators are expensive and are relatively inflexible
to changes in process operating conditions
❑ Working Principle: Particles are charged, then an electric field is imposed on the region through which the
particle-laden gas is flowing, exerting an attractive force on the particles and causing them to migrate to the
oppositely charged electrode at right angles to the direction of gas flow.
❑ Electrostatic precipitation differs from mechanical methods of particle separation in that the external force is
applied directly to the individual particles rather than indirectly through forces applied to the entire gas
stream (e.g., in a cyclone separator).
❑ Collected liquid particles flows down the electrode by gravity and removed at the bottom of the device, and
solid particles collected on the electrode can be removed by rapping (vibrating) the electrode.
❑ Particle charging is achieved by generating ions by means of a corona established surrounding a highly
charged electrode like a wire.
❑ The electric field is applied between that electrode and the collecting electrode. If the same pair of
electrodes serves for particle charging and collecting, the device is called a single-stage electrostatic
precipitator.
❑ In a two-stage electrostatic precipitator, separate electrode pairs perform the charging and collecting
functions.
Electrostatic Precipitator
Generation of Corona:
Positive Corona:
•
Electron Avalanche
Negative Corona:
•
Absorption of free electrons by gas molecules (1015 ions/m3)
•
Absorbing gases such as SO2, CO2, H2O and O2
•
O3 is the by-product so it can not be used in domestic applications
•
Negative Corona is used industrial applications like thermal power
plants and positive Corona in domestic applications
Collecting Electrode:
▪
Particle deposits on the precipitator collection surface must possess at least a small degree of electrical conductivity in order to conduct the ion currents
from corona to the ground
▪
Minimum conductivity required is 1010 (ohm cm)-1
▪
Dust particles resistivity (108 ohm cm) and for fly ash 108 - 1013 ohm cm
▪
Can not be used for BC aerosols (resistivity is 10-3 ohm cm)
Electrostatic Precipitator
Plate precipitator: This is the most basic precipitator type that consists of rows of thin vertical wires and stack of vertically arranged large flat metal plates
that are placed at a distance of 1 cm to 18 cm apart.
Dry electrostatic precipitator: This precipitator is used to collect pollutants like ash or cement in a dry state.
Wet electrostatic precipitator: This precipitator is used to remove resin, oil, tar, paint that are wet in nature. They are more efficient than dry ESPs.
Tubular precipitator: This precipitator is a single-stage unit consisting of tubes with high voltage electrodes that are arranged parallel to each other such that
they are running on their axis. They find applications where sticky particles are to be removed.
Advantages of electrostatic precipitator:
❑ Durability of the ESP is high.
❑ Used for the collection of both dry and wet impurities.
❑ Low operating costs.
❑ Collection efficiency of the device is high even for small particles.
❑ Can handle large gas volumes and heavy dust loads at low pressures.
Disadvantages of electrostatic precipitator:
❑ Can’t be used for gaseous emissions.
❑ Space requirement is more.
❑ Capital investment is high.
❑ Not easily adaptable to change in operating conditions.
Tubular Precipitator
Plate Precipitator
Particle Charging
Trajectory of ions
Total number of charges by field charging during time t
with ion concentration Ni
Repelling like charged ions
for partially charged
particles
Charge buildup
“Saturation charge”
Mean thermal speed of ions
Hinds 1999, Aerosol Technology
Particle drift velocity
Hinds 1999, Aerosol Technology
Electrostatic Precipitators
1) Low pressure drop (~ 100Pa)
2) High for even dp < 1µm
Two step process
(1)Particle are electrically charged
(2)Electric field is applied to drift across the stream
High Voltage Wires
Gravity Settler → 20 µm
Cyclone → 23 µm
ESP → 1 µm
Grounded plates (Collector)
U
d
L
~ 104 V is applied
So, ion pair are created that collide with air molecules ⇒ More ions
Particle acquire charge through diffusion charging or field charging
Large particles
Submicron particles
Large particles are more efficiently charged, small particles acquire less charge
U
Ve
X=0
Mass Balance
Vol/s
d
Ve in direction
of depth
X X+ΔX
X=L
#/wt
𝑼𝑾𝒅 × 𝑪 𝑿 = 𝑼𝑾𝒅 × 𝑪 𝑿 + ∆𝑿 + 𝑽𝒆 𝑾 ∆𝑿 × 𝑪( 𝑿 +
∆𝑿
𝟐)
W= width of plate, normal to flow
Advection flow in
F Balance
𝑭𝒆 = 𝒒𝑬 = 𝑭𝒅 =
Advection flow out
𝟑𝝅𝝁𝝁𝒅𝒑 𝑽𝒆
𝑪𝒐𝒓𝒓
Electrostatic drift out
𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟
= (1 + 𝐴𝑑λ )
𝑝
(1)
𝑉𝑒 = 𝑞𝐸.𝐶𝑜𝑟𝑟
3𝜋𝜇𝑑
𝑝
𝑉𝑒 = 𝑉𝑒 (𝑑𝑝 )
Then (1)
𝑪 𝑿 + ∆𝑿 −𝑪(𝑿)
∆𝑿
𝑑𝐶
𝑑𝑥
=−
𝐶 𝑋 =𝐶 0
η=1−
𝑆𝐶𝐴 =
𝒄(𝑿 +
𝑉𝑒
= − 𝑈𝑑
𝐶
C= C(0) at X=0, So
SCA * Ve >> 1
𝑽𝒆
𝑼𝒅
𝐶 𝐿
𝐶 0
𝐿
𝑈𝑑
∆𝑿
𝟐
)
𝑓𝑜𝑟 𝐿𝑖𝑚 ∆𝑥 → 0
𝑉𝑋
−( 𝑒 𝑈𝑑 )
𝑒
= 1 − exp(−𝑆𝐶𝐴 × 𝑉𝑒 )
= 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐶𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝐴𝑟𝑒𝑎
implying
For
𝑳
good 𝜼, 𝑼𝒅
>> 𝑽𝒆 -1
Deutsch Equation
Dp = 0.3 µm, q= 10e =10 × 1.6 × 10-19C,
Typical E = 2 × 103 V cm-1,
Ve = 0.0099 ms-1 = 1cm s-1
Ve> Vsettl, gravity ⇒ SCA (L/Ud) > 1s cm-1
Design values -0.2- 2 S cm-1
U= 1.2- 2.5 m/s
d= 15- 40 cm
Compute η for an ESP with ε =6 and L/Ud = 0.2 min/ft
𝑞 = 3𝜋
6
8
𝐶
8.85 × 10−12 𝑉.𝑚
(10−6 )2 300 𝐾𝑉
𝑚 = 300𝑒
qE=3𝝅𝝁𝒅𝒑 𝑽𝒆
𝑞𝐸
𝑉𝑒 =
3𝜋𝜇𝑑𝑝
300 × 1.6 × 10−19 × 300 × 103 × 1.17
=
3 × 3.14 × 1.8 × 10−5 × 1 × 10−6
9 × 104 × 1.6 × 1.7 × 10−19
=
9 × 1.8 × 10−11
10−4 𝑚
𝑐𝑚
~1.7 ×
~0.01
𝑠
𝑠
Dia
𝜼
0.1
0.5
1
0.12
0.48
0.73
3
5
0.98
0.998
η = 1 − exp(− 𝑉𝑒 . 2 𝑚𝑖𝑛/𝑓𝑡)
= 1 − exp −1.7 × 10−4 . 2×60
3
−4
= 1 − exp(1.7 × 10 . 4)
Fabric filters
❑ A variety of filters operate on the principle that the particulate-laden gas is forced through an assemblage of collecting elements, such as a fiber or a filter
mat. As the gas passes through the assemblage, particles accumulate on the collectors.
❑ Fabric filters tend to have very high efficiencies but are expensive and are generally limited to dry, low-temperature conditions.
❑ Mechanisms of particle collection by a single fiber: Interception, Brownian diffusion, Impaction, Gravitational settling and Electrostatic force
Baghouse filter
Fabric filters offer the following advantages:
(1) They can achieve very high collection efficiencies even for very small particles;
(2) They can be used for a wide variety of particles;
(3) They can operate over a wide range of volumetric flow rates; and
(4) They require only moderate pressure drops.
Limitations of fabric filters are:
Similar to the ones used in
conventional vacuum cleaner
(1) Operation must be carried out at temperatures lower than that at which the fabric is destroyed, or its life is shortened to an uneconomical degree;
(2) Gas or particle constituents that attack the fabric or prevent proper cleaning, such as sticky particles difficult to dislodge, are to be avoided; and
(3) Baghouse filters require large floor areas. The advantages of fabric filter baghouses clearly outweigh their limitations, as they currently represent close to
50% of the industrial gas-cleaning market.
Filtration
Df = fiber diameter
Gamma = E *t/4df
Single fiber efficiency can be used to compare filters with different thickness as it is independent of filter thickness “t”.
Here, ER is Efficiency due to Interception
EI is Efficiency due to Inertial impaction
ED is Efficiency due to Diffusion
EDR is enhanced Efficiency due to Interception
of diffused particles
EG is Efficiency due to Gravitational settling
Hinds 1999, Aerosol Technology
Deposition mechanisms
These 5 deposition mechanisms are responsible for all types of particle deposition such as deposition in lung, sampling tube or in air purifiers.
1. Interception:
❖ Particle is small and its inertia is not strong enough to cause the
particle to continue in a straight line.
❖ Follow streamline that happens to come within one particle radius
of the surface of a fiber and moves unless it comes in contact with
filter.
❖ Dominant for dp>100 nm and increases with dp.
2. Inertial impaction
❖ Particles deviate from their initial fluid streamline, due to their inertia,
as the fluid curves to flow around the filter structure.
❖ Dominant mechanism for larger particles (particle mass), and
higher fluid velocities
❖ Dominant for dp>300 nm.
3. Diffusion
❖ Particles collide with the filter structure due to their random
Brownian motion, thereby altering the particle trajectory around the
filter structure
Dominant at low velocity sampling and dp <100 nm.
4. Gravitational settling
❖ Large particles (dp >10 mm), at relatively low velocities,
can be captured via gravitational sedimentation
5. Electrostatic attraction
❖ Electrostatic deposition happens when the electrical charges on
either the particle or the filter, or both, create attractive electrostatic
forces of sufficient magnitude to attract the particle to the filter surface
Hinds 1999, Aerosol Technology
Filtration efficiency (%)
Filtration efficiency as function of diameter
Particles of this size are too small
to be directly intercepted by fibers
and too large to undergo
substantial Brownian motion.
governed by aerodynamic diameter
Particles in the range 0.05–0.5 µm
will pass through the filter more
easily than others.
governed by physical diameter
Hinds 1999, Aerosol Technology
Selection of filters
Hinds 1999, Aerosol Technology
Other Filter Types
Different types of substrate:
1.
Fibrous filters : Highly porous, low cost (e.g. Whatman filters); more depth; Made of cellulose or glass fiber
2.
Porous membrane: Expensive, controlled pore size, less depth; Mesh of interconnected fiber like strands; Uniform pore size (0.3 – 2 µm); Made of nylon,
PVC, cellulose ester etc.
3.
Capillary pore membrane: Very thin (10 µm) polycarbonate sheets; Cylindrical holes of uniform diameter; Pores are parallel to airflow. Pores act as tubes
hence diffusion filtration happens.
4.
Combination or composite filters : Combination of fibrous and porous membrane filters.
Surface Filters
Screen having holes of 0.75 in in dia. Collect a layer of Ping Pong balls then cherries
between spaces of ball, then peas, then rice and sand. This is called cake.
Relation for Laminar flow
𝑽𝒔 = 𝑸𝑨 = −
∆𝑷
𝝁
𝑲
∆𝒙
(1)
K= Permeability of filter medium
determined experimentally
𝑽𝒔 =
𝑷𝟏 −𝑷𝟐
𝝁
𝑲
∆𝒙 𝑪𝒂𝒌𝒆
=
𝑷𝟐 −𝑷𝟑
𝝁
𝑲
∆𝒙 𝑭. 𝒎𝒆𝒅𝒊𝒂
(2)
Solving for P2
𝑷𝟐 = 𝑷𝟏 − 𝝁𝑽𝒔
∆𝒙
𝑲 𝑪𝒂𝒌𝒆
So, 𝑉𝑠 =
Cake resistance
= 𝑷𝟑 + 𝝁𝑽𝒔
𝑃1 −𝑃3
∆𝒙
∆𝒙
𝜇[
+
𝑲 𝑪𝒂𝒌𝒆
𝑲
]
𝒇𝒎
.
=𝐴
∆𝒙
𝑲 𝒇. 𝒎
𝑄
𝑓𝑖𝑙𝑡𝑒𝑟
Cloth resistance
(3)