Particle Control Technologies

Lecture notes adapted from Prof. Dr. Benoit Cushman-Roisin

Thayer School of Engineering at Dartmouth

Design Criteria• Design of a system which remove solid or liquid

particulate matter from a gaseous medium• Chacacteristics of gaseous medium and

particulate matter to consider in the design:

• Size T,P,Q,• Chem. Composition Chem. Composition• Resistance Pressure Drop

PM Control Device

Collection Efficiency• Considering the wide range of size of

particulates, efficiency will be different for each size.

• The overall efficiency (can be calculated on a basis of total number (or mass) of particles

• Generally regulations are written based on mass, and efficiencies are calculated on mass basis.

Collection Efficiency

• Efficiencies calculated on mass basis: i

ei

i

ei

L

LL

M

MM

: overall collection efficiency (fraction)

Mi: total mass input rate (g/s or equivalent)

Me: total mass emission rate (g/s or equivalent)

Li: particulate loading in the inlet gas to the device (g/m3)

Le:particulate loading in the exit gas stream, (g/m3)

Collection Efficiency

• When the particulate size distribution is known, and the efficiency of the device is known as a function of particle size, the overall collection efficiency can be calculate:

jjm where

j: collection efficiency for the jth size

mj: mass percent of particles in the jth size

Example 3.1 from the book

1. Gravity Settler

2. Cyclones

3. ESP

4. Filters and Baghouses

5. Wet Scrubbers

PM Control Devices

Settling Chamber

• Efficient for particles with diameter of 10-50 m (depending on its density)

• Velocity through chamber < 0.3-3 m/s (to prevent reentrainment)

V

H

L

Settling Chamber

• Settling time < transit time through chamber

• t = H/vt = L/v

v

H

L

Lw

Qdv

L

vHdv

pt

pt

)(

)(

Settling chambers are cheap to build and operate but not preferred due to their large space requirement

Settling Chamber• Assuming unit density sphere at STP, vt and chamber Lw

are tabulated below: Assumed flow rate Q = 150 m3/min

Dp (um) Vt (m/s) Required Area

0.1 8.6 (10)-7 3 km2

0.5 1.0 (10)-5 0.25 km2

1.0 3.5 (10)-5 71000 m2

5 7.8 (10)-4 3200 m2

10 3.1 (10)-3 810 m2

Lw

Qdv pt )(

Settling Chamber

• Baffled Settling Chamber– Large particles can not

make sudden direction change and settle into dead space of chamber

– Baffle chambers are used as precleaners

Cyclones

• :

Cyclones

• :

Cyclone Geometry

Cyclone Geometry

Cyclone Theory

Cyclone Theory

Cyclone Theory

2/3

2

D

mVVddragforce itp

Cyclone Theory

Collection Efficiency

Collection Efficiency

(i) increase Vt (expensive, since DP Vt2, as we will see in the next slides

Collection Efficiency

Collection Efficiency

Pressure Drop

K: a constant depends on cyclone configuration and operating conditions. Theoretically K can vary considerably but for air pollution work with standard tangential-entry cyclones values of K are in the range of 12 to 18

Cyclone pressure drops range from about 0.5 to 10 velocity heads (250 to 4000 Pa)

Cyclone Analysis

Example

Example

Conventional Type (No:3) N=(1/H) (Lb+Lc/2) = (1/0.5)(2+2/2)=6

•Vi=Q/WH =150 /(0.25*0.5) =1200 m/min 20 Vi=20 m/s

ExampleCalculate efficiency for each size range witch dpc = 5.79 um:

2)/(1

1

pjpcj dd

Example 4.5

Example 4.5

Example 4.5

ESP

ESP

ESP Geometry

ESP Theory

Corona Power vrs Efficiency

ESP Theory

ESP THEORY

ESP Theory

ESP Theory

ESP Theory

ESP Theory

ESP Theory

ESP Theory

ESP Theory

ESP Theory

ESP Theory

Efficiency

Efficiency

Effect of Resistivity

ResistivityResistivity (P) is resistance to electrical conduction and can vary widely

P of a material is determined experimentally by establishing a current flow through a slab (of known geometry) of the material

P = (RA/L)=(V/i)(A/L) [ohm-cm]

R:resistance, ohm

A: area normal to the current flow, cm2

L:path length in the direction of current flow, cm,

V: voltage, i: current, A

Resistivity

Resistivity

Sparking

Internal Configuration• Internal configuration design is more art than

science• The even distribution of gas flow through the

ducts is very important to the proper operation of an ESP

• The number of ducts (Nd) is equal to one less than the number of plates (n-1)

Nd = Q/uDH (eq 5.15)

u: linear gas velocity (m/min)D Channel width (plate separation), mH: plate height, m

Internal Configuration• At the start of the design, use 5.15 to

estimate Nd by assuming a value for H and choosing representative values of u and D

Typical Values for the Fly-Ash ESP

• Table 5.1

Parameter Range of Values

Drift velocity 1-10 m/min

Channel (Duct) Width, D 15-40 cm

Specific Collection Area Plate area/Gas Flow

0.25-2.1 m2/(m3/min)

Gas velocity u 1.2-2.5 m/s

Aspect Ratio (R)

Duct Length/Plate Height

0.5-1.5

Corona Power Ratio Pc/Q 1.75-17.5 W/(m3/min)

Corona Current Ratio (Ic/A) 50-750 A/m2

Plate area per electrical set As 460-7400 m2

Number of electrical sections 2-6

Internal Configuration• The overall length of the precipitator (Lo)

– Lo=NsLp + (Ns-1)Ls + Len +Lex

• Lp:: length of plate

• Ls: spacing between electrical sections (0.5-1.0 m)

• Len: entrance section in length (several meters)

• Lex:exit section in length (several meters)

• Ns: number of mechanical fields

Ns ranges between 2 and 6.

Ns=RH/Lp R is the aspect ratio

Internal Configuration• When the numbers of ducts and sections

have been specified, the actual collection area (Aa) can be calculated as:

Aa=2HLpNsNd

• During the design process several plate sizes and numbers of ducts are tried until one combination is found such that Aa is equal to the required collection area.

Collection Efficiency vrs Particle Diameter

An Example

Example

Example

POWER REQUIREMENT

POWER REQUIREMENT

k: an adjustable constant in the range of 0.5-0.7 for we in ft/sec and Pc/A in W/ft2

Problem 5.10Provide a reasonable design for a 99.4% efficient ESP treating 30,000 m3/min of gas. The dust has a resistivity of 7.1 (10)10 ohm-cm. Specify the total plate area, channel width, number and size of plates, number of electrical sections (total and in the direction of flow), and total corona power to be supplied, and estimate the overall dimensions.

SOLUTION

SOLUTION

SOLUTION

SOLUTION

Video Demonstration on Electrostatic Precipitation

http://www.youtube.com/watch?v=y5w0IGuLR3A

http://www.youtube.com/watch?v=9kauc7OmmLQ

http://www.youtube.com/watch?v=x5YFK8mmeRQ

http://www.youtube.com/watch?v=BdRk3op2zpE

http://www.youtube.com/watch?v=iUXHzYLgrB0