Air Pollution Control – Design Considerations:

General Objectives:

• Background for equipment design

• To be conversant with equipment manufacturers

• Understand expectations for normal operation

Specific objectives & expectations:

• Develop models for air pollution control equipment and implement these models in spreadsheet-based calculations.

• Capability and reliability of these models will vary depending on the system complexity, but factors affecting equipment performance should be evident.

Unit operations of interest:

• Gravity settling chambers (GSC)• Cyclones• Electrostatic precipitators (ESP)• Filtration units – baghouse filters• Wet scrubbers

Basic input information:

• Gas properties – temperature, pressure and composition.

• Derived gas properties – density, viscosity, continuum conditions () and dew point.

• Particulate properties – size, shape and density.• Derived particulate properties – aerodynamic drag

and behaviour in non-continuum conditions.

Basic assumptions:

• Particulates are transported to the collection units in the process – requires minimum transport velocities and “pick-up” velocities.

• Gas streams, even with heat recovery systems, will probably be above the dew point temperature of the gas (this reduces corrosion in equipment).

Single Collection Systems:

Collectionsystem,

fP

Captured stream

mp

Qf

mp

InputQf

mp(1 - )

Penetrating stream

A single collection system will process a total gas flow rate, Qf (m

3/s), and particulate loading, mP (g/s). It will be

characterized by a collection efficiency, (mass fraction-weighted for all particle sizes), and pressure drop, Pf .

Overall collection efficiency (mass-fraction weighted):

Overall collection efficiency is a function of:

(i) Particle size mass fraction, and

(ii) Grade efficiency ( for each particle size)

Multiple collection systems in series:

Collectionsystem 2

12 mp(1 - )

system 1Collection

mp

Qf Input

(1 - ) mp

11

Captured stream

mp1

fP

Penetrating stream

(1 - )2

Captured stream

fP

2mp

(1 - )1

• The overall collection efficiencies for the individual systems will not be equal and generally, 1 > 2 (and this inequality continues for additional

units in the series sequence). The reason for this is that larger, easier to remove particles are captured in the upstream systems. • A drawback for this series arrangement is that the pressure drops add, increasing the overall operating costs.

If a single system does not give adequate collection efficiency, we may use systems in series:

Multiple collection systems in parallel:

When pressure drop considerations are important, the inlet flow stream can be split into parallel streams. Since Pf ~(Qf)

2, this can lead to substantial

cost savings.

Collectionsystem,

m /2p

m /2p

Qf

mp

Q /2f

system,Collection

m /2p

Q /2f

m /2p

fP

Qf

mp(1 - )

Increased efficiency can be realized when the collection efficiency increases with lower gas flow rates through the units (true for most collection devices except cyclones)

Multiple collection systems in series and parallel: fP

mp

Qf

mp

mp

Qf

(1 - )

A combination of series and parallel arrangements might be used to give optimum collection efficiency and pressure drop. This method often involves components of the same type of collection device (or elements of the same type of device). The parallel lines can be turned off (or isolated) in a regular timed sequence for periodic cleaning (baghouse filtration units) or routine maintenance.

Gravity Settling Chambers – Chapter 7

Dust collection hoppers

Gas Inlet Gas

Outlet

Relies on the terminal velocity of a particle to settle out of a gas stream …

A conventional design based on flow cross section expansion and dust collection hoppers.

Novel designs based on multiple chambers in parallel

L

H

Design analysis similar to a problem previously examined (pg. 34-36)

pUx

s

s

y

Uf

pUx

Up

x

pUy

L