Air Pollution Control – Design Considerations by zie20290


									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
• 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
  • Derived gas properties – density, viscosity,
    continuum conditions (l) 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:
                        P f
   Q   f            C o lec ton
                         l i
                                      Q   f
           npu t                                          a i
                                                  P ene tr tng
                    sys e , 
                         tm                           em
                                                   s tr a
   m   p                           ( - m
                                   1    )     p

                            t e
                      C ap u r d
                       s tr a
                       m      p

A single collection system will process a total gas flow
rate, Qf (m3/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:
If a single system does not give adequate collection efficiency, we may
use systems in series:
                    P f                          P f
Q       I pu t
        n               l  i
                 C o le c ton                  C o le c ton
                                                      l  i                      a i
                                                                       P e n e tr t n g
    f                 e
                 sy s t m 1                         e
                                               sy s t m 2                   e
                                                                         s tr am
m                     1        ( - 1 m
                                1      )             2
    p                                      p
                                                                     ( -  )( - 1 m
                                                                     1      1      )      p

                  C ap t r d                           ue
                                                C ap t r d
                   s tr am                          e
                                                 s tr am
                    m      p                2 ( -
                                               1             )
                                                             m   p

    • 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.
 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.
                                        P f
                        Q f /
                                         l i
                                    C o lec ton
                        m       2
                                /   sys e , 

       Q   f                                              Q   f
                                         m p /2
       m   p                                               
                                                        (1 - m) p
                        Q f /
                                         l i
                                    C o lec ton
                                    sys e , 
                        m   p

                                         m p /2

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:
                                      P f

             Q   f                                         Q   f

             m                                            (1 - m
                                                                )   p

                                      m     p

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

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

G as
 In e t
    l                                              G as
                                                   O u te t

                          l     i
             D u s t c o le c t o n h o p p e rs

A conventional design based on flow cross section
     expansion and dust collection hoppers.
Novel designs based on multiple
      chambers in parallel


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


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