Page 165 - Air Pollution Control Engineering

03_chap_wang.qxd 05/05/2004 12:48 pm Page 144
144 José Renato Coury et al.

Example 8
An initial survey of the air inside the production building at an industrial site revealed that
the ambient was loaded with the particulate matter they produced, which has a density of
3
2750 kg/m . The TSP was determined and a concentration of 298 mg/m was detected.
3
Also, the particle size distribution was measured with an apparatus that utilized Stokes
diameter as the operating principle. The distribution is presented in Table 23. After analy-
sis of the results, it was decided that a systematic monitoring of the PM concentration
2.5
would be carried out, and the AN 3.68 minicyclone was purchased, to work at a flow rate
of 28.1 L/min. Considering that the balance available has the sensibility of 10 −5 g, estimate
the minimum sampling time for the cyclone.
Solution
The first step is to transform the given diameter (Stokes) into the aerodynamic diameter
utilized in all monitoring equations. By definition, the Stokes diameter is the equivalent
diameter of a sphere that has the same terminal velocity as the particle. Therefore, the two
diameters can be related by

ρ
D = p D St (76)
ae
ρ
unit
where ρ is the unit density. Therefore, in this case, the aerodynamic particle diameter is
unit
D = 2.75 D = 1.658 D
ae St St

Table 24 lists the mean aerodynamic diameters for each particle size range. It can be noted
that the particles smaller than 2.5 µm correspond to the four smaller ranges. This means
that, according to Table 23, 15.6% of the TSP is constituted of particles smaller than 2.5
3
µm, which corresponds to a concentration of 46.5 µg/m , well above the maximum of 15
µg/m recommended by the EPA for an healthy ambient. This implies that some protection
3
measures need to be taken.
As far as monitoring is concerned, the next step consists of determining the mass con-
centration in the cyclone exit, once this is the mass of PM collected by the membrane
2.5
placed after it. This membrane is assumed to have 100% collection efficiency. The pro-
cedure here is similar to the one adopted in Example 2 in Section 2.5.1: the collection
efficiency of each range, η , is determined and multiplied by the size fraction x . Here, the
i i
efficiency is calculated utilizing Eq. (74) that, for the AN 3.68 minicyclone working at
28.1 L/min, can be written as (see Table 24)
    1    −0.6688 

  D + 0.1945ln 2 0.6688 − 1  − 2.33  
     ae       
 



η=  1 − 1 + exp      
   0.1945   
    
     


Table 24 lists the calculated efficiencies as well as the total mass collected, which was 91%.
The next steps are straightforward:
Concentration of PM in the cyclone exit = ( 1 0.91) × 298mg m = 26.82mg m 3
−
3
2.5